Hydrogen Bonding and Transfer in the Excited State (2 Volume Set)

Hydrogen Bonding and Transfer in the Excited State

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

Hydrogen Bonding and Transfer in the Excited State Volume I

Editors

Ke-Li Han State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

Guang-Jiu Zhao State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

Hydrogen Bonding and Transfer in the Excited State Volume II

Editors

Ke-Li Han State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

Guang-Jiu Zhao State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

This edition first published 2011 Ó 2011 John Wiley & Sons Ltd Registered office John Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book, please see our website at www.wiley.com The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The publisher and the authors make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including, without limitation, any implied warranties of fitness for a particular purpose. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for every situation. In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. The fact that an organization or website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or website may provide or recommendations it may make. Further, readers should be aware that Internet websites listed in this work may have changed or disappeared between when this work was written and when it is read. No warranty may be created or extended by any promotional statements for this work. Neither the publisher nor the authors shall be liable for any damages arising herefrom. Library of Congress Cataloging-in-Publication Data Hydrogen bonding and transfer in the excited state / editors, Ke-Li Han, Guang-Jiu Zhao. p. cm. Includes bibliographical references and index. ISBN 978-0-470-66677-7 (cloth) 1. Hydrogen bonding. I. Han, Ke-Li. II. Zhao, Guang-Jiu. QP517.H93E93 2010 572’.33–dc22 2010015107 A catalogue record for this book is available from the British Library. ISBN 9780470666777 Set in 10/12pt, Times by Thomson Digital, Noida, India Printed in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire

Contents

Editors’ Biographies

xv

Reviewer Comments

xvii

List of Contributors

xxxiii

Preface

xxxix

Volume I 1

2

3

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs Yun-an Yan and Oliver Ku¨hn 1.1 Introduction 1.2 Hydrogen Bonding and Nonlinear Infrared Spectroscopy 1.3 Correlated Vibrational Dynamics of an Adenine–Uracil Derivative in Solution 1.4 Conclusion Acknowledgement Appendix References Vibrational Energy Relaxation Dynamics of XH Stretching Vibrations of Aromatic Molecules in the Electronic Excited State Takayuki Ebata 2.1 Introduction 2.2 IR Spectra of 2-Naphthol and its H-Bonded Clusters in S1 2.3 VER Dynamics of Bare 2-Naphthol 2.4 VER Dynamics of H-Bonded Clusters of 2-Naphthol 2.5 Comparison of the cis ! trans Barrier Height Between S0 and S1 2.6 Conclusion References Hydrogen Bond Basicity in the Excited State: Concept and Applications Attila Demeter 3.1 Introduction 3.2 Experiment

1 1 3 9 22 23 23 24

29 29 30 31 31 36 37 37 39 39 40

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3.3 Results and Discussion 3.4 Summary Acknowledgements References 4

5

6

7

Solute–Solvent Hydrogen Bond Formation in the Excited State. Experimental and Theoretical Evidence Iulia Matei, Sorana Ionescu and Mihaela Hillebrand 4.1 Introduction 4.2 The Prerequisite Conditions for Hydrogen Bond Formation 4.3 Diagnosis Criteria and Quantitative Treatment of Hydrogen Bonds 4.4 Design of the Experiments 4.5 Theoretical Modelling of the H-Bonds 4.6 Conclusions References Electronic-Excited-State Structures and Properties of Hydrated DNA Bases and Base Pairs Manoj K. Shukla and Jerzy Leszczynski 5.1 Introduction 5.2 Ground-State Structures of Nucleic Acid Bases and Base Pairs 5.3 Excited-State Structures of Nucleic Acid Bases 5.4 Excited States of Base Pairs 5.5 Excited-State Dynamics and Non-Radiative Decays 5.6 Conclusions Acknowledgements References Insight from Singlet into Triplet Excited-State Hydrogen Bonding Dynamics in Solution Guang-Jiu Zhao and Ke-Li Han 6.1 Introduction 6.2 Theoretical Methods 6.3 Results and Discussion 6.4 Conclusion Acknowledgements References Probing Dynamic Heterogeneity in Nanoconfined Systems: the Femtosecond Excitation Wavelength Dependence and Fluorescence Correlation Spectroscopy Shantanu Dey, Ujjwal Mandal, Aniruddha Adhikari, Subhadip Ghosh and Kankan Bhattacharyya 7.1 Introduction 7.2 Solvation Dynamics in Nanoconfined Systems 7.3 Fluorescence Resonance Energy Transfer (FRET): lex Dependence

41 76 77 77

79 79 80 82 98 104 117 119

125 125 128 129 138 142 143 143 144

149 149 151 151 156 156 156

159

159 160 166

Contents

7.4 Excited-State Proton Transfer (ESPT) 7.5 Diffusion of Organic Dyes by Fluorescence Correlation Spectroscopy (FCS) 7.6 Conclusions Acknowledgements References 8

9

10

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12

Fluorescence Studies of the Hydrogen Bonding of Excited-State Molecules Within Supramolecular Host–Guest Inclusion Complexes Brian D. Wagner 8.1 Introduction 8.2 Hydrogen Bonding Involving Excited States of Fluorescent Probes in Solution 8.3 Hydrogen Bonding of Excited States of Included Guests 8.4 Conclusions References Hydrogen Bonding on Photoexcitation Debarati Dey, Manas Kumar Sarangi and Samita Basu 9.1 Introduction 9.2 Intermolecular Excited-State Hydrogen Bonding 9.3 Concluding Remarks References Effect of Intramolecular H-Bond-Type Interactions on the Photochemistry of Aza-Stilbene-Like Molecules Giampiero Bartocci, Ugo Mazzucato and Anna Spalletti 10.1 Introduction 10.2 Control of the Conformational Equilibria in the Ground State 10.3 Control of Radiative and Reactive Relaxation 10.4 Unusual Adiabatic Photoisomerization in the E ! Z Direction References Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces Rajib Kumar Mitra, Pramod Kumar Verma, Debapriya Banerjee and Samir Kumar Pal 11.1 Introduction 11.2 Materials and Methods 11.3 Results and Discussion 11.4 Conclusion Acknowledgements References Intermolecular Hydrogen Bonding in the Fluorescence Excited State of Organic Luminophores Containing Both Carbonyl and Amino Groups Ilijana Timcheva and Peter Nikolov 12.1 Introduction 12.2 Experimental 12.3 Results and Discussion 12.4 Conclusion References

vii

167 170 172 172 172

175 175 177 180 187 188 193 193 194 202 202 205 205 206 210 211 214 217 217 222 237 259 260 260 269 269 270 270 284 284

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13

Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds David Lee Phillips 13.1 Introduction 13.2 Experimental and Computational Methods 13.3 Hydrogen-Bonding Effects on the Excited States of Selected Phenacyl Model Compounds 13.4 Hydrogen-Bonding Effects on the Excited States of Selected Para-Hydroxyphenacyl Ester Phototriggers and the Role of Water in the Deprotection and Subsequent Reactions References

14

15

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17

Hydrogen-Bonding Effects on Intramolecular Charge Transfer Govindarajan Krishnamoorthy 14.1 Introduction 14.2 Polarity and Viscosity 14.3 Hydrogen Bonding with the Donor Moiety 14.4 Hydrogen Bonding with the Acceptor Moiety 14.5 Conclusion Acknowledgements References Chemical Dynamics in Room-Temperature Ionic Liquids: the Role of Hydrogen Bonding Souravi Sarkar, Rajib Pramanik and Nilmoni Sarkar 15.1 Photoinduced Electron Transfer in a Room-Temperature Ionic Liquid 15.2 Dynamics of Solvent Relaxation in Room-Temperature Ionic Liquids Containing Mixed Solvents Acknowledgements References Vibrational Spectroscopy for Studying Hydrogen Bonding in Imidazolium Ionic Liquids and their Mixtures with Cosolvents Johannes Kiefer 16.1 Introduction 16.2 Experimental Approaches 16.3 Hydrogen Bonding in Ionic Liquids 16.4 Potential, Challenges and Future Applications Acknowledgements References Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids: a Time-Resolved Fluorescence Study Luca Nardo, Alessandra Andreoni and Hanne Hjorth Tønnesen 17.1 Introduction 17.2 Experimental Set-Up and Data Analysis Methods 17.3 Results and Discussion 17.4 Conclusions References

287 287 288 289

302 310 313 313 317 318 320 327 327 327 331 331 335 339 339

341 341 342 345 349 349 350

353 353 363 366 373 373

Contents

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19

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21

Hydrogen Bonds of Protein-Bound Water Molecules in Rhodopsins Hideki Kandori 18.1 Introduction 18.2 Detection of Water Under Strongly Hydrogen-Bonded Conditions in Bacteriorhodopsin 18.3 Hydration Switch Model as a Proton Transfer Mechanism in the Schiff Base Region of Bacteriorhodopsin 18.4 Time-Resolved IR Study of Water Structural Changes in Bacteriorhodopsin at Room Temperature 18.5 Role of the Water Hydrogen Bond in a Chloride-Ion Pump 18.6 Strongly Hydrogen-Bonded Water Molecules and Functional Correlation with the Proton-Pump Activity 18.7 Conclusion Acknowledgements References Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives Carmen Carmona, Manuel Balo´n, Marı´a Asuncio´n Mun˜oz, Antonio Sanchez-Coronilla, Jose Hidalgo and Emilio Garcı´a-Fern andez 19.1 Introduction 19.2 MBC–HFIP and MHN–HFIP 19.3 BCA–HFIP 19.4 BC–HFIP 19.5 BC–BC and BC–PY 19.6 Concluding Remarks Acknowledgements References

ix

377 377 379 380 382 384 386 388 388 389

393

393 396 403 406 409 415 416 416

Effect of H-bonding on the Photophysical Behaviour of Coumarin Dyes Sukhendu Nath, Manoj Kumbhakar and Haridas Pal 20.1 Introduction 20.2 Effect of Intermolecular H-bonding 20.3 Effect of Intramolecular H-bonding on ICT to TICT Conversion 20.4 Summary References

419

Role of Hydrogen Bonds in Photosynthetic Water Splitting Gernot Renger 21.1 Introduction 21.2 Photosystem II: Overall Reaction Pattern and Cofactor Arrangement 21.3 Hydrogen Bonds and the Thermal Stability of PS II 21.4 Reaction Sequences of PS II and the Role of Hydrogen Bonds 21.5 Concluding Remarks and Future Perspectives Acknowledgements References

433

419 421 426 429 430

433 434 436 437 452 452 452

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Volume II 22

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Proton Transfer Reactions in the Excited Electronic State Vladimir I. Tomin 22.1 Introduction 22.2 ESIPT in 3-Hydroxyflavones and Some Related Compounds 22.3 Dynamic Quenching of Fluorescence as a Simple Test for Study of Photochemical Reaction Character 22.4 Use of Dynamic Quenching of Fluorescence for Study of Reactions from Higher Excited States 22.5 ESIPT from the S2 Singlet State in 3-Hydroxyflavone 22.6 Concluding Remarks Acknowledgements References Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires Carine Tanner Manca, Christian Tanner and Samuel Leutwyler 23.1 Introduction 23.2 Prototype System 23.3 What Favours/Prevents ESHAT 23.4 Conclusion Acknowledgements References Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase Cheng-Chih Hsieh, Chang-Ming Jiang and Pi-Tai Chou 24.1 Introduction 24.2 Biprotonic Transfer Within Doubly H-bonded Homo- and Heterodimers 24.3 Proton Transfer Through Host/Guest Types of Hydrogen-Bonded Complexes 24.4 Solvation Dynamics Coupled into the Proton Transfer Reaction 24.5 Conclusions References QM/MM Study of Excited-State Solvation Dynamics of Biomolecules Tetsuya Taketsugu, Daisuke Kina, Akira Nakayama, Takeshi Noro and Mark S. Gordon 25.1 Introduction 25.2 Applications 25.3 Concluding Remarks Acknowledgements References Excited-State Intramolecular Proton Transfer Processes on Some Isomeric Naphthalene Derivatives: A Density Functional Theory Based Computational Study Sankar Prasad De and Ajay Misra 26.1 Introduction 26.2 Theoretical Calculations

463 463 467 475 483 509 518 520 520 525 525 527 540 551 551 552

555 555 556 563 567 574 575 579 579 580 587 587 587

589 589 591

Contents

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28

29

30

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26.3 Results and Discussion 26.4 Conclusions Acknowledgements References

591 606 607 607

Conformational Switching Between Acids and Their Anions by Hydrogen Bonding Taka-aki Okamura, Hitoshi Yamamoto and Norikazu Ueyama 27.1 Introduction 27.2 pKa Shift of Acids by Neighbouring Amide NH 27.3 Coordination of Anion Ligand to Metal Ion 27.4 Conclusions References

609

Charge Transfer in Excited States: ab initio Molecular Dynamics Simulations Maciej Kolaski, Anupriya Kumar, Han Myoung Lee and Kwang S. Kim 28.1 Introduction 28.2 Charge-Transfer-to-Solvent-Driven Dissolution Dynamics of I(H2O)2–5 Upon Excitation 28.3 Dynamics of Water Photolysis: Excited-State and Born–Oppenheimer Molecular Dynamics Study 28.4 Photodissociation of Hydrated Hydrogen Iodide Clusters: ab initio Molecular Dynamics Simulations 28.5 Excited-State Dynamics of Pyrrole–Water Complexes: ab initio Excited-State Molecular Dynamics Simulations 28.6 Conclusions References

627

Competitive ESIPT in o-Hydroxy Carbonyl Compounds: Perturbation Through Solvent Modulation and Internal Torsion Sivaprasad Mitra 29.1 Excited-State Proton Transfer: An Overview 29.2 Excited-State Intramolecular Proton Transfer (ESIPT) 29.3 ESIPT in o-Hydroxy Carbonyl Compounds 29.4 Concluding Remarks Acknowledgements References Excited-State Double Hydrogen Bonding Induced by Charge Transfer in Isomeric Bifunctional Azaaromatic Compounds Dolores Reyman and Cristina Dı´az-Oliva 30.1 Introduction 30.2 Pyrrolo-Quinoline Derivatives (PQ, DPC, TPC) 30.3 Methylene-Bridged 2-(20 -Pyridyl)indoles and Pyrido[2,3-a]carbazole (PC) 30.4 Fluorescence Quenching by Electron Transfer in Pyrroloquinolines and PyIn-n 30.5 Betacarboline Derivatives 30.6 Conclusions References

609 610 615 623 625

627 628 630 633 633 636 638

641 641 646 650 657 658 658

661 661 662 673 678 680 705 705

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31

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology: Photochemistry and Photophysics of Hydroxyaromatic Dopants Moazzam Ali and Swapan K. Saha 31.1 Introduction 31.2 Microstructural Transition of Micelles in the Presence of Inorganic and Organic Salts 31.3 Microstructural Transition of Micelles in the Presence of Neutral Aromatic Dopants 31.4 Photochemistry and Photophysics of Hydroxyaromatic Compounds 31.5 Excited-State Proton Transfer (ESPT) of Hydroxyaromatic Compounds 31.6 ESPT of Hydroxyaromatic Compounds in Organized Media and Some Unusual Emission Phenomena 31.7 Perspectives Acknowledgements References

32

33

34

35

Excited-State Intramolecular Proton Transfer in 2-(20 -Hydroxyphenyl)benzoxazole Derivatives Yi Pang and Weihua Chen 32.1 Introduction 32.2 Intramolecular Proton Transfer in 2,5-bis(20 -hydroxyphenyl)benzoxazole Derivatives 32.3 Summary and Future Prospects References Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation Dipak K. Palit 33.1 Introduction 33.2 Identification and Characterization of Hydrogen-Bonded Complex 33.3 Vibrational Dynamics of the C¼O Stretching Mode of Fluorenone 33.4 Dynamics of the Excited States of Hydrogen-Bonded Complex 33.5 Summary and Conclusion Acknowledgement References Volume Changes Associated with Solute–Solvent Reorganization Following Photoinduced Proton Transfer in Aqueous Solutions of 6-Methoxyquinoline Stefania Abbruzzetti and Cristiano Viappiani 34.1 Introduction 34.2 Materials and Methods 34.3 Results and Discussion References Molecular Recognition and Chemical Sensing of Anions Utilizing Excited-State Hydrogen-Bonding Interaction Ashutosh S. Singh and Shih-Sheng Sun 35.1 Introduction 35.2 Recognition and Sensing of Anions by Intramolecular Hydrogen Bonding in Excited States

711 711 712 716 730 735 737 743 743 743

747 747 756 758 759 761 761 762 772 775 790 792 792

797 797 798 799 802

805 805 806

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Recognition and Sensing of Anions by Intermolecular Hydrogen Bonding in Excited States 35.4 Recognition and Sensing of Anions by Conjugated Polymers through ESIPT 35.5 Concluding Remarks References

xiii

35.3

36

37

38

39

Theoretical Studies of Green and Red Fluorescent Proteins Hong Zhang, Qiao Sun, Sufan Wang, Seth Olsen and Sean C. Smith 36.1 Introduction 36.2 Method of Calculation 36.3 Results and Discussion 36.4 Conclusions and Future Work Acknowledgements References Changes in Active Site Hydrogen Bonding upon Formation of the Electronically Excited State of Photoactive Yellow Protein Wouter D. Hoff, Zhouyang Kang, Masato Kumauchi and Aihua Xie 37.1 Central Importance of Light in Biology 37.2 Possible Importance of Excited State Hydrogen Bonding in Photoreceptors 37.3 Introduction to Photoactive Yellow Protein 37.4 Hydrogen Bonding in the Initial State of PYP 37.5 Assignment of Vibrational Modes in PYP 37.6 Identification of Vibrational Structural Markers 37.7 Changes in Hydrogen Bonding During the Initial Stages of the PYP Photocycle 37.8 Sub-Picosecond Time-Resolved Transient Spectroscopy of PYP 37.9 Changes in Active Site Hydrogen Bonding upon the Formation of the S1 State of PYP 37.10 Excited State Proton Transfer in the Y42F Mutant of PYP Acknowledgements References Excited State Dynamics in the Light-Driven Enzyme Protochlorophyllide Oxidoreductase (POR) Marie Louise Groot and Derren James Heyes 38.1 Introduction 38.2 Protochlorophyllide Oxidoreductase (POR) 38.3 Catalytic Mechanism of POR 38.4 Ultrafast Catalytic Processes of the Isolated Pchlide Species 38.5 Ultrafast Catalytic Processes of the Enzyme-Bound Pchlide Species 38.6 Conclusions References Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters in the Excited State Michal F arnı´k, Petr Slavı´cek and Udo Buck

808 810 813 813 815 815 820 824 834 835 835

839 839 840 840 841 843 843 844 846 848 850 851 851

857 857 858 859 860 861 862 863

865

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Contents

39.1 Introduction 39.2 Experiment 39.3 Aqueous Photochemistry from the Cluster Perspective 39.4 Hydrogen Bonded Clusters of Nitrogen Heterocycles 39.5 General Conclusions and Outlook Acknowledgements References Index

865 866 868 880 888 889 889 893

Editors’ Biographies

Ke-Li Han was born in 1963 in Shandong Province, China. He received his doctorate in 1990 from the State Key Laboratory of Molecular Reaction Dynamics at the Dalian Institute of Chemical Physics and subsequently became an assistant professor at the Dalian Institute of Chemical Physics. He pursued postdoctoral studies at the Emory University and the University of California at Davis in the years 1993–1995. In 1995, he became a full professor of Chemical Physics at the State Key Laboratory of Molecular Reaction Dynamics at the Dalian Institute of Chemical Physics. He was also an adjunct professor at the Dalian University of Technology and Shandong University and a visiting professor at the University of Melbourne, the City University of Hong Kong, the National University of Singapore, the University of California at Berkeley, New York University, the University of Bristol, and so on. Professor Han received the Outstanding Young Scientist award from the National Natural Science Foundation of China in 1998 and the Natural Science Prize (first class) of the Chinese Academy of Sciences and the Young Chemist Prize of the Chinese Chemical Society in 1999, as well as the Natural Science Prize (first class) of Liaoning Province in 2005. His own achievements have been published in over 300 publications. Professor Han’s current research interests involve experimental and theoretical chemical dynamics, including non-adiabatic reaction dynamics of small molecules, the photodissociation dynamics of gas-phase molecules, the excited-state hydrogen-bonding dynamics of large molecules in solution, biochemical reaction mechanisms and dynamics catalysed by enzymes.

Guang-Jiu Zhao was born in 1980 in Hebei Province, China. He received his bachelor’s degree in Material Engineering in 2003 at the Dalian University of Technology. He received his doctorate in Chemical Physics in 2008 from the State Key Laboratory of Molecular Reaction Dynamics at the Dalian Institute of Chemical Physics. Subsequently, he became an assistant professor at the Dalian Institute of Chemical Physics. In 2009, he was promoted to associate professor at the Dalian Institute of Chemical Physics. He has won the Chinese Academy of Sciences Director Award in 2009, the Natural Sciences Research Award of Liaoning Province in 2008, the Lu-Jiaxi Award for Chinese Excellent Graduate Student in 2007, and so on. His research interests are focused on excited-state hydrogen bonding and hydrogen transfer in photophysics, photochemistry and photobiology by the use of combined experimental and theoretical methods.

Reviewer Comments

Professor Richard N. Zare Chair of the Department of Chemistry, Stanford University, USA Hydrogen bonding has always been a bit of a mystery to me, it having the character of directionality but being an order of magnitude or more weaker than a typical covalent bond. Hydrogen bonding can occur between molecules or between different parts of the same molecule. At last, we have a compilation of studies concerning hydrogen bonding and hydrogen transfer reaction in excited-state species, a most welcome addition to the literature on this important topic. I commend the reading of this monograph to all chemists. Professor Donald G. Truhlar Associate Editor of the Journal of the American Chemical Society, Regents Professor of Chemistry, Chemical Physics, Nanoparticle Science and Engineering, and Scientific Computation, Department of Chemistry, University of Minnesota, USA I have just completed looking at the preface, contents and abstracts of the new book on excited-state hydrogen bonding. Although this area is very important in both biological and technological chemistry, the field has been hampered by the lack of a monograph. The book you have assembled is very impressive, with contributions from a remarkably broad set of groups working in this kind of research. I was especially pleased to see that the coverage includes both standard topics and unusual ones, such as hydrogen bonding in triplet states, which is a very interesting subject, and hydrogen bonding in ionic liquids. The book is sure to become a classic in the field. Professor Wolfgang Domcke Chief Editor of Chemical Physics, Chair of Theoretical Chemistry, Department of Chemistry, Technical University of Munich, D-85747 Garching, Germany I have read the tables of contents and the abstracts of the book chapters. This book gives an impressively broad overview of the current research on excited-state hydrogen bonding in chemistry. Numerous organic chromophores and their intramolecular as well as intermolecular hydrogen and/or proton transfer dynamics are discussed in detail. DNA bases, base pairs and photoactive proteins are considered, as well as basic features of the photosynthetic reaction centre and of the photochemistry of water itself. Interesting aspects that are somewhat underrepresented are the role of hydrogen bonds in the excited-state dynamics of peptides and of protonated peptides, the zwitterionic forms of amino acids in water, as well as hydrogen transfer reactions in hydrogen-bonded chromophore–solvent clusters in supersonic jets. Overall, the book represents a good balance of experimental and computational research. The book provides an excellent introduction to an important contemporary research topic for graduate students as well as for experienced researchers.

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Reviewer Comments

Professor Andrjez Sobolweski Institute of Physics, Polish Academy of Science, Poland Thank you very much for your invitation to review the book. As I was in touch with Wolfgang Domcke at the time he was reviewing this book, I am already familiar with this proposal, and my opinion is in line with his comments, including his reservations. Generally, I think the book represents a really good introduction to the topic for a broader readership. Professor C.N.R. Rao Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India It is nice that we have a much-needed book on excited-state hydrogen bonding. This is most welcome and will be useful to workers in the field. Professor Kankan Bhattacharyya Senior Editor of The Journal of Physical Chemistry, Director of the Indian Association for the Cultivation of Science, Kolkata, India This is a comprehensive text that summarizes the latest developments in hydrogen bonding and its role in many fundamental issues. I liked the wide range of topics covered. The 39 chapters spread over nearly 1200 pages dealt with many systems that range from proteins, ionic liquids and micelles to ultracold vapour in supersonic jets. Many spectroscopic (electronic and vibrational) and microscopic techniques with very high temporal and spectral resolution have been used. The primary aim of these volumes is to focus on hydrogen bonding. Implications of this in many issues, such as solvation dynamics, proton/charge transfer and FRET, have been discussed. This will be an excellent textbook and reference material for graduate students and research scientists. Professor Jun Zeng Guest Professor, Sichuan University, China, and Chief Scientific Officer, Qubist Molecular Design, Australia This is probably the first book that presents comprehensive reviews on the recent theoretical and experimental investigations on the nature of excited-state hydrogen bonding and hydrogen transfers and their influences on many aspects of photophysics, photochemistry and photobiology. From this book, readers will gain much insightful information on the structure, dynamics and spectroscopic properties of hydrogen bonding in the excited states of many important chemical and biological systems. A very useful reference book! Professor Steven D. Schwartz Biophysics and Biochemistry, Albert Einstein College of Medicine, USA This volume promises to be of significant value. Proton transfers are ubiquitous in both complex condensed phases as hydrogen bonds and in biological systems both as hydrogen bonds and as (one of) the chemical steps in enzymatic reactions in biology. In addition, biotechnology through such reactants as GFP is critically dependent on hydrogen transfer. This volume, containing both experiment and theory, promises many useful reviews and new results. In addition, for a western audience, the volume has the advantage of including authors well known to the American audience and others whose work will be new. Professor Hans Lischka Institute for Theoretical Chemistry, University of Vienna, Waehringerstrasse 17, A-1090 Vienna, Austria I have read the Table of Contents of your book Hydrogen Bonding and Transfer in the Excited State with great interest. It contains excellent chapters written by leading world scientists. I am sure that the book will be a great success.

Reviewer Comments

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Professor Chang-Guo Zhan Department of Pharmaceutical Sciences, University of Kentucky, USA I am pleased to read your detailed plan for a book entitled Hydrogen Bonding and Transfer In the Excited State. I think this will be an important book that covers all aspects of hydrogen bonding and hydrogen transfer in excited states. The book will be very interesting for all scientists in the field of chemistry, biochemistry and biophysics who are interested in hydrogen bonding or hydrogen transfer in excited states. I hope this book will be published as soon as possible. Professor Jeffrey R. Reimers ARC Professorial Research Fellow, School of Chemistry F11, The University of Sydney, Sydney NSW 2006, Australia Biochemical structure and function always involve a delicately controlled balance of hydrophobic and hydrophilic forces. While the hydrophobic force is non-specific and always present, the hydrogen-bonding interactions that empower the hydrophilic forces are specific and directed. They are critical to molecular recognition, driving the secondary structure of proteins and the helix formation of DNA. But life is more than biological structure – it is dynamics and motion, metabolism and vitality. What happens to hydrogen bonds in systems with excess energy? Can molecular recognition be modified and a cascade of biological processes ensue? How are proteins and DNA modified when molecules absorb light? Sometimes a change just happens from one possible tautomeric form to another, sometimes whole new motifs like strong hydrogen bonding to aromatics occurs. How quickly do these processes occur, how quickly is the energy dissipated and how quickly does the system return to normal? This is the first book to review excited-state hydrogen bonding, detailing the great variety of consequences found. It provides new insights into the very nature of the forces that create secondary structure in chemistry and biology. Professor Zhi-Ru Li State Key Laboratory of Theoretical and Computational Chemistry, Jilin University, China Hydrogen bonding plays an important role in chemistry, biology and physics. Research on excited-state hydrogen bonding and hydrogen transfer is a novel field. Excited-state hydrogen bonding and hydrogen transfer play significant roles in many photophysical processes and photochemical reactions. This book includes 39 chapters covering various frontier areas of excited-state hydrogen bonding. The contents of this book are very rich. This is very beneficial for researchers and graduate students who are interested in the fields of molecular and supramolecular photochemistry, photobiology and photophysics. Professor Shengli Zou Chemistry Department, University of Central Florida, USA Hydrogen bonding is one of the most important and complex interactions between different molecules or different groups in a big molecule, especially a biomolecule. Understanding the roles of hydrogen bonding in chemical reactions, proton transfer and charge transfer is crucial in revealing the mechanism of these processes. The authors address hydrogen-bonding-induced charge transfer, conformational switching between acids and their anions and controlled intramolecular proton transfer. The importance of hydrogen bonding in photosynthetic water splitting and green fluorescence protein is also discussed. The investigation of hydrogen bonding involving electronically excited molecules is a substantial challenge both experimentally and theoretically. There are few books focusing on hydrogen bonding of molecules in excited states owing to the complexity of the system, especially for theoretical calculations. The proposed book will be a helpful

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reference book for research groups interested in understanding hydrogen bonding in different environments and processes. The book is highly recommended for publication. Professor Anna Spalletti Dipartimento di Chimica, Universit
a di Perugia, 06123 Perugia, Italy Thank you for the information about the new book on hydrogen bonding. My coauthors and I congratulate you on your success in collecting, in a relatively short time, such abundant material (39 contributions!) on a variety of aspects of HB effects on the spectral, photophysical and photochemical properties of so many different organic compounds. From a glance through the abstracts we did not notice any omission. Some discrepancies (for example, in the length of the chapters) and repetitions will certainly be present, but this is bound to happen in such a large review work. Best wishes for the success of the book. Professor Noam Agmon Institute of Chemistry, Hebrew University, Jerusalem The skeleton of the new book looks very impressive in its scope: 39 chapters by world experts covering different aspects of excited-state dynamics within hydrogen-bonded systems. At this stage, when only abstracts are available, it is hard to say more, but I am definitely waiting eagerly for this project to appear in print, as I believe it will be an important milestone for those working in the field and those considering doing so. Professor Gernot Renger Max-Volmer-Laboratorium f€ ur Biophysikalische Chemie, Technische Universit€at Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany Hydrogen bonds are the most important structural determinants in nature. Striking examples of the paramount role of hydrogen bonds are the unique properties of water and the structure of DNA and proteins. The functional relevance of hydrogen bonds is clearly illustrated by their participation in proton transfer mechanisms (e.g. the Grotthus mechanism, proton-transfer-coupled electron transfer, etc.). Of special interest in basic research are the properties of hydrogen bonds in electronically and vibronically excited molecules. This book is an excellent summary of our current stage of knowledge on the different facets of hydrogen bonding which plays a central role for the interaction between molecules. It covers, in 39 chapters, a wide field of topics ranging from the basic properties of hydrogen bonds in comparatively simple electronically excited pigments to the role in complicated biological systems like rhodopsins and photosynthetic water splitting. This book will find a broad audience. It is of great value for scientists working on various aspects of hydrogen bonding. It provides, in a single publication, a nice overview of such a wide field of different topics. Professor Jingwen Chen Key Laboratory of Industrial Ecology and Environmental Engineering, Department of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China Hydrogen bonding determines the properties and activities of many compounds, which is of great importance in chemistry, biology, physics and environmental science. Electronically excited-state hydrogen bonding and hydrogen transfer play an increasingly important role in many photophysical processes and photochemical reactions. In the field of environmental science, studies in recent decades have proved that photodegradation is an important transformation or degradation pathway for toxic organic compounds in aquatic and atmospheric environments, and environmental media have great effects on the photodegradation kinetics and pathways. In some cases, environmental media were observed to influence the photodegradation via hydrogen bonding or hydrogen transfer. Excited-state hydrogen bonding and hydrogen transfer may determine the indirect/direct

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photodegradation kinetics and pathways of many organic pollutants, including halogenated aromatic compounds (e.g. polychlorinated dibenzo-p-dioxin/dibenzofurans, polychlorinated biphenyls, polybrominated diphenyl ethers), pesticides, pharmaceutical and personal care products, etc. Excited-state hydrogen bonding and hydrogen transfer may also have great impacts on the photoinduced toxicities of organic pollutants. This monograph will be the first to deal with hydrogen bonding in excited states, presenting an extensive description of the research progress on excited-state hydrogen bonding and hydrogen transfer in recent years. Both experimental and theoretical investigations on excited-state hydrogen-bonding structures and dynamics of many organic and biological chromophores are included. There are also several chapters describing the influences of excited-state hydrogen bonding and hydrogen transfer on various photophysical processes and photochemical reactions. Thus, this book will be very helpful in understanding the nature of hydrogen bonding in relevant areas and in understanding the photochemical transformation/photoinduced toxicity of environmental organic pollutants. Professor Brian D. Wagner 3M Canada National Teaching Fellow, Department of Chemistry, University of Prince Edward Island, Charlottetown, PE C1A 4P3, Canada I am very impressed by the comprehensive coverage represented by the many chapters in this two-volume set. All of the major topics and considerations involving hydrogen bonding of excited states have been covered. This will be a very useful set of books for a wide range of researchers. I am proud to have been able to make a contribution to the book. Professor Hiroshi Sekiya Department of Chemistry, Faculty of Science, Kyushu Unviversity, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan This book covers a very wide range of topics on hydrogen-bonding and excited-state proton/hydrogen transfer reactions in various molecules and molecular clusters developed by spectroscopic meaasurements and theoretical studies. Many of the results are quite new and interesting for physists, chemists and biologists. I would like to recommend this book for many young and senior researchers interested in the intriguing field of hydrogen bonds and proton/hydrogen transfer reactions. Professor James C. Crabbe Professor of Biochemistry, Dean of the Faculty of Creative Arts, Technologies and Science, University of Bedfordshire, Park Square, Luton LU1 3JU, UK This is an exciting new publication on one of the key elements of life – the hydrogen bond. The authors have produced an array of exciting chapters on hydrogen bonding and hydrogen transfer, covering many aspects of chemistry and biochemistry. This will be an important reference work for many years to come. Professor Takayuki Ebata Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan This book covers recent experimental and theoretical studies on the dynamics of H-bonded systems from the simple aromatic molecules to real biomolecules. An interesting point is that it concentrates on the topic of the electronic excited state, which is different from other books published so far. In this sense, I think (and hope) this book will attract people in a variety of fields.

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Professor Jianzhang Zhao State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning, China This book focuses on excited-state hydrogen bonding and proton transfer. The chapters cover a wide range from the formation of the hydrogen bond in the excited state to the fate of the hydrogen bond in the excited state, such as ESIPT (excited-state intramolecular proton transfer) and vibration dissipation of excited-state energy. Experimental as well as theoretical methods are employed to elucidate hydrogen bonding in the excited state, such as time-resolved vibrational spectra and ab initio or DFT calculations. The subjects involved in the discussion are very diverse, ranging from small organic molecules (such as fluorescent dyes) to biological systems (such as DNA). Therefore, I believe this book addresses most of the research topics of excited-state hydrogen bonding, and the publication of the book will be of significance for the scientific community. Professor Mihaela Hillebrand Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest, Bd. Regina Elisabeta 4-12, Bucharest, Romania The book encompasses the latest achievements in excited-state hydrogen-bonded systems by means of experimental and computational methods. The papers collected provide a good insight into how advances in ultrafast spectroscopic techniques and state-of-the-art quantum chemical calculations have opened up new perspectives on excited-state processes, namely hydrogen bond formation and hydrogen bond transfer in a wide range of chemical and biochemical hydrogen-bonded systems, from molecules, clusters or complexes to biopolymers. It is the first monograph devoted to this subject, and its publication is worthwhile from two points of view – the overall subject and the content. Firstly, considering the importance of hydrogen bond formation in many chemical and biochemical processes and the difficulties related to a good understanding of the excitedstate photophysics, a comprehensive treatment of the subject is necessary. Secondly, the book covers most of the aspects of the topic and is characterized by a good balance between a review of up-to-date literature data and some new results. The book benefits from contributions by renowned scientists with acknowledged results in the field. The editors, remarkably, have succeeded in putting together theoretical aspects involved in excitedstate hydrogen photodynamics and possible applications. The book Hydrogen Bonding and Transfer in the Excited State will be a good tool both for researchers in the field and for graduate students. Professor Samir Kumar Pal Unit for Nano Science & Technology, Department of Chemical, Biological & Macromolecular Sciences, S.N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098, India The book proposal, entitled Hydrogen Bonding and Transfer in the Excited State and edited by Ke-Li Han and Guang-Jiu Zhao, consists of 39 chapters contributed by eminent scientists in this field from all over the world. The contributions embodied here are mostly based on experimental results, along with six papers based on theoretical calculations and simulation results. The theme of the proposed book is very interesting, as many of the fundamental processes in photophysics and photobiology occur in the excited states and involve formation and/or rupture of the hydrogen bonds (HB). A very popular example of such a process is the water splitting in photosynthesis. Ultrafast proton transfer (PT) also serves as the key reaction in many important processes. Unfortunately, there has been no such monograph in the present literature that discusses the various aspects of HB and PT in the excited state. In this regard, this attempt to gather important information on HB and PTwithin a single cover is very encouraging. The proposed book mainly consists of experimental results obtained from steady-state and time-resolved fluorescence studies, as this technique extracts the maximum valuable information on the excited state. Other experimental techniques dealt within the book are UV-IR double-

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resonance excitation, time-resolved resonance Raman spectroscopy, time-resolved FTIR, etc. The related changes in basicity, solvation, hydrogen bond dynamics and other fundamental photophysical and photochemical properties of fluoroprobes upon excitation are discussed and reviewed in many chapters (e.g. Chapters 3, 5, 6, 7, 20, 31, etc.). Intermolecular charge transfer (ICT) is discussed in Chapters 4 and 14. The excited-state photochemistry and photophysics of many organic molecules are discussed in Chapters 12, 13, 19, 25, 27, 30 and 32. Excited-state PT and energy transfer (ET) in different molecules and solvents are discussed in Chapters 10, 17, 19, 22, 23, 24, 27, 29, 32, 33 and 34. Chapters 15 and 16 deal with the hydrogenbonding dynamics in room-temperature ionic liquids (IL). HB barrier-crossing dynamics in nanoconfinement is discussed in Chapter 11. Excited-state HB in biologically important molecules like nucleic acids (Chapters 1 and 9), bacteriorhodopsin (Chapter 18), green fluorescent protein (Chapter 34) and photoconductive yellow protein (Chapter 35), as well as in some complex systems like host–guest complexes (Chapter 8), worm-like micelles (Chapter 29) and clusters (Chapters 1, 2, 26 and 37), is also discussed. Some very interesting topics involving excited-state HB and PT, like the water splitting process in photosynthesis (Chapter 21), excitedstate H-atom transfer (Chapter 24), phototautomerization (Chapter 28) and the catalytic process in light-driven enzymes (Chapter 36), are also included in this monograph. All the contributions embodied in this proposed book are supported by state-of-the-art experimental and computational results, and the topics cover the wide range of diversity in this field. In my opinion this monograph will serve as a very fundamental tool for understanding excited-state HB and PT processes for researchers in the field of photophysics, photochemistry and photobiology. I strongly recommend the publication of this monograph. Professor Soo Young Park School of Materials Science and Engineering, Seoul National University, Korea Congratulations on your excellent publication. It seems that your book covers all aspects of excited-state H-bonding and H-transfer. This book will draw the attention of scientists in many different disciplines such as the organic, physical, as well as materials chemistry fields. Professor Swapan K. Saha Department of Chemistry, University of North Bengal, Darjeeling-734 013, India Thank you for the mail and the attachments. You have done a great job! Congratulations! The coverage of the proposed book is wide and impressive. The authors are mostly of international standing and the topics covered are up to date and relevant to current interest. Professor Weiqun Zhou College of Chemistry and Chemical Engineering, Soochow University, Suzhou 215123, China The studies on the hydrogen bond have been one of the most important research areas in materials chemistry, chemistry and bioscience. The hydrogen bond has a special significance for biomacromolecules; it is part of the reason why protein and level II, level III and level IV nucleic acid can be stable. The excited-state hydrogen bond structure and dynamics play an important part in many chemical, physical and biological procedures. The fluorescence emission behaviours of organic and biological chromophores are often influenced by the interactions of hydrogen bonds between chromophore molecules and protic solvents or biological surroundings. The ultrafast deactivation processes of the photoexcited molecules and the supramolecular systems are also likely to be easier under the influence of excited-state hydrogen bonding. The hydrogen bond in the excited state and hydrogen transfer are becoming an increasingly important subject in the realm of photochemical and photophysical reactions. The publishing of this book will help us to learn more comprehensively the characteristics of the hydrogen bond and also help us to realize the

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importance of the hydrogen bond in photochemistry, photobiology and photophysics. We sincerely hope that this book, with systematic description of the hydrogen bond in its excited state and hydrogen transfer, can be published soon. Professors Sean C. Smith and Hong Zhang Centre of Computational Molecular Sciences, University of Queensland, Australian Institute of Bioengineering & Nanotechnology, ARC Ctr Funct Nanomat, Brisbane, Qld 4072 Australia The publication of the book Hydrogen Bonding and Transfer in the Excited State is a timely landmark contribution to the field, drawing together a wide range of theoretical and experimental contributions that collectively provide a comprehensive picture of recent advances in the field. It covers the recent important work of the experts in this field from all over the world and coherently links the theoretical studies with the experimental developments in this important area. The 39 contributing chapters are well written and thematically organized. The book is of high quality and will no doubt become a mandatory part of library and personal collections for institutions and individuals – researchers and students alike – engaged in this fascinating area of molecular science. We look forward to its publication as soon as possible. Professor Attila Demeter Institute of Materials and Environmental Chemistry, Chemical Research Centre of Hungarian Academy of Sciences, 1525 Budapest, P.O. Box 17, Hungary The proposed book Hydrogen Bonding and Transfer in the Excited State, edited by Ke-Li Han and Guang-Jiu Zhao, is a stop-gap issue that may reckon with considerable interest in the field. There is no really well-known monograph on this discipline from the classical books of Pimentel (1960) and Vinogradov (1971), although the subject is widely studied. The 39 studies cover a very wide area, indicating that the understanding of the influence of hydrogen bonds on photoprocesses is crucial almost everywhere. Most expert readers will find half a dozen studies touching upon their close interests; however, the book will be a valuable tool for obtaining knowledge on topics slightly further afield. One rarely has time to collect such scientific studies from journals, and it is certainly valuable to have them gathered together by expert editors. Professor H. H. Limbach Institut f € ur Chemie und Biochemie, Freie Universit€at Berlin, Takustr. 3, 14195, Berlin This is a timely book in two volumes and 39 chapters, to which many well-known authors from all over the world have contributed. The systems studied are dyes, water wires, ionic liquids and nanoconfined and self-assembled systems up to biomolecules and large proteins. The appetite of the reader is whetted by chapters covering excited-state phenomena such as vibrational dynamics, acid–base interactions, proton and charge transfers, H-bond-induced conformational switching and molecular recognition, as well as the function of complex proteins. The experimental and theoretical techniques used are adapted to the systems and phenomena studied. It will be an important piece in the canon of books on hydrogen transfer and bonding. Professor Johannes Kiefer University Erlangen Nurnberg, LTT, Weichselgarten 8, D-91058 Erlangen, Germany I very much like the fact that a broad range of aspects is discussed in this book concerning both the analytical methods and the systems under investigation. Therefore, it will be of interest for a large readership in the classical fields of physics and chemistry, but also for rather new areas like life science and biophysics.

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Professor Giuseppe Buemi Dipartimento di Scienze Chimiche, Universit
a di Catania, Viale A. Doria nr. 6, 95125 Catania, Italy I have read the summary of the papers enclosed in the new book you have coedited with Prof. Han. Even if I have little experience with excited states, I think that such a book could be very interesting and very useful for collegues working in this field, and so I think the book must be published. Professor Dipak K. Palit Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India This monograph, presented in two volumes, provides a very timely update on the recent developments in the field of hydrogen-bonding interactions in the excited states of different kinds of molecular system in both homogeneous solutions as well as heterogeneous media, including micelles, vesicles and ionic liquids. As rightly mentioned by the editors in the preface, hydrogen-bonding structures and dynamics in the excited states of molecules play important roles in determining many chemical, physical and biochemical processes. In addition to the most popular fluorescence spectroscopic techniques, recent developments of ultrafast, both linear and nonlinear, time-resolved infrared spectroscopic techniques have provided a great opportunity to understand the microscopic structures and functions in many complex hydrogen-bonded systems. While there are quite a good number of monographs published describing the hydrogen-bonding interactions in the ground state of molecules, to the best of my knowledge there is none to deal with the same aspect exclusively in the excited states of molecules. This book presents an extensive review of the progress of research, both experimental and theoretical, on hydrogen bonding and hydrogen transfer, both intramolecular and intermolecular, in the excited states of a wide variety of molecular systems. This book comprises 39 chapters, most of which are written by experts and provide authoritative overviews of each area. Overall, the editors have fulfilled their primary objective of creating a reference volume valuable to both experts and beginners or students who are engaged in investigation of the dynamics of hydrogen-bonding interactions in the excited states of molecular systems forming hydrogen-bonded complexes. This book will provide an excellent entry to the literature of hydrogen bonding and hydrogen transfers in the excited state. Professor Andong Xia The State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, P.R. China The book entitled Hydrogen Bonding and Transfer in the Excited State, edited by Keli Han and Guangjiu Zhao, is a timely and important work for researchers working on the excited-state hydrogen-bonding structure and dynamics. Reading this book will help the reader understand the basic concepts of complex excited-state hydrogen-bonding processes. There are at least three advantages of this book: 1.

2. 3.

The topics covered are extensive and comprehensive. Volume I introduces the structure and dynamics in excited-state hydrogen-bonding systems, and the influences of excited-state hydrogen bonding on photophysical and photochemical processes. Volume II then focuses the attention on the dynamics and control of the excited-state hydrogen proton transfer process. A series of organic chromophores and biomacromolecues in different systems, as well as their inter- and intramolecular hydrogen proton transfer dynamics, are discussed in detail. It has a ‘handbook’ character to some extent, and it is easy to understand the basic concepts of hydrogen bonding for systems specific to researchers. It is very authoritative. The contributed authors are distinguished scholars in this field. Their studies are sufficiently representative of the current overall level in this field. It pays attention to both experimental and theoretical studies. This will be helpful and welcome to experimental researchers seeking theoretical support, and vice versa.

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Professor Laszlo Biczok Hungarian Academy Sciences, Chemistry Research Centre, POB 17, H-1525 Budapest, Hungary This book provides a unique comprehensive overview of the photoinduced processes of hydrogen-bonded systems. The chapters, written by internationally recognized experts, cover the latest developments and fundamental aspects of this rapidly evolving research field. In view of the ubiquitous nature of hydrogenbonding and light-initiated processes, this excellent reference book is likely to be of interest to members of a wide scientific community. It serves as a valuable source of information and inspiration for newcomers and experienced researchers alike. Professor G. Krishnamoorthy Department of Chemistry, IIT Guwahati, Guwahati 781039, India Hydrogen bonding is a fundamental phenomenon that plays a key role in various chemical and biological processes. The hydrogen-bonding effects may be altered significantly upon molecular excitation owing to redistribution of electron density in the excited state. This will have a drastic effect on the photochemistry and photophysics of the system. Thus, excited-state hydrogen bonding and hydrogen transfer are very important subjects of study. In this book, recent advances on both experimental and theoretical studies of hydrogen bonding in photochemistry and photophysics are reviewed. The effect of hydrogen bonding on various phenomena, such as proton transfer, charge transfer, isomerization and photodissociation by conventional solvents to complex proteins, clusters and ionic liquids, are discussed. The book will be a useful reference to active researchers and graduate students. Professor Luca Nardo University of Insubria, Dept. of Physics and Mathematics & C.N.I.S.M.-C.N.R., U.d.R. Como Via Valleggio 11, 22100 Como, Italy Concerning the book as a whole, we sincerely think that it has resulted in a really successful editorial project and are proud of having taken part in it. The book is a detailed and comprehensive compendium on all the principal aspects of H-bonding and H-transfer in the excited state, both from the experimental and from the theoretical point of view. The approach of the book sounds intriguingly interdisciplinary. In this regard, we believe that the readership could be quite broad, and that it could be helpful to divide the book into parts/sections, each section collecting chapters on similar topics and being opened by the chapter offering the most complete introduction on both the theory and the experimental techniques eviscerated in the section itself. This should simplify the task of finding specific information within the very wide spectrum of contents. Moreover, the readers would best appreciate the unity and the development of the arguments, and the monographic character of the work. To this purpose, we also suggest that the editors write a short summary/abstract for each section. We see that the order in which the chapters are presented already seems to match these suggestions, and only wish to highlight the opportunity to take up these suggestions. Professor Oliver Kuhn Institut f € ur Physik, Universit€ at Rostock, D-18051 Rostock, Germany As regards the overall impression of the book you have compiled (based on the Table of Contents), I was very excited. You have managed to bring together a very broad range of people essentially covering many of the fascinating subjects of this field. Congratulations!

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Professor Pi-Tai Chou Department of Chemistry, National Taiwan University, Taipei, Taiwan 106 I thank the editors, Dr Ke-Li Han and Dr Guang-Jiu Zhao, for the invitation to contribute a chapter to this new book. The book extensively covers the hot topics of hydrogen bonding and the associated proton (hydrogen) transfer reaction, from theoretical and experimental approaches to the fundamentals to current-interest biological and material applications. I found that this book may be particularly suited to those readers who are at the stage of initiating proton (hydrogen) transfer research and need a broad spectrum of current/ previous progress in various aspects. The reader can specifically pick a few chapters for his/her own interest and treat other chapters as key references. This would make the reading more convenient and comfortable. Evidently, the contents of this book are very rich and provide an in-depth discussion of various theories on spectroscopy and dynamics. The reader will be able to discern differences, for example in applications, between similar topics, and, conversely, similarities, for example in theory, between different topics. I thus believe that by reading this book the reader will gain deep and broad insights into hydrogen-bonding phenomena and the associated excited-state proton (hydrogen) transfer reaction and perhaps latent applications in several cutting-edge areas. As for the contemporary research progress in hydrogen-bonding studies, the book is indeed a significant milestone for studying excited-state hydrogen bonding and/or hydrogen transfer reactions. Professor Ricard Gelabert Unitat de Quimica Fisica, Departament de Quimica, Edifici Cn Universitat Autonoma de Barcelona, 08193 Bellaterra, Spain First of all, I have not had access to the contents of the book, either in full or in part, except for the index, the foreword and the abstracts of the different chapters, and as such all I can provide is a general overview or impression of the coverage of the topics related to excited-state proton transfer, as far as my knowledge of the field permits, but not a view on the quality of each of the chapters. The book is made up of 39 chapters, each authored by an author or group of authors active in the general field of hydrogen bonding and hydrogen/proton transfer in excited states. The list of chapters is quite extensive and covers several current topics in ESIPT and hydrogen bonding. In a broad sense, the publication of a book devoted to excited-state hydrogen transfer/bonding is good news, as there has been a certain lack of specialized texts on the state of research. This is a collection of specialized papers on current research in this area, and may be of help in particular to researchers in the field, as a guide to the state of the topic at the beginning of this century. The Table of Contents shows the contributions from experimental and theoretical researchers, which should provide a balanced view of research in an area where synergy between theory and experiment has proven to be paramount. Even though the detailed contents of each chapter are unavailable to me, a list of sections and authors is. Thus, a rather balanced and wide spectrum of topics greets the eye. The interested reader will find chapters dealing with vibrational dynamics aspects of hydrogen bonding, acid–base character, solvent effects, supramolecular chemistry, hydrogen bonding in ionic liquids, properties and behaviour of many landmark systems, including fluorescent proteins, rhodopsin and others. Last but not least, a number of the contributions are written by well-known researchers in their field, as is the case, for instance, with no pretence of completeness, with the chapters devoted to GFP/RFP and the vibrational analysis of hydrogen bonds in nucleic base pairs. As a downside, I have found a certain omission in the chapters, namely systems displaying intramolecular double-proton transfers (or multiple ones). These systems have proved to be more complex than expected, as different mechanisms can be envisaged, usually involving zwitterionic intermediates. I could only find one chapter where homodimers of 7-azaindole are described. Another issue that might have deserved specific coverage is that of nonradiative pathways, ubiquitous in excited-state proton transfer and usually a highly competitive process to

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fluorescence. Certainly, the broad topics covered in the book make it necessary to limit the scope covered within the limited confines of the book. Professor Samita Basu Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India This book is an attempt to meld together theory and experiment in the fundamental aspect of hydrogen bonding, especially in the excited state. It is a collaborative effort, made possible by the willingness of a group of leading figures in the subject to write about their own areas of expertise. It contains 39 chapters, the themes of which are summarized very elegantly in the Preface. This book will aid those who wish to understand the structural, physical, chemical and biological basis of hydrogen bonding and plan their own experiments. It should be of value for graduate students and advanced undergraduates. Professor Satoshi Minakata Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871 Japan The book deals with the recent achievements in exited-state hydrogen bonding and hydrogen transfer. Hydrogen bonding and transfer are very important interactions not only in chemistry but also in biology and physics. In particular, the studies of hydrogen bonding in the exited state should contribute to the development of these fields. This book should have a beneficial effect on the development of various investigations and provide valuable information to research scientists as well as graduate students. Professor Rafael Escribano Inst. de Estructura de la Materia, CSIC Serrano 123, 28006 Madrid, Spain This book presents a very comprehensive collection of interesting contributions to the subject of hydrogen bonding in excited-state molecules, plus a number of chapters dealing with hydrogen transfer processes. Whereas there have been many articles and books on the general subject of hydrogen bonding over the years, it is true that the number of scientific communications on the specific area of excited-state hydrogen bonding is small. In this regard, this book fulfils an important task in providing high-level, state-of-the-art contributions to this particular topic. The chapters cover a wide range of scientific communications on different applications that have in common the existence of this type of bonding in the molecular systems under study. Many of the subjects considered deal with biological species, which are of course of high interest in current science. Perhaps an introductory chapter on the general aspects of hydrogen bonding, such as the spectroscopic effects that this kind of bonding has on bandwidths, bandshifts and intensities, and a basic description on the computational techniques applied to its study, even at the ground-state level, could be added at the beginning of the book. Similarly, there do not seem to be any contributions dealing with hydrogen bonding in solid-state systems, which are of course much less common than in the liquid phase. On the whole, the book is excellent and can be wholeheartedly recommended for researchers and university teachers interested in this field. Professor Shih-Sheng Sun Institute of Chemistry, Academia Sinica, 128 Sec2 Academia Road, Taipei, Taiwan I think you have done a fabulous job in bringing together all these topics related to excited-state H-bonding. The subtopics cover many different aspects of H-bonding properties in the excited states. It will be a nice contribution to this area as a valuable reference book.

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Professor Chong Rae Park School of Materials Science and Engineering, Seoul National University, San 56-1, Shinlim-dong, Kwanak-ku, Seoul 151-744, Korea Hydrogen bonding is perhaps the most interesting and complex bonding that prevails in the material world, ranging from natural lives to man-made organic and/or inorganic materials. In spite of the many excellent review papers and monographs already available, there still remain many phenomena due to the presence of hydrogen bonds that require deeper understanding. Apart from hydrogen bonds themselves, understanding these bonds in the excited state and related phenomena is even more intriguing and absolutely necessary if we are to deepen our understanding of the mysterious materials world and phenomena that are created by excited-state hydrogen bonding. In this sense, the book will definitely be a good guide to the yet unknown world of lives and materials. As mentioned in the Preface, this book would be a good reference and/or textbook for postgraduates and experts in materials science who are dreaming of designing new functional materials. Professor Sivaprasad Mitra Department of Chemistry, NEHU, Shillong 793 022, India I appreciate your idea of editing a whole new book (of two volumes) on this topic. The effect of HB is well known and has been studied in detail in the ground state over the years. However, recent developments in ultrafast laser spectroscopy, as well as theoretical chemistry, have helped scientists to explore this phenomenon in the excited state with more certainty. In spite of the vigorous activity (more than 1 150 000 hits resulted from a search in google scholar with hydrogen bond + excited state!), the results are scattered and some compilation was absolutely necessary. I think and sincerely hope that your effort (with the compilation of 39 chapters from different parts of the world) will help immensely in this regard. Professor Mikhail V. Vener Department of Quantum Chemistry, Mendeleev, University of Chemical Technology, Moscow 125047, Russia I was impressed by the Table of Contents of the book. It contains the results of experimental investigations of proton/hydrogen atom transfer in excited-state hydrogen-bonding systems of different type and theoretical studies of the proton-transfer dynamics in biomimicking molecules. The book bridges the gap between the well-developed concepts of the hydrogen bond phenomenon in the ground electronic state and hydrogen bonding in the excited electronic states. Summing up, the book provides valuable insights into the nature of hydrogen bonding and proton dynamics in supramolecular photochemistry and photobiology. Professor Jim Jr-Min Lin Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 10617 It is a comprehensive book. There would be no life without hydrogen bonding. Although focusing on hydrogen bonding, the broad coverage of this book makes it also a nice reference book for studying reactions and dynamics, especially for those who are interested in excited-state chemical dynamics. Professor Satinder Kumar Sikka Off. Principal Sci. Adviser Govt India, Vigyan Bhawan Annexe, New Delhi, India The book is perhaps the first one on the novel topic of excited states of hydrogen bonds, a field that has applications ranging from biology to climate change. It will serve as an excellent resource book for researchers in this field. Personally, I learnt a lot of things about this subject by reading through the abstracts.

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Professor Tetsuya Taketsugu Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan This book is well organized, and a lot of good scientists have contributed to the chapters. I believe this book should play a significant role in this field. Professor Sergio Trasatti University of Milan, Dept Phys Chem & Electrochem, Via Venezian 21, I-20133 Milan, Italy The editors of this new book have to be congratulated for the impressive amount of information gathered from internationally renowned groups, which fills a gap in the dedicated literature. The inclusion of recently developed topics such as ionic liquids is particularly interesting. Hydrogen bonding is relevant also to the field of electrochemistry, where heterogeneous electron and ion transfer are influenced by interfacial hydrogen bonding. The reaction of heterogeneous proton transfer is one of the more studied topics in electrochemistry. Professor Udo Buck Max-Planck-Institut fuer Dynamik und Selbstorganisation, delivery: Bunsenstr. 10, 37073 Goettingen, Germany The new book is an important contribution to the rapidly evolving field of processes in the excited state of hydrogen-bonded systems, with crucial applications to biological systems. These include photophysical and photochemical reactions, the behaviour of many chromophores and hydrogen transfer reactions in the excited state. Most of the presented examples take place in solutions. But some of them also deal with specially prepared nanoconfined surfaces and clusters of different sizes. These systems allow for a microscopic description of some of the key processes and form an important source for the understanding of what is going on in these systems. Professor Yi Pang Department of Chemistry & Maurice Morton Institute of Polymer Science, University of Akron, Akron, OH 44325, USA This much needed monograph places the subject of H-bonding and H-transfer in the excited state in its most modern context. With a broad range of applications in chemistry, biology and material sciences, this authoritative resource will certainly benefit researchers and graduate students who wish to acquire an insight into H-bonding in the excited state. Professor Alenka Luzar Department of Chemistry, Virginia Commonwealth University, 1001 West Main St., Richmond, VA 232842006, USA I have just finished reading the Preface, Table of Contents and the 39 abstracts of your new book proposal. My first impression is that you have gathered an impressive cohort of researchers working in exited-state hydrogen bonding and proton transfer and covering a variety of applications and approaches. I could not notice any obvious omission. There is a proper mix of experimental and theoretical abstracts. The book, when it comes out, will be an excellent source of information gathered in one place for all researchers working in this very important topic of hydrogen bonding and hydrogen transfer in excited states. Congratulations!

Reviewer Comments

xxxi

Professor Peter Hamm Institute of Chemical Physics, University of Zurich, Winterthurerstr 190, CH-8057 Zurich, Switzerland This is a very impressive collection of articles on hydrogen bonding and hydrogen transfer in excited states. The book is impressive in terms of the number of contributions that have been collected, and in particular in terms of the breadth of themes and molecular systems. Hydrogen bonding and hydrogen transfer (or proton transfer), per se, play a pivotal role in many chemical and in particular biological processes. Studying them in excited states adds a twist to the story; for example, it opens up an opportunity to study these processes in a time-resolved manner. For sure this will become an important collection for people working in the field. Professor Laurence Noirez CNRS Research Director, Laboratoire Leon Brillouin (CEA-CNRS), CE-Saclay 91400 Gif-sur-Yvette Cedex, France Finally – a book that gathers and sums up the current state of knowledge on hydrogen bonding and its role in various media. This fascinating and rapidly changing field expands considerably from year to year. This book meets the difficult challenge of upgrading our knowledge of hydrogen bonding. The contents and the abstracts show that this new book covers an impressively broad domain of studies describing the dynamic properties of hydrogen bonding, from the nano to macroscopic length scales, gathering investigations by experts in domains as different as rheology, nonlinear IR spectroscopy, photoexcitation and acoustic and densitometry studies. This book offers a very complete approach to the state-of-the-art in H-bonding research (39 chapters). Such a contribution has become essential because of the necessary specialization of the current research. It is an invitation to open our minds to complementary methods, to multiscale properties, making it possible to establish a synthetic view so essential and yet so often missing. Professor Igor Pugliesi Univ Munich, Lehrstuhl BioMol Opt, D-80538 Munich, Germany I find that the contributions are interesting and cover many of the areas and topics of excited-state hydrogen bonding and hydrogen transfer. I also find it good that you have given a voice to up-and-coming researchers in Asia. However, I think the book would benefit from contributions by authors who are well known in this field, to create a balance between new and established, between east and west.

List of Contributors

Stefania Abbruzzetti, Dipartimento di Fisica, Universita` degli Studi di Parma, Parma and Dipartimento di Biotecnologie, Universita di Verona, Verona, Italy Aniruddha Adhikari, Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Moazzam Ali, Department of Chemistry, University of North Bengal, Darjeeling 734 013, India Alessandra Andreoni, University of Insubria, Department of Physics and Mathematics and C.N.I.S.M.C.N.R., U.d.R. Como Via Valleggio 11, 22100 Como, Italy Manuel Balo´n, Department of Physical Chemistry, Faculty of Pharmacy, University of Seville, Spain Debapriya Banerjee, Unit for Nano Science and Technology, Department of Chemical, Biological and Macromolecular Sciences. S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098, India Giampiero Bartocci, Dipartimento di Chimica, Universita` di Perugia, 06123 Perugia, Italy Samita Basu, Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India Kankan Bhattacharyya, Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Udo Buck, Max-Planck-Institut fu¨r Dynamik und Selbstorganisation, Bunsenstrasse 10, D-37073 Go¨ttingen, Germany Carmen Carmona, Department of Physical Chemistry, Faculty of Pharmacy, University of Seville, Spain Weihua Chen, Department of Chemistry and Maurice Morton Institute of Polymer Science, The University of Akron, Akron, OH 44325, USA Pi-Tai Chou, Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, R.O.C. Attila Demeter, Institute of Materials and Environmental Chemistry, Chemical Research Centre, Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri u. 59-67, Hungary Debarati Dey, Department of Chemistry and Environment, Heritage Institute of Technology, Chowbaga Road, Anandapur, P.O. East Kolkata Township, Kolkata 700 107, India

xxxiv

List of Contributors

Shantanu Dey, Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Cristina Dı´az-Oliva, Departamento de Quı´mica-Fı´sica Aplicada, Facultad de Ciencias, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain Takayuki Ebata, Department of Chemistry, Graduate School of Science, Hiroshima University, HigashiHiroshima 739-8526, Japan Michal Fa´rnı´k, J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences, Dolejsˇkova 3, 182 23 Prague 8, Czech Republic Emilio Garcı´a-Ferna´ndez, Department of Physical Chemistry, Faculty of Pharmacy, University of Seville, Spain Subhadip Ghosh, Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Mark S. Gordon, Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA Marie Louise Groot, Department of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands Ke-Li Han, State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China Derren James Heyes, Manchester Interdisciplinary Biocentre, University of Manchester, 131 Princess Street, Manchester M1 7DN, UK Jose Hidalgo, Department of Physical Chemistry, Faculty of Pharmacy, University of Seville, Spain Mihaela Hillebrand, Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest, Bd. Regina Elisabeta 4-12, Bucharest, Romania Wouter D. Hoff, Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, OK 74078, USA Cheng-Chih Hsieh, Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, R.O.C. Sorana Ionescu, Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest, Bd. Regina Elisabeta 4-12, Bucharest, Romania Chang-Ming Jiang, Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, R.O.C. Hideki Kandori, Department of Frontier Materials, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan Zhouyang Kang, Department of Physics, Oklahoma State University, Stillwater, Oklahoma 74078, USA Johannes Kiefer, Lehrstuhl fu¨r Technische Thermodynamik (LTT) and Erlangen Graduate School in Advanced Optical Technologies (SAOT), Universita¨t Erlangen-Nu¨rnberg, Am Weichselgarten 8, 91058, Erlangen, Germany. Present address: School of Engineering, University of Aberdeen, Fraser Noble Building, King’s College, Aberdeen AB24 3UE, Scotland, UK Kwang S. Kim, Centre for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea

List of Contributors

xxxv

Daisuke Kina, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Maciej Kolaski, Centre for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea, and Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia, 9 Szkolna Street, 40-006 Katowice, Poland Govindarajan Krishnamoorthy, Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India Oliver Ku¨hn, Institut fu¨r Physik, Universita¨t Rostock, D-18051 Rostock, Germany Anupriya Kumar, Centre for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea Masato Kumauchi, Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, OK 74078, USA Manoj Kumbhakar, Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Han Myoung Lee, Centre for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea Jerzy Leszczynski, NSF CREST Interdisciplinary Nanotoxicity Centre, Department of Chemistry and Biochemistry, Jackson State University, Jackson, MS 39217, USA Samuel Leutwyler, Department fu¨r Chemie und Biochemie, Universita¨t Bern, Freiestrasse 3, 3012 Bern, Switzerland Carine Tanner Manca, Laboratorium fu¨r Physikalische Chemie, ETH Zu¨rich, CH-8093 Zu¨rich, Switzerland Ujjwal Mandal, Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Iulia Matei, Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest, Bd. Regina Elisabeta 4-12, Bucharest, Romania Ugo Mazzucato, Dipartimento di Chimica, Universita` di Perugia, 06123 Perugia, Italy Ajay Misra, Department of Chemistry and Chemical Technology, Vidyasagar University, Midnapore 721 102, WB, India Rajib Kumar Mitra, Unit for Nano Science and Technology, Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098, India Sivaprasad Mitra, Department of Chemistry, North-Eastern Hill University, Permanent Campus, Shillong 793022, India Marı´a Asuncio´n Mun˜oz, Department of Physical Chemistry, Faculty of Pharmacy, University of Seville, Spain Akira Nakayama, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

xxxvi

List of Contributors

Luca Nardo, University of Insubria, Department of Physics and Mathematics and C.N.I.S.M.-C.N.R., U.d.R. Como Via Valleggio 11, 22100 Como, Italy Sukhendu Nath, Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Peter Nikolov, Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Takeshi Noro, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Taka-aki Okamura, Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Seth Olsen, The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, Centre for Computational Molecular Science, QLD 4072, Brisbane, Australia Haridas Pal, Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Samir Kumar Pal, Unit for Nano Science and Technology, Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098, India Dipak K. Palit, Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Yi Pang, Department of Chemistry and Maurice Morton Institute of Polymer Science, The University of Akron, Akron, OH 44325, USA David Lee Phillips, Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong Rajib Pramanik, Department of Chemistry, Indian Institute of Technology, Kharagpur, PIN-721302, WB, India Sankar Prasad De, Department of Chemistry and Chemical Technology, Vidyasagar University, Midnapore 721 102, WB, India Gernot Renger, Max-Volmer-Laboratorium fu¨r Biophysikalische Chemie, Technische Universita¨t Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany Dolores Reyman, Departamento de Quı´mica-Fı´sica Aplicada, Facultad de Ciencias, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain Swapan K. Saha, Department of Chemistry, University of North Bengal, Darjeeling 734 013, India Antonio Sa´nchez-Coronilla, Department of Physical Chemistry, Faculty of Pharmacy, University of Seville, Spain Manas Kumar Sarangi, Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India Nilmoni Sarkar, Department of Chemistry, Indian Institute of Technology, Kharagpur, PIN-721302, WB, India

List of Contributors

xxxvii

Souravi Sarkar, Department of Chemistry, Indian Institute of Technology, Kharagpur, PIN-721302, WB, India Manoj K. Shukla, NSF CREST Interdisciplinary Nanotoxicity Centre, Department of Chemistry and Biochemistry, Jackson State University, Jackson, MS 39217, USA Ashutosh S. Singh, Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, ROC Petr Slavı´cˇek, Department of Physical Chemistry, Institute of Chemical Technology, Technicka´ 5, Prague 6, Czech Republic Sean C. Smith, The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, Centre for Computational Molecular Science, QLD 4072, Brisbane, Australia Anna Spalletti, Dipartimento di Chimica, Universita` di Perugia, 06123 Perugia, Italy Shih-Sheng Sun, Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, ROC Qiao Sun, The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, Centre for Computational Molecular Science, QLD 4072, Brisbane, Australia Tetsuya Taketsugu, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 0600810, Japan, and Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan Christian Tanner, TOFWERK AG, Uttigenstrasse 22, CH-3600 Thun, Switzerland Ilijana Timcheva, Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Vladimir I. Tomin, Institute of Physics, Pomeranian University, 76-200, Słupsk, Poland Hanne Hjorth Tønnesen, School of Pharmacy, University of Oslo, PO Box 1068 Blindern, 0316 Oslo, Norway Norikazu Ueyama, Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Pramod Kumar Verma, Unit for Nano Science and Technology, Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098, India Cristiano Viappiani, Dipartimento di Fisica, Universita` degli Studi di Parma and NEST Istituto NanoscienzeCNR, Parma, Italy Brian D. Wagner, Department of Chemistry, University of Prince Edward Island, Charlottetown, PE, C1A 1Z5 Canada Sufan Wang, The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, Centre for Computational Molecular Science, QLD 4072, Brisbane, Australia Aihua Xie, Department of Physics, Oklahoma State University, Stillwater, OK 74078, USA Hitoshi Yamamoto, Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

xxxviii

List of Contributors

Yun-an Yan, Institut fu¨r Physik, Universita¨t Rostock, D-18051 Rostock, Germany Hong Zhang, The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, Centre for Computational Molecular Science, QLD 4072, Brisbane, Australia Guang-Jiu Zhao, State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China

Preface

Hydrogen bonding is of universal importance in chemistry, biology and physics. Hydrogen bonding is central to the understanding of the microscopic structures and functions in many complex systems, for example hydrogen-bonded water or alcohol networks, organic compounds in solution, crystal engineering, selfassembled supramolecular architectures, proteins and DNA building blocks of life. Moreover, hydrogenbonding structures and dynamics in the excited states also play important roles in determining many chemical, physical and biochemical processes. In general, fluorescence emission behaviours of organic and biological chromophores can be significantly influenced by the intermolecular hydrogen-bonding interactions between chromophores and protic solvents or biological surroundings. Furthermore, ultrafast deactivation processes of photoexcited molecular and supramolecular systems can be remarkably facilitated by excited-state hydrogen bonding. In particular, excited-state hydrogen transfer is closely related to excited-state hydrogen-bonding structures and dynamics. Hydrogen bonding in the ground state for various types of molecular and supramolecular systems has been described systematically in an abundance of published monographs. However, to the best of our knowledge, there are no monographs on hydrogen bonding in excited states until now. It has been found that excited-state hydrogen bonding and hydrogen transfer are playing an increasingly important role in many photophysical processes and photochemical reactions. Therefore, a new book presenting hydrogen bonding and hydrogen transfer in excited states is urgently needed. This scientific book will be very helpful in extensively understanding the nature of hydrogen bonding and its key roles in photochemistry, photobiology and photophysics. This book gives an extensive description of research progress on excited-state hydrogen bonding and hydrogen transfer in recent years. First of all, both experimental and theoretical investigations on excited-state hydrogen-bonding structures and dynamics of many organic and biological chromophores are presented: coumarin and its derivates, fluorenone and its derivates, diazines, quinones, b-carbolines, harmane derivatives, substituted phthalimides, 4-aminoindandiones, hydroxyphenacyl compounds, diazaromatic betacarboline derivatives, imidazolium ionic liquids, supramolecular host–guest complexes, nanoconfined systems, rhodopsins, hydrated DNA bases and base pairs, and so on. After that, several chapters will describe the influences of excited-state hydrogen bonding on various photophysical processes and photochemical reactions. For example, the effects of hydrogen bonding on fluorescence emission behaviours and photoisomerization; intramolecular-hydrogen-bond-formation-mediated de-excitation of curcuminoids; the role of hydrogen bonding in photosynthetic water splitting; ultrafast catalytic processes in the light-driven enzyme protochlorophyllide oxidoreductase (POR); photoinduced electron transfer and solvation dynamics in roomtemperature ionic liquids; hydrogen-bonding barrier crossing dynamics at biomimicking surfaces; the effects of hydrogen bonding on intramolecular charge transfer, vibrational energy relaxation and the ICT-to-TICT conversion; hydrogen bond basicity in excited states and dynamic heterogeneity. Finally, in the final chapters

xxiv

Preface

we will focus our attention on excited-state hydrogen transfer. Some experimental and theoretical studies on excited-state hydrogen transfer in some isomeric naphthalene derivatives, benzoxazole derivatives, pyridoindole and pyrrolo-quinoline derivatives, green and red fluorescent proteins, hydrated halides, o-hydroxy carbonyl compounds, hydroxyl aromatic dopants and photoactive yellow protein will be presented. Moreover, investigations on controlling excited-state hydrogen transfer along hydrogen-bonded wires, excited-state double-proton transfer, ab initio QM/MM excited-state molecular dynamics, the reaction volume for photoinduced proton transfer in aqueous solutions of 6-methoxyquinoline, conformational switching between acids and their anions by hydrogen bonding, molecular recognition and chemical sensing of anions utilizing excited-state hydrogen bonding, photodissociation of hydrogen-bonded clusters and proton transfer reactions for dynamic quenching of fluorescence are also reported. The readers of this book will be faculties and researchers in the fields of molecular and supramolecular photochemistry, photobiology and photophysics. It will also serve as a good reference book for graduate students for the study of recent topics and progress in excited-state hydrogen bonding and hydrogen transfer. Finally, we would like to express our sincere thanks to all the authors who have contributed with their excellent chapters to the realization of this monograph. We greatly acknowledge the financial support by NSFC (Nos 20833008 and 20903094) and NKBRSF (Nos 2007CB815202 and 2009CB220010) and the assistance of co-workers from the group in the Dalian Institute of Chemical Physics (DICP) in the editorial process. Also, we thank the team at John Wiley and Sons Ltd, in particular Mr Paul Deards and Mr Mingxin Hou, for their helpful guidance during the entire project. Ke-Li Han Guang-Jiu Zhao State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China November 2010

Plate 1

The power spectrum of the velocity ACF for the atoms in the QM region of the A–U pair

Plate 2 A sequence of 2D IR photon echo spectra given by the real part of equation (1.8) of a 0.1 M solution of A–U in CDCl3 in the NH stretching region for different population times T. Each panel has been separately normalized to the maximum value of the signal

15

0.8

10

0.6

5 0.75

0.80

0.85

0.90

0.95

(1-A /A)/[HFIP]

absorbance / arbitrary

1.0

1.00

A /A

0.4

0.2

0.0 15000

20000

25000

30000

35000

wavenumber / cm-1

Plate 3 Absorption (full line) and fluorescence spectra (dotted line) of I (black line) and of the complex of I with HFIP (red line) in n-hexane. (At 25 000 cm
1 the increasing absorbance corresponds to 0, 0.0027, 0.0054, 0.011, 0.019, 0.027, 0.040, 0.053 and 0.080 mol dm
3 [HFIP], and the derived complex species spectrum.) Inset: linearized plot (in accordance with equation (3.3)) of the 400 nm absorbance results. Reprinted with permission from [6]. Copyright 2005 American Chemical Society

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

0.8

3

absorbance

0.6

0.4

h

i

j

o n m l k

g f e

0.2

[HFIP] / mol dm (a) 0.0 (b) 2.2E-4 (c) 4.4E-4 (d) 0.0011 (e) 0.0022 (f) 0.0033 (g) 0.0054 (h) 0.01 (i) 0.016 (j) 0.023 (k) 0.044 (l) 0.066 (m) 0.087 (n) 0.108 (o) 0.129

d

0.0 300

c b a

310

320

330

340

350

360

370

380

390

400

wavelength / nm

Plate 4 Absorption spectra of DMPN (8  10
5 mol dm
3) with various [HFIP] in n-hexane. Reprinted with permission from [8]. Copyright 2004 American Chemical Society

absorbance

2.0

1.5

N

2.0

absorbance

N

1.5 1.0 0.5 0.0 200

220

240

260

280

300

wavelength / nm 1.0

0.5

0.0 200

220

240

260

280

300

wavelength / nm

Plate 5 Absorption spectra of DMAP (6.14  10
5 mol dm
3) with and without HFIP additive in n-hexane. (At 280 nm, the increasing absorbance corresponds to 0, 0.00006, 0.00013, 0.00026, 0.00052, 0.0009, 0.0013, 0.0019, 0.0033, 0.0059, 0.011, 0.018, 0.028, 0.039, 0.050, 0.062, 0.076, 0.090, 0.105, 0.121, 0.137 and 0.154 mol dm
3 HFIP concentrations.) Inset: the resolved absorption spectra of the uncomplexed (black line), singly (red line) and doubly (blue line) complexed species. Reprinted with permission from [17]. Copyright 2007 American Chemical Society

2.0

A absorbance

3.5 3.0 2.5

1.5

N

1.0 0.5

2.0

0.0 200

220

240

260

280

300

320

wavelength / nm

1.5 1.0

0.0 3.5 3.0 2.5

1.5

B

N absorbance

absorbance

0.5

1.0

0.5

2.0 0.0 200

1.5

220

240

260

280

300

320

wavelength / nm

1.0 0.5 0.0 200

220

240

260

280

300

320

wavelength / nm

Plate 6 Absorption spectra of pyridine (4.3  10
4 mol dm
3) (A) and N,N-dimethyl aniline (9.3  10
5 mol dm
3) (B) with and without HFIP additive in n-hexane. (The increasing absorbance corresponds to 0, 0.0034, 0.0069, 0.010, 0.017, 0.025, 0.036, 0.052, 0.069, 0.104, 0.175, 0.25, 0.36 and 0.54 mol dm
3 [HFIP] at 250 nm (A), and to 0, 0.0033, 0.0065, 0.010, 0.016, 0.023, 0.033, 0.043, 0.052, 0.066, 0.082, 0.099, 0.13, 0.17, 0.20 and 0.27 mol dm
3 [HFIP] at 275 nm (B).) Insets: the resolved absorption spectra of the uncomplexed (black line), singly (red line) and doubly (blue line) complexed species. Reprinted with permission from [17]. Copyright 2007 American Chemical Society

Plate 7 The calculated IR spectra of fluorenone in both the S0 and T1 states. The ground-state IR spectrum of the hydrogen-bonded FN
MeOH complex is also shown for comparison

Plate 8 Calculated IR spectra of the hydrogen-bonded FN
MeOH complex in both the S0 and T1 states

O. D.

fl. intensity (a. u.)

2.0x106

1.5x106

1.0x106

wavelength (nm)

5.0x105

0.0 390

420

450

480

510

540

wavelength (nm)

Plate 9 Steady-state fluorescence spectra of DBPZ (1  10
5 M) (lex ¼ 350 nm) in cyclohexane (—) and in MeCN (----). The inset shows absorption spectra of DBPZ (1  10
5 M) in MeCN (black), 63% (v/v) water–MeCN mixture (red), 2% MeCN–water mixture (blue) and MeCN–HClO4 mixture (green). (The inset is Reprinted with permission from [70]. Copyright 2007 American Chemical Society)

Plate 10 Geometrically optimized structures of DBPZ–1H2O (Ort 1) and two possible structures of DBPZ–2H2O (Ort 2 and Ort 3) systems. Colour code: red ¼ oxygen; blue ¼ nitrogen; yellow ¼ carbon; white ¼ hydrogen. Reprinted with permission from [70]. Copyright 2007 American Chemical Society

1599

1357

1481

#

1167

363

808

(b) 400ns 200ns 100ns 80ns 60ns 40ns

Intensity (arb. units)

20ns

10ns

(a) 6000ps 3000ps 1000ps 500ps 100ps 50ps 10ps 5ps 2ps 0ps

400

600

800

1000 1200 1400 -1 Raman Shift (cm )

1600

1800

Plate 11 Picosecond-TR3 (a) and nanosecond-TR3 (b) spectra of HA in water solution obtained at various time delays with 267 nm pump and 400 nm probe wavelengths for the picosecond spectra and 266 nm pump and 416 nm probe wavelengths for the nanosecond spectra. The band labelled by # is due to a stray laser line. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

(b)

# 500ns 300ns 200ns 100ns 50ns

Intensity (arb. units)

20ns 10ns

(a)

6000ps 3000ps 1000ps 500ps 100ps 50ps 10ps 5ps 2ps 0ps

400

600

800

1000

1200

1400

1600

1800

-1

Raman Shift (cm )

Plate 12 Picosecond-TR3 (a) and nanosecond-TR3 (b) spectra of HA in buffered water solution with pH ¼ 9.0, obtained at various time delays with 267 nm pump and 400 nm probe excitation wavelengths for the picosecond spectra and 266 nm pump and 416 nm probe wavelengths for the nanosecond spectra. The band labelled by # is due to a stray laser line. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

1527 1567 1617

1074

843

705

367

700ns 500ns

Intensity (arb. units)

300ns

150ns 100ns 60ns 40ns 20ns 10ns 0ns

400

600

800 1000 1200 1400 1600 1800 -1

Raman Shift (cm )

Plate 13 Nanosecond-TR3 spectra of HA in water solution obtained at various time delays with 267 nm pump and 341.5 nm probe wavelengths. Features in blue are due to the neutral HA triplet state. See the text for details of the attributions of other features. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

1527 1302 1322

1168

843 367

1617

1074

705

1567

(a)

Intensity (arb. units)

(b)

(c)

400

600

800

1000 1200 1400 -1 Raman Shift (cm )

1600

1800

Plate 14 Comparison of the ns-TR3 spectrum (a) obtained at a 100 ns time delay in Figure 13.7 with the 341.5 nm resonance Raman spectrum of the ground-state HA anion (b) obtained in water/NaOH (0.1 M) solution. B3LYP/6311G(d, p) DFT-calculated normal Raman spectrum of the HA anion ground state (c) is also shown to compare and help with the assignment of the experimental spectra. Features in black shown in spectrum (a) are due to the unidentified species ‘X’ (see the text for details). Reprinted with permission from [26]. Copyright 2005 American Chemical Society

Intensity (a.u.)

4

Time Delay (ps)

(b)

100 70 49 35 24 17 12 8 6 4 3

(a)

3

2

1

0

(c)

*

#

Intensity (a.u.)

(d)

*

#

(e)

* #

*

(f)

# 200

250

300

350

400

450

500

550

Wavelength (nm)

Plate 15 (a) Femtosecond-KTRF contour of HPA obtained with 267 nm excitation in MeCN. (b) Normalized fluorescence decay at 330 nm (circles in blue) and 440 nm (circles in red) for HPA in MeCN. The solid lines show two exponential fittings to the experimental data; the dotted line is the instrumental response function (see Ref. [24] for details). (c) to (f) steady-state absorption spectrum (in black) and typical fluorescence profile of the blue (in blue) and red (in red) fluorescence for HPA in MeCN (c), THF (d), MeOH (e) and 90% H2O/10% MeCN mixed solvent (f). The absorption spectra were normalized to the blue fluorescence spectra. Sharp features indicated by # and  are the solvent Raman band and the second harmonic generation of the 800 nm gating pulse from the Kerr medium respectively. Reprinted with permission from [24]. Copyright 2005 American Chemical Society

Viscosity (cP)

70 (a) RTIL-acetonitrile mixture RTIL-methanol mixture 60 50 40 30 20 0.10

0.15

0.20

0.25

0.30

0.35

0.40

τ avs(ns)

80

RTIL-acetonitrile mixture R=0.99171 RTIL-methanol mixture R=0.95902

70

(b)

Viscosity(cP)

60 50 40 30 20 1.0

1.5

2.0

2.5

3.0

3.5

τrot(ns)

Plate 16 The plot of (a) average solvation time versus bulk viscosity of RTIL þ cosolvents mixtures and (b) average rotational relaxation time versus bulk viscosity of RTIL þ cosolvents mixtures. Reprinted with permission from the American Chemical Society. Copyright 2009 0

10

420 nm 532 nm -1

Intensity

10

-2

10

-3

10

0

500

1000

time (ps)

Plate 17

Instrumental pulse response at 420 nm (violet) and 532 nm (green)

10000 CURC in Ethyl Acetate: 1000

Experimental decay pattern Single exponential fit

Counts

100

Residuals

10

1 200 100 0 -100 -200

0

5000

10000

15000

20000

Plate 18 Upper panel: experimental fluorescence decay pattern of CURC in ethyl acetate (black dots) and single exponential fitting curve (red line; t ¼ 468 ps). Lower panel: residual plot

Plate 19 The K minus BR (A), L minus BR (B), M minus BR (C) and N minus BR (D) difference infrared spectra in the 2750–1930 cm
1 region. The spectra are compared between hydration with D2O (red lines) and D218O (blue lines). The K, L, M and N intermediates were produced by illuminating BR films at 77, 170, 230 and 270 K respectively. The grey curve in the 2700–2000 cm
1 region represents O
D stretching vibrations of D2O. Greenlabelled frequencies correspond to those identified as O
D stretching vibrations of water. Purple-coloured tags represent O
D stretch of Thr89 [22, 23], while the underlined tags represent N
D stretch of the Schiff base [24, 53]. Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC

L-BR

SIMILAR!!

artifactual cytoplasmic water cavity at 170 K

3700

3600

No water cluster deprotonation at 230K

293K 230K strongly weakly H-bonded H-bonded water water

293K 170K 3800

M-BR

3500 -1

wavenumber / cm

3400

3800

3600

2800

2600 -1

wavenumber / cm

293K 230K 2200

2000

1800 -1

wavenumber / cm

Plate 20 Spectral comparison of water signals in the L minus BR (left) and M minus BR (middle, right) difference FTIR spectra between room temperature (red lines) and low temperature (blue lines). Modified with permission from [25]. Copyright 2008 American Chemical Society

Plate 21 (A) The K minus D85S difference infrared spectra of the absent halide (a) and containing Cl
(b), Br
(c) or I
(d) bound form in the 2700–2000 cm
1 region. The sample was hydrated with D2O (red curves) or D218O (blue curves), and spectra were measured at 130 K. Green-labeled frequencies correspond to those identified as O
D stretching vibrations of water. Modified with permission from [37]. Copyright 2006 American Chemical Society. (B) The K minus BR difference infrared spectra of the wild-type (a), halide-free (b), Cl
-bound (c), Br
bound (d) and I
-bound (e) D212N in the 2380–2020 cm
1 region. The samples were hydrated with D2O (red curves) or D218O (blue curves), and spectra were measured at 77 K. Green-labeled frequencies correspond to those identified as O
D stretching vibrations of water. Underlined frequency (2171 cm
1) in the wild type also contains the N
D stretch of the Schiff base [24]. Modified with permission from [39]. Copyright 2007 American Chemical Society

Rhodopsins Having strongly H-bonded water D85N and D212N BR

Having proton-pump activity Bacteriorhodopsin (BR) various BR mutants including D212N(Cl)

D85S(Cl) BR 13-cis,15-syn BR Halorhodopsin (HR)

Azide-bound Halorhodopsin Anabaena Sensory Rhodopsin (ASR)

Salinibacter Sensory Rhodopsin I Sensory Rhodopsin II (SRII) without HtrII

Sensory Rhodopsin II (SRII) with HtrII

Neurospora Rhodopsin (NR)

Proteorhodopsin (PR) Leptosphaeria Rhodopsin (LR)

Visual Rhodopsins

Plate 22 Various rhodopsins are classified in view of (i) proton-pump activity and (ii) whether they have strongly hydrogen-bonded water molecules (O
D stretch in D2O at <2400 cm
1). Blue, green and red characters indicate proton-pump, chloride-ion pump and sensor function respectively, while black characters represent non-functional proteins

Plate 23 Types of reaction sequence (top panel) and cofactor arrangement (bottom panel) in the PS II core (view is along the membrane normal). In the bottom panel the cofactors are coloured in green (Chl), yellow (Pheo), magenta (PQ-9), red (carotenoids) and yellow-grey (YZ) respectively. Ca (yellow), Fe (blue) and Mn (red) are shown as spheres. The coordinating protein subunits D1 and D2 are indicated by a dotted line. Red arrows symbolize the direction of light-induced charge separation (sequence 1); blue and green arrows symbolize the pathway of oxidative water splitting (sequence 2); orange arrows symbolize the steps leading to plastoquinol formation (sequence 3)

Plate 24 Extended Kok cycle of oxidative water splitting. Photo-oxidation of P680þ is marked by red arrows labelled with hn, the redox component Yz is symbolized by a yellow dot, Si states are marked in the following way: the dark stable redox state S1 by black, the metastable redox states S2 and S3 by green (without and with a green background respectively), the transient ‘elusive’ state S4 and the ‘superreduced’ S
i states by grey symbols. The dark reactions of S0 with YDox (marked in purple) and S2 and S3 with YD (marked in black) are symbolized by purple arrows. Reprinted with kind permission from [1]. Copyright 2008 Springer Science and Business Media *

Plate 25 Reaction scheme (left-hand panel) and energetics (right-hand panel) of P680þ reduction by YZ in PS II complexes with an intact water-oxidizing complex (WOC) in redox state S1. The initial state I and the two relaxed states R, 1 and R, 2 are marked in red (I), green (R, 1) and blue (R, 2). For the sake of simplicity, the panel on the righthand side presents only DDE values for the different relaxation states because the absolute energy gap between
[YZ1 P680* Pheo QA] and [YZox P680 Pheo Q
A ] is not exactly known. Likewise, energetic relaxations around the QA
site and the energy loss due to partial reoxidation of Q
by Q (Q ) in the microsecond time domain are omitted (for B A B further details, see the text). Reprinted with kind permission from [1]. Copyright 2008 Springer Science and Business Media *

*

*

*

*

Plate 26 Structural model where a part of Chl PD1 (marked in green), the redox active TyrZ and the metal ions of the  Mn4OxCa cluster are shown as derived from the 3.0 A structure [48]. The view is approximately along the membrane plane, with the lumenal side at the bottom. Mn (red) and Ca (green) are shown as spheres, amino acids and structural elements from D1 (yellow) and CP43 (magenta) in the vicinity of the Mn4OxCa cluster are labelled. The figure was generated by J. Kern using Pymol (Delano, 2003). Reprinted with kind permission from [1]. Copyright 2008 Springer Science and Business Media

Plate 27 Coupling of ET and PT steps in the reactions of Y ox Z with the redox states Si of the WOC. Left-hand panel: mechanisms for separate pathways of ET and PT in the reduction of Y ox Z and oxidation of the manganese cluster with coordinated substrate water. Right-hand panel: concerted ET and PT (‘H atom abstraction’ type mode.) The neutral YZ radical is marked in red, His 190 and the coupled hydrogen bond network (XHB) in green, water (the actual protonation state is not specified) coordinated to manganese in blue, manganese in orange (the symbols Mnx and Mny symbolize different manganese of the Mn4OxCa cluster) and H-transfer channels from the water ligand(s) in purple. For further details, see the text. Reprinted with permission from [150]. Copyright Elsevier *

Plate 28 Schematic representation of proposed oxywater–hydrogen tautomerism of the multistate redox level S3 of the WOC. The presumed hydrogen peroxide state and the coordinating metal centres S3(P) are labelled in green, with the state S3(W) of substrate water coordinated to metal centre M (marked in blue) asymmetrically bound to oxomanganyl (marked in red). M symbolizes a not yet assignable metal centre (either Mn or Ca). Dynamic changes of the protein matrix coupled with the transitions between different forms of the S3 equilibria are symbolized by differently shaped grey areas. Reprinted with permission from [17]. Copyright Elsevier

Plate 29 The two tautomeric forms of 7-hydroxyquinoline. Arrows indicate the hydrogen bond directionality of the solvent wires, from Hþ /H atom donor to acceptor group

Plate 30 7HQ(NH3)3 CIS fully optimized stationary points along the ESHAT pathway. Each line corresponds to a single step in the enol ! keto tautomerization reaction path. The circle indicates the H atom that moves during each step

Plate 31 Structure of the fluorescent moiety of the green fluorescent protein: the chromophore responsible for the fluorescence is hydrogen bonded to a water molecule, followed by a serine residue and a glutamate residue that connects to the scaffold chromophore. The amino acid backbone has been removed to emphasize the analogy of this system with 7HQ(NH3)3

Plate 32 CIS/6-31(þ)G(d, p) optimized structures of (a) the bare GFP chromophore, (b) system (a) hydrogen bonded to a water molecule, (c) system (b) with a histidine-like residue hydrogen bonded to the hydroxyl group of the chromophore and (d) system (c) with a threonine-like residue hydrogen bonded to the hydroxyl group of the chromophore

Plate 33 2C-R2PI spectra of 7HQ(NH3)3 (AAA), 7HQNH3H2ONH3 (AWA), 7HQNH3(H2O)2 (AWW) and 7HQ(H2O)3 (WWW). The structure of the clusters is shown on the right; the 7HQ scaffold is partially cut away. The AWA isomer is separated from the AAW isomer by UV-UV hole burning; this spectrum is shown here instead of the 2C-R2PI

Plate 34 Time evolution of the relative energy of S0 and S1 states ((a) 7AI–H2O and (b) 7AI–(H2O)2) and the interatomic distances related to the proton transfer ((c) 7AI–H2O and (d) 7AI–(H2O)2) along the AIMD simulation. Reprinted with permission from [15]. Copyright 2008 American Chemical Society

Plate 35 A time evolution of the energy of S0 and S1 states relative to the S0 energy at the equilibrium geometry along the AIMD trajectory for (a) C151 and (b) C151 þ EFP. The S0 energies calculated by the state-specific CASSCF method for the S0 state are shown by the dashed line (S00 ). Reprinted with permission from [16]. Copyright 2009 Wiley Periodicals, Inc., A Wiley Company

UV-vis in benzene FL in benzene FL in ethanol

O O N

Fluorescence (a.u.)

Absorbance (a.u.)

HO

O N HO

300

350

400

O

450

500

550

600

650

Wavelength (nm)

Plate 36 Absorption and fluorescence (FL) spectra of HBO derivatives 3 (bottom) and 4 (top). Adapted with permission from [9]. Copyright 2007 The Chemical Society of Japan

H

0.84 Å

1.888Å

O

O

H

O N

N H

1. 97Å

O R

O

N

O

O

R

0. 82Å

1a

1b

N O

29

H

(a: R =H; b: R = t-Bu) O(2A)

O

C(10) C(9)

O

C(11)

O N(1) C(7)

C(8)

O C(12)

C(13)

C(6)

O

(a)

O C(3)

O(2B)

O

C(5) C(4)

C(1) C(2)

O(1)

O

O

O

O

O O

O

O(2A) OO

O

O

O(1)

O

N(1A)

OO

O

O

O

O O O

O(1A) O

N(1) O

O O(2)

O

O

O O

O

(b)

Plate 37 (a) The crystal structure of 1 reveals that the intramolecular hydrogen-bonded rotamers 1a and 1b exist in about 1:1 ratio. (b) The crystal structure of 29b shows that both hydroxyl groups are hydrogen-bonded to N-atom of oxazole rings. Adapted with permission from [29]. Copyright 2007 American Chemical Society

Absorbance (arbitrary unit)

5

0.4

4

0.2

0.0

3

350

400

450

d 2

c

b

1 a 0 300

350

400

450

Wavelength (nm)

Plate 38 Steady-state absorption spectra of the ground electronic state of fluorenone in cyclohexane (a), acetonitrile (b), methanol (c) and TFE (d) [55]. Adapted with permission from [55]. Copyright 2005 American Chemical Society

Plate 39 (A) Absorption spectra of fluorenone in cyclohexane (black) and in 1-octanol (red). The concentrations are 25 mM, and the optical path length is 0.5 mm. (a) Free fluorenone, (b) fluorenone complex with one alcohol molecule and (c) complex with two alcohol molecules. (B) Temperature dependence of the absorption spectrum of 9-fluorenone in 1-octanol [59]. Reprinted with permission from [59]. Copyright 2007 Elsevier

Plate 40 Schematic depiction of the single color pump–probe spectroscopy. The pump produces a ground state bleach and excited state emission. For systems with large anharmonicities (D), no excited state absorption is observed. The “molecule” in the excited state and the “hole” in the ground state result in the amplification (from stimulated emission) and reduced absorption of the probe pulse, respectively. As the excited state decays, the absorption of the probe increases, resulting in a reduced pump–probe signal. Reprinted with permission from [59]. Copyright 2007 Elsevier

Plate 41 IR pump–probe signals of 9-fluorenone in (a) cyclohexane at 1727 cm1, (b) 1-octanol at 1723 cm1 and (c) 1-octanol at 1712 cm1. Red lines are fitting results by a single exponential [59]. Reprinted with permission from [54a]. Copyright 1993 Elsevier

16 0.15 ps 0.4 ps 5 ps

12 Δ Absorbance (mOD)

15 ps 30 ps 100 ps a 8

b

1 4

0 500

550

600

650

0 500

550

600

650

Wavelength (nm)

Plate 42 Time-resolved transient absorption spectra of fluorenone in 1-propanol, constructed for different delay times following photoexcitation using 400 nm laser pulses. Inset: comparison of transient spectra constructed at 0.15 ps delay time following photoexcitation of fluorenone in acetonitrile (a) and 1-propanol (b) [55]. Reprinted with permission from [111]. Copyright 2005 American Chemical Society

6

0.15 ps 0.4 ps 1 ps 2 ps 5 ps 10 ps 20 ps

(A)

4 a'

Δ Absorbance (mOD)

2

4

a 0

0

500

550

600

650 6

(B) 6

b'

3

b 4

20 ps 40 ps 60 ps 100 ps

2

0

500

550

0 500

600

550

600

650

Wavelength (nm)

Plate 43 Time-resolved transient absorption spectra constructed for different delay times following photoexcitation of fluorenone in TFE using 400 nm laser pulses. (A) Time-resolved spectra in sub-20 ps time domain. Inset of (A): comparison of the transient spectra constructed for 0.15 ps delay-time in TFE (a) and in 1-propanol (a0 ). (B) Timeresolved spectra in post-20 ps time-domain. Inset of (B): comparison of the transient spectra constructed for 100 ps delay-time in TFE (b) and in 1-propanol (b0 ) [55]. Reprinted with permission from [111]. Copyright 2005 American Chemical Society

a

(A)

(B) g

0 g

-3 a

-6

-9

500

Delay times (ps): (a) 0.2, (b) 0.5, (c) 1, (d) 2, (e) 10, (f) 20 and (g) 30

600 700 Wavelength (nm)

2

Δ Absorbance (mOD)

Δ Absorbance (mOD)

3

Delay-times (ps) : 0.15, 0.3, 0.5, 1, 5, 20, 40, 80 and 150 ps

0

-2

500 550 600 650 700 750 800

Wavelength (nm)

Plate 44 Time-resolved transient absorption spectra constructed at different delay times following photoexcitation of KCD in DMSO (a) and 1-propanol (b) using 400 nm laser pulses [111]. Adapted with permission from [53]. Copyright 2003 American Chemical Society

Plate 45 Absorption (solid line) and emission (dotted line) spectra (a) of polymers 6 and 7, in anhydrous THF, and UV–vis (solid line) and fluorescence (broken line) spectra (b) of polymer 7 and its anion complexes. Reprinted with permission from [24]. Copyright 2007 American Chemical Society

Plate 46 Titration spectra of compound 8 with Bu4NOH in THF/EtOH (30 : 1). Reprinted with permission from [24]. Copyright 2007 American Chemical Society

Plate 47 Titration spectra (a) and Benesi–Hildebrand plot (b) of 8 with Bu4NF in THF/EtOH (100 : 1). Reprinted with permission from [24]. Copyright 2007 American Chemical Society

Plate 48 Photodissociation of HBr molecules on water clusters. Deuteration experiments pointing to the generation and dissociation of the H3O radical

Plate 49 Measured KEDs of Py (a, b) and Im (c, d) clusters at 243 nm. The top panels correspond to small clusters of   8 (b) and pure mean size n  3, while the bottom spectra correspond to mixed PynArm clusters with n  4 and m Imn clusters with n  6 (d). The spectra are analyzed for a slow (blue) and fast (red) component

Plate 50 Fast-to-slow fragment ratios evaluated from the measured KEDs for Py and Im clusters at 243 nm plotted as a function of the cluster mean size. The F/S ratio obtained from Figure 19.17(b) for Py is plotted at n ¼ 12, which corresponds to the total mean size of the mixed PynArm cluster

Fast/Slow H-fragment ratio

Plate 51 Measured KEDs of Py (a), Im (b) and Pz (c) clusters with n  3 at 193 nm. The spectra are analyzed for a slow (blue) and fast (red) component

Plate 52

0.3

Pz Py

0.2

Im 0.1 0.0

0

5 Mean cluster size n

10

Fast-to-slow fragment ratios evaluated from the measured KEDs for Py, Im and Pz clusters at 193 nm

1 Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs Yun-an Yan and Oliver K€ uhn Institut f€ur Physik, Universit€at Rostock, D-18051 Rostock, Germany

1.1 Introduction The structural selectivity of complementary pairing of nucleic acid bases in DNA signifies the importance of hydrogen bonding in biology [1, 2]. This fact has triggered a host of investigations, starting with gasphase and microsolvation studies of the stability and spectroscopy of nucleic acid bases as the fundamental building blocks [3, 4]. Here, the specific changes in the infrared (IR) spectrum owing to hydrogen bonding provide a sensitive means of identification of bonding patterns, not only in gas but also in condensed phase [5, 6]. For isolated adenine–thymine pairs, for instance, which were studied in gas phase using the IR–UV double-resonance technique [7], evidence was found that Watson–Crick pairing is not very likely under these conditions. In order to identify the dominant tautomer, it was necessary to perform quantum dynamical simulations of IR spectra, including anharmonicity [8]. Anharmonicity of the potential energy surface is characteristic of hydrogen-bonded species. Together with the quantum nature of the problem, it makes hydrogen bond (HB) dynamics a challenging task even in gas phase [9–11]. The issue of preferential association of nucleobases carries over to solution-phase studies. In a seminal work, Rich and coworkers [12] studied the homo- and heteropairing of various adenine and uracil derivatives in deuterochloroform solution using IR spectroscopy. This work triggered investigations of a number of nucleic acid base pairs [13–17]; a review of IR bands in the 800–1800 cm
1 range in aqueous solution can be found in Ref. [18]. Linear IR spectroscopy cannot disentangle the rich information on the HB dynamics that is encoded in the broad and often structured lineshapes of IR spectra, especially in the condensed phase. Ultrafast nonlinear IR spectroscopy, on the other hand, has been proven to provide the means for addressing this dynamical information, thus giving access to anharmonic couplings, fluctuation timescales and pathways of vibrational

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

2 Hydrogen Bonding and Transfer in the Excited State

energy flow [9, 19–21]. As far as single base pairs are concerned, Woutersen and Cristalli reported an ultrafast IR pump–probe study of modified adenine–uracil pairs in deuterochloroform solution [22]. Most notably, they found that the lifetime of the N–H  N H-bonded NH stretching fundamental transition at 3185 cm
1 decreased by a factor of 3 down to 340 fs as compared with the monomer case. This is a clear signature of HB-mediated anharmonic couplings. Further, analysis of transient spectra revealed a two-step relaxation process, with the HB acting as an intermediate accepting mode, which subsequently relaxes on a 2.4 ps timescale. This finding is in accord with the picture obtained from vibrational cooling studies of nucleobases in aqueous solution following electronic excitation and subsequent internal conversion back to the ground state [23]. For the symmetric NH2 stretching vibration at 3315 cm
1, which is involved in a N–H  O HB, an analogous behaviour can be anticipated. Although this band was not the focus of the study in Ref. [22] and was excited by the wing of the pump pulse spectrum only, a similar subpicosecond relaxation is seen in the transient spectrum. Very recently, several ultrafast IR studies of DNA oligomers have been reported. Heyne and coworkers have been the first to show how the anharmonic coupling between the NH2 stretching mode and vibrations of the fingerprint region in an adenine–thymine pair can be exploited to assign the fundamental transition of the NH2 stretching vibration which is completely masked by water absorption [24, 25]. Subsequently, the assignment has been confirmed by anisotropy measurements [26, 27]. For vibrational relaxation of the NH2 stretching vibration, a mechanism involving first ultrafast energy transfer to the NH stretching vibration and then a 600 fs decay into the NH-bending region has been inferred from IR pump/anti-Stokes Raman probe measurements [28]. For the decay of these fingerprint modes, a timescale of several picoseconds was obtained. Direct excitation of the fingerprint region with the NH2-bending and CO stretching modes yielded a biexponential decay with time constants of 0.4 and 1.4 ps [24, 25]. As discussed by Baerends and coworkers, a purely electrostatic description of the HBs in Watson–Crick base pairs is questionable, and charge transfer along the HB as well as resonance assistance needs to be accounted for [29]. This necessitates a quantum treatment of the electrons involved in the hydrogen bonding of nucleic acid base pairs for simulating the dynamics, which adds to the complication already arising from the quantum nature of the proton motion mentioned above. Different approximate strategies are usually followed when dealing with condensed-phase IR spectra. In order to capture the electronic structure, density functional theory (DFT) seems to be most appropriate, as long as stacking interactions are of minor importance [30]. Neglecting quantum effects, the stationary IR spectrum can be obtained from the Fourier transform of the dipole autocorrelation function (ACF) along classical trajectories computed with on-the-fly DFT forces. Car–Parrinello molecular dynamics (CPMD) [31] has been proven to be particularly well suited for simulating condensed-phase dynamics, including H-bonded systems. An important contribution was made by Silvestrelli et al., who determined the IR spectrum of liquid water [32]. Subsequent applications to aqueous solutions, e.g. of uracil, have been summarized in Ref. [33]. More recent investigations have focused on strongly H-bonded systems such as the Zundel cation in the crystalline phase [34]. Different strategies have been developed in order to account for quantum effects in nuclear motion. A popular method makes use of so-called quantum correction factors [35]. Note, however, that quantum correction factors may have a different influence on quantities related to linear and nonlinear correlation functions. Skinner and Lawrence showed this for the exemplary case of HOD in D2O [36]. Quantum effects for the fast proton stretching motion can also be recovered by calculating its potential energy curve for selected configurations along the CPMD trajectory (snapshot potentials). Solution of the stationary Schr€ odinger equation for the motion in these potentials yields a set of fundamental transition frequencies. The combination of CPMD and snapshot potentials for the proton stretching motion has been put forward in Ref. [37]. In this reference, an intramolecular N–H  O HB in a Mannich base in gas and crystalline phase was considered. The IR spectrum was calculated by convolution of the stick spectrum obtained along the trajectory with a Gaussian. A related simulation in CCl4 solution can be found in Ref. [38]. In Ref. [39] the

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

3

method was applied to an N-oxide that exhibited two rather strong non-equivalent O–H  O HBs in the unit cell. Here, two-dimensional (2D) snapshot potentials, including the bending vibration of the HB, were considered as well. Finally, a different N-oxide in the crystalline phase with a rather strong HB was studied in Ref. [40]. Quantum mechanics/molecular mechanics (QM/MM) hybrid methods have been developed to cope with situations where only a part of a large system needs to be treated at the quantum mechanical level (for a review, see Ref. [41]). Here, the critical point is the interface between the QM and MM parts, and, in the context of CPMD, different schemes for the non-bonding electrostatic interactions have been developed [42, 43]. A series of applications to the linear IR spectroscopy of various forms of water cluster in a model of bacteriorhodopsin has been presented in Refs [44–46]. Recently, we have applied the QM/MM approach to the determination of the IR lineshape of the uracil NH stretching vibration in a modified adenine–uracil (A–U) pair in deuterochloroform solution [47]. Instead of calculating snapshot potentials, an empirical NH-frequency–N  Ndistance correlation was used, which gave access to the fluctuating transition frequency along the trajectory (see also Refs. [48, 49]) and therefore to the lineshape function. This chapter gives an account of our work on the QM/MM simulation of the IR spectroscopy of A–U base pairs in CDCl3 solution [47]. The theory of linear and nonlinear IR spectroscopy is briefly reviewed in Section 1.2 on the basis of the response function formalism [50]. After introducing the QM/MM protocol in Section 1.3, we will discuss correlations of HB parameters obtained along a classical trajectory. Special attention is paid to the lineshape function, which contains information on the correlated fluctuations of the two HBs in the A–U pair. These frequency fluctuations are discussed for snapshot-based potentials, as well as for the case of empirical frequency–distance correlations. Further results for pump–probe and 2D IR photon echo spectroscopy of the NH stretching region are given. 2D spectroscopy is known to provide access to the correlated motion of different chromophores [51–53], and we will discuss the signatures of correlated N–H  N and N–H  O HB dynamics based on the QM/MM lineshape function. Finally, Section 1.4 summarizes this chapter.

1.2 Hydrogen Bonding and Nonlinear Infrared Spectroscopy 1.2.1 Four-wave mixing nonlinear IR spectroscopy The response of the molecular system to IR radiation is determined by different sources of nonlinearities, which include anharmonicity in the potential energy surface, a nonlinear dependence of the transition dipole moment on the vibrational coordinates and nonlinear coupling in the system–bath interactions [54–57]. In the traditional linear IR measurements, the spectra are ensemble averaged and fail to differentiate between the various contributions. Nonlinear IR spectroscopies adopt the strategy of multipulse nuclear magnetic resonance spectroscopy to disentangle the interactions of the system under investigation. This forms the basis for modeling the transient molecular structure and solvation dynamics [58]. Here, the above-mentioned effects are reflected in the positions, amplitudes and lineshapes of the nonlinear correlation spectrum. The positions of the peaks describe the transition frequencies of the system’s vibrational eigenstates. The peak amplitudes reflect the relative magnitudes and orientations of the transition dipole moments. Specifically, the 2D lineshapes are determined by the details of the system–bath interactions, which may cause a statistical variation in the eigenenergies in systems with coupled transition dipoles. In the following, we will investigate the linear and nonlinear vibrational spectroscopy of a system of two coupled high-frequency vibrational modes; an extension to more modes is straightforward. These modes will be embedded in and coupled to an environment (bath), Q, of lower frequency modes. In the spirit of the Born–Oppenheimer approximation, we can write the Hamiltonian of the total system under the interaction of

4 Hydrogen Bonding and Transfer in the Excited State

an incident light field E(t) in dipole approximation as follows: Htot ¼

X a;b

  ðdab Ha ðQÞ
Eðr; tÞmab Þa bj

ð1:1Þ

    where a ¼ i; j is that vibrational state where the first mode is in the ith and the second mode in the jth vibrational state, Ha(Q) is the adiabatic Hamiltonian associated with state jai and mab are the matrix elements of the dipole moment operator in Condon approximation. Further, we do not consider the vector character of dipole moments and incident fields. Note that, by virtue of the dependence on the bath modes Q, the adiabatic Hamiltonian becomes time dependent. In a third-order nonlinear spectroscopic measurement, there are three incident fields Eðr; tÞ ¼ E1 ðr; t þ t1 þ t2 Þexpðik1  r
iv1 tÞ þ E2 ðr; t þ t2 Þexpðik2  r
iv2 tÞ þ E3 ðr; tÞexpðik3  r
iv3 tÞ þ c:c:

ð1:2Þ

that generate the outgoing signal field. If the fields are well separated in time, the following picture applies. After interaction with the first pulse with wavevector k1, the system will interact with a second pulse with wavevector k2 after an evolution time t1. Then, a third pulse with wavevector k3 will interact after a waiting time t2. A signal field with wavevector ks, determined by the phase-matching condition ks ¼  k1  k2  k3, will be detected after time t3. The calculation of the polarization by means of third-order perturbation theory yields the third-order nonlinear response functions (in the following we set h ¼ 1) [50]   Sð3Þ ðt3 ; t2 ; t1 Þ ¼ i3 uðt3 Þuðt2 Þuðt1 Þ ½½½mðt3 þ t2 þ t1 Þ; mðt2 þ t1 Þ; mðt1 Þ; mð0Þreq

ð1:3Þ

where u(t) is the Heaviside step function, req is the equilibrium density matrix and h  i denotes the ensemble average. Expanding the commutators leads to Sð3Þ ðt3 ; t2 ; t1 Þ ¼ i3 uðt3 Þuðt2 Þuðt1 Þ

8 X

Ra ðt3 ; t2 ; t1 Þ

ð1:4Þ

a¼1

where Ra are multiple time correlation functions of the dipole moment operator.   R1 ðt3 ; t2 ; t1 Þ ¼ mðt1 Þmðt2 þ t1 Þmðt3 þ t2 þ t1 Þmð0Þreq ;   R2 ðt3 ; t2 ; t1 Þ ¼ mð0Þmðt2 þ t1 Þmðt3 þ t2 þ t1 Þmðt1 Þreq ;   R3 ðt3 ; t2 ; t1 Þ ¼ mð0Þmðt1 Þmðt3 þ t2 þ t1 Þmðt2 þ t1 Þreq ;   R4 ðt3 ; t2 ; t1 Þ ¼ mðt3 þ t2 þ t1 Þmðt2 þ t1 Þmðt1 Þmð0Þreq ; Ra ðt3 ; t2 ; t1 Þ ¼
R*a
4 ðt3 ; t2 ; t1 Þ;

ð1:5Þ

a ¼ 5--8

These eight nonlinear response functions are the basis for understanding the molecular dynamics contributing to a certain signal. Their evolution is usually depicted in Liouville space employing a diagrammatic

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

5

Figure 1.1 Double-sided Feynman diagrams describing the third-order response of a multilevel system that initially is in its ground state. Here, 0 stands for the ground state, a, b and c for excited states

approach [50]. The eight possible double-sided Feynman diagrams are shown in Figure 1.1. A particular phasematching condition can be selected on the basis of the rotating wave approximation. As an example, let us consider the pump–probe spectroscopy. Here, the field is written as Eðr; tÞ ¼ E1 ðt þ TÞexpðik1  r
iv1 tÞ þ E2 ðtÞexpðik2  r
iv2 tÞ þ c:c:

ð1:6Þ

Here, the system interacts with the first pump pulse twice (k1 ¼ k2 ¼ kpump in equation (1.2)), and the population changes that it triggers are interrogated by the probe pulse (k3 ¼ kprobe) which follows after a delay time T. The signal in the kprobe direction can be obtained from [50] Ð1 Ð1 Ð1 Ð1 SPP ðv1 ; v2 ; TÞ ¼ 13 2v2 Re
1 dt 0 dt3 0 dt2 0 dt1 E2 ðt
T
t3 ÞE2 ðt
TÞ   E1 ðt
t2 ÞE1 ðt
t2
t1 Þexpðiv2 t3 þ iv1 t1 Þ ½R1 ðt3 ; t2 ; t1 Þ þ R4 ðt3 ; t2 ; t1 Þ þ R6 ðt3 ; t2 ; t1 Þ þ E1 ðt
t2 ÞE1 ðt
t2
t1 Þexpðiv2 t3
iv1 t1 Þ  ½R2 ðt3 ; t2 ; t1 Þ þ R3 ðt3 ; t2 ; t1 Þ þ R5 ðt3 ; t2 ; t1 Þ :

ð1:7Þ

In the context of general four-wave mixing spectroscopy, the diagrams R2, R3 and R5 (ks ¼ k3 þ k2
k1), as well as R7 (ks ¼
k3 þ k2 þ k1), contribute to the so-called rephasing signals, as the signs of the phase of the first and third pulse are opposite during the interaction (i.e. combinations E1* E3 or E1 E3* ). The diagrams R1, R4 and R6 (ks ¼ k3 þ k2 þ k1), as well as R8 (ks ¼
k3 þ k2
k1), give the non-rephasing signal, where the signs of the phase of the first and third pulse are the same during the interaction (i.e. combinations E1E3 or E1* E3* ). Below, we will consider the photon echo rephrasing signal in the phase-matched direction ks ¼ k3 þ k2
k1 only. Moreover, we will adopt the impulsive limit, i.e. the temporal envelopes Ej(t) will be approximated by d-functions. This assumes that the pulse duration is much shorter than any molecular timescale. Within the impulsive limit, the signal for the rephrasing processes can be expressed by the Fourier transforms with respect to the time intervals t1 and t3, taken for a certain interval t2 ¼ T. This yields SPE ðv3 ; T; v1 Þ ¼

Ð1

Ð1 dt3
1 dt1 expðiv3 t3
iv1 t1 Þ ðR2 ðt3 ; T; t1 Þ þ R3 ðt3 ; T; t1 Þ þ R5 ðt3 ; T; t1 ÞÞ
1

ð1:8Þ

6 Hydrogen Bonding and Transfer in the Excited State

From equations (1.7) and (1.8) it can be seen that the molecular response functions, Ra, determine the thirdorder nonlinear IR spectroscopies. Thus, the central task of the following theoretical simulation will be to calculate the eight third-order nonlinear response functions. 1.2.2 Second-order cumulant expansion for linear and nonlinear spectroscopy The linear IR spectrum is determined by the first-order response function given by the dipole ACF (assuming that the initial density matrix factorizes with respect to the system and bath parts): 1 Re sðvÞ ¼ 2p 1 Re ¼ 2p

ð1
1

ð1

  dt eivt mðtÞmð0Þreq dt eivt


1

X a;b

  raa mab ðtÞmba ð0Þ Q

ð1:9Þ

where raa is the initial system density matrix and h. . .iQ denotes the thermal average with respect to the bath. Within the Condon approximation, the necessary matrix elements can be calculated as iHa t

mab ðtÞ ¼ e

mab e

 ðt  ~ ¼ mab T exp
i dt1 Uba ðt1 Þ


iHb t

ð1:10Þ

0

where T~ denotes time ordering and Uba(t1) is the gap coordinate, Uba(t1) ¼ exp{iHat}(Hb
Ha) exp{
iHat}. As the bath coordinates will be treated classically, equation (1.10) simplifies to   ðt mab ðtÞ ¼ mab exp iWab t þ i dt1 dvab ðt1 Þ

ð1:11Þ

0

where Wab ¼ hvabiQ is the thermal average of the transition frequency, and the fluctuations are given by dvab(t) ¼ vab(t)
Wab. Assuming Gaussian statistics, a second-order cumulant expansion can be performed, which gives the spectrum in terms of the lineshape function gab(t) as follows [50] (raa ¼ r00 ¼ 1): 1 X sðvÞ ¼ Re jma0 j2 p a

ð1

dt expfiðv
Wa0 Þt
gaa ðtÞg

ð1:12Þ

0

The lineshape function itself is defined as gab ðtÞ 

ðt

ðt dt

0

dt0 Cab ðt0 Þ

ð1:13Þ

0

where we have introduced the correlation function of the gap fluctuations   Cab ðtÞ ¼ dva0 ðtÞdvb0 ð0Þ Q

ð1:14Þ

The eight nonlinear response functions Ra(t3, t2, t1) (a ¼ 1–8) can be calculated along similar lines. To this end, we introduce an auxiliary fourth-order response function as follows:   Fðt4 ; t3 ; t2 ; t1 Þ ¼ mðt4 Þmðt3 Þmðt2 Þmðt1 Þreq

ð1:15Þ

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

7

Comparing equations (1.5) and (1.15), we find the following relations: R1 ðt3 ; t2 ; t1 Þ R2 ðt3 ; t2 ; t1 Þ R3 ðt3 ; t2 ; t1 Þ R4 ðt3 ; t2 ; t1 Þ

¼ ¼ ¼ ¼

Fðt1 ; t2 þ t1 ; t3 þ t2 þ t1 ; 0Þ; Fð0; t2 þ t1 ; t3 þ t2 þ t1 ; t1 Þ; Fð0; t1 ; t3 þ t2 þ t1 ; t2 þ t1 Þ; Fðt3 þ t2 þ t1 ; t2 þ t1 ; t1 ; 0Þ

ð1:16Þ

Inserting the completeness relation repeatedly, the four-point correlation function is expressed as X

Fðt4 ; t3 ; t2 ; t1 Þ ¼

a;b;c;d

0

  raa mad ðt4 Þmdc ðt3 Þmcb ðt2 Þmba ðt1 Þ Q

ð1:17Þ

P Note that 0 denotes the summation all over a, b, c, d except a ¼ b, b ¼ c, c ¼ d and d ¼ a. Substituting equation (1.10) into equation (1.17) yields Fðt4 ; t3 ; t2 ; t1 Þ ¼

X0 a;b;c;d

raa mad mdc mcb mba expfiðWad t4 þ Wdc t3 þ Wcb t2 þ Wba t1 Þg

   Ðt Ðt Ðt Ðt  exp i 04 dt dvad ðtÞ þ i 0 3 dt dvdc ðtÞ þ i 02 dt dvcb ðtÞ þ i 0 1 dt dvba ðtÞ Q ð1:18Þ

Carrying out a second-order cumulant expansion [50] of the above equation leads to X0 raa mad mdc mcb mba eiðWad t4 þ Wdc t3 þ Wcb t2 þ Wba t1 Þ expf
gbb ðt2
t1 Þ Fðt4 ; t3 ; t2 ; t1 Þ ¼ a;b;c;d

þ gcb ðt2
t1 Þ
gcb ðt3
t1 Þ þ gcb ðt3
t2 Þ
gcc ðt3
t2 Þ þ gdb ðt3
t1 Þ
gdb ðt3
t2 Þ
gdb ðt4
t1 Þ þ gdb ðt4
t2 Þ þ gdc ðt3
t2 Þ
gdc ðt4
t2 Þ þ gdc ðt4
t3 Þ
gdd ðt4
t3 Þg:

Using equation (1.16), we obtain the final expression for the nonlinear response functions: X0 R1 ðt3 ; t2 ; t1 Þ ¼ raa mad mdc mcb mba expfiWcb t3 þ iWdb t2
iWba t1 þ f1 ðt3 ; t2 ; t1 Þg; a;b;c;d X0 R2 ðt3 ; t2 ; t1 Þ ¼ raa mad mdc mcb mba expfiWcb t3 þ iWdb t2 þ iWda t1 þ f2 ðt3 ; t2 ; t1 Þg; a;b;c;d X0 R3 ðt3 ; t2 ; t1 Þ ¼ raa mad mdc mcb mba expfiWcb t3 þ iWca t2 þ iWda t1 þ f3 ðt3 ; t2 ; t1 Þg; a;b;c;d X0 R4 ðt3 ; t2 ; t1 Þ ¼ raa mad mdc mcb mba expf
iWda t3
iWca t2
iWba t1 þ f4 ðt3 ; t2 ; t1 Þg a;b;c;d

where fa are auxiliary functions given by f1 ðt3 ; t2 ; t1 Þ ¼
gbb ðt1 þ t2 þ t3 Þ
gcb ðt1 þ t2 Þ þ gbc ðt3 Þ
gcc ðt3 Þ þ gcb ðt1 þ t2 þ t3 Þ
gdb ðt1 Þ þ gdb ðt1 þ t2 Þ þ gbd ðt2 þ t3 Þ
gbd ðt3 Þ þ gcd ðt2 Þ
gcd ðt2 þ t3 Þ þ gcd ðt3 Þ
gdd ðt2 Þ; f2 ðt3 ; t2 ; t1 Þ ¼
gbb ðt2 þ t3 Þ
gcb ðt2 Þ þ gbc ðt3 Þ þ gcb ðt2 þ t3 Þ
gcc ðt3 Þ
gbd ðt1 Þ þ gdb ðt2 Þ þ gbd ðt1 þ t2 þ t3 Þ
gbd ðt3 Þ þ gcd ðt3 Þ þ gcd ðt1 þ t2 Þ
gcd ðt1 þ t2 þ t3 Þ
gdd ðt1 þ t2 Þ;

ð1:19Þ

ð1:20Þ

8 Hydrogen Bonding and Transfer in the Excited State

f3 ðt3 ; t2 ; t1 Þ ¼
gbb ðt3 Þ
gbc ðt2 Þ þ gbc ðt2 þ t3 Þ þ gcb ðt3 Þ
gcc ðt2 þ t3 Þ
gbd ðt1 þ t2 Þ þ gbd ðt2 Þ
gbd ðt2 þ t3 Þ þ gbd ðt1 þ t2 þ t3 Þ þ gcd ðt1 Þ þ gcd ðt2 þ t3 Þ
gcd ðt1 þ t2 þ t3 Þ
gdd ðt1 Þ; f4 ðt3 ; t2 ; t1 Þ ¼
gbb ðt1 Þ þ gcb ðt1 Þ þ gcb ðt2 Þ
gcb ðt1 þ t2 Þ
gcc ðt2 Þ
gdb ðt2 Þ þ gdb ðt1 þ t2 Þ þ gdb ðt2 þ t3 Þ
gdb ðt1 þ t2 þ t3 Þ þ gdc ðt2 Þ þ gdc ðt3 Þ
gdc ðt2 þ t3 Þ
gdd ðt3 Þ:

ð1:21Þ

The approach presented here has been pioneered by Mukamel et al. [50, 59]. Note, however, that in Ref. [59] a symmetric lineshape function was assumed which follows in the classical limit only. Equation (1.20) is, however, equivalent to the result given by Cho et al. [60, 61] in terms of the correlation functions Cab(t) (instead of gab(t)). So far we have tacitly assumed that the adiabatic approximation, equation (1.1), holds, that is, non-adiabatic transitions between system modes triggered by the bath mode dynamics have been neglected. Therefore, the present approach does not take into account the effect of vibrational relaxation. Although progress has been made in this respect [62, 63], we will adopt an empirical correction in the spirit of the Bloch model. For the linear response, this amounts to adding a Bloch relaxation term to the lineshape function, i.e. we use gaa ðtÞ ! gaa ðtÞ þ

t 2T1;a

ð1:22Þ

  in equation (1.12), with T1,a being the population relaxation time of state a . The nonlinear response functions, Ra, are supplemented by multiplicative factors, as suggested in Refs [50, 64]. These factors are given by

F1 F2 F5 F6 F7

8 <

9 t3 ðT1;a þ T1;b Þt2 t1 = ; ¼ exp


: 2T1;a 2T1;a T1;b 2T1;a ; 8 9 < t3 ðT1;a þ T1;b Þt2 t1 = ; ¼ F3 ¼ F4 ¼ F8 ¼ exp


: 2T1;b 2T1;a T1;b 2T1;a ; 8 9 < ðT þ T Þt = ðT þ T Þt t 1;a 1;c 3 1;a 1;b 2 1 ; ¼ exp


: 2T1;a T1;c 2T1;a T1;b 2T1;a ; 8 9 < ðT þ T Þt = ðT þ T Þt t 1;b 1;c 3 1;a 1;b 2 1 ; ¼ exp


: 2T1;b T1;c 2T1;a T1;b 2T1;a ; 8 9 < ðT þ T Þt t2 t1 = 1;a 1;c 3 ¼ exp


: 2T1;a T1;c T1;c 2T1;a ;

ð1:23Þ

In the following, we will discuss classical schemes for obtaining the time-dependent transition frequencies vab(t) along a molecular dynamics trajectory. This incorporates quantum effects in the system modes only. As far as the bath modes are concerned, different quantum corrections are usually applied [35]. Furthermore, ergodicity is assumed to obtain the thermal average by means of a time average along the trajectory.

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

9

1.3 Correlated Vibrational Dynamics of an Adenine–Uracil Derivative in Solution 1.3.1 Simulation details Our focus is on linear and nonlinear IR spectroscopy of the double HBs in nucleic acid base pairs. By way of example, we will consider the adenine–uracil base pair containing 9-ethyl-8-phenyladenine (A) and 1-cyclohexyluracil (U) in deuterochloroform solution. The ethyl substitution at the N9 atom of adenine and the cyclohexyl substitution at the N1 site of uracil are adopted to prevent tautomerism of adenine and uracil respectively. Phenyl substitution at the C8 site of adenine is essential to avoid Hoogsteen pairing (see Figure 1.2 for the structure of the A–U pair) [65, 66]. The simulation is based on the hybrid QM/MM method provided by the CPMD/Gromos interface [42]. The CDCl3 solvent molecules are treated with Gromos, adopting the Gromos96 force field [67–69]. All atoms of the A–U pair are treated quantum mechanically via the CPMD package [70], using the Becke exchange and Lee–Yang–Par correlation functional (BLYP) [71, 72]. Further, the Troullier–Martins (TM) pseudopotential [73] with a wavefunction cutoff of 70 Ry is adopted to describe the interaction between the core and valence electrons in the quantum region. Note that BLYP, together with the plane wave base set, has been reported to yield good results for the simulation of a hydrogen-bonded system, such as liquid water [74], liquid HF [75] and strong HB in crystalline picolinic acid N-oxide [40]. In the simulation, one A–U pair is dissolved in 100 CDCl3 molecules, and the solution is put into a box with dimensions 30.0 A  23.5 A  23.5 A (density 0.1 M [22]). The quantum part is placed in a 30.0 A  15.9 A  13.2 A box. The initial configuration is taken from a previous QM/MM simulation [47]. Before the QM/ MM simulation, an MM solvent equilibration with Gromos is carried out for 1 ns at 300 K with the solute fixed using the SHAKE algorithm. Subsequently, a trajectory is propagated for up to 9 ps at 300 K with a Nose–Hoover chain thermostat and by using a time step of 0.04838 fs (2 a.u.). Assigning the first 1 ps for equilibration, an 8 ps production trajectory is available for data analysis. 1.3.2 Geometry and geometric correlations Geometric correlation is one of the vital factors for the DNA/RNA to fold into its three-dimensional structure [76]. As a starting point to understand the bending and torsion in DNA/RNA, it is necessary to examine the geometric correlations induced by the HBs in a single base pair. For this purpose, the HB length is

Figure 1.2 Structure of the Watson–Crick pair formed by 9-ethyl-8-phenyladenine and 1-cyclohexyluracil. In the following analysis we will use the angle a(N6–O4–C4) and b(C4–N3–N1) and the angle g between the two vectors pointing from N3 to N1 and from O4 to N6, as well as the dihedral angle f defined by the atoms N1, N3, O4 and N6

10

Hydrogen Bonding and Transfer in the Excited State

Figure 1.3 Geometric parameters for the two HBs along the QM/MM trajectory. Panels (a) and (b): HB length for N–H  N and N–H  O HB respectively. Panels (c) and (d): distance between the hydrogen atom and the HB centre for N–H  N and N–H  O HB respectively. Panels (e) and (f): the out-of-line motion of the hydrogen measured by the difference L(N–H) þ L(N  H)
L(N  N) and L(N–H) þ L(O  H)
L(N  O) respectively. The horizontal lines indicate the respective time averages

one of the most prominent parameters for a single HB. Furthermore, when two or multiple HBs are present, correlations related to the various angles defining their geometry are of great relevance. As a reference for the MD simulation, geometry optimization and frequency analysis of the A–U pair in gas phase are performed using the DFT/BLYP/6-31Gþ þ (d,p) model chemistry as implemented in GAUSSIAN03 [77]. In the optimized structure, the heterocycles of A and U are planar, and the two HBs are close to being linear. The gas-phase HB length parameters are: ff(NHN) ¼ 178.60 , L(N  N) ¼ 2.90 A , L(N–H) ¼ 1.06 A and L(H  N) ¼ 1.84 A for the N–H  N HB; ff(NHO) ¼ 174.97 , L(N  O) ¼ 2.94 A , L(N–H) ¼ 1.03 A and L(H  O) ¼ 1.91 A for the N–H  O HB. Without the substitution groups, the above parameters are ff(NHN) ¼ 178.89 , L(N  N) ¼ 2.89 A , L(N–H) ¼ 1.06 A, L(H  N) ¼ 1.83 A for the N–H  N HB, and ff(NHO) ¼ 174.74 , L(N  O) ¼ 2.94 A, L(N–H) ¼ 1.03 A and L(H  O) ¼ 1.91 A for the N–H  O HB. This calculation agrees with the observation that the HB length is only slightly affected by the substituent at N9 of adenine and at N1 of uracil [78, 79]. The solvated system deviates from the optimized gas-phase structure. Figure 1.3 depicts the time evolution of the geometries of the two HBs. Panels (c) and (d) show that, in the course of the trajectory, no H-atom transfer occurs. The average HB lengths are L(N  N) ¼ 2.97 A and L(N  O) ¼ 3.02 A. As geometry-based correlations are often discussed assuming linear HBs, we illustrate in panels (d) and (e) of Figure 1.3 the deviation of the H atoms from linear motion. Upon examination of panels (a) and (e), as well as panels (b) and (f), we see that the nonlinearity of the HB is correlated with the HB length. That is, the larger the HB length, the more strongly the HB deviates from linearity. This agrees with the asymptotic case where the HB is broken and all effects induced by hydrogen bonding disappear. The average deviations from linearity are 0.021 A for the N–H  N bond and 0.031 A for the N–H  O bond. Thus, it is safe to say that in both cases it can be assumed that the HBs are approximately linear. The planarity of the gas-phase optimized heterocycle rings is not preserved along the trajectory in solution; the rings are bent and distorted during the dynamics. For a more detailed analysis, we use the dihedral angle f as defined in the caption of Figure 1.2, which measures the torsion of the two HBs in the base pair under the linear HB assumption. Figure 1.4 reveals how this torsion is related to other geometrical changes in the HBs. From

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

11

Figure 1.4 Geometric correlations of the N–H  N and N–H  O HBs along the QM/MM trajectory between the absolute value of the dihedral angle f and (a) the sum of the angles a and b and (b) the angle g (for definitions, see the caption of Figure 1.2)

panel (a) we find that the non-planarity of the two HBs comes with a decrease in the sum of the angles a þ b, which describes a compression of the two HBs owing to N3H- and/or C4O4-bending vibrations. Figure 1.4(b) shows that there is an almost perfect linear correlation between the torsion of the two HBs and the angle g describing the relative direction of the two HBs. This indicates that the number of internal degrees of freedom of the base pair decreases by one owing to hydrogen bonding. Besides the geometric correlation between the two HBs, there also exist correlations for each HB separately. In this respect, Limbach and coworkers promoted Badger’s rule [80–82] which gives a simple explanation of this correlation in terms of valence bond orders pj [83], viewing the HB as being composed of two diatomic units A–H and B–H with bond lengths R1 and R2 respectively [84]: pj ¼ expf
ðRj
Req j Þ=bj g

ð1:24Þ

where Req j is the equilibrium bond length of the hypothetical non-bonded diatomic and bj is a parameter describing the change in the bond valence upon bond stretching. If we assume that the total bond order is unity, i.e. p1 þ p2 ¼ 1, the two bond lengths cannot be changed independently. This can be alternatively expressed in terms of the deviation from the HB centre (R1
R2)/2 and the HB length R1 þ R2. The correlation curve thus obtained from numerous structural data expresses a fact well known from quantum chemical studies of potential energy surfaces, namely that the HB is compressed upon H transfer while passing the transition state [9]. This correlation for a given type of HB – although expressed in equation (1.24) by four parameters only – yields a rather robust description of the HB geometries (see, for example, the case of N–H  N bonds in Refs [84] and [85]). It turns out, however, that, in particular for short HBs, the original formulation requires some modification. In Ref. [86], Limbach et al. suggested an empirical correction as follows: 5 2 pH 1=2 ¼ p1=2  cðp1
p2 Þðp1 p2 Þ
dðp1 p2 Þ ;

ð1:25Þ

H with pH 1 þ p2 < 1. Here, c and d are parameters to be determined by fitting geometric correlations to experimental data. Figure 1.5 gives a test of the empirical correlation, equation (1.25), between the N–H and HB along the QM/ MM trajectory. As anticipated from the robustness of the correlation reported before [84–86], geometries along the trajectory are remarkably well described by this simple expression. This holds true, irrespective of the deviation from linearity and planarity mentioned above. Moreover, there seems to be little variance with respect to the class of molecules, as shown by plotting the correlation curve with parameters obtained by Limbach et al. for intramolecular N–H  N HBs (dashed line in panel (a)) [85].

12

Hydrogen Bonding and Transfer in the Excited State

Figure 1.5 Geometric correlations between HB length and H-atom position following from a fit to equation (1.25) eq (solid line) with the points sampled along the QM/MM trajectory. (a) N–H  N (1 ¼ 2 ¼ NH) with R1 ¼ 1.025 A , eq b1 ¼ 0.3629 A , c ¼
916.1 and d ¼ 0.2638; (b) N–H  O (1 ¼ NH, 2 ¼ OH) with R1 ¼ 1.014 A, b1 ¼ 0.4269 A, eq R2 ¼ 1.245 A, b2 ¼ 0.2347 A, c ¼ 300 and d ¼ 0.52. In panel (a) we also show the correlation curve from Limbach et al. (dashed line) [86]:

1.3.3 Velocity autocorrelation function A global picture of the vibrational dynamics can be obtained from the classical vibrational density of states (DOS), which is given by the power spectrum of the velocity autocorrelation function [87]: ~ vv ðvÞ ¼ C

ð1

dt eivt Cvv ðtÞ

ð1:26Þ

0

with X 1 1 Cvv ðtÞ ¼ P  2  k vk req ðTf
Ti
tÞ j

ð Tf
t Ti

dt nj ðt þ tÞnj ðtÞ

ð1:27Þ

with vj(t) being the velocity of the different atoms in the system. In the following, we will focus on the atoms of the QM region only, placing emphasis on the hydrogen-bonded H atoms. The vibrational DOS for all QM atoms is displayed in Figure 1.6. To guide the assignment of the bands, the analysis for the hydrogen-bonded

Figure 1.6 The power spectrum of the velocity ACF for the atoms in the QM region of the A–U pair (See Plate 1)

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

13

and non-bonded H atoms and the heavy atoms are also presented. From the total DOS we observe several prominent bands around 3369 cm
1, 3160 cm
1, 3000 cm
1, 2800 cm
1 and 1578 cm
1 in the region 1500–3500 cm
1. The narrow high-frequency band peaking at 3369 cm
1 is found to have the main contribution from the non-hydrogen-bonded H atoms and a small contribution from the hydrogen-bonded H atom of the NH2 group. Thus, it represents asymmetric stretching of the amino group of adenine. This frequency assumes a red-shift compared with the experimental result of 3485 cm
1 for adenine in a 1:1 mixture of 1-cyclohexyluracil and 9-ethyladenine in CDCl3 solution [16]. Inspecting the contributions of individual H atoms, we find that the broad bands around 3160 cm
1 are due to the motion of the HA atom. It can be assigned to NH stretching in the N–H  O HB. The strong broad band around 3000 cm
1 and 2800 cm
1 is mostly due to the CH stretching vibrations of the heterocycles and the substituents. Note that the band corresponding to NH stretching in the N–H  N HB is between 2800 cm
1 and 3000 cm
1 and is covered by the CH stretching vibrations. A detailed analysis of the band around 1578 cm
1 reveals contributions of about equal magnitude from the two hydrogen atoms in the NH2 group and is assigned to the NH2 scissoring vibration. This transition was measured at 1642 cm
1 in Ref. [16]. Note that the theoretical results are systematically red-shifted by 3–5%, which is due to the mass renormalization CPMD and is discussed in Ref. [88]. 1.3.4 NH stretching–HB length correlation In the following, we aim to establish a model for the fluctuations of the quantum mechanical energy gaps of the hydrogen-bonded NH stretching vibrations. On account of the fact that the out-of-line motion of the H atom in the HB is small, it can be regarded as moving in a one-dimensional potential. This implies that there exists a correlation between the geometry and the fundamental transition frequency v10 of each HB. This fact was first recognized by Rundle and coworkers when examining crystalline compounds containing intermolecular HBs [89, 90]. Later, it was systematically explored by Novak [91] and by Mikenda et al. [92, 93]. These studies established an empirical stretching–frequency correlation. In the present case, for both the N–H  N and N–H  O HBs, the frequency correlations can be fitted with a Gaussian error function [47]: 2 X v1 f ðrÞ ¼ erf aj r j 2pc j¼0

! ð1:28Þ

where v1 is the free NH stretching frequency in gas phase [94], which is used to fix the asymptotic behaviour corresponding to the dissociated base pair, and r is the HB bond length (for parameters, see the caption of Figure 1.9). When using equation (1.28), however, it should be borne in mind that the NH stretching motion is affected not only by the HB length but also by the overall configuration of the A–U pair and solvent molecules. In other words, a quantitative result would not be expected from this approach. An alternative approach is the on-the-fly calculation of NH stretching snapshot potentials in a frozen environment for some representative configurations along the trajectory [37–40]. To accomplish this goal, geometry snapshots in the time interval 1.0–9.0 ps are sampled at typical long, short and intermediate base-pair separations for the computation of potential curves. In the potential calculation, the N–H  N HB is assumed to be linear, and the H atom is stretched along the N  N line with 11 sampling points in the region 0.7–2.2 A. For the N–H  O HB, the 2D snapshot potential energy surfaces are calculated. For each snapshot, the H atom is moving within the C6–N6–O4 plane characterizing the NH bond stretching and bending motion. The NH bond stretching along the N6–O4 direction is sampled with seven grid points in the region 0.7–2.0 A. The NH bending motion within the C6–N6–O4 plane is sampled with seven points from
45 to 45 . The energy curves/surfaces are interpolated employing a B-spline. Eigenenergies are calculated using the hydrogen mass as the particle mass and using the Fourier grid Hamiltonian method [95]. This gives the frequency of the

14

Hydrogen Bonding and Transfer in the Excited State

Figure 1.7 Normal mode displacement vectors for the gas-phase A–U pair obtained using the DFT/BLYP/631Gþ þ (d,p) level of quantum chemistry. The harmonic frequency of the NH2 and NH stretching mode is 3264 cm
1 (a) and 2834 cm
1 (b) respectively. Note that the adenine symmetric NH2 stretching vibration acquires a local NH stretching character owing to the H-bonding

fundamental transition v10, as well as that of the 1 ! 2 transition v21. It turns out that the bending vibration has no noticeable influence on the considered stretching transitions. Therefore, the bending motion will not be analysed in the following. Finally, we note that the local treatment of the normal mode vibrations is supported by the nature of the gas-phase normal mode displacements shown in Figure 1.7. Figure 1.8 presents potential energy curves for three representative snapshots for the N–H  N case. From the figure it can be seen that all potentials are rather anharmonic. When the A–U separation is large, the curve exhibits a double minimum feature, although the H transfer from the U side to the A side is energetically not favourable at room temperature. When the HB length is small, the double-well feature disappears. Note that the barrier for H transfer is about the same as found in a semi-empirical QM/MM study of a guanine–cytosine pair [96]. Figure 1.9 shows empirical data for the correlation between the HB length and the fundamental transition frequency for both N–H  N and N–H  O HBs and displays the fit according to equation (1.28). We find that, upon approaching the free NH stretching limit, the fitted curves change very slowly in the HB length region 3.2–3.6 A. In Figure 1.9 we also show results from snapshot frequency calculations sampled along the trajectory. They also have been fitted to equation (1.28), using the asymptotic frequency as a free parameter. The on-the-fly data reproduce the expected qualitative dependence on the HB length for the N–H  N HB and

Figure 1.8 Potential energy curves for the N–H  N HB of the on-the-fly QM/MM calculation with representative HB length L(N  N). The horizontal lines indicate the position of the lowest energy levels

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

15

Figure 1.9 Correlation between HB lengths and fundamental transition frequencies (hollow squares: QM/MM onthe-fly calculations). (a) Correlation for the N–H  N HB (bullets: data from crystalline compounds containing intermolecular N–H  N HB; solid line: fitted curve according to equation (1.28) with v¥/2pc ¼ 3436 cm
1 [94],
1
2 a0 ¼ 7.0911, a1 ¼
5.7941 A and a2 ¼ 1.2711 A ; dashed line: fitted curve for the QM/MM on-the-fly calcula tion with v¥/2pc ¼ 3022 cm
1, a0 ¼ 2.9892, a1 ¼
2.7482 A
1 and a2 ¼ 0.7055 A
2. (b) Correlation for the N–H  O HB (bullets: data from crystalline compounds containing intermolecular N–H  O HB, as compiled in the Appendix: solid line – fitted according to equation (1.28) with v¥/2pc ¼ 3434 cm
1 [94], a0 ¼
0.5345,
1
2 a1 ¼
0.8332 A and a2 ¼ 0.5044 A ; dashed line – fitted curve for the QM/MM on-the-fly calculation with v¥/2pc ¼ 3070 cm
1, a0 ¼ 5.4938, a1 ¼
4.2755 A
1 and a2 ¼ 1.0005 A
2

predict a weaker correlation for the N–H  O HB, but are red-shifted with respect to the empirical correlation curve obtained from crystal data by about 415 cm
1 for the N–H  N HB and 360 cm
1 for the N–H  O HB (difference in v1). This disagreement deserves some attention. It is well appreciated that DFT-based PES are too soft [97], and therefore DFT/BLYP predicts longer bond lengths for nucleic base pairs than obtained from X-ray crystal diffraction (see, for example, gas-phase calculation in Ref. [98]). By combining ab initio PES calculations with quantum simulation of anharmonic infrared absorption spectra in gas phase, we found an NH transition frequency in the adenine–thymine pair of 2608 cm
1 using the DFT/B3LYP/6-31Gþ þ (d,p) method. A RI-MP2/TZVP correction increased this value to 2688 cm
1 only [8]. Similar results with frequencies below 3100 cm
1 were reported in Refs [10, 99–103]. Yagi and coworkers obtained an anharmonic frequency of 2488 cm
1 based on a vibrational self-consistent-field method in conjunction with MP2 perturbation theory [104, 105]. Therefore, we conclude that present DFT- and MP2-based calculations consistently give excessively low NH stretching frequencies if compared with experimental data (e.g. 3185 cm
1 in Ref. [22]). Note that the agreement between empirical and on-the-fly calculations is slightly better in the N–H  O case. 1.3.5 Frequency fluctuation correlation function With the help of the QM/MM on-the-fly stretching frequency versus HB length correlation, the time evolution of the fundamental transition frequencies v10(t) is obtained and displayed in Figure 1.10. The time-dependent lengths of the N–H  N and N–H  O HBs are also given so as to emphasize the correlation. The time averages of the frequencies are 2585 cm
1 for the N–H  N HB and 3011 cm
1 for the N–H  O HB. As anticipated from the discussion in the foregoing section, these values are significantly smaller than the experimental results (3185 cm
1 and 3315 cm
1 [22]). We also notice the red-shift as compared with the assignment based on the

16

Hydrogen Bonding and Transfer in the Excited State

Figure 1.10 Time-dependent fundamental transition frequencies for the N–H  N (a) and the N–H  O (b) HB. The dotted lines are the average values hv10i, which are equal to 2585 cm
1 in (a) and to 3011 cm
1 in (b). The dashed lines are the N  N and N  O distances scaled and shifted to manifest the correlation with the timedependent frequencies

power spectra of the velocity autocorrelation functions. In this respect, we should note that the zero-point energy of the NH stretching modes is about 1500 cm
1, that is, much larger than kBT at room temperature. In other words, the on-the-fly vibrational eigenstates sample considerably higher regions of the potential energy surface, which are apparently too soft. In principle, a solution would be to use the empirical correlation derived from crystal structure data (see also Ref. [47]). They miss the structural flexibility of the solution phase, but implicitly include the correct potential as well as zero-point energy effects. Indeed, use of the empirical correlation for mapping HB distances to frequencies yields averages of 2986 cm
1 and 3300 cm
1, i.e. a considerable improvement. The drawback is that empirical correlations exist for fundamental transitions only, but, in order to model nonlinear spectroscopy information about overtone, excitations are needed as well (including transition dipole matrix elements). Therefore, we will use the on-the-fly results and shift the fundamental transitions by 600 cm
1 and 304 cm
1 for the N–H  N and N–H  O HB, respectively, to yield the correct transition frequencies. Now we are in a position to obtain the frequency fluctuation correlation function Cab(t) and the associated IR lineshape function gab(t). The correlation function is calculated from the production part of the trajectory in the interval [Ti ¼ 1 ps, Tf ¼ 9 ps] as 1 Cab ðtÞ ¼ Tf
Ti
t

ð Tf
t Ti

  dt dva0 ðt þ tÞdvb0 ðtÞ Q

ð1:29Þ

The results for the frequency fluctuation correlation function involving the fundamental frequencies are plotted in Fig. 1.11. Panels (a) and (b) show a fast initial decay during the first 100–200 fs and a subsequent oscillation for the ACFs of both HBs. For the N–H  N HB there are at least two timescales for the initial decay: the faster one dominates the first 50 fs and the other one plays its role in the region 50–150 fs. The correlation function for the N–H  O HB has only one decay time, which is shorter than that of the N–H  N HB. The cross-correlation between the frequency fluctuations of the N–H  N and N–H  O HBs displayed in Fig. 1.11c shows that the fluctuations of the two HBs are correlated owing to the hydrogen bonding. Below, the 2D photon echo signal is presented, which sheds some light on the nature of this correlation.

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

17

Figure 1.11 Frequency fluctuation correlation functions of the fundamental transition, as obtained from QM/MM simulation. ACF for (a) the N–H  N and (b) the N–H  O HB; (c) cross-correlation function between the N–H  N and N–H  O HBs. Here, Cav denotes the geometric average of the initial ACFs, C(0), for the N–H  N and N–H  O HBs

1.3.6 Correlation between transitions 0 ! 1 and 1 ! 2     In the following we consider only transitions i; j ! i0 ; j 0 in the NH stretching spectral region for i ¼ i0 ¼ 0, j0 ¼ j þ 1 or j ¼ j0 ¼ 0, i0 ¼ i þ 1 (see Figure 1.13). For the simulation of nonlinear spectra, it is required to have at hand the fluctuating 1 ! 2 transition frequencies. The question to be asked is whether there is a simple relation between v21 and v10 that could be employed as an efficient way to obtain v21(t) on the basis of v10(t). For this purpose we have used the transition frequencies obtained from the on-the-fly simulation (cf. Figure 1.8) and plotted their correlation in Figure 1.12. The available experimental data are also shown in this figure. Panel (a) reveals a linear correlation between the frequencies of 1 ! 2 and 0 ! 1 transitions for the N–H  N case. For the N–H  O HB, a correlation can still be observed, although it has a greater deviation from the experimental data. Thus, we will assume a linear correlation to hold for both HBs and use v21 ¼ gv10 þ D

ð1:30Þ

where the two parameters characterize the anharmonicity of the vibrations. Note that the available experimental points [22] are rather well reproduced by the linear correlation fit. Based on the linear correlation, the average value for the 1 ! 2 transition frequency along the trajectory is found to be W21 ¼ 3073 cm
1 and W21 ¼ 3209 cm
1 for the N–H  N and N–H  O HB respectively. Thus, the anharmonicity calculated with on-the-fly snapshot extraction is 112 cm
1 for the N–H  N HB and 106 cm
1 for the N-H  O HB. Compared with the experimental values (230 cm
1 for the N–H  N HB and 215 cm
1 for the N–H  O HB [22]), we once again encounter the problem with the on-the-fly PES. To correct for this deficiency, we will use g from on-the-fly results and change D to yield the experimental

18

Hydrogen Bonding and Transfer in the Excited State

Figure 1.12 Frequency correlation between 1 ! 2 and 0 ! 1 transitions for (a) the N–H  N HB and (b) the N–H  O HB. Circles – QM/MM on-the-fly calculations; bullets – experimental results from Ref. [22]; lines – linear regression for the calculated values, equation (1.30). The fitted parameters are g ¼ 1.5986 and D ¼
2018.5 cm
1 for the N–H  N HB and g ¼ 1.2436 and D ¼
913.57 cm
1 for the N–H  O HB

anharmonicity upon averaging. In this way, the lineshape function for an arbitrary pair of states for a single vibrational mode reads gab ðtÞ ¼ g a
1 gb
1 g11 ðtÞ

ð1:31Þ

where a and b are 1 for the first excited states and 2 for the second excited states. This equation can be straightforwardly extended to more vibrational modes. From the on-the-fly calculation it is also possible to obtain the ratio between the transition dipole moment matrix elements m10 for 0 ! 1 transition and m21 for 1 ! 2 transition. We find that the ratio is quite independent of the overall configuration and close to the value for a harmonic oscillator. The average value is m21/m10 ¼ 1.48 for the N–H  N HB and m21/m10 ¼ 1.41 for the N–H  O HB. The relative ratio of m10 of the NHO two HBs is calculated to be 0.64. However, we will use mNHN ¼ 0:47, which gives a better agreement 10 =m10 concerning the relative height of the two fundamental transition bands in the experimental measurement. A summary of the model is given in Figure 1.13. 1.3.7 Quantum correction for the bath modes So far the correlation function has been a classical quantity not satisfying detailed balance, and therefore a quantum correction is needed. There are various approaches for completing this task. The most popular one is

Figure 1.13 Schematic representation of the energy levels for the N–H  N and N–H  O HBs. The average values for the four frequencies are W10 ¼ 3185 cm
1 and W21 ¼ 2955 cm
1 for N–H  N and W10 ¼ 3315 cm
1 and W21 ¼ 3100 cm
1 for N–H  O

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

19

to introduce a quantum correction factor, which is multiplied by the correlation function [36]. The method to be used in the following maps the dynamics onto that of an analytically solvable model such as the Brownian oscillator [50]. This phenomenological model assumes that the environment coupling to the NH stretching transitions can be mapped onto a harmonic oscillator bath with bilinear coupling. In this case the correlation function can be expressed as [50] 00

0 CðtÞ ¼ C PðtÞ þ2iC ðtÞ; 0 C ðtÞ ¼ j Sj vj cothðbhvj =2Þcosðvj tÞ; P 00 C ðtÞ ¼ j Sj v2j sinðvj tÞ

ð1:32Þ

where Sj is the dimensionless Huang–Rhys factor representing the strength of the system–bath coupling. In the present case, equation (1.32) is used to fit the ACFs shown in Figure 1.11 within the frequency range 0–1770 cm
1. This covers almost all the low-frequency bond bending and distorting modes coupled to the HB. Its interval is discretized into 20 modes, with the mode frequencies and coupling strengths used as fitting parameters. The fitted coupling strength is a smooth function of the frequency, as shown in Figure 1.14. We find that the coupling between the NH stretchings and the bath modes is dominated by low frequencies. We also observe smaller peaks around 400 cm
1 for the N–H  N HB and around 500 cm
1 for the N–H  O HB. It is tempting to assign these peaks to intermolecular vibrational modes. Indeed, gas-phase harmonic analysis of the A–U pair gives several intermolecular modes modulating the N  N and N  O distances in the region 150–400 cm
1. Modes in the range 400–600 cm
1 are mostly modulating the N  O distance, which is consistent with Figure 1.14.

Figure 1.14 Coupling strength between the NH vibration and the bath as a function of the bath frequency for (a) the N–H  N HB, (b) the N–H  O HB and (c) the cross-correlation between the two HBs. Points are fitted results and the line is a guide for the eye

20

Hydrogen Bonding and Transfer in the Excited State

Figure 1.15 IR lineshape of the NH stretching transitions using the QM/MM on-the-fly approach (dotted), the empirical mapping (dashed) and a population relaxation model (solid). (a) N–H  N HB compared with experimental data [22]; (b) N–H  O HB. Note that we have not included the shift of the transition frequency owing to the imaginary part of the correlation function. It amounts to
2 cm
1 for the N–H  N HB and to
1 cm
1 for the N–H  O HB

1.3.8 IR lineshape From the correlation functions and the fitted lineshape function we obtain the IR lineshapes, equation (1.12), that are shown in Figure 1.15. For the N–H  N HB depicted in panel (a), the overall agreement with the experimental data is fairly good given the simple model for assigning transition frequencies. The FHWM is calculated to be 34 cm
1, i.e. 35% below the experimental value of 53 cm
1. Note also that the lineshape is slightly asymmetric; this is due to the coupling to the bath modes, which gives rise to the pronounced oscillations in the correlation function of this transition. Panel (b) presents the result for the N–H  O HB. It can be seen that the FHWM is only 5 cm
1, that is, significantly narrower than the experimental result (about 41 cm
1, estimated from the experimental data, as no detailed analysis of this band was given in Ref. [22]). For comparison we have also included results obtained using the empirical correlation. This gives FWHMs of 42 cm
1 and 7 cm
1 for the N–H  N and N–H  O case respectively. Overall, however, it can be concluded that the description of the frequency fluctuations is of similar quality in the two approaches. So far we have accounted for the effect of pure dephasing only. Lineshape broadening due to a finite population relaxation time of the fundamental transition and the hot ground state are not considered. In Ref. [22] Woutersen and Cristalli examined the relaxation timescale of the N–H  N HB and found that it is reduced to 0.34 ps as compared with the U monomer in CDCl3 solution. They did not carry out the same analysis for the N–H  O HB. However, from their data it is possible to extract a relaxation time of 0.43 ps by comparing the relative changes in the transient absorption at different delay times, as will be shown below. These timescales can be incorporated in the spirit of the phenomenological Bloch model, equation (1.22). The results are also plotted in Figure 1.15, and we find that the spectrum is broadened to 48 cm
1 for the N–H  N HB and to 14 cm
1 for the N–H  O HB. Still, in the latter case it is substantially too narrow. Additional broadening mechanisms such as coupling to the other amine NH stretching vibration might be effective. On the other hand, the experimental spectrum is distorted by solvent contributions in this range, i.e. the estimated FWHM of 41 cm
1 has to be taken with caution as well. 1.3.9 Pump–probe spectra For the model of Figure 1.13 the pump–probe spectrum has been calculated according to equation (1.7). The pump frequency was taken as the average for the fundamental NH stretching transition of the N–H  N HB. The pulses are Gaussians with an FWHM bandwidth of 80 cm
1 for the pump and 150 cm
1 for the probe [22].

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

21

Figure 1.16 Transient absorption spectra, equation (1.7), of a 0.1 M solution of A–U in CDCl3 in the NH stretching region at different delay times between pump and probe pulses. The pump frequency is taken as the theoretical average value for the NH stretching in the N–H  N HB, which is 3185 cm
1. The probe pulse frequency v2 is scanned in the given interval

The result is given in Figure 1.16 and shows two negative bands centred at 3315 cm
1 and 3185 cm
1, which are due to ground-state bleaching and stimulated emission of the N–H  O HB and the N–H  N HB respectively. A positive band due to the excited-state absorption of the N–H  N HB is found at around 2966 cm
1. The excited-state absorption of the N–H  O HB at 3100 cm
1 is masked by the negative bands related to the fundamental transitions. The pump–probe signal is shown for different delay times, revealing the decay of the excited fundamental transitions. For the vibrational relaxation time of the N–H  N HB we have used the experimental value of 0.34 ps. For the N–H  O the relaxation time has been fitted to give the decay of the respective peak, which yields a value of 0.43 ps. As a nearly harmonic value has been obtained for the ratio of transition dipole matrix elements between the 0 ! 1 and 1 ! 2 transitions, we have used the harmonic approximation to the relaxation time as well, i.e. the second excited state decays twice as fast as the first excited state. 1.3.10 Photon echo spectra: dynamical correlations In order to scrutinize the dynamical correlations between the bath-induced fluctuations of the two fundamental transitions, we have calculated the 2D IR photon echo spectrum in rephasing configuration, equation (1.8). The result is shown in Figure 1.17 for different delay times T. First we discuss the spectrum at zero delay time, which contains six bands. The diagonal positive absorption band centred at (v1 ¼ 3183 cm
1, v3 ¼ 3186 cm
1) is the transition corresponding to the N–H  N HB and is due to contributions from diagrams of the ground-state bleaching (R3 in Figure 1.1) and stimulated emission (R2) with a ¼ b belonging to excitation of the N–H  N HB. The diagonal peak centred at (v1 ¼ 3318 cm
1, v3 ¼ 3319 cm
1) is the respective feature due to the N–H  O HB. Both peaks are essentially elongated along the diagonal, thus indicating the inhomogeneity of the two HB vibrations. Owing to the strong anharmonicity of the HBs, the excited-state absorption features are well separated in the spectrum. Specifically, we observe negative absorption bands centred at (v1 ¼ 3192 cm
1, v3 ¼ 2964 cm
1) and (v1 ¼ 3322 cm
1, v3 ¼ 3108 cm
1). They correspond to the transient excited-state absorption (R5) with a ¼ b belonging to the N–H  N and N–H  O HB respectively. Note that, owing to the interference between contributions from different transitions having different signs, the positions of the peaks are not exactly the same as those given in Figure 1.13.

22

Hydrogen Bonding and Transfer in the Excited State

Figure 1.17 A sequence of 2D IR photon echo spectra given by the real part of equation (1.8) of a 0.1 M solution of A–U in CDCl3 in the NH stretching region for different population times T. Each panel has been separately normalized to the maximum value of the signal (See Plate 2)

Most interesting, however, are the two positive off-diagonal peaks, which manifest the correlations between the fluctuations of the N–H  N and N–H  O HBs. Again, ground-state bleaching and stimulated emission contribute to these two peaks, with (a ¼ N–H  O, b ¼ N–H  N) for the peak around (v1 ¼ 3314 cm
1, v3 ¼ 3191 cm
1) and (a ¼ N–H  N, b ¼ N–H  O) for the peak around (v1 ¼ 3189 cm
1, v3 ¼ 3316 cm
1). The type of correlation can be inferred from the shape of these peaks, especially for the better resolved one around (v1 ¼ 3189 cm
1, v3 ¼ 3316 cm
1). Clearly it is oriented parallel to the diagonal line, which indicates that the stretching vibrations of the N–H  N and N–H  O HBs in the A–U pair in solution are positively correlated [106]. Finally, we have investigated the behaviour of the spectrum as a function of the waiting T. As expected for the simple Bloch model for population relaxation, we do observe a faster decay of the peak related to the N–H  N HB, which is due to the faster population relaxation of the fundamental transition in this HB.

1.4 Conclusion Hydrogen bond dynamics is characterized by the strong anharmonic coupling of the fast hydrogen stretching motion to the surrounding molecular scaffold comprising hydrogen bond donor and acceptor moieties. As a consequence there are pronounced correlations between geometrical parameters, most notably between the hydrogen bond length and the hydrogen position between donor and acceptor. Such correlations are highly valuable, as they reduce the apparently complex dynamics of a thermal ensemble to a simple functional form. Its use, especially in nuclear magnetic resonance spectroscopy, has been extensively documented. A similar role is played in the context of linear absorption spectroscopy by the correlation between the hydrogen bond length and the fundamental transition frequency of the hydrogen’s stretching vibration. Giving access to the

Vibrational Dynamics of the Double Hydrogen Bonds in Nucleic Acid Base Pairs

23

energy gap fluctuations, it makes it possible not only to calculate the absorption lineshape but also to unravel its microscopic origin. Furthermore, the lineshape function carries over to nonlinear spectroscopies, which provide a more detailed look behind the usually broad absorption bands of hydrogen-bonded species. In particular, 2D IR spectroscopy has the capability to unravel correlated dynamics, for instance, between multiple hydrogen bonds. In this chapter we have presented results on the various correlations between the two hydrogen bonds in a modified adenine–uracil base pair in solution. Our results have been derived from a hybrid QM/MM trajectory. Geometries were observed to sample a range that is restricted by the hydrogen bond scaffold. Simple distance relations were found for each hydrogen bond that were not much different from those reported for rather different, e.g. intramolecular, hydrogen bonds. The study of gap fluctuations of the NH stretching vibrations yielded an IR lineshape, which was almost quantitative for the case of the N–H  N bond. Furthermore, the comparison between calculated and measured pump–probe spectra allowed us to pinpoint the vibrational relaxation times, which enter our simulation in the form of an empirical Bloch model. Equipped with this information, we have been able to predict the 2D photon echo spectrum, which shows clear signatures of the correlated dynamics of the two hydrogen bonds.

Acknowledgement This work has been financially supported by the Deutsche Forschungsgemeinschaft (Project Ku952/5-1).

Appendix

Table A1.1 Correlation between N  O distances L(N  O) and NH stretching frequencies, nNH, of crystals containing intermolecular N–H  O HBs. The compounds considered in Table 1 of Ref. [107] are not listed here but have been included in the fit Compound 1 2 3 4 5 6 7 8 9

Ethyl-N-phenylurethane Ethyl-N-(1-naphthyl)urethane Methyl-N-(1-naphthyl)urethane 1,5-Naphthylenediethylurethane 1-Methyl-N-naphthylurethane 1-Ethyl-N-naphthylurethane Urea ACP ACPSE

10 11

L-Arginine

12

Glycine

L-Alanine

maleate dihydrate



L(N  O) (A)

Ref.

nNH (cm
1)

Ref.

2.9 2.86 2.94 2.868 2.86 2.939 2.977 2.95 2.806 2.873 2.921 2.862 2.853 2.832 2.813 3.075 2.855

[108] [110] [111] [112] [112] [112] [113] [115] [117]

3320 3244 3292 3282 3290 3285 3342 3313 3235 3288 3351 3226 3106 3030 2748 3178 2950

[109] [109] [109] [112] [112] [112] [114] [116] [117]

[118] [119] [121]

[118] [120] [122] ðcontinuedÞ

24

Hydrogen Bonding and Transfer in the Excited State

Table A1.1 (Continued) Compound

13

L-Glutamin

14

DL-Serine

15

L-Tyrosine

16

L-Histidine

17

L-Threonine



L(N  O) (A) 2.770 2.772 2.937 2.911 2.948 2.866 2.876 2.772 2.821 2.877 2.884 2.826 2.85 2.76 2.73 3.050 2.869 2.786

Ref.

[123]

[124] [125] [126] [127]

nNH (cm
1) 2680 2746 3374 3319 3139 2975 3105 3043 2970 3215 3124 3061 3128 3043 2916 3160 3068 2927

Ref.

[122]

[122] [122] [122] [122]

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2 Vibrational Energy Relaxation Dynamics of XH Stretching Vibrations of Aromatic Molecules in the Electronic Excited State Takayuki Ebata Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan

2.1 Introduction The vibrational energy relaxation (VER) of polyatomic molecules has been the central issue in many of the chemical reactions in condensed phase [1–17] as well as in gas phase [17–41]. The vibrational energy introduced to the molecule is immediately redistributed within the molecule or to the surrounding molecules. Among many vibrations investigated, the VER of the OH and NH stretching vibrations may be most important from the viewpoint of understanding the dynamics of the H-bonding of protic solvent molecules as well as biologically relevant molecules [42–48]. VER of the electronically excited OH stretching vibration of aromatic molecules and their hydrogen-bonded cluster is especially interesting. Most of the aromatic molecules having an OH group become more acidic upon electronic excitation, leading to hydrogen/proton transfer reactions [49–62]. We expect that vibrational excitation may lead to additional effects. In this review, we report the study of the intramolecular and intracluster vibrational energy redistribution (IMVR and ICVR), vibrational predissociation (VP) and isomerization of molecules and their clusters in the S1 state by using UV–IR double-resonance (DR) spectroscopy (Figure 2.1) [63–67]. In this spectroscopy, the molecule or the cluster in a supersonic jet is excited to the zero-point level of S1 by UV laser light, and is further excited to the XH stretch vibrational level by a tunable IR laser light. VER of vibrationally excited molecules and clusters is observed by dispersed fluorescence spectroscopy. After the UV–IR DR excitation to the XH stretch level, the molecule or the cluster emits a broad fluorescence in a wide energy region owing to fast IMVR or ICVR. If the internal energy is high enough to break the cluster, it dissociates by VP and emission of the fragment is observed. We can distinguish these processes by measuring the dispersed

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

30

Hydrogen Bonding and Transfer in the Excited State

Figure 2.1 Energy level diagram of a 2-naphthol–B cluster and the excitation scheme of UV–IR DR spectroscopy

fluorescence spectra with a monochromator. We apply this spectroscopy to 2-naphthol and its H-bonded clusters [66, 67]. The interesting point about 2-naphthol is that this molecule has two rotational isomers, cis and trans, depending on the orientation of the OH group with respect to the naphthalene ring. Thus, if the energy is high enough to overcome the barrier height, 2-naphthol undergoes cis $ trans isomerization, and the process will complete with VP. In the electronic ground state (S0), the cis-isomer is 140 cm1 more stable than the trans-isomer [68] while in the electronic excited state (S1) the trans-isomer becomes 174 cm1 more stable [69]. The cis $ trans isomerization occurs through the torsional motion of the OH group, and the barrier height in S0 is estimated to be 1500 cm1 by DFT calculation [67], while that in S1 is thought to be much higher, as the torsional barrier height of phenol S1 is reported to be 4710 cm1 [70]. UV–IR doubleresonance spectroscopy is a very powerful tool for studying the energetics and dynamics of the cis $ trans isomerization of 2-naphthol in S1.

2.2 IR Spectra of 2-Naphthol and its H-Bonded Clusters in S1 Figure 2.2 shows the UV–IR DR spectra of (a) 2-naphthol and its H-bonded clusters with (b) H2O, (c) CH3OH and (d) NH3. In these measurements, the UV laser frequency is fixed to the (0, 0) band of each species, and the laser frequency of the IR light, introduced at a delay time of a few nanoseconds after the UV laser, is scanned with the wavelength of the monochromator fixed to the emission from the IVR redistributed levels or the fragment. The OH stretching vibration of bare 2-naphthol in S1 emerges at 3609 cm1. This frequency is 48 cm1 lower than that in S0 [71]. In the H-bonded clusters, the OH stretching frequency decreases with increase in the proton affinity of the proton acceptors, which means that the H-bonding energy increases in the order of magnitude of the proton affinity. In the IR spectra shown in Figure 2.2, other vibrational bands also appear. Some of them are assigned to the CH stretching vibration of 2-naphthol and CH3OH, the OH stretching vibrations of CH3OH and the NH stretching vibrations of NH3. For 2-naphthol–CH3OH, two bands are seen on the higher-frequency side of the H-bonded OH stretch. There is no band assignable to the fundamental in this region, so that they are assigned to the intermolecular vibrational bands associated with the H-bonded OH stretch. In addition to the red-shift, the OH stretch band is significantly

Vibrational Energy Relaxation Dynamics 31

Figure 2.2 IR spectra of cis-2-naphthol and its H-bonded clusters of S1 observed by UV–IR DR spectroscopy

broadened, indicating very rapid relaxation of this level. We fixed the IR frequency to each band and measured the dispersed fluorescence (DF) spectra.

2.3 VER Dynamics of Bare 2-Naphthol Figure 2.3(a) shows the DF spectra of bare cis- and trans-2-naphthol from their band origins of S1 [67]. Figure 2.3(b) shows enlarged portions near the (0,0) band with and without the UV–IR DR excitation of the OH stretching vibration in S1. The UV and IR frequencies are fixed to the (0,0) band of each isomer and its OH stretching vibration (3609 cm1), respectively. In both spectra, a broad emission is seen beneath the (0,0) band of each isomer when IR laser is introduced, which is attributed to the emission from the redistributed levels generated by IMVR of the OH stretching vibration. We examine whether IR-laser-induced isomerization occurs from the broadened DF spectra. In Figure 2.3, the (0,0) band of the cis-isomer is located on the higherfrequency side of the trans-isomer. Then, if IR-induced trans ! cis isomerization occurs, we expect the (0,0) band of cis-isomer emission to appear in the DF spectrum of the trans-isomer. In Figure 2.3(b), however, although a broad emission is extended to the (0,0) band region of the trans-isomer in the DF spectrum, no peak assignable to the trans-isomer is seen. Thus, at this energy (3609 cm1), only IMVR occurs in the bare molecule, and the barrier height for isomerization is higher than this energy.

2.4 VER Dynamics of H-Bonded Clusters of 2-Naphthol Figures 4(a), (b) and (c) show the DF spectra of 2-naphthol–H2O, 2-naphthol–CH3OH and 2-naphthol–NH3 clusters, respectively, after the UV–IR DR excitation. They are compared with the DF spectra of the (0,0) bands of each isomer of bare 2-naphthol. The numbers shown on the left are the frequencies of the IR laser (in cm1).

32

Hydrogen Bonding and Transfer in the Excited State

Figure 2.3 (a) Dispersed fluorescence (DF) spectra of cis- and trans-2-naphthol from the band origin. (b) Enlarged portion of DF with the IR laser on and off. Reprinted with permission from [67]. Copyright 2006, American Institute of Physics

2.4.1 VER of a 2-naphthol–H2O H-bonded cluster In the DF spectra of cis-2-naphthol–H2O with UV–IR excitation, a new band (band C) emerges at the shorter wavelength of the (0,0) band of cis-2-naphthol–H2O. By comparing these bands with the DF spectra of the bare cis-2-naphthol, band C can be assigned to the emission of the cis-2-naphthol fragment generated by the VP of the parent cluster with the same isomeric form. Thus, VP occurs after the IR excitation of cis-2-naphthol–H2O. As can be seen in the spectra, band C becomes broader and the position of the maximum intensity shifts to red with increase in the IR excitation frequency. Thus, the cis-2-naphthol fragment becomes internally hot with increasing excitation energy. 2.4.2 VER of a 2-naphthol–CH3OH H-bonded cluster The DF spectra of cis-2-naphthol–CH3OH after the UV–IR DR excitation in Figure 2.4(b) show two new bands (bands C and T) on the shorter wavelength side of the (0,0) band of the clusters. Band C is due to the cis-2naphthol fragment, while band T is assigned to the trans-2-naphthol fragment. Thus, not only VP within the same isomeric form but also the cis ! trans isomerization accompanied with VP occur in 2-naphthol– CH3OH. Very interestingly, the generation of the trans-2-naphthol fragment occurs in a limited energy region, 2978–3303 cm1, and, at 3679 cm1 excitation, only the cis-2-naphthol fragment is observed.

Vibrational Energy Relaxation Dynamics 33

Figure 2.4 DF spectra of (a) cis-2-naphthol–H2O, (b) cis-2-naphthol–CH3OH and (c) cis-2-naphthol–NH3 H-bonded clusters after UV–IR DR excitation, compared with the DF spectra from the (0,0) band of each isomer of bare 2-naphthol. Reprinted with permission from [67]. Copyright 2006, American Institute of Physics

By examining the DF spectra in more detail, we realize two important features. The first is that the relative intensity of the two bands changes with the IR excitation frequency. At the lowest energy of ~nIR ¼ 2978 cm1, the isomerized fragment, the trans-fragment, is generated more than the cis-fragment. With increase in energy, the intensity of the trans-fragment (band T) rapidly decreases, and only the cis-fragment (band C) is identified at the highest-frequency excitation ~nIR ¼ 3679 cm1. Figure 2.5 shows the plots of I(C)/[I(C) þ I(T)] and I(T)/ [I(C) þ I(T)] as a function of IR frequency. Here, I(C) and I(T) are the fluorescence intensities of the cis- and trans-2-naphthol fragments in the (0,0) band respectively. It can be seen that at 3000 cm1 excitation the cisand trans-fragments are equally generated. The barrier height of the isomerization can be determined to be 2900 cm1 by extrapolating the ratio to lower IR frequency. The second important feature is that the widths of bands C and T are much narrower than in the case of cis-2naphthol–H2O, although the width increases with IR excitation energy similarly to cis-2-naphthol–H2O. This means that the available energy in VP of cis-2-naphthol–CH3OH is much less than that of cis-2-naphthol–H2O owing to the higher H-bonding energy. 2.4.3 VER of a 2-naphthol–NH3 H-bonded cluster The DF spectra of cis-2-naphthol–NH3 after UV–IR excitation in Figure 2.4(c) show another feature in addition to those observed in 2-naphthol–H2O and 2-naphthol–CH3OH. In Figure 2.4(c), at high IR frequency excitation such as ~nIR ¼ 3435 cm1, the two isomeric fragments, bands C and T, are equally produced. On the other hand, at low IR frequency such as ~nIR ¼ 3260 cm1, only trans-2-naphthol (band T) is generated. In the ~nIR ¼ 3090 cm1 excitation, no sharp peak due to the fragment is observed. Instead, the DF spectra show steplike broad features at the position of the (0,0) bands of bare 2-naphthols, which are indicated as arrows. The

34

Hydrogen Bonding and Transfer in the Excited State

Figure 2.5 Plots of I(C)/[I(C) þ I(T)] and I(T)/[I(C) þ I(T)] after UV–IR DR excitation of cis-2-naphthol–CH3OH as a function of the IR frequency. Reprinted with permission from [67]. Copyright 2006, American Institute of Physics

step-like emissions are attributed to the emission of internally hot cis- and trans-2-naphthol–NH3 clusters. At ~nIR ¼ 3260 cm1 excitation, on the other hand, a sharp band T due to the trans-fragment and step-like emission due to the cis-2-naphthol cluster are observed. At ~nIR ¼ 3367 and 3435 cm1 excitations, bands C and T emerge, meaning that both the cis- and trans-fragments are generated in this energy. 2.4.4 Energetics and the dynamics of ‘H-bond dissociation’ versus ‘cis ! trans isomerization’ of the H-bonded cluster of 2-naphthol In the present system, three VER processes occur after UV–IR excitation of the XH stretching vibrations, that is ICVR, cis ! trans isomerization and dissociation of the H-bond (VP), and the rate constant of each process exhibits different energy dependence. Here, we discuss the competition of these processes for the three clusters. Figure 2.6 shows the energy diagrams for (a) 2-naphthol–H2O, (b) 2-naphthol–CH3OH and (c) 2naphthol–NH3. A major difference between them is the H-bonding energy, and we discuss how the energy difference affects the VER dynamics of the clusters. 2.4.4.1 Low-Energy IR Excitation As was described in the introduction, the trans-isomer is 174 cm1 more stable than the cis-isomer in S1. When the energy put into the cis-2-naphthol H-bonded cluster is larger than the barrier but less than the H-bonding energy, the cis-form cluster isomerizes to the trans-form after IVR from the XH stretch. This situation corresponds to excitation scheme (1) of the 2-naphthol–NH3 cluster shown in Figure 2.6(c). The generated isomer is internally very hot and emits a broad fluorescence. A typical example is the step-like feature in the DF spectra of 2-naphthol–NH3 with ~nIR ¼ 3090 cm1 excitation in Figure 2.4(c). 2.4.4.2 Intermediate-Energy IR Excitation When the IR energy is greater than the H-bonding energy, the cluster dissociates to generate bare 2-naphthol as a fragment. This situation corresponds to excitation scheme (2) in Figure 2.6(c). As the energy level of the

Vibrational Energy Relaxation Dynamics 35

Figure 2.6 Energy level diagrams of S1, excitation schemes and VER processes for (a) cis-2-naphthol–H2O, (b) cis-2-naphthol–CH3OH and (c) cis-2-naphthol–NH3. Reprinted with permission from [67]. Copyright 2006, American Institute of Physics

trans-isomer is lower than that of the cis-isomer, the trans-2-naphthol fragment will first be generated in a lowenergy region. A typical example is seen in the DF spectra of the cis-2-naphthol–NH3 cluster at ~nIR ¼ 3139 and 3260 cm1 excitations. The spectra exhibit a sharp peak due to the trans-2-naphthol fragment, which is marked by T, and the step-like broad fluorescence of the hot cis-2-naphthol–NH3 cluster. Thus, in these excitation energies, part of the internally excited cis-2-naphthol–NH3 isomerizes to the cis-isomer and dissociates. In the spectra with the higher-frequency excitations, such as ~nIR ¼ 3367 and 3435 cm1, of cis-2naphthol–NH3, both peaks of the cis- and trans- fragments appear in the DF spectra. Thus, at these energies, both dissociation channels are possible. This situation corresponds to excitation schemes (3) in Figure 2.6. The production of the two fragments is also seen in cis-2-naphthol–CH3OH, as shown in Figure 4(b), with a much wider range of the nIR excitation frequency. The interesting point is that the trans/cis fragment ratio rapidly decreases with IR energy. Thus, with increase in energy, the rate constant of the H-bond dissociation within the isomer becomes greater than that of the isomerization. 2.4.4.3 High-Energy IR Excitation In the higher-energy IR excitation, H-bond dissociation within the isomer becomes dominant, as seen in the DF spectrum with ~nIR ¼ 3679 cm1 of cis-2-naphthol–CH3OH in Figure 4(b). This situation corresponds to excitation schemes (4) in Figure 2.6. In the case of cis-2-naphthol–H2O, an exclusive H-bond dissociation

36

Hydrogen Bonding and Transfer in the Excited State

occurs even at lower IR frequency excitations because of the smaller H-bond energy of cis-2-naphthol–H2O (2400 cm1) compared with that of cis-2-naphthol–CH3OH (2900 cm1). The excess energy after H-bond dissociation is distributed into the internal and translational motion of the fragments. The broad bandwidth of 2-naphthol fragment emission indicates that the generated fragment is internally very hot. Although it is not clear whether the isomerization/dissociation of H-bond processes occur statistically, the exclusive production of the cis-fragment at higher energy can be explained from the viewpoint of statistics. That is, the density of states for the dissociation channel is larger than that of the isomerization channel. The density of states of the latter channel is composed of the quantum levels of the free internal rotations and vibrations of the fragments, while that of the former channel is composed of the quantum levels of the inter- and intramolecular vibrations of the clusters. As the energy spacing of the rotational motions is much smaller than that of the intermolecular vibrations, the density of states of the former (dissociation) channel increases more rapidly with energy than the latter (isomerization) channel, leading to dissociation as the main channel at high energy.

2.5 Comparison of the cis ! trans Barrier Height Between S0 and S1 As was described in the Introduction, the torsional barrier height of the OH group of phenol is 1500 cm1 in S0, while it increases to as high as 4500 cm1 in S1. A similar feature will be seen for 2-naphthol, in which the torsional motion corresponds to the isomerization. In the UV–IR DR measurement of bare cis-2-naphthol, we did not observe emission assignable to trans-2-naphthol at the IR frequency of 3609 cm1, and the barrier height was higher than this energy in S1. Although the barrier height in S0 is not reported, it may be very similar to that of phenol. Figure 2.7 shows the calculated potential energy curves along the torsional coordinate of 2-naphthol and 2-naphthol–NH3 in S0 and S1 obtained by DFT calculation for S0 and TD-DFT calculation for S1 at the B3LYP/aug-cc-PVDZ level. The barrier height in S0 is found to be 1500 cm1 for both species, which is similar to that of phenol, as expected. In S1, the height increases to 3100 cm1 for both species. This value is roughly in good agreement with the observed heights of 2900 cm1 of 2-naphtol–NH3. However, the calculation does not predict the observed decrease in barrier height upon H-bonding. As the change in barrier height is considered to be caused by the interaction with the higher electronic excited state(s), a higherlevel calculation may be necessary for its origin.

Figure 2.7 Potential energy curves of 2-naphthol and 2-naphthol–NH3 along the torsional coordinate in S0 and S1, obtained by DFT and TD-DFT calculation at the B3LYP/aug-cc-PVDZ level. Reprinted with permission from [67]. Copyright 2006, American Institute of Physics

Vibrational Energy Relaxation Dynamics 37

2.6 Conclusion In this review, we described VER dynamics of the XH stretching vibration of H-bonded clusters of 2-naphthol in S1 promoted by UV–IR DR excitation. The UV–IR DR excitation has the capability of conformer and site selectivity, that is, we can selectively excite the vibration of the conformer specified molecule at different sites, for example the donor site OH stretch and the acceptor site OH stretch in the H-bonded system. As such, the detailed study will be very important for a full understanding of the VER mechanism. After IR pulse excitation, the energy is immediately relaxed within the cluster, and two processes, VP and isomerization, compete with each other. At low energy, isomerization effectively occurs, and the cis ! trans isomerization barrier height is found to be 2900 cm1. With increase in IR energy, the VP rate constant rapidly increases and becomes a dominant process. In general, the XH stretching vibration is IR active, but not in the electronic transition owing to small Franck–Condon activity. Thus, the combination of tuneable IR laser light and UV probe is very powerful for the spectroscopic study of these vibrations and the elucidation of their VER dynamics. For the future aspect, a time-resolved pump–probe study will provide us with more detailed dynamics.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

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32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71.

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3 Hydrogen Bond Basicity in the Excited State: Concept and Applications Attila Demeter Institute of Materials and Environmental Chemistry, Chemical Research Centre, Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri u. 59-67, Hungary

3.1 Introduction Hydrogen-bonded complex formation often modifies significantly the physicochemical, spectroscopic and kinetic properties of organics. This observation is even more straightforwardly valid if the properties of the electronic excited states are considered. When the solvents have protic character, the absorption and emission maxima of the fluorophores shift out from the usual range; in addition, the quantum yields and the lifetimes of the excited molecules are often very dissimilar to those expected in solvents of comparable polarity. The first step to account for these phenomena is to construct a proper thermodynamic description of the excited-state hydrogen bonding process; however, relatively little effort has been made to develop a concise handling. The difficulties probably originate from the complex nature of the interaction (weak, competing solute–solvent and solvent–solvent interactions, where the distribution of the interaction energies are comparable), and from the fact that the lifetimes of the excited states are in the same range than the characteristic time of the processes in question. In this chapter, an inchoative but consistent methodology for dealing with this phenomenon is shown. Model systems where the effects are great enough for clear conclusions to be drawn and where disturbing side processes are probably negligible were chosen for examination. The hydrogen bond forming reactants were aliphatic alcohols, mostly fluorinated ones. By using a homologue series of alcohols, the effect of reaction enthalpy can be kept under control. The solvents used in this study were exclusively aprotic, first and foremost n-hexane. In paraffin solvents, not only are the solvent–solute and solvent–reactant interactions negligible but also the examined effects and interactions are much more pronounced.

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

40

Hydrogen Bonding and Transfer in the Excited State

For description of the thermodynamics (equilibrium properties) of ground-state hydrogen bond formation, Abraham’s hydrogen bond acidity–basicity model [1] is used. With a simple and more or less trivial extension, the methodology can be applied for the description of excited-state processes too. The empirical formalism constructed by M. H. Abraham is relatively unostentatious, but it has proved to be very practical and useful in very different areas of chemistry and biology.

3.2 Experiment Detailed experimental information is given in the relevant references, cited at the beginning of the corresponding chapter. Here, only the most important data are shown. The methanol and carbon tetrachloride for UV spectroscopy were from Fluka, while the n-hexane for spectroscopy and the ethanol (EtOH) and 2-propanol (IPA) were of Merck Uvasol quality and were used as received. Fluorinated alcohols (perfluorotert-butanol (PFTB), 1,1,1,3,3,3-hexafluoro-propan-2-ol (HFIP), 1H,1H,7H-dodecafluoro-heptan-1-ol (DFH), 2,2,3,3,3-pentafluoro-propan-1-ol (PFP), 2,2,2-trifluoroethanol (TFE)) were purchased from Fluorochem Limited, except for 2,2-difluoroethanol (DFE), which was delivered by Apollo Scientific Ltd, and were used without further purification. All other solvents were received from Merck, and purified on an aluminaactivated charcoal column, just prior to use. Absorption spectra were recorded on a Unicam UV/VIS spectrophotometer with a resolution typically of 0.2 nm (and exceptionally of 0.5 nm). When necessary, correction was made for dilution caused by the addition of alcohol and for density change due to variation in temperature. In all measurements (except some absorption ones), deaerated samples were used. The fluorescence spectra were measured with a homemade photon-counting spectrofluorimeter equipped with a Princeton Applied Research 1140 A/B detection system of 1 nm, and on a Jobin-Yvon Fluoromax photon-counting spectrofluorimeter with 0.5 nm resolution. Room-temperature fluorescence quantum yields were determined relative to that of quinine sulfate (Ff ¼ 0.546) [2]. Time-resolved fluorescence measurements were made by the nanosecond single-photoncounting technique using an Applied Photophysics SP-3 instrument. When the molecules absorbed in the 400 nm region, time-resolved fluorescence measurements were made by another single-photon-counting set-up with Picoquant diode laser excitation at 404 nm. The typical half-width of the detected pulse was approximately 570 ps. In most of the triplet measurements, 308 nm (XeCl) light pulses from a Lambda Physics EMG 101 or COMPex 201 excimer laser were used. Transient absorption signals were recorded on an optical line (Xe lamp–thermostated sample–Applied Photophysics monochromator–RCA 928 photomultiplier–Hitachi VC6041 digital oscilloscope) mounted perpendicular to the excitation line. Triplet yields were determined using the energy transfer method with perylene as the energy acceptor [3]. These measurements were made relative to the 1.00 triplet yield of benzophenone if the solvent was acetonitrile, and N-methyl-1,8naphthalimide (FISC ¼ 0.96) in other solvents [3]. Molar absorption coefficients of the triplets were determined relative to that of benzophenone (6600 mol1 dm3 cm1) at 525 nm in acetonitrile [4]. The triplet yields (FISC) of compounds not absorbing at 308 nm were measured by laser flash photolysis using the energy transfer method with anthracene as the energy acceptor [5]. The excitation wavelength was 266 nm from a frequencyquadrupled Nd:YAG laser (Continuum Surelight). The synthesis of the compounds prepared for this study is indicated when these first appear; other materials and precursors were obtained from Aldrich. The crude compound was purified by column chromatography and crystallization from an n-hexane–chloroform mixture. Further purification was carried out by preparative thin-layer chromatography (Merck PLC Silica Gel) with an n-hexane–ethyl acetate solvent mixture as the eluant.

Hydrogen Bond Basicity in the Excited State: Concept and Applications

41

3.3 Results and Discussion 3.3.1 Absorption and fluorescence spectra of the complexed species Certainly the equilibrium properties (and thus the thermodynamics) of hydrogen bond complexation can be investigated by several methods. However, in this study most of the experiments were performed using UV-visible absorption–emission spectroscopy, which choice can be consider more or less natural, taking into account that the primary aim of this inquiry was to understand the influence of hydrogen-bonded complex formation on the photophysical properties of the electronic excited state. On the basis of our spectroscopic results, in most of the cases the complexation mechanism given in Scheme 3.1 can be suggested: K1

ð3:1Þ

K2

ð3:2Þ

N þ X !  NX NX þ X !  NX2 Scheme 3.1

where K1 and K2 are the equilibrium constants for the reversible formation of the singly (NX) and doubly complexed (NX2) species respectively. N denotes the uncomplexed substrate in common, while X represents the hydrogen-bond-donating alcohol. 3.3.1.1 Influence of Alcoholic Additives on the Fluorescence and Absorption Spectra in the Simple Case of Isoindolo[2,1-a]indole-6-one [6] In the simplest case shown in this study, the effect of alcoholic additives on the spectrum of isoindolo[2,1-a] indole-6-one (I, Figure 3.1) has been investigated in n-hexane with methanol and four fluorinated alcohols. The results obtained with 1,1,1,3,3,3-hexafluoro-propan-2-ol (HFIP) are given in Figure 3.1. The addition of alcohol caused slight red-shift of the spectrum; a more significant shift is observed at the longer wavelengths. The shift is explained as a consequence of the formation of a hydrogen-bonded complex IX from the interaction of I with the alcohol X, as shown in equation (3.1). (In the alcohol concentration range examined it was unnecessary to presume a more complex mechanism.) If X is in large excess of I, the change in absorbance can be analysed by the equation derived by Mataga [7]: A0 A ¼ K þ K «IX A0 1 1 ½X0 «I A

1

ð3:3Þ

where A0 and A are the absorbance at a given wavelength in the absence and presence of alcohol respectively, and «I and «IX are the molar absorption coefficients of I and IX respectively. For the I–HFIP–n-hexane system, the plot of absorbance according to equation (3.3) is shown in the inset of Figure 3.1. The good straight-line character of the plot supports the singly complexed nature of the new absorbing species. From the intercept of the plot, the equilibrium constants can be directly obtained. These results are given for different alcohols in the third column of Table 3.1. The temperature dependence of the equilibrium constant for complex formation was studied for the I–HFIP–n-hexane system in the temperature range 15–60  C. The reaction enthalpy (DH ) and reaction

Hydrogen Bonding and Transfer in the Excited State 1.0

15

O N

absorbance / arbitrary

0.8

10

5

0.6 0.75

0.4

0.80

0.85

0.90

0.95

(1-A0/A)/[HFIP]

42

1.00

A0/A

0.2

0.0 15000

20000

25000 30000 -1 wavenumber / cm

35000

Figure 3.1 Absorption (full line) and fluorescence spectra (dotted line) of I (black line) and of the complex of I with HFIP in n-hexane. (At 25 000 cm1 the increasing absorbance corresponds to 0, 0.0027, 0.0054, 0.011, 0.019, 0.027, 0.040, 0.053 and 0.080 mol dm3 [HFIP], and the derived complex species spectrum.) Inset: linearized plot (in accordance with equation (3.3)) of the 400 nm absorbance results. Reprinted with permission from [6]. Copyright 2005 American Chemical Society (See Plate 3)

entropy (DS ) were obtained from the measured temperature dependence of the equilibrium constants in accordance with the van’t Hoff equation: ln K ¼

DH  DS þ RT R

ð3:4Þ

A reaction enthalpy of DH1 ¼ (23.9  1.3) kJ mol1 and a reaction entropy of DS1 ¼ (50.5  3.4) J mol1 K1 were derived from the van’t Hoff plot. The knowledge of the K1 equilibrium constant allows us to derive the spectrum of the complexed species IX. Using the absorption spectrum of I, as well as the spectrum measured in the presence of an adequate concentration of alcohol, the absorption spectrum of the complexed IX species is obtained. The spectrum of the hydrogen-bonded complex between I and HFIP is presented as an example in Figure 3.1. The absorption spectra of I and IX are similar; however, that of IX is red-shifted relative to I by about 5–10 nm. The extent of the red-shift increases with the hydrogen bonding capability of the alcohol. The molar absorption coefficient at the maximum is similar for the uncomplexed and complexed species. In addition to the absorption, the fluorescence spectra for the complexed species were also derived. This derivation is complicated by the fact that the singlet excited IX species can be formed via two different routes, i.e. by the excitation of the ground state IX IX þ hn ! 1 IX

ð3:5Þ

PFTB HFIP DFH PFP TFE DFE MET ETOH IPA None

0.88 0.77 0.65 0.64 0.57 0.53 0.43 0.37 0.33

Alcohol aH 2

15.9  0.5 14.2  0.5 11.3  0.5 8.8  0.9 7.1  0.9

98  5 35  4

7.9  1.5 5.6  1.5 3.0  1.5

D1E K1 1 3 (mol dm ) (kJ mol1)

I

28 163

2.8 28 459

28 003 28 016

27 912 27 995

26.5 21

173 94

0--0 K1 n~f1 1 3 (mol dm ) (cm1)

1.2

2.6 2.7

29 13

27 600

27 860 27 780

27 270 27 910

0--0 K2 n~f2 1 3 (mol dm ) (cm1)

DMPN

19.8 10.9 6.8

270 000 33 000 1570 720 398

1.9 1.7 0.5

250 21 5 5 3.5

39 220 39 210 39 234 39 870

38 920 38 997 39 098 39 104 39 115

K1 K2 n~max abs 1 3 1 3 (mol dm ) (mol dm ) (cm1)

DMAP

24 500 24 100 24 370 24 670

22 552 24 387 24 470 24 550 24 300

n~max f (cm1)

0.006 0.014 0.006 0.002

0.029 0.028 0.019 0.013 0.012

Ff

Table 3.1 Thermodynamic and spectroscopic data for the substrate–alcohol–n-hexane system at room temperature. Reprinted with permission from [8]. Copyright 2004 American Chemical Society

Hydrogen Bond Basicity in the Excited State: Concept and Applications 43

44

Hydrogen Bonding and Transfer in the Excited State

and by the complexation of singlet excited I 1

K6

1 I þ X !  IX

ð3:6Þ

In order to determine the fluorescence spectrum of the complexed species IX, fluorescence spectra were determined for I without additive and for samples with various alcohol concentrations. By means of the known K1 equilibrium constant, the a percentage of the uncomplexed species that corresponds to the fraction of the light intensity absorbed by I was calculated. (The excitation was at the isobestic point of the absorption spectra.) Using the experimentally determined decay parameters of the uncomplexed species 1 I (see below), in the presence and absence of alcohol (t1 and t0 respectively), it is possible to calculate the fluorescence intensity of IX at a given wavelength:  if ðIXÞ ¼

t1 if ðsampleÞif ðIÞa t0

  t1 1a t0

ð3:7Þ

where if ðIÞ, if ðIXÞ and if ðsampleÞ are the fluorescence intensities emitted by I, IX and the sample containing alcohol respectively. The fluorescence spectrum of I complexed with HFIP is shown, as an example, in Figure 3.1. Both the uncomplexed and the complexed species show almost structureless fluorescence. The spectrum of IX is red-shifted compared with the uncomplexed spectrum. The extent of the shift is about 40 nm with HFIP and perfluoro-tert-butanol (PFTB) complexing agents, and decreases with decreasing hydrogenbond-forming capability of the alcohol. Compared with the significant fluorescence quantum yield of I (Ff ¼ 0.51), the fluorescence yields of the complexed species are small. The value slightly depends on the applied alcohol and decreases significantly with increasing alcohol concentration. (For example, with HFIP, the yield decreases from 0.03 to 0.01 when the concentration of alcohol increases from 0.004 to 0.08 mol dm3.) The spectrum in the region of the first absorption band is more or less structureless; therefore, the following procedure has been adopted in the determination of the singlet energy: the scale of the fluorescence spectrum relative to the absorption spectrum is chosen in such a way as to ensure mirror symmetry in the low-energy range of the absorption spectrum. Singlet energy is obtained as the energy of the crossing point of the two spectra. In this way the D1 E ¼ 1 EðIXÞ1 EðIÞ differences in the singlet energy of the complexed and uncomplexed species (given in the fourth column of Table 3.1) are obtained. Note that the singlet energy for I is 1 E ¼ 283 kJ mol1 (23 650 cm1). 3.3.1.2 Influence of Alcoholic Additives on the Fluorescence and Absorption Spectra in the Case of Consecutive Two-Step Complexation [8] The UVabsorption spectrum of N-(2,6-dimethylphenyl)-2,3-naphthalimide (DMPN) in n-hexane was studied with different alcoholic additives in the 0–0.15 mol dm3 concentration range [8]. In Figure 3.2, the UV absorption spectra of DMPN are given in the presence of HFIP of different concentration. The alcohol concentration was kept low, and therefore the solvent polarity could be considered constant and dimerization of the alcohol could be neglected. The UV absorption spectrum of DMPN has a strong vibronic progression with 705 cm1 spacing. The half-width of the vibrational band is relatively narrow (305 cm1 or 3.8 nm). At low [HFIP], increasing alcohol concentration causes a continuous decrease in the 351.4 nm band intensity, and simultaneously a new absorption maximum appears at 358.5 nm. An isobestic point is observed around 353.9 nm. Further increase in the HFIP concentration results in the disappearance of the isobestic point, a decrease in absorption at 358.5 nm and the appearance of an absorption band at 368.8 nm. These observations

Hydrogen Bond Basicity in the Excited State: Concept and Applications 0.8

3

0.6

absorbance

45

0.4

h

i

j

[HFIP] / mol dm (a) 0.0 (b) 2.2E-4 (c) 4.4E-4 (d) 0.0011 (e) 0.0022 (f) 0.0033 (g) 0.0054 (h) 0.01 (i) 0.016 (j) 0.023 (k) 0.044 (l) 0.066 (m) 0.087 (n) 0.108 (o) 0.129

o n m l k

g f e

0.2

d

0.0 300

c b a

310

320

330

340

350

360

370

380

390

400

wavelength / nm

Figure 3.2 Absorption spectra of DMPN (8  105 mol dm3) with various [HFIP] in n-hexane. Reprinted with permission from [8]. Copyright 2004 American Chemical Society (See Plate 4)

can be explained by assuming a two-step consecutive complexation reaction mechanism. The absorption maxima at 351.4, 358.5 and 368.8 nm correspond to the (0, 0) transitions of the uncomplexed (N), singly complexed (NX) and doubly complexed (NX2) species respectively. (Here, X stands for the hydrogendonating alcohol molecule, in the present case HFIP.) The strong red-shift observed in the N, NX, NX2 series indicates the pp character of the uncomplexed and the singly complexed species. On the basis of spectroscopic results, the complexation mechanism given in Scheme 3.1 can be suggested. Assuming that dimerization of the alcohol is negligible, the expressions for the equilibrium concentrations of species N, NX and NX2 are given as ½N ¼ ½N0 ½NX½NX2  ½NX ¼

½NX2  ¼

K1 ½N0 ½X 1 þ K1 ½X þ K1 K2 ½X2 K1 K2 ½N0 ½X2 1 þ K1 ½X þ K1 K2 ½X2

ð3:8Þ ð3:9Þ

ð3:10Þ

The initial concentration of DMPN, [N]0, can be determined easily from the absorbance of the sample that contains no alcohol. In the case of samples containing alcohol, the absorbance (A) at a given wavelength is described by the equation A ¼ «N ½N þ «NX ½NX þ «NX2 ½NX2 

ð3:11Þ

where «N , «NX and «NX2 are the molar absorption coefficients of species N, NX and NX2 respectively. In an iterative nonlinear fitting procedure, using the Marquardt algorithm, K1, K2 and the molar absorption coefficients of complexes NX and NX2 were optimized. In the fitting procedure, the absorbance measured at

46

Hydrogen Bonding and Transfer in the Excited State 0.8

367.0 nm

absorbance

0.6

358.5 nm 347.5 nm 351.5 nm

0.4

376.0 nm 0.2

0.0 0.00

0.02

0.04

0.06

0.08

[HFIP] / mol dm

0.10

0.12

-3

Figure 3.3 Fitted DMPN (8  105 mol dm3) absorbance as a function of HFIP concentration in n-hexane at five representative wavelengths. Reprinted with permission from [8]. Copyright 2004 American Chemical Society

five selected characteristic wavelengths in samples with various alcohol concentrations was used. A representative fit is presented in Figure 3.3. The fits are good in general; however, it is to be noted that, at high alcohol concentrations (e.g. concentrations higher than 0.15 mol dm3 in the case of HFIP in n-hexane), calculated curves deviate from the measured points. This may be the result of side reactions, such as the formation and reactions of dimer alcohol. Compartment analysis of the dependence of absorbance on the alcohol concentration at several wavelengths makes the determination of equilibrium constants and absorption coefficients more reliable. The optimized room-temperature equilibrium constants and molar absorption coefficients determined for the DMPN–HFIP–n-hexane system are given in Table 3.1. The temperature dependence of the complexation equilibrium constants K1 and K2 was studied in the temperature range between 50 and þ 65  C. The temperature dependence of the equilibrium constants 8

-1

3

ln(K / mol dm )

6

4

2

0

3.0

3.5

4.0

4.5

-1

1000 (T / K)

Figure 3.4 van’t Hoff plots for complexation in the DMPN–HFIP–n-hexane system. Full and open circles refer to the first and the second complexation steps respectively. Reprinted with permission from [8]. Copyright 2004 American Chemical Society

Hydrogen Bond Basicity in the Excited State: Concept and Applications

47

for HFIP complexation in n-hexane (see equation (3.4)) is presented in Figure 3.4. From these plots, DH1 ¼ 27.2  0.5 kJ mol1 and DH2 ¼ 19.7  0.5 kJ mol1 are obtained. Similar plots for complexation with PFTB in n-hexane yield DH1 ¼ 28.5  2.5 kJ mol1 and DH2 ¼ 21.8  2.1 kJ mol1. It can be seen from Figure 3.4 that the K values at the lowest temperature are lower than expected from the rest of the data; these data were not taken into account in determining the thermodynamic parameters. Similar deviations were also observed at the lowest temperature in the PFTB complexation system, where only approximate values could be derived. The deviation indicates a change in mechanism with temperature, such as, for instance, a change in the kinetics of the complex formation step from an activation-controlled rate at higher temperature to a diffusion-controlled rate at low temperature [9]. As expected, the second complexation step is less exothermic than the first one. Our DH1 values determined in n-hexane can be compared with the DH ¼ 24.7 kJ mol1 obtained by Kivinen et al. [10] for the acetone–HFIP system in carbon tetrachloride and the DH ¼ 33.5  3.4 kJ mol1 reported by Sherry and Purcell [11] for the acetone–PFTB system in n-hexane. The reaction entropies DS1 ¼ 51.9  7.5 J mol1 K1 and DS2 ¼ 47.3  4.2 J mol1 K1 were determined for the complexation processes in the DMPN–HFIP–n-hexane system. Similar values were obtained by us for the DMPN–PFTB–n-hexane system (DS1 ¼ 51.5  8.8 J mol1 K1 and DS2 ¼ 47.3  8.8 J mol1 K1) and were reported in the literature [10, 12] for the other hydrogen bond formation processes where three translational and three rotational degrees of freedom are lost in the reaction. Absorption and Fluorescence Spectra of Complexed Species in the Case of Consecutive Two-Step Complexation The knowledge of K1 and K2 equilibrium constants of complex formation allows us to derive the spectra for the singly complexed and doubly complexed species. Using the DMPN absorption spectrum, as well as the DMPN spectra measured in the presence of a small alcohol concentration (where the singly complexed species dominates) and a relatively high alcohol concentration (where the doubly complexed compound prevails), the spectra of the complexed NX and NX2 species are obtained by an iterative procedure. The fluorescence spectra of the complexed species were obtained by a similar iterative procedure from the fluorescence spectra of DMPN and DMPN spectra measured in the presence of small and relatively high alcohol concentrations. The absorption spectra of DMPN and of the complexed species in the DMPN–HFIP–n-hexane system are presented in Figure 3.5. It is clear from the figure that the characteristics of the spectra of the complexed species are very similar to those of the DMPN. All three spectra are structured with similar progression and relative intensities of vibronic bands. Slight broadening of the bands can be seen in the series N, NX and NX2. The molar absorption coefficients increase by about 35% from N to NX2. The (0, 0) absorption band is shifted considerably as a result of complex formation: the shift is 7.1 nm and 6.9 nm in the first and second complexation step respectively. (For comparison, the solvatochromic shift of the (0, 0) absorption band of DMPN is 7.1 nm when changing the solvent from n-hexane to acetonitrile.) The fluorescence spectra of DMPN and of the complexed species formed in the DMPN–HFIP–n-hexane system are also shown in Figure 3.5. The fluorescence spectrum of DMPN is mirror symmetric to the absorption spectrum, with a very small Stokes shift (less than 1 nm), indicating that the relaxation in the excited state is almost negligible. Mirror symmetry is also observed for the complexed species; however, the Stokes shift is somewhat greater, the structure is less pronounced and the vibronic bands are broader than those of DMPN. The fluorescence quantum yields increase considerably with the strength of complexation (see Table 3.2). The complexation of DMPN was studied also with a number of alcohols other than HFIP. These alcohols included PFTB, 1H,1H,7H-dodecafluoro-heptan-1-ol (DFH), 2,2,3,3,3-pentafluoropropan-1-ol (PFP), 2,2,2trifluoroethanol (TFE) and methanol (MET). Equilibrium constants of complex formation and spectroscopic properties of complexed species were determined, and the main results are given in Table 3.1. In the table, the alcohols are listed in order of increasing hydrogen-bond-donating capability. (See the hydrogen bond acidity values in the second column.) In this sequence, the equilibrium constants for the first as well as for the second

48

Hydrogen Bonding and Transfer in the Excited State wavelength / nm 500

450

400

350

300

N

10000

3

molar absorbance / mol dm cm

0 10000

fluorescence

-1

NX

-1

5000

5000

0

NX2

10000

5000

0 20000

25000

30000 -1

wavenumber / cm

Figure 3.5 Absorption and fluorescence spectra of DMPN and the complexed species in the DMPN–HFIP– n-hexane system. Reprinted with permission from [8]. Copyright 2004 American Chemical Society

complexation step increase, which indicates a decrease in the Gibbs energy change (i.e. progressively more negative DG1 and DG2 values) of complexation in the series. In case of the first complexation step, there is significant red-shift observed in the location of the absorption and fluorescence maxima. A moderate increase in the Stokes shift is also found. It appears from these observations that the spectroscopic properties of the complexed species are significantly influenced by the Gibbs energy change in the reaction. Probably, similar conclusions are valid for the second complexation step too. 3.3.1.3 Absorption Features of the Alcoholic Complex of Dual-Luminescent Naphthalimides in n-Hexane [13] Understanding the spectroscopy of the LE state model compound of naphthalimide (DMPN) and its complexes, it was expected that the absorption and luminescence properties of similar dual-luminescent compounds could now be handled more properly. As a model for further study, N-(3-fluorophenyl)-2,3naphthalimide (MFPN, where the LE ! ICT reaction is reversible [14]) and N-(4-methoxyphenyl)-2,3naphthalimide (PMPN, where the ICT state has clear intramolecular charge transfer character [15]) were selected. Similarly to the LE model compound DMPN, the UV-visible spectrum was studied in n-hexane with different alcoholic additives: the change in the UV spectra, as a function of added fluorinated alcohol concentration, shows similar characteristics to that observed with DMPN.

Hydrogen Bond Basicity in the Excited State: Concept and Applications

49

Table 3.2 Room-temperature equilibrium constants and photophysical parameters for the DMPN–HFIP–n-hexane (carbon tetrachloride) systems. Reprinted with permission from [8]. Copyright 2004 American Chemical Society Species

1

3

K(S0) (mol dm ) (n-hexane) K(S0) (mol1 dm3) (CCl4) K(S1) (mol1 dm3) (n-hexane) K(S1) (mol1 dm3) (CCl4) K(T1) (mol1 dm3) (n-hexane) 1 E (kJ mol1) «(0–0) (mol1 dm3 cm1) Ff FISC FIC t (ns) kf  108 (s1) kISC  108 (s1) kIC  108 (s1)

N

NX

NX2

94  3 39  4 1500  130 340  60 51  4 340.2 6250 0.016  0.002 0.42  0.03 0.56  0.05 0.46  0.02 0.35  0.06 9.1  1.1 12.2  1.5

13  3 2.5  0.5 200  60 30  10 92 332.1 7990 0.18  0.03 0.53  0.04 0.29  0.07 1.9  0.2 0.95  0.26 2.8  0.5 1.5  0.5

324.5 8450 0.45  0.06 0.47  0.05 0.08  0.11 4.7  0.2 1.0  0.2 1.0  0.2 0.2  0.3

The global nonlinear fit, based on the consecutive two-step complexation model [8], of the absorption of MFPN in n-hexane is shown in Figure 3.6 as a function of HFIP concentration (for results, see Table 3.3). Similar measurements were made with perfluoro-tert-butanol (PFTB) additive, resulting in K1 ¼ 92  14, K2 ¼ 22  4 and K1 ¼ 125  8, K2 ¼ 20  4 mol1 dm3 equilibrium constants for MFPN and PMPN respectively. Fluorescence Properties of Complexed Dual-Luminescent Naphthalimides In n-hexane, the MFPN compound shows dominant ICT emission, in addition to the weak but well observable structured LE

O

F

362 nm 0.4

N O

354 nm

absorbance

0.3

350 nm 0.2

370 nm

0.1

376 nm

0.0 0.00

0.02

0.04

0.06 -1

0.08

0.10

3

[HFIP] / mol dm

Figure 3.6 Global nonlinear fit of the absorption of MFPN in n-hexane as a function of HFIP concentration at five representative wavelengths. (Fitting is made by assuming a consecutive two-step complexation model.) Reprinted with kind permission from [13]. Copyright 2005 Springer Science and Business Media

50

Hydrogen Bonding and Transfer in the Excited State

Table 3.3 Physicochemical parameters for the dual luminescent N–HFIP–n-hexane systems. Reprinted with kind permission from [13]. Copyright 2005 Springer Science and Business Media Compound N

MFPN

PMPN

Equilibrium constant (mol1 dm3)

K1

K2

K1

K2

Ground state LE state ICT statea

19 420 350

5 90 130

35 600 1350

6 330 1270

N

NX

NX2

N

NX

NX2

n~max (ICT) (cm1) f

28 270 21 780

27 680 21 210

27 160 20 620

28 390 19 000

27 890 18 250

27 230 17 150

p-Electron charge on the O atom

S0 1.525

LE 1.61

ICT 1.616

S0 1.525

LE 1.61

ICT 1.65

Species n~0--0 (cm1) abs

a

Probably underestimated.

contribution. The ICT/LE ratio changes only slightly as a result of complexation (see Figure 3.7(a)), but both the LE and the ICT emission are shifted to the red. The change with complexation in the energy of the LE (0, 0) band and the change in the fluorescence maximum of the ICT emission are similar, i.e. approximately 500 cm1 and 600 cm1 in the first and second complexation step respectively (Table 3.3). This observation is expected on the basis of our proposed model [15]: the emission from the LE and ICT states corresponds to transition from the LUMO orbital to the HOMO and HOMO-1 orbital respectively. The electron density on the oxygen atom in the case of the HOMO and HOMO-1 orbitals is very similar [16], and therefore no significant difference is expected in the case of complex formation. The fluorescence yield increased by complexation from the moderate 0.007 value of the uncomplexed species to 0.021 and 0.028 values for the singly and doubly complexed species respectively. A similar effect was observed for the yield of triplet formation: for a sample containing 0.0475 mol dm3 HFIP, where the singly complexed species dominates, the triplet yield increases from the 0.30 value of the uncomplexed MFPN to 0.40. The effect is analogous to that observed for DMPN (see below): the decrease in the energy of the lowest excited singlet state results in an increase in the activation energy of the non-radiative singlet-consuming processes (first of all, of internal conversion), causing a pronounced increase in the singlet lifetime and fluorescent intensity and a moderate increase in the triplet yield as well. For PMPN the fluorescence properties are partly different from MFPN: the emission spectrum consists of a single ICT band, independently from the complexation. The shifts in maxima of the ICT emissions of the complexed species and the uncomplexed species are definitively greater than those observed for the (0–0) vibronic bands of the absorption spectra (see Figure 3.7(b) and Table 3.3). If, instead of HFIP, a stronger hydrogen bond donor alcohol (PFTB) was used, the shift in ICT maxima with complexation was even greater: the ICT maxima are 18 110 and 16 660 cm1 for the singly and doubly complexed species respectively. H€uckel calculations [16] suggest that the change in electron density on the oxygen atom is higher in case of the ICT–ground state transition than for the LE–ground state transition, and consequently the hydrogen bond formation will stabilize the ICT state more, causing longer wavelength ICT emission (Table 3.3). The LE ! ICT transition, even for the non-complexed species, is so fast that the LE emission is not observable [15], and consequently it is not surprising that, with the fluorescence of the other two species, where the transition is even more exothermic, the emission spectrum consists only of the ICT band. Contrary to the MFPN, the fluorescence yields decreased with complexation from 0.0058 [15] to 0.0015 and 0.0008 values

Hydrogen Bond Basicity in the Excited State: Concept and Applications

51

wavelength [nm] 600

550

500

450

400

350 F

O

a

N

absorbance [arbitrary]

fluorescence [arbitrary]

O

O

b

N

OMe

O

16000

20000

24000

28000

32000

-1

wavenumber [cm ]

Figure 3.7 The absorption and fluorescence spectra of MFPN (a) and PMPN (b) in n-hexane, using HFIP as additive. The uncomplexed, singly complexed and doubly complexed species are indicated by full, broken and dotted lines respectively. Reprinted with kind permission from [13]. Copyright 2005 Springer Science and Business Media

for the singly and doubly complexed species respectively. Simultaneously (for PMPN), the yield of triplet formation decreases from 0.27 for the uncomplexed molecule [15] to 0.07 (which belongs dominantly to the singly complexed species in the sample containing 0.0475 mol1 dm3 HFIP), indicating that the decrease in fluorescence yield is probably related to the strong red-shift of the ICT emission, which is often connected with an effective temperature-independent internal conversion [3]. 3.3.1.4 Absorption Properties of the Complexes of DMAP in n-Hexane [17] The effect of alcoholic additives on the absorption spectrum of 4-(N,N-dimethylamino)pyridine (DMAP) has been investigated in n-hexane using aliphatic alcohols and some fluorinated alcohols. The results obtained with HFIP are given in Figure 3.8. The addition of alcohol caused no considerable shift in the longer-wavelength bands; it only increased the oscillator strengths. Such observations have been reported for partly forbidden transitions [6, 13]. However, a very significant red-shift is observed around 250 nm for the maximum of the absorption spectrum related to the ICT transition. The shift is explained as a consequence of the

Hydrogen Bonding and Transfer in the Excited State

N

absorbance

2.0

1.5

N

2.0

absorbance

52

1.5 1.0 0.5 0.0 200

220

240

260

280

300

wavelength / nm 1.0

0.5

0.0 200

220

240

260

280

300

wavelength / nm

Figure 3.8 Absorption spectra of DMAP (6.14  105 mol dm3) with and without HFIP additive in n-hexane. (At 280 nm, the increasing absorbance corresponds to 0, 0.00006, 0.00013, 0.00026, 0.00052, 0.0009, 0.0013, 0.0019, 0.0033, 0.0059, 0.011, 0.018, 0.028, 0.039, 0.050, 0.062, 0.076, 0.090, 0.105, 0.121, 0.137 and 0.154 mol dm3 HFIP concentrations.) Inset: the resolved absorption spectra of the uncomplexed (black line), singly and doubly complexed species. Reprinted with permission from [17]. Copyright 2007 American Chemical Society (See Plate 5)

hydrogen-bonded complex formed during the interaction of DMAP with HFIP. An isobestic point is observed around 251.4 nm. An increase above 0.0033 mol dm3 HFIP concentration results in the disappearance of the isobestic point, a decrease in the absorption band at 256.5 nm and the appearance of a new absorption band at 275.0 nm. These observations can be explained by assuming a two-step consecutive complexation reaction mechanism. The absorption maxima at 251.0, 256.5 and 275.0 nm correspond to ICT transitions of the uncomplexed, singly complexed and doubly complexed molecules respectively. The treatment of the measured spectra was completely analogous with what was described before in the case of DMPN, except that, in the case of the strongest hydrogen-bonding alcohols, the consumption of the alcohol had to be taken into account. The knowledge of the K1 and K2 equilibrium constants of complex formation allows us to derive the spectra for the singly complexed and doubly complexed species. Using the DMAP absorption spectrum, as well as the DMAP spectra measured in the presence of small alcohol concentrations (selected where the singly complexed species dominates) and relatively high alcohol concentration (selected where the contribution of the doubly complexed compound is most significant), the spectra of the singly and doubly complexed species are obtained by an iterative procedure (see, for example, the inset in Figure 3.8). The maximum of the absorption band is shifted considerably to the red as a result of complex formation: the shift is 5.6 nm and 18.5 nm caused by the first and second complexation step respectively. The molar absorption coefficients increase moderately by about 20% from DMAP to the DMAP–HFIP complex. The shape of the absorption spectrum of the doubly complexed species has a different character: the oscillator strength of the symmetry forbidden lowenergy transitions seems to increase considerably. Location of Hydrogen Bonding in Singly and Doubly Complexed DMAP In order to reveal where hydrogen bond formation occurs in the first and second complexation steps, the complexation of pyridine and N,N-dimethylaniline (DMAN), considered as models of the two functional groups in DMAP, were studied with alcohols in n-hexane. Cazeau-Dubroca and coworkers [18] stated that complexation with ethanol on the

Hydrogen Bond Basicity in the Excited State: Concept and Applications 2.0

A absorbance

3.5 3.0 2.5

53

1.5

N

1.0 0.5

2.0

0.0 200

220

240

260

280

300

320

wavelength / nm

1.5 1.0

0.0 3.5 3.0 2.5

1.5

B

N absorbance

absorbance

0.5

1.0

0.5

2.0 0.0 200

1.5

220

240

260

280

300

320

wavelength / nm

1.0 0.5 0.0 200

220

240

260

280

300

320

wavelength / nm

Figure 3.9 Absorption spectra of pyridine (4.3  104 mol dm3) (A) and N,N-dimethyl aniline (9.3  105 mol dm3) (B) with and without HFIP additive in n-hexane. (The increasing absorbance corresponds to 0, 0.0034, 0.0069, 0.010, 0.017, 0.025, 0.036, 0.052, 0.069, 0.104, 0.175, 0.25, 0.36 and 0.54 mol dm3 [HFIP] at 250 nm (A), and to 0, 0.0033, 0.0065, 0.010, 0.016, 0.023, 0.033, 0.043, 0.052, 0.066, 0.082, 0.099, 0.13, 0.17, 0.20 and 0.27 mol dm3 [HFIP] at 275 nm (B).) Insets: the resolved absorption spectra of the uncomplexed (black line), singly and doubly complexed species. Reprinted with permission from [17]. Copyright 2007 American Chemical Society (See Plate 6)

dimethylamino group is responsible for the formation of ICT emission, while Waluk and Herbich [19] reported that complexation by butyl alcohol in n-hexane occurs on the pyridine nitrogen. In the case of both pyridine and DMAN, the influence of the HFIP additive on the spectra in n-hexane indicates a complexation mechanism consisting of two consecutive reversible reactions (see Figure 3.9). The equilibrium constants are relatively large for pyridine (K1 ¼ 560 mol1 dm3, K2 ¼ 11 mol1 dm3) and much lower for DMAN (K1 ¼ 56 mol1 dm3, K2 ¼ 8.2 mol1 dm3). In DMAP, the ICT character implies a relatively large negative charge on the pyridine moiety and a small electron density on the amino nitrogen. Thus, greater hydrogen-bond-accepting capability on the aromatic nitrogen atom is expected for DMAP than for pyridine in accordance with the experimental results (note the much larger K1 value of DMAP compared with that of pyridine). DFT calculations [20] support this hypothesis (see below). It turned out that the calculated energy change of the complexation reaction is much more negative if the alcoholic hydrogen of methanol

54

Hydrogen Bonding and Transfer in the Excited State

Scheme 3.2

interacts with the aromatic nitrogen (DE ¼ 30 kJ mol1) than if binding occurs at the amino nitrogen (DE ¼ 11.7 kJ mol1). The binding location of the second interacting alcohol molecule raises an interesting problem: the second alcohol may attack (i) the oxygen atom of the singly complexed species [21] (as observed in the case of complexation of pyridine) or (ii) the nitrogen of the dimethylamino group. Spectral evidence favours the second possibility, as the UV spectrum of the doubly complexed species differs significantly from the singly complexed and uncomplexed species (see the inset of Figure 3.8). Note that, if binding at the oxygen atom were to dominate (case (i)), similar spectra would be expected for the doubly and singly complexed species, with a small further red-shift of the former. Fluorescent Properties of the DMAP and DMAP–HFIP Complexes The character of the fluorescence spectrum of DMAP changes considerably with polarity of the solvent. In apolar n-hexane, only the ‘locally excited’ (LE, 1Lb) emission is observable, and the presence of a very weak ‘intramolecular charge transfer’ (ICT, 1La) emission becomes apparent only by comparison with the fluorescence spectrum of 4-methylaminopyridine (MAP) in n-hexane. The mechanism and rate coefficients of the reversible two-excited-state system are shown in Scheme 3.2, where ka and kd are the rate constants of the forward and reverse reactions 0 of ICT formation, to (¼ 1/(knr þ kf)) and ðko0 Þ1 ¼ t0o ð¼ 1=ðknr þ kf0 ÞÞ are pseudolifetimes, whereas kf and kf0 are the radiative rate constants for the LE and ICT states respectively. In more polar solvents than paraffin, the dual-luminescent feature of the DMAP emission is apparent (see Figure 3.10). In contrast to that observed for the fluorescence of the uncomplexed DMAP, the singly complexed species shows well developed dual luminescence in n-hexane. Moreover, the overall fluorescence yield increases by more than an order of magnitude as a result of complexation. Derivation of the fluorescence spectrum of the singly complexed species was straightforward: owing to the very low concentration of the alcohol applied (often much less than 0.01 mol dm3), the excited-state processes could be neglected and the concentration ratio of the different species could be calculated easily by means of the equilibrium constants and from the absorption properties of the ground-state system. The fluorescence spectra of the species in question were derived at a minimum of three different alcohol concentrations in order to check the concentration independence. The emission spectrum of the DMAP–HFIP singly complexed species in n-hexane is shown in Figure 3.10(a). The fluorescence quantum yield is 14 times greater than that for the uncomplexed molecule. The long-time component of the fluorescence decay (1.5–2.5 ns, depending on the circumstances) increases considerably. The ICT fluorescence maximum is independent of the nature of the most complexing alcohols; however, it shows definite red-shift for the strongest ones (see PFTB in Table 3.1). The fluorescence yield of the doubly complexed species is smaller by a factor of 5 than that of the singly complexed species; the yield decreases further with increasing alcohol

Fluorescence Spectra of Hydrogen-Bonded Complexes of DMAP in n-Hexane.

Hydrogen Bond Basicity in the Excited State: Concept and Applications

55

wavelength / nm 800

600

400

200

n-hexane

A

DEE

fluorescence

B

C

butyronitrile

D

acetonitrile

20000

30000

40000

50000

-1

wavenumber / cm

Figure 3.10 Fluorescence spectra of DMAP (dashed line) and the DMAP–HFIP complex (full line) in different solvents at room temperature. (Dotted line indicates the resolved LE and ICT emission of the complex.) Reprinted with permission from [17]. Copyright 2007 American Chemical Society

concentration. The ICT fluorescence maximum of the doubly complexed molecule is red-shifted by about 47 nm compared with that of the singly complexed species (which is around 21 900 cm1). With increasing polarity of the solvent, the ICT emission of the uncomplexed DMAP becomes more and more visible, and in strongly polar solvents the ICT fluorescence is the dominant component of the emission. The singly complex DMAP emits principally from the ICT singlet state in all solvents (Figure 3.10), while the fluorescence yield of the complexed species decreases with increasing polarity. The equilibrium constant K1 decreases dramatically between n-hexane and the less polar ethers; however, thereafter it becomes practically independent of the solvent polarity (Table 3.4). In all solvents, the maximum of the ICT emission for the singly complexed molecule is strongly red-shifted compared with that observed for the uncomplexed molecule. The smaller fluorescence yield of the singly complexed molecule (when 0.05 mol dm3 HFIP is added to butyronitrile) is in good agreement with the appearance of a new 0.5 ns component in the fluorescence decay of the ICT band. In accordance with this, the triplet yield decreases by more than an order of magnitude from 0.66 to 0.04  0.03 as a result of complexation with HFIP in acetonitrile (see below).

Fluorescence Spectra of the DMAP–HFIP Complexes in Polar Solvents.

56

Hydrogen Bonding and Transfer in the Excited State

Table 3.4 Spectroscopic and equilibrium parameters of DMAP (N) and of DMAP–HFIP complex (NX) in different solvents. Reprinted with permission from [17]. Copyright 2007 American Chemical Society Solvents n-Hexane Dihexyl ether Dibutyl ether Dipropyl ether Diethyl ether Tetrahydropyrane Ethyl acetate Tetrahydrofuran Methyl formate Valeronitrile Butyronitrile Propionitrile Acetonitrile

K2 n~max ~max ~max ðNÞ n~max ðNXÞ D1E Ff(N) Ff(NX) K1 abs ðNÞ n abs ðNXÞ n f f 1 3 1 3 1 1 1 1 (mol dm ) (mol dm ) (cm ) (cm ) (cm ) (cm ) (cm1) 33 000 160 119 88 39 19.5 33 15 60.5 39.3 34.3 25.4 23.1

21

39 832 39 637 39 585 39 575 39 570 39 275 39 220 39 210 38 760 38 990 39 007 38 945 38 965

3.3 1.1 1.6 1.1 5.8

38 997 38 800 38 720 38 735 38 715 38 485 38 505 38 460 38 380 38 390 38 265 38 270 38 150

24 670 24 115 23 727 23 700 23 489 22 943 23 048 22 906 22 098 22 750 22 750 22 358 22 114

24 387 23 400 23 060 22 870 22 170 21 200 20 405 19 970 18 130 18 190 17 972 17 209 16 567

837 720 711 703 679 659 674 652 692 676 675 667 664

0.002 0.0275 0.008 0.024 0.012 0.013 0.020 0.0031 0.015 0.0009 0.017 0.0008 0.012 0.0009

3.3.2 Hydrogen bond basicity of the ground and singlet excited state 3.3.2.1 Hydrogen Bond Basicity of the Ground and Singlet Excited State of DMPN and HFIP-Complexed Derivatives in n-Hexane In Figure 3.11 the singlet excitation energy difference of the complexed and uncomplexed species of DMPN (D1E) is plotted as a function of the Gibbs energy change in complex formation (DG ¼ RT ln K) at room

-6

1

E / kJ mol

-1

-4

-8

-20

-15 o

-10

-5 -1

3

0 -1

G = -RTln(K / mol dm ) / kJ mol

Figure 3.11 Singlet excitation energy difference of complexed and uncomplexed species (D1E) as a function of the Gibbs energy change in complex formation in the DMPN–alcohol–n-hexane system at room temperature. The circles (black) show the data derived from the first complexation step, and the squares (red) represent the second complexation step. The sequence of alcohols from right to left is: MET, TFE, PFP, DFH, HFIP and PFTB (full symbols). The temperature dependence is illustrated by the HFIP-complexed derivatives at 86, 25, 0, 25 and 50  C (open symbols). Adapted with permission from [8]. Copyright 2004 American Chemical Society

Hydrogen Bond Basicity in the Excited State: Concept and Applications

57

1

N+X 1

1

NX

D( N-X)

1

EN

1

ENX

D(N-X)

N+X NX

Figure 3.12 Energy cycle involving ground-state NX and singlet excited-state 1NX hydrogen-bonded complexes as well as their dissociation products. 1EN and 1ENX are the singlet excitation energies of N and NX respectively, and D(N–X) and D(1N–X) are the corresponding bond dissociation energies. Reprinted with permission from [8]. Copyright 2004 American Chemical Society

temperature in n-hexane. The data indicate that linear correlation exists between these two quantities. Similarly good linearity is obtained in carbon tetrachloride. In order to explain this linearity, the energy diagram for complex formation in the ground state and in the excited state (Figure 3.12) has to be considered, in a similar manner to that presented for heterolytic dissociation processes by F€orster [22, 23]. The basic thermodynamic equation for the Gibbs energy change in the formation of ground-state NX from the components may be given as RT ln K1 ¼ DGo1 ¼ DH1o TDSo1

ð3:12Þ

Similarly, for the formation of singlet excited 1NX from excited 1N and X (general form of equation (3.6)) K13

1 N þ X !  NX

ð3:13Þ

o T DSo13 RT lnK13 ¼ DGo13 ¼ DH13

ð3:14Þ

1

the Gibbs energy change is

An analogous relationship holds also for the formation of doubly complexed singlet species: K15

1 NX þ X !  NX 2

ð3:15Þ

o RTðln K13 ln K1 Þ ¼ DGo13 DGo1 ¼ DH13 DH1o TðDSo13 DSo1 Þ

ð3:16Þ

1

From equations (3.12) and (3.14) we obtain

Abraham [1, 24] has expressed logK of a complexation process, in carbon tetrachloride at 298 K, as a function H of the product of the solute hydrogen bond acidity (aH 2 ) and the hydrogen bond basicity (b2 ):

58

Hydrogen Bonding and Transfer in the Excited State H log K ¼ 7:354aH 2 b2 1:094

ð3:17Þ

Using this type of relationship, lnK1 and lnK13 may be given in generalized form H ln K1 ¼ C2 aH 2 b2 ðNÞC1

ð3:18Þ

H 1 ln K13 ¼ C2 aH 2 b2 ð NÞC1

ð3:19Þ

and

where C2 ¼ 2.303  7.354 ¼ 16.933 and C1 ¼ 2.303  1.094 ¼ 2.519 for carbon tetrachloride solvent at 298 K temperature. Substituting aH 2 from equation (3.18) into equation (3.19), an expression is obtained for lnK13 that may be used in rewriting equation (3.16): o DH13 DH1o ¼ RT

H 1 bH 2 ðNÞb2 ð NÞ ðln K1 þ C1 Þ þ TðDSo13 DSo1 Þ bH 2 ðNÞ

ð3:20Þ

Considering the energy cycle involving the ground states and excited states of N and NX (see Figure 3.12), o it can be shown that DH13 DH1o (which equals D(N–X)  D(1N–X)) may be replaced with the difference of the singlet excitation energies of NX and N, i.e. D1E ¼ 1ENX  1EN. Moreover, the entropy change in complexation is expected to be similar for the singlet excited- and ground-state species, and therefore DSo13 DSo1 is negligible, and T(DSo13 DSo1 ) may be omitted on the right-hand side of the equation. Thus, equation (3.20) can be replaced with the equation 1

ENX 1 EN ¼

1 1 H H bH bH o 2 ð NÞb2 ðNÞ 2 ð NÞb2 ðNÞ DG C1 RT 1 bH bH 2 ðNÞ 2 ðNÞ

ð3:21aÞ

The analogous equation for the second complexation step is 1

ENX2 1 ENX ¼

H H 1 1 bH bH o 2 ð NXÞb2 ðNXÞ 2 ð NXÞb2 ðNXÞ DG C1 RT 1 bH bH 2 ðNXÞ 2 ðNXÞ

ð3:21bÞ

These equations demonstrate that, at constant temperature, linear correlation is expected between the singlet excitation energy difference of the complexed and uncomplexed species on the one hand and the Gibbs energy change in the complexation process (or alternatively the logarithm of the equilibrium constant for complex formation) on the other hand. This agrees with what has been found experimentally (see Figure 3.11). Equation (3.16) and the analogous equation for the formation of the doubly complexed species offer some further application. Namely, a treatment based on an energy cycle, analogous to the well-known F€orster cycle [22, 23], can be used to derive K13 and K15, the equilibrium constants for the formation of singlet excited o singly and doubly complexed species. Neglecting T(DSo13  DSo1 ), replacing DH13  DH1o with (1ENX  1EN), as discussed above, and using the known values of the ground-state equilibrium constant and the singlet excitation energy difference (1ENX  1EN), we can calculate the equilibrium constant for the singlet excited species. Such calculated K values are given in Table 3.2. Equation (3.18) or the generalized equation (3.19) offers a simple technique for characterizing complexation equilibria, provided that the hydrogen bond solute parameters are known for the hydrogen donor and

Hydrogen Bond Basicity in the Excited State: Concept and Applications

59

hydrogen acceptor species. Following earlier studies [11, 25], Abraham and coworkers [1, 24] established the H H H aH 2 (hydrogen bond acidity, short notation of Sa2 ) and b2 (hydrogen bond basicity, short notation of Sb2 ) scales, which became widely accepted. In addition, considerable theoretical efforts were made to predict these hydrogen bond solute parameters [26]. Moreover, it was shown [27] that the aH 2 is in excellent correlation with the maximum of the electrostatic potential on the surface of the corresponding molecule. H Originally, the aH 2 and b2 solute scales had been set up using log K values for complexation measured in carbon tetrachloride (see equation (3.18)), and no hydrogen bond basicity parameter has been reported so far for DMPN. In order to obtain bH 2 values for DMPN and its complexed species, equilibrium constants for complex formation with methanol and fluorinated alcohols (listed previously) were determined in CCl4. The K values were determined in the same way as described for the experiments in n-hexane. The hydrogen bond basicity values of N and NX were calculated from equation (3.18) using the K1 and K2 equilibrium constants, respectively, obtained in carbon tetrachloride solvent in this work, and the aH 2 values of the alcohols (MET, TFE, DFH, HFIP and PFTB) taken from the literature [1]. The aH value of PFTB, for which no data were 2 available, was estimated from literature results [11] to be aH ¼ 0.88. The calculated hydrogen bond basicities 2 showed no dependence on the complexing alcohols, and the derived average values were bH 2 ðNÞ ¼ 0.47  0.02 and bH ðNXÞ ¼ 0.29  0.02. As expected, the hydrogen bonding ability of NX reacting in the second 2 H complexation step is lower than that of N participating in the first step (i.e. bH ðNXÞ < b ðNÞ). However, 2 2 it is interesting to note that bH ðNXÞ hardly depends on what kind of alcohol is attached to the other hydrogen2 bonding site of the naphthalimide structure. Since equation (3.18) is based on measurements made in carbon tetrachloride, we used relationships of the equation (3.19) type, with non-fixed C1 and C2 coefficients, to interpret the results of equilibrium studies carried out in n-hexane. Accordingly, the logarithm of the experimentally determined equilibrium constants for the DMPN–HFIP system is plotted against the appropriate aH 2 values in Figure 3.13. The results of equilibrium studies carried out in carbon tetrachloride are presented for comparison too. The intercepts of the straight lines corresponding to the first and second complexation steps are 0.98  0.11 and 1.20  0.06,

2.5

-1

3

log(K / mol dm )

2.0

1.5

1.0

0.5

0.0

-0.5 0.4

0.5

0.6

0.7

0.8

0.9

H 2

Figure 3.13 Plot of the logarithm of the equilibrium constant for complexation of DMPN as a function of hydrogen bond acidity, in accordance with equations (3.18) and (3.19). Circles and squares indicate results obtained in n-hexane and carbon tetrachloride respectively. Full and open symbols refer to the first and second complexation steps respectively. The sequence of alcohols from left to right is MET, TFE, DFH, HFIP and PFTB. Reprinted with permission from [8]. Copyright 2004 American Chemical Society

60

Hydrogen Bonding and Transfer in the Excited State

respectively, in reasonably good agreement with the 1.094 parameter of equation (3.18), which is based on a large number of equilibrium measurements. From the slopes of the straight lines of the CCl4 measurements, H bH 2 ðNÞ ¼ 0.46 and b2 ðNXÞ ¼ 0.30 are derived by means of the C2 ¼ 7.354 coefficient taken from equation (3.18). For the DMPN–HFIP system in n-hexane, similarly good straight lines are obtained both for the first and second complexation steps (see Figure 3.13); however, discussion of these results requires some assumptions H to be made. A reasonable assumption is that the aH 2 and b2 scales are independent or are only slightly dependent on the solvent (at least in the case of apolar and non-associative solvents). From the intercepts of the log K versus aH 2 plots of the data of the first and second complexation steps in n-hexane, we obtain C1 ¼ 1.08  0.3 and 1.3  0.2 respectively. Within the limits of error, these figures agree with the 1.094 parameter of equation (3.18) determined in carbon tetrachloride [1]. Assuming that the bH 2 ðNÞ ¼ 0.47 value determined in carbon tetrachloride can be used to interpret equilibrium results obtained in n-hexane, from the slopes of the plots of the straight lines determined in n-hexane a C2 value about 14% higher than that determined in carbon tetrachloride is obtained. This result is in quantitative agreement with the 13% difference that is obtained from the comparison of the equilibrium constant measurements made for pyridine-Noxide–alcohol complexation in cyclohexane and in carbon tetrachloride [24b]. The knowledge of bH 2 for N and NX allows us to estimate the hydrogen bond basicities of electronically excited N and NX by means of equations (3.21a) and (3.21b) respectively. The plots of the singlet energy difference of the complexed and uncomplexed species against the Gibbs energy change, in the DMPN–nhexane system, yield straight lines (see Figure 3.11) with slopes and intercepts summarized in Table 3.5. H H 1 According to equations (3.21a) and (3.21b), the ratios of ½bH and 2 ð NÞb2 ðNÞ=b2 ðNÞ H 1 H H ½b2 ð NXÞb2 ðNXÞ=b2 ðNXÞ, respectively, are directly obtained from the slopes and can be derived, with a known value of C1, from the intercepts. The ratios originating from the intercepts and the slopes agree within the limits of experimental error, however, in the forthcoming discussion we use the more accurate H slope values. With these ratios, and the above-derived ground-state bH 2 values for N and NX (i.e. b2 ðNÞ ¼ 0.47 H and b2 ðNXÞ ¼ 0.29), the excited-state hydrogen bond basicities given in Table 3.5 are obtained for the system. For the studied system, the hydrogen bond basicities of the excited states are equal or close to each other in the two solvents, as expected. Moreover, a higher value is obtained for the excited species compared with the ground-state ones. As can be seen in Figure 3.11, although the increase in temperature greatly influences the Gibbs energy change of the complexation process, it has no significant effect on the singlet energy variation. This phenomenon is in accordance with expectation: the entropy change in the complexation process is expected to be more or less independent of the chemical nature of the reactants. Table 3.5 Parameters of equation (3.21) and derived hydrogenbond basicity values. Reprinted with permission from [8]. Copyright 2004 American Chemical Society Solvent

Carbon tetrachloride

bH 2 ðNÞ bH 2 ðNÞ

0.47  0.02 0.29  0.02 0.40  0.16 0.9  0.3 0.64  0.08 0.55  0.35 0.76  0.27 0.46  0.12

Slope (equation (3.21a)) Intercept (equation (3.21a)) 1 bH 2 ð NÞ Slope (equation (3.21b)) Intercept (equation (3.21b)) 1 bH 2 ð NXÞ a

Assumed to be the same as in carbon tetrachloride.

n-hexane (0.47)a (0.29)a 0.362  0.007 0.67  0.02 0.64  0.03 0.52  0.02 0.86  0.03 0.45  0.03

Hydrogen Bond Basicity in the Excited State: Concept and Applications

61

-1 3

log(K1 / mol dm )

2

-1

1 0

E / kcal mol

-1

-2

-1 0.2

0.4

0.6

0.8

1.0

H

1

2

-3

-4 -3.0

-2.5

-2.0 o

-1.5

-1.0

-0.5

-1

G 1 / kcal mol

Figure 3.14 Singlet excitation energy difference of complexed and uncomplexed species (D1E) as a function of the Gibbs energy change of complex formation in the I–alcohol–n-hexane system at 25  C (see equation (3.21a)). The sequence of alcohols from right to left is: MET, DFE, TFE, HFIP and PFTB. Inset: plot (in accordance with equation (3.17)) of the logarithm of the equilibrium constant for complexation as a function of hydrogen bond acidity in n-hexane. Reprinted with permission from [6]. Copyright 2005 American Chemical Society

3.3.2.2 Hydrogen Bond Basicity of the Ground and Singlet Excited State of Isoindolo[2,1-a]indole-6-one (I) The equilibrium constant (K1 ¼ 19  3 mol1 dm3) measured for the I–HFIP system in carbon tetrachloride and the aH 2 (HFIP) ¼ 0.77 hydrogen bond acidity, from the database of Abraham [1], is used to derive from equation (3.17) the hydrogen bond basicity bH 2 ðIÞ ¼ 0.42. In the inset of Figure 3.14, the log K1 values, measured in n-hexane, are plotted against aH 2 . The intercept of the straight line is 1.02  0.10 which agrees within the limits of experimental error with the 1.094 value reported by Abraham [1] from numerous measurements made in carbon tetrachloride. The slope of the plot is 3.40  0.17. In the previous chapter it was shown that the slope parameter of equation (3.17) is 14% higher for n-hexane than for carbon tetrachloride, i.e. 8.384 instead of 7.354. With this higher parameter and the experimentally determined slope of 3.40, the hydrogen bond basicity bH 2 ðIÞ ¼ 0.41  0.02 is obtained. This agrees closely with the hydrogen bond basicity of I derived in carbon tetrachloride. A plot of D1E versus D1 Go1 ¼ RT ln K1 for the I–fluorinated alcohol–n-hexane system is shown in Figure 3.14 to give a reasonably good straight line. With the known ground-state bH 2 ðIÞ value, the hydrogen bond basicity of the 1I can be obtained using either the slope or the intercept of the plot. The bH 2 values derived 1 from the slope and intercept are in good agreement and yield an average of bH ð IÞ ¼ 0.81  0.04. This is almost 2 double the hydrogen bond basicity of the ground-state species and shows that the amide group in 1I is an 1 excellent hydrogen bond acceptor. The bH 2 ð IÞ obtained for a heterocyclic amide can be compared with the H 1 b2 ð DMPNÞ ¼ 0.64 determined previously. The knowledge of the basicity of I in the excited state allows us to calculate the equilibrium constant of complexation of 1 I with various alcohols (by means of equation (3.17)), and to determine the DGo6 Gibbs energy change for the complexation processes (which are given in the fourth column of Table 3.6).

0.88 0.77 0.57 0.53 0.43 0.37 0.33

PFTB HFIP TFE DFE MET ETOH IPA

11.2 9.0 5.1 4.3 2.3 1.1 0.31

DGo1 (kJ mol1) 27.6 23.5 15.9 13.8 10.5 8.0 6.7

DGo6 b (kJ mol1)

b

The given uncertainties are the statistical errors at the 2s level. 1 Derived by means of equation (3.18) using bH 2 ð IÞ.

a

aH 2

Alcohol

k23a (108 s1) 2.3  0.8 1.7  0.8 2.0  0.8 3.8  0.5 7.9  0.8 9.7  1.0 22  3

k6a (mol1 dm3 s1) (1.33  0.02)  1010 (1.24  0.10)  1010 (1.16  0.07)  1010 (0.54  0.04)  1010 (0.18  0.01)  1010 (0.18  0.01)  1010 (0.095  0.006)  1010

1.1  0.2 1.2  0.2 1.5  0.3 0.54  0.04 0.17  0.03 0.16  0.05

k24a/1010 (mol1 dm3 s1)

41 22 6.1 4.6 2.3 1.5 1.1

k1/108 (mol1 dm3 s1)

4.4 5.8 7.8 8.2 9.0 9.5 9.9

k1 (107 s1)

Table 3.6 Thermodynamic, spectroscopic and kinetic data for the I–alcohol–n-hexane system at room temperature. Reprinted with permission from [6]. Copyright 2005 American Chemical Society

62 Hydrogen Bonding and Transfer in the Excited State

Hydrogen Bond Basicity in the Excited State: Concept and Applications 8

63

-1

log(K1 / dm mol )

3

-2.5

1

-2.0

6

E / kcal mol

-1

-1.5

4

-3.0 0

2

4 3

6 -1

8 -1

RT ln(K1 / dm mol ) / kcal mol

2

0 0.3

0.4

0.5

0.6

0.7

0.8

0.9

H 2

Figure 3.15 Logarithm of the equilibrium constant for complexation of DMAP as a function of hydrogen bond acidity of the alcohol in n-hexane. Inset: singlet excitation energy difference of complexed and uncomplexed species (D1E), derived from the absorption spectra, as a function of the Gibbs energy change of complex formation (DG ¼ RT lnK1) in the DMAP–alcohol–n-hexane system at 25  C. The sequence of alcohols from left to right is: IPA, EtOH, MeOH, TFE, PFP, DFH, HFIP and PFTB. Reprinted with permission from [17]. Copyright 2007 American Chemical Society

3.3.2.3 A Special Case: Hydrogen Bond Basicity of the Ground and Singlet Excited State of DMAP In Figure 3.15, the log K1 value measured for DMAP–alcohol systems in n-hexane is plotted against the aH 2 parameter of the alcohols. The intercept of the straight line is 2.2  0.2, which is significantly lower than the 1.094 value reported by Abraham [1] from numerous measurements made in carbon tetrachloride. (Similarly, there are other known exceptions, mainly with pyridine bases [1].) The experimentally determined slope of the plot is 8.53  0.32, and the hydrogen bond basicity derived from equation (3.18) is bH 2 ðDMAPÞ ¼ 1.02  0.04, using the C2 ¼ 8.384 multiplier factor (see before). The bH 2 ðDMAPÞ value is one of the highest hydrogen bond basicity values ever published. (See some comparable values among phosphoric acid esters [1].) This means that DMAP is an exceptionally good hydrogen bond acceptor. In the inset of Figure 3.15, the differences in the absorption maxima (D1E) of the singly complexed and uncomplexed DMAP species are plotted as a function of RT lnK1 at room temperature in n-hexane. The data indicate that a good linear correlation exists between D1E and the Gibbs energy change in the complex formation reaction, just as found in the case of the above-cited system. From equation (3.21a) we can 1 calculate the excited-state hydrogen bond basicity of the species in question (bH 2 ð DMAPÞ) if the H 1 corresponding ground-state value (b2 ðDMAPÞ ¼ 1.02  0.04) is known. Using this, bH 2 ð DMAPÞ ¼ 1.16  0.05 can be derived from the slope (0.14  0.01) of the inset of Figure 3.15. This value is considerable 1 bigger than the otherwise large ground-state parameter. (The bH 2 ð DMAPÞ is derived from the absorption data; consequently, it is an ICT excited-state property corresponding to the ground-state equilibrium configuration.)

64

Hydrogen Bonding and Transfer in the Excited State

3.3.3 Reaction rate of hydrogen-bonded complex formation in the excited state Some advantageous properties of the model compound isoindolo[2,1-a]indole-6-one (I), such as a relatively strong tendency towards complexation, which increases considerably with excitation, high fluorescence yield, long lifetime and simple complexation mechanism, made it a proper candidate for investigation of the kinetics of hydrogen bond formation. 3.3.3.1 Determination of the Rate Coefficients of Excited-State Reactions of 1I and 1IX Time-resolved fluorescence decay measurements were made in order to study the mechanism and kinetics of singlet excited IX formation and decay. Excitation was at 404 nm, and 520  20 nm was selected as the monitoring wavelength for the 1IX species. (At this wavelength, 1IX is the major contributor to the emission.) Without added alcohol, single-exponential decay was found, while in the presence of alcohol two-exponential kinetics was observed: if ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ

ð3:22Þ

1 In this equation, the pre-exponential factors and decay parameters, A2, t1 2 and A1, t1 refer to the short-lived and long-lived fluorescence decay components respectively. In Figure 3.16, the fluorescence decay curves of 5.3  105 mol dm3 I observed in the absence and presence of various amounts of DFE in n-hexane are 1 presented. Both the t1 1 and the t2 decay parameters depend on the alcohol concentration for all alcohols investigated. In the case of DFE additive, the pre-exponential factor A2 is negative, and the A2/A1 ratio is found to decrease from 0.31 to 0.69, with the alcohol concentration increasing from 0.003 to 0.047 mol dm3. The negative value for A2 indicates that the emitting 1IX is mainly formed via reaction (3.6) by the complexation of 1I. Moreover, an A2/A1 ratio greater (more positive) than 1 may arise from two effects: (i) additional direct formation of 1IX by light absorption of the ground-state complex IX via reaction (3.5), and/or (ii) the contribution of 1I to the fluorescence at the monitoring wavelength. Similar observations regarding the A2/A1 ratio were also made in experiments with other alcohols such as 2-propanol, ethanol and methanol. However, in the case of the best hydrogen-bond-donating alcohols (TFE, HFIP and PFTB), positive A2/A1 ratios were found, which indicates that in these cases a dominant part of the emitting singlet excited IX is formed via reaction (3.5) from the ground-state complex by light absorption. The Gibbs energy change in the complexation of 1I is between 6.7 and 27.6 kJ mol1 (DH6o is approximately between 21.8 and 42.7 kJ mol1). Therefore, the complexation process is expected to be practically irreversible. As a test of this conclusion, an experiment was made with HFIP in which the monitoring wavelength of 425 nm was used where only the uncomplexed species emits. Using this monitoring wavelength, a single-exponential decay was observed with a lifetime closely agreeing with the long-lived component of the double-exponential fluorescence decay. (In case of two-exponential fluorescence decays, it is the shorter component that always has a negative pre-exponential parameter – if one of them has one at all – independently of the kinetic sequence.) Assuming irreversible complexation processes and taking into account the dependence of the decay parameters on alcohol concentration (see Figure 3.16), it follows that the longer lifetime (t1) can be associated with the complexation of 1I (reaction (3.6) in the forward direction), while the shorter lifetime (t2) is related to the reactions of 1IX via processes (3.23) and (3.24):

IX ! products

ð3:23Þ

IX þ X ! 1 IX2 and=or IX þ X

ð3:24Þ

1

1

Hydrogen Bond Basicity in the Excited State: Concept and Applications 5000 0

4000

/ ns

23.23

Ai

3000

65

a

2

6229 1.057

2000 1000 0 0

20

40

60

80

100

120

10000 8000

/ ns

i

12.48 2.48

Ai

6000

15796 -4890

b

2

1.179

counts

4000 2000 0 0

20

40

60

80

100

120

4000

c

/ ns i

3000 2000

8.65

2.23

2

Ai

7170 -3335 1.135

60

80

1000 0 0

20

40

100

120

12000 10000 i

8000

/ ns

Ai

6000

4.45

1.81

d

2

33441 -20749 1.131

4000 2000 0 0

20

40

60

80

100

120

time / ns

Figure 3.16 Fluorescence response functions of 5.3  105 mol dm3 I (a) and I with 0.0032 mol dm3 (b), 0.0126 mol dm3 (c) and 0.0316 mol dm3 (d) DFE in n-hexane at 25  C. Reprinted with permission from [6]. Copyright 2005 American Chemical Society

Reaction (3.24) is either a complexation reaction or a quenching process. A comparison of the fluorescence spectra of 1IX determined at various alcohol concentrations showed no definite sign of the formation of the doubly complexed singlet species (1IX2). The dependence of the decay parameters on the DFE concentration is presented in Figure 3.17. Good straight lines are obtained with DFE and with other alcohols too. Thus t1 1 ¼ k0 þ k6 ½X

ð3:25Þ

66

Hydrogen Bonding and Transfer in the Excited State

8

5x10

8

4x10

-1

/s

-1

8

3x10

8

2x10

8

1x10

0 0.00

0.01

0.02

0.03

0.04

0.05

-3

[DFE] / mol dm

1 Figure 3.17 The dependence of t 1 1 (*) and t 2 (*) decay parameters on the DFE concentration. Reprinted with permission from [6]. Copyright 2005 American Chemical Society

and t1 2 ¼ k23 þ k24 ½X

ð3:26Þ

7 1 The rate parameter k0 ¼ t1 0 is 4.31  10 s ; moreover, the rate coefficients obtained for k6, k23 and k24 are summarized in Table 3.6. When considering the reliability of the kinetic parameters given in Table 3.6, we have to take into account the way in which these parameters are derived. At the wavelengths of the measurements (i.e. at 520 nm or above), the emission spectra of 1I and 1IX overlap. The more reliable rate coefficient is 1 k6 because it is derived from the dominant decay parameter t1 1 , while k23 and k24 are obtained from t2 , which 1 corresponds to the weaker IX emission (see equations (3.25) and (3.26)). These uncertainties are reflected by the statistical errors of rate coefficients given in the table. In the above derivation of the rate coefficients, the assumption of irreversible complex formation is a critical question. Therefore, validation of this assumption is required. If complex formation is reversible, we have a two-state system with kinetics similar to that of reversible exciplex formation. Using the known equations for such systems [28], the rate coefficients k6, k8 and (k23 þ k24 [X]) can be derived from t0, t1, t2 and A2/A1 (determined at the wavelength where only 1I emits). Such considerations have been made using experimental data for the I–IPA–n-hexane system where DGo6 is the least negative. As the A2/A1 ratio cannot be determined with sufficient accuracy for this system, we followed a different route. An iterative process has been used to find the A2/A1 ratio, which yields (by the use of the appropriate equations) the k6 and k6 rate coefficients, which are in accordance with the K6 ¼ k6/k6 equilibrium constant (derived from DGo6 ). Using this A2/A1 ratio and the experimentally determined decay parameters, the rate coefficient of k6 ¼ (9.5  0.3)  108 mol1 dm3 s1 is obtained. This is in excellent agreement with the rate coefficient k6 ¼ (9.4  0.3)  108 mol1 dm3 s1 derived by assuming irreversible complexation.

3.3.3.2 Kinetics of Complexation Reactions Analysis of the kinetic data presented in Table 3.6 shows that the rate coefficients of the two complexation processes (3.6) and (3.24) are very similar, and the rate parameters decrease with decreasing

Hydrogen Bond Basicity in the Excited State: Concept and Applications

67

-1

3

-1

ln(k6 / mol dm s )

23

22

21

20 -30

-25

-20

-15 o

-10

-5

-1

G6 / kJ mol

Figure 3.18 Plot of ln k6 as a function of the Gibbs energy change in the complexation reaction of 1I (i.e. DGo6 ). The full line is calculated using equation (3.29) (see text). Reprinted with permission from [6]. Copyright 2005 American Chemical Society

hydrogen-bond-donating character of the alcohol (for which aH 2 is a measure). With alcohols of the highest hydrogen-bonding ability (i.e. TFE, HFIP and PFTB), a diffusion-controlled rate was observed. This means that in n-hexane the activation-controlled process changes to a diffusion-controlled process at around DG ¼ 17 kJ mol1. (Diffusion-controlled hydrogen-bonded complex formation in the excited state was also observed by Inoue and coworkers [29] for aminoanthraquinone–ethanol systems in benzene, and by Mataga et al. [30] for the 1-aminopyrene/1-pyrenol – pyridine/methyl-substituted pyridine systems in nhexane.) In the case of reaction (3.6), the dependence of the rate of complexation on the hydrogen-bonding ability of the alcohol was studied in detail. In Figure 3.18, the logarithm of k6 is plotted against DGo6 , the Gibbs energy change in the complexation process of excited I. The covered DGo6 range includes results showing diffusioncontrolled kinetics as well as data obtained in the transition region between diffusion-controlled and activation-controlled kinetics. In order to account for the observed dependence of k6 on DGo6 , we express the rate coefficient of complexation of 1I with alcohols by means of the well-known expression [31] k6 ¼

kdiff kact kdiff þ kact

ð3:27Þ

where kdiff and kact are the rate coefficients for the diffusion-controlled and the activation-controlled processes respectively. As a first approximation, the activation Gibbs energy of the activation-controlled process (DG#6 ) may be estimated by using a linear free energy relationship: DG6# ¼ g0 þ d0 DGo6

ð3:28Þ

where g0 and d0 are constants. With this approximation k6 ¼

kdiff f1 þ kdiff =½g expðdDGo6 Þg

ð3:29Þ

68

Hydrogen Bonding and Transfer in the Excited State

where g ¼ expðg 0 =RTÞ and d ¼ d0 =RT. The experimental data given in Figure 3.18 were fitted with equation (3.29). (In the fitting, the experimental point for complexation with methanol was omitted, as it is not in agreement with the rest of the data for the homologous series. Anomalous behaviour has also been observed [8, 13] in another complexation reaction of methanol in n-hexane.) The optimized parameters used to calculate the full line are kdiff ¼ 1.32  1010 mol1 dm3 s1, g ¼ 1.01  108 mol1 dm3 s1 and d ¼ 0.363 kJ1 mol. The optimized kdiff is somewhat less than the value calculated from the Einstein–Stokes equation (kdiff ¼ 2  1010 mol1 dm3 s1) [32]; however, it is in excellent agreement with experimental determinations [30a, 32]. Equation (3.29) with the optimized parameters derived for the complexation of the excited 1I species can be used [33] to estimate the rate coefficient of the complexation of the ground-state I with alcohols (k1), as the appropriate DGo1 values are available from the measured K1 equilibrium constants. The estimation of such ground-state complexation rate coefficients is particularly important, as no good method is available for their experimental determination. As an example, we calculate from equation (3.30), with the known value of DGo1 ¼ 8.8 kJ mol1, k1 ¼ 2.2  109 mol1 dm3 s1, the rate coefficient of the complexation of I with HFIP in n-hexane. Moreover, with K1 ¼ 35 mol1 dm3, k1 ¼ 5.8  107 s1 is obtained. In comparison with the diffusion-controlled complexation rate of 1I with HFIP (i.e. k6 ¼ 1.24  1010 mol1 dm3 s1), the corresponding ground-state value (k1) is lower by a factor of about 6. The rate parameters are derived for the other alcohols seen in Table 3.6. 3.3.3.3 Photophysical Process of Excited Complexed Species of DMPN Singlet lifetimes were measured for N, NX and NX2 in the temperature range 230–340 K. In these experiments, the excitation wavelength was chosen at the maximum of the corresponding (0, 0) absorption band. The room temperature results are presented in Table 3.2. Using the singlet lifetime (tf), the fluorescence quantum yield (Ff), the intersystem crossing quantum yield (FISC) and the internal conversion yield (FIC ¼ 1  Ff  FISC), the rate coefficients of singlet-state-depopulating photophysical processes are obtained. The room-temperature rate coefficients are given in Table 3.2, and the Arrhenius plots of the photophysical processes of the singlet excited species 1N, 1NX and 1NX2 are shown in Figure 3.19. Although the rate coefficients for the reactions of 1NX show significant uncertainties (owing to the fact that samples used for ‘NX’ measurements always contain N and NX2 ‘impurities’), however, it is clear from the available data that the characteristic tendencies of the temperature dependence of the rate coefficients of 1NX are of intermediate nature between those observed for 1N and 1NX2. At room temperature, as well as at higher temperature, the excited-state lifetime increases considerably with complexation. This increase determines the trend seen in the fluorescence quantum yields, although the fluorescence rate constants show only moderate increase with complexation. At room temperature, the increase in the radiative rate coefficients is in accordance with the observed increase in the oscillator strengths of the lowest-lying absorption band. As expected, kf is practically temperature independent. The temperature dependences of the rate coefficients of uncomplexed 1N and complexed 1NX2 show completely different characteristics. Non-radiative rate coefficients for 1N are temperature dependent, while those of 1NX2 are not. Intersystem crossing from 1N to a higher-lying triplet state may be responsible for the observed temperature dependence of kISC (with A ¼ (4.7  0.3)  109 s1 and Ea ¼ 4.15  0.13 kJ mol1), in agreement with the observations made in the case of N-methyl-2,3-naphthalimide [34]. For NX2, where the singlet excitation energy is lower by 13 kJ mol1 (and with an increase in the corresponding np tripletstate energy [15]), temperature-enhanced singlet ! triplet transition is not possible energetically, and therefore only a slower temperature-independent intersystem crossing process occurs. The temperature dependence of the internal conversion rate coefficient shows a complex character: for 1N, a temperature-independent process dominates at low temperature (k0 ¼ (2.8  0.2)  108 s1) and a significant

Hydrogen Bond Basicity in the Excited State: Concept and Applications 1

1

1

NX

N

22

69

NX2

kISC

ln(k / s )

20 -1

kIC kISC kf

kf

18

kISC

kf

kIC

kIC

16 3.0

3.5

4.0

4.5

3.0

3.5

3.0

3.5

-1

1000 (T/K)

Figure 3.19 Arrhenius plot of the rate coefficients of photophysical processes for uncomplexed DMPN (1N) and singly complexed (1NX, tentative data) and doubly complexed (1NX2) species in n-hexane. The hydrogen bond donor is HFIP. Adapted with permission from [8]. Copyright 2004 American Chemical Society

temperature-enhanced contribution is observed at high temperature (with A ¼ (2.8  0.9)  1013 s1 and Ea ¼ 26  1 kJ mol1). This fast, temperature-enhanced internal conversion process causes the short singlet lifetime of singlet excited DMPN, which is characteristic also for the local excited singlet state of Nphenyl-2,3-naphthalimide and its derivatives [15]. The efficient internal conversion of the N-aryl-naphthalimides has been explained [15] by the crossing of the S1(1A2, LE) and S2(1B1, ICT) excited-state potential energy surfaces. Complexation decreases the energy of the S1 surface; however it has less influence on the S2 state; as a result, little or no temperature-enhanced internal conversion occurs from 1NX2. 3.3.4 Solvatochromism of DMAP and its singly complexed derivative: estimation of the dipole moment of singlet excited complexes Solvatochromic measurements were carried out to determine the LE and ICT state dipole moments of DMAP and its singly complexed derivative with HFIP [17]. The energies corresponding to the maxima of the LE and ICT fluorescence bands can be described [35] by equations (3.30) and (3.31) respectively: 1 2 m ðm mFC Þð f f 0 Þ þ constant 4p«0 hcr3 e e g

ð3:30Þ

1 2 m ðm mFC Þð f 1 =2 f 0 Þ þ constant 4p«0 hcr3 e e g

ð3:31Þ

~nmax ¼ f

~n0max ¼ f

where r is the equivalent spherical radius of the solute (Onsager radius) and «0 is the vacuum permittivity of the solvent, me(LE) and me(ICT) are the dipole moments of the LE and ICT states, respectively, and mFC g ðLEÞ and

70

Hydrogen Bonding and Transfer in the Excited State

mFC g ðICTÞ represent the dipole moments of the Franck–Condon (FC) ground states reached upon emission FC from the LE and ICT states respectively. (It is assumed here that mFC g ðLEÞ and mg ðICTÞ are equal to the ground-state dipole moment mg of the relaxed molecules.) The solvent polarity parameters ( f f 0 ) and ( f 1 =2 f 0 ) are defined by equations (3.32) and (3.33): ð f f 0 Þ ¼ ð«1Þ=ð2« þ 1Þðn2 1Þ=ð2n2 þ 1Þ

ð3:32Þ

and ð f 1

 2

f 0 Þ ¼ ð«1Þ=ð2« þ 1Þ1



2 ðn

2

1Þ=ð2n2 þ 1Þ

ð3:33Þ

where « and n are the dielectric constant and the refractive index of the solvent respectively. In Figure 3.20, the energies of the LE band, ~nmax , of DMAP and DMAP–HFIP are plotted against the solvent f polarity function ( f f 0 ), and the energies of the ICT fluorescence maxima (Table 3.4) are plotted against ( f 1 =2 f 0 ) [35]. In order to derive the dipole moments from the slopes of these plots by using equations (3.30) and (3.31), the ground-state dipole moment, mg, and the Onsager radius, r, have to be known. For the mg value of DMAP, the experimentally determined [36] 4.22 D value was used, which is in reasonable agreement with the calculated values: 4.3 D [19], 4.55 D [37] and 4.83 D [20]. For the DMAP–HFIP complex, mg ¼ 11 D was estimated from our DFT calculations [20], and was multiplied by the experimental/calculated ratio of DMAP to obtain 9.6 D for the dipole moment of the ground-state complex. The Onsager radius of DMAP and the DMAP–HFIP complex were derived relative to that of DMABN. The Onsager radius for DMABN was taken from the literature [35] to be 4.20 A, which corresponds to a 17 D dipole moment for the ICT excited

32000

0.00

0.05

0.10

f- f' 0.15 0.20

0.25

0.30

0.35

fluorescence maximum / cm

-1

30000 28000 26000 24000 22000 20000 18000 16000 0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

f- ½ f'

Figure 3.20 Plot of the wavenumber of the emission maxima of LE fluorescence (empty symbol) against the solvent polarity parameter f f 0 and of ICT fluorescence (full symbol) against f 1 =2 f 0 respectively. Data points: DMAP (&, &), DMAP-HFIP (*,*). The half-filled circle symbol indicates the corrected maximum of the ICT fluorescence of the complex (see text). Reprinted with permission from [17]. Copyright 2007 American Chemical Society

Hydrogen Bond Basicity in the Excited State: Concept and Applications

71

state [38, 39]. The ratio of molecular volumes of DMAP and DMABN, estimated by the atomic increment method of Edward [40], is 259.7/310.2 ¼ 0.837. With this ratio and the above-cited Onsager radius of DMABN, r ¼ 3.96 A is obtained for DMAP. Szydlowska and coworkers [41] obtained lower values from DFT (B3LYP/6-311G(d, p)) calculations: their  estimated r parameters were 3.3 and 3.8 A for the planar and perpendicular conformers respectively. According to this calculation, the molecular volumes of the two conformers differ by about 50%; however, we find it more reasonable to use the same Onsager radius for both structures. For the DMAP–HFIP complex,  a r value of 4.8 A is calculated using Edward’s method cited above [40]. The solvatochromic shift of the absorption maximum belonging to the ICT transition can be analysed by the equation [35] ~nmax abs ¼

1 2 m ðmFC mg Þð f f 0 Þ þ constant 2p«0 hcr3 g e

ð3:34Þ

From the solvatochromic shifts of the absorption and fluorescence spectra of DMAP, summarized in Table 3.7 and presented in Figure 3.20, dipole moments of 8.9 and 9.2 D, respectively, are derived for the ICT excited state. The smaller value corresponds to the ICT state dipole moment at the geometry of the ground state (i.e. at the planar structure). This value is in excellent agreement with the 9.8 D [19] and 8.6 D [37] results derived from semi-empirical calculations. The experimental observation that the excited-state dipole moment derived from the solvatochromism of the absorption and emission spectra are practically the same, together with the moderate magnitude of these data, further supports the hypothesis that the ICT state of the DMAP most probably has a PICT [42] structure. The dipole moment of the relaxed ICT excited state was also determined by Herbich and coworkers [41] from the solvatochromic measurements of ICT fluorescence. Using only data measured in the most polar solvents, they found a greater slope. When plotting their measurements according to equation (3.31), and applying a smaller Onsager radius than in the present work, a higher me value was derived. Our measurements show that the dipole moment of the ICT excited state changes only to a small extent owing to relaxation (see Table 3.7). Using the same procedure to derive the dipole moments, different results are found with DMABN, where the dipole moment of the ICTexcited state increases from 12 to 17 D as a result

Table 3.7 Ground-state dipole moment and Onsager radius, along with LE and ICT state properties related to solvatochromic measurements. Reprinted with permission from [17]. Copyright 2007 American Chemical Society DMABN mg (D) r (A) Slope (equation (30)) (cm1) Slope (equation (34)) (cm1) Slope (equation (31)) (cm1) me(LE) (D) mFC e ðICTÞ (D) me(ICT) (D) a

a

6.6 4.20a 5700  530a 4800  400 24 000  2000a 10.6a 12 17a

Reference [35]. Reference [36]. Parameters derived in apolar solvents. d Parameters derived in more polar solvents than DEE. b c

DMAP b

4.22 3.96 5380  400 3180  190 7360  700 8.2  1.5 8.9  1.0 9.2  1.0

DMAP-HFIP 9.6 4.8 4670  420 2970  190 14 700  530c 36 600  1800d 13.5 13 18.5c 25.5d

72

Hydrogen Bonding and Transfer in the Excited State

of relaxation. This is in excellent agreement with the DFT/MRCI calculations of Parussel [39], where the value changes from 13.2 to 17.3 D. This observation may indicate that the structural relaxation plays a less important role in the formation of the ICT state in DMAP than in DMABN. The solvatochromic shift of the ICT absorption and the emission maxima of the DMAP–HFIP complex is definitively greater than that of the uncomplexed DMAP. However, for a quantitative treatment, we have to take into account the singlet energy change of complex formation (D1E) with the polarity of the solvent. The quantity D1E can be calculated from equation (3.21a) using the previously determined equilibrium constants (Table 3.4). Calculating by equation (3.34), from the absorption data a 13 D dipole moment is derived for the unrelaxed ICT state. From the fluorescence measurements made in apolar solvents (i.e. from n-hexane to diethyl ether), we may derive an 18.5 D dipole moment for the relaxed ICT excited state of the DMAP–HFIP complex, while the fluorescence maxima determined in more polar solvents than the ethers (see Figure 3.20) imply a much greater value: me(ICT) ¼ 25.5 D. All the dipole moments of the HFIP-complexed species are definitively larger than those of the uncomplexed DMAP (i.e. mg, me(LE), meFC(ICT) and me(ICT)). Hydrogen bonding on the heterocyclic nitrogen of DMAP markedly increases the dipole moment compared with that of the uncomplexed molecule both in the ground and the excited states (Table 3.7). The value obtained in polar solvents is so high that it needs explanation. The different slopes of the Lippert–Mataga plot for less polar and more polar solvents than diethyl ether indicate a remarkable change. We consider two possible explanations: (i) if an upper excited state exists with good hydrogen-bonding ability and a very large dipole moment, this may release enough energy by relaxation in polar solvents to become the lowest energy excited state (lower than the ICT state) (unfortunately, neither absorption spectroscopic nor theoretical evidence is so far available for the existence of such a state); (ii) the conformational change in the ICT excited state from a planar geometry in apolar solvent (PICT, [42]) to a perpendicular structure in polar solvent (TICT, [43]) could induce a greater dipole moment and as a consequence an increased red-shift in the fluorescence spectrum (note that the dipole moment of the TICT state may be greater than that of the PICT state in the case of calculated data of DMABN [39]). There are indications that the later explanation is more probable. Fub and coworkers [44] in their recent femto-second study of DMABN discussed in detail the occurrence of the double minimum on the potential energy surface of the 1La(ICT) state. Moreover, semi-empirical [19] as well as DFT calculations [37] for DMAP suggest that the ICT excited state has a smaller minimum at the planar geometry (see Figure 4 of Ref. [19] and Figure 20 of Ref. [37]). The possible change in conformation observed in the case of the DMAP–HFIP complex may occur for other systems (such as DMABN-related compounds, for example) when energetically favorable conditions exist. It may be expected that apolar solvents (e.g. paraffin) or a crystalline environment [45] would favour the planar geometry, while polar solvents would promote the formation of a more perpendicular structure [43]. In the emission spectrum of DMAP in n-hexane, the ICT fluorescence is very weak; the F0f ðICTÞ=Ff ðLEÞ ratio is around 0.05. Similarly to DMABN [42], the kf0 ðICTÞ value is lower than the kf(LE) by a factor of 8. This ratio is estimated from the fluorescence quantum yield and lifetime data measured in n-hexane and butyronitrile (see Chapter 1), where the dominant emission comes from the LE and the ICT state respectively. 1 Consequently, the LE !  ICT reaction is endothermic by 2.1 kJ mol if the pre-exponential factors are equal or very similar (which is a reasonable assumption). The same endothermicity is obtained from the Steven–Ban plot, derived from measurements in DEE (see Figure 2 in Ref. [17]), when the different energies in the two solvents are taken into account (see Figure 3.20). As estimated from the location of the crossing point of the absorption and emission spectra (Figures 1, 3 and 6(a) in Ref. [17]), it can be deduced that the singlet LE energy decreases from 404.1 to 402.8 kJ mol1 owing to HFIP complexation. From the difference in the absorption spectra it is derived that the ICT state energy decreases by 10.5 kJ mol1. Thus, we establish that the LE !  ICT reaction for the complex is exothermic by 7.1 kJ mol1, compared with 2.1 kJ mol1 endothermicity for the uncomplexed species.

Hydrogen Bond Basicity in the Excited State: Concept and Applications

73

3.3.5 Triplet-btate properties of the complexed species Determination of the triplet formation yield of the complex species does not involve a characteristically different procedure from that applied for the uncomplexed species. Using the energy transfer method, tripletstate complexation (and dissociation) processes do not influence the final data significantly: irrespective of the origin of the energy (uncomplexed or complexed triplets), the same triplet excited quencher molecule is produced and detected in the measurements. Of course, the ratio of the light absorption by the different species and the influence of the singlet-state processes must be optimized and considered as well. On the other hand, the lifetime of the triplet states is long enough to establish a new equilibrium and makes it possible to investigate the hydrogen-bonding properties of the triplet excited molecules on proper grounds. 3.3.5.1 Determination of the Triplet Formation Yield of Isoindolo[2,1-a]indole-6-one and Its HFIP-Bonded Complex; Photophysical Consequences Measured in n-hexane against quinine sulfate, the fluorescence quantum yield of I is Ff ¼ 0.51  0.04, which is significantly lower than the value (Ff ¼ 0.9) reported by Disanayaka and Weedon [46] from measurements made in n-hexane against indole (for which the quantum yield is not given in the referred literature). A relatively low triplet yield of FISC ¼ 0.080  0.01 was determined, and consequently FIC ¼ 0.41  0.05 was obtained. The lifetime of the singlet state is found to be long, t0 ¼ 23.2  0.2 ns. The first-order decay rate coefficient of 1IX (i.e. k23), given in the eighth column of Table 3.6, seems to be significantly higher than that of 1I (i.e. k0 ¼ 4.31  107 s1). The rate coefficient k23 is a composite quantity – the sum of the rate coefficients for fluorescence emission, singlet–triplet transition (ISC) and internal conversion (IC) from the singlet excited state of IX: 1

IX ! IX þ hn

ð3:23aÞ

IX ! 3 IX

ð3:23bÞ

IX ! IX

ð3:23cÞ

1

1

Experimental fluorescence quantum yield and triplet yield values are available for the most studied I–HFIP–n-hexane system: F0f ðIXÞ ¼ 0:028  0:005, F0ISC ðIXÞ ¼ 0:008  0:004 at [HFIP] ¼ 0.01 mol dm3. It is to be noted that the experimentally measured fluorescence and intersystem crossing yields depend on the alcohol concentration as a result of the 1IX-consuming bimolecular reaction (3.24). These experimentally measured yields are denoted by F0f ðIXÞ and F0ISC ðIXÞ in order to distinguish from the hypothetical yields of Ff(IX) and FISC(IX) corresponding to zero alcohol concentration. The latter values are obtained from the experimentally determined fluorescence and intersystem crossing yields, respectively, by multiplying these 0 by the t02 =t2 ratio, where t2 and t02 ¼ k23 are the decay parameters determined at the given alcohol concentration and at zero alcohol concentration respectively. Next, with Ff(IX) and FISC(IX), the quantum yield of internal conversion is calculated: FIC(IX) ¼ 0.94  0.02. Finally, from the quantum yields and the t02 value, the rate coefficients for the first-order reactions depopulating the singlet excited IX state are obtained: k23a ¼ 8.5  106 s1, k23b ¼ 2.4  106 s1 and k23c ¼ 1.6  108 s1. A comparison of the kinetic results for the first-order reactions of the excited I and IX species shows that complexation with the alcohol increases by almost one order of magnitude the rate of internal conversion to the ground state, and makes IC the dominant decay process of 1IX. Moreover, hydrogen bonding with the alcohol has no significant effect on the fluorescence and intersystem crossing rates at the experimental conditions studied.

