Electronic materials: the oligomer approach

K. Mullen, G.Wegner

Electronic Materials: The Oligomer Approach

@ WILEY-VCH

Further Reading of Interest:

S. Roth One-Dimensional Metals ISBN 3-527-26875-8

H. S. Nalwa (ed.) Handbook of Organic Conductive Molecules and Polymers 4 Volume Set Fundamentals and Applications ISBN 0-471-96275-9

K. Mullen, G.Wegner

Electronic Materials : The Oligomer Approach

@ WILEY-VCH -

Weinheim . New York Chichester Brisbane . Singapore Toronto

Prof. Dr. Klaus Mullen und Prof. D r . Gerhard Wegner Max-Planck-Institut fur Polymerforschung Ackermannweg 10 D-55128 Mainz iermany

This book was carefully produced. Nevertheless, authors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Every effort has been made to trace the owners of copyrighted material; however, in some cases this has proved impossible. We take this opportunity to offer our apologies to any copyright holders whose rights we may have unwittingly infringed.

Library of Congress Card No. applied for A catalogue record for this book is available from the British Library. Deutsche Bibliothek Cataloguing-in-Publication Data: Miillen, Klaus:

Electronic materials: the oligomer approach / K. Mullen ; G. Wegner. - Weinheim ; New York ; Chichester ; Brisbane ; Singapore ; Toronto : Wiley-VCH, 1998 ISBN 3-527-29438-4

0 WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1998 Printed on acid-free and chlorine-free paper. All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means nor transmitted or translated into a machine-readable language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to b e considered unprotected by law. Composition: Alden Bookset, Oxford Printing and Bookbinding: Bookcraft (Bath) Ltd. Printed in Great Britain

Introduction: What can Materials Science Learn from Conjugated Oligomers?

When polymers are made up of a large number of building blocks linked in a repetitive fashion, oligomers constitute their lower homologs, since they will contain only one or a few of these units. It appears senseless to define a borderline between oligomers and polymers in terms of their molecular weights, but it is certainly a key feature that increasing the size of an oligomer changes its physical properties - until a convergence limit is reached. It follows that a major motivation of oligomer research is to establish relations between chain length and physical properties. Oligomers can be polydisperse, but monodisperse oligomers are assumed to allow a more precise structure-activity relationship to be determined and also to allow extrapolation of these relationships toward those expected for polymers. Researchers with different backgrounds are involved with the study of oligomers: polymer scientists have prepared oligomers as models for polymers, while organic chemists have made oligomers as higher homologs of monomers - with the understanding that 'real' compounds are monodisperse. This book is concerned with conjugated oligomers and their role as electronic materials. Since oligomers have long been a topic of polymer research [ 1, 4, the introduction will first outline some general aspects of the chemistry and physics of oligomers, then describe the role of 7r-conjugation in oligomers and finally consider the function of oligomers in materials science, for example, as active components of devices.

1. General Aspects of Oligomer Research The controlled synthesis of oligomers with well-defined endgroups and chain lengths by a condensation mechanism requires a step-wise approach. Consider the reaction of terephthalic acid 1 with an alkanediol2 to yield oligomers with the repeating unit 3. The first condensation step between the two bifunctional components requires one acid and one $coho1 function, respectively, to carry different protection groups, say P' and P-. What is important for further transformations of the resulting monoester 4 is that the protecting groups can be cleaved separately and the next condensation be initiated at either end. While this oligomer synthesis is conceptually straightforward, it becomes increasingly tedious for higher oligomers. Not surprisingly, therefore, oligomers are also made using a random approach, e.g., through the direct coupling of bifunctional starting compounds such as diols and diacids. While the stoichiometry of reactions can be varied to favor oligomer formation,

VI

Introduction: What cun Muterids S&we Lrurn frorn Conjugutecl Oligomers.?

HOOC

G-

COOH

-

HO - (CH,),

2

1

-O(CH,),-

- OH

OOC

3 pi

- O(CH2),

- OOC

-0

\ - COOP2

4

the weakness of this protocol lies in the need to separate oligomers of different size and in the presence of different endgroups. One expects that increasing the size of the building blocks will render a separation of the oligomeric coupling products easier. Oligomers play an important role in elucidating mechanisms of polymer formation. It might be useful to recall that oligomers occur as intermediate species in repetitive condensation (or addition) reactions leading to polymers, and that the resulting polymers still contain appreciable amounts of oligomers. Oligomers are involved in equilibria not only between different chains, but also between chains and rings. The understanding of the kinetics and thermodynamics of ring formation in polycondensation reactions has, indeed, been further developed by considering transformations between linear and cyclic oligomers. One obvious reason for the important role of cyclic oligomers as polymer models lies in the fact that they do not contain endgroups. While the significance of oligomers for mechanistic discussions is obvious for polycondensation and polyaddition reactions, the situation is slightly different for the polymerization of vinyl monomers, owing to the different reaction kinetics, in particular, owing to the different dependence of the molecular weight upon time and conversion. If anionic polymerization is selected as an example, oligomers demand attention, for example, for the detection of possible back-biting processes, the elucidation of tacticity, or the identification of endgroups. Oligomers adopt a useful role as building blocks in polymer synthesis, and beautiful examples have been presented by Carothers r t al. [3], Kern et al. [4,5]and Zahn et al. [6]. A full account of their work is beyond the scope of this text; however, Kern’s ‘duplication concept’ [7] may serve as one instructive example. More recent work considers the use of oligomers as macroinitiators in radical polymerization and of oligomers with well-defined endgroups in block copolymer synthesis, e.g., the formation of the hard and soft segments of thermoplastic elastomers [l]. A final aspect concerns the significance of oligomers in elucidating polymer structures. Oxymethylene oligomers played a crucial role in Staudinger’s pioneering work toward establishing the nature of macromolecules [8,9]. He demonstrated by X-ray diffraction studies that the packing of oxymethylene oligomers is identical to that of the corresponding polymers and does not depend on the degree of polymerization or on the nature of the endgroups. Accordingly, the crystal structure of the polymer can be adequately described by referring to that of a relatively

small oligomer. When looking at the packing of increasingly large chains the question of chain folding arises, and. indeed, oligomers and cyclooligoniers are referred to when elucidating the chain-fold length and the character of the fold of polymers. The chain length dependence of the melting points of n-alkanes and cycloalkanes should be mentioned as a concluding example, since it is among the first pieces of information conveyed in classes of fundamental chemistry.

2. Conjugated Oligomers The simplest conjugated oligoiner is an oligoene chain consisting of an alternating sequence of double and single bonds with the 7r-7r interaction extending over the whole molecule. Other examples include aromatic building blocks such as benzene, thiophene or pyrrole, or constitute 'hybrids' of olefinic and aromatic units such as stilbene and higher phenylenevinylenes. The general aspects outlined above for the synthesis of oligomers hold also for conjugated oligomers. A troublesome disadvantage arises from the rigid nature of the 7r-systems, which severely limits their solubility. While it is true that the materials science of oligomers is mostly centered around solid-state properties, sufficient solubility is important for synthesis, structure elucidation, and processing (see below). Solubilization can be achieved by attaching alkyl substituents to the molecules and thus providing them with their own 'solvation shell'. This approach, although having proved of great value, has several disadvantages. Alkyl substitution can weaken 7r-conjugation by inducing torsion about formal single bonds or inhibit a tight packing of molecules in the solid state. Further, alkyl substitution will 'dilute' the electronically active function of the molecules. While structural homogeneity of the products is an important requirement of oligomer synthesis in general, this criterion is particularly severe for conjugated molecules. Consider, for example, the synthesis of an oligophenylenevinylene with C=C double bond formation through final elimination steps. A failure in this process leaves sp'-hybridized centers in the chain, which will interrupt the 7rconjugation and give rise to smaller subunits. While such defects will not remain undetected in tests of the structure of oligomers, the situation may become troublesome in the related polymers. The great interest that conjugated polymers such as polypyrrole and polyaniline have attracted within materials research stems. naturally. from their attractive physical properties, but also from the fact that they are easily available in sufficient quantities - even in laboratories without pronounced synthetic expertise. What is sometimes overlooked. however, is that there is not just one polypyrrole: each sample made by electrochemical oxidation must be regarded as an individual sample whose character depends sensitively upon the conditions of the experiment. Further, its structure cannot be represented by that of an idealized chain, but rather the structure comprises crosslinking and ring fusion. Accordingly, one would not necessarily argue against a 'practical' synthesis, but then it should be quick, not

VIIT

Introduction: What can Materials Science Learn f r o m Conjugated Oligomers?

dirty. A strong plea is made to reliably define the molecular structure before proceeding to develop structure-property relationships. There seems to be an uncertainty principle in the science of electronic materials according to which structures arouse the more interest, the more poorly they are defined. Theory predicts, for example, that polymers with low bandgaps have attractive optical and electrical properties. One anticipates from the prevailing bonding situation, however, that such species, after having been formed, are readily susceptible to various kinds of follow-up reactions. This would, of course, destroy the desired structure; also, a reliable optical detection of the bandgap would and undetected - doping were to occur. become impossible if some unwanted There are many other examples of conjugated polymers in which ‘wishful thinking’ rather than sound scientific reasoning has been the guideline of design and synthesis. It is obvious that oligomers adopt a key role in attempts to put materials science on a firm structural basis since a detailed analysis of their molecular structure is more straightforward than that of polymers. Further, the measurement of phenomena such as transport of charge carriers in photoconductivity and of excitons in photoluminescence requires scrupulous purification of samples, since impurities may produce false or at least misleading results. Oligomers are important, therefore, because they can be purified more easily than polymers, whereby quite demanding techniques such as zone melting or vacuum sublimation have been used. The contribution of the endgroups to the overall properties has to be taken into account appropriately. Work centered around oligomers can also stimulate advances in synthesis. Thus methods of C-C bond formation using organometallic intermediates, which were originally proposed for the synthesis of compounds with low molecular weight, have been tested in repetitive processes and then successfully incorporated into polymer synthesis. Key criteria among such design processes are the yield available in each elementary step, the occurrence of structural defects, and the nature of endgroups. While we shall examine the physical properties of conjugated structures in the next section, it is clear that the availability of a homologous series of monodisperse oligomers allows one to reliably follow their behavior as a function of size. Building homologs of conjugated oligomers is also among the fundamental concepts of organic chemistry; questions such as achieving a bathochromic shift for a given class of chromophores, or increasing the number of interacting spins in redoxactive molecules are closely related to progress in oligomer synthesis by which one makes the active component larger. Not surprisingly, conjugated oligomers are rewarding subjects that have often confirmed theoretical considerations and become textbook cases: typical examples are the Kuhn model of the electron-in-the-box, the fusion of a small 7r-chain to a ring with either a Huckel or Mobius topology, or the symmetry distortion in molecules with degenerate orbitals in the ground state. While small organic T systems are treated within molecular orbital theory and conjugated polymers as one-dimensional solids with periodic electron density fluctuations, research centered around oligomers as ‘medium-sized’ links between the two extremes strongly suggests a unified view. To that end, however, various terminology problems have -

to be clarified. Thus, the charging of a conjugated chain can be described by the polaron concept, which means 'charge plus the accompanying lattice distortion', or by looking at an electron-transfer-induced structural change. One concludes that oligomers provide, again, a critical test example for determining the polaron width or the extra-stabilization of di-ionic states (bipolarons). A good case can be made when considering the partial reduction or oxidation of a conjugated polymer such as a poly( pum-phenylene); this process is described as favoring a quinoid bonding situation over the benzenoid one. One must be aware, however, that such models generally consider infinite chains, in which the influence of endgroups can be ignored. In oligomers the influence of endgroups cannot be ignored, and the latter determine the relative importance of benzenoid and quinoid structures. In the language of chemistry this means the differentiation between mesomeric and isomeric states.

3. Physical Properties of Conjugated Oligomers: From Compounds to Materials The extended 7r-systems of conjugated oligomers qualify them as chromophores with a broad range of optical properties and as electrophores, with the ability to accept or donate extra charges. Interestingly, many physical properties relevant for materials science are related to the formation, transport, annihilation, or storage of charge. It is the challenge of oligomer research to systematically and comprehensively investigate these processes under structurally well-defined conditions. It has been stressed above that, by definition, the properties of oligomers are chainlength dependent - until one reaches a borderline length at which further extension will no longer affect their behavior. This aspect defines clearly the role of conjugated oligomers as models for the related polymers: at the heart of oligomer research lies the extrapolation of physical properties toward infinite chain lengths and the description of a conjugated polymer in its 'true' state. The 'effective conjugation length' will be developed in this book from different points of view. Although lacking a generally valid theoretical basis, this empirical concept has proved to be of great value in characterizing the nature of extensively conjugated chains with aromatic, olefinic, and acetylenic subunits. Accordingly, a major concern of this book is the synthesis and physical characterization of conjugated oligomers and, with them, of polymers at the highest possible level of structural precision and reliability. An oligomeric compound whose characterization is restricted to the recording of spectra because of lack of an efficient synthesis and therefore lack of quantity will never become a material. Accordingly, the ucfivr p/zj,sicul,fuizctioiz of conjugated oligomers and polymers is a key prerequisite when proceeding from chemistry and physics to materials science. This transition also requires creation of a specific macroscopic state of matter, and this need highlights the crucial role of processing. It follows that the description of oligomers as electronic materials cannot be

X

In trotluction: Wliur in11 Matcritr1.y S~,iivirc~ Learn ,/i.om Conjugated Oligomers?

confined to properties of individual molecules in a dilute solution, but always deals with ensembles of molecules and their mutual interactions. It is clear that properties such as conductivity in macroscopic samples depend upon charge-transport mechanisms between molecules and, subject to the morphology of the solid, between different structural organizations at various length scales. Therefore, supramolecular ordering, which occurs under the influence of weak intermolecular forces and which depends upon the conditions of processing, must also be included when considering oligomers as electronic materials. The charge carrier mobility of oligomeric semiconductors in organic field-effect transistors may be mentioned as a convincing example. If the limit of convergence of a particular physical property is already reached for a rather low oligomer size, and if oligomers have a high degree of structural homogeneity, one may regard oligomers as 'better' materials. There are indeed cases, such as the above mentioned field-effect transistors or light-emitting diodes, where oligomers serve as electronic materials in their own right. Nevertheless, emphasizing the significance of oligomers should not be misunderstood as an argument against conjugated polymers, since discussing the competition between oligomers and polymers is debilitating. One obvious reason for this is that the performance of an electronic material as an active component of a device depends on a great variety of different, sometimes even conflicting requirements, not the least of which are chemical and morphological stability, as well as processability, and thus the lifetime of the system. The present text is therefore intended to complement the literature on conjugated polymers, not to compete with it. We hope, however, that the oligomeric approach will contribute to a better understanding of electronic materials, provide better access for researchers about to enter the field, and further encourage fruitful interdisciplinary interactions.

References I . V. Percec, C. Pugh. in Enncycloperli~io f P o l j m e r Science arid Engineering (Eds: H . F. Mark, N . M. Bikales, C. G. Overberger. G. Menges), Wiley, New York 1987, Vol. 10, p. 432. 2. M. Rothe, J. Rothe, in P o l w w r Handbook, 3rd ed. (Eds: J. Brandrup, E. H. Iinmergut), Wiley, New York, 1989, Vol. IV, p. 1. 3. W. H . Carothers, J. A. Arvin, G. L. Dorough, J . Am. Chem. Soc. 1930, 52, 3292. 4. W. Kern, Chem. Ztg. 1952, 76, 661. 5. W. Kern, M . Seibel, H. 0. Wirth, Makrotnol. Chenz. 1959. 29, 164. 6. H . Zahn, P. Rathgeber, E. Rexroth el at., Angeiv. Chem. 1956, 68, 229. 7. W. Kern, K. J. Rauterkus, Mukromol. C'liem. 1958, 28, 221. 8. H. Staudinger, M . Luthy, Helv. Chim. Acta 1925, 8, 41. 9. H . Staudinger, Die Hochmoli~kularenOrganisclieri Vc>rhindungen,Springer, Berlin, 1932.

Contents

1

Hydrocarbon Oligomers Y. Geerts, G. Kliirner nnd K. Miillen

1.1 1.2 1.2.1 1.2.2 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.4 1.4.1 1.4.2 1.5

Introduction 1 Olefinic Structures 3 Oligoenes 3 Oligoynes, Oligoenynes 11 Aromatic Structures 24 Oligoarylenes 24 Oligo[n]acenes 48 Cycloarenes 5 1 Polycyclic Aromatic Hydrocarbons of the Clar Type Olefinic-Aromatic Structures 58 Oligoarylenevinylenes 58 Oligoaryleneethynylenes 86 Conclusions 97 References 98

2

Sulfur-Containing Oligomers

2.1

Oligothiophenes P . Bauerle

2.1.1 2.1.2 2.1.2.1 2.1.2.1.1 2.1.2.1.2 2.1.2.1.3 2.1.2.1.4 2.1.2.2 2.1.2.2.1 2.1.2.2.2 2.1.2.2.3 2.1.2.2.4 2.1.2.2.5 2.1.2.2.6 2.1.3

Introduction 105 Synthesis of Oligothiophenes 109 Unsubstituted Oligothiophenes 109 Arene/arene-coupling Methods by Oxidative Couplings 109 Transition Metal Catalyzed Coupling Methods 1 13 Ring Closure Reactions from Acyclic Precursors 120 Physical Properties of a-Oligothiophenes and Isomers 127 Substituted Oligothiophenes 134 /3,P’-Substituted Oligothiophenes 135 a,a’-Substituted Oligothiophenes 155 a,P-Substituted Oligothiophenes 161 Functionalized Oligothiophenes 171 Amphiphilic Oligothiophenes 186 Transition Metal Complexes of Oligothiophenes 187 Conclusions 188 Acknowledgement 189 References 189

53

XI1

Contents

2.2

Oligotetrathiafulvalenes J . Becher, J . Luu and P. Mork

2.2.1 2.2.2 2.2.3 2.2.4 2.2.4.1 2.2.4.2 2.2.4.2.1 2.2.4.3 2.2.5

Introduction 198 Redox properties of Tetrathiafulvalenes 198 Bis-tetrathiafulvalenes, Connected through One Linker 204 Bis-tetrathiafulvalenes,Connected through Two Linkers 212 Two Linkers, u-type 212 Two Linkers, u-type, Annelated Systems 21 3 TTF-Vinylogs 216 Cyclic bis-tetrathiafulvalenes (tetrathiafulvalenophanes) 216 Tetrathiafulvalenes with Three or More TTF-Units, TTF-Dendrimers etc. 222 Polymers 223 Synthesis 224 One or All TTF-Units are Formed in the Oligomerization Step 224 Linking/cyclization of Preformed Tetrathiafulvalenes 226 Conclusion 229 References 23 I

2.2.6 2.2.7 2.2.7.1 2.2.7.2 2.2.8

3

Nitrogen-Containing Oligomers L. Groenedaal, E.- W. Meijrr and J . A . J . M . Vekemans

3.1 3.2 3.2.1 3.2.2 3.3

Introduction 235 Oligo(pyrrole-2,4-diyl)s 237 Synthesis 237 Structural Characterization 242 Mixed Oligomers Consisting of Pyrrole and other (Hetero)aromatics 249 Synthesis 249 Structural Characterization 257 Oligoanilines 263 Synthesis 263 Structural Characterization 268 References 270

3.3.1 3.3.2 3.4 3.4.1 3.4.2

4

Oligomeric Metal Complexes E. W. Constable

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2

Introduction 273 Non-Programmed (Spontaneous) Assembly 275 Carboxylate Ligands 275 Heterocycles 277 Other Bridging Ligands 279 Programmed (step-wise) Assembly 280 Bridging Heterocycles - bpy Domains 281 Bridging Heterocycles - tpy Domains 285

Contents

4.4 4.5

Characterization and Properties of New Materials Conclusions 292 References 293

5

Crystal Structure

5.1

Oligomers as Structural Models for Polymers V. Enkelniun

5.1.1 5.1.2 5.1.2.1 5.1.2.2 5.1.2.2.1 5.1.2.3 5.1.2.3.1 5. I .2.4 5.1.2.5 5.1.2.6 5.1.2.7 5.1.2.8

Design of Endgroups 296 Structural Families and Types of Disorder of Conjugated Polymers Models for Poly(acety1ene) 303 Models for Poly(p-phenylene) 304 Substituted Poly(p-phenylenes) 308 Models for Polythiophene 309 Substituted Polythiophenes 309 Polypyrrole 309 Models for PPV 313 Models for Poly(ani1ine) 3 16 Models for Conductive Polymer Salts 316 Models for ‘Hairy Rod’ Polymers 318 References 322

5.2

Packing Calculations Based on Empirical Force Fields R.Hen tsclike

5.2.1 5.2.2 5.2.2.1 5.2.2.2 5.2.2.3 5.2.3

Introduction 329 Molecular Interactions via Phenomenological Force Fields 329 The Functional Form of Empirical Force Fields 329 Parameterization of Empirical Force Fields 332 Computational Aspects of Packing Calculations 333 Finding the Proper Packing Structure a Multiple Minimum Problem 335 Approaches for Zero Temperature 335 Approaches for Temperature Greater than Zero 337 Crystal Packing, Fibers and Surface Induced Order 339 Crystal Structure of Pentamethyl Ferrocene 339 Fibrous and Globulan Proteins 339 Surface Induced Order 340 References 342

5.2.3.1 5.2.3.2 5.2.4 5.2.4.1 5.2.4.2 5.2.4.3

289

~

6

Structure and Optical Properties of Conjugated Oligomers from their Vibrational Spectra G. Zerhi, C. Castiglioni atid M . Del Zoppo

6.1 6.2

Introduction 345 Frequency and Intensity Spectroscopy

346

XI11

301

XIV

Contiwts

Frequency Spectroscopy 346 Intensity Spectroscopy 347 Dynamics and Spectra of One-Dimensional Lattices 349 From One-Dimensional Crystals to Finite Molecular Chains 351 Survey of the Electronic and Optical Properties of Conjugated Oligomers and Polymers 359 Survey of the Vibrational Spectra of Conjugated Molecules 362 6.6 Infrared and Raman Spectra of Undoped (Pristine) 6.6.1 Materials 363 Infrared and Raman Spectra of Doped (or Photoexcited) 6.6.2 Conjugated Materials 363 The Amplitude Mode or the Effective Conjugation 6.7 Coordinate 366 6.8 Electron-Phonon Coupling, Confinement Length and Pinning Potential 376 6.9 The 5I Mode and the Infrared Spectrum of Doped Species 379 The Raman Spectra of Doped Species 380 6.10 Evidence of Large Charge Fluxes from Oriented Samples in 6.1 1 Polarized Light 382 What do we Learn from Vibrational Spectra? 383 6.12 All Truns-Oligoenes and Trans-Polyacetylene 383 6.12.1 Oligomers and Polymers of Heteroaromatic Building Blocks 386 6.12.2 6.12.2.1 Oligo- and Polypyrroles 387 6.12.2.2 Oligo- and Polythiophenes 388 6.12.2.3 Oligo- and Poly( p-phenylenevinylene) 392 Nonlinear Optical Responses with Intensity Spectroscopy 392 6.13 Acknowledgment 399 References 399

6.2.1 6.2.2 6.3 6.4 6.5

7

Electronic Excitation

7.1

Electronic Excitations of Conjugated Oligomers H . Bassler

7.1.1 7.1.2 7.1.3 7.1.3.1 7.1.3.2 7.1.3.3 7.1.3.4 7.1.4

Introduction 403 Concepts 403 Experimental Results 406 Polyenes 406 Oligothiophenes 410 Oligoarylenevinylenes 414 Oligo-p-phenylenes 424 Conclusions 428 Acknowledgement 429 References 429

7.2

A Quantum Chemical Approach to Conjugated Oligomers: The Case of Oligothiophenes J . Coriiil, D . Brljonne mid J . L . BrtYrrs

7.2.1 7.2.2 7.2.3 7.2.3.1 7.2.3.2 1.2.3.3 7.2.4 7.2.5

Introduction 432 Theoretical Approach 433 Neutral Oligomers 434 Chain-length Evolution of the Lowest Excited States 434 Intersystem Crossing 436 Relaxation Phenomena in the Lowest Excited States 438 Charged Oligomers 440 Conclusion 445 Acknowledgements 446 References 446

8

Nonlinear Optical Properties of Oligomers C. Birheck

8.1 8.2 8.2.1 8.2.2 8.3 8.4 8.5 8.5.1 8.5.2 8.5.3 8.5.4 8.6

Introduction 449 Nonlinear Optical Phenomena 449 Physical Background 449 Third-Order Phenomena and Measurement Techniques 452 Experimental Results 453 Survey of Theories 460 Structure-Property Relations 462 Size Dependence of the Hyperpolarizability 462 Electronic Excitations and Characteristic Lengths 463 Comparison of Polymers, Oligomers and Dyes 469 Scaling Laws for One-Dimensional Conjugated Systems 473 Conclusions 474 Acknowledgments 475 References 475

9

Electrochemical Properties J . Heirire rrrid P. Tschurickj.

9.1 9.2 9.2.1 9.2.1.1 9.2.1.2 9.2.1.3 9.2.1.4 9.2.1.5 9.2.1.6

Introduction 479 Charge Storage Mechanism of Conjugated Oligomeric Systems 480 Redox Behavior in Solution 480 Oligo-(aryleneviny1ene)s 480 Oligoenes 483 Linear Oligoarylenes and Double-Stranded 7r-Systems 485 Oligothiophenes 488 Oligopyrroles 493 Oligoanilines 496

XVI 9.2.2 9.3 9.3.1 9.3.2

Solid-state Measurements on ‘Short Chain’ Oligomers Electropolymerization 504 Oligomerization in solution 504 Solid-state electropolymerization of Oligomers 5 10 References 5 1 1

10

Optical Applications M . G. Harrison and R. H. Friend

501

Overview 515 Preparation of Thin Film Devices 5 18 Sublimation 5 19 Solution-Processing 5 19 Substitution with Side-Chains 520 Using a Soluble Partially-Conjugated Precursor Polymer 522 Blends within Polymers 522 Langmuir-Blodgett Technique 523 Electronic Excitations 523 Intra-molecular Non-Radiative Decay Channels 523 Internal Conversion 525 Intersystem Crossing 526 Singlet Fission 526 Intermolecular Non-Radiative Decay Channels in Thin Films 526 Aggregation and Davydov Splitting 527 Charge-Transfer Excitons 528 Effects of Inter-Ring Torsion and Coplanarity of Oligomers 529 Solution 529 Solid State 530 Concluding Remarks 53 1 Electroluminescent Devices 53 1 Historical Survey of Oligomeric LEDs 532 LEDs Based on Molecular Semiconductors 532 LEDs Based on Oligothiophenes 534 LEDs Based on Oligomers Blended with Polymer Matrices 536 LEDs Based on Polymers with Pendent Oligomeric Side-Chains 539 10.4.1.5 Polarized Electroluminescence from Oriented Oligomers 540 10.5 Photoconductive and Photovoltaic Devices 541 10.5.1 Mechanism of Photoconductivity in Sexithiophene 542 10.5.2 Photovoltaic Applications (Solar Cells) 543 10.6 Field-Effect Devices 545 10.6.1 Electro-Optical Modulation 546 10.6.2 Optical Probing of Field-Induced Charge in Sexithiophene 550 10.7 All-Optical Modulator Devices 553 References 555 10.1 10.2 10.2.1 10.2.2 10.2.2.1 0.2.2.2 0.2.3 0.2.4 0.3 0.3.1 0.3.1.1 0.3.1.2 0.3.1.3 0.3.2 0.3.2.1 0.3.2.2 0.3.3 0.3.3.1 0.3.3.2 0.3.4 0.4 0.4.1 0.4.1.1 0.4.1.2 10.4.1.3 10.4.1.4

11

Field-Effect Transistors Based on Conjugated Materials F. Garnirr

11.1 11.2 11.3 11.4 1 1.4.1 1 1.4.2 I 1.4.3 11.4.4 11.4.5 11.5 11.6 11.7

Introduction 559 Fabrication and Mode of Operation of FETs 560 Conjugated Materials Used in Organic-Based FETs 566 Device Characteristics 568 Conjugated Polymers and Amorphous Materials 569 Conjugated Oligomers, Role of Structural Organization 570 Experimental Conditions of Film Deposition 57 1 Chemical Engineering of Molecules 574 Single Crystals 577 Charge Transport in Conjugated Materials 579 State of the Art of Organic FETs 580 Conclusion 58 1 References 582

Index 585

List of Contributors

H. Biissler Fachbereich Physikal. Chemie & Zentrum fur Materialwissenschaften Universitiit Marburg D-35032 Marburg Germany

C. Bubeck Max-Planck-lnstitut fur Polymerforschung Ackermannweg 10 D-55021 Mainz Germany

P. Biiuerle Abt. Organische Chemie 2 Univeritiit Ulm Albert-Einstein-Allee 11 D-89081 Ulm Germany

C. Castiglioni Dipartimento di Chimia Industriale Politechnico Piazza L. Da Vinci 32 1-20133 Milano Italy

J. Becher Department of Chemistry Odense University Campusvej 55 DK-5230 Odense M Denmark

E. W. Constable Institut fur Anorganische Chemie Universitiit Basel Spitalstr. 5 1 CH-4056 Basel Switzerland

D. Beljonne Chiniie des Materiaux Nouveaux Centre de Recherches en Electronique et Photonique Moleculaire Universite de Mom-Hainant Place du Pare, 20 B-7000 Mons Belguim

J. Cornil Chimie des Materiaux Nouveaux Centre de Recherches en Electronique et Photonique Moleculaire Universite de Mons-Hainant Place du Pare, 20 B-7000 Mons Belguim

J.-L. Bredas Chimie des Materiaux Nouveaux Centre de Recherches en Electronique et Photonique Moleculaire Universite de Mons-Hainant Place du Pare, 20 B-7000 Mons Belguim

M. Del Zoppo Dipartimento di Chimia Industriale Politechnico Piazza L. Da Vinci 32 1-20133 Milano Italy

V. Enkelmann Max-Planck-Institut fur Pol ymerforschung Ackermannweg 10 D-55021 Mainz Germany

J . Heinze Institut fur Physikalische Chemie Universitgt Freiburg Albertstr. 21 D-79 104 Freiburg Germany

R. H. Friend University of Cambridge Department of Physics Optoelectronics Group Cavendish Laboratory Madingley Road Cambridge CB3 OHE United Kingdom

R. Hentschke Max-Planck-Institut fur Polynierforschung Ackermannweg I0 D-55091 Mainz Germany

F. Garnier CNRS Laboratoire de Photochimie Solaire 2, Rue Dunant F 94320 Thiais France Y. Geerts Max-Planck-lnstitut fur Pol ymerforschung Ackermannweg 10 D-55021 Mainz Germany L. Groenendaal Laboratory of Organic Chemistry Eindhoven University of Technology PO Box 513 NL-5600 MB Eindhoven The Netherlands M. G. Harrison University of Cambridge Department of Physics Optoelectronics Group Cavendish Laboratory Madingley Road Cambridge CB3 OHE United Kingdom

G . Klarner Max-Planck-Institut f u r Pol y merforschung Ackermannweg 10 D-5502 1 Mainz Germany

J. Lau Department of Chemistry Odense University Campusvej 55 DK-5230 Odense M Denmark

E. W. Meijer Laboratory of Organic Chemistry Eindhoven University of Technology PO Box 513 NL-5600 MB Eindhoven The Netherlands P. M ~ r k Department of Chemistry Odense University Campusvej 55 D K 5230 Odense M Denmark

xx

List

of

Conrrihutors

K . Mullen Max-Planck-Institut fur Pol ymerforschung Ackermannweg 10 D-55021 Mainz Germany

G . Wegner Max-Planck-Institut fur Pol ymerforschung Ackermannweg 10 D-55021 Mainz Germany

P. Tschuncky Institut fur Physikalische Chemie Universitat Freiburg Albertstr. 21 D-79 104 Freiburg Germany

G . Zerbi Dipartimento di Chimia Industriale Politechnico Piazza L. Da Vinci 32 1-20133 Milano Italy

J. A. J. M. Vekemans Laboratory of Organic Chemistry Eindhoven University of Technology PO Box 513 NL-5600 MB Eindhoven The Netherlands

List of Symbols

Q

ffmax

P PO

6 66 E

x @

Vjkl

R

electronic wave function overlap low-energy absoption maxima quadratic hyperpolarizability hyperpolarizabilit y chemical shift bond distortions wavelength emission dielectric constant extinction coefficient dielectric permittivity of free space microscopic polarizabilities external quantum efficiency interring angles of torsion phase shift between the displacements of two translationally equivalent internal coordinates laser wavelength longest wavelength absorption maxima emission maxima long-wavelength absorption field effect mobility molecular dipole moments molecular dipole moments per unit vibrational frequency equilibrium density electrical conductivity circular frequency angular frequency frequency of waves nonlinear optical susceptibility coefficient quantum yield phase shift phase angle fluorescence quantum yield quantum yield of fluorescence quantum yield of fluorescence valence angles diagonal matrix angle of rotation internal transition frequencies

XXlI

.F

C E En E

Ep:,

EO F FR

Id

K L L, Ld M Kbij

M N P PN

Q R

54 S

Sd T

total thickness of semiconducting film approximate intermetal distance force constant interaction force constant between CC bonds at distance s Lorentz local field factor wave vector nonradiative decay rate radiative decay constant absolute electron charge equilibrium atomic charge distance/displacement of two adjacent atoms for each mode concentration of the sample electrical field strength transition energy electrochemical potential internal energy of the system half-wave redox potential oxidation potential redox potential free energy potential energy matrix drain current equilibrium constant chain length spatial extension on a ID chain 7r-electron delocalization length mass bond stiffness total molecular electric dipole moment Avogadros number doping level macroscopic polarization of a medium polymerization degree quinodal resonance structure vibrational displacements vibrational internal coordinate hopping distance vibrational space coordinate entropy Huang Rhys factor ground state lowest singlet excited state relaxation energy kinetic energy temperature

TI Tm Tc U Uvalence V V vd

vg

W

lowest triplet excited state melting temperature clearing temperature superconductivity transition temperature internal energy valence potential potential energy oxidation potential drain voltage gate voltage energy difference between initial and final electronic states

This Page Intentionally Left Blank

List of Abbreviations

ADMET AM 1 A0 B BBN Boc bPY PC CP

cv

CY DDQ DFWM DIBAL DIIRS DMF DMP2 dmso DOS DPOE DPC DVS E EB ECC ECCF ECL EFISH El-MS EL ENDOR EPR ESCA ESR ET FAB FD-MS FET FTIR

acyclic diene metathesis Austin model one allo-ocimene benzene 9-borabicyclo[3.3. llnonane tert-butoxycarbon yl 2,2'-bipyridyl trans-p-carotene pentamethyl cyclopentadienyl cyclic voltammogram cyanines dichlorodicyanoquinone degenerate four wave mixing diiso butylaluminum doping induced infrared spectrum N,N-dimethylformamide 2,3-dimethyl- 1,3-butadiene dimethyl sulfoxide distribution of excited states a pdipheny loligoenes dodecapreno-p-carotene divin y lsulfone ethylene emeraldine base effective conjugation coordinate equilibrium change and charge flux effective conjugation length electronic field induced second harmonic generation electron impact mass spectrometry electroluminescence electron-nuclear double resonance electron paramagnetic resonance electron spectroscope for chemical analysis electron spin resonance energy transfer fast atom bombardment field desorption mass spectrometry field-effect transistor Fourier transform infrared

XXVI

List of’ Ahhwvicrtions

GPC HCM HMO HOMO HOPG HPLC INDO IP ISC L.R. LC LDA LEB LED LEED LOP LPPP LUMO MALDI TOF MIS MNDO MRD-CI n-Boc NBS n-BuLi NDDO NLO NMR OASLM OAV ODMR OE OE2 OE3 OMP OP OPC OPn = H-P,-H OPV ORY OTn = H-T,-H PA PAH PAT PBN PDMPV

gel phoresis chromatography hydroquinonemethylether Hiickel molecular orbital highest occupied molecular orbital highly oriented pyrolytic graphite high pressure liquid chromatography intermediate neglect of differential overlap ionization potential inter-system crossing Lawesson’s reagent liquid crystal lithium diisopropylamidee leucoemeraldine base light-emitting diodes low energy electron diffraction ladder oligo-p-phenylene ladder PPP lowest unoccupied molecular orbital mass spectrometer method metal-insulator-semiconductor modified neglect of differential overlap multireference double configuration interaction n-butoxycarbonyl N-bromosuccinimide n-butyl lithium neglect of differential diatomic overlap nonlinear optics nuclear magnetic resonance optically addressed spatial light modulators oligoant hryleneviny lene optically detected magnetic resonance oligoenes butadiene hexatriene oligo(m-phenylene) oligo( p-phenylene) oligomeric bridged phthalocyaninato Ru complex oligo( p-pheny1ene)s oligo( p-phenyleneviny 1ene)s oligorylenes oligothiophenes polyacetylene polycyclic aromatic hydrocarbons poly(3-alkylthiophene) pernigraline base poly(2,2’-dimethyl- 1,l ’-biphenylene-4,4’-vinylene)

List

PDPV PFV PIRS PL PMMA PPAn PPP PPPV PPT PPV PT PTFE PTV PVK QP R6G RMS ROHF R-OMP R-OPV

sc

SCF SEC SHG SSF SS-FPV STM T TCNQ tfP THF THG THP TLC TMP2 TMP3 TMP4 TMS TOSMIC TPA tPY TTF UPS

uv

VEH

if

Abbreviations

4,4’-biphenyl( 1,2-diphenyIvinylene) pol y(2,7-fluorenylenevinylene) photoinduced infrared spectrum photoluminescence polymethylmethacrylate polyphenylacet y lenes poly( p-phenylene) poly(phenylpheny1enevinylene) polypheno thiazinobisthiazole pol y( p-phenylenevinylene) polythiophene poly ytetrafluorethylene poly (thienylvinylene) poly(vinylcarbazo1e) quaterphenyl rhodamine 6G root mean square restricted open shell Hartree-Fock alkyl substituted OMP alkyl substituted OPV semiconductor self-consistent field size exclusion chromatoeaphy second harmonic generation site selective fluorescence solution spray flash vacuum pyrolysis scanning tunneling microscope thiophene 7,7,8,8-tetracyanoquinodimethane trifuryl phosphane tetrahydrofuran third harmonic generation tetrah ydropyran yl thin layer chromatography

2,5-dimethyl-2,4-hexadiene 2,7-dimethyl-2.4,6-octatriene 2,9-dirnethyl-2,4,6,8-deca-teraene trimethylsilyl ( p-toluenesulfony1)methyl isocyanide all-trans polyacetylene 2,2‘: 6’2’’-terpyridine tetrathiofulvalene ultraviolet photon spectrometer ultraviolet valence effective Hamiltonian

XXvII

This Page Intentionally Left Blank

1 Hydrocarbon Oligomers Y. Geerts, G. Klarner and K. Miillen

1.1 Introduction Conjugated hydrocarbon oligomers constitute a link between classical organic 7rsystems, such as linear or cyclic olefins, stilbene, biphenyl or distyrylbenzene on the one hand, and conjugated polymers like polyacetylene, poly(para-phenylene) (PPP) and poly( para-phenylenevinylene) (PPV), on the other. The latter group is known to possess attractive electrical, optical and nonlinear optical properties that also qualify them as active components of electronic devices. In attempts to tune such physical properties by way of synthesis, conjugated hydrocarbons play a special role: while representing a great structural manifold, they are made from a limited amount of basic building blocks, as there are olefins, acetylenes and aromatic moieties like benzene, naphthalene and anthracene. Within the present oligomeric approach the combination of these structural units leads to the target structures: oligoenes 1, oligoynes 2, oligoenynes 3, oligoarylenes 4, oligoarylenevinylenes 5, oligoaryleneethynylenes 6, oligo[n]acenes 7, oligorylenes 8, cycloarenes, e.g. Kekulene 9, and polycyclic aromatic hydrocarbons of the Clar type, e.g. hexabenzocoronene 10 (Scheme 1). When the outstanding role of oligomers is discussed, the physical and synthetic aspects should be considered in a unified manner: 0

0

Oligomers serve as well-defined molecular species for a reliable correlation of structure and properties. Moreover, many optical and electrical features of conjugated polymers may closely correspond to those of oligomers containing only a few repeat units. When monitoring the physical properties as a function of chain length, extrapolation towards the behavior of a defect-free polymer becomes possible. Accordingly, increasing the size of a -ir-systemwithin a homologous series of oligomers constitutes a fundamental tool in probing the nature of 7r-conjugation. Oligomers serve as model systems to optimize polymer-forming reactions. A particular challenge is the evaluation of novel C-C-coupling reactions. In their own right oligomers constitute attractive materials and this becomes particularly obvious when searching for chromophores, which display a potential for use in light emitting devices (see Chapter 10).

An important criterion in the design of conjugated oligomers is their processability, i.e. high solubility in organic solvents and good film forming properties. Processability is required not only for the characterization of the oligomer structure, but also for many applications in material science. Unfortunately, low solubility is generally observed for unsubstituted conjugated oligomers due to their rigid and planar structures. This inherent disadvantage of conjugated oligomers is suppressed by the incorporation of solubilizing n-alkyl or t-butyl groups.

1

2

3

4

5

n

Q + q H

6

7

9

8

10

Scheme 1. Oligoenes 1, oligoynes 2, oligoenynes 3, oligophenylenes 4, oligophenylenevinylenes 5 , oligophenyleneethynylenes 6, acenes 7, oligorylenes 8, Kekulene 9, hexabenzocoronene 10.

The electronic properties of conjugated hydrocarbons can be fine-tuned by (i) changing the conjugation pathway (topology), (ii) varying the geometric demands, (iii) incorporating saturated centers into the main chain or (iv) inserting electron withdrawing or donating groups. These factors raise the question of which is the best synthetic strategy to produce a homologous series of extended oligomeric hydrocarbon ?r-chains. The step-bji-step synfhesis of oligomers affords well-defined products when each step is followed by a work-up procedure and isolation of the intermediate pure compound. Within the step-by-step sjntlzesis of oligomers there exists a subcategory, the riiohrlcrr synrhesis. This conserves the essential feature of the step-by-step approach, i.e. the synthesis of well defined oligomers with discrete molecular weight, but the modular approach brings also the additional advantage of an exponential growth weight.

1.-7 Olefitiic Structitres

3

The runrlom sjw/hesi.s includes a 'one-pot'-polycondensation in which the 1 : 1stoichiometry of the bifunctional AA- und BB-components is violated. Tuning of the stoichiometric relation of the bifunctional monomeric building blocks enables the synthesis of particular oligomers. Furthermore, a monofunctional end-cappingreagent can be used to unbalance the stoichionietry when a bifunctional AB-type monomer is subjected to a polycondensation. The addition of an end-capper at the end of the reaction helps to provide well-defined functional groups. The necessary separation of the oligomeric mixtures obtained in the random approach is generally achieved by preparative size exclusion chromatography (SEC) or high pressure liquid chromatography (HPLC). Naturally, the separation of different oligomers is expected to be greatly facilitated if the size of the repeat unit is increased. Any side reactions or incomplete end-capping may well cause problems during the separation procedure. An N pviori evaluation of the advantages and disadvantages of the step-by-step and random approach is not feasible. The discussion of the following examples of the synthesis of olefinic and aromatic hydrocarbon oligomers will, however, help to determine which is the best method for their synthesis. This discussion will be closely linked to aspects of materials science and will finally provide a helpful guide for dealing with future synthetic challenges.

1.2 Olefinic Structures 1.2.1 Oligoenes The major interest in linear oligo- and polyenes lies in their ability to serve as defectfree model compounds for electrically conducting materials such as polyacetylene [ l , 21. Fundamental research concerning the stepwise build-up of a polyene chain has been contributed by Kuhn e/ al. [3]. The applied aldol condensation within this work is still the key step in many oligoene syntheses. Christensen et al. prepared 1,3,5,7,9,11,13-tetradecaheptaene14 from the autocondensation of crotonaldehyde 11 in the presence of piperidinium acetate [4] (Scheme 2). followed by subsequent Grignard reaction of the resulting

11

12

- H20 13

14

Scheme 2. Tetradecaheptaene 14 by autocondensation of crotonaldehyde 11 and subsequent chain elongation.

it

NaNH2I C5H1jON0 Et2O

0

H 16

15

H

?."* H 0 17

16

t-BuOK MeOH

___c

Scheme 3. Oligoenes 18 with carbonyl-acceptor groups by an aldol condensation route

2,4,6,8,10-dodecapentaenal12 with vinyl magnesium bromide and dehydration under acidic conditions to the target compound in only moderate yield [5]. The step-by-step synthesis of oligoenes via the Wittig reaction has attracted much attention, especially within the broad field of carotenoids [6, 71. Recent work by Martin et 01. involved the aldol condensation as a route to carotenoids with up to 13 conjugated double bonds 18 (Scheme 3) [8].Within this approach the effect of terminal carbonyl acceptor groups such as violerythrin dioxime on the optical properties of the carotenoids was investigated. It has been shown that = 460 nm) can the longest wavelength absorption band of @-carotene 19,,A,( significantly be shifted to longer wavelengths, up to 566 nm for 18,by introducing carbonyl acceptors. Bohlmann and Mannhardt synthesized polyenes with up to 8 double bonds 23 by Wittig reaction of bisaldehydes 21 with monophosphonium salts 20 (Scheme 5). The resulting conjugated oligoenyne 22 could be partially hydrogenated by use of a Lindlar catalyst [9]. Sondheimer et 01. chose a different synthetic route, using the prototropic rearrangement of linear 1,5-enynes 24 to conjugated polyenes 25 (Scheme 6). It

19

Scheme 4. @-carotene19.

1.2 Olefinic Structures

20

H2

21

5

20

~

Lindlar

catalyst 23

Scheme 5. Oligoene by Wittig reaction.

was found that linear 1,5-enyne 24 and IJ-diyne hydrocarbons can in fact be isomerized conveniently to the fully conjugated systems by means of potassium t-butoxide in t-butyl alcohol. The prototropic rearrangement presumably proceeds via the allenes [lo]. Recently, Schrock and Knoll described the synthesis of a homologous series of oligoenes 32a,b with up to 15 double bonds [ll]. The t-butyl terminal groups of the oligoenes provide the steric protection for the chain ends to prevent oxidation and cycloaddition reactions and ensure good solubility. In contrast to the carotenoids, the main chain of the polyenes should remain unsubstituted to prevent a substituent induced twisting of the -ir-electron system. The synthesis is based on the ring opening metathesis polymerization of cyclobutene derivatives. Using a 'random' concept [ l l ] (Scheme 7), the initial step of this reaction sequence is the ring opening of the cyclobutene unit in monomer 27 followed by transfer of the t-butyl vinyl group of the catalyst 26 to the end of the chain. The new alkylidene complex can then react with another equivalent of 27 to give a

24

26

Scheme 6. Oligoene by the prototropic rearrangement of a linear 1,5-enyne 24.

F q C F 3 W(CHtBu)(NAr)(OtBu)z +

A

(Ar = 2,6 - C6H3iPr2) 26

initiation

3 -W(NAr)(GtBu)2 I

27

28

termination + RCHO

addition

_______)

-O=W(NAr)(OtBu)p

29

F3C A

CF3

M

30a,b

i) chromatographic separation 32a,b

ii) isomerization

33

a : R =t-BU, b R =

Scheme 7. Synthesis of t-butyl-capped all-trans-oligoenes 33 containing up to 15 double bonds by metathetic, elimination, and isomerization process.

new tungstacyclobutane complex 29. This is repeated until a polymer results that contains IZ equivalents of 27 in a ring-opened form. Each of the propagation steps can produce either a cis- or a trans-double bond, although trans-propagation dominates. The oligomer is finally cleaved from the metal in a Wittig-like reaction between the W=C bond and an aldehyde. The last step is a retro-Diels-Alder reaction that generates the bis-trifluoromethyl benzene 31. If pivaldehyde is used in the Wittig-like reaction, the result is a series of polyenes containing an odd number of double bonds. The use of (E)-4,4-dimethyl-2-pentenal provides the corresponding series of polyenes with an even number of double bonds. The separation of oligoenes 32a,b containing an odd and even number of repeat units was achieved by column chromatography on silica gel at -40°C to

7

1.2 OlrJiriic Srructitres

Table 1. I3C- and 'H-NMR Data for all-trrn~sOligoenes 33 (CDC13). la I1

= 1

I1

=2

I1

=3

n=4 n=5 t1 =

6

I?

=

7

I1

=

8

I?

=9

3

3

135.70 (5.30) 143.62 125.42 (5.62) (5.93) 145.22 125.28 131.40 (5.66) (5.96) (6.09) 145.90 125.46 132.72 (5.70) (5.99) (6.14) 146.37 125.47 133.29 (5.72) (6.00) (6.17) 146.59 125.50 133.65 (5.73) (6.01) (6.18) 146.74 125.52 133.85 (5.74) (6.02) (6.19) 146.83 125.53 133.98 (5.74) (6.02) (6.19) 146.86 125.56 134.06 (5.75) (6.02) (6.19)

4

5

6

131.10 (6.15) 131.14 132.35 (6.17) (6.19) 132.37 132.91 (6.21) (6.22) 131.14 132.36 132.92 (6.18) (6.22) (6.24) 131.13 132.37 132.90 (6.18) (6.23) (6.25) 131.14 132.36 132.90 (6.18) (6.34) (6.26)

7

8

9

133.26 (6.23) 133.26 (6.24) 133.23 (6.25)

133.45 (6.24) 133.45 133.57 (6.25) (6.25)

'H-NMR data are in parentheses. This number refers to the olefinic carbon atom or proton in the chain with the carbon atom to which the r-butyl group is bound defined as 1.

avoid undesirable oxidation and crosslinking reactions. Oligoenes 32a,b with n = 4,5,.. . , 8 , containing ris- and trans-double bonds were converted to all transisomers 33 by UV-irradiation, whereas thermal isomerization at 2 5 3 0 ° C in the presence of a trace of iodine was preferred to provide longer polyenes (I?= 9, 10,. . . , 12). Since these methods failed for the oligoene 33 (n = 13), it was thermally converted to its all trans-isomer at 180°C. Interestingly, the I3C- and 'H-NMR data, as well as the UV-VIS spectra, were investigated as a function of the number of double bonds (n).The 13C-and 'H-chemical shifts of t-butyl endgroups are rather insensitive to chain length whereas that of polyene chains vary largely (Table 1). In polyenes constituted of four or more double bonds, the chemical shift of the innermost carbon atom increases smoothly and can be extrapolated to Slnfinlte = 133.95ppm for a hypothetical trans-polyacetylene of infinite length. Meanwhile the proton chemical shifts converge on a value of 6.25-6.26ppm as the proton moves towards the center of the chain and as the truns-polyacetylene becomes longer. Extrapolation to infinite length predicts a proton chemical shift Sinfinite = 6.29 P P ~ . UV-VIS data and tentative transition assignments for all-trans-oligoenes 33 in n-pentane are collected in Table 2. Extrapolation of these data for a hypothetical all-trans-polyene of infinite length afforded a band gap of about 1.8eV in accordance with values found in the literature [12]. Mullen et al. developed a 'warehouse' of olefin precursors that allows the synthesis of stable and soluble polyenes of defined chain length in good yield by a step-by-step procedure [ 131 (Scheme 8). It is necessary to tailor-make mono- and

8

1 Hydrocarbon Oligorners

Table 2. UV-VIS data and tentative transition assignments for oliogoenes 33 (in n-pentane)a l'B, n

+

0-0

~

~~

2 3 4 5 6 7 8 9 10

11 13

a

3'4

237.2 (0.688) 275.6 (0.766) 311.4 (0.897) 343.0 (0.991) 371.2 (1.00) 396.2 (1 .OO) 418.8 ( I .OO) 438.8 (1 .OO) 456.4 (0.977) 468.8 (0.921) 494 (0.921)

0- 1

0-2

0-3

0-0

0- 1

2'B, l'A, 0-0

227.8 (1 .OO) 264.8 (1 .OO) 297.4 (1 .OO) 325.8 (1 .OO) 351.0 (0.931) 373.6 (0.906) 394.0 (0.869) 411.2 (0.921) 427.8 (1 .OO) 439.4 (1 .OO) 462 (1 .OO)

219.8 (0.948) 255.6 (0.779) 284.8 (0.683) 31 1.0 (0.615) 334.2 (0.558) 355.6 (0.520) 374.0 (0.531) 390.2 (0.574) 405.2 (0.638) 414.4 (0.657) 438 (0.757)

274.6 (0.369) 297.8 (0.294) 3 19.0 (0.2 58) 338.0 (0.239) 354.2 (0.254) 371.8 (0.290) 382.6 (0.310) 393.4 (0.340) 412 (0.450)

237.8 (0.062) 258.2 (0.037) 277.6 (0.035) 296.4 (0.046) 313.6 (0.080) 330.8 (0.099) 346 (0.106) 370 (0.214)

250.0 (0.032) 267.8 (0.026) 285.6 (0.033) 301.8 (0.059) 317.6 (0.056) 330 (0.071) 352 (0.143)

239.0 (0.123) 253.0 (0.179) 267.8 (0.165) 282 (0.159) 306 (0.186)

l'A,

+

I'A,

t

~~

UV-VIS data are listed in nanometers and relative extinction coefficients are listed in parentheses.

bifunctional building blocks with the necessary substitution pattern. The key step of this approach is the Stille coupling reaction [14]. The best catalyst systems for the coupling of the vinyl iodides with mono- and bifunctional organotin compounds proved to be [PdCI2(CH3CN),]/DMF and [ p d ( t f ~ ) ~ ] / D M (tfp F = trifurylphosphane, D M F = dimethyl formamide). The key building block 35 was synthesized by stannylation of 1,4-dichlorobutadiene 34 (Scheme 8 and Table 3). Compared with the 'random' approach of Schrock and Knoll, this step-by-step build-up is preferred, as significantly smaller amounts of catalyst are needed, and the overall yield of each monodisperse oligomer is higher (ca. 30% compared with ca. 10%). The lower yields of individual oligomers in the metathesis can be explained by the difficult chromatographic separation. The crystal structure of dodecahexaene 40 is depicted in Fig. 1 in two projections [13]. In the layered structure the molecules are tilted both relative to the plane of the layer and to the plane of projection with an angle of about 90" between the oligoene chains in adjacent layers. Remarkably, this packing accommodates the bulky t-butyl groups in the space between the layers in such a way that each polyene chain is surrounded by six neighbors, and an arrangement with a subcell structure very similar to that of trans-polyacetylene results. Therefore, analogously to other polyenes, 40 can be regarded as a model for the packing in trans-polyacetylene. The subcell is indicated at the top of Fig. la, and as a projection in the direction

1.2 Olejinic Structures

9

36

37

38

36

41

39

40

Scheme 8. Oligoenes by successive Stille coupling.

of the chain at the bottom. As in trans-polyacetylene, the lengths of the double and single bonds in 40 are not completely equivalent. Instead it shows an alternating structure. The average length of the double bonds is 134pm and that of single bonds, 144 pm. A contraction of the bond lengths towards the center of the molecule Table 3. Characteristic spectroscopic data for oligoenes 39-41. II

‘H-NMR (200 MHz)

“C-NMR (50 MHz)

UV X (nm) in cyclohexane

EI-MS M+

4

1.04 (s, 18H. CH1) 5.72 (d, 2H. 3 J = 14.9 Hz) 5.94-6.25 (m, 6H) 1.04 (s. 18H, CH3) 5.75 (d, 2H. 3 J = 15.2 Hz) 5.94-6.30 (m, 10H) 1.04 (s, 18H, CH3) 5.78 (d. 2H, 3 J = 15.4Hz) 6.02-6.36 (m, 14H)

30.04. 33.77. 125.98, 131.56. 133.19, 146.30

315 (0.895). 301 (1.0) 288 (0.657). 277 (0.327)

m/z=218

30.02, 33.86, 126.05, 131.60, 132.84. 133.37, 134.13, 147.00 29.31, 33.44, 125.53, 131.12, 132.36, 132.89, 146.30, 133.25, 133.45, 133.98, 146.82

376 (0-0), 355 (0-1) 338 (0-2), 322 (0-3) 262 (0-0), 254 (0-1) 425 (1.000). 399 (0.906) 379 (0.580), 359 (0.285) 301 (0.075), 289 (0.058) 241 (0.151)

m / z = 270

6 8

m / z = 322

10

1 Hjw’rocaibon Oligorrieis

Figure 1. (a) Unit cell of oligoene 40 (hydrogen atoms omitted). In addition the subcell of transpolyacetylene is indicated; (b) view in the direction of the molecular axis of 40.

is not observed; the lengths of both double and single bonds are subject only to statistical variations. The dodecahexaene 40 and its higher homologs can also serve as models for doped polyacetylene, because polyenes containing six to ten double bonds are comparable with those regions of the polyacetylene chain that become charged on oxidation or reduction. As in polyacetylene, the oligomers 39-49 have a high redox activity [13]. The cyclic voltammogram of even the tetraene 39 reveals two oxidation and two reduction peaks (Table 4). The HOMO-LUMO energy difference AE can be estimated at 3.5 eV from the first reduction and oxidation potentials. The hexaene 40 does not show a more extended redox sequence, but the energy gap AE decreases to 2.9eV. Finally, in octaene 41 seven successive charge transfer steps from tetraanion to trication can be detected, and AE is decreased even further to 2.6eV. Extrapolation of A E to hypothetical polyene of infinite length affords a band gap of 1.7eV. This value agrees quite well with that determined experimentally and theoretically for polyacetylene itself, i.e. 1.4-1.8 eV [12]. Clearly, the physical and electrochemical characteristics of oligoenes cast some light on the properties of polyacetylene and underline the role of well-defined conjugated oligomers as models for conjugated polymers.

1.2 Olefiriic Srrircrztrc..r

11

Table 4. Cyclovoltammetric data of 39, 40 and 41”. El,: in volt. reference electrode: saturated calomel electrode. Compd.

Reduction in THF $2

33 34 35 35‘

-2.9gb -2.75h

Oxidation in dichloromethane E:,? AE [ ~ V I

EBl2

Efl,

E1,2

~ f , ?

~f,,

-2.79b -2.60h

-2.Mb -2.46b -2.14b -1.08’

-2.52 -2.20 -1.96

0.99 0.79 0.63

2.0b 1.11’ 0.83b

1.45b

3.5 2.9 2.6

-1.80

T = -2O‘C. electrode material gold.

‘ Irreversible charge transfer as a result of the deposition of films on the electrode surface. In DMF at -3O’C.

1.2.2 Oligoynes, Oligoenynes The linear polyynes represent one of the possible modifications of carbon and exhibit alternating single and triple bonds along the main chain [15]. These linear polyynes, called ‘carbynes’, are expected to show interesting electrical properties [16, 171. A reliable correlation of structure and properties is so far lacking, as the materials described in the literature do not show reproducible structures [18, 191. A linear oligomeric or polymeric sp-hybridized carbon structure can be derived from a chain with alternating single and triple bonds, with calculated bond lengths 120.7 and 137.9pm or, alternatively, from a less stable chain of only double bonds (128.2 pm) [ 11. Quantum chemical calculations performed for infinite chain length suggest that the acetylenic form is somewhat preferred over the cumulenic form. The calculated excitation energy is in the range of 2 to 5eV [20-231. In contrast to infinite carbon chains, the oligomers with a number of repeat units (n) of eight or fewer tend to exhibit a cumulene-like structure with only a slight tendency towards bond alternation [24]. For oligomers with an even number of repeat units a linear and a cyclic (rhombic) ground state structure are very close in energy and may even coexist. Oligoynes with I I = 3-10 are presumably linear [25, 261. The following examples are concerned with polymer synthesis, and the controversies in their characterization will fuel interest in defined oligomeric materials. It will become clear that the oligomers play an important role as model compounds for carbyne synthesis and also for questions related to characterization. Although polymers are not the topic of this chapter, it is appropriate to cast light on the challenging questions and problems concerning the correlation of structure and properties of the carbynes. Various synthetic strategies towards carbynes are described in the literature, including the oxidative dehydropolymerization of acetylene with Cu’+ ions [I], condensation of carbon vapor or liquid carbon, phase transitions of diamond or graphite, and the shock transition of glassy carbon [I]. Other routes describe the synthesis of carbynes via dehydrohalogenation of poly(viny1idene halides) [ 19, 271 and dechlorination of chlorinated polyacetylene [28] in alkaline medium. The dehalogenation of perhalogenated precursors is reported to proceed at room

12

1 Hydrocarbon Oligoniers

1) CU*, NH40H 2) air 42

43

Scheme 9. Glaser coupling of acetylenes.

temperature by the action of some anion radicals, alkali metals and alkali metal amalgams or electrochemically at a Pt cathode in aprotic electrolyte solutions [29-381. The reaction of poly(tetrafluoroethy1ene) (PTFE) with alkali metal amalgams is ‘electrochemical’ in nature, since it is controlled by a discharge of a galvanic cell formed in situ with an amalgam anode and continuously renewed carbon cathode. Various direct and indirect methods have been used to investigate the structures of carbynes. X-Ray diffraction [37], electrical conductivity [36], UVjVIS spectra [34] and solid state I3C-NMR studies [30] indicate a superposition of graphitic, diamond-like and also carbynoid structures for the products derived from dehalogenation of PTFE. Carbynes are further characterized by IR-spectroscopy [27, 281, especially by the C-C stretching mode (vCGc) at 2100-2200 cm-’. The C=C stretching mode was clearly detected in the products of dehydrohalogenation of chlorinated polyacetylene and poly(viny1idene fluoride). Excluding air, oxygen and humidity, the products of the dehalogenation of PTFE also show the characteristic stretching mode. At a first glance, formation of a polyyne was considered, but polycumulene or a mixture of both linkages were subsequently found [27]. Kuzmany et ul. were able to confirm the occurrence of a C=C stretching mode by in situ Raman spectroscopic measurements during the reductive treatement of PTFE with alkali metal (Li, Na, K) amalgams [39]. All of the aforementioned examples are related to mostly ill-defined polymeric materials. However, defined all-carbon structures have achieved great interest in recent times because of the discovery of the fullerenes by Kratschmer, Smalley and Kroto [40] and of the major efforts towards carbon-cycles and carbon-networks by Diederich et al. [ 151. In order to include stabilizing, solubilizing and electronically active substituents into the oligoyne frame, it was important to find preparative methods that would enable the coupling of acetylenes and some of their derivatives. Glaser [41] has observed the oxidative coupling of phenylacetylene in which the cuprous derivative formed in the first stage is oxidized to the diyne 43 (Scheme 9). This reaction has been applied to the symmetric coupling of a large number of acetylenes [42]. The oxidation of acetylenes by cupric salts in pyridine was first accomplished by Eglington and Galbraith [43] (Scheme lo). Hay et al. found that tertiary amine

RClCH 44

cu2+ pyridlne

RCEC-CECR 45

Scheme 10. Eglington coupling of acetylenes

1.2 Olejiiic Structures

RCECH 44

+

BrCECR'

cu+

GiiZ

13

RCfC-CfCR 47

46

Scheme 11. Cadiot-Chodkiewicz coupling.

complexes of copper(1) salts are by far the most effective catalyst systems known for the oxidative coupling of acetylenes [44]. In the Cadiot-Chodkiewicz coupling, haloacetylenes 46 react with terminal acetylenes 44 in the presence of cuprous salts and amines [49] (Scheme 11). This method also enables unsymmetrical coupling reactions Walton and Eastmond introduced silylation as a protective method in CadiotChodkiewicz couplings, allowing arylacetylenes to react with bromoethynyl(triethy1)silane [50] (Schemes 12, 13). This coupling yields the silylated diynes 50, from which the terminal diynes are quantitatively liberated by treatment with aqueous methanolic acid. This product 51 serves as starting material for the further build-up of a defined series of oligoynes with up to 16 conjugated acetylene units when using the protecting procedure described above. Gladysz et al. applied the Eglington coupling reaction for the successive construction of symmetrical oligoynes 54a with terminal { Re)-groups (where {Re} = Cp* Re(PPh3)NO;Cp* = pentamethylcyclopentadienyl) [51, 521. To accomplish the synthesis of the analogous compounds with terminal iron instead of rhenium complexes, Lapinte et al. alkynylated [Cp*(C0)2FeI]with Li-C=.C-C=C-SiMe3. Subsequent photochemical ligand exchange with ethylenebis(dipheny1phosphine) and desilylation with Bu4NF allow dimerization following the Eglington coupling procedure to 54b with n = 4 in 80% yield. These oligomers enable the investigation of the interaction of two organometallic centers separated by a C,,-bridge with constant length, and in particular, the effect of the spacer length on the oxidation potentials and on the UV absorption spectra.

52

53

Scheme 12. Silylation as a protective method in Cadiot-Chodkiewicz couplings.

Scheme 13. Oligoynes 54a,b(n) with terminal organometallic complexes.

14

1 Hydrocurbon Oligomers

Table 5. Redox potentials and characteristic spectroscopic data for oligoynes 94(n).

I2 = 2 n=3 I? = 4

0.1 1, 0.64 0.20, 0.58 0.34, 0.62

90, 90 70, 70 70, 70

350 354 390

17 000 37 000 60 000

Surprisingly, the first oxidation of 54a(n) (see Table 5) becomes thermodynamically less favorable with longer oligoyne bridges while the second oxidation is rather independent of the length of the oligoyne bridge. Gladysz et al. [51] have rationalized this finding by invoking the repulsive interactions between occupied orbitals on each metal. These interactions decreased in longer chains, and gave lower ionization potentials and rendered the oxidation more difficult. Oligoynes 54a(n) exhibit strong UV absorptions which are bathochromically shifted and intensified with increased n. These UV maxima are also bathochromically shifted by >35 nm from those of corresponding end-methylated oligoynes and are 100-500 times more intense. This observation underlines the influence of organometallic endgroups on UV absorption spectra [ S l , 521. Recent work from Grosser and Hirsch introduced new dicyanooligoynes by reaction of dicyane and carbon vapor under the conditions of the fullerene synthesis [52]. Actually, this work was meant to enable the synthesis of heterosubstituted fullerenes. Many applications of the promising electronic, optical and catalytical properties of fullerenes [ 15, 41 -451 require their incorporation into soluble polymers and oligomers to reach good film forming behavior [46]. Giigel and Miillen have reported fullerene-containing ladder-type oligomers (Scheme 14) [54]. OHex

r

OHo

67(n)

Scheme 14. Reaction sequence for the preparation of 57(n).

1.2 Olefinic Structures

15

The oligomerization relies on the cycloaddition reaction of [60]fullerene with the in situ generated orrho-quinodimethane. According to matrix assisted laser desorption ionization, time of flight mass spectrometry, and analytical size exclusion chromatography of 57(n), up to seven fullerene molecules ( n = 0-5) are linked by hexyloxy-substituted spacers. The oligomers 57 ( n = 0) and 57 ( n = 1) could be isolated by preparative liquid chromatography on polystyrene gel with chloroform as eluent. Comparison of the solubility of the various oligomers reveals noteworthy differences. The oligomeric mixture is very soluble in chloroform and can therefore be separated by preparative chromatography on polystyrene gel. After separation, the 'dumb-bell' compound 57 (n = 0) precipitated. Subsequently, it could be redissolved only in halogenated aromatics (e.g. 1,2-dichIorobenzene). Without doubt, this poor solubility can be attributed to a weak intermolecular interaction between the electron-rich bis-hexyloxy-substituted bridge in 57 ( n = 0) and the electron poor [60]fullerene unit of a second molecule 57 ( n = 0). In contrast, the higher oligomers remain dissolved for an unlimited period. Presumably, this can be traced back to the increasing 'irregularity' of the molecule, which is determined by the increasing number of regioisomeric structures. Recently, Diederich et a/. have described the synthesis of diethynylmethanofullerene 58, the extremely carbon-rich, but soluble, bis(butadiyny1)methanofullerene 59 and butadiynyl-linked 'dimeric' methanofullerene 67 by the dimerization of diethynylmethanofullerene 60 via Hay coupling (Scheme 15) [44]. These authors have also reported the more soluble fullerene tetraadduct 62 which was successfully cyclized via oxidative coupling to afford the trimers and tetramers 63 [48] (Scheme 16). Along the route toward new carbon-rich structures, Diederich rt a/. introduced the 3,4-dialkynyl-3-cyclobutene-l,2-dione 65 as the key compound for oligoyne synthesis. Solution spray flash vacuum pyrolysis (SS-FVP) was applied as a new method for pyrolysis of thermally unstable compounds with high molecular weight (Scheme 17). SS-FVP of 65 gave linear oligoynes 66 up to the hexamer with various endgroups in about 35% yield [53]. Hybrid structures of carbynes and polyacetylenes created new synthetic challenges. Wegner, Bassler, Sixl, and Enkelmann have applied a topochemical reaction to synthesize the polydiacetylenes 69 [55] (Scheme 18). The interest in these polymers arises from the mechanistic features related to their formation, their conjugated structure and their intriguing optical and electrical properties. Unlike polyacetylene, polydiacetylenes can be prepared as macroscopic and perfect single crystals allowing an unambiguous characterization of their optoelectronic properties. Upon exposure to heat, UV- or y-radiation, diacetylenes are converted from a soluble, colorless, monomer crystal which is transparent, into a deeply colored crystal. With a few exceptions polydiacetylenes are insoluble in common organic solvents. The color arises from the lowest 7r-electron transition of the conjugated polymer backbone, and the absorption maximum is near 600nm. However, insolubility and optical absorption can be used to monitor the degree of conversion as a function of reaction time. Conversely to polyacetylene, polydiacetylenes cannot be doped, i.e. oxidized or reduced to give a polymeric salt of metal-like conductivity. Nevertheless, polydiacetylenes exhibit high charge carrier mobilities (pe z lo3cm2V-' s-I) in the

16

1 Hydroccirbon Oligoniers

58

59

60

Scheme 15. Diethynyl-rnethanofullerene 58 and 60, bis(butadiyny1)methanofullerene A6, and butadiynyl-linked 'dirneric' rnethanofullerene 67.

chain direction. As expected from their fully conjugated structure, the third-order nonlinear susceptibility coefficient x(3) of polydiacetylenes is rather high and comparable to that of GaAs below the absorption edge; moreover, switching times s have been reported [55]. in nonlinear absorption experiments of the order of The nature of the topochemical polymerization of monomers with conjugated triple bonds can be explained as a diffusionless solid state transformation of a single crystal of a suitable monomer into the corresponding single crystal of the polymer which contains very long well ordered conjugated chains. The polymer chains grow via carbenes as the active intermediate, as first suggested by Takeda and Wegner [56] and verified by Bloor and Schwoerer using ESR spectroscopy as a quantitative tool [57, 581. The corresponding polytriacetylenes are not known so far. Diederich et al. provided oligomers revealing the structural element of the polytriacetylenes [59] (Scheme 19). Hay coupling of the tetrayne 70 with phenylacetylene yields a homologous series of oligomers 71 up to the hexamer. The triisopropylsilyl groups in each unit ensure the necessary stability and solubility of these substances.

1.2 Olejinic Structitres

17

62

63

Scheme 16. Fullerene based cyclic trimers and tetramers 63.

-+

0

SS-FVP

K -A.

CI

64

R

k"+I

R CEC R

66

65

Scheme 17. Solution-Spray-Flash-Vacuum-Pyrolysis (SS-FVP) of 3,4-dialkinyl-3-cyclobutene1,2-dione 65.

18

1 Hidrocurbon Oligoniers

69

66

67

Scheme 18. A topochemical reaction for the synthesis of polydiacetylenes.

TIPS

d 70

71

Scheme 19. Hay coupling of tetrayne 70 with phenylacetylene.

Indeed, oligomers 71 are amazingly stable, high-melting materials that remain unchanged for months at room temperature when exposed to air and light. Reducof oligomers 71 are given in tion potentials and longest wavelength absorption A,, Table 6. When, , ,A is plotted against the inverse of the oligomeric length, a straight line was obtained with an intercept at 540 nm (2.3 eV), which corresponds to longest wavelength absorption of an infinite polymer in solution, in other words, to its solution band gap. This band gap is comparable to that of many polydiacetylenes (-2.1 eV). Table 6. Reduction potentials and longest wavelength absorption A,,, of oligomers 71. I1

A,",, (nm)

Reduction potentials E l l z (V vs ferrocene) in THF

1 2 3

422 418 506 51 1

-1.57

4

-1.60, -1.32 -1.96, -1.43. -1.19 - 1.99. - 1.76, - 1.32. - 1. I4

1.2 Olejiriic Structirres

728: n = 2

19

7 2 d : (2,E.Z). R =n-propyl

72b: n = 3 72c: n = 5

Scheme 20. Substituted and non-substituted oligoenynes 72.

The synthesis of all-rrans-polyenynes was first published by Wudl and Bitler [60]. Recent work from Schulz et al. pursues this strategy for oligomeric unsubstituted enynes [61]. The oligomers 72a,b,c (Scheme 20) were synthesized by a stepwise approach including Kumada coupling with a homogenous Ni catalyst and Pdcatalyzed coupling similar to Negishi’s procedure. The key step of this reaction sequence is the selective protection and deprotection of the functional groups. The synthesis of the alkyl substituted enynes 72d was achieved by using the Peterson olefination. Preliminary attempts to extend the cross-coupling method to the substituted oligoenynes failed, probably due to steric hindrance of the alkyl side groups [61] The literature contains a considerable number of publications dealing with oligoand poly-alkynes and enynes, but only very few examples allow detailed characterization of structures and endgroups [12, 13, 51, 67-74]. The role of oligoenynes as models for the corresponding polyenynes is less obvious than for oligoenes and oligoynes. In the case of the polyenynes (‘polydiacetylenes’) this is due to the fact that these polymers can be obtained as perfect single crystals in a topochemical polymerization; thus, the perfect polymer reference state exists. Note that the topochemical polymerization of a diacetylene derivative has also been investigated in restricted geometries, namely in Langmuir-Blodgett [63] and chemisorbed monolayers [64]. Very recently, De Schryver, in collaboration with Mullen, has reported the photopolymerization of a physisorbed monolayer containing diacetylene groups, self-assembled from solution at the liquid/graphite interface [65]. The monolayer structures before and after the polymerization have been revealed with submolecular resolution using scanning tunneling microscopy (STM). Specifically, the diacetylene containing isophthalic acid derivative 73 (Scheme 21) in solution in 1-undecanol forms a physisorbed monolayer spontaneously on graphite surface. Imaging this monolayer with STM results in a structure like the one presented in Fig. 2 where the diacetylene containing isophthalic acid

HOAO

73

Scheme 21. Molecular structure of the diacetylene containing isophthalic acid derivative 73.

20

1 Hj>drocarhonOligonzers

Figure 2. (a) STM image of a physisorbed monolayer of diacetylene (DIA) containing isophthalic acid (ISA) derivatives 73 from a solution of I-undecanol (SOLV). (b) Corresponding model of the area indicated in the STM image in (a).

derivatives 73 can be seen to be codeposited with solvent molecules. After UVirradiation, STM contrast indicates the formation of a polydiacetylene, formed along the lamellar direction (Fig. 3). Scanning on a larger scale reveals the presence of domains of polymerized and unpolymerized diacetylene containing isophthalic acid derivatives 73.

Figure 3. (a) STM image of a polymerized monolayer of diacetylene containing isophthalic acid derivatives 73. (b) Corresponding molecular model including unit cell of the imaged area in (a). (c) STM image of a domain boundary separating an unpolymerized (domain in the lower left) and a polymerized (rest of the image) region. (d) Zoom of the domain boundary present in (c).

1.2 Okejiiic Structures

21

Acceptor substituents (A)

74

D\

A 75

\

D-

?A

CN e

Donor substituents (D)

Scheme 22. a,w-Donor and acceptor substituted oligoenes.

The synthesis of end-functionalized oligoenes and oligoenynes has also attracted considerable attention. The incorporation of a,w-donor and acceptor substituents, or more simply of identical functional groups (Scheme 22), allows the optoelectronic properties of oligoenes and oligoenynes [51, 521 to be fine-tuned. The introduction of a,w-donor and acceptor substituents on oligoenes and oligoenynes has been mainly driven by the high demand for second order nonlinear optical materials [62]. Note that second order nonlinear optical response is not a molecular property but relates to the supramolecular order of chromophores since centrosymmetric aggregates must be avoided. The synthesis of donor and acceptor substituted oligoenes 74-77, illustrated in Scheme 23, relies principally on Wittig and Wittig-Horner reactions similarly to the previously mentioned oligoenyne syntheses. Specifically, benzodithiolyltriphenylphosphonium tetrafluoroborate 78 was first reacted with butyllithium or triethylamine and then with various polyene-dialdehydes to yield the benzodithia-polyene-aldehyde 79. This product was then engaged in a Wittig-Horner reaction to afford the donor and acceptor polyene 81. Meanwhile, the dicyano-methylene polyene 80 resulted

22

I Hjdrocurhon Oligoiners S

i) BuLi or Et3N

pph3+BF;

_____)

a S x H

ii) OHC-R-CHO

/

78

*o

H

79

0

Ph.11 P-CH2-Ar Ph'

malononitrite piperidine

18crown-6, NaH, THF

'CN

80

CH3

01

?Ar

CH3

Scheme 23. Synthesis of donor and acceptor substituted oligoenes

from the reaction of benzodithia-polyene-aldehyde79 with malononitrile in the presence of piperidine. Finally, treatment of benzodithia-polyene-pyridine 82 with methyl iodide afforded the donor and acceptor polyene 83 [73]. Lehn and coworkers have reported on the push-pull oligoenes and oligoenynes depicted in Scheme 22 [67]. The electric-field induced second-harmonic generation technique in solution has been applied to measure the quadratic hyperpolarizability (p)of the oligomeric series 74a,g-77a,g and 74a,h-77a,h. Three observations at the molecular level are noteworthy. Firstly, the value of the hyperpolarizability increases with the length of the conjugated bridge separating donor and acceptor endgroups [68]. Secondly, the dimethylamino substituent gives rise to higher /3 values than the benzodithia group, although the difference in efficiency between the two donor groups tends to vanish for the longest measured compounds. Thirdly,

84

Scheme 24. Schematic drawing of ru,w-disubstituted oligoenes introduced in a membrane as molecular wire.

the triple bond acts as a barrier which tends to decrease the hyperpolarizability. Zyss and Lehn have also investigated supramolecular and material aspects by incorporating donor and acceptor functionalized carotenoids 77c,g-77d,g in noncentrosymmetrical Langmuir-Blodgett films [69]. Oligoenes a,w-difunctionalized with polar functional groups have been incorporated in membranes to serve as molecular wires (Scheme 34). This was demonstrated by an acceleration of the electron transfer through a membrane containing a small amount of caroviologen 85 (Scheme 25) [71f]. The design of tram-membrane molecular wires has been further extended by Lehn to a molecule made of three parallel carotenoid chains a,w-terminated by complexing sites for metal ions 86 [72]. A complementary approach consists of the introduction of metal centers such as ferrocene and Ru-complex endgroups, to benefit from their rich electrochemical behavior while the polyene bridge provides a pathway for electron transfer [70]. It is of particular importance to understand how electronic coupling between redox partners persists over large distances. Lehn and Harriman have demonstrated a weak electronic coupling between metal centers of a mixed valence complex (Ru"/Ru"') derived from 88, although the metal centers were separated by a distance as long as 240 pm [73]. Tolbert has contributed to this field with the synthesis of a,w-bis(ferroceny1)oligomethine cations 91(n) which were prepared by the Wittig method (Scheme 26). The oligomethine cation was prepared by hydride abstraction with triphenylcarbenium tetrafluoroborate. Interestingly, the polymethine cation 91(n) exhibits distinct first and second oxidation potentials with up to 13 carbon atoms between metal centers indicating a significant electronic coupling between the two ferrocenes separated by a distance up to 200pm for 91 ( 1 2 = 13) (Table 7).

24

1 Hyhocnrhon Oligorizers

85

R = C&i$303@

87

Scheme 25. a,w-Disubstituted oligoenes.

Table 7. Half-wave redox potentials in CHzClz and approximative intermetal distance for bis(ferrocenyl)oligoniethine cations 91(n).

0.39 0.72 0.33

El,?( 1) E1/2(2) DE dFe-Fc

3

1

I2

(A,

-5

0.42 0.60 0.18 -8

5

9

13

0.37 0.51 0.14 -10

0.36 0.43 0.07 -15

0.34 0.38 0.04 -20

1.3 Aromatic Structures

25

09

91 (n)

Scheme 26. Synthesis of a,w-bis(ferroceny1)oligomethyne cation 91(n).

1.3 Aromatic Structures 1.3.1 Oligoarylenes Oligo- and polyarylenes are chemically stable and exhibit attractive electro-optical properties which render these materials suitable candidates as components of light emitting diodes (LED) or nonlinear optic (NLO) devices (see Chapter 10) [75, 761. In the case of poly( p-phenylene) (PPP), a detailed structure-property analysis has been hampered by its insolubility, and by the fact that most synthetic routes lead only to ill-defined, defect-rich products or to short chain lengths. PPP was synthesized by Kovacic from benzene 92 with AIC13/CuC12using an oxidative coupling reaction and contained branches and higher condensed aromatic structures [75]. PPP synthesized by the so-called ICI-route using a soluble precursor polymer 94 and subsequent aromatization contains 1,2-phenylene linkages [77]. Recent work from Grubbs et al. [78] introduced a synthetic strategy to high molecular weight, structurally regular PPP via a stereoregular precursor polymer 94 made by transition-metal-catalyzed polymerization. Polymer 94 is converted into PPP via the Lewis or Brernsted acid-catalyzed thermal elimination of acetic acid. PPP synthesized by Yamamoto from dihalobenzenes 95 using Ni" salts as catalyst is characterized by a low degree of polymerization [79]. 2,5-Dialkylated PPPs 97 with high degrees of polymerization have been prepared using a Suzukitype Pdo catalyzed aryl-aryl cross-coupling from 2,5-dialkylated 4-bromophenylboronic acid 96 [80] (Scheme 27).

AIC13 / CuC12

+I92

90

x

x X = e.g. -0-CO-CH3

4

93 94

95

s n 96

97

Scheme 27. Synthetic routes leading to PPP-type structures.

Many activities have been directed towards soluble, well-defined oligomers that allow optimization of the polymer-forming reactions and that provide an analysis of physical properties as a function of chain length. The first series of soluble oligo(ppheny1ene)s 108 were synthesized by Kern and Wirth [81] and shortly afterwards by Heitz and Ulrich [82] using alkyl substitution in each repeat unit (Scheme 28). Various synthetic methods, like the Ullmann coupling, the addition of an organometallic intermediate 100 to a cyclic diketone 101 to yield a dihydro precursor 103, and the oxidative coupling of lithium aryls by CuC12, have been investigated [81, 821. Oligomers up to the hexamer have been synthesized by stepwise procedures using starting compounds such as 98, 100, 105, 106 and 107. The oxidative polycondensation of dilithium tetraphenylene 107 with CuC12 yielded an oligomeric mixture 108 with oligophenylenes containing up to 12 phenylene rings. In a random approach the dodeciphenyl was isolated by preparative TLC [82].

1.3 Aromatic. Strirctrrres

98

99

101

100

102

103

104

105

Scheme 28. A homologous series of soluble oligo(p-pheny1ene)s.

108

27

28

1 Hjdrocar.bon Oligorners

R

109: n = 0,l; R * n-hoxyl, ndodecyl

R

110: n = 0,l; R = nhexyl, ndodecyl

Scheme 29. Soluble oligo( p-phenylene)s

Recently Rehahn et al. presented the synthesis of constitutionally homogeneous, n-alkyl-substituted oligophenylenes 109 and 110 based on the Suzuki cross-coupling reaction (Scheme 29). These oligophenylenes are constituted of three to fifteen benzene rings exclusively coupled in the para-fashion. The step-by-step approach includes a great variety of monomer building blocks with and without alkyl substituents in the 2,5-positions of the central phenylene units [83]. The modular synthesis of oligomers is an iterative procedure based on a sequence of protection-deprotection reactions which leads to defined products. In this procedure, rather than having only one unit growth per iteration, the length of the oligomeric chain doubles per iteration enabling rapid synthesis of large oIigomers with defined end groups (Scheme 30). Using a modular approach, Hensel and Schliiter reported the synthesis of monodisperse oligophenylene rods l l l ( n ) with up to 16 phenylene rings and with welldefined functional endgroups [84]. Their synthetic strategy (Scheme 30) is based on an exponential growth methodology using the Suzuki cross-coupling reaction. It is important that iodoarenes couple significantly faster than the corresponding bromo compounds. Consequently, oligomers containing both bromo and iodo endgroups undergo the coupling at the latter site first, leaving the unreacted bromo site available for further functionalization. The authors took advantage of the monodisperse character of their oligophenylene rods to establish reference standards for GPC measurement of rigid-rod oligomers. Retention times were overestimated by a factor of 1.6 compared to a polystyrene standard of equivalent mass, corroborating the common view that rigid-rod macromolecules have a larger hydrodynamic volume than flexible coils of equivalent molecular weight. While the introduction of alkyl substituents increased the solubility of the oligophenylenes, the electronic properties of the n-system were disturbed by the mutual distortion of the phenylene units. It is, therefore, important that the incorporation of t-butyl groups only in the 3,5-positions of the terminal phenyl rings leads to a soluble series of oligophenylenes without disturbance of the conjugated system. The compounds were first synthesized by Liittke et al. using a Grignard reaction as the key step [85].

1.3 Aromatic Structitres

29

/ CSH13

112(2)

:i&sMT

1 1 l(4)

J 112(4)

dH13

Scheme 30. Modular approach to oligophenylene rods (telechelics) l l l - l 1 3 ( n ) using an (Continued overleaf) exponential growth strategy (n = number of phenyl rings).

Two new strategies for minimizing the mutual distortion of the phenylene rings and to insert solubilizing groups leading to soluble, defect-free oligomers and high molecular weight polymers of the PPP-type have been presented by Mullen and Scherf [86]. In the soluble ladder oligomers 118 and the related polymers (LPPP) the complete n-systems are rendered planar under the influence of the methylene bridges between two adjacent benzene rings. The model oligomers for the ladder poly( p-phenylene) (LPPP) were synthesized by a random approach using the Suzuki coupling as key step (Scheme 31). The bifunctional aromatic dibromodiketone 116 was allowed to react with an aromatic diboronic acid 115.

30

1 Hyrlrocnrboii Oligoniers

J 112(8)

1 13(8)

11l(l8)

Scheme 30. (Cont).

By adding a monofunctional compound 114 as an end-capping reagent and by varying the stoichiometry of the two bifunctional compounds the oligomeric mixture was enriched with the target structure. To guarantee sufficient solubility of the primary condensation products, namely oligo(pheny1ene)s with two benzoyl side-groups, solubilizing alkyl chains were introduced into both monomers [87]. The above mentioned Suzuki-type condensation is a very efficient method for coupling starting materials carrying functional groups. Thus, monomers containing nitro-, keto- or ether functions can be condensed without difficulty [90]. In addition, the electron-accepting keto-substituents appear to increase the rate of the aryl-aryl coupling considerably. The single-stranded intermediates 117 thus formed can then be converted into soluble ladder-type oligophenylenes via formation of methylene bridges, by performing a simple sequence of two polymer-analogous reaction steps. First, to guarantee a regioselective and quantitative conversion of the functional groups, it is necessary to reduce the ketone to an alcohol function with lithium aluminum hydride, then the corresponding oligoalcohol undergoes ring closure to the desired target structure under very mild conditions. The doublestranded, completely soluble oligo(fluoreneacene)s 118 are generated in a few seconds using boron trifluoride as catalyst. The strongly fluorescent ladder-type oligophenylenes 118 with odd numbers of aromatic rings ( n = 3, 5 , 7) were isolated from oligomeric mixtures using conventional column or preparative size exclusion chromatography. From a synthetic point of view, these oligomers serve as test examples to solve the key problem of 'perfect' intramolecular cyclization. The analogous ladder polymer is available with a molecular weight M , of 20000 (about 50 phenylene rings) and

31

1.3 Aromatic Structirres

2

& 0 114

Br

+

9 +B;r$r

2 (Hop

0

B(0H)z

115

116

118

n

0, I,2

Scheme 31. Multi-step route to planar and soluble ladder-type oligo( p-pheny1ene)s.

