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 (Va