74

Hydrogen Bonding and Transfer in the Excited State

3.3.5.2 Determination of the Triplet Formation Yield of DMAP and Its HFIP Complex: a Special Case The triplet yields (FISC) of the DMAP and its complexed forms were measured by laser flash photolysis using the energy transfer method with anthracene as energy acceptor [5]. The excitation wavelength was 266 nm, from a frequency-quadrupled Nd:YAG laser (Continuum Surelight). In view of the relatively short triplet lifetime of the DMAPs (0.7–5 ms), a series of measurements was carried out with different concentrations of the quencher. The unbiased triplet yields were obtained by extrapolating the experimental data to infinite quencher concentration (see Figure 3.21). The triplet yield is moderately low in n-hexane (FISC ¼ 0.18  0.02), which is an indication of the presence of an effective internal conversion channel. This is often observed when two low-lying, vibrationally coupled excited states exist. In accordance with expectation, the triplet formation yield is considerably higher in acetonitrile (FISC ¼ 0.66  0.05). The lifetime of the triplet excited molecule is short: 0.6 and 5.5 ms in n-hexane and acetonitrile respectively. For N,N-dimethylaniline, similarly short triplet decays were reported [47]. The triplet yield of the complexed species is significantly bigger than the uncomplexed one. For instance it is FISC ¼ 0.76  0.05 for the DMAP-HFIP complex in n-hexane. The triplet lifetime is not changed noticeably with complexation (it is around 0.8 ms). As it was written before, the smaller fluorescence yield of the singly complexed molecule (when 0.05 mol dm3 HFIP is added to butyronitrile) is in good agreement with the appearance of a new 0.5 ns component in the fluorescence decay of the ICT band. Accordingly, the triplet yield decreases dramatically from 0.66 to 0.04  0.03 as a result of complexation with HFIP in acetonitrile. 3.3.5.3 Equilibrium Constants and Triplet–Triplet Absorption Spectra in the 3 DMPN–HFIP–n-Hexane System Triplet-state properties of complexed species (including triplet yields, equilibrium constants of complex formation and triplet spectra) were studied in detail for the DMPN–HFIP–n-hexane system. The triplet yields were measured in n-hexane, relative to that of N-methyl-1,8-naphthalimide, by the energy transfer method using perylene as energy acceptor and 308 nm laser excitation. In a system containing alcohol, the energy 0.7 0.6 0.5

(DMAP) = 0.18 ± 0.02

isc

(apparent)

isc

(DMAP-HFIP) = 0.76 ± 0.05

0.4

isc

0.3 0.2 0.1 0.0 0.8

1.0

1.2

1.4

1.6

1.8

-1

{A(366 nm)}

Figure 3.21 Determination of the triplet formation yield of DMAP and the DMAP–HFIP complex in n-hexane, extrapolating to an infinite quencher concentration

Hydrogen Bond Basicity in the Excited State: Concept and Applications

75

transfer method measures the overall triplet yield (3 Foverall ) of various naphthalimide species, i.e. N, NX and NX2: 3

Foverall ¼ rN 3 FN þ rNX 3 FNX þ rNX2 3 FNX2

ð3:35Þ

where ri stands for the fraction of light absorbed by the ith species in the ground state, i.e. rN ¼ «N ½N/f«N ½N þ «NX ½NX þ «NX2 ½NX2 g, etc. The triplet yield for the uncomplexed N was measured directly using samples prepared without added alcohol, while 3 FNX and 3 FNX2 were obtained by an iterative procedure from the overall yields measured with samples containing the appropriate alcohol concentrations corresponding to optimum NX and NX2 concentrations respectively. The ri parameters were calculated using the known equilibrium constants of complexation as well as the molar absorption coefficients at 308 nm taken from Figure 3.5. The results of triplet yield determination are given in Table 3.2. The determination of the equilibrium constants of complex formation of the triplet excited N and NX with HFIP 3

3

K36

ð3:36Þ

K37

ð3:37Þ

3 N þ X !  NX

3 NX þ X !  NX 2

was carried out in an analogous way to that of the ground-state species. The microsecond timescale of the transient absorption measurements is long enough for the development of equilibrium distribution of the triplet species. On the basis of preliminary experiments, four wavelengths (433, 440, 455 and 465 nm) were selected as characteristic ones for the triplet species 3N, 3NX and 3NX2. Measurements were made with added alcohol varying from 0 to 0.23 mol dm3 concentration. (The overall absorbance of all samples was set to the same value.) The transient absorbance, extrapolated back to zero time, is corrected for the small difference of the triplet yields of the three species in order to obtain constant overall triplet concentration in the series of the experiments. (The corrections were typically 10–15% and never exceeded 25%.) The corrected absorbance is plotted as a function of HFIP concentration in the inset of Figure 3.22. An iterative nonlinear fitting procedure, using Marquardt’s algorithm, is employed to obtain K36 and K37 as well as the molar absorption coefficients of the triplet species. Calculated curves are indicated in the figure. The equilibrium constants of complexation of the triplet species (i.e. K36 and K37), obtained from the optimization procedure, are given in the last row of Table 3.2. The comparison of the equilibrium constants of the excited- and groundstate species shows that the singlet-state values are much higher while the triplet-state values are lower than the equilibrium constants of the ground-state species (i.e. they can be characterized by the values H 3 H 1 bH 2 ð NÞ ¼ 0:64, b2 ð NÞ ¼ 0:43 and b2 ðNÞ ¼ 0:47 respectively). This may be explained by the difference in negative charge density on the oxygen atom of the carbonyl group of the ground state, the excited singlet state and the excited triplet state respectively. Semi-empirical AM-1 computations support this explanation, as they show that the negative charge on the oxygen atom is higher for the singlet and lower for the triplet state compared with the ground state. The triplet–triplet absorption spectrum of DMPN and the overall triplet spectra of samples with added alcohol (at small and relatively high HFIP concentrations respectively) were measured in n-hexane. From the overall spectra and the known K36 and K37 equilibrium constants, the spectra of 3NX and 3NX2 were obtained by an iterative procedure analogous to that used for deriving the ground-state NX and NX2 spectra. The spectra

Hydrogen Bonding and Transfer in the Excited State 0.06

433 nm

20000

455 nm

15000

0.02

465 nm

0.05

10000

0.04

440 nm

-1

3

molar absorbance / mol dm cm

-1

25000

transient absorbance

76

0.10

0.15

0.20

0.00

-3

[HFIP] / mol dm

5000

0 360

380

400

420

440

460

480

wavelength / nm

Figure 3.22 Triplet–triplet absorption spectra of DMPN (full line) and singly complexed (broken, red line) and doubly complexed (dotted, blue line) triplet excited species in n-hexane. Inset: dependence of transient absorbance on HFIP concentration measured in n-hexane at four selected wavelengths. Reprinted with permission from [8]. Copyright 2004 American Chemical Society

are shown in Figure 3.22. Complexation is seen to cause moderate red-shift and a decrease in the vibronic structure. The oscillator strengths (the integrals of the spectra) are the same for the three species, within measurement uncertainty. The comparable oscillator strengths indicate that the character of the triplet–triplet transition does not change with complexation.

3.4 Summary The excited-state hydrogen bond basicity is an easy-to-derive, empirical molecular quantity. Using this information, in apolar, non-protic solvent, it is possible to calculate the equilibrium constant of the excitedstate hydrogen bond complexation (mostly directly related to the Gibbs energy change) with acceptably good accuracy even in cases when the interactions are weak. This is a fundamental physicochemical measure that can be used in every further consideration related to the phenomenon in question. As has been demonstrated, the derivation procedure shows good stability in all cases considered. The value depends only on the chemical structure of the molecule and on the nature of the excited state, and seems to be a useful experimental descriptor of the given excited state. It was shown that, by using these data, a correlation can be found between the kinetics (rate constants) and the thermodynamics of the excited-state process; moreover, we can make a reasonable estimate of the kinetic parameters of the ground-state complexation and dissociation reaction rate. It would be very difficult to determine these parameters by other methods, although they are crucial values for understanding the solvent effect on the condensed-phase kinetics. In another study, hydrogen bond basicity data were used for the correction of solvatochromic shifts of absorption and fluorescence spectral values. These changes, although not huge (except for paraffin solvents), are necessary to obtain an unbiased estimate of the dipole moment change of the complexed molecules as a result of electronic excitation.

Hydrogen Bond Basicity in the Excited State: Concept and Applications

77

Acknowledgements This work was supported by the Hungarian Science Foundation (OTKA T33102, T43601 and T45890). The figures in the paper are reproduced with kind permission of Springer Science and Business Media and of the American Chemical Society.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

M. H. Abraham, Chem. Soc. Rev., 22, 73 (1993). J. N. Demas, and G. A. Crossby, J. Phys. Chem., 75, 991 (1971). A. Demeter, T. Berces and K. A. Zachariasse, J. Phys. Chem. A, 105, 4611, and references therein (2001). A. Demeter and T. Berces, J. Photochem. Photobiol. A: Chem., 46, 27 (1989). T. Yoshihara, S. I. Druzhinin, A. Demeter et al., J. Phys. Chem. A, 109, 1497 (2005); K. Suzuki, A. Demeter, W. K€ uhnle et al., Phys. Chem. Chem. Phys., 2, 981, and references therein (2000). A. Demeter and T. Berces, J. Phys. Chem. A, 109, 2043 (2005). N. Mataga, Bull. Chem. Soc. Jpn, 30, 375 (1957). A. Demeter, L. Ravasz and T. Berces, J. Phys. Chem. A, 108, 4357 (2004). T. Yatsuhashi and H. Inoue, J. Phys. Chem. A, 101, 8166 (1997); T. Yatsuhashi, Y. Nakayima, T. Shimada et al., J. Phys. Chem. A, 102, 8657, and references therein (1998). A. Kivinen, J. Murto and L. Kilpi, Suomen Kemistilehti B, 40, 301 (1967). A. D. Sherry and K. F. Purcell, J. Am. Chem. Soc., 94, 1853 (1972); J. Phys. Chem., 74, 3535 (1970). S. N. Vinogradov and R. H. Linnell, Hydrogen Bonding. Van Nostrand Reinhold Company, New York, NY (1971). A. Demeter, React. Kinet. Catal. Lett., 85, 331 (2005). V. Wintgens, P. Valat, J. Kossanyi et al., J. Photochem. Photobiol. A: Chem., 93, 109 (1996); P. Valat, V. Wintgens, J. Kossanyi et al., Helvetica Chimica Acta, 84, 2813 (2001). A. Demeter, L. Biczo´k, T. Berces et al., J. Phys. Chem., 100, 2001 (1996). A. Sreitwieser, Jr, Molecular Orbital Theory for Organic Chemistry. John Wiley & Sons, Inc., New York, NY (1961). A. Demeter, V. Mile and T. Berces, J. Phys. Chem. A, 111, 8942 (2007). C. Cazeau-Dubroca, G. Nouchi, M. Ben Brahim et al., J. Photochem. Photobiol. A, 80, 125 (1994). J. Herbich and J. Waluk, Chem. Phys., 188, 247 (1994). V. Mile, A. Demeter and G. To´th, Mol. Phys., 107, 1987 (2009). H. S. Frank and W. Y. Wen, Discuss. Faraday Soc., 24, 133 (1957). Th. F€orster, Z. Electrochemie, 54, 42 (1950); Z. R. Grabowski, J. Luminescence, 24/25, 559 (1981). S. G. Schulman, Fluorescence and Phosphorescence Spectroscopy: Physico-chemical Principles and Practice. Pergamon Press, Oxford, UK (1977). (a) M. H. Abraham, P. P. Duce, D. V. Prior et al., J. Chem. Soc. Perkin Trans. II, 1355 (1989); (b) J.-L. M. Abboud, K. Sraidi, M. H. Abraham and R. W. Taft, J. Org. Chem., 55, 2230 (1990). R. S. Drago, and B. B. Wayland, J. Am. Chem. Soc., 87, 3571 (1965); R. W. Taft, D. Gurka, L. Joris et al., J. Am. Chem. Soc., 91, 4801 (1969); R. S. Drago, G. C. Vogel and T. E. Needham, J. Am. Chem. Soc., 93, 6014 (1971). J. A. Platts, Phys. Chem. Chem. Phys., 2, 973 (2000); Phys. Chem. Chem. Phys., 2, 3115 (2000). H. Hagelin, J. S. Murray, T. Brick et al., Can. J. Chem., 73, 483 (1995). Yu. V. Il’ichev, W. K€uhnle and K. A. Zachariasse, J. Phys. Chem. A, 102, 5670 (1998); I. R€ uckert, Photoinduzierter Elektonentransfer und Interne Konversion. Cuvillier Verlag, G€ ottingen, Germany, p. 18. (1998). T. Yatsuhashi and H. Inoue, J. Phys. Chem. A, 101, 8166 (1997); T. Yatsuhashi, Y. Nakayima, T. Shimada et al., J. Phys. Chem. A, 102, 8657, and references therein (1998). (a) H. Miyasaka, A. Tabata, K. Kamada and N. Mataga, J. Am. Chem. Soc., 115, 7335 (1993); (b) H. Miyasaka, A. Tabata, S. Ojima et al., J. Phys. Chem., 97, 8222 (1993). M. J. Pilling and P. W. Seakins, Reaction Kinetics. Oxford University Press, Oxford, UK, Chapter 6, p. 143 (1995). P. W. Atkins, Physical Chemistry, 4th edition. Oxford University Press, Oxford, UK, p. 848 (1990).

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33. This approximation is based on the assumption that I and 1I behave similarly with respect to complexation; however, they differ in Gibbs free energy change of complexation (DGo) and consequently in hydrogen bond basicity (bH 2 ). 34. V. Wintgens, P. Valat, J. Kossanyi et al., J. Chem. Soc. Faraday Trans., 90, 411 (1994). 35. T. Yoshihara, V. A. Galievsky, S. I. Druzhinin et al., Photochem. Photobiol. Sci., 2, 342, and references therein (2003). 36. E. Litonska, Z. Proba, I. Kulakowska and K. L. Wierzchowski, Acta Biochim. Polon., 26, 39 (1979). 37. I. Szydlowska, A. Kyrychenko, A. Gorski et al., Photochem. Photobiol. Sci., 2, 187 (2003). 38. W. Schuddeboom, S. A. Jonker, J. M. Warman et al., J. Phys. Chem., 96, 10 809 (1992). 39. A. B. J. Parussel, Phys. Chem. Chem. Phys., 2, 5545 (2000). 40. J. T. Edward, J. Chem. Edu., 47, 261 (1970). 41. I. Szydlowska, A. Kyrychenko, J. Nowacki and J. Herbich, Phys. Chem. Chem. Phys., 5, 1032 (2003). 42. S. I. Druzhinin, N. P. Ernsting, S. A. Kovalenko et al., J. Phys. Chem A, 110, 2955 (2006); K. A. Zachariasse, Chem. Phys. Lett., 320, 8 (2000). 43. Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rew., 103, 3899, and references therein (2003). 44. W. Fub W. E. Schmid, K. K. Pushpa et al., Phys. Chem. Chem. Phys., 9, 1151 (2007). 45. S. Techert and K. A. Zachariasse, J. Amer. Chem. Soc., 126, 5593 (2004). 46. B. W. Disanayaka and A. C. Weedon, Can. J. Chem., 65, 245 (1987). 47. S. Tobita, R. Kamiyama, K. Takehira et al., Anal. Sci., 17, s50 (2001).

4 Solute–Solvent Hydrogen Bond Formation in the Excited State. Experimental and Theoretical Evidence Iulia Matei, Sorana Ionescu and Mihaela Hillebrand Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest, Bd. Regina Elisabeta 4-12, Bucharest, Romania

4.1 Introduction Hydrogen bonds (H-bonds) are of great importance in biological systems, as they are responsible for the secondary structure of proteins and nucleic acids and are also involved in the mechanisms of enzyme catalysis. Supramolecular chemistry makes use of H-bonds in building up novel compounds of various architectures. The identification of these interactions and the analysis of their influence on the photophysical properties are also important in designing new fluorophores for different applications, the processes involved being rather complex in nature and often only partially understood. The main feature reflecting H-bond interaction in both the ground and excited states is the solvatochromic effect on the absorption and fluorescence spectra. This implies that changes in the solvent nature or composition produce shifts in the position, shape and intensity of the spectral bands. The source of solvatochromic shifts is the different solvation of the ground and excited states: depending on the relative stabilization of these states by solvents, bathochromic (positive)or hypsochromic (negative)shifts of the maximum of the band can be observed experimentally. Thus, such changes are indicative of the interactions occurring between the solute and the solvent in its immediate vicinity. The solute–solvent interactions are classified into: 1. non-specific interactions caused by polarity/polarizability effects; 2. specific interactions, such as H-bonds.

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

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Hydrogen Bonding and Transfer in the Excited State

Hydrogen bonds can occur in ground and/or excited states and can have intramolecular or intermolecular character. The intermolecular H-bonds occur between the solute and the solvent or between the solute and a second partner present in solution. In what follows, we will focus mainly on the H-bond formation between the solute and the solvent. Evidence of excited-state H bonds can be obtained by both steady-state and time-resolved fluorescence spectroscopy. In the case of steady-state fluorescence, the experimental data correlated with H-bond formation concern mainly the solvent effects on the fluorescence band and/or the appearance of dual fluorescence. In time-resolved experiments, the occurrence of excited-state H-bonds is attested by the analysis of fluorescence decays, bi- or multiexponential decays being assigned to the presence of multiple emitting species, solvated species, exciplexes, tautomers, etc. The aim of this chapter is to review the basic mechanisms by which H-bonds influence the photophysical properties of molecules, making use of the large volume of experimental and theoretical data accumulated to date. The chapter is organized as follows: (i) the prerequisite conditions for H-bond formation; (ii) analysis of the diagnosis criteria for identifying the presence of excited-state H-bonds and their quantitative treatment; (iii) the design of a reliable experiment; (iv) theoretical modelling of H-bonds.

4.2 The Prerequisite Conditions for Hydrogen Bond Formation One of the prerequisites for H-bond formation is a well-separated charge distribution in the molecule, allowing some atoms, either H-bond donors or acceptors, to have large enough positive or negative charge densities to interact with the solvent molecules. Although a charge separation and, consequently, a charge distribution pattern suitable for H-bond formation can be found in the molecular ground state (S0), usually the effect is enhanced in the excited states (S1, S2, . . .). Such states are called intramolecular charge transfer (ICT) states, and their presence is characteristic of the molecules containing distinct moieties with donor (D) and acceptor (A) character. In these molecules, the highest occupied molecular orbital (HOMO) is mainly localized on the D fragment, while the lowest unoccupied molecular orbital (LUMO) has a predominant contribution from the A part. The electronic transition leading to the first excited singlet state, S1, practically corresponds to an ICT between these two molecular fragments. The change in the charge density pattern is reflected by an increase in the excited-state dipole moment, me. If the two moieties are joined by a single bond allowing an unhindered intramolecular rotation, the D–A conjugation is reduced, the separation of charges is enhanced and the system can be stabilized in a twisted conformation by solvation, leading to a twisted intramolecular charge transfer (TICT) state [1–3]. A qualitative scheme for the formation of the ICT and TICT excited states is given in Figure 4.1. Several classes of compounds were found as most suitable for analysing solute–solvent interaction in the excited S1 state and are known as probes for studying solvation dynamics (Figure 4.2). As can be seen, the presence of several groups that are either H-bond acceptor or donor potential contributors enables different types of H-bond to be formed. An example of H-bond formation in the presence of alcohols is given in Figure 4.3 for molecules containing pyridinic and/or pyrrolic nitrogens and phenolic and/or carbonyl oxygens. The labelling of the H-bonds in Figure 4.3 is usually done for the bonds occurring in coumarin derivatives [4]. When the solute presents either a p-conjugated system or heteroatoms with lone electron pairs, two types of H-bond can be formed, after the type of interaction, which determines the position of the solvent relative to the plane of the solute. The ‘classical’ type is an in-plane (quasi-)linear bond, in which the solvent interacts with an H-bond donor or H-bond acceptor group of the solute. Another type of H-bond was suggested on grounds of both experimental [5, 6] and theoretical data [7, 8], in which the solvent has an out-of-plane position with respect to the molecular plane.

Solute–Solvent Hydrogen Bond Formation in the Excited State 81

LUMO

LUMO

h HOMO

HOMO

S0

S1 A–

D D

A

ICT

A–

D

TICT

Figure 4.1 Formation and features of the ICT and TICT excited states R

O NH2

R R1

N

O

N

O

O

O O

R2 (a)

(b)

(c)

O NH2 N

R

N N O

O

R (d)

(e)

(f)

Figure 4.2 Most used fluorescence probes for solvation processes and H-bond formation: (a) flexible substituted aminocoumarin derivatives; (b) rigid coumarin derivatives: R ¼ CH3, C102; R ¼ CF3, C153; (c) aminoanthraquinones; (d) aminofluorenones; (d) harmane derivatives; (e) substituted phthalimides R-O-H (Type A)

..NR

2

N H

C=O

H-O-R (Type B)

..N

R-O-H

R-O-H

(Type C)

(Type A)

Figure 4.3 Alcohol involvement in hydrogen bonds. The circle symbolizes a generic (hetero)aromatic system. Notations as in Ref. [4]

82

Hydrogen Bonding and Transfer in the Excited State R out-of-plane attack

O H O

* C

R

H

in-plane attack

O

H O R

Figure 4.4 Two possible modes of attack of the alcohol molecule on a carbonyl-containing compound

In a study of aminoanthraquinone in alcohols, the authors found the presence of two H-bonded species, one fluorescent and the other non-emissive [5]. The presence of the two species was explained by a different geometry of the solvent attack on the solute. It was assumed that the alcohol molecule could attack the carbonylic hydrogen of either anthraquinones or fluorenones in two ways, i.e. an in-plane mode and an out-ofplane mode. In the in-plane mode the hydroxylic hydrogen attacks the lone pair of the carbonylic oxygen, whereas in the out-of-plane mode the interaction is via the antibonding p-orbital of the carbonyl. A qualitative picture is given in Figure 4.4. The out-of-plane species was shown to be non-emissive by transient absorption spectroscopy for a related compound, 2-piperidinoanthraquinone [6].

4.3 Diagnosis Criteria and Quantitative Treatment of Hydrogen Bonds The experimental identification of specific interactions can be performed by steady-state and time-resolved experiments.Additionally,theuseoftransientabsorptionspectroscopycanhelptoidentifytheintermediatespecies appearing prior to or post H-bond formation. The change in the charge distribution upon excitation determines a reorganization of the solvent molecules around the solute, a process called dynamic solvation. An aspect of this general process is the H-bond dynamics, the reorganization of the solute–solvent H bonds, a process that occurs even when an H-bond is pre-existent in the ground state. The process occurs on an ultrafast timescale and can be determined by femtosecond experiments. Detailed analysis of such processes is beyond the scope of this chapter. The main aspects in fluorescence spectroscopy that can be associated with the presence of excited-state specific solute–solvent interactions, i.e. the formation of H-bonds, are summarized in Table 4.1. Table 4.1 Solvent-induced changes in the steady-state and time-resolved fluorescence features that can be associated with the formation of hydrogen bonds Steady state

Time resolved

Shift of the fluorescence maximum Change in the fluorescence band shape Appearance of a new band (dual fluorescence) Change in the fluorescence quantum yield

Bi- or triexponential decays Modified lifetime in protic solvents

Solute–Solvent Hydrogen Bond Formation in the Excited State 83

4.3.1 Solvatochromic analysis Literature data show that, in going from non-polar to either aprotic or protic polar solvents, the fluorescence emission undergoes a bathochromic shift, accompanied with a loss of the vibrational structure. The width of the fluorescence band increases, and a new fluorescence band may be observed. Several models were developed for the analysis of these solvent effects on the electronic spectra of organic molecules. Depending on the type of interactions accounted for, they can be divided into: 1. models that consider the solute–solvent interaction as driven solely by polarity/polarizability; 2. models that also account for specific interactions, allowing a separate evaluation of the polarity effect and H-bonding contribution. Within the framework of the first solvatochromic models, solute–solvent interaction was considered to be due to the solvent acting as a dielectric continuum, the strength of the interaction depending on the dielectric constant («) and refractive index (n) of the solvent and on the dipole moment (m) of the solute. The earliest such models were developed by Kirkwood [9] and Onsager [10]. In 1958, Kosower was the first to use solvatochromism for probing solvent polarity, developing the Z-scale of solvent polarity based on the solvatochromic shift of 4-methoxycarbonyl-1-ethylpyridinium iodide [11]. A large number of general empirical solvent polarity scales have been reported since. The xR and x B [12], p [13], Py [14], S0 [15] and SPP [16] scales are just a few examples. In spite of the rather rudimentary information offered on specific interactions, the great utility of these models resides in the possibility of estimating the variation in the excited-state dipole moment upon excitation, Dm, or in the separate determination of the ground- and excited-state dipole moments, mg and me [17, 18]. Of the numerous experimental ways of determining me, the solvent shift method is the simplest and most widely used. The relationships employed for this purpose are linear correlations of absorption (na) and emission (nf) wavenumbers or Stokes shifts, DnSt, with a solvent polarity function [19, 20]. The expression most used in fluorescence spectroscopy is that given by Lippert and Mataga [21, 22]. This dependence was constructed using Onsager’s reaction field theory, under the assumption that the fluorophore is a point dipole residing in the centre of a spherical solvent cavity with radius a (the Onsager radius), in a homogeneous and isotropic dielectric. Considering equal dipole moments in the Franck–Condon (FC) and relaxed excited states, and assuming that a remains unchanged upon excitation, equations describing the solvatochromic shifts arising from dipole–dipole interactions could be derived: DnSt ¼ 

2 ðm m Þ2 eð«; nÞ þ constant hca3 e g

ð4:1Þ

where h is Planck’s constant, c is the velocity of light and e(«, n) is a solvent polarity function, i.e. the orientation polarization, defined by Lippert as eð«; nÞ ¼ eð«Þeðn2 Þ ¼

«1 n2 1  2 2« þ 1 2n þ 1

ð4:2Þ

where e(«) describes the total polarization and e(n2) is the induction polarization. The e(«, n) values range from 0.001 (cyclohexane, CHX) to 0.320 (water). As was previously discussed, numerous polarity probes undergo ICT upon excitation. In such cases, solvent relaxation generates an emitting state of minimum energy different from the locally excited state; as the emissive state is not attainable by direct excitation, its absorption spectrum is unknown. Therefore, only

84

Hydrogen Bonding and Transfer in the Excited State

a correlation of nf can be performed, and e(«, n) is modified accordingly: eð«; nÞ ¼

«1 n2 1  2 2« þ 1 4n þ 2

ð4:3Þ

Numerous studies of solvent effects have been carried out, leading to a variety of such expressions for the solvent polarity function [23–28]. The Lippert–Mataga equation is no longer applicable when, in addition to non-specific interactions, specific interactions such as H-bonds contribute significantly to the overall solute–solvent interaction. If solvents without H-bonding capabilities are chosen, the predicted linear behaviour of DnSt is observed, and Dm can be determined [29]. If protic solvents are also included in the analysis and they participate in specific interactions with the solute, they will appear as deviations from linearity [30–36] (Figure 4.5). Dimroth and Reichard used the large negative solvatochromism of the absorption band of betaine dyes to elaborate the ET(30) scale [37, 38], defining the ET(30) value of a solvent as the transition energy (in kcal mol1) for the longest wavelength p–p absorption band of a pyridinium N-phenolate betaine dye dissolved in that solvent. ET(30) values range from 30.7 (tetramethylsilane, TMS) to 63.1 (water). The ET(30) parameters for a variety of binary solvent mixtures have also been determined. A compilation of these studies can be found in [39]. In addition to ET(30), for an easier handling, a normalized ETN value has been introduced, considering water and TMS as extreme polar and non-polar reference solvents respectively [40]: ETN ¼

ET ðsolventÞET ðTMSÞ ET ðwaterÞET ðTMSÞ

ð4:4Þ

The ETN scale ranges from 0 (TMS) to 1 (water). Literature data report that the Reichard–Dimroth scale takes into account some extent of specific interactions with the solvent [17, 41, 42]. This arises from the fact that the betaine dye is involved in specific interactions with H-bond donor solvents [39]. Thus, correlations of solvent-dependent properties (positions and intensities of the absorption and emission bands) with the ET(30) parameter often follow two distinct lines, one for aprotic and another for protic solvents [43–46] (Figure 4.5). Therefore, in the Lippert–Mataga and Dimroth–Reichardt plots, obtaining two lines that differ in slope, for protic and aprotic solvents respectively, can indicate the presence of H-bonds. If plotting nf versus e(«, n) yields distinct slopes for aprotic and protic solvents, this suggests the presence of H-bonds in the latter. If a single linear dependence is found in a plot of DnSt versus e(«, n), this indicates that the extent of stabilization due to H-bond formation is similar for both S0 and S1 states [33]. Dimroth-Reichardt model

f,

f,

, kr ,

, kr ,

F

F

Lippert-Mataga model

f ( , n)

ET (30)

Figure 4.5 Schematic representation of the dependence of several solute properties in various solvents, (&) nonpolar, polar aprotic and (&) polar protic, on the Lippert–Mataga and Dimroth–Reichardt functions

Solute–Solvent Hydrogen Bond Formation in the Excited State 85

As an example, we mention here the case of 4-hexylresorcinol [44]. Using the Lippert–Mataga model, nonlinear plots were obtained, with low correlation coefficients (r ¼ 0.70 for nf and 0.44 for DnSt), implying that the shifts in nf and DnSt cannot be explained solely in terms of changes in m. Whereas on the basis of the Lippert–Mataga model we can only suppose that the deviation from linearity is due to specific interactions, the Dimroth–Reichardt model treats such data in a quantitative way. Owing to the additional effect of H-bonding in protic solvents, further correlations of nf and DnSt with ETN followed two distinct lines, one for protic (r  0.92) and one for aprotic solvents (r  0.88). Therefore, it was concluded that, in protic solvents, increasing proticity stabilizes the molecule through H-bonds, while in aprotic solvents the dipole–dipole and dipole-induced dipole interactions are responsible for the stabilization of S1. Kamlet, Taft and collaborators [47], and more recently Catalan [48], developed solvatochromic models that quantify the specific interactions due to H-bond donor (HBD) and H-bond acceptor (HBA) contributions of the solvents, and separate them from non-specific ones. According to the Kamlet–Taft model, a given steady-state absorption or fluorescence spectral observable XYZ (nf, na, DnSt, the bandwidth, Dn1/2, or the fluorescence quantum yield, F) can be parameterized using a linear solvation energy relationship (LSER) of the general form XYZ ¼ XYX0 þ pðp* þ ddÞ þ aa þ bb

ð4:5Þ

The p parameter is a solvent dipolarity/polarizability function measuring the ability of the medium to stabilize the solute by virtue of its dielectric effect, thus resembling e(«, n) and ET(30) [13]. p ¼ 0 is attributed to CHX, a solvent unable to stabilize the dipolar solute, while p > 1 corresponds to solvents such as dimethylsulfoxide (DMSO) and water, able to promote such stabilization. Complementary to the p scale are the a and b scales, describing the Br€ onsted acid/base character of the solvent respectively. Thus, a shows the ability of the solvent to donate a proton in a solvent-to-solute H-bond (HBD ability), while b shows the solvent ability to accept a proton in a solute-to-solvent H-bond (HBA) [49]. The p, a and b coefficients are interpreted as solute properties and measure the effect of each process on XYZ. XYZ0 is the spectral observable independent of solvent effects (taken in the reference solvent, CHX, for which p , a and b are 0). d is a polarizability correction term for aromatic (d ¼ 1.0) and chlorinated aliphatic (d ¼ 0.5) solvents [50]. The validity of the Kamlet–Taft approach can be tested by plotting the XYZ values estimated by the model versus the XYZ values determined experimentally. Good correlations are generally observed, attesting that the model can indeed account for the experimental trends. Within the framework of the Kamlet–Taft model, protic solvents can provide the proton to form an H-bond with the HBA functions in the solute, but can also accept a proton to form an H-bond with HBD solute moieties. The greater the a and/or b values, the stronger are the H-bond(s) formed. As regards aprotic solvents, they can be cast into three categories: HBAs like CCl4, dioxane, ethyl acetate, acetone, tetrahydrofurane (THF), N,Ndimethylformamide (DMF) and DMSO (low a, high b values); HBDs like dichloromethane and chloroform (low b, high a values); solvents like acetonitrile (ACN), characterized by high a and b values. A collection of a, b and p values for different solvents as well as for some solvent mixtures can be found in [47]. Another advantage of the Kamlet–Taft model worth mentioning is that, as opposed to the e(«, n) and ET(30) parameters described above, the p , a and b parameters are obtained by averaging solvent effects on a variety of indicators rather than on a single probe [50], thus being much more reliable. The p scale was first based on the solvent-induced shifts of the longest wavelength p–p absorption band of seven nitroaromatic indicators, but was then improved by multiple least-squares correlations with more solvatochromic probes [39]. A similar model was proposed by Catalan, who developed his solvent polarity (SPP) [16], solvent acidity (SA) [51] and solvent basicity (SB) [52] scales on the basis of the solvatochromic behaviour of fluorophores and their homomorphs, i.e. compounds possessing similar but not identical structure to the fluorophores, thus allowing cancellation of many of the spurious effects involved in measurements of solvent properties. The SPP scale was obtained from the solvatochromic shifts undergone by the longest-wavelength absorption maximum of two indicators: 2-(dimethylamino)-7-nitrofluorene (DMANF) and its homomorph, 2-fluoro-7-nitrofluorene (FNF).

86

Hydrogen Bonding and Transfer in the Excited State

Similarly, the SB and SA scales are based on the probe–homomorph couples 5-nitroindoline and 1-methyl-5nitroindoline, and o-tert-butylstilbazolium (TBSB) and o,o0 -di-tert-butylstilbazolium (DTBSB) betaine dyes respectively. The TBSB/DTBSB pair is unsuitable for evaluation of solvents more acidic than methanol (MeOH) owing to the protonation of the indicator, and therefore 3,6-diethyltetrazine is employed instead. The dependence of XYZ on the SPP, SA and SB parameters is given by XYZ ¼ XYZ0 þ pSPP þ aSA þ bSB

ð4:6Þ

More recently, Catalan parameters for mixed solvents became available as well [53]. Mancini et al. determined the microscopic solvent properties – dipolarity/polarizability SPP, basicity SB and acidity SA – for binary mixtures of ethyl acetate with chloroform, ACN or MeOH as cosolvent, where specific intersolvent interactions by H-bonds are involved [54]. The results were then correlated to the Kamlet–Taft dipolarity/ polarizability p, acidity a and basicity b scales in order to evaluate the concordance between the two models. The good correlation showed that the Catalan scale is also appropriate for interpreting the effects of the various processes occurring in mixed solvents. These models provide an insight into the solute–solvent interactions in the ground and first excited singlet states. The magnitudes and signs of the Kamlet–Taft p, a and b parameters are indicative of the relative stabilization or destabilization effect of the respective solvent property on the spectral feature of the solute. The magnitude of the coefficients, sometimes expressed as percentage contributions, allows us to identify the dominant solvent effect in S0 and S1 states. As regards the signs, positive p coefficients for the absorption data mean that increasing the solvent polarity/polarizability produces a blue-shift in na, i.e. a stabilization of the ground state of the solute as compared with the FC state. By contrast, for the fluorescence data, negative p coefficients correlate with the bathochromic shift of nf upon increasing solvent polarity, indicating that the relaxed excited state is more stabilized in polar solvents. The relative magnitudes of a and b coefficients reflect the dominant contribution in H-bond interactions. The literature contains numerous investigations that make use of the Kamlet–Taft and/or Catalan models in order to explain the effects of pure solvents and solvent mixtures on the photophysical properties of molecules and to demonstrate the role of excited-state H-bonds [55–61]. Some examples of studies employing the Kamlet–Taft model are presented in Table 4.2. Aaron et al. [66] correlated the values/signs of the estimated Kamlet–Taft coefficients with the trends of the experimental spectra of a series of fused benzothiophenes. It can be seen that for BTT (Table 4.2), while the solute–solvent interaction in S0 is governed by solvent polarity (p ¼ 67%), in S1 the HBA of the solvent becomes the predominant contribution (b ¼ 79%), indicating the formation of H-bonds. The positive sign for p correlates well with the hypsochromic shift experimentally observed for the absorption spectra in polar media, while the negative p sign for the excited state reflects the bathochromic shift of the fluorescence maximum. A similar correlation was performed for F, which resulted in negative values for the p and b coefficients, showing the increase in F with solvent polarity and HBA ability, as observed experimentally (Fmax ¼ 0.15 was found in DMSO). The F values of 4-hexylresorcinol are strongly influenced by the nature of the solvent: the highest values are recorded in alcohols (0.8), while the smallest one is in CHX (0.05) [44]. A plot of F versus ETN was found to be linear for protic solvents (r ¼ 0.88), and non-linear for aprotic solvents. The Kamlet–Taft analysis suggested that the solvent’s HBA is responsible for the F enhancement. This was explained in terms of an H-bond donated by the hydroxylic hydrogen atom of the dye to the solvent, which becomes stronger upon excitation. The best Kamlet–Taft correlation for F was obtained when only protic solvents were employed (r ¼ 0.99). As can be seen from Table 4.2, DMAC presents solvent-dependent photophysical properties, explained by polar as well as by specific H-bond interactions: 66% polar and 30% HBD contribution in S0; 63% polarity effect and 21% HBA in S1 [68]. Correlations of the absorption and emission energies, Ea and Ef, reinforce the

O3S

-

Dye

Table 4.2

N+

I

NHR

Rose Bengal

O

O

SO3Na

OH

Heptamethine cyanine dye

I

I

COO-Na+

Cl

HO

2-Naphthol

1-Naphthol-5-sulfonate

Na-O

+

I

Cl

Cl

Cl

N

SO3-



na ¼ na,0  70p þ 270a  510b (r ¼ 0.98) nf ¼ nf,0  450p þ 0a  800b

nf ¼ 26 325 þ 0p  1198a þ 471b (r ¼ 0.95)

nf ¼ 12831 þ 410p þ 612a  236b (r ¼ 0.99)

na ¼ 19 262  1458p þ 267a  1012b

Kamlet–Taft LSER

HBA (64%)

HBA (60%)

HBD (72%)

HBD (49%)

Polarity (53%), HBA (37%)

[65]

[64]

[63]

[62]

Ref.

ðcontinuedÞ

Predominant solvent effect (%)

Influence of the solvent polarity and HBD and/or HBA ability on the spectral features of various dyes in S0 and S1 states

Solute–Solvent Hydrogen Bond Formation in the Excited State 87

S

O

O

H

H

N

Benzothieno[3,2b]-thiophene (BTT)

S

OH

OH

CH3

CH3

N

N

N

N

1,4-Bis[ -(2-quinoxalyl)-vinyl]benzene

N

1-(2-Pyridyl)-5-(4-dimethylaminophenyl)penta-2,4-diene-1-one (DMAC)

N

O

3-[4-Di(2-hydroxyethyl) amino]phenyl-l-(2-furyl)-2-propene-1one

Dye

Table 4.2 (Continued) 

Fprotic ¼ 1.305  0.927p  0.161a (r ¼ 0.98) Faprotic ¼ 0.334 þ 0.335p þ 0.195a (r ¼ 0.99)

Dna,1/2 ¼ 3799 þ 757p þ 967a (r ¼ 0.88) Ea ¼ 67.97  5.09p  2.34a  0.31b (r ¼ 0.91) Ef ¼ 59.58  12.48p  3.11a  4.16b (r ¼ 0.92)

na ¼ 25071  1315p  679a  715b (r ¼ 0.98) nf ¼ 22988  2750p  2251a  2366b (r ¼ 0.92)

na ¼ 37037 þ 233p  19a þ 97b (r ¼ 0.91) nf ¼ 30309  185p  1946a  342b (r ¼ 0.90)

Kamlet–Taft LSER

Polarity (63%)

Polarity (85%)

Polarity (63%), HBA (21%)

Polarity (66%), HBD (30%)

HBD (56%)

HBD (31%), HBA (32%)

Polarity (49%)

HBA (79%)

Polarity (67%)

Predominant solvent effect (%)

[69]

[68]

[67]

[66]

Ref.

88 Hydrogen Bonding and Transfer in the Excited State

Solute–Solvent Hydrogen Bond Formation in the Excited State 89

HBD solvent effect in S0 and show an HBA effect in S1, which means a different stability of the H-bonds in the ground and excited states. Other important observations are an increased width of the emission band, decreased fluorescence quantum yield and occurrence of dual fluorescence in polar protic solvents (BuOH, PrOH, EtOH, MeOH). Moreover, the comparison of the excitation and fluorescence spectra of DMAC led to the assumption that two emitting species, one non-H-bonded and another H-bonded, are present in alcohols, the solventcomplexed molecule emitting at longer wavelength. As we will see in the next sections, these are quasi-general observations for potential H-bond formatting probes in protic solvents and can be used as diagnosis criteria. Excited-state H-bond formation often means an increase in the energy of the H-bonds already present in the ground state owing to the change in the charge distribution upon excitation. 7-Amino coumarins are particularly interesting for studying H-bonding interactions in ground and excited states [70–72]. These compounds undergo electron transfer upon excitation, from the amino to the carbonyl group. Moreover, they are capable of forming H-bonds by participation of either the amino nitrogen (type A in Figure 4.3) or the carbonyl oxygen (type B) with HBD solvents, or by participation of the amino hydrogens (type C) with HBA solvents. Das et al. reported a study on the influence of the substituent at the 7-amino group on H-bond formation ability [4]. A Kamlet–Taft analysis was performed on three structurally related coumarins: C151 is the unsubstituted compound, while C500 and C35 are monoethyl and diethyl N-substituted respectively. The polarity contribution to S1 was found to be similar for all dyes (50%). The probability of H-bonding between the carbonyl oxygen or the lone pair of the amino nitrogen and the solvent (type A and B) increases in S1 compared with S0, as revealed by the a values, C151 showing the most significant effect (a is 1% in S0 and 20% in S1). The efficiency of the H-bonding of the amino hydrogens (type C) decreases upon excitation (for C151 the value of the b coefficient decreases from 73% in S0 to 33% in S1). Moreover, as the number of amino hydrogens decreases on going from C151 to C35, so does the magnitude of the interaction. The results show that, although the HBD and HBA solvent polarities make different contributions to the stabilization of the ground state, in excited state their effects become comparable. A comparison between the results of Kamlet–Taft and Catalan treatments can be found in the study of Brooker’s merocyanine, 1-methyl-4(40 -hydroxystyryl) pyridinium betaine, a compound characterized by strong hypsochromic absorption energy shifts (of 16.63 kcal mol1) and moderate hypsochromic fluorescence energy shifts (of 4.57 kcal mol1) [73]. Equations (4.7) and (4.8) present the correlations of the energy of the fluorescence maxima with the p and a Kamlet–Taft parameters and with the Catalan SA: f DEmax ðkcalÞ ¼ 44:75 þ 2:31p* þ 3:33a f DEmax ðkcalÞ ¼ 46:89 þ 4:31SA

ðr ¼ 0:97Þ

ðr ¼ 0:98Þ

ð4:7Þ ð4:8Þ

The polarity/polarizability of the solvents stabilizes the dipolar solute by electrostatic interactions, while the protic solvents interact through H-bonds to stabilize the negative oxygen atom of the dye. The a coefficient is slightly higher than p, suggesting that the H-bonding factor might play the main role in the stabilization process in S1. The two models yield similar conclusions, as the good Catalan correlation also shows the importance of the oxygen atom of the dye as a strong basic centre, easily forming an H-bond with protic solvents. A special type of H-bonding must also be mentioned, which involves the system of p-electrons of the aromatic rings that can act as H-bond acceptors in the presence of strong proton donors. This is the case of indole derivatives, including here the tryptophan-containing compounds. Experiments showed that the photophysical processes of these derivatives in protic solvents are rather complex in nature [74]. Two hypotheses were given for the mechanism of H-bonding of indole in protic solvents, assuming either the implication of the pyrrolic hydrogen or that of the p-electronic cloud of the heteroring. In order to discard the first hypothesis, experiments were performed on the methyl derivative. Its spectral properties were studied in n-hexane in the presence of increasing amounts of trifluoroethanol (TFE). The fluorescence spectra show the dependence on the TFE

90

Hydrogen Bonding and Transfer in the Excited State

concentration. At low alcohol concentration (104–102 M) the effect is insignificant, whereas at higher concentration a red-shift of the maxima, a broadening of the band and the presence of an isoemissive point can be noted, reflecting the formation of a new emitting species. The deconvolution of the spectra allows us to determine the emission maxima of the two species, at 310 nm for indole and for the ground-state H-bonded complex (HBC), and at about 330 nm for the new emitting species. This new species was assigned to an exciplex between the solute and TFE, further assigned to a proton-transfer H-bonded complex (PTC). Another experimental observation was the fluorescence quenching at large concentrations of TFE, as in the previously discussed case of DMAC. Moreover, lifetime measurements have shown that PTC is formed from the excited HBC, the decay of fluorescence being analysed in terms of triexponential functions, one of them with a negative pre-exponential factor, suggesting the formation of new fluorescent species from an excited precursor. This brings us to the complementary data of quantum yield and lifetime measurements and how they can be used as other diagnosis criteria in H-bond formation. For various fluorescence probes prone to specific interaction with the solvent, important changes in the emission efficiency, together with a decrease in the lifetime, were observed in protic solvents [75]. Performing time-resolved experiments, the occurrence of excited-state H-bonds can be attested by the analysis of fluorescence decays, bi- or multiexponential, correlated with the presence of multiple emitting species, solvated species, exciplexes, tautomers, etc. As stated above, a negative value for the pre-exponential factor in the fluorescence decay analysis indicates the presence of a species formed from another excited species. In protic solvents, in the case of biexponential decays, one of the lifetimes has a larger value and is usually assigned to the H-bonded species [76]. Correlating the experimental data on the excited-state lifetime (tf), the fluorescence quantum yield (w) and, when possible, the intersystem crossing quantum yield (wISC), the deactivation constants kf, kIC and kISC can be calculated using the formulae: ð4:9Þ kf ¼ w=tf kISC ¼ wISC =tf kIC ¼ ð1wwISC Þ=tf

ð4:10Þ ð4:11Þ

Linear dependencies of w, tf, knr and kf on the solvent polarity function were found, with different behaviour in protic and aprotic solvents [36, 77, 78]. The conclusion of such studies was that, for the majority of fluorophores, H-bond formation modifies the rate of the non-radiative deactivation processes through different mechanisms, which can lead either to a decrease or to an increase in w. Literature data attest that H-bond formation can either facilitate the ICT process, increasing the fluorescence emission from the ICT state [79, 80], or can quench this emission [81]. Possible mechanisms for the quenching or enhancement of the fluorescence quantum yield assisted by excited-state H-bond interactions were described, some of them correlated with the observation of dual fluorescence. They are summarized in Table 4.3 and will be extensively discussed in the following. Table 4.3 Mechanisms by which the intermolecular H-bond formation can influence the fluorescence quantum yield Fluorescence quenching Internal conversion due to the high-frequency accepting mode of H; most common Changes in excited state geometry Fluorescence enhancement Reversal of 1n–p , 1p–p states Suppression of the ISC process Increase in the energy barrier to conical intersection Inhibition of other deactivation channels (PET)

Solute–Solvent Hydrogen Bond Formation in the Excited State 91

4.3.2 Hydrogen-bond-assisted fluorescence quenching In most reported cases, a decrease in the emission intensity in protic solvents was found that was due to the predominant effect of the non-radiative deactivation processes [82–87]. Although the protic solvents can also act through non-specific polarity interactions, the main process leading to low quantum yields is the internal conversion (IC) induced by H-bonding. Owing to the high vibrational frequency of the OH group, the H-bond formation in alcoholic media can act as an effective accepting mode of non-radiative deactivation. Although the quantum yield of trans-ethyl-p-(dimethylamino)cinnamate is positively influenced by the polarity of the solvents, it decreases drastically in protic solvents, i.e. w ¼ 0.003 in water versus 0.010 in benzene and 0.022 in ACN [88]. The deviation from linearity in the Lippert–Mataga plots and the lifetime values support the correlation of the influence of protic solvents with the occurrence of a new deactivation channel due to H-bond formation. Another example is the highly solvent-sensitive fluorescence quantum yield of a quinoxalyl vinyl benzene dye [69]. In aprotic solvents, w increases strongly with increasing solvent polarity owing to a decrease in the vibronic coupling between the lowest 1 p–p and 1 n–p states. By contrast, in protic solvents, w decreases with increasing polarity, an effect rationalized in terms of an efficient IC by extensive vibronic mixing of the close-lying n–p and p–p states, enhanced by H-bonding with the solvent. The corresponding Kamlet–Taft fits are given in Table 4.2. It must be pointed out that no significant correlation of w was obtained considering all solvents (protic and aprotic, r ¼ 0.26). Similarly, one of the mechanisms proposed for explaining the experimental steady-state and lifetime measurements on cyano-substituted indolines in protic solvents assumes a deactivation channel due to Hbonds. For 5-cyano-N-methylindoline (CMI), unlike for N-methylindoline (MI), a short lifetime in EtOH was observed (0.82 ns versus 3.6 ns in ACN and 7.1 ns for MI in EtOH), but the most striking feature is the more than 10 times higher IC rate: 95  107 s1 in EtOH versus 9.4  107 s1 in ACN and 4.0  107 s1 for MI in EtOH. A similar behaviour is exhibited by 5-cyanoindoline (CI). As none of the other rates of the photophysical processes changed to a great extent in protic solvents, it was proposed that H-bond formation favours IC and is the main reason for lifetime shortening. Moreover, both CMI and CI have the same behaviour, so that it is not the NH group that is involved in this process, but the CN group [89]. This mechanism is also supported by theoretical calculations (see Section 4.4). As shown in Figure 4.3, more than one H-bond can be formed when the solute molecule presents two or more H-bond donor and/or acceptor groups. Furthermore, their geometry can be influenced by the conformation of the solute molecule, in which case more than one H-bonded excited species can be formed. An interesting process of fluorescence quenching in alcohols is reported for 7-(30 -pyridyl)indole and 7-(40 -pyridyl)indole, for which there is no possibility of intramolecular H-bond formation [90]. It was found that the rapid IC in these compounds is due to the simultaneous formation of two H-bonds implicating the indolic and pyridinic nitrogen atoms. For 7-(30 -pyridyl)indole, which can be present as anti and syn rotamers, characterized by the position of the pyridinic nitrogen on the opposite side or on the same side of the indole NH group respectively, the two H-bonds are either separated or form a bridged structure through the water molecule (Figure 4.6). Although a mechanism described in many biological systems, the photoinduced electron transfer (PET) favoured by H-bond formation is less known in organic molecules. It was found for an oxazine derivative [91] whose emission quenching in protic solvents can be explained by this mechanism. The different pattern of the H-bonds in the S1 state can also be due to a change in the molecular geometry upon excitation. Thus, it was found that the fluorescence of monocyanoanilines is practically quenched in water and TFE, although, in other solvents, w can reach values of about 0.34 [92]. The decrease in the quantum yield is associated with a decrease in lifetime, the non-radiative deactivation constant being very large, 2.3  1010 s1. The similitude of the behaviour in water and in the strong HBD solvent TFE supports the explanation that the formation of H-bonds involving the solute amine group is the cause of fluorescence quenching. In the case of 3-cyanoaniline, the variation in knr with temperature allows us to estimate an

92

Hydrogen Bonding and Transfer in the Excited State 7-(3'-pyridy1)indole syn

anti

O H R

N pyridyl

indole NH

pyridyl N

O H

indole NH

H

R

H

O

R

O R

Two separated H bonds

Cyclic structure of two H bonds

Figure 4.6 Hydrogen bond formation for the anti and syn conformations of 7-(30 -pyridyl)indole. Adapted with permission from [90]. Copyright Elsevier

activation energy for H-bond formation of about 11 kJ mol1. It was found that the non-radiative constant correlates with the difference, Du, in the pyramidal angle of the amine group in S0 and S1 states. The changes in geometry upon excitation are reduced for the dicyanoanilines, explaining the reduced fluorescence quenching in water. In the study of several anthraquinone, fluorenone, phthalimide and coumarin derivatives, Inoue et al. have observed a different behaviour on increasing the amount of ethanol (EtOH) in benzene [93]. The anthraquinone and fluorenone derivatives presented only a quenching of the fluorescence, for the phthalimides both a quenching and a shift of the maximum position were observed, while for the coumarin derivatives no effect was noticed. In order to explain these differences, the authors introduced a concept similar to that of hard and soft acid/base, considering as hard anions the compounds in which the negative charge is significantly localized on the carbonylic oxygen in the excited state, and as soft anions the compounds in which the charge is delocalized on the whole molecule. The anthraquinone and fluorenone derivatives for which a maximum effect of the alcohol was obtained behave as hard anions, the carbonylic oxygens having a significant negative charge to allow a strong H-bonding interaction. The coumarin and benzoxazine derivatives are characterized as soft anions, the negative charge on oxygens being low and not sufficient to ensure formation of a strong H-bond. As regards the phthalimide derivatives, the authors conclude that, although they can be classified as strong anions, other factors must be considered as well for explaining the weak fluorescence quenching by alcohol. 4.3.3 Hydrogen-bond-assisted fluorescence enhancement There are some cases in which the formation of H-bonds produces an enhancement of the fluorescence emission [30, 94, 95]. The most encountered mechanism consists in reducing either the ISC or IC non-radiative deactivation channels by modifying the order and/or the energy gaps between the first two excited states, S1 and S2, and the corresponding triplets. In aromatic compounds containing pyridinic nitrogens or carbonyl groups, the relative position of the 1 n–p and 1 p–p excited states (S1, S2) is strongly influenced by the substituents and by the solvents. A low S1–S2 energy gap determines, by the ‘close proximity effect’, a vibronic coupling that enhances the IC process. In cases in which the lowest singlet is 1 n–p , the quantum yield is rather low. The protic solvents determine, by both non-specific and specific interactions, a stabilization of the 1p–p state, as well as an increase in the energy of the 1 n–p state, reversing their energetic order and leading to a larger energy gap between them, hence causing a fluorescence enhancement. A qualitative scheme of the energy levels in non-polar and polar protic solvents is given in Figure 4.7. Several cases are reported underlying this effect of the reversal of the n–p and p–p excited states by increasing the energy of the weak emissive n–p state. In the case of 3-chloro-7-methoxy-4-methylcoumarin, the increase in w on going from hydrocarbon solvents to alcohols (0.10 in hydrocarbons versus 0.80 in

Solute–Solvent Hydrogen Bond Formation in the Excited State 93 E (S2) - *

1

3

-

*

(S2) n- *

1

n- * (S1)

1 1 3

n-

nonpolar

*

- * (S1)

3

n-

3

* *

polar (protic)

Figure 4.7 Qualitative scheme of the excited-state energy levels in non-polar and polar protic solvents for compounds containing both n and p electrons

alcohols and water) is due to a decrease in the non-radiative deactivation constant knr, kf being quasiconstant [95]. Another example is the comparative study of 4-phenoxy-N-methyl-1,8-naphthalimide and the unsubstituted N-methyl-1,8-naphthalimide [96]. The experimental data on 4-phenoxy-N-methyl-1,8-naphthalimide showed, on increasing the solvent polarity, a bathochromic shift of the fluorescence maximum, an increase in the bandwidth and a decrease in the emission. A Lippert–Mataga plot was quasi-linear if the points in the protic solvents, MeOH and EtOH, were excluded. All these effects attested the presence of an ICT excited state and of specific interactions in protic solvents. To gain a better understanding of the solvent influence, experiments were performed in dioxane in the presence of increasing amounts of water. Surprisingly, in the case of the unsubstituted naphthalimide, the presence of water determined an enhancement of the emission owing to a suppression of the ISC deactivation pathway, while for the 4-phenoxy derivative the same quenching as in pure solvents was noted [97–103]. Han et al. reported the synthesis of new fused phenothiazine derivatives with high solvent-dependent sensitivity (Figure 4.8, compounds a and b) [30]. Compound a is characterized by very low fluorescence quantum yields in protic solvents, i.e. in MeOH and water w < 0.0001. In spite of the very similar structure, compound b is highly fluorescent in alcohols and water as compared with the other solvents, i.e. w ¼ 0.048 (hexane), 0.132 (DMF, ACN) versus 0.481 (MeOH) and 0.554 (water). The authors rationalized the different behaviour by the change in the sequence of the first excited singlets. In aprotic solvents, the transition is of n–p nature, leading to low w. Owing to the close-lying n–p and p–p states of the dye, increasing the HBD ability of the solvent modifies the transition from an n–p type to a p–p type. In the case of a, the H-bonds formed in the excited state increase the probability of non-radiative deactivation, leading to quenching of the fluorescence in protic solvents. This behaviour is very specific, and the presence of a double bond in compound b of the series changes this ‘reversed polarity sensitivity’. Kamlet–Taft and Catalan treatment of the data supported this explanation. The experimental studies of the photophysical properties of substituted alloxazines (Figure 4.8, compound c) in ACN, dichloroethane, MeOH and water show a different behaviour in the first two solvents as compared with the protic solvents [104]. In protic solvents, beside a red-shift of the fluorescence band, an enhancement of the emission was noticed, the larger value being obtained in water. As the alloxazines have several centres that could be involved in H-bonding with proton donors, it is assumed that the behaviour in protic solvents is due to specific interactions. As it was previously shown that H-bonding to the pyrrolic NH group is of little importance, there remain as potential H bonding centres the pyridinic nitrogen atoms in the median rings and the two carbonyl groups. Taking into account the position of the fluorescence band in protic solvents, the

94

Hydrogen Bonding and Transfer in the Excited State S

S

N

N O

O (b)

(a)

10 N

HN

NH

O NH

N

NH

N

N

O

O

O (c)

(d)

R O

O

N

R'

R N

O H

H

H

R' (e)

Figure 4.8 Selected compounds showing fluorescence enhancement upon hydrogen bond formation

authors considered that the H-bond at N10 has a major influence on the p system, leading to a flavin-like structure. Although some experimental and theoretical data attest the p–p nature of the first excited singlet state, the low quantum yield of the alloxazines was explained by the energetically close n–p and p–p states. Considering the ‘proximity effect’, it can be assumed that the vibronic coupling between these states is strongly solvent dependent, the gap between the two states being increased in protic solvents. The n–p character of S1 in alloxazines as compared with the p–p nature of S1 in isoalloxazines (Figure 4.8, compound d) explains the lower quantum yields and the larger non-radiative deactivation constants of the former [105]. 1-Hydroxyfluorenone allows us to study three types of H-bond: an intramolecular bond between the OH group and the carbonylic oxygen, an intermolecular bond of the OH with the protic solvents and an intermolecular H-bond between the hydroxylic hydrogen and an H-bond acceptor [106]. The fluorescence maximum is red-shifted on going from CHX to more polar solvents. The results show a different behaviour in non-protic solvents as compared with that in the protic solvents; thus, as reflected by the deactivation constants kisc and kic, the predominant deactivation pathway changes from the ISC process in the former solvents to IC in alcohols (Table 4.4). The important role of the ISC process was explained by the close value of the energies of the S1 (p–p ) and T3 (n–p ) states. Any factor that modifies the difference in energy between these states, solvent or substituent, influences the kisc value [107]. Another explanation for the increase in fluorescence emission in protic media is the increase in these solvents of the barrier to reach a conical intersection (CI) that favours an IC deactivation process.

Solute–Solvent Hydrogen Bond Formation in the Excited State 95 Table 4.4 The different deactivation channels of 1-hydroxyfluorenone in aprotic and protic solvents. Data from Ref. [106] Solvent Cyclohexane CF3CH2–OH (TFE)

kIC (107 s1)

kISC (107 s1)

72  14 2  0.5

18  9 89  9

Unfortunately, this explanation requires elaborate calculations, an example being the theoretical study of 2-aminopurine [108]. The understanding of the photophysical properties of 2-aminopurine represents a good model for the behaviour of purine, a constitutional isomer of adenine. The experimental data show that, unlike adenine, the fluorescence quantum yield of 2-aminopurine is strongly enhanced in water as compared with CHX. The theoretical model used to explain this behaviour showed that the water molecules surrounding the solute increase the barrier of the transition state, leading to a CI that is an efficient IC funnel. The calculated statistical number of water–solute H-bonds is 0.81, 0.94 and 0.86 for the excited-state minimum, the transition state and the CI respectively, mainly implicating N3 and N7. Besides IC, other non-radiative deactivation channels can be suppressed by the formation of H-bonds, i.e. intramolecular photoinduced electron transfer (PET). N-[2-(2-hydroxylethylamino)-ethyl]-1,8-naphthalimide (HEAN) (Figure 4.8, compound e) presents potential H-bond acceptor groups [109]. Its photophysical properties are strongly solvent dependent. In non-polar or low-polarity solvents, or in aprotic polar solvents such as DMF, ACN and ethylacetate, the fluorescence quantum yield is very low, at the limit of detection. A strong enhancement of the emission was observed in polar protic solvents, maximum w being found in water (0.453). A plot of w versus the solvent polarity function (Lippert–Mataga model) or ETN parameter resulted in a regular curve. These experimental observations show a dependence of the photophysical properties on the proticity of the solvent. The special behaviour in protic solvents was explained by the possibility of H-bond formation between the proton of the solvent and the lone electron pairs of the strong acceptors of the molecules, the alkoxide oxygen and the secondary amine nitrogen. The formation of these intermolecular H-bonds suppresses a deactivation channel via a PET mechanism from 2-hydroxyethylamino-N to the dicarboximide group. 4.3.4 Dual fluorescence In certain conditions, the fluorescence emission of some compounds is characterized by the occurrence of two bands, more or less resolved. The band at shorter wavelength (SW) is usually ascribed to the FC excitation leading to a locally excited state (LE), and it is also referred to as ‘normal’ fluorescence, Fn. The band located at longer wavelength (LW) is considered as ‘abnormal’ fluorescence, Fa, being correlated with relaxed ICT or TICT excited states, or with the presence of new emitting states, H-bonded complexes, tautomers, zwitterionic structures and exciplexes. The positions and the intensities of these bands are strongly solvent dependent, and the phenomenon can be enhanced by interactions with the protic solvents [110]. An interesting case is represented by three related compounds, 2-(40 -N,N-dimethylaminophenyl)imidazo [4,5-b]pyridine (DMAPIP-b), 2-(40 -N,N-dimethylaminophenyl)imidazo[4,5-c]pyridine (DMAPIP-c) and 2-(40 -N,N-dimethylaminophenyl)imidazo[4,5-d]pyridine (DMAPIP-d), which differ only in the position of the pyridine nitrogen (Figure 4.9). All compounds show dual fluorescence in protic solvents, with the most significant effect being observed in the case of DMAPIP-b (more red-shifted and better-resolved LW band) [111, 112]. The LW emission has been assigned to the formation of a TICT state assisted by Hbonding. This explanation is supported by several facts: (i) the dependence of the ratio of the two band intensities on the HBD ability of the solvent; (ii) the decrease in or even the lack of an LW band in viscous

96

Hydrogen Bonding and Transfer in the Excited State CH3

N

CH3

N

N NH

N

N N

CH3

NH

DMAPIP-b

H3C

H 3C

O

DMAPIP-c

H3C

NH

CH3

N

CH3

O

O

H3C

N

O N

CH3

MAPAEE

MDMANA

O

CH3

DMANAN

H O

N

O

H

3-HQ-Bf

Figure 4.9 Selected compounds presenting dual fluorescence in protic solvents

solvents which reduce the possibility of intramolecular rotation of the electron donor and acceptor moieties; (iii) a biexponential fluorescence decay in protic solvents. From the three nitrogen centres of the molecule, the experimental data attest implication of the pyridine nitrogen in the H-bond. In an acidic medium, only a single emission band is observed, as the nitrogen from the imidazole ring is protonated. Furthermore, H-bondinduced TICT is not observed in the related compound (N,N-dimethylaminophenyl) benzimidazole, where the pyridine nitrogen is absent. The H-bond between the solvent and the electron acceptor fragment determines a better planarity of the acceptor group with the aromatic cycle and favours charge transfer towards the electron donor part. In water, the strongest H-bond donor, the TICT state is more stabilized, and another non-radiative deactivation channel towards the ground or the proximal triplet states becomes active. Three related compounds containing a secondary or tertiary amine as a donor group and an ester or nitrile as an acceptor group all present dual fluorescence in protic solvents. The presence of a double bond between the acceptor and the aromatic ring ensures, for all compounds, a supplementary p-conjugation. For (E)-3-(4methylamino-phenyl)-acrylic acid ethyl ester (MAPAEE in Figure 4.9), three possible positions for H-bond formation in protic solvents can be assumed: two sites involving the oxygen atoms of the acceptor and one implicating the lone pair of the nitrogen which can fix a hydrogen from the solvent [83]. In non-polar solvents

Solute–Solvent Hydrogen Bond Formation in the Excited State 97

such as methylcyclohexane (MCHX), a single band is observed, assigned to the LE state. Adding increasing quantities of MeOH to MCHX determines the appearance of a new band, bathochromically shifted, and a slight decrease in the intensity of the former band; the family of spectra shows a clear isoemissive point. The new band was assigned to an ICT solvated state due to the solute–MeOH H-bonded complex involving the nitrogen lone pair. The decrease in the quantum yield in MeOH as compared with the value in an aprotic solvent of similar polarity, ACN, together with the linear dependence of the fluorescence band maximum on the a parameter, supports the role of H-bond formation in the overall photophysical properties. Similarly, the methyl ester of N,N-dimethylaminonaphthyl-(acrylic)-acid (MDMANA) also presents dual emission in polar and protic solvents and a single fluorescence band in non-polar solvents [113, 114]. The LW band shifts from 461 nm in CCl4 to 521 nm in water and was ascribed to a CT band. The addition of EtOH to MCHX shifts continuously the band from 440 nm (MCHX) to 480 nm at a content of 70% EtOH. The linear dependence of nf versus a shows the H-bond formation in protic solvents. On the other hand, the lack of mirror symmetry of the absorption and fluorescence spectra attests a change in the geometry upon excitation. The experimental data supported by DFT calculations are rationalized in terms of three coexisting excited states: an LE state, a TICT state and an H-bonded state. Unlike MAPAEE and MDMANA, N,N-dimethylaminonaphthyl-(acrylo)-nitrile (DMANAN) can be implicated in only one type of H-bond, between the amino nitrogen lone pair and the hydrogen of the alcohol [115]. The LE maximum is located at about 425 nm, and the LW (CT) band is in the range 473–498 nm. The involvement of the H-bond in the LW band is evidenced by the linear dependence of nf on a. Additional support for the assumption that the H-bond implicates the nitrogen lone pair was obtained from the experiments in acidic medium. Thus, the protonation of the diethylamino group and consequently the fixation of the nitrogen lone pair hinder the charge transfer and determine the decrease in the LW (CT) band. N,N-dimethylbenzodiazepine exhibits a single fluorescence band in aprotic solvents (normal fluorescence, Fn band) and two bands in protic solvents (an abnormal band, Fa, and the normal one, Fn) [116]. The abnormal fluorescence is assigned to an intermolecular interaction between the solute and solvent molecules in the excited state, with the formation of an exciplex. Plotting the Stokes shift for both bands, Fa and Fn, against the solvent polarity function gives two lines with different slopes, allowing us to estimate the variation in the dipole moment upon excitation for the two emitting states. In protic solvents, fluorenone also presents a dual fluorescence owing to the presence of two excited forms, a free form and an H-bonded complex, with different S1 photophysical features. In the strongly HBD solvent TFE, only the H-bonded complex is evidenced [117]. Some experiments were conducted in mixtures of solvents with similar polarity (close « values) but different basicity. An example is the study of 3-hydroxyquinolones in mixtures of ACN (« ¼ 35.7, b ¼ 0.32) and DMF (« ¼ 37.2, b ¼ 0.74) [110]. 2-Benzofuryl–3-hydroxy–4(1H)-quinolone (3-HQ-Bf) (Figure 4.9) presents a dual fluorescence with good resolved bands and a separation of the maxima in the range 2700–4200 cm1. It was found that the solvent effect is better reflected by the intensities of the two bands observed, IN /IT, than by the changes in the maximum positions. This ratio was found to be linearly dependent on the b parameter (log(IN /IT ) ¼ 1.59 þ 1.62b, r2 ¼ 0.9). Time-resolved experiments were also performed on 3-HQ-Bf in MeOH as protic solvent and in aprotic solvents of different basicity. The fluorescence decays monitored on the two bands, N and T, were biexponential, with similar lifetimes but different pre-exponential factors. The negative sign for the T factor attests its formation from N by an excited-state intramolecular proton transfer (ESIPT) mechanism. The overall mechanism is characterized by the following steps. Firstly, the molecule in the ground state is solvated at the OH group by a basic solvent; upon excitation, the charge densities are changed and the carbonylic oxygen, now with a high negative density, competes with the solvent for H-bonding with the hydroxylic hydrogen, generating the N species. This species goes to the T form through the ESIPT mechanism. The T state is further deactivated by radiation emission. The overall mechanism is presented in Figure 4.10.

98

Hydrogen Bonding and Transfer in the Excited State C-O-

C-OO-H

solvent

C-O-H

H

ESIPT

+

+

S1-solvated N state

+

N*state

C=O N* fluorescence O-H

O-

O

*

T state

T* fluorescence

solvent

S0-solvated N state

S0-N state

S0 -T state

Figure 4.10 Schematic representation of the ESIPT mechanism. Adapted with kind permission from [110]. Copyright 2009 Springer Science þ Business Media

4.4 Design of the Experiments For a good understanding of solvent effects, and especially for separating the non-specific and specific interactions involved, the experimental conditions must be carefully chosen. As was previously shown, the classification of solvents into polar and non-polar is made in terms of the polarity/polarizability functions, which depend on the dielectric constant « and refractive index n. The polar solvents can be further separated into protic and non-protic ones. The Kamlet–Taft parameters, a and b, characterize the ability of the solvents to act as HBD or HBA respectively. A survey of the conditions required for performing reliable experiments leads to the following: . . .

selection of the fluorescence probes; selection of the solvents and mixtures of solvents; use of isotopic effects.

4.4.1 Selection of fluorescence probes The fluorescence probes used for describing solvent effects and excited-state deactivation processes due to H-bond formation must be selected in such a way as to avoid the occurrence of other non-radiative pathways such as ISC and the close positioning of the 1 p–p , 1 n–p , 3 n–p excited states [118, 119]. A convenient structural parameter in such probes is a rigid molecular skeleton that does not favour any conformational or configurational relaxation processes [120]. The most important fluorescence probes for investigating H-bonding interactions in the excited state were presented in Figure 4.2. Performing experiments on related compounds differing only in a single structural element or in a substituent brings about valuable information by comparison of the results [5, 107, 118]. A special case is the substitution of the hydrogen by a methyl radical in compounds containing several atoms that can participate in H-bond formation, i.e. pyrrolic or amine nitrogen/hydrogen (see, for instance, the previously discussed case of CMI versus CI, which proves that NH is not implicated in the H-bond decisive for the

Solute–Solvent Hydrogen Bond Formation in the Excited State 99

deactivation processes [89]. Another case in which N-methyl substitution allows us to identify the H-bond centre is with some pyrrole derivatives. The photophysical properties of non-methylated compounds are strongly influenced by the presence of pyridine as a solvent, unlike those of the N-methylated derivatives. The suggested mechanism comprises, in the first step, the formation, in the ground state, of an H-bond between the NH group as H-bond donor and the pyridine nitrogen as H-bond acceptor [121]. In compounds containing an amine group that can be involved in an intramolecular H-bond, the progressive alkyl substitution of the amino group is useful in differentiating intermolecular H-bonds from intramolecular ones. Thus, the solvatochromism of o-nitroaniline and N-alkyl-o-nitroaniline probes was studied in solvent mixtures involving an ‘inert’ non-polar cosolvent (CHX) and an HBA solvent (THF) [122, 123]. An interesting study was performed on a symmetrical molecule that has no permanent dipole moment in the ground state, 2,6-diaminoanthraquinone [124]. The observed experimental effects in such compounds are due only to intermolecular H-bonds with the solvents; the presence of two carbonyls that can act as independent dipoles explains this behaviour. The linear dependence of DnSt against the b solvent parameter confirms this explanation. The presence of a solvated species is also attested by the hypsochromic shift of the emission maximum in EtOH at increasing excitation wavelength. 4.4.2 Selection of solvents Some general rules for the selection of the most appropriate solvents can be extracted from a survey of the literature data. The experiments must firstly be performed in non-polar solvents or solvents with low polarity in which specific interactions are not expected. Quantitative treatment in such solvents through the Lippert– Mataga or Dimroth–Reichardt analysis allows us to estimate the change in the dipole moment upon excitation. Starting with this result, the use of polar and/or protic solvents permits the identification of the nature of the excited state and the contribution of the specific interactions to the overall solvent effect. In order to identify the role of H-bond formation in the general pattern of the solvation processes, the use of solvents with similar polarity but different H-bonding capabilities is recommended. The selection of solvents also depends on the expected type of H-bonds. Valuable results are obtained by using mixtures of solvents with different characteristics. Several examples will be discussed in the following. A good fluorescence probe for analysing the role of H-bonds in the excited state is 4-aminophthalimide, studied by Krystkowiak et al. [34]. Containing both an electron donor and an electron acceptor group, this compound represents a good model for the change in the electron density pattern upon excitation and, consequently, for the change in the solute–solvent interactions in the S1 state. However, with more than one group prone to form H-bonds, choosing the solvent is very important for evidencing the various possible effects. The authors consider that the best solvents for evidencing only the non-specific interactions are the 1-chloro-n-alkanes. Although they cover only a small range of e(«, n) values (0.11–0.23), both their HBD (a) and HBA (b) capabilities in the Kamlet–Taft model are zero. The second condition was to choose solvents that can form only a single type of H-bond. Thus, for studying the H-bonds of the NH2 group, the authors used DMSO, characterized by b ¼ 0.76 and a ¼ 0. For investigating the H-bonds of the oxygen atom of the carbonyl group, solvents with a  0 and b ¼ 0, i.e. polyfluorinated alcohols (hexafluoroisopropanol (HFIP), a ¼ 1.96, b ¼ 0) were used. Comparing the absorption, emission and excitation spectra in all these solvents, the authors succeeded in evaluating the energy of the H-bonds formed by the different groups in the S0 and S1 states, and to establish the excited species present in each case and the main deactivation pathways. In some cases the alcohols do not have sufficient H-bonding capability to induce H-bond formation, and strong HBD solvents are necessary. In the case of fluorenone, the presence in alcohols of H-bonds implicating the oxygen of the carbonyl group was extensively reported [60, 84–86, 125–127]. However, the related ketone benzo[b]fluorenone was not quenched by alcohols [128], and H-bond formation was observed only in the strong HBD solvent TFE. This was explained by the structural differences between the two compounds, and

100 Hydrogen Bonding and Transfer in the Excited State

especially by the changes in the overall aromaticity that determine different charges on the carbonyl group. In the case of benzo[b]fluorenone, the fact that its emission spectrum has been found to be independent of the excitation wavelength indicates that the emission occurs from a single excited species. The use of mixtures of solvents allows extension of the properties of the media. The separation of the different kinds of interaction can be done using increasing concentrations of a potential HBD or HBA solvent in a non-polar, non-protic solvent [129]. The choice of the particular solvent mixtures depends on the fluorescence probe used. An interesting case is the class of b-carbolines [76, 130, 131]. They present very attractive photophysical properties, dependent on the solvent features. Owing to their specific structure, the simultaneous presence of the pyridinic and pyrrolic nitrogen, b-carbolines can act as both HBD and HBA. The studies were performed in CHX, in the presence of increasing amounts of HBD solvents, i.e. HFIP, 2-chloroethanol (2-ClEtOH), tert-butyl alcohol (t-BuOH) and HBA solvents, i.e. THF, DMF and hexamethyl-phosphoramide. Differing from the 1- and 1,4-disubstituted anthraquinones [132, 133] which can be involved in both intraand intermolecular H-bonds, the 2- or 2,6-disubstituted anthraquinones are only involved in intermolecular H-bonds in protic media [124]. The solvent influence on the excited-state molecular interactions was studied using either pure solvents or several solvent mixtures combining a polar solvent, DMF, with a non-polar (benzene), a protic (EtOH) and two other polar solvents with similar polarities but different b values, DMSO (HBA) and ACN (HBA and HBD). In all mixtures, a deviation from the ideal behaviour was found, the most interesting results being obtained for the system DMF–EtOH. It was found that the solute–solvent H-bonding interactions can break the H-bonds between the self-associated alcohol molecules. The presence of solute–solvent H-bonds was attested by the linearity of the plot DnSt versus b and by the dependence of the emission band in EtOH on the excitation wavelength. A hypsochromic shift of the fluorescence spectrum was obtained for an increase in the excitation wavelength from 458 to 515 nm; such behaviour was not observed in the polar aprotic DMF. The differences in the photophysical properties in mixtures of solvents as compared with an ideal solution are due to preferential solvation in one of the solvents. Some models were described [134, 135] to treat this behaviour. A good example of their applications is the study of fluorenone and 4-hydroxyfluorenone using two mixtures of different solvents, non-polar–polar aprotic (CHX–THF) and non-polar–protic (CHX–EtOH). The last system allows the detection of possible H-bonding effects [136, 137]. The plot of the fluorescence shift against the molar fraction of one component shows a non-linear dependence. According to Bakshiev [23], the averaged molar fraction of the polar component in the first solvation shell of the solute is given by the formula

B «eff «n xp ¼ «p «n

ð4:12Þ

where <«eff> represents the effective dielectric constant


 «eff ¼ «n xBn þ «p xBp

ð4:13Þ

The constant <«eff> can be estimated from the shift of the absorption (d~nA ) or fluorescence (d~nF ) maxima in the binary mixtures with respect to the non-polar solvent, as follows: d~nA;F

!
   «eff 1 n2 1 2n2 þ 1 nþp n
  ¼ D~nA;F D~nA;F ¼ m1;2 2 n þ2 «eff þ 2 n2 þ 2

ð4:14Þ

Solute–Solvent Hydrogen Bond Formation in the Excited State 101

where m1 and m2 depend on the ground- and excited-state dipole moments, mg and me, and on the Onsager cavity radius of the solute, a: m1 ¼

~ m g ð~ m e ~ mgÞ ; 2p«0 hca3

m2 ¼

~ m e ð~ m e ~ mgÞ 2p«0 hca3

ð4:15Þ

The plot <«eff> versus x p deviates from linearity, indicating a preferential solvation. The preferential index of solvation, Z, is calculated using the equation Z¼

1 Cm2 M Df ð«np Þ 4p«0 RTdr6ss

ð4:16Þ

where C is estimated as 3=8p, considering the solute and the solvent as spherical molecules, De(«np) ¼ e(«n)  e(«p), the solvent functions being calculated for both types of solvent by the usual formula, considering the respective dielectric constants «n or «p: eð«Þ ¼

2ð«1Þ 2« þ 1

ð4:17Þ

M represents the mean molecular mass for the two solvents, d is the mean molecular density of the polar and non-polar solvents and r is the solvent–solute distance (rs–s ¼ a þ r). Coumarin 153 was studied in toluene–ACN and toluene–MeOH mixtures in order to evidence the difference between the polar (ACN) and the protic (MeOH) solvent [87]. Although in the ground state C153 does not show preferential solvation, a non-ideal dependence of the photophysical properties on the solvent composition was found in the excited state. The behaviour observed in the presence of MeOH was explained by H-bond formation. The interaction of harmane with strong HBD solvents leads to two species in the S0 state, both implicating the pyridinic nitrogen: an H-bonded complex (HBC) and a second species, labelled as the proton transfer complex (PTC), in which a proton is transferred to the nitrogen via the assistance of a second molecule of solvent [76]. In fact, the excitation process determines the excitation of the free harmane molecule and of these two H-bonded species. In the presence of HFIP, the excited PTC species undergoes other processes leading to a cation-like and a zwitterionic species. It was found that the best conditions to evidence all these species were when a mixture of CHX–toluene (90:10 v/v) was used. Steady-state and time-resolved experiments in the presence of increasing amounts of HFIP allow us to estimate the kinetic aspects of the overall process. In order to isolate the H-bond donor of the heteroring, the 9-methyl derivative of harmane was used. The fluorescence decays in alcoholic solvents were fitted to a biexponential function; the shortest lifetime, t1 ¼ 2.1 ns, similar to that in pure CHX, was assigned to the free molecule, and the longer lifetimes, t2 ¼ 3.4 ns (ClEtOH) and t2 ¼ 3.7 ns (HFIP), to the H-bond-complexed species. Starting with the non-methylated derivative, a similar behaviour was firstly observed, followed by the appearance in ClEtOH and HFIP of a new band assigned to a zwitterionic structure. A separation of the polarity and H-bond interaction by using mixtures of THF and CHX (non-polar), ACN (polar, non-protic), trichloroacetic acid (TCAA) and several alcohols, MeOH, EtOH, PrOH and BuOH (polar, protic), is described by Yamamoto et al. for 4-phenyl-1-N,N-dimethylaminobutane (PDAB) [138, 139]. The fluorescence spectrum of PDAB consists of three bands labelled in order of their increasing wavelength as A (285 nm), B (343 nm) and C (385 nm), and assigned to the phenyl chromophore, to the excited amine group and to an intramolecular exciplex respectively. The last two bands are overlapped and can be separated only by temperature effects. Considering the polar solvents, ACN only has a polar effect, and TCAA only an

102 Hydrogen Bonding and Transfer in the Excited State

H-bonding effect, whereas the alcohols provide both non-specific and specific interactions. The presence of increasing amounts of ACN determines a decrease in bands B and C, without a change in the intensity of band A. In the THF–BuOH mixtures, the addition of the protic solvent determines an increase in the intensity of band A, at the expense of B and C intensities. The same effect, but more pronounced, was noticed in the presence of TCAA. In this case it can be assumed that the changes in the fluorescence spectrum are due exclusively to H-bond formation, i.e. to specific interactions. Defining the ratios X and Y, equations (4.18) and (4.19), and plotting their values against the percentage of BuOH in the mixture, we can separate the two effects, because the X versus BuOH% dependence reflects only the contribution of H-bonding to the change in the fluorescence emission: X¼

Y¼

IA  IA0 ½TCAA ¼ 0 excess IA  IA ½PDAB 0 IB;C  IB;C 0 IB;C

¼

ð4:18Þ

½TCAA ½PDAB

ð4:19Þ

The dependence of the ratio of the H-bonding effect to the polar effect versus the volume percentage of alcohol is linear for BuOH and presents a maximum deviation from linearity for MeOH. For up to 10% vol. MeOH the effect is predominantly due to H-bonding interaction; the plot shows saturation at a value of 0.8 for the H-bonding to polar effect ratio. 4.4.3 Use of deuterated alcohols One of the experimental methods for checking the non-radiative deactivation pathway via H-bond formation is the use of deuterated alcohols, in which case H-bond formation increases the IC rate via a vibrational mechanism. The reduction in the vibrational frequency upon deuteration and the lower ability of the deuterated compounds to be involved in H-bonds determine a decrease in the non-radiative deactivation constant. The fluorescence emission of amino-substituted fluorenones, except 1-aminofluorenone, is strongly quenched in polar protic solvents. Performing experiments in deuterated alcohols, a decrease in the nonradiative deactivation constant was found in going from EtOH to EtOD, i.e. a ratio knr(EtOH)=knr(EtOD) ¼ 1.2, whereas for other aminofluorenones this ratio reaches values as large as 1.9 (Table 4.5) [118]. Some explanations for this behaviour are the presence of an intramolecular H-bond between the carbonylic oxygen and the amine hydrogen, an enhanced decay by an ISC mechanism or the formation of a TICT state. The other compounds in the series, 3- and 4-aminofluorenones, are also strongly quenched in the presence of alcohols, the ratio of the non-radiative deactivation constants in EtOH to those in benzene being in the range between 125 (4Table 4.5 Comparative effects of water, alcohols and the corresponding deuterated species on the ratio of the non-radiative deactivation constants knr(R–OH)/knr(R–OD) knr(Et–OH)/knr(Et–OD) Compound Ref.

F 1.9

3MAF 1.3 [118]

knr(H–OH)/knr(H–OD)

2-PAQ

4-AP

Cum

R6G

R128

R101

RB

1.7 [140]

5.7 [34]

1.1 [36]

5.3

2.9

3.3

1.3

[141]

a F: fluorenone; 3-MAF: 3-(methylamino)-9-fluorenone; 2-PAQ: 2-piperidino-9,10-anthraquinone; 4-AP: 4-aminophthalimide; Cum: coumarin derivatives; R: rhodamine.

Solute–Solvent Hydrogen Bond Formation in the Excited State 103

aminofluorenone) and 43 (3-aminofluorenone). The same behaviour has been previously discussed for some aminoanthraquinones, for which this ratio reaches larger values (158 for 2-aminoanthraquinone and 263 for 2piperidine-anthraquinone (2-PAQ)) [140]. Aromatic thioketones present fluorescence from the S2 excited state [142], the non-radiative deactivation mechanism in fluorinated hydrocarbon solvents being S2–T1. The fluorescence from the S1 state is not observed owing to the more rapid ISC process S1–T1 than the IC S2–S1. The same evolution of the excited states was also postulated in solvents presenting interaction with the solute, such as hydrocarbons and ACN, for which a shortening of the lifetime was observed. The shortest lifetime was obtained in water (1 ps), reflecting the de-excitation of a benzopyranthione–water complex [143]. Support for this assumption was obtained by repeating the measurements in deuterated water. It was found that t(S2) in D2O is twice the value in water. There are also cases in which the use of deuterated water rules out the hypothesis of the H-bonds influencing the solute photophysics in protic solvents. Thus, in a recent study on coumarin 7, Pal et al. found that in MeOH–water and MeOH–deuterated water mixtures, the values for the fluorescence quantum yield and the lifetime are practically unchanged [36]. The lack of any isotope effect reflects the fact that the peculiar photophysical properties of the compound in protic media are not due to H-bond formation. 4.4.4 Other experimental parameters (temperature, excitation wavelength) Emission experiments can be performed in some other experimental conditions as well, such as different temperatures or excitation wavelengths. Increasing the temperature, H-bond formation is disfavoured, so that the rate of the processes it determines should be modified. Steady-state fluorescence and lifetime determinations at variable temperatures can provide useful information about the different processes occurring in the excited states. Although the fluorescence emission is deeply influenced by temperature, it was stated that the effect is merely due to changes in the non-radiative deactivation constants, the radiative processes being practically uninfluenced. Temperature effects were useful in evidencing the presence of excited-state H-bonds. Two kinds of experiment are reported in the literature data: measurements in an interval of temperatures around room temperature [36, 115, 134, 144–147] and measurements in glassy solutions at 77 K [29, 83, 120, 148]. The measurements of fluorescence lifetimes at variable temperatures in short interval ranges around 298 K in protic solvents was used to differentiate between some possible mechanisms such as electron ejection [89], participation of TICT states in the deactivation pathway, flip-flap motion of the amine group in non-polar and polar solvents, 1-N-methylamino- and 1-N,N-dimethylamino-9,10-anthraquinone dyes and coumarin [36, 115, 144, 145], the ESIPT mechanism [146], etc. Experimental determinations were also used for estimation of the activation energy of the various processes and for estimation of the rotational diffusion time [147]. Measurements at several temperatures can thus be used as additional support in identifying excited-state Hbonds. Experiments performed for DMANAN [115] in ACN and MeOH at several temperatures in the range 273–323 K showed a different behaviour in the two solvents. With increasing temperature, in MeOH, both SW and LW bands decreased in intensity, while in ACN only the LW band was affected (strongly decreased). These observations are explained assuming different mechanisms of non-radiative deactivation: in ACN, the normal effect of temperature on the non-radiative process is operating, while, in MeOH, increasing temperature produces the breaking of H-bonds. The influence of temperature on H-bond formation can also be outlined by lifetime measurements, i.e. by investigating the temperature effect on tf. It was found that, in protic solvents with e(«, n) < 0.17, the tf of a coumarin derivative increases with temperature, while, in solvents with e(«, n) > 0.17, tf decreases with temperature [144]. In order to explain this behaviour, H-bonding interactions with the formation of a solute–protic solvent complex were inferred. As depicted in Figure 4.3, the coumarin derivatives can be involved in three types of H-bond (A, B and C) with protic solvents. It was assumed that the H-bonds

104 Hydrogen Bonding and Transfer in the Excited State

correspond to the B type. Increasing the temperature, the equilibrium is shifted towards the free, uncomplexed dye, and tf increases. Experiments at 77 K, sometimes correlated with measurements in viscous media, are performed for compounds presenting dual fluorescence at room temperature, for reinforcing the assignment of the LW band to a solute–H-bonded species. It is assumed that, in glassy solutions, the solvent molecules have reduced the possibility of undergoing a reorientation and the spectra reflect the predominant species in the system. As was previously discussed, the widely used fluorescence probe fluorenone [120] exhibits dual fluorescence (LW and SW bands), at room temperature, in all solvents, but only the SW band in MeOH and the LW band in TFE at 77 K. The experimental data at room temperature were rationalized in terms of an equilibrium between the free fluorenone and fluorenone–H-bonded species, equilibrium dependent on the proton donor capability of the solvent. In MeOH the equilibrium is shifted towards the free fluorenone (SW band), and in the strong proton donor, TFE, towards the fluorenone–H-bonded species. The comparison of these spectra supports the assignment of the LW band to a fluorene–H-bonded excited species, the main species in TFE. A similar case is presented for 4-N,N-dimethylamino cinnamaldehyde [29], which presents at room temperature a single fluorescence band in hydrocarbon solvents but distinct dual fluorescence in both polar aprotic and protic solvents. The main difference between the spectra in aprotic and protic solvents consists in a larger Stokes shift for the LW band in the latter solvents. This observation, together with a good linearity of the plot DnSt versus a HBD parameter, attests the presence of H-bonded species. Additional support for H-bond formation was obtained from experiments in EtOH:water mixtures; it was found that the addition of small water quantities produces significant modifications, the spectrum being similar to that in pure water. On the other hand, the emission spectra do not represent the mirror image of the absorption spectrum, pointing to a different geometry of the S1 state, i.e. to the presence of a possible TICT excited state. To support the assignment of the band to an H-bonded species, measurements at 77 K were performed, assuming in this condition a possible reduction in molecular flexibility. The fluorescence spectrum consists of a single band in hydrocarbon glasses, but of two bands in hydrocarbon glasses with traces of water or in EtOH glasses. At the same time, as compared with the room-temperature results, a hypsochromic shift of the LW band and a fluorescence enhancement were noticed. As the TICT states present phosphorescence at low temperature, the presence of the LW band in EtOH glasses supports the assumption concerning the role of H-bonds against TICT processes. Experimental measurements at different excitation wavelengths can indicate if there are more than one excited species in the system. When the shape of the spectrum depends on the excitation wavelength, multiple species are present. Several excitation wavelengths, lex, are usually selected, characteristic to the absorption of two or more possible species in the ground state, in order to obtain different populations of these species in the excited state, leading to different emission spectra. An example in which the use of several lex chosen towards the red edge of the absorption spectrum allows us to identify the excited species formed from coexistent H-bonded and non-H-bonded ground state species is reported in [149]. Thus, the fluorescence spectra of 40 -dialkylamino-3-hydroxyflavones obtained at different lex can provide a measure of the ground-state distribution of H-bonded and non-H-bonded species, making the compounds useful as sensors for detecting the H-bonding potential of the environment. The sequence of experimental determinations that can be used to evidence the formation of new species in the excited state and can also be applied to investigate H-bond formation, together with the most important information they provide, are summarized in Table 4.6.

4.5 Theoretical Modelling of the H-Bonds As already discussed, it is difficult to gain direct experimental insight into the processes that take place in the excited states of molecules. This is why many authors rely on quantum chemical data as a complementary

Solute–Solvent Hydrogen Bond Formation in the Excited State 105 Table 4.6 Experimental steps in the study of excited-state H-bonds Method

Information

Recording of absorption spectra

Characterization of the electronic transitions (n–p , p–p )

Recording of fluorescence spectra

Solvent influence on: emission maximum (maxima for dual fluorescence) bandwidth fluorescence quantum yield

* * *

Analysis of experimental data: Lippert–Mataga plot Reichardt–Dimroth plot Kamlet–Taft or Catalan models

Change in the dipole moment upon excitation Separation of non-specific and specific interactions Quantification of HBD and HBA effects

Deconvolution of the emission band

Number and position of the more or less overlapped bands

Recording the fluorescence spectrum using several excitation wavelengths, lex (at the red edge of the absorption spectrum)

Evidence of the number of excited species: lack of influence of lex ! single species spectrum modifications dependent on lex ! multiple species

Recording of the excitation spectrum, comparison with the absorption spectrum

Perfect match of the spectra ! same species in S1 as in S0 Differences in the two spectra ! new species in S1

Time-resolved experiments

* *

Parameters of fluorescence decay: lifetime amplitude of the exponential function(s) determination of kr, knr

* * *

Variable temperature determinations

Breaking of hydrogen bonds Determination of activation energy

Comparison of absorption, emission and excitation spectra

Evaluation of the energies of hydrogen bonds

source of information. Different models have been used to characterize the H-bond formation between the solvent and the solute in its excited state. In what follows we will span the recent literature concerning this aspect. It is worth mentioning that the theoretically studied molecules are usually model molecules, with a relatively small number of atoms, in order to use state-of-the-art quantum chemical methods capable of yielding accurate results. This section focuses on the application of theoretical models to excited-state solute–solvent H-bond formation, without a detailed description of the computational methods used, which can be found elsewhere [150, 151]. 4.5.1 Modelling the system For non-H-bond formatting solvents, where no specific interactions with the solute molecule can occur, a continuum model constitutes a reasonable idea with good correlation with the experiment [152, 153], although there are studies that show a better estimation of some spectral features by explicitly taking into account the solvent molecules [154]. The problem of an excited-state solute that forms H-bonds with the solvent molecules comprises a complex model system formed by the solute molecule in its first excited singlet and a number of accessible/relevant solvent molecules adequately oriented in order to form H-bonds, surrounded by the bulk solvent molecules, which are not implicated in the specific interactions. In quantum chemical terms, such a system has at least two components, the solute–solvent molecule(s) complex, which

106 Hydrogen Bonding and Transfer in the Excited State

treats the relevant solvent molecules explicitly, and the bulk solvent, usually a global component in which the solvent is treated implicitly, by means of some macroscopic parameters derived from experimental data, such as the dielectric constant, density, molar volume, etc. Another way of treating the solvent is by statistical methods. The accuracy of the results depends on the adequacy of the model and, of course, on its complexity. It was implied that quantitative correlation with the experimental data can be acquired if all the solvent molecules in the first solvation shell are treated explicitly by an ab initio quantum chemical method and the bulk solvent effect is taken into account within the framework of a continuum solvation model [155–159]. Continuum solvation models alone are scarcely used and account poorly for the specific interactions [155, 160], while the simplified model of a solute–solvent complex gives good results. No matter whether the polarizable continuum is part of the model, when the complex is modelled, it should be noted that the solute can either play the role of H-bond acceptor or act as a donor, or both, if it comprises different heteroatomic groups of H-bond acceptor and donor character, such as OH or NH2. On the other hand, the solvent itself, be it water or MeOH, to give but two examples, is prone to H-donor or acceptor behaviour, depending on its orientation relative to the solute molecule and/or the geometrical parameters of the latter. For a molecule that contains only a carbonyl group, an H-bond will be formed with one solvent molecule (Figure 4.11(a)). When a carbonyl and an amino group, i.e. both an H-donor and an acceptor, are present, and furthermore the amino group can form up to two H-bonds (as in Figure 4.11(c)), there are several solute–water complexes to be considered, from 1:1 to 1:3, in all possible combinations, but quantitative results are obtained for the 1:3 complex [161]. As regards the position of the solvent molecule relative to the plane of the solute, there are two possible geometries to be considered: in-plane and out-of-plane (see Section 4.2 for a thorough discussion). Note that, in Figures 4.11(a) and (b), in-plane H-bonds are formed, whereas in Figure 4.11(d) they are out-of-plane. Thus, choosing the number and orientation of solvent molecules implicated in the specific interactions is not straightforward and should be made on a case-by-case basis. Starting from these considerations, building of the theoretical model has to take some factors into account, such as the electronic structure of the solute and solvent molecules, the nature, number and topology of the H-bond acceptor or donor groups, the possible relative orientations of the solute and solvent molecules in order to find the optimal H-bond donor–acceptor ‘matches’, the choice of the quantum chemical method to treat the solute–explicit solvent subsystem and the choice of the continuum or statistical model for the bulk solvent. The final model should be a balance between a good correlation with the experimental available data and the computational resources needed. The literature comprises a multitude of solvation models, some of which consider the solvent explicitly, on a molecule-by-molecule basis, while others do it implicitly, as a polarizable continuum characterized by some global parameters. Here are some of the possibilities: QM treatment of a solute–solvent molecule(s) complex [162–166]; QM for the solute molecule with a continuum model for the solvent [155–157, 160]; QM for the solute which is placed in an electrostatic potential generated by the solvent [167, 168]; QM for a solute–solvent complex, combined with a continuum model for the bulk solvent [156, 157, 169, 170]; a QM/MM approach, in which the solute molecule or the solute–solvent complex is treated at the QM level and the bulk solvent within the framework of MM; MD for the solute–solvent complex [171] or in a solvent molecule box/droplet [172, 173]. Although the continuous solvation models take into account the polarity of the solvent, treating the solute–solvent specific interaction explicitly yields more complex and reliable results. Barone et al. [160] proved that the solvent shift for the FC state can be theoretically predicted with good accuracy for acetone in water only if a combined explicit/implicit model for the solvent is considered. If only the continuum model is used, the cyclohexane to water shift is 1184 cm1. When two water molecules are added to acetone in two different conformations, values of 2812 and 1550 cm1 are obtained, which are closer to the experimental value of 2000 cm1. It must be noted, however, that the explicit solvent molecules add to the system treated by a quantum chemical method, so their number is limited, especially for large solute molecules. The largest model includes the first solvation shell or all those molecules directly involved in the

Solute–Solvent Hydrogen Bond Formation in the Excited State 107 uracil 6

5 8

O

fluorenone

N

O

O

O

H O

2

3

H

H H

1N

4

H

H

O

H

H

O H

H

H O

CH3

(b)

(a) H3C

CF3

O H

H O H

H3C

N

O

O H

H

N

O CH3

in-plane H bonds

out-of-plane H bond

(c)

O

H

H O

H

H O H H

N

N N N H H

O

dibenzo[a,c]phenazine (d)

Figure 4.11 Models of some solute–solvent complexes: (a) 1:1 fluorenone–MeOH [163]; (b) 1:4 uracil–water [156]; (c) 1:3 C151–MeOH [161], the rectangle represents the plane of the solute; (d) 1:2 dibenzophenazine–water [219]. Solvent molecules in bold

specific interaction with the solute molecule [155–157, 170]. An example in which four water molecules are considered is given in Figure 4.11(b) for uracil [156]. The respective number and configuration of these molecules were assigned on the grounds of experimental data from techniques such as NMR, laser-induced fluorescence or theoretical MD results, which had indicated that no water molecules interact with C5 and C6 and that O7 and O8 interact with two and one water molecules respectively. 4.5.2 Computational methods The quantum mechanical part of the system is treated within the framework of methods usually used for excited states: the configuration interaction (CI) formalism [174], with single (CIS) or single and double

108 Hydrogen Bonding and Transfer in the Excited State

excitations (CISD); the complete active space self-consistent field (CASSCF) [175, 176] method, as such or combined either with a second-order perturbative correction, i.e. CASPT2 [177], or with a configuration interaction scheme, i.e. MRCI [178]; the time-dependent density functional theory (TDDFT) method [179]; coupled cluster-based methods (CC) [180]. As regards the bulk solvent, it can be mimicked by continuum solvation models as the polarizable continuum model (PCM) of Tomasi and Barone [152] and the conductorlike screening model (COSMO) [181]. A different approach is by statistical methods, such as MD or MC, which consider the bulk solvent explicitly, as a number (usually hundreds) of molecules, treated at a less accurate level of theory, MM [182]. The other is an intermediate between the continuum and MD models, which mimics the bulk solvent as an electrostatic potential generated by the atomic charges, i.e. the reference–interaction site model self-consistent field (RISM-SCF) method [183, 184]. The CIS method represents an acceptable compromise between the cost and accuracy for the excited-state geometry optimization, but no quantitative energies can be obtained. In recent years, the commonly used ab initio method for excited states has been the CASSCF method, for which solvation models are available [185]. CASPT2 and MRCI can treat transition energies with quantitative accuracy, and the first can be used to locate CIs [186–188]. The CC method was also found to give good results on transition energies for a large set of molecules, but, statistically speaking, the best theoretical estimate and closest to the experimental values remain the CASPT2 results [189]. In the last decade, the TDDFT method has gained tremendous importance in theoretical studies of excited states. It is an accurate and relatively fast method developed by Runge and Gross [179] and for which continuum models were adapted by the group of Barone [190]. As regards the excited states, both FC and relaxed geometries can be calculated, along with the above-mentioned molecular parameters. From the many functionals present in the literature, the PBE0 [191] functional gives the most reliable results for spectroscopic properties [192]. There are also documented shortcomings of TDDFT in the description of long-range interactions, charge transfer processes [193] and CI [194]. Nevertheless, being a relatively low-resourcedemanding method, it allowed the most complex models of the excited-state H-bond formation process to be constructed, comprising a large number of solvent molecules, up to the number found in the first solvation shell, which were treated quantum mechanically, to which the bulk solvent effect was added within the framework of the PCM model [156–158]. For thorough tests on the performance of different computational methods, the reader is referred to Refs [189], [195] and [196]. After the model and the method have been chosen, there are some parameters of interest for an H-bonded excited state that can be calculated: excitation energies, geometric parameters, atom charges, nature of the excited state, dipole moment, emission spectrum, oscillator strength, vibrational spectrum, H-bond length, H-bond energy. We will focus here only on the electronic and energetic properties of the excited singlet states, which can be correlated with the experimental electronic spectra. They characterize the excited-state photophysical properties or the H-bond and can be used to explain and complete the experimental data. There are two excited states that can be calculated: the FC and the fully relaxed S1 states. Although the latter is the one implied in the emission properties, there are authors that study only the FC state (especially for large systems) and draw conclusions on the emission properties, based on the different electron distribution in the excited state and its effect on the solute–solvent H-bond. Relaxation does not modify the electron distribution to a great extent, but the nature of the excited state does. Such results are not quantitative, but give an insight into the excited-state properties. Once the model and the method have been chosen, there are some computational procedures to be followed, depending on the complexity of the results needed. Let us suppose that the molecule is directly excited to the first excited singlet. Subsequent to excitation, the molecule arrives in the FC state, with the same geometry as the ground state. We can compute the vertical transition energy, denoted by Ea in Figure 4.12, at the optimized geometry of the ground state. It can be correlated with the absorption maximum. Another parameter is the

Solute–Solvent Hydrogen Bond Formation in the Excited State 109 S1 TS

E FC

S0/S1 CI

S1 Ea

Ef

S0

Figure 4.12 Schematic representation of the various electronic states important in the photophysical processes and of the radiative (solid arrows) and non-radiative (dashed arrows) deactivation pathways

emission transition energy, Ef, computed at the optimized geometry of the excited state and correlated with the emission maximum. In order to acquire some information on the decay processes, more features of the potential energy surface (PES) should be known. These include the critical points involved and the minimum energy path (MEP) that connects them. Apart from state minima, they are transition states (TS) on S1 and S1/S0 CIs, which are responsible for ultrafast non-radiative decay by IC to S0, as in Figure 4.12, where the nonradiative path is shown by dashed arrows. Various relative positions of the critical points on the PES are possible (e.g. S1 minimum on the path to the CI, no barrier, etc.). 4.5.3 H-bond-induced changes in the excited-state properties Excited-state optimization of the solute–solvent complex constitutes the simplest model for treating the H-bond explicitly. It is, in fact, the model most often used. As early as the 1970s, del Bene developed an MO theory of the H-bond in a series of some 20 articles (see, for instance, [197–199]), some being devoted to the solute–solvent H-bond in the excited state, but only as far as the FC state is concerned [198]. The author treated the solvent explicitly and calculated the vertical transition energies of some amides and their complexes with one or two water molecules at the CIS/STO-3G level. When the N–H group is the proton donor, the change in the transition energy is not significant. The blue-shift of the n–p band determined experimentally in protic solvents could be explained only by an H-bond formation in which the oxygen atom acts as a proton acceptor. A second H-bond formed with another water molecule, with nitrogen as a proton donor, determines an additive effect on the band shift. In this paper, the information on the excited-state H-bond is somehow indirect. As the blue-shift is very close in value to the calculated ground-state H-bond energy, the author concluded that its energy in the excited state is very small. Another argument for the weakening of the H-bond subsequent to excitation is the calculated atomic charge on the oxygen atom, which decreases owing to the electron redistribution during the n–p transition. We would like to conclude here that the net atomic charge on the atoms implicated and the bond energy are useful in characterising the specific interaction. On the other hand, when the solute molecule has multiple H-bond donor and/or acceptor centres, care must be taken as to the relative orientation of the solute and solvent molecule(s), in order to find the significant geometries. The presence of H-bonds can modify the nature of the excited state and its properties, as, for instance, the geometry. Dahiya et al. [75] considered the case of a diaminoanthraquinone for which the experimental Lippert–Mataga plots of Stokes shift, lifetime, fluorescence quantum yield and radiative and non-radiative constants indicated a different behaviour for polar solvents than for non-polar solvents. They modelled the polar solvent as eight water molecules located out-of-plane, in the vicinity of the two carbonyls and two amino

110 Hydrogen Bonding and Transfer in the Excited State

groups, and arranged two by two, above and below the plane of the solute. Eight H-bonds are thus formed. No details are presented as to why the exact number and relative geometry of the water molecules were chosen. By optimizing the solute in vacuo and in the presence of the water molecules, the authors find different planarity and charge distribution of the respective states, indicating the presence of a non-planar LE state in vacuo/nonpolar solvents and a planar ICT state in water/polar solvents. On the other hand, when the in-plane type of H-bond is considered [161] for C151 (see Figure 4.11 for the molecular structure), the geometry of the amino fragment in the excited state becomes more pyramidal owing to H-bond formation with MeOH (17.7 versus 5.7 in the isolated molecule for the angle that the NH2 fragment makes with that of the aromatic system). In the case of fluorenone in MeOH, where the carbonylic oxygen acts as an H-bond acceptor for one MeOH molecule, the length of the C¼O bond changes from 1.250 to 1.259 A [163]. The effect of solvent molecules on the geometry of the excited states can be more dramatic and even influence some photodissociation processes [171] or intramolecular proton transfer [200]. This is, for instance, the case for pyrrole in water, where N–H dissociation in the excited state is inhibited by the presence of water molecules, as experimental data indicate [201]. Franck and Damianos [171] studied pyrrole clusters formed with up to six water molecules by means of an MD method. They found theoretical proof that, owing to an electron transfer from pyrrole to the solvent (what is called a solvated electron) in the first excited state, the N–H distance is kept at a quasi-normal value of 1.1 A up to a simulation time of 0.4 ps, whereas for the isolated pyrrole the dissociation occurs at about 0.2 ps after the excitation. Moreover, the excited-state PES presents a minimum for the same 1.1 A N–H distance only in the presence of a water molecule. It seems that this stabilization constitutes a general mechanism, as was found for indole as well. As stated in the previous sections, for electron donor–acceptor molecular systems, an ICT excited state can be attained, which will be more polarized and by consequence stabilized in polar solvents. The extent of polarization can be monitored on the grounds of calculated dipole moments, but also by counting the location of frontier molecular orbitals implicated in the respective transition on different fragments of the molecule. We recall here C153 (see Figure 4.1), for which a difference between the dipole moment in excited and ground states of 6.0 D was found from the Lippert–Mataga plot. ATDDFT/PCM study [156] found a very good value of 6.1 D in CHX and 6.6 D in DMSO for the same property. Secondly, the first electronic transition has an essentially HOMO–LUMO character. While the HOMO is delocalized on the whole molecule, with significant contribution by the orbitals of the ‘central’ benzenic ring and of the nitrogen atom, the LUMO is mainly localized on the ‘quinone-like’ terminal ring, with a significant contribution of the p orbital of the carbonyl group. As a consequence, the S0 ! S1 transition has a partial intramolecular charge transfer character – from the nitrogen atom to the carbonyl group – and S1 has a partial zwitterionic character, with the nitrogen atom and the oxygen of the carbonyl group bearing formal positive and negative charges respectively. This results in a significant solvatochromic red-shift: 0.21 eV from CHX to DMSO, which correlates well with the experimental value. In the case of a 9-oxo-imidazopurine derivative containing a pyridine fragment as well (see Figure 4.13), a red-shift of the emission band was observed, along with a decrease in the fluorescence quantum yield and lifetime in alcohols [202]. In the meantime, if a phenyl replaces the pyridine, there are no significant changes in alcoholic media. Theoretical studies by means of the TDDFT method revealed that HOMO is located on the imidazopurine fragment, which is the electron donor, while LUMO is located mostly on the pyridine ring, the acceptor (Figure 4.13). That determines an ICT character for the first excited singlet, which is further proved by the dipole moment difference between the ground and excited states, Dm ¼ 15.2 D (in good correlation with the 11.6 D value found experimentally). The solute–solvent specific interaction was modelled as a 1:1 complex with one MeOH molecule located near the C¼O, N1, N4 and the pyridine N respectively. The bulk solvent effect was considered within the conductor-like PCM [203] model. By calculating the energy needed for such an H-bonded complex to be formed, both in the ground and excited state, the authors found that the pyridine N HO bond is the most stable and the only one that stabilizes in the excited state. This is not surprising

Solute–Solvent Hydrogen Bond Formation in the Excited State 111 O 1

N

9

N N

3

N H

5

N H

N4

O

O

N

N

N

N

N

N N H

N H

N H

N

N H

N

LUMO

HOMO

Figure 4.13 Molecular structure and frontier orbital location for a 9-oxo-imidazopurine derivative. Adapted with permission from [202]. Copyright 2006 American Chemical Society

considering the purine-to-pyridine charge transfer, resulting in an increase in the electronic density for the pyridine N, thus increasing its H-bond acceptor ability. So the solute–solvent H-bonding interaction in the first excited singlet state is responsible for the fast radiationless decay rates determined in protic solvents. Moreover, the nitrogen atom of the pyridine moiety is involved in the interaction, explaining the different behaviour of the phenyl-substituted analogue. Let us now consider the case of a merocyanine dye [169] (see Figure 4.14), which possesses an indolenine fragment as electron D and a barbituric acid fragment as A, linked by an alternant hydrocarbon bridge. It can be proved on theoretical grounds that it is an atypical D–A molecule, in which the p–p HOMO–LUMO transition decreases the polarization of the molecule. In MeOH solution, the localization of the electron in the HOMO is shifted towards the acceptor fragment compared with benzene. Not only the electron in the HOMO but also the distribution of other electrons vary with solvent polarity; therefore, the dipole moment of the S0 state is substantially increased with increasing solvent polarity from benzene (19.78 D) to MeOH (34.63). Similarly, LUMO migrates to the donor part. Consequently, the dipole moment in S1 decreases, i.e. 17.27 D in benzene and 24.16 D in MeOH. The more prominent decrease in MeOH can explain the blue-shift of the emission band in protic solvents found experimentally (0.155 eV) and theoretically (0.091 eV). A different type of CT state, more seldom studied, is the intermolecular CT state between a solute and a solvent molecule. The PET process is assisted by solute–solvent H-bond formation and results in a severe

-

O

11

N 13 9 6

22

14 7

23

21

20

17

12

4

16

N

5 26

24

N+

18

2

3

8

O

25 19

1

Figure 4.14

Merocyanine dye

10

O

15

112 Hydrogen Bonding and Transfer in the Excited State

fluorescence quenching. This was proved in the case of an oxazine on the grounds of electron transition contributions to the excited states and MO location for the 1:2 solute–EtOH complex [165]. This trimer is first excited to the S2 state (LE in nature) owing to the zero oscillator strength of S1 and then passes to S1, an intermolecular CT state, corresponding to a transition between an occupied MO located on the EtOH and a vacant MO located on the oxazine molecule. This functions as a ‘donor state’ for the ET process. It is a state that corresponds only to the H-bonded complex, and cannot be found for the solute molecule, which means that the ET process is indeed assisted by the H-bond formation. Specific interaction with solvent molecules determines changes in the energy gap between electronic states and sometimes even state interchanges. This is correlated with the greater stabilization of a strong H-bonded state versus a weak or non-H-bonded state. This state interchange was experimentally suggested for many compounds that possess non-bonding electron pairs and first excited singlets of n–p character in non-polar solvents [30, 104, 107]. This was proved for uracil [156], for which S1 has an n–p character in the gas phase, but, owing to the strong interaction with the water molecules, the p–p state lowers in energy and in the FC region becomes S1. Considering both four water molecules directly linked to the solute (see Figure 4.11) and the bulk solvent effect, it is possible to account for the Stokes shift (0.6 versus 0.8 eV found experimentally), the fluorescence energy (31 357 versus the experimental value of 32 051 cm1) and the state interchange in uracil. Neither by considering up to six water molecules [204] nor by using a PCM model alone [156] was this state interchange found. As regards the excitation energy, the most dramatic effect of specific interaction with the solvent is for the n–p state. In this type of interaction, the dipole moment value plays an important role. Because for the n–p state the dipole moment is lower than in the ground state, there will be a blue-shift of S1 in protic solvents, whereas S2 will be lowered in energy by the interaction with the solvent [204]. Another case is that of the TICT-forming compounds [83, 196, 205, 206]. Here again, the charge transfer was proved by monitoring the location of the frontier orbitals and the dipole moments in the ground and excited states. In Figure 4.15 the HOMO and LUMO orbitals of methyl p-dimethylaminobenzoate at the planar and twisted geometry are presented [207]. The charge separation appears only for the twisted conformation, confirmed by the large dipole moment in the TICT state (12.43 D versus 6.16 D in the LE state and 4.00 D in S0). Although there are many papers that deal with solvent effects on these twisted excited states, the great majority only consider polar aprotic solvents such as ACN, and in an implicit manner in any case [113, 115]. They were capable of showing that the excited state has a minimum on the PES corresponding to the twisted conformation (D–A torsion dihedral of 90 ) and that the shape of the PES is modified in polar solvents owing to greater relative stabilization of this twisted state, characterized by D–A charge separation and a large dipole moment compared with the LE state. However, studies of the well-known TICT-forming compound dimethylaminobenzonitrile (DMABN) and its complex with MeOH [208] revealed that H-bond formation may be involved in the photophysics of these systems. Comparing the calculated bond lengths and energies in the ground and excited states, we can see that the LE and TICT states behave differently, although in both cases the complexes are of the in-plane type. While the H-bond in the LE state becomes slightly weaker than in the ground state (21.12 versus 22.76 kJ mol1 respectively), it strengthens in the TICT state to 29.48 kJ mol1. This means that the TICT state is relatively stabilized by the H-bond formation, and in the meantime that the IC non-radiative deactivation process between the TICT and ground state is thus favoured, in accordance with the experimental data for this and other compounds [208]. The spectral features of H-bonded systems can be explained on the grounds of relative H-bond strength in different electronic states. Zhao and coworkers [209] studied two related thiocarbonyl compounds, i.e. thiocoumarin (TC) and 4H-1-benzopyrane-4-thione (BPT). They proved that for TC the H-bond is weakened in all the excited states, whereas for BPT it is weakened, too, except for the S2 state. At the same time, in the calculated emission spectrum, a blue-shift relative to the isolated molecule was observed for all the electronic states, with a weaker H-bond compared with the ground state, i.e. S1, S2, T1 for TC and S1 and T1 for BPT,

Solute–Solvent Hydrogen Bond Formation in the Excited State 113

N

N

O

O

O

O

LUMO

N

O

N

O

O

O

HOMO

Figure 4.15 Schematic representation of the frontier molecular orbitals for p-dimethylaminobenzoate: planar conformation (left), twisted conformation (right). Adapted with permission from [207]. Copyright Elsevier

except for the S2 state of BPT, for which the shift is to the red edge of the spectrum. Hence, a strengthening in the H-bond upon excitation determines a lowering of the excitation energy and thus a spectral red-shift, while H-bond weakening induces an increase in the excitation energy and thus a spectral blue-shift. Moreover, the energy gap between different electronic states with different H-bond strengths depends on the relative H-bond energy, so it was inferred that both photophysical and photochemical processes for this type of compound can be tuned by the intermolecular H-bond formation. The presence of solvent molecules can change both the photochemistry and the photophysics of the excited state. This can be best evidenced by calculating other critical points on the excited-state PES besides the minimum, as, for instance, TS. A somewhat exotic feature of excited states, but a feature that has gained increasing importance in elucidating their dynamics, is localization of CIs [186, 210, 211]. They are considered to provide an ultrafast non-radiative decay path to the ground state. Quite recently, conical intersections for solute–water complexes started to be studied [155–157, 212]. They can give important information on the mechanism of excited-state deactivation by non-radiative processes, in conjunction with

114 Hydrogen Bonding and Transfer in the Excited State

the calculation of the MEP followed by the molecule starting with the FC state, passing through the S1 minimum, a TS (if this is the case) and then deactivating to S0 through the CI. Sobolewski and Domcke [213] discussed generic mechanisms of excited-state deactivation via hydrogen atom dynamics in some model systems of isolated aromatic chromophores (indole), their complexes with amphoteric solvent molecules (indole–ammonia, pyridine–ammonia), H-bonded pairs of aromatic chromophores (indole–pyridine) and bifunctional intramolecularly H-bonded aromatic systems. The paper shows that solute–solvent H-bond formation in an ICT electronic state facilitates access to the S1/S0 CI, which determines ultrafast decay kinetics of the excited state. This mechanism may be implied in the concerted decrease in the fluorescence quantum yield and lifetime found experimentally in protic solvents for a large pool of CT states. 2-Aminopurine, an adenine analogue, has a great importance as a fluorescence probe in DNA. Unlike adenine, it presents high emission efficiency, with a fluorescence quantum yield varying from 0.01 in non-polar CHX up to 0.68 in water. Here also, the S0/S1 CI is responsible for its photophysical properties [108, 214]. To prove that, it is necessary to build the MEP in the first excited state, pp in character, which is directly accessed by the molecule subsequent to excitation [214]. Starting from the FC sate, the relaxation process to S1 minimum is barrierless, but, in order to reach the S0/S1 CI, the molecule passes through a TS, as in Figure 4.16. The calculated activation energy at the CASPT2 level in the gas phase is 2.4 kcal mol1 [108], whereas in aqueous solution, within the framework of the QM/MC method, it is 5.5 kcal mol1, over 3 kcal mol1 higher. So the solvent effect on the various critical points on the first excited singlet surface, implicated in the photophysical processes, resides in a greater stabilization of the S1 minimum compared with the TS, which leads to an increase in the energy barrier. In this way, reaching the CI is unfavoured by solvation, and thus the fluorescence quantum yield increases. A similar theoretical treatment can be applied to uracil and some of its derivatives [156, 157, 170, 212], which have a different experimental behaviour, i.e. short lifetimes in water and protic solvents, which were correlated with the H-bond formation ability of the solvent. The difference between uracil and 2-aminopurine is that S1 has an n–p character, so the initial excitation takes place to the bright S2 state, which has to be computed as well. The models used to study the effect of H-bond formation were: (1) different 1:1 complexes with water, for all possible types of H bond in uracil, at the MRCI level [212]; (2) 1:4 uracil–water complex embedded in a continuum, at the TDDFT level [156, 157, 170] (see Figure 4.11). The less demanding TDDFT method allowed for a more complex model to be used and for entire 1D and 2D regions of the hypersurfaces for the electronic states involved to be calculated. The results of the two models are in good correlation with one another and with experimental data. Here, the IC process is favoured by the presence of water owing to the TS

E

5.5

2.4

S0/S1 CI MIN

geometry change

Figure 4.16 Schematic representation of the relative energy of the S1 critical points in gas phase (dotted line) and in aqueous solution (continuous line) for 2-aminopurine. Adapted with permission from [108]. Copyright Elsevier. Activation energy in kcal mol1

Solute–Solvent Hydrogen Bond Formation in the Excited State 115

decrease in the barrier to the TS that links the S1 minimum with the S1/S0 CI. The second model confirmed the state interchange in the FC region in water compared with the gas phase, previously stated on the grounds of vertical transition energy and emission transition energy [156]. The PES shape is modified by specific interaction with water. Whereas uracil in the gas phase and ACN is excited to the p–p state, which crosses the np state in a region far from the FC state and then decays to S0, in water the crossing is in the FC region, as the p–p state is stabilized in water by solvation and the gap between the two states is very small. Nonetheless, of equally great importance in tuning the photophysical properties by solvation are the effects of substituents, in this case the substituent in position 5 [170]. Pyramidalization and out-of-plane motion of C5 is the predominant vibrational mode in passing from the S1 minimum to the CI, and in this way inductive and/or hyperconjugative effects of the substituent determine the barrier energy on this path. 4.5.4 Characterizing the H-bond The calculated molecular parameters that can give information on the H-bond are the relative solute–solvent geometry, the bond length, the charge distribution and the bond energy. Starting from the model(s) chosen, an optimization of the respective complex should be made in order to find the minimum conformations. We have already mentioned the possible types of H-bond theoretically found: (1) in-plane with the solute molecule, what would be called the ‘classical’, quasi-linear type; (2) out-of-plane. After optimization, for the in-plane geometry there are two conformations that correspond to a minimum: the all-planar complex and/or the complex in which the O–H fragment is coplanar to the solute molecule, but the rest of the solvent molecule is out-of-plane (see Figure 4.11). The two of them are quasi-isoenergetic, the most stable being the second one, as, for instance, in the case of fluorenone, where the energy difference is 1.7 kJ mol1 in the excited state, with an even larger difference in the ground state, 15.1 kJ mol1 [120]. The out-of-plane complex type predicted from experimental data [5, 6] was found on the grounds of theoretical results. This is the case of the indole–(H2O)2 complex. Fang [162] studied the ground- and excitedstate complexes of indole with one or two water molecules. In the 1:1 complex, the N–H group of indole is the H-bond donor, while the oxygen atom from the water molecule acts as the acceptor. It is an in-plane H-bond. In the 1:2 complex, besides this bond, a second bond is formed between the H-atom of a second water molecule and the p-electron cloud of the benzene ring in indole. Another case of an out-of-plane complex found to be a minimum in the first excited state is with thiocarbonyls such as TC and BPT [209]. Although in-plane conformers in which, again, only the OH group of MeOH is coplanar to the solute molecule are found in other electronic states (S0, S2, T1), S1 is characterized by a non-planar solute–solvent complex. The most important geometrical parameter is the length of the H-bond. The H-bond strength increases with decreasing bond length. It also increases with energy for the H-bond formation. This can be calculated by means of the equation Eb ðS1 Þ ¼ Ecomplex ðS1 ÞðEsolute ðS1 Þ þ Esolvent ðS0 ÞÞ

ð4:20Þ

where Eb is the energy required for H-bond formation, S1 stands for the first excited state and S0 for the ground state, Ecomplex(S1) is the calculated total energy of the optimized S1 state of the complex, Esolute(S1) is the energy of the optimized excited state of the isolated solute molecule and Esolvent(S0) represents the total energy of the equilibrium conformation of the isolated solvent molecule in its ground state. This formula refers to the case of locally excited states of the complex, where the solvent electronic structure is not perturbed by the excitation, which is entirely located on the solute. Whether this hypothesis is real or not can be decided on the grounds of the frontier molecular orbitals, which should both be located entirely on the solute and have no participation from the solvent. See Ref. [163] for a useful discussion. An alternative method is to calculate Eb as the dissociation energy of the complex [8, 195].

116 Hydrogen Bonding and Transfer in the Excited State Table 4.7 Calculated molecular parameters for the H-bond in the excited state. Ground-state values in parentheses

Compound/solvent

Stoichiometry

H-bond D/A groupa

H-bond typeb

Distance (A)

Energy (kJ mol1)

Method

Ref.

Formamide/water

1:1

C¼O

1

— (—)

— (26.94)

CIS

[198]

Fluorenone/MeOH

1:1

C¼O

1

1.903 (1.909) 1.802 (1.906)

42.3 (38.3) 42.62 (27.85)

CIS TDDFT

[120] [163]

Fluorenone/TFE

1:1

C¼O

1

1.803 (1.909)

— (47.5)

CIS

[120]

Dibenzophenazine/ water

1:1 1:2

N¼C (ring)

1 2

2.05 (2.19) 2.05 (2.21)

26.94 (20.29) 52.97 (39.20)

CASSCF

[217]

C151/MeOH

1:3

C¼O NH2 NH2 (D)

1 2 1

1.888 (1.908) 2.060 (2.017) 1.834 (1.925)

32.68 (23.95) 9.89 (11.17) 36.67 (27.34)

TDDFT

[161]

Indole/water

1:1 1:2

N–H (D) p-cloud

1 2

2.077 (2.087) 2.601 (2.719)

— (—) — (—)

CASSCF

[162]

TC/MeOH

1:1

C¼S

2

2.849 (2.442)

14.6 (24.35)

TDDFT

[210]

Cyano-Nmethylindoline/ water

1:1 1:1

CN N (ring)

1 2

2.24 (2.23) —c (2.16)

12.6 (12.5) —c (5.6)

CIS

[89]

a

H-bond acceptor if not otherwise (D) stated. 1 ¼ in-plane; 2 ¼ out-of-plane. c Unstable. b

H-bond distances and energies are presented in Table 4.7. In general, based on comparing the values of bond length and energy in ground and excited states, the most recent literature states that in-plane H-bonds with different types of acceptor or donor group strengthen upon excitation [120, 161, 163, 216]. In this way, the equilibrium in the excited state is shifted towards formation of the H-bonded species. The explanation of this strengthening is that the electronic distribution changes in the excited state, usually shifting towards the H-bond acceptor. So the charge density on the donor or acceptor heteroatom is very useful as a parameter to characterize the H-bond. Even if the most widely used solvents are water and MeOH, there are papers that consider some other solvents such as phenol [163] or TFE [120]. They deal with a coumarin derivative, C102, and fluorenone, respectively, both having a carbonyl as H-bond acceptor in the molecule. The type of conformation is in-plane, with the solvent molecule, other than the OH group, non-coplanar to the solute molecule. In both cases, again, the H-bond is strengthened in the excited state. So it could be concluded that this may be a general hypothesis for the in-plane solute–solvent H-bond complexes, at least as far as the carbonyl functions as the H-bond acceptor are concerned. Furthermore, it was demonstrated that the H-bond in complexes with TFE is stronger than that in the complex with MeOH, as found experimentally [120]. Regarding the out-of-plane complex, literature data indicate an opposite trend, the H-bond being weakened upon excitation, as can be seen in Table 4.7 from the values of bond length in the ground and excited states.

Solute–Solvent Hydrogen Bond Formation in the Excited State 117



Compare 2.060 versus 2.017 A for C151 in MeOH and 2.601 versus 2.719 A for indole in water, to take but two examples. Moreover, the H-bond energy is generally lowered in S1 compared with the ground state. For instance, it decreases from a value of 24.35 to 14.65 kJ mol1 upon excitation for TC in MeOH. Information on the bond length can be acquired by QM/MD combined studies in a different way. It is characterized by the distribution function of the H atom around the acceptor centre, as a function of the donorto-hydrogen distance. A pyrimidine in water MCSCF/MD study [218], which considered the pyrimidine molecule at the CASSCF level and more than 200 solvent molecules treated by MD, revealed that the N H distance is 1.9–2.0 A. Furthermore, it was possible to illustrate the polarization of the solute molecule by the surrounding medium, by way of calculating the induced dipole moment during the MD run, i.e. about 1 D. One of the advantages of this method is that it takes into account the statistically averaged possible conformations. This is also the case for another statistical method, QM/MC, which gives an average O H distance of 1.9–2.0 A for the acetone–450 water molecule system [219]. In the case of the 1,2,3-triazine–water system, all three N atoms are implicated in H-bonds, both in the ground and excited state [220, 221]. The distribution function of the H atom around all three N atoms presents a sharp maximum at around 1.8 and 2.0 A in the ground state. In the excited state there is a shallow peak for the N2–H distance of 2.6 A, meaning a very weak H-bond and a sharp peak for the H-bond located at the symmetric N1, N3 atoms at 1.8 A, indicating a strong, well-structured H bond. It is thus proved that both methods can describe H-bond formation in the ground and excited states of solute molecules in liquid media. A second geometrical issue found in the literature is that the in-plane bond length is smaller than the out-ofplane bond length. Considering the indole–water complexes again, it can be seen from Table 4.7 that the in-plane H-bond is 2.077 A in length, and the out-of-plane H-bond 2.601 A in length. The trend is maintained also for C151 and TC in MeOH. For cyano-N-methylindoline (CMI) in water and MeOH, both conformations are possible. Moreover, there are also in-plane stable complexes with non-linear geometry (see Ref. [89]), but weaker than the linear geometry both in the ground and excited state. Only the in-plane complexes are stable in the excited state, the out-of-plane bond being broken. On the basis of the respective H-bond energies, as well as bond length, the inplane conformation is obviously more stable for all the compounds. This points to the stronger interaction of the H atom with an H-bond acceptor on a quasi-linear geometry than with an n- or p-cloud on a nonlinear geometry. It was inferred that S1 ! S0 IC is favoured by coupling to the vibrational modes of the H-bond. Calculated values of different possible vibrations for CMI are relatively low (20–100 cm1) [89], thus increasing the density of states and consequently the probability of vibronic coupling between the ground and excited states. Another argument is the comparison with N-methylindoline, for which ultrafast non-radiative decay was not observed experimentally. The quantum chemical calculations show that the only H-bond possible is the weak out-of-plane one, which is broken in the excited state. Therefore, a direct correlation can be made between the formation of a stable H-bond in the excited state (CN OH in this case) and an effective IC deactivation pathway. To summarize, theoretical computations can explain experimental data and many times confirm hypotheses made by experimentalists. It is possible nowadays thoroughly to characterize various electronic states of molecules and the regions of the PES implied in the photophysical and photochemical processes, even for complex systems such as a solvated molecule involved in specific interactions with the solvent.

4.6 Conclusions A survey of literature data on excited-state H-bonds allows us to draw some general conclusions. As regards experimental data, two aspects can characterize the continuously increasing number of papers devoted to this subject. One aspect refers to H-bond studies performed on newly synthesized compounds. These papers contain steady-state fluorescence or time-resolved experiments with the aim of characterizing the photochemical

118 Hydrogen Bonding and Transfer in the Excited State Experimental part Solvent selection Recording of the absorption, excitation and emission spectra

Analysis of the data Identification of the H bonds; Separation of nonspecific and specific interaction

Molecular modelling Calculations of the optimized S0, S1 structures of the isolated molecule Analysis of the MO features Calculations for the electronic spectra Calculations of solute–solvent clusters Characterization of the H bonded species

Correlation of experimental and theoretical results

Change in the experimental conditions Change in the theoretical part

Figure 4.17 interaction

Main steps in designing a combined experimental and theoretical study on excited-state specific

properties and to evidence the presence of excited-state H-bonds. The main data used for assuming the presence of H-bonds are the spectral modifications in protic solvents as compared with non-polar or aprotic solvents, bathochromic shift of the emission band, changes in the lifetime and quantum yields. One of the problems in obtaining reliable experiments is a good design of the experiment, comprising the selection of the probes, solvents or mixtures of solvents, experimental excitation and emission conditions and temperature, in order to distinguish the H-bonding process from all the other possible processes occurring in the excited states. The other aspect is correlated with more precise studies on the already well-described fluorescence probes, aiming to obtain a deeper understanding of excited-state behaviour. Generally, the new studies are correlated with the dynamical aspects of the excited-state solvation process or with a more quantitative analysis of the excited-state kinetics. An increasing interest is being shown in the correlation of experimental data with molecular modelling results. In the last part of our chapter, a description is given of how computational complex methods that combine a QM treatment for the solute and the solvent molecules involved in specific interactions with continuum or statistical models to address the bulk solvent effect are applied to H-bond formation in different electronic states of solvated molecules. These theoretical calculations aim to obtain characterization of the excited-state geometry and nature of the solute–solvent ensemble and the H-bond energy, along with the energetic of the electronic states, which provide an insight into the photophysical processes that take place subsequent to excitation. As a general conclusion, the main steps reflecting the interplay between the experimental and theoretical approaches in the study of excited-state HB interactions are displayed in Figure 4.17.

Solute–Solvent Hydrogen Bond Formation in the Excited State 119

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5 Electronic-Excited-State Structures and Properties of Hydrated DNA Bases and Base Pairs Manoj K. Shukla and Jerzy Leszczynski NSF CREST Interdisciplinary Nanotoxicity Center, Department of Chemistry and Biochemistry, Jackson State University Jackson, Mississippi 39217, USA

5.1 Introduction Computational chemistry methods have been established as an attractive and reliable alternative to the costly and time-consuming experiments used to determine structures, properties and reactivities of molecular species. Although the modelling of the exact environmental conditions at the high theoretical level is still very expensive and may not be possible in several cases, computational methods are capable of providing reliable predictions in many ways, which can be useful to experimentalists and to the general scientific community [1, 2]. Theoretical methods are especially attractive in areas such as the determination of excitedstate geometries of complex molecules, where experiments are not yet possible. Another example of the role of computational studies is the quantitative prediction of amino group pyramidalization of nucleic acid bases. The neutron diffraction study of adenine crystals has suggested a non-planar amino group [3]. Using the ab initio quantum chemical method, the amino groups of bases were suggested to be non-planar more than a decade ago [4, 5]. However, only recently, an experimental method has verified such non-planarity in the gas phase of adenine and cytosine, using the measurement of the vibrational transition moment angles [6]. We strongly believe that theoretical and experimental methods are complementary to each other, and a judicious decision is needed for their efficient applications to determine the structures and properties of different systems. Hydrogen bonding is ubiquitous. It plays an important role in different aspects of all living organisms. Hydrogen bonds can be classified as weak, moderate and strong, depending upon their energies [7]. Hydrogen

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

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126 Hydrogen Bonding and Transfer in the Excited State

bonds can also express unique features that allow them to be classified as blue-shifted hydrogen bonds and dihydrogen bonds [8–11]. In blue-shifted hydrogen bonds, the stretching vibration frequencies associated with CH or NH vibrations are blue-shifted with respect to the corresponding isolated species after hydrogen bond formation. In dihydrogen bonds the two hydrogen atoms are hydrogen bonded to each other. The hydrogen bonds formed between the complementary purine (adenine and guanine) and pyrimidine (thymine and cytosine) bases (see Figure 5.1 for the structures of bases and base pairs) are responsible for the double helical structure of deoxyribonucleic acid (DNA). Importantly, the specific sequences of these hydrogen-bonding patterns in DNA define the genetic code, the carrier of heredity. Alteration in DNA structure may lead to mutation by producing a permanent change in the genetic code. The exact cause of mutation is not known, but several factors, e.g. environment, irradiation, etc., may contribute towards such phenomena. The formation of pyrimidine dimers between adjacent thymine bases on the same strand results in the most common UVinduced DNA damage. Based on a femtosecond time-resolved IR spectroscopic study of thymine oligodeoxynucleotide (dT)18 and thymidine 50 -monophosphate (TMP), Kohler and coworkers [12] have recently shown that thymine dimerization is an ultrafast process that usually occurs in the femtosecond timescale, where the formation of photodimer from the initially excited singlet pp state of thymine is barrierless. However, proper geometrical orientation of stacked thymine pairs is the necessary requirement for the formation of the photodimer. H62

H61

O6

N6

N7

N1

C5

C2

C4

N1 C8 H8

H2

C6

H1

C6

C8

H21 C2

C4

N2

N9

H8 N9

N3

N3 H9

N7

C5

H22

H9

Adenine (A)

Guanine (G) H42

H41

O4

N4

C4

H3 N3

R5 C4

C5 N3

C2 O2

C6

H6

C5

C2

N1 O2

H5

C6 N1

H6

H1 H1

Thymine (T)/Uracil (U)

Cytosine (C)

Adenine-Thymine (Uracil) Base Pair

Guanine-Cytosine Base Pair

R5’

Figure 5.1 Structures and atomic numbering schemes of nucleic acid bases and Watson–Crick base pairs. In thymine, R5/R50 ¼ CH3, and in uracil, R5/R50 ¼ H

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Nucleic acid bases in DNA exist in their respective canonical tautomeric forms. However, minor tautomers, when formed for some reason, will induce mispairing among bases, and this can cause mutation if left unrepaired. In nucleic acids, the number of minor tautomers is reduced owing to the presence of sugar at the N9 and N1 sites of purines and pyrimidines respectively. However, the possibilities of keto–enol and amino–imino tautomerism remain there. The tautomeric analysis of nucleic acid bases, especially guanine, is further complicated by recent experimental and theoretical results where comparatively less stable imino tautomers of guanine have been assigned in the supersonic jet-cooled spectra [13–22]. Water is one of the most important ingredients of our planet and an essential part of our life. In vivo, DNA is heavily hydrated, and the percentage relative humidity determines the degree of hydration. Further, it plays a prominent role in determining the three-dimensional structures and functions of nucleic acids [23, 24]. Nuclear magnetic resonance (NMR) investigations have suggested that the relative humidity and temperature control the movement of the backbone as well as the bases in nucleic acids [25–27]. Schneider et al. [28] have performed an extensive analysis of crystallographic data of hydration of DNA bases. They found that, in general, the degree of hydration depends upon the types of DNA, the location of the minor and major grooves and the bases. Some minor tautomers of nucleic acid bases were found to become more stable under hydration [29–32]. Further, water molecules were also found to increase stacking interaction in base pairs [33– 35]. Our group has shown that the presence of a water molecule in the proton transfer reaction path of the keto–enol tautomerization reaction of nucleic acid bases and related molecules reduces the proton transfer barrier height significantly [29, 36–39]. The transition states of such water-assisted proton transfer reactions have been found to have a zwitterionic structure [37–39]. Transfer of a proton corresponding to the keto–enol tautomerization of such hydrated species is characterized by the collective process. The investigation of polyhydration of adenine, uracil, thymine and cytosine with 11–16 water molecules in the ground state shows significant geometrical deformation of bases [40–42]. Korter et al. [43] have performed an experimental investigation on the interaction of a water molecule with the N–H site of indole, which could be considered as a simplified model of a DNA base, in the ground and the lowest singlet excited state, and found that, consequent to electronic excitation, the position and orientation of the water molecule were changed with respect to the ground state. Nucleic acid bases have an ultrashort lifetime, of the order of subpicoseconds [44]. Although they absorb ultraviolet (UV) radiation efficiently, nucleic acid bases have a very poor radiative quantum yield. Most of the absorbed energy is dissipated in the form of heat mainly through the conical intersection of the groundand excited-state potential energy surfaces [44–48]. Several theoretical calculations supplemented with experimental data have suggested that structural non-planarity in bases facilitates the conical intersection among different electronic states, thus providing paths for efficient non-radiative deactivation [44, 45]. Recently, we have found that the electronic singlet excited-state geometry of guanine significantly depends on the mode and degree of hydration [49, 50]. It was also concluded that excited-state dynamics of guanine will also depend upon the mode and degree of hydration. In another recent combined experimental and theoretical study, the dynamics of isolated thymine was revealed to be different from that of the hydrated form [51]. Recently, several review articles have described the excited-state structures, properties and mechanism of non-radiative deactivations in nucleic acid fragments using both experimental and theoretical techniques [44–46, 52–57]. An excellent collection of lucid descriptions of excited-state phenomena in nucleic acid fragments, including recent developments of theoretical methods to study the excited states of a variety of molecules, can be found in a recent book entitled Radiation Induced Molecular Phenomena in Nucleic Acids [1]. This chapter describes the recent investigation of excited-state structures of nucleic acid fragments under a hydrogen-bonding environment. Brief information on ground- and excited-state properties of isolated nucleic acid bases is also presented.

128 Hydrogen Bonding and Transfer in the Excited State

5.2 Ground-State Structures of Nucleic Acid Bases and Base Pairs Nucleic acid bases can exist in various tautomeric forms, and such tautomeric distribution strongly depends upon the environment. In nucleosides and nucleotides, the N9 $ N7 prototropic tautomerism in purines (adenine and guanine) and the N1 $ N3 prototropic tautomerism in pyrimidine (cytosine) is blocked. However, bases can still show keto–enol and amino–imino tautomerism. The scope of the present review is not to discuss the ground-state properties of DNA fragments in detail. Therefore, ground-state properties are only very briefly discussed here. For detailed discussion of ground-state structures and properties of nucleic acid fragments, the reader is referred to review articles in this area of research [2, 8, 29, 58, 59]. Theoretically, it has been found that the six-membered ring of NABs has significantly large conformational flexibility [60, 61]. It is well known that the amino groups of NABs are non-planar, and such pyramidalization occurs owing to the partial sp3 hybridization of the amino nitrogen. Among NABs, guanine exhibits the largest degree of pyramidalization [4, 29, 58]. Dong and Miller [6] have indicated experimentally the pyramidal nature of the amino group in adenine and cytosine in the gas phase. Recent experimental and theoretical analysis has really complicated our understanding of tautomeric distributions of guanine in particular, and therefore nucleic acid bases in general, under different environmental conditions [13–22]. These complications arise owing to the presence of relatively less stable imino tautomers of guanine under the supersonic jet-cooled condition [16]. Initially, based upon the results of resonance-enhanced multiphoton ionization spectroscopy, the existence of the keto-N9H, keto-N7H, enolN9H and enol-N7H tautomeric forms of guanine has been suggested [13, 14]. However, Choi and Miller [15] have assigned the keto-N9H, keto-N7H and cis and trans forms of the enol-N9H tautomer of guanine trapped in helium droplets. This assignment was based on the comparison of the infrared (IR) data of guanine trapped in helium droplets with theoretically computed vibrational frequencies of guanine tautomers at the MP2 level using the 6-311 þþ G(d, p) and aug-cc-pVDZ basis sets. Mons et al. [16] reassigned their previous R2PI data and found that the enol-N9H-trans, enol-N7H and two rotamers of the keto-N7H-imino tautomers of guanine are present in the supersonic jet-cooled beam. The imino tautomers of guanine are about 8.0 kcal mol1 less stable than the most stable keto-N7H tautomer in the gas phase at the MP2/6-311 þþ G(d, p)//B3LYP/6311 þþ G(d, p) level [17]. Recent experimental and theoretical investigations suggested the presence of three tautomers, namely N9H, N7H and N3H, of adenine in the dimethylsulfoxide solution, with N9H being the major tautomer, while N7H and N3H are the minor tautomeric forms [62]. However, in earlier experimental investigations, the presence of only N9H and N7H tautomers of adenine has been suggested [63–65]. The N9H form was the major tautomer, while the relative population of the minor N7H form was found to be environmentally dependent [63–65]. Recent theoretical study shows that the N7H and N3H tautomers of adenine have a similar stability, and the N9H tautomer represents the global minimum [66, 67]. Cytosine is the only pyrimidine base that has several tautomeric forms. For example, it is present as a mixture of amino-oxo (N1H) and amino-hydroxy forms in the argon and nitrogen matrices, but tautomeric equilibrium is shifted towards the latter form [68, 69]. Microwave spectroscopic investigation has shown the presence of three (amino-oxo, imino-oxo and amino-hydroxy) tautomers of cytosine [70]. However, in water solution only amino-oxo forms (N1H and N3H) have been revealed [71]. Nir et al. [72] have shown the existence of keto and enol tautomers of jet-cooled cytosine. The imino-oxo tautomer has been suggested for 1-methyl and 5-methylcytosine in a matrix isolation study [73, 74]. However, in the crystal environment, only the amino-oxo-N1H form is present [75]. Theoretically, various methods up to the CCSD(T) level of theory have been used to determine the relative stability among different tautomers of cytosine [31, 76]. Thymine and uracil exist mainly in the oxo-tautomeric form [29, 45, 58, 77]. The Watson–Crick (WC) base pairs are found to be planar at the HF and DFT levels, including the amino group [29, 58, 78–81]. However, amino groups of the WC AT and GC base pairs are found to be pyramidal at the electron-correlated MP2 level [80, 82]. Interestingly, the amino group of the WC AT base pair at the MP2

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level was found to be non-planar only at the lower basis set, but planar at the larger basis set. The non-planarity in the GC base pair has been suggested to increase the stacking interaction and may increase the stability of the helix [82]. The geometries of some reverse Watson–Crick (RWC), Hoogsteen (H) and reverse Hoogsteen (RH) base pairs have also been computed to be non-planar [5, 29, 58]. In addition, the energetics of hydrogenbonded and stacked base pairs were studied up to the CCSD(T) level of theory [83–87]. Several theoretical and experimental investigations were also performed to determine the electron affinity, ionization potential and protonation and deprotonation properties of purine and pyrimidine bases and base pairs [88–103]. Based on the experimental and theoretical data, the adiabatic valence electron affinity for pyrimidine bases has been estimated to be in the range 0–0.2 eV, while those for adenine and guanine amount to about 0.35 and 0.75 eV respectively [88]. In general, purines have lower and pyrimidines have higher ionization potentials [89–94]. Guanine has the lowest ionization potential among NABs. Therefore, guanine is the most susceptible among NABs to one electron oxidation under irradiation. Podolyan et al. [95] computed proton affinities of all nucleic acid bases up to the MP4(SDTQ) level and found that the computed proton affinities are very close to the experimental data. Schaefer and coworkers [101] have recently investigated the structures and properties of deprotonated GC base pairs. Kumar et al. [102, 103] have recently investigated the adiabatic electron affinities of GC, AT and hypoxanthine–cytosine base pairs at the DFT level. These authors have found that polyhydration leads to significant increase in the electron affinity of the AT base pair.

5.3 Excited-State Structures of Nucleic Acid Bases 5.3.1 Electronic transitions in nucleic acid bases In general, the absorption bands of purines near 260 and 200 nm are composed of two transitions whose transition dipole moments are non-parallel [48, 104, 105]. Usually, guanine shows five transitions near 4.51, 4.96, 5.51, 6.08 and 6.59 eV (275, 250, 225, 204 and 188 nm) in the UV region [47, 48, 106–108]. Clark [107] has tentatively suggested the existence of the np transitions near 5.21, 6.32 and 7.08 eV (238, 196, and 175 nm) in guanine. The spectral origin of the first electronic singlet pp transition of guanine tautomers is measured using R2PI spectroscopy in the laser-desorbed jet-cooled beam [13, 14, 16]. The reassigned R2PI spectra [16] have suggested the spectral origin of enol-N9H-trans, keto-N7H-imino-cis, keto-N7H-imino and enol-N7H tautomers [17] at 4.31, 4.20, 4.12 and 4.07 eV respectively. Adenine also shows several electronic transitions in the UV region. A photoacoustic spectroscopic study has shown four electronic transitions in the region 180–300 nm [109]. The main absorption peak of adenine at 4.77 eV (260 nm) generally shows two components – the stronger one at 4.75 eV (261 nm) is short-axis polarized, while a weak shoulder near 4.64 eV (267 nm) is long-axis polarized [110]. The existence of np transitions near 5.08 and 6.08 eV was suggested in the crystal of 20 -deoxyadenosine [111] and near 5.38 eV in the stretched polymer film of 9-methyladenine [112]. Based upon an REMPI investigation, Kim et al. [113] have suggested that the first electronic transition of adenine with the spectral origin at 35 503 cm1 (281.7 nm, 4.40 eV) has an np character, while the second transition with the spectral origin at 36 108 cm1 (276.9 nm, 4.48 eV) has a pp character. However, the np assignment was not supported by Luhrs et al. [114], and these authors have speculated the involvement of some other tautomer of adenine. Luhrs et al. [114] have predicted that the spectral origins of the first pp transition of adenine and 9MA are located at 36 105 cm1 (277 nm, 4.48 eV) and 36 136 cm1 (276.7 nm, 4.48 eV) respectively, and these results are in accordance with the observation made by Kim et al. [113]. The R2PI study by Nir et al. [115] on laser-desorbed adenine has also provided similar results. The absorption spectrum of cytosine is generally solvent dependent and shows peaks or shoulders near 4.66, 5.39, 5.85 and 6.29 eV [48, 116–123]. There is significant experimental and theoretical evidence for the existence of an np transition near 5.3 eV (232 nm) in cytosine [45]. Zaloudek et al. [120] have suggested the existence of

130 Hydrogen Bonding and Transfer in the Excited State

another np transition near the 5.6 eV (220 nm). The spectral features of uracil and thymine are generally similar, and both bases have absorption bands near 4.77, 6.05 and 6.89 eV (260, 205 and 180 nm respectively) [44–48]. Several investigations have suggested the presence of an np transition within the first absorption envelope of uracil, thymine and their analogues [48, 109, 124, 125]. In the gas phase and in an aprotic solvent, the np state is the lowest, but in the protic environment it has higher energy than that of the pp state [48, 124, 125]. Transition moment directions in nucleic acid bases have also been assigned by Clark and other researchers in series of investigations [107, 112, 120, 126–128]. Various theoretical methods such as CASPT2/CASSCF [129–131], TDDFT [132–143], coupled cluster [144–147] and CIS [47, 138, 148–152] have been used to compute electronic transition energies of nucleic acid bases, base pairs and their stacked complexes. In one of the TDDFT calculations, several sets of diffuse functions were also used [139]. Computed transition energies were generally found to be in good agreement with the corresponding experimental data. Detailed analysis of experimental and theoretical electronic transitions of nucleic acid bases can be found in a recent review article [45]. Ritze et al. [153] have studied the effect of base stacking on the electronic transitions at the SAC-CI and RI-CC2 levels of theory by considering cytosine–cytosine and thymine–thymine stacked dimers in the A- and B-DNA configuration. It was predicted that the spectral splitting in thymine–thymine stacked dimers in the A-DNA is significantly (6 times) larger than that in the B-DNA. 5.3.2 Excited-state geometries of nucleic acid bases The N9H and N7H tautomers of adenine have planar ground-state geometry. However, the amino groups of both tautomers show pyramidal character, and such pyramidalization is more pronounced for the N7H form. The N9H tautomer has an almost planar structure in the electronic lowest singlet pp excited state, while the N7H tautomer has non-planar geometry in the same state. However, the amino group has been predicted to be pyramidal for both tautomers in the excited state [47]. In the electronic lowest singlet np excited state, the N9H tautomer has a non-planar structure, and such non-planarity is localized around the N1C2N3 fragment of the six-membered ring. For the N7H tautomer in the electronic lowest singlet np excited state, the molecular geometry is reminiscent of twisted intramolecular charge transfer states [151, 154, 155]. The geometry of the molecule has Cs symmetry, and amino hydrogens make dihedral angles of 61 with respect to the ring plane. However, no significant intramolecular charge transfer was revealed for this tautomer in the considered state [47]. The geometries of both tautomers were also optimized in the ground and electronic lowest singlet pp excited state under hydrated conditions where three water molecules were in the first solvation shell. It was revealed that water molecules induce planarity in the system. Consequently, the ground- and electronic lowest singlet pp excited-state geometries of both tautomers were found to be almost planar, including the amino group [47]. Significant non-planarity around the C6N1C2N3 fragment was revealed for the keto-N9H tautomer of guanine in the electronic lowest singlet pp excited state [47]. In the electronic lowest singlet np excited state, which is characterized by the excitation of the carbonyl group lone pair electron, the C6O6  bond length was predicted to be increased by about 0.1 A with respect to the ground-state value. Further, in this state the O6 and H1 atoms were found to be displaced away from the ring plane and located opposite to each other. The geometrical distortions in the keto-N7H tautomer in the excited states were computed similarly to the keto-N9H tautomer, but the amount of distortion was generally smaller than that in the keto-N9H tautomer [47]. The excited-state hydration of guanine will be discussed in detail in the next section. Ground state geometry of the keto-N1H tautomer of cytosine is planar, while that in the electronic lowest singlet pp excited state was found to be non-planar, mainly around the N1C6C5C4 fragment. The amino group is pyramidal in both states. In the electronic lowest singlet np excited state, the amino group was revealed to be considerably rotated, and the N3 atom was located appreciably out-of-plane [148, 152]. The structural deformation in the np state was attributed to the excitation of the N3 lone pair electron. The rotation of the

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amino group was speculated to be due to the partial contribution of orbitals due to the amino lone pair electron in the electronic excitation. Under hydration with three water molecules, the amino group was found to be more planar. The mode of hydration in the electronic lowest singlet np excited state was predicted to be completely modified. The N3 site, the lone pair electron of which was responsible for the np excitation, was revealed to provide a repulsive potential to hydrogen bonding [152]. The ground-state amino group pyramidalization in the keto-N3H tautomer of cytosine was revealed in a more pronounced manner compared with that in the keto-N1H tautomer, while theground-state ring geometry was found to be planar. The electronic lowest singlet pp excitedstate geometry of the keto-N3H tautomer was found to have a boat-type structure where N1C2C4C5 atoms were approximately in one plane and the N3 and C6 atoms were out of this plane. A twisted structure around the N1C2 bond was revealed in the electronic lowest singlet np excited state of the keto-N3H tautomer. The ground-state amino group pyramidalization of the keto-N3H tautomer was found to be decreased, while the ring geometry was found to be slightly non-planar after hydration with three water molecules. In the np excited state, the hydrated structure was revealed to be completely modified. Such changes in the hydrated structures in the np state are in accordancewith the established fact that hydrogen bonds are largelydestabilized under np excitations [43, 156– 158]. The amino group of the enol tautomer of cytosine is found to be more pyramidal than the keto-N1H tautomer, while it shows a lower degree of pyramidization than the keto-N3H tautomer in the ground state. The electronic lowest singlet pp excited-state geometry of the enol tautomer was found to be largely distorted,  especially around the C5C6 bond, and length of this bond was found to be increased by about 0.092 A. Hydration of the enol tautomer significantly reduced the amino group pyramidal character in the ground state. Further, the geometrical deformation of the hydrated enol form in the electronic lowest singlet pp excited state was predicted to be similar to that in the isolated tautomer [152]. The ground-state geometries of uracil and thymine were found to be planar, while the electronic lowest singlet np excited-state geometries were revealed to be slightly non-planar [159]. Further, in the electronic lowest singlet np excited state, the excitation being characterized by the promotion of the C4O lone pair electron to the antibonding p orbital, the length of the C4O bond was increased by about 0.1 A with respect to the ground-state value. Further, the C4O bond was found to be appreciably out of the approximate ring plane. Thymine in the lowest singlet pp excited state has been predicted to adopt a boat-type structure with the N1, C2, C4 and C5 atoms being approximately in one plane, while the N3 and C6 atoms are out of this plane. The geometries of the hydrated forms of thymine and uracil in which three water molecules were considered in the first solvation shell were also optimized in the ground and singlet excited states. It was revealed that hydration generally induces planarity in the system. However, structural deformation in the excited state was generally found to be similar to that in isolated species. Detailed discussion about hydrated species can be found in our earlier paper [159]. In jet-cooled studies, the geometrical deformation of the pp excited states of thymine and uracil has been suggested to be responsible for the diffuseness of spectra of these compounds [160, 161]. 5.3.3 Hydration of guanine 5.3.3.1 Structure We have recently performed systematic computational studies of the interaction of water molecules with guanine in the ground and the corresponding electronic lowest singlet pp excited state [49, 50]. In these investigations, 1, 3 and 5–13 water molecules were considered in the solvation shells of guanine. Ground-state geometries were optimized at the HF level, while excited-state geometries were optimized at the CIS level and the 6-311G(d, p) basis set was used in all calculations. The nature of the potential energy surfaces was ascertained through harmonic vibrational frequency analysis; all computed geometries were revealed minima at the respective potential energy surfaces. The selected calculated geometrical parameters are shown in Table 5.1. The ground-state geometries of the isolated and hydrated guanine were found to be planar, except for

119.1 120.5 119.8 0.6 46.5 10.2 26.5 28.0 7.1 177.3 10.9 178.2

H21N2C2 H22N2C2 H21N2H22 360-SHNH C6N1C2N3 N1C2N3C4 C2N3C4C5 N3C4C5C6 N1C6C5C4 N2C2N3C4 H21N2C2N1 H22N2C2N1

119.1 120.2 120.0 0.7 34.9 0.9 29.4 27.6 5.0 175.9 1.7 172.6

G þ 8W

117.1 114.3 113.8 14.8 64.7 39.2 2.0 18.1 7.3 159.7 31.0 167.9

G þ 1W

a

Values in parentheses correspond to CASSCF [18] and TDDFT [20] results.

G þ 7W3

115.3 (153.3, 117.1) 112.7 (113.3, 115.4) 111.4 (113.0, 115.7) 20.6 (18.4,11.8) 64.0 (70.3, 64.9) 44.2 (65.1, 67.3) 2.4 (14.4, 20.6) 18.5 (26.9, 21.8) 0.6 (23.2, 27.1) 161.4 (151.9, 139.5) 42.3 (54.9, 45.8) 171.8 (172.6, 172.7)

H21N2C2 H22N2C2 H21N2H22 360-SHNH C6N1C2N3 N1C2N3C4 C2N3C4C5 N3C4C5C6 N1C6C5C4 N2C2N3C4 H21N2C2N1 H22N2C2N1

Parameters

Ga

Parameters

122.8 118.6 116.4 2.2 32.6 0.2 31.4 32.5 1.2 174.9 21.8 175.8

G þ 9W

116.7 114.4 113.3 15.6 64.8 42.4 0.0 18.8 3.3 160.5 34.8 170.4

G þ 3W

122.4 118.6 116.1 2.9 32.4 0.6 31.8 32.7 1.1 174.6 23.1 177.0

G þ 10W

119.2 120.4 119.8 0.6 38.1 1.7 32.2 32.2 2.6 177.3 10.2 178.7

G þ 5W

114.5 114.2 114.2 17.1 67.2 51.4 6.9 20.9 5.9 164.0 32.2 166.6

G þ 11W1

119.2 120.4 119.9 0.5 43.0 6.5 29.0 29.7 5.4 179.7 10.6 177.4

G þ 6W

115.7 114.4 112.5 17.4 65.4 35.4 6.5 15.3 15.5 161.8 41.9 175.1

G þ 11W2

119.2 120.1 119.9 0.8 37.1 2.3 30.6 31.0 2.2 177.2 14.8 175.8

G þ 7W1

114.7 114.3 114.3 16.7 67.3 51.2 6.8 20.7 5.4 164.1 31.6 166.4

G þ 12W

119.1 120.2 119.9 0.8 43.9 7.3 28.6 29.5 5.8 179.1 11.2 178.5

G þ 7W2

116.9 116.1 113.4 13.6 63.7 31.5 9.7 16.6 15.9 162.9 33.2 171.4

G þ 13W

Table 5.1 Selected dihedral angles (deg) and amino group angles (deg) of guanine and different hydrated complexes in the lowest singlet pp excited state obtained at the CIS/6-311G(d, p) level. Reprinted with permission from [50]. Copyright 2008 American Chemical Society

132 Hydrogen Bonding and Transfer in the Excited State

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the amino group, which was found to be pyramidal in the isolated and some hydrated forms of guanine. The water molecules, when involved in direct interaction with amino hydrogens, were found to induce planarity in the amino group. Electronic excitation of guanine and hydrated guanine complexes to the electronic lowest singlet pp excited state (S1(pp )) was revealed to be characterized by the HOMO ! LUMO configuration. The geometry of the isolated guanine in the S1(pp ) excited state was found to be significantly non-planar. The C6N1C2N3, N1C2N3C4 and N3C4C5C6 dihedral angles were predicted to deviate by 64.0 , 44.2 and 18.5 , respectively, from the planarity at the CIS/6-311G(d, p) level. Mennucci et al. [162] have also optimized the lowest singlet pp excited-state geometries of the keto-N9H and keto-N7H tautomers in the gas phase and in water solution at the CIS/cc-pVDZ level; the aqueous environment was considered using the integral equation formalism continuum model. A significant solvent effect was revealed in the excited-state geometries, mainly affecting bond lengths and bond angles of the molecules. The excited-state properties of guanine tautomers have also been investigated at the CASSCF [18, 19] and TDDFT [20] levels. Selected excited-state geometrical parameters of guanine obtained at the CASSCF and TDDFT levels are shown in Table 5.1. It is clear from this table that the CIS/6-311G(d, p) predicted excitedstate geometry of guanine is reasonably similar to that obtained from the CASSCF and TDDFT methods. The major difference is obtained for the N1C6C5C4 dihedral angle, which has been predicted to deviate about 23 from planarity at the CASSCF level, while it is planar at the CIS level. Further, the C2 atom is also displaced significantly more out-of-plane at the CASSCF level than at the CIS level. On the other hand, the TDDFT predicted geometry for guanine is qualitatively similar to that obtained from the CASSCF method. However, it should be noted that, for the keto-N7H tautomer of guanine, the TDDFT method predicted significantly less non-planar excited-state geometry [20]. On the other hand, the corresponding geometry at the CASSCF level was found to be significantly non-planar, similar to that obtained for the keto-N9H tautomer [18, 19]. Nevertheless, the CIS method also predicted significantly non-planar excited-state geometry for the keto-N7H tautomer, although the amount of non-planarity was smaller than that of the keto-N9H tautomer [47]. The plots containing the variations in selected important dihedral angles, namely C6N1C2N3, N1C2N3C4, C2N3C4C5, N3C4C5C6 and N1C6C5C4, of guanine with respect to the degree of hydration in the electronic lowest singlet pp excited state are presented in Figure 5.2. Based upon structural data and the dihedral angle plot, the excited-state geometries were arranged in three groups [49, 50]. Hydrated complexes of guanine with 5–10 water molecules were in group I, the isolated guanine and its hydrated complexes with 1, 3, 11 (one configuration – the G þ 11W1 complex) and 12 water molecules were in group II and complexes with 11 (other configuration – the G þ 11W2 complex) and 13 water molecules were in group III. The structures of selected species as a representative of each group in the ground and excited state are shown in Figure 5.3. Excited-state geometries of guanine revealed remarkable differences among these three groups. For example, the complexes belonging to group I had a planar N1C2N3C4 dihedral angle. However, this angle is about 50 and 35 away from the planarity for complexes belonging to groups II and III respectively. The C6N1C2N3 dihedral angle is found to be about 30 for complexes belonging to group I, but for complexes belonging to groups II and III the magnitude of the same angle is predicted to be about 65 . Significant variations, though smaller in magnitude, were also revealed for other dihedral angles. Generally, the excited-state structural deformation of guanine was found to be similar for groups II and III, except for the N1C2N3C4 and N1C6C5C4 dihedral angles. The magnitude of these two dihedral angles is found to be about 15 smaller and 10 larger, respectively, for the group III complexes. The HOMO and LUMO for guanine and hydrated complexes representing each of the groups are presented in Figure 5.4. It is evident that orbital contamination from water is not present in hydrated guanine complexes. The relaxed electronic lowest singlet pp excited state of guanine complexes was revealed to be characterized by the HOMO ! LUMO configuration. Further, the HOMO is p-type and has a similar nature for both the isolated and hydrated guanine. The LUMO has been found to be localized mainly on the six-membered ring and revealed to have a p -type nature. The distributions of LUMO orbitals among the three groups were found

134 Hydrogen Bonding and Transfer in the Excited State

Figure 5.2 Variation in selected important dihedral angles of guanine with the degree of hydration in the electronic lowest singlet pp excited state [49, 50]. Reprinted with permission from [50]. Copyright 2008 American Chemical Society

to be significantly different, as shown in Figure 5.4. Thus, the change in the nature of LUMO orbitals clearly depends upon the mode and degree of hydration of guanine. It has been suggested that the difference in the LUMO distribution is responsible for the remarkably different excited-state structural deformation of guanine in the electronic lowest singlet pp excited state [49, 50]. 5.3.3.2 Ground- and Excited-State Stretching Vibrational Frequencies Table 5.2 shows the computed ground and electronic lowest singlet pp excited-state stretching vibrational frequencies, at the HF/6-311G(d, p) and CIS/6-311G(d, p) levels respectively, corresponding to the N9H (nN9H), N1H (nN1H), symmetric NH2 (nsymNH2) and asymmetric NH2 (nasymNH2) vibrational modes (symmetric and asymmetric NH2 assignments are approximate owing to the structural non-planarity) of guanine in isolated and hydrated forms. The variation in these vibrational modes with respect to the degree and mode of hydration is depicted in Figure 5.5. It has been revealed that stretching vibrations of different protondonating sites of guanine are affected by the degree of hydration and the structural non-planarity in the excited state. As ab initio computed vibrational frequencies need to be scaled for comparison with the corresponding experimental data, a scaling of 0.9051 [163] has been used for ground-state vibrations. The same scaling factor was used for the excited-state vibrations – this selection was justified because the CIS method is the HF analogue for the excited state. A significant spectral shift consequent to the hydration of guanine and that of the electronic excitation was revealed. Further, it was inferred that generally the arrangement of water molecules around the hydrogen-bond-donating sites of guanine in hydrated complexes has an important role in the corresponding NH stretching vibrations. However, in the electronic lowest singlet pp excited state, the nN9H vibrational frequencies of guanine in isolated and hydrated forms are generally similar to those of

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Figure 5.3 Ground- (S0) and electronic lowest singlet pp excited-state (S1(pp )) geometries of complexes of isolated guanine and its hydrated complexes with ten and 13 water molecules. Reprinted with permission from [49]. Copyright 2005 American Chemical Society

the corresponding ground-state values. Similarity among ground- and excited-state nN9H vibrational frequencies of guanine and hydrated complexes was suggested owing to the fact that the five-membered guanine fragment remains planar in the excited state. 5.3.4 Ground- and excited-state proton transfer in guanine The ground- and electronic singlet pp excited-state proton transfer barrier height of the guanine corresponding to keto–enol tautomerism has also been investigated [39]. The effect of a water molecule in the proton

136 Hydrogen Bonding and Transfer in the Excited State

Figure 5.4 HOMO and LUMO orbitals corresponding to the excited-state optimized geometry of selected hydrated complexes of guanine. Reprinted with permission from [49]. Copyright 2005 American Chemical Society

transfer reaction path was also studied. In this investigation, the ground-state geometries, including transition states, were also optimized at the B3LYP/6-311 þþ G(d, p) level, and geometries including the transition state in the electronic lowest singlet pp excited state were optimized at the CIS/6-311G(d, p) level. The TDB3LYP/6-311 þþ G(d, p) approach was used to compute the vertical transition energies using the CIS/6-311G (d, p) optimized geometries. The effect of bulk water solution was investigated using the PCM solvation model. The computed proton transfer barrier heights are shown in Table 5.3. It is clear that the ground- and excited-state proton transfer barrier height has been predicted to be significantly large both in the gas phase and in water solution. However, in the presence of a water molecule in the proton transfer reaction path, the barrier heights are significantly reduced compared with the unhydrated species. Interestingly, the excited-state barrier heights are generally found to be slightly larger than the corresponding ground-state values. Theoretical calculations suggested that the singlet electronic excitation of guanine may not facilitate the keto–enol tautomerization either in the gas phase or in the water solution. The geometries of the transition states were also found to be significantly non-planar. Geometries of the hydrated transition states in the ground and lowest singlet pp excited states were found to adopt a zwitterionic form in which the water molecule is in the form of a hydronium cation (H3Oþ ) and the guanine is in the anionic form, except for the N9H form in the excited state, where the water molecule is in hydroxyl anionic form (OH) and the guanine is in cationic form.

3758 3750

3702

3700 3781 3845 3748 3839

G þ 7W3 G þ 8W

G þ 9W

G þ 10W G þ 11W1 G þ 11W2 G þ 12W G þ 13W

3349 3422 3480 3392 3475

3351

3401 3394

3479 3386 3366 3391 3398 3401 3400

B

3733 3675 3815 3676 3814

3734

3793 3773

3809 3748 3738 3783 3790 3788 3790

A

B

3379 3326 3453 3327 3452

3380

3433 3415

3448 3392 3383 3424 3430 3429 3430

S1(pp )

3704 3684 3744 3690 3766

3731

3718 3717

3900 3900 3799 3884 3715 3712 3823

A

S0

3352 3334 3389 3340 3409

3377

3365 3364

3530 3530 3438 3515 3362 3360 3460

B

A

3683 3637 3718 3643 3753

3752

3727 3722

B

3333 3292 3365 3297 3397

3396

3373 3369

3530 3533 3435 3497 3371 3372 3458

S1(pp )

3900 3903 3795 3864 3724 3726 3821

nN9H

a A represents unscaled and B represents scaled (scaling factor 0.9051, Ref. [163]) frequencies. *Bold numbers show significant mixing with other stretching vibrations.

3844 3741 3719 3746 3754 3758 3757

G G þ 1W G þ 3W G þ 5W G þ 6W G þ 7W1 G þ 7W2

A

S0

nN1H

3648 3644 3762 3629 3730

3662

3735 3699

3805 3819 3817 3714 3731 3730 3734

A

S0

3302 3298 3405 3285 3376

3314

3381 3348

3444 3457 3455 3362 3377 3376 3380

B

3654 3726 3724 3729 3697

3663

3714 3700

B

3307 3372 3371 3375 3346

3315

3362 3349

3288 3353 3333 3327 3354 3350 3358

S1(pp )

3633 3705 3682 3676 3706 3701 3710

A

nsymNH2

3888 3911 3871 3922 3861

3894 3822 3830 3861

3919 3940 3937 3885 3890 3873 3890

A

S0

3519 3540 3504 3550 3495

3524 3459 3467 3495

3547 3566 3563 3516 3521 3505 3521

B

3838 3854 3859 3848 3826 3850 3824

B

3488 3470 3493 3483 3463 3485 3461

3486 3513 3508 3468 3479 3469 3480 3485 3486 3470

S1(pp )

3852 3881 3876 3832 3844 3833 3845 3850 3851 3834

A

nasymNH2

Table 5.2 Selected stretching vibrational frequencies (cm1) of guanine and hydrated guanine in the ground and excited state, obtained at the  HF/6-311G(d, p) and CIS/6-311G(d, p) levels respectivelya . Reprinted with permission from [50]. Copyright 2008 American Chemical Society

Electronic-Excited-State Structures and Properties of Hydrated DNA Bases and Base Pairs 137

138 Hydrogen Bonding and Transfer in the Excited State

Figure 5.5 Variation in different stretching vibrational frequencies (unscaled) of guanine with the degree of hydration in the ground and excited state. Reprinted with permission from [50]. Copyright 2008 American Chemical Society

5.4 Excited States of Base Pairs The electronic singlet excited-state properties of the Watson–Crick GC, AT and AU base pairs were evaluated theoretically [78, 79]. Ground-state geometries of base pairs were optimized at the HF level, and electronic singlet excited states were computed at the CIS level. These calculations were performed under the assumption of a planar symmetry, and the 6-31 þþ G(d, p) basis set was used in the study. Computed vertical singlet pp and np transitions of base pairs suggested that electronic excitations were due to the contribution of molecular Table 5.3 Computed ground- and excited-state barrier height (kcal mol1) for the keto–enol guanine tautomerism at the B3LYP/6-311 þþ G(d, p) and TD-B3LYP/6-311 þþ G(d, p)//CIS/6-311G(d, p) level, respectively, in the gas phase and in water solution. Reprinted with permission from [39]. Copyright 2005 American Chemical Society Species

Keto-N9H ! TS-N9H Enol-N9H ! TS-N9H Keto-N7H ! TS-N7H Enol-N7H ! TS-N7H Keto-N9HH2O ! TS-N9HH2O Enol-N9HH2O ! TS-N9HH2O Keto-N7HH2O ! TS-N7HH2O Enol-N7HH2O ! TS-N7HH2O

Ground state

Excited state

Gas

Water

Gas

Water

37.5 36.3 40.6 35.9 15.9 12.8 17.3 12.0

45.2 38.2 46.5 38.5 16.7 10.4 17.1 10.2

42.9 36.9 36.8 36.8 19.8 12.8 13.9 13.4

45.7 40.8 41.4 39.3 18.3 13.5 13.5 11.2

Electronic-Excited-State Structures and Properties of Hydrated DNA Bases and Base Pairs

139

orbitals localized on one or other monomeric unit. These results are supported by the experimental observation of AT and GC polymers and natural DNA where the electronic transitions were assigned to the corresponding monomer bases [164]. Some charge-transfer-type states, characterized by the excitation of electrons from the occupied orbitals of one base to the virtual orbitals of the complementary base of the base pair, were also revealed, and these states are located slightly higher in energy. A significant effect on the singlet pp transition energies of monomers consequent to the base pair formation was not revealed. However, the np singlet states of monomers were found to be significantly blue-shifted as a result of base pair formation. Such a blue-shift in np states is in agreement with the experimental findings that such transitions are blue-shifted in hydrogenbonding environments [48, 165]. The excited-state geometrical deformation (under planar symmetry) was found to be localized at the excited monomer under base pair excitation. The interaction energies of base pair formation in the ground and electronic singlet excited states were also calculated. The interaction energy between two interacting monomers X and Y can be defined as Eint ¼ EðXYÞEðXÞEðYÞ

ð5:1Þ

where E(XY) is the total energy of the XY dimer, and E(X) and E(Y) are the energies of the interacting monomers. However, this type of interaction energy calculation is contaminated by the basis set superposition error caused by the different number of basis functions used to describe the monomers and dimer. The basis set superposition error (BSSE)-corrected interaction energies of nucleic acid base pairs were estimated using the Boys–Bernardi counterpoise correction scheme [166]. The interaction energy (Eint) in the ground state was calculated using the formula Eint ¼ EðXYÞEðXXY ÞEðYXY Þ

ð5:2Þ

where E(XY) is the total energy of the XY base pair in the ground state, and E(XXY) and E(YXY) are the total energies of the X (adenine in the case of the AT (AU) base pair and guanine in the case of the GC base pair) and the Y (thymine (uracil) in the case of the AT (AU) base pair and cytosine in the case of the GC base pair) monomeric moieties using the optimized XY base pair geometry and ghost atoms in place of the complementary base. It should be noted that the deformation energy of monomers (Edef ¼ E(X)  E0 (XXY); E0 (XXY) is the energy of X monomer in the dimer geometry) is ignored in the above equation. However, in the case where deformation energy is significant, it should be added to the equation. We have utilized the Boys–Bernardi counterpoise correction schemes developed for the ground state [166] to compute the BSSE-corrected interaction energy for excited states. The interaction energy in the excited state ðnÞ (Eint ), where the excitation is localized at the X monomeric moiety, was estimated using the formula ðnÞ

0

Eint ¼ EðnÞ ðXYÞEðn Þ ðXXY ÞEð0Þ ðYXY Þ

ð5:3Þ

For the excited state where the excitation is localized at the Y monomeric moiety, the interaction energy was estimated using the formula ðnÞ

0

Eint ¼ EðnÞ ðXYÞEð0Þ ðXXY ÞEðn Þ ðYXY Þ

ð5:4Þ

In equations (5.3) and (5.4), E(n)(XY) is the total energy of the XY base pair in the nth excited state, 0 E (XXY) and E(n )(YXY) are the total energies of the X and Y monomeric moieties, respectively, in the n0 th excited state, which corresponds to the nth state of the XY base pair (as the nth state of the XY base pair may not necessarily correspond to the nth state of excited X or Y [78, 79]), and E(0)(XXY) and E(0)(YXY) are the (n0 )

140 Hydrogen Bonding and Transfer in the Excited State

ground-state total energies of the X and Y monomeric moieties respectively. In these calculations, the geometries of the X and Y monomeric moieties were those of the bases in the optimized geometry of the XY base pair in the nth excited state, while the ghost atoms were added in place of the complementary base. The hydrogen bond parameters and interaction energies of the base pairs in the ground and excited states are shown in Table 5.4. As expected, the hydrogen bond parameters and interaction energies for the AT and AU base pairs are similar. The analysis of the hydrogen bond angles shown in Table 5.4 clearly suggest that hydrogen bonds are more nonlinear in the singlet np states than those in the ground and singlet pp states. Further, it is also clear from the values of the interaction energy of base pairs in the ground and excited states that base pairs would have a similar stability in the ground and electronic singlet pp states, but would be significantly destabilized in the np states. The geometries of the guanine–cytosine (GC) and guanine–guanine (GG) base pairs in the ground and the electronic lowest singlet pp excited state were also investigated at the HF and CIS levels using the 6-311G(d, p) basis set. The computed results were compared with those for isolated guanine obtained at the same level of 

Table 5.4 Hydrogen bond lengths (A), hydrogen bond angles (deg) and interaction energies (Eint, kcal mol1) of the AT, AU and GC base pairs in the ground and different singlet excited statesa. Reprinted with permission from [79 & 78]. Copyright 2002 American Chemical Society AT base pair N6. . .O40 N1. . .N30 H61(N6). . .O40 N1. . .H30 ffO40 H61N6 ffN30 H30 N1 Eint AU base pair N6. . .O40 N1. . .N30 H61(N6). . .O40 N1. . .H30 ffO40 H61N6 ffN30 H30 N1 Eint GC base pair O6. . .N40 N1. . .N30 N2. . .O20 O6. . .H410 (N40 ) H1(N1). . .N30 H21(N2). . .O20 ffO6H410 N40 ffN1H1N30 ffN2H21O20 Eint

S0

S2(p–p )

S3(p–p )

S6(n–p )

S10(n–p )

3.083 3.023 2.090 2.010 172.6 177.7 9.9

3.011 3.071 2.016 2.063 171.1 174.8 10.0

3.027 3.025 2.029 2.014 174.5 177.2 10.6

3.778 3.090 2.778 2.086 161.8 183.2 5.8

3.094 3.152 2.106 2.149 170.2 174.3 7.1

S0

S2(p–p )

S4(p–p )

S5(n–p )

S10(n–p )

3.082 3.019 2.088 2.007 172.6 177.6 10.1

3.011 3.066 2.016 2.059 171.0 174.6 10.2

3.029 3.022 2.032 2.010 174.2 177.0 10.7

3.742 3.087 2.786 2.083 161.9 183.0 5.9

3.092 3.146 2.104 2.143 170.2 174.1 7.3

S0

S3(p–p )

S4(p–p )

2.932 3.053 3.028 1.926 2.048 2.028 175.7 175.3 176.9 24.8

2.937 3.023 3.037 1.929 2.021 2.034 177.7 176.3 177.4 22.9

2.958 3.066 3.098 1.954 2.059 2.100 177.3 176.4 177.7 20.4

States are given in ascending energy order (see Refs [78] and [79]).

a

Electronic-Excited-State Structures and Properties of Hydrated DNA Bases and Base Pairs

141

Figure 5.6 Molecular geometries in the electronic lowest singlet pp excited state of GC, GG17 and GG16 base pairs. The top indices correspond to the ground state obtained at the HF/6-311G(d, p) level, and the bottom indices correspond to the excited state obtained at the CIS/6-311G(d, p) level. Reprinted with permission from [167]. Copyright Elsevier

theory [167]. For the GG base pair, two different hydrogen-bonding configurations, namely GG16 and GG17, were studied. The GG16 structure was obtained by the formation of the cyclic and symmetrical hydrogen bonds in between the N1H site of the first guanine monomer and the carbonyl group of the second guanine monomer, and vice versa. On the other hand, the GG17 base pair was obtained by the formation of hydrogen bonds between the amino group and the N1H site of the first guanine monomer acting as hydrogen bond donors (GD) with the carbonyl group and the N7 site of the second monomer acting as the hydrogen bond acceptor (GA). The electronic excitation to the lowest singlet pp excited state of the GC base pair was predicted to be dominated by orbitals mainly localized at the guanine moiety. In the GG17 base pair, electronic excitation was found to be localized at the GA monomer. In the case of the GG16 base pair, the orbitals involved in the lowest singlet pp electronic excitation were found to be delocalized [167]. The optimized ground- and excited-state base pair geometries are depicted in Figure 5.6, and selected geometrical parameters are shown in Table 5.5. It was found that the amino groups of guanine belonging to the GC and GG16 base pairs are almost planar, but the corresponding amino group in the GG17 base pair was revealed to be significantly pyramidal both in the ground and in the electronic lowest singlet pp excited state. The excited-state geometries of the isolated Table 5.5 Selected parameters of the guanine monomer in the GC, GG16 and GG17 base pairs and GC þ 5W complex in the ground and lowest singlet pp excited state obtained at the HF/6-311G(d, p) and CIS/6-311G(d, p) levels respectivelya. Reprinted with permission from [167]. Copyright Elsevier GC S0 H21N2C2 H22N2C2 H21N2H22 360-SHNH C6N1C2N3 N1C2N3C4 C2N3C4C5 N3C4C5C6 N1C6C5C4 N2C2N3C4 H21N2C2N1 H22N2C2N1

122.8 117.1 120.1 0.0 0.0 0.0 0.0 0.0 0.0 180.0 0.0 180.0

GG16 pp

122.8 117.4 119.7 0.1 36.4 3.3 28.8 29.6 2.6 177.7 3.7 178.5

S0 120.5 117.4 119.3 2.8 0.1 0.4 0.3 0.1 0.5 178.5 10.9 171.7

GG17 pp

120.7 118.1 119.9 1.3 3.1 0.7 1.3 1.1 1.0 179.8 8.2 175.2

S0 118.4 114.2 115.3 12.1 0.5 0.7 1.1 1.3 0.9 177.4 29.3 170.1

GC þ 5W pp

116.6 113.5 112.2 17.7 64.7 44.4 2.2 18.1 1.9 156.9 41.6 174.3

S0 122.0 117.1 119.0 1.9 0.1 0.7 1.0 0.4 0.5 179.4 8.0 172.0

pp 122.8 118.0 119.0 0.2 37.0 4.8 27.1 27.4 4.1 178.4 7.1 177.9

In the GG17 base pair, the data correspond to the GA guanine moiety which acts as a hydrogen bond acceptor, while in the GG16 base pair both monomers have similar geometry owing to the symmetry.

a

142 Hydrogen Bonding and Transfer in the Excited State

Figure 5.7 Ground- (S0) and electronic lowest singlet pp excited-state (S1(pp )) geometries of the complex of the GC base pair with five water molecules

guanine and those of the GC and GG17 base pairs were found to be non-planar. The predicted structural nonplanarity in these base pairs was located at the excited guanine monomer. A remarkable difference was revealed in the mode of non-planarity, and it was found to be significantly influenced by the hydrogen bonding in the base pair. The geometrical deformation of isolated guanine and the GA monomer of the GG17 base pair in the excited state were similar but found to be significantly different to the structural deformation of guanine monomer of the GC base pair in the excited state. The geometry of the GG16 base pair was predicted to be almost planar in the excited state, except that both guanine monomers are folded with respect to each other. It was inferred that hydrogen bonding environments will have significant influence on excited-state dynamics of bases and DNA. Effects of specific water solvation of the Watson–Crick GC base pair in the ground and electronic lowest singlet pp excited state were studied at the HF/6-311G(d, p) and the CIS/6-311G(d, p) levels respectively. In this investigation, five water molecules were used to hydrate the guanine, and the resulting structure hereafter will be referred to as GC þ 5W. The optimized geometries of the GC þ 5W complex in the ground and in the electronic lowest singlet pp excited state are shown in Figure 5.7, and selected geometrical parameters are shown in Table 5.5. The harmonic vibrational frequency analysis suggested that these structures correspond to minima at the respective potential energy surfaces. Similarly to the isolated GC base pair, the electronic lowest singlet pp excited state of the GC þ 5W complex is found to be dominated by orbitals mainly localized at the guanine moiety. Further, no orbital contamination from water molecules was revealed in the electronic excitation to the lowest singlet pp excited state of the complex. In addition, the ground- and excited-state geometrical distortion of the GC þ 5W complex was found to be similar to the isolated GC base pair in the respective electronic states.

5.5 Excited-State Dynamics and Non-Radiative Decays It is unclear as to what was the basis of the selection of purine and pyrimidine bases (adenine, guanine, cytosine, thymine and uracil) as genetic material by nature and whether their selection was absolute or they have evolved through some natural selection processes over the period of time. However, it is certain that these molecules are highly photostable, and this extraordinary photostability stems from the ultrashort excited-state lifetime [44, 52]. Consequently, the excitation energies are released in the form of non-radiative deactivation processes. Fortunately, the non-radiative relaxation abilities of nucleic acid bases are generally intact in nucleic acids also, although the excited-state dimers formed owing to the presence of stacking of bases also

Electronic-Excited-State Structures and Properties of Hydrated DNA Bases and Base Pairs

143

show decay of radiation in a longer time domain [53]. Here it should be noted that molecular environments for nucleic acid bases are significantly different in the nucleic acids compared with those of isolated species. The non-canonical forms of NAB have a significantly longer excited-state lifetime than the canonical forms [53]. Long back it was suggested that the N7H tautomeric forms of guanine and adenine are fluorescent species, and this conclusion was based upon comparison of the fluorescence spectra of these molecules with the respective methylated forms [48, 168]. The isomeric forms of bases also show an entirely different photostable character. One of the most cited examples is 2-aminopurine, an isomeric form of adenine (6aminopurine). 2-Aminopurine has very strong fluorescence [169], and in fact this property is utilized for a fluorescence probe in DNA dynamics by substituting it in place of adenine [170]. Substitutions on these bases also generally significantly affect photostability, with the exception of some methylated forms, which generally have similar characteristics to unmethylated canonical nucleic acid bases. The most glaring example is uracil (which is present in RNA) and its 5-methylated form, thymine, which is present in DNA, and both species are highly photostable. Recent state-of the-art theoretical and experimental work has convincingly shown that conical intersections between the excited-state and ground-state potential energy surfaces are responsible for non-radiative deactivation in nucleic acid bases [1, 18–22, 44, 45, 51–57, 171–179]. Thus, excited-state structural non-planarity plays an import role in the photophysics of nucleic acid bases, and the molecule at the point of conical intersection has a biradical character [178, 179]. Neither can the role of dark state in conical intersections in pyrimidines be ruled out [52]. As it is not the main objective of the present article to discuss the details of non-radiative deactivation processes, interested readers can find more detailed information in recently published review articles [52–54]. Following the previous discussions, it can be concluded that the mode and degree of hydration significantly influence the excited-state geometrical deformation of guanine [49, 50], and therefore it can be argued that excited-state dynamics of guanine and other bases will exhibit a dependency on hydration.

5.6 Conclusions Ground-state geometries of nucleic acid bases and base pairs in isolated and hydrated forms are generally almost planar, except for the amino groups, which usually have a pyramidal character. However, electronic singlet excited-state geometries of bases and base pairs are very often strongly non-planar, and such nonplanarity is localized in the six-membered ring. The advance of superfast computers and state-of-the-art computational algorithms have enabled researchers to study complicated excited-state phenomena of complex molecules such as nucleic acid bases and base pairs at the multireference theoretical level. Coherent efforts by both experimentalists and theoreticians have convincingly shown that conical intersections between excitedand ground-state potential energy surfaces are responsible for the ultrafast non-radiative decays in nucleic acid bases. Although most of these theoretical calculations have been performed in the isolated condition, we believe that in the near future it will be possible to consider the effect of solvents in such calculations. Further, it will also be possible to consider a larger system where hydrogen bonding and stacking interaction are both present. As real DNA is extremely complex, consideration of such a system will provide better information related to the photophysical and photochemical properties of the genetic system.

Acknowledgements The authors are grateful for financial support from NSF-CREST grant No. HRD-0833178 and NSF EPSCoR grant No. 440900362427-02. They are also grateful to the Mississippi Center for Supercomputing Research (MCSR) for generously providing computer facilities.

144 Hydrogen Bonding and Transfer in the Excited State

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6 Insight from Singlet into Triplet Excited-State Hydrogen Bonding Dynamics in Solution Guang-Jiu Zhao and Ke-Li Han State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China

6.1 Introduction Solute–solvent interactions play a fundamental role in the photochemistry of organic and biological chromophores in solution [1–14]. In addition to non-specific dielectric interactions between solute and solvent, site-specific intermolecular hydrogen bonding interaction between hydrogen donor and acceptor molecules is another important type of solute–solvent interaction and is central to the understanding of the microscopic structure and function in many molecular systems [15–29]. Moreover, the dynamical behaviour of intermolecular hydrogen bonds in electronic excited states plays an important role in determining the rates of many chemical, physical and biochemical processes that occur in hydrogen-bonded surroundings [30–35]. Therefore, investigation of the hydrogen bonding dynamics of photoexcited chromophores in hydrogenbonded surroundings is very valuable and helpful in understanding the interesting photophysical and photochemical behaviours of these chromophores, as well as in designing and synthesizing organic functional materials by using hydrogen bonding interactions [36–51]. In previous studies we have theoretically investigated the electronic-excited-state structures and dynamics of hydrogen bonding for a variety of organic and biological chromophores, such as coumarin, fluorenone, oxazine, thioketones, novel D

p

A systems, dihydrogen-bonded phenol-BTMA, protochlorophyllide a, etc. [52–59]. The ground state and electronic excited states were investigated using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods respectively. It has been theoretically demonstrated for the first time that the intermolecular hydrogen bonds formed between these chromophores and the solvents can be significantly strengthened or weakened in the electronic excited states of

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

150 Hydrogen Bonding and Transfer in the Excited State

chromophores [52–59]. At the same time, we also confirmed that excited-state dynamical behaviours of intermolecular hydrogen bonds contribute importantly to the photophysics and photochemistry of these chromophores [52–59]. It should be noted that all the investigations of electronic-excited-state hydrogen bonding dynamics only relate to the singlet electronic excited states. However, as we know, intersystem crossing (ISC) is a very important non-radiative transition process in many organic compounds and functional materials. Hence, the triplet electronic excited states that can be reached after the ISC process would be very important for understanding the photochemistry of these molecules. How does intermolecular hydrogen bonding influence the triplet electronic excited states of these chromophores? The answer to this question will be closely associated with the triplet electronic-excited-state hydrogen bonding dynamics. On the other hand, it is very difficult in experiments to monitor the hydrogen bonding dynamics in triplet excited states using timeresolved spectroscopic techniques owing to the relatively weak signals for triplet excited states. Therefore, it is a good choice to study the intermolecular hydrogen bonding in the triplet electronic excited states of chromophores by using theoretical methods. As we know, some spectral shifts of the characteristic vibrational modes involved in the formation of intermolecular hydrogen bonds can be induced by changes in intermolecular hydrogen bonding interactions [60–63]. Thus, femtosecond time-resolved vibrational spectroscopy has shown the potential to give good insight into the microscopic dynamics and provide information on local structures [52]. In addition, the time-dependent density functional theory (TDDFT) method has been demonstrated to be a reliable tool for calculating the vibrational spectra in electronic excited states [52–59, 64–69]. Consequently, in the present work, the TDDFT method will be used to study triplet excited-state hydrogen bonding dynamics by monitoring the spectral shifts of some characterized vibrational modes involved in the formation of intermolecular hydrogen bonds in different electronic states. The photophysical and photochemical properties of fluorenone (FN) and its derivatives have recently attracted considerable attention because they are excellent model compounds for investigating the molecular structures in both the ground state and electronic excited states, microscopic solvation dynamics and the ultrafast radiationless deactivation mechanism for photoexcited molecules [70–85]. To identify the nature of the electronic excited states of chromophores, several criteria such as absorption intensity, transition energy shift with increasing solvent polarity, direction of transition moment, vibrational structures, fluorescence and phosphorescence lifetime and quantum yields, singlet–triplet splitting, heavy atom effect on the S0 ! T1 transition and the electron spin resonance have been proposed [70–85]. It has been revealed by experimental investigation of the above criteria that the lowest S1 state of the fluorenone chromophore and its many derivatives in polar solvents is of pp nature. Very recently we have also theoretically demonstrated, by analysing the frontier molecular orbitals (MOs) of the hydrogen-bonded FN

MeOH complex [53], that the S1 state of fluorenone in polar alcoholic solvents is of a distinct pp character. In addition, during the p ! p transition, the electron is delocalized from the benzene rings to the carbonyl group of fluorenone [53]. The electronegativity of the carbonyl group in the S1 state of the hydrogen-bonded FN

MeOH complex would be greatly increased. Therefore, it has been demonstrated in our previous work [52–54] that the intermolecular hydrogen bond C¼O  H

O between fluorenone and methanol molecules is significantly strengthened in the S1 state upon photoexcitation of the hydrogen-bonded FN

MeOH complex. Furthermore, hydrogen bond strengthening in the S1 state upon photoexcitation of fluorenone in the alcoholic solvent can be used to explain well all the spectral features of fluorenone chromophore in alcoholic solvents. At the same time, we have demonstrated that the radiationless deactivation of the fluorescent state via internal conversion (IC) can be facilitated by hydrogen bond strengthening in the S1 state [53]. So the IC from the fluorescent state to the ground state becomes the most important dissipative process for the S1 state of fluorenone in polar alcoholic solvents [53]. As a result, the dynamic behaviour of intermolecular hydrogen bonding in the triplet electronic excited state is difficult to monitor by experimental techniques, although the intersystem crossing (ISC) from the S1 state to the T1 state is also an important radiationless deactivation channel [70–75]. Therefore, theoretical study of the intermolecular hydrogen bonding in the T1 state of the hydrogen-bonded FN

MeOH

Insight from Singlet into Triplet Excited-State Hydrogen Bonding Dynamics in Solution 151

complex will be helpful for understanding the deactivation of the populated triplet electronic states after the ISC process. The nature of the triplet electronic state can also be elucidated by vibrational spectroscopy [82–85]. The different nature of the T1 state for benzophenone and 4-phenylbenzophenone has been revealed by transient resonance Raman spectral studies. It has also been demonstrated that, for the T1(np ) state of benzophenone, the carbonyl group has a single-bond character, while for the T1(pp ) state of 4-phenylbenzophenone the carbonyl group retains a double-bond character [82–85]. Moreover, the time-resolved infrared absorption spectra of fluorenone and its carbonyl 18O-substituted analogues have also been studied to determine the nature of the T1 state of fluorenone in different solvents [85]. The stretching frequencies of the T1 state in both polar and nonpolar solvents were found to be located in the double-bond region, indicating that the T1 state of fluorenone is of pp character [85]. Thus, it could be expected that the intermolecular hydrogen bond C¼O  H

O between fluorenone and methanol molecules in the T1 state of the hydrogen-bonded FN

MeOH complex should also be stronger than that in the ground state, as the T1 state is of the same nature as the S1 state for fluorenone in polar alcoholic solvents. Moreover, it has also been reported that the degree of electronic delocalization between the benzene rings and the carbonyl group for fluorenone in acetonitrile-d3 is smaller in the T1(pp ) state than in the S1(pp ) state [85]. So it could also be expected that the intermolecular hydrogen bonding in the T1 state of the hydrogen-bonded FN

MeOH complex may be weaker than that in the S1 state. In the present work, to delineate the detailed aspects of triplet excited-state hydrogen bonding dynamics, the hydrogen-bonded FN

MeOH complex and the isolated fluorenone chromophore have been theoretically studied using the DFT and TDDFT methods. Only the solvent molecules in the inner solvation shell can be attributed to the hydrogen bonding dynamics occurring in the ultrafast timescale. The hydrogen-bonded complex proposed here is a good model for studying the ultrafast hydrogen bonding dynamics in solutions [53]. Moreover, the vibrational absorption spectra of the hydrogen-bonded complex in the triplet excited states have also been calculated by the TDDFT method. It has been demonstrated that the intermolecular hydrogen bond is significantly strengthened in both the S1 and T1 states of the fluorenone chromoophore.

6.2 Theoretical Methods Ground-state geometry optimizations of isolated monomers and the hydrogen-bonded FN

MeOH complex were performed using density functional theory (DFT) with Becke’s three-parameter hybrid exchange function with the Lee–Yang–Parr gradient-corrected correlation functional (B3-LYP functional) [86–89]. The triple-z valence quality with one set of polarization functions (TZVP) was chosen as the basis set throughout. The excited-state electronic structures were calculated using time-dependent density functional theory (TD-DFT) with the B3-LYP hybrid functional and the TZVP basis set. Fine quadrature grids 4 were also employed. Both the convergence thresholds for the ground-state and excited-state optimization were reset to 10
8 (default settings are 10
6). The excited-state Hessian was obtained by numerical differentiation of analytical gradients using central differences and default displacements of 0.02 Bohr [90–92]. The infrared intensities were determined from the gradients of the dipole moment. All the electronic structure calculations were carried out using the Turbomole program suite [86–92].

6.3 Results and Discussion The electronic excitation energies for the triplet electronic excited states of the isolated fluorenone and the hydrogen-bonded FN

MeOH complex are calculated using the TDDFT method and listed in Table 6.1. In addition, the electronic excitation energies for the singlet electronic excited states are also listed for

152 Hydrogen Bonding and Transfer in the Excited State Table 6.1 Calculated electronic excitation energies (in nm) and the corresponding oscillator strengths of the singlet and triplet excited states for isolated fluorenone and the hydrogen-bonded FN

MeOH complex FN

State 1 State State State State State State State State

2 3 4 5 6 7 8 9

FN

MeOH

Singlet

Triplet

Singlet

Triplet

392 (0.004) H ! L 98.5% — 390 (0.000) 305 (0.016) 276 (0.032) 255 (0.000) 249 (0.869) 242 (0.043) 233 (0.005) 226 (0.003)

497 (0.004) H ! L 84.4% H ! L þ 1 6.0% 458 (0.000) 381 (0.045) 358 (0.023) 342 (0.136) 299 (0.007) 282 (0.162) 280 (0.030) 258 (0.000)

411 (0.003) H ! L 98.7% — 375 (0.000) 345 (0.003) 314 (0.036) 281 (0.034) 250 (0.137) 250 (0.719) 247 (0.058) 239 (0.000)

518 (0.029) H ! L 86.6% H ! L þ 1 4.9% 430 (0.000) 386 (0.044) 372 (0.045) 348 (0.005) 345 (0.104) 306 (0.018) 282 (0.138) 279 (0.044)

comparison. Note that the electronic excitation energy for the T1 state of the hydrogen-bonded FN

MeOH complex is shifted to the lower energy in comparison with that of the isolated fluorenone. Thus, the energy level of the T1 state of the fluorenone chromophore can be induced to undergo a red-shift by intermolecular hydrogen bonding, which is similar to the hydrogen bonding effects on the singlet electronic excited states of the fluorenone chromophore [53]. As we know, it has been demonstrated that, if hydrogen bonding interaction induces an electronic spectral shift to the red, the intermolecular hydrogen bond will be strengthened in comparison with that in the ground state [59]. Hence, the intermolecular hydrogen bond may also be strengthened in the T1 state of the fluorenone chromophore in alcoholic solvents. It is also noted that the T1 state of both the isolated fluorenone and its hydrogen-bonded complex is lower in energy than the corresponding S1 state, which is consistent with the energy levels for the T1 and S1 states obtained in spectral studies [70–85]. Moreover, the contributions of orbital transition to the T1 and S1 states are also listed in Table 6.1. It has been demonstrated in a previous study [53] that the transition from HOMO to LUMO is the dominant orbital transition for the S1 state of both the isolated fluorenone and the hydrogen-bonded FN

MeOH complex. It can be established that the T1 state of both the isolated fluorenone and the hydrogen-bonded FN

MeOH complex is also dominantly contributed to by the orbital transition from HOMO to LUMO. In addition, the orbital transition from HOMO to LUMO þ 1 also contributes to the T1 state. Thus, three orbitals will be considered to discuss the nature of the T1 state. The three frontier molecular orbitals (HOMO, LUMO and LUMO þ 1) of both the isolated fluorenone and the hydrogen-bonded FN

MeOH complex are shown in Figure 6.1. It is clear that the HOMO orbital of the isolated fluorenone is of p character, while both the LUMO and LUMO þ 1 orbitals of the isolated fluorenone are of p nature. Consequently, the T1 state of the isolated fluorenone, which corresponds to the orbital transition from HOMO to LUMO and LUMO þ 1, is demonstrated to be of distinct pp nature. Moreover, it can be noted that the nature of these three orbitals is nearly unchanged by intermolecular hydrogen bonding. The HOMO of the hydrogen-bonded FN

MeOH complex is also of p character, while both LUMO and LUMO þ 1 orbitals of the hydrogen-bonded FN

MeOH complex are again of p nature. Thus, the pp nature of the T1 state for the hydrogen-bonded FN

MeOH complex is also elucidated. From frontier molecular orbital analysis for the isolated fluorenone and the hydrogen-bonded FN

MeOH complex it can be theoretically confirmed that the T1 state of the fluorenone chromophore in both polar and non-polar solvents is of pp nature, which is consistent with previous studies [70–85].

Insight from Singlet into Triplet Excited-State Hydrogen Bonding Dynamics in Solution 153

Figure 6.1 The frontier molecular orbitals HOMO, LUMO and LUMO þ 1 of both the isolated fluorenone and the hydrogen-bonded FN

MeOH complex

Furthermore, it is clear that the electron density of the LUMO orbital is strongly localized on the carbonyl group, while that of the LUMO þ 1 orbital is strongly localized on the benzene rings. Thus, the degree of electron delocalization between the benzeneringsand the carbonylgroup for the orbital transition from HOMO to LUMO is much stronger than that for the orbital transition from HOMO to LUMO þ 1. As a result, it can also be confirmed that the degree of electron delocalization from the benzene rings to the carbonyl group for fluorenone in alcoholic solvents is smaller in the T1(pp ) state than in the S1(pp ) state. So the electronic dipole moment of both the isolated fluorenone and the hydrogen-bonded FN

MeOH complex in the T1 state should be smaller than that in the S1 state [53]. In addition, the electronegativity of the carbonyl group in the T1 state should be weaker than that in the S1 state. Therefore, it could be expected that the intermolecular hydrogen bonding in the T1 state of the hydrogen-bonded FN

MeOH complex may be weaker than that in the S1 state. Calculated IR spectra of fluorenone in both the S0 and T1 states are shown in Figure 6.2. In addition, the ground-state IR spectrum of the hydrogen-bonded FN

MeOH complex is also shown for comparison. Note that the C¼O stretching vibrational mode is calculated to be 1780 cm
1 for the isolated fluorenone in the ground state, while the calculated C¼O stretching mode is 1756 cm
1 for the hydrogen-bonded FN

MeOH complex in the ground state. Thus, the formation of the intermolecular hydrogen bond C

O  H

O can induce a slight red-shift of 24 cm
1 for the C¼O stretching vibrational mode. Moreover, it is noted that the C¼O stretching mode of the isolated fluorenone is drastically red-shifted to 1678 cm
1 in the T1 state from 1780 cm
1 in the ground state. It is evident that the vibrational frequency of the C¼O stretching mode in the T1 state of fluorenone is located in the double-bond region. So the pp character of the T1 state for fluorenone can be confirmed again [82–85]. At the same time it can be noted that both the electronic excitation to the triplet states and intermolecular hydrogen bonding interactions can shift the stretching vibrational mode of the C¼O group to the red. However, the electronic excitation from the ground state to the T1 state can induce a larger redshift for the C¼O stretching mode than the intermolecular hydrogen bonding interaction. Therefore, the stretching mode of the C¼O group is not a sensitive vibrational mode for monitoring intermolecular hydrogen bonding [52, 53]. However, it has been demonstrated that the stretching mode of the O

H group is very sensitive to the intermolecular hydrogen bonding interaction [53]. On the other hand, the methanol moiety remains in its electronic ground state upon photoexcitation to the T1 state of the hydrogen-bonded FN

MeOH complex. So the stretching vibrational mode of the O

H group cannot be strongly influenced by the electronic excitation. Thus, changes in intermolecular hydrogen bonding can be distinctly reflected by monitoring the stretching vibrational mode of the O

H group in different electronic states.

154 Hydrogen Bonding and Transfer in the Excited State

Figure 6.2 The calculated IR spectra of fluorenone in both the S0 and T1 states. The ground-state IR spectrum of the hydrogen-bonded FN

MeOH complex is also shown for comparison (See Plate 7)

The calculated IR spectra of the hydrogen-bonded FN

MeOH complex in both the S0 and T1 states are shown in Figure 6.3. For comparison, the free O

H stretching mode of the ground-state methanol molecule is also shown here. It can be noted that the O

H stretching mode of the hydrogen-bonded FN

MeOH complex in the ground state is significantly red-shifted from 3817 to 3641 cm
1 owing to the formation of the intermolecular hydrogen bond C¼O  H

O. In the T1 state of the hydrogen-bonded FN

MeOH complex, the O

H stretching mode is further red-shifted from 3641 to 3541 cm
1. The larger spectral red-shift of the O

H stretching mode in the T1 state of the hydrogen-bonded FN

MeOH complex in comparison with that in the S0 state indicates that the intermolecular hydrogen bond C¼O  H

O is significantly strengthened in the T1 state of the fluorenone chromophore. Therefore, we have demonstrated that the intermolecular hydrogen bond C¼O  H

O between fluorenone chromophore and methanol solvents can also be strengthened in the

Figure 6.3 Calculated IR spectra of the hydrogen-bonded FN

MeOH complex in both the S0 and T1 states (See Plate 8)

Insight from Singlet into Triplet Excited-State Hydrogen Bonding Dynamics in Solution 155 

Table 6.2 The calculated hydrogen bond binding energies Eb (in kJ mol
1) and the related bond lengths L (in A), as
MeOH well as the dipole moment m (in D) in the ground state and S1 and T1 states of the hydrogen-bonded FN
complex and isolated fluorenone FN

MeOH

S0 S1 T1

FN

MeOH

Eb

LC¼O

LO  H

LH

O

m

LC¼O

m

LH

O

27.85 42.62 37.74

1.219 1.259 1.244

1.906 1.802 1.833

0.972 0.981 0.978

4.061 6.759 5.653

1.212 1.250 1.234

3.520 5.982 4.892

0.963 — —

triplet electronic excited states. It should be noted that the O

H stretching vibrational frequency is calculated to be 3482 cm
1 in the S1 state of the hydrogen-bonded FN

MeOH complex [53]. Thus, the O

H stretching mode in the T1 state is blue-shifted in comparison with that in the S1 state of the hydrogen-bonded FN

MeOH complex. That is to say, the intermolecular hydrogen bond C¼O  H

O in the T1 state may be weaker than that in the S1 state. The calculated hydrogen bond binding energies and the related bond lengths as well as the dipole moment in the T1 state of the hydrogen-bonded FN

MeOH complex and isolated fluorenone are listed in Table 6.2. The corresponding values in both the S0 and S1 states are also listed here for comparison. The bond length of the  C¼O group in the T1 state of isolated fluorenone is calculated to be 1.234 A. The calculated C¼O bond length  in the T1 state of the hydrogen-bonded FN

MeOH complex is slightly lengthened to 1.244 A. It can be noted that the bond length of the C¼O group in the T1 state of the isolated fluorenone and the hydrogen-bonded complex are of a double-bond character. It can also be noted that the hydrogen bond binding energy for the intermolecular hydrogen bond C¼O  H

O in the T1 state of the hydrogen-bonded FN

MeOH complex is calculated to be 37.74 kJ mol, which is significantly larger than that in the ground state. Thus, it is clear that the intermolecular hydrogen bond C¼O  H

O in the T1 state is significantly strengthened in comparison with that in the ground state. Thus, intermolecular hydrogen bond strengthening in the triplet electronic excited states is evidently confirmed. At the same time, the intermolecular hydrogen bond length is shortened to  1.833 A in the T1 state from 1.906 A in the ground state. Both the hydrogen-bonded C¼O and O

H groups are correspondingly lengthened in the T1 state. On the other hand, it can also be noted that the hydrogen bond binding energy of the intermolecular hydrogen bond C¼O  H

O in the T1 state is smaller than that in the S1 state. Thus, it is reconfirmed that the intermolecular hydrogen bond C¼O  H

O in the T1 state of the hydrogen-bonded FN

MeOH complex is weaker than that in the S1 state. In addition, it can also be noted that the electronic dipole moments of both the isolated fluorenone and the hydrogen-bonded FN

MeOH complex in the T1 state are smaller than those in the S1 state. As discussed above, intermolecular hydrogen bonds formed between fluorenone and alcoholic solvents can be dynamically changed following important photophysical processes. Upon photoabsorption, fluorenone is initially photoexcited to the S1 state, in which the intermolecular hydrogen bond is significantly strengthened in comparison with that in the ground state. After internal conversion (IC) from the S1 state to the ground state, the intermolecular hydrogen bond will be weakened in the ground state. On the other hand, the fluorenone molecules in the S1 state can also get to the T1 state through intersystem crossing (ISC). After the ISC process, the intermolecular hydrogen bond is also weakened in comparison with that in the S1 state. The different changes in intermolecular hydrogen bond in the triplet and singlet electronic excited states may be closely associated with the photophysics and photochemistry of fluorenone in alcoholic solvents. More experimental and theoretical studies will be needed to gain a good understanding of the relationship between the photophysics of fluorenone and the hydrogen bonding dynamics in the electronic excited state.

156 Hydrogen Bonding and Transfer in the Excited State

6.4 Conclusion The time-dependent density functional theory (TDDFT) method was used to investigate the triplet electronicexcited-state hydrogen bonding dynamics of the fluorenone chromophore in alcoholic solvents. In the present work, the geometric structures of the hydrogen-bonded FN

MeOH complex as well as the isolated fluorenone in the first triplet excited state (T1) were fully optimized using the TDDFT method. Moreover, the vibrational absorption spectra of the hydrogen-bonded FN

MeOH complex in the triplet excited states were also calculated by the TDDFT method. At the same time, the calculated triplet electronic-excited-state IR spectra were compared with those in the ground state and singlet electronic excited states. Consequently, the triplet excited-state hydrogen bonding dynamics of fluorenone in alcoholic solvents was investigated for the first time by monitoring the spectral shifts of some characterized vibrational modes involved in the formation of intermolecular hydrogen bonds in different electronic states. As a result, it was demonstrated that the intermolecular hydrogen bond can be significantly strengthened in both the S1 and T1 states of the fluorenone chromoophore in comparison with that in the ground state. In addition, we also found that the intermolecular hydrogen bond in the T1 state is weaker than that in the S1 state. Therefore, the strengthened intermolecular hydrogen bond in the S1 state will become weakened in the T1 state after the intersystem crossing (ISC) process from the S1 state to the T1 state. The hydrogen bond strengthening in the triplet excited state of fluorenone is consistent with the triplet excited-state spectral red-shift due to the formation of an intermolecular hydrogen bond. Furthermore, frontier molecular orbital (MO) analysis also confirmed that the intermolecular hydrogen bond can be strengthened in the T1 state in comparison with that in the ground state. The excited-state hydrogen bonding dynamics in the T1 and S1 states of the fluorenone chromophore in alcoholic solvents may play an important role in the competition between IC and ISC processes during S1 state deactivation.

Acknowledgements This work was supported by NSFC (Nos 20903094 and 20833008) and NKBRSF (Nos 2007CB815202 and 2009CB220010).

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7 Probing Dynamic Heterogeneity in Nanoconfined Systems: the Femtosecond Excitation Wavelength Dependence and Fluorescence Correlation Spectroscopy Shantanu Dey, Ujjwal Mandal, Aniruddha Adhikari, Subhadip Ghosh and Kankan Bhattacharyya Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

7.1 Introduction Liquids confined in a nanocavity play a key role in many natural processes. Prominent examples of nanoconfined liquids are water in the hydrophobic pocket of a protein or in a biological cell or many liquids in a nanoporous catalyst and in other materials. In recent years, femtosecond spectroscopy and computer simulations have generated a considerable amount of new knowledge on the dynamics of liquids confined in a nanocavity [1]. The nanoassemblies are heterogeneous on a molecular length scale. As a result, the spectra and dynamics of a fluorescent probe in different regions of such a nanoassembly are distinctly different. In an attempt spatially to resolve ultrafast dynamics, we have recently applied the excitation wavelength (lex) dependence. This method is based on the simple fact that molecules in different environments are spectrally distinct, and, as a result, at different lex, different subsets of molecules are excited. This is the basis of the so-called red-edge excitation shift (REES) which is observed in many organized assemblies [2–4]. Using this strategy, we have been able to delineate the difference in several ultrafast phenomena in different regions of a micelle, reverse micelle, gel and lipid vesicle. The most interesting observation is that the lex dependence shows even a neat ionic liquid to be heterogeneous. The spatial resolution of a microscope is
l/2, i.e.
200 nm for 400 nm excitation. As a fluorescent probe of
1 nm dimension probes its immediate neighbourhood, the spatial resolution achieved by lex variation exceeds the spatial resolution of a microscope. Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

160 Hydrogen Bonding and Transfer in the Excited State

In this review, we will concentrate on ultrafast solvation dynamics, excited-state proton transfer (ESPT), fluorescence resonance energy transfer (FRET) and fluorescence correlation spectroscopy (FCS) in several nanoconfined systems. The latter include protein, micelles, ionic liquids, a cyclodextrin host–guest complex and others.

7.2 Solvation Dynamics in Nanoconfined Systems Solvation in a nanocavity is fundamental in many biological and catalytic processes. We begin with solvation dynamics in a few nanoconfined assemblies.

7.2.1 Solvation dynamics: basic features Solvation dynamics probes the time evolution of interaction between a solute dipole with polar solvent molecules. On excitation of certain solutes by an ultrafast laser, a dipole is created suddenly. With the passage of time, as the solvent molecules reorganize, the energy of the solute dipole decreases. This causes a gradual shift of the fluorescence maximum to lower energy, i.e. towards a longer wavelength. This is known as the dynamic Stokes shift (DSS). The gradual change in solvation (i.e. solvation dynamics) is monitored by the decay of the solvation time correlation function, C(t), which is defined as [5] CðtÞ ¼

nðtÞnð1Þ nð0Þnð1Þ

ð7:1Þ

where n(0), n(t), and n(1) are the observed emission frequencies at time zero, t and infinity respectively. Solvation dynamics in bulk water and several other polar liquids (e.g. methanol, acetonitrile, etc.) may be described by a major component on a 0.1 ps (100 fs) timescale and a minor component of 1 ps [6]. Interestingly, the solvation dynamics of these polar liquids confined in many organized and biological assemblies displays a component on a 100–1000 ps timescale [1]. Many aspects of the ultraslow component have been reviewed earlier [1]. In this account, we focus only on the spatial or lex variation. 7.2.2 Solvation dynamics in ionic liquid A room-temperature ionic liquid (RTIL) consists of an extended positive ion with a large organic moiety and a relatively smaller negative ion. The steric hindrance between the ions frustrates the formation of crystals and consequently makes the melting point quite low, while the interionic attraction raises the boiling point and lowers the vapour pressure. Recent experimental studies and computer simulations suggest that an ionic liquid forms nanostructural aggregates if the alkyl group is sufficiently large [7, 8]. Very recent small-angle X-ray scattering and optical Kerr effect studies have indicated the formation of nanosized aggregates (1.3–2.7 nm) in imidazolium ionic liquids containing an alkyl group with more than three carbon atoms [9]. In such an aggregate there is a clear segregation of the polar (ionic) domain and the non-polar (alkyl group) domains. The structure and dynamics of the bulky ions in an RTIL play a key role in polar chemical reactions. The solvation dynamics in ionic liquids has been studied by ultrafast laser spectroscopy and large-scale computer simulation [10–13]. The early studies [10–13] focused mainly on the role of the counterions on solvation dynamics, but did not consider the presence of spatial heterogeneity in an RTIL.

Probing Dynamic Heterogeneity in Nanoconfined Systems

161

7.2.2.1 Neat Ionic Liquid: kex Dependence and Dynamic Heterogeneity Adhikari et al. have recently studied the lex dependence of solvation dynamics and anisotropy decay in a neat room-temperature ionic liquid (RTIL), 1-pentyl-3-methyl-imidazolium tetrafluoroborate ([pmim][BF4]) [4a]. In neat IL, heterogeneity may arise from clustering of the pentyl groups surrounded by a network of the cationic head group and anions. Using the excitation wavelength dependence, they spatially resolved the dynamics in different regions of neat [pmim][BF4]. The solvation dynamics of C480 in neat [pmim] [BF4] was found to depend on lex (Figure 7.1). For lex ¼ 375 nm, the decay of the solvent correlation function, C(t), exhibits an average solvation time of 860 ps. However, with increase in lex the average solvation time decreases by a factor of
6 to 135 ps at lex ¼ 435 nm. The faster solvation dynamics at long lex may arise from the polar region containing the BF4 anion, while the slower solvation at short lex may be ascribed to the nonpolar domain around the alkyl chains. A similar lex dependence of solvation dynamics was also reported by Maroncelli and coworkers [11a]. In contrast to solvation dynamics, the fluorescence anisotropy decay does not exhibit any lex dependence in neat [pmim][BF4]. In neat [pmim][BF4], C480 exhibits a very slow rotational dynamics with a single exponential decay of time constant 3800 ps. It may be noted that in bulk water the time constant of fluorescence anisotropy decay of C480 is much faster,
70 ps [14]. The slow anisotropy decay in neat ionic liquid may obviously be ascribed to the high viscosity of [pmim][BF4]. The rotational diffusion of the large (
1 nm) probe molecule sweeps almost the entire ([pmim][BF4]) aggregate (1.5 nm in size). During this motion, the probe experiences an average friction, and hence there is no lex or location dependence in neat RTIL. 7.2.2.2 RTIL Microemulsion An ionic liquid may be sequestered inside a reverse micelle in the form of a ‘pool’. Using small-angle neutron scattering studies (SANS), Eastoe and coworkers showed that the ionic liquid pool is ellipsoidal in shape with a semi-minor radius of 2.4 nm and a length 11 nm for an equimolar ratio of ([bmim][BF4]) and the surfactant

Neat [pmim][BF4]

0.8

λ ex

375 nm 405 nm 435 nm

C(t)

0.6

0.4

0.2 0

50

100

150

200

Time (ps)

Figure 7.1 Decay of the solvent response function, C(t), of C480 in neat [pmim][BF4] for lex ¼ 375 nm (D), lex ¼ 405 nm (*) and lex ¼ 435 nm (&). The points denote the actual values of C(t), and the solid lines denote the best fit. Reprinted with permission from [4a]. Copyright 2007 American Chemical Society

162 Hydrogen Bonding and Transfer in the Excited State

(TX-100) [15a]. Gao et al. reported that as much as 6 wt% water may be encapsulated in the polar nanodomain of such a microemulsion [15b]. Solvation dynamics of C480 in the [pmim][BF4]/TX-100/benzene microemulsion is found to exhibit a much stronger lex dependence compared with neat RTIL [pmim][BF4]. For all lex, the decay of C(t) in this microemulsion may be described by a triexponential decay – a very slow (2200 ps), a slow (200 ps) and a very fast (2 ps) component [4a]. Apart from this, there is an ultrafast component (<0.3 ps) that was missed owing to the resolution of the femtosecond set-up. The different components may originate from the different regions of the heterogeneous microemulsion. The long 2200 ps component may be assigned to the highly constrained region within the interior of the TX-100 surfactant chains, while the 200 ps component may be ascribed to the interface region near the polar head group of TX-100. The ultrafast components (<0.3 and 2 ps) may arise from the core of the ionic liquid pool where mobility of particularly the counterion is quite fast. In this microemulsion, the contribution of the ultrafast components (<0.3 and 2 ps) increases from 19% at lex ¼ 375 nm to 84% at lex ¼ 435 nm respectively and the average solvation time decreases
50 fold from 1480 ps at lex ¼ 375 nm to 30 ps at lex ¼ 435 nm. The addition of water to the [pmim][BF4]/TX-100/benzene microemulsion results in slightly slower solvation dynamics (longer average solvation time at all lex) [4a]. As revealed through dynamic light scattering (DLS) studies, this may be attributed to the smaller size of the ‘pool’ for the water-containing microemulsion (diameter
9 nm) compared with the large (diameter
31.8 nm) microemulsion without water. In the smaller water-containing microemulsion, the distance of the polar entities (water and RTIL) from the head group of the surfactant (TX-100) is shorter, and this closer confinement makes the dynamics slower. 7.2.2.3 RTIL Mixed Micelle Triblock copolymers are known to interact with ionic liquids to form mixed micelles. Liu et al. investigated the interaction of the triblock copolymer (PEO)27–(PPO)61–(PEO)27 (Pluronic P104) with a hydrophilic ionic liquid, 1-butyl-3-methyl-imidazolium tetrabromide ([bmim][Br]) [16]. They showed that the size of the P104 micelles increases on gradual addition of [bmim][Br]. Dey et al. studied a mixed micelle containing 1-pentyl-3-methyl-imidazolium bromide [pmim][Br] and a triblock copolymer P123 [4b]. Their dynamic light scattering (DLS) study suggests the formation of micronsized aggregates containing P123 and the RTIL [pmim][Br], which are much bigger than a P123 micelle (18 nm). In 0.9 M [pmim][Br] and 5 wt% P123, the major (
70%) contribution to the DLS signal arises from a particle of 40 nm diameter, and
30% from a P123 micelle-like particle. In 3 M [pmim][Br] there is a negligible contribution of the P123 micelle, and
90% of the DLS signal originates from a giant particle of 3.5 mm (3500 nm) diameter. The substantial red-edge excitation shift observed in the emission spectra of probe C480 in such a system made it possible to employ variation of lex spatially to resolve dynamics in different regions. For the mixed micelle containing 0.9 M [pmim][Br] and 5 wt% P123, the total dynamic Stokes shift (DSS ¼ n(0)  n(1)), decreases 2.5-fold from 1400 to 570 cm1 with an increase in lex from 375 to 435 nm. This is accompanied with a nearly 25-fold decrease in the average solvation time from 1900 to 75 ps. For lex ¼ 375 nm (i.e. for the core region of the copolymer micelle), almost the entire amount of solvation is captured. The decrease in DSS and average solvation time with increase in lex implies that the solvation dynamics is faster in the corona region (probed at lex ¼ 435 nm) than that in the core (selected at lex ¼ 375 nm). For the micron-sized aggregate (3 M [pmim][Br] and 5 wt% P123, diameter 3500 nm), the solvation dynamics is found to be faster than that in the 40 nm particle. The lex dependence of solvation dynamics in the micron-sized particle is less prominent than that in the 40 nm particle (0.9 M RTIL). For the micron-sized particle, the DSS decreases
1.5-fold from 850 cm1 at lex ¼ 375 nm to 525 cm1 at lex ¼ 435 nm, while the average solvation time decreases about 3 times from 500 to 175 ps.

Probing Dynamic Heterogeneity in Nanoconfined Systems

163

The different magnitude of lex variation of the 3500 nm and the 40 nm particle may be ascribed to their structures. In the 40 nm particle (RTIL:P123 molar ratio
100) the difference in the core and corona regions is quite large and the ultraslow core is still retained. In the micron-sized particle (RTIL:P123 molar ratio
340) the core is affected to a very large extent and the contribution of the ultraslow component (
4000 ps) arising from the core is very small. Thus, the average solvation time is very short in the micronsized particle. In order to study the effect of a counterion on the solvation dynamics of such a system, Adhikari et al. studied a mixed micelle comprising the RTIL 1-pentyl-3-methyl-imidazolium tetrafluoroborate, [pmim] [BF4] and P123 [4c]. The ionic liquid [pmim][BF4] is sparingly soluble in water. However, in the presence of P123, as much as 0.9 M [pmim][BF4] dissolves in water. Dynamic light scattering (DLS) data indicates the formation of aggregates involving both P123 and [pmim][BF4]. On addition of [pmim][BF4], the size of the aggregate is found initially to increase slightly to 21 nm in 0.3 M RTIL, but subsequently it shrinks to a diameter of 13.5 nm with further addition of the RTIL at 0.9 M. Incorporation of a large number of [pmim][BF4] molecules within P123 appears to affect the structure of the micelle. As [pmim][BF4] remains mostly as an ion pair within the P123 micelle, it preferentially goes to the core (PPO block) of the P123 micelle owing to its hydrophobic nature. The presence of a large number of ion pairs in the core and their electrostatic repulsion may be responsible for the expansion of the core and hence the observed slight increase in size in 0.3 M [pmim][BF4]. However, at a higher [pmim][BF4] concentration, the corona region is also affected (as evidenced by a decrease in the red-edge excitation shift (REES)). Thus, the nature of the micelle changes above 0.3 M [pmim][BF4], and this may be the reason for decrease in the size of the mixed micelle. It is interesting to note that, in sharp contrast to the case of interaction between the RTIL [pmim][BF4] and P123 micelles, there is a gradual increase in the size of the aggregates when the former is replaced with [pmim] [Br]. The discrepancy may probably be traced to the difference in relative hydrophilicity of the two ILs involved, i.e. [pmim][BF4] and [pmim][Br], with the bromide ion being more hydrophilic. For lex ¼ 375 nm, the solvation dynamics in a P123 micelle in the presence of 0.3 M [pmim][BF4] displays a very fast component (
4 ps) and a very long component (4000 ps), along with an intermediate component (200 ps). With increase in lex, the solvation dynamics becomes faster in both cases. For lex ¼ 435 nm, in both systems a major part of the solvation dynamics is missed in our femtosecond set-up (resolution 0.3 ps) and a 2–4 ps component is detected. In the presence of 0.3 M ionic liquid, the contribution of the ultraslow component decreases gradually from 55% at lex ¼ 375 nm to 30% at lex ¼ 405 nm and finally disappears at lex ¼ 435 nm. The average solvation time is
2270 ps at lex ¼ 375 nm and 20 ps at lex ¼ 435 nm. Similarly, sharp acceleration in solvation dynamics with increase in lex and the complete disappearance of the ultraslow component at lex ¼ 435 nm are also observed in P123 micelle in the presence of 0.9 M ionic liquid. The very fast solvation dynamics in 0.9 M ionic liquid may be attributed to the almost complete absence of the slow core region because of penetration of the ionic liquid. In 0.9 M RTIL and 5 wt% P123, the average solvation time decreases from 260 ps at lex ¼ 375 nm to 5 ps at lex ¼ 435 nm. Thus, the structural differences incurred owing to the gradual addition of an RTIL are also reflected in the changes observed in the dynamics. 7.2.3 Solvation dynamics in bile salt aggregate Bile salts are bioactive natural surfactant molecules that are synthesized in the liver and stored in the gall bladder. Micellization of bile salts and their interaction with biological membranes play an important role in biliary secretion and solubilization of cholesterol, lipid, bilirubin, fat-soluble vitamins and many other species in living organisms [17]. A bile salt exhibits facial amphiphilicity. The non-planar bile salt sodium deoxycholate (NaDC) consists of a convex hydrophobic surface (the steroid ring) and a concave hydrophilic

164 Hydrogen Bonding and Transfer in the Excited State

C(t)

surface (hydroxyl groups and carboxylate ions). In a slightly alkaline medium (pH > 7.5), NaDC displays two critical micellar concentrations (CMC1 and CMC2) at 10 and 60 mM respectively. Above CMC1, primary aggregates are formed, which consist of very few (2–10) monomers [17a]. In the primary aggregate, the polar face of NaDC (hydroxyl group and ions) points outwards. The secondary aggregate (formed at a concentration of >60 mM) resembles an elongated hollow cylinder with a central water-filled tunnel [17, 18]. For NaDC, the   cylinder is about 40 A long, and the radius of the hollow core is
8 A [18]. The microenvironment and binding dynamics of many fluorescent probes at different sites of a bile salt aggregate has long been the subject of study [19]. Adhikari et al. studied the solvation dynamics in 105 mM NaDC, employing C480 as the fluorescent probe [20]. The emission maximum of C480 displays a 8 nm red-shift as lex increases from 345 to 435 nm. The 8 nm red-edge excitation shift (REES) suggests that probe C480 is distributed over regions of varied polarity. Excitation at a short wavelength (375 nm) preferentially selects a probe molecule in the buried locations and exhibits slow dynamics with a major component (84%), a slow component (3500 ps) and a small (16%) contribution of an ultrafast component (2.5 ps). Excitation at lex ¼ 435 nm (red-end) corresponds to the exposed sites where solvation dynamics is very fast with a major (73%) ultrafast component (2.5 ps) and a relatively minor (27%) slow (2000 ps) component (Figure 7.2). The fluorescence anisotropy decay of C480 in bile salt aggregate also exhibits a lex dependence. However, the trend is opposite. For lex ¼ 375 nm (shorter wavelength), the anisotropy decay of C480 in NaDC is faster with an average rotational time 1300 ps (Figure 7.3). At lex ¼ 435 nm the average rotational time is 2100 ps. It is apparent that, with increase in lex, the anisotropy decay of C480 in NaDC aggregate becomes 1.5 times slower (Figure 7.3). Inside the bile salt aggregate the non-planar sheet-like structure and lack of many hydrogen bonding sites of the NaDC molecule may give rise to ‘void’ regions with very few water molecules. Thus, part of the probe C480 may experience very low friction. This may render anisotropy decay relatively faster inside the cavity (lex ¼ 375 nm).

1.0

0.9

0.8

0.6

0.6

0.3 0

50

100

150

0.4 0.2 0.0 0

3000

6000

9000

12000

Time (ps)

Figure 7.2 Complete decay of the solvent response function C(t) of C480 in NaDC for lex ¼ 375 nm (D), lex ¼ 405 nm (&) and lex ¼ 435 nm (). The points denote the actual values of C(t), and the solid line denotes the best fit. Initial portions of the decays are shown in the inset. Reprinted with permission from [20]. Copyright 2008 American Chemical Society

Probing Dynamic Heterogeneity in Nanoconfined Systems

165

0.4 C

0.3 0.2

0.3

r(t)

C

0.0

0.2

A

0.1 0

2

4

6

A

0.1

0.0

0

2

4 Time(ns)

6

8

Figure 7.3 Fluorescence anisotropy decays of C480 (lem ¼ 460 nm) along with fitted curves in NaDC at (A) lex ¼ 375 nm (*), (B) lex ¼ 405 nm (~) and (C) lex ¼ 435 nm (). The fitted curves are shown in the inset. Reprinted with permission from [20]. Copyright 2008 American Chemical Society

7.2.4 Solvation dynamics in P123 triblock copolymer Water-soluble triblock copolymers have recently received a great deal of attention because of their complex aggregation behaviour and widespread industrial applications [21]. The symmetric (A–B–A) triblock copolymers composed of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) are denoted by (PEO)20–(PPO)70–(PEO)20 (Pluoronic P123). The PPO block is soluble in water below 288 K. The hydrophobicity of the PPO block above 288 K and consequent dehydration lead to the formation of a micelle consisting of PEO–PPO–PEO triblock copolymers. According to SANS studies, such a micelle consists of a hydrophobic core presumably containing the PPO block and a hydrophilic corona of the PEO block [21a, b]. 7.2.4.1 P123 Gel In a 30 wt% aqueous solution, the triblock copolymer Pluronic P123 forms a cubic gel phase [21d, e]. In the gel,  the micelles associate to form an interconnected network with an intermicellar distance (140 A ) of less than the  sum (160 A) of the radii of the micelles [21d, e]. Ghosh et al. studied the lex dependence of solvation dynamics in P123 gel [22]. It is observed that, even in the solid phase, there is a bulk water-like ultrafast component (2 and 0.3 ps). The ultrafast component is attributed to the water molecules in voids of the gel. The PPO core in a gel gives rise to a very long component of 4500 ps. There is a third component of 500 ps which is ascribed to chain–chain entanglement. With increase in lex from 375 to 435 nm, the contribution of the ultrafast bulk water-like region increases from 6 to 87%, while that of the ultraslow component (4500 ps) due to the PPO core region decreases from 90 to 7%. 7.2.4.2 P123–SDS Aggregate In the P123–SDS aggregate, SDS penetrates the core of the P123 micelle. This increases the polarity of the core and reduces the difference in the polarity between the core and the corona region [23]. Recently, Mandal et al.

166 Hydrogen Bonding and Transfer in the Excited State

studied the lex dependence of solvation dynamics in a P123–SDS aggregate [24a]. In a P123–SDS aggregate, REES is much smaller (5 nm) than the P123 micelle (25 nm), which suggests a reduced difference between the core and the corona. Solvation dynamics in a P123 micelle displays a bulk-like ultrafast component (<0.3 and 1–ps) in the PEO corona region, a 200 ps component arising from dynamics of polymer segments and a very long component (5000 or 3000 ps) due to the highly restricted PPO core. In a P123–SDS aggregate, at lex ¼ 375 and 405 nm the solvation dynamics is found to be faster than that in the P123 micelle. In this case, the component (3000 ps) arising from the core region is faster than that (5000 ps) in the P123 micelle. In both the P123 micelle and the P123–SDS aggregate, the relative contribution of the core region decreases and that of the corona region increases with increase in lex. At lex ¼ 435 nm, which probes the hydrophilic corona, the solvation dynamics for both the P123 micelle and the P123–SDS aggregate are almost similar. 7.2.4.3 P123–CTAC Aggregate Dey et al. studied the lex dependence of solvation dynamics in a P123–CTAC aggregate [24b]. In a P123–CTAC aggregate, with increase in lex from 335 to 445 nm, the emission maximum of C480 exhibits a significant red-edge excitation shift (REES) by 22 nm. This suggests that the P123–CTAC aggregate is quite heterogeneous. For lex ¼ 375 nm, the probe molecules in the buried core region of P123–CTAC are excited, and the solvation dynamics displays three components – 2, 60 and 4000 ps. It is argued that the insertion of CTAC in the P123 micelle affects the polymer chain dynamics, and this leads to a reduction in the 130 ps component of the P123 micelle to 60 ps in P123–CTAC. For lex ¼ 435 nm, which selects the peripheral highly polar corona region, the solvation dynamics in P123–CTAC and P123 is extremely fast, with a major component of <0.3 ps (
80%) and a 2 ps (
20%) component.

7.3 Fluorescence Resonance Energy Transfer (FRET): kex Dependence Fluorescence resonance energy transfer (FRET) refers to the non-radiative energy transfer from an excited donor molecule (D) to an acceptor (A). FRET inside an organized assembly is expected to be much faster than that in bulk water because of the proximity of the donor and the acceptor. At a very short D–A distance, the electron clouds of the donor and acceptor may overlap, and basic assumptions of Forster theory (product wave functions and point–dipole approximation) may not be valid. FRET between a donor and an acceptor both enclosed in the same micelle is a simple system to study ultrafast FRET at a short distance. Fluorescence resonance energy transfer (FRET) [25, 26] may also exhibit a location, and hence a lex, dependence for two reasons. First, the donor–acceptor distance (RDA) may be different for a donor molecule residing in different locations with a fixed location of the acceptor. Second, spectral overlap between donor emission and acceptor absorption may be different in different regions. 7.3.1 FRET in ionic liquid microemulsions: kex dependence Recently, Adhikari et al. studied the lex dependence of FRET in different regions of an RTIL microemulsion [27]. The microemulsion comprises the RTIL, [pmim][BF4] in TX-100/benzene, with and without water. They used an ionic dye (rhodamine 6G, R6G) as the acceptor. The acceptor, being an ion, preferentially stays in the core of the ionic liquid pool of the microemulsion. The donor (C480), being a neutral molecule, is distributed over different regions of the entire microemulsion. This results in a wide distribution of the donor–acceptor distances (RDA). They studied FRET from the donors residing in different regions of the microemulsion by variation of lex. At a short lex, a donor (C480) molecule in a relatively non-polar region near the surfactant chains is excited, while at a long lex a donor in the polar region (core of the ionic liquid pool) is

Probing Dynamic Heterogeneity in Nanoconfined Systems

167

excited. In the first case (short lex) RDA is large, while in the second case (long lex) RDA is small. This is expected to affect the rate of FRET. The time constants of FRET were obtained from the rise time of the acceptor (R6G) emission. In the absence of water the rise components observed are 1, 250 and 3900 ps, while in the presence of water the rise components are quite similar – 1.5, 250 and 3900 ps. The 1 and 1.5 ps components are assigned to FRETat a close contact of donor and acceptor (RDA
12 A). This occurs within the highly polar (RTIL/water) pool of the microemulsion. With increase in lex from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (
1 ps) increases from 4 to 100% in the RTIL microemulsion and from 12 to 100% in the water-containing RTIL microemulsion. It is suggested that, at lex ¼ 435 nm, mainly the highly polar IL pool is probed, where FRET is very fast owing to the close proximity of the donor and the acceptor. The  very long 3900 ps (RDA
45 A) component may arise from FRET from a donor within the surfactant alkyl chains of the microemulsion to an acceptor residing in the polar IL pool. 7.3.2 FRET in a P123 triblock copolymer micelle and gel: kex dependence Ghosh et al. studied the lex dependence of FRET in the micelle and the gel phase of a triblock copolymer P123 [28]. In a P123 micelle, FREToccurs on multiple timescales – 2.5, 100 and 1700 ps. In the gel phase, three rise components are observed – 3, 150 and 2600 ps. According to a simple F€orstermodel, the ultrafast (2.5 and 3 ps) components of FRET correspond to the donor–acceptor distance RDA ¼ 13 A. The ultrafast FRET occurs between a donor and an acceptor residing at close contact in the corona (PEO) region of a P123 micelle. With increase in lex from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (
3 ps) increases from 13 to 100% in P123 micelle and from 1 to 100% in P123 gel. It is suggested that, at lex ¼ 435 nm, mainly the highly polar peripheral region is probed, where FRET is very fast owing to the close proximity of the donor and the acceptor. The 100 and 150 ps components correspond to RDA ¼ 25 A and are ascribed to FRET from C480 deep inside the micelle to an acceptor (R6G) in the peripheral region. The very long component of FRET (1700 ps in micelle and 2600 ps component in gel) may arise from diffusion of the donor from outside the micelle to the interior, followed by fast FRET. 7.3.3 FRET in bile salt aggregate: kex dependence Recently, Mandal et al. also studied FRET from coumarin 153 (C153) to rhodamine 6G (R6G) in a secondary aggregate of a bile salt, NaDC [29]. The emission spectrum of C153 in NaDC is analysed in terms of two spectra – one with an emission maximum at 480 nm, which corresponds to a non-polar and hydrophobic site, and another with a maximum at
530 nm, which arises from a polar hydrophilic site. In the NaDC aggregate, FRET occurs on multiple timescales – 4 and 3700 ps. The 4 ps component is assigned to FRET from a donor to  an acceptor held at a close distance (RDA
17 A) inside the bile salt aggregate. The 3700 ps component corresponds to a donor–acceptor distance of
48 A. The long (3700 ps) component may involve diffusion of the donor. With increase in lex from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (
4 ps) increases from 3 to 40% with a concomitant decrease in the contribution of the ultraslow component (
3700 ps) from 97 to 60%. The lex dependence is attributed to the presence of donors at different locations. At a long lex (435 nm), donors in the highly polar peripheral region are excited. A short lex (375 nm) selects donors at a hydrophobic location.

7.4 Excited-state Proton Transfer (ESPT) Excited-state proton transfer (ESPT) in bulk liquids and confined assemblies provides valuable information about the mechanism and nature of acid–base reactions. The acidity of many molecules markedly increases in

168 Hydrogen Bonding and Transfer in the Excited State kdiss

kPT (ROH)* + H2O

(RO

*…

+

RO * + H3O

H3O )

krec

kp[H+]w

kRO

kROH ROH

kRO RO

RO

Scheme 7.1 Photophysical processes of an ESPT probe (ROH)

the excited state. For instance, pKa of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) decreases from 7.4 in the ground state to 0.4 in the first excited state [30a]. Thus, an excited HPTS molecule rapidly transfers a proton to a water molecule even in a highly acidic medium (e.g. pH
1). Excited-state proton transfer (ESPT) in an organized assembly differs markedly from that in bulk water. The local pH or pKa of the acid may be very different from that in the bulk because of the differences in polarity and the presence of counterions and consequently electrostatic interactions. The pH at the surface of a cationic micelle (CTAB) has been determined using HPTS as a probe [30b]. It is observed that at a bulk pH 7, the pH at the surface of CTAB micelle is
9.5. The ESPT process in a photoacid (ROH) may involve three steps: initial proton transfer (kPT), recombination of the geminate ion pair (krec) and dissociation of the geminate pair into a solvent-separated ion pair (kdiss) (Scheme 7.1): The time evolution of the different species in Scheme 1 is described by the following coupled differential equations: 2

ROH

3

2

X

7 6 d6 6 RO    H þ 7 ¼ 6 kPT 4 5 4 dt  0 RO

krec Y kdiss

0

3

2

ROH

3

7 7 6  þ 7 6 0 7 5  4 RO    H 5 Z RO

ð7:2Þ

where X ¼ kPT þ kROH  kPT , Y ¼ krec þ kdiss þ kRO and Z ¼ kRO . Fayer and coworkers studied ESPT of HPTS in various solvents to elucidate the nature of proton transfer in water [30c]. They employed a global fitting procedure to explain both the spectral shift (Stokes shift) caused by solvent reorganization and deprotonation of HPTS in water. They assigned the
1 ps component to dynamic Stokes shift and the two other components (3 and 88 ps) to the deprotonation process. Rini et al. used femtosecond visible pump and mid-infrared (mid-IR) probe spectroscopy to study ESPT of HPTS to acetate [31a, b]. They monitored the rise of the carbonyl stretching band (at 1720 cm1) of acetic acid, which clearly marks the arrival of the proton to acetate. They detected an ultrafast (<150 fs) time component, which is attributed to the deuteron transfer in ‘tight’ or directly hydrogen-bonded complexes without intermediate water molecules. They further reported a 6 ps component for deuteron transfer between HPTS and acetate. This is almost two orders of magnitude slower than the sub-150 fs component in a ‘tight’ acid–base complex. They attributed the 6 ps component to the acid–base contact pairs which are separated by D2O [31a, b]. Mohammed et al. showed that proton transfer from HPTS to monochloroacetate ( OAc-Cl) in D2O involves ‘loose complexes’ with D2O bridges separating HPTS and  OAc-Cl [31c]. They proposed that proton transfer involves three steps. In the first step, the deuteron is transferred to the D2O to form an intermediate (HPTSD3Oþ  OAc-Cl). This photoacid dissociation process occurs within 150 fs. Then, the deuteron is transferred to the acetate in 25 ps to form the ‘loose’ product complex (HPTSD2ODOAc-Cl). Finally, the product complex dissociates in 50 ps in a diffusion-controlled process.

Probing Dynamic Heterogeneity in Nanoconfined Systems

169

ESPT in an organized assembly differs markedly from that in bulk water. Firstly, the local pH or pKa of the acid may be very different from that in the bulk because of the differences in the polarity and the presence of counterions and consequently electrostatic interactions. The absorption spectra of the protonated (ROH) and deprotonated (RO) form of HPTS are very different. Thus, HPTS may be utilized as a pH indicator. Using the absorption spectra of HPTS, Roy et al. estimated the pH at the surface of a cationic micelle (CTAB) [32a]. They found that, at a bulk pH 7, the pH at the surface of a CTAB micelle is
9.5. The higher pH (9.5) at the surface of the cationic micelle CTAB suggests a lower concentration of proton. This may be ascribed to the repulsion experienced by the protons from the cationic CTAB. Using picosecond and femtosecond fluorescence spectroscopy, Mondal et al. investigated ESPT of HPTS in a g-cyclodextrin (g-CD) cavity [32b]. They showed that the ESPTof HPTS is slowed down inside a g-CD cavity compared with bulk water because of retardation of the initial proton transfer step, slowing down the dissociation of the geminate ion pair. As, inside the cyclodextrin cavity, there is little scope for diffusion of the geminate ion pair, the main consequence of confinement seems to force the anion and the hydronium ion to stay in close proximity. Several groups used MD simulations to understand ESPT in a nanocavity [33]. MD simulation and electrostatic potential calculations suggest that, at the rim of the cyclodextrin, because of its interaction with the CD cavity, the proton donor/acceptor property of water molecules is seriously impaired [33a]. Sahu et al. showed that deprotonation, recombination and dissociation of the geminate ion pair in the lysozyme–CTAB aggregate are faster than those in a CTAB micelle [34a]. At the surface of the micelle, emission of HPTS is quenched by an acetate ion with a bimolecular quenching constant of 3.5  107 M1 s1 [32a]. This is markedly slower than the ESPT in a solvent-separated HPTS–water–acetate complex in bulk water. It is interesting to note that, even at very high local concentration of acetate (
7.75 M) in the Stern layer, the rate of proton transfer from HPTS to acetate occurs on a 1300–2050 ps timescale [32a]. This is an order of magnitude slower than the time constant (6 ps) of ESPT in a solvent-separated HPTS–water–acetate system in bulk water [34b]. The slow ESPT at the micellar surface compared with bulk water may be ascribed to the following. First, the rigidity of the water hydrogen-bonded network in the Stern layer of the micelle slows down ESPT. Second, at the surface of a micelle, surfactant chains may be inserted between HPTS and acetate and thus prevent direct ultrafast proton transfer. Mondal et al. reported that ESPT from HPTS to acetate inside a g-cyclodextrin (g-CD) and a hydroxypropyl g-nanocavity (Hp-g-CD) occurs in
90 ps, which is much slower than that in bulk water [35a]. The acetate  group does not make a direct hydrogen bond to the OH group of HPTS (distance
4 A). Instead, the acetate group remains hydrogen bonded to the two OH groups of the g-CD (Figure 7.4). The OH group of HPTS is also hydrogen bonded to an OH group of g-CD. Inside the g-CD cavity, the acetate is separated from the OH group of HPTS by two water molecules as bridges [35a]. In this case, proton transfer from HPTS to acetate is not direct and is mediated by water bridges and thus resembles the Grotthuss mechanism. Obviously, in the cavity, ESPT from HPTS to acetate requires rearrangement of the hydrogen bond network and the cyclodextrin cavity. In 40 mM g-CD, the rate of the initial proton transfer process (kPT) increases
3-fold from 4.0 0.4  103 ps1 at 0 M acetate to 11 2.0  103 ps1 at 2 M acetate. In contrast, in the case of Hp-g-CD, the initial proton transfer rate (kPT) remains almost unaffected on the addition of acetate. It seems that the hydroxypropyl group of Hp-g-CD shields the encapsulated HPTS molecule from the acetate. Hence, it is more difficult for the acetate to access HPTS in Hp-g-CD than in unsubstituted g-CD. As a result, ESPT in Hp-g-CD is much slower [35a]. Ghosh et al. studied the excited-state proton transfer of HPTS in a polymer–surfactant aggregate (P123–CTAC) [35b]. Both steady-state and time-resolved studies suggest that ESPTof HPTS in a P123–CTAC aggregate is much slower than that in bulk water, or in the individual micelle. The ratio of the steady-state emission intensities (ROH/RO) in a P123–CTAC aggregate is
50, 12 and 2 times higher than that respectively in water, in P123 micelle and in CTAC micelle. Retardation of ESPT causes an increase in the rise

170

Hydrogen Bonding and Transfer in the Excited State

CH3

O

O 2.78 A

( -CD)

HO

O (W) 2.63 AO

2.71 A (W)

HO

(HPTS)

OH ( -CD) 2.86 A

OH( -CD)

Figure 7.4 Hydrogen bonding in the immediate neighbourhood of the hydroxyl (OH) group of HPTS inside the g-CD cavity. O(W) denotes the oxygen atom of water. Reprinted with permission from [35a]. Copyright 2006 American Chemical Society

time of the RO emission of HPTS. In a P123–CTAC aggregate, RO displays three rise times: 30, 250 and 2400 ps. These rise times are longer than those in a CTAC micelle (23, 250 and 1800 ps), in bulk water (0.3, 3 and 90 ps) and in P123 micelle (15 and 750 ps). The rate constants for initial proton transfer, recombination and dissociation of the ion pair are found to be slower in a P123–CTAC aggregate than in bulk water.

7.5 Diffusion of Organic Dyes by Fluorescence Correlation Spectroscopy (FCS) Single-molecule spectroscopy and fluorescence correlation spectroscopy (FCS) can be used to study the motion (diffusion) of a single molecule in a liquid or in an organized assembly [36–43]. FCS is frequently used to determine the local concentrations, diffusion coefficient or various physical parameters of biomolecules in nanomolar concentrations. Webb and coworkers first applied FCS to measure the diffusion and chemical dynamics of DNA–drug interaction [44]. Using farfield microscopy, Moerner et al. monitored the Brownian motion of a single dye molecule in a polyacrylamide gel [36]. Recently, many groups have used FCS to study the mobility and diffusion constants of fluorescent probes in different systems, including vesicles and membranes [37], colloidal particles [38], agarose gel [39], polymer gel [40a], a supramolecular complex [40b], micelles [41] and microemulsions [42], and also to study protein aggregation [43]. Recently. Ghosh et al. have applied FCS to study the translational diffusion of a fluorescence probe in P123 and F127 [(PEO)100–(PPO)70–(PEO)100] micelle and gel [45]. They used a non-covalent probe instead of a covalent one because the motion of a dye molecule covalently attached to a macromolecule is dominated by the superimposed motion of the macromolecule. In contrast, a non-covalent fluorescent probe faithfully reports the motion of the dye molecule in a micelle or a gel. Owing to the large hydrodynamic radius, the diffusion of a micellar aggregate is expected to be much smaller than that of an isolated organic molecule. In a gel, motion of the micellar aggregate is largely suppressed, and the temporal evolution of the fluorescence autocorrelation arises solely from the motion of the molecule through the highly restricted and entangled medium.

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171

On a molecular length scale, micelles, gels and most surfactant assemblies are heterogeneous in nature. In such a system (micelle or gel), the corona region (PEO blocks) involves large-scale penetration of water. This leads to high mobility of a probe in the corona region. In contrast, the core (PPO block) consists of the highly compact and entangled polymer chains which offer large friction to molecular motion. In such a micelle or gel, the mobility (diffusion) of a dye molecule may be location dependent, with fast mobility in the corona region and slow mobility in the core region. 7.5.1 Diffusion in P123 and F127 micelles A fluorescent molecule remains attached to a particular micelle only if there is a strong electrostatic attraction between the two (e.g. cationic dye for an anionic surfactant) or if the probe is covalently attached to the surfactant. For a neutral micelle (like P123 and F127) there is no such electrostatic attraction between the organic dye and the micelle. Zettl et al. have previously reported that, for a neutral micelle (C12E5), the observed Dt is not small and is marginally lower (
2-fold) than that of the same dye in water [41a]. The hydrodynamic radii of both P123 and F127 micelles are
9 nm, which is nearly 13 times larger than that of the hydrodynamic radius of DCM (
0.7 nm). This indicates that DCM remains attached to a particular micelle throughout its passage through the confocal volume (0.3 fL), and thus the observed Dt corresponds to that of the micellar aggregate. DCM, being a highly hydrophobic dye, remains confined to a particular micelle (core). As a result, DCM reports slow diffusion of the large micellar aggregate with a diffusion coefficient (Dt ¼ 12–14 mm2 s1)
25 times slower compared with water (Dt ¼ 300 mm2 s1).45 For C480 and C343 (hydrophilic probe), the Dt values in the two micelles are found to be lower than those in water by a much small factor (
2–6-fold). The large value of Dt in this case (and also for other probes in neutral micelles) may arise from the occasional escape of the dye from a particular micelle into bulk water during diffusion through the confocal volume. As both C480 and C343 are hydrophilic (and water soluble), and as a large fraction of them reside in the corona region of the micelles (P123 and F127), such a transfer of dye molecule from micelle to bulk water is quite likely. As a result, for C343 and C480 the Dt is
20 times higher than that for DCM in the F127 micelle and
10 times higher in the P123 micelle. The relatively small value of red-edge excitation shift (REES) of the emission maximum suggests that DCM is confined within the core of the triblock copolymer micelles and gels. The hydrophilic probes (C343 or C480) exhibit fast diffusion in the micelles and gels. However, their REES is very different. The large REES of C480 suggests that it is distributed over a large region of the micelle, while the low REES of C343 indicates that it is located primarily in the peripheral corona region. 7.5.2 Diffusion in P123 and F127 gels In a gel, diffusion of a micellar aggregate is completely suppressed. Temporal evolution of the fluorescence autocorrelation arises from the motion of the dye through the pores of the network. In both the gels, the diffusion coefficients of all the three probes are found to be substantially slower than those in bulk water. In the case of F127 gel, Dt of DCM, C480 and C343 are respectively 1, 13 and 29 mm2 s1. These are respectively 300, 45 and 20 times slower compared to that for the same dye in bulk water. The dramatic retardation of diffusion may be attributed to the slow diffusion of the probe through the porous network of the gel. A similar trend is also observed in the case of P123 gel, with the Dt of DCM, C480 and C343 being 1, 7 and 7 mm2 s1 respectively. For two hydrophilic dyes (C480 and C343), the Dt values are very close in the P123 and F127 gels. In P123 gel, the Dt values are identical, but they differ by a factor of 2 in F127 gel. However, the Dt of DCM in the gel is much smaller than that of C343 (29-fold smaller in F127 gel and sevenfold smaller in P123 gel). Such different

172 Hydrogen Bonding and Transfer in the Excited State

Dt values suggest that the microenvironment of DCM and C343 are quite different. On the basis of the structure of the gels, Ghosh et al. propose that DCM is confined within the highly restricted core region. In contrast, C343 is located in the mobile peripheral corona region.

7.6 Conclusions In this article we have attempted to give a short overview of how the lex dependence may be utilized to study the location dependence in neat ionic liquid, in ionic liquid microemulsion and also in micelles and reverse micelles. Our results clearly show that even a neat ionic liquid is heterogeneous. This is attributed to nanostructural organization. This may be useful for an understanding of the unusual chemical reactivity in an ionic liquid. In the case of micelles and reverse micelles, heterogeneity arises from the structure of this aggregate. We have studied four ultrafast processes: solvation dynamics, anisotropy decay, FRET and ESPT. However, the strategy is quite general and may be used spatially to resolve other processes in these systems. Finally, we have studied translational diffusion in different regions of triblock copolymer micelle and gel using FCS.

Acknowledgements Thanks are due to the Department of Science and Technology, India, and to the J.C. Bose Fellowship for generous research support. SD, UM, AA and SG thank CSIR for awarding fellowships.

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8 Fluorescence Studies of the Hydrogen Bonding of Excited-State Molecules Within Supramolecular Host–Guest Inclusion Complexes Brian D. Wagner Department of Chemistry, University of Prince Edward Island, Charlottetown, PE C1A 1Z5, Canada

8.1 Introduction Supramolecular host–guest inclusion complexes are formed when a small guest molecule becomes included within the internal cavity of a large, cage-like host molecule. As represented in Figure 8.1 for a complex with 1:1 host:guest stoichiometry, the formation of such complexes is a dynamic process, with the guest able to enter and exit the host cavity. The strength of the interaction and the stability of the complex are indicated by the binding constant K, the equilibrium constant for the 1:1 complex formation. (Although higher-order complexes can also form, including those with 1:2, 2:1 and 2:2 host:guest stoichiometry, in this chapter the complex stoichiometry is 1:1 unless otherwise indicated.) As this is a supramolecular process, there is no covalent bond formation between the host and guest; rather, the complex is held together by intermolecular forces, including van der Waals forces, charge–dipole interactions and hydrogen bonding [1, 2]. In this chapter, the role of hydrogen bonding in the binding of excited-state guests, studied through guest (or host) fluorescence, will be reviewed and discussed. For the purposes of this discussion, hydrogen bonding will be defined as ‘an intermediate long-range intermolecular interaction between an electron-deficient hydrogen and a region of high electron density’ [3]. Most commonly, the region of high electron density is an oxygen or nitrogen atom with lone pairs of electrons (hydrogen bond acceptor), and the electron-deficient hydrogen atom is bonded to a relatively highly electronegative atom, such as oxygen or nitrogen (hydrogen bond donor). In

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

176 Hydrogen Bonding and Transfer in the Excited State h F h F hA

K

+

Guest

Host

hA

Host-Guest Inclusion Complex

Figure 8.1 A depiction of the formation of a host–guest inclusion complex. Reproduced by permission of Bentham Publishers Ltd., from Wagner, B.D. Curr. Anal. Chem. 207, 3, 183, Copyright 2007 Bentham Scientific Publishers Ltd.

order for hydrogen bonding to contribute to the stability of a host–guest inclusion complex, either the host must be a hydrogen bond donor and the guest a hydrogen bond acceptor, or vice versa. Fluorescence spectroscopy [4, 5] provides an excellent experimental technique for studying hydrogen bonding of excited states. Fluorescence is defined as the emission of light upon the relaxation from a higher to a lower energy electronic state of the same multiplicity; typically, this occurs from the first excited state S1 to the ground state S0. As this emission occurs from an excited state, it provides a direct method for probing excited-state properties. Furthermore, there are numerous experimental measurements of fluorescence, all of which are very sensitive to the environment, properties and interactions of the emitting state. The wavelength of maximum emission, lF,max (alternatively, the frequency of maximum emission, nF,max) is indicative of the energy gap between the excited and ground states. The Stokes shift is equal to the difference in wavelength or frequency units of the band maxima of the fluorescence and absorption spectra, and is indicative of the degree of relaxation of the initially excited state prior to emission. The intensity of the emission at a given wavelength, Il, depends on both the concentration of the excited state and the efficiency of emission as a decay pathway. The fluorescence quantum yield, fF, is a direct measure of the efficiency of the emission (radiative decay) relative to the total of all available decay paths, including non-radiative relaxation (such as internal conversion (IC) and intersystem crossing (ISC) [4, 5]). The fluorescence lifetime, t F, is a direct measure of the lifetime of the excited state, based on an exponential fit of the fluorescence decay curve, the fluorescence intensity measured as a function of time, I(t). Both fF and t F can be related to the rate constants for radiative (kR) and total nonradiative (kNR) relaxation: fF ¼ kR=(kR þ kNR); t F ¼ 1=(kR þ kNR). Finally, if a polarized excitation source is used, then the fluorescence anisotropy decay r(t) can be measured, where r is defined in terms of the intensity parallel and perpendicular to the plane of the polarized excitation light: r ¼ (I2  Iz)=(I2 þ 2Iz). This provides a direct measure of the fluorophore rotational lifetime. Excitation of a ground-state guest molecule within a host cavity can lead directly to the formation of locally excited (LE) singlet states, including S1 and S2. Fluorescence emission can then occur from the excited state (usually S1); measurement of this fluorescence allows for the study of the inclusion process itself, as well as the environment of the excited state within the host cavity [6]. In competition with fluorescence, intersystem crossing (ISC) from S1 to the first triplet state T1 can occur; this can result in phosphorescence emission. Alternatively, in the case of some specific probes, charge transfer can occur in the LE S1 state; if this process is accompanied with an intramolecular twisting of the guest conformation, this process results in the formation of a twisted intramolecular charge transfer (TICT) state [7]. Such TICT states often show distinctive, red-shifted fluorescence emission. Hydrogen bonding within the host cavity can affect the energy (and thus the spectroscopy) of all of these emissive states (S1, S2, T1 and TICT). It should be noted that excited-state molecules included within host cavities are typically created there, as the lifetimes of most singlet states are very short (of the order of ns), and thus guest molecules excited in solution will likely decay faster than the time required for them to enter the host cavity. This does depend, however, on the relative timescales of the

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excited-state lifetime and the kinetics of the particular complex formation, and relatively long-lived excited states may be able to move into and out of a host cavity during their lifetime. Inclusion of a guest molecule within a host cavity can have significant effects on its excited states [6]. The major effects come from the change in environment experienced by the guest. Most host–guest inclusion is carried out in aqueous solution, to maximize the hydrophobic driving force for inclusion. Thus, the guest experiences a significantly lower polarity within the host cavity as compared with free in solution. Typically, the S1 excited state is more polar than the S0 ground state, in which case S1 will be destabilized relative to S0 inside the host cavity, resulting in an increased S1–S0 energy gap. Such an increase in excited-state energy will have significant effects, including a blue-shift in the fluorescence spectrum. In addition, the increased energy gap will result in a decreased rate of internal conversion kIC, according to the energy gap law [4]; this will result in an increase in both fluorescence quantum yield (intensity for a given solution) and lifetime. In such cases, significant enhancement of the guest fluorescence is observed upon host inclusion. Other effects of inclusion are also possible, including prevention of quenching, disruption of solvent–guest hydrogen bonding and intramolecular rotational restriction (increased microviscosity). Steady-state fluorescence spectroscopy can be used to measure the effect of the host on the guest by performing a fluorescence titration experiment, a measurement of the guest fluorescence spectrum as a function of the host concentration. The intensity versus host concentration data can be used to extract the binding constant for a specific host–guest stoichiometry (in the case of 1:1 complexation, the value of K in M1) [6]. This chapter will describe some recent steady-state fluorescence-based literature studies on the hydrogen bonding of excited-state guests within supramolecular host–guest complexes, involving in particular three widely used molecular hosts, namely cyclodextrins, calixarenes and cucurbiturils. It is not meant to provide a comprehensive review, but rather a representative one, and is furthermore concerned only with systems involving discrete molecular hosts and guests (thus, hosts with tethered fluorescent probes will not be considered). There have been a number of reviews published on the role that hydrogen bonding plays in the formation of supramolecular structures [1, 2, 8–12]. Similarly, there have also been a number of reviews involving hydrogen bonding of excited-state molecules [13–15] and its role in excited-state proton transfer [16–18]. However, none of these has involved hydrogen bonding of excited states of guests included in supramolecular host–guest complexes (although Chou reviewed double excited-state proton transfer in host–guest pairs [18], this term referred to hydrogen-bonded heterodimers, not inclusion complexes).

8.2 Hydrogen Bonding Involving Excited States of Fluorescent Probes in Solution Before dealing with hydrogen bonding of excited-state guest molecules included within molecular hosts, it is useful first briefly to review the effects of hydrogen bonding in solution on the excited states of fluorescent molecules, and to consider the effects of specific solute–solvent hydrogen bonding interactions on the fluorescence properties of a few representative probe molecules in protic solvents. (For an extensive recent discussion of the effects of hydrogen bonding on the photochemical dynamics of fluorophores in solution, see Ref. 15.) The fluorescence properties of many molecules are strongly solvent dependent; Lakowicz provides a comprehensive recent discussion of the effects of solvent on fluorescence [4]. These solvent effects can be divided into two categories: general environmental effects resulting from the bulk solvent properties, including polarity and viscosity, and specific solute–solvent interactions, including hydrogen bonding. General solvent effects can typically be well described using the Lippert–Mataga equation, which relates the frequency of absorption and emission to the refractive index and dielectric constant of the solvent, as well as the dipole moment of the ground and excited states of the fluorescent molecule (see equation (6.1) in

178 Hydrogen Bonding and Transfer in the Excited State

Ref. [4]). Deviations from this equation (non-linear Lippert plots [4]) occur when specific solvent–solute interactions such as hydrogen bonding are important. For example, deviation from the Lippert equation in protic versus aprotic solvents (typically larger Stokes shifts in protic solvents [4]) indicates significant solvent hydrogen bonding between the solvent and the emissive state. There is no single theory available that accounts for specific solvent–solute effects such as hydrogen bonding, as the effects depend on each specific solute–solvent pair. Wehry provides a useful list of potential effects of intermolecular hydrogen bonding on the fluorescence of excited states [19]; besides spectral shifts, these can include changes in the relative energy of close-lying excited states; changes (usually increases) in the rate of non-radiative decay of the excited state, and promotion of excited-state proton transfer. Of particular relevance to this chapter is the work of Zhao and Han [15], who have demonstrated that specific hydrogen bonds between chromophores in solution and solvent molecules are significantly strengthened upon excitation of the chromophore, and furthermore that internal conversion of the excited state is enhanced as a result of this hydrogen bond strengthening, leading to significantly reduced fluorescence emission. To illustrate further some of these potential effects of hydrogen bonding on fluorescence properties, a number of representative examples will be discussed, including coumarins, indole and anilinonaphthalene sulfonates. Coumarins are a family of polarity-sensitive fluorescent molecules that have been widely used as probes of heterogeneous systems [20]. Hydrogen bonding of the emissive state is an extremely important contributor to the large sensitivity of coumarin fluorescence to its local environment; numerous studies of the effects of hydrogen bonding on coumarin fluorescence have been reported [20–25]. Lo´pez Arbeloa et al. [21] studied the fluorescence of 7-aminocoumarin derivatives (including C1 shown in Figure 8.2(a)) in water and alcohols, and observed a much greater solvent effect on the excited state than on the ground state. They showed that general solvent effects could not explain the observed spectral shifts, and attributed these large shifts to specific solvent–solute interactions, involving different modes of hydrogen bonding of the excited TICT state than that which occurs in the ground state. Kro´licki et al. [22] studied the role of hydrogen bonding in the preferential solvation of coumarin 153 in toluene–acetonitrile and toluene–methanol mixtures. They showed not only that preferential solvation through hydrogen bonding occurs only in the excited state, not the ground state, but also that specific hydrogen bonding to methanol results in an unusual dependence of the fluorescence lifetime on the mole fraction of methanol, arising from the difference in lifetime of the hydrogen-bonded versus nonhydrogen-bonded excited state. Pal et al. [23, 24] showed that hydrogen bonding of the excited state of C1 [23] and two other coumarin dyes 152 and 481 [24] in protic solvents results in enhanced formation of a TICT state, resulting in increased non-radiative decay; this TICT formation did not occur at all in even highly polar aprotic solvents. Wells et al. [25] studied the dynamics of hydrogen bonding of coumarin 102 (C102, shown in Figure 8.2(b)) in acetonitrile–water mixtures. They found that this coumarin dye is hydrogen bonded to two water molecules, both as donors to the carbonyl group, and that, upon excitation, one of these bonds is

(a)

(b)

CH3

N

O

O

CH3

N

O

O

Figure 8.2 The structure of two representative coumarin fluorescent probes for which hydrogen bonding of the fluorescent excited state has been studied: (a) C1; (b) C102

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strengthened while the other is weakened. In fact, there have been conflicting reports on the dynamics of hydrogen bonding of coumarin dyes upon excitation. Chuboda et al. [26] reported the cleavage of site-specific solvent hydrogen bonds of C102 within 200 fs upon excitation, using femtosecond vibrational spectroscopy. In contrast, Zhao and Han [27] used DFT calculations to show that the intermolecular hydrogen bonding between C102 and solvent molecules is in fact strengthened in the early time after excitation. Again, they concluded that this transient strengthening of the hydrogen bonding of the excited state results in an increase in the internal conversion rate (as discussed above), and therefore explains the observed decrease in fluorescence emission of this coumarin in the presence of hydrogen bonding solvents. They further proposed that this phenomenon could also explain the quenching of other fluorophores in hydrogen bonding solvents. Most recently, Liu et al. [28] also studied the dynamics of the hydrogen bonding of excited-state coumarins using theoretical methods, and found that three types of hydrogen bond can form between methanol and coumarin-151, and that all are strengthened upon electronic excitation, as are those in the case of C102. They assert that solvent hydrogen bonding of coumarin dyes is ‘undoubtedly strengthened’ upon dye excitation. Another common and very important family of fluorescent probes known to be highly sensitive to excitedstate hydrogen bonding is indole (Figure 8.3(a)) and its derivatives [4, 29–32]. This family includes tryptophan, a highly fluorescent amino acid, and the most common intrinsic probe for protein fluorescence studies. Indole exhibits a large shift in its emission spectrum upon hydrogen bond formation between solvent molecules and the imino nitrogen [4]. Indole is an interesting fluorophore, as there are two closely lying excited singlet states, 1 La and 1 Lb , either of which can be the lowest excited state, depending on the solvent medium. For example, in pure cyclohexane, indole exhibits a structured emission spectrum that can be assigned to the 1 Lb emissive state, but, in the presence of small amounts of ethanol, specific hydrogen bonding interactions result in an unstructured emission spectrum that can be assigned to 1 La as the emissive state [4, 31]. This possibility of emission from either state, with 1 La emission being more sensitive to solvent than 1 Lb , makes the emission properties of indole (and tryptophan) complex. In fact, Gryczynski et al. have shown that the fluorescence decay of indole in cyclohexane/ethanol mixtures is highly non-exponential and consists of an underlying distribution of lifetimes; they attributed this heterogeneity of the indole emissive state to a range of hydrogen bonding configurations between the solvent and the indole excited state [31]. Tubergen and Levy used van der Waal complexes of indole with solvent molecules to demonstrate the role of solvent hydrogen bonding in determining the degree of mixing of 1 La and 1 Lb in the emissive state [32]. They reported that, in these complexes, hydrogen bonding was occurring between protic solvents and the p electrons of the indole excited state.

(a)

(b)

N H

N H

(d)

(c)

NH

SO3-

O

Figure 8.3 The structure of selected hydrogen-bond-sensitive fluorescent probes: (a) indole; (b) carbazole; (c) fluorenone; (d) 1,8-ANS

180 Hydrogen Bonding and Transfer in the Excited State

A few other specific probes will be mentioned in closing this section, to illustrate other effects of the hydrogen bonding of excited states on fluorescence properties. The quenching of carbazole (Figure 8.3(b)) by CCl4 and CH2Br2 was found to decrease significantly in ethanol as compared with 3-methylpentane [33]. This effect was explained as a result of the lowering of the S0–S1 energy gap via hydrogen bonding with the ethanol (strongly red-shifted emission), which increased the singlet ionization potential of the solute–solvent hydrogen-bonded pair, reducing the intermolecular quenching rate constant. As mentioned earlier, hydrogen bonding of the excited state by solvent molecules can result in increased non-radiative decay efficiency, resulting in a significant reduction in fluorescence quantum yields. In addition to coumarin C102 described above, this has also been reported to be the case for N-substituted acridones [34] and fluorenone (Figure 8.3(c)) [35]. Morimoito et al. showed this was also the case for a range of fluorophores that form TICT excited states; these were effectively quenched in the presence of alcohols [36]. Fluorescence anisotropy rates can be strongly affected by hydrogen bonding with solvent, as a result of increased microfriction of solvent molecules through hydrogen bonding with the solute [37]. Fluorescence decays can also be affected by hydrogen bonding with the solvent, not only in terms of decreased lifetimes due to increased non-radiative decay rates, as mentioned above, but also in terms of the heterogeneity of the measured fluorescence decays [38]. In fact, hydrogen-bonded and non-hydrogen-bonded excited states can act as independent fluorophores, yielding fluorescence decays that are biexponential or even higher order. The final example of the effect of hydrogen bonding on molecular fluorescence in solution is 1-anilinonaphthalene-8-sulfonate (1,8-ANS) (Figure 8.3(d)), a highly sensitive fluorescence probe that is extensively used for fluorescence studies of proteins, host–guest inclusion complexes and other heterogeneous systems [6, 39]. Once again, hydrogen bonding with solvent has a major effect on 1,8-ANS fluorescence, as indicated by highly curved Lippert plots, with much larger Stokes shifts in protic polar solvents than in aprotic polar solvents [40]. However, in addition to this intermolecular hydrogen bonding with solvent, 1,8-ANS can undergo intramolecular hydrogen bonding between the amino nitrogen and the sulfonate group, and exist in a range of hydrogen-bonded conformations in solution, which further complicates its fluorescence behaviour [40].