*

the cyclization to give five-membered rings proceeds quantitatively, as shown by Hand I3C-NMR spectroscopy; no indication of structural irregularities, such as incomplete cyclization or intermolecular cross-linking, was found [88]. The UV spectra reveal a bathochromic shift starting from the trimer at 328nm to the polymer at 450 nm (Fig. 4). Interestingly, the fluorescence emission spectra of ladder-type oligophenylenes 118 show a very small Stokes shift of a few nanometers resulting from the rigid structure of the planar ribbon polymer. In the case of the oligo(tetrahydr0pyrene)s 125 [89], the solubilizing alkyl side chains are introduced into the peripheral 4,9-positions of the tetrahydropyrene unit so that there is no additional steric hindrance to conjugation between the phenylene units. The oligo(tetrahydr0pyrene)s 125 represent a soluble structure composed of doubly ethano-bridged biphenyl building blocks. Within this approach the synthesis of functionalized 4,5,9,10-tetrahydropyrene120 moieties is crucial. According to Scheme 32, 2,2’-bis(alky1-1”-eny1)biphenyl 119, synthesized by a Wittig reaction of either biphenyl 2,2’-dicarbaldehyde and an alkylphosphonium

32

1 Hydrocarbon Oligorners

2,5x105

2,ox1o5

I

i,ox1o5

5,OxlO'

0.0

350

450

Wavelrnath [nm] Figure 4. UV-VIS spectra of ladder-type oligo@-pheny1ene)s 118.

salt or of an aliphatic aldehyde and 2,2~-bis(triphenylphosphoniomethyl)biphenyl dibromide, were photocyclized to 4,9-dialkyl-4,5,9,10 tetrahydropyrene 120 in 90-95% yield. Compound 120 was obtained as a mixture of cisltrans-isomers, since the alkyl chains at the 4- and 9-positions can be arranged either above or below the tetrahydropyrene plane. The bromination of 120, catalyzed by 5% palladium or platinum on activated charcoal, afforded, after purification by column chromatography, 2,7-dibromo-4,9-dialkyl-4,5,9,lO-tetrahydropyrenes 123 and the corresponding monobromo compounds 122 as colorless oils in 90-95% yield. The homologous model oligomers (n = 4, 6, 8, 10, 12) 125 were synthesized by a random approach using the Yamamoto procedure. Specifically, a mixture of 4,9-dialkylated 2,7-dibromo-tetrahydropyrene derivatives 123 and of the corresponding monobromo compounds 122 was reacted in presence of nickel(0)cyclooctadiene and 2,2/-bipyridyl in toluene/dimethylformamide (Scheme 33). The well-defined oligomers 125 (Table 8) were then isolated from the oligomeric mixture by means of size-exclusion chromatography. The corresponding polymer was also synthesized by Yamamoto coupling and had a number average molecular weight M , of about 16 000 (degree of polymerization of about 40; ca. 80 1,Cphenylene rings). The UV spectra reveal a bathochromic shift with growing chain length from the dimer at 329 nm to the octamer at 379 nm (Fig. 5 ) and up to 385 nm for the polymer. Fukuda et al. have reported a simple synthesis of soluble oligofluorenes 127 by the random approach [91]. The oligomerization of 9-alkyl-fluorene 126 is based on an oxidative coupling using FeC13 (Scheme 34). The fluorene moieties were mainly linked in the 2,7/-fashion and the M , of the oligofluorenes 127 correspond to a

1.3 Aromatic Structures

P R

33

P 90-95%

R

119

120

(r = n -octyl)

R

, I Pd I C or Pt I C

90.95% 122

P d l C or Pt l C

R = n -0clyl

9045% 123

Scheme 32. 2,7-Dibromo-4,9-dialkyl-4,5,9,1O-tetrahydropyrene 123.

number of repeat units of the order of 10. UV-VIS absorption and emission spectra , , ,A z 380 nm and, , ,A z 420 nm, respectively. No in chloroform solution revealed systematic synthesis of a series of oligofluorenes has been reported to date. These homologous series of planar and structurally defined oligomers serve not only as model compounds in terms of optimization of synthesis, but also provide a very important tool to explain 7r-conjugation in polymers [88]. To understand the degree of conjugation in the linear 7r-systems, the transition energy En, determined from the longest wavelength absorption maxima of the particular oligomers, is plotted against the reciprocal chain length l/n (Fig. 6) and extrapolated to infinite chain length. The resulting information on the so-called

Ni(C0D)z 2,2'-Blpyridyl COD L DMF 75 "C 99 % 122

124

Ar 122

123

125

Scheme 33. Oligo-tetrahydropyrenes (n = 4, 6,8,10, 12).

effective conjugation length can then be related to the torsional angles in the systems investigated [91] (see also Chapters 6, 7, and 10). Table 9 contains the inter-ring torsional angles for the various PPP-type structures determined by X-ray analysis. The studies on well-defined, monodisperse model oligomers of PPP-type show that the convergence of optical properties has been reached for ladder-type poly( p-phenylene)s and for poly(tetrahydr0pyrene)s. The planarity of the whole 7r-system in ladder-type oligo( p-phenylene)s 118 decreases the effective conjugation length of 11-12 benzene rings compared to oligo(tetrahydr0pyrene)s 125 which exhibit an effective conjugation length of 20 benzene rings. The optical properties of poly(p-pheny1ene)s cannot be correlated with absorption measurements on oligo( p-pheny1ene)s in dilute solution. Common PPP is characterized by strong intermolecular interactions of the 7r-systems in the solid state (high crystallinity). This leads to inconsistencies between the transition energies of the oligo( p-pheny1ene)s and the transition energy of the pristine PPP [92]. The effective conjugation length of PPP would be expected to be close to the value of poly(tetrahydropyrene)s, based on the experimental results on the ladder-type oligo(p-pheny1ene)s 118 and

35

I .3 Aroriiatic Structirres Table 8. Characteristic spectroscopic data of oligo (tetrahydropyrene)s 125 I1

'H-NMR (200 MHz)

UV X (nm) in cyclohexane ( E [mol-' cm-'1)

7.38-7.44 (4s. 4H). 7.12-7.13 (m. 4H). 7.01-7.20 (m. 2H), 2.75-3.11 (m. 8H). 2.92-2.95 (m, 4h). 1.23-1.80 (m, 56H), 0.89-0.97 (m, 12H) 7.40-7.48 (m. 6H), 7.13-7.20 (m. 4H). 7.06-7.08 (m. 2H), 2.77-3.15 (m,18H). 1.24-1.85 (m, 84H), 0.90-0.92 (m. 18H) 7.39-7.46 (m. IZH), 7.11-7.18 (m, 4H), 7.09-7.11 (m, 2H), 2.76-3.15 (m. 24H). 1.24-1.78 (m. 112H). 0.85-0.89 (m, 24H) 7.38-7.46 (m. 16H). 7.16-7.18 (m. 4H). 7.08-7.1 1 (m, 2H). 2.77-3.1 5 (m, 30H), 1.26-1.85 (m, 140H). 0.85-0.89 (m, 30H) 7.40-7.46 (m, 20H), 7.16-7.18 (m, 4H). 7.08-7.1 1 (m, 2H), 2.76-3.15 (m, 36H). 1.27-1.85 (m, 168H), 0.86-0.89 (m, 36H) 7.38-7.47 (m, 24H). 7.16-7.18 (m. 4H), 7.08-7.1 1 (m, 2H). 2.72-3.15 (m 42H), 1.27-1.86 (m, 196H). 0.86-0.89 (m, 42H) 7.37-7.45 (m. 30H). 7.16-7.18 (m, 4H). 7.08-7.1 1 (m, 2H). 2.81-3.15 (m. 48H), 1.26-1.85 (m. 224H), 0.85-0.89 (m, 48H)

329 (28800). 223 (33400)

858.5

352 (44900), 277 (12700), 225 (48700)

1287.8

364 (67400). 298 (15200). 224 (68800)

1716.8

370 (SSIOO), 316 (22700), 274 (16300), 225 (88700)

2145.3

375 (11200). 285 (15100), 224 (116100)

2574.5

377 (127200), 224 (133400)

3004.1

379 (138000). 224 (148200)

3432.4

FD-MS M+

Oc t a m e r

I

215

-

I

I

240

265

'

I

290

'

I

315

'

I

340

Wavelength / nm Figure 5. UV-VIS spectra of the oligo(tetrahydr0pyrene)s 125.

'

1

365

'

r

390

-

7

415

36

I Hydrocarbon Oligomers

FeCI,, CHC13 ____)

126

127

Scheme 34. Oligornerization of fluorene 126. R, = H, n-alkyl; R2 = n-alkyl.

oligo(tetrahydr0pyrene)s 125. Particular attention should be drawn to this aspect since structurally well-defined PPPs, without alkyl substituents, containing more than 15 benzene rings have not yet been described [87]. In this context, it is worth mentioning the 2,2'-bipyrenyls 131 andp-terphenyl 132 (Scheme 35). The syntheses of 131 and 132 are based on the Yamamoto coupling of monobromo pyrene 129 and of a mixture of monobromo pyrene 129 and dibromopyrene 130 [93]. Chromatographic purification was used to separate p-terphenyll32 from side products. The absorption maxima of the monomer A85,,A(, = 344 nm), of the dimer 131,,A,( = 350nm), and of the trimer 132 ,,A,( = 353nm) are shifted relative to one another by only a few nanometers (0-9nm). Accordingly, the cyclic voltammetry investigations do show that the first reduction potentials El are virtually independent of the number of pyrene units 131: El = -2.24eV; 132: El = -2.27eV). These findings are due to a characteristic stereoelectronic

2

v

W'

p 4,5 Q)

5 C

.-0

-

,...

..." ........

n-6

............

LPPP-Series

..'

I)-7

)imu: 441 nm (LPPP. exp.)

U

5

Reciprocal Chain Length l/n

Figure 6. Optical absorption energies of p-phenylene-type structures as a function of reciprocal chain length.

1.3 Aromatic Structures

37

Table 9. The inter-ring angles of torsion for differing PPP-type structures (from ref. 65).

4

PPP-type structure PPP alkylated PPP polytetrahydropyrene LPPP

23" >45' 20" 0'

situation. The molecular orbital coefficients of the bridgehead centers in the two frontier orbitals are almost zero, and consequently the rings are electronically decoupled to a first approximation. Thus, oligopyrenyls which fulfill this requirement differ significantly from oligoarylenes with a similar steric arrangement by the weak dependence of the absorption maximum of the number of repeat units. Consider for the sake of comparison biphenyl:, , ,A = 249 nm and terphenyl: , , ,A = 353 nm with dipyrenyl 131:, , ,A = 350 nm and terpyrenyl 132:, , ,A = 353 nm. The low electronic coupling between monomer units in oligopyrenyls resembles that of oligo-9,lO-anthrylenes 173-174 in which a conjugation barrier is achieved by positioning the subunits orthogonally (see below).

128

129

130

R

D

-q-p h

i) Ni(COD)p, 129

-*

=/

2,2'-bipyridine \

R

A 131

R

+ 130

R

A

i) Ni(COQ2, 2,2'-bipyridine

129

ii) chromatographic separation

R

R

R = n - OCtyl

R

132

Scheme 35. Synthetic pathway t o 2,2'-bipyrenyl 130 and p-terpyrenyl 131

38

1 Hydrocurbon Oligornrrs

133

R

R

R

R

R

Q

134

R

Scheme 36. Synthetic pathway to hexakis(terpheny1)benzene 134, R = C(CH3)2C,4H29

Recently, Miillen and coworkers have also reported the construction of star-shape oligomers such as the hexakis(terpheny1)benzene 134, and hexakis(quaterpheny1)benzene 139 [94] The synthesis of oligomer 134 is based on the cyclotrimerization of 133 (Scheme 36) while the synthesis of the higher star-shape homolog 139 involved the Pd coupling of six terphenyl boronic acids 137 with hexakis(iodopheny1)-benzene 138. Remarkably enough, the hexakis(terpheny1)benzene 134 and the hexakis(quaterpheny1)benzene 139 are composed of six oligophenyl chromophores centered closely together. The solubility of 134 and 139 is excellent, unlike that of the starting compound 133. The expected twisting of the terphenyl units at an angle of about 65" to the central benzene ring in 134 hinders close packing of the molecules in the solid state, and this explains the good solubility of 134 compared with that of 133. The question arises as to whether or not the terphenyl units are electronically coupled. Comparison of cyclovoltammetric studies of 134 with p-terphenyl indicates that essentially terphenyl redox units are present in 134. However, the absorption and emission spectra brings a rather different view, the bathochromic shift in the

1.3 Aromatic Strirctirres

39

Table 10. Long wavelength absorption and emission maxima, extinction coefficient and quantum yield of fluorescence for alkyl-substituted p-terphenyl 135. p-quaterphenyl 140. and star-shaped 134 and 139. Compound 134 139 135 140

Ax,,(em)

(nm)

(l/mol.cm)

(nm)

Quantum yield of fluorescence

305 316 285 299

3.5 x 3.2 x 3.6 x 4.7 x

360-380 425, 3x5, 374 364, 346, 332 390, 361. 354

0.66 0.93 0.99 0.96

, , ,A

(abs) lo4 10'

lo4 10'

absorption and emission spectra of 134 and 139 relative to the respective model substances 136 and 140, is not consistent with the picture of completely independent chromophores in 134 and 139 (Table 10) (Scheme 37). Note that the molecular extinction coefficients of 134 and 139 are remarkably high due to the density of chromophores. Star-shape oligomers 134 and 139 are fluorescent in solution and in the solid state. The similarity between the fluoresence spectra of the film and the solution excludes the formation of aggregates in the solid state. The good solubility, the low crystallization tendency, the high fluorescence quantum yields, and the absence of excimer formation make 134 and 139 ideal materials for LEDs. Both oligo- and poly(m-phenylene)s have been reported [95]. Early syntheses of linear and cyclic oligo(m-phenylene)s (OMPs) were reported by Staab and coworkers (Scheme 38) [96]. Cyclohexa-n~phenylene 142, cycloocta-m-phenylene 143, and cyclodeca-m-phenylene 144 were obtained in poor yields from the Grignard reagent, prepared from 3,3'-dibromobiphenyl, in the presence of copper( 11) chloride. An improved synthesis of cyclohexa-m-phenylene 142 involved the preparation of the linear dibromohexa-m-phenylene 147 and its subsequent cyclization. Similarly, cyclopenta-m-phenylene 150 and cyclodeca-m-phenylene 144 were obtained from the linear dibromopenta-m-phenylene 149. More recently, the OMPs 153 have been synthesized via a random approach from 1,3-dichlorobenzene in the presence of Zn, nickel(I1) chloride and triphenylphosphine [97]. The molecular weight of OMP 153 was controlled by the amount of chlorobenzene added. The mixture of OMPs has been separated by HPLC. The X-ray structure of deca-m-phenylene 153 ( 1 2 = 8) revealed an all-cis conformation yielding an apparently infinite helical chain with five aromatic rings to each turn of the helix (Scheme 39). The corresponding high molecular weight poly(m-phenylene) 155 has been synthesized by Yamamoto et al. by reaction of the Grignard reagents from 1,3-dichlorobenzene, 1,3-dibromobenzene and 1,3-diiodobenzene with nickel(I1) chloride in bipyridine [98]. However, presumably due to crystallinity, only 35% of the polymer formed was soluble in hot toluene. Extended 7r-conjugation in oligo- and poly(m-pheny1ene)s is prevented due to the lack of resonance beyond two phenylene units. This is clearly demonstrated by the fact that the absorption maxima of cyclopenta-m-phenylene 150, cyclohexa-m-phenylene 142, cycloocta-m-phenylene 143, cyclodeca-m-phenylene 144

40

1 Hydrocarbon Oligoniers

135

'

138

3

140

Scheme 37. Synthetic pathway to hexakis(terpheny1)benzene 139, R = C(CH3)2C14H29.

and biphenyl are all centered around 250 nm [96]. Consequently, UV-VIS spectra cannot be used to determine the length of oligomers. The synthesis of substituted oligo(o-pheny1ene)s 157 using the random approach has been reported [76,96]. Specifically, toluene, bromo-, chloro-, and fluorobenzene have been subjected to oxidative coupling using a mixture of aluminum trichloride and copper(I1) chloride yielding substituted oligo(o-pheny1ene)s 157 containing 10- 12 repeat units. Some polycyclic aromatic substances were also presumably formed. Extended 7r-conjugation does not occur in oligo(o-pheny1ene)s because of the large sterically induced twist angle between benzene rings and, similarly to OMPs, UV-VIS spectroscopy cannot be used to determine the length of oligomers (Scheme 40).

1.3 Aromatic Structures

41

1. Mg.THF 2. cuc12 ___)

b'

Br 191

1921

+

t

144

143

I . Condensation 2. Dehydration 3 . Aromatiration

n 145

146

1. Mg.THF

2. cuc12

B~

142

2

y p o+ 146

___)

142

-

I . Condensation 2 Dehydration 3 Aromatization B r M g a M g B r

142

149

150

+

144

Scheme 38. Synthesis of cyclopenta-rn-phenylene, cyclohexa-rn-phenylene, cycloocta-mphenylene, and cyclodeca-rn-phenylene.

42

1 Hjdrocarhon Oligoi?iers

-

151

162

*ax 1)

Mg

2) NiCI,.

154 : X = CI,

153

(by)

Br, I

155

Scheme 39. Synthesis of oligo- and poly(m-pheny1ene)s.

To learn about the scope and limitations of the above methods of ary-aryl coupling it is necessary to apply the random and the step-by-step approaches to different starting compounds like, for example, naphthalene or anthracene. Within these structural units the incorporation of solubilizing groups is straightforward. The oligo( 1,4-naphthylene)s 160 were synthesized from 2,7-di-t-butylnaphthalene with monobromo or mono boronic acid functions 158 and 1,4-dibromonaphthalene 159 or 4,4’-dibromo-l,l’-binaphthylusing the palladium catalyzed coupling procedure described by Suzuki. This method allows the generation of open-chain oligo(naphthy1ene)s 160 with I I = 1, 2, 3 (Scheme 20). The t-butyl substituents at the terminal positions of the oligomers provide sufficient solubility for detailed spectroscopic characterization and for processing of the materials from solution. Further, to gain a better understanding of inter-ring conjugation in oligoarylenes it is necessary to change the nature of the 7r-system and the bridgehead position of the inter-ring bond. Aromatic hydrocarbons that consist of double-stranded, peri-fused naphthalene subunits (so called ‘rylenes’) have been synthesized as r-butyl substituted oligorylenes 161 up to the pentarylene (11 = 3) [loo, 1011. However, the insolubility of higher members of the homologous series hinders their synthesis and characterization. To obtain structurally defined, higher homologs of the rylene series, Miillen and coworkers synthesized t-butyl substituted oligorylenes 161 using a stepwise process, which is based on the soluble oligonaphthylenes described above [102,103](Scheme 41). The key step of this method involves the ring-closure of open-chain oligo(naphthy1ene)s 160 leading directly to the aromatic ribbon-type compounds. The reaction can be described as an electron-transfer induced electrocyclic rearrangement followed by a

6-

AICI,, CUClp

156 R

R = Br, CI, F, CH3

157

n = 10-12

Scheme 40. Synthesis of oligo(o-pheny1ene)s.

1.3 Aromutic Structures

Br 158

43

n

159

160

161

Scheme 41. Oligo(naphthy1ene)s 160 and oligo(ry1ene)s 161.

dehydrogenation [ 1041. The reductive process only allows the generation of terrylene units as the most extended peri-fused naphthalenes. The analogous mild oxidative (Kovacic) coupling (aluminum trichloride, copper(I1) chloride) provides a pathway to the higher oligorylenes. Mullen and coworkers have also described the synthesis of higher oligorylenes bearing dicarboximide functions at the peri position. The aim was to combine the outstanding photostability, the chemical inertness and the high quantum yield of fluorescence of perylene tetracarboxdiimide 162a (Scheme 42) with the long wavelength absorption and emission of higher rylenes. The synthesis of quaterrylene tetracarboxdiimide 162c with four naphthylene units is depicted in Scheme 43. Two different synthetic pathways are nessecary to reach rylene tetracarboxdiimides containing an even or an odd number of naphthylene subunits.

R-N*@-R

0

' I no

162a : n = 2 162b:n=3 162c : n = 4

Scheme 42. Tetracarboximide rylene oligomers, perylene tetracarboximide 162a, terrylene tetracarboximide 162b, quaterrylene tetracarboximide 162c.

44

1 Hvdrocarhon Oligotners

Ni(C0D)z 2,2'-bipyridine

COD

E%

glucose

1

163

1 162c

R

= alkyl, aryl

Scheme 43. Synthetic pathway to quaterrylene tetracarboxdiimide 162c.

It relies on the Yamamoto coupling of bromo functionalized perylene dicarboximide 163 followed by an oxidative cyclization [ 1051. However, the cyclization conditions leading to oligorylenes 161 cannot be applied for the ring closure of tetracarboxdiimide quaterrylene precursors 164. Ring closure is therefore carried out by heating 164 in a KOH/ethanol/glucose melt to afford quaterrylene tetracarboxdiimide 162c in good yield. The synthetic pathway leading to terrylene tetracarboxdiimides with three naphthylene units requires one more step and is exemplified in Scheme 44 [106]. The presence of the dicarboximide function on perylene 163 prevents its transformation into the boronic acid, since it requires the use of butyllithium. This obstacle is avoided via the stannylation of 163 affording the perylene 165 which is then reacted with the bromonaphthalene 166. The resulting terrylene tetracarboxdiimide precursor 167 is easily fused in a KOH/ethanol/glucose melt to afford terrylene 162b. The stannane-based synthetic pathway also allows the synthesis of rylene oligomers peri-substituted with different endgroups 168 (Scheme 45). The UVjVIS absorption spectra of the soluble oligorylenes 161 exhibit distinct bathochromic shifts of the longest wavelength absorption band as the number of naphthylene rings increases (Fig. 7). Pentarylene ( n = 3 ) shows a, , ,A of 745 nm. Convergence of the electronic properties in the rylene series is, however, not yet

1.3 Aromatic Structures

/

163

166

45

R=Bu

166

1

167

162b

KOH

EtOH Glucose

R = alkyl, aryl

Scheme 44. Synthetic pathway to terrylene tetracarboxdiimide 162b.

reached with pentarylene. This fact motivates the synthesis of the higher members of the homologs series (hexa-, heptarylene), since the absorption maxima should be shifted into the near infrared region. Extrapolation of the transition energy towards the polymer poly(peri-naphthylene) or polyrylene predicts a value of about 1 .O eV. Considering the high thermal stability of the oligorylenes, these compounds represent particularly attractive examples of materials with low band gap. The members of the oligorylene tetracarboxdiimide series 162a-c exhibit a further bathochromic shift compared to the corresponding members of the oligorylene

168

R = alkyl, aryl

Scheme 45. Terrylene peri-substituted with different endgroups 168.

I Hydrocarbon Oligomers

I

I0

375 550 725 Wavelength [nm]

00

Figure 7. UV-VIS-NIR spectra of rylene oligomers 161.

series 161 (Fig. 8). The convergence of the longest wavelength absorption band of oligorylene tetracarboxdiimide is not yet attained in the case of the higher oligomer, the quaterrylene 162c, which displays a, , A, of 762nm ( E = 1 6 2 0 0 0 ~ - cm-'). ' Terrylenes 162b and 168 are characterized by their long wavelength emission band at 650 nm ( E = 93 000 M-' cm-I) and 676 nm ( E = 62 000 M - ' cm-I), respectively. The high extinction coefficients make the terrylenediimides 162b and 168 and quaterrylenediimide 162c promising candidates as functional dyes. The emission band of terrylenes 162b and 168 are bathochromically shifted by 23 nm and 25 nm from their absorption bands, respectively. The most remarkable feature of terrylenes 162b and 168 is their high quantum yield of fluorescence ($nUo) reaching 160000

-,

300

16%

400

500

600

700

800

h [nml Figure 8. UV-VIS-NIR spectra of the rylene oligomers 162a-c.

900

1.3 Aromatic Structures

47

0.6 zt 0.1 and even 0.9 i0.1 for 168 and 162b, respectively. Therefore, their application as active component of electroluminescent diodes may be envisaged. Quaterrylene tetracarboxdiimide 162c is, in contrast, only very weakly fluorescent. A homologous series of oligo(9,lO-anthrylenes) 173, 174 up to the heptamer was synthesized by reductive coupling of quinones 172 with lithioanthrylenes 169, followed by reductive aromatization of the intermediate hydroxy species 171 (Scheme 46) [107]. This method allows the introduction of various alkyl substituents, which ensure sufficient solubility of the anthrylene systems in common organic solvents. The interruption of conjugation by a significant torsion about the formal g-bonds leads to electronically decoupled n-subunits. It was shown by

&6Hf3

000 C6H13

169

a:X=H b: X=Br c: XnLi

171

173

170

a: X=H b: X=Br c: X=Li

172

174

Scheme 46. Oligo(9,lO-anthry1ene)s; R,, R, R, = alkyl, aryl.

48

1 Hydwcarbori Oligoniers

electron paramagnetic resonance that a relationship exists for oligo(9,IO-anthry1ene)s between formation of higher spin states in polyradicals, and effective localization versus delocalization of spin density in monoradical anions. The intramolecular electron transfer in monocharged species of oligo(9,lO-anthrylene)~ between anthracene moieties depends on the particular substitution pattern, on the radical concentration and on the ion pair conditions. In successive reduction processes the oligoanthrylene 173 can be charged with one electron per anthracene unit. The electron spin resonance spectrum of the tetra-anionic tetramer derived from 173, taken in glass, leaves no doubt as to the existence of a quintet state. However, temperature dependent ESR studies indicated that this state, as all other spin multiplicities in general, is thermally activated. The results for the synthesis of the oligoarylenes can be summarized as follows: (i) Most of the synthetic work has been devoted to oligo(p-ary1ene)s. In particular, modern aryl-aryl coupling methods like the Suzuki and the Yamamoto reactions enable a simple random approach whereas, for obvious reasons, the step-by-step approach is preferentially realized using the Suzuki reaction. The incorporation of differing aromatic building blocks as well as functional or solubilizing groups in the monomer units and in the terminal units of oligomers, with the intention of gaining tailor-made electro-optical properties, underlines the high potential of the applied synthetic methods. (ii) The defect-free structures of the resulting oligo( p-ary1ene)s enable transfer of reaction conditions to the polymer-forming synthesis in some cases. (iii) The electronic properties of conjugated PPPs can be analyzed by using a series of soluble oligo(p-arylene)s, and a reliable correlation of structure and properties can be established. (iv) The synthetic methods for oligo(ni-ary1ene)s and poly(m-ary1ene)s resemble those for oligo( p-ary1ene)s and poly( p-ary1ene)s. Specifically, organometallic aryl-aryl coupling methods have been used and both random and step-bystep approaches have been applied. Conversely, little has been reported on the synthesis of oligo(o-ary1ene)s.

1.3.2 Oligo[n]acenes Oligo[n]acenes are linear ladder-type oligomers composed of laterally fused benzene rings (Scheme 47). The lower homologs from benzene to anthracene are extracted from coal or from crude oil while the higher homologs such as pentacene 175(5), hexacene 175(6), and heptacene 175(7) are obtained by stepwise syntheses [ 1081. The synthetic pathways involve the preparation of a suitable non-aromatic precursor and its subsequent aromatization to yield the target acenes (Schemes 48-50). An efficient pentacene synthesis was reported by Hart and Luo and is depicted in Scheme 48 [ 1081. The first step involves a Diels-Alder reaction between benzocyclobutene 176 and anthracene 1,4-endoxide 177 yielding 178. Dehydration of 178 with acid affords the 5,lCdihydropentacene 179 which was dehydrogenated under the catalytic influence of Pd on carbon to yield pentacene 175(5).

1.3 Aromatic Strirctitres

175 (1)

175 (2)

175 (3)

175 (4)

175 (5)

175 (6)

175 (7)

175 (8)

175 (9)

Scheme 47. Representative oligomers of the acene series 175(n).

H

176

170

177

HCI, MeOH ___) toluene

179

PdK; ___)

175

Scheme 48. Synthetic pathway to pentacene 175(5).

49

(yJ$ 180

0

0

U

181 184

-

EtS. SEt

EtSH, ZnC1,

EtS SEt 182

183

Pd-C ____)

175(6)

Scheme 49. Synthetic pathway to hexaacene 175(6).

The synthesis of hexacene 175(6) and of heptacene 175(7) reported by Bailey and Liao [ 1091relies on two closely related reaction Schemes 49, 50. The first step consisted of a Diels-Alder reaction affording a dione which was reacted with ethanethiol and zinc chloride in a second step to yield a tetraethyltetrathioketal, which was not isolated but directly converted using Ni into the non-aromatic acene precursor. Catalytic dehydrogenation of this compound afforded the desired acene. The absorption spectra and extinction coefficients of oligoacenes 175(1-7) are collected in Table 11. Clearly, the absorption maxima increase with the length of the oligomers but convergence could not be reached within the limit of existing oligomers. Octacene 175(8), nonacene 175(9) or even higher homologs of the acene series are unknown [l 101. This is due to the chemical instability with size, due to a gradual loss of the benzonoid character, according to Clar’s sextet concept [l 1 11. In 1996, Mullen et a/. reported an efficient pentacene 175(5) synthesis via a retro Diels-Alder reaction at 140°C (Scheme 51) [112]. A decisive advantage of this

-

1.3 Arornaric Structures

ax:::

51

0

1,4-benzoquinone

l

I 0

184

185

186

Ni ___)

-

187

Pd-C

175

--

Scheme 50. Synthetic pathway to heptacene 175(7).

synthetic procedure is that the extrusion reaction can be carried out in bulk. Spin coated thin films from the soluble precursor 188 were easily converted into high quality pentacene films which were used as semiconductors in metal-insulatorsemiconductor field effect transistors with high charge-carrier mobility [ 1 121. Sublimation of pentacene does not give films of comparable charge carrier mobility.

CI

CIQCI

CI

CI

A

188

+

175

Scheme 51. Retro Diels-Alder synthetic route to pentacene 175(5)

52

1 Hydrocurbon 0ligonier.r

Table 11. Wavelength of absorption of oligomers of the acene serie 55(n). Compound (nmf (l/mol.cm)

Amax

E

55m

55U)

5W)

55(3)

55(4)

55(5)

256 200

312 290

370 6300

475 12 590

582

695

-

-

1.3.3 Cycloarenes According to Staab, cycloarenes are defined as polycyclic aromatic compounds in which, by combination of angular and linear annellations of benzene units, fully annellated macrocyclic systems are present, enclosing a cavity into which point carbon-hydrogen bonds [ 1 13,1141. Examples of cycloarenes are given in Scheme 52. Kekulene, also named cyclo[d.e.d.e.d.e.d.e.d.e.d.e.]dodecakisbenzene 9, and cyclo[d.e.d.e.e.d.e.d.e.e.]decakisbenzene189 have been synthesized by Staab [ 1 15, 1161. The synthetic pathway to Kekulene is summarized in Scheme 53. The sulfurcontaining macrocycle 192 was obtained from the reaction of the bis(bromomethy1) compound 190 and the bis(mercaptomethy1) compound 191 under high dilution. The resulting sulfur-containing macrocycle 192 was easily converted into the disulfone 193 by oxidation with nz-chloroperbenzoic acid. The conjugated macrocycle 194 was obtained in several steps from the sulfur-containing macrocycle 192 since the application of the Ramberg-Bgcklund reaction to the disulfone 193 did not proceed successfully. The conjugated macrocycle 194 was either directly converted into Kekulene 9 by photo-cyclodehydrogenation in good yield or first oxidized to the dehydrogenated conjugated macrocycle 195 which was subsequently photodehydrogenated to yield Kekulene 9, but in very poor yield. The difference in reactivity between the octahydrodibenzoanthracenophanediene 195 and dibenzoanthracenophanediene 194 under photodehydrogenation conditions could not be rationalized in terms of electronic effects, but was explained in terms of steric hindrance. Obviously, the conjugated macrocycle 194 has a much higher conformational mobility than the dehydrogenated conjugated macrocycle 195. The flexible macrocycle 194 could adopt more easily a favorable cyclization geometry than the stiff macrocycle 195.

9

189

Scheme 52. Examples of cyloarenes: cyclododecakisbenzene also referred to as 'Kekulene' 9, and cyclodecakisbenzene 189.

1.3 Aromatic Srructures

194

53

I

195

Scheme 53. Synthetic pathway to Kekulene 9.

1.3.4 Polycyclic Aromatic Hydrocarbons of the Clar Type Another field in which the Mainz group is actively engaged is the preparation of aromatic polycyclic aromatic hydrocarbons (PAH) of the Clar type. In the n-sextets model the n-electrons are assigned to single six-membered rings such that the maximum number of n-electron sextets is formed. With increasing number of T sextets the resonance energy of the PAH and the thermodynamic stability increase. Typical examples are rhombus 199 (Scheme 54), hexaalkyl substituted hexabenzocoronene also called ‘superbenzene’ 202 (Scheme 5 9 , and ‘supernaphthalene’ 203 (Scheme 56) [117]. Several new synthetic routes to aromatic polycyclic hydrocarbons of the Clar type have been developed. They all involve the synthesis of soluble oligophenyl precursors and their subsequent cyclodehydrogenation in high yield. The less conventional is probably that leading to the rhombus 199 [I 181. The substituted terphenyl 196 was quantitatively transformed into 197 via an intramolecular

Scheme 54. Synthesis of rhombus 199.

Diels- Alder reaction. The product 197 was already composed of eleven cycles and afforded the completely unsaturated rhombus 199 after aromatization with 2,3dichloro-5,6-dicyano- 1,4-benzoquinone (DDQ) and the subsequent loss of 12 hydrogens under the influence of a mixture of A1Cl3 and CuClz in CS2 [118]. Similar reaction conditions have been applied by Kovacic to the synthesis of poly( y-pheny1ene)s (see above) [81, 821. In the intermolecular case, the reaction was not particularly selective and yielded material which was crosslinked and insoluble and was never well-characterized. In contrast, the intramolecular version of the Kovacic reaction is highly selective and gives the desired cyclodehydrogenated product 199 in nearly quantitative yield. Rhombus 199 was insoluble in common organic solvents, but was successfully sublimed at 550°C in ultrahigh vacuum. The microcrystalline material obtained was bright yellow and, despite its considerable size, nothing like the black and lustrous graphite. Figure 9 shows an STM image of rhombus 199.

1.3 Aromatic Strucrures

Figure 9. STM image of vacuum deposited rhombus 199

J

A

Cu (OTf),/AICI,

Scheme 55. Synthesis of hexabenzocoronene 202, R = alkyl R

R

R

R

R

R R

R 203

Scheme 56. Supernaphthalene 203, R = t-butyl

201

55

In this context, it is worth mentioning hexaalkyl-substituted hexabenzocoronene 202 because it is available in large quantities due to its efficient and facile synthesis as outlined in Scheme 55 [I 191. The tolane derivative 200 was cyclotrimerized under the catalytic action of [Co,(CO),]. The readily soluble hexa(4-alkylpheny1)benzene201 was then cyclodehydrogenated under oxidative conditions. Hexaalkyl-substituted hexabenzocoronene 202 has the additional advantage of forming columnar mesophases [120]. The phase width depends on the length of the alkyl substituents and can reach 339°C for hexadodecyl hexabenzocoronene. This is one of the largest phase widths among the known columnar systems. Moreover, the very extensive 7r-system of 202 suggests that charge transport along 7r-stacks should be particularly rapid. This is confirmed experimentally by Warman and coworkers who have investigated the charge transport properties in the liquid-crystalline phase of hexaalkyl-substituted hexabenzocoronene 202 using the pulse-radiolysis time resolved microwave conductivity technique [121]. In fact, a large mobility of 0.1 3 x lop4m2/Vs is observed and represents the largest ever determined for a discotic liquid crystalline material. Moreover, the mobility is constant over a very wide range of temperature from 75°C up to at least 200°C. The combined properties of high, one dimensional mobility, liquid crystallinity and good thermal stability are well-suited to the use of such materials as vectorial charge transport layers in, for example, xerography and electrophotography. Remarkably enough, hexaalkylsubstituted hexabenzocoronene 202 adsorbs on a graphite surface into a regular lattice. Taking advantage of the large, but defined aromatic core of hexabenzocoronene to which six long alkyl chains have been attached, Rabe and coworkers, utilizing a scanning tunneling microscope, have successfully measured a diode-like current-voltage curve for a single molecule [119]. In the STM images of the alkylated peri-condensed hexabenzocoronene 202 (Fig. 1O(a)), the bright areas (high tunneling current) correspond to the aromatic cores, while the dark areas (low

Figure 10. (a) STM image of hexadodecylsubstituted hexabenzocoronene 202 on graphite with unit cell depicted. (b) Close packed model of a two dimensional crystal of hexadodecylsubstituted hexabenzocoronene 202.

1.3 Aronintic Structures

57

0

t

-1-5 0

I [nAI -1 5

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

U[VlFigure 11. Current voltage curve hexadodecylsubstituted hexabenzocoronene 202 on graphite. (a) Symmetric for the alkyl part. (b) Diode-like curve for the aromatic part of the molecule.

tunneling current) correspond to the aliphatic part of the two-dimensional lattice. As indicated in Fig. 10(b), the adsorbed alkyl chains have a preferential direction parallel to one of the main axes of the underlying graphite lattice. Because the depicted packing results in some free volume, the alkyl chains fill the space dynamically by utilizing the thermal energy, KT, at room temperature. The unit cell drawn in Fig. 11 reveals that the sixfold symmetry of the molecule has been reduced to a twofold symmetry of the adsorbate lattice. The current-voltage curve ‘a’ in Fig. 11 was taken above the aliphatic part (marked by A in Fig. 10). The broken black lines show the I-V curves with the bias values scanned from minus to plus; the grey lines document the backscans. Curve ‘a’ is symmetric with respect to the origin. It can therefore not be distinguished from an I-V curve taken on pure graphite covered only by a thin film of solvent. The curve ‘b’ in Fig. 11 was taken above the aromatic core of a molecule of 202 (marked by B in Fig. 10). Contrary to curve ‘a’, it exhibits a strong asymmetry. The diode-like shape reveals a factor of asymmetry of approximately one order of magnitude with respect to the maximal current values. A detailed explanation has still to be developed; nevertheless, two factors seem to be responsible for the observed asymmetric behavior. Firstly, the position of the highest occupied molecular orbital within a range of f 1 . 5 V about the Fermi level of the electrodes makes a resonant tunneling current possible. Secondly, the asymmetric position of the benzocoronene layer in the tunneling gap plays an important role. At relatively high tunneling resistance, the adsorbate layer is closer to the substrate than to the tip. The break in geometric symmetry in this experiment entails an asymmetric current response of the system. Stimulated by these results, the Mainz team has extended the synthetic concepts towards even larger polycyclic aromatic hydrocarbons such as supernaphthalene 203 (Scheme 56) which is only obtained with less Lewis acidic cyclization conditions, i.e. FeC13 at room temperature in dichloromethane, than for rhombus 199. This is

58

1 Hyhocurbon Oligotiiers

due to the reactivity of the r-butyl side groups to the Kovacic conditions yielding the formation of a complex mixture of products [117]. The polycyclic aromatic hydrocarbons approach, developed by Mullen and coworkers, deviates from the mainstream of conjugated oligomers because of the extension of conjugation in two dimensions rather than in one. As a consequence of their two dimensional structure, these large polycyclic aromatic hydrocarbons tend to organize spontaneously in bulk and on the surface into supramolecular assemblies like discotic mesophases and monomolecular adsorbate layers [ 118- 1201.

1.4 Olefinic- Aromatic Structures 1.4.1 Oligoarylenevinylenes The chemical stability of arylenevinylenes combined with their optical and electrical properties enable application of these materials as laser dyes, NLO-phores, photoconductors, photoresists and photocrosslinking compounds [ 122- 1241. From a structural point of view, poly( p-phenylenevinylene) (PPV) can be regarded as a hybrid of poly( p-phenylene) and polyacetylene. PPV can be oriented by stretch alignment of its precursor and shows remarkable optical nonlinearities [ 125, 1261. From the synthetic point of view there are two basic approaches to oligophenylenevinylenes (OPV): (i) the 'organic approach', trying to transfer the reaction conditions of stilbene synthesis to the higher homologs; (ii) the 'polymeric approach', trying to adapt the polymer-forming reactions to the oligomer synthesis.

I

Ar- CH2Hal

f

Ar'-CH-'NC&Hs Siegrist (ORe I DMF)

Ar-CH2$R3 Ha?

Heck Ar-Hal

I H*C=CH-Ar'

Scheme 57. Classical routes to stilbene used in organic chemistry.

1.4 Olefiriic-Armmitic Srructitrrs

59

205

Scheme 58. Metathetical route to symmetrical stilbene.

The classical methods in organic chemistry used to synthesize C=C-bonds in stilbenes 204 are the Wittig-Horner reaction [127], the Siegrist method [ 1281, the McMurry reaction [129], the Heck reaction [130], and the acyclic diene metathesis reaction (ADMET). These methods, summarized in Schemes 57 and 58, can be classified as follows:

206

207

208

P

206

209

210

Scheme 59. Poly( p-phenyleneviny1ene)s by polycondensation of dialdehydes R, R' = alkyl.

60

1 Hydr.ocarbon Oligoniers

21 1

212

213

214

Scheme 60. Polycondensation of diketones 211 with PO-activated methylene compounds 212.

(i) the pre-formed double bonds already exist in the starting compounds, for example the Heck reaction, and the Stille reaction as a Pdo-catalyzed coupling of olefinic bonds with haloaryls; (ii) the carbon skeletons are constructed together with the olefinic double bonds [ 131, 1321, for example the Siegrist, the Wittig-Horner and the McMurry reactions. These methods, which display a high potential on the level of stilbene synthesis, have been adapted to oligomer and polymer synthesis. A relevant question is whether or not efficient methods of polymer synthesis can also lead to oligomers. A first group of polycondensations comprises the reactions of an aldehyde or a ketone component with the activated methylene groups of xylene derivatives. The Wittig reaction [ 133-1351 and the Knoevenagel condensation [ 136, 1371 represent this class of step-growth methods leading only to moderate molecular weights (10-20 repeat units) due to insolubility and possible side-reactions (Scheme 59). Another member of this class of step-growth polymerization is the Wittig-Horner polycondensation involving the reaction of aromatic diketone monomer 211 with phosphine oxide-activated aromatic methylene compounds 212 leading to phenylated PPV (Scheme 60) [ 1381 A second group of polycondensation is based on the McMurry reaction as exemplified in Scheme 61. Specifically, dialdehyde monomers 215 are reacted in the presence of titanium trichloride to yield moderate molecular weight PPV 216

215

Scheme 61. McMurry route to PPVs.

216

1.4 Olefinic-Aromatic Structures

217

61

218

Scheme 62. Synthesis of poly(p-phenylenevinylene) by Wurz reaction

characterized by olefins with cis- and trans-configuration in the ratio 4:: 10 and an average degree of polymerization of 30 [139]. The Wurtz-type polymerization by reductive coupling of bis(gemina1)xylene tetrachloride (Scheme 62) represents a third group of polycondensation. The reductive coupling of xylene tetrachlorides 217 catalyzed by chromium diacetate was shown to occur in a step-growth process [ 1381. A fourth group of polycondensation reactions constituted by the transition-metal catalyzed Heck and ADMET reactions involves monomers containing one and two pre-formed C-C double bonds, respectively. Heitz, Greiner et al. [140, 1411 examined the suitability of the Heck reaction for poly(ppheny1enevinylene) synthesis. Although there is almost no restriction in the use of the olefins, the use of the halogen compounds is limited to bromo- or iodoarenes (Scheme 63). Palladium acetate or palladium chloride serve as catalysts, but palladium on charcoal and tetrakis(tripheny1phosphine)palladium are also applied. There are various side reactions like reductive dehalogenation and attack of the halogen compound not only at the /?-position of the double bond, but also at the a-position, leading to exo-methylene groups which easily react with a second halogen compound. These side reactions cause structural defects and limited molecular weights. Recent work demonstrates the importance of the Heck reaction for oligomer synthesis, the suitability for polymer synthesis, nevertheless, has yet to be shown [131]. The ADMET polycondensation depicted in Scheme 64 involves a two-step elimination of ethylene catalyzed by a stable molybdenum carbene complex and affords only oligo(phenyleneviny1ene)s 221 although the reaction should, in principle, lead to materials of higher molecular weight [142]. The number of repeat units does not exceed ten, presumably because of the low solubility of the higher molecular weight oligomers, although two heptyl solubilizing side chains are grafted on each monomer unit. The defect-free structure, i.e. all-trans configuration, observed by H-NMR spectroscopy for the low molecular weight soluble oligomers, was invoked to explain the poor solubility of the higher molecular weight oligomers. Ar-Br

Pd(0)

b B r

Ar-Pd-Br I

I

A r ‘ A

I Br-Pd-H

I

+ At-CH=CH-Ar

Scheme 63. Mechanism of the Heck reaction.

I

-P,d-Br Ar ‘-C H-C H2-Ar

62

I Hydrocarbon Oligomers

n < 10

219

22 1

Scheme 64. Synthesis of soluble, all-trans poly(2,5-diheptyl-p-phenylenevinylene) 221

1) Yiid Formation:

222

223

2) 1,6-Eiimination:

224

225

3) Addition:

226

227

4) Elimination: R R Q , + 2 H - c H k 228

+ OHe-

*H=c+” 229

+

nR2S

+

nHzO

Scheme 65. Synthesis of poly(p-phenylenevinylene) by the method of Wessling-Zimmermann.

1.4 OleJiriic-Aromatic Structures

63

Interestingly, OPVs 221 can be considered as telomers for other metathetical reactions because the vinyl endgroups are much more reactive than the inner vinylidene groups. This paves the way to block copolymers containing defined OPV blocks [142]. To tackle the question whether or not efficient polymer synthesis is also relevant for oligomer synthesis, and for the sake of comparison between step-growth and chain-growth polymerizations, it is worth mentioning the method of Wessling and Zimmermann which is the most common chain-growth polymerization for the synthesis of PPV [143] (Scheme 65). In a first step a thioether reacted with an cx,cu’-dihalogenated-xylene, leading to the bis-sulfonium salt 222. Base-induced elimination of one mole of thioether gave an unsaturated quinodimethane intermediate 224, which polymerized to a polyelectrolyte 225 containing one sulfonium group in each monomer unit. The soluble pre-polymer 225 allowed the formation of films. Insoluble PPV 226 resulted from the thermally induced extrusion, at high temperature (up to 300°C), of a second equivalent of thioether. The insolubility of PPV has hampered detailed characterization. Sonoda and Kaeriyama have applied the Wessling/Zimmermann synthetic procedure for the preparation of alkyl substituted PPV (Scheme 66) [142]. The poly( p-phenylenevinylene) 235 is soluble in common organic solvents and thus full structural characterization is facilitated. The main features of alkyl-substituted PPVs 235 are: (i) the high ratio of trans to cis olefinic units (found to be 93:7 from ‘H-NMR spectroscopy) and (ii) the presence of 2,5- and 3,6-diheptylphenylene isomers in the 9: 1 ratio (Scheme 67).

230

CH30H

231

,

I“

H,d r , Q -

233

300 “C ____)

- (CH&S,

HCI

H3C CH3 GIQ 234

Scheme 66. Synthesis of soluble PPV 235.

235

232

1 Hydrocarbon Oligomers

Scheme 67. Two structures existing in alkylsubstituted PPV 235.

The formation of high molecular weights was confirmed by GPC indicating a M , up to 4.7 x lo4 corresponding to a degree of polymerization of 160 repeat units. A rather large polydispersity of 3.8 was observed. These findings could give some indication of the structure of the widely used insoluble PPV obtained by the Wessling-Zimmermann procedure, i.e. cisltrans ratio, degree of polymerization, and molecular weight distribution. Poly(pphenyleneviny1ene)s containing various substitutents, such as 237, 239 and 241 were obtained from the chain-growth polymerization of monomers 236, 238 and 240, respectively, in the presence of base (Scheme 68). The polymerization mechanism is comparable to that taking place in the Wessling-Zimmermann procedure and involves sequential elimination-polymerization-elimination reactions [ 138- 1401.

dehydrochlorination NaH. DMF

236

237

230

239

pyridine

n 240

241

Scheme 68. Synthesis of poly(pphenyleneviny1ene)s by dechlorination reaction

1.4 OleJnic-Aromatic Structures

65

In general, polycondensation reactions including Wittig, Knoevenagel, WittigHorner, Wurtz, Heck and ADMET step-growth polymerizations are favorable for the formation of oligomers because molecular weight can be controlled and defined endgroups can be incorporated. These condensations facilitate the synthesis of tailor-made oligomers by establishing a defined relation of functional groups. The polymerization-type reactions, in which an active intermediate like the unsaturated quinodimethane polymerizes via a chain-growth process, are difficult to stop at a certain degree of polymerization. Therefore, they are not suited for the synthesis of defined oligomers. The Wittig-Horner reaction, as a step-growth process, is often used for the synthesis of homologous series of phenylenevinylenes because the monomers are easily available and the structure is flexibile, which also provides the necessary modification in terms of solubility. Most importantly, this reaction proved to be highly efficient for the synthesis of various stilbenes [I 311. The first oligomeric series of unsubstituted phenylenevinylenes was synthezised by Drefahl and Horhold using the Wittig reaction starting from p-bromomethylbenzaldehyde 242 and triphenylphosphonium chloride [ 144, 1451 (Scheme 69). Repetetive in situ generation

r

X Y C - Q C H O 242

+ [(C&)JP-C&-C&I~-R]

1

243

Cle

1

1 +247

,

244

+ (CeHs)rP

245

+ PhCHO 246

I

+ PhCHO

247

248

Scheme 69. Oligo( p-phenylenevinylene) by Wittig reaction.

66

I Hjldrocarbon Oligorners

WR R

0

R 249

P

R 250

R

251

d

Scheme 70. Oligo( p-phenylenevinylene) 249; oligo(m-phenylenevinylene) 250; oligo(o-phenylenevinylene) 251.

of the triphenylphosphonium group enables the successive formation of oligomers 247 and 248 with up to 8 repeat units.

The poor solubility of these materials motivated efforts to increase the solubility by inserting phenyl rings at the double bond. These two additional phenyl rings at the double bond cause twisted structures that decrease the electronic interactions along the main chain. The synthetic approach by Mullen et a/. to insert t-butyl groups in the 3,Sposition of the terminal phenyl rings leads to more soluble and fully conjugated materials 249 that provide a sound correlation of structure and physical properties [ 146, 1471 and have prompted extensive investigations related to electronic structures (Scheme 70). In particular voltammetric solid-state investigation of the oligomeric series 249 has shown that the number of accessible redox states increases with increasing chain length of the oligomer, resulting in the superposition of redox states over a broad potential range for long chain length [146, 1471. These findings cast some light on the redox behavior of the corresponding PPV where the presence of impurities and defects prevent unambiguous structure-property relationship to be drawn (see Chapter 9). Steady state photoconduction studies on oligo(ppheny1eneviny1ene)s 249 (n = 2) led to the conclusion that several physical processes contribute to photoionization. An extrinsic process is excited state dissociation at the positively biased electrode whereas intrinsic processes involve field-induced exciton dissociation

1.4 Olrfinic-Aromatic Structures

67

as well as optical charge transfer transition both generating a geminate electron-hole pair that can dissociate by field and temperature assisted diffusion [148] (see Chapter 7.1). The issue of the coplanarity and conjugation length of OPV 249 and of the corresponding PPV has been studied by UV-VIS absorption, infrared and Raman spectroscopy. Information relevant to such issue is that the PPV chain has a conjugation length of 8-10 repeat units [149] (see Chapter 6). Mathy et a/., using third harmonic generation, have investigated the third-order nonlinear optical susceptibility x(3)(-3w; w , w,w , w ) oligo(p-phenyleneviny1ene)s 249, poly(p-phenylenevinylene) and other one dimensional conjugated .ir-electrons such as polyacetylene and polythiophene systems in their neutral form. From this study a general scaling behavior has been deduced for these materials: their values follow an empirical scaling relationship 2 ( 1 3 ) / ~ m ,,,A:a x where a,,, and, , ,A denote the absorption coefficient and wavelength of the low-energy absorption maximum [ 1501 (see Chapter 8). The photophysical behavior of oligo( ppheny1enevinylene)s 249 and poly( p-phenylenevinylene) in isotropic and anisotropic polymer films has been investigated by means of polarized fluorescence spectroscopy [ 1511. The optical absorption and emission properties of OPV 249 and PPV in dilute blends and in concentrated films have been compared to distinguish the intrinsic properties of isolated molecules from those effects involving intermolecular interactions [ 1521. A comparative site-selective fluorescence study of OPV 249 (n = 3) and comparison with related oligomers led to the conclusion that the observed Stockes shift for PPV is due to spectral diffusion which becomes smaller with improving structural perfection [ 1531 (see Chapter 7.1). With the aim of better understanding of the efficiency of PPV based electroluminescent diodes, the photoluminescence of OPV 249 ( n = 3) films have been recorded with and without coverage of a monolayer of Ca. A dramatic quenching of photoluminescence has been observed upon Ca deposition [ 1541. A joint experimental and theoretical study of the vibronic structure in the optical absorption spectra of OPV 249 has been conducted [155] (see Chapter 9). UVphotoelectron spectroscopy has been used to determine the absolute binding energies of the occupied molecular orbitals of OPV 249 (n = 1, 2 , . . . , 5). The energetic position of the frontier orbitals in this oligomeric series in the solid state as a function of the conjugation length shows only small changes, 0.40eV for the ionization potential and 0.18 eV for the electron affinity. These data explain the small changes in the redox potentials of OPV 249 [156]. The extension of the negative polaron (radical anion) in OPV 249 ( n = 1 , 2 , . . . , 5 ) has been studied as a function of chain length. The spin density was determined by an analysis of the hyperfine interactions obtained from solid state pulsed electron nuclear double resonance spectra. The extension of the polaron increases with chain length up to ii = 7 in accordance with Huckel-type calculations. A linear extrapolation to infinite chain length, however, does lead to a finite extension of the polaron [157]. The simultaneous detection of polaronic and bipolaronic states in reduced OPV 249 has been carried out by UV-VIS-NIR spectroscopy as a function of chain length. Two absorptions were found for the monoanions and one for the dianions. All absorption maxima show a bathochromic shift with increasing chain length. Extrapolation to infinite chain length that the delocalization of an excess charge within PPV affects about nine to ten repeat units [158].

-

68

1 Hydrocarbon Oligoniers

The members of the homologous oligophenylenevinylene series can be formally regarded as being constructed from two 3,5-di-t-butylphenyl endgroups and a xylylidene chain with different length. Starting from the phosphonium salt 252a the chain could be lengthened by a styryl unit via a Wittig reaction with 4-methylbenzaldehyde (Scheme 7 1). Subsequent functionalization was achieved by bromination of the methyl group with N-bromosuccinimide (NBS). The phosphonium salt 252b was obtained by subsequent reaction of 254a with triphenylphosphine in toluene. The larger phosphonium salt 255 was accessible in an analogous reaction sequence. The target molecules were obtained in a final Br 0

4-rnethylbenraldehyde LiOEt I EtOH, DMF, r.t.

6t*F&4<Bu

-

252 a,b : x 0 , i

‘“WcH3 NBS, CC14, T = BO’C

cI_L___)

PPb, toluene, T = 110°C

tBu

264 a,b : x = 0 , l

tB;

255 a,b : x = 0 , l

tB;

256 a-c : x = 0-2

257 a,b : y = 0 , l

LiOEt I EtOH. 7tBUt

e:n=5 f: n = 6

258

Scheme 71. Soluble oligo( p-phenyleneviny1ene)s with terminal alkyl substitution.

69

1.4 Okefitiic-Aromatic Stritctirres

1. i a

v

640

1040

1440

1840 h(nm) 2240

Figure 12. Absorption spectra of the oligo( p-phenylenevinylene) 258 radical monoanions where (a) n = 2. (b) 12 = 3, (c)11 = 4.

bis-Wittig reaction of the phosphonium salts 252a,b and 255 with terephthalaldehyde 257 or 4,4‘-stilbenedicarbaldehyde. The pure all-trans isomers of 258 were prepared by heating the crude product with a catalytic amount of iodine in toluene. The products were purified by column chromatography on silica gel and subsequent recrystallization. The extended T conjugation is revealed in the UV spectra with a bathochromic shift for the homologous series of oligomers (Fig. 12 and Table 12). Previously Horhold proposed that in PPVs a subunit consisting of about four styryl units is charged independently [145]. This would imply that in oligomers containing more than four styryl units the absorption maxima of the monoanions no longer depend on chain length. The optical results from the alkyl-substituted materials 258 clearly show that even in the heptamer a limit of convergence is not yet attained. The discrepencies between the conjugation length derived from PPV and OPV studies can be attributed to the presence of impurities and structural Table 12. Characteristic spectroscopic data of oligo( pphenyleneviny1ene)s 258 Iz

~~

1

2

3 4 5 6

‘H-NMR (200 MHz)

UV , , ,A (nm) EI-MS in CHCl3 M+ (log €1

~

7.56 (s, 4H), 7.44 (d, ‘ J = 1.8Hz, 2H). 7.37 (d, ‘ J = 1.8Hz, ‘H), 7.27, 7.14 (2d, 3 J = 16.3Hz. 2H). 1.37 (s. 36H) 7.56 ( s , 4H). 7 44 (d. ‘ J = 1.8Hz. 4H) 7.37 (d. 4J = 1.8 Hz, 2H). 7.26. 7.14 (2d, ’ J = 16.3 Hz. 2H). 1.37 (s. 36H) 7.57 (s. 12H), 7.44 (d, ‘ J = 1.8Hz, 4H) 7.37(d.‘J= 1 . 8 H ~ , ’ H ) , 7 . 2 7 , 7 . 1 4 ( 2 d , ~ J16.3Hz,2H), = 1.37 (s, 36H) 7.48 (br s. 16H). 7.30 (br s. 6H), 7.08 (s, 6H), 7.13, 7.04 (2d. ’ J = 16.3 Hz, 6H). 1.34 (s, 36H) -

359.8 (4.75)

I??/:

= 506

386.9 (4.94)

HI/:

= 710

402.7 (5.05)

HI/:

= 710

412 (5.05)

??I/:

= 812

417.9 (5.13) 415

I??/:

= 914

defects in PPV generating a n-electron localization. The effectively conjugated segment of a PPV chain can be concluded from extrapolation from OPV to comprise about eight to ten styryl units. This conclusion has to be considered when describing a polaronic state in PPV [ 1591 (see Chapters 6, 7.1 and 9). Recently, Meier et d.have contributed to the question of the effective conjugation length in PPV with the synthesis and the spectral characterization of soluble OPV 259a(n) depicted in Schemes 7 2 - f [ 1601. Four different but closely related synthetic pathways were conceived for the preparation of OPV 259a(n) (Scheme 72a-1). They have in common that the final step involves the formation of one or two carbon-carbon double bonds by applying the Wittig-Horner, the Siegrist or the McMurry reactions. The OPV 259a(1) and 259a(2) were prepared according to Scheme 72b. Specifically, the Rieche-Gross formylation procedure was applied to 259bA, and alkaline condensation of the corresponding N-phenylaldimine 259d with 259bB yielded the stilbene 259a(l). Compound 259bA was chloromethylated twice and then converted into the bisphosphonate 259e by a Michaelis-Arbuzow reaction. OPV 259a(2) was obtained by a Wittig-Horner reaction. The synthesis OPV 259a(1) and 259a(2) is given in Scheme 72b. A twofold bromination of 259bB with two equivalent of NBS was performed. One bromine atom was attached on the phenyl ring whereas the other bromine atom was introduced on the methyl group. Subsequent transformation into the phosphonate 259f, followed by a Wittig-Horner reaction of 259f with 259c. yielded the bromostilbene 2598. The bromine atom of 2598 was converted into a formyl function by the Bouveault reaction. The synthetics OPV 259a(I) and 259a(2) were obtained by a McMurry reaction of two equivalent of 25911 and by a twofold WittigHorner reaction of 25911 with 259e, respectively. The preparation of 259a(6) required a synthetic pathway of seven steps (Scheme 72c). A Rieche-Gross formylation of 259bB followed by a Wittig-Horner reaction of 2593 with the previously mentioned 259f yielded the bromostilbene 259j. This one was then converted into the aldehyde 259k by the Bouveault process. A Siegrist reaction of the corresponding acetal 2591 with the 259d afforded 259111after acidic deprotection. Finally, the OPV 259a(6) was obtained by a twofold Wittig-Horner reaction of 259m and 259e. The preparation of 2S9a(8) and 259a(11) required two synthetic pathways of three steps each (Scheme 72d). The Schiff base 2590 was obtained by the conversion of 25911 with aniline. The Siegrist reaction of 2590 with 2591 followed by a twofold Wittig-Horner reaction of 25913 with 259e afforded the OPV 259a(8). The Schiff base 259q derived from 259p was subjected to the Siegrist reaction with 2591 yielding the aldehyde 259r. This one was finally converted into the OPV 259a(l1) by a McMurry reaction. The stereoselective formation of pure trcins isomers was achieved for OPV 259a(n) either by the Siegrist reaction or by elimination of minute amounts of cis isomers by recrystallization. This extensive synthetic work allowed the investigation of UV-VIS absorption characteristics of OPV 259a(n) and their correlation with the oligomer length. The long-wavelength absorption maxima (A,) of 259a(n) and their extinction

I .4 Olefiriic-Aromatic Structures

OC3H7

-

Scheme 72a. Dial koxy-substituted oligo(phenyleneviny1ene)s 259(n).

CeH5

C6H5-NHZ

W3H7

4

259c

0w7 259d

259b A

k3H7

2596

259e

+ 25913 B

259e

KWCH3)3 DMF

+ 259c

259a (2)

Scheme 72b. Synthetic pathway to oligo(pheny1enevinylene)s 259a(1) and 259a(2).

2599

259h

TiCI,

Zn

259h

+ 25gh

-+

250h

+ 259e

-+DMF

259a (3)

KWCH& 259a (4)

Scheme 72c. Synthetic pathway to oligo(phenyleneviny1ene)s 259a(3) and 259a(4).

71

72

1 Hydrocarbon 0ligomer.s

DMF 2591

259k

CH30H (H+) ___L)

OCnH7

i 259e

259a (6)

KOC(CHd3 DMF

Scheme 72d. Synthetic pathway to oligo(phenyleneviny1ene)s259a(6)

coefficients ( E ) are collected in Table 13. The tendency of, , ,A of approaching a limit of convergence with n is clearly observed in Fig. 13. Meier rt al. have also with n is better described by an demonstrated that the dependence of the, , ,A exponential function Amax(n) = X i - (A; - Al)'- h ( n - I ) than by the conventional function Xmax(n)= f( 1/ ? I ) . Thereby, b is a dimensionless empirical parameter indicating how fast the limit of convergence is reached and X i is the limiting value when I I tends to infinity. The excellent fit of the measured absorptions by the proposed functions provided the values of b = 0.461 and of Ai = 481 nm. The value of A; matches the A; of OPV 259a(11) indicating that the effective conjugation length of OPV 259a(n) is reached for it = 11. In a first approximation, these findings are comparable to the aforementioned effective conjugation length of PPV (n = 9-10).

1.4 Olefinic-Arornatic Strirctures

259p

+ 259e

73

259a(8)

+

259r _____)

259a(ll)

TiCI., IZn

Scheme 72e. Synthetic pathway to oligo(phenyleneviny1ene)s 259a(8) and 259a(ll).

Yu and coworkers have reported an efficient stepwise synthesis of OPV via an orthogonal approach. This means that two non-interacting reaction types, i.e. Wittig-Horner and Heck reaction, are used to construct the same functionality i.e. a C-C double bond (Scheme 72f ). This strategy eliminates the need for protecting groups and allows sequential growth of OPV. Specifically, monomer 260a possesses an aldehyde at one end and a vinyl group at the other end while monomer 260b possesses an iodo substituent at one end and a phosphonate ester group at the other end. The Heck reaction was used to couple the iodo arene with the vinyl group. Similarly, the Wittig-Horner reaction allowed coupling of the phosphonate with the aldehyde function. The application of the sequence of alternating Wittig-Horner and

74

1 Hydruoccrrbon Oligomers

260d

260e

i

+

___)

260a

260(1)

+

260b

ii

CHO

H3C 260f(n)

RO

RO

CHO

H3C RO

260f(n)

RO

OR

I, ii ___) ___)

CHO RO

260f(n)

RO

Scheme 721. Stepwlse synthesis of substituted oligo(phenyleneviny1ene) 26Of(n), n = 1-5 via an orthogonal approach. (I) Pd(OAc),, P(o-tolyl),, NBu3, DMF. (ii) NaH/DME. R = C8Hl7.

Heck reactions, as depicted in Scheme 72f, leads to the series of OPV 260f(n). As previously mentioned in the work of Meier et al. minor amounts of oligomers containing cis isomers were eliminated by flash column chromatography. Yu et al. have also investigated the phase behavior and the UV-VIS absorption spectra of OPV 260f(n) (Table 14). A careful look at the, , ,A of OPV 260f(n) indicates a tendency to converge to a value of, , ,A similar to that of OPV 259a(n) (Table 13). However, it should be noted that OPV 260f(n) and 259a(n) have different substitution patterns, rendering a more accurate comparison difficult. Interestingly, OPV 260f(n), with the exception of the shortest one 260f(l), display a reversible thermotropic liquid crystalline behavior. The melting temperature ( T,), the clearing temperature (T,) and the temperature range over which the LC phase persists have been found to increase roughly with the length of the oligomers.

Table 13. Long-wavelength absorption maxima of 259a(n) measured in chloroform.

259a( 1) 259a(2) 259a(3) 259a(4) 259a(6) 259a(8) 259a(11)

1

2 3 4 6 8 11

354 40 1 43 I 450 466 415 48 1

16800 40 700 58 500 84 800 117200 146 200 I96 300

Stimulated by the intense research activities on donor and acceptor p-substituted stilbene 261 for NLOs (Scheme 73) nonlinear optics, Klarner and Miillen have reported a stilbene 265 where the donor and acceptor substituents are located on the vinylene unit [161]. Stilbene 265 exists as mixtures of E and Z oligomers with a E/Z ratio of 57: 43 at 15°C. as determined by 'H-NMR spectroscopy. The synthesis is based on a cation-anion coupling reaction as outlined in Scheme 74. The high isolated yield (98%) allowed this reaction to be successfully applied to the synthesis of higher oligomers and even polymers. Within the homologous series of the oligomers 265-270 the increase of size is reflected in a bathochromic shift of the longest wavelength absorption maximum in the UV spectrum. However, starting from the stilbene analog 198 with 340 nm, via the next oligomers in the series 268-269 and 269b (with 370, 374 and 378 nm, respectively), a convergence of the absorption maximum can be observed in solution. Compared to unsubstituted PPV, which possesses an effective conjugation length of 8-10 units, convergence of the value of ,,A,, occurs after only 5-6 units for the donor/acceptor system. This lower conjugation length is due to the weak electronic interaction between the olefinic and aromatic moieties. The key step in

350

1

1

1

3

Figure 13. Plot of the, , ,A ny1ene)s 259a(n).

5

n

7

9

11

values as a function of the number of repeat units of oligo(pheny1enevi-

76

1 H~~rlrocurhon Oligoiners

Table 14. Thermal transition and long-wavelength absorption maxima of OPV 260f(n). 260f(l)

T,, ('0 T, ('C)" A,, (nm) a

97 -

43 1

260f(2)

260f(3)

260f(5)

260f(6)

61 87 44 1

90 158

113

105 185 463

176 460

457

Clearing temperature from the liquid crystalline phase to the isotropic phase.

the characterization of the donor and acceptor substituted oligomers is the continuation of the localized dipole units on the vinylene units on increasing chain length. The dipole moment increases with increasing number of repeat units (Table 15). A rigorous additivity of the individual dipoles and a constant dipole moment per dipole center ( p / n ) cannot be expected in solution, because configurational and conformational isomers can appear and a Coulombic repulsion between the individual dipole centers must be taken into account. The Coulombic repulsion between individual dipoles apparently can be reduced by incorporation of biphenyl units or alkanediyl chains and the subsequent increase in separation of the dipole (compare 266,267 with 268,269a and 269b or with 270) (Scheme 75). The high dipole density in these materials and the largely suppressed Coulombic repulsion of the polar units represent a promising property profile for the adjustment of the hyperpolarizable building blocks [ 1611. The synthesis of oligo(m-phenyleneviny1ene)s 250 was achieved by an analogous Wittig approach with 3,5-di-t-butylphenyl endgroups [ 1591. The m-phenylene subunits in 7r-chains provide, on the one hand, highly improved solubility; on the other hand, these units interrupt the n-conjugation and induce a localization of charges on one stilbene unit (stilbene polaron). The I??-bridgingleads to nonKekulC structures in the charged n-systems. As a result of the topology-induced charge localization, the absorption spectra of the charged species reveal an additional long wavelength charge-transfer band. Interestingly enough, the assumption made by Fukutome [162] that on doping n.1-phenylene systems such as 250, ferromagnetism can arise if the two side chains are sufficiently long for bipolaron

261

262

Scheme 73. 4,4-and a,P-donorlacceptor-substituted stilbene 261-262.

+

@KN

m::e-% 2

R2N

263

264

265

Scheme 74. Synthesis of a-dialkylamino-,B-stilbene 265.

\ /

1.4 Olejinic-Arornatic Structures

77

Table 15. Molecular dipole moments p and dipole moment per stilbene unit p/r7 for oligo(pheny1eneviny1ene)s265-270. Oligomer

265

266

267

268

269a

269b

270

P (D) p l n (D)

6.12 6.12

6.24 3.12

7.73 2.58

7.10 3.55

7.91 2.64

9.35 2.34

7.5 1 3.75

formation was not verified experimentally [160]. Indeed ESR studies have shown that dianions of 250 are diamagnetic [159, 1601. Oligo(o-phenyleneviny1ene)s 251 constitute an intermediate case between those of 249 and 250: similar to 249 the topology allows an extended conjugation interaction which is, however, inhibited by the non-planar geometry (Scheme 30). A hornologous series of monodisperse oligo(o-phenylenevinylenejs275-278 was synthesized

266

267

268

269a n = 1 269b n = 2

‘CN 270

Scheme 75. Donor/acceptor-substituted phenylenevinylene oligomers.

by using the Wittig, McMurry and Heck reactions [132]. First, the Wittig reaction between o-phthaldialdehyde and benzyl triphenylphosphonium afforded 1,2-distyrylbenzene in 5 5 % yield. By using only 0.8 equivalents of the phosphonium salt, 19% of the bis-coupling product and up to 27% stilbene-2-carbaldehyde 271 were isolated as a mixture of cis- and frcrns-isomers. Starting from stilbene-2carbaldehyde 271 as a basic building block, the titanium-induced McMurry coupling gave the o-trimer 276 in 80% yield (Scheme 76). The corresponding tetramer 277 of this series could be synthesized by Wittig reaction of 271 with the 1,2-bisphosphonium salt 272 in only 24% yield, a result of the high steric demand of two neighboring benzyl positions during the olefination. Wittig reaction of the bisphosphonium salt 272, which could be prepared in three steps from 2methylbenzaldehyde, with the aldehyde 271 gave the pentamer 278 in 12% yield. Transformation of the aldehyde 271 into the vinylic compound 273 enables Heck coupling of 273 with 2’-bis(bromo)distyrylbenzene 274, yielding the hexamer in 21% yield. This example emphasizes how a stepwise build-up of homologous series of oligomers can be achieved starting from one basic building block and by using various coupling methods. The procedure described above is limited to oligomer synthesis. Another approach to oligo(o-pheny1enevinylene)s and also to the correspanding polymers

T

o

o

0

274

273

\

276

271

278

\

277

Scheme 76. Oligo(o-phenyleneviny1ene)s by Wittig, McMurry and Heck reactions.

1.4 Olejiriic-Aroniatic Sfrrtcfirrrs

79

implies the use of the Pdo-catalyzed coupling reaction according to Stille [14]. The reaction conditions for the oligomers and polymers were, initially, tested for stilbene synthesis. Thus, reaction of iodobenzene and tributylstyrylstannane in D M F with palladium dibenzylidene-acetone/triphenylarsine as catalyst yielded 98% stilbene. The corresponding reaction of 1,2-diiodobenzene 279 with bistributyl-stannylethylene 281 gave higher oligomers 282 and also polymers (Scheme 7 7 ) [132]. The intermediate position of the o-phenylenevinylenes 251 between the meta- and para- compounds is also reflected by the optical absorption spectra [163]. On the other hand, the radical anions 251- show surprisingly small hyperfine couplings with increasing chain length, suggesting even better delocalization of electron spin density than in 249-' and 250W'. A reason for this deviation can only be found by taking into account the localization of spin density in particular segments of the chains and an electron transfer between charged and uncharged domains that is fast on the time scale of the EPRiENDOR experiment (10-7-10-9 s-I), but slow on the time scale of optical absorption measurements ( 10-"-10-'3 s-l). Hitherto, para- and meta-divinyl substituted benzenes have been mainly investigated in photopolymerizations [ 131, 1641, whereas the corresponding use of orrho-disubstituted compounds in topochemical reactions is largely unexplored. '

< 279

+

280

i i Hexyl

+

283

Hexyl

Bu3Sn*SnBu3

281

WO)

A 284

Scheme 77. Oligo(o-phenyleneviny1ene)s by Stille reaction.

80

I Hdrocurbon 0ligoiner.r R

205

286

Scheme 78. Topochemically controlled photodimerization reaction of 2,2’-distyrylbiphenyl. R = H, t-butyl.

A noticeable exception is given by the topochemical reactions of 2,2’-distyrylbiphenyl derivatives 285 and of the corresponding polymer 289 [165]. The photoreactivity of 2,2’-distyrylbiphenyl derivatives 285 in the crystal-state is governed by the nature of the substituents, i.e. for the unsubstituted derivative (R = H) no reaction was observed upon UV-irradiation whereas the corresponding derivative with R = r-butyl reacted quantitatively to the cyclobutane product 286 (Scheme 78). The difference in the topochemical behavior was explained by the molecular alignment of the substituted and unsubstituted 2,2’-distyrylbiphenyl derivatives 285 in the crystal state. In the case of the unsubstituted derivative, according to the crystal structure analysis, the two stilbene moieties of the molecule are twisted by an angle of 114.9” against each other; also, the intramolecular and intermolecular distances between the qlefinic double bonds are far above the maximum interaction radius of 4.0-4.1 A [165]. In single crystals derived from the t-butyl substituted derivative, the angle of torsion in the central biphenyl unit only amounts to 59.1”, thus allowing for the intramolecular approach of the reactive vinylene units and for the observed intramolecular [2+ 21 cycloaddition. The corresponding highly soluble poly( 1,4-phenylene-vinylene-2,2’-biphenylylenevinylene) 289 has been prepared by the Heck reaction of 2,2’-divinylbiphenyl with 1.4-dibromobenzene. Interestingly enough, a slight modification in the functionality of the building blocks has a strong influence on the reaction products formed by the Heck reaction. Thus, the reaction of 2,2’-dibromobiphenyl291 with 1,4-divinylbenzene 292 afforded not the expected polymer 289, but a mixture of the fluorenyl derivatives 293 and 294 (Scheme 79). Poly( 1,4-phenylene-vinylene-2,2’-biphenylylenevinylene) 289 has, , ,A = 35 1 nm and has been shown to be photoreactive in solution and the solid state. Specifically, a significant bleaching process and a hypsochromic shift of the long wavelength absorption maximum were taking place during exposure to light. UV absorption and ‘H-NMR spectroscopy indicated the formation of a substructure analogous to 286 in the polymer during irradiation process. This showed that strongly twisted conformations were available allowing the olefinic units to approach each other to react. Irradiation of a thin film through a mask demonstrated the photostructuring potential of poly( 1,4-phenylene-vinylene-2,2’biphenyl ylenevinylene).

-

1.4 Olefiiiic-Aroi?iatic Structures

81

Pd(OAC),. NEt,,

tris(o-toly1)phosphine

Br 287

n

288

289

hv ___)

290

Q

Br

Pd(0Ac)p. NEt3, tris(o4olyl)phosphine

291

292

“

\

4

0

293

294

Scheme 79. Synthesis and topochernically controlled photodirnerization reaction of poly(l,4phenylenevinylene-2,2’-biphenylenevinylene).

This study on 2,2’-biphenylylenevinylene derivatives illustrates the fundamental role of oligomers in the understanding of the reactions taking place during polymerizations. Another interesting approach to oligo( pphenyleneviny1ene)s is presented by Wennerstrom et al. [166, 1671. This work introduces large ring compounds 296 with extended 7r-systems (Scheme 80). The paracyclophanes 296 have been prepared by fourfold Wittig reaction between aromatic dialdehydes 295 and bisphosphonium salts from bis(halomethy1)arenes. This one-pot reaction sequence provides a simple method for the synthesis of a large variety of materials with different sizes and building blocks like furan, biphenyl and naphthalene. These macrocycles helped to reach a better understanding of photoinduced Z/E-isomerization and cyclization of stilbenes. Mullen and coworkers have synthesized, by Wittig reaction, a series of conjugated cyclophane 299a-h as model compounds for conjugated polymers (Scheme 8 1)

82

I Hydrocarboil Oligoniers

296

296

Scheme 80. Paracyclophanes by fourfold Wittig reactions.

[ 1681. The 299a-h series was reduced with alkali metals to yield the corresponding dianions and (in part) tetra-anions. A spectacular outcome arose from the pronounced ring current effect which appears from the 'H-NMR spectra of the ions. In particular for the inner protons of the benzene rings of cyclophane 299d, the chemical shift varies from 6 = 7.37 for its neutral species to 5 = -7.07 for its dianion CH,P Ph,Br VHO CHO

LiOEt

TGE+

CH,P Ph,Br 297

298

299

b

a

C

f

++% g h Scheme 81. Synthesis of conjugated cyclophanes 299a-h.

1.4 Ol~firiic.-.4roniatic.Strircturrs 4.48

7.37 6.48

83

2.09

t

(2.09)

@ 7.37

\

/

\ /

2994

Scheme 82. Structure and proton chemical shifts of cyclophane 29961 and of its dianion and tetraanion.

and to 6 = 12.76 for its tetra-anion [I691 (Scheme 82). The diatropism of the dianions and paratropism of the tetra-anions were ascribed to the formation of perimeter-type structures involving rr-conjugation via aromatic moieties [ 1681. The incorporation of naphthalene subunits into the polymer main chain is motivated by the search for structures with a low excitation energy. 26-Di-f-butylnaphthalene 301a appears as a suitable precursor for the synthesis of the soluble oligo(naphthaleneviny1ene)s 304 via Wittig and McMurry reactions as the key step [ 1701. Selective bromination of the 2,6-di-f-butylnaphthalene 301a and subsequent generation of the dialdehyde 301e gave one monomer for the Wittig reaction. The corresponding bis-phosphonium salt 301c was synthesized by hydroxymethylation of 2,6-di-f-butylnaphthalene 301a in the presence of HBr and reaction with triphenylphosphine. This reaction sequence also provided the monofunctional phosphonium salts 302b by using equimolar amounts of formaldehyde. These monomers enabled the synthesis of oligomers up to the trimer. The tetramer was synthesized by McMurry reaction of the aldehyde 303 (TiC14, Zn/Cu, THF, 60‘C) (Scheme 83).

a: R=H b: R=CH2Br c: R=CH2PPh3+Bre d: R= Br e: R= CHO 301

a: R=CH2Br b: R=CH2PPh3+Brc: R= Br d: R= CHO

2 w

m” 302

304

Scheme 83. Oligo(naphthyleneviny1ene)s.

303

84

I Hydrocarbon Oligoniers

306

308

Scheme 84. Cyclic oligo(naphthylenevinylene)s, R,, R P , R,, R4 = H, 0-alkyl

Meier et al. have also contributed to the field of oligo(naphthyleneviny1ene)s with the synthesis of various cyclic oligomers 306-308 by the Siegrist reaction (Scheme 84) [131, 1711. Adequate substitution at the periphery of cyclic oligo(naphthy1enevinylene)s 307-308 leads to the formation of thermotropic discotic mesophases [131]. By incorporating 9,lO-anthracene building blocks into 7r-conjugated chains, it should be possible to obtain polymers with potentially high charge-storage capacities because of efficient minimalization of Coulomb repulsion (Scheme 85). Defined oligo(anthryleneviny1ene)s 309 were synthezised by using Horner-Emmons olefination [ 1721. The reason for the inaccessibility of oligo- and poly(anthryleneviny1ene) by means of the Heck reaction lies in the lack of reactivity of 9,lO-divinylanthracene. This can be explained by the sterically demanding situation at the reaction center; the presence of the peri-hydrogens in the anthracence derivative (1/4 respectively, 5/8 position) prevents an addition of the organometallic intermediate to the olefinic

1.4 Ol&iiic-Arotiirrtic Strucriwes

85

310

309

Scheme 85. Oligo(9,lO-anthryleneviny1ene)s.

double bond. In the 9,10-bis(1,3-butadienyl)anthracene,the points of attack, i.e. the terminal double bonds, are separated from the anthracene by two carbons. Not surprisingly, therefore, the formation of both oligomers 310 and polymers via the Heck reaction becomes possible. Optical studies of oligo(9,lO-anthryleneviny1ene)s 309 do indeed provide some support for a lower excitation energy (2.0eV) compared to that of the oligo(pheny1eneviny1ene)s 260 (2.4eV) when extrapolated to the polymer although in the former the steric hindrance between the subunits is distinctly larger [ 1631. The outstanding redox and photochemical properties of the benzene homolog cylcooctatetraene (COT) make the successive replacement of the phenylene units in the linear 7r-system by cyclooctatetraenylene attractive [ 1731. Extended 7rconjugation can be generated by starting from functionalized cyclooctatetraene derivatives such as 311. The known cyclooctatetraene carbaldehyde and its easily accessible phosphonium salt serve as building blocks for olefination by the Wittig or McMurry reaction (Scheme 86). The suitably functionalized cyclooctatetraenes were prepared by thermolysis of semibullvalene precursors. The stepwise construction of higher oligomers via dialdehydes also required the protection of one aldehyde function by ketal formation.

a: Rf=R2=H b: Rq=CH3, R2= CHO

a: R= CH3

31 1

312

313

Scheme 86. Oligo(cyclooctatetraenylenevinylene)s.

The construction of the chain via the Wittig reaction gives better yields than the McMurry coupling. Another characteristic feature of this reaction sequence is that chains with terminal phenyl units are more easily accessible than those with terminal isomeric mixtures were formed. Attempts cylcooctatetraenyl units and c'i,~/t~ui~.s at iodine-catalyzed formation of the trms-isomers led to decomposition, but separation of the isomers by chromatography was possible. The reduction of the oligo(cyclooctatetraenyleneviny1ene)s convincingly documents the role of these compounds as unusual redox systems. In spite of the extended Ti.-conjugation,each COT subunit is able to accept two electrons, whereby the charge is largely localized on the COT rings, as can be shown by a combination of NMR and cyclic voltammetric measurements. During the course of the charging process, however, an electronic interaction between COT units definitively exists. The dianion formation in one COT subunit slightly influences the neighboring rings causing these to become at least partially flattened. A summary of the merits of the step-by-step versus the random approach syntheses towards homologous series of oligoarylenevinylenes follows:

(i) Stepwise Wittig or Wittig-Horner reactions appear most straightforward for the step-by-step approach. Alkyl-substitution of the aromatic subunits or endgroups is required for soluble and processable materials. There is no transition-metal catalyzed reaction, except the ADMET polycondensation, that enables random approaches with easily accessible monomers. The Heck reaction is suitable for the stepwise synthesis of defined oligomers, but this synthetic procedure is inhibited by extensive side reactions. The McMurry method gives poor yields compared with those of the Wittig reaction [129]. The most efficient synthetic method relies on the orthogonal of WittigHorner and the Heck reactions. (ii) The oligoarylenevinylenes and the oligoarylenes represent the two groups of materials around which most of the research on highly efficient electro-optical devices with long term stability is centered. When comparing the two structures it becomes clear that the oligomers provide the necessary information on conjugation length and on stability and efficiency of electro-optical devices. From a synthetic point of view, conclusions are significantly different for the class of arylenes, on the one hand, and that of arylenevinylenes, on the other. In the former, oligomer and polymer synthesis rely on the same group of synthetic methods. In sharp contrast, the polymer forming reactions that provide high molecular weight arylenevinylenes (like the Wessling-Zimmermann route) are not suitable for oligomer synthesis. Here, an extension of the well-established methods of stilbene synthesis provides the best access to oligoarylenevinylenes.

1.4.2 Oligoaryleneethynylenes The controlled synthesis of phenyleneethynylene chains is motivated by the need for .ir-conjugated rod-like oligomers and polymers in the construction of nanoarchitectures, in particular for use as molecular wires [174, 17.51.It is important to synthesize

I

+

H-CEC-H

(PhjP),PdCIz Cul Et2NH

314

316

315

Scheme 87. Hagihara coupling

tailor-made oligoaryleneethynylenes of controllable length and with functionalized endgroups [ 1761. The first homologous series of oligophenyleneethynylenes was synthesized by Drefahl and Plotner by elimination of hydrobromic acid from the corresponding stilbene bromides with alcoholic potassium hydroxide [ 1771. More recently, various modern transition metal-catalyzed coupling reactions have been applied for the formation of oligomers and polymers containing arylene and ethynylene units. The Stephens-Castro reaction of copper( 1)arylacetylenes with iodoarenes or iodoalkenes has been reported to be a useful route for the synthesis of acetylenes [178, 1791. The major limitations of this coupling method are the violent reaction conditions and the difficulties in the preparation of cuprous acetylides. Hagihara et a/. reported that an acetylenic hydrogen as in 315 can be easily substituted by iodoarenes 314, bromoalkenes or bromopyridines in the presence of a catalytic amount of bis(triphenylphosphine)palladium dichloride-cuprous iodide in diethylamine under very mild conditions [180]. Cassar [181] and Heck [ 1821 independently introduced the same substitution reaction using similar catalysts. The Hagihara reaction proceeds under milder conditions and gives higher yields, according to the presence of the co-catalyst cuprous iodide (Scheme 87). Terminal acetylenes 320 represent valuable synthetic intermediates for the introduction of ethynyl groups into organic structures [ 1831. In applications of the Hagihara method to the synthesis of terminal acetylenic compounds, aryl iodides were allowed to react with a large excess of acetylene, but the major product was always the disubstituted acetylenic compound. However, coupling of the monoprotected acetylene 318 with a suitable halide led to the desired structures 320 (Scheme 88). This is an excellent method for the step-wise construction of phenyleneethynylene oligomers. Tour ef al. describcd the synthesis of a stable, oligo( p-phenyleneethynylene) 327 with a length of 128A as part of an attempt to achieve molecular wires [184]. The X

R 317

+

IC-CX-Si(CH&

(Ph3P)2PdC12 Cul

EC-Si(CH3)3

R

318

319

hydrolysis

CX-H

320

Scheme 88. Synthesis of phenylacetylenes.

88

1 Hydrocarbon Oligotners

326

Scheme 89. Modular synthesis of the oligo( p-phenyleneethyny1ene)s.

key step of this modular synthetic method is the Hagihara reaction. This sequence is characterized by the use of an AB-monomer 321 in which both functional groups are protected. The AB-monomer 321 was deprotected and allowed to react repetitively, alternating on either side (Scheme 89). This method enables a fast synthesis of oligomers up to the 16-mer 327. Solubilizing alkyl groups were attached to the phenylene unit. Of particular interest for molecular electronics are conjugated rigid-rod polymers end-functionalized with thiol groups 335. These polymers were designed to serve as molecular wires to bridge the gap in nm lengths between two electrodes [185]. The synthetic pathway leading to polymers 335 relies on Hagihara cross-coupling and on the twofold end-capping of oligomers 330 by the thiol precursors 331 and 333 (Scheme 90). Specifically, diiodo monomer 328 and diethynyl monomer 329 were polymerized by Pdo cross-coupling in the presence of a base and of Cu' affording the polyphenyleneethynylene 330 with a degree of polymerization reaching 22. The polyphenyleneethynylene 330 has two different endgroups which can be used for further functionalization. The ethynyl endgroup of 330 was further reacted with the thiol precursor 331 by Hagihara cross-coupling affording 332. Subsequently, the iodo arene of 332 was reacted with the thiol precursor 333 by Hagihara cross-coupling yielding the symmetrically end-functionalized polyphenyleneethynylene 334. In the next step, the thiol groups were deprotected affording the target structure 335. Monodisperse end-functionalized oligomers 335 could be separated from the polydisperse oligomers 335 by gel permeation chromatography [ 1861. Kratz et al. introduced a new set of conjugated oligomers 348-349 built up from ethynylene and 1,2-phenylene units [185]. This strategy employs the well-documented

I .4 Olejtiic-Arot?iatic Structures

328

329

89

330

Hex

Scheme 90. Synthetic pathway to dithio end-functionalized polyaryleneethynylene 335.

Pd-mediated coupling of terminal acetylenes with aryl halides as the essential step. The necessary components are trimethylsilylacetylene as a means of introducing a singly-protected triple bond, and 1,2-dibromo- 189 or 1,2-diiodobenzene to achieve the desired ortho-connectivity. The reaction sequence outlined for the stepwise synthesis of the oligomers up to the nonamer in Scheme 91 is thus straightforward. The two triple bonds of the 1,2-diethynylbenzene moiety are at a distance that allows cyclization processes to take place. One aim of this synthesis was to transform the linear 7r-chains into conjugated arenes by domino-type folding of the closely spaced acetylenic units. This cyclization could be achieved only for model systems with a few repeat units. The series of oligomers also enables the investigation of the effect of chain length on the extent of conjugation. The UV data reveal a bathochromic shift of the longest wavelength absorption from the dimer to the nonamer of more than 80 nm (from 303 to 388 nm) (Table 16). The design of materials possessing large second-order nonlinear response has recently attracted much attention because of their potential applications in electro-optic modulation and second harmonic generation (SHG). Nicoud et al.

340

341

342

343

--

344

345: 346: 347: 340: 349

.-D

345-349

n=5 n=6 n=7 n=8 n=9

Scheme 91. Oligo(o-phenyleneethyny1ene)s.

reported a new class of highly hyperpolarizable 1,2-disubstituted systems of zig-zag chromophores 359-360 to obtain off-diagonal tensor components which are useful in electric-field poled polymers [ 1871. These zig-zag chromophores are basically composed of an oligomeric chain as a conjugation core, an electron-donating group at one end and an electron withdrawing group at the other. The synthetic pathway for the oligomers 359-360 is presented in Scheme 92. The step-by-step formation starting from nitrophenylacetylene 351 is characterized by the standard Hagihara coupling reaction and a protecting/deprotecting sequence. Table 16. Characteristic spectroscopic data of the oligo(o-phenyleneethyny1ene)s 344, 348-351. 11

'H-NMR (200 MHZ) in C D ~ C I ~

UV, , ,A (nm) in ethanol

1

7.4 (8H), 7.62 (6H) 7.35 (2H). 7.37 (6H), 7.58 (8H) 7.31 (2H), 7.35 (6H). 7.53 (4H), 7.61 (4H) 7.26 (2H), 7.31 (8H). 7.36 (2H). 7.55 (8H). 7.64 (2H) 7.26 (2H). 7.30 (2H), 7.54 (6H). 7.59 (6H) 7.22 (2H), 7.29 (14H), 7.52 (8H), 7.39 (6H) 7.22 (4H). 7.29(14H), 7.55 (16H) 7.20 (8H), 7.29 (12H). 7.54 (18H)

to to to to

303 340 356 364

to 380 to 464 to 480 to 486

478.17 12

to to to to

371 376 384 388

to to to to

506 516 529 543

578.2014 678.2334 778.26 10 878.2930

2 3 4 5 6 7 8

Fluorescense in ethanol Amax (nm)

HRMS M+

-

Br' 350

351

353

352

354

f

356

358

369

360 355 a: trlmethylsllylacetylene,PdCl 2, Cul, PPh), NEt). 70% b: n-BuqF, aqueous THF. rt c: 2-bromolodobenzene,PdCl2, Cul, PPh3, NEt), 70°C d: trlm~thylsllylacetylene,PdCl 2, Cul, PPh 3, plperldlne. 85'C 0: PdC12, Cul, PPh3, NEt3, 70°C

Scheme 92. Oligo(o-phenyleneethyny1ene)s with donor and acceptor substituents

Heitz et 01. described a synthetic approach to poly( p-phenyleneethyny1ene)s and the corresponding oligomers by Pd-catalyzed arylation of 2-methyl-3-butyn-2-01 as a protected commercial acetylene derivative [188]. By varying the structure of p dihaloarenes, soluble materials could be obtained. The Hagihara coupling method has also been used as a synthetic route towards phenylacetylene macrocycles. Thus, Moore et (11. succeeded in synthesizing macrocycles 362 with an inside diameter on a nanometer scale by a stepwise, repetitive approach in which chain growth follows a pattern of geometric progression using the coupling/protection/ deprotection procedure described above [ 1891 (Scheme 93). This route leads to

362

Scheme 93. Cyclo(phenyleneethyny1ene).