8.3 Hydrogen Bonding of Excited States of Included Guests In this main section of the chapter, the focus will be on the effects of hydrogen bonding between the host cavity and the excited guest fluorophore on the measured fluorescence properties in host–guest inclusion systems, and on the information that can be obtained about the inclusion complex based on such studies. The focus will be on three important families of molecular hosts: cyclodextrins, calixarenes and cucurbiturils. 8.3.1 Cyclodextrins Cyclodextrins (CDs) are the best-known and most widely used family of molecular hosts [41]. These compounds are cyclic oligomers of gluocopyranose; the chemical structure of b-cyclodextrin (b-CD), which has seven gluocopyranose units, is shown in Figure 8.4. In aqueous solution, intramolecular hydrogen bonding between hydroxyl groups results in the adoption of a truncated cone-shape solution (also shown in Figure 8.4), with a relatively low-polarity internal cavity (compared with the bulk aqueous solvent) in which smaller hydrophobic molecules can become encapsulated. In addition to b-CD, a-CD (six sugar units) and g-CD (eight sugar units) are also commonly used; this provides a range of cavity diameters (5.7, 7.8 and 9.5 A for a-, b- and g-CD respectively) to match a particular guest. In addition, CDs are readily chemically modified via the hydroxyl groups [42], allowing targeted modification of the host properties, such as solubility and cavity polarity. It is the excellent ability of CDs to bind a broad range of guest molecules in aqueous solution,

Fluorescence Studies of the Hydrogen Bonding of Excited-State Molecules

181

OH 7.8 Å

O HO

O

O O HO

O OH HO HO

O

HO HO

OH OH O OH OH

HO

O

HO

OH

O

OH

RH OH O

O

O OH

HO O O HO

Figure 8.4 The chemical structure (left) and cartoon depiction of the host shape in aqueous solution (right) of b-cyclodextrin

combined with their ready availability and ease of modification, that has made them the most widely used hosts for the formation of host–guest inclusion complexes. Several reviews of the role of intermolecular hydrogen bonding between CD hosts and included guests in the host–guest inclusion process, and its contribution to the size of the binding constant K for complex formation, have been previously published [9–12]. Hydrogen bonding to guests tends to involve the primary CD hydroxyls, as a result of their greater flexibility, and, in addition, water-mediated hydrogen bonding can occur [10]. The effects of hydrogen bonding on the complexation thermodynamics of CDs have also been previously reviewed [43, 44], as have the effects of CD inclusion on guest fluorescence [45]. There have also been recent relevant reviews published on excited states of guests in higher-order host:guest complexes (i.e. involving more than one guest or one host) [46], as well as the triplet states of guests included in CDs [47]. In the following, specific research reports on the effects of hydrogen bonding on excited states of guests within CD complexes will be reviewed and described. Although direct hydrogen bonding between the host and guest can lead to host–guest complex stabilization, Park and Nah reported a very important study on the role of hydrogen bonding in the binding of a series of organic guests in b-CD in aqueous solution, and came to a very interesting conclusion, namely that increased hydrogen bond acceptor ability of the guest can actually lead to lower stability of the b-CD complex relative to similar guests with lower hydrogen bond acceptor abilities [48]. They attributed this decreased complex stability to increased hydrogen bonding between the guest and the water solvent molecules, which increases the dissociation of the complex; thus, the equilibrium lies further to the left in Figure 8.1. This phenomenon occurs because water is a stronger hydrogen bond donor than b-CD. This result has important implications for excited states in CD complexes: if the excited state is a better hydrogen bond donor than the ground state, it is expected that the stability of the complex will decrease upon excitation. Hydrogen bonding between the hydroxyl groups along the CD rims and an included guest can have significant effects not only on the stability of the host–guest complex but also on the excited states of the guest, and thereby on the guest fluorescence. For example, Scypinski and Drake [49] reported that hydrogen bonding between b-CD and the coumarin probe C540 prevents quenching of the coumarin excited state; this, in addition to the effect of the decreased polarity experienced by the guest within the CD cavity, results in strongly enhanced guest emission. Such fluorescence enhancement of guests within CD complexes is very commonly reported [45]. Another related way in which inclusion by CDs affects hydrogen bonding of guest excited states

182 Hydrogen Bonding and Transfer in the Excited State OH (a)

OH

HO (b)

HO

NH

O

OH

O

N H

HO

S

H (d)

(c)

CH3 CO2H H3CO

O

Figure 8.5 The structure of selected hydrogen-bond-sensitive fluorescent probes studied in cyclodextrins: (a) bis(2,4,6-trihydroxy-phenyl(squaraine)); (b) tetrahydro-betacarboline; (c) naproxen; (d) 4H-1-benzopyran4-thione

is through disruption of guest–solvent hydrogen bonding. Das et al. [50] reported a very large fluorescence enhancement of the anion of bis(2,4,6-trihydroxy-phenyl(squaraine) (neutral form shown in Figure 8.5(a)) upon inclusion into b-CD of ca 90-fold. In addition to the polarity and microviscosity effects, they determined that a major contribution to this incredibly large enhancement was intramolecular hydrogen bonding, which greatly stiffened the molecule, thereby decreasing the efficiency of the non-radiative decay of the excited state. In free solution, this intramolecular hydrogen bonding is replaced by solvent hydrogen bonding, which results in a more flexible molecule and a more efficient non-radiative decay, and hence a much less fluorescent molecule. By contrast, Biczo´k et al. [51] found that fluorenone (Figure 8.3(c)), which they proposed as a probe for hydrogen bonding interactions, is still significantly hydrogen bonded to solvent water even when included inside a b-CD cavity. They eliminated the possibility of hydrogen bonding between the fluorenone carbonyl group and the CD hydroxyls, based on the symmetrical inclusion within the CD cavity, and the similarity between the results in b-CD and heptakis-(2,6-di-O-methyl)-b-CD, in which two-thirds of the hydroxyl groups has been replaced by methyl groups. They proposed that the fluorenone is hydrogen bonded to water, either co-included or outside the cavity, and directly correlated the internal conversion rate to the degree of hydrogen bonding. Murphy et al. [52] also studied fluorenone inclusion into CDs, as well as the related ketone xanthone, and also concluded that, while these ketones were included within the b-CD cavity, they were still somewhat exposed to the aqueous medium. In the case of the smaller a-CD, however, 2:1 host:guest complexes were formed, in which the ketone guests were fully protected from solvent hydrogen bonding. El-Kemaray et al. [53] also concluded that hydrogen bonding between the guest and solvent water occurs in the case of the inclusion of 2-amino-4,6-dimethylpyrimidine within b-CD, in both the ground and excited state. In other cases, direct effects of hydrogen bonding to CD hydroxyls on the excited-state properties of guests have been reported. It is interesting to note from studies of ground-state guests that hydrogen bonding between guests and CDs is often characterized by relatively shallow inclusion of the guest into the CD cavity [54] to allow optimal interaction between the guest hydrogen bonding group (such as a hydrogen bond donor) and the hydroxyl groups around the CD cavity rim. In addition, there is the possibility that the guest will be tilted off the cavity central axis [54, 55], again to maximize the hydrogen bonding interaction. ¨ rstan and Ross [56] found that the fluorescence spectrum of indole included within the cavity of b-CD was O very similar to that in glycerol, while the absorption spectrum was similar to that of indole in an alcohol, or in cyclohexane hydrogen bonded to butanol, and thus concluded that the N-1 hydrogen of indole is hydrogen bonded to one of the CD hydroxyl groups. The hydrogen bonding by the CD was found to have a significant role

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in the relative energies of the 1 La and 1 Lb excited states, as was discussed in Section 8.2 for indole in solution and in van der Waals complexes with solvents. The authors were unable to determine how large a contribution this host–guest hydrogen bonding made to the overall stability of the complex. Velasco et al. [57] studied the b-CD inclusion complex of an alkaloid indole derivative, tetrahydrobetacarboline, shown in Figure 8.5(b). This compound is a naturally fluorescent alkaloid (owing to the presence of the indole moiety) of interest for its physiological properties. It was found that were two types of inclusion complex formed in competition, one with the piperidinic ring extending outside the b-CD cavity (type I), and the other with the opposite guest orientation, i.e. with the piperidinic ring included inside the cavity (type II). In the case of type I complexes, a large perturbation of the excited state of the guest was observed, manifested as very large increases in both the fluorescent quantum yield and lifetimes. However, minimal effects were obtained with type II. This was explained as the result of hydrogen bonding between the indole N-1 hydrogen ¨ rstan and Ross [56] for indole itself in b-CD. The indole and a CD hydroxyl, in agreement with the report of O fluorophore is well included inside the cavity only in the type I complex, so significant hydrogen bonding with the CD can only occur in that complex. Interestingly, there was no effect of CD inclusion on the absorption spectrum for either complex. Sadlej-Sosnowska et al. [58] reported a very interesting study on the inclusion complexes of the nonsteroidal anti-inflammatory drug naproxen (Figure 8.5(c)) in native and modified CDs. Significant enhancement of naproxen fluorescence was observed. NMR studies indicated strong hydrogen bonding between the guest carboxylate group and the CD hydroxyls in this complex. Furthermore, they found that the binding constants for b-CD that they measured using fluorescence studies were consistently and significantly larger than those reported in the literature using phase solubility experiments. For example, for the acid, they measured a value of K of 1950 M1, which is approximately a factor of 2 larger than those reported in the literature using phase solubility measurements. They proposed that in this case the complexation equilibrium is established very rapidly, on the timescale of the excited state, and that therefore their fluorescence experiment measured the binding constant for the excited state, whereas the solubility study measured that of the ground state. They therefore proposed that naproxen is more strongly complexed to b-CD in the excited state than in the ground state. Hydrogen bonding of a number of other drug or drug precursors by CD hosts has also been described in the literature, including tryptamine [59], a series of three related non-steroidal antiinflammatory drugs [60], ibuprofen [61] and acetaminophen [62]. Nau et al. introduced a unique azoalkane fluorescent probe, 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO), which has an exceedingly long-lived singlet excited state [63, 64], with a fluorescence lifetime of up to 1 ms [63]. In addition, the emissive state is n,p , which is distinguished from the more usual p,p state of typical aromatic fluorescent probes. They used this probe to study the host–guest complexation mechanism by CDs, as the long lifetime allows kinetic information on rates of entry and exit from the CD cavity to be obtained. The fluorescence lifetime of DPO was found to be dramatically reduced upon CD inclusion; this was attributed to a quenching of the DPO fluorescence, arising from an ‘aborted hydrogen abstraction’ by DPO [63]. The authors also studied a series of substituted DPO, and found significant differences in complexation with b-CD, which was attributed to differences in hydrogen bond donor ability of the substituted DPO guests [64]. The guests with hydroxyl and ammonium substituents (and thus expected to have the strongest hydrogen bonding interactions with the CD hydroxyls) were found to have weaker binding constants than those with an amine group; thus hydrogen bonding was not found to make a significant contribution to the thermodynamic stabilization of these complexes [64]. There have been a number of reports on the effects of hydrogen bonding of TICT states of guests within CD cavities [65–77]. Nag and Bhattacharyya studied the TICT emission of dimethylaminobenzonitrile (DMABN) in a-CD [65]. They found a significant enhancement of the TICTemission, which they attributed to the reduced polarity of the CD cavity. However, Shayira Banu et al. studied this same guest in modified b-CDs [66] and found two distinct absorbing and emitting states for the free and bound DMABN. They proposed that TICT

184 Hydrogen Bonding and Transfer in the Excited State

emission originates with the aqueous DMABN, while the complexed molecule exhibited LE emission. They attributed this effect of the CD cavity to an absence of hydrogen bonding to the solvent water and the tight fit of the cavity, which restricts intramolecular rotation. Yoon et al. also reported solvent hydrogen bonding effects on the TICT state of p-(N,N,-diethylamino)benzoic acid (DMABA) in a- and b-CD [67]. In the case of a-CD, inclusion was proposed to involve the carboxyl group of the guest, which prevented hydrogen bonding to water, and greatly reduced the TICT emission. In b-CD, however, a different orientation of the guest was proposed, in which the carboxyl group sticks out into the aqueous medium, allowing hydrogen bonding to water, and strong TICT was observed. The authors concluded that specific hydrogen bonding of the carboxylate group to water plays an important role in TICT formation. Similar conclusions regarding the enhancement of TICT state formation through hydrogen bonding of partially exposed complexed guests in CDs have been reached for other guests, including 4-acetylbiphenyl [68], 4-hydroxy-3,5-dimethoxybenzaldehyde [69] and 2-naphthylamine-6-sulfonate [70]. Sainz-Rosas et al. however reported that inclusion of dibenzofuran-2-carboxylic acid resulted in a dramatic quenching of its ICT fluorescence (the CT state does not involve twisting in this case), owing to efficient shielding of the guest from hydrogen bonding with the aqueous solvent, which provides significant stabilization of the ICT state in free solution [71]. There have been numerous reports of direct hydrogen bonding between bound TICT guests and CD hydroxyl groups. Yoon et al., who previously attributed hydrogen bonding to solvent effects on the TICT emission of b-CD-bound DMABA [67], later studied the effects of silver colloids as absorbates for DMABA on the TICT emission from DMABA included in CDs [72]. They concluded that, in the case of the smaller a-CD, the DMABA carboxylic group is hydrogen bonded to water, as previously reported, but, in the case of b-CD, direct hydrogen bonding between the DMABA carboxyl group and the rim secondary hydroxyl groups occurs. This host–guest hydrogen bonding, together with the lower cavity polarity, results in a large enhancement of the observed TICT emission. Effects of direct hydrogen bonding interactions between an included guest and the CD hydroxyls on guest TICT or ICT emission have also been reported for Nile Red (g-CD) [73, 74], 2-aminodiphenylsulphone (b-CD) [75], L-tyrosine (b-CD) [76] and 2-(4-(dimethylamino) styryl)-1-methylpyridinium iodide (g-CD) [77]. The hydrogen bonding of higher excited singlet states of guests included in CDs has also been reported. Milewski et al. [78–80] investigated the effects of CD encapsulation on the photophysical properties of 4H-1benzopyran-4-thione (BPT) (Figure 8.5(d)). This molecule is of interest as it is an aromatic thione, one of two common groups of molecules that exhibit fluorescence from the S2 state (the other being azulenes). They found that the S2 emission of BPT in b-CD had a low quantum yield and short lifetime very similar to those in pure water, and suggested that hydrogen bonding was occurring between the bulk solvent and the included guest, leading to a shortening of the excited-state lifetime [79]. Whereas the CD provides some reduction of the effect of solvent hydrogen bonding on the S1 band, the much more sensitive S2 band of the included guest remains effectively quenched by the solvent water molecules. However, in the presence of alcohols, a ternary CD inclusion complex forms, containing both BPT and an alcohol as guests [80]. In this case, interaction with the alcohol shields the BPT guest from the quenching interaction with water, and a very dramatic increase in the S2 fluorescence results. Hydrogen bonding has also been shown to have significant effects on the properties of triplet states included within CD cavities, using phosphorescence and time-resolved absorbance studies [81–85]. Bohne et al. [81– 83] have shown that triplet xanthone is much more weakly bound in b-CD than is ground-state xanthone, with measured binding constants K of 1100 and 48 M1 for the ground and triplet states respectively [81]. They proposed this to be a result of stronger hydrogen bonding abilities of the triplet as compared with the ground state [83], in agreement with the findings described earlier that indicated that increased hydrogen bond abilities of a guest will lead to lower binding [48]. (In this same study [83], they also found a lower binding of 1-naphthyl-1-ethanol in b-CD than that of corresponding alkylnaphthalenes, which they also attributed to its hydrogen bond acceptor ability.) They measured the dynamics of triplet xanthone inclusion into b-CD [81] and

Fluorescence Studies of the Hydrogen Bonding of Excited-State Molecules

185

found a much faster exit rate for the triplet as compared with ground-state xanthone, supporting the idea of hydrogen bonding by the water solvent competing with binding into CDs. Other researchers have shown that inclusion of guests within CDs can lead to strongly enhanced room-temperature phosphorescence (RTP) [84, 85]. Hartmann et al. showed that the addition of hydrogen bonding substrates such as bulky alcohols or amines significantly enhances the RTP of 1-bromonaphthalene via hydrogen bonding to the CD rim hydroxyls, thus forming a lid on the CD cavity, and providing increased protection of the guest from O2 quenching [84]. Mun˜oz de la Pen˜a et al. have provided a useful review of the analytical applications of this hydrogen-bonding CDenhanced RTP [85]. Finally, a discussion of the effects of hydrogen bonding on the excited states of guests in CD complexes would not be complete without at least a brief consideration of excited-state proton transfer (ESPT), or photoacidity, within CD cavities. Obviously, hydrogen bonding of the excited state can play a major role in the ESPT process; this role in solution has been extensively reviewed [16–18]. In general, inclusion of a guest into a CD results in a significant decrease in the rate of ESPT [86–90]; this has been attributed to hydrogen bonding between the guest and the CD hydroxyls, which disrupts the hydrogen bonding of the guest to the solvent, and thus inhibits proton transfer to solvent [88]. Mondal et al. showed that ESPT from pyramine to a non-solvent proton acceptor acetate is also greatly reduced upon inclusion of the pyramine in either g-CD or HPb-CD [89]. They attributed this rate decrease to the existence of a network of hydrogen-bonded bridging waters separating the donor and acceptor in these complexes; this network would need to undergo significant rearrangement for ESPT to occur. In addition to such intermolecular proton transfer to solvent, excited-state intramolecular proton transfer (ESIPT) can also occur for specific molecules, and this process has also been extensively studied within CD cavities [91–95]. Once again, inclusion into a CD cavity disrupts the hydrogen bonding of the excited guest with the solvent; however, in this case, disruption of solvent hydrogen bonding actually favours intramolecular hydrogen bonding, and hence the rate of ESIPT can be enhanced upon CD inclusion [91–94]. In other cases, direct hydrogen bonding between the guest and CD hydroxyls reduces intramolecular hydrogen bonding, resulting in a decreased rate of ESIPT [95]. Douhal et al. have proposed the use of ‘proton transfer fluorescence’ to probe the hydrogen bonding abilities of CD cavities [96]. 8.3.2 Calixarenes Calixarenes are cyclic oligomers of phenol [97, 98]. The general calix[n]arene structure is shown in Figure 8.6, with R0 ¼ H in typical cases. Intramolecular hydrogen bonding between the calixarene hydroxyl groups, as well as intermolecular hydrogen bonding with solvent and/or included guest molecules, are major contributors to the determination of the shape of calixarenes in solution, and their cavity properties. Hydrogen bonding interactions between calixarene hosts and included guests in the solid state have been previously reviewed [99]. There have been a few reports on the effects of hydrogen bonding on the excited states of guests included within calixarene hosts [100–103], but this family of hosts has been much less studied than cyclodextrins. Tao and Barra reported a detailed thermodynamic study of the formation of p-sulfonated calixarene (n ¼ 4, 6, 8; R

CH2 n OR'

Figure 8.6 The general chemical structure of a calix[n]arene

186 Hydrogen Bonding and Transfer in the Excited State

R ¼ SO3Na; R0 ¼ H in Figure 8.6) complexes in solution, and discussed the contributions to complex stability [100]. They determined that hydrogen bonding between the calixarene hydroxyls and the polar excited states of bound guests results in a large enthalpic contribution to the complex. Liu et al. presented a direct comparison of the thermodynamics of inclusion of calixarenes versus cyclodextrins as hosts, based on the fluorescence of bound dyes [101]. They found that, whereas all CDs studied increased the fluorescence of the dyes of interest, some p-sulfonated calixarenes decreased the fluorescence; they attributed this to a different mode of binding of guests by calixarenes as compared with CDs. Umadevi et al. reported a study of the inclusion of 2-methyl-1,4-naphthylquinone in p-tert-butylcalix[8] arene (n ¼ 8, R ¼ C(CH3)3; R0 ¼ H in Figure 8.6) [102]. Intermolecular hydrogen bond formation between the calixarene hydroxyl groups and the quinoid carbonyl oxygen of the guest was postulated to reduce the energy of the ground but not excited state, resulting in a significant blue-shift of the absorption and fluorescence spectra. Furthermore, the fluorescence quantum yield was found to decrease with increasing concentration of calixarene; this result was partially attributed to the hydrogen bonding of the excited state, which enhances the non-radiative excited-state decay. The authors used these hydrogen bonding results to predict the orientation of the guest within the calixarene cavity. These authors also reported a later study on the inclusion of the related guest 2,3-bis(chloromethyl)-1,4-anthraquinone [103] in the same calixarene. Again, direct hydrogen bonding between the calixarene hydroxyls and the guest quinoid carbonyl oxygen was reported, with similar resulting effects on the guest fluorescence (blue-shifted spectrum and reduced emission). The authors also concluded that there was no change in the hydrogen bonding interactions between the calixarene and the guest in the ground and excited states. 8.3.3 Cucurbit[n]urils Cucurbit[n]urils (CB[n]) are macrocycles composed of n glycoluril units linked by pairs of methylene bridges [104, 105]; the structure of CB[7] is shown in Figure 8.7. The paired methylene bridging makes CB[n] much more rigid hosts than cyclodextrins or calixarenes, with very well-defined cavities. Binding of guests in cucurbit[n]urils is dominated by ion–dipole interactions, and CB[n] are well established as extremely effective hosts for cations, such as alkylammonium [104, 105]. However, hydrogen bonding between guests and the carbonyl portals shown in Figure 8.6 can also play a role, although a relatively minor one compared with the cases of cyclodextrin and calixarene binding. In this section, a few illustrative examples of the effects of hydrogen bonding on excited states of guests included in CB[n] cavities will be briefly discussed. Wang et al. have reported significant effects of the interactions of guest hydrogens with CB[7] carbonyls on guest properties [106, 107]. In the case of 2-aminoanthracene, the excited-state pKa is significantly increased upon inclusion into CB[7] through interactions with the carbonyls; this results in a switching in the observed guest fluorescence, from the green fluorescence of the neutral form to a blue fluorescence of the protonated O

O O

O

O

O

O

N

N N

N N

N N

N N

N N

N N

N

N

N N

N N

N N

N N

N N

N N

N

O O

O

O O

O

O

Figure 8.7 The structure of the host molecule cucurbit[7]uril

Fluorescence Studies of the Hydrogen Bonding of Excited-State Molecules

187

form [106]. In the case of a bis(imidazolium) dication, the observed deuterium exchange of one of the guest hydrogens was found to be significantly inhibited upon inclusion into CB[7]; this was attributed to direct hydrogen bonding between C–H groups on the guest and the carbonyl host portals [107]. Mohanty et al. also studied the effect of inclusion in CB[7] on the pKa and binding of an excited guest, in this case the fluorescent dye Neutral Red, and directly compared the effects of CB[7] with those of b-CD [108]. They found that the neutral form is preferentially bound by b-CD, while the protonated form is preferentially bound by CB[7]. They also observe an opposite temperature dependence of the dye fluorescence intensity in the presence of CB [7], which they attributed to disruption of the hydrogen bonding of the dye with water. Barooah et al. found that hydrogen bonding of specific coumarins with the carbonyl portals of CB[8] results in a templating effect, with a favoured orientation of two coumarins inside one CB[8] host as 1:2 host:guest complexes [109]. Upon excitation, a photochemical adduct of the two guests forms, the stereochemistry of which was found to depend on the hydrogen bonding abilities of the guest. Isaacs et al. have reported a fluorescent analogue of CB[6], with incorporated bis-phthalhydrazide walls and an oval-shaped cavity [110–112]. In this case, the host itself is fluorescent, and the fluorescence has been found to be sensitive to the inclusion of guests [111, 112], allowing the study of non-fluorescent guests. Inclusion of guests such as benzene [111] and alkylammonium cations [112] resulted in a significant increase in the host fluorescence. A number of guests have been proposed to form direct hydrogen bonds with this host, particularly those containing NH3þ and OH groups [112]. This example is distinguished from all of the previous cases described in cyclodextrins, calixarenes and cucurbiturils, as in this case it is the host excited state that is involved, as opposed to that of the guest. This direct hydrogen bonding between the guest and the host excited state is one of the contributing factors to the large effect of guest binding on the fluorescence intensity and photophysical properties of this host. 8.3.4 Other molecular hosts In this final section, two reports of the hydrogen bonding of excited-state guests in molecular hosts other than cyclodextrins, calixarenes or cucurbiturils will be briefly discussed. Chung et al. have studied the effect of the binding of amines by a series of fluorescent bis(oxazolinyl)phenol sensors [113] (this is the second example in this chapter of a fluorescent host and the effects of hydrogen bonding on a host excited state). The fluorescence of this sensor was found to be enhanced in the presence of butylamine and arylethylamines, but quenched in the presence of secondary and branched amines. This drastically different response was attributed to conformational restriction of the host excited state owing to the formation of a very stable complex in a tripodal hydrogen bonding mode in the case of the latter types of amine, involving the phenol hydroxyl and two nitrogens of the host with the guest amine group. Gassensmith et al. studied the inclusion of squaraine dyes within anthracenecontaining tetralactam macrocycles in cyclohexane solution [114]. Extremely strong complexes resulted, with logK ¼ 5.2. They were able to obtain an X-ray crystal structure of the complex, which showed hydrogen bonding occurring between the squaraine guest carbonyls and the amide NH residues of the macrocyclic host. The complexation and hydrogen bonding of the excited state also resulted in a significant red-shifting of the squaraine fluorescence spectrum.

8.4 Conclusions Hydrogen bonding of the excited states of guests included within the internal cavities of molecular host molecules can have dramatic effects on both the stability of the complex and the photophysics and spectroscopy of the included guest (or host). However, this host–guest hydrogen bonding occurs in competition with guest–solvent as well as host–solvent and host–host hydrogen bonding interactions. Thus,

188 Hydrogen Bonding and Transfer in the Excited State

the formation of supramolecular host–guest inclusion complexes in aqueous solution is a highly complex phenomenon, and the specific role that hydrogen bonding plays in this complexation process will vary greatly, depending on the specific guest and host pair, as well as the solvent. Upon reviewing the literature, three main ways in which host inclusion can have a significant impact on the hydrogen bonding of excited guest molecules can be distinguished: 1. 2. 3.

Disruption of guest–solvent hydrogen bonding interactions by the screening effect of the host. Disruption of intramolecular hydrogen bonding interactions in the guest molecule. Direct hydrogen bonding interactions between the host and the excited guest.

Disruption of the guest–solvent hydrogen bonding often leads to an increase in guest fluorescence owing to a decrease in the rate of the internal conversion decay pathway which is otherwise facilitated by hydrogen bonding to the solvent. Disruption of intramolecular hydrogen bonding of the excited guest can have a significant impact on excited-state intramolecular charge transfer within the guest molecule, as well as effects on the observed guest fluorescence. Direct hydrogen bonding interactions between the host and the excited guest can have various effects on the guest fluorescence, with enhanced fluorescence observed in some cases (for example, via prevention of external quenching) but reduced fluorescence observed in others (usually a result of increased internal conversion of the hydrogen-bonded excited state). In the case of TICT states, TICT fluorescence is in general increased by hydrogen bonding to the host, as a result of stabilization of the charge transfer state, with a corresponding decrease in LE emission. In addition, the stability of the complex can change significantly in the excited state for some guests as a result of changes in hydrogen bonding upon excitation. In summary, hydrogen bonding of the excited states of guest molecules included within the internal cavities of molecular hosts can play a significant role in the formation and stability of host–guest inclusion complexes, and has major and varying effects on the fluorescence properties of the complexed guest (or host). The specific contributions and effects of hydrogen bonding will depend on the particular host–guest pair and solvent involved.

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191

9 Hydrogen Bonding on Photoexcitation Debarati Dey1, Manas Kumar Sarangi2 and Samita Basu2 1

Department of Chemistry and Environment, Heritage Institute of Technology, Chowbaga Road, Anandapur, P.O. East Kolkata Township, Kolkata 700 107, India 2 Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India

9.1 Introduction For nearly a century, hydrogen bonding has been the subject of contemporary research interest owing to its prevalence and importance in various branches of science [1–7]. The hydrogen bonds are the most important for holding the three-dimensional structures of the major biological macromolecules, such as proteins and DNA. Recent structures of different functional RNA molecules, such as ribozyme, ribosome, tRNA, RNA switches, etc., also indicate that these are stabilized by a huge number of hydrogen bonds [8]. Hydrogen bonds, along with a few other types of interaction, such as van der Waals, charge–charge or charge–dipole, are also very important in macromolecular folding and recognition, be it specific or non-specific. Hence, hydrogen bonds are considered the most important when designing structure-specific drug molecules in pharmaceutical companies. Although a great deal of information is available on hydrogen bonding, identification of new types of hydrogen bond in solid-state supramolecular chemistry and biology has triggered intense research on the nature of hydrogen bonding [6, 7]. Hydrogen bonding involving heteroaromatic rings such as azines, diazines, quinones, etc., is very important as it plays an important role in the structure and function of many biological systems [3, 9, 10]. Hydrogen bonding to molecules in their ground electronic state has been widely investigated by different spectroscopic [11–26] and theoretical [8, 25, 27–45] methods; however, much less is known about hydrogen bonding to molecules in their excited states. There are two possible electronic transitions that may occur on photoexcitation in the case of heterocyclic compounds: np
and pp
transitions. The former type of transition shows blue-shift and the latter transition shows red-shift on ground-state hydrogen bonding. The blue-shift for the np
transition is due to the fact that the electronic transition removes one of the lone pairs of electrons that directly participate in hydrogen

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

194 Hydrogen Bonding and Transfer in the Excited State

bonding [46]. However, on photoexcitation, this ground-state hydrogen bonding might become stronger or weaker, depending on the nature of the molecule. Two motifs have been identified for the hydrogen bonding of water with excited states of azines. For pyridine, the simplest azine, the hydrogen bond is broken by the removal of one of the lone pairs of electrons from the nitrogen, leading to spontaneous dissociation of the vertically excited complex [44]. Nevertheless, strong hydrogen bonds are predicted to form between water and the electron-rich p cloud of pyrazine. N

N

N

N

N N

N

pyridine pyridazine pyrimidine pyrazine

For diazines such as pyradazine, pyrimidine and pyrazine the absorption spectra show a large blue-shift in the np
transition in hydrogen bonding solvents, but only small changes are observed in the corresponding fluorescent spectra. Baba et al. qualitatively interpreted experimental data as indicating a large dipole moment in the ground state and a nearly zero dipole moment in the excited state [11]. They concluded that hydrogen bonding is broken in the np
singlet excited state of pyridine and diazines. This analysis seems to be valid for pyridine, but for diazines it is incomplete. In the diazines there are two nitrogen lone pairs that can either localize or delocalize on photoexcitation. Liquid structure simulations indicate that two hydrogen bonds are formed in the ground state of diazines [30, 33, 34]. In the excited state, the particular nitrogen atom of the molecule on which the electron density localizes becomes analogous to the nitrogen in the pyridine, and the other nitrogen remains unaffected. One would thus expect the hydrogen bond to the unaffected nitrogen atom to remain intact, and the other hydrogen bond to break. However, if on excitation the electron density is delocalized over both the nitrogen atoms, then the environment of both nitrogen atoms will be changed and they may or may not form hydrogen bonds. We will discuss the hydrogen bonding of different diazine derivatives later in detail.

9.2 Intermolecular Excited-State Hydrogen Bonding 9.2.1 b-carbolines An interesting class of compounds having a polyfunctional hydrogen bonding character is b-carboline rings. This includes pyrrolic and pyridinic rings that have an acidic and a basic nitrogen atom respectively, which allow b-carboline to act as a hydrogen bond donor as well as an acceptor molecule in the ground state. The hydrogen bonding properties of b-carboline are considerably modified in the first singlet excited state. Upon photoexcitation, the charge density on the nitrogen atoms of the b-carboline ring changes, which makes the pyridinic nitrogen much more basic and the pyrrolic nitrogen much more acidic than those in their ground states [47]. Thus, under appropriate conditions, b-carboline phototautomerizes to its corresponding zwitterionic species. Aromatic donor and acceptor molecules form hydrogen-bonded complexes that quench the fluorescence. The quenching occurs possibly owing to electron transfer in the excited state induced by hydrogen binding interaction and formation of a non-fluorescent exciplex [48]. 9.2.2 Coumarins Significant change in hydrogen bonding between the laser dye coumarin 102 and various hydrogen bond donors has been observed by Elsaesser, Yoshihara and their coworkers following photoexcitation of coumarin

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102 in solution [49–53]. They have assigned transient absorption behaviour of the IR active vibrations at the hydrogen bonding site and the electronic transition in coumarin 102. They concluded that the time of cleavage of the hydrogen bond at the carbonyl is less than 200 fs. Although spectroscopic evidence is not questionable, more recent time-dependent density functional theory calculations indicate an increase in the hydrogen bond strength upon excitation of coumarin 102 in isolated gas-phase complexes [50, 54, 55]. Very recently, Monte Carlo simulations of coumarin 102 and a binary mixture of acetonitrile and water (in different volume ratios) have been performed by Wells et al. to quantify the number of hydrogen bonding interactions between coumarin 102 and water [56]. These simulations indicate the probability of coumarin 102 solute being hydrogen bound with two water molecules, both as donors at the carbonyl site. In all the binary mixtures, they observed a change in the solvent–solute electrostatic interaction that followed a large increase in the coumarin 102 dipole moment. However, for solutions with a greater water content, they found an additional component of opposite sign, which is corroborated by the previously assigned ultrafast hydrogen bond cleavage in the excited state between coumarin 102 and non-aqueous donors [49–53]. Considering these two apparently opposite observations, a model has been proposed concerning the hydrogen bonding dynamics between coumarin 102 and water on excitation of coumarin 102. Coumarin 102 can participate in two types of hydrogen bonding: single- and double-bond configurations, as given by Monte Carlo simulations, with one and two water molecules respectively. The single-hydrogen-bond configuration prefers a hydrogen bonding structure with the water occupying quarter spheres centred about the lactone group of coumarin 102. When the second hydrogen bond forms, the distribution of hydrogen bonds becomes a half-sphere about the carbonyl of coumarin 102. Thus, in the ground electronic state, one water molecule, hydrogen bonded to coumarin 102, is removed from the solute molecule with two water molecules preferably from the a-carbon side of the lactone, which is energetically favourable. The non-equivalence of the two hydrogen bonding sites for coumarin 102–water hydrogen bonds suggests that, upon electronic excitation, the water present opposite the a-carbon side is bound more strongly at the expense of the hydrogen bond to the water on the a-carbon side, which is substantially weakened and perhaps broken. This offers potential clarification for the previously unexplained hydrogen bond disruption. 9.2.3 Diazines Intermolecular hydrogen bonding is a site-specific local interaction. Fast and very sensitive detection of intermolecular hydrogen bonding can be based on fluorescence quenching, which occurs on formation or disruption of these bonds with fluorescence probe molecules [57, 58]. This quenching is frequently difficult to quantify and distinguish from other quenching effects. A different solvatochromic dye can also be used for this purpose [46, 59]. The primary condition for a molecule to act as an efficient dye is that it must undergo redistribution of electron density on photoexcitation. Examples of these kinds of probe are 6-propionyl-2-(N, N-dimethylamino)naphthalene (PRODAN) [60], aminonaphthalimide [61], 1-anilino-naphthalene-8-sulfonate (ANS), fluorenones [62], hemicyanine dye, etc. These compounds contain a free electron pair capable of interacting with hydrogen bond donor (protic) solvents or substrates. This intermolecular bonding becomes stronger in the excited state because excited-state charge transfer increases the electronic charge on the donor atom. This leads to red-shift in absorption and fluorescence spectra [63]. Moreover, owing to excited-state charge transfer, the dipole moment in the excited state increases, and thus the emission maxima and quantum yields of these compounds are often very sensitive to the polarity of the medium [60, 61, 63]. The enhanced stability of such compounds in polar aprotic and protic solvents is reflected in their fluorescence spectra, quantum yield and lifetime. A large Stokes shift is observed in the fluorescence maximum with an enhanced quantum yield and lifetime with increase in polarity of the solvent. However, in protic solvents such as ethanol and methanol, although the Stokes shift is much greater than that in an aprotic solvent of similar polarity, the quantum yield and lifetime decrease abruptly. The large red-shift of the fluorescence peak in protic media

196 Hydrogen Bonding and Transfer in the Excited State

might be due to the encapsulation of the charge transfer species by hydrogen bonding in the solvent cage, which enhances non-radiative internal conversion and hence decreases the fluorescence quantum yield and lifetime. The effect of hydrogen bonding is also reflected in the triplet spectrum and lifetime. This sensitivity is often exploited to determine the polarity of unknown solvent mixtures [64] and protic impurities in apolar liquids [65] and oils [66] or to estimate the polarity of the microenvironments in organized assemblies such as micelles [67], phospholipid bilayers and biomembranes [68, 69]. The effect of polarity is thus similar to that of hydrogen bonding in the direction and magnitude of the spectral shifts [63]. Consequently, the selectivity of these dyes as hydrogen bonding sensors is low, and it is almost impossible to distinguish between the polarity and hydrogen bonding effects in unknown media. The problem in differentiation between intrinsic polarity and hydrogen bonding capacity of a solvent has been partly solved by a polarity-insensitive hydrogen bonding probe, dibenzo[a,c]phenazine (DBPZ) [70]. The parent phenazine molecule shows almost no change in absorption spectra with solvents of different polarity and hydrogen bonding capacity. However, its fluorescence spectra show only 30 nm blue-shift with water compared with acetonitrile, indicating that the first excited singlet state is of np
character [71]. In acetonitrile and in ethanol, phenazine does not show any shift in the fluorescence maxima, and only the quantum yield increases in the latter. Thus, the parent phenazine molecule is not very informative about the polarity and hydrogen bonding capacity of its residing medium (Figure 9.1). On the other hand there are quite a large number of phenazine derivatives that show significant differences in their dipole moments in the ground and excited states, and hence can be used as polarity probes [72]. However, DBPZ is a molecule that shows no change in its absorption spectra with different polarity of the solvents, e.g. cyclohexane, acetonitrile and methanol, and even with water up to a certain concentration

6

2.0x10

0.30

O. D.

0.25

6

fl. intensity (a. u.)

1.5x10

0.20

0.15

0.10

6

1.0x10

360

380

400

420

440

460

480

500

520

wavelength (nm)

5

5.0x10

0.0 390

420

450

480

510

540

wavelength (nm)

Figure 9.1 Steady-state fluorescence spectra of DBPZ (1  105 M) (lex ¼ 350 nm) in cyclohexane (—) and in MeCN (----). The inset shows absorption spectra of DBPZ (1  105 M) in MeCN (black), 63% (v/v) water–MeCN mixture, 2% MeCN–water mixture and MeCN–HClO4 mixture. (The inset is Reprinted with permission from [70]. Copyright 2007 American Chemical Society) (See Plate 9)

Hydrogen Bonding on Photoexcitation

197

7

fl. intensity (a. u.)

1.6x10

increasing water up to 58%(v/v)

7

1.2x10

6

8.0x10

6

4.0x10

0.0 400

450

500

550

600

650

wavelength (nm)

Figure 9.2 Steady-state fluorescence spectra of DBPZ (1  105 M) (lex ¼ 350 nm) in MeCN with an increasing amount of water (v/v): 0% (---), 23%, 48% and 58%. Reprinted with permission from [70]. Copyright 2007 American Chemical Society

(Figure 9.2). The fluorescence maxima and the fluorescence quantum yield also show no change in cyclohexane and acetonitrile. This leads to the conclusion that DBPZ is insensitive to the polarity of its environment. Interestingly, with hydrogen-bond-donating solvents the fluorescence spectra of DBPZ change drastically. As the hydrogen-bond-donating capacity of the solvents increases, the fluorescence maximum shows a red-shift with increasing quantum yield. The bathochromic shift of the fluorescence maxima and increase in quantum yield not only depend on the hydrogen-bond-donating capacity of the solvents but also on the steric factor around the hydrogen bond donor site. For example, although trifluoroethanol has a greater hydrogen-bond-donating ability compared with that of water [74], the latter shows greater red-shift in the fluorescence maxima and quantum yield than the former. Thus, DBPZ forms much stronger hydrogen bonds in its first excited singlet state compared with those in the ground state. Theoretical studies reveal that the dipole moment of the molecule changes severely in the first excited state from that in the ground state, so how can it be insensitive to the polarity of the medium? The answer remains hidden in the structure of the molecule. The two benzene rings in the ‘a’ and ‘c’ positions of the parent phenazine molecule shield the two nitrogen lone pairs, evident from 1H NMR study [75], from the solvent environment. Hydrogen, being smallest of all the elements, can interact with the nitrogen lone pairs, and thus the DBPZ molecule is sensitive only to the hydrogen-bond-donating solvents. The steric bulk around the hydrogen-bond-donating site is also important because, as the steric bulk increases, it becomes more difficult for the solvent to approach the DBPZ molecule. Ab initio calculations also suggest that the hydrogen bond length decreases on photoexcitation of the above-mentioned solute–solvent system and accordingly hydrogen bond energy increases. The hydrogen bond energy suggests that the DBPZ–water system forms moderate to strong hydrogen bonding in the excited state (Table 9.1 and Figure 9.3) [76]. Excited-state hydrogen bonding is also evident during photoinduced electron transfer between DBPZ and organic amines, even in the triplet state [77]. The transient radical cations and anions formed by electron transfer from amines to DBPZ were monitored by the laser flash photolysis technique, where a nanosecond Nd–YAG laser was used to excite DBPZ and the non-fluorescent transients were probed by a pulsed xenon light source through absorption. The radical ion pairs containing free electrons are susceptible to a magnetic

198 Hydrogen Bonding and Transfer in the Excited State Table 9.1 Hydrogen bonding parameters in the ground state (g.s.) and in the excited state (e.s.) Reprinted with permission from [70]. Copyright 2007 American Chemical Society Systems

Ort 1 Ort 2 Ort 3 a



HB length (A) g. s.

e. s.

2.19 2.21 2.20

2.05 2.05 2.06

EHB (kcal mol1) g. s. 4.8481 9.3662 9.4955

e. s. 6.4420 12.6638 12.6675



OH bond length (A)a

n–OH (cm1)b

g. s.

e. s.

g. s.

e. s.

0.9477 0.9474 0.9474

0.9535 0.9530 0.9529

4096.07 4065.56 4063.49

4005.75 3959.27 3958.37



The OH bond length for free water is 0.95 A . The n–OH for free water is 4027.09 cm1.

b

field. The partners of the initially formed geminate spin-correlated radical ion pairs undergo diffusion into the bulk solvent. When the diffusive separation between the partners reaches an optimum distance where exchange interaction between the free electrons becomes negligible, the maximum intersystem crossing between singlet and triplet spin states of the geminate pairs may occur if hyperfine interaction is present in the molecules as an internal magnetic field. Now, the application of an external magnetic field of the order of hyperfine interaction (0.02 T) or higher lifts the degeneracy between singlet and triplet states by introducing Zeeman splitting in triplet sublevels, and hence reduces intersystem crossing. Thus, the population of geminate radical ion pairs with the initial spin state increases. Therefore, a magnetic field can serve as an efficient tool for identification of the initial spin state of the geminate radical ion pairs formed by electron transfer. The essential features for observation of magnetic field effects are the diffusion, spin flipping and recombination or free ion formation, depending on the singlet or triplet spin states, respectively, of the spin-correlated geminate radical ion pairs. If the participating radical ions are very close to each other, the exchange interaction will hinder spin conversion, whereas a large distance of separation between them will destroy the spin correlation and their geminate characteristics. Both phenomena will reduce the

Figure 9.3 Geometrically optimized structures of DBPZ–1H2O (Ort 1) and two possible structures of DBPZ–2H2O (Ort 2 and Ort 3) systems. Reprinted with permission from [70]. Copyright 2007 American Chemical Society (See Plate 10)

Hydrogen Bonding on Photoexcitation

199

magnetic field effect. Therefore, the magnetic field effect indirectly serves as a tool for estimating the separation distance between geminate radical ions, and the maximum field effect is obtained at an optimum interradical distance where maximum spin flipping and consecutive phenomena could take place. Most of the magnetic field effect studies on triplet-born radical ion pairs using laser flash photolysis are carried out in viscous or confined media rather than in a homogeneous non-viscous medium to enhance the lifetime of the spin-correlated geminate radical ion pairs. Surprisingly, a prominent magnetic field effect is observed for radical ion pairs formed by electron transfer between DBPZ and amines in a homogeneous acetonitrile/water mixture, and the effect increases on the addition of water up to its 0.15 M concentration in the mixture. Further addition of water decreases the field effect. This observation could be explained only by considering interradical hydrogen bonding via the intervening water molecules, which helps to sustain the geminate characteristics and hence the spin correlation in the radical pairs to show the maximum field effect up to that particular concentration of water, beyond which the spin correlation is lost and the magnetic field effect decreases. The occurrence of interradical hydrogen bonding is also supported by the following observation. In the case of water-soluble amines such as triethylamine, the estimated hyperfine interactions from the observed magnetic field effect is lower than the calculated hyperfine interactions obtained from the electron density of the radical ion pairs. As the amine is water soluble, its cation can come very close to the DBPZ anion through intervening water molecules (Structure 9.1), which induces very fast electron exchange in the geminate radical ion pair and decreases the effective magnetic field effect. Thus, excited-state hydrogen bonding between DBPZ and water is also reflected in the triplet DBPZ.

N

N H O H N

Structure 9.1 Intermolecular hydrogen bonding mediated by water molecules

9.2.4 Quinones Similar experiments, i.e. laser flash photolysis and magnetic field effects, also reveal the presence of excitedstate hydrogen bonding which leads to hydrogen atom transfer in competition with electron transfer in drug–DNA interactions. It was observed that more or less all the DNA and RNA bases, i.e. adenine, thymine, guanine, cytosine and uracil, and their corresponding nucleosides, adenosine, thymidine, guanosine, cytidine and uridine, undergo hydrogen atom transfer in micellar media and electron transfer in polar homogeneous media while interacting with a model quinone drug, 2-methyl 1,4-naphthoquinone or menadione (MQ), used in cancer chemotherapy [78–81]. This implies that DNA bases enhance hydrogen atom transfer more in the hydrophobic interior of a micellar medium. The question arises as to why the bases participate in hydrogen atom transfer in a micelle, while electron transfer is the dominating pathway in an organic solvent and in water. There is more than one contributing factor. A radical ion is more stabilized in a polar solvent (acetonitrile or water) than in the hydrophobic interior of a sodium dodecyl sulphate (SDS) micelle. However, in a less polar solvent such as in tetrahydrofuran, neither electron transfer nor hydrogen atom transfer was evident.

200 Hydrogen Bonding and Transfer in the Excited State

The micellar environment is essential for hydrogen atom transfer. It was reported earlier that interbase proton transfer could compete with electron transfer along the DNA helix. This was attributed to hydrogen bonding between the bases that bring the molecules in suitable geometry where a little displacement of the bridging proton leads to proton transfer. A similar mechanism is operative for hydrogen atom transfer between the bases and MQ. These molecules may form a hydrogen bond where the participating groups are NH2 (base) and CO (MQ). It is well known that two molecules forming a hydrogen bond in a less polar environment lose the strength of hydrogen bonding in an aqueous medium. In aqueous solution, the solvent–MQ and solvent–base interactions predominate, and hydrogen bonding with the solvent is favoured. Therefore, hydrogen atom transfer is not significant in water. The solvent acetonitrile is also polar where dipole–dipole interactions with MQ and base exist. This may reduce the possibility of hydrogen bonding between MQ and base. It has been found that the base pairing between adenine and thymine derivatives occurs in SDS owing to hydrogen bonding and hydrophobicity. In an aqueous medium, no such base pairing was evident. The micelle not only provides the non-polar environment but also enhances the local concentration of MQ and base in its interior. It is quite plausible that the higher concentration makes possible the hydrogen bonding interaction and hydrogen atom transfer. In other words, the micellar environment forces the MQ and base to form a hydrogen bond that breaks in polar and homogeneous environments owing to solvent interaction and dilution. Moreover, it may be that the base transfers a hydrogen atom to MQ indirectly through SDS. The MQ and base molecules enter the micelle by hydrophobic interaction, where the closeness between them makes possible hydrogen bond and hydrogen atom transfer. It should be mentioned that the mode of interaction is highly dependent on the structure of the participating molecules. Unlike MQ, its higher homologue, 9,10-anthraquinone (AQ), prefers electron transfer in both polar and micellar media because the bulky structure of AQ hinders DNA bases from coming very close to it to form a hydrogen bond [78]. However, there are exceptions. The sugar unit of adenosine, a nucleoside of purine base, increases hydrogen abstraction in a micellar medium, irrespective of the structure of the quinone molecules, owing to its higher hydrogen donation ability [78]. Again, the sugar unit of thymidine, a nucleoside of pyrimidine base, enhances the extent of hydrogen abstraction just by inhibiting keto–enol tautomerism, which is responsible for electron transfer by the generation of an aromatic sextet in enol form, even in a homogeneous medium for all the quinones [79]. O H3C

OH

N3H N1 H Keto form

O

H3C

N3 N1

OH

Enol form

The interactions of these two quinones with other pyrimidine DNA and RNA bases, cytosine and its nucleoside cytidine and uracil and its nucleoside uridine, in the same pair of media, acetonitrile/water and SDS, have strong resemblances to those of the earlier pyrimidine base, thymine and thymidine. Moreover, 1,3dimethyl uracil undergoes two-step hydrogen abstraction with AQ in the excited state [80]. It is well established that water plays an important role in the stabilization of biomolecular systems. The hydrogen bonding capability of water is essential in the interactions between water and biomolecules. Moreover, hydrating water molecules are necessary for charge transport in DNA. Among all the DNA bases, guanine and its nucleoside guanosine are the most easily oxidizable bases and play a crucial role in charge transport in DNA. However, guanine undergoes hydrogen abstraction instead of electron transfer with both the quinones, which could be attributed to the absence of hydration owing to its low solubility in aqueous phase of the micellar medium [81]. Amada et al. reported that, for naphthoquinone (NPQ), a small amount of hydrogen

Hydrogen Bonding on Photoexcitation

201

atom transfer is possible from the solvent acetonitrile, which is known as an inert solvent for a substrate like benzophenone [82]. For dimethylnaphthoquinone, no such hydrogen atom transfer was obvious. Becker and Natarajan reported that hydrogen atom transfer occurs in the triplet n,ð
state of quinones even from benzene [83]. The interactions of individual nucleoside 50 -monophosphates of all five nucleic acid bases, adenosine 50 -monophosphate, guanosine 50 -monophosphate, thymidine 50 -monophosphate, cytidine 50 monophosphate and uridine 50 -monophosphate, have also been studied with quinones MQ and AQ. Although electron transfer is a predominant phenomenon involving the nucleotides in a DNA molecule, these experiments have revealed a totally opposite scenario. In homogeneous medium, except for guanosine 50 monophosphate, all four monophosphates have failed to favour electron transfer, which has been attributed to a semicircular conformation of the monophosphate. This brings the phosphate moiety closer to the electron donor centre, the nitrogenous base, resulting in an overall non-occurrence of electron transfer. The monophosphate and quinone have been brought closer by repeating similar experiments in SDS. However, in micelles, even guanosine 50 -monophosphate fails to transfer electrons completely towards quinones. Simultaneous observations of a low magnetic field effect and failure of electron transfer in SDS have been associated with a rapid spin exchange interaction among the partners of a radical pair, i.e. the monophosphates and quinone radicals, which is a very rare phenomenon. Observation of MFE has also shown a triplet-state reaction for the nucleotides and quinones. So the reaction mechanism among monophosphates and quinones is mainly based on either the adoption of a semicircular conformation (Structure 9.2) in a free homogeneous medium or spin exchange among triplet radical pairs by encapsulation of the interacting molecules in SDS micelles. However, both processes are manifested by less electron transfer. O

O

OH

N7

P

N9

O

N1

N3

NH2

O O

HO

OH

Structure 9.2 Semicircular conformation of guanosine 50 -monophosphate

Thus, it is evident that micellar media are unique in the sense that they provide a cage environment and create a special situation, as shown by Sakaguchi and Hayashi, where SDS transfers an H atom to MQ [84]. Later, the same research group observed ET or hydrogen atom transfer to MQ in SDS in the presence of another radical, 4-(lauroylamino) TEMPO [85]. Intermolecular hydrogen atom abstraction by photoexcited ketones has also been studied by several groups because it is helpful in understanding the complicated mechanistic pathways with different types of substrate. For example, Balakrishnan and Umapathy studied the fluoranil/isopropanol system [86], Arimitsu and Tsubomura investigated the interaction of benzophenone with aromatic amines [87] Didirjean et al. reported about exceptionally fast intermolecular hydrogen abstraction by 4,40 -bipyridine from alcohols [88], etc. Intermolecular hydrogen bonding can be an efficient radiationless deactivation pathway. Schultz et al. reported experimental and theoretical evidence for an excited-state deactivation mechanism specific to hydrogen-bonded aromatic dimers, which may account, in part, for the photostability of Watson–Crick base pairs in DNA. Using femtosecond time-resolved mass spectroscopy of 2-aminopyridine clusters, they

202 Hydrogen Bonding and Transfer in the Excited State

confirmed that the excited-state lifetime of a near-planar hydrogen-bonded dimer is significantly shorter than the lifetime of either monomer or the three- or four-membered non-planar clusters [89].

9.3 Concluding Remarks Besides the intermolecular excited-state hydrogen bond formation or hydrogen atom abstraction, there exists a vast field of excited-state intramolecular hydrogen bonding, proton transfer, etc. Organic bifunctional molecules containing both hydrogen bond donor and acceptor groups in close proximity may form an intramolecular H-bonded structure, which on photoexcitation leads to a intramolecular redistribution of electronic charge, and the proton is transferred from the hydrogen bond donor to the hydrogen bond acceptor group. This phenomenon is commonly termed excited-state intramolecular proton transfer (ESIPT). There are numerous works in this field, and numerous reviews also [90–93]. There have been some observations indicating that intramolecular hydrogen bonding competes with excited-state proton transfer, e.g. flavonoids while interacting with proteins, DNA and membranes [94]. However, the evidence for excited-state intermolecular hydrogen bonding is comparatively rare. We presume that this field is still emerging, and in the near future the intra- and intermolecular phenomena in the excited state will be very helpful in elucidating different excited-state phenomena and the mechanism of different complicated photochemical, photobiological and photosynthetic reactions.

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C. E. M. Carvalho, I. M. Brinn, A. V. Pinto and M. C. F. R. Pinto, J. Photochem. Photobiol., A, 136, 25 (2000). M. J. Kamlet, J.-L. M. Abboud, M. H. Abraham and R. W. J. Taft, Org. Chem., 48, 2877 (1983). J. R. Dias and B. Liu, Monatsh. Chem., 121, 13 (1990). R. Parthasarathi, V. Subramanian and N. Sathyamurthy, J. Phys. Chem. A, 110, 3349 (2006). D. Dey, A. Bose, M. Chakraborty and S. Basu, J. Phys. Chem. A, 111, 878 (2007). (a) T. Sengupta, S. Dutta Choudhury and S. Basu, J. Am. Chem. Soc., 126, 10589 (2004); (b) A. Bose, A. K. Sarkar and S. Basu, Biophys. Chem., 136, 59 (2008); (c) A. Bose, D. Dey and S. Basu, J. Photochem. Photobiol. A: Chem., 201, 197 (2009). (a) A. Bose, D. Dey and S. Basu, Sci. Technol. Advd Mater., 9, 024205 (2008); (b) A. Bose, A. K. Sarkar and S. Basu, J. Lumin., 129, 1186–1191 (2009). (a) A. Bose and S. Basu, J. Phys. Chem. A, 112, 12045 (2008); (b) A. Bose and S. Basu, Biophys. Chem., 140, 62 (2009). A. Bose, D. Dey, and S. Basu, J. Phys. Chem. A, 112, 4914 (2008). I. Amada, M. Yamaji, M. Sase and H. Shizuka, J. Chem. Soc., Faraday Trans., 91, 2751 (1995). R. S. Becker and L. V. Natarajan, J. Phys. Chem., 97, 344 (1993). Y. Sakaguchi and H. Hayashi, J. Phys. Chem., 88, 1437 (1984). J. Chen, Y. Mori, Y. Sakaguchi and H. Hayashi, Mol. Phys., 100, 1355 (2002). G. Balakrishnan and S. Umapathy, Chem. Phys. Lett., 270, 557 (1997). S. Arimitsu and H. Tsubomura, Bull. Chem. Soc. Jpn, 45, 1357 (1972). C. Didirjean, G. Buntinx and O. Poizat, J. Phys. Chem. A, 102, 7938 (1998). T. Schultz, E. Samoylova, W. Radloff et al., Science, 306, 1765 (2004). S. M. Ormson and R. G. Brown, Prog. React. Kinet., 19, 45 (1994). D. LeGourrierec, S. M. Ormson and R. G. Brown, Prog. React. Kinet., 19, 211 (1994). J. W. Petrich, Int. Rev. Phys. Chem., 19, 479 (2000). J. T. Hynes, J. P. Klinman, H.-H. Limbach and R. L. Schowen (eds), Hydrogen-Transfer Reactions, Vol. 1. WileyVCH, Weinheim, Germany (2007). B. Sengupta, A. Banerjee and P. K. Sengupta, J. Photochem. Photobiol. A: Chem., 80, 79 (2005).

10 Effect of Intramolecular H-Bond-Type Interactions on the Photochemistry of Aza-Stilbene-Like Molecules Giampiero Bartocci, Ugo Mazzucato and Anna Spalletti Dipartimento di Chimica, Universit
a di Perugia, 06123 Perugia, Italy

10.1 Introduction It is well known that molecular photochemistry can be affected significantly by both intermolecular and intramolecular interactions, and that hydrogen bonding (HB) can play an important role in this respect. The abundant experimental and theoretical investigations reported in the literature on the role of the intramolecular hydrogen bond (IHB) in controlling the dynamics of excited molecules mainly refer to the process of hydrogen or proton transfer to a heteroatom acceptor under irradiation (a few representative examples are reported in Refs [1] to [6]). When the interaction of H with the accepting group is only moderately strong, the effect can be limited to favour specific conformations, geometrical configurations and deactivation channels of the excited molecule. Early work on the effect of intramolecular interactions on the cis–trans (Z–E) photoreaction of compounds containing isomerizable double bonds showed how IHB of moderate strength can control the direction and the extent of the photoreaction [7–11], inhibiting photoisomerization and enhancing competing relaxation processes, or vice versa. Later, detailed studies were carried out on diarylethenes bearing aza-aromatic groups, such as pyridyl, quinolyl, pyrrolyl and indolyl groups [12–17]. When rotation around single bonds is operative, different conformations may display a different behaviour. It is well known that flexible molecules containing double bonds, such as 1,2-diarylethenes, can be found in fluid solution as a mixture of different conformational geometries (conformers or rotamers) that exist in dynamic equilibrium in the ground state (S0). The competition between conjugative and steric effects leads

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

206 Hydrogen Bonding and Transfer in the Excited State

to a type of diastereoisomerism and determines the stability of the conformers (s-trans and s-cis), with the most stable species generally being that species characterized by the less distorted geometry [18, 19]. The IHB effect on the rotamer distribution in S0 can be reflected in the photobehaviour of the lowest excited states. Within the framework of a long-term project on the study of internal rotation around single and double bonds in stilbene-like compounds, intramolecular N


H interactions were found to play an important role in the relative stability of the conformers and in the photoreactivity and reaction mechanism of the geometrical isomers in the lowest excited states of singlet and triplet multiplicity (1;3 Z * and 1;3 E*) [20–28]. This behaviour was observed for specific positional isomers of compounds bearing one (or more than one) isomerizable double bond(s) when a suitable distance of the heteroatom favours the interaction with the closest hydrogen atom of the ethene bridge. Electronic and NMR spectrometry and theoretical calculations were used to obtain information on this phenomenon. Important evidence came from the fluorescence spectrum, which markedly shifts to the red, indicating the presence of N


H interactions and an increase in the weight of planar conformations in non-protic solvents. The protic solvent causes a breaking of the IHB, favouring the presence of a set of conformations with more distorted geometries and blue-shifted absorption spectra. Also the NMR spectrum proved to be particularly useful for the study of IHB because it shows absorption peaks typical of bonds of this kind. In non-protic solvent the less shielded proton implied in the IHB-type interaction resonates at lower fields (higher d values) with respect to the other H atoms, whereas in a protic solvent the peak resonates again at higher fields because the formation of intermolecular HBs destabilizes the IHB structure. This short review article mainly deals with some typical examples encountered in our laboratory. As IHB can substantially affect the relative abundances of the species present in the conformational equilibrium in the ground state [18, 19], stabilizing specific conformers, and thus modifying the excited-state behaviour under irradiation, the first part of the next section will describe the effect of IHB on the relative abundances in the conformer mixture.

10.2 Control of the Conformational Equilibria in the Ground State It has been shown that the presence of rotamers in solution of diarylethenes and related compounds can have a relevant role in affecting the selective relaxation of the excited geometrical isomers of these molecules, particularly of the Z isomers. In fact, the s-trans rotamers generally undergo the Z ! E photoisomerization only, whereas the s-cis conformations are mainly involved in the photocyclization reaction [18, 19, 29]. The introduction of nitrogen heteroatoms in the ortho position to the ethenic bridge of 1,2-diarylethenes and related compounds, where one or both side phenyl groups of stilbene are replaced by aza-aryl groups, produces an unexpected spectral, photophysical and photochemical behaviour in some of these flexible molecules. In the following, some significant examples are illustrated where IHB produces relevant changes in the equilibrium composition of the rotamers in the S0 state and then in the photobehaviour of these molecules. Trans-3-styryl-2-azaphenanthrene. In inert solvents, the photobehaviour of the trans isomer of 3-styryl-2azaphenanthrene (3St-2AP) is very different from that observed for both its hydrocarbon analogue, 3-styrylphenanthrene (3StP), and its positional isomer, 3St-7AP, as shown in Table 10.1. In fact, the fluorescence decay of 3St-2AP was monoexponential, and no effect of the excitation (lexc) and emission (lem) wavelengths on its emission and excitation spectra, respectively, was observed [28]. On the contrary, polyexponential decay and wavelength effects on the spectral properties of 3StP [30] and 3St-7AP [28] were observed owing to the presence of rotamers with similar abundance in the ground state (see Table 10.1). This unexpected behaviour of 3St-2AP was explained by the formation of IHB-type interactions between the nitrogen atom and the nearest H atom of the ethenic bridge. Such N


H interaction is expected to force the

Effect of Intramolecular H-Bond-Type Interactions on the Photochemistry of Aza-Stilbene-Like Molecules

207

Table 10.1 Relative energy values, corresponding abundances and photophysical parameters of the conformers of E-3St-2AP, showing N


Hb interactions, as compared with 3StP and 3St-7AP, in 9/1 methylcyclohexane/ 3-methylpentane (MCH/3MP) at room temperature DE (kcal mol1)

Abundance (%)

s-trans s-cis s-trans s-cis s-trans

0 3.1 0 0.19 0

99.5 0.5 58 42 66

s-cis

0.4

34

Compound

Rotamer

3St-2APa 3StPb 3St-7APa

kf (108 s1)

wf

tf (ns)

0.44

4

1.1

0.45 0.94 0.35c 0.40d

18.3 7.7 7.8c 7.5/1.5d

0.25 1.22 0.45

a

From Ref. [28]. In n-hexane, from Ref. [30]. c At lexc ¼ 316 nm, where the s-trans rotamer largely prevails. d Parameters of the rotamer mixture. b

molecule into a transoid (s-trans) conformation, while weaker interactions are present in the s-cis conformer (Scheme 10.1). Theoretical ab initio calculations, used to optimize the geometries of the different conformers of these compounds in S0, provided the relative energy values reported in Table 10.1, which confirmed that the conformation proper for the HB interaction (s-trans) is largely stabilized (ffi3 kcal mol1) with respect to the other ones and practically the only expected species in solution ( > 99%). On the other hand, the corresponding 3StP shows a mixture of the expected conformational isomers of comparable energies (derived by kinetic and statistical fluorescence analysis [30]), as also obtained by calculations for 3St-7AP [28], where the interactions are not present. The calculations also showed that only in the s-trans conformation of 3St-2AP are the values of  ^ angles (93.3 ) close to those expected for HB formation. the N


Hb distances (2.44 A) and CHN This explanation was confirmed by the solvent effect on the 1 H NMR spectra, which showed a net shift to higher d values (deshielding effect) of the peak assigned to the H atom (Hb) responsible for the intramolecular interaction on going from methanol to benzene. For the related positional isomer (3St-7AP), the d increase on going from methanol to benzene was not observed. Moreover, the fluorescence excitation spectrum of 3St-2AP in methanol depends on lem, and its emission spectrum and quantum yield on lexc. These results confirm that, in protic solvents, stronger intermolecular HBs are favoured which destabilize the s-trans conformation responsible for the IHB. This leads to the presence of a rotamer mixture in the ground and excited states, thus making the behaviour of 3St-2AP in methanol similar to that observed for 3StP and 3St-7AP in non-polar solvents.

Hb

N

Hb N

Ha s-trans

Scheme 10.1

Ha s-cis

Conformational equilibrium of E-3St-2AP

208 Hydrogen Bonding and Transfer in the Excited State

H

H

H

N

N

N N

N

H

s-cis,s-trans

s-trans,s-trans

N H

H

s-cis,s-cis

Scheme 10.2 Conformational equilibrium of E-(3IQ)2E

Trans-1,2-di-(30 -isoquinolyl)ethene. In non-polar solvents, the photobehaviour of the trans isomer of 1,2di-(30 -isoquinolyl)ethene, (3IQ)2E, is quite different from that observed for both its hydrocarbon analogue, 1,2-di-(20 -naphthyl)ethene, (2N)2E, and its positional isomer, 1,2-di-(30 -quinolyl)ethene, (3Q)2E, where the HB interactions are not expected. Such differences are even larger than those described for 3St-2AP. Even in this case the anomalous behaviour of (3IQ)2E was explained by the presence of HB-type interactions in inert solvents [28], which lead to preferential stabilization of the s-trans,s-trans conformer, as shown in Scheme 10.2. The relative energy values, corresponding abundances and photophysical parameters of the conformers of E-(3IQ)2E are collected in Table 10.2 and compared with those of (2N)2E and (3Q)2E in inert solvent at room temperature. The relative energy values for the rotamers of (3IQ)2E, provided by theoretical ab initio calculations, confirmed that the s-trans,s-trans conformation is largely stabilized ( > 3 kcal mol1) with respect to the other ones and is the only expected species in solutions (99.9%) [28]. On the other hand, the corresponding hydrocarbon, (2N)2E, exists in S0 as a mixture of the expected conformational isomers of comparable energies (derived by kinetic and statistical fluorescence analysis [31]). This was also found by calculations for (3Q)2E [28], in agreement with the observed wavelength effects on the spectral properties and with a polyexponential decay. The calculations also showed that only in the s-trans,s-trans conformation of (3IQ)2E ^ angle (97.3 ) suitable for HB-type interaction. are the values of the N


H distance (2.49 A) and CHN Table 10.2 Relative energy values, corresponding abundances and photophysical parameters of the conformers of E-(3IQ)2E, showing N


H interactions, compared with (2N)2E and (3Q)2E, in MCH/3MP at room temperature DE (kcal mol1)

Abundance (%)

s-trans,s-trans s-cis,s-cis s-trans,s-cis s-trans,s-trans s-trans,s-cis s-cis,s-cis s-trans,s-trans s-trans,s-cis

0 3.9 5.9 0 0.43 1.1 0 0.43

99.9 0.1 <0.1 60 29 11 56 27

s-cis,s-cis

1.1

17

Compound

Rotamer

(3IQ)2Ea (2N)2Eb (3Q)2Ea

a

From Ref. [28]. From Ref. [31]. c Parameters of the rotamer mixture at lexc ¼ 315 nm. d Parameters of the rotamer mixture at lexc ¼ 375 nm. b

wf

tf (ns)

kf (108 s1)

0.45

4.3

1.04

1.0 0.97 0.58

7.2 2.1 1.4

1.4 4.6 4.1

0.38c/ 0.60d

1.9c/ 1.5d

Effect of Intramolecular H-Bond-Type Interactions on the Photochemistry of Aza-Stilbene-Like Molecules

209

Even in this case this explanation was confirmed by the solvent effect on the 1 H NMR spectra and on the spectral properties of (3IQ)2E. In fact, its 1 H NMR spectra showed a net shift to higher d values of the peak assigned to the ethenic hydrogen on going from methanol to benzene. The double interaction, which is expected in this symmetric molecule, leads to an even stronger increase (0.80 ppm) of the d value of the singlet ethenic peak in benzene [28]. Such stronger stabilization of the s-trans,s-trans conformer makes it particularly planar and rigid, thus explaining the absence of photoisomerization (see below). As expected, for the related positional isomer (3Q)2E, no d increase on going from the protic to non-polar solvent was observed [28]. The effect of protic solvents on the absorption and emission spectra was also very informative about the preferential stabilization of the s-trans,s-trans rotamer with respect to the other ones. Figure 10.1 shows the fluorescence excitation and emission spectra of E-(3IQ)2E in MCH/3MP and methanol at room temperature [32]. In non-polar solvent, the excitation spectrum is independent of lem and overlaps the absorption (not shown); moreover, the emission spectrum and quantum yield are independent of lexc, as expected when only one rotamer is present in solution. On the other hand, in protic solvent, a bathochromic shift of both absorption and fluorescence spectra, due to the polarity effect, and less structured emission spectral shapes were observed. Furthermore, the excitation spectrum becomes dependent on lem and does not overlap the absorption, particularly in the 260–310 nm

ExcitationEmission λem=362,380,440nm

λexc=332,370nm

Intensity

MCH/3MP

CH3OH 3 4,5

1absorption 2 λem=397nm 3 λem=365nm 4 λexc=332nm 5 λexc=370nm

1

2

300

350

400

λ/nm

450

500

Figure 10.1 Normalized fluorescence excitation and emission spectra of E-(3IQ)2E in two solvents at room temperature (the absorption spectrum in methanol is also shown for comparison)

210 Hydrogen Bonding and Transfer in the Excited State

spectral region, where a dependence of the fluorescence quantum yield on lexc is observed (0.27 and 0.46 at 280 and 316 nm respectively), even if the shape of the emission spectrum does not change with lexc, as shown in Figure 10.1. These results confirm the presence of a rotamer mixture in methanol, where the bathochromic s-trans,s-trans is the most fluorescent and abundant species. The stronger intermolecular HBs destabilize the s-trans,s-trans conformer responsible for the IHB, thus favouring the presence of a rotamer mixture in the ground state and making the behaviour of (3IQ)2E in methanol more similar to that observed for (2N)2E and (3Q)2E in non-polar solvents.

10.3 Control of Radiative and Reactive Relaxation In Section 10.2 some significant examples have been described where IHB interactions produced relevant changes in the equilibrium composition of the rotamers in the S0 state; the consequent effects on the photobehaviour of these molecules will now be illustrated. It has to be recalled that diarylethenes mainly photoisomerize by the well-known diabatic mechanism widely accepted for stilbene and many related molecules. Accordingly, activated twisting of the E or Z isomer in the potential energy surface of the lowest excited states S1 and T1 leads to a perpendicular configuration (1;3 P*) of minimum energy. From there, internal conversion of 1P or intersystem crossing (ISC) of 3 P* to the ground-state potential energy curve (1 P) and relaxation to the E and Z isomers with comparable probabilities takes place [33]. Accordingly, the kinetic parameters reported in Table 10.3 were derived as kf ¼ wf/tf and kE ! Z ¼ 2  wE ! Z/tf without taking into account the multiplicity of the excited state responsible for photoisomerization. The most evident effect of IHB in S1 or T1 is the change in the efficiency of the competitive relaxation pathways of the excited isomer. In several cases we noticed that the more planar conformation induced by IHB led to a decrease in the isomerization quantum yield and an increase in the yield of the radiative deactivation. A simple example is that of two aza-derivatives of trans-1-styrylnaphthalene, namely 4-styrylquinoline (4StQ) and its positional isomer 8StQ (Scheme 10.3), whose behaviour is not complicated by the presence of different conformers because only one species (s-cis) is largely prevalent in solution for steric reasons [18, 19], as well established for the corresponding hydrocarbon [34, 35]. This was supported by theoretical calculations that showed an energy difference larger than 1.5 kcal mol1 between the two conformers, and by the monoexponential fluorescence decay measured for these compounds (see Table 10.3). Scheme 10.3 shows that only in the case of 8StQ is the heteroatom position suitable for the establishement of HB-type interactions, and that such interaction is possible, in principle, for both conformers. Recent ab initio calculations (6-31G basis set) of the energy levels showed that the most stable conformer of 8StQ is again the s-cis species [36]. The s-trans species would be expected to be the most favourable for the formation of IHB, but its stabilization is not enough to invert the corresponding energy levels, so that the largely prevailing conformer remains the s-cis species. Nevertheless, the stabilization of s-cis by IHB leads to a huge increase in the emission quantum yield and lifetime and a decrease in reactivity if compared with 4StQ (Table 10.3). The global effect is reflected by the rate constants, which show an important decrease in the rate of the torsional process. This behaviour recalls that reported for the isomeric 1-styrylisoquinoline, another case where only the s-cis rotamer Table 10.3 Fluorescence lifetimes and quantum yields and rate constants of the radiative and reactive deactivations of the main conformer of the trans isomer of 4StQ and 8StQ in MCH/3MP [21] Compound

tf (ns)

wf

kf (108 s1)

wE ! Z

kE ! Z (108 s1)

4StQ 8StQ

0.07 2.6

0.011 0.67

1.6 2.6

0.26 0.14

74 1.1

Effect of Intramolecular H-Bond-Type Interactions on the Photochemistry of Aza-Stilbene-Like Molecules

Hb

Hb

Ha

N

Ha

N

s-cis

Hb

211

8StQ

s-trans

Hb

N

N

Ha

Ha

s-cis

s-trans 4StQ

Scheme 10.3 Conformational equilibrium of two E-styrylquinolines [24]. Reproduced by permission of the PCCP Owner Societies

is expected for steric reasons. The excited rotamer, where an HB-type interaction is expected, decays mainly by internal conversion, the radiative and reactive quantum yields being <0.0001 and 0.005 respectively (the latter shows a tenfold increase in protic solvents) [11]. However, a deactivating role of the n,p states cannot be excluded in the photobehaviour of these positional isomers. An even more interesting case of the role of IHB in the relaxation of S1, which results in complete inhibition of E ! Z photoisomerization, is that of the symmetric (3IQ)2E [28], whose conformational properties were discussed in Section 10.2. Its behaviour can be usefully compared with that of (3Q)2E. As seen in Section 10.2, a different conformational behaviour was found for the two compounds, leading in both cases to a rather high emission yield but to the absence of the lexc effect on the fluorescence spectrum and clear mono-exponential fluorescence decay for (3IQ)2E only. Moreover, while the 3Q derivative has a substantial photoisomerization yield (0.28 at lexc ¼ 315 nm), the 3IQ derivative is not reactive at all. This behaviour, probably owing to a large torsional barrier in the lowest excited states, consistent with the experimental results, resembles that reported for analogous compounds with the N atom in ortho position to the ethenic bridge. The occurrence of one-way photoisomerization (only in one direction) because of the preferential stabilization of one of the two geometrical isomers was first reported in 1973 [7] and then found in several dihetarylethenes [8–17, 20–28]. In addition to the case of 1-styrylisoquinoline cited above [11], other interesting examples are 1-(20 -pyridyl)-2-phenylbutadiene [21, 22] (see Section 10.4) and 1-(20 -pyridyl),2-(200 -indolyl)ethene [13, 15, 16].

10.4 Unusual Adiabatic Photoisomerization in the E ! Z Direction It has to be recalled that stilbene-like molecules can photoisomerize in favourable cases by the less common adiabatic mechanism first proposed for trans-2-styrylanthracene in the triplet state [37] and then found to be operative also in the singlet manifold in several systems [29, 38]. According to this mechanism, the photoisomerization occurs on the same potential energy surface of S1 and T1, leading to the excited product. As the process is generally less activated, or even barrierless, when starting from the Z side, the adiabatic mechanism has been mainly evidenced for the reaction in the 1;3 Z * ! 1;3 E* direction, where the formation of

212 Hydrogen Bonding and Transfer in the Excited State Ha

Ha

Hc

Ha

Hc

Hb

N

Hd

EE B rotamer

Hb

Hd

hv

N

Hb Hd

N Hc

N Hc

Hd

hv

EE A rotamer

Scheme 10.4

Ha

Hb

ZE A rotamer

ZE B rotamer

Conformational equilibria of the EE and ZE isomers of 2PyPhBu

1

E* is easily recognized from its fluorescence, and that of 3 E* from the transient absorption spectrum obtained by laser flash photolysis. Adiabatic photoisomerization in the E ! Z direction is quite uncommon for stilbene-like molecules, where very often the energy level of 1;3 Z * is higher than that of 1;3 E*. Very few examples are reported in the literature [23, 39–42]. As stated above, in some aza-diarylethenes, IHB may preferentially stabilize the Z geometrical isomer, inducing in some cases energy inversion of the 1;3 Z * and 1;3 E* species. Such stabilization in the ground and particularly in the excited state can modify the potential energy surfaces of the electronic states associated with these stereoisomers, thus controlling the photoisomerization mechanism. An interesting case is represented by the adiabatic EE ! ZE photoisomerization of 2-pyridylphenylbutadiene (2PyPhBu), whose marked deviations from the normal spectral behaviour and the reduced photoisomerization yield suggested a preferential stabilization of specific species by IHB involving the nitrogen in ortho position with respect to the adjacent ethenic bridge [23, 24], as shown in Scheme 10.4. Indeed, the absorption spectrum of the isomer with one cis double bond adjacent to the pyridyl group (ZE) is slightly structured and anomalously red-shifted with respect to the corresponding EE isomer. The fluorescence spectrum is more structured and markedly red-shifted (by about 14 nm) with respect to EE (Figure 10.2). The absorption spectrum of ZE does not show the red-shift expected for p ! p bands in polar solvents, but its spectral shape changes, and a blue-shift of the maximum is observed in protic solvents (methanol and 9/1 water/acetonitrile). This behaviour is indicative of the presence of IHB between the nitrogen atom and the ethenic hydrogen (Hc) (see Scheme 10.4), which increases the weight of planar conformations. The polar and protic solvent induces the breaking of such N


H bonds, thus favouring the presence of a set of conformations with more distorted geometries and leading to a more convoluted and blue-shifted absorption spectrum. The emission

EE ZE

Intensity

absorption

300

350

400

450

λ (nm)

Figure 10.2 Normalized absorption and emission spectra of the EE and ZE isomers of 2PyPhBu in MCH-3MP at room temperature

Effect of Intramolecular H-Bond-Type Interactions on the Photochemistry of Aza-Stilbene-Like Molecules

213

presence of IHB in the EE and ZE isomers of 2PyPhBu was also evidenced by NMR spectroscopy. Indeed, the Hc atom in ZE shows strong deshielding in benzene (d ¼ 9.01 ppm) with respect to methanol (7.96 ppm), whereas the other protons are shifted towards minor d (around 6.5 ppm in benzene). This behaviour is in agreement with the presence of an IHB interaction that is reduced or broken in polar and protic solvents in favour of intermolecular HBs. A similar but less marked shift was also found for the EE isomer (d ¼ 7.82 and 7.39 ppm in benzene and methanol respectively). The effect observed for the ZE isomer (more than 1 ppm of shift) is indicative of a stronger N


H bond because of a shorter distance between the interacting atoms. This was confirmed by ab initio calculations of the total molecular energy of rotamers of Scheme 10.4, using the optimized geometry obtained at 3-21G level [26]. The theoretical results showed that A is the most stable rotamer for both EE and ZE isomers, and that, in ZE-2PyPhBu(A), the calculated intramolecular N


H  ^

N angle is 122.75 , thus suggesting the formation of a ‘real’ IHB. The distance is 2.266 A and the C–H
preferential stabilization of ZE with respect to EE in the excited singlet state is responsible for the unusual adiabatic photoisomerization in the trans ! cis direction. The photobehaviour of the four stereoisomers of 2PyPhBu is reported in Table 10.4, which shows the EE compound to be photoreactive, giving both ZE and ZZ. The formation of ZZ from EE clearly indicates the occurrence of a ‘one photon – two bonds’ mechanism, which implies the 1 EE* ! 1 Z E* adiabatic step. Likewise, the high stabilization of the A rotamer of 1 Z Z* by IHB with respect to that of 1 EZ* (one cis double bond adjacent to the phenyl group) causes the ‘one photon – two bonds’ process from EZ to ZE, implying the 1 EZ* ! 1 Z Z* adiabatic step (always in the trans ! cis direction). The quantum yield of the adiabatic 1 EE* ! 1 Z E* process, 1 wad EE* ! ZE* , was estimated by the equation 1 ad wEE* ! ZE*

¼ wEE ! ZZ =wZE ! ZZ

ð10:1Þ

and found to be 0.23. The corresponding yield of the ground-state ZE formation could be derived by the equation 1 ad wEE ! ZE

¼ 1 wad EE* ! ZE* ð1wZE ! EE wZE ! ZZ Þ

ð10:2Þ

which takes into account the deactivation processes of 1 Z E* to the ground state. The comparison between the EE ! ZE photoisomerization yield derived for the adiabatic pathway (0.18) and the experimental quantum yield value (0.19) evidenced that this rotation takes place through an adiabatic mechanism only. The adiabatic quantum yields for EZ ! ZZ and ZZ ! ZE (isomerization in the common cis ! trans direction), estimated by analogous equations, account for the overall experimental quantum yields. The obtained results are shown in Table 10.4. Measurements of the sensitized (by biacetyl) isomerization quantum yields in benzene showed substantial reactivity for the isomers with cis double bonds only and gave information on the involvement of adiabatic Table 10.4 Photophysical and photochemical parameters of the geometrical isomers of 2PyPhBu in MCH/3MP at room temperaturea (from Refs [23] and [24]) Isomer (X) EE ZE EZ ZZ a

wf 0.0085 0.0057 0.0041 0.0019

tf/ns 0.042 <0.01 <0.01 <0.01

wX ! EE 0.14d 0.079d 0.027

The superscripts ad and d refer to an adiabatic and diabatic mechanism respectively.

wX ! ZE 0.19

wX ! EZ

ad

0.035 0.23ad

0.011d

wX ! ZZ 0.014 0.061d 0.087ad

214 Hydrogen Bonding and Transfer in the Excited State

Figure 10.3 Qualitative sketch of the potential energy curves as a function of the rotation around the two double bonds for the four geometrical isomers of 2PyPhBu in the singlet manifold [24]. Reproduced by permission of the PCCP Owner Societies

processes in the triplet manifold in the more common cis ! trans direction, indicating that the preferential stabilization of the compounds with a double bond in cis configuration adjacent to the pyridyl group (ZE and ZZ), with respect to EE and EZ respectively, is not operative in the triplet state [24]. The sketch of the potential energy curves of the excited singlet state, shown in Figure 10.3, reflects the observed behaviours, which obviously depend on the shape of the energy surfaces as a function of the rotation around the ethenic bonds. As an example, the S1 state of 2PyPhBu is characterized by higher torsional barriers for the cis double bond adjacent to the phenyl group (EZ isomer), while lower energy barriers in T1 favour the isomerization of all its geometrical isomers.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

A. Weller, Progr. React. Kinet. Mech., 1, 187–214 (1961). N. Mataga, Pure Appl. Chem., 56, 1255–1268 (1984). E. M. Kosower and D. Huppert, Ann. Rev. Phys. Chem., 37, 127–156 (1986). L. G. Arnaut and S. J. Formosinho, J. Photochem. Photobiol. A: Chem., 75, 1–20 (1993). S. J. Formosinho and L. G. Arnaut, J. Photochem. Photobiol. A: Chem., 75, 21–48 (1993) and references therein. A. L. Sobolewski and W. Domcke, Science, 302, 1693–1694 (2003) and references therein. J. A. Eenkhoorn, S. Osamund de Silva and V. Snieckus, Can. J. Chem., 51, 792–810 (1973). D. A. Lightner and Y.-T. Park, J. Heterocycl. Chem., 14, 415–422 (1977). J. A. de Groot, H. Jansen, J. R. Fokkens and J. Lugtenburg, Rec. Trav. Chim. Pays Bas, 102, 114–118 (1983). F. D. Lewis, D. K. Howard, J. D. Oxman et al., J. Am. Chem. Soc., 108, 5964–5968 (1986) and references therein. G. Gennari, G. Galiazzo and P. Bortolus, J. Photochem. Photobiol., 43, 293–302 (1988). G. Galiazzo, G. Gennari and P. Bortolus, Gazz. Chim. It., 121, 67–71 (1991). T. Arai, T. Iwasaki and K. Tokumaru, Chem. Lett., 691–694 (1993). T. Arai, M. Moriyama and K. Tokumaru, J. Am. Chem. Soc., 116, 3171–3172 (1994). F. D. Lewis, B. A. Yoon, T. Arai et al., J. Am. Chem. Soc., 117, 3029–3036 (1995). T. Arai, M. Obi, T. Iwasaki et al., J. Photochem. Photobiol. A: Chem., 96, 65–69 (1996). M. Obi, H. Sakuragi and T. Arai, Chem. Lett., 169–170 (1998). U. Mazzucato and F. Momicchioli, Chem. Rev., 91, 1679–1719 (2001).

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19. G. Bartocci, A. Spalletti and U. Mazzucato, Conformational Analysis of Molecules in Excited States, ed. by J. Waluk. Wiley-VCH, New York, NY, pp. 237–296 (2000) and references therein. 20. G. Bartocci, U. Mazzucato and A. Spalletti, Rec. Trav. Chim. Pays Bas, 14, 459–464 (1995). 21. A. Spalletti, G. Bartocci, F. Elisei et al., Proc. Indian Acad. Sci. (Chem. Sci.), 110, 297–310 (1998). 22. L. Giglio, U. Mazzucato, G. Musumarra and A. Spalletti, Phys. Chem. Chem. Phys., 2, 4005–4012 (2000). 23. G. Bartocci, G. Galiazzo, U. Mazzucato and A. Spalletti, Phys. Chem. Chem. Phys., 3, 379–386 (2001). 24. G. Bartocci, G. Galiazzo, L. Latterini et al., Phys. Chem. Chem. Phys., 4, 2911–2916 (2002). 25. A. Spalletti, G. Cruciani and U. Mazzucato, J. Mol. Struct., 612, 339–347 (2002). 26. I. Baraldi, A. Spalletti and D. Vanossi, Spectrochim. Acta Part A, 59, 75–86 (2003). 27. E. Marri, G. Galiazzo, U. Mazzucato and A. Spalletti, Chem. Phys., 312, 205–211 (2005). 28. S. Ciorba, F. Fontana, G. Ciancaleoni et al., J. Fluoresc., 19, 759–768 (2009). 29. G. Bartocci, U. Mazzucato and A. Spalletti, Trends Phys. Chem., 12, 1–36 (2007) and references therein. 30. A. Spalletti, G. Bartocci, F. Masetti et al., Chem. Phys., 160, 131–144 (1992). 31. G. Bartocci and A. Spalletti, J. Phys. Chem. A, 106, 7068–7074 (2002). 32. S. Ciorba, unpublished results. 33. J. Saltiel and Y.-P. Sun, Photochromism: Molecules and Systems, ed. by H. D€ urr and H. Bouas-Laurent. Elsevier, Amsterdam, The Netherlands, pp. 64–162 (1990) and references therein. 34. E. Fischer, J. Photochem., 17, 331–340 (1981) and references therein. 35. G. Bartocci, F. Masetti, U. Mazzucato and G. Marconi, J. Chem. Soc., Faraday Trans. 2, 80, 1093–1105 (1984). 36. A. Spalletti, unpublished results. 37. T. Arai, T. Karatsu, H. Sakuragi and K. Tokumaru, Tetrahedron Lett., 24, 2873–2876 (1983). 38. G. Bartocci, G. Galiazzo, E. Marri et al., Inorg. Chim. Acta, 360, 961–969 (2007). 39. M. F. Budyka, O. D. Laukhina and V. F. Razumov, Chem. Phys. Lett., 279, 327–332 (1997). 40. S. P. Kazakov, V. F. Razumov and M. V. Alfimov, High En. Chem., 38, 249–255 (2004) and references cited therein. 41. P. Bortolus, G. Galiazzo, G. Gennari et al., Photochem. Photobiol. Sci., 3, 689–694 (2004). 42. M. F. Budyka, N. I. Potashova, T. N. Gavrishova and V. M. Lee, J. Photochem. Photobiol. A: Chem., 203, 100–104 (2009).

11 Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces Rajib Kumar Mitra, Pramod Kumar Verma, Debapriya Banerjee and Samir Kumar Pal Unit for Nano Science and Technology, Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098, India

11.1 Introduction Water has anomalous properties that emanate from its hydrogen-bond-induced structure. Most of the unique properties of water are related to the network of strong three-dimensional hydrogen bonds that interconnect the water molecules [1, 2]. According to Stillinger [3], water has a preferential three-dimensional tetrahedral structure containing a few free or single-bonded water molecules. A simulation study [4] challenges this concept and reports some distorted hydrogen-bonded structure in which one hydrogen atom is attached with two oxygen atoms, and these bifurcated bonds play a central role in the molecular mobility in the liquid state by lowering the Gibbs energy barrier of diffusion. In pure water, hydrogen bonds have a lifetime of about 1 ps. Defects in this hydrogen-bonded network seem to produce very short-lived hydrogen bonds (<200 fs) [5]. In biological interfaces, however, certain water molecules in the hydration shell have dynamics very different from that of bulk water, with a residence time of up to 100 ps [6, 7]. In proteins, the internal water molecules exchange with external water molecules typically on a timescale of 0.1–10 ms [8–10]. Hydrogen bonds are in a continuous process of breaking and reformation, with a continuous change in hydrogen bond length and strength. Pure liquid water represents a disordered ensemble of highly polar molecules linked through a fluctuating network of intermolecular hydrogen bonds on femtosecond to picosecond timescales, as shown by the pioneering work by Elsaesser et al. [11–13], Wiersma et al. [14, 15], Voehringer et al. [16], Fayer et al. [17], Tokmakoff et al. [18], using ultrafast vibrational spectroscopy and MD simulation studies, and Bakker et al. [19, 20], using time-resolved pump–probe laser spectroscopy measurements. The slowest component of the fluctuations is associated with the globular structural rearrangement of the hydrogen bond

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

218 Hydrogen Bonding and Transfer in the Excited State

network [21]. These results have greatly enhanced the understanding of hydrogen bond dynamics in pure water. At the molecular level, most of the biology occurs at the interfaces where water makes contact with the macromolecules or molecular aggregates. So it is important to understand how it behaves at the interface. Taking bulk water as the reference, it is interesting to observe in what way and to what extent the physical properties of water are changed at the interface. The high cohesive energy of water tends to minimize the effect on hydrogen bond network made by any solute. The dense hydrogen bond network makes water structurally robust without compensating for the fluidity. Molecular mobility remains high in liquid water because the hydrogen bond network is restructured by a cooperative mechanism where hydrogen bond partners are interchanged in a concerted way, thereby avoiding the high energy barrier that has to be overcome if several hydrogen bonds are broken simultaneously [4, 22, 23]. An interface interferes with this cooperative mechanism, leading to a dynamic perturbation [24]. However, when a macromolecule such as protein, DNA, etc., is dissolved in water, the physical properties of the water in the vicinity of the macromolecular surface become altered relative to those in the absence of the macromolecule [24–31]. The water molecules enclosed within the solvation shell of a biomolecule are known as ‘biological water’ [32, 33] or ‘vicinal water’ [34] or ‘lubrication of life’ [30, 35, 36]. This effect is due to the fact that the hydrogen bond network is locally disrupted at the interface and differs significantly from that in the bulk. The high density of particles inside a cell (cytoplasm typically contains up to 400 g L
1 macromolecules) makes the water hydrogen bond structure inside the cell highly perturbed and reduces the average coordination number to 2.2, as opposed to 3.6 in the bulk [37–41]. This is an important issue, because water molecules are found in abundance at the interfaces of proteins and DNA, and they control the structure, function and reactivity of many natural and biological systems [42–44]. Micelles and reverse micelles (RMs), supramolecules involving cage-like hosts (e.g. cyclodextrins), microporous solids (e.g. nanoporous silica, zeolites), semirigid materials (e.g. hydrogels, polymers, etc.) and lipid vesicles are some examples of self-organized assemblies that can mimic several important and essential features of biological processes and are less complex than the biological systems they mimic. The physical and chemical properties of the entrapped water are markedly different from those of the bulk water, but similar in several aspects to those of biological interfacial water as found in membrane or protein surfaces [45–48]. The perturbing effects of interfaces and confinements on the dynamics of water and hence the hydrogen bonding structure have been studied extensively in a number of systems, using numerous techniques [45, 49–65]. Molecular simulation studies [66–69] reveal that water is largely immobilized at the RM interface by strong interactions with polar head groups and counterions. Water trapped or bound in this ionic layer at the surfactant surface shows a decreased mobility. A series of measurements using ultrafast IR vibrational echo and pump–probe experiments performed directly on water confined in bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) RMs have provided evidence of the slow dynamics of water [70–75]. Surface force measurements using mica surfaces [76, 77] show that the viscosity and density of very thin films of water are higher than those of the bulk water owing to structuring of water near the confining surfaces. Magnetic resonance dispersion experiments indicate that the dynamics of water in the first hydration layer on a protein, averaged over the entire protein, are slowed down relative to the bulk water [78]. However, recent studies of protein and nucleic acid suggest that the hydration effects as measured by volumetric techniques extend beyond the first hydration layer [79, 80]. The hydration layer of many proteins does not freeze even at subzero temperatures and thus sustains life at low temperature [81–83]. Mutual polarization of the hydrogenbonded water molecules increases the dipole moment and dielectric constant of bulk water [84]. However, the absence of such polarization for a water molecule hydrogen bonded to a biological system makes biological water less polar than bulk water. It is evident from the results obtained on this wide variety of systems that the dynamics of water that is nanoscopically confined or in contact with interfaces differs from that of bulk water. Both theoretical and experimental studies prove the existence of a very slow component in the dynamics of water in such restricted

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environments. The slow component decays in hundreds to thousands of picoseconds, depending upon the systems involved, whereas pure water molecules exhibit solvation dynamics with a subpicosecond timescale [85]. The origin of this slow component in micelles is due to a dynamic equilibrium between the bound and free water molecules, in which the water molecules at the interface are hydrogen bonded with the polar head group of the micelle [86, 87]. Some of the water molecules are directly hydrogen bonded to the polar head group of the surfactant molecules, and some are either free or intermolecularly hydrogen bonded among themselves [88, 89]. The equilibrium between bound and free water molecules is sensitive to the change in the microenvironment of the microheterogeneous systems, e.g. temperature, pressure, additives, etc., as it can induce changes in different physicochemical properties of biomimetics and hence of biomacromolecules. Thus, it is important to study the effect of different external conditions on the solvation dynamics of water in a restricted environment. The dynamics of solvation at a macromolecular interface can be exploited to extract information on the energetics of the participating water molecules [27, 88, 90–92]. As mentioned above, we can consider two types of water to be present at the interface, that bound to the surface and that which is free. In the water layer around the surface, the interaction with water involves hydrogen bonding to the polar and charged groups of the surface. When strongly bonded to the biomacromolecules or biomimicking surfaces, the water molecules cannot contribute to solvation dynamics because they can neither rotate nor translate. However, hydrogen bonding is transient, and there exists a dynamic equilibrium between the free and the bound water molecules. The potential of interaction can be represented by a double-well structure to symbolize the processes of bond breaking and bond forming. In general, the bonded water molecules become free by translational and rotational motions. The equilibrium between bound and free water can be written as [27, 86, 93] Waterfree state K Waterbound state

ð11:1Þ

Using the dynamic exchange model, an expression for this equilibrium can be derived. In a coupled reactiondiffusion equation the rate constant k can be written as k ¼ 0:5½
B  ðB2
4DR kbf Þ1=2 

ð11:2Þ

where B ¼ 2DR þ kbf þ kfb, DR is the rotational diffusion constant, kbf is the rate constant of the free to bound transition and kfb is that of the reverse process. Typically, the rate constant of the free to bound reaction is larger than that for the reverse process. It can be shown that, when the rates of interconversion between ‘bound’ and ‘free’ water molecules are small compared with 2DR, then
1 tslow  kbf

ð11:3Þ

kbf  ðkb T=hÞexpð
DGo =RTÞ

ð11:4Þ

and from activated complex theory we have

If the transition process (reaction (11.1)) follows a typical Arrhenius-type energy-barrier-crossing model, we can write t
1 slow  kbf ¼ A expð
Ea =RTÞ

ð11:5Þ

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where Ea is the activation energy for the transition process and A is the pre-exponential factor. A plot of ln(1/tslow) against 1/T produces a straight line, and, from the slope of the line, Ea can be calculated. Bagchi et al. [88, 90, 94] conducted simulation studies in order to understand the temperature-dependent water dynamics at the surface of Cs-pendeca-fluorooctanoate micelles at 300 and 350 K. They found that the reorientational motion of water molecules around the micelles slows down as temperature is reduced from 350 to 300 K, which is due to the formation of bridge hydrogen bonds that water molecules form with polar head groups. Sen et al. [95] studied the solvation dynamics of 4-aminophthalimide (4-AP) in Triton X-100 (TX-100) micelles at three different temperatures of 283, 303 and 323 K using picosecond-resolved fluorescence Stokes shift measurement. They observed an overall acceleration of the dynamics of solvation with increase in temperature. They used an activation-energy-barrier-crossing-type model (equation (11.5)) and plotted 1/htsolvationi against 1/T to obtain a linear plot, and calculated the corresponding activation energy (Ea) to be 9 kcal mol
1. From the linearity of such an Arrhenius-type plot they concluded that the changes in structure and hydration number of micelles have a minor role in the observed temperature dependence of solvation dynamics. In the same year, Kumbhakar et al. [96] published their study on the temperature dependency of solvation dynamics of coumarin 153 and coumarin 151 in TX-100 and Brij-35 micelles at 288, 298 and 308 K temperatures. They found that, while temperature change has a significant effect on the solvation dynamics of the TX-100 micellar system, the effect is insignificant for the Brij-35 system. They found an unusual inversion of solvation time constant at 298 K for both micellar systems. In particular, in TX-100 micelle the average solvation time constants at 288, 298 and 308 K were found to be 1281, 1981 and 1044 ps respectively. The irregular trend of the solvation dynamics with temperature reported by Kumbhakar et al. [96] is attributed to the change in the hydration number and size of the micelles. Their results [96] do not support the activationenergy-barrier-crossing-type model as employed by Sen et al. [95]. They criticized the work of Sen et al. [95] in respect of the choice of probe. According to them, as the probe 4-AP is highly hydrophilic in nature, it generally resides at the interfacial region and in the bulk, and not in the interior of the palisade layer of the micelles. Thus, the probe was unable to sense any change due to the structure and hydration of the micelles. The same group has recently reported [97] the temperature-dependent solvation of coumarin dyes in aqueous block copolymer micelles in the temperature range 293–313 K and interpreted their results in the light of structural and hydration studies. Although a thorough understanding of the effect of temperature on the solvation dynamics in microheterogeneous systems is strongly demanding, only a few reports [98–101] are available in the literature. To the best of our knowledge, there are only a handful of such temperature dependency studies for micellar systems [95–97]. These previous studies, however, disagree as to whether the temperature-induced change in solvation dynamics is due to an activation-energy-barrier crossing or due to a change in the size and hydration of micelles. Such discrepancies in previous studies led us to initiate the present series of studies in order to determine the effect of temperature on the solvation dynamics at biomimicking surfaces, and to recheck the validity of the barrier-crossing model. We chose two different self-organized biomimicking assemblies, namely micelles and RMs. For the micellar system we used sodium dodecyl sulfate (SDS) micelles, and as the probe we chose DCM (4(dicyanomethylene)-2-methyl-6(p-dimethylamino-styryl) 4H-pyran), as the dye has negligible water solubility and the most probable location of it is within the micellar interface, [102, 103], making the solvation response come predominantly from the interface itself. A wider range of temperature (298–348 K) in comparison with previous studies [95, 96] was chosen to carry out the study. The structural integrity of SDS micelles in this temperature range was investigated using the dynamic light scattering (DLS) technique. A temperature-dependent time-resolved anisotropy study was made to explore the location of the probe at different temperatures. The hydration number of the micelles at different temperatures was determined using ultrasound velocity measurements. Our aim in this study was to understand the effect of temperature on the

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solvation dynamics of a probe at a micellar surface, which retains its structural integrity, and its probable correlation with the hydration of the micelle–water interfacial layer, and to find out the correctness of identifying the problem as an activation-energy-barrier crossing type. RMs being our second choice, we studied the solvation dynamics of RMs as a function of temperature. The temperature dependence of RM solvation is not a well-studied phenomenon owing to the fact that the structural integrity of RMs is not assured over a temperature range. It is known that several RM systems are very sensitive to temperature [104, 105]. Depending upon the nature of the oil and water content, the flexibility of the interfacial film changes with temperature. Oils that penetrate deep into the interfacial layer make the film rigid and thus insensitive to temperature, whereas long-chain alkanes that do not penetrate deep into the interfacial layer make the interface more fluid with increasing temperature at a fixed w0 (the ratio of the molar  concentration of water to that of the surfactant, where the radius of the water pool (r, in A) is empirically defined as r ¼ 2  w0 [106]) value [107–109]. It is not clearly known whether this also changes the physicochemical characteristics of the entrapped water. A recent report [110] suggests that the hydrogen bond strength of interfacial water increases as the temperature is lowered, although water in RMs is less susceptible to temperature-related perturbation than bulk water. Thus, it is of prior importance to understand the nature of the temperature-dependent solvation dynamics of water entrapped in RMs. As a primary step towards an exploration in this arid field, we studied the solvation dynamics of the probe coumarin 500 (C-500) in AOT/isooctane (i-Oc) RMs with varying degrees of hydration (w0 ¼ 5, 10 and 20) at four different temperatures (293, 313, 328 and 343 K) using the picosecond-resolved TCSPC technique. The choice of w0 values was justified by the fact that AOT/i-Oc RMs form a well-defined water pool at w0¼10. At a lower hydration level the water molecules only hydrate the polar head groups of AOT, and at w0 10 the added water goes into the water pool to increase its size. The structure of these RMs at different temperatures was determined using the DLS technique. The choice of the probe was justified by its unique property that, when excited at 409 nm, the probe molecules residing at the polar interface (water–AOT head group) are selectively excited [111] and information is generated solely from the interface and its immediate vicinity. Rotational relaxation dynamics of the dye in different RM systems at different temperatures was also determined by using picosecond-resolved fluorescence anisotropy decay. Having validated the Arrhenius-type behaviour at the surface of micelles and RMs, our goal was then to check whether it was valid for mixed surfactant systems with preferential attachment of the probe to the interface. In this regard, we investigated the temperature-dependent reorganization of entrapped water molecules at the interface of AOT/lecithin (1:1) mixed RMs in a wide temperature range (293–343 K). The presence of cosurfactants in mixed surfactant systems often gives rise to enhancement of solubilization capacity by altering the interfacial rigidity of the RMs [109]. To explore the temperature effect on the solvation dynamics in mixed RM systems, we used two fluorescent probes: ANS (1-anilino-8-naphthalenesulfonic acid, ammonium salt) and C-500. The most probable location of ANS is close to the RM interface [112]. It should be noted that the probe ANS is expected to have less affinity for AOT head groups compared with that of the lecithin, as both ANS and AOTare anionic in nature. The probe C-500, being neutral in nature, should approach the interface of RMs more closely. The structural integrity of RMs in the temperature range 293–343 K was examined by the DLS technique. A time-resolved anisotropy study was carried out to explore the location of the probe at different temperatures. Knowing the validation and divergence of the activation-energy-barrier-crossing transition at the micellar and RM interface, we then moved to a real application. The interface between biological molecules (biointerface) and its immediate environment has attracted the attention of researchers for over a decade [28, 86, 91, 92, 113–117]. Many biologically important processes take place at biointerfaces. These include transport, oxidation and reduction of molecules at cell membranes and the recognition of proteins and DNA by drugs. As biomolecules are functionally active in their hydrated state [118], the hydration at the biological interface has received due attention [28, 86, 91, 92, 113–115]. Many naturally occurring biointerfaces contain

222 Hydrogen Bonding and Transfer in the Excited State

charged molecules with compensating counterions dissolved in the adjacent aqueous phase, like the cell membrane and DNA. It is to be noted that favourable charge interactions such as the interaction of DNA with the protein histone [119] and anti-helmenthic minor-groove-binding drugs such as daunomycin and H258 [120] dictate molecular recognition at a charged biointerface. The ligand resides in the hydration layer of the interface and reports environmental dynamics associated with the equilibrium between water molecules present in the different energy states. Exploration of the interplay between electrostatic attraction and hydration in ligand binding and the related energetics as shown by an Arrhenius model at a biointerface was the motive of this work. We used an AOT/i-Oc RM to replace the complicated biomolecule by more simple biomimetics to study various fundamental properties [121]. In the present study we explored the environmental dynamics reported by positively charged H258, which acts both as a model ligand and as a fluorescence reporter at the negatively charged AOT RM interface at different temperatures to characterize the dominant forces in molecular recognition. The residence of H258 at the interface of AOT RMs had been shown in a previous study [122]. Picosecond-resolved fluorescence and polarization-gated anisotropy were used to characterize the binding of the H258 to the interface at different temperatures. The dynamics at the interface was constructed from the time-resolved emission spectrum (TRES) at different temperatures. Our studies explored the role of surface hydration in ligand interaction at a model charged interface.

11.2 Materials and Methods 11.2.1 Systems 11.2.1.1 Self-Organized Assemblies (Biomimetics) Amphiphilic molecules, such as surfactants, aggregate to form macromolecular assemblies such as micelles and RMs. As these assemblies closely resemble certain structural and dynamical properties of biomolecules, theyare widely used as mimics of the actual biological systems. In the following section we will discuss these systems. Micelles Micelles (Figure 11.1(a)) are spherical or nearly spherical aggregates of amphiphilic surfactant molecules formed in aqueous solution above a certain concentration known as the critical micellar concentration (CMC) [123, 124]. Micellar aggregates have diameters varying within 10nm, and the aggregation number, i.e. the number of surfactant molecules per micelle, ranges from 20 to 200. Micelles can be neutral (TX-100, obtained from Sigma-Aldrich) or ionic, SDS (anionic, obtained from Fluka) and cetyltrimethylammonium bromide (CTAB) (cationic, obtained from Sigma-Aldrich). The core of a micelle is essentially ‘dry’ and consists of hydrocarbon chains with the polar and charged head groups projecting towards the bulk water. The Stern layer, surrounding the core, comprises the ionic or polar head groups, bound counterions and water molecules. Between the Stern layer and the bulk water there exists a diffused Guoy–Chapman (GC) layer (Figure 11.1(a)) that contains the free counterions and water molecules. In non-ionic polyoxyethylated surfactants, e.g. TX-100, the hydrocarbon core is surrounded by a palisade layer, which consists of polyoxyethylene groups hydrogen bonded to water molecules. For SDS micelles, the CMC, the thickness of the Stern layer and the overall radius of   the hydrophobic core are reported to be 8.6 mM, 33 A and 5 A respectively [125]. Reverse Micelles Reverse micelles (RMs) or water-in-oil microemulsions (Figure 11.1(b)) are nanopools of polar solvent protected by a monolayer of surfactant molecules at the periphery with polar head groups pointing inwards into the polar solvent, and the hydrocarbon tails directed towards the non-polar organic solvent [126, 127]. RMs with water nanopools resemble the water pockets found in various bioaggregates such as proteins, membranes and mitochondria. Thus, these systems are very often considered as templates for the

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

Figure 11.1

223

Schematic representation of the structure of (a) a micelle and (b) a reverse micelle

synthesis of nanoparticles and as excellent biomimetics for exploration of biological membranes and biologically confined water molecules [128, 129]. Aqueous RMs are generally characterized by the degree of hydration (w0). The shapes and sizes of the surfactant aggregates depend strongly on the type and concentration of the surfactant, and on the nature of the counterion [130] and external solvent. The AOT–alkane–water (AOT was obtained from Fluka) system is interesting, as the solution is homogeneous and optically transparent over a wide range of temperatures, pressures and pH [127, 131]. The AOT RM can compartmentalize a large amount of water in its central core, and the nanoscale aggregation process is fairly well characterized with respect to the size and shape at various w0. These studies have shown that water inside the RM is generally of two types: (i) interfacial (bound) and (ii) core (free) water. One of the studies [45] has shown the existence of a third type of water (trapped) molecules present between the polar head groups of the individual surfactant molecules. Thus, the interior of RMs is extremely heterogeneous. 11.2.1.2 Molecular Probes In this section we will discuss the different probe molecules that have been used in the course of study.

224 Hydrogen Bonding and Transfer in the Excited State

Scheme 11.1 Molecular structures of the anionic surfactant AOT and SDS, C-500, ANS, DCM, H258 and zwitterionic surfactant lecithin

4-(Dicyanomethylene)-2-Methyl-6-(p-Dimethylamino-Styryl) 4H-pyran (DCM) The laser dye DCM (commercially available from Exciton) (Scheme 11.1) is completely insoluble in water and has selective binding affinity to the micellar surface [102]. The dye is completely hydrophobic (non-polar) in the ground state. However, UV excitation increases the dipole moment of the probe, making it polar, and hence increases

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

225

its hydrophilicity in the excited state. Thus, the excited DCM diffuses from the micellar surface (relatively nonpolar) towards the polar bulk water phase, revealing a fluorescence emission signature (temporal line width) of the excursion through multiple environments in the excited state [103]. Coumarin 500 (C-500) The solvation probe C-500 (commercially available from Exciton) (Scheme 11.1) is sparingly soluble in water and shows reasonably good solubility in isooctane. In bulk water, the absorption peak (400 nm) is significantly red-shifted compared with that in isooctane (360 nm). The emission peak of C-500 in bulk water (500 nm) also shows a 90 nm red-shift compared with that in isooctane (excitation at 350 nm). The significantly large solvochromic effect (solvation) in the absorption and emission spectra of C-500 makes the dye an attractive solvation probe for microenvironments. The photophysics of the probe has also been studied in detail [132]. 1-Anilino-8-Naphthalenesulfonic Acid, Ammonium Salt (ANS) ANS (commercially available from Sigma) (Scheme 11.1) is a well-known solvation probe [133] that binds selectively to the native state of certain proteins and enzymes at their hydrophobic as well as their polar sites. In aqueous solution, the emission quantum yield of ANS is very small (0.004), with an emission peak at 520 nm and a lifetime of 0.25 ns. The steady-state emission is quenched dramatically in polar solvents. Because of its bichromophoric structure, ANS is known to undergo charge transfer (CT) from one aromatic moiety to the other ring. In non-polar solvents, the emission is strong and is mostly from the locally excited state, i.e. before charge separation. In polar solvents, the fluorescence intensity decreases and is dominated by the emission from the CT state. The solvent polarity and rigidity determine the wavelength and yield of emission, and that is why ANS is a useful biological probe. In the protein solutions, the steady-state fluorescence intensity is much larger than that in water. ANS is known to bind rigidly at a single site on the surface of the enzyme protein bovine pancreatic a-chymotrypsin (CHT) near the cysteine-1–122 disulfide bond. This ANS binding site is almost opposite in position to the enzymatic centre of CHT. With femtosecond time resolution, Zewail and coworkers have reported ultrafast hydration dynamics at the surface of the enzyme CHT when the protein is in its physiologically active or inactive state, using ANS as the fluorescent probe [134]. 20 -(4-Hydroxyphenyl)-5-[5-(4-Methylpiperazine-1-yl)]-Benzimidazo-2-yl Benzimidazole, Hoechst 33258 (H258) The commercially available probe H258 (commercially available from Molecular Probes) (Scheme 11.1) is widely used as a fluorescent cytological stain of DNA. As it has affinity for double-stranded DNA, H258 can affect transcription/translation and block topomerase/helicase activities. The dye is also used as a potential antihelminthic drug. X-ray crystallographic and NMR studies of the dye bound to a dodecamer DNA show that the dye binds to the A-T-rich sequence of the DNA minor groove. The binding constant of the dye to double-stranded DNA at low [dye]:[DNA] ratio is found to be 5  105 M
1. The solvochromic properties of the dye [135] can be used to report the hydration dynamics [120] as well as the dynamics of restricted systems [122]. 11.2.2 Methodology 11.2.2.1 Solvation Dynamics Solvation dynamics refers to the process of reorganization of solvent dipoles around a dipole created instantaneously or an electron/proton injected suddenly into a polar liquid. In order to understand the meaning and scope of solvation dynamics, let us first visualize the physical essence of the dynamical process involved for a solute molecule in a polar solvent [136–138]. A change in the probe (solute) is made at time t ¼ 0 by an excitation pulse, which leads to the creation of a dipole. This dipole gives rise to an instantaneous electric field on the solvent molecules. The interaction of permanent dipoles of the solvent with the instantaneously created electric field shifts the free energy minimum of the solvent to a non-zero value of the polarization. The solvent

226 Hydrogen Bonding and Transfer in the Excited State

Figure 11.2 Schematic illustration of solvation of an ion (or dipole) by water. The neighbouring molecules (numbered 1 and 2) can either rotate or translate to attain the minimum energy configuration. On the other hand, distant water molecule 3 can only rotate to attain the minimum energy configuration. The field is shown as E0. The springs connected to the molecules are meant to denote hydrogen bonding. Reprinted with permission from [91]. Copyright 2002 American Chemical Society

motion is crucial (Figure 11.2) because the probe is excited instantaneously (a Franck–Condon transition as far as the nuclear degrees of freedom are concerned), and the solvent molecules at t ¼ 0 find themselves in a relatively high-energy configuration. Subsequently, the solvent molecules begin to move and rearrange themselves to reach their new equilibrium positions (Figure 11.2). The nuclear motion involved can be broadly classified into rotational and translational motions. When the solvent is bulk water, rotational motion would also include hindered rotation, libration, while translation would include the intermolecular vibration due to the extensive hydrogen bonding. The two specific motions, libration and intermolecular vibration, are relatively high in frequency and are expected to play a dominant role in the initial part of solvation [85]. The molecular motions involved are shown schematically in Figure 11.2, and in Figure 11.4 we show a typical solvation time correlation function. For clarity, we approximate the motions responsible for the decay in different regions. A simple way to address the dynamics of polar solvation is to start with the following expression for the solvation energy, Esolv(t): ð 1 drE0 ðrÞ:Pðr; tÞ ð11:6Þ Esolv ðtÞ ¼
2 where E0(r) is the instantaneously created, position-dependent electric field from the ion or the dipole of the solute, and P(r, t) is the position and time-dependent polarization. The latter is defined by the following expression: ð Pðr; tÞ ¼ dW:mðWÞ:rðr; W; tÞ ð11:7Þ

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

227

where m(W) is the dipole moment vector of a molecule at position r, and r(r,W,t) is the position, orientation and time-dependent density. Therefore, the time dependence of the solvation energy is determined by the time dependence of polarization, which in turn is determined by the time dependence of the density. If the perturbation due to the probe on the dynamics of bulk water is negligible, then the time dependence of polarization is dictated by the natural dynamics of the liquid. The theoretical analysis of the time-dependent density is usually carried out using a molecular hydrodynamic approach that is based on the basic conservation (density, momentum and energy) laws and includes the effects of intermolecular (both spatial and orientational) correlations. The latter provides the free energy surface on which solvation proceeds. The equation of motion of the density involves both orientational and translational motions of the solvent molecules. The details of the theoretical development are reported in the literature [138]; here, we will present a simple physical picture of the observed biphasic solvation dynamics. Within linear response theory, the solvation correlation function is directly related to the solvation energy as       dEð0Þ dEðtÞ EðtÞ
Eð1Þ      CðtÞ ¼ ¼ dE2 Eð0Þ
Eð1Þ

ð11:8Þ

where dE is the fluctuation in solvation energy from the average equilibrium value. Note that the equality in equation (11.8) indicates the direct relation for the average of the fluctuations over the equilibrium (left) and the non-equilibrium (right), which relates to observables; without distribution   function  Eð1Þ the correspondence is clear, and Eð1Þ is rigorously the result of the equilibrium term in the numerator and for normalization in the denominator. The ultrafast component in the solvation time correlation function (Figure 11.4(a)) originates from the initial relaxation in the steep collective solvation potential. The collective potential is steep because it involves the total polarization of the system [136, 138]. This initial relaxation couples mainly to the hindered rotation (i. e. libration) and the hindered translation (i.e. the intermolecular vibration), which are the available highfrequency modes of the solvent; neither long-amplitude rotation nor molecular translation are relevant here. The last part in the decay of the solvation correlation function involves larger amplitude rotational and translational motions of the nearest-neighbour molecules in the first solvation shell. In the intermediate time we get contributions from the moderately damped rotational motions of water molecules. Figure 11.3 shows a schematic of the solvation potential and the orientational motions for the water molecules involved. From the shape of the potential it can be seen that the transient behaviour for the population during solvation should be a decay function on the blue edge of the spectrum and a rise function on the red edge. These wavelength-dependent features can be explained nicely within a generalized model of relaxation in which a Gaussian wave packet relaxes on a harmonic surface. The relaxation is non-exponential, and a Green’s function can describe the approach of the wave packet along the solvation coordinate, X, to its equilibrium value. For the general non-Markovian case it is given by [139] " # 1 ½X
X0 CðtÞ2 GðX; tjX0 Þ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp
 2    2 X ½1
C 2 ðtÞ 2p X 2 ½1
C 2 ðtÞ

ð11:9Þ

  where X 2 is the equilibrium mean-square fluctuation of the polarization coordinate on the excited state surface, C(t) is the solvation correlation function described in equation (11.8) and X0 is the initial value of the packet on the solvation coordinate. Equation (11.9) describes the motion of the wave packet (polarization density) beginning at t ¼ 0 (X0) as a delta function and according to the solvation time correlation function.

228 Hydrogen Bonding and Transfer in the Excited State

Figure 11.3 Schematic representation of the potential energy surfaces involved in solvation dynamics, showing the water orientational motions along the solvation coordinate together with instantaneous polarization P. In the inset we show the change in the potential energy along the intramolecular nuclear coordinate. As solvation proceeds, the energy of the solute comes down, giving rise to a red-shift in the fluorescence spectrum. Note the instantaneous P, e.g. P(1), on the two connected potentials. Reprinted with permission from [28], [91]. Copyright 2002, 2004 American Chemical Society

As t ! 1, C(t) ! 0 and we recover the standard Gaussian distribution. Initially (t ! 0), the exponential is large, so the decay is ultrafast, but at long times the relaxation slows down, ultimately to appear as a rise. In Figure 11.4(b) we present calculations of G(X, t|X0) at different positions along the solvation coordinate, giving decays at X1 and X2, but with different time constants, and a rise at X3, as demonstrated experimentally. In order to study the solvation stabilization of a probe in an environment, a number of fluorescence transients are taken at different wavelengths across the emission spectrum of the probe (a detailed description of the instrumental set-up and measurement techniques can be found elsewhere [140]). As described earlier, the blue and red ends of the emission spectrum are expected to show decay and rise, respectively, in the transients. The observed fluorescence transients are fitted by using a nonlinear least-squares fitting procedure to a function:   ðt 0 0 0 XðtÞ ¼ Eðt ÞRðt
t Þdt

ð11:10Þ

0

comprising convolution of the IRF (E(t)) with a sum of exponentials RðtÞ ¼ A þ

N X i¼1

! Bi expð
t=ti Þ

ð11:11Þ

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

229

Figure 11.4 (a) A typical solvation time correlation function for water is shown here. The time correlation function exhibits three distinct regions: the initial ultrafast decay, an intermediate decay of about 200 fs and the last slow decay with a time constant of 1 ps. The physical origin of each region is indicated on the plot itself (see text). (b) Green’s function G(X, t|X0) for population relaxation along the solvation coordinate (X) is plotted against time in femtoseconds. In G, X0 is the initial position at t ¼ 0. This figure shows the position and time dependence of the population fluorescence intensity. At early times (when the population is at X1) there is an ultrafast rise followed by an ultrafast decay. At intermediate times (when the population is at X2) there is a rise followed by a slow decay. At long times, when the population is nearly relaxed (position X3), we see only a rise. Reprinted with permission from [91]. Copyright 2002 American Chemical Society

with pre-exponential factors (Bi), characteristic lifetimes (ti) and a background (A). The relative concentration in a multiexponential decay is finally expressed as an ¼

Bn N P Bi i¼1

ð11:12Þ

230 Hydrogen Bonding and Transfer in the Excited State

The relative contribution of a particular decay component (fn) in the total fluorescence is defined as fn ¼

t n Bn  100 N P Bi t i

ð11:13Þ

i¼1

The quality of the curve fitting is evaluated by reduced chi-square (0.9–1.1) and residual data. The purpose of the fitting is to obtain the decays in an analytical form suitable for further data analysis. To construct time-resolved emission spectra (TRES), we follow the technique described in Refs [141] and [142]. As described above, the emission intensity decays are analysed in terms of the multiexponential model: N X Iðl; tÞ ¼ ai ðlÞexpð
t=ti ðlÞÞ ð11:14Þ i¼1

where ai(l) are the pre-exponential factors, with Sai(l) ¼ 1.0. In this analysis, we compute a new set of intensity decays, which are normalized so that the time-integrated intensity at each wavelength is equal to the steady-state intensity at that wavelength. Considering F(l) to be the steady-state emission spectrum, we calculate a set of H(l) values using FðlÞ Iðl; tÞdt

ð11:15Þ

FðlÞ a i i ðlÞti ðlÞ

ð11:16Þ

HðlÞ ¼ Ð 1 0

which for multiexponential analysis becomes HðlÞ ¼ P

Then, the appropriately normalized intensity decay functions are given by I 0 ðl; tÞ ¼ HðlÞIðl; tÞ ¼

N X

a0i ðlÞexpð
t=ti ðlÞÞ

ð11:17Þ

i¼1

where a0i ðlÞ ¼ HðlÞai ðlÞ. The values of I0 (l, t) are used to calculate the intensity at any wavelength and time, and thus the TRES. The values of the emission maxima and spectral width are determined by nonlinear leastsquares fitting of the spectral shape of the TRES. The spectral shape is assumed to follow a lognormal lineshape [141] ( "

  #) lnða þ 1Þ 2 Ið
nÞ ¼ I0 exp
ln 2 b with a¼

2bð
n

nmax Þ
1 D

ð11:18Þ

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

231

where I0 is amplitude,
nmax is the wave number of the emission maximum and the spectral width is given by   sinhðbÞ G¼D b The terms b and D are asymmetry and width parameters respectively, and equation (11.14) reduces to a Gaussian function for b ¼ 0. The time-dependent fluorescence Stokes shifts, as estimated from TRES, are used to construct the normalized spectral shift correlation function or the solvent correlation function, C(t), and is defined as CðtÞ ¼


nðtÞ

nð1Þ
nð0Þ

nð1Þ

ð11:19Þ

where n(0), n(t) and n(1) are the emission maxima (in cm
1) of the TRES at time zero, t and infinity respectively. The
nð1Þ value is considered to be the emission frequency beyond which insignificant or no spectral shift is observed. The C(t) function represents the temporal response of the solvent relaxation process, as occurs around the probe following its photoexcitation and the associated change in the dipole moment. 11.2.2.2 Fluorescence Anisotropy Anisotropy is defined as the extent of polarization of the emission from a fluorophore. It provides information about the size and shape of macromolecular aggregates or the rigidity of various molecular environments. These measurements are based on the principle of photoselective excitation of those fluorophore molecules whose absorption transition dipoles are parallel to the electric vector of polarized excitation light. In an isotropic solution, fluorophores are oriented randomly. However, upon selective excitation, a partially oriented population of fluorophores with polarized fluorescence emission results. The relative angle between the absorption and emission transition dipole moments determines the maximum measured anisotropy (r0). The fluorescence anisotropy (r) and polarization (P) are defined by r¼

Ik
I? Ik þ 2I?

ð11:20Þ

Ik
I? Ik þ I?

ð11:21Þ

P¼

where Ik and I? are the fluorescence intensities of vertically and horizontally polarized emission when the fluorophore is excited with vertically polarized light. Polarization and anisotropy are interrelated as r¼

2P 3
P

ð11:22Þ

P¼

3r 2þr

ð11:23Þ

and

Although polarization and anisotropy provide the same information, anisotropy is preferred because the latter is normalized by total fluorescence intensity (IT ¼ Ik þ 2I? ), and, in the case of multiple emissive

232 Hydrogen Bonding and Transfer in the Excited State

species, anisotropy is additive while polarization is not. Several phenomena, including rotational diffusion and energy transfer, can decrease the measured anisotropy to values lower than the maximum theoretical values. Following a pulsed excitation, the fluorescence anisotropy, r(t), of a sphere is given by rðtÞ ¼ r0 expð
t=wÞ

ð11:24Þ

where r0 is the anisotropy at time t ¼ 0 and w is the rotational correlation time of the sphere. For a radiating dipole, the intensity of light emitted is proportional to the square of the projection of the electric field of the radiating dipole onto the transmission axis of the polarizer. The intensity of parallel and perpendicular projections is given by Ik ðu; cÞ ¼ cos2 u

ð11:25Þ

I? ðu; cÞ ¼ sin2 u sin2 c

ð11:26Þ

where u and c are the orientational angles of a single fluorophore relative to the z- and y-axes respectively (Figure 11.5(a)). In solution, fluorophores remain in random distribution and the anisotropy is calculated by excitation photoselection. Upon photoexcitation by polarized light, the molecules having absorption transition moments aligned parallel to the electric vector of the polarized light have the highest probability of absorption. For the excitation polarization along the z-axis, all molecules having an angle c with respect to the y-axis will be excited. The population will be symmetrically distributed about the z-axis. For experimentally accessible molecules, the value of c will be in the range from 0 to 2p with equal probability. Thus, the c dependency can be eliminated: Ð 2p 2  2  sin c d c 1 ð11:27Þ sin c ¼ 0 Ð 2p ¼ 2 dc 0

and Ik ðuÞ ¼ cos2 u

ð11:28Þ

1 I? ðuÞ ¼ sin2 u 2

ð11:29Þ

Consider a collection of molecules oriented relative to the z-axis with probability f(u). Then, measured fluorescence intensities for this collection after photoexcitation are Ik ¼

I? ¼

ð p=2

  f ðuÞcos2 u du ¼ k cos2 u

ð11:30Þ

k 2  sin u 2

ð11:31Þ

0

ð p=2 0

f ðuÞsin2 u du ¼

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

233

Figure 11.5 (a) Emission intensity of a single fluorophore (ellipsoid) in a coordinate system. (b) Schematic representation of the measurement of fluorescence anisotropy

234 Hydrogen Bonding and Transfer in the Excited State

where f(u) du is the probability that a fluorophore is oriented between u and u þ du and is given by f ðuÞdu ¼ cos2 u sin u du

ð11:32Þ

and k is the instrumental constant. Thus, the anisotropy (r) is defined as   3 cos2 u
1 r¼ 2

ð11:33Þ

when u ¼ 54.7 , i.e. when cos2u ¼ 1/3, complete loss of anisotropy occurs. Thus, the fluorescence taken at u ¼ 54.7 with respect to the excitation polarization is expected to be free from the effect of anisotropy and is known as magic angle emission. For collinear absorption and emission dipoles, the value of hcos2 ui is given by the equation 



Ð p=2

cos u ¼ 2

0

cos2 uf ðuÞdu Ð p=2 f ðuÞdu 0

ð11:34Þ

Substituting equation (11.32) in equation (11.34) yields hcos2 ui ¼ 3/5 and an anisotropy value of 0.4 (from equation (11.33)). This is the maximum value of anisotropy obtained when the absorption and emission dipoles are collinear and when no other depolarization process takes place. However, for most fluorophores, the value of anisotropy is less than 0.4 and it is dependent on the excitation wavelength. It is demonstrated that, as the displacement of the absorption and emission dipole occurs by an angle g relative to each other, it causes further loss of anisotropy (reduction by a factor 2/5) [142] from the value obtained from equation (11.33). Thus, the value of fundamental anisotropy, r0, is given by r0 ¼

  2 3cos2 g
1 5 2

ð11:35Þ

For any fluorophore randomly distributed in solution, with one-photon excitation, the value of r0 varies from
0.2 to 0.4 for g values varying from 90 to 0 . For time-resolved fluorescence anisotropy decay (r(t)) measurements (Figure 11.5(b)), the emission polarization is adjusted to be parallel and perpendicular to that of the excitation polarization. Spencer and Weber [143] have derived the relevant equations for the time dependence of Ik ðtÞ (equation (11.36)) and I? ðtÞ (equation (11.37)) for single rotational and fluorescence relaxation times, w and tf respectively: Ik ðtÞ ¼ expð
t=tf Þð1 þ 2r0 expð
t=wÞÞ

ð11:36Þ

I? ðtÞ ¼ expð
t=tf Þð1
r0 expð
t=wÞÞ

ð11:37Þ

The total fluorescence is given by FðtÞ ¼ Ik ðtÞ þ 2I? ðtÞ ¼ 3 expð
t=tf Þ ¼ F0 expð
t=tf Þ

ð11:38Þ

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

235

The time dependent anisotropy, r(t), is given by rðtÞ ¼

Ik ðtÞ
I? ðtÞ ¼ r0 expð
t=wÞ Ik ðtÞ þ 2I? ðtÞ

ð11:39Þ

F(t) depends upon tf and r(t) depends only upon w, so that these two lifetimes can be separated. This separation is not possible in steady-state measurements. It should be noted that the degree of polarization (P) is not independent of tf and is therefore not as useful as r. For reliable measurement of r(t), three limiting cases can be considered: (a) If tf < w, the fluorescence decays before the anisotropy decays, and hence only r0 can be measured. (b) If w < tf, in contrast to steady-state measurements, w can be measured in principle. Equations (11.36) and (11.37) show that the decay of the parallel and perpendicular components depends only upon w. The experimental disadvantage of this case is that those photons emitted after the lapse of a few rotational correlation times, w, cannot contribute to the determination of f, but can be avoided with a good signalto-noise ratio. (c) If w  tf, then it becomes the ideal situation because almost all photons are counted within the time (equal to several rotational relaxation times) in which r(t) shows measurable changes. For systems with multiple rotational correlation times, r(t) is given by rðtÞ ¼ r0

X

bi e
t=wi

ð11:40Þ

i

where Sbi ¼ 1. It should be noted that the instrument monitoring the fluorescence, particularly the spectral dispersion element, responds differently to different polarizations of light, and therefore a correction factor is needed. For example, the use of diffraction gratings can yield intensities of emission that depend strongly upon orientation with respect to the plane of the grating. It is necessary when using such instruments to correct for the anisotropy in response. This instrumental anisotropy is usually known as the G-factor (grating factor) and is defined as the ratio of the transmission efficiency for vertically polarized light to that for horizontally polarized light (G ¼ Ik =I? ). Hence, values of fluorescence anisotropy, r(t), corrected for instrumental response would be given by [144] rðtÞ ¼

Ik ðtÞ
GI? ðtÞ Ik ðtÞ þ 2GI? ðtÞ

ð11:41Þ

The G-factor at a given wavelength can be determined by exciting the sample with a horizontally polarized excitation beam and collecting the two polarized fluorescence decays, one parallel and the other perpendicular to the horizontally polarized excitation beam. The G-factor can also be determined following the long-time tail-matching technique [144]. If w < tf, it can be seen that the curves for Ik ðtÞ and I? ðtÞ should become identical. If in any experiment they are not, it can usually be assumed that this is due to a non-unitary G-factor. Hence, normalizing the two decay curves on the tail of the decay eliminates the G-factor in the anisotropy measurement. A more detailed picture of the ease of the rotational motion at the macromolecular interface could be obtained from wobbling-in-cone analysis [145–148]. According to this model, the rotational anisotropy decay

236 Hydrogen Bonding and Transfer in the Excited State

function, r(t), is expressed as rðtÞ ¼ r0 ½be
t=tslow þ ð1
bÞe
t=tfast 

ð11:42Þ

where b ¼ S2, and S is the generalized-order parameter that describes the degree of restriction on the wobblingin-cone orientational motion. Its magnitude is considered to be a measure of the spatial restriction of the probe and can have a value from 0 (for unrestricted rotation of the probe) to 1 (for completely restricted motion). Assuming that the slow and the fast motions are separable, the slow (tslow) and fast (tfast) rotational time constants can be related as 1 1 1 ¼ þ tslow tL tM

ð11:43Þ

1 1 1 ¼ þ tfast tW tslow

ð11:44Þ

where tL and tW are the time constants for the lateral diffusion and the wobbling motion of the probe respectively, and tM is the time constant for the overall rotation of the RM and is given by the Stokes– Einstein–Debye (SED) equation hVh ð11:45Þ tM ¼ kb T where Vh is the hydrodynamic volume of the RM, and h is the viscosity of the dispersing solvent. Note that tM values are an order of magnitude higher than the tfast and tslow values. Hence, the overall rotation of the RM does not contribute to the decay of the anisotropy. In view of this, tfast and tslow essentially represent the time constants for wobbling motion and lateral diffusion respectively. The semicone angle uW that the probe makes with the surface is related to the ordered parameter as 1 S ¼ cos uW ð1 þ cos uW Þ 2

ð11:46Þ

The diffusion coefficient for wobbling motion DW can be obtained from the following relation: " DW ¼

1 ð1
SÞ2 tW

#  

x2 ð1 þ xÞ2 1þx 1
x 1
x ð6 þ 8x
x2
12x3
7x4 Þ ln þ þ 2 2 24 2ðx
1Þ

ð11:47Þ

where x ¼ cos uW. 11.2.2.3 Acoustic and Densimetric Study The adiabatic compressibility (bs) of a solution can be determined by measuring the solution density (rs) and the sound velocity (us) and applying Laplace’s equation bs ¼

1 rs u2s

ð11:48Þ

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

237

The apparent specific volume of the solute, wv, is given by wv ¼

1 r
rs þ solv rsolv cs rsolv

ð11:49Þ

where cs is the concentration of solute, and rsolv and rs are the densities of the solvent and the solution respectively. The partial apparent adiabatic compressibility (wk) is obtained from the following relation:   1 wk ¼ bs 2wv
2½u
rsolv

ð11:50Þ

where [u], the relative specific sound velocity increment, is given by ½ u ¼

us
usolv usolv cs

ð11:51Þ

and usolv and us are the sound velocities in solvent and solution respectively. To estimate wk, we have used the effective medium theory [149], where solubilized water (corresponding to w0 ¼ 5) has been considered as a solute in AOT/i-Oc environment.

11.3 Results and Discussion 11.3.1 Activation energy barrier at the micellar surface [150] As discussed in the introduction, there has been some controversy in the literature concerning the applicability of the Arrhenius model for the transition of different types of water molecule at the micellar interface [95, 96]. The major drawbacks of the earlier studies lay in their choice of surfactants and fluorescent probes, as the structural stability of the micelles over the studied temperature range and the location of the probe were not taken into consideration. Therefore, to establish the validity of the Arrhenius model at the micellar interface, the major focus needs to be on confirming the stability of the micellar structure and the location of the probe over the studied temperature range. In order to determine the structural stability of the micelles, we measure the temperature-dependent hydrodynamic diameter (dH) of 50 mM SDS micelles using the DLS technique. A typical DLS signal at 298 K is shown in the inset of Figure 11.6(a). The signal has been found to be highly monodispersed in nature, and the measured hydrodynamic diameter of 4.3  0.4 nm is consistent with previous studies [151]. The high monodispersity of the DLS signal is retained at all the studied temperatures and, with increase in temperature, the hydrodynamic diameter decreases marginally from 4.3 nm at 293 K to 3.4 nm at 343 K. Note that, for the non-ionic surfactant (TX-100) used in the previous studies, the radius increases substantially on increase in temperature [152]. In the present study, however, structural integrity of the SDS micelles in the experimental temperature window is assured. The marginal change in the hydrodynamic diameter in the DLS measurement is consistent with loss of the hydration layer (see below). The hydration number (nh), i.e. the number of water molecules associated per SDS head group, can be obtained from the ultrasound velocity measurements following the model used by Bockris et al. [153]: nh ¼

  nw b V 1
SDS


w nSDS bw Nw V

ð11:52Þ

238 Hydrogen Bonding and Transfer in the Excited State

Figure 11.6 (a) Dynamic light scattering measurement of 50 mM SDS in aqueous solution at different temperatures. Note that the hydrodynamic diameter of the micelle decreases marginally with increase in temperature. A typical size distribution at 298 K is shown in the inset to show the monodispersity of the distribution. (b) Hydration number per SDS molecule (nh) calculated from equation (11.54) at different temperatures. nh decreases gradually with temperature. Reprinted with permission from [150]. Copyright 2007 American Chemical Society

where b is the compressibility calculated from equation (11.44), bw and bSDS are the compressibility of water and SDS micellar solution respectively, nw is the number of water molecules, nSDS is the number of SDS
w is the molar volume of molecules, V is the total volume, Nw is the number of moles of water in solution and V water. As in the solution the concentration of SDS is very low (50 mM) as compared with that of water, V can be

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

239


w and equation (11.52) takes the form taken to be equal to Nw V   nw bSDS nh ¼ 1
nSDS bw

ð11:53Þ

The number of water molecules and number of SDS molecules can be replaced with the concentration of the species in the solution, and the final form of the equation is   cw bSDS nh ¼ 1
cSDS bw

ð11:54Þ

from which nh could be calculated. The nh values at different temperatures are plotted in Figure 11.6(b). At 293 K, the obtained nh is 17.3, which is in good agreement with the reported value of 20 by Kunz et al. [154] using thedielectric relaxation technique. As shown by the figure, the hydration number (hydration layer) gradually decreases with increase in temperature, which signifies that SDS head-groupbound water is converted into free water with increasing temperature. The gradual decrease in nh with temperature leads us to employ the energy-barrier-crossing model in which stable polar head-group-bound water crosses the energy barrier at elevated temperature and becomes free. Here, we consider a very simplified model, assuming a Boltzmann kind of distribution between free and bound water, which follows the relation E*

nb ¼ nf e
RT

ð11:55Þ

where nb and nf are the bound and free water molecules, respectively, at temperature T, and E is the energy difference between the two types of molecule. With increase in temperature, the nb population decreases with a concomitant increase in the nf population. We further assume that nf is large compared with nb and does not change appreciably with respect to nb (this assumption is further supported from our time-resolved solvation experiments (see below)). Thus, E can be calculated using the equation 

   1 1 nb1
E* ¼ R ln T1 T2 nb2

ð11:56Þ

where nb1 and nb2 are the number of bound water molecules at temperature T1 and T2 respectively. We calculate E for different temperature intervals, and the values vary in the range 2.7–3.5 kcal mol
1. This energy can be empirically equated to the difference between the bound and free states of water at the micellar surface, which is in close agreement with that obtained by Pal et al. [88, 90] using molecular dynamics (MD) simulation studies. After confirming the structural integrity of the micelle at elevated temperatures, our next focus should be to ascertain the location of the probe in the micelle in this temperature window. Figure 11.7(a) depicts the fluorescence spectra of DCM excited at 409 nm in 50 mM SDS at 298 and 348 K. The emission peak, which is obtained at 623 nm at 298 K, is slightly (2–3 nm) blue-shifted upon increase in temperature to 348 K. Such an insignificant blue-shift in the emission spectra of the probe indicates that the microenvironment of DCM does not change appreciably with increase in temperature. It is known that DCM is completely insoluble in water and dissolves only in the interfacial layer of micelles [102]. Thus, it might be inferred that no significant change in probe location occurs upon increase in temperature, and it resides at the interface.

240 Hydrogen Bonding and Transfer in the Excited State

Figure 11.7 (a) Emission spectrum of DCM in 50 mM SDS micellar solution at two different temperatures. The spectrum produces insignificant change with increase in temperature. (b) r(t) of DCM in 50 mM SDS micellar solution at different temperatures. The slow r(t) indicates that the probe resides in the micellar surface at elevated temperatures. Reprinted with permission from [150]. Copyright 2007 American Chemical Society

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

241

Table 11.1 r(t) parameters of DCM in 50 mM SDS at different temperatures. Reprinted with permission from [150]. Copyright 2007 American Chemical Society Temperature (K) 298 323 348

r0

Offset

tr1 (ns)

tr2 (ns)

0.32 0.32 0.30

0.10 0.11 0.10

0.40 (23%) 0.28 (40%) 0.20 (46%)

1.6 (45%) 1.2 (27%) 1.0 (20%)

The binding of the probe with the micelle is further clarified with r(t) measured at 620 nm at three different temperatures (Figure 11.7(b)). For all the studied temperatures, r(t) is characterized by a slow decay, with the presence of a considerable offset. The decays fit well biexponentially, and the rotational time constants (tr1 and tr2) estimated at different temperatures are listed in Table 11.1. The presence of an offset value is indicative of a rotational motion of the micelle that is not complete within the experimental time window. The r(t) decay is associated with two time constants, one of the order of hundreds of picoseconds, and the other of the order of 1 ns. The rotational relaxation times obtained in this study are much slower than those of rotational times reported in bulk water [155, 156]. This concludes that the dye experiences much higher microviscosity in micellar systems than in bulk water, as the probe resides in the interfacial layer of the micelle at all the studied temperatures. The rotational time constants become faster upon increase in temperature, which reflects the thermal effect on the microviscosity and consequently the wobbling rate of the probe, and also the increase of free water molecules at the interface [157]. Thus, the anisotropy result proves that DCM is highly efficient for probing the dynamics of water located in the interfacial layer at all the studied temperatures. We now investigate the effect of temperature on the solvation dynamics of water at the micellar interface. The fluorescence decay transients of DCM in SDS micelles at three representative wavelengths at 298 K are shown in Figure 11.8(a). It is evident from the figure that the decay patterns are strongly wavelength dependent. At 520 nm, the transient is very fast, with decay components of 100 ps (37%), 350 ps (60%) and 1800 ps (3%). The transient becomes slower with increase in wavelength and at the extreme red end (700 nm); a decay component of 1370 ps along with a distinct rise component of 175 ps is obtained. Similar wavelength dependency is registered at the other two temperatures, with a gradual decrease in average lifetime with increasing temperature (figures not shown). The time-resolved emission spectra (TRES) of DCM in SDS are constructed at different temperatures. Typical TRES for DCM in SDS, as obtained at 298 K, are presented in the inset of Figure 11.8(b). The timedependent fluorescence Stokes shifts, as estimated from TRES, are used to construct the normalized spectral shift correlation function or the solvation correlation function C(t) as defined in equation (11.19). The C(t) curves obtained at three different temperatures are depicted in Figure 11.8(b). All the decay curves are fitted biexponentially, and the fitted parameters are presented in Table 11.2. The average solvation time htsi in the system is calculated using the equation   ð11:57Þ t s ¼ a1 t 1 þ a2 t 2 where a1 and a2 are the relative concentrations corresponding to the solvation time constants t1 and t2 respectively. It can be seen from Table 11.2 that t1 and t2 are of the order of several tens of pisoseconds and several hundreds of picoseconds respectively. Both components are slower in comparison with the 1 ps timescale of bulk water [85]. The component t1 is comparable with that obtained for the NATA–TX100 system by Pal et al. [134] using femtosecond-resolved decay. The possibility of a significant loss of dynamical Stokes shift cannot be ruled out owing to the limited time resolution of our experimental set-up. Previously, femtosecond-resolved solvation experiments [158] with coumarin 153 in SDS micelles reported a dynamical Stokes shift of 450 cm
1, which is comparable with those observed in our study (550–650 cm
1). The

242 Hydrogen Bonding and Transfer in the Excited State

Figure 11.8 (a) A typical decay transient of DCM in 50 mM SDS micellar solution at three different wavelengths at 298 K. (b) Decay transient of C(t) of DCM 50 mM SDS micellar solution at three different temperatures. The solid lines are biexponential fitting. Note that C(t) becomes faster with increase in temperature. A representative timeresolved emission spectrum (TRES) at 298 K is shown in the inset. Reprinted with permission from [150]. Copyright 2007 American Chemical Society Table 11.2 Decay parameters of C(t) for DCM in 50 mM SDS at different temperatures. Reprinted with permission from [150]. Copyright 2007 American Chemical Society Temperature (K) 298 323 348

a1

t1 (ns)

a2

t2 (ns)

htsi (ns)

0.77 0.80 0.39

0.10 0.08 0.04

0.23 0.19 0.60

0.68 0.48 0.14

0.24 0.16 0.10

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

243

observed spectral shift is estimated to be only 37% of the total solvation shift. The significantly higher contribution of the faster component in the C(t) decay (Table 11.2), which might be due to the solvation by free water molecules present at the interfacial Stern layer, supports our earlier assumption that nf is considerably higher than nb. It is evident from Table 11.2 that the magnitude of both t1 and t2 decreases gradually with increase in temperature. This phenomenon is similar to that obtained by Sen et al. [95] using 4-AP in TX-100 micelles. We assume an Arrhenius dependency of the rate constant (equation (11.5)) and plotted ln(1/htsi) against 1/T. A good linear fit is obtained (Figure 11.9), and the corresponding Ea value obtained from the slope is 3.5  0.3 kcal mol
1, which is in good agreement with the 2.4–4 kcal mol
1 obtained by Pal et al. [88] using MD simulation studies and is attributed to the barrier energy of bound to free water transition at the micellar interface. Earlier, Sen et al. [95] obtained a somewhat higher value of 9 kcal mol
1. They attributed this high value of Ea to the energy difference between head-group-bound water and bulk water, which is of the order of 7–8 kcal mol
1. The simulation study of Pal et al. [88] and Bruce et al. [159] showed that penetration of water molecules into the SDS micelle is restricted to the head-group region and there exist three different types of water molecule at the micellar interface. Some water molecules (IBW-1) are hydrogen bonded to the polar head group of the surfactant, while a few water molecules (IBW-2) make two hydrogen bonds with surfactant head groups. The third kind of water molecule (IFW) is not directly hydrogen bonded to any polar head group, rather they form hydrogen bonds with other water molecules. All three kinds of water molecule have an energy lower than that of the bulk water. Simulation studies reveal that IBW-1 and IBW-2 are progressively more stable than the IFW species, and the energy difference between IBW-1 and IFW is of the order of 2.4 kcal mol
1. This value is comparable with the estimated Ea value of 3.5 kcal mol
1 in the present study. It is to be noted that DCM can only sense solvation by IBW-1, IBW-2 and IFW molecules in the interfacial Stern layer. Thus, the Ea value obtained in the present study signifies the activation energy barrier crossing between these interfacial water molecules. On the other hand, Sen et al. [95] used a hydrophilic probe 4-AP, which is highly soluble in bulk water. The probable location of the probe is at the edge of the palisade layer facing the bulk water and thus can only sense the conversion of the interfacially bound water to bulk water, with the corresponding energy barrier of the order of 7–8 kcal mol
1, which is in close approximation to the Ea value obtained by them. Unlike the

Figure 11.9 Arrhenius plot of ln(1/htsi) against 1/T for DCM in 50 mM SDS micellar solution. The solid line represents a linear fit. Reprinted with permission from [150]. Copyright 2007 American Chemical Society

244 Hydrogen Bonding and Transfer in the Excited State

previous reports [95, 96], in the present report we have studied the solvation dynamics within the structural integrity of the micelles, which might have resulted in a gradual decrease in htsi with increasing temperature. Thus, the barrier-crossing model might hold good within this condition only. It can also be noted that, in the present study, the activation energy of 2.7–3.5 kcal mol
1 calculated from the hydration measurements is in excellent agreement with that obtained from solvation dynamics (3.5 kcal mol
1). Although the value obtained from the hydration study is only an approximate one, its resemblance to that from the solvation study leads us to conclude that the change in solvation dynamics with temperature is primarily associated with an activation energy barrier crossing within the framework of structural integrity of the micelle, in which the ratedetermining step is the transition between bound and free water molecules. 11.3.2 Arrhenius model in another self-organized reverse micellar system [160] After the confirmation of the applicability of the Arrhenius model to determining the activation energy associated with the transition of water molecules at the micellar interface, let us now further proceed to find its applicability at the RM interface. RMs are unique microheterogeneous systems wherein the entrapped water molecules in the water pool show properties very different from those of the bulk water and thus provide a unique platform to mimic the biological environment inside the living cell. In this study we will investigate the temperature-induced solvation dynamics of water at the AOT/i-Oc interface using C-500 as a fluorescent probe at three different levels of hydration (w0 ¼ 5, 10 and 20). We first investigate the structural perturbation of the RM systems as a function of temperature. Figure 11.10 depicts the size variation of the AOT/i-Oc RMs of various w0 values at different temperatures as obtained from DLS measurements. As evident from the inset of Figure 11.10, the RMs are essentially monodispersed in nature. The diameter of the RMs increases with increasing w0. This finding is in accordance with the previous DLS measurements reported from our group [156]. From the temperature-dependent DLS measurement, it is

Figure 11.10 Hydrodynamic diameter of AOT/i-Oc reverse micelles of different hydration (w0) as a function of temperature. The size remains unchanged for w0 ¼ 5 and 10, but increases for w0 ¼ 20. A typical size distribution at 298 K is shown in the inset to show the monodispersity of the distribution. Reprinted with permission from [160]. Copyright 2008 American Chemical Society

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

245

evident that temperature has little or insignificant effect on the RMs with w0 ¼ 5 and 10. However, in the RM with w0 ¼ 20, the micelles grow in size with increase in temperature, and dH increases from 9.6 to 23.5 nm when temperature is increased from 293 to 343 K. In the present case, the temperature-induced increase in the droplet size of the w0 ¼ 20 RM is a signature of the onset of percolation (droplet coalescing) process [109, 161]. For the other two systems (i.e. w0 ¼ 5 and w0 ¼ 10), the non-interacting hard sphere nature of the droplets prevails at all the studied temperatures, retaining their structural integrity. After checking the structural stability of the RM systems, we now move on to determine the solvation dynamics at different temperatures. We choose a well-known solvation probe C-500 which is sparingly soluble in water and shows reasonably good solubility in non-polar solvents such as i-Oc and 1,4-dioxane. The absorption and fluorescence spectra of C-500 in different solvents indicates that the photophysical properties of C-500 are strongly dependent on the polarities of the solvents, and the absorption and emission maxima of C-500 suffer a blue-shift on moving from a polar water medium to a non-polar medium [132, 162]. Figure 11.11(a) depicts the difference absorption spectra of C-500 in AOT/i-Oc RMs of different w0 values at 293 K (inset of Figure 11.11(a)) and with w0 ¼ 10 at different temperatures, in which the absorption spectrum of C-500 in i-Oc has been subtracted from that of C-500 in AOT/i-Oc RMs. As shown by the figure, the appearance of the peak at 400 nm, which is close to the absorption peak of the probe in bulk water, signifies absorption by the C-500 molecules present at the polar AOT/i-Oc interface [111]. The spectral feature and peak position do not change with temperature, indicating that no ground state phenomenon is associated at elevated temperatures. It is to be noted that C-500 in i-Oc produces an absorption peak at 360 nm and shows very weak absorption at 410 nm at all the studied temperatures (figure not shown). Thus, the red edge excitation [163, 164] at wavelength 409 nm used in our experiment only excites those populations of C-500 that are in the close vicinity of the water/surfactant interface, and thus the emission spectra report only the microenvironment of the interface and its immediate polar vicinity. Temperature-dependent emission spectra of C-500 in AOT/i-Oc RMs excited at 409 nm at different w0 values and temperatures are measured, and one such representative illustration for w0 ¼ 5 at 293, 313, 328 and 343 K is presented in Figure 11.11(b). The corresponding emission peaks (lmax) are presented in Table 11.3. Insignificant spectral change of C-500 in AOT/i-Oc RMs with various w0 values, as can be seen from Table 11.3, indicates that the absorption spectrum reaches an equilibrium position even in the RM with lowest hydration (w0 ¼ 5). As shown by Table 11.3, the w0 ¼ 5 RM produces an emission peak of 490 nm at 293 K, which suffers 3 nm red-shift when the temperature is increased to 343 K. For the w0 ¼ 10 and 20 systems, the emission maxima are obtained at 495 and 498 nm respectively and remain unchanged upon increase in temperature. The progressive red-shift of the emission peak with increasing w0 signifies that the probe senses a more polar environment at higher hydration owing to an increase in the number of bulk-type water molecules in the water pool [50, 165]. The peak shift is significantly small for w0 ¼ 10 and 20 RMs. Water molecules present in the small RM (w0 ¼ 5) hydrate the surfactant head groups only. With progressive loading of water, bulk-type water begins to form, and any further addition of water increases the water pool size by increasing the amount of bulk-type water. Previous studies using different techniques [64, 166–168] and our present steady-state measurements also confirm the fact that the interface can hardly differentiate the water polarity beyond w0 ¼ 10. We study the solvation dynamics of the probe in AOT/i-Oc RMs at different levels of hydration (w0) and temperature. We construct the TRES from the decay transients, and a representative TRES at 293 K with w0 ¼ 5 is presented in Figure 11.12(a). It can be seen that the emission peak undergoes a red-shift with time. The C(t) curves obtained for all the systems at four different temperatures are fitted with biexponential functions; two representative curves are shown in Figures 11.12(b) and (c), and the fitting parameters are presented in Table 11.3. Note that Riter et al. [169] reported an ultrafast component (of the order of several hundreds of femtoseconds to a few picoseconds) of coumarin 343 (C-343) in AOT/i-Oc RMs. They found that such a fast component is missing in a relatively dry RM (w0 ¼ 1.1) and increases progressively with increasing hydration. At a high hydration level, the ultrafast solvation is essentially governed by the hundreds of

246 Hydrogen Bonding and Transfer in the Excited State

Figure 11.11 (a) Difference absorption spectra of C-500 at different temperatures for AOT/i-Oc reverse micelles with w0 ¼ 10. A peak at 400 nm indicates the selective excitation of the probe molecules residing at the interface. The insignificant effect of hydration on the absorption spectrum at 298 K is shown in the inset. (b) Emission spectrum of C-500 at different temperatures for AOT/i-Oc reverse micelles with w0 ¼ 5. A marginal red-shift (2–3 nm) is observed with increase in temperature. Reprinted with permission from [160]. Copyright 2008 American Chemical Society

femtosecond component, which is identical to the solvation of the probe C-343 by water as reported by Jimenez et al. [85]. Owing to our limited instrumental resolution (IRF  80 ps), we miss such an ultrafast component, especially at elevated temperatures. The solvation obtained in the present study is broadly due to the bound-type water molecules (with different strengths of hydrogen bonding interactions) present in the RM,

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

247

Table 11.3 Decay parameters of C(t) for C-500 in AOT/i-Oc RMs at different w0 and temperature (ai are the relative contribution of ti). Reprinted with permission from [160]. Copyright 2008 American Chemical Society Temp. (K) w0 ¼ 5 293 313 328 343 w0 ¼ 10 293 313 328 343 w0 ¼ 20 293 313 328 343

Fl. peak (nm)

a1

t1 (ns)

a2

t2 (ns)

htsi (ns)

fr

htsicorr (ns)

490 493 493 493

0.31 0.46 0.49 0.44

0.11 0.25 0.21 0.16

0.67 0.52 0.48 0.54

1.72 1.30 1.21 1.03

1.18 0.79 0.68 0.63

0.73 0.70 0.66 0.59

0.86 0.55 0.45 0.37

495 497 496 496

0.55 0.77 0.73 0.64

0.42 0.29 0.17 0.16

0.44 0.23 0.27 0.35

1.34 1.06 0.90 0.55

0.82 0.47 0.37 0.33

0.68 0.64 0.57 0.48

0.55 0.30 0.21 0.16

498 499 499 498

0.48 0.77 0.78 0.85

0.22 0.21 0.15 0.15

0.51 0.22 0.22 0.15

0.86 0.73 0.60 0.70

0.54 0.32 0.25 0.23

0.61 0.56 0.52 0.39

0.33 0.18 0.13 0.09

as has been observed for the micellar system. In this regard, one could average out the slow and fast components, both of which are in turn slower compared with the bulk-water solvation. The time constants obtained in the present study are consistent with the reported results on coumarin 480 in AOT/i-Oc RMs [170]. Such slow time components have recently been reported from our group using the water-soluble dye acridine orange (AO) in AOT RMs [156]. As shown by Table 11.3, the htsi value is 1.18 ns for the w0 ¼ 5 system at 293 K, and with increase in temperature it decreases – first rapidly and then moderately up to 343 K. This trend is more prominent for the w0 ¼ 10 and 20 systems. This observation is supported by our steady-state experiments, where only a marginal red-shift of lmax occurs with increase in temperature (Table 11.3). The gradual decrease in htsi with temperature for all three systems indicates that an increase in temperature accelerates the solvation process at the interface irrespective of the water pool size. Numerous experimental data indicate that, even in a large water pool (w0 10), the properties of the confined water differ from those of the bulk water [171–173]. Thus, almost bulk-like behaviour of the water inside the pool is not expected even at higher w0 values. The observed slow timescale (of the order of a few nanoseconds) of the present system is due to the slow-moving interfacially bound water molecules, which become faster upon increase in temperature. As mentioned earlier, we miss a considerable fraction of the Stokes shift owing to our limited instrumental resolution. We determine the loss in the dynamic Stokes shift using the procedure developed by Fee and Maroncelli [174], where n(0) can be calculated by the equation np npem ð0Þ ¼ npabs
ðnnp abs
nem Þ

ð11:58Þ

np where npabs , nnp abs and nem are the absorption peak in a polar solvent, the absorption peak in a non-polar solvent and the emission peak in a non-polar solvent respectively. In the present study we use cyclohexane as the non-polar solvent, with absorption and emission maxima of C-500 at 360 and 410 nm respectively. Water is used as the polar solvent, in which C-500 produces an absorption peak at 390 nm. We calculate a 27% loss in the dynamical Stokes shift for w0 ¼ 5 at 293 K. The loss is 41% at 343 K. For the more hydrated systems the loss is higher, and for w0 ¼ 20 the loss is 39% at 293 K and 61% at 343 K. The fraction of Stokes shift recovered from our experimental set-up (fr) (i.e. the ratio of Dn (¼n(0)
n(1)), obtained by using n(0), to that obtained by

248 Hydrogen Bonding and Transfer in the Excited State

Figure 11.12 (a) A representative time-resolved emission spectrum (TRES) of C-500 in AOT/i-Oc reverse micelles with w0 ¼ 5 at 298 K. (b), (c) Decay transient of C(t) of C-500 in AOT/i-Oc reverse micelles with w0 ¼ 5 (b) and 20 (c) at 298 and 343 K. The solid lines are biexponential fitting. The decay becomes faster with increase in temperature. (d) Arrhenius plot of ln(1/htsicorr) against 1/T for C-500 in AOT/i-Oc reverse micelles with w0 ¼ 5, 10 and 20. The solid lines represent linear fits. Reprinted with permission from [160]. Copyright 2008 American Chemical Society

employing npem ð0Þ using equation (11.58)) has been presented in Table 11.3 for all the systems. Sen et al. [175] previously reported a 31% loss in Stokes shift using 2,6-toluidinonapthalene sulfonate (TNS) in AOT/heptane RM with w0 ¼ 56 at room temperature. As the contribution of the subpicosecond component of the solvation dynamics to the average solvation times is very small, the corrected solvation time (htsicorr) could be obtained from    corr ts  fr ts

ð11:59Þ

The temperature-induced decrease in htsicorr in AOT/i-Oc RM (Table 11.3) must be associated with bound-type to free-type transition of water molecules with temperature, which in turn is governed by an Arrhenius-type activation energy barrier-crossing model [86, 95, 150]. We fit an Arrhenius-type plot using the htsicorr values listed in Table 11.3 for all three systems, and all three systems exhibit good linear fit (Figure 11.12(d)); the estimated Ea values are 3.4  0.3, 4.9  0.5 and 5.1  0.5 kcal mol
1 for the w0 ¼ 5, 10 and 20 systems respectively. The 3.4 kcal mol
1 value obtained for the w0 ¼ 5 system is in good agreement with that obtained for the interfacial water transition process in SDS micelles as discussed in the earlier section. It is to be noted that, at w0 ¼ 5, the water molecules present in the RM is essentially of interfacial type

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

249

and a temperature-induced transition between the different kinds of water present at the surfactant interface is associated with an energy barrier of 2.4–4 kcal mol
1. For the hydrated systems (w0 10) in which the RM water pool is formed the transition is bound to bulk type, which is associated with a higher energy barrier (7–8 kcal mol
1). In the present study, we obtain an Ea value of 5 kcal mol
1, which might be due to an averaging out effect, as a significant fraction of the dye molecules are still associated with the interface even at elevated temperatures. Note also that the Ea values are identical for the w0 ¼ 10 and 20 systems, which corroborates the steady-state results that the probe can hardly differentiate the water type in these two RMs. DLS study reveals that, for the w0 ¼ 20 system, micellar size increases with increase in temperature (Figure 11.10). This implies an increase in the radius of curvature of the AOT interfacial monolayer and a corresponding decrease in the rigidity of the film. However, the similar effect of temperature on the solvation dynamics of this system as that for the w0 ¼ 10 system and the identical Ea values indicate that the mechanical properties of the interfacial monolayer has an insignificant effect on the observed solvation dynamics. We measure r(t) of the probe in the RMs with different w0 values and temperatures, and a representative diagram for the w0 ¼ 5 system at 293 and 343 K is presented in the inset of Figure 11.13. The rotational correlation time constants are given in Table 11.4. It can be seen from Table 11.4 that the slower component decreases with increasing w0 at a constant temperature, which is identical to that reported by Shaw et al. [156] for the AO in the AOT/i-Oc RM system. The faster time constant with increasing temperature certainly indicates that the probe is freer to move in larger micelles and at elevated temperatures. To understand the effect of temperature on the rotational relaxation process of the probe inside the RM in a more quantitative manner, biexponential anisotropy decay has been analysed using the two-step and wobbling-in-cone model [145–148]. The results obtained from the analysis are summarized in Table 11.4 and Figure 11.13. It can be observed that the wobbling-cone angle (uW) for all the systems increases with increasing temperature. The diffusion coefficient (DW) values are of the same order of magnitude as reported by Shaw et al. [156] for AO in AOT RM and increases with increasing temperature for all the systems (Figure 11.13).

Figure 11.13 Diffusion coefficient of water (Dw), calculated using equation (11.47), in AOT/i-Oc RMs at different hydrations as a function of temperature. Dw gradually increases with temperature for all the systems. The broken lines are a guide for the eyes. A representative r(t) of C-500 in AOT/i-Oc RM with w0 ¼ 5 at two different temperatures is shown in the inset. Reprinted with permission from [160]. Copyright 2008 American Chemical Society

250 Hydrogen Bonding and Transfer in the Excited State Table 11.4 r(t) and wobbling-in-cone data of C-500 in AOT/i-Oc RMs at different w0 and temperatures. The figures in parentheses in the tfast and tslow columns signify the relative percentage of the components in the total anisotropy. Reprinted with permission from [160]. Copyright 2008 American Chemical Society Temperature (K) w0 ¼ 5 293 313 328 343 w0 ¼ 10 293 313 328 343 w0 ¼ 20 293 313 328 343

r0

tfast (ns)

tslow (ns)

uW (deg)

DW  108 (s
1)

0.26 0.25 0.24 0.24

0.20 0.18 0.13 0.12

(35%) (45%) (49%) (54%)

1.70 (65%) 1.26 (55%) 0.92 (51%) 0.72 (44%)

30.15 35.26 37.29 40.92

3.25 4.64 7.09 8.70

0.27 0.26 0.24 0.24

0.18 0.13 0.11 0.11

(43%) (48%) (54%) (63%)

1.27 (57%) 0.77 (52%) 0.58 (46%) 0.46 (37%)

34.24 36.79 39.87 44.68

4.41 6.70 8.85 9.97

0.26 0.27 0.25 0.24

0.18 0.13 0.11 0.11

(46%) (50%) (53%) (62%)

1.20 (54%) 0.65 (50%) 0.47 (47%) 0.44 (38%)

35.77 37.81 39.35 44.13

4.72 6.76 8.18 9.64

This signifies that, with increasing temperature, the probe experiences less restricted rotation in the micelle. This might be due to the smaller restriction imposed on the rotation of the probe in trapped water at elevated temperatures and/or diffusion of the probe towards central bulk-type water of the RMs. It can be seen from Table 11.4 that the uW and DW values are comparable in w0 ¼ 10 and 20 systems at all the studied temperatures. Thus, the probe experiences a similar kind of microenvironment in these two systems, which corroborates well with the steady-state and solvation dynamics results. As observed from Tables 11.3 and 11.4, the average solvation time constant htsi and average rotational time constant htri become faster with increase in temperature for all the RM systems. Thus, the change in microviscosity at the micellar interface might be responsible for the observed temperature-induced change in the solvation dynamics. The diffusion coefficient for wobbling motion (DW) of the probe increases with increase in temperature (Table 11.4). Such an increase in DW is associated with a decrease in microviscosity at the micellar interface [176, 177], which corroborates with the conversion of surface-bound water to free-type water [86, 88, 94] and the corresponding Ea calculated from the solvation dynamics measurements. 11.3.3 Activation energy barrier-crossing transition at an AOT/lecithin mixed reverse micellar interface [178] In the previous section we have shown that the Arrhenius model of activation energy barrier crossing holds good for AOT/i-Oc RM systems at different levels of hydration. We now intend to check whether the same holds good in the case of a mixed surfactant system in which one of the surfactants is ionic (AOT) while the other one is non-ionic (lecithin), and the probe might have different affinities towards the different charge groups of the surfactants. Here, we use a 1:1 (molar ratio) blend of AOT and lecithin to prepare the RMs at a fixed hydration of w0 ¼ 10. We ensure structural stability of the mixed RM within the temperature window 293–343 K by measuring the size of the RMs using the DLS technique (Figure 11.14). It can be seen from the figure that, with increase in temperature, the reverse micellar diameter changes marginally, and the measured dH is 9.4  0.6 nm, which in turn is larger than the single AOT RM system (Figure 11.10). Thus, in the present

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

251

Figure 11.14 Hydrodynamic diameter of AOT/lecithin/i-Oc mixed RM with w0 ¼ 10 as a function of temperature. Typical size distributions at 293 and 343 K are shown in the inset to show the monodispersity of the distribution. Reprinted with permission from [178]. Copyright 2008 American Chemical Society

study the structural integrity of w0 ¼ 10 AOT/lecithin mixed RMs in our experimental temperature window is assured. To establish the effect of temperature on the solvation dynamics, we use two probes here, C-500 and ANS. In comparison with C-500, ANS is completely insoluble in bulk i-Oc but soluble in water, where it produces an absorption peak at 360 nm and, when excited at 375 nm, gives an emission maximum at 520 nm. As in the case of C-500, ANS emission also depends strongly on the polarity of the solvent [112, 134]. Excitation of the RM system with C-500 at 409 nm produces an emission peak at 495 nm with an insignificant change in the peak position at elevated temperatures, confirming the location of C-500 to be in the close vicinity of the RM interface, as has already been discussed in the case of AOT/i-Oc RM. On the other hand, ANS is extremely hydrophilic because of its anionic nature, and when dissolved in AOT/lecithin RM it is expected to reside in the water pool of the RM. The steady-state absorption and fluorescence spectra of ANS (excitation l ¼ 375 nm) in w0 ¼ 10 AOT/lecithin RM in the temperature range 293–343 K (Figure 11.15) shows that ANS emission suffers a blue-shift of 25 nm relative to water at room temperature and even at elevated temperatures. The observation is consistent with the fact that a significant population of the probe resides in the interfacial region of the RM, where the polarity of the water molecule and the dynamical freedom of the probe are much lower than those in the centre of the water pool. In the RM interface region, the anionic character of ANS is expected to make its interaction with the negatively charged head group of AOT less favourable. Thus, the probe ANS in the RM interface is expected to interact with the lecithin polar head groups. Figure 14.15 shows quenching in the ANS emission at elevated temperatures, indicating increase in the polarity around the local environment of the interface-bound ANS at higher temperature owing to the barriercrossing transition of bound to free water at the reverse micellar interface. The insignificant change in the spectral shape of the absorption and fluorescence spectra of ANS in the w0 ¼ 10 RM (Figure 11.15) rules out the possibility of ANS migration in various environments with increase in temperature [179, 180]. In order to investigate the alteration of the dynamical properties of water at the RM interface with change in temperature, we have examined picosecond-resolved solvation dynamics of both probes in the RM.

252 Hydrogen Bonding and Transfer in the Excited State

Figure 11.15 Absorption and emission spectrum of ANS in AOT/lecithin/i-Oc mixed RM with w0 ¼ 10 at different temperatures. The emission peak does not change appreciably with temperature, whereas the intensity decreases. Reprinted with permission from [178]. Copyright 2008 American Chemical Society

Figure 11.16(a) depicts the C(t) decay constructed from TRES of ANS and C-500 at two representative temperatures. The C(t) of ANS are fitted biexponentially, and the fitting parameters are presented in Table 11.5. The origins of the shorter and the longer time components in ANS are due to the motion of bulk-like water and interface-bound water of RM respectively [112]. With increase in temperature, the faster component shows an insignificant change, whereas the slower component decreases considerably (Table 11.5), indicating enhancement of the solvation rate for the interface-bound ANS owing to the barrier-crossing transition of bound to free water at the RM interface. For probe C-500, selective excitation at 409 nm has been employed to unravel the dynamics of the trapped interfacial water of AOT/lecithin RM with variation in temperature, similarly to the case of AOT RM. The C(t) curves are fitted triexponentially, and the time constants are presented in Table 11.5. The faster time constants obtained (0.17 and 1 ns) are similar to those obtained in the case of ANS. The shorter time component (0.2 ns) may be attributed to the fast-moving trapped water [170] in the interfacial Stern layer of the RM. The intermediate time constant of 1 ns is assigned to the bound-type water molecules at the interface [128]. However, the possibilities of the contribution of counterions [181, 182] (sodium ions) in the dynamics of bound-type water cannot be completely ruled out. We recognize the slowest time component of 4.5 ns to be the contribution from the surfactant head groups, because of the close proximity of the probe C-500 to the interface. As can be seen from Table 11.5, the magnitudes of both slower and faster time components decrease gradually with increase in temperature. However, the slowest solvation time constant (from surfactant head groups) is found to increase with increase in temperature. At elevated temperature there is a possibility that some population of C-500 molecules interacts more with the polar head groups of the surfactant owing to the dehydration of the head groups, thus increasing the slowest solvation time constant. In the present study we are interested in exploring the nature of temperature-dependent solvent relaxation, and hence the longest solvation time constant of C-500 could be excluded in the estimation of htsi.

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253

Figure 11.16 (a) Decay transients of C(t) of C-500 and ANS in AOT/lecithin/i-Oc mixed RMs with w0 ¼ 10 at 293 and 343 K. The solid lines are exponential fittings. The decays become faster with increase in temperature. (b) Arrhenius plot of ln(1/htsi) against 1/T for C-500 and ANS in AOT/lecithin/i-Oc mixed reverse micelles with w0 ¼ 10. The solid lines represent linear fits. Note that good linear fit is obtained for C-500, but there is deviation from linearity for ANS at higher temperatures. Reprinted with permission from [178]. Copyright 2008 American Chemical Society

The total dynamic Stokes shifts for both ANS and C-500 are observed to be more than 1000 cm
1 at all the experimental temperatures. The observed Stokes shift of C-500 emission in the w0 ¼ 10 RM is found to be 1690 cm
1. Following Fee and Maroncelli, we may calculate the amount of solvation missed in a picosecond set-up using equation (11.58). With the use of i-Oc as the non-polar solvent, npem ð0Þ for C-500 is calculated to be 21 212 cm
1. From our time-resolved data for C-500 in the w0 ¼ 10 RM, npem ð0Þ and npem ð1Þ are found to be 20 362 and 18 672 cm
1 respectively. The total estimated Stokes shift is thus 2540 cm
1 compared with the observed Stokes shift of 1690 cm
1. Thus, in our picosecond set-up, we have missed 33% of the total dynamic spectral shift, identical to what was obtained for the AOT RM system (Table 11.3).

254 Hydrogen Bonding and Transfer in the Excited State Table 11.5 Decay parameters of C(t) for ANS and C-500 in AOT/lecithin/i-Oc mixed RM with w0 ¼ 10 at different temperatures. The figures in parentheses are the relative contribution of the time constants. Reprinted with permission from [178]. Copyright 2008 American Chemical Society Temperature (K)

t1 (ns)

t2 (ns)

t3 (ns)

ANS 293 313 328 343

0.10 0.10 0.07 0.07

(0.50) (0.39) (0.41) (0.45)

1.82 1.70 1.40 0.55

(0.50) (0.61) (0.59) (0.55)

— — — —

C-500 293 313 328 343

0.17 0.10 0.13 0.14

(0.25) (0.43) (0.55) (0.61)

1.00 0.60 0.58 0.56

(0.63) (0.52) (0.36) (0.32)

4.48 6.27 10.25 9.62

htsi (ns) 0.96 1.07 0.85 0.33

(0.12) (0.05) (0.09) (0.07)

0.67 0.35 0.28 0.26

Figure 11.16(b) displays the Arrhenius plot of ANS and C-500 in the mixed RM in the temperature range 293–343 K. The most striking observation in the case of ANS is the initial increase in the value of ln(1/htsi) with increase in temperature (in the 293
328 K range), followed by a sudden decrease at 343 K (Figure 11.16(b)). On the other hand, a gradual increase in the value of ln(1/htsi) with increase in temperature is evident for C-500 within the experimental temperature window, identical to what was obtained for the AOT RM (Figure 11.12(d)). The obtained magnitude of Ea for ANS is 7.4  0.5 kcal mol
1 after fitting the three collinear points (Figure 11.16(b)). This energy is associated with the barrier-crossing transition from bound to free water at the interface of RM, and the value is very close to the difference between water–micelle and water–water hydrogen bond energies (7–8 kcal mol
1) reported earlier [88] and is identical to the Ea value reported by Sen et. al. [95] (9 kcal mol
1) for the hydrophilic probe 4-AP located at the palisade layer of SDS micelles. The corresponding Ea value for the C-500 system is 3.9  0.5 kcal mol
1, which is close to the value of 2.4–4.0 kcal mol
1 obtained from a molecular dynamic simulation study by Pal et al. [88] for interfacial water hydrogen bonding. The activation energy (Ea) values of 7.4 and 3.9 kcal mol
1 obtained for the probes ANS and C-500, respectively, also indicate that the proximity of ANS to the RM interface is comparatively less than that of the probe C-500. Thus, from our study it is evident that the probe location is extremely important to following the barrier-crossing transition. We now investigate the observed deviation from the activation energy barrier-crossing dynamics at 343 K for ANS. It is known that the initial location of probe C-500 (at 293 K) is at the interface of the RMs. This enables us to obtain the slowest component (corresponding to solvation from head groups) widely separated from the other two faster solvation time constants (free-type water and interface-bound water) at all temperatures. Hence, in the calculation of htsi for C-500, the contribution of solvation from the head group of the surfactant can be easily discarded, giving the solvation contribution from solvent molecules alone. Hence, as expected, the probe C-500 follows the Arrhenius model with good linear fit (Figure 11.16(b)). However, for probe ANS the situation is slightly different, as it is distributed in a range of locations in the RM interface. Hence the solvation of ANS reveals shorter and longer time components corresponding to bulk-like water and the interface-bound water of RM respectively. At higher temperatures, when the dehydration of the head group takes place, the contribution to the solvation from the head group also comes into play, which is convoluted with the contribution from the interface-bound water molecules of the RM. Owing to the incapability of separating the contribution of the head group from interface-bound water, the average solvation time constant gives a value that does not fit with the Arrhenius equation. Thus, the spatial heterogeneity in the location of ANS in RM, which becomes prominent at 343 K owing to the proximity of a certain population of the probe to the surfactant head group, is responsible for the observed deviation.

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Table 11.6 r(t) for ANS and C-500 in AOT/lecithin/i-Oc mixed RM with w0 ¼ 10 at different temperatures. The figures in parentheses are the relative contribution of the time constants. Reprinted with permission from [178]. Copyright 2008 American Chemical Society Temperature (K)

tr1 (ns)

tr2 (ns)

tr3 (ns)

r0

ANS 293 313 328 343

0.11 0.14 0.16 0.09

(0.19) (0.35) (0.30) (0.33)

1.35 1.07 0.91 0.71

(0.40) (0.31) (0.35) (0.39)

37.90 (0.41) 35.00 (0.34) 28.30 (0.35) 26.07 (0.28)

0.38 0.39 0.38 0.39

C-500 293 313 328 343

0.10 0.08 0.10 0.06

(0.32) (0.47) (0.55) (0.59)

1.17 0.75 0.67 0.43

(0.42) (0.40) (0.35) (0.35)

40.47 (0.26) 34.78 (0.13) 31.82 (0.10) 26.90 (0.06)

0.38 0.38 0.36 0.35

Time-resolved r(t) of ANS and C-500 in RM, measured at their corresponding emission peaks, are characterized by a slow decay that fits well triexponentially with a considerable offset (Table 11.6, Figure 11.17). The rotational time constants reflect the temperature effect on the local environment around the probes, and the anisotropy decays for both probes are significantly slower than those in bulk water. The faster relaxation time constant of 0.1 ns does not show a regular temperature dependence and can be recognized as inertial/wobbling motion of the probe [183]. The time constant, which is of the order of 1.5 ns in the anisotropy decay, shows a reasonably good temperature dependence and may be due to the rotation of the excited-state species of the probe in RM, close to the head group of lecithin, and the longer time component

Figure 11.17 Plot of the slow rotational time constant, tslow, for C-500 and ANS in AOT/lecithin/i-Oc mixed RMs with w0 ¼ 10 as a function of temperature. The solid lines are the Stokes–Einstein–Debye fitting. A representative r(t) of ANS at two different temperatures is shown in the inset. Reprinted with permission from [178]. Copyright 2008 American Chemical Society

256 Hydrogen Bonding and Transfer in the Excited State

(40 ns) depicts the residence of the dye at the interface, revealing the overall rotation of the RM [184]. These results can be interpreted as indicative of the dye residing in a range of locations in the interface. With increase in temperature, the overall rotational dynamics becomes faster, revealing the lability of the microenvironment with temperature. The temperature-dependent rotational time constant of 1.5 ns reveals that the microviscosity around probe ANS varies from 3.3 to 2.1 cP in the temperature range 293–343 K using SED equation (11.45). The value of microviscosity near room temperature is in close agreement with the reported value (5 cP) in AOT RM [185]. On the other hand, the temperature dependence of the slower relaxation time can be fitted using the SED equation (solid line) (Figure 11.17), revealing the average diameter of the RM to be 8.9 nm, which is in excellent agreement with DLS measurement of the RM diameter (9.4 nm). Similarly to ANS, the decrease in the slower time constant of C-500 with increasing temperature is an indication of the probe becoming labile at elevated temperature. The microviscosity around probe C-500 is found to vary from 4.3 to 1.9 cP in the above-mentioned temperature range, which is consistent with the values reported in the literature [185]. As shown in Figure 11.17, the slower component of the anisotropy decay of C-500 in RM also follows the SED model, revealing the similar diameter (9.0 nm) of the RM obtained from DLS studies. 11.3.4 Importance of activation barrier-crossing transition at the reverse micellar interface in the electrostatic attachment of ligand molecules [186] Here we use AOT/i-Oc RM with w0 ¼ 5, and the structural integrity of the RM is retained at higher temperatures, as described earlier (Figure 11.10). H258 is a positively charged dye at neutral pH. The dye binds to the negatively charged surface of AOT RMs [122] and also fits tightly in the minor groove of a duplex DNA [187, 188]. Figure 11.18 shows the absorption spectrum of the dye in the RM at different temperatures. It can be seen from the figure that the peak corresponding to the maximum absorption suffers progressive blueshift with increasing temperature, which indicates that H258 moves towards the interface with increasing temperature. The location of the absorption dipole moment of the probe molecule makes it insensitive to the increased electrostatic interactions. The emission maximum remains constant in the temperature range 293–313 K but suffers a blue-shift as the temperature is further increased (Figure 11.18), which indicates that

Figure 11.18 Emission and absorption spectrum of H258 in AOT/i-Oc RM with w0 ¼ 5 at different temperatures. The peak position suffers a blue-shift with increase in temperature. Reprinted with permission from [186]. Copyright 2007 American Chemical Society

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Figure 11.19 Plot of the rotational time constant, tr, for H258 in AOT/i-Oc RMs with w0 ¼ 5 as a function of temperature. The values show good agreement with the r calculated from the SED equation. A representative r(t) of H258 at 298 K is shown in the inset. Reprinted with permission from [186]. Copyright 2007 American Chemical Society

the excited state of the dye is destabilized owing to the decreased environmental polarity at higher temperatures. In order to confirm that the dye remains in the RM at higher temperatures, we measure the rotational anisotropy (Figure 11.19), and the time components are presented in Table 11.7. H258 in bulk buffer shows a rotational lifetime of 500 ps, indicative of its free rotational motion [122]. At the interface of the RM, the twisting dynamics of H258 is frozen, and the tr obtained reflects the overall rotational dynamics of the RM [122] and it becomes faster with increase in temperature (Table 11.7). The rotational lifetime of the RM is independently calculated (Table 11.8) using the SED equation and assuming the hydrodynamic radius (rH) of the RM to follow the relation rH ¼ 0.2w0 þ length of the surfactant chain. Considering the length of the surfactant chain to be 1.1 nm [149], rH ¼ 2.1 nm is obtained, which is in close agreement with that obtained from the DLS measurements (Figure 11.10). The excellent agreement of the experimental and calculated values (Table 11.7, Figure 11.19) of tr suggests that H258 remains as an integral part of the RM at higher temperature and successfully reports its dynamics. Table 11.7 Average time constant of solvent correlation decay (htsi) and r(t) time constant (tr (experimental)) for H258 in AOT/i-Oc RM with w0 ¼ 5 at different temperatures. The tr values calculated from the SED equation are also given for comparison. Reprinted with permission from [186]. Copyright 2007 American Chemical Society Temperature (K) 288 298 308 318 328 338 348

htsi (ns)

tr (experimental) (ns)

tr (SED equation) (ns)

1.66 1.46 1.33 1.24 1.25 1.40 1.57

5.10 4.08 3.52 3.27 2.55 2.51 2.45

4.51 3.84 3.39 2.97 2.64 2.42 2.21

258 Hydrogen Bonding and Transfer in the Excited State Table 11.8 Apparent specific volume (wv) and apparent specific adiabatic compressibility (wk) of solubilized water in AOT/i-Oc RM with w0 ¼ 5 at different temperatures. Reprinted with permission from [186]. Copyright 2007 American Chemical Society Temperature (K) 293 303 313 323 333 343

wv  103 (m3 kg
1)

wk  1013 (Pa
1 m3 kg
1)

8.63 8.69 8.78 8.93 9.10 9.30

7.5 7.6 7.8 8.4 9.6 16.0

Figure 11.20 (a) A representative decay transient of C(t) of H258 in AOT/i-Oc RMs with w0 ¼ 5 at 298 K. The inset shows the apparent specific adiabatic compressibility (wk) as a function of temperature. Note that wk increases drastically beyond 313 K. (b) Arrhenius plot of ln(1/) against 1/T for H258 in AOT/i-Oc RMs with w0 ¼ 5. The solid line represents linear fit. The system deviates from linearity beyond 313 K. Reprinted with permission from [186]. Copyright 2007 American Chemical Society

Hydrogen Bonding Barrier-Crossing Dynamics at Biomimicking Surfaces

259

To explore the environmental dynamics of H258 at different temperatures, the temporal decay of C(t) is constructed at different temperatures. A representative decay of C(t) at 15  C is presented in Figure 11.20(a), and the htsi at different temperatures are presented in Table 11.7. With increase in temperature, up to 318 K, htsi gradually becomes faster and reaches a plateau in the range 318–328 K. After 328 K, htsi decreases (Table 11.7). We plot an Arrhenius equation from the htsi values listed in Table 11.7 (Figure 11.20(b)). It is evident from the figure, that up to 328 K, the htsi values agree well with the Arrhenius model, yielding an energy barrier of 1.9 kcal mol
1, which is close to the value for dynamical transition between head-groupbound water and interfacially bound water, obtained from previous simulation studies [88]. This result indicates that H258 is located in the hydration shell at the interface of the RM, and the excited-state relaxation of H258 essentially involves the transition between interfacially bound waters up to this temperature. It could be argued that, at higher temperatures, an increased fraction of the bound water molecules at the interface cross an activation energy barrier to reach a less bound state. Densimetric and acoustic studies on the RM at different temperatures show that the apparent specific adiabatic compressibility (wk) of solubilized water molecules in the RM increases drastically after 313 K (Table 11.8, inset of Figure 11.20(a)). The increased compressibility of hydration water at higher temperatures is also observed at the interface of anionic SDS micelles [150]. These observations suggest that both the rigidity and the number of bound water molecules in the interfacial hydration shell are lost at higher temperatures, making the hydration shell soft at higher temperatures. According to Omta et al. [189], individual ions along with their first solvation shells are like rigid spheres rotating in bulk water, and water present only in the first solvation shell screens the charge of the central ion. In the present system, at lower temperatures, the intact hydration shell surrounding the charged surface screens the charge at the interface and effectively dilutes the electrostatic interaction between H258 and the oppositely charged interface. However, at higher temperatures, the soft hydration shell improperly screens the charge of the interface, which causes an increased electrostatic attraction between the charged interface and the probe, making it diffuse towards the interface. The diffusion of H258 to more hydrophobic regions of the interface explains the observed blue-shift in emission spectra (Figure 11.18). As the probe approaches the interface at higher temperature, the motions of the surfactant head groups contribute towards the solvation stabilization of the probe, making the dynamics slower. The observed dynamics therefore deviates from the Arrhenius model, which depicts the transformation of head-group-bound water to interfacially bound water.

11.4 Conclusion Water molecules in confined biomimicking systems like micelles and reverse micelles (RMs) provide a useful platform for understanding the complex nature of water within a biological environment. The water molecules in the close vicinity of these systems carry physical/chemical properties similar to those in the proteins and DNA and are known as ‘biological water’. The biological water is physically different from bulk water, and these two types of water remain in thermodynamic equilibrium, which is associated with an Arrhenius-type energy-barrier-crossing model. Picosecond-resolved studies of the probes within micelles or RMs elucidate the key timescales involved in the complex rigidity and solvation. These studies also attempt to link structural and dynamical features to account for the validity of as well as the deviation from the Arrhenius model. A detailed temperature-dependent solvation study at the surface of an SDS micelle has validated an activation energy barrier-crossing model in which interfacially bound water molecules are converted into free molecules at a constant size of the micelle. The decrease in the hydration number at the micellar surface with increase in temperature is also found to be a consequence of the transition. Secondly, we report, for the first time, a detailed study of the temperature-dependent solvation dynamics of a probe fluorophore, C-500,

260 Hydrogen Bonding and Transfer in the Excited State

in well-studied AOT RM with varying w0 at different temperatures. The observed temperature-induced faster solvation dynamics is associated with a transition of bound to free water molecules, and the corresponding activation energy value for the w0 ¼ 5 system has been found to be 3.4 kcal mol
1, whereas for the w0 ¼ 10 and 20 systems it is 5 kcal mol
1; all these studies were done within the structural integrity of the RM. Next, to mimic a complicated biological system more closely, we used AOT/lecithin mixed RMs and revisited the validity and divergence of the activation energy barrier-crossing model for the bound to free water transition at the interface of the mixed RMs using picosecond-resolved solvation dynamics of two fluorescent probes, ANS and C-500. Using the DLS technique, the size of the mixed RMs at different temperatures is found to show an insignificant change. The activation energies, Ea, calculated for ANS and C-500 are 7.4 and 3.9 kcal mol
1 respectively, which are in good agreement with those obtained by MD simulation studies. However, deviation from the regular Arrhenius-type behaviour is observed for ANS around 343 K, which has been attributed to the spatial heterogeneity of the probe environments. Finally, we expressed the importance of the barrier-crossing transition at the reverse micellar surface and addressed the interplay between electrostatic attraction and the dynamics of hydration in molecular recognition of a negatively charged AOT RM interface by positively charged H258. Up to 318 K, the solvation dynamics reported by the interface-binding probe H258 becomes progressively faster with increasing temperature and follows the Arrhenius equation. Above 318 K, the observed dynamics slows down with increasing temperature, thus deviating from the Arrhenius equation. This deviation indicates proximity of the probe to the interface at higher temperatures owing to the enhanced contributions from the motions of the surfactant head groups. This suggests an increased electrostatic attraction between the ligand and interface at higher temperatures and is attributed to the change in hydration. Our study indicates that the hydration layer at a charged interface acts both as a physical and as an energetic barrier to electrostatic interactions of small ligands at the interface.

Acknowledgements P. K. V and D. B thank CSIR for providing research fellowships. We thank DST (SR/SO/BB-15/2007) for a financial grant. We are grateful to colleagues in our laboratory at S.N.B.N.C.B.S., whose contributions over the years, acknowledged in the references, have been instrumental in the successful evolution of work in this area. In particular, we thank Dr Rupa Sarkar and Dr S. S. Narayanan.

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169. R. E. Riter, D. M. Willard and N. E. Levinger, Water immobilization at surfactant interfaces in reverse micelles. J. Phys. Chem. B., 102, 2705 (1998). 170. N. Sarkar, K. Das, A. Datta et al., Solvation dynamics of coumarin 480 in reverse micelles. Slow relaxation of water molecules. J. Phys. Chem., 100, 10523 (1996). 171. A. Amararene, M. Gindre, J.-Y. Le Huerou et al., Water confined in reverse micelles: acoustic and densimetric studies. J. Phys. Chem. B, 101, 10751 (1997). 172. M. D’Angelo, D. Fioretto, G. Onori et al., Dynamics of water-containing sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles: a high-frequency dielectric study. Phys. Rev. E, 54, 993 (1996). 173. G. Carlstroem and B. Halle, Water dynamics in microemulsion droplets. A nuclear spin relaxation study. Langmuir, 4, 1346 (1988). 174. R. S. Fee and M. Maroncelli, Estimating the time-zero spectrum in time-resolved emission measurements of solvation dynamics. Chem. Phys., 183, 235 (1994). 175. S. Sen, P. Dutta, D. Sukul et al., Solvation dynamics in the water pool of aerosol sodium dioctylsulfosuccinate microemulsion: effect of polymer. J. Phys. Chem. A, 106, 6017 (2002). 176. G. B. Dutt, Rotational diffusion of hydrophobic probes in Brij-35 micelles: effect of temperature on micellar internal environment. J. Phys. Chem. B, 107, 10546 (2003). 177. G. B. Dutt, How critical micelle temperature influences rotational diffusion of hydrophobic probes solubilized in aqueous triblock copolymer solutions. J. Phys. Chem. B, 109, 4923 (2005). 178. S. S. Narayanan, S. S. Sinha, R. Sarkar and S. K. Pal, Validation and divergence of the activation energy barrier crossing transition at AOT/lecithin reverse micellar interface. J. Phys. Chem. B, 112, 2859 (2008). 179. G. B. Dutt, Rotational diffusion of nondipolar probes in Triton X-100 micelles: role of specific interactions and micelle size on probe dynamics. J. Phys. Chem. B, 106, 7398 (2002). 180. M. Kumbhakar, T. Mukerjee and H. Pal, Temperature effect on the fluorescence anisotropy decay dynamics of coumarin-153 dye in Triton-X-100 and Brij-35 micellar solutions. Photochem. Photobiol., 81, 588 (2005). 181. M. Kumbhakar, T. Goel, T. Mukherjee and H. Pal, Nature of the water molecules in the palisade layer of a Triton X-100 micelle in the presence of added salts: a solvation dynamics study. J. Phys. Chem. B, 109, 14168 (2005). 182. D. Huppert, V. Ittah and E. M. Kosower, Static and dynamic electrolyte effects on excited-state behavior. Chem. Phys. Lett., 159, 267 (1989). 183. A. B. Myers, M. A. Pereira, P. L. Holt and R. M. Hochstrasser, Rotational dynamics of electronically excited aniline in solution from picosecond fluorescence anisotropies. J. Chem. Phys., 86, 5146 (1987). 184. A. K. Shaw, R. Sarkar, D. Banerjee et al., Direct observation of protein residue solvation dynamics. J. Photochem. Photobiol. A, 185, 76 (2007). 185. E. M. Corbeil, R. E. Riter and N. E. Levinger, Cosurfactant impact on probe molecule in reverse micelles. J. Phys. Chem. B, 108, 10777 (2004). 186. D. Banerjee, S. S. Sinha and S. K. Pal, Interplay between hydration and electrostatic attraction in ligand binding: direct observation of hydration barrier at reverse micellar interface. J. Phys. Chem. B, 111, 14239 (2007). 187. M. C. Vega, I. G. Saez, J. Aymami et al., Three-dimensional crystal structure of the A-tract DNA dodecamer d (CGCAAATTTGCG) complexed with the minor-groove-binding drug Hoechst 33258. Eur. J. Biochem., 222, 721 (1994). 188. A. Fede, A. Labhardt, W. Bannwarth and W. Leupin, Dynamics and binding mode of Hoechst 33258 to d (GTGGAATTCCAC)2 in the 1:1 solution complex as determined by two-dimensional 1H-NMR. Biochemistry, 30, 11377 (1991). 189. A. W. Omta, M. F. Kropman, S. Woutersen and H. J. Bakker, Negligible effect of ions on the hydrogen-bond structure in liquid water. Science, 301, 347 (2003).

12 Intermolecular Hydrogen Bonding in the Fluorescence Excited State of Organic Luminophores Containing Both Carbonyl and Amino Groups Ilijana Timcheva and Peter Nikolov Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

12.1 Introduction The radiationless deactivation of the excited states of organic luminophores as a result of specific intra- or intermolecular interactions (intramolecular charge transfer; formation of excimers and exciplexes; formation of hydrogen bonds) has been studied by many authors (see, for instance, [1–20]). Some of these investigations are devoted to the possible formation of intermolecular hydrogen bonds in the excited state between the molecules of the substance and the protic solvents [5, 7–20]. The conclusions are based on analysis of the specific photophysical properties of the studied compounds in solvents of different polarity and protondonating ability. The present review is a generalization of our previously published investigations on the deactivation processes of the excited states of 5-aminoindan-1,3-diones (AID) [11], 5-amino-3-hetarylmethylene-1(3H)isobenzofuranones (HAB) [12], 10-decyl- and 10-propenyl-9-acridanones (ACR) [13], 3-arylmethylene1(3H)-isobenzofuranones (benzylidenephthalides, BPH) and 5-amino-3-arylmethylene-1(3H)- isobenzofuranones (aminobenzylidenephthalides, ABPH) [14] and synthetic chalcones [17]. All these groups of compounds are characterized by the presence both of a carbonyl group and an electron-donating substituent in a common conjugated system.

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

270 Hydrogen Bonding and Transfer in the Excited State

The conclusions for the possible formation of intermolecular hydrogen bonds in the fluorescence excited state of the investigated compounds are based on: .

. . . .

interpretation of experimental data for the dependencies of the fluorescence Franck–Condon transition energy and the fluorescence quantum yield on the semi-empirical solvent polarity parameter ET(30) [21] at room temperature; analysis of the fluorescence decay curves in solution; the deuterium isotope effect on the spectral characteristics; the matrix effect on the fluorescence quantum yield in frozen glasses at 77 K; quantum chemical PM3 calculations for the negative charge distribution in the first singlet excited state.

Increasing the polarity and proton-donating ability of the solvent (passing from hexane through acetonitrile to ethanol) does not exert a substantial influence on the position and shape of the longest-wavelength absorption bands of the studied compounds; only a slight bathochromic shift of the absorption maxima is observed. The dependence between the energy of the longest-wavelength absorption maxima and the Df constant of the solvent is described by one linear correlation only, regardless of whether the solvent is protic or aprotic. Hence, there is no specific interaction between the substance and the proton-donating solvents in the ground state.

12.2 Experimental The investigated compounds were synthesized by a standard procedure [22–26]. They were recrystallized until a constant melting point was obtained and were characterized by elemental analysis, NMR, IR, UV-VIS absorption and fluorescence spectra. The UV-VIS absorption spectra were recorded on a Lambda 25 spectrophotometer (Perkin Elmer). The corrected fluorescence spectra were taken on an LS55 spectrofluorimeter (Perkin Elmer). The fluorescence quantum yields (QF) were measured relative to 3-aminophthalimide (QF ¼ 0.6 in ethanol) [27], to 3-p-methoxyphenylmethylene-1(3H)-isobenzofuranone (QF ¼ 0.12) [28] and to rhodamine 6G (QF ¼ 0.95 in ethanol) [29]. The solvents used were of fluorescence grade. The natural lifetimes were measured on a nanosecond PRA 2000 spectrofluorimeter at room temperature. The low-temperature luminescence measurements were performed at 77 K using the standard phosphorescence accessory of the Perkin Elmer MPF44 spectrofluorimeter equipment and quartz tubes of 4 mm diameter.

12.3 Results and Discussion 12.3.1 5-amino-2-aryl-2-carboxymethylindan-1,3-diones (AID) The 2-aryl- and 2-aryl-2-substituted indandiones do not fluoresce in solution [30, 31]. Their diketo tautomeric form is responsible for the photoisomerization to the corresponding isobenzofuranones upon steady-state or flash UV irradiation [32]. The 2-arylindan-1,3-diones, amino-substituted in the phthaloyl fragment (Chart 12.1), are photostable and fluoresce in the 20 000–25 000 cm1 energy region [33]. For 4-aminoindandiones, the fluorescence maxima shift to the red with increasing solvent polarity and proton-donating ability (compound 3.1.7 in Chart 12.1). No evidence for differences in the nature of the emitting states in protic and aprotic solvents is present. On the other hand, the polarity effects on the fluorescence characteristics of the 5-aminoindan-1,3-diones (AID) depend strongly on the hydrogen-bonding ability of the solvent. Figure 12.1 shows a plot of the energy of the fluorescence maxima nf of compound 3.1.5, a typical representative of the AID, against the solvent polarity

Intermolecular Hydrogen Bonding in the Fluorescence Excited State 271 X

O 7

1

6 5 4 Y

2

3

R

O

Compounds 3.1 Number 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.1.6 3.1.7

X H H CH3 OCH3 Cl H CH3

Y 5-NH2 5-NH2 5-NH2 5-NH2 5-NH2 4-NH2 4-NH2

R H CH2COOH CH2COOH CH2COOH CH2COOH CH2COOH CH2COOH

Chart 12.1 Investigated amino-2-aryl-2-carboxymethyl-indan-1,3-diones – Figure 1. Reprinted with permission from [11]. Copyright Elsevier

parameter ET(30). Two different linear correlations are observed in aprotic solvents (1 to 6) and protic solvents (7 to 12). In hydroxylic solvents, the Franck–Condon transition energies are lower than expected from extrapolation of the data and protic solvents (7 to 12). In hydroxylic solvents, the Franck–Condon transition energies are lower than expected from extrapolation of the data obtained in aprotic solvents with higher polarities. This suggests the existence of specific interactions of these compounds with hydroxylic solvent molecules, leading to an additional stabilization of the emitting state. Similar effects have been observed previously in the cases of polarized enones [7], dihydroquinolinones [8], 3- and 4-aminophthalimides [9] and acylanthracenes [10, 18–20]. The conclusion concerning a difference in the emitting states of AID in protic and aprotic solvents is supported by the observed dependence of the fluorescence quantum yields on the ET(30) constants (Table 12.1, Figure 12.2). In aprotic solvents the Qf values are very low and increase slightly with increase in solvent polarity; the Qf values in protic solvents are much higher, but just the opposite tendency is

26

24

1 4 2

-3

-1

V f * 10 [cm ]

25

5 3

6

23 22 21

7 8

9 10

20

11 12

19 18 30

35

40

45

50

55

60

65

ET(30)

Figure 12.1 Energy of the fluorescence maxima nf of compound 3.1.5 (Chart 12.1) versus the solvent polarity parameter ET(30) – Figure 2. Reprinted with permission from [11]. Copyright Elsevier. The numbers of the solvents correspond to those given in Table 12.1

272 Hydrogen Bonding and Transfer in the Excited State Table 12.1 Fluorescence characteristics of compounds 3.1.1, 3.1.5 and 3.1.7 (see Chart 12.1); nf is the energy of the fluorescence maximum (in cm1); Qf is the fluorescence quantum yield – Table 1. Reprinted with permission from [11]. Copyright Elsevier No.

1 2 3 4 5 6 7 8 9 10 11 12

Solvent

3.1.1

Diethyl ether Dioxane Tetrahydrofuran Ethyl acetate Acetone Acetonitrile 2-Butanol 2-Propanol 1-Propanol Ethanol Methanol Water

3.1.5

3.1.7

nf

Qf

nf

Qf

23 800

<0.001

22 700 23 450 23 200 23 000 21 150 21 100 20 900 20 800 20 700 18 900

<0.001 <0.001 <0.001 0.002 0.10 0.07 0.05 0.02 0.01 0.002

25 000 24 400 24 100 24 250 23 700 23 050 21 400 21 400 21 050 21 000 20 350 19 600

<0.001 <0.001 0.002 0.002 0.007 0.006 0.34 0.27 0.23 0.24 0.08 0.02

nf

Qf

23 000 22 050 22 800 22 750 22 050

0.01 0.01 0.02 0.03 0.29

21 800 21 650 20 650

0.25 0.23 0.36

observed: the fluorescence quantum yield decreases with increase in the ET(30) constant of the medium. Similar results have been obtained for all the studied AID. The observed dependences of spectral shifts and fluorescence quantum yield on solvent property gives good evidence for a significant influence of specific intermolecular interactions on the S1(pp ) deactivation of the investigated 5-aminoindandiones. Bearing in mind that hydrogen-bonding interactions may cause significant variations in fluorescence properties (see, for example, Ref. [34]), it may be assumed that the observed specific interactions are due to hydrogen bond formation with alcohols and water in the excited singlet states of the investigated AID. Such intermolecular interactions have also been claimed for other carbonyl-containing aromatic compounds [7–10, 18–20]. 0,4 7

0,3

8

Qf

9 10

0,2

0,1

11

30

35

40

12

6

5

1 2 34

0,0

45

50

55

60

65

ET(30)

Figure 12.2 Fluorescence quantum yield Qf of compound 3.1.5 (Chart 12.1) versus the solvent polarity parameter ET(30) – Figure 3. Reprinted with permission from [11]. Copyright Elsevier. The numbers of the solvents correspond to those given in Table 11.1

Intermolecular Hydrogen Bonding in the Fluorescence Excited State 273 Table 12.2 Fluorescence maxima nf, fluorescence quantum yields Qf and natural lifetime 1 t of compound 3.1.5 (see Chart 12.1) in an acetonitrile/ethanol mixture – Table 2. Reprinted with permission from [11]. Copyright Elsevier 1

1

EtOH (%)

nf (cm1)

Qf

t1 (ns)

t2 (ns)

0 10 20

21 530 21 850 23 050

0.046 0.019 0.007

2.9 1.5 0.77

12.4 14.9

When ethanol is added to acetonitrile solutions of AID, the fluorescence maximum shifts bathochromically and the fluorescence quantum yield gradually increases (see Table 12.2). Similar effects in solvent mixtures have been observed in other cases, such as that of acylanthracenes [10], and are attributed to a combination of the non-specific hydrogen-bonding interactions with protic solvents. In acetonitrile–ethanol solutions, the measured fluorescence decay of the studied 5-aminoindandiones could be fitted with good precision to a double exponential function (Table 12.2). This behaviour can be interpreted in terms of two differently emitting species: one corresponds to the species responsible for the high emission in the protic solvents, and the other to the weakly fluorescent species in the aprotic environment. This supports the assumption that the different emission properties in these two classes of solvents can be attributed to solute–solvent complexation in hydroxylic media. Upon deuterium isotope substitution of the protic solvents, the energy of the fluorescence Franck–Condon transition remains practically unchanged, while fluorescence intensity is significantly enhanced (see Table 12.3). In rigid glasses at 77 K the fluorescence quantum yield becomes equal in ethanol and ethanol-d1, in contrast to liquid solutions, where a significant isotope effect is found. The results (Table 12.3) show that the radiative rate constant kr is invariant on deuteration, but the radiationless rate constant knr diminishes. In frozen solutions the spectral shift of the fluorescence band between ethanol and methyltetrahydrofuran glasses becomes minimal, and this shows that excited-state solvent reorientation is important in the hydrogen-bonding environment. There are two main sites for hydrogen bonding in the indandione chromophore: the amino group and the carbonyl group. Upon excitation, charge is shifted from the amino group to the carbonyl moiety. Hydrogen bonds where the amino group is the donor should be weakened, but the carbonyl group becomes a much better proton acceptor. Hence, the formation of intermolecular hydrogen bonds of the type C=O    H--O in the excited state can be expected. Excited-state complexation should be responsible for the different fluorescence characteristics in hydrogenbonding and non-hydrogen-bonding solvents. The fluorescence quenching of the investigated AID in protic Table 12.3 Fluorescence maxima nf, fluorescence quantum yields Qf, natural lifetime 1 t of compound 3.1.5 (Chart 12.1) and rate constants of radiative (kr) and radiationless (knr) deactivation of S1(pp ) state, calculated from the fluorescence decay curve, fitted to a monoexponential linear function I(t) ¼ A exp(t/1 t) – Table 3. Reprinted with permission from [11]. Copyright Elsevier 1

Solvent

nf (cm1)

Qf

t (ns)

kr (ns1)

knr (ns1)

Ethanol Ethanol-d1 Water Deuterium oxide

21 000 21 000 19 600 19 600

0.24 0.34 0.02 0.10

13.4 17.7 3.3 15.0

0.018 0.019 0.006 0.007

0.057 0.037 0.297 0.060

274 Hydrogen Bonding and Transfer in the Excited State

solvents with increasing ET(30) constant is connected with the H--O vibrational motions of the solvent, and this is supported by the deuterium isotope effect. The C=O    H--O vibrations may act as accepting modes in the non-radiative deactivation of the studied compounds [5, 7, 8, 35, 36]. A possible explanation of the absence of evidence for the formation of intermolecular hydrogen bonds in the case of 4-aminoindandiones could be the formation of intramolecular hydrogen bonds between the carbonyl and the amino groups, similar to those in anthraquinone derivatives [5], which hinders the specific interactions of the carbonyl group with protic solvents. 12.3.2 3-hetarylmethylene-1(3H)-isobenzofuranones (heterocyclic analogues of benzylidene phthalides, HAB) The investigated HAB are structural analogues of indandiones (compounds 3.1), in which the aryl substituent is a heteroaromatic cycle – pyrrole, furane, thiophene or indole (Chart 12.2). In contrast to AID, in these molecules the phthaloyl fragment, containing an amino and a carbonyl group, and the heteroaromatic substituent in position 3, which has weak electron-donating properties (saþ between 0.8 and 1.2) [37], are conjugated. However, the studied HAB are unsubstituted in the heteroaromatic cycle, and that is why it is impossible to estimate how the presence of the substituent in the aryl part of the molecule might influence the distribution of the electron density in the excited state and thus the possibility of specific interactions with the solvents.

H

C

H

N H

C

O

N H O

H2N

O

O

Compound 3.2.6

Compound 3.2.7

H

C

X O

Y O

Compounds 3.2 Number 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5

X S S O O NH

Y H NH2 HNCOCH3 NH2 NH2

Chart 12.2 Investigated 3-hetarylmethylene-1(3H)-isobenzofuranones – Figure 1. Reprinted with permission from [12]. Copyright Elsevier

Intermolecular Hydrogen Bonding in the Fluorescence Excited State 275

All HAB, except compound 3.2.1, fluoresce in hexane in the region 20 000–24 000 cm1. In ethanol, where compound 3.2.1 also exhibits fluorescence, the emission is shifted bathochromically relative to that in hexane by 1500–2000 cm1; the fluorescence maxima are between 19 000 and 22 000 cm1. The presence of an amino group in the phthaloyl fragment of HAB reduces the energy of the fluorescence Franck–Condon transition in ethanol by about 1000 cm1. In hexane this effect is less pronounced. The energy of the fluorescence Franck–Condon transitions of compounds 3.2.1, 3.2.3 and 3.2.6 (with no amino group in the phthaloyl fragment) decreases monotonically with increasing polarity and proton-donating properties of the solvent (Table 12.4). The fluorescence maxima of the amino-substituted HAB in the phthaloyl fragment (compounds 3.2.2, 3.2.4, 3.2.5 and 3.2.7) in aprotic solvents, like those of compounds without an amino group, move monotonically to the red with increasing solvent polarity. A sharp drop in energy of the fluorescence Franck–Condon transition of about 1000 cm1 is observed between acetonitrile and isobutanol, in spite of their similar polarity. No linear correlation between the fluorescence frequency of these compounds and the Df constants for protic solvents is observed, while such a correlation is present for the aprotic solvents, including polar ones. This fact shows that the significant red-shift of the fluorescence maxima of amino-substituted HAB in protic solvents could not be due only to their higher polarity. Two different linear correlations are obtained for the relation between the fluorescence frequency of the amino-substituted HAB and the ET(30) constants of the solvents, one for protic and the other for aprotic solvents (Table 12.6 and Figure 12.3). As in the case of aminoindandiones, the presence of two correlation lines shows that the nature of the emitting states in protic and aprotic solvents is different. Such a spectral behaviour is explained, as mentioned above, by the possible formation of intermolecular hydrogen bonds in the fluorescence excited state between the substance and the protic solvent. The dependence of Qf of the amino-substituted HAB on the ET(30) constants is similar to that described in Refs [8] and [11], and also supports the assumption of the formation of intermolecular hydrogen bonds. In aprotic solvents, Qf increases monotonically with increase in the ET(30) value from hexane to acetonitrile. The opposite tendency is observed in alcohols, where the Qf value decreases with increasing ET(30) value (Figure 12.4). The fluorescence quantum yield is highest in n-propanol and diminishes in ethanol, methanol and water (Table 12.4, Figure 12.4). Principally, at room temperature, intermolecular hydrogen bonds may be formed between the nitrogen of the amino group in the molecule of HAB and the protic solvent. Thus, bonds of the H--N    H--O and C=O    H--O type could be present. Quantum chemical PM3 calculations [38] show that in the first excited singlet state the negative charge is localized on the oxygen of the carbonyl group, which increases its protonaccepting properties, while the nitrogen atom has a partial positive charge. The charges in the S1 state of the nitrogen and the oxygen atoms in compound 3.2.4 (X ¼ O, Y ¼ NH2) are 0.1238 and 0.2418, and in compound 3.2.5 (X ¼ NH, Y ¼ NH2) they are 0.1257 and 0.2944 respectively. That is why it could be assumed that, in the case of the investigated HAB containing an amino group in the phthaloyl fragment, intermolecular hydrogen bonds of the type C=O    H--O are formed similarly to the case of compounds described in [7, 11, 13, 35, 36]. The dependence of the fluorescence frequency on the ET(30) value for the HAB without amino substituent in the phthaloyl fragment (compounds 3.2.1, 3.2.3 and 3.2.6) is described by one linear correlation only. The Qf/ET(30) dependence differs from that observed for compounds with Y ¼ NH2 (Table 12.4). Consequently, no intermolecular hydrogen bonds are formed between the substance and the protic solvent in this case. 12.3.3 3-arylmethylene-1(3H)-isobenzofuranones (benzylidene phthalides, BPH) In [11, 12] it was shown that 5-aminoindan-1,3-diones and their structural analogues 5-amino-3-hetarylmethylene-1(3H)-isobenzofuranones form intermolecular hydrogen bonds of the type C=O    H--O with

Solvent

n-Hexane Diethyl ether Ethyl acetate Acetone Acetonitrile 2-Butanol 2-Propanol 1-Propanol Ethanol Methanol Water

No.

1 2 3 4 5 6 7 8 9 10 11

— — 22 500 22 240 22 260 21 340 22 390 21 740 21 940 21 850 21 280

nf

Qf — — 0.001 0.002 0.003 0.014 0.014 0.012 0.012 0.014 0.028

3.2.1

22 520 22 470 22 390 22 270 22 200 20 850 20 830 20 850 20 720 20 660 20 100

nf

Qf 0.003 0.12 0.13 0.14 0.16 0.34 0.36 0.40 0.39 0.33 0.02

3.2.2

22 550 21 730 21 960 21 540 21 520 21 550 21 510 21 510 21 490 21 430 20 580

nf

Qf 0.001 0.004 0.01 0.02 0.05 0.07 0.08 0.08 0.08 0.09 0.11

3.2.3

22 360 22 000 22 100 21 830 21 770 20 610 20 630 20 630 20 400 20 450 19 880

nf

Qf 0.003 0.08 0.08 0.13 0.13 0.21 0.30 0.40 0.37 0.24 0.08

3.2.4

23 970 22 260 21 790 20 810 20 530 19 800 19 940 20 070 19 760 19 840 18 950

nf

Qf 0.004 0.05 0.05 0.05 0.07 0.18 0.18 0.19 0.15 0.13 0.09

3.2.5

21 000 20 820 20 540 19 830 19 880 19 690 19 350 19 250 19 320 19 160 18 080

nf

3.2.6

— 0.28 0.45 0.37 0.28 0.24 0.31 0.32 0.35 0.33 —

Qf

20800 20680 20 520 20 370 19 900 19 200 19 280 19 250 19 190 19 050 18 200

nf

3.2.7

— 0.40 0.56 0.40 0.40 0.43 0.57 0.68 0.55 0.47 —

Qf

Table 12.4 Fluorescence characteristics of compounds 3.2 (see Chart 12.2): nf is the energy of the fluorescence maximum (in cm1); Qf is the fluorescence quantum yield – Table 3. Reprinted with permission from [12]. Copyright Elsevier

276 Hydrogen Bonding and Transfer in the Excited State

Intermolecular Hydrogen Bonding in the Fluorescence Excited State 277 23 1 3 4

2

5

-3

-1

V * 10 [cm ] f

22

21

7

8

10

6 9

20

19 30

35

40

45

11

50

55

60

65

ET(30)

Figure 12.3 Energy of the fluorescence maxima nf of compound 3.2.4 (Chart 12.2) versus the solvent polarity parameter ET(30) – Figure 3(a). Reprinted with permission from [12]. Copyright Elsevier. The numbers of the solvents correspond to those given in Table 12.4

protic solvents in the excited state. It has been established that the probability of formation of the hydrogen bonds via the carbonyl group depends decisively on the presence of a strong electron-donating substituent in the phthaloyl fragment. The above-described investigations have been extended on two groups of differently substituted 3-arylmethylene-1(3H)-isobenzofuranones (benzylidene phthalides, BPH). The first group includes BPH, substituted in the p-position of the phenyl ring with electron-donating substituents – compounds 3.3.1 and 3.3.2 (Chart 12.3). The second group consists of BPH that contain an amino group in position 5 of the phthaloyl fragment and simultaneously an electron-donating or electron-withdrawing substituent in the p-position of the phenyl ring (compounds 3.3.3, 3.3.4 and 3.3.5) (Chart 12.3). In all these structures, the phthaloyl fragment and the phenyl ring substituted in the p-position are in a common conjugated system, which makes it possible to

8

0.4

9

0.3

Qf

10

0.2 4

0.1

5

2

11

3

0.0 30

1

35

40

45

50

55

60

65

ET(30)

Figure 12.4 Fluorescence quantum yield Qf of compound 3.2.4 (Chart 12.2) versus the solvent polarity parameter ET(30) – Figure 3(b). Reprinted with permission from [12]. Copyright Elsevier. The numbers of the solvents correspond to those given in Table 12.4

278 Hydrogen Bonding and Transfer in the Excited State X H

C O

Y O

Compounds 3.3 Number 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5

X OCH3 NH2 N(CH3)2 CN OCH3

Y H H NH2 NH2 NH2

Chart 12.3 Investigated 3-arylmethylene-1(3H)-isobenzofuranones – Table 1. Reprinted with permission from [14]. Copyright Elsevier

follow the influence not only of the substituents in the phthaloyl fragment (as in the case of AID and HAB) but also of the substituents in the p-position of the phenyl ring on the photophysical properties of BPH. 12.3.3.1 Effect of the Amino Group in the p-Position of the Phenyl Ring The energy of the fluorescence transitions of BPH with a p-OCH3 group in the phenyl ring (compound 3.3.1 in Chart 12.3) decreases monotonically, going from non-polar to polar solvents in spite of their proton-donating properties (Table 12.5). The dependencies of the fluorescence transitions energy nf and the fluorescence quantum yield Qf on the ET(30) constant of the solvent are both monolinear. Consequently, in the case of compound 3.3.1 there is no indication of the presence of emitting states of different nature in protic and aprotic solvents. Similar results have been obtained with BPH unsubstituted in the phenyl ring. The presence of an amino group in the p-position of the phenyl ring of BPH (compound 3.3.2, Chart 12.3) leads to significant changes in the interaction of the molecules and the protic solvents in the excited state in comparison with BPH in which X ¼ H and OCH3. Two different linear correlations are obtained for the dependencies nf/ET(30) and Qf/ET(30) in protic and aprotic solvents. Consequently, similar to the 5-aminosubstituted indandiones and HAB in the phthaloyl fragment, in the case of the BPH amino-substituted in the phenyl ring the nature of the emitting states in protic and aprotic solvents is different owing to the formation of intermolecular hydrogen bonds in the excited state. The differences in the spectral behaviour of the above BPH could be explained by the significantly stronger electron-donating properties of the amino group (sp ¼ 0.66) in comparison with those of the methoxy group (sp ¼ 0.27). Therefore, the increase in the negative charge on the carbonyl oxygen in the excited state will be less pronounced in the case of p-OCH3-substituted BPH in comparison with p-NH2-substituted BPH, and this is the reason for the absence of specific interactions with protic solvents via the carbonyl group in the case of compound 3.3.1. 12.3.3.2 Effect of the Amino Group in the Phthaloyl Fragment The other group of BPH in which the formation of intermolecular hydrogen bonds with protic solvents in the fluorescence state was found are those with an amino group in the phthaloyl fragment (ABPH). These compounds are similar to 5-aminoindandiones and 5-amino-3-hetarylmethylene isobenzofuranones. In this case, the substituents in the p-position of the phenyl ring were varied. The experimental results show that,

Intermolecular Hydrogen Bonding in the Fluorescence Excited State 279 Table 12.5 Fluorescence characteristics of compounds 3.3 (see Chart 12.3): nf is the energy of the fluorescence maximum (in cm1); Qf is the fluorescence quantum yield – Table 2. Reprinted with permission from [14]. Copyright Elsevier No.

1 2 3 4 5 6 7 8 9

Solvent

Diethyl ether Ethyl acetate Acetone Acetonitrile 2-Butanol 2-Propanol 1-Propanol Ethanol Methanol

3.3.1

3.3.2

3.3.3

3.3.4

3.3.5

nf

Qf

nf

Qf

nf

Qf

nF

QF

nF

QF

23 040 22 800 22 100 21 920 22 130 21 750 21 700 21 770 21 770

0.001 0.008 0.01 0.03 0.07 0.10 0.13 0.14 0.14

20 580 19 860 25 640 18 250 18 260 18 240 18 190 18 100 18 100

0.23 0.19 0.14 0.08 0.18 0.16 0.12 0.13 0.13

22 480 19 200 18 620 17 800 18 420 18 180 17 860 17 600 17 600

0.40 0.37 0.26 0.24 0.36 0.31 0.28 0.22 0.22

22 480 21 380 20 740 20 600 19 910 20 160 20 070 19 890 19 860

0.031 0.047 0.05 0.062 0.27 0.16 0.14 0.13 0.11

22 560 22 470 22 420 22 210 21 060 20 780 20 720 20 600 20 570

0.11 0.20 0.21 0.38 0.44 0.40 0.46 0.34 0.29

independently of the type of substituent X in the phenyl ring of ABPH – electron-donating (N(CH3)2 and OCH3, compounds 3.3.3 and 3.3.5, Chart 12.3) or electron-withdrawing (CN, compound 3.3.4, Chart 12.3) – the linear correlations nF/ET(30) are different for protic and aprotic solvents (Table 12.5, Figure 12.5). Hence, even the presence of a second electron-withdrawing centre in the molecule of ABPH – the cyano group in the pposition of the phenyl ring – does not lead to a remarkable decrease in the partial negative charge localized on the carbonyl oxygen as a result of the influence of the amino group in the phthaloyl fragment, on account of which the possibility of the formation of complexes with protic solvents in the excited state is preserved. The hypothesis of the formation of intermolecular hydrogen bonds in the excited state in the case of the investigated BPH containing an amino group both in the phenyl ring and in the phthaloyl fragment is supported by the different linear correlations Qf/ET(30) obtained in protic and aprotic solvents (Figure 12.6).

23 1

Vf * 10-3 [cm-1 ]

22 2

21 3

4 6

20

7

9

5 8

19 30

35

40

45 ET(30)

50

55

60

Figure 12.5 Energy of the fluorescence maxima nf of compound 3.3.4 (Chart 12.3) versus the solvent polarity parameter ET(30) – Figure 2. Reprinted with permission from [14]. Copyright Elsevier. The numbers of the solvents correspond to those given in Table 12.5

280 Hydrogen Bonding and Transfer in the Excited State 0.20 5 6 7

0.15

8

Qf

9

0.10

0.05

1

0.00 30

35

3

2

40

4

45

50

55

60

ET(30)

Figure 12.6 Fluorescence quantum yield Qf of compound 3.3.4 (Chart 12.3) versus the solvent polarity parameter ET(30) – Figure 1. Reprinted with permission from [14]. Copyright Elsevier. The numbers of solvents correspond to those given in Table 12.5

The fact that Qf always decreases in protic solvents with increasing ET(30) constants shows that the vibrations of the formed intermolecular hydrogen bonds of the C=O    H--O type define a new possibility of radiationless deactivation of the excited states of the studied compounds. 12.3.4 N-substituted acridanones The photophysical properties of 10-decyl- and 10-propenyl-9-acridanones, which are important emitters in electro- and chemiluminescence [39, 40] and are also used as photoinitiators in polymerization reactions [41], are investigated as a function of the solvent polarity and hydrogen-bonding ability at room temperature, as well as in a frozen matrix at 77 K [13]. The experimental results show that, in non-polar solvents at room temperature, their fluorescence is inefficient owing to a fast intersystem crossing, but increases with solvent polarity, which is connected with the inversion of the S1(pp ) and the T1(np ) states. Chart 12.4 shows the structures of the investigated compounds. Changing the proton-donating ability of the solvent, i.e. replacing highly polar acetonitrile with hydrogen-bonding solvents such as ethanol, does not result O R

N X Compounds 3.4

Chart 12.4 Elsevier

Number 1

X –CH–CH=CH2

R H

2

–CH2–(CH2)8–CH3

H

3

–CH2–(CH2)8–CH3

–CHO

4

–CH=CH–CH3

H

Investigated N-substituted acridanones – Scheme 1. Reprinted with permission from [13]. Copyright

Intermolecular Hydrogen Bonding in the Fluorescence Excited State 281 Table 12.6 Experimental spectral characteristics of compounds 3.4 (see Chart 12.4); na is the energy of the absorption maxima; nf is the energy of the fluorescence maxima; Qf is the fluorescence quantum yield; « is the molar absorptivity – Table 1. Reprinted with permission from [13]. Copyright Elsevier Parameters

3.4.1

3.4.2

3.4.3

3.4.4

Solvent

na (cm ) nf (cm1)

25 640 25 340

25 420 25 180

25 430 25 163

25 790 25 410

Cyclohexane

Qf na (cm1) nf (cm1) Qf

0.002 25 210 24 420 0.28

0.004 24 910 24 270 0.47

0.003 24 870 24 430 0.43

0.003 25 320 24 600 0.18

na (cm1) « (L mol1 cm 1) nf (cm1) Qf

24 930 11 800 23 990 0.55

24 690 18 500 23 520 0.56

24 690 15 600 23 880 0.62

25 000 15 900 24 070 0.046

1

Acetonitrile Ethanol

in a measurable change in the spectral characteristics of the absorption bands, which indicates that specific solute–solvent interactions are either weak or absent in the ground state. The spectral characteristics of the investigated acridanones in cyclohexane, acetonitrile and ethanol at room temperature are presented in Table 12.6. Increasing the solvent polarity leads generally to a lowering of the energy of the fluorescence transition and to an increase in Stock’s shift. While nf of compounds 3.4.1 to 3.4.3 decreases monotonically with increasing solvent polarity, nf of compound 3.4.4 shifts additionally by 500 cm1 by the use of acetonitrile instead of 2butanol, irrespective of their similar polarity (Figure 12.7). Qf of compound 3.4.4 is also smaller by one order of magnitude in protic solvents in comparison with the other investigated compounds or with solutions in

0.20 0.18

6

0.16 0.14

5

0.12 f

4

0.10

7 3

0.08

89 10

0.06 0.04

2 11

0.02 1

0.00 30

35

40

45

50

55

60

ET(30)

Figure 12.7 Fluorescence quantum yield Qf of compound 3.4.4 (Chart 12.4) versus the solvent polarity parameter ET(30) – Figure 4. Reprinted with permission from [13]. Copyright Elsevier. The numbers of the solvents correspond to those given in Table 12.7

282 Hydrogen Bonding and Transfer in the Excited State Table 12.7 Fluorescence characteristics of compound 3.4.4 (see Chart 12.4); nf is the energy of the fluorescence maximum (in cm1); Qf is the fluorescence quantum yield No.

Solvent

nf

Qf

1 2 3 4 5 6 7 8 9 10 11

Heptane Diethyl ether Dioxane Cl-benzene Chloroform Acetonitrile 2-Propanol 1-Butanol 1-Propanol Ethanol Methanol

24 600 25 030 24 550 24 750 24 340 24 600 24 170 24 100 24 020 24 070 23 880

0.004 0.02 0.09 0.12 0.14 0.18 0.09 0.08 0.08 0.06 0.02

non-hydrogen bonding polar solvents (Table 12.7). A similar lowering of Qf for compound 3.4.4 in comparison with the other compounds is observed for all protic solvents used (2-propanol, 1-propanol, butanol, ethanol, methanol), and this is demonstrated in Figure 12.8 by plotting Qf versus the ET(30) constant of the solvent. This behaviour indicates some specific influence of the proton-donating ability of the solvents upon the processes of non-radiative deactivation of the S1(pp ) state in the case of 9-[10-(1-propenyl)]-acridanone (compound 3.4.4). As already mentioned above, similar solvent effects on Qf in protic solvents, as well as different linear correlations in aprotic and protic solvents of the Stock’s shift and Df(«, n), of nf and ET(30) (see Figure 12.4) and of Qf and nf (see Figure 12.4), are usually related to the formation of intermolecular hydrogen bonds in the fluorescence excited state with solvent molecules. The observed fluorescence quenching with increasing ET(30) values in protic solvents should be related to an increase in the probability of non-radiative deactivation of the excited state. The O--H vibrations in the hydrogen-bonded complex could act as promoting modes for 23

1

21

-3

-1

νf*10 [cm ]

22

20 2

19 3

4

18 30

35

40

45 ET (30)

5

6 7

50

8

55

60

Figure 12.8 Energy of the fluorescent maxima nf of compound 3.5.5 (Chart 12.5) versus the solvent polarity parameter ET(30). Solvents: 1 – benzene; 2 – ethyl acetate; 3 – acetone; 4 – acetonitrile; 5 – n-Butanol; 6 – ipropanol; 7 – ethanol; 8 – methanol

Intermolecular Hydrogen Bonding in the Fluorescence Excited State 283

such radiationless deactivation processes. Like the other investigated structures – AID, HAB and BPH – the studied acridanones are acceptor–donor systems, where the carbonyl group is the acceptor and the N atom substituted by 10-decyl or 10-propenyl at the 9-position is the electron donor centre. It is worth mentioning here that a measurable influence of hydrogen bonding on the photophysical parameters is thus only found for 9[10-(1-propenyl)]-acridanone, i.e. only in the compound where a C¼C double bond is directly connected to the acridanone nitrogen atom. The hypothesis of intermolecular hydrogen bond formation in the excited state of compound 3.4.4 is also supported by the results of analysis of fluorescence decay data. The fluorescence decay curves for the other compounds both in ethanol and acetonitrile do not show anomalies and are described by a monoexponential decay function; in the case of compound 3.4.2, the corresponding lifetimes are 11.7 and 7.4 ns respectively (the excitation was at 354 nm and the emission was monitored at 430 nm). These values are in good agreement with values published for acridane and unsubstituted acridanone in ethanol – 11.3 and 10 ns respectively were reported [42, 43]. On the other hand, the fluorescence decay of 9-[10-(1-propenyl)]-acridanone in ethanol is biexponential. The decay function is described by a predominating fraction (93%) with a short lifetime (0.7 ns) and a small fraction (7%) with a longer lifetime (14.8 ns). This result should indicate the presence of two different excited molecular structures of compound 4 in protic media: the small fraction of the excited molecules with a lifetime of 14.8 ns represents most likely non-hydrogen-bonded molecules, the decay of the excited state of which is not affected, and their lifetime corresponds to the respective values measured for compounds 3.4.1 to 3.4.3. The larger fraction with a short lifetime is due to molecules forming intermolecular hydrogen-bonded solute– solvent exciplexes with ethanol molecules. Non-radiative deactivation of the fluorescence excited state is strongly enhanced in these complexes, leading to a short fluorescence lifetime and a low quantum yield. Nevertheless, the formation of the complexes must be fast – faster than the time resolution of the equipment used, which is 400 ps. With freezing of ethanolic solutions, the fluorescence quantum yield increases concomitantly and becomes equal for all four acridanones investigated. In a frozen matrix, the geometrical rearrangements are strongly hindered, which reduces the possibility of intermolecular hydrogen bond formation during the excited-state lifetime of 9-[10-(1-propenyl)]-acridanone. 12.3.5 Synthetic chalcones substituted in the styril fragment The unsubstituted chalcone (compound 3.5.1, Chart 12.5) does not fluoresce in solution at room temperature owing to the fact that its longest wavelength absorption transition lies at around 27 000 cm1 and is of the O B

A

X Compounds 3.5 Number 3.5.1 3.5.2 3.5.3 3.5.4 3.5.5

X H CH3 OCH3 OH N(CH3)2

Chart 12.5 Investigated synthetic chalcones – Table 1. Reprinted with permission from [17]. Copyright 2008 Bulgarian Chemical Communications

284 Hydrogen Bonding and Transfer in the Excited State 0,25 4

Qf

0,20

3

0,15

0,10 2

5

0,05

6 7 8

1

0,00 30

35

40

45

50

55

ET(30)

Figure 12.9 Fluorescent quantum yield Qf of compound 3.5.5 (Chart 12.5) versus the solvent polarity parameter ET(30) – Figure 4. Reprinted with permission from [17]. Copyright 2008 Bulgarian Chemical Communications. Solvents: 1 – benzene; 2 – ethyl acetate; 3 – acetone; 4 – acetonitrile; 5 – n-butanol; 6 – i-butanol; 7 – i-propanol; 8 – ethanol

S0–S1(np ) type. As is well known from the literature, radiation S1(np )–(S0) transitions in carbonyl compounds are forbidden to a great extent [44]. The presence of an electron-donating substituent in the p-position of ring B (compounds 3.5.2, 3.5.3 and 3.5.4) leads to a bathochromic shift of the longest wavelength absorption transition and to the appearance of fluorescence in the visible region of the spectrum. In the case of the chalcone p-dimethylamino-substituted in ring B (compound 3.5.5), a specific interaction with protic solvents in the excited state is observed. The dependencies of the fluorescence transition energy and the fluorescence quantum yield on the ET(30) constants of the solvents are described by two separate linear correlations in protic and in aprotic media (Figures 12.9) owing to the formation of hydrogen bonds in the excited state.

12.4 Conclusion The analysis of the experimental steady-state and dynamic photophysical characteristics of the investigated organic luminophores containing simultaneously carbonyl and amino groups indicate that the character of the fluorescence S1(pp ) state strongly depends on the proton-donating ability of the solvent as a result of the formation of intermolecular hydrogen bonds in the excited state in protic solvents. It is shown that both the nature and the position of the electron-donating substituents in the investigated structures are of decisive importance for the possibility of solute–solvent complexation.

References 1. N. Mataga and T. Kubota, Molecular Interactions and Electronic Spectra. Marcel Dekker, New York, NY (1970). 2. T. Foerster, Fluoreszenz Organisher Verbindungen. Vandenhoeck and Ruprecht, G€ ottingen, Germany (1951). 3. T. Miwa and M. Koizumi, Bull. Chem. Soc. Jpn, 39, 2603 (1967); K. Kaneta and M. Koizumi, Bull. Chem. Soc. Jpn, 40, 2254 (1967).

Intermolecular Hydrogen Bonding in the Fluorescence Excited State 285 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

P. Nikolov and H. Goerner, J. Photochem. Photobiology, A: Chem., 101, 137 (1996). H. Inoue, M. Hida, N. Nakashima and K. Yoshihara, J. Phys. Chem., 86, 3184 (1982). S.-I. Nagaoka, J. Photochem. Photobiol., A: Chem., 40, 185 (1987). Y. Wang, J. Chem. Phys., 89, 3799 (1985). S. Bakalova, Z. Naturforschung, 46a, 823 (1991). D. Noukakis and P. Suppan, J. Luminesc., 47, 285 (1991). T. Tamaki, Bull. Chem. Soc. Jpn, 53, 577 (1980). G. Koehler, S. Bakalova, N. Getoff et al., J. Photochem. Photobiol., A: Chem., 81, 73 (1994). I. Timtcheva, P. Nikolov, N. Stojanov and S. Minchev, J. Photochem. Photobiol., A: Chem., 101, 145 (1996). P. Nikolov, I. Petkova, G. Koehler and S. Stojanov, J. Molec. Struct., 448, 247 (1998). P. Nikolov and I. Timtcheva, J. Photochem. Photobiol., A: Chem., 131, 23 (2000). V. Zelmene and G. Vanag, Izv. Akad. Nauk Latv. SSR, Chim. Ser., 103 (1960). P. Hrnciar, M. Hrnciarova and V. Kovalcik, Ceskosl. Farm., 17, 118 (1968). D. Ivanova, I. Timtcheva, B. Stamboliyska and D. Batovska, Bulgarian Chem. Commun., 40, 440 (2008). S. A. El-Daly, M. Gaber, S. S. Al-Shihry and Y. S. El Sayed, J. Photochem. Photobiol., A: Chem., 195, 89 (2008). S. Ghosh, A. Chakraborty, S. Kar and N. Guchhait, J. Luminesc., 129, 482 (2009). A. Sakar, P. Banerjee, Sk. U. Hossain et al., Spectrochim. Acta A, Mol. Biomol. Spectrosc., 72 (5), 1097–1102 (2009) K. Dimroth and C. Reichard, Ann. Chem., 1, 661 (1963). M. Lacova, Chem. Zvesti, 23, 450 (1969). R. Weiss, J. Jonson and H. Snyder, Organic Syntheses, Coll. Vol. II. John Wiley & Sons, Inc., New York, NY, p. 61 (1943). K. Papadopouls, J. Hadjianestis and J. Nikokavouras, J. Photochem. Photobiol., A: Chem., 75, 91 (1988). USSR Patent, 1977, Reg. No. 578 574. J. Stoclet, T. Chataigneau, M. Ndiaye et al., Eur. J. Pharm., 500, 299 (2004). N. Borisevitch, V. Zelinskii and B. Neporent, Dokl. Akad. Nauk USSR, 94, 37 (1954). P. Nikolov, F. Fratev and S. Minchev, Z. Naturforschung, 38a, 200 (1983). G. Reynolds and K. Drexhage, Optics Commun., 13, 222 (1975). J. Zechner, G. Grabner, G. Koehler et al., J. Photochem., 23, 61 (1983). I. Timtcheva, P. Nikolov, J. Zechner et al., Z. Naturforschung, 43a, 59 (1988). I. Timtcheva, P. Nikolov, J. Zechner et al., Z. Naturforschung, 42a, 490 (1987). I. Timtcheva, P. Nikolov, S. Minchev and N. Sofroniev, Z. Naturforschung, 42a, 289 (1988). G. Koehler and K. Rechthaler, Pure Appl. Chem., 65, 1647 (1993). S. Okajima and E. C. Lim, Chem. Phys. Lett., 70, 283 (1980). K. F. Freed and F. K. Fong (eds), Topics in Applied Physics, Vol. 15. Springer, Berlin, Germany, p. 22 (1970). N. Chapman and J. Shorter (eds), Correlation Analysis in Chemistry. Plenum, New York, NY, p. 495 (1978). J. J. P. Stewart, J. Comput. Chem., 10, 221 (1989). S. Albrecht, H. Brandl and T. Zimmermann, Chemilumineszenz. H€ uthig Verlag, Heidelberg, Germany (1996). B. M. Krasovskii and B. M. Bolotin, Organicheskie Luminofori. Khimiya, Moscow, USSR (1984). D. N. Shigorin, Dokl. Akad. Nauk USSR, 238, 641 (1978). J. J. Aaron and M. Maafi, Spectrochim. Acta, 51a, 603 (1995). M. Siegmund and J. Bendig, Ber. Bunsenges. Phys. Chem., 82, 1061 (1978). B. Valeur, Molecular Fluorescence Principles and Applications. Wiley-VCH Verlag GmbH, Weinheim, Germany, p. 57 (2002).

13 Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds David Lee Phillips Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong

13.1 Introduction There is much interest in developing efficient phototriggers that can be utilized for real-time monitoring of physiological responses in biological systems [1–13]. The p-hydroxyphenacyl (pHP) protecting group has gained particular interest because it has demonstrated practical potential as a very fast and efficient ‘cage’ for the release of various biological effectors [14–19]. The photodeprotection reaction of pHP caged compounds appears to occur only in aqueous or aqueous-containing solutions and does not appear to occur in neat organic solvents such as acetonitrile (MeCN) [14–18, 20, 21]. In aqueous or aqueous-containing solvents, in addition to the photodeprotection reaction, photolysis also leads to a photosolvolytic rearrangement of the pHP cage into a p-hydroxyphenylacetic acid (HPAA) final product [14, 18]. Although the products and conditions for pHP deprotection have been determined in previous work [14–22], the reaction mechanism is not well understood, and there is still some uncertainty as to the events and reactive intermediates involved in the photochemical pathway for the deprotection of pHP phototriggers. In this book chapter we present experimental results from a combination of ultrafast time-resolved spectroscopy and time-resolved resonance Raman spectroscopy on the picosecond and nanosecond timescales, as well as ab initio and density functional theory computational results, better to elucidate the photodeprotection reaction of these important phototriggers. These studies found that explicit hydrogen-bonding effects on the excited states of p-hydroxyphenacyl compounds play a key role in their photochemistry and photodeprotection and rearrangement reactions. The effects of hydrogen bonding on the structure and properties of the excited states of p-hydroxyphenacyl compounds were examined, and their role in the photodeprotection reactions was elucidated. The results presented here enable us to gain a better understanding of the overall mechanistic details of the pHP

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

288

Hydrogen Bonding and Transfer in the Excited State

photochemistry and the role of the hydrogen bonding effects of water molecules in the photodeprotection reactions of this important class of phototrigger compounds. D

O

O

O

D

13

HO

HO

D

C

HO

D HA

HA-13C

HA-D4

13.2 Experimental and Computational Methods The natural abundance of p-hydroxyacetophenone (HA) and HPAA were purchased from Aldrich Chemical Company and utilized after recrystallization. HA-13 C and HA-D4 isotopically labelled compounds (displayed in Scheme 13.1 of Section 13.3) were synthesized and characterized as previously detailed in Ref. [23], and spectroscopic-grade acetonitrile and deionized water were employed as solvents to make samples for the TR3 measurements with pH 9.0 (acetate þ phosphate þ borate, I ¼ 0.1 M) used in the aqueous samples. The HPA, HPDP and HPPP phototriggers were synthesized using the procedures detailed in Refs [24] and [25]. The identity and purity of these compounds were determined from analysis of MS, NMR and UVabsorption spectra obtained for the compounds. Spectroscopic-grade MeCN, CF3CH2OH and DMSO, as well as deionized water, were used as solvents for the time-resolved experiments performed on the phototrigger compounds shown later in the chapter. The femtosecond transient absorption (fs-TA), Kerr gated time-resolved fluorescence (fs-KTRF) and psTR3 experiments were all performed employing a Ti:sapphire regenerative amplifier laser system that also contained a home-made OPA system to give a tunable femtosecond/picosecond light source (the details of this laser system are given in Refs [24] and [25]). An optical delay line was used to control the difference in time between the pump (267 nm) and probe pulses employed in the different femtosecond/picosecond spectroscopy experiments. The time resolutions of the fs-TA and fs-KTRF experiments were approximately 150–200 fs, and 1 mM samples were employed in these experiments. Fs-TA experiments were performed for the compounds of interest in neat MeCN and H2O/MeCN (1:1 by volume) mixed solvents. In some cases the experiments were also performed in solvents DMSO, CF3CH2OH and a mixture of these two solvents, so as to investigate specific

D

O

O

O

D

13

C

HO

HO

D

HO

D HA

HA-D4

HA-13C

Scheme 13.1 Schematic structures of HA, HA-13 C and HA-D4. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds

289

solvent effects. In the ps-TR3 experiments, a 267 nm laser pulse was used to photolyse the samples (of 1–1.5 mM sample concentration), and a probe laser pulse (200, 342 or 400 nm wavelength, depending on the species or process that was under investigation) was employed to obtain the resonance Raman spectra of the species of interest. The time-resolution of the ps-TR3 experiments was about 2–3 ps. Nanosecond time-resolved resonance Raman (ns-TR3) experiments using a 266 nm pump laser pulse and either a 341.5 nm or a 416 nm probe laser pulse were also performed for some of the model and phototrigger compounds reported in this chapter. In all of the ps- and ns-TR3 experiments, the Raman signal was acquired in a backscattering geometry, and its photons were detected by a nitrogen-cooled CCD detector. Each TR3 spectrum presented in this chapter was acquired by first obtaining a pump–probe spectrum and then subtracting appropriately scaled pump only and probe only spectra from it. Details of the experimental equipment and methods employed for the ns- and ps-TR3 experiments are the same as those described in Refs [23] to [28]. The optimized geometry, frequency and Raman activity of the vibrational modes for the ground-state species of interest were computed from B3LYP/6-311G(d, p) density functional theory (DFT) calculations, and open-shell UB3LYP functionals were employed for the triplet-state species investigated here. Analogous calculations were also performed for the corresponding H-bonded complexes, with one, two and three water molecules being H-bonded with the hydroxy hydrogen and lone pairs of the carbonyl oxygen and hydroxy oxygen, respectively, in the species of interest. For all of the H-bond complexes, the H-bond stabilization energy needed for the complexes to dissociate into free molecules and water molecules were determined on the basis of the total energy (including zero point energy, ZPE) computed for the relevant species. Vibrational analysis was done to confirm that all of the stationary points determined were minima, and frequency computations were also performed for the triplet states of HA-13 C and HA-D4 to estimate the isotope effect and to compare with the experimental spectral results. All of the computations were performed using the Gaussian 98 program suite [29].

13.3 Hydrogen-Bonding Effects on the Excited States of Selected Phenacyl Model Compounds In order to learn more about the para-hydroxyphenacyl chromophore and its excited-state properties and interaction with water molecules, we first examined several model compounds of p-hydroxyacetophenone (HA), including several isotopically labelled derivatives HA-13 C and HA-D4 that were synthesized and characterized as described in Ref. [23]. The schematic structures of these compounds are shown in Scheme 13.1 below. Figure 13.1 displays the absorption spectrum of HA in neutral water, which is predominantly due to the neutral form of HA, and also an absorption spectrum of 0.1 M NaOH/water solution, which is predominantly due to the ionized HA anion ground-state species. Figure 13.2 presents representative ps- and ns-TR3 spectra of HA acquired in water solvent, where the ps-TR3 spectra were obtained with 267 nm pump and 400 nm probe excitation wavelengths and the ns-TR3 spectra were taken with 266 nm pump and 416 nm probe wavelengths [26]. The Raman bands in the ps-TR3 spectra are broad because of the convolution of the intrinsic Raman bandwidth with the broad line width of the picosecond laser pulses (15 cm1), and the ns-TR3 spectra have higher resolution because of the smaller line width of the nanosecond laser (1 cm1). Examination of Figure 13.2 shows that the 400 nm or 416 nm probe wavelength TR3 spectra observe two different transient species in water solvent, and this differs from the corresponding TR3 spectra for HA in acetonitrile solution, where only one transient determined to be the triplet state of HA was seen [23]. A femtosecond-time-resolved fluorescence (fs-TRF) investigation found that, for HA and p-HP phototrigger

290

Hydrogen Bonding and Transfer in the Excited State 2.5

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ε (104 M-1cm-1)

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0.5

0.0 220

240

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280 300 320 Wavelength (nm)

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Figure 13.1 UV absorption spectra of HA in water (solid line, lmax ¼ 274.5 nm, « ¼ 13 000 M1 cm1) and in water/NaOH (0.1 M) (dashed line, lmax ¼ 325 nm, « ¼ 22 000 M1 cm1) solution. The 267 nm pump wavelength used in the TR3 experiments is indicated by an arrow in the figure. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

compounds, the intersystem crossing (ISC) rates are extremely fast, with time constants of about 1–2 ps, with the conversion rates in water and water-mixed acetonitrile being similar to those measured in neat acetonitrile [24]. Based on this, the first species seen in Figure 13.2 (0–1000 ps spectra) is assigned to the HA triplet state. It is interesting to note that a comparison of the triplet spectra here in water solution in Figure 13.2 with those reported in acetonitrile solvent [23] shows some distinct differences in the frequencies of many of the vibrational features as well as spectral profiles of some Raman bands that appear to be indicative of hydrogenbonding effects of the water solvent molecules on the triplet excited state. To distinguish the H-bonded triplet species from the normal free HA triplet species (denoted as 3HA hereafter), the H-bonded HA triplet species is denoted as 3 HA0 . Similar to previous Raman spectroscopic measurements for the para-methoxyacetophenone (MAP) ground-state species [27], the ground-state HA molecule is H-bonded with the solvent water molecules at both the hydroxy and carbonyl moieties in water solution, and the free and H-bonded ground-state HA species are denoted as HA and HA0 respectively. The H-bonding interaction between the carbonyl group and the solvent water molecule(s) results in a substantially broader C¼O stretching Raman bandwidth in the Raman spectra observed in water compared with spectra acquired in acetonitrile solvent. The first few picosecond spectra in Figure 13.2 indicate a very rapid growth of the H-bonded 3 HA0 triplet species. A previous TR3 study on HA found that the free 3 HA triplet has a lifetime of 40 ns in acetonitrile, and the reader is referred to Ref. [23] for details of the vibrational assignments of the 3 HA TR3 spectra. Examination of Figure 13.2 shows that the 3 HA0 species decays within 10 ns in water and directly converts into another species with new Raman bands at, for example, 1167, 1357, 1481 and 1599 cm1 (the features in red as shown in Figure 13.2). The time constant for the transformation of the 3 HA0 triplet to the new species can be crudely estimated to be 10 ns owing to the limited time resolution of the present TR3 experiment, and the decay of this new transient species has a time constant of 95 ns under open-air conditions. Nanosecond-TR3 measurements were also performed with purging of the sample solution with nitrogen, and this caused the decay of the new species to be noticeably slower, with a 140 ns decay time constant determined in these experiments, which indicates that the new species likely has a triplet character [26].

Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds

291

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Figure 13.2 Picosecond-TR3 (a) and nanosecond-TR3 (b) spectra of HA in water solution obtained at various time delays with 267 nm pump and 400 nm probe wavelengths for the picosecond spectra and 266 nm pump and 416 nm probe wavelengths for the nanosecond spectra. The band labelled by # is due to a stray laser line. Reprinted with permission from [26]. Copyright 2005 American Chemical Society (See Plate 11)

Analogous TR3 experiments [26] were acquired in a basic water solution (pH ¼ 9.0 buffer), and these spectra are shown in Figure 13.3. The ns-TR3 spectra (Figure 13.4(b)) show only the new triplet species, and comparison with analogous spectra in neutral water solvent (Figure 13.2(b)) indicates that the new triplet species is produced faster in basic solution than under neutral conditions consistent with the ps-TR3 spectra shown in Figure 13.4(a). One could expect that deprotonation of the 3 HA0 species may be quicker in basic solution than in a near neutral solution, and thus the new triplet species is tentatively attributed to the triplet anion of HA formed from the 3 HA0 deprotonation (or ionization) reaction. This attribution is consistent with previous nanosecond laser flash photolysis (LFP) results reported by Givens et al., who found that the TA spectra of HA triplet and HA triplet anion are very similar to one another [20]. It appears that the direct

292

Hydrogen Bonding and Transfer in the Excited State (b)

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Figure 13.3 Picosecond-TR (a) and nanosecond-TR3 (b) spectra of HA in buffered water solution with pH ¼ 9.0, obtained at various time delays with 267 nm pump and 400 nm probe excitation wavelengths for the picosecond spectra and 266 nm pump and 416 nm probe wavelengths for the nanosecond spectra. The band labelled by # is due to a stray laser line. Reprinted with permission from [26]. Copyright 2005 American Chemical Society (See Plate 12) 3

excitation of the ground-state anion to form the anion triplet may be excluded because the ground-state anion exhibits only a very small degree of absorption at the pump (267 nm) and probe (400 or 416 nm) wavelengths employed in the TR3 measurements, as indicated by the ultraviolet absorption spectrum of HA in NaOH/water solution presented in Figure 13.1. Since the TR3 spectra in Figures 13.2 and 13.3 indicate that the HA triplet anion is produced from the H-bonded triplet complex 3 HA0 , it is reasonable that the triplet anion also has the form of an H-bonded complex.

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UB3LYP/6-311G(d, p) density functional theory (DFT) calculations were performed to determine the optimized geometry, vibrational frequencies and predicted Raman spectrum for free HA triplet anion (designated as 3 HA hereafter) and anion triplet complexes (3 HA0  ) with the water molecule(s) H-bonded to the oxygen lone pairs of the carbonyl and deprotonated hydroxy groups [26]. Figure 13.4 presents a simple schematic of the calculated optimized geometry of the free HA triplet anion and the anion triplet H-bonded with two water molecules (designated as 3 HA0 –2H2O hereafter), with the atoms numbered, and selected structural parameters for the 3 HA and 3 HA0  –2H2O species are listed in Table 1S in the supporting information of Ref. [26]. The optimized structures of the anion triplet complex H-bonded with one water molecule at the carbonyl and hydroxy oxygen atoms, respectively, are depicted in Figure 5S and the corresponding structural data are listed in Table 2S in the supporting information of Ref. [26]. The triplet anion has a planar conformation, with the H-bonding perturbations causing mainly local structural changes near the H-bonding  sites. For example, the H-bonds result in only a very modest increase in the bond lengths (by less than 0.01 A) for the C1O1 and C7O2 bonds that have direct H-bond interactions, and there is very little change in the other bond lengths and bond angles [26]. The computed Raman spectra for the free triplet anion and the anion triplet complexes studied show some similarities with each other, and this is consistent with the structural similarity for the free triplet anion and the H-bonded triplet anion complexes. The DFT computations estimate the stabilization energy to be 14.2 and 12.5 kcal mol1 for the H-bond interaction with the oxygen of the

Figure 13.4 Optimized geometry of the free HA triplet anion and the anion triplet complex containing two water molecules H-bonded with the HA oxygen lone pairs of the carbonyl and deprotonated hydroxy moieties. The structure was obtained from DFT calculations using the UB3LYP method with a 6-311G(d, p) basis set. Bond lengths  (in A) are labelled for the CC, CO, and H-bond-associated bonds. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

294

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351

#

1590

1461

carbonyl and hydroxy moieties respectively, and these H-bonding energies are larger than the analogous energies for the neutral HA triplet H-bonded complexes, which are 6.8 and 4.9 kcal mol1 respectively [26]. The triplet anion appears to be in, or very near to, the form of an H-bonded complex with a structure similar to that of the 3 HA0  –2H2O displayed in Figure 13.4. Figure 13.5 compares the experimental TR3 spectrum acquired at 40 ns in Figure 13.2(b) with the computed normal Raman spectrum of the 3 HA0  –2H2O triplet anion complex, where a Lorentzian function with a 10 cm1 bandwidth was employed to produce the calculated Raman spectrum from the vibrational frequencies and relative Raman intensities from the UB3LYP/6-311G(d, p) computation [26]. Nanosecond-TR3 spectra of isotopically substituted HA-13 C and HA-D4 acquired under the same experimental conditions as the spectra displayed in Figure 13.2(b) are also shown in Figure 13.5 to assist the attribution of the vibrational bands [26]. Examination of Figure 13.5 shows that the computed normal Raman spectrum is in reasonable agreement with the experimental TR3 spectrum for the species produced from the decay of the HA triplet state. This

804

1602

1473

#

1167

1599

1357

1481

(c)

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1355

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Figure 13.5 Comparison of the experimental ns-TR3 spectrum (b) of the HA triplet anion obtained at 40 ns displayed in Figure 13.2(b) with the UB3LYP/6-311G(d, p) DFT-calculated normal Raman spectrum (a) of the HA anion triplet complex. The # in the experimental spectrum marks a stray laser line. Nanosecond-TR3 spectra of the HA triplet anion obtained for the isotopically substituted HA-13 C (c) and HA-D4 (d) with 267 nm pump and 416 nm probe wavelengths are also shown to help with the assignment of the experimental spectrum. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

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295

comparison indicates that the second transient species in Figure 13.2 can be attributed to the triplet anion species, and that the TR3 spectra directly observe the deprotonation reaction of the HA triplet (in the H-bonded form) to form the HA triplet anion. The experimental (Figure 13.5(b)) and computed (Figure 13.5(a)) spectra for the HA anion triplet exhibit some differences in their relative Raman intensity pattern, as the experimental spectrum is resonantly enhanced while the computed spectrum is a non-resonant Raman spectrum. Tentative Raman band assignments of the experimental bands were determined on the basis of the isotopic shifts in frequencies and the direct correlation between the experimental and computed spectra displayed in Figure 13.5, and Table 1 in Ref. [26] lists these vibrational assignments with nominal descriptions of their normal modes. Table 1 in Ref. [26] also lists the isotopic shifts seen in the spectra of HA-13 C (Figure 13.5(c)) and HA-D4 (Figure 13.5(d)), along with the computed frequencies for the triplet anion of HA-13 C. The vibrational assignments and the computed frequencies for HA-D4 are listed separately in Table 3S in the supporting information of Ref. [26] because of the obvious differences in the vibrational constituents for many vibrational modes and the different order of the nominal modes. The TR3 spectra of the triplet anion have Raman bands predominantly due to vibrations affiliated with the ring CC stretching and CH bending motions. Examples include the 1599, 1481, 808 and 363 cm1 bands, which are mainly due to the various ring CC vibrations, and the bands at 1371 and 1167 cm1 are mostly due to the ring CH in-plane bending motions; these attributions are consistent with the isotopic shift observations in the spectra from HA-13 C and HA-D4 (Figure 5(c) and (d)). The 1481 cm1 band in the HA spectrum (1473 cm1 in the spectrum from HA-13 C) exhibits an obvious isotopic shift upon carbonyl 13 C substitution, and this agrees with the computed results, which indicate that the vibration for this feature has a significant component from the carbonyl stretching motion [26]. Ring deuteration causes the ringrelated vibrational bands, such as the 1599, 808 and 365 cm1 features in the normal HA spectrum, to shift down to 1590, 771 and 351 cm1, respectively, in the HA-D4 spectrum, and the degrees of these shifts are diagnostic for the attributed ring CC stretching vibrations [23, 30–33]. The absence of 1167 cm1 bands in the ring-deuterated spectrum coincides with their ring CH in-plane bending assignment because the ring CH in-plane bending vibrations are generally seen to shift down by 300 cm1 upon ring deuteration [30–33], and hence the moderate feature at 842 cm1 in the HA-D4 spectrum may correspond to the 1167 cm1 band in the normal HA spectrum. The different resonance enhancement pattern between the spectrum from HA-D4 and the spectrum from the normal HA and HA-13 C molecules may result from changes in the vibrational coupling pattern and redistribution of potential energy of the relevant modes caused by the ring deuteration [23, 30–33]. Nanosecond-TR3 spectra shown in Figure 13.6 were acquired after 266 nm photoexcitation of HA in water with a different probe wavelength (341.5 nm), and these spectra saw the growth and decay of another species. The time dependence of the Raman bands shown in Figure 13.6 indicates that there are two groups of bands that have noticeably different kinetics that can be attributed to two different transient species. One species has distinct Raman bands at 705, 843, 1074, 1527, 1567 and 1617 cm1, which are shown in red in Figure 13.6 and appear to have a maximum intensity near 120 ns and then decay over the next 600 ns or so [26]. The 705 cm1 Raman band integrated area was employed to find the dynamics of this transient species that can be fitted well by a two-exponential function consisting of a growth constituent with a time constant of around 90 ns and a decay constituent with a time constant of approximately 200 ns [26]. The 341.5 nm probe TR3 experimental results are consistent with results from a previous transient absorption investigation by Givens et al. [20], which indicated that ultraviolet photolysis of HA in neutral water solution formed an intermediate with lmax ¼ 325 nm absorption band, which is close to the 341.5 nm probe wavelength for the TR3 spectra shown in Figure 13.6. This species yield and lifetime are not influenced by oxygen, and the absorption spectrum is essentially identical to that of the HA ground-state anion in aqueous base, which indicates that the new intermediate seen in the 341.5 nm TR3 spectra is the anion ground-state species [26]. A 341.5 nm resonance Raman spectrum of the HA anion in a water/NaOH (0.1 M) basic solution

1527 1567 1617

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Hydrogen Bonding and Transfer in the Excited State

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296

700ns 500ns

Intensity (arb. units)

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150ns 100ns 60ns 40ns 20ns 10ns 0ns

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Raman Shift (cm )

Figure 13.6 Nanosecond-TR3 spectra of HA in water solution obtained at various time delays with 267 nm pump and 341.5 nm probe wavelengths. See the text for details of the attributions of other features. Reprinted with permission from [26]. Copyright 2005 American Chemical Society (See Plate 13)

was acquired to confirm this attribution, and Figure 13.7 presents a comparison of this spectrum with that of the 100 ns transient 341.5 nm TR3 spectrum observed in the neutral water solution shown in Figure 13.6 [26]. Comparison of the spectra shows they are essentially identical to each other, and this confirms that this intermediate is the HA ground-state anion. The unambiguous assignment of the new species to the HA ground-state anion in conjunction with comparison of the anion ground-state kinetics with that for the anion triplet state seen for the same sample and corresponding 416 nm probe wavelength TR3 spectra in Figure 13.3 indicate that the growth of the anion ground state interrogated with the 341.5 nm probe wavelength correlates well with the decay of the anion triplet state seen with the 416 nm probe wavelength. This demonstrates that the anion ground state is formed directly from the anion triplet state, and the 90 ns time constant is due to the ISC conversion of the anion triplet state to the anion ground state [26]. DFT B3LYP/6-311G(d,p) calculations were done to estimate the structure of the anion ground state and the influence of the H-bond interactions on its structure and properties. Similarly to the DFT computations for the H-bonded complex of the HA anion triplet, the H-bonded ground-state computations were also performed with the solvent water molecules H-bonded with the oxygen lone pairs of the carbonyl and deprotonated hydroxy groups [26]. Like the computed structures for the anion triplet complexes, a planar geometry was predicted for the anion ground state, and the H-bonding conformations in the ground state were similar to those for the analogous

Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds 1567

(a)

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Intensity (arb. units)

(b)

(c)

400

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Figure 13.7 Comparison of the ns-TR3 spectrum (a) obtained at a 100 ns time delay in Figure 13.7 with the 341.5 nm resonance Raman spectrum of the ground-state HA anion (b) obtained in water/NaOH (0.1 M) solution. B3LYP/6-311G(d, p) DFT-calculated normal Raman spectrum of the HA anion ground state (c) is also shown to compare and help with the assignment of the experimental spectra. Features in black shown in spectrum (a) are due to the unidentified species ‘X’ (see the text for details). Reprinted with permission from [26]. Copyright 2005 American Chemical Society (See Plate 14)

triplet state, with the H-bonding leading to only modest and localized perturbation of the anion structure in the ground state. The computed structural parameters for the free HA anion ground state (designated as HA hereafter) and the anion complex with two H-bonded water molecules (designated as HA0 –2H2O hereafter) and also the total energies and the H-bond stabilization energy are given in Table 4S of the supporting information of Ref. [26], and the analogous data for the ground-state complexes involving one water molecule are listed in Table 5S of the supporting information of Ref. [26]. From these tables (e.g. Table 5S), the stabilization energies are 12.1 and 15.1 kcal mol1, respectively, for the H-bonding at the carbonyl and deprotonated hydroxy sites [26]. Comparison with the analogous triplet state results shows that the H-bond interaction is modestly weakened for the carbonyl site but becomes stronger for the deprotonated hydroxy oxygen. The HA anion ground state likely exists in the form of an H-bonded complex. Accompanying the structural similarities, the computations lead to similar Raman spectra for the free and H-bonded anion ground states, and a comparison of the computed spectrum for the HA0 –2H2O species with the experimental resonance Raman spectra acquired for the groundstate HA anion is presented in Figure 13.7; the reasonable agreement between the computed and experimental

298 Hydrogen Bonding and Transfer in the Excited State

spectra suggests a straightforward assignment of the vibrational bands seen in the experimental spectra, and Table 2 of Ref. [26] presents the experimental and computed frequencies and their assignments. Examination of Figure 13.6 indicates that, in addition to the Raman bands from the HA ground-state anion species, there are residual Raman bands at 810, 945, 965, 1370 and 1455 cm1 due to another intermediate that has a single-exponential decay kinetics with a lifetime of 88  7 ns [26]. Although the formation and decay times of this intermediate are similar to those of the HA anion triplet state seen in the 416 nm probe wavelength ns-TR3 spectra (Figures 2(b) and 3), this assignment can be discounted by the results from TR3 experiments performed for HA in buffered acid water solution employing a 266 nm pump and 341.5 or 416 nm probe wavelengths, which show a total absence of the Raman bands from the anion species (both the triplet and ground state) and the exclusive appearance of the above-mentioned residual Raman bands seen in the 341.5 nm probe spectra. This indicates that the intermediate is directly produced from the HA triplet state. A representative ns-TR3 spectrum acquired for HA in pH ¼ 1 buffered water solution with the 341.5 nm probe wavelength is shown in Figure 7S in the supporting information of Ref. [26] and illustrates the sole observation of this transient species in the acidic environment. This is in contrast to the spectra in neutral water, which observe both the HA ground-state anion and this intermediate, which indicates that the new intermediate is preferentially formed by excess protons in the water solution. A TA study by Givens et al. found a protonated species at low pH values that exhibited an absorption band with a maxima around 350 nm [20], and, because the 341.5 nm probe wavelength is resonant with this absorption, it is likely that the Raman bands (810, 945, 965, 1370 and 1455 cm1) seen in Figure 13.6 are from the same species as those detected in the TA investigation. This intermediate has been proposed to be due to a quinonoid enol triplet of HA [20–22] and the cationic species (most possibly in its triplet) produced by protonation of the carbonyl oxygen of the neutral HA triplet [21, 22]. Both of these assignments are based on the increased basicity and acidity of the carbonyl oxygen and phenolic hydrogen, respectively, in the triplet relative to that of the ground state. A preliminary comparison of the DFT-computed Raman spectra with the experimental spectrum suggests that neither of these two species seems to give a reasonable correlation with the experimental spectra. Further experimental and theoretical study is needed better to identify this species, and it is provisionally designated as ‘X’ hereafter. The work presented so far indicates that both the triplet and ground states of the HA anion species have planar structures (Cs symmetry) with the hydroxy and carbonyl groups lying in the plane defined by the phenyl ring, and they also both exist as H-bonded complexes of some form with two solvent water molecules Hbonded, respectively, to the oxygen atom of the carbonyl and deprotonated hydroxyl moieties. The geometries predicted from the DFT calculations of the H-bonded complexes of these anion species are typically very similar to those of their free counterparts, and this is noticeably different from the case of the neutral HA triplet state, where previous TR3 and DFT work on HA in neat acetonitrile showed that the free HA triplet 3 HA (3 pp* nature) is non-planar (C1 symmetry), with the C(O)CH3 group being twisted out of the molecular plane by about 16 and the CH3 group rotated around the C(O)C bond by about 37 [23]; this deviation of the aromatic carbonyl pp triplet from a planar structure has been linked with strong vibrational coupling between the pp triplet and a nearby np triplet [34–36]. In contrast to the case of HA species in aceotonitrile, UB3LYP/6-311G (d, p) DFT computations of the H-bonded complex of the neutral HA triplet (3 HA0 ) containing up to three water molecules H-bonded, respectively, with the hydroxy hydrogen and oxygen lone pairs of the carbonyl and hydroxy moieties lead to a planar structure and a significantly increased quinoidal nature of the phenyl ring compared with that of the free 3 HA in acetonitrile [23, 26]. The optimized geometry of the 3 HA0 –3H2O complex is presented in Figure 13.8, while the structures of the 3 HA0 complexes with one water molecule at different sites are available in Figure 8S of the supporting information of Ref. [26]. Similar DFT computations were also performed for the corresponding ground-state HA–water complexes, and the H-bond effects were found to be comparable with those of the HA triplet complex [23, 26]. Similarly to the examples of the HA anion triplet and the ground state, the H-bond interactions result in only very modest and localized structural changes to the HA ground state but lead to significant changes to the HA

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Figure 13.8 Optimized geometry of the H-bonded HA triplet complex containing three water molecules, obtained  from DFT calculations using the UB3LYP method with a 6-311G(d, p) basis set. Bond lengths (in A) are labelled for the CC, CO, and H-bond-associated bonds. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

triplet state. Table 6S of Ref. [26] gives the computed geometry data and energies for the free 3 HA and the 3 HA0 –3H2O, and the corresponding results for the HA triplet complexes containing one water molecule and those for the HA ground-state H-bonded complexes are available in Tables 7S and 8S, respectively, in the supporting information of Ref. [26]. The H-bond topologies in the neutral triplet complexes are like those of their corresponding ground-state species, and the H-bond strengths of the carbonyl oxygen (6.8 kcal mol1), hydroxyl oxygen (4.9 kcal mol1) and hydroxyl hydrogen (9.9 kcal mol1) of the triplet complexes are larger than those of the corresponding ground-state species (6.7, 4.5 and 9.2 kcal mol1 respectively) [26]. The increased strength of the H-bond at the carbonyl oxygen (as an H-bond acceptor) and the hydroxy hydrogen (as an H-bond donor) is consistent with the carbonyl oxygen of the aromatic carbonyl compound becoming more basic in the pp triplet than in the ground state and the hydroxyl hydrogen becoming more acidic in the excited state than in the ground state. Based on the values of the H-bond stabilization energy, the carbonyl oxygen and hydroxy hydrogen triplet complexes appear more stable than the hydroxy oxygen complex, and this is consistent with the degree of structural perturbation due to the respective H-bond interactions. The computed geometries indicate that the carbonyl oxygen and hydroxy hydrogen H-bonds result in a similar quinoidal planar structure that is significantly different from that of the 3 HA free triplet. It is interesting that the hydroxy oxygen H-bond results in only a small change, and the analogous complex has a twisted structure like that of the 3 HA free triplet. The computed Raman spectra for the H-bonded triplets containing the carbonyl oxygen and hydroxy hydrogen H-bonds consistently exhibit significant differences from that of the free HA triplet [23, 26]. However, the spectrum of the complex involving only the hydroxy oxygen H-bond is close to that of the free triplet spectrum, and this suggests the importance of the carbonyl and hydroxy hydrogen H-bond interactions in describing the structural perturbations of the H-bonding that are mostly responsible for the spectral differences seen experimentally in the solutions of water compared with spectra acquired in acetonitrile, and the good agreement between the computed Raman spectrum of the 3 HA0 –3H2O complex with the HA triplet TR3 spectrum obtained in water (e.g. the 50 ps spectrum in Figure 13.2) in Figure 13.9 appears to confirm this. Table 3 of Ref. [26] gives the vibrational frequencies and assignments for the spectra in Figure 13.9. A characteristic and noteworthy change in the TR3 spectra is the frequency upshift (by 25 cm1) of the ring centre CC stretching vibration as the solvent changes from acetonitrile (at 1594 cm1) [23] to that in water solution (at 1619 cm1), and this is diagnostic of the increased quinoidal ring conformation in the H-bonded

Hydrogen Bonding and Transfer in the Excited State

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Figure 13.9 Comparison of the ps-TR3 spectrum (a) obtained at a 50 ps time delay in Figure 13.2(a) with the UB3LYP/6-311G(d, p) DFT-calculated normal Raman spectrum of the HA H-bonded triplet complex containing three water molecules. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

triplet complex compared with that in the free triplet. The computed structural data from Table 6S of Ref. [26]  show that the H-bonded carbonyl CO bond length is smaller (by 0.05 A) in the H-bonded triplet complex than in the free triplet, and this is significant because the H-bond typically results in an increase in the bond length for the H-bonded acceptor group [37–42] such as for the H-bonded complexes of the HA ground state and anion species (see Table 8S in the supporting information of Ref. [26]). The specific difference in the carbonyl bond length between the H-bonded and free HA triplet is diagnostic of the different electronic state property for these two types of triplet state, where the free HA has a delocalized pp triplet, with the p electron populated on both the ring and carbonyl moieties [23], and the H-bonded HA complexes have a ring-localized biradical pp triplet. As there are significant differences in the structural and electronic properties of these species, the H-bonded HA triplet complexes can be looked upon as a distinct triplet species from that of the free HA triplet. This description is similar to what we found for the closely related H-bond effect for the triplet state of p-methoxyacetophenone (MAP) reported in Ref. [27]. The very similar H-bond effects found for HA and MAP indicate that there is a similar electronic effect of the para-substituted group hydroxy and methoxy to the phenacyl chromophore. For the MAP triplet, the H-bonding interaction results not only in a geometry change but also in a significantly longer triplet lifetime than that of the free triplet [27]. In contrast to MAP, the presence of a hydroxy group that can ionize/deprotonate in the HA appears responsible for the shorter triplet lifetime in water (10 ns) than in acetonitrile (40 ns) solution [23]. This also suggests that the HA triplet deprotonation reaction is correlated with the hydroxy-hydrogen-sited intermolecular H-bonding interaction. The importance of the H-bonding interaction on the anions of the HA triplet and ground state is also shown by the greater H-bonding stabilization energies computed for these anions than those for their corresponding neutral species. Because the leaving group has little effect on the triplet property of the pHP cage [23, 43], the H-bonded effect on the triplet structure presented here for HP and MAP is also expected to be applicable generally to the pHP phototrigger compounds. The triplet that appears to be the precursor for the photodeprotection reaction of the pHP phototrigger compound is an H-bonded triplet complex with a planar and ring-localized biradical structure. It is likely that the water solvent influence on assisting heterolytic cleavage and the nucleofuge

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character of the leaving group have an important role in driving the deprotection reaction for pHP phototrigger compounds. The TR3 spectra acquired with 400 nm (Figure 13.2) and 341.5 nm (Figure 13.6) probe wavelengths after ultraviolet photolysis of HA in neutral water solutions show that the HA triplet is produced with a 2 ps time constant by ISC from the S1 excited state and subsequently decays with a 10 ns time constant to give the simultaneous formation of the HA triplet anion by the deprotonation reaction and an unidentified X species by an unknown protonation-related process, and then these species decay with time constants of about 95 ns and 88 ns respectively. The decay of the triplet anion species results in the production of an anion ground-state intermediate that has a lifetime of 200 ns. Given that the ground-state HA has a pKa of 7.9 [20], it is thought that, in the neutral water solution, the HA anion ground-state molecules are further protonated by the solvent water molecules to go back to the HA neutral species. It appears that all of the relevant species exist in the form of H-bonded complexes. The hydroxy-hydrogen-sited H-bond interaction helps promote the triplet deprotonation reaction. The X species probably decays back to the ground state of the neutral HA because UV absorption experiments of the HA water solution immediately after the TR3 experiment show no noticeable sample degradation. The following scheme for reaction pathways observed after ultraviolet photolysis of HA in neutral water solution was developed on the basis of the results described above. The species that are in the form of H-bonded complexes have apostrophe labels in this scheme. It is noteworthy that the HA triplet deactivation resulting in the deprotonation and parallel production of the X species in neutral water solution is fairly slow (10 ns) in comparison with the decay of the analogous triplets of the p-hydroxyphenacyl acetate (HPA) and p-hydroxyphenacyl diphosphate (HPDP) phototrigger compounds, which have time constants of 2130 and 420 ps, respectively, in 50% acetonitrile/50% H2O solution [24]. The rate of HA triplet deprotonation and relevant deactivation pathways is dependent on the water concentration and is thus slower in the 50% MeCN/50% H2O mixed solvent than in neat water solution [20]. This and the likelihood that the triplets of HPA and HPDP have similar reaction times to the HA triplet for their triplet deprotonation reaction indicate that some other competing process, most likely involving the deprotection reaction, causes the observed leaving-group-dependent decay of the triplets of HPA and HPDP in water-containing solvents. The TR3 data here for the HA model compound in conjunction with previous results for the HPA and HPDP phototriggers give no evidence for the deprotonation reaction being associated with the leaving-group deprotection reaction. The spectra in Figure 13.2 show that the HA triplet anion can be observed selectively with the 400 nm probe wavelength, but TR3 spectra for the HPA and HPDP phototrigger compounds with the same probe wavelength [24] show no observation of vibrational bands due to the triplet anion intermediate at

~88 ns

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HA’(S0)

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HA’ (S3, ππ*)

1

267 nm

IC 1 HA’ (S1, nπ*) ~80 fs

Protonation (+ H+) ~200 ns

ISC

HA’ (T 1, ππ*)

3

~2 ps

Deprotonation (-H+) ~10 ns HA’- (S0)

ISC ~90 ns

HA-’ (T1, ππ*)

3

Scheme 13.2 Proposed reaction pathways for ultraviolet excitation of HA0 in aqueous solutions, based on the experimental and theoretical results presented in this chapter. Reprinted with permission from [26]. Copyright 2005 American Chemical Society

302 Hydrogen Bonding and Transfer in the Excited State

all, and only Raman bands due to the neutral triplets that exhibit single-exponential decay kinetics were observed in the 50% MeCN/50% H2O solvent for these two phototrigger compounds. A transient absorption investigation of HPDP in the 50% MeCN/50% H2O solvent also observed the same single-exponential decay of the HPDP triplet absorption with lmax at 400 nm [20]. However, an additional transient absorption band with a maximum at 330 nm with a longer nano- to microsecond lifetime (likely due to the anion ground state and the X species) has been observed for photolysis of HPA in 50% MeCN/50% H2O solvent [21]. As the pHP photorelease reaction appears to take place exclusively in a water-containing environment, the H-bonding effects examined here indicate that the precursor triplet species to the leaving-group deprotection reaction is the H-bonded triplet complex with a planar structure instead of the twisted free triplet observed for these compounds in acetonitrile solvent in analogous studies.

13.4 Hydrogen-Bonding Effects on the Excited States of Selected Para-Hydroxyphenacyl Ester Phototriggers and the Role of Water in the Deprotection and Subsequent Reactions Figure 13.10(a) presents a three-dimensional topology of the KTRF spectra for HPA acquired with 267 nm photolysis in acetonitrile (MeCN) with a time delay of up to 4 ps after photolysis. The fluorescence has a strong emission band with a maximum near 340 nm that is seen at very early times and that decays within 0.5 ps to generate a weaker emission band that has a maximum 420 nm and has a lifetime of a few picoseconds. The fluorescence decay kinetics at 330 and 440 nm and the instrument response function (IRF) are shown in Figure 13.10(b). Both of the decay kinetics can be fitted well by convolution with the IRF and a sum of two exponential components with time constants of 0.08 and 1.78 ps respectively but employing different weighting factors for the two components [24]. The decay kinetics at other wavelengths can also be simulated in the same way, and the spectra at 0.2 and 0.9 ps (shown in Figure 13.10(c)) were chosen to be representative of the net fluorescence spectrum for the short lifetime blue and longer lifetime red emissive excited states [24]. The pHP part of the phototrigger is the chromophore excited with the 267 nm photolysis of these compounds, which results in direct population of the strongly allowed pp (La type) S3 state associated with the lowest strong absorption band displayed in Figure 13.10(c). Previous work on aromatic carbonyl compounds has demonstrated that the energy levels of the excited pp and np states are influenced noticeably, and increasing the H-bonding strength or polarity of the solvents in sample solutions results in stabilization of the pp states and destabilization of the np states at the same time [38, 44–55]. These solvent-dependent energy level perturbations are responsible for the diagnostic blue-shift of the np and redshift of the pp spectral bands in solution phase fluorescence spectra [44–50, 55, 56]. The KTRF spectra acquired in solvents of varying polarity and H-bonding ability were used to determine the character of the fluorescence seen in the experiments. Examination of Figure 13.10(c) to (f) reveals that, as the solvent employed goes from MeCN to MeOH to H2O/MeCN mixed solvent, the blue fluorescence moves to the red and the red fluorescence moves to the blue, which indicates that there are two distinct fluorescence components with one component associated with a pp character state emitting blue fluorescence and the other component associated with the np state emitting red fluorescence [24]. The spectra displayed in Figure 13.10 were the first ultrafast time-resolved fluorescence spectra obtained for pHP phototrigger compounds and also for related aromatic carbonyl compounds. Figure 13.11(A) presents a comparison between the ps-TR3 spectra acquired in 50% H2O/50% MeCN (v:v) mixed solvent with spectra acquired in neat MeCN for the HPDP phototrigger; a 400 nm probe wavelength [24], which is close to the maximum position of the T1 ! Tn absorption band, was employed to obtain all of the spectra shown [21]. The close resemblance between the spectra in the two kinds of solvent suggests that the spectra in the H2O/MeCN mixed solvent can be assigned to the HPDP 3 pp* state.

Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds

303

Intensity (a.u.)

4

Time Delay (ps)

(b)

100 70 49 35 24 17 12 8 6 4 3

(a)

3

2

1

0

(c)

*

#

Intensity (a.u.)

(d)

*

#

(e)

* #

*

(f)

# 200

250

300

350

400

450

500

550

Wavelength (nm)

Figure 13.10 (a) Femtosecond-KTRF contour of HPA obtained with 267 nm excitation in MeCN. (b) Normalized fluorescence decay at 330 nm and 440 nm for HPA in MeCN. The solid lines show two exponential fittings to the experimental data; the dotted line is the instrumental response function (see Ref. [24] for details). (c) to (f) steadystate absorption spectrum (in black) and typical fluorescence profile of the blue and red fluorescence for HPA in MeCN (c), THF (d), MeOH (e) and 90% H2O/10% MeCN mixed solvent (f). The absorption spectra were normalized to the blue fluorescence spectra. Sharp features indicated by # and  are the solvent Raman band and the second harmonic generation of the 800 nm gating pulse from the Kerr medium respectively. Reprinted with permission from [24]. Copyright 2005 American Chemical Society (See Plate 15)

Figure 13.11(B)(a) and (b) exhibit kinetic comparisons for the 3 pp* states in neat MeCN and H2O/MeCN mixed solvent for HPDP and HPA respectively, and these spectra indicate that the early-time 3 pp* evolution is similar for both the compounds in the two solvents with 7–12 ps time constants for the triplet formation [24]. However, the 3 pp* lifetime is substantially shorter in the H2O/MeCN mixed solvent than

3 ns 1 ns 500 ps 200 ps

Intensity (a.u.)

100 ps

Intensity (a.u.)

Hydrogen Bonding and Transfer in the Excited State

(a)

Intensity (a.u.)

304

(b)

50 ps 10 ps 6 ps 4 ps 3 ps 2 ps 1 ps 0 ps

500

1000

1500

1000

Raman Shift (cm-1)

1500

2000

0

1

2

3

4

5

6

Time Delay (ns)

Figure 13.11 (A) Picosecond Kerr gated time-resolved resonance Raman spectra of HPDP obtained with 267 nm pump and 400 nm probe wavelengths in 50% H2O/50% MeCN mixed solvent (left) and neat MeCN (right). (B) Time dependence of the triplet 1600 cm1 band areas for HPDP (a) and HPA in 50% H2O/50% MeCN mixed solvent (circles) and neat MeCN (squares), obtained in 400 nm probe ps-KTR3 spectra. Solid lines show an exponential fitting of the experimental data. Reprinted with permission from [24]. Copyright 2005 American Chemical Society

in the neat MeCN solvent, and this solvent- and leaving-group-dependent quenching of the triplet suggests that subsequent reaction(s) occur in the H2O-containing solvent [24, 25]. Figure 13.12(a) displays early-time ps-KTR3 spectra in the 1450–1800 cm1 spectral range for HPDP in 50% H2O/50% MeCN mixed solvent, acquired with 400 nm and 342 nm probe wavelengths after 267 nm photolysis. The Raman feature at 1610 cm1, growing in at 2 ps and later at both probe wavelengths, is due to the same 3 pp* triplet also observed in spectra acquired in neat MeCN solvent in Figure 13.11 [24]. Figure 13.12 has an additional Raman feature at about 1550 cm1 that is only observed at very early times up to 4 ps, and this band decays quickly to generate the 3 pp* triplet state; the absence of the early-time 1550 cm1 Raman feature in the 400 nm probe ps-TR3 spectra indicates that this feature is due to another species that is not the 3 pp* triplet state. It is important to note that the 1550 cm1 Raman feature decay tracks the decay of the red fluorescence of the 1 np* S1 state observed in the preceding KTRF spectra. This indicates that the new earlytime 1550 cm1 Raman feature can be assigned to the 1 np* S1 state. The spectra here clearly demonstrate that the ISC takes place from the 1 np* S1 state even with S3 excitation and indicate that the main decay pathway of the photopopulated S3 state is internal conversion to the S1 state. In Figure 13.12, the growth of the intensity of the triplet Raman feature tracks a frequency upshift of 40 cm1 and a bandwidth that decreases by 40 cm1 for HPA and HPDP in both of the H2O/MeCN mixed and neat MeCN solvents (see Refs [23] and [43] for details), and this kind of short-time behaviour in Raman band positions and bandwidths is typical of an excess energy relaxation process [23, 43] that generally occurs on a timescale of around 10 ps, which correlates well with the 7–12 ps time constant seen in the spectra of Figure 13.12. The short-time behaviour of the 3 pp* Raman spectra appears to be due to the combined dynamics associated with the growth of the triplet population and the relaxation of the excess energy (11 000 cm1) deposited into the system by 267 nm photolysis and subsequent very fast ISC conversion to populate the triplet.

Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds (a)

305

(b) 0 ps 1 ps 2 ps 3 ps 4 ps 5 ps 6 ps 7 ps

10 ps

20 ps 1500

1650

Raman Shift

1500

1650

(cm-1)

Figure 13.12 Picosecond Kerr gated time-resolved resonance Raman spectra obtained for HPDP in 50% H2O/ 50% MeCN mixed solvent with 267 nm pump and 342 nm probe (a) and 400 nm probe (b) wavelengths respectively. Reprinted with permission from [24]. Copyright 2005 American Chemical Society

Figure 13.13(A) displays the fs-TA spectra HPDP (a) (from 0.1 to 12 ps) and (b) (from 12 to 2000 ps) picosecond times obtained after 267 nm photoexcitation in an H2O/MeCN (1:1) mixed solvent [25]. The righthand part of Figure 13(A)(a) shows that two absorption bands (an intense band located at 400 nm and the other broader and weaker band located in the 470–620 nm region) appear and become more intense, while simultaneously the middle intense band at 320 nm decays in intensity and an isobestic point at 330 nm between these absorption bands is an indicator of a dynamical conversion between two distinguishable electronic states [25]. These changes in the transient absorption spectra can be attributed to the ISC conversion from the lowest singlet excited state (S1) to the nearby triplet (T1) excited state, which indicates that the very short-time spectra (like the 0.25 and 0.1 ps spectra in Figure 13.13(A)(a)) can be attributed to a S1 ! Sn absorption from the S1(np ) singlet state, and the later spectra (like the 12 ps spectra in Figure 13.13) are due to the T1 ! Tn absorption from the T1 triplet (pp ) state [25]. Additional fs-TA spectra given in Ref. [25] examined the kinetics of the decay of the T1 triplet (pp ) state when different water concentrations are employed in the mixed solvents and also when some other solvents are utilized in making the sample solutions. A Stern–Volmer analysis using the triplet decay data obtained at different water concentrations in mixed solvents results in a nonlinear and approximately square dependence on water concentration that suggests that two or more water molecules are associated with the deprotection reaction [25]. It is important to note that water is an unusual solvent that is able simultaneously to be a hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) solvent. In addition, fs-TA experiments were also performed for 267 nm photoexcitation of

306

Hydrogen Bonding and Transfer in the Excited State 250 (a)

4 ps

200 -1

ΔOD(10 )

12 ps

150

2

4 ps

1

100 0

400

nm

500

-3

ΔOD (10 )

50 0 250

0.1 ps 12 ps

(b)

200 150 100 50 0

2000 ps 350

400

450

500

550

600

650

Wavelength (nm)

Figure 13.13 Transient absorption spectra of HPDP at early (a) (from 0.1 to 12 ps) and late (b) (from 12 to 2000 ps) picosecond times recorded with 267 nm excitation in H2O/MeCN (1:1) mixed solvent. The right-hand part in (a) shows the profile change of the transient absorption spectra recorded over 4–12 ps. (B) The triplet decay kinetics observed for HPDP by transient absorption measurements in water/MeCN mixed solvent with a water concentration of 0% (filled circles), 10% (filled diamonds), 15% (open diamonds), 25% (open triangles) and 50% (open circles). Data labelled by filled triangles are obtained in a DMSO/CF3CH2OH (1:1 by volume) mixed solvent. Solid lines indicate dynamic fittings using a one-exponential decay function to the experimental data points. Reprinted with permission from [25]. Copyright 2006 American Chemical Society

HPDP in DMSO, CF3CH2OH and a mixed solvent consisting of DMSO/CF3CH2OH (1:1 by volume). These solvents were chosen because they have distinctly different properties; for example, DMSO is a typical HBA solvent with close to zero ability to be a HBD solvent, and CF3CH2OH is an effective HBD solvent that does not behave as an effective HBA solvent [57]. This allows selective H-bonding with the HPDP triplet at either the acidic site (phenolic proton) in the DMSO solution or at the basic site(s) (the carbonyl oxygen and phosphate anion as the leaving group) in the CF3CH2OH solution. Both the acidic and basic sites can have H-bonding by the respective components in the DMSO/CF3CH2OH (1:1) mixed solvent, and hence roughly model the simultaneous HBD and HBA ability of water as a solvent. The TA data in DMSO show no appreciable triplet quenching, and the triplet decay kinetics is about the same as that observed in MeCN [25]. A very modest amount of triplet quenching was seen in the CF3CH2OH solvent, but a much greater quenching was observed in the DMSO/CF3CH2OH (1:1) mixed solvent [25]. This demonstrates that the triplet can be quenched reasonably well in the DMSO/CF3CH2OH mixed solvent owing to its simultaneous HBD and HBA ability by its respective components, but it is not quenched that well in the respective neat solvents [25]. This indicates that the presence of a solvent with both hydrogen-bond-donating and hydrogen-bond-accepting capabilities is

Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds

307

required for the triplet quenching process, and suggests that concerted solvation at both the acidic and basic sites is essential for the deprotection reaction [25]. The double-bonded oxygen of the phosphate leaving group and the hydrogen atom of the hydroxy group are probable basic and acidic sites respectively, associated with the concerted triplet solvation and related deprotection reaction. The importance of the leaving-group solvation is supported by the strong leaving-group dependence of the triplet decay dynamics for the pHP compounds examined (HPPP, HPDP and HPA), where the decay rate correlates with the stability of the leaving-group anion (see Figure 13.14) [24, 25]. The ability of a good leaving group to stabilize the associated solvent-solvated anion and assisting the photoinduced dissociation is diagnostic of an excited-state heterolytic cleavage kind of reaction [57]. The solvent and leaving-group dependence seen for the triplet decay dynamics for the pHP phototriggers is a strong support for a triplet quenching reaction pathway resulting in direct heterolytic cleavage assisted by the water solvation of the leaving-group anion. The requirement for leaving-group solvation for the pHP photodeprotection reaction also makes it possible to account for the complete lack of deprotection reactivity for HPDP in MeCN [20, 21] and DMSO solvents, because these two solvents are highly dipolar but are unable to be effective H-bond donors, so they are bad at effectively solvating the leaving group [57]. Water solvation of the hydroxy proton is also needed to account for the results of previous studies that found that photolysis of MPDP (the p-methoxy counterpart of HPDP) in a H2O/MeCN (1:1) solvent leads to very little if any deprotection products [14, 21]. Because the hydroxy and methoxy moieties have similar electronic effects on the intrinsic properties of the triplet states [43, 57], the lack of deprotection reactivity for MPDP indicates that there is not just a simple primary step involving only CO bond heterolysis in the HPDP triplet, and suggests that the proton-donating ability of the hydroxy group in the pHP triplet is associated with the different photochemical deprotection reactivity of MPDP from HPDP in the same water/MeCN mixed solvent. Ps-TR3 spectra were acquired to find the dynamics for the production of the rearrangement product HPAA for photoexcited HPDP and HPPP in an H2O/MeCN (1:1) mixed solvent. The ultraviolet absorption spectrum Dependence of the triplet quenching dynamics on the type of leaving group τ

Leaving group

pKa

O CH3

~ 4 ns

4.89

~3

ns

4.76

~370 ps

1.39

~130 ps

0.4

O

O

O O

P

OCH3

OEt

O

O

OEt

O O

P

Oph

O

P

OEt

Oph

OEt O O

0

500 1000 1500 2000 2500 3000 3500 4000 4500 5000

P Oph

Oph

Time delays (ps)

Figure 13.14 (Left) Time dependence of the transient absorption intensity for the triplet states of HPPP, HPDP, HPA and HPH in H2O/MeCN (1:1) at 400 nm. Solid lines indicate kinetics fitting employing a one-exponential function to fit the experimental data points. (Right) The pKa values of the different leaving groups and the time constants for the lifetime of the triplet states of HPPP, HPDP, HPA and HPH in H2O/MeCN (1:1) from the data shown in the left-hand part of the figure are listed. See the text for more details

308

Hydrogen Bonding and Transfer in the Excited State

of HPAA displayed in Figure 13.15(a) shows that its two lowest energy absorption bands are weak, while HPAA absorbs intensely in the 190–200 nm region and a 200 nm probe wavelength was chosen as a suitable probe wavelength for the ps-TR3 experiments to detect the formation dynamics of HPAA. Figure 13.15(b) displays representative 200 nm probe ps-TR3 spectra of HPDP at different delay times, obtained after 267 nm photolysis in a H2O/MeCN (1:1) solution, and a 200 nm resonance Raman spectrum for an authentic sample of HPAA obtained under the same conditions is also displayed in the figure to compare with and to clearly identify the transient species observed in the TR3 spectra [25]. The TR3 spectra show the appearance and growth of new Raman bands at 200 ps and afterwards that are obviously a result of the formation of a new species. It is clear that the new transient spectra are essentially identical to the 200 nm resonance Raman spectrum of an authentic sample of HPAA, and this shows that the new species can be attributed unambiguously to the HPAA rearrangement product. The spectra in Figure 13.15(b) represent to our knowledge the first direct timeresolved detection of the solvolytic rearrangement reaction for pHP phototrigger compounds. A crude fit of the spectrum kinetics in Figure 13.15(b) by a one-exponential growth function leads to a time constant of 1100 ps, and comparison with the 350 ps time constant triplet decay time observed for HPDP in the same solvent suggests that the rearrangement reaction occurs some time after the triplet quenching process. Because the triplet decay time can probably be taken as an indicator of the time constant for the release of the phosphate leaving group, the delayed rearrangement dynamics suggests that there is a consecutive reaction mechanism and there is probably some intermediate between these two processes [24, 25]. Based on the time-resolved spectroscopy results briefly described in this chapter, a reaction mechanism for the photodeprotection reaction that releases the phosphate leaving group and the rearrangement reaction that forms the HPAA side product following photolysis of pHP phototriggers is proposed, and a simple diagram of this mechanism is presented in Figure 13.16. The 267 nm photolysis of the pHP phototrigger compound excites the molecule from its ground state to the S3 (1 pp*) excited state responsible for the very short-lived blue fluorescence seen in the fs-KTRF experiments, and this state then proceeds by very fast internal conversion (of

OH

50000

O

HO

40000 30000

O

200 nm

O

O P EtO

OEt

HO

20000 10000

180 200 220 240 260 280 300 320 340

Wavelength (nm)

200 nm chosen as the probe wavelength for the TR3 experiment

HPAA 6000 ps 4000 ps 2000 ps 1500 ps 1000 ps 700 ps 500 ps 300 ps 200 ps 150 ps 85 ps 50 ps 30 ps 15 ps 10 ps 5 ps 2 ps 0 ps 600 800 1000 1200 1400 1600 1800

Intensity (a.u.)

60000

-1

-1

Extinction coefficient (Lmol cm )

Ps-TR3 spectroscopy employed to directly detect the kinetics of the pHP rearrangement reaction

-1

Wavenumber (cm )

Figure 13.15 (a) UV-VIS absorption spectra of the HPAA product (solid line) and the HPDP substrate (dashed line) in a H2O/MeCN (1:1) mixed solvent. (b) Picosecond-time-resolved resonance Raman (ps-TR3) spectra of HPDP obtained with 267 nm pump and 200 nm probe wavelengths in a H2O/MeCN (1:1) mixed solvent. Resonance Raman spectra of an authentic sample of HPAA recorded with 200 nm excitation is displayed in red at the top. Reprinted with permission from [25]. Copyright 2006 American Chemical Society

Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds

309

Solvent assisted triplet cleavage and stepwise solvolysis rearrangement reaction pathway for HPDP in aqueous solutions

S3

IC (~80 fs) ISC (~2 ps)

(1ππ∗)

S1

(1nπ∗)

H2O O

O

+ -

(~370 ps)

OP(OEt)2

+ H2O T1(3ππ∗) O

contact ion pair

H

solvent separation

H2O

267 nm hν

on ir nati a i p b n Io com re

H2O O +

O OH

O

O

O

solvolysis

+

P EtO

OEt

O HO

HO

S0

solv

HPAA

rearrangement (~1100 ps) H

- OP(OEt)2

solv

O

H2O

Figure 13.16 Proposed mechanism for the deprotection and rearrangement reactions that take place subsequent to ultraviolet excitation of pHP phototrigger compounds, based on the results of time-resolved spectroscopy experiments presented here. See the text for more details

the order of 80 fs) to produce the S1(1 np*) state that is responsible for the 2 ps lifetime red fluorescence seen in the fs-KTRF experiments [24]. Intersystem crossing (ISC) then takes the S1(1 np*) state to the T1(3 pp*) state, and this was directly seen in both the ps-TR3 (see Figure 13.12) and fs-TA (see Figure 13.13A) experiments [24, 25]. The TA data presented in Figures 13.13 and 13.14 found that the solvent effects are consistent with a solvent-assisted triplet heterolytic cleavage process for the deprotection of the phosphate leaving group [25]. The solvent and leaving-group dependences observed in the experiments suggest that strong coupling of water occurs as site-specific near-simultaneous solvation of the hydroxy proton and the phosphate-leaving-group anion through their respective intermolecular H-bonding with solvent water molecules, and this assists the deprotection process [25]. Correlation of the dynamics of the deprotection process and the rearrangement reactions for HPDP in the same 50%H2O/50%MeCN solution indicates that the deprotection process takes place with a time constant of 350 ps, while the rearrangement has a time constant of 1100 ps, as shown in Figure 13.16. This suggests that there is a stepwise mechanism, with the rearrangement taking place after the deprotection, and that there is some kind of intermediate between the two reactions. The dependences of the solvent and the leaving-group dynamics indicate that the intermediate is a solvation complex with a contact ion pair character, as shown in Figure 13.16 [25]. The results presented in this chapter give important dynamical and structural data that enable an overall mechanistic picture to be developed for the photophysical and photochemical processes taking place subsequent to photolysis of pHP caged phosphates in various solvent environments. This kind of direct time-resolved information will be helpful in designing and developing pHP caged compounds and related species for use as phototriggers in particular applications. While much has been learned about the effects of hydrogen bonding on the excited-state properties and chemical reactions involving pHP compounds and phototriggers, there remains much to learn in the future. More detailed information may be extracted in the future from complementary time-resolved spectroscopy

310 Hydrogen Bonding and Transfer in the Excited State

methods, such as time-resolved infrared absorption spectroscopy, which may reveal more about the hydrogenbonding environment around the reacting excited states, as well as from reaction pathway theoretical calculations that explicitly include hydrogen-bonding solvent molecules. In addition, there is much to learn about substituent effects on the hydrogen bonding of the excited states and their properties and chemistry. We anticipate that these pHP and related aromatic carbonyl compounds will remain an area of study in order to learn more about hydrogen-bonding effects on the properties and chemistry of electronic excited states.

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Hydrogen-Bonding Effects on Excited States of Para-Hydroxyphenacyl Compounds 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57.

311

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14 Hydrogen-Bonding Effects on Intramolecular Charge Transfer Govindarajan Krishnamoorthy Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India

14.1 Introduction Bichromophoric organic molecules composed of directly attached electron donor and electron acceptor moieties have received considerable attention as possible models to investigate the intramolecular charge transfer (ICT) process [1, 2]. Several experimental and theoretical studies have been made to understand these systems, and their prospective role in photochemistry and photobiology has been analysed. Similarly to excited-state proton transfer or exciplex formation, dual fluorescence associated with the existence of two different excited states has been observed in numerous electron donor–acceptor molecules owing to the formation of the ICT state [1–5]. However, the formation of a non-emissive ICT state is also proposed in some cases [6, 7]. In spite of the fact that the photochemistry and photophysics of different ICT molecules have been well studied, the nature and the mechanism for the formation of the ICT state are still under active discussion. Lippert et al. first observed dual fluorescence from p-(dimethylamino)benzonitrile (DMABN) in polar solvents [3]. The origin of this dual luminescence has been much debated [1, 2]. Later, Grabowski et al. explained this phenomenon by introducing the twisted ICT (TICT) model [8]. The occurrence of dual fluorescence was explained in the TICT model by means of an adiabatic photoreaction or horizontal radiationless transition leading to a twisted conformer of the same molecule. According to the model, fluorophore excitation induces transfer of charge from donor to acceptor and is accompanied with rotational relaxation of the electron donor from within the molecular plane to a position perpendicular (twisted conformation) to the rest of the molecule, forming the so-called TICT state. It brings the initially generated locally excited (LE) state to the TICT state, the new minimum on the excited-state potential energy surface (Figure 14.1). In this configuration, complete decoupling of the groups leads to full charge transfer, which

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

314 Hydrogen Bonding and Transfer in the Excited State LE state

TICT state

S1

H3C

N

CH3

E

CN S0 Reaction Coordinate

Figure 14.1 Potential energy diagram of DMABN; the reaction coordinate contains both solvent relaxation and rotation of the donor group

results in a large dipole moment, and hence a greater stability is achieved in polar solvents. While shorterwavelength emission (normal emission) originates from the LE state, longer-wavelength emission arises owing to the TICT state, thus resulting in dual fluorescence. The TICT mechanism is supported by the study of a series of model compounds. Only normal emission was observed from molecules (Figure 14.2) with a fixed molecular framework where the donor rotation towards the twisted geometry was prevented by molecular bridging [9–12]. On the other hand, only longer-wavelength emission was reported in molecules (Figure 14.3) that were pretwisted and where planar conformation was made unattainable by steric hindrance [13–16]. CH3 N

N

CN

CN

Figure 14.2

H3C

N

CN

Molecular bridged compounds

N

H3C

CH3

N

CH3

CN

Figure 14.3

CH3

CN

Sterically hindered compounds

Hydrogen-Bonding Effects on Intramolecular Charge Transfer

315

Although TICT is the commonly accepted structure for the ICT state, it has been vigorously challenged. Zachariasse et al. questioned the TICT model, based on the fact that longer-wavelength emission of DMABN appears not only in polar solvents but also in toluene, where no dielectric solvent relaxation is expected [17]. Zachariasse et al. propose pseudo-Jahn–Teller (PJT) coupling of the 1 La and 1 Lb states via the N-inversion mode (rehybridization) of the amino group, which leads to a pyramidal geometry in the ICT state. It decouples, even without rotational isomerization, the nitrogen lone pair from the p electrons of the phenyl ring [18, 19]. The condition for this coupling is a small energy gap between the 1 La and 1 Lb states. The donor rehybridization model is sometimes called wagging ICT (WICT). The acceptor rehybridization hypothesis was put forward by Sobolewski and Domcke [20]. Based on ab initio calculations, they suggested that sp ! sp2 rehybridization of the carbon atom of the cyano group is responsible for stabilization of the ICT state in donor-substituted benzonitriles, and proposed the term rehybridization by ICT (RICT) [20]. Later, they compared DMABN and isoelectronic analogue 4-dimethylaminobenzethyne and predicted the presence of TICT and RICT states in both molecules (Figure 14.4). However, the RICT state had lower energy than the TICT in 4-dimethylaminobenzethyne, and the order of states was reversed in DMABN. Accordingly, they predicted longer-wavelength emission from TICT in the case of DMABN and from RICT in the case of 4-dimethylaminobenzethyne. However, only one emission from the LE state with single exponential decay was reported for 4-dimethylaminobenzethyne [21]. Further, the RICT hypothesis would not be able to explain the dual fluorescence observed in other analogues of DMABN, where the cyano group is replaced by other acceptors such as aldehyde, acid or ester. Zachariasse suggested that his PJT scheme is applicable only to molecules with weakly perturbing substituents, such as alkyl-substituted aromatic hydrocarbons with singlet excited states without an appreciable CT character, and does not hold for donor- and acceptor-substituted benzene such as DMABN, for which CT interactions play an important role. He modified his model to propose the planar ICT (PICT) state [22]. In the PICT model, the energy of the highly polar S2 (1 La , CT) state preferentially decreases compared with the S1 (1 Lb ) state with increase in solvent polarity. Upon state reversal, a highly dipolar planar quinoid structure is formed (Figure 14.5). Both TICT and PICT are leading models, but, owing to experimental difficulty in direct measurement of the structure of the emitting species, the experimental evidence advanced for both models has been largely circumstantial. However, equally convincing indirect evidence has been reported for both models. Dobkowski et al. [23] demonstrated that syn–anti isomerization around the C--N bond accompanied the dual fluorescence of desymmetrized analogue 2-(N-methyl-N-isopropylamino)-5-cyanopyridine in methanol, whereas its ordinary fluorescence in tetrahydrofuran is not accompanied with isomerization (Figure 14.6). This implies the intermediacy of a perpendicular moiety in methanol where dual fluorescence occurs. On the other hand, Yoshihara et al. showed that a planar rigidized fluorazene molecule, which cannot attain perpendicular H3C

H3C

N

CH3

N

CH3

C

C N

C H

Figure 14.4 Acceptor rehybridization (RICT) model

316 Hydrogen Bonding and Transfer in the Excited State

Figure 14.5

D

A

A

Normal and quinoid canonical structures

H3C

i-Pr

i-Pr N

N

CH3

N

N

CN

Figure 14.6

D

CN

Syn–anti isomers of 2-(N-methyl-N-isopropylamino)-5-cyanopyridine

geometry, undergoes fast reversible ICT in the excited state [24]. This result indicates that large-amplitude motions such as those necessary for the formation of the TICT state are not required for the formation of the ICT state. Recently, Cogan et al. [25], based on theoretical calculation, suggested that there are three low-lying electronically excited states for DMABN and its analogues, two of which are of charge transfer character and the third being the LE state. Dual fluorescence may arise from any two of these states, as each has a different geometry at which it attains a minimum. They proposed that the ICT state is formed by the transfer of an electron from a covalently linked donor group to an antibonding orbital of the p-electron system of benzene. The change in charge distribution of the molecule in the ICT leads to distortion of the benzene ring to a quinoid and an antiquinoid structure. As the dipole moment is larger in the perpendicular geometry than in the planar one, this geometry is preferred in polar solvents. This supports the TICT model; however, they also proposed, in cases where the planar conformation of ICT states is lower in energy than that of the LE state, that dual fluorescence can be observed also from the planar structures. Apart from general interactions such as polarity and viscosity, specific interaction such as hydrogen bonding of solvent with electron donor or acceptor is reported to influence the formation of the ICT state. Although the emission maxima are approximately linearly dependent on various forms of solvent polarity functions [26–28], in some cases exceptions from the correlation are found in protic solvents [29–31]. In addition, a Kamlet and Taft [32] multiregression analysis [33] also indicates that hydrogen bonding affects the energy level of the ICT state along with non-specific interactions. The hydrogen bonding effect may be an important subject in exploring the coupling of proton motion to charge separation. The proton-coupled charge transfer phenomenon is crucial for the most important processes of biological energy conversion such as photosynthesis, vision, transmission of the nervous impulses and respiration [34]. For instance, in PS II, photoinduced charge separation is coupled to proton motion within the quinine pool [35]. Here, our recent studies of the hydrogen-bonding effect on ICT emission, along with a few literature reports, are reviewed. Different hypotheses have been proposed concerning the role of hydrogen bonding in ICT emission. However, before going on to the hydrogen-bonding interaction, the role of specific interactions in ICTemission

Hydrogen-Bonding Effects on Intramolecular Charge Transfer

317

is outlined in Section 14.2. Reports that favour the role of hydrogen bonding with the charge donor moiety of fluorophore in the formation of the ICT state are discussed in Section 14.3. Studies that support the role of hydrogen bonding with the acceptor moiety in ICT emission are discussed in Section 14.4. The conclusions are summarized in the final section.

14.2 Polarity and Viscosity Dual emission of ICT molecules is strongly influenced by the energy gap of the first two excited states, and the ICT state is populated under the condition [1] EðICTÞEðLEÞ < 0 and the energy of the ICT state can be approximated to [36, 37] EðICTÞ ¼ IPðDÞEAðAÞ þ C þ DEsol where IP(D), EA(A) and C are the ionization potential of the donor moiety, the electron affinity of the acceptor moiety and the Coulombic energy, respectively, and DEsol is the solvation energy. As the energy gap between the LE and ICT states depends on the donor and acceptor strength, the energy gap can be tuned to get dual emission. For example, unlike DMABN, its amino analogue p-aminobenzonitrile did not show dual fluorescence even in highly polar solvent [38]. As expected, owing to the large dipole moment in the ICT compared with the LE state, solvent polarity also strongly influences the energy between these states. For instance, DMABN emits single emission in non-polar solvents, but emits dual emission in polar solvents, and the ICT emission band undergoes a strong red-shift with increase in polarity of the medium. It has also been demonstrated that, with an increase in solvent polarity, the energy barrier for the ICT process decreases, which increases the solvent relaxation time and consequently the rate of ICT [39]. With increase in polarity, the quantum yield and lifetime of normal emission decreases monotonically with a small red-shift. However, the ICT emission quantum yield initially increases with increase in polarity to reach a maximum, but decreases on further rise in polarity [1, 2]. This is due to the fact that the polarity of the medium affects both LE and ICT species in different ways. Because of the large dipole moment, the ICT state is preferentially more stabilized than the LE state in polar environments. As a consequence, the energy of activation for the LE to TICT transition is decreased in polar media, thus enhancing the formation of the ICT state and thereby decreasing the normal fluorescence quantum yield. As far as the ICT state is concerned, the polarity of the environment modifies the decay and the formation channels in quite opposite ways. In a highly polar medium, although the formation of the ICT state is favoured, it also affects the non-radiative decay from the ICT state to the low-lying triplet and/or ground state. The lowest triplet state of DMABN does not have an appreciable charge transfer character as does the ICT singlet state [40]. Hence, the dipole moment of DMABN in the triplet state is less than that in the ICT state [41]. Thus, a rise in polarity leads to preferential solvation of the ICT state over the triplet state, which reduces the energy gap between the two states. According to the energy gap law, this would lead to an increase in non-radiative decay, i.e. the intersystem crossing between the ICT and triplet states. Owing to enhanced intersystem crossing between the ICT and triplet states, the triplet yields of DMABN and related molecules increase as the polarity of the medium increases [42]. Evidently, polarity affects the fluorescence yield of ICT emission in two opposing ways. While acceleration of the ICT process tends to increase the ICT emission, the increase in non-radiative rate from the ICT state tends to lower it. Because of these opposite effects, the ICT emission exhibits a rise and fall in fluorescence quantum yield with rise in the polarity of the environment.

318 Hydrogen Bonding and Transfer in the Excited State

As the TICT process is supposed to involve internal rotational motion, i.e. twisting motion, besides solvent polarity, the viscosity of the medium is also expected to play a role in the formation of the TICT state. An increase in viscosity is likely to hinder the twisting motion. However, for DMABN, the rotating group, i.e. the dimethylamino group, is quite small and the longer-wavelength emission was found to be almost independent of viscosity up to moderate viscosity [43, 44]. At very high viscosity, however, friction does play a role [45–49]. On the other hand, for molecules that have bulky donor groups, for example N, N-dimethylaminophenyl diphenylphosphine oxide, which has a bulky anilino group as donor, increase in viscosity is found to retard the ICT process even at low viscosity [50]. Thus, the energy and the emission yield of ICT fluorescence is strongly influenced by the polarity of the medium, and the viscosity effect depends on the bulkiness of the rotating donor groups. These sensitivities of ICT emission towards the polarity and viscosity of the microenvironment make it a useful tool to probe polymers [51], surfaces [52] micelles [53, 54], cyclodextrins [55–57], etc.

14.3 Hydrogen Bonding with the Donor Moiety Cazeau-Dubroca et al. were first to suggest that hydrogen bonding of solvents with the donor group is responsible for the TICT emission in DMABN and its analogues. They proposed ground-state complex formation by hydrogen bonding of the solvent with the donor moiety (i.e. the amine) in the ground state to form a twisted conformer that is otherwise planar in the isolated condition [58–61]. Such a pretwisted conformer was essential for the formation of the TICT state. They first studied the spectral properties of DMABN in rigid polymer matrices having different polarity [51]. They observed dual emission only in protic poly(vinyl alcohol), and the intensity ratio of shorter wavelength to longer wavelength was independent of DMABN concentration. The longer-wavelength fluorescence disappeared at liquid nitrogen temperature, and the normal fluorescence was accompanied with phosphorescence. As dual emission was not observed in more polar poly(vinyl chloride), it was concluded that hydrogen bonding is more important in the formation of longer-wavelength emission than the dipole–dipole interaction. It was also reported that the absorption spectrum of DMABN is red-shifted in a protic polymer matrix compared with a more polar aprotic polymer matrix. This was attributed to the formation of twisted conformers by hydrogen bonding in the ground state, which was supposed to be responsible for the TICT emission of DMABN in poly(vinyl alcohol). The study was extended to other hydrogen-bond-donating polymers, nylon and polyurethane, and also to a few other molecules that emit from the TICT state [62]. The intensities of absorption spectra as well as the longerwavelength emissions were reported to increase with the strength of the hydrogen bond of the solute with the matrix. However, Cazeau attributed the longer-wavelength emission to a delayed ICT fluorescence due to its very long lifetime (t  1 s). According to Cazeau et al., excitation of free molecules (untwisted) led to a planar singlet state from which normal fluorescence occurs. The excitation of the hydrogen-bonded molecules, i.e. socalled twisted molecules, led to an unrelaxed TICT state that populated the triplet state. The singlet TICT state was reached from the triplet state by thermal activation at room temperature. They ascribed the absence of dual emission at low temperature to unpopulation of higher vibrational levels of the triplet state. However, the model of specific hydrogen bonding of solvent with solute being the promoter for the formation of TICT states, as suggested by Cazeau et al., was challenged by Al-Hassan and Azumi on the basis of their observation of longer-wavelength emission in non-hydrogen-bonding polymer matrices, and a free volume model was put forward to account for the dual emission of DMABN and related molecules in rigid matrices [63]. However, Cazeau et al. attributed the presence of dual fluorescence in non-hydrogen-bonding polymers reported by AlHassan and Azumi to the presence of water traces introduced in the matrices. Based on the significant decrease in longer-wavelength emission by softening the matrix, they ruled out the free volume model as being solely responsible for the TICT emission in polymeric matrices and fixed on their specific solute–solvent interaction

Hydrogen-Bonding Effects on Intramolecular Charge Transfer

319

model. Cazeau further extended the argument of water traces to explain the ICT fluorescence in aprotic solvents by assigning it to the deforming effect of hydrogen bonds on amine conformation by traces of water [58]. Kobayashi et al. suggested the formation of a 1:1 complex between DMABN and H2O/CF3H mainly by the interaction of solvent molecules with the benzonitrile part [64]. They did not observe any ICT fluorescence. It was speculated that there exist two kinds of complex between a polar molecule and DMABN: one at the cyano site that absorbs the light but does not form an ICT state; the other at the amino site hardly absorbs light, yet favours ICT-state formation. ICT-state fluorescence was observed in solution phase by further solvation so that the amino site complex could absorb light. Although a few others [65, 66] supported the Cazeau hypothesis, no clear unambiguous evidence was provided to substantiate the hypothesis. For instance, the twisted intramolecular charge transfer emission of ethyl and methyl esters of N, N-dimethylaminonaphthyl(acrylic)-acid in polar protic solvents was reported to vary linearly with the hydrogen-bonding parameter a [65, 66]. The authors referred to the Cazeau model of the ground-state tetragonal arrangement of the amine side owing to the formation of a hydrogen bond with protic solvents, which is supposed to favour the TICT process. But they did not provide any clear evidence to support this. On this basis, it cannot be ruled out that hydrogen bonding of solvent with acceptor is responsible for the observed hydrogen bonding of the TICT process (see Section 14.3). Even the authors included such hydrogen bonding in their hydrogen-bonded clusters (Figure 14.7). In addition, unlike Cazeau et al., the absorption spectra of N, N-dimethylaminonaphthyl acrylates were blue-shifted in aqueous medium compared with that in polar aprotic solvents. On the other hand, the role of the pretwisted conformer formed by hydrogen bonding of solvent with donor moiety in the formation of the TICT state in DMABN is severely challenged. The role of water traces in aprotic solvents reported by Cazeau could not be reproduced by others [2]. Further, it was shown, contrary to the results of Cazeau, that the intensity ratio of TICT emission to normal emission decreases with the addition of water. It was also reported that specific association of water leads to quenching of both normal and TICT emissions. Ground-state hydrogen bonding at the amino nitrogen is hardly detectable in DMABN and seems not to play a promoting role in the formation of the ICT state. However, a few authors have suggested that it rather inhibits the formation of the ICT state [67, 68]. Being the electron donor, the amino group in DMABN and its analogues is expected to form a hydrogen bond with protic solvents in the ground state. The hydrogen bond is expected to break in the excited ICT state from the positively charged amino group, and it is supposed to form at the electron-density-rich sites: either at

H3C

HOR' CH3 N

C C C RO

O

R'OH

HOR'

R = CH3, C2H5

Figure 14.7 Hydrogen-bonded cluster of N,N-dimethylaminonaphthyl acrylates

320 Hydrogen Bonding and Transfer in the Excited State ROH

CH3 CH3 N

H3C

CH3

H3C

N

CH3 N

H3C

CH3 N

HOR

ROH

H

C

C

C

C

N

N

N

N

HOR

Figure 14.8

Hydrogen-bonded structures of DMABN

the acceptor moiety or in the ring (Figure 14.8) [2]. This bond breaking and bond formation can be inferred from the slow kinetics of LE ! ICT in protic solvents [69–71]. Recently, theoretical calculations predicted that the hydrogen bond between methanol and the cyano group of DMABN is significantly strengthened in the TICT compared with the ground state and thus facilitates fluorescence quenching [72]. More recently, based on a computational study, it was suggested that the isomer that is hydrogen bonded with water through dimethylamino nitrogen is responsible for the TICT emission in 4-(dimethylamino)methylbenzoate in gas phase, and not the isomer that is hydrogen bonded through the oxygen atom of the acceptor [73]. The argument was that the excited molecules have to lose excess internal energy only radiationlessly by fragmentation to localize in equilibrium structures. In the ICT state, immediate breaking is possible only in a hydrogen bond between amino nitrogen and water because of strong electrostatic repulsion between the now positively charged amino nitrogen and the hydrogen atom. On the other hand, the hydrogen bond between the carbonyl group and water strengthened owing to increased negative charge on the acceptor. In summary, experimental evidence is against the Cazeau model of pretwisted conformer by hydrogen bonding of solvent with donor as a cause of formation of the TICT state in solutions. However, theoretical calculation prediction of the donor hydrogen-bonded complex responsible for TICT emission in isolated conditions suggests that the Cazeau model cannot be completely ruled out in gaseous phase.

14.4 Hydrogen Bonding with the Acceptor Moiety For some molecules, ICT emission is not observed even in highly polar aprotic solvents, but, by enhancing the electron affinity of the acceptor through a hydrogen bond, a protic solvent induces the formation of ICT states. Herbich et al. demonstrated that 4-(N, N-dimethylamino)pyrimidine exhibits dual fluorescence only in protic solvents, and in aprotic solvents it emits ICT fluorescence only in the presence of a Zn2þ ion (Figure 14.9) [67]. However, they observed dual emission from 4-(N, N-diethylamino)pyrimidine in sufficiently polar aprotic solvents and only ICT emission in sterically hindered ortho-substituted 4-(N, N-dimethylamino)pyrimidine, where the amino group is pretwisted (Figure 14.9). Furthermore, it was shown that in 4-dialkylaminopyrimidines the decay of the TICT emission is monoexponential, whereas that of normal fluorescence is multi- or nonexponential [68]. It was gleaned from this that, of the several hydrogen-bonded species, only one induces the formation of the ICT state and at the other sites leads to fast proton transfer resulting in a non-fluorescent cation. Fasani et al. reported that 2-(40 -aminophenyl)-pyrido-thia-, oxa- and imidazoles exhibit dual fluorescence only in alcoholic solvents, and the long-wavelength emission has been attributed to TICT emission [74]. Fasani et al. proposed that the hydrogen bonding of protic solvents with nitrogen of pyridoimidazole moiety twists the

Hydrogen-Bonding Effects on Intramolecular Charge Transfer H3C

N

CH3

C2H5

N

N

C2H5

N

N

CH3 CH3

N N

N

H3C

321

N

Figure 14.9 4-(N, N-Dialkylamino)pyrimidines

HOR ROH

N

H

N N X

H

HOR

X = S, O, NH

Figure 14.10

Acceptor hydrogen-bonded complex of 2-(40 -aminophenyl)-pyrido-thia-, oxa- and imidazoles

pyridoimidazole group with respect to the rest of the molecule, which favours the formation of the TICT state (Figure 14.10). Yoon et al. also found that H-bonding of the solvent with the acceptor plays a major role in the formation of TICT in p-diethylamino- and p-dimethylaminobenzoic acids [75, 76]. For instance, they observed a reduction in the ratio of TICT fluorescence to normal fluorescence in p-(diethylamino)benzoic acid when the carboxylic group was buried inside the a-cyclodextrin cavity and was not available for hydrogen bonding. On the other hand, the ratio was enhanced with substantial increase in TICT fluorescence in b-cyclodextrin, where the carboxylic group was exposed to aqueous media [75]. However, they suggested that H-bonding with the solvent causes the acceptor to adopt a more coplanar geometry with the benzene ring, thereby increasing charge flow from the benzene ring and acceptor and thus enhancing the formation of the TICT state. 2-(40 -N, N-dimethylaminophenyl)pyrido[3,4-d]imidazole (DMAPPI) also emits dual fluorescence only in protic solvents [77]. In methanol and water, ICT emission appears as a clear shoulder, and the fluorescence decays measured at ICT emission and normal emission are different. In ethanol and propanol, the fluorescence spectra have a long tail, but no clear shoulder as in methanol or water was observed. However, the fluorescence decays measured at longer wavelengths are biexponential (Table 14.1). This clearly indicates that in ethanol and n-propanol the longer-wavelength ICT emission is buried underneath the normal fluorescence. This was manifested by the fact that the shorter-wavelength emission for DMAPPI in ethanol and propanol was broader compared with that observed in methanol. The intensity of the TICT emission increases as the protic nature of the solvents increases, except in alkaline water (pH 9.0), where it decreases. The decrease in TICT emission in water was attributed to the greater stabilization of the TICT state, which results in an enhanced non-radiative rate owing to decrease in the energy gap with low-lying states. The intensity of TICT fluorescence increases with increase in the lexc from 310 to 350 nm; for example, the fluorescence quantum yield ratio of TICT emission to normal emission increases from 0.28 to 0.38 in methanol. On lowering the temperature from 293 to 203 K, the fluorescence intensity of normal fluorescence is enhanced by 8%, whereas that of ICT emission decreases by 30%. In non-polar and polar aprotic solvents, the fluorescence excitation band maxima are independent of lem, whereas in protic solvents the fluorescence excitation spectra shift towards red with increase in lem. But only single-exponential decays were observed from both normal and TICT emission. Gradual addition of water to Triton X-100 in cylohexane and n-hexanol results in a red-shift in the absorption spectra with a clear isosbestic point [78]. All these indicate the presence

322 Hydrogen Bonding and Transfer in the Excited State Table 14.1 Absorption maximum (lab, nm), fluorescence maximum (lf, nm), quantum yield (wf) and fluorescence lifetime (tf, ns) of DMAPPI in different mediaa Medium

lab

lf (wf)

Cyclohexane Ethyl acetate Acetonitrile n-Propanol

327, 341 (sh) 332 335 339

350, 367, 384 (0.57) 379 (0.60) 391 (0.69) 393 (0.76)

Ethanol

340

395 (0.76)

Methanol

341

Water (neutral form)

335

Sodium dodecyl sulphate

340

Cetyl trimethyl ammonium bromide

343

Triton X-100

342

398 (0.22) 475 (0.07) 415 (0.011) 506 (0.003) 400 (0.027) 505 (0.031) 398 (0.37) 495 (0.031) 397 (0.41) 495 (0.34)

tf 1.02 0.96 1.10 2.50 0.95 2.56 0.35 1.85 0.22 0.49 0.47 1.04 0.82 1.69

From Refs [77] and [82].

a

of different solvated structures (in protic solvents) in equilibrium. To identify the role of hydrogen bonding in the formation of the TICT state, AM1 calculations have been performed on DMAPPI and different hydrate DMAPPI (Figure 14.11). In the isolated condition, the semi-empirical calculations predict 32 between the phenyl ring and the pyridoimidazole ring and 16 between the phenyl ring and the dimethylamino group. Upon hydration at different acidic and basic centres of the pyridoimidazole ring, the dihedral angle between it and the phenyl ring decreases to 19 , but the angle between the donor and the phenyl ring is nearly unaffected. Accordingly it was suggested that hydrogen bonding of the solvent with the pyridoimidazole ring (acceptor) makes it more planar with the benzene ring, thereby increasing the charge flow and thus inducing its TICT emission. On the other hand, hydration at the dimethylamino group twists it (67 ) more with respect to the phenyl ring. The prototropic equilibria in DMAPPI are very interesting (Figure 14.12) [79]. At pH 14, DMAPPI is deprotonated at the > NH proton of the imidazole ring to form a monoanion. In monoanonic form, the TICT R R

O O

H H

H

N

N

N R

O

N

CH3 CH3

H R

Figure 14.11

O H

Hydrated 2-(40 -N,N-dimethylaminophenyl)pyrido[3,4-d]imidazole

Hydrogen-Bonding Effects on Intramolecular Charge Transfer CH3

N

N

323

N N H

CH3

Neutral (N) N

CH3

N

N

N N

N H

CH3

Monoanion (MA) H

N

N N H

CH3

N

N

CH3

N

N H

CH3

Monocation 2 (MC2)

CH3

Monocation 3 (MC3)

N*

MC2*

(dual emission in protic solvents)

(normal emission)

hv hv

H CH3

Monocation 1 (MC1)

H N

CH3

N

N

Biprotonicphototautomerism

MC1 + MC3

H+

hv

N - H+

MC3* (TICT emission)

MA

hv MA* (dual emission with enhanced TICT)

Figure 14.12

2-(40 -N,N-dimethylaminophenyl)pyrido[3,4-d]imidazole and its ionic forms

emission was enhanced substantially compared with the neutral form. A similar increase in TICT emission was also reported for dimethylaminobenzoic acid [80]. AM1 calculations predicted that in both DMAPPI and dimethylaminobenzoic acid the acceptor becomes more planar in the anionic form than in the neutral form. From this it was gleaned that increase in planarity between the acceptor and the phenyl ring is responsible for the enhanced TICT emission. At pH 4.0, DMAPPI forms two kinds of monocation: (i) protonated at the dimethylamino nitrogen (MC1); (ii) protonated at the pyridine nitrogen (MC3) in the ground state. Upon excitation, the monocation formed by the protonation of dimethylamino nitrogen is deprotonated and undergoes biprotonic phototautomerism to protonate the imidazole nitrogen (MC2). However, the other monocation, formed by the protonation of pyridine nitrogen, was also observed in the excited state. However, it emits from the TICT state and not from the LE state. Again based on prediction of semi-empirical calculation, the argument concerning planarity between the acceptor and the phenyl ring promoting TICT formation was extended to the monocation also. However, all three dications and trication (Figure 14.13) show only normal fluorescence. Since solvatochromic and prototropic studies of DMAPPI did not yield a definite conclusion concerning the role of hydrogen bonding of the solvent with the donor of DMAPPI, the studies were extended to

324 Hydrogen Bonding and Transfer in the Excited State H N

N

CH3 N

HN

N H H N

H CH3 CH3

N

HN

N H

CH3

N

CH3 N

HN

N H H N

H CH3

CH3 N

N H

H CH3

Figure 14.13 Dications and trication of 2-(40 -N,N-dimethylaminophenyl)pyrido[3,4-d]imidazole

cyclodextrins, with an expectation that encapsulation of either donor or acceptor by cyclodextrins may lead to a conclusion concerning the role of hydrogen bonding of the solvent with the donor in the formation of the TICT state (Table 14.1) [81]. DMAPPI forms a 1:1 inclusion complex with cyclodextrins. The red-shift observed in the absorption spectra in cyclodextrins clearly indicates that DMAPPI enters the cavity by breaking the hydrogen bond between the dimethylamino nitrogen and water (Figure 14.14). Owing to its size, DMAPPI was only partially encapsulated in cyclodextrins. The pyridoimidazole ring of DMAPPI was outside the cavity. The water molecules form hydrogen bonds with acidic and basic centres of the pyridoimidazole ring. Such hydrogen bonding ensured the presence of different solvated structures and was confirmed by the red-shift in the fluorescence excitation spectra with lem. There is an enormous increase in TICT emission of DMAPPI in cyclodextrins, for instance it was enhanced by a factor of 45 in b-cyclodextrin with respect to an aqueous medium. The destabilization of the TICT state in the non-polar cyclodextrin cavity increased the energy gap between the TICT state and the low-lying states, thereby lowering the non-radiative rates. Blue-shift in the normal and TICT emissions band maxima substantiates this. Observation of TICT emission in the cyclodextrin inclusion complexes suggests that it is the hydrogen bonding of protic solvents with the pyridoimidazole ring, i.e. the acceptor, that is responsible for its protic-solvent-induced TICT emission. The donor (the dimethylamino group) was buried inside the cyclodextrin cavity and was not able to form hydrogen bonds either with the protons of the hydroxyl rim of the cavity or with water molecules at the interface. This rules out the role of hydrogen bonding of solvent with donor in the formation of the TICT state in DMAPPI. Destabilization of the TICT state inside the cyclodextrin cavity indicates that the donor plays a major role in the dipole–dipole

H H N

H

N N H N H

Figure 14.14

H H

2-(40 -N,N-dimethylaminophenyl)pyrido[3,4-d]imidazole b-cyclodextrin inclusion complex

Hydrogen-Bonding Effects on Intramolecular Charge Transfer

325

interaction solvation stabilization of the TICT state. The conclusions were further substantiated by the observed enhancement of TICT emission of MC3, the monocation formed by the protonation of pyridine nitrogen of DMAPPI in cyclodextrins. There, too, the donor was buried inside the cavity. One interesting aspect about DMAPPI is that, owing to charged interactions, its TICT species has different binding constants with ionic micelles compared with normal species, and the binding constants are the same with non-ionic micelles [82]. Different lifetimes observed for TICT and normal species of DMAPPI suggest that the equilibrium was not established between the two states. This may be due to greater stabilization of the TICT state by H-bonding with the acceptor, which should increase the activation energy barrier for the reverse process. Like DMAPPI, 2-(40 -N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP-b) (Figure 14.15) also shows longer-wavelength emission only in protic solvents [83]. The ratio of normal emission to TICT emission increases with increasing H-donating capacity of the solvent. However, in water the TICT emission is almost completely quenched. Lifetime data suggested that equilibrium is not established between the two emitting states (Table 14.2). The intensity ratio of longer-wavelength emission to shorter-wavelength emission decreases with increase in viscosity of the protic solvent, suggesting the presence of a viscosity-dependent barrier in the formation, i.e. common for TICT emission. On decrease in pH, unlike DMAPPI, only one fluorescence band is observed for DMAPIP-b in water at pH 4.0. This is due to the normal emission of a monocation formed by protonation on imidazole nitrogen of DMAPIP-b. The fluorescence spectra of DMAPIP-b are more red-shifted than those of DMAPPI, indicating a better charge transfer interaction when the position of the nitrogen changes. This is supported by the larger change in the dipole moment between the excited state and ground state, calculated by a Lippert–Mataga plot [84] for DMAPIP-b (7.1 D), than that calculated for DMAPPI (5.3 D) [77]. The longer-wavelength emission is also shifted to red in DMAPIP-b. The fluorescence intensity of the band is also enhanced enormously. For example, in contrast to DMAPPI, in DMAPIP-b the longer-wavelength emission in 1-butanol, 1-propanol, 2-propanol and ethanol appears as a clear shoulder. This shows that the long-wavelength emission is also strongly influenced by the position of the pyridine nitrogen. This was further substantiated by ab initio calculations showing that the electron density on

H N

CH3 N

N H (Normal emission)

N H

+

CH3

CH3

N N N

N H (Normal emission)

CH3 ROH

CH3

N N N HOR

Figure 14.15

N H

CH3 (Normal and TICT emission)

2-(40 -N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine and its monocation

326 Hydrogen Bonding and Transfer in the Excited State Table 14.2 Absorption band maximum (lab, nm), fluorescence band maximum (lf, nm) and lifetimes (t, ns) of DMAPIP-b in different solventsa Solvent

lab

lf

t

Cyclohexane Ethyl acetate Acetonitrile Dimethylformamide 1-Butanol

336, 352 338 (4.49) 345 (4.50) 346 (4.50) 348 (4.47)

1-Propanol

349 (4.48)

Ethanol

350 (4.48)

Methanol

350 (4.50)

Glycol

358 (4.50)

Glycerol Water

360 345

359, 379, 398 390 407 428 407 475 410 486 413 494 414 506 428 526 446 451

1.99 1.28 1.50 1.49 0.98 (42.48) 2.45 (57.52) 0.90 (24.22) 2.27 (75.78) 0.71 (24.22) 1.95 (75.78) 0.29 (67.70) 1.12 (32.30) 0.48 (78.17) 1.11 (21.83) 0.16 (98.75) 2.13 (01.25)

From Ref. [83].

a

dimethylamino nitrogen is less in DMAPIP-b than in DMAPPI and on pyridine nitrogen is more in DMAPIP-b than in DMAPPI (Table 14.3). The ab initio calculation performed predicted that both in DMAPPI and DMAPIP-b the donor (dimethylamino group), the phenyl ring and the acceptor (pyridoimidazole moiety) are coplanar in the ground state [83]. DFT calculations performed later for DMAPIP-b also predicted that the phenyl ring and the pyridoimidazole moiety are in same molecular plane [85]. This virtually rules out the earlier prediction based on the semi-empirical calculations that hydrogen bonding of solvent with acceptor makes it more planar with the phenyl ring, increasing the charge flow from phenyl ring to acceptor and being responsible for the formation of TICT emission in these molecules. Nonetheless, the hydrogen-bond-induced TICT emission in DMAPPI and DMAPIP-b can be explained as follows: hydrogen bonding of protic solvents with pyridine nitrogen enhances the electron affinity of the acceptors, thereby favouring the formation of the TICT state in DMAPPI and DMAPIP-b. The following evidence substantiates the explanation: (i) H-bonding-induced TICT emission is observed only in DMAPIP-b and in DMAPPI, but not in (N,Ndimethylaminophenyl)benzimidazole, where pyridine nitrogen is absent; (ii) when imidazole nitrogen is protonated, no TICT emission is observed in DMAPIP-b or in DMAPPI. On the other hand, the monocations formed by the protonation of pyridine nitrogen of DMAPPI and DMAPIP-b emit only TICT emission, and (iii) Table 14.3 Theoretical parameters obtained by ab initio calculation on DMAPPI and DMAPIP-b

Phenyl ring/NMe2 dihedral angle Phenyl/PI dihedral angle Charge on pyridine nitrogen Charge on dimethylamino nitrogen mg (D)

DMAPPI

DMAPIP-b



0.0 0.0 0.716 0.844 4.5

0.0 0.0 0.690 0.955 7.2

Hydrogen-Bonding Effects on Intramolecular Charge Transfer

327

huge enhancement in TICT is observed in cyclodextrin inclusion complexes of DMAPPI, where pyridine nitrogen is involved in hydrogen bonding with water [77, 79, 81, 83, 85]. Interestingly, unlike DMAPPI, DMAPIP-b emits dual emission only in sodium dodecylsulfate and Triton X-100 micelles, and not in cetyltrimethyl ammonium bromide micelle, which has a relatively poor hydrogen-bond-donating capacity [86]. Thus, the suggestion by Fansi et al. that twisting of the acceptor by hydrogen of the solvent is a mechanism for protic-solvent-induced TICT emission was not supported by theoretical calculation. Although semiempirical calculation predicted that hydrogen bonding of the solvent with the acceptor makes it planar with the benzene ring, this is not supported by ab initio calculations. Most experimental evidence suggests that hydrogen bonding of the solvent with the acceptor enhances its electron affinity, thereby inducing TICT emission.

14.5 Conclusion Like general interaction, hydrogen bonding also plays a major role in the formation and stabilization of ICT states in many molecules. The hydrogen bonding of protic solvents with acceptor groups of the molecules plays a major role in the formation and stabilization of ICT states. Hydrogen bonding of the solvent with the acceptor moiety increases the electron affinity of the acceptor group, thereby favouring ICT emission. In some molecules, such hydrogen bonding is essential for lowering the energy level of the ICT state. The hydrogen bond between protic solvent and donor breaks in the excited state to form the ICT state. Most experimental evidence suggests that it does not play an important role in the formation of ICT emission in solution phase. This was substantiated by the enormous increase in longer-wavelength emission of molecules like DMAPPI in cyclodextrins, where the donor is buried inside the cavity and is not available for hydrogen bonding. However, dipole–dipole interaction between donor and polar solvent is also one of the important factors that is responsible for solvent stabilization of ICT states. In some cases, equilibrium is not established between the LE state and the ICT state in protic solvents. This is due to greater stabilization of the ICT state by hydrogen bonding of protic solvents with acceptor moieties, which increases the barrier for the ICT ! LE reverse process.

Acknowledgements The author expresses his gratitude to Prof. S. K. Dogra who introduced him to this exciting research area. The author thanks the Department of Science and Technology (DST), New Delhi, and the Council for Scientific and Industrial Research (CSIR), New Delhi, for financial support. The author also thanks Prof. A. K. Mishra and Dr U. Subuddhi for their valuable suggestions and comments.

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15 Chemical Dynamics in Room-Temperature Ionic Liquids: the Role of Hydrogen Bonding Souravi Sarkar, Rajib Pramanik and Nilmoni Sarkar Department of Chemistry, Indian Institute of Technology, Kharagpur, PIN-721302, WB, India

15.1 Photoinduced Electron Transfer in a Room-Temperature Ionic Liquid The term ‘ionic liquid’ has come to indicate a class of molten organic salts that are liquid at room temperature and may contain bulky aromatic moieties such as imidazolium or heterocyclic pyridinium as cation and an inorganic anion such as PF6
, BF4
, [(CF3SO2)2N]
, NO3
, etc. The room-temperature ionic liquids (RTILs) have some unique properties such as good electrical conductivity, negligiblevapour pressure, high ionic mobility, excellent electrochemical and thermal stability and a wide liquidus temperature range (
96 to 300 C) [1–4]. Various photophysical, theoretical and ultrafast spectroscopic studies have been done on RTILs [5–13]. Photoinduced electron transfer (PET) is one of the most fundamental reactions in biological, physical, inorganic and organic chemical systems. The theory regarding electron transfer was first proposed by Marcus [14], and the electron transfer rate can be written as "

kET


ðDGo þ lÞ2 ¼ n exp 4lkB T

# ð15:1Þ

where l is the energy required structurally to reorganize the donor and acceptor, kET is the electron transfer (ET) rate constant, n is the frequency of the motion in the reactant potential well and T and kB are the temperature and Boltzman constant respectively. From this theory it is clear that, with increase in free energy change (
DGo), the ET rate initially increases, reaches a maximum at
DGo ¼ l and then decreases when
DGo > l. The experimental evidence for the bell-shaped dependence of rate constant on
DGo has been established only for a first-order reaction having fixed donor–acceptor separation. However, for bimolecular electron

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

332 Hydrogen Bonding and Transfer in the Excited State

transfer reactions, the observed rate has the form of a consecutive reaction mechanism consisting of the diffusion (kdiff) and activated rate constant (kact) for electron transfer: kET ¼

kact kdiff kdiff þ kact

ð15:2Þ

The electron transfer process is extremely fast at
DGo ¼ l. The rate constant for a bimolecular ET reaction shows an increase with increase in the free energy, reaches a maximum and finally becomes independent of the free energy of the reaction. Recently there have been some simulation studies of model systems of two ionic liquids and comparison with acetonitrile to judge the applicability of Marcus’ theory in ionic liquids [15]. Our group has already studied PET in several confined systems [16]. In this study our aim is to understand how the dynamics of PET is affected in RTILs owing to its unique feature. In order to investigate the dynamics, mechanism and how the PET process is affected by viscosity, polarity and ionic constituents of the RTILs, we have used N,N-dimethyl ethanol ammonium formate (DAF) as a protic non-aromatic room-temperature ionic liquid. We have used several coumarin dyes as the acceptor and dimethyl aniline (DMA) as the donor for PET studies in DAF using steady-state (SS) and time-resolved (TR) fluorescence spectroscopy. We have carried out fluorescence quenching studies by gradual increase of the DMA concentration in the solution of coumarin dyes in DAF. It is observed that, with the addition of DMA, both steady-state fluorescence intensity and the fluorescence lifetime of coumarin dyes in DAF are quenched (Figure 15.1). The fluorescence-quenching constant is determined by the well-known Stern–Volmer equation I0 t0 ¼ 1 þ KSV ½Q ¼ 1 þ kq t0 ½Q ¼ I t

ð15:3Þ

where I0 and I and t0 and t are the fluorescence intensity and lifetime of the coumarin dyes in the absence and in the presence of the quencher, Ksv is the Stern–Volmer constant and [Q] is the quencher concentration. The I0/I versus [Q] and t0/t versus [Q] plots for different probes in the presence of different concentrations of DMA are shown in Figures 15.2(a) and (b). For all coumarin–amine pairs, the plots are linear in nature. The fluorescence quenching constant kq values are determined by dividing the Stern–Volmer constant by the lifetime of the coumarin dyes in the absence of quencher. The measured kq values for different coumarin– amine systems in DAF from steady-state fluorescence are listed in Table 15.1. In this case we can see that the kq values obtained from time-resolved measurement are somewhat smaller than the kq values obtained from steady-state measurement, which indicates a contribution of static quenching to the kq values obtained from steady-state measurement, even though there is no indication of ground-state complex formation from the absorption spectra; also, there is an ultrafast decay component that remains undetected in our time-resolved set-up using picosecond (90 ps) time resolution. The ET rate depends on the free energy change of the system. So we have calculated DGo for each coumarin–amine system. The common expression for calculating DGo is given by the famous Rehm–Weller equation [17] DGo ¼ EðD=D þ Þ
EðA=A
Þ
EIPS
E00

ð15:4Þ

where E(D/Dþ ) and E(A/A
) denote the oxidation potential of the donor and the reduction potential of the acceptor respectively, EIPS is the Coulombic attraction term and E00 is the energy difference between the S0 and S1 states. The respective DGo value for all the systems are listed in Table 15.1 and shown in Figure 3(b).

Chemical Dynamics in Room-Temperature Ionic Liquids: the Role of Hydrogen Bonding

Fluorescence intensity (normalized)

1.0

(a)

(i)

0.8

333

(ii)

0.6

(iii) (iv)

0.4

(v) (vi) (vii)

0.2 0.0

450

500

550

600

Fluorescence intensity (Counts)

Wavelength (nm)

10000

(b) (ii)

8000

(iii)

6000 4000

(iv)

2000 (i) 0

4

8 Time (ns)

12

Figure 15.1 (a) Steady-state fluorescence spectra of C-151 in the ionic liquid DAF in the presence of total DMA concentration: (i) 0 mM; (ii) 12.62 mM; (iii) 25.25 mM; (iv) 37.87 mM; (v) 50.49 mM; (vi) 63.11 mM; (vii) fluorescence spectra of neat DAF. (b) Time-resolved fluorescence decays of C-153 in the presence of DMA concentration: (i) lamp profile; (ii) 0 mM; (iii) 41.02 mM; (iv) 78.89 mM. Reprinted with permission from the American Chemical Society. Copyright 2009

In a homogeneous solvent, the electron transfer process is mainly governed by the diffusive motion of the reactants, as diffusion is the rate-determining step; in a higher free energy region, the electron transfer rate ultimately levels off with the diffusion rate. We have determined the diffusion constant by the Smoluchowski equation: 8RT ð15:4Þ kdiff ¼ 3000h where R is the gas constant (J K
1 mol
1), T is the temperature (K) and h is the viscosity of the medium (P). In our case the kdiff value for the ionic liquid DAF is approximately 0.9  108 m
1 s
1 and the kq values are 10 times higher than the kdiff value. This difference is due to the existence of voids in RTIL, and the solute

334 Hydrogen Bonding and Transfer in the Excited State 1.3

2.5

1.2

τ0/τ

2.0 I0/I

(b)

(a)

1.5

1.1 1.0

1.0

0.9 0

20

40

60

80

0

20

[Q] (mM)

40

60

80

[Q] (mM)

Figure 15.2 (a) Stern–Volmer plots for C-152A (*), C-151 (~) and C-522 (&) systems in DAF with increasing quencher concentration using steady-state intensity. (b) C-152 (~), C-480 (*) and C-522 (&) using time-resolved data. Reprinted with permission from the American Chemical Society. Copyright 2009 Table 15.1 Lifetime, quenching constants, redox potentials, E00 values and DGo for different coumarin–amine systems studied in neat DAF System

t0 (ns)

SS kq (109M
1 s
1)

TR kq (109 M
1 s
1)

E(A/A
) V/vs SCE

E00 (eV)

E(D/Dþ ) V/vs SCE

DGo (eV)

3.59 2.94 2.72 0.84 0.76 1.72

1.621 3.946 3.765 13.607 11.987 11.645

0.549 1.303 1.162 2.952 3.026 3.087

2.10 1.685 1.653 1.660 1.626 1.565

2.920 2.586 2.657 2.734 2.756 2.846

0.756


0.254
0.315
0.428
0.498
0.554
0.705

C-480 þ DMA C-153 þ DMA C-522 þ DMA C-152A þ DMA C-152 þ DMA C-151 þ DMA 0.4

(b)

(a) 3.00E+009

2.00E+009

0.2

kq

r(t)

0.3

0.1

0.0 0.0

1.00E+009

2.5

5.0

Time (ns)

7.5

10.0

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

0

ΔG (eV)

Figure 15.3 (a) Decays of fluorescence anisotropy r(t) of C-522 (*) and C-153 (!) in DAF. (b) The plot of kq versus DGo for several coumarin–DMA systems in DAF. Reprinted with permission from the American Chemical Society. Copyright 2009

molecules diffuse rapidly from one void to another through the movement of the segment of the ions. We have measured the viscosity of neat DAF, and also, by using the rotational relaxation time of the probe molecules, the microviscosity h of the surrounding environment in the ionic liquid DAF was estimated by using the Stokes–Einstein relation [18]

Chemical Dynamics in Room-Temperature Ionic Liquids: the Role of Hydrogen Bonding

  hV trot ¼ kT

335

ð15:6Þ

where h is the viscosity, V is the volume of the fluorophore, k is the Boltzmann constant, T is the absolute temperature and trot is the rotational relaxation time. We have observed that the measured viscosity of neat DAF is different from the calculated microviscosity of the probe molecules. This also supports the notion that the microviscosity sensed by the solute molecules is different from the macroscopic viscosity. The macroscopic viscosity depends on the movement of the whole ion [19]. We have compared the electron transfer rate constants of the two probes C-153 and C-522 in CH3CN with the rate constant in DAF (Table 15.1) [20]. It is found that the rate constant decreases by an order of magnitude from CH3CN to the ionic liquid DAF. This is a real effect of the ionic liquid on the rate of the electron transfer reaction and is not due to the limiting effect of diffusion. We have plotted kq values obtained from time-resolved measurement with
DGo; here, kq increases with increasing
DGo and approaches the diffusion limit. So, in this PET reaction in ionic liquid DAF we have observed Rehm–Weller [16] behaviour like that of other homogeneous solvents such as acetonitrile. In the PET reaction, solvent polarity has a major effect on the free energy of activation and solvent reorganization energy. The dynamic solvent effect, i.e. the friction between the reactants and polar medium, play a major role in the rate of PET. Therefore, the rate of PET is dependent on the solvent relaxation time. We have reported the solvent relaxation time of C-153 in neat DAF [8]. The solvent relaxation is very fast in DAF, and, using our TCSPC set-up, we are unable to capture the solvation dynamic at 298 K owing to the limited time resolution [8]. We have reported the solvent relaxation time of C-153 in neat DAF at different low temperatures (278, 283 and 288 K). From this study, the rate constant of solvation dynamics was calculated at 2  109 s
1. In the present work we have observed that the rate constant of the PET reaction is of the order of 109 s
1. Therefore, in the present system, the solvation dynamics and PET are competitive in nature. Although the appearance of dynamic heterogeneity in ionic liquid is well demonstrated [6, 9, 13], the ET rate (kq) versus
DGo plot shows saturation to a diffusion-controlled value in the higher exergonicity region, which is observed in other neat solvents [20]. In the present work, the photoinduced electron transfer rate in the ionic liquid DAF decreases by an order of magnitude compared with the conventional solvent acetonitrile. Moreover, the electron transfer rate constants in DAF are significantly higher than the values predicted from the measured viscosity on the basis of the Smoluchowski equation. This may possibly be due to the fact that the microviscosity sensed by the solute molecules is different from the macroscopic viscosity. We have observed a saturation that shows a Rehm–Weller type of behaviour in the electron transfer rate in the correlation of the quenching constant (kq) with the free energy change (DGo) of the reaction.

15.2 Dynamics of Solvent Relaxation in Room-Temperature Ionic Liquids Containing Mixed Solvents The dynamics of the solvent and rotational relaxation of 153 (C-153) has been investigated in N,N, N–trimethyl-N-propylammonium bis(trifluoro-methanesulfonyl) imide (abbreviated as [N3111][Tf2N]) as an RTIL. Methanol and acetonitrile were used as the primary polar cosolvents to study the effect on solvation dynamics in the RTIL. [N3111][Tf2N] is an aprotic ionic liquid. Furthermore, for [N3111][Tf2N] the optical density at 410 nm and the fluorescence emission are very low compared with other aromatic RTILs. For these reasons, we have chosen [N3111][Tf2N] for this solvation and rotational dynamics study. Moreover, we optimized the geometry of [N3111][Tf2N]–methanol by density functional theory (DFT) methods [21]. C-153 in neat [N3111][Tf2N] shows an absorption peak at 425 nm. With the gradual addition of methanol, the emission

336 Hydrogen Bonding and Transfer in the Excited State

peak is red-shifted and finally reaches a value of 530 nm after the addition of 0.4 mol fraction of methanol in neat [N3111][Tf2N]. Similarly, the addition of 0.4 mol fraction of acetonitrile in neat [N3111][Tf2N] leads to a red-shift of the emission maximum to 525 nm. There were several studies on solvation dynamics in neat RTILs [22–24] after the first report of Karmakar and Samanta [25]. The emission obtained from neat [N3111][Tf2N] is only <1%, as opposed to the emission of C-153 in [N3111][Tf2N] at 523 nm, as shown in Figure 15.4(b). The estimated average solvation time and rotational relaxation time in neat [N3111][Tf2N] are found to be 0.35 ns and 2.95 respectively at 298 K. The average solvation and rotational relaxation time of the RTIL decreases with gradual additions of cosolvents. At 298 K the average solvation time decreases 58 and 29% owing to the addition of 0.33 mol fraction of acetonitrile and methanol respectively. The variation in fast components is very small, and the slow components gradually decrease owing to the addition of cosolvents. The presence of cosolvents reduces the electrostatic attraction between the ions and lowers the overall cohesive energy, resulting in a decrease in viscosity. The motion of the cations and anions causes solvation in RTILs. 0.3 (a) C153 in [N3111][Tf2N] absorbance

0.2

0.1

0.0 350

400

450

500

550

Fluorescence intensity (a.u)

wavelength (nm)

1.0

(b)

0.8 0.6

(ii)

0.4 0.2

(i) 0.0 450

500

550

600

650

700

wavelength (nm)

Figure 15.4 (a) Absorption spectrum of C-153 in neat [N3111][Tf2N]. (b) Emission spectra of (i) neat [N3111] [Tf2N] and (ii) C-153 in neat[N3111][Tf2N]. Reprinted with permission from the American Chemical Society. Copyright 2009

Chemical Dynamics in Room-Temperature Ionic Liquids: the Role of Hydrogen Bonding

337

1.0 (a) 0.8

C(t)

0.6 0.4 0.2 0.0 0

500

1000 1500 2000 Time (ps)

2500

1.0 (b) 0.8

C(t)

0.6 0.4 0.2 0.0 0

500

1000

1500

2000

Time (ps)

Figure 15.5 (a) Decay of the solvent correlation function, C(t), of C-153 in a [N3111][Tf2N]–methanol mixture: (i) 0.00 (*); (ii) 0.25 (~); (iii) 0.40 (&) mol fraction methanol at 298 K.(b) Decay of the solvent correlation function, C(t), of C-153 in a [N3111][Tf2N]–acetonitrile mixture: (i) 0.00 (*); (ii) 0.14 (~); (iii) 0.33 (&)mol fraction acetonitrile at 298 K. Reprinted with permission from the American Chemical Society. Copyright 2009

Thus, with the gradual addition of cosolvents, the viscosity of the RTIL–cosolvent mixture decreases. Consequently, decay of the solvent correlation function C(t) is faster owing to the gradual addition of cosolvents in RTIL (see Figure 15.5). This is because the addition of methanol separates the anion of RTIL, thus making its movement faster (similar to that of acetonitrile), but at the same time restricting its motion through hydrogen bonding. Also, ion dipole interaction is greater in RTIL–acetonitrile than in RTIL–methanol mixtures [26]. So, the decrease in solvation time is greater owing to the addition of acetonitrile to RTIL compared with methanol addition to RTIL (see Table 15.2). Therefore, we can say that acetonitrile screens the direct Coloumbic interactions between cations and anions of RTIL more efficiently than methanol, which facilitates rotational and translational motion. The variation in average solvation time versus viscosity of RTIL–acetonitrile and RTIL–methanol mixtures is almost linear. For accurate observation, we fitted the data points to a straight line and generated the error bar (see Figure 15.6). The figure clearly shows that the fitting accuracy value of the RTIL–methanol mixture (R ¼ 0.998) is greater than the fitting accuracy value of the RTIL–methanol mixture (R ¼ 0.975). The greater deviation in the case of the RTIL–methanol mixture is due to

338 Hydrogen Bonding and Transfer in the Excited State Table 15.2 Decay parameters of C-153 in neat [N3111][Tf2N] and [N3111][Tf2N] þ cosolvents mixture at 298 K. Reprinted with permission from the American Chemical Society. Copyright 2009 Systems mol mol mol mol mol mol mol

a1

t1 (ns)

a2

t2 (ns)

htiav (ns)

298 298 298 298 298 298 298 298

0.52 0.51 0.59 0.58 0.62 0.45 0.44 0.45

0.13 0.11 0.11 0.10 0.09 0.09 0.06 0.05

0.48 0.49 0.41 0.42 0.38 0.55 0.56 0.55

0.60 0.54 0.52 0.46 0.46 0.41 0.32 0.27

0.35 0.32 0.28 0.25 0.23 0.26 0.21 0.15

fraction methanol fraction methanol fraction methanol fraction methanol fraction acetonitrile fraction acetonitrile fraction acetonitrile

Viscosity (cP)

NEAT RTIL RTIL þ 0.14 RTIL þ 0.25 RTIL þ 0.33 RTIL þ 0.40 RTIL þ 0.14 RTIL þ 0.25 RTIL þ 0.33

Temp. (K)

70 (a) RTIL-acetonitrile mixture RTIL-methanol mixture 60 50 40 30 20 0.10

0.15

0.20

0.25

0.30

0.35

0.40

τ avs(ns)

80 70

RTIL-acetonitrile mixture R=0.99171 RTIL-methanol mixture R=0.95902

Viscosity (cP)

(b)

60 50 40 30 20 1.0

1.5

2.0

2.5

3.0

3.5

τrot(ns)

Figure 15.6 The plot of (a) average solvation time versus bulk viscosity of RTIL þ cosolvents mixtures and (b) average rotational relaxation time versus bulk viscosity of RTIL þ cosolvents mixtures. Reprinted with permission from the American Chemical Society. Copyright 2009 (See Plate 16)

Chemical Dynamics in Room-Temperature Ionic Liquids: the Role of Hydrogen Bonding

339

Figure 15.7 Optimized structure of [N3111][Tf2N]–methanol, calculated at the B3LYP/6-31G(d) level. Reprinted with permission from the American Chemical Society. Copyright 2009

the associating behaviour of methanol with the RTIL molecule. For further confirmation, we have added Figure 15.7, which shows a DFT-optimized structure of RTIL in the presence of a methanol molecule. The optimized structure shows a non-bonded interaction between the hydrogen atoms attached to the oxygen atom of methanol and nitrogen atoms of the anion of the RTIL. In the B3LYP/6-31G(d) level optimized  structure of RTIL–methanol, the distances between N. . .H, O. . .H and N. . .O are 2.05, 0.97 and 3.01 A. The van der Waals N. . .H and O. . .H distances are 2.75 and 2.72 A respectively [27]. In the B3LYP/6-31G(d) level  ... optimized structure of the RTIL, the N O distance is 3.01 A , which is shorter than the N. . .H. . .O distance  (3.02 A). Moreover, in the B3LYP/6-31G(d) level optimized structure of RTIL–methanol, the N. . .H. . .O cone angle is 172 . This supports the conclusion that the N. . .H. . .O cone angle is very close to 180 , confirming the presence of linear hydrogen bonding. The variation in solvation dynamics of C-153 in neat [N3111][Tf2N] has been investigated with the addition of two different cosolvents (methanol, acetonitrile). In neat RTIL and all other mixed solvents, solvation occurs in two well-separated time regimes within our instrumental time resolution. A substantial amount of solvation is too fast to be detected in our instrument. Solvation and rotational relaxation time decrease owing to the addition of cosolvents. The cosolvent additions to RTIL screen the direct Coloumbic attractions between cations and anions, facilitate rotational and translational motion and decrease the viscosity. Hence, both the motion of anions and the rearrangement of the ions around the photoexcited system become faster, and consequently the solvation time decreases. From the optimized structure of [N3111][Tf2N] it was shown that a hydrogen bond is formed between the hydrogen atoms attached to the oxygen atom of methanol and nitrogen atoms of the anion of RTIL. The addition of the same amount of acetonitrile has a more pronounced effect on solvation of RTIL than methanol.

Acknowledgements NS is grateful to the Department of Science and Technology (DST), CSIR and BRNS, Govt of India, for generous research grants. SS and RP are grateful to CSIR for research fellowships. SS is also grateful to BRNS for a senior research fellowship.

References 1. (a) N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 37, 123 (2008); (b) R. D. Rogers and K. R. Seddon, Science, 302, 792 (2003). 2. M. J. Earle, K. R. Seddon and P. B. McCormac, Green Chem., 2, 261 (2000).

340 Hydrogen Bonding and Transfer in the Excited State 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

M. J. Earle, J. M. S. S. Esperanca, M. A. Gilea et al., Nature, 439, 831 (2006). Y. Shim, D. Jeong, S. Manjari et al., J. Acc. Chem. Res., 40, 1130 (2007). A. Samanta, J. Phys. Chem. B, 110, 13 704 (2006). A. Paul and A. Samanta, J. Phys. Chem. B, 111, 4724 (2007). D. Chakrabarty, P. Hazra, A. Chakraborty et al., Chem. Phys. Lett., 381, 697 (2003). D. Seth, S. Sarkar and N. Sarkar, J. Phys. Chem. B, 112, 2629 (2008). H. Jin, X. Li and M. Maroncelli, J. Phys. Chem. B, 111, 13 473 (2007). H. Jin, B. O’Hare, J. Dong et al., J. Phys. Chem. B, 112, 81 (2008). H. Shirota, H. Pal, K. Tominaga and K. Yoshihara, J. Phys. Chem. A, 102, 3089 (1998). A. Adhikari, K. Sahu, S. Dey et al., Phys. Chem. B, 111, 12 809 (2007). Z. Hu and C. Margulis, J. Acc. Chem. Res., 40, 1097 (2007). R. A. Marcus, J. Chem. Phys., 24, 966 (1956). R. M. Lyndel-Bell, J. Phys. Chem. B, 111, 10 800 (2007). A. Chakraborty, D. Seth, P. Setua and N. Sarkar, J. Chem. Phys., 124, 074512 (2006). D. Rehm and A. Weller, Isr. J. Chem., 8, 259 (1970). A. S. Holmes, D. J. S. Birch, A. Sanderson and G. G. Aloisi, Chem. Phys. Lett., 266, 309 (1997). (a) A. Skrzypczak and P. Neta, J. Phys. Chem. A, 107, 7800 (2003); (b) D. Rehm and A. Weller, Isr. J. Chem., 8, 259 (1970). S. Nad and H. Pal, J. Phys. Chem. A, 104, 673 (2000). M. J. Frisch, G. W. Trucks, H. B. Schlegel et al., Gaussian 98 Revision A.6. Gaussian, Inc., Pittsburgh, PA (1998). A. Paul and A. Samanta, J. Phys. Chem. B, 111, 4724 (2007). H. Jin, G. A. Baker, S. Arzhantsev et al., J. Phys. Chem. B, 111, 7291 (2007). D. Seth, S. Sarkar and N. Sarkar, Langmuir, 24, 7085–7091 (2008). R. Karmakar and A. Samanta, J. Phys. Chem. A, 106, 4447 (2002). M. T. Z. Moattar and H. Shekaari, J. Chem. Eng. Data, 50, 1694 (2005). A. Bondi, J. Phys. Chem., 68, 441 (1964).

16 Vibrational Spectroscopy for Studying Hydrogen Bonding in Imidazolium Ionic Liquids and their Mixtures with Cosolvents Johannes Kiefer Lehrstuhl f€ur Technische Thermodynamik (LTT) and Erlangen Graduate School in Advanced Optical Technologies (SAOT), Universit€at Erlangen-N€urnberg, Am Weichselgarten 8, 91058, Erlangen, Germany School of Engineering, University of Aberdeen, Fraser Noble Building, King’s College, Aberdeen AB24 3UE, Scotland, UK

16.1 Introduction Ionic liquids (ILs) are substances consisting of anions and cations, but, in contrast to ordinary salts, ILs are liquid at temperatures below 100
C according to a well-accepted definition [1]. Many of them are characterized by extremely low vapour pressures, non-flammability, electric conductivity and unique solubility properties, which explain the growing interest in this class of liquid material from both the science and engineering communities [2]. Over the past years, numerous concepts for IL applications have been developed as media for, for example, catalysis [3], biotechnology [4], separation technology [5], electrochemistry [6] and particle synthesis [7]. As ILs are interesting for so many different fields, they are nowadays intensively studied by all kinds of experimental and theoretical methods with the aim of gaining a detailed understanding of their molecular behaviour as well as their macroscopic properties. Among these methods, spectroscopic techniques play an important role in better understanding the special nature of ionic liquids and their interactions with dissolved components or interfaces. A large and important class of ILs is based on cations involving an imidazolium ring (see Figure 16.1). The electronic structure of the imidazolium ring can be described as follows: a delocalized three-centre–fourelectron configuration can represent the configuration across the N1--C2--N3 moiety; at the opposite side of the

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

342 Hydrogen Bonding and Transfer in the Excited State

Figure 16.1 Schematic structure of the imidazolium-based cation

ring, a double bond between C4 and C5 is present, resulting in a weak electron delocalization in the centre of the imidazolium ring. The peripheral hydrogen atoms carry most of the positive charge. The C2 atom is slightly positively charged owing to the electron deficit in the C¼¼N bond, whereas C4 and C5 are practically neutral. Further details have been discussed by Hunt et al. [8, 9]. The mentioned electron distribution results in a moderate acidity of the C2 hydrogen atom, and, as a consequence, this moiety can act as a hydrogen bond donor and plays an important role in molecular interactions with anions as well as cosolvent molecules. Typical dialkylimidazolium-based ILs contain a methyl group at the R2 position, and hence in most cases 1-alkyl-3-methylimidazolium [AMIM]þ cations are considered and investigated. At the R1 position, the alkyl chain length is often varied in order to vary the specific chemical and thermophysical properties of the resulting IL. Often, methyl [MMIM]þ , ethyl [EMIM]þ , butyl [BMIM]þ and hexyl [HMIM]þ are of interest. As mentioned above, the hydrogen atom at the C2 position is moderately acidic and therefore has a strong tendency to form hydrogen bonds either towards anions or towards polar cosolvent molecules that are present in the system. Such interionic and intermolecular hydrogen bonds can exert influences on the vibrational structure of the participating molecules in terms of a change in vibration intensity as well as vibration frequency. Therefore, in turn, this hydrogen bonding can be studied by means of vibrational spectroscopy. In this article, the literature on vibrational spectroscopic studies for the investigation of hydrogen bonding in pure ionic liquids and their mixtures with organic cosolvents is reviewed without claiming to be comprehensive, as this would go far beyond its scope. The article is structured as follows: firstly, the most common experimental methods, i.e. infrared absorption and Raman scattering spectroscopy, are introduced and further techniques are briefly mentioned; secondly, an overview of hydrogen bonding studies using IR and Raman techniques is given, focusing on cation–anion interactions and mixtures of ILs with water; thirdly, a concluding section describes the potential, challenges and possible future applications of vibrational spectroscopy in this field regarding both ground and excited states.

16.2 Experimental Approaches The most common methods for experimental study of molecular vibrations are absorption spectroscopy in the infrared region and spontaneous Raman scattering. Although similar transitional energy ranges are investigated in IR and Raman spectroscopy, the signal intensities are governed by different selection rules. Therefore, both methods can deliver complementary information, and as a consequence both types of spectrum may be needed for a detailed characterization of the system under investigation. Furthermore, nonlinear optical techniques, e.g. in terms of coherent anti-Stokes Raman scattering (CARS) and sum-frequency generation (SFG), are sometimes used. Details about all techniques mentioned can be found in the common textbooks (see, for instance, Refs [10] and [11]). 16.2.1 Infrared absorption spectroscopy Infrared (IR) spectroscopy is based on the direct absorption of electromagnetic radiation by a molecule. In order to make this happen, the energy of the photon to be absorbed has to match the energy difference

Vibrational Spectroscopy for Studying Hydrogen Bonding

343

between two molecular rovibrational energy levels. In other words, the frequency of the electromagnetic wave must be equal to the oscillating frequency of a molecular vibrational mode. As a result, the molecule is in an excited rovibrational state after the absorption process. Moreover, a necessary requirement for IR absorption is that an oscillating dipole moment be produced during the vibrational motion of the molecule. This can be expressed by   qm 6¼ 0 ð16:1Þ qq where m is the dipole moment and q is the normal coordinate. As an absorption-based technique, the intensity I of the light transmitted by a sample of thickness d can in principle be described by the Beer–Lambert relation   I E ¼ log ¼ «ð lÞ  c  d ð16:2Þ I0 with the incident light intensity I0, the extinction coefficient « as a function of wavelength l and the concentration c. The parameter E is usually named either absorbance or extinction. In most cases in which IR spectroscopy is applied, the sample is placed in-between two transmittive plates, and the transmitted light intensity in the infrared region is recorded as a function of wavelength, yielding the characteristic IR spectrum. Typical sample thicknesses lie in the range 25–500 mm. As the material of the plates must be transmittive for the IR radiation, conventional borosilicate or fused silica quartz glasses cannot be employed. Crystalline materials such as KBr, CaF2 and NaCl are better suited. However, one serious problem arises, as these salt materials may be affected and be irreversibly damaged by water-containing samples or by the pure ionic liquids themselves. An alternative for performing IR spectroscopy is employing an arrangement for attenuated total reflection (ATR). In such an experiment, the IR radiation is propagating in a transmittive material (often ZnSe or diamond crystals) with a high refractive index ncrystal. The sample, which has a lower refractive index nsample, is in contact with a surface of the crystal. The radiation is reflected at an angle a at this surface (total internal reflection) so that the evanescent field can interact with the sample under investigation and is partly absorbed. As a consequence, the reflected beam is attenuated. An advantage of the transmission arrangement is its well-defined sample thickness. In ATR experiments, the penetration depth dp (1/e) dp ¼

l   2 1=2 nsample 2 2p  ncrystal  sin a ncrystal

ð16:3Þ

of the evanescent wave is a function of wavelength and must be taken into account when ATR IR spectra are interpreted quantitatively. 16.2.2 Spontaneous raman scattering In contrast, Raman spectroscopy is based on an inelastic light scattering process. This means that the scattered radiation is frequency shifted referring to the incident monochromatic light. In other words, an energy exchange takes place during the light–matter interaction. In the case of Stokes Raman scattering, the scattered light is shifted towards lower frequency (red-shift), which means that energy is transferred from the incident light to the molecule. As a result, the molecule is in an excited state. In the case of vibrational

344 Hydrogen Bonding and Transfer in the Excited State

Raman scattering, the molecules reaches a higher vibrational level. In the case of anti-Stokes Raman scattering, the opposite effect can be observed as energy is transferred from the molecule to the electromagnetic wave and hence the scattered light is shifted towards higher frequency (blue-shift). Of course, the latter effect can only occur when molecules in excited states are already present. Note that at room temperature the number of molecules in excited vibrational states (recall the Boltzmann distribution) is more or less negligible; hence, the Stokes Raman spectrum is typically recorded. A necessary requirement for Raman scattering is a change in the polarizability a of the molecule during the vibrational motion. This can be expressed by   qa 6¼ 0 qq

ð16:4Þ

where q is the normal coordinate again. A commonly used picture for the illustration of light scattering processes is shown in Figure 16.2. The final energy state is reached via an intermediate, so-called virtual level, which is not allowed from the quantum mechanical point of view. Therefore, the entire process can be considered to be instantaneous. In general, the integrated signal intensity for a Stokes Raman line is often simplified and considered linearly dependent on the species number density, the intensity of incident light, the solid angle of the signal collection optics and the Raman scattering cross-section. For gas-phase diagnostics in engineering applications this can be sufficient (see, for example, Refs [12] and [13]). However, such a simple relation does not take into account the behaviour of larger molecules or the influences of intermolecular interactions. In this context, the polarization properties of the Raman signal can play an important role for the interpretation of Raman data. When linearly polarized monochromatic light is used as the excitation source, the Raman signal is mainly linearly polarized in the same way. This typically holds for small molecules. When the molecules are large and the individual vibrating systems are non-symmetric, a strong depolarization of the Raman signal may be observed. In ionic liquids, such depolarization effects can be of interest for the assignment of spectral lines to vibrational modes and also for the interpretation of spectral changes which suggest influences from hydrogen bonding. A detailed description of the Raman signal characteristics can be found in the edited book of Schrader [10], and an example of depolarization ratio measurements in ionic liquids has been given in Ref. [14].

Figure 16.2

Energy level schemes of linear elastic and inelastic light scattering processes

Vibrational Spectroscopy for Studying Hydrogen Bonding

Figure 16.3

345

Energy level schemes of the CARS and SFG processes

16.2.3 Further techniques Other methods for vibrational spectroscopy that can be employed to study hydrogen bonding effects in ionic liquids may be found in the field of nonlinear optics. In order to describe the behaviour of a medium in an oscillating electric field E, the induced polarization P of the medium is given by P ¼ «0 w1 E þ «0 w2 E 2 þ «0 w3 E 3 þ   

ð16:5Þ

where «0 is the vacuum dielectric constant and w is the susceptibility. The first term describes linear effects such as Rayleigh and Raman scattering, the second term includes the so-called three-wave mixing processes (recall the second harmonic generation in a laser) and the third term is assigned to four-wave mixing phenomena. It should be noted that processes of even order are only possible in optically anisotropic media (like birefringent crystals). Therefore, third-order effects are the simplest ones that can be observed from typical bulk gases and liquids. However, second-order effects may be used at the surface of liquids, as an optical anisotropy is present there. As regards vibrational spectroscopy, the most common technique is coherent anti-Stokes Raman scattering (CARS) spectroscopy, which utilizes a third-order process. A frequently employed nonlinear method for vibrational surface studies is sum-frequency generation (SFG), which is based on a second-order effect. Both energy level schemes are shown in Figure 16.3, revealing the possibility of investigating molecular vibration. However, these techniques are only mentioned for the sake of completeness and will not be further discussed herein. For details, see, for example, Refs [10], [11], [15] and [16].

16.3 Hydrogen Bonding in Ionic Liquids A large number of vibrational studies have been performed to investigate the conformational isomerism of pure ionic liquids or even isolated ions. In this context, typically, vibrational spectra of ILs with systematically varied anions or cations while keeping one ion constant are recorded and compared with ab initio and semiempirical molecular orbital model calculations. These methods allow us to predict the IL chemical structures and derive their vibrational spectra. The basics of such quantum chemical calculations can be found, for example, in Refs [17] and [18]. Moreover, the use of Raman spectroscopy in combination with ab initio calculations has recently been summarized in an excellent review by Berg [19].

346 Hydrogen Bonding and Transfer in the Excited State

In the following, an overview of vibrational spectroscopic investigations of hydrogen bonding in pure ionic liquids and their mixtures with cosolvents is given. Seeing as a comprehensive summary would go beyond the scope of this article, only a number of selected works are covered. 16.3.1 Interionic hydrogen bonding Hydrogen bonds play an important role in the interactions between cations and anions in ILs. Strong and directional H-bonds between cations and anions can destroy the charge symmetry and thus can fluidize ionic liquids. H-bonds introduce ‘defects’ into the Coulomb network of ILs and increase the dynamics of the ions, resulting in decreased melting points and reduced viscosities. Thus, the properties of ILs can be altered by adjusting the ratio between Coulomb forces and van der Waals interactions [20–22]. The low-frequency vibrational bands between 50 and 120 cm1 can be assigned to the bending and stretching modes of cation–anion interactions. Measurements in this spectral region are only possible with special equipment suitable for the far infrared region. Note that the materials mentioned in Section 16.2.1 on IR spectroscopy are not suited for this purpose. The cation–anion interactions are represented in a general way by C--H. . .A between an imidazolium ring C atom and an arbitrary A anion. The strength of the interactions and thereby the strength of the H-bonds between cation and anion differ significantly with changing anion. An increase in the strength of the interionic hydrogen bond is accompanied with a lengthening of the covalent C--H bond and a shortening of the C--H. . .A hydrogen bonds. The weaker force constant of the covalent C--H bond leads to a lower vibrational frequency and as a consequence to a red-shift of the corresponding vibrational band. The opposite behaviour is expected for the stretching modes of hydrogen bonds themselves. A stronger hydrogen bond means a shorter intermolecular distance and a larger force constant. Thus, the stronger the H-bond, the higher is the vibrational frequency and the higher is the intensity of the corresponding vibrational band. In conclusion, one appropriate way to measure the strength of H-bonds between cation and anion in ILs is to investigate the frequency blue-shift in the far infrared region 50–120 cm1, or to investigate the resulting redshift of the covalent C--H bonds in the ring C--H stretching region 3050–3200 cm1. In imidazolium-based ILs, a fairly large number of different anions may be involved. Along with rather simple ones – such as the halogenides (Cl, Br and I), [BF4], [PF6]  and [SCN] – more complex anions are also common. Some examples are given in Figure 16.4. Fumino et al. [21] measured the low-frequency IR spectra of [EMIM][SCN], [EMIM][N(CN)2], [EMIM] [EtSO4] and [EMIM][TFSI] and compared them with the calculated binding energies of isolated ion pairs (note that this is often referred to as ‘gas phase’). The strength of the hydrogen bonds has been found to decrease in the following order: [SCN], [N(CN)2], [EtSO4] and [TFSI]. This is indicated by decreasing wave numbers and peak intensities. The experimentally determined frequencies of the interionic vibrational frequencies have been compared and correlated with the calculated binding energies. The obtained relationship indicates that the measured low-frequency vibrational bands really describe the interionic forces. The frequency shift of the interionic vibrational bands only results from a decreasing force constant, and not from a different mass of the anion. This finding is supported by measuring the symmetric and antisymmetric stretching vibrations of water in similar ILs. Water is also interacting with the anion via hydrogen bonds. So a change in the anion strength also affects the vibrational motions of water molecules. The intermolecular O--H stretching frequencies show the same anion dependency as the interionic vibrational modes, although the intermolecular vibrational frequencies of water are completely independent of the anion masses [21]. The redshift of the stretching frequencies of water in the order [TFSI] to [SCN] can also be explained by the strength of the hydrogen bonds between anion and water molecules. The hydrogen bonds of [TFSI] are the weakest, and thus the covalent O--H bonds of the water molecule are stronger and the O--H stretching modes appear at higher frequencies.

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Figure 16.4 Chemical structure of typical anions. Note that bis(trifluoromethylsulfonyl)imide is often abbreviated as [TFSI], [NTf2] or [Tf2N]

Yokozeki et al. [23], K€ oddermann et al. [24] and Fumino et al. [20] investigated the temperature dependency of the ring C--H vibrational region in the range 3050–3200 cm1. Their results give a more detailed look at the hydrogen-bonding interactions between the ring C--H bonds (C2--H and C4,5--H) and the corresponding anion. Their results indicated the existence of free ions and ion-pair aggregates, depending on the hydrogen bond type. K€ oddermann et al. distinguished between imidazolium cations that are completely H-bonded to anions and those that are singly H-bonded in ion pairs. This conclusion has been supported by the spectroscopic results. Two major peaks were observed in the ring C--H stretching region. The deconvolution of the C--H stretching spectrum revealed four peaks instead of three peaks as predicted for an isolated molecule owing to the three covalently bonded H atoms at the ring. Moreover, the hydrogen atoms in positions 4 and 5 are nearly symmetric. Consequently, the antisymmetric stretching modes would be nearly degenerated and eventually only two peaks would be expected. Yokozeki et al. speculated that the four peaks suggest the possibility that the imidazolium ring exists in two different configurations: as dissociated ions and in associated ion pairs. Two of the C--H stretching modes could be assigned to the C4--H and C5--H vibrations. The other two originated from the two configurations of the C2--H, i.e. the free ion and the hydrogen-bonded ion. These findings and conclusions have been further supported by experiments in ionic liquids where the C2 hydrogen atom was replaced with a methyl group, as well as by density functional theory (DFT)-based calculations. A detailed study of the conformational isomerism in [BMIM][BF4] has been carried out by Holomb et al. [25]. They utilized DFT methods in combination with Raman and IR spectroscopy and identified characteristic spectral features revealing the coexistence of four cation conformers by considering isolated cations first. In order fully to understand the vibrational spectra, the interionic hydrogen bonds between the different cation conformers with the anion needed to be taken into account. Similar approaches of combining theoretical and experimental methods are a kind of standard nowadays, and they are frequently applied to ILs (see, for example, Refs [19] and [26]–[28]).

348 Hydrogen Bonding and Transfer in the Excited State

16.3.2 Intermolecular hydrogen bonding Besides contributing to the interactions between anions and cations, hydrogen bonds can be formed between the ions and polar cosolvent molecules as well. The presence of cosolvents can strongly influence the molecular behaviour of an IL and, in addition, the macroscopic properties of the mixture. In recent years the determination of thermophysical properties of IL/cosolvent mixtures has been a very active field (see, for example, Ref. [29] and the references therein). The most important species in this context is water, as numerous ILs are hygroscopic owing to their ionic nature. For this reason there is often a small water content present, and as a consequence IL/water mixtures are the most frequently investigated ones. The presence of water in ILs may affect many of their solvent properties such as polarity, viscosity and conductivity [30]. Moreover, water may also react with the constituents of ILs, e.g. in ILs involving the [PF6] anion a chemical reaction may take place to form HF [31]. Nevertheless, usually water interacts with ILs via hydrogen bonds, as already mentioned in the previous subsection. The symmetric (n1) and antisymmetric (n3) stretching frequencies of water are sensitive probes in vibrational spectroscopy for the intermolecular interaction of water with ionic liquids. The role of water in ILs is complex and depends on the molecular structures of ILs. For instance, water can modify the self-organization pattern of ILs. In this context, interactions between the anion and water molecules are investigated in particular, but there are also studies dealing with the interaction between the C--H groups of imidazolium and H2O. As the imidazolium ring C--H bonds also interact with water by hydrogen bonds, similar approaches to the ones described in the previous subsection are employed to study these phenomena between the water molecules and the imidazolium cation. It should be mentioned that in practice it is often difficult to deduce information concerning the molecular state of water from the symmetric and antisymmetric stretching frequencies of water because there is a strong coupling between n1 and n3. To overcome this problem, in many studies heavy water D2O is used. A similar study to Fumino et al. [21], which revealed the interaction strength of various anions with the imidazolium cation, was published by Cammarata et al. [32], who investigated the interaction strength of various anions with water and additionally showed the minor cation influences on the molecular states of water in the O--H stretching region. K€ oddermann et al. [33] gave exact molecular states of water in [EMIM][TFSI] and [EMIM][EtSO4]. Moreover, this group was able to show that the vibrational modes of single water molecules dissolved in ILs are a viable probe for their polarity. Chang et al. [34, 35] analysed the structural organization in aqueous solutions of 1-butyl-3-methylimidazolium halides under high pressure. Their study elucidates that the imidazolium C--H groups seem to be more favourable sites for hydrogen bond formation compared with the C--H groups of the alkyl chains. Influences of varying water contents on the molecular state of the 1-butyl-3-methylimidazolium cation in [BMIM][BF4] and [BMIM][I] have been discussed by Jeon et al. [36, 37]. The far infrared spectral region relating to the liberation bands of water and corresponding to the interaction strength of water and anion has been reported by DominguezVidal et al. [38]. Looking at the existing experimental vibrational studies of IL/water mixtures, it seems reasonable to divide them into studies in ‘water-rich’ regions with a high content of water and ‘water-poor’ regions with low water contents. In the water-poor region, the experimental results suggest that water solely interacts with the anion of an IL, whereas, in the water-rich region, interactions between the alkyl side chain and the imidazolium ring are also observed. However, to the best of our knowledge, no concrete criteria have been reported so far for distinguishing between the two regions. K€ oddermann et al. [33] investigated IR spectra from mixtures with water concentrations below 1 wt%. At these concentrations, the spectra did not show further changes with changing concentration and temperature. Thus, they assumed that single water molecules are embedded in the IL environment and that no water clusters exist. Water molecules are isolated from each other and interact with the ions of the ionic liquid by hydrogen bonding. In contrast, in the water-rich region it can be assumed that all water/anion interaction sites are already

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occupied and further water molecules are left to interact with the alkyl side chain, the imidazolium ring and water clusters.

16.4 Potential, Challenges and Future Applications In recent years, vibrational spectroscopy has been proven to be a versatile tool for studying ionic liquids and their mixtures with cosolvents. In particular, methods such as IR absorption and spontaneous Raman scattering spectroscopy are frequently employed to investigate interionic and intermolecular interactions in terms of hydrogen bonds. In addition, quantum chemical methods are used in order to get a clear picture of the interactions and their influences on the molecular structure. This is a very active field today and will facilitate obtaining new insights into and a deeper understanding of the nature of ILs. In turn, a detailed understanding of the molecular structure and its influences on the behaviour on a molecular scale (e.g. responsible for chemical behaviour) will allow the tailoring of ILs from theoretical considerations for technical applications. Efforts in this field have already been reported [39, 40]. In the third section of this article, rather simple systems of either pure ionic liquids or their binary mixtures with small cosolvent molecules have been mentioned. However, there are efforts going on to make vibrational spectroscopy a tool for more complex systems. For instance, IR spectroscopy has been employed for quantitative measurements of carbohydrates or a Wilkinson catalyst dissolved in ionic liquids [41, 42]. In the case of the catalyst experiments, the spectra revealed a red-shift of imidazolium ring bending mode with increasing catalyst concentration. This was attributed to the hydrogen bond between the C2--H group and the chloride ligand of the catalyst. Minnich et al. [43, 44] used IR techniques for reaction monitoring in IL synthesis in order to obtain information about the reaction progress. This approach could also be utilized in future applications for further study of the processes in technical systems at the molecular scale. Further fields of development are related to the determination of macroscopic properties of ILs and their mixtures with cosolvents. The thermophysical properties such as density, viscosity, speed of sound, sound attenuation, surface tension, etc., can be determined with high accuracy and precision nowadays. In this context, there are ongoing efforts to find correlations between macroscopic properties and microscopic behaviour, e.g. in terms of the vibrational structure. First results show that certain spectral features revealing intermolecular interactions of ILs and cosolvents can be correlated with the change in selected thermophysical properties [45]. Along with the linear techniques (mainly IR and Raman spectroscopy), the nonlinear optical methods are experiencing an increasing interest. In particular, sum-frequency generation (SFG) is a useful tool for investigating interfaces between ionic liquids and gases [46, 47], liquids [48] and solids [49, 50]. Moreover, time-resolved coherent anti-Stokes Raman scattering (CARS) will facilitate studying ionic liquids in excited vibrational states. Other nonlinear pump–probe techniques will allow a closer look into ILs in excited electronic states.

Acknowledgements The author would like to thank K. Noack and J. Hackner for proofreading the manuscript and helping with the literature search. Support from and helpful discussions with A. Leipertz, A. P. Fr€oba, K. Obert, A. B€osmann and P. Wasserscheid over the past years are gratefully acknowledged. Furthermore, financial support of the Erlangen Graduate School in Advanced Optical Technologies (SAOT) by the German Research Foundation (DFG) within the framework of the German excellence initiative is acknowledged.

350 Hydrogen Bonding and Transfer in the Excited State

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25. R. Holomb, A. Martinelli, I. Albinsson et al., Ionic liquid structure: the conformational isomerism in 1-butyl-3methyl-imidazolium tetrafluoroborate ([bmim][BF4]). J. Raman Spectrosc., 39, 793–805 (2008). 26. Y. Umebayashi, T. Fujimori, T. Sukizaki et al., Evidence of conformational equilibrium of 1-ethyl-3-methylimidazolium in its ionic liquid salts: Raman spectroscopic study and quantum chemical calculations. J. Phys. Chem. A, 109, 8976–8982 (2005). 27. J. C. Lassegues, J. Grondin, R. Holomb and P. Johansson, Raman and ab initio study of the conformational isomerism in the 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide ionic liquid. J. Raman Spectrosc., 38, 551–558 (2007). 28. S. Hayashi, R. Ozawa and H. Hamaguchi, Raman spectra, crystal polymorphism, and structure of a prototype ionicliquid [bmim]Cl. Chem. Lett., 32, 498–499 (2003). 29. A. P. Fr€oba, P. Wasserscheid, D. Gerhard et al., Revealing the influence of the strength of Coulomb interactions on the viscosity and interfacial tension of ionic liquid co-solvent mixtures. J. Phys. Chem. B, 111, 12 817–12 822 (2007). 30. L. A. S. Ries, F. Amaral, K. Matos et al., Evidence of change in the molecular organization of 1-n-butyl-3methylimidazolium tetrafluoroborate ionic liquid solutions with the addition of water. Polyhedron, 27, 3287–3293 (2008). 31. H. Weing€artner, Understanding Ionic Liquids at the Molecular Level: Facts, Problems, and Controversities. WileyVCH, Weinheim, Germany (2008). 32. L. Cammarata, S. G. Kazarian, P. A. Salter and T. Welton, Molecular states of water in room temperature ionic liquids. Phys. Chem. Chem. Phys., 3, 5192–5200 (2001). 33. T. K€oddermann, C. Wertz, A. Heintz and R. Ludwig, The association of water in ionic liquids: a reliable measure of polarity. Angew. Chem. – Int. Edn, 45, 3697–3702 (2006). 34. H. Chang, J. Jiang, C. Chang et al., Structural organization in aqueous solutions of 1-butyl-3-methylimidazolium halides: a high-pressure infrared spectroscopic study on ionic liquids. J. Phys. Chem. B, 112, 4351–4356 (2008). 35. H. Chang, J. Jiang, Y. Liou et al., Effects of water and methanol on the molecular organization of 1-butyl-3methylimidazolium tetrafluoroborate as functions of pressure and concentration. J. Chem. Phys., 129, article 04452 (2008). 36. Y. Jeon, J. Sung, D. Kim et al., Structural change of 1-butyl-3-methylimidazolium tetrafluoroborate þ water mixtures studied by infrared vibrational spectroscopy. J. Phys. Chem. B, 112, 923–928 (2008). 37. Y. Jeon, J. Sung, C. Seo et al., Structure of ionic liquids with different anions studied by infrared vibration spectroscopy. J. Phys. Chem. B, 112, 4735–4740 (2008). 38. A. Dominguez-Vidal, N. Kaun, M. J. Ayora-Canada and B. Lendl, Probing intermolecular interactions in water/ionic mixtures by far-infrared spectroscopy. J. Phys. Chem. B, 111, 4446–4452 (2007). 39. M. B. Turner, S. K. Spear, J. D. Holbrey et al., Ionic liquid-reconstituted cellulose composites as solid support matrices for biocatalyst immobilization. Biomacromolecules, 6, 2497–2502 (2005). 40. S. Katsyuba, E. Zvereva, A. Vidis and P. Dyson, Application of density functional theory and vibrational spectroscopy toward the rational design of ionic liquids. J. Phys. Chem. A, 111, 352–370 (2007). 41. J. Kiefer, K. Obert, A. B€osmann et al., Quantitative analysis of alpha-D-glucose in an ionic liquid using infrared spectroscopy. ChemPhysChem, 9, 1317–1322 (2008). 42. J. Kiefer, K. Obert, S. Himmler et al., Infrared spectroscopy of a Wilkinson catalyst in a room-temperature ionic liquid. ChemPhysChem, 9, 2207–2213 (2008). 43. C. B. Minnich, P. Buskens, H. C. Steffens et al., Highly flexible fiber-optic ATR-IR probe for inline reaction monitoring. Org. Process Res. Dev., 11, 94–97 (2007). 44. C. B. Minnich, L. K€upper, M. A. Liauw and L. Greiner, Combining reaction calorimetry and ATR-IR spectroscopy for the operando monitoring of ionic liquids synthesis. Catalysis Today, 126, 191–195 (2007). 45. J. Kiefer, J. Lehmann, A. Leipertz and A.P. Fr€oba, Intermolecular interactions between acetone and the roomtemperature ionic liquid [EMIM][EtSO4]. Proceedings of the 17th Symposium on Thermophysical Properties, Boulder, CO (2009). 46. T. Iimori, T. Iwahashi, K. Kanai et al., Local structure at the air/liquid interface of room-temperature ionic liquids probed by infrared-visible sum frequency generation vibrational spectroscopy: 1-alkyl-3-methylimidazolium tetrafluoroborates. J. Phys. Chem. B, 111, 4860–4866 (2007).

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17 Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids: a Time-Resolved Fluorescence Study Luca Nardo1, Alessandra Andreoni1 and Hanne Hjorth Tønnesen2 1

University of Insubria, Department of Physics and Mathematics and C.N.I.S.M.-C.N.R., U.d.R. Como Via Valleggio 11, 22100 Como, Italy 2 School of Pharmacy, University of Oslo, P.O. Box 1068 Blindern, 0316 Oslo, Norway

17.1 Introduction 17.1.1 Fluorescence decay studies in drug development In a drug formulation, light may have effects on both the active principle and the excipients [1]. New compounds are frequently added to the list of photolabile drugs: some of these drugs will decompose by only a small percentage after several weeks exposure, while other substances (e.g. nifedipine derivatives) are extremely photolabile: their photodecomposition rate may be comparable with the radiative decay rate. Obviously, light-sensitive drug substances frequently raise formulation problems. The most obvious result of drug photodecomposition is loss of potency. Moreover, even trace amounts of certain photodecomposition products formed in the formulation during administration, which for certain drugs (e.g. intravenous solutions) may last for hours or even days, can lead to undesirable side effects. A drug that displays photochemical reactivity in vitro may also give rise to adverse photosensitivity effects in patients after administration. Sunlight might catalyse interactions between any of the drug formulation components and endogenous substrates, generating potentially (photo)toxic products. It is therefore important to stabilize the drug substance or product against photodegradation in order to increase the drug shelf life and, if possible, reduce side effects in vivo. A first step in this direction is to identify the potential photoreactivity of all the compounds that are intended to be used in the drug formulation at an early stage of the preformulation work. The drug

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

354 Hydrogen Bonding and Transfer in the Excited State

photoreactivity describes how efficiently and by which reaction pathways the compound or formulation responds to optical radiation. Unfortunately, a relationship between the chemical structure of a compound and its photoreactivity can be difficult to predict, although certain structural features are expected to cause photoreactivity. Evaluation of drug photoreactivity includes identification of degradation products by means of, for example, HPLC, mass spectrometry and NMR, evaluation of photoxidation potential by, for example, detection of singlet oxygen luminescence and detection of reactive intermediates generated during irradiation. The latter involves time-resolved techniques such as laser flash photolysis, pulse radiolysis and time-resolved fluorescence (TRF). Upon absorption of light, the drug molecule is excited to a higher energy state from which a number of photochemical and photophysical reactions may occur. When the products of these reactions are permanent, they can be analysed and quantified by steady-state techniques. However, these stable products are most likely formed via short-lived excited states and possibly free radicals. These species can further react with other components in a drug preparation or with biomolecules in vivo. Identification and characterization of the short-lived intermediates are therefore very important in order to estimate the overall photoreactivity of a drug substance (i.e. both within the formulation and in vivo). In the present work, TRF is applied to investigate the influence of aromatic substituents and microenvironment on the photoreactivity of potential photosensitizers containing a b-diketone moiety. Time-resolved fluorescence is a convenient method for elucidating the singlet-excited-state deactivation pathways. Moreover, time-resolved fluorescence methods are valuable tools for investigation of photodecomposition reactions in fluorescent drug molecules, in that the shape of the fluorescence decay pattern is not affected by variations in the excitation intensity. Photodecomposition time constants can be directly revealed as exponential components in the drug fluorescence decay both in the case where photodecomposition is caused by direct cleavage of the drug intramolecular chemical bonds (e.g. cleavage of a halogen bond in a substituted aromatic ring) and in the case where photodegradation is mediated by dynamic quenching of molecules in the substrate of the drug. If photodecomposition is triggered by interactions between the drug and its environment that are different from dynamic quenching, the photodecomposition rate may still be determined as the difference between the total decay rates measured in the absence and in the presence of the substrate reactants involved in the photodegradation reaction. Furthermore, the influence of either excipients or microenvironment on the initial photodecomposition reaction step can be elucidated by performing in vitro measurements on samples containing the drug compound combined with selected excipients or solvents. Finally, if either the intermediates or the final photoproducts are fluorescent, they can be recognized by means of multiwavelength excitation of the sample. The lifetime of the intermediates, and thus the kinetics of the dark (thermal) reactions, may be assessed by means of fluorescence correlation/coincidence methods. If the excited-state reaction pathways of a compound have been elucidated, it is possible to modify the chemical structure to improve photostability while preserving the biological activity. Enhancement of intramolecular H-bonding by modification of molecular moieties involved in the H-bond formation results in changes to the photoreactivity of the compounds studied in the present work. Alternatively, the drug photoreactivity might be changed by modification of the microenvironment, e.g. through complexation with suitable drug carriers. The ability of the drug molecule to form intermolecular H-bonds with either the solvent or the carrier is of primary importance. For instance, instauration of H-bonds within the cage carrying the drug is the basis for the formation of inclusion complexes between a drug and, for example, cyclodextrins. In the case of some drug molecules, the formation of inclusion complexes has been reported significantly to reduce the photodecomposition rate of the drug. TFR can provide useful information on how minor modifications in the drug molecule or drug delivery system will alter the deactivation pathways of the singlet excited state. TRF techniques are also useful in the development of photosensitized drug substances or drug delivery systems [2]. Many drug substances show enhanced therapeutic potential, or even display new biological activities, upon excitation to their (fluorescent) first excited singlet state S1. Intersystem crossing can occur from the first excited singlet state, bringing the drug to the first excited triplet state T1. During the long T1

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 355

lifetime (phosphorescence decay time), the excitation energy carried by the molecule can be exchanged with substrates to trigger reactions featuring a high activation potential. This often leads to the formation of free radicals and subsequent chain reactions. Molecular oxygen can effectively quench molecules in their T1 state. In fact, the ground state of the oxygen molecule, that is, a spin triplet, can be efficiently excited to a highly reactive singlet state by collision with the drug in T1. The singlet oxygen formed in the process may exhibit a phototoxic effect on living cells. This principle can be used to design photochemotherapeutic antitumour drugs (examples are porphyrins, phthalocyanines and some antracycline derivatives), or antibacterial/antiviral drugs. Drugs whose therapeutic action is mediated by light exposure are called photosensitizers, and their clinical use is referred to as photodynamic therapy, although strictly speaking this name is correct only when oxygen is involved in the therapeutic process. A great advantage of photodynamic therapy is that the drug (i.e. photosensitizer) needs to be exposed to light in order to trigger the cytotoxic action. Thus, unlike classical antibiotics and chemotherapeutic drugs being cytotoxic in the ground state, and thus killing indistinctly pathogenic and healthy cells, photosensitizers can be target specific, assuming that a suitable drug delivery system is designed and that the irradiation source can be directed to the actual tissue to be treated via, for example, fibre optics. The mechanisms responsible for the phototherapeutic activity of a photoactivated drug substance have to be clarified in order fully to exploit its therapeutic potential. Moreover, once the deactivation pathway of S1 relevant to the biological activity of the molecule (usually intersystem crossing to T1) has been elucidated, it would be convenient to inhibit all the other possible S1 deactivation pathways that compete with this process. Apart from undergoing S1 decay via photophysical pathways, such as internal conversion or static and dynamic quenching, a photosensitizer might de-excite by means of a wide range of either reversible or irreversible photochemical reactions. As discussed above, photochemical degradation reactions are examples of irreversible and undesirable photochemical deactivation pathways. Most photochemical reactions occur through a series of simple steps known as primary photochemical processes. Very often, the primary photochemical process involves the transfer of protons or electrons either from one to another moiety of the same fluorophore molecule by excited-state intramolecular charge transfer [3] or from the fluorophore to one solvent molecule (in vitro) or the substrate (in vivo) by excited-state intermolecular charge transfer [4]. In some instances, charge transfer occurs spontaneously owing to the extensive changes in the molecular charge distribution experienced by the fluorophore upon excitation to S1. In such cases, the charge transfer is very fast. Sizeable residual fluorescence emission, occurring in competition with the photochemical reactions triggered by charge transfer, is usually observed. The fraction of fluorophores that decay through fluorescence emission undergo the reverse charge transfer spontaneously and recover the ground-state charge distribution. In other systems, the charge transfer can be a thermodynamically unfavourable process, or it can be inhibited by the presence of a sizeable activation potential barrier. In such systems, the excitation energy can be used to trigger charge transfer. The charge transfer step thus occurs simultaneously to de-excitation and may or may not be followed by thermal dark reactions. Moreover, owing to the presence of a sizeable activation potential barrier, the molecule may preserve the new charge distribution once it has returned to S0. The majority of charge transfer reactions (both intramolecular and intermolecular) involve the transfer of a proton from an oxygen donor to an oxygen or nitrogen acceptor [3], although a few cases are known in which a nitrogen atom acts as the donor and a carbon atom as the acceptor [3]. Intramolecular H-bonding between the two moieties involved in the proton transfer significantly enhances both the proton transfer probability and its rate by lowering the activation potential barrier. The most common example of H-bond-mediated excited-state proton transfer in the literature is the tautomerization between the keto and the enol moiety in the six-membered ring of methyl salicylate, which occurs through an excited-state intramolecular proton transfer (ESIPT) reaction [5]. Other molecular structures prone to ESIPT are five-membered rings of the type C2OHO and six-membered rings such as C3OHN, NC2OHN and C3OHC, in which the carbon that acts as the acceptor moiety is alkenic or alkynic in nature [3]. In these instances, the dynamics of tautomerization encompasses a large range of time domains,

356 Hydrogen Bonding and Transfer in the Excited State

ranging from femtoseconds to microseconds. The tautomerization rate depends on the height of the activation potential barrier and thus on the strength of the H-bond connecting the donor with the acceptor moiety. As ESIPT is facilitated by intramolecular H-bond formation, the process is strongly affected by the fluorophore environment. For instance, in solution the solvent polarity and H-bonding properties have major effects on the ESIPT probability and rate. However, the majority of the photochemically and photophysically relevant ESIPT processes occur in the picosecond domain. As the intrinsic radiative decay time of a fluorophore is usually several nanoseconds, the fluorescence quantum yield of the primary excited form is usually very low. Thus, the fluorescence of this form is often hardly detectable by means of standard steady-state fluorescence detection techniques. On the other hand, TRF methods offer suitable sensitivity. Indeed, although overall very faint, the fluorescence is emitted by the primary excited form in a very brief time lapse, and thus the fluorescence photon count rate is measurable on a sufficiently fast timescale. Moreover, the rate of the ESIPT process, and consequently the strength of the intramolecular H-bond, can be quantified by TRF measurements. A class of compounds that are likely to undergo ESIPT is that of b-diketones. The b-diketones display tautomerization in multiple enol and diketo structures, which are shown in Figures 17.1 and 17.2 respectively. The closed cis-enol forms are characterized by a strong intramolecular H-bond between the keto oxygen and the enol proton (keto–enol H-bond, KEHB). When the enol group forms the intramolecular H-bond, the b-diketone system undergoes changes towards the total p-system delocalization [6]. There is a strong correlation between the strength of KEHB and the molecule p-system delocalization. Charge delocalization on the enol–keto structure in turn favours ESIPT from the enol to the keto moiety. Electronically excited b-diketones can also deactivate by undergoing photochemical reketonization [7, 8]. This process, accompanying the enol excitation, is postulated to start with cis–trans isomerization, so that the intramolecular H-bonding stabilization of the cis-enol is lost, followed by decay via another ESIPT mechanism

Figure 17.1

Possible structures of a b-diketone in its enol conformer

Figure 17.2 Possible structures of a b-diketone in its diketo conformer

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 357

in which the accepting moiety is the vinylic CH. It is worth noting that, as in this instance the donor and the acceptor moieties are not connected by any intramolecular H-bond, the reketonization process is expected to be both less probable and much slower with respect to ESIPT between the enol and keto moiety. The photosensitized therapeutic potential of a drug molecule might be improved either by means of the rational design of synthetic substituted molecular analogues or by modification of the microenvironment. The knowledge of the decay mechanisms at different environmental conditions of various substituted molecular analogues and the quantification of the rates at which such mechanisms take place are important in order to implement either of the two approaches. TRF techniques are suitable for obtaining such information, and should be combined with other methods applied in the photoreactivity screening in order to achieve the optimal drug molecule or formulation. 17.1.2 Curcumin and its derivatives as photosensitizers Curcumin, bis(4-hydroxy-3-methoxyphenyl)-1,6-diene-3,5-dione (see the chemical structure in Figure 17.3), is a natural yellow-orange pigment derived from the rhizome of the plant Curcuma longa L., popularly called turmeric, a member of the Zingiberaceae family. Curcumin belongs to the group of b-diketones and exhibits the usual tautomerism between enol and diketo structures of Figures 17.1 and 17.2. The isolated curcumin molecule in the ground state adopts any of the closed cis-enol forms (Figure 17.1), which are stabilized by KEHB formation. In solution, curcumin can form intermolecular H-bonds with the solvent molecules. This fact strongly influences its intramolecular H-bonding and physicochemical properties in both the ground and excited states. The compound is the main constituent of curry, and it is widely employed in traditional Indian and Arabic cooking. Curcumin is also applied worldwide as a dye for the industrial colouring and conservation of food, and it is an ingredient of several cosmetic products [9–14]. Considering the therapeutic potential of curcumin, the dried and powdered rhizome of turmeric has been used as a traditional medicine since ancient times[15–17]: its wound healing and anti-inflammatory properties are even described in the sacred books of ayurveda. In the last 20 years, a constantly increasing number of publications have shown that, in vitro, curcumin displays notable effects, not only as an anti-inflammatory compound [15–20] but also as a potent antioxidant [19–29], chemopreventive [30–37] and chemotherapeutic [38–41] agent. Moreover, curcumin can inhibit the metabolic action of aflatoxin B1 [42], of aminopeptidase N [43], of lipoxygenase [44], of cycloxygenase [45], of ornithine decarboxylase [37] and of the efflux transporters MRP1 and MRP2 [46]. Further in vitro studies demonstrate curcumin efficiency against Alzheimer’s disease [47] and cystic fibrosis [48]. Finally, curcumin is considered to be a drug or model in the treatment of HIV infections [49–51] and as an immunestimulating agent [49]. Instauration/disruption of both intramolecular and intermolecular H-bonds, together

methoxy groups O

O OCH3

H3CO

-diketo moiety HO

OH

Phenolic hydroxyl groups

Figure 17.3

Chemical structure of the curcumin molecule

358 Hydrogen Bonding and Transfer in the Excited State

with charge delocalization, combine to determine drug interactions with biomolecules (e.g. binding to amyloid B plaques [47] and proteins [52]), and thus its therapeutic potential. Additional biological activity can be triggered in curcumin by light exposure. Indeed, while curcumin in its ground state, S0, is essentially non-toxic to animals and humans [53–58], and cytotoxicity to selected bacteria has been reported only at impractically high drug concentration [59], upon excitation to the S1 state, curcumin becomes remarkably phototoxic to bacteria [60, 61] and to mammalian cells, both cancerous [13] and healthy [62]. Phototoxicity is at least partly mediated by oxygen [13, 60, 61] and is apparently not induced by the stable curcumin photoproducts vanillin and ferulic acid [61]. There has been speculation concerning the generation of reactive oxygen species (ROS) such as singlet oxygen [61], hydroxyl radical [63, 64] or hydrogen peroxide [60] as a result of curcumin excitation. Indeed, singlet-oxygen photosensitization has been reported for curcumin [14, 60, 61, 65]. However, the singlet-oxygen yield, although strongly dependent on the environment, is generally quite low [65]. Moreover, the primary cytotoxic reactant exhibits a lifetime significantly longer than that typical for ROS [13]. Alternatively, curcumin radicals [66] have been proposed as either the phototoxic species or the long-lived transients capable of interacting with oxygen to generate the toxic species without further illumination [13]. Although the mechanisms responsible for the phototoxic activity of curcumin are not clear [13, 14], it would be useful to inhibit all S1 deactivation pathways that compete with the one that has photosensitizing potential for full exploitation of the therapeutic potential of this molecule. As for any tentative photosensitizer, photochemical degradation of curcumin, which has been reported to be significant in certain environments [67, 68], is a particularly undesirable deactivation pathway. Instauration of KEHB in the closed cis-enol conformer potentially brings about other S1 decay mechanisms that might enhance the non-radiative decay rate. Hydrogen-stretching vibrations, e.g. a lengthening of the O
H bond and a shortening of the hydrogen bond in the closed cis-enol form of S1, could lead to radiationless decay. As the minimum Stokes shift was observed in non-polar solvent [13, 65], where KEHB should be the tightest, deactivation by H-stretching vibrations is apparently not efficient. Charge-transfer interactions provide other radiationless decay pathways to the closed cis-enol conformer. As for any b-diketone, three proton transfer mechanisms can occur: reketonization, intermolecular exchange of the labile enol proton and H-bondmediated ESIPT from one oxygen to the other of the cis-enol conformer. The last mechanism (outlined in Figure 17.4) is the fastest one and hence the most relevant [69], as it is favoured by KEHB formation [70–73]. Indeed, for other b-diketones, ESIPT rates as high as 1011 s
1 were reported [71]. As curcumin has both electron-donating and electron-accepting properties, S1 deactivation by intermolecular electron transfer should also be considered in H-bonding environments [9]. Optimizing both photosensitized therapeutic action and photostability might be achieved by means of rational design of synthetic curcuminoids [74]. 17.1.3 Methods for the time-resolved analysis of fluorescence: principles and advantages of time-correlated single-photon counting The above discussion makes it evident that being able to perform time-resolved measurements on very low fluorescence signals emitted by tentative drug substances, or on short-lived transient species generated upon

Figure 17.4

Excited-state intramolecular proton transfer in a b-diketone closed cis-enol conformer

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 359

excitation, is of importance in a rational drug development. The principal time-resolved fluorescence analysis techniques devised to this purpose will be briefly described in the following. Two complementary classes of techniques allow assessment of the parameters of a fluorescence decay (amplitudes and decay times of the decay components): the frequency domain techniques and the time domain techniques. In the case of measurements in the frequency domain, the fluorescent sample is excited by a source of light whose intensity varies in time with a high-frequency sinusoidal modulation [75]. The intensity of the fluorescence signal emitted by the sample oscillates with the same frequency of the exciting beam, but it is phase shifted and partially demodulated with respect to excitation. Both demodulation (M) and phase shift (w) are determined by the fluorescence decay times of the fluorophores in the sample. Thus, the decay times themselves can be inferred by measuring M and w. Frequency domain techniques are also capable of assessing the singleexponential or multiexponential nature of a decay [76]. In fact, for a single-exponential decay, the same lifetime should be calculated by using the observed w and M at any given frequency. On the other hand, if more than one decay component exists, w is biased towards the faster decay components and M is weighted towards the slower components. Even in this case, M and w constitute two separate observable parameters that are both directly related, via a Fourier transformation, to the initial relative fluorescence intensities, ai, and decay times, ti, of the fluorescence decay components. When multiexponential behaviour is displayed, the sample is usually subsequently excited with light modulated over a range of frequencies (typically ten or more) because of the intrinsic limitations of resolving the components of a multiexponential decay using only one frequency. In time domain techniques, a short pulse of light is made to impinge on the sample, and the temporal distribution of the photons re-emerging from the sample is directly recorded [77]. The time and the frequency domain techniques theoretically yield equivalent data. However, time-resolved methods allow straightforward data analysis and feature relevant advantages over frequency domain methods when the light to be timed is very dim. There are many ways to record time domain data. For instance, temporal resolution of the order of a few picoseconds, or even better, can be achieved by employing streak cameras as the light detectors [78]. A streak camera is an instrument for measuring the variation in time of the intensity of a pulse of light. Streak cameras are used to measure the pulse duration of picosecond-pulsed lasers. They also find applications in the development of LIDARs (remote sensing devices analogous to RADARs, but employing light rather than radio waves) and in time-resolved spectroscopy. A streak camera operates by transforming the temporal profile of a light pulse into a spatial profile of the photoelectrons emitted by a photocathode. Namely, the photoelectrons are accelerated in a cathode ray tube and pass through an electric field produced by a pair of plates, which deflects the electrons sideways. By modulating the electric potential between the plates, the electric field is quickly changed to give a time-varying deflection of the electrons, sweeping the electrons across a phosphor screen at the end of the tube. A linear detector, such as a charge-coupled device (CCD) array, is used to measure the streak intensity pattern on the screen, and thus the temporal profile of the light pulse. Most agree, however, that the method of time-correlated single-photon counting [79] is by far superior. Conceptually, the time domain profile of a light pulse could be directly acquired by means of single-photon counting (SPC) techniques by building a histogram of the number of photons detected in successive sampling time intervals much shorter than the pulse duration. Unfortunately, the light signals relevant to fluorescence studies typically have pulse durations of a few nanoseconds or less. Thus, sampling time interval widths in the picosecond range would have to be used in order to obtain decent profiles. Consequently, ‘count and store’ data acquisition electronics operating at the unconceivable rate of several terahertz would be needed. However, in TRF applications, the fluorescent sample can be repeatedly excited by a train of equally spaced ultrashort laser pulses in order to generate a corresponding number of virtually equal fluorescence emission pulses. In such a case, the time profile of a typical pulse can be very effectively recovered by applying a reconstruction technique called time-correlated single-photon counting (TCSPC), which, although based on SPC, makes it

360 Hydrogen Bonding and Transfer in the Excited State

possible to overcome the operational limits of the detection/storing electronics by approaching the data acquisition process in a way slightly different from ‘count and store’ methods, namely: 1. 2. 3. 4.

For each pulse of the pulse train to be analysed, at most one fluorescence photon must be detected. The arrival of each pulse of the train must be heralded by a signal triggering the timing device. The time elapsing from the heralding signal to detection of the photon (lag time) must be measured. The lag times must be digitalized into sampling intervals, and the photons being detected within each lag interval must be counted.

It is worth noting that the output data of TCSPC are histograms of the number of detected photons versus time, analogous to those that would be ideally obtained by applying a ‘count and store’ procedure. It should be noted that the detection/timing/storing electronics cannot reset within a few nanoseconds. Thus, only one photon per pulse can be timed. Thus, it is very important that more than one photon belonging to the same pulse impinges on the detector sensitive area with negligible probability. Otherwise, only the least-lagged photon data would be acquired, and the pulse temporal profile would be distorted. This means that no photon will be detected in the majority of the excitation periods. The diagram in Figure 17.5 shows the fundamental blocks of a TCSPC system. This diagram will be used in the following to outline the basic functions of the major components in such a system, and their mutual interactions. The present introduction constitutes an oversimplification of the technical solutions devised in modern laboratories. However, from a conceptual point of view the block diagram of Figure 17.5 substantially describes what actually occurs in a TCSPC experiment. The first essential component of any TCSPC apparatus is the light source. The light source has two main functions in a TCSPC apparatus: the first is to deliver onto the sample d-like pulses of light to probe its fluorescence properties; the second is to send a START signal to the time-to-amplitude converter (TAC). The light source of a TCSPC system must deliver pulses of very brief duration at a very high rate (in order to register reliable fluorescence decay distributions in reasonable acquisition times).

Figure 17.5 Scheme of a TCSPC set-up. BS: beam splitter; ND: neutral density filters; CFD: constant-fraction discriminator; TAC: time-to-amplitude converter; MCA: multichannel analyser

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 361

The second component of a TCSPC set-up is the single-photon detector. Cost-effective detectors, endowed with considerable temporal resolution (a few tens of picoseconds), are photomultiplier tubes (PMTs) [79]. PMTs intrinsically show better overall detection quantum efficiencies than streak cameras and are suitable for use in TCSPC. On the other hand, such detectors are very fragile and have to be stored and operated in the dark. Finally, they are subject to several operational drawbacks: first, they must be fed with high voltage (1000–3000 V); second, to assure reasonably low dark signals, they are often operated at low temperature (<
15  C). Another class of relatively low-cost single-photon detectors are the reach-through-mode avalanche photodiodes (APDs), which are much more user friendly than PMTs in that they are to be fed at lower voltage and can be used at room temperature [80]. APDs can be regarded as the semiconductor analogue to PMTs. They are endowed with higher detection quantum efficiency and characterized by lower dark count rates (a few hundreds of counts per second). On the other hand, in general they display a poor time resolution (>100 ps). The APD operating gain is the mean number of electrons generated in the depletion region by impact ionization from a single photoelectron, and primarily depends on the reverse bias potential applied to the APD. The higher the gain, the higher is the APD quantum efficiency. For this reason, some silicon APDs employ alternative doping and bevelling techniques compared with traditional APDs. Such techniques allow much greater voltage to be applied (>1500 V) before breakdown is reached, and hence a greater operating gain. Doped APDs are therefore more sensitive, but also more fragile compared with other semiconductor photodiodes. Single-photon avalanche diodes (SPADs) are a particular class of APD that are not only able to detect extremely low intensity signals (down to the single photon) but also to signal the time of the photon arrival with high temporal resolution (a few tens of picoseconds) [81–86]. The SPADs, like any other APD, exploit the photon-triggered avalanche current of a reverse biased p–n junction to detect an incident radiation. The fundamental difference between SPADs and APDs is that SPADs are specifically designed to operate with a reverse bias voltage well above the breakdown voltage (whereas APDs operate at a bias voltage slightly below the breakdown voltage). This kind of operation is also called the Geiger mode in the literature, by analogy with the Geiger counter. At this bias, the electric field is so high (>3  105 V cm
1) that a single charge carrier injected in the depletion layer can trigger a self-sustaining avalanche. The current rises swiftly (subnanosecond rise time) to a macroscopic steady level in the milliampere range. If the primary carrier is photogenerated, the leading edge of the avalanche pulse marks (with a picosecond time jitter) the arrival time of the detected photon. The current continues to flow until the avalanche is quenched by lowering the bias voltage down to, or below, the breakdown value: the lower electric field is no longer able to accelerate the carriers to impact-ionize with lattice atoms, and therefore the current ceases. In order to be able to detect another photon, the bias voltage must be raised again above breakdown. These operations require a suitable circuit, which has to sense the leading edge of the avalanche current, generate a standard output pulse synchronous with the avalanche build-up, quench the avalanche by lowering the bias down to the breakdown voltage and restore the photodiode to the operative level. This circuit is usually referred to as a quenching circuit. The simplest quenching circuit is commonly called the passive quenching circuit and is composed of a single resistor in series with the SPAD. This experimental set-up has been employed since the early studies on the avalanche breakdown in junctions. The avalanche current self-quenches simply because it develops a voltage drop across a high-value resistance (>100 kW). After quenching of the avalanche current, the SPAD bias slowly recovers to the operational value, and therefore the detector is ready to be ignited again. A more advanced quenching scheme is called active quenching. In this case a fast discriminator senses the steep onset of the avalanche current across a 50 W resistor and provides a digital output pulse synchronous with the photon arrival time. Any detector must be shielded, and the light to be analysed must be suitably filtered in order to eliminate non-time-correlated stray light from the source or the environment, and to depress the probability of more than one photon per excitation pulse impinging on the detector. In fact, as anticipated, none of the existing single-photon detectors can reset instantaneously after detecting one photon. The characteristic time during which a detector remains blind after revealing a photon

362 Hydrogen Bonding and Transfer in the Excited State

is called ‘dead time’. The function of the detector in the TCSPC system is to convert each photon impinging on its sensitive area into a photoelectron, and subsequently to amplify the microscopic photoelectric current in order to obtain a macroscopic current pulse. To accomplish this task, the detector must feature the highest possible ‘detection quantum efficiency’, which is the fraction of impinging photons converted into measurable current pulses. Moreover, the current pulse generated by a detected photon should be much higher than the parasitic currents intrinsic to the detector circuitry, in order to avoid ‘fictitious photon’ detection. In other words, the detector ‘operational gain’ should be as high as possible. Finally, in any singlephoton detector there are thermal and/or electronic processes leading to dark count events. Dark counts are processed as real photodetection events. This causes the detector to be blind for a whole dead time period following each false photon detection event. Moreover, detection of dark counts diminishes the signal-tonoise ratio, especially when very weak light pulses are to be analysed. For these reasons it is desirable for the detector of an efficient TCSPC system to show a low dark count rate. The properties that a detector must have in order to assure a high time resolution in TCSPC measurements will be discussed after explaining the operating principles of the time-to-amplitude converter (TAC). Before triggering the TAC, both the START and the STOP signals are evaluated by a discriminator. The discriminator is a key element in any TCSPC system, as it is the unit that makes TCSPC essentially insensitive to the noise problems that plague analogical methods of light analysis. A discriminator is an electronic circuit designed to differentiate between current intensity levels. If the input signal to the discriminator is below a specified threshold level, the signal is completely ignored; conversely, an input signal greater than the threshold level is recognized, causing the discriminator to produce a very neat output pulse which in turn is delivered to the TAC. By setting the discriminator threshold level to a current value substantially greater than the detector mean parasitic current level but smaller than the detector photocurrent pulse amplitude, the detector noise can be removed from the data. The central element in a TCSPC system is the TAC. A TAC can be viewed as a very precise stopwatch, with the light source providing the START signal and the emitted light providing the STOP. When it receives the START signal, the TAC begins to charge the plates of a capacitor by means of a precisely controlled constant current. When it receives the STOP signal, the TAC suddenly stops the current flow through the capacitor plates and generates an analogical voltage pulse whose amplitude is exactly equal to the potential difference DV between the capacitor plates. The latter is proportional to the time lapse between the START and the STOP signal. In other words (hence the instrument name), the TAC associates a voltage pulse of welldefined amplitude to any START/STOP time interval. As the TAC current starts flowing with a fixed delay from production of the START pulse by the START discriminator and stops flowing with a fixed delay from production of the STOP pulse by the STOP discriminator, the precision with which a photon is timed does not depend on the actual pulse width of the detector photocurrent pulse, as would happen in any analogical technique, but rather on the time jitter between detection of a photon and emission of the corresponding photocurrent pulse. This jitter must be the smallest possible to assure good timing performances in TCSPC experiments. The timing accuracy can be up to 10 times better than the half-width of the detector pulse response. It should be noted here that TCSPC provides a differential time measurement that is virtually unaffected by drifts and instabilities in the light source pulse period. The analog-to-digital converter (ADC) measures the amplitude of the voltage pulse coming from the TAC to determine into which slot of the histogram approximating the detected photon temporal distribution a particular detected photon should be recorded. It sends that ‘time slot information’ to the multichannel analyser (MCA) in the form of a digital channel number. Upon receiving the channel number, which is really just a memory address, the MCA adds 1 to the contents of that memory cell to record the fact that a photon was just detected with that specific START/STOP time lag. This process, which typically overall takes only a few microseconds, or even less, is then repeated over and over again until the events being recorded yield a reliable approximation of the actual temporal distribution to be studied.

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 363

17.2 Experimental Set-Up and Data Analysis Methods 17.2.1 Curcuminoids and solvents Curcumin (CURC) and dicinnamoylmethane (DCMeth) were synthesized as previously described [65, 87]. Their chemical structures are reported in Figures 17.3 and 17.6 respectively. Studies on the crystal structures [10, 88, 89] of the two compounds show that they display different chemical affinities with respect to the formation of KEHB in the solid state. In CURC, the enol proton is localized with equal probability around either of the two carbonyl oxygens [88], indicating the formation of a fairly weak KEHB, while in the DCMeth crystal structure the enol proton is equidistant from the two carbonyl oxygens [89], indicating complete charge delocalization and the formation of a semi-aromatic ring at the diketo structure, which is necessarily mediated by an extremely tight KEBH. The solvents used in the present study were divided into: non-polar (cyclohexane), polar non-H-bonding (chloroform, ethyl acetate, acetone and acetonitrile), H-bond-accepting (dimethylformamide (DMFA) and dimethylsulfoxide (DMSO)) and alcohols (isopropanol, ethanol and methanol). Note that the alcohols display both H-bond-donating and H-bond-accepting properties. To assess the polarity, we use the dielectric constant. The dielectric constants of the above solvents, together with the hydrogen-bonding donor parameter of the H-bond donors (acidity parameter a) and the hydrogen-bonding acceptor parameter (basicity parameter b) of the H-bond acceptors, are reported in Table 17.1. All the solvents were of 99.5% purity and used as received, except ethyl acetate, which was dried over sodium sulfate. Samples in the solvents were prepared the same day they were used for measurements.

O

O

Figure 17.6 Chemical structure of the dicinnamoylmethane molecule

Table 17.1 Solvent properties: hydrogen-bonding donor parameter a; hydrogen-bonding acceptor parameter b; dielectric constant at 25  C « Solvent Non-polar Polar non-H-bonding

H-bond acceptors Alcohols

Cyclohexane Chloroform Ethyl acetate Acetone Acetonitrile DMFA DMSO Isopropanol Ethanol Methanol

«

a

b

2.02 4.81 6.02 20.60 38.8 37.6 48.9 19.92 25.07 33.62

0 0.44 0 0.08 0.19 0 0 0.78 0.83 0.93

0 0 0.45 0.48 0.31 0.69 0.76 0.95 0.77 0.62

364 Hydrogen Bonding and Transfer in the Excited State

17.2.2 Our TCSPC set-up Our TCSPC experimental set-up is sketched in Figure 17.7. For the measurements presented here, as the light source we used a continuous-wave SESAM-mode-locked Ti:sapphire laser (fundamental emission wavelength 840 nm) emitting pulses at 48 MHz repetition rate with a 3.9 ps pulse width (Tiger-ps SHG; Time Bandwidth Products, Zurich, Switzerland). The samples were excited at 420 nm by the Ti:sapphire built-in second harmonic. The light exiting the excitation source is partially reflected by means of a microscopy cover glass. The principal part of the beam is transmitted through the glass and conveyed to the sample, while the small fraction of reflected light is delivered to the detector via a multimode optical fibre (Fsync), without interacting with the sample, to provide a time reference [90–93] stable against electronic drifts. In fact, the start pulses due to photons reaching the SPAD through the Fsync fibre give rise to two sharp peaks which are separated in time by exactly a pulse period. By selecting a temporal acquisition window slightly longer than the pulse period, they can be positioned at the first and last channels of the MCA, as depicted in Figure 17.7 (bottom right), by adjusting the delay (see below). The time distance between these peaks is extremely stable over time (it was measured to be constant within one channel over several hours of monitoring), while the absolute peak positions drift by as much as 100 channels. Superimposing the reference peaks of two different

Figure 17.7 Our TCSPC set-up. BS: beam splitter; ND: neutral density filters; Fsync: multimode fibre conveying the reference peaks to the detector; L: collimating lens; PIN: fast photodiode for synchronization with the excitation pulse; CFD: constant-fraction discriminator; TAC: time-to-amplitude converter; MCA: multichannel analyser; OBJ: 40 microscope objective; SPAD: single-photon avalanche diode; AQC: active quenching circuit

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 365

fluorescence decay patterns provides a common absolute timescale for the distributions to be analysed, as the difference in time between detection of photons conveyed to the reference branch of the TCSPC apparatus (sent to Fsync) and impingement onto the sample cell of photon pulses conveyed to the sample branch is constant. Collection of stray light by Fsync was prevented by inserting a black plastic tube (3 cm diameter) in front of the Fsync input end. A neutral density filter (OD ¼ 4) was attached to the tube entrance. Our TCSPC system features a passive electronic delay line for rough positioning of both the reference peaks and the temporal distribution of the photon emerging from the sample inside the temporal window. However, in order to be able to shift the reference peaks with respect to the measured distribution so as to obtain time distributions perfectly centred into the time window and reference peaks at its extremes, an optical delay has been put on the reference branch. The emitted light is collected at 90 with respect to the excitation beam and selected from excitation stray light by means of suitable cut-off (long-wavelength pass) filters. It is focused by means of a 40 microscope objective on the sensitive area of the detector. This is a SPAD (PDM50, Micro Photon Devices, Bolzano, Italy) featuring the following technical properties: 50 mm diameter of the sensitive area, <50 Hz dark count rate; built-in active quenching circuitry, assuring 70 ns dead time. The TCSPC system was run in a reversed START/STOP configuration, which makes it possible to prevent the delivery to the TAC of an abnormal number of START signals when the TCSPC device is used to study very rare (single-photon) events, such as fluorescence emission by low fluorescence quantum yield molecules. In these conditions, most START signals from the reference branch of the timing set-up would not be followed by the detection of photons on the probe branch within a temporal acquisition window. In the subsequent TAC dead time, any detection event would not be time correlated. At best this means that the experiment has to be run for a very long period of time to gather an amount of data assuring reliable statistics; at worst (e.g. if the sample is photolabile) it means that a reliable distribution histogram will never be obtained. The START/STOP reversal method consists in inverting the START and STOP connections of the TAC, so that the timing process is started by the detection of a fluorescence photon by the single-photon detector on the probe branch. In this way, the TAC is ready to accept a START all the time, as fluorescence photons are rarely detected. Moreover, the TAC is stopped for certain within the temporal width of the acquisition window by the laser pulse subsequent to that producing the fluorescence photon. Thus, in our TCSPC system the avalanche current pulses from the SPAD serve as the START signals to the TAC (mod. 2145, Canberra), while the STOP is given by a PIN photodiode internal to the laser. In order to assure optimal timing performance of the TCSPC system, the PIN output pulse was analysed via a constant-fraction discriminator (CFD, mod. 2126, Canberra). The CFD splits the pulse into two parts. One of them is anticipated/delayed by about one-half of the rise time with respect to the other, reversed and attenuated by an arbitrary factor, the so-called ‘constant fraction’ (optimal values of the attenuation factor range approximately from 0.3 to 0.7). The two pulses are then subtracted one from the other, and a comparator is used to detect the instant at which the current zero level (that is, the mean dark current level) is crossed by the difference pulse. It can be demonstrated that such a zero point is independent of the original pulse amplitude value. In other words, the zero point provides a jitter-free reference for the discriminator to send the STOP signal to the TAC. The TAC output is elaborated by an Acuspec B-Genie 2000MCA (Canberra). The MCA is endowed with a 5.35 ps bin width in a 30 ns time window. We recall that the time spacing between the reference peaks is equal to the laser pulse period. Thus, by measuring daily the laser repetition rate with a spectrum analyser, we were able to assign a precise value to the MCA channel width. As the MCA operates at 19 bits, excellent differential nonlinearity (<1%) is assured. At the other end, the output of the TAC can be digitalized at the fairly low rate of 16 kHz. The overall time resolution of our TCSPC apparatus is 30 ps (full width at half the maximum of the detected laser pulses). In Figure 17.8 the instrumental pulse responses of our apparatus to laser pulses at 420 nm and 535 nm are reported. Note that the fluorescence peaks of all the curcuminoids studied in this chapter fall either in the blue or in the green portion of the spectrum.

366 Hydrogen Bonding and Transfer in the Excited State 0

10

420 nm 532 nm -1

Intensity

10

-2

10

-3

10

0

500

1000

time (ps)

Figure 17.8 Instrumental pulse response at 420 nm and 532 nm (See Plate 17)

17.2.3 Data analysis method The CURC fluorescence decay data were fitted, without deconvolving the system pulse response (full width at half the maximum duration 30 ps), with either single, double or triple exponentials above a constant background by minimizing the chi-square value through a Levenberg–Marquardt algorithm with Origin 7.0 software. The multiexponential decays of DCMeth were fitted with a self-made program written with Mathemathica software (Wolfram Research). The program automatically performs fitting with 3–10 exponential components and yields x 2 values, the residual plots and the residual autocorrelation plots of each fit. The DCMeth decays were optimally fitted by either four or five exponentials. Fitting with additional components resulted in determination of more than one component with the same decay constant, with no improvement of the x 2 value and of the residuals. Six decay curves were acquired for each sample. The means of the values obtained from the corresponding fits, with errors given by the standard deviations, were assumed as the time constant, ti, and initial relative amplitude, Ai, of the ith decay component, being the Ai values calculated at the peak channel of the experimental data. An exemplary decay pattern is plotted in Figure 17.9, together with its fitting curve and residuals.

17.3 Results and Discussion 17.3.1 Curcumin The fluorescence decay times and relative amplitudes obtained for CURC dissolved in each of the solvents of Table 17.1 are reported (Table 17.2) and discussed first. Three decay components are identified in cyclohexane. Two fast components, with decay times t1 ¼ 57 ps and t2 ¼ 256 ps, account for 99% of the fluorophores (A1 ¼ 83% and A2 ¼ 16% respectively), while a much slower component with decay time t1 ¼ 1405 ps accounts for the remaining 1%. Single-exponential decays are observed in polar non-H-bonding solvents. The decays are double exponential in the polar H-bond-accepting solvents. In alcohols the decays are also double exponential, the only exception being isopropanol, in P Pwhich a single-exponential decay is observed. The average fluorescence lifetime tav (i.e. tav ¼ i ti Ai = i Ai ) was calculated for curcumin in each solvent by disregarding the decay components with Ai < 1%. The tav values are also reported in Table 17.2. The longest tav values are those obtained in polar non-H-bonding solvents (ranging between tav ¼ 467 ps in ethyl acetate

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 367 10000 CURC in Ethyl Acetate: 1000

Experimental decay pattern Single exponential fit

Counts

100

Residuals

10

1 200 100 0 -100 -200

0

5000

10000

15000

20000

Figure 17.9 Upper panel: experimental fluorescence decay pattern of CURC in ethyl acetate (black dots) and single exponential fitting curve (t ¼ 468 ps). Lower panel: residual plot (See Plate 18) Table 17.2 Decay times t (ps),  standard deviation, and relative initial amplitudes Ai, that fit the experimental fluorescence decays of CURC in the different solvents (i ¼ 1–3). Reprinted with permission from Elsevier. Copyright 2009 Solvent Non polar Polar non-H-bonding

H-bond acceptors Alcohols

Cyclohexane Chloroform Ethyl acetate Acetone Acetonitrile DMFA DMSO Isopropanol Ethanol Methanol

t1 (A1)

t2 (A2)

t3 (A3)

tav

57  1 (0.83) 596  4 (1) 467  3 (1) 702  4 (1) 695  2 (1) 246  3 (0.98) 117  4 (0.79) 476  1 (1) 221  6 (0.89) 138  5 (0.85)

256  3 (0.16)

1405  78 (0.01)

89 596 467 702 695 251 155 476 260 162

708  98 (0.02) 299  13 (0.21) 579  10 (0.11) 302  11 (0.15)

and tav ¼ 702 ps in acetone), while the shortest value is obtained in cyclohexane (tav ¼ 89 ps). In both H-bondaccepting and alcoholic solvents, tav (between tav ¼ 155 ps in DMSO and tav ¼ 260 ps in ethanol) is shorter than in polar non-H-bonding solvents, but longer than in cyclohexane. Once again, isopropanol is an exception, as in this solvent tav ¼ 476 ps, a value similar to those found in polar non-H-bonding solvents. It is reasonable to assume that the intrinsic radiative decay time of CURC is independent of the molecular environment. This is consistent with the observation that tav is proportional to the fluorescence quantum yield wf [65]. In order to explain the photophysics of a symmetric b-diketone such as CURC, all three diketo conformers and six enol conformers of the molecule should in principle be taken into account [94]. However, in solution, curcumin mainly adopts either of the cis-enol forms (Figure 17.1) [9, 13, 95]. This is confirmed by the lack of structures in both the absorption and fluorescence spectra [13]. The trans (anti) diketo conformer (Figure 17.2), whose dipole moment has been calculated to be much smaller than that of cis-enol [9], should not be ruled out

368 Hydrogen Bonding and Transfer in the Excited State

Figure 17.10 (A) Perturbation of intramolecular H-bonding by H-bonding solvents; (B) perturbation of intramolecular H-bonding by polar non-H-bonding solvents

in the case of non-polar solvents even if the enol form dominates in both polar and non-polar solvents at room temperature [96]. The presence of KEHB in the CURC excited state would mediate the S1 decay by means of ESIPT. Intermolecular H-bonding with solvent molecules perturbs KEHB formation. In the enol tautomer, the H-bond-accepting moieties of both alcohols and H-bond acceptors interact with the enol proton, while the H-bond-donating moieties of the alcoholic solvents can interact with the keto moiety (see Figure 17.10(A)). Perturbation by polar non-H-bonding solvents is also likely to occur by polarity effects (Figure 17.10(B)). The tav values reported in Table 17.2 indicate that the non-radiative pathways are most efficient in cyclohexane, where the tightness of the intramolecular H-bond ensures the occurrence of ESIPT. This indicates that ESIPT is the leading mechanism in the deactivation of the curcumin singlet state. However, the three-exponential fluorescence decay demonstrates the existence of other pathways with comparable rates. The shortest decay can be ascribed to the fraction of closed cis-enol curcumin molecules that are excited to the fluorescent state without breaking the H-bond, thus decaying through direct ESIPT. The intermediate decay can be ascribed to the closed cis-enol conformers that experience cis–trans isomerization upon excitation and deactivate by reketonization. The small molecular population emitting fluorescence with the longest lifetime might be the ground-state trans (anti) diketo conformers. The tav values are higher in polar solvents than in non-polar solvents. As stated above, the fluorescence decays in all the H-bonding solvents, except isopropanol, are biexponential. The fastest decay time values are not of the order of a few tens of picoseconds, as expected for direct ESIPT-driven decays, but approximately 200 ps. Possibly, the instauration of intermolecular H-bonds results in almost complete KEHB breaking, with consequent blocking of direct ESIPT. Proton transfer can no longer occur directly owing to the lack of any electronic connection between the separate solvated groups. Desolvation is required to form the fast-decaying intramolecular H-bonded enol structure. Because the ESIPT step is rapid once an intramolecular H-bond is formed, the decay time observed for this deactivation path will depend essentially on the rate of the desolvation process. Anyway, alternative decay mechanisms involving delivery of the excitation energy (or intermolecular charge transfer) to the solvent molecules are expected to be highly enhanced by the instauration of intermolecular H-bonds. On the other hand, even if a large number of excited molecules in the trans-enol conformer should be found in H-bonding solvents, reketonization is not probable, as the trans (anti) diketo conformer is much less polar compared with the two enol tautomers. Double-exponential fluorescence decays can thus be explained by assuming

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 369

a fast, intermolecular-H-bond-mediated S1 deactivation by transfer of energy to the solvent molecules and a slow solvent-reorganization-moderated ESIPT. In the case of isopropanol, the first mechanism might be prevented somehow. The tav values are the highest in polar non-H-bonding solvents. This indicates that polar non-H-bonding solvents also efficiently inhibit deactivation through ESIPT, but their interactions with curcumin do not bring about new decay pathways. This hypothesis is supported by the single-exponential decays we observed in all polar non-H-bonding solvents. Interestingly, the t1 values measured in the non-H-bonding solvents are in the same range as the longer decay times, t2, in H-bonding solvents that we associated with molecules decaying through solvent-rearrangement-moderated ESIPT. A model capable of explaining all the relevant features in the S1 dynamics of CURC can be devised by considering the above observations [65]. The model only requires the following two assumptions, which are supported by extensive literature on the behaviour of b-diketones: 1. ESIPT is the fastest possible non-radiative decay mechanism for the S1 state [71, 94] of CURC, and takes place only if KEHB is formed. 2. CURC in solution at room temperature is virtually found in its enol conformers only (Figure 17.1) [9, 14, 95, 96]. The H-bonded closed cis-enol structure is dominant in non-polar environments, while either the open cis-enol or the trans-enol conformers, which cannot form KEHB, are dominant in polar H-bonding and non-H-bonding solvents [97]. Tiny amounts of the least polar conformer, the trans (anti) diketo, can be found in non-polar environments [9].

17.3.2 Dicinnamoylmethane We now report the fluorescence decay data of DCMeth, elucidate the S1 decay mechanisms of this curcuminoid and compare its excited-state dynamics with that of CURC [14, 65]. It is worth remembering that KEHB is much stronger in DCMeth than in CURC in the solid state. The decay times ti, together with the relative initial amplitudes (see Ai in brackets), as determined by the multiexponential fits of the fluorescence decays measured for DCMeth, are reported in Table 17.3. The DCMeth fluorescence decay is multiexponential in all the solvents. Four decay components were detected in cyclohexane, in the polar non-H-bonding solvents and in the alcohols, except methanol, while five decay components were detected in DMFA, DMSO and methanol. The average fluorescence lifetime, tav, was calculated for each solvent. The tav values (see Table 17.3) do not show any trend with respect to the solvent dielectric constant. In fact, the tav that we find in cyclohexane (tav ¼ 104 ps), the solvent having the lowest dielectric constant, is only slightly longer than the value observed in DMSO (tav ¼ 101 ps), the solvent having the highest dielectric constant, but much shorter than the value tav ¼ 232 ps measured in DMFA, whose dielectric constant is intermediate. However, there seems to be a correlation between tav and the hydrogen-bonding donor and acceptor parameters of the solvents. A decrease in tav is indicated by an increase in the b value of the H-bondaccepting solvents, while an increase in tav is indicated by an increase in the a value of the alcohols. In an attempt to elucidate the S1 decay of DCMeth by the concurrence of the same mechanisms that were postulated for CURC, we first organize the data in Table 17.3 according to the ranges of values of the decay times ti (i ¼ 1–5). The decay components are grouped as indicated by the symbols and footnotes in Table 17.3, and the corresponding decay times are named as follows: .

ð1Þ

ð1Þ

ð1Þ

tESIPT , with 25 ps tESIPT 49 ps (relative amplitude 0.28 AESIPT 0.8), which is detected in the decays of DCMeth in all the investigated solvents (symbol ).

ð2Þ

tESIPT (AESIPT );

127  5 (0.43) 107  21 (0.20)† 90  2 (0.24)† 141  16 (0.39)† 152  19 (0.15)† 98  1 (0.36)† 78  3 (0.33)† 166  28 (0.18)z 221  47 (0.18)z 245  27 (0.15)z

(0.63)* (0.28)* (0.74)* (0.49)* (0.51)* (0.80)* (0.72)* (0.60)*

15  1 25  5 35  1 26  1 29  1 40  3 49  4 40  5

Ethyl acetate Acetone Acetonitrile DMFA

DMSO Isopropanol Ethanol Methanol

††

tREKETO (AREKETO ); tANTI (AANTI ); zz tcis (Acis )

**

tS:M:ESIPT (Aalcoh: S:M:ESIPT ) and

H
b
acc: (tS:M:ESIPT ) (AH
b
acc: S:M:ESIPT );

xc

xb alcoh:

tS:M:ESIPT (Anon
H
b S:M:ESIPT );

xa non
H
b

†

20  2 (0.53) 22  3 (0.62)*

t2 (A2)

Cyclohexane Chloroform

t1 (A1) *

tS:M:ESIPT (AS:M:ESIPT ) (further split into):

x

tsolvent (Asolvent );

z

ð1Þ

† ð2Þ

tESIPT (AESIPT );

* ð1Þ

Alcohols

H-bond acceptors

Non-polar Polar non-Hbonding

Solvent **

189  3 (0.11)z 526  59 (0.02)xb 703  54 (0.09)xb 549  3 (0.04)xb

442  4 (0.12)xa 392  9 (0.32)xa 480  0 (0.10)xa 311  1 (0.12)z

494  11 (0.03) 332  21 (0.17)xa

t3 (A3) ††

913  3 (0.04)xc 2301  16 (<0.01)†† 4007  161 (0.01)†† 1038  1 (0.12)xc

2022  86 (0.01)†† 2327  182 (0.01)†† 3745  160 (0.01) 977  3 (0.02)xc

2552  268 (0.01) 2547  112 (0.01)††

t4 (A4)

4388  1 (0.09)zz

5032  3 (0.01)zz

3911  11 (0.01)zz

t5 (A5)

101 75 178 602

84 211 117 232

104 91

tav

Table 17.3 Decay times ti (ps),  standard deviation, and relative initial amplitudes Ai that fit the experimental fluorescence decays of DCMeth in the different solvents (i ¼ 1–5). The decay components were associated with the following decay mechanisms. Reprinted with permission from Elsevier. Copyright 2009

370 Hydrogen Bonding and Transfer in the Excited State

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 371 . . .

. . .

ð2Þ

ð2Þ

ð2Þ

tESIPT , with 78 ps tESIPT 152 ps (0.15 AESIPT 0.43), detected in all the solvents except in alcohols (symbol †). tSOLVENT , with 166 ps tSOLVENT 311 ps (0.11 ASOLVENT 0.18), detected in all H-bond acceptors and alcohols (symbol z). tS:M:ESIPT , divided into three subgroups: 332 ps tnon-H-b S:M:ESIPT 480 ps in the polar non-H-bonding solvents; 703 ps in the alcohols; 913 ps tH-b-acc: 526 ps talcoh: S:M:ESIPT S:M:ESIPT 1038 ps in the H-bond-accepting solvents DMFA and DMSO, and in methanol. The corresponding amplitudes are in the ranges: 0.01 Anon
H
b S:M:ESIPT xa xb xc H-b-acc: 0.32, 0.02 Aalcoh: 0.09 and 0.02 A 0.12. The symbols are , and respectively. S:M:ESIPT S:M:ESIPT tREKETO , that is, detected in cyclohexane (tREKETO ¼ 494 ps with AREKETO ¼ 0.03, symbol

); tANTI , with 2022 ps tANTI 2552 ps (AANTI 0.01), detected in all the solvents having dielectric constant «ANTI 20.6 (symbol ††). tcis , with 3745 ps tcis 5032 ps (Acis 0.09), detected in all the solvents having dielectric constant «cis  24.3 (symbol zz). ð1Þ

ð2Þ

The decay components having time constants tESIPT and tESIPT can be ascribed to two ESIPT mechanisms occurring with different rates. In fact it is known that, for simple b-diketones [94], the proton can be exchanged between the two oxygen atoms by two different routes in the ESIPT process: the reaction coordinate path (having an activation energy barrier of 48.5 kJ mol
1) or the direct transfer path (having an activation energy barrier of 66.7 kJ mol
1). The former mechanism, being more probable than the latter, should be responsible ð1Þ ð2Þ for tESIPT, which is smaller than tESIPT . Consistent with the higher affinity of DCMeth compared with CURC for the KEHB formation process as reported in the solid state, a decay component having a time constant in the ð1Þ ð2Þ range of either tESIPT or tESIPT is revealed in the decay distributions of DCMeth in all the solvents. In the case of CURC (see Section 17.3.1), only one of the decay components measured in cyclohexane was attributed to S1 deactivation by direct ESIPT. This component, having a lifetime t1 ¼ 57  1 ps (see Table 17.2), falls in the ð1Þ ð2Þ range of the tESIPT values rather than in the range of tESIPT values. Indeed, molecular symmetry is essential for the occurrence of ESIPT through the direct transfer path: even the slightest perturbation of either the structural symmetry or the intramolecular charge distribution symmetry may inhibit or even suppress this process. Symmetry perturbations are likely to be induced by rotations of the hydroxyl and methoxy groups on the aromatic rings of CURC. Moreover, of the two closed cis-enol tautomers shown in Figure 17.1, the second one, called (2) in the following, in which the enolic proton is delocalized between the two oxygen atoms and a semiaromatic ring is formed, displays a more symmetric charge distribution compared with the first tautomer in Figure 17.1, called (1) in the following, in which the hydrogen atom is bound to either of the oxygen atoms. Molecular dynamics indicates that, in the S0 state, CURC adopts conformation (1) rather than (2), both in the gas and in the solution phases [9]. Finally, CURC has been demonstrated to adopt structure (1) in the solid state [10, 88]. All these elements support the exclusion of ESIPT through the direct transfer pathway for CURC. On the other hand, DCMeth has been shown to adopt conformer (2) rather than conformer (1) in the solid state [89]. It is possible that in the case of DCMeth the closed cis-enol tautomers (1) and (2) have comparable energies in the gas phase, and that, in solution, the relative abundance of (1) and (2) depends on the ð1Þ ð2Þ solvent properties. In fact, AESIPT  AESIPT for DCMeth in cyclohexane (Table 17.3), which is the most inert ð1Þ ð2Þ solvent. In the non-H-bonding solvents, with the exception of acetone, AESIPT was found to increase and AESIPT was found to decrease almost symmetrically at increasing solvent dielectric constant «. Indeed, according to ab initio calculations [9], the highest occupied molecular orbital (HOMO) of conformer (1) is characterized by a relevant residual negative charge localized at the keto oxygen, while the enol hydrogen carries a relevant residual positive charge. On the other hand, in the HOMO of conformer (2), the intramolecular charge is delocalized on the semi-aromatic ring. Thus, (1) should be better stabilized than (2) in the case of DCMeth owing to more efficient and thermodynamically favourable interactions with the solvent dipole moments. Conversely, an increase in the H-bond-accepting character of the solvent (increasing b) seems to stabilize (2) at

372 Hydrogen Bonding and Transfer in the Excited State ð1Þ

ð2Þ

the expense of (1). In DMFA we found AESIPT ¼ 0.49 and AESIPT ¼ 0.36, while in acetonitrile, a solvent with ð1Þ ð2Þ very similar « and much smaller b, we found AESIPT ¼ 0.74 and AESIPT ¼ 0.15. Furthermore, even if DMSO has ð1Þ

ð2Þ

an « value higher than that of DMFA, the AESIPT and AESIPT values in the two solvents are almost equal (see Table 17.3). In the last case the effect of the increase in « is likely to be compensated for by an increase in b. In ð2Þ this context the high AESIPT value measured for DCMeth in acetone can be ascribed to the sizeable b value of ð2Þ

this solvent. The explanation for the observed correlation between AESIPT and b might be that, for the HOMO of conformer (2), the acidic character of the enolic hydrogen is higher than that for the HOMO of conformer (1); that is, deprotonation is more likely for (2) than for (1). Finally, conformer (2) is absent when DCMeth is dissolved in alcohols. These solvents interact with the keto oxygen. A higher residual charge is localized on this oxygen in the HOMO of conformer (1) than in the HOMO of conformer (2). This might play a role in shifting the equilibrium towards conformer (1). Moreover, because the enol proton is equidistant from the two oxygens in the HOMO of conformer (2), the two oxygen atoms are equally shielded from interaction with the solvent by its positive charge. According to the above interpretation, at least 60% of DCMeth is found in either of the tautomers of the closed cis-enol structure. This indicates that KEHB is much tighter in DCMeth than in CURC, also in solution, independent of the solvent properties. In the case of DCMeth, KEHB formation is perturbed only in the alcohols. This is supported by the observation that an increase in the H-bond-donating character of the solvent correlates with the decrease in the relative amount of DCMeth molecules in the closed cis-enol conformer (1). Perturbation ð1Þ is also emphasized by the slight increase in the tESIPT values reported in Table 17.3 that is observed with increasing acidity parameter. However, even in alcohols KEHB formation is not totally suppressed. The decay component denoted tSOLVENT , because it is detected in all H-bond acceptors and in the alcohols, can be ascribed to decay mechanisms involving excited-state charge/energy transfer from DCMeth in the trans-enol conformer to the solvent, by analogy with the case of CURC. The tSOLVENT values indicate that such a transfer is a relatively efficient S1-quenching pathway, although not as efficient as the ESIPT mechanisms. S1 decay by means of this mechanism is revealed even in isopropanol in the case of DCMeth. This behaviour is different from that displayed by CURC. Based on the results on CURC [65], the decay components for DCMeth having time constants denoted alcoh: H-b-acc: tnon-H-b S:M:ESIPT , tS:M:ESIPT and tS:M:ESIPT (Table 17.3) can be ascribed to decay by solvent-rearrangement-moderated ESIPT, in which the desolvation dynamics determine the decay rate. It is worth noting that desolvation is the alcoh: H-b-acc: fastest in polar non-H-bonding solvents and in isopropanol (tnon-H-b S:M:ESIPT < tS:M:ESIPT < tS:M:ESIPT ), where the interaction between DCMeth and the solvent molecules is weakest and the formation of intermolecular H-bonds plays a minor role. The solvent rearrangement kinetics is slower in alcohols than in polar nonH-bonding solvents, reflecting the tighter H-bond-mediated interaction of DCMeth with these solvents. The slowest rearrangement kinetics is displayed in H-bond-accepting solvents. For DCMeth decay in methanol, a relevant contribution of tH-b-acc: S:M:ESIPT was also observed, which indicates that this solvent behaves both as an H-bond donor and as an acceptor with respect to DCMeth. The decay component detected for DCMeth in cyclohexane, with a time constant tREKETO ¼ 494 ps, is ascribed to decay by the reketonization mechanism. The relative amplitude of this component is much smaller for DCMeth (AREKETO ¼ 0.03) than for CURC (AREKETO ¼ 0.16). This indicates that cis–trans isomerization of the enol conformer upon excitation is much less probable for DCMeth than for CURC, which further confirms that KEHB is much tighter for the non-substituted curcuminoid. The decay component denoted tANTI is ascribed to the decay of the non-polar trans (anti) diketo conformer [9]. The decay component denoted tcis is ascribed to the decay of the polar cis-diketo conformer. This conformer was reported to be unstable in the case of CURC [9] owing to the combination of two effects: (i) electrostatic

Intramolecular H-Bond Formation Mediated De-Excitation of Curcuminoids 373

repulsion between the valence electrons, which are highly localized at keto oxygens (residual charge
0.7), and (ii) steric interaction with the phenolic substituents. In DCMeth the latter interaction is obviously absent because of the lack of aromatic substituents. This is possibly sufficient to allow traces of the cis-diketo conformer to exist in a highly polar environment. According to this interpretation, a substantial amount of cisdiketo conformer of DCMeth is found in methanol (Acis ¼ 0.09). Methanol is the smallest molecule among the selected H-bond-accepting solvents. Possibly, the methanol molecular dimensions are compatible with insertion of its H-bond-accepting moiety in the narrow space between the keto oxygens of the cis-diketo without destabilization of the diketo system through steric repulsion. The consequent shielding of the oxygen residual negative charge and prevention of electrostatic repulsion, together with solvation, might result in particularly enhanced thermodynamic stability of the cis-diketo structure of DCMeth compared with all the other solvents. The present results demonstrate that the phenolic substituents in CURC weaken the KEHB, thereby inhibiting ESIPT, which is the most efficient non-radiative S1 decay pathway of CURC. The phototoxic potential should therefore be higher for CURC than for the non-substituted DCMeth.

17.4 Conclusions In this chapter, the excited-state dynamics of potential photosensitizers were discussed. We focused on the role of excited-state intramolecular and intermolecular H-bonding in enhancing or depressing the probability and rate of tautomerization and charge-transfer photochemical reactions, thus affecting non-radiative deactivation pathways alternative to that triggering the photosensitized therapeutic activity. The correlation between the ability of b-diketones in their closed cis-enol conformers to form an intramolecular H-bond connecting the enol with the keto moiety, on the one hand, and their decay by means of excited-state intramolecular proton transfer or reketonization photoreactions, on the other hand, were particularly emphasized. The relevance of time-resolved fluorescence measurements in elucidating the S1 decay mechanisms of photolabile drug substances was stressed, and the time-correlated single-photon-counting measuring technique was described in details. Time-correlated single-photon-counting measurements of the fluorescence decays of curcumin and its non-substituted analogue dicinnamoylmethane in several solvents differing in polarity and H-bonding properties were reported. The closed cis-enol conformers of these two b-diketones differ in their intramolecular H-bond formation chemical affinity. The decay data were interpreted on the basis of the expected effects of intramolecular H-bonding in the excited state on the excited-state intramolecular proton transfer and reketonization probability and rate.

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18 Hydrogen Bonds of Protein-Bound Water Molecules in Rhodopsins Hideki Kandori Department of Frontier Materials, Nagoya Institute of Technology, Showa-ku, Nagoya, 466-8555, Japan

18.1 Introduction Rhodopsins convert light into energy or a signal [1]. Such a function is initiated by photoisomerization of their chromophore, a retinal molecule, followed by structural changes of protein. There are four archaeal rhodopsins in Halobacterium salinarum: bacteriorhodopsin (BR), halorhodopsin (HR), sensory rhodopsin I (SRI) and sensory rhodopsin II (SRII, also called phoborhodopsin, pR). BR and HR are light-driven ion pumps, which act as an outward proton and an inward chloride-ion pump respectively [2–4]. SRI and SRII are light sensors in the archaea, which act for attractant and repellent responses in phototaxis respectively [5, 6]. Recent genomesequencing projects have shown the presence of archaeal rhodopsins not only in archaea but also in other biological kingdoms. Proteorhodopsin (PR) [7] and Anabaena sensory rhodopsin (ASR) [8] found in marine bacteria and cyanobacteria, respectively, are typical members in eubacteria, while Neurospora rhodopsin (NR) and Leptosphaeria rhodopsin (LR) found in fungi [9] are typical members in eukaryotes. On the other hand, visual rhodopsin is a member of the G protein-coupled receptor family that has evolved into a photoreceptive protein in visual cells of vertebrates and invertebrates [1, 15]. There is no sequential homology between archaeal and visual rhodopsins, which possess an all-trans and 11-cis retinal as the chromophore respectively. Nevertheless, both rhodopsins are composed of 7-transmembrane a-helices, and a retinal molecule is attached to a lysine residue in the seventh helix via protonated Schiff base linkage, as shown in Figure 18.1 (except for UV-sensitive pigments that have a deprotonated Schiff base). The protonated Schiff bases have a positive charge, which is stabilized by its counterion, deprotonated Asp or Glu. Deprotonation of the Schiff base is coupled to the function in many rhodopsins. Based on these similarities, it is believed that there is a common mechanism for the function of archaeal and visual rhodopsins.

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

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Figure 18.1 X-ray crystallographic structures of the Schiff base region in BR, ppR, sHR, and bovine rhodopsin (PDB entries are 1C3W [55], 1JGJ [56], 1E12 [57] and 1L9H [58] respectively). Each membrane normal is approximately in the vertical direction of the figures. Upper and lower regions correspond to the cytoplasmic and extracellular sides respectively. Green spheres represent water molecules. Hydrogen atoms and hydrogen bonds (dotted lines) are
supposed from the structures, while the numbers are the hydrogen-bonding distances in A [53]. Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC

Internal water molecules are presumed to play important roles in the functional process of archaeal rhodopsins [16]. They could stabilize the ion-pair structure between the Schiff base and its counterion. In addition, internal water molecules must assist the transport of ions through the hydrophobic membranespanning region of the protein in the case of ion-pump proteins such as BR and HR [16]. Figure 18.1 indeed shows the presence of water molecules in the Schiff base region of various rhodopsins. What is the role of these water molecules in the Schiff base region? How are these hydrogen bonds altered accompanying the proton transfer reaction in the Schiff base? Although the Schiff base does not deprotonate during the photocycle of HR, the role of water molecules in the chloride-ion pump is also interesting. In this article, I review our study on internal water molecules of rhodopsins by means of Fourier transform infrared (FTIR) spectroscopy. I first show a recent development to detect water stretching vibrations in BR under strongly hydrogen-bonded conditions by means of low-temperature FTIR spectroscopy. The extended study led to the proposal of a model of the proton transfer mechanism from the Schiff base to Asp85 (hydration switch model). Water structures in BR at cryogenic temperatures were then examined by time-resolved FTIR spectroscopy. In Section 18.5, I review the hydrogen bonds of water molecules in HR and their role in the chloride-ion pump. Finally, functional correlation of strongly hydrogen-bonded water molecules is described. On the basis of measurements of the BR mutants and various rhodopsins, an interesting correlation between strongly hydrogen-bonded water molecules and proton-pump function has been found, where rhodopsin molecules appear to possess strongly hydrogen-bonded water molecules if they pump protons.

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18.2 Detection of Water Under Strongly Hydrogen-Bonded Conditions in Bacteriorhodopsin FTIR spectroscopy is a powerful tool for detecting protein structural changes, where spectral changes are normally detected at <1800 cm1. Although XH (OH or NH) stretches are a direct probe of the hydrogen bond, strong absorption of water involved in the sample easily masks such information [16]. It is, however, known that films of rhodopsins exhibit a normal photocycle under suitable hydration conditions. In addition, protein conformation changes are conveniently obtained as difference IR spectra, as rhodopsins are a photoreceptive protein. These facts allowed differences in IR spectra not only in the frequency region <1800 cm1 but also in the water stretching region to be obtained. A water molecule has two OH groups, and their frequencies are distributed in a wide 3700–2700 cm1 region, depending on their coupling and hydrogen-bonding strength [17]. Gaseous water exhibits asymmetric and symmetric stretching modes at 3756 and 3657 cm1 respectively [16], and the stretching frequency is lowered in the condensed phase such as liquid and protein as its hydrogen bonding becomes stronger [17]. It is, however, noted that the hydrogen-bonding strengths of the two OH groups are probably not equivalent in the restricted protein environment, which breaks the C2n-type symmetry. In such CS-type symmetry, one OH is hydrogen bonded and the other is unbonded, and their frequencies are widely split. The symmetry of actual internal water molecules would be between the two extremes, C2n and CS types. I reviewed the details of the historical aspects of FTIR studies on BR 10 years ago [16]. OH stretching vibrations of water molecules were identified by comparing hydrations of H2O and H218O. Then, the locations of bound water molecules were analysed by measuring various BR mutants [16]. However, the observed frequency region of water molecules in the previous FTIR studies was limited to >3450 cm1 for the following reasons: (i) the spectral accuracy is less because of intense absorption by water; (ii) many protein bands other than water OH stretches overlap this region; (iii) strongly hydrogen-bonded water possesses broad OH stretching bands. These facts have disturbed observations of clear isotope shifts of water. In particular, an isotope shift between OH and 18OH is about 10 cm1, and such a small shift could be hidden in complex spectral features in the 3450–3000 cm1 region. In 1998, we optimized low-temperature FTIR spectroscopy of BR at 77 K, which made it possible to detect water stretching vibrations under strong hydrogen bonds [18]. Figure 18.2 illustrates the experimental set-up of polarized FTIR spectroscopy of hydrated films of BR. One of the important issues was to hydrate samples with D2O, by which D2O-insensitve stretching vibrations are separated in frequency. In fact, the K minus BR difference spectra in Figure 18.3(A) show the isotope shift of water not only for weak hydrogen bonds (2700–2500 cm1) but also for strong hydrogen bonds (<2400 cm1) [19]. There are at least six negative bands of water OD stretches at 2690, 2636, 2599, 2323, 2292 and 2171 cm1, located at both edges in broad absorption spectra of D2O (grey spectra in Figure 18.3). Because the structural changes of protein upon formation of the K intermediate are limited at around the retinal chromophore, these water molecules are presumably located in the Schiff base region. Of particular note is the OD stretch at 2171 cm1, which is of much lower frequency than those for the fully hydrated tetrahedral water molecules [17], suggesting that the hydrogen bond acceptor of this water molecule is negatively charged, such as oxygen of Asp85 and Asp212 (Figure 18.1) [19]. The origin of the water OD stretches in the K minus BR spectra was investigated by the use of various mutant proteins [20, 21]. The water bands were not affected by mutations at the cytoplasmic side, such as T46V, D96N and D115N, implying that water molecules in the cytoplasmic domain do not change their hydrogen bonds in the BR to K transition. In contrast, significant modifications of the water bands were observed for mutations in the Schiff base region and at the extracellular side, such as R82Q, D85N, T89A, Y185F, D212N, R82Q/D212N and E204Q. From these results we concluded that the six OD stretches of BR

380 Hydrogen Bonding and Transfer in the Excited State

Figure 18.2 Experimental set-up and typical data for polarized FTIR difference spectroscopy

originate from three water molecules, water401, 402 and 406, involved in the pentagonal cluster (Figure 18.4). Two stretching modes of each water molecule are highly separate (300–470 cm1 for OD stretches, and 500–770 cm1 for OH stretches), which is consistent with the previous QM/MM calculation [25]. The small amplitudes of vibrational coupling are presumably due to strong association of the water molecules with negative charges of Asp85 and Asp212. It should be noted that the hydrogen-bonding interaction of water402
is stronger with Asp85 than with Asp212 [20], although the distance between water402 and Asp85 (2.6 A) is
similar to that between water402 and Asp212 (2.8 A) (Figure 18.1). This suggests that the geometry of the hydrogen bond of water402 is ideal with Asp85, but not with Asp212. As the bridged water interacts more strongly with Asp85, Asp85 is presumably a stronger counterion than Asp212. Another interesting issue is that D85N and D212N without strongly hydrogen-bonded waters do not pump protons, while other mutants do [21]. Therefore, the proton-pump activity seems to be correlated with the presence of internal water molecules under strongly hydrogen-bonded conditions. This hypothesis is further examined by testing the water bands of other rhodopsins below.

18.3 Hydration Switch Model as a Proton Transfer Mechanism in the Schiff Base Region of Bacteriorhodopsin The structural changes of water molecules during the proton pumping of BR were studied by measuring photointermediates [26, 27]: the L minus BR (Figure 18.3(B)), M minus BR (Figure 18.3(C)) and N minus BR (Figure 18.3(D)) spectra. According to previous mutation studies for the OH stretching region [16], the negative band at 2690 cm1 in all difference spectra is assignable to a water molecule near Asp85. In the L minus BR spectra, an additional negative band at 2669 cm1 and a broad positive feature in the 2650–2560 cm1 region originate from water molecules located in the cytoplasmic region [16]. In the M minus BR spectra, the positive

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Figure 18.3 The K minus BR (A), L minus BR (B), M minus BR (C) and N minus BR (D) difference infrared spectra in the 2750–1930 cm1 region. The spectra are compared between hydration with D2O and D218O. The K, L, M and N intermediates were produced by illuminating BR films at 77, 170, 230 and 270 K respectively. Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC (See Plate 19)

water bands at 2715 and 2640 cm1 were also assigned to water molecules in the cytoplasmic region [16]. These spectra show no isotope effect of water in the <2500 cm1 region, being in clear contrast to the K minus BR (Figure 18.3(A)) spectra. It is particularly notable that the 2171 cm1 band as the OD stretch of water402 hydrating with Asp85 was not observed for the L minus BR and M minus BR spectra.

Figure 18.4 Schematic drawing of the assignment of the water molecule in the Schiff base region of BR. Modified with permission from [16]. Copyright 2005 American Chemical Society

382 Hydrogen Bonding and Transfer in the Excited State

On the basis of these IR observations, we proposed a hydration switch model as a proton transfer mechanism from the Schiff base to Asp85 [26, 27]. In BR, water402 forms a strong hydrogen bond with Asp85 (Figure 18.5), whose OD stretch is located at 2171 cm1. Upon photoisomerization, the N D group of the Schiff base orients parallel to the membrane [20], which perturbs the water-containing pentagonal cluster structure and results in weakening of the interaction between water402 and Asp85 (Figure 18.5). This scheme based on our FTIR results is consistent with the structural [28, 29] and theoretical [30, 31] studies for the K intermediate. The water band of BR at 2171 cm1 is changed in K, whereas the band is restored in L, M and N (Figure 18.3). As the frequency (2171 cm1) is extremely low for a water stretch, we postulate that the hydrogen-bonding acceptor of water is negatively charged as it is for BR. This means that the acceptor would be either Asp85 or Asp212 in L, and Asp212 in M. We proposed that water402 forms a strong hydrogen bond (i) with Asp85 in L and (ii) with Asp212 in M (Figure 18.5) [26, 27]. The hydration switch of water402 from Asp85 to Asp212 is likely to be coupled with the proton transfer reaction from the Schiff base to Asp85 upon formation of the M intermediate. The hydration switch seems to work in the proton transfer reaction in pharaonis phoborhdopsin (ppR) as well [32].

18.4 Time-Resolved IR Study of Water Structural Changes in Bacteriorhodopsin at Room Temperature Low-temperature studies of internal water molecules in BR are based on the assumption that positions and properties of water molecules are similar at room and low temperatures. It is generally accepted that BR intermediates trapped at low temperatures should display a protein structure and a retinal conformation essentially equivalent to their room-temperature counterparts [1–4]. But can such an argument be applied to internal water molecules? Even if the time-averaged BR protein structures and retinal conformations for the different intermediates are truly conserved at low temperature, internal water molecules could be significantly affected in their interactions, or even in their location, by the reduced protein and/or solvent dynamics when the BR ground state and BR intermediates are studied at low temperatures. We recently performed time-resolved room-temperature FTIR difference spectra for the BR photocycle [33, 34] at 8 cm1 spectral and 5 ms temporal resolution, from 4000 to 900 cm1. An in situ hydration method allowed a controlled and stable sample hydration (92% relative humidity), reducing the bulk water absorbance to a tolerable level, largely improving the data quality without affecting the functionality of BR. Experiments in both H216O and H218O were conducted to assign bands to internal water molecules. The room-temperature M minus BR spectrum was almost identical to that at 230 K (Figure 18.6). On the other hand, an intense positive broad band in the low-temperature L minus BR spectrum (170 K), assigned to the formation of a water cavity in the cytoplasmic domain, is absent in the room-temperature L minus BR spectrum (Figure 18.6). This water cavity, proposed to be an essential feature for L formation, seems to be a low-temperature artefact caused by restricted protein dynamics at 170 K [33]. Maeda and coworkers argued that the water signals in the L state are identical between room and low temperatures [35], but they neglected transient heat in the room-temperature measurements. Correction of transient heat is important, particularly in the analysis of the L state, which was previously stated by Gerwert and coworkers [36], because the lifetime in the submillisecond time domain is coincident with that of the transient heat. We showed that the heat relaxation kinetics is not affected by the sample temperature, in contrast to the relaxation steps of the BR photocycle. On the other hand, the heat relaxation kinetics is delayed when the sample thickness increases, which never happens for the BR photocycle. Thus, the heat released in the primary photoisomerization step dissipates from the sample via a conventional thermal diffusion process, proceeding uncoupled with further steps of the BR photocycle [33].

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Figure 18.5 Hydration switch model proposed for the proton transfer mechanism from the Schiff base to Asp85 in BR. The structure of the Schiff base region in D2O is schematically drawn. The strong hydrogen bond of water402, whose OD stretching frequency is at 2171 cm1, is highlighted in BR, L and M. The hydrogen-bonding acceptor is Asp85 in BR and L, but switched to Asp212 in M. In this model, the hydration switch of water402 from Asp85 to Asp212 is correlated with a proton transfer from the Schiff base to Asp85. The interaction of Thr89 and Asp85 is from our FTIR results [22, 23]. The orientation of the ND group in K is from a theoretical calculation [30], which is consistent with our FTIR results [24]. On the other hand, the orientation of the ND group in L is arbitrary [53]. Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC

384 Hydrogen Bonding and Transfer in the Excited State

L-BR

SIMILAR!!

artifactual cytoplasmic water cavity at 170 K

3700

3600

No water cluster deprotonation at 230K

293K 230K strongly weakly H-bonded H-bonded water water

293K 170K 3800

M-BR

3500 -1

wavenumber / cm

3400

3800

3600

2800

2600 -1

wavenumber / cm

293K 230K 2200

2000

1800 -1

wavenumber / cm

Figure 18.6 Spectral comparison of water signals in the L minus BR (left) and M minus BR (middle, right) difference FTIR spectra between room temperature and low temperature. Modified with permission from [25]. Copyright 2008 American Chemical Society (See Plate 20)

As the water structures in the L intermediate differ between room and low temperatures, the model in Figure 18.5 may have to be reconsidered, particularly in the L state. However, the key issue in the hydration switch model is the hydrogen bond of water in the unphotolysed BR and M intermediate states, but not in the L intermediate. As thewater structures in the M intermediate are identical between room and low temperatures, the model probably works for proton transfer under physiological conditions. Then, what about water structures in the unphotolysed BR state? Figure 18.3 shows that strongly hydrogen-bonded water molecules are only for the K minus BR spectrum, but this time resolution was not sufficient for detecting the room-temperature K minus BR spectra in previous studies [33, 34]. Nevertheless, the hydrogen-bonding structure in the Schiff base region is probably identical between room and low temperatures, which is also supported by theoretical calculations [25, 31]. In addition, functional correlation of strongly hydrogen-bonded water and proton-pump activity provides strong support of the water structure in Figures 18.4 and 18.5 at physiological temperatures. Additional important information was gained for the proton release group (PRG) in the extracellular domain. A proton is released from BR by lowering the pKa value of the PRG from 9 to 5.8 in the M intermediate. Although two glutamic acids at the extracellular surface, Glu204 and Glu194, were good candidates for the PRG, a deprotonation signal of carboxylic acid was not clearly detectable upon formation of the M intermediate, suggesting that the PRG is not formed by these glutamic acids. New insight was gained from time-resolved FTIR studies by Gerwert and coworkers, who reported a broad negative IR continuum band at 2000–1800 cm1 in the M minus BR difference spectrum [37]. This observation indicates that the continuum vibration is only present in BR, but not in the M state, which is consistent with what is expected from the vibration of the PRG. It has been proposed that the continuum band originates from a protonated water cluster, and our time-resolved FTIR study of BR indeed observed an isotope effect of 18O water on the continuum signal, indicating that the continuum band contains vibrations of water molecules [33]. Thus, it is now established that the PRG in BR is a protonated water cluster, although the structure of the water cluster and the mechanism of its high pKa are still under investigation. Interestingly, the continuum band was not observed at low temperatures [33].

18.5 Role of the Water Hydrogen Bond in a Chloride-Ion Pump As already mentioned, extensive studies have been performed to reveal the mechanism of unidirectional proton translocation in BR, whereas the molecular mechanism of the chloride pump in salinarum HR (sHR)

Hydrogen Bonds of Protein-Bound Water Molecules in Rhodopsins

385

and a homologous HR from Natronomonas pharaonis (pHR) is less understood [1–4]. It is believed that the interaction between the protonated Schiff base and Cl plays a crucial role in the vectorial transport of the chloride ion. Therefore, it is essential experimentally to monitor the hydrogen-bonding strength of the Schiff base during the photocycle of HR. Earlier resonance Raman spectroscopy of sHR revealed that the hydrogen bond of the Schiff base is weaker in sHR than in BR [38], but it becomes much stronger in the L intermediate [39]. As the Schiff base changes the orientation from the extracellular side to the cytoplasmic side by isomerization, it is reasonable to postulate that Cl is a hydrogen-bonding acceptor in the L state by moving to the cytoplasmic region. However, the hydrogen-bonding interaction between the Schiff base and Cl had never been experimentally evidenced. The NH stretching mode is a direct probe of the hydrogen-bonding strength of the Schiff base, whose frequencies of the all-trans retinal protonated Schiff base in CDCl3 are located at 2600, 2735 and 2957 cm1 in the presence of Cl, Br and I respectively [40]. In solution, the halide ion is located at the most stable position relative to the Schiff base, where the Schiff base NH group presumably forms a direct hydrogen bond with the halide ion. Thus, we can state that, if the Schiff base forms a hydrogen bond with the halide directly, the NH mode of the Schiff base is upshifted as the size of the halide increases, even in a protein environment. Based on highly accurate low-temperature FTIR spectroscopy, we successfully identified the ND stretching vibration of the Schiff base and OD stretching vibration of internal water molecules in pHR containing Cl, Br and I. We found that the hydrogen bonds of the Schiff base and water molecules are weak in the unphotolysed state, whereas they are strengthened upon retinal photoisomerization [41]. Halide dependence of the stretching vibrations enabled us to conclude that the Schiff base forms a direct hydrogen bond with Cl only in pHRK. The hydrogen bond of the Schiff base is further strengthened in pHRL1, whereas halide dependence revealed that the acceptor is not Cl but presumably a water molecule [41]. On the basis of the present results, we propose a model for the early stage of the chloride-ion pump (Figure 18.7). According to the model, removal of hydrogen bonds of Cl with the Schiff base and water(s) makes the environment around Cl less polar in pHRL1, which drives the translocation of Cl from its binding site to the cytoplasmic domain. Why is a chloride ion transported only towards the cytoplasmic domain, even though it is hydrophobic in the unphotolysed state? This suggests that the cytoplasmic domain becomes transiently less hydrophobic. Similarly, there may be a mechanism for unidirectional transport in the extracellular domain. We observed that Glu234 becomes deprotonated in the L2 intermediate, suggesting that the appearance of a negative charge at the extracellular surface prevents a chloride ion from translocation towards the extracellular side [42] (Figure 18.7).

Figure 18.7 Schematic drawing of the mechanism of light-driven chloride-ion pump halorhodopsin (HR). Proteinbound water molecule(s) play an important role in HR, but in a different manner from light-driven proton pumps. Reprinted with permission from [34]. Copyright Elsevier

386 Hydrogen Bonding and Transfer in the Excited State

18.6 Strongly Hydrogen-Bonded Water Molecules and Functional Correlation with the Proton-Pump Activity Through the BR study it has been found that strongly hydrogen-bonded water molecules play a crucial role in its function, for which highly accurate low-temperature FTIR spectroscopy is used. The importance of alterations in hydrogen bonding in energy storage provided an unexpected finding, i.e. a positive correlation between the strong hydrogen bond of water and proton-pump activity. As mentioned above, we measured FTIR difference spectra of internal water molecules at 77 K, where measurement in D2O is advantageous, because XH and XD stretching can be separated. Water OD stretching vibrations appear at 2700–2100 cm1, depending on their hydrogen-bonding strength, and we define a strong hydrogen bond at <2400 cm1. A mutation study of BR showed that D85N and D212N only exhibit no water bands under strongly hydrogenbonding conditions [20, 21], suggesting that such water molecules may be a prerequisite for the proton-pump function. Since then, we have tested various rhodopsins, including visual Rh, to ascertain whether the protein contains strongly hydrogen-bonded water in the unphotolysed state (OD stretch at <2400 cm1). Typical data are shown in Figure 18.8, where water signals under various halide-bound forms are shown for D85S (A) and D212N (B). As shown in Figures 18.1 and 18.4, the Schiff base region in BR has a quadrupolar structure with positive charges located at the protonated Schiff base and Arg82 and the counterbalancing negative charges located at Asp85 and Asp212. The quadrupole inside the protein is stabilized by three water

Figure 18.8 (A) The K minus D85S difference infrared spectra of the absent halide (a) and containing Cl (b), Br (c) or I (d) bound form in the 2700–2000 cm1 region. The sample was hydrated with D2O or D218O, and spectra were measured at 130 K. Modified with permission from [37]. Copyright 2006 American Chemical Society. (B) The K minus BR difference infrared spectra of the wild-type (a), halide-free (b), Cl-bound (c), Br-bound (d) and Ibound (e) D212N in the 2380–2020 cm1 region. The samples were hydrated with D2O or D218O, and spectra were measured at 77 K. Underlined frequency (2171 cm1) in the wild type also contains the ND stretch of the Schiff base [24]. Modified with permission from [39]. Copyright 2007 American Chemical Society (See Plate 21)

Hydrogen Bonds of Protein-Bound Water Molecules in Rhodopsins

387

molecules, forming a roughly planar pentagonal cluster composed of these water and two oxygen molecules of Asp85 and Asp212 (one from each carboxylate side chain). It is known that BR lacks proton-pumping activity if Asp85 or Asp212 is neutralized by mutation, but binding of Cl has different functional effects in mutants at these positions. Binding of Cl to D85T converts into a chloride-ion pump [43]. On the other hand, photovoltage measurements suggested that binding of Cl to D212N restores the proton-pumping activity at low pH [44]. Figure 18.8(A)(a) shows the absence of strongly hydrogen-bonded water molecules (<2400 cm1) upon mutation of Asp85 to Ser, as is the case in D85N [20, 21]. Halide binding converts the D85S mutant into a halide-ion pump, whereas strongly hydrogen-bonded water molecules were never observed [45]. The absence of strongly hydrogen-bonded water molecules is also the case in HR, a chloride-ion pump [41, 46]. Figure 18(B)(b) also shows the absence of strongly hydrogen-bonded water molecules (<2400 cm1) upon mutation of Asp212 to Asn [20, 21]. Nevertheless, strongly hydrogen-bonded water molecules appear at 2350 2310 cm1 when halide binds to the D212N mutant (Figure 18.8(B)(c to e)) [47]. As these mutants pump protons, the results in Figure 18.8 are consistent with our hypothesis that strongly hydrogen-bonded water molecules are a prerequisite for proton-pumping activity of rhodopsins. It should also be noted that HR pumps protons in the presence of azide, and we observed strongly hydrogen-bonded water molecules in the azidebound HR [48]. Figure 18.9 summarizes the results, which clearly show a strong correlation between the presence of strongly hydrogen-bonded water molecule(s) and proton-pumping activity. For example, strongly hydrogenbonded water molecules are observed for BR [19–21], D212N(Cl) BR [47], azide-bound HR [48], salinibacter SRI [49], SRII [50], proteorhodopsin [51, 52] and Leptosphaeria rhodopsin [53], which all pump protons. Strongly hydrogen-bonded water molecules were not observed for D85S(Cl) BR [45],

Rhodopsins Having strongly H-bonded water D85N and D212N BR

Having proton-pump activity Bacteriorhodopsin (BR) various BR mutants including D212N(Cl)

D85S(Cl) BR 13-cis,15-syn BR Halorhodopsin (HR)

Azide-bound Halorhodopsin Anabaena Sensory Rhodopsin (ASR)

Salinibacter Sensory Rhodopsin I Sensory Rhodopsin II (SRII) without HtrII

Sensory Rhodopsin II (SRII) with HtrII

Neurospora Rhodopsin (NR)

Proteorhodopsin (PR) Leptosphaeria Rhodopsin (LR)

Visual Rhodopsins

Figure 18.9 Various rhodopsins are classified in view of (i) proton-pump activity and (ii) whether they have strongly hydrogen-bonded water molecules (OD stretch in D2O at <2400 cm1) (See Plate 22)

388 Hydrogen Bonding and Transfer in the Excited State

HR [41, 46], Anabaena sensory rhodopsin [54, 55], Neurospora rhodopsin [56], bovine Rh [57] and squid Rh [58], which have no proton-pumping activity. Thus, comprehensive FTIR analysis revealed that there is a strong correlation between strongly hydrogen-bonded water molecules and proton-pump activity. Figure 18.9 shows two exceptions. The 13-cis form of BR [59] and the SRII complex with the transducer protein [60] possess strongly hydrogen-bonded water but no proton-pump activity. However, the former has the 13-cis chromophore, and it is known that only the all-trans chromophore has proton-pump activity. In the latter case, the other region of the SRII–transducer complex may determine its functionality. Consequently, the presence of a strong hydrogen bond of water is a prerequisite for proton pumping in rhodopsins. It is likely that destabilization of the water-containing hydrogen-bonding network plays an important role for light-energy storage in this case. Thus, the hydrogen-bonding strength of water molecules is likely to bevery important for the proton-pumping function [14]. It should be noted that photon energy is stored first in its isomerized form. In addition to the distorted chromophore, the importance of the hydrogen-bonding network has to be emphasized for light-energy storage. As only about 10% of light energy is spent on the electrochemical potential of transporting a proton [61], small energy differences in hydrogen-bonding stabilization may be crucial for the pumping function. Our hypothesis and its successful application to various rhodopsins has provided a concept for the new roles of internal water molecules. One of the important roles of internal water molecules is to occupy the empty space inside a protein which may be energetically unfavourable. Internal water molecules may assist the transport of ions inside a protein by raising the dielectric constant. In addition, we showed the possibility that the hydrogen bond of water carries energy for the proton-pumping function, which can be monitored by FTIR spectroscopy.

18.7 Conclusion FTIR spectroscopy is now an established method for investigating internal water molecules during the functional processes of rhodopsins. In particular, accurate spectral detection in the entire water stretching frequency region in D2O enabled us to capture the water molecules under strongly hydrogen-bonded conditions. On the basis of the measurements of BR, a new mechanism (hydration switch model) has been proposed for proton transfer from the Schiff base. Time-resolved FTIR spectroscopy allowed the comparison of hydrogen-bonding conditions of internal water molecules between room and low temperatures. In addition, comprehensive studies of the BR mutants and archaeal and visual rhodopsins have provided an interesting correlation between the presence of strongly hydrogen-bonded water molecules and proton-pump activity. It is likely that the proton-pumping rhodopsins possess strongly hydrogen-bonded water molecules. The mechanism of the proton pump in BR has been extensively studied, where the main issue of discussion is the switching mechanism between extracellular (proton release) and cytoplasmic (proton uptake) sides in the L and M intermediates [1, 2]. Our finding comes only from the hydrogen-bonding structure of water in the unphotolysed state. We hypothesize that strongly hydrogen-bonded water molecules are necessary for the proton-pump function of rhodopsins, where transient weakening of such hydrogen bonds plays an important role. We agree that the correlation between the presence of strongly hydrogen-bonded water molecules and proton-pump activity is empirical, and that examples may not be sufficient at this moment. Therefore, we will continue further experimental efforts, which will lead to a better understanding of the role of internal water molecules in rhodopsins.

Acknowledgements I thank many collaborators in the References. Some of the research described herein was supported by grants from the Japanese Ministry of Education, Culture, Sports, Science and Technology.

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19 Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives Carmen Carmona, Manuel Balo´n, Marı´a Asuncio´n Mun˜oz, Antonio Sanchez-Coronilla, Jose Hidalgo and Emilio Garcı´a-Fernandez Department of Physical Chemistry, Faculty of Pharmacy, University of Seville, Spain

19.1 Introduction The dynamics of the excited-state reactivity of intra- and intermolecular hydrogen-bonded systems has been attracting considerable attention in recent years owing to great interest in the photochemistry field [1–10]. Although hydrogen bond interactions in the ground electronic state have been widely investigated by different spectroscopic and theoretical methods, much less is known about these hydrogen bond interactions in the excited state [11–13]. As is well known, photoexcitation of a molecule in solution generally induces significant changes in its electronic and geometrical structures. Therefore, the spectral and photophysical properties of intra- or intermolecular hydrogen-bonded chromophores can be profoundly affected upon photoexcitation. These changes can also modify the reactivity in the electronic excited state of these hydrogen-bonded systems, inducing a variety of photophysical and photochemical processes such as excited-state intra- and intermolecular proton transfer (ESPT), intramolecular charge transfer (ICT) and intermolecular photoinduced electron transfer (PET). One of the most challenging problems in this field is the study of excited-state hydrogen bond interactions in molecules possessing both proton acceptor and donor centres [14–19]. Thus, the excited-state dynamics of these hydrogen bond systems is usually complicated by the fact that multiple equilibria and different species can appear. In this sense, the individual or simultaneous photoinduced proton transfer suffered by these centres can give rise to the observation of different exciplexes or phototautomers respectively.

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

394

Hydrogen Bonding and Transfer in the Excited State

N

N

N

N

H

CH3

BC

MBC

N

N N

N CH3

CH3

CH3

MHN

BCA

Scheme 19.1

Among the great variety of substrates possessing potential hydrogen bond donor/acceptor sites, the betacarboline ring, 9H-pyrido[3,4-b]indole (BC) [20] (Scheme 19.1), has been widely studied [21–33]. These rings are the structural units of numerous naturally occurring alkaloids that possess a wide range of biological and pharmacological properties [34–45]. Also, some of them have been proposed as fluorescence standards [26, 27], their fluorescence activity being highly sensitive to the solvent [46, 47]. In these molecules, upon excitation by light absorption, the charge density on the nitrogen atoms changes considerably, the pyridinic nitrogen becoming a better hydrogen bond acceptor and the pyrrolic nitrogen a better hydrogen bond donor in the first singlet excited state than in the ground state [22, 23]. A consequence of these photoinduced changes in the acceptor/donor capabilities of the betacarboline nitrogens is the complex excited-state acid/base properties of their hydrogen bond complexes. Thus, in the excited state the hydrogen bond interaction can induce the protonation or deprotonation of the pyridinic and pyrrolic nitrogen atoms of the betacarboline ring respectively. These ESPT reactions render the corresponding excited-state pyridinic cations (C
) and pyrrolic anions (A
). Moreover, the concerted or stepwise coupling of these ESPT reactions can induce the photautomerization of the betacarboline ring. Indeed, more than 20 years ago, Sakurovs and Ghiggino [21] attributed to a betacarboline phototautomer of zwitterionic structure the weak strongly red-shifted emission band, around 520 nm, that they detected in aqueous basic solutions. Thus, depending on the nature of the betacarbolinic substrate, the donor or acceptor molecule forming the hydrogen bond complexes and the media, the simultaneous presence of different species in the excited state can be observed. As an example, it has been reported that, for betacarboline in benzene, only the neutral species (N
) emit, lem  360 nm, while N
and cations C
, lem  450 nm, and zwitterions Z
, lem  500 nm, have been observed in pure methanol by Dias et al. [22]. Also, for BC in dichloromethane–acetic acid mixtures, Reyman et al. [26] observed the emission of N
, C
, Z
and a novel species, which they called a phototautomer, P
, lem  400 nm. Therefore, the simultaneous presence of several species, which in principle can be formed from the same or different precursors, makes it difficult to separate the different equilibria and to characterize the responsible species. This is the main reason why there is not a complete description of the dynamics of these processes and even controversy between different authors [48–51]. In our opinion, the interpretation of the photophysical acid/base behaviour of the betacarbolines is so complicated because the different authors have mainly worked with the parent compounds. As the donor and acceptor centres are free to interact through hydrogen bonds, different hydrogen-bonded complexes can be simultaneously formed. Thus, to get a consistent picture of the excited-state reactivity of hydrogen-bonded

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 395

systems, one of the crucial steps is the recognition of the hydrogen bond complexes in the ground state that, after photoexcitation, can act as the species responsible for the occurrence of excited-state processes. In this sense, and within our interest in the betacarboline photophysics, we have designed a strategy to broach the study of the ground- and excited-state hydrogen bonding of these compounds. It consists in the detailed analysis of ground- and excited-state hydrogen bond interactions of 9H-pyrido[3,4-b]indole (BC), N9-methyl9H-pyrido[3,4-b]indole (MBC), N9-methyl-1-methyl-9H-pyrido[3,4-b]indole (MHN) and N2-methyl-9Hpyrido[3,4-b]indole (BCA) (Scheme 19.1) with 1,1,1,3,3,3-hexafluoro-propan-2-ol (HFIP) in the low polar aprotic solvent cyclohexane [52–58]. Because the last substrates have blocked the pyridinic or the pyrrolic nitrogen atoms by methylation, they make it possible selectively to analyse the hydrogen bond interactions of each one of the centres. On the basis of these results, the photophysics of the hydrogen bond complexes of the parent BC, in which both nitrogen atoms are free for hydrogen bonding, can be safely interpreted [59–61]. One of the main conclusions reached from these studies is the existence of a general model for the groundstate interactions of the different BCs with HFIP. Thus, our experimental results clearly indicate that the hydrogen bond interactions of the BCs with HFIP give rise to the sequential formation of two ground-state hydrogen bond complexes. At low donor concentrations, 1:1 hydrogen bond complexes (HBC), N  H  O, are formed. Upon increasing the donor concentration, a second hydrogen bond complex formed from HBC is observed. These second complexes, with a stoichiometry of at least 1:2, are called the proton transfer complexes (PTC), Ndþ   H  Od. The existence of a charge separation in the PTC complex has been confirmed by FTIR studies on the fundamental stretching frequency of the hydroxylic group of the alcohols [52]. Thus, upon the addition of the betacarboline derivative, the OH stretching band of HFIP decreases and a continuum absorption in the middle region of the spectra is observed. These changes are typical of hydrogen bonds involving hydroxylic groups. The IR continua arise because hydrogen bonds with double- or multiminimum proton potentials show so-called proton polarizabilities owing to proton shifts within hydrogen bonds [62–64]. Because of the large polarizabilities, hydrogen bonds strongly interact with their environments, which gives rise to the continua observed in the IR spectra. It is worth noting that, while HBC complexes can be observed with alcohols of different strengths, PTC complexes are only observed in the presence of strong hydrogen bond donor substrates such as HFIP. The formation of the HBC and PTC complexes is explained by postulating a double-minimum potential for the position of the proton in the NHO bridge, one minimum centred near to the hydroxylic oxygen of the alcohol and the other near to the nitrogen atom of the BC derivative. The stoichiometry assumed for the PTC complex suggests the specific solvation of the oxygen atom of the HBC by a second alcohol molecule, i.e. the so-called cooperative effect [65]. Another interesting conclusion reached from our studies is the different chemical, photophysical and photochemical behaviour of the excited HBC and PTC complexes. Thus, neither the spectral nor the photophysical properties of the betacarbolinic substrate appreciably change upon the formation of the HBC complexes. Moreover, these complexes do not react in the singlet excited state. Therefore, the excited HBC complexes behave as independent fluorophores, their quantum yields and fluorescence lifetimes remaining practically unaffected. Conversely, the formation of PTC complexes notably modifies the spectra of the betacarbolinic substrates. Typically, PTC formation produces bato- or hypsochromic shifts and changes in the intensities of the absorption and fluorescence spectra. These PTC complexes are, on the other hand, very reactive in the excited state. In fact, our studies have demonstrated that the PTC of the betacarbolines can dynamically tune a great variety of photophysical processes. In the present review we elaborate upon some illustrative examples of the photophysical processes in which the betacarboline PTC complexes are involved. For this purpose, the review has been organized as follows. Section 19.2 deals with our studies of the hydrogen bond interactions of MBC and MHN with HFIP in cyclohexane. These systems provide excellent examples of the hydrogen-bond-assisted ESPT reactions of the PTC
complexes [52–56].

396 Hydrogen Bonding and Transfer in the Excited State

Section 19.3 is dedicated to our studies of the hydrogen bond interactions between BCA and HFIP in cyclohexane. As will be seen, in this system the PTC
hydrogen bond interactions can play an important role in the ICT processes of the betacarboline ring [57, 58]. The study in Section 19.4, on the hydrogen bond interactions between BC and HFIP in cyclohexane [59], serves as a paradigmatic example of how the simultaneous hydrogen bond interactions through the donor and acceptor centres can mutually influence the ESPT and ICT processes of the betacarboline ring. Finally, the study in Section 19.5, on the hydrogen bond interactions between BC and pyridines [60] and proper BC self-aggregation [61], provides examples of the PET processes suffered by the excited PTC complexes of these systems. As we are not going to describe in detail the experimental techniques used, some remarks seem pertinent here. The most important tools for studying hydrogen bond formation and proton transfer in the ground and the excited state are UV-vis absorption spectroscopy and fluorescence measurements. Their combination can prove the occurrence of a reaction and identify the responsible species. Moreover, the use of time-resolved techniques allows the rates of the excited-state processes to be extracted.

19.2 MBC–HFIP and MHN–HFIP While the formation of HBC complexes of MBC and MHN slightly modifies the absorption spectra, bathochromic shifts and isosbestic points are observed upon PTC formation [54–56]. Figure typically shows these changes for the MBC–HFIP system. The ground-state association constants for the HBC/PTC equilibria (Figure 19.1(a)) can be calculated from the Benesi–Hildebrand plots of the absorbance data for PTC formation versus the reciprocal of HFIP concentration. One of these plots is typically shown in the inset of Figure 19.1(a). These association constants measured at different temperatures, together with the calculated thermodynamic parameters, are recorded in Table 19.1. Further increase in HFIP concentration up to its solubility limit in cyclohexane, 9  102 M, produces new changes in the long-wavelength side of the absorption spectra of MBC (Figure 19.1(b)). These changes, not observed in the MHN–HFIP system in cyclohexane, are attributed to the formation of cationic species. This assignment is confirmed by the changes observed in the absorption spectra of MBC–HFIP and MHN–HFIP systems in cyclohexane–toluene mixtures [55, 56]. In these media, because higher HFIP concentrations can be managed, the changes observed in Figure 19.1(b) clearly evolve to the appearance of the typical cationic band, around 380 nm, of the BC derivatives (spectra not shown). From the corresponding Benesi–Hildebrand plots (see the inset in Figure 19.1(b)), the values reported in Table 19.1 can be estimated for the ground-state PTC/C equilibrium constants of the MBC–HFIP system in cyclohexane. The thermodynamic parameters for this equilibrium have also been recorded in this table. As can be seen in Table 19.1, in contrast to the behaviour observed for HBC/PTC equilibrium, PTC/C equilibrium is an entropy-controlled process. This uncommon behaviour might be related to the complete proton transfer and ion-pair separation needed for cation formation. The changes in the fluorescence emission spectra of MBC upon changing the HFIP concentration are shown in Figure 19.2. For the sake of clarity, the spectral changes have been analysed at different ranges of HFIP concentrations. The behaviour of the MHN–HFIP system in cyclohexane is the same, except the changes shown in the spectra of Figure 19.2(c) are never observed. The region of the lowest HFIP concentration, where ground-state MBC(MHN)/HBC equilibrium is established, has been omitted because the dynamics results indicate that the free derivative and its HBC behave as independent fluorophores and therefore do not participate in the excited-state reactions [56] Figure 19.2(a) shows the changes in the emission spectra produced by the ground-state formation of the PTC complexes. In a higher HFIP concentration range, where ground-state cationic species are still not being formed (Figure 19.2(b)), the spectra show a neat isoemissive point and the development of a band centred around 420–430 nm. Because the appearance of this band depends

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 397

25

1/(A-A0 )

(a)

20 15 10 0

1500

3000

4500

Absorbance

1/[HFIP] (M -1)

0.1 [HFIP]

0.0 320

340

360

380

(nm)

0.2 (b) 1/(A-A0 )

60 50 40 30 20

Absorbance

0

20

40

60

80

1/[HFIP] (M -1)

0.1 [HFIP]

0.0 320

340

360

380

400

(nm)

Figure 19.1 Changes in the absorption spectra of the MBC–HFIP system in cyclohexane with increasing HFIP concentration: (a) formation of PTC complexes; (b) formation of C species [MBC] ¼ 2  105 M. The arrows indicate the spectral shifts with increasing HFIP concentration. Reprinted with permission from [56]. Copyright 2004 American Chemical Society

398 Hydrogen Bonding and Transfer in the Excited State Table 19.1 Apparent association constants and thermodynamic parameters for the formation of the ground-state hydrogen-bonded complexes (HBC), PTC and C of MBC–HFIP and MHN–HFIP systems in cyclohexane and in cyclohexane–toluene mixtures. [MBC/MHN] ¼ 2  105 M. Reprinted with permission from [56]. Copyright 2004 American Chemical Society MBC HBC K20 (M1) K25 (M1) K30 (M1) K35 (M1) K40 (M1) DH0 (kJ mol1) DS0 (J mol1 K1)

>8000

PTC

MHN C

Cyclohexane 3134 24 2016 32 1092 61 736 100 125 74 68 187 258

K25 (M1)

HFIP-d in cyclohexane 2564

K25 (M1)

Cyclohexane/toluene 1760 0.78

HBC

>4000

PTC

C

3533 2488 1866 1265 1073 47 92

1873

0.84

on HFIP concentration and, in this HFIP concentration range, PTC complexes are the ground-state species, it is obvious that, upon excitation, PTC
interacts with HFIP to produce an exciplex. We ascribe this band to a hydrogen bond exciplex in which the extent of the proton shift is still greater than in the PTC complex, i.e. a cation-like exciplex, CL
. This assumption is in agreement with the red-shift of its emission band and, as will be seen later on, with the long lifetime component obtained in the time-resolved fluorescence studies at these HFIP concentrations. It should be realized that the shift of the fluorescence maximum brought about by the addition of HFIP is quite large, but smaller than that of the cationic species upon ionic dissociation in aqueous solutions. Under these conditions, the dynamics of the MBC–HFIP system differs from that observed for MHN–HFIP system. Thus, while for the latter the fluorescence decays are always biexponential, for the former the decays can be fitted to bi- or triexponential functions, depending on the selected excitation or emission wavelengths. These discrepancies are probably due to the fact that, as mentioned before, in the case of MHN–HFIP the experimental data are not contaminated by the appearance of cationic species. Therefore, we select this system to analyse the dynamics of PTC
/CL
transformation. As typically shown in Table 19.2, the decays recorded at 440 nm emission wavelengths and different HFIP concentrations are biexponential, with a long decay component increasing and a short decay component decreasing on increasing HFIP concentration. Moreover, the short component appears as a rise time, i.e. with a negative pre-exponential. The appearance of the rise time confirms the above assumption concerning the excited-state formation of a CL
exciplex from excited PTC. For the MBC–HFIP system, the decays in the medium concentration range of HFIP, where ground-state cations are not still being formed, are always triexponential functions (see Table 19.3). In this case, apart from PTC
and CL
complexes, another excited-state species emits. This third decay component, not observed in the MHN–HFIP system, can be tentatively ascribed to the emission of cationic species. Undoubtedly, these cations must be formed in an excited-state reaction from the CL
exciplexes. However, when the sample of the MBC–HFIP system in cyclohexane in the highest HFIP concentration range is excited at 380 nm wavelength, where only the incipient cationic species absorbs (Figure 19.1(b)), a broad band at 440–450 nm is observed in the spectra (Figure 19.2(c)). This band is characteristic of the emission of cationic betacarboline derivatives. Thus, the excited-state cations can be formed in an excited-state reaction from CL
and/or, when possible, by

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 399 2500 (a)

2000 IR

1500

[HFIP]

1000 500 0 350

400

450

500

(nm)

1000 (b)

800 IR

600

[HFIP]

400 200 0 350

400

450

500

550

(nm)

500

(c)

400 IR

[HFIP]

300 200 100 0 400

450

500

550

(nm)

Figure 19.2 Changes in the emission spectra of the MBC/HFIP system in cyclohexane with increasing HFIP concentration: (a) low concentration range, lexc ¼ 335 nm; (b) medium concentration range, lexc ¼ 335 nm; (c) high concentration range, lexc ¼ 380 nm. [MBC] ¼ 2  105 M. The arrows indicate the spectral shifts with increasing HFIP concentration. Reprinted with permission from [56]. Copyright 2004 American Chemical Society

400

Hydrogen Bonding and Transfer in the Excited State

Table 19.2 Fluorescence lifetimes and pre-exponential factors (in parentheses) at 440 nm emission wavelength and different HFIP concentrations for MHN in cyclohexane, lexc ¼ 340 nm. Reprinted with permission from [54]. Copyright 2001 American Chemical Society 100[HFIP] (M)

t1 (ns)

2.0 3.0 4.0 5.0 6.0 7.0

x r2

t2 (n)

14.15 (0.121) 14.31 (0.117) 14.47 (0.116) 14.7 (0.151) 14.91 (0.144) 15.05 (0.137)

2.82 2.53 1.98 1.89 1.66 1.44

(0.043) (0.062) (0.081) (0.084) (0.083) (0.085)

1.262 1.222 1.150 1.168 1.180 1.157

Table 19.3 Lifetimes and pre-exponential factors (in parentheses) obtained by global analysis of the fluorescence decays, at different excitation and observation wavelengths, of MBC–HFIP in cyclohexane, [HFIP] ¼ 0.0864 M, x g2 ¼ 1.198, [MBC] ¼ 2  105 M. Reprinted with permission from [56]. Copyright 2004 American Chemical Society lexc (nm) 350 325 360 350 350 350

lem (nm)

t1 (ns)

t2 (ns)

t3 (ns)

420 430 430 430 450 470

18.1  0.5 (0.091) 18.0  0.4 (0.111) 18.1  0.4 (0.114) 17.9  0.4 (0.110) 18.0  0.4 (0.132) 18.1  0.3 (0.145)

2.4  0.9 (0.075) 3  1 (0.047) 2.0  0.9 (0.070) 3  1 (0.037) 2  3 (0.023) 3  2 (0.015)

0.6  0.5 0.6  0.4 0.9  0.6 0.7  0.9 1.2  0.8 1.1  0.7

(0.052) (0.073) (0.088) (0.031) (0.066) (0.053)

direct excitation of ground-state cations. These observations enable us to propose the general kinetic mechanism in Scheme 19.2 for the reactivity of betacarbolines with HFIP in the ground and the first singlet excited state. According to this mechanism, and because in the case of the MHN–HFIP system in cyclohexane the absorption and/or emission of the cationic species is not observed, only the PTC
/CL
transformation must be considered for this system. Adequate treatment of the data in Table 19.2 makes it easy to determine the kinetic parameters k1 and k1 in Scheme 19.2. These values are recorded in Table 19.4. From this analysis, lifetimes of 5 and 16 ns are obtained for the PTC
and CL
species respectively. This mechanism also accounts for the more complicated behaviour observed for the MBC–HFIP system in cyclohexane – mainly that related to its excitedstate dynamics. Thus, it explains the impossibility of finding an adequate concentration range of HFIP for independent study of the PTC
/CL
transformation by dynamic measurements. Also, the triexponential behaviour of the decays in the medium concentration range of HFIP (Table 19.3), where no ground-state cations are observed, is in agreement with the formation of excited-state cations. k2

k1 N*

HBC *

PTC * k-1

1/τN

N

1/τHBC

HBC

1/τPTC

PTC

Scheme 19.2

CL*

C* k-2

1/τCL

1/τC

C

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 401 Table 19.4 Excited-state kinetic parameters for MBC/HFIP and MHN/HFIP systems in cyclohexane and in the cyclohexane/toluene mixtures. Reprinted with permission from [56]. Copyright 2004 American Chemical Society MBC k1 t0PTC (M1)

MHN

k2 t0CL (M1)

k2 t0C

73  3 77  2

28  2 29  3

1.4  0.1 1.4  0.1

67  3 80  2

139  31 142  30

95  3 110  3

6.6  0.6 6.6  0.4

k1 t0PTC (M1)

k2 t0CL (M1)

k2 t0C

1.7  0.1 1.7  0.1

0.90  0.01 0.90  0.01

Cyclohexane 33 k1 t0CL ¼ 0:24

HFIP-d in cyclohexane 5.0  1.0 5.0  0.9 Cyclohexane/toluene 0.90  0.06 46  1 0.90  0.04 60  3

In an attempt to obtain quantitative information on the excited-state reactivity of the MBC–HFIP system, we have used a distinct strategy for this substrate, consistent in the detailed analysis of the emission fluorescence spectra, in order to search for the contribution of the different species at the different HFIP concentrations. To this end, we have deconvolved the emission spectra, as typically shown in Figure 19.3, for the different HFIP concentration ranges. As can be seen in Figure 19.3(a), initially the experimental profiles can be deconvolved into the three bands corresponding to the PTC
spectra and the band of the CL
exciplex [56]. An entirely similar deconvolved spectrum is obtained for the MHN–HFIP system in the higher concentration range of HFIP. With increasing HFIP concentration, the fitting process clearly shows that a band at 450 nm must be taken into account (Figure 19.3(b)). This new band corresponds to the betacarboline cationic species. Thus, as mentioned, emission from the cations appears even in a HFIP concentration range where ground-state cations are still not observed. That is, as shown in Scheme 19.2, cations are formed in an excited-state reaction. Finally, at the highest HFIP concentrations, when the excitation is selected at a wavelength where only cations absorb, the sole CL
and cation C
emissions are observed (Figure 19.3(c)). Interestingly, the maximum and the shape corresponding to the deconvolved bands are always the same, irrespective of the HFIP concentration. This confirms the goodness of fit, and also that PTC
, CL
and C
are the only emitting excited-state species. The deconvolved emission spectra at different HFIP concentrations make it possible to obtain the relative quantum yields of the three species [56]. From the dependence of the relative quantum yields on the HFIP concentration, the kinetic parameters in Scheme 19.2 can be obtained for the MBC–HFIP system in cyclohexane. These values are reported in Table 19.4. Finally, it should be mentioned that the mechanism in Scheme 19.2 is corroborated when MBC–HFIP and MHN–HFIP are studied in higher-polarity cyclohexane–toluene mixtures. Under these conditions, both systems fulfil the steps in Scheme 19.2 and cationic species are also observed for MHN. The kinetic parameters in this mixture are recorded in Table 19.4. In this table, the corresponding parameters for the interaction of MBC with deuterated HFIP in cyclohexane are also included. The results in Table 19.4 indicate that the reactivity of MBC, although similar, is higher than that of MHN. Thus, for MBC, the k2 rate constant is higher than k1, and therefore the CL
back step is not operative [56]. The results also show that the kinetic isotope effect is negligible in the formation of CL exciplexes, but it affects the rate of the step for the formation of cationic species. This fact can be taken as an indication that the mechanisms for both steps must be significantly different. While in the formation of the C exciplexes the proton movement can be the rate-controlling step, in the case of the CL exciplexes it should play a secondary

402

Hydrogen Bonding and Transfer in the Excited State 2500

(a)

2000 1500

IR 1000 500 0 350

400

450

500

550

(nm)

(b)

2000 1500

IR 1000 500 0 350

400

450 (nm)

500

550

600 (c) 500

IR 400 300 200 100 0 400

450

500

550

600

(nm)

Figure 19.3 Deconvolution of the corresponding spectra in Figure 19.2: PTC (dotted curve), CL (dashed curve) and C (dotted solid curve). Reprinted with permission from [56]. Copyright 2004 American Chemical Society

role. In this sense, it can be suggested that, in the formation of the CL exciplexes, a tautomerization process, which requires a geometrical redistribution, can be involved. Thus, as has been reported for pyridine [66, 67], we assume that, upon excitation, the PTC complexes acquire a non-planar boat-shaped geometry in their singlet-excited state. This excited state sp2 ! sp3 rehybridization of the PTC pyridinic nitrogen atom makes

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 403

this centre prone to hydrogen bonding with an additional donor molecule. This very attractive hypothesis allows us tentatively to assign to the CL
exciplex a non-planar quinoid structure. So, two important contributions to the knowledge of the ground- and excited-state proton transfer equilibria of betacarbolines can be determined from these results. First, we have demonstrated that a stepwise mechanism, as shown in Scheme 19.2, operates for both substrates. Second, the novel contribution of these results is the characterization of cationic exciplexes of MHN and MBC, and the establishment of the mechanism of their formations. Thus, although emission from C exciplexes has been previously reported, the corresponding neutral forms have always been supposed to be their precursors. In this work, we have shown that C exciplexes are only observed when ground-state PTC and their corresponding hydrogen bond CL exciplexes have been formed. Under the appropriate experimental conditions, these CL
complexes, assisted by another donor molecule, give the cation exciplexes.

19.3 BCA–HFIP In a recent study on the influence of solvent polarity on the spectral properties of BCA [57] we observed the existence of two ground-state isomers of supposed quinoid, Q, and zwitterionic, Z, structures whose relative proportions vary with solvent polarity (Scheme 19.3). Thus, the Q isomer is the predominating species in cyclohexane and the Z isomer in toluene. Very convincing proof of the existence of both isomers is provided by the changes that the addition of HFIP produces in the absorption spectra of BCA [57, 58]. Thus, bathochromic or hypsochromic shifts are observed in cyclohexane and toluene, respectively, upon HFIP addition. Upon excitation, both isomers give rise to a dual fluorescence simultaneously emitting from their locally excited (LE) state, around 360 nm, and from their relaxed intramolecular charge transfer (ICT) excited state, around 520 nm. The photoinduced charge transfer process operates from the pyridinic to the pyrrolic nitrogen atoms for the Q form, rendering these centres partially charged, and in the reverse direction for the Z form, Z

Q N

N

CH3

N

HOR

N

N

(HBC) CH3

H O R HOR + N H O R

N

CH3

(PTC)

-

Scheme 19.3

N

CH3

404

Hydrogen Bonding and Transfer in the Excited State

reducing the negative and positive charges of these centres. Since this photoinduced charge transfer process is small in the case of the Q form, the emission from the ICT band is practically negligible. We suppose that the dynamics of the LE ! ICT relaxation processes is related, owing to the structural relationship, to that of the np
and pp
excited states of pyridine [66, 67]. Thus, the pyramidalization of the pyridinic nitrogen atoms of the isomers in their ICT excited states is proposed. The addition of HFIP increases and decreases the intensity of the ICT band of the Q and Z form respectively. Because, interestingly, the long red-shifted emission of the ICT band is completely similar to that attributed to the BC zwitterionic phototautomer, next we will describe the hydrogen bond interaction of BCA with HFIP in cyclohexane. The addition of HFIP to BCA in cyclohexane produces changes in the absorption spectra that are attributable to the formation of two hydrogen bond complexes [58]. Again, the formation of an HBC complex is followed by the appearance of the corresponding PTC complex (Scheme 19.3). While the formation of the HBC complexes only produces slight changes in the absorption spectra, isosbestic points and a long tail of weak absorbance at the longer wavelengths are observed upon PTC formation (see Figure 19.4). The Benesi– Hildebrand plot of absorbance data against HFIP concentration (the inset of Figure 19.4) allows us to calculate a value of 12  1 M1 for the HBC/PTC equilibrium constant. Paralleling the absorption spectra, two sequences of changes in the fluorescence spectra that correspond to the successive formation of HBC and PTC complexes are observed with increasing HFIP concentration. Thus, the formation of HBC complexes shifts to the red the main fluorescence band, the LE band, but does not significantly affect the ICT band. However, as can be seen in Figure 19.5, PTC formation quenches the fluorescence of the LE band and clearly enhances the emission of the ICT band. The spectra in the inset of Figure 19.5 show that quenching slightly modifies the profile and position of the LE band. Also, comparison of the spectra in this inset highlights the small fluorescence quantum yield of the PTC complex of BCA in cyclohexane. The Benesi–Hildebrand plots of the areas under the emission bands at short (LE) and long (ICT) wavelengths (Figure 19.5) against HFIP concentration are linear. Furthermore, the values of the formation

400

Absorbance

1/(A-A0 )

0.15

0.10

200

0 0

25

50

75

1

1/[HFIP] (M )

0.05

0.00 250

300

350

400

(nm)

Figure 19.4 UV-vis absorption spectra of BCA–HFIP (4.8  103–8.6  103 M) system in cyclohexane. The arrows indicate the spectral shifts with increasing HFIP concentration. The inset shows the Benesi–Hildebrand plot of absorbance data against HFIP concentration. Reprinted with permission from [58]. Copyright Elsevier

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 405 220

200 165 4,8 e-3 M 8,6 e-2 M 8,6 e-2 M (I x8)

IR 110

150 IR

55

100

0

350

400

450

500

550

600

(nm)

50

0 350

400

450

500

550

600

(nm)

Figure 19.5 Fluorescence emission spectra, lexc ¼ 325 nm, of BCA–HFIP (4.8  1038.6  102 M) system in cyclohexane. The arrows indicate the spectral shifts with increasing HFIP concentration. In the inset, the normalized emission spectra before and after the quenching process are compared. Reprinted with permission from [58]. Copyright Elsevier

constants, obtained from the slope and intercept of these plots, are practically the same, 15  1 M1 and 17  2 M1. Therefore, it seems obvious that the changes in the emission intensities of both bands are coupled. Moreover, the excellent agreement between these equilibrium constants and that calculated from absorption measurements for PTC formation, 12  1 M1, clearly indicates that the intensity changes in the LE and ICT bands are due to the formation of the ground-state PTC complex. As expected, the higher positive density charge on the pyrrolic nitrogen in this PTC complex (Scheme 19.3), as compared with the HBC complex, favours the charge transfer from the pyridinic to the pyrrolic nitrogen. Thus, the LE ! ICT transformation of the PTC complex is responsible for the decrease and increase in the emission intensities in the LE and ICT bands respectively. Time-resolved fluorescence measurements confirm this statement. Thus, under the experimental conditions where the ground-state PTC complex is being formed, the fluorescence measured in the ICT band, at 520 nm (lexc ¼ 340 nm), decays biexponentially. A medium constant decay time component, t1  4.2 ns, and a short component, t2, whose decay time decreases as the HFIP concentration increases, are recovered from the decays. The pre-exponential factors of these decay times are practically equal but of different sign. These experimental results bear out that, upon excitation, the PTC complex emits from its LE state and from its relaxed ICT state. The appearance of a rise time, when the decays are followed in the ICT band, indicates that the ICT state is not directly reached from the excitation of the ground-state PTC. On the contrary, the PTC complexes in their excited LE states react with new HFIP molecules to give the ICT state. Also, the constancy of the medium decay time is in agreement with the LE ! ICT transformation being irreversible, as shown in Scheme 19.4. The kinetic parameters in this scheme can be calculated from the dependence of the reciprocal of the decay times with HFIP concentration. Thus, values of (1.2  0.3)  1010 M1 s1 and 6.6  0.4 ns for the bimolecular rate constant, k, and for the decay time of the LE state, t0LE , can be obtained [58]. The constant component in the decays, 4.2 ns, corresponds to the lifetime of the ICT state, t0ICT . This lifetime is of a similar LE

k 1/τ0LE

ICT 1/τ0ICT

Scheme 19.4

406 Hydrogen Bonding and Transfer in the Excited State

order of magnitude to those reported for the ICT states of other compounds, such as 9-aminoacridinium derivatives and N-phenylpyrrole [68, 69]. Interestingly, t0ICT is also in excellent agreement with the values reported by different authors for the decay time of the zwitterionic phototautomer of betacarboline [22, 28, 50]. As mentioned, we have postulated that the dynamics of the transformation of the LE state into the ICT state involves the pyramidalization of the pyridinic nitrogen atoms of BCA [57]. As shown above, in the presence of the proton donor, the HFIP molecules assist this transformation. Thus, Scheme 19.4 shows an oversimplified mechanism, because both processes, pyramidalization and hydrogen bonding to the rehybridized pyridinic nitrogen atom, must participate in the formation of the ICT state. The observed dependence of the dynamics on HFIP concentration allows us to postulate that the hydrogen bonding to the pyramidalized nitrogen has to be the rate-controlling step [58]. Again, as concluded in Section 19.2, the formation of the PTC complexes is a prerequisite for the observation of excited-state processes. Interestingly, the photophysical behaviour of PTC complexes of the BCA–HFIP system differs from that observed for the corresponding PTC complexes of MBC/MHN–HFIP systems. Moreover, as will be seen later on, a close similarity, in both shape and position, exists between the ICT emission band of BCA and the long red-shifted band observed for BC in different media. Although from the pioneering work of Sakuros and Ghiggino [21] this BC band has been universally attributed to the emission of a zwitterionic phototautomer, formed through a double photoinduced proton transfer, the present results place this interpretation in some doubt. We deal with this theme in the following section.

19.4 BC–HFIP The initial shifts of the absorption and emission bands of the BC–HFIP system in cyclohexane [59] closely resemble those described in Section 19.2 for the MBC–HFIP and MHN–HFIP systems in cyclohexane. Because the BC pyridinic and pyrrolic nitrogens are moderately basic and extremely weak acid, respectively, and because HFIP is a strong donor but a weak acceptor, it seems reasonable to assume that the initial hydrogen bond interactions start through the pyridinic nitrogen atom. Therefore, we attribute these spectral changes, as for the MBC–HFIP system, to the successive formation of pyridinic HBC and PTC complexes, as shown in Scheme 19.5. However, the evolution of these PTC complexes with further HFIP addition is different from that observed in MBC and MHN–HFIP systems. Thus, there is no evidence of the formation of the CL
hydrogen bond exciplexes described in Section 19.2. Instead, with increasing HFIP concentration, the less energetic band of the BC absorption spectra broadens and shows a long tail that extends to nearly 450 nm (Figure 19.6). The linearity of the Benesi–Hildebrand double reciprocal plot of the absorbances measured at this absorption tail against HFIP concentration suggests the formation of a new ground-state BC–HFIP hydrogen bond complex with a formation constant of 63  2 M1 (inset of Figure 19.6). Also, HFIP addition quenches the BC fluorescence of the short wavelength band and gives rise to the appearance of a weak emission band at 520 nm (Figure 19.7). The Benesi–Hildebrand plot of the areas under the fluorescence spectra against HFIP concentration yields an equilibrium constant value of 67  1 M1, very close to that obtained from the absorbance data. The excellent agreement between these equilibrium constants clearly indicates that a static process is responsible for the intensity changes in the short- and long-wavelength emission bands (Figure 19.7) that is, the formation of a ground-state hydrogen bond complex with a low fluorescence quantum yield. In the inset of Figure 19.7 it can be seen that the quenching slightly shifts to the red the fluorescence band at shorter wavelengths. The difference between the behaviour of the pyridinic PTC of the parent BC and MBC lies in the fact that, while for MBC the pyrrolic group is blocked by methylation, in BC the N–H pyrrolic group is free for hydrogen bonding. Thus, once the pyridinic PTC complexes have been formed, the positive charge density created on the

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 407

N N H HOR

N N H

(HBC) HO R

HOR

N H

N H

(PTC) OR

HOR

NH N H

(DHBQ) OR

HOR

Scheme 19.5

pyridinic nitrogen atom (Scheme 19.5) favours the charge transfer process from the pyrrolic to the pyridinic nitrogen, the former becoming more acid. This facilitates, with further addition of HFIP, the formation of double hydrogen bonds (DHB) in which the HFIP molecules interact with both betacarboline nitrogens. Interestingly, it is worth noting that the changes observed in the absorption and fluorescence spectra of BC–HFIP at high HFIP concentrations are very similar to those previously described for BCA–HFIP in Section 19.3. In fact, there is an excellent correspondence between the spectral behaviours shown in Figures 19.4 and 19.6 and Figures 19.5 and 19.7. Thus, we propose that the DHB complexes in cyclohexane evolve to a structure (DHBQ in Scheme 19.5) similar to the PTC structure of the quinoid form of BCA (see Scheme 19.3). Like the PTC complexes of BCA, these DHBQ complexes give rise, upon excitation, to a dual fluorescence. They emit from their LE excited state and from their relaxed ICT excited state. Thus, we assume that the same mechanism for the LE ! ICT transformation must operate for both systems (see Scheme 19.4). The time-resolved fluorescence data at these HFIP concentrations fully confirm this assumption. These measurements, carried out at 520 nm, show biexponential decays with a short lifetime component that decreases with increasing HFIP concentration, and a constant lifetime of 4.0 ns. Moreover, pre-exponential factors of similar magnitudes and different signs are obtained. Also, the constancy of the 4.0 ns lifetime is in agreement with the LE ! ICT transformation being irreversible, as shown in Scheme 19.4. That is, the dynamic behaviour of BC–HFIP in cyclohexane in this HFIP concentration range is exactly the same as that mentioned for the BCA–HFIP system in cyclohexane [58]. Moreover, the values of (1.3  0.4)  1010 M1 s1

408

Hydrogen Bonding and Transfer in the Excited State 26

Absorbance

0.20 1/(A-A0 )

24

0.15

22 20 18 16 10

0.10

20

30

40

50

1/[HFIP] M −1

0.05

0.00 250

300

350

400

450

(nm)

Figure 19.6 UV-vis absorption spectra of the BC–HFIP (8.6  1038.6  102 M) system in cyclohexane. The arrows indicate the spectral shifts with increasing HFIP concentration. The inset shows the Benesi–Hildebrand plot of absorbance data against HFIP concentration. Reprinted with permission from [59]. Copyright Elsevier

and 6.0  0.8 ns calculated, as before, for the bimolecular rate constant, k, and for the lifetime of the LE state, respectively (see Scheme 19.4), are in excellent agreement with those obtained for the BCA–HFIP system in cyclohexane. Furthermore, as mentioned, the lifetime of the ICT state, 4.0 ns, coincides with the values published by different authors for the lifetime of the presumed zwitterionic phototautomer of BC [22, 28, 50]. Thus, the origin of the large Stokes-shifted fluorescence band of betacarbolines, around 520 nm, is not due to 250 200

200

9,6 e-3 M

IR 150

8,6 e-2 M 8,6 e-2 M (IR x 4,7)

100

150 50

IR

0 350

100

400

450

500

550

(nm)

50

0 400

500

600

(nm)

Figure 19.7 Fluorescence emission spectra, lexc ¼ 325 nm, of the BC–HFIP (8.6  1038.6  102 M) system in cyclohexane. The arrows indicate the spectral shifts with increasing HFIP concentration. In the inset, the normalized emission spectra before and after the quenching process are compared. Reprinted with permission from [59]. Copyright Elsevier

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 409

the emission of a zwitterionic phototautomer but to the ICT emission of the double non-cyclic hydrogen bond adduct (DHBQ) of betacarboline. As for the BCA–HFIP system, the HFIP molecules assist the LE ! ICT transformation, and therefore the hydrogen bonding to the pyramidalized nitrogen is again postulated as the rate-controlling step. At this point it should be mentioned that double hydrogen bond complexes of BC have already been postulated by other authors as the responsible species for the emissions at 520 nm. Tapia et al. ascribe the longwavelength emission to a complex formed between BC (with a quinoid structure) and two acetic acid molecules [51]. One of the donor molecules binds the pyridinic nitrogen, while the other interacts with the pyrrolic nitrogen. This complex can be formed, in the excited state, from a 1:1 BC/acetic acid complex or from the neutral form through a double proton transfer with two acid molecules. On the other hand, according to Chou et al., this emission is due to a ground-state 1:2 BC/acetic acid complex with a structure of triple hydrogen bonding formation [50]. In this complex, the two acid molecules form hydrogen bonds with the BC nitrogen atoms, and they are also linked to each other by an additional hydrogen bond. Once this cyclic complex is excited, a negligibly small geometry adjustment is required in the complex to achieve the correct geometry for a triple proton transfer to proceed. However, on the basis of the present results, the emission at 520 nm is not directly due to the double hydrogen bond complexes, but to the species formed in the ICT excited-state process suffered by these complexes. Thus, the greater the charge separation between the nitrogen atoms, the higher the emission at 520 nm will be. We therefore postulate that the formation of a PTC complex through the pyridinic nitrogen, that is, the creation of a partial positive density charge on this atom, is the driving force for the formation of these DHB complexes that further evolve to the quinoid, DHBQ, structure.

19.5 BC–BC and BC–PY The changes in the UV-vis absorption spectra of BC in 2MB with decreasing temperature are shown in Figure 19.8. The initial temperature decrease, from 258 to 208 K, produces a slight hyperchromic effect 0.8

Absorbance

0.8

Absorbance

0.6

208 K

0.6

0.4

258 K

0.2

0.4

0.0 260

280

300

320

340

360

380

(nm)

208 K 0.2

298 K 0.0 260

280

300

320

340

360

380

(nm)

Figure 19.8 UV-vis absorption spectra of BC (2  104 M) in 2MB from 298 to 208 K. The inset shows the UV-vis absorption spectra from 258 to 208 K [61]. Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC

410 Hydrogen Bonding and Transfer in the Excited State

without significant modifications of the wavelength maxima. Further decrease in temperature shifts the absorption bands to the red. As can be seen in the inset of Figure 19.8, at temperatures ranging from 258 to 208 K, clear isosbestic points appear in the spectra. This experimental behaviour is observed in the 2  1053  104 M concentration range studied, and in other solvents such as n-hexane, n-octane and methylcyclohexane. Finally, at the lowest temperatures, 200 K, only minor modifications of the absorption spectra are observed with the temperature decrease. All these spectral changes are reversible [61]. Owing to the peculiarities of this system, the changes in the absorption spectra, originated by the decrease in temperature (Figure 19.8), must be ascribed to the formation of self-associated hydrogen-bonded BC aggregates. These aggregates should be formed by the interaction between the NH pyrrolic donor group of one BC molecule and the pyridinic acceptor nitrogen atom of another BC. At this point it must be stated that, although other weaker interactions such as non-conventional NH–p hydrogen bonds or p–p stacked interactions can also be operative, they could hardly compete with the conventional hydrogen bonds. In fact, according to the results of previous work [70–72], the formation of such complexes only produces minor changes in the absorption spectra. Therefore, they can be ruled out as the main interactions responsible for the changes observed in the absorption spectra of BC with decreasing temperature. The changes in the absorption spectra of Figure 19.8 resemble those previously mentioned for MBC [56], MHN [54], BCA [58] and BC [59] in the presence of HFIP in cyclohexane that have been ascribed to the sequential formation of the corresponding HBC and PTC complexes. However, in this system the HBC/PTC transformation must be envisaged in a somewhat different way. In this sense, and to model this type of hydrogen bond, we have studied the interactions between BC and several pyridine derivatives with different reduction potentials and hydrogen bonding acceptor strengths in cyclohexane [60]. The changes in the BC–pyridine absorption spectra are entirely similar to those shown in Figure 19.8. That is, hyperchromic effects and further bathochromic shifts are observed upon changing the pyridine concentration and/or acceptor strength. Interestingly, as the hydrogen bond acceptor properties of the pyridine derivative increase, the hyperchromic effect becomes less perceptible and the bathochromic shift more relevant [60]. The hyperchromic effect, clearly observed with pyridines of low hydrogen bond acceptor strength, is attributed to the formation of HBC complexes. The later bathochromic shift, only appreciable with the strongest hydrogen bond acceptor derivatives, with increasing pyridine concentration is ascribed to PTC complex formation. Thus, although HBC/PTC transformation also occurs in these systems, it can, as mentioned, hardly be explained by the hydrogen bond cooperative effect produced by the increase in acceptor molecule concentration. However, as is known, the position of the HBC/PTC equilibrium can depend not only on the concentration and the proton donor and acceptor properties of the solutes but also on a variety of factors such as the polarity and nature of the solvent and the temperature [62–64]. In the present case, because of the non-polar character of the solvent, the increase in medium polarity produced by the gradual addition of polar pyridine molecules would necessarily help to shift the tautomeric equilibrium towards the PTC. On the other hand, it is evident that the increasing basicity of the pyridinic substrate would also strongly contribute to moving the BC pyrrolic proton closer to the pyridine nitrogen atom. Accordingly, the temperature decrease must favour the formation of HBC complexes, which further evolve to PTC complexes with decreasing temperature. Thus, as expected, temperature decrease produces an effect similar to increase in the hydrogen bond donor or acceptor concentration, that is, it favours the proton transfer process. Although, at first, only the formation of hydrogen-bonded dimeric structures can be assumed, higher aggregates could also account for the experimental behaviour. However, the fact that the influence of temperature on the absorption spectra of BC is the same as that produced by the addition of pyridine derivatives indicates that dimeric species should be responsible for the observed changes. Moreover, we have also analysed the temperature influence on BC plus MBC and BC plus PY systems in 2MB [61]. For the first system, the MBC concentration was 4 times greater than that of BC to minimize BC self-association. In the case of the

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 411

BC-PY at 298 K BC-PY at 138 K BC at 138 K

Relative Absorbances

0.16

0.12

0.08

0.04

0.00 300

320

340

360

380

(nm) 4

Figure 19.9 UV-vis absorption spectra of BC (10 M) plus PY (103 M) in 2MB at 298 and 138 K and BC (104 M) in 2MB at 138 K [53]. Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC

BC plus PY system, the PY concentration has been selected in order to have HBC hydrogen-bonded complexes at room temperature. Owing to the structural characteristics of these systems, they can be taken as models for the formation of hydrogen-bonded dimeric aggregates. In both cases, the behaviour is the same as that observed for BC in 2MB. Figure 19.9 typically shows the change in the absorption spectra for BC plus PY in 2MB at two extreme temperatures, 298 and 138 K. In this figure, and for the sake of comparison, the absorption spectrum of BC in 2MB at 138 K has also been included. As can be seen, the temperature decrease produces the same shifts of the absorption spectra in both systems. Therefore, as the mentioned experimental results suggest, it can be assumed that the observed hyperchromic effect and further red-shift of the absorption bands of BC in 2MB (Figure 19.8) can certainly be ascribed to the formation of dimeric BC hydrogen-bonded complexes. Near to room temperature, the BC molecules easily dimerize, giving rise to the initial formation of HBC complexes, which evolve, with decreasing temperature, to give their proton transfer tautomers, PTC. From the changes in the absorption spectra, the apparent equilibrium constants for the transformation of HBC complexes into PTC complexes at each temperature can be calculated. For the calculations, the absorbances have been measured in the temperature range where, as mentioned, an isosbestic point is clear in the spectra. Under these conditions we assume that the concentration of free BC can be considered negligible. From the van’t Hoff plot of the equilibrium constants, a mean value for the enthalpy of this process of 46 kJ mol1 can be obtained. As observed for MBC–HFIP and MHN–HFIP systems in cyclohexane (see Table 19.1), PTC formation is an exothermic process. Figure 19.10 typically shows the changes in the fluorescence emission spectra of BC (104 M) in 2MB with decreasing temperature at lexc ¼ 291 nm. As can be seen in Figure 19.10(a), the initial temperature decrease, down to around 208 K, strongly quenches the fluorescence and shifts the emission band to the red by around 20 nm. This behaviour is again completely similar to that observed in the study of the interactions of BC with pyridine derivatives [60]. Thus, in the presence of the weakest hydrogen bond acceptor pyridine derivative, a hyperchromic effect followed by an intensity decrease is obtained in the emission spectra with increasing derivative concentration. For the strongest hydrogen bond acceptor pyridines, only the quenching effect is observed. Thus, the hyperchromic effect is easily ascribed to the formation of HBC complexes, while the strong quenching of fluorescence must be attributed to PTC complexes. Therefore, an interesting difference

412

Hydrogen Bonding and Transfer in the Excited State (a)

248 K 238 K 228 K 208 K 198 K

6000

IR

4000

2000

0

400

(b)

148 K 300

IR

3000

200

188 K 100 0 460

IR

1500

480

500

520

(nm)

540

560

148 K

188 K

0 350

400

450

500

550

(nm) 4

Figure 19.10 Fluorescence emission spectra of BC (10 m) in 2MB (a) from 248 to 198 K and (b) from 188 to 148–K. The inset shows the emission spectra in the 460–560 nm wavelength range, lexc ¼ 291 nm [53]. Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC

exists between these systems and those described in the above sections. BC forms hydrogen-bonded complexes with non-aromatic acceptor molecules, which shift to the red its absorption and fluorescence spectra, but fluorescence quenching is never detected. On the other hand, as the present results show, when the acceptor is an aromatic molecule, a red-shift and quenching of the fluorescence are observed upon PTC formation. Because the spectral characteristics of this PTC complex do not provide any evidence of the formation of BC anions, we assume that the quenching of fluorescence must be due to a different factor. Indeed, it must also be noted that, in the complexes of MBC with hydrogen-bond aromatic donors such as phenol, a quenching effect is also observed [73]. The origin of the low fluorescence quantum yield of these PTC complexes must be related to the well-known phenomenon of fluorescence quenching typically observed in intermolecularly hydrogen-bonded aromatic chromophores. In fact, in structurally related systems such as pyrrole or carbazole chromophores, a dramatic quantum yield decrease has been observed in the presence of pyridine [74–77]. This highly effective deactivation of fluorescent states has been explained by Mataga in terms of curve-crossing dynamics involving a dark charge-transfer state [78]. Moreover, recent computational studies fully confirm the qualitative model proposed by Mataga [79, 80]. According to the authors, a common feature of the photochemistry of various systems, among them indole–pyridine and pyrrole–pyridine systems, is an electron-driven proton-transfer mechanism. Thus, highly polar charge-transfer states of 1 pp*, 1 np* or 1 ps* character drive the proton transfer, which leads, in most cases, to a conical intersection of the S1 and S0 surfaces, and thus to ultrafast

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 413 kq

PTC *

Nonfluorescent

kr PTC

Scheme 19.6

internal conversion. The formation of the charge-transfer state, which involves a rather small barrier, should be the rate-limiting step in the deactivation process. In the present case, this phenomenon must accompany the transformation of HBC complexes into PTC complexes. This quenching mechanism is schematically shown in Scheme 19.6. At this point it is interesting to state that, for a solution of MBC in 2MB, it is found that the fluorescence intensity slightly increases with decreasing temperature and the emission wavelength maxima do not change [61]. Because this substrate has blocked its pyrrolic nitrogen, it cannot form self-associated hydrogen bond complexes. Therefore, it shows the typical behaviour of organic molecules. On the other hand, the temperature decrease only produces a negligible quenching in the emission spectra of BC in toluene. This effect may be ascribed to the solvation of the BC molecules by this polar solvent, which weakens the hydrogen bond interactions. Further decrease in temperature in the BC in 2MB system intensifies the emission without significant modification of the wavelength maxima. Finally, as shown in Figure 19.10(b), below 178 K the intensity of the emission again decreases, and, concomitantly, a new emission band, at around 500 nm, together with clear isoemissive points, appears in the spectra. The emission intensity increase in Figure 19.10(a) is also observed for BC plus MBC and BC plus PY systems in 2MB [61]. However, in these systems, unlike BC, no further changes are observed, and emission at 500 nm does not appear. This is clearly seen in Figure 19.11, where the normalized spectra of BC and BC plus PY systems in 2MB at 138 K are compared. On the other hand, the spectral behaviour of a more dilute BC solution, 2  105 M, is identical to that of the 104 M solution, except

9000

IR

BC plus PY BC

6000

3000

0 350

400

450

500

550

(nm)

Figure 19.11 Fluorescence emission spectra of BC (104 M) and BC (104 M) plus PY (103 M) in 2MB at 138 K, lexc ¼ 291 nm. [53] Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC

414 Hydrogen Bonding and Transfer in the Excited State

the more dilute solution exhibits neither further intensity decrease nor appreciable emission at 500 nm (spectra not shown). It apparently behaves like BC plus MBC and BC plus PY systems in 2MB. As has been mentioned, at a certain temperature the emission intensity starts to increase upon decreasing temperature (see Figure 10(a)) without significant changes in the emission band maxima. Because the quenching rate constant, kq, is expected to decrease with decreasing temperature, it can be assumed that, as the temperature decreases, kq diminishes, becoming similar to or smaller than the radiative rate constant, kr, in Scheme 19.6. Thus, while at high temperatures kq kr, the temperature decrease produces an inversion of the relative magnitude of these rate constants. As a consequence, fluorescence quenching decreases with temperature decrease, and the emission intensity of the dimeric PTC complexes increases. On the basis of the mechanism depicted in Scheme 19.6, the kq values at the different temperatures can be calculated [61]. From the Arrhenius plot of these rate constants, a value of 16 kJ mol1 for the activation energy of the quenching process is obtained. This low energy barrier is, on the other hand, in qualitative agreement with computational studies, which predict a rather small barrier for the formation of the charge-transfer state in pyrrole–pyridine complexes [80]. According to these results, the quenching rate constant estimated at, for example, 268 K, 2.0  1010 s1, is, as expected, much higher than the PTC radiative rate constant, 1.7  108 s1. As shown in Figure 19.10(b), further temperature decrease produces an emission intensity decrease of the 104 M BC solution, with the concomitant appearance of a new band at around 500 nm. Also, clear isoemissive points are observed in the spectra (Figure 19.10(b)). This dual fluorescence emission, previously observed for BC–HFIP in cyclohexane (Section 19.4), has been attributed to the formation of ground-state double hydrogen bond complexes (DHB) in which both nitrogen atoms of the BC molecule are involved. These complexes emit from an LE state and from an ICT state. The band at 500 nm corresponds to the ICT emission. These previous results, together with the fact that dual emission is not observed for BC plus PYand BC plus MBC systems, where further hydrogen bond aggregation is not possible, enable us to propose that, at the lowest temperatures, BC dimeric PTC hydrogen-bonded complexes further aggregate. This is also supported by the fact that, in a more diluted BC solution, higher-order aggregates are not formed to any appreciable extent. Because BC is forming dimeric adducts, it is reasonable to assume that the dimers aggregate to give cyclic tetrameric species in which four hydrogen bonds are involved. These are the simplest kind of complex with both BC nitrogen atoms of each BC unit being involved in hydrogen bonds. Moreover, DFT(B3LYP) quantum mechanical calculations fully support the formation of these tetrameric species at low temperatures [61]. Thus, while the formation of the tetramer in the gas phase at room temperatures is a non-spontaneous process, it is accompanied with a negative Gibbs free energy change at low temperatures. As an example, the generation of the tetramer from two dimers in the gas phase at 158 K is an exergonic process with DGo158 K ¼ 7.5 kcal mol1. The optimized model for such cyclic tetramer at 158 K is shown in Scheme 19.7. Like the double hydrogen bond complexes of the BC–HFIP system in cyclohexane, these cyclic tetrameric aggregates emit from their LE state and from their ICT state. Therefore, we conclude that the influence of the temperature in the emission spectra of BC in non-polar aprotic solvents is due to the formation of intermolecular hydrogen bond BC aggregates. This contradicts the statements by Olba et al. on the temperature influence in the emission spectra of BC [81]. According to the authors, the observed changes were due to a change in the nature of the emitting state. However, we have shown that, in the temperature range analysed by the authors mentioned, the formation of dimeric hydrogen-bonded BC adducts, with a proton-transfer structure, satisfactorily explains the modifications suffered by the absorption and emission bands with decreasing temperature. Moreover, a change in the nature of the emitting state could hardly explain the dual fluorescence emission observed at the lowest temperatures. These double emissions are nicely ascribed to a further aggregation of the dimeric hydrogen bond PTC complexes to produce ground-state cyclic tetrameric aggregates. Thus, we postulate that the quadruple proton transfer in these tetramers produces the tautomeric form that emits from its LE state and from its ICT state [61].

Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 415

Scheme 19.7

19.6 Concluding Remarks The results presented in this chapter concerning the excited-state reactivity of hydrogen bond complexes of BC may give an idea of the inherent complexity involved in studies of the photoinduced processes occurring in hydrogen bond complexes of diazaromatic compounds. One of the more relevant issues is related to the importance of investigating the hydrogen bond effect on electronic spectra, the assignment of absorption bands and also the nature of the hydrogen bond itself. These considerations must especially be taken into account in studies of pure protic solvents such as alcohols. As shown, in the presence of hydrogen bond donor/acceptor compounds, the reactive excited-state species are never free substrates but hydrogen bond complexes. Thus, because two potential sites for hydrogen bond interactions exist in this kind of substrate, it is especially important, and extreme care must be taken, to establish the nature of the ground-state complexes responsible for the excited-state processes. This can only be safely achieved by the separate analysis of the hydrogen bond capabilities of each centre and the influence that the different adducts have on the luminescence spectra. In this sense, the different excited-state reactivity of ground-state HBC and PTC complexes should be highlighted. Thus, whereas HBC complexes behave as independent fluorophores and therefore do not participate in excited-state reactions, the formation of PTC complexes is a necessary prerequisite for observation of excitedstate processes. This is of extreme importance because these processes will only be observed in the presence of those donor compounds able to form PTC complexes. On this basis, the behaviour of hydrogen bonds in the parent compound, in which both centres are free for hydrogen bond interaction, can be reliably interpreted. Moreover, it has also been shown that the hydrogen bond excited-state interactions of photoexcited PTC
give rise to distinct excited-state processes, depending on the electronic structure of the substrate. Thus, PTC
complexes of MBC hydrogen bond in the excited state to produce CL
exciplexes, which are the excited-state precursors of C
, that is, for such PTC
complexes an excited-state intermolecular proton transfer (ESPT) process is observed. On the other hand, PTC
complexes of BCA–HFIP and double hydrogen bond complexes of BC–HFIP systems (DHBQ) suffer a hydrogen-bond-assisted intramolecular charge transfer (ICT) process. Finally, the observed excited-state phenomenon of PTC
complexes formed between BC and PYor in BC selfaggregates is an intermolecular photoinduced electron transfer (PET) process. As shown, in all these situations

416 Hydrogen Bonding and Transfer in the Excited State

the different steps can frequently be preceded by the achievement of a proper structure. Thus, only an exhaustive analysis of the excited-state dynamics allows correct interpretation of the excited-state reactivity of these systems.

Acknowledgements We gratefully acknowledge financial support from the Direccio´n General Cientıficay Tecnica MCE (CTQ2006-13539) and Junta de Andalucıa (2005/FQM-368).

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Ground- and Excited-State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives 417 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81.

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20 Effect of H-bonding on the Photophysical Behaviour of Coumarin Dyes Sukhendu Nath, Manoj Kumbhakar and Haridas Pal Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

20.1 Introduction The Lippert–Mataga [1, 2] expression is the most generalized equation used to describe the solvent-dependent spectral shifts of many chromophoric molecules. The Lippert–Mataga equation considers mainly two characteristic factors of the solvents, namely the refractive index (n) and the static dielectric constant («). According to the Lippert–Mataga representation, a chromophore should show bathochromic shifts in the absorption and emission spectra and a linear increase in the Stokes shift with increasing solvent polarity parameter, Df (Df ¼ [(«  1)/(2« þ 1)  (n2  1)/(2n2 þ 1)]). This general solvent effect predicted by the Lippert–Mataga equation is expected to be independent of the chemical properties of the chromophore and the solvent used. Besides its extensive use, the Lippert–Mataga equation fails to predict the spectral properties of several chromophores in different solvent environments [3–8]. This failure is mainly due to the presence of specific interactions of the chromophores with their surrounding solvent molecules. These specific interactions can arise owing to different mechanisms such as preferential solvation [9–11], charge transfer interaction [12], hydrogen bonding interaction [13–15], etc. The changes in spectral characteristics experienced by a chromophore owing to its specific interaction with the surrounding solvent can sometimes be much more prominent than those expected from the general solvent effect as predicted by the Lippert–Mataga equation. For example, N-phenyl-2-aminonaphthalene shows a structured emission spectrum in a non-polar solvent such as cyclohexane. However, the structures in the emission spectrum disappear, and the emission maxima undergo a large shift on the addition of just 0.2% of ethanol in the cyclohexane solution of the dye [16]. Further, it is seen that the spectrum of N-phenyl-2-aminonaphthalene in 3% ethanolic solution is only marginally different from that observed in the neat ethanol solution. Such a small amount of ethanol in

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

420 Hydrogen Bonding and Transfer in the Excited State

cyclohexane is not sufficient to alter the bulk polarity of the medium to any significant degree, such that it can cause a large spectral change of the dye. Specific interaction, namely the H-bonding interaction, is undoubtedly responsible for the observed changes in the spectral properties of N-phenyl-2-aminonaphthalene in cyclohexane solution in the presence of a small amount of alcoholic solvent. Another interesting example of specific interaction is reported by Saroja et al. for 4-aminophthalimide [17]. Only a small shift is expected in the emission spectra of 4-aminophthalimide with increasing solvent polarity, because the difference in the dipole moment between the ground and the excited state of the dye is not large (4 D). Thus, a shift of 1700 cm1 is observed in the emission maxima of the dye on changing the solvent from diethyl ether (« ¼ 4.2) to acetonitrile (« ¼ 35.9). However, a very large shift of 4220 cm1 is observed in the emission maxima when the solvent system is just changed to methanol (« ¼ 32.66), a solvent having polarity lower than that of acetonitrile. This large change in the emission maxima in alcoholic solution clearly indicates that specific interaction takes place between 4-aminophthalimide and the alcoholic solvent molecules. As these results indicate, even a small amount of cosolvent, capable of forming a hydrogen bond with the chromophore, can cause a significant change in the spectral characteristics of the chromophoric molecules. Such specific interactions can take place in many of the chromophoric molecules, and the effect of these interactions should be considered critically in interpreting the spectral properties of these chromophores in different solvents. Although such specific interactions often cause some confusion regarding the polarity of the solvent medium, they do help, on many occasions, to reveal whether a fluorophore is exposed to some specific microenvironment that can lead to some specific interaction for the dye with the surrounding media. Coumarins, the 1,2-benzopyrone dyes, number among the solvatochromic probes, which have been investigated extensively for many years. Among the coumarin dyes, the 7-aminocoumarins are known to have a large difference between their ground-state and excited-state dipole moments [18–21]. Because of this large difference in the dipole moments, these coumarin dyes show very large solvent-polarity-dependent Stokes shifts. Most of the coumarin molecules also have relatively high fluorescence quantum yields [22–26]. Because of these properties, the 7-aminocoumarin dyes have been used extensively as fluorescence probes to study different processes in the condensed phase, namely solvatochromic studies [27, 28], to determine the microenvironment of different types of microheterogeneous medium [29–31], solvent relaxation studies [29–31], etc. Because of their extensive use as fluorescence probes, it is extremely important to understand the nature of interactions that take place with the coumarin dyes in different solvent systems. For about one decade or so, our research group has been actively involved in studying the photophysical properties of coumarin derivatives in different solvents, and also in different microheterogeneous media [32–50]. The aim of all these studies is to understand the specific interactions pertaining to these systems in homogeneous media, as well as to understand the nature of the solvation behaviour in different microheterogeneous media. These studies demonstrate that the photophysical behaviour of some of the coumarin molecules is not only sensitive to solvent polarity but also highly sensitive to solute–solvent specific interaction, namely H-bonding interaction. Along with the photophysical properties, photoinduced chemical reactions, such as electron transfer (ET) processes, are also found to be greatly affected by the presence of intra- and intermolecular H-bonding in the system. For example, Zhao et al. [51] have shown from both experimental studies and theoretical calculations that intermolecular H-bonding largely facilitates ultrafast ET reaction in the photoexcited state of the molecules. Coumarins have been used extensively as electron acceptors as well as electron donors in many ET studies [52–58]. Thus, understanding of the effect of H-bonding on the photochemical and photophysical behaviour of coumarin dyes is a very important aspect that needs to be investigated thoroughly. In this mini review, we give a brief account of the results obtained from our work on the effect of H-bonding on the photophysical behaviour of some of the important 7-aminocoumarin dyes.

Effect of H-bonding on the Photophysical Behaviour of Coumarin Dyes

H

H N

O

O

H

H N

O

421

O

CF3

CH3

Coumarin 151 (C151)

Coumarin 120 (C120)

Scheme 20.1

20.2 Effect of Intermolecular H-bonding Intermolecular H-bonding between the solute and the solvent molecules causes a change in the electronic distribution in the solute molecule. If the H-bonding interaction is reasonably strong, then this may cause a substantial change in the electronic energy levels of the solute molecule. Because of this change in the electronic energy levels, there will be substantial change in the photophysical behaviour of the solute molecule. H-bonding interaction takes place both in the ground state and in the excited state of the molecule. If the electronic charge distributions in these two states are quite different, a substantial difference in the strength of the H-bond in these two states may be observed. For example, through detailed quantum chemical calculations, Zhao and Han [59] have shown that the strength of intermolecular H-bonding between coumarin and alcoholic solvent increases on photoexcitation. In the following, we discuss the changes in the photophysical properties of some of the coumarin molecules (see Scheme 20.1 for molecular structures) owing to intermolecular H-bonding with the solvent molecules. 20.2.1 Effect on photophysical properties

-1

-1

Abs. Maxima (cm )

(A) 30000

Em. Maxima (cm )

Photophysical properties such as absorption maxima, emission maxima, Stokes shifts, fluorescence quantum yields, fluorescence lifetimes, etc., have been measured for C120 dye in a number of solvents with different polarities [35]. All these photophysical parameters are seen to follow, as expected from the Lippert–Mataga representation, a linear correlation with the solvent polarity function (Df). Interestingly, however, two different linear correlations have been observed for aprotic and protic solvents. Figure 20.1 shows these correlations for the absorption and emission maxima for C120 in several solvents. The appearance of two different linear correlations clearly indicates that the photophysical properties of C120 in protic solvents are quite different

29000

28000

(B)

25500 24500 23500 22500

0.0

0.1

0.2

f

0.3

0.0

0.1

0.2

0.3

f

Figure 20.1 Variation in the absorption (A) and the emission (B) maxima of C120 with Df in different solvents. Aprotic solvents are represented by an open circle and protic solvents by a filled circle. Reprinted with permission from [35]. Copyright 2003, American Institute of Physics

422 Hydrogen Bonding and Transfer in the Excited State % of Acetonitrile in Ethylacetate 0

20

40

60

80

100

80

100

-1

Emission Maxima (cm )

27000

26000

25000

Ethylacetate +Acetonitrile

24000

23000 3-methylpentane+ Methanol 0

20

40

60

% of Methanol in 3-Methylpentane

Figure 20.2 Changes in the emission maxima of C120 dye in a 3-methylpentane–methanol mixture (*) and in an ethylacetate–acetonitrile mixture (D) with the volume fraction of the polar cosolvent [60]

from those in aprotic solvents. This difference in the photophysical behaviour of C120 in protic and aprotic solvents is attributed to the presence of H-bonding between C120 and the protic solvents. The effect of H-bonding interaction on the photophysical properties of the dye in protic solvents can be easily understood from Figure 20.2 [60]. Figure 20.2 shows the variation in the emission maxima of C120 dyes in two solvent mixtures, namely 3-methylpentane (3MP)–methanol and ethylacetate–acetonitrile mixtures. It can be seen that, in the case of the aprotic solvent mixture, the ethylacetate–acetonitrile mixture, the emission maximum follows a near-linear correlation with the volume percentage of the polar cosolvents. However, in the 3MP–methanol solvent mixture, the emission maximum follows a highly non-linear correlation with the volume percentage of methanol. It can be seen that the addition of only 1% methanol, which is a very small amount to change the bulk polarity of the medium, causes about 77% of the total Stokes shift that is observed between 3MP and the methanol solvent. About 94% of the total Stokes shift is observed on the addition of only 4% methanol. Further addition of methanol causes a nominal shift in the emission maximum. This result clearly indicates that a small amount of protic solvent can cause a very large change in the photophysical properties, and this large change is undoubtedly due to the formation of H-bonds between the coumarin dye and the protic solvent molecules. A bathochromic shift in the absorption as well as in the emission spectra is observed with an increase in the Df value for all aprotic solvents. However, a small but definite hypsochromic shift in the absorption spectra is observed with increasing Df for protic solvents (cf. Figure 20.1). It should be mentioned that the emission spectra show a bathochromic shift with Df even in protic solvents. This differential effect of protic solvents on the absorption and the emission spectra of C120 indicates that the strength of H-bonding is different in the ground state and in the excited state of the dye molecule. This differential nature of H-bonding in two electronic states can be explained by considering the different types of H-bond that are possible for the coumarin dyes in protic solvents. Scheme 20.2 shows all possible H-bonding between the coumarin and protic solvent as proposed by Kamlet et al. [61]. C120 can have three different types of H-bonding with protic solvents. The protic solvent molecules can act as donors in type-I and type-II interactions, whereas it acts as an acceptor in type-III H-bonding. It is well known that the excited state of coumarin dyes is of intramolecular charge transfer (ICT) character [18–21]. Thus, on photoexcitation, an electron from the amine site is partially transferred to the carbonyl site. Because

Effect of H-bonding on the Photophysical Behaviour of Coumarin Dyes H

R O

H

H O

N R

Type-I

O

h

H

423

N

R

Type-III

O H O

R

O

Type-II

O R R

R

Scheme 20.2 Different types of H-bonding interaction between coumarin and protic solvents

of such charge separation, type-II and type-III hydrogen bonding are preferable in the excited state of the dye. However, type-I interaction is more important in the ground state of the dye. The observed small hypsochromic shift in the absorption maxima of C120 in protic solvents is due to the variation in the strength of the type-I H-bonding in different protic solvents. As we increase the H-donating ability of the solvents, the strength of the type-I interaction will increase, and consequently there will be an increased stabilization for the ground state of the dye molecule. This extra stabilization with increase in the H-donating ability of the solvent is mainly responsible for the observed hypsochromic shift in the absorption spectrum. Similarly, with increase in the H-donating ability of the solvent, type-II and type-III interactions will increase, stabilizing the excited state of the molecule. This extra stabilization of the excited state results in a bathochromic shift of the emission spectra with an increase in the H-donating ability of the solvents. The observed Stokes shifts in C120 are also seen to be affected by H-bonding with the solvent molecules, as indicated in Figure 20.3 [35]. The changes in the Stokes shifts with Df follow the linear Lippert–Mataga equation quite satisfactorily for aprotic solvents. However, the Stokes shifts in protic solvents deviate considerably from the correlation observed in the aprotic solvents. Thus, as in the absorption and emission maxima, the Stokes shifts of the dye also show two different correlations, one for the aprotic solvents and the other for the protic solvents. Such dual correlations are well known in the literature for many other coumarin dyes [61, 62]. It is to be noted from Figure 20.3 that the increase in the Stokes shifts with Df is much steeper in the case of protic solvents than in the case of aprotic solvents. This comparatively large increase in Stokes shifts in protic solvents is due to the increase in the H-bonding ability of the solvents. As mentioned earlier, with

-1

Stokes' Shifts (cm )

5200

4800

4400

4000 0.0

0.1

0.2

0.3

f

Figure 20.3 Variation in the Stokes shifts of C120 with Df in different solvents. Aprotic solvents are represented by open circles and protic solvents by filled circles. Reprinted with permission from [35]. Copyright 2003, American Institute of Physics

424 Hydrogen Bonding and Transfer in the Excited State

increase in H-donating ability there is a small hypsochromic shift in the absorption spectra and a bathochromic shift in the emission spectra. On account of these changes in the absorption and emission maxima in two opposite directions, Stokes shifts are found to increase substantially with increase in the H-donating ability of the solvents. Coumarin-151 (C151), which has a very similar structure to that of C120, has also shown similar behaviour in protic solvents in comparison with aprotic solvents [32]. Thus, like C120, C151 also shows a small but definite hypsochromic shift in the absorption spectra with an increase in the H-donating ability of the solvent molecules. One difference in C151 compared with C120 is that the Stokes shifts for the former in protic solvents follow a quite similar correlation to that in aprotic solvents. This result indicates that the extents of stabilization of the ground and the excited state owing to the formation of intermolecular H-bonding are quite similar. The difference in the behaviour of Stokes shifts for C151 and C120 dyes is possibly due to the presence of a strong electron-withdrawing group (trifluoromethyl) in the former molecule. 20.2.2 Effect on the ICT to TICT conversion It is known that the coumarin dyes having a 7-N,N-dialkylamino substituent, such as C1, C152, C481, etc. (see Scheme 20.3 for molecular structures), behave quite unusually in solvents with relatively high polarity. Thus, in highly polar solvents, these molecules show a marked reduction in fluorescence quantum yields and lifetimes, and subsequently a sharp increase in their non-radiative decay rate [22, 23, 36, 37, 39–41, 63–65]. For aminocoumarin dyes, intramolecular charge transfer (ICT) takes place even in their ground states owing to the presence of the electron-donating alkylamino groups and the electron-withdrawing carbonyl group. However, on photoexcitation, the electron from amino group is transferred further more to the carbonyl group, resulting in a much larger ICT character in the excited state. The enhancement of the charge transfer character on photoexcitation is supported by the fact that the difference in dipole moment between the ground and the excited state for these molecules is very large [18–21]. It is now believed that the emission of these coumarin dyes is mainly originated from their planar ICT state. As the increased solvent polarity stabilizes the charge transfer state, it is likely that in highly polar solvents there could be a complete transfer of an electron from the amino nitrogen to the carbonyl group, and subsequently there is a rotation of the dialkylamino group to adopt a perpendicular structure with respect to the planar aromatic moiety of the dye (to minimize the steric factor). In this twisted ICT (TICT) state, total charge separation between the dialkylamino and carbonyl moieties takes place because the system is decoupled with a zero overlap of the orbitals involved in the conjugation of the ICT state (see Scheme 20.4). There are enormous numbers of studies reported in the literature to support the formation of the TICT state in coumarin dyes in high-polarity solvents [22, 23, 36, 37, 39–41, 63–65]. As the TICT state is highly polar in nature, in very high-polarity solvents the TICT state can become lower in energy than the planar ICT state. To ought to be mentioned, however, that, unlike other TICT molecules, the TICT state of coumarin dyes is found to be non-emissive in nature. It is understood that the TICT state primarily introduces a new non-radiative deactivation channel for the excited coumarin molecules in highpolarity solvents. C2H5 C2H5 C2H5

N

O

O

CH3 Coumarin 1 (C1)

C2H5

N

O

O

CF3 Coumarin 481 (C481)

Scheme 20.3

H3C N H3 C

O

O

CF3 Coumarin 152 (C152)

Effect of H-bonding on the Photophysical Behaviour of Coumarin Dyes

R

R

R

R O

N

O

h

R

O

N

O

Bond Twisting

N

R

R

R

Ground State

425

O

O

R

Planner ICT

TICT

Scheme 20.4

The transformation of the planar ICT state to the TICT state in the excited state of the N,N-dialkylaminocoumarins in high-polarity solvents is mainly responsible for the large reduction in the fluorescence quantum yields and lifetimes through the introduction of a new non-radiative decay channel for the dyes. Detailed photophysical studies have shown that, like other TICT molecules [66–68], a critical solvent polarity is also required to induce the ICT to TICT conversion for these coumarin molecules. Thus, a plot of any of the photophysical parameters, such as the fluorescence quantum yield, fluorescence lifetime, etc., with the Df function shows a sudden break at a certain value of Df, which is specific to the specific coumarin molecule. Detailed investigation shows that the ICT to TICT conversion is also affected by the specific interaction of the solute with the solvent molecules. The following example will show how intermolecular H-bonding between the coumarin dye and the solvent molecules can affect the induction of the TICT process from the ICT state. Figure 20.4 shows the variation in the fluorescence quantum yield and lifetime of C1 dye as a function of Df of different solvents. The variation in emission yield and lifetime shows very nice linear correlation up to Df ¼ 0.28. However, the emission yield and lifetime of C1 are seen to show a sharp decrease in solvents with Df > 0.28. From Stokes shift measurement it has been shown that the nature of the emissive state for C1 remains the same in all the solvents [33, 39]. Thus, a sudden decrease in the emission yield and lifetime in solvent with Df > 0.28 clearly indicates the appearance of a new non-radiative decay channel for this dye when the Df value of the medium exceeds 0.28. Through a detailed photophysical study it has been established that the formation of the TICT state from the planar ICT state is the cause of a new non-radiative channel that appears in the solvent with high polarity, and accordingly the emission yield and lifetime decrease significantly in these solvents. The variation in the fluorescence quantum yield and lifetime of C1 dye in polar aprotic solvents is also shown in Figure 20.4. It is clearly evident from this figure that, even when the Df value is significantly higher than 0.28 in aprotic solvents, there is no indication of the appearance of any new non-radiative decay channel for C1 dye. 0.8

(A)

0.6

3 f (ns)

f

(B)

4

0.4

2

0.2

1

0.0

0 0.20

0.24

0.28

f

0.32

0.20

0.24

0.28

0.32

f

Figure 20.4 Variation in the fluorescence quantum yield (A) and fluorescence lifetime (B) of C1 dye with Df in different solvents. Aprotic solvents are represented by open circles and protic solvents by filled circles. Reprinted with permission from [33]. Copyright 2003, American Institute of Physics and Reprinted with permission from [39]. Copyright Elsevier

426 Hydrogen Bonding and Transfer in the Excited State

This clearly indicates that in aprotic solvents the formation of the TICT state from the planar ICT state is not favourable even in solvent with a Df value of 0.305 (acetonitrile). This difference in the formation of the TICT state in protic and aprotic solvents undoubtedly suggests that the specific interaction, namely the H-bonding interaction, favours the formation of the TICT state from the planar ICT state. The fact that intermolecular H-bonding between solute and solvent often favours TICT state formation is also indicated in other fluorophores [69–72]. Thus, if the solvent has H-bonding ability, the TICT state can be formed even at a solvent polarity much lower than would have been the case in aprotic solvents. Intermolecular H-bonding favours charge separation in the excited state of the molecules. This assumption is supported by the fact that the dipole moment measured by the solvatochromic method for C1 dye in protic solvents (10.9 D) [39] is higher than that measured in aprotic solvents (9.1 D) [33]. Owing to this extra charge separation, the stabilization of the excited state is relatively greater in protic solvents compared with aprotic solvents with a similar solvent polarity parameter, Df. The extra H-bonding results in much greater stabilization of the TICT state compared with that of the planar ICT state. This extra stabilization of the TICT state owing to H-bonding is mainly responsible for the shifting of the critical polarity (Df) required for TICT state formation in protic solvents towards lower Df values compared with that in aprotic solvents. A similar effect of intermolecular H-bonding on the ICT to TICT conversion in other coumarin dyes, namely C481 and C152, is also reported by our group [36, 40]. Unlike C1, coumarins C481 and C152 in aprotic polar solvents show the formation of the TICT state from the planar ICT state at a Df value of 0.2, whereas in protic solvents the formation of the TICT state takes place at a Df value of 0.18. The lower polarity required for TICT formation in C481 and C152 dyes compared with that in C1 is supposed to be mainly due to the presence of a stronger electron-withdrawing group, trifluoromethyl, in the former molecules. Owing to the presence of the stronger electron-withdrawing group in C481 and C152, the extent of charge separation in the planar ICT state is much greater in these molecules compared with that in C1. Because of this extra charge separation, the stabilization of the excited state is much greater in C481 and C152 compared with that in C1, and hence less polarity of the solvent is required to induce TICT state formation in the case of the former molecules.