1.4 Olefiriic- Arormfic Strirctirres

93

preoriented oligomers 361 that are cyclized in a final intramolecular Pd-catalyzed coupling reaction, in which the oligomers are slowly added to the solution containing the active catalyst. Hoger and Enkelmann have reported shape-persistent macrocycles containing amphiphilic functions 367 [ 1901. The synthesis involved the Hagihara coupling of compound 363 with 3,5-diiodotoluene, followed by removal of triisopropylsilyl protecting groups, yielding the tetra-yne 364. Coupling of 364 with 3-bromo-5-iodotoluene gave the dibromide 365. The formation of oligomeric side products was prevented by the much higher reactivity of aryl iodides over aryl bromides. Coupling of 365 with trimethylsilyl-acetylene, and subsequent removal of the trimethylsilyl group, generated the bisacetylene 366. Finally, the shape-persistent macrocyclic amphiphile 367 was obtained by a modified Eglington-Glaser coupling under high dilution followed by the acid catalyzed deprotection of the tetrahydro-(2H)pyranyl groups. Note that compound 367 is the first example of an amphiphilic shape-persistent macrocycle in which the arrangement of the amphiphilic functions of the macrocycle depend on the nature of the surrounding solvent or included guest molecules (Scheme 94). In conclusion, the Hagihara coupling reaction is the method of choice for synthesis of oligophenyleneethynylenes. Both the step-by-step and the random approach are successfully used for the construction of the oligo(pheny1eneethyny1ene)s. The step-by-step approach benefits from efficient protection/deprotection chemistry of the acetylene function rendering the synthesis of large macrocycles possible. The modular divergent/convergent route to duplicate the monomer units provides fast growth of the chain length and enables the necessary substitution of solubilizing or electronically active groups. However, beside the widely used Hagihara coupling for the synthesis of oligophenyleneethynylenes, an alternative random synthetic approach route exists, based on the acyclic diyne metathesis reaction in the presence of tungsten carbyne (Scheme 95). The reaction is driven by removing butyne (bp=27'C). The only product remaining in the reaction is the dimer 369. Weiss and coworkers have applied this principle to the synthesis of polyphenyleneethynylenes 371 containing up to 150 repeat units (Scheme 45) [191]. The nature of the propynyl endgroups allows the selective end-functionalization with thiocarbamate 375 of the polymer to yield the a,w-dithiolprecursor 376. This synthetic route can also be applied to the synthesis of oligomers (Scheme 96) The potential applications of monodisperse end-functionalized conjugated oligomers are elegantly illustrated by the works of Sita et al. [I921 and by the work of Andres et al. [ 1931 Sita and Guyot-Sionnest have demonstrated the self-assembly of oligo(pheny1eethyny1)benzenethioIs 377-379 on a gold surface [ 1921. They have observed by STM that the resolution on a molecular level depends on the length of the monothiol terminated oligomers. No molecular periodicity has ever been observed for 377 suggesting that this molecule does not form an ordered self-assembled monolayer. In contrast, the longest oligomer of the series 379 self-assembles into a well-ordered monolayer where each molecule stands perpendicular to the surface. Not surprisingly, the intermediate oligomer 378 displays a pattern with a lower degree of

94

I Hj&ocnrhoti 0ligotiier.s

-

i)3,5-diiodotoluene

TIPS

=

=

\ / OPr

ii) Bu4NF,THF

363

OR

-

364

OR

3-bromo-5-iodotoluene

[PBCI2(PPhS)] Cul, piperidine

i) TMS-acetylene [PdCWPhdl Cul, piperidine

ii) CH2C12I MeOH. H+

Scheme 94. Synthesis of shape-persistent macrocylic amphiphile, R = tetrahydro-(2H)-pyranyl.

order than 379. This work bridges the gap between self-assembled monolayers and conjugated oligomers. and paves the way to a variety of optical and electronic studies related to nanostructured materials (Scheme 97). Andres et al. have reported a process that make use of molecular self-assembly to fabricate a two-dimensional superlattice of monodisperse metal nanocrystals linked B -t(uOa )W,

1-BU

Catalytic amount

368

369

Scheme 95. Example of the acyclic diyne metathesis reaction.

1.4 Olefinic-Aroniutic Str.irc.tures

95

R = alkyl, aryl

370

371

(t-BuO),W+-t-Bu Catalytic amount

H3C

H3C 372

CH3 373

+

374

(t-BuO)3W+t-BU Catalytic amount

2 - H&TCH~ 375

Scheme 96. Application of the acyclic diyne metathesis reaction to polymerization and endcapping of polymers.

379

Scheme 97. Monothiol oligophenyleneethynylene 377-379 self-assembled on a gold surface

-

= molecular interconnect

= Au cluster covered by a dodecanethiol shell

Figure 14. Schematic drawing of the self-assembly of Au cluster (a) in the absence and (b) in the presence of molecular interconnect 377-379.

by organic interconnects (Fig. 14) [193]. Their process involves a total of four steps. The first two steps are the synthesis of gold clusters and subsequently the absorption of a self-assembled monolayer of dodecanethiol on the gold clusters to facilitate their manipulation. The third step consists of the formation of a closely packed monolayer film of dodecanethiol coated clusters on a flat substrate. The fourth step deals with the displacement of the organic surfactant by a molecular interconnect that covalently bonds adjacent particles to each other without destroying the order in the monolayer film (Scheme 98).

1.5 Coizclirsioiis

97

380

381

Scheme 98. Dithiol molecular interconnect 380, diisonitrile molecular interconnect 381.

The role of oligo(phenyleneethyny1ene) molecular interconnects 377-379 are twofold. Due to their rigid structure and their covalent bonds with the gold clusters, the molecular interconnects provide a physical reinforcement of the lattice. Molecular interconnects also serve as molecular wires and provide controlled electronic coupling between adjacent gold clusters. The ordered structure of two dimensional superlattice of interconnected gold cluster was proven by transmission electron microscopy. Interestingly, the electrical conductance through gold clusters interconnected by dithiol and diisonitrile oligophenyleneethynylene has been measured by STM and a Coulomb charging behavior has been observed. The results given by Sita et 01. [192] and by Andres et 01. [I931 demonstrate the central role played by conjugated oligomers in molecular and nanoelectronics.

1.5 Conclusions Conjugated oligomers and in particular monodisperse oligomers play a central role in understanding the chemistry and the physics of conjugated polymers notably for optoelectronic properties. The synthesis of monodisperse conjugated oligomers can be achieved via a step-by-step synthesis, via modular synthesis or via a random synthesis followed by a chromatographic separation, both strategies presenting their own advantages and disadvantages depending on the target molecular architecture and on the nature of the chemical reaction involved. Whatever the synthetic strategy used, a large part of the conjugated oligomer synthesis relies on organometallic coupling reaction involving palladium (Suzuki), nickel (Yamamoto), copper (Hay), titanium (McMurry), and tungsten (ADMET, ROMP) as catalysts, although the role of more conventional organic reactions should not be overlooked. With respect to their physical characterization, the key advantage offered by monodispersed oligomers lies in the study of relevant properties such as transition energies or redox potentials as a function of chain length together with the reliable extrapolation towards the corresponding polymers (see Chapters 7.1 and 9). The hydrocarbon oligomers discussed in the present chapter share this advantage with

related heteroaromatic structures such as oligothiophenes and oligopyroles. The former, however, allow a broader structural variation and a systematic control of factors relevant for an effective conjugation along the extended 7r-systems (see Chapters 2.1 and 3 ) . Practical applications of conjugated oligomers as electronic materials are hampered by the fact that the synthesis of related polymers is often less demanding and that polymers have a superior film forming ability. A typical example comes from all-optical signal processing which requires thin films with large nonlinear refractive indices and low losses. These amorphous films are difficult to achieve with oligomers due to their tendency towards crystalline formation. Not surprisingly, therefore, oligomers serve as model systems for relating NLO data with structure, but are less significant as materials (see Chapter 8). The chemical properties which do qualify oligomers as appropriate materials should not be overlooked: the higher purity of oligomers is a key issue in light emitting diodes since it helps to avoid traps for non-radiative decay. Another important feature for LEDs is the high photostability of oligomeric dye-stuff materials whereby films are produced via deposition from the gas phase (see Chapter 10). Oligomers with their highly regular structures may be better suited for supramolecular ordering increasing charge carrier mobility in oligomer biased field effect transistors [ 1 121 and photoconducting discotic mesophases are typical examples [ 1211. Finally, major breakthroughs in visualizing and handling single molecules or small aggregates, e.g. by STM have become possible in immobilizing well-defined oligomers in regular 2D-patterns [65, 1 191.

References I . (a) Skotheim, T. A. Handbook qf' Conducting Pol~wwrs,Marcel Dekker, New York, 1986; (b) Kuzmany, H., Mehring, M., Roth, S. Electronic Properties of Conjugated PolJwzers. Springer Series in Solid State Sciences, Vol. 76, Springer, New York, 1988. 2. Levine, I. N. Qu~intumC h c w i s f r ~3rd ~ . Edn, Allyn and Bacon, Boston, 1983. 3. Kuhn. R. Aiigecv. Chem., 1937. 50, 703. 4. Snyder, R., Arvidson, E., Foote, C., Harrigon, Christensen, R. L. J . A m . Cheni. Soc. 1985, 107, 4117. 5. D'Amico. K. L., Maurs. C.. Christensen, R. L. J . A m . Cliern. Soc. 1980, 102. 1777. 6. Isler, 0. (ed.) Carotetzoid.?,Birkhiiuser Verlag. Basel, 1971. 7. Britton, G., Goodwin, T. W. (ed.) Carotenoid Cheniistrjj (mi Biochemistry, Pergamon Press, London, 1983. 8. Beutner, S., Graf, O., Schaper, K., Martin, H. D. Pure Appl. Cheiii. 1994, 66. 955. 9. Bohlmann, F., Mannhardt, H. J. Ckem. Ber. 1956, 89, 1307. 10. Sondheimer, F., Efraim, D. A., Wolovsky, R. J . A m . Chem Soc. 1961, 83. 1675. 11. Knoll, K., Schrock, R. R. J . Am. Chem. Soc. 1989, 111, 7989. 12. Lathi, P. M., Obrzut, J., Karasz, F. E. Macroniolerules. 1987, 20, 2023. 13. Kiehl, A., Eberhardt, A . , Adam. M.. Enkelmann, V., Miillen, K. Angew. Chem., 1992, 104, 1623; Angecv. Cheiii. b i t . Ed. Engl. 1992, 31. 1588. 14. Stille, J. K. Angew. Chem., 1986, Y8, 504; Angel$'. Chrni. Iiit. Ed. Engl. 1986. 25, 208. 15. Diederich, F.. Rubin, Y. Atigew. Chem. 1992, 104, 1123; Angew. Chern. Int. Ed. Engl. 1992,31, 1101.

Reference.?

99

16. Heimann, P. B., Kleimann, J., Salensky, N . M. Carbon 1984. 22, 147; Nature 1983, 306, 164. 17. Kudryavtsev. Yu. P., Evsyukov, S. E., Babaev, V. G., Guseva, M. B., Khvostov, V. V., Krechko, L. M. Carbon 1992,30, 213. 18. Sladkov, A. M., Kudryavtsev, Yu. P. Usp. Khim. 1963, 32, 509. 19. Smith, P. P. K., Buseck, P. R. Science (Washington, D. C.) 1982, 216, 984. 20. Kertesz, M., Koller, J., Azman, A. J . Cliem. P h j x 1978, 68, 2779. 21. Karpfen, A. J . Plij,.~.C: Solid Strite Phyc.. 1979, 12, 3227. 22. Rice, M. J., Phillpot, S. R., Bishop, A. R., Campbell, D. K. Phys. Rev. 1986, B34,4139; 1987, B36, 1735. 23. Springbory, M., Drechsler, S. L., Malek, J. Phys. Rev. 1990, B41, 11954. 24. Weltner, W., vanZee, P. J. Chem. Rev. 1989, 89, 1713. 25. (a) Shen, L. N., Withey, P. A., Graham, W. R. J . Chem. Pkys 1991,94,2395;(b) Martin, J. M., Francois, J. P., Gijbels, R. J . Chem. Phys. 1991, 94, 3753. 26. Bjarnov, E., Christensen, D. H., Nielsen, 0. F., Aughdal, E., Kloster-Jensen, E., Rogstad, A. Spectrochim. Acta 1974, 30A, 1255. 27. Korshak, V. V., Kudravtsev, Yu. P., Evsyukov, S. E. e t d . Dokl. Akad. Nuuk. SSSR 1988,298, 1421; Makrornol. Chem. Rupici Commun. 1988, 9, 135. 28. Akagi, K., Furukawa. Y., Harada, I., Nishiguchi, M., Shirahawa, H. Synth. Met. 1987, 17, 557. 29. Kavan, L. In Elecrroclien~icnlCarboni:atiori qf Fluoropolymers, Thrower, P. A. (Ed.), Cliemistry und PIij,sics qf'Curbon, Marcel Dekker, New York, 1991, Vol. 23, 69. 30. Costello, C. A,, McCarthy, T. J. Macromolecules 1987, 20, 2819. 31. Iqbal. Z., Ivory. D. M., Szobota, J. S . , Elsenbaumer, R. L., Baughman, R. H. Macromolecules 1986, 19, 2992. 32. Jansta, J., Dousek, F. P. Curbon 1980, 18. 433. 33. Kijima, M., Sakai, Y., Shirakawa, H. In Proc. Int. Conf: Sci.Technol. Synth. Met. 1994, Seoul, Korea. 34. Kavan, L.. Dousek, F. P. Sjwth. Met. 1993, 58, 63. 35. Kavan, L., Dousek, F. P., Micka, K. Solid State lonics 1990, 38, 109. 36. Kavan, L., Dousek, F. P., Micka, K. J . Phys. Chem. 1990, 94, 5127. 37. Cervinka, L., Dousek, F. P., Jansta, J. Philos. Mug. 1985, B51, 603. 38. (a) Nakamizo. M., Kammereck, R., Walker, P. L. Carbon 1974, 359, 259; (b) Nishihara, H., Harada, H., Kaneko, S., Tateishi, M., Aramaki, M. J . Chem. Soc., Furaday Trans. 1991, 87, 1187. 39. Kastner, J., Kuzmany, H. Macromolecules 1995, 28, 344. 40. Kroto, H. W., Heath, J. R., O'Brien, S. C., Curl, R. F., Smalley, E. E., Nature 1985, 318, 162; Kratschnier, W., Lamb, L. D.. Fostiropoulos, K., Huffman, D. R., Nature 1990, 347; 354. 41. Glaser, C. Lieb. Ann. 1870, 154, 137. 42. Meier, H. Synthesis 1972, 248. 43. Eglington, G., Galbraith, A. R. Cheni. Incl. 1956, 727. 44. Hay, A. S. J . Org. Chem. 1960, 25, 1275-1276; ibid, 27, 3320. 45. (a) Kroto, H . W., Allaf, A. W., Balm. S. P. Chem. Rev. 1991, 91, 1213; (b) Kroto, H. W. Angew. Chem. 1992, 104, 113; (c) ibid, Angew. Chem. In!. Ed. Engl. 1992, 31, 1101. 46. Geckeler, E. Trends Polym. Sci. 1994, 2, 355. 47. Anderson, H . L., Faust, R., Rubin, Y., Diederich, F. Angew. Chem. 1994, 106, 1427; Angew. Chem. I n t . E d Engl. 1994, 33, 1366. 48. Isaacs, L., Seiler, P., Diederich, F. Angew. Chem. 1995, 107, 1636; Angew. Chem. Inr. E d Engl. 1995, 34, 1466. 49. Chodkiewicz, W., Alhuwalia, J. S., Cadiot, P., Willemat, A. Compt. rend. 1957, 245, 322. 50. Eastmond, R., Walton, D. R. M. Tetruhrdron 1972, 28, 4591-4599. 51. (a) Gladysz, J. A., Bartik, T. A. Angeic. Chem. 1996, 108,467; (b) Angew. Chem. I n t . Ed. Engl. 1996, 35,414; (c) Brady, M., Wenig, W., Gladysz, J. A. J . Chem. Soc., Chem. Commun. 1994, 2655; (d) Coat, F., Lapinte, C. Organometullics 1996, 14, 634. 52. Grosser, T., Hirsch, A. Angew. Cliem. 1993, 105, 1390; Angen,. Chem. Int. Ed. EngI. 1993, 32, 1340. 53. Rubin, Y . , Lin, S. S., Knobler, B., Anthony, Y., Boldi, A. M. J . Am. Chrm. Soc. 1991, 113,6943. 54. Giigel, A,, Belik, P., Walter, M., Kraus, A,, Harth, E. Tetrahedron, 1996, 52, 5007.

100

I Hydrocarbon Oligomers

55. Wegner. G. Z. Nritirrforsch. 1969,24b, 824; Wegner, G. Molecular Metals, Plenum Press, New York & London, 1979; Bassler. H., Enkelmann, V., Sixl, H. Adv. Po1vn. Sci. 1974, 63, I . 56. Takeda. K.. Wegner, G. Makromol. Chem. 1979. 160, 349. 57. Stevens, G. C.. Bloor, D. Chern. Phq’s. Letters 1976, 40, 37. 58. Eichele, H.. Schwoerer. M., Huber, R., Bloor. D. C/iem. Phys. Letters 1976. 42, 342. 59. Anthony, J., Boudon, C., Diederich, F. et cil. Angew. Clieni. 1994. 106, 794: Angew. Chem. Int. Ed. Engl. 1994. 33, 763. 60. Giesa, R., Schulz, R. C. Po1jon. I n t . 1994, 33. 43-60. 61. Wudl, F., Bitler. S. P. J. Am. Cliem. Soc. 1986. 108, 4685. 62. Materials for second order non-linear optics have been several times reviewed, see for example: Marks. J., Ratner, M. Angew. Clzem. 1995, 107, 167; Angew. Chern. In/. Ed. Engl. 1995. 34. 155. 63. Lio, A., Reichert. A., Nagy. J. O., Salmeron, M., Charych, D. H. J . Vac. Sci. Technol. 1996. n . 1995, A33. 2455. 814, 1481; Nezu, S., Lando. J. B. J . P o l ~ ~ tSci. 64. Batchelder. D. W.. Evans, S. D., Freeman, T. L., Haussling. L., Ringsdorf, H., Wolf, H. J . Am. Clwn. Soc. 1994, 116, 1050. 65. Grim. P. C. M., De Feyter, S . , Gesquiere, A. et a/. Angew. Chem. 1997, 109, 2713; Angew. Chern. Int. Ed. Eng1. 1997, 36, 2601. 66. Blanchard-Desce, M.. Ledoux. I., Lehn, J.-M., Malth2te J., Zyss J. J . CIzeni. Soc. Chetn. Cornmitn. 1988, 737. 67. Slama-Schwock, A.. Blanchard-Desce. M.. Lehn, J.-M. J . P h j x Ckem. 1990. 94, 3894. 68. Barzoukas, M.. Blanchard-Desce. M., Josse. D., Lehn, J.-M. C/ien?.Phys. 1989. 133. 323. 69. Dentan. V., Blanchard-Desce. M., Palacin, S.. Ledoux, I.. Barraud, A., Lehn, J.-M., Zyss, J. Thin Solid Fibns 1992, 210/211, 221. 70. Lehn, J.-M. Supramolecular cliemistry: concepts and perspectives: a personal account, VCH, Weinheim, 1995. 71. Kugimiya, S.-l., Lazrdk. T.. Blanchard-Desce. M., Lehn, J. M. J . Ckem. Soc., Chern. Conimun. 1991, 1179; Slama-Schwock, A., Blanchard-Desce. M., Lehn, J. M. J . Pliys. Chon. 1992, 96, 10559. 72. Lehn. J.-M., Vigneron, J.-P.. Bkouche-Waksman, I., Guilhem, J., Pascard, C. Helv. Ckim. Actri 1992, 75, 1069. 73. Barzoukas, M., Blanchard-Desce, M., Josse, D.. Lehn, J.-M., Zyss, J . Cliem. Pliys. 1989,133,323, 74. Tolbert, L. M. Arc. Clzeni. Res. 1992, 25, 561; Tolbert. L. M., Zhao, X., Ding, Y., Bottomley, L. A. J . Am. Clietn. Soc. 1995, 117, 12891. 75. (a) Kovacic, P., Jones, M. B. Clwrn. Rev. 1987,87,357; (b) Wegner, G. Angew. Cheni. 1981.93, 352; Angew. Clwni. Int. Ed. Engl. 1981, 20, 361. 76. (a) Grem, G., Leditzky. G., Ulrich, B., Leising, G. Adv . Mater. 1992, 4, 36; (b) Huber, J., Miillen, K., Salbeck, J., Schenk, H., Scherf, U. et a/. Acta Po1ytn. 1994. 45, 244. 77. Ballard, D. G. H., Courtis, A., Shirley, I. M., Taylor, S. C. J . Clietn. Soc., C l i m . Commirn. 1983. 954. 78. Gin, D. L., Conticello, V. P.. Grubbs, R. H. 1’01ym. Muter. Sci. Eng. 1992. 67. 87. 79. Yamamoto, T., Hayachi. Y.. Yamamoto, A. Bull. Clzeni. Soe. Jpn. 1978, 51, 2091. 80. Rehahn, M.. Schluter, A.-D., Wegner, G.. Feast. W. J. Po1vmer 1989, 30. 1054, 1060. 81. Kern, W., Seibel. M.. Wirth, H.-0. Makromol. Cliem 1959, 29. 164. 82. Heitz. W., Ulrich, R. Makroniol. C h i . 1966, 98, 29. 83. Galda, P., Rehahn, M. S!~rithesis1996, 614. 84. Liess, P., Hensel, V., Schliiter, A. D. Licd~igsAnn. 1996. 1037. 85. Liittke, W., Gerhardt, H., unpublished results; Gerhardt, H. Dissertrition, University of Gottingen, 1984. 86. Scherf. U., Miillen, K. Makrotnol. C/iem. Rapid Commirn. 1994. 12, 489. 87. Grimme, J.. Kreyenschmidt, M., Uckert, F.. Miillen, K.. Scherf, U. Adv. Mater. 1995. 7, No. 3. 88. Scherf, U., Miillen, K. Synthesis 1992, 23. 89. Kreyenschmidt. M., Uckert, F., Miillen, K. i2lacromolec.ules 1995. 28, 4577. 90. Scherf, U . , Miillen. K. At/\,. Po1~vn.Sci., 1995. 123, 1. 91. Fukuda. M., Sawada, K.. Yoshino, K. J . Po1vm. Sci., Polym. Cheni. 1993. 31, 2465. 92. Gin, D. L.. Avlyanov, J. K., MacDiarmid. A. G . Syntli. Met. 1994, 66, 169.

References

101

93. Kreyenschmidt. M.. Baumgarten. M.. Tyutyulkov. N.. Mullen. K . Arige1il. Clieni 1994. 106. 2062; Angew. Chw7. Int. Ed. Engl. 1994. 33. 1957. 94. Keegstra, M. A.. De Feyter. S . , De Schryver. F. C.. Mullen. K. Atigew. Chenz 1996, 108. 830: Angew C/iet?t.h7t. Ed. Etzgl. 1996, 35. 774. 95. Tour, J. M. Adv. Muter. 1994, 6. 190. 96. Braunling. H., Binnig, F.. Staab. H. A. Cliem. Ber. 1967, I W , 880; Staab, H. A., Binnig. F. Chetn. Ber. 1967, 100, 293. 889. 97. Williams, D . J.. Colquhoun. H . M.. O'Mahoney. C. A. J . Cl7eni. Soc., Chetii. Conitmrn. 1994, 1643. 98. Yamamoto, T.. Hayashi. Y.. Yamamoto. A. Bull. CAeni. Sor. Jpti. 1978. 51, 2091. 99 Noren. G. K.. Stille. J. K. Mrrc,romtl. Re18. 1971, 5 , 385. I00 Clar, E.. Kelly, W., Laird, R. M. Mottatslt. Clietnie 1953. 57, 391. 101, Kerr. K. A.. Ashmore, J. P., Speakman, J. C. Pror. R. Sue. 1975. 311, 199. 102 Bohnen, A.. Koch. K.-H.. Luttke, W., Mullen, K. .4tigei1'.Cltetri. 1990. 102. 548: Attgen.. Cltetn. h i t . Ed. E n d . 1990. 29. 525. 103 Koch. K.-H., Mullen. K. Clietti. Ber. 1991, 1-74.3091. 104 Solodovnikov. S. P.. loffe, S. T.. Zaks. Y. B.. Kabachnik. M. I . Birll. Acnd. Sci. U D S S R D;v. Client. Sci. 1968. 442. 105. Quante. H.. Muilen, K. .4ngeii'. Chent. 1995, 107, 1427; Atigrw. Ch~n7.h i , Ed. Etig. 1995. 34, 1323. 106. Holtrup. K.. Muller, G., Quante. H., De Feyter. S.. De Schryver, F.. Mullen, K . Clintz. Eur. J . 1997, 3, 219. 107. Muller. U., Adam, M., Miillen, K . Cl7en7. Ber.. 1994. 127. 437. 108. Hart, H., Luo. J. J . Org. Cheni. 1987. 30. 4833. 109. Bailey, W. J., Liao, C.-W. J . Am. Clietii. Sor. 1954. 77, 992. 110. Graham. R. J., Paquette. L. A. J . Org. Cheni. 1995. 60, 5770. 111. Biermann, D.. Scmidt. W. J . .4n7. Clzem. Soc. 1980. 102, 3163. I I?. Brown, A. R.. Pomp. A., de Leeuw. D. M. et a / . J . Appl. Pkys. 1996. 79, 2136. 113. Staab, H. A.. Diederich, F. Angeii.. Clieni. 1978. YO, 383; Angew. Cliem. h i t . Ed. Etigl. 1978. 17. 372. 114. Staab. H. A., Diederich, F. Clzetn. Ber. 1983. 116. 3487. 115. Funhoff, D. J. H.. Staab, H. A. i i t i g ~ ~Cl7m. i.. 1986. 98, 757; Atigew. C/iem. I t i t . Ed. Etig/. 1986, 25, 742. 116. Krieger. C.. Diederich, F.. Schweitzer. D., Staab, H. A. Attgew. Cliern. 1979. 91, 733; Angew. Chem. I t i t . Ed. Etigl. 1979. 18, 699. 117. Bunz, U.. Francke. V., Klapper. M., Uckert, F.. Mullen, K., Symposium proceedings of OSM 5. Heidelberg. 1996. 118. Miiller, M.. Mauermann-Dull, M., Wagner, M., Enkelinann, V.. Mullen. K. Angeiv. Ckrm. 1995, 107. 1751: Angeii.. Cl7m7. b i t . Ed. Engl. 1995. 34, 1583; Muller, M . , Petersen J., Strohmaier. R., Gunter. C., Karl, N., Mullen, K. Angew. Cl7en7. 1996, 108, 947: Angeiij. Clietii. Int. Ed. EngI. 1996. 35. 886. 119. Stabel. A., Herwig, P., Mullen. K., Rabe. J. Aiigew. Clieni. 1995. 107. 1768: Angehis. Clietn. h t . Ed. Etigl. 1995, 34, 1609. 120. Herwig, P., Kayser, C., Mullen. K.. Spiess. H. W. Adis. Muter. 1996, 8. 510. 121. van de Craats. A. M.. Warman. J. M.. Brandt. J. D.. Geerts, Y.. Mullen. K . Adv. Muter. 1998. IO, 36. 122. Ito. T.. Shrakaw, H., Ikula. S. J . Polwi. S i . Polwt. ClietTi. Ed. 1974, 12, 1 I . 123. Baughman, R. H., Bredas. J. L.. Chance, R. R.. Elsenbaunier, R. L., Shacklette, L. W. Chem. Rev. 1982, 82. 209. 124. Friend, R. H.. Bradley, D. D. C.. Townsend, P. D. J . Pkys. D: Appl. Phy.~.1987, 20. 1367. 125. Gourley. K. D.. Lillya, C. P., Reynolds, J. R., Chien. J. C. W. Macrot?iolecztles 1984. 17, 1025. 126. Gagnaon. D. R., Karasz, F. E.. Thomas. E. L.. Lenz, R. W. S p r k . h4et. 1987, 20, 85. 127. Bestman, H. J.. Vostrowsky, 0. Top. Citrr. Chen7. 1983. 109, 85. 128. Siegrist, A. E. Hehi. Cliitn. Actn 1967, SO, 906. 129. McMurry, J. E. Ace. Clieni. Rrs. 1983, 16. 405. 130. Heck, R. F. Org. React. 1981, 27, 345.

102 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151.

152. 153. 154. 155.

156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172. 173.

1 Hjdrocarhon Oligonzers

Meier, H. Angew. Cheni. 1992. 104, 1425; Angeiv. Chem. Inr. Ed. Engl. 1992, 31, 1399. Mauermann-Dull, H., Adam, M., Bohm, A,, Reuter, R., Miillen, K., to be published. Horhold, H.-H., Opfermann, J. Makroniol. Chem. 1970, 131, 105. Horhold, H.-H. Z . Chern. 1972, 12, 41. Kossmehl. G., Hlrtel, M., Manecke, G. Makrotnol. Chem. 1970, 131, 37. Lenz, R. W., Handlovits, C. E. J . Org. Chem. 1960, 25, 813. Horhold, H.-H., Grlf, D., Opfermann, J. Plustc Knutscii. 1970, 17, 84. Horhold, H.-H., Helbig, M. Makroniol. Chem., Mucromol. Synp. 1987, 12, 229. Rehahn, M., Schluter, A . D. Mrrkrotnol. Cliem., RupidCornmun. 1990, 11, 375. Heitz, W., Greiner A. Mukrotnol. Cliem. Rapid Cornmun. 1988, 9, 581. (a) Heitz, W., Greiner, A,, Briigging, W. Mukromol. Chem. 1988. 189, 119; (b) Thorn-Csanyi, E., Kraxner, P. Macromol. Rapid Cornmun. 1995, 16, 147. Sonoda, Y., Kaeriyama, K. Bull. Chem. Soc. Jpn. 1992, 65, 853. Wessling, R. A. J . Polym. Sci. Polym. Syrnp. 1986, 72, 55. Drefahl G., Plotner, G. Chem. Ber. 1961, 94, 907. Drefahl, G., Kuhmstedt, R., Oswald, H., Horhold, H.-H. Mukronzol. Cheni. 1970, 131, 89. Schenk, R., Gregorius, H., Meerholz, K., Heinze, J., Miillen, K. J . A m . Chenz. Soc. 1991, 113, 2635. Meerholz, K., Gregorius, H., Mullen, K., Heinze, J. Adv. Muter. 1994, 6 , 671. Barth, S., Bassler, H., Wehrmeister, T., Miillen, K. J . Cham. P h j ~ 1997, . 106, 321. Tian, B., Zerbi, G., Schenck, R., Mullen, K. J . Clzem. Plzys. 1991, 95, 3191; Tian, B., Zerbi, G., Miillen, K. J . Chem. Phys. 1991. 95, 3198. Mathy, A,, Ueberhofen, K., Schenk, R. c'f ul. Phys. Rev. B. 1996, 53, 4367. Hennecke, M., Damerau, T., Mullen, K. Macromolecules 1993, 26, 341 1. Heller, C. M., Campbell, I. H., Laurich, B. K. et ul. Phys. Rev. B. 1996. 54, 5516. Pauck, T., Biissler, H., Grimme, J., Scherf, U.. Mullen, K. Cliem.Phys. 1996, 210, 219. Choong, V., Park, Y., Gao, Y. el a/. Appl. Phys. Lett. 1996, 69, 1492. Cornil, J., Beljonne, D.. Shuai, Z. et a/.C h ~ mPhys. . 1995, 247, 425. Schmidt, A,, Anderson, M. L., Dunphy, D., Wchrmeister, T., Mullen, K. Ad,,. Muter. 1995, 7 , 722. Brendel, P., Grupp, A., Mehring, M.. Schenck, R., Miillen, K.. Huber, W. Synth. Met. 1991, 45, 49. Schenck, R., Gregorius, H., Mullen, K. A h , . Muter. 1991, 3, 492. Gregorius, H., Baumgarten, M., Reuter, R., Tyutyulkov, N., Mullen, K. Angen. Chem. 1992, 104, 1621; Angew. Chem. Int. Ed. Engl. 1992, 31, 1653. Stalmach, U., Kolshorn, H., Brehm, I.. Meier, H. Liehigs Ann. 1996, 1449. Kliirner, G., Former, C., Yan, X., Richert, R.. Mullen, K. Adv. Mater. 1996, 8, 932. Fukutome, H., Takahashi, M., Ozaki, M . Cliem.Phys. Lett. 1987, 133, 34. Baumgarten, M., Bunz, U., Scherf, U., Mullen, K. Organic synthesis and materials science, in Moleculur Engineering,#or An'vunced Materiuls, Becher, J., and Schaumburg, K. (eds.), Kluwer Academic Publishers, Amsterdam, 1995. Mauermann, H., Bohm, A,, Miillen, K. in preparation. Bohm, A., Adam, M., Mauermann, H., Stein, S., Miillen, K. Tetrahedron Lett. 1992,33, 2795; Mauermann, H., Bohm, A., Fieser, G., Mullen, K. Mucrornol. Chem. Phys. 1996, 197, 413. Thulin, B., Wennerstrom, 0. Actn C h n . Scund. 1977, B31, 135. Raston, Wennerstrom, 0. Actu Cliem. Scmncl. 1982, B36, 655. Miillen, K., Unterberg, H., Huber, W. et a/.J . A m . Cheni. Soc. 1984, 106, 7514. Huber, W., Mullen, K., Wennerstrom, 0. Angrw. Chem. 1980, 92, 636; Angeit.. Chem. Int. Ed. Engl. 1980, 19, 624. Ohlenmacher, A., Schenk, R.. Weitzel, H. P., Tyutyulkov, N., Tasseva, M., Mullen, K. Mukrotnol. Cheni. 1992. 193. 81. Meier, H., Miiller, K. Angew. Chem. 1995, 107, 1598; Angew. Chern. Int. Ed. EngI. 1995, 34, 1437. Weitzel, H.-P., Miillen, K. Mukromol. Cheni. 1990, 191, 2837. Auchter-Krummel, P., Mullen, K. Angew. Clzem. 1991, 103, 996; Angcw. Chem. Int. Ed. Engl. 1991, 30, 1003.

174. Aviram, A. (ed.) Molecular Eluctronics: Science and techno log!^, Conf. Proc. 262, American Institute of Physics, New York, 1992. 175. Miller. J. S. Arb'. M(J/w. 1990. -7, 378. , Press, San 176. Kirk. W. P.. Reed. M. A. (ed.) Nmiostrr~ctiiresund Me,so.sc~opicS w e n ~ sAcademic Diego, 1992. 177. Drefahl. G., Plotner, G. Chet~r.Bey. 1958. 91, 1174. 178. Stephens. R . D., Castro, C. E. J . Org. Ckeriz. 1963. 28, 3313. 179. Burdon, J., Coe, P. L., March, C. R., Tatlow. J . C. Chern. Conimun. 1967, 1259. 180. Sonogachira, K.. Tohda, Y; Hagihara, N . Tet. Lett. 1975, 50, 4467. 181. Cassar. L. J . Organonw/u/. Cheni. 1975. Y3. 253. 182. Dieck, H. A,, Heck, F. R. J . Orgnnonielnl. C/retu. 1975, 93. 259. 183. Takahashi. S., Kuroyama, Y., Sonogachira. K., Hagihara. N. S~nr1resi.s 1980. 627. 184. Schurnm, J. S.. Pearson, D. L.. Tour, J. M. Angew. Clzern. 1994, 106. 1445; Angew. Clirm. Int. Ed. Engl. 1994, 33, 1360. 185. Grubbs. R. H.. Kratz, D. Cheni. Ber. 1993, 126, 149. 186. Mangel, T., Eberhardt, A., Bunz. U.. Miillen, K., manuscript in preparation. 187. Wong. M. S.. Nicoud, J.-F. Tet. Lett. 1994. 35, 6113. PIiIx 1994. 195, 303. 188. Solomin, V . A.. Heitz. W. Mrrcrortiol. Clic~t~i. 1992, 114, 2773. 189. Zhang. J. S., Moore. J. S.. Xu. Z.. Aquirre. R. A. J . h i . Cheni. SOC. 190. Hoger, S., Enkelmann, V. Angen,. Clieni. 1995. 107. 2917: Angeiv. Clien?.Int. Ed. Engl. 1995, 34. 2713; Morrison, D. L., Hoger. S. J . Chem. Soc. Ckem. Coniniloi. 1996. 2313. 191. Weiss, K., Michel. A,, Auth, M.-E., Bunz, U. H . F., Mangel, T., Miillen. K. AI7gPI.I'. C/let?l. 1997, 109. 522; Angew. C1iem hi/. Ed. Enngl. 1997. 36. 506. 192. Dhirani, A.. Zehner, R., Hsung. R.. Guyot-Sionnest. P.. Sita. L. R. J . A n i . Cheni. SOC.1996, 118, 3319. 193. Andres, R., Biefeld, J. D., Henderson, J. I. et 01. Science, 1996, -773. 1690.

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2 Sulfur-Containing Oligomers P. Bauerle

2.1 Oligothiophenes 2.1.1 Introduction The discovery of highly conducting polyacetylene in 1977 [ l ] prompted the synthesis of other polymers with conjugated n-systems such as polypyrrole [2], poly-p-phenylene [3], poly-p-phenylenevinylene [4], polyphenylenesulfide [5, 61, polyaniline [7], as well as of other polyheterocycles such as polythiophene [8], polyfuran [8], polyselenophene [9], or more extended polyaromatics such as polyazulene [ 101. Because they can have a variety of structural variations, polythiophenes have become the type of conducting polymer most frequently investigated because conductivity is mostly unaffected by substituents [l I]. In addition, both conducting and semiconducting polythiophenes are very stable and readily characterized. Although the polymer properties can be influenced and tailored by the variation of monomeric building blocks to a certain extent, precise predictions about unequivocal structure/property relationships are not possible. The physical properties of such conducting polymers cannot be correlated directly to the structural parameters, which frequently depend on the preparation conditions. Due to statistical chain length distribution and interruption of the conjugated chain by mislinkages and other defects, these materials, like all polymers, lack a rigidly defined structural principle. Therefore, the conjugation and conducting pathways are interrupted and severely disturbed. Figure 1 depicts the ‘real’ structure of a polythiophene schematically. The synthesis and investigation of well-defined model oligomers has therefore recently become useful to gain insight into the structural and electronic pecularities of the corresponding polymers. In the meantime, for nearly all basic conducting polymers homologous series of defined oligomers have been synthesized. Especially oligorhiophenes have reached more and more prominence in recent years [l 11. By assembling defined mono- and oligomers step by step, via well understood organic reaction sequences, materials are obtained where both chain and conjugation length are well controlled and rigorously defined [12]. Since oligomers may serve as model compounds for the respective polymers, the painstaking synthetic procedure appears well justified. Depending on their size and substitution pattern they are usually more soluble than polymers and are stable in various redox states. The precise characterization of the electronic and geometric structure succeeds both in solution and in the solid state. The physical properties are now well correlated to the (conjugated) chain length and thus ‘real’ structure/property relationships become available. The data may be compared to those of the corresponding polymers in order to estimate

106

2 Sulfur-Containing Oligomrrs

Figure 1. Typical ‘real’ structure of polythiophene including mislinkages and defects in comparison with a defined and all-a-linked oligothiophene. Conjugation pathways are drawn with bold lines.

their mean conjugation length or to extrapolate to a (hypothetical) infinite chain length, This information is not accessible from investigations on the polymeric systems [ 131. The study on defined oligomers also provides information which may be used to improve strategies for the development of novel materials. Some of their physical properties even surpass those of the polymers. For this reason, oligothiophenes have been recently advanced as components for molecular electronics [ 141 and optical devices [ 151. In fact, an all-a-linked sexithiophene has been successfully employed as an active component in an organic field-effect transistor [14] and in a light-modulating device [16]. The mobility of the charge carriers and the transistor characteristics were found to be superior to those of an analogous poly(bithiophene) transistor and even approach those of transistors based on amorphous silicon. Prior to 1960, bi- and polythienyls and their derivatives were not easily attainable, and consequently only few investigations on these compounds had been made. It is intriguing to see that even in the 1930s a whole series of a-oligothiophenes or apolythienyls (as they were also called) up to a-septithiophene were laboriously isolated and well characterized, by Steinkopf and coworkers. They were interested to discover the difference between thiophenes and corresponding benzenes on the one hand, and with aliphatic conjugated chains, polyenes, on the other hand. Since both series were already known up to septi-p-phenylene [I 71 and dimethyldodecahexaene [18], respectively, Steinkopf et al. synthesized a whole series of aoligothiophenes 2-7 up to the heptamer, and expected thiophenes to be the missing link between olefins and benzene [19]. Ullmann’s biaryl synthesis was applied, and 2iodothiophene 8 was treated with copper bronze. The main product, a-bithiophene 2, could be isolated by steam distillation (<20% yield), and also after lengthy

2. I Oligotliiophenes

107

purification through extraction and vacuum sublimation, the whole series of aterthiophene 3 up to a-septithiophene 7 was obtained in very small amounts , [Eq. ( l ) ] [ I ~ c201.

m,

Cu-bronze,

S n 2-7 (n 0-5)

8

By comparing melting points, absorption maxima, and the qualitative assessment of fluorescence and ‘halochromie’ (reaction with concentrated sulfuric acid) the various oligomers were characterized and, for the first time, their structures were qualitatively correlated with their properties. Remarkably, 60 years after the first report on a-oligothiophenes, these properties are of high current interest with respect to the application of conjugated materials in organic light emitting diodes [21]. After several a-oligothiophenes had been isolated in small amounts from yellow marigold [22], Gronowitz and coworkers started a new era of oligothiophenes in 1960. 2,2’-Bithiophene 2 and the isomeric 3,3’-derivative 11 thus became easily available by symmetric coupling of the corresponding lithiated thiophenes 9 and 10 with cupric chloride [Eq. ( 2 ) ] [93]. This breakthrough gave rise to a vast amount of synthetic work [24]. In the following years, the main interest has been devoted to bi- and terthiophenes also because they were found to occur frequently as acetylenic derivatives in certain plants (family of Conipositae and Astrwcear) [25]. Many of the a-conjugated oligothiophenes which have been isolated and synthesized exhibit biological activity: some of them generate skin pigmentation, act as herbicides or inhibit feed germination, while others are phototoxic against nematodes, algae, human erythrocytes, insect larvae and eggs [26]. This phototoxic activity rests on the ability of the thiophene oligomers to develop singlet oxygen when illuminated. This was proven by many mechanistic and photophysical studies [27].

2

8

Li

d S

10

cuc’2

943

s /

11

Finally, the third and current era of oligothiophenes was initiated in 1982 with the first synthesis of polythiophene P1 as an electrically conductive polymer [8]. Somewhat later, a-bithiophene 2 and a-terthiophene 3 were used as monomers, instead of thiophene 1 itself. as substrates for electropolymerization in order to control the

108

2 Suijur-Containing Oligomers

structure of the polymer [Eq. (3)] [28]. Owing to their lower oxidation potential, electropolymerization can be performed under milder conditions [29]. Furthermore, since the thiophene rings are exclusively a-linked in the starting molecule, a polymer with less cu,P-defects is to be expected [12, 301. Nevertheless it was found in most studies, that electropolymerization of thiophene on the one hand, and of thiophene oligomers on the other hand, does not lead to the same polymer and that, contrary to what could be expected, the use of conjugated precursors yields less conjugated and less conducting material [30b, 3 11.

- $3 OXidatiCfl

H m : 1-3 ( f J = 1-3)

(3)

Pi

Conversely, due to the defined structure of a-oligothiophenes it was quickly realized that they constitute interesting models of the electronic properties of polythiophene [32]. The first spectral characterization of oligothiophene radical cations and dications in 1989 [32c, 331 and the use ofa-sexithiophene 6 as active component in an all-organic electronic device one year later [34] have triggered a renaissance of interest and intensive research work in the synthesis and characterization of these materials that had been neglected for so many years [35]. Several types of nomenclature have been established for the oligothiophenes. Short notations like the original proposed by Bredt [36] (e.g. a,a,a-quaterthiophene) and later on by Steinkopf [ 19a] (e.g. a-quaterthiophene) were later followed by the general and more convenient abbreviations nT or a-nT (e.g. 4T or cu-4T). However, only the much more complicated systematic nomenclature used by Chemical Abstracts allows the precise denotation of linkages between rings and positions of substituents. The numbering scheme of thiophene, of the three isomers of bithiophene, and of a substituted terthiophene is given in Fig. 2. Since for conjugated materials only oligothiophenes connected in a-positions are relevant, and a consistency with other oligomeric series should be given, in this chapter we will use a more precise but also convenient short notation which considers the repeating units according to polymer nomenclature and the endgroups: X-T,-Y (e.g. H-T4-H). The early synthesis and characterization of oligothiophenes entailed an immense number and variety of derivatives and related compounds. Due to their structural variability, stability, and well-balanced properties, it seems likely that oligothiophenes have developed to be ideal model compounds for the corresponding conjugated polymers. This chapter intends to give an overview of the synthesis of various classes of oligothiophenes. They will be arranged by the different methods

1

2

I1

12

13

Figure 2. IUPAC-nomenclature of thiophene 1, the isomeric bithiophenes 2, 11, 12, and 3’-substituted terthiophene 13.

2.1 Oligotl~iophenes

109

of preparation, in contrast to the three extensive reviews which have been published up to now and cover the literature up to 1988 [37].

2.1.2 Synthesis of Oligothiophenes 2.1.2.1 Unsubstituted Oligothiophenes

In the short time in which oligothiophenes have been investigated with respect to conjugated materials, several general categories have been established. Unsubstituted all-a-linked oligothiophenes (this section) and substituted derivatives (section 2.1.2.2) represent the most interesting derivatives. Regioisomeric oligothiophenes including $-linkages have a strongly decreased conjugation and are only discussed to a minor extent. The main interest of unsubstituted oligothiophenes lies in their solid-state properties and in their application as organic semiconductors in electronic devices. Oligothiophenes may be synthesized via two different routes, either by C-C-linkage reactions between thiophenes and/or oligothiophenes or by ring closure reactions from acyclic precursor molecules. One of the most useful procedures in the formation of C-C-a-bonds which is also effective in the synthesis of oligothiophenes is the metal-promoted coupling reaction of organic halides. The reactions of active organometallic reagents, such as organolithium or Grignard reagents with salts of transition metals, are described below. These reactions, however, have the disadvantage of requiring at least stoichiometric amounts of transition metals. More recently, not only very effective, but also purely catalytic transition metal-catalyzed methods have been developed which provide oligothiophenes of different chain lengths and of different connectivities. Mainly, low-valent transition metals and their complexes are important coupling agents for such processes. Representative examples are the already mentioned Ullmann reaction, and the Ni(0)- and Pd(0)-catalyzed cross coupling of organometallics with organic halides (see below). Ring closure reactions of acyclic precursor molecules like e.g. diacetylenes, 1,4diketones, or 1,4-dithiins to form thiophene rings are also widely used to synthesize various types of oligothiophenes. These reactions are described in section 2.1.2.1.3. 2.1.2.1.1 Arene/arene-Coupling Methods by Oxidative Couplings Copper(IJ)-proinoted oxidutive couplirig The oxidative coupling of organolithium compounds with cupric chloride [38] was the first reaction that was found to be considerably more effective than the Ullmann analogous reactions utilized by Steinkopf [I91 and Sease rt al. [22] [Eq. (4)]. In 1930 Steinkopf reported that the reaction of (2-thieny1)magnesium bromide with CuCI: resulted in 2,2’-bithiophene H-T2-H 2. in a 44% yield [39]. Lipkin could increase the yield up to 50% [40]. Gronowitz [41] and later Kauffmann [42] and Kagan [43], further improved the yields of oligothiophenes by using the corresponding organolithium compounds rather than the Grignard

1 10

2 SuIjur-Containing Oligonzers

reagents. Lithiation of thiophenes can be effected either by simple deprotonation or by halogen-metal interconversion. Thus, the reaction of monothiophene H-TI-H 1 with n-butyl lithium (n-BuLi) followed by the addition of one equivalent of CuCI2 afforded H-TI-H 2 in 54% yield [41, 421. Application of two equivalents of CuCI2 even raised the yield to 8S%, indicating that the effect of the salt is more than catalytic [42]. Copper salts also surpass the effect of other transition metals: the use of catalytic amounts of cobalt chloride either with Li-TI-H 9 [44] or the corresponding Grignard derivative [45] led only to 30% and 26% formation of H-T2-H 2, respectively. The oxidative coupling of organolithium compounds is not restricted to bithiophenes, but can also be applied to higher, even-numbered oligothiophenes up to a-sexithiophene H-T6-H 6. While the coupling of H-Tz-H 2 to a-quaterthiophene H-T4-H 4 succeeds in 64% yield, the attempt of the repetitive coupling of the latter to result in a-octithiophene H-Tx-H 14 failed [42]. Probably the reduced solubility of the polyaromatic precursors in etheral solvents made the dimerization of H-Tx-H 14 to an a-sedecithiophene impossible [46]. Meanwhile, the coupling of much better soluble a-octi-(N-methylpyrrole) to the corresponding hexadecamer became possible [47].

9,15,16 (n=1,2,3)

1,2,3 (n=1,2,3)

2,4,6 (fl=1,2,3)

When lithiumdiisopropylamide (LDA) was used as base the yield of the oxidative dimerization of H-T2-H 2 to H-T4-H 4 was raised to 86%. In the same way, H-T6-H 6 was obtained from a-terthiophene H-T3-H 3 in 65% [48] and 73% yield [43] after purification. Nevertheless, lithiation of oligothiophenes always includes the problem, that with the use of equimolar amounts of base a mixture of the desired product and its dimer is always found. Thus, the reaction of Li-TI-H 9 with CuClz gave 41% of H-T2-H 2 and 30% of H-T4-H 4. The equilibrium obtained in a mixture of H-T2-H 2 and Li-T1-H 9 evidently favors the lithium salt of the Li-T2-H 15 [Eq. (S)]. This clearly indicates that the a-protons of a-oligothiophenes H-T, -H exhibit greater acidity compared to those of H-TI -H 1. The use of half equimolar amount of base, however, led to the nearly exclusive formation of the desired oligomer. The excess of unreacted H-TI-H 1 could mostly be recovered [43].

9

2

15

1

This method has furthermore been used to synthesize a quaterthiophene regioisomer. Thus, the reaction of 3,3/-bithiophene 11 with one equivalent of n-BuLi affords the 2-lithio compound 17. Coupling of the latter with CuCI2 led in 6% yield to 3,3': 2',2": 3",3"'-quaterthiophene 18 which includes one a-a-linkage and two P-P-linkages [Eq. (6)]. A crystallographic study showed that the thiophene

2.1 Oligotliiopheries

111

rings were not coplanar. The angle between the adjacent rings in the 3,3/-bithienyl moieties was found to be about 20” [49].

Cyclopolyarenes have also been synthesized in moderate yields by oxidative coupling of Grignard or lithium derivatives with halides of transition metals (CuCI2, NiCL) [50]. In this manner, Kauffmann et a/. described the only examples of cyclo(o1igothiophenes) reported so far [42, 46, 5 13. The required dilithiated thiophene species were obtained by metal/halogen exchange of the corresponding dibromobi thiophenes. Interestingly, both the coupling of 3,3’-dilithio-2,2/-bithiophene 19 and 2,2’-dilithio-3,3/-bithiophene 20 with CuCI? or FeCI3 afforded the cyclo(tetrathiophene) 21 in 23% and 24% yield, respectively [Eq. (7)]. The macrocyclic oligothiophene (cycloocta [ 1,2-b:4,3-b1:5,6-b”:8,7-b”’]tetrathiophene) 21 comprises two a-a-linkages and two /I-P-linkages between the thiophene rings involved. As side product a cyclohexathiophene was isolated in 4% yield in one experiment. 4,4’Dilithio-3,3’-bithiophene 22 was converted to the isomeric all-&linked cyclo(tetrathi0phene) 23 (cycloocta [1,2-c:4,3-c’:5.6-c”: 8,7-~”’]tetrathiophene)[Eq. (S)].

fi s

s

s

s 21

19

GS

s /

a

23

22

s

CUCI*

n-BuU

s

21

s

s 24

(9) LI 25

S

112

2 Sulfur-Containing Oligomers

Its structure was determined by X-ray crystallography [42, 511. The planes of the corresponding macrocyclic adjacent thiophene rings form an angle of 53.7" which is smaller than that in cyclotetrabenzene. Even the oxidative coupling of two macrocycles 21 with the system n-BuLi/CuCI2 can be achieved in 28% yield forming the corresponding bis-macrocycle 25 [Eq. (9)] [42, 511. The synthesis of the first all-a-linked cyclo(o1igothiophenes) are presently under way [52]. Finally, the oxidative coupling with CuC12 has also been used for the chemical synthesis of various polythiophenes. Thus the reaction of the bifunctional Li-T2-Li 26, obtained in 92% yield by deprotonation of H-T2-H 2, with two equivalents of n-BuLi, with cuprous chloride in anisole lead to poly(bithiophene) P2, an insoluble brown precipitate [Eq. (lo)]. After extraction, the polymer was obtained in yields ranging from 25 to 50%. Its 'doping' with AsFs afforded a polymer with a conductivity of 5 S cm-' which is somewhat lower than that determined for films grown electrooxidatively [53].

Chemical and electrochemical oxidative coupling Another synthesis of H-T6-H 6 is described in a patent where the chemical oxidation of H-T3-H 3 with iron(II1) chloride via the dimerization of the radical cation results in H-T6-H 6 in 84% yield [54]. In a very recent paper the synthesis of H-T8-H 14 from H-T4-H 4 is described by the same oxidation procedure, but no yield is given [55].This method is also used for the chemical polymerization of thiophenes to polythiophenes [54]. The related electrooxidative dimerization of H-T4 -H 4 and a-quinquethiophene H-T5-H 5 extended the series of a-conjugated oligothiophene up to H-T8-H 14 and a-decithiophene H-Tlo-H 27, respectively [Eq. ( 1 l)]. Since in this homologous row the increase in chain length goes along with a dramatic decrease in solubility, it is doubtful and not reported whether or not these derivatives were easy to purify and characterize [56].

"1 4,s (n=4,5)

14,27 (1745)

Oxidative coupling of organoboranes Kagan reported in 1983 the synthesis of the whole series of H-T,-H via organoboranes [26d]. The oligomers can be prepared separately in acceptable yields with organoborane reagents. To a solution of Li-T,-H 9, 15, 16, 28, the 9-methoxy derivative of 9-borabicyclo[3.3.Ilnonane is added and a corresponding boronated thiophene 29-32 is formed [Eq. (1 2)]. After neutralization with boron trifluoride etherate, a second Li-T,-H, which may differ from the previous one, is added to boranes 33-36 and the resulting complex 37-41 is oxidized with iodine in order to

2.1 Oligothiophenes

113

couple the two thiophene units attached to the boron moiety. Thus, 81% H-T2-H 2, 37% H-T3-H 3, 50% H-T4-H 4, 55% H-T5-H 5, and 59% H-T6-H 6, could be synthesized in a particularly simple procedure which allows the choice of reagent in each of the steps [Eq. (1 3)].

9,15,16,28 @-1,2.3,4)

,2.3,4) 33,34,35,36 @I

29,30,31,32 (p1.2.3.4)

33,34,35,36 (p1.2.3.4)

37,38,39,40,41@+q=2.3,4,5,6)

2,3,4,5,6 (fl=2.3.4.5,6)

Davies et al. synthesized H-T2-H 2 in 50% yield reacting the ethanolamine ester of di(2-thieny1)borinic acid with N-bromosuccinimide (NBS) as brominating agent. The investigation aimed at the use of the easily isolable borinic acids as intermediates in synthetic organic chemistry [57]. 2.1.2.1.2 Transition Metal Catalyzed Coupling Methods

The 'Ullmann reaction' Although several improvements to the original Ullmann procedure have been made, for instance: using DMF as solvent to avoid the formation of higher oligomers [58], using copper acetate instead of copper [59], or using directly organocopper derivatives [43, 601; the Ullmann biaryl synthesis is still best suited for the coupling of acceptor substituted and electron deficient arenes [61]. Also, the Ullmann reaction has recently been employed with great success in the synthesis of well-defined oligopyrroles from N-protected a,@-dibromo(oligopyrro1es)and elemental copper in D M F [62]. Mechanistic studies supported the evidence that arylcopper compounds are intermediates in the synthesis of biaryls [60, 6 11. This observation smoothed the way for the efficient synthesis of unsymmetric biaryls in a two-step procedure consisting, first, in the preparation of an, e.g. thienylcopper derivative followed by the treatment with different aryl halides. 2-Thienylcopper 42, for example, is prepared from the corresponding Li-T,-H 9 or Grignard derivative and a copper(1)halide and is then reacted with iodo- or bromoarenes in pyridine or quinoline. By this procedure e.g. H-T2-H 2 is formed in 42% yield by the reaction of Cu-TI -H 42 and 2-iodothiophene 8 [611 [Eq. (14)]. Analogously, 2-( p-nitropheny1)thiophene is obtained in 70% when Cu-TI-H 42 is treated with p-iodonitrobenzene. Note that the formation of undesired symmetric biaryl products is avoided [60, 61, 631.

9

42

8

2

1 I4

2 Sulfur-Contciining Oligotnrrs

Nickel- and palladium-catalyzed cross coupling reactions - the ‘Kumada reaction ’ The real breakthrough in oligothiophene synthesis was achieved when very effective transition metal catalyzed cross coupling reactions were applied to the C-C-bond formation between heterocyclic systems. Cross-coupling means the formation of C-C-single bonds on the basis of the reaction of an organometallic compound with an organic halide. The classical ‘Wurtz reaction’ includes a possible metalhalogen exchange and hence, the formation of homocoupled products. Unsaturated organohalides with CsP’-Hal bonds do not enter non-catalyzed C-C-bond formation. Although it has been known for a long time that catalytic amounts of transition metals induce the coupling of Grignard reagents with organic halides, the so-called ‘Kharash reaction’ was seldom employed in synthesis because complex mixtures of cross-coupling, homo-coupling, and disproportionation products were formed [64]. More attention was paid in 1971 to a publication of Kochi et al. who reported that soluble catalysts of silver, iron, or copper were very effective for the selective coupling of Grignard reagents with organic halides [65]. One year later Corriu et al. [66] and Kumada et al. [67] discovered independently that phosphine complexes of nickel catalyze the selective cross-coupling of Grignard reagents with aryl and alkenyl CsP2-halides.Murahashi found later, that also phosphine complexes of palladium exhibit catalytical activity in the reaction of alkyllithium compounds with haloolefins [68]. This opened up the possibility for the preparation of a wide variety of unsaturated organic compounds from two different organic halides [69] [Eq. (15)]. These reactions, which are described in more detail in the following paragraphs, allow the synthesis of even- and odd-numbered all-a-linked oligothiophenes and regioisomers with various chain lengths and substitution patterns [70]. R-MQX

+

R’-X’

IML2C$I [M=Ni.Pq

R-R‘

+ MgXX’

In 1982, at a time when much attention was already paid to conducting polymers, and polythiophene was first synthesized [8], an extensive report of Kumada et al. was published describing Grignard cross-coupling reactions including heterocyclic compounds, mainly thiophene and pyridine derivatives [71]. It was shown that some oligothiophenes are most effectively and conveniently prepared by the reaction of the Grignard reagent of 2-bromothiophene 42 with various bromo- or dibromothiophenes. By adding the Grignard solution to a mixture of the organic halide and catalytic amounts of nickel-complexes (0.1-1 mol.%) in ether solution, the reaction proceeds under very mild conditions and gives the coupling product in high yields. The ‘Kumada reaction’ has become the most frequently used method in the synthesis of various types of thiophenes. Since its mechanism is basically valid also for other reactions catalyzed by transition metals, it will be elucidated more thoroughly [69, 701. The catalytic cycle is depicted in Fig. 3. (1) In the introductory step, the dihalophosphinenickel L2NiX2 reacts with two equivalents of the Grignard reagent to form the intermediate bis-organo complex L2NiR2. Its formation is easily observed by the dissolution of the insoluble catalyst to a reddish-brown solution.

2. I Oligotliiophet ics

115

i

I RW

FT

&Ni(

I R bNi( R

R

/

RX’

Figure 3. Catalytic cycle of the organonickel complex promoted cross-coupling reaction of Grignard reagents with haloarenes by Kumada [69.70].

(2) On the action of an organic halide R’X’ the bis-organo complex L2NiR2releases the homo-coupling product R-R to form L2NiR’X’, the actual catalytic species. (3) The halogen X’ is readily replaced by the organic group R of another equivalent of Grignard reagent. The resulting complex L2NiRR’ comprises the two coupling components. (4) A second haloarene is taken up to form the fivefold coordinated intermediate L2Ni(. . .R’X’)RR’. (5) The catalytic cycle closes when the cross-couplinp product R-R’ is set free and the catalytic species L2NiR’X’ recovered. Due to the starting reaction in the catalytic cycle the homo-coupling product may be found to a certain extent which is typically around 0.5-1.5% [72]. Nevertheless. the reaction is usually very selective and gives the cross-coupling products in high yields. The reaction is applicable to various types of Grignard reagents (e.g. aryl, alkyl) and organic halides bearing a C,,,:-carbon (e.g. aryl, vinyl). The reactivity order of the halide component was found to be Ar-I > Ar-Br > Ar-CI > Ar-F. Grignard reagents are equally prepared either in E t 2 0 or in THF, however, the

116 Rap\

2 Sulfur-Containing Oligoriiers ,P%

M. ‘3 CI CI

4

Ph,P-(CH,),-PPh,

PA,:

PPh,

n-2, dppe n = 3 , dppp

dppf

43 (M = Ni, Pd)

Figure 4. Structures of organonickel and -palladium complexes which are active catalysts in crosscoupling reactions of Grignard reagents with haloarenes.

reaction proceeds considerably faster in Et,O than in T H F and the dehalogenation product is formed in larger amount in THF. For poorly soluble compounds, either the Grignard or the halogenated reagent of oligothiophenes, benzene works very successfully as cosolvent [73]. From the large variety of catalysts 43 examined for thiophene synthesis, Ni(dppp)C12 [74], Ni(dppf)C12 [75], and Ni(dppe)C12 [74] were the most effective (Fig. 4). However, in some cases where these Ni-catalysts are not very reactive or side reactions occur, the analogous Pd-complexes L2PdX2 gave much better results [71]. In general, the Pd-complexes are less reactive, but more selective. Rossi et al. discovered that depending on the ratio of the reacting compounds, the Pd(dppf)C12-promoted reaction of the Grignard reagents of 2- or 3-bromothiophene with dibromothiophenes results in either mono-coupling to synthetically very valuable bromobithiophenes or in a twofold reaction to terthiophene isomers [76]. Thus, H-T2-H 2 (9Ooh), 3,3’-bithiophene 11 (100% GC), H-T3-H 3 (80% GC), and H-T4-H 4 (64%) were synthesized by the reaction of the Grignard 42 with 2-bromothiophene reagent of 2-bromothiophene BrMg-TI-H Br-TI-H 44, 3-bromothiophene, 2,5-dibromothiophene Br-TI -Br 45, or 5,5’dibromobithiophene Br-T2-Br 46, respectively [Eq. (16, 17)] [71]. In a systematic study by Zimmer et al., some yields were increased (2: 81 %; 3: 86%; 4: 89%) and the series was extended to H-T5-H 5 (60-70% [49], 91% [77]). The synthesis of H-T6-H 6 failed due to the inherent insolubility of a,a-dibromoquaterthiophene Br-T4-Br 48 in ether or tetrahydrofuran (THF). Nevertheless, H-T6-H 6 could then be obtained in 56% yield by the ‘Kumada reaction’ of the Grignard reagent BrMg-T2-H 49 with Br-T2-Br 46 [Eq. (IS)]. However, the synthesis of monobrominated oligothiophenes, the precursors of the Grignard reagents, is still rather difficult (see section 2.3) and Br-T2-H 50 could finally be obtained by selective metal/halogen exchange of Br-T2-Br 46 with one mole of n-BuLi and successive aqueous work-up [77].

42

45,46,47,40 In=1,2,3,4)

3,4,5 (nd.2.3)

2.1 Oligothiopherie.~

49

46

117

6

Naarmann et 01. used exactly the same reactions to synthesize the whole series of a-oligothiophenes up to H-T6-H 6. No yields are given, but the authors paid very much attention to the purification procedure and the physical constants of the various thiophene oligomers [12]. The use of iodo compounds proved to be even more effective, since H-TI-H 2 was obtained by the nickel-catalyzed coupling [Ni(dppe)C12]of I-T1-H 8 and its Grignard derivative in 90% yield [78]. The fourteen regioisomers of terthiophene, which exhibit an interesting photoenhanced toxic activity, have been satisfactorily synthesized mainly by nickel- or palladium-catalyzed Grignard cross-coupling reactions. The synthesis of these isomers has been reviewed very thoroughly [37b,c] and their significance with respect to conjugated materials is rather incidental. Table 3 (section 2.1.2.1.4) gives only a short overview including yields and physical properties. Generally, the yields are somewhat higher for the phosphine complexes of nickel as catalyst than for the palladium derivatives. The Ni-catalyzed homo-coupling of 2-bromo-3,3'-bithiophene to form the ,&a,,& linked quaterthiophene isomer 18 also proved to be far more effective (84% yield) than the oxidative coupling of the corresponding lithiated bithiophene 17 with CuC12 (6% yield; see section 2.2.1) [49]. A method originally developed by Colon and Kelsey for the symmetric coupling of arenes was used for the synthesis of even numbered a-oligothiophenes [79]. The catalyst is prepared in sit24 by the reduction of NiClz in the presence of PPh3 or Ni(PPh3)?CL2with zinc in DMF as solvent. Thus, Br-T,-H 44. Br-T2-H 50, and the corresponding terthiophene derivative Br-T3-H 51 have been coupled to H-T2-H 2. in 41% [80], to H-T4-H 4 in 66% [81] and 87% [82], and to H-T6-H 6 in 48% yield [12, 811, respectively [Eq. (19)]. Compared to the oxidative coupling of the corresponding lithiated oligothiophenes to the a-oligothiophenes 2 85% [42], 4 86% [43], 6 73% [43] the yields in this type of reaction turn out to be somewhat lower. With this procedure, however in only 18% yield, even H-Ts-H 14, could be obtained from 5-bromo-n-quaterthiophene Br-T4-H 52 which was synthesized from H-T4-H 4 in 51% yield [81]. Besides H-Tlo-H 27 whose existence could not be proven unequivocally [57], H-Ts-H 14 is the highest member of a-oligothiophenes synthesized so far. The octamer is practically insoluble in all solvents which prevents closer examination. e.g. of its UV spectrum in solution. However, a UV spectrum of an evaporated thin film of H-Tg-H 14 could be investigated [56].

44,50,51,52 (n=1,2,3.4)

2,4,6,14 (fl=1.2.3.4)

Finally, both methods of Ni(0)-catalyzed reactions were also used for the chemical synthesis of various polythiophenes. For instance, reaction of the bifunctional

118

2 Sulfur-Containing Oligornrrs

Br-T,-Br 45 either with Ni(dppe)CI2 and Zn in HMPT at 150°C or with Mg and Ni in T H F led to polythiophene P1 as insoluble powders [Eq. (20)] [83].

45

PI

A limitation of the applicability of the nickel-catalyzed reactions may occur either when the heterocyclic ring does not effectively add the Grignard reagent, or the halide does not give efficient halogen-magnesium exchange which then would lead to symmetric by-products [84]. The most serious limitation of the reactions described is, however, that substituents on both the organic halide and the Grignard reagent, are restricted to those which do not react with Grignard reagents. Thus e.g. the reaction of carbonyl groups with Li- or Mg-reagents leads to side products [71, 851. In order to circumvent this problem, either protected components [86] or, more versatile, organo derivatives of less electropositive metalloids may be used. The cross-coupling reactions especially for aryl/arylcouplings became applicable to compounds with a wide variety of functional groups by using less reactive, since less nucleophilic, tin-, boron-, or zincorganyls, in combination with appropriate palladium catalysts. These metal-organic arylating agents are moreover advantageous since they are relatively stable against oxygen and water and may be synthesized from the corresponding lithiated derivative and purified seperately. Copper-, zirconium-, or aluminum-organic compounds are less favorable since their synthetic availability is restricted or low catalytic activity is found. The ‘Stille reaction’ The palladium-promoted coupling of organotin compounds turned out to be especially fruitful because of the possible use of various organic electrophiles, the mild reaction conditions, the regioselectivity, and the tolerance of many functional groups (e.g. C 0 2 R , CN, OH, CHO, NOz). General aspects of this so-called ‘Stille reaction’ and of its applicabilty have been extensively reviewed [87]. With respect to heterocyclic compounds the literature has been compiled up to 1991 [88]. In the ‘Stille-type reaction’ the organometallic component R’SnR’j may be synthesized from the corresponding lithiated derivative R’Li by the reaction with trialkylstannylchloride R”SnX or by the reaction of organic halides R’X with hexaalkyldistannane R’jSn-Sn3R’’ [89]. R’ may be alkinyl, alkenyl, aryl, benzyl, or ally1 and R” methyl or butyl which are typically not transferred [Eq. (21)]. In general, aryliodides, -bromides, and -triflates add even at moderate temperatures to the Pd(0)-complex, whereas arylchlorides must be activated by electron withdrawing substituents. IP%I R-X

+

R-SnR’,

___+

R-R’

+ R;SnX

Pd2(dba)3. As a rule, catalytically active complexes of Pd(0) are derived from Pd(I1) in situ. The catalytic cycle is basically similar to that of the Ni-catalyzed ‘Kumada reaction’. It involves the conversion of Pd(0) to Pd(II), begins with the oxidative addition of the electrophilic reagent R-X to Pd(0). continues with transmetalation of R’ from the tin to the palladium compound, and closes with the reductive elimination of R-R’ from the palladium complex [Eq. (22)] [SS]. XSnW3

R‘SnR3

R-X

Pd4,

__*

L2RPdR’

L2PdRX

t

isom.

LZPdRR’

I

(22)

4 R-R‘

An acceleration of the reaction rates has been achieved by the use of AsPh3 or P(furyl)3 as ligands [90] or cocatalytic Cu(1)- [91] and Ag(1)-species [92]. With respect to oligothiophenes the ‘Stille reaction’ has mostly been applied to the synthesis of substituted oligothiophenes (section 2.1.2.2). However, Crisp describes the synthesis of H-T2-H 2 (80%) and H-T3-H 3 (61 YO)by the Pd(PPh3)?C12catalyzed coupling of tributyl(2-thieny1)tin 53 with [-TI -H 8 and 2.5-diiodothiophene I-T, -I 54, respectively [Eq. (33)]. [93]. The yields are somewhat lower than for the corresponding nickel-catalyzed coupling reactions. Reaction of the organotin compound 53 with Br-T1-Br 45 and Pd(PPh3)4 as catalysts yields H-T,-H 3 in 59% yield [82].

53

54

3

The ‘Stille reaction’, however, is limited by a side reaction which has sometimes been observed. The detection of coupling products including a phenyl group derived from the palladium catalyst indicates a rearrangment in which a phenyl group of the phosphine ligand migrates to the metal. A systematic investigation showed that the use of triarylphosphines with electron withdrawing substituents prevents this side reaction [94]. The ‘Suzukireaction’ Suzuki [85a] and Miller [95] have described the palladium-catalyzed coupling of various bromobenzene derivatives with benzene boronic acid or its cyclic esters. These cross-couplings which use the less electropositive boron instead of magnesium or tin proceed with good to excellent yields even in sterically demanding positions. Gronowitz et al. modified the palladium-catalyzed ‘Suzuki coupling’ so that it became useful to the heterocyclic series [96]. This carbon-carbon coupling method, as does the ‘Stille-coupling’, tolerates a variety of functional groups both in the organometallic reagent and in the heterocyclic halide. In the series of unsubstituted oligothiophenes, 2-thiopheneboronic acid 55, easily prepared from Li-TI -H 9 and boronic acid trimethylester, reacts with Br-TI -Br

120

2 Sulfur-Contaitring 0ligomer.r

45 to H-T3-H 3 in 40% yield [Eq. (24)]. Without any necessary precautions against oxygen, the reaction is performed in a mixture of DME and aqueous sodium bicarbonate, using [Pd(PPh3)4] as catalyst [96]. Similarly, 3,3’-bithiophene 11 and 2,3’-bithiophene 12 were synthesized by Gronowitz et al. using 2- and 3-thiophene boronic acid as organometallic component [97, 981. 3,2’: 5’,3’’terthiophene 56 has been obtained in 48% yield from 3-thiopheneboronic acid and Br-T1-Br 45 [96].

55

45

3

Typically, an excess of 20% boronic acid is used in the coupling reaction. Less excess results in the formation of mono-coupled by-products which are difficult to separate. A typical side reaction that may occur is deboronation. Gronowitz observed the greatest tendency for deboronation in the case of electron rich heteroaromatics [97]. Only few examples are known in which organozinc or organomercuric derivatives instead of the more reactive Grignard reagents are used in palladium catalyzed heteroaryl/heteroaryl-coupling reactions. In most cases, they are used for couplings of substituted thiophenes [99] or mixed thiophene/(hetero)arene compounds [ 1001. The various examples of mixed heteroarene compounds, including oligomers of thiophenes combined with pyrroles [loll, furans [102], pyridines [98b, 1021, or pyrimidins [99, 103, 1041, are not considered in detail in this chapter. 2.1.2.1.3 Ring Closure Reactions from Acyclic Precursors

Aside from the arene-coupling methods, oligothiophenes can also be synthesized by ring closure reactions from acyclic precursor molecules. Since there have been many reports using ring closure methods for the synthesis of mainly bi- and terthiophene derivatives [37], in the following only those examples which are related to conjugated materials are described.

Aromatization of tetvahydrotliiophenes (thiophanes) One of the first reports using the aromatization of saturated precursor molecules was the preparation of 3,3’-bithiophene 11 from the tetrasodium salt of 1,2,3,4butanetetracarboxylic acid by twofold cyclization with phosphorous hexa- or heptasulfide in only 5% yield [36]. Gronowitz et al. improved the yield to 17% by reaction of 3-ketotetrahydrothiophene with 3-thienyllithium followed by dehydration and aromatization of the tertiary alcohol with chloranil [105]. In the following, dehydration procedures were used for the synthesis of several terthiophene isomers, however, always in low yields. A similar reaction of 3-ketotetrahydrothiophene with the Grignard reagent of 5-iodo-2,2’-bithiophene affords a tertiary alcohol in 8 1% yield. Nevertheless, the aromatization to 2,3’:4’,2”-terthiophene proceeds less effectively (18% yield) [106]. This is also the case for the reaction of 5-(2-thienyl)-3-ketotetrahydrothiophene57 with BrMg-T1 -H 42 which results

121

2. I Oligo tli iophenes

in 2,2’:4’,2”-terthiophene 59 in 19% after aromatization of the tertiary alcohol 58 [Eq. (251 [1071.

42

59

1

59

60

The same terthiophene isomer 59 has been obtained by the treatment of thiophene with orthophosphoric acid under mild conditions. The trimer 60 comprising a saturated central tetrahydrothiophene ring was successively oxidized with chloranil to form the terthiophene 59 [Eq. (26)] [log]. Cylizatioii of I ,4-diketones Another more convenient pathway for the synthesis of thiophenes proceeds via 1,4diaryl-substituted 1,4-diketones which are cyclized to thiophene rings by treatment with hydrogen sulfide and hydrochloric acid. Some methods for the synthesis of thiophene-substituted 1,4-diketones begin with the transformation of acetylthiophene. 2-Acetylthiophene 61 is reacted in a ‘Mannich reaction’ with formaldehyde and dimethylamine to yield the corresponding Mannich base 62 in 70% yield. The Mannich base 62 is then subjected to a ‘Stetter reaction’ [ 1091 which results in 1,4-di-(2’-thienyl)-1,4-butanedione 63 in 70% yield via the cyanhydrine of 2-thiophenecarbaldehyde [ 1 101. Reaction of Mannich base 62 with the isomeric 3-thiophenecarbaldehyde under the same conditions results in 1-(2’thienyl)-4-(3‘-thienyl)-1,4-butanedione64 in lower (35%) yield [Eq. (27)] [110].

Q

cno

wNMez NaCN I DMF

HCHo HNMe,

%CH3 S

0 01

03

0

cno

(27)

02 A

NaCN I DMF

Another approach to 1,4-diketones is the oxidative coupling of the lithium enolate of 2-acetylthiophene 61 with CuCll in DMF. 1.4-Di-(2’-thienyl)- 1.4-butanedione 63 is formed in 85% yield [ l 1 13.

122

2 Sulfur-Contuining Oligomer.7

Similarly, the silyl ether of 2-acetylthiophene 65 obtained by the reaction of 61 with trimethylsilychloride in DMF is oxidized with A g 2 0 to yield the dithienyl1,4-diketone 63 in 71% yield [Eq. (28)] [112]. (-&,0SiMe3

,

PhlO

BF,

Me,SiCI

. EGO

CH,

0 65

61

63

The isomeric 1,4-di-(3/-thienyl)-l,4-butanedione 68 is synthesized in 60% yield by oxidation of the silyl ether of 3-acetylthiophene 66 with phenyl periodate and BF3-etherate [Eq. (29)] [I 131.

JCHa

s

LDA

Me,SiCI

66

‘acH2 ws ___, PhlO BF3. EbO

(29)

S

67

68

The cyclization of the 1,4-dithienyl-substituted1,4-diketones to oligothiophenes proceeds by treatment with H2S and HC1 (classical Paal-Knorr synthesis [114]). Phosporous(V) sulfide or Lawesson’s reagent (L.R.) are also used as sulfurization reagents. H-T3-H 3 was obtained from 1,4-bis-(2’-thieny1)-1,4-butanedione63 and H2S/H+ in 70% yield [115], with P2S5 in 66% yield [110, 1121, and with L.R. in 85% yield [110], respectively [Eq. (30)]. The isomeric 3,2’: 5/,3”-terthiophene 56 is similarly synthesized by the ring closure of 1,4-di-(3/-thienyl)-1,4-butanedione 68 with P2S5 in 75% yield [Eq. (31)] [113]. In the same way the unsymmetric 1(2’-thienyl)-4-(3/-thienyl)- 1,4-butanedione 64 is closed to 2,2/: 5/,3”-terthiophene 69 in 84% yield [Eq. (32)] [l lo].

@&2

HpS / HCI M

P2S5

or L.R.

63

3 ‘2’5

68

56 ‘2%

~

64

I

Lawesson’s Reagent

69

I

(32)

2.1 Oligothioplieiies

123

More recently Ellinger and Merz described the synthesis of H-T3-H 3 and H-T5-H 5 via acylation by the FriedelLCrafts method. Thiophene H-TI -H 1 and H-T2-H 2 were converted into 1,4-di(2’-thienyl)-lq4-butanedione 63 and the corresponding bis(dithieny1)-substituted diketone 70 by acylation with succinyl chloride and aluminum chloride in 55% and 25% yield, respectively [116]. Reaction of the diketones 63 and 70 with L.R. gave H-T3-H 3 and H-T5-H 5 in high yields (87%, 92%) [Eq. (33)]. However, the Friedel-Crafts route was not successful for higher homologs since analogous acylation of H-T3-H 3 with succinyl chloride resulted in the formation of 5,5’-bis-(2,2’:5’,2”-terthien-5-yl)tetrahydrofuran-2-one 71. The formation of this cyclic lactone may be explained via rearrangement of the intermediate 4-OX0acid.

a”

n

Cl-(

a:+,

)-Cl

0 0

H

H-

H

1,2 (n4.2)

63,70 (npl.2)

3,5 (n=l,2)

(33)

71

Cjdiiarion of diacetj,lenes The cyclization of thienyl-substituted diacetylenes with hydrogen sulfide or sodium sulfide is also a very successful method for the synthesis of oligothiophenes. The key point here is the synthesis of the precursor diynes. Symmetrical diacetylenes can be obtained by the oxidative coupling of acetylenes with copper(1) or copper(I1) salts (‘Glaser, Hay, or Eglington coupling‘). The synthesis of unsymmetric diynes proceeds best via the ’Cadiot-Chodkiewicz procedure’ in which an acetylene is reacted with an acetylic halide in the presence of copper(1) salts [117].Therefore, thienylsubstituted acetylenes and bromoacetylenes are important starting materials. Ethinyl-substituted (o1igo)thiophenes are e.g. obtained by the Wittig reaction of the corresponding aldehyde 72-74 with tetrabromomethane and triphenylphosphine which results in 1,l-dibromoethylene substituted thiophenes 75-77. Successive elimination of HBr and halogen/metal exchange with rr-BuLi leads, after aqueous work-up, to the desired acetylenes 78-80 in good overall yields [Eq. (3411”2W.

72,73,74 (fl=1.2,3)

75,76,77 (fl=1.2.3)

78,79,80 (nsl.2.3)

Another procedure starts from I-TI-H 8 which is reacted with the Grignard reagent of bromo( trimethylsi1yl)acetylene and [Pd(PPh3)4]as catalyst to yield a TMS-protected thienylacetylene 81. Deprotection is effectively feasible by treatment

124

2 Sulfur-Contuining Oligomers

with base and results in the desired thienylacetylene 78 in good overall yield (84%) [Eq. (35)1 WI. BrMgC3CSIMe,

6’

KOH or F

[Pd(PPh3)J

H

81

8

(35)

SiMe, 78

2-Ethinylthiophene 78, 5-ethinyl-2,2‘-bithiophene 79, and 5-ethinyl-2,2’:5/,2”terthiophene 80, have been dimerized with cuprous or cupric chloride under ‘Glaser conditions’ to the corresponding diacetylenes 82 in 87% [SO, 1181, 83 in 73% [26d], and 84 in 99% yield [26d], respectively [Eq. (36)]. Similarly, 3-ethinylthiophene 85 gives the corresponding 1,4-di-(3/-thienyl)-1,3-butadiyne 86 in 73 YO yield [Eq. (37)] [106]. The unsymmetric 1-(2’-thienyl)-4-(3’-thienyl)-1,3-butadiyne 87 is obtained by coupling 3-ethinylthiophene 85 and bromo(2-thieny1)acetylene under ‘Cadiot-Chodkiewitz conditions’ in 91 YO yield [Eq. (38)] [ 1061. Similarly, 1-(2’thieny1)-4-(2,2’-bithien-5-~1)1,3-butadiyne 88 is synthesized by the coupling of 5-ethinyl-2,2’-bithiophene 79 and bromo(2-thieny1)acetylene in 96% yield [Eq. (W1 WdI. cu+cf C$+ TMEDA 78,79,80 (n=l,2.3)

- 82,83,84 (n=1.2,3)

Cu+ or Cuz+

(37)

TMEDA 86

86

88

87

WH

(39)

\ I 79

88

Organoboranates of the type (T, -BR2 -T,)- Li’ 37-41 have already been mentioned with respect to their significance in the preparation of oligothiophenes (see above). Similarly, the coupling of two acetylenes to a butadiyne via an organoborane intermediate which is treated with iodine can be utilized to synthesize thiophene-substituted diacetylenes. Kagan et al. reacted 5-(lithioethyny1)2,2’-bithiophene 89 successively with 9-borabicyclo[3.3.llnonane and BF3-etherate

2.1 Oligothiopheties

125

to form borane 90 and 2-(1ithioethynyl)thiophene to yield the above mentioned unsymmetric 1 -(2’-thienyl)-4-(2,2’-bithien-5-yl)1,3-butadiyne 88 in 69% yield in a one pot procedure [Eq. (40)] [26d]. R

09

90

The final step in the synthesis of oligothiophenes is now the cyclization of the thienyl-substituted diynes with hydrogen sulfide or sodium sulfide. 1,4-Di-(2’thienyl)-l,3-butadiyne 82 was cyclized to H-T3-H 3 with H2S in ethanol by Schulte et al. in 51 YOyield [ I 191, and with NazS in methanol by Kagan rt ul. in 84% yield [118]. H-TS-H 5 and H-T7-H 7 were synthesized by cyclization of the corresponding 1,3-butadiynes 83 and 84 in 74% and 98% yield, respectively [Eq. (41)]. H-T4-H 4 could be obtained in quantitative yield from the unsymmetric 88 [Eq. (42)] [26d]. Terthiodiyne, 1-(2’-thienyl)-4-(2,2’-bithien-5-yl)-l,3-butadiyne phene isomers 69 and 56 are formed by the reaction of 1-(2’-thienyl)-4-(3’-thienyl)1,3-butadiyne 87 [119] and 1,4-bis-(3’-thienyI)-1,3-butadiyne86 with sodium sulfide in 87% and in quantitative yield, respectively [Eqs. (43. 44)] [80, 1181.

02,03,04 (n=12.3)

3,5,7 (n.l.2.3)

88

07

86

4

69

56

126

2 Sulfur-Containing OligomerA

Oligothiophenes ,from I ,4-dithiins Nakayama et al. used thienyl-substituted 1,4-dithiins which are obtained from easily accessible diketosulfides for the preparation of a-oligothiophenes and isomers up to the heptamer [37b, 1201. The dithienyldiketosulfides 91-93 are prepared by the reaction of chloroacetyl-substituted (0ligo)thiophenes and sodium sulfide in almost quantitative yield and are further cyclized to the corresponding 1,4-dithiins 94-96 with L.R. in 60% yield. The extrusion of sulfur from the dithiin moiety via ylide intermediates is achieved by refluxing the dithiin in o-dichlorobenzene and results in a mixture of two possible isomers [Eq. (45)]. In the case of 2,6-di-(2/thienyl)-l,4-dithiin 94, a ratio of 13: 1 of H-T3-H 3 and the 2,3’:4’,2/1-isomer97 in 85% yield is obtained. The separation of the compounds by recrystallization turns out well since the a$-connected terthiophene is better soluble in hexane. Oxidation of the dithiin with m-chloroperoxybenzoic acid and extrusion of SO from the resulting sulfoxide in the presence of DMSO afford a mixture of H-T3-H 3 and the isomer 97 in a ratio of 22: 1 and in a total yield of 90% [120].

\& 91,92,93 (n=1.2,3)

94,95,96 (m1.2.3)

3,5,7 (k1-3)

100

(45)

97,98,99 (n=1.2.3)

59

The synthesis was extended to the corresponding penta- and heptathiophenes. Thus starting from the chloroacylated bi- and terthiophene, the corresponding diketosulfides 92 and 93 are both obtained in 98% yield. Cyclization with L.R. and extrusion of sulfur results in H-T5-H 5 and H-T,-H 7 in an overall yield of 33% and 38%, respectively. In these cases, the amount of isomeric by-products 98 and 99 are small (<2%). Interestingly, the 2,5-di-(2/-thienyl)-1,4-dithiin 100 which forms only one ylide intermediate gives the 2,2’: 4’,2”-terthiophene 59 in 12% yield upon heating to 200°C [Eq. (46)] [107]. Various diketosulfides 91-93 were also transformed by Nakayama and coworkers in a ‘McMurry reaction’ to the corresponding cis-hydroxydithiolane 101 or dihydrothiophenes 102 and 103. The reaction proceeds via intramolecular reductive coupling released by a low-valent titanium reagent (83-89% yield).

2.1 Oligothiophenes

127

Acid-catalyzed dehydration of hydrothiophenes 101-103 or oxidation with dichlorodicyanoquinone (DDQ) give rise to the isomeric oligothiophenes 97-99 including a central thiophene ring which is connected in the ,&positions to other thiophenes (83-95% yield) [Eqs. (47, 48)] 11211.

91

101

97

DW

DlOXanS

92,93 (n=2,3)

H

102,103 (n=2,3)

When the complete reaction sequence is applied to 2,3’: 4’,”’-terthiophene 97, the a,P-linked septithiophene 106 which includes alternating a$- and /?,a-bonds is obtained in excellent overall yield (62%) via the diketosulfide 104 and the dihydrothiophene 105 [Eq. (49)] [121]. In contrast to the isomeric H-T7-H 7 which is red and only poorly soluble in organic solvents, the heptamer 106 is a white crystalline solid, easily soluble, and exhibits a melting point 200°C lower than its linear counterpart. The attempted synthesis of a corresponding pentadecamer from the isomeric septithiophene 106 led to a glassy solid which could not be characterized.

____,

1)CICH,COCI

2) Na,s

I \ S

w:w S

07

I \

I \

104

Tiii,/Zn THF. r.t.’