20.3 Effect of Intramolecular H-bonding on ICT to TICT Conversion Like intermolecular H-bonding, intramolecular H-bonding is also seen to affect the photophysical properties of many chromophoric molecules [73–78]. Detailed photophysical studies of coumarin-7 (C7) and coumarin-30 (C30) (see Scheme 20.5 for the molecular structure) in different protic solvents indicate that intramolecular H-bonding has a substantial influence on the excited-state dynamics of these dyes [37, 41, 42]. Variation in the fluorescence lifetime and non-radiative rate constant for C7 and C30 dyes with the Df parameter of the solvents is shown in Figure 20.5. For both of these dyes there is a sudden decrease in the fluorescence lifetime and a sudden increase in the non-radiative rate in high-polarity solvents [37, 41]. This sudden change in lifetime and non-radiative rate constant in solvents of very high polarity has been attributed to the formation of the non-emissive TICT state from the emissive ICT state. As in C1, it is again seen that, Et

Et N

O

N

O

O

O

Et

Et

N

N N

N H

CH3

Coumarin-7 (C7)

Scheme 20.5

Coumarin-30 (C30)

Effect of H-bonding on the Photophysical Behaviour of Coumarin Dyes 4.0

(B)

-1

knr /10 (S )

C7 3.0

C30 3.0

8

2.4

8

-1

2.0

knr /10 (S )

Lifetime (ns)

4.0

(A)

2.8

427

2.0 1.6

2.0 0.20

0.24

0.28

f

0.32

0.20

0.24

0.28

0.32

f

Figure 20.5 Variation in the fluorescence lifetime (A) and non-radiative rate constant (B) of C7 (D) and C30 (*) dyes with Df in different solvents [37, 41]. Data points for acetonitrile solvent are shown by two concentric circles. Reprinted with permission from [41 and 37]. Copyright 2009 John Wiley & Sons, Inc.

owing to intermolecular H-bonding, protic solvents favour the formation of the TICT state in both of these dyes over aprotic solvents. For example, in acetonitrile (Df ¼ 0.304, the corresponding data point is encircled in Figure 20.5), the lifetime (2.57 ns) is reasonably longer than that in a mixed protic solvent (80% methanol þ 20% isopropanol, lifetime ¼ 1.88 ns) having the same Df value. Similarly, the non-radiative rate constant in acetonitrile is much lower than that in mixed solvents. From Figure 20.5 it is evident that the critical Df value required for ICT to TICT conversion is 0.31 for C7 dye and 0.27 for C30 dye. Although C7 and C30 have similar chemical structures, this difference in the critical Df values for the formation of the TICT state indicates that a higher polarity is required to form the TICT state in C7 compared with that required in C30. The observed difference in the critical Df values for the formation of the TICT state in C7 and C30 dyes has been attributed to the presence of intramolecular H-bonding between the NH group of the 3-benzimidazole moiety and the carbonyl group of the C7 molecule. Owing to the formation of such intramolecular H-bonding in C7, the planar ICT state gains an extra stability over its twisted configuration. This is the reason why C7 dye requires significantly higher solvent polarity (Df ¼ 0.31) for its TICT state formation than C30 dye (Df ¼ 0.27), although the two dyes are structurally very similar to each other. The support for the formation of intramolecular H-bonding between the NH group of the 3-benzimidazole moiety and the carbonyl group in C7 is also obtained from quantum chemical calculation. The optimized structures of C7 and C30 dyes in the presence of three water molecules are shown in Figure 20.6. The formation of intra- as well as intermolecular H-bonds is shown by dotted lines, along with their bond distances. The dihedral angle between the benzimidazole and benzopyrone moieties (i.e. N1--C1--C2--C3 dihedral angle) for the optimized geometry are 0.609 and 56.75 for C7 and C30 respectively [41]. The close-to-zero N1--C1--C2--C3 dihedral angle in C7 dye clearly indicates that the benzimidazole and benzopyrone moieties lie in the same plane. However, the dihedral angle value of 56.75 between the benzimidazole and benzopyrone moieties in C30 indicates that these two moieties are not in the same plane. This non-planar geometry in C30 is due to the steric factor introduced by the methyl group on the N1 atom. It is also evident from the optimized structures of C7 that the water molecule containing the O1 atom can form two hydrogen bonds with the C7 molecule. However, the same water molecule can form only one H-bond with the C30 molecule. Owing to the presence of two extra H-bonds (O1  H1 and O2  H1) in C7 compared with C30, the rotation of the benzimidazole moiety with respect to the benzopyrone moiety is more hindered in the former dye than in the latter. Because of this additional H-bonding, the formation of the TICT state is less favourable in C7 than in C30, and hence a higher solvent polarity is required for the formation of the TICT state in C7 dye than that required in C30 dye.

428 Hydrogen Bonding and Transfer in the Excited State

Figure 20.6 Optimized geometry of C7 and C30 in the presence of three water molecules. Reprinted with permission from [41]. Copyright 2009 John Wiley & Sons, Inc.

All coumarin molecules that form the TICT state in high-polarity solvent are found to show a strong temperature-dependent excited-state dynamics [36, 37, 39, 41]. This temperature dependence indicates that the relaxation from the excited state in these molecules is activation controlled. Another surprising thing that has been observed is that the estimated activation energy increases with increase in the solvent polarity [36, 37, 39–41]. Note that, in most other molecules that can form the TICT state [79–81], the activation energy is usually seen to decrease with solvent polarity. This decrease in activation energy is quite expected, because, with increase in solvent polarity, the TICT state is supposed to be stabilized more than the ICT state. Thus, the activation energy for the conversion of the ICT state to the TICT state should decrease with increase in solvent polarity. For such systems, the ICT to TICT conversion process is the rate-determining step. However, the observation that in coumarin dyes the activation energy increases with solvent polarity indicates that the ICT to TICT conversion process is not the rate-determining step for this class of molecules. Through detailed temperature-dependent studies, it is proposed that for coumarin molecules the actual activation-controlled process is TICT to ground state conversion [36, 37, 39, 41]. With increase in solvent polarity, not only does the TICT state become more stable but also the potential energy surface of this state becomes steeper with respect to the solvent polarization axis as compared with those of the relatively less polar ICT state. Because of this increase in steepness of the TICT potential energy surface, the crossing between the ground-state and the TICT-state potential energy surfaces takes place at higher energy, and consequently there is an increase in the activation energy for the TICT to ground state conversion process. Interestingly, it is seen that the activation energy (DEa) measured for C7 dye is always much lower (DEa  0.3–1.6 kcal mol1) than that of C30 dye (DEa  2.3–2.9 kcal mol1) in all the solvents [37, 41]. Such an observation, as well as the fact that the onset of TICT state formation occurs for C7 dye at much higher

Potential Energy

Effect of H-bonding on the Photophysical Behaviour of Coumarin Dyes

C30

( Ea)C30

C7

ICTex

429

( Ea)C7

TICT

ICTgr Solvent Polarization Axis

Figure 20.7 Schematic potential energy diagram for the ICT ground state (ICTgr), ICT excited state (ICTex) and TICT state for C7 and C30. Reprinted with permission from [41]. Copyright 2009 John Wiley & Sons, Inc.

polarity than for C30 dye, has been rationalized on the basis of the qualitative potential energy (PE) diagrams shown schematically in Figure 20.7 [41]. As mentioned earlier, in the case of C7 dye, owing to the formation of intramolecular H-bonding between the N--H proton of the 3-benzimidazole group and the carbonyl group of the benzopyrone moiety, the planar ICT structure receives extra stabilization. Thus, it is quite expected that, for a similar stabilization of the TICT state of the two dyes with respect to their ICT states, the C7 dye will require a much higher solvent polarization than C30. Alhough there will be many other interactions for the stabilization of the different electronic states of the dyes in different solvents, for simplicity we consider the similar PE curves for the ICT ground (ICTgr) and ICT excited (ICTex) states of both C7 and C30 dyes, and the relative changes in the PE curves for the TICT states of the two dyes are presented qualitatively considering the effect of the NH group of C7 dye on the destabilization of its TICT state in comparison with the TICT state of C30 dye. As indicated in Figure 20.7, owing to intramolecular H-bonding, for similar stabilization of the TICT states, the PE curve for the TICT state of C7 dye will be shifted to higher solvent polarization compared with that of C30 dye. On account of this drifting of the PE curve for the TICT state of C7 dye along the solvent polarization axis compared with that of the C30 dye, PE crossing from the TICT state to the ICTgr state for the former dye will occur with a much lower activation barrier in comparison with that of the latter dye. This result indicates that intramolecular H-bonding not only affects the ICT to TICT conversion but also the relaxation from the TICT state to the corresponding ground state.

20.4 Summary Both intermolecular and intramolecular H-bonding have a profound effect on the photophysical properties of coumarin dyes. The effect of H-bonding on the spectral properties of coumarin dyes can be used to determine the nature of the surroundings of an entrapped dye in different microheterogeneous media. For example, if a bound coumarin in any macromolecule, such as a protein, is buried inside, then any change in the solvent outside the macromolecule will not be sensed by these probe molecules. However, if the bound probe can access the solvent environment to any extent, then a change in the solvent environment will be reflected in the photophysical properties of the probe. Further, specific H-bonding interaction can cause a greater change in the

430 Hydrogen Bonding and Transfer in the Excited State

photophysical properties of the coumarin dyes and hence can be easily observed in order to understand the changes in the microenvironments. It is shown that solute–solvent intermolecular H-bonding often favours the ICT to TICT conversion process for most of the coumarin dyes. In some coumarin dyes, intramolecular H-bonding is seen to act in opposition to intermolecular H-bonding, hindering the formation of the TICT state of the molecules.

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431

21 Role of Hydrogen Bonds in Photosynthetic Water Splitting Gernot Renger Max-Volmer-Laboratorium f€ur Biophysikalische Chemie, Technische Universit€ at Berlin, Strasse des 17 Juni 135, 10623 Berlin, Germany

21.1 Introduction Photosynthetic water splitting is the key step in the exploitation of solar radiation as the unique Gibbs free energy source of all higher forms of life on earth. The ‘invention’ of a machinery enabling the performance of this process can be considered as the ‘big bang’ in the evolution of the biosphere. This event, which occurred 2–3 billion years ago, at the level of ancient cyanobacteria (for a review, see Ref. [2]), made the huge water pool available as hydrogen source for biological organisms and simultaneously led to the formation of an aerobic atmosphere. The latter effect is the cornerstone of the highly efficient extraction of Gibbs free energy from food through aerobic respiration (for thermodynamic considerations, see Refs [3] and [4]). Both fundamental bioenergetic processes, i.e. photosynthetic water splitting and oxidative respiration, use the same redox system H2O/O2, where H2O is the substrate in the former and the terminal product in the latter case. The reaction H2O ! H2 þ 1/2O2 is highly endergonic, with DG ¼ 237.17 kJ mol1 [5], i.e. this reaction requires a strong driving force, whereas the process in the opposite direction offers a powerful Gibbs free energy source. The overall process can be separated into two redox reactions: H2 O > 1=2 O2 þ 2H þ þ 2e

ð21:1aÞ

2H þ þ 2e > H2

ð21:1bÞ þ

In biological systems the hydrogen is chemically bound to compounds like NADP or NADþ , and equation (21.1b) is modified to T þ 2e þ 2Hþ > T * H2, where T * H2 is the metabolically bound hydrogen.

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

434 Hydrogen Bonding and Transfer in the Excited State

Scheme 21.1

The most important step of water splitting is reaction (21.1a). The sequence of the individual one-electron steps of the redox chemistry of the redox couple H2O/O2 can be represented by Scheme 21.1 An inspection of Scheme 21.1 readily shows that the chemical reaction is coupled with the release (oxidative water splitting) or uptake (oxygen reduction) of four protons. It is clear that, regardless of the mechanistic details, electron transfer (ET) and proton transfer (PT) are closely interrelated reactions. The mode of coupling between ET and PT is a general problem of high mechanistic relevance for many redox reactions (for reviews, see Refs [6] to [8]) and in particular for ‘handling’ the redox couple H2O/O2 in bioenergetics, where the reactions take place in the protein environment of enzymes. In this respect it is important to note that the protein matrix including hydrogen bond networks plays a key role, and that marked differences exist between the pathways in the forward and backward directions of Scheme 21.1: in the highly endergonic reaction of oxidative water splitting, four protons are released per O2 molecule formed (see Section 21.4.2), while the strongly exergonic reduction of O2 to H2O is often energetically coupled to pumping of protons against the electrochemical potential difference of protons across the coupling membrane. In this way, the DG of the oxidation of T * H2 by O2 is transformed into a proton motive force (pmf), which can be used as the driving force for the chemical reaction of ATP synthesis [9] (for a review, see Ref. [10]) or the active transport of ions. Photosynthetic water splitting into T * H2 and O2 is energetically driven by two light reactions that transform solar radiation into electrochemical Gibbs free energy. These two reactions take place in pigment–protein complexes (designated photosystem I (PS I) and photosystem II (PS II)), which act in series and give rise to water splitting into NADPH (T * H2) and O2 (for a general review, see Ref. [11]). PS II acts as a light-driven water:plastoquinone-oxido:reductase, which generates the moderately reducing bound hydrogen in the form of plastoquinol (PQH2), thus providing the redox equivalents for the light-driven NADPþ reduction at PS I (for a review on PS I, see Ref. [12]). This pattern shows that the key steps of photosynthetic water splitting are localized in PS II. Therefore, the present communication on the role of hydrogen bonds in photosynthetic water splitting is entirely focused on PS II.

21.2 Photosystem II: Overall Reaction Pattern and Cofactor Arrangement PS II is a multimeric integral membrane protein complex anisotropically incorporated into the thylakoid membrane and complemented by extrinsic regulatory units (for a review, see Ref. [13] and references therein). The system is characterized by a striking complexity of the polypeptide composition, i.e. it consists of at least 20 subunits [14]. This feature markedly contrasts with that of type II reaction centres of anoxygenic photosynthetic bacteria which contain only two or three subunits [15]. Furthermore, it is very intriguing that PS II evolved in a rather narrow geological period, and that the composition of cofactors and polypeptides remained surprisingly invariant to evolutionary development [16]. These findings indicate that PS II is a

Role of Hydrogen Bonds in Photosynthetic Water Splitting

435

Figure 21.1 Types of reaction sequence (top panel) and cofactor arrangement (bottom panel) in the PS II core (view is along the membrane normal). The coordinating protein subunits D1 and D2 are indicated by a dotted line (See Plate 23)

specially tailored molecular machine for performing the highly demanding oxidative water splitting into molecular oxygen and four protons [17]. The overall reaction pattern, which comprises three different types of reaction sequence, is schematically summarized in the top panel of Figure 21.1. In the first sequence, the lowest electronically excited singlet state of a pigment complex denoted P680 is transformed into electrochemical free energy by the formation of the radical ion pair P680 þ Q A (see Section 21.4.1). This process provides the driving force for oxidative water splitting into molecular oxygen and four protons according to sequence 2 (see Section 21.4.2) and reductive plastoquinol formation via sequence 3 (see Section 21.4.3). The bottom panel of Figure 21.1 presents X-ray diffraction crystallography (XRDC) data on the structural array of the cofactors that are involved in the three types of reaction (the pathways are marked by red, blue and orange arrows for sequences 1, 2 and 3 respectively). All these cofactors are embedded into a heterodimeric protein matrix that consists of polypeptides D1 and D2. This protein environment functionalizes the cofactors by tuning their reaction behaviour through fixing the mutual distance and orientation and regulating the energetics via effects on the midpoint redox potentials, in particular through the dielectric microenvironment including hydrogen bonds. In the following section, effects of hydrogen bonds on the properties of PS II will be outlined in more detail. *

*

436 Hydrogen Bonding and Transfer in the Excited State

21.3 Hydrogen Bonds and the Thermal Stability of PS II Whole plants, isolated thylakoids, PS II membrane fragments and PS II core complexes are prone to loss of activity under thermal stress. Oxygen evolution capacity was found to be the most sensitive part of PS II (see Ref. [18] and references therein). In order to study the possible effects of hydrogen bonds on thermal stability, thylakoids suspended either in H2O or D2O buffer solutions were exposed in the dark for an incubation time (tinc) of 3 min to elevated temperatures T, and after this treatment the samples were injected into a cuvette where the average oxygen yield per flash Yav(T, tinc ¼ 3 min) was determined in aqueous (H2O) buffer suspensions at 20  C (for details, see Ref. [19]). The quantity Yav(T, tinc ¼ 3 min) is a measure of PS II centres, which are functionally competent in oxidative water splitting after 3 min dark incubation at temperature T. The results obtained are shown in Figure 21.2. Inspection of these data reveals a shift of the curve Yav(T, tinc ¼ 3 min) by about 5 deg towards higher temperatures when thylakoids are suspended in buffer solutions containing D2O instead of H2O. This finding indicates that the thermal stability is enhanced owing to the replacement of exchangeable protons by deuterons. The thermal degradation of oxygen evolution capacity was found to be satisfactorily described by monoexponential kinetics, i.e. Yav(T, tinc)  Yav(T, tinc ¼ 0) * exp(kTapp * tinc) (data not shown). Therefore, the activation energy of this detrimental process can be gathered from the experimental data by using the Arrhenius plot of ln kTapp versus 1/T, where kTapp is obtained by the relation kTapp ¼ ðtinc ¼ 3minÞ1 ln½Yav ðT; tinc ¼ 0Þ=Yav ðT; tinc ¼ 3 minÞ

ð21:2Þ

when the activity loss is approximated by first-order decay kinetics. The inset of Figure 21.2 shows the Arrhenius plot obtained. Activation energies of 140 and 143 kJ mol1 were found for thermal inactivation of the O2 evolution capacity in thylakoids heated for 3 min in the presence of H2O and D2O respectively. In spite of the approximation used for calculation of kTapp , some interesting

Figure 21.2 Average oxygen yield per flash as a function of temperature at a 3 min dark incubation (main panel) app app and ln kT as a function of reciprocal incubation temperature (inset); kT is calculated according to equation (21.2) (for details, see the text)

Role of Hydrogen Bonds in Photosynthetic Water Splitting

437

conclusions emerge from these data if it is assumed that the H/D exchange before starting the heat treatment affects only the reaction coordinates of the degradation process (i.e. the partition functions of all other internal modes remain invariant). In this case the activation energy is expected to be changed solely owing to the differences in the zero-point energies of the hydrogen bonds with exchangeable protons. The difference in the zero-point energies between pffiffiffi hydrogen bonds with a proton (H  X–A) and deuteron (D  X–A) in proteins is DE0 ¼ 1/2hnH(1  1/ 2) (the mass of hydrogen bond acceptor X–A is very large compared with the mass of the proton (H) and deuteron (D)), where nH is the frequency of a normal hydrogen bond containing a proton. Values of (5–10)  1022 J are obtained for DE0 of a hydrogen bond owing to replacement of H by D when using typical values of 200–300 cm1 for nH [20, 21]. Accordingly, the difference of about 3 kJ mol1 in activation energy of thermal degradation between samples heated in H2O (140 kJ mol1) and D2O (143 kJ mol1) corresponds to the breaking of 5–10 neutral hydrogen bonds during thermal inactivation. Regardless of quantitative uncertainties, these considerations reveal that hydrogen bonds are relevant for thermal stability of the oxygen evolution capacity of thylakoids. Qualitatively similar results were obtained for PS II membrane fragments from spinach [22].

21.4 Reaction Sequences of PS II and the Role of Hydrogen Bonds 21.4.1 Light-induced charge separation 21.4.1.1 Type II Reaction Centres of Anoxygenic Photosynthetic Bacteria and PS II The photosynthetic transformation of light into electrochemical Gibbs free energy comprises two types of reaction (for a review, see Ref. [23]): (i) population of electronically excited singlet states through light absorption by pigment protein complexes acting as antennae and excitation energy transfer (EET) to the photochemically active pigment PRC in the reaction centres; (ii) ejection of an electron from 1 P*RC and transfer þ to an acceptor molecule, thus creating the primary cation–anion radical pair PRC Acc 1 , which is ‘stabilized’ by subsequent electron transfer (ET) step(s). In anoxygenic purple bacteria, the reaction centres (PBRCs) are structurally well-defined operational units that can be isolated by detergent treatment and provide most suitable sample material for detailed studies using sensitive spectroscopic methods of high time resolution (for a review, see Ref. [15] and references therein). The charge separation in PBRCs can be summarized by the following equation (for a review, see Ref. [15]): *

*

þ PRC BChlA BPheoA QA ! 1 P*RC BChlA BPheo QA > PRC BChl A BPheo QA hv

*

þ þ  > PRC BChl BPheo A QA > PRC BChlA BPheoA QA *

*

*

*

*

ð21:3Þ

where PRC is a special pair of two excitonically strongly coupled bacteriochlorophyll (BChl) molecules, BChlA is a monomeric BChl acting as primary electron acceptor, BPheoA is a bacteriopheophytin and QA is an ubiquinone (menaquinone) molecule (EET steps are omitted because the special pair P can be selectively excited by light absorption). The structural array of the four BChl and two BPheo molecules and of QA resembles that of the corresponding Chl, Pheo and QA in PS II (for a review, see Ref. [24]). An analogous reaction pattern emerges in PS II. However, compared with PBRCs, details of the mechanisms are significantly different. First, the photoactive pigment P680 is not a special pair like PRC. In fact, a unit of six excitonically coupled pigments exists (four Chl a and two Pheo molecules), referred to as RC–PC (see Figure 21.1 and, for reviews, Refs [1] and [25]), where the excited singlet state is distributed among all six

438 Hydrogen Bonding and Transfer in the Excited State

pigments, with the lowest exciton state being dominated by the contribution from the monomeric ChlD1 (for details, see Ref. [1]). Second, ChlD1 most likely acts as the primary electron donor rather than as the primary electron acceptor as in the case of the analogous BChlA in PBRCs (see equation (21.3); for an alternative þ hypothesis on the function of ChlD1, see Refs [26] and [27]). ChlD1 rapidly oxidizes the ‘dimer’ (PD2PD1), þ where the spin density of the cation radical (PD2PD1) is located on PD1 in intact PS II [1, 28] (see Section 21.4.1.2.2). The triplet-state 3 P680 resides on ChlD1 [1]. On the basis of our current state of knowledge, the light-induced charge separation in PS II can be summarized by the equation *

*

þ  þ ðPCRCÞ ! 1 ðPCPCÞ* > ðPD2 PD1 ÞChlD1 Pheo A QA > ðPD2 PD1 ÞChlD1 PheoA QA hn

*

þ > ðPD2 PD1 ÞChlD1 PheoA Q A *

*

*

*

*

ð21:4Þ

(the components ChlD1, PD1, PheoA and QA are shown in Figure 21.1; for the sake of simplicity, EET processes from the antenna to PC–RC are omitted). 21.4.1.2 Effects on Hydrogen Bonding on Light-Induced Charge Separation The cofactors of light-induced charge separation of both PBRCs and PS II contain donor and acceptor groups for forming hydrogen bonds that can affect the spectroscopic and kinetic properties. The kinetics of the individual ET steps of equation (21.3) have been shown to be retarded by factors of 2–2.5 when exchangeable protons are replaced by deuterons in PBRCs (see Ref. [29]), whereas in PS II the analogous effects on the reactions of equation (21.4) are rather small (a factor of about 1.1) (see Ref. [30]). Detailed information on hydrogen bond effects was obtained by using time-resolved spectroscopy on PBRCs, where the individual reaction steps can be spectrally separated. Analogous analyses cannot be performed on PS II owing to spectral congestion of the cofactors in PC–RC (see Refs [1] and [25]). Protein Relaxation due to Electronic Excitation of Pigments. Studies on isolated PBRCs from Rhodobacter(Rb) sphaeroides revealed that replacement of exchangeable protons by deuterons does not affect the stationary absorption spectrum of the sample but leads to a red-shift of the emission maximum by about 7 nm. This finding indicates that rearrangement of the microenvironment in response to population of electronically excited singlet states gives rise to relaxation processes of the protein that are affected by hydrogen bonds and cause a small energetic stabilization of the relaxed 1 P*RC state when the PBRCs are suspended in buffer containing D2O rather than H2O [31]. Time-resolved spectroscopic studies were performed in order to resolve the kinetics of this effect. Excitation of the QX transition with 70 fs (FWHM) laser pulses at 600 nm revealed that the bleaching maximum of 1 P*RC is shifted to the red by about 4 and 5 nm with time constants of about 250 and 375 fs when the same sample material is suspended in buffer solutions containing H2O and D2O respectively [32]. This marked kinetic isotope effect indicates that the red-shift is caused by protein relaxations which comprise rearrangements of hydrogen bonds. The red-shift is followed by a reversal back to the original position with lifetimes of 3.5 and 8.5 ps in the presence of H2O and D2O respectively [32]. These kinetics, which are ascribed to the primary charge separation step from 1 P*RC;relaxed to the acceptor BChlA (see equation (21.3)), reflect a striking kinetic isotope effect on the primary charge separation step. Accordingly, rearrangement of hydrogen bonds is involved in this process. Effects on the Midpoint Potential of P and P680. Hydrogen bonds can affect the midpoint potential by stabilizing the oxidized and reduced forms of a redox component to different extents. This effect has been

Role of Hydrogen Bonds in Photosynthetic Water Splitting

439

clearly demonstrated in mutants of PBRCs from Rb sphaeroides. Each BChl molecule has two possible acceptor sites for formation of hydrogen bonds (the acetyl group of ring I and the keto carbonyl group of ring V). Although P contains four potential sites, only one hydrogen bond exists (between P and His 168 of the L submit) in the WT [24]. Additional hydrogen bonds can be genetically engineered at the remaining three positions through replacement with His of residues Leu 131 in the L subunit and Leu 160 and Phe 197 in the M subunit. The effects are virtually additive, i.e. each hydrogen bond gives rise to an incremental increase by 60–125 mV in þ the Em value of PRC/PRC , which is about þ 500 mV in the WT (for a review, see Ref. [33]). Likewise, the elimination of the single hydrogen bond to P in the WT causes a downshift of the Em value by about 80 mV. Furthermore, a correlation was found between the increase in Em and the strength of the hydrogen bond to the C113 keto carbonyl group of ring V of the BChl molecules at the PM position [34]. An exchange of protons by deuterons is expected to have no significant effect on the Em value because the chemical nature of the microenvironment is not changed. Redox titration experiments of P confirmed this to be the case [31]. The situation is quite different for P680 in PS II (see above). The cation radical P680 þ is one of the strongest oxidants in biological organisms. The reduction potential of P680 with an estimated value of about þ þ 1.25 V [35] exceeds that of Chl a in solution [36] by more than 400 mVand the Em values of PRC =PRC of all other synthetic reaction centres by at least 700 mV. This finding clearly illustrates the paramount importance of the protein matrix in functionalizing the properties of cofactors. Inspection of the chemical structure reveals that the Chl a molecule contains only a single group (the keto carboxyl of ring V) for forming a hydrogen bond, in contrast to BChl a which carries two different groups (see above). As a consequence, manipulation via hydrogen bonding could not be the most appropriate evolutionary ‘tool’ for achieving the indispensable (for oxidative water splitting) drastic upshift of the P680 reduction potential. Theoretical calculations showed that establishing a microenvironment with a low dielectric constant is the key parameter, i.e. nature used this mode of tuning for achieving the goal of building up a strongly oxidizing species P680 þ [37]. The critical role of the protein matrix is clearly illustrated by FTIR measurements by Okubo et al. on PS II membrane fragments, PS II core complexes and isolated D1/D2/Cyt b559 preparations [28]. This study revealed that the positive charge of P680 þ is localized on PD1 only in the former two complexes but delocalized over PD1/PD2 in isolated D1/D2/Cyt b559 preparations. Density functional theory (DFT) calculations on the PD1/PD2 dimer were reported that suggest a downshift of the redox potential by about 140 mV owing to charge delocalization of the state P680 þ [38]. *

*

*

*

*

*

Effects on BPheo and Pheo. Light-induced charge separation leads to transient formation of the cation–anion þ þ radical pairs PRC BChlA BPheo Pheo A QA in PBRCs and P680 D1 QA in PS II (see equations (21.3) 2þ and (21.4)). The Mg free base molecules BPheo a and Pheo a have the same hydrogen-bond-forming acceptor groups as BChl a and Chl a. A well-resolved characteristic of BpheoA is the hydrogen bonding of its ring V keto group by Glu 104 of the L subunit (for a review, see Ref. [24]). An analogous hydrogen bonding of PheoD1 was found for Pheo D1 in PS II from plants [39, 40], where Glu 130 of polypeptide D1 acts as proton donor to interact with the keto carbonyl of ring V [41]. Cyanobacteria contain Gln instead of Glu, and the optical transition of the Bx band is shifted to the blue by 2–3 nm compared with plants, but the band position returns back to that of plants when Gln 130 is replaced by Glu in mutants of Synechocystis PCC 6803 [42]. This finding indicates that the hydrogen bond is likely to be responsible for this energetic shift. The effect of hydrogen bonds on the Em value of Pheo A /PheoA in PS II is less pronounced than that reported for the special pair of PBRCs (see Section 21.4.1.2.2). Replacement of D1-Glu 130 with Gln in Synechocystis PCC 6803 was shown to give rise to an increase by about þ 30 mV. On the other hand, a change to Leu causes a decrease by about 75 mV, but this shift is not necessarily only due to elimination of the hydrogen bond [43]. *

*

*

*

*

*

440 Hydrogen Bonding and Transfer in the Excited State

Changes in the Em value of PheoD1 might be of physiological relevance (for further discussion, see Refs [25], [44] and [45]). þ The kinetics of light-induced ET leading to PRC QAþ formation in PBRCs were found to remain virtually unaffected in mutants, where the hydrogen bond to BPheoA is disrupted owing to replacement of Glu 104 by other residues [46]. On the other hand, replacement of exchangeable protons by deuterons gives rise to a kinetic isotope effect of 1.5–2 [32, 47]. 1 No significant kinetic effects on the P680 þ Pheo A formation were found in D1/D2/Cyt b559 preparations from Synechocystis PCC 6803, where site 130 of polypeptide D1 is mutated by replacement with other amino acid residues [43]. Likewise, the kinetics of QA reduction by Pheo A in PS II remains almost unaffected by H/D exchange [30]. Based on these findings it can be concluded that the hydrogen bond to the keto carboxyl of ring V of either BPheoA in PBRCs or PheoD1 in PS II is not an indispensable structural motif for the function of light-induced charge separation. It seems more likely that the hydrogen bonding could induce some fine tuning of the properties. The possible functional relevance of this tuning, which is somehow different between PBRCs and PS II, is not yet clarified. *

*

*

*

*

Effects on QA and Q A . The QA molecules ubiquinone (menaquinone) in PBRCs and plastoquinone in PS II contain two keto carbonyls as potential hydrogen bond acceptor groups. Accordingly, these molecules form hydrogen bonds with the protein environment of their binding sites within the M subunit of PBRCs and the polypeptide D2 in PS II (for reviews on the structure of QA binding sites, see Refs [13] and [48]). QA acts as a one-electron component under normal conditions, thereby switching between the redox levels of quinone and semiquinone. Both states, QA and Q hydrogen bonds. In PS II the keto groups of QA are A , form  connected via hydrogen bonds with D2-His 214 (2.6 A ) and the backbone amide nitrogen D2-Phe 261  (2.8 A) [13, 48]. For the semiquinone from Q A , a similar hydrogen bond pattern has earlier been gathered from electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopic studies [49, 50]. This striking similarity might indicate that the hydrogen bond pattern does not change when QA is reduced to Q A . However, it cannot be excluded that the X-ray radiation used for XRDC studies leads to the formation of Q A as was demonstrated for PBRCs [51], and therefore the similarity could be a trivial phenomenon because it might reflect in both cases the hydrogen bonding of the same species, i.e. of Q A . In addition to the problem of unambiguous assignment, the XRDC data do not provide information on details of hydrogen bonding (see also Section 21.4.2.2.3). Spectroscopic methods (FTIR, ENDOR) offer much more sensitive tools to study the mode and strength of hydrogen bonds (for a review, see Ref. [52]). ENDOR studies performed on isolated PBRCs, where the high spin non-heme iron is replaced with the paramagnetic Zn, revealed that the hydrogen bonding of Q A and QB  is asymmetric. This feature affects the spin densitiy distribution and the electronic structure of QA . As a consequence, the Em could be shifted owing to stabilization of the Q A form, thus hindering its reduction to the quinol form [53]. *

*

*

*

*

*

*

*

*

*

21.4.2 Oxidative water splitting 21.4.2.1 General Reaction Pattern: the Kok Cycle The general reaction pattern of oxidative water splitting was unravelled by the discovery of the periodfour oscillation of oxygen yield induced by excitation of dark adapted algae and chloroplasts with a train of single turnover flashes [54]. This feature, which is virtually invariant to the number of competent 1

By analogy to PBRCs, the D1/D2/Cyt b559 preparations are often misleadingly designated PS II reaction centres because they neither contain QA nor the non-heme iron and do not permit stable charge separation (see Ref. [25]).

Role of Hydrogen Bonds in Photosynthetic Water Splitting

441

water-oxidizing complexes (WOCs), was interpreted by a four-step sequence of oxidation reactions [55], referred to as the Kok cycle. The individual oxidation steps are energetically driven by P680 þ . Kinetic studies revealed that the reduction of P680 þ is much faster (see Section 21.4.2.2.1) than the oxidation steps of the WOC (see Section 446), i.e. a redox carrier functionally connects P680 with the WOC. This component was identified as the redox active tyrosine (YZ) located in polypeptide D1 [56–58] and later structurally assigned by XRDC studies (see Figure 21.1 and Refs [48] and [59]). It was also found that the original Kok scheme comprising the redox states S0,. . . , S4 has to be extended because the WOC can attain redox levels below S0 (states down to S3 have been identified [60], and even the possible existence of S4 and S5 has been discussed [61]). An extended Kok scheme is presented in Figure 21.3, including the reactions of YD/Yox D with the WOC (see Section 21.4.2.2.4). Inspection of this pattern shows that the overall process of oxidative water splitting comprises two types of reaction: (i) oxidation of YZ by P680 þ and (ii) stepwise abstraction of electrons from the WOC by Yox Z , mostly coupled with proton transfer steps. *

*

*

21.4.2.2 Oxidation of YZ by P680 þ and the Properties of YD *

Although PS II contains two redox active tyrosines (YZ and YD) in an almost symmetrical array to PC–RC (see Figure 21.1), only YZ is sufficiently close to the Mn4OxCa cluster to mediate the rapid turnover of the WOC. Compared with YZ, the tyrosine residue YD exhibits markedly different properties (Section 21.4.2.2.4; for further details, see Refs [62] and [63]). Energetics and Kinetics of YZ Oxidation by P680 þ . The reduction kinetics of P680 þ were found to be multiphasic [64, 65] and to exhibit a striking dependence on both the redox state and the integrity of the wateroxidizing complex (WOC) [65–68]. In PS II complexes with intact oxygen evolution capacity, the multiphasic kinetics of P680 þ reduction by YZ can be satisfactorily described by a pattern that comprises three components: (i) ‘fast’ nanosecond kinetics (20–50 ns); (ii) ‘slow’ nanosecond kinetics (300–600 ns); (iii) ‘35 ms kinetics’, which is an appropriate approximation of components in the ms range (for discussion, see Ref. [69]). In competition with these kinetics, P680 þ also becomes reduced by Q A , with half-lifetimes of 100–200 ms [70, 71], thus giving rise to *

*

*

*

*

Figure 21.3 Extended Kok cycle of oxidative water splitting. Reprinted with kind permission from [1]. Copyright 2008 Springer Science and Business Media (See Plate 24)

442 Hydrogen Bonding and Transfer in the Excited State

misses in the advancement of the Kok cycle. This back reaction does not exhibit a significant kinetic H/D exchange effect [72]. At first glance, the multiphasic kinetics appear to be puzzling because P680 and YZ are bound to the same polypeptide D1 in a structurally well-defined manner (see Figure 21.1). In general, the typical time course, which has been found in all studies thus far performed on different samples from higher plants and thermophilic cyanobacteria with a functionally competent WOC [64, 65, 69, 73–75], can be explained either by a static structural heterogeneity or a sequence of dynamic relaxation processes, thus giving rise to progressive shifts of the equilibrium P680 þ YZ > P680Yox Z towards the right side (for a detailed discussion, see Ref. [69]). A critical survey of the published data favours the model of dynamic relaxations, which includes in its simplest form three different conformational states: (i) initial state I; (ii) first relaxed state R1 due to local dielectric response; (iii) second relaxed state R2, which is attained after rearrangement of a hydrogen bond network. The left-hand panel of Figure 21.4 shows the reaction scheme, and the right-hand panel presents the energetics of this pattern derived from the kinetic data and the total Gibbs free energy gap between states (P680 þ YZ)I and (P680Yox Z )R2 (for further details, see Ref. [69]). Thermodynamic considerations reveal that the oxidation of YZ by P680 þ requires a deprotonation of the OH group of the tyrosine residue YZ [76, 77]. Therefore, questions arise as to the mode of coupling between the proton transfer (PT) and electron transfer (ET) step. As the OH group of YZ is able to form hydrogen bonds, this structural motif is anticipated to be of key mechanistic relevance for the reaction, i.e. details of the reaction should essentially depend on the nature of the hydrogen bond. *

*

*

Hydrogen Bonding of YZ in PS II with Intact WOC. Based on the finding of a small activation energy of about 10 kJ mol1 for the ‘fast’ nanosecond kinetics [68], we first proposed that the OH group of the redox active tyrosine YZ forms a hydrogen bond with an amino acid residue acting as acceptor base X, and that this array permits a rapid PT step within a double well potential concomitant with the electron transfer (ET) to P680 þ [68]. *

Figure 21.4 Reaction scheme (left-hand panel) and energetics (right-hand panel) of P680þ reduction by YZ in PS II complexes with an intact water-oxidizing complex (WOC) in redox state S1. For the sake of simplicity, the panel on the right-hand side presents only DDE values for the different relaxation states because the absolute energy gap  between [YZ1 P680* Pheo QA] and [Yox Z P680 Pheo QA ] is not exactly known. Likewise, energetic relaxations around   the QA site and the energy loss due to partial reoxidation of Q A by QB(QB ) in the microsecond time domain are omitted (for further details, see the text and Ref. [1]). Reprinted with kind permission from [1]. Copyright 2008 Springer Science and Business Media (See Plate 25) *

*

*

*

*

Role of Hydrogen Bonds in Photosynthetic Water Splitting

443

Figure 21.5 Scheme of proton-coupled electron transfer of P680 þ reduction by YZ with His as hydrogen-bonding partner (for details, see the text). Reprinted with kind permission from [1]. Copyright 2008 Springer Science and Business Media *

The idea of a hydrogen-bonded YZ has been confirmed by site-directed mutagenesis studies, and base X was identified as His 190 of polypeptide D1 [78–81]. Structural analyses of PS II support this assignment [48, 59]. With respect to the mechanism, generally two different modes of reactions can be distinguished: (i) concerted proton-coupled electron transfer (PCET) and (ii) sequential steps of PT and ET. Regardless of these two types, the YZ oxidation by P680 þ comprises in any case different sites for electron transfer (ET) and proton transfer (PT), as illustrated in Figure 21.5. This mode of multiple-site electron and proton transfer (MS-PET) is of general relevance for many redox processes, as outlined in Ref. [8]. An analysis of the ‘fast’ nanosecond kinetics within the framework of the Marcus theory [82] of nonadiabatic electron transfer (NET) led to a comparatively small reorganization energy of about 0.5 eV [83] and  an edge-to-edge distance between YZ and P680 of 9  2 A [84], which is in perfect agreement with the XRDC data [48, 59]. This finding clearly shows that the reaction is kinetically limited by the ET step. Accordingly, the PT step either occurs synchronously in a PCET mode reaction or is kinetically faster. Depending on the fine structure of the system, two basically different configurations have to be taken into consideration for the nature of the hydrogen bond: (i) a ‘conventional’ hydrogen bond or (ii) a low-barrier hydrogen bond (LBHB). In the former case, the difference of the pKa values, DpKa, of the OH group of YZ and the nitrogen of His 190 has to be smaller than 4.0 in order to satisfy the requirements of the transition state theory for a proton shift to be faster than 20 ns (for details, see Ref. [85]). Interestingly, the difference between the pKa values of His (pKa 6.0) and Tyr (pKa 10.0) in solution (see Biochemistry textbooks) is about 4.0, so that only a slight relative downshift of DpKa in the D1 protein matrix is required to satisfy this condition (the absolute shifts of Tyr and His can be significantly larger). If the bond distance is sufficiently short, an LBHB configuration will arise that is characterized by specific properties and most likely of high functional relevance in several enzymatic reactions [86–89]. Recent experimental evidence, however, does not support the LBHB concept for a serine-type protease from the chymotrypsin family [90]. The possibility of an LBHB configuration for the YZ–His 190 pair was first discussed in Ref. [72], butwas not favoured in spite of the striking coincidence of the calculated activation energy of about 10 kJ mol1 [76] for an LBHB array with the experimental value for the ‘fast’ nanosecond kinetics of P680 þ reduction by YZ [68]. Later, however, the existence of an LBHB configuration was proposed on the basis of low-temperature EPR measurements [91], and this idea was further extended by the assumption that the nature of the hydrogen bond switches from an LBHB configuration in state YZHis 190 to a normal (weak) hydrogen bond in state YZ His 190Hþ [92]. The formation of a structural LBHB requires proper tuning of the pKa values of the donor (YZ) and acceptor group (His 190) to a small difference DpKa. It is not easy to rationalize that this condition is satisfied in PS II (but see Ref. [92]), and therefore the exact nature of the hydrogen bond remains to be clarified. FTIR (see Refs [52] and [93]), UV-resonance Raman [94] and high-field EPR [95] spectroscopy offer most powerful tools to unravel the nature of hydrogen bonds. However, so far no detailed studies could be performed on PS II samples with an intact WOC because Yox Z (this symbol will be used for the YZ radical with a proton in *

*

*

*

444 Hydrogen Bonding and Transfer in the Excited State

its vicinity without specifying the exact site of the proton) decays in the microsecond–millsecond time domain (see Section 446). Hydrogen Binding of Yz in PS II Lacking an Intact WOC. The lifetime of Yox Z can be drastically increased when PS II preparations are deprived of the WOC by Mn depletion. Several FTIR studies have been reported on hydrogen bonding of YZ [96, 97]. Recently, a detailed analysis was performed on Mn-depleted PS II membrane fragments isolated from mutants, where the DG value of YZ oxidation by P680 þ was varied by modulation of the reduction potential of P680 (DE 80 mV), and also the pKa of YZ was changed through replacement of Tyr with the 3-fluoro-Tyr (DpKa 1.5) [98]. A switch of the reaction mechanism was deduced from these data, i.e. at pH < 7.5 a concerted PCET takes place, while at pH > 7.5 a sequential PT/ET process occurs. It was also speculated that the reaction at pH > 7.5 might reflect the mechanism of intact systems [98]. Unfortunately, all of these data do not really reflect the situation of the intact PS II because the reaction coordinate of P680 þ was found to be drastically changed in samples deprived of the WOC, as reflected by a marked increase in the activation energy (by a factor of about 3, see Refs [99] to [101]) and a significant kinetic H/D isotope effect of 2.7–3.3 [100–103], which is completely absent for the ‘fast’ and ‘slow’ nanosecond kinetics in intact PS II complexes [104]. Figure 21.6 shows a comparison of the Arrhenius plots and the H/D effects of the ‘fast’ nanosecond kinetics of PS II with an intact WOC and the dominating microsecond kinetics in samples deprived of the WOC. An evaluation of these data revealed that the reorganization energy of about 0.5 eV [83, 100] increases by a factor of about 3 and reaches a value of 1.6 eV after destruction of the WOC [84, 100, 105]. The latter value closely resembles the number of 1.4 eV reported for model systems and YZ oxidation by P680 þ in Mn-depleted samples [77]. These findings reflect a significant alteration of the microenvironment of YZ in PS II when the WOC is destroyed. One major change is assumed to be due to the presence of several water molecules in the neighbourhood of YZ [100, 105, 106] in these preparations, whereas the YZ in the native PS II complex is most likely almost ‘dry’ [84, 100, 106]. This modification of the microenvironment rationalizes the largely different reorganization energies in both sample types [107]. In summary, a critical review of the literature clearly shows that sound conclusions on the hydrogen bonding of YZ in vivo cannot be drawn from data gathered on samples with a destroyed WOC. *

*

*

Figure 21.6 Flash-induced 830 nm absorption changes (top panels) and rate constants of the ‘fast’ nanosecond kinetics as a function of reciprocal temperature (bottom panels) in systems with (left-hand side) and without (righthand side) a fully competent WOC (for details, see the text). Reprinted with permission. Copyright Elsevier

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Likewise, existing XRDC data do not provide reliable structural information on the hydrogen bonding of YZ because the WOC was shown to undergo severe damage due to the comparatively high X-ray doses [108, 109], and this effect will necessarily modify the mutual array between YZ and His 190. Therefore. models on the nature of the hydrogen bond between YZ and His 190 are questionable when based on these structural data. A recent analysis of the X-band EPR spectrum of state S2Yox Z suggests that the hydrogen bonding of YZ is stronger and its environment might be more electropositive in PS II complexes with an intact WOC compared with Mn-depleted samples [110]. However, the resolution of the g-values is poor, and no detailed information can be gathered from these data on the hydrogen bonding in native PS II complexes. Regardless of the unresolved exact structure, a protonation of His 190 distorts the hydrogen bond to the OH group of YZ, and the ‘fast’ nanosecond kinetics is expected to disappear. A pKa value of <5.0 was estimated from titration experiments on PS II membrane fragments from spinach [72]. Measurements of the amplitude of the ‘fast’ nanosecond component as a function of pH in PS II core complexes from T. elongatus revealed that the decrease in the acidic region can be described by a single pKa of 4.6 [111]. This effect could also comprise a Ca2þ release (for a discussion of this effect, see Ref. [72]), which is known to modulate the kinetics of reduction [83, 112]. Interestingly, virtually the same pKa value ( 4.7) was found for blockage of the lightinduced split signal [113], which is ascribed to the formation of S1Yox Z at 5 K [91, 113–115]. This finding suggests that a well-defined hydrogen bond between the OH group of YZ and the nitrogen atom of His 190 is the structural prerequisite for ‘fast’ nanosecond kinetics at room temperature and oxidation of YZ by P680 þ at 5 K. *

Hydrogen Bonding of YD. Compared with the limited knowledge on the properties of native YZ, more information is available on hydrogen bonding of YD because it forms a radical that is rather stable in the dark in PS II complexes with an intact WOC [116, 117]. The redox active tyrosine residue YD, which was identified as Y160 of polypeptide D2 [118, 119], is characterized by a strikingly lower reduction potential of about þ 750 mV [120, 121] and does not participate in the normal forward reactions of the Kok cycle. On the other hand, YD reduces S2 and S3 via slow reactions in the time domain of a few seconds, and the oxidized form Yox D can only react with the redox state S0 of the WOC to form S1 with a very slow kinetics of about 10 min at room temperature (see Refs [122] and [123] and references therein). This reaction pattern of YD is a general feature of PS II because it was found in thylakoids from spinach [123] and whole cells of T. elongatus [124] and the Chl d-containing cyanobacterium Acaryochloris marina [125]. As a consequence, the WOC of dark adapted samples populates the redox state S1 with a high probability. The physiological role of this phenomenon is not yet clarified (for a discussion, see Ref. [63]). The nature of the hydrogen bond of Yox D has been analysed by magnetic resonance techniques (X-band, highfield EPR, ENDOR) and FTIR spectroscopy [126–129]. It was shown that YD forms a hydrogen bond with His 189 of polypeptide D2 [130]. Furthermore, ENDOR measurements revealed that three (or fewer)  protons are located within a shell between 4.5 and 8.5 A [131]. Based on an FTIR study, two water molecules were shown to interact with YD most likely through a hydrogen bond network. One of these H2O molecules might even directly interact with YD [132]. Interestingly, no significant differences between intact and Mn-depleted samples have been reported for the hydrogen bonding of YD, in marked contrast to the behaviour of YZ (see Section 21.4.2.2.3). This finding is not surprising because the distance of the WOC to YD is much longer than the distance to YZ (see Figure 21.1 and Refs [48] and [59]). The localization of the proton that is released upon YD oxidation is not yet clarified, but water molecules around YD, giving rise to comparatively high dielectric values, could stabilize the positive charge in the vicinity of YD and also contribute to the surprisingly low midpoint potential (for a discussion, see Ref. [132]). It was proposed that Yox D with the positive charge trapped in its environment might affect the midpoint potential of P680/P680 þ and the spin density distribution of P680 þ [133]. However the replacement of Tyr 160 with Phe had only a marginal effect of 10 mV [134]. *

*

446 Hydrogen Bonding and Transfer in the Excited State

At present the functional role of YD is not yet clarified (for further discussion, see Refs [62], [63] and [133]) and is a topic for future research. 21.4.2.3 Oxidation of the WOC by Yox Z Oxidative water splitting to molecular oxygen is necessarily coupled to the net release of four protons. The paramount role of hydrogen bond dynamics for the mechanism of this process is obvious because water molecules per se are well known as potent species for forming hydrogen bonds. A deeper understanding at the molecular level of the reaction steps in the WOC, which are energetically driven by Yox Z as the direct oxidant, requires detailed information on: (i) the structure of the catalytic site, including the positions of all water molecules in the microenvironment; (ii) the nuclear geometry and electronic configuration of the catalytic site in each redox state Si (see Figure 21.3); (iii) the reaction coordinates of the individual redox steps and their coupling with protolytic reactions; (iv) the pathways for substrate (H2O) uptake and product (O2, Hþ ) release. The catalytic site of the WOC was shown to be a Mn4OxCa cluster (x symbolizes the number of m-oxobridges), and its structure has been described in several recent research reports and review articles [13, 48, 59, 135–139]. Likewise, the kinetics and energetics of oxidative water splitting have been thoroughly discussed [1, 17, 63, 140–142]. Therefore, the present description will mainly address point (iii), with special focus on our fragmentary knowledge concerning the possible existence and function of hydrogen bonds. Redox State Transitions and Coupling to Deprotonation Reactions. An analysis of the kinetic data revealed that, in marked contrast to the behaviour of ‘fast’ nanosecond kinetics of P680 þ reduction by YZ (see Section 21.4.2.2.2), the rate of the redox steps between Yox Z and the WOC is not limited by non-adiabatic electron transfer [1, 17]. Therefore, these reactions were inferred to be triggered most likely by proton shift(s), but conformational gating might also be involved, at least in some steps of the Kok cycle [1, 63]. The Si state transitions in the WOC comprise proton movements coupled with both the reduction of Yox Z and the oxidation of the Mn4OxCa cluster, including the participation of substrate/ligand molecules (see Section 21.4.2.3.3). The Mn4OxCa cluster is coordinated by amino acid residues of polypeptide D1 and Glu 354 of polypeptide CP 43, as illustrated in Figure 21.7. *

Figure 21.7 Structural model where a part of Chl PD1, the redox active TyrZ and the metal ions of the Mn4OxCa  cluster are shown as derived from the 3.0 A structure [48]. The view is approximately along the membrane plane, with the lumenal side at the bottom. The figure was generated by J. Kern using Pymol (Delano, 2003). Reprinted with kind permission from [1]. Copyright 2008 Springer Science and Business Media (See Plate 26)

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Although probably affected by X-ray damage [108, 109], this structure clearly shows that, apart from the substrate water (these molecules cannot be seen at the currently available structural resolution of XRDC data), the Mn4OxCa cluster is surrounded by protonatable amino acid residues with pKa values that are expected to be changed owing to ET transfer processes which are not balanced by protolytic reactions. This effect gives rise to a non-integer proton release pattern, as was first discussed in Ref. [143] and later confirmed by experimental data [144–146]. Recent FTIR measurement showed that the stoichiometry of proton release from T. elongatus PS II core complexes in highly buffered suspensions is (0.8–1.0):(0.2–0.3):(0.9–1.2):(1.5–1.6) for the sequence Yox Z Si ! YZSi þ 14di3 þ di3O2 þ niHlumen, where i ¼ 0, . . ., 3 and di3 ¼ 0 for i 6¼ 3 and 1 for i ¼ 3. The non-integer stoichiometry is ascribed to contributions from Arg, Lys or Tyr in the vicinity of the WOC, whereas carboxylic groups, His and Cys are not involved (or only marginally at most). The ‘intrinsic’ release pattern from water is assumed to be 1:0:1:2 [146]. A reliable mechanism of the individual redox steps of the Kok cycle cannot be presented without knowledge of the detailed structure, in particular of the water molecules and the hydrogen bond pattern. Unfortunately, at present direct information is not available on the array of water molecules (see above), and even the number of these molecules coordinated to the Mn4OxCa cluster and located in the environment is not known (see above). A total of 6–12 water molecules were gathered from indirect lines of evidence [147–149]. The two substrate water molecules must be deprived of four electrons and four protons in order to form molecular dioxygen, but the exact mechanism of PT during the Kok cycle is not yet resolved. Because of this lack of information, any reliable modelling of a detailed mechanism is futile, and therefore only general principles of the mode of coupling between ET and PT steps will be discussed. Two basically different modes of reaction pathways have to be taken into consideration: (i) separate ET and PT pathways and (ii) PCET, including the possibility of a hydrogen atom abstraction chain via bridging water molecules. Figure 21.8 schematically illustrates these two reaction pathways (see Ref. [150]). In the separate ET/PT sequence (left-hand panel of Figure 21.8) the initial shift of the proton to His 190 upon YZ oxidation (see Section 21.4.2.2.2) is followed by transfer into a hydrogen bond network (XHB), where it can be transiently ‘stored’. The subsequent reduction of Yox Z occurs via ET from the Mn4OxCa cluster with its

Figure 21.8 Coupling of ET and PT steps in the reactions of Y ox Z with the redox states Si of the WOC. Left-hand panel: mechanisms for separate pathways of ET and PT in the reduction of Y ox Z and oxidation of the manganese cluster with coordinated substrate water. Right-hand panel: concerted ET and PT (‘H atom abstraction’ type mode.) For further details, see the text. Reprinted with permission from [150]. Copyright Elsevier (See Plate 27)

448 Hydrogen Bonding and Transfer in the Excited State

coordinated substrate water (for the sake of simplicity, the actual protonation state is not specified), while the proton transiently stored in (XHB * H)þ is transferred back to YZ via His 190. The oxidation step in the WOC gives rise to the concomitant shift of a proton, which is either released into the lumen or could be transiently trapped in the microenvironment (see Section 21.4.2.3.3). In the alternative mode of a PCET pathway, both the electron and proton are extracted from the Mn4OxCa cluster, including the substrate water, and transferred to Yox Z , as illustrated in the right-hand panel of Figure 21.8. This type of reaction pathway, originally proposed as an ‘H-atom abstraction model’ by Babcock and coworkers [105, 151], could comprise water molecules as intermediate bridges. In this case the proton ultimately released into the lumen is funnelled via the His 190–(XHB) channel. As the Mn4OxCa cluster probably contains at least two Mn centres which undergo ‘stable’ redox transitions Mn(III) ! Mn(IV) during the Kok cycle ([152], for details, see Section 21.4.2.3.3), it is reasonable to assume that different modes of coupling between ET and PT steps take place. þ It was discussed that the steps Yox Z Si ! YZSi þ 1 þ niH for i ¼ 0 and 1 might occur via pathway (i), while þ ox ox reactions YZ S2 ! YZS3 þ n2H and YZ S3 ! YZS0 þ n3Hþ þ O2 could take place through pathway (ii) [150, 153]. The nature of the bond changes involving protons were addressed by studying kinetic H/D isotope effects (KIEs). Replacement of exchangeable protons by deuterons revealed that the oxidation steps of the WOC are characterized by comparatively small values with ki(H)/ki(D) ratios of 1.3–1.4 for i ¼ 1, . . ., 3 in PS II membrane fragments [22, 84, 100, 154] and slightly higher values of 1.5–2.5 in PS II core complexes from spinach [84, 155]. Interestingly, similar numbers in the range of about 1.4–2.5 have been found in cytochrome c oxidase [156, 157], which catalyses the reverse reaction, i.e. O2 reduction to water [158]. The KIE values exclude a break of O--H or N--H bonds as rate-limiting steps in the WOC and rather reflect interactions between redox intermediates and protons with basic group(s) [142, 150], in line with suggestions for cytochrome c oxidase [156]. These findings show that, in addition to the mode of coupling between ET and PT steps, all mechanistic considerations necessarily need to take into account that water molecules certainly form hydrogen bonds and that this pattern changes during the redox chemistry of the WOC. Therefore, the unravelling of the mode of hydrogen bonding is of crucial relevance for a deeper understanding of the mechanism. Hydrogen Bonding of Water Molecules in the WOC. First sound information on hydrogen bonding of substrate and associated water molecules were obtained by the pioneering work of Noguchi and coworkers using FTIR difference spectroscopy [159–161]. This method permits the detection of vibrations that are changed in their frequency owing to the redox transitions of the WOC, thus reflecting variations in the hydrogen bond strength during the Kok cycle. For technical reasons, OH groups of water molecules with weak hydrogen bonds can be analysed in a more straightforward manner than in the case of strong hydrogen bonds [161, 162]. Marked band shifts and negative bands were observed in the region of OH vibrational modes (3588–3617 cm1 and 3612–3634 cm1 respectively). Furthermore, the region of carboxylate stretching modes (1300–1600 cm1) exhibits complex spectral features owing to the individual Si state transitions [161]. The OH groups of the detected water molecules were inferred to form asymmetric hydrogen bonds, and a model was proposed where in the S0 ! S1 transition the proton is released from an asymmetrically hydrogen-bonded water molecule that is coordinated to manganese in S0 [161]. During the S1 ! S2 transition, the hydrogen bonding of another water molecule that is ligated to a different manganese of the Mn4OxCa cluster is changed without the release of a proton, which is probably transiently trapped in the neighbourhood. Simultaneously, an additional water molecule approaches the binding site [161]. A recent study on a Synechocystis sp. PCC 6803 mutant, where Glu 354 of polypeptide CP 43 was replaced with Gln, resolved a water molecule bound to the

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manganese that is oxidized in the S1 ! S2 transition and ligated by Glu 354 (monodentate ligation in S1 in the form of a carboxylate bridge with another metal centre and bidentate ligation in S2) [163]. þ In the transition Yox Z S2 ! YZS3 þ n2H , one proton is released, and probably a concomitant rearrangement of water molecules near the Mn4OxCa cluster takes place, including the possibility of the population of a peroxidic configuration (see Section 21.4.2.3.3). In the last sequence of steps during the reaction Yox Z S3 ! YZS0 þ n3Hþ þ O2, two protons are released (for further details, see Ref. [161]). Nature of the Redox Transitions and the Functional Role of Hydrogen Bond Shifts for the WOC. A consensus exists that redox transitions S0 ! S1 and S1 ! S2 are metal-centred reactions giving rise to Mn (III) ! Mn(IV) oxidation steps at two different manganese centres and no redox change of the substrate molecules takes place (for reviews, see Refs [136], [139], [142], [150] and [152]). Alternatively, the oxidation of only a single manganese was also proposed, i.e. Mn(II) ! Mn(III) and Mn(III) ! Mn(IV) [164]. þ þ ox On the other hand, the nature of the transitions Yox Z S2 ! YZS3 þ n2H and YZ S3 ! YZS0 þ n3H þ O2 þ ox is a matter of controversy. The redox step YZ S2 ! YZS3 þ n2H was earlier proposed to include the formation of a binuclearly complexed peroxide [165]. This idea was further elaborated by the postulate that S3 is a multistate redox level of the WOC that comprises redox isomerism and proton tautomerism equilibria. The peroxidic configuration acts as an ‘entatic state’, which is oxidized by Yox Z , thus leading to the subsequent formation of molecular dioxygen (for reviews, see Refs [1], [17] and [63]). Currently, two alternative and contrasting points of view for the S2 ! S3 are favoured by the majority of research groups: (a) a metal-centred reaction [141, 153, 166] and (b) a ligand-centred reaction leading to the formation of an oxo radical [136, 139, 142]. These models generally assume that the essential O--O bond formation can only take place in redox state S4 (for reviews, see Refs [1] and [139] to [142] and references therein). Figure 21.9 illustrates the tautomerism equilibrium of the ‘multiple S3 state’ model, which implies that proton shifts in S3 give rise to a rapid switch between at least two states (for the sake of simplicity, the redox isomerism equilibrium is omitted, see Ref. [17]): (i) coordinated water interacting with a manganyl-oxo group and (ii) hydrogen peroxide. The two states, symbolized by S3(W) and S3(P) respectively, form hydrogen bonds with the protein environment, as schematically illustrated in Figure 21.9. Accordingly, a controlled shift of these hydrogen bonds is postulated to be essential for the formation of the O--O bond which is the mechanistic cornerstone of the whole process.

Figure 21.9 Schematic representation of proposed oxywater–hydrogen tautomerism of the multistate redox level S3 of the WOC. M symbolizes a not yet assignable metal centre (either Mn or Ca). Dynamic changes of the protein matrix coupled with the transitions between different forms of the S3 equilibria are symbolized by differently shaped grey areas. Reprinted with permission from [17]. Copyright Elsevier (See Plate 28)

450 Hydrogen Bonding and Transfer in the Excited State

This idea consequently ascribes a catalytic role to the protein matrix of the WOC, which acts as a ‘director’ for a regulated proton shift towards the side of the hydrogen peroxide configuration. Therefore the WOC is considered to be a specially tailored ‘molecular machine’ with a functional mechanism that cannot be described within the framework of the classical ‘cofactor–apoprotein’ concept (for discussion, see also Refs [17] and [150]). With respect to the role of protons, three mechanistic assumptions are introduced: (a) a local proton gradient in the neighbourhood of the two substrate oxygen atoms rH þ ð~ r; t; S3 Þ supports their linkage in forming the key O--O bond; (b) the dependence on the space vector~ r determines the population probability of the different S3 states; (c) protein dynamics modulate the equilibration kinetics through time-dependent changes of the hydrogen bond network (see also Refs [1] and [63]). Indirect lines of experimental evidence illustrate the role of protein dynamics for the turnover of the WOC because redox steps S2 ! S3 and S3 ! (S4) ! S0 þ O2 þ nsHþ are ‘frozen’ below threshold temperatures of 10 to 20  C in thermophilic cyanobacteria [167] and about 50  C in PS II membrane fragments and PS II core complexes from spinach [122, 168]. Likewise, these transitions are blocked upon dehydration [169]. Proton Release from the WOC via Hydrogen Bonds. Regardless of the detailed coupling mechanism of ET and PT steps (see Section 21.4.2.3.1), the products of oxidative water splitting are ultimately released into the aqueous bulk phase of the luminal space (Hþ ) and the aquatic environment in equilibrium with the gas atmosphere (O2). Hydrogen bonds are of minor relevance for transport of hydrophobic O2 molecules, which is assumed to occur through channels in the protein matrix [137, 170–173], but are essential constituents of exit pathway(s) for protons. The chemical protons released from the WOC are probably funnelled via Asp 61 [141, 159] and Glu 65 [137] of polypeptide D1 into the extrinsic PsbO protein, which covers up the WOC together with other extrinsic subunits (for a review, see Ref. [174]). Carboxylic groups of Glu and Asp residues of PsbO were assumed to form the exit pathway into the lumen [175, 176]. This idea is supported by theoretical calculations [177].  A recent analysis of 2.9 A resolution XRDC structural data [178] led to the characterization of nine possible channels that connect the Mn4OxCa cluster with the aqueous bulk phases for substrate/ product transfer: (i) five narrow channels are potential candidates for facilitating proton transport via a Grotthuss-type mechanism, including water molecules, and (ii) four wider channels could mediate substrate water and oxygen product funnelling. Lipids might be essential constituents of the oxygen channels (for details, see Ref. [178]). 21.4.3 Plastoquinol formation The reduction of plastoquinone to plastoquinol takes place in a special binding pocket: the QB site, which is shown to be formed exclusively by polypeptide D1 [13]. The reaction pattern closely resembles that of ubiquinol formation in anoxygenic purple bacteria. It has been described in several review articles and book chapters [1, 13, 179–181], and therefore only a brief discussion will be presented, with a focus on the formation and function of hydrogen bonds. 21.4.3.1 Reaction Pattern of Plastoquinol Formation The formation of PQH2 occurs via a sequence of two one-electron steps, with Q A acting as reductant, which is formed as a result of light-induced charge separation (see Section 21.4.1.2.4 and Figure 21.1). Figure 21.10 summarizes the reaction sequence. For the sake of clarity, the donor side reactions are omitted, and the formation of Q A is symbolized by an arrow labelled with hn to indicate the driving force for this reaction; QB, *

*

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Figure 21.10 Simplified reaction scheme of PQ reduction to PQH2 at the QB site by the one-electron reductant Q A that is formed as a result of light-induced charge separation. The square marked in yellow symbolizes an empty QB site. For the sake of clarity, protolytic steps are explicitly shown only for the PQ-9 molecule in its different redox states, i.e. protonation/deprotonation reactions of amino acid residues are omitted. For further details, see the text. Reprinted with kind permission from [1]. Copyright 2008 Springer Science and Business Media *

 Q B , QBH and QBH2 represent the PQ-9 molecule in different redox and protonation states bound to the QB site. In addition to the binding site of QA in polypeptide D2 and the QB site in polypeptide D1, a third binding site of PQ was predicted to exist in the neighbourhood of cytochrome b559 and is symbolized by QC [182]. The existence of the QC site is now confirmed by XRDC data and discussed to be possibly involved in the PQ/PQH2 exchange mechanism (for further details, see Ref. [137]). Inspection of the molecular structure of substrates (PQ and 2Hþ ), intermediates and product (PQH2) readily shows that these species are potent candidates for the formation of hydrogen bonds (see Section 21.4.1.2.4), and therefore the mode of these bonds is expected to be of high functional relevance. *

21.4.3.2 Hydrogen Bonds and Plastoquinol Formation The formation of PQH2 comprises the uptake of two protons at different oxygen atoms of the quinone head group upon its sequential reduction by two electrons from Q A . Accordingly, it seems likely that different hydrogen bonds are involved in this process. Recent progress in resolution of XRDC analysis of PS II core complexes from Thermosynechococcus (T.) elongatus provides a deeper insight into the structure of the Q B site [48, 137]. These data show that the head group of the substrate molecule PQ-9 is connected via hydrogen bonds to amino acid residues Ser 264 and His 215 and the backbone amide of Phe 265 (for further details, see Refs [13] and [137]). The pathways of the protolytic reaction of ubiquinol formation in anoxygenic purple bacteria comprise carboxylic amino acid residues (Asp, Glu) and connecting water molecules [183, 184]. A new theoretical study was performed on the basis of quantum mechanical/molecular mechanics calculations on Rb. sphaeroides RCs. The results obtained suggest that the first proton located at Glu 212 of subunit L is directly transferred to   Q B just after the reduction of QB by QA . The transfer of the second proton to the other oxygen atom of the  QBH head group occurs from Asp 210 of subunit L and comprises a long-range hydrogen bond network that is occasionally formed with about three water molecules and acts as a proton channel [185]. Although the overall reaction scheme is similar in PBRCs and PS II, the pathways of protonation are markedly different because FTIR studies in PS II core complexes of T. elongates have shown that carboxylic amino acid residues are not involved in the protonation steps leading to plastoquinol formation [186]. This phenomenon is probably *

*

*

*

452 Hydrogen Bonding and Transfer in the Excited State

related to the different types of axial coordination of the non-heme iron located between QA and QB (see Figure 21.1). The Glu ligand in PBRCs is replaced by carbonate in PS II (see Ref. [187] and references therein). This mode of ligation appears to be responsible for the marked ‘bicarbonate effect’ on the electron transport at the PS II acceptor side (for a review, see Ref. [188]). At present, detailed information is not available on the proton pathways to the QB site. The role of the protein dynamics for the reoxidation of Q A by QB has been studied using different methods. These results are described in recent reviews [1, 189] and will not be discussed here. *

21.5 Concluding Remarks and Future Perspectives This contribution presents an attempt to summarize our current knowledge of the role of hydrogen bonds for the process of light-driven water splitting in photosynthesis. Hydrogen bond networks are shown not only to be structural determinants of the apparatus but, even more importantly, to have essential functions. In this respect, water molecules are of special relevance because they act as both (i) the substrate for molecular dioxygen formation and (ii) indispensable constituents of functionally relevant hydrogen bond networks and ‘plasticizers’ for tuning the flexibility of the protein matrix and its dynamics (see Ref. [189] and references therein). At present, only limited information is available on the details of the hydrogen bonding patterns, in particular of the WOC. During the last decade, enormous progress has been made in our knowledge of the structure and the reaction mechanism of photosynthetic water splitting, but some fundamental problems still remain challenging tasks of future research. Among these, the unravelling of hydrogen bond networks and their functional role for tuning of the reactions in the WOC are of special relevance. Site-directed mutagenesis, isotope labelling and further advanced vibrational spectroscopy in combination with new computational methods (quantum chemistry, molecular mechanics and molecular dynamics) will pave the way for a deeper understanding of the most important topic of the role of hydrogen bond networks and their dynamics for water splitting by solar radiation. It is the author’s hope that this contribution will entice young scientists to enter this fascinating field of research in life sciences.

Acknowledgements I thank T. Noguchi for critical reading of the manuscript, R. Jordan (Figure 21.8), J. Kern (Figures 21.1 and 21.7), P. K€ uhn (Figures 21.3 to 21.7), T. Renger (Figure 21.10), R. Steffen (Figure 21.9) and C. Theiss (Scheme 21.1 and Figure 21.2) for preparing electronic versions of the figures, and S. Nothing for competent typing of the manuscript. Financial support by Deutsche Forschungsgemeinschaft (Sfb 429 TP A1) is gratefully acknowledged.

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22 Proton Transfer Reactions in the Excited Electronic State Vladimir I. Tomin Institute of Physics, Pomeranian University, 76-200, Słupsk, Poland

22.1 Introduction Proton transfer reactions undoubtedly belong to the most important transformations in chemistry that contribute to fundamentals of this science. Proton transfers (PT) refer to acid–base reactions and include such reactions as dissociation of molecular acids and bases in water and non-aqueous solvents, selfdissociation of amphoteric solvents, neutralization, hydrolysis of salts, reactions of Lewis acids and acid–base catalysis (isomerization of olefins, keto–enol tautomerism) [1]. They were studied initially for chemical compounds that exist in stable states or occupy the ground energy levels. Later, owing to the development of experimental techniques and the emergence of a new field of chemistry – photochemistry, these interesting transformations were discovered and studied in the electronic excited state of some classes of molecules. The acid and base definition known as the Brønsted–Lowry definition [2] is based on the PT process. This definition has far-reaching consequences in the understanding of a wide range of phenomena and in the stimulation of chemical engineering. The definition is as follows: an acid is a species having a tendency to lose a proton, and a base is a species having a tendency to gain a proton: A , B þ Hþ where A and B are a conjugate acid–base pair. In such a pair, A must have one more positive charge (or one less negative charge) than B, but there is no other restriction on the sign or magnitude of the charges. Such reactions do not actually occur in any solution processes – they are only a chain of more complex chemical transformations. This is because Hþ , the bare proton, has a strong tendency to add to almost all chemical species and cannot exist in any detectable concentrations except in vacuum. Apart from any specific

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464 Hydrogen Bonding and Transfer in the Excited State

chemical interaction, the very small size of the proton (about 1015 m) means that it generates a highintensity electric field that will polarize and therefore attract any particles (molecules or ions) in the nearest environment. Excited-state proton transfer (ESPT) reactions belong to one of the main kinds of reaction in photochemistry and refer to relatively new classes of reactions first observed in 1952 by Weller, who discovered dual fluorescence of naphtholes [3]. Weller observed that some naphthols having one form in the ground state emit two broad bands of fluorescence. The short-wavelength band in the UV was assigned to the initially excited neutral molecules, and the longer blue band to the anions of the same molecules which were formed in the excited singlet state by deprotonation. This finding provided experimental proof of the fact that acid–base properties of photoexcited molecules can be different from those in the ground state, and thus that proton transfer can occur just after transition of molecules to the excited state. Since that time, ESPT reactions have been studied extensively, generally for cases where the molecule and its product have measured fluorescence, which enables the photoreaction to be monitored and the main characteristics to be obtained. The second important fact that facilitates study of ESPT reactions is that they can be triggered easily by photoexcitation, and spectra and other parameters describing the dye and photoproduct, as well as their mutual transformations in time, can be determined by methods of steady-state and kinetic spectroscopy. At present the understanding of ESPT is a fundamental part of photochemistry, some fields of nanotechnologies and units of bio- and chemical sensing as well. ESPT reactions can be divided into two large groups – intermolecular and intramolecular, often abbreviated in the literature as ESIPT. The first group includes well-known reactions such as deprotonation with the creation of anions in solutions (naphtholes, methylumbelliferons and some others). The field of the study of intermolecular proton transfer reactions in excited electronic states (ESPT) was initiated by F€ orster in Ref. [4]. He came to this conclusion in explaining the results of Weber [5], who reported that the shift of an acid–base equilibrium of some organic compounds is different in absorption and fluorescence. Such changes may be described in terms of equilibrium constants in proper electronic states. For determination of the equilibrium constants in the excited states, Forster proposed [6] the method of shift of chemical equilibrium upon excitation, the so-called Forster cycle. The method is based on the experimental determination of pure electronic transitions in absorption and fluorescence, which are assumed to belong to the normal form and the anion of molecule. This cycle combines thermodynamic and spectroscopic data and enables chemical equilibrium constants in the excited electronic state to be calculated. Assuming that the entropy changes are the same in the two states, the shift of chemical equilibrium upon excitation can be expressed as DpKa ¼ pK*a pKa  0:0021ð~nA nBoo Þ oo  ~ where pK*a and pKa denote the values of dissociation constants in the excited and ground state respectively, and ~nA nBoo are the wave numbers of the pure electronic transitions of the initial and deprotonated forms oo and ~ respectively. The cycle is applicable to the simplest reversible reaction of deprotonation for some species BHþ in the ground state and excited states: BH þ , B þ H þ ðBH þ Þ*  B* þ H þ A large decrease in pK*a in the lowest excited singlet state, about seven units, is observed, for instance, for solutions of both 1- and 2-naphtholes [7], which are the archetypal molecules revealing deprotonation in the

Proton Transfer Reactions in the Excited Electronic State 465

excited state. Many authors noted strong changes in pKa when molecules occur in the excited state [8]. The success of the method can best be appreciated by comparing the literature pK*a value with the values presently available from direct measurements of protonation and deprotonation rates. Such comparative analysis has been fulfilled in the review of Arnaut and Formosinho [9], where data on almost 40 different molecules were collected. PT quite often is treated as one of the simplest reactions. Seemingly, one of the simplest cases is when we have intramolecular or intrinsic reaction that takes place between proximate groups of the same molecule, and the latter has a rigid skeleton. However, in this case the PT is usually an extremely fast reaction occurring on a timescale of a few dozen femtoseconds, which is comparable with periods of vibration of some groups of atoms [10]. Measurements of such short times are quite often limited by the apparatus capabilities of the experimental set-up. Even in the case of an intrinsic reaction, a few mechanisms determining the PT are debatable; among these are the role of the preformed hydrogen bond, the influence of inner vibrations and proton tunnelling. It is not so simple to reveal the contribution of individual effects in light of the very fast PT process. When a molecule has a non-rigid skeleton enabling a molecule to be involved in conformational rearrangements, the reaction rates will be controlled by conformational relaxation as well. Conformational aspects of intra- and intermolecular excited-state proton transfer were reviewed by Waluk [11]. Obviously, the situation is more complex for intermolecular excited-state proton transfer reactions, as the reaction partners must encounter each other prior to the proton transfer step, and also diffusion is involved in the process. The acid–base properties of various molecules in electronically excited states, as well as multiple examples of intramolecular and intermolecular photoinduced proton transfer reactions, have been the subject of numerous papers and reviews (see Refs 7 and 9–26 and references therein). Among these we may recommend two comprehensive reviews on excited-state proton transfer reactions by Arnaut and Formosinho which address the most important fundamental aspects and the theoretical models of intermolecular [9] and intramolecular reactions [10]. Many papers report the study of dual fluorescence and ESIPT reactions in a number of organic molecules, including methyl salicylate, tropolone, 9-hyfroxyphenalenone, anthraquinones, salicylaldehyde, salicylidenaniline, methyl-2-hydroxy-6-methylnicotinate, 2-(20 -hydroxyphenyl)benzothiazole (see Ref. 10 and references therein). Here we would like to concentrate on some new results of proton transfer study in 3-hydroxyflavone (3HF) derivatives, which are treated now as perspective molecular multiparametric probes for many physicochemical and life science applications [26–29]. Recently, a new group molecules with dual fluorescence due to ESIPT – 3-hydroxyquinolones, which are structural analogues of 3HFs – were synthesized and studied [30, 31]. These dyes, in comparison with 3HF dyes, exhibit a higher fluorescence yield and increased photostability, which makes them promising for multiparametric probing and other applications. For some aromatic molecules and complexes in the excited state, simultaneous transfer of two protons can take place. Such a reaction, called excited-state double-proton transfer (ESPDT), was observed and studied, for example, in dimers and complexes of 7-azaindoles and related compounds [11] in solutions and 2-aminopiridine/acetic acid systems [32]. PT reactions feature a variety of applications. A number of them explore a very strong (and probably the largest possible!) Stokes shift, by 9000 cm1, in low-polarity media. This allows them to be excited in the UV, and to obtain visible green (T ) or orange emission even on incorporation into solid-state (e.g. polyethylene [33]) matrices. Intermolecular reactions have been applied in plastic high-energy scintillators [34–36], in chemical and dye lasers [37–40], in energy storage systems and information storage at a molecular level [41], in high-energy radiation detectors [42], in polymer stabilizers [43, 44], to enlarge the sensitive spectral window of silicon photodiodes by converting near UV into visible light [45] and as a fluorescent solar energy concentrator [46]. Proton transfer reaction–mass spectroscopy is a very sensitive method for express monitoring of organic compounds [47], the use of which has now become widespread.

466 Hydrogen Bonding and Transfer in the Excited State

An extremely important field is the study and application of PT reactions in molecular biology. Proton transfer at the surfaces of biological systems is one of the most prevalent reactions in the biosphere – it is used in the aerobic generation of ATP and oxygen [48]. Proton transfer into and out of proteins is important both for many enzyme reaction mechanisms and for proton pumping across membranes [49]. C  H bonds of biological molecules should be stable in water, a property necessary for the viability of living systems. The heterolytic cleavage of stable CH bonds occurs through proton transfer, which is the first step of many enzyme-catalysed processes [50, 51]. Among the many PT reactions, multi-PT reactions in hydrogen-bonded networks are of particular interest. Typical examples are proton-pump relay in biosystems [52] and proton relay in water [53]. Single and double PT reactions are a good starting point for understanding multi-PT reactions. In this chapter we will review original results of our study of ESIPT in 3HFs, obtained at selective excitation of fluorescence in the range of different singlet bands of absorption, at different temperatures and applying the method of dynamic quenching of excited states. We will describe the study of recently discovered ESIPT reactions from higher excited states and also methods, based on the ratiometric approach, for evaluation of the process rates using spectra of fluorescence and excitation of fluorescence. General spectral properties and characteristics of all forms of 3HF and the formalism for ESIPT reaction description are presented in Section 22.2. Then, in Section 22.3, a theory describing effects of dynamic quenching within the framework of a two-state excited-state reaction model that predicts interesting, non-trivial behaviour of two bands in steadystate spectra is given. In line with predictions of the theory, experiments demonstrate that quenching greatly changes the distribution of intensities between bands of dual fluorescence for the kinetic type of reaction (3hydroxyflavone) but does not change it for the novel compounds (FA, F – see formulae in Figure 22.9 in Section 22.3), where ESIPT is under thermodynamic control. These findings suggest that the quenching of fluorescence by an efficient collisional quencher can be a simple and convenient method using only the steadystate regime of registration for distinguishing the excited-state reactions occurring under thermodynamic or under kinetic controls. The developed theoretical description is not limited to ESIPT; it can be useful in analysis of quenching effects on other photochemical reactions that can be described by two-state excited-state reaction formalism. The effects of high-lying excited states and dynamic quenching on the fluorescence intensity of both forms of ESIPT dye and on the peculiarities of the definition of Stern–Volmer constants of solutes and their photoproducts are considered in Section 22.4. For products arising as a result of photoreaction in the excited state, these constants could not be correctly obtained from characteristics of product emission at different quencher contents; new methods for determining collisional characteristics of dyes in solution, which differ from methods commonly used in the case of single-band fluorescence, are substantiated. Generally, upon excitation of molecular objects via high-lying singlet states, the yield of photoreaction products shown can be increased in a number of cases. Then, if fluorescence of fluorophores and their photoproducts is detectable, the probabilities of reactions via high-lying singlet states can be determined by means of fluorescence measurements. Experimental data are presented that demonstrate the role played by high-lying singlet states at various temperatures and dynamic quenchings of fluorescence in solutions of 3-hydroxyflavone. The treatment of experimental results enables us to obtain valuable information concerning the change in the proton transfer rates with temperature, with the addition of a quencher and with a change in energy of excitation quanta. The ESIPT reaction from the S2 state of 3HF, which was found recently, is discussed in detail in Section 22.5. Results of the experiments obtained by means of steady-state and picosecond laser spectroscopy methods are presented. Spectra of the dual fluorescence of 3-hydroxyflavone in different solvents were obtained with picosecond time resolution when excited within the S1 and S2 absorption bands by wavelength-tunable laser pulses. Spectra dynamics reveals time development of the internal proton transfer in the excited state of the molecule from the hydroxyl to the carbonyl group of the solute. The relative contribution of the tautomer form

Proton Transfer Reactions in the Excited Electronic State 467

to integral emission remains essentially higher during the entire interval of emission observation. The obtained data directly evidence an additional channel of internal proton transfer from the S2 singlet state of 3-hydroxyflavone with a high kinetic rate of 0.2–0.3  1012 s1. Some new applications of ESIPT reactions from the Sn states are discussed.

22.2 ESIPT in 3-Hydroxyflavones and Some Related Compounds 22.2.1 Dual fluorescence of 3HF Proton transfer may occur rapidly after excitation to form a tautomer when either acidic or basic moieties of the same molecule become stronger acids or bases in the excited state. The majority of reactions of this type involve the transfer of a proton from an oxygen donor to an oxygen or nitrogen acceptor, although a few cases are known where an N atom can function as a donor and a C atom as an acceptor. Usually, an intramolecular hydrogen bond between the two moieties of the molecule facilitates the PT. In 1979, Sengupta and Kasha [54] discovered and studied one of the most interesting, attractive and until now widely studied ESIPT reaction that occurs in 3-hydroxyflavone and results in dual fluorescence with emission in the violet and green of the visible spectrum. The chemical structure of the 3-hydroxyflavone family is shown in Figure 22.1. We have the structure of the parent 3HF molecule when fragments R1, R2 and R3 are hydrogen atoms. Nowadays, ESIPT is often considered as a prototype of proton transfer (or, more precisely, hydrogen atom transfer) reactions, which are basic reactions in chemistry and biochemistry. A variety of organic fluorophores exhibiting ESIPT have been described in the literature [7, 9–26], but 3-hydroxyflavones are unique in many respects. In contrast to ESIPT in systems with symmetric proton transfer (e.g. 9hydroxyphenalenone and tropolone [10, 28, 55, 56]), in 3HFs this reaction occurs between structurally and energetically asymmetric states and therefore results in dual emission, so that the two emission bands are observed and are strongly separated on a wavelength scale. It is very important that in 3HFs the reaction is not accompanied with changes in the dye conformation in the ground or excited state, which causes a reorganization of hydrogen bonding. The two effective groups 3-OH (proton donor) and 4-carbonyl (proton acceptor) are attached to a rigid skeleton and are permanently connected by a hydrogen bond, which makes possible the fastest pathway of the ESIPT reaction along this bond (see Figure 22.1). Being isomeric to the initially excited normal (N) form, the tautomer (T) form is well fluorescing, with the emission spectrum strongly shifted to the red, up to 5000–6000 cm1 [10, 25–29], with respect to the fluorescence of the N form having a band with a maximum near 390–400 nm. In addition to the intramolecular H-bond, the 4-carbonyl group can form an additional intermolecular bond with the proton donor group of the solvent, and then carbonyl becomes less acidic and the ESIPT reaction is less favourable energetically. If the intramolecular H-bond is disrupted (when 3-OH and 4-carbonyl create

R2 R1

O

O

R3

O

H

Figure 22.1 The dyes of the 3-hydroxychromone family exhibiting the ESIPT reaction – 3-hydroxyflavones. For parent 3HF, R1, R2 and R3 are hydrogens. For dye F, R2 is dimethylamino and R1 and R3 are hydrogens. For dye F2, R1 is hydrogen, R2 is diethylamino and R3 is dimethyloctyl ammonium bromide

468 Hydrogen Bonding and Transfer in the Excited State

intermolecular bonds with solvent molecules), the ESIPT reaction should be affected strongly [10, 28, 57–60]. It was observed in Refs [61] to [63] that the N band of 3HF, which is not seen in an apolar solvent at room temperature, becomes dominant in cryogenic conditions. On incorporation of H-bond proton donor molecules into a Spol’skii matrix with 3HF, a retardation of ESIPTwas observed [64]. With account taken of the results of steady-state and time-resolved studies in protic solvents, a number of 3HF–solvent configurations have been suggested, some allowing ESIPT through a solvent linker [25, 61, 65–68]. In such solvents, solvates or molecular clusters with different structures are formed, and a slower proton transfer occurs, which enhances the violet fluorescence. Kasha [25] distinguished four different classes of reactions: (i) those in which there is an H bond between the H atom of the donor group and the acceptor (intrinsic intramolecular transfer); (ii) those in which hydrogen is located far away from the acceptor and requires a mediator for the PT (concerted biprotonic transfer); (iii) static and dynamic catalysis of the PT; (iv) proton relay transfers. In fact, only the intrinsic process is a direct ESIPT reaction taking place between proximate groups in the 3HF molecule; all the others are more complex than one-step processes, and molecules of solvent within microsolvent clusters are involved in forming the tautomer. Distinction between these PT cases is extremely important and useful for treating their behaviour, but requires special study. In general, ESIPT reaction kinetics reveals itself in the rise and decay of the tautomer fluorescence. In most cases the fluorescence of the initial forms does not appear at all or is very weak. The dynamics of the tautomer arising covers a large time domain, from the femtosecond to the nanosecond range, although the majority of the relevant processes occur in the picosecond domain. The transfer of a proton having a positive charge between groups of a molecule causes changes in molecular geometry and large electronic and structural rearrangements, which are accompanied with significant changes in the dipole electric moments, the latter being the main reason for large Stokes shifts of fluorescence (5000–6000 cm1) in polar solvents. As a consequence, the dynamics of such processes can be essentially affected by the nature of the solvent, namely with respect to the formation of H-bonds and dielectric properties. The photochemical stability of molecules indicates that there is a reverse isomerization in the ground state after decay of the excited tautomer, so that the entire excitation–emission process has a cyclic character. The emission not only of the T band but also of the N band is very strongly red-shifted by about 4200–4800 cm1 in comparison with the main absorption band with a maximum at 340 nm. This suggests some intramolecular relaxation events after initial excitation to Franck–Condon N*F--C state prior to emission [10, 28], probably owing to an intramolecular charge transfer (ICT) reaction leading to a wellstabilized N state associated with a redistribution of electronic density. Such changes in electronic structure and the energies of electronic states, as well as electronic transitions between these states, are relatively well described by ab initio and also by simpler semi-empirical quantum chemistry calculations [10, 28, 69]. For 3HF the quantum yield h values in aprotic nucleophylic solvents, such as ethyl acetate and acetonitrile, are quite low (h  3–7%), as opposed to 20–30% in low-polarity solvents such as toluene and chloroform [70, 71]. This may be explained by the formation of an intermolecular H-bond with the chromone 3-OH group, which favors the out-of-plane conformation of the phenyl ring [10, 70]. Unfortunately, quantum yields h cannot be predicted so easily. Since the classical works of Kasha [25, 54, 61], 3-hydroxyflavones and their derivatives have become very popular for studies of ESIPTand applications of these molecules for probing the physicochemical properties of different sites of incorporation [10, 26–29]. The simpler structure of 3HF without a phenyl ring in position 2 is called 3-hydroxychromone, which has no ability to emit fluorescence quanta. If the 3-OH group is substituted, for example by methylation, as was done in Ref. [72], we will have a 3-methoxyflaovone compound with the same spectroscopic properties of the N band but with the complete absence of the T band, which means that the ESIPT donor has been liquidated and the new molecule cannot appear in the ESIPT state. A number of attempts have been described at substituting the phenyl ring with other groups in order to improve the fluorescence

Proton Transfer Reactions in the Excited Electronic State 469

properties and construct molecular probes with better solvatochromic features and sensitivity to variations in the physicochemical characteristics of their incorporation sites. Here, it is necessary to mention the results in this field of Demchenko and coauthors, reviewed in Refs [26] to [29]. Among the achievements in spectroscopic properties of these compounds are increases in the quantum yield, shifts of bands of emission and absorption to longer wavelengths and higher sensitivity of the ratio of intensities of the N and T bands to a change in environment characteristics. Most interesting are modifications of 3HFs with the addition of electron-donor substitutes, such as dialkylamino groups. They have increased electric dipole moment differences in the ground and excited states of N forms up to 7 D, as opposed to 2 D for 3HF, which results in stronger solvatochromic effects of this form. A changed electronic structure of these compounds also results in a stronger influence of the environment on proton transfer rates, and hence a greater effect of the environment on the ratio of intensities of the N and T bands. 22.2.2 S2 fluorescence The well-known rule stating that ‘emission always occurs from the lowest electronically excited state of a given multiplicity’ (the Kasha rule) and the rule stating the independence of the fluorescence spectrum of complex molecules on a wavelength of excitation (Vavilov’s rule) may [73–75] be exceptions for some aromatic molecules, 3HF among them. The physical background of these rules is directly associated with high probabilities of excitation energy relaxation before emission. The S2 ! S1 relaxation that follows very rapidly after excitation to the S2 state for some reason may be as slow as the emissive S1 ! S0 conversion. Recent publications on 3HF behaviour in supercritical CO2 demonstrate this possibility [76]. Observations of anti-Kasha fluorescence are reported for different dyes. For instance, for diphenylpolyenes, rhodamines, polymethyne dyes and some others, emission from S2 can be detected [75, 77, 78]. The fluorescence from the second excited state S2 of 3HF has been observed only in a supercritical (sc) carbon dioxide environment. The registered absorption spectrum consists of bands with maxima at 333 and 300 nm. These bands, corresponding to the S1 and S2 bands respectively, are also observed in gas phase and in liquid solvents. Excitation at 333 nm in sc-CO2 leads to two emission bands, one structured band in the 370–450 nm region (the violet band of the normal form) and one broad band peaking at 520 nm (the tautomer emission). Excitation at 291 nm leads to the appearance of a new broad emission spectrum centred at 278 nm (36 000 cm1) and a quantum yield of this band that is even higher than that of the violet band. The lifetime of this emission is less than 200 ps (time resolution of the experimental set-up). A similar band is not observed in solutions, and the authors proved that this new band cannot be assigned to any impurities, as there is no such emission in neat sc-CO2. The authors suppose that S2 fluorescence comes from the 3HF in a solvent cluster where the intramolecular H-bond required for the PT is hindered. The inertness of sc-CO2 contributes to the enhancement of the S2 fluorescence yield. We think that S2 fluorescence can probably also be observed in some other matrices if to provide for ESIPT solute conditions to interrupt an H-bond between active proximate groups. 22.2.3 Ground-State anionic form If the 3HF environment possesses strong proton-acceptor properties, the 3-OH group can dissociate, yielding the ground-state anionic (proton-dissociated) form. This form is easy excited at the long-wavelength slope of the S1 absorption band, in the region of 410–420 nm. Figure 22.2 shows a scheme of energy levels that describes the fluorescence of all three known forms of 3HF and the PT reaction. This scheme reflects the existence of six energy levels: the ground N and excited N levels of the normal form (N), the excited T and ground T levels of the PT or tautomeric form (T), and the ground A and excited A levels of the anionic form (A). The A form can be excited to an excited-state dissociated form (A ), which does not transform to T form

470

Hydrogen Bonding and Transfer in the Excited State

N*

A*

T*

νΑ

ν ex

νΤ

νΝ

A N

T

Figure 22.2 Scheme of energy levels of the normal (N) and anionic (A) form and the tautomer (T) of the 3HF molecule. Reprinted with kind permission from [81]. Copyright 2007 Springer Science and Business Media

and behaves as a distinct species. 3HF anions can be detected in absorption/excitation and emission spectra respectively [10, 28]. Its luminescence has been studied and analysed in most detail for 3HF [79, 80] in methanol and in a mixture of methanol with acetonitrile. In addition, this form was observed in 2-propanol, 1-butanol and ethanol, but was absent in the methoxy derivative of 3HF in the same alcohols [79]. This form was not observed in water. In Refs [81] and [82] the A form was studied in acetonitrile and a mixture of acetonitrile with water. Figure 22.3 shows the absorption and fluorescence spectra of 3HF in acetonitrile, obtained upon UV irradiation at the wavelength lex ¼ 280 nm, and the spectrum of excitation of the violet fluorescence. One can see the fluorescence bands of the N and T forms at 395 and 525 nm respectively, and a rather intense band near 470 nm. Hence, we observe a multiband fluorescence of the solution. The third band between the blue and green spectra belongs to the fluorescence of the A form. The parameters of this band are well determined by approximation of the complex spectrum shown in Figure 22.3 by a sum of three spectral components. The results of this approximation and the parameters of the components are also shown in Figure 22.3. The fluorescence spectra of the N, A and T forms have maxima at 385, 465 and 525 nm and half-widths (FWHI) of 62, 62 and 54 nm respectively.

Figure 22.3 (1) Fluorescence excitation (lreg ¼ 390 nm), (2) absorption and (3) fluorescence (lex ¼ 280 nm) spectra of 3HF in acetonitrile. The characteristics of expansion of the fluorescence spectrum into the spectral components corresponding to the normal N, tautomeric T and anionic A forms are, respectively, lm ¼ 385, 465 and 525 nm and D lm ¼ 62, 63 and 54 nm. The spectral range of N and A fluorescence is shown on an enlarged scale. Reprinted with kind permission from [81]. Copyright 2007 Springer Science and Business Media

Proton Transfer Reactions in the Excited Electronic State 471

The absorption spectra have three intense maxima lying near 340, 305 and 240 nm. The positions of the first and second maxima can be assigned to the S1 and S2 bands, as they agree with the results of semi-empirical quantum mechanical calculations [10, 69], which yielded energies of 26 100 and 31 700 cm1 for the S1 and S2 states of a free 3HF molecule with the optimal geometric configuration in the ground state. Taking into account the accuracy of such calculations, which varies rather strongly for different methods, we can attribute the maxima at 340 and 305 nm to the transitions to the S1 and S2 states. It has been found that there exist such regions of excitation wavelengths in which only the band at 470 nm is selectively excited. Figure 22.4 shows the fluorescence spectrum recorded upon excitation in the region from 390 to 440 nm. In this case, we clearly see only the wide (with a half-width of 61 nm) third luminescence band peaking at about 467 nm with a profile typical for complex organic molecules. It is noteworthy that both the peak and the half-width of this spectrum agree well with the corresponding parameters of the component assigned to the A form in the expansion of the complex spectrum in Figure 22.3. To study the properties of the third luminescence band of 3HF in acetonitrile in more detail, we measured its excitation spectrum, which, as well as the absorption spectrum, is shown in Figure 22.4. The spectrum of excitation of the fluorescence at 470 nm is strongly different from the absorption spectrum also presented in Figure 22.4: the long-wavelength excitation maximum lies at 410 nm, and two maxima are seen in the UV region, near 320 and 270 nm. At the same time, the excitation spectrum of N fluorescence presented in Figure 22.3 has two maxima corresponding to the S1 and S2 bands of absorption. This directly indicates that the fluorescence at 470 nm belongs neither to the parent 3HF molecule nor to its tautomer. A very characteristic fact is that the fluorescence at 470 nm completely vanishes with the addition of water, the quenching occurring gradually with increasing water concentration. Of great interest is the fact that the fluorescence spectrum of solutions with a rather high concentration of water (1–10 M) has, again, only violet and green bands, and that the excitation spectrum of the green fluorescence band is similar to the corresponding spectrum for 3HF molecules in pure acetonitrile. Therefore, only normal and tautomeric forms of 3HF exist and spectrally manifest themselves in this solution. These data directly point to efficient quenching of the A form in the ground state by water, which is known to be an effective donor of protons. Hence, the A form in the ground state undergoes quenching of the first kind, or static quenching [73, 74], which means that it virtually vanishes. An interesting result of such quenching is the transformation of the A form to the normal parent

200 180

120 100

40

3 1,0

0,8

0,8

0,6

80 60

1

0,4

0,6 0,4

Ifl / a.u

140

2 1

ka /сm-1

Excitation /a.u.

160

0,2

0,2

20 0 150

250

350

450

550

0,0 650

λ /nm

Figure 22.4 Spectra of 3HF in acetonitrile: (1) fluorescence excitation of the A form (lreg ¼ 470 nm); (2) absorption; (3) fluorescence of the A form (lex ¼ 410 nm). Reprinted with kind permission from [81]. Copyright 2007 Springer Science and Business Media

472 Hydrogen Bonding and Transfer in the Excited State

molecule. Thus, the two reactions leading to the formation of the A form in pure acetonitrile and to its quenching with the addition of water can be written as 3HF þ S ! 3HF þ SH þ 3HF þ H2 O ! 3HF þ OH

ð22:1Þ ð22:2Þ

In the first reaction, the solvent molecules S, which have proton-acceptor properties, form a cation at the expense of a proton of the hydroxyl group of 3HF. As a result, the fluorophore remains in the A form, which exhibits the spectral properties demonstrated in Figure 22.4. Reaction (22.2) occurs with the addition of water to the acetonitrile solution. In this case, the efficient proton donor H2O gives a proton to the 3HF anion, as a result of which the latter transforms to a neutral molecule. The lifetime of the A form is 2 ns [79] in methanol and 3.7 0.2 ns [81] in acetonitrile. Thus, it is clear that, to describe the fluorescent properties of 3HF and related molecular compounds in some solvents, it is also necessary to take into account the levels of the A form. In most cases, the role of the A form and the reactions responsible for its formation in pure solvents are disregarded, and the properties of this form remain inadequately studied. Sytnik and Litvinyuk [83] studied the luminescence of 3HF in human serum albumin and showed that there exist two probe penetration or binding sites: the first one in the region of subdomain IIA, and the second in the region of subdomain IIIA. The neutral 3HF molecules are localized at the first site, and the molecules of the A form of this probe are localized at the second site. In addition, both types are acceptors of energy nonradiatively transferred from tryptophan. This makes it possible to determine the critical distances and some other parameters important for photobiological and biotechnological applications. Thus, as shown in Refs [10] and [83], it is important to know the spectral properties not only of the parent 3HF molecule and its tautomer but also of the A form, especially for investigating the properties of penetration sites in proteins and other complex biostructures. Nowadays, the great body of information on the properties of flavonoles that has been obtained using steadystate and kinetic spectroscopy contains mainly data on the properties of the parent molecule and its tautomer in pure solvents, their mixtures and some biological structures. Hence, deeper investigation of the spectral properties of all three forms of flavonole (normal, tautomeric and anionic) is not only of fundamental but also of practical interest. The anionic form is evident in alkaline alcohol solutions and as a minor component in neat alcohols, but not in water. The results of experimental and quantum mechanical calculations allow us to conclude that the electronic transitions of the N, A and T forms may be ascribed the values 27 500, 22 230 and 19 500 cm1, respectively, in the energy levels of Figure 22.2 [10, 69, 81, 82]. 22.2.4 Excited-State transformations and their modelling The mechanism of the ESIPT reaction between two excited species in 3HFs has been studied extensively [10, 25–29, 54, 61]. In studies of isolated molecules and in condensed media, the emission of the T form can be easily observed (usually it is dominant and sometimes it is the only form in emission), the ground-state T form is unstable and no traces of this form was detected even by transient spectroscopic techniques. Thus, as a basic model, very often researchers consider the four-level model originally suggested by Kasha [25, 54] (Figure 22.5). In this model, the time-dependent evolution of the initially excited N form should involve forward transition to the T state (ESIPT) with a kinetic rate constant k þ , together with emissive and nonN emissive conversion to the ground N state with kinetic constants kRN and knR respectively. Likewise, the decay of the T form resulting from the ESIPT reaction can be characterized by reverse transition to the N state with a rate constant k, and by radiative (with rate constant kRT ) and non-radiative (with rate constant

Proton Transfer Reactions in the Excited Electronic State 473 k+

N*

P*

knRP

νN

kRP

kRN

knRN

ν ex

k_

νP P N

N

P

Figure 22.5 Scheme of energy levels for description of the ESIPT reaction and dual fluorescence of the fluorophore (N) and photoproduct (P). Excitation of fluorescence with frequency nex at the main band of the fluorophore. T knR ) conversions to the ground T state. Emission from the N and T excited states forms the contour of dual fluorescence. An analysis of experimental data based on the four-level scheme allows us to introduce a hypothetical path of the ESIPT reaction. Experiments in various solvents and in jets [10, 84, 85] showed clearly that the ESIPT reaction in 3HF is extremely fast (high kþ ) and may proceed in the femtosecond time range (40–100 fs) even in cryogenic conditions and supercritical jets. The excited-state dynamics of 3HF has attracted significant interest in the literature. Following an initial report [54], the photophysics of 3HF was studied by a number of research groups. Most attempts to measure ESIPT in non-interacting solvents have been proved to be instrument limited, but Ernsting and Dick [85a] were able to calculate the ESIPT rate constant as 7.4  1011 s1 on the basis of lineshapes in the jet-cooled emission of 3HF molecules. For these, in methylcyclohexane and acetonitrile, the ESIPT was found [85b] to be so rapid that it was only possible to assign a time constant of 35 fs to the process. In ethanol, however, a time constant of 60 fs was determined. In addition to the femtosecond kinetics, there was also a picosecond component of the kinetics. To explain the simultaneous observation of two bands in emission, it is necessary to accept that the fluorescent N state is not the initially excited Franck–Condon (F–C) state but a relaxed state having a local minimum [10, 28]. The minimum in the N state can be the result of the charge transfer character of the N state with the essential dipole electric moment and its interactions with the reaction field of the surroundings. Thus, we have to consider the presence of at least two minima on the excited-state reaction coordinate and two possible mechanisms (see Figure 22.6) of transitions between them:

(i) Kinetic. In this case, owing to a strong energy gap, the equilibrium in the ESIPT reaction is shifted towards the T form, which makes the reaction practically irreversible. However, the transition from N to T form should be slow owing to the energy barrier in the case where transition takes place from the relaxed sublevels at the bottom of the potential curve (see the upper scheme in Figure 22.6). During this N ! T transition, the N form should have enough time to emit light, and this emission gives the N band of fluorescence.

474 Hydrogen Bonding and Transfer in the Excited State

Figure 22.6 Scheme of two mechanisms of the ESIPT reaction: upper scheme – kinetic (slow irreversible) reaction; lower scheme – fast reversible reaction. Reprinted with permission from [87]. Copyright Elsevier

(ii) Thermodynamic. In this case (see the lower scheme in Figure 22.6), the ESIPT reaction (the N ! T transformation) can be very fast. However, the presence of the reverse reaction on the same timescale, T which is faster than decay with probability (kRT þ knR ) of the T state, enables equilibrium to be established between the N and T forms, so that two emission bands of comparable intensities can be observed. In this case the distribution of intensities between two bands is determined by the Boltzmann distribution of excited-state species between these forms [10, 27, 28]. Minima of potential curves in Figure 22.6 correspond to positions of energy levels in the four-level scheme in Figure 22.5. In time-resolved experiments in the picosecond domain, several authors [57–59, 84, 85b, 86] clearly showed that in parent 3HF the ESIPT reaction is kinetically irreversible (the rise time of the T band fits well the decay time of the N band). A kinetic barrier may appear owing to the change in the relative configuration (see Figure 22.1) of the phenyl and chromone rings (which may be coupled with the solvation coordinate [28, 59]),

Proton Transfer Reactions in the Excited Electronic State 475

and, in protic environments, owing to competition between intramolecular and intermolecular hydrogen bonds [58]. This probably slows down the ESIPT reaction, and emission from the N form occurs in the course of its transition to the T form. The intermediate cases are also possible. Moreover, a switching of the reaction mechanism may be generated by variation in medium conditions (solvent, quenchers), which influence the interplay of the kinetic constants describing the reaction and also of the emissive and nonemissive decays of both forms [10, 28]. The reaction mechanism can be elucidated using time-resolved spectroscopy because, when the equilibrium is established, the decay rates of the two emissions become indistinguishable [10, 28]. This approach is still not very common in chemistry laboratories, and a simpler method based on the steady-state spectroscopic technique is needed. In Ref. [87] an original method based on the dynamic quenching of fluorescence by the nitric oxide spin compound TEMPO was proposed. For different dyes, TEMPO exhibits very high bimolecular quenching constants that are close to the diffusional limit. Tomin et al. [87] developed the theory, describing the effects of dynamic quenching within the framework of two-state excited-state reaction formalism, which predicts interesting, non-trivial behaviour of two-band emission in steady-state spectra, and carried out experiments on the quenching by TEMPO of the fluorescence of several 3-hydroxychromone dyes that exhibit excited-state intramolecular proton transfer reaction and on which the two types of reaction mechanism are observed. These results are in line with the predictions of theory. This method will be discussed in the next section.

22.3 Dynamic Quenching of Fluorescence as a Simple Test for Study of Photochemical Reaction Character [87] 22.3.1 Theory 22.3.1.1 Kinetics in Two-State Excited-State Reaction The simplest ESIPT reaction can be approximated by the four-level model presented in Figure 22.5. In this model, the N form is the only initially excited species, and its time-dependent evolution involves forward transition to the T state (the ESIPT reaction) with a kinetic rate constant kþ , together with radiative and nonN radiative decay to the ground N state with kinetic constants kRN and knR respectively. Likewise, the decay of   the T state takes place owing to reverse transition k to the N state and by radiative conversion with T constant kRT and non-radiative conversion with constant knR to the ground T state. Therefore, kinetic N description of the ESIPT reaction includes the determination of six kinetic constants (kþ , kRN , knR , k, kRT and T knR ). Interplay of these constants determines the time-dependent and steady-state characteristics of emission of both forms. For the four-level reaction scheme, the differential rate equations for the change in concentration of the N and T forms, N and T, after excitation by a d-pulse at t ¼ 0, with time are given by equations (3.1) and (3.2). N þ k þ ÞN* þ k T* dN* =dt ¼ ðkRN þ knR

ð22:3Þ

T dT* =dt ¼ ðkRT þ knR þ k ÞT* þ k þ N*

ð22:4Þ

Solution of these equations, under the condition of fast establishment of excited-state equilibrium (k þ ; N T k kRN , knR , kRT , knR ) shows that, in this limit, the two forms will exhibit identical rates of fluorescence decay that will be the weighted average of the decay rates of two excited-state forms [88]. If this condition is not met, the decay rates will be different.

476 Hydrogen Bonding and Transfer in the Excited State

22.3.1.2 The Effects of a Collisional Quencher on Excited-State Depopulation Rates Consider the action of a collisional fluorescence quencher of concentration Q added to the solution. It influences the decays of both forms, making them shorter. In general, this influences the two decays in a nonequal manner, which depends on all the rate constants involved. Equations (22.3) and (22.4) can be rewritten with account taken of the bimolecular rate constant kQ describing this quenching: N þ k þ ÞN* þ k T* kQ QN* dN*=dt ¼ ðkRN þ knR

ð22:5Þ

T dT*=dt ¼ ðkRT þ knR þ k ÞT* þ k þ N* kQ QT*

ð22:6Þ

Consider, then, a common case where an ESIPT molecule is excited by continuous radiation with density uex in the absorption band of the N form. In this case, with account taken of collisional quenching, we can write the balance equation for a steady-state regime of fluorescence: N fðkRN þ knR þ k þ Þ þ kQ QgN* ¼ k T* þ ka uex N0 T k þ N* ¼ fðkRT þ knR þ k Þ þ kQ QgT*

ð22:7Þ ð22:8Þ

where N0 is the concentration of species in the ground state SN 0 , and ka is the absorption coefficient. Denoting the figured bracket in equation (22.7) as { }1 and the figured bracket in equation (22.8) as { }2, these equations can be rewritten in a more compact way: f g1 N* ¼ k T* þ ka uex N0 k þ N* ¼ f g2 T*

ð22:9Þ ð22:10Þ

The obtained system of linear algebraic equations is resolved with respect to two concentrations N and T as follows: N* ¼ n

ka uex N0 f g1  kf gk þ

o¼

f g2 ka uex N0 f g1 f g2 k  k þ

ð22:11Þ

2

T* ¼

kþ f g2 k þ ka uex N0  ka uex N0 ¼ f g2 f g1 f g2 k  k þ f g1 f g2 k  k þ

ð22:12Þ

From these equations we can obtain all information about dependences of N and T on all rate constants, and consequently about appropriate intensities of the N and T bands of fluorescence, taking into account the values of all constants describing our ESIPT system. Let us estimate the simple but quite common case where the constant of the direct ESIPT reaction is essentially higher than that for the sum of deactivation constants of the N state, i.e. N þ kQ Q k þ kRN þ knR

ð22:12aÞ

Proton Transfer Reactions in the Excited Electronic State 477

For this case, equations (22.11) and (22.12) can be presented in a more transparent way:   ka uex N0 k N ¼ 1þ T T þk Q kþ kR þ knR Q *

T* ¼

ka uex N0 T þk Q kRT þ knR Q

ð22:13Þ ð22:14Þ

As can be seen from the latter equations, the concentrations of species in both N and T states may be found to decrease with growth in quencher concentration Q. Let us analyse equations (22.13) and (22.14) in detail. We can select three different but characteristic situations: 1. The first case is where the rate of the reverse ESIPT reaction is very low in comparison with full deactivation of the T state, i.e. T k  kRT þ knR þ kQ Q

ð22:14aÞ

In this case, as follows from equation (22.13), there is no dependence of N concentration on quencher, as the second member in brackets can be neglected with respect to unity. This is not a trivial case; it appears when the lifetime of the N state is very low owing to the high rate of direct ESIPT reaction k þ depopulating this state. For example, for 3HF, the lifetime of the N form may be estimated as 0.1 ns [10, 28], and it is obvious that for such short times there are practically no diffusional collisions of dye with molecules of quencher which can deactivate the N species at usual quencher concentrations of 103 M. On the contrary, the tautomer form reveals quenching, which does not depend on the rate of the ESIPT reverse reaction k, and concentration T decreases in agreement with equation (22.14). 2. The second interesting situation concerning the influence of quencher on N and T concentrations is where the rate constant of the reverse reaction k is higher than in the previous case and will be close to the sum of constants describing deactivation of the T form, i.e. T þ kQ Q k  kRT þ knR

ð22:14bÞ

In this case, as can be seen from equation (22.13), the second term in the brackets will be of the same order as unity, and the N concentration must feel the presence of the quencher. The higher the efficiency of collisional deactivation of the excited state, kQ, the stronger is the sensitivity of the N state to the presence of the quencher. Let us then also take into account the fact that in real experimental conditions the rate of diffusional quenching is lower or of the same order of magnitude as the rate of excited-state T deactivation owing to radiative and non-radiative transitions: T kQ Q kRT þ knR

ð22:14cÞ

Further, for the sake of simplicity, we suppose that the sum of radiative and non-radiative rates of N and T state deactivation should not differ essentially, i.e. 

T N  kRN þ knR kRT þ knR

ð22:14dÞ

478 Hydrogen Bonding and Transfer in the Excited State

Using the latter condition, expression (22.14a) can be rewritten as N þ kQ Q k  kRN þ knR

Comparing this expression with inequality (22.12a), we establish that the rate of the direct ESIPT reaction is essentially higher than the rate of the reverse reaction: k þ k Hence, the assumptions that were made with respect to the rate values, i.e. equations (22.12a) and (22.14a) to (22.14d) are consistent and they directly indicate the kinetic control of the ESIPT reaction. As follows from equations (22.13) and (22.14), the tautomer form must be quenched more strongly than the normal form if the condition (22.14b) is valid. Such a statement follows directly from equation (22.13), as the term in the brackets will be comparable with unity. This situation is characteristic for the kinetic type of ESIPT reaction, when additionally condition (22.14b) is satisfied. 3. Finally, a third interesting case is anticipated. The strong sensitivity of N and T concentrations to the presence of quencher can be observed at relatively high values of the reverse reaction rate, i.e. when k  k þ In this case, submitting k into equation (22.12a) yields N þ kQ Q k kRN þ knR

and then, using expression (22.14d), the latter inequality can be rearranged as T þ kQ Q k kRT þ knR

Using this expression, we can present equation (22.13) as N* ¼

ka uex N0 k T T k þ kR þ knR þ kQ Q

ð22:14fÞ

Therefore, comparing this last expression and equation (22.14), we can conclude that the concentrations of N and T species respond to the presence of quencher in the same way, i.e. for both forms the effect of quenching is identical. This conclusion is a consequence of the fact that the ESIPT reaction becomes thermodynamically controlled if k  k þ , and the latter reaction rates are higher than the emission decay rates; in such cases, the excited-state equilibrium between the N and T states is reached before the acts of emission. 22.3.1.3 The Effects of a Collisional Quencher on the Intensities in Steady-State Spectra Among the most interesting parameters in applying of ESIPT probes in sensing of various samples are the relative intensities of the N and T bands of fluorescence. Let us discuss the collisional quencher effect on this

Proton Transfer Reactions in the Excited Electronic State 479

important parameter. As can be seen from equations (22.13) and (22.14), the ratio of the concentrations of the two excited species, N /T, can be expressed as  N* 1  T T ¼ ðkR þ knR þ k Þ þ kQ Q * kþ T

ð22:15Þ

It is important that the ratio N /T does not depend on the density of excitation light, in contrast to N populations N and T, which are dependent on this parameter. Thus, when the condition k þ kRN þ knR þ kQQ expressed by equation (22.12a) is satisfied, the quenching effect depends only on the deactivation rates of T the T state (kRT þ knR þ k ). Now we can express the ratio of steady-state emission intensities of N and T bands as a function of quencher concentration Q, which will be useful for comparison with experiment. Equation (22.15) can be presented in the form  IN cN kRN nI N* cN kRN nI 1  T T ¼  T ¼ ðkR þ knR þ k Þ þ kQ Q T * IT cT kR nII T cT kR nII k þ

ð22:16Þ

where cN and cT are the sensitivities in the N and T band detection channels respectively. The factors in equation (22.16) before the brackets are independent of Q and in this respect can be treated as constants. The term in brackets will determine the quencher effects. It follows that the role of the last term in brackets, kQQ, is essential when the rate of quenching will be comparable or higher than the sum of other rate constants describing deactivation of the T state, i.e. T þ k kQ Q kRT þ knR

ð22:17Þ

It can be seen that in this case the ratio IN/IT exhibits a linear dependence on the concentration of quencher Q, which is similar to the Stern–Volmer equation. Inequality (3.15) will be valid if the constant k is T small enough to satisfy expression (22.17) (for example, k kRT þ knR þ ka Q), which is in line with relations between the same rates (see inequality (22.14a)) and thus corresponds to the case of kinetic control of ESIPT. Finally, there will be no dependence of the ratiometric parameter IN/IT on the content of quencher at high enough magnitudes of k when k  k þ and the rate constant k þ is very fast, as in equation (22.12a). In this case the role of the last term in the brackets of equation (22.16) can be neglected, and the ratio of the two intensities remains unchanged for different concentrations of quencher. This is characteristic for the conditions of thermodynamic control of the ESIPT reaction. Thus, concluding this section, we emphasize that, in collisional quenching of species exhibiting the ESIPT reaction, two main cases may be distinctly selected: (i) The kinetic type of ESIPT reaction (k þ k), which shows a linear dependence of the relative intensities of both bands of dual fluorescence IN/IT on quencher concentration. Within this case there could be two situations found with respect to the value of k: . if k  kT þ k T þ kQ Q there is no dependence of N concentration on quencher; on the contrary, the R nR tautomer form reveals quenching; . if k  kT þ k T þ kQ Q, both N and T forms are liable to quenching, and moreover the tautomer form R nR is quenched faster.

480 Hydrogen Bonding and Transfer in the Excited State

Figure 22.7

Structural formula of the G+;C? quencher (2,2,6,6-tetramethylpiperidine 1-oxyl)

(ii) The thermodynamic type of ESIPT reaction (k þ  k) shows identical dependences of populations and intensities of both bands of dual fluorescence on quencher concentration. Because of this, the relative band intensity IN/IT does not depend on the presence of quencher. Experimental study of dual fluorescence of 3HF and its derivatives when the excited states of dyes were subjected to quenching by strong collisional quencher 2,2,6,6-tetramethylpiperidine 1-oxyl, or TEMPO for short, was carried out in Refs [87] and [89]. The structural formula of the TEMPO quencher is presented in Figure 22.7. Materials and methods are described in detail elsewhere [87]. All experiments were performed at room temperature (295 K), which was stabilized by a thermostat. Intensities of fluorescence for calculation of the IN/IT ratio were measured at maxima of appropriate N and T bands of emission. 22.3.2 Experimental results and discussion 22.3.2.1 The Effect of Quencher on Fluorescence of Parent 3HF 3HF is the classical object in studies of the mechanism of the ESIPT reaction and of the dependence of this reaction on different factors such as temperature and solvent. The rigid skeleton of the chromone ring makes the 3-hydroxy group (proton donor) and 4-carbonyl group (proton acceptor) fixed in space and connected to the hydrogen bond, which excludes the involvement in this reaction of other factors such as conformational isomerizations [10, 26–28]. However, this intramolecular H-bond can be subjected to intermolecular hydrogen bonding perturbations, which change the ESIPT characteristics. Time-resolved studies in solutions [57–59, 65, 90] and high-resolution spectra in cryogenic Spol’skii matrices [86] demonstrated that the ESIPT reaction is very fast and can be completed on a timescale of femtoseconds. And, indeed, in most experiments and in most solvent media only the strongly Stokes-shifted T band was observed in 3HF emission. The presence of N band in emission was observed in protic solvents and in highly polar aprotic solvents such as acetonitrile [10, 28, 87, 89]. The time-resolved studies in polar protic solvents showed clearly that the long components of emission decay of N and T forms differ [57, 58], which indicated the absence of excited-state equilibrium and suggested the slow kinetics origin of emission from the N state. It was suggested that in these solvents there is an Arrhenius-type barrier to the ESIPT reaction, which probably appears owing to (i) non-planar mutual orientation of the phenyl and chromone rings (Figure 22.1) in the molecule (in agreement with quantum chemical simulations in Refs [69] and [91], this angle is 25–40 in the ground state and near zero in the excited state), (ii) reorganization of intramolecular/intermolecular hydrogen bonds [57, 58] or (iii) reorientational reorganization of polar solvent molecules [91]. The experiments were performed in highly polar solvent acetonitrile, in which the N band is clearly observed, with the predominance of the T band (Figure 22.8). The increase in quencher concentration results in a decrease in intensity of the major T band with a simultaneous decrease in the N band, so that the intensity ratio IN/IT is increased. The Stern–Volmer plot for both bands are linear. This plot of the N and T bands has a positive slope, indicating quenching as seen in inserts A and B. According to the theory (Section ), this behaviour is a strong indication of the occurrence of ESIPT under kinetic control, so that the reaction is slow and irreversible, and the excited-state equilibrium with Boltzmann distribution of forms is not reached.

Proton Transfer Reactions in the Excited Electronic State 481

Figure 22.8 The influence of quencher TEMPO on the fluorescence spectra of 3HF in acetonitrile. The excitation wavelength is 340 nm. The effect of sequential additions of TEMPO is shown by arrows. Presented are the spectra obtained at quencher concentrations of 0, 2, 5, 7.5, 10 and 12.5 mM. Insert A: the dependence of band intensity ratio IN/IT on quencher concentration. Insert B: The Stern–Volmer plots for the effect of quencher on the intensities of the N and T bands. Reprinted with permission from [87]. Copyright Elsevier

22.3.2.2 The Effect of Quencher on the Fluorescence of Novel 3HF analogues The group of other molecules studied in Ref. [87] – F, F2 (the structural formula is given in Figure 22.1) and FA (the structural formula is presented in Figure 22.9) – are the fluorophores, in which the dipole moment in the N state is strongly increased in comparison with 3HF owing to electronic charge transfer from the dialkylamino group (at site R2) to the chromone moiety [26–28]. This allows this state to achieve substantial stabilization in energy on interaction with polar solvent molecules. Because of this, these dyes are very strong solvent polarity sensors, reacting to change in polarity not only by a spectral shift of the N band but also by redistribution of intensity between the two bands; increase in polarity increases the relative contribution of emission from the N state. The substitutions at sites R1 and R3 modulate the dye response to a lesser extent [26], and these sites are used, for example, for generation of functional derivatives and for covalent modifications of proteins [27]. In experiments, the dye F was studied in ethyl acetate (Figure 22.10). Like 3HF, it exhibits two bands, with a predominance of the T band in emission. However, the behaviour of this dye in response to quenching by

R2 R1

O

O

O

R3

O

H

Figure 22.9 The dyes of the 3-hydroxychromone family exhibiting the ESIPT reaction – 3-hydroxybenzofuranochromones. For dye FA, R1 and R3 are hydrogens and R2 is diethylamino

482 Hydrogen Bonding and Transfer in the Excited State

Figure 22.10 The influence of quencher TEMPO on the fluorescence spectra of dye F in ethyl acetate. The excitation wavelength is 395 nm. The effect of sequential additions of TEMPO is shown by arrows. Presented are the spectra obtained at quencher concentrations of 0, 0.95, 1.94, 2.9, 3.85, 4.7 and 5.7 mM. Insert A: the dependence of band intensity ratio IN/IT on quencher concentration. Insert B: the Stern–Volmer plots for the effect of quencher on the intensity of the T band. Reprinted with permission from [87]. Copyright Elsevier

TEMPO is quite different; both N and T bands are quenched in such a way that the IN/IT ratio remains unchanged for different contents of quencher. Similar behaviour is observed for dye F2 in much more polar solvent dimethylformamide, although in this case the N band dominates in emission. The studies of the third member of this series, dye FA, were performed in toluene, for which the two bands are of comparable intensity. They show the same trend: both bands are quenched proportionally. The IN/IT ratio does not change in the entire range of studied quencher concentrations, and the Stern–Volmer plots are linear with positive slope. Moreover, the latter are identical when plotted for N and T bands. This quencher effect is in line with the model of the fast equilibrium mechanism of ESIPT. Direct and indirect evidence for the achievement of this equilibrium can be found in the literature. In earlier studies on dialkylamino-3-hydroxyflavone derivatives, a correlation was observed between the shift of the N band (to the red) and increase in the relative magnitude of this band in emission [92, 93], which suggested the thermodynamic control of the ESIPT reaction with a small solvent reorganizational energy barrier. In a more detailed study [71], this correlation was expressed in quantitative terms because in experiments performed in a significant number of solvents it conforms to the Boltzmanm distribution of populations of N and T states, which depends on their relative energies. Direct time-resolved studies of emission decays and reconstruction of time-resolved spectra of dyes F and F2 [94] showed the establishment of excited-state equilibrium in ethyl acetate and dichloromethane on a timescale of tens of picoseconds. Similar results on dye F were obtained in other studies [95]. Thermal quenching was studied for dye FA, and it was shown that variation in temperature, which changes strongly the absolute intensity, does not change the IN/IT ratio [96]. The present results describe the first application of an efficient collisional quencher in neat solvents and demonstrates the conformance of obtained results to the model of the fast equilibrium ESIPT reaction. Thus, we can conclude that, in line with predictions of the theory, quenching changes strongly the distribution of intensities between bands of dual fluorescence for the kinetic type of reaction (3HF) but does not change it for the novel compounds (F, FA, F2), the excited states of which exhibit a strong charge transfer

Proton Transfer Reactions in the Excited Electronic State 483

character. These findings suggest that the quenching of fluorescence by an efficient collisional quencher can be a simple and convenient method using only a steady-state experiment for distinguishing the excited-state reactions occurring under thermodynamic or under kinetic controls. Here, it is worth noting that, in agreement with the theory presented above, any kind of dynamic quenching could be used for establishing the kinetic or thermodynamic character of reaction. Therefore, both special quenchers, such as TEMPO, and natural processes, such as, for example, thermal quenching or quenching by molecular oxygen dissolved in solvents at normal atmospheric pressure, can be employed for study of photoreactions by the method described above. Thus, the theory of dual fluorescence quenching makes it possible to use changes in fluorescence emission, arising for various physical reasons, to determine the character of excited-state reactions. Finally, it is worth noting that the developed theoretical description is not limited to ESIPT; it can be useful in analysis of quenching effects on other photochemical reactions that can be described by two-state excited-state reaction formalism. Based on these findings, a simple test can be suggested for the mechanisms of these reactions, allowing discrimination of two important cases – thermodynamic and kinetic control. As shown in this section, dynamic quenching is very efficient and at the same time a simple method for the study of PT and other reactions in the excited state. In the next section we describe results that we obtained by applying this method together with changes of other physical conditions such as temperature of solution and energy of excitation.

22.4 Use of Dynamic Quenching of Fluorescence for Study of Reactions from Higher Excited States Various photophysical and photochemical processes in organic molecules, such as the formation of excimers and exciplexes, electronic density redistribution, transitions to the triplet metastable states, molecular geometry changes, phototautomerization and some others, are directly related to the singlet S1 state. As a rule, such processes are studied without taking into account highest excited states. At the same time, as was shown, for example, in Refs [97] and [98], the role of high-lying singlet states is not confined only to processes of intramolecular internal conversion. Very frequently, both initial excited molecules and products of their photoreactions exhibit pronounced luminescence; in this case, luminescence methods are very effective and are widely used in various fields of physics, chemistry and biology [73–75, 88, 99]. In recent years, interest in excited states has stimulated the development of new experimental methods for their study. These methods are stepwise excitation of Sn states using two or more radiation photons, n-photon excitation, T–T annihilation, etc. [73–75]. Recent publications have demonstrated photoreactions occurring through the S2 singlet state in 3-hydroxyflavone [100–103] and 2-butylamino-6-methyl-4-nitropyridine N-oxide [104]. This stimulates interest both in processes that involve the participation of the S2 state of such systems and in experimental methods that are used for their investigation. An important method for investigating highest singlet states is studying fluorescence transitions from these states. However, the frequently very low quantum yield of such fluorescence creates certain experimental difficulties in its investigation, especially using standard steady-state instruments (spectrofluorimeters). A valuable property of dually fluorescing systems is the mutual change in intensity of fluorescence bands in relation to intermolecular interactions. This property forms the basis for the procedure of self-calibration of the response to the properties of the environment, which is widely used [26–28] in studies of various physicochemical objects with the help of molecular probes that experience excited-state internal proton transfer. In connection with this, it is also useful to know the properties of such probes under their excitation via Sn (n 2) states, which can extend the scope of their use. In this section we consider in general how the properties of dually fluorescing luminophores are affected by dynamic fluorescence quenching in the case

484

Hydrogen Bonding and Transfer in the Excited State

where the second fluorescence band of luminophores is formed as a result of photoreaction in their excited state and where their fluorescence is excited by radiation in the range of the S1 and Sn absorption bands. The effects of high-lying excited states and quenching on the fluorescence intensity of a luminophore and its photoproduct, and on the peculiarities of the definition of the constants of bimolecular quenching of initial reagents and products of reaction, are considered. For products, methods for determining these constants, which differ from methods commonly used in the case of single-band fluorescence, are substantiated. It is shown that, upon excitation of molecular objects via high-lying singlet states, the yield of reaction products can increase in a number of cases. Then, if fluorescence of initial molecules and their photoproducts is detectable, the probabilities of reactions can be determined via high-lying singlet states. Experimental data are presented that demonstrate the roles played by high-lying singlet states and dynamic quenching in a solution of 3-hydroxyflavone, in the excited state of which reactions of formation of tautomers as a result of internal proton transfer take place. It is shown that, for fluorescence excited via different Sn states, the Stern–Volmer constants strongly differ. 22.4.1 Theoretical 22.4.1.1 Effect of Quenching on Populations of Excited States Figure 22.11 presents the scheme of energy levels for description of ESIPT reactions, taking into account highlying singlet states Sn. The intensity of either of the two fluorescence bands is directly proportional to the population of the corresponding energy state. Taking this into account, it can be assumed that, upon excitation of fluorophore molecules via different absorption bands (see Figure 22.11), the relation between the P populations N and P of the SN 1 and S1 levels, respectively, can change if the change in the relative intensity of the corresponding bands is measured. In the simplest case of a fluorophore in solution with a quencher of the excited states, upon standard steady-state excitation in the range of the first absorption band, the detailed equilibrium equation for the SP1 level has the form P þ k þ kQP QgP* k þ N* ¼ ðkRP þ knR

Sn

N

kn2

p n2 S1

N

k+

S1

S0

N

N

P

knRP

kR P

kRN

knRN

νex

ν’ex

k_

S0

ð22:18Þ

P

P

Figure 22.11 Scheme of the energy levels describing excited-state reaction in a molecule (N) having a photoN product (P) upon excitation into the SN 1 and Sn absorption bands respectively. Reprinted with kind permission from Springer Science and Business Media

Proton Transfer Reactions in the Excited Electronic State 485

From this expression, the relative population of N and P states can be written as P þ k þ kQP Qg N* fkRP þ knR ¼ kþ P*

ð22:19Þ

If we denote the lifetime of the SP1 state as tP ¼

1 P þ k þ k P Qg fkRP þ knR  Q

ð22:20Þ

and, for the case where Q ¼ 0, we have a formula for the lifetime of the product fluorescence in a neat solution: tP0 ¼

kRP

1 P þk þ knR 

ð22:21Þ

In view of equation (22.20), expression (22.19) takes the form N* 1 ¼ * k þ tP P

ð22:22Þ

This expression is convenient, as tP can be determined by methods of kinetic fluorescence spectroscopy. Equalities (22.19) and (22.20) show that quenching will increase the population ratio N /P and decrease the lifetime tP of the reaction product. However, it should be noted that this regularity will occur if the following inequality holds: P þ k kQP Q kRP þ knR

ð22:23Þ

We will complicate the problem and will excite the fluorophore in a solution with quencher via high-lying 0 singlet state SN n by radiation at a frequency of n ex (see Figure 22.11). In this case, the balance equations for the P N N S1 and S1 levels acquire a somewhat different form, as particles now pass to the SN 1 state via the Sn state by P means of internal conversion, the rate constant of which is pn2. Simultaneously, the excited S1 state of the photoproduct can additionally be populated by particles that pass to this state directly from the SN n state with the interconversion rate constant kn2. Then, obviously, the corresponding steady-state equations for the populaP tions N and P of the SN 1 and S1 levels can be written, respectively, as N þ k þ þ kQN QgN* ¼ k P* þ pn2 N*n fkRN þ knR

ð22:24Þ

P þ k þ kQP QgP* k þ N* þ kn2 N2n ¼ fkRP þ knR

ð22:25Þ

In equation (22.24), kQN is the rate constant of the bimolecular fluorescence quenching of the luminophore. This constant can differ from the analogous constant kQP for the reaction product in equation (22.25). It can be seen that the left-hand side of equation (22.25) differs from expression (22.18) by the term kn2 N*n , which describes the transition of molecules to the fluorescence state of the photoproduct as a result of the nonradiative transition kn2 from a high-lying excited state of the normal form of the fluorophore whose population

486 Hydrogen Bonding and Transfer in the Excited State

is N*n . It follows from equations (22.24) and (22.25) that the properties of dual fluorescence and reaction of the N P photoproduct formation are described by the eight kinetic constants k þ , k, kRN , knR , kRP , kQ, kn2 and knR , and also that the relation between them will significantly affect the properties and behaviour of processes. Consider equation (22.24). We can explicitly express from this equation the population of the singlet state Sn of the initial form of the fluorophore as Nn* ¼

  1  N N kR þ knR þ k þ þ kQ Q N* k P* pn2

ð22:26Þ

By substituting this expression into equation (22.25), we obtain k þ N* þ

o  n kn2  N N P fkR þ knR þ k þ þ kQ QgN* k P* ¼ kRP þ knR þ k þ kQP Q P* pn2

ð22:27Þ

By collecting the similar terms with N and P, we can rewrite this expression as 

n o k k  * kn2  N n2  N P P P kþ þ k þ knR þ k þ þ kQ Q N ¼ kR þ knR þ k þ kQ Q þ P* pn2 R pn2

ð22:28Þ

From this equation, we can find the following expression for the ratio N /P : P pn2 fkRP þ knR þ k þ kQP Qg þ k kn2 N* ¼ N þ k þ k Qg P* k þ pn2 þ kn2 fkRN þ knR þ Q

ð22:29Þ

Therefore, equations (22.24) and (22.25) allowed us to find the ratio of the populations N and P of the fluorescing states of the fluorophore and its photoproduct in the case of excitation via an arbitrary electronic level SN n . This expression also takes into account the possibility of the dynamic quenching of excited states. As can be seen from expression (22.29), the ratio N /P does not depend on the photoexcitation intensity; however, it is directly affected by possible formation of the photoproduct from the SN n state with the probability kn2 and dynamic quenching processes, which are characterized by the quenching rate constants kQN Q and kQP Q. These processes can change the population ratio N /P or the intensity ratio of the corresponding fluorescence bands. Let us analyse expression (22.29) for the frequently occurring case where the photoreaction efficiency is high, i.e. where the photoproduct formation rate is very high compared with the sum of the constants determining the fluorescence decay of the normal form N k þ kRN þ knR þ kQ Q

ð22:30Þ

Under these conditions, we can represent the sum in parentheses in the denominator of expression (22.29) as N fkRN þ knR þ k þ þ kQ Qg1  k þ . Therefore, we can write expression (22.29) in a simpler form: P þ k þ kQP Qg þ k kn2 N* pn2 fkRP þ knR ¼ * k þ ðpn2 þ kn2 Þ P

ð22:31Þ

It can be seen from this equation that the population ratio N /P is inversely proportional to the probability k þ of the direct reaction of the photoproduct formation (i.e. the higher the probability k þ , the higher is the

Proton Transfer Reactions in the Excited Electronic State 487

yield of the reaction product). This ratio also depends on the contribution kn2 of the assumed reaction of the photoproduct formation from the Sn singlet state. In the trivial case where the probability kn2 is very small (kn2 ! 0), we arrive at the well-known variant where the ratio N /P is the same (see formula (22.22)), as in the 0 case where fluorescence is excited via the SN 1 band by radiation at a frequency of n ex . In this case, the quenching causes an increase in the population of the N form compared with the concentration of the product P., Such dependences were studied experimentally in Res [87] and [89]. Consider in more detail the general case of the effect of quenching on the population ratio N /P for excitation via an arbitrary singlet state Sn. Here, two limiting cases are possible. Clearly, we can assume that the effect of the interconversion process kn2 will be possible if this constant is comparable with the constant pn2 of internal conversion (see the factor (pn2 þ kn2) in the denominator of expression (22.31)). If, in this case, the probability k of the reverse reaction is small, so that the relation P pn2 fkRP þ knR þ k þ kQP Qg k kn2

ð22:32Þ

holds, then N /P will increase with the quencher concentration according to the linear law P þ k þ kPQ Qg N* pn2 fkRP þ knR ¼ k þ ð pn2 þ kn2 Þ P*

ð22:33Þ

This expression differs from expression (22.19) in that the slope of the dependence of the ratio N /P on the quencher concentration in (22.33) is smaller than in (22.19) because of the occurrence of the factor pn2=(pn2 þ kn2). Therefore, if relation (22.32) holds, which is the case at small rates k of the reverse reaction, the ratio N /P linearly depends on the quencher concentration. Another limiting case is where the rate constant k of the reverse reaction dominates over the remaining deactivation rate constants of the singlet state SP1 of the product P þ kQP Q k kRP þ knR

ð22:34Þ

Then, we can assume that P þ k þ kQP Qg  k fkRP þ knR

ð22:34aÞ

In view of this equality, expression (22.28) obviously yields N* k ¼ P* k þ

ð22:35Þ

Clearly, in this case, all dependences on the concentration of the quencher Q and on the product formation rate kn2 vanish. As was discussed in Section 22.2.4, primary photoreactions can be divided into thermodynamic and kinetic. Thermodynamic schemes are characterized by a fast energy exchange between states of the normal form and its product, which clearly should occur if singlet levels lie comparatively close to each other, i.e. if the energy gap is small and the energy barrier is low (Figure 22.4, lower scheme). Energy exchange is intense if the rate constant of the reverse transition is sufficiently large; i.e. the implementation of inequality (22.34) can be considered to be the condition for such exchange. In this case, because of the strong energy exchange, the two fluorescing states decay similarly. Taking into account the condition (22.34a), expression (22.35) is identical to

488 Hydrogen Bonding and Transfer in the Excited State

expression (22.19) for the population ratio upon excitation in the main absorption band. Therefore, we obtained an important result, namely that, in the thermodynamic case, the population ratio of excited states of the normal form and its photoproduct does not depend on which absorption band was used for excitation. This particular feature is the fundamental distinction of this case from the kinetic one. Photoreactions with weak interactions (energy exchange) between fluorescing levels of the normal form and its photoproduct pertain to the second, i.e. kinetic, type. This is determined by the considerable energy gap between the excited states of the fluorophore and the product and by an energy barrier that separates these states. Such reactions are characterized by an equilibrium shifted towards the product, which makes them almost irreversible. In connection with this, the probability of the reverse transition for such reactions will be considerably smaller than the rate constant k þ of the direct reaction, and, in certain cases, k can be even smaller than the sum of the radiative and non-radiative rate constants of the SP1 ! SP0 electronic transition, i.e. P k  kRP þ knR þ kQP Q

ð22:36Þ

In this case, the quenching affects the population ratio N /P, i.e. we can use expression (22.33). Previously, in Section 22.3, the behaviour of dual fluorescence of some derivatives of flavones on dynamic quenching of their excited states was described. Based on substantiated criteria for their different behaviour in reactions of different character, the kinetic and thermodynamic reactions proceeding in solutions were determined. We note here another interesting option with respect to possible change of the character of a reaction with the help of quenching. Indeed, inequality (22.34) shows that, if quenching is sufficiently strong, this condition can be violated. As a result, the reaction will be forced to proceed according to the kinetic scheme without changing the arrangement of the excited levels of the luminophore and product! Usually, if the quenching of excited states is weak or absent, the lifetime of the fluorescing state of the photoproduct is tP  1 ns [10, 28]. The reciprocal of this quantity can be equated to the sum of the rate constants on the right-hand side of inequality (22.36) as follows: P kRP þ knR þ kQP Q 

1 tP

ð22:37Þ

In view of (22.36) and (22.37), equation (22.31) can be written in the simpler form 1 pn2 þ k kn2 N* pn2 þ k tP kn2 tP ¼ ¼ k þ ðpn2 þ kn2 Þ k þ tP ðpn2 þ kn2 Þ P*

ð22:38Þ

which can be used in the kinetic case. Because, according to (22.36) and (22.37), the product k tP should be considerably smaller than unity, i.e. k tP  1

ð22:38aÞ

we can assume that the reaction of the photoproduct formation via a high-lying singlet state will mainly affect the population ratio N /P via the second term in the denominator of fraction (22.38). Clearly, this will occur if the rate constant kn2 of the reaction of the photoproduct formation via the state Sn is comparable with or higher than the interconversion rate constant pn2, i.e. if the condition kn2 pn2 is satisfied. Then, it is obvious that the occurrence of such reactions characterized by an appreciable constant kn2 will lead to a relative increase in the yield of reaction products. In spectra of dual fluorescence, the role played by the

Proton Transfer Reactions in the Excited Electronic State 489

rate constant kn2 will lead to an increase in the relative contribution of the long-wavelength fluorescence band when higher-lying states are excited. These conclusions are similar to those obtained in Ref. [105], where the effect of photoreactions proceeding via Sn states on the product yield was considered without taking quenching into account. 22.4.1.2 Effect of Quenching on the Fluorescence Intensities Let us now consider how the relations obtained can be used for processing experimental data and extracting useful information on probabilities of reaction [106]. It is impossible experimentally to measure the populations of levels, because, in luminescence experiments, the intensities of corresponding signals are measured, which are proportional to the populations. Thus, let us take into account that the fluorescence intensity of the ith form is directly proportional to the concentration of molecules in the corresponding state Ni, the probability of the transition kRi , the average quantum energy hni and the detection sensitivity ci: Ii ¼ ci Ni kRi hni

ð22:39Þ

then the fluorescence intensity of the initial form is expressed as IN ¼ cN N* kRN hnN

ð22:40Þ

Here, the population N of the fluorescence level in the general case of excitation via the SN n singlet state and in a steady-state regime is given by the expression N* ¼ tN B1n ðnex Þuðnex ÞN1 pn2 =ðpn2 þ kn2 Þ

ð22:41Þ

where the factor pn2 =ðpn2 þ kn2 Þ is the fraction of excited particles that passes from the level SN n to the N fluorescing level SN . If the internal conversion from the S state p dominates, then the above factor turns to n2 1 n unity. By analogy with (22.20), we will write the lifetime of the state N as tN ¼

fkRN

N þ knR

1 þ k þ kQN Qg

ð22:42Þ

Formulae (22.40) to (22.42) show that the fluorescence intensity and the state population N , as well as the lifetime of the N form, decrease with increasing quencher content Q; i.e. they are quenched by impurities. These expressions also yield the well-known Stern–Volmer formula N ðI0 =IÞ ¼ ðh0 =hÞ ¼ 1 þ kSV Q

ð22:43Þ

N N ¼ tN for the relative changes in the fluorescence intensity I and the luminophore quantum yield h. Here, kSV 0 kQ N is the Stern–Volmer constant, and I0, h0 and t0 are, respectively, the fluorescence intensity, the quantum yield and the lifetime of the excited state in the absence of quenching. This formula is widely used in physicochemical and biological applications in studies of the excited state and mechanisms by which various molecular objects interact in solutions [73, 74, 99]. From the experimental dependence of the ratio I0/I on the N quencher concentration Q, we can easily determine the product tN 0 kQ , and, if the constant of bimolecular N N collisions kQ is known, we can calculate the lifetime t0 of the excited state.

490 Hydrogen Bonding and Transfer in the Excited State

Taking into account relation (22.40), we may obtain the following expression for the ratio of the fluorescence intensity of the N and P forms: IN cN N* kRN hnN ¼ IP cP P* kRP hnP

ð22:44Þ

By substituting the ratio N =P from (22.38) into expression (22.44), and taking into account the conditions (22.37) and (22.38a), we can represent the intensity ratio of the N and P forms as P þ k þ kQP Qg IN cN nN kRN pn2 fkRP þ knR ¼  P IP cP nP kR k þ ðpn2 þ kn2 Þ

ð22:45Þ

This expression contains two characteristic dependences that make it possible to determine useful properties of excited states. Let us analyse ratio (22.45) in more detail. We can see from this equation that the intensity ratio grows linearly with the quencher concentration. This proportionality makes it possible to determine from experimental data the bimolecular quenching rate constant kQP for the excited state of the reaction product if the remaining constants (i.e. cN, cP, nN, nP, kRN , kRP and k þ ), which determine the slope of the dependence of IN=IP on the quencher concentration, are known. To eliminate the constants kn2 and pn2, the determination of which is an independent problem, it is necessary to carry out additional measurements of the ratio IN=IT dependence on the quencher Q in such a way that the excitation is performed via the S1 state. Then, the rate constant kn2 of internal conversion from this state can be equated to zero. In this case, we obtain the following relation from (22.45): P þ k þ kQP Qg IN cN nN kRN fkRP þ knR ¼ IP cP nP kRP kþ

ð22:46Þ

which makes it possible to obtain the set of constants cN nN kRN 1 cP nP kRP k þ determining the slope of the IN=IP(Q) function. Substituting (22.42) into (22.46), we have IN cN nN kRN 1 ¼ IP cP nP kRP tP k þ

ð22:47Þ

Let us return to the general case described by expression (22.45). It is convenient to eliminate the constants from this relation altogether. To do this, we will use the values of the ratio IN=IP at Q ¼ 0 and Q 6¼ 0. We denote the signal in the pure solvent (without quencher) as I0NP ¼

  P IN cN nN kRN pn2 kRP þ knR þ k ¼  P IP 0 cP nP kR pn2 þ kn2 kþ

ð22:48Þ

Proton Transfer Reactions in the Excited Electronic State 491

Then, taking this equation into account, expression (22.45) can be written in a more compact form: I NP P ¼ 1 þ kSV Q I0NP

ð22:49Þ

P ¼ t0P kQP kSV

ð22:50Þ

where

and t0P ¼

1 P þk kRP þ knR 

ð22:51Þ

is the fluorescence lifetime of the product in the solution without quencher (Q ¼ 0). The relation (22.42) is similar to the Stern–Volmer formula, the difference being that, instead of the intensity or quantum yield, the left-hand side contains the relative intensities I NP ¼ IN =IP , and their initial values appear in the denominator rather than in the numerator as in the Stern–Volmer formula (22.43). The coefficient kSV has the physical meaning of the Stern–Volmer constant for diffusion collisions of quencher molecules with product molecules and has already been denoted accordingly. Clearly, the dependence on the quencher concentration will be reliably observed only if the quenching rate constant is comparable with the rate constants of the remaining processes of deactivation of the excited P level see inequality (22.23). 22.4.2 The Stern–Volmer constant for excited states of photoproduct Here, we should make the following important points with respect to the Stern–Volmer constants considered. In the general case, the bimolecular quenching rate constant kQP of the reaction products should not coincide with the similar quenching constant kQN of the excited level of the luminophore, which is found from the Stern–Volmer equation (22.43). More precisely, this applies only to reactions of the kinetic type. In thermodynamic reactions, fast energy exchange will make it impossible to observe these differences, and the constants should coincide. Indeed, as was shown in Figure 22.10 for FA molecules appearing in ESIPT under thermodynamic control, the slopes of INP(Q) dependences are the same for both bands of fluorescence, N and T, which means equality of the Stern–Volmer constants. In contrast to (22.43), the constant kQP cannot be directly calculated from the dependence of the fluorescence intensity of the product on the quencher content. Let us show this. Indeed, the fluorescence intensity of the product is given by IP ¼ cP P* kR hnP

ð22:52Þ

In the simplest case of excitation via the S1 state, the concentration P of excited products is given by the expression P* ¼

kRP

k þ N* þ k þ kQP Q

P þ knR

ð22:53Þ

492 Hydrogen Bonding and Transfer in the Excited State

Upon population of the upper state of the normal state of the solute by radiation, the balance equation yields N* ¼

B12 uex N1 N þk þk Q kRN þ knR þ Q

ð22:54Þ

where uex is the energy density of the excitation radiation, and N1 is the concentration of molecules in the ground state. Then, from (22.52) to (22.54), we obtain I P ¼ cP

fkRN

N þ knR

k þ B12 uex N1 P þ k þ k P Qg kR hnP þ k þ þ kQ QgfkRP þ knR  Q

ð22:55Þ

It can be seen from this expression that the fluorescence intensity IP of the product depends on the quencher concentration in a more complicated way than the fluorescence intensity of the reagent does, and this dependence is affected by the bimolecular quenching rate constants of both the reagent kQN and the product kQP . Let us represent this dependence in a form more convenient for discussion and processing. We denote the fluorescence intensity in the solution without quencher (Q ¼ 0) as IP0 ; then, the intensity ratio can be written as N P þ k þ þ kQ QgfkRP þ knR þ k þ kQP Qg IP0 fkRN þ knR ¼ N þ k gfk P þ k P þ k g IP fkRN þ knR þ  R nR

ð22:56Þ

Further, this expression may be represented in a more compact form: IP0 N P ¼ ð1 þ kSV QÞð1 þ kSV QÞ IP

ð22:57Þ

N P ¼ t0N kQN and kSV ¼ t0P kQP are, as above, the Stern–Volmer constants for the dynamic quenching of where kSV the luminophore and its product respectively. Expression (22.57) is convenient for processing experimental data on quenching and for seeking the Stern–Volmer constant for photoreaction products. To do this, we need only know the corresponding constant for quenching the excited level of the N form of the solute; however, it can be written in a mor˛e convenient and simpler form if we take into account the Stern–Volmer formuła (22.43). Accordingly

IP0 IN0 P ¼ ð1 þ kSV QÞ IP IN

ð22:58Þ

where IN0 =IN is the relative luminescence quenching of the luminophore, which is simultaneously measured in the dual fluorescence spectrum in the course of the same experiment. The obtained expression is practically P identical to expression (22.49), the only difference being that the rate constant kSV is sought now using the intensity ratios IN0 =IN and IP0 =IP separately for each of the detected luminescence bands rather than the change in the ratio I NP =I0NP during quenching. With careful and correct experimental data processing and stability of the sample, formulae (22.49) and (22.58) should yield coinciding results; however, this will be the case only if the measurement procedure is identical to that used upon the detection and recording of the relative signal IN=IP of the two fluorescence bands. When applying such a processing procedure, we use intensities that are measured either simultaneously if the measurements are performed with a high time resolution or with a

Proton Transfer Reactions in the Excited Electronic State 493

minimal delay between recording the second fluorescence band with respect to the first one if the fluorescence spectrum is recorded using equipment operating in the steady-state regime. The separated-in-time measurements of fluorescence bands of luminophore and photoproduct can be incorrect and fraught with errors because of possible effects of photothermal and photochemical decomposition and complexation processes, which almost always occur in solutions of complex organic compounds. Because the effects of all these factors, together with apparatus instability, can be reduced to a minimum or even completely eliminated if the relative (or ratiometric) signal is correctly measured, it seems we can state that data processing with this method is obviously favourable. This means that it is favourable to determine the Stern–Volmer constant using expression (22.49). Note another interesting feature of the study of quenching processes in systems with dual fluorescence that is clearly seen precisely because of expression (22.58). For correct determination of the Stern–Volmer constant for products, it is necessary to take into account the fluorescence quenching of the normal form of the luminophore. However, it is formally seen from (22.58) that, if there is no such quenching, the Stern–Volmer constant can also be correctly determined directly from formula (22.43). Such a case can indeed take place in N experiments with small lifetimes of the luminophore, because, according to the relation kSV ¼ t0N kQN , the Stern–Volmer constant for the excited state of the luminophore will be small. Consequently, at quencher concentrations markedly quenching fluorescence of the product, a decrease in the fluorescence intensity of the luminophore will still not be observed. Physically, this result is clear: if the lifetime is small, the number of diffusion collisions in the excited state between luminophore and quencher molecules is also small. This interesting case allows selective quenching only of the long-wavelength form of fluorescence belonging to the photoproduct. Expression (22.49) was written for a rather general case where fluorescence can be excited via an arbitrary singlet state. It is clear that the physical microcharacteristics of solvates (local temperature, viscosity, etc.) experiencing non-radiative relaxation Sn ! S1 will noticeably differ from the characteristics of solvates directly excited to the S1 state before a spontaneous emission event. From this it immediately follows that the Stern–Volmer constants should depend on the energy excess released in the solvate before fluorescence; i.e. they should depend on the excitation wavelength of the luminophore. The corresponding experimental evidence for this conclusion will be presented in the experimental section. The second useful consequence of equation (22.45), which opens up new possibilities, will only manifest itself upon excitation via a high-lying singlet state Sn. This implies that, according to (22.45), the ratio IN=IP is related to the product formation process via the excited Sn level, which is described by the constant kn2. The latter reaction contributes to this ratio upon excitation via higher states. This dependence allows us to estimate the rates kn2 of direct photoreactions via the SN n state using experimental data on the intensity ratio IN=IP for the two fluorescence bands that are obtained at different excitation frequencies. In this case there is no need to take into account different sensitivities of the measurement equipment for the frequencies at which the fluorescence of the N and P forms was measured; i.e. there is no need to take into account differences between sensitivity coefficients cN and cP, which can differ considerably when the absolute values of the measured signals are used. Clearly, the constant kn2 will significantly affect the ratio IN=IP only if it is comparable with the rate of interconversion: pn2 kn2

ð22:59Þ

In this case, we can take into account the contribution of higher states to photoreactions that proceed through the SN n state if experimental data on the intensity ratio IN=IP for the two fluorescence bands are obtained. It can be seen from expression (22.45) that the photoproduct yield is directly affected by the application of a quencher. If all the remaining parameters do not change, the addition of a quenching impurity increases the ratio IN=IP upon excitation in any absorption band. However, in principle, the quencher can also affect the

494

Hydrogen Bonding and Transfer in the Excited State

probabilities of reactions of formation of the product that proceed from the S1 state (as will be shown in the next section) and from the Sn state (the probability pn2). We can state that the proposed general description of the contribution of high-lying states to photoreactions covers a large body of physicochemical processes that proceed after the singlet excitation. These reactions include the formation of excimers and exciplexes (classical examples are pyrene in non-polar solvents and anthracene with dimethylaniline [107]), electron transfer (a strong electron density redistribution) in the first excited singlet state of complex polyatomic molecules (p-dimethylaminobenzonitrile and its derivatives [73, 74, 108]), internal proton transfer (molecules of the 3-hydroxyflavone and 3-hydroxychromone families) and hydrogen bonding. It is likely that the formation of a triplet metastable state of complex molecules and a number of other classical reactions, such as, for example, cis–trans isomerization, can also be attributed to these reactions. The conclusions formulated were used in processing the data that were obtained in experiments with solutions of 3-hydroxyflavone molecules, in the excited state of which reactions of internal proton transfer are observed upon excitation of fluorescence via different singlet states. 22.4.3 Dynamic quenching of fluorescence under different physical conditions 22.4.3.1 Experimental Spectra of absorption of a 3HF solution in acetonitrile for different temperatures in the range 20–80  C are presented in Figure 22.12. As can be seen, the absorption spectrum consist of two intense electronic bands peaked at about 340 and 300 nm, assigned to the S0 ! S1 and S0 ! S2 singlet bands respectively. It is also found that the TEMPO quencher, in contrast to the ionic quencher KJ [109], does not affect the intrinsic absorption of the 3HF molecules [110]. It is evident that absorption changes with temperature, and the coefficient of absorption in maxima of both bands decreases by about 20% with heating of the solution to 80  C, but the contour of the bands reveals no noticeable changes while heating. The values of the absorption coefficient changes on the wavelength of excitation (at the maximum of the band) are presented in Table 22.2. In Refs [109] and [110] the fluorescence, absorption and excitation spectra of 3HF in solutions with the addition of the classical quencher potassium iodide KJ and the spin quencher 4-oxo-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) with different concentrations were studied in the UV and visible regions upon 20C

1,2

60C

T

80C

absorbance

1,0 0,8 0,6 0,4 0,2 0,0 250

300

350

400

450

500

λ /nm

Figure 22.12 Absorption spectra of 3HF in acetonitrile at different temperatures 20, 60 and 80  C. The arrow shows the direction of absorption change with temperature

Proton Transfer Reactions in the Excited Electronic State 495

excitation into the main absorption band. It was found that the intensities and quantum yields of both bands have rather complex dependences on the quencher concentration, the quenching is stronger for the longwavelength fluorescence band, which belongs to the tautomeric form, and the quenching laws are considerably different for the two quenchers. In this section, we present the properties of the dual fluorescence of 3HF in acetonitrile under conditions of dynamic quenching by the spin quencher TEMPO in the temperature region from 20 to 80  C. Selective excitation at wavelengths of 290, 304 and 340 nm, which lie in different absorption bands of the luminophore, were used. The temperature dependences of the fluorescence intensity in the maxima of the two bands for solutions in pure acetonitrile and in acetonitrile with the addition of TEMPO are shown in Figures 22.13 and 22.14. As can be seen from comparison of these two figures, the behaviour of the fluorescence intensity with increasing temperature is considerably different for the two bands. The fluorescence of the normal form is characterized 25

2

20

IN /a.u.

6

15 4

10

1 3

5

5

0 15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

T / °C

Figure 22.13 Intensity at the maximum of the normal fluorescence band (395 nm) of the 3HF solution in (1, 2, 4) pure acetonitrile and (3, 5, 6) acetonitrile with the addition of TEMPO as a function of the solution temperature for excitation wavelengths of (1, 5) 290 nm, (3, 4) 305 nm and (2, 6) 340 nm. Reprinted with kind permission from Springer Science and Business Media 1000

I T /a.u.

800

600

3

6

400 2

200

5 1

4

0 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

T / °C

Figure 22.14 Intensity at the maximum of the tautomer fluorescence band (525 nm) of the 3HF solution in (1, 2, 3) pure acetonitrile and (4, 5, 6) acetonitrile with the addition of TEMPO as a function of the solution temperature for excitation wavelengths of (1, 4) 290 nm, (2, 5) 305 nm and (3, 6) 340 nm. Reprinted with kind permission from Springer Science and Business Media

496 Hydrogen Bonding and Transfer in the Excited State

by a decrease with shift of the excitation wavelength from 340 to 290 nm. As can be seen from Figure 22.13, at the standard excitation in the region of the first absorption band (at a wavelength of 340 nm), the intensity of the short-wavelength fluorescence band continuously decreases with heating, this decrease occurring somewhat more slowly in the temperature region from 50 to 70  C (curve 2). The addition of a quencher causes a stronger fluorescence quenching at all temperatures, which is most pronounced at higher temperatures (curve 6). In particular, at room temperature the decrease in the fluorescence intensity caused by the quencher corresponds to 0.6 arbitrary units on the scale of Figure 22.13, while at 80  C this decrease reaches 5.6 arbitrary units. The behaviour of the fluorescence excited at a wavelength of 305 nm (curve 4) is unusual. In this case the fluorescence is almost not quenched with temperature and even increases in the range from 50 to 80  C. The addition of TEMPO strongly changes the temperature dependence of the fluorescence, which becomes almost independent of temperature in the entire region (curve 3). An even stronger enhancement of fluorescence with temperature is observed upon excitation at a wavelength of 290 nm (curve 1), when the fluorescence increases more than twofold at the maximum temperature of the experiment (80  C). At this excitation wavelength, the addition of the quencher causes an even more pronounced reduction in the fluorescence than in the previous case, and the fluorescence intensity shows only a very slight increase with temperature (curve 5). Let us now consider the temperature dependences of the tautomeric fluorescence at a wavelength of 525 nm (Figure 22.14). Similarly to the case of the short-wavelength fluorescence, the efficiency of the tautomeric fluorescence decreases as the excitation wavelength shifts from 340 to 290 nm. The maximum fluorescence signal is recorded for the excitation at a wavelength of 340 nm (curve 3) and rapidly drops with temperature. The addition of the quencher considerably decreases the fluorescence intensity in the entire temperature region. A similar behaviour of fluorescence is also observed in the case of excitation at 304 and 290 nm; the fluorescence intensity decreases with temperature in the absence of the quencher (curves 2 and 1) and the addition of TEMPO efficiently quenches the fluorescence at all temperatures (curves 5 and 4). Thus, a comparison of Figures 22.13 and 22.14 shows that the green fluorescence band is more strongly reduced by the quencher than the violet band, i.e. the tautomeric band is more sensitive to the action of quenching impurities. It should be noted that the profiles and positions of the N and T fluorescence bands do not noticeably change in the process of quenching with all of the TEMPO concentrations used. The dependences of the intensity ratio of the fluorescence in the maxima of the N and T bands, IN/IT, on the presence of TEMPO are shown in Figure 22.15. The curves are plotted using the data shown in Figures 22.13 and 22.14. As can be seen from Figure 22.15, the parameter IN/IT grows with heating of the solution in all cases, but its sensitivity to temperature strongly depends on the excitation wavelength. Higher sensitivity is observed for excitation at the shortest wavelength (290 nm, curve 7), when the intensity ratio increases from 0.018 to 0.132, while this parameter in the case of excitation at 340 nm (curve 5) increases from the same 0.018 to 0.06. Further, we can see that the addition of the quencher always decreases the IN/IT parameter, most strongly in the case of excitation at 290 nm (curve 2). Thus, we can conclude that the intensity ratio is most sensitive to the action of the quencher at a temperature of 80  C and with excitation at a wavelength of 290 nm. 22.4.3.2 Discussion of Temperature Quenching Some impurities in a solution can lead to a reduction in the spontaneous emission of the luminophore, i.e. to fluorescence quenching. The quenching occurs as a result of intermolecular interactions and can be of the first or second kind [73, 99, 111]. Quenching of the first kind, or static quenching, is a process related to the action on the molecules in the ground state. In fact, in this case we deal with complexation involving the ground state of the dye. In the case of quenching of the second kind, the impurity interacts with the fluorophore in the excited state, which is the principal difference between the two kinds of quenching. In the second case, the fluorescence yield is decreased owing to the non-radiative deactivation of excited molecules, which can occur via different channels, namely by energy transfer from the excited fluorophore molecules either to

Proton Transfer Reactions in the Excited Electronic State 497 0,140 1 0,120 2

IN / IT

0,100

3 6 5 4 3 2 1

0,080 0,060

4 5 6

0,040 0,020 0,000 15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

T / °C

Figure 22.15 Intensity ratio of the violet and green fluorescence bands IN/IT of 3HF molecules in neat acetonitrile (1, 3, 5) and with the addition of TEMPO (2, 4, 6) as a function of the solution temperature for excitation wavelengths of (1, 2) 290 nm, (3, 4) 305 nm and (5, 6) 340 nm. Reprinted with kind permission from Springer Science and Business Media

unexcited molecules or to quencher particles, where the energy is converted to the thermal energy of excited vibrations. In the case of dynamic quenching, which is a pure physical process, the absorption and fluorescence spectra do not change, while the fluorescence quantum yield decreases in parallel with a shortening of the excited-state lifetime [73, 99, 111, 112]. The absorption spectra of 3HF in acetonitrile consist of two intense S1 and S2 electronic bands peaked at about 340 and 300 nm (Figure 22.12). It was also found that the TEMPO quencher does not affect the intrinsic absorption of the 3HF molecules in the vicinity of these bands [110]. Let us now turn to the interpretation of the temperature dependences of the fluorescence spectra of solutions with the quencher. The fluorescence quenching in neat solvents without impurities with increasing temperature is a conventional effect and is explained by non-radiative deactivation of the excited states by the solvent [74, 99, 112]. Such a behaviour is observed for the short-wavelength fluorescence band upon excitation to the first absorption band of the 3HF molecule at a wavelength of 340 nm. The addition of the quencher accelerates the quenching at all temperatures, because in this case the non-radiative relaxation of the probe molecule occurs owing to diffusion interactions with both solvent and TEMPO molecules (curves 2 and 6 in Figure 22.13). In the case of excitation in the region of the second absorption band, at 305 and 290 nm, we observe a different pattern; in particular, the fluorescence of the normal form in the absence of the quencher is enhanced with heating of the solution. In our opinion, such an anomaly can be explained by the stimulation of the reverse proton transfer (the rate constant k in Figure 22.11), i.e. of the reverse transitions from the SP1 to the SN 1 excited state under the action of heat released owing to the S2 ! S1 relaxation after the absorption of a photon with frequency n0 ex (Figure 22.11) in the 3HF solvate before the emission event (this process is similar to the thermal activation of the delayed fluorescence of organic solutions [73, 99, 112]). The specific features listed above are not observed in the fluorescence of the tautomeric form. In this case, the band intensity behaves like ordinary temperature quenching for all three excitation wavelengths, and TEMPO efficiently quenches the fluorescence in all cases. In this connection, it is natural to expect that the intensity ratio of the violet and green bands, IN/IT, should strongly depend on temperature. The fluorescence quenching with temperature is mainly an intramolecular process and occurs in an excited state, which is directly evidenced by parallel changes in the quantum yield and the excited-state lifetime [73, 74, 99, 112]. Similar fluorescence quenching also exists in rarefied vapours of aromatic compounds, i.e. in the complete

498

Hydrogen Bonding and Transfer in the Excited State

absence of any intermolecular interactions in an excited state. It is clear that the effect of a solvent is considerable but does not determine the nature of the process. The role of the solvent consists in a specific kind of quenching, quenching by a solvent, which can depend on temperature and thus complicate the process of quenching. Usually, the fluorescence intensity in moderately viscous solutions decreases almost twofold with heating from 20 to 70  C, which corresponds to the case observed in our experiment. Indeed, as can be seen from Figure 22.15, the parameter IN/IT increases with temperature in all cases and is almost linear for excitation in the region of the first absorption band (340 nm). For excitation in the region of the second absorption band (at 305 nm and shorter), the increase in the ratio IN/IT at temperatures above 50  C is steeper than linear. The strongest increase is characteristic for lex ¼ 290 nm. It was proposed in Ref. [113] to use the temperature dependences of the fluorescence intensity ratio for absolute measurements of the temperature of the environment of 3HF molecules in the sample. Obviously, the highest accuracy of such measurements will be achieved in the region of higher temperatures. The addition of a quencher decreases the sensitivity of the IN/IT parameter to temperature, which can be used to reveal the occurrence of quenching by foreign impurities. The data presented allow us to conclude that these dependences can be used for measuring absolute temperatures of the environment of 3HF molecules. Obviously, the best accuracy of such measurements will be achieved in the region of higher temperatures. Taking into account the measurement error of the fluorescence intensity of the two bands determined by the Hitachi F-2500 spectrofluorimeter used, we estimate that, without special measures, the measurement error in the case of a sufficiently intense single signal may be 3–5%. For a series of 20 measurements, this accuracy is higher and depends on the band; in this case, the rms deviation can be as low as 1–2%. The determination errors of real temperature owing to the error in the measured intensity ratios are shown in Figure 22.16 for a solution of 3HF in methanol; this solution was chosen because in this solvent the sensitivity of the ratio IN/IT to temperature is higher. All the estimations are fulfilled for an excitation wavelength of 280 nm, at which the sensitivity of the relative signal to the solution temperature is maximum. As can be seen from Figure 22.16, if the error in the ratio IN/IT is 5%, a sufficiently low error in temperature (of about 2%) is achieved only at high temperatures (60–70  C), whereas, if the error in the intensity ratio is about 2%, the temperature is determined with an error of 3% even in the region of room temperature. With increasing T, the accuracy continuously improves and DT decreases to 0.5%. Such accuracy is rather high; hence, the

6 3

Δ t /°C

4

2 2 1 0 10

20

30

40

50

60

70

t /°C

Figure 22.16 Temperature dependences of the absolute error DT in the measurement of the temperature of the 3HF solution in methanol for lex ¼ 280 nm and the errors in the intensity ratio D(IN/IT) ¼ (1) 0.5, (2) 2 and (3) 5%

Proton Transfer Reactions in the Excited Electronic State 499

dependences shown in Figure 22.16 can be used for non-contact determination of the temperature of samples from the measured intensity ratio of the two fluorescence bands of molecular probes with ESIPT. We believe that the accuracy of determination of temperature can be considerably improved if the operation of the spectrofluorimeter is well stabilized. In Refs [105] and [114] we considered the behaviour of the IN/IT ratio as a function of the TEMPO concentration and showed that this ratio is always higher than in the pure solution and monotonically increases almost by an order of magnitude with increasing TEMPO concentration. Based on these experiments, we concluded that the fluorescence intensity ratio depends on the presence of a quencher in the solution and is rather sensitive to its concentration. Here it can be seen that this sensitivity grows dramatically with temperature. It was shown in Section 22.3 that an increase in the IN/IT ratio with quencher concentration indicates that the PT in the solution of 3HF in acetonitrile occurs according to the kinetic model. This means that the reverse PT constant k is small in comparison with the direct transition probability k þ ; i.e. the SP1 energy level lies significantly lower than the SN 1 level (Figure 22.11). It can now be added that the increase in the intensity ratio with temperature also points to the kinetic character of the reaction, as temperature quenching leads to the same influence on the ratiometric parameter as the quencher. Hence, temperature quenching of dual fluorescence also reveals the character of reaction and can be used for this purpose. An increase in the ratio IN/IT with T and with concentration of quencher evidences the kinetic character of the photoreaction, in full agreement with the conclusions of Section 22.3. Consequently, the PT has a kinetic character under all excitation conditions represented in Figure 22.15. Finally, one more important circumstance is that the visually observed colour of the dual fluorescence depends on the intensity ratio of the two bands. Thus, one can anticipate that the temperature or other quenching factors can be determined using simple visual or photographic recording of the fluorescence after preliminary calibration of its colour. 22.4.4 Stern–Volmer constants for 3-hydroxyflavone under different excitation conditions Since the absorption of the solution almost does not change with the addition of a quencher [110], the dependences shown in Figures 22.13 and 22.14 reflect the behaviour of the quantum yields of the corresponding fluorescence bands, and hence the Stern–Volmer constants kSV can be estimated by the Stern–Volmer formula (22.43) on the assumption of the diffusion mechanism of bimolecular collisions. Thus, calculated Stern–Volmer constants for the two fluorescence bands under different excitation conditions and temperatures are listed in Table 22.1. In the calculations, we used expression (22.43) directly applied to the tautomer fluorescence. Note, first of all, that heating of the solution leads to an increase in the constants kSV in most cases, excluding only the case of excitation at a wavelength of 290 nm in the temperature range from 30 to 60  C. At the standard excitation at 340 nm, the quenching constants for the normal fluorescence band increase from 13.3 M1 at room temperature to 88 M1 at T ¼ 80  C. For the tautomeric band, these constants are considerably higher at room temperature and increase with heating within the same temperature range from 30.7 to 52.2 M1. Such values of kSV and their temperature enlargement agree with the data usually obtained on the assumption of a collisional mechanism of quenching of excited states [73, 74, 99], taking into account that the diffusion coefficient increases with temperature. Let us consider the Stern–Volmer constants calculated for a solution of 3-hydroxyflavone under different conditions, or, more precisely, at different excitation frequencies. Upon the excitation of a molecule and after its subsequent relaxation, certain excess energy is always released into the environment, which causes heating of solvates. The instantaneous heat release depends on the excitation frequency and can be rather significant (30–50  C). However, as this thermal energy excess is rapidly dissipated within a time period much shorter than the lifetime of the excited state, it is hardly manifested at all in the spectra. Therefore, this excess energy and its effect on the environment can be taken into account only for such characteristics of the solvate that

500 Hydrogen Bonding and Transfer in the Excited State Table 22.1 Stern–Volmer quenching constants (in M1) for the fluorescence of 3HF (c ¼ 5  105 M) in acetonitrile, quenched by TEMPO for different temperatures T and fluorescence excitation wavelengths lex (cTEMPO ¼ 5  103 M). Reprinted with kind permission from Springer Science and Business Media Form

N T N T N T

T,  C

lex, nm

290 290 305 305 340 340

20

30

40

50

60

70

80

430. 387. 61.6 65.9 13.3 30.7

355.5 439.8 80.6 98.7 27.2 43.0

447.5 409.7 101.9 86.7 34.1 34.5

351.5 414.6 74.8 88.5 46.8 37.6

756.8 458.7 229.8 109.9 87.9 58.8

526.5 475.6 184.6 107.2 92.5 53.9

595.8 478.0 249.2 109.8 87.6 52.2

retain the memory of this excess for longer. An elevated mobility of molecules in solvates that have absorbed large energy will be retained longer than the heat release from the luminophore to the solvate shell (these are time intervals comparable with vibrational periods of molecules). Therefore, one should expect an increase in the diffusion coefficient in a solvate of a luminophore absorbing light with a sufficiently large heat release increasing with excitation frequency. This effect can be detected fairly easily by comparing the Stern–Volmer constants obtained upon excitation of different Sn states. Processing the data on fluorescence of 3-hydroxyflavone with the addition of this quencher to the solution allowed us to determine the Stern–Volmer constants in different excitation conditions. Analysis of the data in Table 22.1 reveals a very important feature, namely that these constants strongly increase with energy of the level to be excited, i.e. with decreasing wavelength of exciting light. As can be seen from the table, upon excitation in the range of the second absorption band (305 nm), the Stern–Volmer constant acquires a value of 61.6 M1 at T ¼ 20  C and continuously increases to reach a value of 249 M1 at 80  C, in comparison with values of 13.3 M1 and 87 M1 for the same temperatures, respectively, upon excitation in the main band. Finally, upon excitation by radiation with the shortest wavelength used in the experiment, 290 nm, which corresponds to the edge of the S2 absorption band, the value of the Stern–Volmer constant for the N band increases to 430 M1 at room temperature, and, as the temperature of the solution increases to 80  C, it reaches a value of 596 M1. To explain this behaviour, let us consider in more detail the definition and the physical meaning of the Stern–Volmer constant. According to the classical notions, this constant characterizes the number of contacts of a molecule residing in an excited state with molecules of a quencher per unit time. During such a contact, the luminophore molecule loses its excitation energy and passes to the ground state. The energy transferred to the quencher is non-radiatively dissipated in the solution. The temperature dependence of the constant N kSV ¼ t0N kQN , and therefore of the constant of bimolecular collisions kQN , is primarily determined by the temperature dependence of the diffusion coefficient, which is defined as [73, 74] D¼

kT 6phðTÞR

ð22:60Þ

Here, T is the temperature, R is the collisional radius, which is usually assumed to be the sum of the radii of the luminophore and quencher molecules, and h(T ) is the viscosity of the medium, which also strongly depends on temperature for many liquid solvents (usually, it decreases with increasing T ). It can be seen from equation (22.60) that the diffusion coefficient should strongly increase with temperature, more rapidly than

Proton Transfer Reactions in the Excited Electronic State 501

according to a linear law. Therefore, under steady-state detection conditions, particles of the solution should rather markedly respond to any energy excess in the molecular motion in the solution (owing to either temperature increase or the excitation energy excess); in turn, this energy excess should strongly affect the collision frequency between luminophore molecules and other particles in the solution. For completeness, we note that the lifetime of the excited state can also vary with temperature. From the data of Table 22.1 we can estimate the bimolecular quenching rate constant kQ, or the rate constant of bimolecular collisions, at different temperatures, based on its relation to the Stern–Volmer constant N according to the expression kSV ¼ t0N kQN . The lifetime of the normal form of 3-hydroxyflavone is no longer than 0.1 ns [10, 102], and then, at room temperature, kQ is 13.3  1010 L M1 s1. It is natural to assume that the lifetime will remain practically the same upon excitation of high-lying absorption bands. Then, for excitation in the range of the second band, this constant is already greater by more than an order of magnitude, kQ ¼ 6.16  1011 L M1 s1. The passage to an even shorter-wavelength excitation yields an even greater value of this constant at room temperature, 4.3  1012 L M1 s1, which increases with T further to 6  1012 L M1 s1. Therefore, there is direct evidence of the effect of the heat released as a result of non-radiative transitions on the dynamics of molecules in the nearest environment of the luminophore during the lifetime of the solute. In principle, such data make it possible to calculate microlocal values of the diffusion coefficients using the diffusion theory of bimolecular collisions. 22.4.5 Temperature effect on proton transfer Processing the set of experimental data presented in Figures 22.13 and 22.14 makes it is possible not only to calculate the Stern–Volmer constants but also to obtain more interesting information concerning the proton transfer rate k þ and its temperature dependence. For such study we will use the results of the theory presented early in this section, experimental dependences of fluorescence intensities of both forms of 3HF, photoproduct lifetimes/P and absorption coefficient ka [114]. The set of all these data for excitation of solute in the main band of absorption at 340 nm is presented in Table 22.2. The values of the ratio IN =IP are seen to increase essentially with heating both in neat (from 0.023 to 0.062) and quenched (from 0.025 to 0.054) solutions, i.e. we have the function IINP ðTÞ. At the same time, the lifetime of the fluorescing state of the tautomer tP drops from 0.9 ns at room temperature to 0.37 ns at 60  C (see Table 22.2). However, taking into account such decrease in tP with temperature, the dependence IINP ðTÞ can only partially be explained by equation (22.47). This feature can be explained better if we additionally assume that the rate constant k þ of direct proton transfer depends on temperature, i.e. k þ (T). Relations (22.46) and (22.47) were derived without invoking such an assumption, because, at present, there is no experimental evidence to support it. The remaining channels for the influence of the quencher on the photophysics of processes (see the scheme in Figure 22.11) – increase in the probability of non-radiative transitions for the normal form and for the product (the probabilities kQN Q and kQP Q) – were already taken into account upon derivation of formula (22.47). Therefore, in view of the assumed dependence k þ (T), the intensity ratio given by formula (4.29) is rewritten for two arbitrary temperatures T1 and T2 as   P þ k þ kQP Qg IN cN nN kRN fkRP þ knR  T1 ¼ ðk þ ÞT1 IP cP nP kRP

ð22:61Þ

  P þ k þ kQP Qg IN cN nN kRN fkRP þ knR ¼  IP T2 cP nP kRP ðk þ ÞT2

ð22:62Þ

502 Hydrogen Bonding and Transfer in the Excited State Table 22.2 Temperature and dynamic quenching of the excited-state effect on the proton transfer rates in a solution of 3HF in acetonitrile (5  105 M), lex ¼ 340 nm. CTEMPO ¼ 5  103 M. IN =ka is the fluorescence intensity at the maximum of the violet band of fluorescence, normalized on absorption coefficient ka; t P is the lifetime of the green band of fluorescence; IN =IP is the ratio of fluorescence intensities of the N and T forms in solution with TEMPO; ðIN =IP Þ0 is the ratio of fluorescence intensities of the N and T forms in neat solution; kSV is the Stern–Volmer constant calculated by formula (22.43) for the violet band of fluorescence; ðk þ ÞT2 =ðk þ ÞT1 is the ratio of proton transfer rates at temperature T2 and at room temperature T1; k þ ðQÞ=k0þ is the relative change in the proton transfer rate with the addition of quencher to the solution at temperature T Temperature,  C

Parameters 20 IN/ka

18

IQ N

22

30

40

50

17.4 18

16.5

60

70

18 15

18.4 12.5

12

tT (ns)

0.9

IN =IP

0.025

0.028

0.032

0.035

0.040

0.046

0.054

ðIN =IP Þ0

0.023

0.027

0.032

0.037

0.045

0.053

0.062

kSV (M1)

13

ðk þ ÞT2 =ðk þ ÞT1

1.0

k þ ðQÞ=k0þ

1.16

06

13

80

27

34

0.37

46

1.16 1.1

1.17

88

92

88

1.24 1.3

1.62

1.68

1.65

By dividing expression (22.62) by expression (22.61), we can determine the expression for the relative change in the rate constant of the proton transfer as follows: ðIN =IP ÞT1 tP2 ðk þ ÞT2 ¼ ðIN =IP ÞT2 tP1 ðk þ ÞT1

ð22:63Þ

where tP1 and tP2 are the lifetimes for temperatures T1 and T2 respectively; they are determined by expression (22.42) and are presented in Table 22.2. The numerator and denominator on the left-hand side of this equality contain the intensity ratios of the two fluorescence bands that can be experimentally determined for the two different temperatures. The expression allows us to find the relative change in the proton transfer rate with temperature. The results are presented in Table 22.2 by the values of the ratio ðk þ ÞT2 =ðk þ ÞT1 (where T1 is room temperature and temperature T2 is varied from 20 to 80  C). It can be seen that, in the temperature range 20–60  C, this ratio increases from 1.16 to 1.24. The variation in this ratio is minimum at the beginning of this temperature range and maximum in its end. From the behaviour of this dependence it follows that, in a first approximation at room temperature, we can ignore this dependence, while at elevated temperatures it is necessary to introduce the corresponding correction. The mechanisms of the PT are ruled both by intramolecular processes in the dye molecule and by intermolecular interactions with the environment, leading to perturbations of H-bonding between active groups. A change in temperature affects these processes (a distribution of molecules over vibrational degrees of freedom, intermolecular interactions and relaxation processes), and they, in turn, influence the rates of the PT. Therefore, it is reasonable to suppose a dependence of the PT on these factors for interpretation of experimental data. Therefore, our assumption that the proton transfer rate constant k þ depends on temperature

Proton Transfer Reactions in the Excited Electronic State 503

is very plausible. Exact data on constants k þ at different temperatures can be independently obtained by the methods of kinetic spectroscopy with a subpicosecond resolution by measuring photoproduct fluorescence arising under sufficiently fast excitation of the normal form of the luminophore. The Stern–Volmer constant of the tautomeric band calculated directly by formula (22.43) also increases upon heating (Table 22.1). As the temperature was increased from 20 to 80  C, this constant increased from 20.7 to 52 M1 at lex ¼ 340 nm. We have shown already in this section that such a calculation is incorrect and that this constant should be determined by formula (22.58) using the values of the ratio IN/IP in the pure solvent and in the solvent with quencher. Furthermore, expression (22.58) can be used only if the intensity ratios for the quenched and pure solvents obey the inequality IN > IP

  IN IP 0

ð22:64Þ

The values of the ratio IN =IP , also given in Table 22.2, always increase with heating of the solution; however, inequality (22.64) holds only for temperatures below 40  C. For room temperature we obtained values of the Stern–Volmer constant of 13.3 and 30.7 M1 for the N and T forms respectively. A comparison of these values shows that the constant for the tautomer is significantly (almost by a factor 2.3) larger, whereas the same quenching constants determined from the concentration dependence of IN =IP according to equation (22.49) is 17 M1. Hence, we see that the difference between the constants is very large. In Ref. [109] the same calculations are made for 3HF in acetonitrile with KJ as quencher, and, again, the results demonstrate the difference for constants obtained from formulae (22.43) and (22.49). 22.4.6 Effect of a quencher on the proton transfer rate Another important inference that follows from the analysis of the data in Table 22.2 is that the occurrence of a quencher should affect the reaction rate of proton transfer. At room temperature, the quencher increases the ratio IN/IP from 0.023 to 0.025; at T ¼ 30  C, this ratio increases from 0.027 to 0.028; at 40  C, no increase is observed. Beginning at 40  C, the introduction of the quencher causes this ratio to decrease rather than increase. It is logical to assume that, as in the preceding case, the quencher affects the establishment of the proton transfer reaction constant because quencher molecules (as well as solvent molecules) are in direct collisional contacts with excited luminophore molecules. As is known, the intensity ratio IN/IP of the two bands strongly depends on the polarity and other characteristics of a solvent. It is natural to expect that quencher molecules will also affect the ratio. Quencher molecules much more efficiently quench excited states than solvent molecules do; one such quenching channel may be non-radiative transition with the rate constant k þ . Therefore, we can tentatively assume that this constant is a function not only of temperature but also of the quencher concentration. Accordingly, we assume that, as a first approximation, the rate constant k þ of proton transfer is a linear function of Q: k þ ðQÞ ¼ k0þ þ kQþ Q

ð22:65Þ

where kQþ is the rate constant of bimolecular transitions with proton transfer that occur under the action of the quencher. In view of this relation, formula (4.31) can be written as k0þ I NP P ð1 þ kSV ¼ QÞ I0NP k þ ðQÞ

ð22:66Þ

504 Hydrogen Bonding and Transfer in the Excited State

This expression can be used if the dependence k þ ðQÞ is known. For estimation, we can assume as a first P N approximation that kSV  kSV  kSV (if the properties of the luminophore and product do not differ strongly). In this expression, the ratio I NP =I0NP can be determined from the experimental data, and the Stern–Volmer P constant kSV can be taken from Table 22.2. The relative values of k þ ðQÞ=k0þ determined in this way are given in Table 22.2. It can be seen from Table 22.2 that values of k þ ðQÞ=k0þ increase with temperature from 1.16 to 1.65 in the temperature range 20–80  C, which is equivalent to increasing the rate of PT by an appropriate amount. Up to T ¼ 50  C this dependence is weak, the ratio is 1.3, later on it grows sharply to 1.6 and then, with increase in T, it behaves as in a saturation regime. Therefore, for 3HF molecules, the ratio k þ ðQÞ=k0þ should be taken into account at temperatures higher than 40  C, but at room temperature it is possible to apply expression (22.49). Thus, if we take into account the quencher effect on the rate of PT, expression (22.45) for relative intensities should be written as P þ k þ kQP Qg IN cN nN kRN ðpn2 þ k tP kn2 ÞfkRP þ knR ¼ IP cP nP kRP k þ ðQÞðpn2 þ kn2 Þ

ð22:67Þ

Note that all expressions discussed herein are written for the general case where the photoreactions are excited via an arbitrary singlet state. If we apply them upon excitation in the main band of absorption, when there is no creation of product through the SN n state, then it is necessary to insert into the above equations a rate kn2 equal to zero. Therefore, the following conclusion can be drawn from our calculations: when the solution is heated, the rate of the PT may grow, which obviously must be taken into account when determining molecular parameters using signals of dual fluorescence.

22.4.7 Separation of a weak fluorescence signal of 3-hydroxyflavone using dynamic quenching In this subsection, for the example of a 3HF solution, we show that the method of dynamic fluorescence quenching can be used for probes with dual fluorescence when the spectrum of one of the two bands (of the normal form in the case of 3HF) used for the measurement of the intensity ratio is overlapped by an unwanted signal that appears during experiment owing to a change in the physicochemical conditions in the sample [115]. To demonstrate this possibility, we will use the spectral and time characteristics of the dual fluorescence of a 3hydroxyflavone probe in a solution with a fluorescence quencher in the temperature range 20–80  C. At room temperature, upon excitation in the main band of absorption at 340 nm, the fluorescence consists of two bands that belong to the normal and tautomer forms of the luminophore (see Figure 22.8), while heating of the solution results in the appearance of an additional fluorescence band belonging to the anionic form of the luminophore (see below). The additional band is strongly overlapped by the band of normal form, and its intensity rapidly increases with temperature to exceed the intensity of the normal band. A contribution from the A form depends on the wavelength of excitation, as the latter and the N form have different spectra of excitation. The introduction of the TEMPO spin quencher of excited states into the solution completely quenches the fluorescence of the anionic form, which makes it possible to record the pure dual fluorescence of 3-hydroxyflavone in the entire temperature range studied. The fluorescence of all three known forms of 3HF and the proton transfer (PT) reaction are described by the energy level diagram shown in Section 22.2 (Figure 22.2). The diagram reflects the existence of the following six energy levels: the ground N and excited N levels of the normal form (N), the excited T and ground T levels of the PT form or tautomer (T), and the ground A and excited A levels of the anionic (A) form. An important property of 3HF is its ability to respond to various intermolecular interactions by changing the main

Proton Transfer Reactions in the Excited Electronic State 505

characteristics of the two fluorescence bands (Section 22.2). A valuable feature of ESIPT systems is the possibility of self-calibration of the luminescence response because the environment causes a change in the intensity ratio of the two bands, owing to which these probes are also called ratiometric. The possibility of selfcalibration considerably simplifies absolute measurements using such probes and increases the sensitivity of the method. The existence of a signal overlapping with the signal of the normal form (as, for example, the signal of the anionic form in our case) will reduce the accuracy of the measurement of the intensity ratio of the normal and tautomeric forms. The detection of probe signals in pure form is important for applications of proton transfer molecular probes using the intensity ratio of the fluorescence bands as the main sensitive parameter. Hence, it is also important to have methods and approaches that make it possible correctly to take into account this circumstance. Figure 22.17 shows the fluorescence spectra of 3HF in neat acetonitrile (a) and acetonitrile with the addition of 5  103 M of the TEMPO quencher (b) measured upon UV irradiation at a wavelength of 340 nm at temperatures of 20, 50 and 80  C. Vertical arrows show the direction of change in the fluorescence intensity with increasing temperature. At room temperature, the fluorescence spectrum consists of a short-wavelength violet band of the N form peaked near 390 nm and a more intense green band of tautomer with the maximum at about 525 nm. An increase in temperature to 50  C is accompanied with the appearance of one more band near 450 nm, which overlaps with the fluorescence bands of the normal and tautomer forms. With increasing temperature, the intensity of this band increases and, at a temperature of 80  C, noticeably exceeds the fluorescence intensity of the normal form. This fluorescence was studied previously and was found to belong to the anionic form of the luminophore, which is formed in the ground state owing to the detachment of a proton from the hydroxyl group and its transfer to the solvent (reaction (22.1), Section 22.2). The characteristics of this fluorescence band are clearly seen in Figure 22.3, which presents the decomposition of the complex fluorescence spectrum into three components. The average lifetime of the A form in acetonitrile is equal to 3.7 0.2 ns. The lifetime of the normal form is as short as 60 ps (probably limited by the time resolution of the apparatus) [81]. Measurements using femtosecond flash photolysis in highly purified acetonitrile showed the existence of two time constants (35 fs and 5 ps), which characterize the times of the PT from the carboxyl to the carbonyl group of the molecule [84, 85b]. The lifetimes of the tautomer of 3HF are longer, about 0.9 ns, the same value as that obtained in Ref. [116]. As can be seen from Figure 22.17(b), the addition of TEMPO at a small concentration (5  103 M) to the solution completely quenches the emission of the A form, which does not appear even at the highest temperature used (80  C). At the same time, the main fluorescence bands change only slightly with the introduction of TEMPO (the intensity ratio IN =IP increases from 0.024 to 0.026). This result is physically understandable because the lifetime of the normal form is very short and the Stern–Volmer quenching constants for the A form are considerably higher than for the N form. Hence, solutions with the quencher allow us to achieve a better accuracy in recording the fluorescence of the N form and measuring the intensity ratio IN=IT of the N and T forms, which is one of the main characteristics of probes with dual fluorescence. As is well known [26–29], the use of the intensity ratio has some advantages in comparison with the measurement of absolute intensities of individual fluorescence bands owing to the possibility of self-calibration. This enables direct monitoring of the solution’s microcharacteristics, such as microviscosity, permittivity, the existence of hydrogen bonds, the distribution of local fields [26–29] and temperature [113]. An important advantage of the dual fluorescence of 3HFs and some other luminophores with well-spaced bands in the visible region, which is still not used widely enough, is the possibility of visual monitoring by a change in the colour of the integrated emission. This monitoring may considerably simplify and accelerate measurements with the help of these probes. All of these advantages can be successfully used in the absence of signals that overlap with the recorded signal of the probe and distort the parameter IN =IP. Hence, our results on the separation of the emission of the

506 Hydrogen Bonding and Transfer in the Excited State

Figure 22.17 Fluorescence spectra of 3HF (c ¼ 105 M) in (a) pure acetonitrile and (b) acetonitrile with the addition of the TEMPO quencher with a concentration of 5  103 M at temperatures of 20, 50 4 80  C. The vertical arrows show the direction of change in the fluorescence intensity with increasing temperature. Spectra of the N band are also shown in enlarged 22 scale

N form are important for the application of PT probes when the main sensitive parameter is the intensity ratio of the fluorescence bands. 22.4.8 Effect of the excitation frequency on dual fluorescence It is evident from the data in Tables 22.1 and 22.2 that the properties of the 3HF molecule, even in the same excited state, essentially depend on the excitation wavelength. Let us discuss the properties of fluorescence that were observed at excitation in the second singlet band at a wavelength of 300 nm. These data are collected in Table 22.3.

Proton Transfer Reactions in the Excited Electronic State 507 Table 22.3 Temperature and dynamic quenching of the excited-state effect on proton transfer of 3HF in acetonitrile, lex ¼ 300 nm. CTEMPO ¼ 5  103 M. IN is the fluorescence intensity at the maximum of the blue band of fluorescence; ka is the absorption coefficient at the maximum of the S2 band at 300 nm; IN =ka is the fluorescence intensity at the maximum of the violet band, normalized on absorption at a wavelength of 300 nm; t P is the lifetime of the green band of fluorescence; IN =IP is the ratio of fluorescence intensities of the N and T forms in solution with TEMPO; ðIN =IP Þ0 is the ratio of fluorescence intensities of the N and T forms in neat solution; kSV is the Stern–Volmer constant calculated by (22.43) for the violet band of fluorescence; ðk þ ÞT2 =ðk þ ÞT1 is the ratio of proton transfer rates at temperature T2 and room temperature T1; k þ ðQT2 Þ=k0þ is the relative change in the proton transfer rate with addition of the quencher to the solution at temperature T2 Temperature,  C

Parameters 20 10

IN 1

ka (cm ) IN/ka

30 10

0.97

40 10,1

50 9

0.94

60 13

70 12.8

0.84

80 15 0.78

10.3

10.7

tT (ns)

0.9

0.5

IN =IP

0.02

0.022

0.03

0.037

0.048

0.059

0.07

ðIN =IP Þ0

0.019

0.022

0.03

0.042

0.054

0.073

0.11

kSV, (M1)

61

ðk þ ÞT2 =ðk þ ÞT1

1.0

k þ ðQÞT2 =k0þ

1.24

81

102

15.4 0.37

75

2.84 1.41

1.51

19.2

230

185

249

6.9 1.56

2.4

2.38

3.5

First of all it is necessary to pay attention to the fact that in neat solutions (without TEMPO) the intensity of the normal band IN increases with temperature (except for one point, at T ¼ 50  C), by 50% at T ¼ 80  C, in relation to room temperature. A more pronounced effect takes place for the absorption-normalized intensity, IN=ka. This parameter grows with heating almost by 100%. This may be explained by an increase in the internal P conversion probability pn2 (see the scheme in Figure 22.12) when there is a non-zero process SN n ! S1 . The quencher eliminates this rise, and a stable decrease in intensity with heating can be observed, but the effect is considerably weaker than at excitation in the main band at 340 nm. The relative intensities IN =IP grow faster both in neat solvent and in solution with TEMPO from 0.02 to 0.07 and from 0.019 to 0.11 respectively. It is important to note that, in every case, the ratio IN =IP is lower at an excitation of 300 nm than at 340 nm. A change in the PT rate with temperature, calculated by formula (22.63) as in the previous case for excitation at 340 nm (Table 22.2), is essentially higher and increases to 6.9 at T ¼ 60  C, i.e. faster than in the previous case at excitation in the main band. The quencher effect on the rate of the PT is also considerably higher: at first there is a growth from 1.24 at 20  C to 1.56 at T ¼ 50  C, and later an even stronger growth to 3.5 at T ¼ 80  C. Comparison of these values with those obtained earlier (Table 22.2) allow us to conclude that these features are due to at least two factors: (i) The enhanced heat release in solvates of luminophore as a result of non-radiative conversion pn2 between singlet states S2 ! S1 before the event of fluorescence emission and the PT. This heat generation reaches a value as high as 3400 cm1, and is about 17 times greater than the thermal energy kT at room temperature. A considerable part of this energy does not manage to dissipate before the act of spontaneous

508 Hydrogen Bonding and Transfer in the Excited State

emission from the SN 1 state, and the quenching processes take place in heater solvates, i.e. under different physical conditions to the relatively ‘soft’ excitation in the main band of absorption. The Stern–Volmer constants for the violet band of emission are essentially higher at such excitation than at the standard excitation in the main band and grow from 61.6 to 249 M1 in the same temperature range (Table 22.1). (ii) the excitation at the second band of absorption results in switching on of the second channel of the PT directly from the S2 state with probability kn2 (a lower ratio IN =IP at excitation in the S2 band). Separation of these two mechanisms requires additional simulations and discussion. An analysis of expression (22.45) provides us with a similar conclusion to (ii). The latter shows that the excitation in the range of different singlet bands leads to a change in the relative populations N and P (of the P levels SN 1 and S1 respectively in Figure 22.18), which determine the intensities of the corresponding band changes. In accordance with the empirical Kasha rule for complex polyatomic molecules, the radiative state is always the lowest excited state of the same multiplicity; therefore, the properties of the emitted radiation should not depend on the excitation wavelength. Consequently, in our case, the radiative states of the violet and green forms are the same irrespective of whether the excitation was performed via the S1 or Sn singlet state, and N P none of the constants of radiative and non-radiative transitions (kRN , knR , k þ , k , kRP and knR in the scheme of Figure 22.11) should be affected essentially by the excitation. Therefore, it was assumed that the dependence of the ratio IN =IP on lex arises because the populations N and P are affected by other processes involved in P degradation of high-lying excited states. In particular, it can be expected that, apart from the SN 1 ! S1 transition

Figure 22.18 Two-dimensional matrices of 3HF in ethyl acetate fluorescence obtained at excitation by 300 and 340 nm. Pulses of fluorescence measured at the maximum of the tautomer form near 530 nm are presented in the lower graph. The wavelength bands Dl ¼ 4.7 nm (10 pixels  0.47 nm) were used for plotting of the fluorescence pulses, and they are normalized on unity for better comparison of their shapes. Reprinted with permission from [102]. Copyright Elsevier

Proton Transfer Reactions in the Excited Electronic State 509

with the constant k þ (Figure 22.11), some additional transitions are involved in the population of the SP1 state, or that the probability of the reaction of internal proton transfer from the S2 and S3 singlet states is rather high. Such assumptions concerning the nature of the observed dependences can be additionally supported by analysis of the corresponding excitation spectra of the violet and green fluorescence bands. Such dependences were first studied and discussed in Refs [100] and [101]. Later, by means of methods of picosecond kinetic laser spectroscopy, the ESIPT reaction from the S2 state of 3HF was directly observed. In the next section we present the results of such study and discuss methods for evaluation of such reaction rates.

22.5 ESIPT from the S2 Singlet State in 3-Hydroxyflavone 22.5.1 Introduction The general scheme of ESIPT accounting for reaction via the Sn singlet state was shown in Figure 22.11. This scheme accounts for all important radiative and non-radiative transitions for describing properties of dual fluorescence arising owing to PT in the dye molecule. It was shown in Section 22.4 that, according to the present view of the concepts suggested in Refs [25] and [54], after absorption of light with a frequency nex in the N main absorption band, the 3HF molecule passes from the ground SN 0 state to the excited S1 level, which is responsible for the short-wavelength (violet) fluorescence. The second green band in emission is due to fluorescence of the tautomer. The dual fluorescence of 3HF derivatives, as well as other dyes exhibiting PT, has been extensively studied before, but practically always upon excitation at a frequency nex corresponding to the N main absorption band of SN 0 ! S1 . Obviously, the fluorescence of the N and T forms may also be excited via higher excited singlet states SN n, and such a possibility was discussed elsewhere [105, 117] and in Section 22.4. In this case there may be an P additional path for the ESIPT reaction, i.e. the direct transition from the SN n to the S1 state with the probability kn2 shown in Figure 22.1, and, of course, in this case all the main characteristics of dual fluorescence may be changed. First, the ratio of the fluorescence intensities of the N and T forms, IN =IP , will drop at such proton transfer, and second, in the excitation spectra of both forms, changes must also be observed in the region of the S2 band. The changes in the steady-state spectra of 3HF in acetonitrile were recently reported in Refs [100] and [101], and excitation of the tautomer form was registered to be more efficient in the vicinity of the second absorption band. The excitation band positions and shapes were similar to those at registration of fluorescence of the N and T forms. However, the ratio IN =IP was lower at excitation of the fluorescence through the second band. These features are evident from the experimental data gathered in Tables 22.2 and 22.3. Similar features were obtained for 2-butylamino-6-methyl-4-nitropyridyne-N-oxide in Ref. [104]. Observed features were explained in Refs [100] [101] and [104] by ESIPT photoreactions flowing via S2 states. However, direct observation of such reactions, which may be performed by kinetic spectroscopy methods, were carried out later in Refs [102] and [103]. It is important to underline that the existence of transitions with probability kn2 means a possibility of realization of photoreactions through the highest singlet states and, additionally, new possibilities and methods of study of these levels. In this section we describe the results of fluorescence study with picosecond time resolution of 3HF molecules. Spectra of the fluorescence of 3-hydroxyflavone were obtained at excitation in the range of the S1 and S2 singlet bands of absorption by 45 ps pulses of the optical parametric generator. The dynamics of obtained instant spectra demonstrates tautomer creation from excited states of the normal form, and the reaction has a higher yield if we excite molecules in the S2 band rather than in the S1 band. Differences in the two spectra indicate a larger contribution of tautomer fluorescence in integral emission at all instants of time of fluorescence observation. The obtained data directly evidence the additional channel of ESIPT from the S2 singlet state of the 3-hydroxyflavone molecule.

510 Hydrogen Bonding and Transfer in the Excited State

22.5.2 Experimental Solutions of 3HF (Indofile Chemical Co., additionally purified by recrystallization) in ethyl acetate (Sigma Aldrich, pure grade 99.5%, for spectrophotometry) with a concentration 104 M were taken for experiments [102, 103]. As the excitation source, the EKSPLA (Lithuania) laser system was used, which consists of a PL 2143 A/SS laser and an OPG 401/SH parametric generator. The latter is a pulsed Nd:YAG laser with an active Pockels cell and passive mode synchronization. The control system with negative feedback ensures the stability of output pulses of light as far as energy and time are concerned. Fluorescence light was collected from the same front plane of the cuvette with the solution under study, which was illuminated by the OPG. The detection part consisted of a 2501S spectrograph (Bruker Optics, USA) and a C4334-01 streak camera (Hamamatsu, Japan). The spectrograph ensured spatial resolution of the analysed light (wavelength axes), whereas the streak camera allowed temporal resolution of the light beam coming out of the spectrograph. The streak camera enabled recording of incoming temporal light signals separated spectrally (the entire spectrum was obtained at the same time). The resolution of the image converter – 640  480 pixels (wavelength  time) – ensured a maximum point-to-point spectral resolution of 0.5–0.02 nm, depending on the grating used. The maximum point-to-point temporal resolution amounted to 2 ps for the 1 ns range. The key point for proper apparatus work was synchronization between the streak camera and light pulses coming out of the laser and OPG. For the fastest ranges, 1–20 ns, synchronization was ensured by the ultrafast light detector (photodiode) directly from the laser. It yielded the smallest jitter of 30 ps. A more detailed description of the experimental set-up can be found in Ref. [118].

22.5.3 Results and discussion In our experiment we chose 300 and 340 nm wavelengths for fluorescence excitation, which correspond well to the maxima of the S1 and S2 singlet bands (see Figure 22.12, where the absorption spectra of 3HF in the region 270–390 nm are shown). Figure 22.18 presents two spectrochronograms of 3HF emission, taken at excitation with wavelengths of 300 and 340 nm respectively. The results of the measurements comprise a coloured picture of 640  480 pixels, where the x-axis corresponds to wavelength and the y-axis to time. For better legibility, this result is presented in Figure 22.18 as a negative of the real picture on the computer screen in greyscale. Both spectrochronograms in Figure 22.18 show the dynamics of two bands of fluorescence: the first one, violet and weak, is situated at 400 nm and belongs to the N form; the second one, green, is concentrated near 525 nm and can be attributed to the tautomer of 3HF. It can be seen that the violet band decays faster than the green one. The lifetime of the N form was found to be about 60 ps. In the bottom of this figure, the pulses of the tautomer fluorescence are presented for both cases of excitation. The vertical lines in the spectrochronograms in Figure 22.18 denote the column of pixels (10 pixels  0.47 nm, Dl ¼ 4.7 nm) that were chosen for plotting of the fluorescence pulses. As can be seen, at 340 nm excitation, the T form emission reaches its maximum intensity at a time of 380 ps and then decays (Figure 22.18, bottom), obeying the monoexponential law with a time constant of 1.0 ns. Excitation by shorter wavelengths of 300 nm leads to a faster rise of the tautomer emission, which peaks at 260 ps, the decay again obeying the monoexponential law with a time constant of 1.0 ns. The quantum yield of 3HF in ethyl acetate is 7% [70]; fluorescence was collected over a period of a few minutes to provide the required accuracy of registration. Absorption and fluorescence spectra of the solute overlap weakly (see, for example, Figure 22.3), and the inner filter effect does not take place. The same conclusion follows from the fact that there is no effect of concentration of the solute on observed differences in the spectra at various lex in the range 105–104 M. Absorption of 3HF at wavelengths of excitation of 300 and 340 nm were quite similar (with a difference of 20%), and fluorescence of solvent at the same excitation conditions was not observed. All stages of the registered dual spectra transformation show only the appearance of the N form of the solute spectra and consequent emergence of the emission of the tautomer; there are no

Proton Transfer Reactions in the Excited Electronic State 511

fluorescent admixtures in the registered instant spectra. The gathered data led us to conclude that in the latter case there is obviously some additional channel populating the excited states of the tautomer. More information can be obtained from treatment of instant spectra of emission, which are easy to obtain by plotting signals of horizontal pixels in a proper time interval. Figure 22.19 shows instant spectra of the dual fluorescence of 3HF at different instants of time at excitation by laser pulses with wavelengths of 300 nm (Figure 22.19(a)) and 340 nm (Figure 22.19(b)). The pictures demonstrate the evident difference in the spectra at the chosen wavelengths of excitation. The first row of the spectra relates to emission at a time of 140 ps and is seen to demonstrate the ratio IN =IP ¼ 0.163 when a wavelength of 300 nm excites the sample, whereas at excitation by 340 nm we have higher values of IN =IP ¼ 1.446. Thus, we have two different spectra (Figures 22.19(a) and (b)). To reveal their difference, we have subtracted the second spectrum from the first spectrum, with both normalized at the maximum of the N band (400 nm). The difference in the spectra upon excitation by different wavelengths is shown in Figure 22.19(c), revealing a spectrum that is the same (in position of the maximum and half-width) as the green spectrum of the tautomer. At the second chosen instant of time, 190 ps, at each excitation wavelength we observe a development of the ESIPT reaction as the role of green emission belonging to the tautomer grows. Now the ratio IN =IP reaches 0.054 and 0.336 when fluorescence is excited at 300 and 340 nm respectively. The difference spectrum displays again exactly the same contour as the green band of the tautomer. The last, third set of spectra was taken at a time of 500 ps, and, as in the previous cases, we observe that ratio IN =IP has the lower value of 0.004 when lex ¼ 300 nm is applied, as opposed to 0.007 at lex ¼ 340 nm. The two spectra demonstrate once more an essential difference that coincides with the tautomer spectra in the previous cases. The time evolution of the instant spectra for both excitation wavelengths apparently shows the proton transfer process from hydroxyl to carbonyl groups of 3HF with different time constants. The ratio IN =IP gives us relative intensities of both bands; this ratio is a simple and important parameter that evidences the stage and contribution of the ESIPT reaction in the integral spectra of dual fluorescence. The difference between the two spectra can be seen in more detail in Figure 22.20, which shows the intensity ratios IN =IP at different time instants, obtained by complete processing of the signal matrices shown in Figure 22.18. As can be seen, the curves for different excitation wavelengths differ greatly. In the case of excitation in the second absorption band, the ratio IN =IP reaches a maximum sooner (0.8 at time 80 ps) and then, beginning from t ¼ 170 ps (approximately at the level of 0.1 from the maximum 0.8), asymptotically approaches a level of 0.004. Excitation into the region of the main band demonstrates slower dynamics of the ratio IN =IP , which reaches a maximum value of 1.87 at an instant of 100 ps and then, beginning from 210 ps, tends towards a level of 0.007. Thus, such behaviour of the relative intensities evidences faster population of the tautomer excited state when we excite our luminophore in the S2 band of absorption as well. These data directly point to the existence of an additional channel of population of the 3HF tautomer upon excitation into the S2. It is obviously interesting to evaluate the probability of the ESIPT reaction via the S2 state, and in the next subsection we present such an approach. 22.5.4 Evaluation of the ESIPT rate from the S2 state Figure 22.21 illustrates fluorescence spectra of 3HF at two different excitation energies in the ranges of the main and second singlet bands of absorption. It can be seen that solution fluorescence spectra consist of two luminescence bands: violet with a maximum at 400 nm and green with a maximum at 525 nm. Both spectra are normalized at the maxima of green emission. In the same figure, excitation spectra of the same solution at registration of luminescence in the 400 and 525 nm field are shown (curves 3 and 4). It can be seen that excitation of green luminescence becomes more effective in the field of the second singlet band (curves 1 and 2). Figure 22.22 illustrates the ratio of fluorescence intensities IN =IP of 3HF (measured at the maxima of appropriate bands of emission), which are registered at different excitation wavelengths in the field of the first

400

400

0 400 450

Intensity /a.u.

Intensity /a.u. 160

Intensity /a.u.

Intensity /a.u.

(a)

Intensity /a.u.

Intensity /a.u.

λex = 300 nm

500 550

160

0 400

300

0 400

(b)

t = 0,14 ns IN / IT = 0,163

120

t = 0,19 ns IN / IT = 0,054

t = 0,5 ns IN / IT = 0,004

600

450 400

500

λ /nm 450

550

λex = 340 nm

t = 0,14 ns IN / IT = 1,446

500

λ /nm λ /nm

(c) difference

t = 0,14 ns

120

80

40

t = 0,19 ns

300

200

100

0

t = 0,5 ns

200

100

600

16

12

80 8

40 4

0

0 80

t = 0,19 ns IN / IT = 0,336

300 60

200 40

100 20

0 0

t = 0,5 ns IN / IT = 0,007

400

300 300

200 200

100 100

550 600 0

Figure 22.19 Instant spectra of 3HF emission at various times, indicated, together with the ratio IN/IP, in the left-hand corners in each frame, at excitation by 300 nm (a) and 340 nm (b); (c) differences in the emission spectra normalized at the maximum of the N form. The signals presented were averaged over 2 pixels of the spectrochronograms, which corresponds to the time interval Dt  22 ps. Reprinted with permission from [102]. Copyright Elsevier

512 Hydrogen Bonding and Transfer in the Excited State

Proton Transfer Reactions in the Excited Electronic State 513 2,0 340 nm 300 nm

3HF in EA -4 c = 10 M

1,6

0,012

I N / IT

1,2 0,008 0,0072

0,8

0,004

0,000 0,0

0,4

0,0 0,0

0,0039

0,1

0,2

0,2

0,3

0,4

0,6

0,4

0,8

1,0

0,5

t /ns

Figure 22.20 Intensity ratio IN/IT at different instants of recording of the fluorescence of 3HF in ethylacetate (104 M) upon excitation in the region of the main (340 nm, light circles) and the second (300 nm, dark circles) singlet bands. The inset shows the signal tails on an enlarged scale; the dashed lines corresponds to their asymptotes. Reprinted with permission from [102]. Copyright Elsevier

Figure 22.21 Fluorescence lex ¼ 340 nm (1) and 300 nm (2) and fluorescence excitation lreg ¼ 385 nm (3) and 525 nm (4) spectra of 3HF in acetonitrile. Reprinted with permission from [102]. Copyright Elsevier

and second band of absorption. Apparently, this ratio is considerably lower at the excitation in the second singlet band at 300 nm than in the main band at 340 nm. Registered changes in spectra of excitation and fluorescence indicate the additional channel of the photoreaction with some probability kn2. Let us try to estimate this probability using, for example, spectra of excitation [119]. We are going to use equation (22.21) obtained in Section 22.4 for the ratio of population of the N form of fluorophore and its product P in a solution without a quencher (Q ¼ 0) under the condition of a fast reaction of proton transfer (22.12a). Assuming that the sum of excited singlets of luminophore, N , and tautomer, P (see again the scheme of energy levels in Figure 22.11), can be expressed as N* þ P* ¼ Ntn Buex

ð22:68Þ

514

Hydrogen Bonding and Transfer in the Excited State IN / IT

0,030 0,025 0,020 0,015 0,010 0,005 0,000 220

320

λ ⁄ nm

Figure 22.22 Dependence of the intensity ratio of the violet and green fluorescence bands IN/IT on the excitation wavelength. Reprinted with permission from [102]. Copyright Elsevier

where N is the concentration of solute molecules and uex is the density of excitation light. This expression can be derived considering the stationary equation for any excited singlet population N*n ¼ tn Buex N and further distribution (as a result of decay) of this population between the N and P states N*n ¼ N* þ P* The lifetime of the Sn state in equation (22.68) is given by tn ¼

1 pn2 þ kn2

ð22:69Þ

Let us take expression (22.31) for the relative population of N and P states, and the latter, together with equation (22.68) for the same unknown values (N and P ), gives us a system of two linear equations that can be easily solved. From expressions (22.31), (22.68) and (22.69) we will obtain

N*1 ¼

P P pn2 fkRP þ knR þ k g þ k kn2 * pn2 fkRP þ knR þ k g þ k kn2 P1 ¼ ðBuex NtP N*1 Þ k þ ðpn2 þ kn2 Þ k þ ðpn2 þ kn2 Þ

where 

 1 P þ k ¼ kRP þ knR tP

ð22:70Þ

Proton Transfer Reactions in the Excited Electronic State 515

Resolving this equation with respect to the population of the fluorescent level of our fluorophor N*1, we will obtain N*1 ¼ Buex NtP

pn2 fkRP

P pn2 fkRP þ knR þ k g þ k kn2 P þ knR þ k g þ k kn2 þ k þ ðpn2 þ kn2 Þ

ð22:71Þ

From equation (22.68), taking into account this last expression for N*1, we have, for the population P of the excited state of the photoproduct, the equation P*1 ¼ NtP Buex N*1 ¼ NtP Buex

k þ ðpn2 þ kn2 Þ P þ k g þ k k þ k ðp þ k Þ pn2 fkRP þ knR   n2 þ n2 n2

ð22:72Þ

The ratio of populations of reaction product singlet levels received at excitation through the first and second bands of absorption P*1 and P*2 , respectively, is as follows: P21 ¼

P*2 B2 uex2 k þ ðpn2 þ kn2 Þ ¼ P P * P1 B1 uex1 pn2 fkR þ knR þ k g þ k kn2 þ k þ ðpn2 þ kn2 Þ

ð22:73Þ

This equation gives us the ratio of excitation intensities of the second to the first absorption bands at registration of product emission. From this equation it can be seen that, at excitation in the first band, when it is possible to consider that kn2 ¼ 0, the ratio will be equal to unity, i.e. P*2 =P*1 ¼ 1, as should apparently follow from the equation in agreement with general physical conditions. Let us now write the ratio of populations, or the ratio of intensities equal to them, of the first singlet SN 1 emission (short wavelength band) at excitation in different bands of absorption: N21 ¼

P N*2 tP B2 uex2 pn2 fkRP þ knR þ k g þ k kn2 ¼ P P * N1 tN B1 uex1 pn2 fkR þ knR þ k g þ k kn2 þ k þ ðpn2 þ kn2 Þ

ð22:74Þ

It can be seen from this expression that, at excitation in the main band, when it is possible to consider that kn2 ¼ 0, the ratio N*2 =N*1 ¼ 1, as it really should be at such a condition of excitation. With respect to experimentally measured characteristics, N21 is the ratio of intensities of the maxima of the second to the first band in the spectrum of excitation of the initial N form of the solute. If measured intensities of the excitation spectrum are obtain with a non-calibrated device, then, to take into account the change in intensities at different wavelengths of the excitation source, the corresponding factors a have to be introduced into expressions (22.73) and (22.74), considering various intensities in the ranges of excitation (indices 2 and 1) a2 =a1 . Finally, let us present the ratio of our parameters N21 and P21 from expressions (22.73) and (22.74) for the sake of simplicity in the evaluation procedure:

C¼

1 P N21 tP pn2 fkRP þ knR þ k g þ k kn2 tP pn2 tP þ k kn2 pn2 þ k kn2 tP ¼ ¼ ¼ P21 tN k þ ðpn2 þ kn2 Þ tN k þ ðpn2 þ kn2 Þ tN k þ ðpn2 þ kn2 Þ

where tN us defined by expression (22.42).

ð22:75Þ

516 Hydrogen Bonding and Transfer in the Excited State

As a first approximation, in the case of a treated fast reaction (condition (22.12a)) it is possible to suppose for an estimation that the product tN k þ  1. Then, taking into account the equation for tN (expression (22.42)), we have C¼

N21 pn2 þ k kn2 tP ¼ P21 pn2 þ kn2

ð22:76Þ

There are on the left-hand side of the derived equation the relative intensities measured in excitation spectra, and on the right-hand side the probabilities defining the characteristics of dual fluorescence of our sample. In equations (22.75) and (22.76) it is possible to use the spectra of excitation obtained with non-calibrated apparatus over different intensities of the excitation source on different wavelengths, as the ratio a2 =a1 , which should be in the numerator and in the denominator, will be reduced; in our opinion, this is a very significant fact. The majority of standard devices, even those of well-known companies, are being produced without such calibration (for example, the HITACHI F-2500 spectrofluorimeter), and, when this calibration is performed independently in a laboratory, there are essential errors in the use of obtained correction coefficients, especially in the UVareas, because of an instability and the ‘broken’ (non-smooth) character of the emission spectrum of excitation lamps. It is possible to determine from equation (22.76) the probability of a photoreaction via the S2 state kn2 . Having performed corresponding transformations, we have kn2 ¼ pn2

1C CtP k

ð22:77Þ

This equation makes it possible to calculate the probability of reaction kn2 from the Sn singlet state if the characteristics of a particular solution are used. 22.5.5 Processing of experimental data Let us see how the derived equation (22.77) works. Take, again, the well-known molecular system 3HF with dual fluorescence arising because of ESIPT. The latter reaction from the S2 state with probability kn2, in standard steady-state characteristics and in the time-resolved spectra, manifests itself as follows: .

.

.

In fluorescence spectra, the ratio of intensities of the main and the product band IN =IP changes according to the excitation band; it decreases with shift of excitation wavelength from the first to the second band of absorption. In fluorescence excitation spectra, the second singlet band of excitation of the tautomer has different intensities at luminescence registration in the ranges of the N form and tautomer bands; it increases in intensity in relation to the main band at registration of product emission. At excitation by short laser pulses and registration of luminescence with sufficient time resolution, excitation in the second band leads to a higher product contribution to the total emission of fluorescence.

All three mentioned features of the proton transfer reaction from the Sn state open the door for an estimation of reaction probability. The techniques of measurement of steady-state fluorescence spectra and excitation are described in Refs [100] and [101], and spectra with picosecond time resolution are described in Refs [102] and [103]. More detailed estimation can be made on the basis of the above-mentioned equations using the experimental spectra of excitation presented in Figure 22.21. Let us consider that, for studied 3HF, the lifetime tP is 0.9 ns at

Proton Transfer Reactions in the Excited Electronic State 517 Table 22.4 The main parameters of dual fluorescence of 3HF in acetonitrile, obtained from excitation and fluorescence spectra, and the results of probability kn2 evaluation using these data. The first row gives the ratio of intensities in maxima of fluorescence excitation spectra of the N form, N21, and the tautomer, P21, and their ratio C ¼ (N21/P21). The second row gives the ratio of fluorescence signals of the N and T forms ðIN =IP Þ340 and ðIN =IP Þ300 obtained at excitation in the main and second bands of absorption, respectively, and their ratio ðIN =IP Þ300 =ðIN =IP Þ340 ; values taken from the fluorescence spectra in Figure 22.21 1

P21 0.68

N21 0.89

C ¼ (N21/P21) 0.764

kn2  1012, s1 0.3

2

ðIN =IP Þ300 0.023 0.019

ðIN =IP Þ340 0.028 0.023

ðIN =IP Þ300 =ðIN =IP Þ340 0.82 0.82

0.22 0.22

T ¼ 293 K. As for the probability of the reverse reaction k, it can be estimated as 108 s, as there are no nanosecond components in pulses of the normal form and tautomer. The pulses of these forms are approximated by monoexponential curves, as a result of fitting with constants of 0.06 and 0.9 ns, in which there are no long components that could be associated with the constant of the reverse reaction k. In this respect, our data [102, 103] correlate with the data of study [116], where, likewise, no features were registered in the range from 50 ps to 1 ns in kinetics of both components of 3HF emission, which corresponded well to the monoexponential law. Table 22.4 illustrates the basic parameters of 3HF dual fluorescence, obtained from characteristics of the excitation and fluorescence spectra, and also the results of estimation of probability kn2 using the data presented in the table. The first row of Table 22.4 presents the ratio, measured from 3HF excitation spectra, of the maxima in the spectra of the fluorescence excitation of fluorescing probe N21 and the tautomer P21, and also their ratio C ¼ (N21/P21), which is equal to 0.764. Then we can conclude that in equation (22.77)C tP k , and then equation (22.77) becomes even simpler: kn2 ¼ pn2

1C C

ð22:78Þ

The second line presents data on relative fluorescence intensities at excitation in the main ðIN =IP Þ340 and second ðIN =IP Þ300 bands of absorption and their ratio ðIN =IP Þ300 =ðIN =IP Þ340 , which were obtained from the fluorescence spectra presented in Figure 22.22 and other independent series of experiments. It has recently been shown (equation (27) in Ref. [117]) that the ratio ðIN =IP Þ300 =ðIN =IP Þ340 is expressed by the same equation as the right-hand side of equation (22.76). Therefore, with correct measurements of the fluorescence and excitation spectra, we should obtain similar or the same results. A comparison of the values presented in the two rows in Table 22.4 shows their good agreement, which provides direct evidence for the good working capacity of the ratiometric measurements. The final column in Table 22.4 contains values of the proton transfer probability kn2, calculated on the assumption that probability p21 is equal to 1012 s1. Obviously, it is possible to consider the contribution of the reaction of product formation through the SN n state when the following inequality is valid: pn2 kn2 For a great number of molecules, similarly to 3HF, that have modest energetic gaps DES2 S1 (3000 cm1 for 3HF), the probability of interconversion pn2 is 1012 s1 (for example, see Refs [73] to [75]), and then we have the value of the probability of ESIPT for our experiments, kn2  0.2–0.3  1012 s1. Apparently, that from the second singlet is high enough and comparable with the probability of proton transfer from the first singlet of

518 Hydrogen Bonding and Transfer in the Excited State

3HF, the average value of which can be estimated as 1012 s1 [10, 85]. More precise estimation of the probability kn2 can be obtained with the help of equation (22.77) if independently measured values of the probability pn2 for the chosen probe are used; however, it is not yet clear how these measurements can be made by standard methods, considering the high rate of the S2 state decay and parallel existence of the proton transfer channel with rate kn2. Hence, we can conclude that the use of relative or ratiometric signals in dual fluorescence spectra can be used successfully for estimation of the reaction rates from the highest energy levels, in spite of the doubts that sometimes arise because of the unreliability and instability of standard spectrofluorimeter measurements in the near UV. The optional measurements with direct subpicosecond resolution are still quite unique and not very common. Proton transfer through the S2 state is also directly registered reliably in the steady-state spectra of fluorescence excitation, and we have shown in this section how it is possible to estimate the rate of this reaction by means of ratiometric measurements. We believe that the ESIPT reaction from the Sn singlet states may also take place for other compounds undergoing the proton transfer process. Some of these are specially synthesized and tested as multiparametric sensors and possess unique properties in determination of the polarity of the environment, local electric fields, detection of single molecules of water in membranes and visicules and various properties of H-bonds. It is possible with their help to detect molecular oxygen in live cells, the different subdomains of DNA, the ions of different metals, the cholesterol in proteins and apoptosis of cells [26–30].

22.6 Concluding Remarks Nowadays it is clear that probes with dual fluorescence, especially those appearing in ESIPT states, open the door, in a number of applications, for the most rationalized and precise way of fluorescence reporting. The main advantage of such probing is based on the self-calibration response signal at two-band ratiometric recording. The importance of such calibration for sensor technology and other applications has been discussed in detail in the recently published monograph of Demchenko [27], where it was shown that two-band ratiometry is insensitive to instrumental factors, concentration of sensor and uncontrolled quenching by impurities: 1. Using the two-band ratiometric fluorescence signal, it becomes possible to achieve an efficient calibration of the sensor unit and to provide a simple account (or even full elimination) of all instrumental factors that appear in the measurement of fluorescence, such as the intensity and spectra of the light source, the geometry of optical registration and the sensitivity of the detector. 2. It is especially important to avoid any dependence on the concentration of probe molecules, especially in view of the fact that organic solutes tend to make complexes and degrade at illumination by light as a function of time. In biomedical applications (imaging and some others), the local dye concentrations cannot easily be measured, and variation in the probe concentration in the illuminated volume may become the limiting factor for quantitative assay. 3. In sensor technologies, at a given probe concentration the fluorescence response also depends on many poorly identified or variable factors on the molecular level that cannot be eliminated or easily accounted for during experiments. The most important of these is fluorescence dynamic quenching, which cannot be accounted for in lifetime or anisotropy sensing (they influence these parameters directly), and the twocolour fluorescence recording remains seemingly the only possibility for fluorescence sensors that are insensitive to their action. However, as we observe in Section 22.3 for the example of 3HC dye fluorescence quenching by TEMPO, it is possible to satisfy this property in the case of a fast and reversible excited-state reaction; only then, owing to rapid establishment of ESIPT equilibrium, will the action of the quencher not

Proton Transfer Reactions in the Excited Electronic State 519

change the band intensity ratio. These facts give rise to great concern about the prospects for application of new molecular sensor technologies based on the switching of light intensities between the two bands. We believe that the approach described in Section 22.3, using the effects of collisional quenchers in steady-state spectra, will serve this purpose. Different ratios of band intensities of dyes such as 3HFs, 3-hydroxychromones or 3-hydroxyquinolones, having fluorescence in the range from the violet to the green or even the orange part of spectrum, are perceived by the human eye as a change in fluorescence colour between both bands of emission through practically the entire visible spectrum. Hence, in some cases, even relatively simple visual control is possible owing to the use of such excited-state reactions as ESIPT, as was shown in Refs [27] and [29]. It features a broad-range applicability on the molecular, nanoscale and whole-cell levels. The functional derivatives of 3-hydroxyflavone and 3-hydroxybenzofuranochromone dyes have found application as molecular sensors in ion sensing [120], micelle [121] and phospholipid vesicle [122, 123] research, in ligand binding [124] and the studies of conformational changes in proteins [125] and in cell imaging [126]. For these dyes, on change in their physical environment (polarity, hydrogen bonding, local electric fields), the switching between two different colours of emission, usually violet–green and orange–red, is commonly observed, and this can provide a strong response in sensing. Of course, these properties may be improved, and new attempts to synthesize the dyes with better characteristics are in progress. Recently, a new group of molecules – 3-hydroxyquinolones with dual fluorescence due to ESIPT – have been synthesized and studied [30, 31]. These dyes, in comparison with 3HFs, exhibit a higher fluorescence yield and tenfold increased photostability, which makes them promising for multiparametric probes. The proton transfer rate in some of them is slower, about 1010 s1, than in 3HFs, which makes them promising for further detailed study of the PT mechanism fundamentals. We have described the theory and applied it to determination of the kinetic or thermodynamic character of photoreactions and various physicochemical parameters using dynamic quenching; this approach is not limited to ESIPT; it can be useful in analysis of quenching effects on other photochemical reactions that can be described by two-state excited-state reaction formalism. Based on these findings, a simple test was suggested for the mechanisms of these reactions, allowing discrimination of two important cases – thermodynamic and kinetic control. In this chapter we have presented some new possibilities for determination of molecular characteristics by means of the ratiometric method. This approach is very convenient and in many cases seems to be the most reliable way to obtain the desired information by fluorescence reporting. Thus, we demonstrate in Section 22.3 that direct measurement of the IN/IT dependence on quencher concentration gives us a precise answer concerning the kinetic or thermodynamic character of the proton transfer reaction flowing in the sample. The same band fluorescence ratio dependences on temperature and excitation frequency enable us to obtain interesting information concerning the changes in proton transfer rates with temperature, the addition of a quencher and absorbed quantum energy. In all cases, the simulation procedures are shown to be constructed in such a way that it would be possible not to take into account all apparatus factors. Therefore, it may probably be concluded that there were suggested new applications of ratiometric measurement with ESIPT probes, such as determination of the change in the proton transfer rates with temperature, the addition of a quencher and a change in the initial electronic state for ESIPT. It has been shown that, generally, upon excitation of molecular objects via high-lying singlet states, the yield of photoreaction products can increase in a number of cases. Then, if fluorescence of fluorophores and their photoproducts is detectable, the probabilities of reactions via high-lying singlet states can be determined by means of fluorescence measurements. The ESIPT reaction from the S2 state of 3HF in solutions, which was discovered recently, was discussed in detail in Section 22.5. The results of experiments, obtained by means of steady-state and picosecond laser spectroscopy methods, provide good evidence for the additional channel of

520 Hydrogen Bonding and Transfer in the Excited State

internal proton transfer from the S2 singlet state of 3-hydroxyflavone with a high kinetic rate of 0.2–0.3  1012 s1, which is comparable with the rate of the same reaction from the S1 state. The fact of registration of a photoreaction from a state higher than S1 reveals a new channel for such processes, and, probably, in some cases this channel may be explored as the additional one. The ESIPT reaction from Sn states may be used as an additional channel in two-photon selective excitation of probes in biological membranes and cells, where excitation may be carried out in the red or infrared wavelength range to avoid simultaneous excitation of organic or biological matrices in which probes are incorporated. It is worth mentioning finally that the creation of photoreaction products through the SN n states may also concern some other primary photoreactions, such as the redistribution of electronic density (charge transfer) in the excited state, protolitic reactions, intramolecular proton transfer (phototautomerization), Hbond creation and the creation of excimers and exiplexes.

Acknowledgements We thank the Pomeranian University (Slupsk, Poland) for financial support (project BW 8/1239/08), A. P. Demchenko for helpful discussions, the Regional Laboratory of Luminescence Kinetics (University of Gdansk) and R. Jaworski for kinetic measurements and technical support.

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23 Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires Carine Tanner Manca1, Christian Tanner2, and Samuel Leutwyler3 1

Laboratorium f€ur Physikalische Chemie, ETH Z€ urich, CH-8093 Z€ urich, Switzerland 2 TOFWERK AG, Uttigenstrasse 22, CH-3600 Thun, Switzerland 3 Dept. f€ur Chemie und Biochemie, Universit€ at Bern, Freiestrasse 3,3012 Bern, Switzerland

23.1 Introduction Hydrogen-bonded wires are involved in a large variety of chemical and biological processes [1–16]. The main reaction associated with hydrogen bonds is proton or H-atom transfer, in which a charge accompanies the transferring proton. A topic of specific interest is proton transport through transmembrane ion channels (‘proton wires’), because many transmembrane proteins create, control and use the proton gradient across biological membranes. Hydrogen-bonded wires of water molecules have been identified in numerous membrane-spanning proteins, such as bacteriorhodopsin [17–22], the bacterial photosynthetic reaction centre of Rhodobacter sphaeroides [23], the transmembrane channel formed by gramicidin [24–26], cytochrome oxidase [27] and other voltage-gated proton channels [28]. The enzymes carbonic anhydrase [29, 30] and alcohol dehydrogenase [30] also contain proton relays along chains of water molecules embedded in the interior of the protein. Recently, hydrogen-bonded ammonia wires have been detected in ammonia/ammonium transporter (Amt) transmembrane proteins, which are essential for nitrogen metabolism in all species [31, 32]. The translocation mechanisms along these ‘proton wires’ have become fields of intense theoretical study [5–8, 24–26, 29, 30, 33, 34]. Most of the time, the assumed mechanism is Grotthuss type, after the inventor (1806) of the process. The modern version of the Grotthuss mechanism consists of successive transfers of an excess proton along a hydrogen-bonded wire. These transfers require sequences of reorientation of the wire molecules, which constitutes the rate-limiting step of the mechanism [35]. The excess proton diffuses through a hydrogen bond network via the formation/breaking of covalent bonds, as shown schematically in

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

526 Hydrogen Bonding and Transfer in the Excited State

Figure 23.1 Schematic representation of the modern Grotthuss mechanism for proton transfer along a unidirectional hydrogen-bonded wire of four water molecules (from top left to bottom right)

Figure 23.1 in the case of water molecules. Because of the turn steps, it is never the same proton that is transferred. The existence of hydrogen-bonded wire and, more specifically, transmembrane water wire, however, does not always result in proton translocation: the aquaporins allow efficient single-file transfer of water molecules through cell membranes [36–38], whereas the translocation of protons along this water wire is blocked. This proton/water selectivity has been attributed to orientational effects that are believed to destroy the perfect translocation path along the water wire [39–42], but it has since been pointed out that a large part of the barrier is the (mostly electrostatic) desolvation penalty of moving the proton charge from bulk solution to water molecules in the channel interior [43, 44]. Hence, the understanding and modelling of proton and H-atom transfer reactions, and factors favouring or preventing them, are of fundamental interest but also challenging for physical chemistry: direct experimental observation of these reactions along water or ammonia wires would allow us to understand the factors controlling these processes. This is difficult in biological samples because of the short timescales, the local inhomogeneities of the solvent and the rapid solvent fluctuations. This complexity can be partially avoided by studying isolated and supersonically cooled model systems, which also allow precisely controlled preparation of reactants and detection of products [45–49]. Indeed, clusters represent a suitable transition regime between molecules and small liquid drops. Isolating them in a supersonic expansion allows us to focus on the intrinsic properties of the cluster and to reduce/prevent environment interactions. Another aspect of hydrogen-bonded systems has been constantly developed over the last three decades: the excited-state proton or H-atom transfer (ESPT or ESHAT). It offers the possibility of experimentally controlling the onset of these reactions. Figure 23.2 shows the number of publications pertaining to excited-state transfer along hydrogen bonds. The overall increase in publication activity in computational as well as experimental areas is correlated with the growing interest in such potential reactivity. One of the most famous examples of ESPT is the green fluorescent protein (GFP). This component was found in numerous bioluminescent organisms, such as the jellyfish Aequorea victoria [50] and the sea pansy Renilla reniformis [51]. This protein fluoresces green when exposed to blue light and is therefore widely used as a fluorescent marker in cell biology. Many studies are devoted to understanding its complex photochemistry.

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

527

Figure 23.2 Number of publications pertaining to excited-state proton or H-atom transfer along a hydrogenbonded wire between 1980 and 2008

The chromophore responsible for green fluorescence is involved in a hydrogen-bonded wire and exhibits two main components in its absorption spectrum, assigned to the neutral and the anionic (deprotonated) forms [52], highlighting the proton transfer to the hydrogen-bonded wire. In addition, excited-state H-atom transfer reactions from organic chromophores to small solvent clusters have been studied intensively in recent years, both spectroscopically and theoretically [53–58]. This chapter will address the reactivity of intermolecular hydrogen bonds in the excited state and, more specifically, treat the understanding and control of the ESHAT reaction along a hydrogen-bonded wire. In Section 23.2, we will introduce experimental techniques and theoretical approaches to describe our prototype systems. We will also provide a detailed description of the ESHAT reaction path and the first step of the global reaction. Section 23.3 will provide some examples of parameters that can influence the reactivity. For almost every parameter studied, we base our investigation on experimental results and appropriate calculations.

23.2 Prototype System 23.2.1 Experimental observations 23.2.1.1 Techniques Used The experimental techniques we used are closely related to the properties of the systems we want to study. Figure 23.3 shows the 7-hydroxyquinoline (7HQ) and the tautomeric 7-ketoquinoline (7KQ) forms. The molecule is a relatively rigid aromatic chromophore with two functional groups: a proton/H-atom donor group (OH) and a proton/H-atom acceptor group (quinolinic N) spaced far apart. This molecule represents an ideal scaffold to connect, stretch and constrain a unidirectional (homodromic) hydrogen-bonded wire (see Figure 23.3). Furthermore, the acidity/basicity of the two functional groups increases in the electronic excited state: the pKa value of the equilibrium in aqueous solution at room temperature between the protonated cation (7H2 Q þ ) and the neutral form increases in the S1 electronic excited state compared with the S0 ground state, while the pKa value of the equilibrium between the neutral form and the deprotonated anion (7Q) decreases (see the values in Table 23.1). In the excited state, the more acidic hydroxyl group can

528 Hydrogen Bonding and Transfer in the Excited State

Figure 23.3 The two tautomeric forms of 7-hydroxyquinoline. Arrows indicate the hydrogen bond directionality of the solvent wires, from Hþ /H atom donor to acceptor group (See Plate 29)

inject a proton/H atom into the connected hydrogen-bonded wire. As the quinolinic N site is also rendered more basic, the conduction of the excess proton/H atom along the wire results in an excited-state enol ! keto tautomerization reaction, yielding 7KQ with a unidirectional hydrogen bond pattern inverted compared with that of 7HQ, as schematically shown in Figure 23.3. The two tautomers of 7-hydroxyquinoline as well as the anion 7Q and the cation 7H2Qþ have been extensively characterized by absorption and fluorescence spectroscopy in non-polar, polar and protic solvents [59–70]. 7HQ emits in the 370–390 nm range, depending on the solvent, while 7KQ emits in the 525–580 nm range, strongly Stokes shifted relative to 7HQ. The emission bands of both 7H2Qþ and 7Q lie in a 430–450 nm intermediate range. We base our experimental work on laser spectroscopy and use these four emission bands for experimental probing of ESPT or ESHAT along the hydrogen-bonded wire connected to the scaffold. Figure 23.4 shows a scheme of fluorescence emission and two-colour resonant two-photon ionization (2CR2PI) experiments. These laser experiments are used to characterize the structure of the cluster through the identification of the vibrational modes in the electronic ground and excited states by comparison with ab initio calculations. The UV-UV hole burning (UV-UV HB) consists of an HB laser selectively burning a population before a 2C-R2PI measurement, and is designed to investigate the presence of different isomers (see Figure 23.5, left). These three laser experiments have been extensively used to identify the isomers of 7HQ(H2O)n and 7HQ(NH3)n, n  1 [46, 71–75]. As soon as an isomer is fully characterized, two further laser experiments are used to investigate the possible dynamic of the cluster in the excited state. In the fluorescence action spectroscopy scheme (Figure 23.6), the fluorescence of 7KQ is collected as a function of the excitation energy provided to 7HQ. It may make it possible to estimate the energy required for the onset of a process and to check from which isomer the keto form is built. The UV-UV depletion scheme is similar to the UV-UV HB, except that the HB laser is scanned before a fixed 2C-R2PI measurement (see Figure 23.5, right). This measurement actually probes all vibrational levels in

Table 23.1 The pKa values in aqueous solution from Ref. [59]. For the S1 state, a range is indicated instead of a single value because different methods have been employed Equilibrium þ

7H2 Q > 7HQ=7KQ þ H 7HQ=7KQ > 7Q þ H þ

pKa þ

S0 state

S1 state

5.64 8.67

8.813.5 2.73.5

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

529

Figure 23.4 The resonant two-photon ionization 2C-R2PI (left) and the fluorescence emission (right) measurement schemes

the S1 state independently of further non-radiative processes, contrary to 2C-R2PI measurements, for which the signal intensity depends on the lifetime of the vibrational level in the excited state. Over the last decades, supersonic expansions have emerged as an ideal tool to investigate cluster structures and dynamics using laser spectroscopy for three main reasons: (i) the absorption and fluorescence spectra are dramatically narrowed owing to the low rotational/vibrational temperatures (1–5 K); (ii) the low translational temperature (around 1 K) achieved in supersonic expansions makes it possible to generate and preserve weakly bound clusters; (iii) the collision-free regime allows the depletion of specific energy levels by optical pumping.

Figure 23.5 Scheme of UV-UV hole burning (left) and UV-UV depletion (right) measurements

530 Hydrogen Bonding and Transfer in the Excited State

Figure 23.6 Scheme of the fluorescence action measurement

The molecular clusters probed experimentally and discussed in this chapter are all formed during the adiabatic expansion of 7-hydroxyquinoline heated to 170  C and expanded in a gas mixture of Ne and less than 1% of the desired solvent molecule. 23.2.1.2 The 7HQ(NH3)3 Cluster Figure 23.7 shows the 2C-R2PI spectra of the 7HQ(NH3)n clusters, 1  n  4. These structures have been previously analysed and unambiguously assigned to wire-type cluster structures, as indicated in Figure 23.8 and discussed in Refs [46, 71–75]. Regarding the n ¼ 4 cluster 2C-R2PI spectrum, the bands have quite a large width (25–70 cm1) and are superimposed on a broad background. This is probably due to an ultrarapid excited-state process such as proton or H-atom transfer [76]. Conversely, the n ¼ 13 cluster spectra exhibit discrete and sharp bands with rotational contour widths from 1 to 2 cm1. The n ¼ 1 and n ¼ 2 spectra show bands at least 1500 and 900 cm1 above the electronic origin, respectively. The n ¼ 3 spectrum shows narrow bands up to 200 cm1 above the electronic origin. Figures 23.9 (a) and (b) show the low-frequency part of the 2C-R2PI spectrum of 7HQ(NH3)3 in more detail and the corresponding UV-UV depletion spectrum obtained by scanning a HB laser over the same spectral region, 300 ns before the 2C-R2PI measurement on the electronic origin of 7HQ(NH3)3. The latter spectrum reveals the S1 vibrational levels above 200 cm1 which are not observable in the 2C-R2PI spectrum, indicating that the cluster does not ‘lose’ its vibrational structure, but the initially excited form of the cluster is lost: n ¼ 3 is the critical size for the onset of a fast process in the S1 state. The fluorescence emission spectra measured at several excitation energies are shown in Figure 23.10. When the cluster is excited at the electronic origin, one only observes the UV fluorescence due to the enol form of the cluster. This fluorescence, however, drops off rapidly 200 cm1 above the electronic origin, in qualitative agreement with the disappearance of the 2C-R2PI signal and confirming the loss of the excited 7HQ (NH3)3 population. Simultaneously and concurrent to the loss of the UV fluorescence, a yellow

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

Figure 23.7

531

Two-colour resonant two-photon ionization (2C-R2PI) spectra of 7HQ(NH3)n, 1  n  4

fluorescence emission appears and increases. Such a yellow fluorescence has already been observed in bulk protic solvents [59, 60, 68, 77] and been assigned to the emission from the S1 state 7-ketoquinoline tautomer. It is worth noting that we did not observe any other fluorescence in our measurements; this point will be discussed later. As fluorescence emission experiments are not size selective, we cannot definitely identify the origin of the yellow fluorescence (i.e. the cluster size of the emitted keto form). Therefore, we recorded fluorescence action spectra by monitoring the yellow fluorescence of the newly formed keto at the maximum of the fluorescence emission spectra while scanning the excitation laser over the UV absorption bands of the enol form of 7HQ(NH3)3. This spectrum, shown in Figure 23.9(c), exhibits the same characteristic vibronic band pattern of 7HQ(NH3)3 as observed in UV-UV depletion (see Figure 23.9(b)). Moreover, the onset of the discrete band pattern in the fluorescence action spectrum coincides with the fall-off of the 2C-R2PI spectrum, 200 cm1 above the electronic origin. These results allow us to conclude that at least a part of the yellow fluorescence due to the keto tautomer comes from the excitation of the n ¼ 3 cluster. All these results have been confirmed by performing the same set of measurements with the deuterated isomer 7DQ(ND3)3: the lowfrequency part of the 2C-R2PI spectrum is similar to that of 7HQ(NH3)3, and a decrease in vibrational band intensities also occurs, simultaneously with the rise of the yellow fluorescence [46]. The difference lies in the onset of the fast excited-state process: for the deuterated isotopomer, it occurs at higher energies, over 300 cm1, as will be discussed later.

532 Hydrogen Bonding and Transfer in the Excited State

Figure 23.8

Minimum energy structures of the 7HQ(NH3)n, 1  n  4 clusters

Figure 23.11 summarizes the information obtained from the laser experiments: the excitation of the enol 7HQ(NH3)3 at energies higher than 200 cm1 above its electronic origin leads to excited-state tautomerization and the formation of the excited keto 7KQ (NH3)3 cluster. We have managed to build a prototype system for which we can observe translocation along the hydrogen-bonded wire. The experiments with a time resolution of 5–10 ns cannot provide further information on the mechanism: they only prove that the tautomerization occurs, but we cannot reach a definitive conclusion about the mechanism itself. Therefore, we now turn to ab initio calculations to obtain a deeper understanding of the reaction. 23.2.2 Theoretical calculations 23.2.2.1 Choice of Model Chemistry We assume that the first step of the 7HQ ! 7KQ tautomerization reaction involves dissociation of the O--H bond. This dissociation can be heterolytic, leading to the formation of the 7-quinolinate (7Q) anion and a proton, or homolytic, leading to the formation of the 7-quinolyl radical and an H atom. Sobolewski and Domcke investigated the excited-state reactivity of the heteroaromatic chromophore indole [78] in the late 1990s using complete active space self-consistent field (CASSCF) theory. They showed that the initially excited pp state is coupled to a repulsive ps state along the N--H coordinate. The ps state can be mainly described as the excitation of an electron from the p HOMO to a diffuse Rydberg-type s orbital and is not

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

533

Figure 23.9 Spectra of 7HQ(NH3)3 in the excited state: (a) 2C-R2PI; (b) UV-UV depletion; (c) fluorescence action spectrum recorded at 18 360 cm1 (the slowly rising background in the fluorescence action spectrum arises from the fluorescence of larger clusters also produced in the supersonic expansion)

accessible from the electronic ground state by direct absorption. As the N--H bond is stretched, the ps state crosses the pp and the electronic ground state, leading to an N--H homolytic bond cleavage. Later on, analogous behaviour was predicted for chromophores such as pyrrole [79] and phenol, as well as hydrogenbonded complexes such as phenolH2O and phenolNH3 [54, 80]. This represents a new type of photochemical reaction pathway. An important point concerning calculations of such ps states is that the standard basis sets need to be augmented with very diffuse functions. In order to investigate the possible importance of ps excited states for 7HQ photochemistry, we calculated potential energy curves for the S0 and the lower pp and ps excited states. The O--H distance was fixed at  different values between 0.9 and 2.4 A, and all remaining degrees of freedom were allowed to relax for each state individually. All geometry optimizations were performed with the CASSCF method correlating eight electrons in eight orbitals ((8,8)-CASSCF). The standard 6-31G(d, p) basis set was augmented with diffuse s and p functions (as ¼ ap ¼ 0.02) centred on a floating dummy centre positioned between the O and the H atom. The location of the dummy centre along the O--H bond was optimized together with the other degrees of freedom. For the ground-state and the pp excited-state surfaces, the four p and four p orbitals that dominate the pp state wave function formed the active space, whereas for the ps state the least important p orbital was replaced by the s . The three orbitals that mainly describe the pp and ps states are shown in Figure 23.12. To account for dynamical correlation effects, multireference second-order Møller–Plesset (MRMP2) calculations were performed at the (8,8)-CASSCF optimized geometries. The MRMP2 potential energy

534 Hydrogen Bonding and Transfer in the Excited State

Figure 23.10 Fluorescence emission spectra of 7HQ(NH3)3, excited at the electronic origin O00 ; þ155; þ187; þ200; þ256 and þ461 cm1. The visible fluorescence is scaled 10 times. The UV fluorescence for excitation at þ461 cm1 is not shown because of the strong contribution of the 7HQ(NH3)2 cluster overlapping that of 7HQ(NH3)3

curves along the O--H bond dissociation coordinate are shown in Figure 23.13. The ps potential energy curve crosses both the pp excited-state and ground-state curves, similarly to phenol [80] and indole [78]. The ps curve crosses the pp curve 11 500 cm1 above the pp minimum, significantly higher than in the case of indole [78] or phenol [80]. If one takes out-of-plane vibrations into account, an avoided crossing occurs instead of a conical intersection, as shown in Figure 23.14. Configuration interaction single (CIS) calculations can also qualitatively reproduce the potential energy curve crossings, like the high-level CASSCF or MRMP2 methods, provided that the basis set used contains functions that are sufficiently diffuse [45]. While CIS lacks in contributions from double, triple and higher excitations and hence overestimates barriers and excitation energies, it offers a large reduction in the computational cost of our investigations on a medium-size system such as 7HQ(NH3)3. In the following sections, it will be shown that CIS calculations with appropriate basis sets provide results that are in good agreement with both the MRMP2 calculations and the experimental observations. Excited-state calculations of the two systems used as examples in this chapter have been performed with several methods; here we chose to present the results of the CIS calculations only in order to compare all the results on an equal footing. In this chapter we will also discuss calculations involving a hydrogen-bonded wire only; considering that the excitation in our experiments is initially located on the chromophore, calculations concerning the properties of the hydrogen-bonded wires were performed in the ground state at the B3LYP level. Experience has shown that

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

535

Figure 23.11 Schematic of the spectroscopic information: vibrational levels in the S0 (from UV fluorescence emitted) and S1 (from the UV-UV depletion spectrum) states, and onset of a fast process in the S1 state (from 2C-R2PI and UV-UV depletion spectra), i.e. the enol ! keto tautomerization (from the yellow fluorescence action spectrum)

the B3LYP method can reproduce the electronic ground-state vibrational pattern experimentally observed, and it represents a good compromise between computational cost and accuracy. Because these calculations are performed in the electronic ground state, a good representation of the ps state is not essential, and we used the standard basis set 6-311þþG(2d, 2p) with the B3LYP method.

Figure 23.12 (a) Dominant one-electron excitation of the pp and ps excited states of 7-hydroxyquinoline. For a given excitation, the same contour value is used for the two orbitals, but a lower contour value was used in the case of the ps excited state in order to properly visualize the spatial diffuseness of the s orbital

536 Hydrogen Bonding and Transfer in the Excited State

Figure 23.13 MRMP2 potential energy curves for O--H bond breaking in the electronic ground state and the lowest excited states of A0 (pp ) and A00 (ps ) symmetry of 7-hydroxyquinoline

Figure 23.14 Schematic representation of a reaction path with a conical intersection (a) or an avoided crossing (b)

23.2.2.2 ESHAT We have investigated the stationary points along the excited-state proton transfer (ESPT) reaction path, assuming a Grotthuss-like mechanism, i.e. series of proton translocation steps. Figure 23.15 shows the schematic S1 potential connecting the fully optimized CIS stationary points obtained along the enol ! keto tautomerization path. The ground-state enol form is taken as reference for these calculations, and the excitedstate potential has been arbitrarily offset to match the experimental excitation energy at the electronic origin (28 798.4 cm1). The S1  S0 energy difference for 7KQ after S1 state offset is predicted to be 18 020 cm1, in

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

537

Figure 23.15 CIS fully optimized stationary points for the excited-state proton transfer (dotted line) and excitedstate H-atom transfer (full line) of 7HQ(NH3)3. The calculated energies of the enol and keto forms in the ground state are also indicated. For each minimum of the ESHAT path, the orbital that contributes dominantly to the excited state is shown

good agreement with the experimentally determined fluorescence maximum at 18 360 cm1. This enol ! keto tautomerization pathway exhibits one single minimum, PT2, corresponding to a translocation of two protons, one from the hydroxyl group to the first ammonia molecule of the wire, and one from this molecule to the second of the wire. The form corresponding to the proton transfer from the hydroxyl group to the first molecule of the wire, i.e. PT1, is a transition state, as well as the form for which three protons have already been transferred along the wire, i.e. PT3. Taking the experimental reaction threshold of only 200 cm1 and a larger threshold for a deuterated cluster (350 cm1) into account, one would assume a tunnelling process and consequently a quite narrow barrier. In the case of the ESPT pathway, the first step of the reaction produces significant geometrical changes, which implies a wide barrier. Also, the intermediate form is less stable in energy than the enol form, and here one would expect a higher activation energy. Finally, this form should fluoresce, but we did not observe any fluorescence except that from the enol and the keto forms. This does not prove that the reaction occurring is not ESPT, but seems annoying. We have also investigated the excited-state H-atom transfer (ESHAT) reaction with the same method; this was motivated by the results on phenolH2O and phenolNH3 [80], as well as our investigation of the bare chromophore 7-hydroxyquinoline (see above). The schematic reaction path is shown in Figure 23.15 for comparison with ESPT. In this case, three minima have been found between the enol and the keto forms, corresponding to the H-atom-transferred intermediates on the first, second and third ammonia molecule of the wire and denoted HT1,HT2 and HT3 respectively. These minima are separated by four transition states labelled TSe/1,TS1/2,TS2/3 and TS3/k, so that the global path is composed of four steps. All the stationary points are depicted in Figure 23.16, each line representing one step of the H-atom translocation steps. The calculated barrier at the first transition state (TSe/1) connecting enol and HT1 is 3790 cm1 (45.3 kJ mol1), which is probably too high because the CIS method is known typically to overestimate such barriers [54, 80, 81]. This transition state originates from an avoided crossing between the pp and ps

538 Hydrogen Bonding and Transfer in the Excited State

Figure 23.16 7HQ(NH3)3 CIS fully optimized stationary points along the ESHAT pathway. Each line corresponds to a single step in the enol ! keto tautomerization reaction path. The circle indicates the H atom that moves during each step (See Plate 30)

excited states, as indicated in Figure 23.15 (top left). The enol ! HT1 step is predicted to be exoergic by about 2400 cm1 (28.7 kJ mol1) and provides the driving force for the global reaction. The HT1, HT2, HT3 and 7KQ forms lie close in energy; once the system passes through the TSe/1 barrier, the following two barriers are surmounted. The last barrier TS3/k between the HT3 and 7KQ forms is calculated to be nearly as high as TSe/1 (see Figure 23.15, top right) and originates from a reverse crossing between the ps and pp states, leading to the pp excited 7KQ (NH3)3 tautomer. A comparison of the ESPT and ESHAT reaction paths reveals that the HT1 minimum is 28.7 kJ mol1 (2400 cm1) more stable than the enol, while the PT2 minimum is 48.9 kJ mol1 (4100 cm1) less stable. The PT reaction path always lies above the HAT reaction path, and thus the electronic excitation provides no driving force for the proton transfer but does so for the H-atom transfer; hence, there is no competition between PT and HAT in the excited state for 7HQ(NH3)3. The first step of the complete reaction path is seen to play a key role, and we therefore focus our investigation on it. In the ESHAT reaction path, the enol and HT1 minima are separated by the transition state TSe/1 on the potential energy surface. From this first-order saddle point, the system was propagated on steepest descent paths in mass-weighted coordinates backwards and forwards to the enol and HT1 minima. The resulting reaction profile is visualized in Figure 23.17 as a function of two coordinates: (i) the distance R between the O

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

539

Figure 23.17 Potential energy along the intrinsic reaction coordinate (IRC) connecting the enol and the HT1 minima of 7HQ(NH3)3. Projections of the potential energy are plotted as a function of the s ¼ r(O  P)  r(P  NA) coordinate and R ¼ r(O  NA) coordinate. See the text for the detailed definition

atom of the quinoline and NA, the N atom of the nearest ammonia molecule of the wire, and (ii) the coordinate s, which is defined as the difference r(O  P)  r(P  NA), P being the projection of the moving H on the O  NA axis. The R coordinate involves only heavy atoms and is denoted the ‘heavy’ coordinate, whereas s involves the movement of the H atom and is the ‘light’ coordinate. The potential energy curve as a function of s (grey projection in Figure 23.17) is very similar to the potential energy profile V(rO--H) published previously, in which the O--H distance was used as the driving coordinate [46], and one can assume that s provides a good description of the reaction coordinate. Conversely, the potential energy curve as a function of R (projection on the left in Figure 23.17) unveils the importance ofthe separation into a ‘light’ and a ‘heavy’ coordinate: the  reaction starts at the enol minimum with R ¼ 2.80 A and s ¼ 0.87 A . In a first step, R contracts by more than   0.3 A and s increases by 0.3 A. Up to this point the energy has risen 1500 cm1 (18 kJ mol1) above the enol reference. During the second step the O  NA distance stays essentially constant, whereas s dramatically increases (see the projection of the potential energy curve on the (s, R) plane in Figure 23.17); this section of the path accounts for 60% of the barrier height (27.5 over 45.5 kJ mol1). The last step occurs mainly along the R coordinate, which increases, bringing the cluster to a geometry close to the original one before the transfer. The fact that most of the energy barrier is passed along the light coordinate agrees with a tunnelling mechanism, as described above. Figure 23.18 schematically summarizes the information about the first step of the ESHAT reaction path. In the next part we will investigate some parameters that can influence the ESHAT reaction path and the competition between ESPTand ESHAT. We consider the effect of the parameter weak if it slightly modifies the height of the barrier around TSe/1 or renders the enol ! HT1 pathway more favourable. On the other hand, we consider the effect strong if there is a possibility of preventing the ESHAT reaction and favouring ESPT. For some parameters (mode selectivity and solvent effect), we have combined experimental measurements and ab initio calculations. For other parameters, our calculations agree with published experimental data (chromophore local solvation) or are only predictive (wire solvation) and would benefit from confirmation by experimental measurements.

540 Hydrogen Bonding and Transfer in the Excited State

Figure 23.18 Schematic representation of the pp and ps potential energy curves along the H-atom transfer coordinate from the enol to the HT1 forms. In Cs symmetry, the curve crossing gives rise to a conical intersection (indicated by dotted lines). In C1 symmetry, the lower sheet of the conical intersection forms a barrier near which the electronic configuration switches from pp to ps . The n ¼ 0 vibrational energy level of the O--H (or O--D) vibrational stretching mode of 7HQ (or 7DQ) and the action integral controlling the tunnelling through the barrier are indicated

23.3 What Favours/Prevents ESHAT 23.3.1 Weak effects 23.3.1.1 Wire Solvation In the late 1970s, Aue et al. defined the hydrogen affinity of alkylamines as the difference between the proton affinity of a species M and the ionization potential of M and H [82]. Later, Jouvet et al. suggested a generalization of the concept to characterize the ability of a neutral species to be linked to an H atom [53]. Therefore, analogous to the proton affinity PA, we define the H-atom affinity HA in the gas phase of a species M as the negative of the enthalpy change for the reaction M þ H  ! MH 

ð23:1Þ

If HA is positive, the reaction is exothermic. In the following, we neglect the zero-point energy and thermal corrections and consider the energy change DHATE for reaction (23.1); if DHATE is negative, then the reaction is exoergic. For the first H-atom transfer step from 7HQ to the solvent wire, we have to consider De ! HT1 E ¼ DHAT EðwireÞDHAT Eð7Q ÞDD0

ð23:2Þ

where DD0 is the difference in binding energy between 7Q and (wire H) and that between 7HQ and the wire. We believe that this term is smaller than the two others in the expression for De ! HT1 E in a first approximation, and it will be neglected in the following. According to B3LYP/6-311þþG(2d, 2p) calculations, the dehydrogenation of 7HQ is endoergic by þ 382.0 kJ mol1. This energy is mainly supplied by the electronic excitation of the 7HQ chromophore in our experiment, and the rest has to be compensated for by the exoergicity of the hydrogenation of the wire in

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

541

Table 23.2 DHATE (in kJ mol1) calculated at the unrestricted B3LYP level with the 6-311þþG(2d, 2p) basis set System DHATE

7Q þ382.0

NH3 24.7

(NH3)2 40.5

(NH3)3 51.0

(NH3)4 54.8

order to observe any reaction without providing too much excess energy to the cluster. We have performed unrestricted B3LYP calculations with the same basis set as for 7HQ, optimizing the wire without the scaffold molecule while constraining the distance between the heavy atoms of the connecting molecules of the wire to be that in a 7HQ(NH3)n cluster. In Table 23.2 we report the DHATE values for (NH3)n clusters with 1  n  4. DHATE predicts that the NH3 þ H ! NH4 reaction is exoergic (24.7 kJ mol1). Care should be taken with this value, as the calculation does not include zero-point vibrational energy and thermal corrections. Our results, however, are in good agreement with previous calculations [83–87]. The potential energy curves of ground and low-lying excited states for the dissociation of Rydberg NH4 radicals into (NH3 þ H) have been calculated with different levels of theory, namely MP2, MP4, CIPSI, SDCI and CCSDT. The potential energy gap between NH4 and (NH3 þ H) is very small, and the conclusions about the formation of NH4 from (NH3 þ H) are not straightforward: some calculations predict it to be very slightly endothermic, and others exothermic. Park [87] thus concluded that NH4 and (NH3 þ H) are nearly isothermic. Spectroscopic measurements have shown that NH4 has a very short lifetime [88, 89]. Conversely, in small NH4(NH3)n clusters, the radical is stabilized extensively and its lifetime is up to 106 times longer than that of the monomer [90, 91, 92]. In our B3LYP calculations, the capture of an H atom is predicted to be exoergic for (NH3)n with n > 1, as already observed with calculations at the MP2 level [93]. Moreover, the larger the wire, the larger is the affinity for H atom transfer. Increasing the wire length stabilizes the (NH3)nH radical, in agreement with experimental observations. From the B3LYP calculations, one should at least provide 28 000 cm1 to 7HQ(NH3)3, but more than 30 000 cm1 in the case of 7HQNH3, which could explain why we did not observe any of the 2C-R2PI spectrum break-off with our experimental conditions in the case of this cluster. We can, of course, not provide a quantitative match with experimental values, as our calculations do not take the height of the barrier of the reaction path into account, nor do they account for any tunnelling effect and binding energy of the cluster. Nevertheless, the minimum energy required seems to be in agreement with our experimental data. We have also performed another series of DHATE calculations for ‘solvated’ three-ammonia-molecule wire, adding ammonia molecules hydrogen bonded to the wire. Note that these series of calculations are only speculative, and no experimental measurements have been performed to confirm them yet. The molecules of the wire were used as H-bond donor to new H-bond acceptor solvent molecules. To distinguish hydrogen bonds on the first molecule of the wire from those on the second and the third, we introduce the following notation: the first number indicates the total number of ammonia molecules (including the three of the initial wire), and the indices in the subscript refer to the number of hydrogen-bonded molecules on the first (A), second (B) and third (C) molecules of the wire respectively. For example, the 4100 structure describes a three-ammonia-molecule wire with a fourth molecule hydrogen bonded to the first molecule of the wire. Figure 23.19 shows the 5101 and 7211 structures as examples. We also introduce the concept of the solvation index dS for a hydrogen-bonded cluster by assigning different weights to the hydrogen-bonded molecules on the wire: a molecule hydrogen bonded to the first molecule of the wire contributes 1 to dS, whereas those hydrogen bonded to the second and the third molecules contribute 0.33 and 0.11, respectively, to dS. The three molecules of the wire do not contribute to dS, so that each structure has an unambiguous solvation index. The advantage of dS is that it represents the cluster connectivity by a single number.

542 Hydrogen Bonding and Transfer in the Excited State

Figure 23.19 and 2.44 (b)

Optimized structures for three-ammonia-molecule wire with a solvation index dS of 1.11 (a)

Some values of DHATE are provided in Table 23.3. From Table 23.3(a), DHATE decreases as the first wire molecule is solvated: the decrease in DHATE is between 15 and 18 kJ mol1 on the addition of a first molecule (or an increase in dS by 1), and it is between 18 and 32 kJ mol1 on the addition of a second molecule, depending on the degree of solvation in the initial wire (further results available in reference [94] show the same behavior when two molecules are hydrogen bonded to the second or the third wire molecule). In other words, the solvation of the first wire molecule favours H-atom transfer to the wire. As regards Table 23.3(b), the influence of the solvation of the second and third wire molecules is not as straightforward as in the case of the first wire molecule. The first reason is that the changes in DHATE are much smaller, i.e. between 6 and þ 6 kJ mol1. The second reason is that we were not able to find a clear monotonic change in DHATE by increasing the solvation index. Moreover, these values are small enough to be of the same order of magnitude as the accuracy of the calculations. DHATE might slightly increase with the solvation of the second and the third wire molecules, which means that the solvation of the last two wire molecules does not favour the ESHAT reaction; nevertheless, this effect would be compensated for by the solvation of the first wire molecule. All in all, the solvation of the wire favours the ESHAT reaction in that it makes the first step of the reaction path more exoergic. Table 23.3 Wire solvation effect on (a) the first molecule of a three-ammonia wire, and (b) the last two molecules of the ammonia wire: DHATE (in kJ mol1) calculated at the B3LYP level for three-ammonia-molecule wire. The 3000 structure (dS ¼ 0) is used as reference for comparison. See the text for the symbol used to characterize a structure and the definition of the solvation index dS (a) Influence of solvation on (NH3)A Structure dS DHATE

3000 0.0 51.0

4100 1.0 65.6

5200 2.0 83.6

Structure dS DHATE

4001 0.11 45.1

5101 1.11 61.0

6201 2.11 92.4

Structure dS DHATE

4010 0.33 48.0

5110 1.33 66.0

6210 2.33 91.5

(b) Influence of solvation on (NH3)B and (NH3)C Structure dS DHATE Structure dS DHATE

3000 0.0 51.0

4010 0.33 48.0

4001 0.11 45.1

5020 0.66 53.9

5002 0.22 47.2

5011 0.44 51.8

6012 0.55 49.7

6021 0.77 54.0

7022 0.88 56.8

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

543

23.3.1.2 Mode Selectivity As regards the 2C-R2PI spectrum of 7HQ(NH3)3, all overtones and combinations of the intermolecular ammonia wire vibrations, i.e. s1, s2, s3 and s4 (see reference [46] for labeling and detailed assignment), have very short lifetimes: they appear only in the UV-UV depletion spectrum, and not in the 2C-R2PI spectrum (see Figure 23.20). However, several intramolecular vibrational fundamentals, such as n4 and n7, are still weakly observed in the 2C-R2PI spectrum above the threshold [48]. We have also recorded 2C-R2PI and UV-UV depletion spectra for the deuterated cluster 7DQ(ND3)3. These are shown in Figure 23.20. The disappearance of the vibronic bands in the 2C-R2PI spectrum of 7DQ(ND3)3 is observed above 300 cm1. The threshold is less sharp than for the non-deuterated cluster, and several vibronic bands appear above 300 cm1. These bands are assigned to intramolecular vibrations and appear more intensely in the 2C-R2PI spectrum of the deuterated cluster (see Figure 23.20(b)). These observations imply that the ESHAT reaction is vibrational mode selective, and that this selectivity is more pronounced for the deuterated cluster. The vibrational analysis shows that the intramolecular vibrations of the 7HQ chromophore accelerate the ESHAT reaction less than the intermolecular ammonia wire stretching modes [46, 49, 95].

Figure 23.20 2C-R2PI and UV-UV depletion spectra of 7HQ(NH3)3 (a) and the deuterated cluster 7DQ(ND3)3. Intramolecular vibrational excitations that lie above the ESHAT threshold are marked with asterisks

544 Hydrogen Bonding and Transfer in the Excited State

Figure 23.21 Comparison of the 6-31( þ )G(d, p) eigenvectors of the s4 and n4 normal modes to the difference vector between the S1 state enol minimum and the TSe/1 transition state geometries

Within the framework of non-adiabatic proton transfer theory, several models have been developed that predict a vibrational state dependence of the tunnelling rates [3, 9, 10, 96–100]. These models predict that excitation of the intermolecular stretching vibrations allow the cluster to approach the barrier around TSe/1, thereby increasing the tunnelling rate. We will discuss here one example of this effect, the results being similar for all the other vibrations (s1, s2 and d1 for the intermolecular modes, n1, n2, n7 and n14 for the intramolecular modes) [49]. Figure 23.21 shows the eigenvectors of the intermolecular normal mode s4 (the O--H  NA stretching mode) and the intramolecular mode n4 (7HQ in-planevibration). These are compared with the CIS-calculated difference between theexcited-stateenol andTSe/1 geometriesof7HQ(NH3)3.Asmentionedpreviously, the main changein geometry at the TSe/1 transition state is a contraction of the O  NA distance of the 7HQ--O--H  NH3 hydrogen bond. The s4 vibrational mode modulates the O  NA distance and therefore modulates the tunnelling path for the ESHAT reaction. On the other hand, the n4 intramolecular vibrational mode distorts the 7HQ framework, but this does not act on the O  NA bond and does not propel the system towards the TSe/1 barrier [48]. The mode selectivity is closely connected to the pp /ps state crossing: the modes that accelerate ESHAT are those that reduce the energy difference between the pp and ps states. For each vibrational mode, the pp and ps potential energy curves were calculated for small displacements along the pp state normal-mode eigenvectors (details of the calculations are given in reference [49]). Figure 23.22 shows the resulting curves for the s4 and n4 modes. As far as the pp state is concerned, the potential curves as a function of the s4 and n4 coordinates are close to harmonic, albeit with different curvatures. On the other hand, as regards the ps state, displacement along the s4 intermolecular coordinate leads to monotonic energy decreases, as shown in Figure 23.22. In other words, displacement along the intermolecular coordinate leads towards regions with a lower ESHAT barrier arising from the crossing between the two states. In contrast, along the n4 coordinate the potential energy curve for the ps state resembles that for the pp state. Vibrational excitation along this coordinate does not reduce the pp ps energy difference significantly. The PE curves of d1, n1 and n2 (not shown here) are similar to that of n4, whereas the PE curves of s1 and s2 are similar to that of s4 [49]. Figure 23.23 shows the calculated pp  ps energy difference DErel relative to that for the enol form versus the excess vibrational energy. The energy difference is highly specific to the vibrational mode: vibrational displacements along the intramolecular modes n1, n2 and n4 increase the pp  ps energy difference compared with the enol form by less than 100 cm1. Hence, they do not markedly influence the tunnelling barrier. On the other hand, the intermolecular modes s1, s2 and s4 and the intramolecular modes n7 and n14 decrease the pp  ps energy difference by 300–450 cm1, i.e. excitation of these modes in the pp state leads to considerable decreases in the barrier height. The most striking example of mode selectivity is the difference in DErel for the 2s4 and n7 excitations: they have almost the same excess vibrational energy above

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

545

Figure 23.22 Potential energy Erel of the pp and ps states along intermolecular normal coordinate s4 (a) and intramolecular normal coordinate n4 (b). The energies are relative to the minimum of the pp state. Note the break in the vertical scale

the ESHAT threshold, but the intermolecular excitation reduces the pp  ps energy difference 40% more than the intramolecular excitation. Similar effects are predicted for the deuterated clusters, and the calculated mode selectivity effects as well as the changes in mode selectivity upon deuteration are in agreement with experimental observations [46].

Figure 23.23 Calculated energy differences DErel between the pp and ps potential energy surfaces (in cm1) of the 7HQ(NH3)3. For each vibrational mode, the displacement along the positive vibrational coordinate (as shown in Figure 23.21) is chosen to yield the vibrational n ¼ 0 ! 1 excitation energy in the pp state. The pp  ps vertical energy difference is calculated at this displacement

546 Hydrogen Bonding and Transfer in the Excited State

This is one of the first examples of excited-state vibrational mode selectivity on tunnelling in such a large system. We note that the tunnelling rate is enhanced by a factor of 5–10, even though the pp  ps energy difference decreases by only 2–4% of the calculated ESHAT barrier (12 500 cm1). 23.3.2 Strong effects 23.3.2.1 Chromophore Local Solvation A similar but larger system has also been investigated: the fluorescent moiety of the green fluorescent protein (GFP) [101]. Its chromophore consists of an aromatic phenol-type ring and a five-membered heterocycle. It is involved in a hydrogen-bonded wire involving a water molecule hydrogen bonded to a serine residue, which in turn connects a glutamate residue that is hydrogen bonded back to the chromophore (see Figure 23.24). With these considerations, the system is analogous to 7HQ(NH3)3, the chromophore exhibiting acidic and basic functional groups and acting as a scaffold for the hydrogen-bonded wire. The GFP chromophore also exhibits the particularity of pp /ps crossing when the OH bond of the hydroxyl group is stretched. An H-atom transfer between the excited green fluorescent protein chromophore and the water molecule has been predicted to arise from this state crossing, and leads to an ESHAT reaction, similarly to phenolH2O [102, 103]. This crossing can be described with CIS calculations and a diffuse basis set (see Figure 23.25). In this case, after the homolytic OH bond breaking, the system is in a dark state that does not fluoresce. On the other hand, fluorescence of the anionic form of GFP (after heterolytic OH bond breaking) has been observed [52], which indicates that ESPT occurs and not ESHAT. This underlines the importance of the interaction between the chromophore and its protein environment. If the chromophore intrinsically possesses the same property as phenol or 7HQ, i.e. pp /ps crossing when the OH bond is stretched, the question of competition between proton and H-atom transfer arises. Completing the wire and connecting it back to the chromophore does not significantly affect the pp /ps crossing [101]. Finally, the solvation around the hydroxyl group of the chromophore was investigated. Figure 23.26 shows the four structures that were optimized at the CIS level of theory with a basis set analogous to that used for 7HQ(NH3)3. System (a) is the bare chromophore, while system (b) is the chromophore hydrogen bonded to the water molecule. System (c) is composed of the chromophore hydrogen bonded to a water molecule and an imidazole (representing the histidine residue in GFP) hydrogen bonded to the O atom of

Figure 23.24 Structure of the fluorescent moiety of the green fluorescent protein: the chromophore responsible for the fluorescence is hydrogen bonded to a water molecule, followed by a serine residue and a glutamate residue that connects to the scaffold chromophore. The amino acid backbone has been removed to emphasize the analogy of this system with 7HQ(NH3)3 (See Plate 31)

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

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Figure 23.25 CIS/6-31( þ )G(d, p) single occupied molecular orbitals that mostly contribute to the pp and ps excited states of the system composed of a chromophore hydrogen bonded to a water molecule; the p orbital is located on the chromophore, while the s orbital is located on the water molecule

the chromophore hydroxyl group. System (d) is composed of system (c) and a further hydrogen bond on the O atom of the hydroxyl group, with a carboxylic acid representing the threonine residue in the GFP environment. Figure 23.27 shows the energy difference between the pp and ps states for the hydroxyl OH bond  stretched to 1.35 A. For the bare chromophore (system (a)) and for system (b), the lowest excited state is ps , because the state crossing occurs at shorter OH bond length. The pp state lies at least 30 kJ mol1above the ps state. When the hydroxyl group of the chromophore is further solvated, the pp state is stabilized relative to the ps state: for systems (c) and (d), the pp state lies 30 kJ mol1 and more than 90 kJ mol1below the ps state respectively. For system (d), the pp state stabilization is so large that no state crossing occurs even at a longer OH bond length. This shows that solvation of the OH group of the chromophore and the stabilization of the nascent anion play a key role in the ESPT/ESHAT competition. It also explains why the fluorescence of the deprotonated form is observed experimentally in solution, i.e. when the chromophore is fully solvated.

Figure 23.26 CIS/6-31(þ)G(d, p) optimized structures of (a) the bare GFP chromophore, (b) system (a) hydrogen bonded to a water molecule, (c) system (b) with a histidine-like residue hydrogen bonded to the hydroxyl group of the chromophore and (d) system (c) with a threonine-like residue hydrogen bonded to the hydroxyl group of the chromophore (See Plate 32)

548 Hydrogen Bonding and Transfer in the Excited State

Figure 23.27 CIS/6-31(þ)G(d, p) relative energy (in kJ mol1) of the pp and ps states for systems (a), (b), (c)  and (d) for an O--H bond length stretched to 1.35 A. The ps state energy is taken as reference. See the text and Figure 23.26 for the labels of the systems

23.3.2.2 Solvent Effect The last point we would like to discuss in this chapter pertains to the effect of replacing ammonia with a water molecule in a hydrogen-bonded wire. As in Sections 23.2 and 23.3.1.2, we have used 7HQ hydrogen bonded to a wire as an example, and we have combined experimental measurements and ab initio calculations to model the observations. In the following, the notation for the 7HQ(NH3)n(H2O)m clusters designates the monomer sequence in the chain: the ‘pure’ ammonia and water wire clusters are denoted by AAA and WWW respectively. The cluster with one water molecule in the middle of the wire is denoted by AWA, and so on. Except for WWW, all clusters studied exhibit an ammonia molecule at the beginning of the wire. As previously, the structures of the isomers have been identified by vibrational pattern and electronic origin assignments [95, 104]. Figure 23.28 shows the 2C-R2PI spectra of 7HQ(NH3)n(H2O)m, n þ m ¼ 3. All spectra are plotted relative to the respective electronic origin. They exhibit the same characteristics: narrow vibrational bands, followed by an abrupt decrease in the signal towards higher excess vibrational energy. The band structure extends to increasingly higher vibrational frequency with increase in the number of H2O molecules in the solvent wire. For 7HQ(NH3)3, the vibronic structure breaks off at 200 cm1 above the electronic origin. For 7HQ (NH3)2H2O, the vibronic band structure disappears at 350 cm1 above the electronic origin, and for 7HQNH3(H2O)2 at 500 cm1. In contrast, the spectrum of 7HQ(H2O)3 exhibits sharp vibronic bands at least up to 1200 cm1 above the electronic origin. It is worth noting that no yellow fluorescence was observed from any cluster except 7HQ(NH3)3. We can draw several conclusions from these observations. First, the energy threshold for H-atom transfer depends on the chemical composition of the wire, and stepwise replacement of NH3 by H2O increases the energy threshold for H-atom transfer. Second, no excited-state reaction occurs for 7HQ(H2O)3 up to an excess vibrational energy of 1200 cm1. Finally, even a single NH3 molecule in the solvent wire can induce excitedstate reactivity. In other words, the experimental results show that the introduction of any water molecule blocks the complete ESHAT reaction, i.e. the enol ! keto tautomerization. Our interpretation of the effect of the chemical composition of the solvent wire on the ESHAT reaction is based on CIS calculations using a diffuse basis set as for 7HQ(NH3)3. Figure 23.29 compares the CIS/6-31(þ)G (d, p) calculated ESHAT reaction profiles of all the probed clusters, that for AAA being shown for comparison. The profiles exhibit the same types of stationary point: the successive minima correspond to subsequent

Controlling Excited-State H-Atom Transfer Along Hydrogen-Bonded Wires

549

Figure 23.28 2C-R2PI spectra of 7HQ(NH3)3 (AAA), 7HQNH3H2ONH3 (AWA), 7HQNH3(H2O)2 (AWW) and 7HQ(H2O)3 (WWW). The structure of the clusters is shown on the right; the 7HQ scaffold is partially cut away. The AWA isomer is separated from the AAW isomer by UV-UV hole burning; this spectrum is shown here instead of the 2C-R2PI (See Plate 33)

Figure 23.29 ESHAT potentials of the clusters 7HQ(NH3)3 (AAA), 7HQNH3H2ONH3 (AWA), 7HQNH3 (H2O)2 (AWW) and 7HQ(H2O)3 (WWW)

Fluorescence emission, 2C-R2PI, UV-UV depletion 2C-R2PI, UV-UV depletion Yellow fluorescence excitation 2C-R2PI, UV-UV depletion,

Fluorescence emission

2C-R2PI 2C-R2PI

7HQ(NH3)3

7HQ(NH3)3

7HQ(NH3)3, 7DQ(ND3)3

GFP

7HQ(NH3)n, 1  n  4

7HQ(NH3)n(H2O)m, n þ m¼3

b

a

Not discussed in this chapter. See reference [52].

Experiments

System

IRC

Enol ! keto tautomerization Deuteration effect on excess vibrational energy

Wire length effect on the dynamic in S1 No break-off of 2C-R2PI signal in the case of WWW

CIS, B3LYP

B3LYP

CIS

CIS

CIS

Difference in relative intensities of vibrational modes Anion protonated form of GFPb

Assignment of vibrational modesa ESHAT reaction path

SCF, B3LYP, CIS

Vibrational pattern in S0 and S1 states 2C-R2PI break-off, Yellow fluorescence arising,

ESPT/ESHAT competition induced by chromophore solvation H-affinity concept, wire length effect Solvent effect on H-affinity

Decomposition of the first step of the reaction, tunnelling Mode selectivity

Determination of cluster structurea

TDB3LYP

Measurement of electronic origin

Interpretation

Calculations

Observations

[104]

[76, 94]

[101]

[46, 49, 95]

[46, 49]

[45]

[46]

[46, 95]

Ref.

Table 23.4 Summary of experiments and calculations we have performed to understand the reactivity of hydrogen-bonded wires connected to a scaffold molecule

550 Hydrogen Bonding and Transfer in the Excited State

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551

H-atom translocated structures of the Grotthuss type. However, if one compares the two ‘pure’ wires AAA and WWW, the HT1–HT3 intermediates all lie 6400–6800 cm1 (77–81 kJ mol1) above the enol minimum for WWW, whereas they lie 2300–3250 cm1 (28–39 kJ mol1) below the enol form for AAA. For the mixed clusters AWA and AWW, the replacement of any ammonia molecule by water pushes the potential energy upwards by about 6500 cm1, far above that of AAA wire and close to that of WWW. The 2C-R2PI spectra of these mixed species break off at increasing vibrational energies between 300 and 700 cm1, whereas there is no such indication for the pure water wire. As the first step of the ESHAT reaction exhibits similar behaviour for the mixed wires and the pure ammonia wire but differs from the pure water wire (Figure 23.29), we conclude that the break-off in the 2C-R2PI signal is linked to the initial enol ! HT1 step. All spectra that show a break-off correspond to clusters with an NH3 molecule hydrogen bonded to the O--H group of 7HQ, allowing for the formation of a stabilized HT1 form. On the other hand, because no yellow fluorescence is observed for the mixed ammonia/water clusters, we infer that the cluster is trapped in the HT1 structure.

23.4 Conclusion Table 23.4 lists concepts used for our investigations on excited-state proton or H-atom transfer along a hydrogen-bonded wire. The experimental measurements are the ‘starting point’ and cornerstones for almost all aspects of our research. The calculations help to understand and interpret the experimental results. The current experimental results by themselves do not make it possible to determine the existence of ESHAT but only the enol ! keto tautomerization. On the other hand, the mode selectivity calculations would not be reliable if no experimental observation had confirmed this happening. The first striking idea is that the well-known and widely used concept of proton affinity (PA) or acidity/ basicity of a solvent did not play any role in the gas-phase reactivity for such clusters, as the reaction that may occur in the excited state for 7HQ(NH3)3 or 7HQ(H2O)3 is an H-atom transfer and not a proton transfer. Under our experimental conditions, the systems shown in this work were insensitive to PA. Nevertheless, the reaction is strongly solvent dependent in terms of H-affinity. Because the ESHAT reaction is connected with the capacity of the chromophore to exhibit a pp /ps state crossing, it is also strongly dependent on the chromophore environment, which actually helped us to interpret and qualitatively describe the difference in behaviour in the gas phase and in solution: in the case of the green fluorescent protein, solvation of the chromophore prevents the stabilization of the ps state and then the ESHAT process. Without any state crossing, the system stays in the initially excited state pp and the reaction occurring is ESPT. This is actually the only parameter studied where ESHAT and ESPT effectively compete. Some weaker effects, like exciting specific intermolecular modes or increasing the hydrogen bond system on the wire connected to the chromophore, can favour the ESHAT reaction by lowering the barrier height of the first step of the global reaction or by rendering it more exoergic. We have managed to build medium-size clusters in cold gas phase that allow us to investigate ESHAT along a hydrogen-bonded wire, both experimentally and theoretically. As already mentioned, it would be interesting to obtain more diverse experimental data to confirm and develop the H-affinity concept. We are currently investigating further parameters that might influence the ESHAT process; in particular, we are investigating other solvent wire types.

Acknowledgements CMT would like to thank Karen Keppler Albert for fruitful discussions and invaluable comments while this chapter was being written.

552 Hydrogen Bonding and Transfer in the Excited State

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24 Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase Cheng-Chih Hsieh, Chang-Ming Jiang and Pi-Tai Chou Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, R.O.C.

24.1 Introduction Proton transfer represents one of the most fundamental processes involved in chemical reactions as well as in living systems [1]. Vast numbers of scientists have been participating in relevant research, leading to thousands upon thousands of publications and numerous books on the topic. Accordingly, various types of proton transfer reaction, depending on, for example, the reaction in the ground or excited state, adiabatic versus non-adiabatic, strong and weak hydrogen bonding, acidity (proton donor) and basicity (proton acceptor), have been identified [2–8]. At this time, proton transfer reactions seem to have the potential for unlimited extensions and aspects in terms of fundamentals and applications. Thus, to cover the broad spectrum of proton transfer research within a limited number of pages seems to be an unachievable task. Rather than trying to achieve the impossible, this chapter focuses mainly on areas related to excited-state proton transfer (ESPT) [9] with pre-existing intramolecular hydrogen bonds. Under this constraint, the photoinduced process involving excited-state proton dissociation and/or protonation in bulk solvents or clusters, also a currently active topic that has received considerable attention [10–14], unfortunately cannot be addressed in this chapter. One particular focus of this chapter is bifunctional molecules possessing a proton donor group and a proton acceptor group, such that ESPT takes place via self-associated hydrogen-bonded (HB) dimers and/or solvent bridge relay. In spite of numerous explorations and elegant research on guest-molecule-assisted ESPT reactions, the results of which have provided great insight into the fundamentals as well as perspectives in applications, review articles focused on the relevant ESPT reactions have unfortunately been scant over the

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

556 Hydrogen Bonding and Transfer in the Excited State

past decade. Hence, one of the goals of this chapter is systematically to explicate prototype examples for host/ guest coupled ESPT reactions from both fundamental and application viewpoints, among which contemporary progress on the extensively studied host/guest ESPT molecules, namely 7-azaindole (7AI) and its corresponding analogues, will receive special attention. The following sections are organized in a sequence of steps, where we first review the excited-state doubleproton transfer (ESDPT) dynamics of 7AI analogue homo- and heterodimers in solution and solid state. Subsequently, the mechanism of ESDPT in biomimetic heterodimers is reviewed, with a focus on their fundamental differences from chemical aspects. Owing to the similarity of its double hydrogen bonds to the Watson–Crick DNA base pair [15], the results addressing the importance of the associated ESDPT mechanism of 7AI dimers has been deemed to be a paradigm for the study of DNA mutation. In addition, the mechanisms of protic-solvent-assisted ESDPT have been reviewed, where a plausible resolution is deduced. The results address the importance of the strength and geometry (i.e. configuration) for conjugated hydrogen bonds, especially multiple hydrogen bonding systems, in view of proton transfer. Particular attention has been paid to excited-state proton transfer dynamics in alcohol and aqueous solutions, with an eye on its future potential in biological applications. Finally, the differences in mechanism between solvent diffusive reorganization and solvent relaxation that affect the ESPT dynamics will be reviewed in detail.

24.2 Biprotonic Transfer Within Doubly H-Bonded Homo- and Heterodimers 24.2.1 ESDPT dynamics in a 7AI dimer in condensed phase ESDPT, or more generally ‘excited-state multiple proton transfer’, requires HB molecules or complexes possessing bifunctionality, for which the conduction between proton-donating (O
H or N
H) and accepting (C¼O or pyridinic nitrogen) sites might be either geometrically hindered or distant from each other. These types of bifunctional molecule may undergo self- or solvent-molecule-assisted ESDPT if this process is both thermodynamically and dynamically allowed within the excited-state lifespan. In this sense, the dualhydrogen-bonding dimer of 7AI has long been recognized as a simplified model for the HB base pair of DNA. Upon singlet p ! p excitation, the 7AI dimer demonstrated the first documented prototype of the biprotonic transfer reaction, resulting in a large Stokes-shifted tautomer emission (e.g. lmax  480 nm in cyclohexane) [16]. Over the past three decades, spectroscopic measurements and theoretical approaches have also revealed that the intrinsic structure of 7AI facilitates excited-state proton transfer via either solvent catalysis [17–24] or self-dimerization [25–45]. Early picosecond studies at ambient temperature have revealed that the timescale of ESDPT on a 7AI dimer is less than 5 ps in the condensed phase [25]. Experiments with subpicosecond resolution further resolved a time constant of 1.4 ps for the rate of 7AI dimer ESDPT in non-polar solvents [26]. In view of the ESDPT dynamics of the 7AI dimer, ever since a non-concerted, two-step reaction model was proposed by Zewail and coworkers [27], there has been a longstanding and stimulating debate regarding the cooperative motion of two protons in a stepwise [27–34] versus concerted [35–45] double-proton transfer pathway (Figure 24.1). Based on the femtosecond fluorescence upconversion technique, Zewail and coworkers [29] were able to resolve a 1 ps decay time of 7AI dimer normal emission (i.e. 360 nm), and a biexponential rise of the proton transfer tautomer emission (e.g. 480 nm) fitted to be 1 and  12 ps. The normal decay rate is sensitive to N
D isotope substitution (4.5 ps), while a much smaller deuterium isotope effect is observed for the tautomer rise kinetics (1.6 and 12 ps), suggesting a lack of correspondence between the initial decay of the normal and the rise of the tautomer emission. Accordingly, a stepwise reaction mechanism was proposed, incorporating the first proton transfer to form an ion-pair-like intermediate, followed by second proton transfer to produce the

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

557

Figure 24.1 Two mechanisms proposed for the double-proton transfer reaction of the 7-azaindole dimer: (a) concerted mechanism; (b) stepwise mechanism. Reprinted with permission from [44]. Copyright 2007 National Academy of Sciences

tautomer species (see Figure 24.1(b)). The barrier associated with the first proton transfer is non-negligible, as indicated by the salient deuterium isotope effect. Subsequently, Takeuchi and Tahara [35] were able further to resolve biexponential components (t1 ¼ 0.2 ps, t2 ¼ 1.1 ps) associated with the decay of 7AI dimer normal species, in which the decay of the 1.1 ps component is consistent with the rise of the tautomer emission reported previously [29]. Alternatively, however, they reassigned the previously observed 12 ps rise dynamics of the tautomer emission to vibrational cooling of the tautomeric excited state. Accordingly, Takeuchi and Tahara concluded a concerted ESDPT, with the proton transfer proceeding exclusively from the lowest La excited state with a time constant of 1.1 ps after the occurrence of the internal conversion (t1  0.2 ps) from the initially populated 1Lb (S2) to the 1La (S1) state (Figure 24.1(a)). Later, Zewail and coworkers [30] reinvestigated the ESDPT dynamics of the 7AI dimer, based on complementary methods combining transient absorption and fluorescence upconversion techniques. In accordance with Takeuchi and Tahara’s conclusion, they reinterpreted the slow-rise component of 12 ps time constant to be vibrational relaxation of the ‘hot’ tautomer species. However, through a detailed investigation of the deuterium isotope effect, they also pointed out a common flaw arising from inefficient deuteration. Once the percentage of deuterium exchange is low, the dynamic results may become complicated, such that correct isotope-dependent dynamics must be carefully extracted from the complicated deuterated and undeuterated mixture. Nevertheless, both tautomer and normal species were still found to undergo excitationwavelength-dependent dynamics. When the excitation frequency was near the zero-point energy (ZPE), biexponential rise dynamics (130 fs for the first transfer and 1 ps for the second one) (Figure 24.1(b)) were observed for the tautomer species, which was not previously recognized, as the excitation energy was well

558 Hydrogen Bonding and Transfer in the Excited State

above the ZPE [35]. Accordingly, Zewail and coworkers concluded that the biprotonic transfer takes place on a  1 ps timescale in a non-polar solvent when the internal energy is near or above the barrier height. Only when the internal energy is low, such as in a molecular beam or possibly in a low-temperature experiment, can one examine the tunnelling processes and stepwise pathways. Recently, Takeuchi and Tahara reconfirmed the concerted reaction in 7AI ESDPT in non-polar solvents, based on comprehensive excitation-wavelength-dependent ESDPT dynamics [44]. The relaxation dynamics exhibits remarkable changes, depending on excitation wavelength, at the blue versus red edge of the dimer absorption band. The observation shows that the 0.2 ps component is not correlated with the ESDPT. The absence of any intermediate species in between the normal and tautomer excited states indicates that the double-proton transfer reaction is essentially a single-step process. Subsequently, however, Zewail and coworkers [33] reported that the rate of proton transfer is significantly dependent on solvent polarity and on the isotope composition in the pair. Considering the global potential energy surface, they also pointed out that both trajectories of the symmetric and asymmetric vibrational motions must be considered for the overall proton transfer reaction. Their experimental results support the presence of an ionic intermediate species that forms on the femtosecond timescale and decays to the final tautomeric form on the picosecond timescale. They concluded that the evidence supported the stepwise mechanism, which is in agreement with their previous proposal [30] and is consistent with the recent theoretical study by Serrano-Andr
es and Merch
an [32]. It is worth noting here that another theoretical approach reported by Chen and Chao [40] was not consistent with the stepwise model claimed by Serrano-Andr
es and Merch
an. Also, by definition, the striking feature in a stepwise mechanism is the appearance of intermediate species, where only one proton is transferred. The concerted pathway, on the other hand, supposes a cooperative translocation of two protons. Note that this does not necessarily translate to synchronous motion of the two protons, in which they would always move exactly the same distance, thereby preserving the C2h symmetry. For the motion of dual protons without constraint of C2h symmetry, it is believed that differentiation between stepwise and concerted is extremely challenging. The resolution, in our view, may become more feasible via the chemical approach, i.e. chemical modification of the studied system, in combination with technique advancement. 24.2.2 ESDPT dynamics of 7AI analogue homodimers in the solid state On the above basis, it appears that temperature-dependent studies of the 7AI dimer may provide further insight into the ESDPT dynamics as well into reaction thermodynamics [30]. Unfortunately, most studies of ESDPT on 7AI HB complexes in solution phase are limited at ambient temperature, while detailed temperaturedependent studies on the ESDPT dynamics of 7AI are scant. In our experience, the major obstacle is the formation of thermodynamically more favourable HB oligomers at low temperature, in which cooperative double-proton transfer is prohibited. In this regard, it is noteworthy that a drastic temperature-dependent effect on ESDPT has been reported [46]. Upon observation of steady-state tautomer-like emission (lmax  460 nm), even in 77 K methylcyclohexane, the dynamics of ESDPT is proposed to incorporate a proton tunnelling mechanism. However, the well-resolved so-called ‘tautomer’ emission in the low-temperature glassy form was subsequently reassigned as phosphorescence resulting from the dominant 7AI aggregate species, where proton transfer is prohibited during the excited-state lifetime [47]. Also, applying a moderate concentration of 7AI (10
4 m) in 2-methylbutane, a pure dimeric form and its corresponding ESDPT dynamics were resolved at <227 K [48]. The results of the drastic deuterium isotope effect for the tautomer emission intensity indicate tunnelling as the principal mechanism for ESDPT at low temperature. In yet another approach, in a carefully prepared non-polar polymer matrix, the oligomer formation in 7AI may be dynamically prohibited at cryogenic temperature, as the rigid environment prevents diffusive motion of the pre-existing 7AI dimer at ambient temperature [49]. Nevertheless, such careful

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

559

preparations, in our experience, still involve several sensitive parameters, such as concentration, which causes a possible microcrystalline formation owing to low solubility, temperature gradient and protic solvent contamination, resulting in intrinsic difficulty in achieving an ideal 7AI dimeric formation. Alternatively, one might expect that a study of the crystal form of 7AI may provide indisputable information on the intrinsic biprotonic transfer dynamics [50] if a cyclic, dual-HB dimeric structure persists in a perfect crystal. Unfortunately, the crystal structure of 7AI reveals a very unusual hydrogen-bonding configuration, consisting of a tetrameric unit of approximate S4 symmetry, in which the molecules are associated through four complementary N
H  H hydrogen bonds [51] (Figure 24.2(a)). The lack of a dual-hydrogen-bonded dimeric form makes the study of intrinsic ESDPT dynamics in the 7AI single crystal unfeasible. Intuitively, introducing steric hindrance at the C-3 position of 7AI (Figure 24.2) may prohibit the tetrameric arrangement. Accordingly, through a nucleophilic substitution at the C-3 position, forming, for example, 3-methyl-7azaindole (3MAI) (Figure 24.2(b)), a dimeric structure is exhibited in the single crystal. In contrast to the

(a)

hυ ESDPT prohibited

(b)

normal emission (λmax~370 nm)

H3C 3

N hυ ESDPT

N H

H N

N CH3

tautomer emission λmax ~ 500 nm

3MAI

(c)

hυ ESDPT

N

N H

H N

N

6HIQ tautomer emission λ max ~ 560 nm

Figure 24.2 Structures of (a) the tetramer formation of 7AI. Reprinted with permission from [52]. Copyright 2002 American Chemical Society

560 Hydrogen Bonding and Transfer in the Excited State

Figure 24.3 (a) The absorption (– – –) at room temperature and the fluorescence (––) of the non-deuterated 3MAI crystal as a function of temperature at a 298 K, b 240 K, c 195 K, d 90 K, e 50 K and f 8 K. Inset: the plot of ln[kobs
(kr þ knr)] versus the reciprocal of temperature. (b) The fluorescence of a deuterated 3MAI crystal as a function of temperature at a 298 K, b 200 K, c 165 K, d 135 K, e 110 K, f 85 K, g 35 K, h 12 K and i 8 K. The excitation spectrum (– – –) of deuterated 3MAI monitored at the F2 (*) and F1 bands (o). Inset: the plot of tautomer emission intensity at 500 nm versus the temperature (in K). Reprinted with permission from [52]. Copyright 2002 American Chemical Society

normal fluorescence (lmax  370 nm) observed in 7AI, 3MAI in a single crystal exhibited a unique, large, Stokes-shifted fluorescence (lmax  500 nm) throughout the 298–8.0 K range [52]. Both steady-state and timeresolved measurements down to 8.0 K have revealed a remarkable deuterium isotope effect on the rate of ESDPT in the 3MAI single crystal (Figure 24.3). Accordingly, a barrier height of ESDPT of 1.73 and 0.58 kcal mol
1 has been estimated for the N(1)-deuterated and non-deuterated dimers, respectively, in a single crystal, and the ESDPT rates are mainly governed by a proton/deuterium tunnelling mechanism. In a continuous effort regarding chemical modification of ESDPT systems, very recently, 11-propyl-6Hindolo-[2,3-b]quinoline (6HIQ) (Figure 24.2(c)) was strategically designed and synthesized [53]. 6HIQ possesses a fused four-ring aromatic system, while the core chromophore credited for ESDPT, that is, 7AI, remains intact. In non-polar solvent, the concentration-dependent absorption and emission spectra of 6HIQ indicate that the dimerization occurs via dual-hydrogen-bonding formation. Ultrafast relaxation dynamics revealed that the <150 fs ESDPT rate constant of 6HIQ dimer in cyclohexane [54] is much faster than that (1.1 ps) of the 7AI dimer [35]. This leads to the conclusion that ESDPT in the 6HIQ dimer may have a very small barrier, or even be barrierless, as opposed to the finite-rate, barrier-containing ESDPT for the dimer of 7AI and its analogues. The sharp contrast in the 6HIQ dimer is fundamentally intriguing. One plausible rationalization lies in the stronger photoacidity for the 6HIQ pyrrolic N
H.

Relative Intensity (arb. units)

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

561

80 K

260 K

400

450

500

550

600

650

Wavelength (nm)

Figure 24.4 The temperature dependence fluorescence spectra of 6HIQ in a single crystal from 260 to 80 K, where lex ¼ 380 nm (note that the temperature interval between each spectrum is 20 K) [55]

In a single crystal, 6HIQ also reveals a dual-hydrogen-bonded dimeric (6HIQ)2 configuration (Figure 24.2(c)). Both steady-state and time-resolved measurements reveal a remarkable temperature dependence on the rate of ESDPT in the 6HIQ single crystal. However, in contrast to the tautomer fluorescence (lmax  500 nm) observed in the 3MAI dimer down to 77 K, the 6HIQ dimer in a single crystal exhibits dual fluorescence emission (lmax  440 and 550 nm respectively) throughout the 260–80 K range (Figure 24.4). Time-resolved fluorescence decay also indicates a much slower ESDPT rate of 6HIQ (tpt  0.6 ns) at 77 K, as compared with that of 3MAI in a single crystal. The low-temperature crystal structure of 6HIQ shows that the perpendicular  distance between two parallel planes of the 6HIQ dimer is approximately 0.50 A (Figure 24.5(a)), much longer  than the distance between the 3MAI dimer (0.22 A) (Figure 24.5(b)). These results clearly indicate that configuration (i.e. geometry) plays a key role in accounting for the ESDPT dynamics in a solid [55]. 24.2.3 The ESDPT dynamics on the heterodimer In spite of those elegant studies on the 7AI dimer, however, from the biological point of view, the key for photoinduced mutation to be mimicked in nature lies in an intrinsic heterodimeric structure, such as in A
T

Figure 24.5 The perpendicular distances between two parallel planes of (a) 6HIQ and (b) 3MAI molecules are  approximately 0.5 and 0.2 A, respectively, at 77 K [55]

562

Hydrogen Bonding and Transfer in the Excited State

and G
C base pairs. Watson and Crick called attention to the possible tautomerism of base pairs, in which the transfer of protons (or hydrogen atoms) between two partner bases may perturb the genetic code because the resulting tautomers are capable of pairing with different counterparts. Thus, as for mimicking biorelevant approaches, it should be more appropriate to provide a model base pair consisting of, for example, 7AI and 7AI analogues, forming a heterodimer [56–58]. In the seminal ESDPT study on 7AI [16–45], a variety of 7AI derivatives have been designed and synthesized to probe either fundamentals or applications, among which 3-methyl-7-azaindole [52, 56–58] 1-azacarbazole [59–61] and 3-cyano-7-azaindole [62] are prototypes and have been intensively investigated. Unfortunately, these derivatives possess a S0 ! S1 absorption band, which is very close to that of the parent 7AI. Accordingly, studies on the reaction dynamics of the heterodimers of 7AI analogues in solution as well as in the cool jet phase are not feasible owing to serious interference from the homodimers, even though a number of studies have focused on the spectroscopy of the 7AI analogue/7AI heterodimer [56–58]. In an aim to resolve the above issue, an ingenious design of the 7AI analogue should possess a greatly separated S0–S1 energy gap from that of 7AI, while its core 7AI chromophore remains intact, to facilitate ESDPT. To achieve this goal, 6HIQ/7AI heterodimer (Figure 24.6) provides a way to demonstrate unambiguously for the first time an ESDPT reaction scheme from femtosecond dynamics [54]. The results reveal a stepwise ESDPT process in the 6HIQ/7AI heterodimer, in which 6HIQ (deuterated 6HIQ) delivers the pyrrolyl proton (deuteron) to 7AI (deutrated 7AI) in less than 150 fs, forming an intermediate with a charge-transfer-like ion pair, followed by the transfer of pyrrolyl proton (deuteron) from cation-like 7AI (deuterated 7AI) to the pyridinyl nitrogen of anion-like 6HIQ (deuterated 6HIQ) in 1.5  0.3 ps (3.5  0.3 ps). The barrier of second proton transfer has been estimated to be 2.86 kcal mol
1 for the 6HIQ/7AI heterodimer. The proposed ion-pair intermediate in which the greater photoacidic moiety (6HIQ) forms an anionic-like species, while the counterpart 7AI acts as a photobasic moiety, forming a cationic-like species, confirms recent reports that proton transfer in an A
T (or G
C) pair may undergo a charge-transfer state [63–67]. It is believed that the results should pave a new way to mimic ESDPT dynamics in biorelevant systems.

N*

M*

*

T* *

*

N

N

N

N

N

H

H N

H N

H N

H

N

<150 fs

N H

N

N

(N-H) 1.5 0.3ps (N-D) 3.5 0.3ps

400 nm

~10-20ps

560 nm (τ~1.7 ns)

6HIQ/7AI heterodimer

Figure 24.6 Structure of a 6HIQ/7AI heterodimer and the associated ESDPT pathway. Reprinted with permission from [54]. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

563

24.3 Proton Transfer Through Host/Guest Types of Hydrogen-Bonded Complexes 24.3.1 Catalytic versus non-catalytic proton transfer As elaborated in Section 24.2, the spectroscopy and dynamics of host/guest HB-type ESPT reaction have long received considerable attention. Besides 7AI [16–45], 7-hydroxyquinolines [68–76], b-carbolines [77–81] and pyrrole-containing heteroaromatics [82–86] are other prototypes where the ESPT tautomerism is mediated either by self-association or by adding guest molecules (including solvents) forming host/guest types of HB complex. From the structural viewpoint, the proton-donating and proton-accepting sites can be adjacent to each other, such that a complex possessing intact dual hydrogen bonds is formed through a perfectly fitted 1:1 (host:guest) stoichiometric ratio in the ground state. For example, the conjugated dual hydrogen bonding (CDHB) effect [87] in the case of a 1:1 7AI/acetic acid complex mutually induces p electron charge transfer from the pyrrolic nitrogen to the pyridinic nitrogen (Figure 24.7), resulting in additional stabilization energy. As a result, an association enthalpy of
14 kcal mol
1 has been reported [88]. Upon electronic excitation, the 7AI/guest CDHB complex undergoes an ultrafast rate of double proton transfer (e.g. 1 ps
1 in the case of 7AI dimer [29, 30, 35]), resulting in an imine-like tautomer fluorescence. Based on the concept of the CDHB effect, stable HB complexes have been experimentally observed among 7AI and many other guest molecules possessing bifunctional HB sites [89]. It was then subsequently recognized that, depending on the properties of the guest molecules, the thermodynamics and/or dynamics of ESDPT can vary drastically. The guest/host types of complex, based on their chemical aspects upon ESDPT, have been classified into two categories [89]. Applying 7AI as a host, on the one hand, acid-, alcohol- and H2O-assisted ESDPT has been specified as a catalytic process, as the molecular structure of the guest species remains unchanged during the ESDPT reaction. Studies of 7AI complexes to catalytic types of single molecule, such as various carbon-chain monocarboxylic acids and phosphoric acids, have revealed rapid ESDPT reactions based on either the steady-state or time-resolved approach [88]. On the other hand, ESDPT

ESDPT

hυ

480 nm

N

N

H

H O

O C

N H

N

Ka ~ 2.0x104

+ CH3COOH

N H

N H O

O C

Figure 24.7 Conjugated dual-hydrogen-bonding (CDHB) formation for 7AI, its associated redistribution of the electron density and the corresponding ESDPT reaction in cyclohexane. Reprinted with permission from [79]. Copyright 2001 American Chemical Society

564

Hydrogen Bonding and Transfer in the Excited State (a) *

N

N

hυ

H H

ESDPT

O

N

N H

catalytic process

H

O

R

R

(b)

*

N H

N H O

hυ ESDPT

O

N

N

H

H O

O

C

catalytic process

C

CH3

CH3 *

(c) N H

N H N

hυ ESDPT

N

N

N

H

H

N

N

non-catalytic process

*

(d) N H

N H N

O

hυ ESDPT

N H

N H

non-catalytic process

O N

Figure 24.8 Types of ESDPT reaction: (a, b) catalytic and (c, d) non-catalytic types. While (d) can be generalized to various lactams, the d-lactam/7AI hydrogen-bonded complex is shown. Reprinted with permission from [89]. Copyright 1995 American Chemical Society

in 7AI dimer, 7AI/lactam and 7AI/amide HB complexes results in 7AI(T) /7AI(T), 7AI(T) /lactim and 7AI(T) /imine forms respectively ((T) denotes the electronically excited tautomer species), where the guest molecules have been tautomerized simultaneously. This type of reaction has been defined as a non-catalytic process (Figure 24.8). From an energetic point of view, the catalytic type of ESDPT requires only the tautomerization of the host molecule, while the non-catalytic ESDPT depends not only on the 7AI ! 7AI(T) proton transfer but also on the tautomerization of the guest molecule. In the case of the 7AI dimer, the unexcited 7AI can be treated as a guest that does not act as a catalyst, but rather as a reactant. The tautomerization of the guest molecule should be an endergonic process for 7AI (in the case of the 7AI dimer), lactams and amides. Evidently, the 7AI ! 7AI(T) proton transfer reaction is highly exothermic and renders a driving force that leads to a simultaneous tautomerization of the guest molecules. With the same host molecule (e.g. 7AI), the non-catalytic type of ESDPT in 7AI HB complexes is therefore less exergonic than that of the catalytic type of ESDPT. Accordingly, the relative thermodynamics between normal and tautomer HB complexes may be of importance in fine-tuning the dynamics of ESDPT.

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

565

24.3.2 p-Electron conjugation tuning ESDPT From the molecular structure viewpoint, the reaction centre of the ESDPT process in 7AI and its corresponding analogues, such as 6HIQ [54] and pyrrole-containing heteroaromatics [82–86], are essentially the amine/imine proton transfer tautomerism, in which a proton (or hydrogen atom) in the pyrrolic nitrogen transfers to the pyridinic nitrogen, forming an imine-like isomer. Such a process, in general, is highly endergonic in the ground state. For example, a theoretical calculation estimates the free energy of 7AI dimer ! 7AI(T) dimer tautomerism to be  23.5 kcal mol
1 [90]. It thus becomes crucial that the S00 ! S10 transition gap of the tautomer species (the prime denotes the imine tautomer state) has to be significantly smaller than that of the normal species (i.e. the S0 ! S1 gap) in order to proceed as a thermally favourable ESDPT reaction. A simple and empirical approach based on the p electron conjugation predicts that the C2
C3 double bond (Figure 24.9) in the imine-like tautomer of 7AI should actively be involved in the delocalization of p electrons and hence play a key role in determining the S00 –S10 energy gap. Conversely, theoretical approaches also indicate that the relative thermodynamics between amino and imino forms in the ground state depend mainly on the destabilization of the aromaticity of the pyridine ring, and hence are less affected by the C2
C3 double bond in the pyrrolic ring. On this basis, 7-azaindoline (7AZD) (Figure 24.9) has been synthesized via the

Figure 24.9 Proton transfer tautomerism tuned by relative thermodynamics. Note that the S0 state has been normalized to an equivalent energy level. The energy level is in kcal mol
1, with its corresponding wavelength (in parentheses) in nm. Reprinted with permission from [91]. Copyright 2000 American Chemical Society

566 Hydrogen Bonding and Transfer in the Excited State

hydrogenation of the C2
C3 double bond in 7AI. On the one hand, 7AZD is potentially capable of generalizing an amine–imine type of proton transfer tautomerism. On the other hand, owing to the lack of a C2
C3 double bond, the imine-like tautomer of 7AZD apparently possesses a shorter length of the p electron conjugation than 7AI, and hence a larger S00 –S10 energy gap is expected. Therefore, 7AZD serves as an ideal model to examine the catalytic versus non-catalytic ESDPT reaction fine-tuned by the length of the p electron conjugation. The results show that, for the catalytic type of ESDPT, such as the 1:1 7AZD/acetic acid CDHB complex, photoinduced double-proton transfer is energetically favourable and may only require a small displacement of the hydrogen atom and/or molecular skeleton, resulting in a negligible energy barrier. The rate of such a cooperative proton transfer reaction, which takes place either stepwise or concertedly, is much faster than the spontaneous decay rate of the electronically excited 7AZD/acetic acid complex, as supported by the lack of the normal 7AZD/acetic acid emission and the unresolved rise time of the imine tautomer fluorescence (trise < 3  10
11 s) [91]. In contrast, in spite of the dual-hydrogen-bonding association, both experimental and theoretical approaches conclude that ESDPT is prohibited in the case of the 7AZD dimer and 7AZD/lactam complex (Figure 24.9), unambiguously illustrating that the catalytic versus noncatalytic type of ESDPT is able to be mediated by the p electron conjugation length. 24.3.3 Multiple hydrogen bonds tuning the guest/host excited-state proton transfer reaction The above concept of catalytic versus non-catalytic ESDPT can be further exploited towards biorecognition. Among the various approaches for biorecognition, the principle of using hydrogen bonds to confer binding strength and selectivity is of particular interest [92–96], mainly owing to their relative flexibility in geometry compared with rigid covalent bonds [97, 98]. Because guest molecules of biological interest may possess various numbers of proton-donating and/or proton-accepting groups, the design and syntheses of host receptors providing multiple hydrogen-bonding sites are necessary to maximize the recognition capacity. Furthermore, the redistribution of p electrons might result in unusual photophysical properties; for example, the host/guest type of ESPT reaction induced by multiple hydrogen bonds can provide a superb opportunity for enhancing signal transduction [91]. In an effort to apply this concept to molecular recognition, numerous multiple-hydrogen-bonding systems have been examined, among which 3,4,5,6-tetrahydrobis(pyrido[3,2-g]indolo)[2,3-a:30 ,20 -j]acridine (TPIA) (Figure 24.10), designed and synthesized by Thummel and coworkers, is an exquisite case in point [99, 100]. TPIA was designed so that the pyrrole and pyridine moieties function as the proton donor and acceptor respectively. Two symmetric push-pull conjugated units form a V-shaped framework with a cleft of appropriate size to provide as many as five HB sites. This, in combination with a flexible dimethylene skeleton, renders a preorganized motif particularly suitable for multiple-HB recognition. Depending on the properties of guest molecules, the photophysics of the TPIA/guest complex varies drastically. On the basis of the aforementioned chemical aspects of guest-molecule-assisted ESPT, the TPIA/guest HB complexes are also classified into two categories: catalytic and non-catalytic. As depicted in Figure 24.10, the carboxylic-acid-assisted ESPT in TPIA can be specified as a catalytic process, for, following the ESPT reaction, the molecular structure of the carboxylic functional group remains unchanged. Conversely, with the guest molecule replaced by a urea derivative such as 2-imidazolidone, both TPIA and 2-imidazolidone would have tautomerized (i.e. lactam ! lactim) simultaneously if ESPT in TPIA/urea HB complexes had taken place. This type of reaction is defined as a non-catalytic process in which ESPT depends not only on the host but also on the isomerization of the guest molecule and is thus expected to be energetically less favourable. Both spectroscopic and theoretical approaches indicate the occurrence of catalytic ESPT in TPIA/acetic acid. In contrast, it is prohibited in the case of a non-catalytic TPIA/2-imidazolidone HB complex because of the endergonic energy required for simultaneous tautomerization of TPIA and 2-imidazoline [101]. Such catalytic versus non-catalytic reaction thus provides a recognition concept in that one may envisage utilizing a

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

567

Figure 24.10 Proposed proton transfer mechanism for TPIA/guest HB complexes. Reprinted with permission from [101]. Copyright 2004 American Chemical Society

multiple-hydrogen-bonding site as a pocket (or cleft) in which the guest–host hydrogen-bonding strength becomes maximized while the water interference is minimized. Moreover, the switch of the fluorescence signal from water-catalysed ESDPT, i.e. proton transfer tautomer emission, to, for example, urea-prohibited ESDPT, i.e. normal fluorescence, acts as the signal transduction. Such a multiple-hydrogen-bonding system should have great potential with respect to, for example, urea recognition.

24.4 Solvation Dynamics Coupled into the Proton Transfer Reaction 24.4.1 Theoretical background In this section we turn to a contemporary topic regarding solvation dynamics coupled into the ESPT reaction. One relevant, important issue of current interest is the ESPT-coupled excited-state charge transfer (ESCT) reaction. Some seminal theoretical approaches taken by Hynes and coworkers revealed the key features, with description of the dynamics and electronic structures of non-adiabatic [102–104] and adiabatic [105–107] intermolecular proton transfer reactions in protic solution. The most recent theoretical advancement has incorporated both solvent reorganization and proton tunnelling through barriers and made the framework resemble electron transfer [102–111], such that the proton transfer rate kpt can be categorized into two regimes: (a)

For the non-adiabatic limit [102]: C2 kpt ¼ h 

  rffiffiffiffiffiffiffiffiffiffiffi p DG6¼ exp
Es RT RT

ð24:1Þ

ðDGrxn þ Es Þ2 4Es

ð24:2Þ

where the free energy barrier DG6¼ ¼

568 Hydrogen Bonding and Transfer in the Excited State

DGrxn and DG6¼ denote the reaction free energy and reaction barrier respectively, Es is the solvent reorganization energy, C represents the proton coupling quantum average over the vibrational modes associated with the proton motion and  h is the Planck constant. (b) For the adiabatic limit [106]: ! vs
DG6¼ ad kpt exp ð24:3Þ 2p RT where vs is the solvent fluctuation frequency in the reactant well, and the adiabatic reaction activation energy is expressed as 6¼ 0 DG6¼ ad ¼ DG0 þ a0 DGrxn þ a0

ðDGrxn Þ2 2

ð24:4Þ

where DG6¼ onsted coefficient, is the derivative of DG6¼ 0 represents the intrinsic barrier at DGrxn ¼ 0, a0, the Br€ 0 with respect to DGrxn, evaluated at DGrxn ¼ 0, and a0 is the derivative of a0 with respect to DGrxn, evaluated at DGrxn ¼ 0. To which category the proton transfer reaction is ascribed strongly depends on coupling between reactant and product electronic states, which has a significant dependence on the distance between proton donor and acceptor, i.e. the hydrogen bonding length. A larger electronic coupling not only reduces the barrier height but also narrows the width. Note that, compared with non-adiabatic electron transfer, the tunnelling probability in non-adiabatic proton transfer should be much more sensitive to interatomic separation simply because a proton is much heavier (2000 times) than an electron. In a real system, the separation between proton donor and acceptor would not be fixed, but fluctuating, owing to any vibrational motion associated with changes in hydrogen bonding. As a result, the proton tunnelling probability varies with respect to types of vibration. Hynes and coworkers then extended the non-adiabatic proton transfer model [102–104] by incorporating a low-frequency vibrational motion between proton donor and acceptor, i.e. the Q mode, indicating that the rate constant for non-adiabatic proton transfer is the sum of kn ! m , the rate constant from the nth vibrational state of the Q mode in the reactant:   XX X X C 2 rffiffiffiffiffiffiffiffiffiffiffi p DG6¼ n!m exp
kpt ¼ Pn k n ! m ¼ Pn nm ð24:5Þ Es RT h RT n m n m where Pn is the thermal distribution of the nth excited vibrational state of the Q mode, Cnm is the quantum average of electronic coupling and DG6¼ n ! m is the activation barrier for n ! m transition. With explicit evaluation of Cnm, and considering reaction symmetry within the Q mode, a new term, Ea ¼ h2 a2 =2mH , whose physical interpretation is the coupling term between Q mode and solvent polarity, is introduced, accompanied with removal of the summation term in (5): (   2  sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ) ~a 2 DGrxn þ Es þ E C p   exp
  kpt ¼ ~ a RT ~ a RT h  4 Es þ E Es þ E

ð24:6Þ

  ~ a ¼ Ea hvQ coth hvQ E 4pRT 4pRT

ð24:7Þ

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

569

Note that the rate constant expression (24.5) resembles (24.1); the only differences are that the coupling probability is thermally averaged and the Q-mode vibrational reorganization energy has a contribution as well as solvent reorganization energy. In general, other intramolecular modes could also be incorporated by similar treatment. One aspect of note in this modern model is the quantum character of the proton in a proton transfer reaction, that is, that the proton motion is quantum rather than classical. Accordingly, the motion is commonly faster than the rearrangement of protic solvent molecules, and thus Born–Oppenheimer approximation can be analogously made for the correlation between proton and solvent molecules in this reaction. As a consequence, the equilibrium between the moving proton and the surrounding solvent molecules was established at each instant, and the reaction activation free energy was thus essentially dominated by the solvent reorganization, rather than being given by the height of proton migration barrier, as charge redistribution was involved. Furthermore, the solvent coordinate, rather than the proton coordinate, serves as the reaction coordinate within the proton transfer process. If we approximate the free energy along the solvent coordinate by a simple parabola, the amplitude of the reaction activation energy could be analytically determined by three factors: (1) reaction energy (vertical displacement); (2) dipole moment difference between normal and tautomer forms (horizontal displacement); (3) solvent polarity (curvature). In the following sections, we will elaborate several ESPT systems that are strategically designed to probe solvent-polarization-coupled reaction dynamics. The experimental results firmly support the theoretical model. 24.4.2 Excited-State charge-transfer-coupled proton transfer reaction To probe as well as to signify the solvent polarity effect, it is important fully to comprehend the occurrence of ESPT accompanied with large changes in dipole moment, such that a proton transfer event is greatly subject to the solvent polarity effect [112–114]. To avoid bimolecular complexity, a unimolecular system, i.e. a system invoking excited-state intramolecular proton transfer, has received particular attention. In such situations, ESCT/ESPT describes the proposed systems more appropriately. A conceptually designed ESCT/ESPT system is depicted in Figure 24.11, in which D and A denote the electron donor and acceptor respectively. In most cases, D and A are separated by a chromophore (represented by a benzene-like structure), and their relative positions are suited for charge transfer reactions via, for example, p electron delocalization in the excited state. In most designed systems, a hydroxyl (or amino) hydrogen forms an intramolecular hydrogen bond with A [115–126]. On the one hand, electronically exciting PC to PC may cause charge transfer, forming a charge transfer species CT. On the other hand, the hydrogen-bonded H atom may act as a strong photoacid, such that proton transfer takes place upon excitation,

Figure 24.11 A generalized ESCT/ESPT system and its possible reaction patterns ( denotes the electronically excited state). Reprinted with permission from [136]. Copyright 2008 American Chemical Society

570

Hydrogen Bonding and Transfer in the Excited State

resulting in a proton transfer tautomer denoted by PT (Figure 24.11). Thus, depending on the reaction time domain, studies of ESCT versus ESPT can be classified into two categories: when the rate of ESCT is faster than the rate of ESPT, following PC ! CT charge transfer, the CT ! CPT proton transfer reaction then takes place; when ESPT takes place prior to ESCT, the overall reaction may be described as a PC ! PT proton transfer, followed by a PT ! CPT charge transfer process. Bearing the above concept in mind, over the past few years, a number of potential ESCT/ESPT systems have been ingeniously designed and investigated. Prototypical examples include N,N-dialkylamino-3-hydroxyflavones [113, 117–126], 2-hydroxy-4-(di-p-tolyl-amino)benzaldehyde [114] and 2-(20 -hydroxy-40 -diethylaminophenyl)benzothiazole [112] in which the N,N-dialkyl group is strategically designed to act as an electron donor, while the carbonyl oxygen or the nitrogen group within the parent ESPT moiety serves as an electron acceptor. Studies conclude that most systems studied so far can be ascribed to case (A), namely an ultrasfast (<<150 fs) ESCT prior to ESPT (Figure 24.12). Such an adiabatic type of ESCT, i.e. an optical electron transfer process, can be rationalized by a strong p electron overlap between donor and acceptor moieties, such that the electronic coupling matrix is much larger than that of the Marcus type of weak-coupling electron transfer process [127–130]. For this case, following ultrafast ESCT, it is found that ESPT always takes place and that its rate is complicated by the competitive solvation relaxation process, i.e. the CT ! CTeq process (the subscript ‘eq’ denotes the solvent equilibrated state). After reaching the solvent equilibration, owing to the difference in equilibrium solvent polarization between CTeq and CPTeq , the CTeq ! CPTeq proton transfer reaction is associated with a solvent-induced barrier (Figure 24.13). The above approach is mainly in polar, aprotic solvents. As for the protic solvent, which serves as an ideal model to mimic the biosystem, the solvent hydrogen-bonding perturbation leads to the rupture of the solute intramolecular hydrogen bond and hence makes the study extremely complicated. For approaching the ESCT/ESPT coupled reaction in protic solvents, strategic design lies in molecules lacking an intramolecular hydrogen bond, such that ESPT takes place via the assistance of protic solvent molecules, i.e. the catalytic type of proton transfer described in Section 24.3. Under this criterion, a case in point is the 7AI type of ESPT system. Bearing this goal in mind, the electron donor/acceptor functionality has been incorporated into 7AI, such that charge transfer influencing ESPT can be examined in protic solvents. Prototypes of these designated 7AI

N(C 2H 5) 2

O N O O

H

H O

4'-N,N-diethylamino-3-hydroxyflavone

O H

2-hydroxy-4-(di-p-tolyl-amino)benzaldehyde (C 2H 5) 2N

H

O

O

N N(C 2H 5) 2 O O

S

H

2-(2'-hydroxy-4'-dietheylaminophenyl)benzothiazole 7-N,N-diethylamino-3-hydroxyflavone

Figure 24.12 Molecular structures of some representative ESCT/ESPT compounds

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

571

N N O O O O

O H

CPT*

solvent relaxation

O

H

PC* (CT*)

Free Energy

proton transfer

proton transfer

CTeq*

CPT eq* N

hv

O

O O

H

Solvent polarization coordinate P

pr ot

on tra ns

fe rc

oo rd i

na te

PC

Figure 24.13 Relaxation processes for the case (A) ESCT/ESPT system, using 40 -N,N-diethylamino-3-hydroxyflavone as an example

analogues consist of 5-cyano-7-azaindole (5CNAI), 3-cyano-7-azaindole (3CNAI)[62], 3,5-dicyano-7-azaindole (3,5CNAI) and dicyanoethenyl-7-azaindole (DiCNAI) (Figure 24.14) [131]. In this approach, on the one hand, the cyano moiety serves as an electron-withdrawing group, such that ESCT may take place from pyrrolic nitrogen to the cyano substituent. On the other hand, the pyrrolic hydrogen and the pyridinyl nitrogen act as a proton-donating and a proton-accepting group respectively, with a lack of intramolecular hydrogen bond, such that proton transfer takes place with the assistance of the solvent relay (Figure 24.15). As a result, similarly to 7AI, 3CNAI and 3,5CNAI undergo methanol-catalysed ESDPT, revealing dual (normal and proton transfer) emission. However, proton transfer is prohibited for 5CNAI and DiCNAI in methanol, supported by a unique normal (non-proton transfer) emission. The normal emission of each derivative reveals a significant charge transfer property by its prominent solvatochromism, although to different extents. The ESPT rate, which is determined by the rise/decay of tautomer/normal form emission through dynamical experiments, is shown to be quite different through this series of 7AI derivatives (Table 24.1). The widely accepted 7AI ESDPT mechanism, which involves equilibrium between free 7AI and a 1:1 solvent complex [22], cannot explain the above-mentioned dynamical data adequately, for the main chromophore in these compounds remains almost unchanged. The difference in the solvent complex equilibrium constant also could not account for the large proton rate difference. Thus, it is proposed that the solvent-induced barrier plays a significant role in this system. According to the theoretical approach, the dipole moment differences between normal form and tautomer in the first excited state were calculated to follow the trend 5CNAI > 3,5CNAI > 3CNAI  7AI. The trend of the observed rate of the ESPT process is in good correlation with the proposed solvent-polarity-induced barrier resulting from the difference in the

572

Hydrogen Bonding and Transfer in the Excited State

3,5CNAI

ΔG+ 5CNAI

ΔG+ Energy

DiCNAI

ΔG+

R1

R1

R2

R2 N

N

N*

N

N

H

3CNAI R1=CN, R2=H 3,5CNAI R1=CN, R2=CN 5CNAI R1=H, R2=CN DiCNAI R 1=H, R2=

H

T*

eq

CN CN

eq

Solvent Coordinate

Figure 24.14 The proposed mechanism of ESPT incorporating a solvent-polarity-induced barrier in protic solvents following optical charge transfer and solvent relaxation. Reprinted with permission from [131]. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA

N

N

*

C

C

hυ N H

N H

O R

adiabatic ESCT

N

*

C

ESPT N H

N H

H

O R

N

N

H O R

normal emission

tautomer emission

Figure 24.15 The proposed ESPT/ESCT coupled mechanism of 5CNAI in protic solvents

changes in dipole moments between the equilibrium polarization of the normal (Neq ) and tautomer species (Teq ) in the solvent coordinate (Figure 24.13) [131]. Moreover, as shown in Figure 24.14, it is obvious that the barrier is increased upon increase in the difference in dipole moment (either magnitude or direction) between the normal and tautomer forms, consistent with the theoretical prediction regarding horizontal displacement of dipole separation described in Section 24.4.1.

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

573

Table 24.1 Photophysical properties of 7AI and its correlated cyano analogues in methanol. Reprinted with permission from [131]. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA labs (nm)

lem (nm) (F)a

7AIc

288

N: 374 T: 503 (0.07)

3CNAI

285

N: 343 T: 480 (0.02)

3,5CNAI

294

N: 377 T: 515 (0.03)

5CNAI DiCNAI

297 351

N: 395 (0.20) N: 600 (0.0014)

t (ns)b t: 0.146 t1: 0.134 (
0.44) t2: 0.654 (0.56) t: 0.23 t1: 0.24 (
0.49) t2: 5.88 (0.51) t: 0.69 t1: 0.71 (
0.52) t2: 1.13 (0.48) t: 4.8 t: 0.20

a

The reported F is the sum of the normal (N) and tautomer (T) emission bands. Data in parentheses are the fitted pre-exponential factors. The photophysical properties of 7AI are taken from Ref. [17].

b c

The above 7AI analogues serve as one of a few experimental proofs for the solvent-induced barrier in proton transfer reaction. It is thus believed that, through other ingenious design, the systematic investigation of the ESCT/ESPT coupled reaction becomes possible in protic solvents, which may be crucial to gaining fundamental insights into the current research fields regarding, for example, proton-coupled electron transfer in a living system. 24.4.3 Excited-State proton transfer coupled with charge transfer Both theoretical and experimental progress elaborated in Sections 24.4.1 and 24.4.2 has fundamental importance in that it clearly addresses the role of solvent polarity channelling into the proton transfer dynamics. Unfortunately, up to this stage, for most experimental model systems applied, the ESCT/ESPT dynamics is ascribed to case (A) in which ESCT takes place prior to ESPT. Thus, upon Franck–Condon excitation, the associated reaction dynamics has been complicated by competitive solvent relaxation to reach equilibrium polarization. As such, the study of early ESCT/ESPTreaction dynamics is commonly limited to the rate of solvent relaxation. To overcome this hurdle, it is of great fundamental interest to seek an ideal system to probe ESCT/ESPT coupling reactions free from early solvent relaxation process. In theory, an ideal case in point stems from case (B), for which a molecule is designed such that it undergoes ESPT prior to the ESCTreaction. Recently, via the strategic design and synthesis of molecule 2-((2-(2-hydroxyphenyl) benzo[d]oxazol-6-yl)methylene)malononitrile (diCN–HBO) [132], the study of ESPT-coupled ESCT, i.e. process (B), becomes feasible. From the molecular structure point of view, for diCN
HBO, the lone-pair electrons of the benzo nitrogen atom are intrinsically involved in p electron resonance to establish aromaticity, such that its electron-donating strength, compared with that of alkyl and aryl amines, is negligibly weak. Thus, upon Franck–Condon excitation of diCN
HBO, the degree of charge transfer should be negligible. On the other hand, similarly to its parent molecule 2-(20 -hydroxyphenyl)benzoxazole (HBO) [133–135], ESPT is expected to take place from the hydroxyl proton to the N1 nitrogen, resulting in a proton transfer tautomer, i.e. a keto form. Once the proton transfer tautomer is formed, the N1 nitrogen atom becomes the secondary alkyl amino nitrogen and thus should act as a good electron donor (Figure 24.16). Unlike most of the ESCT/ESPT systems, in which ESCT takes place prior to ESPT, diCN
HBO undergoes ESPT, concomitantly accompanied with the charge transfer process, such that the ESPT reaction dynamics is directly coupled with solvent polarization effects. The long-range solvent polarization interactions result in

574

Hydrogen Bonding and Transfer in the Excited State H

H

O

O

N

N

hv ESPT

O

O

HBO

H 1

H

O

N

O

N

hv ESPT

O

O

NC

CN

diCN-HBO

NC

CN

adiabatic ESCT

proton motion coupled with charge transfer

H

O

+δ N

O

NC

-δ

CN

Figure 24.16 Proposed ESPT reaction for HBO and ESPT/ESCT coupled reaction for diCN
HBO. Reprinted with permission from [136]. Copyright 2008 American Chemical Society

a solvent-induced barrier that affects the overall proton transfer reaction rate. According to the spectroscopic measurements, dual emission was observed, and the proton transfer tautomer emission peak moves drastically in solvents bearing different polarities. In cyclohexane, the rate constant of ESPT of diCN
HBO was determined to be 1.1 ps, which is apparently slower than the 150 fs for the parent molecule HBO. Upon increase in solvent polarity, the ESPT rate constants were also determined to be 1.00  0.13 ps in benzene, 0.60  0.05 ps in CH2Cl2 and 0.31  0.03 ps in CH3CN, values revealing that an increase in solvent polarity tends to produce an increase in the rate of ESPT [136]. The overall reaction dynamics can be described by a mechanism incorporating both solvent polarization and proton- transfer reaction coordinates (Figure 24.17). In the solvent coordinate, the proton transfer tautomer possessing a high degree of charge transfer character is obviously stabilized upon increase in solvent polarity, while the PC state is not influenced, and thus the corresponding solvent-induced barrier is reduced. This diCN
HBO system serves as the first ESPT/ESCT example in which ESPT occurs prior to the ESCT process. And, again, the existence and importance of the solvent-induced barrier are revealed.

24.5 Conclusions In this chapter, we have succinctly reviewed three pivotal topics relevant to the excited-state proton transfer reaction. In the first section, on biprotonic transfer within doubly H-bonded homo- and heterodimers, we have

Excited-State Proton Transfer via Hydrogen-Bonded Dimers and Complexes in Condensed Phase

H

575

O

N H O

O

CN

NC

Free Energy

ESPT/CT

PC*

O

N

NC

CN

solvent relaxation

C6H12 CH2Cl2 CH3CN

increasing solvent polarity CPT*

PC Solvent polarization coordinate P

Figure 24.17 Proposed ESPT/ESCT reaction/relaxation dynamics using diCN
HBO as a model. Reprinted with permission from [136]. Copyright 2008 American Chemical Society

described the concerted versus stepwise type of excited-state double-proton transfer for hydrogen-bonded dimers. For the case of the 7-AI dimer, although a definitive mechanism of double proton transfer is still debatable at the time of writing, both pro and con research groups present their viewpoints, all based on fundamental and state-of-the-art techniques, from which the readers should gain profound knowledge. Thus, this issue, although disputable, should attract a broad spectrum of readership. Section 24.3 fully extends the dimer or heterodimer hydrogen bonding concept (in Section 24.2) to any host/guest types of hydrogen-bonded complex. Their corresponding proton transfer reaction may thus be either dynamically or thermodynamically controlled, depending on the tautomerism for host, guest or both. Thus, the proton transfer reaction can be fine-tuned on the basis of chemical structures designed for both host and guest molecules. Last but not least, once in the polar solvents, owing to possible changes in the dipole moment during the proton transfer reaction, solvent polarity is expected to play a crucial role in channelling into the reaction dynamics. In this section, theory on solvent-polarity-induced reaction dynamics is first elaborated, followed by its verification based on two reaction types, namely the charge-transfer-induced proton transfer reaction (case A) and the protontransfer-induced charge transfer reaction (case B). Both prove to be prototypical models for proof of the concept. We thus hope that, through reading this chapter, the reader can gain a more fundamental knowledge of the excited-state proton transfer reaction and perhaps its potential in future applications.

References 1. For example, see A. M€uller, H. Ratajack, W. Junge and E. Diemann, Studies in Physical and Theoretical Chemistry, Vol. 78, Electron and Proton Transfer in Chemistry and Biology. Elsevier, Amsterdam (1992). 2. J. Waluk, Conformational Analysis of Molecules in Excited States. Wiley-VCH (2000). 3. T. H. Elsaesser and H. J. Bakker, Ultrafast Hydrogen Bonding Dynamics and Proton Transfer Processes in the Condensed Phase. Springer, Netherlands (2002).

576 Hydrogen Bonding and Transfer in the Excited State 4. 5. 6. 7. 8. 9.

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25 QM/MM Study of Excited-State Solvation Dynamics of Biomolecules Tetsuya Taketsugu1,2, Daisuke Kina1, Akira Nakayama1, Takeshi Noro1 and Mark S. Gordon3 1

Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan 3 Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA

2

25.1 Introduction Excited-state hydrogen transfer (ESHT) is a fundamental reaction that plays an important role in a variety of biological processes [1–3]. The microscopic mechanism and dynamics of ESHT can be elucidated only through a combined knowledge of experimental and theoretical studies. The theoretical approach can be separated into a static examination of excited-state potential energy surfaces (PES) and dynamics simulations on excited-state PES. The excited-state PES can be obtained through ab initio electronic structure or density functional theory (DFT) calculations. Taking into account the applicability to reaction processes accompanying bond cleavage and formation, one should employ, at least, the multiconfigurational self-consistent field (MCSCF) method. In dynamics simulations, an ab initio molecular dynamics (AIMD) approach has proven to be a powerful tool [4]. AIMD is a classical trajectory method in which the force acting on each atom is calculated ‘on the fly’ by ab initio electronic structure methods and enables one to perform a more reliable molecular dynamics simulation. Recently, this methodology has been extended to dynamical processes in electronic excited states [5–7] by including a non-adiabatic surface-hopping scheme, and it can be employed to predict physicochemical properties in the condensed phase by combining QM calculations with some appropriate treatment of solvent effects. For chemical processes in the condensed phase, a hybrid quantum mechanics/molecular mechanics (QM/ MM) method [8] has been developed. If solute–solvent interactions are strong in solution, they could have a large influence on the electronic structure of the solute molecule, including the excitation spectrum [9]. In a

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

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QM/MM approach, each solvent molecule is represented explicitly by classical MM potentials. The solvent molecules that participate directly in the chemical process should be included in the QM part, while the environmental solvent molecules can be included in the MM part. The quality of the QM/MM calculations depends on the model potential used in the MM part, and a number of sophisticated model potentials have been developed. The effective fragment potential (EFP) method [10, 11] is a very sophisticated QM/MM approach that has been implemented in the program package GAMESS [12, 13]. The method accounts for three important solute–solvent and solvent–solvent interaction terms: (a) Coulomb interactions; (b) polarization interactions; (c) exchange repulsion þ charge transfer interactions. In the presence of an ab initio solute, these terms are added as one-electron terms to the ab initio Hamiltonian. The Coulomb term is treated with a distributed multipolar analysis (DMA) of the solvent molecule, expanded through octopoles. The polarization term is treated by a self-consistent distributed finite field model with localized molecular orbital (LMO) polarizability tensors. These two terms have been calculated for many points on the water dimer potential energy surface and subtracted from the total Hartree–Fock interaction potential. Then, this remaining (exchange repulsion þ charge transfer) interaction is fitted to a functional form. In GAMESS, the EFP option can be used with a state-specific MCSCF wave function, where the electron density used to determine EFP-induced dipoles is obtained from the orbitals and CI coefficients of the selected MCSCF state [14]. Thus, the modelling of excited-state dynamics in solution is feasible at the MCSCF-EFP level. Very recently we have developed a general AIMD code for excited-state dynamics in solution using the EFP code in GAMESS, and reported two applications of solvated excited-state dynamics, i.e. ESHT of 7-azaindole (7AI) plus a water cluster [15] and excited-state dynamics of coumarin 151 [16]. In this review, we introduce these two applications to provide insight into the excited-state dynamics in solution, including ESHT of a DNA-base model molecule.

25.2 Applications 25.2.1 Excited-state H-transfer in 7-azaindole-(H2O)n Tautomerization of 7AI, accompanying the ESHT from the five-member ring to the six-member ring, has been studied with much attention both experimentally [17–25] and theoretically [26–29], as 7AI can be regarded as a simple model for a DNA base. The 7AI molecule in its ground electronic state S0 has a delocalized (‘aromatic’) six-member ring attached to a partially saturated five-member ring that contains an N–H bond. In the higher-energy tautomer, the N–H hydrogen is transferred to the six-member ring, thereby breaking the strong delocalization in that ring. In the first excited S1 state, the relative energies of the two isomers is reversed, since the excitation is essentially a p–p excitation in the six-member ring. Sakota et al. measured the dispersed fluorescence (DF) spectra and resonance-enhanced multiphoton ionization (REMPI) spectra for the 7AI–(H2O)n (n ¼ 2, 3) cluster at low temperature in the gas phase and found that the tautomerization of 7AI–(H2O)n (n ¼ 2, 3) occurs in the S1 state [24]. It was also reported [22, 23] that excited-state triple-H transfer occurs in the 7AI–(MeOH)2 cluster, based on the measurement of the DF and REMPI spectra. They pointed out that ESHT in 7AI–(MeOH)2 proceeds via a tunnelling mechanism in their experimental conditions. An early theoretical effort on the ESHT reaction of 7AI–H2O was performed by Chaban and Gordon [26, 27], in which the intrinsic reaction coordinate was calculated for the tautomerization in an isolated 7AI molecule and the 7AI–H2O complex in the singlet ground (S0) and first-excited (S1) states at the complete active space MCSCF (CASSCF) level of theory, with improved energetics obtained using multireference second-order perturbation theory (MCQDPT2) [30]. It was shown that the normal form of 7AI is more stable than the tautomer in the S0 state, while the relative energies are reversed in the S1 state. The activation energy for tautomerization in 7AI is significantly reduced by the complexation with water, because the H transfer occurs via the 7AI–H2O hydrogen-bonded network. In addition to experimental observation in

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the gas phase, tautomerization in 7AI was also observed in the condensed phase in alcohol or water solution [17–19], where the tautomerization exhibits a strong dependence on the water concentration in mixtures of water and aprotic solvents [19]. It is possible that ESHTwould exhibit different mechanisms for the reactions in the gas phase and in the condensed phase. Spectroscopic experiments in the gas phase have been carried out at very low temperatures, so quantum-mechanical tunnelling could contribute significantly to the ESHT reaction. On the other hand, the experiments in solution have been carried out at room temperature, where ESHT reactions could occur classically, because there is sufficient energy in the system to overcome the small activation barrier. In our previous study [15], geometry optimizations and normal mode analyses were carried out for 7AI and 7AI–(H2O)n (n ¼ 1, 2) in the ground and first excited singlet states by the state-specific CASSCF method with the segmented DZP basis set [31–34], using GAMESS [12, 13]. The CASSCF active space includes ten electrons in nine orbitals, which comprise all of the 7AI p orbitals and electrons. Following the examination of the energetics of the reactions, AIMD simulations were performed for the ESHT processes in 7AI–(H2O)n (n ¼ 1, 2). The AIMD code used here is based on the leapfrog algorithm [35]. The initial conditions for the dynamics simulations were the equilibrium structures in the ground state, with the atomic velocities given in the directions of two normal modes of vibration related to the H transfer where the kinetic energy corresponds to three quantum numbers of the respective modes. The time step was taken as 0.5 fs throughout the simulation. Figure 25.1 shows the calculated equilibrium geometries for the normal and tautomer forms of 7AI–H2O and the transition state (TS) geometries for tautomerization in both the S0 and S1 states, where the interatomic

Figure 25.1 Equilibrium geometries of the normal form, the tautomer form and the transition state (TS) in the S0 and  S1 states of 7AI–H2O. Interatomic distances are given in A . Relative CASSCF energies are also shown, where the numbers in parentheses are vibrational zero-point corrected values. Reprinted with permission from [15]. Copyright 2008 American Chemical Society

582 Hydrogen Bonding and Transfer in the Excited State

distances and relative energies are given. Comparing the S0 and S1 equilibrium structures, it is observed that most CC and CN bond distances increase upon photoexcitation. As a result, vibrational excitation of the 7AI ring skeleton may be induced by photoexcitation. The normal form is lower in energy than the tautomer by ca 13 kcal mol1 in the S0 state, while the tautomer is lower in energy than the normal form by ca 32 kcal mol1 in the S1 state. The activation barriers relative to the normal isomer are 39.7 and 16.9 kcal mol1, respectively, in the S0 and S1 states, so the H transfer reaction occurs more easily in the S1 state. The energy profiles for the tautomerization in 7A–(H2O)2 are almost the same as those in 7AI–H2O. The AIMD trajectory simulations were started on the S1 excited state from the ground-state equilibrium structure for the normal form of 7AI–(H2O)n. Figure 25.2 displays the time evolution of the relative energy between the S0 and S1 states ((a) 7AI–H2O and (b) 7AI–(H2O)2) and the interatomic distances related to the H transfer ((c) 7AI–H2O and (d) 7AI–(H2O)2) along the AIMD trajectory. In the case of 7AI–H2O, the molecule stays in the normal form region of the potential energy surface during the initial 40 fs, with fluctuations in the excitation energy with a period of 10 fs (Figure 25.2(a)). The excitation energy in Figure 25.2(a) decreases rapidly at 40–60 fs, and the hydrogen bond distances N  H and O  H decrease to covalent bond lengths, indicating that ESHT occurs through the hydrogen-bonded network. The N  H and O  H hydrogen bond lengths initially exhibit in-phase vibrational motions, but their relative phases gradually change, and, just before the H transfer at t ¼ 40 fs, they become completely out of phase, indicating an asynchronous H transfer. The H is transferred from H2O to 7AI via the N  H hydrogen bond, resulting in 7AI–Hþ   OH around t ¼ 50 fs. The net charge on OH is 0.49 using charges based on the electrostatic potential (ESP) fitting method. So, the transferring H carries a significant positive charge, but it is not a true proton transfer. Following

Figure 25.2 Time evolution of the relative energy of S0 and S1 states ((a) 7AI–H2O and (b) 7AI–(H2O)2) and the interatomic distances related to the proton transfer ((c) 7AI–H2O and (d) 7AI–(H2O)2) along the AIMD simulation. Reprinted with permission from [15]. Copyright 2008 American Chemical Society (See Plate 34)

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this process, the second H transfer occurs promptly from the NH on the five-member ring to OH, completing the tautomerization of 7AI. After the H transfer, the molecular system acquires considerable energy owing to the stabilization of the tautomer in the S1 state, and the 7AI and H2O fragments separate, breaking the two hydrogen bonds. This dynamical behaviour indicates that the ESHT reaction in 7AI–H2O proceeds in a concerted manner via the TS structure shown in Figure 25.1, but rather asynchronously. It is important to consider the experimental study of Takeuchi and Tahara [25] for the double-proton transfer in the 7AI dimer in solution, in which the 7AI dimer converts to the tautomer configuration in the excited state. They investigated this process by excitation wavelength dependence in steady-state and femtosecond timeresolved fluorescence spectroscopy, and concluded that it proceeds in a concerted manner. They noted, however, that this conclusion does not necessarily translate to a synchronous motion of the two protons. A concerted mechanism does not imply strict simultaneity, only that the motions of the two protons are correlated. In the case of 7AI–(H2O)2, changes in the interatomic distances (Figure 25.2(d)) indicate that tripleH-transfer relays occur in the time range t ¼ 40–60 fs, accompanying the gradual decrease in the excitation energy as shown in Figure 25.2(b). A detailed picture of this process is summarized in the following three steps: (1) H2 moves from N1 to O3; (2) H4 moves from O3 to O5; (3) H6 moves from O5 to N7. In the first step, the sum of the net atomic charges on the H5O2 moiety is þ 0.50 according to the ESP population analysis. The reaction mechanism is a concerted asynchronous process. As shown in Figure 25.2(b), the adiabatic energies of the S0 and S1 states approach each other in the tautomer region, suggesting the possibility of a non-radiative decay through a conical intersection of the potential energy surfaces. In order to examine this possibility, the minimum energy conical intersection (MECI) point between the S0 and S1 states was determined for an isolated 7AI molecule. In the MECI structure, the participating H atom attached to the six-member ring is out of the molecular plane. Therefore, when an H is transferred from the out-of-plane side to the six-member-ring N atom, 7AI could easily reach this conical intersection point, suggesting a non-radiative decay process after the phototautomerization in the 7AI–(H2O)n (n ¼ 1, 2) cluster. The activation barrier for tautomerization in an isolated 7AI molecule is much higher than that in 7AI–H2O because, without a bridge of water molecules, an H needs to be directly transferred from the five-member-ring N atom to the six-member-ring N atom [27]. Note that the addition of dynamic correlation via CASPT2 significantly reduces the activation barrier for the reaction in both the S0 and S1 states. In particular, the CASPT2 activation barrier for 7AI–H2O in the S1 state is predicted to be only 2.5 kcal mol1 at the state-averaged CASSCF geometries. This small activation barrier indicates that, at room or higher temperatures, the H transfer reaction does not require quantum tunnelling to proceed. Now, consider aqueous solvation effects on the ESHT dynamics of 7AI–H2O and 7AI–(H2O)2. To simulate the ESHT processes in water, 100 Hartree–Fock (HF)-based effective fragment potential (EFP1/HF) water molecules were distributed around 7AI–(H2O)n. The RHF/STO-3G-AIMD simulations for 7AI–(H2O)n–EFP were performed in the ground state for 10 ps at a constant temperature of 300 K with a time step of 1 fs, where velocity scaling was applied for a time interval of 0.25 ps, to control the temperature of EFP–waters at 300 K, and the geometry of the solute 7AI–(H2O)n was fixed. The temperature undergoes a small fluctuation around 300 K for t > 1.0 ps. Twenty different configurations and velocities of EFPs were chosen from the above AIMD trajectories and used as the initial conditions for the AIMD simulations in the excited state. The AIMD simulations for phototautomerizations in 7AI–(H2O)n–EFP were performed at the CASSCF level of theory with the segmented DZP basis set. During the simulations, each EFP–water maintains a hydrogen bond with other EFP or ab initio waters. When one hydrogen bond breaks, a new hydrogen bond forms. In the AIMD simulations for 7AI–H2O–EFP, CASSCF does not converge in one trajectory; among the remaining 19 trajectories, the ESHT reaction occurs in 15 trajectories, while no reaction occurs in three trajectories. In one trajectory, the ESHT reaction occurs once, but promptly the system returns to the

584 Hydrogen Bonding and Transfer in the Excited State

normal-form region, which is an example of transition-state recrossing. This suggests that the ESHT of the 7AI–H2O cluster takes place asynchronously in both the gas phase and in solution. In most of the reactive trajectories, the ESHT occurs faster compared with that in the gas phase by one period of the N  H bondlength oscillation, presumably owing to the thermal energy of the surrounding solvent molecules. ESHT was not observed in three trajectories, suggesting that the solvent water molecules could diminish the probability of ESHT, depending on the configuration of the surrounding water molecules. AIMD simulations were also carried out for 7AI–(H2O)2–EFP, starting from 20 different initial conditions. The triple-proton transfer occurs in 14 trajectories, while no reaction occurs in five trajectories. Recrossing behaviour is seen in one trajectory. Unlike 7AI–H2O–EFP, the surrounding solvent waters affect the ESHT dynamics more significantly, resulting in different proton transfer mechanisms, depending on the initial solvent configurations. In 12 trajectories, the ESHT dynamics proceeds asynchronously via an H2O–OHd  7AI-Hdþ species in nearly half of the trajectories, while the other half of the trajectories exhibit an H2O–Hdþ –H2O  7AId species. In two trajectories, an almost-synchronous proton transfer is observed. This work reports classical dynamics simulations that do not include tunnelling contributions, which could play an important role in the ESHT process. One needs a quantum dynamical approach to include such quantum effects in the excited-state dynamics simulation. In solution at standard thermodynamic conditions, however, the total system consists of a large number of solute and solvent molecules, and is expected to have sufficient energy to surpass the activation barrier upon photoexcitation. Although the present results are obtained from classical dynamics simulations, they nonetheless provide useful information related to ESHT reaction dynamics. 25.2.2 Excited-state dynamics of coumarin 151 7-Aminocoumarins are a well-known important group of chromophores that emit in the blue-green spectral region, with their fluorescence quantum yields often close to unity [36–38]. As the structures contain an electron-donating amino group at the 7-position and an electron-withdrawing carbonyl group, 7-aminocoumarins are polarized in the ground state. For most of the 7-aminocoumarins there is a large change in dipole moment upon photoexcitation to the first singlet state [36–38]. All of the 7-aminocoumarins show very large Stokes shifts between their absorption and fluorescence maxima. These Stokes shifts are again very sensitive to the solvent polarities [38–40]. In addition, the 7-aminocoumarins are capable of forming hydrogen bonds with solvent molecules; this can influence the solvation dynamics. Owing to these interesting properties, many 7aminocoumarins have been widely used as probes to elucidate a variety of physicochemical processes in the condensed phase. These properties include solvatochromic behaviour, polarities of different environments and measurement of solvent relaxation times using the dynamic Stokes shift method [39, 40]. Although many theoretical [41–44] and experimental [45, 46] studies have been carried out on photophysical properties of 7-aminocoumarins, there remains unusual behaviour that has not yet been understood. Rechthaler and K€ ohler [45] investigated the photophysical properties of several 7-aminocoumarins in diverse organic solvents and found that, unlike other 7-aminocoumarins, the fluorescence quantum yields are very low for coumarin 120 (C120) and coumarin 151 (C151) in non-polar solvents, such as hexane and heptane. Later, Nad and Pal [46] examined this process using picosecond laser flash photolysis and pulse radiolysis techniques, and suggested that the non-radiative decay process of C151 is attributed not to transition to the triplet state but to the ground state (S0) via the umbrella motion of the 7-amino group. Cave et al. [41, 42] performed a detailed theoretical study on the ground and first-excited singlet states of C120 and C151. They found that time-dependent (TD) DFT gives good agreement with the experimental gas phase S0–S1 excitation energies; however, TDDFT combined with a dielectric continuum model overestimates the excitation energies in protic solvents, suggesting that an explicit description of solute–solvent interactions is important in these systems. In another survey with hybrid QM/MM TDDFT-MD simulations [43, 44] the red-shifts of three

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aminocoumarins (C151, C35 and C153) caused by water and acetonitrile solvents were successfully reproduced. In these studies, however, the unusually strong quenching of C120 and C151 in non-polar solvents is not well understood. In our previous study [16], geometry optimizations were carried out for C151 in the ground and first excited singlet (S0 and S1) states by the state-specific CASSCF method with segmented DZP basis sets [31–34] using GAMESS [12, 13]. Preliminary calculations determined the CASSCF active space to be six electrons in six orbitals of C151. RHF/DZP-AIMD simulations were performed for C151 in the S0 state for 10 ps at 300 K with a time step of 1 fs. Then, eight different sets of atomic positions and velocities were taken from the above trajectory with a fixed time interval and used as the initial conditions for the subsequent AIMD simulations in the S1 state. CASSCF/DZP AIMD simulations were performed for isolated C151 in the S1 state, with a time step of 0.5 fs. To simulate the solvation processes of C151 in water, 150 EFP1/HF water molecules were distributed around C151. The preliminary RHF/DZP-AIMD simulations were performed for the C151–EFP system in the ground state for 10 ps at 300 K with a time step of 1 fs, and eight different configurations and velocities were chosen as the initial conditions in the same way as in the simulations of the isolated C151 molecule. The CASSCF/DZP AIMD simulations were performed for excited-state solvation processes of C151 in water, and the solute–solvent interactions were investigated from a dynamical point of view. The time step was taken as 0.5 fs throughout the simulation. Figure 25.3 shows the equilibrium geometry for C151 in the S0 and S1 states with geometrical parameters and dipole moments, calculated by the state-specific CASSCF method. In the S0 state, C151 has an almost planar structure, except for the amino group. Owing to the electron-donating amino group and electronwithdrawing carbonyl group, C151 has a relatively large dipole moment of 5.0 D in the S0 state. The CASSCF geometry optimization located two minimum energy structures on the S1 potential energy surface, denoted as I and II. In structure I, the amino group stays almost in the molecular plane while the six-member ring with the carbonyl group is deformed from the plane. In structure II, the six-member ring with the carbonyl group maintains planarity, but the amino group rotates relative to the molecular plane. Structures I and II have similar energies relative to the S0 minimum, while the S0 energy at the two excited-state structures is



Figure 25.3 Equilibrium structures of C151 in the S0 and S1 states. Bond lengths (in A ), the distance of N1 atom relative to the C5H1H2 plane, r(N1–C5H1H2), the dihedral angle d(C1C2C3O1), dipole moments (indicated by the arrow) and relative energies to the S0 equilibrium point are given. Reprinted with permission from [16]. Copyright 2009 Wiley Periodicals, Inc., A Wiley Company

586 Hydrogen Bonding and Transfer in the Excited State

76.0 kcal mol1 (structure I) and 38.2 kcal mol1 (structure II). This indicates that the energies of the S1 and S0 states are close to each other near structure I. Dipole moments were calculated to be 11.1 and 8.3 D for structures I and II respectively. Their orientations are almost in parallel to that of the dipole moment in the S0 state. In both structures, the S1 state has a HOMO–LUMO single-electron excitation as the dominant configuration. The electron density distributions in the HOMO and LUMO are consistent with the direction of the dipole moment in the S1 state. AIMD simulations were performed for an isolated C151 in the S1 state, starting from eight different initial conditions. These simulations may be regarded as approximate excited-state dynamics simulations of C151 in the presence of non-polar solvents. Analyses of the AIMD simulations show that trajectories can be classified into two types: in three trajectories, C151 exhibits vibrational motion around structure I and reaches the crossing point of the S0 and S1 states, while in five other trajectories, C151 remains near structure II with vibrational motions. Figure 25.4 illustrates the time evolution of the energy of the S0 and S1 states along the AIMD trajectories for (a) C151 and (b) C151–EFP. Figure 25.4(a) illustrates that the adiabatic energies of the S0 and S1 states approach each other around t  340 fs, indicating the possibility of a non-radiative decay through a crossing point of the two potential energy surfaces. The energy of the S1 state is almost constant, with a small fluctuation, while the energy of the S0 state increases by ca 40 kcal mol1 in the initial stage, and again starts to increase around t  220 fs, reaching the crossing point. Here, the S0 energy is estimated using the statespecific CASSCF wave function that was obtained for the S1 state. State-specific CASSCF calculations for the S0 state were also performed at selected points along the S0 trajectory, and the energies obtained in this manner are depicted in the same figure. As expected, the S0 energy obtained in this manner is lower than the ones obtained using the state-specific CASSCF wave function for the S1 state, but the energies for the two calculations show a similar trend in changes along the trajectory. The dipole moment in the S1 state remains close to the large value of 10 D, and then starts to decrease near the crossing point. As shown in Figure 25.4(b), the variations in the S1 and S0 energies are essentially parallel to each other, and this is very different from the AIMD simulations on isolated C151 shown in Figure 25.4(a). The energies of the two states become close to each other occasionally, but no crossing point is observed during the 500 fs of the trajectory. The C151–EFP S1 potential energy exhibits strong fluctuations compared with those in Figure 25.4(a), suggesting a strong electrostatic interaction between C151 and the surrounding EFP waters. The S1 dipole moment is very large owing to the strong polarization effects from the solvent, which could be responsible for the large fluctuations of the S1 potential energy. The solvated S0 dipole moment is also large, 12 D, pointing in almost the same direction as the S1 dipole moment. This finding suggests that the electronic

Figure 25.4 A time evolution of the energy of S0 and S1 states relative to the S0 energy at the equilibrium geometry along the AIMD trajectory for (a) C151 and (b) C151 þ EFP. The S0 energies calculated by the state-specific CASSCF method for the S0 state are shown by the dashed line (S00 ). Reprinted with permission from [16]. Copyright 2009 Wiley Periodicals, Inc., A Wiley Company (See Plate 35)

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wave functions of the S1 and S0 states are similar to each other in this highly polar solvent. This could partly explain the parallel variation in the potential energies.

25.3 Concluding Remarks In this review we have described two applications of an AIMD approach with a combination of the EFP method, which can be used to examine excited-state reaction mechanisms and dynamics in solution. In both 7AI–H2O and 7AI–(H2O)2 clusters, an asynchronous hydrogen transfer occurs via water molecules at t  50 fs in a concerted manner, after the photoexcitation. While the ESHT mechanism for 7AI–H2O in water does not change appreciably compared with that in the gas phase, AIMD simulations on 7AI–(H2O)2 in water solution exhibit two different mechanisms. Using the results of the AIMD trajectories, the minimum energy conical intersection point in the tautomer region has also been located. In AIMD simulations for isolated C151, two patterns are observed for the dynamics: (a) C151 decays from S1 to S0 via a crossing point near structure I (charge transfer state); (b) C151 vibrates on the S1 state. In AIMD simulations for C151 in the presence of 150 EFP waters, the S1 and S0 energy variations are very similar in all of the trajectories, so no crossing point is observed. Assuming that the gas-phase simulation does correspond to a non-polar solvent, this indicates that C151 in a polar solvent is likely to remain on the S1 potential energy surface for a longer time than in a nonpolar solvent. This is in agreement with experimental observations.

Acknowledgements This work was supported in part by a grant-in-aid for scientific research from the Ministry of Education, Culture, Sports, Science and Technology, and in part by a grant from the US Department of Energy via the Ames Laboratory.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16. 17.

L.G. Arnaut and S.J. Formosinho, J. Photochem. Photobiol. A, 75, 1 (1993). S.J. Formosinho and L.G. Arnaut, J. Photochem. Photobiol. A, 75, 21 (1993). A. Douhal, F. Lahmani and A.H. Zewail, Chem. Phys., 207, 477 (1996). M. S. Gordon, G. Chaban and T. Taketsugu, J. Phys. Chem., 100, 11512 (1996). T. Taketsugu, A. Tajima, K. Ishii and T. Hirano, Astrophys. J., 608, 323 (2004). M. Kayanuma, T. Taketsugu and K. Ishii, Chem. Phys. Lett., 418, 511 (2006). M. Kayanuma, T. Taketsugu and K. Ishii, Theor. Chem. Acc., 120, 191 (2008). A. Warshel and M. Levitt, J. Mol. Biol., 103, 227 (1976). S. Yoo, F. Zahariev, S. Sok and M.S. Gordon, J. Chem. Phys., 129, 144 112 (2008). P.N. Day, J.H. Jensen, M.S. Gordon, et al. J. Chem. Phys., 105, 1968 (1996). M.S. Gordon, M.A. Freitag, P. Bandyopadhyay, et al. J. Phys. Chem. A, 105, 293 (2001). M.W. Schmidt, K.K. Baldridge, J.A. Boatz, et al. J. Comp. Chem., 14, 1347 (1993). M.S. Gordon and M.W. Schmidt, “Advances in Electronic Structure Theory: GAMESS a Decade Later” Theory and Applications of Computational Chemistry, Ch. 41, C.E. Dykstra; G. Frenking; K.S. Kim; G.E. Scuseria Eds., Elsevier, 2005. M. Krauss and S.P. Webb, J. Chem. Phys., 107, 5771 (1997). D. Kina, A. Nakayama, T. Noro, et al. J. Phys. Chem. A, 112, 9675 (2008). D. Kina, P. Arora, A. Nakayama, et al. Int. J. Quantum Chem., 109, 2308 (2009). C.A. Taylor, M.A. El-Bayoumi and M. Kasha, Proc. Natl. Acad. Sci. USA, 63, 253 (1969).

588 Hydrogen Bonding and Transfer in the Excited State 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.

K.C. Ingham and M.A. El-Bayoumi, J. Am. Chem. Soc., 96, 1674 (1974). P.-T. Chou, M.L. Martinez, W.C. Cooper, et al. J. Phys. Chem., 96, 5203 (1992). A. Douhal, S.K. Kim and A.H. Zewail, Nature, 378, 260 (1995). A. Nakajima, M. Hirano, R. Hasumi, et al. J. Phys. Chem. A, 101, 392 (1997). K. Sakota, Y. Komoto, M. Nakagaki, et al. Chem. Phys. Lett., 435, 1 (2007). K. Sakota, N. Inoue, Y. Komoto and H. Sekiya, J. Phys. Chem., 111, 4596 (2007). H. Sekiya, private communication. S. Takeuchi and T. Tahara, Proc. Natl. Acad. Sci. USA, 104, 5285 (2007). M.S. Gordon, J. Phys. Chem., 100, 3974 (1996). M.G. Chaban and M.S. Gordon, J. Phys. Chem. A, 103, 185 (1999). R. Casadesu´s, M. Moreno and J.M. Lluch, Chem. Phys., 290, 319 (2003). T. Taketsugu, K. Yagi and M.S. Gordon, Int. J. Quant. Chem., 104, 758 (2005). H. Nakano, J. Chem. Phys., 99, 7983 (1993). This basis set consists of a Hartree-Fock set (¼ Tatewaki-Koga’s all-electron, non-relativistic, segmented contraction] plus a correlating set (¼ natural orbital based segmented cgtf, including valence correlation only] that is referred to as TK/NOSeC-V-DZP. See http://setani.sci.hokudai.ac.jp/sapporo/ H. Yamamoto and O. Matsuoka, Bull. Univ. Electro. Comm., 5, 23 (1992). T. Noro, M Sekiya and T. Koga, Theor. Chem. Acc., 98, 25 (1997). T. Noro, M. Sekiya and T. Koga, Theor. Chem. Acc., 109, 85 (2003). R.W. Hockney and J.W. Eastwood, Computer Simulations Using Particles, McGraw-Hill, New York, 1981. E.J. Schimitschek, J.A. Trias, P.R. Hammond, et al. Opt. Commun., 16, 313 (1976). R.L. Atkins and D.E. Bliss, J. Org. Chem., 43, 1975 (1978). Jones G., II, W.R. Jackson and A.M. Halpern, Chem. Phys., 72, 391 (1980). K. Tominaga and G.C. Walker, J. Photochem. Photobiol. A, 87, 127 (1995). J.A. Gardecki and M. Maroncelli, J. Phys. Chem., 103, 1187 (1999). R.J. Cave, K. Burke and Castner E.W. J. Phys. Chem. A, 106, 9294 (2002). R.J. Cave and Castner E.W., J. Phys. Chem. A, 106, 12117 (2002). M. Sulpizi, P. Carloni, J. Hutter and U. Rothlisberger, Phys. Chem. Chem. Phys, 5 4798 (2003). M. Sulpizi, U.F. R€ohrig, J. Hutter and U. Rothlisberger, Int. J. Quantum Chem., 101, 671 (2005). K. Rechthaler and G. K€ohler, Chem. Phys., 189, 99 (1994). S. Nad and H. Pal, J. Phys. Chem. A, 105, 1097 (2001).

26 Excited-State Intramolecular Proton Transfer Processes on Some Isomeric Naphthalene Derivatives: A Density Functional Theory Based Computational Study Sankar Prasad De and Ajay Misra Department of Chemistry and Chemical Technology, Vidyasagar University, Midnapore 721 102, W.B, India

26.1 Introduction The proton transfer reaction has been found to occur extensively in both chemical and biological processes. Proton transfer may be of intra- or intermolecular nature, and each intra- and intermolecular process can occur either in the ground or in the excited state. Among the various types of proton transfer process, excited-state intramolecular proton transfer (ESIPT) has received much attention owing to its importance in both chemical and biological processes. Numerous ESIPT molecules have been strategically designed and synthesized with the aim of shedding light on the fundamentals of the proton transfer mechanism and/or exploring their potential applications. ESIPT reactions are of great scientific and technological interest. Since its introduction, the photoinduced excited-state intramolecular proton (or hydrogen) transfer reaction, which generally incorporates transfer of a hydroxyl (or amino) proton to the carbonyl oxygen (imine nitrogen) through a pre-existing intramolecular hydrogen bonding (IMHB) configuration, has received considerable attention because it has led to a wide range of applications, such as laser dyes [1, 2], polymer stabilizers [3, 4], environmental probes in biomolecules [5], etc. The main requirement of the ESIPT reaction is that the molecule must have acid and basic groups and at the same time a strong intramolecular hydrogen bond between the two groups. The large number of molecules belonging to this class include, for example, o-hydroxybenzoyl [6–16], o-hydroxy Schiff bases [17–20] and so on. Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

590

Hydrogen Bonding and Transfer in the Excited State

Weller [21, 22], in his pioneering work, had pointed out the dual emission in the fluorescence spectra of salicylic acid and methyl salicylate and attributed it to the asymmetric double well potential arising from proton transfer in the ground as well as in the excited state (Scheme 26.1). The two wells in the ground-state potential energy curve represent the primary (N) and tautomeric (T) forms, and the two wells in the excitedstate curve represent the corresponding excited states N
and T
respectively. It is clear from Scheme 26.1 that the N form is the most stable in the ground state, and T
in the excited state. Since the original work of Weller [21] on the ESIPT of methyl salicylate (MS), a large number of experimental [23–30] and theoretical [31–37] studies on the ESIPT of a variety of systems have been reported. Among them, MS [38–44] and its related compounds o-hydroxyacetophenone (OHAP) [45–47] and o-hydroxybenzaldehyde (OHBA) [48–51] have been well studied as prototypes of molecules showing the ESIPT process. Although theoretical studies on ESIPT on MS and related compounds are quite abundant, similar information about their naphthalene analogue is scant. Evidence for the existence of an intramolecular hydrogen bond in methyl-2-hydroxy-3-naphthoate (MHN23) has been reported by Bergmann et al. [52]. The dual emission of MHN23 was first reported by Naboikin et al. [53]. MHN23 possesses a strong IMHB in the ground electronic state and also shows ESIPT upon excitation to its excited singlet states. On the other hand, the lack of ESIPT emission from methyl-1-hydroxy-2-naphthoate (MHN12) and methyl-2-hydroxy-1naphthoate (MHN21) had been explained by Catalan et al. [54] in terms of the non-radiative dynamics of their respective normal tautomers. Shizuka et al. [55] carried out a comprehensive study on ESIPT of 1-hydroxy-2-acetonaphthone (1H2AN), 1-hydroxy-2-naphthaldehyde (1H2NA) and methyl-1-hydroxy2-naphthoate (1H2MN) by means of laser photolysis, time-resolved thermal lensing and the fluorometry method. They found that both 1H2NA and 1H2AN show ESIPT, whereas 1H2MN gives no ESIPT emission. They also observed a structured ESIPT emission band (lem ¼ 476 nm) for 1H2NA with very little Stokes shift. They showed that distinct relaxation properties of 1H2NA, 1H2AN and 1H2MN are responsible for the relative stabilities between the parent enol (N) form and tautomeric keto (T) form in the lowest excited singlet states of these compounds. On the other hand, laser-induced fluorescence studies by Wu et al. [56] showed a weak tautomeric emission band maximized at 715 nm for 2-hydroxy-3-naphthaldehyde (2H3NA) in cyclohexane at 298 K, with the fluorescence quantum yield as low as 6.8  106. 1H2NA and 2H3NA differ only in the relative

N*

T*

P.E.

T N

Proton transfer coordinate

Scheme 26.1

Excited-State Intramolecular Proton Transfer Processes 591

position of the OH and CHO groups, but they show a wide difference in their fluorescence properties. This wide difference in fluorescence properties of these two compounds motivates us to carry out the present extensive quantum mechanical investigations in order to gain some insight into their electronic structure. Hybrid HF/DFT methods have been proposed as a reliable tool for electronic computation in a general protocol for studying the static and dynamic properties of hydrogen-bonded systems [54, 57, 58]. One such method, B3LYP [59], predicts molecular data that match with the available experimental data as well as with results obtained using the highest post-HF method [60]. In view of its widespread success for the calculation of large molecules [58], we decided to choose the density functional approach for the present calculation of the excited-state proton transfer process in 1H2NA and 2H3NA.

26.2 Theoretical Calculations All ab initio calculations reported in this chapter were carried out using the Gaussian 03 suite of programs [60]. We compared the results for a number of methods and basis sets and found the DFT-based calculations using a hybrid functional (B3LYP) with the 6-31G basis set to be optimal in terms of price–performance ratio for carrying out elaborate electronic structure calculations within our limited computational resources. Analytic vibrational frequency computations at the optimized structure were done to confirm the optimized structure to be an energy minimum or a transition structure. The strength of the intramolecular hydrogen bond (IMHB) of each molecule studied was evaluated as the difference between the energy of the fully optimized structure of the non-hydrogen-bonded form (hydroxyl group rotated by 180 , i.e. open-form Scheme 26.2) and the energy of the N tautomer. Ground-state intramolecular proton transfer (GSIPT) curves were calculated with energies of the B3LYP/6 31G fully optimized structures at fixed OH distances over the 0.8–2.0 A range. Information on the ESIPT mechanism was obtained by calculating the Franck–Condon (FC) transition energies for the DFT (B3LYP)/631G ground-state structures at the TD-DFT(B3LYP)/6-31G level. The Franck–Condon (FC) curves for the proton transfer processes were obtained by adding the TD-DFT(B3LYP)/6-31G excitation energies to the corresponding GSIPT curves. Free energy calculations on the optimized ground-state N and T forms of both compounds were done using RHF/6-31G(d) level of theory.

26.3 Results and Discussion The ground-state optimized structure of both 1H2NA and 2H3NA shows that the enol (N) form is the stable structure having strong intramolecular hydrogen bonding (Scheme 26.2).

1' 1.005

H

O

O 1.409

4'

C 1.442

O

1.370

6'

1.266

1.360

3'

2'

1.705

2'

1.439

5'

H

3'

4' 5'

C H

1H2NA (enol form)

2H3NA (enol form)

Scheme 26.2

1'

1.783 1.451

H

0.995

O 6' 1.258

592 Hydrogen Bonding and Transfer in the Excited State 

Table 26.1 Calculated bond length (in A), bond angle (in deg) and dihedral angle (in deg) at the DFT-B3LYP(631G) level for the IMHB ring (Scheme 26.2) of the N form of 1H2NA 1H2NA Bond length H(1)O(2) 1.005 O(2)C(3) 1.360 C(3)C(4) 1.409 C(4)C(5) 1.442 C(5)O(6) 1.266 O(6)H(1) 1.705

Bond angle

Dihedral angle

H(1)O(2)C(3) 109.262 O(2)C(3)C(4) 121.570 C(3)C(4)C(5) 119.846 C(4)C(5)O(6) 123.926 C(5)O(6)H(1) 100.979

H(1)O(2)C(3)C(4) 0.000 C(3)C(4)C(5)O(6) 0.000 C(5)O(6)H(1)O(2) 0.012

Bond length, bond angle and dihedral angle of the six-member ring system containing an intramolecular hydrogen bond of both 1H2NA and 2H3NA, as shown in Tables 26.1 and 26.2, give some idea about the ground-state geometry of these two compounds. The ground-state bond angle and dihedral angle data given in Tables 26.1 and 26.2 suggest that the sixmember rings formed by IMHB for 1H2NA and 2H3NA are planar and are in the same plane as the naphthalene ring. A critical analysis of the bond length data shows that the C(30 )C(40 ) and O(60 )H(10 ) bonds are much shorter in 1H2NA than in 2H3NA. A shorter O(60 )H(10 ) distance is a measure of stronger IMHB in 1H2NA. Again the double-bond character at C(30 )C(40 ) in 1H2NA and the single-bond character at C(30 )C(40 ) in 2H3NA support the fixed double-bond character of the naphthalene ring. In other words, IMHB in 1H2NAwill be more conjugated, and hence the strength of hydrogen bonding will be greater compared with 2H3NA. In order to get some idea about the relative strength of IMHB in 1H2NA and 2H3NA, we compare the C¼O and OH stretching frequencies of these two compounds with some model compounds like 1-naphthol, 2-naphthol and 2-naphthaldehyde (Table 26.3). We used B3LYP/6-31G(d) level of theory for frequency calculations, and 0.9613 was used as the scale factor for frequencies, as given in Ref. [61]. Table 26.3 shows that our methodology for calculation of vibrational frequency works nicely, as our calculated C¼O and OH stretching frequencies agree well with the experimental results. It is reasonable to infer from both the experimental as well as theoretical calculations that the position of OH absorption in these monosubstituted compounds (1-naphthol and 2-naphthol) is independent of substitution. Hunsberger [62] showed that, for the disubstituted compounds (1H2NA and 2H3NA), the displacement of the OH band (DnOH) from its average



Table 26.2 Calculated bond length (in A), bond angle (in deg) and dihedral angle (in deg) at the DFT-B3LYP (6-31G) level for the IMHB ring (Scheme 26.2) of the N form of 2H3NA 2H3NA Bond length H(1)O(2) 0.995 O(2)C(3) 1.370 C(3)C(4) 1.439 C(4)C(5) 1.451 C(5)O(6) 1.258 O(6)H(1) 1.783

Bond angle

Dihedral angle

H(1)O(2)C(3) 109.981 O(2)C(3)C(4) 121.146 C(3)C(4)C(5) 120.696 C(4)C(5)O(6) 124.094 C(5)O(6)H(1) 100.965

H(1)O(2)C(3)C(4) 0.000 C(3)C(4)C(5)O(6) 0.010 C(5)O(6)H(1)O(2) 0.014

Excited-State Intramolecular Proton Transfer Processes 593 Table 26.3 Theoretical and experimental carbonyl and hydroxyl stretching frequency values of 1H2NA, 2H3NA and some model compounds Compounds

Expa.

Theor. a-naphthol b-naphthol 2-Naphthaldehyde 1H2NA 2H3NA

O–H stretching frequencies (incm1)

C¼O stretching frequencies (cm1)

1729 1654 1675

1702 1637 1670

Theor.

Exp.a

3608 3609

3618 3618

3097 3280

3178 3249

DnC¼O

DnO-H

Theor.

Exp.a

Theor.

Exp.a

75 54

64 31

511 329

440 369

a

Experimental values of IR frequencies (in 0.02 molal CCl4) are obtained from Ref. 62.

position in 1-naphthol and 2-naphthol and the displacement of the C¼O band (DnC¼O) from its average position in the corresponding monosubstituted compound, i.e 2-naphthaldehyde, are taken as a quantitative measure of the strength of IMHB in the disubstituted compounds. Greater red-shift of the band positions from its monosubstituted compound, i.e. larger values of DnOH and DnC¼O, are taken as evidence for corresponding stronger IMHB. Both the experimental and theoretical data of DnOH and DnC¼O in Table 26.1 suggest that 1H2NA has stronger IMHB than 2H3NA. The strength of the intramolecular hydrogen bond of the enol form (N) was calculated by rotating the phenolic OH group out of the hydrogen-bonded conformation and computing the difference in energy between the closed and open form for 1H2NA and 2H3NA and is shown in Figures 26.1 and 26.2 respectively. The corresponding calculated values are given in Table 26.4. The calculated IMHB strength for 1H2NA and 2H3NA are found to be 17.14 and 12.34 kcal mol1 respectively. Figures 26.1 and 26.2 also show that the barrier for phenolic OH rotation is 19.08 kcal mol1 in 1H2NA and 15.25 kcal mol1 in 2H3NA. Thus, the relative strength of IMHB is 5 kcal mol1 greater in 1H2NA than in 2H3NA.

Energy / Kcal mol - 1

20

15

10

5

0 0

20

40

60

80

100 120 140 160 180 200

Dihedral angle (H1'-O2'-C3'-C4') / Degree

Figure 26.1 Energetics of the transformation from IMHB from (N) to the non-hydrogen-bonded form (hydroxyl group rotated by 180 ) of 1H2NA. For each value of the dihedral angle (H(10 )O(20 )C(30 )C(40 )), the geometry has been optimized using the DFT-B3LYP(6-31G) level of theory. Reprinted with permission from [65]. Copyright Elsevier

594

Hydrogen Bonding and Transfer in the Excited State 18

Energy / Kcal mol - 1

16 14 12 10 8 6 4 2 0 0

20

40

60

80

100

120

140

160

180

200

Dihedral angle (H1'-O2'-C3'-C4') / Degree

Figure 26.2 Energetics of the transformation from IMHB from (N) to the non-hydrogen-bonded form (hydroxyl group rotated by 180 ) of 2H3NA. For each value of the dihedral angle (H(10 )O(20 )C(30 )C(40 )), the geometry has been optimized using the DFT-B3LYP(6-31G) level of theory. Reprinted with permission from [65]. Copyright Elsevier

Table 26.4 Energy (in kcal mol1) at various dihedral angles (H10 O20 C30 C40 ) of 1H2NA and 2H3NA, as obtained using DFT-B3LYP(6-31G) level of theory 1H2NA Dihedral angle (H10 O20 C30 C40 ) (deg) 0.0000 8.0429 16.2478 24.8341 34.1085 44.6711 56.1993 68.2001 80.2491 92.0792 103.5461 114.5939 124.9997 134.7501 144.0421 153.0322 161.9035 170.8967 180.0000

2H3NA Energy (kcal mol1)

Dihedral angle (H10 O20 C30 C40 ) (deg)

Energy (kcal mol1)

0.00000 0.47561 1.85288 3.99800 6.69213 9.62155 12.42010 14.81030 16.68163 17.99987 18.78274 19.08348 18.98950 18.62970 18.15252 17.70268 17.37548 17.19588 17.14143

0.0000 8.8077 17.7134 26.9077 36.5782 46.9904 58.1623 69.8019 81.4787 92.9985 104.3356 115.2993 125.8265 135.8144 145.3008 154.3749 163.1061 171.6157 180.0000

0.00000 0.36711 1.43833 3.11904 5.24561 7.59349 9.90706 11.95480 13.60097 14.79426 15.50799 15.75039 15.57133 15.05793 14.34672 13.58978 12.93528 12.49629 12.34152

Excited-State Intramolecular Proton Transfer Processes 595

The conversion from N to T in the ground electronic state can be thought of as arising from proton transfer from Od(20 ) to Oa(60 ) with simultaneous redistribution of electron density within the six-member hydrogenbonded ring. Some authors have considered the OdOa distance as fixed and have varied the OdH bond distance to get an idea about the potential energy curve for both GSIPT and ESIPT processes. A plot of Od(20 )Oa(60 ) distance as a function of rOd _H of 1H2NA, as shown in Figure 26.3 (corresponding data are given in Table 26.5), reveals that, as the proton shifts from Od to Oa, the Od—Oa distance changes significantly. At smaller OdH distance it increases slowly. In the close vicinity of a stable OdH distance, the OdOa distance falls sharply. However, as the proton shifts further from Od, it decreases sharply, passes through a minimum and then enlarges to a distance comparable with that in the T form. This variation is almost identical in the case of 2H3NA (Figure 26.4). Figure 26.5 shows the variation in OdHOa angle as a function of rOH distance. At smaller OdH distance, it increases slowly. The OdHOa angle increases sharply in the near vicinity of the stable OdH distance. It increases with increase in rOH distance, reaches a maximum and then shows a sudden fall with further increase in rOH distance. We obtained similar variation in OdHOa angle with rOd --H in the case of 2H3NA (Figure 26.6). The corresponding data are given in Table 26.6. Therefore it becomes clear that, by freezing the geometry or by fixing the OdOa distance at a particular value, one ends up introducing artificial constraints on the system and hence a barrier for the enol (N) to keto (T) conversion. In this chapter we used the ‘distinguished coordinate’ approach as proposed by Sobolewski et al. [63], where the OdH bond distance is varied and the rest of the structural parameters are allowed to relax for each choice of rOH. Maheswari et al. [64] conducted an extensive theoretical study on salicylic acid and showed that the variation in OdH bond length can be used as a reaction coordinate in order to get some idea about the PE curve for the ground-state as well as for the excited-state proton transfer processes. Catalan et al. [54] used a similar reaction coordinate (rOH) for the ESIPT processes of some naphthalene derivatives.

2.65

2.60

2.50

d

a

rO -O / A0

2.55

2.45

2.40 0.50

0.75

1.00

1.25

1.50

1.75

0

rO-H / A

Figure 26.3 Variation in the Od(20 )Oa(60 ) distance of 1H2NA with rOd --Ha , as obtained from DFT-B3LYP(6-31G) level of theory. Reprinted with permission from [65]. Copyright Elsevier

596

Hydrogen Bonding and Transfer in the Excited State 

Table 26.5 Value of OdOa bond distance at various OdH distances (in A) of 1H2NA and 2H3NA, as obtained using DFT-B3LYP(6-31G) level of theory 



rOH (A)

rOd --Oa (A)

0.50 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.40 1.50 1.70 1.75 2.00

1H2NA

2H3NA

2.6000 2.6358 2.6373 2.6350 2.6281 2.6146 2.5928 2.5608 2.5161 2.4643 2.4212 2.3969 2.3919 2.4149 2.4608 2.5777 2.6090

2.6237 2.6676 2.6706 2.6713 2.6682 2.6606 2.6460 2.6237 2.5914 2.5477 2.4978 2.4567 2.4355 2.4404 2.4780 2.5887 2.6190 2.7771

26.3.1 Ground- and excited-state potential energy curve of 1H2NA The ground-state potential energy curve, as shown in Figure 26.7, the corresponding data of which are given in Table 26.7, reveals that the enol (N) form is the most stable one. Surprisingly, there is no shallow minimum for the keto form. The barrier for the enol (N) to keto (T) form is about 6.26 kcal mol1, and this is large enough to

2.75

2.70

2.60

d

a

r0 -O (A0)

2.65

2.55

2.50

2.45 0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

rO -H(A0) d

0

0

Figure 26.4 Variation in the Od(2 )-Oa(6 ) distance of 2H3NA with rOd --Ha , as obtained from DFT-B3LYP(6-31G) level of theory

Excited-State Intramolecular Proton Transfer Processes 597 160

150

Od -H-Oa angle / 0

140

130

120

110

100

90 0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

0

rO -H / A d

Figure 26.5 Variation in the Od—H—Oa angle of 1H2NA with rOd --Ha , as obtained from DFT-B3LYP(6-31G) level of theory. Reprinted with permission from [65]. Copyright Elsevier

make any GSIPT under thermal conditions. Again, the calculated free energy change for the keto–enol tautomerization of 1H2NA gives a large positive value (9.201 kcal mol1), and the equilibrium constant obtained from the free energy change, i.e 1.7  106, suggests that the above equilibrium lies towards the enol form. On the basis of the equilibrium constant, the population ratio in the gas phase for the enol form versus 152 150

Od-H-Oa (Degree)

148 146 144 142 140 138 136 134 0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0

rO -H (A ) d

Figure 26.6 Variation in the Od—H—Oa angle of 2H3NA with rOd --Ha , as obtained from DFT-B3LYP(6-31G) level of theory

598

Hydrogen Bonding and Transfer in the Excited State 

Table 26.6 Value of the OdHOa angle (in deg) at various OdH distances (in A) of 1H2NA and 2H3NA, as obtained using DFT-B3LYP(6-31G) level of theory 

rOH (A)

OdHOa angle (deg)

0.50 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.40 1.50 1.70 1.75 2.41

1H2NA

2H3NA

139.11581 139.82649 140.34876 140.99463 141.82576 142.91872 144.22973 145.94132 147.98119 150.06545 151.67221 152.36214 152.25716 150.76805 148.30341 142.28409 140.70802 94.33674

139.20331 139.73050 140.21231 140.72809 141.39689 142.21755 143.24496 144.53978 146.12801 148.00281 149.88770 151.64820 151.41013 148.50879 142.74887 141.16524 133.41114

keto form in the ground state is 6  106:1. This again confirms that there is hardly any possibility of GSIPT at normal temperature in 1H2NA. The GSIPT curve for the keto (T) form is almost flat, which implies that the proton transfer fluorescence may not show a broad structureless band. Interestingly, in their experimental work, Tobita et al. [55] observed a structured and somewhat less broad emission band of 1H2NA in cyclohexane having lem  476 nm. They also 100

Energy (kcal./mol)

95 90 85

S2

80

S1

14 12 10 8 6 4 2 0

S0

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1. 8

0

r(O -H)(A ) d

Figure 26.7 GSIPT curve (S0) and ESIPT Franck–Condon curves of 1H2NA, as obtained from DFT-B3LYP(6-31G) and TDDFT-B3LYP(6-31G) levels of theory. Reprinted with permission from [65]. Copyright Elsevier

Excited-State Intramolecular Proton Transfer Processes 599 

Table 26.7 Values of relative energy (in kcal mol1) at various OdH distances (in A) of 1H2NA, as obtained using DFT-B3LYP(6-31G) level of theory 

Relative energy (kcal mol1)

rOH (A)

0.50 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.40 1.50 1.70 1.75

S0 state

S1 state

S2 state

506.51003 53.38376 29.33885 14.28628 5.58148 1.29799 0.00000 0.55610 2.11823 3.83657 5.16336 5.93043 6.25985 6.27911 6.15437 6.69000 7.01953

589.91244 135.56460 111.17397 95.69962 86.47393 81.53817 79.40583 78.94320 79.28839 79.83127 80.29374 80.57219 80.67804 80.44608 80.26372 80.81779 81.17037

590.29273 137.91782 114.00660 98.75121 89.86663 85.34574 83.71355 83.83173 84.82227 85.87451 86.65506 86.76064 86.51616 85.68725 84.96556 84.53086 84.68983

observed that the red-shifted absorption band of 1H2NA is p–p
in nature, with lmax ¼ 368 nm in cyclohexane. On the other hand, our gas-phase calculation shows lmax at 360 nm for the enol (N) form of 1H2NA. Thus, our calculated lmax is in good agreement with the experimental findings of Tobita et al. [55]. 26.3.2 Ground- and excited-state potential energy curve of 2H3NA For the calculation of the ground-state potential energy curve, we used the same technique as before, i.e the ‘distinguished coordinate’ approach as proposed by Sobolewski et al. [63], and obtained a minimum in the  PE curve at an rOH distance of about 1 A, which is due to the N form of 2H3NA. Surprisingly, there is no shallow minimum for the T form; rather, the ground-state potential energy curve (Figure 26.8) increases  steadily as the rOH distance increases from 1 to 2 A. The corresponding data are given in Table 26.8. The FC  potential energy curve for the S1 state shows two minima, one at rOH  1 A and the other, which is much lower in energy, at rOH  1.5 A. The former minimum is due to the excited enol form (N
), and the latter minimum is due to the excited keto tautomer (T
). The repulsive nature of the GSIPT curve and the energy  gap between the S0 and S1 curves at the keto tautomer position (rOH  1.5 A) implies that the keto tautomer emission will be broad, structureless and largely red-shifted. In their laser-induced fluorescence measurements, Wu et al. [56] showed a large Stokes-shifted, extremely weak emission band of 2H3NA maximized at 715 nm for the keto tautomer resulting from ESIPT. We believe that the nanosecond UV pulse excites the normal species to a vibronic S1 state via a Franck–Condon transition, where the nuclear coordinates remain unchanged. The rapid charge distribution after excitation results in an electronic potential surface possessing a substantial gradient and an energy minimum shifted towards the proton position in the keto tautomer. The potential energy surface of the S1 state shows that some of the normal vibrational modes are displaced from the equilibrium position. As a result, the system begins to evolve along these normal coordinates on the excited-state energy surface towards a new equilibrium position. This motion may be conceived as

600

Hydrogen Bonding and Transfer in the Excited State

Energy (kcal./mol)

90 S2

80

70

S1

20

S0

10

0 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 0

r(O -H)(A ) d

Figure 26.8 GSIPT curve (S0) and ESIPT Franck–Condon curves of 2H3NA as obtained from DFT-B3LYP(6-31G) and TDDFT-B3LYP(6-31G) levels of theory. Reprinted with permission from [65]. Copyright Elsevier

the propagation of a wave packet made up of the superposition of wave functions of various vibronic states that are involved in the temporal development. In particular, those vibrational coordinates coupled with the proton displacement should deviate significantly from the equilibrium position, which is eventually reached by the formation of the excited keto tautomer. 

Table 26.8 Values of relative energy (in kcal mol1) at various OdH distances (in A) of 2H3NA, as obtained using DFT-B3LYP(6-31G) level of theory 

Relative energy (kcal mol1)

rOH (A)

0.50 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.50 1.70 1.75 2.00

S0 state

S1 state

S2 state

505.29500 52.38250 28.44140 13.52560 5.00210 0.96190 0.00000 1.06360 3.32270 6.08320 8.73930 10.86930 12.38530 13.43310 14.18480 15.28630 17.38900 18.00060 21.58020

581.90969 126.57021 101.97224 86.30045 76.87577 71.73155 69.37985 68.66643 68.63453 68.53705 68.04706 67.44354 67.04423 66.81515 66.70254 66.73230 67.84623 68.32646 71.57647

586.08373 132.33688 108.19295 93.05359 84.27425 80.10498 78.75818 86.05172 80.99530 82.96755 85.05206 86.95388 87.28750 85.66171 84.36672 82.58719 81.26722 81.23347 82.32847

Excited-State Intramolecular Proton Transfer Processes 601

The repulsive nature of the ground-state PE curve outright ruled out the possibility of ground-state intramolecular proton transfer in 2H3NA. Again, our free energy calculation for the ground-state enol–keto equilibria of 2H3NA gives a positive free energy change (DG ¼ 29.70 kcal mol1), and the calculated equilibrium constant is 1.40  1022. On the basis of the equilibrium constant, the population ratio in the gas phase for the enol vs. keto form in the ground state is 7  1021:1. This clearly explains that the GSIPT is thermodynamically unfavourable. Experimental investigations of Wu et al. [56] shows that lmax of 2H3NA in cyclohexane is nearly 390 nm. On the other hand, our calculated value of lmax in the gas phase is about 404 nm, and this is in good agreement with their experimental findings. 26.3.3 Comparison of IMHB and ESIPT of 1H2NA and 2H3NA Both compounds 1H2NA and 2H3NA contain a naphthalene ring and two other chromophore OH and CHO. However, they differ only in the relative position of the OH and CHO groups. Our calculation suggests that, for both these molecules, the N form is the most stable in their ground state. The strength of IMHB is nearly 5 kcal mol1 greater in 1H2NA than in 2H3NA. As far as the ring skeleton of the N form is concerned, 1H2NA and 2H3NA resemble phenanthrene and anthracene respectively. We then decided to compare the energy of phenanthrene and anthracene using the same quantum mechanical method (DFT/ B3LYP with 631G basis), and, to our complete surprise, we found that, energetically, phenanthrene is nearly 5 kcal mol1 more stable than anthracene. As significant characteristic differences are not observed between the S2 states of 1H2NA and 2H3NA, this state is not shown in the potential energy surfaces, although the corresponding values are given in Tables 26.7 and 26.8. Figure 26.7 shows that the excited singlet state (i.e 1(pp
)) potential energy curve of 1H2NA has a minimum at the equilibrium distance of the N tautomer and a wide minimum near the keto tautomer position. Figure 26.7 also shows that the barrier of the N tautomer and T tautomer is very small. If the proton transfer process or any other non-radiative deactivation channel is quite fast compared with the lifetime of the excited N tautomer, as observed by Tobita et al. [55] for 1H2NA, it will be very difficult to observe emission from the N tautomer. On the other hand, owing to the faster formation rate, the population of the T tautomer will be high enough to observe emission. Whereas the 1(pp
) potential energy curve of 2H3NA exhibits two minima and an exothermal behaviour of the potential energy curve of the T tautomer explains the occurrence of proton transfer fluorescence. Figures 26.9(a), (b) and (c) show the PES with simultaneous variation in OdH and OdOa distances in the ground (S0) state, the first excited singlet (S1) state and the second excited singlet (S2) state, respectively, of compound 1H2NA. The contour levels are given in relative energies and expressed in kcal mol1. In the S0 state (Figure 26.9(a)), the presence of a single valley corresponding to the enol (N) form indicates that in the ground state the enol form is the most stable one. By contrast, in the S1 (Figure 26.9(b)) and S2 (Figure 26.9(c)) states, two valleys are observed, corresponding to the excited enol (N
) and excited keto (T
) tautomers. Figures 26.9(b) and (c) also illustrate a narrow potential well for N
and a much wider and deeper well for T
, thus indicating a greater possibility of its existence over the N
form in the S1 as well as in the S2 excited state. As S1 is the lowest excited state, proton transfer emission will come from the S1 (according to Kasha’s rule) state only. An almost identical contour curve is obtained in the case of 2H3NA, as shown in Figures 26.10(a), (b) and (c), which represent the S0, S1 and S2 states respectively. In Figures 26.11(a) and (b), the oscillator strength f for the S0–S1 transition increases with increase in OH bond distance in the case of both 1H2NA and 2H3NA. It indicates that the red-shifted emission can be substantial. The corresponding data are given in Table 26.9. A detailed analysis of the electron density of HOMO and LUMO of these two compounds can throw some light on ground- and excited-state electron transfer processes. Both HOMO and LUMO are of the p type, but their phases are quite different in 1H2NA and 2H3NA. The HOMO orbital on the IMHB ring of 1H2NA

602

Hydrogen Bonding and Transfer in the Excited State

(a)

1.8

1.8

(b) 2.632

72.22

1.6

1.6

-10.53 1.4

1.4

61.11

2.632

15.79 42.11

0.8

68.42 81.58 94.74

55.26

94.44

d

1.0

0.4

0

1.2

28.95

r(O -H) / A

d

r(O -H) / A0

1.2

0.6

72.22

83.33

107.9 121.1 160.5 226.3 265.8 134.2 147.4 278.9 239.5 292.1 357.9 186.8 213.2 305.3 384.2 331.6 173.7 252.6 200.0 2.40

2.45

2.50

2.55

1.0

0.8

0.6

0.4

105.6 127.8 138.9 116.7 161.1 150.0 172.2 194.4 183.3 238.9 283.3 316.7 205.6 216.7 305.6 350.0 416.7 338.9 261.1 294.4 227.8 383.3 461.1 250.0 272.2 327.8 361.1 427.8 2.40

2.60

2.45

0

2.60

r(O -O )/A

a

(c)

2.55 0

r(O -O )/A d

2.50 d

a

1.8

1.6

83.33

d

r(O -H) / A0

1.4

72.22

1.2

105.6 94.44 1.0

172.2

0.8

0.6

0.4

127.8 116.7 150.0 138.9 161.1 183.3 194.4

238.9

205.6 283.3

327.8 316.7 294.4 216.7 227.8 350.0 427.8 272.2 305.6 338.9 383.3 261.1 461.1 250.0 372.2 416.7 2.40

2.45

2.50

2.55

2.60

0

r(O -O )/A d

a

Figure 26.9 2D PES for the (a) ground (S0) state, (b) the first excited singlet (S1) state and (c) the second excited singlet (S2) state of 1H2NA, plotted as a function of OdH and OdOa distances. The contour levels are the relative energies of the respective states (in kcal mol1)

(Figure 26.12) is primarily of bonding character over the C(30 )C(40 )C(50 ) atoms, whereas C(30 )O(20 ) and C(50 )O(60 ) show antibonding character. Both the hydroxyl oxygen and aldehyde oxygen have bonding character, with a larger electron density over the hydroxyl oxygen. Analysis of the HOMO electron density after ground-state electron transfer (Figure 26.13) still shows a larger density on the hydroxyl oxygen and also a shift of electron density over the C(40 )C(50 ) bond. The HOMO electron density around the IMHB ring (Figure 26.14) of the N tautomer of 2H3NA shows a node at C(40 ). Again, there is no electron density contribution over the aldehyde group of 2H3NA. The HOMO electron density of the ground-state proton

Excited-State Intramolecular Proton Transfer Processes 603 (a)

(b)

2.0

1.8

18.75

1.8

1.6

1.6

5.000

r(O -H) / A0

1.4

1.2

0.8 0.6 0.4 2.40

87.50

101.3

142.5 156.3

73.75 60.00 128.8 115.0

121.4

2.50

2.55

1.0

92.86 78.57

64.29

135.7

107.1

178.6 192.9 0.8

170.0 197.5 238.8 183.8 211.3 293.8 266.3 321.3 225.0 307.5 376.3 335.0 252.5 280.0 2.45

1.2

d

46.25 32.50

d

r(O -H) / A0

1.4

1.0

2.0

2.60

2.65

2.75

0.4 2.40

2.80

207.1

164.3

235.7 250.0 278.6 321.4 364.3 264.3 292.9 335.7 407.1 378.6 450.0 307.1 392.9 350.0 221.4

0.6

2.70

150.0

2.45

2.50

2.55

0

d

2.60

2.65

2.70

2.75

2.80

0

r(O -O )/A

r(O -O )/A

a

d

a

2.0

(c)

1.8

78.57

1.6

d

r(O -H) / A0

1.4 1.2

135.7

150.0

1.0

192.9 0.8 0.6 0.4 2.40

78.57 107.1 92.86 121.4 164.3

207.1 221.4

178.6

250.0 264.3 292.9 335.7 278.6 307.1 378.6 350.0 421.4 321.4 392.9 407.1 464.3 364.3 235.7

2.45

2.50

2.55

2.60

2.65

2.70

2.75

2.80

0

r(O -O )/A d

a

Figure 26.10 2D PES for the (a) ground (S0) state, (b) the first excited singlet (S1) state and (c) the second excited singlet (S2) state of 2H3NA, plotted as a function of the OdH and OdOa distances. The contour levels are the relative energies of the respective states (in kcal mol1)

transfer form of 2H3NA (Figure 26.15) shows less electron density on C(40 )C(50 ) and aldehyde oxygen than that of 1H2NA. Thus, the lower conjugation through the IMHB ring in 2H3NA weakens its IMHB strength. Again, the less effective electron transfer along the proton transfer coordinate makes the GSIPT process less probable. The opposite effect was found for 1H2NA, thereby strengthening the IMHB ring system and stabilizing the GSIPT potential energy curves with respect to 2H3NA. Nevertheless, this stabilization is not sufficient to produce a GSIPT process. For both 1H2NA (Figure 26.12) and 2H3NA (Figure 26.14), LUMO is p
in nature. If we look into the electronic charge distribution of LUMO within the IMHB ring (N tautomer) for both 1H2NA and 2H3NA, the C(40 )C(50 ) position has a bonding character, whereas the C(30 )(20 ), C(30 )C(40 ) and C(50 )O(60 ) positions have an antibonding character. The LUMO of the enol form of 2H3NA possesses a high electron density on the O(60 )

604

Hydrogen Bonding and Transfer in the Excited State 0.105

(a)

Oscillator strength(f)

0.100

0.095

0.090

0.085

0.080

0.4

0.6

0.8

1.0

1.2 (Od -H)

0.048

1.4

1.6

1.8

0

r

(A )

(b)

Oscillator strength(f)

0.046 0.044 0.042 0.040 0.038 0.036 0.034 0.032 0.4

0.6

0.8

1.0

1.2

r

(Od -H)

1.4

1.6

1.8

2.0

2.2

0

(A )

Figure 26.11 Variation in the oscillator strength with OdH distance in the S0–S1 transition of 1H2NA (a) and 2H3NA (b) computed using DFT (B3LYP)/6-31G level of theory

atom, and there is no electron distribution on the O(20 ) atom. On the other hand, the LUMO of 1H2NA possesses high electron density at O(60 ) and comparatively lower electron density at O(20 ) than that of the corresponding HOMO. After tautomerization, the LUMO of the keto tautomer of 2H3NA still shows high electron density on the O(60 ) atom and much lower electron density on the O(20 ) atom. Thus, it favours the

Excited-State Intramolecular Proton Transfer Processes 605 

Table 26.9 Values of oscillator strength f at various OdH distances (in A) of 1H2NA and 2H3NA, as obtained using DFT-B3LYP(6-31G) level of theory 

rOH (A)

0.50 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.40 1.50 1.70 1.75 2.00

Oscillator strength f 1H2NA

2H3NA

0.0797 0.0818 0.0823 0.0828 0.0833 0.0841 0.0849 0.0858 0.0873 0.0894 0.0921 0.0941 0.0949 0.0979 0.0992 0.1006 0.1009

0.0353 0.0349 0.0347 0.0346 0.0344 0.0341 0.0338 0.0335 0.0332 0.0331 0.0336 0.0348 0.0362 0.0389 0.0409 0.0438 0.0444 0.0471

Figure 26.12 HOMO and LUMO orbitals of 1H2NA (enol form), as obtained with DFT-B3LYP(6-31G). Reprinted with permission from [65]. Copyright Elsevier

transfer of a proton from O(20 ) to O(60 ). However, in the case of the keto tautomer of 1H2NA, O(20 ) and O(60 ) have comparable electron density with that of the HOMO. So, the ESIPT process in 1H2NA is less favourable compared with that in 2H3NA. Our PES calculations along the proton transfer coordinate also suggest the exothermal behaviour of ESIPT processes. Our PES calculations of 1H2NA suggest that the N and T forms have comparable energy in the first excited singlet state. We believe that, owing to the faster rate of formation of the excited T form and also the presence of other faster non-radiative deactivation channels from the excited singlet form of the N tautomer, it is very difficult to observe normal emission in 1H2NA. This type of extensive study on the intramolecular proton transfer process has been conducted by the present authors – see Refs [65] to [67].

606 Hydrogen Bonding and Transfer in the Excited State

Figure 26.13 HOMO and LUMO orbitals of 1H2NA (keto form), as obtained with DFT-B3LYP(6-31G). Reprinted with permission from [65]. Copyright Elsevier

Figure 26.14 HOMO and LUMO orbitals of 2H3NA (enol form), as obtained with DFT-B3LYP(6-31G). Reprinted with permission from [65]. Copyright Elsevier

Figure 26.15 HOMO and LUMO orbitals of 2H3NA (keto form), as obtained with DFT-B3LYP(6-31G). Reprinted with permission from [65]. Copyright Elsevier

26.4 Conclusions The relative position of the OH and CHO groups in 1H2NA and 2H3NA determine the strength of the intramolecular hydrogen bond and the nature of ESIPT emission. In the ground state, the strength of IMHB in 1H2NA is greater than in 2H3NA. Free energy calculation, HOMO electron density, ground-state PE calculation and also contour diagrams for both 1H2NA and 2H3NA support the non-viability of GSIPT processes. On the other hand, excited-state potential energy, LUMO electron density and oscillator strength calculations support the ESIPT for both 1H2NA and 2H3NA. The nature of the ground- and excited-state

Excited-State Intramolecular Proton Transfer Processes 607

potential energy curves nicely explains the red-shifted broad structureless emission band of 2H3NA, and also the less broad and less Stokes-shifted ESIPT band of 1H2NA. Analysis of the LUMO electron density suggests that ESIPT is more favourable in 2H3NA. As all the calculations have been carried out by the DFT method with hybrid functionals (B3LYP/6-31G), they again support the potential of the DFT method for calculations of ESIPT processes.

Acknowledgements We gratefully acknowledge the financial support received from DSTand UGC, New Delhi, for carrying out this research work.

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27 Conformational Switching Between Acids and Their Anions by Hydrogen Bonding Taka-aki Okamura, Hitoshi Yamamoto and Norikazu Ueyama Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

27.1 Introduction In general, pKa shifts for acids, such as thiols, carboxylic acids, phenols and phosphoric acid monoanions, are considered to occur under homogeneous conditions and change with dielectric constant. Theoretical calculations have been performed for homogeneous environments [1–4], and some improved theoretical studies have considered anisotropic and heterogeneous environments [4–8] to deal with the particularly large shift for thiols, phenols and carboxylic acids located inside proteins. This paper discusses pKa shifts for acids having neighbouring amide NHs. A relationship between the pKa shift and the formation constant is observed for metal complexes in hydrophobic environments. The influence of intramolecular NH


X hydrogen bonds on the metal–X bond character is also observed for metal complexes. The presence of NH


S hydrogen bonds in iron–sulfur proteins observed by crystallographic studies has been suggested by Adman and coworkers [9]. Various synthetic metal–thiolate complexes having chelating Cys-containing oligopeptides and synthetic simple intramolecularly NH


S hydrogen-bonded thiolate ligands exist. NH


S hydrogen bonds in metal–thiolate complexes play an important role in the following functions. Intramolecular hydrogen bonds contribute to the following: increase in the stabilization constant, e.g. Fe(II), Zn(II), Cd(II), Hg(II) and Ca(II) complexes; positive shift of the redox potential, e.g. Fe(II), Fe(III), Mo(IV) and W(IV) complexes; and increase in air stabilization, e.g. Fe(II) and Fe(III) complexes. In addition, the NH


S hydrogen bond increases the formation constant for metal–sulfur complexes formed by substitution reaction between non-hydrogen-bonded and hydrogen-bonded thiolates [10]. The large formation constant is due not only to the pKa-lowering shift of thiol but also to a pp–dp interaction by the NH


S hydrogen bond. Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

610 Hydrogen Bonding and Transfer in the Excited State

27.2 pKa Shift of Acids by Neighbouring Amide NH 27.2.1 pKa shift of thiol, phenol and carboxylic acid derivatives by prelocated hydrogen bond Thiol, phenol, carboxylic acid and phosphate monoanions exist as relatively weak acids under hydrophobic and neutral conditions, although their deprotonated anion states are strong anions that coordinate to metal ions. When the anion has an amide NH in the neighbourhood, it forms a strong NH


X (X¼S, O) hydrogen bond (Figure 27.1). The extent of deprotonation in these acids reflects the lowering of pKa by the prelocated amide NHs. Crystallographic analyses of thiol (RSH) and its thiolate anion (RS) in the solid state, and 1H NMR and FT-IR analyses in solution, indicate that the S atom of the SH group in an amidated thiophenol derivative does not interact with the neighbouring amide NH in the ground state (Figure 27.2(a)), but the thiolate anion does this strongly. If the thiophenolate has an amide NH at the p-position, the thiolate anion strongly interacts with other thiolate molecules, intermolecularly (Figure 27.2(b)). Therefore, when a solvent has an amide NH, the amide NH in the solvent strongly interacts with the thiolate anion. Similarly, the S atom in the disulfide does not interact with the neighbouring amide NH in bis[2,6-di(pivaloylamino)] phenyl disulfide (Figure 27.2(c)), as shown by crystallographic analysis in the solid state and 1H NMR and IR spectroscopic analyses in solution [11].

R''

R

H

S

H

R

R'

O

N

O

R''

R'' S

S

H

H

R

R'

O

N

+ H+

H N R'

Figure 27.1 Deprotonation of RSH assisted by prelocated amide NH

(a)

H

(c) S

H

-H

N O

H

S

+

R

R

O

+ H+

R

R

N

R R

O

O

H N N H

(b)

H

S

O

S

- H+

S

H

S

N

H N N H

H N

O

O

O

+ H+ S H N

O

R R R

R

R

R

No NH---S interaction (R = H, Me, Ph)

Figure 27.2 Deprotonation processes: (a) 2-t-BuCONH-C6H4SH; (b) 4-t-BuCONH-C6H4SH. After deprotonation, the thiolate anion intermolecularly interacts with amide NH. (c) Disulfide structure determined by crystallographic analysis. No interaction is observed between amide NH and sulfur

Conformational Switching Between Acids and Their Anions by Hydrogen Bonding

611

The pKa shifts by prelocated amide NH in thiols were examined in aqueous micellar solution using bulky thiol. The shift for RSH is known to occur because of the electron-withdrawing character of the R group. Electronic effects for the amide group on the phenyl ring are negligible by Hammet’s parameter (sp ¼ 0). The shift for the N-methylated derivative, which cannot form hydrogen bonds, is slight, while that for 2,6-(t-BuCONH)2C6H3SH with double amide NHs is large (pKa ¼ 4.4) (Table 27.1). Intermolecular NH


S hydrogen bond formation is detected for the anion form of 4-t-BuCONHC6H4SH, which has a relatively high

Table 27.1 pKa values for various thiols in Triton X-100 aqueous micellar solution at room temperature Thiol

pKa 8.0

5.4

5.7

6.1

4.9

4.4

4.9

9.6 8.8(Cys1), 10.0 9.3(Cys1), 9.8(Cys2)

612 Hydrogen Bonding and Transfer in the Excited State

pKa (7.6) compared with that for 2-BuCONHC6H4SH. It turns out that the high value is due to a lack of interaction with the remote amide NH even at the transition state because of the lower encounter probability in intermolecular collisions between thiolate and amide NH. The pKa value for Cys-containing oligopeptides in aqueous micellar solution depends on the location of the Cys residue in the peptide chain [12]. For a short Cys-containing tripeptide, pKa is 9.6 in aqueous micellar solution. This peptide in the thiolate state forms a strong NH


S hydrogen bond, detected by the IR and 1 H NMR shifts of amide NH not only in chloroform but also in aqueous micellar solution. For Z-Cys(1)-ProLeu-Cys(2)-OMe, the values for Cys(1) and Cys(2) are pKa ¼ 8.8 and 10.0 respectively. For Z-Cys(1)-Ala-ProCys(2)-OMe, the values for Cys(1) and Cys(2) are high, pKa ¼ 9.3 and 9.8 respectively. The solution structures determined by 1H NMR indicate that the former tetrapeptide has a hairpin turn structure, which readily forms an NH


S hydrogen bond after easy deprotonation from Cys(1) thiol, whereas the latter tetrapeptide has an extended structure that yields similar pKa values for Cys(1) and Cys(2) after deprotonation. The pKa shift with the neighbouring amide NH is due to the lowering of the energy barrier for the transition state during deprotonation. In the ground state the thiol is not affected by the neighbouring amide NH, whereas in the transition state it interacts with the adjacent amide NH. The pKa shift corresponds to the extent of energy lowering for deprotonation. The pKa values for phenol [13–15] and carboxylic acid derivatives [16] show a similar tendency to the prelocated amide NH in aqueous micellar solution. Singly and doubly hydrogen-bonded phenol and benzoic acid derivatives exhibit a clear pKa shift. For example, although phenolic OH oxygen is known to have a weak negative charge, the O atom of the OH group does not interact with the neighbouring amide NH, even being closely located to the O atom in the ground state. The NH group begins to interact with the O atom of the phenol under the transition state for deprotonation (Figure 27.3(a)). This effect contributes to the pKa shift through deprotonation [13]. Thus, only the prelocated neighbouring amide NH affects the pKa shift. When the amide NH is in the p-position of the phenol, the pKa does not shift. Of course, solvent having an amide NH also does not significantly affect the pKa shift. The intramolecularly prelocated amide NH increases the probability that the amide NH will be near the OH group of phenol.

(a)

H

(b) H

O

O

O

N H O

R

O N H

pKa lowering

R conformational change

O H

O

H

O

H R

N O H

O

H N

R O

prelocated amide NH

H O

O

H N

R O

H O

R

N H non-prelocated amide NH

O

N

R O

Figure 27.3 Deprotonation reaction coordinates: (a) for phenol and 2-acylaminophenol with prelocated amide NH; (b) for 2-carbamoylphenol, accompanied with conformational change

Conformational Switching Between Acids and Their Anions by Hydrogen Bonding

613

In contrast, the carbamoyl derivative forms an intramolecular hydrogen bond between the phenolic OH and amide CO in the ground state (Figure 27.3(b)). The amide NH does not prelocate near the O atom of phenolic OH. Therefore, this phenol has a large pKa. When deprotonation occurs, the phenolate anion obtained is unstable without the NH


O hydrogen bond and becomes an energetically stable conformer with amide NH by rotation of the amide plane. However, in biological systems, it has been proposed that the pKa also decreases when the SH group undergoes RSH


base interaction [17].

27.2.2 Proton-driven conformational switching in asp- oligopeptide and model complexes We mentioned previously that a prelocated amide NH decreases the pKa value for weak acids, such as thiols, phenols, carboxylic acids, and phosphoric acid monoanions, and destabilizes their anions. In contrast, a remote amide NH around thiol and phenol does not cause prompt deprotonation of their acids (Table 27.1; Figure 27.2), although the anion form interacts intermolecularly with amide NHs in other molecules. If the conformationally stabilized structure of the anion differs from that of thiols, phenol or carboxylic acids, preferable conformational switching occurs intramolecularly between conformers of the acid and the anion such as 2-acylaminophenol (Figure 27.3(b)). Such intramolecular conformational change is relevant to the construction of switching devices. Various conformationally restricted molecules have been synthesized using oligopeptides, Kemp’s acids, salicylamide and maleic acid derivatives. As a simple conformationally switching compound, doubly amidated Kemp’s acid derivative r-1,c-3,c-5(CH3)3-3,5-(Ph2CHNHCO)2C6H6-1-COOH has been synthesized [18]. A chair form of the carboxylic acid converts to a twist-boat form with deprotonation (Figure 27.4). This transformation is observed over a relatively high energy barrier, approximately 40–80 kJ mol1, in acetonitrile at room temperature. The twistboat form is extremely stabilized by the two NH


O hydrogen bonds between the carboxylate anion and the two amide NHs in the solid state and in acetonitrile solution. A proton-driven conformational change in Asp-containing oligopeptides occurs from the carboxylic acid to the carboxylate anion. An Asp-containing tripeptide, AdCO-Asp(COOH)-Val-Gly-NHCH2Ph (Ad ¼ adamantyl), has an inverse g-turn structure in the acid state, whereas the tripeptide carboxylate anion converts to a b-turn-like conformer with intramolecular NH


O hydrogen bonds in chloroform or aqueous micellar solution (Figure 27.5) [19]. These structures were determined by 1H NMR. Any conformational change in the Asp-containing dipeptide is not detected between AdCO-Asp(COOH)- Val-NHAd and its anion state, AdCO-Asp(COO)-Val-NHAd. The model peptide study suggests that the hydrogen bond stabilizes

H H

N O

H

H O

H N

H

O H

O

- H+ +

+H

Carboxylic acid

O N

NH O H

O

O

Carboxylate anion

Figure 27.4 Proton-driven conformational change between an amidated Kemp’s acid, r-1,c-3,c-5-(CH3)3-3,5(Ph2CHNHCO)2C6H6-1-COOH and its anion form

614 Hydrogen Bonding and Transfer in the Excited State (a)

O

O

Asp

O

H

Val

O

H N

H

N H

O

Gly H

N

-H+

O

N O

N H

O H H

O

+H+

H N

N

O

O

Carboxylic acid

N

O

Carboxylate anion

(b)

Val O

O

O

N H

N

H

H Asp

O O

H

O Gly -H+

N

+H+

H

Carboxylic acid

N

O

N H

O

O

N O

H H

O

H

N

N

O

Carboxylate anion

Figure 27.5 Proton-driven conformational changes of AdCO-Asp(COOH)-Val- Gly-NHCH2Ph and its anion form: (a) in chloroform; (b) in aqueous micellar solution

the anion state and thus decreases the basicity of the carboxylate anion, presumably resulting in decreased nucleophilicity. An Asp-containing peptide, benzyloxycarbonyl-Phe-Asp(COO)-Thr-Gly-Ser- Ala-NHCy (Cy ¼ cyclohexyl) anion, has a hairpin-turn structure in acetonitrile. This fragment has been reported to function as a nucleophile in the active centre of proteases, such as pepsin, which contains a –Phe-Asp- Thr-Gly-Ser-Ser– fragment as well as the invariant amino acid fragment –Asp-Thr-Gly– in HIV-1 proteases [20, 21]. Crystallographic analyses of these proteins in the resting state indicate that the invariant Asp-containing peptide fragments of Asp-X-Gly (X ¼ Thr, Ser) form a similar hairpin turn with NH


O hydrogen bonds. It is likely that conformational switching of this fragment is related to the formation of a strong nucleophile. Conformational switching is observed in unsymmetrically linked phenolic oligoamides having an intramolecularly NH


O hydrogen-bonded structure. Crystallographic analysis indicates that the salicylamide unit changes the conformation around the amide plane (Figure 27.3(b)) [13]. Deprotonation of this molecule causes a linear-to-turn conformational switch (Figure 27.6) [14, 15]. In general, oligophenols have a complicated pKa because an increase in neighbouring phenolate anions prevents the residuary phenols from deprotonating. The decrease in basicity of the anion due to the NH


O hydrogen bond allows accumulated phenolate anions to move close to each other. In fact, p-nitrocalix[4]arene is known to have a wide pKa range (2.9–13) because of increasing anionic repulsion [22, 23]. Diarylazomethine carboxylic acid and its carboxylate have an amide group linked to an azomethine moiety, which introduces photoinduced switching supported by the intramolecular NH


O hydrogen bond. The ciscarboxylate compound forms a stronger intramolecular NH


O hydrogen bond than does the cis-carboxylic acid compound (Figure 27.7(a)) [24]. (E)-3-(2-Pivaloylaminophenol) acrylic acid switches the intramolecular

Conformational Switching Between Acids and Their Anions by Hydrogen Bonding t-Bu

t-Bu H N

t-Bu

O

H

H N

O N H

O

O

615

O

H

N H

O H

H N

O O

O

t-Bu

H

t-Bu

t-Bu

4H+ t-Bu O O t-Bu

N

O

N H

H

t-Bu O H N

O

O H N

t-Bu

H O

t-Bu

N O

O t-Bu

Figure 27.6 Linear-to-turn conformational switching induced by the deprotonation of unsymmetrically linked phenolic tetraamide

distance between amide group and carboxylate O atoms by E/Z photoisomerization of the cinnamate framework (Figure 27.7(b)). An intramolecular NH


O hydrogen bond is formed predominantly in the Z-carboxylate form not only in solution but also in the solid state. The pKa value for the carboxylic acid is lowered because of E/Z photoisomerization [25]. ortho-Coumaric acid derivatives with an amide group linked to an olefin moiety also show photoinduced switching accompanied with intramolecular hydrogen bonding [26]. Another type of intramolecular OH


O¼C hydrogen bond in the Z-phenol compound switches to an intramolecular NH


O hydrogen bond in the Z-phenolate state by deprotonation (Figure 27.7(c)). The pKa value for the Z-phenol derivative is lower than for the E-phenol derivative. Thus, a new photocycle system involving protonation and deprotonation processes has been achieved using carboxylic acid and phenol derivatives with remote amide NHs.

27.3 Coordination of Anion Ligand to Metal Ion 27.3.1 Increase in the stabilization constant by pka shift in metal complexes The lowering of pKa for thiols, phenols, carboxylic acids and phosphoric acid monoanions as precursor acids for a ligand anion is significant for complexation, especially for soft metal ions. A hydrophobic layer prevents ionic ML (M ¼ metal ion; L ¼ ligand anion) interactions in metalloproteins [27]. The conventional

616 Hydrogen Bonding and Transfer in the Excited State

Figure 27.7 Photoisomerizations of (a) diarylazomethines with thermal reversion, (b) cinnamic acid derivatives and (c) ortho-coumaric acid derivatives

complexation formation constant b is described by equation (27.1) under hydrophobic conditions, where K is the equilibrium constant. When the L anion is a strong base, i.e. when LH has a high pKa, a newly proposed formation constant b0 depends on the pKa value for LH, as in equation (27.2): log b ¼ log K

ð27:1Þ

log b0 ¼ log KpKa

ð27:2Þ

Conformational Switching Between Acids and Their Anions by Hydrogen Bonding

617

In general, a high anion basicity is preferable for complexation, although it promotes hydrolysis with water in aqueous solution. The ML bond depends on the pKa value for LH, and the L anion competes with the water molecule. Protection of the ML bond from hydrolysis by water has been demonstrated for NH


S hydrogen-bonded ferredoxin model complexes, such as (Et4N)2[Fe4S4(S-2-cholylNHC6H4)4], in aqueous micellar solution. The low pKa value for thiol prevents hydrolysis and air stability owing to the positive shift in redox potential [28]. Similarly, dissociation of the HgS bond was observed by monitoring Hg(0) formation during reduction of Hg(II) complexes having NH


S hydrogen-bonded ligands [29]. HgS dissociation is shown to affect the pKa value for thiol but not the covalency of the HgS bond in aqueous micellar solution. The formation of NH


S hydrogen bonds can be employed for conventional synthesis of NH


S hydrogenbonded metal complexes. For example, reaction (27.3) proceeds quantitatively: ½FeðSPhÞ4 2- þ 4
2-t-BuCONHC6 H4 SH ! ½FeðS-2-t-BuCONHC6 H4 Þ4 2- þ 4
PhSH ½FeðSPhÞ4 2- þ 2
ð2-t-BuCONHC6 H4 SÞ2 ! ½FeðS-2-t-BuCONHC6 H4 Þ4 2- þ 2
PhSSPh

ð27:3Þ

The addition of 2-t-BuCONHC6H4S anion to [Fe(SPh)4]2 results in the formation of mixed-ligand complexes in equilibrium [10]. The complete reaction (27.3) proceeds over the high energy barrier for the deprotonation of thiol and for redox reaction of disulfide. Similarly, for synthesis of Ca, Zn and Cd complexes of carboxylate with intramolecular NH


O hydrogen bonds [30–32], the carboxylate ligand with a hydrogen bond promotes ligand-exchange reaction (27.4). Ligand-exchange reaction (27.5) between a phosphate Ca(II) complex and NH


O hydrogen-bonded phosphoric acid also proceeds quantitatively [33]: MII ðOCO-2; 4; 6-Me3 C6 H2 Þn ðOH2 Þ4-n þ 2; 6-ðt-BuCONHÞ2 C6 H3 COOH ! MII ðOCOC6 H3 2; 6-ðt-BuCONHÞ2 Þn ðOH2 Þ4-n þ 2; 4; 6-Me3 C6 H2 COOH

ð27:4Þ

(where M ¼ Ca(II), Zn(II), Cd(II)) CaII ðO2 PðOHÞO-2; 6-i-Pr2 C6 H3 Þn ðOH2 Þ4-n þ 2; 6-ðt-BuCONHÞ2 C6 H3 OPO3 H2 ! CaII ðO2 PðOHÞOC6 H3 -2; 6-ðt-BuCONHÞ2 Þn ðOH2 Þ4-n þ 2; 6-i-Pr2 C6 H3 OPO3 H2

ð27:5Þ

Sulfonate ligands like RSO3 are known to have a low pKa owing to their highly conjugated structure. Then, NH


O hydrogen-bonded sulfate (PPh4)(SO3-2-t-BuCO-NHC6H4) has a weak NH


O hydrogen bond, as determined by IR and 1H NMR analyses in solution and by crystallographic analysis in the solid state [34]. The amide NH does not direct towards one of the sulfonate O atoms; rather, it directs to the middle position between the two O atoms. The strength of the NH


O hydrogen bond depends on the basicity of the anion state, and increases in the following order: phenoxides, thiolates, carboxylates, phosphates and sulfates. Thus, the low energy barrier for the deprotonation of these acids lowers the pKa value of the precursor acid for metal ligands. When the coordinating anion form has a high pKa value, the formation and dissociation of a metal–ligand bond depend on that value. Dissociation is prevented by the NH


O hydrogen bonds from coordinating carboxylate groups, because these NH


O hydrogen bonds lower the pKa value for the corresponding carboxylic acid [16, 30, 31, 35]. The hydrogen bonds involving the coordinating O atom of phenolate [36] or thiolate function similarly [37, 38].

618 Hydrogen Bonding and Transfer in the Excited State

Figure 27.8 Molecular structures: (a) [CaII{O2C-C6H3-2,6-(NHCO-t-Bu)2}4]2; (b) CaII{OCO-2,6-(t-BuCONH)2 C6H3}2(H2O)2

27.3.2 pp–dp covalent interaction affected by hydrogen bonds A tetrakis carboxylate Ca(II) complex, Ca{OCO-2,6-(t-BuCONH)2C6H3}42, has a regular CaO bond  distance of 2.4 A with an acute CaOC angle in the bidentate mode (Figure 27.8(a)). Thus, a totally anionic Ca(II) complex has a relatively long CaO bond distance, and the bond is presumably weak because of the typical cationic bond character. A neutral Ca(II) complex, Ca{OCO-2,6- (t-BuCONH)2C6H3}2(H2O)2, has a  relativelyshorterCaO bonddistance of2.278(2) Awithan obtuseCaOC angle of164.4(2) intheunidentate mode (Figure 8(b)). The covalency of the CaO bond has been discussed by Einspahr [39]. The relationship between the CaO bond distances and the CaOP bond angles for various Ca(II) complexes with O2POR, O2P(OH)R, O3PR and O2P(OH)R ligands suggests that the covalent CaO bond characters are similar [33]. Our theoretical analysis of the relationship between the CaO bond distance and the CaOP bond angle in Ca phosphate complexes supports the belief that the obtuse CaOP angle increases the d-orbital occupation number for Ca(II) to form a bonding pp–dp interaction with oxygen pp [33]. The NH


O hydrogen bond increases the bonding orbital in the CaO bond. NH


O hydrogen bonds to the coordinated O atoms prevent the CaO bonds from dissociating by lowering not only the pKa value of the ligands but also the CaO covalent bond strength. Stabilization of metal–oxygen bonds by NH


O hydrogen bonds was established for a Tb(III) complex, the ionic radius of which is similar to that of Ca(II) [40]. A Tb(III) complex of 2,6-bis(acetylamino)benzoate exhibits a higher emission intensity than does non-substituted benzoate in aqueous solution. In general, the Tb3þ ion exists as an aqua complex [Tb(OH2)9]3þ in the absence of anion ligands [41, 42]. These coordinated water molecules efficiently quench intrinsic Tb(III) luminescence; the emission intensity of an aqueous terbium solution is normally very weak under these conditions. In the presence of 2,6-bis(acetylamino) benzoate ligand, the carboxylate ligand forms a stable Tb(III) complex assisted by the NH


O hydrogen bond and displaces water molecules. Benzenesulfonate Ca(II) complexes have a common ionic CaO bond character without any correlation between CaO bond strength and oxy anion basicity. Intermolecular NH


O hydrogen bonds to the sulfonate O atom have been observed in the solid state and in solution [43, 44]. Intramolecular NH


O hydrogen bonds between the amide NH and SO have been observed for the Ca(II) arylsulfonate complex, [Ca2(SO3-2-t-BuCONHC6H4)2(H2O)4]n(2-t-Bu-CONHC6H4SO3)2n, sulfonate anion, (HNEt3)(SO3-2-tBuCONHC6H4), (PPh4)(SO3-2-t-BuCONHC6H4), (n-Bu4N)(SO3-2-t-BuCONHC6H4) and the sulfonic

Conformational Switching Between Acids and Their Anions by Hydrogen Bonding

619

acid, 2-t-BuCONHC6H4SO3H by IR and 1H NMR analyses both in the solid state and in solution [34]. The sulfonic acid, the sulfonate anion and its Ca(II) complex have a substantially weak (electrostatic) intramolecular NH


O hydrogen bond between the sulfonate O atom and the amide NH. A weak NH


O hydrogen bond forms between them because of the strong conjugation of the sulfonate group which decreases the basicity of the oxy anion. In this case, the CaO bond possesses an ionic character. Thus, carboxylates and phosphate monoanions, which have high basicity, differ from sulfonates in the properties of their hydrogen bonds. Systematic investigation reveals that strong NH


O hydrogen bonds form only with an oxy anion atom in the high basicity of a ligand coordinating to the metal ions, namely that the ligand is a weak acid such as thiol, phenol, carboxylic acid or phosphoric acid monoanion. 27.3.3 Regulation of thiolate and phenolate ligands in metal complexes by NH


X hydrogen bonds The chemical functions of intramolecular NH


S hydrogen bonds between thiolate and neighbouring amide NH have been elucidated using simple thiolate and Cys-containing peptide complexes. The hydrogen bond provides several clear functions because the sulfur pp -orbital lobe on the thiolate is remarkably large compared with that on carboxylate oxygen. When the MS bond has a covalent character due to sulfur pp electrons, the NH


S bond is extremely weak even if the amide NH is near the thiolate S atom. The amide NH IR band for Hg(S-t-BuCONHC6H4)2 in the solid state shows the absence of NH


S hydrogen bonding owing to the consumption of sulfur pp electrons for the covalent HgS bond. A tetrakis Hg(II) complex, (Et4N)2[Hg(S-2-CH3NHCOC6H4)4], has a clear NH


S hydrogen bond (Figure 27.8(a)) [45]. Thiolate in a tetrahedral Hg(II) complex can form a relatively strong NH


S hydrogen bond with a large amide NH (IR shift Dn ¼ 215 cm1). Formation of the hydrogen bond has been proposed by solution structure analysis of a trigonal complex, Hg(S-t-Bu)(t-buty-oxycarbonyl-cys-Pro-Leu-cysOMe) [46].The NH


S hydrogen bond presumably involves geometrical switching between linear Hg(II) and tetrahedral Hg(II) structures. The tetrakis Cd(II) complex has a similar electron configuration in (Et4N)2[Cd(S-2-t-BuCONHC6H4)4], which forms a relatively strong NH


S hydrogen bond with shortening CdS bond distance. Theoretical calculations suggest that the NH


S hydrogen bond decreases the sulfur pp electron density in the HOMO and thus weakens the related CdS antibonding pp–dp interaction. However, Hg(II) does not exhibit shortening of the HgS bond with NH


S hydrogen bonding because of its strong covalency. 199 Hg and 113Cd NMR analyses show a stabilized four-thiolate-coordinated structure for the tetrakis complexes. The chemical shifts show the influence of the NH


S hydrogen bonds on pp(Hg)–pp(S) interactions. NH stretching in the IR bands for amide NH in Cd(II) and Hg(II) complexes shows that the NH


S hydrogen bonds are stronger than in the corresponding Zn complex. Experimental and theoretical results suggest that the NH


S hydrogen bond influences the efficient capture of toxic Cd and Hg ions by metallothioneins [47]. [PtII(bpy)(S-t-BuCONHC6H4)] (bpy ¼ 2,20 -bipyridine) [48] shows the presence of relatively weak NH


S hydrogen bonds owing to strong Pt–S covalency. Mononuclear metal–thiolate complexes including Cu(I) [49], Mo(IV) [50], Fe(II) [51] and Co(II) ions [10], exhibit shortening of the MS bond distances with NH


S hydrogen bonding when compared with the corresponding thiophenolate complexes without hydrogen bonding. For example, a Co(II)–thiolate complex has a tetrahedral geometry with a strong intramolecular NH


S hydrogen bond (Figure 27.8(c)). [MoIVO(S-2t-BuCONHC6H4)4]2 also has a weak NH


S hydrogen bond, as detected by IR shift. The MoSe bond distance in [MoIVO(Se-2-t-BuCONHC6H4)4]2 is almost the same as the MoS distance in [MoIVO(S-2-tBuCONHC6H4)4]2. A clear COSY cross-peak is observed in the 77Se1H COSY spectrum (J(77Se1H) ¼ 5.4 Hz). The coupling constant indicates that the covalency of the NHSe bond in the NH


Se hydrogen bond is approximately 10% of that of the SeH single bond in CH3SeH (J(77Se1H) ¼ 41.7 Hz) [52].

620 Hydrogen Bonding and Transfer in the Excited State

Figure 27.9

Molecular structures of intramolecularly hydrogen-bonded metal–thiolate complexes

Mo(IV), W(IV), Mo(VI) and W(VI) complexes having intramolecular NH


S hydrogen bonds, for example, [MoIVO{S2C2(CONH2)2}]2 and [MVIO2{3,6- (Ph3CCONH)2-1,2-bdt} 2]2 (M ¼ Mo, W) with a 3,6-diamide-1,2-dithiolene ligand, were synthesized as models of the M(IV) and M(VI) states in molybdoand tungsten enzymes (Figure 27.9(a)) [53, 54]. In the WVIO2 complexes, the NH


S hydrogen bond trans to the oxo ligand is stronger than that cis to the ligand. The hydrogen bond stabilizes the ligand by trans influence and regulates O-atom transfer in molybdo- and tungsten enzymes. Probably, the hydrogen bond on the trans S atom is a crucial trigger for activation of M¼O transfer. In contrast, the FeS bond distance in P450 model porphyrin complexes slightly elongates with NH


S hydrogen bonding, as it does for the similar Ga(III) complex [55, 56]. IR shifts for [FeIII(OEP)(S-tBuCONHC6H4)] (OEP ¼ octaethylphorphinato) and [GaIII(OEP)(S-t-BuCONHC6H4)] indicate the presence of a relatively weak NH


S hydrogen bond. Thus, the NH


S hydrogen bond controls the properties of the MS bond, which affects the reactivity of a trans-coordinating substrate by mutual trans influence [57]. Such regulation is significant for stabilization and activation of the metal centre towards redox reactions in metalloproteins, metalloenzymes and their model complexes. In transition metal complexes, the NH


S hydrogen bond shifts the redox potential towards the positive side [50, 58–60]. Hydrogen bonding causes easy reduction of these electron-rich metal–thiolate complexes by a mild reductant. A similar hydrogen bond is found to the NH


O hydrogen bond in metal complexes of phenolate with neighbouring amide groups. The axial tyrosine of heme in catalase is known to have double NH


O hydrogen bonds with a neighbouring arginine residue fixed by hydrogen-bonding networks [61, 62]. The crystal structures of [FeIII(TPP){O-2,6-(CF3CONH)2C6H3}] and [FeIII(TPP) (O-2-CF3CONHC6H4)] suggest that NH


O hydrogen bonds contribute to the positive shift in redox potential of Fe(III)/Fe(II) [63]. OEP Fe(III) complexes with these ligands at the axial position, [FeIII(OEP)(O-2-CF3CONHC6H4)], [FeIII(OEP)(O-2,6(CF3CONH)2C6H3)], [FeIII(OEP)(OPh)] and [FeIII(OEP) (O-2,6-(i-Pr)2C6H3)], were synthesized and compared with the corresponding TPP Fe(III) complexes [36]. The NH


O hydrogen bond elongates the FeO

Conformational Switching Between Acids and Their Anions by Hydrogen Bonding

621

bond distance and widens the FeOC bond angle compared with those in [FeIII(OEP)(OPh)] without the hydrogen bond. Perturbation by the hydrogen bond towards the S atom, presumably associated with a bonding FeO in the LUMO, weakens the FeO bond and decreases the dp–pp overlap. 27.3.4 Switching by an NH


S hydrogen bond during the catalytic cycle in p450 and CPO Non-hydrogen-bonded model Fe(III) complexes for the resting state of native P450 and chloroperoxidase (CPO), for example, [FeIII(OEP)(SMe)], are known to be unstable, whereas NH


S hydrogen-bonded model Fe(III) complexes are thermodynamically stable. However, reduction of these model complexes results in decomposition. A reported Raman spectral analysis of the complex between P450ox and oxidized [2Fe–2S] putidaredoxin suggests a shortening of the FeS bond upon complexation [64]. Probably, suitable regulation of the FeS bond occurs during the redox reaction. Cys-containing oligopeptide Fe(III) model complexes are also stable owing to the formation of some NH


S hydrogen bonds in less polar solvents such as acetonitrile. Reduction of the peptide complex yields a relatively stable Fe(II) form. The NH


S hydrogen bond can be controlled by rotation of the amide plane, because breaking the bond stabilizes the Fe(II) species. The orientation angle of the amide planes is affected by other hydrogen bonds, depending on the amide C¼O in the same plane. The addition of multiamide compounds, such as t-butoxycarbonyl-Gly-m-anthranyl methyl ester, to [FeIII(OEP)(Z-cys-Leu- Gly-LeuOMe)] and [FeIII(OEP)(Z-cys-Pro-Ala-Leu-OMe)] in acetonitrile results in a negative shift in redox potential for the former complex and decomposition for the latter complex. Complexation leads to a negative shift in redox potential (Figure 27.10). It is likely that the peptide ligand in the former is flexible and in the latter rigid. The ligand in the former can be used to tune the structure so as to be stable for the reduced state upon oneelectron reduction. Reduction of [FeIII(OEP)(Z-cys-Pro-Ala-Leu-OMe)] having a rigid ligand also results in decomposition. It is consistent that the active centre in CPO does not involve the Fe(II) state. In contrast, reduction of [FeIII(CO)(OEP)(Z-cys-Leu- Gly-Leu-OMe)] with (Et4N)(BH4) under carbon monoxide yields [FeII(CO) (OEP) (Z-cys-Leu-Gly-Leu-OMe)]. The tetrapeptide ligand can switch the structures during both redox states. Solution structures of 6-coordinated ruthenium complexes show that the invariant fragments maintain a b-III turn-like structure and then form weak NH


S hydrogen bonds between Cys S and NH (third and fourth amino acid residues) that differ from those in 5-coordinated [GaIII(OEP)(cys-peptide)]. Thus, for the ruthenium complexes, elongation of the N


S distance in the NH


S hydrogen bond is induced by steric hindrance between porphyrin ring and peptide side chain. This fact suggests that dynamic conformational

Figure 27.10 Molecular structures: (a) [WVIO2{3,6-(CH3CONH)2-1,2-bdt}2]2; (b) [MII{3,6-(CH3CONH)2-1,2bdt}2]2 (M ¼ Zn, Cd, Hg)

622 Hydrogen Bonding and Transfer in the Excited State

switching of the peptide chain induces a change in coordination geometry, which regulates reactivity through the FeS bond character during the catalytic cycle in the native enzymes. However, a Cys-containing hairpin-turn fragment, Cys-Leu-Gly-Leu, supported by the subsequent a-helix, is not able to support an Fe(II) state by the reduction. Presumably, the rigid peptide structure cannot convert to a suitable conformer for the Fe(II) state over a high energy barrier. A simple switching model ligand, such as t-BuNHCOC6H4SH, can provide both states, although even the NH


S hydrogen-bonded Fe(III) form gradually decomposes, because the rotation energy barrier between aromatic and amide planes is not high. Furthermore, P450 porphinate model complexes with Cys-containing a-helical peptides have shown that FeS bond properties can be regulated by the NH


S bonds supported by the hairpin-turn conformation and the successive NH


O¼C hydrogen bond networks in the helix [65]. The above-mentioned hairpin turn structure involving NH


S hydrogen bonds is supported by the subsequent a-helix fragment with the same direction of amide dipoles. Actually, the peptide complex [FeIII(OEP)(Ac-LcPAF-LLLLL- ALFL-OMe)] has a larger positive shift (D 130 mV) than does the corresponding tripeptide complex, as the NH


S hydrogen bond is stabilized by the a-helix [65]. The solution structure of the peptide model ligand in Fe(III) complexes for P-450 and CPO was determined using [GaIII(OEP)(Ac-LcPAF-LLLLL- ALFL-OMe)] in chloroform-d. The structure has a shorter NH


S distance than does the corresponding Cys-containing tetrapeptide complex (Figure 27.11). Thus, an a-helix following the coordinating cysteinyl residue strengthens the NH


S hydrogen bond. In contrast, [FeIII(OEP)(AcLcLAF-LLLLL-ALFL-OMe)] has a positive shift of only 70 mV because the hydrogen bond of the Cys-Leu-Ala fragment is not stabilized by the a-helix. The redox reactions seem to be regulated by the cooperating effect of the a-helix and the NH


S hydrogen-bonded invariant fragment in the native enzymes. Thus, the NH


S hydrogen bond positively shifts the redox potential, which actually contributes to easy reduction of these Fe(III) complexes to Fe(II) complexes with a mild reductant such as (Et4N)(BH4) in acetonitrile. This suggests that the local conformational change of a peptide ligand is crucial for functional switching of the metal centre. 27.3.5 Structural transformation of calcium phosphate clusters involving rearrangement of inter- and intramolecularly hydrogen-bonding networks Ca(II) complexes of phosphate (ROPO3) and phosphonate ligands (RPO3) such as Ca(O3POCH2CH2NH3) have a linear structure [66]. Others, such as Ca(O3PMe), have layered structures [67]. Small ligands predominantly form polymeric structures. On the other hand, extremely bulky and less bulky amide ligands can control coordination geometry. A phosphate monoanion complex, (NMe4)[CaII{O2P(OH)OC6H3-2,6-

Figure 27.11 reduction

On-off switching of NH


S hydrogen bonding in [FeIII(OEP)(Z-cys- Leu-Gly-Leu-OMe)] upon

Conformational Switching Between Acids and Their Anions by Hydrogen Bonding

Figure 27.12 1 H NMR

623

Solution structure of [GaIII(OEP)(Ac-LcPAF-LLLLL-ALFL-OMe)] in chloroform-d, determined by

(NHCOCPh3)2}3(NCMe)3], and a phosphate dianion complex, [CaII{O3POC6H3-2,6-(NHCOCPh3)2} (H2O)3(MeOH)2], with an extremely bulky triphenylacylamino group are forced by steric congestion to form a mononuclear Ca(II) core. Less bulky ligands enable restriction of coordination to the side of the Ca cluster. A less-bulky phosphoric acid, 2,6-(PhCONH)2C6H3OPO3H2, gives three novel polynuclear Ca(II)–phosphate and Na(I)–phosphate complexes [68]. The first is the zigzag-chain Ca cluster [CaII{O3POC6H3-2,6-(NHCOPh)2}(H2O)4(EtOH)]n. The second is the cyclic octanuclear form [CaII8{O3POC6H3-2,6-(NHCOPh)2}8(O¼CHNMe2)8(H2O)12]. The third is the hexanuclear complex (NHEt3)[Na3{O3POC6H3-2,6-(NHCOPh)2}2(H2O)(MeOH)7. Crystallographic structures reveal that all have an unsymmetric ligand position owing to the less bulky amide groups. Dynamic transformation of the zigzag-chain Ca structure to the cyclic octanuclear Ca complex is induced by the addition of N,N-dimethylformamide (DMF) owing to the coordination of the DMF molecules (Figure 27.12(a)) [69]. Transformation occurs with reorganization of the intermolecularly and intramolecularly hydrogen-bonding networks. DMF coordination breaks one of the hydrogen bonds to rearrange the network (Figure 27.12(b)). Biopolymer and synthetic polymer ligands successively connected as the less bulky phosphate and carboxylate ligands can precisely coordinate to the surface of Ca clusters, calcium carbonate or hydroxyapatite, which has an unsymmetrical surface coordination geometry [35, 70–72].

27.4 Conclusions Amide NH, prelocated towards the S atom of thiols or the O atom of carboxylic acids, phenols or phosphoric acid monoanions, lowers the pKa value by deprotonation under hydrophobic conditions. The anion easily forms a strong intramolecular NH


X (X¼S, O) hydrogen bond that is thermodynamically stabilized. When thiols, phenols or carboxylic acids do not have a prelocated amide NH but have a conformationally switchable remote amide NH, proton-driven conformational switching can be achieved. We have demonstrated

624 Hydrogen Bonding and Transfer in the Excited State

Figure 27.13 (a) Transformation of a Ca(II)–phosphate cluster from zigzag to octanuclear by the addition of DMF. (b) Rearrangement of hydrogen-bond networks by the addition of DMF

twist-boat-to-chair switching on a partially amidated Kemp’s acid derivative, linear-to-turn conformational switching on an unsymmetrically linked phenolic tetraamide, extended-form-to-turn switching on Aspcontaining oligopeptides and photoisomerization switching on azomethine, cinnamic and ortho-coumine carboxylic acid derivatives. These switchings involve a crucial intramolecular hydrogen bond between amide NH and carboxylate O atom. These hydrogen bonds are associated not only with an increase in the formation constant for metal–thiolate, metal–carboxylate, metal–phenolate and metal–phosphate complexes by pKa shift but also with an increase in the covalent character of the metal ligand. The NH


S hydrogen bond in P450 model complexes contributes to stabilization of the Fe(III) state, but not of the Fe(II) state. This switching is presumably related to the conformational switching of the Cys-containing peptide chain in P450. Structural transformation occurs on the calcium phosphate cluster between the cyclic octanuclear Ca complex and the zigzag-chain Ca complex, involving rearrangement of inter- and intramolecular hydrogen-bond networks.

Conformational Switching Between Acids and Their Anions by Hydrogen Bonding

625

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47.

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28 Charge Transfer in Excited States: ab initio Molecular Dynamics Simulations Maciej Kolaski1,2, Anupriya Kumar1, Han Myoung Lee1 and Kwang S. Kim1 1

Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea 2 Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia, 9 Szkolna Street, 40-006, Katowice, Poland

28.1 Introduction Dissociation of a chemical compound by light is called photodissociation or photofragmentation, which plays an important role in many chemical [1, 2] and atmospheric phenomena [3]. In the earth’s atmosphere, photolysis occurs as a part of a series of reactions in which primary pollutants such as hydrocarbons and nitrogen oxides react to generate secondary pollutants [4]. In the stratosphere, ozone is formed through photolysis by ultraviolet (UV) light [5]. Chlorofluorocarbons (CFCs) are broken down by photolysis in the uppermost atmosphere to form halogen-free radicals, which are responsible for destroying the ozone layer [6]. Ion–water cluster interactions are important for understanding solvation/desolvation phenomena in chemical processes, designing efficient ionophores for biological molecular recognition and engineering of selfassembled nanomaterials. Photoexcitation of halide anion–water clusters causes the charge transfer of an electron from the halogen anion to the solvent (water), i.e. the charge-transfer-to-solvent (CTTS) phenomenon. This photoexcitation results in the dissociation of hydrated halide anions into very active free halide radicals and an excess electron–water cluster [e
(H2O)n] in which the excess electron is stabilized by the electron dipole interaction with the water network. The photolysis of water is essential in neutron irradiation to the cooling water in nuclear reactors and in the destruction of living cells caused by radiation. A possible fuel source may be obtained via the photolysis of water to hydrogen and oxygen gases. The mechanism of water photolysis is still not well understood. The experimental ionization energy of water in the gas phase is around 12.6 eV, which is significantly larger than

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

628 Hydrogen Bonding and Transfer in the Excited State

the corresponding excitation energies in the UV/VIS spectrum. Thus, if the photon energy is above the ionization threshold, the mechanism of dissociation is driven by the ionization process. Photoexcitation of hydrated iodic acids breaks the H
I bonds and releases both hydrogen and iodine radicals, which leads to the possible utilization of hydrogen radicals in generating hydrogen by dissociating water with hydrogen iodide. Pyrroles are one of the important compounds for studying aromatic systems, as they are the components of various functionally significant biological molecules, such as amino acids, nucleotide bases and aromatic rings such as porphyrins of heme, chlorins/bacteriochlorins of chlorophyll and corrin rings of vitamin B12. In photochemical reactions in water, a universal solvent and quencher, the electron transfer process from aromatic compounds, such as optically active amino acid tryptophane and indole, has been confirmed to trigger the formation of a hydrated electron. In spite of the importance of photolytic reactions in environmental chemistry and biology, as well as its potential role in harnessing nature’s energy source, the excited-state dynamics of hydrated ions, radicals and aromatic compounds is hardly understood. Here, by using excited-state ab initio molecular dynamics (ES-AIMD) simulations based on complete-active-space self-consistent field (CASSCF) formalism, we have investigated the difference in the photoexcitation dynamics between I
(H2O)n¼2–4 and I
(H2O)n¼5 clusters, water photolysis, the photoinduced charge transfer process of hydrated hydrogen iodide [(H2O)nHI (n ¼ 1–6)] and the photoexcitation of pyrrole–water clusters. However, the lack of information available for reliable excited-state geometries, as well as the detailed experimental data, has made the study challenging.

28.2 Charge-Transfer-to-Solvent-Driven Dissolution Dynamics of I
(H2O)2–5 Upon Excitation The excitation of hydrated halides results in the transfer of an excess electron from the anionic precursor to the water cluster [7–12] which is then stabilized by the network of water molecules. This causes the dissociation of hydrated halides into halogen radicals and wet electron–water clusters. In this review, we report the CTTSdriven dissolution dynamics for I
(H2O)n¼2–5 complexes using ES-AIMD simulations employing the CASSCF method. This analysis shows that, after the iodine radical is detached from the I
(H2O)n¼2–5 complex, a simple population decay is observed for small water clusters (n ¼ 2–4), while a substantial reorganization of the water network to form an entropy-driven structure is observed for n ¼ 5. These results are in very good agreement with ultrafast pump–probe experiments [13]. In this work, O and H atoms are treated with the aug-cc-pVDZ þ (2s2p/2s) basis set [14], where the extra diffuse 2s2p and 2s functions are added to all oxygen and hydrogen atoms in order to properly stabilize the excess electron. In the case of hydrated iodine anion clusters, the extended basis sets are necessary to study the excited states, and the set of highly diffuse functions are needed for the study of the dissociation of the anionic species, including excess-electron–water clusters. As the aug-cc-pVDZ basis set is not available for iodine, the CRENBL effective core pseudopotential (ECP) basis set was employed [14]. An active space of six electrons and six active orbitals was used (CAS[6, 6]) for clusters with up to three water molecules. As ES-AIMD simulations are computationally very demanding for clusters comprising four and five water molecules, the optimum choice of the active space for the reduction of computational resources is required, even though numerical accuracy is slightly sacrificed. Thus, a reduced active space (CAS[4,4]) was used for the practical study of larger complexes (n ¼ 4, 5). The ES-AIMD simulations of the I
(H2O)n¼2–5 complexes were carried out for 800 fs (n ¼ 2–4) and 600 fs (n ¼ 5) with a time step of 0.2 fs [15]. The ES-AIMD simulations for an excited state of I
(H2O)2–4 exhibit

Charge Transfer in Excited States: ab initio Molecular Dynamics Simulations

629

Figure 28.1 Snapshots of the evolving process of the I
(H2O)n¼2,5 clusters upon excitation, taken from ab initio molecular dynamics simulations. Reprinted with permission from [15b]. Copyright 2008 American Chemical Society

simple population decay, while that of I–(H2O)5 shows drastic rearrangement of the water network. Snapshots of the time evolution of the structures of I
(H2O)n¼2,5 clusters upon excitation are shown in Figure 28.1. In the ground state, the most stable structure of the water pentamer cluster in the I
(H2O)5 complex is a planar cyclic water tetramer ring with a water molecule attached through an H-bond. For the I
(H2O)5 complex, five different ES-AIMD simulations with KE0 ¼ 0, 100, 200, 300, and 400 K were performed. The simulation results are almost identical, but there is a substantial difference in structures and electronic properties between ES-AIMD simulations carried out at 0 K and at higher temperatures. In the latter, the pentamer has a quasi-linear structure due to the entropy effect (which favours more flexible structures), while in the former it has a deformed quasi-tetragonal ring stabilized by an additional hydrogen bond. In this regard, all the simulations are almost identical, except for KE0 ¼ 0 K, regardless of the initial geometry and initial velocities. We find that this is due to the fact that, above 100 K, the linear structure of e
(H2O)5 is more stable in terms of the free energy than the quasi-tetragonal ring structure. As compared with the tetragonal ring structure, the quasi-linear structure is 1.0 kcal mol
1 less stable at 0 K, but becomes more stable above 100 K. For instance, at 298 K, it is 4.2 kcal mol
1 more stable at the B3LYP/6-311þþG level of theory.  The experimental VDEs of I
(H2O)n are evaluated to be 0.03, 0.08 and 0.16 eV for clusters n ¼ 2, 3 and 4  respectively [16]. The abnormal shift from 0.15 to 0.4 eV in the VDE of I
(H2O)5 was observed in the pump–probe experiment to be around 600 fs after excitation. This is demonstrated by the computed VDEs, which increase from 0.15 to 0.38 eV during 600 fs of ES-AIMD simulations. This indicates complex dynamics for the cluster n ¼ 5, which involves significant reorganization of the water cluster to more effectively accommodate the excess electron. In contrast, smaller clusters exhibit a simple population decay. Our study clearly shows how the detachment process of I
(H2O)n¼2–5 evolves upon excitation and why the evolution process of I
(H2O)5 is significantly different from the other smaller-sized clusters I
(H2O)2–4. To reproduce experimental VDE values for I
(H2O)5, it was necessary to perform ES-AIMD simulations above 100 K

630 Hydrogen Bonding and Transfer in the Excited State 0.50 IW2 IW3 IW4 IW5 300K

0.45 0.40

VDE [eV]

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0

200

400

600

800

Time [fs]

Figure 28.2 Time evolution of the vertical detachment energy (VDE) for the I
(H2O)n¼2–5 clusters. Reprinted with permission from [15b]. Copyright 2008 American Chemical Society

(Figure 28.2). The ES-AIMD study gives an insight into the process of the dehydration/dissolution of halogen atoms by photoexcitation phenomenon and the rearrangement mechanism of the excess-electron–water clusters.

28.3 Dynamics of Water Photolysis: Excited-State and Born–Oppenheimer Molecular Dynamics Study In spite of the importance of water photolysis in atmospheric chemistry, the mechanism of photodissociation is still not well understood [17]. Two competitive mechanisms of water photolysis have been suggested: *

*

(i) Upon excitation, a water molecule is defragmented into two radicals: OH and H , and the excess energy is transferred mainly to the H radical. Consequently, the H radical has extremely large kinetic energy and promptly reacts with a water molecule in the first hydration shell, forming H3Oþ and a hydrated electron [, 18–20]: *

*

H2 O þ hn ! H2 O* ! H þ OH *

*

H ðhotÞ þ H2 O ! H3 O þ þ e
ðhydratedÞ *

(ii) It has been proposed that ionization above the Born–Oppenheimer threshold leads to the creation of H3Oþ , an OH radical and a hydrated electron [19, 21–24]: *

H2 O þ hn ! H2 O þ þ e
ðhydratedÞ H2 O þ þ H2 O ! H3 O þ þ OH

*

Both proposed schemes lead to identical products, in spite of their significantly different reaction mechanisms.

Charge Transfer in Excited States: ab initio Molecular Dynamics Simulations

631

Figure 28.3 Snapshots of the evolving process of the (H2O)n¼1,2 clusters upon excitation (ES-AIMD simulations). Reprinted with permission from [26]. Copyright 2008 American Chemical Society

In this study, we report on the photoexcitation-driven mechanism by employing ES-AIMD simulations based on the complete-active-space self-consistent field (CASSCF) approach, and on the photoionizationdriven mechanism by using the ground-state unrestricted Møller–Plesset second-order perturbation theory (UMP2) based on Born–Oppenheimer molecular dynamics (BOMD) simulations [25]. The CAS[6, 6] active space was used for the water molecule and the CAS[12, 12] active space for the water dimer. In ES-AIMD simulations, the molecular orbitals involved in the excitation should be present in the active space. In this ES-AIMD simulation, we considered the first singlet excited state (i.e. HOMO–LUMO excitation) [26]. The initial structures having the ground-state minimum-energy geometry (optimized at the CASSCF/augcc-pVDZ level of theory) were vertically excited. The ES-AIMD simulations of the water clusters were carried out for 40 fs with a time step of 0.1 fs. In this case, it was necessary to decrease the time step because of convergence problems. We also performed longer ES-AIMD simulations, but all important changes take place during the first 20 fs of the MD run. Figure 28.3 shows the time evolution of the conformational changes of the water clusters (H2O)n¼1,2. At the beginning of the ES-AIMD simulation, the dynamics of a single water molecule differs from the dynamics of larger water clusters, showing very strong O
H stretching motions. Both OH bonds start to break at 5 fs. At 10 fs, one hydroxyl group is formed again, and the hydrogen radical is released. The detached hydrogen radical has extremely large kinetic energy (it is very hot), which increases up to 55 kcal mol
1, and immediately reacts with a water molecule from the first hydration shell. This is consistent with the photodissociation mechanism of water photolysis. For the water dimer, the OH bond starts to stretch at 4 fs, and subsequently the hydrogen radical is released from the molecular system with very large kinetic energy. While the OH bond breaks, the kinetic energy of the released hydrogen grows very rapidly. After 10 fs, the kinetic energy of the hydrogen radical increases to 55 kcal mol
1 and remains constant until the end of the molecular dynamics run. When the hydrogen radical is finally detached, the hydroxyl group and the adjacent water molecule form a stable structure. In the case of UMP2-BOMD simulations for ionized water clusters (Figure 28.4), the initial UMP2optimized minimum energy structures of neutral clusters were employed. We set the initial KE to zero, as in the former case. The UMP2-BOMD simulations were carried out for 1000 fs with a time step of 0.2 fs. The proton strongly oscillates between the hydroxyl group and the neighbouring water molecule in the ionized water dimer. The mobile proton has a relatively large kinetic energy; however, it does not exceed 10 kcal mol
1, which holds the proton between the two oxygen atoms. The time evolution of OH distances clearly shows that the Eigen (H3Oþ -like water cluster) and Zundel (H2O. . .Hþ . . .OH2-like water cluster) forms compete [27]. After 500 fs of UMP2-BOMD simulations, an Eigen form is finally created, which is

632 Hydrogen Bonding and Transfer in the Excited State

þ Figure 28.4 Snapshots of the evolving process of the (H2O)n¼2 clusters upon photoionization of (H2O)n¼2 (UMP2-BOMD simulations). Reprinted with permission from [26]. Copyright 2008 American Chemical Society

3.6

-152.09

+

(H2O)2

3.2

+

(H2O)2 -152.10

r(A...B) [A]

2.8

H

-152.11

2.4

2

H-O1 H-O2 O1-O2

2.0

1

-152.12

1.6 -152.13

1.2 0.8

-152.14 0

200

400

600

Time [fs]

800

1000

0

200

400

600

800

1000

Time [fs]

Figure 28.5 Time evolution of distances and potential energy for the ionized water dimer. Reprinted with permission from [26]. Copyright 2008 American Chemical Society

energetically more favourable. While the Eigen conformer is formed, the kinetic energy of the mobile proton significantly decreases. Figure 28.5 shows that the distance between O1 and O2 atoms oscillates highly at the beginning of the simulations, but, after 500 fs, it does not change considerably. The time evolution of potential energy, presented in Figure 28.5, for an ionized water dimer proves that the resulting structure is the minimum energy conformer. This study reported the first ab initio excited-state molecular dynamics results to explain the mechanism of water photolysis. The first mechanism is driven by the photoexcitation process. By carrying out ES-AIMD simulations based on the CASSCF approach, we proved that, upon excitation, water clusters release very hot hydrogen radicals and (hydrated) hydroxyl radicals within 15 fs. In the case of water photolysis driven by ionization, all structural changes are much slower in comparison with the dynamics controlled by the photoexcitation phenomenon. The hydrogen atoms that are responsible for hydrogen bond formation determine the structural rearrangement of ionized water clusters. At the beginning of UMP2-BOMD simulations, these mobile protons have relatively large kinetic energy (although much smaller in comparison with that in ES-AIMD simulations), and thus Eigen and Zundel forms compete. After 500 fs, one proton is attached to the adjacent water molecule, and the hydronium and OH radical are finally created. *

Charge Transfer in Excited States: ab initio Molecular Dynamics Simulations

633

28.4 Photodissociation of Hydrated Hydrogen Iodide Clusters: ab initio Molecular Dynamics Simulations In various chemical, biochemical and atmospherical systems, it is important to note that photochemical reactions often produce hydrogen radicals [28]. As the addition of an electron to a proton, which involves hydrogen transfer, proton transfer [29] and solvent-driven charge transfer [30], can form a hydrogen radical, the charge-transfer-to-solvent-driven hydrogen radical chemistry is an interesting subject. Hydrogen halide acids produce hydrogen and halogen radicals through the photodissociation phenomenon [31]. In spite of its importance in widespread environmental and biological issues, as well as in the possibility of harnessing nature’s energy source, the hydrogen radical reaction mechanism and dynamics are not well understood. As one of the important issues in atmospheric chemistry, we have studied the photoinduced excitation dynamics of hydrated hydrogen iodide [32]. To investigate the dynamic behaviour of the dissociation of hydrogen iodide and the release of radicals from the cluster, we show in Figure 28.6 the structural changes and NBO charge evolution from the ES-AIMD simulations for HI(H2O) and HI(H2O)2 complexes with initial kinetic energies KE0 of 0 and 200 K. In the simulations, the transition state of the proton between HI and H2O was observed, and the kinetic energy of the released H atom was very large (35 kcal mol
1). The r(I
H) distance monotonically increases, showing the release of iodine atom at both KE0 ¼ 0 K and KE0 ¼ 200 K for HI(H2O). The H radical in HI(H2O) was released at the KE0 of 200 K, but not at the KE0 of 0 K. For HI(H2O), the NBO charge of H in HI is þ 0.09 au before excitation and þ 0.08 au at the time (t ¼ 0) of excitation, but þ 0.34 au after 60 fs (at 0 K) [0.0 au after 70 fs for 200 K]. Then, the formation of the neutralized H3O is noted along with the complete release of the iodine radical. To study the release of the hydrogen radical from the neutralized H3O, ground-state AIMD simulations for H3O and H5O2 were carried out. In this case, the H radical was easily released at the KE0 above 1000 K (for H3O) and 2500 K (for H5O2) to cross over the small activation energy barrier (0.1 eV) (Figure 28.7). However, it was not released below the KE0 of 1000 K. Even though the energy barrier from H3O to H þ H2O is relatively small (0.1 eV ¼ 2 kcal mol
1), a high KE0 is required for the H radical to get over the barrier towards the dissociation process. Nevertheless, it should be emphasized that, even at very low temperatures, the release of the H radical occurs because in a system consisting of molecules of the order of Avogadro’s number (1024), a large number of molecules would have enough KE0 (according to the Maxwell–Boltzmann distribution) to release H radicals. Thus, hydrated iodic acids, upon excitation, release hydrogen radicals or hydrogen molecules as well as iodine radicals. This photoexcitation process involving CTTS takes place in four steps: (i) hydration of the acid; (ii) charge transfer to water upon excitation of hydrated acid; (iii) release of the neutral iodine atom; (iv) detachment of the hydrogen radical [26]. The detachment of iodine from excited hydrated hydro-iodic acids is highly exothermic. The detachment of hydrogen radicals from hydrated hydronium radicals is spontaneous. It occurs only if the initial kinetic energy of the cluster is large enough to cross over the small activation energy barrier.

28.5 Excited-State Dynamics of Pyrrole–Water Complexes: ab initio Excited-State Molecular Dynamics Simulations Investigation of hydration- and photoexcitation-driven charge transfer phenomena [33, 34] is essential for understanding the solvation phenomena. Studies of solvated clusters involving hydrogen bonds, as well as

634 Hydrogen Bonding and Transfer in the Excited State

Figure 28.6 ES-AIMD simulations of HI(H2O)1,2 with an initial kinetic energy of 0 or 200 K (CASSCF calculations with aug-cc-pVDZ and CRENBL ECP basis sets). The time evolutions of the H
I, H
O and I
O distances and the NBO charges in the dissociation process are depicted. Reprinted with permission from [32]. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA

Charge Transfer in Excited States: ab initio Molecular Dynamics Simulations

635

Figure 28.7 Ground-state AIMD simulations of H3O and H5O2 (CASSCF with the aug-cc-pVDZ basis set). The time evolutions of interatomic distances and the NBO charges in the dissociation process are depicted. Reprinted with permission from [32]. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA

those involving weak van der Waals interactions with p systems, have been useful for mimicking and understanding solute–solvent interactions and s/p hydrogen bonding networks with p systems [35–38]. For the pyrrole–(H2O) cluster, an ES-AIMD simulation was carried out with an initial KE0 of 0 K [39]. For the pyrrole–(H2O)2 cluster, it was necessary to perform ES-AIMD simulations with KE0 values of 0 and 300 K.

636 Hydrogen Bonding and Transfer in the Excited State

The initial structures having the ground-state minimum-energy geometry (optimized at the CASSCF/aug-ccpVDZ þ (2s2p/2s) level of theory) were vertically excited at 0 fs. The experiments were performed at very low temperatures, so we set up the initial kinetic energy of the systems to 0 K for both pyrrole–(H2O) and pyrrole–(H2O)2 complexes. For the ES-AIMD simulations of pyrrole–(H2O)2 at 300 K, the initial velocities were set according to the Maxwell–Boltzmann distribution. The ES-AIMD simulations demand enormous computing time, so it is not possible to do more than one or two trajectories. Even a single trajectory in an ES-AIMD simulation performed at 300 K is likely to represent what would be observed near the peak in the distribution of trajectory results; therefore, simulations with a few different initial KE0 values should provide a reasonable understanding of the dynamics. The results of ES-AIMD simulations for pyrrole–(H2O) and pyrrole–(H2O)2 are presented in Figure 28.8. It shows the changes in the structures of pyrrole–(H2O) (KE0 ¼ 0 K) from the 400 fs ES-AIMD and those of pyrrole–(H2O)2 (KE0 ¼ 0 and 300 K) from the 200 fs ES-AIMD simulation trajectories. In the case of pyrrole–(H2O), the KE of the pyrrole increases up to 7 kcal mol
1 at the beginning of ES-AIMD simulations (5 fs). On the other hand, the KE of the water molecule smoothly increases up to 2 kcal mol
1 around 40–80 fs, and then starts to fluctuate. Pyrrole–(H2O) undergoes charge transfer upon excitation and forms a highly polarized state during the ES-AIMD run. The CTTS is maximized around 100 fs, after which it strongly fluctuates and reaches another maximum around 300 fs. In the case of a pyrrole–(H2O)2 cluster with KE0 ¼ 0 K, the KE of pyrrole increases up to 5 kcal mol
1 at the beginning of ES-AIMD simulations. The second water molecule (denoted as W2) involved in the p–H interaction picks up the KE up to 3 kcal mol
1, while the first water molecule (denoted as W1) s H-bonded to the pyrrole molecule picks up the KE up to 2 kcal mol
1. Therefore, the water molecule involved in the p H-bond formation is more accessible to excitation than the other water molecule involved in the s H-bond. The distance between oxygen atoms (r(O1. . .O2)) increases after 100 fs, and the hydrogen bond between two water molecules is broken after 150 fs of simulations. In the meantime, the electron cloud is completely reorganized (the total charge is localized on one water molecule). At 0 K, a substantial charge transfer occurs from pyrrole to W2 through space, while W1 is neutral, i.e. charge transfer is not observed for the s H-bond. During ES-AIMD simulations, the negative charge begins to build up even for W1. At 100 fs, the net charges localized on pyrrole and W2 are 0.53 and
0.29 au respectively, which exhibits the maximum charge transfer during simulations. Consequently, the cluster was kept as a highly polarized state complex, showing dynamics very similar to the pyrrole–H2O system (as W2 is isolated from the complex). The simulation results of pyrrole–(H2O)2 with KE0 ¼ 300 K show the dissociation of the cluster into pyrrole and two completely isolated water molecules. For the latter simulations, the charge transfer takes place at the beginning of simulations, however, and almost disappears as time elapses, because the pyrrole–(H2O)2 cluster turns into pyrrole and two isolated water molecules. In the excited state, at t ¼ 0 fs, the charge density is almost completely transferred to the farthest water molecule, which is repelled by the depleted p-electron cloud of the aromatic ring. During ES-AIMD simulations, the electron charge density is distributed over all the water molecules (but more on the nearest water molecule). In the excited-state dynamics of pyrrole–(H2O) and pyrrole–(H2O)2, the charge transfer occurs through a hydrogen bond between pyrrole and one water molecule upon excitation, and the charge transfer is maximized at 100 fs, resulting in a significantly polarized state of the complex. However, a CTTS complex is never formed. As the temperature increases, the charge transfer becomes less important.

28.6 Conclusions Excited-state molecular dynamics based on the complete-active-space self-consistent field approach is a valuable tool for studying the charge transfer phenomena in various molecular systems. Our study

Charge Transfer in Excited States: ab initio Molecular Dynamics Simulations

637

Figure 28.8 Time evolution for the first excited states of a pyrrole–(H2O) complex with a KE0 of 0 K (a) and of a pyrrole–(H2O)2 complex with a KE0 of 0 K (b) and 300 K (c) in CASSCF[6,6]/aug-cc-pVDZ ES-AIMD simulations. Here, both the ground and the excited states at 0 fs have the same geometry. (d) Time evolution of NBO (natural bond orbital) charges localized on the pyrrole and water molecules for pyrrole–(H2O) and pyrrole–(H2O)2 with a KE0 of 0 K. Reprinted with permission from [39]. Copyright 2008, American Institute of Physics

638 Hydrogen Bonding and Transfer in the Excited State

demonstrates how the detachment process of I
(H2O)n¼2–5 evolves upon excitation and why the evolution process of I
(H2O)5 is different from the other smaller-sized clusters I
(H2O)2–4. The I
(H2O)n dynamics is important for the design of novel dynamic receptors that can selectively bind ions and then release them by the CTTS mechanism upon excitation. This concept could open a new field of the dynamic host–guest chemistry involved in the capture–transport–release mechanism of smart receptors. We reported the first ab initio excited-state molecular dynamics results to unravel the mechanism of water photolysis. We studied the photoexcitation mechanism by using ES-AIMD simulations based on the CASSCF approach and the photoionization mechanism by using the ground-state unrestricted Møller–Plesset second-order perturbation theory based on BOMD simulations. We performed ES-AIMD simulations for HI(H2O)n¼1,2 clusters. This study clearly shows how the hydrogen and halogen radicals can be dissociated and released from their hydrated acids. Indeed, we were able to demonstrate the predicted process from very simple experiments. H radicals released from iodic acid in water by UV light are useful as a reducing agent. In the excited-state dynamics of pyrrole with one and two water molecules, charge transfer occurs through a hydrogen bond between the pyrrole molecule and one water molecule upon excitation, which is maximized at 100 fs, resulting in a significantly polarized state of the complex. As the temperature increases, the charge transfer becomes much less significant.

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29 Competitive ESIPT in o-Hydroxy Carbonyl Compounds: Perturbation Through Solvent Modulation and Internal Torsion Sivaprasad Mitra Department of Chemistry, North-Eastern Hill University, Permanent Campus, Shillong 793022, India

29.1 Excited-State Proton Transfer: An Overview The transfer of a proton from one group to another is probably one of the most fundamental reactions in chemistry. Owing to the light mass of the proton, the transfer process usually occurs on an ultrafast timescale. As experimental techniques have advanced up to the femtosecond timescale, it has now been possible to study the different aspects of proton transfer (PT) processes that were previously inaccessible. The molecules undergoing excited-state proton transfer (ESPT) have attracted considerable attention in recent times owing to their potential application in chemistry and biology [1, 2]. In a typical ESPT reaction, AHþ þ B ! A þ BHþ , where A and B denote different molecular species for intermolecular (ESIerPT) or different sites of a single molecular species for intramolecular (ESIPT) proton transfer process (Figure 29.1). These processes depend strongly on the nature of the probe molecules and also the surroundings in the given experimental situation. In the following sections we give a brief review on different aspects of the ESPT reaction, followed by a detailed discussion of the ESIPT process in specific cases and its perturbation through solvent modulation as well as internal torsion. 29.1.1 General background In 1931, Weber [3] first reported the difference in acid–base equilibrium constant (Ka) of organic photoacids in the ground and excited states. In 1949, F€ orster [4] proposed the correct explanation for the observation that an

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

642 Hydrogen Bonding and Transfer in the Excited State

Figure 29.1

Schematic diagram showing intermolecular (i) and intramolecular (ii) proton transfer

organic acid having a large pKa difference in the ground and excited states may undergo protonation and deprotonation in the excited state, thus initiating the field of ESIerPT. The excited prototropic species thus produced relaxes to the ground state, with a subsequent reverse process (deprotonation or reprotonation), completing a four-level proton transfer cycle, the so-called F€orster cycle (Figure 29.2). On the other hand, Weller [5] in 1955 first proposed the formation of a proton transferred isomer (Ia) in the excited state, to explain the unusually large Stokes-shifted fluorescence for intramolecularly hydrogen-bonded methyl salicylate (I) (Figure 29.3). He also showed that only UV fluorescence with a mirror-image relationship with its absorption counterpart appears when the phenolic proton of I is methylated. With this pioneering work, the field of ESIPT was studied extensively.

A* ΔH*

ΔEA

(AH+)* ΔEAH+

A ΔH

AH+

Figure 29.2

A schematic representation of the F€ orster cycle for the acid–base equilibrium AHþ $ A þ Hþ

OCH3

OCH3

C

C O

O

H

H O

O I

Figure 29.3

Ia

Tautomerization in methyl salicylate (I)

Competitive ESIPT in o-Hydroxy Carbonyl Compounds

643

29.1.2 Description of ESPT in terms of the potential energy (PE) diagram In most cases, ESPT reactions comprise the following steps: (a) excitation of the normal photoacid; (b) rapid, non-radiative isomerization for ESIPT (or, ionization in the case of ESIerPT) in the excited state; (c) radiative decay of the newly formed excited species; (d) reverse isomerization (or reprotonation) in the ground-state surface. Thus, the entire process is cyclic and consists of a double-minimum potential in both the ground and excited states. Depending on the stability of different species formed during the ESPT process, the nature of the double-minimum potential can vary from symmetric to quasi-symmetric to fully asymmetric, both in the ground- and excited-state surfaces [6, 7]. 29.1.2.1 Symmetric and Slightly Asymmetric Double Minima If the isomerized structure following proton transfer is exactly identical to the initial photoacid, then this process must be characterized by a symmetric double-minimum potential, as in the cases of 9-hydroxy phenalenone and tropolone (structures II and III) (Figure 29.4). In these cases, the proton-transferred products IIa and IIIa are exactly identical to II and III respectively, and there is no physical property that can monitor the isomerization in real time. However, in the case of 2-(20 -hydroxy phenyl) benzothiazole (IV), the isomerized product (IVa) is a physically different molecule and the rate can be monitored by the fluorescence spectrum. The potential energy diagram is an asymmetric double minimum assuming that both the chemical structures are local minima. The other examples of this type of molecule are V and VI (Figure 29.5). In the asymmetric case, two possibilities, namely the deepest minimum in (a) the same atom (common asymmetry) or (b) the opposite atom (reverse asymmetry) may occur (Figure 29.6).

Figure 29.4

Examples of systems undergoing ESIPT in a symmetric double-minimum potential

644 Hydrogen Bonding and Transfer in the Excited State

Figure 29.5 Examples of systems undergoing ESIPT in an asymmetric double-minimum potential

Figure 29.6 Three schematic possibilities of the potential energy curves for proton transfer in the ground and excited states

29.1.2.2 Strongly Asymmetric Double Minima There are numerous examples of ESIPT systems that do not possess substantial amplitude in either local well in the excited singlet surface above the lowest vibrational level. The classical example that shows strongly reverse asymmetry is the case of methyl salicylate (I). 29.1.3 Thermodynamics of excited-state proton transfer The direct thermodynamic approach to measuring the electron density distribution in different energy states is to measure the respective chemical acidity constants (Ka). Most studies use the general method proposed by

Competitive ESIPT in o-Hydroxy Carbonyl Compounds

645

F€orster [4], which involves a thermodynamic cycle (the F€orster cycle) (Figure 29.2) combining thermodynamic and spectroscopic data. If DH and DH are the respective enthalpies of proton dissociation in the ground and the excited states, then DH ¼ DG þ TDS

ð29:1Þ

DH * ¼ DG* þ TDS*

ð29:2Þ

If DS ffi DS , then DHDH * ¼ DGDG* ¼ RTðln Ka ln Ka* Þ where Ka and Ka* are the acid–base equilibrium constants in the ground and excited states respectively. On rearrangement of the above equation, we have DpKa ¼ pKa pK*a ¼

DHDH * 2:303RT

ð29:3Þ

From Figure 29.2 we can write DEAH þ þ DH * ¼ DH þ DEA or DHDH * ¼ DEAH þ DEA ¼ NA hcðn0 AH þ n0 A Þ Then we can write DpKa ¼ pKa pK*a ¼

NA hcDn0 2:303RT

ð29:4Þ

where, Dn0 is the frequency difference (in wave numbers) of the lowest absorption bands between the acid and the basic forms. This method, although very successful in interpreting the acid–base properties of several organic compounds, involves several assumptions and limitations [8–10]. A more direct use of spectroscopic data to determine the pKa values of the organic acids was proposed by Weller [11, 12]. In this method, the relative fluorescence quantum yields (wf) of the excited-state species are plotted against the pH of the solution. However, a major disadvantage is that this method requires prototropic equilibrium to be established in the excited state, and also makes some assumptions regarding the fluorescence lifetime of the excited state. 29.1.4 Kinetics of excited-state proton transfer Although proton transfer reactions seem to be very simple at first sight, the kinetics of these reactions often becomes complex with the involvement of solvent molecules during the transfer process. With certain approximations, a number of models have been proposed to account for such a fast process, such as (a) the

646 Hydrogen Bonding and Transfer in the Excited State

Eigen model [13], (b) the bond energy bond order (BEBO) model proposed by Johnston et al. [14], (c) the Agmon–Levine model [15], (d) the intersecting state model [16] and (e) even Marcus theory, which was originally developed to interpret the rates of electron transfer reactions [17]. Although detailed discussion of all these models is out beyond the scope of this review, interested readers are advised to go through the original papers mentioned above to get a complete picture of the developments in this fascinating field of research.

29.2 Excited-State Intramolecular Proton Transfer (ESIPT) Weller [5, 11, 12] was the first to propose an intramolecular proton transfer mechanism in the excited state to account for the unusually large Stokes shift in the fluorescence properties of the methyl salicylate parent molecule. Since this first observation and its subsequent explanation, the field of ESIPT has expanded rapidly owing to its potential application in different areas of science and technology [18–21] and has been extensively reviewed by several authors [22–25]. In this section we will briefly discuss some of the aspects of ESIPT reactions. ESIPT usually involves systems having six-membered rings with an intramolecular hydrogen bond. The proton moves in between two highly electronegative heteroatoms of the type –O–H. . .O, –N–H. . .O, –S–H. . .O, etc. The five-membered intramolecularly hydrogen-bonded ring systems also undergo ESIPT. Excitation with an ultrashort laser pulse causes rapid redistribution of charge within the molecule, resulting in ultrafast ESIPT occurring within a 1012 s (ps)–1015 s (fs) timescale. 29.2.1 Different types of ESIPT Kasha [26] was the first to describe different types of ESIPT systematically. In this section we will give a brief description of these types: (a)

The symmetrical intramolecular proton transfer process as described earlier is observed in the cases of tropolone, 9-hydroxy phenalenone etc. In these cases, the transfer rates are usually very high and explained in terms of the tunnelling mechanism. (b) The molecules possessing a five- or six-membered intramolecular hydrogen bond (including the donor as well as the acceptor groups) usually undergo highly asymmetric proton transfer to show unusually large Stokes-shifted fluorescence. Depending on the strength of the internal hydrogen bond, more than one species (tautomer, different rotamers as well as proton-dissociated anions, etc.) may generate in the solution to give solvent-dependent fluorescence spectra. This particular type of ESIPT is intrinsic in nature. There are numerous examples of this category in the literature, but the most important are 3-hydroxy flavones (VII), o-hydroxy benzaldehyde (VIII) (Figure 29.7) and methyl salicylate (I). (c) Some of the systems, which may not have an internal hydrogen bond in suitable form to undergo intrinsic ESIPT, can undergo concerted double-proton transfer in the dimer form. The intensity of the longwavelength tautomer emission in these cases is found to increase with increase in solute concentration or with lowering of experimental temperature. The examples include 7-azaindole (IX) and N-phenyl benzamide (X) (Figure 29.7) [27]. Another type of ESIPT process, which may be termed solvent assisted intramolecular proton transfer, involves relay of the proton from one part of the molecule to the other, mediated by solvent bridges. Examples are 7-hydroxy quinoline (with two methanol molecules) and 3-hydroxy xanthone (with three methanol molecules). Some of the recent literature reports the importance of this type of ESIPT phenomenon, particularly in biological systems [28].

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647

Figure 29.7 Structures of the compounds 3-hydroxy flavone (VII), o-hydroxy benzaldehyde (VIII), 7-azaindole (IX) and N-phenyl benzamide (X)

29.2.2 Mechanism of ESIPT: tunnelling or vibrational relaxation? ESIPT reactions are characterized by a double-well potential energy surface having the reactant on one side and the product (phototautomer) on the other. Depending on the barrier height between these stationary points, ESIPT reactions may proceed either as proton tunnelling or as part of an intramolecular vibrational redistribution (IVR) in a barrierless adiabatic potential. Indication of proton tunnelling can be observed by the kinetic isotope effect (KIE) during the proton transfer process [29]. The extent of KIE dependence for the proton tunnelling is limited to certain factors. These can be either the degree to which the reaction is nonadiabatic and characterized by tunnelling through the potential barrier or if the reaction occurs by means of IVR, then the role of vibrational motions other than the O–H stretch becomes important. There is even precedence for the ESIPT process not to exhibit the KIE [30]. In these cases, excitation of the initial molecules causes a vibationally hot state, from which intramolecular relaxation caused by the change in geometrical parameters leads to the formation of the product. Hynes et al. [31] have presented a theory of proton transfer in both the adiabatic and the non-adiabatic limit. The theory assumes that three coordinates play the key role during ESIPT; the coordinate for the proton motion itself, the intramolecular separation of the two heavy atoms between which the proton is transferred and a collective solvent coordinate. Here, the electrons are always treated adiabatically, and the proton transfer process is considered to be adiabatic or non-adiabatic depending on the separation of two heavy  atoms. For typical O–H. . .O proton transfer reactions, an O. . .O distance greater than 2.7 A causes the wave function for the proton to be localized about one of the oxygen atoms and a large barrier to exist that proton transfer must overcome through a non-adiabatic tunnelling process. Here, the rate of tunnelling is modulated  by the O. . .O separation and the solvent fluctuations. If, however, the separation is less than 2.7 A, then the barrier of the proton transfer is greatly decreased and IVR becomes dominant over tunnelling in the PT mechanism. 29.2.3 Some examples of ESIPT As an active area of contemporary research over the last three decades, a large number of molecules have been found to undergo ESIPT that are so complex and so diverse that detailed discussion of all of them is beyond the scope of this review. Here we give a brief description of some of the well-studied examples of

648 Hydrogen Bonding and Transfer in the Excited State

ESIPT systems that have found scientific and technological applications. However, the most important cases of ESIPT in o-hydroxy carbonyl compounds and its perturbation are described separately and not included in this section. 29.2.3.1 3-Hydroxy Flavones 3-Hydroxy flavones (VII) are one of the most interesting and widely studied systems among ESIPT molecules. Since the first observation of ESIPT in 3HF by Sengupta and Kasha [32], numerous studies have been devoted to unravelling the intricacies of ESIPT in these molecules. In highly purified hydrocarbon solvents, VII undergoes very fast ESIPT at room temperature to give green fluorescence emission of the tautomer (lmax  520 nm), with a rise time of about 250 fs (k  1012 s1). The shift of the tautomer band with respect to the absorption onset is about 8500 cm1. The fluorescence lifetime of the excited tautomer is approximately 4 ns. After fluorescence, ground-state recovery of the normal structure occurs within 40–60 fs [33, 34]. The OCCOH moiety of VII forms a five-membered ring and thus has very weak internal hydrogen bond strength. This bond can be easily broken by interacting solvents such as water, ether and/or alcohol. Detailed discussion on the ESIPT of VII in different environments is available in the literature [35]. 29.2.3.2 2-(20 -Hydroxy Phenyl) Benzothiazole Cohen and Flavian [36] first proposed the large Stokes-shifted fluorescence for 2-(20 -hydroxy phenyl) benzothiazole (IV) towards the formation of proton-transferred tautomer. Since then, a number of studies have been devoted to this system [37]. It was suggested that the ground-state stable conformer of IV had the proton predominantly on the phenolic oxygen and the excited state had the proton on the nitrogen atom. The rate of ESIPT was found to be very high (6  1012 s1), faster than vibrational and torsional relaxation. Again, the absence of a significant isotope effect indicates that the proton transfer is essentially a barrierless process rather than tunnelling [38]. 29.2.4 Perturbation to ESIPT Ultrafast excited-state proton transfer can be perturbed in many ways. The most common causes of perturbation arise from (i) solvent interaction, (ii) the structure of the molecule undergoing ESIPT and (iii) competition with other possible non-reactive deactivation channels. In this section we will describe all these points in brief, with some representative examples. 29.2.4.1 Solvent Perturbation As ESIPT is a direct manifestation of a preformed hydrogen bond between donor and acceptor atoms bridging through the transferring proton, any solvent that ruptures this bond can cause perturbation to ESIPT. Purely intrinsic ESIPT can be achieved only in solutions of analytically pure samples with dried hydrocarbon solvents. The time dependence of ESIPT dynamics has frequently been found to vary systematically with the nature of the solvent [39, 40]. In general, the ESIPT rate decreases to a large extent in hydrogen-bonding solvents owing to the formation of intermolecular hydrogen bonding with the solvent and consequent increase in the barrier height for proton transfer. Hochstrasser et al. [41] showed a good correlation of the proton transfer rate in 2(20 -hydroxy-50 -t-octyl phenyl) benzotriazole (HOPB) with a longitudinal relaxation time (tL) of the 1-alkanol solvent series. These

Competitive ESIPT in o-Hydroxy Carbonyl Compounds

649

data are particularly intriguing because, in spite of this dependence on tL, no time-dependent Stokes shift is observed in this system. The ESIPT time of 240 fs in the case of 3HF in hydrocarbon solvents changes to more than 10 ps in hydrogen-bonding alcohol solvents. Petrich and coworkers [42] reported that the ESIPT time in hypericin, a naturally occurring polycyclic quinone that has received interest for its ability to deactivate the HIV virus, is about 10 ps, which is independent of the nature of the solvent. They argued that the O–H. . .O intramolecular hydrogen bonding in hypericin is much stronger than any potential hydrogen bonding in solution. Sytnik et al. [43] reported that 4-hydroxy-5-azaphenanthrene (HAP) exhibits ESIPTwhich appears to be unaffected by hydrogen-bonding solvent perturbation, and fluorescence emission is found to appear from the proton-transferred form even in ethanol. 29.2.4.2 Structural Perturbation The presence of only an internal hydrogen bond is not sufficient for ESIPT to occur. The nature of the product formed in the excited state, tautomer emission, quantum yield and PT rate are strongly dependent upon the structure of the molecule concerned. Nagaoka et al. [44] showed that ESIPT in VIII occurs only in the S1(p) state but not in the S0 and S2(p) states. They explained this behaviour of OHBA and other related molecules (Figure 29.8) such as methyl salicylate (I), o-hydroxy benzophenone (XI) and 2-(20 -hydroxy phenyl) benzothiazole (IV) in terms of the nodal pattern of the wave function in different states. The idea of the nodal plane dependence of ESIPT has also been extended to other systems. For example, methyl-3-hydroxy 2-naphthoate (XIII) is found to show ESIPT in the S1(pp ) state; however, phenyl-1-hydroxy-2-naphthoate does not show any proton transfer [45]. Furthermore, in the case of 2,5-bis(2-benzoxazolyl) hydroquinone (XIV), only one proton is transferred in the excited state, although this molecule has two intramolecular hydrogen-bonded sites [46]. 29.2.4.3 Competition with Other Non-Radiative Processes The low quantum yield of proton-transferred tautomer emission as observed in most of the ESIPT cases is mainly due to the competition among different deactivating channels other than proton transfer. A number of processes may be involved, such as intersystem crossing (ISC) from the initially excited state of the normal

Figure 29.8 Structures of the compounds o-hydroxy benzophenone (XI), methyl-3-hydroxy-2-naphthoate (XII), phenyl-1-hydroxy-2-naphthoate (XIII) and 2,5-bis(2-benzoxazolyl) hydroquinone (XIV)

650 Hydrogen Bonding and Transfer in the Excited State

conformer to the triplet, twisted intramolecular charge transfer (TICT), the formation of different rotamers in the excited state, etc., which results in multiple fluorescence bands. Among all these processes, competition of ESIPT with TICT processes was studied in great detail by Kasha and coworkers [47]. 29.2.5 Applications of ESIPT As an active area of contemporary research for the last three decades, ESIPT has found a large number of applications in the scientific and technological arena. Prototype examples are the development of laser dyes [48], molecular scintillators [49], special Raman filters [50], switches for dye laser pulse shortening [51], fluorescence probes for biomolecules [52], the conversion of solar into electrical energy [53] and the design of new bistable units in optical digital memory storage devices [54]. Again, as most of the important ESIPT reactions occur in solutions and are restricted to the subpicosecond timescale, ESIPT probes are widely used for novel formulations of solvation dynamics [55].

29.3 ESIPT in o-Hydroxy Carbonyl Compounds 29.3.1 o-Hydroxy benzaldehyde and its derivatives o-Hydroxy benzaldehyde (VIII) or salicylaldehyde is the simplest compound with six-membered internal hydrogen bonding in the ground state that undergoes intrinsic ESIPT. A detailed study of the spectroscopic properties of VIII and its derivatives was conducted by Nagaoka and coworkers [44]. The absorption spectra of the closed form of VIII covers the range 375–280 nm in non-polar solvents. The large Stokes-shifted fluorescence emission due to the formation of OHBA tautomer arises with a maximum at ca 510 nm. Nagaoka and Nagashima [44c] also performed ab initio calculations on different states of VIII and explained the cause of ESIPT only in the S1(p) state in terms of the nodal pattern of the wave function. The observed behaviour concerning proton transfer in other hydrogen-bonded molecules was explained similarly. However, in polar protic solvents such as ethanol, VIII gives dual fluorescence, one in the 520 nm region and the other at 420 nm, arising from different ground-state species, as confirmed by excitation spectra corresponding to these two emission bands, indicating the existence of two ground-state conformers. The emission peak at 420 nm is assigned to an intermolecularly solvent-mediated hydrogen-bonded open conformer (Figure 29.9). 29.3.2 Symmetrically substituted o-hydroxy carbonyl compounds In spite of all the observations for classic ESIPT systems such as o-hydroxy benzaldehyde, o-hydroxy acetophenone, o-hydroxy propiophenone and methyl salicylate described in the previous sections, there remains some questions regarding the nature of the fluorescent species, external control of the final photoproduct, etc. For the past few years we have been making a systematic effort to understand some of these questions in symmetrically substituted o-hydroxy carbonyl compounds. The representative structure of the systems investigated is given in Figure 29.10. 29.3.2.1 ESIPT and Solvent Participation in Excited-State Photophysical Processes The absorption spectra of XV in non-polar hydrocarbon solvents show a single broad band with a maxima at 350 nm due to pp (S1 S0) transition from the normal enol structure. On the other hand, the fluorescence spectrum exhibits a large Stokes-shifted emission in the 440–680 nm regions, with a

Competitive ESIPT in o-Hydroxy Carbonyl Compounds

651

Figure 29.9 Possible structural forms during proton transfer of o-hydroxy benzaldehyde (VIII) and its derivatives

maximum at 535 nm corresponding to the proton-transferred keto structure. However, the emission spectra for XV at 77 K consist of an additional band in the 460–470 nm region owing to the formation of open conformers (Figure 29.11), along with 535 nm tautomer emission. Both these open conformer structures can give rise to 3np -type phosphorescence in non-polar solvents because this spectrum is characterized by a progression of the C¼O stretching mode in other benzaldehyde types of molecule [56]. This indicates that the formation of an open conformer competes effectively with proton transfer in XV at 77 K in non-polar solvents. However, in moderately interactive solvents such as 1,4-dioxane and tetrahydrofuran, the open conformer emission arises even at room temperature. The nature of the excitation spectra for 535 and 460 nm emissions are different and appear at 360 and 420 nm respectively, which conclusively points to the formation of different conformers responsible for these emission bands. The steady-state spectral properties at different temperatures, along with the fluorescence lifetime of the excited species, are summarized in Table 29.1; the structures of the corresponding species having specific emission properties are also given in Figure 29.11. However, the solvent-dependent fluorescence spectra of XVI [57] indicate that the properties of intramolecularly hydrogen-bonded compounds change considerably, depending upon the substitution in the vicinity of intramolecularly hydrogen-bonded sites (Table 29.2). While the formyl group in XV rotates freely to give 3np -type phosphorescence at lower temperature, this is not observed in XVI owing to the restricted rotation of the acetyl group. The temperature and added electron donor act in a similar way on the decay properties of XVI, as in XV. The activation energy for the non-radiative processes is dependent on the group attached to the carbonyl carbon and indicates the importance of carbonyl torsion in the non-radiative decay path. The proton transfer dynamics of XVI is mainly controlled by the proton-accepting ability or basicity of the solvent, but orientational motion of the solvent molecule has a minor effect on it [57c]. The ESIPT dynamics of XV and XVI was studied in detail by femtosecond transient experiments as well as by semi-empirical and ab initio level of theory [58]. Ultrafast

652 Hydrogen Bonding and Transfer in the Excited State CH3

CH3

H

H C O

O

H3C

CH3

C

C

O

O

C O

XV

XVI CH3

CH3

OH

HO C O

O H

H

O

H3CO

OCH3

C

C

O

O

C O

XVII

O H

H XVIII

Figure 29.10 Structure of the compounds having a symmetrically substituted proton transfer site: 4-methyl-2, 6-diformyl phenol (XV), 4-methyl-2,6-diacetyl phenol (XVI), 3-methyl-6-hydroxy-m-phthalic acid (XVII) and 4-methyl-2,6-dicarbomethoxy phenol (XVII)

ESIPT occurs at about 200 fs in XV, followed by an IVR component of about 2.8 ps before undergoing fluorescence decay from the excited-state proton-transferred keto structure (Figure 29.12). The corresponding time constants for XVI are in the range of 250 fs and 1.5 ps respectively. The construction of the potential energy surface (PES) for ESIPT, as well as the estimation of different vibrational levels and the corresponding eigenfunctions supported at the ground and excited PESs using the Fourier grid Hamiltonian (FGH) method, showed that Franck–Condon excitation from the S0 state would take the system almost over the barrier and eventually into the potential well representing the tautomeric form; however, ESIPT through tunnelling could not be ruled out completely [58a]. In fact, FGH-based complex coordinate rotation calculation estimates a tunnelling rate constant (ktunnel)  1011 s1, which is almost an order of magnitude slower than the experimentally measured rate constant, indicating the importance of the over-barrier mechanism in these systems. On the other hand, the ESIPT systems XVII and XVIII are symmetrically substituted analogues of salicylic acid and methyl salicylate respectively. These systems can exist in two different hydrogen-bonded conformers in the ground state (Figure 29.13); one of these is capable of undergoing ESIPT, leading to tautomer emission, and the other will show normal emission. Extensive studies of these systems indeed indicate that there exist at least two conformers in the ground state, whereas in the excited state at least three species are formed. Moreover, the species responsible for tautomer and open conformer emission originate directly from the respective ground-state structures. Further, as observed in the other cases, the non-radiative decay process dominates again in the excited-state photophysics [59].

Competitive ESIPT in o-Hydroxy Carbonyl Compounds

Figure 29.11

653

Different structural forms of XV produced in the excited state

29.3.2.2 Modulation of Excited-State Phenomena by Solvent Dielectric Interaction As ESIPT involves movement of a hydrogen atom over a very small distance, in most cases the solvent dielectric does not play a major role in determining the excited-state phenomena. However, judicious substitution in and around the PT site can have a detrimental effect on the fluorescence properties of these systems. A recent example of this type of system, where the ESIPT phenomenon is modulated by internal torsion as well as by the solvent dielectric, has been the case of substituted o-hydroxy acetophenone derivatives (Figure 29.14) [60]. The absorption spectrum of both XIX and XX in non-polar solvent shows a single peak at around 355 nm. However, the fluorescence emission spectrum corresponding to this absorption consists of two bands. The high-energy, structured emission at around 430 nm has less Stokes shift (DnSS  5000 cm1) compared with the broad, low-energy, unstructured emission at 505 nm which has unusually large Stokes shift (DnSS  8300 cm1). The excitation spectra corresponding to both these emissions match very closely the ground-state absorption spectra and appear at about 375 and 360 nm respectively. In accordance with previous studies of ESIPT in o-hydroxy acetophenone derivatives, the largely Stokes-shifted emission arises owing to proton

654 Hydrogen Bonding and Transfer in the Excited State Table 29.1 Summary of absorption (labs), emission (lem) and excitation (lexc) spectral maxima, along with the fluorescence quantum yield (wf) and excited-state lifetime (tf), of different ground- and excited-state species of XV (for structure, see Figure 29.11) Solvent

labs(nm)

Cyclohexane CCl4 1,4-Dioxane

350 350 350

THF Acetone Acetonitrile

350 350 350

DMF

350 460d 350 480 350 350

DMSO Methanol Water

a

lem(nm)

lexc(nm)

wf

tf (ns)

535 535 535 460c 460 460 460 520d 460 520 460 520 520 520

360 360 360 400 400 430 400 440 400 460 400 480 440 440

0.10 0.12 0.18 0.20 0.10 0.20 0.24 0.30 0.24 0.32 0.40 0.48 0.35 0.26

1.2 1.3 1.4 4.2 3.9 2.2 4.7 4.9 3.9 4.1 4.6 5.0 4.5 4.8

b

Corresponding to structure XV. Corresponding to structure XVa. Corresponding to structure XVc. d Corresponding to a proton-dissociated anionic structure. a

b c

Table 29.2 Summary of fluorescence quantum yield (wf) and excited-state lifetime (tf) of XVI (for structure, see Figure 29.11) in different solvents at room temperature (298 K) and 77 K Solvent

3-Methyl pentane CCl4 DMSO DMF Acetonitrile

Fluorescence yield (wf)

Fluorescence lifetime (tf,ns)

298 K

77 K

298 K

77 K

0.04 0.06 0.28 0.21 0.08

0.32 0.34 0.82 0.76 0.18

0.40 0.40 4.3 3.3 0.3

4.1 4.5 6.8 5.1 3.2

transfer (ESIPT fluorescence) in the S1(pp ) state of the primary enol (E) form, where the intramolecular hydrogen bond involves the phenolic hydrogen and carbonyl oxygen of the acetyl group in the ortho position (Figure 29.15, structure (a)) to form the keto (K) conformer (structure (b)). On the other hand, the structured emission with a lower DnSS value may arise from another conformer of the enol structure where the hydrogen bonding partners involve the phenolic hydrogen and oxygen of the nitro group in the ortho position. As shown in Figure 29.15, two structural forms can be presumed for this conformer (E1 and E2, structures (c) and (d) respectively). However, from the geometry of these two conformers, it can be seen that in E1 the proximity of oxygen atoms in neighbouring acetyl and phenolic groups may cause additional non-bonding interaction. Therefore, the relative abundance of E2 should be greater in the solution compared with E1. The results of theoretical calculation indeed predict the same. On the other hand, the relative energy difference, as obtained from DFT calculation for XX, between E and E2 structures varies from 0.5 to 0.02 kcal mol1 in the different solvents studied here. The very small energy difference in solution indicates that there is equilibrium among

Competitive ESIPT in o-Hydroxy Carbonyl Compounds

655

Figure 29.12 Schematic views of ESIPT phenomena and corresponding time parameters for XV, as obtained by femtosecond transient absorption (TrA) experiments. The shaded arrows indicate the origin of TrA at different time delays (adapted from Ref. 58c)

different enol conformers in the ground state, and ESIPT emission from E is in competition with normal fluorescence from E2. The calculated excited-state energy difference between E and E2 is 5.6 kcal mol1 in CCl4 and decreases substantially to 3.5 kcal mol1 in polar solvents such as ethanol or acetonitrile. The large energy difference in non-polar solvents ensures that ground-state equilibrium between these structures is perturbed in the excited state, and respective emission appears from individual conformers. However, the situation is different in polar solvents and demands careful attention. The interaction of excited fluorophore and solvent dipole reduces the energy difference substantially. Furthermore, ESIPT is known to be a very efficient non-radiative deactivation channel for intramolecularly hydrogen-bonded compounds and occurs on an ultrafast timescale. Also, the keto

CH3

CH3

HO

OH C O

O

HO

O

C

C

O

O

C O H

H

Figure 29.13

O

Possible hydrogen-bonded structural forms of XVII

H

656 Hydrogen Bonding and Transfer in the Excited State

Figure 29.14 phenol (XX)

(a)

Structural representation of 2-acetyl-4-methyl-6-nitrophenol (XIX) and 2-acetyl-4-chloro-6-nitro-

O

(b)

O

O

N

O N

O

O H

H

O

O

C

R

C

R CH3

(c)

O

CH3

(d)

O N

O

O N

H

H O

O

CH3

O R

C

CH3

R

C

O

Figure 29.15 Structure of possible conformers of XIX (R ¼ CH3). The enol (a), keto (b) and non-ESIPT enol structures (c) and (d) are represented as E, K, E1 and E2 in the text (adapted from Ref. 60)

Competitive ESIPT in o-Hydroxy Carbonyl Compounds

657

Fluo. Intensity (arb. units)

250

200

8 150

1

100

50

0 425

450

475

500

525

550

575

600

Wavelength /nm

Figure 29.16 Fluorescence emission spectra of XX with increasing concentration of b-cyclodextrin in aqueous solution. [b-CD] (mM) ¼ 1 (0.0), 2 (2.2), 3 (3.6), 4 (4.9), 5 (6.8), 6 (9.0), 7 (17.6) and 8 (22.2) respectively (adapted from Ref. 61a)

conformer (K) is the most stable geometrical entity in the excited state. Therefore, it was argued that ESIPT acts both as a thermodynamic and as a kinetic sink in the excited-state potential energy surface, and the equilibrium is tilted heavily towards the pro-ESIPT conformer (E) in these solvents for both XIX and XX. As a result, the fluorescence spectra show only ESIPT emission at around 510 nm in these solvents. The argument concerning the difference in photophysical properties of XX in polar solvents to those in non-polar solvents like CCl4 was further substantiated by observing the fluorescence spectra of a probe encapsulated in cyclodextrin nanocavities. As the probe experiences a relatively more hydrophobic environment inside the host cavity, it is expected that the ESIPT fluorescence of XX in pure water will be changed to both ESIPT and non-ESIPT fluorescence under encapsulation with cyclodextrin (CD). As shown in Figure 29.16, this is indeed the case in the presence of CDs [61].

29.4 Concluding Remarks In this chapter, different aspects of ESIPT phenomena have been reviewed, with a special emphasis on the nature of participation of solvent molecules to cause specific perturbations in intrinsic ESIPT. Examples were presented to emphasize the most common cause of perturbation in ESIPT through direct participation of solvent to rupture the internal hydrogen bond; however, thermodynamically favourable internal torsion and solvent dielectrics can also play a major role in determining the nature and number of fluorescing species arising from these systems. Some of the concepts are well established; others are more speculative, and a welldocumented conclusion is still lacking. However, the present vigorous research activity in the field of ESIPT with ultrafast laser experiments, as well as high-level theoretical calculation, will undoubtedly unravel many more examples of this type and the associated mechanisms. The present results address fundamental issues of competitive ESIPT and solvent-modulated excited-state photophysics from structurally similar ground-state

658 Hydrogen Bonding and Transfer in the Excited State

conformers resulting from torsional motion. These motions are very important for the construction, stability and function of macromolecular architecture.

Acknowledgements Thanks are due to Professors S. Mukherjee and S. P. Bhattacharyya, both from the Department of Physical Chemistry, the Indian Association for the Cultivation of Science, Jadavpur, Calcutta, and to Professor N. Tamai, Department of Chemistry, Kwansei Gakuin University, School of Science, Sanda, Japan, for their guidance, help and constant enthusiasm on this topic. The author also expresses heartfelt thanks to all those who have collaborated on this problem at different times. Partial financial support from the University Grants Commission (UGC), Government of India, on ‘Spectroscopy and Dynamics of Photoinduced Processes in Homogeneous and Biomimetic Environments’ (34-299/2008 (SR)) is gratefully acknowledged.

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Competitive ESIPT in o-Hydroxy Carbonyl Compounds

659

24. For a review on the biologically relevant excited-state double-proton transfer process, see P. T. Chou, J. Chin. Chem. Soc., 48, 651 (2001). 25. S. M. Ormson and R. G. Brown, Prog. Reac. Kinet., 19, 45 (1994). 26. (a) M. Kasha, J. Chem. Soc., Faraday Trans. II, 82, 2379 (1986); (b) J. Heldt, G. Gormin and M. Kasha, Chem. Phys., 136, 321 (1989). 27. (a) C. A. Taylor, M. A. El-Bayoumi and M. Kasha, Proc. Natl Acad. Sci. USA, 63, 253 (1969); (b) G.-Q. Tang, J. McInnis and M. Kasha, J. Am. Chem. Soc., 109, 2531 (1987). 28. (a) E. J. de Berker, J. D. Geerlings and C. A. G. O. Varma, J. Phys. Chem. A, 104, 5916 (2000); (b) M. K. Nayak and P. Wan, Photochem. Photobiol. Sci., 7, 1544 (2008). 29. (a) T. Carrington and W. H. Miller, J. Chem. Phys., 84, 4364 (1986); (b) N. Shida, P. F. Barbara and J. Alml€ of, J. Chem. Phys., 92, 4061 (1989); (c) M. Kijack, Y. Nosenko, A. Singh et al., J. Am. Chem. Soc., 129, 2738 (2007). 30. (a) W. Frey, F. Laermer and T. Elsaesser, J. Phys. Chem., 95, 10391 (1991); (b) B. J. Schwartz, L. A. Peteanu and C. B. Harris, J. Phys. Chem., 96, 3591 (1992); (c) S. Lochbrunner, K. Stock, C. Schriever and E. Riedle, Ultrafast phenomena XIV, Part VI. Springer Ser. Chem. Phys., 79, 491 (2005). 31. (a) J. T. Hynes, Ann. Rev. Phys. Chem., 36, 573 (1985); (b) D. Bogris and J. T. Hynes, J. Chem. Phys., 94, 3619 (1991); (c) H. Azzouz and D. Bogris, J. Chem. Phys., 98, 7361 (1993). 32. P. K. Sengupta and M. Kasha, Chem. Phys. Lett., 68, 382 (1979). 33. (a) P. M. Felker, W. R. Lambert and A. H. Zewail, J. Chem. Phys., 77, 1603 (1982); (b) D. McMorrow and M. Kasha, Proc. Natl Acad. Sci. USA, 81, 3375 (1984); (c) P. T. Chou, M. L. Martinez and J. H. Clements, J. Phys. Chem., 97, 2618 (1993); (d) K.-Y. Chen, Y.-M. Cheng, C.-H. Lai et al., J. Am. Chem. Soc., 129, 4534 (2007). 34. (a) M. Itoh and Y. Fujiwara, J. Phys. Chem., 87, 4558 (1983); (b) M. Itoh, Y. Tanimoto and K. Tokomura, J. Am. Chem. Soc., 104, 4164 (1982); (c) R. de Vivie-Riedle, V. D. Waele, L. Kurtz and E. Riedle, J. Phys. Chem. A, 107, 10591 (2003). 35. (a) P.-T. Chou, C.-H. Huang, S.-C. Pu et al., J. Phys. Chem. A, 108, 6452 (2004); (b) A. N. Bader, F. Ariese and C. Gooijer, J. Phys. Chem. A, 106, 2844 (2002); (c) S. Ameer-Beg, S. M. Ormson, X. Poteau et al., J. Phys. Chem. A, 108, 6938 (2004). 36. M. D. Cohen and S. J. Flavian, J. Chem. Soc. B, 321 (1967). 37. (a) W. Al-Soufi, K. H. Grellmann and B. Nickel, Chem. Phys. Lett., 174, 609 (1990); (b) F. Laermer, T. Elsaesser and W. Kaiser, Chem. Phys. Lett., 148, 119 (1988); (c) S. Lochbrunner, A. J. Wurzer and E. Riedle, J. Phys. Chem. A, 107, 10580 (2003). 38. P. F. Barbara, P. M. Rentzepis and L. E. Brus, J. Am. Chem. Soc., 102, 2786 (1980). 39. (a) A. U. Acun˜a, F. Amat-Guerri, J. Catalan and F. G. Tables, J. Phys. Chem., 84, 629 (1980); (b) M. Itoh, Y. Fujiwara, M. Sumitani and K. Yoshihara, J. Phys. Chem., 90, 5672 (1986); (c) S. Nagaoka, M. Fujita, T. Takemura and H. Baba, Chem. Phys. Lett., 123, 489 (1986). 40. (a) O. K. Abou-Zied, R. Jimenez, E. H. Z. Thompson et al., J. Phys. Chem. A, 106, 3665 (2002); (b) S. J. Schmidtke, D. F. Underwood and D. A. Blank, J. Am. Chem. Soc., 126, 8620 (2004). 41. Y. R. Kim, J. T. Yardley and R. M. Hochstrasser, Chem. Phys., 136, 311 (1989). 42. F. Gai, M. J. Fehr and J. W. Petrich, J. Phys. Chem., 98, 8352 (1994). 43. (a) A. Sytnik and J. C. D. Valle, J. Phys. Chem., 99, 13028 (1995); (b) A. Sytnik and M. Kasha, Proc. Natl Acad. Sci. USA, 91, 8627 (1994). 44. (a) S. Nagaoka, U. Nagashima, N. Ohta et al., J. Phys. Chem., 92, 166 (1988); (b) S. Nagaoka, K. Sawada, Y. Fukumoto et al., J. Phys. Chem., 96, 6663 (1992); (c) S. Nagaoka and U. Nagashima, Chem. Phys., 136, 153 (1989). 45. G. J. Woolfe and P. J. Thistlethwaite, J. Am. Chem. Soc., 103, 3849 (1981). 46. A. Mordzin´ski, A. Grabowska, W. Kuˆhnle and A. Kro´wezyn´ski, Chem. Phys. Lett., 101, 291 (1983). 47. (a) J. Heldt, D. Gormin and M. Kasha, Chem. Phys., 136, 321 (1989); (b) C. A. S. Potter, R. G. Brown, F. Vollmer and W. Rettig, J. Chem. Soc., Faraday Trans. I, 90, 59 (1994). 48. (a) A. U. Khan and M. Kasha, Proc. Natl Acad. Sci. USA, 80, 1767 (1983); (b) P. T. Chou, D. McMorrow, T. J. Aartsmma and M. Kasha, J. Phys. Chem., 88, 4596 (1984); (c) A. U. Acun˜a, A. Costela and J. M. Mun˜oz, J. Phys. Chem., 90, 2807 (1986). 49. A. Sytnik and M. Kasha, Radiat. Phys. Chem., 41, 331 (1993). 50. P. T. Chou, S. L. Studer and M. L. Martinez, Appl. Spectr., 145, 513 (1991).

660 Hydrogen Bonding and Transfer in the Excited State 51. N. P. Ernsting and B. Nikolaus, Appl. Phys. B, 39, 155 (1986). 52. (a) A. Sytnik, D. Gormin and M. Kasha, Proc. Natl Acad. Sci. USA, 91, 11698 (1994); (b) M. Gutman and D. Huppert, J. Am. Chem. Soc., 103, 3709 (1981). 53. F. Vollmer and W. Rettig, J. Photochem. Photobiol. A: Chem., 95, 143 (1996). 54. R. W. Munn, Chem. Br., 517 (1984). 55. (a) M. Maroncelli, E. W. Castner, B. Bagchi and G. R. Fleming, Faraday Disc. Chem. Soc., 85, 199 (1988); (b) P. J. Rossky and J. D. Simon, Nature, 370, 263 (1994). 56. (a) S. Mitra, R. Das and S. Mukherjee, Chem. Phys. Lett., 202, 549 (1993); (b) S. Mitra, R. Das and S. Mukherjee, Spectrochim. Acta, 50A, 549 (1994); (c) S. Mitra, R. Das and S. Mukherjee, J. Photochem. Photobiol. A: Chem., 79, 49 (1994); (d) R. Das, S. Mitra and S. Mukherjee, J. Photochem. Photobiol. A: Chem., 76, 33 (1993); (e) R. Das, S. Mitra and S. Mukherjee, Bull. Chem. Soc. Jpn, 66, 2492 (1993). 57. (a) S. Mitra, R. Das and S. Mukherjee, Chem. Phys. Lett., 228, 393 (1994); (b) A. Mandal, D. Guha, R. Das et al., J. Chem. Phys., 114, 1336 (2001); (c) D. Guha, A. Mandal, R. Das et al., Isr. J. Chem., 39, 375 (1999). 58. (a) S. Mitra, R. Das, S. P. Bhattacharyya and S. Mukherjee, J. Phys. Chem. A, 101, 293 (1997); (b) S. Mitra and S. Mukherjee, J. Lumin., 118, 1 (2006); (c) S. Mitra, N. Tamai and S. Mukherjee, J. Photochem. Photobiol. A: Chem., 178, 76 (2006). 59. (a) R. Das, S. Mitra, D. Guha and S. Mukherjee, J. Lumin., 81, 61 (1999); (b) A. Mandal, S. Mitra, D. Banerjee et al., J. Chem. Phys., 118, 3154 (2003); (c) A. Mandal, D. N. Nath, S. Mukherjee et al., J. Chem. Phys., 117, 5280 (2002). 60. S. Mitra, T. S. Singh, A. Mandal and S. Mukherjee, Chem. Phys., 342, 309 (2007). 61. (a) S. Mitra, ISRAPS Bull., 20, 23 (2008); (b) S. Mitra and T. S. Singh, unpublished results.

30 Excited-State Double Hydrogen Bonding Induced by Charge Transfer in Isomeric Bifunctional Azaaromatic Compounds Dolores Reyman and Cristina Dı´ıaz-Oliva Departamento de Quı´mica-Fı´sica Aplicada, Facultad de Ciencias, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain

30.1 Introduction Proton transfer is one of the most important processes involved in both chemical reactions and biological systems. Consequently, a large number of studies on hydrogen bonding and proton transfer processes in ground and excited states have been carried out [1]. A special class within the systems presenting proton transfer is represented by heteroaromatic molecules containing both hydrogen-bonding donor and hydrogen-bonding acceptor groups. The reaction may occur both as an intramolecular or intermolecular process. Examples of the former are provided by numerous molecules such as salicylic acid and its derivatives [2–6], 2-hydroxybenzophenone [7], 5-hydroxy-flavone [8, 9], 1,5-dihydroxyxanthraquinone [10], 2-hydroxy-4,5-naphthotropone [11], 2-(20 -hydroxyphenyl)benzoxazole, 2-(20 -hydroxyphenyl)-benzothiazole [12–14] and 2-(20 hydroxy-50 -methylphenyl)benzotriazole [15–18]. Several reviews are based on these molecules [19–22]. Conversely, in another class of molecules, the possibility of an internal H-bond is hindered either for geometrical reasons or because of a large spatial separation between proton-donating and proton-accepting sites. The prototype of these molecules is exemplified by 7-azaindole (7AI), which represents the first documented example of dual-proton transfer in dimers and alcohol complexes [23]. However, in spite of decades of research on 7AI [23–44], the phototautomerization process continues to arouse much controversy. Based on time-resolved experiments, it was proposed that the transfer occurs in a stepwise fashion [25, 27, 45–51]. Some authors, however, postulate a concerted simultaneous double-proton transfer mechanism [52–57].

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

662 Hydrogen Bonding and Transfer in the Excited State

7AI belongs to N-heteroaromatic compounds, which possess two or more nitrogen atoms in their structure, and are of great interest [58] owing to their being ubiquitous in nature and comprising the basic elements of most macromolecules of biological interest. Thus, some are components of DNA (such as purine bases), others are prototypes of photoprotectors (e.g. Tinuvin P) and others are parts of the active sites of all heme proteins involved in the biological functions of oxygen transport (hemoglobin), electron transport (e.g. cytochrome in respiration) and biocatalysis (e.g. enzymes such as catalases and peroxidases, and cytochrome P450). Usually, these heteroaromatic molecules can be structurally viewed as condensation products of simpler individual Nheteroaromatic rings, a p-deficient pyridine ring fused to a p-excessive indole ring. As a general rule, the acidity/ basicity of the nitrogen atoms in these condensed molecules greatly differs from that observed in the individual rings [59]. The changes in molecular structures and electronic charge distribution accompanying the annelation process can greatly affect the intrinsic acidity/basicity of these nitrogen centres [60]. These molecules possessing both hydrogen-bond donor and hydrogen-bond acceptor groups are able to form dimers or complexes with hydrogen-bonding solvents. In many cases, the excited-state properties of such intermolecular hydrogen-bonded species are totally different from those of non-bonded molecules in non-polar or polar aprotic solvents leading to tautomerization processes [19, 61–65]. The driving force for excited-state proton transfer is the electron density redistribution occurring upon excitation. This redistribution causes an increase in the excited-state pKa of the proton acceptor, a decrease in the excited-state pKa of the proton donor or a combination of both. This review focuses on analysing ground- and excited-state effects induced by hydrogen-bonding compounds in two series of isomeric bifunctional azaaromatic chromophores based on pyrrolo-quinoline, such as pyrido[3,2-g]indole (PQ), dipyrido[2,3-a:30 ,20 -i]carbazole (DPC), 7,8,9,10-tetrahydropyrido[2,3-a]carbazole (TPC) and pyrido[2,3-a]carbazole (PC), and on pyrido-indole, such as methylene-bridged 2-(20 -pyridyl) indoles (PyIn-n) and 9H-pyrido[3,4-b]indole (BC) (or norharmane), 1-methyl-9H-pyrido[3,4-b]indole (HN) (or harmane), N9-methyl-harmane (MHN) and N2-methylharmane (T-MeHN). In the betacarbolinic derivatives (BCD) the pyridine and pyrrol rings are fused, while in the others they are separated by a benzo ring spacer, such as in pyrrolo-quinolines, or by a single bond and a methylene bridge, such as in 2-(20 -pyridyl)indoles.

30.2 Pyrrolo-Quinoline Derivatives (PQ, DPC, TPC) 1-H-pyrrolo[3,2-h]quinoline (or pyrido[3,2-g]indole) (PQ) and related compounds (Chart 30.1), can be considered as the 7AI modified by a benzo ring spacer, which separates the pyrido and the pyrrolo rings. The geometry of these molecules does not enable an internal H-bond from the pyrrolo-H to the aza-N atom. PQ has been cited in numerous places in the literature in connection with potentially valuable chemical and biomedical applications. In chemical applications, some derivatives of 1-methyl-pyrrolo[3,2-h]quinoline have been shown to be good stabilizers for polymers [23]. The molecule PQ has also been proposed as a host molecule in molecular recognition ab initio studies of ring motions [66]. Biomedical reports propose its use as a potential anticancer drug [67]. It exhibits tuberculostatic activity [68] at 4 mg mL1 against tubercule strain H-37, and has also been tested as an antimalarial drug [69]. Furthermore, DPC has been considered as a probe of hydrophilic/hydrophobic surface character owing to its sensitivity towards hydroxilic groups [70]. It is thus of substantial interest to understand the electronic structural characteristics and consequent physical and chemical behaviour of these compounds in order to gain a full understanding of their properties [71]. 30.2.1 Steady-state measurements Absorption spectra for these compounds show a red-shift of all transitions upon increasing solvent polarity and a distinct band on the red energy side. The addition of small amounts of alcohol (c < 102 M) to a solution

Excited-State Double Hydrogen Bonding

N

N

N

N

PQ

N

N

N

H

H

663

H

DPC

TPC (CH2)n

N

N

N

N H

H PC

PyIn-n

N N

R2

R1 BC HN MHN

Chart 30.1

R1, R2 = H R1 = H, R2 = Me R1, R2 = Me

Formulae and acronyms of the compounds

of these compounds in n-hexane (non-polar solvent) leads to the appearance of isosbestic points, which are indicative of the equilibrium between two ground-state species (Figure 30.1). Initially, one of the two species was assigned to the alcohol complex and the other to the ‘bare’ molecule (uncomplexed) [72]. The equilibrium constant, K, for a reaction involving n alcohol molecules B þ nA ¼ BAn

K ¼ ½BAn =ð½B½An Þ

may be expressed as K¼

ðOD  OD0 Þ ðOD¥  ODÞ  ½An

ð30:1Þ

where OD0 and OD¥ denote the optical density measured when only the uncomplexed or complexed forms are present, and OD is the optical density measured at an alcohol concentration [A]. From the plot of ln [(OD  OD0)/[(OD¥  OD)] versus ln[A], the number of alcohol molecules in a complex can be obtained. The values extracted from this plot point to the 1:1 stoichiometry at a low concentration of alcohols [72, 73] (see Figure 30.1). In emission, all the molecules considered reveal intense fluorescence both in non-polar and polar aprotic solvents [74]. The spectra consist of a single emission, labelled as F1, which has been assigned as the radiative decay of the initially excited state [71, 72]. The change from an aprotic to a protic solvent causes a decrease in the fluorescence intensities of this band, F1, and the appearance of a new red-shifted fluorescence band, labelled as F2 (Figure 30.1). As can be seen in Figure 30.2, the spectral locations and shapes of these

664 Hydrogen Bonding and Transfer in the Excited State

Figure 30.1 Titration of the solution of PQ in n-hexane with 1-butanol at 293 K. Top, changes in absorption; bottom, evolution of both fluorescence bands. The arrows show spectral changes accompanying the addition of alcohol. The alcohol concentration varied from 3.0  104 to 2.2  102 M. The insert in the top part shows the determination of the equilibrium constant and stoichiometry of the complex from the absorption data recorded at 28011 cm1. Reprinted with permission from [73]. Copyright 1999 American Chemical Society

low-energy emissions (F2) coincide almost exactly with the fluorescence recorded for tautomeric model structures (Chart 30.2) [73]. These findings leave no doubt that low-energy fluorescence originates in the tautomeric species, obtained as a result of a double-proton transfer reaction occurring in complexes of the photoexcited chromophore with alcohol. However, the fact that the decrease in the intensity of the F1 fluorescence was much greater than the concomitant increase in the tautomeric emission led the model that assumes the presence of only two species, and thus only one form of the alcohol solvate, to be considered as probably excessively simple [73]. This finding has been explained by the presence of at least two different forms of alcohol complexes. A cyclic, doubly hydrogen-bonded 1:1 complex that can undergo phototautomerization, and a non-cyclic 1:n complex (n being the number of alcohol molecules associated with the substrate) that is also efficiently deactivated, but by a pathway that is different from proton transfer. Another argument for the presence of 1:1 cyclic, doubly hydrogen-bonded solvates in the ground state is the fact that the F2 band can be still observed in rigid glasses at 77 K for PQ and DPC [74, 75]. Lowering the temperature leads to an increase in both F1 and F2 emission intensities, where F1 is the most affected. For instance, the quantum yield for DPC, in n-butanol, increases by an order of magnitude between 293 and 183 K (from 0.0005 to 0.005), while the corresponding values for F2 are 0.002 and 0.007. Moreover, the F1 emission becomes structured and very similar to the emission observed at 293 K in non-polar solvents. However, in spite of the recovery of the fluorescence intensities, the sum of F1 and F2 quantum yields in alcohols at low

Excited-State Double Hydrogen Bonding

665

Figure 30.2 Fluorescence of PQ, TPC, PC and PyIn-2 (solid lines, a) and of the corresponding tautomeric model structures (dotted lines, b). The spectra were recorded in 1-butanol at 293 K. Reprinted with permission from [73]. Copyright 1999 American Chemical Society

temperature is still much lower than the fluorescence quantum yield at 293 K in non-polar or polar aprotic solvents [72]. For these compounds, phosphorescence was also detected, and was located between the F1 and the F2 fluorescence bands. The excitation spectra taken at 77 K showed that the ‘normal’ F1 emission and phosphorescence have the same precursor, whereas the tautomeric emission occurs from another type of complex [72] (contrary to the behaviour observed at room temperature, where the fluorescence excitation spectra of F1 and F2 coincide with each other and with the absorption spectrum). Once more, these findings show that, in the ground state, at least two different forms of the solvates exist, only one of which is capable of undergoing phototautomerization at low temperatures (see Scheme 30.1). The F2 emission has been attributed to the cyclic 1:1 complexes, and the high-energy fluorescence and phosphorescence to ‘incorrectly solvated’ species, or ‘open’ form, with only one intermolecular hydrogen bond, which requires structural rearrangement prior to tautomerization. Such rearrangement should obviously be a function of viscosity and temperature, and therefore should be blocked at low temperatures. The behaviour of these compounds in alcohols differs from that observed in 7AI and 1-azacarbazole (1AC). In the latter two molecules, lowering the temperature causes the disappearance of the tautomeric emission band and the recovery of the radiative properties (a strong

666 Hydrogen Bonding and Transfer in the Excited State

N

N

N

N

Me

Me T-MePQ

T-MeTPC (CH2)n

N

N

N

N

Me

Me

T-MePC

T-MePyIn-n

N Me N Me T-MeHN

Chart 30.2

Chemical models for the tautomers

increase in F1 fluorescence). This has been interpreted as the DPC and PQ solvates being ‘better prepared’ for photoinduced tautomerization [72]. The driving force for excited-state proton transfer is the electron density redistribution occurring upon excitation. This redistribution leads to an increase in the excited-state pKa of the proton acceptor and a decrease in the excited-state pKa of the proton donor. Changes in pKa in the excited state have been obtained from the ‘F€ orster cycle’ [76, 77], employing the formula nA nB00 DpKa ¼ pK*a  pKG a   0:00207 ~ 00 ~




ð30:2Þ

where pK*a and pKG a denote values in the excited and ground state respectively; the electronic transition energies in the acid ~nA nB00 forms are expressed in wave numbers. Thus, for PQ it has been shown 00 and the base ~ that, upon excitation, the pyridine nitrogen becomes much more basic (DpKa ¼ þ9.6), and the acidity of the N–H group is also strongly enhanced (DpKa ¼ 6) [73]. Similar results have also been predicted by calculation using as reactivity indices [78, 79] the effective valence electron potentials [79–82]. 30.2.2 Time-resolved kinetics studies Subsequently, time-resolved kinetics studies led to the detection of a faster non-radiative process. Photophysical parameters for PQ, DPC and TPC measured at room temperature are set out in Tables 30.1 to 30.3. The time dependence of the F1 and F2 band emissions was measured, and the behaviour of the two bands was found to be quite different [83]. For instance, for detection within the F1 band, the fluorescence transient of PQ consisted of an instantaneous rise followed by a decay on the picosecond timescale. The transients were fitted to a triexponential decay function. Transients detected within the F2 emission band exhibited initially a

Excited-State Double Hydrogen Bonding

N

N

N

H H O

N

N

O H O R

H

H O

O

R

N

N

H

H

O R H

R

N H

ESDPT

H

H O R

R

N

N

R

IC

667

H

H O R

F2 h~

F'1

h~

F1''

h~

F1'

F1''

h~

N

N BDPT

H

H O R

N

N

N

H O R

O H O R

N

N

H

H

O R H

N H

H

H R

O

H O

N

N

H

H O R

R

R

Scheme 30.1 Equilibria between various types of complex for PQ, and related compounds, with protic solvents, and excited-state deactivation channels. IC: internal conversion; ESDPT: excited-state double-proton transfer; BDPT: back double-proton transfer

biexponential rise, with time constants equal to the decay constant of the F1 band, followed by a decay of several hundred picoseconds. Evidently, the time t1 and t2 are somehow related to the proton transfer process. On the other hand, the slower decay component of F1 had no counterpart in the F2 rise. The discovery of these two time parameters (t1 and t2) have made it possible to establish that the double-proton transfer process for PQ and related compounds does not occur in a simple one-step or two-step mechanism. Both mechanisms would give rise to a single decay step for the initially excited species and thus lead to a single exponential decay of the F1 band emission. These findings supported the proposal of two distinct solute–solvent species emitting the ‘normal’ F1 fluorescence. One of these is capable of undergoing tautomerization (the cyclic complex), while the other is a ‘blocked’ complex that can suffer both tautomerization and radiationless deactivation (the non-cyclic one). In other words, the excited complex, which was already cyclic in the ground state, avoids the region of rapid deactivation and undergoes tautomerization at a rate approximately an order of magnitude faster than internal conversion. However, for the non-cyclic species, an efficient radiationless deactivation channel was located on the excited-state energy surface, along the path leading from non-cyclic to cyclic species. The comparison of the photophysical parameters obtained for the phototautomer with those of its N-methylated chemical model provided more arguments for the presence of two kinds of alcohol complex deactivated via different channels (see Table 30.4). For instance, for PQ, in spite of the room temperature, the

668 Hydrogen Bonding and Transfer in the Excited State Table 30.1 Photophysical parameters for PQ in various solvents, measured at room temperature: absorption (~ nabs ) and fluorescence (~ nfl ) maxima, fluorescence quantum yields (wfl ) and decay times (t) [83]. Pre-exponential factors in parentheses n~abs (cm1)a

Solvent Methanol

d

Ethanold

30 100

1-Propanold 1-Butanola

29 700

Decanold H2Ob,e,h ACN þ H2Od,f,i Et2O þ H2Od,g n-Hexanek DMSOk ACNb Butyronitrilek Pyridinek

28 900 30 500

F1 F2 F1 F2 F1 F2 F1 F2 F1 F2 F1 F2 F1 F2 F1 F2

n~fl (cm1)b

wfl b;c

25 300 16 000 25 500d 17 200d 25 800 16 250 25 850 16 250

0.0003 0.0008

23 300 15 900 25 200 16 000 26 000 16 000 27 300a

0.00085 0.0003 0.0025 0.00055 0.01 0.001 0.25k 0.23k 0.16 0.16 0.014

25 850

0.00035 0.0011 0.0004 0.0014

t1 (ps) 0.6 0.6 0.7 0.7 0.7 0.7

(0.14) (0.31) (0.19) (0.62) (0.16) (0.46)

0.9 (0.37) 0.9 (0.66)

60e,f,g 290e,f <300 450 <400 <400 <300 600 6300 5300 4300 4300 320

t2 (ps)

t3 (ps)

6.0 (0.46) 6.0 (0.64) 9.0 (0.56) 9.0 (0.38) 7.0 (0.37) 7.0 (0.5)

41 (0.39) 170 (1.0) 76 (0.24) 230 (1.0) 87 (0.47) 270 (1.0)

9.0 (0.23) 9.0 (0.34)

300 (0.40) 413 (1.0)

a

Ref. [73]. Ref [84]. Estimated error: 20% for wfl > 0:01 and 40–50% for wfl < 0:01. d Ref [83]. e Accuracy: 20. f Measured with higher time resolution (single picoseconds). g Biexponential decay, the longer component reported. h Water plus NaOH, pH  12. i cwater ¼ 1.84 m. j cwater ¼ 0.28 m. k Ref. [95]. l [ACN: acetonitrile; Et2O: diethyl ether, DMSO: dimethyl sulfoxide.] b c

decay times of tautomeric fluorescence and of the model compound emission (T-MePQ) were found to be very similar (700 ps), while the quantum yields were not, with those of the model compound being higher (0.0008 and 0.0024 respectively [75]). Under the reasonable assumption that the radiative rates are similar in both emitting species, these results indicated that the phototautomerization yield is lower than unity; that is, there is a radiationless deactivation channel before the molecule reaches the minimum corresponding to the tautomeric structure. In other words, the incorrectly solvated complex attempts to reach the cyclic structure needed for tautomerization to occur, but it is deactivated before this structure is attained. Thus, for PQ, the fraction of the initially excited species that become tautomers has been estimated at about 40%, hence only approximately one-half of the excited alcohol complexes of PQ undergo tautomerization. The other half become deactivated in a different way. For TPC and DPC, values of 50 and 99% were obtained. The latter is approximately twice as high as the values obtained for PQ and TPC, and is explained by the presence of twice the number of hydrogen-bond acceptor centres [73].

Excited-State Double Hydrogen Bonding

669

Table 30.2 Photophysical parameters for DPC in various solvents, measured at room temperature: absorption nfl ) maxima, fluorescence quantum yields (wfl ) and decay times (t). Pre-exponential factors (~ nabs ) and fluorescence (~ in parentheses n~abs (cm1)

Solvent Methanol

c

Ethanolc

28 800

1-Propanolc 1-Butanola Decanolc ACN þ H2Oa Et2O þ H2Oa n-Hexanee DMSOe Acetonitrilea Butyronitrilee Pyridinee

n~fl (cm1)a F1 F2 F1 F2 F1 F2 F1 F2 F1 F2 F1 F2 F1 F2

wfl b

24 400 14 500 25 060c 15 100c 24 800 14 600 24 800 14 650

0.0002 0.0013 0.0004 0.0016 0.0005 0.002 0.0005 0.002

25 100 14 700 25 600 14 800

0.0034 0.0012 0.007 0.002 0.27 0.29 (0.35) 0.30 0.30 0.004

25 400

t1 (ps) 0.7 0.7 0.7 0.7 0.8 0.8

(0.21) (0.38) (0.23) (0.4) (0.28) (0.5)

0.9 (0.40) 0.9 (0.41)

t2 (ps)

250d 250d <500 500 600 700 14 000 5300 9200 9200 72

t3 (ps)

7.0 (0.56) 7.0 (0.62) 7.0 (0.40) 7.0 (0.6) 6.0 (0.36) 6.0 (0.5)

75 (0.22) 150 (1.0) 77 (0.17) 178 (1.0) 170 (0.36) 206 (1.0)

7.0 (0.34) 7.0 (0.59)

400 (0.26) 330 (1.0)

a

Ref. [84]. Ref [72]. c Ref. [83]. d Multiexponential decay. e Ref [95]. [ACN: acetonitrile; Et2O: diethyl ether; DMSO: dimethyl sulfoxide.] b

30.2.3 Theoretical results Studies performed for alcohol and water complexes with the use of molecular dynamics (MD) simulations and ab initio/density functional theory (DFT) [73, 85] have shown that, at low solvent concentration, the formation of a 1:1 cyclic, doubly hydrogen-bonded complex is predicted, and such complexes also persist in bulk solvent, along with other species such as 1:2 complexes. These studies confirm that the topology of the hydrogenbonding centres for PQ and related compounds, possessing the same structural motif as the solute ‘active’ sites, is favourable for the formation of the cyclic species in bulk alcohol and water, unlike what occurs with 7AI. It has been estimated that the relative fractions of cyclic and non-cyclic species for PQ and 7AI in bulk methanol yield values of 68 and 1% respectively [73]. This difference has been traced to the ease of formation of cyclic hydrogen bonds. As can be seen in Scheme 30.2, the two bonds involving PQ are more linear than those involving 7AI. For PQ complexes, the intermolecular hydrogen bonding between the solute and the solvent was analysed in terms of hydrogen bonding distance distribution (HBD distribution), which displays a relative frequency of the simultaneous occurrence of a pair of distances RN–H. . .O and RN. . .H–O within a hydrogen-bonded complex. The values of these distances provide a criterion to distinguish between cyclic and non-cyclic solvates. The complex is considered to be cyclic when both distances are simultaneously smaller than a certain value

(2.2 and 2.5 A have been used as the established values [85]; the population maxima occur at around 1.8–2.0 A).

670 Hydrogen Bonding and Transfer in the Excited State Table 30.3 Photophysical parameters for TPC, in various solvents, measured at room temperature: absorption nfl ) maxima, fluorescence quantum yields (wfl ) and Decay Times (t) [83]. Pre-exponential (~ nabs ) and fluorescence (~ factors in parentheses Solvent

n~abs (cm1)a

Methanol Ethanol

28 550

1-Propanol 1-Butanola

27 800

Decanol n-Hexanec DMSOg Acetonitrilec Butyronitrilef Pyridinef

28 750

n~fl (cm1)a F1 F2 F1 F2 F1 F2 F1 F2 F1 F2

28 650

wfl a,b

23 500 16 050 23 850 15 850

0.0006 0.0017

25 450

0.26 0.26 0.17 0.17 0.019

23 000

t1 (ps) 0.9 (0.19) 0.9 (0.30) 0.8 (0.35) 0.8 (0.44) 0.7 (0.48) 0.7 (0.46) 0.7 (0.46) 0.7 (0.42)

t2 (ps)

60c,d,e 190c,d 6700 6200 5000 5000 450

11.0 11.0 11.0 11.0 10.0 10.0

t3 (ps)

(0.45) (0.7) (0.37) (0.6) (0.30) (0.5)

30 (0.36) 138 (1.0) 50 (0.28) 157 (1.0) 58 (0.20) 187 (1.0)

8.5 (0.32) 8.5 (0.85)

133 (0.21) 300 (1.0)

a

Ref. [73]. Accuracy: 20–30%. c Accuracy: 20. d Measured with higher time resolution (single picoseconds). e Biexponential decay, the longer component reported. f Ref. [95]. b

Table 30.4 Photophysical parameters for the tautomer models of PQ, TPC, PC and PyIn-2 [73] Solvent T-MePQ

T-MeTPC

T-MePC T-MePyIn-2 a

n-Hexane ACN 1-Butanol Water n-Hexane ACN 1-Butanol Water ACN 1-Butanol Water ACN 1-Butanol

n~abs (cm1)

n~fl (cm1)

wfla

tflb (ps)

20 350 22 420 22 450 24 240 18 800 20 550 20 725 22 750 19 270 19 420 20 830 20 160 19 880

15 800 16 150 16 650 16 750 15 600 16 000 16 650 16 800 14 600 14 150 14 600 14 500 14 900

0.013 0.0011 0.0024 0.0009 0.0010 0.0014 0.0036 0.0032 0.0003 0.0006 0.0001 0.0001 0.0005

<700 <600 350 20c <700 <200 <400 200 20c <700 <600 60 10c <500 <200 <200

Estimated error: 40–50%. Estimated error: 200 ps. c Measured with higher time resolution (single picoseconds). b

Excited-State Double Hydrogen Bonding

H

O

R N ... H_ O

R N_H...O

H H

O

R N _H...O

R

R

Scheme 30.2

N

N

N H

N

R N ... H_ O

671

Hydrogen-bonding geometries in 7AI and PQ, and definition of RN-H. . .O and RN. . .H–O

This HBD distribution has already been shown to be useful [38]. For a PQ–(MeOH)1 complex, the cyclic complex has been found to persist for 69–88% of the time. The results obtained by addition of a second methanol molecule show that, in the case of the PQ–(MeOH)2 solvate, approximately 26–39% of the hydrogen-bonded species still corresponds to the 1:1 cyclic, doubly hydrogen-bonded complex. However, they also show the appearance of a small population of hydrogen-bonded complexes with a non-cyclic structure. From the HBD distribution [85], at least three different hydrogen-bonded complexes could be revealed: one cyclic and two non-cyclic, separately hydrogen-bonded complexes. In the two latter cases, PQ is hydrogen bonded through only one of its ‘active sites’. The relative fractions of the two non-cyclic complexes were estimated to be approximately equal. In the case of a PQ–(MeOH)2 solvate, competition between the two molecules of methanol results in loss of rigidity of the 1:1 cyclic hydrogen-bonded complex. The HBD distribution for PQ in bulk methanol is very similar to that obtained for PQ–(MeOH)2. Upon passing from a dilute solution to bulk methanol, the equilibrium between the 1:1 cyclic and various non-cyclic hydrogenbonded species is not significantly shifted towards the latter. The result obtained by simulation corroborates what is usually postulated on the basis of experimental data generated by the addition of small amounts of alcohol to a hydrocarbon solution [72, 73, 86]. When a bifunctional heteroazaaromatic molecule interacts with a hydrogen-bonding partner at 1:1 stoichiometry, the solute usually prefers to form a cyclic double hydrogen-bonded complex. For PQ–(MeOH)2 complexes, two kinds of hydrogen-bonded species were examined on the DFT level. The first was a cyclic, triply hydrogen-bonded complex containing two methanol molecules arranged in a closed ring structure (Scheme 30.3(b), left). The second was a complex also containing two methanol molecules, but in which one molecule was doubly hydrogen bonded to a solute in a cyclic way, with another methanol hydrogen bonded to the first methanol molecule [85] (Scheme 30.3(b), right). The triple hydrogen-bonded structure arranged in the closed hydrogen bond network ring was found to be more stable than the other possible hydrogen-bonded configuration. It is known that a multiple, cyclic hydrogen bonding, even including (a)

(b)

N

N

N

R

O H O R

H

H

H O

N

N

H

H

H

N

O R

R

H O R

Scheme 30.3

Geometries of (a) 1:1 and (b) 1:2 cyclic complexes of PQ with alcohol

672 Hydrogen Bonding and Transfer in the Excited State

a pseudo hydrogen bond with a hydrogen atom attached to an aromatic carbon, is in many cases found to be energetically more favourable than a separate hydrogen bond at the same stoichiometry [31, 86–88]. For PQ–water complexes, the corresponding HBD distribution for PQ–(H2O)1 and PQ–(H2O)2 are very similar to those obtained for methanol. However, in bulk water, analysis shows that the equilibrium between the 1:1 cyclic, doubly hydrogen-bonded complex and the two other separately hydrogen-bonded species (independent hydrogen bondings via both solute ‘active sites’ –N and N–H) is clearly shifted towards a decrease in the relative fraction of the cyclic hydrogen-bonded structure. The equilibrium population of the 1:1 cyclic, doubly hydrogen-bonded species for PQ in bulk water is found to be 3.5 times smaller than that of the cyclic complex in bulk methanol [85]. By combining infrared/femtosecond multiphoton ionization (IR/fsMPI) and fluorescence-detected infrared (FDIR) spectrometry in the region of hydrogen-bonded N–H and O–H stretch vibrations with DFT calculations [89, 90], three different types of H-bonded solvate for the PQ–(H2O)2 complex were examined (see Scheme 30.4): a 1:2 complex in which two water molecules form a cyclic H-bond chain connecting the pyrrole N–H hydrogen and the quinoline-like nitrogen atoms (in such an H-bonded solvate, cooperative triple proton transfer through water bridges is possible); two other isomers with a configuration corresponding to a modified 1:1 complex in which the second water molecule is hydrogen bonded to the water molecule of the cyclic 1:1 cluster either as a hydrogen-bonding acceptor (1:1 þ 1a) or as a donor (1:1 þ 1d). Only very weak IR activity was detected for the region of free O–H vibrations. However, three intense bands at 3140, 3310 and 3411 cm1 have been observed, which correspond to the hydrogen-bonded N–H and O–H stretching vibrations. These bands are significantly red-shifted in their values from those observed for the isolated molecules: 3507 cm1 for PQ90 and 3657 and 3756 cm1 for water. This demonstrates that the fluorescent species possesses three hydrogen bonds, and that at least two water molecules must be involved in the cluster. Of the three complexes containing two water molecules, the calculations for the 1:2 tallies best with the experiment, and a significant amount of the additional complexes of the same stoichiometry (1:1 þ 1a and 1:1 þ 1d) can therefore be ruled out. The calculated binding energies indicate that these clusters should be less stable by several kcal mol1 when compared with the 1:2 complex. Hence, the bands at 3411, 3310 and

N

N

N

N

H

H

H

H O H O

O H

H

N

N

H

H

N

N O H

H O H H

H

H

O

1:1 + 1a

H

1:2

1:1

1:1 + 1d

H

O H

Scheme 30.4 Schematic structures of the considered hydrogen-bonding complexes of PQ with water

Excited-State Double Hydrogen Bonding

673

3140 cm1 are assigned to the donor O–H stretch of the water dimer, the out-of-phase N-H  O and O–H  N and the in-phase ‘proton transfer’ vibrational modes respectively.

30.3 Methylene-Bridged 2-(20 -Pyridyl)indoles and Pyrido[2,3-a]carbazole (PC) 30.3.1 Steady-state measurements A different case is represented by methylene-bridged pyridylindoles [PyIn-n (n ¼ 1–4)] and PC. As can be seen in Figure 30.3, adding alcohol to a solution of PyIn-2 in a non-polar solvent also results in spectral changes that point to the formation of ground-state complexes. A new band appears on the low-energy side of the electronic spectrum, and isosbestic points are observed. This effect is quite strong for PyIn-1 and PyIn-2 and weaker for PyIn-3 and PyIn-4. Nonetheless, the same result is obtained when, instead of alcohol, dimethyl sulfoxide (DMSO), which can only act as a hydrogen bond acceptor, is added. This led us to consider that the bonding with alcohol in the ground state occurs preferentially at the N–H proton. The same conclusion was reached from IR measurements. For PyIn-2 in CCl4, the N–H stretch of the unbound molecule is located at 3473 cm1. Adding alcohol leads to a decrease in the intensity of this peak and to the appearance of a broad band at 3288 5 cm1. However, the ratio of the intensity of the O–H stretching band of the alcohol (3644 cm1) to the intensities of the other bands (not involving the O–H vibrations) does not change in mixed solutions of pyridylindoles and alcohols in CCl4 and remains the same as observed in the absence of PyIn-2. This appears to indicate that the hydroxylic proton of the alcohol and the pyridine nitrogen atom of pyridylindole are not linked by hydrogen bonding in the ground state [78]. At low alcohol concentrations, the value obtained by plotting ln[(OD  OD0)/[(OD¥  OD)] versus ln[A] (see equation (30.1)) pointed to the 1:1 stoichiometry for the complex. For higher alcohol concentrations, however, the slope of ln[(OD  OD0)/[(OD¥  OD)] versus ln[A] deviated from the values close to 1 towards larger values. This was interpreted as a sign of formation of 1:2 (or even higher-order) complexes. Moreover, these 1:2 complexes prevail over 1:1 species at relatively small alcohol concentrations. In bulk alcohol, less than 1% for the 1:1 complex was determined [78]. Again, these results, and the fact that (i) no significant spectral shifts were observed varying from mixed solutions of non-polar solvents with alcohols (where mostly 1:1 species are present) to bulk alcohols and (ii) the absorption spectra of the N-methylated derivatives in aprotic solvents are completely insensitive to the addition of alcohols, suggested that addition of alcohols to

Figure 30.3 Absorption changes observed upon adding 1-butanol to a 105 M solution of PyIn-2 in n-hexane at 293 K. The concentrations of 1-butanol ranged from 0 to 1.76  102 M. The arrows indicate increasing alcohol concentrations. Reprinted with permission from [78]. Copyright 1996 American Chemical Society

674 Hydrogen Bonding and Transfer in the Excited State

non-polar solutions of pyridylindoles does not lead to the formation of cyclic ground-state 1:1 complexes. As the alcohol concentration increases, 1:n species (n  2) begin to predominate, but in such species the second alcohol molecule is bound to the first one, which was previously involved in the complex. The absence of ground-state hydrogen bonding to the pyridine nitrogen atom can be explained from the pKa values of these PyIn-n, obtained by using spectrophotometric titration [78]. These values, which are practically the same for all the PyIn-n studied (i.e. pKa ¼ 4.4 0.2 for PyIn-2), are significantly lower than the pKa value of pyridine (5.2). The most important implication of these outcomes is that the cyclic, doubly hydrogen-bonded 1:1 complex, crucial for the understanding of the fluorescence quenching process, must be formed in the excited state. In emission, as with pyrroloquinolines (PQ, DPC and TPC), adding alcohol or water to a solution of pyridylindoles in an aprotic solvent gives rise to very strong fluorescence quenching [78, 91] (see Figure 30.4). Thus, the PyIn-n fluorescence was found to be nearly two orders of magnitude weaker in alcohols than in nonpolar and polar aprotic solvents (Table 30.5). In alcohols and other hydrogen-bonding solvents, such as pyridine, the bridged pyridylindoles exhibit very similar spectra [93]. However, lowering the temperature leads to the recovery of the emissive properties in all pyridylindoles. At low temperatures, both quantum yield (higher than 60%) and lifetimes (2.3–2.8 nm) are very similar for all the compounds, including the methylated derivatives, and practically solvent independent. Hence, the quenching process in alcohols is strongly temperature dependent. For the N-methylated derivatives, the fluorescence yields are already very high at room temperature. This confirms that the origin of the fluorescence quenching lies in the capability to form complexes with alcohol molecules [72, 73, 75, 78, 83, 91–93]. Other possible channels of excited-state deactivation, such as out-of-plane motion or 1 Lb 1 La state inversion, could be excluded on the basis of (i) comparison of the properties of pyridylindoles with those of 2-phenylindole, (ii) measurements of the same chromophore in non-polar and polar aprotic solvents and (iii) studies of various chromophores with different 1 Lb 1 La separation [94]. For a quantitative description of excitation-induced changes in hydrogen-bonding abilities, the values of effective valence electronic potentials [79–82] have been calculated using the INDO/S method [78]. These

Figure 30.4 Fluorescence changes occurring during titration of acetonitrile solutions of PyIn-2 with water at 293 K. The arrow indicates increasing water content. The water concentration ranged from 0 to 1.759  102 M. Reprinted with permission from [94]. Copyright 2000 American Chemical Society

Excited-State Double Hydrogen Bonding

675

Table 30.5 Photophysical parameters for PyIn-n (n ¼ 1–4) in various solvents, measured at room temperature: nfl ) maxima, fluorescence quantum yields (wfl ) and decay times (tfl ) [95] absorption (~ nabs ) and fluorescence (~ Solvent PyIn-1

PyIn-2

PyIn-3

PyIn-4

n~abs (cm1)

n-Hexane DMSO Butyronitrile Pyridine 1-Propanolb

29 050

1-Butanol

29 050

n-Hexaneb DMSO Acetonitrileb Butyronitrile Pyridine n-Hexane DMSO Butyronitrile pyridine n-Hexane DMSO Butyronitrile Pyridine

29 600

F1 F2 F1 F2

29 650

n~fl (cm1)

wfl a

27 800 26 000 26 700 26 200 25 500 14 900 25 500 14 900 26 350 24 900 25 700 25 600 24 900 26 500 24 900 25 700 25 300 26 200 24 400 25 200 24 900

0.42 0.71 0.48 0.03 0.006 0.0004 0.009 0.0003 0.50 0.74 0.38 0.8 0.06 0.39 0.69 0.45 0.15 0.04 0.52 0.14 0.02

tfl (ns) 1.4 2.4 1.8 0.1 <0.2 <0.2 <0.2 <0.2 1.3 2.8 1.8 1.75 0.23 1.0 3.0 1.5 0.65 <0.3 3.8 0.6 0.00014

a

Accuracy: 20–30%. Ref. [94].

b

calculations predict that pyridylindoles must be less basic than pyridine by about one pKa unit, which tallies well with the experimental results. Again, this should explain the absence of ground-state hydrogen bonding to the pyridine nitrogen atom in pyridilindoles. Another relevant result, obtained from calculations, is the prediction of a sharp increase in the basicity of the pyridine nitrogen and in the acidity of the indole proton upon electronic excitation, which appears to provide the driving force for excited-state proton transfer. The results of ground- and excited-state studies enabled the following sequence to be proposed [78]: prior to excited-state proton transfer, the 1:n (n > 1) complexes of pyridylindoles with alcohol, which are predominant in the bulk solvent, must rearrange to form a suitable conformation of the complex, a 1:1 cyclic structure. The first step, solvent reorientation, should involve expulsion of one or more alcohol molecules from the complex. The reaction may be depicted as k1

k2

* * A* !  B ! C k1

where A , B and C correspond to the primarily excited complex, the structure obtained after solvent reorientation and the phototautomer respectively (see Scheme 30.5). The rate constants k1 and k1 refer to solvent rearrangement, and k2 describes the deactivation of the ‘prepared’ structure involving proton transfer. Thus, k2 appears to depend on deuterium substitution, whereas k1 and k1 does not. The second step actually involves two processes, proton transfer and internal conversion. The population of molecules capable of undergoing proton transfer seems to be solvent controlled and to decrease rapidly upon increasing viscosity or decreasing temperature. However, at high temperatures, where solvent reorientation is rapid, the rate-limiting

676 Hydrogen Bonding and Transfer in the Excited State A*

B* C* N

N

H H O

N

N

ESDPT

H

H R

IC

N

N

O

H O

H

R

R

H O R

F2 h~

F1'

F1''

C

N

N BDPT

N

N

H H O

H O

H O

B

A

H R

N

N

H

H O R

R

R

Scheme 30.5 Mechanism proposed for the fluorescence quenching of methylene-bridged pyridylindoles. IC: internal conversion; ESDPT: excited-state double-proton transfer; BDPT: back double-proton transfer

step would correspond to proton transfer. The activation energy of the quenching process determined for various alcohols has been correlated with the activation energy of viscous flow and does not change in deuterated alcohols. This has been interpreted in terms of a solvent-controlled quenching mechanism. It predicts than the rate-determining step in the deactivation of the lowest excited singlet state involves reorientation of the alcohol molecules around an excited chromophore, necessary for the formation of a 1:1 cyclic complex with alcohol. Once such a structure has been achieved, the movement of the hydrogenbonded protons towards the tautomeric structure leads to an efficient internal conversion. An important point in this model is that deactivation is occurring during proton movement rather than after formation of the excited tautomeric form. The arguments initially supporting this hypothesis were: (i) lack of observation of tautomeric fluorescence and (ii) pre-exponential factors, obtained from the temperature dependence fluorescence, of the order of 1013 s1, typical of internal conversion [78]. 30.3.2 Time-resolved kinetics studies Initially, in transient picosecond absorption spectra of PyIn-n, no band was found that could be attributed to the tautomeric species. This was again interpreted as the deactivation of the S1 state taking place via internal conversion directly to the ground state. However, a broad absorption in alcohol solvents, extending over the whole visible range, and lasting much longer than the fluorescence, could be detected. This band, which was not observed by fluorescence spectroscopy, was assigned to the radical cation of 2-(20 -pyridyl)indoles, obtained as a result of photoionization, and pointed to yet another channel of excited-state deactivation – photoionization, occurring faster than S1 decay [93].