S

(49)

101

108

It can be concluded that a-oligothiophenes are available by many powerful methods. For bi- and terthiophenes not only the a-linked, but also the remaining regioisomers have been prepared, whereas for the higher homologs only few oligothiophenes comprising a,/?-or /?,/?-bonds have been synthesized so far. The next section summarizes the physical constants of oligothiophenes. The last paragraph gives experimental details of some selected examples. 2.1.2.1.4 Physical Properties of a-Oligothiophenesand Isomers

Modern chemistry can describe the electronic and structural features of conjugated materials in general and of oligothiophenes in particular more specifically. The

128

2 Sulfur-Contuining Oligomers

Figure 5. Mesomeric resonance forms of polyenes, oligothiophenes, and oligo-p-phenylenes.

general structural principle of these systems is represented by a one-dimensional chain comprising conjugated double bonds which may also include heteroatoms. Therefore, oligo- and polythiophenes can be regarded as conjugated chains consisting of sp2p,-carbon atoms which have an analogous structure to cis-polyacetylenes and are stabilized by sulfur atoms. The capability of a conjugated system to transduce electronic effects depends on the delocalization of the charge carriers which are created along the molecular chain due to electronic or optical excitation. Delocalization of charge carriers is represented by resonance structures. In the case of conjugated systems an aromatic and a quinoid structure can be drawn (Fig.5). Among conjugated chains, resonance is most pronounced for polyenes. Due to the energetic adequacy of the resonance forms, polyenes have a degenerate ground state. In contrast, r-systems consisting of linked aromatic or heteroaromatic units exhibit a reduced conjugation between these moieties. This is on one hand caused by aromaticity and on the other hand by the reduced planarity due to the steric hindrance of o-hydrogen atoms. Therefore, in the case of aromatic systems like oligo- and polythiophenes, the quinoidal resonance form is of only minor significance and leads to non-degenerate ground state [ 1221. A diagnostic criterion for the efficiency of delocalization is the bond length alternation [ 1231. Furthermore, physical properties directly correlated to conjugation phenomena are optical transitions, redox potentials and nonlinear optical effects. The physical properties (melting point, absorption, fluorescence, and oxidation potentials) of the various oligothiophenes are summarized in Table 1. The highest melting points found are noted in the table. With increasing chain length of the oligothiophene the melting point increases as expected. With respect to the electronic properties, the longest wavelength absorptions [ 131, emissions [ 1241 and also the oxidation potentials [12,77] gradually shift to lower energies with increasing number of thiophene rings. This reflects, as expected, an increasing conjugation which is also observed by the colors of the homologous row. The color deepens from colorless for bithiophene 2, to pale and chrome yellow, to orange, to red, and finally to wine red for septithiophene 7 and octithiophene 14 [81]. However,

-7. I Oligotliioplienes

129

only a rough correlation of the transition energies with the (inverse) chain length can be determined. This type of correlation is predicted theoretically [ 1351 and confirmed in several oligomer series [13]. The optical data and oxidation potentials of the higher hornologs are rather unreliable due to their very low solubility. Nor are the redox data of the smaller members in the series precise, since the cyclic voltammograms are irreversible due to rapid follow-up reactions of the radical cations created. Therefore, precise determination of the redox potential is difficult. Nevertheless, a reasonably linear relationship of the oxidation potential versus the inverse chain length was obtained by Diaz et nl. for oligothiophenes H-TI -H 1 to H-TS-H 5, oligophenylenes and oligopyrroles (Fig. 6) [126]. The values for polythiophene, however, are clearly blue-shifted with respect to the extrapolated potential of an (hypothetic) infinite chain length. Despite these drawbacks, Fichou ef ul. were able to determine absorption spectra and magnetic properties of the radical cations and dications, respectively, of H-Ts-H 5 and H-T6-H 6 in highly diluted solutions [127]. The charged oligothiophenes are considered to be ideal model compounds for the investigation of the charge carriers responsible for the charge transport in conducting polymers. ESR investigations of radical cations and anions of H-T2-H 2 to H-T5-H 5 reveal that the oligomers exist not exclusively in an all-tram conformation of the thiophene rings [ 1281. Absorption spectra of oligothiophene radical cations were also obtained by oxidation of the corresponding oligothiophenes in the channels of zeolites where the subsequent reaction of the reactive cationic species is suppressed by the local environment. However, only mixtures of cations were created in the presence of large excess of unreacted neutral species. so that assignment of the absorption bands is rather difficult [32b, 1291. The development of rectifying Schottky diodes and field effect transistors (FET) which contain thin films of oligothiophenes as the active semiconducting component, has stimulated many investigations of the solid state properties of oligothiophenes. Investigations on silicon dioxide/oligothiophene FETs clearly reveal that the charge transport properties of the oligothiophenes correlate with the inverse chain length [ 1301. Thus, the conductivity and mobility increase gradually with increasing chain length of the oligomer and considerably exceed those of polythiophene [ 1311. The photophysics of ter- and quaterthiophenes were thoroughly investigated against the background of their phototoxic activities [ 1321. Transient absorption spectra which are identical to those of the corresponding radical cations and fluorescence kinetics in the row H-T2-H 2 to H-Ts-H 6 exhibit an increasing fluorescence life time with increasing chain length [ 1331. Fluorescence spectra and quantum yields of thin films of oligothiophenes reveal clearly that in the solid state the fluorescence efficiency is dramatically diminished by more than three orders of magnitude [ 1341. Due to their extended conjugated system, oligothiophenes are also promising materials for non-linear optical (NLO) applications [16]. In this respect, it was shown that the third order susceptibilities correlate with the chain length. For example, H-T6-H 6 exhibits a Xj-value which is higher than this of H-T3-H 3 by a factor of ten [135]. The electronic structure of oligothiophene films has also been investigated by electron loss spectroscopy [136], UPS and XPS [137].

+

Table 1. Preparation and physical properties of a-oligothiophenes H-T,-H.

w

0

Method of preparation (paragraph)

Yield ["/.I

2.1.2.1.1' 2.1.2.1.1' 2.1.2.1.1e 2.1.2.1.2' 2.1.2.1.2h 2.1 .2.1.1e 2.1.2.1.1' 2.1.2.1.2s 2.1.2.1. l e 2.1.2.1.2' 2.1.2.1.2g 2.1.2.1.2' 2.1.2. I.le 2.1.2.1.2'

26 30 41 41 42 44 50 50 54 80 81 81 85 90

HPT3-H 3

2.1.2.1.2g 2.1.2.1.2k 2.1.2.1.3" 2.1.2.1.3m 2.1.2.1.2' 2.1.2.1.2' 2.1.2.1.3'" 2.1.2.1.3O 2.1.2.1.3" 2.1.2.1.2' 2.1.2.1.3" 2.1.2.1.3"

37 40 51 55 59 61 66 70 70 80 84 85

H-T4-H

2.1.2.1.1g 2.1.2.1.1' 2.1.2.1.2' 2.1.2.1.2'

55 64 64 66

Oligothiophene

inT) H-T2-H

2

4

Ref.

m.p." ["CI

Ref.

Absorption ~'",,b(lg€1 [nml

Fluorescence ~ m a xbmIC

390 (4.66)

437, 478

Oxidation potentials

PId

0.97 (0.12)

h,

H-TS-H

5

H-TB-H 6

2.1.2.1. I C 2.1.2.1.2' 2.1.2. I.?' 2.1.2.1.3" 2.1.2.1 .3m 2.1.2.1 .1g 2.1.2.1.3" 2.1.2.1.2' 7.1.2.1.3" 7.1.2.1.2' 2.1.2.1.2' 2.1.2.1.2' 7.1.2.1.1g

H-T7-H 7 H-Tg-H H-TI0-H

14

27

2.1.2.1 .I5 2.1 2.I . I r 2.1.2.1 .I' 2. I .2.1.3" 2.1.2.1.1g 2.1 . ? . I , I " 2.1.2.1. I' 2.1.2.1.2' 2.1.2. I . I'

86 87 89 99 25 55 56 60-70 74 91 48 56 59 65 73 84 38 98

[431 [821 [771 [26d] 11 161 [26d] [120b] 1481 P6dl [771 [12, 811 [771 [26d] [481 1431 j54j [ 120bl

258- 259

[i21

416 (4.74)

482, 514

0.70 (0.08)

307-309

[i21

432 (-4.78)

510"

0.46 (0.04)

328

[ 120b]