Excited-State Double Hydrogen Bonding

677

Subsequently, repeating the measurements on an instrument of higher sensitivity made it possible to detect an extremely weak emission that was significantly red-shifted with respect to the normal band (see Figure 30.2). This emission coincided with fluorescence recorded for 1-methyl-3–30 -dimethylene-2-(20 indolenylidene)-1,2-dihydropyridine (T-MePyIn-2), the chemical model of the phototautomer (Chart 30.2). These results leave no doubt that the low-energy fluorescence corresponds to the product of the excited-state double-proton transfer reaction in alcohol complexes. The excitation spectra obtained by separately monitoring emission intensity in the region of each band were very similar and agreed with the absorption spectra [94]. What could be discarded as a deactivation channel is the excited-state protonation on the pyridine nitrogen. Thermodynamically, such a process should be very favourable owing to the large variation in pKa upon S0 ! S1 excitation (DpKa ¼ 12.2 2.5 for PyIn-2 [94]). However, measurements performed for the protonated species reveal weak fluorescence in the region 18 000–19 000 cm1, and no such emission is observed in alcohol solutions. On the other hand, this excited-state protonation could be detected in water. Fluorescence of the monocation appeared even when only the neutral form was observed in absorption [94]. 30.3.3 Theoretical results MD and ab initio/DFT studies show that, for PyIn-2 in bulk methanol, three forms of the complex are present: one cyclic and two non-cyclic ones with comparable populations [85]. Simulations of diluted mixtures with methanol in n-hexane reveal that, both for stoichiometry 1:1 and for 1:2, a 1:1 cyclic structure is favoured, just as occurs for PQ. For PyIn-2–(MeOH)1 the equilibrium populations of the cyclic species calculated in the ground state are 50 and 72%, that is, the cyclic complex persists for 50–72% of the time, in contrast to the value obtained for PQ (69–88%). The HBD distribution for PyIn-2–(MeOH)2 reveals, by analogy with PQ, at least three different stable hydrogen-bonded solvates: a cyclic doubly hydrogen-bonded species, found, on average, 15–26 % of the time, and two non-cyclic, separately hydrogen-bonded complexes. The predominant hydrogen bonding occurs between the hydrogen atom of the solute pyrrole moiety and the methanol oxygen. This result agrees with the previous experimental observation, i.e. that ground-state hydrogen bonding occurs preferentially to the N–H group [78]. The second non-cyclic complex is mainly due to hydrogen bonding to the PyIn-2 pyridine-type nitrogen atom. The fraction of this complex is roughly twice as small as that of the former one. The hydrogen-bonding dynamic behaviour within the PyIn-2–(MeOH)2 solvates shows that neither 1:1 cyclically bonded species nor a 1:2 ‘nine-member ring’ structure is found as a long-lived stable hydrogenbonded complex. The hydrogen bonding was described in this case by continuous and frequent breaking and reforming of separate hydrogen bonds between the PyIn-2 ‘active sites’ and one of the two methanol molecules. Methanol–methanol hydrogen bonding also occurs, but the main tendency is to form two independent strong hydrogen bonds with the PyIn-2 solute. Thus, PyIn-2 can be treated as an intermediate structure between 7AI and PQ with regard to the dynamic manner of hydrogen-bonding solvation, when two molecules of methanol are available. At 1:2 stoichiometry, the 1:1 cyclic, doubly hydrogen-bonded complexes already become less favourable. However, stabilization due to solvation via a network of hydrogen bonds, which would result in a ‘nine-member ring’ structure, is not reached [85]. In bulk methanol, the equilibrium population of the 1:1 cyclically doubly hydrogen-bonded complexes of the PyIn-2 solute becomes reduced to 9–15% [73, 74, 85]. For PyIn-2, as in PQ, the triple hydrogen-bonded structure, arranged in the closed hydrogen-bonded network ring, was found to be more stable than the other possible hydrogen-bonded configuration [85]. The fact that the population of the cyclic species in bulk alcohol for PQ is at least 4 times greater than the corresponding population for PyIn-2 [73] has been explained by PyIn-2 not being flat. The calculated angle of twisting between pyridine and pyrrole moieties is approximately 12 . This twisting can facilitate the formation

678 Hydrogen Bonding and Transfer in the Excited State Table 30.6 Photophysical parameters for PC: absorption (~ nabs ) and Fluorescence (~ nfl ) Maxima, Fluorescence Quantum Yields (wfl ) and Decay Times (tfl ) Solvent

n~abs (cm1)

Methanolc Ethanolc

29 400

1-Propanolc 1-Butanold

29 150

n-Hexaned DMSOg Acetonitriled Butyronitrileg Pyridineg

29 250

n~fl (cm1) F1 F1 F2 F1 F1 F2

29 500

23 400 14 440 23 600 14 400 25 600 23 200

wfl a

tfl b (ps) 15 30

0.0005 0.0015 0.0002 0.32 0.27 0.15 0.15 0.0034

50 60 20e,f 60 10e 10 100 8900 7100 7100 110

a

Estimated error: 20% for wfl > 0:01 and 40–50% for wfl < 0:01. Estimated error: 200 ps. Ref. [83]. d Ref. [73]. e Measured with higher time resolution (single picoseconds). f Biexponential decay, the longer component reported. g Ref. [95]. [DMSO: dimethyl sulfoxide.] b c

of separate hydrogen bonds with the solvents. For PyIn-2–(H2O)1, the corresponding HBD distribution is in many aspects very similar to that obtained from methanol. In bulk water the equilibrium population of the 1:1 cyclic, doubly hydrogen-bonded species is 3 times smaller than that of the cyclic complex in bulk methanol [84]. On the other hand, pyrido[2,3-a]carbazole (PC), a molecule similar to DPC, but which lacks one pyridine ring, also has a very low fluorescence quantum yield in alcohols (0.0005 in n-propanol) compared with the values in polar and non-polar aprotic solvents (see Table 30.6). However, it exhibits an extremely weak tautomeric emission in alcohol solutions, making PC analogous to PyIn-n. All experimental evidence shows that photoexcited PC behaves in the same way as pyridylindoles (see Scheme 30.4). At first, the different behaviour of DPC with regard to PyIn-n and PC was justified by the presence of two pyridine-type nitrogen atoms in DPC. However, the fact that 7,8,9,10-tetrahydropyrido[2,3-a]carbazole (TPC), which possesses the same topology of –N and N–H groups as pyridylindoles and PC, emits double fluorescence in alcohols, in the same way as DPC and PQ, indicates that the situation is more complex [92].

30.4 Fluorescence Quenching by Electron Transfer in Pyrroloquinolines and PyIn-n It is worth mentioning that it is not only hydroxylic solvents that are efficient in fluorescence quenching. As can be seen in Figure 30.5, the addition of pyridine to a solution of these compounds in non-polar solvents causes a red-shifting in absorption spectra and the appearance of isosbestic points, indicating the presence of two ground-state species in equilibrium. No changes are detected, however, upon adding pyridine to solutions of Nmethylated derivatives, which makes it possible to conclude that the spectral changes are due to the formation of hydrogen-bonded complexes with pyridine [95]. Moreover, a comparison of the photophysical parameters, determined in n-hexane (non-polar solvent), DMSO (polar aprotic solvent) and pyridine for these compounds,

Excited-State Double Hydrogen Bonding

679

Figure 30.5 Spectrophotometric titration of PyIn-2 (left, c ¼ 2  105 M) and PQ (right, c ¼ 8  105 M) in nhexane with pyridine. The arrows show spectral changes accompanying the addition of pyridine. The pyridine concentration increased up to 0.08 M for PyIn-2 and up to 0.4 M for PQ. Reproduced in part with permission from [95]. Copyright 2002 American Chemical Society

reveals that, in spite of the fact that the shape and location of the fluorescence maxima in DMSO and pyridine are similar, for pyridine the quantum yield is much lower and the excited-state lifetime much shorter (Tables 30.1 to 30.6). Furthermore, the decrease in fluorescence intensity is greater when quinoline is added. The pKa values of pyridine and quinoline are very similar (5.2 and 4.9 respectively [96]), whereas their reduction potentials are not. Quinoline is a much better electron acceptor than pyridine (the reduction potentials are 2.0 and 2.66 V respectively [97]). None of these changes is observed for N-methylated derivatives. This result eliminates the possibility of quenching being due to the formation of a non-hydrogen-bonded exciplex, as with indole [98], and supports the model of quenching by electron transfer occurring as a result of intermolecular hydrogen bonding [99–103]. According to this mechanism, hydrogen bonding may lower the ionization potential of the donor and increase the electron affinity of the acceptor, thus facilitating photoinduced electron transfer (ET) between the two moieties (see Scheme 30.6). The presence of low-lying CT states was predicted by calculations [95]. However, transient picosecond studies for pyridine solutions did not detect the presence of any transient band in the visible region that could be assigned to a CT state [93]. Also, no spectral changes were observed that would indicate a proton transfer along the hydrogen bond. It is thus likely that the back electron transfer rate from a CT state is larger than the rate of the CT state formation. In summary, the addition of compounds capable of forming hydrogen bonding in solutions of the abovementioned N-azaaromatic derivatives leads to fluorescence quenching resulting from changes in the electronic distribution upon excitation, which alter the acid/basic properties in both nitrogen atoms. This quenching is mainly caused by two rapid processes, phototautomerization and internal conversion. Excited-state double-proton transfer is favoured in cyclic species, while non-cyclic complexes are more easily deactivated via internal conversion. For the pyrroloquinolines PQ, DPC and TPC, the presence of cyclic 1:1 species is dominant even in the ground state; however, for pyridylindoles and PC, such structures are more readily formed upon electronic excitation, and thus pyridylindoles can be treated as structures intermediate between 7AI and pyrroloquinolines. Accordingly, one can conclude that the ‘successful’ and ‘aborted’ excited-state proton transfers are complementary to each other. Which of them is dominant depends on the relative fractions of differently hydrogen-bonded ground-state solvates [74].

680 Hydrogen Bonding and Transfer in the Excited State

(+) N

N

H

H N (_)

N

S1(LE)

h~

N

N

ET

1

CT

BET 10 >10 s-1

N

N

S0

H N

Scheme 30.6 Mechanism of fluorescence quenching by pyridine and derivatives. ET: electron transfer; BET, back electron transfer

30.5 Betacarboline Derivatives Betacarboline derivatives (H-pyrido[3,4-b]indol derivatives) (BCDs) make up a group of drug-binding alkaloids, widely distributed in nature [104–111] with interesting biological and photophysical properties. Their biological activities, such as intercalating into DNA [112, 113] and inhibiting CDK [114], topisomerase [115, 116] and monoamine oxidase [117, 118], produce a broad spectrum of pharmacological properties, including anxiolytic, hypnotic, anticonvulsant [119–121], antiviral [122] as well as antimicrobial activities [123]. The interactions of BCDs with biological receptors are not yet well understood. These receptors have both proton-donating and proton-accepting groups, hence it can be assumed that their biological activity occurs through hydrogen bond formation. The interesting photophysical properties that these derivatives present were always associated with photoinduced changes in their electronic distribution, which modify the acidic and basic properties of the two nitrogen atoms at the betacarboline ring and lead to a variation in its reactivity. Upon excitation to the S1 state, the pyridinic nitrogen (Npd) and pyrrolic group (NprH) become more basic and more acidic respectively. This may give rise to a double proton transfer when both hydrogen-bonding donors and hydrogen-bonding acceptors are present in the environment surrounding this molecule. This double transfer induces a phototautomerization process, thus complicating the photophysicochemistry of these derivatives. Scheme 30.7 summarizes some of the species whose existence has been proposed to date to explain this photophysicochemistry.

Excited-State Double Hydrogen Bonding 4

5

6

3

N

7 8

N9 H

681

+ N H

2

1

+ N H

N

Me

NEUTRAL

N _

Me

H CATION

Me

ZWITTERION (Z)

N N H

BCD-

N

N

N

N _

H

H

N

N + Me

Me Me BCN-N

Scheme 30.7 derivatives

Me PTC-N

Some of the proposed species involved in the photophysicochemistry of the betacarboline

The mechanism of formation of these species has evolved through research into the spectroscopic properties of different methylated BCDs. The main controversy involves establishing the structure, kinetics and formation mechanism of species with an emission band at around 520 nm. Initially, this emission was observed in aqueous solutions of BC at pH values greater than 12. A zwitterionic structure (Scheme 30.7) was associated with this emission, in which the pyridinic and pyrrolic nitrogen atoms are protonated and deprotonated respectively. Obviously, it is easy to predict that this compound will be stable only in highly polar media, and that its formation requires the presence of both hydrogen-bonding donors and acceptors in the medium surrounding this molecule. However, this emission has been observed in the presence of one single hydrogen donor, as in the case of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP). In this review, we have considered four BCDs: BC (9H-pyrido[3,4-b]indole), HN (1-methyl-9H-pyrido[3,4b]indole), MHN (N9-methyl of HN) and T-MeHN (N2-methyl of HN) (Charts 30.1 and 30.2). Firstly, the interactions of the pyrrolic group with different aromatic and non-aromatic bases are described. There are two different interactions that appear in these systems and must be considered: NprH–p and NprH–N. The spectroscopic properties of BCDs and these interactions can be correlated by means of steady-state and timeresolved techniques. Npd interactions can be studied with different hydrogen donors, and, to gain further insight into the hydrogen-binding ability of the BC ring, the photophysicochemistry of these derivatives can be

682 Hydrogen Bonding and Transfer in the Excited State

performed in polar and low-polarity media in the presence of a hydrogen-bonding donor-acceptor, such as acetic acid. 30.5.1 Spectral changes induced by the interactions between BCDs and hydrogen-bond acceptors 30.5.1.1 Interactions of the NprH–p Type Benzenoid-p Bases Mun˜oz et al. [124] conducted UV-visible and FTIR studies of the interactions of BC and HN with benzenoid-p bases such as benzene (BN), naphthalene (NP) and phenanthrene (PN) (Chart 30.3). These derivatives are very adequate substrates with regard to studying p-stacking and NprH–p interactions of BC without the interference of NprH–N hydrogen bonding. These polycyclic compounds possess similar p-electron density; nevertheless, the size difference will help to measure the steric hindrance produced by the 1-methyl group of HN in the interaction of the NprH group with the different benzene rings. The addition of increasing amounts of benzenoid derivatives causes a slight hypochromic effect on the UVvisible spectrum of BC (Figure 30.6). This effect has often been related to a molecular pairing caused by p–p stacking interactions. However, more significant changes are observed in the FTIR spectra of BC. Upon the

BN

CH3

CH3

CH3

CH3 toluene

PN

NP

m-xylene

H3C

CH3

H3C

CH3

H3C

CH3

mesitylene

durene CH3 CH3

N

N

PY

QN

N

N

N

N LPY

PD

F N N PM

N

N PZ

PDA

Chart 30.3

F F

F N FPY

F

Excited-State Double Hydrogen Bonding

683

Figure 30.6 Hypochromic effects in the UV-visible absorption spectrum of BC (8.4  103 M) with increase in NP from 0 to 0.705 M. Path length 0.5 mm. Reprinted with permission from [124]. Copyright 1999 American Chemical Society

addition of increasing amounts of BN, NP or PN, the N–H stretching band at 3461 cm1 decreases in intensity, and simultaneously a new red-shifted band appears together with an isosbestic point (Figure 30.7). This behaviour was related to the interaction of benzene derivatives and BC, in which the NprH group is engaged. Typically, when the XH bond of the donor molecule interacts with the lone electron- pair of the B acceptor, the XH bond weakens, and consequently its stretching vibration band in the IR spectrum shows low-frequency shifts that strongly depend on the nature and the strength of the molecular acceptor. In this case, similar spectral shifts were obtained, 36–39 cm1, in agreement with the similar electronic density of BN, NP and PN.

Figure 30.7 Changes in the IR spectrum of BC with increase in (a, left) BN (0.141–0.846 M), [BC] ¼ 8.16  103 M, (b, middle) NP (0.141–0.705 M), [BC] ¼ 8.44  103 M, and (c, right) PN (0.094–0.567 M), [BC] ¼ 8.49  103 M. Reprinted with permission from [124]. Copyright 1999 American Chemical Society

684 Hydrogen Bonding and Transfer in the Excited State

Considering the formation of a complex (C) according to equilibrium M þ nB $ C (where M represents the uncomplexed BCD and B is the hydrogen-bonding acceptor), using the relation for the stability constant of the complex Ks ¼

½C ½M½Bn

ð30:3Þ

and the mass conservation law (where [M]0 is the initial concentration of M) ½M0 ¼ ½M þ ½C and considering that the absorbance is proportional to the concentrations (in accordance with the Lambert–Beer law), the well-known Benesi–Hildebrand equation can be derived:  0  AM log  1 ¼ log Ks þ nlog½B AM

ð30:4Þ

where A0M and AM are the initial absorbance of BC at 3461 cm1 and after the addition of varying concentrations of the benzene derivative B respectively, Ks is the association constant and n is the stoichiometry of the complex. The plots according to this equation were linear up to at least 102 M of benzenoid [124] with slopes close to unity (1:1 stoichiometry). Moreover, the molar extinction coefficients of the complexes were obtained [124] from the equation log

AC «C ¼ log Ks þ n log½B AM «M

ð30:5Þ

where AC represents the absorbance at maximum wavelength of the associated bands and «C and «M are the extinction coefficients of the complexed and uncomplexed BC respectively. The plots, according to this equation, showed that the extinction coefficients of the complexes are of the same order of magnitude as those of the free BC (225 7 M1 cm1). The association constants are reported in Table 30.7. These data show that the interactions increase almost linearly with the number of benzene rings of the p-bases. Therefore, apart from the strength of the base, another effect must exist that affects the stability of the complex. Semi-empirical calculations (AM1/MOPAC) [125] showed a minimum energy (1.2 kcal mol1) at equilibrium distances of 2.3 A for the T-shaped hydrogen-bonded complexes (BCD–p, Scheme 30.7). Conversely, no minima were found for stacking structures. Additionally, these calculations also showed that the interaction energy of the pyrrolic hydrogen with the different benzenoids is the same. These results do not corroborate the data obtained experimentally, where Ks increases with the number of benzene rings of these derivatives. Mun˜oz et al. [124] ascribed these higher constants to an enhancement of the entropic contribution to the free energy on increasing the number of benzene rings in the molecule. Thus, the increase in the experimental association constant values from BN to PN (Table 30.7) is related to the chance of interaction as the number of rings increases, rather than a strengthening of the binding forces. In the case of the complexes of HN with these benzenoids, smaller values of the association constants were obtained for the NprH–p hydrogen-bonded complexes (Table 30.7). It is reasonable to associate these results

Excited-State Double Hydrogen Bonding Table 30.7

685

Ground-state association constants and quenching data for the interaction of BCDs with different bases BC D~ n (cm1)

Benzene Naphthalene Phenanthrene Toluene m-Xylene Mesitylene Durene Pyridine Quinoline Phenantridine Pyrimidine Pyrazine Pyridazine

39 36 37 47 55 64 73 320 303 314

Ks (M1) 0.16 0.06 0.42 0.06 0.65 0.04 0.22 0.03 0.26 0.01 0.34 0.02 0.29 0.01 1.86 0.03 1.86 0.09 2.04 0.09

HN KSV

D~ n (cm1)

Ks (M1)

0.18

30 29 28 36 42 47 51 320 322 318

0.145 0.006 0.20 0.01 0.23 0.01 0.12 0.04 0.17 0.05 0.17 0.03 0.2 0.1 2.5 0.2 1.86 0.09 1.86 0.09 4 5.0 0.8

0.82 1.7 7.7

kq (109 M1 s1)

5.1 5.4 9.9

with a steric hindrance of the 1-methyl group of HN. Thus, the suppression of this entropic factor should have a levelling effect on the values of the association constants of the complexes. Methylbenzene Bases Balo´n et al. [126] also studied the interactions of BCDs in the presence of another series of p-bases such as benzene, toluene, m-xylene, mesitylene and durene (see Chart 30.3). In this case, these methylbenzene derivatives represent a series of homologue compounds with different electronic densities, while the ‘p-surface’ remains practically unchanged. The addition of increasing amounts of these derivatives also showed a slight hypochromic shift in the UVvisible absorption spectra, and showed more significant changes in the FTIR spectrum. As the number of methyl groups in the benzene ring increases, the red-shifts associated with the N–H stretching band become more pronounced. One could therefore establish a correlation between the p-electron density of the acceptor and the red-shift (Table 30.7). That is, a correlation between the frequency shift and the basicity of the acceptor can be applied. The plots according to equation (30.5) are linear. A 1:1 stoichiometry was obtained for these complexes, with extinction coefficients of the same order of magnitude as the aforementioned free BC, and the association constants progressively increase with the number of methyl groups in the benzene ring. The NH–p interactions also affected the fluorescence of BC. The fluorescence profile and maxima did not appreciably change when pure methylbenzene derivatives were used as solvents. However, fluorescence quantum yields appreciably diminish as the hydrogen bonding p-acceptor character of the solvent increases. On the other hand, fluorescence decays monoexponentially in all these solvents, with lifetimes close to that of cyclohexane, i.e. 2.9 ns. The addition of benzene to the solutions of an N9-methyl-BC that has blocked the NprH hydrogen-bonding donor centre produces a very slight quenching. Balo´n et al. [126] also studied the influence exerted by the progressive addition of benzene and its methyl derivatives on the fluorescence spectrum of BC in cyclohexane. At low benzene concentrations ( up to 0.05 M), fluorescence intensity increased (Figure 30.8). At higher concentrations, however, fluorescence is quenched and slightly red-shifted. Fluorescence decay can be described by a linear combination of two exponentials with very similar lifetime values. This behaviour is consistent, assuming a system formed by a mixture of two independently emitting species, described in Scheme 30.8.

686 Hydrogen Bonding and Transfer in the Excited State

Figure 30.8 Changes in the fluorescence spectrum of BC (4  105 M) in cyclohexane with increase in benzene concentration: (—) 0 M, (   ) 0.01 M, (– –) 0.03 M and (––) 0.05 M. Reprinted with permission from [126]. Copyright 2001 Elsevier

The quenching of BC fluorescence, induced by the methylbenzene derivatives, increases with the number of methyl groups in the benzene ring. A mechanism proposed for this quenching involves an electron transfer in the excited state, induced by hydrogen bonding interactions, which leads to a non-fluorescent exciplex. According to this mechanism, Balo´n et al. [125] related the quenching efficiency of the methylbenzene derivatives to both the hydrogen-bonding acceptor and the electron-donor properties, which are enhanced by the increase in p-electronic density. This quenching can be fitted to the Stern–Volmer equation I0 ¼ 1 þ Ksv ½acceptor I

ð30:6Þ

where I0 and I are the fluorescence intensity in the absence and in the presence of the benzenoid acceptor. In Scheme 30.8, assuming that static quenching is due only to the formation of a ground-state non-fluorescent complex, and considering that the fluorescence intensity is proportional to concentration, KSV represents the stability constant of the complex, Ks. KSV values obtained by this method also increase with hydrogen-bonding strength of the benzenoid (Table 30.7). The 1-methyl derivative of BC, HN, interacts with the methylbenzenes similarly to BC. However, the redshifts of their associated bands and Ks are smaller than those of BC complexes owing to the fact that the methyl group in HN sterically hinders hydrogen bond formation.

BCD*

BCD-

KS BCD + -acceptor Scheme 30.8

BCD-

*

Excited-State Double Hydrogen Bonding

687

30.5.1.2 Interactions of the NprH–N Type Benzopyridinic Bases. In low-polarity and weakly or non-hydrogen-bonding solvents, such as cyclohexane, chloroform, carbon tetrachloride, toluene and benzene, BCD and pyridine (PY) form a 1:1 hydrogen-bonding complex in both the ground and singlet excited state. The formation constants are greater in the excited state than in the ground state. The stabilities of the hydrogen-bonding complexes diminish as the polarity and hydrogen-bonding ability of the solvent increase. To demonstrate these conclusions, Balo´n et al. [127] compared the spectroscopic behaviour of two BCD: HN and MHN. The UV-visible absorption spectrum of HN in cyclohexane in the presence of increasing amounts of PY is red-shifted (10–15 nm). Conversely, MHN only showed a very slight hypochromic effect in the absorption. Ks was evaluated by using FTIR spectra, and the influence of PY addition on the infrared stretching band of N–H (3472 cm1) was examined. Because of the low solubility of HN in cyclohexane, these studies were conducted in chloroform as a solvent. The free N–H stretching band at 3472 cm1 decreases in intensity, and simultaneously a broad new band grows at approximately 3100–3200 cm1. From the plots of equation (30.5), the slope confirms the 1:1 stoichiometry of this complex. Ks was also evaluated from the UV-visible absorption spectra using equation (30.7), which assumes 1:1 stoichiometric binding [128]: 1 1 1 1 1 1 ¼ þ DA ð«C  «M Þ ½M0 Ks ð«C  «M Þ ½M0 ½PY

ð30:7Þ

where «C and «M are the extinction coefficients of the complexed and uncomplexed BC, respectively, at the titration wavelength, and DA represents the change relative to the completely free HN at this wavelength. The plots according to this equation provide straight lines. From the slopes and intercepts, the Ks values were calculated in cyclohexane. The values obtained from FTIR (1.9 M1) are very close to those obtained by UVvisible spectrometric titrations (2.5 M1 at 25 C). The fluorescence spectra of different HN–PY mixtures were used to obtain the value of this association constant in the first excited state, KE. The fluorescence was strongly quenched, even at very low PY concentrations. At a very high PY concentration, 1 M, where only the complex is present in the solution, a very weak and red-shifted fluorescence is observed. The constant KE in the excited state could be estimated by using the F€ orster equation: log

KE 0:625 D~n ¼ T Ks

ð30:8Þ

where D~n is the average of the spectral shifts, in cm1, between the maxima at the longest wavelength of the absorption and the fluorescence spectra of the hydrogen-bonded complex and the free HN respectively. KE was estimated at 549 M1. Comparing KE with Ks ( 2 M1) provides an insight into the strength of interactions between HN and PY in both states. The strengthening of the HN–PY hydrogen bonding in the excited state indicates a significant decrease in charge density on the pyrrole nitrogen atom when HN is excited to S1. To obtain information on the fluorescence quenching mechanism, HN–PY solutions in cyclohexane were excited at the isosbestic point of their absorption spectra. Single exponential fluorescence decay was also gathered (361 or 371 nm). The change in the HN fluorescence lifetime owing to increasing PY concentration follows the Stern–Volmer equation: t0 ¼ 1 þ KD ½PY t

ð30:9Þ

688 Hydrogen Bonding and Transfer in the Excited State BCD* + N-acceptor

Kq

BCD- *

K_q

BCD + N-acceptor

KS BCD-

Scheme 30.9

where t0 and t are the fluorescence lifetimes in the absence and in the presence of PY respectively, and KD is the dynamic quenching constant of the complex in the S1 state. The above results indicate the simultaneous presence of static and dynamic contributions to the total quenching process. The quenching data were analysed according to the kinetics proposed in Scheme 30.9, commonly used to describe fluorescence quenching through complexation processes in both the ground state and the excited state. The complex formed is very weakly fluorescent. The observed single exponential decay can be explained [129] by assuming that kq kq, and therefore, in equation (30.9), KD ¼ kqt. When both static and dynamic quenching occur simultaneously, a deviation from linearity of the plot of I0/I versus concentration is observed. Furthermore, under photostationary conditions, and when exciting at an isosbestic point, the above assumptions lead to equation (30.10) for the steady-state intensities of fluorescence:
1 ¼ ðKs þ KD Þ þ Ks KD ½PY ½PY

I0 I

ð30:10Þ

The Ks and KD values determined from the slopes and intercepts of these plots are in good agreement with those independently obtained from absorption and time-resolved fluorescence measurement respectively. Interestingly, PY weakly quenches the fluorescence intensity of the MHN, but does not affect its fluorescence lifetime. Hypochromism has been frequently attributed to ‘vertical’ or p–p stacking interactions between two chromophores [130]. The small magnitude of these interactions and the hypochromic effects observed in the UV-visible absorption spectra of this system point to the formation of weak p–p stacked complexes bonded by dispersive van der Waals forces. The HN–PY hydrogen-bonding interactions are markedly dependent on solvent nature. Thus, in weakly hydrogen-bonded solvents, such as chloroform, carbon tetrachloride, chlorobenzene and toluene, the UVvisible spectra are entirely similar to cyclohexane. Conversely, polar aprotic solvents such as acetonitrile, or polar solvents such as water, produce slight shifts in the UV-visible absorption spectrum of HN, even at PY concentrations higher than 2 M. In acetonitrile, PY also quenches the HN fluorescence, although differently; the quenching data do not follow equation (30.10), but rather the simplest Stern–Volmer equation I0/I ¼ 1 þ KD[PY], which is derived from the assumption the Ks ¼ 0. This indicates that solvation by polar solvent molecules hinders the groundstate hydrogen-bonding interaction. For instance, the greater destabilizing influence of acetonitrile could be related to its hydrogen-bond-accepting ability, which competes with the HN–PY interaction. The changes observed in the UV-visible spectrum of BC by the addition of the benzopyridine derivatives quinoline (QN) and phenanthridine (PD) [124] are similar to those observed for PY and different from those shown by the benzenoide derivatives BN, NP and PN. Thus, the UV-visible spectrum is broadened and bathochromically shifted as the concentration of benzopyridine increases. These changes are expected for the formation of NprH–N hydrogen-bonding

Excited-State Double Hydrogen Bonding

689

complexes. In the FTIR spectrum, the N–H stretching band of BC diminishes in intensity, while a broad new band grows around 3100–3200 cm1. The association constants estimated from equations (30.4) and (30.5) are reported in Table 30.7. From these data, Balo´n et al. concluded that the strength of the hydrogen-bonding interaction does not appreciably change when benzene rings are added to the active pyridine centre. This behaviour is to be expected if we take into account the similar hydrogen-bonding acceptor properties reported for PY, QN and PD [131]. The same study was also conducted for HN derivative. The results of this study show that HN interacts with benzene and pyridine in a similar fashion to BC. However, whereas the 1-methylation of BC has an important destabilizing effect on the benzenoid complexes, it has only a minor effect on the benzopyridinic ones. Taking into account the steric hindrance of the 1-methyl group, these results suggest that the two types of complex have different geometrical constraints. Diazine Bases. Pyrimidine (PM), pyrazine (PZ) and pyridazine (PDA) are part of a very important group of compounds that have been extensively studied owing to their occurrence in living systems. The hydrogen bonding-acceptor b parameters for these diazines [131] are 0.65, 0.62 and 0.81 respectively. Similarly to observations in the presence of PY, in the ground state, the absorption spectrum of HN in cyclohexane, toluene or chloroform is red-shifted and slightly broadened with increased diazine concentrations [132]. The FTIR spectra obtained were similar to those observed with PY. Regarding the steady-state and time-resolved fluorescence measurements, neither the shape nor the maxima of the UV-visible spectra changed. Fluorescence intensities progressively diminish with a monoexponential decay, decreasing the lifetime, with an increase in diazine concentration. It is inferred that HN interacts with PM and its isomers (PZ and PDA) in its ground state and in its lowest singlet state. These results reveal the simultaneous presence of static and dynamic components in the quenching mechanism. Therefore, quenching data were analysed by the model depicted in Scheme 30.9, leading to equations (30.9) and (30.10). The values of Ks and kq obtained from these equations are reported in Table 30.7. Other Benzopyridinic Bases. A photophysical study of BC was conducted [133] with three pyridine derivatives (PYD) with increasing hydrogen-bond donor properties, pentafluoropyridine (FPY), pyridine (PY) and 3,4-dimethylpyridine (LPY), the Marcus hydrogen-bonding acceptor parameters [134] of which are 0.16, 0.64 and 0.78 respectively. In this case, when using cyclohexane, the UV-visible absorption spectrum initially undergoes a hyperchromic shift at low PYD concentrations, which is further followed by a batochromic one at higher PYD concentrations (Figure 30.9). These changes suggest the sequential formation of two different BC–PYD hydrogen-bond complexes. This behaviour can be explained by assuming a double-minimum potential for the position of the proton in the NprH–N bridge. These two minima can be represented by the tautomeric equilibrium between two species: HBC (NprH–N) and its ion-pair proton transfer complex PTC (Npr–Hþ N). At low PYD concentrations the slight hyperchromic shift can be attributed to the formation of the HBC, whereas at high PYD concentrations the further batochromic shift is attributable to the formation of the PTC. The absorbance data obtained in different regions of the absorption spectra and at different PYD concentrations were analysed by the standard equation (30.11) [102] based on the 1:1 hydrogen bonding complex:
1 AA0 « M A0 ¼ Ks þ ð30:11Þ Ks ½PYD «C A where A0 and A are the absorbance at the monitored wavelength of the solutions before and after PYD addition respectively. At low PYD concentrations, Ks1 ¼ 299 9 M1, which is 10 times greater than that obtained at

690 Hydrogen Bonding and Transfer in the Excited State

Figure 30.9 Changes in the absorption spectra of BC upon the addition of increasing concentrations of: (a) FPY: 0, 4  102 and 0.1 M; (b) PY: 0, 4  103, 1  102, 2  102, 5  102 and 0.1 M; (c) LPY: 0.4  103, 1  102, 2  102 and 5  102 M. Reprinted with permission from [133]. Copyright 2007 Elsevier

higher PYD concentrations, 24.1 0.4 M1. In any case, comparison of different Ks values showed that absorbance increases with the hydrogen-bonding acceptor strength of the pyridinic substrate. Furthermore, all the PYD studied quench BC fluorescence (Figure 30.10). Quenching by FPY is preceded by an initial slight increase in fluorescence intensity. Stern–Volmer plots of BC relative quantum yields, measured as the areas under the fluorescence spectra versus the concentration of PY and LPY derivatives, show upward deviations. This reveals the presence of static and dynamic components in the quenching process. The extended Stern–Volmer plots for simultaneous static and dynamic quenching according to equation (30.10) provide meaningless results.

Excited-State Double Hydrogen Bonding

691

Figure 30.10 Changes in the emission spectra of BC, lexc ¼ 325 nm, upon the addition of increasing concentrations of: (a) FPY: 0–0.45 M; (b) PY: 0–0.1 M; (c) LPY 0–0.1 M. In the insets, plots of the areas under the fluorescence spectra against FPY concentration (a) and normalized fluorescence spectra of free (—) and quenched BC ([PY] ¼ 0.9 M) (– –) (b) are shown. Reprinted with permission from [133]. Copyright 2007 Elsevier

Regarding time-resolved fluorescence measurements, the PYD additions modify the fluorescence decays and constitute bi- or triexponential functions depending on the nature of PYD and on its concentration. BC in cyclohexane presents a monoexponential decay at around 2.9 ns. Upon the addition of FYP it becomes biexponential, and, at higher FYP concentration, triexponential. Conversely, for the BC–LPY system, triexponential functions were fitted for most of the LPY concentration range. From the biexponential decays, a medium lifetime at around 3 ns and a very short lifetime (0.2 ns) almost in the resolution limit of the device were measured. Upon increase in the acceptor concentration, the medium lifetime decreases, whereas the very short lifetime shows no appreciable change. The third lifetime is a minor component (ca 4 ns), which is constant with an increase in LPY. These experimental results suggested an equilibrium between two different BC–PY hydrogen-bonding complexes: HBC and PTC. It is well known that the equilibrium position of this complex type depends on factors such as solvent polarity, temperature,

692 Hydrogen Bonding and Transfer in the Excited State

HBC- *

BCD* + N-acceptor

1/

1/

BCD

KS1

BCD + N-acceptor

PTC-N*

BCD-N

1/

PTC-N

KS2

HBC-

PTC-N

Scheme 30.10

concentration and donor and acceptor properties of the solute. HBC slightly modifies the absorption and fluorescence spectra of BC, whereas PTC shifts the absorption and fluorescence spectra batochromically. A model as represented in Scheme could explain these time-resolved results. At low PY concentrations, when PTC was negligible, the fluorescence decay was fitted to a biexponential function. At higher PY concentrations, once PTC formation had become appreciable, a triexponential decay was gathered for most of the monitored emission wavelengths. The new lifetime component, ca 4 ns, unobserved in the biexponential fits, was associated with PTC emission. Non-aromatic Acceptor. Interactions of HN with hydrogen-bond acceptors such as tetrahydrofuran (THF), N,N-dimethylformamide (DMF) and hexamethylphosphoramide (HMPA) produced a similar effect to the ones previously studied [135]. Thus, the absorption and fluorescence spectra of HN were red-shifted (around 10–15 nm) as the concentration of acceptor was increased. Moreover, the hydrogen-bonded complexes of N–H with this non-aromatic acceptor fluoresced as intensely as free HN. The lifetimes of the monoexponential decays were very close to the non-bounded HN (2.9 ns) and rose slightly with increase in acceptor concentration. Attempts to fit to a double-exponential decay led to unexplained negative contributions to the overall decay. To explain such results, Balo´n et al. [135] considered the artificial contribution of attempting to separate two lifetimes differing by less than 1 ns. Hence, this system is interpreted by assuming a system formed by a mixture of two independent emitting species with very similar fluorescence lifetimes, as in Scheme 30.8. As expected, MHN did not show any variation in their spectra. From all these studies, mainly reported by Balo´n et al., it can be established that NprH–p and NprH–N interactions quench the fluorescence of BC and HN. In this quenching process, the hydrogen bonding between the bases and the pyrrolic hydrogen of the betacarboline ring plays an essential role, as blocking by methylation of the betacarboline pyrrole group suppresses the quenching process. This interaction leads to the formation of a very weak fluorescent exciplex with a very short lifetime that cannot be measured with a device of nanosecond resolution. 30.5.2 Spectral changes induced by the interactions of BCDs with hydrogen-bond donor derivatives (OH–Npd) The lone electron pair of the pyridinic nitrogen atom has excellent hydrogen bonding properties and allows BCDs to interact with a variety of hydrogen-bond donors. Spectroscopic studies of the interactions of HN, MHN and T-MeHN derivatives with different hydrogenbonding acceptor/donor molecules in non-polar solvents such as cyclohexane and non-polar/polar mixtures have been reported in different papers [136–144]. Among these, an interesting and systematic study of the interactions of HN and MHN with different hydrogen-bonding donors such as HFIP, 2,2,2trifluoroethanol (TFE), 2-chloroethanol (ClEtOH) and tert-butyl alcohol (t-BuOH) has been performed in cyclohexane solutions [135]. TFE and HFIP are strong proton donors with very weak acceptor properties. The hydrogen-bonding donors were selected on the basis of their hydrogen-bonding donor capabilities measured

Excited-State Double Hydrogen Bonding

693

by the a descriptors of Marcus [134]: 1.96, 1.51, 1.28 and 0.42 for HFIP, TFE, ClEtOH and t-BuOH respectively. For MHN in cyclohexane, the addition of strong hydrogen-bonding donors in low concentrations had a slight hypochromic effect on the absorption maximum, with no appreciable shifts in its position. At higher donor concentrations, the absorption bands were red-shifted, and an isosbestic point was observed. These progressive changes suggested that two different complexes were formed at low and high donor concentrations. Related changes were also observed in the fluorescence spectra. The addition of small amounts of hydrogenbonding donors caused a progressive increase in fluorescence intensity together with a slight shift to the red. At higher hydrogen-bonding donor concentrations, a new band appeared with a maximum at approximately 414 nm (Figure 30.11). The latter emission was only observed in the presence of a strong hydrogen-bonding donor such as HFIP. In solvent mixtures of cyclohexane with ClEtOH or TFE, this phenomenon was not observed. The fluorescence decays of MHN–HFIP and MHN–ClEtOH mixtures were measured. In cyclohexane they were found to be monoexponential, t0 ¼ 2.9 ns, and became clearly multiexponential in the presence of a hydrogen-bonding donor. Table 30.8 shows the results of a biexponential global analysis, with t1 and t2 fixed at 2.1 and 3.7 ns respectively, for the fluorescence decays at low HFIP concentration (around 104 M). With an increase in alcohol concentration, a1 decreased while a2 increased. The longer lifetime of around 3.7 ns was assigned to the fluorescence of a hydrogen-bonded complex (HBC). These results were interpreted on the assumption of a system formed by a mixture of two independently emitting species. At the highest HFIP concentrations, and monitoring the fluorescence at 440 nm, the decays were biexponential, with a long lifetime (14–15 ns) component increasing and a short lifetime component decreasing with increase in HFIP concentration. Moreover, the short component appeared as a negative pre-exponential (see Table 30.8). The behaviour of the MHN–t-BuOH system was simpler. The decays were biexponential in the entire range of t-BuOH concentrations (0–1 M) and similar to those mentioned above for HFIP and ClEtOH at low concentration.

Figure 30.11 Changes in the fluorescence spectrum of MHN in cyclohexane with increase in the HFIP concentration: (—) 0.01 M, (  ) 0.02 M, (– –) 0.03 M and (– –) 0.09 M [135]. Reproduced by permission of the PCCP Owner Societies

694 Hydrogen Bonding and Transfer in the Excited State Table 30.8 Fluorescence lifetimes and pre-exponential factors (in parentheses) of MHN at different low (lem ¼ 366 nm) and high (lem ¼ 440 nm) HFIP concentrations in cyclohexane. lexc ¼ 340 nm Low HFIP (104 M) 1.0 1.5 2.0 2.5 3.0 3.5 4.0

366 nm t1 (a1) 2.1 2.1 2.1 2.1 2.1 2.1 2.1

High HFIP (102 M)

t2 (a2)

(0.045) (0.044) (0.036) (0.033) (0.030) (0.028) (0.026)

3.7 3.7 3.7 3.7 3.7 3.7 3.7

(0.008) (0.009) (0.016) (0.019) (0.021) (0.023) (0.025)

440 nm

2.0 3.0 4.0 5.0 6.0

t1 (a1)

t2 (a2)

14.1 (0.121) 14.3 (0.117) 14.5 (0.116) 14. 8 (0.116) 14.9 (0.144)

2.8 (0.043) 2.5 (0.062) 2.0 (0.081) 1.9 (0.084) 1.7 (0.083)

The enhanced MHN fluorescence observed at low concentration of HFIP made it possible to find the stoichiometry of the hydrogen-bonding complexes and to determine the association constant. Within Scheme 30.11, where uncomplexed and HBC species are not connected in S1, the fluorescence intensity reflects only the ground-state equilibrium concentration. Considering this situation, it is possible to use the Benesi–Hildebrand equation (30.12) to obtain the association constant, Ks1 1 1 1 1 ¼ þ I  I0 I1  I0 ðI1  I0 ÞKs1 ½donor0

ð30:12Þ

where I0 is the initial fluorescence intensity at the titration wavelength of the free MHN, I1 is the intensity when the substrate is completely bounded and I is the observed fluorescence intensity of the MHN–donor mixtures. Analysis of the fluorescence data showed a linear dependence at low donor concentration that deviates from linearity at higher concentration. The fits of the experimental data obtained only at low concentrations enabled calculation of the ground-state formation constants of the 1:1 complex. From the data obtained, the association constant of the complexes increases with an increase in hydrogen-bonding parameter a of the different donors: 7.5, 77, 448 and >4000 M1 for t-BuOH, ClEtOH, TFE and HFIP respectively [135]. The spectral changes at high HFIP and ClEtOH concentration suggested the formation of a 1:2 complex. In this case, it is possible to analyse Ks2 from the UV-visible absorbance data and to employ the Benesi–Hildebrand equation:

HBC*

BCD*

1/

PTC* + donor

1/

BCD

PTC

1/

KS2

K S1

BCD + donor

1/

HBC

Z*

HBC + donor

low donor concentration Scheme 30.11

PTC

Z

high donor concentration

Z

Excited-State Double Hydrogen Bonding

1 1 1 1 ¼ þ A  A1 A2  A1 ðA2  A1 ÞKs2 ½donor0

695

ð20:13Þ

where A1 and A2 represent, respectively, the absorbance of the 1:1 and 1:2 complexes at the titration wavelength. During the formation of the 1:1 complex, the absorption spectra remained practically unchanged; it is therefore assumed that the absorbance changes were exclusively due to the 1:2 complex. The Ks2 values (20, 60 and 1200 M1 for ClEtOH, TFE and HFIP respectively) are clearly smaller than those obtained for Ks1. This photophysical behaviour can be rationalized by postulating a double-minimum potential for the position of the proton in the HO–Npd bridge. These minima can be represented by an equilibrium involving two hydrogen-bonding complexes, denoted by Balo´n et al. [136] as HBC (OH  N), and the ion-pair proton transfer complex, denoted as PTC (O  Hþ N) (see Scheme 30.12). The equilibria in heteroconjugated systems have been dealt with in numerous works [145–149]. These studies reveal that the position of this equilibrium depends on the polarity and nature of the solvent, the temperature and concentration and the proton donor and acceptor properties of the solvent. In cyclohexane and at low HFIP concentrations, the equilibrium is shifted towards HBC. A higher concentration of HFIP implies an increase in the polarity of the mixture, resulting in the formation of a 1:2 hydrogen bonding complex and shifting of the equilibrium towards PTC. The 1:2 stoichiometry assumed for PTC suggests the specific solvation of the oxygen atom of HBC by a second alcohol molecule. HBC and PTC present similar lifetimes (around 4 ns), and the lifetime around 14 ns, gathered at high HFIP concentration (Table 30.8), could be related to hydrogen approaching pyridinic nitrogen (in pure HFIP, the MHN cations fluoresce at 450 nm with a lifetime of 22 ns). From these results, Carmona et al. [138] proposed a novel species called the ‘cation-like exciplex’ (CL ) (see scheme 30.12). Upon excitation, PTC interacts with another HFIP molecule to produce this CL exciplex. CL only appears in the highest concentration range 103–102 M of HFIP. The appearance of negative pre-exponential factors indicates that this exciplex is only created during the lifetime of PTC. As mentioned, Carmona et al. believe that the emission at 410 nm is due to a hydrogen-bonded exciplex in which the extent of the proton shift is still greater than in the PTC complex, hence the name cation-like species. When these experiences were repeated with HN, the emission spectra, in the HFIP concentration range between 0 and 9  102 M, showed a fluorescence maximum at approximately 520 nm that was not observed in the MHN case (Figure 30.12). Time-resolved fluorescence of the HN–HFIP system showed that, at low HFIP concentration, the biexponential fluorescence decays at 366 nm, with lifetimes of 2.9 and 4.2 ns, remaining almost independent of HFIP concentration. This result is compatible with a mixture of two independently absorbing and emitting species. At higher HFIP concentration (102 M), the fluorescence at 366 nm decayed monoexponentially, with lifetimes decreasing with increased HFIP concentrations. At 520 nm, the decays were biexponential, with a short rise time similar to that measured at 366 nm and a lifetime independent of HFIP concentration. These results are shown in Table 30.9. Scheme 30.11 was proposed in order to explain time-resolved fluorescence data. From these, Balo´n et al. [135] concluded that the species responsible for the emission at 520 nm is formed during the lifetime of the HN–HFIP complex. At first, this long lifetime, around 10.9 ns, was related to a zwitterionic species. However, recent work of Balo´n et al. [142–144] associates this long lifetime with the 1:2 complex through pyridinic and pyrrolic nitrogens. Within Scheme 30.11, the decays of the PTC and Z can be expressed as ½PTC* ¼ ½PTC*0 expðl1 tÞ ½Z* ¼

k1 ½PTC*½HFIP ½expðl2 tÞexpðl1 tÞ l1 l2

696 Hydrogen Bonding and Transfer in the Excited State

N

N

H

N

N

O

H

H

O

O

N

N

H

N H O H

H

N

O

O

OH

D1B

D1A

O

Me

H

O

O

O

H O

O H O D2

1:3

N

+ N

HOR

N

N

H

H

HBC

N

+

O H R _O

CL

N

H

_R O H _

R

_ H

OR

H PTC

_ N H OR

+ N

N

H H

O

_ + N H OR CH3

H R

H

DHBQ

O

R DHBZ

Scheme 30.12 Some of the proposed species involved in the photophysicochemistry of the betacarboline derivatives

l1 ¼

1 þ k½HFIP tPTC

ð30:14Þ

According to equation (30.14), the plot of the experimental l1 values against HFIP concentration is linear. From the slope and intercept of this plot, the values of k and tPTC can be estimated as (2.7 0.3)  109 M1 s1 and 4.5 0.2 ns respectively. These results confirmed the correlation between the increase in the lifetime of the 1:1 complex with the approach of the hydroxylic hydrogen to pyridinic nitrogen. Thus, these authors concluded that the formation of species with fluorescence around 520 nm involves an initial attack of a donor molecule on the pyridinic nitrogen atom of HN and the formation of a PTC complex.

Excited-State Double Hydrogen Bonding

697

Figure 30.12 Changes in the fluorescence spectrum of HN in cyclohexane with increase in the HFIP concentration from 0 to 9  102 M [135]. Reproduced by permission of the PCCP Owner Societies

Z formation involves a hydrogen bonding interaction of both pyridinic nitrogen (HPN–donor) and pyrrolic nitrogen (NprH–acceptor). The most surprising result of this study was the fact that this species was generated with only a strong hydrogen donor such as HFIP, whereas in the presence of weaker donors such as t-BuOH the emission at around 520 nm was not detected. Thus, the question that arises is whether the precursor of Z is CL or PTC . In order to provide clarity to the formation mechanism of this species with emission at approximately 520 nm (Z), Carmona et al. [139] studied the spectroscopic behaviour of HN and MHN in different cyclohexane–toluene mixtures. Thus, it was possible to observe the effect of medium polarity on the formation mechanism of Z. In 10% (v/v) cyclohexane–toluene mixtures, and in the range of around 103 M of HFIP, the fits of the decays at 355 nm undoubtedly showed the appearance of a lifetime of around 5 ns, together with another component with a lifetime of around 4 ns. In this concentration, three species are present in the ground state, which emit independently upon excitation to the first excited state. These lifetimes, around 3, 4 and 5 ns, can be associated with the emission of free HN, HBC and PTC. In a higher concentration range (102 M), Carmona et al. [139] recorded fluorescence decays at two emission wavelengths. At the shortest one, they estimated the lifetimes from the monoexponential fits Table 30.9 Fluorescence lifetimes and pre-exponential factors (in parentheses) of HN at different HFIP concentrations in cyclohexane [HFIP] (M) 0.01 0.02 0.03 0.04 0-05 0-06 0.08

t (ns) (lem ¼ 366 nm) 4.04 3.72 3.65 3.03 2.78 2.70 2.30

t (ns) (lem¼ 520 nm)

3.59 (0.124)

10.96 (0.131)

2.70 (0.216) 2.68 (0.164) 2.36 (0.190)

10.90 (0.224) 10.90 (0.186) 10.89 (0.198)

698 Hydrogen Bonding and Transfer in the Excited State Table 30.10 Fluorescence lifetimes and pre-exponential factors (in parentheses) of HN and different HFIP concentration in 10% (v/v) cyclohexane–toluene at lexc ¼ 355, 390 and 530 nm 355 nm 2

10 [HFIP] (M) 2.0 2.5 3.0 4.0 5.0 6.0 7.0 9.0 10.0 15.0 20.0 25.0

t2 3.4 2.9 2.6 2.3 2.1 2.1

390 nm

530 nm

t2

t3

3.4 (0.044) 3.2 (0.049) 2.8 (0.050) 2.6 (0.053) 2.2 (0.060) 2.2 (0.197) 1.8 (0.241) 1.7 (0.243)

1.2 1.2 1.1 0.9 0.7 0.7 0.6 0.5

t2 (0.019) (0.020) (0.022) (0.030) (0.031) (0.152) (0.129) (0.104)

2.0 1.7 1.4 1.1 0.9

t3

(0.146) (0.141) (0.131) (0.116) (0.101)

11.9 (0.138) 11.9 (0.137) 11.8 (0.123) 12.0 (0.110) 11.8 (0.102)

(Table 30.10). These lifetimes decrease with increasing HFIP concentration. At 390 nm, where the emission of CL is dominant, the decays were always biexponential. Attempts to fit the decays to triexponential functions were unsuccessful. A negative pre-exponential factor was observed, and both lifetimes decreased upon increase in proton donor concentration. Time-resolved measurements at concentrations from 0.09 to 0.25 M were also recorded at 530 nm. The biexponential fits with a short rise time exhibited a decrease on increase in HFIP concentration and are presented in Table 30.10. From these results, Carmona et al. associated a lifetime of 11.9 ns to Z and concluded that CL and Z are two different exciplexes formed in the first excited state. These compounds are not independent. They are formed via parallel reactions of the photoexcited PTC and they are coupled with CL , the precursor of Z. In this case, the CL lifetime decreases sharply by deactivation to Z. This mechanism is represented in Scheme 30.13. Balo´n et al. [141] recently hypothesized that Z might also present a quinoide structure. With this in mind, they studied the photophysics of N2-methylcarboline, which can be considered to be the prototype of the betacarboline phototautomer. The absorption spectra of this compound were similar to BC, with the novelty that a weak and wide absorption band at 450 nm was observed. The position, shape and low intensity of this band suggested a forbidden charge transfer transition. The addition of increasing amounts of HFIP shifts the absorption spectra batochromically in cyclohexane and hypsochromically in toluene. Two different groundstate isomers were proposed. One predominates in cyclohexane and the other in toluene. Sanchez-Coronilla et al. [143] proposed that these isomers possess quinoid (DHBQ) and dipolar zwitterionic structures (DHBZ) (see Scheme 30.12). Upon excitation, each isomer gives rise simultaneously to two emissions: one from a locally excited state (LE) at 360 nm, and the other from a relaxed intramolecular charge transfer excited state (ICT) at 520 nm. The BCD*

1/

HBC*

1/

BCD

BCD + donor

KS1

PTC* + donor

1/

HBC

HBC + donor

PTC

CL* + donor

1/

Z*

1/

CL

K S2

PTC-N

Scheme 30.13

CL

Z

Z

Excited-State Double Hydrogen Bonding

699

LE states of DHBQ and DHBZ isomers present pp and np characters respectively. For the quinoid form, the photoinduced charge transfer process operates from the pyridinic to the pyrrolic nitrogen atoms. In the zwitterionic form, the charge transfer process operates in the opposite way. The simultaneous presence of partially charged donor and acceptor nitrogen atoms in these complexes allows the charge transfer process between these centres. Thus, they can acquire a quinoid (DHBQ) or zwitterionic (DHBZ) type of structure, depending on the polarity of the media. 30.5.3 Spectral changes induced by the interactions of BCD with a hydrogen-bonding donor–acceptor The photophysics of BCD was also investigated in polar and non-polar, protic and non-aprotic media in the presence of a hydrogen-bonding donor–acceptor such as acetic acid (AcOH) [150–161]. Dioxane, dichlomethane, chloroform, benzene and toluene were mainly used as solvent. Similar spectroscopic behaviour was observed in all solvents [157]. Dioxane hardly solvates to the anions, and thus the dissociation of acetic acid is not favoured. On comparing the absorption of BC in dioxane and benzene, it becomes quite clear that the behaviour is qualitatively similar, although in benzene the cationic form is observed for AcOH concentrations 100 times lower than in dioxane. In dioxane, at AcOH concentrations below 1% (v/v), the free BC absorbs and a small shoulder appears at approximately 360 nm (Figure 30.13). A cationic form (pyridinic nitrogen protonated) (see Chart 30.3)

Figure 30.13 Absorption spectra of BC in dioxane with variable concentrations of acetic acid (AcH). (Above) 0.02, 0.04, 0.06, 0.08, 0.1, 0.5 and 1% (v/v). (Below) 5, 10, 20, 30 and 40% (v/v) [157]. Reproduced by permission of the PCCP Owner Societies

700 Hydrogen Bonding and Transfer in the Excited State

begins to absorb at an AcOH concentration above 5%, although some absorbance is still found at 40% (v/v) in the region where free BC absorbs. In the interval 0–0.1% AcOH, the fluorescence intensity shows a slight decrease and loses vibrational structure. For concentrations higher than 0.5%, the decrease is greater and a new fluorescence band around 515 nm simultaneously appears. At an AcOH concentration of >5%, a new fluorescence band is recorded with its maximum around 450 nm (cation species) (see Figure 30.14). In relation to fluorescence decays, monoexponential decay is recorded in dioxane (lexc ¼ 337 nm) with a lifetime of around 2.9 ns. Upon addition of AcOH, this decay begins to decrease at concentrations higher than 0.5%. In all cases, a monoexponential decay was obtained, which indicates that no back reaction leading to free BC was taking place. The decays at 440 nm are triple exponentials at AcOH concentrations of 5 and 10%. The shortest decay obtained for this triexponential fitting (0.8 ns) is similar to that observed at 360 nm. Upon excitation at 370 nm, the decays at 440 nm are also triexponential for 5 and 10% AcOH. In these cases, the longest decay time (17 ns) also shows the highest amplitude, which is always positive and increases with acid concentration (see Table 30.11). The decays at 520 nm, exciting at 337 or 370 nm, are also triexponential, the shortest lifetime being a rise time; their contribution decreases with increase in AcOH concentration. In all cases there is a decay time of approximately 3.5 ns with a positive amplitude and a decay of around 17 ns with a positive amplitude, which can be attributed to the cation emission. On recording gated fluorescence spectra of BC for time intervals from 0 to 1.5 ns, a fluorescence profile with a maximum about 420 nm and a shoulder at about 500 nm was obtained. For the interval of between 3 and 5 ns, a maximum around 500 nm was obtained, and for times higher than 14 ns the maximum was recorded at 450 nm (cation emission). Hence, for BC in mixtures of AcOH with benzene, p-dioxane, dichloromethane and

Figure 30.14 Emission spectra of BC at an excitation wavelength of 337 nm in (A) dioxane with variable concentrations of acetic acid (AcH), from 0 to 1% (v/v), and (B) dioxane in the interval between 5–50% [157]. Reproduced by permission of the PCCP Owner Societies

Excited-State Double Hydrogen Bonding

701

Table 30.11 Fluorescence lifetimes and pre-exponential factors (in parentheses) of the BC at different AcOH concentrations (v/v) in dioxane lex ¼ 337 nm

% ACOH 360 nm 0 0.5 5.0

2.9 2.8 0.9

10.0

0.5

440 nm

0.8 2.2 16.1 0.5 2.3 16.1

(90) (2) (8) (78) (4) (18)

lex ¼ 360 nm 520 nm

0.8 (0.133) 3.2 (0.115) 17.1 (0.002) 0.5 (0.033) 3.8 (0.075) 17.1(0.001)

440 nm

0.6 1.9 17.3 0.3 2.6 17.4

(27) (2) (71) (17) (3) (80)

520 nm

0.4 3.4 17.2 0.5 3.7 17.0

(0.128) (0.075) (0.005) (0.100) (0.078) (0.006)

chloroform, three different complexes are formed in the presence of AcOH: D1A (AcOH–Npd), D1B (NprH–AcOH) and D2 (NprH–AcOH, AcOH–Npd) (see Scheme 30.12). The effect of temperature on these prototropic equilibria was also considered [156]. The fluorescence of BC in mixtures of dichloromethane with 0.4% AcOH presents the emission of at least three species with maxima of approximately 360, 390 and 520 nm at 310 K. These emissions are converted into one, located around 450 nm, when the temperature is lowered to 225 K. In polar solvents such as acetonitrile or methanol there is no emission at 520 nm when AcOH is added to the solution. The fluorescence spectra present only two bands corresponding to free and cationic species, the cationic band being enhanced when temperature decreases. When temperature is lowered, the dielectric constant shows an increase. That is, the density of the environment increases and the weak interactions between BC and AcOH are favoured. The dielectric enrichment of the solvation shell blocks the p indole ring and prevents the formation of species other than the cationic one. Thus, the emission at around 520 nm is only observed in solvents with low polarity and at high temperatures. Other studies of BC in cyclohexane [162] in the presence of AcOH showed an equilibrium in the ground state between three different species: uncomplexed and 1:1 and 1:2 complexes (BC:AcOH). The latter consists of a cyclic triple hydrogen-bonding complex for which, once excited, only a small adjustment of the geometry is required for the triple-proton transfer to occur (see Scheme 30.12). Therefore, this fast excited-state proton transfer (ESPT), which takes place through a conduit of relayed hydrogen bonds, is responsible for the emission at around 520 nm. It is clear that weak hydrogen bonding interactions take place between hydrogen donors or acceptors with BCDs, and these weak interactions become strong when BCDs are excited. These properties are due to photoinduced changes in electronic distribution, greatly increase the pyridinic nitrogen basicity and pyrrolic nitrogen acidity in the S1 state and thus vary the reactivity of BCDs. NMR spectroscopy has contributed to the understanding of the formation of hydrogen bonding, especially by means of low-temperature experiments [163–168] which enable separate analysis of the chemical environment of each nucleus of the molecule rather than observation of the molecule as a whole. NMR spectroscopy [161] confirmed the proposed structures for these complexes in the ground state. The chemical shifts of H-6, H-7 and H-8 (Figure 30.15) are almost constant in the presence of AcOH, showing that the addition of AcOH does not significantly change their environment. H-3, H-4 and H-5 undergo greater variation, moving to a lower frequency with an increase in AcOH concentration owing to the fact that these hydrogen atoms become more shielded. This can be explained if we assume that the hydrogen atom of AcOH

702 Hydrogen Bonding and Transfer in the Excited State 8,4

8,0

H5

1

Chemical shift H (ppm)

8,2

7,8

H3 H7

7,6

H8 7,4

H4 H6 0

2

4

6

8

10

12

Molar ratio (AcH/Harmane)

Figure 30.15

Changes in chemical shifts as a function of the AcOH/HN molar ratio

forms hydrogen bonds with the pyridinic nitrogen atom of HN. Formation of a hydrogen bond –(N(2). . .H–O), with delocalization of the charge density of the pyrrolic ring towards the pyridinic ring, shields the hydrogen atoms of this pyridine ring. The presence of this hydrogen bond is supported by a new signal that appears when AcOH is added. This signal is delocalized downfield with an increase in AcOH concentration. At the same time, the NprH signal also shifts to a higher frequency with an increase in AcOH concentration. This progressive shift finalizes when the two signals overlap. From this point, both signals shift slowly and together. Figure 30.15 shows that the H-3, H-4 and H-5 chemical shifts versus AcOH concentration are at a minimum when the HN/AcOH molar ratio is 3. Above this ratio there is no great variation in the chemical environment of the HN hydrogen atoms. This effect suggests the formation of a 1:3 hydrogen bond complex between HN and AcOH. From a molar ratio of 3 (AcOH/HN) (see Figure 30.16), Npr–H (N9H) and HO–Npd (N2H) signals join and do not separate on increase in the AcOH concentration in solution, the bridging protons producing one single signal. This implies an exchange process between them. This effect can only be explained if a cyclical complex is considered (see Scheme 30.14). This cyclical structure is in accordance with the stoichiometric ratio and structure for the hydrogen-bonding formation proposed by Chou et al. [162] and is responsible for the excited-state proton transfer tautomerism proposed for these compounds in different solvents using timeresolved measurements and syntheses of different BCDs. Chemical shifts were measured at different temperatures between 233 and 323 K. Again, the greatest effect upon chemical shifts was observed for H-3, H-4 and H-5. Reducing the temperature causes an increase in the electronic density of H-3, H-4 and H-5. These results agreed with those obtained for this system using other spectroscopic techniques: lowering of temperature produces the same effect as an increase in the amount of AcOH in solution. This can be explained by considering a shortening, in both cases, of the hydrogen bond between HN and AcOH. That is, an increase in AcOH concentration is accompanied with an increase in the number of AcOH molecules in the chain, which decreases the OH–Npd distance. Moreover, when temperature is lowered, the dielectric constant increases, and the solvation shell around HN favours cationic species formation, which can be related to a shortening of the OH–Npd distance.

Excited-State Double Hydrogen Bonding

Figure 30.16 298 K

1

703

H NMR spectra of HN with different AcOH/HN molar ratios (0.4, 1.3, 2.7 and 4.5) in CDCl3 at

N9 H

N 2 H

O

Me O H

O O H O

O

A

Scheme 30.14

_ N H

+ N

H

N O

Me O H

O O H O

B

O

N H

H

O

Me O H

O O H O

C

Different forms for the 1:3 HN/AcOH complex

O

704 Hydrogen Bonding and Transfer in the Excited State

To conclude, HN binds AcOH with 1:3 stoichiometry through hydrogen bonding to Npr–H and pyridinic Npd groups. For BC, the stoichiometry of this complex is 1:2. Low concentrations of AcOH in solutions of HN and BC produce a charge transfer from the indole ring to the pyridine ring, similar to that produced by excitation up to the first excited state by light absorption. For HN and BC, this transference stops when 1:3 and 1:2 complexes are formed respectively. From this moment, H-3 and H-4 begin to be unshielded, and a conduit of conjugated hydrogen bonds occurs, which is reflected by the overlapping, and from this point there is no separation of NprH and OH–Npd1 H NMR signals. With this conduit, a monotonous transformation of the type dþ þ Npr --H    O ! Nd ! N pr    H    O pr    H--O

concomitant with þ Npd    H--O ! Ndpdþ    H    Od ! Npd --H    O

might explain the fluorescence spectra with a maximum at 520 nm. With this, an increase in AcOH concentration drives a gradual transformation of a molecular hydrogen-bonded complex (A in Scheme 30.14) to a zwitterionic complex (B in Scheme 30.14). The latter form is also favoured with a reduction in temperature (higher dielectric constant). Nevertheless, the formation of the cationic form [156, 157, 160] is favoured when working at a high dielectric constant, such as an increasing concentration of acetic acid. This behaviour can be explained if we consider that this zwitterionic form (B) resonates with C (Scheme 30.14), and C is an extremely basic species that acts as an acid sponge [155]. When this species is formed in S0, it reacts immediately with an acid, and the cationic species is thus formed. This explanation confirms the results of recent research [142] in which the authors propose the existence of two ground-state isomers with quinoid (C) and dipolar zwitterionic (B) structures, the equilibrium concentrations of which change with medium polarity. It has also been considered that the quinoid and dipolar zwitterionic forms proposed by Carmona et al. are not isomers but rather mesomeric structures that originate a single emission band (around 520 nm). Following this trend, the emission maxima at 360 and 520 nm can be associated with a tautomeric equilibrium between species T1 (Npy, Npr–H, 360 nm) and T2 (Npy–H, Npr, 520 nm). In the case of T1, formation of the hydrogen bond in Npy creates a partial positive charge density on the pyridinic nitrogen, favouring photoinduced charge transfer from the pyrrolic to the pyridinic nitrogen [143]. Formation of the hydrogen bond in Npr of T2 creates a partial positive charge density on the pyrrolic nitrogen, favouring photoinduced charge transfer from the pyridinic to the pyrrolic nitrogen. The key to understanding this system is to consider an oscillating system between these two tautomers. This balancing occurs by means of the following process: upon excitation, a redistribution of the p-electron cloud is produced via intramolecular charge transfer from the pyrrolic to the pyridinic ring. This charge transfer reduces the electronic density in the Npr, weakening the Npr–H bond and providing more facile proton dissociation, whereas the excess electronic density in the Npy increases its basicity, favouring interaction between the lone electron pair of the latter nitrogen with the hydrogen donor molecule. When the Npy–donor bond is sufficiently short, the lone-pair orbital of Npy is coupled to the p-cloud, resulting in a structure of the quinoid type associated with the T2 tautomer. From this moment, charge transfer begins to occur from the pyridinic to the pyrrolic ring. Once the charge is transferred to Npr, this nitrogen becomes more basic and Npd becomes less basic. In this case, coupling occurs between the lone-pair orbital of Npr and the p-cloud, resulting again in T1. This balancing between T1 and T2 only occurs in low-polarity media and in the absence of protons. The presence of protons stabilizes the system and prevents charge transfer.

Excited-State Double Hydrogen Bonding

705

Although some questions have been answered, the mechanism of Z formation is still unclear and requires further investigation. Subnanosecond time-resolved experiments and effects of temperature and viscosity of the solvent on excited-state proton transfer could provide more information on the photophysicochemistry of these derivatives. It is clear that interaction between the pyrrolic group of BCDs and hydrogen-bond acceptors quenches fluorescence through the formation of a very short-lived non-fluorescent complex. By contrast, interaction between pyridinic nitrogen and hydrogen-bond donors gives rise to complexes whose lifetime increases as hydrogen comes closer to the nitrogen, which occurs as the donor concentration is increased.

30.6 Conclusions As a consequence of all this, it can be concluded that all the N-azaaromatic compounds considered undergo phototautomerization processes as a result of the electronic phototransfer induced by hydrogen-bonding interactions. This phototransfer process, in pyrroloquinolines and pyridilindoles (PyIn-n), always occurs from the pyrrolic to the pyridinic ring. However, for betacarboline derivatives this electronic transfer can take place in both directions: from the pyrrolic to the piridinic ring for the zwitterionic structure isomer or from the pyridinic to the pyrrolic ring for the quinoide structure. However, the photophysicochemistry of betacarboline derivatives, where the two moieties are fused, is more complicated owing to the existence of species not detected in pyrroloquinolines and pyridylindoles (PyIn-n). Thus, whereas in the latter the hydrogen-bonding interaction gives rise to fluorescence quenching, mainly caused by phototautomerization or internal conversion, in the former the hydrogen-bond acceptors quench fluorescence, but the hydrogen-bond donors increase it. In all cases, these hypo- and hyperchromic effects result from the formation of hydrogen-bonding complexes.

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31 Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology: Photochemistry and Photophysics of Hydroxyaromatic Dopants Moazzam Ali and Swapan K. Saha Department of Chemistry, University of North Bengal, Darjeeling 734 013, India

31.1 Introduction Amphiphilic molecules have been a realm of interest in chemistry for over a 100 years, not only in respect of the pure science but also in respect of their wide applications in industry. They are used as detergents, cleaning agents, emulsifiers in food, pharmaceuticals and cosmetics. Surfactant molecules in aqueous solution can self-assemble into aggregates (so-called micelles) in which their immiscible parts clump together. However, the shape and size of the micelles depend on the molecular structure of the surfactant, the nature of solvent and additives and their molar composition. Specifically, ionic surfactant molecules form spherical micelles in water when their concentration exceeds the critical micelle concentration (CMC). The presence of an ionic or cosurfactant additive reduces the repulsions between micelle head groups and thereby affects the structural transitions. In addition, the micellar aggregates can grow anisotropically under appropriate conditions, changing their shapes from spheres to rods or highly flexible worm-like aggregates. Such evidence provided some analogies between giant flexible cylindrical micelles and conventional polymeric solutions [1]. However, unlike ordinary polymers, micellar chains possess the unique ability reversibly to break and then recombine. They reform by addition and loss of individual amphiphiles or by the scission and recombination of

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

712 Hydrogen Bonding and Transfer in the Excited State

entire micelles. Meanwhile, when immersed in a mean flow, surfactant micelles show various flow-induced microstructures, and the flow properties of these systems are of interest in technological processes. Owing to their viscoelastic properties, worm-like micelles have found applications in many areas, such as home and personal care products and in the oilfield industry. Frequently they are used as drag-reducing agents for district heating. Fluids that viscosify or gel upon heating are of high interest for biomedical and drug delivery applications, for flow control and separation using microfluidic devices and as hydraulic fracturing fluids in enhanced oil recovery. The simplicity, low cost and ease of preparation of these systems might make it attractive for some of these applications [2].

31.2 Microstructural Transition of Micelles in the Presence of Inorganic and Organic Salts The type of self-assembly (Figure 31.1) at a certain concentration depends on the intrinsic surfactant geometry, which can be understood on the basis of the critical packing parameter (CPP) given by the equation

Figure 31.1 Schematics showing the connection between the self-assembly of amphiphiles and their molecular geometry. The hydrophilic heads are shown in dark shade and the hydrophobic tails in light shade. Adapted from Ref [3]

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 713

CPP ¼ ahg =atail where ahg is the effective area of the amphiphile head group and atail is the average area of the amphiphilic tail. The larger the head group area compared with the tail area, the more curved the aggregate is. Thus, a CPP of 1/3, corresponding to a cone shape, leads to spherical micelles, while a CPP of 1/2 (truncated cone) corresponds to cylindrical micelles. Finally, molecules shaped like cylinders, i.e. having atail  ahg and CPP ¼ 1, tend to assemble into bilayer structures (vesicles). Therefore, the respective requirements of the hydrophilic head group on the side of the water interface and those of the hydrophobic tails on the other side of the interface determine an optimal curvature, also called the spontaneous curvature. The morphology sequence and phase behaviour of surfactant aggregates is driven by the spontaneous curvature of the hydrophobic/hydrophilic interface and may be tuned by various external factors, such as the amount and nature of added electrolyte, the presence of other species in solution, as has been mentioned already, the pH or the temperature. For example, if the head groups are charged, they repel each other, which increases the effective head group area and favours the formation of small spherical micelles. The addition of electrolyte tends to screen the electrostatic interactions, even more efficiently in the case of a strongly binding salt, which allows the head groups to come closer to each other and can lead to the formation of cylindrical structures. Surfactant systems show an impressive polymorphism of structures in aqueous solutions, which, in turn, influences a large variety of physical properties, in particular rheological properties. The polymer-like micelles that are formed by certain cationic surfactants in solution exhibit very interesting rheological properties. At high concentrations, these solutions show typical viscoelastic behaviour, while at very low concentrations more complex and unusual rheological phenomena are observed. The pioneering works in this field were those of Rehage and Hoffmann [4–6], Shikata [7–10] and Candau [11, 12] and their coworkers. Rehage and Hoffmann have used rheology to demonstrate that the micellar growth results in an increase in the fluid viscosity. Figure 31.2 displays static viscosity data for cetylpyridinium chloride at 3.6 wt% (100 mmol) as a function of sodium salicylate (NaSal) content. The steep

Figure 31.2 Static viscosity h0 for cetylpyridinium chloride–sodium salicylate solutions as a function of R (¼ [Sal-]/[CPþ ]). The CPCl concentration is 100 mmol. [4] Adapted with permission from Taylor & Francis Ltd, http:// www.informaworld.com

714 Hydrogen Bonding and Transfer in the Excited State

increase seen at molar ratio R  0.3 is interpreted as a transition between spherical and worm-like micelles. Similar viscosity behaviours were observed with cetyltrimethylammonium bromide (CTAB) and NaSal. Alkyl trimethylammonium and alkylpyridinium surfactants are the most extensively studied surfactant systems in this respect. Rehage and Hoffmann [4–6] reported that the viscosity of a 0.9 mM cetylpyridinium salicylate solution slowly increases with time (rheopexy) when subjected to shear flow with a sufficiently high shear rate, and that it takes an unexpectedly long time, several minutes, for the system to reach steady state. In a detailed study of the tetradecyltrimethylammonium salicylate system, they also reported that an increase in flow birefringence accompanies the stress growth. Actually, both the stress and flow birefringence curves show an induction period before rapid growth commences. In addition to the general features as described, the induction time is shown to be inversely proportional to the applied shear rate and is independent of the flow direction. On the basis of this information, a kinetic coagulation mechanism, first proposed by Rehage et al. [5], was favoured to account for the rheopectic phenomenon. According to this model, the initial small micelles collide with each other more frequently in shear flow than in quiescence, resulting in the formation of large micelles. The same results are also obtained when the influence of sodium salicylate and sodium bromide concentration on the shear-thickening behaviour of aqueous micellar solutions of (CTAB) and (NaSal) is studied experimentally. The realization that there could be micelles and aggregates of different types is recent, although manifestations of unusual properties of such systems have been known for some time. The classic example of such an ‘abnormal’ system is a solution containing cationic surfactant cetyl pyridinium chloride (CPC) with sodium salicylate (NaSal) as the additive [13]. These systems show strong viscoelastic effects even at extremely low volume fractions – a few millimolar concentrations. Notable features are observation of high viscosity even at extremely low concentrations and the existence of two peaks in the viscosity (Figure 31.2). Electron microscopic pictures of these highly viscoelastic solutions show the existence of entangled polymeric micelles with a length of several microns. In general, the addition of organic or inorganic counterions [14, 15], uncharged compounds such as aromatic hydrocarbons [16] or an oppositely charged surfactant [17] can transform spherical micelles into worm-like micelles [11, 12]. Halide counterions bind moderately to cationic surfactant aggregates, and therefore micellar growth is gradual. On the other hand, with aromatic counterions, which usually display higher counterion binding, micellar growth occurs at low surfactant and counterion concentrations [18]. High counterion binding is not the only prerequisite for micellar growth, and other factors such as the orientation of substituents on the aromatic ring are also important. 1 H NMR studies reveal that þ N(CH3)3 proton signals are shifted to higher fields and that they are broadened upon addition of salicylate counterions. It was shown that the aromatic ring of salicylate is located between the head groups, and that the OH and COO- substituents protrude out of the micellar surface [19]. Theoretical studies showed that worm-like micelles are long and flexible and that they undergo transformations on relatively short timescales [20]. This was confirmed by negative staining [21] and cryogenic electron microscopy [22], which showed that worm-like micelles can become several hundreds of nanometres in length. With increase in the surfactant concentration, an entangled network of worm-like micelles is formed, which displays viscoelastic behaviour. As has already been mentioned, the rheological behaviour observed for these surfactant systems is similar to that of solutions of flexible polymers, and therefore aqueous solutions of entangled worm-like micelles are often called ‘living’ polymer systems. Lin et al. [23a] studied the aggregation behaviour of mixtures of CTAB and 5-methylsalicylic acid. Changing the molar ratio of 5-methylsalicylic acid to CTAB from 0.1 to 1.1, a gradual change from spherical micelles to vesicles via worm-like and entangled worm-like micellar phases was observed. The additional methyl group of the promoter molecule increases the size of the hydrophobic portion, resulting in the formation of vesicles. Also, sodium 3-hydroxynaphthalene-2-carboxylate (NaHNC) is able to induce the formation of vesicles in aqueous solutions of CTAB [7]. This system is compared with cationic surfactants, as the phase behaviour

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 715

shows similarities to that of mixtures of cationic and anionic surfactants. The same system without excess salt (C16TAHNC) has also been studied. The critical aggregation concentration (CAC) of C16TAHNC is 0.03 mM, which is significantly smaller than that of C16TAB (1.0 mM) [24]. Aqueous solutions of C16TAHNC show interesting temperature-dependent phase behaviour. Increase in the temperature causes the system to undergo a transition from a turbid vesicular phase to a clear viscoelastic phase containing a network of entangled wormlike micelles. Fluorescence anisotropy and NMR spectroscopy showed an increase in fluidity of the aggregate surface upon increase in temperature [25]. The phase transition has also been studied by differential scanning calorimetry (DSC) and conductivity experiments [26]. It was shown that increasing the temperature results in a decrease in the HNC–counterion binding; concurrently the head group repulsions between C16TAþ moieties in the bilayer increase, which eventually leads to a change in aggregate shape from vesicles to worm-like micelles. Avesicle-to-micelle transition in aqueous solutions of C16TAHNC can also be induced by shear or by adding CTAB or NaHNC. Thus, so far, most studies have focused on mixtures of CTAB and N-methylsalicylic acid or NaHNC. When a worm-like micellar solution is heated, the micellar contour length decays exponentially with temperature [27, 28]. The reason for this is that, at higher temperatures, surfactant unimers can hop more rapidly between the cylindrical body and the hemispherical end-cap of the worm (the end-cap is energetically unfavourable over the body by a factor equal to the end-cap energy). Thus, because the end-cap constraint is less severe at higher temperatures, the worms grow to a lesser extent. The reduction in micellar length, in turn, leads to an exponential decrease in rheological properties such as the zero-shear viscosity h0 and the relaxation time tR. Accordingly, an Arrhenius plot of ln h0 versus 1/T (where T is the absolute temperature) falls on a straight line, the slope of which yields the flow activation energy Ea. Values of Ea ranging from 70 to 300 kJ mol-1 have been reported for various micellar solutions [28–30]. Mixing surfactants of opposite charges, cationic and anionic, but with varying chain lengths, one can control the degree of precipitation of the surfactants to produce different supramolecular structures like vesicles and polymeric micelles [31–42]. One can also produce tubules, ribbons, etc., by controlling the solubility of surfactants [43, 44]. This facilitates an easy control over the aggregate structure, and hence it is possible to induce transformations from vesicles to micelles by a proper choice of additives that are cationic [45–47], anionic [48–51] or neutral [52–54]. Although a number of early investigations were carried out on surfactantmediated solubilization of vesicles owing to its important implications in biochemistry, there are very few studies describing such vesicle–micelle transition induced by temperature [55, 56]. It is apparent that shearthickening occurs at low surfactant-hydrotropic concentration, because free worm-like micelles join a transient network under shear, and the microstructures have been broadly named as shear-induced structure or phase (SIS or SIP). It is particularly interesting that, while a wide variety of worm-like ionic micellar solutions display identical rheological responses, a common element in most of these systems is the presence of salt anions such as NaSal. Although a few examples are available in the literature where additives other than NaSal have been used, these molecules have never been considered as highly as promoters like NaSal. However, a number of studies on micellar shape transition in cationic, anionic and catanionic surfactant systems, induced by polar and nonpolar organic species under comparatively high concentration conditions, have been reported in the literature. While hydrophobic molecules with either an aromatic ring or a small polar group have shown better efficiency, no unusual rheological feature was apparent under this condition. For a long time, the presence of an anionic charge on the promoter molecule has been considered to be pivotal in achieving a low concentration shape transition of cationic micelles via charge screening because it decreases the average area per surfactant head group, allowing the packing parameter to exceed the critical value of 1/3. Studies on microstructural modifications by neutral aromatic compounds are rather recent. In the following section, microstructural modifications of micelles by neutral aromatic compounds are discussed.

716 Hydrogen Bonding and Transfer in the Excited State

31.3 Microstructural Transition of Micelles in the Presence of Neutral Aromatic Dopants As has already been mentioned, the self-assembly of amphiphilic molecules results in a variety of structures of diverse shapes and sizes [57]. Among the most intriguing of these are the worm-like micelles, which are flexible cylindrical chains with radii of a few nanometres and contour lengths of up to several micrometres [58]. The past two decades have witnessed strong interest in worm-like micelles among scientists because these systems are similar to polymer chains in their ability to entangle into viscoelastic networks. At the same time, the micellar chains are held by weak physical bonds, unlike the covalent bonds in polymers; consequently, the chains break and recombine, and their contour length is not fixed by chemical synthesis but by solution thermodynamics [59]. From a rheological point of view, worm-like micellar systems are interesting because they can behave like Maxwell fluids (i.e. as model viscoelastic fluids having just a single relaxation time [60]). Neutral dopants cannot affect charge screening in ionic surfactants but may reduce surface curvature and induce microstructural transition by increasing the effective local tail volume and not allowing the surfactant head group to undergo any rectilinear displacement on dopant permeation. However, neutral aromatic dopants are sometimes able to screen the surfactant head group by p-electron shielding. Changes in microstructure of surfactant aggregates that are induced by the addition of a series of phenolic organic dopants (phenol, cresol and alkyl-substituted phenols, namely 4-ethyl phenol, 4-n and i-butyl phenol) to a micellar solution of CTAB have been studied using cryogenic transmission electron microscopy (cryoTEM) [61–63]. As the concentration of each of the dopants is increased, there is a systematic reduction in the curvature of the observed microstructure. For phenol and cresol, the microstructure changes from globular micelles to rod-like micelles and then to long worm-like micelles. For 4-ethyl phenol and 4-butyl phenol, the microstructure transforms from globular to worm-like micelles to unilamellar vesicles and then bilamellar vesicles. The concentrations at which these morphological transitions take place decrease as the number of methyl substitutions on the phenolic ring is increased. These microstructure transitions are attributed to changes in the packing parameter of the surfactant palisade layer, resulting from a balance of interaction between the surfactant tails and the dopant aromatic chain and the hydrogen bonding of the hydroxyl group with surrounding water. It is also claimed that the alignment of the hydroxyl dipoles reduces the electrostatic field around the CTAB head groups, decreases the interlayer repulsive interactions and allows membrane fluctuations to stabilize the bilayer vesicles. These highly tunable microstructures can be exploited for templated material synthesis and have applications for the micellar-enhanced Ultrafiltration process. An aqueous mixture of CTAB and 2,3-dihydroxynaphthalene (2,3-DHN), shown in Figure 31.3, gives a viscous solution of rod-like nanoaggregates, as confirmed by TEM. These supramolecular assemblies are used as templates for sol–gel synthesis, providing an aqueous route for tube silicates [64]. It has also been shown by cryo-TEM and small-angle neutron scattering (SANS) studies that, as the dopant concentration is increased, transformation from spherical to worm-like and subsequently to globular and then to tubular vesicles takes place. Hydrogen-bonding interactions among amphiphilic molecules can also change the head group area, resulting in a change in the microstructure of the self-assembly. For example, amide hydrogen bonding near the surfactant head group of N-acyl amino acid (NAA) surfactants is found to be the driving force for the formation of bilayer aggregates [65–69]. It has been observed that sodium salts of long-chain fatty acids form micelles at alkaline pH. However, when the solution pH is lowered to acidic range, the surfactants form closed bilayer vesicles as a consequence of hydrogen bonding between the –COO- and –COOH groups at the surfactant head. The direct entrapment of phenolic compounds in mesoporous silica provides a route to synthesizing useful nanostructured composite materials that allow transition from hexagonal to lamellar with increasing

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 717

Figure 31.3 TEM images of (a) 2,3-DHN/CTAB aggregate and (b) catechol/CTAB aggregate. Adapted with permission from [64]. Copyright 2005 The Chemical Society of Japan

solubilisate content. This transition is gradual and can be correlated roughly with the varying micellar template shapes upon increase in the dopant concentration. The spherical CTAB micelles originally elongate one-dimensionally, forming worm-like micelles up to a certain point before two-dimensional growth occurs to form multilamellar vesicles (Figure 31.4). These vesicles template multilamellar structures in silica. The technique of incorporating phenols in surfactants followed by material synthesis provides a feasible and easy method to separate toxic phenolic pollutants from aqueous waste streams. Silica precipitation of contaminant-containing micelles is almost instantaneous, following which the surfactant can be removed from the contaminant stream. Streams with high surfactant and high contaminant concentrations are usually the most difficult to treat using the stripping technique. Silica encapsulation may offer a fast and efficient method of removing the contaminant-containing surfactant micelles (Figure 31.5). A second potential application is based on the concept that phenolic compounds in CTAB micelles create confined environments for the enzymatic synthesis of polyphenolics. Coupling enzymatic polymerization with mesoporous silica synthesis introduces the possibility of creating such novel polymer–ceramic nanocomposites [70].

Figure 31.4 Transitions from spherical micelles to worm-like micelles and then to vesicles upon doping CTAB with increasing levels of 4-ethylphenol. Adapted with permission from [62]. Copyright 2004 American Chemical Society

718 Hydrogen Bonding and Transfer in the Excited State

Figure 31.5 Schematic of the process of phenol solubilization in CTAB micelles, followed by encapsulation in mesoporous silica. Adapted with permission from [62]. Copyright 2004 American Chemical Society

However, the dopants just discussed act at high-concentration conditions only (above 60 mM). Therefore, these systems cannot be considered as model systems for studying stimulus-responsive fluids. Another neutral dopant that acts as cosurfactant is hexanol. It is reported that micelle–vesicle transition is induced by hexanol into a worm-like micelle solution of gemini cationic surfactant. This transition was investigated with rheology measurements as well as direct imaging techniques. The micelles first become elongated with an increase in hexanol, which increases the viscosity. As the hexanol concentration is increased, another more fluid phase appears where micelles are highly branched and coexist with small vesicles. With further increase in hexanol, these vesicles grow in size and some of them show tubule-like shapes, typically a few micrometres thick and up to several hundred micrometres long [52]. The most efficient uncharged hydroxyaromatic promoters potentially comparable with NaSal include 1- and 2-naphthols [71]. Stimulus-responsive viscoelastic gels of long worm-like CTAB and CPB micelles are formed at low surfactant concentrations in the presence of 1- and 2-naphthols. These classes of promoters, which trigger the formation of giant molecular aggregates, have generated multifield interest. Firstly, these promoters are effective under salt-free conditions (low ionic strengths), and secondly the mechanism of transformation of micellar aggregates is altogether different and the role of non-covalent interaction is dominant. It is also interesting to note that the self-assembly of p-conjugated molecules into nanostructural materials is potentially facile and an effective route towards the development of functional materials for electronic and biological applications.

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 719

31.3.1 Shear-induced viscosity and fluorescence intensity displayed by 1- and 2-naphthols

Viscosity (mP.s)/ Fluorescence intensity (arbitrary unit).

Much like the CTAB–NaSal system, CTAB–naphthols and CPB–naphthols also display maximum viscoelasticity at a 1:1 molar ratio of surfactant and promoter. The argument that an excess or deficiency of charge on the micelles owing to adsorption of hydrotrope anions (e.g. NaSal) would shorten the micellar lifetime and size is not apparently true for the present system because, under the present experimental condition of solution pH (6.5), the naphthols are mostly protonated, i.e. uncharged (pKa > 9.0). Therefore, it seems that the symmetrical distribution of surfactant and promoter molecules leading to highly compact spherical micelles facilitates the formation of an optimum surface curvature in the presence of H-bonding (discussed later), and this results easily in sphere-to-rod transition. At low concentrations (<2 mM), CTAB–naphthol or CPB–naphthol solutions show shear-thinning properties, typically observed in the case of a non-Newtonian fluid. However, at higher concentrations (>2 mM; 25 C), these systems display interesting rheological phenomena. Up to the applied shear rate of 52 s-1 (which is concentration dependent) for the CPB-2–naphthol system, solutions shear thin (Figure 31.6). An onset of viscosity rise is observed thereafter as a function of applied shear, and the viscosity shear rate profile passes through a maximum, e.g. at 102 s-1 for the above system. The system recoils after the applied shear is withdrawn and takes a very long time (e.g. the half-life period of viscosity decay of a CTAB-1–naphthol (7.0 mM) system equals 56 min; samples were sheared in a rotational viscometer at 100 s-1 for 5 min to ensure that the high-viscosity regime was reached) to recoil completely and to return to an equilibrium unsheared state. Shear rates at which viscosity transition takes place and the shear rates at which maximum viscosity is displayed by various systems are shown in Table 31.1. Systems that display shear-induced nonlinear rheological changes (such as the present systems) bring about a formidable problem in measuring unperturbed solution viscosity because the measuring techniques (e.g. torsional shear rheometry) often apply considerable stress on the system during measurement, and thus the zero-shear viscosity becomes obscure. Both the naphthols are well-known fluophores, and, significantly, the quantum yields of emission of the naphthols is found to be very sensitive to the solution viscosity of the present

80 Fluorescence Viscocity 70

60

50

40

30 35

70

105

140

175

210

-1

Shear rate/s

Figure 31.6 Variation in viscoelasticity of a CPB-2–naphthol (10 mM) system with applied shear rate. Adopted from Ref [71]

720 Hydrogen Bonding and Transfer in the Excited State Table 31.1 naphthols

Shear-induced viscoelastic characteristics of CTAB and CPB micelles in the presence of 1- and 2-

Viscoelastic system (10 mM, 1:1) 1-Naphthol–CTAB 2-Naphthol–CTAB 1-Naphthol–CPB 2-Naphthol–CPB

Transition shear rate (s-1)

Shear rate at maximum viscosity (s-1)

60 42 63 52

115 100 106 102

systems. This offers an interesting route for fluorescence monitoring of unperturbed viscosity as a function of applied shear. In a viscous medium, a fluophore cannot transfer energy efficiently by non-radiative means because of delayed collisions with the surrounding molecules, resulting in an increased emission quantum yield. Moreover, the dipole moment of the probe in the excited state is greater than in the ground state, and hence interaction of the excited probe molecule with its surrounding molecules is different from that before absorption. Reorientation and translation of nearest-neighbour molecules allow the probe molecule to relax gradually to its equilibrium excited singlet state (S1). In solutions of low viscoelasticity, where these relaxations are very fast, fluorescence practically takes place from this equilibrium excited state S1. In highly viscoelastic solutions, the relaxation of molecules surrounding the probe may be slow, and the probe molecules may emit before reaching their equilibrium excited state S1; a blue shift of the fluorescence spectrum may also be observed, accompanied with intensity enhancement. A similar situation is also encountered in a twisted intermolecular charge transfer state (TICT) formation, where, in a less viscous environment, the probe molecules also display internal rotation and charge transfer, which results in a lower emission quantum yield than that in a highly viscous environment [72, 73]. Furthermore, naphthols are weak acids in the ground state. In aqueous solution (pH  6.0–7.0), they exist almost completely in the acid forms. On excitation into the lowest singlet excited state, the pKa values drop by several units (2-naphthol: pKa  2.78; 1-naphthol: pKa  0.40) [74, 75], i.e. they undergo deprotonation in the excited state (DES) [76, 77]. As a result, the emission from the neutral forms of 1- and 2-naphthols at 360 and 357 nm, respectively, exhibits a very much lower intensity than that of the anion forms near 450 or 420 nm respectively. However, on binding to micelles, the DES process is restricted significantly, causing a 20–90-fold increase in the intensity and lifetime of the neutral emission, as well as in the rise time of the anion emission [78]. This is probably because of the unavailability of an adequate number of water molecules in the vicinity of the naphthol molecules embedded inside the micelle to hydrate the proton released during photolytic deprotonation [78]. [This point is discussed further at the end of the chapter.] Therefore, at low surfactant concentrations (<2 mM; DES is significant), emissions from the deprotonated anion forms of the naphthols were monitored at higher wavelengths, whereas, in the presence of high concentrations of surfactant, emissions from the neutral form of 2-naphthol were monitored at lower wavelengths (where DES is insignificant) in the present experiments (1-naphthol shows a very low quantum yield for neutral emission). Figure 31.6 also compares shear-induced viscosity data with fluorescence intensity data for the CPB-2–naphthol system. While the overall feature of the shear-induced viscosity profile is identical to that of the emission, they are not exactly superimposed on one another, possibly because of the perturbation imposed on the system during viscosity measurement. (The near-identical nature of the two profiles is important; one should not compare viscosity and fluorescence intensity in absolute terms). However, any direct effect of applied shear on the DES process is not observable. This is apparent from the non-variant ratio of emission intensities of protonated to deprotonated naphthols as a function of applied shear. This also indicates that the shear does not influence the availability of water molecules to hydrate the

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 721

liberated protons, i.e. the microstructure around the naphthol molecules in the worm-like micelles remains unchanged in SIS. However, as shown in Figure 31.6, it seems likely that, with the high applied shear rate (>180 s-1), the viscosity and fluorescence have opposite variation. Partial modification/disruption of wormlike micelles under high shear may change the compactness, causing redistribution of naphthols in the micellar gel and expelling some of the inner-site naphthols to the outer site (more accessible to water molecules), resulting in a slight increase in the fluorescence intensity due to the modified DES process [79]. In an experiment where the hydrotropic promoter for micellar shape transition is not a fluophore, a probe must be added from the outside for the above measurement. This, in turn, may alter the hydrophobic trait of the system and affect the rheology. Therefore, one should be careful in using external fluorescence probes for monitoring viscosity. 31.3.2

1

H NMR study

To ascertain the location and orientation of the additive naphthol molecules in the micelles, and to understand the nature of interaction in micellar shape transitions, 1 H NMR experiments may be helpful, along with absorption and emission spectroscopies. The NMR spectrum of 2-naphthol in D2O (in the absence of CTAB) shows clusters of signals centred at d values of 7.850 and 7.382, respectively, owing to the resonance of the aromatic ring protons (Figure 31.7). These two sets of signals are shifted upfield, broadened and merged to give two broad signals at d values of 7.337 and 6.991, respectively, if D2O solutions of CTAB and naphthols are mixed in 1:1 molar ratio (1.0 mm; Figure 31.7, b). This large shift of aromatic proton resonance to low d values clearly indicates the location of naphthol rings in a less polar environment than water. Previous studies with the CTAB–NaSal system also showed a similar upfield shift of proton resonance of the aromatic moiety of the NaSal molecule, and it was argued that this was due to insertion of NaSal molecules into the micelles [18]. On the other hand, CH3 protons of the CTAB head groups and the adjacent CH2 protons, which resonate at 3.132 and 3.289, respectively, in D2O (Figure 31.7, c), are shifted upfield and resonate at 2.746 and 2.397, respectively, in the presence of 2-naphthol (Figure 31.7, d).

Figure 31.7 1H NMR spectra of the CTAB-2–naphthol system. (a) 1H signal from 2-naphthol; (c) 1H signal from CTAB; (b, d, e) NMR spectrum of CTAB-2–naphthol (1 mM, 1:1). Adopted from Ref. [71]

722 Hydrogen Bonding and Transfer in the Excited State

Figure 31.8 1H NMR spectra of the CTAB-2–naphthol system. (a, b, c) NMR spectrum of CTAB-2–naphthol (10.0 mM, 1:1). Adopted from Ref. 71

However, CH2 protons adjacent to the CTAB head groups are affected most in the presence of naphthols, and, unlike pure CTAB, the signal from CH2 protons emerges on the other side of CH3 protons of the CTAB head groups in the presence of naphthols. This identification is important because it indicates the presence of an aromatic ring of naphthol near the surfactant head groups and close to adjacent CH2 groups. Signals from protons of other parts of the hydrocarbon chain, however, remain unaffected in the presence of naphthols (Figure 31.7, e). The NMR spectra of 10 mM CTAB-2–naphthol (1:1) have further subtle features (Figure 31.8). While the signals from water protons remain well resolved (not shown), the signals from the aromatic protons of the naphthol molecules are broadened dramatically (Figure 31.8, a). This means that, on the NMR timescale, the motion of the naphthol molecules is highly restricted in viscoelastic phase, but water molecules rotate freely [80]. The signals from CTAB protons are, however, broadened to a lesser extent but appear structureless, preventing further analysis (parts b and c in Figure 31.8). It seems that the naphthol molecules are held tightly in the micelles by means of strong hydrophobic interaction and H-bonding (discussed later). The above observation conclusively proves that the solubilized naphthol molecules do not penetrate deep inside the micellar core but are present near the surface, probably with a well-defined orientation in which the OH groups are protruded from the micellar surface towards the polar aqueous phase. A previous study on measuring the ‘apparent’ shift of pKa of 1-naphthol at the micellar surface of CTAB yielded an effective dielectric constant value of 45, indicating that the location of OH groups of naphthol at the micellar surface is fairly polar in nature [81, 82].

31.3.3 Shear-Induced viscoelasticity and the role of OH groups of the dopants Figure 31.9 shows the rheological responses for a representative viscoelastic system, namely the aqueous CTAB-1–naphthol system, as a function of concentration (1:1 molar ratio; this composition yields strongest viscoelasticity) at 25 C (pH5.0). At low concentrations (<2 mM) this system shows a shear-thinning property up to a shear rate of 25 s-1, and then the shear-thickening phenomenon starts to occur, but above a shear rate of 60 s-1 the fluid shows a Newtonian-type behaviour. However, an overall non-Newtonian nature is apparent as the concentration of the CTAB–naphthols (1:1) system is raised above 2.0 mM. At still higher concentrations (>5.0 mM), the rheological response changes dramatically and the system starts displaying an unusual rheology as a function of shear rate. Up to a shear rate of 60 s-1 the fluid shear thins. An onset of viscosity rise

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 723 60

Viscosity/Pa.s

50 5 40 30

4 3

20

2 10

1 7 6

8

Co nc .

5

/m M

2 60

180

120 -1

Shear rate/s

Figure 31.9 Steady shear viscosity as a function of the applied shear rate for dilute and concentrated solutions of 2-naphthol/CTAB at 25 C. The molar concentration ratio [2-naphthol]/[CTAB] was fixed at unity. (1) 1.0 mM; (2) 1.5 mM; (3) 2.0 mM; (4) 5 mM; (5) 10 mM. 6 and 7 are for 1- and 2-methoxynaphthalene respectively with CTAB (5 mM, 1:1). Adopted from Ref. [83]

is observed at a shear rate of 60 s-1, and the system again shear thins, passing through a maximum at 70 s-1. At even higher concentrations (8.5 mM), the viscosity–shear rate profile again changes feature; the initial shear-thinning characteristics disappear. The overall behaviour is consistent with the build-up of long wormlike micellar bundles at relatively high concentrations. Therefore, it appears that the shear-thinning viscosity at low shear rates is clearly indicative of flow-induced alignment towards the flow directions. Meanwhile, when the CTAB concentration is above 10.0 mM in the equimolar CTAB/naphthol solutions, the micelles are much longer and entangled with each other in the solution. In this case the shear viscosity increases much more and the micellar solution behaves like an entangled polymer solution, exhibiting typical non-linear viscoelastic behaviour such as a stress plateau. The contour length of the worm-like micelles is highly dependent on the concentrations of the surfactant and the promoter. The methoxynaphthalene–CTAB system, on the other hand, neither displays an ability to develop viscoelasticity (6 and 7 in Figure 31.9) in the system nor exhibits any viscosity modification with applied shear; it behaves entirely like a Newtonian liquid. This result is quite surprising in view of the fact that, much like 1- and 2-naphthols, both 1- and 2-methoxynaphthalenes (MN) are expected to embed into the micelles of CTAB [84]. Therefore, to ascertain the location and orientation of the additive methoxynaphthalene molecules in the micelles, and to understand the nature of interaction with micelle, 1 H NMR experiments were performed (Figure 31.10). The NMR spectrum of 1-MN in D2O (in the absence of CTAB) signals are centred at d values of 8.151, 7.853, 7.482 and 6.947, respectively, owing to the resonance of the aromatic ring protons. All four sets of signals are shifted upfield, remain well resolved and appear at d values of 8.013, 7.492, 7.147 and 6.487, respectively, when D2O solutions of CTAB and 1-MN are mixed in 1:1 molar ratio (7.5 mM) (Figure 13.10(B)). Similarly, the methoxy protons, which resonate at a d value of 3.953 in water (not shown), are also shifted upfield and resonate at a d value of 3.561 in CTAB. This large shift of aromatic proton resonance to low d values clearly indicates the location of naphthalene rings in a less polar environment than water. Previous study with the CTAB–naphthol system also showed similar upfield shift of proton resonance

724 Hydrogen Bonding and Transfer in the Excited State

Figure 31.10 1H NMR spectra of 1-methoxynaphthalene in the absence (A) and in the presence (B) of CTAB (7.5 mM, 1:1). Adopted from Ref. [83]

of the aromatic moiety of the naphthol molecule, and it was argued that this was due to insertion of naphthol molecules into the micelle [71]. Unlike naphthol–CTAB systems, the absence of line broadening and the well-resolved structures of the NMR signals clearly indicate fast rotation of naphthalene rings in the CTAB–MN systems (on the NMR timescale). However, the degree of upfield shift of the signals is less in 1-MN than that in naphthols; this indicates a stronger partitioning of naphthol molecules in the micelles. In this context, comparison of surface activities of methoxynaphthalenes and naphthols may also be interesting. Figure 31.11 clearly shows that both 1- and 2-naphthols reduce the surface tension (ST) of water to a considerable extent, indicating that naphthols are surface active. Previously it has been shown that 3-hydroxynaphthalene-2-carboxylate (SHNC) also reduces surface tension in a similar manner, and it also effectively promotes micellar shape transition. Interestingly, while SHNC reduced the surface tension of water from a value of 75 to 60 N m-1 in the presence of a 60 mM concentration of SHNC, an identical decrease in surface tension has been brought about by naphthols in the presence of only 0.4 and 0.5 mM concentrations. On the other hand, the surface tension of water is reduced to a similar extent by an even smaller amount of 1- and 2-MNs. While 1-MN reduces the ST of water to 58 N m-1 in the presence of 0.31 mM concentration,

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 725

-1

68 66 64 62 60

A

58

1-methoxynaphthalene 2-methoxynaphthalene

72

1-Naphthol 2-Naphthol

70

Surface Tension / mNm

Surface Tension / mNm

-1

72

70 68 66 64 62 60 58

B 56

1

2

3

4

5

6

7 4

Concentration X 10 M

8

9

10

11

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4

Concentration x 10 M

Figure 31.11 Surface activity of naphthols (A) and methoxynaphthalenes (B) as a function of concentration in water at 25 C. Adopted from Ref. [83]

2-MN has the same effect in the presence of only 0.16 mM concentration. This result indicates that 1-and 2-methoxynaphthalenes are stronger surface-active agents than 1- and 2-naphthols, and therefore these molecules are expected to be strongly embedded into the micelle. Thus, the findings of the NMR experiment are further strengthened by this fresh evidence of surface activities exhibited by methoxynaphthalene molecules. However, in spite of the stronger surface activities shown by 1- and 2-MN, they fail to induce microstructural transition in CTAB micelles. It seems that OH groups in naphthalenes play an important role in this respect. The results show that the naphthols are involved in stronger and different kinds of interaction with the CTAB micelle compared with the methoxynaphthalenes. The surfactant, namely CTAB, forms spherical micelles in the presence of methoxynaphthalene (2–10 mM; 1:1), whereas in the presence of naphthols the CTAB forms long worm-like micelles and even vesicles. 31.3.4 FTIR and cryogenic transmission electron microscopy (TEM) study The FTIR spectra of 2-naphthol in the presence and absence of CTAB micelles are shown in Figure 31.12(A) (inset). The IR spectra of vacuum-dried samples (25 C) provided interesting results (in KBr pellets). (Studying the spectral feature of the OH group of naphthols in aqueous solution was not possible (in the CaF2 cell) owing to overlapping of IR peaks with that of water.) The broad band around 3354 cm-1, which is assigned to the OH stretching of 2-naphthol (typically observed in phenols) is shifted to 3163 cm-1 owing to partitioning in the micelles. The comparatively sharper peak at higher wavelength confirms the presence of well-defined and stronger H bonds in naphthols, which are embedded inside CTAB micelles. The peak at 3050 cm-1, which remains almost unchanged upon gelation, may be assigned to aromatic CH stretch. The above shifting of OH stretching frequency is very much reproducible and consistently displayed by a wide variety of worm-like micellar systems promoted by naphthols. It seems apparent that H-bonding plays an important role in micellar shape transition. The result also shows that vacuum drying at room temperature did not destroy the microstructure completely, although it may have modified it to some extent. This observation is also supported by cryo-TEM experiments. The TEM micrograph looks like a condensed, isotropic and continuous network (Figure 31.12(A)). The structure represents the transition in shape from spherical to rod-like micelles. Interestingly, this cryo-TEM picture is almost identical to that of the microstructure observed under cryo-TEM for cetyltrimethylammonium hydroxide (100 mM) in the presence

726 Hydrogen Bonding and Transfer in the Excited State

Figure 31.12 Cryo-TEM micrographs of the CTAB-2–naphthol system (10 mM, 1:1) at normal pH (6.5) (A) and at high pH (9.8) (B). Inset: FTIR spectra of CTAB-2–naphthol: A – in the absence of CTAB; B – in the presence of CTAB. Adopted from Ref. [83]

of 2-hydroxy-1-naphthoic acid (55 mM) by a previous worker [84]. Freeze-fracture electron microscopy, done with the CTAB–NaSal system, also indicated the formation of isotropic worm-like micelles similar to the one shown in Figure 31.12 along with other morphologies. This result clearly suggests that specific and stronger H bonds are formed via naphthol molecules, which are embedded in micelles because of their favourable orientation. In the absence of charge screening, the surface area per surfactant head group is decreased owing to charge shielding by p-electron systems of naphthols causing micellar shape transition to occur. In the presence of hydrotropes like NaSal, however, both phenomena, namely charge screening as well as H-bonding, may occur simultaneously. Figure 31.12(B) depicts the cryo-TEM micrograph at high pH (9.8). At high pH, ionization of OH groups takes place, and the packing parameter exceeds the critical value of 1/2 via enhanced charge screening by naphtholate ion, leading to unilamellar vesicle formation along with long worm-like micelles.

31.3.5 Ground- and excited-state H-bonding: strength of micelle–dopant interaction In view of the differences in the viscoelastic responses and the morphological transitions of CTAB micelles induced by neutral naphthols and the methoxy naphthalenes, UV absorption spectra of these dopants are interesting in micellar media. To understand the kind of interactions that occur in micelle–dopant systems, the key element of the approach would be to compare the spectral characteristics of naphthols (HN, which contain OH) with those of methoxy naphthalenes (MN, which do not contain OH) under various conditions, and to eliminate the untenable propositions in order to visualize a consistent molecular picture. Aromatic compounds, e.g. naphthalene, generally have two strongly overlapped bands in the near UV region, namely the longitudinally polarized 1 La 1 A band and the transversely polarized 1 Lb 1 A band. While the vibrational structure of these bands appears to be different in different substituted compounds, effects of extension of the conjugation in the 1 and 2 positions by the OH or CH3O group in naphthols and methoxy naphthalene molecules, respectively, are interesting. Both in 1-naphthol and 1-methoxy naphthalene, conjugation is

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 727

extended in the transverse direction and therefore affects the transverse polarized 1 La band. In 2-naphthol and 2-methoxy naphthalene, on the other hand, conjugation is primarily extended in the longitudinal direction, affecting both the intensity and the frequency of the longitudinally polarized 1 Lb band compared with the unsubstituted naphthalene. It is well known that near-UV spectra of aromatic compounds are affected by specific interactions such as hydrogen bonding. Non-covalent interactions such as p–p and cation–p also cause shifts in the electron distributions of the molecule. The OH group of naphthols can act both as proton donors and as proton acceptors in forming intermolecular hydrogen bonding. A hydrogen bond in which the hydroxyl group of naphthols is a proton donor releases electron density from the O–H bond towards the oxygen and hence, by an inductive effect, towards the aromatic ring. This causes a red-shift of the p–p transition. Conversely, if a hydrogen bond is formed in which the hydroxyl oxygen is a proton acceptor, electrons are withdrawn from the naphthalene ring and an opposite shift is anticipated. If both bonds could form at the same time, and with equal ease, as their effects on the partial charges of the oxygen are opposite, the net change on the oxygen and hence on the aromatic ring may be small. Therefore, in such a situation the spectral shift relative to the position of the band in a non-hydrogen-bonding situation ought to be small. The near-UV absorption of 1-naphthol, which arises from two strongly overlapped p–p transitions, remains unaffected in the presence of submicellar aqueous CTAB solution, indicating the absence of any appreciable interaction (Figure 31.13(A)). Interestingly, however, significant red-shift is observed (6.4 nm at the lmax (293 nm)) in the presence of CTAB just above its CMC (0.96 mM), with a well-defined isobestic point at 296 nm. Such shifting of lmax continues until most of the naphthol molecules have been partitioned in the micellar phase. The spectral feature, nature and degree of shift undoubtedly resemble the spectra of naphthol/isoctane at various dioxane concentrations, where naphthol acts as the hydrogen bond donor and dioxane as the acceptor (Figure 31.13(C)). This striking resemblance of spectral features indicates that, like the naphthol–dioxane system, the micelle-embedded naphthols also act as H-donors in forming hydrogen bonds with the interfacial water molecules. On the other hand, the spectra of methoxynaphthalenes exhibit a significant drop in intensity in the presence of 0.33 mM aqueous CTAB, but the intensity gain is displayed on subsequent addition of CTAB until the concentration reaches 5 mM (Figure 31.13(B)) [85]. Previously, it has been shown that, in the ground state, 1-naphthol interacts with water via oxygen, whereas with alcohols (ethanol and isopropanol) and acetonitrile it interacts via hydrogen from the hydroxyl group. The nature of spectral modification encountered by micelle-free naphthol molecules in the presence of water is shown in Figure 31.13(D). The figure shows that, on every addition of water (up to 10% v/v), a substantial gain in intensity is displayed by 1-naphthol spectra (in acetonitrile) with little change in wavelength. It is known that the water molecules at the micellar interface have some strange properties. The solvation dynamics slowed down by several orders of magnitude relative to bulk water. The reorientational motion is also restricted. The dynamics of water molecules near an aqueous micellar interface is the subject of intense current interest because such a system serves as a prototype of complex biological systems. However, although the dielectric constant at the CTAB micellar surface was shown to be 30, the location of the protruded OH groups of the embedded naphthol molecules experiences a dielectric constant of 45 [81, 82]. Therefore, like alcohols and acetonitrile, water at the interface acts as an H- acceptor at a low dielectric constant. Furthermore, the H-bonds that are formed at the interface are strong and remain stable even at temperatures of 70 C (Figure 31.14(B)). Oxygen K absorption and emission spectra of water molecules at the micellar interface also show that local electronic structure of water molecules is dramatically different from that of bulk water [86]. The relatively less polar and less mobile water molecules compared with bulk water form strong H-bonds with the OH groups of embedded naphthols, which act as H-donors and result in an optimum orientation of aromatic p-electron systems in the micelles to shield the surfactant head group charges efficiently; perhaps via cation–p interaction, i.e. the cation charge of the surfactant head groups interacts with the quadrupole moment of the aromatic p-system of naphthols [85].

728 Hydrogen Bonding and Transfer in the Excited State

Figure 31.13 (A) Absorption spectra of 1-HN (0.25 mM) in water at varying concentrations of CTAB at 25 C. [CTAB]: (1) 0.0 mM; (2) 0.44 mM; (3) 0.55 mM; (4) 0.75 mM; (5) 1.00 mM; (6) 1.25 mM; (7) 1.50 mM; (8) 1.75 mM; (9) 2.00 mM; (10) 2.50 mM; (11) 3.00 mM; (12) 3.50 mM; (13) 4.00 mM; (14) 5.00 mM; (15) 5.00 mM; (16) 0.00 mM. (B) Absorption spectra of 1-MN (0.25 mM) in water at varying concentrations of CTAB at 25 C. [CTAB]: (1) 0.33 mM; (2) 0.50 mM; (3) 0.75 mM; (4) 1.00 mM; (5) 1.50 mM; (6) 2.00 mM; (7) 2.50 mM; (8) 3.00 mM; (9) 3.50 mM; (10) 4.00 mM; (11) 5.00 mM. (C) Absorption spectra of 1-HN (0.25 mM) in isooctane at various concentrations of 1,4-dioxane at 25 C. [Dioxane]: (1) 0.00 mM; (2) 13 mM; (3) 20 mM; (4) 40 mM; (5) 50 mM; (6) 80 mM; (7) 100 mM; (8) 160 mM; (9) 200 mM. (D) Absorption spectra of 1-HN (0.25 mM) in acetonitrile at different % of water at 25 C. % of water: (1) 0.00%; (2) 4%; (3) 6%; (4) 8%; (5) 10%. Adopted from Ref. [83]

Figure 31.14 (A) Absorption spectra of 1-MN (0.25 mM) in water at varying concentrations of CTAB at 50 C. [CTAB]: (1) 0.52 mM; (2) 0.78 mM; (3) 1.75 mM; (4) 2.63 mM; (5) 3.95 mM; (6) 5.92 mM; (7) 8.88 mM; (8) 13.33 mM; (9) 20.00 mM. (B) Absorption spectra of 1-HN (0.25 mM) in water at varying concentrations of CTAB at 70 C. [CTAB]: (1) 0.0 mM; (2) 0.73 mM; (3) 0.98 mM; (4) 1.31 mM; (5) 1.63 mM; (6) 2.04 mM; (7) 2.56 mM; (8) 3.2 mM; (9) 4.00 mM; (10) 5.00 mM; (11) 7.50 mM; (12) 10.00 mM; (13) 15.00 mM; (14) 20.30 mM. Adopted from Ref. [85]

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 729

As the H atom of OH is replaced by a CH3 group (namely a methoxynaphthalene molecule), the ability of intermolecular H-bond formation disappears. Instead, the H-accepting tendency from a potential donor is enhanced. The nature of changes encountered in the UV spectra of methoxynaphthalenes on the addition of CTAB indicates the permeation of the dopant molecules in the micelles. The small red-shift compared with that in naphthols indicates a weaker non-covalent interaction to take place. The large drop in intensity on the first addition of 0.33 mM CTAB is the signature of breaking of H-bonds with bulk water molecules. At high temperature (50 C), methoxynaphthalene spectra show a gain in intensity as a function of the CTAB concentration only (no shifting of spectra; an H-bond is present in neat water at 50 C) owing to a non-polar environment in the micellar phase. Electrostatic interaction between the cationic charge of surfactant head groups and the quadrupole moment of the aromatic p-system of the methoxynaphthalenes ceases to occur at high temperature (50 C) (Figure 31.14(B)). The result of unusual H-bonding of CTAB–naphthol systems may be relevant not only when considering the H-bonding of the interfacial water molecules in the specific micelle and dopants studied here but also for the H-bonding interaction of other micelle–dopant systems [86]. The result of a fluorescence spectral study as a function of shear rate (Figure 31.6) suggests that H-bonding which is effective in the ground state of naphthols occurs in the excited state also. The binding constant Ks (and surfactant CMC) of naphthols and methoxynaphthalenes with CTAB micelles are determined from the study of the effect of added surfactant on the absorption spectra of the dopants using the following relationships: f =ð1f Þ ¼ Ks f½D½St f gKs CMC where f ¼ [Sm]/[St] and Dm ¼ [Dt]  CMC (subscript t refers to total). The above equation is derived assuming that the following equilibrium holds between the aqueous solubilisate (Sw) and the CTAB micelles (Dm) to form the micelle-embedded substrate (Sm): Sw þ Dm ¼ Sm Experimentally, f is calculated from f ¼ (A  Aw)/(Am  Aw), where A, Aw and Am are absorption intensities in surfactant, in water and at complete micellization of the substrate respectively (insets of Figure 31.13(A) and (B)). The binding constant value for naphthol–CTAB is about threefold higher than that of methoxynaphthalene–CTAB systems (Table 31.2). This result is quite pertinent in view of the strong H-bonding interaction in the former system. Owing to strong interaction of CTAB with naphthols, CMC is also modified (literature value 0.96 mM) to an extent unlike methoxynaphthalene, which does not alter the CMC of CTAB. Therefore, unlike methoxynaphthalenes, naphthols interact with micelles strongly, and the UV spectra of naphthols are modified, showing significant red-shifting, and display a sharp isosbestic point owing to strong H-bonding interaction with interfacial water molecules. On account of their directionality and spatial arrangement, complementary multiple H-bonding interactions at the micellar interface lead to the engineering Table 31.2 Binding constant values for 1-naphthol and 1-methoxynaphthalene Dopant 1-Naphthol 1-Methoxynaphthalene

Ks

CMC

1325 (r ¼ 0.9981) 448 (r ¼ 0.9918)

1.20 mM 0.96 mM

730 Hydrogen Bonding and Transfer in the Excited State

Figure 31.15 Schematic representation of the microstructures found in worm-like micelles formed by naphthols and spherical micelles formed by methoxynaphthalenes with CTAB. Adopted from Ref [83]

of a well-defined supramolecular structure via the micellar head group charge shielded by p-electron systems of naphthols (Figure 31.15).

31.4 Photochemistry and Photophysics of Hydroxyaromatic Compounds [87] The polar and apolar characteristics of solvents are known to control the reactivity as well as the physicochemical characteristics of a process at the molecular level. As a result, solvents play a significant role in chemical and biological processes. Electronic absorption and emission spectra of a molecule dissolved in a solvent medium in general are shifted in energy relative to the spectra of the isolated molecule. In some cases, these solvent shifts amount to more than 30% of the energy of the isolated-molecule transition, although shifts of the order of 102–103 cm-1 are more common. These effects have been the subject of many investigations directed towards defining the molecular bases of the shifts. Many, especially the larger, shifts are attributed to specific chemical effects of the solvent on one or both electronic states of the chromaphore. Some important specific effects are: hydrogen bond formation; proton or charge transfer between solvent and solute; solvent-dependent aggregation, ionization and isomerization equilibria. The solvent effects in general have been known to cause spectral shifts in fluorophores that are sensitive to the environment. It is well known that, owing to the general solvent effects, fluorescence spectral shifts occur. However, this theory is often insufficient for explaining the behaviour of fluorophores in different environments because in many cases the interaction of the fluorophore with the local environment can also shift the spectra by an amount comparable with the general solvent effect. Although the above theory cannot account for the detailed behaviour of many fluorophores, a valuable framework for consideration of solvent-dependent spectral shifts can still be obtained.

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 731

A better description of the spectral displacement of absorption and emission bands due to solvent effects both in the ground and the excited state can be accounted for by the dependence of the Stokes shift with the solvent. The general effects of the solvent are described by considering the fluorophore as an electric dipole residing in a cavity of radius a in a continuous medium of uniform dielectric constant « and refractive index n. According to the model developed by Lippert [88] and Mataga et al. [89], the energy difference between the ground and the excited state, in wave numbers, is given by the equation
nA 
nF ¼

2Df ðm m Þ2 þ const hca3 E G

ð31:1Þ

where
nA and
nF are the wave numbers (in cm-1) of absorption and emission respectively, h is Planck’s constant, c is the speed of light, mG and mE are the dipole moments of the solvent in the ground state and excited state respectively and a is the radius of the cavity in which the fluorophore resides. From Lippert’s equation, a linear dependence between the Stokes shift, represented by the difference in wave numbers D
n, and the orientation polarizability, expressed as Df , is expected. The term Df is expressed as Df ¼

D1 n2 1  2 2D þ 1 2n þ 1

The first term i.e. (D  1)/(2D þ 1) accounts for the spectral shifts due to the reorientation of the solvent dipoles as well as the redistribution of the electrons in the solvent molecules. The second term, (n2  1)/ (2n2 þ 1), accounts only for the redistribution of electrons. The variation in Stokes shift with the solvent polarity function, i.e. Df , of different solvents is a useful way to determine the dipole moment of the fluorophore both in the ground state as well as in the excited state. The variation can also be used to estimate the polarity of a new solvent. The value of Df in a non-polar solvent would be zero and expected to increase with the polarity of the solvent used. The maximum absorption band of 1-naphthol in water appears at 292–300 nm, while the maximum absorption bands in different alkanols with increasing alkyl chain length appear in the range of 320–331 nm, and, upon excitation, the fluorescence maximum of 1-naphthol in water appears at 460 nm, but in alkanols, with increasing alkyl chain length, the fluorescence maxima appears in the range 350–352 nm. The intensities of both the absorption and the fluorescence bands also increase with increase in alkyl chain length of the alkanols. The results obtained using the above equation for 1- and 2-naphthols in the presence of different alkanols, namely methanol, ethanol, isopropyl alcohol, tert-butyl alcohol, pentanol, hexanol, heptanol and octanol, and in 1,4-dioxane–water mixtures of different compositions, namely 10, 20, 30, 50, 60 and 80% (w/v), are shown in Figure 31.16. For comparison, the corresponding value in pure water is also shown. A linear relationship between D
n and Df has been observed for both 1- and 2-naphthols in all the alkanols, showing evidence of the general solvent effect. However, in dioxane–water mixtures the plots are not linear. The change in the position of the OH group in the naphthalene ring has a profound effect on the sensitivity of the probe towards solvent polarity. The study in alkanols shows that 1-naphthol experiences higher D
n values, and thus seems to be more sensitive than 2-naphthol to changes in the polarity of the solvent. In dioxane–water mixtures, the difference in the sensitivity of 1-naphthol is very large. The nonlinearity of the plots for both probes in dioxane–water mixtures shows in general the dominance of the specific solvent effect, not included in Lippert’s model, in the spectral shifts, and also points to the dual nature of the solvents, hydrophilic as well as hydrophobic.

732 Hydrogen Bonding and Transfer in the Excited State 10000

A

5

6

3

4

2800

1

2

B

8000

1

Water and Alkanols Dioxane-water mixture

2600

1

2 3

−1

Water and Alkanols Dioxane-water mixture

6000

Δν / cm

Δν / cm

−1

2400

4000

8

9

2000

4

5

6

7

2

3

2200

1 2

2000

3

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0.22

0.24

6

8 9

1600

0.20

0.26

0.28

0.30

4

4

0.32

0.20

6

0.22

5

5

7

0.24

0.26

Δf

0.28

0.30

0.32

Δf

Figure 31.16 Dependence of Stokes shift with orientation polarizability D f for (A) 1-naphthol and (B) 2-naphthol in pure solvents (&) and 1,4-dioxane–water mixtures (~). Solvents (&): (1) water; (2) methanol; (3) ethanol; (4) isopropyl alcohol; (5) tert-butyl alcohol; (6) pentanol; (7) hexanol; (8) heptanol; (9) octanol. Dioxane–water mixtures (dioxane %, v/v) (~): (1) 10%; (2) 20%; (3) 30%; (4) 50%; (5) 60%; (6) 80%

The solvent parameters such as the dielectric constant (D), the refractive index (n), the Kosower Z-value, etc., can be correlated with the Stokes shifts D
n for both the naphthols in water, different alkanols and in dioxane–water mixtures. The plot of Stokes shift D
n against the refractive index function (n2  1)/(2n2 þ 1) and the dielectric constant function (D  1)/(2D þ 1) of the different solvents and in 1,4-dioxane–water mixtures are shown in Figures 31.17 and 31.18. Figure 31.19 shows the linear variation of the Stokes shift with the Kosower Z-value for both the dopants. The linearity of these plots is often regarded as evidence for the dominant importance of the general solvent effect in spectral shifts. The plot of Stokes shift D
n versus solvent polarity function [(D  1)/(2D þ 1)  (n2  1)/(2n2 þ 1)] is also found to be linear where the general solvent effect occurred. The non-linearity of the Lippert plot (plot of Stokes shift versus solvent polarity function) is due to the specific solvent effects. This is (n2-1) / (2n2+1)

(n2-1) / (2n2+1) 0.16

0.18

0.20

0.22

0.17

0.24

10000

0.19

0.21

0.23 1

1 2

1 2000

8000

2

3

4000 1 2000

A 0 0.40

2

3 4 54 3 2 5

9 8 766 7 0.45

(D-1) / (2D+1)

4

4

1800

6000 – cm-1 Δv/

– cm-1 Δv/

3

1600 8

5 5 68 6 7 7

9

1400 8

B 0.50

0.42

0.44

0.46

0.48

(D-1) / (2D+1)

Figure 31.17 Plot of Stokes shift D
n against the dielectric constant function and refractive index function for (A) 1-naphthol and (B) 2-naphthol in pure solvents. Solvents: (1) water; (2) methanol; (3) ethanol; (4) isopropyl alcohol; (5) tert-butyl alcohol; (6) pentanol; (7) hexanol; (8) heptanol; (9) octanol

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 733 (n2-1) / (2n2+1)

(n2-1) / (2n2+1) 0.16

0.17

0.16

0.18

0.17

0.18

0.19

1

0.19

0.20 1

9500 1

1 2

2500

2

2

2 3

4

– / cm-1 Δv

– / cm-1 Δv

9000

4 5

2000 4

4 5

5

5 6

6

1500 6

6

A

3

3

3

B

8500 0.42

0.44

0.46

0.48

0.42

0.44

(D-1) / (2D+1)

0.46

0.48

0.50

(D-1) / (2D+1)

Figure 31.18 Plot of Stokes shift D
n against the dielectric constant function and refractive index function for (A) 1-naphthol and (B) 2-naphthol in 1,4-dioxane–water mixtures. (Dioxane %, v/v): (1) 10%; (2) 20%; (3) 30%; (4) 50%; (5) 60%; (6) 80%

because there is a possibility of hydrogen bonding between the polar solvent molecule and polar group on the fluorophore. However, the major reason may be the modified excited-state proton transfer (ESPT) in non-aqueous solvents. Stokes shifts are higher in water than in other solvents, which can be interpreted as being due to the higher dielectric constant of water compared with other solvents, but principally as being due to the ESPT process. While comparing all plots of Stokes shift against orientation polarizability, it is assumed that the Stokes shifts are approximately proportional to the orientation polarizability, i.e. solvent polarity function.

2200 1 Naphthol

1

1

2 Naphthol

8000 2 3

-1

6000

Δν /cm

Δ ν / cm-1

2000

4000

2000

9

8 7

6

54

4

1800

2

3

6 8

9

300

5

1600

320

340

360

Z KJ mol

-1

380

400

300

7

320

340

360

Z KJ mol

380

400

-1

Figure 31.19 Plot of Stokes shift D
n against the Kosower Z-value for 1-naphthol and 2-naphthol in pure solvents. Solvents: (1) water; (2) methanol; (3) ethanol; (4) isopropyl alcohol; (5) tert-butyl alcohol; (6) pentanol; (7) hexanol; (8) heptanol; (9) octanol

734 Hydrogen Bonding and Transfer in the Excited State

In the case of interactions between the fluorophores and dioxane–water mixtures, excess Stokes shifts are observed in the order 10% > 20% > 30% > 50% > 60% > 80%. This may also be due to ESPT and the involvement of hydrogen bonding between the protic group of the fluorophore and the solvents. The 1- and 2-naphthols contain OH groups that are capable of forming hydrogen bonds. In all these cases, the Stokes shift increases with increase in the solvent polarity function. Immediate spectral shifts, attributed to the polar nature of both solvent and fluorophore, occur. They are polar and are associated in the ground state. If they were associated in the excited state, only these properties would have been dependent on the rates of diffusion of the polar solvent and the fluorophore. In these cases, the dependence on the concentration of the polar solvent would be similar to that for quenching reactions. From the plot of Stokes shift as a function of dielectric constant (D  1)/(2D þ 1) in different solvents and in the presence of the indicators, the Stokes shifts become larger as the dielectric constant function increases. Maximum shift is found in water with a maximum dielectric constant function owing to its large dielectric constant. Here, the spectral shifts are due both to the orientation of the solvent dipoles and to the redistribution of electrons in the solvent molecules. While moving towards the other solvents, namely pentanol, hexanol, heptanol and octanol, the shift is less owing to the dielectric constant values. In the plots of Stokes shift versus refractive index function (n2  1)/(2n2 þ 1), the opposite effects are found, i.e. Stokes shift decreases with increase in the refractive index function as well as the refractive index of the solvents itself. The term refractive index function accounts only for the redistribution of electrons. In the case of polar solvent water, the largest shift is found to be due to the minimum refractive index value of water. Actually, the refractive index function affects the Stokes shift less than the dielectric constant function because, according to Lippert’s theory, only solvent reorientation is expected to result in a substantial Stokes shift. Considering the similar plots of all the additives in dioxane–water mixtures, the maximum shift is observed in a 10% dioxane–water mixture owing to its maximum dielectric constant value. In all cases, the variation in Stokes shift with the dielectric constant function is linear. As the solvent properties near the interface of the micelles and reverse micelles of surfactants compare the solvent properties of dioxane–water mixture, these plots are useful for determining the dielectric constant and refractive index of the micelles. It has also been observed that the variation in Stokes shift with the solvent polarity function, i.e. orientation polarizability [(D  1)/(2D þ 1)  (n2  1)/(2n2 þ 1)], of different solvents is a useful way to determine the dipole moment of the solute (fluorophore) both in the ground state as well as in the excited state [89, 90]. The variation often can also be used to estimate the polarity of a new solvent. On the basis of the solvent effect on ultraviolet spectra, Kosower [90, 91] established the scale of solvent polarity, which he called the Z-values. The scale is based on the particularly solvent-dependent absorption band, the charge transfer band of the 1-alkyl pyridinium iodides. The high solvent dependence of this band is understandable in terms of the foregoing arguments, as the transition, as charge transfer transition, involves a large change in polarity. The scale of the Z-value obtained by Kosower is parallel to the Grunwald and Winstein Y-values, a kinetic scale of solvent polarity, as far as data are available for comparison [92]. The plot of Stokes shift versus Kosower Z-values for 1- and 2-naphthols in water and different alkanols is found to be in good agreement with the Kosower Z-value scale. The absorption maxima are transformed to Kosower Z-values (kcal mol-1) using the following relation: Z ¼ 2:859 105 =l 

where l is the wavelength of the absorption maxima (in A). The transition energies (both absorption and fluorescence) of 1- and 2-naphthols with Stokes shift in different media (in cm-1) are represented in Tables 31.3 and 31.4.

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 735 Table 31.3 Transition energies of absorption and emission of 1-naphthol in water and different alkanols along with the solvent parameters at 298 K Solvent

Water Methanol Ethanol Isopropyl alcohol tert-Butyl alcohol Pentanol Hexanol Heptanol Octanol

Transition energy (
n) (cm-1) Absorption

Emission

31 447 31 250 31 056 30 864 30 769 30 675 30 488 30 303 30 211

22 727 28 571 28 490 28 490 28 490 28 409 28 409 28 409 28 409

Stokes shift (D
n) (cm-1)

Dielectric constant (D)

Refractive index (n)

Kosower Z (kJ mol-1)

8720 2679 2566 2374 2279 2266 2079 1894 1802

78.4 32.6 24.3 20.1 17.1 15.2 13.4 11.2 10.0

1.3333 1.3288 1.3576 1.3859 1.3993 1.4101 1.4198 1.4249 1.4509

395.8 349.8 333.1 327.6 325.1 315.5 311.3 304.2 303.3

Table 31.4 Transition energies of absorption and emission of 2-naphthol in water and different alkanols along with the solvent parameters at 298 K Solvent

Water Methanol Ethanol Isopropyl alcohol tert-Butyl alcohol Pentanol Hexanol Heptanol Octanol

Transition energy (
n) (cm-1) Absorption

Emission

30 303 30 120 30 030 29 940 29 674 29 586 29 499 29 412 29 326

28 169 28 090 28 090 28 090 28 011 27 933 27 933 27 778 27 778

Stokes shift (D
n) (cm-1)

Dielectric constant (D)

Refractive index (n)

Kosower Z (kJ mol-1)

2134 2030 1940 1850 1663 1653 1566 1634 1548

78.4 32.6 24.3 20.1 17.1 15.2 13.4 11.2 10.0

1.3333 1.3288 1.3576 1.3859 1.3993 1.4101 1.4198 1.4249 1.4509

395.8 349.8 333.1 327.6 325.1 315.5 311.3 304.2 303.3

31.5 Excited-State Proton Transfer (ESPT) of hydroxyaromatic compounds Excited-state molecular processes are those that change the structure of the excited-state fluorophore and take place subsequent to excitation phenomena. Such reactions occur because light absorption frequently changes the electron distribution within a flurophore, which in turn changes its chemical and physical properties. The best-known example of an excited-state reaction is that of phenols and naphthols, which in neutral solution can lose the phenolic or naphtholic proton in the excited state. Deprotonation occurs more readily in the excited state because the electrons on the phenolic/naphtholic hydroxyl groups are shifted into the aromatic ring, making this hydroxyl group more acidic.

736 Hydrogen Bonding and Transfer in the Excited State

Intermolecular proton transfer is perhaps the most important elementary reaction in the condensed phase, especially as it is critical for many biological processes. How gas-phase work contributes to our understanding of acid–base reactions in the condensed phase has been convincingly demonstrated. As the making and breaking of hydrogen bonds are involved in the proton transfer reaction, one can learn much about the properties of the most important non-covalent bond. In addition, the current level of both theory and experiments has become high enough to yield synergies in developing a detailed description of this bimolecular reaction. Thus, by studying proton transfer reactions, one obtains information on all three of the issues mentioned above. 1-Naphthol was one of the first molecules identified as an excited-state acid. When F€orster [93] and Weller [94] studied the photochemistry of 1-naphthol in aqueous solution, they observed a strong red-shifted emission over a large pH range in which the ground state was not dissociated. By comparison with deprotonated naphthol at high pH, this was assigned as originating from electronically excited naphtholate anions. The red-shifted naphtholate emission is still considered to be the major hallmark of ESPT (Figure 31.20). Photoinduced ESPT can be considered to be a reaction induced by a pH jump of the system, and is thus a modern example of a relaxation method, introduced to study the kinetics of fast reactions. Excited-state reactions are not restricted to deprotonation or ionization only. Many dynamic processes that affect fluorescence can be interpreted in terms of excited-state reactions. These processes include spectral relaxation, resonance energy transfer and excimer formation. Excited-state reactions occur in biochemical systems, such as energy transfer between fluorophores in phycobiliproteins and during photosynthesis. In spite of the diversity of phenomena, excited-state processes display characteristic time-dependent decays that can be unambiguously assigned to the presence of an excited-state process [73].

Figure 31.20 A schematic representation of excited-state proton transfer, similar to a F€ orster diagram. 1-Naphthol (1-NpOH) is more acidic in its excited electronic state; in aqueous solution, proton transfer to the solvent M and formation of the naphtholate anion (1-NpO) are energetically favourable. As the charge separated ground state is energetically higher than the covalent ground state, emission from the naphtholate anion is strongly red-shifted relative to the excitation wavelength

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 737

Phenomena that display the characteristics of an excited-state reaction Phenomenon

Reaction

Compound

pKa

pKa

Proton loss

ROH > RO þ H þ

Phenol 1-Naphthol 2-Naphthol 2-Naphthylamine 2-Naphthylamine (protonated) Acridine Benzoic acid 1-Naphtholic acid Anthracene Carboxylic acid

10.0 9.2 9.5 7.14 4.1 5.1 4.2 3.7 3.7

4.1 2.0 0.4 12.2 1.5 10.6 9.5 7.7 6.9

Proton gain

RNH2 > RNH þ þ H þ RNH3 þ > RNH2 H þ ArH þ H þ > ArNH þ RCO2  þ H þ > RCO2 H

While a variety of phenomena can be described as excited-state reactions, excited-state chemical reaction is the loss or gain of protons. Whether a fluorophore loses or gains a proton in the excited-state reaction is determined by the directions of the change in pKa in the excited state. If the pKa increases (pKa > pKa, where the asterisk denotes the excited state), then the fluorophore would tend to lose a proton in the excited state [73]. The best-known examples are naphthols and acridines. Phenols undergo excited-state deprotonation and acridines undergo excited-state protonation. Electron donors such as –OH, –SH and –NH2 have a lone pair of electrons, and these electrons tend to become more conjugated to the aromatic ring system in the excited state, resulting in pKa < pKa. Electron acceptors such as –CO2 and –COOH have vacant p-orbitals into which electrons can be transferred in the excited state. This increased electron density results in weaker dissociation in the excited state; pKa > pKa [95]. Borgis and Hynes [96] suggested a distinction between adiabatic and non-adiabatic proton transfer reactions, as illustrated in Figure 31.21. In phenols and naphthols, two close-lying excited states are generally involved in the ESPT process, termed La and Lb, the latter generally being energetically lower. The coupling between these naphthalene-like states is mediated by the solvent or by intramolecular motions. If the excited states of the naphthol are (initially) only weakly coupled and largely retain their character, the ESPT process is termed non-adiabatic. Owing to various effects, to be discussed below, the two states will move with respect to each other during the reaction and become more mixed, or inverted, during ESPT. If, on the other hand, the solvent induces a strong coupling between the reactant excited states prior to excitation, two new adiabatic potential energy surfaces are formed. These are no longer well described as only La or Lb, and must be more generally denoted as S1 and S2 states. The barrier for motion of the proton can, as a result, be low enough for the first vibrational eigenstates to be above the barrier, permitting ESPT to occur spontaneously (i.e. without thermal activation) after excitation. This situation is termed adiabatic ESPT, proceeding on a single surface that changes its character (from Lb- to La-like) along the reaction coordinate.

31.6 ESPT of Hydroxyaromatic Compounds in Organized Media and Some Unusual Emission Phenomena Among hydroxyaromatic compounds, 1- and 2-naphthols and their derivatives stand out owing to their extremely fast deprotonation rates. As a result of ultrafast deprotonation in an aqueous medium, the intensity of the neutral emission of 1-naphthol is extremely low and one observes almost exclusively the anion emission. Equilibrium, as well as the dynamics in a confined environment and at various interfaces, plays a vital role in many biological and natural processes. Proton transfer reactions in the ground and excited states have been

738 Hydrogen Bonding and Transfer in the Excited State

Figure 31.21 When the excited electronic states in naphthol initially retain their original character, either La or Lb, proton transfer follows a non-adiabatic mechanism (upper trace). However, the relative positions and natures of the states are modulated by vibronic coupling and solvent reorganization. In adiabatic proton transfer (lower trace), the two states are strongly mixed prior to excitation. Proton transfer proceeds on a single surface that is not well described as either Lb or La

studied in various microheterogeneous systems such as micelles, liposomes, Langmuir–Blodgett films, etc. Many ultrafast processes are significantly retarded in confined environments. For example, the rate of the intermolecular charge transfer process of different probes is reduced by 1–2 orders of magnitude in microemulsions and in other organized media. Some excellent references on this subject are available in the literature [76–78, 97, 98]. Photoinduced proton transfer reactions can be used to investigate the local properties and structure of various organized molecular systems and to determine the nature and mechanism of biologically important proton transfer phenomena [96]. These reactions have also been studied in organized molecular systems using hydroxyaromatic compounds (1- and 2-naphthols) of moderate hydrophobicity whose location inside the microphase was defined. Significant slowing down of photodissociation is observed in micellar media. Interestingly, experimental results are rationalized in terms of the effect of the specific location of the naphthol and naphtholate moieties of different hydrophobicity in the micelle. Ultrafast intermolecular proton transfer in excited states is also markedly slowed inside cyclodextrin cavities. More dramatic retardation has been observed in the case of solvation dynamics. While the solvation dynamics of water molecules occurs on the subpicosecond timescale in ordinary water, it is retarded by at least three orders of magnitude to the nanosecond timescale inside cyclodextrin cavities, micelles and microemulsions [97]. It has been demonstrated that in aqueous solution the rate of deprotonation of protonated aminopyrene increases nearly 3 times when the probes bind with cyclodextrin, while for 1-naphthol the deprotonation becomes almost 20 times slower. The marked increase in the intensity and lifetime of neutral emission and in

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 739

the rise time of anion emission for 1-naphthol in SDS, CTAB and TX-100 (R) has been demonstrated [78]. Whereas for cationic CTAB the rise time of anion emission is similar to the decay time of neutral emission, for SDS and TX-100 the rise times are faster than the decay time of the neutral form. This observation has been attributed to the different locations of the probe molecules inside the micelle. This result is consistent with the earlier observation that, even in alcohol–water mixtures with a high alcohol content, the rise time of the anion emission is faster than the decay time of the neutral form. For anions of the probe in CTAB and TX-100 (R), a large increase in intensity as well as lifetime is observed. For SDS, however, the intensity of anion emission decreases while its lifetime remains very close to that of the free probe in water. It was believed that for SDS the anion emission arises mainly from the free probe molecules in bulk water. For long-chain alkyl-substituted 2-naphthols, the ESPT rate constants are shown to decrease and the pKa to increase in CTAB micellar solutions in comparison with 2-naphthol, although they are almost the same in homogeneous solution [98]. Similar effects of alkyl substitutions were also observed for pKa in the ground state. These phenomena were also rationalized on the basis of the different locations of neutral and anions in the micelles. The effect of alkyl substitution is discussed in terms of the Gibbs energies of the different steps of the photolytic reaction at different location sites of alkyl-substituted and non-substituted naphthols inside the micelle. The authors argued that the difference in the ESPT rate constants is too large to be explained by the difference in the energy of transformation of excited naphthols to the ion pair in media with various dielectric constants in the range 16–36. A very low polarity (dielectric constant value of 4–8) of the localization site of alkyl naphthol is associated with the observed value of the rate constant. To explain the phenomenon, the authors suggested different energies of water cluster formation at the different localization sites of 2-naphthol and its derivatives. Similarly, the change in pKa is mainly relative to the large energies of water cluster formation in the more hydrophobic regions of localization of the alkyl-substituted naphthols. An additional increase in the activation energy for alkyl-substituted naphthols can be attributed to the larger energy of charge separation at less polar localization sites. In contrast to aqueous solutions, fluorescence decay of naphthol in micelles of dodecyltrimethylammonium bromide (DTAB) was found to be biexponential. The rise time of the fluorescence of the anion of naphthol, almost in all cases, coincides with the lifetime of the faster component of the excited naphthol molecules [77]. The fluorescence kinetic curves for 2-naphthol and the anion of 1-naphthol are shown in Figures 31.22 and 31.23. The rate constant of ESPT of hydroxyaromatic compounds in Brij 56 and in the alkyl sulphates also decreased as compared with those in aqueous solutions because of the lower polarity and higher microviscosity of the micelles [99]. For CTAB micelles, the retardation effect of the micelles is compensated for by the catalytic effect of the micellar potential. Contrary to the earlier idea, it is argued that the photoprotolytic dissociation process does not need any exit of the reactant from the micellar phase because the product is formed initially within the micellar phase. However, the rate constant of photolytic dissociation decreases and the rate constant of exit of the product from the micelle increases with growth of the length of the micellized surfactant. Protolytic photodissociation of 1- and 2-naphthols and substituted naphthols was also studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. The authors believed that there were two different localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes or vesicles there were two comparable fractions of naphthol molecules, one of which underwent photodissociation while the other did not dissociate. In micelles, minor fractions (a few percent) of naphthol molecules, which were probably located in the core of the micelles, did not take part in the ESPT reaction. These phenomena also reflect the heterogeneous structure and dynamics of lipid bilayer membranes and micelles. Partial dissociation of naphthol in the excited state in liposome membranes was observed by Sujatha and Mishra [100]. The authors found that 1-naphthol distributes between two different sites of different excited state reactivity in liposome, and the relative population at the two different sites changes with any perturbation

740 Hydrogen Bonding and Transfer in the Excited State

Figure 31.22 Fluorescence decay curve (0.125 ns channel-1) of 2-naphthol in DTAB micelles. Adapted with permission from [77]. Copyright 1991 American Chemical Society

Figure 31.23 Fluorescence decay curve (0.125 ns channel-1) of the anion of 1-naphthol in DTAB micelles. Adapted with permission from [77]. Copyright 1991 American Chemical Society

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 741

in membrane fluidity. The microenvironment of the naphthol probe in a poly(vinylpyrolidone)–SDS couple was very different from that in SDS micelles [78]. In PVP–SDS complexes there are broadly two different microenvironments. In one of them, the polymer chain shields the probe 1-naphthol molecule from bulk water, resulting in almost total suppression of the deprotonation process. In the other environments, the probe remains partially exposed to the water molecules, and, as a result, the ESPT process occurs on a timescale much lower than that in bulk water. To evaluate the hydroxyaromatic compounds, namely 1-naphthol, as a convenient fluorescence probe for monitoring ethanol-induced interdigitation in lipid bilayer membranes, it was observed that the partition coefficient values and the quenching fractions obtained supported the process of redistribution of naphthol that takes from the inner core to the surface site of interdigitation [79]. 31.6.1 Fluorescence quenching by H-bond strengthening It has been reported by Zhao et al. for the first time that fluorescence quenching can be induced by hydrogenbonding interactions for fluophores in hydrogen-bonding surroundings and is explained by the hydrogenbonding dynamics in the fluorescence state [101]. Both theoretically and experimentally it has been shown that solute–solvent intermolecular photoinduced electron transfer reactions occur much faster than the solution dynamics. In these cases, the important role of the hydrogen bonds for facilitating such an ultrafast intermolecular electron transfer is demonstrated. Fluorescence quenching of oxazine 750 dye in protic solvents has been attributed to solute-solvent intermolecular photoinduced electron transfer from protic alcohols to the OX 750 chromophore, which is in turn facilitated by the intermolecular H-bond. It has been found theoretically that ultrafast intermolecular electron transfer exhibits selectivity towards the intermolecular H-bonding sites. It is believed that the ultrafast intermolecular ET reaction can be facilitated by strengthening of the selected H-bonds in the excited states. Transient enhancement of the H-bonding interactions for facilitating ultrafast intermolecular ET reactions may be extended to other ET reaction systems accounting for fluorescence quenching [101]. The calculated H-bond binding energies for H-bonded coumarin 102–(phenol)1,2 complexes and the corresponding H-bond lengths in the excited states have been determined [102]. It is seen that the hydrogen bond OH OH between the phenols remains nearly unchanged upon electronic excitation. However, the binding energy of the hydrogen bond C¼O–HO between coumarin 102 and phenol in the excited state is substantially larger that in the ground state for both the C102–phenol and C102–(phenol)2 complexes. Therefore, the intermolecular hydrogen-bonding interactions between C102 and phenols are evidently strengthened in the electronically excited states. Thus, intermolecular early-time hydrogen-bond strengthening behaviour for chromophores in H-bonding solvents upon electronic excitation is displayed. The novel transient hydrogen-bond strengthening behaviour upon photoexcitation may take place widely in many other systems in solutions. As the fluorescence of a chromophore in hydrogen-donating environments can be strongly quenched via H-bonding interactions, ultrafast hydrogen-bond strengthening may play an important role in directly influencing the early-time radiationless deactivation processes associated with fluorescence quenching. Steady-state fluorescence and time-resolved absorption measurements in the pico- and femtosecond time domain have been used to investigate the dynamics of hydrogen bonds in the excited singlet (S1) state of fluorenone in alcoholic solvents. A comparison of the features of the steady-state fluorescence spectra of fluorenone in various solvent systems demonstrates two spectroscopically distinct forms of fluorenone in the S1 state, namely the non-H-bonded (or free) molecule and the H-bonded complex. However, in 2,2,2-trifluoroethanol, a strong H-bond-donating solvent, emission from only the H-bonded complex is observed. The time-dependent density functional theory (TDDFT) method also demonstrates that the intermolecular hydrogen bond C¼O H–O between fluorenone and methanol molecules is significantly strengthened in the

742 Hydrogen Bonding and Transfer in the Excited State

electronically excited state upon photoexcitation of the hydrogen-bonded fluorenone–methanol complex. The hydrogen-bond strengthening in electronically excited states can be used to explain well all the spectral features of a fluorenone chromophore in alcoholic solvents. Furthermore, the radiationless deactivation via internal conversion (IC) can be facilitated by hydrogen-bond strengthening in the excited state. At the same time, the quantum yields of excited-state deactivation via fluorescence are correspondingly decreased. Therefore, the total fluorescence of fluorenone in polar protic solvents can be drastically quenched by hydrogen bonding [103]. It has been demonstrated that the intermolecular hydrogen bonds between coumarin 102 and hydrogen-donating solvents are strengthened in the early time of photoexcitation to the electronically excited state by theoretically monitoring the stretching modes of C¼O and H_O groups. The transient hydrogen-bond strengthening behaviour in the excited state of chromophores in hydrogen-donating solvents may take place widely in many other systems in solution and is very important to explaining the fluorescence quenching phenomena associated with some radiationless deactivation processes, for example the ultrafast solute–solvent intermolecular electron transfer and the internal conversion process from the fluorescent state to the ground state [102]. Alcohols and weak acids, with high oxidation potentials, quenched the singlet excited state of fluorenone, but not the triplet state, at rates that parallel H-bonding power. This is attributed to a physical mechanism involving vibronic coupling to the ground state via the H-bond. This is much stronger in the excited state than in the ground state and provides efficient energy dissipation in the radiationless transition via efficient intersystem crossing. Phenol, with hydrogen-bonding power comparable with that of the alcohols but with much lower oxidation potentials, quenches both singlet and triplet. Kamat et al. observed that, even though the excited state is only weakly bound to alcohol by a hydrogen bond, this interaction brings about a significant increase in nonradiative decay [104]. From the foregoing discussion pertaining to a number of studies in organized media, a number of common points have been gleaned. Owing to lack of water availability inside the micelles to hydrate the released proton, the ESPT process is significantly retarded in micelles and cyclodextrin cavities. However, CTAB micelles catalyse the ESPT process owing to the electron potential and at the same time retard it owing to the unavailability of water. A number of sites of organized media are treated differently by the neutral and anionic naphthols, and their emission characteristics are different at different sites. The molecular naphthols are also embedded not at one site but at more than one site in the organized media. 31.6.2 Role of the interfacial H-bond in unusual emission phenomena In view of the presence of an unusual and strong H-bond at the interface of micelle–naphthol systems, an emendation may be suggested to the conjecture of the above multisite theory in microheterogeneous systems and the ultrafast mobility of probe species in such an environment. The experimental observation that, at long time, the neutral and anion emissions of naphthols do not decay with the same lifetime may be due to discrepancies with respect to their H-bonding capabilities with interfacial water molecules, where neutral hydroxyaromatic compounds form H-bonds but anions cannot. This may affect the emission decay rate also. The H-bonding may be the route of energy transfer, and therefore neutral emission becomes complete long before anion emission extinguishes in micelles. Similarly, the enhancement of anion emission could be related to the reduction in the non-radiative rate of the anion in micellar phase. This is because the H-bond that is present in bulk water with a naphtholate ion (H2O acts as H-donor) and water molecule is not present when naphthols are partitioned in micellar phase. Fluorescence quenching, which occurs in bulk water, is absent in micelles. On the other hand, the difference in the ability of naphthols in different orientations and positions to form interfacial H-bonds with H2O may induce differences in emission characteristics of micelle-embedded probe molecules as well. It may be pointed out that, in view of fresh evidence of the unusual H-bond at the interface, the overall microstructure scenario that controls some of the emission characteristics of

Hydrogen-Bonded Large Molecular Aggregates of Charged Amphiphiles and Unusual Rheology 743

hydroxyaromatic dopants in micellar media has been changed considerably at least for some systems, and this fact must be taken into consideration along with the multisite theory. In some other systems (e.g. SDS micelles), enhanced normal emission arises from naphthol molecules in the interior of micelles, where the lower local hydroxyl ion concentration (around anionic SDS) causes significant suppression of the deprotonation process and thus gives almost exclusively neutral emission at the expense of anion emission [78].

31.7 Perspectives Owing to its directionality and spatial arrangement, complementary multiple H-bonding at the micellar interface leads to the engineering of a well-defined supramolecular structure via surfactant head group charge shielding by the p-electron system of hydroxyaromatic dopants. These long supramolecular aggregates (worm-like micelles) have fascinating rheological properties. Alhough they are complex fluids, they are characterized by a single relaxation time. The low dielectric constant value of the micellar interface is attributed to a low interfacial water activity, which is related to the unusual H-bonding property of water at the interface. This result of unusual H-bonding may be relevant not only when considering the H-bonding of the interfacial water molecules in the specific micelle and dopant mentioned here but also for the H-bonding interaction of other micelle/biomembrane–dopant systems. The unusual spectral emission behaviour of the dopant species is related to fluorescence quenching via H-bond strengthening in the excited state.

Acknowledgements The authors are grateful to the American Chemical Society, the Chemical Society of Japan and Taylor & Francis for kindly permitting them to use the figures and images in the text.

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32 Excited-State Intramolecular Proton Transfer in 2-(20-Hydroxyphenyl)benzoxazole Derivatives Yi Pang and Weihua Chen Department of Chemistry & Maurice Morton Institute of Polymer Science, The University of Akron, Akron, OH 44325, USA

32.1 Introduction 32.1.1 Acid and base properties of chromophores in excited state The acidic or basic properties of a chromophore in the ground state are often different from those in the excited state. This is because the light absorption frequently changes electron distribution within a molecule, which in turn leads to changes in chemical and physical properties. The loss or gain of protons in the excited-state reactions has been the subject of many studies [1, 2]. Whether a chromophore loses or gains a proton in the excited-state depends on the direction of the change in pKA in the excited state. If the pKA decreases on going to the excited state (i.e., pKA < pKA, where the asterisk denotes the excited state) then the chromophore has a higher tendency to lose a proton in the excited state. The best known example, which exhibits different chemical properties in the excited state, is phenol. In neutral solution, phenols can lose the phenolic proton in the excited state. This is because the OH in phenol has a lone pair of electrons, and these electrons tend to become more conjugated to the aromatic ring system in the excited state, resulting in pKA < pKA to aid deprotonation. In other words, the phenol tends to lose a proton in the excited state, as the phenolic proton becomes more acidic (decreasing pKA) upon photon absorption. If the pKA increases in the excited state (pKA > pKA), the chromophore may pick up a proton upon photon absorption. As a typical example (Table 32.1), acridine undergoes excited-state protonation. An electron acceptor, such as C¼N, CO2H and C:N, permits the electrons to be transferred into the p -orbital of

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

748

Hydrogen Bonding and Transfer in the Excited State

Table 32.1 Known excited-state deprotonation and protonation reactionsa Deprotonation reaction

Compound

ArOH ! ArO þ Hþ

Phenol

Structure OH

pKA

pKA

10.6

3.6

9.2

2.0

OH

1-Naphthanol SO3

OH

2-Naphth-6,8-disulfonate

9.3

<1

5.5

10.6

5.2

11.8

O3S

Protonation reaction ArN þ Hþ ! ArNHþ

Acridine N

MeO

6-Methoxyquinoline N a

Dissociation constants in the ground state (pKA) and excited state (pKA ) are from Ref. [3].

the excited state to increase the local electron density, thereby leading to pKA > pKA. The C¼N group, therefore, is widely used as a proton acceptor in partnership with a proton donor (such as phenol) to realize the proton transfer in the excited state. 32.1.2 Intramolecular proton transfer in 2-(20 -hydroxyphenyl)benzoxazole 2-(20 -Hydroxyphenyl)benzoxazole (HBO) 1 is an interesting example, in which the phenol acts as a proton donor and the nitrogen atom in the benzoxazole fragment acts as a proton acceptor. The hydrogen bonding in HBO holds the proton donor and acceptor in permanent contact and creates an ideal situation for excited state intramolecular proton transfer (ESIPT). During the ESIPT process, a proton on the hydroxyl group of 1 migrates to the neighboring hydrogen-bonded nitrogen atom to give the keto tautomer (2). Many studies have been carried out to understand the nature of the ESIPT process in the HBO system [4]. Earlier work on the ESIPT reaction [1], ESIPT to nitrogen atom [5], and the complexity of the reaction [6] can be found in review articles. O

O

proton transfer

N 1

N H O

2

H O

Notably, the proton transfer does not occur in the ground state of 1, as the phenolic proton is not sufficiently acidic to warrant deprotonation. The redistribution of the electronic density upon excitation, however, simultaneously increases the acidity of the phenolic proton and the basicity of the proton acceptor (Table 32.1), thereby enabling the proton transfer process. This unique feature, that both the acid and base are stronger in the excited state than in the ground state, makes HBO derivatives a very interesting system to study, because excitation may trigger a photoinduced proton transfer. A distinctive feature for the HBO derivatives is that their fluorescence is well separated from their absorption maxima, leading to an unusually large Stokes shift [7]. As an example, the fluorescence of 1 displays a major

Excited-State Intramolecular Proton Transfer in 2-(20 -Hydroxyphenyl)benzoxazole Derivatives 749 keto

Absorbance (a.u.)

Fluorescence (a.u.)

UV-vis Fluorescence

enol

300

350

400

450

500

550

600

650

Wavelength (nm)

Figure 32.1 Absorption and fluorescence of 2-(20 -hydroxyphenyl)benzoxazole (1) in 3-methylpentane. Adapted with permission from [4]. Copyright 1994 American Chemical Society

emission peak at 485 nm (Figure 32.1), which is about 155 nm redshifted from its absorption maximum (lmax  330 nm). The unusually large Stokes shift (155 nm), in comparison with those observed in aromatic molecules, is attributed to the ESIPT mechanism, which occurs in the electronically excited state (Figure 32.2). O

Sn (n>1)

N HO

S1

ESIPT O TS* of N H

Absorption

O

Stokes-shifted low energy fluorescence O

O

TS of

S0

N HO

1 enol tautomer

N H

O

2 keto tautomer

Figure 32.2 Electronic transitions in the photoluminescent process of 1. The excited tautomer 2 in the enol form (S1 state) is transformed into its tautomeric structure (keto form) through “excited state intramolecular proton transfer” (ESIPT). The observed low energy fluorescence is attributed to the relaxation from the excited transition state (TS of keto form) to its ground state

750

Hydrogen Bonding and Transfer in the Excited State UV-vis in benzene FL in benzene FL in ethanol

O O N

Fluorescence (a.u.)

Absorbance (a.u.)

HO

O N HO

300

350

400

O

450

500

550

600

650

Wavelength (nm)

Figure 32.3 Absorption and fluorescence (FL) spectra of HBO derivatives 3 (bottom) and 4 (top). Adapted with permission from [9]. Copyright 2007 The Chemical Society of Japan (See Plate 36)

In a non-polar solvent, HBO 1 typically gives long wavelength fluorescence (490 nm) from the keto tautomer, which is generated from the intramolecularly hydrogen-bonded enol via the fast ESIPT process. The intensity of the enol emission at the short wavelength (360 nm), however, gradually increases with solvent polarity [4, 8]. 32.1.3 Effect of substitution on the ESIPT The photophysical behavior of HBO derivatives depends on the substitution position on the phenol fragment, as a substituent can influence the hydrogen-bonding capability. In the benzoxazole derivatives 3 and 4 [9], the methoxy substituent is placed at 3- and 4-positions respectively of the phenyl moiety. In a non-polar solvent, such as benzene, HBO 3 revealed two emission peaks, at about 363 and 525 nm (Figure 32.3), which have been attributed to the enol and keto tautomers, respectively. HBO 4 in benzene, however, gives only keto emission at about 480 nm, indicating a higher tendency for ESIPT. The ESIPT process in 4 is even competitive in the protic solvent EtOH. The decreased tendency of ESIPT in 3 is likely due to the competitive OHO bonding involving the adjacent methoxy group.

N

O

O

O 4

1 2

HO

3

1

Solvent: hexane labs ¼ 321 nm lem ¼ 506 nm ffl ¼ 0.017 ffl ¼ 0.0026(methanol)

O

N HO

O

3

N HO

4

Solvent: benzene labs ¼ 312 nm lem ¼ 525 nm ffl ¼ 0.002

Solvent: hexane labs ¼ 340 nm lem ¼ 480 nm ffl ¼ 0.018

Excited-State Intramolecular Proton Transfer in 2-(20 -Hydroxyphenyl)benzoxazole Derivatives 751

As a useful property, the emission of HBO derivatives can be tuned by substituents over a wide spectral range. In the aprotic solvent CHCl3, the ESIPT fluorescence of 5 is found to be dependent on the nature of the substituent [10]. From compounds 5a ! 5g, the emission peak lem shifts gradually to a longer wavelength. For 5a and 5b, the emission is predominantly from the enol tautomer. For 5c and 5d, predominant emission is from keto tautomer (>450 nm), accompanied with emission from the enol (at 350–400 nm) tautomer (i.e., dual emission). For 5e–5g, the emission is exclusively from the keto tautomer. Clearly, a donor substituent at the 4position of the phenol fragment has a negative effect on the ESIPT process. An acceptor substituent at the 60 position of the benzoxazole fragment, however, plays a positive role on the ESIPT process. In reference to the parent system 5e, an electron-donor substituent causes a hypsochromic shift in fluorescence, while an electron acceptor causes a bathochromic shift. 7'

A

8'

O

9'

N

6' 5' 4'

5 2' 4

1 2

HO

D

3

5 Compound a b c d e f g

lmax (nm) A ¼ -H; D ¼ -NEt2 A ¼ -CO2Et; D ¼ -NEt2 A ¼ -H; D ¼ -OMe A ¼ -COOEt; D ¼ -OMe A ¼ -H; D ¼ -H A ¼ -COOEt; D ¼ -H A ¼ -CHO; D ¼ -H

lem (nm)

362 381 336 349 335 345 352

382 421 466 479 481 492 512

(enol) (enol) (keto) (keto) (keto) (keto) (keto)

Note: The absorption lmax is obtained from cyclohexane solution, while the emission maxima lem is from chloroform solution (from Ref. [10]).

Further comparison among 3, 4 and 5c reveals that the substitution at 5-position is probably a suitable position to enhance ESIPT through the phenol segment. The methoxy substituent at the para-position of the phenol in 4 might stabilize the keto tautomer, thereby making the ESIPT process more competitive. In compounds 6–8 [11], the phenol segment of the HBO unit is part of the rigid dibenzofuran, dibenzothiophene or carbazole structures, respectively. Absorption and emission data from their toluene solutions shows that these compounds have strong fluorescence from ESIPT. In contrast to other HBO derivatives whose ffl values decrease sharply in protic solvents or at an elevated temperature, compounds 6–8 are reported to retain most of their ffl under such conditions. The heteroatom at the para-position of phenol is thought to play a positive role in the enhanced ESIPT process.

HO

O

N O

O 6

lmax (toluene) (nm) lem (toluene) (nm) ffl (toluene) Stokes shift (nm)

HO

HO N

N

377 545 0.41 168

O

7

388 565 0.36 177

S

8

N C6 H 13

362 (« ¼ 41,000), 420 (« ¼ 8200) 598 0.17 178

752

Hydrogen Bonding and Transfer in the Excited State

Figure 32.4 Absorption and fluorescence of 10 in various solvents. Adapted with permission from [22], [66]. Copyright 2001, 2002 American Chemical Society

Introduction of a fluoro-substituent on the phenol segment leads to 9–11. 2-(30 ,40 ,50 ,60 -Tetrafluoro-20 hydroxyphenyl)benzoxazole 9 exhibits similar long-wavelength fluorescence as 1 at around 500 nm in a nonpolar solvent [12]. In polar solvents, however, compound 9 exclusively emits intermediate-wavelength fluorescence at around 440 nm, which is assumed to originate from the excited state of the phenolate anion of 9. It appears that the electron-withdrawing substituents significantly increase the acidity of phenol in the polar media, leading to early deprotonation, ArOH ! ArO, in the ground state and eliminating ESIPT. Replacing one of the fluoro atoms with a methoxy group gives 10, whose fluorescence is also sensitive to solvent polarity [13]. As seen from Figure 32.4, emission in toluene is primarily from the keto tautomer 10a (at 513 nm), which is accompanied by a minor emission from the enol tautomer (at 350 nm). In a polar solvent (acetonitrile), the major emission band shifts to 447 nm, which is attributed to phenoxide anion 10b. The assignment appears to be reasonable as both enol and keto emissions are still observable as shoulders. The tendency to generate luminescent phenoxide makes compound 10 quite responsive to pH change (between pH 7 and 8) [13]. The imidazole-containing compound 11 further expands the pH response range to pH 5–9 [14], since the protonated imidazole substituent can electronically perturb the HBO to influence the ESIPT process. HO

HO

F

ffl

O F

F

9

295, 316 500 (in toluene) 440 (in CH3CN) 0.004 (cyclohexane)

N

OMe O

F

F

N

F O

lmax (nm) lem (nm)

HO

F

N

N

F

10

350 (in toluene) 513 (in toluene) 447 (in CH3CN) 0.03 (CH3OH, pH  7) 0.23 (CH3OH, pH 10)

11

F

N

F

315 (in CHCl3) 500 (in CHCl3) 0.23 (CHCl3) 0.37 (CH3OH)

The following examples illustrate the impact of substitution on the benzoxazole fragment. As seen from 12 and 13, modification of benzoxazole causes a notable redshift in absorption, but negligible response in

Excited-State Intramolecular Proton Transfer in 2-(20 -Hydroxyphenyl)benzoxazole Derivatives 753

emission wavelength [15, 16]. Modification of both phenol and benzoxazole segments, as shown in 14 [11], leads to relatively large bathochromic shift in both absorption and emission. HO

HO

O

HO

H N

N

N

O

O

O

N

N

13

12

14

lmax (nm) lem (nm) ffl

O

350 (in DMF) 491 (in DMF) 0.03

1

N

4 2

HO

385 (in benzene) 485 (in benzene) —

375 (in toluene) 598 (in toluene) 0.21

O

O

N

N

O N HO

15

16

Solvent: CHCl3 labs ¼ 355 nm; lem ¼ 495 nm ff ¼ 0.48

Solvent: toluene labs ¼ 382 nm; lem ¼ 524 nm ff ¼ 0.16

Compound 15 can be considered as the derivative of HBO 1 formed by introduction of an additional benzoxazole on the phenol segment. Although the absorption and emission lmax of 15 are comparable with those of 1, its higher quantum yield suggests that the additional benzoxazole substituent could play a positive role in facilitating the keto emission [17]. With the aid of a larger aromatic ring, both absorption and emission lmax values of 16 are redshifted by about 30 nm. Introduction of an electron-withdrawing substituent leads to derivatives 17 and 18, whose absorption is at about 370 nm [18]. The substituent perturbs the excited state from the ESIPT, and induces strong solvatochromic fluorescence (from keto tautomer). The CHCl3 solution of 18 gives bright orange emission with lem ¼ 580 nm. The HBO derivative 19 bears substituents on both phenol and benzoxazole fragments [19]. The keto emission of 19 exhibits a blue-shift from 541 to 519 nm as a result of gel formation, which is accompanied by a remarkable enhancement in emission intensity. Incorporation of HBO into a dendrimer structure, as illustrated by 20 (lem ¼ 480 nm, ffl  0.038), enhances the keto emission by a factor of about two in comparison with 1 [20]. Compound 21 connects two HBO units together via a conjugated fluorenevinylene unit, whose DMF solution exhibits absorption lmax at 404 nm, and emission at 499 and 559 nm (ffl ¼ 0.09) [21]. The molecule has been shown to be useful as two-photon-absorbing sensor for Zn(II) and hydroxide ions. These examples illustrate the great potential for the use of ESIPT in material science. HO

HO

N

N R

O NC

CN

17

HO

O NC

H

N

N

HO N

O

O

O

O N

19

H

18

OR

C6H13 C6H13

H

OH

N

Dendrimer

HN R'

HO O

N

N O

O

O O

21

20

Dendrimer

754

Hydrogen Bonding and Transfer in the Excited State

Table 32.2 Impact of heteroatom substitution on the optical characteristics of oxazole derivatives S

H N

O

N

N

N

HO

S

HO

HO

H N O

N

23 (HBI)

22 (HBT)

O

O HO

RO 2 C

24 a : R=Et; b: R=H

RO 2 C

N HO

RO 2 C

25 a: R = Et; b: R = H

26 a: R = Et; b: R = H

lmax (nm) lem (nm) ffl Stokes shift

332a 526a 0.0006a 194

317a — 0.15a —

318 (R ¼ Et)b 360, 464 (R ¼ Et)b 0.26 (R ¼ H)b 42, 146

335 (R ¼ Et)b 375 (R ¼ Et)b 0.07 (R ¼ H)b 40

319 (R ¼ Et)b 435 (R ¼ Et)b 0.25 (R ¼ Et, H)b 116

a

Data acquired from methanol solution (from Ref. [24]). Data acquired from ethanol solution (from Ref. [22]).

b

32.1.4 Effect of heteroatom substitution on ESIPT Substitution of the oxygen atom in HBO with other heteroatoms also has a notable effect on the optical properties. In comparison with HBO, 2-(20 -hydroxyphenyl)benzothiazole (HBT) (22) shows a notable redshift in the absorption by 15 nm, while the benzimidazole (HBI) 23 typically exhibits a slight blue-shift (Table 32.2). The fluorescence quantum yield of HBI appears to be higher than that of the HBO derivative [4, 22]. The emission of 24a–26a further reveals the effect of the heteroatoms on the ESIPT (Figure 32.5). The HBO derivative 24a in ethanol gives two emission bands at 360 and 464 nm, corresponding to the enol and keto tautomers [22]. Interestingly, the fluorescence of HBT 25a in ethanol exhibits a single band at 375 nm, indicating emission solely from the enol tautomer. In sharp contrast, the HBI derivative 26a gives emission predominantly from the keto tautomer. The increased ESIPT in the HBI derivative is related to the electrondonating capacity of the heteroatoms, which is in the order of N > O > S, as revealed from the relative

2.7

HBO HBT HBI

Normalized Fluorescence (a.u.)

2.4 2.1 1.8 1.5 1.2 0.9 0.6 0.3 0.0 300

350

400

450

500

550

600

Wavelength (nm)

Figure 32.5 Fluorescence spectra of HBO (24a), HBT (25a) and HBI (26a) in ethanol at 25  C. Adapted with permission from [4]. Copyright 1994 American Chemical Society

Excited-State Intramolecular Proton Transfer in 2-(20 -Hydroxyphenyl)benzoxazole Derivatives 755

reactivity of five-membered heterocycles on electrophilic substitution [23].1 The increased electron density makes the C¼N in HBI a more suitable acceptor for hydrogen bonding, thereby leading to higher ESIPT. 2-(2-Tosylaminophenyl)benzimidazole 27 is an interesting system, where the NH is used as a proton donor partner to undergo ESIPT to give 28 [25]. In a non-polar solvent such as cyclohexane, the compound gives an absorption lmax at 318 nm, and emission lem at 507 nm (ffl ¼ 0.53).

O

S

O O

N H

S

O N

H

N

N

N H

N H

27

28

32.1.5 Effect of temperature on ESIPT In general, the fluorescence intensity decreases as the temperature increases, because the rate of dynamic quenching is increased. At room temperature, significant emission from the enol tautomer of HBO 1 is observed, partially attributable to the competitive intermolecular hydrogen-bonding with the polar solvent. At low temperature, both the rotational and vibrational modes are lowered, thereby leading to a narrower emission with resolved band structure. Emission from the keto tautomer at 470 nm is significantly increased at 77 K (liquid nitrogen temperature) (Figure 32.6) [4]. Low temperature-enhanced fluorescence is also

Fluorescence Intensity (a.u.)

room temp 77 K

350

400

450

500

550

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650

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Figure 32.6 Emission spectra of HBO 1 in alcohol excited at 320 nm at room temperature (298 K) and at 77 K. Adapted with permission from [29]. Copyright 2007 American Chemical Society

1

The reactivity of five-membered heterocycles on electrophilic substitution is in the order pyrrole > furan > thiophene.

756

Hydrogen Bonding and Transfer in the Excited State H O

0.84 Å

1.888 Å

O

H

O

O

N

N H

1. 97Å

O

R

1a

0. 82Å

(a)

R

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O

1b

O

N O

H

29 (a: R=H; b: R= t-Bu)

(b)

Figure 32.7 (a) The crystal structure of 1 reveals that the intramolecular hydrogen-bonded rotamers 1a and 1b exist in about 1:1 ratio. (b) The crystal structure of 29b shows that both hydroxyl groups are hydrogen-bonded to N-atom of oxazole rings. Adapted with permission from [29]. Copyright 2007 American Chemical Society (See Plate 37)

observed from other HBO derivatives such as 3 and 4 in methylcyclohexane [9]. It is likely that the content of rotamer 1a is increased at the lower temperature, as the intramolecular O--H    N hydrogen bond is energetically more favorable than the O--H    O bond in the rotatomer 1b (Figure 32.7).

32.2 Intramolecular Proton Transfer in 2,5-bis(20 -hydroxyphenyl)benzoxazole Derivatives x-Ray diffraction of HBO reveals that the intramolecular hydrogen-bonded rotamers 1a and 1b exist in about 1: 1 ratio in the crystalline state [26], with the hydroxyl group pointing to either N- or O-atom side of the oxazole ring (Figure 32.7). It has been demonstrated that only conformer 1a undergoes ESIPT process [8, 27], which leads to the keto tautomer 2 to give the emission with large Stokes shift. The rotamer 1b is likely the one responsible for the enol emission. It is thus desirable to minimize the content of 1b for optimal performance of ESIPT-based physical properties. An interesting example is 2,5-bis(benzoxazol-2-yl)benzene-1,4-diol derivative 29, bis(HBO), in which two intramolecular hydrogen-bonds reinforce each other to facilitate the desirable ESIPT. The crystal structure of 29b (Figure 32.7b) shows that both hydroxyl groups are oriented to form intramolecular O--H    N hydrogen bonds with the adjacent oxazole rings [28]. The hydrogen bond length   for H    N in 29b is 1.888 A, which is notably shorter than that in mono(HBO) 1a (1.97 A) [26]. Each HBO group in 29b adopts the rotamer conformation of 1a, which provides an ideal molecular geometry for ESIPT. Observation of the O--H    O hydrogen bond in 1 (as seen in 1b) but not in 29b, in addition to the shorter H    N bond distance, suggests that the O--H    N bond is energetically more favorable in the latter than in the former. The known intermolecular hydrogen bond strength [29] for O--H    N (DH ¼ 6.5 kcal mol1, measured from phenol/pyridine) is slightly stronger than that for O--H    O bond (DH ¼ 5.0 kcal mol–1, measured from phenol/ether). The exclusive formation of O--H    N bond in 29, in sharp contrast to the 1:1 ratio of O--H    N to O--H    O in 1 [26], could be attributed to the cooperative interaction of two intramolecular hydrogen bonds in para-positions. Resulting from the enhanced O--H    N hydrogen bonding, the fluorescence of bis(HBO) 29b gives primarily keto emission, at 600 nm, in a wide range of solvents (Figure 32.8). In sharp contrast to 1, which gives significant enol emission in ethanol (Figure 32.6), the content of enol emission between 425 and 475 nm remains low even in the protonated solvent (EtOH).

Excited-State Intramolecular Proton Transfer in 2-(20 -Hydroxyphenyl)benzoxazole Derivatives 757

1.0

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Figure 32.8 Fluorescence spectra of 29b at 25  C in different solvents. Adapted with permission from [29]. Copyright 2007 American Chemical Society H

O O

N

H

t-Bu

O O

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t-Bu

O

NEt2

N O

30

lmax (nm) ¼ 532 (in THF) lem (nm) ¼ 620 (in THF) ffl ¼ 0.38

Et2N

N

O O

31

H

lmax ¼ 468 (in DMF) lem ¼ 573 (in DMF) ffl ¼ 0.79

Despite the favorable hydrogen bonding for ESIPT in 29b, its fluorescence intensity is rather weak (ffl ¼ 0.021 in THF). Addition of excess acetate to 29b leads to immediate appearance of a red color, which is attributed to the formation of mono anion 30 [28]. Although the fluorescence intensity is enhanced by as much as 20-fold, the emission arises exclusively from the enol form of 30 (Figure 32.9). Clearly, the presence of negative charge on the phenol ring prevents the proton transfer in 30, as the ESIPT requires the formation of a highly energetic dianion (via removal of the remaining phenolic proton). Recently, the metal binding is found to reduce the impact of the phenoxide anion in 30, thereby re-enabling the ESIPT to give the emission near 700 nm [30]. The electron donor substituent on the benzoxazole fragment can also exhibit a large impact on the ESIPT process, as bis(HBO) 31 with two amino substituents is reported to give emission from its enol tautomer [31]. The fluorescence arising from ESIPT depends on the content of O--H    N hydrogen in HBO. A recent study has revealed a new strategy, as illustrated in 32, in which the presence of the second hydroxy group creates dual hydrogen bonding (i.e., O--H    N and O--H    O) in every molecule [32]. Since every molecule is capable of undergoing the ESIPT process, the keto emission from 32 (lem ¼ 543 nm) is dramatically increased in comparison with that from 1 (lem ¼ 499 nm) (Figure 32.10).

Hydrogen Bonding and Transfer in the Excited State 6000000

1.0

29b 29b + OAc– 0.8

5000000

Absorption (a.u.)

4000000 0.6 3000000 0.4 2000000

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1000000

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Figure 32.9 UV–Vis and fluorescence spectra of 29b (broken line) and mono anion 30 (solid line) in THF at room temperature. The mono anion 30 is formed by addition of excess NaOAc to the 29b solution

O N HO

1.0x10

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OH

1

6

O N OH

32

5

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Figure 32.10

Fluorescence spectra of 1 and 32 in DMF

32.3 Summary and Future Prospects Studies on 2-(20 -hydroxyphenyl)benzoxazole derivatives have made significant progress towards understanding electronic (via substitution) and environmental impact on the excited-state intramolecular proton transfer (ESIPT). Optimization of fluorescence from the keto tautomer remains a challenge in molecular design, with

Excited-State Intramolecular Proton Transfer in 2-(20 -Hydroxyphenyl)benzoxazole Derivatives 759

success depending on further advances in the comprehensive understanding of the ESIPT process. The aim of this chapter is to correlate various chemical structural features with the properties associated with the ESIPT process, and to identify promising design principles for the rational design of new molecules with optimized properties. In the HBO derivative, there exist two different rotamers with O--H    N and O--H    O hydrogen bondings (as seen in HBO 1a and 1b). The rotamer ratio identified in the crystalline form, however, may not reflect the ratio in solution, which remains a poorly understood subject. Analytical tools need to be developed to accurately characterize the rotamer ratio in solutions, which will facilitate optimization of the O--H    N hydrogen bonding content for ESIPT. Currently, the application of HBO derivatives is limited mainly by two factors: (1) unpredictable quantum yield since the emission from the keto tautomer is dependent not only on the efficiency of the proton transfer in the excited state (to generate the tautomer) but also on the chemical structure of the tautomer (to be an efficient emitter) – simultaneous optimization in both proton transfer and radiative decay processes remains a challenge; and (2) lack of emission wavelength in the NIR region. The emission of HBO derivatives is currently limited to the visible region (below 620 nm). Extending the emission wavelength to the near-infrared (NIR) region would make HBO a more attractive tool in the biological field, where both a large Stokes shift and NIR emission are desirable.

References 1. S. J. Formosinho and L. G. Arnaut, Excited-state proton transfer reactions II. Intramolecular reactions, J. Photochem. Photobiol. A: Chem., 75, 21–48 (1993). 2. L. M. Tolbert and K. M. Solntsev, Excited-state proton transfer: from constrained systems to “super” photoacids to superfast proton transfer, Acc. Chem. Res., 35, 19–27 (2002). 3. B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley-VCH Verlag GmbH, Weinheim, Chapter 4, pp. 99–106 (2001). 4. K. Das, N. Sarkar, A. K. Ghosh, et al. Excited-state intramolecular proton transfer in 2-(20 -hydroxyphenyl) benzimidazole and -benzoxazole: effect of rotamerism and hydrogen bonding, J. Phys. Chem., 98, 9126–9132 (1994). 5. S. M. Ormson and R. G. Brown, Excited state intramolecular proton transfer part 1: ESIPT to nitrogen, Progr. React. Kinet., 19, 45–91 (1994). 6. J. Waluk, Conformational aspects of intra- and intermolecular excited-state proton transfer, in Conformational Analysis of Molecules in Excited States; ed. by J. Waluk, Wiley-VCH, Verlag GmbH, New York, pp. 57–111, (2000). 7. D. L. Williams and A. Heller, Intramolecular proton transfer reactions in excited fluorescent compounds, J. Phys. Chem., 74, 4473–4480 (1970). 8. G. J. Woolfe, M. Melzig, S. Schneider and F. Doerr, The role of tautomeric and rotameric species in the photophysics of 2-(20 -hydroxyphenyl)benzoxazole, Chem. Phys., 72, 213–221 (1983). 9. A. Ohshima, M. Lkegami, Y. Shinohara, et al. The effect of meta-substitution on the photochemical properties of benzoxazole derivatives, Bull. Chem. Soc. Jpn., 80, 561–566 (2007). 10. J. Seo, S. Kim, S. Park and S. Y. Park, Tailoring the excited - state intramolecular proton transfer (ESIPT) fluorescence of 2-(20 -hydroxyphenyl)benzoxazole derivatives, Bull. Korean Chem. Soc., 26, 1706–1710 (2005) 11. J. M. Kauffman and P. T. Litak, Synthesis and photophysical properties of fluorescent dibenzofurans, a dibenzothiophene, and carbazoles substituted with benzoxazole and hydroxy groups to produce excited state intramolecular proton transfer, J. Heterocycl. Chem., 32, 1541–1556 (1995). 12. K. Tanaka, M. Deguchi, S. Yamaguchi, et al. Solvent- and concentration-sensitive fluorescence of 2-(3,4,5,6tetrafluoro-2-hydroxyphenyl)benzoxazole, J. Heterocycl. Chem., 38, 131–136 (2001). 13. K. Tanaka, T. Kumagai, H. Aoki et al., Application of 2-(3,5,6-trifluoro-2-hydroxy-4-methoxyphenyl)benzoxazole and -benzothiazole to fluorescent probes sensing pH and metal cations, J. Org. Chem., 66, 7328–7333 (2001).

760 Hydrogen Bonding and Transfer in the Excited State 14. K. Tanaka, K.-I. Sano, T. Katoh et al., Proton- and metal cation-enhanced excited state intramolecular proton transfers of 2-(20 -hydroxyfluorophenyl)benzoxazole having imidazole moiety, J. Fluorine Chem., 127, 1073–1078 (2006) 15. A. O. Doroshenko, E. A. Posokhov, V. M. Shershukov et al., Spectral and luminescence properties of derivatives of 2-aryl[9,10]phenanthroxazole, Chem. Heterocycl. Compd., 31, 492–499 (1995). 16. A. M. Osmans, A. M. Metwallayn and M. S. K. Youssef, Heterocyclic compounds. VIII. Studies on oxazolophenoxazines, Can. J. Chem., 54, 37–43 (1976). 17. J. M. Kauffman and G. S. Bajwa, Synthesis and photophysical properties of fluorescent 2,5-dibenzoxazolylphenols and related compounds with excited state proton-transfer, J. Heterocycl. Chem., 30, 1613–1622 (1993). 18. J. Seo, S. Kim and S. Y. Park, Strong solvatochromic fluorescence from the intramolecular charge-transfer state created by excited-state intramolecular proton transfer, J. Am. Chem. Soc., 126, 11154–11155 (2004). 19. T. H. Kim, M. S. Choi, B.-H. Sohn et al., Gelation-induced fluorescence enhancement of benzoxazole-based organogel and its naked-eye fluoride detection, Chem. Commun. 2364–2366 (2008). 20. A. Ohshima, A. Momotake, R. Nagahata and T. Arai, Enhancement of the large Stokes-shifted fluorescence emission from the 2-(20 -hydroxyphenyl)benzoxazole core in a dendrimer, J. Phys. Chem. A, 109, 9731–9736 (2005). 21. Y. Tian, C. Y. Chen, C. C. Yang et al., 2-(20 -Hydroxyphenyl)benzoxazole-containing two-photon-absorbing chromophores as sensors for zinc and hydroxide ions, Chem. Mater. 20, 1977–1987 (2008) 22. M. M. Henary and C. J. Fahrni, Excited state intramolecular proton transfer and metal ion complexation of 2-(20 hydroxyphenyl)benzazoles in aqueous solution, J. Phys. Chem. A, 106, 5210–5220 (2002). 23. L. I. Belenkii, I. A. Suslov and N. D. Chuvylkin, Substrate and positional selectivity in electrophilic substitution reactions of pyrrole, furan, thiophene, and selenophene derivatives, Chem. Heterocycl. Compd., 39, 36–48 (2003). 24. S. R. Vazquez, M. C. R. Rodriguez, M. Mosquera and F. Rodriguez-Prieto, Excited-state intramolecular proton transfer in 2-(30 -hydroxy-20 -pyridyl)benzoxazole. Evidence of coupled proton and charge transfer in the excited state of some o-hydroxyarylbenzazoles, J. Phys. Chem. A, 111, 1814–1826 (2007) 25. C. J. Fahrni, M. M. Henary and D. G. VanDerveer, Excited-state intramolecular proton transfer in 2-(2GC€ ¸ ytosylaminophenyl)benzimidazole, J. Phys. Chem. A, 106, 7655–7663 (2002). 26. Y. P. Tong, 2-(20 -Hydroxyphenyl)-1,3-benzoxazole, Acta Crystallogr., Sect. E, 61, o3076–o3078 (2005). 27. K. Das, N. Sarkar, D. Majumdar and K. Bhattacharyya, Excited-state intramolecular proton transfer and rotamerism of 2-(20 -hydroxyphenyl)benzimidazole, Chem. Phys. Lett., 198, 443–448 (1992). 28. Q. Chu, D. A. Medvetz and Y. Pang, A polymeric colorimetric sensor with excited-state intramolecular proton transfer for anionic species, Chem. Mater., 19, 6421–6429 (2007). 29. Selected hydrogen bond strength can be found in (a) E. V. Anslyn and D. A. Dougherty, Modern Physical Organic Chemistry, University Science Books, pp. 171–180 (2006); (b) M. D. Joesten and L. J. Schaad, Hydrogen Bonding, Marcel Dekker, New York (1974). 30. Y. Xu and Y. Pang, Zinc binding-induced near-IR emission from excited-state intramolecular proton transfer of a bis(benzoxazole) derivative, Chem. Commun., 46, 4070–4072 (2010). 31. E. M. Vernigor, E. A. Luk’yanets and L. P. Savvina, Spectral-luminescence properties of some 2,5-dihydroxy-1,4di(azol-2-yl)benzenes, Chem. Heterocycl. Compd., 29, 212–215 (1993). 32. W.-H. Chen and Y. Pang, Excited-state intramolecular proton transfer in 2-(20 ,60 -dihydroxyphenyl)benzoxazole: effect of dual hydrogen bonding on the optical properties, Tetrahedron Lett., 51, 1914–1918 (2010)

33 Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation Dipak K. Palit Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

33.1 Introduction In solutions, solute and solvent molecules interact to minimize the free-energy of the solute–solvent system [1–5]. Each solute molecule is surrounded by a shell of more or less tightly bound solvent molecules because of intermolecular forces between them; the phenomenon is well-known as “solvation” and “solvation energy” can be defined as the change of Gibb’s free energy when a molecule is transferred from the gas phase into the solvent. Solvation interaction mainly arises due to the columbic forces between the dipoles of the solute and solvent molecules [6]. This is a long-range interaction, also known as a “non-specific” interaction, and for a particular solute molecule, the solvation energy is solely determined by the dielectric properties of the solvent. However, in addition to the dipolar solvation energy, the free energy of the solute–solvent system may be further lowered if a hydrogen bond is formed between the solute and a solvent molecule [7, 8]. Since two specific solute and solvent molecules are involved in formation of the hydrogen bond, this kind of solvation interaction is known as “specific” interaction [9], which will be described here as formation of hydrogenbonded complex. Following photoexcitation of a solute molecule, the electronic charge distribution is changed. In such a case, in which the molecule is excited directly to an excited state having intramolecular charge transfer (ICT) character, the instantaneous and significant change in the polarity of the molecule following photoexcitation drives the dipoles of the surrounding solvent molecules to reorganize themselves to minimize the free energy of the solute–solvent system. This phenomenon is popularly known as “dipolar solvation” [10–15]. Following photoexcitation of the hydrogen-bonded complex formed in the ground electronic state of the solute, the

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

762 Hydrogen Bonding and Transfer in the Excited State

hydrogen bond may also need to be reorganized in the excited state of the solute. In some cases, the hydrogen bond formed in the ground electronic state of the solute molecule may even break and form a new one at a different site of the molecule in the excited state. Dynamics of the excited states of the intermolecular hydrogen-bonded complex are of special interest to photochemists for several reasons. Firstly, it is wellestablished that both intra- as well as intermolecular hydrogen bonds act as accepting modes for the nonradiative electronic energy of the excited state, providing an efficient radiationless deactivation pathway, and its lifetime is strongly quenched [16–23]. The nature of coupling between the solute and the solvent molecules also governs the rate and mechanism of the transfer of excess vibrational energy of the excited state of the solute to the solvent bath [24–27]. Secondly, the hydrogen-bonding interaction between the solute and the solvent molecules in the first solvation cell also plays an important role in the dynamics of solvation, as well as the relaxation processes undergone by the excited states [10, 28–33]. Additionally, most protic solvents, such as water, alcohols, phenols and amines, that form hydrogen-bonded complex with a solute molecule are also associated among themselves via three-dimensional (water) or two-dimensional (alcohols, phenols and amines) hydrogen bond network structure [34–36]. Owing to the relative weakness of the interaction, hydrogen bonds continuously rupture and reform at room temperature in hydrogen bonding solvents. This occurrence makes up a significant part of the solvent’s overall dynamics. Understanding these dynamics is essential to develop an enhanced picture of how these solvents behave around reactive substrates or complex biological molecules. Since the solvent molecule, which is associated with a particular solute molecule via hydrogen bond formation, is also associated with the hydrogen bond net work structure of the solvent, the hydrogen-bonding interaction in the first solvation cell is likely to perturb the hydrogen bond network structure of the solvent, which needs to be reorganized. Hence, a detailed understanding of the dynamics of chemical reactions involving the excited states of the molecules in such kind of interacting solvents obviously needs information regarding the microscopic motions of the solvent molecules directly engaged in interaction with the solute molecules as well as the response of the bulk liquid [11, 13, 37, 38]. For the last few decades, dielectric relaxation of different kinds of solvents, including alcohols, has attracted much attention and has been studied by theoretical simulation and ultrafast spectroscopy [9–15, 31, 32, 39]. Numerous efforts have been devoted to understanding the dynamics of polar solvation in alcoholic solvents using time-resolved fluorescence techniques. In these studies, the alcohols were shown to behave as simple non-interacting dipolar solvents for the excited probe molecule [40–49]. However, in recent times, numerous studies, both theoretical and experimental, have been devoted to understanding the dynamics of specific interaction as well as the structural and spectroscopic properties and the relaxation dynamics of the excited state of the hydrogen-bonded complex formed via interaction of the excited states with the solvent [4, 5, 29, 36, 50–80]. In these cases, the alcoholic solvents behave both as a dipolar liquid as well as a quencher of the excited state due to specific interaction. Ultrafast time-resolved, both visible and infrared, pump–probe spectroscopic techniques have been used extensively for investigating both the structural and temporal dynamics of the hydrogen-bonded complex in the excited state [29, 51–59, 74, 80]. In this chapter, we provide a brief account of recent studies on the photophysical and photochemical properties of the hydrogen-bonded complex and the dynamics of the hydrogen bond reorganization process in the excited states of a few selectively chosen probe molecules using ultrafast time-resolved visible and infrared absorption spectroscopic techniques.

33.2 Identification and Characterization of Hydrogen-Bonded Complex Hydrogen bonds are of particular importance since these interactions are present in many chemical and biological systems. Hydrogen bonds greatly influence the structure and dynamics of many important materials, including water, alcohols, DNA, RNA and proteins. A hydrogen bond, A--H


B, is an attractive

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

763

interaction between a hydrogen bond donor, A--H, and a hydrogen bond acceptor, B. A and B are usually the highly electronegative N, O and F atoms, for which the molecule gains a large dipole moment and the hydrogen bond can be attributed to a simple electrostatic interaction between the positive end of the bond dipole of A--H and the negative end of the dipole associated with B. Moreover, because of the small size of the hydrogen atom, AH and B can approach very closely. Since the strength of dipole–dipole interaction falls off as the fourth power of the distance, this close interaction is usually so strong that it can be considered as a bond [7, 8]. The energy of a hydrogen bond typically varies in the range 5 to 30 kJ mol1 and can be compared to that of weak covalent bonds (155 kJ mol1). However, hydrogen bonds can be extremely strong (>155 kJ mol1), as in the ion HF2 [8]. The strength of a hydrogen bond has a direct correlation with the length of the bond. The shorter the bond ˚´ represents a very weak length the stronger is the hydrogen bond. A hydrogen bond length of about 2 A 1 ˚´ long is very strong, bond with the bond-energy of about 1 kcal mol , whereas a hydrogen bond about 1.2 A 1 with a bond energy as large as 25 kcal mol . The length of a hydrogen bond also depends on temperature, pressure, bond angle and environment (usually characterized by the local dielectric constant). The typical  length of a hydrogen bond in water is 1.97 A. The ideal bond angle depends on the nature of the hydrogen bond donor [4, 5, 7, 8]. As described earlier, the hydrogen bond is often described as an electrostatic dipole–dipole interaction. However, it also has some features of covalent bonding: it is directional, strong and produces interatomic distances shorter than the sum of van der Waals radii. It usually involves a limited number of interaction partners, which can be interpreted as a kind of valence. These covalent features are more significant when acceptors bind with the hydrogen atoms from more electronegative donors. A hydrogen atom attached to a relatively stronger electronegative atom, for example, fluorine, oxygen, or nitrogen, is a hydrogen bond donor, whereas another electronegative atom can act as a hydrogen bond acceptor, regardless of whether it is bonded to a hydrogen atom. In our discussion, we examine examples of hydrogen bond donors that are alcohols, amines and phenols, which have a hydrogen atom bonded to oxygen or nitrogen atom, and an example of hydrogen bond acceptor is the carbonyl group. 2,2,2-Trifluoroethanol (TFE), 1,1,1,3,3,3-hexafluoropropanol (HFIP) and perfluoro-tert-butyl alcohol (PFTB) are commonly used as strong hydrogen bonding alcoholic solvents [69, 81, 82]. Prior to investigating the dynamics of the hydrogen bond in the excited state of a probe molecule it is essential to identify and characterize the hydrogen-bonding interactions between the solute and solvent molecules and characterize the spectroscopic properties of the hydrogen-bonded complexes formed in the ground and excited states. There are several techniques to identify the specific intermolecular hydrogen bonding interactions between the solute and the solvent molecules. Below is the brief description of the spectroscopic techniques applied to characterize the hydrogen-bonding interaction between coumarin 102 (C102) or fluorenone (Figure 33.1) and alcohols, which are the most well-studied systems because of their hydrogen bond accepting properties and have been considered here as model systems [51, 53, 55, 59, 73, 74, 83]. 33.2.1 Electronic absorption and fluorescence spectroscopy The most preliminary observation leading to a prediction about the hydrogen-bonding interaction is the increased absorption and redshift of the long wavelength absorption band upon addition of strong hydrogenbonding alcoholic solvents. Larger bathochromic shifts of the fluorescence maximum, as well as larger Stokes shift in protic or alcoholic solvents, as compared to those in solvents of comparable dielectric constants, for example, methanol and acetonitrile, further confirms the hydrogen bonding interaction with the hydrogen bond donating solvents. As a consequence, the Lippert–Mataga plot shows different slopes in the cases of protic and aprotic solvents (vide infra).

764 Hydrogen Bonding and Transfer in the Excited State

Figure 33.1 Chemical structures of coumarin 102 (A) and fluorenone (B). Adapted with permission from [55]. Copyright 2005 American Chemical Society

Absorbance (arbitrary unit)

Figure 33.2 shows the electronic absorption spectra of fluorenone in the 270–500 nm region in four nonaqueous solvents, namely cyclohexane, acetonitrile, methanol and TFE. The spectroscopic properties of the lower lying electronically excited states of fluorenone have been well investigated by several groups [55, 83–86]. In this wavelength region, the electronic absorption spectrum recorded in each of these solvents consists of a few well-resolved absorption peaks. Careful examination of the effect of polarizability and hydrogen-bond forming ability of the solvents on the positions and intensity of the individual absorption bands suggests that the excited electronic states corresponding to the 300–350 nm region are substantially different from those corresponding to the 270–300 and 350–500 nm regions and the changes are more significant in hydrogen-bonding solvents [55, 85]. Biczo´k et al. have assigned these changes in the absorption spectrum to the formation of hydrogen-bonded complex and applied steady-state and time-resolved fluorescence techniques to characterize the hydrogen-bonded complexes formed in the ground and excited states of fluorenone with alcohols and phenols [85]. They suggested formation of a 1 : 1 complex between fluorenone and the hydrogen-bonding alcohols, and the binding constant, K, were determined to be 3.5, 10.7, 2.8 and

5

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Figure 33.2 Steady-state absorption spectra of the ground electronic state of fluorenone in cyclohexane (a), acetonitrile (b), methanol (c) and TFE (d) [55]. Adapted with permission from [55]. Copyright 2005 American Chemical Society (See Plate 38)

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

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a

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Figure 33.3 Steady-state fluorescence spectra of fluorenone in cyclohexane at 298 K (a); in acetonitrile at 298 K (b) and 77 K (b0 ); in methanol at 298 K (c) and 77 K (c0 ); in TFE at 77 K (d0 ); in dimethylformamide (e), dimethylformamide (50%) þ formamide (50%) mixture (f) and formamide (g) at 298 K [55]. Adapted with permission from [19a]. Copyright 1991 American Chemical Society

0.7 M1 for HFIP, 4-cyanophenol, phenol and TFE, respectively, in CH2Cl2. This order parallels their relative hydrogen bonding power as measured with respect to common acceptors [81]. Figure 33.3 shows the steady-state emission spectra of fluorenone in solvents of different kinds, recorded at two different temperatures, i.e., in solution at room temperature (298 K) and in rigid matrices at 77 K. In each of the aprotic solvents, the fluorescence spectra recorded in solution and in rigid matrix at 77 K show no difference in shape and no shift in position of the maximum. However, in methanol (and other normal alcohols), the fluorescence spectra recorded under these two conditions are significantly different. The fluorescence spectrum recorded in methanol solution has two distinct bands. One has a maximum at ca 565 nm, and the other appear as a shoulder at ca 514 nm. The position of the shoulder band coincides with the maximum of the fluorescence spectrum recorded in acetonitrile and DMF, either in solutions or in rigid matrices. However, the fluorescence spectrum recorded in the rigid matrix of methanol is characterized by a single band with a maximum at ca 514 nm. DMF and formamide have similar polarity on the p scale of solvent polarity, but formamide is a hydrogen bond donor while DMF is not. In pure DMF, the fluorescence maximum appears at 514 nm, which is due to emission from the free form. Although, unlike the spectra in alcoholic solvents, the bands due to the free molecule and the hydrogen-bonded complex are not resolved, the broadening and shifting characteristics of the new spectral component due to the hydrogen-bonded form with increasing concentration of formamide are apparent. The fluorescence spectrum with a maximum at 562 nm recorded in pure solvent of formamide suggests that emission from the hydrogen-bonded complex has a significant contribution to the total emission. In TFE, fluorescence is significantly quenched by TFE in solution and hence could not be recorded. However, comparison of the emission spectra of fluorenone in methanol solution and in rigid matrices at 77 K with that recorded in TFE in rigid matrices clearly reveals the fact that fluorenone in methanol solution exists as both free and hydrogen-bonded complex form in equilibrium, whereas in TFE the equilibrium shifts almost entirely in favor of formation of the hydrogen-bonded complex form.

766 Hydrogen Bonding and Transfer in the Excited State

Because of electron-donating properties of the amino group, the presence of this functional group often enables a large charge separation in the molecule and hence the photophysical properties of differently substituted aminofluorenones have been the subject of detailed investigation by different groups [16–20]. Moog et al. have studied the solvent effects on the excited state properties of 1-aminofluorenone (1AF) and 3aminofluorenone (3AF) [19a]. These two molecules were chosen because of their similar geometrical and electronic structure but only 1AF could form an intramolecular hydrogen bond and thus a comparison of the excited state photophysics of these two fluorenones in various aprotic and protic solvents lent insight into the role of specific interactions between the solvent and a solute molecule containing an amino group. A comparison between the steady state absorption and fluorescence spectroscopic characteristics of 1AF and 3AF in polar aprotic (DMSO) and protic (methanol) solvents revealed significant differences in the absorption and fluorescence spectroscopic characteristics of 3AF but not in the case of 1AF [19]. This clearly suggested formation of an intermolecular hydrogen-bonded complex in the excited state of 3AF. However, both molecules exhibited a solvent-dependent Stokes shift, which increased significantly with increasing solvent polarity, suggesting substantial charge-transfer characteristics in the excited state. Within the dielectriccontinuum approximation of the solvent, the Lippert–Mataga equation predicts a linear correlation between the Stokes shift and the dielectric function (F) [equation (33.1)]. The slope of such a plot is proportional to the square of the change in dipole moment, Dm2, between the ground and the excited electronic states of the probe: F¼

«1 n2 1  2 2« þ 1 2n þ 1

ð33:1Þ

In case of only non-specific interaction between the probe and the solvent molecules, data for both aprotic and protic solvents are expected to fall on a single line in a Lippert–Mataga plot. However, for 1AF and 3AF, the Stokes shift could best be approximated by a linear function, if the solvents were divided into two groups – aprotic and protic (Figure 33.4). These results suggested the occurrence of specific solute and solvent interaction (hydrogen-bonding) in protic solvents, thus influencing the Stokes shift [19].

STOKES SHIFT (CH-1)

1AF APROTICS 6500

1AF ALCOHOLS 3AF APROTICS 3AF ALCOHOLS

5500

4500

3500

0.05

0.1

0.15

0.2

0.25

0.3

F

Figure 33.4 Lippert–Mataga plots for 1AF and 3AF. The steady-state Stokes shift is plotted against F, the dielectric function, as defined in equation (33.1). For 3AF, the data in alcohols and in aprotic solvents fall on two separate, but approximately parallel, lines. For 1AF, the data also fall on separate lines for aprotics and alcohols (excluding TFE). The difference between protics and aprotics is much less for 1AF than for 3AF. The least-squares fits to these data produce values for Dm of about 4 and 5 D for 1AF and 3AF, respectively [19a]. Reprinted with permission from [19b & c]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation 5

1AF

767

12

Slope: 1021 cm -1 N= 11, R = 0.96

3 -1 Stokes Shift (10 cm )

4 1

5

3

6 7

11

8

2

3 0.0

7

10

9

4

0.2

0.4

0.6

3AF

0.8

10 6

4

12

8 7

11

9

3

6

5 Slope = 2247 cm 2 0.2

C. C. = 0.93 0.4 0.6

-1

0.8

ET(N)

Figure 33.5 Plots of Stokes shift vs. ET(N) parameter of the solvents. Solvents are: cyclohexane (1), benzene (2), acetone (3), DMSO (4), acetonitrile (5), 1-pentanol (6), 1-butanol (7), 1-propanol (8), ethanol (9), methanol (10), ethylene glycol (11) and TFE (12). In the case of 1AF in TFE, point 12 is excluded from the fit [19b,c]. Adapted with permission from [51d]. Copyright 2004 American Chemical Society

A significant role of the hydrogen-bonding interaction influencing the energetic and photophysical properties of the excited states of the aminofluorenones is evident from a good correlation between the Stokes shift and the ET(N) values of the corresponding solvents (Figure 33.5) [19b,c]. The ET(N) or ET(30) parameter, which is a popular measure of interaction between the electronic state and the solvent, is known to be as much a measure of the hydrogen-bonding ability of the solvent as a measure of interaction with the solvent dipoles [81]. It is well known that the electronic transition in molecules involving ICT kinds of excited state is very sensitive to the nature of the microenvironment around the solute and that the spectral parameters can be used to study the nature of solute–solvent interactions at the microscopic level [9–15]. In general, to obtain an insight into the relative contributions of specific and non-specific modes of interaction towards the stabilization of the S1 states of the solute molecule, it is customary to write an observed parameter in a solute–solvent system in terms of linear solvation energy relationship. Considering the structures of the aminofluorenones and the nature of electronic transition, the contribution of three solvent parameters, namely, a, b and p , have been considered here to correlate the energies associated with the Stokes shift in different kinds of solvents using the Kamlet–Taft (K-T) solvatochromic scale [87]. Here a is a measure of the hydrogen bond or the proton donating ability of the solvent and b is a measure of the hydrogen bond accepting ability of the solvent; p represents the dipolarity or the polarizability term, which is a measure of nonspecific interaction [81]. Linear regression analyses of the Stokes shift values (
n) for 1AF recorded in ten solvents yield the results represented by equation (33.2): D
nð103 cm1 Þ ¼ 3:260:45 b þ 0:75 a þ 3:26p*

ðN ¼ 10; R2 ¼ 0:96Þ

ð33:2Þ

768 Hydrogen Bonding and Transfer in the Excited State

In equation (33.2), the relative magnitudes of the coefficients associated with the solvent parameters suggest that the hydrogen bond donating ability and the dielectric polarity of the solvent [both these properties determine the ET(N) of the solvent] make major contributions to the energy stability of the S1 state of 1AF, whereas the hydrogen bond accepting solvents destabilize (or increase the energy of) the S1 state. Linear regression analyses of the Stokes shift values for 3AF recorded in ten solvents yield the results represented by equation (33.3): D
nð103 cm1 Þ ¼ 5:21 þ 0:14 b þ 1:03 a þ 0:68p*

ðN ¼ 10; R2 ¼ 0:80Þ

ð33:3Þ

In equation (33.3), the relative magnitudes of the coefficients associated with the solvent parameters again suggest that the hydrogen bond donating ability and the dielectric polarity of the solvent make major contributions to the energy stability of the S1 states of 3AF, as compared to the hydrogen bond accepting ability of the solvents. Different kinds of effects of aprotic and protic solvents on the fluorescence lifetimes of fluorenone and its derivatives confirm hydrogen-bonding interaction in the excited state [85]. In aprotic solvents, the lifetime has been shown to increase smoothly with increasing ET(N). However, the reverse trend is observed in alcohols and the lifetime is sharply decreased with increasing ET(N). The lifetime of the singlet state of fluorenone has been shown to decrease linearly with increasing concentration of alcohols in CH2Cl2, within the limit of the concentration range of alcohols, at which there is little or no hydrogen bonding to the fluorenone in the ground electronic state. Fluorescence decay remained single exponential in all cases. Moreover, close agreement in the values of Stern–Volmer constants determined for fluorescence quenching of fluorenone by alcohols from both steady-state and time-resolved fluorescence measurements suggested the dynamic nature of quenching. In addition, a parallelism between the decrease in fluorescence yield and the shortening of the fluorescence lifetime suggested that deactivation of the fluorescence singlet by alcohols occurred via induced internal conversion, not enhanced intersystem crossing. Biczo´k et al. have found a clear correlation between the quenching rate constant and the relative hydrogen-bonding power of the alcohols as measured directly by the values of the equilibrium constants for hydrogen bond formation or by the hydrogen bond donating ability parameter, a, of the Kamlet–Taft scale [87]. 33.2.2 IR absorption spectroscopy In the cases of hydrogen-bonding interaction between the hydrogen donor group A–H and the acceptor atom B, attractive interaction between them modifies the molecular potential energy surface and has dramatic consequences for the vibrational spectra [87–90]. In particular, the hydrogen stretching band of the hydrogen donor group undergoes a spectral shift to lower frequency and a substantial reshaping of its spectral envelope. A substantial fraction of the observed redshift of the hydrogen stretching band has been attributed to an increase of anharmonicity of the potential energy surface along the stretching coordinate [51d]. The redshift of the stretching vibration reflects a decrease of the effective force constant of the hydrogen stretching oscillator. Empirical relations between the peak position of O--H stretching band and the O--H


O distance can be taken as a measure of the strength of the hydrogen bond [91–93]. Such relations predict O


O distances between 0.26 and 0.3 nm for stretching frequencies between 2800 and 3500 cm1. However, this type of correlation may be useful for the solid-state systems with a well-defined linear hydrogen-bonding geometry, for example, in crystals. In hydrogen-bonded liquids, the O--H stretching frequency may depend on other geometrical parameters and on interactions with the fluctuating environment as well [94, 95]. On the other hand, the width of the IR absorption spectrum gives a picture of the equilibrium distribution of hydrogen bond strengths in a hydrogen bonded system. Moreover, new modes related to A


B motions occur at low frequencies of up to several hundreds of wavenumbers (cm1). Such drastic changes of the vibrational spectra have made

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

769

Figure 33.6 Infrared absorption spectra showing O--H stretching absorption bands of (a) the free O--H group of phenol in C2Cl4 and (b) hydrogen-bonded O--H group in H2O [51d]. Adapted with permission from [51a]. Copyright 1999 American Chemical Society

vibrational spectroscopy one of the most powerful techniques with which to identify and analyse hydrogen bonds [51, 53]. This basic effect has been studied most extensively for O--H stretching vibrations, in particular in O--H


O hydrogen bonds [81–83]. Free O--H groups, for example, in diluted phenol in C2Cl4, give rise to an absorption band around 3600 cm1 with a spectral width (full width at half-maximum) of 15–20 cm1 (Figure 33.6) [51d]. However, the O--H stretching band of water, which forms an extended network of O--H


O hydrogen bonds, has an absorption maximum for the O--H stretching band at 3400 cm1, a bandwidth of 235 cm1 and an average hydrogen bond distance of 0.285 nm [91, 96]. Hence the most direct and convincing method for establishing the specific interaction via formation of hydrogen bonded complex between the solute molecule in the ground state and the solvent is the change of the stretching modes of all the bonds involved in the hydrogen bond formation. For example, in the case of hydrogen-bonding interaction between molecules containing >C¼O group and alcohols or amines, the frequencies of the stretching modes for both the O--H or N--H bonds as well as the C¼O bonds are shifted to the lower energy (or frequency) region and the amount of frequency shift with respect to that of the stretching mode of the free molecule (or uncomplexed) is indicative of the strength of the hydrogen bond formed between the solute and the solvent [51, 53]. C102 has been considered as an ideal probe molecule to study hydrogen bond dynamics, since the C¼O group is the only site to serve as the acceptor of hydrogen bond from alcohols, phenols and amines [51, 53, 72, 74, 97]. The dynamics of the hydrogen-bonded complex between C102 and the hydrogen-bond donating phenol have been investigated both theoretically by Zhao et al. [72, 73] and experimentally by Elsaesser, Nibbering and coworkers [51a,b]. The stretching mode of the free carbonyl (C¼O) group of C102 dissolved in non-hydrogen bonding solvent, C2Cl4, appears at around 1735 cm1 (Figure 33.7a). Addition of phenol leads to a downshift of the band to 1695 cm1. This redshift by about 40 cm1 was assigned to the formation of a strong C¼O


H--O hydrogen bond between the C¼O group and an O--H group of phenol, with a binding energy on the order of 60 kJ mol1. Formation of the C¼O


H--O hydrogen bond strongly affects the O--H stretching bands of the phenol molecules. The spectrum of phenol in C2Cl4 displays a stretching band at 3610 cm1, which has been assigned to free O--H groups, not being part of a hydrogen bond. However, a broader band in the 3450–3550 cm1 region, which arises predominantly due to 1 : 1 phenol–phenol complexes, because of formation of intermolecular O


H--O hydrogen bond between two phenol molecules appears, too

770 Hydrogen Bonding and Transfer in the Excited State

Figure 33.7 (a) Ground-state C¼O stretching bands of C102 in pure C2Cl4 (dashed line) and of C102–(phenol)n complexes in C2Cl4 (solid line). The C102 and phenol concentrations were 5 and 40 mM, respectively. (b) Steady-state O–H stretching bands of phenol dissolved in C2Cl4 (concentration 30 mM). (c) Ground-state infrared spectrum of C102–(phenol)n complexes [51a]. Adapted with permission from [53]. Copyright 2003 American Chemical Society

(Figure 33.7b). Addition of C102 leads to the formation of an additional broad band that is strongly redshifted to around 3380 cm1. This band is assigned to the O--H group in the C¼O


H--O hydrogen bonds (Figure 33.7c). We have studied the hydrogen-bonding interaction between C102 and aniline [53]. Curve “a” in Figure 33.8 shows the steady state FTIR spectrum of a solution of C102 (1.5  102 mol dm3) in tetrachloroethylene (TCE). It shows a very strong absorption band having the maximum at ca 1738 cm1 due to the stretching vibration of the free C¼O group. The vibrational spectrum of C102 recorded in neat aniline has an absorption maximum due to C¼O group at 1698 cm1 (curve b). The appearance of the new absorption band indicates the formation of an association complex between C102 and aniline via formation of a hydrogen bond between the C¼O group of C102 and the H--N group of aniline (C¼O


H--N). For several reasons (vide infra), fluorenone is another ideal probe molecule for studying the dynamics of hydrogen-bonded complex and has been investigated both theoretically and experimentally [21, 55, 59, 98, 99]. Tominaga and coworkers have recently studied the hydrogen-bonding interaction between fluorenone and primary alcohols (Figure 33.9) [59]. Figure 33.10 (A) below shows the IR absorption spectrum of fluorenone in cyclohexane, a non-hydrogen bonding solvent. The CO stretching mode of fluorenone shows a sharp band with a

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

1.5

771

a

Absorbance

b 1.0

0.5 1680

1700

1720

1740

-1 Wavenumber (cm )

Figure 33.8 Steady state FTIR spectra of C102 (1.5  102 mol dm3) in tetrachloroethylene (TCE) (curve a) and in neat aniline (curve b) [53]. Reprinted with permission from [59]. Copyright 2007 Elsevier

peak wavenumber of 1725 cm1 and a full-width at half maximum (fwhm) of 4.5 cm1. This figure also shows the IR absorption spectrum of fluorenone in 1-octanol. The spectrum shows multiple bands with two maxima at around 1713 cm1 (a) and 1721 cm1 (b), and a shoulder at around 1700 cm1 (c). The oxygen atom has two lone pairs, which may act as hydrogen-bonding sites to form two possible hydrogen-bonding complexes with one and two methanol molecules. Optimization of geometries and normal mode coordinates using density functional theory provided the frequency of the CO stretching mode of the free fluorenone (FL) as 1784 cm1, and those for the hydrogen-bonded complexes formed with methanol, i.e., FL : MeOH and FL : (MeOH)2, appear at 1761 and 1736 cm1, respectively (Figure 33.9). By correlating this with the fact that the peak frequency of the mode shifts to a lower frequency with increasing number of hydrogen bonds, the observed bands in the IR spectrum have been assigned to different complexes of fluorenone and solvent molecules, that is, the (a), (b) and (c) bands correspond to free fluorenone, a fluorenone complex with one alcohol molecule, and a complex with two alcohol molecules, respectively. Figure 33.10(B) displays the temperature dependence of the IR spectrum of fluorenone in 1-octanol from 293 to 323 K. The relative intensities of the three bands at 1713, 1721 and 1700 cm1 depend on the temperature. This indicates that the three bands result from three different species in equilibrium and not from an intramolecular effect, such as Fermi resonance, and therefore support the spectral assignment.

Figure 33.9

Structures of fluorenone–methanol complexes

772 Hydrogen Bonding and Transfer in the Excited State

Figure 33.10 (A) Absorption spectra of fluorenone in cyclohexane and in 1-octanol. The concentrations are 25 mM, and the optical path length is 0.5 mm. (a) Free fluorenone, (b) fluorenone complex with one alcohol molecule and (c) complex with two alcohol molecules. (B) Temperature dependence of the absorption spectrum of 9-fluorenone in 1-octanol [59]. Reprinted with permission from [59]. Copyright 2007 Elsevier (See Plate 39)

33.3 Vibrational Dynamics of the C¼O Stretching Mode of Fluorenone In hydrogen-bonded complexes, various dynamical properties, such as reactivity and energy relaxation, are strongly influenced by intramolecular as well as intermolecular hydrogen bonds [24–29, 59]. The effects of the OH stretching mode of alcohols or water on vibrational energy relaxation have been investigated by timeresolved infrared (IR) spectroscopy extensively; the vibrational energy relaxation of the OH stretching mode is accelerated by more than one order of magnitude [24, 25, 100–105]. To understand the effects of hydrogen bonds on the vibrational dynamics such as vibrational frequency fluctuations or excitation energy transfer, Tominaga and coworkers have investigated the vibrational energy relaxation of the CO stretching mode of the fluorenone molecule in alcohol solvents by sub-picosecond IR pump–probe spectroscopy [59]. IR pump–IR probe spectroscopy measures transient changes in the vibrational populations, as schematically shown in Figure 33.11. It involves the use of two ultrashort pulses of IR light; the pump pulse causes a reduction in the ground state population and an increase in the excited state emission, while the probe pulse monitors the evolution of these changes in population as a function of time (i.e., the delay between the two pulses).

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

773

Figure 33.11 Schematic depiction of the single color pump–probe spectroscopy. The pump produces a ground state bleach and excited state emission. For systems with large anharmonicities (D), no excited state absorption is observed. The “molecule” in the excited state and the “hole” in the ground state result in the amplification (from stimulated emission) and reduced absorption of the probe pulse, respectively. As the excited state decays, the absorption of the probe increases, resulting in a reduced pump–probe signal. Reprinted with permission from [59]. Copyright 2007 Elsevier (See Plate 40)

Figure 33.12(a) displays the IR pump–IR probe signal of fluorenone in cyclohexane at 1727 cm1 which is close to the peak wavenumber. The signal corresponds to the recovery of the ground state bleach and the decay of the stimulated emission. The signal shows a sharp spike at around t ¼ 0 ps, owing to a coherent artifact, which persists up to about 0.3 ps. Hence the pump–probe signals beyond 0.3 ps delay-time were fitted with a monoexponential decay with a time constant of 4.07  0.07 ps. Lim and Hochstrasser have studied the CO stretch of methyl acetate in carbon tetrachloride and found that the vibrational energy relaxation of the CO stretching mode is biexponential with time constants of 0.23 and 8.2 ps [106]. In this study, it was not possible to resolve the ultrafast component. A pump–probe signal at 1723 cm1, which is close to the peak of the CO stretching vibration of free fluorenone (Figure 33.12b), decays exponentially with a time constant of 4.3  0.1 ps. The time constant is similar to that in cyclohexane. Additionally, Figure 33.12(c) displays a pump–probe signal at 1712 cm1, close to the peak of the CO stretching vibration of the 1 : 1 complex of fuorenone–1-octanol. The signal decays exponentially with a time constant of 1.6  0.1 ps. Figure 33.13 displays the time-resolved differential absorption spectra of fluorenone in 1-octanol constructed at the delay-times of 0.45, 1.05 and 5.10 ps. Considering the three bands in the static IR spectrum of fluorenone in 1-octanol, which have been attributed to free fluorenone, the 1 : 1 complex and the 1 : 2 complex, it becomes evident that more than one species gives rise to signal intensities in the IR spectrum. The fact that

774 Hydrogen Bonding and Transfer in the Excited State

Figure 33.12 IR pump–probe signals of 9-fluorenone in (a) cyclohexane at 1727 cm1, (b) 1-octanol at 1723 cm1 and (c) 1-octanol at 1712 cm1. Reprinted with permission from [54a]. Copyright 1993 Elsevier (See Plate 41)

the signals at both 1723 and 1712 cm1 can be reproduced well with a single exponential decay suggests that at these wavenumbers a single species, that is, the free fluorenone and 1 : 1 complex, respectively, is the dominant contributor to the signal. However, the positive absorption band in the 1680–1710 cm1 region may have a contribution from all three species. Hence, assuming that the decay time constant of the 1 : 2 complex is faster than those of the free fluorenone and 1 : 1 complex, each spectral component was extracted using a global fitting analysis with the decay time constants 4.7  0.1, 2.3  0.1 and 0.27  0.02 ps. Therefore, the fast component may be due to a mixture of the coherent artifact and the 1 : 2 complex. The major factors determining the vibrational relaxation rate are the vibrational density of states, strength of couplings with overtone of the bending modes, and energy gap [107, 108]. Formation of a hydrogen bond could

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

775

Figure 33.13 Time-resolved differential spectra obtained at 0.45, 1.05 and 5.10 ps of fluorenone in 1-octanol. The green and purple lines are at 1712 and 1723 cm1, corresponding to the peak wavenumbers of the 1 : 1 complex and the free fluorenone, respectively [59]. Adapted with permission from [55]. Copyright 2005 American Chemical Society

influence all these factors. Since the energy shift due to formation of a hydrogen bond in this case is about only 10 cm1, it is unlikely that the energy gap change is a major factor for the increased rate of vibrational relaxation time in the hydrogen-bonded complex. However, the increase in density of states may play an important role in the vibrational relaxation process. Theoretical calculation using density functional theory reveals the presence of intermolecular vibrational modes with frequencies of 15, 37 and 49 cm1 for the fluorine–methanol complex and these modes may be accepting modes in the relaxation process. Another important point to note is that the time constants of the free fluorenone and the 1 : 1 complex have different values. This suggests that interconversion between the two conformers is not faster than vibrational relaxation of the two complexes.

33.4 Dynamics of the Excited States of Hydrogen-Bonded Complex 33.4.1 Electronic spectroscopy Despite the widespread importance of hydrogen bonding in both biological and non-biological systems, surprisingly little was known about the dynamics of hydrogen-bond formation and breakage until Berg and coworkers introduced a new approach that allowed the direct, time-resolved measurement of the lifetime of

776 Hydrogen Bonding and Transfer in the Excited State

Figure 33.14 Chemical structure of resorufin. Adapted with permission from [55]. Copyright 2005 American Chemical Society

hydrogen bonds between various solvents and resorufin (Figure 33.14), a hydrogen-bond accepting dye molecule [54]. This paper reported the first systematic study of the hydrogen-bond breaking and forming process. The results demonstrated that the rate of bond breaking was strongly affected by solvent dynamics and led to a multistep model for the bond-breaking process. This report has inspired the examination of several chemical systems to study the dynamics of hydrogen bonds using visible pump–probe spectroscopy. 33.4.1.1 Resorufin–Alcohols (Ref. [54]) Comparison of the ground state electronic absorption spectra of resorufin in a polar aprotic solvent, say propylene carbonate, and in protic solvents of varying hydrogen-bond donating ability has revealed the formation of two distinct kinds of hydrogen-bonded complexes, having absorption maxima at 583.5 and 592 nm, which have been assigned to two different forms of the molecule, namely “B” (blue) and “R” (red), respectively. These two forms remain in equilibrium in solution. In weaker hydrogen bond-donating solvents, say ethanol, the equilibrium shifts in favor of the B form and in a very strong hydrogen-bond donating solvent (TFE) only the B form is seen. However, the fluorescence spectra are only weakly affected [54a]. The interconversion of R and B forms has been studied using time-resolved measurements of the absorption change following photoexcitation of resorufin (Figure 33.15). In the polar aprotic solvent, DMSO, the transient spectra recorded at 1.5 and 300 ps are virtually identical (Figure 33.15a), suggesting that the polar solvation of resorufin is very weak, because of a small change in dipole moment on excitation, between 0.2 and 0.4 D. However, following photoexcitation of resorufin in ethanol using 577 nm light, the short-wavelength portion of the transient spectrum does not change, indicating that the absorption bleach does not change. However, the excited-state emission on the long-wavelength side of the spectrum undergoes a shift to longer wavelengths with time. At 577 nm, the R and B forms are excited in approximately the proportion found in the ground state at equilibrium. Thus, there is little rearrangement of the ground state populations after excitation. In the excited state, on the other hand, the populations are out of equilibrium. The B form converts into R to restore equilibrium, and the change in the emission frequency is seen in the transient spectrum. The existence of an isosbestic point indicates that the relaxation is between two distinct species, and is inconsistent with the continuous shift that results from polar solvation. Spectral evolution represents the conversion of the B form into the R form. Figure 33.15(c) shows the transient spectra after excitation at 590 nm; the spectral evolution suggests the conversion of the R form into the B form. Excitation at 590 nm will bleach primarily the R form out of the ground state, disturbing the ground state equilibrium. As in the ground state B converts into R to restore the equilibrium, and the frequency of the absorption bleach should shift from the frequency of R to the frequency of B. Assuming that the process of interconversion between the R and B forms follows first-order kinetics, the equilibration time in ethanol excited at 577 nm, where the excited-state dynamics predominate (Figure 33.15b), or that in ethanol excited at 590 nm, where ground state dynamics predominate (Figure 33.15c), is nearly the same and the ground-state equilibration rate constant in ethanol has been determined to be 42 ps. The equilibrium constant in the ground state has been estimated to be 1.86 and the hydrogen-bond formation time and the lifetime of hydrogen bond have been determined to be 65 and 120 ps, respectively.

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

777

Figure 33.15 Transient spectra of resorufin solutions. (a) Spectra are the sum of absorption and fluorescence components (dotted). In the polar aprotic solvent DMSO there is little spectral change between 1.5 ps (solid) and 300 ps (dashed). In the hydrogen-bonding solvent ethanol [(b) excited at 577 nm, (c) excited at 590 nm], spectral evolution with an isosbestic point occurs: 1.5 ps (solid), 20 ps (long-dashed), 50 ps (dotted), 300 ps (short-dashed). The vertical lines indicate the integration regions used in determining rates [54a]. Adapted with permission from [55]. Copyright 2005 American Chemical Society

The lifetime is similar to the reported 250 ps lifetime of hydrogen bonds in ethanol oligomers isolated in a non-polar solvent [109], but is much longer than the subpicosecond lifetime reported in pure water [110]. The lack of a strong viscosity dependence of the equilibration time suggested that the bond formation/breaking event was a very local motion that did not involve motion of the center of mass of the solvent molecule or largescale rearrangement of the solvent hydrogen-bond network but, rather, that only a simple rotation around the O--C bond might be involved. 33.4.1.2 Fluorenone–Alcohols (Ref. [55]) Fluorenone has been considered to be an ideal molecule for studying the hydrogen bond dynamics for several reasons. First, the primary photophysics of fluorenone has been well studied [82–85]. In fluorenone, like all other molecules with an active C¼O group, hydrogen bonding and polarity are the key factors in controlling the pathways of energy dissipation following electronic excitation. Second, as discussed in Section 33.2, fluorenone forms intermolecular hydrogen-bonded complex with alcohols in the ground state and both the S1 and T1 states are dynamically quenched due to hydrogen-bonding interaction with the alcoholic solvent molecules [73, 85]. Third, fluorenone is a planar molecule with a rigid framework, and, hence, no other

778 Hydrogen Bonding and Transfer in the Excited State

relaxation process, such as conformational or configurational relaxation, than the relaxation process arising due to solvent motions is important in the S1 state. Fourth, the change of dipole moment (Dm ¼ 2.2 D) upon photoexcitation of fluorenone to its S1 state is not very large [43]. Hence, the difference in spectroscopic properties between the Franck–Condon (FC) state and the relaxed excited state following dipolar solvation is expected to not be very significant. However, we may expect a significant change in the spectroscopic properties of the excited state due to hydrogen-bonding interaction between the carbonyl group and the molecules of the protic solvents. Finally, since fluorenone is very weakly fluorescent, the transient absorption spectroscopic technique should be a very useful tool to reveal the role of hydrogen bond dynamics in the excited-state relaxation processes of fluorenone [17]. Figure 33.16 shows the time-resolved absorption spectra of the transient species formed upon photoexcitation of fluorenone in acetonitrile and DMSO using 400 nm laser pulses of 70 fs duration. In each of these solvents, the time-resolved transient absorption spectra recorded in the sub-5 ps time domain show two distinct absorption bands in the 470–530 and 530–700 nm regions. Within this time-domain, evolution of spectral characteristics is insignificant but a slight decrease of absorbance occurs in the 570–700 nm region. Each of the temporal profiles recorded in this wavelength region consists of an ultrafast decay component, followed by another very long-lived component, which arises as a residual absorption in sub-500 ps time domain and can be assigned to the S1 state. Two such typical temporal profiles recorded at 630 nm in acetonitrile and at 610 nm in DMSO are shown in the insets of Figure 33.16. The lifetimes of this short component are 1.4 and 2.1 ps in acetonitrile and DMSO, respectively, and have been assigned to the vibrational relaxation process. Acetonitrile

20

0.15 ps 0.8 ps 2 ps 10 ps 50 ps

15 5

630 nm τ1 (d)= 1.4 ps

10

Δ Absorbance (mOD)

5

τ2 (d) = long

0

0 0 15

5 10 Time, ps

15

6

610 nm τ1(d)= 2.1 ps

3

τ2(d) = long

0

10

DMSO 5

0 500

0.2 ps 2 ps 5 ps 10 ps 50 ps 100 ps

0

5

550 600 Wavelength (nm)

10

15

Time, ps

650

Figure 33.16 Time-resolved transient absorption spectra of fluorenone in acetonitrile and DMSO constructed for different delay times following photoexcitation with 400 nm laser pulses. Insets: temporal absorption profiles recorded at 630 and 610 nm following photoexcitation of fluorenone in acetonitrile and DMSO, respectively. Solid lines represent the best-fitted dual exponential functions. The lifetimes of the shorter component, which are given in the figure, are assigned to the vibrational energy relaxation process happening in the S1 state and the long component arises due to long lifetime (19 ns) of the S1 state of fluorenone in these solvents [55]. Reprinted with permission from [111]. Copyright 2005 American Chemical Society

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

779

16 0.15 ps 0.4 ps 5 ps

12 Δ Absorbance (mOD)

15 ps 30 ps 100 ps a 8

b

1 4

0 500

550

600

650

0 500

550

600

650

Wavelength (nm)

Figure 33.17 Time-resolved transient absorption spectra of fluorenone in 1-propanol, constructed for different delay times following photoexcitation using 400 nm laser pulses. Inset: comparison of transient spectra constructed at 0.15 ps delay time following photoexcitation of fluorenone in acetonitrile (a) and 1-propanol (b) [55]. Reprinted with permission from [111]. Copyright 2005 American Chemical Society (See Plate 42)

The spectroscopic and dynamical features of the transient species have been investigated in normal alcohols as well as in ethylene glycol. The spectroscopic and the dynamical features of the transient species have been observed to be very similar in these solvents. As a typical example, Figure 33.17 shows time-resolved spectra of the transient species produced upon photoexcitation of fluorenone in 1-propanol using 400 nm laser pulses. The transient spectrum constructed for the 0.15 ps delay time, that is, immediately after photoexcitation, has features that are very similar to those of the transient spectrum recorded in acetonitrile (inset of Figure 33.17), although the relative intensities of two bands in these spectra are somewhat different. However, evolution of the transient spectra with increasing delay time is more significant as compared to that in acetonitrile, suggesting a stronger solute–solvent interaction. Despite the fact that 1-propanol (dielectric constant, «  21.1) is less polar than acetonitrile («  37), on comparing the features of the time-resolved spectra recorded in acetonitrile and 1-propanol the significant difference in evolution of the spectral characteristics of the transient species in these two solvents cannot be assigned to the simple dipolar solvation but, obviously, to the hydrogen-bonding interaction between the excited state of fluorenone and the solvent. The temporal dynamics of transient absorption have been seen to be wavelength dependent, because of overlapping of absorption bands due to more than one differently hydrogen-bonded species (vide infra). In a series of normal alcoholic solvents, the viscosities of the solvents increase as the length of the linear hydrocarbon chain increases from methanol to 1-pentanol. The spectral and temporal characteristics of the transient species generated in these solvents as well as in ethylene glycol (EG) have been seen to be very similar to those

780 Hydrogen Bonding and Transfer in the Excited State

6

0.15 ps 0.4 ps 1 ps 2 ps 5 ps 10 ps 20 ps

(A)

4 a'

Δ Absorbance (mOD)

2

4

a 0

0

500

550

600

650 6

(B) 6

3

4

0

b' b

20 ps 40 ps 60 ps 100 ps

2

0

500

550

500

600

550

600

650

Wavelength (nm)

Figure 33.18 Time-resolved transient absorption spectra constructed for different delay times following photoexcitation of fluorenone in TFE using 400 nm laser pulses. (A) Time-resolved spectra in sub-20 ps time domain. Inset of (A): comparison of the transient spectra constructed for 0.15 ps delay-time in TFE (a) and in 1-propanol (a0 ). (B) Time-resolved spectra in post-20 ps time-domain. Inset of (B): comparison of the transient spectra constructed for 100 ps delay-time in TFE (b) and in 1-propanol (b0 ) [55]. Reprinted with permission from [111]. Copyright 2005 American Chemical Society (See Plate 43)

in 1-propanol. Hence the dynamics of solute–solvent interaction is not controlled by the viscosity of the solvents. Relaxation dynamics of the S1 state of fluorenone in TFE have also been investigated (Figure 33.18). The features of the spectrum recorded at 0.15 ps are very similar to those of the spectra recorded in acetonitrile and 1-propanol at the same delay-time. Following the spectral evolution in sub-20 ps time domain, the transient spectrum constructed at 20 ps delay-time is seen to consist of an ESA band with maximum at 550 nm and a shoulder at 510 nm. At longer delay times, beyond 20 ps, this entire band decays without any further evolution of the spectral features. The features of the transient spectrum recorded at 20 ps delay time have been compared to those of the transient spectrum recorded at 100 ps delay time in 1-propanol (inset of Figure 33.18B). This shows that while the transient spectrum recorded in 1-propanol consists of two ESA bands with maxima at 520 and 570 nm, the transient spectrum recorded in TFE has only a single band with maximum at ca 540 nm and the band with maximum at 570 nm is missing in the latter. Both steady-state electronic and IR spectroscopic study have clearly revealed that in solutions of fluorenone in normal alcoholic solvents both the free fluorenone molecule and the hydrogen-bonded complex exist in equilibrium, although the equilibrium remains in favor of the free molecule [55, 59, 75]. However, in TFE, which is a strong hydrogen bond-donating solvent, the equilibrium shifts in favor of the hydrogen-bonded complex. Hence, photoexcitation of fluorenone molecules in normal alcoholic solvents using 400-nm laser light creates the excited states of both the free fluorenone molecule as well as its hydrogen-bonded complex.

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

781

On the other hand, in TFE, the excited state of only the hydrogen-bonded complex is created. Considering the similar positions of the maxima in the transient spectra recorded at 0.15 ps delay time in these solvents (insets of Figures 33.17 and 33.18) the transient spectrum in1-propanol or TFE may be assigned to the excited state of the free form of fluorenone. This might have been formed either upon photoexcitation of the free fluorenone molecule existing in the ground state (in 1-propanol) or by ultrafast photodissociation of the hydrogen bond in the excited state of the hydrogen-bonded complex (in 1-propanol and TFE). However, the solvent molecules cannot reorganize rapidly to incorporate the newly released alcohol molecule into its two-dimensional hydrogen bond network structure; that is, there is a “dangling” hydrogen bond still present [54b]. This non-equilibrated state of the excited fluorenone molecule is associated with a completely non-hydrogen-bonded solvent molecule, or a solvent molecule bonded into a chain to form a branch point, or some other unfavorable hydrogen bond configuration. This is a poorly solvated state, which is sufficiently unstable and tends to undergo geminate reformation of fluorenone–alcohol hydrogen bond with high probability. This reformation process is accompanied by the requisite reorganization of the hydrogen bond structure of the solvent to fully equilibrate and incorporate the dangling hydrogen bond into the hydrogen bond network structure of the solvent. With increasing delay time, the evolution of the time-resolved absorption spectra, which is associated with the rapid decrease of transient absorption in the 590–700 nm region and concomitant increase in absorption in the 510–590 nm region, can be assigned to the geminate reformation of the hydrogen bond, possibly with a new equilibrium geometry, accompanied by equilibration of the hydrogen bond network structure of the solvent. The significant difference in evolution of the spectral characteristics of the transient species in these two solvents cannot be assigned to the simple dipolar solvation but obviously to the hydrogen bonding interaction between the excited state of fluorenone and the solvent. In other words, the hydrogen bond dynamics are responsible for the observed evolution of the time-resolved spectra of the S1 state of fluorenone in alcohols. However, wavelength-dependent dynamics might have arisen due to both hydrogen reorganization process and the presence of more than one kind of hydrogen-bonded species [59]. Zhao and Han have demonstrated that the intermolecular hydrogen bond between fluorenone and methanol is significantly strengthened in the S1 state of the hydrogen-bonded complex [72]. This conclusion is quite reasonable and justified considering the fact that the excited state of fluorenone has an ICT character and, because of the increased charge density on the oxygen atom, the hydrogen bonded complex reformed following hydrogen-bond reorganization process is expected to have stronger hydrogen bond in the excited state. 33.4.1.3 Ketocyanine Dyes–Alcohol Systems (Refs [111,112]) The pronounced solvent effects in both absorption and emission spectra of the isosbestic dyes 2,5-bis[(2,3dihydroindolyl)propylene]cyclopentanone (KCD)and 2,5-bis(N-methyl N-1,3-propdienylaniline)cyclopentanone (MPAC) (Figure 33.19) make them promising probes for monitoring micro-polarity and hydrogenbond donating interactions. Bagchi and his coworkers have made a systematic study on the solvation characteristics of several ketocyanine dyes using both the steady state absorption and fluorescence and the time-resolved fluorescence measurements [113]. They showed that while non-specific dipolar solvation is O

O

N

N

N H 3C

I

N CH 3

II

Figure 33.19 Chemical structures of KCD (I) and MPAC (II). Adapted with permission from [55], and reprinted with permission from [111]. Copyright 2005 American Chemical Society

782 Hydrogen Bonding and Transfer in the Excited State

responsible for the solvatochromic properties of these dyes in aprotic solvents, specific interaction involving formation of intermolecular hydrogen-bond provides stronger solvatochromic behavior of these dyes in protic solvents. Workers from different groups have also shown that the ketocyanine dyes form strong hydrogenbonded complexes both in the ground state and in the excited state [113–116]. However, the fluorescence efficiency of these dyes is seen to be reduced significantly in protic solvents due to hydrogen-bonding interaction with the solvent. One very interesting result regarding the photophysics of KCD reported by Bagchi’s group is that both the fluorescence quantum yield (ff) and the fluorescence lifetime (tf) of KCD and MPAC, unlike in the case of many other ketocyanine dyes, increase with increasing solvent polarity. Additionally, in protic solvents, both ff and tf are remarkably higher than in aprotic solvents of comparable polarity. This is in contrast to the properties of other ketocyanine dyes [113]. The change in dipole moment (Dm) between the ground state and the fluorescing S1 state of KCD has been determined to be 3.6 D [105]. The lowest energy absorption (S1 S0) band arises due to p–p transition, involving an intramolecular charge transfer (ICT) from the electron donating indolyl group to the electron accepting carbonyl group through the intervening conjugated system [113–116]. The cause of increased ff and tf of the S1 state of these dyes could be predicted by correlating the fluorescence energy with the various measures of the solvent parameters (Figure 33.20). We find very poor correlation between the fluorescence energy and the reaction field parameter of the solvent, F. The correlation between the fluorescence energy and the Kamlet–Taft solvatochromic parameters, a and p , of the solvents are also very poor. In contrast, the fluorescence energies show good correlation with the ET(N) values of the solvents. These facts suggested formation of a hydrogen-bonded complex in the S1 state of these molecules. 20 C. C. = 0.84

C. C. = 0.79

Fluorescence Energy (103 cm-1)

18

18

16 16 0.0

0.1

0.2

0.3

0.0 0.4 0.8 1.2 1.6

Δ F( ε ,n)

20

α -Scale

20 C. C. = 0.59

C. C = 0.99

18

18

16

16

14

14 0.0

0.4 0.8 π *-Scale

0.0

0.3

0.6

0.9

ET(N) Scale

Figure 33.20 Correlation of fluorescence energy (corresponding to the maximum of fluorescence spectrum) with different solvent parameters. The fluorescence energy shows good linear correlation with ET(N) values of the solvents [111]. Adapted with permission from [51a]. Copyright 1999 American Chemical Society

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

783

a

(B) g

0 g

-3 a

-6

-9

500

Delay times (ps): (a) 0.2, (b) 0.5, (c) 1, (d) 2, (e) 10, (f) 20 and (g) 30

600 700 Wavelength (nm)

2

Δ Absorbance (mOD)

Δ Absorbance (mOD)

3

(A)

Delay-times (ps) : 0.15, 0.3, 0.5, 1, 5, 20, 40, 80 and 150 ps

0

-2

500 550 600 650 700 750 800

Wavelength (nm)

Figure 33.21 Time-resolved transient absorption spectra constructed at different delay times following photoexcitation of KCD in DMSO (a) and 1-propanol (b) using 400 nm laser pulses [111]. Adapted with permission from [53]. Copyright 2003 American Chemical Society (See Plate 44)

In Figure 33.21, we compare the time-resolved differential transient absorption spectra recorded in DMSO and 1-propanol following photoexcitation of KCD using 400 nm excitation. Since, 400 nm light excites the KCD molecules in the S2 state in both these solvents, early time dynamics have been assigned to that of the S2 state and will not be discussed here in detail. Lifetimes of the S2 state have been determined to be about 0.5 and 0.33 ps in DMSO and 1-propanol, respectively. Decay of the negative absorption band in the 470–530 nm region in the early time-domain could be assigned to the recovery of the ground state bleaching and decay of the stimulated emission associated with the S2 state. Simultaneous to the decay of the S2 state in DMSO, the growth of the SE band at 550 nm indicates the formation of the S1 state, which is long lived with a life time of about a few nanoseconds [113]. However, the nature of the time-resolved spectra and the dynamical behavior of the transient species in 1-propanol are significantly different from that observed in DMSO. At longer delay times, ranging from 1 to 150 ps, we observe the development of the stimulated emission band in the 570–800 nm region and an ESA band in the 530–570 nm region as well as the reduction of the negative absorption band in the 490–530 nm region. On increasing the delay-time, the maximum of the stimulated emission band is shifted from 590 nm (observed at 1 ps delay time) to 650 nm (observed at 150 ps delay time). The dynamic Stokes shift of the maximum of the time-resolved stimulated emission band from 590 nm to 650 nm could not be assigned merely to solvation, since the stimulated emission band also grows simultaneously with increasing delay time. This leads to the presumption of involvement of a precursor state, which has been designated as the S10 state, prior to formation of the long-lived S1 state. The dynamics of the excited states of KCD in straight-chain alcohols with varying length of the hydrocarbon chain, for example, methanol to 1-octanol, as well as in ethylene glycol (EG) have been investigated. The spectral and temporal characteristics of the transient species produced in these alcoholic solvents have been observed to be very similar to those in 1-propanol. Figure 33.22 shows time-resolved spectra of the transient species produced due to photoexcitation of KCD in TFE with 400 nm laser pulses. The spectrum constructed for 0.15 ps delay-time consists of two negative absorption bands in the 490–530 nm (bleaching band) and 530–620 nm regions (SE band), and an ESA band in the 620–800 nm region. All these features are characteristics of the S2 state of KCD. The time-resolved spectra constructed at longer delay-times reveal the rapid decay of the SE band in the 490–630 nm region and the

784 Hydrogen Bonding and Transfer in the Excited State 4 2

Delay-times (ps) (a) 0.15, (b) 0.3, (c) 0.5, (d) 1 & (e) 2

(A)

0 a f -2

Δ absorbance (mOD)

f

a

-4 -6

Delay-times (ps)

2

(B)

(f) 4, (g) 10, (h) 20 & (i) 50

0 f -2 -4 -6

500

550

600 650 700 Wavelength (nm)

750

Figure 33.22 Time-resolved transient absorption spectra constructed in sub-5 ps (A) and sub-50 ps (B) time domains following photoexcitation of KCD in TFE [111]. Copyright American Chemical Society, reproduced with permission

development of another stimulated emission band with a maximum at 600 nm. Following the line of assignments of the different transient species in other solvents, the emission bands with maximum at ca 0 0 500, 600 and 650 nm have been assigned to the S2, S1 and S1 states, respectively. The lifetimes of the S2 and S1 states could be assigned as 0.5  0.1 and 9.0  0.5 ps, respectively. As in other solvents, the lifetime of the S1 state is too long to measure here. 33.4.1.4 Hydrogen Bond Dynamics Versus Solvation (Refs [54, 55, 111, 112]) The standard picture of non-specific or dipolar solvation dynamics assumes relatively weak intermolecular interactions between the excited solute and the surrounding solvent molecules [10–15]. The nature of interaction between the solute and each of the large number of solvent molecules is equally important, and the dynamics are associated with the motion of many solvent molecules along a collective coordinate. The barriers to these motions are small compared to thermal energies. However, although the energy barrier for the hydrogen bond breaking or forming is low, it is not negligible as compared to the thermal energies. In reality, hydrogen-bond dynamics are intermediate between non-specific solvation and covalent bond breaking. Additionally, interaction of the solute with the single hydrogen-bonded solvent molecule is stronger than its interaction with other solvent molecules. However, the strength and dynamics of this interaction may not differ too much from the hydrogen bonding interaction between the solvent molecules, and hence the latter may

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

785

influence the relaxation dynamics of the excited solute molecule in a significant way [54]. Finally, hydrogen bonding, which has been described here as a “specific” interaction, is similar to non-specific interactions such as dipole–dipole or van der Waals interactions [117]. From this viewpoint hydrogen-bond making or breaking is just a change in solute–solvent interaction resulting from the reorganization of the solvent, that is, it is a type of solvation dynamics. The possible reason behind the fact that dielectric relaxation of alcoholic solvents has attracted more attention than the dynamics of specific interaction may be the choice of the probe molecules, which do not display much difference in fluorescence behavior in protic and aprotic solvents because of weak hydrogenbonding interaction between the solute and the solvent. Morimoto et al. have studied the quenching of fluorescence of a series of aromatic carbonyl compounds by alcoholic solvents [20]. This work revealed that the S1 states of 3-amino- and 4-aminofluorenones are more efficiently quenched by alcohols than those of two coumarin dyes, namely, coumarin 153 and coumarin 151. These dyes are popular fluorescent probes for studying the dipolar solvation dynamics [118]. To explain this difference in behavior, Morimoto et al. have invoked the concept of “hard and soft acid–base” (HSAB) behaviors of the solvent and the excited state, respectively. Both the amino and the carbonyl groups are present in both the kinds of molecules and the values of Dm of the aminofluorenones are comparable to those of the coumarin dyes (Dm  5.8 and 3.1 D for C153 and C151, respectively). However, theoretical calculations have revealed that in the S1 state with ICT character the negative charge is mostly localized on the carbonyl oxygen atom in the case of aminofluorenones, whereas in the case of the coumarin compounds the negative charge is substantially delocalized over the whole aromatic moiety [20]. Following the creation of the S1 state, the local charge density on the amino group is decreased by about 21% in both kinds of molecules but the charge density on the carbonyl oxygen atom is increased by about 9% in the case of the aminofluorenones, whereas it is increased by only about 2% in the case of the coumarin molecules. Hence, the S1 states of the aminofluorenones have been considered as hard anions (or bases), since the negatively charged oxygen atom interacts strongly with the hydroxyl hydrogen of the alcoholic solvent, which has been considered as a hard cation (or acid). On the other hand, the S1 states of the coumarin molecules are soft anions (or weak bases) and do not have appreciable interaction with the hydroxyl hydrogen of the alcohols. This is why the coumarin dyes have proven to be ideal probes for studying the dynamics of polar solvation in both aprotic and protic solvents without any quenching of fluorescence of the probe via specific interaction between the probe and the solvent [118]. In cases of the chemical systems described above, while the hydrogen bond dynamics investigated in TFE can be thought of as bond-breaking and bond-making chemical reactions rather than a solvation process, in normal alcohols the wavelength-dependent dynamics of the hydrogen bond lead us to consider it as merely a solvation process. In hydrogen-bonding solvents, the Debye dielectric relaxation process follows multiexponential dynamics and the longest component is generally assumed to be connected with the rate of hydrogen bond reorganization in the solvent [9]. The most common and conventional method of determining the lifetime of dipolar solvation process is to follow the time-dependent redshift of the emission maximum of the emission or SE spectrum or blue-shift of the ESA spectrum during the relaxation of the excited state of a probe molecule [10–15, 118]. The timeevolution of the fluorescence spectrum due to solvation is quantified by the time correlation function, C(t), for the dynamic shift of the emission maximum observed in the time-resolved fluorescence experiment, and is expressed by equation (33.4): CðtÞ ¼

nðtÞnð1Þ nð0Þnð1Þ

ð33:4Þ

where n(0) is the optical frequency of the maximum of the emission of the probe at zero time (i.e., just after excitation), n(1) at infinite time (when the solvent dipoles have relaxed to equilibrium) and n(t) at

786 Hydrogen Bonding and Transfer in the Excited State

intermediate time (t) (i.e., during the solvent relaxation). However, unfortunately, in the present cases, the values of C(t) could not be determined with reasonable accuracy, because of significant mutual overlap between the ESA and SE (or fluorescence) bands [111, 112] and/or involvement of multiple excited states [54, 55]. However, a solvent relaxation process has clearly been revealed by the more rapid growth and decay of the intensity of SE band at the blue-edge of the emission spectrum and slower growth and decay of the same on the red-edge of the emission spectrum [111, 112]. Hence, considering these facts, application of equation (33.4) in determining the lifetime of the solvent reorganization process does not seem to be justified and may lead to erroneous results. Therefore, we have adopted the “single or linear wavelength” method to determine the average solvation time [119, 120]. We observe that, in protic solvents, both the growth and decay lifetimes of the intensity of the SE band increase as the monitoring wavelength approaches the lower energy region of the SE band. The largest value of this lifetime has been obtained at wavelengths in the lower energy region beyond the maximum wavelength of the SE band. This value has been considered here as the average lifetime of the solvent reorganization process, tR. In the case of solvent-controlled barrier crossing reactions, in which the reactant and the solvent are strongly coupled, dynamical solvent effects are usually discussed in terms of “friction”, z, to account for the effect of the solvent on the lifetime (t) of the reactant [121–123]. Berg and coworkers conceived the fact that the dynamics of a specific interaction affecting hydrogen bond reorganization time of the solvent are well correlated with the dielectric relaxation time, and the hydrogen bond lifetimes are very similar to the longitudinal relaxation time, tL [54]. Based on these arguments, equation (33.5) could be rewritten by equating the solvent friction, j, to the longitudinal relaxation time (tL) of the solvent as [equations (33.6) and (33.7)]: t ¼ Aj expðDH=RTÞ

ð33:5Þ

t ¼ AðtL ÞexpðDH=RTÞ

ð33:6Þ

lnðtÞ ¼ lnðtL Þ þ lnðAÞ þ DH=RT

ð33:7Þ

Assuming that the enthalpy of the transition state, DH, does not vary significantly due to a change of solvents belonging to the same class, for example, primary alcohols, we expect a linear correlation between ln(tR) and ln(tL) [equation (33.7)]. As shown in Figure 33.23, we find a perfect linear correlation between these two parameters in the cases of fluorenone and KCD in normal alcohols and reconfirm the postulation that hydrogen bond reorganization around the excited solute molecule is solely responsible for the dynamics of the relaxation process of the S1 state in the alcoholic solvents observed here. 33.4.2 Time-Resolved IR absorption spectroscopic technique The examples presented above show that the electronic transitions are strongly broadened owing to coupling with the fluctuating solvent and are relatively featureless due to overlapping of different transitions as well. Spectral shifting and reshaping caused by solvent reorganization leads to wavelength-dependent dynamics, and ensemble-averaged time-correlation functions for liquid motion provide very limited information regarding microscopic solvent structure. However, despite the many disadvantages of probing electronic transitions of the transient species, valuable information could be obtained regarding the dynamics of chemical reactions by detailed analyses of the transient spectra and the dynamics monitored at different wavelengths using ultrafast visible spectroscopy. The following two examples show that time-resolved vibrational spectroscopy is a more powerful technique for monitoring hydrogen-bond dynamics in real time. The vibrational spectroscopic technique also has the added advantage that it is possible to observe changes in distinct functional groups involved in hydrogen bond formation, which provides site-specific insight into local dynamics.

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

787

4.0 3.5

A. Fluorenone C. C. = 0.95

2.5

R

)

3.0

ln(τ

2

3

4

5

4 2 B. KCD

0

C. C. = 0.97 -2 -2

-1

0

1

2

3

4

5

ln (τL)

Figure 33.23 Correlation between the solvent relaxation time, tR, and the longitudinal relaxation times, tL, of the alcoholic solvents [55, 111]. Copyright American Chemical Society, reproduced with permission

33.4.2.1 C102–Phenol (Ref. [51]) In Section 33.3, we discussed the hydrogen-bonding interaction and formation of a hydrogen-bonded complex between C102 and phenol in the ground electronic state. To investigate the ultrafast response of the complexes to an electronic dipole, the C102 chromophore was excited by a 100 fs pulse at 400 nm, resonant to the purely electronic S0–S1 transition, whereas the phenol molecules remain in their electronic ground state. The resulting changes of vibrational absorption of both the C102 and phenol molecules are monitored by mid-infrared probe pulses tunable in the range of the C¼O and O--H stretching bands. The time-resolved data gave the first direct insight into the microscopic dynamics of the complexes and revealed two distinct time scales of structural response, relating to the different types of hydrogen bonds in the system. Upon photoexcitation of C102, the stretching band of the C¼O group of C102 at 1695 cm1 disappears and is replaced by a weaker broad band with a maximum at about 1740 cm1 (Figure 33.24a). This position is characteristic of a free C¼O group for both the ground state (Figure 33.7a) and the S1 state of C102 [51b]. Since these changes take place within the time resolution of the experiment of 200 fs, these observations lead to the conclusion that the C¼O


H--O hydrogen bond in both the C102-phenol complexes broke within 200 fs after excitation of C102. The fast cleavage of the C¼O. . .H--O hydrogen bond is manifested in the step-like increase of free O--H absorption at 3610 cm1 (Figure 33.24b). The cleavage of this bond has been explained by the changes of the local charge distribution of C102. In particular, the polarity, and thus the hydrogen affinity, of the C¼O group decreases in the S1 state of C102, as is suggested by semi-empirical calculations of the S1 charge distribution [42]. Cleavage of the hydrogen bond could be an instantaneous process owing to a rearrangement of electronic density upon excitation. It could also involve nuclear motion on a time scale set by the period of low-frequency vibrations of the hydrogen bonded groups in the 100–200 cm1 range. Such frequencies translate into time constants < 200 fs, in agreement with the dynamics found here. Significant changes of the broad O--H band between 3200 and 3550 cm1 has also been found upon electronic excitation of C102 for delay times up to 5 ps. There is a strong increase of infrared absorption

788 Hydrogen Bonding and Transfer in the Excited State

Figure 33.24 (a) Transient C¼O stretching band of C102 in the complexes after excitation to the S1 state of C102. The spectra are shown for different time delays after excitation. (b) Transient vibrational spectra of the (phenol)n units in the complexes after excitation of C102, recorded at the same delays as shown in (a) [51a]. Copyright American Chemical Society, reproduced with permission

in this frequency range, followed by a reshaping of the spectrum. The reshaping of the spectrum occurs with a characteristic time constant of 800 fs. The vibrational band found after about 5 ps stays unchanged for even later times and is very close to the stretching band of the hydrogen bonded O--H group in the phenol dimer. The absorption changes disappear on a time scale of nanoseconds with the decay of the S1 state of C102. Evolution of the transient absorption spectra in the sub-5 ps time-domain presented in Figure 33.24 reflects the non-equilibrium geometry of the complexes immediately after cleavage of the C¼O. . .H--O hydrogen bond. Cleavage is accompanied by a redistribution of electron density in the phenol molecule close to C102, and it acts back on the neighboring C¼O group of C102, thereby changing the strength of the C¼O band. However, the hydrogen bond with the other phenol molecules in C102–(phenol)n complex remains intact and hence the cleavage leads to changes in hydrogen bond strength and the vibrational transition moments of the bridged OHgroups. Subsequently, the released (phenol)n moiety reorganizes itself to a new equilibrium configuration, mainly by reorientation of the phenol units relative to each other and to the excited C102 molecule. This structural reorganization of (phenol)n with a time constant of 800 fs has been directly monitored in this study (Figure 33.24b). The new band between 3400 and 3550 cm1 has a shape very close to the infrared absorption of 1 : 1 phenol–phenol complexes (Figure 33.7b), leading to the conclusion that the contribution of larger complexes (n > 2) towards the observed dynamics is not significant. The reorientation time of 800 fs corresponds to a low-frequency motion of about 40 cm1, a frequency typical for librational motions that could well represent the relative tilting of the rings in (phenol)2. Importantly, this technique has provided an

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

789

insight into the dynamics involving the phenol unit not directly bonded to C102, demonstrating the potential of femtosecond vibrational spectroscopy for probing dynamics in hydrogen bonding networks at a finite distance from the directly excited groups. Wells et al. have recently reported the intermolecular response of solvent molecules following photoexcitation of C102 in acetonitrile–water binary mixtures using a technique measuring the time-domain Raman response [74]. At low water concentrations, the solvent response was consistent with a dipolar solvation process. However, with increasing water concentration, an additional response was found subsequent to dipolar solvation, exhibited as a rapid gain in the solvent’s polarizability on the 250 fs time-scale. Simulation studies indicated that the probability of the C102 solute being hydrogen bound with two water molecules simultaneously, both as donors at the carbonyl site, increases in a correlated fashion with the amplitude of the additional response in the measurements, leading to the conclusion that the excitation of C102 simultaneously weakens and strengthens hydrogen bonding in complexes with two inequivalently bound waters. 33.4.2.2 C102–Aniline (Ref. [53]) Time-resolved infrared absorption spectroscopic studies have been performed with solutions containing C102 in neat aniline. Upon photo-excitation of C102–amine systems by ultrashort (duration of about 150 fs) laser pulses of 400 nm, which is resonant to the electronic S1 S0 transition of C102, only the C102 chromophore is excited. Aniline molecules do not absorb at 400 nm and, hence, remain in the ground electronic state. Figure 33.25 presents the temporal profile of the transient absorption monitored at 1742 cm1. The hydrogenbonded complex has no absorption at 1742 cm1 but the free C¼O group absorbs strongly at this frequency. The probe is resonant to C¼O stretching and causes the v ¼ 0 to v ¼ 1 transition in the S1 state of C102. We observe the instrument response time-limited rise of transient absorption followed by a tri-exponential decay. The lifetimes of the two components decaying in the early time domain were determined to be 0.5  0.1 and 6.8  0.4 ps and the third component is very long and has a lifetime of about a few nanoseconds.

Absorbance, mOD

1.5

C-102 in neat AN ν probe

= 1742 cm-1

τ 1 (d) = 0.5 ps τ 2 (d) = 6.8 ps

1.0

τ 3 (d) = long

0.5

0.0 -5

0

5

10

15

20

25

Time, ps

Figure 33.25 Time-resolved change of vibrational absorption monitored at 1742 cm1 in neat aniline. The lifetimes obtained by two exponential fittings of the data are given [53]. Copyright American Chemical Society, reproduced with permission

790 Hydrogen Bonding and Transfer in the Excited State

Since the lifetime of the S1 state of C102 in neat aniline is 1.4 ns, the time dependent absorbance changes, as shown in Figure 33.25, indicate the role of a hydrogen bond in the excited state dynamics of C102 in aniline. The appearance of a transient absorption signal at 1742 cm1, which indicates the formation of free C¼O group, immediately after the electronic excitation of the C102 chromophore in C102–aniline hydrogenbonded complex, indicates the instantaneous dissociation (<250 fs) of the hydrogen bond between C102 and aniline. Cleavage of the hydrogen bond is driven by the changes of the local charge distribution in the excited state of C102. The impulsive enhancement of the transient absorption following optical excitation represents a non-equilibrium geometry of the cleaved hydrogen bond. Since 85% of the transient infrared absorption signal, which is characteristic of the C¼O group, decays within a few tens of a picosecond (this is much shorter than the lifetime of the S1 state of C102 in aniline) it could be correlated with the process of reformation of hydrogen bond after its cleavage in the S1 state. However, this new hydrogen bond, reformed between C102 in the S1 state and aniline molecules in the ground state, has an equilibrium geometry and electronic structure that differ from those formed when both components are in the ground state. Based on these arguments, we have proposed a model in which hydrogen bond breaking is followed by solvent reorganization on two time scales. After initial bond breaking, the fragments are rapidly, but incompletely, solvated to leave a dangling hydrogen bond. The component with 0.5 ps lifetime, associated with the temporal profiles shown in Figure 33.25, which represent the vibrational dynamics of the C¼O group of C102 in neat aniline, may have been arisen due to this rapid non-diffusive component of solvation. To complete the equilibration of the product state, the dangling bond must be fully incorporated into the hydrogen-bond structure of the solvent. This process occurs on a longer time scale related to the rate of hydrogen-bond reorganization in the bulk solvent. In hydrogen-bonding solvents, the longest component of the Debye dielectric relaxation is generally assumed to be connected with the rate of hydrogen-bond reorganization in the solvent. Thus the slower decay process having a lifetime of about 6.8 ps could be correlated to the diffusive restructuring of the first solvation shell around the C¼O group of C102 in the S1 state. The longitudinal relaxation time (tL) in liquid aniline, which is predicted to be the solvation time by the simplest continuum theory, has been determined to be 8.1 ps. Recently, Liu et al. have published their results on an investigation of hydrogen-bonding interaction between the electronically excited state of C102 and aniline using time-dependent density functional theory [75]. They demonstrated that the intermolecular hydrogen bond between C102 and aniline molecules is strengthened in the electronically excited state, since the calculated hydrogen bond energy increases from 25.96 kJ mol1 in the ground state to 37.27 kJ mol1 in the excited state. However, this conclusion is not in contradiction with the experimental observation that the hydrogen bond, which is formed between C102 and aniline in the ground state, is cleaved following photoexcitation of C102. Temporal dynamics clearly reveal that the cleaved hydrogen-bond is reformed via geminate recombination and hydrogen bond reorganization processes. However, the geometry of the hydrogen bond formed in the excited state is different from that of the ground state complex. The experimental results discussed here clearly demonstrate the ability of ultrafast time-resolved vibrational spectroscopy to reveal the dynamics of hydrogen bonds not only involving the solvent molecule directly linked to the initially excited hydrogen-bonded acceptor solute molecule but also the dynamics in hydrogenbonding network further away from it.

33.5 Summary and Conclusion In this chapter, we have reviewed the results of a few recent studies on the intermolecular hydrogen-bonding interactions between a solute (called a probe) molecule and hydrogen-bond donating solvents, namely, primary alcohols, phenols and amines. We have confined our discussion mainly to molecules that contain a

Ultrafast Dynamics of the Excited States of Hydrogen-Bonded Complexes and Solvation

791

carbonyl group as a good hydrogen bond acceptor and form strong hydrogen-bonded complexes with the hydrogen bonding solvents, both in the ground and excited states. Another important characteristic of these probe molecules is that the excited state has weak intramolecular charge transfer characteristics (ICT), but the dipole moment change following photoexcitation is not very large (say, Dm  2–5 D), so that in the solvent relaxation process the contribution of dipolar solvation is not very significant but the spectroscopic properties of the excited state are mainly controlled by the hydrogen-bonding interaction with the solvent. General characteristics of hydrogen-bonding interactions have been described in brief. Applications of steady-state visible and IR absorption spectroscopic techniques have been described in identifying and characterizing the hydrogen-bonded complex formed in the ground electronic state. Steady-state and timeresolved fluorescence spectroscopic techniques provide useful information regarding the hydrogen-bonding interaction in the excited state. The faster non-radiative relaxation rate in a hydrogen-bonding solvent as compared to that in aprotic solvents of similar polarity ensures the solute–solvent interaction via hydrogenbonding. A brief description of the spectroscopic techniques applied to characterize the hydrogen-bonding interaction between coumarin 102 (C102) or fluorenone (Figure 33.1) and alcohols or phenols, which are the most well studied (and considered here as model systems), has been provided. In hydrogen-bonded complexes, various dynamical properties, such as reactivity and energy relaxation rates, are strongly influenced by intramolecular as well as intermolecular hydrogen bonds. It is important to understand the effects of hydrogen bonds on the vibrational dynamics such as vibrational frequency fluctuations or excitation energy transfer. Results of investigations on the vibrational energy relaxation of the CO stretching mode of the fluorenone in alcohol solvents using sub-picosecond IR pump–probe spectroscopy to understand the effect of hydrogen bond on vibrational energy relaxation have been presented. While the vibrational density of states, strength of couplings with overtone of the bending modes, and energy gap are considered to be the major factors determining the vibrational relaxation rate, in the present case the increase in density of states because of hydrogen bonding interaction possibly plays an important role in the vibrational relaxation process. Significant differences in the evolution of the spectroscopic properties of the excited state following photoexcitation of a probe molecule in aprotic and protic solvents can be considered as an indication of intermolecular hydrogen-bonding interaction in the protic solvents. Berg and coworkers first introduced a new approach that allows the direct time-resolved measurement of the lifetime of hydrogen bonds between resorufin and alcohols using time-resolved visible absorption spectroscopy. The same technique has been used subsequently to study the dynamics of hydrogen bond in alcohols using fluorenone and ketocyanine dyes. A perfect linear correlation between the relaxation time of the singlet excited state and longitudinal relaxation time of the normal alcohols establishes the fact that the hydrogen bond reorganization around the excited solute molecule is solely responsible for the dynamics of the relaxation process of the S1 state in the alcoholic solvents. Since the electronic transitions are strongly broadened owing to the coupling with the fluctuating solvent, and are relatively featureless due to overlapping of different transitions leading to wavelength-dependent dynamics, they provide very limited information regarding microscopic solvent structure. However, the timeresolved vibrational spectroscopic technique is a more powerful way to monitor the hydrogen-bond dynamics in real time, because it is possible to observe changes in distinct functional groups involved in the hydrogen bond formation, which provides site-specific insight into local dynamics. Application of femtosecond time-resolved visible pump–IR probe absorption spectroscopy to coumarin 102–phenol and coumarin 102–aniline systems has been discussed to demonstrate the potential of this technique in gaining an insight into the microscopic details of the ultrafast structural response of hydrogen bonded complexes to an electronic dipole excitation as well as probing dynamics in hydrogen bonding networks at a finite distance from the directly excited groups. More recently, ultrafast time-resolved infrared spectroscopy has proven to be an important tool for investigation of the dynamics of hydrogen bond in neat liquids, say water. The vibrational lifetime, the

792 Hydrogen Bonding and Transfer in the Excited State

time-constant for hydrogen bond breaking and the rate of orientational relaxation have been determined [124– 126]. The most advanced version of time-resolved IR spectroscopy is ultrafast 2D IR vibrational echo spectroscopy, which is an ultrafast analog of 2D NMR that directly probes the structural degrees of freedom of molecules [66, 127–130]. 2D IR vibrational echo spectroscopy has several characteristics that make it a useful tool for investigation of problems involving rapid dynamics under thermal equilibrium in condensed phases, for example, fast chemical exchange reactions, solute–solvent interactions, water dynamics and intramolecular interactions. Such problems are important and ubiquitous in nature and difficult to study by other means. 2D IR vibrational echo experiments have a temporal resolution <50 fs, which is sufficiently fast to study the fastest chemical processes.

Acknowledgement I gratefully acknowledge the co-authors of my original papers, colleagues and students for their active contribution. I also acknowledge the constant encouragement of Professors S. K. Sarkar, Head, RPCD, and T. Mukherjee, Director, CG, BARC.

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34 Volume Changes Associated with Solute–Solvent Reorganization Following Photoinduced Proton Transfer in Aqueous Solutions of 6-Methoxyquinoline Stefania Abbruzzetti1 and Cristiano Viappiani2 1

Dipartimento di Fisica, Universita degli Studi di Parma, Parma and Dipartimento di Biotecnologie, Universita di Verona, Verona, Italy 2 Dipartimento di Fisica, Universita degli Studi di Parma and NEST, Istituto Nanoscienze-CNR , Parma, Italy

34.1 Introduction Several light induced inter- and intramolecular proton transfer reactions are known to be at the basis of several photochemical and photobiological processes [1–5]. Several organic molecules exhibit a marked change in their acid–base equilibria when electronically excited by absorption of light, becoming either strong acids or bases [2, 6]. Since the acid and its conjugated base generally exhibit different spectroscopic properties, timeresolved absorption and/or fluorescence have been applied to give a detailed picture of the kinetic events following absorption of a photon. Besides their inherent interest, these photochemical reactions have found several applications. Photoinduced intermolecular proton transfer has been applied to study the kinetics of ground state proton transfer reactions, otherwise too fast to be followed in conventional rapid mixing experiments [3, 4]. Strong acids in the excited state can serve as photoactivatable caged protons whereas strong bases can be used as photoactivatable caged hydroxide [3]. Alternatively, photoactivatable (caged) proton compounds can be used to impose a rapid change in pH. The most widely used photoprotecting groups are based on 2-nitrobenzyl photochemistry [7, 8]. Using a pulsed UV laser it is possible to trigger proton transfer reactions in aqueous solutions containing a suitable caged compound and the chemical species under investigation. Applications include the study of

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

798 Hydrogen Bonding and Transfer in the Excited State

protonation reactions for pH indicators [9], green fluorescent protein (GFP) chromophore [10, 11] and unfolding reactions [12, 13]. Taking advantage of the large volumetric effects accompanying proton transfer reactions [14–16] we have applied time-resolved photoacoustics [17, 18] to the characterization of the kinetics of several proton transfer reactions. The volume changes accompanying the photoinduced deprotonation of the excited states of 1-naphthol, 2-naphthol and 1-naphthol-3,6-disulfonate (and the back recombination reaction) in aqueous solutions have been determined. For the recombination reaction with the conjugated base it was also possible to determine the bimolecular rate constants [19]. Using 2-nitrobenzaldehyde as an irreversible caged proton, we have induced a rapid proton concentration jump in solution and measured the reaction volume and the rate constant for the formation of water from protons and hydroxyls [20–22]. The same experimental methodology has also been used to induce proton transfer reactions in more complex systems, including model polypeptides [23, 24] and proteins [25]. The method has proved useful also for the determination of the pKa of the acinitro intermediate in the photochemical reactions of 2-nitrobenzyl compounds [21]. The strong alkaline character gained by aromatic ethero-substituted compounds in their excited state can be exploited to introduce alkaline pH changes in solution [3, 4, 26]. Prior to kinetic investigations on the volumetric effects induced on the target molecules (simple molecules or proteins), it is necessary to characterize the volumetric response of the photoactive compound. In this work we have investigated the reaction volume for the photoinduced transient protonation of the ethero-substituted aromatic compound 6-methoyquinoline (6MQ). This compound has an excited state pKa (pK*a ) much higher than the corresponding ground state pKa. In particular, the pK*a of 6MQ is 11.8 [1]. Photoexcitation of aqueous solutions of 6MQ, at pH below the pK*a and above the pKa, leads to the rapid abstraction of a proton from water. The rate of the process is 2.9
108 M1 s1 [1], resulting in apparent rates in the sub-nanosecond range.

34.2 Materials and Methods 34.2.1 Chemicals 6-Methoxyquinoline was obtained from Sigma-Aldrich and used without further purification. Brilliant black BN was used as photocalorimetric reference compound [27]. Solutions were freshly prepared before use. No buffer was used in any of the experiments. Solutions were nitrogen saturated to avoid carbonate buffering. The pH of the solutions was adjusted by addition of concentrated HCl or NaOH. 34.2.2 Photoacoustics The photoacoustic setup has been described previously [20, 25]. Photoexcitation of the samples was achieved either by a XeCl excimer laser (EMG 50, Lambda Physik, 308 nm) or by a frequency tripled, Q-switched Nd: YAG laser (Surelite II-10, Continuum, 355 nm). The pressure wave induced in solution was detected by a PZT piezoelectric transducer (Panametrics V-103). The signal was amplified (60 db) and recorded by a digitizing oscilloscope (LeCroy 9450A, 400 Ms/s). A quartz cuvette was mounted inside a temperature controlled sample holder (Quantum Northwest, Inc. TASC 300) and degassed with nitrogen. Data acquisition and analysis were performed by means of dedicated software (Sound Acquisition and Sound Analysis, Quantum Northwest, Inc.). Deconvolution of experimental data to obtain the time profile of the volume changes in solution was performed using the program Sound Analysis 3000 (Quantum Northwest Inc., Spokane, WA). Volume changes were estimated by means of a two-temperature method [18, 28]. The sample waveform was acquired at Tb¼0 (3.9  C in water [29]) and was compared to a reference waveform acquired at a slightly higher temperature, Tb„0 ¼ 5.0  C. At Tb¼0 the thermal expansion coefficient, b, is zero and the signals of thermal origin vanish. The sample waveforms measured at Tb¼0 originate purely from structural changes in the solution and include no enthalpic contribution.

Volume Changes Associated with Solute–Solvent Reorganization 799

The extent of the structural volume change DVi is calculated from the amplitude of the exponential decays wi as:  DVi ¼ El

 b w Cp r i

ð34:1Þ

where El is the molar energy content of the laser pulses, b=Cp r is the thermoelastic parameter of the solution at the temperature at which the reference waveform is acquired. The reaction volume DVR,i can be obtained from the measured volume change DVi and the quantum yield Fi as DVR,i ¼ DVi/Fi.

34.3 Results and Discussion Photoexcitation of aqueous solutions of 6MQ is accompanied by a contraction occurring with sub-resolution kinetics, that is, with a lifetime below a few nanoseconds. This contraction is associated with the abstraction of a proton from water by the excited state of 6MQ, occurring on sub-nanosecond time scales [30]. The establishment of the excited state ionic equilibrium and the relaxation of the electronic excited states occur on a short time scale and are integrated into a prompt, sub-resolution transient in the photoacoustic experiment. The reaction scheme of this fast phase can be sketched as follows: FN þ H2 O þ hn ! FNH þ þ OH

ð34:2Þ

The yield for the overall reaction is indicated with FH þ . The prompt negative volume change is followed by an expansion, associated with the relaxation of the coupled chemical equilibria: þ FNH þ !  FN þ H

ð34:3Þ

H þ þ OH !  H2 O

ð34:4Þ

FNH þ þ OH !  FN þ H2 O

ð34:5Þ

leading to the neutralization of two net charges (FNHþ and OH). Figure 34.1 shows the photoacoustic signals measured for 6MQ at pH 7.7 and 10.5 at T ¼ 3.9  C. The signals are compared to the signal of the photocalorimetric reference compound brilliant black BN. The fast contraction is almost identical for both curves whereas a larger expansion appears at longer times for the waveform acquired at pH 10.5. Figure 34.2 shows the pH dependence of the measured volume changes for an aqueous solution of 6MQ at T ¼ 3.9  C. When the pH is increased above 10, the extent of the fast, sub-resolution contraction decreases sigmoidally, reflecting the excited state ionic equilibrium. The absolute values of the fast expansion and the slower contraction are identical at above pH 10.5 but differ substantially at pH below this value. In particular, the expansion is systematically smaller than the corresponding value of the contraction at all pH below 10.5. Both volume changes are constant between pH 7 and 10.5.

800

Hydrogen Bonding and Transfer in the Excited State

PA signal

2 0 ref pH105 pH77

-2 -4

2

4

6

8

time (μs)

Figure 34.1 Photoacoustic signal of a solution of 6MQ in water at pH 7.7 (dashed line) and pH 10.5 (dotted line) at T ¼ 3.9  C. The reference waveform for the aqueous solution of brilliant black BN was acquired at T ¼ 5  C

The lifetime of the expansion is independent of pH between pH 7 and 10.5. Increasing the pH above this value results in shorter lifetimes. The shortest lifetime we could measure reliably is 58 ns at pH 12.5; above this pH the lifetime is too short and the amplitude too small to give reliable results. The pH dependence of the lifetime is plotted in Figure 34.3. The pH dependence of the amplitude and the lifetime strongly support the idea that the reaction associated with the expansion we detect at pH < 10.5 is the unimolecular deprotonation reaction of FNHþ [equation (34.3)]. This reaction is not affected by the concentration of free OH and has a pH-independent lifetime. The following reaction [equation (34.4)] is slow on the time scale of the experiment at pH below 10, due to the low hydroxide concentration, and is not sensed by the photoacoustic instrument. Given a bimolecular rate constant of 5
1010 M1 s1 [3, 20], apparent lifetimes for reaction (34.4) to occur are above 1 ms below pH 10. Reaction (34.5) is equally not detectable due to the low concentration of the reactants at pH < 10, resulting in long relaxation times. When the pH is raised above 10, relaxation of equations (34.4) and (34.5) falls within the detection range of the technique. At pH above 10.5, the positive volume change has the same absolute value as

15

ΔV (ml/mol)

10 5 0 -5 -10 -15 8

10

12

pH

Figure 34.2 pH dependence of the photoinduced volume changes for an aqueous solution of 6MQ. Lifetime of the fast contraction (open circles) is below the experimental resolution (approx. 20 ns)

Volume Changes Associated with Solute–Solvent Reorganization 801

τ2 (ns)

1000

100

8

10 pH

12

Figure 34.3 pH dependence of the lifetime of the expansion following photoexcitation of an aqueous solution of 6MQ

the fast contraction at all pH values, but opposite sign. This finding is consistent with direct relaxation of reaction (34.5), giving essentially the backward relaxation of equation (34.2). Alternatively, the relaxation can be pictured as a result of equations (34.3) and (34.4), which lead to a volume change indistinguishable from the volume change for equation (34.5). The lifetimes at pH above 10 allow us to obtain an estimate for the bimolecular binding rate constant of reaction (34.5). We retrieved a forward rate constant of approx. 1010 M1 s1, a value that is substantially lower than the known value for reaction of proton with hydroxide (5
1010 M1 s1). The above data can be used to estimate the protonation quantum yield. At pH below 10, DV2 can be written as: DV2 ðpH < 10Þ ¼ V ðH þ Þ þ V ðFNÞV ðFNH þ Þ FH þ

ð34:6Þ

DV2 ðpH 10:5Þ ¼ V ðH2 OÞ þ V ðFNÞV ðFNH þ ÞV ðOH Þ FH þ

ð34:7Þ

while at pH 10.5 we have:

From the above equations it is straightforward to obtain: FH þ ¼

DV2 ðpH 10:5ÞDV2 ðpH < 10Þ V ðH2 OÞV ðH þ ÞV ðOH Þ

ð34:8Þ

Using the value 24.5 ml mol1 [20, 31] for the reaction volume of water formation and the measured volume changes at pH < 10 (5.4  0.2 ml mol1) and pH 10.5 (13.6  0.7 ml mol1) we obtain FH þ ¼ 0:33  0:04. Finally, from the pH dependence of the two volume changes at pH above 10.5 we could estimate the pK*a of 6MQ. The solid curves in Figure 34.1 are fits to a Henderson-Hasselbalch equation [32]. The resulting pK*a ¼ 11:8  0:1 is identical for both the contraction and the expansion and is in perfect agreement with the available literature data of 11.8, determined by time-resolved fluorescence emission [26].

802 Hydrogen Bonding and Transfer in the Excited State

The values of DV1 and DV2 for the investigated compound show that the protonation and the subsequent relaxation to the prepulse equilibrium lead to substantial rearrangements of the solute-solvent assembly. Using the previously determined value of FH þ ¼ 0:33  0:04 we can estimate the reaction volume for reaction (34.6) from the low pH expansion (5.4  0.2) as 16.4  0.6 ml mol1. The fast contraction observed at pH below 10 consists of both reaction volumes for (34.6) and (34.7), and after correction for FH þ it shows a reaction volume of 36.1  2.7 ml mol1. Using the value for the reaction volume determined for reaction (34.6), we obtain for the deprotonation of water a value of 20  3 ml mol1, a value that is fairly consistent with the literature data (25.4 ml mol1) [14, 15]. The contraction measured for the protonation of 6MQ (16.4  0.6 ml mol1) is intermediate between the contraction observed for the ionization of water and that measured for a typical aliphatic carboxylic acid (about 11 ml mol1) [14–16]. This value is similar to what we previously measured for the photoinduced deprotonation of 1-naphthol (12.7  1.5 ml mol1) and 2-naphthol (16.2  1.7 ml mol1) [19]. Qualitatively, this difference can be rationalized in terms of the different electrostrictive effects induced in solution by the formation of charged species. In the ionization of water, the negative charge is localized on the oxygen atom of OH whereas in the case of the carboxylate anions it is equally shared between the two atoms of the group CO2. In the case of 1-naphthol and 2-naphthol and 6MQ, delocalization of the charge is possibly not as high as for carboxylate anions, and thus the contractions resulting from electrostriction are enhanced. Electrostrictive effects induced in solution by proton transfer can be roughly modelled using continuum dielectric models [18]. However, a detailed understanding of the mechanism for the observed volume changes will require careful modelling the solute-solvent system, taking into account specific interactions (e.g. hydrogen bonds) between the ionized molecules and the surrounding solvent. Nevertheless, it is expected that the modelling of specific solute-solvent interactions will benefit from experimentally determined quantities such as the volume changes associated with formation of charged species, as this will provide a guidance for testing the predictions from molecular modelling.

References 1. L. G. Arnaut and S. J. Formosinho, J. Photochem. Photobiol., A: Chem., 75, 1–20 (1993). 2. P. Wan and D. Shukla, Chem. Rev., 93, 571–584 (1993). 3. M. Gutman and E. Nachliel, Biochim. Biophys. Acta, 1015, 391–414 (1990). 4. M. Gutman and E. Nachliel, Annu. Rev. Phys. Chem., 48, 329–356 (1997). 5. T. Gensch, J. Heberle and C. Viappiani, Photochem. Photobiol. Sci., 5, 529–530 (2006). 6. L. M. Tolbert and K. M. Solnsev, Acc. Chem. Res., 35, 19–27 (2002). 7. Caged Compounds, ed by G. Marriott, Academic Press, San Diego, Volume 291 (1998). 8. A. P. Pelliccioli and J. Wirz, Photochem. Photobiol. Sci., 1, 441–458 (2002). 9. C. Viappiani, G. Bonetti, M. Carcelli et al., Rev. Sci. Instrum., 69, 270–276 (1998). 10. S. Abbruzzetti, E. Grandi, C. Viappiani et al., J. Am. Chem. Soc., 127, 626–635 (2005). 11. R. Bizzarri, R. Nifosı`, S. Abbruzzetti et al., Biochemistry, 46, 5494–5504 (2007). 12. S. Abbruzzetti, C. Viappiani, J. R. Small et al., J. Am. Chem. Soc., 123, 6649–6653 (2001). 13. S. Abbruzzetti, S. Sottini, C. Viappiani and J. E. T. Corrie, Photochem. Photobiol. Sci., 5, 621–628 (2006). 14. T. Asano and W. J. L. Noble, Chem. Rev., 78, 407–489 (1978). 15. R. VanEldick, T. Asano and W. J. LeNoble, Chem. Rev. 89, 549–688 (1989). 16. A. Drljaca, C. D. Hubbard, R. vanEldik et al., Chem. Rev., 98, 2167–2289 (1998). 17. S. E. Braslavsky and G. E. Heibel, Chem. Rev., 92, 1381–1410 (1992). 18. T. Gensch and C. Viappiani, Photochem. Photobiol. Sci., 2, 699–721 (2003). 19. A. Losi and C. Viappiani, Chem. Phys. Lett., 289, 500–506 (1998). 20. G. Bonetti, A. Vecli and C. Viappiani, Chem. Phys. Lett., 269, 268–273 (1997).

Volume Changes Associated with Solute–Solvent Reorganization 803 21. M. Carcelli, P. Pelagatti and C. Viappiani, Isr. J. Chem., 38, 213–221 (1998). 22. S. Abbruzzetti, M. Carcelli, D. Rogolino et al., Photochem. Photobiol. Sci., 2, 796–800 (2003). 23. C. Viappiani, S. Abbruzzetti, J. R. Small et al., Biophys. Chem., 73, 13–22 (1998). 24. S. Abbruzzetti, C. Viappiani, J. R. Small et al., Biophys. J., 79, 2714–2721 (2000) 25. S. Abbruzzetti, E. Crema, L. Masino et al., Biophys. J., 78, 405–415 (2000). 26. S. J. Formosinho and L. G. J. Arnaut, Photochem. Photobiol. A: Chem., 75, 21–48 (1993). 27. S. Abbruzzetti, C. Viappiani, D. H. Murgida et al., Chem. Phys. Lett., 304, 167–172 (1999). 28. T. Gensch and S. E. Braslavsky, J. Phys. Chem., 101, 101–108 (1997). 29. CRC Handbook of Chemistry and Physics, 52nd edn, ed. by R. C. Weast, CRC Press, Boca Raton, FL (1971). 30. E. Pines, D. Huppert, M. Gutman et al., J. Phys. Chem., 90, 6366–6370 (1986). 31. F. J. Millero, E. V. Hoff and L. Kahn, J. Solution Chem., 1, 309–327 (1972). 32. T. Gensch, C. Viappiani and S. E. Braslavsky, J. Am. Chem. Soc., 121, 10573–10582 (1999).

35 Molecular Recognition and Chemical Sensing of Anions Utilizing Excited-State Hydrogen-Bonding Interaction Ashutosh S. Singh and Shih-Sheng Sun Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, ROC

35.1 Introduction Hydrogen bonding is a weak, non-covalent interaction that plays crucial roles at the interface between physics, chemistry, biology and various multidisciplinary areas of science. The hydrogen bonding interactions determine the physical properties of molecules such as the solubility of solids or the organization of amphiphilic molecules in large aggregates such as membranes, micelles and vesicles that have well-defined structures (helices, molecular grids, molecular containers, molecular capsules, etc.). In general, the nature and strength of a hydrogen bond depends upon the electronegativity of connected atoms, solvent polarity and physical environmental conditions (temperature, pressure, etc.). The nature, role and importance of H-bonding based on structural and mechanistic consequences have been well studied and plenty of literatures and books are available, along with its applications in sensor, catalysis, selective recognition of molecular species and its transport and so on [1]. The existence of hydrogen-bonding interaction in the excited state has been known for decades. Its structural and mechanistic consequences, however, were less explored until the advancement of modern spectroscopic techniques. In recent years, the number of literature publications has grown, revealing the mechanism in depth and its paramount importance through many examples [2]. Seminal studies on the excited state intramolecular proton transfer (ESIPT) phenomenon were conducted by Weller, who reported very large Stokes shifts in the fluorescence spectra of salicylic acid and its methyl ester [3]. Hydrogen bonding in the excited state not only depends upon the electronegativity of atom(s) attached to the molecule and the solvent polarity but also upon

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

806 Hydrogen Bonding and Transfer in the Excited State

the molecular structure and microscopic structural differences in the excited state [4a]. ESIPT generally requires hydrogen-bonding formation between the vicinal proton donor and acceptor groups. After photo-excitation, a molecule changes the charge distribution and, consequently, dipole moment. Thus, its intermolecular interaction with solvent molecule(s) determines the fate of the excited molecular species. Some molecules have a larger dipole moment in the electronic excited state than in ground state1c and this dynamic change of dipole moment induces strong hydrogen bond formation in the excited state [4b] in polar solvents [4c5]. Excited-state hydrogen bond dynamics are typically fast on the order of 10 fs or less. The available ultrafast laser techniques and computational methods have revealed very interesting photophysical and photochemical properties. After electronic excitation, both the basicity of the proton acceptor and the acidity of the proton donor in a molecule increase because of the redistribution of electronic charges [4b]. When basic and acidic groups are in close proximity, with a suitable geometry where the hydrogen bond formation is possible, then excited state inter/intramolecular proton transfer may take place (Scheme 35.1) [6]. Excited state proton transfer plays a critical role in the photostability of Watson–Crick base pairs in DNA [7] and UV photostabilizers (protection of organic polymers and UV-protecting pigments from degradation by the UV component of the sunlight [8a,b]); these characteristics also open up future possibilities for fast memory data storage, high-energy radiation detectors and fluorescent probes [8c,d]. A large Stokes shift emission originating from ESIPT would appear in a molecule with intramolecular (or intermolecular) H-bonding interaction between a proton donor and a proton acceptor. If this excited state proton transfer can be interrupted by external stimuli such as anion addition to promote the deprotonation then this process may be utilized for ratiometric anion sensing. Several fluorescence methods are available today for sensing purposes, such as intramolecular charge transfer (ICT) [9a], fluorescent resonance energy transfer (FRET) [9b], excimer/exciplex formation [9c], chemodosimetry [9d], luminescent lanthanide-anion complexation [9e], anion-sensitive dual chromophore recognition [9f], and ratiometric fluorescent sensing [9g]. One would easily envision that the ratiometric response through perturbing the ESIPT process represents one of the most reliable methods for biological, polymeric and sensory materials chemistry because of the insensitivity to (i) instrumental factors, (ii) the amount/concentration of sensor molecules and (iii) environmental factors such as fluorescence quenching and so on. Moreover, since the efficiency of proton transfer interruption in the excited state will depend upon the basicity [10] and charge density of anions [11], anion sensing by this process could be highly selective. In this chapter, we focus primarily on the applications of excited-state hydrogen-bonding interactions for anion recognition and sensing.

35.2 Recognition and Sensing of Anions by Intramolecular Hydrogen Bonding in Excited States The polarized aromatic –OH group [12] and –NH group, including imidazole [13], amide [6a, 14], pyrrole [15], urea [16], benzothiazole [17], azaindole [18] and azacarbazole [19], have been utilized for

Scheme 35.1

Molecular Recognition and Chemical Sensing of Anions Utilizing Excited-State Hydrogen-Bonding Interaction 807

their unique proton transfer properties for molecular sensing purposes. Fabbrizzi et al. have shown through a number of colorimetric anion receptors possessing –NH interacting sites that deprotonation of NH group can be enhanced by increasing the –NH acidity and also the basicity of anions [20]. Peng et al. have used the polarized –NH group in sulfonamide (1) and urea (2) in conjunction with the 2-(2-hydroxyphenyl) benzoxazole moiety to explore anion recognition and sensing based on the ESIPT mechanism [21]. The basic principle behind the receptor design is that in the presence of basic anion the deprotonation of the sulfonamide unit in 1 or the urea unit in 2 may takes place and disturbs the ESIPT to achieve a ratiometric fluorescence response.

The ratio of quantum yield of the tautomer and normal emission (FT/FN) of 1 decreases significantly in polar solvents such as DMSO and methanol in comparison to in cyclohexane. This can be attributed to the partial proton dissociation in polar solvents due to the sufficient acidity of the sulfonamide –NH group. Thus, anions with similar basicity and electron density such as F
, CH3COO
and H2PO4
cannot be differentiated by using 1. On the other hand, a small decrease in the ratio of quantum yield of tautomer and normal emission (FT/ FN) of 2 indicates the low intrinsic acidity of the urea donor group, which will make the proton transfer process resistant to solvent disturbance in the ground state. The 1 H NMR titration and UV–vis absorption spectra of 2 with F
suggests a two-step equilibrium. The first step is slow with the formation of hydrogen bond between incoming F
anion with the urea moiety in 2, producing minimum disturbance of dipole associated with charge-transfer transition of 2 and hence a small change observed in the UV–vis absorption spectra (Figure 35.1a). The second step is a neat proton transfer process from urea to F
to form a deprotonated 2 and HF2
. Subsequent redistribution of charge density takes place and hence UV–vis absorption spectra shows redshifted charge-transfer (CT) band (Figure 35.1b). The

Figure 35.1 Changes in the UV–vis absorption spectra for 2 (1.0  10
5 M) in DMSO with the addition of [(Bu)4N]F (a) from 0 to 0.002 M and (b) from 0.002 to 0.022 M. Reprinted with permission from [21]. Copyright 2007 American Chemical Society

808 Hydrogen Bonding and Transfer in the Excited State

Figure 35.2 Changes in fluorescence spectra (lex ¼ 322 nm) for 2 (1.0  10
5 M) in DMSO with the addition of [(Bu)4N]F (a) from 0 to 0.002 M and (b) from 0.002 to 0.022 M. Reprinted with permission from [21]. Copyright 2007 American Chemical Society

corresponding fluorescence of deprotonated 2 appeared upon F
addition in concomitance with the decrease of both the normal and tautomer emission (Figure 35.2). In contrast, the anion titration experiment of 2 with CH3COO
showed a one-step change, a slight blue-shift as well as a decrease in absorption band and no redshift CT band appeared even after addition of excess CH3COO
anion. This observation suggests the formation of double hydrogen bonds between the CH3COO
anion and 2 and, thus, prevents the intramolecular H-bonding interaction in the ground state and subsequent ESIPT. The ratiometric responses in emission spectra also support the mechanism, with observation of the decrease of the tautomer emission at 554 nm accompanying the increase of normal emission at 414 nm (Figure 35.3).

35.3 Recognition and Sensing of Anions by Intermolecular Hydrogen Bonding in Excited States Hamilton et al. have reported an example showing ratiometric fluorescent sensor for H2PO4
anion through ESPT by intermolecular hydrogen bonding in the excited state based on the concept shown in Figure 35.4 [6a]. They have prepared rigid macrocyclic receptor with convergent amide H-bond arranged in skew fashion and

Figure 35.3 Changes in the fluorescence spectra (lex ¼ 330 nm) for 2 (1.0  10
5 M) in DMSO with the addition of [(Bu)4N]CH3COO from 0 to 0.025 M. Reprinted with permission from [21]. Copyright 2007 American Chemical Society

Molecular Recognition and Chemical Sensing of Anions Utilizing Excited-State Hydrogen-Bonding Interaction 809

Figure 35.4 Two possible fluorescence emission channels from a CT excited state. An anion receptor (R) is connected to the electron-donating terminal of a fluorophore (F). Anion binding stabilizes the CT excited state by electrostatic interaction. The proton that becomes acidic by the localized positive charge on F is transferred to the anion and the second emission channel is opened. Reprinted with permission from [6a]. Copyright 2001 Wiley-VCH Verlag GmbH & Co. KGaA

another derivative (3) possessing amide functionality bearing a coumarin fluorophore that participates in ESPT process. The shape and size of this receptor is suitable for binding tetrahedral oxyanions. The binding constant measurement by fluorescence titration shows that this receptor was selective for H2PO4
anion.

The fluorescence intensity of 3 is enhanced upon addition of H2PO4
, which is ascribed to the restricted conformational flexibility upon anion binding inside the macrocycle through the formation of multiple hydrogen bonding interactions (Figure 35.5). The appearance of an additional emission band at a longer wavelength and

Figure 35.5 Emission spectra (lex ¼ 320 nm) of receptor 3 (3 mM in 1 : 1 DMSO/1,4-dioxane) with different anions. Reprinted with permission from [6a]. Copyright 2001 Wiley-VCH Verlag GmbH & Co. KGaA

810 Hydrogen Bonding and Transfer in the Excited State

the emission intensity followed the trend of H2PO4
> PhPO3H
> pTsO
. Furthermore, the same excitation spectra for both emission bands confirmed that the two excited states originate from the same ground state. The correlation of the second emission band with bound anion basicity suggested that excited-state proton transfer was occurring with the increased acidity of the fluorophores in the CT excited state upon anion binding.

35.4 Recognition and Sensing of Anions by Conjugated Polymers through ESIPT Conjugated polymers possess unique electrical and optical properties due to their extended p-conjugation in the main chain. With appropriately functionalized receptors incorporated into conjugated polymers, it is possible to detect, transduce and amplify chemical information into an optical signal after analyte–receptor interactions by the so-called “molecular-wire effect” [22]. There are several examples reported in the literature of anion sensing by conjugated polymers [23]. Recently, an interesting example has been reported by Pang et al., showing the application of a conjugated polymer incorporating 2-(2-hydroxyphenyl)-1,3-benzoxazole (HBO) derivative for anion sensing through H-bonding in excited state followed by an ESIPT process [24]. It is known that HBO undergoes ESIPT with transformation of enol into keto form (dependent upon solvent [25] and temperature [26]). HBO exists in two rotamers (4a and 4b) in a 1 : 1 ratio in the crystalline state: one with the –OH group oriented towards N-atom and the other with the –OH group towards the O-atom of the oxazole ring [27]. However, only one conformer (4a), in which the –OH group is oriented towards N-atom, undergoes the ESIPT process to give emission with a large Stokes shift [28]. For sensitive performance by HBO-type moiety in sensing terms, it is highly desirable to reduce the formation of conformer 4b. In this regard, the 2,5-bis(benzoxazol-2-yl)benzene-1,4-diol derivative (5) is a suitable system to study in which two benzoxazole units remain in the same plane as the central para-catechol moiety through intramolecular hydrogen bonding in the ground state (Scheme 35.2) [24]. With a series of polymers and corresponding monomers based on the 2,5-bis(benzoxazol-2-yl)benzene-1,4-diol derivative, Pang et al. have shown that polymer 7, obtained after de-alkylation of parent polymer 6, exhibited much weaker fluorescence

Scheme 35.2 Schematic representation of the ESIPT process in bis(HBO). Reproduced with permission from Ref. [24]. Copyright 2007 The American Chemical Society

Molecular Recognition and Chemical Sensing of Anions Utilizing Excited-State Hydrogen-Bonding Interaction 811

than polymer 6. A large redshift (200 nm) of the emission band, however, was observed at lmax 619 nm for 7, despite the bathochromic shift (40 nm) in absorption spectra compared to 6 (Figure 35.6a). This large Stokes shift (200 nm) in polymer 7 was attributed to an ESIPT process through bis(HBO) moiety, which was not possible in polymer 6.

The absorption spectra of polymer 7 shows a disappearance of the bands at 400 and 421 nm along with the appearance of a new band at 510–540 nm (Figure 35.6b) upon addition of certain anions. The large shift of 120 nm in absorption spectra after addition of anions shows significant perturbation in the ground state. After

Figure 35.6 Absorption (solid line) and emission (dotted line) spectra (a) of polymers 6 and 7, in anhydrous THF, and UV–vis (solid line) and fluorescence (broken line) spectra (b) of polymer 7 and its anion complexes. Reprinted with permission from [24]. Copyright 2007 American Chemical Society (See Plate 45)

812 Hydrogen Bonding and Transfer in the Excited State

Figure 35.7 Titration spectra of compound 8 with Bu4NOH in THF/EtOH (30 : 1). Reprinted with permission from [24]. Copyright 2007 American Chemical Society (See Plate 46)

addition of fluoride or acetate anion to polymer 7 in THF/ethanol (50 : 1), it turned from a very weakly fluorescent to a strong fluorescent solution and the result clearly showed the inhibition of ESIPT process because of the deprotonation of the –OH functional group. Because of the poor solubility of compound 7, the authors have employed a monomeric derivative 8 to study in more detail. The titration spectra of 8 with Bu4NOH in mixture of THF and ethanol show the appearance of new band at 496 nm with disappearance of bands at 400 and 410 nm (Figure 35.7). With a gradual increase in concentration of Bu4NOH, the intensity of the band at 496 nm increases and shows a characteristic isosbestic point at 432 nm and no new band appears, which suggests the formation of monoanion species after deprotonation of the bis(HBO) moiety. This was also confirmed through 1 H NMR titration and ESI-MS spectra. A similar observation was made in the titration spectra of 8 with Bu4NF (Figure 35.8a); the Benesi– Hildebrand plot (Figure 35.8b) showed that complex formation involved two fluoride anions and it was

Figure 35.8 Titration spectra (a) and Benesi–Hildebrand plot (b) of 8 with Bu4NF in THF/EtOH (100 : 1). Reprinted with permission from [24]. Copyright 2007 American Chemical Society (See Plate 47)

Molecular Recognition and Chemical Sensing of Anions Utilizing Excited-State Hydrogen-Bonding Interaction 813

suggested that fluoride anions first interact with 8 to form H-bonding in the ground state to form 9 followed by second fluoride anion induced deprotonation to form 10 with elimination of [F  H  F]
anion.

35.5 Concluding Remarks In conclusion, for anion sensing through ESIPT process the receptors should possess the following characteristics: (i) the receptor should form intramolecular hydrogen bonding in the ground state within neighboring proton donor and acceptor groups, (ii) the proton donor should be acidic enough to become deprotonated upon interaction with basic anions but should be resistant to solvent disturbance and (iii) the proton donor should be strong for fast ESIPT process and increase the quantum yield of tautomer emission. The ESIPT process provides a new platform for a fast and selective route for anion sensing. Notably, some literature examples have reported anion sensing based on an ESIPT mechanism but, instead, they were simply ground-state acid–base reactions. It is, therefore, crucial to carefully judge all the experimental evidence before claiming the anion recognition and sensing mechanism. Although the number of examples in the literature for anion sensing based on the ESIPT mechanism is still very low, the judicious choice of molecular receptor with appropriate complementary structures for anions and incorporation of a suitable moiety for ESIPT process would surely open up the way for detection of biologically and environmentally important anions in the future.

References 1. (a) P. Schuster, G. Zundel and C. Sandorfy, The Hydrogen Bond - Recent Developments in Theory and Experiments. II. Structure and Spectroscopy, North Holland, Amsterdam (1976); (b) G. A. Jeffrey, An Introduction to Hydrogen Bonding, OUP, New York, (1997); (c) G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond in Structural Chemistry and Biology, OUP, Chichester, (1999); (d) N. J. Turro, Modern Molecular Photochemistry, University Science Books, Mill Valley, CA, p. 132 (1991). 2. (a) A. Jarczewskia and C. D. Hubbard, J. Mol. Struct., 649, 287 (2003); (b) L. M. Tolbert and K. M. Solntsev, Acc. Chem. Res., 35, 19 and references cited therein (2002); (c) J. T. Hynes, T.-H. Tran-Thi and G. Granucci, J. Photochem. Photobiol. A: Chem., 154, 3 (2002); (d) J. Waluk, Conformational aspects of intra- and intermolecular proton transfer, in Conformational Analysis of Molecules in Excited States, ed. J. Waluk, Methods in Stereochemical Analysis Series, Wiley-VCH, New York, Chapter 2, pp. 57–111 (2000); (e) L. G. Arnaut and S. J. Formosinho, J. Photochem. Photobiol. A: Chem., 75, 1 (1993); (f) S. J. Formosinho and L. G. Arnaut, J. Photochem. Photobiol. A: Chem., 75, 21 (1993). 3. (a) A. Weller, Z. Elektrochem., 60, 1144 (1956); (b) A. Weller, Prog. React. Kinet., 1, 188 (1961). 4. (a) A. Morimoito, T. Yatsuhashi, T. Shimada et al., J. Phys. Chem. A, 105, 10488 (2001); (b) H. Inoue, M. Hida, N. Nakashima and K. Yoshihara, J. Phys. Chem., 86, 3184 (1982); (c) M. Maroncelli, J. Macinnis, G. R. Fleming, Science, 243, 1674 (1989). 5. (a) H. Shirota, H. Pal, K. Tominaga and K. Yoshihara J. Phys. Chem., 100, 14575 (1996); (b) M. Sugita, T. Shimada, T. Tachibana and H. Inoue, Phys. Chem. Chem. Phys., 3, 2012 (2001). 6. (a) K. Choi and A. D. Hamilton, Angew. Chem., Int. Ed., 40, 3912 (2001); (b) H. Tong, G. Zhou, L. Wang et al., Tetrahedron Lett., 44, 131 (2003); (c) X. Zhang, L. Guo, F. Y. Wu and Y. B. Jiang, Org. Lett., 5, 2667 (2003). 7. (a) T. Schultz, E. Samoylova, W. Radloff et al., Science, 306, 1765 (2004); (b) A. L. Sobolewski and W. Domcke, Europhys. News, 37, 20 (2006).

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36 Theoretical Studies of Green and Red Fluorescent Proteins Hong Zhang, Qiao Sun, Sufan Wang, Seth Olsen and Sean C. Smith The University of Queensland, Australian Institute for Bioengineering and Nanotechnology, Centre for Computational Molecular Science, QLD 4072, Brisbane, Australia

36.1 Introduction The green fluorescent protein (GFP) of the Aequorea victoria jellyfish (and its structural analogues, e.g., red fluorescent protein) has emerged as a unique fluorescent label and has evolved into an extraordinarily important platform for biotechnological and cell biology applications due to its amazing ability to generate a highly visible, efficiently emitting internal fluorophore (for an overview, see review articles [1, 2]). Its unique photophysical properties are nowadays being commonly exploited for imaging studies of protein folding, gene expression, protein trafficking and cell development, since the gene in GFP contains all the information necessary for the post-translational synthesis of the chromophore and expression of the gene in other organisms can create fluorescence. Structurally, GFP is an 11-stranded b-barrel, which is nearly a perfect cylinder, threaded by an a-helix running up the axis of the cylinder (Figure 36.1) [3, 4]. The chromophore is attached to the a-helix and is buried almost perfectly in the centre of the cylinder. The chromophore (p-hydroxybenzylideneimidazolinone) is auto-catalytically generated by the post-translational modification of a three-amino-acid sequence. This characteristic structure of GFP can both constrain the motions of the chromophore and shield it from the surrounding water solvent that would otherwise quench its fluorescence. A surprising number of polar groups and structured water molecules are buried adjacent to the chromophore and appear to play a crucial role in the functionality of the system through their participation in hydrogen bonding network. Owing to the importance of GFP as a bio-imaging reagent, there is substantial interest in understanding the photophysics of the embedded chromophore and in particular in determining the structural changes and the dynamics in and around the chromophore following the photo-excitation. Various

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

816 Hydrogen Bonding and Transfer in the Excited State

Figure 36.1 Structure of green fluorescent protein and its chromophore. Reprinted with permission from RCSB PDB

spectroscopic techniques as well as theoretical tools have been employed to elucidate the fluorescent mechanisms in progressively greater detail [3, 5–15]. The wild-type (wt) GFP has been widely regarded to exist under ambient conditions in two forms that are related by deprotonation of the chromophore through a proton shuttle mechanism that favors the neutral form in vivo. The absorption spectrum of wtGFP is characterized by two bands at 395 nm (band A) and at 475 nm (band B) [16]. Excitation into either band results in highly efficient generation of green fluorescence. The excited state dynamics of wtGFP have been studied using ultrafast fluorescence and absorption spectroscopies [16–18]. Based upon their picosecond spectroscopy studies, Boxer et al. concluded that light-driven conversion between the neutral (A state) and anionic (B state) chromophore proceeds via excited state proton transfer (ESPT) and passes through an intermediate state I [18]. Using hole-burning spectroscopy, Creemers et al. have characterized the A, B and I states in wtGFP and demonstrated that they have distinct photophysical properties [19]. Thus, the generally accepted picture about wild-type GFP [18] is that upon excitation at 395 nm there is proton transfer on the excited A
state to form an intermediate excited state I
of GFP, from which state fluorescence of 508 nm (the characteristic intense green fluorescence) is emitted and GFP returns to the ground intermediate state I. Figure 36.2 shows one mechanism for the photo-isomerization of GFP in which proton transfer processes play a vital role [3]. The three-proton relay includes chromophore (in green), water W22, and Ser205 and Glu222 residues, which is connected using the arrow symbols. Upon excitation at 475 nm, there is no proton transfer on the excited state, and the emission of 503 nm (at room temperature) is from the B
to B state of GFP. The I state is electronically very similar to B state the (chromophore is in its anionic state), but environmentally very similar to A state (i.e., structurally not relaxed). During most light absorption/emission cycles, the proton transfer eventually reverses on the ground state. However, occasionally the proton does not return to the chromophore, so the neutral chromophore is photo-isomerized to the anionic form, which involves a slower structural relaxation (e.g., side chain of Thr203 rotates to solvate and stabilize the phenolate oxyanions [3]). Of course, it is still arguable about how the proton moves and how many intermediate (I) states exist [20–25]. As pointed by Boxer et al. [18], there are several sites within the chromophore that could accept or donate a proton, and the transfer could occur within the chromophore or between the chromophore and a nearby residue of the protein. The mechanism in Figure 36.2, as proposed by Brejc et al. [3], is supported by most experimental evidence, and in the following discussions we will focus on this mechanism, although other mechanisms derived from the experimental evidence have also been proposed [21, 23–25]. Evidence of the A
to I
proton pathway and the nature of the I
state have been confirmed by transient infrared absorption spectroscopy, which has allowed the detection of protonated Glu222 after photoconversion [26–28]. For the

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Figure 36.2 (a) Hydrogen bonding network and the three-proton relay mechanism surrounding the chromophore. (Reprinted with permission from [3]. Copyright 1997 National Academy of Sciences). The three-proton transfer chain includes chromophore, water W22, and Ser205 and Glu222 residues. (b) Energetics for ground and excited state proton transfers in GFP. Reprinted with permission from [35]. Copyright 2002 National Academy of Sciences

electronic ground state, Kennis et al. have found that there exist two distinct anionic ground intermediate states (denoted as I1 and I2) [7]. They absorb at 500 and 497 nm, respectively, and interconvert on a picosecond time scale. The I2 intermediate has a lifetime of 400 ps, corresponding to a proton back-transfer process that regenerates the neutral ground state. Hydrogen/deuterium exchange of the protein leads to a significant increase of I1 and I2 life time, indicating that proton motion underlies their dynamics. More recently there is even evidence pointing to an extended proton wire linking Glu222 to Glu5 on the protein surface [23]. In addition, based upon the asymptotic behavior of the fluorescence, Agmon et al. have proposed a mechanism involving a conformational change enabling the rotation of Thr203, which eventually allows the proton, via the backbone carbonyl of His148, to escape to the exterior solution [24, 25]. Theoretically, very important work has been performed for GFP, which includes electronic structure calculations of the GFP chromophore in different protonation states (some including its immediate surrounding residues) [29–31], mixed quantum mechanics/molecular mechanics calculations [8, 10] and molecular dynamics (MD) simulations [32–35]. For example, Langhoff and coworkers [31] have carried out exploratory ab initio calculations to investigate the mechanism of internal conversion via torsional motions within the chromophore. Patnaik et al. have [36] studied the relationship between molecular structure and the redshift in absorption spectra of S65G and S65T green fluorescent protein through a combination of molecular dynamics simulations with time-dependent density functional theory (TDDFT) method. Also notable is the development of a CHARMM force field for representation of the chromophore in GFP by Thiel and coworkers [34], which will help in the implementation of more reliable molecular dynamics studies. While much interest has been placed on the possible rapid deactivation pathways of the excited state due to transit through non-adiabatic crossings in GFP and its mutants, specific studies on the proton relay energetics and dynamics are rare. A dynamical simulation of the operation of the proton wire would provide insight into matters such as feasibility of the operation of this mechanism, its rate and nature (e.g., whether is concerted or sequential, and in the latter case what the order of the motion is). On the ground state, our group [12, 13] and others [37] have performed quantum DFT calculations and explored several cluster models for the ground-state proton chain transfer pathways. Mechanistic aspects of the proton transfers have been revealed, indicating a combination of both concerted and sequential aspects of the proton motions. Despite the overall “concerted” nature of the

818 Hydrogen Bonding and Transfer in the Excited State

potential profile, the configurational evolution along the reaction coordinate involves sequential movement of the protons. On the excited state, Vendrell et al. [14] have explored the overall topology of the PES for the proton wire in a sightly different cluster model for both the S0 ground state and 1pp
excited state, utilizing complete active space self-consistent field (CASSCF) and complete active space with second-order perturbation theory (CASPT2) methods. Importantly, they found that the most energetically favourable pathways on both the ground and excited states follow the same ordering of movement of the protons as illustrated in Refs [12, 13, 37]. However, despite the qualitative consistency there are quantitative discrepancies in terms of the barrier height and overall endothermicity, which might be due to different methods used, different models and different symmetry constraints. While important, proper dynamical simulation is extremely complicated in GFP, since the basic system is very large and non-adiabatic transition on the excited state cannot be disregarded. In addition, the quantum effects for proton transfers are strong due to the lightness of the protons involved. Thus only very limited dynamical studies have been reported so far. Zhang and Smith [38] used a simplified quantum dynamical model to investigate the photo-absorption and excited state proton transfer processes in wild-type GFP recently. Lill and Helms [35] have implemented molecular dynamics together with a stochastic jump mechanism for incorporating the quantum proton transfer rates in an approximate manner. In their simulations it was suggested that proton-wire operation is triggered by the phenolic proton transfer from chromophore to the captive water molecule, and afterward the remaining proton transfers would occur very rapidly. Recently, Vendrell et al. [15] have performed a six-dimensional variational multi-configurational time-dependent Hartree (MCTDH) quantum dynamics investigation for the proton transfers in GFP using an empirical valence bond (EVB) fitted potential energy surface based upon 338 ab initio data [39]. Their ab initio calculations were performed at CASSCF/CASPT2 theory, assuming a planar geometry (Cs symmetry). From analysis of the energetics it seems that the first transfer of protons is from Ser205 to Glu222, which is opposite to the assumption of the phenolic proton transfer movement. Their quantum dynamics (QD) simulations indicate that proton motion in the wire is essentially concerted due to the very small potential barrier, led (as noted above) by movement of the last proton onto the Glu222 acceptor group and overall very fast, with multiple wavepacket recurrences apparent on the picosecond timescale. In a very recent study, we adopted a contrasting quantum mechanical approach from the work of Vendrell et al. [15] towards modeling the nuclear dynamics of proton chain transfer. As noted by those authors, the multiple and persistent wavepacket recurrences observed on the ps timescale in their simulations may indeed be due to the lack of any dissipative component in their model. Similarly, over tens of picoseconds the PES experienced by the protons would be expected to modulate due to relaxation effects of other modes in the chromophore and immediate surroundings. In light of these considerations, we adopt a quantum kinetic framework for the ground state I ! A proton shuttle in which the cluster is assumed to be thermalized and we compute directly the cumulative reaction probability for “first passage” multidimensional transmission from the I state to the A state. We have extended our earlier ab initio DFT calculations [12] to complete a full 3D relaxed potential energy surface for the ground state proton transfer. Dissipative conditions are explicitly incorporated in the form of absorbing boundary conditions (i.e., a complex absorbing potential) on both the I-state side and the A-state side of the proton transfer barrier. In addition to the dynamical motivations outlined above for our adoption of the present dissipative time-independent quantum framework, the approach also facilitates direct calculation of the kinetic isotope effect (KIE) for the ground state I ! A proton shuttle, a quantity of direct experimental interest that has been measured recently by Kennis et al. [7] In their femtosecond multipulse control spectroscopy study, intermediate ground states in the GFP photocycle were uncovered for the first time and the relevant kinetic lifetimes measured for both normal and deuterated proteins as mentioned above. Two distinct anionic ground state intermediates, I1 and I2, were unveiled (“anionic” here referring to the charge state of the chromophore). The KIE on the ground state for the I1 ! I2 conversion was 2, implying a structural rearrangement of some sort, while the KIE for the subsequent I2 ! A conversion was 12.

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The large KIE for the I2 ! A conversion suggests that this stage intimately involves the proton chain transfer as a rate-determining step. Hence, our model quantum mechanical calculations address directly the measured KIE for the I2 ! A conversion, and we will compare our calculated KIEs from this work with the measured ones later in this chapter. Following the success of GFP as a fluorescent marker, there has been a concerted research effort worldwide into engineering new fluorescent proteins (FPs) with enhanced properties for various types of applications, particularly those with far-red emission [40]. Red fluorescent proteins (RFP) provide unique opportunities for noninvasive labeling and tracking of specific cell types in living organisms in real time. Together with the development of new systems for whole-body imaging, red fluorescent proteins allow visualization of changes in target-gene promoter activity, tracking cellular movement in embryogenesis and inflammatory processes, monitoring migration of small parasites within a host, and studying important aspects of cancer, such as tumor cell trafficking, invasion, metastasis and angiogenesis [41, 42]. Furthermore, photoactivatable fluorescent proteins [43, 44] and fluorescent timers [45] expand the scope of temporally and spatially controlled tracking experiments. Thus, as a complement to the emission color palette of GFPs, intrinsic red fluorescent proteins (RFPs) with origins in Anthozoa species potentially have very significant advantages. Firstly, far-red FPs provide an additional color for multi-color labeling, as their emission is well separated from the green-yellow autofluorescence of cells. Secondly, the reduced light scattering at longer wavelengths facilitates imaging of thick tissues because animal tissues are almost translucent to far-red light. These optical properties make RFPs potentially invaluable tools for deep tissue in vivo imaging in biomedical research. However, the application of naturally occurring RFPs is complicated by oligomerization, slow maturation and low quantum yield. As a result, much experimental work has been put forward to generate improved RFPs [46]. Far-red fluorescent proteins whose emission maxima reach the 650-nm barrier have been developed, such as HcRed [47], mPlum [48] and AQ143 [49]. In particular, a recently reported far-red fluorescent protein, named Katushka, which is seven- to tenfold brighter than the spectrally close HcRed or mPlum, is characterized by fast maturation and a high pH-stability and photostability [50]. Our group is at the forefront of efforts to theoretically characterize RFP chromophore and their electronic states [51–54]. Prior to our work, the number of theoretical studies on isolated RFP chromophores is very limited [55]. There are currently few whole-protein modeling studies of RFPs present in the literature, in particular for the excited state studies. Thus RFP physics on the molecular level is poorly understood. In recent years we have calculated the bond rotation profiles for model DsRed chromophore using DFT method [52], and have performed TDDFT calculations on model RFP Rtms5H146S chromophore [51]. We also employed complete active space self-consistent field (CASSCF) and multireference Rayleigh–Schr€odinger second-order perturbation theory (MRPT2) methods to characterize the bridge photoisomerization pathways of a model red fluorescent protein (RFP) chromophore (see, for example, Figure 36.3 for the structure of DsRed) [53]. Going beyond the cluster model, it is essential to incorporate the solvated protein environment to gain the most realistic representation of the potential energy surface (PES). Very recently we have initiated quantum mechanical/molecular mechanical (QM/MM) modeling for RFP through collaboration with Thiel’s group, which will help gain a better fundamental understanding of how RFPs’ fluorescent properties are controlled and mediated by the protein environment. To develop insights into the mechanistic aspects that govern the function of promising FP candidates, we have also been collaborating with structural biologists for the fluorescent protein and related studies. This has led to a series of publications [51–54, 56–58] that have elaborated both key structural and mechanistic issues relating to the RFP HcRed [57] and pocilloporin Rtms5H146S [59, 60] in recent years, and provides an important experimental context within which the knowledge gained from our calculations can aid in furthering experimental development of new engineered proteins. In this regard simulations and modeling can add fundamental understanding to the direction aspect of a directed evolution approach, since the physical interactions and mechanisms introduced by mutations that yield improved RFPs are unknown, and

820 Hydrogen Bonding and Transfer in the Excited State

Figure 36.3 Structure of red fluorescent protein (DsRed) and its chromophore. Reprinted with permission from [53]. Copyright 2007 American Chemical Society

mutagenesis studies alone cannot reveal them. Thus an understanding of these interactions would streamline development and lower the associated cost. For example, our computational studies have tackled the basic question of how structural rigidity of the chromophore varies between the GFP and RFPs and which residues impact most on this, and the recent benchmark studies from our collaborative team provide strong evidence that this is a crucial factor in controlling fluorescent quantum yield. The rest of this chapter is arranged as follows: the description of the method of calculation is given in Section 36.2, and a discussion of the results follows in Section 36.3. Conclusions and future outlook are summarized in Section 36.4.

36.2 Method of Calculation We investigate the fluorescence mechanism in green fluorescent protein (GFP) and in red fluorescent protein (RFP) through a combination of computational methods. For GFP, most of our studies are based on cluster models that consider only the residues close to the chromophore. For RFP, we focus on the truncated chromophore model initially; extension to include the whole protein environment using QM/MM method is currently in progress. 36.2.1 Computational methodology for GFP proton wire 36.2.1.1 Cluster Model for GFP Proton Wire Three-proton transfer models have been built and one example is shown Figure 36.4, where the water and methanol have been chosen as the bridge molecules and acetic ion as the terminal acceptor. The model is based on the GFP protein data file 1emb.pdb [3], and all other residues and water molecules have been removed. The cluster model was employed to mimic the proton chain transfer within GFP as shown in Figure 36.2, in which the methanol represents the residue Ser205, whereas the acetic ion represents residue Glu222. The acetate moiety as acceptor group is more basic in character than the more heavily truncated formate, and thus more representative for the residue Glu222. The geometry optimization of the model system has been carried out for the electronic ground state using the cc-pVDZ basis set at the hybrid density-functional theory (DFT) level of Becke and Lee, Yang and Parr (B3LYP) [61–63]. The three O–H bond lengths are labeled as r1 for O–H bond of phenol in chromophore, r2 for O–H bond of water and r3 for O–H bond of methanol, which represent the proton coordinates in the proton transfer chain. We noticed that there are several different types of cluster models being explored for GFP, and we focus on two of them, namely, fully relaxed model and rigid model, since we have performed calculations using the two

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Figure 36.4 One cluster model for the GFP chromophore and proton wire as used in quantum dynamics simulations [82]. Reproduced by permission of the PCCP Owner Societies

kinds of cluster models in our work. In the fully relaxed model, all other degrees of freedom except three O–H bond lengths are fully relaxed, whereas in the rigid model the geometry of the model system is held fixed at their values in the minimum in the ground electronic state when moving the three protons between donor and acceptor O atoms in order to construct three-dimensional (3D) PESs. The rigid model has also been employed by Vendrell et al. [14] in a slightly different context (see below for comparison). They also explored a partially relaxed model very recently in which only three O–H bond lengths and the three donor–acceptor distances are allowed to move (six-dimensional model) [15]. Such model calculations are necessary and practical in a quantum mechanical sense, given the sheer size of the system. 36.2.1.2 Ground State DFT Calculations in Fully Relaxed Model In our calculations of the three-dimensional PES grid data for ground state proton transfers, the grid points are defined by constraining three O–H bond lengths along the chain, namely, the chromophore phenolic O–H bond (labeled as r1), the breaking/forming O–H bond of water (labeled as r2) and the O–H bond of methanol (labeled as r3). All other degrees of freedom in our model are fully relaxed. This relaxation of all other modes in the cluster most likely leads to some degree of underestimation of the effective proton transfer barrier, whereas the “frozen” approach in which no other modes are relaxed [14, 15] most likely overestimates it (see also discussion below). Density function theory (DFT) has been used to complete the 3D potential energy surface scan for the ground electronic state, as implemented in the Gaussian 03 package [64]. This level of theory has been discussed and compared against HF and MP2 methods in relation to a two-step model of the GFP proton chain transfer [12, 13]. We carried out the DFT calculations for three-dimensional PES in order to construct a realistic PES for the quantum dynamics calculations for the three proton transfer reactions, which need to span a much larger configuration space. The constraint geometries have been optimized under B3LYP/ 6-31g
theory level followed by the single point energy calculation under B3LYP/6-31 þ g

level. The step  size for the scan is 0.05 A with 16 steps for each O–H bond, and the initial bond length is 0.95 A and the final length is 1.70 A. In total 16  16  16 (¼ 4096) ab initio potential energy points have been obtained. We have not found it necessary to fit this PES with analytic forms. Rather we evaluate the potential as necessary for the construction of the discrete variable representation (DVR) Hamiltonian using cubic spline interpolation.

822 Hydrogen Bonding and Transfer in the Excited State

36.2.1.3 Excited State ab initio Calculations in a Rigid Model Utilizing the cluster model for the proton wire for determination of the excited state PES for proton chain transfer, we have performed high-level ab initio calculations to model the GFP excited state proton transfer (ESPT), which constitutes the first stage of the photocycle after excitation of the chromophore. To determine the molecular orbitals (MOs), complete active space self-consistent field (CASSCF) calculations have been carried out. After determining the MOs, we have performed multi-reference (MR) configuration interaction (CI) calculations to obtain low-lying potential energy curves. In this method, multi-reference (MR) single- and double-excitation (SD) CI is employed, in which the configuration state functions (CSFs) were generated by single and double excitations with respect to the reference configuration used in the CASSCF calculations. Since the ab initio MRCI calculations for a model of this size (Figure 36.4) are quite challenging, we have used a moderate basis set, that is, Dunning’s cc-pVDZ (correlation consistent, polarized valence, double zeta) basis set [65]. As for CASSCF active space, six MOs from 83a to 87a should be included in the active space, since they are important to describe the low-lying electronic states and should be occupied by six electrons in CASSCF (6 MOs/6 electrons). The MOLPRO 2002.6 program package [66] was used to obtain the potential energy curves and surfaces of electronic ground and excited states. Our ab initio calculations span a larger configuration space, to construct a 3D PES for the quantum dynamics calculations for the three proton transfer  reactions later. The step size for the scan is 0.1 A for most data points with 12 steps for each O–H bond, and the  initial bond length is 0.7 A and the final length is 1.70 A. In total 1728 ab initio potential energy points have been obtained. 36.2.1.4 Quantum Dynamics (QD) Calculations Two kinds of QD methods have been employed in our simulations, namely, time-independent (TI) and timedependent (TD) QD methods. In the former method, we solve an eigenvalue problem to obtain the cumulative reaction probabilities for proton transfer reactions, from which the rate constants and thus kinetic isotope effects (KIE) can be extracted. In the latter method, we use the TD wavepacket split-operator propagation technique (the Hamiltonian itself is time-dependent when laser field is explicitly treated) to simulate the real time proton transfer dynamics, which is especially useful for simulating the femtosecond pump–probe spectroscopy. While TI and TD methods have their own advantages and disadvantages in terms of describing proton transfer reactions, we focus on the TI method in this chapter. The TI method we have implemented in the current modeling of the kinetic isotope effect (KIE) for the GFP proton chain transfer is based on a three-dimensional model, which includes the line coordinates for the three protons moving between the donor and acceptor oxygen atoms (e.g., Figure 36.4). We calculated a fully relaxed potential energy surface as a function of these three specific coordinates as mentioned above, which feeds into the Hamiltonian for the quantum dynamics calculation. Then we compute the cumulative reaction probability as a function of energy, N(E), for this system and then Boltzmann average to obtain the temperature-dependent equivalent, N(T). An important component of our model Hamiltonian is the use of a complex absorbing potential (CAP) in order to impose dissipative boundary conditions on either side of the proton transfer barrier. This allows us to directly access the rate of proton chain transfer connecting the neutral and anionic chromophore states without the complication of wavepacket recurrences. Within our formulation, then, the rate constants and the KIE are given by:


kH=D ðT Þ ¼ 2p hQH=D ðT Þ

1

ð þ¥ ¥

dEeE=kB T NH=D ðEÞ ¼

NH=D ðT Þ 2phQH=D ðT Þ

ð36:1Þ

Theoretical Studies of Green and Red Fluorescent Proteins

823

and:   k H ðT Þ QD ðT Þ NH ðT Þ KIEðT Þ ¼ ¼ k D ðT Þ QH ðT Þ ND ðT Þ

ð36:2Þ

Our modelling shows that it is crucial to adopt a direct numerical approach to calculation of the partition functions also, since the PES for the proton wire is highly anharmonic and this has a very important impact on the predicted KIE. We emphasize that the key components in our approach involve exact three-dimensional quantum dynamical calculation of the energy-dependent reaction probability [67], given by equation (36.3), for the triple-proton chain transfer in the cluster model followed by Boltzmann averaging to obtain the thermal quantities, including full quantum evaluation of the three-dimensional reactant partition functions:   1=     1 1 ^ «E 1 ^«p H ^ þ i^«E 1 ^«r=2 N ðEÞ ¼ Tr « ^r 2 Hi^ 4

ð36:3Þ

36.2.2 Computational methodology for model chromophores in FP 36.2.2.1 Ground State Calculations We employ truncated models for FP chromophores (e.g., DsRed, Rtms5H146S). For ground state calculations, the models were optimized with B3LYP density functional theory using a 6-31 þ G

basis set. For coordinatedriving potential scans, we optimized the models under the constraint of a constant angle of the dihedral. At each of the optimized geometries we calculated the energy using B3LYP DFT and a 6-31 þþ G

basis set. All calculations were performed using the GAUSSIAN03 package [64] 36.2.2.2 Excited State Calculations For the excited state calculations for model chromophores in several fluorescent proteins (e.g., green fluorescent protein-GFP, kindling fluorescent protein-KFP and red fluorescent protein-RFP), we also use a model of the FP chromophore that is truncated to include only the chromophore. Connections that would be made to the protein backbone are terminated by hydrogen atoms. For excited state calculations, we have analysed the electronic structure of the S0 and S1 states of more substantial models and determined that truncation does not change their nature. The structures of these chromophores are considered in their Z and E configurations. The reader should note that the Z isomer is often referred to as cis in the fluorescent protein literature, and the E isomer as trans. Our electronic wave functions were generated via dynamically correlated multistate electronic structure computations built upon a state-averaged [68] complete active space self-consistent field [69] (SA-CASSCF) reference wave function, using geometries optimized at the SA-CASSCF level. To evaluate energies at these geometries, we make use of multireference Rayleigh–Schr€odinger second-order perturbation theory (MRPT2) [70]. In this perturbation theory, the reference space is remixed as the perturbation is applied, and the states are obtained by diagonalization of a symmetrized perturbed effective Hamiltonian. MR-MS-RSPT2 provides size-consistent energies. Our choice of orbitals was guided by orbital energies and occupation numbers that were obtained in a preliminary battery of self-consistent field calculations. We are interested in bridge torsion. To obtain a broader view of the photoisomerization, we calculated energies and properties along coordinate-driven slices through the potential energy surfaces. Coordinatedriving potential surface scans were generated by fixing one (or both) of the two bridge dihedrals (the driven

824 Hydrogen Bonding and Transfer in the Excited State

coordinates) of the model chromophore and minimizing all other degrees of freedom subject to this constraint. All of the results for the excited state calculations were obtained with the MOLPRO program [66]. 36.2.3 QM/MM method for RFP 36.2.3.1 SCC-DFTB/MM Molecular Dynamics 

The crystal structure of HcRed protein (protomer B) with a resolution of 2.1 A provided the starting point of our calculations [57]. The addition of missing hydrogen atoms was determined using the HBUILD facility in CHARMM at pH 7 and the definition of protonation states of titratable residues were performed using the PROPKA method [71, 72]. The systems consisting of protein and crystallographic water molecules were solvated in a sphere of radius 30 A formed of TIP3P water molecules [73] and the water molecules too close to existing atoms were deleted. We performed 12 hydration cycles until the numbers of water molecules were approximately constant. Finally, the MD production runs without restrains were performed for 500 ps to complete the preparation of the complexes. The systems were heated from 50 to 300 K through increasing the temperature by 0.1 K each step. In the MD simulations the chromophore was described by the semi-empirical SCC-DFTB (self-consistent charge density-functional tight-binding) method [74], while the protein environment has been treated using the CHARMM force field [75]. The QM/MM boundary at the two covalent bonds was treated by generalized hybrid orbital (GHO) method [76]. 36.2.3.2 DFT/MM Calculations Four snapshots for the initial structures for QM/MM optimization were randomly taken from the 500 ps MD trajectories of cis chromophore in HcRed. The structures with the trans conformation of chromophore were derived from the optimized cis isomer by a manual rotation of the hydroxyphenyl group and complete reoptimization of the geometry parameters at the same calculational level. In the QM/MM calculations, the QM part was treated by the B3LYP density functional method with the basis sets of SV(P) and TZVP, and the MM part was described by the CHARMM force field. An electronic embedding scheme was adopted in the QM/MM calculations [77]. Hydrogen linker atoms with the charge shift model were employed to treat the QM/MM boundary. The TURBOMOLE program was used for the QM treatment in the QM/MM as well as in the pure QM calculations. The CHARMM force field was run through the DL_POLY program to handle the MM part of the systems. The QM/MM calculations were performed with the ChemShell package [78, 79] that integrates the TURBOMOLE and DL_POLY programs and also performs geometry optimization with the HDLC optimizer [80].

36.3 Results and Discussion 36.3.1 Proton transfers in GFP Our work in this regard focuses on the proton transfer dynamics in green fluorescent protein (GFP) using rigorous quantum mechanical methods, based on several cluster models. We performed density functional theory (DFT) calculations for the ground-state proton chain transfer pathway in model GFP [12, 13, 81]. The mechanistic conclusions arising from studies in our laboratory [12, 13] and others [14, 15, 37] may be summarized as follows: (i) Based on all the cluster models explored so far, proton transfer on the ground state as well as on the excited state is predicted to occur via a single barrier, implying a concerted mechanism. (ii) Despite the presence of a single barrier, implying a concerted kinetic process, analysis of the minimum energy pathway (MEP) showed clear signatures of largely sequential movement of the protons. The ordering of

Theoretical Studies of Green and Red Fluorescent Proteins

825

movement of the protons along the reaction pathway corresponds to a “pulling” rather than a “pushing” mechanism – for example, for the neutral to anionic proton chain transfer, the first proton movement is from the bridging Ser 205 moiety to the accepting Glu 222 group, followed by the second proton moving from the bridging water to Ser 205, and the phenolic proton on the chromophore being the last one in the chain to move. In other words, the mechanism of proton chain transfer is impacted heavily by the presence of the charged acceptor group at the end of the chain. The mechanistic picture arising from analysis of the MEPs is hence a blend of the classic “concerted” and “stepwise” mechanisms. As highlighted in the sections above, the actual dynamical processes in GFP are very complicated, and quantum-mediated kinetics of proton chain transfer have been found to play a crucial role in the photocycle of the GFP. Thus, moving beyond the static reaction pathway picture, to explore the true proton transfer dynamics we have performed nuclear quantum dynamics (QD) simulations, which is the only reliable and accurate way to describe the ultrafast dynamics involved. We have used both time-dependent split-operator propagations to simulate the ultrafast femtosecond pump–probe dynamics [38] and time-independent methods to address the thermal rates and KIEs for proton chain transfer on the longer timescale of 1 ps to 1 ns. The TD method can directly probe in real time the primary proton transfer processes within the chromophore and its immediate environment. In a recent preliminary study [38], we explored the time-dependent idea for the one-dimensional excited state proton transfer case in GFP. This work is also partly motivated by the femtosecond experiments performed for GFP [6, 7] since femtosecond experiments can provide new insights into the microscopic mechanisms of the photodynamics in GFP. In the frequency-resolved femtosecond pump–probe experiment in wt-GFP, V€ ohringer et al. [5] studied the microscopic origin of the dispersive kinetics and the molecular mechanism of the primary events involved in the excited state proton transfer (ESPT) dynamics. They proposed the energetic scheme with additional two higher-lying electronic configurations, upon which we have built the model potential energy surfaces for quantum dynamics calculations. While our model quantum dynamics calculations can explain the origin of the early-time stimulated emission observed in experiment, the predicted KIE for the ESPT is too small (about 1.25) compared with the experimental one of about 6.0 [18]. Following the TD quantum dynamics work, we have very recently performed TI quantum dynamics simulations to predict the rate constants of the ground state proton transfer reactions and thus the KIEs. Our main goal is to compare the simulations with experimental results (in particular KIE) in order to understand the underlying reaction mechanisms. We use a three-dimensional potential energy surface for the ground state proton movements generated using DFT. Using the TI approach we have achieved a remarkable nearquantitative agreement with the very large experimental KIE of 12 at room temperature for the ground state proton chain transfer (Figure 36.5) [82]. Ours and others previous attempts to model the KIE have generally predicted KIEs in the range 1–3 (see Table 36.1 for a comparison). For example, in a very recent report [15], the true movements of the proton transfers have been explored through MCTDH quantum dynamical calculations. In the MCTDH simulations, within the initial phase of short-time dynamics, a clear isotope effect of about 1.5 appears for the H/D substitution on the excited state. However, an inverse KIE in the long time range was found, which we believe is artificial due to the reflections of the wave-packets without appropriate absorptions in the product region. Thus our latest result suggests that our reduced-dimensional time-independent quantum dynamics model offers considerable predictive power not only for the GFP but potentially also for other important biological proton wires. While the KIE predicted by our first implementation of this TI approach for the GFP is in very good accord with the experimental measurement (and is the only model to have successfully achieved this), the absolute rate constants for proton and deuteron transfer are still two orders of magnitude too fast. This implies that our PES, which allows all other degrees of freedom in the cluster to relax as a function of the proton coordinates, is predicting absolute barriers that are too low (although the overall shape and anharmonicity of the surface are apparently reasonably well represented). In practice, it is unlikely that slower modes will be able to fully relax on the timescale of operation of the proton wire. However, we know already that freezing all other coordinates

826 Hydrogen Bonding and Transfer in the Excited State

Figure 36.5 Predicted KIE for GFP ground state proton chain transfer. Quantum model results: solid line. Experimental result: circle. Standard translational model: triangle [82]. Reproduced by permission of the PCCP Owner Societies

Table 36.1 Comparison of kinetic isotope effects (KIEs) from both calculations and experiments. MCTDH result is from Ref. [15], whereas the experimental data is from Table 1 in Ref. [7]. Other data are from our work TI QD Ground state (I2 to A) Ground state (I1 to I2) Excited state

MCTDH

TD QD

Exp

1.25

12 2 6

15.5 1.5 (early)/inverse (later)

yields barriers that are much too high (see below for the results from rigid model). This implies that a static averaging approach – whereby the proton PES is calculated with all other modes frozen and then averaged by configuration sampling over the other modes – might result in barriers that are too large. Hence, an intermediate approach involving only partial relaxation of other modes will be necessary – and this strategy must necessarily be informed by dynamical studies. Our proposal is to implement ab initio molecular dynamics calculations for the cluster model in order to explore this question of how to determine the most appropriate way of computing an effective PES for the proton motions in a biological proton wire (using GFP as our prototype). This will be an essential prerequisite for the quantitative calculation of absolute rates of proton chain transfer in such systems. To illustrate this more clearly, we have very recently performed a series of constrained quantum chemical minimum energy pathway calculations for the ground and first excited electronic states of a cluster model of the GFP photocycle; each MEP corresponding to a different assumption as to which modes are relaxing along the reaction coordinate. Figure 36.6 presents the ground state energy profiles computed by density functional theory (DFT) at the B3LYP/cc-pVDZ level for a set of four different relaxation models: (i) in which all coordinates of the cluster model have been allowed to relax, yielding the true (fully relaxed) MEP for this cluster model; (ii) in which all cluster modes except the internal modes of the chromophore are allowed to relax (the chromophore is constrained to remain in a configuration corresponding to the optimized neutral geometry – excepting of course the phenolic proton which is transferred to W22); (iii) in which only six coordinates are relaxed, corresponding to the donor–acceptor oxygen atoms of the water, Ser205 and Glu222 and the three protons (all other modes being fixed at the optimized neutral geometry); and (iv) in which only the

Theoretical Studies of Green and Red Fluorescent Proteins 48.9

50

Relative energy (KJ/mol)

827

43.9 iv

40

34.7

33.7

iii

30 20

8.4

10 0

0.0

4.9

4.2

ii

neutral -11.8

-10

i

-20

Reaction Path

Figure 36.6 Energy profiles for the ground state proton transfer computed at the B3LYP/cc-pVDZ level for the cluster model

three protons (linear coordinates along the lines between donor and acceptor oxygens) are allowed to relax. The fully relaxed calculation (i) in Figure 36.6 corresponds to plot (1c) in Figure 2 of our earlier work [12], with the only difference being a somewhat larger basis in the present calculations. Figure 36.7 presents an analogous set of energy profiles for the first excited state proton transfer computed at the modest level of CIS/cc-pVDZ by constrained optimizations to locate the saddle-points and final anionic geometries. Clearly, while the absolute values of the barriers would likely decrease substantially with a higher level of theory and a larger basis, a similar trend of sensitivity in the barrier height to the details of the relaxation model is seen as for the ground state results of Figure 36.6. Thus the effective barrier for proton chain transfer is found to be markedly sensitive to the choice of the closely dynamically coupled group of degrees of freedom. The complexity of this choice is a particularly vexatious feature of proton wire systems, as exemplified by the GFP. These results demonstrate that the choice of closely coupled degrees of freedom that will be explicitly incorporated in quantum simulations (or tunneling-corrected transition state theory calculations) must be carefully made and will need to be informed by dynamical studies in appropriate cluster models. This group of

Relative energy (KJ/mol)

120

110.9

100

101.8

80

80.9

66.0 70.6

60

35.3

40

36.6

iv iii

ii

20 0

0.0 neutral

-20

-36.5 -40

i

Reaction Path

Figure 36.7 Energy profiles for the first excited state proton transfer computed at the CIS/cc-pVDZ level for the cluster model

828 Hydrogen Bonding and Transfer in the Excited State

Figure 36.8

Molecular orbitals (MOs): 86a and 87a

closely dynamically coupled modes will then be treated explicitly in the quantum simulations, based on a computed ab initio potential energy surface (ideally with subsequent configurational averaging over the “environmental” modes). This is currently under investigation. Lately, we have also extended our investigations into the excited state proton transfers using high level ab initio methods and exact nuclear quantum dynamics simulations. We employ rigid cluster model based on the optimized structure for ground state A with neutral chromophore as shown in Figure 36.4. We performed high level complete active space self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) calculations to generate three-dimensional potential energy surfaces for the three proton transfers. In total 12  12  12 ab initio data points are used for the construction of the 3D PES. Our CASSCF active space consists of six molecular orbitals (MOs): 83a to 88a, with 87a being HOMO, and 88a being LUMO. As examples, in Figure 36.8 we have plotted some selected molecular orbitals (MOs) corresponding to optimized state A structure with neutral chromophore. The electronic structure is characterized by the 86a, 87a and 88a MOs, and in particular the 86a and 87a MOs feature the p complex conjugate system. Analysis of the molecular orbitals indicate that the first excited state is of 1pp
character, which is the photoactive state. If further excitations (e.g., 1ps
) are of interest, we will need to include more MOs in our calculations. Figure 36.9 presents some selected 2D contour plots of the 3D potential energy surfaces for three proton transfers in the cluster model for the excited state. The plots correspond to the proton transfer leading coordinate r3 ¼ 1.5, 1.2 and 1.0 A for (a)–(c), respectively. Following the pictures from reactant state to product state from (a) to (c), we can see that as the third proton moves from the reactant state to transitional state and finally to product state, the well depths of the product state are becoming deeper and deeper, thus making it easier for other two protons to transfer from reactant state to product state. The results of 3D potential energy

(b) r3=1.2 angstrom

1.4

1.2

r2 (ang

1.0

strom )

0.8 0.8

4 1.6

1.4

1.2

r2 (ang

1.0

strom

)

0.8 0.8

4 6 8 10 12 14

14 12 10 8 1.8 1.6 1.4 1.2 1.0

6

m)

Ve(eV) m)

m)

tro

1.6

an gs

4

1.8 1.6 1.4 1.2 1.0

6

tro

1.8 1.6 1.4 1.2 1.0

6

8

an gs

8

10

r1 (

Ve(eV)

12

10

r1 (

Ve(eV)

12

4 6 8 10 12 14

14

tro

14

4 1.6

1.4

an gs

4 6 8 10 12 14

(c) r3=1.5 angstrom

1.2

r2 (ang

1.0

strom

0.8

r1 (

(a) r3=1.0 angstrom

0.8

)

Figure 36.9 Potential energy surfaces for three proton transfers in the electronic excited state of the cluster model:  r3 ¼ (a) 1.0, (b) 1.2 and (c) 1.5 A

Theoretical Studies of Green and Red Fluorescent Proteins

829

Table 36.2 Barrier heights DH and relative energies DE in eV;
indicates excited state. CASSCF and CASPT2 results are from Ref. [14], and experimental estimation is from Ref. [6] MRCI CASSCF CASPT2 DFT EXP (GFP)

DE ¼ EI – EA

DH ¼ ETS – EA

DE
¼ EI
– EA


DH
¼ ETS
–EA


0.28 0.73 0.90 0.18 0.07

0.59

0.33 0.24 0.34

0.48

0.24

0.12

0.197 0.24

surface show that the correlations of movement between protons are very important, and the transfer of the more energetically favorable proton (e.g., proton 3) facilitates the transfers of the more energetically unfavorable protons (e.g., proton 2 and 1). The results of the 3D MRCI potential energy surface also further support the mechanistic picture of the concerted proton transfer processes with a single barrier; however, the sequential movement of the protons can be identified within the concerted picture. Our observations in this work agree with previous principle mechanistic conclusions arising from literature of ours [12, 13] and others [14, 15, 37]. This rigid cluster model generates energetics that qualitatively agree with the experimental findings [6], and in Table 36.2 we have listed the barrier heights DH and relative energies DE from different sets of calculations and from some available experimental estimations. In this table the unit is eV, and
represents the excited state. From this table we can see that MRCI calculations produced the relative energy of 0.28 eV between anionic ground state I and neutral ground state A and the barrier height of 0.59 eV for the ground state, both of which are higher than the experimental estimation (the experimental result is roughly 0.24 eV [6] for barrier height and is roughly 0.07 eV for the relative energy [6]). For the excited state a similar trend is found; for example, the calculated barrier height and relative energy are higher than the experimental ones, which might be due to the rigid model employed in which no relaxation is allowed in our MRCI calculations. The experimental estimations for the energetic terms are based upon some available but limited experimental data for the ground and excited state, mainly from three experimental results, that is, picosecond spectroscopy of Boxer et al. [18], hole-burning spectroscopy of Creemers et al. [19] and frequency-resolved femtosecond pump–probe spectroscopy [6]. Based on these experimental results, illustrative PESs have been given by V€ohringer et al. [6], with which our comparisons are made. In this table we also list the relative energies for both ground and excited state from CASSCF and CASPT2 calculation [14], in which a sightly different model and a co-planar Cs symmetry constraint are employed. For the excited state, CASPT2 predicts a fairly similar relative energy with MRCI calculation, whereas CASSCF predicts that the energy in the anionic state I
is higher than the neutral state A
, which disagrees with other calculations and the experimental estimation. For the ground state, both CASSCF and CASPT2 produced larger relative energies than MRCI calculations. We also list the DFT results from our 3D PES calculations with constrained relaxation for the ground state [82], and, as we can expect, it predicts a barrier height that is lower than the MRCI calculation and the experimental estimation. 36.3.2 Internal conversion mechanism in FP chromophores The preferred optical window for deep tissue imaging is in the far-red and near-infrared (650–1100 nm) because this window largely avoids the (optical) absorptions of melanin and hemoglobin at shorter wavelengths and vibrational absorptions of water at longer wavelengths. Thus, following the success of GFP as a unique fluorescent label, a search has been going on for new fluorescent proteins, particularly those with far-red emission, which could enable such in vivo deep tissue imaging. RFPs have been discovered in

830 Hydrogen Bonding and Transfer in the Excited State

numerous coral species and it is now known that RFPs and their homologues are responsible for much of the coloration seen on coral reefs [83]. Current evidence points to a possible photoprotective role for these proteins [84]. Red fluorescent proteins possess a chromophore similar to that of GFP, but with additional chemical modifications that occur auto-catalytically during a complicated maturation process (Figure 36.3). At present, the detailed origin of the redshift of these proteins is not completely understood, and our goal of the RFP modeling project is to further the understanding of the molecular physics of red fluorescent proteins. Through an understanding of the mechanisms leading to the redshift, the design of further redshifted variants may be possible, leading to applications in deep-tissue biomedical imaging. Furthermore, the application of RFPs is quite often hampered by a low quantum yield. Through an understanding of the deactivation processes that compete with fluorescence in these systems, brighter variants may be designed. The tools we used for RFP include ground and excited state electronic structure methods coupled with molecular dynamics methods for multiple states. In our early study [52], we carried out the first comparative examination of internal rotation barriers in acylimine (model R0) and peptide (model G0) substituents in model red fluorescent protein chromophores in DsRed. Model G0 (G-green) and model R0 (R-red) represent the immature and mature DsRed chromophore, respectively. Figure 36.10 shows the results from the potential energy surface scans generated by coordinate driving of the uCNCO dihedral in R0 and G0 models. The definition of the uCNCO dihedral is given on the righthand side. The highest calculated energy for R0 is 5.5 kcal mol1 (uCNCO ¼ 105 ) relative to the cis acylimine optimized geometry and the highest energy for G0 is 16.3 kcal mol1 (uCNCO ¼ 90 ) relative to the trans peptide optimized geometry. These energies represent lower bounds to the true transition state energy. Our results indicate that the barrier to trans–cis isomerization of the substituent is much lower if it is an acylimine instead of a peptide, providing prima-facie evidence that acylimine formation precedes trans–cis isomerization in DsRed chromophores. Our study provides partial evidence for the emerging picture that RFPs with low quantum yield do not maintain the chromophore in a structurally rigid conformation as does the GFP – thus facilitating bridge twisting and internal conversion via conical intersections and a low fluorescence quantum yield. We also performed TDDFT calculations on model RFP Rtms5H146S chromophore [51], which provided strong supporting evidence for the assignment of the protonation state of the chromophore of the yellow form of Rtms5H146S at low pH.

Figure 36.10 Internal rotation profile of models R0 and G0. The internal rotation potential scans about the central dihedral of the acylimine (model R0, filled upward triangles/solid line) or peptide (model G0, open downward triangles/dashed line). Energies were evaluated using B3LYP DFT and a 6-31 þþ G

basis set at geometries optimized with B3LYP DFT and a 6-31 þ G

basis set under the constraint of constant uCNCO. Energies are in kcal mol1 and angles are in degrees. All energies are referenced to the lowest energy conformer for each model. Peak heights for each curve are indicated. Solid (R0) and dashed (G0) lines are simple spline curves. Reprinted with permission from [52]. Copyright 2006 Elsevier

Theoretical Studies of Green and Red Fluorescent Proteins

831

Figure 36.11 Characterization of energies and charge distributions in isomerizing RFP chromophores. Reprinted with permission from [53]. Copyright 2007 American Chemical Society

In a more recent study [53], we used CASSCF and MRPT2 methods to characterize the bridge photoisomerization pathways of the model red fluorescent protein (RFP) chromophore, DsRed. Figure 36.11 shows energy profiles and charge distribution profiles on the anionic RFP chromophore as a function of the key isomerization coordinates that lead to internal conversion via twisted conical intersections. The study indicates that photoisomerization of the imidazolinone-bridge bond is suppressed by the induction of a high barrier on the S1 surface, whereas photoisomerization of the phenoxy-bridge bond is favored via stabilization of the S1 pathway and the convergence of the pathway with an S0/S1 conical intersection seam at intermediate values of the bond torsion. These effects are due to the action of the strongly electronegative acylimine on the twisted intermolecular charge-transfer (TICT) states that are encountered along the pathways. Our results are further evidence of the importance of TICT states and charge-transfer intersections in the control of photoisomerization processes in fluorescent protein chromophores. Indeed, bridge photoisomerization is an important internal conversion mechanism in other fluorescent proteins, as very recently reported by our studies in the anionic green fluorescent protein and kindling fluorescent protein chromophore models [54]. In this report [54], the ground and excited state electronic structures and the potential energy surfaces of two model chromophores [representing green fluorescent protein (GFP) and kindling fluorescent protein (KFP), respectively] have been characterized. The two fluorescent protein chromophores differing by a single substitution demonstrated qualitative differences in the potential energy surfaces that indicate inversion of bond selection in the photoisomerization reaction. Bond selection is also modulated by whether the reaction proceeds from a Z or an E conformation. These configurations correspond to fluorescent and non-fluorescent states of structurally characterized FPs, including some that can be reversibly switched by specific illumination regimes. We explain the difference in bond selectivity via substituent stabilization effects on a common set of charge-localized chemical structures, as different combinations of these structures give rise to both optically active (planar) and twisted intramolecular charge-transfer (TICT) states of the molecules, and offer an experimental proposal to test our hypothesis.

832 Hydrogen Bonding and Transfer in the Excited State

Theory and molecular simulation together with structural characterization have a uniquely powerful role to play in achieving insights into the mechanistic aspects that govern the function of promising FP candidates. Without this knowledge to guide design principles, the strategy for directing evolutionary approaches utilizing random mutagenesis must remain primitive – based on measured properties rather than insight. The recent determination of structure of the RFP HcRed [57] – one of only a handful of RFPs that have been structurally characterized to date – was achieved by a combination of X-ray structure and optical spectroscopy aided by our quantum chemical modeling. More recently, a remarkable enhancement in fluorescence efficiency at high pH of the weakly fluorescent RFP Rtms5 has been reported, structurally characterized and mechanistically rationalized by the same collaborative team [60]. The mechanistic picture that emerges hinges on the protonation state of the chromophore and its surrounding residues, which not only impacts the electronic structure, and consequently the absorption and emission wavelengths, but also the structural stability of its more highly fluorescent isomeric forms. Following the early quantum chemical modeling of these chromophores in our laboratory that lent support to the experimental interpretations, we have performed wholeprotein QM/MM calculations to elaborate in detail the structural properties controlling these protonation states and the isomerization propensity of the chromophore. The insights accruing from this theoretical work have significant potential for aiding future design efforts. 36.3.3 QM/MM studies in RFP Very recently we have investigated the far-red fluorescent protein HcRed using molecular dynamics (MD) and QM/MM calculations, and the preliminary outcomes of this study are summarized in Figure 36.12. Figure 36.12(a) shows the monomeric version of HcRed, embedded within a water cell in silico, which is then relaxed and equilibrated. Figure 36.12(b) shows the hydrogen bonding network surrounding the chromophore, embedded within the protein cavity of HcRed as determined by our calculations. Figures 36.12(c) and (d) show, respectively, snapshots of the two different isomeric structures of the chromophore (cis and trans) as obtained from our QM/MM molecular dynamics simulations. The results of our QM/MM calculations, implemented with density functional theory (DFT) for the QM part and the CHARMM forcefield for the MM part, demonstrate that different protonation states of glutamines (Glu214 and Glu146) nearby the chromophore are crucial in determining the stability of cis and trans isomers, with important ramifications for the fluorescent properties of HcRed [85]. In more detail, firstly, we carried out SCC-DFTB/MM MD simulations for the anionic form chromophore in which Glu214 is protonated and Glu146 is deprotonated (defined as model B). Table 36.3 presents the

Figure 36.12 (a) The far red protein HcRed in solvent, (b) hydrogen network around the cis conformation of chromophore of MD runs; hydrogen network at DFT/CHARMM level of (c) cis and (d) trans conformations in HcRed

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Table 36.3 Important dihedral angle ( ) of QM region (SCC-DFTB method) and bond distances (A) around the cis conformation of chromophore with protonated Glu214 and deprotonated Glu146 of HcRed of the average 500-ps MD production runs 



MD
a (A)

Dihedral angle ( ) N2_CA2_CB2_CG2 CA2_CB2_CG2_CD1  Bond distance (A) O_NE2(Gln107) O2_NH2(Arg93) OH_OG(Ser144) N2_OE2(Glu214) N2_NE2(Gln40)



Dm0 a (A)

6.4 6.2

Exp [57] (A)

6.4 2.2

3.054 2.676 2.856 3.447 3.231

0.0 8.4

0.035 0.514 0.255 0.481 0.048

3.019 3.190 2.601 2.966 3.183

a MD
¼ average MD values from the average 500-ps MD production runs; Dm0 ¼ standard deviations between average MD values and x-ray 1YZW pdb



important dihedral angles ( ) and bond distances (A) obtained from model B. The MD results match well with the available crystallographic data: (1) Most bonds in the QM region are well reproduced and the rootmean-square (rms) deviation between experimental bond distances and average MD data is 0.079 A. (2) For the tyrosyl moiety of the chromophore, the average dihedral angles (i.e., N2_CA2_CB2_CG2 and CA2_CB2_CG2_CD1) obtained from the average 500-ps MD runs are 6.4 and 6.2 . Smaller dihedral angles demonstrate that the trajectories of the cis chromophore during MD runs are nearly co-planar, which is consistent with the experimental observation. (3) The rms deviations of five hydrogen bonds (bond distance is measured between the two heavy atoms) around the chromophore are quite small, and are in the range  0.035–0.514 A. We also performed SCC-DFTB/MM MD simulations with the chromophores of HcRed in different protonation states for the residues of Glu214 and Glu146, which are defined as model A and C, respectively. We then performed QM/MM (DFT/MM) optimizations with the structures, starting with arbitrarily selected snapshots from the MD calculations. Table 36.4 lists the QM energies (E(QM,MM)), MM energies (E(MM,QM)), total energies (Etotal) and relative energies (DE, relative to the energy of the cis isomer) of four snapshots of cis and trans chromophore of model B (i.e., Glu214 is protonated and Glu146 is deprotonated) at the DFT/CHARMM level. The results of the E(QM,MM) of the four snapshots show that cis-conformations are always more stable than the trans-isomers. The relative QM energies (DEQ) of trans Table 36.4 QM energies, MM energies, total energies and relative energies (relative to cis conformation) of four snapshots of cis and trans isomers of model B (Glu214 is protonated and Glu146 is deprotonated) at the DFT(B3LTP/ SV(P))/MM level Snapshot 1 2 3 4

cis trans cis trans Cis trans cis trans

E(QM,MM) (a.u.)

DEQ (kcal mol1)

E(MM,QM) (a.u.)

DEM (kcal mol1)

Etotal (a.u.)

DEt (kcal mol1)

1478.94658 1478.90565 1478.95365 1478.92237 1478.94117 1478.90826 1478.95541 1478.92339

0 25.7 0 19.6 0 20.7 0 20.1

54.24006 54.26645 54.21931 54.23319 53.84091 53.85320 53.67451 53.68634

0 16.6 0 8.7 0 7.7 0 7.4

1533.18664 1533.17210 1533.17296 1533.15555 1532.78208 1532.76147 1532.62992 1532.60973

0 9.1 0 10.9 0 12.9 0 12.7

834 Hydrogen Bonding and Transfer in the Excited State

chromophore [relative to E(QM,MM) of cis conformation] in HcRed are in the range 19.6–25.7 kcal mol1. In contrast, E(MM,QM) energies of cis conformations of the corresponding four snapshots are higher than those of trans isomers, and the relative MM energies (DEM) of the trans isomers (relative to E(MM,QM) of cis conformation) are from 16.6 to 7.4 kcal mol1. Hence, the total energies DEt of cis chromophore are lower than those of trans isomer by about 9.1–12.9 kcal mol1, which means the cis conformation is more stable than the trans counterpart. Moreover, in the case of model C where both Glu214 and Glu146 are deprotonated, the cis is much more stable than the trans, by about 12.4–19.9 kcal mol1. However, in model A, where both Glu214 and Glu146 are protonated, the stability of the cis and trans chromophores of HcRed is reversed. Hence, the different protonation states of Glu214 and Glu146 nearby chromophore control the stability of cis and trans isomers, which can further influence the fluorescent properties of HcRed. The study gains insight into the experimental phenomena that some fluorescent proteins such as mKate and Rtms5 show bright fluorescence at high pH, which might be due to the deprotonation of residues near the chromophores. Therefore, it provides a simple and useful manner to tune the photochemical properties of fluorescent proteins by modifying the protonation state of existing residues near chromophores, for example, by pH-induction.

36.4 Conclusions and Future Work The proton chain transfer event in GFP is crucial to its photophysical functions, through which the neutral chromophore undergoes fast proton transfer on the excited state to yield the green fluorescent anionic chromophore. After fluorescence, the reverse proton transfers on the ground state PES regenerate the neutral chromophore. We have extensively studied structural, energetic, dynamic and spectral properties of GFP chromophores on the basis of several cluster models. Our computational investigations have revealed some mechanistic aspects in green fluorescent protein (GFP) that indicate both concerted and sequential nature for the proton motions. Despite the overall “concerted” nature of the potential profile, the configurational evolution along the reaction coordinate involves sequential movement of the protons. Moving beyond the static reaction pathway picture, we performed nuclear quantum dynamics (QD) simulations using both TI and TD methods. In particular, the time-independent quantum dynamical approach based on a minimal quantum chemical cluster model designed to represent the ground state proton transfer between the neutral and anionic states of the GFP chromophore is very encouraging. The calculated KIE value for the I ! A transition of 15.5 at 300 K is in remarkable agreement with the experimentally measured value of 12, suggesting that this quantum dynamical approach has considerable promise for future investigation of proton chain transfer kinetics in the GFP as well as other biomolecular systems. A key conclusion, which comes out of comparison of the present exact 3D quantum calculations with more simplistic models that would treat the proton coordinates as limiting harmonic or translational modes, is that these limiting approximations fail utterly to approach the experimentally measured KIE – suggesting that correct incorporation of anharmonic features of the PES is crucial to reliably model this complex proton transfer process. Despite the remarkable early success of this new TI approach to modeling biological proton wires, there remain many challenges to a more complete and general development of this strategy, which we discuss below. Very recently we have extended our quantum mechanical studies to complete full 3D potential energy surfaces for the proton transfers on the excited state in a minimal quantum mechanical cluster model for GFP. We employ the MO MRCI methods to compute the ab initio potential energy surfaces for both the ground and the first excited state. 12  12  12 ab initio data points were used to construct the potential energy surfaces, and no symmetry restriction is assumed. Comparison of the energetic calculated from current MRCI calculations using rigid model with available experimental estimations for GFP in the protein environment indicates that while the barrier heights and relative energies on both the ground and excited states are in qualitative agreement, the calculated excitation wavelength and emission wavelength are smaller than the

Theoretical Studies of Green and Red Fluorescent Proteins

835

experimental ones for GFP. These discrepancies might be due to the rigid model employed and the neglect of the protein environment effects. Currently we are exploring the excited state proton transfer nuclear dynamics through the time-independent (TI) quantum dynamics method, in which the dynamic properties such as rate constants and KIEs will be computed using the high-level 3D PESs and will be compared with the available experimental data. For red fluorescent protein (RFP) chromophores, the bridge photoisomerization pathways of the model chromophores have been characterized using electronic structure methods. Our work, carried out in collaboration with experimental studies, has revealed that the embedded chromophore of red fluorescent protein takes both cis-coplanar and trans-non-coplanar conformations due to a degree of inherent mobility that is not displayed in the GFP. The cis-coplanar conformation is suggested to exhibit bright fluorescence whereas the trans conformation is non-fluorescent, and cis–trans isomerization might play a key role for the fluorescence mechanism in RFP. Our work using truncated chromophore models has laid the theoretical groundwork and established the key mechanistic questions that need to be targeted in subsequent wholeprotein computational studies, which will seek to understand the critical role of protein environment on the fluorescent properties. Owing to the truncation of the models and the neglect of environmental considerations, cluster models need to be extended to include the protein environmental effects, and more reliable QD methods need to be developed to describe the proton nuclear dynamics in a more realistic way in the future. In this direction, some preliminary QM/MM work is currently undertaken through collaboration between our laboratory and Thiel (PI)’s group in Germany. For example, combined quantum mechanics/molecular mechanics (QM/MM) calculations are being performed to gain insight into the function of the far-red fluorescent protein (FP) and to seek an understanding of which residues within the protein influence the fluorescent properties of the embedded chromophore. In addition, reliable mixed quantum dynamics/molecular dynamics (QD/MD) methods are being explored to study the proton transfers on both the excited and ground states in green fluorescent protein. Given the lightness of proton and the corresponding quantum effects involved, these proton transfer dynamics should be considered within the framework of quantum mechanics. Thus the full description of the reaction mechanisms in GFP should combine the quantum-dynamical model for the protons with MD simulations for all other atoms. This is one direction that needs further developments.

Acknowledgements We are grateful to the Australian Research Council and The University of Queensland for supporting this work. We also acknowledge generous grants of high performance computer time from both The University of Queensland (the Computational Molecular Science cluster computing facility) and the Australian National Computational Infrastructure (NCI) Facility.

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37 Changes in Active Site Hydrogen Bonding upon Formation of the Electronically Excited State of Photoactive Yellow Protein Wouter D. Hoff,1 Zhouyang Kang,2 Masato Kumauchi,1 and Aihua Xie2 1

Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, OK 74078, USA 2 Department of Physics, Oklahoma State University, Stillwater, OK 74078, USA

37.1 Central Importance of Light in Biology Light plays three main roles in biology. First, light from the sun provides the main source of energy driving the biosphere through chlorophyll-based electron transfer in photosynthetic reaction centers and retinal-based proton pumping in rhodopsins [1]. Second, light is a source of information about the environment for many organisms using photosensory proteins, including animals, plants, fungi and bacteria [2, 3]. Third, light can cause damage to biological systems. Light from the near-UV region can damage DNA [4], and excess visible light can damage the photosynthetic machinery in photoinhibition [5]. Thus, light is of great importance for a wide range of biological processes. Proteins that perform photosynthesis or light sensing consist of an apoprotein and a bound chromophore. Only a small number of chromophores account for most known processes in photobiology: chlorophyll and linear tetrapyrroles, caretoids and retinal, flavins and p-coumaric acid [2]. In many cases the chromophore has strong and functionally important interactions with the protein binding pocket, particularly charge–charge interactions and hydrogen bonding interactions. While many different proteins interact with light in a wide range of organisms, almost all of these systems are based on key types of photochemical processes, including electron transfer, C¼C double bond isomerization and the formation of chemical bonds [2, 6]. These photochemical events often trigger a cascade of thermal reactions in the protein that extends to the millisecond and minute range, and that result in a biologically relevant output. If the cascade of thermal reactions results in the re-formation of the initial state, the process is referred to as a photocycle. It is the initial ultrafast

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

840 Hydrogen Bonding and Transfer in the Excited State

photochemical event that drives the subsequent thermal events in a photocycle through strong chromophore– protein interactions. In this chapter we examine current understanding of ultrafast changes in hydrogen bonding upon photoexcitation of photoactive yellow protein (PYP), a bacterial photosensor.

37.2 Possible Importance of Excited State Hydrogen Bonding in Photoreceptors In many photosensory proteins the primary photochemical event involves isomerization of double and single bonds. These isomerization events can greatly alter hydrogen bonding interactions between the chromophore and protein binding pocket, including the disruption of hydrogen bonds and the formation of new hydrogen bonds. These changes in hydrogen bonding can have important consequences for subsequent functionally important conformational changes in the protein. Evidence for functionally important changes in hydrogen bonding during the initial ultrafast transitions of the photocycle have been reported not only for PYP (see below), but also for other photoreceptors, such as the BLUF domain [7]. In addition, it is possible that hydrogen bonding in the electronically excited state is important in guiding the initial steps of the photocycle. Such functional effects of excited state hydrogen bonding in photosensory proteins have only just been started to be explored, and raise the question as to how these hydrogen bonds are altered in the electronically excited state and how this affects [8] the earliest steps in the function of the protein. Here we review knowledge regarding hydrogen bonding in the electronically excited chromophore in the active site of PYP.

37.3 Introduction to Photoactive Yellow Protein Photoactive yellow protein [9] is a bacterial blue-light receptor. It is a highly accessible model system that exhibits proton transfer between active site groups with shifted pKa values during its light-triggered functional cycle [9, 10]. The PYP from Halorhodospira halophila (Hh PYP) is a highly studied [11–13] member of a growing family of bacterial blue-light receptors [14]. A rich body of information is available on functionally important active site hydrogen bonds and proton transfer events in the PYP from H. halophila, providing an attractive and tractable system to examine these ubiquitous and central aspects of protein function. PYP was first discovered in the extremely halophilic anoxygenic photosynthetic proteobacterium H. halophila [9, 15], where it functions as a photoreceptor for negative phototaxis [16]. It exhibits a light-triggered photocycle [9, 17–19] based on its p-coumaric acid (pCA) chromophore [20, 21]. The pCA is covalently linked to Cys69 in the protein via a thioester bond (Figure 37.1) [22, 23]. Three key structural features of the pCA are that its phenolic oxygen can undergo protonation/deprotonation, its central C7 ¼ C8 double bond can undergo photoisomerization, and both its phenolic and carbonyl oxygen can form hydrogen bonds. In the initial state of PYP the pCA is deprotonated and in the trans configuration. The crystal structure of PYP consists of a central antiparallel six-stranded b-sheet flanked by five a-helices (Figure 37.1a) [24]. PYP shares its fold with a large superfamily of proteins called the PAS domain [25–27]. Absorption of a blue photon by the pCA chromophore embedded in the protein initiates the light sensing process of PYP by chromophore trans to cis photoisomerization [10, 28–31]. This ultrafast isomerization reaction converts the initial pG state into the redshifted pR state via a complex series of ultrafast processes. The initial steps of the PYP photocycle (Figure 37.2) have been studied extensively by sub-ps time-resolved spectroscopy [32], and will be discussed below. The pR state thermally decays to the blue-shifted pB0 intermediate in a sub-millisecond process that involves protonation of the pCA chromophore [21] by proton transfer from Glu46 to the pCA [10, 33, 34]. Large conformational changes [33, 35–43] convert the pB0 state into the pB state on the millisecond time scale. The destabilizing buried negative charge on Glu46 has been identified as a key factor in driving these conformational changes [33, 44]. The large structural changes that

Changes in Active Site Hydrogen Bonding upon Formation of the Electronically Excited

841

Figure 37.1 Crystal structure of PYP. (a) Overall fold of PYP showing the pCA, Glu46, Tyr42 and Cys69 (BDP ID 1NZW). (b) Active site of PYP with three key hydrogen bonds. (c) The thioester linked pCA chromophore in PYP with numbering of its atoms

occur upon pB formation can be described as a protein quake driven by the electrostatic epicenter formed by the negative charge of Glu46 [33]. The pB state is long-lived and is thought to be the active signaling state of PYP that interacts with an as yet unidentified downstream signaling partner. It thermally recovers to the initial pG state in a few hundred milliseconds.

37.4 Hydrogen Bonding in the Initial State of PYP Extensive crystallographic studies have shown that the phenolic oxygen of the negatively charged pCA chromophore of PYP interacts with active site residues Glu46 and Tyr42 via a forked ionic hydrogen bond. The

842 Hydrogen Bonding and Transfer in the Excited State

Figure 37.2 Schematic representation of the PYP photocycle. The pCA isomerization state in the various intermediates is indicated as superscripts and their absorbance maxima as subscripts. The wavy line indicates a photochemical reaction, straight lines indicate thermal transitions. The structure of the pCA and its hydrogen bonds with Tyr42, Glu46 and Cys69 in the intermediates are indicated schematically

side chains of Glu46 and Tyr42 form unusually short hydrogen bonds with the pCA (Figure 37.1b): 2.59 and  2.50 A, respectively [45]. For comparison, the average length of hydrogen bonds in proteins involving O and N  atoms is 3.0 A [46]. Recently the pCA–Glu46 interaction was identified as a low-barrier hydrogen bond based on neutron crystallography [47]. However, an independent neutron diffraction study did not reveal this, and discussed the partial occupancy of deuterions in the hydrogen bonding network at the phenolic oxygen of the pCA [48]. Direct spectroscopic evidence for hydrogen bonding between Glu46 and the pCA has been obtained from FTIR studies (see below). The C¼O stretching mode of the side chain of Glu46 in Hh PYP is at 1736 cm1 in H2O [10]. This frequency corresponds to a single strong hydrogen bond to the Glu46 side chain [49]. In D2O this mode is downshifted by 10 cm1 to 1726 cm1 [10], a typical shift for H/D exchange of a protonated carboxylic acid. Several studies have shown that the hydrogen bond between Glu46 and the pCA is of great functional importance. First, this hydrogen bond is an important contributor to the strong shifts in the pKa values of both the pCA chromophore and Glu46 in PYP compared to their values in solution. The pKa of the thioester-linked pCA is downshifted from 8.8 in solution [50] to 2.8 in PYP [15, 51], while the pKa of Glu46 is strongly upshifted from 4.25 in solution [52] to >10 in PYP. Owing to this “pKa inversion” the ionized [28] pCA (normally a base) is hydrogen bonded to the protonated [10, 33] side chain of Glu46 (normally an acid). Mutations in both Glu46 and Tyr42 significantly shift the pKa of the pCA in the direction of its value in solution [53–55]. This indicates the importance of the hydrogen bonds of Glu46 and Tyr42 to the pCA in downshifting its pKa. The pKa inversion of these Glu46 and the pCA in PYP is of central functional importance since proton transfer between these two groups is a key event during the light-triggered photocycle in PYP [10, 33]. A second functionally important aspect of active site hydrogen bonding in PYP is that the strength of the hydrogen bond between Glu46 and the pCA is a main factor in regulating the absorbance spectrum of PYP. Deprotonated thioester-linked pCA absorbs near 400 nm in solution [50], which is strongly blue-shifted from the absorbance maximum of PYP at 446 nm. Interactions between the pCA and its protein binding pocket cause a 46 nm redshift in its absorbance maximum. In the E46Q mutant, in which the hydrogen bond between  the pCA and residue 46 is weakened and lengthened by 0.3 A [45, 56] the absorbance maximum is redshifted by 16 nm [57, 58], presumably by the reduced localization of p-electrons by the hydrogen bonding interaction.

Changes in Active Site Hydrogen Bonding upon Formation of the Electronically Excited

843

These results illustrate the functionally central role of the hydrogen bond between Glu46 and the pCA chromophore in PYP.

37.5 Assignment of Vibrational Modes in PYP The molecular events that occur during the PYP photocycle have been studied with both resonance Raman and infrared difference spectroscopy. Vibrational signals originating from the pCA chromophore can be identified by resonance Raman spectroscopy; infrared spectra contain contributions from both the protein and the chromophore. In principle such vibrational spectra contain rich information on changes in structure, protonation state and hydrogen bonding interactions during the PYP photocycle. A key first step in extracting such information from the spectra is the assignment of specific vibrational modes to molecular groups in PYP. Three strategies have been used for the assignment of the vibrational modes of the pCA chromophore in PYP. First, the vibrational spectra of model compounds of the pCA chromophore, such as the p-coumaryl phenyl thioester [28], have been studied and compared to the vibrational spectra of PYP. Studies of these model compounds in different states, particularly with a neutral or ionized phenolic oxygen or in the trans and cis states, have yielded valuable information on which vibrational modes are sensitive to which molecular events in the PYP photocycle [28, 59]. Second, computational methods for calculating the vibrational spectrum of molecules are now sufficiently accurate to be a highly valuable tool for assigning vibrational modes, and this strategy has been applied to PYP [60–62]. In such calculations it is critical to have sufficient experimental data to determine if the calculations are reliable. No vibrational calculations on pCA in the electronically excited state have been reported yet. Third, various derivatives of pCA labeled with stable isotopes have been incorporated into PYP [59, 60, 62–64]. This can be a highly valuable tool for the assignment of vibrational signals in PYP to specific vibrational modes in the pCA chromophore. In this approach it is critical to design isotopically labeled pCA derivatives that affect only a small number of vibrational modes. This is a significant issue, since some isotopic substitutions affect many vibrational modes, defeating the original purpose of the isotopic labeling. The C¼O stretching mode of functionally important active site residue Glu46 in light-induced FTIR difference spectra was assigned based on (i) the unusual protonation state of this residue, which is the sole carboxylic acid in PYP that is not exposed to solvent, and (ii) the sensitivity of this signal to pCA photoisomerization at 80 K, indicating its immediate vicinity to the pCA [10]. Subsequent FTIR studies on the E46Q mutant of PYP confirmed this assignment [34]. This still remains the only amino acid side chain for which a firm assignment of signals in FTIR difference spectra of PYP is available. Extensive assignments have been performed for the vibrational modes of the pCA as observed by resonance Raman spectroscopy. These assignments are particularly well established for the pG state [62] and the longlived pB state [63, 64]. Some key pCA vibrational bands in the pR state have also been studied [60]. These assignments were based on strategically chosen isotopic labeling of the pCA in combination with high-level ab initio vibrational calculations. Signals that were found to be structurally most informative (see below) are the C¼O stretching mode at 1631 cm1, the C8–C9 stretching mode at 1052 cm1 and the in-plane pCA ring CH bending mode at 1164 cm1 (stated values are for the pG state in H2O). In FTIR difference spectra a strong and structurally informative band at 1498 cm1 has been assigned to a pCA ring C¼C stretching vibration [33].

37.6 Identification of Vibrational Structural Markers The interpretation of vibrational signals in terms of molecular structure requires the identification of modes that serve as reliable indicators of specific molecular processes. A reliable vibrational structural marker is a vibrational mode whose frequency depends only (or largely) on a single structural feature, for example,

844 Hydrogen Bonding and Transfer in the Excited State Table 37.1

Vibrational structural markers for structural changes during the PYP photocycle. All values are in cm1

Structural change pCA trans to cis isomerization pCA protonation from O to OH pCA O C¼O loss of hydrogen bonding pCA OH C¼O loss of hydrogen bonding Glu46 loss of hydrogen bonding Glu46 deprotonation

Mode

Before event

After event

C8–C9 stretching ring CH bending ring C¼C stretching C¼O stretching C¼O stretching COOH C¼O stretching COO(H) C¼O stretching

1050 1165 1498 1640 1665 1736 (1726)a 1736 (1726)a

1000 1174 1515 1673 1692 1765 (1755)a <1650

a

Values in brackets were measured in D2O

hydrogen bonding to the C¼O group of the pCA or the isomerization state of the central C¼C group of the pCA. The structural interpretation of vibrational modes that are altered by multiple effects (in the case of the pCA in PYP particularly protonation, isomerization and hydrogen bonding) is more complicated and less reliable. In FTIR difference spectra negative bands correspond to vibrational modes in the pG dark state that have disappeared because of photoexcitation. Positive signals are caused by the appearance of new vibrational modes in photoproducts. Thus, information on the structural properties of photocycle intermediates should be obtained from positive vibrational signals. Well-established vibrational markers are available for several functionally important molecular events in PYP (Table 37.1). First, the isomerization state of the central C¼C group of the pCA can be determined based on the shift of the pCA C8–C9 stretching mode from 1052 cm1 for trans-pCA to 1001 cm1 for cis-pCA [60]. This mode is clearly visible in both resonance Raman and FTIR spectra. A signal at 1301 cm1 has also been proposed to serve as a marker for pCA trans to cis isomerization [59]. However, subsequent in-depth computational and isotopic labeling studies indicate that this mode is not as reliable for this purpose [60, 62, 64]. Second, protonation of the pCA can be probed by both the upshift in the pCA ring CH bending mode from 1164 to 1174 cm1 [60] and by the upshift in the pCA ring C¼C stretching mode from 1498 to 1515 cm1 [33]. Third, the C¼O stretching mode of the pCA can be used to examine hydrogen bonding to this oxygen. For ionized pCA the C¼O stretching mode shifts from 1640 to 1670 cm1 upon loss of hydrogen bonding [60]. In general, structural interpretation based on this mode should be performed with care, because it is also sensitive to pCA isomerization and protonation state [60]. In resonance Raman spectra this mode is weak but clearly present; in the case of infrared absorbance spectra the identification of this mode is not straightforward, since it overlaps with contributions from other vibrational modes in PYP. Finally, the C¼O stretching mode of Glu46 at 1736 cm1 (in the pG state in H2O) serves both as a marker for the protonation state of this residue and for its hydrogen bonding strength [49]. Its deprotonation causes a strong downshift in frequency, to below 1650 cm1. The position of this mode in the 1770–1700 cm1 spectral window can be reliably used to determine if the COOH group is involved in two, one or zero hydrogen bonding interactions [49]. This signal is unmistakable in infrared spectra since Glu46 is the only protonated acidic side chain in PYP at pH values above 5. The Glu46 C¼O stretching mode is the only vibrational structural marker in this set that is sensitive to H/D exchange.

37.7 Changes in Hydrogen Bonding During the Initial Stages of the PYP Photocycle The first insights into changes in hydrogen bonding during the PYP photocycle were obtained from cryogenic and steady-state FTIR difference spectroscopy [10]. At 80 K an early redshifted photointermediate is

Changes in Active Site Hydrogen Bonding upon Formation of the Electronically Excited

845

trapped [65]. The light-induced FTIR difference spectrum at 80 K contains clear signals in the C¼O stretching region of protonated carboxylic acids (1700–1770 cm1). A frequency upshift from 1740 to 1732 cm1 was observed upon photoexcitation at 80 K, and assigned to the side chain of Glu46 [10]. This upshift corresponds to the strengthening of a single hydrogen bond to the Glu46 side chain. Considerations regarding the threedimensional structure of the pCA binding pocked showed that these data imply that the hydrogen bond between Glu46 and the pCA that is present in the initial pG ground state remains preserved in the primary photoproduct. This is only possible if C¼C trans to cis isomerization of the pCA is achieved by a flip in the position of the pCA C¼O group that leaves the position of the phenolic ring of the pCA largely unchanged (Figure 37.3). This primary event involves both single and double bond rotations, which convert the initial 7-trans s-cis pCA into its 7-cis s-cis configuration in the primary photoproduct. This model, first proposed and experimentally supported in 1996 [10], was subsequently confirmed by both X-ray crystallographic studies of cryotrapped early intermediates [30, 31] and a range of spectroscopic studies (discussed below). Timeresolved step-scan FTIR spectroscopy revealed that also at room temperature the Glu46–pCA hydrogen bond remains intact during the first part of the photocycle [33, 66]. This conclusion is of great importance for understanding the molecular mechanism of the PYP photocycle, since proton transfer from Glu46 to the pCA chromophore has been identified as a key step in receptor activation in PYP [33, 44]. Such proton transfer requires the presence of a hydrogen bond between the proton donor (Glu46) and proton acceptor (pCA). Resonance Raman data show that in the pR intermediate the pCA chromophore is indeed in the cis conformation and that it remains ionized [60, 61]. In addition, the Raman data revealed an upshift in the pCA C¼O stretching mode from 1631 to 1666 cm1, which is fully consistent with the loss of hydrogen bonding of this carbonyl group by its rotation upon pCA isomerization [60, 61]. The quenching of fluorescence emission from the single Trp in PYP (Trp119) by the pCA chromophore has also been used as a probe to examine changes in chromophore orientation. This study also concluded that in the pR intermediate the chromophore ring does not undergo significant changes in its position [67].

Figure 37.3 Active site structural changes during the formation of the primary photoproduct of PYP. The active site containing the pCA and its hydrogen bonding interactions is shown in the ground state and the primary photoproduct (PDB ID 1OT9)

846 Hydrogen Bonding and Transfer in the Excited State

37.8 Sub-Picosecond Time-Resolved Transient Spectroscopy of PYP The earliest steps in the PYP photocycle have been probed by sub-picosecond visible pump–infrared probe spectroscopic methods [68–70]. To derive information on the structure of specific photocycle intermediates from such time-resolved data, it is essential to separate the contributions from different photocycle intermediates. Since multiple photoproducts can be populated and depopulated in parallel, such deconvolution of the data in general is complex. Notably, different nomenclatures are used in literature for the various PYP photocycle intermediates. Here we use the nomenclature indicated in Figure 37.2. Independent data on the decay kinetics of the electronically excited state of PYP are provided by fluorescence upconversion studies [71–73], which contain information on the lifetime of the electronically excited state without contributions from other photocycle intermediates. These studies have shown that the decay of the excited state of PYP is multiphasic, with 50% of the population decaying with a time constant of 0.5 ps and 30% with a time constant of 4 ps. The remaining 20% of the excited state decays much more slowly (50 ps). These measurements also revealed a clear dependence of the kinetics on detection wavelength: decay in the blue and red flanks of the fluorescence emission band is faster than at the fluorescence maximum. This ultrafast narrowing of the fluorescence emission band [74–76] indicates the possibility that significant vibrational cooling of the electronically excited occurs during the first 3 ps of the PYP photocycle. Interestingly, biphasic kinetics for the change in the width of the fluorescence emission band were observed, with time constants of 0.2 and 1.5 ps [76]. Analysis of the temperature dependence of the three kinetic components in the fluorescence decay yields activation energies of 0, 1.9 and 6.8 kcal mol1, indicating that the fastest transition is barrierless [77]. An additional contribution to the ultrafast fluorescence emission data arises from oscillations that originate from vibrational modes of 100 cm1 and that extend 1.5 ps into the photocycle [75, 76, 78–80]. This time scale is similar to the range of values reported for amide I modes in the electronic ground state of myoglobin [81]. The oscillations at the red and blue edges of the PYP emission spectrum have opposite phases. Remarkably, the oscillatory contribution to the kinetics of fluorescence intensity decay kinetics is restricted to discrete spectral regions of the fluorescence emission band [76]. Visible pump–probe spectroscopic data are in line with a multiphasic decay of the excited state, with time constants of 0.7 and 4 ps for the decay of the excited state [82–86]. Table 37.2 lists the time constants obtained by different spectroscopic approaches. While not all studies reveal identical rates, processes of 0.6 and 5 ps emerge as the most central events on the ultrafast time scale. However, a generally accepted kinetic model for the initial stages of the photocycle has not yet been obtained. The slight differences between the visible and infrared data may be real, and hold valuable information on the processes that occur during the ultrafast events that initiate light sensing in PYP. One possible complicating factor is that excitation in the blue flank of PYP can result in a pCA ionization process in which an electron is ejected from the chromophore into the protein binding pocket [82]. This process (lexc ¼ 395 nm) occurs in parallel with the pCA isomerization reaction, exhibits a strong power dependence and was not observed upon excitation at 470 nm [82]. Thus, experimental studies using excitation wavelengths near 400 nm in combination with high peak powers and small excitation volumes are prone to contain contributions from pCA ionization that will complicate the detection of the functionally important pCA isomerization pathway. The multiphasic decay of the excited state in transient IR measurements could be caused either by distinct intermediates or intramolecular vibrational energy redistribution and vibrational cooling [69]. Such vibrational relaxation processes could result in time-dependent changes in the shape of the spectrum of the state undergoing vibrational cooling, complicating mathematical analysis of the data [69]. In summary, the structural basis for the multiphasic decay of the excited state in PYP is currently not known. A similar situation exists regarding the decay of the electronically excited state of the light-driven proton pump bacteriorhodopsin [87]. One possibility is structural heterogeneity in the electronically excited state [32]. Such excited state heterogeneity has been deduced from polarized visible pump–probe spectroscopy [83] and from

Changes in Active Site Hydrogen Bonding upon Formation of the Electronically Excited Table 37.2 shown Ref.

847

Time constants for the initial stages of the PYP photocycle. Only time constants shorter than 1 ns are

t1 (ps)

Fluorescence emission [91] 11.7 0.5–0.9(46–58%) [71]b [73]b 0.4–1.0(40–60%) [74] 0.7 [75]b 0.46–0.54(38–43) [72] 0.1 and 0.75 [76]b 0.18–0.45(15–44%) Visible pump–probe [86] 0.7 [92] — [85] 0.8 [84] 0.95 [83] 0.7 [93] 1.1d Visible pump–dump–probe [82] 0.6 Visible pump–infrared probe [68] 2 [69] 3 [70] 1.2 UV resonance Raman [90] —

t2 (ps)

t3 (ps)

Resolution (ps)

lexc (nm)

S/Na

63 3.6–10(25–43%) 3.1–4.3(26–35%) 5 3.1–4.8(46–54%) 3.3 2.0–2.5(48–65%)

60 >60(5–19%) 38–69(10–25%) — — 40 —

426 0.22 0.15 0.3 0.11 0.19 0.18

1300 440 420 410 410 400 400

50 11 25 65 25 75

3.6 — — 7.1 6.3 3.6

— 220 — 1300c 220 18

0.2 3–6 0.1 0.17 0.1 1.2

400/464 452 395 390 400/485 428

15 22 15 12 70 75

2.8

40

0.12

395

270

9 — 6.2

950c — 700c

0.18 0.23 0.2

475 445 475

30 60 60

8

—

2.8

446/460

11

a

Signal to noise ratios were estimated from figures in cited references. Values depend on detection wavelength, with faster rates observed at the blue and red flanks of the emission band. These values indicate the kinetics of pR formation from I0. d The somewhat slower rate of this process compared to the 0.7 ps component observed in other studies may be due to the time resolution of this experiment. A fourth time constant of 463 ps was also observed in this study b c

X-ray crystallography of the cryotrapped photoproduct state [30, 31]. However, the small wavelength dependence for the kinetics of dump pulse-induced fluorescence intensity depletion observed by ultrafast fluorescence pump–dump spectroscopic measurements indicates that the electronically excited state is homogeneous with respect to emission transition energy [76]. Since only part of the electronically excited state is converted into the primary photoproduct, a parallel pathway is the decay of the excited state back to the initial pG dark state. The quantum yield of the forward reaction was initially estimated as 0.64 [35], but a later study adjusted this value down to 0.35 [88]. More recent ultrafast pump–dump spectroscopy revealed that the pathway from the excited state to the pG state proceeds via a ground state intermediate (GSI), which decays with a time constant of 3.6 ps [82]. Another complicating factor is that the initial photoproduct has been reported to partly decay back to the initial pG state in a thermal branching reaction [70, 82] (Figure 37.2). The formation of the primary photoproduct containing cis-pCA has been reported to occur on a time scale of 2 ps [68], mainly based on the photoproduct signal at 1289 cm1 [59]. A more recent study used the wellestablished cis-pCA isomerization marker [60] at 1000 cm1 to derive a time constant for the formation of the cis photoproduct as 3  1 ps [69]. The decay of the Glu46 signal at 1747 cm1 follows the same kinetics. The finding that the time constant for the formation of the cis-pCA electronic ground state primary

848 Hydrogen Bonding and Transfer in the Excited State

photoproduct appears to follow the slower 4 ps phase in excited state decay matches an interpretation in which the 0.5 ps time constant for the excited state is caused by its vibrational cooling, not by its decay to the primary photoproduct. In this interpretation the 0.5 ps process is barrierless transition on the excited state energy surface, while the 4 ps process is the conversion of the electronically excited state into the primary photoproduct that proceeds over a barrier on the excited state energy surface of 1.9 kcal mol1 [77]. The occurrence of ultrafast double bond isomerization of the pCA was also concluded based on changes in visible anisotropy, indicating a 24 degree change in transition dipole moment upon the formation of the primary photoproduct [83].

37.9 Changes in Active Site Hydrogen Bonding upon the Formation of the S1 State of PYP

963

1069 1041

1163

1498

1631 1755 1747

0 1438

1740 1725

-2

-4

1800

1557

Amplitude (mOD)

2

1288

Since the fastest time constant in the reported sub-ps infrared spectroscopic studies is a 2 ps time constant associated with the decay of the excited state, positive signals in the infrared difference spectra taken at sub-picosecond time delays are dominated by contributions from the electronically excited state. Figure 37.4 compares the calculated species associated difference spectrum for the formation of the electronically excited state measured in H2O [68] with the difference spectrum at 0.4 ps time delay taken under polarization anisotropy free conditions measured in D2O [69]. For comparison, the steady state resonance Raman spectrum of the pG dark state in H2O is shown to aid the identification of pG state pCA

1600

1400

1200

1000

Wavelength (cm-1)

Figure 37.4 Ultrafast mid-infrared difference spectroscopy for formation of the electronically excited state of PYP. The species-associated difference spectrum for formation of the electronically excited state of PYP in H2O (blue; from Ref. [68]) is shown together with the transient absorbance difference spectrum at 0.4 ps time delay for PYP in D2O obtained under polarization anisotropy free conditions (black, from Ref. [69]). For comparison, the steady state resonance Raman spectrum of the pG dark state of PYP is also shown (green; from Ref. [62]). Solid vertical lines indicate vibrational modes in the pG state that correspond well to negative signals in the transient infrared absorbance difference spectra; dotted vertical lines indicate modes where the match between the Raman and infrared data is less good. For the infrared difference spectra the vertical scale indicates the amplitude of the signals in milliOD

Changes in Active Site Hydrogen Bonding upon Formation of the Electronically Excited

849

signals in the infrared spectra. Below, these two FTIR difference spectra will be interpreted using the assumption that they represent the difference spectrum for the formation of the excited state from the pG ground state. A highly informative pair of positive and negative signals is observed at 1740/1755 cm1 in H2O and 1725/ 1747 cm1 on D2O (Figure 37.4). Based on steady state difference spectra with a high signal to noise ratio [10] the negative peaks corresponding to the pG state would have been expected at 1736 and 1726 cm1 in H2O and D2O, respectively. These data reveal that the side chain of Glu46 in the electronically excited state of PYP is located at 1755 and 1747 cm1 in H2O and D2O, respectively. Thus, the data show that the C¼O stretching mode of the side chain of Glu46 in the excited state of PYP exhibits a typical downshift upon H/D exchange. In general a very good correlation exists between the frequency of the C¼O stretching frequency of protonated acidic side chains and their hydrogen bonding strength [49]. Since the side chain of Glu46 remains in the electronic ground state, this published vibrational spectral marker can be applied to the signals in the electronically excited state of PYP. The value of 1736 cm1 in the pG state indicates the presence of a fairly strong single hydrogen bond, while the value of 1755 cm1 indicates either the absence of hydrogen bonding or a very weak single hydrogen bond. The above correlation between C¼O stretching frequency and hydrogen bonding applies to steady state conformations of proteins. In the case of functional intermediates the correlation is less strong, presumably because of structural perturbations [49]. It has been proposed that a large change in charge distribution in the electronically excited state of the pCA is a major factor in causing the upshift in Glu46 C¼O stretching frequency in the excited state, particularly because a frequency downshift in this mode occurs upon the formation of the primary photoproduct [68, 69]. A large change in permanent dipole moment upon formation of the electronically excited state of PYP has been detected by Stark spectroscopy [89]. Its value of 26 debye  corresponds to the movement of one electron over 5 A, and has been attributed to a displacement of the negative charge on the phenolic oxygen of the pCA towards the thioester bond [89]. However, the extent of the change in Glu46 C¼O stretching mode expected to be caused by this change in charge distribution has not been quantitatively discussed. An additional argument in favor of the continued presence of the Glu46-pCA hydrogen bond in the excited state was obtained from ultrafast polarized infrared measurements, indicating that the orientation of the C¼O group of Glu46 remains the same in the excited state [69]. These data confirm that the phenolic ring of the chromophore does not undergo significant structural changes, but that it is the pCA C¼O group that rotates upon photoexcitation. A second structurally informative vibrational signal is the pCA C¼O stretching mode, which undergoes a 35 cm1 upshift upon disruption of the pCA-Cys69 hydrogen bond [60, 61]. In the primary photoproduct this mode is located at 1663 cm1 in H2O [68] and at 1669 cm1 in D2O [69]. This is clearly upshifted from its position at 1631 cm1 in the pG state, providing direct evidence for the disruption of the pCA–Cys69 hydrogen bond in the primary photoproduct. As explained below, the interpretation of the pCA C¼O stretching in the electronically excited state is not as straightforward. The absence of a strong positive band near 1665 cm1 in the electronically excited state of PYP has been proposed as evidence that the hydrogen bond between the pCA C¼O and Cys69 remains intact in the excited state [70]. The absence of this same band in the ground state intermediate indicates that in PYP molecules that are unsuccessful in entering the PYP photocycle the hydrogen bond between the pCA C¼O and Cys69 remains intact [70]. Thus, the disruption of this hydrogen bond has been proposed to be a key step for the successful entry into the PYP photocycle. A signal at 1640 cm1 has been assigned to the C¼O stretching mode of the electronically excited state, and has been presented as evidence for the presence of the pCA C¼O–Cys69 NH hydrogen bond [70]. The rotation of the pCA C¼O group is a key reaction coordinate during the initial stage of the photocycle, and causes the disruption of this hydrogen bond in the primary photoproduct. Thus, the status of this hydrogen bond in the electronically excited state is of considerable interest. The above assignment depends on several assumptions. First, the signal at 1640 cm1 is present in the difference spectrum between the electronically excited state and

850 Hydrogen Bonding and Transfer in the Excited State

the ground state that was extracted from the time-resolved data by mathematical modeling. Since the calculation of this species-associated difference spectrum is model-dependent and the correct kinetic model for the initial steps in the PYP photocycle has not yet been firmly established, this introduces an additional uncertainty. In addition, since an electron is excited from a bonding p orbital to an anti-bonding p orbital of the pCA upon formation of the electronically excited state, the location of the C¼O stretching mode may be significantly altered. Its definitive identification could be performed experimentally using appropriate isotopic labeling of the pCA. A third issue regards the structural interpretation of the frequency of the C¼O stretching mode in the electronic excited state. The reported interpretation of this mode in terms of hydrogen bonding [60] applies to the electronic ground state. High-level calculations are required to establish the frequency of this mode in the electronically excited state with and without hydrogen bonding. This can yield definitive information regarding the status of the hydrogen bond between the pCA C¼O and the Cys69 NH group in the electronically excited state. Two additional observations can be made based on Figure 37.4. First, the resonance Raman signals for the pG state at 1557, 1438, 1163, 1068, 1041 and 963 cm1 closely match negative signals in the sub-picosecond time-resolved infrared data. This is strong confirmation of the high experimental accuracy of the time-resolved data. However, three negative signals in the infrared spectra do not match well with the pG resonance Raman spectrum. These are located at 1631, 1498 and 1288 cm1. These discrepancies may be due to overlap with signals originating from vibrations in the protein. The resonance Raman signal at 1631 cm1 of the C¼O stretching mode of the pCA [60] is a key part of deriving changes in pCA hydrogen bonding during the pCA photocycle. The slight mismatch between the resonance Raman signal and the negative signal in the infrared difference spectra increases the need for assignment of this important mode by isotopic labeling of the pCA. Similarly, it is not clear from the difference spectra in Figure 37.4 if the C¼O stretching signal is slightly upshifted or downshifted in the electronically excited state. A second observation is that some infrared difference signals show a clear sensitivity to H/D exchange. Even though the pCA chromophore in the pG state does not contain exchangeable protons, the vibrational spectrum of the pCA in PYP is altered upon H/D exchange. The most striking change is that in D2O the doublet of signals at 1059 and 1043 cm1 merges into a single band at 1059 cm1 [28, 62]. Presumably, this effect is caused by vibrational coupling between the pCA chromophore and H/D exchanging groups in the pCA binding pocket, particularly the side chains of Tyr42 and Glu46, and the amide backbone of Cys69. Inspection of Figure 37.4 reveals that a strong negative signal at 1490 cm1 in H2O is shifted to 1510 cm1 in D2O. The direction of this 20 cm1 shift is opposite from what one would expect for H/D exchange, and has not been interpreted. Vibrational signals for probing the third hydrogen bond of the pCA chromophore (to Tyr42) have not yet been identified in infrared difference spectra. However, a recent picosecond time-resolved ultraviolet resonance Raman spectroscopic study reported Tyr signals that were tentatively assigned to Tyr42 [90]. The loss of intensity of a Tyr ring vibration near 1605 cm1 was interpreted to indicate the ultrafast strengthening of the hydrogen bond between Tyr42 and the pCA in the electronically excited state of PYP, and the subsequent slight weakening of this hydrogen bond with an 8 ps time constant [90]. Identification of this signal in ultrafast infrared difference spectra of PYP (Figure 37.4) will likely require isotope editing.

37.10 Excited State Proton Transfer in the Y42F Mutant of PYP Very recently it was reported that the Y42F mutant of PYP exhibits excited state proton transfer [94]. In this mutant the hydrogen bond between active site residue 42 and the pCA (Figure 37.1) is disrupted, while that between Glu46 and the pCA is strengthened. This results in a novel spectroscopic species that is in thermal equilibrium with the pG state. This species absorbs near 390 nm, and was concluded to contain a protonated

Changes in Active Site Hydrogen Bonding upon Formation of the Electronically Excited

851

pCA. Since a very similar photocycle is observed upon excitation of the pG state with an ionized pCA and the 390 nm species with a protonated pCA, it was concluded that photoexcitation of the 390 nm species most likely results in excited state proton transfer [94]. This is in contrast to the situation in wild-type PYP, where photoexcitation triggers pCA photoisomerization, followed by much slower proton transfer on the electronic ground state energy surface. In summary, a range of sub-ps spectroscopic studies have been performed on the electronically excited state of PYP and the initial stages of the PYP photocycle. These studies have fully confirmed a model of PYP isomerization that involves the rotation of the pCA C¼O group, while the phenolic group of the pCA does not significantly move [10]. The data indicate that in the excited state all three hydrogen bonds of the pCA remain intact, while disruption of the pCA C¼O–Cys69 hydrogen bond is a key step for successful photocycle entry. Further studies will be needed to confirm and extend this model and to determine the origin of the multiphasic decay of the electronically excited state. In addition, ultrafast spectroscopic studies on PYP mutants promise to uncover rich information on how the protein binding pocket tunes events in the electronically excited state of the pCA chromophore.

Acknowledgements W. D. H. gratefully acknowledges support from NIH grant GM063805 and OCAST grant HR07-135S, and from startup funds provided by Oklahoma State University. A. X. was supported by funds from OCAST (HR02-137R) and from the Oklahoma State Regents for Higher Education. The authors thank Marie Louise Groot and Masashi Unno for providing ASCI data of their vibrational measurements on PYP, and Delmar Larsen for helpful discussions.

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38 Excited State Dynamics in the Light-Driven Enzyme Protochlorophyllide Oxidoreductase (POR) Marie Louise Groot1 and Derren James Heyes2 1

Department of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands 2 Manchester Interdisciplinary Biocentre, University of Manchester, 131 Princess Street, Manchester M1 7DN, UK

38.1 Introduction Enzymes can catalyse chemical reactions with rate enhancements of up to 1017 compared to the equivalent reaction in solution [1]. However, understanding exactly how enzymes achieve their huge catalytic power remains challenging [1–3] as catalysis is generally limited by diffusion-associated processes, such as the binding of substrates and cofactors, or by conformational changes of the enzyme. Hence, for most enzymes that require mixing strategies to initiate the reaction it is impossible to directly study the real time formation of catalytic intermediates. However, this possibility is afforded in light-driven enzymes, such as photosynthetic reaction centres [4–6], DNA photolyase [7, 8] and protochlorophyllide oxidoreductase [9–11], where the reaction can be initiated with a trigger that is more rapid (i.e. a laser pulse) than the fastest dynamics involved. Reaction centres are transmembrane pigment–protein complexes that are responsible for catalysing lightdriven charge separation and represent one of only a very few systems where electron transfer between redox centres can be monitored with femtosecond time resolution. The initial steps in the electron-transfer reaction take place on a picosecond timescale, leading eventually to the two-electron reduction of a bound quinone molecule on a millisecond timescale, after the subsequent absorption of two photons [4–6]. DNA-photolyase induces the repair of UV-induced lesions in DNA by scission of covalent bonds between neighbouring

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

858 Hydrogen Bonding and Transfer in the Excited State

pyrimidines. Photoactivation of an FAD cofactor occurs via a fast (picosecond timescale) multistep electron transfer through a chain of three tryptophan residues [7, 8]. However, in the present chapter we focus on another light-driven enzyme, protochlorophyllide oxidoreductase (POR), which also provides an opportunity to study initial ultrafast processes that are required for catalysis. Detailed reviews on POR can be found in Refs [9] to [11] but in the present chapter we give a limited overview of the enzyme and provide a detailed description of recent time-resolved transient absorption and fluorescence experiments.

38.2 Protochlorophyllide Oxidoreductase (POR) Chlorophyll is the most abundant pigment on Earth and is essential for life, directly or indirectly, as the cofactor for the photosynthetic proteins that harvest sunlight and convert it into photochemical energy for the cell. POR catalyses one of the latter steps in the chlorophyll biosynthesis pathway, the light-dependent trans addition of hydrogen across the C17–C18 double bond of the D-ring of protochlorophyllide (Pchlide) to produce chlorophyllide (Chlide) (Figure 38.1) [9–11]. In a biological context, the light-driven reaction catalysed by this enzyme provokes a profound change in the morphological development of the plant that is visualized at the ultrastructural level as a modification and reorganization of plastid membranes [9, 10]. In etiolated plants POR is mainly found to exist as a ternary complex in highly organized networks of tubular membranes termed prolamellar bodies [12, 13]. After illumination and subsequent formation of Chlide, there is a disintegration of the POR-pigment aggregates, resulting in the dispersion of the prolamellar bodies [14]. In addition to POR, non-flowering land plants, algae and cyanobacteria possess a light-independent Pchlide reductase, which consists of three separate subunits and allows these organisms to produce Chlide in the dark [15, 16].

Figure 38.1 Light-driven reduction of protochlorophyllide (Pchlide) catalysed by protochlorophyllide oxidoreductase (POR). The trans addition of hydrogen across the C17–C18 double bond of Pchlide to form chlorophyllide (Chlide), catalysed by the light-driven enzyme POR, is a key regulatory step within the chlorophyll biosynthetic pathway. The enzyme requires NADPH as a cofactor and the dashed box indicates the double bond that is reduced during the reaction

Excited State Dynamics in the Light-Driven Enzyme Protochlorophyllide Oxidoreductase (POR) 859

Figure 38.2 Catalytic cycle of POR. An overall scheme for the catalytic cycle of POR showing the stepwise formation of the reaction intermediates together with the rate constants that have been calculated previously [18, 26, 29]

38.3 Catalytic Mechanism of POR The POR enzyme requires NADPH as a cofactor and in the dark it is found in a ternary complex with the two substrates [17]. Various experimental approaches [18–28] have been used to solve the catalytic mechanism of POR and have resulted in a detailed understanding of the reaction cycle (Figure 38.2). The formation of the ternary enzyme–substrate complex is the rate-limiting step in the overall catalytic cycle and involves multiple steps, which are controlled by slow conformational changes in the protein [18]. During the reaction, the Pchlide molecule performs the function of the photoreceptor and upon illumination it is proposed that a hydride is transferred from the pro-S face of NADPH to the C17 position of the Pchlide molecule [19, 20] and a conserved Tyr residue donates a proton to the C18 position [21]. By using low temperature spectroscopy to trap catalytic intermediates the reaction cycle has been shown to consist of an initial light-driven reaction [22] followed by a series of subsequent (slower) dark reactions [23, 24]. The initial light-driven step involves hydride transfer from the NADPH molecule to form a charge-transfer complex, which facilitates the subsequent protonation of the C18 position of Pchlide during the first of the ‘dark’ reactions [25]. Recent laser photoexcitation studies have revealed that these two enzymatic H-transfer reactions occur in a sequential mechanism on the microsecond timescale [26]. By combining studies of the temperature and isotopic dependence of the rate of Pchlide reduction it was shown that both H-transfer reactions proceed by using quantum mechanical tunnelling that is coupled to specific motions (vibrations) in the protein [26]. Further studies on the role of the bulk solvent on catalysis suggested that solvent slaved motions control proton tunnelling but not hydride tunnelling, implying that a long-range ‘dynamic network’ from solvent to the enzyme active site facilitates proton transfer [27, 28]. Furthermore, a series of recent studies, aimed at resolving the remaining ‘dark’ steps in the reaction, have shown that these latter catalytic events represent a series of ordered product release and coenzyme binding processes [23, 24]. Thus, following the rapid formation of the ternary enzyme–product complex, NADPþ is first released from the enzyme and is then replaced by the NADPH coenzyme. This is followed by the release of the Chlide product and subsequent binding of the Pchlide substrate to complete the catalytic cycle [24]. Laser photoexcitation experiments have been used to follow the interconversion of these various bound/ unbound Chlide product species on the millisecond to second timescales, which suggested that conformational changes and/or reorganization of the protein were required to facilitate the release of the products and substrate rebinding [29]. This is also in agreement with previous cryogenic measurements, which revealed that these stages of the reaction could only proceed above the ‘glass transition’ temperature of proteins [23, 24].

860 Hydrogen Bonding and Transfer in the Excited State

38.4 Ultrafast Catalytic Processes of the Isolated Pchlide Species Although the actual chemical steps in the POR-catalysed reaction proceed on a much slower timescale [26], catalysis is completely dependent on excited state processes in the Pchlide molecule, which occur on an ultrafast picosecond timescale. Several recent studies have attempted to characterize some of these short-lived Pchlide species and we now have a more detailed description of the excited-state dynamics that precede catalysis [30–36]. To clarify the complex photochemical reactions that occur in the POR enzyme Dietzek et al. have used time-resolved absorption and fluorescence spectroscopy to study the excited-state chemistry of the isolated Pchlide substrate [30–32]. To mimic the different environmental conditions in the enzyme complex, various solvents were used, which demonstrated that the excited state dynamics of Pchlide strongly depends on the solvent polarity [30, 31]. In polar solvents, such as methanol and acetonitrile, the excited state relaxation dynamics are multiexponential with three distinguishable timescales of 4.0–4.5 ps for vibrational relaxation and vibrational energy redistribution of the initially excited S1 state; 22–27 ps for the formation of an intermediate state, most likely with a charge transfer character; and 200 ps for the decay of this intermediate state back to the ground state [31]. However, in the non-polar solvent cyclohexane, only the 4.5 ps relaxational process can be observed [31]. In addition, by increasing the viscosity of the solvent by adding glycerol to a methanolic solution of Pchlide, two of the former relaxation processes are found to be decelerated [30, 31]. This suggests that not only is the vibrational cooling of the S1 state slowed in the more viscous surrounding, but the formation rate of the intermediate state with charge transfer character can also be reduced. Hence, it is likely that nuclear motions along the reaction coordinate are involved in the charge transfer and that the formation of the intermediate state may be related to the dynamic solvation process of Pchlide in the S1 state [30–32]. Consequently, the site-specific solvation of photoexcited Pchlide in methanol has been investigated by using time-dependent density functional theory [33]. It was shown that intermolecular site-specific coordination and hydrogen-bonding interactions between the Pchlide and methanol molecules play a very important role in the steady-state and time-resolved spectra of the pigment. A coordinated and hydrogen-bonded Pchlide-(MeOH)4 complex was proposed to represent the intermediate state that is formed in 22–27 ps in the time-resolved spectroscopic studies [33]. Moreover, the intermolecular coordination and hydrogen bonds between the Pchlide and methanol molecules can be strengthened in the electronically excited state of Pchlide, which, in turn, induces the site-specific solvation of the photoexcited Pchlide molecule. It was also shown that all of the steady-state and time-resolved spectral features of Pchlide in different solvents can be explained by the formation of this hydrogen-bonded intermediate state after the site-specific solvation [33]. A more recent pump–probe study in the visible and near-IR regions has extended the current model for the excited-state dynamics of Pchlide into the sub-ps and ns-time domains (Figure 38.3) [34]. Following excitation, an initial ultrafast 450 fs process is suggested to represent the motion out of the Franck–Condon region on the excited state surface. In addition, a long-lived photointermediate with absorption bands at approximately 530 and 890 nm builds up in 3.5 ns. This long-lived photointermediate is ascribed to a low-lying excited triplet state and is formed directly from the fully equilibrated excited S1 state [34]. The data on the isolated Pchlide pigment have provided important insights into the mechanism of lightactivation in the enzyme–substrate complex and suggest that the enzyme-catalysed photoreduction of Pchlide is strongly influenced by the protein environment in the substrate binding pocket. It has been proposed that specific chromophore–protein interactions in the enzyme–substrate complex may quench the non-productive and high-risk formation of a triplet state by efficiently promoting the biologically relevant photoreduction reaction [34]. It is likely that the polarity of the protein environment controls the reaction and that the formation of an intramolecular charge transfer complex plays a significant role in the excited-state reaction dynamics of the enzyme-bound Pchlide [37]. The dipolar nature of the intramolecular charge-transfer state is expected to result from the presence of the electron-withdrawing carbonyl group on the D-ring of the Pchlide molecule.

Excited State Dynamics in the Light-Driven Enzyme Protochlorophyllide Oxidoreductase (POR) 861

Figure 38.3 Schematic model of the light-induced relaxation processes in Pchlide. The excited state relaxation processes have been proposed by studies on Pchlide in solution [30–32, 34]. FC refers to the Franck–Condon region, SX denotes a secondary S1 excited state on the S1 potential energy surface and SICT an intramolecular charge transfer state

As the catalytic photoreduction of the C17–C18 double bond involves the initial transfer of a hydride ion to the C18 position of Pchlide [19, 20, 25, 26], a decreased local electron density at this position would enhance this nucleophilic attack process. Hence, it is interesting that the excited-state charge-localization observed in the isolated Pchlide molecule might lead to an electronic configuration that favours the reduction of the C17–C18 double bond in the enzyme active site [37].

38.5 Ultrafast Catalytic Processes of the Enzyme-Bound Pchlide Species To investigate the initial ultrafast steps that are directly associated with the formation of Chlide we have recently used femtosecond transient absorption measurements to study the excited state processes in the enzyme-bound Pchlide species [35, 36]. Immediately after excitation a negative signal was observed due to the bleached absorption and stimulated emission of Pchlide, peaking at approximately 640 nm. After a further few picoseconds a negative signal, corresponding to stimulated emission, appeared at approximately 674 nm, which occurred with two time constants of 3 and 400 ps [35]. The absence of this negative signal in measurements on Pchlide in solution and in a mutant (Y189F) in which the putative proton donor Tyr189 was replaced by a Phe, led to the conclusion that this was a catalytic product [35]. Unfortunately, the exact nature of this excited state species, now referred to as I675, could not be fully determined at this stage. However, with the recent theoretical studies on the Pchlide excited state [33] and the identification of the hydride and proton transfer reactions [26] it is likely that I675 is a precursor species in which Pchlide forms a strongly hydrogen-bonded complex with residues in its direct environment and/or NADPH that is essential for the subsequent hydride and proton transfer steps to proceed on a microsecond timescale [26]. In a subsequent study [36], the ultrafast reaction dynamics of the enzyme–substrate complex were analysed under single pulse conditions. By using a Lissajous sample scanner, in combination with very high detection sensitivity, reaction rates and quantum yields were measured as a function of the total number of laser shots previously seen by the sample. The dynamics of the POR–substrate complex proved to be very strongly dependent on the number of pulses applied to the sample [36]. Following a single laser pulse only minor dynamics in the Pchlide region were observed with no I675 formation, but after further laser pulses stimulated emission from I675 appeared on the same timescale as reported previously [35]. To analyse the data in more

862 Hydrogen Bonding and Transfer in the Excited State

Figure 38.4 Model for the ultrafast catalytic reactions in POR [36]. The laser induced dynamics in complexes that had not been excited before results in the kinetic scheme for inactive enzymes, showing only Pchlide photochemistry (from Pchlide I to Pchlide II to Pchlide III). In complexes that had been previously excited, the intermediate I675 appears on a picosecond timescale and results in the kinetic scheme for active enzymes, including the intrinsic photochemistry from the Chlide product accumulated in previous scans (Chlideaccum )

detail the full set of spectra were fitted to a model in which the POR enzymes were divided into two populations: one that is inactive and shows intrinsic Pchlide photochemistry but does not lead to product, and an active population that also shows the formation of I675 (Figure 38.4). The fraction of active enzymes was found to be dependent on the number of laser pulses and demonstrates that the rate and quantum yield of formation of the I675 intermediate is significantly enhanced after the Pchlide substrate has cycled through the excited state at least once [36]. Therefore, it appears that a first single photon is needed to activate the POR complex, whereas a second photon then induces catalysis. This remarkable effect was suggested to arise from a more favourable catalytic configuration of the enzyme–substrate complex, caused by the changed electron distribution in the Pchlide excited state [36]. Consequently, rapid scan FTIR spectroscopy was used to investigate whether conformational changes in the enzyme are triggered upon the absorption of a photon. The corresponding spectral changes in the mid-infrared showed that initial laser photoexcitation induced absorption changes associated mainly with protein bands, amide I and II, whereas subsequent laser shots induced absorption changes that could be assigned to the disappearance of NADPH and Pchlide together with the appearance of NADPþ and Chlide [36]. This suggested that indeed the first photon produces a conformational change in the POR enzyme that switches it from inactive to active, and that when in the active state a second photon can induce catalysis that leads to Chlide formation with a quantum yield of 0.3.

38.6 Conclusions Light-driven enzymes, such as POR, provide ideal model systems for studying excited state processes that are linked to enzymatic catalysis. In POR it is likely that several excited state Pchlide species are formed within a few nanoseconds [30–37], which are essential for the subsequent chemical steps to occur on the microsecond timescale [26]. An intramolecular charge transfer complex [30–32] and a hydrogen bonded intermediate [33, 36] are proposed to play a significant role in the excited-state reaction dynamics of the enzyme-bound

Excited State Dynamics in the Light-Driven Enzyme Protochlorophyllide Oxidoreductase (POR) 863

Pchlide and are thought to be linked to conformational changes in the protein [36]. We expect that further experiments on POR will lead to a full identification of the reaction pathway at room temperature and the identification of the initial intermediate state(s) involved, together with a better understanding of the structural changes that lie at the origin of the activation process. The resolution of the structure of POR, either by X-ray diffraction or by NMR techniques, will undoubtedly be important in this process.

References 1. S. J. Benkovic and S. Hammes-Schiffer, A perspective on enzyme catalysis, Science, 301, 1196–1202 (2003). 2. J. Villa and A. Warshel, Energetics and dynamics of enzymatic reactions, J. Phys. Chem. B, 105, 7887–7907 (2001). 3. E. Z. Eisenmesser, O. Millet, W. Labeikovsky, et al., Intrinsic dynamics of an enzyme underlies catalysis, Nature, 438, 117–121 (2005). 4. M. Y. Okamura, M. L. Paddock, M. S. Graige and G. Feher, Proton and electron transfer in bacterial reaction centers, Biochim. Biophys. Acta-Bioenerg., 1458, 148–163 (2000). 5. M. L. Paddock, G. Feher and M. Y. Okamura, Proton transfer pathways and mechanism in bacterial reaction centers, FEBS Lett., 555, 45–50 (2003). 6. C. A. Wraight, Proton and electron transfer in the acceptor quinone complex of photosynthetic reaction centers from Rhodobacter sphaeroides, Frontiers Biosci., 9, 309–337 (2004). 7. C. Aubert, M. H. Vos, P. Mathis et al., Intraprotein radical transfer during photoactivation of DNA photolyase, Nature, 405, 586–590 (2000). 8. A. Mees, T. Klar, P. Gnau et al., Crystal structure of a photolyase bound to a CPD-like DNA lesion after in situ repair, Science, 306, 1789–1793 (2004). 9. N. Lebedev and M. P. Timko, Protochlorophyllide photoreduction, Photosynth. Res., 58, 5–23 (1998). 10. T. Masuda and K. Takamiya, Novel insights into the enzymology, regulation and physiological functions of lightdependent protochlorophyllide oxidoreductase in angiosperms, Photosynth. Res., 81, 1–29 (2004). 11. D. J. Heyes and C. N. Hunter, Making light work of enzyme catalysis: protochlorophyllide oxidoreductase, Trends Biochem. Sci., 30, 642–649 (2005). 12. C. Sundqvist and C. Dahlin, With chlorophyll pigments from prolamellar bodies to light-harvesting complexes, Physiol. Plantarum, 100, 748–759 (1997). 13. F. Franck, U. Sperling, G. Frick et al., Regulation of etioplast pigment-protein complexes, inner membrane architecture, and protochlorophyllide a chemical heterogeneity by light-dependent NADPH:protochlorophyllide oxidoreductases A and B Plant Physiol., 124, 1678–1696 (2000). 14. L.B. Zhong, B. Wiktorsson, M. Ryberg and C. Sundqvist, The Shibata shift: effects of in vitro conditions on the spectral blue-shift of chlorophyllide in irradiated isolated prolamellar bodies, J. Photochem. Photobiol. B-Biol., 36, 263–270 (1996). 15. Y. Fujita and C. E. Bauer, Reconstitution of light-independent protochlorophyllide reductase from purified BchL and BchN-BchB subunits - in vitro confirmation of nitrogenase-like features of a bacteriochlorophyll biosynthesis enzyme, J. Biol. Chem., 275, 23583–23588 (2000). 16. M. J. Br€ocker, S. Virus, S. Ganskow et al., ATP-driven reduction by dark-operative protochlorophyllide oxidoreductase from Chlorobium tepidum mechanistically resembles nitrogenase catalysis, J. Biol. Chem., 283, 10559–10567 (2008). 17. W. T. Griffiths, Reconstitution of chlorophyllide formation by isolated etioplast membranes, Biochem. J., 174, 681–692 (1978). 18. D. J. Heyes, B. R. K. Menon, M. Sakuma and N. S. Scrutton, Conformational events during ternary enzyme-substrate complex formation are rate limiting in the catalytic cycle of the light-driven enzyme protochlorophyllide oxidoreductase, Biochemistry, 47, 10991–10998 (2008). 19. V. Valera, M. Fung, A. N. Wessler and W. R. Richards, Synthesis of 4R- and 4S-tritium labeled NADPH for the determination of the coenzyme stereospecificity of NADPH: protochlorophyllide oxidoreductase, Biochem. Biophys. Res. Commun., 148, 515–520 (1987).

864 Hydrogen Bonding and Transfer in the Excited State 20. T. P. Begley and H. Young, Protochlorophyllide reductase. 1. Determination of the regiochemistry and the stereochemistry of the reduction of protochlorophyllide to chlorophyllide, J. Am. Chem. Soc., 111, 3095–3096 (1989). 21. H. M. Wilks and M. P. Timko, A light-dependent complementation system for analysis of NADPH:protochlorophyllide oxidoreductase. Identification and mutagenesis of two conserved residues that are essential for enzyme activity, Proc. Natl. Acad. Sci. USA, 92, 724–728 (1995). 22. D. J. Heyes, A. V. Ruban, H. M. Wilks and C. N. Hunter, Enzymology below 200 K: the kinetics and thermodynamics of the photochemistry catalyzed by protochlorophyllide oxidoreductase, Proc. Natl. Acad. Sci. USA, 99, 11145–11150 (2002). 23. D. J. Heyes, A. V. Ruban and C. N. Hunter, Protochlorophyllide oxidoreductase: “dark” reactions of a light-driven enzyme”, Biochemistry, 42, 523–528 (2003). 24. D. J. Heyes and C. N. Hunter, Identification and characterization of the product release steps within the catalytic cycle of protochlorophyllide oxidoreductase, Biochemistry, 43, 8265–8271 (2004). 25. D. J. Heyes, P. Heathcote, S. E. J. Rigby et al., The first catalytic step of the light-driven enzyme protochlorophyllide oxidoreductase proceeds via a charge transfer complex, J. Biol. Chem., 281, 26847–26853 (2006). 26. D. J. Heyes, M. Sakuma, S. De Visser and N. S. Scrutton, Nuclear quantum tunneling in the light-activated enzyme protochlorophyllide oxidoreductase, J. Biol. Chem., 284, 3762–3767 (2008). 27. G. Durin, A. Delaunay, C. Darnault et al., Simultaneous measurements of solvent dynamics and functional kinetics in a light-activated enzyme, Biophys. J., 96, 1902–1910 (2009). 28. D. J. Heyes, M. Sakuma and N. S. Scrutton, Solvent slaved protein motions accompany proton but not hydride tunneling in light-activated protochlorophyllide oxidoreductase, Angew. Chem. Int. Ed., 48, 3850–3853 (2009). 29. D. J. Heyes, M. Sakuma and N. S. Scrutton, Laser excitation studies of the product release steps in the catalytic cycle of the light-driven enzyme, protochlorophyllide oxidoreductase, J. Biol. Chem., 282, 32015–32020 (2007). 30. B. Dietzek, R. Maksimenka, T. Siebert et al., Excited-state processes in protochlorophyllide a: a femtosecond timeresolved absorption study, Chem. Phys. Lett., 397, 110–115 (2004). 31. B. Dietzek, W. Kiefer, J. Popp et al., Solvent effects on the excited-state processes of protochlorophyllide: a femtosecond time-resolved absorption study, J. Phys. Chem. B., 110, 4399–4406 (2006). 32. B. Dietzek, S. Tschierlei, G. Hermann et al., The excited-state chemistry of protochlorophyllide a: a time-resolved fluorescence study, ChemPhysChem, 7, 1727–1733 (2006). 33. G. J. Zhao and K. L. Han, Site-specific solvation of the photoexcited protochlorophyllide a in methanol: formation of the hydrogen-bonded intermediate state induced by hydrogen-bond strengthening, Biophys. J., 94, 38–46 (2008). 34. B. Dietzek, W. Kiefer, A. Yartsev et al., Protochlorophyllide a: a comprehensive photophysical picture, ChemPhysChem, 10, 144–150 (2009). 35. D. J. Heyes, C. N. Hunter, I. H. M. van Stokkum et al., Ultrafast enzymatic reaction dynamics in protochlorophyllide oxidoreductase, Nat. Struct. Biol., 10, 491–492 (2003). 36. O. A. Sytina, D. J. Heyes, C. N. Hunter et al., Conformational changes in an ultrafast light-driven enzyme determine catalytic activity, Nature, 456, 1001–1004 (2008). 37. M. Schmitt, B. Dietzek, G. Hermann and J. Popp, Femtosecond time-resolved spectroscopy on biological photoreceptor chromophores, Laser Photonics Rev., 1, 57–78 (2007).

39 Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters in the Excited State Michal F
arn
ık1, Petr Slav
ıcek2 and Udo Buck3 1

J. Heyrovsk
y Institute of Physical Chemistry, Academy of Sciences, Dolejsˇkova 3, 182 23 Prague 8, Czech Republic 2 Department of Physical Chemistry, Institute of Chemical Technology, Technick
a 5, Prague 6, Czech Republic 3 Max-Planck-Institut f€ur Dynamik und Selbstorganisation, Bunsenstrasse 10, D-37073 G€ ottingen, Germany

39.1 Introduction The role of hydrogen bonding in the ground state chemistry is widely acknowledged. The most prominent examples of its significance are the structure of water and ice, the pairing of base pairs in nucleic acids, molecular recognition or enzyme catalysis, and so on. All of the above examples are related to essential conditions of life. For this reason, hydrogen bonding phenomena became a subject of extensive studies. Photoinduced processes in hydrogen bonded systems are explored to a much lesser extent. While photochemistry of systems with intramolecular hydrogen bonds have been studied for a relatively long time, for example, in a context of sunscreen protection or fluorescence spectroscopy, photochemistry of intermolecular hydrogen bonds has only recently become a major research subject [1]. There are interesting questions to be addressed: Is there any photochemistry specific for hydrogen bonded systems? And if the answer is yes: Is the different photochemistry related to biophysically relevant processes? The answer to the first question is affirmative, photoinduced processes are controlled by the ground state structure and the X-H


Y structural motif implies ultrafast excited state hydrogen (or proton) transfer as a typical reaction of hydrogen bonded systems.

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

Edited by Ke-Li Han and Guang-Jiu Zhao

866 Hydrogen Bonding and Transfer in the Excited State

The hydrogen bond was suggested to be a key component in protecting nucleic acids against radiation damage [2–4]. Recently, it was also shown that an analogical mechanism can protect another class of biomolecules-peptides [5]. Experimental studies of hydrogen bonding in the ground state are relatively straightforward, for example, using vibrational spectroscopy. Experimental investigation of hydrogen bonding in the excited state is more complicated since the absorbed photon triggers ultrafast dynamical processes. Various time-resolved experiments, based on photoabsorption, fluorescence or photoionization, can then be used to study the excited state dynamics [6]. The dynamical processes occur often on the edge of the time resolution of the respective experiments. In our studies, we have concentrated on experiments in the energy domain. In particular, we detect the kinetic energy distribution of the atomic fragments released from the studied system. The natural choice for the study of hydrogen bonded systems is to detect the kinetic energy of the hydrogen atoms. If the detected hydrogen atom was released from the molecule participating in the hydrogen bonding, we obtain information about the character of this bond both in the ground and in the excited states. Such experiments on the photochemistry of hydrogen bonded systems are a direct continuation of our previous studies of the photodissociation in cluster environments [7]. In our experiment, we observe photoinduced processes in molecular clusters. The cluster approach has many advantages [7, 8]. First, quantities only hardly accessible in the bulk can be monitored (e.g., kinetic energy of the fragments). Second, some control over the size of the particles under study can be imposed. Third, one can study both bulk and surface phenomena within the molecular clusters. Finally, results obtained on molecular clusters can be directly compared with theoretical calculations. In this chapter, we present results of our studies on two types of hydrogen bonded systems: (i) water and aqueous solutions and (ii) hydrogen bonded heterocycles. Both topics are related to the subject of radiation damage of biomolecules. Biological molecules (which are often of heterocyclic character) can be damaged directly by a photon; they can, for example, react in the excited state, fragment or ionize. Biomolecules can be also damaged indirectly: photons can either excite or ionize water, resulting into the production of radicals that then attack the biomolecules. The photochemistry of doped water particles is also interesting from the perspective of atmospheric science. The chapter is organized as follows. First we briefly overview our experimental technique and methods employed. Then we discuss the photochemistry of aqueous systems, that is, first pure water clusters and then water clusters with hydrogen halide molecules. In the second part of our contribution the photochemistry of small heteroaromatic molecules relevant to biological systems is studied. Finally, general conclusions and an outlook are provided.

39.2 Experiment The photodissociation experiments were carried out in a molecular beam apparatus that contains a cluster source, a buffer chamber for manipulating the beams, a laser port for the photoexcitation, and a time-of-flight mass spectrometer for the detection of the dissociated fragments, in this case the H atoms. In addition there is another chamber with a quadrupole mass spectrometer and an electron impact ionizer to mass analyse the cluster beam composition. For details we refer the reader to review articles [7, 9, 10]. The host clusters are produced by isentropic expansions through conical nozzles. By varying pressure and temperature of the source the average size is shifted from n ¼ 2 to more than 1000. The large clusters in this size range usually follow a log–normal distribution that is nowadays directly measured by fragmentation free [11] methods, namely by doping with Na atoms. The resulting average sizes can be correlated with the source parameters based on the ideas of Hagena [12]. The resulting parameters are available for rare gases [13], water and ammonia [11] clusters. The size distributions of small clusters are analysed by a scattering experiment in a

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 867

crossed-molecular beams arrangement [14–16]. The well-defined cluster beam is scattered by a beam of He atoms. In elastic collisions the clusters of different sizes are scattered into different laboratory angles. By measuring the angular and velocity distributions of the scattered clusters the sizes are selected independent from their detection method. To prepare embedded or adsorbed molecules with these clusters two different techniques are applied. The preparation of molecules that are adsorbed on the surface is realized by the so-called pick-up technique introduced by Scoles and coworkers [17]. The cluster is passed through a small scattering cell filled with the molecular vapor with variable pressure. The number of molecules captured depends sensitively on this pressure and follows a Poisson distribution [18]. By a suitable choice of the source pressure, one can easily arrange conditions at which only one molecule is adsorbed on the surface of the cluster. The probability of penetrating inside the cluster depends on the minimum distance and the well depth of the local interaction. From a series of new calculations [19] one can estimate that, for example, HBr on Arn stays near the surface in the first and second shell. To place the molecule inside the clusters a co-expansion with the host gas is used. Since hydrogen bonded molecules form, because of the higher binding energy, much more easily clusters with themselves than with the rare gas atoms, one can either generate the pure hydrogen bonded molecular clusters this way or one can go to more dilute mixtures to generate a small molecular cluster in the rare gas cluster. The biomolecules that are treated in Section 39.4 are produced in this way. By analysis of the measured mass spectra, the laboratory angular and velocity distributions for various fragments, the mean neutral cluster sizes are obtained. The molecules in or on the cluster beam are dissociated by a focussed pulsed laser beam of 243.07 nm and/or 193 nm and a pulse duration between 10 and 20 ns. The ionization takes place with the 243.07 nm laser pulse in a (2 þ 1)-resonant enhanced multiphoton ionization (REMPI) scheme. The ions are extracted into a two-stage time-of-flight mass spectrometer (TOFMS) of the Wiley–McLaren type [20], which is typically used in the low-field mode to detect the protons and measure their TOF distribution. By applying a small electric field we extract those ions already flying in the direction of the detector and turn around those ones that start in the opposite direction. In this way also H atoms with small and even zero velocity are detected. They give rise to a single peak centered between the peaks of the fast fragments. The way to extract from this TOF spectra the kinetic energy distribution is described in detail in Refs [21, 22]. We note that in our experimental arrangement the detection probability is extremely enhanced at small kinetic energies so that we are, in particular, sensitive to the caged atoms [21]. The different contributions that can be derived from the measured kinetic energy of the H atom, Ekin(H), are best discussed by the energy balance of the process. We can illustrate this with an example of HBr molecule dissociation in clusters: hn þ Eint ðHBrÞ ¼ D0 þ Eint ðBrÞ þ Ekin ðBrÞ þ Ekin ðHÞ þ Eclu

ð39:1Þ

where the excitation wavelength hn and the dissociation energy D0 of HBr are known, and Ekin(H) is measured. By conservation of momentum, the kinetic energy of the Br atoms, Ekin(Br), is also known. The excitation of the spin–orbit state Br* in the Br product channel is presented by Eint(Br) and is measured as energy loss in the kinetic energy of the H atom, Ekin(H). These effects would also appear in the dissociation of HBr monomers and indicate the direct cage exit. The influence of the cluster is expressed by the continuous energy loss Eclu of the H atoms caused by the collisions with the cage. This leads, depending on the position, to delayed cage exit or complete caging. The internal excitation of the HBr molecule before the dissociation, Eint(HBr), is observed as energy gain. In general, the molecules will be in the ground state after the expansion. There are, however, various possibilities to observe internally excited molecules. One is

868 Hydrogen Bonding and Transfer in the Excited State

Figure 39.1 Photodissociation of HBr molecules on Arn clusters at two different wavelengths. The spin–orbit states of the Br atom are well resolved in the cage exit contributions

the existence of recombined molecules that are quite hot. The other might occur by the vibrational excitation in collisions within the cluster. A typical example of experimental kinetic energy distributions (KED) is shown in Figure 39.1 for the systems HBr–Ar159 dissociated at two different wavelengths [23]. The distributions exhibit mainly three peaks: one very sharp one at zero energy, which is caused by the completely caged H atoms, and two others that originate from the direct cage exits of H atoms dissociated into the two different spin–orbit states of the Br atom. The higher energy at 193 nm leads to the higher kinetic energy of the H atoms.

39.3 Aqueous Photochemistry from the Cluster Perspective In this section we present the results of our photodissociation studies of pure water clusters [24] and water clusters doped with the hydrogen halide molecules HBr and HCl [25, 26]. 39.3.1 Photoinduced processes in isolated water molecules We start with a description of what is known about the photodissociation of the bare molecule. The photodissociation of the water molecule is a well-studied subject both experimentally and theoretically [27]. A single H2O molecule can undergo two fundamental processes after being irradiated with ultraviolet (UV) ~ state is at photons: photoionization and photodissociation. The maximum of the first absorption band of the A 7.4 eV (168 nm). This state is repulsive with respect to the OH bond, leading to fragmentation into OH and H radicals [28]: ~ ! H þ OH H2 O þ hn ! H2 OðAÞ

ð39:2Þ

Photodissociation on this state has been studied both at 193 nm [29] and 157 nm [30, 31] and found to result in a very low internal excitation of the OH radical product. This process on a single repulsive surface leads to fast and direct dissociation with only a weak excitation of the vibration or rotation of the resulting OH radical.

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 869

~ with the absorption maximum at 9.6 eV At higher energies, water is excited to another dissociative state, B, ~ and B ~ absorption bands are rather broad (full-width at half-maximum, FWHM,  1 eV). (129 nm). Both the A ~ state lead to a far more complex behavior, resulting in a highly rotationally excited OH The dynamics on the B ~ state and the ground X ~ state, which introduces a large fragment. The reason is a conical intersection with the A torque on the OH fragment that is then left in a highly excited rotational state. Most of the population is found in ~ state: the ground X ~ ! H2 OðXÞ ~ ! H þ OH H2 O þ hn ! H2 OðBÞ

ð39:3Þ

~ or The kinetic energy of the resulting H atoms is therefore quite different depending on the excitation to the A ~ state. In the first case they carry nearly all the available energy, while in the latter case the maximum is found B around 1.3 eVafter excitation at 10.2 eV [32, 33]. At higher photon energies, above 12.6 eV, water ionization to H2Oþ starts to compete with the excitation leading to the dissociation. 39.3.2 Photoinduced processes in hydrogen-bonded water: bulk and cluster perspectives The electronic structure and role of hydrogen bonds in irradiated liquid water is also an unresolved issue. The solvation apparently deeply influences both the photodissociation and photoionization [25, 34, 35]. The maximum of the first absorption band of water shifts to higher excitation energies of 8.2 and 8.4 eV for liquid ~ water [36] and ice [37], respectively. The blue shift appears because of the partially Rydberg character of the A state, but also a small red tail has been observed in the absorption spectrum of small water clusters [38]. This suggests a surface preference for the photon absorption at low energies. The effect of solvation on the ionization processes is even more pronounced, leading to dissociation of the protonated water molecule along the hydrogen bond coordinate. In this way the solvated electron and the hydronium cation are formed: 2H2 OðliqÞ þ hn ! H3 O þ þ OH þ e ðaqÞ

ð39:4Þ

The photoelectron spectrum shifts by 1.5 eV (vertical ionization potential) to lower energies upon bulk solvation while the width of the spectrum increases [34]. Thus the ionization and dissociation processes start to overlap below 10 eV. Actually, the two processes might not be distinguished at all in hydrogen bonded systems. The H atom released in the photodissociation can be scavenged by a solvating water molecule to generate the H3O radical. It has been proposed [1, 39, 40] that this species can be interpreted as the solvated electron–hydronium cation pair. Stated differently, the hydronium radical can serve as a cluster model of the solvated electron. The barrier to the H þ H2O limit is only 0.12 eV. However, the H3O radical is stabilized by the solvation with water molecules. This is illustrated in Figure 39.2, which shows the minimum energy path of the H3O (H2O)3 cluster for the H atom detachment. The barrier increased to about 0.3 eV and the released energy is 0.4 eV. We argue here that the hydrated hydronium radical (H3O)aq plays a central role in the hydrogen bonded water systems in general. And on top of that it also plays a central role in the photochemistry of the mixed hydrogen halide–water clusters, as we will demonstrate in Section 39.3.4. We have studied the photodissociation of the pure (H2O)n clusters at 243 nm (5.1 eV) in the range of the the mean sizes between  n ¼ 85 and 670. Figure 39.3 shows a typical example of the KED of the ejected H atoms. Evidently, hydrogen atom fragments observed in our experiment come predominantly from two-photon processes. Neither gas-phase nor bulk water absorbs at 5.10 eV. There are, however, neutral states that are 10.2

870 Hydrogen Bonding and Transfer in the Excited State

Å

Figure 39.2 Potential energy function along the minimum energy path for H atom detachment from the hydronium solvated in three water molecules H3O(H2O)3 [39]. Reproduced by permission of the PCCP Owner Societies

Figure 39.3 Photodissociation of water clusters for n ¼ 670. For comparison a spectrum of water clusters for n ¼ 400 doped with HBr molecules is also displayed [24]. Reproduced by permission of the PCCP Owner Societies

above the ground state, to which the system can be promoted by absorption of two photons. The spectrum is composed of a major contribution from very slow fragments with a maximum around 0.05 eV and kinetic energies up to 0.4 eV and a smaller, broader contribution of faster fragments with energies up to 1.5 eV. The spectral shape reflects various processes taking place upon excitation in the clusters. After the hydrogen atom is formed by the photodissociation, it can follow several scenarios: the hydrogen can leave the cluster without being disturbed (cage exit) and the atom can be trapped within the cluster (caging). Finally, the hydrogen atom can also react with the surrounding water molecules to form either a metastable H3O radical or the OH radical and the hydrogen molecule (H2). Let us first discuss the long tail of the spectrum of fast H atoms, effectively ending at 1.5 eV. The overall excess energy available after the two-photon excitation at 243 nm (5.1 eV) is much higher. The ground state OH bond dissociation energy is 5.1 eV [41], therefore the maximum available energy for hydrogen is also ~ state about 5.1 eV. However, it has been shown previously for the bare water molecule that, when exciting the B in this energy range, most of the excess energy is deposited into the rotational energy of the OH fragment during the internal conversion process [32, 33]. The H-fragment KED of the bare molecule has therefore a maximum at about 1.2–1.4 eV. In addition, this part of the spectrum is generated by surface molecules, a position that favors direct cage exit processes. Thus the experimental findings point to the direct dissociation ~ state. processes originating from the B An explanation of the peak at slow energies that dominates the spectrum is not as straightforward as that for the fast peak. An obvious possibility would be trapped or slowed down H atoms induced by the cluster

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 871

environment. However, direct comparison with experiments of hydrogen halide molecules in rare gas clusters reveals characteristic differences. The peak intensity for the caged H-fragments is always observed at zero energy [7, 23, 42] (see also Figure 39.1). This result is also confirmed by accompanying calculations. This suggests that another mechanism is responsible for the slow part of the KED. It is proposed to be the formation of the H3O radical and the subsequent decay into H þ H2O. In a first step the metastable H3O radical is formed after the photodissociation. This is energetically possible for hydrogen atoms with a kinetic energy of at least 0.8 eV, depending on the environment [39, 43, 44] (see also Figure 39.2). The hydronium molecule then decays within an activated process H3O ! H þ H2O, and H atoms with an energy of at most 0.8 eV are produced. These atoms can then lose their energy again by collisions within the clusters and be detected after emerging from it. Thus, generation of the H3O molecule acts as an effective way of energy thermalization, which leads to the prevailing generation of the slow fragments. This interpretation is supported by direct comparison with the experimental result obtained for water clusters doped with hydrogen halide molecules, as is also shown in Figure 39.3, and will be further discussed in Section 39.3.4. There is a striking similarity between the slow component of the present water spectra up to 0.4 eVand the HBr(H2O)n spectrum. For the HX(H2O)n clusters, the H3O arrangement is already present in the system before the photodissociation takes place, due to an acidic dissociation in the ground state of the system to H3Oþ and Cl. The excitation then proceeds via a charge-transfer-to-solvent process, in which the H3O radical is formed directly. Since the acidic dissociation of pure water (i.e., water autoionization) does not occur to a significant extent, the metastable H3O radical can only be formed after the corresponding photodissociation and reaction. The mechanisms that lead to the measured KED are summarized in Figure 39.4. As for the slow peak the ~ state where they dissociate into H water molecules in the cluster are excited by a two-photon process to the B atoms with moderate velocities. They are further slowed down in the cluster cage and are finally trapped by another water molecule, forming H3O. This radical then decays with a maximum energy release of 0.8 eV. For the energetics we refer to Figure 39.2. The faster fragments originate from the direct photolysis of H2O molecules in the cluster, where most of the 5.1 eV of the available energy is transferred into the internal excitation of the system. This process is more probable for the surface molecules as we argue in the next paragraph.

Figure 39.4

Mechanism of H2O photodissociation in clusters

872 Hydrogen Bonding and Transfer in the Excited State

Figure 39.5 Pure water clusters: relative fraction of slow fragments as a function of the mean cluster size. The dashed line indicates the fraction of interior molecules [24]. Reproduced by permission of the PCCP Owner Societies

The size of the cluster affects the processes triggered by the photon absorption. If the excited water molecule is embedded inside of the cluster, there is a high probablity of hydronium radical formation. With decreasing cluster size the relative contribution of the faster component increases in intensity. This trend is illustrated in Figure 39.5, which shows the relative fraction of the slow fragments in dependence of the mean cluster size. The fraction has been obtained as an integral of the corresponding KED between 0 and 0.4 eV relative to the total KED integral from 0 to 1.5 eV. The dashed line in Figure 39.5 indicates the fraction of the molecules inside the cluster volume compared to the total number of molecules in the cluster as a function of cluster size, obtained assuming an icosahedral cluster structure. This comparison suggests that the slow fragments originate from the photodissociation of molecules inside the cluster volume, while the fast fragments originate from the cluster surface. Our experiments were performed for large clusters. Here, we discuss how the photochemical processes converge with respect to clusters size. The surface effects are seen already for the excitation process, that is, for the absorption spectra. We have calculated the absorption cross section of the first band for the water clusters (H2O)n (n ¼ 1–5) using the TDDFT/BHandHLYP/6-31þþg approach. We were able to estimate both the positions of the peaks and their widths. The latter quantity was obtained using the reflection principle method. Figure 39.6 shows the results. The absorption maxima shift to larger energies with increasing size of the clusters. This can be rationalized due to a Coulombic repulsion between a positive charge of the excited water oxygen and a positive charge of a corresponding ground state water hydrogen [38]. The increasing width of the first absorption band is related to a vibrational delocalization induced by the lowering of the OH bond vibrational frequency and also to the fact that more states are involved, caused by the excitation to orbitals localized at different molecules. Figure 39.6 also shows the experimental absorption spectra for an isolated water molecule [45], liquid water [36] and ice [37]. There is satisfactory agreement between our calculated monomer absorption spectrum and the measured one. Minor discrepancies can be assigned mostly to the electronic structure method used. The absorption spectrum then evolves in the direction of ice and liquid water. Clearly, the ice (bulk water) limit is not fully reached within five molecules. However, the maximum has shifted from 170 nm in the gas phase to 150 nm for (H2O)5. The last question to be discussed here is the dependence of the observed processes on the excitation energy. We have also measured the photodissociation in the clusters at 193 nm (6.43 eV). Figure 39.7 shows the KED from the photodissociation of water clusters for  n ¼ 430 at this wavelength. The top spectrum (a) corresponds to the two overlapping laser pulses: 193 nm for molecule dissociation and 243 nm for H fragment REMPI ionization. The middle spectrum (b) corresponds to the two-photon dissociation with the 243 nm laser only, and the difference spectrum (c) is then due to the action of the 193 nm photon only. Notably, due to the lower

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 873

Intensity (arb. units)

4 3 2 1 0 140

160

180

Wavelength λ (nm)

~ in comparison with the experimental Figure 39.6 Absorption cross section of H2O clusters in the first band (A) results for the bare molecule and the condensed phase of ice and water. Reprinted with permission from [25]. Copyright 2008 American Chemical Society

Intensity (arb. units)

2

1

0 0.0

0.5 1.0 Kinetic Energy (eV)

1.5

 ¼ 430 at 193 nm. Top spectrum (a) corresponds to the two Figure 39.7 Photodissociation of water clusters for n overlapping laser pulses: 193 nm for molecule dissociation and 243 nm for H fragment REMPI ionization, middle spectrum (b) corresponds to the two-photon dissociation with 243 nm laser only, and the difference spectrum (c) is then due to the action of the 193 nm photon only [24]. Reproduced by permission of the PCCP Owner Societies

laser intensity and less tight beam focus the 193 nm spectrum corresponds to single-photon processes. Increasing the laser intensity did not change the character and the intensity of the spectra, suggesting that for the two 193 nm photons (12.86 eV) the competing ionization channel prevails, adding no intensity to the Hfragment dissociation channel. Surprisingly, upon excitation at lower energy of 6.4 eV (versus 10.2 eV), the slow H-fragments are absent in the blue spectrum. The 1.2 eV peak is in good agreement with the available energy after a direct dissociation process, namely 6.4–5.1 ¼ 1.3 eV. This peak and the missing slow component ~ state spectrum, we preferentially excite surface water molecules. suggest that, in the extreme red tail of the A 39.3.3 Photoinduced processes of isolated hydrogen halides ~ state represents another example of direct photodissociation process. Hydrogen halides excitation to the A The reaction is very similar to the photodissociation of water described above. The major differences arise for

874 Hydrogen Bonding and Transfer in the Excited State

the heavier hydrogen halides due to the increasing importance of the spin–orbit interaction. This opens new dissociation channels, leading to two spin–orbit states of the halogen atom, which in turn results in Hfragments with two different kinetic energies in the KEDs (see Figure 39.1). The HCl molecule starts absorbing below approximately 190 nm, while the absorption spectra of the heavier hydrogen halides are shifted to the red [46]. These chromophores were used for studies of the rudimentary manifestations of the solvation in the rare gas clusters, the so-called cage effect [47]. Apparently, electronic processes remain unchanged in the rare gas clusters and the observed effects have only mechanical character. Therefore, these systems can serve as a baseline for observations of more elaborate solvation phenomena such as hydrogen bonding in the water clusters. 39.3.4 Photoinduced processes of hydrogen halides in water The photochemistry of hydrogen halides attached to water particles is significantly more complex than the photochemistry of pure water clusters. The reason stems from the fact that the photochemical channels are controlled by the structural features of the clusters in the ground state, which, however, are not straightforward. Hydrogen halides can form hydrogen bonds with water (XH


OH2). Hydrogen halides can also acidically dissociate, forming ionic bonds of the X


H3Oþ type. In the latter case, we can further distinguish contact ion pairs (the hydrogen halide anion is closely adjacent to the hydronium cation) and solvent separated pairs (the ionic components are separated by a number of water molecules). Which type of structure is present in the systems is controlled by the halogen and by the temperature and structure of the water particles. Despite huge effort both on the experimental and theoretical side, the character of adsorbed hydrogen halides under different conditions remains a somewhat controversial issue. Thus, when we began our experimental study, we did not know in which form the hydrogen halide was present on the cluster. However, as we argue below, the photodissociation experiments also answer this question. First, we briefly review the current status on the structure of hydrogen halides on water/ice surfaces. For small clusters of the type HX(H2O)n a series of theoretical investigations predicted that the formation of stable solvated ions H3Oþ(H2O)n1X first occurs for n ¼ 4 [48–52]. Experiments that explore this transition are still quite rare. In matrix isolation spectroscopy such a transition was claimed, although the assignment of the bands to the correct size is difficult [53]. A pump–probe photoionization experiment with HBr(H2O)n clusters confirmed the transition for n ¼ 5 [54]. On the other hand, vibrational spectroscopy of free HCl(H2O)n clusters could only unambiguously identify covalently bound species up to n ¼ 2 [55, 56]. The situation changes when large particles are considered. The HX dissociation has been theoretically studied on model ice surfaces [57– 62], and it has also been the subject of extensive experimental investigations using temperature controlled desorption, X-ray absorption spectroscopy, reactive Csþ ion scattering, low energy sputtering and Fouriertransform infrared spectroscopy [63–71]. The process of dissociation is strongly temperature dependent and requires collective action of an extended hydrogen bonded network [63]. Despite the partially contradictory results, the following general picture emerges. The amount of intact neutrals decreases with increasing temperature, while that of the dissociated ions increases. The crossover takes place between about 80 and 120 K. Similarly as for water particles, also for the HX–(H2O)n systems the hydronium radical is a central intermediate. In water, this particle is formed in the excited state. This mechanism can again be operational for the doped water clusters. However, the H3O moiety can be also produced by excitation of the acidically dissociated ground state. Two different ways of photochemical production of the hydronium molecule are proposed (Figure 39.8). The first mechanism (1) starts with the acidic dissociation of the HX molecule, forming a zwitterionic structure with X and H3Oþ ions. The resulting species of the H3OþX(H2O)n1 type is then excited by the 193 nm laser pulse. The excited state of this structure is of the charge-transfer-to-solvent

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 875

Figure 39.8 Mechanism of HX photodissociation on H2O clusters. Modified with permission from [26]. Copyright 2007, American Institute of Physics

(CTTS) character. In the S1 state, the system then relaxes into a biradical minimum [40]. The biradical can ultimately decay into X and H radicals and water, which means that dissociation of the H3O is observed [39, 43]. The second possible mechanism (2) of hydronium production starts with exciting an intact HX molecule into its dissociative state. The released high energy hydrogen atom penetrates into the cluster. After it loses part of its energy by inelastic collisions it can form the H3O molecule. This molecule then again decomposes into water and hydrogen. This second mechanism has been proposed above as a way of producing hydronium in water. Below we provide evidence that the two proposed reaction channels can be distinguished based on the different character of the KED spectra. We arrive at the conclusion that the acidic dissociation in the ground state takes place in our clusters followed by excitation into the CTTS state, which than relaxes to the H3O radical and subsequently dissociates to H2O and H. Three arguments are given below to support this claim. 39.3.4.1 Shape and Intensity of the KED Spectrum The first experiments were carried out for HBr(H2O)n clusters in the size range n ¼ 400–500. The mixed clusters are produced by pick-up and photolysed by laser light of 193 nm. The produced H atoms are then detected by photoionization at 243.07 nm. The result of the kinetic energy spectrum is shown in the lower part of Figure 39.3. The spectrum consists of one peak only at low kinetic energies. Compared to the spectrum of pure water clusters, the long tail for larger kinetic energies is completely missing. Actually, if comparable laser beam intensities were used in both experiments, the H-fragment signal from the HBr(H2O)n system is more than a factor of ten higher. In other words, the H-atom signal from pure water clusters represents only an order of magnitude weaker background in these experiments. However, it ought to be mentioned that this signal could be increased by tuning experimental parameters, namely by increasing the 243 nm laser intensity, since it is caused by two-photon processes as discussed above. Thus the signals measured in the experiments with the pure water clusters, discussed in Section 39.3.2, were actually only slightly smaller than the present ones. In comparison with the results obtained for HBrArn clusters of Figure 39.1 both the fast exit contributions and also the completely caged H atom at zero energy are missing. The spectrum resembles the low energy part of KED obtained for pure water clusters (Figure 39.3). This can be rationalized by the fact that in the mixed

876 Hydrogen Bonding and Transfer in the Excited State

clusters the hydrogen atoms come entirely from the H3O radical. This would be consistent with acidic dissociation in the ground state and the subsequent CTTS excitation. Direct HX dissociation can be excluded in this reaction channel. 39.3.4.2 Double Isotope Substitution This argument provides unambiguous experimental evidence for the hypothesis that the detected H-atom originates from the H3O species. We measured, aside from the just shown HBr(H2O)n clusters, also the isotopic variants DBr(H2O)n and HBr(D2O)n. The results for the H atom fragment time-of-flight (TOF) distributions are displayed in the right-hand side of Figure 39.9. The background signal and also signals due to the photodissociation of pure (H2O)n clusters were subtracted from the spectra in the figure. We also note that the (2 þ 1) REMPI process for D atoms occurs at 243.00 nm [72], well outside the 0.04 nm bandwidth of the ionizing laser. Besides, the heavier D fragments would arrive at the detector approximately 2 ms later than the lighter H fragments, that is, in a TOF-spectra region where we do not observe any signal. Thus only H atoms are detected. The TOF spectra for the different isotope variations are very similar in shape. Their one peak structure leads to the type of KED displayed in Figure 39.3. The intensity ratio is 3.0 : 2.1 : 1.0. Since we are only able to detect H atoms, the compelling explanation is that we detect in the three experiments H atoms from H3O, H2DO and HD2O species. These processes are pictorially demonstrated on the left-hand side of Figure 39.9. The origin of the signals from pure water clusters and also from HBr can be ruled out. Indeed, the pure (H2O)n produce signals more than an order of magnitude lower and with a slightly different shape. The signal also cannot be simply due to the direct photolysis of the HX molecule on the cluster, since the pick-up of DBr molecules on (H2O)n also produces an intense Hfragment signal. At the same time, H atoms are also produced by the photodissociation of HBr on (D2O)n clusters. This indicates an exchange of hydrogen atoms between the hydrogen halide and the water cluster.

Figure 39.9 Photodissociation of HBr molecules on water clusters. Deuteration experiments pointing to the generation and dissociation of the H3O radical (See Plate 48)

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 877

Figure 39.10

Photodissociation of HX molecules on water clusters at 243 nm

The same result is obtained when we replace the HBr molecule with HCl. The results for the KEDs are shown in Figure 39.10. We observe the same shape of the distribution within the experimental error bars, but the intensities differ by about a factor of seven. This intensity ratio is a fingerprint of the acidic dissociation, as will be discussed below, but more importantly at this point the similarity of the KED shape for both HBr(H2O)n and HCl(H2O)n clusters suggests that the H-fragments originate from the same species in both systems. This characteristic shape results, as was already discussed for the pure water clusters, from the decay of the hydronium radical according to H3O ! H þ H2O. However, we note that in the present case the generation of H3O is completely different from the result for pure water clusters. Contact Ion Pair or Solvant Separated Structure The estimated internal temperature of 100–130 K of our clusters [73] is beyond the onset of the acid dissociation proposed by the various studies mentioned above. The missing fast H fragments in our measurements also suggest the acid dissociation in the ground state of our clusters. Therefore, the excitation (1) in Figure 39.8 of some form of the zwitterionic species H3Oþ–Cl is the more likely route of H3O generation. The finally considered zwitterionic structure can be either of a contact ion type (ClH3Oþ(H2O)n1) or the ions can be separated by solvent water molecules (Cl (H2O)n1 H3Oþ). The latter option would imply a higher proton mobility leading to a rapid isotopic dilution in the experiment with D2O. The ratio of the H-fragment signal from HBr(D2O)n to HBr(H2O)n clusters with n ¼ 500 would be expected to be 1/1000 if the dilution would take place rather than the measured 1/3. Since the isotopic dilution is not observed, the formation of a contact ion pair is suggested. Proton mobility on ice surfaces has been studied by Park et al. [74]. After adding the HCl, the H/D exchange for the surface layer became almost complete in 10 min. Our experiment provides complementary evidence that the ions do not separate during the 0.65 ms between pick-up and photolysis. 39.3.4.3 CTTS and Direct Excitation Cross Sections Which mechanism is indeed operating was confirmed by comparison of the measured signal intensity ratio from HBr(H2O)n and HCl(H2O)n clusters with calculations on the structure and the absorption cross section of these clusters. We will discuss these options in detail. As discussed earlier, there are three major structural motifs for HX on water clusters. The first structural type consists of the hydrogen halide remaining in a

878 Hydrogen Bonding and Transfer in the Excited State

Figure 39.11 Structures of small HX–(H2O)n clusters, X ¼ Cl,Br, and n ¼ 1–5. Different structural motifs appear for n  4: (a) intact, (b) contact ion pair and (c) solvent separated ion pair. Reprinted with permission from [25]. Copyright 2008 American Chemical Society

covalent state HX(H2O)n, further referred to as “intact.” Upon the acidic dissociation, the oxonium cation and halogenide anion can generate either the contact ion pair or the solvent separated structure. Figure 39.11 shows the optimized structures for HX(H2O)n, X ¼ Cl,Br and n ¼ 4,5. Optimization was performed at the MP2/6-31þþg level, at which the structures seem to be converged from both energetical and structural perspectives. For clusters with n ¼ 1–3, the only local minima found are the intact structures. For clusters with n ¼ 4,5, all three types of structural motifs are present: intact structure (a), contact ions (b) and solvent separated ions (c). From the relative energies at several levels of theory, it is found that HCl and HBr clusters are predicted to support dissociated states starting from n ¼ 4 onwards. The general picture of the HX dissociation process is in agreement with previously reported calculations [50, 75, 76]. Based on the results of the electronic excitation we have calculated the electronic absorption spectra for HX(H2O)4 clusters with X ¼ Br,Cl in both the covalent state and the ionic states. Figure 39.12 shows the results for HCl(H2O)4 and HBr(H2O)4 clusters. The absorption spectra were calculated with the TDDFT/BHandHLYP/6-31þþg method employing the reflection principle. The absorption is significantly redshifted upon the acidic dissociation. The position of the maximum shifts from 153 nm to approximately 180 nm for HCl (H2O)4 and to 210 nm for HBr(H2O)4. Importantly, this shift leads to a huge enhancement of the photoabsorption cross section at 193 nm for HCl. The photoabsorption cross section at 193 nm increases also for HBr but to a lesser extent than in the case of HCl. Chloride and bromide anions are directly involved in the photoexcitation process. For the intact structure, the first excited state is mostly connected with excitations within the HX molecule, after the acidic dissociation the excitation adopts the CTTS character; that is, an electron is promoted from the X moiety to orbitals of the water solvent. This CTTS band falls into the spectral range of our photodissociation experiment and can thus be probed. The major findings of our calculations can be summarized as follows: (i) The addition of HCl, HBr or of another water molecule to the water cluster leads to a slight blue-shift in the electronic absorption spectrum. (ii) The acidic dissociation of HCl and HBr results in a significant redshift in the absorption spectrum; in fact, a new CTTS band appears. Thus the CTTS band provides an important spectroscopically observable clue of the acidic dissociation. There are now two major interconnected questions to be discussed: (1) Are the calculations

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 879 6

Intensity (arb. units)

4 2

0

160

180

200 220 240

260

160

180

200 220 240

260

5 4 3 2 1 0

Wavelength l (nm)

Figure 39.12 Absorption cross sections of HX(H2O)4 clusters for intact, contact ion pairs and solvent separated structures. The geometries of the clusters are shown in Figure 39.11. The long vertical arrows mark the excitation wavelength. Reprinted with permission from [25]. Copyright 2008 American Chemical Society

supported by our recent photodissociation experiments? (2) Are HCl and HBr acidically dissociated under the experimental conditions? The quantity for which the theoretical prediction can be directly compared with the photodissociation experiment is the ratio of hydrogen atom yields produced from HCl and HBr in different environments. These ratios are compared with the values calculated for the model HX(H2O)4 clusters in Table 39.1. The measurement for the bare molecule, which gives 23  5, agrees within experimental errors with the known absorption cross sections [77, 78]. The ratio does not change very much when the halogen halides are complexed with argon, leading to 20  5. The same ratio, however, measured for hydrogen halides deposited on the water clusters gives 7.4  1. The calculations have to be carried out for: jðHBrÞ ½s193 ðHBrÞ þ s243 ðHBrÞ ¼ jðHClÞ ½s193 ðHClÞ þ s243 ðHClÞ

ð39:5Þ

since, in the experiment, photons of the two wavelengths are actually present. The result for water complexes yields a theoretically predicted ratio of the photolysis of about 2.7 for the contact ion and 1.8 for the solvent separated ion structure (Table 39.1). A theoretical estimate of this ratio for the intact structure (as well as for bare HX from our calculations) is difficult because even at the 193 nm we sample only the very red tail of the HCl spectrum. Nevertheless, by extrapolation one can conclude that the ratio exceeds 30 for the bare molecule and 45 upon the complexation with water. Therefore, the comparison strongly suggests that acidic dissociation takes place. The measured ratio still somewhat exceeds the theoretical prediction of the j(HBr)=j(HCl) ratio. The most likely explanation is that the precise quantitative agreement is not achieved due to the comparison of theoretical calculations performed for small water clusters with the experiment conducted on much

880 Hydrogen Bonding and Transfer in the Excited State Table 39.1 Comparison of calculated and experimental ratios of the photolysis rate of HCl to HBr, j(HBr)=j(HCl), in different environments and dissociation states Environment Bare molecule HX HX–Ar100 HX–(H2O)400

Measured 23  5 20  5 7.4  1.0

Motif

Calculated

Bare molecule

30

HX(H2O)4 dissociated HX(H2O)4 intact

2.7 (1.8)a 45

a

Number in parenthesis refers to the solvent separated ions.

larger clusters. In addition, the TDDFT calculations may introduce some errors. The conclusion that the j(HBr)=j(HCl) ratio drops upon an acidic dissociation is, however, quite robust. Summarizing, we state that the photodissociation mainly follows the mechanism presented on the righthand side of Figure 39.8 [indicated by (1)]. In the first step the intact hydrogen halide molecule undergoes acidic dissociation to the zwitterionic state. Then this state is excited by the 193 nm laser pulse to the CTTS state, which relaxes to the biradical state with H3O and Br=Cl. The H3O then decays into H2O and H, which is detected by photoionization. The interaction between the HBr and the HCl molecules with the water molecules of the cluster is of a very local nature within the experimental time scale (<1 ms), otherwise we would have observed a pronounced dilution. 39.3.5 Atmospheric implications Hydrogen halides, in particular HCl, are important species in atmospheric chemistry [79]. Isolated HCl molecules are photochemically intact in both the troposphere and stratosphere. HCl molecules therefore represent an important reservoir species for the production of the reactive Cl radical. Release of the nascent Cl radicals leads to the chain reaction of ozone destruction. It has been known for quite some time that the heterogeneous chemistry of HCl on ice particles in the polar stratospheric clouds (PSCs) is a key step in the chlorine radical production. The commonly accepted mechanism assumes Cl2 molecule generation on the PSC particles. The chlorine molecule is significantly more active photochemically at the relevant UV wavelength region >200 nm penetrating into the stratosphere than the HCl molecule [80]. We have shown that the acidic dissociation itself is sufficient to significantly enhance Cl radical production from HCl on ice nanoparticles. The acidic dissociation shifts the electronic absorption spectrum to the red, so that it can be expected that the photolysis rate will be much enhanced when the hydrogen halide lands on a water cluster and acidically dissociates. To quantify this issue, we have calculated the photolysis rate using an actinic flux measured at an altitude of 50 km. The photolysis rate increased by four orders of magnitude upon the HCl uptake and dissociation on (H2O)4 cluster; the enhancement for HBr was two orders of magnitude. Notably, our dynamical calculations show that the Cl radical is released directly from the ice nanoparticle. Ice nanoparticles thus might catalyse HCl photolysis in a way that can potentially play a non-negligible role in the stratospheric chlorine budget.

39.4 Hydrogen Bonded Clusters of Nitrogen Heterocycles To investigate the influence of different hydrogen bonding patterns on the photostability of molecules, the photodissociation of pyrrole (Py), imidazole (Im) and pyrazole (Pz) molecules in clusters has been studied. All three molecules have a very similar five-membered heteroaromatic ring structure (Figure 39.13), which can be found in many biological compounds. Pyrrole is one of the simplest molecules with biological relevance: the

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 881

Figure 39.13

Structures of the studied molecules

pyrrole structure is present in, for example, hemes and chlorophylls. The imidazole structure can be found in purine, histidine and so on. Purine is a molecular skeletal building block of the nucleic acid bases adenine and guanine. Histidine is a naturally occurring amino acid and imidazole can be found in its side chain. Imidazole is, for example, also an important ligand towards transition metal ions in vitamin B12. Pyrazole is an isomer of imidazole, which is rare in nature; however, due to the different position of the nitrogen atom in the heteroaromatic ring, it generates clusters with hydrogen bond motifs different from those of the imidazole clusters. Therefore, pyrazole clusters have been investigated for the purpose of comparison. The different hydrogen bond motifs for the dimers are schematically represented in Figure 39.14. These different structural motifs can lead to different pathways in photodissociation. In the pyrrole clusters the NH bond of one molecule binds to the p-electron cloud of the neighboring molecule, NH


p. This type of interaction represents the simplest type of solvation, in which the chromophore does not react with the solvent. If, however, the NH bond is involved in hydrogen bonding, the hydrogen atom of the donor molecule can migrate to the acceptor molecule and new phenomena can occur, for example, hydrogen or proton transfer in the excited state. This is the case for pyrrole


ammonia [81], pyrrole


water [82], phenol


ammonia [83, 84] or pyrrole


pyridine complexes [85]. In the imidazole clusters the “normal” NH


N hydrogen bond is observed, and in the pyrazole dimer the NH


N double-bond is present. The latter structure closely resembles the bonding pattern between DNA base pairs. Therefore it is interesting to investigate how these different bonding motifs can influence the photochemistry following a UV excitation and the stability of the molecules in relation to the question of stability and radiation damage of larger biomolecules. The UV photochemistry of isolated nitrogen heterocycles in the gas phase has been studied extensively both experimentally [86–89] and theoretically [90–93]. Photochemical pathways in these compounds are controlled by an interplay between the dissociation channel on the ps* state and ground state recovery via different mechanisms. Especially, the role of the ps* states in the photodissociation of these and similar molecules (phenol) has recently attracted much attention [86]. The potential energy surfaces (PES) for all three molecules Py, Im, and Pz possess qualitatively very similar features, involving low lying excited ps* and pp* states and conical intersections between them and between the ground S0 state. Figure 39.15 shows, schematically, cuts

Figure 39.14

Schematic picture of the different hydrogen bonding motifs represented by the Py, Im and Pz dimers

882 Hydrogen Bonding and Transfer in the Excited State

Figure 39.15 Schematic picture of general PES typical for the studied species based on ab initio calculations for Py molecule. The cuts through PES along the NH stretching and ring-deformation coordinates are shown. The arrows that follow the cuts indicate the possible dissociation pathways. Circles indicate the CI ps* =S0 , pp* =ps* and pp* =S0

along the NH stretching coordinate and the ring deformation coordinate through a general PES for such systems (based on the real calculated PES for the pyrrole molecule [94]). The general picture of the photochemistry, which emerged from the numerous gas phase studies, is schematically represented in Figure 39.16. Upon low energy excitation, the photodynamics is dominated by the ps* state. This state is (asymptotically) dissociative and, as a result, the hydrogen atom is released (hydrogen dissociation, HD, channel). At elongated NH distances, the ps* =S0 intersection occurs. It is therefore possible that frustrated dissociation (FD) takes place, and the molecular ground state is recreated. At higher photon energies, the pp* state is populated. A molecular ring distortion (MRD) reaction channel is thus opened. Subsequently, the molecule quenches into the vibrationally hot ground state where again the hydrogen atom can dissociate or other molecular fragments can be formed. As pointed out above, the role of the ps* states in biomolecules has lately been much discussed for the gas phase [39]. The functionality of biomolecules is, however, controlled by their environment, either by a solvent or by specific interactions with other molecular units (such as hydrogen bonding in DNA base pairs).

Figure 39.16 Schematic picture of photodissociation processes possible in the molecules Py, Im and Pz. Excitation of the ps* state can lead to direct hydrogen dissociation (HD) and fast H-atom production. Alternatively, frustrated dissociation (FD) can occur via a ps* =S0 conical intersection at elongated NH distances. At higher photon energies, the pp* state is populated and molecular ring distortion (MRD) can occur, quenching the molecule into the vibrationally hot ground state, where it can again dissociate, yielding the slow H-fragments or other products. Modified with permission from [102]. Copyright 2009 American Chemical Society

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 883

Therefore, the photochemistry in solvated systems can be quite different. This section summarizes the various effects the solvation and hydrogen bonding in clusters can have on the photodissociation dynamics and stability of these molecules. 39.4.1 Experimental results We have investigated the photodissociation of Py, Im and Pz clusters at two different wavelengths, 243 and 193 nm, and for various mean cluster sizes. Figure 39.17 shows examples of the measured H-fragment KEDs from Py [left-hand panels (a) and (b)] and Im [right-hand panels (c) and (d)] clusters at 243 nm. The top panels (a) and (c) correspond to the clusters generated in expansions with He as a buffer gas, resulting in mean cluster sizes of  n  3 for both Py and Im clusters. The bottom panels (b) and (d) correspond to the larger clusters generated in expansions with Ar as a buffer gas. Our mass spectrometric and scattering experiments have   8 for pyrrole [95], while pure Imn shown that these result in mixed PynArm clusters with n  4 and m clusters with  n  6 were generated for imidazole. No spectra for pyrazole are presented in Figure 39.17 since we did not observe any measurable photodissociation signal at 243 nm for pyrazole at any exploited expansion conditions. It should be mentioned that the KEDs in Figure 39.17 are normalized and the corresponding measured TOF spectra intensity exhibits a strong dependence on the mean cluster size, that is, the signal decreases significantly with the cluster size, as suggested by the increasing error bars on the data in Figure 39.17. All the spectra in Figure 39.17 exhibit a bimodal character with a narrower peak of faster fragments at approximately 0.8 eV, and a slower component with a broad distribution peaking below 0.4 eV. The spectra have been analysed and deconvoluted to the contributions of the two components (red and blue lines in Figure 39.17). Similar spectra have also been measured for the bare pyrrole [96–101] and imidazole [87] molecules. The relative ratio of the two contributions changes with the cluster size. Already from Figure 39.17 it is obvious that the fast component decreases in intensity relative to the slow one with increasing mean cluster size. Figure 39.18 shows this trend quantitatively: here the ratio of fast to slow (F/S) component obtained by integrating the corresponding peaks in the measured KEDs for Py and Im clusters at 243 nm is plotted as a function of the mean cluster size.

Figure 39.17 Measured KEDs of Py (a, b) and Im (c, d) clusters at 243 nm. The top panels correspond to small   3, while the bottom spectra correspond to mixed PynArm clusters with n   4 and m  8 clusters of mean size n   6 (d). The spectra are analysed for a slow and fast component (See Plate 49) (b) and pure Imn clusters with n

884 Hydrogen Bonding and Transfer in the Excited State

Figure 39.18 Fast-to-slow fragment ratios evaluated from the measured KEDs for Py and Im clusters at 243 nm plotted as a function of the cluster mean size. The F/S ratio obtained from Figure 39.17(b) for Py is plotted at n ¼ 12, which corresponds to the total mean size of the mixed PynArm cluster (See Plate 50)

Figure 39.19 shows the KEDs for Py, Im and Pz clusters of the mean cluster size n  3 measured at 193 nm. The spectra for Py and Im closely resemble the spectra measured for bare molecules in other experiments [87, 96–101]. Again, the spectra could be deconvoluted to the contributions of the fast and slow components. Also the dependence on the mean cluster size has been investigated at the various expansion conditions; however, since the relative contribution of the fast component is very small at 193 nm both for the bare molecule (see the references cited above) and for clusters (Figure 39.19) a reliable quantitative F/S ratio dependence is difficult to obtain. Nevertheless, the evaluated F/S ratios for Py, Im and Pz clusters at 193 nm are shown in Figure 39.20. Notably, here at 193 nm, unlike at 243 nm, a good H-fragment signal has been measured for Pz clusters, comparable to the signals from Py and Im clusters at similar conditions. As mentioned above, the comparable error bars on the three normalized spectra in Figure 39.19 suggest comparable signal intensities.

  3 at 193 nm. The spectra are analysed Figure 39.19 Measured KEDs of Py (a), Im (b) and Pz (c) clusters with n for a slow and fast component (See Plate 51)

Fast/Slow H-fragment ratio

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 885

0.3

Pz Py

0.2

Im 0.1 0.0

0

5 Mean cluster size n

10

Figure 39.20 Fast-to-slow fragment ratios evaluated from the measured KEDs for Py, Im and Pz clusters at 193 nm (See Plate 52)

39.4.2 Photochemistry of isolated and solvated heterocycles The two peaks in the KED spectra can be attributed to two distinct processes: (i) The slow peak corresponds to the hydrogen dissociation from the vibrationally excited hot ground state. (ii) The fast peak results from direct dissociation on the ps* state. The shape of the first peak can be modeled using statistical theory as was outlined in Ref. [102]. Note, as a technical point, that the photodissociation resulting in the slow component in our experiment corresponds to processes in which two photons were subsequently absorbed by the cluster within a single laser pulse, resulting in a statistical decay of the system with an energy content corresponding to almost twice the photon energy. Notably, the statistical decay of the imidazole molecule after single photon absorption resulted in a substantially narrower energy distribution of the H-fragment peaking at somewhat lower energies [87, 102]. The remaining part of the signal can be then attributed to the direct dissociation channel. The ratio between these two peaks therefore depends on the relative importance of the different dissociation channels. What we are trying to establish here is how the photodissociation dynamics of the molecule outlined in Figure 39.16 changes in clusters. Alternatively, one can approach this problem as, how the potential energy surfaces in Figure 39.15 change upon solvation. Phrased this way, the theoretical calculations can provide a significant insight. We start with the pyrrole clusters bound with the NH


p bond. The influence of a solvent on the photochemistry of the pyrrole molecule is shown schematically in Figure 39.21. We have extended the photochemical mechanism presented by Barbatti et al. [103] in a theoretical study to situations where the pyrrole molecule is solvated. The upper part of Figure 39.21 represents the cuts through the PES of pyrrole molecule along the NH stretch (right) and ring-deformation (left) coordinates. This includes the calculation of the ps* =S0 conical intersection along the NH stretch coordinate and the pp* =S0 and ps* =pp* conical intersections of the ring deformation. The influence of the solvent was simulated by the presence of an Ar atom at a fixed distance in the plane of the carbon atoms, which resembles the NH


p binding motif of the pyrrole dimer. The presence of the Ar atom has a strong influence on the conical intersection in the NH stretch coordinate (see the lower part of Figure 39.21). It is expected that the crossing between the ps* and the ground state will increase in energy due to the repulsive Pauli interaction with the argon atom. The effect is, however, even stronger – the conical intersection will cease to exist. The reason is as follows. While the S0 and the excited valence states are virtually unchanged, the highly delocalized ps* states, which are of a Rydberg character, are shifted by 0.5 eV in the Franck–Condon region and the interaction between the Rydberg state and argon becomes stronger with increasing NH distance. As a result the ps* =S0 conical intersection is no longer present and the channel for the fast H atoms is closed. The changes in the deformation coordinate upon solvation with Ar are, on the other hand, almost negligible. The pp* =S0 intersection remains open and the population of the slow peak is not hindered. To account for

886 Hydrogen Bonding and Transfer in the Excited State

Figure 39.21 Schematic picture showing the influence of a solvent on the PES of pyrrole molecule based on ab initio calculations. The upper panel shows the bare Py molecule, while the lower panel illustrates the effects of solvation in the NH bond stretching and ring deformation coordinates. Reprinted with permission from [94]. Copyright 2007, American Institute of Physics

solvation effects other than the Pauli repulsion, we have also performed the calculations for the pyrrole dimer. However, the above conclusions hold true even when the argon atom is replaced by another pyrrole molecule. Note that a very similar effect has also been observed at the same time in the group of Kitsopoulos [100, 101] by solvating the pyrrole molecule with a single Xe atom. The results of our photodissociation experiments can be understood in terms of this mechanism. We observe many more H atoms in the slow peak in clusters with respect to the molecule, because the route to the direct dissociation into the fast peak is closed. Furthermore, even when the molecule reaches the ps* =S0 intersection, the solvent can prevent direct dissociation and the system quenches to the bound ground state [104]. In addition, the out-of-plane mode is also operating in clusters and plays an important role, possibly even at 243 nm. The increase of the slow component relative intensity for 193 nm with respect to 243 nm is traced back to the direct excitation of the pp* state. The pp* =S0 conical intersection is in this case diabatically directly accessible and this does not change with complexation. An important message that can be drawn from these results is that conclusions about the photochemical behavior in a confined environment (e.g., in biological systems) that are solely based on gas-phase data may be misleading. In imidazole clusters a somewhat different picture arises from the calculations. Owing to the NH


N bonding, the hydrogen transfer (HT) between the imidazole units can play a role in the excited imidazole cluster. This is illustrated by our calculations for the imidazole dimer in Figure 39.22, which shows the energy profile along the interpolation coordinate between the optimized ground state structure (Franck–Condon point) (right) and the hydrogen transferred structure (left) going through a transition state (center), where the hydrogen is shared in the middle between the two imidazole units. The calculations performed at CAS-PT2 level of theory based on the CAS-SCF wavefunction with six electrons in six active orbitals are shown. The bottom curve always corresponds to the ground state S0 while the lower excited curve corresponds to the ps* state and the upper one to the pp* state, and the conical intersections are indicated. The HT mechanism is opened upon the excitation to the pp* state (at 193 nm in our experiment). Barriereless transition of the H-atom towards the hydrogen bond acceptor unit occurs on the PES until the system reaches the conical intersection pp* =S0 on the left-hand side boundary of Figure 39.22. There the electronic population can be funnelled to the ground state S0, where the H-atom moves back to the former hydrogen bond donor molecule. In this process the excitation energy dissipates between the two participating imidazole molecules and the imidazole dissociation is then much less probable. Thus the HT process in the cluster has a stabilizing

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 887

Figure 39.22 Schematic picture showing the effect of the hydrogen transfer on the photochemistry of imidazole dimer. The CAS-PT2 method based on the CAS-SCF wavefunction with six electrons in six active orbitals was used. The lowest curve corresponds to the ground state S0 while the lower excited curve corresponds to the ps* state and the upper one to the pp* state. The conical intersections are indicated

effect on the imidazole molecule. At the longer wavelength of 243 nm, the HT is still a barriereless process on the ps* state PES, yet the driving force for it is negligible (the PES is essentially flat). Therefore, the excitation energy stays localized on the one imidazole molecule within one complex, which may lead to its fragmentation. It ought to be mentioned that in the dimer all the processes observed for the bare imidazole molecule are still possible with the free NH bond of the hydrogen bond acceptor unit. However, larger species, starting from trimer, generate cyclic structures without the free NH bonds available. In our experiments, we probed the cluster size distributions with  n  3 and found some contributions of dimers and monomers. Therefore, the contribution from processes analogical to the processes in the bare Im molecule can still be non-negligible for  n  3. However, the contribution from these processes decreases with increasing mean cluster size, that is, for  n  6. Figure 39.23 summarizes suggested photochemical pathways for imidazole clusters (compare to Figure 39.16 summarizing the processes possible in the molecule). A similar effect can be expected in pyrazole clusters. It is now interesting to compare the photodissociation mechanism in all the studied clusters: At 243 nm the measured F/S ratio decreases with the complexation for both Py and Im clusters (Figure 39.18). We have shown that the change in PES due to the solvation, which closes the dissociation channel leading to the fast fragments, is responsible for this dependence in pyrrole clusters. In the imidazole clusters the EFD process is suggested to cause the F/S ratio decrease. No photodissociation was observed for pyrazole clusters at this wavelength, presumably due to their larger NH bond stability (the dissociation energies are 4.07, 4.12 and 4.6 eV for Py, Im and Pz, respectively). At the shorter wavelength of 193 nm the fast fragments are much less populated, therefore the F/S ratio trends are difficult to asses. Yet, it seems that this ratio is independent of the cluster size for pyrrole clusters, while it decreases for imidazole and pyrazole (Figure 39.20). This would suggest that a different mechanism operates in Py clusters than in the Im and Pz clusters. In Py the dissociation at 193 nm proceeds mainly along the ring-deformation coordinate, which remains unchanged by the solvation, and therefore the F/S ratio remains independent of cluster size. On the other hand, in imidazole clusters the dissociation at 193 nm is driven to the HT channel, which funnels the population to the ground state and thus decreases the probability for generating the fast fragments. This is indeed not possible for the bare molecule, which leads to the F/S ratio decrease with complexation. A similar situation can be expected in pyrazole

888 Hydrogen Bonding and Transfer in the Excited State

Figure 39.23 Schematic picture of photodissociation processes possible in imidazole clusters (dimer): At 243 nm the ps* state of an imidazole molecule is excited, leading to the direct free hydrogen dissociation. However, the frustrated dissociation may be enhanced by the cluster environment (EFD). At higher photon energies, the pp* molecular state is populated. In addition to the molecular ring distortion (MRD) channel present for the molecule the hydrogen transfer (HT) channel can also occur in the clusters. Modified with permission from [102]. Copyright 2009 American Chemical Society

clusters, which are also bound by the NH


N bonds, allowing for the HT process to occur. Thus this is tentative evidence for operation of the HT process in the clusters bound by the NH


N bonds (Im, Pz) as opposed to the NH


p bound Py clusters.

39.5 General Conclusions and Outlook Generally, by the above examples of aqueous systems and clusters of heteroaromatic ring molecules we have illustrated that the various hydrogen bonded systems provide a large playground for various processes to occur in the excited states: opening new reaction channels such as the generation of the H3O radical or the solvated electron, on the one hand, and closing some dissociation channels upon solvation, on the other hand, either by electronic interaction with the solvent or by hydrogen transfer reactions and subsequent energy dissipation. Now, as the next step, we would like to merge the two areas of our research presented here and investigate the biologically relevant heteroaromatic molecules in the aqueous clusters. In this way, for example, the role of the solvated electron from the solvent environment on the photochemistry of the heteroaromatic molecules can be investigated. The ultimate goal of such research is to shed some light onto the molecular level mechanism of such important and complicated processes as DNA radiation damage. More specifically, we have provided some evidence that the H3O radical is a central species in the photochemistry of the aqueous systems. Two mechanisms of H3O generation were suggested: In the multiphoton excitation of the pure (H2O)n clusters the H3O is generated in the excited state by a reaction of hydrogen atom fragments with water molecules. The H3O radical is stabilized by solvating water molecules and ultimately it decays to the observed H-atom. A completely different mechanism of H3O radical production was found in the HX(H2O)n clusters. Here, the precursor ion H3Oþ is generated by the acidic dissociation already in the ground state, which is then excited to generate the H3O radical via a charge-transfer-to-solvent (CTTS) process. This mechanism seems to be a general pattern in the systems, where acidic dissociation can occur. Thus a logical next step of our research after HBr and HCl molecules on water clusters is to prove experimentally that the mechanism also operates for the analogical system with HI molecules. A very

Photodissociation of Molecules in Pure and Doped Water and in Nitrogen Heterocyclic Clusters 889

interesting case is the photochemistry of the much less acidic HF molecule. As a weak acid, hydrogen fluoride should not undergo the acidic dissociation and the CTTS mechanism should therefore play no role. Recent experiments have, however, suggested [105, 106] that the actual structure of these species can be best described as a proton shared structure. From a more practical point of view, and of relevance to the atmospheric chemistry, an extension towards nitrogen- and chlorine-containing molecules (HNO3, NOx, OClO) in aqueous clusters is of great interest. For the heteroaromatic molecules we have demonstrated how profound an effect the solvation can have on the photochemistry of these species. The major message from these studies is that one should cautiously examine the relevance of gas-phase data when making conclusions about the photochemistry of these molecules solvated in biological systems. In particular, we have demonstrated that the dissociation channel can be closed upon solvation by electronic interaction in pyrrole clusters. On the other hand, in imidazole and possibly also in pyrazole clusters the closing of the fast dissociation channel is suggested to be due to the hydrogen transfer process and subsequent energy dissipation in the system. Further studies of the present molecules in water clusters, which is the natural solvent in biological systems, are planned, as well as the extension of our experiments towards larger biomolecules.

Acknowledgements Support by the special program “Nanotechnology for society” of the Czech Academy of Sciences via grant Nr. KAN400400651, grants Nr. 203/09/0422 and 203/07/P449 of the Grant Agency of the Czech Republic are acknowledged. M. F. acknowledges a special J. E. Purkyneˇ fellowship of the Czech Academy of Sciences.

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Index Bold page numbers indicate tables. Italic numbers indicate figures. ab initio molecular dynamics (AIMD) approach, 579, 580, 587 ABPH, see 5-amino-3-arylmethylene-1(3H)isobenzofuranones Acaryochloris marina, 445 acceptor rehybridization model, 315 acetic acid (AcOH), interactions with betacarboline derivatives, 699–704 acetone-HFIP systems, 47 acetone-PFTB systems, 47 2-acetyl-4-chloro-6-nitrophenol, 653–7, 656 2-acetyl-4-methyl-6-nitrophenol, 653–7, 656 acridanones, 280–3 acridine, 737, 747–8 acridine orange (AO), 247 activation energy barrier-crossing transition, see Arrhenius-type energy-barrier-crossing model 2-acylaminophenol, 613 ADC, see analog-to-digital converter adenine, 126, 128, 129 adenine-thymine base pairs, 1–3, 15, 126 excited-state properties, 138–140 ground state structures, 128–9 adenine-uracil base pairs, 2, 3, 9, 22–3 correlations between transitions 0 ! 1 and 1 ! 2, 17–18 excited-state properties, 138–140 frequency fluctuation correlation function, 15–17 geometric correlations, 9–11 IR lineshape, 20, 23 NH stretching-HB length correlation, 13–15 photon echo spectra, 21–2 pump-probe spectra, 20–1 quantum correction for the bath modes, 18–19 velocity autocorrelation function, 12–13 adenosine, 200

Hydrogen Bonding and Transfer in the Excited State, Volume I & II © 2011 John Wiley & Sons, Ltd. ISBN: 978-0-470-66677-7

adiabatic compressibility, 236–7 adiabatic photoisomerization, 211–14 adiabatic/nonadiabatic proton transfer reaction, 737, 738 Agmon-Levine model, 646 7AI, see 7-azaindole AACID, see 5-amino-2-aryl-2-carboxymethylindan-1,3diones AID, see 5-aminoindan-1,3-diones AIMD approach, see ab initio molecular dynamics approach alcohols deuterated, 102–3 hydrogen-bonding interactions with fluorenone, 770–5, 777–81, 786, 787 hydrogen-bonding interactions with ketocyanine dyes, 781–4 hydrogen-bonding interactions with resorufin, 776–7 involvement in hydrogen bond formation, 80, 81 alloxazines, 93–4 amide NH prelocated, 609–13 remote, 613–15 5-amino-2-aryl-2-carboxymethylindan-1,3-diones (AID), 270–4 5-amino-3-arylmethylene-1(3H)-isobenzofuranones (ABPH), 269, 278–80 aminoanthraquinones, 81, 82, 103 aminobenzylidenaphthalides, see 5-amino-3-arylmethylene-1(3H )-isobenzofuranones (ABPH) 7-aminocoumarins, 89, 178, 420, 584 ICT to TICT conversion 424–6 aminofluorenones, 81, 102–3, 766, 785 5-aminoindan-1,3-diones (AID), 269, 270–4 4-aminophthalimide (4-AP), 220, 243 4-aminophthalimide, 99, 420 2-aminopurine, 95, 114, 143

Edited by Ke-Li Han and Guang-Jiu Zhao

894 Index amphiphilic molecules, 711 analog-to-digital converter (ADC), 362 aniline, complex with coumarin 102 (C102), 770, 771, 789–90 1-anilino-8-naphthalenesulfonic acid (ANS), 221, 224, 225, 251–6 1-anilinonaphthalene-8-sulfonate (1,8-ANS), 179, 180 anion sensing by conjugated polymers, 810–13 through ESIPT process, 806–8, 810–13 through ESPT process, 808–10 anisotropy, 231–6 anoxygenic photosynthetic bacteria, 434, 437, 451, 452 see also purple bacteria reaction centres (PBRCs) 1,8-ANS, see 1-anilinonaphthalene-8-sulfonate ANS, see 1-anilino-8-naphthalenesulfonic acid 9,10-anthraquinone (AQ), 200, 201 anthraquinone derivatives, 81, 92, 100 AO, see acridine orange AOT, see bis(2-ethylhexyl)sulfosuccinate sodium salt APDs, see avalanche photodiodes AQ, see 9,10-anthraquinone AQ143 (fluorescent protein), 819 aromatic molecules, 29 Arrhenius-type energy-barrier-crossing model, 219–20, 237 electrostatic attachment of ligand molecules, 256–9 in micellar systems, 237–44 in mixed reverse micellar systems, 250–6 in reverse micellar systems, 244–50 3-arylmethylene-1(3H)-isobenzofuranones (BPH), 275–80 Asp-containing oligopeptides, 613–14 ATR, see attenuated total reflection attenuated total reflection (ATR), 343 avalanche photodiodes (APDs), 361 7-azaindole (7AI), 465, 556, 646, 647, 661–2 conjugated dual hydrogen bonding (CDHB) formation, 563 crystal form, 559–61 cyano analogues of, 571–3 ESDPT in analogue homodimers, 558–61 ESDPT in dimers, 556–8 ESDPT in heterodimers, 561–2 ESDPT in host/guest-type HB complexes, 563–7 hydrogen-bonding geometries, 671 7-azaindole-(H2O)n clusters, 580–4 7-azaindoline (7AZD), 565–6 7AZD, see 7-azaindoline azines, 194 bacteriorhodopsin (BR), 378, 379, 525 detection of water stretching vibrations in, 379–80 hydration switch model, 380–2, 383 role of strong hydrogen bond of water, 379–80, 386–7 structural changes of water in, 382–4

Badger’s rule, 11 basis set superposition error (BSSE), 139 BC, see betacarboline BCA, see N2-methyl-9H-pyrido[3,4-b]indole BEBO model, see bond energy bond order Beer-Lambert law, 684 Beer-Lambert relation, 343 Benesi-Hildebrand equation, 684, 694–5 benzenoid-p bases, 682–5 benzimidazoles, 225, 754–5 benzo[b]fluorenone, 99–100 2-benzofuryl-3-hydroxy-4(1H)-quinolone (3-HQ-Bf), 97 4H-1-benzopyrane-4-thione (BPT), 112–13, 182, 184 benzopyridinic bases, 687–92 2,5-bis(2-benzoxazolyl) hydroquinone, 649 benzylidene phtalides, see 3-arylmethylene-1(3H)isobenzofuranones beta-turn structure, 613, 614 betacarboline (BC), 394, 663 BC-AcOH system, 699–704 BC-BC system, 409–14 BC-HFIP system, 395, 406–9 BC-pyridine system, 405–14, 689–92 interactions with benzenoid-p bases, 682–5 interactions with benzopyridinic bases, 689–92 interactions with methylbenzene bases, 685–6 betacarboline derivatives, 394–5, 680–2 BCA-HFIP system, 403–6 HN-AcOH system, 702–4 HN-HFIP system, 695–8 HN-pyridine system, 687–8 interactions with hydrogen-bond acceptors, 682–92 interactions with hydrogen-bond donors, 692–9 interactions with hydrogen-bonding donoracceptors, 699–705 MBC-HFIP system, 396–403 MHN-t-BuOH system, 693–4 MHN-CIEtOH system, 693–4 MHN-HFIP system, 396–403, 693–4 MHN-pyridine system, 687 bile salts, 163–5, 167 ‘biological water’, 218 biomimicking systems, 218–22, 259–60 Arrhenius model in micellar systems, 237–44 Arrhenius model in mixed reverse micellar systems, 250–6 Arrhenius model in reverse micellar systems, 244–50 self-organized assemblies, 222–3 biomolecules, radiation damage, 866 biorecognition, 566 biprotonic transfer reactions, 556–62 2,5-bis(benzoxazol-2-yl)benzene-1,4-diol (bis (HBO), 756–8 2,5-bis[(2,3-dihydroindolyl)propylene]cyclopentanone (KCD), 781–4, 786, 787 bis[2,6-di(pivaloylamino)] phenyl disulfide, 610

Index bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT), 218, 221, 222, 223, 224, 244–50 in anion sensing, 810–11 bis(HBO), see 2,5-bis(benzoxazol-2-yl)benzene-1,4-diol 2,5-bis(N-methyl N-1,3-propdienylaniline)cyclopentanone (MPAC), 781–2 bis(2,4,6-trihydroxy-phenyl), 182 blue-shifted hydrogen bonds, 126 bond energy bond order (BEBO) model, 646 Boys-Bernardi counterpoise correction scheme, 139 BPH, see 3-arylmethylene-1(3H)-isobenzofuranones BPheo, 439–40 BPT, see 4H-1-benzopyrane-4-thione BR, see bacteriorhodopsin Brønsted-Lowry definition, 463 BSSE, see basis set superposition error t-BuOH, see tert-butyl alcohol tert-butyl alcohol (t-BuOH), interactions with betacarboline derivatives, 692–4 2-butylamino-6-methyl-4-nitropyridine-N-oxide, 509 Ca(II) complexes, 618–19, 622–3, 624 calcium phosphate clusters, 622–3 calixarenes, 185–6 Car-Parrinello molecular dynamics (CPMD), 2 carbazole, 179, 180, 182–3 b-carbolines, 100, 194 carboxylic acid derivatives, 612–15 CARS, see coherent anti-Stokes Raman scattering CASPT2 method, 108 CASSCF method, 108 Catal
an model, 85–6, 89 ‘cation-like exciplex’ (CL ), 695–8 CBs, see cucurbiturils CC methods, 108 Cd(II) complexes, 619 CDHB effect, see conjugated dual hydrogen bonding effect CDs, see cyclodextrins cetylpyridinum chloride, 713–14 cetyltrimethylammonium bromide (CTAB) CTAB-methoxynaphthalene systems, 723–5, 726–30 CTAB-naphthol systems, 719–30 CTAB-NaSal systems, 714 micelles, 169, 739, 742 CFCs, see chlorofluorocarbons chair structure, 613 chalcones, synthetic, 283–4 charge-transfer-to-solvent (CTTS), 627, 628–30, 633 Chlide, see chlorophyllide chloride-ion pump, 384–5, 387 chlorine radicals, 880 1-chloro-n-alkanes, 99 2-chloroethanol (CIEtOH), interactions with betacarboline derivatives, 692–5 chlorofluorocarbons (CFCs), 627 chloroperoxidase (CPO), 621, 622

895

chlorophyllide (Chlide), 858–9, 861–2 CHT, see chymotrypsin chymotrypsin (CHT), 225, 443 CI, see configuration interaction CIEtOH, see 2-chloroethanol CIS method, 107–8, 534 CISD method, 108 CL , see ‘cation-like exciplex’ ‘close proximity effect’, 92, 94 CMC, see critical micelle concentration Co(II)-thiolate complexes, 619 coherent anti-Stokes Raman scattering (CARS), 342, 345 conductor-like screening model (COSMO), 108 configuration interaction (CI), 107, 108, 109, 113–15 confined environments, 737–8 see also nanoconfined systems conformational switching, 613–15, 621–2 conjugated dual hydrogen bonding (CDHB) effect, 563 conjugated polymers, 810–13 COSMO, see conductor-like screening model p-coumaric acid (pCA) chromophore, 840–51 coumarin 1 (C1), 178, 424, 425–6 coumarin 7, 103, 426–9 coumarin 30 (C30), 426–9 coumarin 102 (C102), 178–9, 194–5 C102-aniline complexes, 770, 771, 789–90 C102-phenol complexes, 102, 741, 769–70, 787–9 chemical structure, 764 in hydrogen-bonded complexes, 769–70, 771, 787–9 coumarin 120 (C120), 421–4, 584, 585 coumarin 151 (C151), 107, 110, 220, 333, 334, 421, 424, 785 excited-state dynamics of, 584–7 coumarin 152 (C152), 178, 334, 424, 426 coumarin 153 (C153), 101, 110, 178, 220, 334, 334 335, 785 solvation dynamics in [N3[1]][Tf2N] ionic liquid, 335–9 coumarin 343 (C343), 245–6 coumarin 480 (C480), 247, 334 coumarin 481 (C481), 424, 426 coumarin 500 (C500), 221, 224, 225, 244–56 coumarin 522 (C522), 334, 334, 335 coumarins, 81, 92, 420 as fluorescent probes, 178–9 ICT to TICT conversion, 424–9 intermolecular hydrogen bonding, 194–5, 421–6 intramolecular hydrogen bonding, 426–9 PET reaction in coumarin-amine systems, 332–5 temperature-dependent solvation, 220 CPB-naphthol systems, 719–21 CPMD, see Car-Parrinello molecular dynamics CPO, see chloroperoxidase CPP, see critical packing parameter cresol, 716 critical micelle concentration (CMC), 711

896 Index critical packing parameter (CPP), 712–13 CTAB, see cetyltrimethylammonium bromide CTTS, see charge-transfer-to-solvent cucurbiturils, 186–7 CURC, see curcumin curcumin as photosensitizer, 357–8 chemical structure, 357 ESIPT, 368–9 hydrogen-bond-mediated de-excitation, 366–9 KEHB formation, 363 cyano-substituted indolines, 91 3-cyanoaniline, 91 cyclodextrins (CDs), 180–5 chemical structure, 181 in host-guest inclusion complexes, 169, 180–5, 324–5 ESIPT, 185, 657 ESPT, 185, 738 Cys-containing oligopeptides, 612 cytosine, 126, 128, 129 DAF, see N,N-dimethyl ethanol ammonium formate ‘dangling’ hydrogen bonds, 781, 790 DBO, see 2,3-diazabicyclo[2.2.2]oct-2-ene DBPZ, see dibenzo[a,c]phenazine DCM, see 4-(dicyanomethylene)-2-methyl-6(p-dimethylamino-styryl) 4H-pyran DCMeth, see dicinnamoylmethane deuterated alcohols, 102–3 deuterated water, 103 DHBQ, 407, 409, 415, 696, 698–9 DHBZ, 696, 698–9 2,3-DHN, see 2,3-dihydroxynaphthalene 40 -dialkylamino-3-hydroxyflavones, 104 4-dialkylaminopyrimidines, 320, 321 diarylazomethines, 614, 616 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO), 183 diazines, 194, 689 dibenzo[a,c]phenazine (DBPZ), 196–9 dicinnamoylmethane (DCMeth) chemical structure, 363 ESIPT, 371 hydrogen-bond-mediated de-excitation, 369–71 KEHB formation, 363, 369, 371–3 diCN-HBO system, 573–4 4-(dicyanomethylene)-2-methyl-6(p-dimethylamino-styryl) 4H-pyran, (DCM), 220, 224–5, 239–43 p-(N,N-diethylamino)benzoic acid (DMABA), 184, 321 diffusion, 170–2 6-diformyl phenol, 650–2, 653, 654, 655 dihydrogen bonds, 126 2,3-dihydroxynaphthalene (2,3-DHN), 716, 717 1,2-di(30 -isoquinolyl)ethene ((3IQ)2E), 208–10, 211 b-diketones, 356–8 dimethyl aniline (DMA), 332 N,N-dimethyl ethanol ammonium formate (DAF), 332–5

4-N,N-dimethylamino cinnamaldehyde, 104 4-(dimethylamino)-pyridine (DMAP) absorption properties, 51–6 DMAP-HFIP complexes, 51–2, 54–6, 70–2, 74 hydrogen bond basicity, 63 solvatochromism, 69–72 triplet formation yield, 74 4-dimethylaminobenzethyne, 315 p-dimethylaminobenzoate, 112, 113 dimethylaminobenzonitrile (DMABN), 70–2, 112 dual fluorescence, 313–16 role of polarity and viscosity in ICT emission, 317–18 TICT emission, 183–4, 318–20 N,N-dimethylaminonaphthyl acrylates, 319 N,N-dimethylaminonaphthyl-(acrylic)-acid (MDMANA), 97 N,N-dimethylaminonaphthyl-(acrylo)-nitrile (DMANAN), 97, 103 2-(40 -N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP-b), 95, 325–7 2-(40 -N,N-dimethylaminophenyl)pyrido[3,4-d]imidazole (DMAPPI), 121–7 inclusion complex with cyclodextrins, 324–5 N,N-dimethylaniline (DMAN), 52–3 N,N-dimethylbenzodiazepine, 97 N-(2,6-dimethylphenyl)2,3-naphtalimide (DMPN) absorption spectra , 43, 44–5, complexation, 68–9 DMPN-HFIP-n-hexane systems, 43, 44–9, 56–60, 75–6 DMPN-PFTB-n-hexane systems, 47 hydrogen bond basicity, 56–9 Dimroth-Reichard model, 84–5 1,2-di(20 -naphthyl)ethene ((2N)2E), 208 dipolar solvation, 761 dipyrido[2,3-a:30 ,20 -i]carbazole (DPC), 662–9 1,2-di(30 -quinolyl)ethene ((3Q)2E), 208, 211 ‘distinguished coordinate’ approach, 595, 599 DMA, see dimethyl aniline DMABA, see p-(N,N-diethylamino)benzoic acid DMABN, see dimethylaminobenzonitrile DMAC, see 1-(2-pyridyl)-5-(4-dimethylaminophenyl)penta-2,4-diene-1-one DMAN, see N,N-dimethylaniline DMANAN, see N,N-dimethylaminonaphthyl-(acrylo)nitrile DMAP, see 4-(dimethylamino)-pyridine DMAPIP-b, see 2-(40 -N,N-dimethylaminophenyl)imidazo [4,5-b]pyridine DMAPPI, see 2-(40 -N,N-dimethylaminophenyl)pyrido [3,4-d]imidazole DMPN, see N-(2,6-dimethylphenyl)2,3-naphtalimide DNA, 126–7, 199–201 see also nucleic acid base pairs; nucleic acid bases DNA photolyase, 858 dodecyltrimethylammonium bromide (DTAB), 739–40 donor rehybridization model, 315

Index double hydrogen bond complexes, 407–9 double-proton transfer, 661, 664, 667, 676 see also excited-state double proton transfer (ESDPT) DPC, see dipyrido[2,3-a:30 ,20 -i]carbazole drugs, fluorescence decay studies, 354–5 DsRed, 819, 820, 830, 831 DSS, see dynamic Stokes shift DTAB, see dodecyltrimethylammonium bromide dual fluorescence, 464, 465, 590 applications, 518–19 betacarboline derivatives, 403, 407, 414 effect of excitation frequency on, 506–9 3-hydroxyflavones, 467–9, 505, 516–17 solute-solvent hydrogen bond formation, 95–8 TICT mechanism, 313–16 dual-luminescent napthalimides, 48–51 dynamic quenching of fluorescence, 475–509 effect of excitation frequency on dual fluorescence, 506–9 effect on excited-state depopulation rates, 476–8 effect on fluorescence intensities, 489–91 effect on populations of excited states, 484–91 effect on proton transfer rate, 503–4 effect on steady-state spectra, 478–80 in 3-hydroxyflavone, 480–1, 504–6 in 3-hydroxyflavone analogues, 481–3 reactions from high-lying excited states, 483–509 Stern-Volmer constants, 491–4, 499–501 temperature effect on proton transfer rate, 501–3 temperature quenching, 496–9 two-state excited-state reaction, 475 under different physical conditions, 494–99 dynamic solvation, 82 dynamic Stokes shift (DSS), 160 Edward’s method, 71 EET, see excitation energy transfer effective fragment potential (EFP) method, 580 effective medium theory, 237 EFP method, see effective fragment potential method Eigen model, 646 electron nuclear double resonance (ENDOR), 440 ENDOR, see electron nuclear double resonance enzymes, light-driven 858–9 see also protochlorophyllide oxidoreductase (POR) ES-AIMD simulations, 628–9, 631–2, 635–6 ESCT, see excited-state charge transfer ESDPT, see excited-state double proton transfer ESHAT, see excited-state H-atom transfer ESHT, see excited-state hydrogen transfer ESIPT, see excited-state intramolecular proton transfer ESPT, see excited-state proton transfer excitation energy transfer (EET), 437 excitation wavelength dependence, 159–66 excited-state charge transfer (ESCT), ESPT-coupled 567–74, 569–73

897

excited-state double proton transfer (ESDPT), 465 catalytic versus noncatalytic reaction, 563–4, 566 in 7-azaindole analogue homodimers, 558–61 in 7-azaindole dimers 556–8 in 7-azaindole heterodimers, 561–2 in host/guest types of hydrogen-bonded complexes, 563–7 in multiple-hydrogen bonding systems, 566–7 p-electron conjugation tuning, 565–6 excited-state H-atom transfer (ESHAT), 526 chromophore local solvation, 546–8 in green fluorescent protein, 546–8 mode selectivity, 543–6 reaction path, 536–40 solvent effect, 548–51 wire solvation, 540–2 excited-state hydrogen transfer (ESHT), 579–80 in 7-azaindole-(H2O)n system, 580–4 excited-state intramolecular proton transfer (ESIPT), 355–6, 589–90, 646 anion sensing based on, 806–8, 810–13 applications, 650 compared in 1H2NA and 2H3NA, 601–5 four-level model, 472–3, 475, 642 from the Sn singlet state, 484, 509–18 in curcuminoids, 368–9, 371 in b-diketones, 356–7, 358 in o-hydroxy carbonyl compounds, 650–7 in 3-hydroxyflavones, 467–75, 509–18, 647, 648 in 2-(20 -hydroxyphenyl)benzothiazole, 648 in 2-(2-hydroxyphenyl)benzoxazole, 748–50, 755 in 2-(2-hydroxyphenyl)benzoxazole derivatives, 750–59 in naphthalene derivatives, 589–607 internal torsion and solvent dielectric, 653–7 kinetic type of reaction, 473, 474, 479 mechanism, 97, 98, 647 perturbation to, 648–50 solvent assisted, 646 thermodynamic type of reaction, 474, 480 types of, 646–7 within cyclodextrin cavities, 185 see also dynamic quenching of fluorescence excited-state proton transfer (ESPT), 464–5, 526–7, 641–2 adiabatic/nonadiabatic, 737, 738 anion sensing based on, 808–10 excited-state charge-transfer-coupled (ESCT/ESPT system), 569–73, 569–73 in nanoconfined systems, 167–70 in Y42F mutant, 850–1 intermolecular/intramolecular, 641, 642 kinetics of, 645–5 of hydroxyaromatic compounds, 736–7 of hydroxyaromatic compounds in organized media, 737–41

898

Index

potential energy diagram, 643–4 thermodynamics of, 644–5 within cyclodextrin cavities, 185, 738 see also excited-state double proton transfer (ESDPT); excited-state intramolecular proton transfer (ESIPT) excited-state reactions, characteristics of, 736–7 F dye, 467, 481–2 F2 dye, 467, 481–2 F127, 171–2 FA dye, 481–2 FCS, see fluorescence correlation spectroscopy Fe(II)) complexes, 622 Fe(III) complexes, 621–2 fluorenone, 92, 100, 104 as fluorescent probe, 179, 182 chemical structure, 764 CO stretching mode, 772–5 fluorenone-methanol complexes, 151–6, 741–2, 771, 781 interactions with alcohols, 741–2, 763–8, 770–5, 777–81, 786, 787 triplet excited states, 150–6 see also aminofluorenones fluorescence definition, 176 magic angle emission, 234 see also dual fluorescence fluorescence anisotropy, 231–6 fluorescence correlation spectroscopy (FCS) 170–2 fluorescence emission, 528, 529 fluorescence enhancement, 90, 92–5 fluorescence lifetime, 90, 176 fluorescence probes, for studying solvation dynamics, 81, 98–9 fluorescence quantum yield, 90, 176 fluorescence quenching, 90, 91–2 by hydrogen-bond strengthening, 741–2 by solvent, 498 in betacarboline derivatives, 411–3 in pyridylindoles, 678–79, 680 in pyrroloquinolines, 678–9, 680 fluorescence-quenching constant, 332 fluorescence resonance energy transfer (FRET), 166–7 fluorescence spectroscopy, 176, 177, 528, 530 and electronic absorption, 763–8 fluorescent proteins (FP), 819 computational methodology, 823–4 internal conversion mechanism, 829–32 TICT states in, 831 see also green fluorescent protein (GFP), red fluorescent proteins (RFPs) N-(3-fluorophenyl)-2,3-naphthalimide (MFPN), 48–51 F€ orster cycle, 464, 642, 645, 666 F€ orster equation, 687 four-wave mixing (IR) spectroscopy, 3–6

FP, see fluorescent proteins FRET, see fluorescence resonance energy transfer FTIR spectroscopy, 379–80, 388 G-factor, 235 Geiger mode, 361 Gibbs free energy, 433–4, 437, 761 green fluorecent protein (GFP), 526–7, 815–19 chromophore local solvation, 546–8 cluster models, 820–22 computational methodology, 820–3 photoisomerization mechanism, 816–17 pp and ps states, 546–8 proton transfer in, 816–18, 820–1, 824–9, 834–5 quantum dynamics calculations, 822–3, 834 structure of, 815–16 wild-type, 816, 825 Green’s function, 227, 229 Grotthus mechanism, 525–6 ground-state intramolecular proton transfer (GSIPT) 1H2NA, 596–9 2H3NA, 599–601 GSIPT, see ground-state intramolecular proton transfer guanine, 126, 128, 129, 200 hydration of, 127, 131–8 guanine-cytosine base pairs, 126 excited-state properties, 138–142 ground state structures, 128–9 hydration, 142 guanine-guanine base pairs, 140–2 guanosine 50 -monophosphates, 201 H258, see 20 -(4-hydroxyphenyl)-5-[5-(4-methylpiperazine-1-yl)benzimidazo-2-yl HA, see p-hydroxyacetophenone HA-13C, 288, 295 HAB, see 3-hetarylmethylene-1(3H)-isobenzofuranones HA-D4, 288, 295 hairpin-turn structure, 614, 622 halide anion-water clusters, 627 halorhodopsin (HR), 378, 379, 387, 388 role of water hydrogen bond, 384–5 Halorhodospira halophila, 840 1H2AN, see 1-hydroxy-2-acetonaphthone HAP, see 4-hydroxy-5-azaphenanthrene hard and soft acid-base behaviour (HSAB), 785 harmane, 101, 662 see also 1-methyl-9H-pyrido[3,4-b]indole (HN) HBC, see hydrogen bond complexes HBI, see 2-(2-hydroxyphenyl)benzimidazole HBO, see 2-(2-hydroxyphenyl)benzoxazole HBO derivatives, see 2-(2-hydroxyphenyl)benzoxazole derivatives Hbr-Arn systems, 867–8, 875 HBT, see 2-(2-hydroxyphenyl)benzothiazole HcRed, 819, 832–4

Index HEAN, see N-[2-(2-hydroxylethylamino)-ethyl]-1,8naphthalimide Henderson-Hasselbalch equation, 801 3-hetarylmethylene-1(3H)-isobenzofuranones (HAB), 274–5, 276, 277 1,1,1,3,3,3-hexafluoro-propan-2-ol (HFIP) complexes with betacarboline, 395, 406–9 complexes with betacarboline derivatives, 395–406, 692–8 complexes with DMAP, 51–2, 54–6, 70–2, 74 complexes with DMPN, 43, 44–9, 56–60, 75–6 complexes with isoindolo[2,1-a]indole-6-one, 41–4, 61–2, 73 hexanol, 718 4-hexylresorcinol, 85 HFIP, see 1,1,1,3,3,3-hexafluoro-propan-2-ol 3HFs, see 3-hydroxyflavones Hg(II) complexes, 619, 620 high-lying excited states, 483–4 6HIQ, see 11-propyl-6H-indolo-[2,3-b]quinoline 1H2MN, see methyl-1-hydroxy-2-naphthoate HN, see 1-methyl-9H-pyrido[3,4-b]indole 1H2NA, see 1-hydroxy-2-naphthaldehyde 2H3NA, see 2-hydroxy-3-naphthaldehyde Hoogsteen base pairs, 129 HOPB, see 2(20 -hydroxy-50 -t-octylphenyl) benzotriazole host-guest inclusion complexes, 175–7, 187–8 calixarenes, 185–6 cucurbiturils, 186–7 cyclodextrins, 180–5 supramolecular, 175, 177, 188 pHP, see p-hydroxyphenacyl HPA, see p-hydroxyphenacyl acetate HPAA, see p-hydroxyphenylacetic acid HPDP, see p-hydroxyphenacyl diphosphate HPPP, 288, 307 HPTS, see 8-hydroxypyrene-1,3,6-trisulfonate 7HQ, see 7-hydroxyquinoline 3-HQ-Bf, see 2-benzofuryl-3-hydroxy-4(1H)-quinolone 7HQ(NH3)n clusters, 530–2, 534, 541 ESHAT, 537–4, 548–51, 549 ESPT reaction path, 537 7HQ(NH3)n(H2O)m clusters, 548, 549 HR, see halorhodopsin HSAB behaviour, see hard and soft acid-base behaviour human serum albumin, 472 hydration number, 237–9 hydration switch model 380–2, 383, 384 hydrogen bond basicity of DMAP 63 of DMPN and DMPN-HFIP systems, 56–60 of isoindolo[2,1-a]indole-6-one, 61–2 hydrogen bond complexation, 39–40 complexation mechanism, 41, 45 effect on absorption and fluorescence spectra, 41–56 effect on triplet-state properties, 73–6

899

reaction rate, 64–9 solvatochromism, 69–72 hydrogen bond strengthening, 741–2 hydrogen-bonded complexes, 761–2 electronic spectroscopy studies, 775–90 formation of, 761–2 hydrogen bond dynamics versus solvation, 784–6 of betacarbolines, 395–415 of coumarin 102 (C102), 769–70, 771, 787–90 of fluorenone, 763–8, 770–5 spectroscopic properties of, 763–71 time-resolved vibrational spectroscopy studies, 786–90 hydrogen-bonded wires, 525–8, 550, 551 see also excited-state H-atom transfer (ESHAT); 7HQ (NH3)n clusters hydrogen-bonding effects, 288–9 p-hydroxyacetophenone (HA), 289–301 p-hydroxyphenacyl acetate (HPA), 301–4 p-hydroxyphenacyl diphosphate (HPDP) p-methoxyacetophenone (MAP), 290, 300 on intramolecular charge transfer, 313–16, 318–27 hydrogen bromide, see Hbr-Arn systems hydrogen chloride, 880 hydrogen fluoride, 889 hydrogen halide-water clusters, 870, 871, 874–5 CTTS and excitation cross sections, 877–80 double isotope substitution, 876–7 KED spectrum, 875–6 hydrogen halides, 871, 873–5, 880 see also hydrogen halide-water clusters hydrogen iodide, 633, 634 hydrogen radicals, 633 hydronium radical (H3O) in doped water clusters, 874–5, 888 in pure water clusters, 869–72, 888 1-hydroxy-2-acetonaphthone (1H2AN), 590 4-hydroxy-5-azaphenanthrene (HAP), 649 o-hydroxy benzaldehyde, 646, 647, 650, 651 o-hydroxy-benzophenone, 649 o-hydroxy carbonyl compounds ESIPT in, 650–7 o-hydroxy benzaldehyde, 650 symmetrically substituted compounds, 650–7 1-hydroxy-2-naphthaldehyde (1H2NA), 590–6, 598 ESIPT, 601–5 HOMO and LUMO orbitals, 601–6 intramolecular hydrogen bonding, 601–3 potential energy curve, 596–9 structure, 591 2-hydroxy-3-naphthaldehyde (2H3NA), 590–6, 598 ESIPT, 601–5 HOMO and LUMO orbitals, 601–6 intramolecular hydrogen bonding, 601–3 potential energy curve, 599–601 structure, 591

900

Index

2(20 -hydroxy-50 -t-octylphenyl) benzotriazole (HOPB), 648 2(20 -hydroxy phenyl)benzothiazole, 648, 649 7-hydroxy quinoline, 646 3-hydroxy xanthone, 646 o-hydroxyacetophenone (OHAP), 590 p-hydroxyacetophenone (HA), 288–301 hydroxyaromatic compounds ESPT, 736–41 ESPT in organized media, 737–41 fluorescence quenching by H-bond strengthening, 741–2 photochemistry of, 730–6 photophysics of, 730–6 role of interfacial H-bonding, 742–3 o-hydroxybenzaldehyde (OHBA), 590, 649 3-hydroxybenzofuranochromones, 481 3-hydroxychromones, 467, 468, 475, 481 3-hydroxyflavones (3HFs), 465–6 chemical structure, 467, 647 dual fluorescence, 467–9, 505–9, 516–17 dynamic quenching of fluorescence, 480–3, 504–6 dynamic quenching under different physical conditions, 494–99 effect of excitation frequency on dual fluorescence, 506–9 ESIPT, 467–75, 509–18, 647, 648 excited-state transformations, 472–5 ground-state anionic form, 469–72 novel analogues, 481–3 S2 fluorescence, 469 separation of a weak fluorescence signal, 504–6 Stern-Volmer constants, 499–501 temperature effect on proton transfer, 501–3 1-hydroxyfluorenone, 94–5 4-hydroxyfluorenone, 100 N-[2-(2-hydroxylethylamino)-ethyl]-1,8-naphthalimide (HEAN), 95 3-hydroxynaphthalene-2-carboxylate (SHNC), 725 p-hydroxyphenacyl (pHP), 288 p-hydroxyphenacyl acetate (HPA), 301–4, 307 p-hydroxyphenacyl diphosphate (HPDP), 301–9 20 -(4-hydroxyphenyl)-5-[5-(4-methylpiperazine-1-yl)]benzimidazo-2-yl (H258), 224, 225, 256–9, 260 p-hydroxyphenylacetic acid (HPAA), 288, 307–8 2-(2-hydroxyphenyl)benzimidazole (HBI), 754 2-(2-hydroxyphenyl)benzothiazole (HBT), 648, 754 2-(2-hydroxyphenyl)benzoxazole (HBO) ESIPT in, 748–50, 755 2-(2-hydroxyphenyl)benzoxazole (HBO) derivatives, 758–9 effect of heteroatom substitution on ESIPT, 754–5 effect of temperature on ESIPT, 755–6 ESIPT in bis(HBO), 756 in anion sensing, 810 8-hydroxypyrene-1,3,6-trisulfonate (HPTS), 168–70

7-hydroxyquinoline (7HQ), 527–8, 532–3, 535–8, 540, 544–5 see also 7HQ(NH3)n clusters; 7HQ(NH3)n(H2O)m clusters 3-hydroxyquinolones, 97, 465 hypericin, 649 IBW-1 molecules, 243 IBW-2 molecules, 243 ice, 872, 873, 874, 880 ICT, see intramolecular charge transfer IFW molecules, 243 IHB, see intramolecular hydrogen bond I(H2O)2–5 complexes, 628–30 imidazole clusters, 880–8 imidazolium ionic liquids, 341–2, 346–9 imidazolium ring, 341–2 in-plane bonds, 80, 81, 115, 117 indoles, 179, 182–3 indole-(H2O)2 complex, 115, 117 indolines, cyano-substituted, 91 infrared (IR) spectroscopy, 342–3 2D, 21–2, 23, 792 characterizing hydrogen-bonded complexes, 768–72 for studying hydrogen bonding in ionic liquids, 342 four-wave mixing, 3–6 linear/nonlinear, 1–2, 3–8 pump-probe, 4–5, 772–3 second-order cumulant expansion, 6–8 intermolecular charge transfer, 355 intermolecular excited-state hydrogen bonding 269–70 acridanones, 280–3 5-amino-2-aryl-2-carboxymethylindan-1,3diones, 270–4 3-arylmethylene-1(3H)-isobenzofuranones, 275–80 b-carbolines, 194 coumarins, 194–5 diazines, 195–9 3-hetarylmethylene-1(3H)-isobenzofuranones, 274–5, 276, 277 quinones, 199–202 synthetic chalcones, 283–4 intersecting state model, 646 intersystem crossing (ISC), 150, 176, 649 intramolecular charge transfer (ICT), 355 conversion to TICT state, 424–9 formation of, 80, 81 hydrogen-bonding effects, 313–16, 318–27 hydrogen-bonding with acceptor moiety, 320–7 hydrogen-bonding with donor moiety, 318–20 planar, 315 rehybridization by, 315 role of polarity and viscosity, 317–18 wagging, 315 see also twisted intramolecular charge transfer (TICT) intramolecular hydrogen bond (IHB), 205–6

Index in stilbene-like molecules 206–14 intramolecular vibrational redistribution (IVR), 647 iodic acids, 628, 633 ion-water clusters, 627 ionic liquid/cosolvent mixtures, 348 ionic liquids, 331, 341–2 FRET in, 166–7 imidazolium, 341–2, 346–9 vibrational spectroscopic studies, 341–9 see also room-temperature ionic liquids (RTILs) (3IQ)2E, see 1,2-di(30 -isoquinolyl)ethane IR spectroscopy, see Infrared (IR) spectroscopy ISC, see intersystem crossing isoindolo[2,1-a]indole-6-one, 41–4 hydrogen bond basicity, 61–2 kinetics of complexation reaction, 64–8 triplet formation yield, 73 isoindolo[2,1-a]indole-6-one-HFIP-n-hexane systems, 41–4, 61–2, 73 IVR, see intramolecular vibrational redistribution Kamlet-Taft model, 85, 87–8, 89 Kamlet-Taft parameters, 98 Kamlet-Taft solvatochromic scale, 767, 768 Kasha rule, 469, 508 Katushka, 819 KCD, see 2,5-bis[(2,3-dihydroindolyl)propylene] cyclopentanone KEHB, see keto-enol H-bond Kemp’s acids, 613, 624 keto-enol H-bond (KEHB), 356, 357, 363, 368–9, 371–3 ketocyanine dyes-alcohol systems, 781–4 7-ketoquinoline (7KQ), 527–8 KFP, see kindling fluorescent protein KIE, see kinetic isotope effect kindling fluorescent protein (KFP), 831 kinetic isotope effect (KIE), 448, 647 Kok cycle, 440–1, 442, 447, 449 Kosower Z values, 735–6 7KQ, see 7-ketoquinoline Laplace’s equation, 236 LBHB, see low-barrier hydrogen bond lecithin, 221, 224, 250–6 light-induced charge separation effects of hydrogen bonding, 438–40 type II reaction centres, 437–8 linear solvation energy relationship (LSER), 85 linear-to-turn conformational switch, 614, 615 Lippert-Mataga equation, 177–8, 419, 423, 731, 766 Lippert-Mataga model, 83–5, 731 low-barrier hydrogen bond (LBHB), 443 LSER, see linear solvation energy relationship 3MAI, see 3-methyl-7-azaindole MAP, see p-methoxyacetophenone

901

MAPAEE, see (E)-3(4-methylamino-phenyl)-acrylic acid ethyl ester Marcus theory, 443, 646 Marquardt’s algorithm, 45, 75 MBC, see N9-methyl-9H-pyrido[3,4-b]indole MCA, see multichannel analyser MCSCF method, 579, 580 MCTDH method, 818, 825 MDMANA, see N,N-dimethylaminonaphthyl-(acrylic)acid menadione, 199–201 merocyanine dye, 111 metal complexes effect of pKa shifts, 615–18 NH  S hydrogen bonds switching, 621–2 pp-dp interaction, 618–19 rearrangement of hydrogen-bond networks, 622–3 regulation of thiolate and phenolate ligands, 619–21 metal-sulfur complexes, 609 metal-thiolate complexes, 609, 619–20 methanol complex with Phlide, 860 complexes with fluorenone, 741–2, 771, 781 p-methoxyacetophenone (MAP), 290, 300 methoxynaphtalene-CTAB systems, 723–5, 726–30 N-(4-methoxyphenyl)-2,3-naphthalimide, (PMPN), 48–51 6-methoxyquinoline (6MQ), 798–802 3-methyl-7-azaindole (3MAI), 559–61 4-methyl-2,6-diacetyl phenol, 651–2, 654 4-methyl-2,6-dicarbomethoxy phenol, 652, 655 N9-methyl-harmane, 662 see also N9-methyl-1-methyl-9H-pyrido[3,4-b]indole (MHN) methyl-1-hydroxy-2-naphthoate (1H2MN), 590 methyl-1-hydroxy-2-naphthoate (MHN12), 590 methyl-2-hydroxy-1-naphthoate (MHN21), 590 methyl-2-hydroxy-3-naphthoate (MHN23), 590 methyl-3-hydroxy-2-naphthoate, 649 3-methyl-6-hydroxy-m-phthalic acid, 652 1-methyl-4-(40-hydroxystyryl) pyridinium betaine, 89 2-(N-methyl-N-isopropylamino)-5-cyanopyridine, 315, 316 N9-methyl-1-methyl-9H-pyrido[3,4-b]indole (MHN), 394, 395, 663 interactions with benzopyridinic bases, 687–9 interactions with hydrogen-bond donors, 692–4 MHN-CIEtOH system, 693–4 MHN-HFIP system, 396–403, 693–4 MHN-pyridine system, 687 MHN-t-BuOH system, 693–4 N-methyl-1,8-naphthalimide, 93 2-methyl 1,4-naphthoquinone (MQ), 199–201 1-methyl-9H-pyrido[3,4-b]indole (HN), 662 chemical structure, 663 HN-AcOH system, 702–4

902 Index HN-HFIP system, 695–8 HN-pyridine system, 687–8 interactions with acetic acid, 702–4 interactions with benzenoid-p bases, 682, 684–5 interactions with benzopyridinic bases, 687–9 interactions with hydrogen-bond donors, 692, 695–8 interactions with methylbenzene bases, 686 interactions with non-aromatic acceptors, 692 N2-methyl-9H-pyrido[3,4-b]indole (BCA) 394, 395 BCA-HFIP system, 403–6 N9-methyl-9H-pyrido[3,4-b]indole (MBC), 394, 395 MBC-HFIP system, 396–403 methyl salicylate (MS), 355, 590, 644, 646, 649 tautomerization, 642 (E)-3(4-methylamino-phenyl)-acrylic acid ethyl ester (MAPAEE), 96 methylbenzene bases, 685–6 N2-methylcarboline, 698 N2-methylharmane (T-MeHN), 666, 681, 692 MFPN, see N-(3-fluorophenyl)-2,3-naphthalimide MHN, see N9-methyl-1-methyl-9H-pyrido[3,4-b]indole MHN12, see methyl-1-hydroxy-2-naphthoate MHN21, see methyl-2-hydroxy-1-naphthoate MHN23, see methyl-2-hydroxy-3-naphthoate micelles, 218, 711–12 Arrhenius model at interface, 237–44 CTAB, 739, 742 DTAB, 739–40 effect of neutral aromatic dopants, 716–30 effect of salt anions, 712–15 interactions with dopant, 726–30 micelle-to-vesicle transition, 714–15, 716–18 polymer-like, 713–15 role of OH group of dopants, 725 SDS, 220, 222, 237–44, 739, 741 shear-induced viscoelasticity, 722–5 shear-induced viscosity, 719–21 solvation dynamics in, 738 structure of, 222, 223 vesicle-to-micelle transition, 715 worm-like 712, 714–15, 716–8, 725–6 mKate, 834 Mn4OxCa cluster, 446–50 molecular aggregates, see micelles molecular bridged compounds, 314 molecular hydrodynamic approach, 227 molecular recognition, 566–7, 805 see also anion sensing molecular-wire effect, 810 MPAC, see 2,5-bis(N-methyl N-1,3-propdienylaniline) cyclopentanone MPDP, 307 mPlum, 819 MQ, see 2-methyl 1,4-naphthoquinone 6MQ, see 6-methoxyquinoline MRCI method, 108 MRPT2 method, 819, 823

MS, see methyl salicylate MS-PET, see multiple-site electron and proton transfer multichannel analyser (MCA), 362 multiple-site electron and proton transfer (MS-PET), 443 mutation, 126–7, 556 (2N)2E, see 1,2-di(20 -naphthyl)ethane NaDC, see sodium deoxycholate NADPH, 859, 861, 862 nanoconfined systems, 159–60 diffusion of organic dyes, 170–2 excited-state proton transfer (ESPT), 167–70 fluorescence resonance energy transfer (FRET), 166–7 solvation dynamics, 160–6 2-naphthaldehyde 592–3 naphthalimides, 48–51 1-naphthol C¼O and O-H stretching frequencies, 592–3 ESPT, 736, 737–41 spectral shifts, 730–6 transition energies, 735 volume changes, 802 2-naphthol cis ! trans barrier height, 36 ESPT, 737–41 H-bonded clusters of, 30–6 IR spectra, 30–1 isomers of, 30 spectral shifts, 730–6 transition energies, 735 VER dynamics, 31–6 volume changes, 802 naphthol-CPB systems, 719–21 naphthol-CTAB systems, 719–30 naproxen, 182, 183 NaSal, 713, 714, 716 NET, see nonadiabatic electron transfer 2-nitrobenzaldehyde, 798 nonadiabatic electron transfer (NET), 443 [N3[1]][Tf2N], see N,N,N-trimethyl-N-propylammonium bis(trifluoro-methanesulfonyl)imide nucleic acid base pairs, 1–3 excited-state properties, 138–42 ground-state structures, 128–9 hydration, 142 structures of 126 see also adenine-thymine base pairs; adenine-uracil base pairs; guanine-cytosine base pairs nucleic acid bases, 125, 127 electronic transitions in, 129–30 geometries of, 130–1, 140–2 ground-state structures of, 128 hydration, 127, 131–8 mutation, 125–7 non-radiative deactivation, 142–3 structures of 126

Index tautomerism, 127, 562 nucleoside 50 -monophosphates, 201 OHAP, see o-hydroxyacetophenone OHBA, see o-hydroxybenzaldehyde oligopeptides Asp-containing, 613–14 Cys-containing, 612 Onsager’s reaction field theory, 83 out-of-plane bonds, 80, 81, 115, 117 oxazines, 91, 111–12, 741 oxidative water splitting general reaction pattern, 440–1 hydrogen bonding of YD, 445–6 oxidation of WOC, 445–51 oxidation of YZ, 441–6 9-oxo-imidazopurine derivative, 110–11 ozone destruction, 627, 880 P123 triblock copolymer FRET in, 167 gel, 165, 171 micelles, 171 solvation dynamics in, 165–6 P123-CTAC aggregate, 166, 169–70 P123-SDS aggregate, 165–6 P450, 620, 621–2 P680, 435, 439, 441–6 PBRCs, see purple bacteria reaction centres PCET, see proton-coupled electron transfer PCM, see polarizable continuum model PDAB, see 4-phenyl-1-N,N-dimethylaminobutane PEO-PPO-PEO triblock copolymers, 162–3, 165–6, 170 perfluoro-tert-butanol (PFTB), 47, 49, 50 perfluoro-tert-butyl alcohol (PFTB), 763 PES, see potential energy surfaces PET, see photoinduced electron transfer PFTB, see perfluoro-tert-butanol PFTBA, see perfluoro-tert-butyl alcohol phenolate-metal complexes, 620–1 phenols change of properties in excited state, 747–8 conformational switching, 613–15 effect of dopants on micelle microstructure, 716 ESPT, 737 incorporation in surfactants, 717, 718 interactions with coumarin 102, 741, 769–70, 787–9 pKa shifts by prelocated hydrogen bond, 612–13 phenothiazine derivatives, 93 N-phenyl 1–2-aminonaphthalene, 419–20 N-phenyl-benzamide, 646, 647 4-phenyl-1-N,N-dimethylaminobutane (PDAB), 101–2 phenyl-1-hydroxy-2-naphthoate, 649 Pheo, 439–40 Phlide, see protochlorophyllide photoactive yellow protein (PYP), 840–1

903

crystal structure, 840, 841 electronically excited state of, 848–50 initial stages of the photocycle, 840–3, 844–8, sub-picosecond time-resolved transient spectroscopy, 846–8 vibrational modes, 843 vibrational structural markers, 843–4 Y42F mutant of 850–1 photocycle, 839–40 photodeprotection reaction, 288, 307–9 photodissociation, 627 of bare water molecule, 868–9 of doped water clusters, 870, 871, 874–80 of hydrated hydrogen iodide clusters, 633, 634 of hydrogen halide-water clusters, 870, 871, 874–80 of isolated hydrogen halides, 873–4 of nitrogen heterocycles, 880–8 of pure water clusters, 869–73 photodynamic therapy, 355 photoexcitation, 193–4 see also intermolecular excited-state hydrogen bonding photoinduced electron transfer (PET) in HEAN, 95 in oxazine 91, 111–12 in room-temperature ionic liquids, 331–5 photoinduced processes, 865–6 photoionization, 869 photoisomerization, adiabatic, 211–14 photolysis, 627 of hydrogen chloride, 880 of water, 627–8, 630–2 photomultiplier tubes (PMTs), 361 photoreactivity, of drugs, 354–5 photoreceptors, 840 see also photoactive yellow protein (PYP) photosensitizers, 355–8 photosynthetic reaction centres, 858 photosynthetic water splitting, 433–4 see also photosystem II (PS II) photosystem I (PS I), 434 photosystem II (PS II), 434 cofactor arrangement, 434–5 light-induced charge separation sequence, 437–40 overall reaction pattern, 434–5 oxidative water splitting sequence, 440–50 plastoquinol formation sequence, 451–2 thermal stability of, 436–7 phthalimide derivatives, 81, 92 pick-up technique, 867 PICT, see planar ICT pigment-protein complexes, 434, 858 PJT coupling, see pseudo-Jahn-Teller coupling pKa shifts, 609 and stabilization constant in metal complexes, 615–18 of carboxylic acid derivatives, 612 of phenols, 612–13

904

Index

of thiol derivatives, 610–12 planar ICT (PICT), 315 plastoquinol (PQH2), 434, 451–2 plastoquinone (PQ), 440, 451 PMPN, see N-(4-methoxyphenyl)-2,3-naphthalimide PMTs, see photomultiplier tubes polarizable continuum model (PCM), 108 poly(vinylpyrolidone)-SDS complexes, 741 POR, see protochlorophyllide oxidoreductase porphyrin complexes, 620 potential energy surfaces (PES), 579 PQ, see plastoquinone 1HPQ, see 1-H-pyrrolo[3,2-h]quinoline PQH2, see plastoquinol PRG, see proton release group PROPKA method, 824 11-propyl-6H-indolo-[2,3-b]quinoline (6HIQ), 559, 560–1 6HIQ/7AI heterodimer, 562 protochlorophyllide (Phlide) 858–9 enzyme-bound species, 861–2 isolated species, 860–1 Phlide-methanol complex, 860 protochlorophyllide oxidoreductase (POR), 857–8 catalytic mechanism of, 859 see also chlorophyllide (Chlide); protochlorophyllide (Phlide) proton-coupled electron transfer (PCET), 443, 447–8 proton-driven conformational switching, 613–15 proton-pumping, in rhodopsins, 380, 384–5, 386–8 proton release group (PRG), 384 proton transfer complex (PTC), betacarbolines 395, 415, 689–92, 695–8 ‘proton transfer fluorescence’, 185 proton transfer reactions, 463–7, 465–6, 555 adiabatic/nonadiabatic, 737, 738 and volume changes, 797–802 see also excited-state double proton transfer (ESDPT); excited-state intramolecular proton transfer (ESIPT); excited-state proton transfer (ESPT) proton tunnelling, 647 ‘proton wires’, 525 pseudo-Jahn-Teller (PJT) coupling, 315 PT reactions, see proton transfer reactions PTC, see proton transfer complex pump-probe spectroscopy, 4–5, 772–3 purple bacteria reaction centres (PBRCs), 437–40 PyIn-n, see 2-(20 -pyridyl)indoles PYP, see photoactive yellow protein 2PyPhBu, see 2-pyridylphenylbutadiene pyrazole clusters, 880–8 pyridine hydrogen-bond complexation, 52–3 interactions with betacarbolines, 405–14, 687–92 interactions with pyridylindoles, 678–9 interactions with pyrrolinoquinolines, 678–9 pyrido[2,3-a]carbazole (PC), 663, 665, 670, 678

pyrido[3,2-g]indole, 662–72 9H-pyrido[3,4-b]indole, see betacarboline 1-(2-pyridyl)-5-(4-dimethylaminophenyl)-penta-2,4-diene-1-one (DMAC) 86–9 7-(30 -pyridyl)indole, 91, 92 2-(20 -pyridyl)indoles (PyIn-n), 663, 665, 670, 673–9 fluorescence quenching by electron transfer, 678–80 2-pyridylphenylbutadiene (2PyPhBu), 212–14 pyrimidine, 117 pyrrole-water complexes, 633–6, 637 pyrroles, 110, 628 clusters, 880–8 1-H-pyrrolo[3,2-h]quinoline, 662 pyrroloquinolines, 662–73 fluorescence quenching by electron transfer, 678–80 tautomerization, 664–8, 670 QA molecules, 440 (3Q)2E, see 1,2-di(30 -quinolyl)ethane QM/MM, see quantum mechanics/molecular mechanics method quantum correction factors, 2, 19 quantum mechanics/molecular mechanics (QM/MM) method, 3, 579–80 for studying fluorescent proteins, 819, 824, 832–4 quenching circuit, 361 quinones, 199–202 radiation damage, 866 Raman spectroscopy, 342, 343–4 for studying hydrogen bonding in ionic liquids, 342 Stokes/anti-Stokes, 343–4 see also coherent anti-Stokes Raman scattering (CARS) reactive oxygen species (ROS), 358 red-edge excitation shift (REES), 159 red fluorescent proteins (RFPs), 819–20, 835 internal conversion mechanism, 830–2 QM/MM studies, 824, 832–4 redox reactions, in photosynthetic water splitting, 433–4 REES, see red-edge excitation shift Rehm-Weller behaviour, 335 Rehm-Weller equation, 332 rehybridization, 315 rehybridization by ICT (RICT), 315 resorufin, 776–7 reverse micelles (RMs), 218, 222–3 Arrhenius model at interface, 244–59 solvation dynamics, 220–1 structure of, 222–3 RFPs, see red fluorescent proteins rheopexy, 714 Rhodobacter (Rb.) sphaeroides, 438, 452, 525 rhodopsins, 378–9 proton-pump activity, 380, 384–5, 386–8 role of strong hydrogen bond of water, 379–80, 386–8 see also bacteriorhodopsin; halorhodopsin

Index RICT, see rehybridization by ICT RISM-SCF method 108 RMs, see reverse micelles room-temperature ionic liquids (RTILs), 160 microemulsion, 161–2, 166–7 mixed micelle, 162–3 neat, 161 solvation dynamics in, 160–3, 335–9 photoinduced electron transfer (PET) in, 331–5 room-temperature phosphorescence (RTP), 185 ROS, see reactive oxygen species RTILs, see room-temperature ionic liquids Rtms5, 819, 830, 832, 834 RTP, see room-temperature phosphorescence ruthenium complexes, 621 SCC-DFTB method, 824 SDS, see sodium dodecyl sulfate SED equation, see Stokes-Einstein-Debye equation serine-type protease, 443 serum albumin, 472 SFG, see sum-frequency generation SHNC, see 3-hydroxynaphthalene-2-carboxylate Silica encapsulation, 717, 718 single-photon avalanche diodes (SPADs), 361 single photon counting (SPC), 359 see also time-correlated single-photon counting (TCSPC) single-photon detectors, 361–2 Smoluchowski equation, 333 sodium deoxycholate (NaDC) FRET, 167 solvation dynamics, 163–5 sodium dodecyl sulfate (SDS), 220, 222, 224, 237–44, 739, 741 solute-solvent complexes, 105–7 solute-solvent hydrogen bond formation, 79–80, 82 changes in excited-state-properties, 109–15 characterizing hydrogen bonds, 115–17 combined experimental and theoretical approaches, 118 design of experiments, 98–104, 105 dual fluorescence, 95–8 fluorescence enhancement, 92–5 fluorescence quenching, 91–2 prerequisite conditions for, 80–2 solvatochromic analysis, 83–90 theoretical modelling, 104–17 solute-solvent interactions, 149, 177–8, 761–2 specific/nonspecific, 79–80, 761, 784–5 see also hydrogen-bonded complexes; solute-solvent hydrogen bond formation solvation, 761–2 ‘dipolar’, 761 versus hydrogen bond dynamics, 784–6 solvation dynamics, 160, 225–31

905

coupled into ESPT reaction, 567–74 in bile salt aggregate, 163–5 in biomimicking systems, 219–21 in confined systems, 738 in ionic liquids, 335–9 in nanocofined systems, 160–6 in P123 triblock copolymer, 165–6 in room-temperature ionic liquids, 160–3, 335–9 solvation energy, 226–7, 761 solvation interactions, see solute-solvent interactions solvatochromic effect, 79 solvent effects, 83–4, 177–8, 730–1 solvent friction, 786 solvent shift method, 83 solvents, 99–102 sound velocity, 236, 237 SPADs, see single-photon avalanche diodes SPC, see single photon counting spinach, 437, 445, 450 spontaneous Raman scattering, see Raman spectroscopy 3St-2AP, see 3-styryl-2-azaphenanthrene 3St-7AP, see 3-styryl-7-azaphenanthrene sterically hindered compounds, 314 Stern-Volmer constant, 489, 768 for excited states of photoproduct, 491–4 for 3-hydroxyflavones, 499–501, 503, 505 Stern-Volmer equation, 332, 489, 491, 499, 686, 687–8 stilbene-like molecules adiabatic photoisomerization, 211–14 IHB effects on conformational equilibria, 206–10 IHB effects on radiative and reactive relaxation, 210–11 Stokes-Einstein-Debye equation (SED equation), 236, 256, 257, 334–5 Stokes shift, 176 3StP, see 3-styrylphenanthrene 4StQ, see 4-styrylquinoline 8StQ, see 8-styrylquinoline streak cameras, 359 3-styryl-2-azaphenanthrene (3St-2AP), 206–7 3-styryl-7-azaphenanthrene (3St-7AP), 206–7 3-styrylphenanthrene (3StP), 206–7 4-styrylquinoline (4StQ), 210, 211 8-styrylquinoline (8StQ), 210, 211 sub-picosecond time-resolved transient spectroscopy, 846–8 sulfonamide, 807 sum-frequency generation (SFG), 343, 345, 349 supersonic expansions, 529 Synechocystis sp. PCC 6803, 449 TAC, see time-to-amplitude converter Tb(III) complexes, 618 TC, see thiocoumarin TCAA, see trichloroacetic acid TCSPC, see time-correlated single-photon counting

906

Index

TDDFT, see time-dependent density functional theory method TEMPO, see 2,2,6,6-tetramethylpiperidine 1-oxyl tetrahydrobetacarboline, 182, 183 3,4,5,6-tetrahydrobis(pyrido[3,2-g]indolo)[2,3-a:30 ,20 -j] acridine (TPIA), 566–7 7,8,9,10-tetrahydropyrido[2,3-a]carbazole (TPC) 663, 665, 666, 668, 670 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), 475, 480, 481, 482, 494–7, 499 TFE, see trifluoroethanol Thermosynechococcus (T.) elongatus, 445, 447, 451–2 thiocoumarin (TC), 112 thioketones, 103 thiols conformational switching, 613 pKa shifts, 610–12 thylakoids, 436–7, 445 thymidine, 200 thymine, 126, 127, 128, 130, 143 TICT, see twisted intramolecular charge transfer time-correlated single-photon counting (TCSPC), 359–62, 364–6 time-dependent density functional theory (TDDFT) method, 108, 149–51, 156 time-resolved emission spectra (TRES), 230–1 time-resolved fluorescence evaluation of drug photoreactivity, 354–7 techniques, 358–62 time-resolved IR absorption spectroscopy, 786–90 ultrafast, 762, 790, 791–2 time-to-amplitude converter (TAC), 360, 362 T-MeHN, see N2-methylharmane TNS, see 2,6-toluidinonaphthalene sulfonate 2,6-toluidinonaphthalene sulfonate (TNS), 248 TPC, see 7,8,9,10-tetrahydropyrido[2,3-a]carbazole TPIA, see 3,4,5,6-tetrahydrobis(pyrido[3,2-g]indolo)[2,3a:30 ,20 -j]acridine TRES, see time-resolved emission spectra 1,2,3-triazine-water complex, 117 triblock copolymers interaction with ionic liquids, 162–3 solvation dynamics in, 165–6 trichloroacetic acid (TCAA), 101–2 trifluoroethanol (TFE), 763 interactions with betacarbolines, 692 interactions with fluorenone, 780–1 N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl) imide ([N3[1]][Tf2N]), 335–9 triplet electronic excited-states, 73–6, 149–56 tryptophan, 89–90, 179 turmeric, 357 twist-boat structure, 613 twisted intramolecular charge transfer (TICT) and dual fluorescence, 313–16 formation of 80, 81, 176, 183–4

in coumarins, 424–9 in DMABN, 318–20 in DMAPPI, 321–7 in fluorescent proteins, 831 TICT-forming compounds, 112 two-colour resonant two-photon ionization (2CR2PI), 528–9 tyrosine D, 445–6 tyrosine Z 441–5 ubiquinol, 451, 452 ubiquinone, 440 UMP2-BOMD simulations, 631–2 uracil, 112, 114–15, 130, 143 see also adenine-uracil base pairs urea, 807–8 recognition, 567 UV-IR DR spectroscopy, 29–30, 37 UV-UV depletation, 528, 529 UV-UV hole burning, 528, 529 van’t Hoff equation, 42 Vavilov’s rule, 469 VER, see vibrational energy relaxation vesicles, 712, 714–15, 716–18 vibrational energy relaxation (VER), 29–30, 37, 786–90 bare 2-naphthol, 31 H-bonded clusters of 2-naphthol, 31–6 ionic liquids, 341–9 viscosity and solvation in ionic liquids, 336–8, 339 shear-induced, 719–21 volume changes, 797–802 wagging ICT (WICT), 315 water ‘biological’, 218 deuterated, 103 dynamics at biological interfaces, 217–8, 221–2 in biomimicking systems, 218–23, 237–60 in rhodopsins, 378–88 ion solvation mechanism, 226 photolysis of, 627–8, 630–2 pure, 217–18 solvation time correlation function, 229 see also photodissociation; photosynthetic water splitting water-oxidizing complexes (WOCs), 441 effect on hydrogen bonding of YD in PS II, 445 effect on hydrogen bonding of YZ in PS II, 442–5 oxidation of, 446–51 Watson-Crick base pairs, 2, 9, 126, 128 excited-state properties, 138–42 reverse, 129 see also nucleic acid base pairs WICT, see wagging ICT

Index wobbling-in-cone analysis, 235–6 WOCs, see water-oxidizing complexes wtGFP, see green fluorescent protein (GFP), wild type

YD, 445–6 YZ, 441–5 Y42F, 850–1

xanthone, 184–5

zigzag-chain structure, 623, 624

907