440q

522. 560

-

18 -

~~~

Highest melting points are given. Maximal absorptions and extinction coefficients in CHCI, (lit. 77). Fluorescence maxima in dioxaneiacetonitrile at 298 K (lit. 124). Irreversible oxidation potentials. differential pulse voltammetry in CH3CN/TBABF vs. Ag/AgCl (lit. 77); in parentheses cyclic voltammetry in propylencarbonat/LiC104 (0.5 M) vs. AgiAg' (lit. 12). Chemical and electrochemical oxidative coupling. ' Copper(l1)-promoted oxidative coupling. 'Ulirnann coup~ing'. Oxidative coupling of organoboranes. ' 'Kuniada coupling'. 'Stille coupling'. Aromatization of thiophanes. 'Suzuki coupling'. "' Cyclation of I .4-diketones. " Predicted Cyclation of diacetylenes. value based on a 1/11 vs. E,,,,, plot. From 1,Cdithiins. Not reliable due to low solubility.

' '

J

Id

I

2 2' %

3

=.

2 2

132

2 Sulfur-Containing 0ligorner.s

Figure 6. Plot of oxidation potentials (Epa)versus inverse chain length (1 / n ) for aromatic oligomers [126].

Since for electronic and optical applications the solid-state structure of the conjugated materials and the interface of the semiconductor to the (metal) substrate are crucial, the bulk and surface properties of oligothiophenes were investigated by several methods. Angle dependent and polarized absorption and IR-measurements on sexithiophene single crystals elucidate the spatial arrangement of the molecules [ 1381. Optical measurements on oriented monolayers of various oligothiophenes reveal a different orientation of the molecules depending on the substrate surface [134]. XRD measurements show that H-T6-H 6 forms a liquid crystalline mesophase at higher temperatures [ 1391. Crystalline charge-transfer complexes are formed by the combination of oligothiophenes with TCNQ [I401 or heteropolyanions [ 1411. In the former case the variation of the oligothiophene components do not affect the relatively low conductivities (lo-'' to Scm-'). The conductivity of a H-T6-H/PMol20;i charge-transfer complex, however, is notably high (2.7 S cm-I). Interestingly, oligothiophenes are oxidized as well in the solid state, as in the gas phase. The simultaneous evaporation of H-T6-H 6 and FeC13 results in thin films whose electronic structure corresponds to the one of 'doped' a-sexithiophene [142]. Table 2 also gives the data for the regioisomeric and cyclic oligothiophenes described above. For the isomeric oligothiophenes which include a,P- or P,P-bonds

133

2.I Oligoth iopheiies

i

2

rQI i

a

I

m

30

I! .-a J -

>f

If) 00

-30

-

w m

m If) m N r- vi w m ct r50 0 If) c mmmmr-mmr-Ximr-wbOmv, If)

---

-I-

ri

-

I

m w

-

o ri r l

o w

w m

.r!. . ..ri'?. . . PI

ri P I

.A

.A .A

-- z N-

I-

GG $ .I .I.-I

.I .i.L .I .i .L .L .I .i.I

I

1

v v

5L

general structure/property-relationships are not as evident. Thus e.g. comparing the three septithiophene isomers 7, 99, 106, the melting points decrease the more Pbonds are involved in the molecule which then becomes less linear (7: 328°C; 99: 203°C; 106: 126- 127°C). Simultaneously, due to the distortion of the corresponding thiophene rings and the weaker overlap of the orbitals involved, the absorption max= 7: imum is shifted to shorter wavelength and higher transition energies ,,A,( 440 nm; 99: 370 nm; 106: 288 nm). This seems to be a general trend and is also observed in the series of bithiophenes (2, 12, 11:, , A, = 2: 302 nm; 12: 283 nm; 11: 260nm) and terthiophenes (e.g. 3, 109, 114:, , ,A = 3: 355nm; 109: 310nm; 114: 250nm). However, in the bithiophene sequence 2, 12, 11 the melting points rise (2: 33°C; 12: 68-68.4"C; 11: 133-134°C).

2.1.2.2 Substituted Oligothiophenes While polythiophene itself is intractable and therefore not processable, in the neutral state poly(3-alkylthiophenes) are soluble in regular organic solvents and still exhibit electrical and optical properties comparable to the unsubstituted derivatives [ 15I]. Since the description of the first poly(3-alkylthiophenes) in 1986, structure examination in solution has become possible and information about the type of connectivity between the thiophene rings and the mean chain length is available. Nevertheless, due to head-to-head couplings, adjacent alkyl substituents cause steric hindrance and diminution of the conjugation via non planar conformations. The degree of regularity and the structure of the polymers also depend strongly on the polymerization conditions. Besides the typical chain length distributions and conjugation defects for poly(3-alkylthiophenes), stereoregular factors play an additional role. Recently, the very effective synthesis of highly regioregular poly(3-alkylthiophenes) via selective metal-catalyzed coupling reactions has been reported independently by McCollough [ 1521 and Rieke [153]. The polymers consist of alkylated thiophene rings which are mostly linked head-to-tail (>95-99%). In comparison to the corresponding random polymers, these polythiophenes exhibit highly crystalline parts and strongly increased conjugation despite moderate molecular masses. Thus, the absorption maxima and the band aps are shifted to lower energies and the conductivities ' are considerably higher [ 1541. reach values (1 350 S cm- ?) which In analogy to the polymers, the solubility of oligothiophenes decreases dramatically with increasing chain length, which is due to the stiffness of the conjugated 7rsystem and the strong interactions between the chains, they are difficult to purify. Especially the long members are virtually impossible to characterize. This problem of low solubility can be overcome by the synthesis of corresponding oligothiophenes which bear alkyl groups in ,&positions. As an example, the solubility of a dialkylated sexithiophene which is described in more detail later on, is higher than 400gl-' whereas that of H-T6-H 6 is lower than 0.O5glp' [155]. This solubility provides an opportunity to synthesize much longer oligomers which would serve as desired models for the better understanding of polymeric systems. They therefore represent ideal model compounds for the investigation of charge carriers which are responsible for the charge transport along the conjugated chains. These properties may be correlated with the (defined) chain length of the oligomers and be compared

-1.1 Oligoiliioplieries

135

to that of the ‘real’ polymer. However, the characterization of radical cations and dications as models for polarons and bipolarons demands the synthesis of longer oligomers because the shorter ones are inherently reactive and undergo follow-up reactions. Thus, the concept of conjugation length plays an important role in the theory of conductivity and the non-linear optical properties of conjugated polymers. The modification of conducting polymers with flexible side chains in order to improve their tractability has in the meantime become widespread, but the influence of such a modification on the conjugation length remains rather subtle. In order to study the interrelation between chemical structure and various physical properties in such polymers, starting in 1991, several series of partially alkyl-substituted oligothiophenes have been prepared as model systems which are described below. Although a variety of differently substituted bi- and terthiophenes have already been synthesized and charaterized before, e.g. with respect to their biological properties, only those which show electronic properties interesting for new organic materials or which served as starting material for polymerizations will be described.

p, P’-Substituted Oligothiophenes The concept of conjugation length was a major motivation for the Dutch group of Wynberg to synthesize in 199 1 the first series of 0.0-coupled oligothiophenes which bear solubilizing alkyl side chains in the free +positions [ 1.561. A series starting from a trimer up to an undecamer including two oligomers with r-butyl endgroups in the terminal o-positions has been developed. The synthetic strategy chosen starts from 1.4-diketones which are prepared vicr the ‘Stetter reaction’, followed by ring closure with L.R. (see above). Thus, a series of oligothiophenes 116-122 including 3 lo 7, 9 and 11 thiophene rings were built up. This synthetic strategy, however, leads in general to a mixture of isomers differing in the position of the ri-alkyl groups in certain thiophene rings. Thus e.g. alkylsubstituted 1,4-diketone 116 is cyclized with L.R. to terthiophene 117. On one hand this is bis-acylated to terthiophene 118 which is further converted into the bis-Mannich base 120. On the other hand, terthiophene 117 is formylated under ’Vilsmeyer-Haack’ conditions to the terthiophene carbaldehyde 119. Both compounds are subjected to a ‘Stetter reaction’ yielding the bis-I .4-diketone 121 which is cyclized by L.R. to the undecamer 122 [Eq. (50)l. The properties depend critically on the effective conjugation length, structure and conformation. since adjacent alkyl substituents give rise to steric hindrance and, hence, to a nonplanar conformation. In this series, certainly the solubility is reduced with increasing chain length, but the different undecamers even exhibit a solubility of 2-1Ogl-’ in chloroform, whereas the conductivity (30Scni-’) is in the same order of magnitude as for doped poly(3-alkylthiophenes) (10- 100 S cm-’) [157]. This is a clear indication that the effective conjugation length in the corresponding polymers is not much longer than 1 I units. The UV-absorption, however, is a more direct measure of the conjugation length. In this series, the absorption maxima are = 345 nm for terthiophene steadily red-shifted with increasing chain length ,A,(, 117 to 462 nm for undecamer 122) and seems to approach saturation, without actually reaching it. The conjugation lengths of the notiamer and undecamer 122 exceed = 430-440 nm) due to less steric interaction of that of poly(3-alkylthiophenes) (A,,, 2.1.2.2.1

": 136

2 Sulfur-Containing Oligorners

.

a

0 116

\ I

H

3

C

117

C

%

Hs*4

0

0

R

\ I

W

R

[rn(%%)I

118 134% (57%)]

R

119 (85%)

R

I

120 170% (&%)I WCN. DMF

A

(50)

k 121

1

R

LR.

R

R

122 145% (44%)1 (last t v a sleps)

A

= C,H, (Cl2H2&

the side groups which are more widely spaced in the case of the oligomers. Absorption spectra of solid films for both oligomers are much like poly(3-alkylthiophenes) and exhibit maxima at lower energies ,A,(, = 520 nm) than in solution. This is an indication of a larger conjugation length in the solid state. In 1992, the group of Garnier published the synthesis of oligothiophenes substituted with solubilizing decyl side chains in P-positions [ 1551. Their synthetic strategy was different from that of the Dutch group and started from the 3'-decylterthiophene 124 which is obtained by Kumada-coupling of two equivalents of the Grignard reagent of 2-bromothiophene 42 and 2,5-dibromo-3-decylthiophene 123 in 92% yield. The corresponding hexamer 125 bearing two alkyl side chains was isolated by the oxidative coupling of the lithiated trimer 124 with CuClz in 50% yield. This in situ homo-coupling is not selective and resulted also in regioirregularly substituted products. Sexithiophene 125 could be purified and separated from longer oligothiophenes by chromatography. The optical spectra in solution and in the solid state, respectively, match with those of the corresponding sexithiophene synthesized by the Wynberg group. An unusually high fluorescence quantum yield of @ = 50% is reported for sexithiophene 125 which is double that for unsubstituted H-T6-H 6. A high solubility of 400 g 1-' (1 moll-') in chloroform is

2. I Oligot li iopheiies

137

given for the dialkylated sexithiophene 125 and the electrochemical coupling of this hexamer is reported. The cyclovoltammograms show two quasi-reversible oxidation waves which are due to the isomeric mixture rather less structured (EP = 0.47V, E: z 0.65V vs. Ag/Ag+) and correspond to the successive formation of radical cations and dications. By repetitive cycling, thin adsorbed films are obtained which exhibit redox waves with a shape typical for poly(3-alkylthiophenes) (Epa= 0.37 V, Epc= 0.20 V vs. AglAg’). The peak potentials correspond to those of the best polythiophene films. Since the narrowness of the peaks (half-height width = 140 mV) suggests homogenous conjugated chains and the optical characterizations revealed a red shift of the maximum absorption in comparison to the undecamer 122 prepared by Wynberg rt nl., it is concluded that by electrocoupling a corresponding dodecamer 126 is formed. These experiments which demonstrate the strength of the ‘oligomeric approach’, showed that stable and soluble radical cationic and dicationic model compounds can even be obtained from intermediate long oligomers and provide insight into the doping and conducting mechanism of polythiophene. Indeed, despite the relatively short conjugation length, they behave much like conducting polymers, and the synthesis of longer members would certainly result in an even better performance.

42

123

C,.Y, 126 (15%)

Consequently, due to the good solubility, the Paris group was able to couple the dialkylated sexithiophene 125 via the same method to a tetraalkylated regioirregular dodecithiophene 126 in 15% yield [Eq. (51)] [158]. Since lithiation of oligothiophenes is not selective, duodecithiophene 126 had to be separated from higher homologs and hexamer 125 by liquid chromatography. Oligothiophene 126 is reasonably soluble ( 3 x lop3mol I-’) allowing optical and electrochemical measurements in solution. The absorption maximum is slightly red-shifted ,A,(, = 465 nm) with respect to undecamer 122. A correlation of the absorption data of the different oligothiophenes available up to this moment with their inverse chain length revealed an almost linear behaviour. In electrochemical experiments, dodecamer 126 showed

138

2 Suifur-Containing OIigonirrs

less pronounced and rather ill-defined quasi-reversible redox waves. The (estimated) peak potentials in comparison to the corresponding sexithiophene 125 are shifted to negative potentials (Ey z 0.33 V, E; = 0.62 V vs. Ag/Ag+). The cyclovoltammograms are stable upon continuous cycling, indicating that no further coupling of the radical cations and dications occurs. The CV of a solid film of duodecithiophene 126 is comparable to that of poly(3-alkylthiophene) films. The redox potentials lie very close to those measured for the material which was produced electrochemically by coupling hexamer 125 (EPa= 0.35 V, Epc= 0.20 V vs. Ag/Ag+). Dodecamer 126 was doped in solution with FeCI3. In contrast to sexithiophene 125 which can be oxidized successively to the monocation and the dication, the absorption spectra of 126 showed a one-step oxidation leading to two near-IR peaks located at E M 0.8 and 1.7 eV. These values are close to those of electrochemically doped polythiophene. Assisting in situ EPR measurement gave only very weak signals on oxidized dodecamer 126, The authors concluded that in the case of very long oligomers immediately a spinless dication as most stable state is formed, which would correspond to bipolarons in conducting polymers. A conductivity of o = 5 S cm-l for iodone-doped solid dodecamer 126 is reported. The problem of regio-irregularity in such p-substituted oligothiophenes was first solved with the synthesis of isomerically pure alkyloligothiophenes by Biiuerle r t al. [ 1591. The decisive path was the regioselective monobromination of alkyl bi- and terthiophenes as key building blocks for the synthesis of higher oligomers. However, direct bromination of oligothiophenes, even under selective conditions, always gives some dibromo derivatives which cannot be separated on preparative scale [160]. The preparation of monobrominated oligothiophenes was done in a painstaking way either by employing protecting-group techniques [161, 1621 or by the use of the palladium-catalyzed cross-coupling of dibromothiophenes with one equivalent of the Grignard reagent of 2- or 3-bromothiophene catalyzed by Pd(dppoCl2. Rossi et al. could obtain monobromobithiophenes in moderate yields after tedious purification [76]. In contrast, bromination of oligothiophenes with the high selective system NBS in D M F resulted in a marked increase in selectivity. Under carefully chosen conditions, a-monobromo bithiophene Br-T2-H 50 and terthiophene Br-T3-H 51 could be isolated in good yields [73]. This technique was now applied to the bromination of 3’-dodecylterthiophene 13 which was obtained analytically pure in 3 1Yoyield by the ‘Kumada-coupling’ of two equivalents of BrMg-TI -H 42 and 2,5-dibromo-3-dodecyIthiophene.Reaction of terthiophene 13 with one equivalent NBS at ambient temperature gave the monobromination products 127-128 in 56% and the dibromo compound in only 9 % yield. The ratio of the two possible isomers 127 and 128 was determined independently to 90: 10 by ‘HNMR and HPLC analysis indicating an unexpectedly high selectivity. Repetitive crystallization afforded the isomer 127 in 97% purity and 26% yield. The definitive structural assignment of bromoterthiophene 127 relied mainly on 2D correlation spectra. The isomerically pure bromo(dodecy1)terthiophene 127 is the key intermediate for the synthesis of the rigidly defined higher oligomers and thus was coupled to the corresponding regioregular didodecylsexithiophene 129 in 33% yield [Eq. (52)]. The nickel catalyst was prepared in situ from nickel dichloride, triphenylphosphine and zinc (see above). As expected, the melting point of hexamer

2. I Oligorl1iop/1el 1e.s

139

vBr

129 ( I 10-1 11'C) was distinctly higher than that reported for the isomeric mixture (80-84°C) prepared by Wynberg et 01. NBSIDMF.

\ I

+

C12Hn

Br

\ I

C12H2,

127 (26%)

13

I

Ct2H2S

128

NiCI, / Zn/PPh, I DMF

C,,H,, 129 (33%)

Characterization of the electronic properties of the regioregular and easily soluble sexithiophene 129 revealed that it is stable even in four different redox states due to its well-defined structure [ 1631. In the cyclovoltamrnogram reversible waves due to the formation of the radical cation and dication (EP = 0.34V, Eg = 0.54V vs. FciFc'), additionally quasireversible waves for the radical anion and dianion (Eg = -2.27V, EY = -2.40V vs. Fc:'Fc+) are found. The stability of the redox states allowed the examination of their optical properties. The maximal absorption in the neutral state ,A,(, = 416nm) is blue-shifted in comparison to the unsubsti= 432 nm) and reflects the steric interaction of the alkyl tuted sexithiophene (A,, chains with the conjugated 7r-system. Accordingly, a crystal structure of sexithiophene 129 reveals a twisting of the relevant thiophene rings by 10.8 '. The transition energies of all the species in different redox states were determined and it was found by variable-temperature measurements that the absorption spectrum of the radical cation is reversibly blue-shifted with decreasing temperature. This phenotnenon was also discovered by Miller et ul. for a terthiophene radical cation and can be explained by a reversible equilibrium between the monomeric and a dimeric radical cation [Eq. (53)] [164]. The 7r-dimerization of radical cations (and anions) has been described before for other large aromatic radical ions like porphyrins or viologens. It was recognized by both groups that the formation of corresponding dimeric species is complementary to polarons and bipolarons and may be a new alternative to the description of the doping behavior of polythiophenes. 2 (H-T,-H)+

C

(H-T,-H)~~+

(53)

In sitzr EPR spectra were taken from the paramagnetic sexithiophene radical cation and anion and allowed the determination of the spin density distributions. In accordance with the postulated -ir-dimerization, for the radical cation the intensity of the EPR signal decreases substantially with decreasing temperature and indicates a spin pairing to the 7r-dimer. This fact easily explains the puzzling observation that the EPR activity of polarons in conducting polymers is only detected at unexpectedly low levels of doping. As mentioned before, for the radical cation of

140

2 Sulfur-Containing OIigomers

Figure 7. STM images (area ca. 8.5 x 5.8 nm) of (a) non-regioregular dialkylsubstituted sexithiophene and (b) the regioregular dialkylsubstituted sexithiophene 129 [ 1651.

tetraalkylated duodecithiophene 126 no ESR activity could be detected [ 1591 and is now easily explained due to this dimerization. The disproportionation of the radical cations to a dication and a neutral species as well as the formation of a 0-bond in the dimer can be excluded from the optical properties. STM-investigations by Rabe and Stabel on adsorbates of a non-regioregular sexithiophene synthesized by Wynberg et al. [I 561 and the directly comparable regioregular sexithiophene 129 of Biiuerle rt al. [I601 on highly oriented pyrolytic graphite (HOPG) showed for the first time the influence of the defined structure on ordered monolayers [165]. The STM images of both materials exhibit similarly ordered structures in which both the sexithiophene backbones and the alkyl chains are oriented in a lamella structure parallel to the graphite surface in such a way that the alkyl side chains can achieve maximal van-der-Waals interactions. The conjugated backbone is given by the white areas and the alkyl side chain by the dark areas (Fig. 7). The difference between the two hexamer monolayers, however, is the spatial demand of the repeating units. For both compounds the area per unit cell contains one molecule. This area is 3.25 nm2 in the case of the non-regioregular sexithiophene and nonuniform spacing is found. In contrast, for the regioselectively substituted hexamer 129 the structure is much more uniform and the demand of area is much smaller (2.38 nm2). More insight into the structural pecularities of /?-substituted oligothiophenes was given by Hadziioannou et al. who synthesized two analogous stereoregular dialkysubstituted sexithiophenes 134 and 135. Their X-ray structures were determined more precisely than before [ 1661. The more straightforward synthesis started with the symmetric dialkyl quaterthiophenes 131 which were obtained in 47-66% yield by the nickel-catalyzed cross-coupling of the Grignard reagents of 3-alkyl-2-iodothiophene 130 and Br-T2-Br 46. Bromination of quaterthiophenes 131 with NBS in chloroform/acetic acid provided the dibromo quaterthiophenes 132 in 56-79% yield and were further coupled under ‘Kumada conditions’ 133, with the Grignard reagent of 2-bromo- or 5-trimethylsilyl-2-bromothiophene respectively, to the regioregular sexithiophenes 134 and 135 which differ by

2.1 Oligorliiopheiies

141

the length of the alkyl side chain and the terminal end group (25-59% yield) [Eq. (541.

131 [47%(66?6)]

R

A‘

A‘

134 [#H. R=C,H9 (59%)] 135 f#SiMe3. R=C,H,, (25x11

X-ray data of oligothiophenes are normally scarce due to the difficulties in crystallization. In the case of bis(trimethylsily1)substituted hexamer 135, obviously the end-substitution helps in the crystallization process. Thus, good refinements could be obtained due to the lower rotational disorder in the molecules. In accordance with sexithiophene 129, butylated sexithiophene 134 shows torsional angles of 9 and 1 l o , respectively, for the thiophene rings bearing the alkyl chain and the connected inner and outer rings. In the case of the octyl-substituted hexamer 135 the deviation from planarity is smaller (5 and 7”). The alkyl chains have a planar zig-zag conformation and are lying almost in the plane of the thiophene backbone. In 1994, Sat0 et al. could increase the chain length of a-oligothiophenes up to 15 thiophene rings in the correct connectivity [167]. A homologous series of oligothiophenes bearing now two alkyl side chains per three thiophene units was synthesized through nickel-catalyzed reductive coupling of 5,5”-dibromo-3,3”-dihexylterthiophene 137 which was obtained by bromination of the corresponding dihexylterthiophene 136 in 95% yield. Due to the bifunctionality of the reagent, coupling with the system NiC12/Zn/PPh3/DMF/2,2’-bipyridine yields a mixture of higher oligomers [Eq. (55)]. The yields of the isolated hexyl-substituted oligothiophenes 138-141 are therefore low. Separation from higher molecular weight products could be achieved through liquid chromatography and afforded hexamer 138 in l.8%, nonamer 139 in 2.8%, dodecamer 140 in 1.4%. and pentadecamer 141 in

1.8%. GPC suggested that the oligomers are nearly monodisperse ( M , / M w = 1.05-1.13). Due to their solubility, their structure could be proven by N M R spectroscopy.

136

137 (95%)

(55) R = C,H,,

“J,

a.DMF

--____*

PP$,

H

H n

138 139 140 141

(n=2; 1.8%) (n=3; 2.8%) (n=4: 1.4%) (n=5; 1.8%)

138-141

In the absorption spectra the n-n* transitions are gradually shifted to lower energies as the chain length increases [138 (A = 410nm), 139 (A = 440nm), 140 (A = 448 nm), 141 (A = 456 nm)]. The conjugation length of the pentadecamer 141 must thus be similar to poly(3,3”-dihexylterthiophene) which exhibits the same absorption maximum. The absorption maxima of solid films also show a decrease of the transition energy with increasing chain length, but the maximum of pentadecamer 141 is conversely located at higher energies [138 (A = 408 nm), 139 (A = 456 nm), 140 (A = 494 nm), 141 (A = 448 nm)]. The authors conclude from this finding that the oligomers up to duodecithiophene 140 have coplanar structures whereas the structure of the longest oligothiophene 141 should be coiled, which was once proposed for polythiophene [168]. The UV/VIS/NIR and ESR spectra of the corresponding cations were investigated by controlled oxidation with FeCI3 [167a,c]. By adding two equivalents of oxidant the optical and ESR spectra showed that the oxidized species were radical cations and the charge seems to be spread over 6 to 8 thiophene units. The absorption spectra were similar to those of doped poly(3-alkylthiophenes) bearing long alkyl chains. Also in these cases, relatively low spin concentrations are detected in ESR experiments which decrease with increasing chain length. The low number of spins per molecule and the simultaneous change in the absorption spectra point also to the formation of diamagnetic n-dimers. The further oxidation of hexamer 138 led to absorption features which were interpreted as being due to the formation of a radical trication instead of the usual dication. The longest oligothiophene so far synthesized as a part of a homologous row of isomerically pure a-linked oligothiophenes 143-146 was recently reported by Bluerle et al. [ 1691. According to calculations, sedecithiophene 146 should be 64 A long when extended. 3,3/”-Didodecylquaterthiophene143 is the main building block and was obtained in 75% yield by the ‘Kumada-coupling’ of two equivalents of the Grignard reagent of 2-bromo-3-dodecylthiophene 142 with Br-T2-Br 46. By oxidative coupling of the lithiated tetramer 143 with CuClz, the corresponding octithiophene 144 and duodecithiophene 145 were obtained in one step due to the formation of dilithiated products. Owing to their good solubility, 144 and 145

2.1 OIigotI~iophenes

143

could be obtained in analytically pure form in isolated yields of 30% and 8%. respectively, by repeated chromatography. Octamer 144 was then coupled again with the system n-BuLi/CuC12 to give the corresponding hexadecamer 146 in 19% yield of pure material after chromatographic work-up [Eq. ( 5 6 ) ] .

1

145 (8%)

146 (19%)

The melting points of the very stable oligothiophenes 143-146 increase with increasing chain length but lie markedly below those of the unsubstituted oligothiophenes. The alkyl side chains lead to increased steric interaction between the conjugated backbones and thus reduce T-T interactions between the molecules in the solid state. Additionally, the solubility is raised by the presence of the side chains in a way that all the homologs are soluble in hexane, aromatic and chlorinated hydrocarbons and could be fully characterized. Absorption spectroscopy of 143-146 showed that, as expected, the energy of the longest wavelength T-T* transition is red-shifted and the extinction coefficient linearly increased with increasing chain length. While a good correlation is normally found between the absorption energy and the reciprocal of the chain length for other oligothiophene series up to a chain length of seven [13, 1701, the relationship in this series is not linear. The longer members of the series, oligomers 145 and 146, deviate from linearity such that the absorption maxima are shifted to higher energies than expected. It is not clear at the moment, if this is due to a winding of the long thiophene backbone which would cause a reduction of the effective conjugation or if the effect observed is a general phenomenon and a saturation is approached. Theory predicts a linear behavior up to infinite chain lengths [ 1711. Temperature-dependent

144

2 Sulfur-Contuining Oiigornws

spectroelectrochemical investigations on this series showed the tendency that the radical cations dimerize more strongly with increasing chain length. Whereas the monomeric and the dimeric radical cations are in equilibrium at approximately -40°C for quaterthiophene 143, equivalent portions are formed at ~ 3 0 ° C for octithiophene 144, at ~ 6 5 ° Cfor duodecithiophene 145, and even >80°C for sedecithiophene 146. Since oligothiophenes are ideal model compounds for investigating the charge carriers responsible for the charge transport in conjugated polymers, the redox properties are of special interest. As expected, the first oxidation potential for oligomers 143-146 is shifted cathodically with increasing chain length. The oxidation potentials of dodecamer 145 (Epd= 0.19 V vs Fc/Fc+) and hexadecamer 146 (Epa= 0.12V vs Fc/Fc+) are more negative than that of the structurally related poly(3-dodecyl-2,2'-bithiophene)(EFd= 0.3-0.35 V vs Fc/Fc+) [169]. This clearly indicates that longer oligomers exceed the average conjugated chain length of the parent polythiophenes which typically lie at 8- 10 properly linked monomer units. In multisweep experiments of all homologs, thin films are deposited on the working electrode. The films exhibit sharp and symmetric redox waves similar to the electrochemical response of redox polymers. Their first oxidation potential is once again at considerably lower potentials than those of the parent oligothiophenes (Epd= -0.1 1 to -0.13 V vs Fc/Fc+)and indicate the presence of fairly well-defined higher oligomerization products. Evidently, a preorganization of the molecules takes place on the metal surface and well-ordered products are formed by oxidation. An investigation of physisorbed monomolecular layers of these oligothiophenes on graphite by STM high resolution images revealed that each compound forms ordered supramolecular structures in which the molecules are oriented in lamellae (Fig. 8). The bright areas are due to the conjugated .ir-system and the darker regions to the alkyl chains. In all four cases the lamellae are separated by roughly 2.2 nm and the degree of overlap between two neighboring thiophene chains decreases from 35" for quaterthiophene 143 to 5" for sedecithiophene 146. Finally, for 146 the molecules are lined up almost linear and are oriented ideally for further oxidative coupling of the adjacent a-positions. This fact may explain the formation of extremely long 'molecular wires' in electrochemically prepared thin films.

Oligorhiophenes synthesized f o r subsequent polymerization Since there are many examples of P-substituted bi- and terthiophenes only those which served for subsequent polymerization to the corresponding poly(alky1thiophenes) and functionalized poly(alky1thiophenes) will be listed. Oligomers including different substitution patterns and varying alkyl substituents up to sexithiophenes have been synthesized in this context and are noted in Tables 3-7. In most cases, the oligomers were obtained by transition metal catalyzed cross-coupling reactions between organometallic reagents and thienyl halides (see section 2.1.2). Optical, electrochemical and X-ray diffraction characterizations of various substituted oligomers provide insight into the influence of the substituents on structural features and consequently on the electronic properties. Despite the alkyl groups impose a weak electron-donating effect on the conjugated 7r-system, e.g. with respect to H-T2-H 2, the absorption maxima of 3- and 3,3'-substituted bithiophenes are blue-shifted and the redox potentials are increased. This is due

2.1 Oligothiophenes

145

Figure 8. STM images of 2D crystals of the whole series ofoligothiophenes on HOPG. (a) Tetramer 143; (b) octamer 144: (c) dodecamer 145; (d) hexndecanier 146 [169].

to a deviation of the thiophene rings from coplanarity which causes a diminution of the 7r-orbital overlap (see e.g. 2, 147, 155). X-ray structure data indeed show that the conformational properties in the solid state are different from those in solution, but they are mainly determined by the system to relieve conformational strain which is dispersed over the whole molecule by succesive and cooperative bond and angle deformation [192]. Following the development in polythiophene chemistry and the development of regioregular poly(3-alkylthiophenes) [ 152- 1541, the first examples of regioregular oligo(3-alkylthiophenes) were very recently reported. The inherent problem in the synthesis of this model compounds is that isonier-free building blocks with a 2,4substitution pattern are necessary. However, 3,4’.4”-trialkyl-2,2’: 5’2’’-terthiophenes 193, 196, 200 are the longest regioregular oligo( 3-alkylthiophenes) reported so far. Barbarella et a/. succeeded in the synthesis and characterization of the corresponding methyl [189] and hexyl derivatives [177, 1781. In order to get insight into the structural pecularities of poly(3-hexylthiophene) all the other possible regioisomeric terthiophenes 197-199 were also synthesized and the properties compared. The methyl-substituted regioregular bithiophene 162 and terthiophene 193 were obtained by successive ’Kumada‘ coupling reactions. However, the coupling of the corresponding brominated bithiophene and the Grignard reagent of 2-bromo4-methylthiophene affords the regioregular terthiophene 193 in only 20% yield [Eq. (57)]. The authors give for terthiophene 193 an (isomeric) purity of >98%. This compound was synthesized before by Zimmer c’t al. [173]. but its melting

P

m

Table 3. Preparation and physical properties of p-substituteda-bithiophenes.

Bithiophenes

Compound number

Yield [%]

81

3,3’-DimethylL2,2’-bithiophene

2 147 148 149 150 151 152 153 154 155

3,3’-DiethyL2,2’-bithiophene 3,3’-DihexyL2,2‘-bithiophene 3-Ethyl-3’-methyl-2,2’-bithiophene 3,3’-Di-[(2-tetrahydropyranyloxy)ethyl]-2,2’-bithiophene 3,3‘-Di-(2-hydroxyethyl)-2,2’-bi thiophene 3,3’-Di-(2-sulfonatoethyl)-2,2’-bithiophene-sodium salt 3,4’-DimethyL2,2‘-bithiophene

156 157 158 159 160 161 162

88

3,4’-DihexyL2,2’-bithiophene 3,4’-Didodecyl-2,2’-bithiophene 3,4’-Di-[(2-tetrahydropyranyloxy)ethyl]-2,2’-bithiophene 3,4’-Di-[(2-hydroxyethyl]-2,2’-bithiophene 3,4’-Di-[6-( p-methoxyphenoxy)hexyl]-2,2’-bithiophene 4,4’-DirnethyL2,2‘-bithiophene

163 164 165 166 167 168

23 75

4,4’-Dihexyl-2,2’-bi thiophene 3,3’,4,4’-TetramethyL2,2’-bithiophene

169 170

6

2,2’-Bithiophene 3-Methyl-2,2’-bithiophene 3-Ethyl-2,2’-bithiophene

3-Hexyl-2,2’-bi thiophene 3-Dodecyl-2,2’-bithiophene 4-Methyl-2,2’-bithiophene 6-(2,2’-bithien-3-yl)hexanoicacid N-[6-(2,2’-bithien-3-yl)hexanoyloxy]pyrrolidin-2,5-dion 2-[2-0xa-7-(2,2’-bithien-3-yl)heptyl]1,4,7,1O-tetraoxacyclododecane

72 89 89

Absorption [nm] in CHCI,

,A,,

302 299 295

85

11 70 80 66 86

76 30 69 95 53 85

309

295 270 268 279 270

Oxidation potential

77,170,172,173 170,172,113 170 174 1.20 73 1.67 77 175 0.88 (FC/FC+)175 176 1.23 77,170,172,173 1.51,1.17 177 1.33 (SCE) 17 1.87 177,178,179 1.53(SCE) 172 180

180

1.13 1.22(SCE) 1.35 (SCE)

310

1.10

31 1

1.21 (SCE) 1.25 (SCE)

310

180 77,172,113 177 177,179 181 180 180

292

50

75

? 3 ::

1.24 1.20 1.75

50

70

ru

Epa[V] vs. Ag/AgCl

250 242 302 299 298

Ref.

182 77,170,172,173 177 177 148

9 5

a. 3.

rz 2 2

; -

point (39‘C) is considerably lower than this obtained by Barbarella et crl. (61 ‘C), indicating a far lower isomeric purity in the former case. X-ray structure determination of regioregular terthiophene 193 revealed two crystallographically independent, but identical molecules, characterized by torsional angles of 6-9‘ around the inter-ring bond. Interestingly. the molecule crystallizes in a chiral space group containing one single enantiomer. Due to the defined head-totail linkages of the thiophene rings in 193 and the resulting non-planarity, a pair of rod-like equienergetic enantioconforniers or atropisomers with +u and -w interring twist angles exists. Single atropisomers cannot be observed in solution, since the rotational barriers around the inter-ring bond are too small. 3,4’.4’’-Trihexyl-2,2’:5’,2”-terthiophene 196 was similarly prepared, but by ‘Stilletype’ cross-coupling reactions [ 1771. In this reaction sequence, the less effective step is the Pd(0)-catalyzed coupling of the organotin thiophene and 2-bronio-3-hexylthiophene to yield 3,4’-dihexyl-2.2’-bithiophene163 in only 23%. Bromination of the latter and reaction with the stannylated thiophene under ‘Stille-conditions’ results in 69% yield of the desired regioregular terthiophene 196 [Eq. (57)]. R

R

- R

+%q]

catalyst +

@Or

M

s

R

R = Me. C,H,,.

A

162-167

(CH,),OC,H,OCH,.

M = BrMg, Me3Sn

A

193,196,200

While there is only one report on regioregular poly(3-alkylthiophenes) functionalized with polar groups [ 1981, the synthesis of corresponding oligomers was attempted recently [ 1801. Analogous ‘Stille-type’ reaction of the related tetrahydropyranyl(THP)-protected derivatives of 3-(?-hydroxyethyl)thiophene revealed another problem not mentioned before. A mixture of the desired cross-coupled 3,4’-disubstituted bithiophene 165 (50%) and the 4,4’-substituted regioisomer (10%). which is formed due to homo-coupling reactions. is obtained. The authors were not able to separate the regioisomers completely, but finally the deprotected hydroxyethyl-substituted derivative 166 could be purified by chromatography. Regioregular bi- and terthiophenes which bear hydroquinonemethylether(HCM)-protected alkyl side chains were synthesized by Crayston P Z crl. using successive ‘Kumada’ coupling reactions [ 1821. Ether cleavage of the HCM-group directly results in the corresponding bromo derivatives which can be replaced by a great variety of functional groups [86]. Although the synthesis of the 3.4j-disubstituted dimer 167 succeeds in moderate yield (50%). the successive coupling of the

e

P 00

lu

2 2 Table 4. Preparation and physical properties of P-substituted a-terthiophenes. Terthiophenes

z

Compound

Yield

Absorption

Oxidation

number

[%I

A, [nm] in CHCl3

potential Eva [Vl

%

vs. Ag/AgCI

5

3

86

355

3-Methyl-2,2’: 5’,2’’-terthiophene

171

70

352

3’-Methyl-2,2’:5/,2’’-terthiophene 3’-Ethyl-2,2’:5‘,2’‘-terthiophene 3’-Butyl-2,2’: 5’,2’’-terthiophene 3’-0ctyl-2,2’: 5’,2’’-terthiophene 3’-Decyl-2,2’: 5’,2’’-terthiophene 3’-Dodecyl-2,2’:5’,2’’-terthiophene 3‘-Phenyl-2,2’:5’,2’’-terthiophene 3’-( p-Methoxyphenyl)-2,2’: 5’,2’’-terthiophene 3’-( p-Cyanophenyl)-2,2’: 5’,2’’-terthiophene 3’-(Thien-2-~1)-2,2’:5’,2”-terthiophene 3’-(5-Methylthien-2-~1)-2,2’:5’,2”-terthiophene 3’-(Pyrid-4-~1)-2,2’:5‘,”’-terthiophene 3’-(3,6-Dioxaheptyl)-2,2’: 5’,2’’-terthiophene

172 173 174 175 176 13 177 178 I79 180 181 182 183

80 53 77 60 92 58 (26) 69 53 68 23 42 33 60

336 345

2,2’: 5’,2”-Terthiophene

1.05 0.98 (SCE) 1.13 1.03 (SCE) 1.11 1.05 (SCE) 1.05 (SCE)

347 345 346 (MeCN) 350 (MeCN) 340 (MeCN) 345 (MeCN) 345 (MeCN) 344 (MeCN)

1.06 (SCE)

1.05 (SCE)

Ref.

5 E. 3.

9 3

77, 172 183 77. 172 183, 184 77, 172, 184 77 156, 183 184 158 156, 73 184, 185 185 185 185 185 185 184

3‘-(3-Sulfonatopropy1)-2,2’: 5’,2’’-terthiophene-potassium salt 6-(2.2’: 5‘,2”-Terthien-3’-yl)hexanoicacid N-[6~(2,2’5’.2‘’-tertl~ienthien-3’-yl)hex~noyloxy]pyrrolidin-2,5-dion

0.90 (SCE)

2-[2-0xa-7-(2.2’: 5’,2’‘-terthien-3’-yl)heptyl]1.4.7.10-tetraoxacyclododecane 3.3‘-Dimethyl-2,2’: 5’,2’’-terthiophene 3,3”-Dimethyl-2,2’: S’,”’-terthiophene 3’.4’-Dimethyl-2,2’: S’.?”-terthiophene 3‘.4’-Dibutyl-2,2’: S’,”’-terthiophene 2.5-Di( thien-2-yl)-cyclopenta[c]thiophene

184 185 186 187 188 189 190 191 192

3.4’.4’’-Trimethyl-2.2‘: 5‘.2”-terthiophene

193

20

3,4’,3’‘-TrimethyI-2,2’: 5’,2”- terthiophene 3.3’.3’’-Trimethyl-2,2’: S’,Z‘’-terthiophene 4‘-Ethyl-3,3‘’-dimethyl-2.2‘: 5’,2”-terthiophene 3.4‘,4”-Trihexyl-2.2’: 5’,2”-terthiophene 4,4’.3”-Trihexyl-2,2’: S’.”’-terthiophene 3,4’,3”-Trihexyl-2.2‘: 5’.2’‘-terthiophene 4.4‘.4”-Trihexyl-2.2’: S’,?’-terthiophene 3,4’,4”-Tris-[6-( p-methoxyphenoxy)hexyl]-2,3’: 5’,3”-terthiophene 3,4,3”,4”-Tetraiuethyl-2.2‘: S’,”’-terthiophene 3.4,3’,~‘,3’’,4’’-Hexaniethyi-3.2/:5’2”-terthiophene

194 195 196 197 198 199 200 20 I 202

87 67 48 95 78

346 344

0.72 (Fc”:’) 0.90 1.09 1.03 (SCE)

350 0.97 (SCE)

76

324 346 324

0.93 0.85 (SCE) 0.98

35 69 51 24 37 36

321 336 336 316 348 354

0.88 (SCE) 0.88 (SCE) 0.88 (SCE) 0.88 (SCE)

183 175 175 176 149 77. 186 148. 183 187. 188 183 173.178. I89 177 77 148 77 177, 178 177. 178 177. 178 177, 178 182 I48 148

. I>

c

P

s s.

Table 5. Preparation and physical properties of []-substituted 0-quaterthiophenes. Quaterthiophenes

2,2’: 5’,2”:5”,2”’-Quaterthiophene 4’,3’’-Dimethyl-2,2’:S‘,”‘: 5”,2’”-quaterthiophene 3,3”’-Dimethyl-2,2’:5’,2”: 5”,2”’-quaterthiophene 4’,3’’-Dimethyl-2,2’:5‘,2“:5”,2”’-quaterthiophene 3,4’,3”,3”’-Tetramethyl-2,2’: 5‘,2‘‘:5”,2”’-quaterthiophene 3.3’,4”3‘’-Tetramethyl-2,2’: 5‘,”‘: 5”,2”’quaterthiophene 4,4’,3’’,4’’’-Tetramethyl-2,2‘: S‘,”‘: 5”,2”’-quaterthiophene

Compound number

4

203 204

m

Yield [“/n]

89 19 80

205

206 207 208

70 9

Absorption , ,A, [nm] in CHCl3 390 348 380 346 342 346 348

Oxidation potential Epa[V] vs. Ag/AgCI

Ref.

1.05

77 77 77, 190 190, 192 77, 173, 190, 192 190, 192 191

0.90

2

2 3

?

Table 6. Preparation and physical properties of /j-substituted wquinquethiophenes Quinquethiophenes

,,. . 5/,-y ,.

_,&

._ . 5”/.2‘“‘-Quinquethiophene ,111.

3.3’”’-Dimethyl-2,2’: 5‘.2“: 5”,2’”:5”’.2””-quinquethiophene 3‘.4”’-Dimethyl-),2’: 5’.2”: 5”,2”‘: 5”’.2””-quinquethiophene 3.3’.1”’.3””-Tetramethyl-9.7’: 5’,2’’: 5”.2”’:5’”,2’”’-quinquethiophene 3”.4”-Dibutyl-2,2‘: 5/,2”:5”.2”‘: 5”’,7””-quinquethiophene

Compound number

5 209 210 21 1 212

Yield

[%I

91

Absorption , , ,A [nm] in CHCI?

416

Oxidation potential E,, [V] vs Ag/AgCI

Ref

0.97 0.80 (SCE) 0.80 (SCE) 0.86 (SCE)

11 193 193 193 194

b‘

5

Table 7. Preparation and physical properties of B-substituted a-sexithiophenes. Sexithiophenes

2,2’: 5‘,2‘‘: 5”,2’”:5’”,2’”’:5m‘,2“‘‘-Sexithiophene 3’(4’),3’’”(4’’”)-Dimethy1-2,2’: 5’,2’’: 5”,2”’: 5”’,2””:5””,2”‘”-~exithiophene(irreg.) 3’(4’),3’’~’(4”‘‘)-Dioctyl-2,2’: 5’,2“:5”,2”’:5“‘,2”‘‘: 5’”’,2”’’-~exithiophene(irreg.) 3’(4‘)”‘’’’(4’”’)-Di-(3,6-oxahepty1)-2,2’: 5‘,2’‘: 5”,2”’:5”’,2””:5/”’,2’”’’-sexithiophene (irreg .) 3‘,4”3’”,4’’’’-Tetrabutyl-2,2‘:5’,2’’: 5”,2’”: 5”’,2””:5’”’,2r”’r-~exithiophene 3‘,4‘,3“’’,4““-Tetrahexyl-2,2‘: 5’,2”: 5“,2“’:5“‘,2“”:5””,2”’”-sexithiophene 3,3’,4”,3”’,4’’”,3’’’’-Hexamethyl-2,2’: 5‘,2’‘: 5”,2”’:5’”,2’”’: 5””,2’”’’-sexithiophene 4,4’,3‘’,4”’,3””,4’‘”-Hexamethyl-2,2’: 5’,2”: 5“,2“‘:5”’,2””:5’”r,2’r”’-~e~ithiophene

3.

Compound number

6 213 214 215 216 217 218 219

Yield

[%I

56

20 8

Absorption , , ,A [nm] in CHCl, 432

368 368

Oxidation potential Epa [V] vs. Ag/AgCl

Ref.

(0.46) 1.06 (SCE) 0.98 (SCE) 0.99 (SCE)

I1

z3. 2

% 3

s 195 195 195 194 196 197 197

2.1 Oligothiopkenes

153

brominated dimer leads in only 26% yield to the regioregular terthiophene 200. Also in this case, a minor amount of homo-coupling product (4,4’-disubstituted bithiophene) was detected. The synthesis of longer regioregular oligothiophenes would be extremely interesting with respect to their model character and their electronic properties in their own right. Evidently, this demands more rigidly defined regioselective and effective synthetic steps with a minimum formation of regioisomeric homo-coupling products. In this respect, the ‘Suzuki-type’ coupling reaction of 3-dodecyl-5-thienylboronic acid 220 and 2-bromo-3-dodecylthiophene 142 proved to be more successful [181]. Since the bororganic component can be obtained pure and free of isomers by recrystallization. optimized coupling with [Pd(PPh3)4]as catalyst in DME/NaHC03 affords 75% dimeric products in which the desired 3,4‘-didodecyl bithiophene 164 is preferably formed in a ratio of 28: 1 (detected by ’ H NMR spectroscopy) in comparison to the corresponding 4,4’-isomer 221 [Eq. (58)]. Pure 3,4/-didodecyL2,2/-bithiophene 164 was effectively separated from the 4,4/-disubstituted homo-coupling product 221 by preparative HPLC.

Bridged oligothiophenes A strategy for controlling regioregularity, planarity and rigidity simultaneously is best realized in P,P’-bridged oligothiophenes including a fixed conformation. Thus, cyclopentadithiophenes originally synthesized by Wynberg et al. as thiophene analogs of fluorene [199] have recently received much attention. In order to prove that the electronic properties of alkylated ologithiophenes are dependent on steric hindrance, Zimmer et al. synthesized and reinvestigated the rigidified 3,3‘-bridged bithiophenes 222 and 223 in which the thiophene rings are kept in a syn-cisconformation [170]. However, the synthesis of this type of compounds is tedious and includes many steps. In comparison to 3,3/-dimethyl-2,2’-bithiophene 155 (A,, = 270 nm; Epa= 1.51 [ 1.171 V vs Ag/AgCl), the maximum absorption of the fully planar 4H-~yclopenta[2,1 -b;3,4-b’]dithiophene 222 is considerably red-shifted (A,, = 31 1 nni) and exhibits the by far lowest oxidation potential in the alkylbithiophene series (Epa= 0.97 V). The less rigid analog 4H,5H-cyclohexa[2,1b;3,4-b’]dithiophene 223 exhibits still a rather long-wavelength absorption (A,, = 305nm) and a somewhat higher oxidation potential (Ep, = 1.2OV). Zerbi et al. investigated this series including the homologous cycloheptadithiophene derivative 224 and their corresponding polymers by means of vibrational spectra [200]. The interpretation of the spectra revealed largest 7r-electron delocalization for the fully planar dithienocyclopentadiene system. A series of 4-alkyl- 225 and 4,4’-dialkyl-substituted cyclopentadithiophenes 226 synthesized and polymerized by Berlin and Zotti et al. resulted in soluble and highly conductive polymers. The partial rigidity of the polythiophene backbone causes anomalously red-shifted absorption spectra in the neutral state which

indicates a high conjugation ,A,(, = 545-680 nm). The monomers are obtained by one- or two-fold deprotonation of the parent bridged bithiophene with n-BuLi and subsequent alkylation with alkyl halides in 31 -47% yield [201]. X-ray structure analyses of 4H-cyclopenta[2, l -b;3,4-b’]dithiophene 222 and the spiro-analog spiro[4H227 [ 1861 confirm the fully cyclopenta[2,1 -b;3,4-b’]dithiophene-4,1f-cyclopentane] planar arrangement of the bridged bithiophenes [202].

fi S

S

222-224 (n = 1-3)

As a further model compound for rigidified polythiophene, Roncali et al. have developed a new synthesis of the first fully planar terthiophene by bridging the internal P-positions [203]. The synthesis is based on the cyclic ketone 228 which is oxidatively dimerized after deprotonation by CuC12 to the 1,4-diketone 229 (30% yield). Intramolecular cyclization of 229 with L.R. affords the rigid 4H,SH-dicyclopenta[2,1-b;3,4-b’;2/1/-a’;3/,4/-b’/]terthiophene 230 in 45% yield. The rigidification induces considerable changes in the optical and chemical properties and leads to a much smaller HOMO/LUMO-gap. Thus, the absorption spectrum of 230 differs from that of H-T3-H 3 by a fine structure typical of rigid conjugated systems and by a bathochromic shift of the absorption maximum of AA,ll,, = 22 nm. A shoulder at X = 387 nm and a weak absorption tail proceeding up to about 850 nm confirms the considerable extension of the effective conjugation in 230. Simultaneously, the oxidation potential of rigid terthiophene 230 is shifted negatively to Ep, = 0.60V (H-T3-H 3: Epa= 1.07V). Surprisingly, rigid terthiophene 230 also can be reduced relatively easily (I& = -0.75V). The estimated HOMO/LUMO-gap is diminished from A E = 3.20eV for H-T3-H 3 to A E x 1.65eV for the rigid analog 230.

228

229 (30%)

230 (45%)

A further diminution of the HOMO/LUMO-gap, respectively the band-gap, in rigid bithiophenes and the corresponding polymers is found when electronwithdrawing groups are linked to the bridging carbon. This is verified in cyclopenta[2,l-b;3,4-b’]dithiophene-4-one] 231 where the carbonyl group should increase the quinoid character of the oligomer [204]. In this compound the longest wavelength absorption is reasonably red-shifted (AA = 161 nm) compared to the parent compound 222. The oxidation potential of ketone 231 is somewhat higher (AE,, = 0.28 V) than that of cyclopentabithiophene 222, indicating that the withdrawing carbonyl group has a moderate effect on the energy level of the HOMO.

-7.1 0ligothioplierie.s

155

Several other substituted bridged bithiophenes 232-240 are in fact derivatives of ketone 231. Thus, the dioxolane 232 and the dithioacetale 233 have been synthesized by Roncali rt al. [205]. Furthermore by condensation reaction of thienocyclopentanone 231 and malonic acid derivatives, the dicyanoniethylene and cyano(nonafluorbuty1)sulfonyl-substituted derivatives 234 [206] and 235 [207], respectively, were recently described. In both compounds again the LUMO energy is lowered and the absorption red-shifted by about (AA = 100 nm) compared to ketone 231. Zotti, Berlin et a/. synthesized similarly p-nitrobenzyl-, p-nitrobenzylidene-, 4-pyridyL. and 4-(N-methylpyridiniun1)-substituted cyclopentadithiophenes 236-239 [208]. In these cases, the substituents cause a decrease in oxidation potential by A E = 0.25-0.30 V compared to the parent compound 222 [209]. Also starting from thienocyclopentanone 231, Roncali et a/. synthesized viu Wittig-Horner and Wittig olefination with phosphonate esters or phosphoniuni salts, respectively, a series of bridged bithiophenes 240 including a 1.3-dithiole moiety [210]. Here also the oxidation potential is decreased by A E = 0.1-0.3V and A,,, red-shifted by 90 nm.

231

232 (R,R' SY -O(CH,)P) 233 (R,R' -S(CHZW) 238 (R = H; R' = CHtC&LNOz) 238 (R = H; R = CH&H,N)

234 ( R R = CN) 235 (R = CN; R' SO,C,F,) f

237 (R s H; R' = C,H,NO,) 239 (R = H; R' = C5H,NMe* CF,S03.)

244 (R.R = -S(R'C=CR')S-)

The above mentioned examples prove that besides other strategies the rigidification of conjugated systems leads to a decrease of the HOMO/LUMO-gap and the extension of 7r-conjugation. However, new synthetic strategies seem to be necessary in order to develop longer rigid oligothiophenes or a totally planar (super)polythiophene.

2.1.2.2.2 a,a'-Substituted Oligothiophenes The different series of P-substituted oligothiophenes described above showed clearly that alkyl side chains in @-positionlead to a strong increase of the solubility properties, particularly for the longer oligomers. The investigation of the electronic structure of different redox states as models for polarons and bipolarons in solution is nevertheless only possible for longer members ( M 2 5). since radical ions of shorter oligomers tend inherently to dimerize or to oligomerize. Therefore, the introduction of solvating substituents at the reactive terminal a,a'-positions of the oligothiophenes should increase the stability of shorter members in the oxidized (and reduced) state and facilitate their investigation. Series of a-alkyl and 0.a'-dialkyl-substituted oligothiophenes were synthesized and characterized by different research groups. Especially, monosubstituted derivatives are attractive candidates since they offer the possibility of dimerizing them to the corresponding cia'-disubstituted oligothiophenes with doubled conjugated

156

2 Sulfiir-Containing Oligomcw

chain length. However, due to the electron richness of the oligothiophene system they are difficult to obtain selectively. Zotti et al. synthesized a-methyl derivatives up to the pentamer by using Pd-catalyzed Grignard cross-coupling reactions [211]. Thus, 5-bromo-5’-methyl-2,2’-bithiophene 242 obtained from 5-methyl-2,2/bithiophene 241 was reacted with BrMg-TI -H 42 to afford 5-methylterthiophene 243 in 93% yield. Bromination of Me-T3-H 243 with NBS led effectively to the 5bromo-5”-methylterthiophene 244 (92%). Successive coupling with the same Grignard reagent led to 5-methylquaterthiophene 245 in 87% yield on one hand and with the reaction with BrMg-T2-H 49 to the corresponding pentamer Me-T5-H 246 in moderate 20% yield, on the other hand [Eq. (59)].

242

243 (93%)

42

245 (87%)

/

244 (92%)

246 (20%)

Zotti et al. then coupled oxidatively the singly blocked a-methyl oligothiophenes Me-T,-H 241, 243, 245, 246 to the corresponding a,a’-disubstituted oligothiophenes Me-T2,-Me 247-250 [Eq. (60)]. The materials with doubled length up to the decamer were found as solids on the working electrode. From in situ EPR and conductivity measurements as a function of the potential it was concluded that the highest conductivity is obtained in a ‘mixed-valence’ state in which the oligothiophenes are partly in a radical cationic and a dicationic state [211].

241,243,245,246

247-250 (P2-5)

(n=2-5)

In an analogous manner, BBuerle et al. synthesized corresponding a-dodecyl- and a,a’-didodecyl oligothiophenes [73]. In order to obtain the monoalkylated derivatives, first the selective synthesis of monobrominated oligothiophenes in high yield and purity was performed. Due to the always present formation of disubstituted products which are difficult to separate, the mono-functionalization of oligothiophenes is an inherent problem. This was solved in this case by the use of the mild and selective brominating system NBS/DMF and by the careful choice

2. I Oligorhiopheries

157

of the reaction conditions in order to suppress the formation of the dibrominating products. Thus. Br-T2-H 50 and Br-T3-H 51 were isolated in pure form in 70% and 86% yield. respectively, starting from the unsubstituted H-T2-H 2 and H-T3-H 3 [Eq. (61)]. Normally, this type of compound has been synthesized by indirect methods and in moderate yields [ 164. The monobromination of H-T4-H 4 was however problematic, since the dibrominated product Br-T4-Br 48 is instantaneously formed. In the same way, the dibromination of these oligothiophenes was performed with two equivalents of NBS in DMF and results in the a,o’dibromooligothiophenes Br-T,-Br 46-48 in 76-88% yield [Eq. (61)]. 0r

w/

N0S

46,47,48 ( n= 2-4)

2,3,4 ( n = 2-4)

H

#?

(611

50,51 ( n = 2,3)

The o-monoalkylated oligothiopheiies 252, 254. 255 were obtained in 59-87% yield by ‘Kuniada-coupling’ of BrMg-TI -H 42 and 2-bromo-5-dodecylthiophene 251 [Eq. (62)] or of the Grignard reagent of the latter with Br-T2-H 50 and Br-T3-H 51, respectively [Eq. (62)].

42

251

50,51 ( n = 2,3)

252

253

254,255 ( n = 2.3)

The same reaction of two equivalents of 5-dodecyl-2-thienylmagnesium bromide 253 with Br-T,-Br 45 and Br-T2-Br 46 led to the corresponding a,&’-dialkylated terthiophene 256 and quaterthiophene 257 in 81 O/O and 70% yield, respectively [Eq. (64)l. a,a’-Didodecylsexithiophene 258 was prepared by oxidative coupling of lithiated terthiophene 16 with CuCI2 [Eq. (65)] [72].

253

51

45.48 (n= 1,2)

16

256,257(n= 1.2)

258 ( n = 4)

Hotta et nl [212] realized a series of corresponding methyl-substituted oligothiophenes up to the hexamer. This homologous row was also synthesized using

158

2 Sulfirr-Containing O/igonzers

Kumada's aryl/aryl-coupling procedure. Thus, the Grignard reagent of 2-halo-5methylthiophene 260 was reacted with Br-T,-Br 45-48 under Ni(0)-catalysis to yield the a,n'-dimethyl-oligothiophenes Me-T,-Me 261, 262, 247, 263, 248 which were purified through recrystallization from alcohols or ketones (dimer to tetramer) and chlorobenzene (penta- and hexamer) [Eqs. (66), (67)]. Even single crystals could be obtained using the purified materials by further slow recrystallization process. An X-ray structure determination of Me-T4-Me 247 could be performed.

259

260

260

45-48 (n = 1-4)

261

262,247,263,248 (n = 1-4)

Furthermore, the doping of thin films and single crystals, respectively, of these oligothiophenes with oxidating agents like iodine, nitrosyl salts NO'X-, or acceptors like TCNQ was investigated and resulted in conductivities in the range of F = lop2 to lo-' Scm-' [213]. Additionally, the neutral and the doped oligothiophene films showed spectroscopic characteristics in the solid state which were different from those in solution. In the neutral state, as for nonsubstituted oligothiophenes the absorption bands show a fine structure due to vibronic couplings. The lowest and the second lowest energy modes are assigned to the 0-0 and 0-1 transitions, respectively. Their energy separation is about 0.2 eV or 1600cmp' and is attributed to the ring-stretching mode in the thiophene rings [214]. In the doped state, the spectra were interpreted by the association of two molecules. For the first time, the same features were observed in the solid state (secondary peaks or shoulders on the high energy side), as was found for the dimerization of radical cations in solution (see above) [215]. With respect to the solid-state properties in organic transistors and light emitting diodes, Garnier et al. synthesized a,cw'-dihexylsexithiophene 267 by oxidative coupling of lithiated 5-hexylterthiophene 266 with CuCI2 (55% yield). The monoalkylated terthiophene 266 was obtained by palladium-catalyzed coupling of 5-hexyl-2-thienylzinc-chloride 264 and Br-T2-H 50 in 70% yield [216]. However, the solubility given for hexamer 267 is surprisingly low due to the large intermolecular interactions in the solid state. Structural characterization of vacuum-evaporated thin films of hexamer 267 by X-ray diffraction revealed molecular organization and layered structures with molecules standing with a tilting angle of 16" on the Si/Si02-substrate surface. Electrical characterization indicates a higher conductivity (factor of 3-6) and higher field-effect mobility (factor of 25) for the a,@'-disubstituted derivative than found for the parent H-T6-H 6. Furthermore, the conductivity in the oriented films is largely anisotropic with a ratio of 120 in favor of the conductivity parallel to the substrate plane.

2.1 Oligotliiopiienes

I59

By a similar strategy, the corresponding (iPr)3Si-T6-Si(iPr)3 269 was obtained [217]. Palladium-catalyzed coupling of the silylated organozinc thiophene 265 and Br-T2-H 50 gave (iPr)3Si-T3-H 268 in 48% yield. This is further dimerized to the a,a’-disubstitiited (iPrj3Si-T6-Si(iPr)3 269 with n-BuLi/CuCI? in 43% yield. Single crystals of this compound could be obtained and X-ray analysis showed that, in contrast to other oligothiophenes and obviously due to the bulky triisopropyl groups, intermolecular interactions play a more important role than intramolecular ones. Thus, in this case, a non-planar anti conformation of the conjugated 7r-system is favored in which a gradual twist of the thiophene rings is observed. The terminal thiophene rings are twisted with even 37.4” in relation to their neighbors which themselves form a dihedral angle of 21.4- with the two inner coplanar thiophene rings. The efficiency of oligothiophene-based light emitting diodes could be enhanced by using a two-layer system of unsubstituted H-T6-H 6 and ( i p ~ ) ~ S i - T ~ - S i ( i P269 r ) ~[2 181. The longest wavelength absorptions of both sexithiophenes are red-shifted in comparison to H-T6-H 6 (AA,,, = 11-12 nm) due the comparable electron-donating character of the alkyl and silyl substitutents.

264,265

50

266,268

p, 48%)

(68)

n-BuLi I CUa,

___, 267,269 (55%. 43%)

R = CeH,*

(iPr)$i

Parakka and Cava have reported long chain a,a’-disubstituted sexithiophenes 277-279, which were either obtained by oxidative dimerization of appropriate monoalkylated terthiophenes 270-272 or by reductive nickel-catalyzed coupling of the corresponding brominated terthiophenes 274-276 [219]. Reaction of monoaldehyde OHC-T3-H 74 with hexadecylmagnesiumbromide yields the corresponding carbinol in 92% yield, which is very effectively further reduced with sodium cyanoborohydride to 5-heptadecylterthiophene 270 in 97% yield. This monoalkylated terthiophene was successively coupled with FeCI3 in benzene to the a,a-diheptadecyl-sexithiophene277. The blue oxidation product is finally reduced by hydrazine to yield 55% of the hexamer. 5-Hexadecyloxymethyl-terthiophene 271 was made by nucleophilic substitution of 5-hydroxymethyl-terthiophene. This was obtained from OHC-T3-H 74 by reaction with hexadecyl bromide in only 7 % yield. The olefinic 5-(heptadec- I-eny1)-terthiophene 272 was synthesized by dehydration of carbinol 5-( I-hydroxyheptadecy1)-terthiophene with p-toluenesulfonic acid. At ambient temperature, 30% of the pure truns derivative is obtained, at 85°C 58% of a cisltrans mixture (85: 15). The bromo compounds 274-276 were prepared by the same procedures as described above starting from 5-bromo-5”formyl-terthiophene Br-T3-CHO 273 which is obtained by bromination of OHC-T3-H 74 with NBS in 20% yield. In one case, direct bromination of

160

2 Suifur-Containing Oligomers

alkylterthiophene 270 to bromoalkylterthiophene 274 was achieved with 1,3dibromo-5,5-dimethylhydantoinin 93% yield. The bromoterthiophenes 274-276 were coupled to the corresponding aa’-dialkylated sexithiophenes 277-279 with the catalytic system [Pd(PPh3)4],zinc, potassium iodide, and triphenylphosphane in D M F in 56% 277, 81% 278, and 82% yield 279 [Eq. (69)].

74

273

274-276

270-272

277-279

Absorption spectra and redox potentials of the monoalkylated terthiophenes and the dialkylated sexithiophenes were determined. Due to the conjugation of the adjacent double bonds in a-position, in comparison to the n-alkyl substituted terthiophenes 270 and 271, the olefinic counterpart 272 exhibited a bathochromic shift of the longest wavelength absorption (Ax = 22-25 nm) and is oxidized at lower potentials (AE,, = 0.12-0.17 V). The same trend is found less pronounced for olefinic sexithiophene 279 in comparison to the alkylated hexamers 277 and 278 (Ax = 10-15 nm; AE,, = 0.04-0.08 V). Despite their solubilizing alkyl side chains, surprisingly the hexamers did not show a much higher solubility in organic solvents than the parent unsubstituted H-T6-H 6. Evidently, besides the T-T interaction of the conjugated system, additional van der Waals attractions of the long alkyl chains cause the low solubility of the rigid-rod type molecules. Some more alkylated and silylated oligothiophenes were synthesized with respect to their electrochemical and EPR properties of the corresponding radical ions, or their biological properties: 5,5/-dimethyl-, 5,5/-diisopropyl-, 5,5’-di-tbutyl- [220], 5-trimethylsilyl-, 5,5’-bis(trimethylsilyl)-2,2~-bithiophene[221], 5-methyl-, 5,5’dimethyl-, 5-tbutyl-, 5,5/-di-tbutyl-, 5-[(H3C)2=CHCH2-]-[24b, 2221, 5,5”-bis(trimethylsily1)-terthiophene, and 5,5/’-bis(trimethylsilyl)-quaterthiophene[221]. In their series of regioirregular P,$-alkyl substituted oligothiophenes, Wynberg et al. also included the septithiophene tBu-T,-tBu 280 which was synthesized similarly

?.I Oligorliiopkeries

161

by ‘Stetter reaction’ of 5-formyl-5”-rbutyl-terthiophene and the corresponding Mannich base (47% yield) and subsequent cyclization of the resulting diketone with L.R. (26% yield) (see above) [156]. Also in this case, the solubility is drastically reduced in comparison to the $-alkyl substituted analogs and the longest wavelength absorption is red-shifted (Ax = 1 1 - 16 nm). Evidently, aa’-disubstitution of oligothiophenes results in a nearly undisturbed conjugated 7r-system and therefore simultaneously in strong intermolecular interactions which on the other hand cause a low solubility.

280

2.1.2.2.3 a@-Substituted Oligothiophenes Several series of oligothiophenes bearing substituents at both the a- and the ppositions have been developed recently. This class of compounds now comprises two factors which affect the properties of oligothiophenes. The substituents at the (terminal) a-positions block the reactive part when the oligothiophenes are transformed into cationic species as models for polarons and bipolarons, which are considered as the charge carriers in conducting polymers. Thus, also the investigation of shorter and normally reactive oligomers becomes available. As was seen in the previous paragraph, alkyl side chains in the a-positions do not cause an increase of the solubility and the longer members are scarcly soluble in common organic solvents due to their rigid-rod character, the additional introduction of solubilizing alkyl side chains in i3-positions is straightforward. On the other hand, certainly, Ihubstituents at inner thiophene rings cause steric interactions with the adjacent rings and thus a certain reduction of the conjugation length should be taken into account. The synthesis and characterization of a complete series of ‘end-capped’ oligothiophenes (ECnT) up to a heptamer by Biiuerle clearly revealed the usefulness of this approach [13, 721. Due to the blocking of the reactive 0-and 0-positions with a cyclohexene ‘cap’, on the one hand a more precise characterization of the oligomers in various oxidation states was possible, and on the other hand, due to the enhanced solubility, excellent correlations of the spectroscopic and electrochemical data with the (inverse) chain length were obtained. 4..5.6,7-Tetrahydrobenzo[b]thiophene281 is the key building block for this series and was synthesized in 75% yield by ether cleavage of 3-( p-methoxyphenoxybutyl)thiophene with boron tribromide under dilution conditions which favors the intramolecular ring closure reaction [86].The smallest member in this series, E C l T 282. could be obtained with the same ether cleavage reactions from 3.4-di( p-niethoxyphenoxybutyI)thiophene in 70% yield. Selective bromination of 281 with NBS yields the 2-bronio derivative 283 in 89% yield which can easily be transformed into the corresponding Grignard reagent. ‘Kumada-coupling’ of the latter with 283 itself, Br-T,-Br 45, Br-T2-Br 46, and Br-T3-Br 47, results in the bithiophene EC2T 284 (47% yield) [Eq. (70)], terthiophene EC3T 285 (64% yield) [Eq. (71)], quaterthiophene EC4T 286 (78% yield) [Eq. (72)], and quinquethiophene ECST 287 (64% yield) [Eq. (73)], respectively. ECST 287 and the higher members 290-291 were synthesized by first

162

2 Sulfur-Containing Oligomers

reacting BrMg-T, -H 42 with 2-bromotetrahydrobenzo[b]thiophene 283 under ‘Kumada-conditions’ to form the ‘mono-capped’ bithiophene 288 in 51 YOyield which was successively brominated with NBS to the other important key component 289 in 67% yield. Transformation of 289 into the Grignard compound and nickelcatalyzed coupling with Br-T,-Br 45, Br-T2-Br 46, and Br-T3-Br 47 gave EC5T 287 in 58% yield [Eq. (73)], the hexamer EC6T 290 in 58% yield [Eq. (74)], and the heptamer EC7T 291 in 44% yield [Eq. (75)], respectively.

u7, 284

(--&+@ \ I

285

\

(72)

di)

S

286

(73)

288

287

dii) n

(74) W 290 289

(75) 291

(i) NBS/DMF/25”C [89%]; (ii) BrMg-TI-H (42)/Ni(dppp)CI,/Et,O [87%]; (iii) NBS/DMF/25“C [67%];(iv) 1. Mg/Et,O; 2. Ni(dppp)C1,/5-bromo-2-(4,5,6,7-tetrahydrobenzo[b]thien-2-yl)thiophene 283 [47%]; (v) 1. Mg/Et,O; 2. Ni(dppp)CI,/Br-TI-Br (45) [64%]; (vi) 1. Mg/Et,O; 2. Ni(dppp)Cl,/ Br-T,-Br (46) [78%]; (vii) 1. Mg/Et20;2. Ni(dppp)CI,/Br-T,-Br (47) [64%];(viii) 1. Mg/Et,O/benzene; 2. Ni(dppp)CI,/Br-TI-Br (45) [58%]; (ix) 1. Mg/Et,O/benzene; 2. Ni(dppp)CI,/Br-T,-Br (46) [58%]; (x) 1. Mg/Et,O/benzene; 2. Ni(dppp)CI,/Br-T,-Br (47) [44%].

Purification of the oligomers was achieved by repeated chromatography and recrystallization. In the case of the higher members, extraction of the crude material and fractional sublimation gave the best results. The final purification of all compounds in this series was achieved by repeated fractional sublimation in a glass tube with temperature gradient.

The enhanced solubility and the blocking of the reactive positions without perturbing the 7r-conjugation allowed, even in the case of the shorter oligomers, the precise determination of the electronic and structural features a t various oxidation levels and their correlation with the chain length (see section 2.1.2.1.4). The longest wavelength absorptions and the emission maxima shift to lower energies as the extent of the 7r-system in the oligomer increases. An excellent correlation a

1.5

4

4

0

0.2

0,4

0.6

0,8

1

Inverse chain length (lh) b

0

0.2

0.4

0.6

0.8

Inverse chain length (lh)

I

Figure 9. (a) Correlation of the absorption and emission energies of the ‘end-capped’ oligothiophenes ECIT-EC6T 282. 284287, 290 with the inverse chain length ( l / t i ) . (b) Correlation of the first and the second oxidation potentials of the ‘endcapped’ oligothiophenes EC IT-EC6T 282. 284-287. 290 with the inverse chain length ( 1 / t i ) [ I 31.

164

2 Sulfur-Containing Oligomers

of these transition energies with the inverse chain length is observed (Fig. 9, a). Extrapolation to a hypothetical infinite chain length models the properties of an ‘ideal’ polymer (in solution) and gives a maximum absorption at, ,A, = 538nm = 704nm (1.76eV). These energies lie (2.30eV) and a maximum emission at, , A, lower than those found experimentally for (solid) polythiophene films as a result of defects and interruptions of the conjugated backbone in the polymer. On the other hand, this correlation allows the estimation of the mean conjugation length in the ‘real’ polymer which is in this case for polythiophene about 10-1 1 a-linked thiophene units and thus differ dramatically from the mean chain length of the polymer. Except the monomer E C l T 282 all compounds fluoresce strongly and [”:: = 7% (EC2T the quantum yield increases with increasing chain length @ 284) to 40% (EC6T 290)]. It was shown by cyclovoltammetry that even the trimer EC3T 285 is reversibly oxidized to the cation radical. Starting with the quaterthiophene EC4T 286 even stable dications can be created. In analogy to the spectroscopic results, the oxidation potentials of both the mono and the dications are gradually shifted to lower energies with increasing size of the .ir-system. Again for both redox potentials an excellent correlation of the energy levels verms the inverse chain length is obtained. The energy difference between the first and the second oxidation potential gradually decreases and finally both regression lines intersect as the ‘ideal’ infinite chain length is approached (Fig. 9b). This result clearly implies that in the case of very long chains a second and probably additional electrons can simultaneously be removed at the same energetic level as long as the charged defects can reside sufficiently separated on the conjugated 7r-system without interaction. In comparison to ‘real’ bulk polythiophene which exhibits a broad reversible redox wave (EO FZ 0.3 V vs. Fc/Fc+) due to the inhomogeneity of the material, the redox potential of an ‘ideal’ polythiophene is estimated to be considerably lower ( E O E 0.07 V vs. Fc/Fc+). Vice versa, the estimation of the mean conjugation length of the ‘real’ polymer from this correlation results in about 5- 10 correctly linked thiophene rings [13, 721. Due to the stability of the radical cations, it was now possible to investigate the reversible dimerization equilibrium of the ‘end-capped’ oligothiophenes by temperature-dependent in situ spectroelectrochemistry combined with EPR [223]. These studies revealed now a clear dependence of the dimerization tendency on the chain length. The dimerization enthalpy which was determined either by UV/VIS/NIR or EPR increases as the chain length increases. The experiments showed that radical cations of very long oligomers are almost completely dimerized at room temperature and show only weak EPR activity (see above) [169]. The correlation of the transition energies obtained for the ECnT monomeric and dimeric radical cations 283-285 also exhibit a linear dependence with the inverse chain length. Extrapolation of both sets to an infinite chain length reveals that at this extreme point, the regression lines for the pair of transitions almost intersect. This result clearly implies that the charge becomes more and more delocalized in very long chains and that Coulomb repulsion decreases with increasing chain length. The electronic structure of the dimeric cation radical thus approaches that of the monomeric cation. There is a considerable variance

2.1 Oligotliiophenes

165

in experimental data for polarons and bipolarons in doped polythiophene ( E l M 1.3-1.4eV; E. z 0.3-0.5eV) and thus an unavoidable uncertainty in the estimation of the conjugation length from this diagram. The extrapolated transition energies for infinite chain length ( E , z 1.25-1.38eV; El M 0.45-0.54eV) do basically correspond to the experimental values and are slightly lower for the high energy transition [223]. The data now permitted construction of an energy level diagram for monomeric and dimeric radical cations which is consistent with the observed transitions and explains their distinct blue-shift upon dimerization. This 'Davidov blue shift' [224] is coherent with a stack-like arrangement of the two oligomeric cations. Recently, from electrochemical measurements on /3alkyl substituted dodecithiophene 126 it was concluded that oxidized species form four-fold charged 7r-dimers [225]. In this context. the analogy between the 7r-dimerization of oligothiophene radical cations and highly conducting charge transfer salts is astonishing. Since increased crystallinity and orientation in e.g. polyacetylene leads to an increase in conductivity [226] and since doped microcrystalline (longer) olieothiophenes approach conductivities of the corresponding polymers (30 S cm- r) [ 1561, it is reasonable that in analogy to the conducting crystalline charge-transfer salts, high conductivity in conducting polymers and even more in well-defined oligomers on the molecular level might be primarily due to charged (micro)crystalline stacks of conjugated segments. However, the measurable macroscopic conductivity is determined and diminished by the transfer of the charge carriers from stacks to stacks and to bigger aggregates. Since in most applications conjugated materials are used in the solid form, an important advantage of oligomers is therefore that the solid state properties can be investigated in crystals or in vapor-deposited (thin) films. However, in the solid state morphological and supramolecular effects may play an important role and lead to different properties than those found in solution. Investigations on the optical and transport properties of the 'end-capped' oligothiophenes ECnT 286, 287, 290, 291 in the solid state have been undertaken. The absorption and emission spectra of thin films (thickness 40-60 nm) show strongly structured bands including several vibronic couplings. The maxima gradually shift to lower energies with increasing chain length. Also for the solid state, excellent correlations of the transition energies with the inverse chain length are obtained. Nevertheless, the fluorescence quantum yields are diminished by three to four orders of magnitude in comparison to those in solution, but are still much higher than in unsubstituted oligothiophenes which form oriented monolayers when evaporated on fused silica [72, 2271. First EPR studies on photoexcited triplet states of oligothiophenes were performed using the 'end-capped' oligothiophenes ECZT to EC6T 284-287, 290 in frozen solutions at 4 K [228]. The characteristic lineshape of the EPR spectra provides evidence that photoexcitation leads to niolecular triplet states in all compounds. The fine structure parameter D could be determined and decrease continuously with increasing oligomer chain length, i.e. the wavefunction becomes more extended the longer the oligomer chain is. The best correlation between D and the number of thiophene rings was found by plotting D against the inverse chain

166

2 Sulfur-Containing 0ligonier.s

length ( l / n ) . The extrapolation to infinite chain length suggests that triplet excitation on an infinite one-dimensional oligothiophene would possess a finite extension. Solid-state in situ ATR, FTIR- and FT-Raman spectra of the whole series of ‘end-capped’ oligothiophenes ECnT have been studied experimentally and theoretically. The spectra in the neutral state show for some bands (C=C double bond vibration) a convergent behavior with increasing chain length and are shifted to lower energies. This clearly indicates that both the bond order and the bond fixation is slightly decreased. Finally the spectra approach the vibrational properties of polythiophene [229]. During doping with iodine vapor, films of EC3T to EC6T 285-287, 290 show broad doping induced bands in the region of 6500-5500 cm-l similar to polythiophene which are due to free charge carriers whereas the bands in the region from 1800-660cm-’ are much narrower. In the doped state no convergence of the bands is found indicating that the oligomers are too short to allow a fully extended defect [230]. Electrical transport properties of non-doped and iodine doped ‘end-capped’ oligothiophenes in thin films were studied by current/voltage measurements between gold microcontacts [231, 2321. The experiments were performed as a function of dimension of the microstructures, film thicknesses, chain length of the oligomers, doping state and time. The devices show Ohmic behavior and a logarithmic dependence of the conductivities on the inverse chain length and furthermore a strong dependence on the in sirtr doping time and geometric parameters. The conductivities of non-doped devices are independent of the film thickness and electrode distance (cl 2 l00nm). Light-emitting diodes (LED) based on conjugated materials are at present possibly the most important application of conjugated materials. However, there are still some drawbacks, e.g. lifetimes necessary for industrial applications. The underlying and limiting physical and chemical processes such as charge carrier injection of holes and electrons at the electrodes or their recombination are therefore of considerable interest and mostly barely understood. In order to study the electroluminescent properties of conjugated systems systematically, the first organic LEDs based on defined conjugated oligomers were developed by Umbach rt al. [21]. The series of ‘end-capped’ oligothiophenes EC4T to EC7T 286,287,290,291was used to prepare LEDs by vacuum sublimation of the active organic material and allowed to investigate the dependence of their transport properties and spectral distributions on the chain length of the oligomers. The devices yield light emission in the yellow/orange color range at relatively low voltages ( 2 2 . 5 V) and moderate current densities. The electroluminescence spectra which possess the same shape and energy positions as the photoluminescence spectra indicate that the same radiative decay process in both cases is valid. The peak maxima are gradually shifted to lower energies with increasing chain length and an excellent correlation with the inverse chain length was obtained. Since these materials are easily oxidized or p-doped, respectively, further investigations on metal/ECnT/ITO LEDs showed that the corresponding current/voltage curves are due to the injection and transport of holes. The electroluminescence, however, is correlated to the injection of electrons at the cathode and light emission

2.1 Oligotliioph~rirs

167

directly arises from a region close to the cathode [233]. Current and intensity of electroluniinescence were also measured as a function of various metal cathodes in a wide range of temperature and thicknesses of the active EC6T film [234]. The external quantum efficiencies ( t i = IO-' to I0-j) significantly depend on the metal contact (Ca, Mg, Al, In. Ag) and the device temperature (4-300K). At room temperature they are found to be in the same order as those reported for LEDs based on various polythiophene derivatives (71 = 3 x lop5 to 1 x lo-') [235]. In comparison with LEDs based on H-T6-H 6 [236] the efficiencies of the EC6T LEDs are about one order of magnitude higher, which is in agreement with the ratio of the respective photoluminescence yields [237]. Even much smaller efficiencies were reported for LEDs based on e.g. Me-T6-Me 248 (11 = 3 x lop9) [238]. On the other hand, the efficiency of a two layer oligothiophene LED conr ) could ~ be remarkably enhanced sisting of H-T6-H 6 and ( i P ~ ) ~ s i - T ~ - S i ( i P269 [218]. Controlled vacuum-deposition of the 'end-capped' oligothiophenes EC3T to EC6T 285-287. 290 in very thin films down to the monolayer regime on Ag( 1 1 I )-surfaces allowed to investigate their supramolecular behavior by means of STM [239]. The STM-images of the oligothiophenes showed in each case extremely large areas with highly ordered 2D crystalline monolayers in which a well-oriented and nearly defectfree stack-like arrangement of the oligomers is observable. This indicates a high purity of the oligomer materials (Fig. 10). Surprisingly and for the first time, images with a mbiizuIeczikm resolution of the oligothiophene units are obtained. Oligomers with an even number of thiophene rings (EC4T, EC6T) form structures with equal stacks and one molecule per unit cell, whereas an uneven number of thiophene rings (ECST) leads to the formation of two different stacks and two molecules per unit cell. The interpretation of the images which are underlined by LEED and theoretical investigations leads

Figure 10. STM image of a monolayer of ECST 287 on Ag( 1 1 I). (a) Scan size 330 x 330 A and (b) 80 x 80A [739].

168

2 Sulfur-Containing Oligornrrs

to the conclusion that each thiophene ring and the ‘end-caps’ are represented by white spots. The comparison of the distances in the STM-images and the calculated geometries in the case of EC5T monolayers revealed that the single molecules include the energetically favorable ail-trans orientation of the thiophene rings. Conformations including cis arrangements are less probable, but cannot be excluded. With respect to their properties in solution and the solid state, the model character and their applications as new materials, the ‘end-capped’ oligothiophenes ECnT seem to be one of the best investigated series up to now. Due to their defined character, their effective purification methods and controllable processability by vacuum evaporation highly pure materials are obtained. Tour et ai. have synthesized and characterized an a,,!%substitutedoligothiophene series up to an octamer in which the reactive a-positions are blocked by trimethylsilyl (TMS) end groups and the solubility is retained by the alkyl substituents regioregularly attached to free @-positions[240]. In contrast to the above mentioned ‘end-capped’ oligothiophenes, in this case the solubilizing alkyl side chains infer some distorsion of the oligothiophene 7r-system and thus induce reduction of the overall conjugation. By stepwise metal-catalyzed coupling of substituted (oligo-)thiophene building blocks the whole series was synthesized. Terthiophene 294 was obtained in 73% yield by ‘Suzuki-reaction’ of 5-trimethylsilyl-Z-thiopheneboronic acid 292 and 2,5-diiodo-3,4-dimethylthiophene293 [Eq. (76)]; quaterthiophene 297 in 42% yield by ‘Stille-coupling’ of two bithiophene units: 5-iodo-3-methyl-5’trimethylsilyl-bithiophene 296 and 5-tributylstannyl-3-methyl-5’-trimethylsilyl-2,2’bithiophene 295 [Eq. (77)]; pentamer 298 in 47% yield by ‘Stille-coupling’ of two equivalents of the latter tinorganic derivative 295 and I-TI-I 54 [Eq. (78)]; hexamer 303 in 52% yield by palladium-catalyzed coupling of two terthiophene units: 5-iodo-3,4’-dirnethyl-5’’-trimethylsilyl-terthiophene 301 and 5-tributylstannyl3,4’-dimethyl-5’-trimethylsilyl-a-terthiophene 302 [Eq. (SO)]; heptithiophene 304 from two equivalents of the latter stannylated terthiophene 302 and I-TI -I 54 in 64% [Eq. (81)] and the heptamer 305 in 58% yield by reacting the organotin reagent 302 with 3,4-dimethyl-2,5-diiodothiophene293 [Eq. (SO)]. Finally, the longest defined oligomer in this series, the octithiophene 307 was obtained by the ‘Stillecoupling’ of the stannylated terthiophene 302 and bithiophene I-T2-I 306 in 52% yield [Eq. (SZ)].

295

296

297

2.1 Oligotliiopheries

169

+

2 Me,Si *SB ,%

+4c 295

296

HF'

302

54

298

300 (X = H) 301 (X = 1 ) 302 (X = SnBug)

299

306

307

The linear optical [240], nonlinear optical [24 11, and electronic properties [242], of these thiophene oligomers were studied. The absorption maxima increase with increasing chain length, but no saturation was reached. Due to the different number of methyl groups in the oligomers and therefore different influence on the conjugation, in this series only a rough correlation with the (inverse) chain length can be obtained. By comparison of the UVjVIS spectra with the spectra of the

110

2 Sulfur-Containing 0ligomer.c

analogous polymers, it was concluded that electrochemically prepared poly(3alkylthiophene) A(,, = 430-440 nm) should have an effective conjugation length of 6-7 correctly linked thiophene units. The longest homolog, octamer 307, has maximum absorption at , , ,A = 458 nm. The third-order non-linear optical studies, determined by third-harmonic generation, on this series of oligomers corroborates well with the results obtained on polymeric systems while refuting data that had been obtained on the less soluble unsubstituted oligothiophenes. The soluble thiophene oligomers with three and more units can be electrooxidized stepwise to the radical cation and the dication. First and second oxidation potential and absorption energy of the radical cations as well as of the dications correlate well with the inverse chain length. The correlation of the electronic transitions of the oxidized oligomers permitted to estimate the delocalization length of the radical cation to 12 units and the dication to 10 units in the corresponding polymer [242]. In a related study directed toward the construction of molecular electronic devices [243] Tour et al. synthesized orthogonally fused oligothiophenes which might include potentially addressable 'on' and 'off' states [244]. First, the spiro core 309 was constructed from tetraalkyne 308 in 41 YOyield, transformed into the tetrabromo derivative 310 in 88% yield, and then the four thiophene 'branching arms' added at one time by metal-catalyzed coupling reactions.'Stille-type' coupling of the brominated spiro core 310 with excess 2-tributylstannyl-5-trimethylsilylthiophene 311 resulted in the orthogonally fused terthiophene 312 in 41% yield [Eq. (83)]. Reaction of the spiro compound 310 with excess of terthiophene 313 yielded the spiro-fused heptathiophene 314 in 86% yield [Eq. (84)]. Me,Si

--

SiMe,

1. BULL CpJrCI, ~~

2 S,CI,

311

308

-

30% X SIMe, (41%) 310, X

312

= Br (86%)

Me

310

+ ME

313 Me '

Me 314

Each oligothiophene unit could be independently charged to the radical cation and dication by means of cyclovoltammetry, indicating that there is no

2.1 Oligothiopheries

171

cross-communication between the orthogonally fused segments which is certainly a prerequisite for the ‘molecular electronic’ device capability [245]. A number of 0.3-methyl-substituted ‘end-capped’ bi-, ter- and quaterthiophenes were synthesized and characterized by Engelmann and Kossmehl in order to have model compounds for the closer elucidation of the polymerization mechanism and kinetics of thiophenes [246]. Nearly all compounds were synthesized by nickelcatalyzed ‘Kumada-coupling’ reactions in yields ranging from 7 to 69%. Investigations on the various radical cations by fast scan voltammetry, ring disc electrode kinetics. and EPR revealed for the first time that the /$positions 3.3‘ and 4,4/ in bithiophenes give different contributions to the reactivity of electrochemically generated radical cations. The chemical oxidation of several ‘monocapped’ bi- and terthiophenes with aqueous FeC13 lead in good yields to perinethylated dinierization products and related quater- and sexithiophenes were isolated. Furthermore, oxidation of compounds like 4,4’,5,5’-tetramethy1-2,2’bithiophene 315 now lead to novel reaction products in which two bithiophene units are linked viii a methylene group. In this particular case (4,4’,5’-trimethyl-2,2’bithien-5-yl)(4,5,4’,5’-tetraniethyl-2,2’-bithien-5-yl)methan 316 was isolated in 25% yield [Eq. (85)l.

In their series of regioirregular $,,!?’-alkyl substituted oligothiophenes (see section 2.1.2.2.1) Wynberg ef al. also included a $alkylated undecithiophene 317 which was synthesized similarly to the oligomers by ‘Stetter reaction‘ and subsequent cyclization of the diketone with L.R. In this case. the terminal groups d o practically not influence the properties in comparison to the 3-alkylated undecithiophene 122. Conductivity, solubility, and absorptions are nearly identical [ 1561.

317

2.1.2.2.4 Functionalized Oligothiophenes

Several series of oligothiophenes bearing functional groups in the /3- and aposition are summarized below as far as these compounds are relevant to conjugated materials. This includes functional groups which might be electrondonating or -accepting. For example, donor-substituted oligothiophenes represent

172

2 Sulfur-Containing Oligomers

ideal model compounds for the corresponding polythiophenes in order to better elucidate the steric and electronic influence of the substituent onto the properties. Especially, poly(a1koxythiophenes) are very promising materials because due to the electron-donating effect of the substituents they show an excellent environmental and electrochemical stability, high conductivity, and transparency in the conducting state. In this respect, doped poly[(3,4-dioxyethylen)thiophene]is one of the most stable polythiophenes known and in the meanwhile is commercialized in antistatic and -electromagnetic shielding layers in photographic document films [247]. Donor- and acceptor-substituted oligothiophenes The synthesis of donor-substituted oligothiophenes as model compounds for the corresponding polymers and as starting monomers for polymerization has become attractive. Besides several examples of 3,3’- and 4,4/-dialkoxybithiophenes and parent mixed alkoxy,alkyl-substituted derivatives for subsequent polymerization [248, 2491, Gronowitz and Peters obtained 3/-methoxy-2,2/:5’,2”-terthiophene 320, which also occurs naturally, in 54% yield by the exchange of the halogen function in 3/-iodo-2,2’:5/,2”-terthiophene 318 with sodium methoxide and cupric oxide in a nucleophilic substitution reaction. As a non-negligible side reaction dehalogenation takes place and the 30% H-T3-H 3 formed could be separated by column chromatography. Iodoterthiophene 319 itself is prepared by the Pd(0)-catalyzed reaction of 2,3,5-triiodothiophene 318 with 2-thiopheneboronic acid

318

In order to evaluate the influence of the regiochemistry on polymer properties, Zotti et al. synthesized two series of donor-substituted oligothiophenes which bear pentoxy groups either in the 3-position of the terminal thiophene rings or in the 4-position [250]. 3-Pentoxythiophene 321 is converted by iodination with iodine and mercury oxide to 2-iodo-3-pentoxythiophene322 (91YO yield) which serves as starting material for the oligothiophenes 323-325. Thus, bithiophene 323 was prepared by Ni(0)-catalyzed homo-coupling in 83% yield. In contrast, terthiophene 324 was obtained by Pd(0)-catalyzed coupling of iodothiophene 322 with thiophene-2,5-diboronic acid in 50% yield. Quaterthiophene 325 was either isolated as by-product in the preparation of the trimer 324 or obtained in a ‘Kumada-type’ coupling of the Grignard reagent of iodopentoxythiophene 322 with I-T2-I 306. However, in this case no yields are given [Eq. (87)]. The regioisomeric 4,4‘-dipentoxy-2,2/-bithiophene 327 bearing the pentoxy groups in the ‘outer’ ,”-positions was synthesized in 70% yield by oxidative coupling of lithiated 3-pentoxythiophene 321 with copper chloride. 4-Pentoxy-2-thiopheneboronic acid 326 which was synthesized from 3-pentoxythiophene 321, lithiumdiisopropylamid and trimethylborate was coupled with I-T1-I 54 and I-T2-I 306,

2.1 Oligotliiopheiies

173

respectively, under [Pd(PPh3)4]-catalysisto result in the corresponding terthiophene 328 (67%) and quaterthiophene 329 (no yield given) [Eq. (SS)]. OR

OR

OR

322

321

326

RO

323-325(n.0-2)

R = C,H,,

327-329(n=0-2) R = C5H,,

Due to the electron-donating character of the alkoxy groups, the redox potentials of these oligomers are diminished in comparison to the non-substituted parent oligomers and as expected decrease with increasing chain length. In the series of oligothiophenes 323-325 which bear the substituents at the ‘inner’ p-positions, reversible cyclovoltammograms were obtained (EP = 0.38-0.26 V vs. Ag/AgCl). In contrast, the members of the other series 327-329 exhibit irreversible redox waves indicating the follow-up reaction of the radical cations to form higher oligomers or polymers (Epa= 0.67-0.40 V vs. Ag/AgCl). Miller et ul. presented a series of structurally defined methoxy-substituted oligothiophenes, dimers through hexamers, symmetrically substituted at the ‘outer’ (330-332) or ‘inner’ P-positions (333-336) and with terminal methyl (337340) or carbonic acid groups (341) [251]. The electron-donating methoxy groups and terminal alkyl groups stabilize cationic species and thus radical cations and protonated oligothiophenes could be investigated. Furthermore, these compounds serve as models for the interchain radical -ir-dimerswhich were found by Miller and others to be an important alternative to bipolarons in oxidized polythiophene [164]. The carboxylic acid endgroups provide the solubility of the hydrophobic oligomer in water which was found to be an ideal medium for the observation of 7r-stacking of radical cations [252]. The oligomers were built up by the cross-coupling of (oligo-)P-methoxy-a-iodothiophenesand (oligo-)a-stannylthiophenes, catalyzed by Pd(0)-complexes. Furthermore, the oxidative homo-coupling of a-lithiated thiophenes by Fe(acac)3 was used to prepare the dimers 330 and 333. Interestingly, V O ,

OCH, R

330-332( n = 0-2)

* n

333-336( n = 0-3; R = H) 337-340( n = 0-3;R = CHd 341 ( n = 2: R = COOK)

R

for these type of compounds ‘Kumada-type’ and ‘Suzuki-type’ couplings were not successful. Corresponding tetramethoxy-substituted pentamers 344, 345 and a hexamer 347 were synthesized starting from 3,3”-dimethoxy-cu-terthiophene342. Pd(0)-catalyzed coupling of the bis-stannylated terthiophene with 2-iodo-3-methoxythiophene343 results in quinquethiophene 344 (62% yield). This is subsequently lithiated and methylated in the terminal a-positions with n-BuLilDMS to form pentamer 345 in 80% yield [Eq. (89)l. Monomethylated terthiophene 346 obtained in 81% yield from 342 with the same procedure was dimerized with the system n-BuLi/Fe(acac)3 to the tetramethoxy,dimethyl-substituted hexamer 347 in 70% yield [Eq. (90)]. The quaterthiophene dicarbonic acid 341 was obtained from the tetramer 335 by reaction with n-BuLi/C02 in nearly quantitative yield. Oxidation of the dicarbonic acid 341 in water produces stable radical cations and aggregated 7r-stacks were demonstrated spectroscopically. Isolation of the latter results in an electrically conducting salt with a conductivity of CT = 2 x lop3S cmp’. Partially oxidized mixed valence salts even exhibited a ten-fold higher conductivity [251b]. Absorption data of the ‘inside’ and ‘outside’-substituted oligothiophenes in comparison to the parent non-methoxylated a,a’-dimethyloligothiophenes showed the clear trend that adding two inside methoxy groups leads to an increase of the maximum absorption by about Ax,,, = 17 nm, adding two outside methoxy groups by about Ax,,, = 22nm. It thus becomes clear that in contrast to alkyl chains these substituents do not much perturb the electronic structure and represent valuable models for non-methoxylated polythiophenes. X-ray crystallographic analysis of the tetramer 335 shows in coherence to the spectroscopic results that the molecule is nearly coplanar in a trans conformation [251a].

!

343 344(R=H).346( R=CH,)

n-BuLi I DMS

346

347

From the ‘capped’ trimer 331 and tetramer 332 Miller et al. prepared the first protonated oligothiophenes 348 and 349 in solution by the treatment with trifluoroacetic acid (TFA) [Eq. (91)] [252]. Since a protonated species could not be obtained from Me-T3-Me 262, the methoxy groups clearly enhance the

2.1 Oligotliiophenes

175

basicity and stabilize the positive charge. These compounds can also be regarded as a-complexes, which are normally reactive intermediates in electrophilic substitution reactions, and which were characterized by H-NMR and absorption spectroscopy. In comparison to the neutral parent compound, the protonated cationic species exhibit a distinct bathochromic shift of the longest wavelength absorptions (AA = 204, 216 nm) which indicates a delocalization of the positive charge. Interestingly, the absorption maxima are located very similar to those of authentic terthiophene dications obtained by oxidation (348” , , ,A = 590 nm; 3492+ A,, = 570nm). These data indicate that the oligothiophene .ir-dimers which absorb in the near IR are not a-dimers or dimeric 0-complexes which e.g. can be isolated as follow-up products of radical cations formed in the oxidation of triaminobenzenes [253].

’

‘“w ‘“w CHl H3

\ I

H3C

A CF COOH

\ I

HC ,

n

H

cF,coo- (911

n

331,332 (n=1,2)

348,349 (n=1,2)

A homologous series of donor-substituted oligothiophenes bearing somewhat weaker electron-donating methylmercapto groups in the ’outer’ ,&positions were synthesized up to a quaterthiophene by Bauerle et a1 [254]. The central building block for the synthesis of the P.P-disubstituted oligothiophenes was 2-bromo-4(methy1mercapto)thiophene 350 which in contrast to many other 2,4-disubstituted thiophenes could be synthesized selectively and free of isomers. It was reacted with magnesium to the corresponding Grignard reagent and homo-coupled under Ni(0)-catalysis to the 4,4’-di(methylniercapto)-2,2’-bithiophene351 in 66% yield. Cross-coupling of the monothiophene with Br-T1-Br 45 led in 35% yield to the homologous /?,P-disubstituted trimer 352 and cross-coupling with Br-T2-Br 46 in 63% yield to the corresponding tetramer 353 [Eq. (92)]. For comparison purposes, 3’-(methylmercapto)-2,2’: 5’,2’’-terthiophene 355 has been synthesized via ‘Kumada-coupling’ of 2,5-dibromo-3-(methyImercapto)thiophene354 and two equivalents of Grignard reagent BrMg-T1 -H 42 in 65% yield [Eq. (93)]. MeS

Mg I Et,O

MeS ,

SMe

350

\

354

355

(93) SMe

176

2 Sulfur-Contuining Oligumers

Physical properties are dependent on the conjugated chain length. With increasing chain length of the oligomers 351-353 the (irreversible) oxidation potentials decrease stepwise (Epa= 1.30 V to 0.80 V vs. Ag/AgCl). Simultaneously, the maximum absorption ,,A,( = 282-393 nm) and emission energies ,,A(, = 392-484 nm) are gradually shifted to lower energies. Relative to unsubstituted oligothiophenes, the fluorescence quantum yield of the methylmercapto derivatives is slightly decreased. Due to the substitution pattern, which allows enough spin density in the ‘outer’ a-positions, even the longer oligomers exhibit good film forming properties and could be electropolymerized to the corresponding donor-substituted polythiophenes including a stereoregular structure. The electronic properties were found to be dominated by the donor strength of the su bstituen ts. The synthesis and structural characterization of all regioisomeric di(methy1thio)substituted bithiophenes 356-358 was reported by Folli rt ul. [255]. Supported by force field calculations, crystal structure, absorption, and H NMR-NOE data, the conformational properties of the regioisomeric bithiophenes were investigated which are head-to-head, head-to-tail, and tail-to-tail repeating units of the corresponding polythiophenes. Despite the great differences in the electronic and steric properties of the methyl and thiomethyl groups, the conformational properties are very similar to each other. The regiochemistry dominates over the intrinsic properties of the substituent which is in fact different to the above mentioned methoxy substitutents. Experimental evidence suggests the fact that syn- or s-cis conformations play a role in solution.

’

5

$

H,CS

H3cs+

SCH, 356

357

SCH, 358

The extension of this work led now to the synthesis of a series of soluble quater- and sexithiophenes, regioregularly substituted with the electron-donating thiomethyl groups in ,&positions [256]. ‘Stille-type’ cross-coupling of the dibromo derivative of bithiophene 356 with 3-methylthio-2-trimethylstannylthiophene 363 and a Pd(0)-catalyst lead to the tetrasubstituted quaterthiophene 360 whereas ferric chloride oxidation of the bithiophene 356 resulted in the regioisomeric quaterthiophene 359. Bis-bromination of both compounds gave the corresponding a,a’-dibromoquaterthiophenes 361 and 362, respectively, which were successively cross-coupled with 2-trimethylstannylthiophene under Pd(0)-catalysis to the regioisomeric tetrasubstituted sexithiophenes 364 and 365, respectively. Crosscoupling of bromoquaterthiophene 362 with 3-methylthio-2-trimethylstannylthiophene 363 under the same conditions give the hexasubstituted sexithiophene 366 [Eq. (94)]. In this paper no yields are given. The maximum optical absorptions = 406-430 nm) are located close to these of the unsubstituted of the hexamers,,A(, sexithiophene A(,, = 432 nm) indicating that the loss of 7r-conjugation due to the steric effect of the P-substituents are nearly compensated by the mesomeric effect

2.1 Oligothiopheries

177

i.e. the delocalization of the electron lone-pairs of the niethylthio group into the aromatic system.

361 (R , R, = SMe. R, = H) 362 (R , RP = SMe. R, = H)

359 (R , R, = SMe. R, = H) 360 (R , Rp = SMe. R, = H)

%Me,

, ;[Pd(PPh,),I

(94)

b

364 (R , R, = SMe. R, = H R, = H) 365 (R , R, = SMe, R , = H, R, = H) 366 (R , R,. R, = SMe. R, = H)

Among the electron-donating substituents. dialkylamino groups exhibit the most pronounced resonance eiTects and the strongest donor character which is reflected in very negative Hammett cr+-constants. While 3-di(alkylamino)thiophenes are long known, due to the excellent stabilization of the corresponding radical cation, poly[di( alkylamino)thiophenes] are not formed. Biuerle et a / . now synthesized and characterized the first series of oligothiophenes 369-371 bearing the strongest known electron-donating substituent, the pyrrolidino group, in the ‘outer’ /3positions [257]. Despite the direct metallation of thiophenes with coordinating substituents in 3-position proceeds normally in the neighbouring 2-position, 4pyrrolidino-2-trimethylstannylthiophene367 could be synthesized as key building block in 76% yield free of isomers by direct metallation of 3-pyrrolidinothiophene with n-butyllithium/trimethylstannylchloride.Pd(0)-catalyzed homocoupling of the stannylthiophene 367 with 2-iodo-4-pyrrolidinothiophene 368 which was created in situ from 367 and iodine resulted in 4,4’-dipyrrolidino-2,2’-bithiophene 369 in 82% yield [Eq. (95)]. Cross-coupling of the stannylthiophene 367 with I-Tl-I 54 gave nevertheless a mixture of the /j$-disubstituted terthiophene 370 (23%) and the homo-coupling product 369 ( 5 5 % ) which could be separated by sublimation. ‘Stille-type coupling’ of the organostannyl compound 367 with I-T,-I 306 finally resulted i n the largest homolog, ~,~j-dipyrrolidino-quaterthiophene371 in 58% yield [Eq. (96)]. Due to the strong electron donating effect of the pyrrolidino groups the three oligomers 369-371 are oxidized at very low potentials. Interestingly, bithiophene 369 exhibits the lowest oxidation potential (EPa= 0.08V vs. Fc/Fc+)so far found for oligothiophenes whereas, surprisingly, it increases again and rests constant on

178

2 Sulfur-Containing Oiigonwrs

c? SnMe,

367

,.‘

370,371[ n= 1 2 )

going to terthiophene 370 (Epa= 0.11 V) and quaterthiophene 371 (Epa= 0.1 1 V). Evidently, in bithiophene 369 already exists the full conjugation between the two donor substituted thiophene rings, whereas in the higher homologs 370 and 371, respectively, the terminal substituted rings are oxidized more or less independently at the same potential, irrespective of the size of the oligothiophene. The independent addressing of the thiophene units could also be observed for the corresponding poly(pyrro1idinothiophenes) which were obtained by electropolymerization of each of the three oligomers and which exhibit practically the same redox potential. An interesting series of a,a’-bis(aminomethy1)-functionalizedoligothiophenes 378-381 has been reported and the properties compared to the corresponding dimethyl derivatives [258]. The amino groups had to be protected before building up the oligomeric 7r-system. Thus, 2-(aminomethy1)thiophene 372 was reacted with tetramethyl- 1,4-dichlorodisilylethylene to yield the corresponding protected monothiophene 373 in 96% yield. Transformation of the latter into the Grignard reagent and cross coupling with dibromo(o1igo)thiophenes under ‘Kumadaconditions’ yielded the silyl-protected oligothiophenes 374-377 (dimer to pentamer) in 38-72% yield. Deprotection with hydrochloric acid leads to water soluble dihydrochlorides which are transformed to the free amines 378-381 with hydroxide in 70-85% yield [Eq. (97)]. The electronic spectra were virtually the same compared to those of Me-T,-Me 261 (n = 2), 262 ( n = 3), 247 (n = 4), 263 (n = 5 ) , 248

I

I

(97)

(n = 6). The absorption maxima were only slightly red-shifted indicating a slightly higher electron-donating effect of the aminomethyl group in comparison to the methyl group. An extended analog of tetrathiafulvalene, a strong donor which is frequently used as a building block for conducting charge transfer salts [259], including an rr-terthiophene spacer group was synthesized by Roncali et ul. in order to get more insight into the influence of the structural parameters on the charge-transport mechanism and the (super)conducting properties "601. The dilithiated 3'mbstituted terthiophenes 13, 172, 175, 183 were formylated by the reaction with N-methylformanilide or acylated with acetic acid and phosphoric acid catalysis to the bisformylated terthiophenes 382-385 in 50-60% yield and the 0,"-diacetyl-terthiophene 386 in 35% yield, respectively. Wittig-Horner olefination of these terthiophenes with anion gave the bis-donor-substituted terthiothe I ,3-dithiol-2-ylidene-phosphonate phenes 387-391 in 40-80% yield [Eq. (98)].

0

___, LDA

x

w

x

HCONPhMe

0

0

R

13,172,175,183

R

382-385[(R = H. Me, C&,.

(CH,CH,O),Me):

X = HJ

386 (R = H: X = Me)

The donor-substitution in terthiophenes 387-391 clearly effects a decrease of the HOMO/LUMO gap. With respect to the corresponding terthiophenes 13, 171, 175, 183, the longest wavelength absorptions are distinctly red-shifted (Ax,,, = 108 nni). The oxidation potentials are decreased by about 500-600 mV and the TTF-analogous terthiophenes 387-391 show two reversible and one irreversible redox waves. Production of charge transfer salts obtained by iodine doping which is far lower yielded conductivities in the range of o = 10-'-lO-'Scmp' than the usual TTF donor/acceptor complexes. Series of a,a'-disubstituted oligothiophenes bearing electron accepting groups have been developed recently [261]. Thus, a,(>'-diformyl-oligothiophenes 395-397 were prepared in a three-step synthesis up to a hexamer. OHC-T*-H 73 and OHC-T,-H 74 were obtained from H-T2-H 2 and H-T3-H 3 by formylation with phosphorous oxychloride and dimethylfornianiide in 9 1'10 and 84% yield, respectively. Formylation thereby is one of the very few reactions of oligothiophenes which leads selectively to monosubstitution products which are deactivated for further electrophilic substitution. This is due to the electron-withdrawing effect of the carboxaldehyde group. Successive bromination of the latter compounds resulted

180

2 Sulfur-Containing OIigornevs

in the unsymmetrical cu-bromo-a/-oligothiophene carbaldehydes 393,394 [Eq. (99)]. 5-Bromothienyl-2-carboxaldehyde 382 is available commercially. The bis-formylated oligothiophenes 395-397 were finally obtained by symmetric coupling of bromoformyloligothiophenes 392-394 in the presence of zinc, NiC12 and triphenylphosphine in DMF (see above) in 70-85% yield [Eq. (loo)]. Interestingly, the attempted synthesis of the desired compounds via ‘Kumada-coupling’ of the acetal protected parent compounds failed according to the authors.

2,3 ( n = 2.3)

73,74 ( n = 7.3)

NiC12 I Zn I PPh3

O

H

393,394 [ fl= 2.3)

C

W

C

H

O

Br

2n-2 395-397 ( n = 1-3)

392-394 (fl=1-3)

OHC-T2-CHO 395 and OHC-T3-CHO 382 could also be obtained in 65 and 60% yield by dilithiation of H-T2-H 2 and H-T3-H 3, respectively, and trapping of the lithioorganic species with dimethylformamide. However, ‘Vilsmeier-formylation’ of H-T3-H 3 led to only 18% of OHC-T3-CHO 382 because OHC-T3-H 74 was formed as main product [262]. An interesting combination of oligothiophenes and electron accepting aromatic tropylium ions as capping substitutent was presented by Takahashi et al. [263]. Dilithiation of H-Tz-H 2 and reaction with two equivalents of tropylium 398 in tetrafluoroborate gave the 5,5/-bis-(lH-~ycIoheptatriene)-2,2’-bithiophene 60% yield. Thermal isomerization by heating in refluxing xylene gave the isomeric 4H-cycloheptatriene 399 and successive hydride abstraction with trityl fluoroborate yielded the stable bis-dicationic bithiophenes 400 in 89% yield [Eq. (lol)].

mHH 2

398

Ph3C+BFi

H H

399

400

Monoreaction of H-T2-H 2 with tropylium tetrafluoroborate gave in 40% yield the monosubstituted bithiophene 401 which was isomerized to the corresponding 4Hderivative 402. Oxidative homo-coupling of lithiated bithiophene 402 with CuC12 gave the disubstituted quaterthiophene 403 in a moderate yield (27%). Following hydride

-7.1 Oli‘ly)I l l iophrr1 es

181

abstraction with tritylfluoroborate yielded the stable bis-dicationic quaterthiophene 404 in 73% yield [Eq. (102)].

2

401

U

n- BuLilCuClp

402

H

403

404

The spectroscopic data and the redox potentials exhibit the influence of the cationic withdrawing substituents. The longest wave length absorption of the dimer 400 ,A,(, = 568 nm) and the tetramer 404 (A, = 652 nm) are considerably red-shifted in comparison to the non-substituted oligothiophenes. With increasing chain length, interestingly, both oligothiophenes exhibit an increased electron affinity and are irreversibly reduced at E,, = -0.15V and -0.09V vs. SCE. Semiempirical calculations reveal that the contribution of a quinoidal resonance structure (Q) increases with increasing chain length in comparison to the aromatic form (A). The quinoidal structure is similar to an oligothiophene dication or a bipolaron in conducting polythiophene [Eq. (103)].

400 A

400 Q

Albers et al. synthesized a series of (4-pyridyl)-‘capped’ oligothiophenes [264]. Thus. the smallest homolog, 5,5’-di(4’-pyridyl)-2,2’-bithiophene407 was prepared by homo-coupling from 5-iodo-2-(4’-pyridyl)thiophe11e 405 i n 49% yield [Eq. (104)]. The synthesis of the corresponding terthiophene 408 was most successful by cross-coupling the organozinc derivative of 2-(4’-pyridy1)thiophene 406 and Br-TI -Br 45 under Pd(dppf)Clz-complex catalysis (66% yield). As a by-product the mono-substituted product 411 was isolated. In contrast, oxidative coupling with CuCI, or ‘Kumada-coupling’ were not successful and only led to very poor yields ( < 2 % ) . The higher homologs, dipyridyl-substituted quaterthiophene 409 and quinquethiophene 410, formed by cross-coupling reaction of pyridylthiophene 405 and the Br-T,-Br however. could only be identified but not separated from

182

2 Sulfur-Containing OIigomerJ

mixtures with the mono-coupling products 412 and 413, respectively. The reaction to the corresponding hexamer did not proceed anymore [Eq. (105)l.

/

406 405 (X = ZnCI) I), Br

407

#$j \ Bl

n

408-410 ( n = 3-5)

411-413 ( n = 3 - 5 )

a,a’-Functionalized oligothiophenes bearing both a donor and an acceptor substituent are very interesting compounds with regard to their strong solvatochromic and second-order nonlinear optical properties. In this context, Effenberger and Wiirthner reported on the synthesis of a series of donor/acceptor-substituted oligothiophenes [265].The oligomers 414-426 were built up either by cross-coupling of organozinc or organotin derivatives bearing the electron-donating endgroup and halogenated thiophenes bearing the accepting substitutent in 8-87% yield (Table 8). The ‘Stille-type’ coupling gave exceedingly higher yields. However in two cases, surprisingly, non-negligible amounts of a phenylated side-product (30% yield) which must stem from the triphenylphosphino ligands of the catalyst were found and had to be separated from the product. Additionally, several donor/acceptor-substituted ter- and quaterthiophenes were synthesized in order to gain series with varying chain lengths. Thus, MeO-TI -ZnCl427 was cross-coupled with Br-T2-H 50 and resulted in 5-methoxy terthiophene MeO-T3-H 428 in 53% yield, the homo-coupling products MeO-T2-OMe 429, and H-T4-H 4 [Eq. (106)l. Subsequent coupling of MeO-T2-ZnC1 430 and MeO-T3-ZnCI 431 with 2-iodo-5-nitrothiophene Table 8. Donor/acceptor-substituted bithiophenes 414-426 [265]. 414

415

416

417

418

419

420

D A

H NO2

Me0 NO2

MeS NO2

MezN NO2

Pyr NO2

Me0 CHO

Me2N CHO

D

421

422

423

424

425

Me0 H

Me2N Pyr H H

DA *

426

~

D A Pyr = Pyrrolidino

Me2N Me2N CN C=C(CN)2

Me2N SOzMe

2.1 0ligoiliiopherir.v

183

I-TI -NO2 432 gave the donor/acceptor-substituted oligothiophenes MeO-T3NO2 433 and MeO-T4-N02 434 in 73% and 74% yield. respectively [Eq. (107)]. Similarly, the corresponding a-pyrrolidino-a'-nitro derivatives 437 and 438 were obtained in somewhat lower yields (48Y0, 5 5 % ) by palladium-catalyzed coupling of metalated 5-pyrrolidino-2,2'-bithiophene 435 and I-TI -NO2 432 or I-T2-N02 436, respectively [Eq. (1OS)J.

427

50

428

430,431 ( n = 2,3)

432

433,434 ( n = 1.2)

435 ( n = 2)

432,436 ( n = 1.2)

437,438 ( n = 1.2)

The longest wavelength absorptions of the novel donor/acceptor oligothiophenes are polarized along the long axis of the molecules and therefore strongly influenced by the endgroups. The bathochromic shift and the increasing intensity with rising donor and acceptor strength is consistent with a charge-transfer character of this transition. All compounds show exceptionally strong solvatochromic properties which are useful for the empirical determination of solvent polarity. Thus e.g. pyrrolidino/nitro-substituted bithiophene 418 is readily soluble in all organic solvents and displays a solvatochromic shift which extends almost over the whole visible range ,,A,[ = 466 nm (n-hexane) to , , ,A = 597 nni (formamideiwater)] indicating a large electronic interaction between the two endgroups and full electron delocalization. X-ray structure analysis shows a coplanar arrangement of the two thiophene rings and an equalization of the C-C bond lengths in the thiophene rings. Second-order non-linear optical properties were also found to be very high. According to EFISH measurements, the static hyperpolarizability of bithiophene 418 is twice as large as that of the corresponding biphenyls and agrees well with the value calculated from the solvatochromic data. This trend is also obtained for other donor/acceptor combinations and different chain length of the conjugated 7r-system [266]. Hutchings et al. also synthesized donor/acceptor-substituted bithiophenes and their measurement of the linear and non-linear optical properties corroborated by theoretical calculations confirm the results found by Effenberger et 01. Thus e.g. bithiophene 441 substituted in the a,cu'-positions by a dimethylhydrazono and a dicyanovinyl group showed a bathochromic shift of the longest wavelength absorption, a slightly reduced dipole moment. and a hyperpolarizability

184

2 Sulfur-Containing Oligomers

(Po = 100 x lop3’ esu) which is twice as high in comparison to pyrrolidino/nitrobithiophene 418 (Po = 54 x lop” esu) [267]. The synthesis was performed by reacting OHC-T2-H 73 with N,N-dimethylhydrazine to yield the corresponding hydrazone 439 in 85% yield. Formylation with n-BuLilDMF gave 5’-formyl-2,2’bithiophene-5-carboxaldehyde dimethylhydrazone 440 in 53% yield. The final transformation to donor/acceptor oligothiophene 441 was achieved by reaction of the hydrazone 440 with malodinitrile in 67% yield [Eq. (109)l.

73

439

n-BuLi / DMF

44 1

(109)

440

A series of more complicated donor/acceptor-substituted oligothiophenes were synthesized by Effenberger et al. as model compounds for intramolecular photoinduced energy transfer [268]. The conjugated triad molecules consist of an oligothiophene as conjugated bridge which is terminally linked to the 9-position of anthracene acting as photoexcitable donor and a meso-position of a porphyrin as acceptor. The synthetic sequence starts from anthrone 442 which is reacted with I-TI -I 54 and n-BuLi to form the donor-substituted 2-(9’-anthryl)-5-iodothiophene 443 in 77% yield. Elongation of the conjugated chain by one and two thiophene rings is achieved by palladium-catalyzed coupling of the latter with the zincorganic derivative of protected thiophene aldehydes 444 and 445, respectively. 5-(9”-Anthryl)-5’-formyl-2,2’-bithiophene 446 and the corresponding terthiophene 447 were formed in 42% and 30% yield, besides homo-coupling products. The final porphyrins 448 and 449 were prepared by condensation of pyrrole and a mixture of the anthryl-oligothienyl aldehydes 446, 447, and l-hexanal. Subsequent oxidation with p-chloranil using ‘Lindsey’s method’ gave mixtures of porphyrins containing different ratios of pentyl and anthryl-oligothienyl meso substituents. MPLC-chromatographic purification allowed one to isolate the desired triad molecules, anthryl-bithienyl-porphyrin448 and anthryl-terthienyl-porphyrin449 in %lo% yield [Eq. (1 lo)]. In addition, anthryl-oligothiophenes and oligothienylporphyrins representing models for the donor-chain and the chain-acceptor, respectively, were included and synthesized by the same methodology [269]. Despite direct connection of the endgroups with the oligothiophene bridge, the electronic interactions of the anthracene and porphyrin moieties are weak. Due to steric reasons, the individual subunits in the supermolecule can be recognized in the absorption spectra by separated bands. Thus, selective excitation of the anthryl unit is possible and a unidirectional and efficient energy transfer to the emitting porphyrin acceptor via the conjugated oligothiophene chain has been studied by

2.1 Oligotkiophenes

ClZn 54

443

185

WOCH,),

n

444, 445 ( n = 1.2)

R

.

448,449 (R = C5H,, n = 1.2)

446,447 ( n = 1,Z)

steady-state fluorescence spectra, fluorescence exitation spectra and picosecond time-resolved fluorescence measurements. Another combination of oligothiophenes with porphyrins was used by Shimidzu et a/. to construct electrochemically one- and two-dimensional porphyrinoligothiophene copolymers as models for ultra-fine nanostructures [270]. The porphyrin moieties are linked either by oligothiophene bridges in the meso-positions to result in two-dimensional net-works or linked in the axial direction via a central phosporous which forms two stable axial bonds with oligothienylalkoxy groups. Thus, rnemtetrakis(bithieny1)porphyrin 450 and the terthiophene analog 451 were synthesized by refluxing the oligothiophene carboxaldehydes with pyrrole in propionic acid. The axially connected derivatives 452 and 453 were synthesized by the reaction of dichlorophosphorus(V)-tetraphenylporphyrin and the corresponding oligothienylmethylalcohols. H

H

H

450,451 ( n = 2.3)

452,453 ( n = 2.3)

186

2 Sulfur-Contuining Oligornrrs

Several metal complexes of these porphyrinoligothiophene copolymers were used to construct sandwich cells which exhibit diode-like rectifying properties.

2.1.2.2.5 Amphiphilic Oligothiophenes Very recently, (oligo-)thiophenes were presented which are functionalized in such a way that they exhibit amphiphilic behavior and form ordered assemblies of conjugated molecules. Ideally, longer fatty acids or alkane thiols may be organized in Langmuir-Blodgett (LB) films or in self-assembled monolayers (SAM), respectively. The incorporation of thiophenes in these ordered systems would possibly allow polymerization in a more ordered way. Thus, the amphiphilic 3thienylpentadecanoic acid forms stable films on the water subphase and can be transferred onto a variety of hydrophilic and hydrophobic substrates as monoand multilayers [271]. The inclusion of oligothiophenes in these assemblies, however, would result in a stack-like arrangement of the conjugated 7r-systems. Such supramolecular structures are very interesting with respect to the conduction mechanism in oxidized oligothiophenes, since conductivity along stacks might contribute to the overall conductivity. From several a-substituted terthiophenes 454-457, Nakahara et al. obtained LB-films with well-defined alignments of the conjugated molecules [272]. Alkylesters of these terthiophenes with a short alkyl chain form stable monolayers on water and can be deposited on substrates in such a way that the long axes of the oligothiophenes stand nearly vertical.

454-457 [(R = CH,,C,H,,,C,,H,,.CH=C(CH,),

]

466-467 (R = H, AC)

Amphiphilic bithiophenes 459, 460 and corresponding terthiophenes 463-465 which bear a longer alkyl chain and terminal polar head groups like carboxyalkyl, alkylthiol or viologen were recently synthesized and their film forming properties investigated. Starting from H-T2-H 2, 5-(1 l-bromoundecyl)-2,2'-bithiophene 458 was synthesized via acylation and by successive reduction of the carbonyl group and the terminal halogen function were replaced by a carbonic acid or a thiol group to yield the amphiphilic bithiophenes 459 and 460, respectively. Elongation to the corresponding terthiophenes was achieved via bromination of bithiophene 458 with NBS to yield 461, nickel-catalyzed cross-coupling with BrMg-TI -H 42 gave the 5-( 1 1-bromoundecyl)-a-terthiophene 462, and analogous exchange of the terminal bromo group by carboxylic acid 463, thiol 464 or 4,4'bipyridinium group 465 [Eq. (1 1 l)] [72]. The viologen-functionalized terthiophene 465 represents a donor/spacer/acceptor-system which forms LB-mono- and multilayers [273]. In these structures, the electron donating units are separated from the electron accepting units by the alkyl chain in a defined manner giving access to a controllable vectorial electron transfer. Undecylthiol-substituted terthiophene 464 forms very stable SAMs on gold surfaces due to the additional 7r-7r-interaction of the conjugated moieties [274]. Tour et al. have reported that (2,2'-bithien-5-yl)thiol 466 and the corresponding thioacetyl compound 467 form also SAMs [275].

2.1 Oligorhioplienes

187

469,480 ( X = COOH. S H )

2

461

462

463,464,466 (X = C W H ,SH,Viologen)

2.1.2.2.6 Transition Metal Complexes of Oligothiophenes Mann et a/. have reported the first metal 7r-complexes of oligothiophenes [276]. Fourteen different cyclopentadienyl (Cp) and pentamethylcyclopentadienyl (Cp') ruthenium complexes of the general form [CpRu(oligothiophene)]PF6 and [Cp*Ru(oligothiophene)]PF6 were synthesized and characterized. Ruthenium is found to bind in the complexes 468, 469 q5 to the outermost thiophene ring of H-T2-H 2, H-T3-H 3, H-T,-H 4, and Me-T3-Me 262, respectively. In the case of Ph-T3-Ph complexes (470, 471) the metal is bound '71 to the pendant phenyl groups. The latter complexes are very stable with respect to decomplexation, whereas complexes in which the ruthenium is bound to a thiophene ring are stable in the solid state and in dichloromethane solutions, whereas in acetone rearrangements occur producing equilibrium mixtures of free oligothiophene, mono- and diruthenated species. By NMR spectroscopy it could be shown that the coordination of ruthenium affects the electronic structure of the complexed oligothiophene and extends over the bound ring and the nearest neighbor rings.

I

468 (n=0,1.2)

470 (n=3)

&

469 (n=0,1,2)

2.1.3 Conclusions Are defined oligomers more than just model compounds jor conducting polymers.? This contribution intended to describe the development in oligothiophene chemistry up to mid 1996 and to cover those compounds which have relevance to the field of conjugated materials and conducting polymers, respectively. The strength of the ‘oligomeric upprouch’ is very evident for oligothiophenes, since they represent the class of defined model compounds which by far is the most frequently synthesized, characterized, and utilized in applications. A great variety of known synthetic methods have been used, but new strategies have also been developed to build up homologous series of oligothiophenes. Methods starting either from acyclic precursors, which are cyclized by specific ring closure reactions, or a variety of mostly transition metal-catalyzed arene/ arene coupling reactions lead to oligomeric systems with defined chain and conjugation length. A broad range and various series of oligothiophenes with varying chain length and substitution pattern have been developed and characterized in recent years, whose structure and consequently properties can be controlled and tailored, due to the introduction of substituents such as alkyl side chains, donors, acceptors or other functional groups. A valuable insight into the relationship between geometric and electronic structure and the resulting properties is thus provided. In some cases, structural studies by X-ray analysis have been possible and gave valuable information about the subtle influence of steric and electronic factors on conformational properties. Very recently, even the structural data of the very long member, H-T8-T 14 were determined [277]. In homologous series of oligomers, the properties can be followed as a function of chain length. Therefore, clear structure/property-relationshipscan be evaluated. Indeed, as can be shown experimentally and theoretically, nearly all of the (electronic) properties depend linearly on the inverse (conjugated) chain length. These correlations now allow the effective conjugation length of a corresponding polymer, and values for a (hypothetical) infinite chain length to be estimated by extrapolation. This information is not normally available from investigations on the corresponding polydisperse polymeric systems. For very long members in oligothiophene series, even some physical properties surpass those of the corresponding polymers which makes them attractive candidates as active materials in (opto)electronic applications. Do defined oligothiophenes represent more than just model compounds for conducting polymers? In fact, there are definitely much more exciting aspects - not known for the corresponding polymers - which entitle them to be an extremely interesting class of compounds in their own right. Thus, if conducting polymers are still considered as ‘molecular wires’, defined oligomers are certainly the much better candidates and components to built up future ‘molecular electronic devices’. Besides the fact that, for example with oligothiophenes a new species contributing to the charge transport in conjugated systems could be found, one of the most intriguing aspects is the possibility of arranging them in well-ordered supramolecular architectures and adsorbing them specifically on substrate surfaces. The

Rqferences

189

ordering of these self-assembled mono- and multilayers is also a function of chain length and substitution pattern of the oligomers. Optical and electric properties and the imaging on a submolecular level, provide totally new information and views about conjugated systems. The most challenging vision, which is to contact a single molecule with a STM-tip and to control the (unidirectional) charge transport along a single conjugated molecule or 'wire' is only possible with welldefined and well-ordered assemblies of oligomers. Furthermore, with the recently achieved synthesis of very long oligomers, dimensions are already approached which in the meanwhile can also be produced in nanostructures by special lithographic techniques. Therefore, we might already have the tools in hand to span two nanoelectrodes with a single conjugated molecule. If simultaneously the charge transport along the 'molecular wire' can be controlled viu attached functional groups which can be addressed from the outside, the way to the realization of 'molecular electronic devices' like a diode or a transistor on the molecular level seems to be opened due to now available well-defined oligomeric systems. Acknowledgement First of all, I would like to thank my coworker Christiane Heim for helping to prepare the manuscript. It is a great pleasure to thank my students and coworkers in our group who did exciting research and contributed to the field of oligothiophenes: Dr. Kai-Uwe Gaudl, Dr. Stefan Scheib, Dr. Markus Hiller, Dr. Thomas Fischer, Ullrich Mitschke, Andreas Emge, Guido Rimmel, Jens Kromer, Alexander Meyer, Gerhard Gruner, Michael Schon, Christiane Heim and Jens Glaser. I'm very indebted to Prof. Eberhard Umbach and his group, Physics Department, University of Wiirzburg, for a very fruitful and intensive cooperation over many years. In many discussions, they opened our eyes and taught us about surfaces and their interaction with organic molecules. Finally, I'd like to thank the editors, Prof. Wegner and Prof. Miillen, Max-Planck-Institut, Mainz, who gave us the opportunity to contribute to this book series.

References 1. (a) H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K . Chiang, A. J. Heeger. J . Clieni. Soc., C/imi. Commioi., 1977, 578; (b) C. K. Chiang, Y. W. Park, A. J. Heeger. H. Shirakawa, E. J. Louis, A. G. MacDiarmid, J . C'/iem. Phys., 1978. 69, 5098; (c) C. K . Chiang. C. R . Fincher. Y. W. Park et al., P k j ~ Rev. . Lett., 1977, 39, 1098. 2. A. F. Diaz. J. I. Castillo. J. Ckem. Soc.. Cliem. Commu~i.,1980, 397. 3. D. M. Ivory, G. G. Miller. J. M. Sowa. L. W. Shacklette, R. R. Chance, R. H. Baughman, J . Cheni. Pk,vs., 1979, 71. 1506. 4. G. E. Wnek. J. C . W. Chien, F. E. Karasz, C. P. Lillja, Poljwter, 1979. 20, 1441. 5. J . F. Rabolt. T. C. Clarke, K. K . Kanazawa, J. R. Reynolds. G. B. Street, J. Cliem. Soc., Chrm. Conirnurt.. 1980. 347. 6. R. R. Chance. L. W. Shacklette. G. G. Miller et nl., J . Chem. Soc., Chem. Conimtn., 1980, 348. 7. A. F. Diaz. J. A. Logan, J . Eleciroririul. Chem.. 1980. 111. 11 I .

190

2 Sulfur-Containing Oiigomers

8. G. Tourillon, F. Garnier, J . Electround. Chem., 1982, 135, 173. 9. H. Guenther, M. D. Bezoari, P. Kovacic, S. Gronowitz, A.-B. Hoernfeldt, J . Polym. Sci., Polym. Lett. Ed., 1984, 22, 65. 10. J. Bargon, S. Mohmand, R. J. Waltman, Mol. C r p t . Liq. Cryst., 1983, 93, 279. 11. P. Bauerle, Adv. Muter., 1993, 5 , 879. 12. F. Martinez, R. Voelkel, D. Naegele, H. Naarmann, Mol. Cryst. Liq. Cryst., 1989, 167, 227. 13. P. Bauerle, Adv. Muter., 1992, 4 , 102. ~ ~ . 1989, 28, C705; (b) D. Fichou, 14. (a) F. Garnier, G. Horowitz, D. Fichou, S J V I Met., G. Horowitz, Y. Nishikitani, F. Garnier, ihid, C723; (c) F. Garnier, G. Horowitz, X. Peng, D. Fichou, Adv. Muter., 1990, 2, 592; (d) B. Xu, D. Fichou, G. Horowitz, F. Garnier, Adv. Muter., 1991, 3, 150. 15. K. Yoshino, Sq'nth. Met., 1989, 28, C669. 16. D. Fichou, J.-M. Nunzi, F. Charra, N. Pfeffer, Aclv. Muter., 1992, 4 , 64. 17. M. Busch, W. Weber, J . prakt. Chern., 1936. 146, 146. 18. R. Kuhn, Ch. Grundmann, Chem. Ber.. 1938, 71, 442. 19. (a) W. Steinkopf, W. Kohler, Lieb. Ann. Chc~m.,1936, 522, 17; (b) W. Steinkopf, H.-J. v. Petersdorf, R. Cording, Lieb. Ann. Chem., 1937, 527, 272; (c) W. Steinkopf, R. Leitsmann, K.-H. Hofmann, Lieb. Ann. Chern., 1941, 546, 180. 20. R. Leitsmann, Dissertation Technische Hochschule Dresden, 1941. 21. F. Geiger, M. Stoldt, H. Schweizer, P. Bauerle, E. Umbach, Adv. Muter., 1993, 5, 922. 22. L. Zechmeister, J. W. Sease, J . A m . Cheni. Soc., 1947, 69, 273. 23. S. Gronowitz, H.-0. Karlsson, Arkiv Krmi, 1960, 17, 89. 24. (a) C. Soucy-Breau, A. MacEachern, L. C. Leitch, J. T. Arnason, P. Mordnd, J . Heterocycl. Chem., 1991,28,41I ; (b) A. MacEachern, C. Soucy, L. C. Leitch, J. T. Arnason, P. Mordnd, Tetrahedron, 1988, 44, 2403; (c) R. Rossi, A. Carpita, M. Ciofalo, J. L. Houben, Gazz. Chirn. Ital., 1990, 120, 793. 25. (a) K. R. Downum, D. Provost, L. Swain, Bioact. Mol., 1988, 7, 151; (b) R. Jente, G. A. Olatunji, F. Bosold, Phytochemistrv, 1981, 20, 2169; (c) F. Bohlmann, C. Zdero, Chem. Ber., 1976, I O Y , 901. 26. (a) R. J. Marles, R. L. Compadre, C. M. Compadre ef al., Pestic. Biocliem. Physiol., 1991, 41, 89; (b) J. C. Scaiano, A. MacEachern, J. T. Arnason, P. Morand, D. Weir, Photocheni. Photohiol., 1987,46, 193; (c) J. T. Arnason, B. J. R. Philogine, C. Berg et a/., Phytochemistry, 1986, 25, 1609; (d) J. Kagan, S. K. Arora, J . Org. Chem., 1983, 48, 4317. 27. (a) J . Bakker, F. J. Gommers, I. Nieuwenhuis, H. Wynberg, J . Bid. Chem., 1979, 254, 1841. (b) J. T. Arnason, G. F. Q. Chan, C. K. Wat, K. Downum, G. H. N. Towers, Photocheni. Photohiol., 1981, 33, 821. 28. (a) J. Roncali, F. Garnier, M. Lemaire, R. Garreau, Synth. Met., 1986, I S , 323; (b) B. Krische, M. Zagorska, Synth. Met., 1989, 28, C263. . 1987, 21, 209. 29. J. Heinze, J. Mortensen, K. Hinkelmann, S j ~ t hMet., 30. (a) R. J. Waltman, J. Bargon, A. F. Diaz, J . Phys. Cheni., 1983, 87, 1459; (b) Y. Yumoto, S. Yoshimura, Synth. Met., 1985, 13, 185. 31. 0. Inganas, B. Liedberg, C. R. Wu, H. Wynberg, Syntii. Met., 1985, I / , 239. 32. (a) A. Alberti, L. Favaretto, G. Seconi, J . Chem. Soc., Perkin Trans. 2, 1990, 931; (b) J. Caspar, V. Ramamurthy, D. R. Corbin. J . Am. Chem. Soc., 1991, 113, 600; (c) D. Fichou, G. Horowitz, F. Garnier, Synth. Met., 1990, 39, 125. 33. D. Fichou, G. Horowitz, B. Xu, F. Garnier. Springer Ser. Solid State Sci., 1989, 91, 386; (b) D. Fichou, G. Horowitz, B. Xu, F. Garnier, Svnth. Met., 1990,39, 243; (c) D. Fichou, B. Xu, G. Horowitz, F. Garnier, Synth. Met., 1991, 41, 463. 34. F. Garnier, G. Horowitz, X. Peng, D. Fichou, Adv. Muter., 1990, 2, 592. 35. U. Schoeler, K.-H. Tews, H. Kuhn, J . Chem. Phys., 1974, 61, 5009. 36. K. Auwers, T. V. Bredt, Chem. Ber., 1894, 27, 1741. 37. (a) H. D. Hartough in The Chemistry of Heterocyclic Compound.7, A. Weissberger, ed.: Thiophene and Its Derivatives, S. Gronowitz, ed., John Wiley, 1952, p.459; (b) J. Nakayama, T. Konishi, M. Hoshino, Heterocycles, 1988, 27, 1731; (c) R. Hakansson in The Chemistry o f Heterocyclic Conzpounds, Volume 44, Part 5: Thiophene und Its Derivatives, S. Gronowitz, ed., John Wiley, 1992, p. 755.

References

191

Y. Ito. T. Konoike. T. Harada, T. Saegusa, J . A n i . Clietii. Soc., 1977. YY, 1487. W. Steinkopf. J. Roch. L i d . ,4nn. Chein., 1930. 482, 251. A. E. Lipkin. J . Gen. Ch~vn.USSR, 1963, 33, 188. S . Gronowitz. H.-0. Karlson, Arkiv K ~ > i n i1961, , 17, 89. T. Kauffmann. Angew. Chin., 1974, 86, 321. J . Kagan. S. K . Arora. Heteroqdes, 1983. 20, 1937. J. P. Morizur. Birll. Soc. Chirn. Fr.. 1964. 1331. J. P. Morizur. R. Pallaud. Conipf. Rend., 1962, 254. 1093. T . Kauffmann, .4ngeu.. Clzeni., 1979, Y1, I . T. Kaufmann, H. Lexy. Clieni. Ber., 1981, 114, 3674. F . Gamier, G. Horowitz, D. Fichou. Sjwth. Met,, 1989. 38. C705. N. Jayasuriya, J . Kagan. D.-B. Huang. B. K. Teo, HeterocJrles, 1988. 27. 1391. ( a ) W. S. Rapson, R. G. Shuttleworth, J. N. van Niekerk. J . CIieni. Soc., 1943, 326; (b) G. Wittig, Q. Rev. C/ieni. Soc., 1966. 20, 205; (c) H. A. Staab, F. Binnig, Chen?.Bw., 1967, 100. 293; ibid. 889. 51. (a) B. Greving, A. Woltermann. T. Kauffinann, Atigeiv. Clzeti7.. 1974. 86. 475; Angew. Cl7en7. Itit. Ed. Engl., 1974. 13, 467; (b) T. Kauffmann, B. Greving, J. Konig, A. Mitschker, A. Woltermann, .4ngew. Chetn., 1975, 87. 745; Angew. Chem. h t . Ed. Engl.. 1975, 14. 713; (c) T . Kauffinann. B. Greving. R. Kriegesmann. A. Mitschker. A. Woltermann, Cliern. Ber., 1978, 111, 1330; (d) T. Kauffmann. H. P. Mackowiak. C/iel?7.Ber., 1985. 118. 2343. 52. C. Heim. Diplomarbeit Universitat Wiirzburg, 1995. 53. A. Berlin, G. A. Pagani, F. Sannicolo. J . Cheni. Soc., Cheni. Cornmuri.. 1986, 1663. 54. D. Fichou, G. Horowitz, F. Gamier, Europ. Patent 402269. 12.12.1990 (Frankreich); Clieni. Abstr., 1990. 114, 186387g. 55. N. Noma, T. Tsuzuki. Y . Shirota, Ad\'. Mater., 1995, 7, 647. 56. Z . Xu, D. Fichou, G . Horowitz, F. Gamier. J . Electronnnl. Cheni., 1989, 267, 339. 57. G. M. Davies, P. S. Davies. W. E. Paget. J. M. Wardleworth, Tetrahedron Lett., 1976, 795. 58. H. Wynberg. A. Logothetis, J . Atti. Cheni. Soc., 1956, 78. 1958. 59. R. E. Atkinson, R. F. Curtis, G. T. Phillips, J . Cheni. Soc. C.. 1967, 2011. 60. M. Nilsson, Ch. Ullenius. .4ctu Chen7. Scarid.. 1970. 24. 2379. 61. P. E. Fanta. Syitlzesi.c, 1974. 9. 62. L. Groenendaal. H. W. I. Peerlings, J. L. J. van Dongen, E. E. Havinga. J. A. J. M. Vekemans. E. W. Meijer. Macroniolecules. 1995, 28, 116. 63. M. Nilsson. Tetrahedron Lerrers, 1966. 679. 64. M. S. Kharash. 0. Reinmuth. Grignartl Reactions sf Non-rnetallic Sirbstances, Prentice-Hall, New York, 1954. 65. (a) M. Tamura. J. K. Kochi, Sj~r7tliesis.1971, 303; (b) M. Tamura. J. K . Kochi, J . Anr. Chenr. Sot,., 1971. 93. 1483. 66. R. J . P. Corriu. J. P. Masse, J . Cliern. Soc.. Cheni. Cotnriiiin.. 1972, 144. 67. K. Tamao, K . Sumitani, M. Kumada, J . Ani. C/ietn. Soc.. 1972. 94, 4374. 68. M. Yamamura, I. Moritani, S . Murahashi. J . Orgtrriorwtnl. Chem.. 1975, 91, C39. 69. K . Tamao. K. Sumitani. Y. Kiso et a/., Biill. Clieni. Soi,. Jpn.< 1976, 4Y, 1958. 70. M. Kumada. Pure Appl. Chenz., 1980, 52, 669. 71. K. Tamao. S. Kodama, I . Nakajima. M. Kumada, Tetrahedron, 1982, 38. 3347. 72. P . Biiuerle, Habilitationsschrift Universitiit Stuttgart, 1994, p. 90. 73. P. Biiuerle. F. Wurthner, G. Gotz, F. Effenberger. S~wtkesis,1993. 1099. 74. (a) G . R. van Hecke, W. de W. Horrocks. Jr.. Inorg. Clieni.. 1966, 5. 1968: (b) S . S . Sandhu, M. Gupta. Ckeni. Ind. (London), 1967, 1876. 75. I. R. Butler, W. R. Cullen. T.-J. Kim. S. J. Rettig, J. Trotter, Orgnnoriietallics, 1985. 4 , 972. 76. A. Carpita. R. Rossi. C. A. Veracini. Termhrdrori, 1985. 41, 1919. 77. C. van Pham. A. Burkhardt, R. Shabana et ul.. Pliosph. SiilJ Silic., 1989, 46, 153. 78. R. Rossi, A. Carpita. A. Lezzi, Tetrahedron, 1984. 40, 2773. 79. I. Colon, D. R. Kelsey, J . Org. Clzenr., 1986, 51, 2627. 80. M. Kumada, M . Zembayas, Tetrnhedron Lett.. 1977. 4089. 81. J. Nakayama. T. Konishi. S. Murabayashi, M. Hoshino. HCveroejdes, 1987. 26, 1793.

38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

192

2 Sulfur-Containing Oligomers

82. K. Yui, Y. Aso, T. Otsubo, F. Ogura, Bull. Chem. Soc. Jpn., 1989, 62, 1539. 83. (a) T. Yamamoto, K. Osakada, T. Wakabayashi, A. Yamamoto, Macromol. Chem. Rupid. Commun., 1985, 6, 671; (b) T. Yamamoto, K. Sanechika, J. Yamamoto, J . Polym. Sci., Polym. Lett. Ed., 1990, 18, 9. 84. S. Gronowitz, Chem. Scr., 1987, 27, 535. 85. (a) N. Miyaura, T. Yanagi, A. Suzuki, Sjnth. Commun., 1981, 11, 513; (b) T. Hayashi, M. Konishi, M. Fukushima et al., J . Am. Chem. Soc., 1982, 104, 180. 86. P. Bauerle, F. Wurthner, S. Heid, Angew. Chem., 1990, 102, 414; Angew. Chem. Int. Ed. Engl., 1990, 29, 419. 87. J. K. Stille, Angew. Chem., 1986, 98, 504; Angew. Chem. Int. Ed. Engl., 1986, 26, 508. 88. V. N. Kalinin, Synthesis, 1992, 413. 89. (a) H. Azizian, C. E. Eaborn, A. Pidcock, J . Organomet. Chem., 1981, 215, 49; (b) A. N. Kashin, I. G. Bumagina, N. A. Bumagin, V. N. Bakunin, I. P. Beletskaya, J . Org. Chem. USSR, 1981, 17, 789. 90. V. Farina, B. Krishnan, J . Am. Chem. Soc., 1991, 113, 9585. 91. V. Farina, S. Kapadia, B. Krishnan, C. Wang, L. S. Liebeskind, J . Org. Chem., 1994, 59, 5905. 92. J. Malm, P. Bjork, S. Gronowitz, A.-B. Hornfeldt, Tetrahedron Lett., 1992, 33, 2199. 93. G. T. Crisp, Synth. Commun.. 1989, 19, 307. 94. D. K. Morita, J. K. Stille, J. R. Norton, J . Am. Chem. Soc., 1995, 117, 8576. 95. R. B. Miller, S. Dugar, Organometallics, 1984, 3, 1261. 96. S. Gronowitz, D. Peters, Heterocycles, 1990, 30, 645. 97. (a) S. Gronowitz, K. Lawitz, Chem. Scr., 1983, 22, 265. 98. (a) S. Gronowitz, V. Bobosik, K. Lawitz, Chem. Scr., 1984, 24, 5; (b) S. Gronowitz, A. Svensson, Isr. J . Chem., 1986, 27, 25. 99. (a) E. Negishi, F. T. Luo, R. Frisbee, H. Matsuchita, Heterocycles, 1982, 18, 117; (b) T. Frejd, T. Klingstedt, Synthesis, 1987, 40. 100. A. Minato, K. Tamao, T. Hayashi, K. Suzuki, M. Kumada, Tetrahedron Lett., 1980,845. 101. L. Groenendaal, H. W. I. Peerlings, E. E. Havinga, J. A. J. M. Vekemans, E. W. Meijer, Synth. Met., 1995, 69, 467. 102. N. A. Bumagin, P. G. More, I. P. Beletskaya, J . Organomet. Chem., 1989, 364, 231. 103. P. Vincent, J.-P. Beaucourt, L. Pichat, Tetrahedron Lett., 1984, 25, 201. 104. S. Gronowitz, A.-B. Hornfeldt, V. Kristjansson, T. M u d , Chem. Scr., 1986, 26, 305. 105. S. Gronowitz, H.-0. Karlsson, Arkiv Kemi, 1960, 17, 89. 106. J. Kagan, S. K. Arora, I. Prakash, A. Uestuenol, Heterocycles, 1983, 20, 1341. 107. H. Wynberg, A. Logothetis, D. VerPloeg, J . Am. Chem. Soc., 1957, 79, 1972. 108. R. F. Curtis, D. M. Jones, G . Ferguson et a/., J . Chem. Soc., Chem. Commun., 1969, 165. 109. H. Stetter, B. Rajh, Chem. Ber., 1976, 109, 534. 110. H. Wynberg, J. Metselaar, Synth. Comm., 1984, 14, I . 11 1. J. Kagan, S. K. Arora, Heterocycles, 1983, 20, 1941. 112. T. Asano, S. Ito, N. Saito, K. Hatakeda, Heterocycles, 1977, 6, 317. 113. R. M. Moriarty, 0. Prakash, M. P. Duncan, Synth. Commun., 1985, 15, 789. 114. D. R. Shridhar, M. Jogibhukta, P. S. Rao, V. K. Handa, Synthesis, 1982, 1061. 115. H. J. Kooreman, H. Wynberg, Rec. Trav. Chirn. Pays-Bas, 1967, 86, 37. 116. A. Merz, F. Ellinger, Svnthesis, 1991, 462. 117. P. J. Stang, F. Diederich, Modern Acetylene Chemistry, Verlag Chemie, Weinheim, 1995. 118. D. M. Perrine, J. Kagan, Heterocycles, 1986, 24, 365. 119. K. E. Schulte, J. Reisch, L. Horner, Chem. Ber., 1962, 95, 1943. 120. (a) J. Nakayama, M. Shimomura, M. Iwamoto, M. Hoshino, Heterocycles, 1985, 23, 1907; (b) J. Nakayama, Y. Nakamura, T. Tajiri, M. Hoshino, Heterocycles, 1986, 24, 637; (c) J. Nakayama, Y. Nakamura, S. Murabayashi, M. Hoshino, Heterocycles, 1987, 26, 939. 121. J. Nakayama, S. Murabayashi, M. Hoshino, Hderocycles, 1987, 26, 2599. 122. J. L. BrCdas, G. B. Street, Acc. Chem. Res., 1985, 18, 309. 123. (a) R. Radeglia, S. Dahne, J . Molec. Struct., 1970, 5 , 399; (b) S. Dahne, Chimia, 1991, 45, 288; (c) S. R. Marder, J. W. Perry, B. G . Tiemann et al., J . Am. Chem. Soc., 1993, 115, 2524; (d) S. R. Marder, C. B. Gorman, B. G. Tiemann, L.-T. Cheng, J . Am. Chem. Soc., 1993, 115, 3006.

Ryftrelices

193

124. R. S. Becker. J. S. de Melo. A . L. Macanita. F. Elise;, Pure & AppL Ckeni.. 1995. 67, 9. 125. P. M. Lahti. J. Obrzut. F. E. Karasz. Mmronio/rczr/e.s, 1987, 20, 2023. 126. A . F. Diaz, J. Crowley. J. Bargon, G. P. Gardini. J. B. Torrance, J . Elecrrounnl. Cliiw., 1981. 1-71. 355. 127. ( a ) D. Fichou, G. Horowitz, B. Xu, F. Garnier. Spririger Ser. SolidS/ute Sci.. 1989, 91, 386: (b) D. Fichou, G. Horowitz. F. Garnier, Sjvth. M c i . . 1990. 39. 125: (c) D. Fichou, G. Horowitz. B. Xu. F. Garnier. Sjvith. M e / . , 1990, 39. 243; (d) D. Fichou, B. Xu. G. Horowitz, F. Garnier. S y d i . Met.. 1991, 41. 463. 128. (a) A. Albert;, L. Favaretto, G. Seconi. J . C'heui. Soc., Perkin Trans. 2. 1990. 931; ( b ) C. E. Brillas. A . G. Davies. L. Fajari et d.. J . Org. Cheni.. 1993, 58. 3091. 129. P. Enzel. T. Bein, Sjvith. Met., 1993. 55, 1238. 130. (a) G. Horowitz. X. Peng, D. Fichou, F. Garnier. J . Mol. Ekwtrori.. 1991, 7. 85; (b) F. Garnier, F. Deloffre, G. Horowitz. R. Hajlaoui, Syrirh. Met., 1993, 57. 4747; (c) H. Akimichi, K. Waragai, S. Hotta. H. Kano, H. Sakaki, Appl. P/i!~s.Lett.. 1991, 58. 1500. hlet.. 1993. 57. 4198. 131. J. Paloheinio, H. Stubb, L. Gronberg. Sj~i//i. 132. (a) J . P. Reyftmann, J. Kagan, R. Santus, P. Morliere, Photochem. Photohiol., 1985. 41. 1; (b) C. Evans, D. Weir, J. C. Scaiano et 01.. Photocheni. PhotohioL, 1986, 44, 441. 133. (a) H. Chosrovian. S. Rentsch. D. Grebner. D. U. Dahm, E. Birckner. H. Naarmann, Sjwt/i. Met.. 1993. 60, 23; (b) S. Rentsch. H . Chosrovian. D. Grebner. H. Naarmann. S m h . Met.. 1993, 57. 4740. . F I ~5,~ ~165. ~P, 134. D. Oelkrug, H.-J. Egelhaaf. J. F / ~ I O P ~ . ' 1995, 135. (a) D. Fichou, F. Garnier. F. Charra, F. Kajzar. J. Messier, Spec. Puhl. Roy. Soc., 1989, 69, 176; (b) M. T. Zhao, M. Sanioc. B. P. Singh. P. N. Prasad. J . Phys. U i e w i . . 1989. 93. 7916. 136. E. Pellegrin. H. Fritzsche. N. Niicker el ul., Sjw/h. Mer.. 1991, 41. 1207. 137. ( a ) H. Fujimoto, U. Nagashima, H. lnokuchi et d., J . Cl7em. Plij,s.. 1990. 92, 4077; (b) H. Fujimoto. U. Nagashima. H. lnokuchi el d.,Phys. Scr.. 1990, 41, 105; (c) D. Jones, M. Guerra, L. Faveretto, A. Modelli. M. Fabrizio, G. Distefano. J. P/ij:v. C/ieni.. 1990, 94, 5761. 138. R. Lazzaroni. A. J. Pal. S. Rossini, G. Ruani, R. Zamboni. C. Taliani, Sjwth. Me!.. 1991, 42, 2359. 139. S. Destri, M. Mascherpa, W. Porzio. Aih. Muter.. 1993. 5. 43. 140. S. Hotta. K. Waragai. S y t h . hlet.. 1989, 32. 395. 141. B. Fabre. G. Bidan, A A . Mrrtu., 1993, 5, 646. 142. D. Oeter, C. Ziegler, W. Gopel, S y i t / i . Met., 1993, 61, 231. 143. H. Nakahara. J. Nakayama, M. Hoshino. K. Fukuda. Thin Solid Filnis. 1988. 160, 87. Chiui. I t d . . 1985. 115. 575. 144. A. Carpita, R. Rossi. Go:. 145. N. Jayasuriya. J. Kagan. H e t m q c l e s . 1986. 24, 2261. 146. N. Jayasuriya. J. Kagan, Hererocjdes, 1986. 24. 9901. 147. J. Kagan, S. K. Arora, 1. Prakash, A. Uestuenol. Heterocycles, 1983. 20. 1341. Y , 79, 1181. 148. J. H. Uhlenbroek. J. D. Bijloo, Rec. Triw. Chini. P N I Y - B ~ .1960. 149. J.-P. Beny. S. N. Dhawan. J. Kagan. S. Sundlass. J . Org. C/teni., 1982, 47, 2201. 150. H. Wynberg. A . Bantjes. J . A m . Ckeni. SOL..,1959, 84, 1421. 151. (a) K. Y. Yen. G. G. Miller, R. L. Elsenbaumer. J . C/zem. Soc, Cliern. Cornniim.. 1986, 1346; ( b ) M. Sato, S. Tanaka, K. Kaeriyama. J . Chem. Soc, C/iew. Cor~imun..1986. 873. 152. R. D. McCul[ough, R. D. Lowe. M. Jayaraman, P. C. Ewbank, D. L. Anderson. S. TristranNagle, Sjwth. Met., 1993. S5, 1198. 153. T.-A. Chen, R. D. Rieke. J . h i . C/ieni. Soc., 1997. 114. 10087. 154. T.-A. Chen, R. D. Rieke, Svith. M e t . . 1993, 60. 175. 155. D. Delabouglise, M. Hmyene, G. Horowitz. A. Yassar, F. Garnier, Adv. hlnter.. 1992, 4, 107. 156. (a) W. ten Hoeve, H . Wynherg. E. E. Havinga. E. W. Meijer. J . A m . C / i m . Soc.. 1991. 113. 5887; (b) E. E. Havinga. 1. Rotte. E. W. Meijer. W. ten Hoeve, H. Wynberg. Sj77th. Met., 1991. 41-43, 473. . 1986, 15, 169. 157. R. L. Elsenbaumer, K . Y. Yen, R. Oboodi, S ~ x t l iMet., 158. A. Yassar. D. Delabouglise. M. Hmyene, B. Nessak. G. Horowitz, F. Garnier. Ado. M u t e r . , 1992, 4, 490. 159. P. Bauerle, F. Pfau, H. Schlupp et id., J . Chem. Soc. Perkiri Trrins. 2, 1993. 489.

194 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 170. 171. 172. 173. 174. 175. 176. 177. 178. 179. 180. 181. 182. 183. 184. 185. 186. 187. 188. 189. 190. 191. 192. 193. 194. 195. 196. 197. 198. 199.

2 Suijiur-Containing Oligomers

J. H. Uhlenbrock, J. D. Bijloo, Reel. Trav. Chim. Pays-Bas, 1960, 79, 1181. R. Kellogg, A. P. Schaap, H. Wynberg, J . Org. Chem., 1969, 34, 343. R. F. Curtis, G. T. Phillips, J . Chem. Soc., 1965, 5134. P. Bauerle, U. Segelbacher, K.-U. Gaudl, D. Huttenlocher, M. Mehring, Angew. Chem., 1993, 105, 125; Angeic. Chem. Int. Ed. Engl., 1993, 32, 76. (a) M. G. Hill, K . R. Mann, L. L. Miller, J.-F. Penneau, J . Am. Chem. Soc., 1992, 114, 2728; (b) M. G. Hill, J.-F. Penneau, B. Zinger, K.R. Mann, L.L. Miller, Chem. Muter., 1992, 4 , 1106; (c) B. Zinger, K. R. Mann, M. G. Hill, L. L. Miller, Chrm. Mater., 1992, 4 , 1113. A. Stabel, J. P. Rabe, Synth. Met., 1994, 67, 47. J. K. Herrema, H. Wildeman, F. van Bolhuis, G. Hadziioannou, Synih. Met., 1993, 60, 239. (a) M. Sato, M. Hiroi, Clzem. Lett., 1994, 745; (b) M . Sato, M. Hiroi, Chem. Lett., 1994, 985; (c) M . Sato, M. Hiroi, Chrm. Lett., 1994, 1649; (d) M. Sato, M. Hiroi, Synth. Met., 1995, 71, 2085. G. Tourillon, in Handbook qf Conducting Polymers, T. A. Skotheim (Ed.), Dekker, New York, 1986. P. Bauerle, T. Fischer, B. Bidlingmaier, A. Stabel, J. P. Rabe, Angew. Chem., 1995, 107, 335; Angew. Chem. Int. Ed. Engl., 1995, 34, 303. A. Amer, A. Burkhardt, A. Nkansah et a/.,Phosph. Sup: Silic., 1989, 42, 63. J. L. Bredas, R. Silbey, D. S. Boudreaux, R. R. Chance, J . A m . Chem. Soc., 1983, 105, 6555. D . D. Cunningham, L. Laguren-Davidson, H. B. Mark, Jr., C. V. Pham, H. Zimmer, J . Chem. Soc., Chem. Commun., 1987, 1021. L. Laguren-Davidson, C. V. Pham, H. Zimmer, H. B. Mark, Jr., J . Electrochem. Soc., 1988, 135, 1406. H. Matsuda, K. Kaeriyama, H. Suezawa, M. Hirota, J . Pol-vm.Sci.: Part A , Polym. Chem., 1992, 30, 945. P. Biuerle, M. Hiller, S. Scheib, M. Sokolowski, E. Umbach, Arlv. Muter., 1996, 7, 214. P. Bauerle, S. Scheib, Adv. Mater., 1993, 5 , 848. C. Arbizzani, A. Bongini, M. Mastragostino, A. Zanelli, G. Barbarella, M . Zarnbianchi, A d v . Mater., 1995, 7, 571. G. Barbarella, A. Bongini, M. Zambianchi, Macromolecules, 1994, 27, 3039. H. Mao, B. Xu, S. Holdcroft, Macromolrcules, 1993, 26, 1163. G. Barbarella, M. Zambianchi, Tetrahedron, 1994, 50, 11249. M. Schon, Diploma Thesis, Universitat Wiirzburg, 1996. A . Iraqi, J. A. Crayston, J. C . Walton, J . Muter. Chem., 1995, 5 , 1831. E. E. Havinga, L. W. van Horssen, Makromol.Chem., Mukronzol. Symp., 1989, 24, 67. J. Roncali, A. Gorgues, M. Jubault, Chem. Muter., 1993, 5 , 1456. J. Kankare, J. Lukkari, P. Pasanen, R. Sillanpiia, H. Laine, K . Harmaa, Macromolecules, 1994, 27, 4327. T. Bennicori, E. Brenna, F. Sannicolo rt a/., J . Chem. Soc. Chem. Commun., 1995, 881. J. C. Horne, G. J. Blanchard, E. LeGoff, J . Am. Clzem. SOL..,1995, 117, 9551. L. DeWitt, G. J. Blanchard, E. LeGoff, M. E. Benz, J. H . Liao, M. G. Kanatzidis, J . A m . Chem. Soc., 1993, 115, 12158. G. Barbarella, M. Zambianchi, A. Bongini, L. Antolini, Adv. Muter., 1994, 6 , 561. G. Barbarella, A. Bongini, M. Zambianchi, A&. Muter., 1991, 3, 494. G. Barbarella, M. Zambianchi, A. Bongini, L. Antolini, Adv. Muter., 1993, 5, 282. G. Barbarella, M. Zambianchi, A. Bongini, L. Antolini, Adv. MatcJr., 1993, 5 , 834. P. Audebert, J.-M. Catel, G. Le Coustumer, V. Duchenet, P. Hapiot, J . Phys. Chem., 1995. 99, 11923. J.-H. Liao, M. Benz, E. LeGoff, M. G. Kanatzidis, Adv. Mater., 1994, 6 , 135. J. Roncali, M. Giffard, M. Jubault, A. Gorgues, J . Elrcirounal. Chem., 1993, 361, 185. K. Faid, M. Leclerc, J . Chem. Soc., Chenz. Commun., 1993, 962. G. Barbarella, A. Bongini, M. Zambianchi, Tetrahedron, 1992, 48, 6701. K. A. Murray, S. C . Moratti, D. R. Baigent et a/., Syntli. Met., 1995, 69, 395. A. Kraak, A. K. Wiersma, P. Jordens, H. Wynberg, Tetrahedron, 1968, 24, 3381.

Rt&rences

195

200. G. Zerbi. R. Radaelli. M. Veronelli. E. Brenna. F. Sannicolo, G. Zotti. J . Clitw. P/I!Y.. 1993, YH. 4531. 201. (a) G. Zotti. G. Schiavon. A. Berlin. G. Pagani, ~~ric~rot~iolrczrli~s, 1994, 27, 1938; (b) G. Zotti, A. Berlin, G. Pagani, G. Schiavon, S. Zecchin. A t h . Mrrter.. 1994, 6 , 231. 202. T. Pilati. Acttr C /.yt.. 1995, C 5 l , 690. 203. J . Roncali. C. Thobie-Goutier, A r h . M l r / t J r . ,1994. 6, 846. 204. T. L. Lambert. J. P. Ferraris. J . C/iwi. Soc. Chrrii. Cotmiitn., 1991. 752. 205. (a) J. Roncali. H. Brisset. C. Thobie-Gautier, M . Jubault, A. Gorgues, J . Cliini. P l i j ~ . .1995. 92. 771: (b) H. Brisset. C. Thobier-Gautier. A . Gorgues, M. Jubault, J. Roncali. J . Chetii. Soc. Chefil. ~ o r ? i / ? i i t t i . 1994. , 1305. 206. J. P. Ferraris. T. L. Lambert, J . C / i m . Snc., C/iwi. Cotiimuri.~1991, 1268. 207. J . P. Ferraris. J. Henderson, D. Torres. D. Meeker, Synth. Me/., 1995, 72, 147. Met.. 1994, 66. 149. 208. G. Zotti. G. Schiavon, S. Zecchin. A. Berlin, G. Pagani, sJ3ti//7. 209. G. Zotti, A. Berlin, G. Pagani. G . Schiavon, S. Zecchin. .4tfi1. Marer., 1995, 7, 48. 210. H . Brisset. C. Thobie-Gautier. M. Jubault, A. Gorgues. J. Roncali. J . Chem. Soc.. Chet71. C ~ ~ / ? I i ~ ?1994, f O l . .765. 211. ( a ) G . Zotti, G. Schiavon. A. Berlin, G. Pagani, C/iwz. hfoter., 1993, 5, 430; ( b ) G. Zotti. G. Schiavon, A. Berlin. G. Pagani, Chem. Muter., 1993, 5, 620; (c) G. Zotti, G. Schiavon. A . Berlin. G. Pagani. .kh. M N I C I . .199 , 212. S. Hotta. K. Waragai. ..lth.. Mom..1993. 5, 896. 213. S. Hotta. K. Waragai, J . Afarer. C/7tvii.. 1991. 1. 835. . 1993, 97, 7427. 214. S. Hotta, K. Waragai, J . P h j ~ Cheni,. 215. K. Tanaka. Y. Mutsuura. Y. Oshima. T. Yamabe. S. Hotta. Syritli. M e / . . 1994. 66. 295. 216. F. Garnier. A . Yassar. R. Hajlaoui C I nl.. J . Arii. C h i i . Sot,.. 1993, 115. 8716. 217. A. Yassar. F. Garnier. F. Deloffre, G. Horowitz. L. Ricard. .4dv. Mnirr.. 1994, 6. 660. 218. G. Horowitz, P. Delannoy. H. Bouchriha et 01.. A t h . Muter., 1994. 6 , 752. 219. J. P. Parakka, M. P. Cava, Tt~trnlrerlrotr,1995, 51. 2229. 220. C . Aleman. E. Brillas. A. G . Davies 01 d . .J . Org, Clitwi.. 1993, 58. 3091. 221. (a)J . L. Sauvaljol, C. Chorl-o, J.-P. Lore-Porte t'/ 1 7 / . . S~wt/i. Met.. 1994, 62.233: (b) P. Hapiot, L. Gaillon, P. Audebert. J . J . E. Moreau. J.-P. Lere-Porte, M. Wong Chi Man, S p r h . Met.. 1995, 73, 139. 222. C. Soucy-Breau. A. MacEachern. L. C. Leitch. T. Arnason, P. Morand, J . f f t . r e r o c j d i c C/ietn., I99 I . 28, 41 I . 223. P. Bauerle. U . Segelbacher. A . Maier. M. Mehring. J . A m Clieni. Soc.. 1993, 115, 10217. 224. A. S. Davidov. Tlieory qf hlol~~culur E.\-citom, McGraw-Hill. New York. 1962. 225. B. Nessakh. G. Horowitz. F. Garnier. F. Deloffre. P. Srivastava. A. Yassar, J . Elec/rorr~itr/. C'/ierii., 1995. 399, 97. 226. H. Haberkorn. H. Naarmann, K. Penzien, J. Schlag, P. Simak. Sjwrh. Me/., 1982. 5. 51. 227. ( a ) H.-J. Egelhaaf, P. Biiuerle, K. Rauer. V. Hoffmann, D. Oelkrug, J . Mol. Strwt.. 1993, 2Y3. 249; ( b ) H.-J. Egelhaaf. P. Biiuerle. K. Rauer. V. Hoffmann, D. Oelkrug. Syutk. Met., 1993. 61, 143. 228. M. Bennati. A . Grupp. M. Mehring. P. Biiuerle. J . Phys. Chcwi., 1996. 100. 2849. 229. (a) C. Ehrendorfer, H. Neugebauer. A. Neckel. P. Bauerle, Sjvith. Me!., 1993, 55, 493: (b) C. Ehrendorfer, A. Karpfen, P. Biiuerle, H. Neugebauer. A. Neckel. J . Molec. S/rirct., 1993. 298. 65. 230. C. Ehrendorfer. H. Neugebauel-. P. Biiuerle, A . Neckel. Sl'ti//i. Me/.. 1995. 69. 393. 231. M. Stoldt, P. Bbuerle. H. Schweizer. E. Umbach. Swtli. M e / . , 1993. 57, 4059. . Techriol.. Sect. 332. M. Stoldt, P. BBuerle, H . Schweizer. E. Umbach. Afol. Crj3st. Liq. C q ~ s tSci. A . 1904, 340. 127. 233. H. Neureiter. W. Gebauer. C . Vaterlein, M. Sokolowski, P. Bauerle, E. Umbach. S,vur/i. M e / , , 1994. 67, 173. 234. C. Vbterlein. H. Neureiter. W. Gebauer et ul.. J . Appl. P h ~ s . submitted. . 335. (a) D. Braun, G. Gustafsson, D. McBranch. A. J. Heeger, J . Appl. Plijx. 1992. 72, 546: (b) M. Bergreen. G. Gustafsson. 0. Inganas. M. R. Anderson, 0. Wennerstrom, T. Hjertberg. Adv. M t r t c . r.. 1994, 6. 488. 236. G. Horowitz. P. Delannoy. H. Bouchriha e / a/.. . 4 h . Matcr.. 1994, 6 , 753.

196

2 Sulfur-Containing 0ligonier.Y

237. H.-J. Egelhaaf, D. Oelkrug, J . Of'SPIE, 1996. 2362, 398. 238. K. Uchiyama, H. Akimichi, S. Hotta, H . Noge, H. Sakaki, Synth. Mer., 1994, 63, 57. 239. A. Soukopp. K. Glockler, P. Bauerle, M. Sokolowski, E. Umbach, Ad)?.Muter., 1996, 7, 902. 240. (a) J. M. Tour, R. Wu, Macromolecules, 1992, 25, 1901; (b) J. M. Tour, Chem. Rev., 1996, 96, 537. 241. L.-T. Cheng, J. M. Tour, R. Wu, P. V. Bedworth, Nonlinear Opt., 1993, 6, 87. 242. (a) J. Guay, P. Kasai, A. Diaz, R. Wu, J. M. Tour, L. H. Dao, Chem. Muter., 1992, 4, 1097; (b) J. M. Tour, J. Guay, A. Diaz, R. Wu, L. H. Dao, Chem. Muter., 1992, 4, 254. 243. A. Aviram, J . Am. Chem. Soc., 1988, 110, 5687. 244. (a) J. M. Tour, R. Wu, J. S. Schumm, J . Am. Chem. Soc., 1990, 112, 5662; (b) J. M. Tour, R. Wu, J. S. Schumm, J . Am. Chem. Soc., 1991, 113, 7064. 245. (a) J. M. Tour, J. Guay, A. Diaz, R. Wu, J . Am. Chem. Soc., 1993, 11.5, 1869; (b) J. R. Diers, M. K. DeArmond, J. Guay et ul., Chem. Mater., 1994, 6, 327. 246. (a) B. Kirste, P. Tian, G. Kossmehl, G . Engelmann, W. Jugelt, Mugn. Res. Chem., 1995, 33, 70; (b) G. Engelrnann, Dissertation, FU Berlin 1995. 247. G. Heywang, F. Jonas, Adv. Mufer., 1992, 4, 116. 248. M. Hasik, J. E. Laska, A. Pron, I. Kulszewicz-Bajer, K. Koziel, M. Lapkowski, J . P o l j ~ ~ . Sci.,A : Polym. Cliem., 1992, 30, 1741. 249. R. Cloutier, M. Leclerc, Synth. Met., 1993, 55-57, 1272. 250. G. Zotti, M. C. Gallazzi, G. Zerbi, S. V. Meille, Synth. Met., 1995, 73, 217. 251. (a) L. L. Miller, Y. Yu, J . Org. Chem., 1995, 60, 6813; (b) L. L. Miller, Y. Yu, E. Gunic, R. Duan, Adv. Mater., 1995, 7, 547. 252. Y. Yu, E. Gunic, L. L. Miller, Chem. Mater., 1995, 7, 255. 253. F. Effenberger, Acc. Chem. Res., 1989, 22, 27. 254. (a) P. BBuerle, G. Gotz, A. Synowczyk, J . Heinze, Lieb. Ann., 1996, 279; (b) H. Theobald, V. Harries, U. Kardorff, P. BBuerle, G. Gotz, Ger. Offen. DE 4.201.305 (Chem. Abstr., 1993, 119, 225805s). 255. (a) U. Folli, D. Iarossi, M. Montorsi, A. Mucci, L. Schenetti, J . Chem. Soc., Perkin Trans. I , 1995, 537; (b) G. Barbarella, M . Zarnbianchi, L. Antolini et al., J . Chem. Soc., Perkin Trans. 2, 1995, 1869. 256. G . Barbarella, M. Zambianchi, R. DiToro, M. Colonna, L. Antolini, A. Bongini, Adv. Muter., 1996. 8, 327. 257. P. Bauerle, G . Gotz, S. Scheib, R. Klose, J. Heinze, Europ. J . Org. Chem., submitted. 258. H. Muguruma, T. Saito, S. Sasaki, S. Hotta, I. Karube, J . Heterocyclic Chem., 1996,33, 173. 259. F. Wudl, G . M. Smith, E. J. Hufnagel, J . Chem. Soc., Chem. Commun., 1970, 1453. 260. (a) J. Roncali, M. Giffdrd, P. Frtre, M. Jubault, A. Gorgues, J . Chem. Soc., Chem. Commun., 1993, 689; (b) J. Roncali, M. Giffard, M. Jubault, A. Gorgues, Synth. Met., 1993, 60, 163. 261. Y. Wie, B. Wang, W. Wang, J. Tian, Tetruhcdron Letters, 1995, 36, 665. 262. J. Nakayama, T. Fujimori, Sulfirr Letters, 1990, 11, 29. 263. T. Nihira, S. Tarutani, K. Takase, K. Takahashi, Heterocycles, 1995, 41, 2169. 264. W. M. Albers, G. W. Canters, J. Reedijk, Tetrahedron, 1995, 51, 3895. 265. (a) F. Effenberger, F. Wurthner, Angeir. Chem., 1993, 105, 742; Angeir. Chem. Int. Ed. Engl., 1993, 32, 719; (b) F. Effenberger, F. Wurthner, F. Steybe, J . Org. Chem., 1995, 60, 2082. 266. F. Wiirthner, F. Effenberger, R. Wortmann, P. KrBmer, Chem. Phys., 1993, 173, 305. 267. M. G. Hutchings, I. Ferguson, D. J. McGeein, J. 0. Morley, J. Zyss, I. Ledoux, J . Chem. Soc., Perkin Trans. 2, 1995, 171. 268. F. Wiirthner, M. S. Vollmer, F. Effenberger et a/., J . Am. Chem. Soc., 1995, 117, 8090. 269. P. Emele, D. U. Meyer, N. Holl et a/., Chem. Phys., 1994, 181, 417. 270. H. Segawa. F.-P. Wu, N. Nakayama et a/., Synth. Met., 1995, 71, 2151. 271. (a) M. Schmelzer, S. Roth, P. Bauerle, R. Li, Thin SolidFilms, 1993,229,255; (b) M . Schmelzer, M. Burghard, P. Bauerle, S. Roth, Synth. Mer., 1993, 61, 97; (c) M. Schmelzer, M. Burghard, P. BBuerle, S. Roth, Thin Solid Filmx, 1994, 243, 620. 272. H. Nakahara, J. Nakayama, M. Hoshino, K. Fukuda, Thin SoliLfFilms, 1988, 160, 87.

References

197

273. M. Schmelzer, M. Burghard, P. Biuerle. S. Roth, Mol. Cryst. Liq. Cryst. Sci. Terhriol., Sect. A , 1994, 252-253, 465. 274. B. Liedberg. Z. Yang, I. Engquist e1 al., J . Chrni. Plzys., 1997, 101. 5951. 275. J. M. Tour, L. Jones 11, D. L. Pearson et a/.. J . Am. Chem. Sor., 1995, 117, 9529. 276. D.D. Graf, N.C. Day, K.R. Mann, Itiorg. Chetn., 1995, 34, 1562. 277. D. Fichou. B. Bachet. F. Demanze. I. Billy, G. Horowitz, F. Gamier. A h . Muter.. 1996, 8, 500.

2.2 Oligotetrathiafulvalenes J. Becher, J. Lau and P. Marrk

2.2.1 Introduction Tetrathiafulvalenes (TTFs) and related tetrachalcogena based heterocycles have been of high interest for more than two decades due to their unique properties as .ir-donors. Although dibenzotetrathiafulvalene had been reported in 1926 [ 11, the research area was not active until Wudl et al. [2] reported the facile reversible oxidation to the TTF dication and the demonstration in 1973 [3] of the unusually high conductivity (500 S cm-' ) for the charge transfer complex between tetrathiafulvalene and the electron acceptor tetracyanoquinodimethane (TCNQ) 2 (Fig. 1). The research field came into focus when superconductivity was discovered in a number of the so-called Bechgaard salts (D2X) [4],which are radical cation salts between a chalcogenafulvalene donor (D) and an anion (X = Cloy, BF,, PF; etc.). The current record of a superconductivity transition temperature T, is 1 1.6 K (1 atm) for a salt of the TTF derivative bis(ethylenedithia)tetrathiafulvalene (BEDT-TTF) 4 (Fig. I ) , namely: (BEDT-TTF),CU[N(CN)~]B~ [5]. A number of reviews [6] covering many aspects of this research area have appeared. The most comprehensive and up to date on TTF chemistry is that by Fanghinels [7] including bis- and oligo-TTFs, while Bryce [8] has given an excellent overview of the tetrachalcogenafulvalene charge transfer complexes and their electronic properties. Khodorkovsky and Becker [9] have given an introduction to molecular design of organic conductors, and include references to a number of bis-TTFs.

2.2.2 Redox Properties of Tetrathiafulvalenes Tetrathiafulvalene is a planar non-aromatic 14 7r-electon system. Oxidation takes place sequentially in two distinct reversible steps with formation of the radical cation (i) and the dication (ii) (see Fig. 2). The redox potentials for the unsubstituted T T F are found at a relative low potential (E1,2 = 0.35 and E:/2 = 0.71 V, for the experimental conditions in the cyclic voltammetry experiments (see Tables 1-5 at the end of this chapter) [7]. Change of substituents in tetrathiafulvalenes allows the redox properties to be 'fine tuned', for example for 5 (R'=COOMe, R2=SMe) the redox potentials are higher than for the unsubstituted tetrathiafulvalene (E:,2 = 0.63 and E;,? = 0.93 V, vs. Ag+/AgCl, MeCN, TBAP) [7], due to the electron withdrawing ester and alkylthio groups. Changing sulfur to other chalcogenes (Fig. 3 ) results

2.2.2 Redo.\- Properties of Tetrathia{ihalenes

1 TTF

2 TCNQ

199

TCNQF4

0

3 dibenzo-TTF

4 BEDT-TTF

DDQ

Figure 1. Fundamental tetrathiafulvalenes and electron acceptors

in enhanced overlap between the chalcogene atoms in charge transfer complexes in the solid state, and both tetraselena- 6 and mixed selena-sulfur fulvalenes 8 have given rise to superconducting salts [7]. The following general scheme (Fig. 4) give an overview of the variety of TTFderivatives and related 7r-donors which have been studied [lo]. There are two main research topics where the TTF-type 7r-donors are of special interest; i the more ‘classical’ reseach topic of molecular conductors and superconductors and ii; the application of TTF-derivatives as x-donors in supramolecular chemistry (Fig. 5). The electronic properties of conductors based on radical ion salts are extremely dependent on the stabilization of a multidimensional chalcogen-chalcogen network in tbe solid state. It is almost impossible to predict and control crystal packing at the molecular level [ 1 11 and the rationale behind the idea of covalent linking of the TTFgroups in oligo-TTFs is that this may increase the dimensionality in the solid state. Another aspect of the ‘oligomer-approach’ is the potential control over the stoichiometry in C T complexes as it has been found [12] that a stoichiometry with two donors to one acceptor does increase the chance of partial electron transfer, which is a prerequisite for metallic conductivity. It is evident that the degree of conformational freedom of a bis-TTF-system will decrease by changing the connection from one to two linkers at the 2,3- or 2,7positions of the TTF. Furthermore such linkers may be either cr- or 7r-bonds, the bis-TTF-systems being classified as in Fig. 6. For oligomeric TTFs with more

1

Figure 2. Redox properties of TTF.

200

2.2 Oli~otetrathiufulvalenes

6

7

8

TSF

lleF

mixed tetrachalcogeneluldelene

Figure 3. Tetrachalcogenafulvalenes.

*/-

change of subsmuents

1'

,'

exchange

t

N covalent spacers,

cattton to nitrogen exchange

conjugated, aromatic or heteroaromatic

Figure 4. Overview of the types of modifications on the basic TTF framework which have been studied (adapted from ref. [lo]).

dendrimen initiator in radical reactions

?

conductina and semicondkting materials

A superconducting materials

4

k~

Langmuir-Blodgett films

molecular shuttles, rotaxanes and catenanes

*-----

k' n-donors

c organic fernmagnets

i sensors

1

conducting polymers

NLO materials

Figure 5. Examples of applications where r-donors derived from 1,3-dithiole-systems (TTFs) have been used.

22.2 Redo.u Properties of' Tetrutlii~firl~~alenes

20 1

Table 1. Characterization by cyclovoltaninietric measurement of bis-tetrathiafulvalenes, connected through one linker.

X

R

R'

-PC&-

H

H

-CH2CH2-S(CH2)nS-

Me Me

H Me

(-SCH?)lChH4

Me

Me

-SCHZS-

S(CH2),S SMe

-S(CH2)nS-

Me Me

-S(CH2)nS-

H

cv

Method (yield) (see section 2.2.7.1)

(a: vs SCE; b: vs Ag/AgCl)

A l (2In/n) B2 (61 YO) A3 (30%) A3 n = I : (24%) n = 2 : (ISYn) I1 = 3: (33%) n = 10: (50%) A3 0: (23%) m : (31%) p : (35%)

0.43; 0.84. PhCN.a 0.35; 0.82. PhCN.b 0.29; 0.35; 0.86, DCM,a 0.22; 0.36; 0.80, DCM.a 0.22; 0.36; 0.74, DCM,a 0.28; 0.75, DCM.a 0.27; 0.77, DCM,a 0.29; 0.79, DCM,a 0.26; 0.75. DCM.a 0.28: 0.76, DCM,a

A2

0.47; 0.55; 0.81*, b

COOMe A3 n = 2: (61%) n = 3: (68%) n = 4: (75%) H A3 n = 2: (48%) n = 3: (55Y") n = 4 : (61%) H B3 n = 3 : (13%) n =4: (5%)

0.60; 0.94, MeCN.b 0.40; 0.48; 0.56; 0.94 0.56; 0.91 0.53; 0.87 0.46; 0.78 0.44; 0.78 0.44; 0.78. DCM.b 0.44; 0.81, DCM.b

-S(CH2)4NHMe

B5 (40%)

Me H H COOR

B5 (92%) A2 ( 1 5%) B2 (62%) BI

0.50; 0.59; 0.85 0.43; 0.84; PhCN,b 0.85; 1.13

H

H

SAlkyl

SAlkyl

B2 X = CH: (77%) X = N: (729'0) B2 X = CH: (55%) X = N: (48%)

0.36; 0.74, PhCN,a 0.29; 0.87. PhCN,a 0.54; 0.80, PhCN.a 0.54; 0.82. PhCN,a

SCloHzr -S(CH2)3CONH(CH?)?NHCO(CH2)3SSC(oH?[ SMe H R1: COOR Rl: H CO(CH2)3S-

*: irreversible

Ref.

202

2.2 Oligotetrathiafulvulenes

Table 1. (Cont.) Method (yield) (see section 2.2.7.1)

cv

B2 (74%)

0.37; 0.75, PhCN,a

TTF TTF

B2 (21%)

0.31; 0.91, PhCN,a

TTF

B2 (19%)

0.40; 0.79, PhCN,a

TTF TTF

8 2 (14%)

0.29; 0.89, PhCN,a

8 3 (78%)

0.46; 0.81, DCM,b

B3 R = H: (84%)

0.47; 0.78, DCM,b

X

R

R’

qQ

w a

(a: vs SCE; b: vs Ag/AgCl)

R TTF.

s,,,~~-s.~~~ 0

0

TTF-iihTTF

0

B3

+ B4 (34%)

0.45; 0.56; 0.79, DCM,b

B3 (10%)

0.45; 0.79, DCM,b

B4 (74%)

0.05’; 0.37*; 0.62*, PhCN 0.44; 0.69; 0.92; PhCN,b 0.51; 0.80, DMF,b

B4 (69%)

0.52; 0.66; 0.99*, PhCN,b 0.61; 0.86, DMF,b

B4 (56%)

0.41; 0.71, DMF,b

B4 (22%)

0.56; 0.78, DMF,b

Ref.

Table 1. (Cmif.) X

R

R'

Method (yield) (see section 2.1.7.1)

cv

B1 ( 5 1 % )

0.35; 0.82, P1iCN.b

B3 (20%)

0.37: 0.71; 0.85, MeCN.b

B4 (3004)

0.45: 0.88, DCM,b

0.31; 0.65; 0.804, MeCN,b

w m

w

o

Ref.

(a: vs SCE; b: vs Ag/AgCI)

0

-Te-

H

H

BI (20%)

-Te-Tee

H

H

BI (28%)

-PPh-

H

H

BI (340;/0)

0.37; 0.47: 0.84. MeCN,b

-S-Se-Hg-Hg-Hg-SiMe2-SiMe2-SiMe2 -PPh-PPh-

H H H Me Me H Me Me Me Me

H H H H Me H H Me H Me

BI (14%) B1 (99") BI (80%) BI (85"n) BI BI (71%) BI ( 2 5 % ) BI BI 160%) BI (68%)

0.49: 0.61. 0.86'. MeCNb 0.49: 0.61; 0.86'. MeCNb 0.35: 0.68. UMF.a 0.27; 0.66. DMF,a 0.28; 0.63. DMF.a 0.29: 0.38; 0.77. DCM.a 0.22: 0.32; 0.82. DCM,a 0.24; 0.34; 0.88. DCM.a 0.28; 0.32; 0.88. DCM,a 0.23; 0.36; 0.76: 0.87,

[74]

204

2.2 Oligotetrathiufulvalenes

Table 2. Characterization by cyclovoltammetric measurement of bis-tetrathiafulvanes, connected through two linkers.

X

R

-S(CH*CH*0)3 CH2 CH2S-SCH*CH?S-SCHZCHZS-S-S-S-S-Te-Te-

SMe SMe -SCH2CH*SSMe SMe H H H Me H SMe Me SMe SMe SMe Me Me

R'

Method (yield) (see section 2.2.7.1)

CV (a: vs SCE; b: vs Ag/AgCl)

A2 (3%) A2 A2 A2 (50-580/,) A2 (50-58%) A2 (50-58Yo) A2 (50-580/,) B1 (26%) B1

0.59; 0.84*, DCM,b 0.60; 0.86, DCM,b 0.47; 0.67; 0.89, PhCN,b 0.43; 0.64; 0.88, PhCN,b 0.49; 0.75; 0.89, PhCN,b 0.47; 0.76; 0.87, PhCN,b 0.57; 0.84, PhCN,b 0.48; 0.83, PhCN,b

Ref.

than two TTF units more combinations are possible, both 2- and 3-dimensional. Only a few such TTF systems have hitherto been prepared.

2.2.3 Bis-Tetrathiafulvalenes, Connected through One Linker The idea of linking multiple .ir-donors was originally introduced by Wudl et al. [ 131 who prepared the first bis-tetrathiafulvalene 9 in 1977, hoping to increase the critical temperature T, of its CT-salts. Cyclic voltammetry of the bis-TTF 9 with the conjugated phenyl group as linker revealed that the two TTF-units do not interact electronically, and each of the two redox waves corresponds to a two-electron oxidation. The fundamental bis-TTF 10 [14] and the extended derivative 11 [15] likewise show two redox couples, implying that there is no coulombic effect between the two T T F moieties. Put in simple terms 'the two TTF-groups do not communicate' in these systems (Fig. 7). Charge-transfer complexes of 11 were prepared with DDQ or TCNQF4 (see Fig. 1) as electron acceptors. In both cases a 1 : 1 complex was formed. Compressed pellet samples of 11 DDQ showed an electrical conductivity of 1.1 x lo-' S cm-' and 11 TCNQF4 a conductivity of 3.6 S cm-' . The redox behavior of the bis-TTF derivatives in a series connected by aliphatic linkers (compound 12, Fig. 7, R = Me and X = -(CH2)n-. n = 1, 2, 3, 10) was investigated by Bechgaard et al. [16]. For IZ = 3 and 10 the usual two redox couples were observed whereas for n = 1 and 2 more complex CV results were obtained; three redox couples with a characteristic lowering of the first redox potential E:,2.

-7.2.3 Bis-Tetratliiufulvale~ies,Connected through One Linker nonconjugatedlinkers

conjugated linkers

one linker

Figure 6. Classification of bis-TTF systems

10

9

12

11

X = -(CH2)”- or e,m,pxytylene n = 1, 2, 3. 10 R = Me. COoMe

Figure 7. Examples of bis-TTFs connected with one linker

205

Table 3. Characterization by cyclovoltammetric measurement of bistetrathiafulvalenes two linkers, .rr-type,annelated systems Method (yield) (see section 2.2.7.1)

CV (a: vs SCE, b: vs Ag/AgCl)

B5

A2

A21 R A2: R A2: R A2: R A2: R A2: R A2: R A2: R A2: R A2: R A2: R

0.78; 0.92; 1.20’. MeCN,a

= R‘ = CF3 (50-75%) = R’ = CN (50-75%) = R’ = COOMe = R’ = SMe (35%)

= R’ = H ( > 28%) = H, R’ = COOMe = H, R’= S(CHZ)I_3S = SMe, R’ = H (17-78%)

SMe, R’ = COOMe (do) = SMe, R’ = SCHzS (do) = SMe, R’ = S(CH2)2_3S(do) =

A2: R = SMe,

R‘ = O ( C H 2 ) 2 0(do)

A2: R = H A2: R = COOMe

A2 (10%)

0.53; 0.72; 0.94; 1.11, PhCN,a 0.44; 0.62; 1.05; 1.13, PhCN,a 0.49; 0.71; 0.58; 0.81; 0.51; 0.70, 0.52; 0.74;

0.99; 1.13’, PhCN,a 1.12; 1.31*, PhCN,a PhCN,a 1.01; 1.19*, PhCN,a

0.51; 0.72; 0.97; 1.12, PhCN,a

0.37; 0.50; 0.81; 1.05, PhCN,a 0.56; 0.70; 1.07; 1.30, PhCN,a

Ref.

2.2.3 Bis- Trtrnrhinfuhaleties. Connected through One Linker

E

i

V

V

9 g ZV

n

dn

II

d

2

Q

207

h,

Table 3. (Con?.)

0

03

Method (yield) (see section 2.2.7.1)

CV (a: vs SCE, b: vs Ag/AgCl)

Ref.

0

rg

R’S R’s~H;+)=?*Hy. OR OR -s

SA.

A2: R’= -(CH2)2-, R = -C,H13 R’ = -C,HI,, R = Alkyl

0.28; 0.42; 0.68; 0.88*, DCM,a 0.34; 0.44; 0.69; 0.78; 1.11; 1.33

t931 ~301

s 2?

$ 5

s

s.

;;+s&&H:& SR‘ SR

SR’ SR.

A2: R’ = Alkyl, R = Isoamyl (30%)

0.41; 0.63; 1.06; 1.22, DCM,a

P51

A2: R = C2H5 (17%)

0.54; 0.69; 1.06; 1.22, DCM,a

t951

A2: R = C,Hg

0.60; 1.02, DCM,a

RS !iR

n

OR

OR

OR

5

2.2.3 Bis-Tetratliiu~ul~~aleiies, Connected through One Liriker

209

These results were explained by a strong ititrrrmoleculnr electronic interaction between the partly oxidized TTF moieties. Bechgaard proposed the model presented in Fig. 8; the first redox potential is about 60mV lower than the potential of the model compound 2,3,6-trimethyl-7-methylthio-TTF (Ell2 = 0.28 and E;,? = 0.73 V). This is explained by the formation of a sandwich-like intraniolecular charge-transfer complex. The second oxidation gives rise to a bis-radical cation, and the third wave corresponds to a two electron oxidation forming the tetracation. On a related bis-TTF system (compound 12, Fig. 7, R = COOMe or R = H and X = -(CH2),,-. n = 2, 3 , 4) Cava el ul. [I71 found a simpler redox behavior i.e. two redox couples in all cases but one. The derivative R = COOMe, n = 3 showed four redox couples. These results show that it is not only the nature of the linker between the donor units that is responsible for the conformation of the donor units, but that the substituents also play a crucial role. Figure 9 shows a system without much conformational freedom, were the TTF-units are substituted on a rigid annulene [18]. In each case two redox waves were observed: for 13(Fig. 9) the values are El,. = 0.40 and Ef,. = 0.79 V and for 14, EtI2 = 0.29 and Efp = 0.89 V. In 14 the two TTFs are able to interact intramolecularly and this interaction results in lowering of El and a higher E’ due to coulombic repulsion between the two radical cations. For 13 such interaction is impossible and since there is no interaction through the 7r-system the result is that a ‘normal’ E’ value is observed. Connection of two TTFs with heteroatoms such as S. Se, Te and Te-Te linkers have also been studied (Fig. 10). For X = S and Se (Fig. 10) three redox waves were observed [19]. The first is due to a one electron oxidation of the first TTF-unit, followed by another one electron oxidation of the next TTF-unit. The third redox wave is due to formation of the tetracationic species. For X = Te three redox waves were likewise observed [20]. In this case the third wave was ascribed to the oxidation of the Te atom. In this series i t was concluded that the two TTF-units do interact electronically. Two CT complexes with different stoichiometries were prepared from 15, X = Te - Te and TCNQ [21]. The 1 : 1 complex (TTFTe), .TCNQ showed a room temperature conductivity of 0.3 S cm-’ while the 1 : 2 complex (TTFTe)? (TCNQ), had a room temperature conductivity of 8.2 S cm-’. -SiMe2- and In an analogus series with covalent linkers, X = -Hg-, -PPh- [22] it was concluded that coulombic repulsion through space was the only observable electronic interaction. The corresponding tris-TTF system (TTF)?P (Fig. 10) only shows two reversible three electron redox waves [23], corresponding to the formation of three cation radicals followed by three dications, because the rigid pyramidal phosphor atom prevented electronic interaction. Calculations [22] on the 7r-type HOMO atom coefficients of the parent T T F show that the electron density is primarily located on the TTF-sulfur atoms and the fulvene double bond, resulting in only small coefficients at the 2. 3 , 6, and 7-carbon atoms. The approximate HOMO values can be depicted as in Fig. 11. In a valence bond picture this simply means that resonance structures where electrons are delocalized from the peripheral carbons in the dithiole rings do not contribute to any significant degree.

-

210

2.2 Oligotetrathiafulvalenes

M r e m o l ~dwpttransfercomplex ~l~

Figure 8. Model for oxidation/reduction of bis-TTF 12 (Bechgaard et al. [16]).

13

Figure 9. TTFs substituted on a rigid annulene system.

15 X =

S,Se, Te, TeTe, HQ,SiM% and -PPh

15A

Figure 10. Bis-TTFs connected through heteroatoms.

2.2.3 Bis- Trtrrithiq~rl~~ali~iii~s~ Coiiiiecretl through Oiic Linker

21 1

Figure 11. HOMOcoeficients calculated for the unsubstituted TTF, the sizes of the circles represent roughly the relative values (adapted from ref. [22]).

16

Figure 12. Bis-TTF with a niethylenedithio bridge.

Twindonor 16 [24] shows three redox waves (Fig. 12). indicating a coulombic interaction between the two TTF moieties. Electrocrystallization of 16 with nBu4NC104 as the supporting electrolyte resulted in formation of two radical salts, 16.C104 and 16.(C104)05which were isolated from the same batch as black blocks and green needles res ectively. 16 C104 is an insulator as the room tempera) - I . The X-ray structure of 16 C104 revealed that ture conductivity is 1 x 10- b Scm the twin donor is taking a sandwich-like shape (Fig. 13). From inter- and intramolecular S-S distances it was concluded that the radical cation is intramolecularly

-

-

Figure 13. Molecular structure of TTF-dimer and its crystal packing, crystal A. 16. CIOJ (Repro. 285). duced from A. Izuoka 6'1 d . .Chew. L ~ i i . 1992.

212

2.2 Oligotetrathiajiilvalenes

B

Q

Figure 14. Molecular structure of TTF-dimer and its crystal packing, crystal B, 16. (C104)o,s (Reproduced from A. Izuoka et al., Chem. Lett., 1992, 285).

-

stabilized in a sandwich-like structure. 16 (C104)0,5(Fig. 14) showed semiconducting properties with a room temperature conductivity of 0.15 S cm-' . The X-ray diffraction analysis of 16 (C104)0,5(Fig. 14) revealed that the C-S bond in the methylenedithiobridge is twisted compared to the previous structure resulting in an open sandwich conformation.

-

2.2.4 Bis-Tetrathiafulvalenes, Connected through Two Linkers There are two types of bis-tetrathiafulvalenes connected through two linkers at the 2,3-positions, either with linkers of 0- or 7r-type (Fig. 6).

2.2.4.1 Two Linkers, a-type Bis-TTFs connected through two linkers of a-type, are illustrated in Fig. 15. Similar to the bis-TTFs 15 (Fig. 10) in the previous section, the dithia-TTF 17 (Fig. 15) show three redox waves in the CV (for a variety of different R-groups), thus there is some intramolecular interaction [25]. The asymmetric derivative 17 (R = H, R' = SMe) forms a charge-transfer complex with TCNQ with the stoichiometry 2: 1 i.e. four TTF units per TCNQ. This complex is a semiconductor with a room temperature conductivity of 6 S ern-'. Spiro compound 18 [26] is an example of a bis-TTF derivative with reduced conformational freedom. 18 reveals four redox couples, i.e. there is a considerable couloumbic interaction between the donor moieties. The room temperature conductivity of the CT complex 18 D D Q is 1.5 x 10-6Scm-'. In case of the bis-BEDT-TTF 19 only two redox waves are

-

17

18

n

19

Figure 15. Examples of bis-TTFs connected with d i n k e n a t the 9,3-positions

seen [27] at values close to those found for the parent BEDT-TTF 4 (Fig. I ) . and therefore the TTF-units are electronically independent in this compound. Electrocrystallization afforded 19 CIOj (Fig. 15) w b c h showed metallic conductivity down to 100 K, with a room temperature conductivity of 2-8 S c m - ' . The tetrathia[l2]crown-4 ring in 19 is quite flexible and the X-ray diffraction study revealed an interesting crystal structure. The TTF-planes are almost parallel in the neutral molecule (Fig. 16) [28], whereas in the radical ion-salt (counterion C l o y ) there is a nearly 90" angle between the planes of the TTF-groups [27]. 19 forms a charge-transfer inclusion complex with DDQ (Fig. 17). The X-ray structure in this case revealed a U-shaped donor molecule with the acceptor sitting in the cavity (Fig. 17). An additional example of the versatility of the bis-TTF as donor in CT complexes is the known 19 C6,) complex [29].

.

-

2.2.4.2 Two Linkers, .rr-type, Fused Systems The rationale for interest in bis- and oligomeric-TTFs connected by .ir-type linkers, in either fused or used systems, is that in such compounds the extended i7-system will result in a planar and rigid system with extended delocalization of the positive charges and hence lower repulsion between the charged species. This may then result in segregated stacking, and therefore stronger intermolecular electronic interactions can be expected. Mullen et a/. [30] have prepared a series of benzo-fused TTF-systems such as 20 (Fig. 18). where the substituents R on the central benzene ring are added for solubility reasons. Cyclic voltammetry studies on compound 20 show four redox waves (for a variety of R-groups), where the first two are equivalent to the values for the parent dibenzo-TTF. This leads to the conclusion that there is no extra stabilization

Figure 16. Molecular structure of BEDT-dimer 19, (a) viewed along the molecular short axis, (b) showing the overlap (taken from T. Tachikawa et ul., J . Chem. Soc., Chem. Commun., 1993, 1227).

12.00

5.0°

Figure 17. Molecular structure of BEDT-dimer 19. D D Q inclusion complex (taken from T. Tachikawa PI al., J . Cliem. Soc., C h ~ mCornnmn., . 1993, 1227).

2.2.4 Bis-Tetmthiufulvulenrs, Connecttd rhrough T11.oLinkrrs

2I 5

I

R

20

Figure 18. Examples of conjugated benzo-fused bis-TTFs.

due to conjugation in the annelated system, i.e. the charge is localized at the central TTF-unit. This is in close agreement with the results described in the previous section. The third and fourth redox waves are found at much higher potentials due to coulumbic repulsion between the positive charges. Related annelated trisTTF systems also show a rather complex electrochemistry. Tris-TTF 21 (Fig. 18) is an interesting example which has been prepared recently, such systems can be regarded as oligomers of dibenzo-TTF and in the case of 21 R = OC6HI3, R - H cyclic voltammetry showed: El,,? = 0.31, Ef,? = 0.61, E;i2 = 0.81. 1.01, Ei12 = 1.28V while for R = SC5HII,R I = SEt these values were: El,2 = 0.44, ET12= 0.62, E:,2 = 0.81, E:,? = 1.01, E:,? = 1.288. Here El,‘? are due to the central TTF-unit. Charge transfer complexes with various acceptors only gave rise to semiconducting materials [31]. Data for related annelated systems can be found in Table 3. A number of directly fused TTF derivatives (BDT-TTP) of type 22 (Fig. 19) have been prepared. Upon electrocrystallization several of these yield radical salts with metallic conductivity [32], [33], [34]. The parent compound (22, R = H) [33] shows four redox couples in CV. Electrocrystallization with different counter ions (IT, ClO,, and AsF;) gave radical cation salts with metallic conductivity down to 115K, l l O K and 95K, with room temperature conductivity of 360Scm-I. 120 S cm-I and 800 S cm-’ respectively. Recently the vinylog of BDT-TTP 23 was prepared [35]. The radical cation salts for a variety of counterions were all metallic

22 BDT-TIP

Figure 19. Examples of thiapentalene derivatives.

23

2 I6

2.2 Oli~otetrathiafulvalenes

24

25

20

Figure 20. Examples of TTF-vinylogs.

down to 1.4 K, one derivative 23. A u ( C N ) ~even showed superconductivity with a critical temperature of 4 K . This is the first bis-TTF derivative combining a T T F and an extended TTF-system that shows superconductivity.

2.2.4.2.1 TTF-Vinylogs Although TTF-vinylogs may not be considered as oligo-TTFs, a few examples will be mentioned here. It has been found that an alternative way to diminish coulombic repulsion in an oxidized TTF-type 7r-donor is insertion of a vinylene-group or similar conjugated spacer groups between the two 1,3-dithiole units (Fig. 20). This resulted in interesting observations, for example the parent system 24 is a strong donor with the following redox waves in CV Eljl = 0.20, E f j l = 0.36 V [36]. Figure 20 also show an example 25 of a heterocyclic extended TTF [37] as well as an example of a so called ‘giant-TTF’ 26 [38].

2.2.4.3 Cyclic bis-tetrathiafulvalenes (tetrathiafulvalenophanes) Cyclophanes are a well known group of macrocycles [39]. The study of cyclophanes has provided a great deal of information on intramolecular interactions, charge transfer reactions and host guest interactions. Until now only few tetrathiafulvalenophanes are known in spite of the obvious importance for the study of interactions of such 7r-donors with electron acceptors. In contrast to their acyclic analogs cyclophanes have less conformational mobility and it is possible to design host molecules incorporating electron donors such as TTF-groups. Staab et al. [40] have prepared the first example in this class via coupling of appropriate bis1,3-dithiolium salts. The tetrathiafulvalenophane 27 (Fig. 21) was obtained as the cislcis isomer (X-ray crystallography). Another and more recent example is the cyclophane 28 [41]. This example revealed strong intramolecular electronic interaction between the two TTFgroups, resulting in two redox waves and a rather low first oxidation potential I = 0.18 with at Ef!,2 = 0.48V. Here we clearly see that the radical cation is stabilised via intramolecular CT-formation. An interesting new class of bis-TTF derivatives is the so-called ‘criss-cross overlapped tetrathiafulvalenophanes’ 29 (Fig. 2 I ) recently reported by Otsubo et al. [42].

2.2.4 Bis-Te~rathiufirl~~aletzes, Coimecied fhrough Tiso Linkers

2 17

29 n = 3 , 4 a n d 5

Figure 21. bis-Tetrathiafulvalenophanes.

In the case of n = 4 no electronic interaction is observed in CV whereas for n = 3 and 5 some interaction is taking place. Electrocrystallization of 29 only gave insulating materials with conductivities lower than 1 x Scm-'. Miillen et a / . [43] have reported another example of a TTF-cyclophane. The bisTTF belt molecule 31 could be isolated after electrochemical oxidation of the strained TTF-cage 30 (Fig. 22). The first CV scan of the TTF-cage 30 showed an irreversible oxidation peak at 0.93V, typical of a distorted TTF-group [44]. However upon repeated CV scans two new reversible oxidation waves appeared. Preparative electrochemical oxidation (at 1.O V/30 min.) gave the bis-TTF-belt 31 in 44-53% yields. The mechanism

l)l.OV, 3(hnin., TBAHFP 2)DMSO,6-8h.

m

R

44-53% R

R = OPr, OBu. &HG

30 Figure 22. TTF-cages and belt molecules.

31

218

2.2 Oli~otetrathiuful~~~~lenrs

m

m-.;i

vi-

0

-

0 0 0

x

"09"

.*

00

0

s 00 0

1

000

2.2.4 Bis- Tetratliiafirlvirleries,Conrircterl tlirwgli Tire Lir1ker.r

m

a

m r-

v

2

u

n

--

.b^

9

. I

9 e 0

+

cI

d

0

J

n

cn

H* 1

m u )

219

Table 5. Characterization by cyclic voltammetric measurement of tetrathiafulvalenes with three or more TTF-units.

h, h,

0

R

R = -CH?STTF

Method (yield) (see section 2.2.7.1)

CV(a: vs SCE, b: vs Ag/AgCI)

Ref.

BI (40%)

0.28; 0.52, DMF,a 0.47; 0.86, MeCN,a 0.35; 0.47; 0.54; 0.86' DCM.a

[231

0.47; 0.81, DCM,b

~461

B3 (17%)

[601 [601

lu

h,

2

2-. 1

e-. ,-. ,e. 5g 3

2 B3 (72%) B2 (72%)

0.45; 0.78, DCM,b 0.29; 0.78, PhCN,a

~461 [701

0.54; 0.81, PhCN,a

~701

A3 (23%)

0.40; 0.83*, DCM,a

[551

B4 (83%)

0.42; 0.84, MeCN,b

B4 (75%)

0.45; 0.86, MeCN,b

B2

"q" X

X = R = COOCH2TTF

+ BI ( 4 9 ~ 5 3 % )

R

22 1

2.2.4 Bis-Tetrnthiafulvalenes.Connected through Two Linkers

rUY

Y

mi

a

Q!? 0 0

A-6; \4?

0 0

hl

s m

-

v

Lo

m

h

c v

8

83 /I II

II

80 II X

N

I

d d

j)

X

X

222

2.2 OIigotetruthir!fuh'aIenrs

for this interesting dimerization is still unexplored, but formation of the TTF-belt is probably a result of release of strain as the TTF-groups in the TTF-belt are less distorted (X-ray crystallography).

2.2.5 Tetrathiafulvalenes with Three or More TTF-Units, TTF-Dendrimers etc. The syntheses and investigation of dendrimeric macromolecules is currently an active research topic at the interface of supramolecular and polymer chemistry [45]. Dendrimers are sometimes called starburst polymers or cascade molecules and because of their hyperbranched structure, with a branching point at each monomer unit, they have a high level of three dimensional order. Using specific linkers it is possible to control molecular weight, interior cavity size, topology, surface functionality etc.. An important aspect of the properties of dendrimers is due to the multiplication of functional components and, especially in the case of TTFdendrimers, their sequential and reversible oxidation to radical cations at relatively low potentials. It can be anticipated that such functional cascade molecules will be useful for a wide area of industrial and practical applications. Recently a growing number of reports on tris-TTFs and higher oligomers have appeared, along with the first examples of TTF-dendrimers (Table 5). In Fig. 23, the tris-TTFs with C3-symmetry 32 [46] the TTF-groups are electronically independent and give rise to two redox waves (E1,2 = 0.47, E;,2 = 0.81 V) equivalent to the oxidation potential for the parent monomeric TTF. Other examples of tris-TTFs can be found in Table 5. The first example of a genuine TTF-dendrimer 33 was reported by Bryce et al. [47]. Dendrimers of these types containing up to 12 TTF-units were prepared in a convergent way. They showed the characteristic redox behavior of an isolated TTF unit with EjI2 2 0.45, E;,2 0.85 V in which each redox couple involves a simultaneous multi-electron transfer, i.e. with no significant interaction between the charges on the TTF-units in the dendrimer. Doping with iodine resulted in a broad absorption at, , ,A = 590 nm, consistent with formation of the TTF cation radical. Dendrimers of type 33 were relative unstable and slowly decomposed at room temperature. Completely stable pentakis-TTFs were prepared in a convergent way using the cyanoethyl protection/deprotection methodology developed by Becher et ul. [48-501. The CV of pentamer 34 [50](Fig. 23) revealed four reversible well resolved redox waves, consistent with a multidonor system without intramolecular electron transfer. A related TTF-pentamer with tetrakis(hydroxyethy1thio)TTF as the central unit, prepared by an alternative protection/deprotection route has also been reported [51], and in contrast to the previous example 34 this dendrimer showed three oxidation waves. All TTF-dendrimers described here are quite soluble in nonpolar solvents such as methylenchloride etc., confirming the expectation that oligo-TTFs having the right linkers are soluble and therefore processable for example for incorporation in polymeric materials.

2.2.6 Polvmers

223

Am

s-m

STlF

32

Figure 23. TTF-dendrimers, examples of tris-, pentakis- and higher oligomeric TTFs.

2.2.6 Polymers This chapter does not include TTF-polymers, but a compilation of these compounds can be found in Fanghanels two reviews [7]. However a soluble and processable TTF-polymer combined from two electroactive molecular units, a molecular conductor and a conjugated polymer, was reported [52].

224

2.2 Oligotetrathiafulvalenes

35

Figure 24. Thiophene TTF-monomer.

Electropolymerization (oxidation) of the thiophene monomer 35 (Fig. 24) in acetonitrile resulted in formation of a processable polythiophene with pendant TTF-groups. The polymer showed two successive well resolved redox waves with Ef1, = 0.31, E f p = 0.14 V.

2.2.7 Synthesis Tetrathiafulvalenes can and have been prepared by a large variety of methods, and a number of comprehensive reviews covers preparations of TTFs [6]. The following section gives typical examples for the most important synthetic methods which have been used for preparation of oligo-tetrathiafulvalenes. The preparative methods compiled in Tables 1-5 are given by the code explained below. Oligomeric tetrathiafulvalenes are usually prepared by one of the two fundamental methods (Fig. 25): A: The TTF-unit(s) are formed during the oligomerization step. B: Linking of preformed tetrathiafulvalenes.

2.2.7.1 One or All TTF-Units are Formed in the Oligomerization Step Coupling of a bis- or tris-1,3-dithiole-derivativewith an excess of a mono-1,3dithiole-derivative. A, COupKflg

8, Linking, akylaUon eb.

Z=O,S,Se

Figure 25. Retrosynthetic scheme for preparation of substituted TTFs, oligomers etc.

2 2 . 7 Synthesis

225

9:2196

Figure 26. Preparation of a bis-TTF via deprotonation of a 1,3-dithiolylium salt.

A1 Deprotonation of 1,3-dithiolylium salts This method was used in the synthesis of the first bis-TTF system 9 [2], (Fig. 26), as well as for the first mono and bis-TTF-macrocycles [40, 531. Yields are modest as dimerization/cyclization is accompanied by polymerization. A2 Phosphite mediated coupling of bis-1,3-dithiol-2-chalcogenones

Trialkylphosphite-mediated coupling of bis- 1,3-dithiol-2-chalcogenones is preferred for syntheses involving intramolecular cyclization, even when the resulting macrocyclic TTF-system results in formation of a bent TTF-unit [ a ] . Thus the starting bis-l,3-dithiole-2-thione(Fig. 27) did not give a bis-TTF macrocycle (such as dimer 31, Fig. 22), but instead TTF-cage 30. The TTF-cage 30 in Fig. 27 was also prepared via method A1 [54]. In general a bis-1,3-dithiole derivative will undergo intramolecular coupling to the corresponding mono-TTF macrocycle rather than intermolecular coupling to a bis-TTF macrocycle. A number of other examples have confirmed this conclusion (see ref. [44]). A3 Coupling of 1,3-dithiolylium salts using a Wittig or a Wittig-Horner reaction This method has been used in several variations, for example [ 161 (Wittig-Horner), [17] (Fig. 28), [55] (Wittig). The yields are in general satisfactory as only the asymmetric product is formed in substantial amounts. A4 Coupling of trithioorthooxalates with 1,3-dithiol-2-thiones The coupling depicted in Fig. 29 gives 30% yield in contrast to 2.5% in the corresponding phosphite-mediated cross coupling [56], see also Mullen et a/. for an overview of these types of compounds [30].

30:34-59 %

Figure 27. Trialkylphosphite mediated coupling of a bis- 1.3-dithiol-2-thione.

226

2.2 Oligotetruthiafulvulene.~

Me

Me

Yield: 61 -75 %

Figure 28. Synthesis of a bis-TTF via coupling of a 1,3-dithiolylium salt, using a Wittig-reagent.

2.2.7.2 Linking/cyclizqtion of Preformed Tetrathiafulvalenes In this method the TTF-units are linked/cyclized via reaction of a suitable functionality on the TTF. Until now the majority of functionalized TTFs have been prepared from the parent TTF by lithiation using Green’s original method [57], which has been developed to give methods Bl-B4. In general, method 1B is best suited for the preparation of monofunctionalized TTFs, as dilithio-TTFs and trilithio-TTFs undergo facile disproportionation to T T F and tetralithio-TTF [58]. However, it is possible to obtain dilithiated TTF by starting from a substituted T T F [59].

QR

heat

OR

Figure 29. Coupling of thioorthooxalates with 1.3-dithiol-2-thione.

2.2.7 Syntliesis

227

LDA

Figure 30. Preparation of bis- and tris-tetrathiafulvalenyl phosphines.

B l Lithiation of TTF and direct reaction with a linker This method (Fig. 30) has been used to link TTFs with a single heteroatom and in general the yields are fair [22, 23, 601.

B2 Pd-Mediated coupling of trialkylstannyltetrathiafulvalenes (Stille-coupling) In contrast to lithio-TTF, trialkylstannyl-TTF can be used at room temperature. The Stille coupling (Fig. 31) seems to give good yields. For example Wudl’s bisTTF 9 can be obtained in 61% yield by this method [14] in contrast to 21% using method A l . B3 Functionalization of a TTF-monothiolate TTF-monothiolate is obtained in excellent yield from TTF by lithiation and subsequent reaction with sulfur (Fig. 32). The stable benzoate can be isolated and used later, or the thiolate can be directly transformed in a one pot reaction. For example, reaction with 2-bromo ethanol gives a 2-hydroxyethylthio-TTF [61], and this monofunctionalized TTF-derivative can then be oligomerized via esterification [46].

B4 Reactions of TTF-carbonyl derivatives TTF-carbonyl derivatives and hydroxymethyl-TTF are also obtained via lithio-TTF in good yields; see [62] for a review of these reactions. For the use of chlorocarbonyl TTF, see [63]. A TTF-carbonyl group shows normal reactivity, for example a Wittig reaction will take place readily (Fig. 33).

B5 Oligo TTFs via protection/deprotection of functionalized TTF-derivatives An effective methodology for oligomerization of preformed TTFs has been reported. This reaction sequence is based on three reactions; ( 1) the easy protection/deprotection nBuLi. THF, - 78 OC

Br

A3SnCI 7585%

52 %

Figure 31. ‘Stille-coupling’ of trialkylstannyl-TTFs.

Pd(PPh3)4 toluene, rsflux

228

2.2 OligotetrathiufulvuIenes

\r

PhCOC 78 %

3

"T-- -oy

ms-w+

O-Sm

NE*3

Cocl O

e

m

72 96

Figure 32. Functionalization of TTF-monothiolate via the lithiation route.

of a cyanoethylthio protected TTF, (2) the selective mono deprotection of cyanoethylthio -TTF, and (3) quantitative alkylation of a TTF-thiolate [48-501. The choice of reagents in Fig. 34 is crucial for the success of these reactions; usually the thiolate building blocks are used as the cesium salts, because such salts are much more stable in air than the corresponding sodium or lithium thiolates. TTF-oligomers can be prepared in excellent yields by selective deprotection of cyanoethylthio TTFs (Fig. 35). The example in Fig. 35 illustrates use of this method in a convergent high yield synthesis of an oligo-TTF system [50]. 1) LDA.

2 TrFCHO

+

-78 OC

pE%@pphs Me

-

LiOEtEtOH 74 %

m $ m Me

Figure 33. Preparation and reactions of TTF-carbonyl derivatives of the parent TTF.

2.2.7 Synthesis

229

Figure 34. TTF-building blocks derived via the cyanoethylthio protection/deprotection method

S-CN

WCN 1) C s 0 H . W 2) Mel, DMF

94%

1

n NC

W

Mesxs~sxsMe

MeS

S-CN

WCN

1

1) KOBU-1 2) W3??)3a 3)NaVacetone

WCN

MeS

Figure 35. The cyanoethylthio protection/deprotection methodology for preparation of TTFoligomers [50].

2.2.8 Conclusion In a bis-TTF with linkers having a molecular architecture allowing close intramolecular contact between the fulvene 7r-bond of the donors, strong electronic interaction results in lowering of the redox potentials. It is evident that small changes in the linker or in the substituents as well as in the spatial arrangements of the 7r-donors will result in major differences in the redox potentials. The overall conclusion for oligo-TTF systems is that, if electronic interaction takes place, the interaction is predominantly mediated through space and it is mainly the geometry of the linker combined with the nature of the substituents which will control the electronic interaction between the two TTF-units. Therefore a rigid framework can effectively prevent any electronic interaction betweenb the TTF’s. The electronic properties of TTF radical cation salts are critically dependent on the stabilization of a sulfur-sulfur network in the solid state. By careful molecular design it may be possible to increase the dimensionality by covalent linking of the electroactive groups. This was recently demonstrated by the preparation of a superconducting radical salt of the vinylog bis-TTF 23 [35] (Fig. 19). The synthetic methods now available enable almost a straightforward synthesis to various oligo-TTF systems. A particular challenge is the pre-alignment of individual TTF-units by a ‘mol&ular backbone’ designed to introduce a specific structure such as segregated stacking. Some suggestions are schematically depicted in Fig. 36. Predesigned spatial arrangements of oligomeric TTFs may also be realized for example via complex formation with metal cations using TTF-groups covalently linked to suitable ligands. The construction of helical oligo-TTF systems can also be anticipated as well as oligomeric systems formed by H-bonding interactions or by T-7r stacking. Apart from the obvious use of oligo-TTFs in new electronic and conducting materials there are other applications such as for example the use of oligo-TTFs

side-on

end-on

Figure 36. Some binding motifs in oligo-TTFs showing 7-stacking (cis-frunsisomers not shown).

References

23 1

(dendrimers) in multielectron redox catalysis. Thus Begley et al. [64] have demonstrated the use of tetrathiafulvalene as a trigger for sequential radical translocation and functionalization in specific substrates. Note added in proof. The following reviews on macrocyclic and oligomeric TTFs have been published recently: M. B. Nielsen and J. Becher, Liehigs Ann./Reciteil 1997. 2177. T. Otsubo, Y. Aso and K. Takiyama, Adv. Muter. 1996. 8, 203.

References 1. W. R. H. Hurtley and S. Smiles, J . Chem. Soc., 1926. 2263. 2. F. Wudl. G . M. Smith and E. J. Hufnagel, J . Chem. Soc., Chem. Commun.. 1970, 1453. 3. J. Ferraris, D. 0. Cowan, V. Walatka Jr. and J. H. Perlstein. J . Am. Cheni. Soc., 1973. 95,948. 4. K. Bechgaard, C. S. Jacobsen. K. Mortensen. H. J. Pedersen and N. Thorup, Solid State Commzm., 1980, 33. 11 19. 5. A. M. Kini. U. Geiser, H. H. Wang et al.. h o r g . Chem., 1990. 29. 2555. 6. M . Narita and C. U. Pittman Jr.. Srnthesis. 1976, 489; G. Schukat. A. M. Richter and E. Fanghanel, Sitlfur Reports, 1987, 7, 155: G . Schukat and E. Fanghanel. Sulfur Reports. 1993, 14, 245; M. Bryce, Cheni. Soc. Rev., 1991, 20, 355: V. Khodorkovsky and J. Y. Becker. chapter 3 in Organic Conductors (J.-P. Farges Ed.) Dekker NY 1994; T . K. Hansen and J. Becher. Advanced. Mat., 1993, 5, 288. 7 G . Schukat, A. M. Richter and E. Fanghiinel, Sdfirr Reports, 1987. 7. 155: G . Schukat and E. Fanghanel, Sulfur Reports, 1993, 14, 245. 8 M. Bryce, Chern. Soc. Rev., 1991. 20. 355. 9 V. Khodorkovsky and J. Y. Becker, chapter 3 in Organic Conditctors (J.-P. Farges Ed.) Dekker NY 1994. 10 T. K. Hansen and J. Becher. Advanced. Mat.. 1993, 5. 288. 11 See for example A. Gaveotti, Acc. Chem. Res., 1994,27, 309 and J. Maddox. Nature, 1988,335, 201.

12 K. Bechgaard, K. Lertrup. M. J~rgensenand J. Christensen. The Physics and Chemistrj) of Organic Superconductors, Springer Berlin, 1990. 51. 349. 13 M. L. Kaplan, R. C. Haddon and F. Wudl, J . Chem. Soc., Chem. Commun., 1977, 388. 14 M. lyoda, Y. Kuwatani. N. Ueno and M. Oda, J . Chem. Soc., Chem. Commun., 1992, 158. 15. T. Outsubo. Y. Kochi. A. Bitoh and F. Andura, Chem. Letr., 1994. 2047. 16. M. Jmgensen, K. A. Lertrup and K. Bechgaard, J . Org. Chem., 1991, 56, 5684. 17. I. V. Sudmale, G. V. Tormos, V. Y. Khodorkovsky, A. S. Edzina, 0. J. Neilands and M. P. Cava, J . Org. Chem., 1993. 58. 1355. 18. U. Kux and M. Iyoda, Chem. Lett., 1994, 2327. 19. M. R. Bryce, G. Cooke, A. S. Dhindsa. D. J. Ando and M. B. Hursthouse. Tetrahedron Lett., 1992, 33.-1783. 20. J. Y. Becker, J. Bernstein, S. Bittner. J. A. R. P. Sarma and L. Shahal. Tetrahedron Lett., 1988, 29, 6177. 21. J. Y. Becker, J . Bernstein. M. Dayan, A. Ellern and L. Shahal. Ad)'. Mat.. 1994, 6 , 758. 22. M. Fourmigue and Y. S. Huang. Orgariometallics, 1993. 12, 797. 23. M. Fourmigue and P. Batail. J . Chem. Soc.. Chem. Commun., 1991, 1370. 24. A. Izuoka, R. Kumai and T. Sugawara. Chem. Letr.. 1992, 285. 25. E. Aqad, J. Y. Becker. J. Bernstein, A. Ellern. V. Khodorkovsky and L. Shapiro, J . C'hem. So(,., Cheni . Comniun ., 1994. 2 775. 26. E. Nishikawa, H. Tatemitsu and Y. Sakata. Chem. Lett., 1986, 2131.

232 27. 28. 29. 30. 31. 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. 51. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71.

2.2 Oligotetrathiujulvalenes

T. Taichikawa, A. Izuoka and T. Sugawara, Solid State Commun., 1992, 82, 19. T. Tachikawa, A. Izuoka and T. Sugawara, J . Chem. Soc., Chem. Commun., 1993, 1227. A. Izuoka, T. Tachikawa, T. Sugawara, Y. Saito and H. Shinohara, Chem. Lett., 1992, 1049. M. Adam and K. Miillen, Advanced Mat., 1994,6, 439. E. Fanghanel, K. Miillen, Yong-Jia Shen and R. Wegner, SjJntheticMetals, 1994, 66, 75. Y. Misaki, H. Nishikawa, K. Kawakami, S. Koyanagi, T. Yamabe and M. Shiro, Chem. Lett., 1992, 2321. Y. Misaki, T. Matsui, K . Kawakami, H. Nishikawa, T. Yamabe and M. Shiro, Chem. Lett., 1993, 1337. Y. Misaki, H. Nishikawa, T. Yamabe et al., Chem. Lett., 1993, 729. Y. Misaki, N. Higuchi, H . Fujiwara et al., Angew. Chem. Int. Ed. Engl., 1995, 34, 1222. Z . Yoshida, T. Hawase, H. Awaji, I. Sugimoto and S. Yoneda, Tetrah. Left., 1983, 24, 3469. T. K. Hansen, M. V. Lakshmikantham, M.-P. Cava and J. Becher, J . Org. Chem., 1991,2720. M. Salle, A. Belyasmine, A. Gorgues, M. Jubeault and N. Soyer, Tetrahedron Lett., 1991, 32, 2897. F. Vogtle, ‘Cvclophane Chemistry’, Wiley, 1993. J. Ippen, C. Tao-pen, B. Starker, D. Schweizer and H. A. Staab, Angew. Chem., Int. Ed. Engl., 1980, 19, 67. F. Bertho-Thoraval, A. Robert, A. Souizi, K. Boubekeur and P. Batail, J . Chem. Soc., Chem. Commun., 1991, 843. K. Takimiya, Y. Shibata, K. Imamura et a/., Tetrahedron Lett., 1995, 36, 5045. M. Adam, V. Enkelmann, H-J. RBder, J. Rohrich and K. Miillen, Angew. Chem. Int. Ed. Engl., 1992, 31, 309. T. K. Hansen, T. Jsrgensen, F. Jensen et al., J . Org. Chem., 1993,58, 1359. J. Issberner, R. Moors and F. Vogtle, Angew. Chem. Int. Ed. Engl., 1994, 33, 2413. M. R. Bryce, G. J. Marshallsay and A. J . Moore, J . Org. Chem., 1992, 57, 4859. M. R. Bryce, W. Devonport and A. J. Moore, Angew. Chem. Int. Ed. Engl., 1994. 33, 1761. N. Svenstrup, K. M. Rasmussen, T. K. Hansen and J. Becher, Synthesis, 1994, 809. J. Becher, J. Lau, P. Leriche, P. Merrk and N. Svenstrup, J . Chem. Soc., Chem. Commun., 1994, 2715. J. Lau, 0. Simonsen and J. Becher, Svnthesis, 1995, 521. G. J. Marshallsay, T. K. Hansen, A. J. Moore, M. R. Bryce and J. Becher, Synthesis, 1994,926. C. Tobie-Gautier, A. Gorgues, M. Jubault and J. Roncali, Macromolecules, 1993, 26, 4094. H. A. Staab, J. Ippen, C. Tau-Pen, C. Krieger and B. Starker, Angew. Chem., Int. Ed. Engl., 1980, 19, 66. J. Rohrich, P. Wolf, V. Enkelmann and K. Miillen, Angew. Chem., 1988, 100, 1429. M. Fourmigue. 1. Johannsen. K. Boubekeur, C. Nelson and P.Batail, J . Am. Chem. Soc., 1993, 115, 3752. M. Adam, A. Bohnen, V. Enkelmann and K. Miillen, Adv. Muter., 1991, 3, 600. D. C. Green, J . Cheni. Soc., Chem. Commun., 1977, 161. D. C. Green, J . Org. Chem., 1979, 58, 1476. C. Wang, A. Ellern, J. Y. Becker and J. Bernstein, Tetrahedron Lett., 1994, 35, 8489. M. Fourmigue and P. Batail, Bull. SOC.Chem. Fr., 1992, 129, 29. A. J. More, M. R. Bryce, J . Chem. Soc., Chem. Commun., 1991, 1638. J. Carin, J. Orduna, S. Uriel et al., Synthesis, 1994, 489. C. A. Panetta, J. Baghdadchi and R. M. Metzger, Mol. Cryst. Liq. Cryst., 1984, 107, 103. M. J. Begley, J. A. Murphy and S. J. Roome, Tetrahedron Letters, 1994, 35, 8679. K. Lerstrup, M. Jsrgensen, I. Johannsen and K. Bechgaard, The Physics and Chemistry of Organic Superconductors (Eds. G. Saito, S. Kagoshima), Springer, Berlin, 1990, 383. A. Izouka, R. Kumai, T. Tachikawa and T. Sugawara, Mol. Cryst. Liq. Cryst., 1992,218, 213. R. P. Parg, J. D. Kilburn, M. C. Petty, C. Pearson and T. G. Ryan, Synthesis, 1994, 613. H. Tatemitsu, E. Nishikawa, Y. Sakata and S. Misumi, Synth. Met., 1987, 19, 565. J. Kreicberga, A. Edzina, R. Kampare and 0. Neilands, Zh. Org. Khim., 1989, 25, 1456. M. Iyoda, M. Fukuda, M. Yoshida and S. Sasaki, Chem. Lett., 1994, 2369. M. Mitzutani, K. Tanaka, K. lkeda and K. Kawabata, Synth. Met., 1992, 46, 201.

Rejerenres

233

72. K. Ikeda, K. Kawabata. K. Tanaka and M. Mitzutani, Synth. Met.. 1993, 55-57, 2007. 73. X. Yang, P. Wu and D. Zhu, Herherig Htrusue. 1993, I , 141. 74. A. J. Moore, P. J. Skabara, M. R. Bryce, A. S. Batsanov, J. A. K . Howard and S. T. A. K. Daley, J . Chetn. Soc.. Chem. Conimun., 1993, 417. 75. G. J. Marshallsay and M. R. Bryce, J . Org. Cheni., 1994, 59, 6847. 76. V. Y. Khodorkovsky, J. Y. Becker and J. Bernstein, Sjmth. Met., 1993, 2 - 5 7 , 1931. 77. M. Salle, M. Jubault. A. Gorgues et al.. Cheni. Muter.. 1993, 5 , 1196. 78. J. Y. Becker, J. Bernstein, M. Dayan and L. Shahal, J . Chem. Soc., Cheni. Comniuti., 1992, 1048. 79. J. D. Martin, E. Canadell, J. Y. Becker and J. Bernstein, Cheni. Muter., 1993, 5, 1199. 80. M. Fourmigue and P. Batail, Bull. Soc. Cheni. Fr., 1992, 129, 29. 81. T. Otsubo and F. Ogura, Bull. Cheni. Soc. Jpn.. 1985, 58. 1343. 82. T. Tachikawa, A. Izouka and T. Sugawara, Solid State Coiiimun., 1993, 88, 207. 83. C. Wang, A. Ellern, J. Y. Becker and J. Bernstein, Tetrahedron Lett., 1994, 35. 8489. 84. C. Wang, A. Ellern, V. Khodorkovsky, J. Y . Becker and J. Bernstein, J . Chenz. Soc., Chem. Cominun.. 1994, 2 1 15. 85. N. M. Rivera and E. M. Engler, J . Climi. Soc., Chctii. Coinmuri., 1979, 184. 86. R. R. Schumaker and E. M. Engler, .I. Am. C h e i ~Soc., 1980, 102, 6651. 87. E. M. Engler and V. V. Patel, J . Chem. Soc., Cheni. Cotnrnim., 1979. 516. 88. T. Mori, H. Inokuchi, Y. Misaki et a/., Chem.Lett., 1993, 733. 89. H. Tatemitsu. E. Nishikawa, Y. Sakata and S. Misumi. Sjvzfh. Met., 1987. 19, 565. 90. P. Wolf, H. Naarmann and K. Miillen, Aiigew. Cheni., 1988, 100, 290. 91. H.-J. Rader. U. Scherer, P. Wolf and K. Miillen. Synth. Met., 1989.31, 15. 92. M. Adam, P. Wolf, H-J. Rader and K. Miillen, J . Chem. Soc., Clieni. Cotnmun.. 1990, 1624. 93. M. Adam, U. Scherer, Y-J. Shen and K. Miillen, Syntli. Met., 1993, 55-57, 2108. 94. U. Scherer, Y-J. Shen. M . Adam. W. Bietsch, J. U. von Schiitz and K. Mullen. Adv. Muter., 1993, 5, 109. 95. R. Wegner. N. Beye, E. Fanghanel, U. Scherer, R. Wirschem and K. Miillen, Syntli. Met., 1993, 53, 353. 96. W. H. Watson, E. E. Eduok. R. P. Kashyap and M. Krawiec. Tetrahedron, 1993. 49, 3035. 97. M. Badri. J. P. Majoral, F. Gonce, A-M. Caminade, M. Salle and A. Gorgues. Tetrahedron Lett., 1990, 31. 6343. 98. T. Jmgensen, J. Becher. J-C. Chambron and J-P. Sauvage, Tetrahedron Lett., 1994, 35, 4339.

This Page Intentionally Left Blank

3 Nitrogen-Containing Oligomers L. Groenendaal, E. W. Meijer and J. A . J. M. Vekemans

3.1 Introduction Pyrrole and aniline are among the most prominent constituents of conducting polymers [ 11. These nitrogen-containing repeating units of polypyrrole 1 and polyaniline 2 are highly susceptible to oxidation and, therefore, the doped conducting polymers are stable. However, this air-sensitivity of the neutral species hampers their synthesis and characterization in the unoxidized form. Hence, it is not a surprise that the number of studies on well-defined oligomers of pyrrole or aniline is very limited and not comparable to the comprehensive studies on the behavior of their hydrocarbon- and thiophene-based analogs (Chapters 1 and 2). Nevertheless. both in the early days of oligomeric synthesis as well as more recently, a considerable number of reports has been published and the insight into the properties of these well-defined nitrogen-containing oligomers is now increasing rapidly. Polypyrrole 1 is available only in its oxidized, intractable form and exhibits conductivities in the range of l-lOOScmp' [l]. The two major preparative routes to polypyrrole are based on chemical or electrochemical oxidative polymerization of pyrrole. Both routes result in a polymer containing many structural defects (Scheme 3) and cannot be applied to the synthesis of well-defined oligomers. However, free-standing films of the polymer with long-term stability are easily obtained by continuous electrochemical synthesis. The methods recently introduced using the metal-catalyzed polymerization of N-protected-2.5-dibromopyrrole derivatives yield structurally perfect precursor polymers. The same methodology is used in a sequence of reactions to prepare well-defined oligopyrroles. From these N-protected oligomers and polymers it is possible to prepare the neutral N-H species under inert conditions and their oxidation behavior can be investigated, giving rise to a detailed structure-property relationship of oligopyrroles. Polyaniline 2 has been investigated in detail because of its potential in device construction and low cost. Aniline black is one of the first polyaromatics known since its first 'synthesis' in 1834 and its oxidation state has been the topic of investigation ever since. Conductivity is only obtained in the protonated, partly oxidized species, a situation generally referred to as 'protonic-acid doping'.

H 1

Scheme 1

2

Scheme 2

236

3 Nitrogemcontaining Oligomers

Scheme 3

Polyaniline is synthesized by the electrochemical polymerization of aniline in acidic media and the oxidation potential is used to modulate the oxidation state, or by the oxidative polymerization of aniline with ammonium persulfate. Processability of polyanilines is obtained by the proper choice of the counterion. Polyaniline doped with all kinds of aliphatic sulfonic acids is soluble in polar solvents and can be film-cast. The green polyaniline film so obtained has been used as a conductor in many prototype devices. In sharp contrast to the extensive studies on the synthesis and properties of the polymer, hardly any work has been expended to the investigation of well-defined oligomeric aniline structures. Both N-unsubstituted and N-substituted oligomers are synthesized by routes much more complicated than the polymerization. For many decades the major emphasis in 7r-conjugated polymers and oligomers has been on homopolymers [l]. More recently, it has been established that systems with an alternating sequence of different repeating units (thiophene, benzene, ethylene and pyrrole) give rise to materials with special properties. It has been shown that alternation of electron-rich and electron-poor units will give oligomers and polymers with narrow band gaps; however, upon doping of these mixed systems the charge is localized. Well-defined oligomers are used to obtain a detailed insight into the behavior of the mixed systems and the synthetic methods used can be applied to the preparation of perfect alternating copolymers as well. Motivation for investigating well-defined oligomers of pyrrole comes both from the area of electronic materials, i.e. polypyrrole [l], and from the interest in biological compounds like heme, vitamin BI2 and extended porphyrins [2-lo]. In almost all cases the coplanarity of the 7r-conjugated system is a prerequisite for an optimal function of the oligopyrrole molecule. This coplanarity is hampered by substitution at either the P-carbon or the nitrogen atom of the pyrrole ring. However, unsubstituted oligopyrroles are very sensitive to oxidation, while electron-withdrawing substituents at pyrrole increase the stability and solubility.

3.2 Oligo(pyrrole-2.4-diyI)s

237

For the past three decades a small number of research groups have prepared well-defined oligomeric pyrrole structures both as models for structurally perfect poly-2,5-pyrrole and for porphyrin synthesis. The most important series are discussed here, namely oligo(N-methy1)pyrroles. oligo(N-t-butoxycarbony1)pyrroles and the oligopyrroles themselves.

3.2 Oligo(pyrrole-2,4-diyl)s 3.2.1 Synthesis Dictated by its important role in naturally occurring compounds, the synthesis of 2,2’-bipyrrole 3 [2, 31 and 2,2/,5’,2’’-terpyrrole 4 [4-101 was investigated in the sixties. The oligomers were prepared by a Vilsmeyer condensation reaction followed by dehydrogenation (Scheme 4). Starting from pyrrole, phosphor oxychloride and 2-pyrrolidinone, 2,2’-( 1’-pyrrolinylj-pyrrole 1 was first prepared. After dehydrogenation with 10% Pd/C, 2,2‘-bypyrrole 3 could be obtained as a white solid. By repeating this reaction sequence starting from 2,2’-bypyrrole 3, 2,2’,5’,2’’-terpyrrole4 could be isolated. The air-sensitivity of oligopyrroles is evident from 3 and 4; these white molecules turn readily into brown materials upon exposure to air. Therefore, this approach is not appropriate for the preparation of larger systems. N-Unsubstituted oligomers that are substituted at the a and/or /3 positions with electron-withdrawing groups exhibit a much higher stability. In porphyrin chemistry this strategy has often been used to prepare substituted bipyrrole units. a-Monoiodo ester-substituted pyrrole derivatives have been coupled in the Ullmann reaction with copper bronze in D M F to afford the corresponding dimers. A significant example is the

H R=H: 1 R=Z-py~t~lyl: 2 Scheme 4

H R=H: 3 R=2-pqrrolyl: 4

238

3 Nitrogen-Contuining Oligomrrs

wc

1

U0,C

Cu-bronze

E4QC

mH,

5

6

I Scheme 5

preparation of an n-linked quaterpyrrole 7 by Sessler et al. (Scheme 5 ) [ 111. Starting from the a-monoiodobipyrrole derivative 5, this compound was first coupled with copper bronze in D M F at 140°C resulting in the hexaester-substituted quaterpyrrole 6, which appeared to be remarkably stable under normal conditions. However, when the stabilizing ester groups were removed in a saponification-decarboxylation step (NaOH in ethylene glycol at 190'C) the product obtained 7 decomposed within a few hours [12]. Hence, in order to prepare longer oligopyrroles without substitution at the carbon-ring atom, it is a prerequisite to protect the oligomer at the nitrogen atom. This principle was demonstrated by Kauffmann et al. in 1981 with the synthesis of a series of oligo(N-methylpyrrole-2,5-diyl)s(Scheme 6) [ 131. N-Methylpyrrole 8 was first lithiated at the a-position using n-butyllithium (n-BuLi) in combination with TMEDA as activator. The lithium derivative 9 was then oxidatively coupled with anhydrous NiC12 to give the dimer 10. Lithiation of this dimer with 9 followed by the NiC12-catalyzed crossed coupling reaction with 9 then gave the trimer 12. Dimerization of lithium derivative 11 with NiC12 resulted in the tetramer 13. Analogous treatment of 12 and 13 with n-BuLilTMEDA followed by reaction with NiC12 gave the hexamer 14 and the octamer 15, respectively. Similarly, even the octamer 15 could be converted into the hexadecamer 16. Although these oligomers represent the first series of oligo(pyrrole-2,5-diyl)s ever prepared, they are not ideal oligomers as models for polypyrrole since the methyl groups cannot be removed. Good models for polypyrroles, therefore, require a labile protecting group at nitrogen, which can easily be removed. For this purpose the thermally labile N-tert-butoxycarbonyl (BOC) group was introduced into pyrrole chemistry [14, 151. However, it was only at the end of the 1980s that the first BOC-protected oligomers of pyrrole were prepared by Martina et af. [16-261. They investigated both the Pd-catalyzed Suzuki coupling [27] and the very successful Stille coupling [28].

3.2 OIigo (p~~rrole-2.4-did) s

239

1) n-BuLi, lMEDA 12

2) ~ i a 2

HK& I Q-13

14

13

1) n-BuLi. RvlEDA 2) Nia2

1) n-Bul& l’MEDA 2) ~ i a ~

*

+

H+-q+: a

15

3

CH3 16

Scheme 6

In the Suzuki coupling reaction to form the N-BOC protected trimer 19, the boronic acid substituted pyrrole derivative 17 and the dibrominated compound 18 are brought into reaction under the influence of tetrakis(tripheny1phosphine)palladium(0) as a catalyst in a two-phase system of toluene and 1 M Na2C03(Scheme 7) [16-191. Only traces of 19 could be detected afterwards whereas the main products were N-BOC-pyrrole, unreacted dibromide 18, and some monosubstituted product. Apparently, proto-deboration occurs faster than the desired coupling reaction. Using N-BOC protected 2-trimethylstannyl pyrrole 20 and 18 in the Stille reaction with the same Pd-catalyst and using almost the same reaction conditions, much better results were obtained (Scheme 8). Repeatedly performing a Pd(PPh3)4-catalyzed (2 mol%) reaction between a dibrominated oligopyrrole (18, 23-25) and two equivalents of the monostannylpyrrole 20 in the two-phase system benzene/aqueous 1 M Na2C03, allowed for the isolation of the oligo(pyrrole-2,5-diyl)s 19 and 21, 22, and 26 in moderate yields. All oligomers were fully characterized and details are given in section 3.2.2. A detailed insight into the structural parameters of these oligomers was obtained by

240

3 Nitrogen-Contuining Oligonwrs

18

17

main product

10%

Scheme 7

an X-ray analysis of several substituted trimeric and pentameric structures [16, 17, 211. The N-BOC-protected oligomers were easily transformed into the parent unsubstituted derivatives by heating to approximately 190°C. In order to avoid uncontrolled oxidation this was performed under inert conditions leading to ultra-pure oligomers, which were fully characterized. The Stille coupling reaction was also applied to polymerize functionalized oligopyrroles (Scheme 9) [ 16, 201. Starting from the AB-monomers 27 and 28 oligomers up to n = 18 and n = 48 were prepared, respectively, all being a-CH3-a’-Sn(CH3)3-terminated as derived from NMR spectra of isolated fractions. A similar AA-BB polymerization using the functionalized trimers 23 and 29 afforded oligomers up to n = 18. However, these contained a variety of different endgroups and were therefore not further investigated. Another method of preparing N-BOC-protected oligo(pyrrole-2,5-diyl)s was recently presented [29, 301. A mixture of oligomers was prepared by the Ullmann

Boc

Boc

20

18

Boc 19

.- -,-- H+o+H

t

BOC -3: 19 m=5: 21 m=7: 22 Scheme 8

m

T01./Na2C03(1M)

I

Boc

ioc m=3:23 m=5: 24 m=7: 25

-3:

21 22 m=7: 26 -5:

mt2

3.2 O l i g o ( p ~ r r o l e - 2 , 4 - ~ i ~ , l ) s 24 1

Scheme 9

coupling after which these oligomers were separated by preparative HPLC. Although this sequence of steps is not new in organic chemistry, and more or less simultaneously an analogous approach to oligo(m-pheny1ene)s [311 was described, it was new in oligopyrrole chemistry. The Ullmann polymerization was performed with three different dibrominated oligopyrroles (18,30,23)which were reacted with Cu bronze in dry DMF at 100°C (Scheme 10). The resulting product mixtures were investigated by analytical HPLC. The HPLC analysis of the polymerization of 18 is shown in Fig. 1. Each oligomer prepared in this polymerization is visible in the HPLC analysis as a separate peak, showing that 25 different oligomers are formed. In the case of the dimeric and the trimeric building block (30 and 23, respectively) only eight different oligomers were traced and contained up to 16 and 25 pyrrole units, respectively. In order to study the molecular structure of the oligomers obtained by polymerizing 18, preparative HPLC separations were performed. The first twenty oligomers (n = 1-20) were isolated on a 2-20mg scale. All oligomers appeared to be hydrogen terminated as could be deduced from 'H-NMR spectra. These terminal hydrogen atoms are probably abstracted from the solvent. In the polymerization of 30 and 23 relatively many monobrominated oligomers are formed, from which it was concluded that the Ullmann coupling is most favorable for monomer 18 due to the high reactivity of the dibrominated monopyrrole. n/m Br-fo)-Br

Cu-bronze

,

DMF,IOO'C

Boc m=l: 18 m=2: 30 m=3: 23 Scheme 10

BOC

242

3 Nitrogen-Containing Oligomers 1

10

9 8

7

l6 6

5

4

4

3 2 1 0 0

10

30

20

40

50

I 60

time (min.)

Figure 1. HPLC analysis of the Ullmann polymerization of 18.

The last series of oligopyrroles discussed here consists of a number of p, P-linked oligomers. Although these oligomers are not of interest as models for conducting polypyrrole, the elegance of their synthesis merits attention (Scheme 11) [32]. Treatment of fumaronitrile with the anion of (p-toluenesulfony1)methyl isocyanide (TOSMIC) in D M F led to the formation of 3,4-dicyanopyrrole 31. NTosylation of this compound using NaH/TsCl followed by reduction of the nitrile with DIBAL afforded the dialdehyde 33. A Wittig-Horner reaction between this compound and diethyl (cyanomethy1)phosphonate using NaH as base finally resulted in 34. Repetition of the same sequence of transformations, TOSMICtreatment, N-tosylation and Wittig-Horner reaction, also gave access to the dicyano substituted trimer 35, pentamer 36 and heptamer 37, respectively. Although the number of investigations on oligopyrroles is limited due to their extreme air-sensitivity, the most important series have now been made and fully characterized, and the details are given in the next section.

3.2.2 Structural Characterization The structural characterization of three series of oligo(pyrrole-2,5-diyl)s, N-unsubstituted, N-methyl substituted and N-BOC substituted, will be discussed in this paragraph (Table 1). The UV data regarding N-unsubstituted oligopyrroles are consistent with a high degree of coplanarity; hence a substantial bathochromic increment is observed upon ranging from 276 nm for n = 2 the incorporation of an additional pyrrole unit ,,A(,

- NaQ TOSMIC

DlBAL

N a T a

I

H 31

__c

I

I

TS

TS

32

33

v a Ts

T

a

2) 1)NaHTOsMIC NaH, T d l

-

Ts

NC

I TS

0H~~@0klP(0m2$

1) DIBAL 2) NaH (whP(OW2OJ 3) Nail TOSMIC 4) NaK TSQ c

I TS

34

35

Ts

Ts 1) DIBAL

& x N

I TS

I \

I \

N I

N

Ts 36

I

Ts

2) NaH. G30~PCOKH20J 3) NaH. TOSMIC

CN

4)

NaH, TsCI

-

NC

Ts

TS

Ts

f-qp$( CN

I

Ts

I

TS

I

TS

37

Scheme 11

to 381 nm for IZ = 7). By contrast the N-methyl and N-BOC oligo(pyrrole-2,5-diyl)s show only a small bathochromic shift upon elongation of the chain (N-Me: 250 nm for 17 = 2,271 nm for 17 = 3 and 287 nm for I I = 8; N-BOC: 270 nm for n = 2,283 nm for n = 3 and 299nm for n = 7), reflecting the lack of coplanarity. The 'H-NMR data of protected oligopyrroles are not easily related due to lack of solubility when ~7 > 2. The N-H absorption is shifted downfield upon introduction of 0pyrrolyl moieties, suggesting that anisotropic deshielding of the pyrrole ring overrules its electron donating properties. From 'H-NMR data of N-Me and N-BOC substituted oligopyrroles it is apparent that the methyl proton absorptions are a useful probe. The signals corresponding to the inner methyl protons undergo an upfield shift of approximately 0.35 ppm, in agreement with anisotropic shielding by two out-of-plane orientated pyrrole neighbors. The 13C-NMR data of the pyrrole ring carbon atoms in N-BOC oligopyrroles are of diagnostic value. Terminal unsubstituted Qcarbons are found at 122ppm while those linking pyrrole units show a relative deshielding of 5ppm (126-128ppm). The /?-carbons at the outer rings feature signals at IlOppm (C-4) and >115ppm (C-3), while the inter /?-carbons are found below 114ppm, indicative of a small shielding effect exerted by a 111pyrrolyl ring (<3 ppm) combined with a small deshielding by an o-pyrrolyl ring (<3 ppm). Finally, besides the different routes to synthesize oligopyrrole structures a number of theoretical studies has been performed on pyrrole oligomers. Worth mentioning in this field is the work of Bredas [33], Tomas [34] and Kopranek [35] whose

Table 1 Structural data of (a) N-H, (b) N-Me and (c) N-BOC substituted oligo(pyrrole-2,5-diyl)s a

I H

IR (cm-I)”

Mol. formula

UV-Vis (CH,CN, nm)

m= 1

C4HSN

208

m=2

C8HRN2

276

m=3

CI2HllN3

317

1521, 11 13,764,720

m=5

C20H17N5

367

1519, 1064, 1035

m=7

C28H23N7

38 1

1.507, 1037, 762

a

IH-NMR (ppm)

(CDCI,) 6.22 (dd, J = 4.2 and 2.2 Hz, (CDC13) 107.8, 1 17.7 2H, H-3,4), 6.65 (dd, J = 4.3 and 2.3Hz, 2H, H-2,5), 7.82 (s, broad, lH, NH) (CDC13)6.21 ( t d , J = 2.5and 1..5Hz,2H, (CDC13) 103.5, 109.4, 117.6, 125.9 H-3,3’),6.24(dd,J=6.1 and2.7Hz,2H, H-4,4’), 6.77, (td, J = 2.6 and 1.5 Hz, 2H, H-5,5’). 8.23 (s, broad, lH, NH) (DMSO-d6) 6.03 (m, 2H), 6.20 (d, (DMSO-d6) 12.5.8, 125.5, 116.9, 108.1, 103.6, 102.7 J = 2.5Hz, 2H), 6.29 (m, 2H), 6.69 (m, 2H), 10.67(broad, lH), 10.83 (broad, 2H) (DMSO-d6) 6.03 (m, broad, 2H), 6.23 (acetone-d6) 126.3, 117.5. 109.0. (b.,2H),6.30(b.,4H),6.70(b.,2H),10.70104.8, 103.5 (b., 3H), 10.86 (b, 2H) (acetone-d6) 6.08 (broad, 2H), 6.29 (b., IOH), 6.72 (b., 2H), 10.17 (b., exchange with solvent)

For more detailed information, including Raman spectroscopy, see Zerbi et al. [23, 25, 261

N

P P

zd

? 3:

1

c:

3:

-'aw r- w

rim'

macnchmo m m m m m -

N P

Table 1 (b) (Cotit.)

Mol. formula

c% M.S (m,e)

El analysis C vs. H vs. N

uv-VlS

Mp ( C)

‘H-NMR (ppm)

178-182

3.35 (m, 18H, I/-, 1”””-CH3) 3.53 (s, 6H, 1-, l”””’-CH~) 6.20-6.30 (m, 16H, H-d) 6.60-6.80 (m, 2H, H - a )

(CHCI;. nm)

~

/I =

n

a

=

8

16

C40H42NX

CXOH22N16

+

636 (13%, M+ 2) 635 (49%, M’ + 1 ) 634 (loo%, Mi) 556, 555, 317.5, 317, 277.43 1268 (23%, M+ + 2) 1267 (92%, M+ 1) 1266 (loo%, M+) 1188, 1187, 1109, 1108, 1030, 1029, 951, 872, 871, 793, 792, 714, 713. 635

According to Gronowitz et al. [94].

+

~3 .-.

u/

Calcd.: 75.69, 6.67. 17.64 Found: 75.71, 6.76, 17.58

287 (4.53)

n

9

K

_‘

Calcd.: 75.83, 6.48, 17.69 Found: 75.83. 6.65, 17.31

240-246

3.30- 3.41 (m, 42H)

5

3.54 (s, 6H) 6.17-6.33 (m, 32H, H-3) 6.60-6.80 (m, 2H, H-a)

B

9.

3.

2

2

z3

BOC Mol. formula

M.S. (m/e)

El. analysis

C vs. H vs. N p= I

C,H,,N02

UV-Vis (CH,CN. nm)

I67 (21%. M+ ) 94. 67, 57

Mp ( C )

'H-NMR (CDCI3. ppm)

80/18 mmHg

148.8. 119.9, 111.7. 83.4. 1.58 (s, 9H, H-BOC). 6.20 27.9 J = 2.2Hz. 2H. H-3,4), 7.23 (t. J = 2.2 Hz, 2H, H-2.5) 1.39 (s, 18H, H-BOC and BOC'). 149.1. 126.0, 121.9. 115.3, 6.20 (d. J = 2.6 Hz. 4H. 110.1, 83.0, 27.8 H-3,4,3',4'). 7.40 (t. J = 2.6Hz. 2H. H-5.5') 149.1, 149.0, 127.6. 126.7. 1.21 (s, 9H. H-BOC'). 1.37 (s, 18H. H-BOC and BOC"). 6.14 121.9. 115.3. 113.8. 110.3, (dd. J = 3.3 and I .9 Hz. 2H. 83.3. 82.5. 27.8. 27.6 H-3.3"). 6.15 (s. 2H. H-3',4'), 6.20 (t. J = 3.3 Hz. 2H. H-4.4"). 7.40 (dd. J = 3.3 and 1.9 Hz, 2H. H-5.5") 1.26(s, 18H.H-BOC'andBOC"). 149.1. 148.9. 128.2. 127.5. 1.42 (s. 18H. H-BOC and BOC"'). 126.8. 121.9. 115.3, 113.9. 6.13-6.22 (m. 8H. H-A), 7.40 113.8. 110.2. 83.2. 82.7. (dd, J = 3.3 Hz. 2H. H-cr) 27.8. 27.7 149.0. 148.9, 148.8. 128.3. 1.22 (s. 18H), 1.24 (s, 9H). 1.39 (s. 18H). 6.14-6.11 (m. IOH. 128.2. 127.5. 126.8. 121.8. H-,]). 7.40 (dd. J = 3.1 and 115.3. 113.9. 113.8, 110.2. 2.0Hz. 2H, H-n) 83.1. 82.8, 82.5. 27.8. 27.7, 27.5 (t,

p=2

C I H H x Nz0

p=3

C2,HINI0,

497 (7%. M') 285 (74%) 196 (51%) 57 (100%)

Calcd.: 65.17, 7.09. 8.44 Found: 65.22, 7.15. 8.65

283

159

p=5

C,,H,,N,O,,,

817 (100%. M')

Calcd.: 65.28. 6.94, 8.46 Found: 65.50. 6.70. 8.08

294

starts at 95

270

4

Table 1 (c) (Con? ) Mol formula

h,

M S (m e)

El a n a l y m C w Hvs N

uv-VlS

M~ (

c)

(CH,CN, nm)

C63H79N7014 1157.3 (M') (FAW

Calcd.: 65.32, 6.87. 8.46 Found: 65.25. 7.05, 8.34

299

starts at 115

p=8

p=9

CXIH101N9018 1487.1 (Mt) ( F W

Calcd.: 65.32, 6.84, 8.47 Found: 65.29, 6.97, 8.46

301

30 1

p = 11-20

C-NM R (CDCI3. P P ~ ) I'

149.1, 148.9. 148.8, 128.4, 128.3, 128.1, 127.4, 126.9, 121.8, 115.3, 113.9, 113.8, 110.2, 83.2, 82.6. 82.5, 28.7. 27.7 (2 peaks) 1.25 (d (s + s), 45H), 1.40 (s, 149.1, 148.9 (2 peaks), 148.8, 128.4, 128.2, 128.0, 18H). 6.15- 6.23 (m, 14H, H-0). 7.40 (dd. J = 3.3 dnd 1.9Hz, 2H, 127.4, 126.8, 121.8, 115.3, 113.9, 113.8, 110.2, 83.2, H-a) 82.6 (2 peaks), 82.5, 27.8 (2 peaks). 27.7 149.1, 148.9, 148.8 (2 1.27-1.43 (m, 72H). 6.15-6.23 (m, 16H. H-h9), 7.40 (dd, J = 3.2 peaks), 128.4, 128.3. 128.1. 128.0, 127.4, 126.8, 121.8, and 1.9 Hz, 2H, H-a) 115.3, 113.9, 113.8, 110.2, 83.2, 82.7, 82.5 (2 peaks), 27.8, 27.7 (2 peaks), 27.6 149.1, 148.9, 148.8 1.25 (d, (s + s), 63H), 1.40 (2 peaks), 128.4, 128.3, (s, 18H), 6.15-6.23 (m, 18H, 128.2, 128.1. 128.0, 127.4. H-,?), 7.40 (dd, J = 3.2 and 126.8, 121.8, 115.3, 113.9. 1.9 Hr, 2H, H-a) 113.8, 110.2, 83.2, 82.7, 82.5 (2 peaks), 27.8, 27.7 149.1, 148.9, 148.8, 128.4, 1.23-1.41 (m, 90H), 6.15-6.23 (m, 20H, H-B), 7.40 (dd, J = 3.2 128.3, 128.2 (2 peaks). 128.0, 127.4, 126.8, 121.8, and 1.9 Hz, 2H, H-a) 115.3, 113.9, 110.2, 83.2, 82.7, 82.5, 27.8, 27.7

1.25-1.43 (m, 54H), 6.15-6.22 (m, 12H. H-P), 7.40 (dd, J = 3.3 and 1.9 Hz, 2H, H-n)

p=6

p=7

% 'H-NMR (CDCI,, PPm)

*

303-305

starts at 143

h

F r o m p = 11 t o p = 20 only 'H-NMR spectra were measured. These proton spectra were comparable to that ofp = 10; just the ratios changed in the expected way. Oligomer p = 13 was investigated with electrospray mass spectroscopy which resulted in a mass of 2186 ( p = 13 (2147) plus K' counterion).

a

-

'

%

i

2 5

~

2:

9

3

g i i

3.3 Mixed Oligoniers Consisting of’Pl>rroleorid other lHetero)ar.oniutics

249

contributions added substantial knowledge about the structure and properties of oligopyrroles.

3.3 Mixed Oligomers Consisting of Pyrrole and other (Hetero)aromatics 3.3.1 Synthesis Despite the large number of variations possible by combining different repeating units into well-defined oligomers, the number of mixed oligomers with pyrrole synthesized so far is surprisingly low. Two different synthetic approaches are elaborated to incorporate pyrrole into mixed oligomers. with thiophene as the most important partner. 1,4-Diketones, made by the Stetter reaction from an aldehyde and a Mannich base, easily undergo ring closure with NH3 [36], ammonium salts [37, 381 or a primary amine [39] to pyrrole, or with PzSs [36-381 or Lawesson’s reagent [40, 411 to thiophene (Scheme 12). In the other approach the Stille reaction, so successfully introduced in the oligopyrrole synthesis, can be applied to mixed aryl-aryl couplings as well (Scheme 13) [16-26, 421. We will discuss the structures made by both routes as well as the scope and limitations of each approach. In the first approach pyrrole containing co-oligomers are prepared by the cyclization reaction of 1,4-diketones with the appropriate reagents (Scheme 12). A commonly used method to prepare a 1,4-diketone is the Stetter reaction which is based on the Michael addition of a cyanohydrin to an a , /i-unsaturated carbonyl compound [43-451. The catalyst is either a cyanide ion or an ylide (Scheme 14). The Stetter reaction can be performed with a non-electron donating as well as an electron-withdrawing substituent attached to the aryl aldehyde [46]. The Michael acceptor is often prepared in situ via a Mannich base (Scheme 15).

Scheme 12

250

3 Nitrogm-C'ontciining Oligorners

Scheme 13

Scheme 14

Starting from formaldehyde and an amine, usually dimethylamine, an aryl ketone can be converted into the corresponding Mannich base. After elimination of the amine and subsequent addition of the cyanohydrin, the a , P-unsaturated 1,4-diketone is generated. An alternative route to 1,Cdiketones makes use of divinyl sulfone (Scheme 16) [47, 481. The aryl aldehyde, in this case 2-formylpyrrole obligatorily protected with a strong electron-withdrawing group (-S02Ph, -S02CH3), adds to divinyl sulfone (DVS) in the presence of a thiazolium salt to give the heteroaromatic 1,4-diketone. In a similar way 2-fury1 and 2-thienyl aldehydes can be converted into the corresponding diketones. The cyclization of 1,6diketones can afford pyrroles, thiophenes, and furans. Furans are obtained from acid catalyzed dehydration [36] while thiophenes are made upon ring-closure with P2S5[36-381 or Lawesson's reagent [40,41]. N-Unsubstituted or N-substituted pyrroles are formed upon the reaction of a 1,4-diketone with ammonia [36], ammonium salts (NH,OAc, (NH4)2C03)[37, 381 or primary amines [39], respectively (Scheme 17).

Scheme 15

3.3 Mixed Oligorners Consisting of Pyrrole arid other (Hetero)uroniutic.s

25 1

Scheme 16

With these tools in hand numerous oligomers have been prepared. Important contributions to the synthesis of co-oligomers came from Wynberg et al. [40], who synthesized 2,5-dithienylpyrrole 41, and Cava et al. [49-541, who synthesized 45 (Scheme 18). In the Stetter reaction formylthiophene 38 was first converted into the 1,4-diketone 40 using the Mannich base 39. After an acid catalyzed cyclization reaction with ammonia, 2,5-dithienylpyrrole 41 was isolated, which may be considered as the smallest oligomer containing pyrrole and thiophene. This molecule is then used as monomer in the polymerization toward alternating pyrrole-bithiophene conducting polymers [49-591. Compound 40 also undergoes ring closure with dodecylamine to yield 42. This compound can then be transformed into the dialdehyde using n-BuLi and D M F after which a second Stetter reaction with two equivalents of the Mannich base 39 leads to tetraketone 44. A final ring-closure with

R

Lawsson's reagent

(W Scheme 17

252

3 Nitrogen-Containing Oligom~rs

38

41

n-BuLi, DMF

OHC

/

42

43

44

45

Scheme 18

dodecylamine affords heptamer 45. Numerous other co-oligomers 46-50 have been prepared using this sequence of steps (Scheme 19) [60]. Worth mentioning is that this ring-closure reaction can also give access to nonsymmetrical oligomers (Scheme 20) [54]. A last interesting point deals with the functionalization of 1,4-diketones. As shown in Scheme 21, these compounds can easily be substituted with alkyl [52, 601 and thioalkoxy [62] groups.

q-04-6 13I,

R

46

R R=CH3: 49 R=CI2H25: 50 Scheme 19

R

3.3 Mixed Oligomers Coiisisting of Pvrrole and other (Heterolaromatics

42

253

51

f

52

Ac20, HCI

53

54

Scheme 20

Oxidative coupling of the anion of ketone 55 with CU'+ allowed for conversion into the symmetrical dimethyl-substituted 1,4-diketone 56. Mono-alkylation of compound 40 was achieved with an alkyl bromide in the presence of base. After mono-chlorination of 40, using S02C12and subsequent substitution with a thiolate, the thioalkoxy substituted diketone 59 was obtained.

.

55

.

56

C12HsBr. KOH

57

58

Scheme 21

59

2 54

3 Nitrogen-Containing Oligomers

The second approach to prepare co-oligomers based on pyrrole makes use of organometallics. Well-investigated for aryl-aryl couplings are organocopper (Ullmann) [63-671, organoboron (Suzuki) [27] and organotin (Stille) [42] compounds. The Stille coupling will be discussed in detail since it represents the method of choice for the preparation of longer pyrrole containing co-oligomers. The Stille coupling or Pd-catalyzed crossed coupling reaction relies on the coupling of an aryl halide and an arylstannane [42]. The reaction is catalyzed by a palladium catalyst (Pd' or Pd2+),can be performed in a wide variety of solvents and is compatible with different functionalities on the reactants. However, for pyrroles the Stille coupling reaction is primarily performed with Pd(PPh3)4as catalyst in the two phase system toluene/aqueous Na2C03 (1 M ) . Although the Stille coupling reaction has already been used previously for preparing oligopyrroles [ 16-26] and oligothiophenes [68,69], the first synthesis of mixed heteroaromatic oligomers has only been performed very recently. We prepared a large number of co-oligomers for different purposes [70-721. In Scheme 22 the total synthesis of mixed oligomer 67 using this Stille coupling reaction is given as an example. The synthesis of the heptamer, consisting of three pyrrole units, two thienyl spacers and two phenyl blockers, starts with the N-BOC-protected pyrrole 60. First this compound was stannylated selectively at one a-position using lithium 2,2,6,6-tetramethylpiperidide followed by trimethylstannyl chloride in T H F at -70°C. Then a first Stille reaction with bromobenzene was performed resulting in N-BOC-2-phenyl-pyrrole 62. Selective bromination of 62 followed by stannylation

60

61

63

41

62

64

65

67

Scheme 22

3.3 Mixed Oligorners Consistirig of Pyrrole and other (Hetero)aromcitics

-1: 68 m269 m370

255

m k 77 -2: 78 -3 79

m 4 71

80

ml:72 m2 73 m3:74

ml: 75 m2 76

Scheme 23

using n-BuLi and trimethylstannyl chloride finally gave compound 64, being a key intermediate in the synthesis of numerous oligomers. Then 2,5-dithienylpyrrole 41 was protected with a BOC-group using di-t-butyl dicarbonate and KOtBu in T H F and the resulting compound 65 was dibrominated at the a-positions. However, instead of direct bromination with 2 equivalents of NBS, which leads to quite some /3-substitution of the central pyrrole unit, 65 was first distannylated (LTMP, (CH3)3SnCl)and then dibrominated (NBS) taking advantage of the easy stannyl-bromo exchange reaction [73]. The resulting dibromide 66 was finally coupled with two equivalents of 64 to give the heptamer 67 after difficult purification (extraction, column chromatography, crystallization). Similarly, numerous other co-oligomers have been prepared (Scheme 23). Among these oligomers are some donor-7r-acceptor molecules 77-80 which have been investigated with hyper Rayleigh scattering (HRS) for their nonlinear optical properties [74]. Furthermore, two N-dodecyl substituted oligomers 81 and 82 have been investigated by scanning tunneling microscopy (STM), and three series of phenyl-blocked oligopyrroles with different spacers have been investigated by (spectro)electrochemistry after thermolytic removal of the t-BOC groups (190°C under inert conditions) [71]. Figure 2 depicts cyclic voltammetry measurements of two series of oligomers, phenyl-blocked oligopyrroles and phenyl-blocked oligopyrrole-thiophenes. A striking difference can be observed in the first oxidation potentials of these two series: in the oligopyrrole series the first oxidation potential decreases with increasing number

256

3 Nitrogen-Containing Oligomers

0 rf

43

0

.;..

..... ......

: ,

: :

.. ..

(I

3.3 M i d Oligoriirrs Corisistitig of P v r o l r ~ n other d (Hetero)aroriiatics

257

of pyrrole units, while in the case of the mixed series this value increases, as expected, with increasing number of thiophene units. In both cases the second oxidation potential decreases with an increasing number of heteroaromatic units. These data were explained by a strong preference of charge to be localized on the pyrrole units. Therefore, mixed oligomers are of more importance in the neutral form, mimicking low band-gap polymers, than as doped species.

3.3.2 Structural Characterization Table 2 shows the structural data of a number of N-substituted and N-unsubstituted co-oligomers. The UV data of N-unsubstituted mixed oligomers reveal that once 5-7 units are reaches an asymptotic value. Furthermore, the band-gap is diminished linked, , ,A more significantly by a-thienyl groups than by phenyl and a-pyrrolyl groups. A comparison of oligomers containing five nuclei nicely illustrates these effects. Quinquepyrrole (Table l), 5,5”-diphenyl-2,2’,5’,2”-terpyrrole85 and 1,4-bis(5phenyl-2-pyrro1e)benzene 90 show comparable,,,A, values (367, 374, and 371 nm, respectively). Replacement of phenyl or pyrrolyl groups by thienyl groups leads to a significant bathochromic shift: 2,5-bis(5-phenyl-2-pyrrolyl)thiophene87 and 2,5-bis(5-thienyl-2-pyrrolyl)thiophene47 feature maxima at 390 and 40 1 nm, respectively. However, one has to take into account that the two compounds were measured in different solvents. values observed for N-methyl and N-dodecyl-substituted thienylpyrThe, , A, roles (45,46,48,49, and 50) are primarily determined by the number of consecutive thiophene units: 5,5’-dipyrrolyl-2,2’-bithiophene 46 (4 units, 379 nm) shows a more bathochromic absorption maximum than the alternating thienylpyrrolyl derivative 45 (7 units, 363 nm). In the N-BOC series a comparable behavior is observed: an increasing number of consecutive N-BOC-pyrrolyl units hardly affects . , , ,A Indeed, that of bipyrrole 69 (295nm) does not differ strongly from that of quaterpyrrole 71 (310nm), while insertion of thiophene units in 69 induces a marked bathochromic effect: terthienyl of 419 nm. Finally, insertion of p-phenylene units in 69 derivative 74 reveals a, , ,A has a minor effect as exemplified by biphenylene derivative 76,,A(, 323 nm). IH NMR-data of mixed N-H oligopyrroles containing in addition phenyl and/or thienyl units, show some general trends: (1) For N-H absorptions a striking but small effect has been observed: while apyrrolyl substitution induces in THF-d8 an upfield shift (10.40 to 10.21 ppm), a-thienylation brings about a downfield shift (to 10.57 ppm). (2) Apart from N-H signals, phenyl protons feature the most downfield signals (7.7-7.1 ppm), b-thienyl protons are found in a narrow range (7.1-7.0ppm) and P-pyrrolyl protons give always the most upfield signals (6.5-6.0 pprn). The mutual influence of phenyl, a-thienyl and 0-pyrrolyl substituents is easily rationalized by electronic and conformational considerations.

Table 2 Structural data for N-H, N-CH3, N - C I 2 H 2 5 and N-BOC substituted pyrrole co-oligomers. Mol. formula

Mp ( ‘ C )

UV-Vis (nm) (log E )

IR (KBr, cm- ’ )

N-H Ph-p-Ph 83

CI~HIIN

(CH3CN) 328

Ph-p-p-Ph 84

C2oHi6N1

(CH,CN) 360

Ph-p-p-p-Ph 85

C14H,,N,

(CHICN) 374

Ph-p-p-p-p-Ph 86

C28HZLN4

(CH,CN) 373

Ph-p-t-p-Ph 87

C2,Hl,NIS

(CHICN) 390

El. analysis (C : H : N)

M.S. (mle)

IH-NMR

(CDCI:. ppm)

(THF-d8) 10.4 (s. IH, H-I), 7.63 (dd. J = 7.4 and 1.1 Hz, 4H, o-Ph), 7.31 (t. J = 7.4Hz. 4H, m-Ph), 7.12 (It. J = 7.4 and 1 . 1 Hz, 2H. p-Ph), 6.51 (d. J = 2.5Hz. 2H, H-3.4) (THF-ds) 10.33 (s. ZH, H-I,I‘), 7 56 (dd. J = 7.4 and 1.1 Hz, 4H, o-Ph), 7.28 (I, J = 7.4Hz, 4H, m-Ph), 7.08 (tt, J = 7.4 and 1 . 1 Hz. 2H, p-Ph), 6.48 (dd, J = 3.5 and 2.5 Hz, 2H, H-3,3‘,’H4.4’). 6.37 (dd, J = 3.5 and 2.5 Hz. 2H, H-3.3’:H-4.4’) (THF-d,) 10.23 (s, 2H. H-1.1”). 10.12 (s. IH, H-1’) 7.55 (m, 4H. o-Ph). 7.28 (m, 4H, m-Ph), 7.08 (m, 2H. p-Ph), 6.47 (m. 2H. H-4.4”). 6 31 (m, 4H, H-3,3”.3’,4’) (THF-d8) 10.21 (s, 2H, H-1,I”’). 10.09 (s, 2H, H-I’,l’‘). 7.56 (m, 4H, o-Ph). 7.28 (m. 4H, m-Ph), 7.08 (m, 2H, p-Ph). 6.48 (m. 2H, H-4,4“‘), 6.28 (m, 4H, H-3,3’.3“,3”‘,4‘,4”) (THF-d8) 10.53 (s, 2H. H-1’). 7.62 (dd, J = 7 4 and 1.1 Hz, 4H, o-Ph). 7.32 (t. J = 7.4 Hz, 4H, m-Ph), 7.13 (tt, J = 7.4 and 1.1 Hz, 2H, p-Ph), 7.06 (s, 2H, ThH-3.4), 6.51 (dd, J = 3.5 and 2.5 Hz, 2H. PyH-4). 6.40 (dd. J = 3 5 and 2.5 Hz, 2H. PyH-3)

-

z

?,

z-

0-=:

c

2 4

F

ci ?. .c

c

.c L

Yf P

Table 2 (Conr.) Mol. formula

Mp (-C)

UV-Vis (nm) (log E )

N-CH, p-t-1-p 46

CIXHI~N~SI

142.5-143.5

(CHCI,) 379 243

t-p-1-t-p-t 49

CL6HZoN1S4

213-215

(CHCI,) 399 (4.5I ) 245 (4.29)

IR (KBr, cm-')

El. analysis (C : H : N )

M.S. (mie)

'H-NMR (CDCI,. ppm)

Calcd.: 66.62 : 4.98 : 8.64 326 (M + 2, 25.6) 7.1 I (d, J = 3.9Hz, 2H), 6.92 Found: 66.36 : 5.01 : 8.52 325 (M I . 47 3) (d, J = 3.9Hz. 2H), 6.72 (m. 324 (M', 100) 2H), 6.37 (dd, J = 3.3 and 309 (31.6). 282 1.6Hz. 2H). 6.18 (m, 2H), (5.6), 218 (9.0). 162 3.77 (s, 6H) (25.9) 161 ( I 1.8) 141 (6.1) 490 (M + 2, 10 4) 7.32 (dd, J = 4.3 and 1.7Hz. 489 (M + I . 35.4) 2H), 7.15 (d. J = 3.8Hz, 2H), 488 ( M i , 100) 7.10 (m, 4H), 6.98 (d, 471 (11.8). 244 J = 3.5 Hz, 2H), 6 38 (dd, (5.6). 163 (7.l), 151 J = 6.8 and 3.8Hz. 4H). 3.79 (9.8), 149 (9.8) (s, 6H)

+

(C : H : N : S) Calcd.: 73.88 : 8.75 : 3.92 : 13.45 Found: 73.85 . 8.79 . 3.88 : 13.39 t-p-t-t-p-t 50

C46H64N2S4

74.5-75.5

(CHCI?) 393 (4.25) 245 (4.33j

(C : H : S) Calcd.: 72.30 : 8.1 I : 16.08 Found: 72.18 : 8.12 : 16.17

1-pt-p-1-p-t 45

C,H,, N,S,

66-77

(CHCI,) 363 (4.51) 242 (4.36)

(C : H : N : S ) Calcd.: 74.56 : 8.92 ' 4.08 : 12.44 Found: 74.30 : 8.88 : 4.09 : 12.51

716(M+2, 11.6) 715 (M I , 24.5) 714 (M+, 48.4) 545 (18.5), 377 (25.7), 165 (28.9), 141 (44.4). 128 (67.3) 798 (M 2,43.5) 797 (M 1, 66.5) 796 (M+, 100) 627 (13.9). 512 (17.7), 459 (l3.l), 458 ( I 3.5), 398 (10.8), 287 (8.0) 1033 (M 3, 13) 1032 (M 2,44) 1031 (M I , 82) I030 (Mi, 100)

+

+

+

+ +

+

7.33 (dd, J = 4.2 and 2.1 Hz, 2H), 7.09 (m, 4H). 7.02 (s. 2H), 6.36 (dd, J = 6.5 and 3.5 Hz, 4H), 4.18 (1, J = 7.5 Hz, 4H). 1.19 (broad m, 40H), 0.87 (I. 6H) 7.33 (dd. J = 4.3 and 2.3 Hz, 2H), 7.15 (d, J = 3.6Hz, 2H), 7.09 (m, 4H), 6.97 (d, J = 3.6Hz, 2H), 6.36 (dd, J = 7.4 and 3.8Hz. 4H). 4.18 (t, J = 7.5 Hz. 4H), 1.20 (broad m, 40H), 0.87 (t, 6H) 7.33 (dd, J = 4.3 and 1.5 Hz, 2H), 7.09 (m, 4H), 7.03 (s, 4H). 6.38 (dd, J = 6.7 and 3.4 Hz, 6H), 4. I8 (broad m, 6H), 1.21 (broad m, 60H), 0.87 (1. 9H)

WNMR (CDCI,, ppm)

N-BOC Ph-p-Ph 68

CZHZINOZ

I20

(CHICN) 290 225

3073-2933, 1746, 1605, 1486, 1444, 1305, 1146, 701643

7.43-7.28 (m.IOH. PhH), 6.23 (s, 2H, H-3,4), 1.17 (s, 9H. CHI) 7.38-7.28 (m.IOH. PhH), 6.27 (d, J = 3.4Hr. 2H. H-R,3'/H-4,4'), 6.24 (d, J = 3.4Hr. 2H. H-3.3':H4.4'). 1.25 (s, 18H, C H I )

Ph-p-p-p-Ph 70

Ph-p-p-p-p-Ph 71

Ph-p-t-t-p-Ph 73

7 40-7.30 (m,IOH, PhH), 6.28 ( s , 2H. H-3',4'), 6 26 (d. J = 3 4 % 2H. H-3.3'W 4.4'). 6.23 (d, J = 3.4Hz. 2H, H-3,3'/H-4,4'), 1.34 (s, 9H, CH3(BOC')), 1.26 (s, IXH, CHl(BOC, BOC"))

CIYH41N10h

7.38 7.26 (m.10H. PhH). 6.26 (d, J = 3.3 Hr, 2H. H3.3"'.'H-4,4"'). 6.22 ( 5 . 4H, H3'.4'.3",4"). 6 19 (d. J = 3.3 Hz, 2H, H-3.3"'lH4.4"'). 1.?9,1.24(s.36H,CHI)

C,,HS4NVaO,

C I , H I ~ N : O ~ S ~ 159

149.8 ( G O ) , 136 2, 134.1 (C-Z,Sjipso-Ph), 1288, 127.8 (a-Ph/m-Ph), 127.2 (p-Ph), 112.1 (C-3,4), 83.9 (C-CH3), 27 1 (CHI) 149.3 (C=O), 1366, 134.6 (C-5,5';ipso-Ph), 128.4, 128.1 (o-Ph!m-Ph), 128.1 (C-2.2'). 127.0 (p-Ph), 114.4, 112.6 (C-4.4'/C-3,3'). 83.4 (C-CH?).

(C=O (BOC, BOC")), 27.4 149 2(CH,) 149.0 (C=O (BOC')). 136.5, 134 7 (C-5.S"/ipso-Ph). 128.7. 121 7 (C-2.2",lC-2',5'), 128.3. 127.8 (o-Phjm-Ph). 126.9 (C-CHl)(BOC, BOC")), 82.8 (C-CH,(BOC')). 27.6 (CH?(BOC')),27.4 (CHI(BOC, BOC")) 149.3, 148.9 (C=O). 136.4, 134.8(C-5,5"'lipso-Ph). 128.8, 128.4. 128.3. 127 7, 127.6. 126.9 (o-Phjm-Ph:p-Ph:C2.2"C-5',2"1C-5",2"'). 114 4, 113.9. 113.8,112.7(C-3.3"'/C4,4"'lC-3',3''/C-4'.4"). 83.2, 82.8 (C-CHI), 27 7. 27.4 (CHI) 149.6 (C=O), 136.7, 134.4. 134.0 (ThC-21PyC-5,'ipsoPh). 128.4, 127.9 (o-Ph/mPh), 127.8, 127 2 , 127 0 (ThC3/p-Ph,'PyC-2), 113.9. 112.1 (PyC-3/PyC-4). 83 4 (C-CH,). 27.1 (CHI) 149 6 (C=O), 137.2. 137.1, 137.0, 134.0. 133.4 (ThC-21 ThC-S,PyC-Z/PyC-5'ipsoPh), 128.4. 128.2. 128.0 (ThC4!o-Ph/m-Ph), 127.3 (p-Ph).

(CH,CN) 33 I

2977. 1750. 1302 1141. 842-698

7.41 7.28 (m, 10H. PhH). 7.07 ( s . 2H. ThH-3.4). 6.38 (d. J = 3.5 Hz, 4H, PyH-3,PyH41, 6.24 (d, J = 3.5Hz. 4H. PyH-3,'PyH-4). 1.25 (s, IXH. CH,)

(CH,CN) 364

2978. 1751. 1295. 841 698

7.41-7.30 (m.10H, PhH), 7.12 (d, J = 3.7Hz, 4H. ThH3,ThH-4).7.04(d, J = 3.7 Hz, 4H. ThH-3.ThH-4). 6.39 (d. J = 3.5Hr. 4H, PyH-3,PyH4 ) , 6 . 2 5 ( d , J = 3 . 5 Hz,4H 123.3(ThC-3).114.1.112.1 PyH-3,PyH-4). 1.24 (s. 18H. (PyC-3,4), 84.4 (C-CH,), 27 2 CHI) (CHI)

Table 2 (Cont ) Mol formula

Ph-p-t-t-t-p-Ph 74

C,IH;,N,O,S,

Ph-p-Ph-p-Ph 75

CI,H3,NI04

Ph-p-Ph-Ph-p-Ph 76

CdZHJONZOI

Ph-p-t-p-t-p-Ph 67

C4iH17N10hSZ

According to Luo p r ill. [60] Accordmz to Cava ('r (11. [491

M p ( C)

139

122- 123

UV-Vis IR ( n m ) ( l o g c ) (KBr cm ' )

(CH,CN) 310

2980. 1751 1707. 1144. 847 698

(CH;CIU) 323

2973. 1748. 1310. 1145. 847-699

(CH,CN) 346

2978. 1750. 1300. 1141. 842-698

tl analbsis (C H N )

M S (me)

'H-NMR (CDCL ppm)

"C-NMR (CDCL ppm)

1490 (C=O). 134.1. 130.9 7.40-7.29 (m. 10H. PhH), 7.1 I (d. J = 3.7 Hz, 2H. ThH- (Ipso-Ph PyC-5). 128.8. 128.7. 3.3" ThH-4.4"). 7.08 (s. 2H. 127.9. 127.4. 127.1 (PyC-2,' ThC-4 ThC-3 ThC-3' p-Ph). ThH-3'.4'). 7.05 (d, 124.4. 124.2. 123.4 (ThC-5 J = 3 7H2, 2H. ThH-3.3'' ThC-2 ThC-2'), 128.6. 128.1 ThH-4.4"). 6.39 (d. J = 3 . 5 H ~ZH. . PyH-3 PyH- (0-Ph m-Ph). 114.1. 112.2 (PyC-3,PyC-4). 84.5 4). 6.24 (d. J = 3.5 H7. 2H. PhH-3 PyH-4). 1 24 (s, 18H. (C-CH1),27.2 (CHI) CH:) 7.43 7.28 (m, 14H. H-phenyl 150 3 (C=O). I36 4 I36 0 114 I 132 8 IC-l C-4 PvC-5 (H-2 H-3)phenyl). 6.28 (d, J = 34H2. 2H. H-3l.4'). 6.25 C-ipso(end-capped phenyl)). 128.8 (C-2. C-3). 128 I , 127 8 (d. J = 3.4Hz. 2H. H-3'.4'), ((0-Ph 'm-Ph) end-capped 1.22 (5. IXH. CH,) phenyl). 127.2 (p-Ph (endcapped phenyl)), I12 3 . 112.2 (PyC-3.4). 84.1 (C-CH,). 27.1 7.76 (d. .I = 8 4H2.4H. H-2), 150.3 (C=O), 139.4 (C-I). 7.49 (d. J = 8.3 Hz. 4H. H-3). 136.5. 135.9 (C-2"C-50. 134.1. 133.1 (C-4, C-ipsolend7.45-7.31 (m. IOH. H-endcapped phenyl)). 129.1. 126.4 capped phenyl). 6.31. 6.27 (2xd. J = 3.4 H2. 4H. H-3' H- (C-2 C-3). 128.8 127 8 ( ( C ortho C-meta) end-capped 4'). 1.20 ( 5 . 18H. CH1) phenyl). 127 2 (C-pard (endcapped phenll)). 112.4. 112.3 (C-3' C-4'). 84.1 (C-CH3), 27 1 (CHI) 149.5 (C=O(BOC')). 149.4 7.41 7.28 (m. IOH. H(C=O(BOC)). 136 7. 134.4. phenyl). 7.08, 7.03 (2xd. 134.4. 134.3. 134.0. 128.7. J = 3.7 Hz, 4H. H-3'. H-4'). 128.3 (C-Z,C-S'C-2"C-5' C6.37 (d. J = 3.6 Hz. 2H. H3"). 6.35 (s. 2H. H-3 H-4). ?"'C-5"' C-ipso(phenq1)). 128.3, 127.8 (C-ortho C6.23 (d. J = 3.3H7. 2H. H4"). 1.36(s. 9H. CHI (BOC)). meta), 127.2. 127.0. 126.6 (C-3' C-4' C-para). 113 9. 1.22 (s. 18H. CH I (BOC')) 113.6. 112 I (C-3 C-4 C-3" C4"). 84.6 (C-CH,). 27.2 (CH? (BOC)). 27.1 (CHI (BOC'))

3.4 Oligourdiries

263

(3) When N-H compounds are involved y-pyrrolyl protons undergo a marked deshielding upon double a-phenylation (+0.25 ppm) a significant deshielding upon a-thienylation ($0.15 ppm) and almost no effect upon a-pyrrolylation. (4) In the series of phenyl-blocked oligopyrroles the outer P-pyrrole protons feature signals a t -6.5 ppm, while the inner protons are found at 6.37 ppm in bipyrrole 84 and at 6.28 ppm in the corresponding quaterpyrrole 86. The latter value is close to that found in homo-oligopyrroles. The phenyl protons are wellseparated: ortho protons at -7.6. iiieta protons at -7.3 and para protons at -7.1 ppm. Finally, p-thienyl protons are only slightly affected by a-pyrrolyl substituents. In N-BOC protected derivatives the position of the methyl protons is of some diagnostic value: while in N-BOC-pyrrole (Table 1) a value of 1.58 ppm is observed, a shielding is operative in the corresponding di-phenyl (68, 1.17 ppm) and di-thienyl (68, 1.36 ppm) derivatives. The influence of additional N-BOCpyrrolyl substituents seems to depend on their own substitution pattern: while in the N-BOC protected terpyrrole the central BOC-methyl protons are the more shielded (1.21 ppm), in the diphenyl-blocked homolog 70 they are the more deshielded (1.34 pprn).

3.4 Oligoanilines 3.4.1 Synthesis The different oxidation states of the first member in the oligoaniline series, p-aminodiphenylamine, have already been investigated in the previous century [75, 761. The intriguing properties of aniline black also initiated the first synthesis of oligomers with eight aniline repeating units as early as 1907 by Willstatter, Green and Woodhead [l, 77-80]. In this pioneering work also the beautiful names, leucoemeraldine, protoemeraldine, emeraldine, nigraniline, and pernigraniline were proposed for the different oxidation states of the octamer (Scheme 24). These names are nowadays also used to describe the different oxidation states of the higher molecular weight polymers. More recently both N-unsubstituted and N-methyl or N-phenyl substituted oligoanilines have been prepared (Scheme 25). The synthesis, characterization and different oxidation states of the oligomers will be discussed from a historical perspective, to give credit to the early work in this interesting area of models for the conducting polymer now so popular around chemists, physicists, and device engineers. p-Aminodiphenylamine as well as the octamers given in Scheme 24 are made by the careful oxidation of aniline in alkaline medium followed by titrated reduction (titanium trichloride) or oxidation (chromic acid or hydrogen peroxide) [75, 76, 78-80]. Detailed analyses revealed that all the structures with amine endgroups as shown in Scheme 24 can be prepared using this oxidative coupling. Furthermore,

264

3 Nitrogen-Containing Oligotners

leucoemenldioe

pmlmrneraldine

emeraldine

perrigraniline

Scheme 24

it was proposed that the principal product of the aniline oxidation is emeraldine containing two quinonediimine units in the octamer. It is now well established that the polymer possesses the same oxidation state. However, from the early work it is not evident that the octamers prepared are free from longer or shorter oligomers as contaminants. Also the partial hydrolysis of a quinonediimine endgroup is possible; a reaction studied in detail for the N-phenyl-p-quinonediimine [75, 761. Nevertheless, the statements made by Willstatter, Green and Woodhead on the principal oxidation state of the octamer and, consequently of polyaniline itself, have been shown to be correct and are now well established. Recent research has been performed to check the characterization of the products as obtained by these pioneers, and the work of Yoffe et al. has been very important in this respect [81, 821. Using both Willstatter's methodology (Scheme 26) and a new approach (Scheme 27), they prepared several oligoanilines.

Scheme 25

3.4 OIigouniIines

265

91

yu

92

Scheme 26

Starting from the HCI-salt of p-aminodiphenylamine 90, an oxidative coupling with FeC13 resulted in the partially oxidized tetraaniline 91, also known as 'blue imine', which was then reduced to the fully aromatic tetraaniline 92 using phenylhydrazine. This same compound was also prepared by a completely different route (Scheme 37). Yoffe's new approach to tetraaniline 92 starts with p-aminodiphenylamine 93. First a coupling with sodium 2-chloro-5-nitrobenzenesulfonateis performed resulting in the trimeric substituted oligomer 94. After reduction of the nitro group with zinc in a NaOH-solution, the sulfonic acid group was removed by boiling in concentrated HCI. The resulting trianiline 96 was then subjected to the same sequence of steps which finally gave what appeared to be the same compound as derived from the oxidative coupling/reduction sequence.

!m Scheme 27

92

266

3 Nitrogen-Containing Oligomers

H

. . .

m=2: 99 m=3: 100 m=4: 101 m=6: 102

m 4 : 101 m=6: 102

Scheme 28

It was not until 1969 that Honzl et al. [83-851 synthesized a first series of phenylblocked N-unsubstituted oligoanilines, which, due to the synthetic scheme, are welldefined with respect to chain length and aryl-substituent pattern. The dimer (n = 2, 99) and the trimer (n = 3, 100) were prepared by a Sandemeyer type of elimination of the amino groups of 96 and 92 using hypophosphorous acid. For the synthesis of the tetramer 101 and the hexamer 102 they modified Liebermann's method [86] of preparing derivatives of 2,5-diaminoterephthalic acid (Scheme 28). By condensation of the diethyl ester of 103 with the oligoanilines 92, 93 and 96, the oligoaniline derivatives of 1,4-dihydroterephthalic acid diethyl ester 104-106 were prepared which, upon oxidation, gave the corresponding derivatives of terephthalic acid diethyl ester 110-112. After hydrolysis to the free acids 113-115, and decarboxylation by high-vacuum sublimation the linear tetramer and hexamer, 101 and 102, were obtained. Unfortunately, the corresponding octamer could not be isolated after decarboxylation. However, almost twenty years later Wudl et al. isolated octamer 117 after coupling of two equivalents of Willstatter's tetramer 92 with one equivalent of dihydroxydihydroterephthalicacid 116, and subsequent reduction with phenylhydrazine (Scheme 29) [87].

3.4 Oligoiiriiliries

267

117 Scheme 29

-1:

118

m2:119 -3: 120

m=l: 121 m=2: 122 m=3: 123

Scheme 30

Following their successful preparation of the unsubstituted oligoanilines, Honzl ul. also prepared a number of N-methyl-substituted oligoanilines [83-85]. Starting from the dimethylamino terminated oligomers 118-120, methylation using phenyllithium and methyl iodide in diethyl ether resulted in the oligomers 121123 (Scheme 30). Later, Strohriegl and Heinze reported on the synthesis and characterization of both N-methyl and N-phenyl substituted oligoanilines [88, 891. The N-methyl substituted oligomers 124-126 were prepared by methylation of Honzl's phenylblocked N-unsubstituted dimer 99, tetramer 101 and hexamer 102 using formaldehyde and NaBH3CN in acetonitrile (Scheme 31). rt

m=2: 99 m d : 101 m=6: 102 Scheme 31

m=2: 124

m 4 : 125 m=6: 126

268

3 Nitrogen-CantuiningOligomers

The N-phenyl substituted oligoanilines were synthesized starting from diphenylamine and N,N'-diphenyl- 1,4-~henylenediamine.After lithiation of these compounds with n-BuLi, the N-phenyl substituted and phenyl-blocked dimer 127, trimer 128 and tetramer 129 could be prepared using iodobenzene and 1,6diiodobenzene in combination with copper(1) iodide as a catalyst (Scheme 32). Thanks to the synthetic work performed over a period of almost a hundred years, we now can rely on convenient routes to prepare most of the interesting oligoanilines. Despite the fact that the octamer possesses most of the optical and electrical characteristics of the corresponding polymer, the octamer is not yet regarded as an interesting electronic material in itself. This is surprisingly different from the sexithiophene case, although there is not an obvious reason.

3.4.2 Structural Characterization Since most of the synthetic work on oligoanilines was performed more than 25 years ago, the structural characterization (e.g. MS, NMR, elemental analysis) is very limited. Data of three series of oligoanilines have been included in Table 3. The UV data on non-oxidized oligoanilines shown in Table 3 indicate that once a N,N'-diphenyl-p-phenylenediamine moiety is present, chain elongation only marginally affects. , ,A, Hence the real conjugation length is limited in the three series represented in the table. A small long-wavelength band, often observed in

Table 3 Structural characterization of three series of N-unsubstituted and N-substituted oligoanilines.

Mol. formula n? = 1 1H = 2 n1 = 3 12 = 1 n=2 n=3 n=4 11 = 6 n=8

M.S. (m/z)

CI?H12N7 CIXH17N3 C24H21N4

c 4 2 H36N6 C.(4H46N8

ClXHlSN

p=3

C42H33N3

p=4

C54H42N4

C30H?4N1

IR (cm-l)

MP ( C )

4.3 eV 4.1 eV 4.0 eV

3372, 1597. 1515 3382, 1599, 1526, 1310, 822 3388. 1598, 1513, 1309, 825

73 --75 154 183- 184

KBr: 1603, 1515, 1500, 1317. 822

53-54 192

286 (ethanol) 306 (dioxane) 320 (dioxane) 321 (dioxane) 323 (dioxane) 334 (DMF)

Cl2HllN C18H16N2 C?4H?IN3 C3nH26N4

p=l p=2

UV-Vis (nm)

413 (M+ + 1, 62%) 412 (M+, l0OYn) 206 (M'+. 26.5%) 580 (M' + 1, 21%) 579 (M'. 100%) 747 (M' + 1,62%) 746 (M'. 100%) 373 (M'+, 27%)

~ 7 1

249 306 330

312 (THF)

KBr: 1590, 1490, 1270, 825, 750. 700

125-127 196 [88], 200 [93]

316 (THF)

KBr: 1587. 1502, 1493. 1271. 748, 694

257

319 (THF)

KBr: 1602. 1522. 1500, 1305, 817

KBr: 1593. 1502. 1493. 1269, 748. 692

297

0

=: 0s

I Scheme 33

oligoanilines especially in acidic medium, is most probably attributable to a partially oxidized species [89,90]. Wudl et al. were able to prove that phenyl-capped octaaniline easily oxidizes in air to the mono-quinonoid material (‘protoemeraldine’), whilst upon treatment with ammonium persulfate the bi-quinonoid species (‘emeraldine’) is formed. The tetra HCI salt of the latter features an IR spectrum superimposable with, and with a conductivity similar to, that of acid-doped polyaniline. This suggests that the conductive unit in oligoaniline and polyaniline may be represented by resonance structures of type I (Scheme 3 3 ) and implies that in both cases an intermolecular mechanism of charge transport is needed. One systematic study of I3C-NMR data on oligoanilines (Table 1, rn = 1-3) and their oxidized adducts revealed the chain structure of these oligomers. It proved, as expected, that the symmetry axis is parallel to the molecular axis in the completely reduced forms, but not in the oxidized forms. In the latter case, therefore, an increased number of signals was observed. More advanced techniques (cyclic voltammetry, I R, conductivity measurements, XPC and even X-ray crystal structure analysis) have also contributed to our present view of oligoaniline and its oxidized states [ l , 77, 90-921.

References Street, G. B. Handhook of Condttcfing Po1ymer.s. 1986, Marcel Dekker, New York. Rapoport, H., Holden, K . G. J . A m . Chem. Soc. 1962, 84, 635. Rapoport, H., Castagnoli, N., Jr. J . A m . Chem. Soc. 1962, 84, 2178. Rapoport, H., Castagnoli, N., Jr., Holden, K. G. J . Org. Chem. 1964, 2Y, 883. Grigg, R., Johnson, A. W., Wasley, J. W. F. J . Chem. Soc. 1963, 359. Bullock, R., Grigg, R.. Johnson, A . W., Wasley, J. W. F. J . Chem. Soc. 1963, 2326 Grigg, R., Johnson, A. W. J . Chem. Soc. 1964, 3315. Webb, J . L., Threlkeld, R. R. J . Org. Clwm. 1953, 18, 1406. Bauer, V. J., Woodward, R. B. J . Am. Cliem. Soc. 1983, 105, 6429. 10. Sessler, J. L., Cyr, M., Burrell, A. K. Tc,traherlron, 1992, 48, 9661. 1. 2. 3. 4. 5. 6. 7. 8. 9.

References

27 1

11. Ikeda, H., Sessler, J. L. J . Org. Chern. 1993. 58, 2340. 12. Sessler, J. L. et al. (Arigew. Chem. 1994,106(14). 1572) also prepared an N-unsubstituted trimer by ring-closure of a diketone with NH40Ac: this method is discussed in See. 3.3.1. 13. Kauffmann, T., Levy. H. Chem. Ber. 1981, 41-43, 403. 14. Carpino, L. A., Barr. D. E. J . Org. Chern. 1966. 31. 764. 15. Hasan, I., Marinelli, E. R.. Lin, L.-C., Fowler, F. W.. Levy, A. B. J . Org. Cheni. 198I , 46, 157. 16. Martina, S. Dissertation, 1992, University of Mainz. Mainz. Germany. 17. Martha, S., Enkelmann, V., Schluter, A.-D., Wegner. G. SJnrh. Met. 1991. 41, 403. 18. Martina, S., Enkelmann, V., Schluter, A.-D.. Wegner, G. Polvrn. P r e p . ( A m Chem. Soc., Div. Poljwi. Chern.) 199I , 32(3),2 15. 19. Martina, S., Enkelmann, V.. Schluter, A.-D., Wegner, G. Sjwthesis 1991, 613. 20. Martina. S., Schlilter, A.-D. Macromolecules 1992. 25. 3607. 21. Martina, S., Enkelmann, V.. Schluter, A.-D., Wegner. G. Syrzrh. Met. 1992, 51, 299. 22. Zotti, G., Martina, S., Schluter, A.-D., Wegner, G. Adv. Muter. 1993, 4(12), 798. 23. Martina, S., Enkelmann, V.. Schluter. A.-D., Wegner, G.,Zotti. G.. Zerbi. G. Synrh. Met. 1993, 55(2-3), 1096. 24. Schluter, A.-D., Wegner. H. Acra Polymericn 1993, 44, 59. 25. Zerbi, G., Veronelli. M.. Martina, S., Schluter. A.-D.. Wegner. G. J . Chern. Phys. 1994, 100(2), 978. 26. Zerbi, G., Veronelli, M., Martina, S., Schluter, A.-D., Wegner, G. Adv. Muter. 1994. 6 ( 5 ) ,385. 27. Hoshino. Y., Miyama, N., Suzuki, A. BUN. Cheni.Soc. Jpn. 1988. 61, 3008. 28. Stille. J. K. Angew. Chern. 1986, 98, 504. 29. Groenendaal, L.. Peerlings. H. W. I.. van Dongen. J. L. J. et a/. Polym. P r e p . 1994, 207, 194. 30. Groenendaal, L.. Peerlings, H. W. I., van Dongen, J. L. J. et a/. Macromolecules 1995,28. 116. 31. Williams, D. J., Colquhoun, H. M., O'Mahoney, C. A. J . Chem. Soc., Chem. Commim. 1994, 1643. 32. Magnus. P., Danikiewics. W.. Katoh. T.. Huffmann, J. C., Folting. K. J . A m . Chem. Soc. 1990. 112, 2465. 33. Beljonne, D.. Bredas, J. L. Phys. Rev. B 1994, 50(5),2841. 34. Orti, E., Sanchez-Marin, J,, Tomas, F. Theor. Chim. Acta 1986. 69. 41. 35. Kofranek, M., Kovar, T., Karpfen. A., Lischka, H. J . Chern. Phys. 1992, Y6(6).4464. 36. Smith, A. J . Chern. Soc. 1890, 57, 643. 37. Scheeren. J. W., Oomes, P. H. J., Nivard, R. J. F. Synthesis 1973, 149. 38. Young, D. M.. Allen, C. F. H. Org. Synrh. Coll. Vol. 11, 8th ed, 1957, 219. 39. Stetter, H., Schreckenberg, S. Cheni. Ber. 1974, 107, 2453. , 1984, 14, 1 . 40. Wynberg, H., Metselaar, J. S y ~ t hComriiun. 41. Peterson, B. S., Scheibyl, S., Nilsson. N. H., Lawesson, S. 0. Bull. Soc. Chini. Belg. 1978, 87, 223. 42. Stille, J. K. Angeiv. Chem. I n t . E d Erg/. 1986, 25, 508. 43. Stetter, H. Arigeiv. Chem. Inr. E d Engl. 1976, 15(11),639. 44. Stetter, H., Rajh, B. Ber. 1976, 109. 534. 45. Stetter, H., Kuhlmann, H. Org. React. 1991. 40, 407. 46. Phillips. R. B.. Herbert, S. A.. Robichaud, A. J. Sjwth. Conimun. 1986. 16, 411. 47. Stetter. H., Bender, H. Cheni. Ber. 1981, 114, 1226. 48. Merrill, B. A., LeGoff, E. J . Org. Chon. 1990, 55. 2904. 49. Niziurski-Mann, R. E., Cava, M. P. Adv. Muter. 1993, 5(7/8).547. 50. Joshi, M. V., Cava, M. P.. Bakker, M. G. e t a / . Synth. Met. 1993, 55-57, 948. 51. Joshi, M. V.. Hemler, C., Cava, M. P. et a/. J . Cheni. Soc., Perkin Trans. 2 1993, 1081. 52. Niziurski-Mann, R. E., Scordilis-Kelley. C., Liu. T.-L. et a/. J . A m . Cheni. Soc. 1993. 115. 887. 53. Cava, M. P., Parakka, J. P., Lakshimikantham, M. V. Mat. Res. Soc. Syrnp. Proc. 1994, 328. 179. 54. Parakka, J. P., Cava. M. P. Synth. Met. 1995, 68, 275. 5 5 . Ferraris. J. P., Skiles, G. D. Polvnier 1987, 28, 179. 56. Ferraris, J. P., Andrus, R. G., Hrncir, D. C. J . Cheni. Soc.. Chern. Cornniun. 1989, 1318 57. Ferraris. J. P., Hanlon. T. R. Po/jwer, 1989, 30, 1319. 58. Ferraris. J. P., Newton, M. D. Poljmer 1992. 33(2), 391.

272 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94.

3 Nitrogen-Containing Oligoniers

Naitoh, S., Sanui, K., Ogata, N. J . Chem. Soc., Chem. Commun. 1986, 1348. Chen, L.-H., Wang, C.-Y., Luo, T.-M.H. Heterocycles 1994, 38(6), 1393. ten Hoeve, W., Wynberg, H., Havinga, E. E., Meijer, E. W. J . Am. Chem. Soc. 1991, 113, 5887. Ssrensen, A. R., Overgaard, L., Johanssen, I. Synth. Met. 1993, 55-57, 1626. Ullmann, F. Ber. 1896, 1878. Ullmann, F. Annulen 1904, 332, 38. Fanta, P. E. Chem. Rev. 1946, 38, 139. Fanta, P. E. Chem. Rev. 1964, 64, 613. Fanta, P. E. Synthesis 1974, 9. Barbarella, G., Bongini, A,, Zambianchi, M. Mucromolecules 1994, 27, 3039. Barbarella, G., Zambianchi, M. Tetruhedron 1994, 50(38), 11249. Groenendaal, L., Peerlings, H. W. I., Havinga, E. E., Vekemans, J. A. J. M., Meijer, E. W. Syntk. Met. 1995, 69, 461. van Haare, J. A. E. H., Groenendaal, L., Havinga, E. E. et al. Chem. Muter. 1995, 7, 1984. Groenendaal, L., PhD thesis, University of Eindhoven, 1996. Groenendaal, L., Van Loo, M. E., Vekemans, J. A. J. M., Meijer, E. W. Synth. Commun. 1995, 25(10), 1589. Groenendaal, L., Bruining, M. J., Hendrickx, E. H. J., Persoons, A,, Vekemans, J. A. J. M., Havinga, E. E., Meyer, E. W. Chetn. Mat. 1998, 10, 226-234. Caro, H. Gesellsch. Deutsch. Natucforscher u. Ante: 68. Vers. Frankfurt a. M. 1896, II, 119. v. Bandrowski, E. Monutsh. fiir Chem. 1888, 9, 134. MacDiarmid, A. G. Conjugated Polyniers and Related Materials (Proc. of the 81st Nobel Symposium, Oxford University Press) 1993, 73. Willstatter, R. W., Moore, C. W. Ber. 1907, 40, 2665. Green, A. G., Woodhead, A. E. J . Chenz. Soc. 1910, 97, 2388. Green, A. G., Woodhead, A. E. J . Ciiem. Soc. 1912, 101, 1117. Yoffe, I. S., Metrikina, R. M. Zh. Russ. Fiz.-Chim. Obshch. 1930, 62, 1101. Yoffe, 1. S., Soloveychik, V. Y. Zh. Obshch. Khim. 1939, 9, 129. Honzl, J . . Ulbert, K., Hadek, V., Tlustakova, M. Collect Czech. Chem. Commun. 1965, 19,440. Honzl, J., Tlustakova, M. J . Poly. Sci. C 1968, 22, 451. Honzl, J., Ulbert, K., Hadek, V., Tlustakova, M., Metalova, M. J . Poly. Sci.C 1969, 16, 4465. Liebermann, H. Ann. 1914, 404, 272. Lu, F.-L., Wudl, F., Nowak, M., Heeger, A. J. J . Am. Chem. Soc. 1986, 108, 8311. Strohriegl, P., Jesberger, G., Heinze, J., Moll, T. Makrornol. Chem. 1992, 193, 909. Moll, T., Heinze, J. Synth. Met. 1993, 55-57, 1521. Cao, Y., Li, S., Xue, Z., Guo, D. Synth. Met. 1986, 16, 305. Shacklette, L. W., Wolf, J. F., Could, S., Baughman, R. H . J . Chem. Phys. 1988, 88(6), 3955. Rodrique, D., Domingue, M., Verbist, J. J. Synth. Met. 1993, 55-57, 4802. Zorn, H., Schindlbauer, H., Hammer, D. Monatsh. Chem. 1967, 98, 731. Gjm, N., Gronowitz, S. Actu Chem. Scand. 1971, 25, 2596.

4 Oligomeric Metal Complexes E. C. Constable

4.1 Introduction Transition metal centres offer an obvious way for the introduction of charge centres or charge-carrying centres into new materials. The coordination of metal centres to ligands containing two or more distinct metal-binding domains provides a phenomenological and methodological approach to the assembly of designed novel materials. The limitations of this approach are currently being probed, and this chapter will provide an overview of the area. The emphasis will be upon synthesis rather than the properties of the materials and, in particular, the controlled assembly of materials of known nuclearity and topology. In order to keep this chapter to a reasonable length, the coverage will be restricted in a number of ways. Firstly, materials which are 'truly' polymeric will only be discussed if there are good solid state structural data to support their formulation and structural integrity. Secondly, the emphasis will be placed upon systems in which the ligand predisposes the assembly towards a particular structure. This latter restriction means that little will be said about halide. oxy and hydroxy bridged systems. Furthermore, it will be assumed for the most part that the ligands involved play some r61e in the electronic communication between metal centres and 'insulating' spacers will not be discussed in detail. In particular, the design of extended molecular materials will be discussed. The methods adopted involve the use of bridging ligands containing two or more metal-binding domains. A metal-binding domain is usually readily recognized as a conventional ligand type (for example, a phosphine, carboxylate, thiolate or oligopyridine). The linking together of the domains utilises conventional organic synthetic methods. In an ideal case, simply mixing the bridging ligand with an appropriate metal source, usually, but not necessarily in homogeneous solution, results in the formation of the desired oligomeric system. This process is shown in Fig. 1 [l]. It is immediately apparent that a number of features need to be carefully controlled if the desired assembly process is to occur. The stability of the desired assembly must be maximised in order to drive the system towards the correct (i.e. wanted) product. The simplest way to achieve this is by the use of multidentate chelating ligands rather than monodentate donors. For this reason, systems based upon chloride, oxy or hydroxy bridges are not covered in detail in this article, because, although they possess extremely interesting properties, it is not usually possible, n priori, to predict exactly what material, morphology and topology will be obtained. The metal-binding domain can be tailored to the desired metal centre. This may be at a gross structural level by simply matching up the number of donor atoms to

214

4 Oligomeric Metal Complexrs

OX-ZO

0 metal

t

ccz

I;< metal-binding domain

terminator

a

m E x l initiator

Figure 1. The basic process used in the assembly of oligomeric metal complexes. A ligand with two or more (in this case two) metal-binding domains interacts with complementary metal ions. In the illustrated process each metal-binding domain possess a total of n donor atoms and the preferred coordination number of the metal ion is 2n.

the number of coordination sites available. Thus, a system with connectivity 121 at the metal would have two domains with n donor atoms coordinated to a metal ion with coordination number 2n. In general, a system with connectivity [m]at the metal ion will involve a metal ion, with coordination number mn,coordinated to m domains presenting n donor atoms [ 2 ] . Variations of this type allow the design of the gross morphology of the material, with the commonest procedures involving the use of six coordinate metal centres bonded to domains presenting two or three donor atoms. By a suitable combination of various domains it is possible to approach heteronuclear systems; for example, ligand 1 is designed such that the three nitrogen donors address a six coordinate centre whilst the two oxygen donors target a square-planar four coordinate centre [3]. A more subtle control involves the matching of the donor atoms of the ligand to the coordination requirements of the metal. This is simply achieved by a consideration of the hardness or softness of the donor and acceptor centres. Soft donor atoms such as phosphorus or sulfur are more likely to interact with centres such as palladium(I1) or platinum(I1) whilst harder donors such as nitrogen or oxygen are

X

I

3 X

1

2

PPh,

4

expected to be found associated with harder acceptors such as first row transition metal ions [ 2 ] . Such an approach will ultimately allow the design of heterometallic systems by the use of ditopic bridging ligands incorporating both hard and soft domains. A range of ditopic systems incorporating oligopyridine domains (hard) in combination with phosphine or alkyne domains (soft) have been reported and multinuclear systems derived therefrom described [4]. Examples of such ligands include 2, 3 and 4. In order to ensure some degree of control over the assembly process, it is best to use multidentate metal-binding domains so that the additional stability imparted by the chelate effect ensures the desired coordination mode is adopted [2]. Ligands containing such multidentate domains are widely used for the designed assembly of novel materials. as will be seen later.

4.2 Non-Programmed (Spontaneous) Assembly Although specifically designed ligands are the key to the assembly of specific novel materials, the serendipitous use of potentially bridging ligands has led to an enormous number of potentially interesting polymeric species. In general, such species only maintain their integrity in the solid state and undergo dissociation into smaller units upon solution. Thus, although the self-assembly process leading to the solid-state material may be very efficient, it is not always possible to predict the precise character of the material that will be obtained.

4.2.1 Carboxylate Ligands Carboxylates contain two potential oxygen donors which may interact with metal centres in a number of ways. Of interest to this article, is the mode in which the two oxygen atoms of a single carboxylate are bonded to two different metal centres, with the carboxylate acting as a bridge between them. Enormous numbers of compounds are known with carboxylates acting as bridging ligands between metal centres [ 5 ] . In general the precise nature of the solid state material is not readily predictable in advance. An example of such a polymeric material is zinc benzoate, which has the solid state structure shown in Fig. 2a [6]. It is quite possible to have carboxylates exhibiting a variety of bonding modes in a material, and this is illustrated in the solid state structure of ruthenium(I1,III) benzoate in which the bridging benzoate ligands support both short Ru-Ru bonded interactions and longer Ru . . . Ru non-bonded interactions as seen in Fig. 2b [7]. Although many such materials have interesting properties, their synthesis is still relatively unsystematic. In order to overcome these problems, it is necessary to turn to more organized ligands. The simplest such approach is to incorporate additional donor atoms into the ligand.

276

4 Oligomeric Metal Coinplexes

(b)

Figure 2. Examples of polymeric complexes involving carboxylate ligands (a) zinc benzoate [6] and (b) ruthenium(I1,III) benzoate [7]. In (b), the phenyl groups have been omitted for clarity.

As expected, the introduction of other donor sites into carboxylic acids allows the formation of complexes with a greater degree of organization and a greater stability. Typical examples of such polymeric systems are seen in the 2-pyridinecarboxylate complex shown in Fig. 3a [8] or the 2,2,3,3-tetrafluorosuccinatospecies seen in Fig. 3b [9]. However, although this approach is increasingly being used to prepare novel species, there is still a general failure, in that it is not usually possible to predict the precise nature of the material which will be obtained. Examples of these problems are well-illustrated by the variety of structures formed with dicarboxylic acids as potentially bridging ligands. This is, in part, associated with the variety of bonding modes which can be adopted by carboxylate ligands and ultimately suggests that programmed assembly must involve the use of different donor types and/or the use of less labile metal centres.

(b)

Figure 3. Examples of polymeric complexes involving carboxylate ligands bearing additional donor atoms ( a ) a cobalt(l1) complex of pyridine-2-carboxylic acid [8] and (b) a zinc coniplex of 2.2J.3tetrafluorosuccinic acid [9].

4.2.2 Heterocycles An obvious choice of bridging ligand is a nitrogen donor heterocycle. Such ligands may be prepared in (relatively) facile and usually high-yielding processes which allow a significant variation of structural motifs giving a method for the finetuning of the properties of the final complex. Furthermore, the complexes of such ligands tend to be significantly more kinetically and thermodynamically stable than those with carboxylates. The design of such ligands simply involves the choice of the metal-binding domains and their linking together by conventional synthetic methods. Ligands which are expected to be of use in such an approach include 5-10. As an example, the structure of a linear polymer obtained from silver(1) and 5 is presented in Fig. 4(a) [lo]. To a certain extent, the structure of this material is predictable, since silver(1) is known to form many complexes with a linear two-coordinate geometry. However, it is also true that a vast number of three and four-coordinate silver(1) complexes are known, and that quite minor changes in the reaction conditions result in the formation of different materials. If we develop the methodology of increasing the number of donor atoms such that the coordination possibilities are reduced at a given metal centre, ligands such as 9 and 10 become increasingly attractive. In principle, ligands containing linked 2,2’-bipyridine metal-binding domains may also be proposed, but as we will see later, there is an inherent stereogenic problem associated with the formation of polynuclear complexes from such ligands. A typical example of the use of such ligands is seen in the polymeric species shown in Fig. 4b, which is formed from the reaction of 9 with cobalt(I1)

218

4 Oligomeric Metal Complexes

Figure 4. Some self assembled polymeric complexes involving heterocyclic ligands (a) a silver(1) pyrazine complex [lo] and (b) a cobalt(l1) complex of ligand 9 [l 11.

5

6

7

8

4.2 Non-Progrumnied (Spontaneous) Assembly

279

chloride. This particular material illustrates rather well the problems which are inherent in this area [ll]. The good news is that the complex (of stoichiometry [{(p-C1)2(MeCN)Co(9)Co(CoC14)},,] is formed spontaneously upon mixing the ligand and cobalt(I1) chloride in acetonitrile containing an excess of sodium chloride. However, the compound contains three different types of cobalt centre, none of which exhibits the expected coordination geometry with six nitrogen donors coming from two tridentate 9 ligands. Furthermore, upon dissolution, the polymeric structure is disrupted and a variety of smaller fragments is observed. Nevertheless, the self-assembly approach does have advantages, and ligands based upon structure 10 have been shown to form insoluble polymers upon reaction with iron(I1) or cobalt(I1) salts. In these cases, the insolubility of the material gives some structural integrity, but also means that it is not necessarily simple to determine the average chain lengths in the metallopolymeric complex. Some of the vagaries of self assembly with such ditopic ligands are overcome by the use of kinetically inert metal centres, and will be developed in section 4.3 However, suffice it to say that this approach has been used with considerable degrees of success in the formation of oligomeric ruthenium(I1) and osmium(I1) complexes of ligand 9 [12]. These arrays are of interest from the point of view of their electronic properties together with the photoactivity associated with such d 6 metal complexes.

4.2.3 Other Bridging Ligands In principle the synthetic chemist is only limited by his imagination and ability to prepare the desired precursors in his choice of bridging ligands. Numerous examples of such ligands are to be found elsewhere in this book. Although the emphasis in this chapter is on oligomeric coordination compounds, a number of extremely interesting organometallic or hybrid organometallic-coordination compounds have recently been described. The formation of dendritic systems containing platinum(1V) centres has been described by Puddephatt, with the key step being the oxidative addition of a platinum(I1) species to an organic halide [13]. Such hybrid materials are likely to be of considerable interest in the future. To date, the largest of the dendritic systems contains a total of 28 metal centres (Fig. 5). Although platinum centres are kinetically inert, the above examples are best considered as genuine self-assembly processes because the key dendrimer growth step involves a low energy oxidative addition to a platinum(I1) centre, and not a ligand displacement reaction. Lewis et ul. have investigated other hybrid systems involving the linking together of palladium, platinum or ruthenium centres by polyacetylene ligands which give M-C bonds in the key oligomerization step. In this case, ligand displacement reactions are involved, but the assembly proceeds readily and complexes of this type have been shown to exhibit useful optical properties [14]. Finally, McCleverty et al. have been exploring the use of [(R2HC2N2)3BH], tris(pyrazoly1) borate complexes of molybdenum as building blocks for a variety of materials with novel electronic or optical properties [15].

280

4 Oligonzeric Metal Complexes

Figure 5. A hybrid organometnllic/coordination polymer formed froin the oxidative addition of platinum(I1) complexes to benzyl bromides [ 131.

4.3 Programmed (step-wise) Assembly The above discussion has outlined the principles of self-assembling materials and has, perhaps, emphasized some of the difficulties associated with such systems. These problems are all associated with the control over the nature of the selfassembled material [16]. In particular, the use of metal-binding domains which are not fully complementary to the target metal centres leads to an ambiguity in the number and type of ancillary ligands and associated uncertainties in the topology and topography of the oligomer that is formed. Although these problems are, in part, overcome by the use of polydentate metal-binding domains which are carefully matched to the coordination requirements of the metal centres, there remains an ambiguity concerning the nuclearity of the product. What factors control the growth and the termination of the oligomers? In many cases, the growth of the oligomer is terminated by the precipitation of insoluble material from the solution. However, this usually leads to rather ill-characterized and variable material. In this section we concentrate upon the use of the properties of metal ions to control the assembly of the oligomer and develop a step-wise methodology. Ligand displacement reactions at transition metal centres encompass a vast range of rates, varying between s-' to l o s s p ' . Many of the compounds that we have considered in the sections above have involved 'labile' metal centres such as cobalt(II), iron(I1) or zinc(I1). Indeed, the self-assembly method relies upon the

4.3 Progmr?inied (.step-wise) Assernhly

28 1

relatively rapid displacement of ligands, allowing the rapid attainment of the thermodynamically favoured arrangement of ligands about the metal centres. The disadvantages discussed above spring directly from this high lability of the ligands. By the use of less labile metal centres, particularly those involving transition metal ions with d 6 electronic configurations, it is possible to use a step-wise methodology in which metal-ligand bonds are formed sequentially, and to all intents and purposes, irreversibly. We will concentrate upon metal-binding domains composed of heterocyclic residues.

4.3.1 Bridging Heterocycles - bpy Domains The ligand 2,2’-bipyridine (bpy, 11) is widely encountered in coordination chemistry and it played a formative role in the development of the subject [17]. In view of its well-documented and extensive coordination chemistry with six-coordinate d 6 metal centres, it is not surprising that this metal-binding domain has been incorporated into bridging ligands for the formation of coordination oligomers. This area of research has been further prompted by the useful photochemical and photophysical properties associated with d 6 metal complexes of this ligand, typified by the complexes [ R ~ ( b p y ) ~ and ] ~ +[Os(bpy)$+ [ 171. The earliest studies centred upon species such as 12 and 13 which contain two metal-binding bpy domains. The ligand 12 bears the same relationship to 11 that 9 does to 3,2’: 6’,2’’-terpyridine (tpy, 14). In general, ligands of this type have been used for the formation of dinuclear complexes, and the extension to higher nuclearities is not common, although the synthesis and photochemical and photophysical properties of dinuclear complexes of 12 are extremely well-studied [18]. The basic structures utilize the two didentate N2 domains of the bridging ligand to bind to two octahedral metal centres, each of which maintains an overall coordination

11

12

13

14

282

4 Oligomevic Metal Complexes

1

4i

L

15

number of six by binding two additional didentate N 2 donor ligands; this structural motif is typified by a complex such as [(bpy)2Ru(12)Ru(bpy)2]4+(15)] Similarly, ligands based upon structure 13 have primarily been investigated as components of dinuclear systems; typical examples involve Y = 4-Me; X = 4,4’-(CH2CH2)) [19] and Y = H, X = direct 4,4’-linkage [20]. The formation of dendritic systems involving 12 is likely to be opposed by the proximity of the charged metal centres and the large build-up of positive charge on the pyrimidine rings which are coordinated to two metal centres. The progressive build-up of complexes from a central {M(12)3} unit, which has, in principal three divergent didentate metal-binding domains available for coordination, is also sterically disfavored. Furthermore, such oligomers are likely to be formed with little or no stereochemical control (see later). However, isolated examples of higher nuclearity species are known, including [R~((l2)Ru(bpy)~},]~+ (Fig. 6 ) [21]. The problem of electrostatic

Figure 6. A representation [Ru{(l2)W b P Y 12 1 ,I8+.

of

one

of

the

diastereomers

of

the

complex

cation

4.3 Programmed (step-wise) Assetiihlj,

283

repulsion between the metal centres is likely to be less important with ligands based upon structure 13, in which the use of spacer groups may be used to control the metal-metal distances. However, unless rigid spacer groups are used. there is an ambiguity in the metal-metal distance and indeed in the overall topology of the complex which is formed. In fact, ligands based upon 13 have been shown to be key components of self-assembling helical complexes [22]. In conclusion, the bulk of the chemistry of ligands related 12 or 13 with octahedral &metals i s based upon dinuclear species. The electrostatic repulsion between the metal centres and the charge build-up on the rings is minimised by the use of ligands such as 16-20. Such ligands also serve to reduce the steric interactions between the other ligands which are attached to each metal centre. Although 16 has proved to be of considerable interest in the design of topologically novel complexes, it does not possess an optimal arrangement of metalbinding domains for the formation of extended oligomeric systems and leads preferentially to closed species [23]. Similar comments apply to the extended ligand 17 [24].

N-N 16

17

18

19

20

284

4 Oligomeric Metal Corriplexes

One notable development is the synthesis of a series of ditopic and tritopic ligands consisting of bpy and tpy metal-binding domains linked by alkyne or polyalkyne spacers [25]. The alkyne serves both to control the topographical properties of the complexes as well as providing a conducting molecular framework; the generic ligand structure is presented in 18. Future developments with these ligands promise to be exciting, although recent results suggest that in many cases large ring, but closed, metallomacrocyclic systems are obtained. A range of other bpy-type ligands with rigid-rod spacers which control the distance between metal sites have recently been introduced [26]. Probably the greatest progress in the bpy system has been made with the hybrid ligands 19 and 20. These ligands do not lead to the severe electrostatic interactions expected with 12, and possess the correct divergent topology for the formation of large metallodendritic systems. There is a large body of work in this area, and some representative entries to the literature are given in reference 27. The most spectacular recent work comes from the groups of Balzani, Denti and co-workers and entries to the literature for the Italian work is to be found in reference 28. The basic assembly process involves the stepwise build-up of multinuclear complexes incorporating various numbers of 19 ligands coordinated to ruthenium(I1) or osmium(I1) centres. The simplest growth step involves [M(19)3I2+ as a starting point. This complex contains three non-coordinated ‘bpy’ domains. Reaction of this species with a complex containing two labile chloride ligands, such as [Ru(bpy),Cl,] yields a tetranuclear complex [ M { ( 1 9 ) R ~ ( b p y ) ~ } ~(Fig. ] ~ + 7). However, if [R~(bpy)~Cl,] were to be replaced by [ R ~ ( l 9 ) ~ C lwhich ~ ] , contains two labile ligands and two non-coordinated ‘bpy’ domains, the tetranuclear species would now have a total of six non-coordinated domains in the surface generation which could be further reacted. This approach has been termed the ‘complexes as metals, complexes as ligands’ strategy.

Figure 7. An illustration of the ‘complexes as metals, complexes as ligands strategy’ in action.

4.3 Progrririinied (step-wise) Assembly

285

21

However, there are a number of inherent difficulties associated with this approach. Firstly, the degree of selectivity in addressing particular reactive sites as the metallodendrimer grows larger must be very high - in essence, the growth at each generation must be complete before the next is commenced. For example, in the simplest case, [ R ~ ( 1 9 ) ~ C lcould ,] react with [ R ~ ( 1 9 ) ~ ] "as is desired or it could react with itself to start building up a series of uncontrolled oligomers and polymers. Secondly, a number of isomers are possible at each metal centre. This is easily seen by considering the central metal site of the complexes in Fig. 7. This could form meridional or facial isomers, depending on the spatial arrangement of the pendant pyridine groups. In addition, each metal centre is chiral and may possess a A or A configuration. This means that, in most cases, the high nuclearity metallodendrimers will not, and cannot, be single materials; they will be a mixture of diastereomers and other isomers. Even in the most favourable case, where single spatial isomers are formed at each site a i d the chirality at each later generation site is predetermined by the chirality at the central site a pair of enantiomers is expected. Such a favourable situation is unlikely. These problems have been addressed in various ways. The latest synthetic method uses a protected ligand 21 in which one of the potential metal-binding domains is protected by methylation. The building block [R~(21)~Cl,]'+can only react with [ R ~ ( 1 9 ) ~ ] "and not with itself. Once reaction has occurred, the methyl group may be removed to leave the desired vacant metal-binding domains at the surface generation. Complexes containing up to 22 metals (as of 1995) have been prepared by this method and higher nuclearities are confidently expected. The representative structure of a decanuclear species is shown in Fig. 8. Finally, it should be noted that substantial advances are being made in the design of chirally pure building blocks for incorporation into such systems [29].

4.3.2 Bridging Heterocycles - tpy Domains The difficulties enumerated above led us [ l , 30, 311 and Sauvage [32] to replace bpy metal-binding domains by tpy domains. As we have rehearsed elsewhere, when tpy domains are functionalized on the central ring, {M(tpy),} motifs are formed which are achiral and consist of a single isomer [ 11. We merely note here, that this analysis is not true if the substituent is attached to a terminal ring. The assembly processes that were initially adopted employed the 'complexes as metals, complexes as ligands' method, and this proved to be highly successful

20+

Figure 8. A decanuclear complex formed using a protection/deprotection sequential assembly.

[ 1, 30-321. In general, rod-like linear systems may be prepared by this method, but the introduction of ligands such as 22 allows some structural variation. More recently, the photochemical and photophysical properties of such systems have been tuned by the introduction of photoactive terminator tpy ligands [33] or by the introduction of cyclometallation sites (leading to individual C N N donor

22

1

2+

Co(ll) or Fe(ll)

Figure 9. A convergent approach to heterometallic metallodendrimers ( r n = Co, Fe).

domains) [30,3 1,341. Although the strategy was successful in circumventing the problems associated with the uncontrolled formation of regio- and stereoisomers, there was still a difficulty in addressing precisely the desired metal-binding domain at the correct time. Furthermore, the synthetic approaches tended to be divergent, based upon a stepwise chain growth assembly and, in addition, the desired ligands were not necessarily readily accessible. It is possible to develop convergent synthetic approaches by using labile first row transition metal centres in combination with kinetically inert ruthenium(I1) centres (Fig. 9) and this method partially overcame the problems associated with the divergent synthesis. More recently, a new method has been developed in which the bridging ligands containing the multiple tpy domains are assembled in siru. This avoids the problems of prior ligands synthesis and makes use of the metal centres in activating the coordinated ligands. The initial approach relied upon the attack of a nucleophile upon a coordinated electrophilic tpy ligand (Fig. 10) [35].This allowed the synthesis of new surface generations, but attempts to develop the method further

1

2*

+ .CI

Figure 10. Zri situ ligand assembly allows the assembly of new multi-domain ligands directly at the metal complex.

288

4 Oligorneric MetuI Conipk>xes

23

were unsuccessful; subsequent reactions resulted in the destruction of the inner generations. A combination of the above strategy with one involving the reactions of coordinated nucleophiles with electrophiles allows the synthesis of starburst systems such as 23 [36]. Many other new developments are presented in the articles cited in reference 30.

4.4 Characterization and Properties of New Materials This section is concerned primarily with the species arising from programmed synthesis. The majority of these new complexes have been prepared within the

4.4 CIiiiru terixtioii

iriid

Properties of New Mrrtt~riirls

289

framework of molecular electronics or molecular devices research programmes. However, the synthesis and characterization have mainly been centred within research groups with experience in molecular rather than solid-state materials. The characterization has therefore tended to rely upon ‘conventional’ chemical methods based upon molecular spectroscopic properties. In many cases, primary characterization of oligomeric species has relied upon mass spectrometric studies. Although mass spectrometric methods are wellestablished for organic oligomers and dendrimers, the science is still in its infancy when applied to cationic polynuclear metal complexes. The problems associated with the formation of multiply-charged ions, with associated m / z values, from solid-state species containing cations with large charges has led to a pragmatic approach to mass spectrometric Characterization. If a parent ion is observed, well and good. If not, then other methods of characterization are investigated. It is certainly true to state that electron impact methods of ionization are of very little utility for the characterization of oligomers, and most success has been obtained by the use of FAB (Fast Atom Bombardment) or MALDI-TOF (Matrix Assisted Laser Desorption Ionisation-Time of Flight) methods. The increasing use of electrospray ionization techniques offers hope for the future, but once again the problem of multiply-charged ions makes this technique perhaps not so useful as it might at first seem. As an example, we can consider the nonanuclear complex 24 which exhibited peaks in its MALDI-TOF mass spectrum at i n / : 7881 (Parent -PF6 F) and 7333 (Parent -2PF6 F) together with a variety of other fragmentation ions [36]. The principal problems associated with characterization are in establishing that the species is genuinely monodispersed and to determine the degree of oligomerization. Of course, in an ideal programmed assembly process, there should be no ambiguity! Conventional elemental analysis is of little use for the characterization of metallodendrimers. Firstly, the addition of each subsequent generation has little effect upon the overall constitution of the material and secondly, metal-rich systems are frequently found to undergo only incomplete combustion with the formation of refractory carbides and nitrides. When we began our own studies of metallodendrimers, one of the most surprising features was the fact that even high nuclearity species (18-20 metal centres) with highly charged (+36 to +40) cations could still be characterized by methods more usually associated with small molecules. Thus, higher nuclearity systems related to 24 still exhibited sharp, well-resolved ‘H-NMR spectra in fluid solution. Although the molecular weights of the cations are comparable to polypeptides and small proteins, the molecules still behave as if they are at the fast tumbling limit in solution. This is partly a consequence of the repeat units - each {M(tpy)?}domain can rotate freely around the C-X bonds which link it to the dendrimer and the end effect is a sequence of quasi-independent {M(tpy)?] groups which are freely rotating about the vector between the two 4’-positions of the two tpy ligands. In the case of other systems, the individual repeat domains are not so free and broadened N M R spectra are observed. Of course, solution NMR spectra require materials of moderate solubility, but the majority of the programmed species discussed above are surprisingly soluble in a variety of solvents. In general, the solubility is determined by the counter ion. For example,

+

+

290

4 Oligorneric Metal Cornple.ues

24

chloride salts are soluble in water or methanol, hexafluorophosphates or tetrafluoroborates in acetonitrile, acetone or dimethylsulfoxide and tetraphenylborates in dichloromethane. In the case of the systems incorporating ligands such as 19 which are discussed above, the reported broad N M R spectra may be more a function of the mixture of isomers present rather than an inherent broadening of the spectrum. If the oligomer contains redox-active metal centres, the various electrochemical techniques may be used, in principle to characterize the metal-containing generations. Many examples of this technique applied to ruthenium- and osmium-containing species are discussed in reference 24. By tuning the redox potentials of different metal sites, redox cascades may be built into metallodendrimers. However, there are a number of complications inherent in this approach. For example, compound 25 is expected to exhibit two different oxidation processes, corresponding to the

4.4 Clinrnc.ier.ixiiori arid Properties of' New Marerials

25

M=Ru

26

M=OS

29 1

central metal ion and the outer generation metal centres. The central metal ion is expected to be oxidized at a slight more positive potential than the outer generation by comparison with model mononuclear species. However, a single three-electron oxidation wave is observed corresponding to the concurrent oxidation of the three outer generation ruthenium(I1) centres at 1.53 V (against calomel). The central metal ion redox potential will be perturbed by the outer generation ruthenium(II1) sites and shifted to higher potential. These considerations make the assignment of redox processes rather difficult. Furthermore the mtrs of electron transfer are expected to vary between the generations as the metal centres are progressively further from the electrode surface. In contrast, the heteronuclear complex 26 shows two redox processes. A single electron process corresponds to the oxidation of the osmium(I1) at +1.25 V (against calomel) and is followed by a three-electron process involving the surface generation ruthenium(I1) centres at $1.55 V [37]. Many of the ruthenium and osmium systems discussed have been prepared as components of photochemical machines or photoconversion systems. For a detailed discussion, the reader is referred to reference 24. However, compound 25 typifies these systems and will be used to indicate the basic information which may be obtained from the absorption and emission spectra. After photochemical excitation a molecule may undergo radiative or nonradiative decay, electron transfer or energy transfer processes. The homonuclear

+

292

4 Oligomeric M e t d Complexes

complex 25 has MLCT absorptions at 545 and 422nm. The bpy ligand is a better electron donor than the 19 ligand and the ligand-centred 7r*-levels of 19 lie to lower energy than those of bpy. Thus, Ru(1I) t 19 charge transfer (CT) absorptions will lie to lower energy than Ru(I1) + bpy CT processes. The 422 nm absorption is the Ru(l1) -+bpy CT whilst the 545 nm process is assigned to Ru(1I) + 19 CT. There are two possible types of Ru(I1) + 19, involving the central and peripheral metals. However, as bpy is a better electron donor than 19, the higher electron density on the outer generation ruthenium centres means that the Ru(I1) + 19 CT will occur at lower energy than that involving the central metal. In principle, we expect three absorption processes corresponding to a high energy Ru(II),,,,, + bpy CT, Ru(II),,,,, + 19 and R u ( I I ) , , ~ ~+~ 19 CT processes. In practice, the latter two absorptions overlap. Each of these MLCT excited states could be luminescent. However, regardless of the site of excitation (428 or 545 nm) a single luminescence centred at 721 nm (77K, MeOH, EtOH glass) is observed. This is characteristic of emission from a Ru(II),,,,, + 19 CT state. In this case, efficient energy transfer occurs from the higher energy CT states centred on the inner metal or the outer metal bpy centred states to the lowest energy Ru(II),,,,, + 19 CT state. This behaviour may be contrasted with that of 26 which shows two MLCT bands at 428 and 549 nm. The 428 nm absorption is the Ru(II),,,,, -+ bpy CT process. The 549nm absorption is very broad with a long tail to higher energy. For a given ligand, Os(I1) --+ L CT process lie to lower energy than Ru --+ L transitions. Thus, the 549 nm absorption is assigned to Ru(I1) --+ 19 and Os(I1) + 19 processes, Os(I1) + 19 CT state lying to lower energy. Excitation of this complex at any of the absorption maxima results in luminescence centred at 798nm (77K, MeOH, EtOH glass) which is characteristic of an Os(I1) t 19 CT state. In this case, there is efficient energy transfer from the outer generation ruthenium to the inner osmium centre - exactly the opposite direction to that observed in 25. These concepts are currently being incorporated into larger metallodendrimers to allow the designed synthesis of antenna molecules. It appears that these interesting polynuclear species may be handled and characterized in much the same ways as smaller metal complexes. In the future, methods of materials science for the characterization of nanostructured systems will be applied to the large oligomers.

4.5 Conclusions Throughout this chapter the emphasis has been placed upon the assembly methodology with little or no discussion of the electronic properties of the materials which are obtained. Indeed, many of the examples used in the preliminary discussion of non-programmed assembly involved metal centres which are unlikely to be of any significant application in the design of new electronic materials. However, the aim has been to present the state of the art as far as the synthetic chemist is concerned. Metal complexes bearing suitable functionality are also proving to be of

Rqferences

293

utility, and a variety of systems involving vinyl or thienyl-substituted ligands have been usefully investigated.

References I . E. C. Constable, A. M. W. Cargill Thompson. D. A. Tocher. in Siipramolecular Chernistrjj (Eds.: V. Balzani. L. De Cola), Kluwer, Dordrecht 1992. p. 219; E. C. Constable in Transition Metals iri Suprarnolecular Chemistry (Eds.: L. Fabbrizzi, A. Poggi), Kluwer, Dordrecht, 1994, p. 81: E. C. Constable, D. R. Smith in The Polymeric Materials Encyclopedia. (Ed.: J. Solomon), CRC Press, Boca Raton, 1996, p. 4237; E. C. Constable, A. M. W. Cargill Thompson, D. A. Tocher. Mukromol. Sjwzp. 1994, 77, 219; E. C . Constable, Makrornol. Sjwp. 1995. 98. 503; E. C. Constable, Pirre Appl. Cheni. 1996, 68, 253. 2. For a basic discussion of coordination chemistry see M. Gerloch and E. C. Constable, The Valence Shell in d-Block Chemistry. VCH, Weinheim. 1993. 3. C. A. Howard, M. D. Ward, Angew. Cheni. Int. Ed. Erigl. 1992, 31. 1028. 4. R. Ziessel, D. Matt, L. Toupet. J . Cheni. Soc., Chern. Cornmun. 1995, 2033; M. Hissler. R. Ziessel, New J . Chern. 1995. 19, 751; M. Hissler, R. Ziessel, J . Chern. Soc.. Dolton Trans. 1995. 893; E. C. Constable, C. E. Housecroft, A. Schneider, Unpublished results. 5. C. Oldham in Comprehensive Coordination Chernistrj~,Volume 2 (Eds.: G. Wilkinson, R. D. Gillard, J. A. McCleverty), Pergamon, Oxford, 1987, p. 435. 6. G. A. Guseinov. F. N. Musaev. B. T. Usubaliev, I. R. Amiraslanov. Kh. S. Mamedov, Koord. Khini. 1984, 10, 117. 7. M. Spohn, J. Strahle, W. Hiller Z . Natiirfor.sch.. Teil B 1986, 41. 541. 8. P. Richard, D. T. Qui, E. F. Bertaut. Aeta Crj*stallogr.. Sect.B 1973, 29, 11 11. 9. A. Karipides, Acta Crj~stallogr.,Sect.B 1980. 36. 1659. 10. R. G. Vranka, E. L. Amma, Inorg. Chem. 1966. 5. 1020. 11. E. C. Constable, A. J. Edwards. D. Phillips, P. R. Raithby, Szipr.arnol. Cheni. 1995, 5, 93. 12. R. P. Thummel, S. Chirayil, Inorg. Chirn. Aeta 1988, 154, 77; R. Rusminski, J. Kiplinger, T. Cockcroft. C. Chase, Inorg. Cheni. 1989, 28. 370; C. R. Arana, H. D. Abrufia, Inorg. Chmi. 1993. 32, 194. 13. S. Achar. R. J. Puddephatt. Arigew. Chern. I n / . Ed. Engl. 1994, 33. 847; S. Achar, R. J. Puddephatt. J . Cheni. Soc., Chern. Cornrnun. 1994, 1895. 14. M. C. B. Colbert, D. Hodgson. J. Lewis. P. R. Raithby, N. J. Long, Polyhedron 1995, 14, 2759; C. W. Faulkner, S. L. Ingham, M. S. Khan et a/.. J . Orgariometul. Cheni. 1994, 482, 139; A. J. Hedge, S. L. Ingham, A. K. Kakkar et a/.. J . Organometal. Chem. 1995, 488. 205; M. S. Khan, A. K. Kakkar, N. J. Long et al., J . Muter. Cheni. 1994, 4. 1227; A. Kohler. H. F. Wittmann. R. H. Friend, M. S. Khan, J. Lewis, Synthet. Metul. 1994, 67, 245. 15. A. J. Amoroso. A. M . W. C. Thompson, J. P. Maher, J. A. McCleverty, M. D. Ward, Inorg. Chem. 1995, 34, 4828 and references cited therein. 16. A discussion of the applications of self-assembly processes is to be found in R. Dagani. Cheni. Eng. News 1996, July 8, 26. 17. E. C. Constable, A d v . Inorg. Chem.. 1989, 34. 1. W. R. McWhinnie and J. D. Miller, Adv. Inorg. Cliem. Radiocheni., 1969, 12, 135. 18. See for example: M. Hunziker, A. Ludi, J . A m . Chern. Soc. 1977, 99, 7370; E. V. Dose, L. J. Wilson. Inorg. Chem. 1977, 17, 2660: D. P. Rillema, G. Allen, T. J. Meyer, D. Conrad, Inorg. Cheni. 1983.22, 1617; X. Hua. A. von Zelewsky, Inorg. Cheni. 1991,30, 3798; J. D. Petersen, s. L. Gahan. s. C. Rasmussen and s. E. Ronco, Coord. Chern. Rev. 1994, 132, 15. 19. X. Song, Y. Lei, S. Van Wallendal et al., J . P h j x Chem. 1993, 97, 3225; M. Furue. T. Yoshidzumi, S. Kinoshita et a/.. Bull. Chem. Soc. Jpn. 1991, 64, 1632. 20. A. J. Downard, G. E. Honey, L. F. Phillips, P. J. Steel, Inorg. Cheni. 1991. 30, 2260; M. D. Ward, J . Chem. Soc.. Dalton Tiatis. 1993. 1321; V. Balzani, D. A. Bardwell, F. Barigelletti et a/.. J . Chem. Soc., Dalton Trans. 1995, 3601,

294

4 Oligorneric Metal Conip1e.ue.c

21. R. Sahai, L. Morgan, D. P. Rillema, Inorx. Clieni. 1988, 27, 3495. 22. E. C. Constable, Tetrahedron 1992,4N, 10013; E. C. Constable, Prog. Inorg. Chem. 1994.42, 67; E. C. Constable in Compreliensive Supramolecular Chemistry, Vol. 9 (Ed.: J.-M. Lehn), Pergamon, 1996; D. B. Amabilino, J. F. Stoddart, Cliem. Rev. 1995, Y5, 2725. 23. M.-T. Youinou, N. Rahmouni. J. Fischer, J. A. Osborn, Arigen,. Chem. Int. Ed. Engl. 1992, 31, 733. 24. P. Baxter, J.-M. Lehn, A. DeCian, J. Fischer, Angeiv. Cl7em. Int. Ed. Engl. 1993, 32, 69; 25.

26. 27.

28.

29.

A. Marquis-Rigault, A. Dupont-Gervais, P. N. W. Baxter, A. Van Dorsselaer, J.-M. Lehn, Inorg. Cheni. 1996, 35, 2307. A. Harriman, M. Hissler, R. Ziessel, A. Decian, J. Fisher, J . Chem. Soc., Dalton Trans. 1995, 4067; M. Hissler, R. Ziessel, Neb$' J . Chem. 1995, 19, 751; J . Chem. Soc., Dalton Trans. 1995, 893; V. Grosshenny, A. Harriman, R. Ziessel, Angew. Chem. Int. Ed. Engl. 1995, 34, 1100; R. Ziessel, J. Suffert, Tetrahedron Lett. 1996, 37, 2011; V. Grosshenny, A. Harriman, R. Ziessel, Angrw. Chem. Int. Ed. Engl. 1996, 34, 2705; F. M. Romero, R. Ziessel, A. Dupont-Gervais, A. Van Dorsselaer, Chern. Cornmun. 1996, 551; F. M. Romero, R. Ziessel, Tetrahedron Lett. 1994, 35, 9203; V. Grosshenny, R. Ziessel, J . Cheni. Soc., Dalton Trans. 1993, 817 and references cited in these papers. M. Frank, M. Nieger, F. Vogtle ~t al.. Inorg. Cliitn. Acta 1996,242, 281; L. Decola, V. Balzani, F. Barigelletti et a[.,Rec. Truv. Chini. 1995, 114, 534. J. B. Cooper, D. B. MacQueen, J. D. Petersen, D. W. Wertz, Inorg. Cliem. 1990, 29, 3701; J. D. Petersen, L. W. Morgan, I . Hsu, M. A. Billadeau, S. E. Ronco, Coord. Chem. Rev. 1991,111,319; A. W. Wallace, W. R. Murphy jr., J. D. Petersen, Inorg. Cliim. Actu 1989, 75, 47; K. J. Brewer, W. R. Murphyjr., S. R. Spurlin, J. D. Petersen, Inorg. Chern. 1986, 25, 882; C. H. Braunstein, A. D. Baker, T. C. Strekas, H. D. Gafney, Inorg. Clzern. 1984, 23, 857; D. P. Rillema, K. B. Mack, Inorg. Chenz. 1982, 21, 3849; M. M. Richter, K . J . Brewer, Inorg. Chirn. Acta 1991, 180, 125; D. P. Rillema, R. W. Callahan, K. B. Mack, Inorg. Chem. 1982, 21, 2589. The key entries to the literature are to be found in the references in the following papers: G. Denti, S. Serroni, S. Campagna et d., in Perspectivrs in Coordination Chemistry (Ed.: A. F. Williams, C. Floriani, A. E. Merbach). VHCA, Basel, 1992, p. 153; S. Campagna, G. Denti, S. Serroni et al., Chem. Eur. J . 1995, 1, 211; V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni, CIiem. Rev. 1996. 96, 759. X. Hua, A. Von Zelewsky, Inorg. Ckem. 1995, 34, 5791; D. Zurita. P. Baret, J. L. Pierre, Nett. J .

Chem. 1994, 18, 1 143. 30. E. C. Constable, Chem. Commun. 1996, 1073; E. C. Constable and C. E. Housecroft in Self' assembly in Synthetic Chen?istry (Ed.: J. D. Wuest), Kluwer, Dordrecht; 1997. 31. E. C. Constable, A. M. W. Cargill Thompson, J . Chem. Soc., C/iem. Commun. 1992, 617; J . Chem. Soc., Dalton Trans.1992, 3467; J . Cheni. Soc., Dalton Truns. 1992, 2947; J . Chern. Soc., Dalton Trans. 1994, 1409; E. C. Constable, A. M. W. Cargill Thompson, J . Chern. Soc., Dalton Tran.~.1995, 1615; E. C. Constable, A. M. W. Cargill Thompson, D. A. Tocher, New J . Chem. 1992, 16, 855; Supramol. Chem. 1993, 3, 9; Polymer Preprints 1993, 34, 110; F. Barigelletti, L. Flamigni, V. Balzani et a/..J . Chrm. Soc., Chem. Commun. 1993, 942; Conrd. Clzem. Rev. 1994, 132, 209; J . Ani. Cheni. Soc. 1994, 116, 7692. 32. See references cited in J.-P. Sauvage, J.-P. Collin, J.-C. Chambron et ai.,Chem. Re!,. 1994, Y4, 993. 33. E. C. Constable, A. M. W. Cargill Thompson. S. Greulich, J . Chem. Soc., Chem. Commun. 1993, 1444; E. C. Constable, A. M. W. Cargill Thompson, New. J . Chem. 1996, 20, 65; Supramol. Cher77. 1994, 4, 95. 34. N. Armaroli, V. Balzani, E. C. Constable, M. Maestri. A. M. W. Cargill Thompson, Polyhedron 1992, 11, 2707; E. C. Constable, D. R. Smith, Supramol. Cliem. 1994, 4, 5 . 35. E. C. Constable, A. M. W. Cargill Thompson, P. Harveson ~t ul., Chem. Eur. J . 1995, 1, 360. 36. E. C. Constable, P. Harverson, Clirwi. Commun. 1996, 33. 37. S. Campagna, G. Denti, L. Sabatino et NI., J . Chcvn. Soc., Chmi. Conimun., lY89, 1500.

5 Crystal Structure 5.1 Oligomers as Structural Models for Polymers V. Enkelmann In one of the first papers [I] which described the use of X-ray diffractions methods to characterize polymers and oligomers, Staudinger noted that in a series of oligo(oxymethylenes), regardless of the degree of polymerization, a certain number of strong reflections was always observed at constant scattering angles. These were also independent of the nature of endgroups. Another set of reflections, however, shifted in a regular manner so that a linear relationship between the number of oxymethylene repeat units and the d-spacing was observed. From the slope of this plot a length increment could be calculated by which the unit cell expands upon addition of one monomer unit. Staudinger correctly deduced that in all samples investigated the packing of chains was identical and independent of the degree of polymerization, so that the crystal structure of the polymer could be represented by the structure of few monomer units. The necessity for structural models of polymers is a consequence of the fact that the state of order which can be achieved in many polymers is comparatively low, so that the diffraction patterns often d o not contain enough information to elucidate the polymer structure unambiguously. This is especially the case with polyconjugated polymers which, owing to the rigidity of their backbones, are often not soluble or processable. Knowledge of the structure, however, is of vital importance for the proper understanding of all physical properties. In this context it should be noted that the progress made in the investigation of the properties of one of the most thoroughly studied examples of this class of polymers, poly(acetylene), is marked by modifications of the synthesis which greatly improved the state of order and quality of the samples [2-41. With the exception of certain biopolymers the unit cell of a polymer is quite small, containing only a small number of monomer units, and thus contains no information on the degree of polymerization. This observation, which at Staudinger's time was a matter of great debate, is a consequence of the crystallization mechanism of these polymers in which the crystallite size is normally smaller than the length of the individual macromolecule. The endgroups are buried in the amorphous regions or located in the crystalline part as lattice defects. Since they are not part of a periodic lattice they d o not contribute to Bragg reflections. Thus in crystallographic terms the unit cell of a synthetic polymer can be regarded as a subcell. This concept is well known in the crystallography of substances containing a periodic arrangement of subunits, e.g. polymethylene chains. As a example in Fig. 1 the crystal structure of HCX-(CH2)20-C=CH is shown [5]. It is characterized by a periodic arrangement of subcells. The projection along the chain axis reveals the typical orthorhombic packing found in paraffins and poly(ethy1ene). The subcell packing

296

5 Crystal Structure

Figure 1. Crystal structure of H C d - ( C H 2 ) 2 0 - C ~ C H ,(a) Projection perpendicular to the (CH2), chain. Subcells of paraffin packing are outlined. (b) Projection along the (CH*), chain.

gives rise to a set of strong subcell reflections which are typical for this particular type of packing. The comparison of the diffractograms calculated for the oligomer and poly(ethy1ene) [6] (Fig. 2) reveals that the subcell reflections of the oligomer model are detected at the angles where strong reflections of the polymer appear. In this particular example the match between the model structure derived from the subcell and the polymer is almost perfect as seen by the almost quantitative agreement of the diffractograms of the model and poly(ethy1ene).

5.1.1 Design of Endgroups It is quite clear that the proper choice of endgroups is the most important parameter in the design of useful structural models for polymers. A subcell packing as described above can develop only if the endgroups allow a close lateral approach of the oligomer backbones. Once this goal has been achieved a quantitative comparison between the diffractogram calculated for the subcell model structure and the

5.1 Oligoniers us Structural Models for Poljwers

297

(a) 100

80

60

40

20

0

Diffraction Angle

(b) 35000

30000 25000

20000 15000 10000

5000

I__IJ I i -I

I

I

I

I

I

10

20

30

40

50

28 Figure 2. Powder diffraction diagram of HCrC-(CH2)20-C=CH in comparison to the diffraction of poly(ethy1ene) [ 6] .

298

5 CrystuI Structure

Figure 3. Crystal structure of N-iodoacetyl-amphotericin N-iodoacetyl-amphoterlcln B tetrahydrofuran solvate mono monohydrate lhydrate [71.

polymer will directly reveal the quality of the model which in favorable cases can be used as a starting point for the refinement of the polymer crystal structure. If, however, bulky endgroups are used or the oligomer is only a comparatively small part in a complex molecule often no subcell packing is observed. In this case only information about the bonding geometry of an isolated chain, e.g. bond lengths, bond angles or torsion angles, can be gathered. This information, which in the field of biological macromolecules is called ‘secondary structure’, is, albeit valuable, normally also available with equal precision from other sources, e.g. spectroscopy or molecular modeling. Examples for this behavior are shown in Figs. 3 and 4. In the first an oligoene chain is isolated as part of a larger complex molecule [7], in the second [5] the structure of a simple triene is dominated by endgroup packing, so that no parallel arrangement of the molecules is observed. The packing of the endgroups also influences the way in which different sets of subcells are oriented with respect to each other. This effect is well known in the field of paraffins where the differences in the physical properties of the series of even and odd paraffins are caused by differences in the packing of the terminal methyl groups [8,9]. As can be seen in Fig. 1 the endgroups in oligomers are often arranged in layers. Packing interaction between the chain ends gives rise to a tilt of the oligomer chains with respect to the crystallographic direction in which the molecules are oriented. The number of monomer units which take part in subcell packing and the relative shifts in neighboring oligomer segments depend on this tilt angle.

Figure 4. Crystal structure of 7-phenylheptatrien(?,4.6)-al [ S ]

Of special interest are endgroups which isomorphically replace a chain segment. Isomorphous replacement may be achieved if an endgroup occupies a space of equal volume and size to a chain segment so that it can replace it without changing the periodicity of the subcell. In this situation the whole structure can be described as a periodic lattice of subcells which, as in the polymer, are all aligned in the same direction. Two limiting cases can be considered: first, the endgroups are placed randomly as defect sites in the crystal. Here only the (small) subcell is observed as the unit cell and no information on the randomly placed defect sites is contained in the Bragg reflections. The subcell found for the oligomer is under these conditions identical to the unit cell of the polymer. Although in many synthetic polymers endgroups may be found as defects in the crystalline part, this behavior is rarely observed in oligomer structures. An example in the field of polyconjugated polymers is the structure of oligo(ni-phenylenes), C5H5-(m-C6H4),,-CbH5with 12 > 8 [ 101. The structures consist of helices built up from ten phenylene units which are packed in a tetragonal cell. The helices exhibit a longitudinal disorder such that the gaps between two molecules are randomly placed in the crystal. Thus ni-deciphenyl and m-undeciphenyl crystallize in identical structures which have the same (although better resolved) diffraction pattern as the polymer. Obviously in these systems the length of the oligomer must exceed a given limit so that the defect density is kept low. A higher concentration of chain ends tends to destabilize the structure in a way that endgroups are concentrated in layers: nquinquephenyl crystallizes in a different structure [ I 11.

300

5 Crystal Structure

Figure 5. Crystal structure of the radical cation salt (trans-l,Z-bis(Z-naphthayl)ethene)(SbF6)[ 12,131 showing a random placement of endgroups by orientational disorder.

Another example of this behavior is the random replacement of a vinylene linkage in arylenevinylenes -Ar-CH=CH-Arby two terminal hydrogen atoms -Ar-H. . . H-Ar-. In the crystal structure of the conductive radical cation salt (trans- 1,2-bis(2-naphthyl)ethene) SbF6) [ 12, 131 vinylene linkages and endgroups are distributed randomly so that the resulting structure reveals a polymeric structure of the SbF6 salt and poly(2,6-naphthylenevinylene).Similarly, the same type of disorder is found in the structure of its neutral isomer, trans-l,2-bis( 1-naphthy1)ethene [ 131 modeling the polymer chain of poly( 1,5-naphthylenevinylene)(Figs 5 and 6).

Figure 6. Crystal structure of trans-1 ,Z-bis(l-naphthy1)ethene [13]. The atoms in the vinylene group have half occupancy.

5.1 Oligoruers

NS

Structural Models f o r Po1jvner.s

301

Another isomorphous replacement which has gained some importance in modeling polymer structures is the replacement of a -(CH& unit by two methyl groups. In crystalline ionenes [ 141 chain segments (-(CH2)6-) and two endgroups (-CH2CH3) statistically occupy the same lattice sites. In this polymer salt the counterions are located on fixed positions and the polymer backbone is assumed to obey random walk statistics. Thus this particular type of defect structure gives rise to the formation of perfect, macroscopic single crystals of the polymers; oligomers and polymers form identical crystal structures. The formation of ordered oligomer structures in which the endgroups are located in layers between the chain segments is much more common. Here also the chain ends can be designed, as described above, to occupy a space of equal volume and size as a chain repeat unit. In this case no tilting of the oligomers will be observed and the unit cell of the oligomer is commensurate with the subcells which all line up perfectly. Good models for polymers having (CH2),,subunits in the repeat unit have been derived using this substitution of -(CH2)4 by two methyl endgroups method as described above [ 15- 171.

5.1.2 Structural Families and Types of Disorder of Conjugated Polymers Most of the conjugated polymers can be regarded with good accuracy as flat and rigid chains. Crystal structures built up from such elements always consist of layers where the axes of the chains run parallel. The molecular packing problem may be broken into two parts: the packing in a layer and the packing of the layers. Two families of crystal structures can be derived which differ in the way adjacent stacks of chains are oriented with respect to each other (Fig. 7). In the first type (in the following called ‘oblique structure’) which belongs to the triclinic and monoclinic system the chains in all rows have identical orientation, the second type is the well know herringbone structure in which the orientation of the molecules in neighboring layers alternate. Structures of this type are monoclinic or orthorhombic. If the main chain assumes a helical conformation or if the cross section of the chain becomes cylindrical by substitution of the chain or by rotational disorder both structures degenerate into a third (pseudo)hexagonal packing. From these structural families other types can be derived. In polymers with a rigid backbone which are substituted with long and flexible sidechains (‘hairy rods’) layer structures often develop which originate from the oblique or herringbone type. Here stacks of chains are separated by a layer of sidegroups which may form an ordered structure or can be disordered. Depending on the conformation of the main chain and the type of substitution also cylindrical (pseudo)hexagonal structures are possible in which all backbones are equidistant and isolated from each other by a circular layer of disordered side chains. Packing calculations on several conjugated polymers have shown that the flat and rigid shape of the chains allows the packing in a variety of different structures which do not differ significantly in their energy [18-211. The possibility for polymorphism

302

5 Crystal Structure

Figure 7. Structural types of rigid polymers. (a) oblique structure, (b) herringbone-type structure.

and polytypism must always be considered in all conjugated polymers. This is of special importance since as stated above the state of order in the real sample is often quite low. Intermediates between the oblique and herringbone type can be found in which no regular alternation of chain orientation in neighboring layers is observed. Structures of this type lack orientational long range order and often give rise to diffractograms which are typical for the pseudohexagonal case. Different experimental techniques also influence the results of the structure determination. X-ray scattering samples quite a large volume so that in structures with restricted long range order lattice parameters of an average structure will be determined. Electron diffraction, however, samples much smaller areas and enables the determination of the structure of one or few morphological units which often exhibit a much higher state of order than the average obtained by X-ray scattering methods. In addition, ordered parts of the sample which constitute only a small volume fraction, and are thus not representative for the bulk sample, can be detected and characterized. It should be emphasized, however, that special sample preparation techniquFs are often used owing to the necessity of small sample thickness (< 1000 A) for electron diffraction studies. Thus the possibility of artefacts originating from the sample preparation should always be considered when comparing results from X-ray scattering and electron diffraction. Another source of misinterpretation of scattering data may be the use of powder diffraction experiments without knowledge of the sample morphology and defect

5.1 0ligomer.s us Strircturul Models,for Polymers

303

structure. Many conjugated polymers have a nematic disorder in which the lateral position of chains is fixed but the relative position in chain direction is random. This gives rise to diffuse scattering which is directly evident in fiber diagrams obtained from oriented samples but is frequently misinterpreted as Bragg reflection or 'amorphous halo' in powder diffraction data. Many of the problems discussed above are a consequence of difficulty in obtaining suitable samples. In order to understand the often limited diffraction data available for most of the conjugated polymers good structural models are necessary. The study of oligomer structures is one way to obtain these models. 5.1.2.1 Models for Poly(acety1ene)

Poly(acety1ene) has been regarded in many respects as a model system for the whole class of polyconjugated polymers [22-261. A large number of structural investigations have been performed both on the polymer and its conducting salts and also on oligomers. Regardless of the details of the preparation which may greatly influence most physical properties, poly(acety1ene) forms two isomers. The product which is formed by catalytic polymerization of acetylene, cis-poly(acety1ene) is thermally unstable and spontaneously transforms into the t r a m form when kept at higher temperature. Owing to the instability of cis-oligoenes no good models for cis-poly(acety1ene) or cis-traits mixtures are available. All structural data for cis-polyacetylene suggest an orthorhombic structure of the herringbone-type [27-401. Units cell parameters for rrans-polyacetylenes prepared by different methods are compiled in Table 1. Like in many other conjugated polymers the predominant packing mode for trans-poly(acety1ene) is the herringbone-type structure (Fig. 5.7). Different monoclinic herringbone-type structures for rraiwpoly(acety1ene) have been proposed (e.g. P2,/a and P2,/n) which do not differ significantly with Table 1 Crystallographic data of rruns-polyacetylene.

3.99 7.38 4.18 7.330 4.08 7.37 1.32 7.20 4.24 4.18 7.26 4.20 7.20 7.32 4.095 5.62 3.73

7.29 4.09 7.34 4.090 7.41 4.065 4.24 4.15 7.32 7.34 4.24 7.28 4.15 4.00 7.386 4.92 3.73

2.5 1 2.457 2.455 2.457 2.47 2.45 2.46 2.44 2.46 2.42 2.47 2.456 2.44 2.42 2.457 2.592 3.44

90 90 90.5 90 91.3 90 90 90 91.5 90.5 90 91.5 90 90 92 90 98

1.18 1.165 1.15 1.174 1.16 1.176 1.13 1.18 1.15 1.16 1.14 1.15 1.18 1.22 1.163 1.20 1.27

Pnam P21Ia Pnam P21 /a Pnam

P21/n

32, 33 35, 31 38 39, 40 41 42 43 44 45 46 47 48 49 50 51 52 52

304

5 Crystal Structure

Table 2 Subcells for polyene chains found in oligomers R, -(CH=CH),-R* RI

R2

IZ

a@)

b(A )

c(A)

p(")

D, ( g ~ m - ~ )

Ref.

H Ph Ph t-Bu CH3 CN

H Ph Ph t-Bu COOH CN

4 4 5 6 3 4

7.38 7.50 7.54 8.10 4.03 3.98

4.12 4.41 4.32 4.56 4.07 6.34

2.47 2.48 2.41 2.47 2.44 2.44

97.8 99.1 100.1 92 99 147

1.162 1.067 1.087 0.95 1.09 1.29

54 55 57 56 53 53

respect to the lateral packing but only in a shift of neighboring stacks of chains by half a repeat unit in chain direction. The symmetry of the orthorhombic space group Pnam also suggested for trans-polyacetylene does not provide for bond alternation so that here the structure must be disordered. In view of the comparatively high defect density found in even the best poly(acety1ene) samples these differences although important for the theory of conjugated polymers seem not to be significant for practical purposes. In addition, Monkenbusch et al. [53] have shown that the energy barrier for the translation of a polyacetylene chain in the lattice is small so that the state of order could probably be best described being laterally disordered, a mixture of both models. The lattice of trans-poly(acety1ene) is pseudohexagonal (a: b = &) which is another indication of disorder. Subcells derived from oligoenes which have been studied as models for poly(acety1ene) are compiled in Table 2. Packing arrangements of the herringbone-type which closely resemble the data found with trans-poly(acety1ene) are frequently observed. Since the cross section of the polyene chain is quite small, endgroups of moderate size, e.g. phenyl or tbutyl groups, tend to increase the lateral distances while preserving the packing scheme. The best approximation of the packing in the polymer is found in the structure of trans-l,3,5,7-octatetraene(Fig. 8). A number of oblique subcells found in oligoenes which differ from the herringbone structure are also given in Table 2. Evidence for the existence of a pseudomonoclinic (triclinic) as well as of a second orthorhombic polymorph was found by Lieser et al. [52]by electron diffraction studies on highly oriented samples of presumably small degree of polymerization which had been prepared by catalytic polymerization of acetylene under a high shear field. The possibility of the existence of different polymorphs was emphasized by the fact that the different packing modes of trans-poly(acety1enes) obtained by structural investigation of the polymer or from oligoene model subcells differ very little with respect of their packing energy [53]. Crystal structures of other oligoenes are found in Ref. [59-621.

5.1.2.2 Models for Poly(p-phenylene) The research on poly(p-phenylene) (PPP) evolved in parallel to that on poly(acety1ene). PPP is insoluble and infusible so that, similar to poly(acetylene), progress made in the understanding of the physical properties is marked by the development of synthetic routes leading to polymers with exclusivepara linkages without branching [63-691. For undoped PPP, oligophenyls can be regarded as the ideal model

5.1 Oligomers as Structuml Models f o r Polwiers

305

. . . . . . . . . . . . . . . . . .. . . . .

.................. .....

Figure 8. Oligoene packing in model structures. (a) rruris-l.3,5,7-octatetraene[54],((i) unit cell and (ii) chain packing). (b) trans-diphenyloctatetraene[%I, (c) trrnzs-di-t-butyl-dodecahexaene [56].

306

5 Crystal Structure

Figure 8. (Cont.).

compounds in which only minor endgroup effects are expected owing to the small volume of the terminal H atoms. Unfortunately, the solubility of oligophenyls decreases rapidly with chain length so that until recently crystal structures were available only for the lower oligomers up to quaterphenyl. In an excellent study Farmer et a/. [70] have been able to extend this series to septiphenyl. All oligophenyls pack in similar structures with molecular organization of the herringbone type. Interaction of the terminal H atoms gives rise to a tilt of the molecular long axis with respect to the longest crystallographic axes. This tilt angle shows a marked odd-even effect. The crystal structure of sexiphenyl is shown in Fig. 9. Farmer et a/. have also addressed the important question of the conformation of the polymer backbone in PPP. According to this study PPP crystallizes as all oligophenyls at room temperature in a planar conformation. For the lower oligomers (biphenyl, terphenyl, quaterphenyl) it is well established that this planar conformation is the average of two twisted forms which can be represented by a shallow

Figure 9. Crystal structure of sexiphenyl. Projections of (a) the unit cell and ( b ) phenylene chain packing.

308

5 Crystal Structure

b)

I I I I Figure 9. (Cont.).

double-well potential thus allowing rapid vibrational motion at room temperature [71-731. At lower temperatures phase transitions into low temperature phases of the twisted form are observed. Farmer et al. have observed similar transitions for the higher oligophenyls but the crystal quality was not sufficient to allow full structure analyses. Isolated oligophenyls are non-planar owing to the steric repulsion of the orthohydrogens on adjacent rings [74,75]. According to Farmer et al. the planarity of the higher oligophenyls and PPP results from the overcompensation of the loss of conformational energy in the planar form by a gain of packing energy which strongly favors the planar chain in the crystalline lattice. In several other studies [76-791 a non-planar structure of PPP in which adjacent rings of PPP are alternately rotated by 20" has been deduced from the analysis of diffraction data. These differences are still unsolved and may be largely attributed to differences of sample preparation and defect structure and do not affect the overall packing scheme found in PPP. 5.1.2.2.1 Substituted Poly(p-phenylenes) Substitution induces a torsion of the phenyl rings in the PPP backbone. A number of substituted oligophenyls are found in the literature [80-891. Most of these structures have not been performed to model the polymer so that in most cases the substitution patterns in the oligomers do not correspond to polymers of interest so that from the

data only trends for the torsion of phenyl rings can be obtained. Oligomers with flexible side chains which can serve as models for 'hairy rod' polymers will be discussed in some more detail in a later section of this review.

5.1.2.3 Models for Polythiophene Since oligothiophenes have gained importance as materials for possible applications in the field of molecular electronics [90-931 a number of crystal structures of oligomers are available. As a common feature most oligothiophenes possess an all-anti configuration which avoid steric interaction of the a-hydrogen atoms of adjacent rings. Unsubstituted oligothiophenes are planar with a fully conjugated carbon skeleton as evidenced by a pronounced bond length alternation. Similar to oligophenyls insufficient solubility hampers the crystallization of the higher oligomers. Despite these difficulties reliable crystal structures of quaterthienyl, quinquethienyl and sexithienyl have recently been determined from X-ray powder data using the Rietveld method of analysis [94]. The predominant packing found in this series as well as in methyl substituted thiophene oligomers [95] is of the herringbone-type. This packing is shown for two representative structures in Fig. 10. The packing found in the oligomers can be regarded as good model fof undoped polythiophene for which a rectangular cell with a = 7.81 A and b = 5.53 A is found [177]. Similar to PPP polythiophene exhibits a nematic disorder of the translation position of the chains.

5.1.2.3.1 Substituted Polythiophenes By substitution with alkyl side chains polythiophene becomes readily soluble in common organic solvents. A large amount of research has been devoted to both the properties of these solutions and the structure of the semicrystalline material. Although in most cases the diffraction data are not well resolved a number of detailed models have been proposed in the literature [96-1011. Most of them can be characterized as layer structures in which stacks of chains are separated by layers containing the sidegroups. Structural data are available for a number of substituted oligothiophenes [ 102- 1 101. As expected, substitution in the center of the oligomer chain tends to induce a twist. In most cases the twist angles are comparatively small in the center of the molecule and increase at the end of the chain. In most cases the all-miri conformation is retained. Two structures of alkyl substituted thiophene oligomers [102,103] are shown in Fig. 11. A large number of substituted bithienyls have been studied [111-134]. They contain no information regarding the packing of the polymer chain and will not be discussed in detail here. For a given substitution pattern twist angles between thiophene rings may be obtained from these data. 5.1.2.4 Polypyrrole In contrast to PPP or polythiophene which are quite stable in the updoped state, polypyrrole is extremely unstable under ambient conditions owing to its low oxidation potential. Polypyrroles are normally salts containing negatively charged

310

5 Crystal Structure

Figure 10. Crystal structure of two oligothiophenes. (a) sexithienyl [94], (b) 5,5”’-dimethyl2.2’ : S’,”’ : S”,2’”-quaterthiophene [9S]. Projections of (i) the unit cell and (ii) phenylene chain packing.

Figure 10. (Coiir.)

counterions. Most of these samples are amorphous. Certain counterions with a high tendency of self-aggregation are able to induce a certain (low) degree of long range order [135-1371. It is not clear whether this is the consequence of lack of steric purity which could be overcome by improved synthetic procedures or is an inherent property of polypyrrole salts. The expected molecular geometry of the unsubstituted polypyrrole chain is very much like polythiophene a flat and rigid chain in the allunfi conformation. Owing to the chemical instability of even small pyrrole oligomers. no reliable data on this class of materials were available until recently. Martina was able to prepare monodisperse structurally perfect oligo(2,5-pyrroles) for n = 2 . . . 15 and polydisperse polypyrrole [ 138- 1401. The synthetic route used Pd-catalyzed coupling of pyrroles which had been stabilized by protection of the nitrogen with f-BOC groups. The protected oligomers are highly twisted with dihedral angles between

312

5 Crystal Structure

b(ii)

Figure 10. (Cont.)

the rings ranging from 65 to 85" and assume a helical conformation (Fig. 12). Full crystal structure determinations on single crystals of two trimers prepared by deprotection could be carried out. Owing to solubility and stability problems the pentamer was obtained as a microcrystalline powder. The structure of this oligomer could be derived from the powder diffraction data. The crystal structures of the pyrrole oligomers are shown in Fig. 13 in comparison. The structures of a dimer and one of the trimers have been previously reported by Street et al. [141] but no data were given in the paper. In all three structures the identical orthorhombic herringbone-type subcell packing is present which closely resembles the polythiophent structure. The lattic? parameters for this polypyrrole model as a = 5.32 A, b = 7.42 A, c = 7.09 A (chain repeat). On the basis of packing calculations Trip$thy et nl. [201]have deriyed structures of lowest energy for polypyrrole (a = 7.32A, b = 5.64A, c = 7.21 A) and polythiophene which are very close to the subcells derived from oligomers.

Figure 11. Crystal structures of substituted oligothiophenes showing the torsion of the backbone. (a) 3’,3’”’.4’.4’’”-tetrabutylhexathione[ 1021, (b) 4’,3”’-di-n-butyl-2,2’ : 5‘2’‘ : 5”,2”’ : 5”’.2”’’: 5’”’,2’’”’sexithiophene [103].

5.1.2.5 Models for PPV

From the viewpoint of molecular structure poly( p-phenylene vinylene), PPV, can be regarded as a copolymer with an alternating sequence of acetylene and phenylene units. PPVs are readily available in good quality and high molecular weights and have the advantage over poly(acety1ene) of an improved environmental stability. Several precursor routes have been developed which allows the production of highly oriented samples by preorientation of the precursor polymer [ 142- 1451. Similar to poly(acety1ene) and PPP, conductive salts of PPV are available by partial

t

&

Figure 12. Crystal structure of a protected pyrrole pentamer.

314

5 C r j , . m l Striicturc

Figure 13. Crystal structures of oligopyrroles. Projections of (a) the unit cells and (b) chain packing.

5.1 Oligorners us Structural Modelsfor Polwiers

315

Figure 13. (c)

oxidation or reduction, Extensive studies have been performed on the alkali salts obtained by reductive doping [146, 1471. Much of the current interest in PPVs is concerned with the electro-optical properties of the undoped state, e.g. the application in light emitting diodes. Substitution of the polymer backbone is used to increase the solubility and processability or to tune the optical properties. Pristing unsubstitutFd PPV crysdallizes in a monoclinic herringbone packing with a = 8.15A, b = 6.07A, c = 6.60A, (Y = 123" [144, 145, 1481. Most of the oligomer models for PPVs are quite small so that in these cases only information about the geometry of the isolated chain can be gathered [149- 15 11. It is assumed that the polymer backbone is not strictly planar but twisted by approximately 10" owing to the steric hindrance between the hydrogen atoms on the phenyl rings and the vinyl group. A deformation of the same order of magnitude is already

3 16

5 Crystal Structure

present in the shortest oligomer, trans-stilbene [152]. Unit cell parameters for a number of substituted oligomers containing three rings have been determined with the aim to correlate electrical conductivity to the n-electron density and substitution effects [153]. A longer model containing five phenyl rings of which the central ring was substituted by two flexible side chains has been studied recently. The molecule is comparatively planar and forms a layer structure [154].

5.1.2.6 Models for Poly(ani1ine) In comparison to other conductive polymers where the generation of charge carriers is achieved by redox reactions, poly(ani1ine) is a more complicated system. Here two independent processes are involved in the formation of the conductive salt. First, by oxidation of the leucoemeraldine base (LEB) chain segments of the poly(parapheny1ene imine) structure are formed. At approximately 50% oxidation the resulting polymer is called emeraldine base (EB), the fully oxidized form is pernigraniline base (PBN). All three pure forms are insulators. Conductive salts are prepared by partial oxidation of LEB, partial reduction of PBN or protonation and reduction of EB. The chemistry of poly(ani1ines) is reviewed in detail by MacDiarmid [I 55- 1591. As a consequence a variety of different conductive poly(ani1ine) salts are formed. Crystal structure and state of order depend on the sample history and the doping process used. Structural models for different polyanilines have been proposed by Pouget and Epstein [ 160- 1621. There is only one crystallographic study of oligomers modeling the poly(ani1ine) chain in its various oxidation states. Shacklette and Baughman [I631 have investigated ‘phenyl end capped’ oligomers. Electrochemical oxidation of the tetramers with subsequent crystallization yielded crystals of the dications with BF; and ClO, counterions in which each nitrogen atom is protonated. The oxidation state in these structures is thus similar to the emeraldine salt. In addition, the dimer was obtained by oxidation in the neutral imide form. The molecular geometry is discussed in some detail and all bond lengths, bond angles and torsion angles agree with the values expected for the different oxidation states. Unfortunately the full details of the crystal structure analyses are not given. It is assumed that no information on the packing of oligomer chains beyond the conformation of the isolated chain can be derived from the data. A number of substituted phenylanilines are also found in the literature from which the geometry of the corresponding leucoemeraldines may be obtained [ 164- 1661. 5.1.2.7 Models for Conductive Polymer Salts

Many conjugated polymers can be transformed by oxidation or reduction in the solid state to derivatives which exhibit metal-like conductivity. By the redox reaction charge carriers are generated on the polymer backbone which are counterbalanced by counterions. Thus conductive polymers are salts built up by polymer chains bearing charges and (usually low molecular weight) counterions. The term ‘doping’ which has been applied to this reaction is misleading in that it suggests that concentration electronic properties are gradually varied by a change of a dopant (i.e. the

S.1 Oligoniers as Structiirril Models for Poljwers

317

counterion). However, it has been shown for many conductive polymers that the doping process gives rise to a series of salt structures with fixed composition and crystal structure ('staging') [49, 146, 1471. In some cases reversible phase transformations between different salts are observed. In most cases the doping proceeds heterogeneously by reaction of the insoluble polymer with the redox reagent or electrochemically with the polymer as electrode in an electrochemical cell. The comparatively high stability of conducting polymer salts probably results from the inaccessability of the unstable charge carriers in the rigid matrix of the polymer salt. In solution the redox states produced by the doping processes are usually quite unstable. This is why not many oligomer models for conducting polymers have been studied. By reduction of oligophenyls with alkali metals in polar solvents, crystals of the alkali salts of radical anions can be obtained [167]. However, here the metal ion is solvated by the solvent. The packing interaction in the oligomer salts is different from the polymers. The alkali salts of conjugated polymers are channel structures in which the metal ions are found unsolvated in channels built up by polymer backbones [49,146,147]. Thus this type of oligomer salt does not provide useful information on the packing in the polymers. Stable radical cation salts of oligophenyls can be prepared by anodic oxidation of solutions of the arene in an inert solvent, e.g. CH2C12,in the presence of a suitable supporting electrolyte, e.g. N B u ~ X -(X- = BF,, Cloy, PF;, AsF;, SbF;). The electrochemical processes which lead to the growth of aromatic radical cation salts can be split into three independent reactions: Aro + AR'+

+e

+ Aro (Ar2)'+ (Ar2)'+ + X- + Ar,X Ar"

(2)

(3)

The key reaction in the electrocrystallization is the dimerization [Eq. (2)] in which the short lived intermediate monomer radical cation is stabilized in a dimer. The equilibrium constant of this reaction determines not only the stability of the reaction intermediate but also the kinetics of crystal growth and the composition of the crystals. Radical cation salts for a variety of aromatic hydrocarbons have been prepared and characterized [ 168- 1741. The resulting crystal structures are columnar structures in which stacks of the arenes (e.g. naphthalene, pyrene, fluoranthene, perylene) are packed in a fashion leaving channels in which the counterions (and in more complex cases also solvent molecules) are located. Metal-like conductivity is observed in the stack direction. It has been proposed that the interaction found between the aromatic rings in the radical cation salts can be regarded as a model for interchain interactions in conducting polymers. The concept which uses the structural principles found in radical cation salts to construct models for conductive polymer salts is illustrated in Fig. 14. In a polymer the stack-forming elements are part of the main chain. Consequently the structural models derived from the packing in aromatic radical cation salts are intercalation structures in which layers of polymer chains and counterions alternate.

318

5 Crystal Structure

0

:Areno

0

x-

OQ

Pa‘ymer Segment

Figure 14. Analogy of the packing in (a) radical cation salts and (b) conducting polymer salts

Radical cation salts of various oligophenyls have been prepared as models for conducting PPP salts [173-175]. The structure of a quaterphenyl salt, QP3 (QP)(SbF&, has been studied in detail. Two projections of its crystal structure are shown in Fig. 15. As predicted by the model considerations the structure consists of layers in which QP molecules are stacked which are separated by rows of counterions. This layer contains also an additional (neutral) Q P molecule. End group packing effects cause a tilting of the layers with respect to the long crystallographic axis. The packing arrangement found in the dashed subcell was used to construct a model for the PP salt (Fig. 161. Both the unit cell dimensions (orthorhombic, a = lO.OA, b = 6.6A, c = 16.8A) and the composition (C6H4)*(SbF& were used without further adjustment and reproduced the experimental diffraction data of a saturation doped AsF6 salt with 40% conversion per phenyl ring almost quantitatively [176].

5.1.2.8 Models for ‘Hairy Rod’ Polymers Polymers with a rigid backbone and flexible side chains are of special interest for many applications since the side chains provide solubility in common organic solvents. This allows the fabrication of samples suitable for applications in the field of thin film microelectronic and optoelectronic devices [ 177- 18 11. In most cases the rigidity of ‘hairy rod’ polymers is the conjugation of the main chain although also other polymers fall in this general class, e.g. helical polyaminoacids (e.g. substituted polyglutamates) or poly(phthalocyaninatosi1oxane)s. The resulting structures depend on several parameters, among others sample preparation (e.g. LB

5.1 Oligomers ns Structural Modelsfor PoIjwiers

3 19

Figure 15. Crystal structure of the radical cation salt QP3 (QP)(SbF,),. (a) Projection in the stacking direction, (b) projection perpendicular to the stacks.

a

b

@ J4

Figure 16. Structural model for conducting PPP salts.

320 a

5 Crystal Structure

b

Figure 17. Schematic plot of different types of hairy rod macromolecules. (a) layered structures, (b) columnar hexagonal packing.

technique, spin coating), conformational flexibility of the main chain, substitution density and length and chemical nature of the side chains. Two types of packing have been reported for conjugated polymers with flexible side chains, as displayed in Fig. 5.17: layered structures consisting of the polymer backbones separated from each other by the side chains are favored if linear, long alkyl-chains are attached to the polymer backbone [182] (Fig. 17a), whereas short

b

Figure 18. Structure of isopentyloxy substituted oligomers along the main chain. (a) layered structure of a trans-stilbene model, (b) structure of a substituted terphenyl.

5.1 0ligornrr.s us Striccrrrrul Models,for Poljwwrs

32 1

and branched side chains force the system into a hexagonal packing with an isotropic distribution of the side chains around the backbone [183] (Fig. 17b). A cylindrical superstructure of the polymer will induce hexagonal packing. If the conjugated backbone tends to be in a planar conformation, the backbones will prefer a layered structure as shown in Fig. 17a. The local conformation of the macromolecule, such

Figure 18. (Corir.)

322

5 Crystal Structure

as the torsion angle between adjacent phenylene units, therefore strongly influences its packing and consequently its optical and electronic properties. Although the packing of conjugated polymers turns out to be a major relevance for the interpretation of the optical and electrical properties, very few X-ray investigations of single crystals of model compounds in which the substitution pattern matches exactly that of the polymer have been reported. The reason for this is probably the difficulty of crystallizing oligomers with high substitution density and long substituents [ 1861. A series of oligomeric model compounds for PPP, PPV and poly(p-phenylene ethynylene) which are substituted with two isopentyloxy side chains on each phenyl ring have been studied recently [184]. The polymers with relatively short branched sidegroups are of interest in applications of polymeric light emitting diodes [ 185,1861. It was found that in the oligomer stibenes and tolanes the conformation of the main chain is planar. This allows a packing arrangement in a layered structure according to Fig. 17a. However, steric hindrance induces a pronounced twist of 60" in the PPP oligomers. Here a packing in which all main chains are separated from each other by a layer of side chains is observed. Projections of the two structure types are shown in Fig. 18 for two examples. The differences found in the packing of the planar and twisted oligomers explain why in the substituted PPPs intermolecular x-71 interactions are far less important for the electro-optical properties [185].

References If applicable the reference code of a crystal structure in the Cambridge Structural Database (CSD) is given, e.g. AMPIABlO 1. H. Staudinger, H. Johner, R. Signer, G. Mie, J. Hengstenberg, Z . Phys. Chem. 1927, 126,425. 2. T. Ito, H. Shirakawa, S. Ikeda, J . Polym. Sci., Polym. Chem. Ed. 1974, 12, 11. 3. W. J. Feast, D. Parker, J. N. Winter, D. C. Bott, N. S. Walker, in Electronic Properties oj'Polymers und Reluted Compounds (Eds: H. Kuzmany, M. Mehring and S. Roth) Springer Series in Solid-state Science, Vol. 63, Springer, Heidelberg, 1985, p. 45. 4. H. Naarmann, N. Theophilou, Sq'nth. Met. 1987, 22, I . 5. V. Enkelmann, unpublished results. 6. G. Avitabile, N . Napolitano, B. Pirozzi, K . D. Rouse, M. W. Thomas, B. T. M. Willis, J . Polymer Sci.. Polym. Lett. Ed., 1975, 13, 351. 7. AMPIABlO N-Iodoacetyl-amphotericin B tetrahydrofuran solvate monohydrate derivative of antifungal agent C49 H74 I1 N 1 018,3(C4 H8 OI), H2 01. P. Ganis, G. Avitabile, W. Mechlinski, C. P. Schaffner, J . Am. Chem. Soc. 1971, 93, 4560. 8. R. Boistelle, in Current Topics in Muteriul Scimce (Ed: E. Kaldis) North-Holland Publ. Co, Amsterdam, 1980, p. 104. 9. K . Sato, M. Kobayashi, Crystuls Vol 13, Springer, Heidelberg, 1981, p. 65. 10. D. J. Williams, H. M. Colquhoun, C. A. O'Mahoney, J . Chem. Soc., Chem. Commun. 1994, 1643 11. HAXRED P. W. Rabindeau, A. Sygula, R. K. Dhar, F. R. Fronczek, J . Chem. Soc., Chem. Comm., 1993, 1795. 12. J. D. Stenger-Smith, R. W. Lenz, V. Enkelmann, G. Wegner, Makromof. Chem. 1989, 190, 2995. 13. J. D. Stenger-Smith, R. W. Lenz. V. Enkelmann. G. Wegner, Mukromol. Chem. 1992,193,575.

Referrtiers

323

109 L. Dominguez. V. Enkelmann, W. H. Meyer, G. Wegner Pol~wier1989, 2030. Blackwell. H. Nefzger. H. Hayen, C. D. Eisenbach, V. Enkelmann, Makroniol. Cheni., 1994, 195, 3325. C. D. Eisenbach. E. Stadler. V. Enkelmann. M ~ e r o n i o l Cliern. . P/j>s. 1995, 196, 833. V. Enkelmann. G. Lieser, M. Monkenbusch, W. Muller, G. Wegner, Mol. C r y t . Liq. Crj,st. 1981, 77. 111. 19. V. Enkelmann. M. Monkenbusch, G. Wegner. Polynier. 1982, 23, 1581. 20 . G. Lieser, M. Monkenbusch, V. Enkelmann. G. Wegner, Mol. Crvst. Liq. Cryst. 1981. 77, 169. 21 . B. J. Orchard. B. Freidenreich, S. K. Tripathy. Poliwier. 1986, 27. 1533. 33 S. Curran. A. Stark-Hauser. S. Roth, in Htnirlhook of'Orgrrriic Conthrctive Molecirles cnid P u l p -... I I I ~ Vol. , 2 (Ed: H. S. Nalwa) Wiley, New York 1997. p. 1, and reference cited therein. 23 E. P. Goodings, C/ieni. Soc. Rrv. 1976, 5. 95. 24 G. Wegner, Angeu,. Cheni. h i t . €d. €rig/. 1981. 20. 361. 25 Hotidhook qf'Conductirig Pol~~tiiers (Ed: T. A. Skotheim) Marcel Dekker, New York, 1986. 26 Cotzji/gmd Poljwiers (Ed: J. L. Bredas, R. Silbey) Kluwer Academic Publishers, Dordrecht, 1991. 27 R. H. Baughman. S. L. Hsu, G. P. Pez. A. J. Signorelli. J . Chrni. P h j x 1978. 68. 5405. 28 R. H. Baughman, S. L. Hsu. J . Poljwi. Sci.. Pol~wi.LPif. E d 1979. 17, 185. 79 G. Lieser. G. Wegner. W. Muller. V. Enkelmann, Mrikromol. C/ieni.,R q i d Coriumoi. 1980. I , 671. 30 J. C. W. Chien. F. E. Karasz, K. Shimamura, J . Polwi. Sci.. Poljwi. Lett. Ed. 1982, 20. 97. 31 J. C. W. Chien. F. E. Karasz, K. Shimamura. Macroniolecirles 1982, 15, 1012. 32 C. Riekel. H. W. Hisslin, K. Menke, S. Roth. J . C / i m . P/q3s. 1982, 77, 4254. 33 H. W. HBsslin, C. Riekel, K. Menke, S. Roth, Mrrkroniol. Chetn. 1984. 185, 397. 34 D. White, D. C. Bott. R. H. Weatherhead, Polwier 1983. 24. 805. 35 G. Perego, G. Lugli, U . Pedretti, E. Cernia. J . P h ~ s .iPcrris) Colloq. 1983. 44, C3-93. 36. Y. Yamashita, S. Nishimura, K. Shimamura, K. Monobe, Makroniol. Cheni. 1986, 187. 1757. 37. G. Perego. G. Lugli. U . Pedretti, Mol. C y w . Liq. C r y s ~ 1985, . 117%59. 38. H. Kahlert, 0. Leitner. G. Leising, Sjwtli. Met. 1987. 17, 467. 39. G. Perego, G. Lugli, U. Pedretti, M. Cesari, Mnkroniol. Climi. 1988, 189. 1657. 40. G. Perego, G. Lugli, U . Pedretti, M. Cesari. Makroniol. Clieni. 1988, 189, 2671. 41. R. H . Baughman, G. Moss, J . C/imi. Ph.13. 1982, 77. 6321. 42. T. Akaishi. K. Miysaka. K. Ishikawa. H . Shirakawa, S. Ikeda. Rep. Prog. Polwi. P h ~ vJapan . 1979, XXII, 175. 43. K. Shimamura. F. E. Karasz, J. A. Hirsch. J. C . W. Chien, M ~ k r o n i o lClieni., . RapidConin~irii. 1981, 2, 473. 44. G. Wegner. M. Monkenbusch, G. Wieners. R. Weizenhofer. G. Lieser. W. Wernet, M u / . Ci.jjs/. Liq. Crj*st. 1985, 118, 85. 45. C. R. Fincher Jr.. C.-E. Chen. A. J. Heeger. A . G. MacDiarmid, J. B. Hastings, Pliys. Rev. Let/. 1982, 48. 100. 46. H. Kahlert, G. Leising. Mol. Crvst. Liq. C r y r . 1985. 117, 1. 47. Y. B. Moon. M. Winokur. A. J. Heeger, J. Barker. D. C. Bott, Mrrcrornolt~c~ules 1987.20.7457. 48. M. M. Sokolowski, E. A. Marseglia. R. H. Friend. Poljw7er 1986. 27, 1714. 49. C. Mathis, R. Weizenhofer, G. Lieser, V. Enkelmann, G. Wegner, Makr.on7ol. Chetn. 1988, 189. 1617. 50. K. Shimamura. Y. Yamashita. F. Yokohama e f d..P o l w e r 1989, 30, 435. 51. Q. Zhu, J. E. Fischer, R. Zuzok. S. Roth, Solid Stcite Coninurn. 1992. 83, 179. 52. G. Lieser, G. Wegner. W. Muller, V. Enkelmann. W. H. Meyer, Makrornol. Clienz. Rapid CO~litJll~~?. 1980. 1. 627. 53. V. Enkelmann, M. Monkenbusch, G. Wegner. Poljwier 1982, 23, 1581. 54. DERWOM R. H. Baughman, B. E. Kohler, I. J. Levy. C. Spangler. Sjxth Met. 1985, I ! , 37. 55. DPHOCE W. Drenth. E. J. Wiebenga, A c t r i Crjwdlogr. 1955, 8, 755. DPHOCEOI W. Drenth, E. H. Wiebenga, Re(,.Trav. Chim. Pa~s-Brrs1953, 72, 39. DPHOCEOI R. H. Baughman. B. E. Kohler. 1. J. Levy, C. Spangler. S w t h . M e / . 1985, 11.37. 56. PATNUT A. Kiehl. A. Eberhardt, M. Adam, V. Enkelmann, K. Mullen, .4ngew. C / i m . 1992, 104, 1623. 14. 15. 16. 17. 18.

324

5 Crystal Structurt’

57. R. H. Baughman, S. L. Hsu, L. R. Anderson, G. P. Pez, A. J. Signorelli, N A T O Conference Series 1979, 6 , 187. 58. DTHOCT truns-l,X-Di(2-thienyl)-I,3,5,7-octatetraene C16 H14 S2. J. F. Buschmann, G. Ruban, Eur. Cryst. Meeting 1979, 5 , 105. 59. EBHEPT Ethyl 7-p-bromophenyl-2,4,6-heptatrienoate C15 H 15 Brl 0 2 . L. G. Vorontsova, B. Tashkhodzhaev, Z h . Strukt. Khim. 1975, 16, 420. 60. BPZHTR 1-(p-Bromophenyl)-6-benzoyl-hexa-l,3,5-triene C19 HI5 Brl 0 1 . L. G. Vorontsova, A. 1 . Isakova, Zh. Strukt. Khim. 1974, 15, 99. C20 H20 0 2 . 61. SAXLOS trans-l,6-bis(o-Methoxyphenyl)-1,3,5-hexatriene orthorhombic form C18 H 16. ZZZQNKO2 trans-1,6-Diphenyl-1,3,5-hexatriene T. Hall, S. M. Bachrach, C. W. Spangler e t ul., Acta Cryst. C (Cr. Str. Comm.) 1989,45, 1541. 62. TDTHTR trans-1,6-Di(2-thienyl)-1,3,5-hexatriene C14 H I 2 S2. J. F. Buschmann, G. Ruban, Acta Crystnllogr. B 1978, 34, 1923. 63. J. G. Speight, P. Kovacic, F. W. Koch, J . Macromd. Sci., Rev. Macromol. Chem. C 1971, 5 , 295. 64. G. K. Noren, J. K. Stille, J . Polym. Sci. D 1971, 5, 385. 65. P. Kovacic, M. B. Jones, Chmn. Rev. 1987, 87, 357. 66. R. L. Elsenbaumer, L. W. Shacklette, in Handbook of Conducting Polymers (Ed: T. A. Skotheim) Marcel Dekker, New York 1986, 213. 67. D. G. H. Ballard, A. Courtis, I. M. Shirley, S. C. Taylor, J . Chem. Soc. Chem. Commun. 1983, 954. 68. T. Yamamoto, Y. Hayashi, A. Yamamoto, Bull. C h m . Soe. Japan 1978, 51, 2091. 69. M. Rehahn, A. D. Schluter, G. Wegner, W. J. Feast, Polymer 1989, 30, 1054. 70. K. N. Baker, A. V. Fratini, T. Resch et ul., Polymer 1993, 34, 1571. 71. J. L. Baudour, Y. Delugeard, P. Rivet, Acta Crystallogr. 1978, B34, 625. 72. H. Cailleau, J. L. Badour, C. M. Zeyen, Acta Crystallogr. 1979, 835, 426 73. H. Cailleau, A. Dworkin, Mol. Cryst. Liq. Cryst. 1979, 50, 21. 74. S. Tzusuki, K. Tanabe. J . Phys. Chem. 1991, Y5, 139. 75. B. L. Farmer, B. R. Chapman, D. S. Dudis, W. W. Adams, Polymer 1993, 34, 1588. 76. S. Sasaki, T. Yamamoto, T. Kanbara, A. Morita, T. Yamamoto, J . Polym. Sci. Polym. Phys. Ed. 1992, 30, 293. 77. C. Ambrosch-Draxl, J. A. Majewski, P. Bogl, G. Leising, Phys. Rev. B 1995, 51, 9668. 78. A. Kawaguchi, J. Petermann, Mol. Cryst. Liq. Crjlst. 1986, 133, 189. 79. F. Teraoka, T. Takahashi, J . Macromol. Sci (Phys.) 1980, B18, 73. 80. DEUTPH Tetradecadeutero-p-terphenyl neutron study, at 200K, simple-well model refinement C18 D14. DEUTPHOl Tetradecadeutero-p-terphenyl neutron study, at 200K, double-well model refinement C18 D14. J. L. Baudour, H. Cailleau, W. B. Yelon, Acta Cr)~stallogr.,Sect. B 1997, 33, 1773. C20 HI 8 0 5 . 81. MXHTPH 1,4-Dimenthoxy-2,4/,4”-trihydroxy-p-terphenyl G. D. Andreetti, G. Bocelli, P. Sgarabotto, Cryst. Struct. Commun. 1974, 3, 145. 82. PARKUO 2/,3””-Dimethyl-p-sexiphenyl C38 H30. G. Subramaniam, R. K. Gilpin, A. A. Pinkerton, Mol. Cryst. Liq. Cryst. 1992, 213, 229. 83. PRTPCX Di-n-propyl-p-terphenyl-4,4/’-dicarboxylate C26 H26 0 4 . D. B. Chung, R. E. Carpenter, A. de Vries, J. W. Reed, G. H. Brown, J . Cryst. Mol. Struct. 1978, 8 , 8 I . 84. QUPHEN p-Quaterphenyl room temperature form C24 H18. Y. Delugeard, J. Desuche, J. L. Baudour, Actu Crystallogr., Sect. B (1976), 32, 702. QUPHENOl p-Quaterphenyl low temperature ordered form, at 110K C24 H18. J.-L. Baudour, Y. Delugeard, P. Rivet, Acta Crystullogr., Sect. B 1978, 34. 625. 85. TCQDTPlO 7,7,8,8-Tetracyanoquinodimethane-tetradecadeutero-p-terphenyl neutron study C12 H4 N4 C18 D14. G. C. Lisensky, C. K. Johnson, H. A. Levy, Actu Crystallogr., Sect. B 1976, 32, 2188. 86. TERPHEOl p-Terphenyl C18 H14. H. M. Rietveld, E. N. M a s h , C. J. B. Clews, A r t a Crystallogr., Sect. B 1970 26, 693. TERPHEOSp-Terphenyl at 113K, C18 H14.

R
325

J. L. Baudour, Y. Delugeard, H. Cailleau. Acra Crjsrallogr., Sect. B 1976. 32, 150. 87. VEFLIB 4-Cyano-4’-n-pentyl-p-terphenyl room temperature phase I C24 H23 NI . W. Hasse, H. Paulus. Z. X. Fan, I. H. Ibrahim. M. Mokhles, Mol. Cryst. Liq. Cr.vst. ( L e t t . ) 1989, 6 , 113. l,1’:3’. 1”:3”.1”’-quaterphenyl phenylmesitylene at 88. VINWEU 2.2”,4.4”.6.6”-Hexamethyl-45°C C30 H30. E. Fischer. H. Hess, T. Lorenz, H. Musso. I. Rossnagel, Cheni. Ber. 1991. 124, 783. 89. WEMGAW 4,4”-Difluoro-p-terphenyl C18 HI 2 F2. C,34 H16 F2. WEMGIE 4.4”’-Difluoro-p-quaterphenyl H. Saitoh. K. Saito, Y . Yamamura et rrl. Bull. Chern. Soc. Jpn. 1993. 66, 2847. YO. F. Garnier, G. Horowitz, X. Peng. D. Fichou, Adv. Mater. 1990, 2, 592. 91. G. Horowitz, D. Fichou, X. Peng. Z. Xu. F. Garnier, Solid State Cornmiin. 1989, 72, 381. 92. C. Ziegler, in Handbook of Organic Conductive Molecules and Polymers. Vol. 3 (Ed: H. S. Nalwa) Wiley, New York, 1997, p. 676 and references cited therein. 93. S. Hotta. in Handbook of Organic Coridiictive Moleculrs and Poljwers, Vol. 2 (Ed: H. S. Nalwa) Wiley, New York, 1997, p. 309 and references cited therein. 94. W. Porzio, S. Destri, M. Mascherpa. S . Briickner. Acta Poljwier. 1993, 44. 266. 95. SOTCEJ 5,5”’-Dimethyl-2,2’ : 5’,2’’ : 5”.2”’-quaterthiophene CH 18 H I4 S4. S. Hotta, K. Waragai, J . Mater. Clieni. 1991, 1, 835. 96. E. J. Samuelsen. J. Mardalen. in Handbook o f Orgoriic Coiihictive Molecules and Polymers. Vol. 3 (Ed: H. A. Nalwa) Wiley, New York, 1997. p. 676 and references cited therein. 97. M. J. Winokur. D. Spiegel. Y. Kim, S. Hotta, A. J. Heeger, Svnth. Met. 1989, 28, C419. 98. A. Bolognesi, M. Catellani, S. Destri, W. Porzio, Makroriiol. Cliem., RapidConirnirii. 191 I . 12. 9. 99. G. Gustaffson, 0. Inganas, H. Osterholm, J. Laasko. Polymer 1991, 32, 1574. 100. J. Mardalen. E. J. Samuelsen. 0. R. Gautun. P. H. Carlsen, Sjwrh. Met. 1992. 48, 363. 101. G. Schopf, G. Kossmehl. .4di#.P o l j w ~Sci. 1997, 129. 1. 102. PIJFOD 3’.3’’”,4’,4””-Tetrabutylhexathione C40 H46 S6. Ju-Hsiou Liao. M. Benz, E. LeGoff, M. G. Kanatzidis, Adv. Mater. 1994. 6 , 133. 103. POGMON 4’,3’”’-Di-n-butyl-2,2‘: 5‘,2“ : 5”,2”’: 51”,2”11 5’”’,17””’-sexithiophene C32 H30 S6. J. K. Herrema. J. Wildeman. F. van Bolhuis, G. Hadziioannou, Synth. Met. 1993, 60, 239. 104. FEPYUU Tetrathiophene tetracyanoquinodimethane C16 H I 0 S4 1+,C12 H4 N4 I-. Qian Minxie. Fu Heng. Cao Yong, Jiegou Huasue ( J . Striict. Cheni) 1986. 5 , 163. 105. LEFZIF 3/.4’-DibutyL2,2’:5’,2‘’-terthiophene C20 H24 S3 L. DeWitt, G. J. Blanchard, E. LeGoff, M. E. Benz. J. H. Liao. M. G. Kanatzidis, J . Am. Ckem. Soc. 1993, 113. 12158. 106. PIJFIX 3”,4”-Dibutylpentathioneat - 1OO’C C28 H28 S5. Ju-Hsiou Liao, M. Benz. E. LeGoff. M. G. Kanatzidis, Adv. Mater. 1994, 6 , 135. : 5‘.2” : 5”.2’” : 5’”,2”” : 5/”’,2’‘”’-sexithio107. POGMUT 5,5’”-bis(Trimethylsilyl)-4’,3””-dioctyl-2,2’ phene at 140K C46 H62 S6 Si2. J. K. Herrema, J. Wilderman. F. van Bolhuis. G. Hadziioannou. Synth. Met. 1993, 60. 239. 108. TAKMAT Trithiophene tetracyanoquinodimethane C12 H8 S3.CI2 H4 N4. Qian Minxie, Cao Yong, Fu Heng. Tang Youqi, Zhang Fugui, Wuli Hunsue Xiiebao (Acro P/ij~.~.-C/iini. Sin.)1990. 6. 277. 109. VAFHAL 3,3’:2’,2’’:3/’.3”’-Tetrathiophene C16 H10 S4. N. Jayasuriya, J. Kagan, De-Bin Huang, B. K. Teo, Heterocycles 1988. 27, 1391. 1 10. VUWTEM 4,4’.3”,4”‘-Tetramethyl-2.7’ : 5‘.2‘‘ : 5”,2’”-tetrathiophene C20 H 18 S4. G. Barbarella, M. Zambianchi. A. Bongini, L. Antolini, Adv. Muter. 1992. 4. 282. 1 1 1. CAPTOC Dithieno(3.2-b:2’,3’-d)thiophene7,7,8,8-tetracyanoquinodimethaneC8 H4 S3, C12 H4 N4. D. Zobel, G. Ruban, Acta Cryst.. B (Srr. Sci.1 1983, 39, 638. 112. CAPTOCOl Dithieno(3.2-b;2’,3’-d)thiophene7.7,8.8-tetracyanoquinodimethane C8H4 S3, C12 H4N4. F. Bertinelli, P. C . Bizzarri, C. D. Casa et al. Mol. Cryst. Liq. Crjist. 1984, 109, 289. 113. CPDTHO 4H-Cyclopenta(2.1-b.3.4-br)dithiopheneat -160’C CY H6 S2. P. B. Koster, F. van Bolhuis, G. J. Visser. Acta Crystallogr.. Sect. B 1970, 26, 1932.

326

5 Crvstal Structirrc

1 14. DITHSB 2,2’-Dithienyl antimony trichloride C X H6 S2,2(C12 Sbl). L. Korte, A. Lipka, D. Mootz, Eur. Cryst. Meeting 1979, 5, 92. I 1 5 . DITHSBlO 2,2’-Dithienyl bis(trich1oro-antimony) C8 H6 S2,2(C13 Sbl). L. Korte, A. Lipka, D. Mootz, Z. Anorg. A&. Chem. 1985, 524, 157. 116. DTENYL 2,2’-Dithienyl at -140°C C8 H6 S2 G. J. Visser, G. J. Heeres, J. Wolters, A. Vos, Actu Crystallogr., Sect. B 1968, 24, 467. 117. DTENYLOl 2.2’-Bithiophene 2,2’-dithienyl at 173K C8 H6 S2 P. A. Chaloner, S. R. Gunatunga, P. B. Hitchcock, A c f a Cryst., Sect. C ( C r . Str. Comm.) 1994, 50, 1941. 118. DTENYLOt 2,2’-Bithiophene 2,2’-dithienyl at 133K, redetermination of G. J. Visser, C. J. Heeres. J. Wolters, A . Vos, Acta Cryt., Sect. B 1968, 24, 467 C8 H6 S2. M. Pelletier, F. Brisse, Acta Cryst., C (Cr. Str. Comni.) 1994, 50, 1942. 119. FEPYOO Bithiophene tetacyanoquinodimethane C8 H6 S2 I+, C12 H4 N4 I-. Qian Minxie, Fu Heng, Cao Yong, Jiegou Htrirxire ( J . Struct. Chem.) 1986, 5, 159. 120. GAKVET 5,5’-Dibromo-2,2’-bithiophene C8 H4 Br2 S2 G. J. Payrka, Q. Fernando, M. B. Inoue, M. Inoue, E. F. Belazquez, Acta Cryst., C (Cr. Str. Comm.) 1988, 44, 562. 121. HAFKII 1,8-bis(5,2’-Dithiophene-2-yl)naphthalene C26 H 16 S4. M . Kuroda, J. Nakayama, M. Hoshino, N. Furusho, T. Kawata, S. Ohba, Tetrahedron 1993. 49, 3735. 122. HAWXUY 3,3,3’-TrichlorobithiopheneC8 H3 C13 S2. HAWYAF 3,3’,5,5’-Tetrachlorobithiophene CX H2 C14 S2. K. F. Mok, S. C. Ng, I. Novak, H. H. Huang, J . Cry’st. Spectrosc. 1993, 23, 799. 123. KEDYAT 4,4’-Dimethoxy-2,2’-bithienyl CIO HI0 0 2 S2. E. F . Paulus, K. Siam, K. Wolinski, L. Schafer, J . Mol. Struct. 1989, 196, 171. C20 H22 S4 Si2. 124. MTSITH 5,5’-bis(Dimethyl(2-thienyl)silyl)-2,2’-bithienyl A. Lipka, H. G. von Schnering, Cliem. Ber. 1977, 110, 1377. C8 H4 N2 0 4 S2 125. NOTHNL 5,5’-Dinitro-2,2’-dithienyl L. V. Panfilova, M. Yu Antipin, Yu. T. Struchkov, Yu. D. Churkin, A . E. Lipkin, Z/i. Strukt.

Khim. 1980, 21, 190-2. 126. SAGSEY 3,3’-Dimethoxy-2,2’-bithiophene CIO HI0 0 2 S2. E. F. Paulus, R. Dammel, G. Kampf et al., Acta Cryst., B (Str. Sci.) 1988, 44, 509. 127. SATHEA 3,3,5’-Tribromo-2,2’bithiophene CX H3 Br3 S2. G. J . Pyrka, Q. Fernando, M. B. Inoue, M. Inoue. Actrr C r j ~ t .C , ( C r . Str. Cornm.) 1988, 44, 1800. 128. VEPKAC 2,2’-Bithiophenium- I , 1‘-bis(bis(methoxycarbony1)methanide) monohydrate. VEPKEG 5,5’-bis(bis(Methoxycarbonyl)methyl)-2,2’-bithiopheneC18 HI8 0 8 S2 C18 H I 8 0 8 S2,H2 01. S. Bien, M. Kapon, S. Gronowitz, A.-B. Hornfeldt, Chem. Scr. 1989, 29, 221. 129. WEPDAW 5.5’-bis(Trimethylsilyl)-2,2‘-bithienylC I4 H22 S2 Si2. WEPDEA 5,5’-Dimethyl-2,2’-bithienyl C10 H10 S2. C . Aleman, E. Brillas, A. G. Davies e f id., J . Org. Chem. 1993, 58, 3091. -b;3,4-b’)dithiophene C12 H6S4. 130. WEPGON 4-( 1,3-Dithiol-2-ylidene)-4H-cyclopenta(2,1 M. Kozaki, S. Tanaka, Y. Yamashita, J . Org. C h e i . 1994, 59. 442. 13 1. WlKClC 2,2’-Bithiophene-S-carbaldehyde C9 H6 01 S2 S . P . Armes, P. A. Chaloner, P. B. Hitchcock, M. R. Simmons, Acta C r ~ s t . C , (Cr. Sir. Comm.) 1994, 50, 1945. 132. YAFMIB 5-(4,5-Benzo-1,3-dithiole-2-ylidene)-4H-cyclopenta(2,1 -b;4-b’)dithiophene C16 H8 s4. M. Kozaki, S. Tanaka, Y. Yamashita, J . Cliem. Soc., Chern. Comni. 1992, 1137. CIO H I 0 N2 0 2 S2. 133. YAPMEH S-Dimethyl-S’-nitro-2,2’-bithiophene F. Effenberger. F. Wurthner, Angew. Cltem.. hit. Ed. Engl. 1993, 32, 719. C36 H22 S4. 134. YEHJAW 5,5’-bis(8-(2-ThienyI)naphthalene-l-yl)-2,2’-bithiophene M. Kuroda, J. Nakayama, M . Hoshino, N. Furusho, S. Ohba, Tetrahedron Lett. 1994, 35, 3957. 135. W. Wernet. M . Monkenbusch, G. Wegner, Miikromol. Cheni., Rapid Conimun. 1984, 5 , 157.

References

136. 137. 138. 139. 140. 141. 142.

321

K . J. Wynne, G. B. Street, Macromolecules 1985, 18, 2361. G. R. Mitchell, Polym. Commun. 1986, 27. 346. S. Martina. Thesis, Mainz, 1992. S. Martina, V. Enkelmann, A.-D. Schluter, G. Wegner, Synth. Met. 1991, 41, 403. S. Martina. V. Enkelmann, G. Wegner, A.-D. Schluter, Synih. Met. 1992, 51, 299. G. B. Street, S. E. Lindsey, A. I. Nazzal, K. J. Wynne, Mol. Cryst. Liq. Cryst. 1985, 118, 137. F. E. Karasz, J. D. Capistan, D. R. Gagnon, R. W. Lenz, Mol. Cryst. Liy. Cryst. 1985, 118,

282. 143. D. R. Gagnon, J. D. Capistan. F. E. Karasz, R. W . Lenz, S. Antoun. Polymer 1987, 28, 587. 144. D. R. Gagnon, J. D. Capistan, F. E. Karasz, R. W. Lenz, Polym. B U N . 1984, 12, 293. 145. T.Granier, E. L. Thomas, D. R. Gagnon, F. E. Karasz, R. W. Lenz, J . Po/ym. Sci., Polyni. Pliys. Ed. 1986, 24, 2793. 146. D. Chen, M. J. Winokur, M. A. Masse, F. E. Karasz, Phys. Rev. B 1990, 41, 6759. 147. M. J. Winokur, D. Chen, F. E. Karasz, Synth. Met. 1991, 41-43, 341. 148. M. A . Masse, J. B. Schlenoff, F. E. Karasz. E. L. Thomas, J . Polym. Sci., Polym. Phys. Ed. 1989, 27, 2045. 149. HESMUN 2,5-Distyryl- 1,4-dimethoxybenzene C24 H22 0 2 . HESNAU 2,5-bis(2-(o-Chlorophenyl)vinyl)-1,4-dimethoxybenzeneC24 H20 C12 0 2 . HESNEY 2,5-bis(2-(3,4-Dichlorophenyl)vinyl)- 1.4-dimethocybenzene C24 H 18 C14 0 2 . HESNIC 2,5-bis(2-(Benzo( 1,3)dioxol-5-yl)vinyl)- 1,4-dimethoxybenzene C26 H22 0 6 . H. Irngartinger, J. Lichtenthaler, R. Herpich, Siruci. Chrm. 1994, 5 , 283. 150. JACBIY trans,trans-l,4-bis(2-(3,4,5-Trimethoxyphenyl)vinyl)benzene C28 H30 0 6 . M. Verbruggen, Yang Zhou, A. T. H. Lenstra, H. J. Geise, Acta Cryst., C (Cr. Str. Comm.) 1988, 44. 2120. 151. PEBRAP Z,Z-1,4-bis(3,5-Di-t-butylstyryl)benzene at 133K C38 H50. M. Hakansson, S. Jagner, M. Sundahl, 0. Wennerstrom, Acta Chem. Scand. 1992,46, 1160. 152. C. J. Finder, M. G. Newton, N. L. Allinger, Actri Crysta/logr. B 1974, 30, 41 1. 153. Z. Yang, H. J. Geise, M. Mehbod, G. Debrue, J. W. Visser, E. J. Sonneveld, L. van’t Dack, R. Gijbels, Synrh. Met. 1990, 39, 137. 154. R. E. Gill, A. Meetsma, G. Hadziioannou. Adv. M u t w . 1996, 8, 212. 155. A. G. MacDiarmid. J. Chiang, M. Halpern e f a/.. Mol. Crysf. Liy. 1985, 121, 173. 156. A. J. Epstein, A. G. MacDiarmid, Springer Series in Solid Stcite Sci. 1989, 91, 282. 157. J. P. Travers, J. Chroboczek, F. Devreux et a/., Mol. Cryst. Liq.Cryst. 1985, 121, 195. 158. W. R. Salaneck, I. Lundstrom, W. S. Huang. A. G. MacDiarmid, Synih. Met. 1986, 13, 291. 159. P. M. McManus, S. C. Yang, R. J. Cushman, J . Chem. Soc. 1985, 1556. 160. M. E. Jozefowicz, A. J. Epstein, J.-P. Pouget et a/., S y t h . Met. 1991, 41-43, 723. 161. J. P. Pouget, M. Laridjani, M. E. Jozefowicz, A. J. Epstein, E. M. Scherr, A. G. MacDiarmid, Synili. Met. 1992, 51, 95. 162. J. P. Pouget, M. E. Jozefowicz, A . J. Epstein. X. Tang, A. G. MacDiarmid, Mucromolwules 1991, 24, 779. 163. R. H. Baughman, J. F. Wolf, H. Eckhardt, L. W. Shacklette, Sjnth. Met. 1988, 25, 121. 164. BASNOY Diethyl-N,N’-diphenyI-2,5-diaminoterephthalateC24 H24 N2 0 4 . BASNUE Diethyl-N,N’-diphenyI-2,5-diaminoterephthalate benzene solvate C24 H24 N2 04,C6 H6. B. J. Mann, I . C. Paul. D. Y. Curtin, J . Chm7. Soc. Perkin Trcins. 1981, 2, 1583. 165. BERJOX Diethyl-bis(2-aminophenyl)-p-diaminoterephthalateC24 H26 N4 0 4 . B. J. Mann, R. B. Wilson, D. Y. Curtin. I. C. Paul, Cryst. Sirucf. Commim. 1982, 11, 163. 166. DPENAM N,N’-Diphenyl-p-phenylenediamineCI 8 HI6 N2. 2 . P. Povet’eva, L. A. Chetkina, V. V. Kopilov, Kristal1ografil.a 1976, 21, 312. 167. BABKUK Disodium diterphenylide terphenyl 1.2-dimethoxyethane solvate at 130K 2(C18 HI4 I-),C18 H14,2(Nal 1+),6(C4 HI0 02). J. H. Noordik, H. M. Doesburg, P. A. J. Prick, A c f n Crj,stdlogr. B 1981, 37, 1659. 168. H. P. Fritz, J. Gebauer, P. Friedrich, P. Ecker, R. Artes, U . Schubert, Z . Naturforsch. B 1978, 33, 498. 169. H. J. Keller, D. Nothe, H. Pritzkow et a/., Mol. Crj,.st. Liy. Crysf. 1981, 62, 181. 170. C . Krohnke, V. Enkelmann, G. Wegner, Angew. Chem. 1980, 92, 941.

328

5 Crystal Structure

171. V. Enkelmann, B. S. Morrd, C. Krohnke, G . Wegner, J. Heinze, Chem. Phys. 1982,66, 303. 172. V. Enkelmann, in Pol-vnuclear Aromatic Compounds (Ed: L. B. Ebert) Advances in Chemistry Series, ACS, Washington 1988, 217, p. 177. 173. V. Enkelmann, K. Gockelmann, G. Wieners, M. Monkenbusch, Mol. Cryst. Liq. Cryst. 1985, 120, 195. 174. W. Grauf, J. U. von Schiitz, H. P. Werner et al., Mol. Cryst. Liq. Cryst. 1988, 158B, 307. 175. K. Gockelmann, Thesis, Mainz, 1987. 176. M. Stamm, J. Hocker, A. Axmann, Mnl. Cryst. Liq. Cryst. 1981, 77, 125. 177. S. Briickner, W. Porzio, Makromol. Chem. 1988, 189, 961. 178. A. Tsumura, H. Koezuka, T. Ando, Appl. Phys. Lett. 1986, 49, 1210. 179. J. H. Burroughes, D. D. C. Bradley, A. R. Brown et al., Nature 1990, 347, 539. 180. N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend, A. B. Holmes, Nature 1993, 365, 628. 181. V. Cimrova, M. Remmers, D. Neher, G. Wegner, Adv. Muter. 1996,8, 146. 182. M. Ballauff, Angew. Chem. 1989, 101, 261. 183. T. Vahlenkamp, G. Wegner, Macrom. Chem. P/IJ~s. 1995, 195, 1933. 184. M. Remmers, V. Enkelmann, G. Wegner, Mol. Cryst. Liq. Cryst., in press. 185. M. Remmers, D. Neher, J. Gruener et al., Macromolecules 1996, 29, 7432. 186. C . Albrecht, V. Enkelmann, G. Lieser et al., in Crystallization of Po1jwwr.Y (Ed: M. Dosiere), NATO ASI Ser. C, 1993, 405, p. 316.

5.2 Packing Calculations Based on Empirical Force Fields R. Hentschke

5.2.1 Introduction Since the 1930s, quantum theoretical methods have been developed to calculate in principle the structure and dynamics of a molecular system, and thus its material properties to any desired accuracy. However, the tremendous computational effort usually prevents the application of such ab iriirio methods in many practical applications. On a simpler level of the molecular modeling hierarchy, one focuses solely on the structure and dynamics of the nuclear skeleton of a molecule or a molecular system. Here the Born-Oppenheimer approximation allows the electrons surrounding the nuclei to be reduced to an effective inter-atomic potential, which is modeled via a simple empirical or phenomenological force field. This approach to molecular structure calculation has been called molecular mechanics [ 1-31 and its dynamic extension is called molecular dynamics [3-81. Inexpensive but powerful workstations, which can currently handle systems consisting of up to several thousand atoms, increasingly make these force field methods indispensable as tools for molecular research. This chapter reviews some of the molecular mechanics and dynamics methods with a special emphasis on molecular crystal packing calculations. (A number of applications of molecular mechanics in this context can be found in a recent review by Glaser [9]). In the first section, the functional form and parameterization of empirical force fields for molecular systems including computational aspects of potential and force calculations are discussed. The second section focuses on techniques and strategies used in the search for the equilibrium packing structure of crystalline molecular systems. It should be noted that the theoretical calculation of molecular packing, whether by empirical force fields or otherwise, is by no means a routine procedure, and detail is given to enable a better understanding of the methods, their future potential, and the problems involved. In the third section, three examples are given, where the aforementioned methods are applied to bulk packing, fibers, and surface induced molecular ordering.

5.2.2 Molecular Interactions via Phenomenological Force Fields 5.2.2.1 The Functional Form of Empirical Force Fields Over the course of time the potentials from which the intra- and inter-molecular

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5.2 Packing Culculutions Bused on Enipiricol Force Fields

forces are derived have converged towards a functional form which consists of a valence part, U,,olmc.e, plus a part describing longer ranged or non-bonded interactions, Unon.hond [2, 10- 121 (For inorganic and organometallic compounds see [13].). The valence potential

is a sum over functionally simple terms involving between two to four atoms. The first sum, extending over all covalent bonds ( i j ) , where i and j indicate the bonded atoms, accounts for bond distortions, 6hii, with respect to some equilibrium value. These bond distortions are usually modeled by harmonic potentials, bond ui/ - Kb,,Sh;, where Kb,, is the bond stiffness. If necessary anharmonicity is included via cubic and quadratic terms in 6bii or by using the Morse function. The second sum extends over all valence angles, $ i j k , between bonds ( i j ) and ( j k ) . The harmonic form is usually used, u;.:?" = K#ilkS&, unless effects due to anharmonicity are considered to be important. The last sum consists of potentials modeling in part (cf. below) the rotation of two molecular fragments with respect to each other around a connecting bond ( j k ) . The angle of rotation, diikl, is the angle between the planes defined by the successive atoms ijk and ,jkl. Analogous to these proper torsions, sometimes improper torsions are introduced to constrain three bonds (il), ( j l ) , and ( k l ) having the atom 1 in common to (for instance) being coplanar. Torsion potentials are commonly expressed as Fourier series, 30

py

=

K~!l,,k,,m cos(m8ijkl)

>

m=O

truncating the sum after two to four terms. Whereas the full rotational freedom is crucial for conformationally unconstrained, flexible molecules, in the crystal near equilibrium (or in the case of improper torsion angles a harmonic description in terms of small angular distortions, 6flUkl)may be sufficient. In addition to the above diagonal terms some force fields include coupling terms [2,1 I], like bond-anglr u.. t i , r/k .. - Kb,l,,!liSb, 6$ok describing the coupling between bond stretch and valence angle distortions, etc. Force fields, which do not include such mixing terms, are often called diagonal force fields. Even though any molecular conformation can be specified solely in terms of the valence coordinates, calculating its energy requires the inclusion of non-bonding interactions potentials Unon-bond =

c(uj,

ourrlap & dispersion

i<j

The pair potential UIIo z f e r l u ~ & c i f s ~ e r s includes fon electronic overlap repulsion between pairs of neutral atoms, which dominates at close inter-nuclear separations r(= r 0 ) , and van der Waals or London dispersion attractions, which dominate at large r . The former are modeled in terms of ALexp(-a,r) or AIIr-" with 9 5 n 5 16 [2,14] (Values between 9 and 10 seem to work best (cf. [lo]). Even

though both the 1.p9 as well as the exp(-or) potential appeared to perform somewhat better in some early alkane packing calculations [ 151 as well as in packing calculations involving amide and carboxylic acids [ 161 (where iz = 9 and n = 12 were compared), one now finds the numerically advantageous rp"-dependence more frequently.). The latter are modeled as B,rp6(induced dipole-induced dipole interaction). Usually the various coefficients are treated as adjustable parameters. Note that the rP6-dependence of the London dispersion energy for neutral atoms is the first term of a series in r-.', with s even [17]. The next term, r-' (induced dipole-induced quadrupole interaction), might contribute significantly, but to some extent its effect can be included in the coefficient of the F6-term [18]. Intraor inter-molecular Coulomb interactions due to (permanent) polarization or ionic groups are most commonly modeled in terms of point charge interactions q i q , ( u p ' . Here qi and yi are the (partial) charges centered on the nuclei i a n d j or sometimes on separate charge sites to better reproduce the molecular multipole moments, and E is the dielectric constant. Drawing the line between the valence and the non-bonded interactions is a matter of definition. Commonly, covalently bonded atoms and atoms covalently bonded to a common third atom, do not interact via non-bonded potentials. It is also worth noting that almost all practical force fields assume pairwiseadditive non-bonded interactions. Including for instance three-body interactions [19] increases the number of non-bonded terms from - N ' / 2 to ""6, where N is the number of interacting sites. Many-body interactions of the overlap and dispersion type may be included partially in the pair-interactions via effective amplitudes. Similarly, the non-additive induced polarization, which may be a significant contribution to the Coulomb interaction in condensed phases is often taken into account by scaling the vacuum values of the partial charges. However, great care has to be taken when calculating the corresponding induction part of the energy [20,211. More satisfying, e.g. with respect to the (phase) transferability of the potential, is the explicit inclusion of induced polarization [22,23]. The above 'basic' interaction potentials may be augmented by 'special' potential terms. Some force fields incorporate an additional (12-10) potential [24-261 to improve the description of hydrogen bonding (sometimes also including a factor depending on the angle between the hydrogen donor, the hydrogen, and the hydrogen acceptor [27,28] and references therein), even though site-site Coulomb interactions alone appear to be sufficient [2,5, 16,29-3 1,321. Finally, for packing problems involving the interactions of molecules with inert solid surfaces, e.g. in the case of physisorption, the effect of the surface may be modeled by adding a potential, C i u Y r f . ziy' describes the surface potential acting on an = (.Y;,,.J'~,,,Z;~,) with respect to adsorbate atom of type o located at the position 6,) some origin on the surface. Here is the perpendicular distance to the surface measured from the topmost layer of atoms in the solid. In the simplest case the interaction between an adsorbate atom of type o and a substrate atom of type separated by a distance rn,j can be approximated in terms of Lennard-Jones pair interactions, i.e. by

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5 . 2 Pucking Calculations Bused on Empirical Force Fields

To a good approximation

uFf

is then given by

which is obtained by ‘smearing out’ the solid’s atoms within identical layer planes parallel to the surface. Here np, denotes the two-dimensional number density of @-atoms in the Ith layer, and d/ is the separation of that layer from the surface. Already the I = 0 term by itself is a significant improvement over the often used (9-3) potential obtained by continuously distributing the atoms within the solid. Employing the solid’s translational symmetry and explicitly summing over all lattice [33], where the right hand sites yields u”“’(Y;,) = u ’ ~ ~ ~ ( ~+) CES1 (z,~J side is a sum over contributions from successively larger reciprocal neighbor shells corresponding to the two-dimensional surface lattice. The terms with m > 0 account for the corrugation of the surface. A detailed discussion including Lennard-Jones and other adsorbate-substrate interactions can be found in [34].

5.2.2.2 Parameterization of Empirical Force Fields The above quantities K,, where x stands for bond lengths, valence angles, and torsion angles, as well as the equilibrium values x, for which Sx = 0, and the corresponding constants A’, a, A , B, . . . appearing in the non-bonded potential usually are adjustable parameters. In order to limit the number of parameters, which is important for the transferability and thus for the usefulness of a force field, identical functional groups within reasonably comparable environments will have identical parameters, e.g. the parameterization of CH2-groups in n-alkanes does not depend on molecular weight. The parameters are determined by comparing the force field results obtained for a selected set of ‘training’ compounds to corresponding experimental data or increasingly via quantum theoretical calculations (Note that the non-bonded cross Lennard-Jones parameters are mostly determined via simple mixing or combining rules, e.g. cr,, = (oLr aJJ)/2 and E ~ ,= For a recent discussion of various combining rules see [35].). Thus, when using an empirical force field one should make sure that its ‘training set’ is sufficiently similar to the system which one wishes to study. An overview over some of the currently more common empirical force fields is provided by table 2.2 in reference [12,36]). A more extensive discussions of force field parameterization can be found in [2, 10,11,37]. Most established force fields are parameterized using a combination of vibrational spectroscopy (mainly for bond length, bond angle, and torsion parameters [24,27,31,38]), elastic constants [39], thermodynamic data (mainly for non-bonded parameters via second virial coefficients, critical point data, diffusion coefficients [17]; heats of sublimation [15,18,40-421 (hydrocarbons) [29,38,43451 (peptides, proteins, and nucleic acids) [39] (others); heats of formation [46,47] (hydrocarbons); heats of vaporization [3I] (hydrocarbons); or heats of adsorption [48] (hydrocarbons on graphite)); and structural data (mainly for equilibrium

m.

+

5.2.2 Moleculur Ititeructions via Pherionienologicd Force Field.7

333

bond length and bond angles as well non-bonded parameters, but also for torsion potentials) [24,28,30,39,47,49-511, obtained via X-ray, electron or neutron scattering techniques (crystal structures of molecular compounds were used at an early stage to calibrate non-bonded potential parameters [15, 18,40-42,461 (hydrocarbons) [ 16,29,43-451 (peptides and proteins)). Today’s force field developers also rely strongly on quantum mechanical nb initio and semi-empirical calculations, which can be used to calculate all force field parameters for small molecules containing the fragments of interest (cf. the C F F force field in [52-551). A discussion of the quality of semi-empirical parameters can be found in [56] (and references therein; see also [57]). Parameter sources for a number of force fields are listed in [58]. Partial charges deserve extra mentioning. Frequently they are computed for isolated molecules via population analysis of ab initio [59] (and references therein; see also [60] for a general discussion) and semi-empirical [61] results, and by empirical methods [62-641. The point charge distributions obtained by different methods may vary considerably. In addition, calculations for isolated molecules neglect polarization effects important in condensed phases (e.g. the gas phase dipole moment of water is 1.85D, whereas in liquid water it increases to -2.5D, and in the solid state it is even higher [65]). Therefore the vacuum charges are often scaled to yield the experimental crystal structures (e.g. [42,54]) or liquid densities, and enthalpies [30]). If, however, predicting the packing structure of condensed phases is what one is interested in, then induced polarization should be included explicitly (cf. above)!

5.2.2.3 Computational Aspects of Packing Calculations Given a force field or rather its potential and the proper parameterization, one can, in principle, calculate the energy of an isolated molecule in any given conformation or the configuration energy of many interacting molecules. Just one energy calculation on today’s workstations is quite fast even for larger systems with up to a few thousand atoms. Nevertheless, configuration or conformation searches as discussed below sometimes require thousands or even millions of energy evaluations. In a large system consisting of N atoms the number of interactions per atom is dominated by the non-bonded interactions and, for pairwise additive forces, increases with N / 2 . Therefore one introduces a cutoff radius, rcu,, reducing the total number of non-bonded interactions from N 2 / 2 to N N,,,,, where N,.,, (<
-

334

5 . 2 Puckirig Calculations Bused on Empirical Force Fields 1.10

-*

do /o

0

O3P*

0

1.56

0

1.54

.

I

I , . ,

1.05

*. I

I

I

1.oo

Figure 1. Reduced equilibrium lattice constant d,>/u(left axis; circles) and reduced equilibrium density c3pt,(right axis; diamonds) vs. rct,,/c for the fcc Lennard-Jones crystal.

the relation between rcu,and the equilibrium density po as well as the equilibrium lattice cocstant do for a fcc Lennard-J?nes crystal. Since cr usually is on the order of -3-4 A, rcufshould be at least 2 10 A (Commonly 'tail corrections' are applied, e.g. by adding i p N,:J uLJd3rto the total packing energy.). Currently the typical linear dimension L of computer models of multi-molecular syst<ms, i.e. a p b i c section cut out of a bulk crystal or a bulk liquid, is between 30A and l00A for workstations. To minimize boundary effects the system is embedded in an infinite periodic lattice of its own translated images (Note that rcuf< L / 2 to avoid interactions between multiple copies of the same molecule.). For non-polar liquids (or solids) this scheme reproduces bulk properties well, unless they involve correlation lengths on the order of L as for instance near critical points. For polar or ionic systems other techniques must be applied [5]. In polar liquids or in ionic solutions long range interactions can be calculated via reaction field methods (e.g. [66]) or the Ewald technique (e.g. [67]), whereas for corresponding solids the latter or modified versions of the latter (e.g. [68]) are most appropriate. The approach is easily applicable to any F2'-potential, i.e. to the summation of non-bonded potential energies

I

# for Z= 0

even though commonly it is applied to the Coulomb case, v = 1/2, only. Here C, is a constant, and the Z are lattice vectors on the lattice of image cells. Thus, all interactions between real atoms i a n d j , as well as between real atoms and their images are included. The evaluation of the above sum proceeds via the identity I?+ Zl-2' = ( 2 / r ( v ) ) dss2'-l exp[-s21r'+ n'I2], where r ( v ) is the Gamma 30 function. Subsequently the integral is split into two parts, i.e. J0 ds.. = J ds..i + JKm ds.., another trick, which in the Coulomb case can be motivated physically in terms of charge screening [67]. The separate evaluation of the integrals

Jr

5.2.3 Finditig the Proper Puckitig Strrrcmre

-

a Mirltiple

Miriitiiitrii

Problem

335

(following the path outlined in [69]) yields the result

where the sum over the lattice vectors $is over the lattice reciprocal to the Z-lattice, and r ( v , .. .) is the incomplete Gamma function [70]. Note that the prime (f 0') indicates that for v 5 3/2 the g = 0-term must be included and of course diverges. However, for v = 1/2, i.e. in the important Coulomb case, the g = 0term does not contribute due to the overall neutrality of the system. Despite the expression's apparent complexity, the reader may convince himself that the convergence in markedly improved (depending on the value of K ! ) . It should be noted also that in the Coulomb case, depending on the boundary conditions, there may be an additional reaction field term, which must be added to the right hand side of the above expression (cf. equation 5.20 in [67], which is the above result for v = 1/2).

5.2.3 Finding the Proper Packing Structure Minimum Problem

-

a Multiple

5.2.3.1 Approaches for Zero Temperature Finding the proper packing structure of crystalline molecular systems means searching for the structure for which the free energy, F = E - TS,of the system is lowest. Here E is the internal energy of the system and S is its entropy. Often the latter is neglected, i.e. the problem is treated at T = 0, which means minimizing E ( T = 0) = U (In some cases entropy effects are 'included' via the parameterization.). The total potential energy U described by the empirical terms discussed above defines an extremely complicated hypersurface in the space of the valence and distance coordinates with a large number of local minima. The optimal packing of molecules is a multiple minimum problem! To the best of the authors knowledge there exists no general algorithm, which solves these problems, i.e. an algorithm which locates the global minimum with certainty (cf. [36,71,72]) - unless the coordinate space is small enough to allow systematic searches or if one begins the search close to the solution by using experimentally determined starting structures. Fortunately, plausibility considerations along the line of the pioneering work of Kitaigorodsky may reduce the extend of the problem. Based on the principle of close packing Kitaigorodsky analyzed which space groups are compatible with irregularly shaped objects, with or without symmetry. This

336

5.2 Packing Calculations Bused on Empiricul Force Fields

analysis yields that only 44 space groups are permissible, only thirteen are probable, and only eight are closest-packed (cf. table 1 in [9] and [73]; see also the first of the example discussed in the last section). A distinction is made between local and global optimization. Local optimization techniques attempt to find the closest minimum (whether local or global) in a downhill fashion, and numerous well-established algorithms exist for this purpose [74]. Local optimization often appears as an ingredient in the structural refinement as part of a global optimization strategy. Because the problem is so difficult, global optimization schemes often assume the character of recipes. Normally only a subset of the structural parameters is variable, whereas the remainder is constrained to either experimental values or ‘reasonable’ guesses (cf. above). In the case of molecular crystals one can, for instance, optimize the molecular packing while keeping the unit cell parameters fixed to their experimental values. One can also constrain the (gas phase) molecular conformations (at T = 0) and optimize the unit cell parameters usually also imposing the proper space group symmetry. Close to the global minimum, one can attempt to optimize the unconstrained system. Various ‘testing and refining’ calculations (In the ‘testing mode’ one compares the calculated packing geometries with the experimental ones, whereas in the ‘refining mode’ the potential parameters are varied to obtain the overall best fit between the calculated and the experimental structures.) along these lines can be found in the above references on force field parameterization via crystal structure calculations and, in addition, in [75] (and refs 1-18 therein) [76] or for polymeric systems in [77]. An extensive overview of early molecular mechanics crystal packing calculations is given in chapter 9 of ref. [2]. Today most commercial molecular modeling programs offer some means for automating such calculations, and there are some programs especially designed for this purpose (cf. [75]; an overview of modeling software can be found in [3]). The development of systematic conformation and configuration search techniques has largely been advanced by people in the biopolymer field and their interest in the conformation and packing of peptides, proteins, etc. Some of these approaches, which are also useful for packing and crystal structure calculations of other molecules are discussed briefly below; for a more extensive discussion, see refs [36,71] In the ‘build-up’ approach one starts from low-energy structures of single residues, i.e. atomic groups, and uses these to build up low-energy structures of two residues put together and so on. This approach, which was developed for peptide residues [36] (and examples therein), can also be applied to molecules instead of residues. This then corresponds to a cluster build-up technique. An example where one molecule is added at a time, is the packing of small benzene clusters [78]. A problem with such a method is, that at an early stage energetically less favorable conformations or configurations are discarded even though in the bulk they are stabilized by the previously missing neighbors. Another method is simulated annealing (discussed in [71] and [36]), where a Monte Carlo search of the conformation space at an initially high temperature is combined with a ‘cooling schedule’, which theoretically ensures that the system eventually ‘freezes’ into the conformation of lowest energy. Therefore this method

5.2.3 Finding the Proper Packing Structure

~

a Multiple Mininiuni Problem

337

is not strictly a T = 0 method, which, if one wants to apply it to the search for the optimal packing structure of a molecular crystal, would require similar enhancement to the molecular dynamics approach discussed in the next section. A further and possibly the most promising [79] approach, the diffusion equation method, is based on smoothing the energy hypersurface U until only the global minimum remains. The method will be discussed in some detail. Let us consider the one-dimensional case U = U , ( x ) , where s is the coordinate. Then U l ( s )= U o ( s )+ S ~ d * U ~ ( s ) / d . .where y ~ , 6~ is a small positive number, is a transformation which diminishes both the minima and the maxima because of the sign of the second derivative in these cases. Replacing of Uo(x), the index 1 by n + 1 and 0 by n we immediately obtain the recursion formula (U,,,, (x)- U,,(.U))/ST= d2Ujl(.x-)/d..y2or in differential form d U ( x ,T ) / ~ = T d’U(s, ~)/d..y.’.This is the diffusion equation with a diffusion coefficient equal to unity. In our multi-dimensional space s can be replaced by .?, whose d components are the atomic coordinates. The corresponding d-dimensional diffusion equation has the solution

(Fourier-Poisson integral), where U(-?!0) = Uo(.?) is the original potential energy function. Thus we should be able to find a ‘time’ r,.,,, beyond which U(.?,T) has only one remaining minimum - presumably the deepest, i.e. global, minimum. A single minimum, however, can be found easily with any local method. At this point we reverse the procedure. Starting from the minimum of U(.?, T,.,,,) we reverse ‘time’ to first obtain U(.?, T,.,,, - 57) and then its slightly shifted global minimum (again with a local method). The last two steps, i.e. calculating U(-?,T - 57) and finding the nearest (and hopefully still global) minimum, are repeated until we are back at Uo(.?). The idea is that if ST is sufficiently small we remain sufficiently close to the global minimum to find it with a local method, even as the ‘bumpy’ landscape of Uo(.?) begins to reappear. This interesting procedure has been applied successfully by Scheraga and coworkers to find the global minimum energy packing structure of Lennard-Jones [80] and water [81J clusters as well as the conformation of oligopeptides [82]. The 55-atom Lennard-Jones cluster of minimal potential energy shown in Fig. 2 illustrates the structural complexity already encountered even for simple interaction potentials.

5.2.3.2 Approaches for Temperature Greater than Zero Already early on in the molecular modeling of crystal packing it was recognized that the neglect of thermal vibrations is an important fdctor, which diminishes the agreement with room-temperature crystal structures [40]. In the worst case, neglecting entropic contributions may lead to entirely wrong predictions of equilibrium structures. A shallow but broad minimum in the potential energy may be favored over a deeper but narrow minimum, because of the constraints on the molecular

338

5.2 Packing Calculations Based on Empirical Force Fields

Figure 2. Different views of a 55-atom Lenndrd-Jones cluster of lowest energy (Mackay-Ikosaeder) obtained via the diffusion equation method taken from reference [80].

conformations imposed by the latter, which therefore is entropically much less favorable. Temperature driven structural phase transitions, for instance, cannot be handled with the above optimization methods at all, and other methods must be used. The molecular dynamics method applied to constant pressure simulations of molecular solids is described here, even though Monte Carlo techniques are certainly equally useful in this context [83]. In its simplest form molecular dynamics comprises the numerical integration of the classical equations of motion of the atoms of the system of interest, i.e. It, = -l/~n,$, U . Here It, is a Cartesian position vector of the ith atom (Here as well as in molecular mechanics calculations certain groups of atoms, e.g. methyl groups, are often condensed into one effective_‘united’ atom.) with the mass m,,where the dots indicate time derivatives, and V, is the gradient with respect to il.The total potential energy, U , again is the sum of Ut,rr,r,l,cr and Ul,on-ho,tL,. Note that the time-step of any numerical method for solving the above equations of motion [67] must be significantly smaller than the inverse of the highest frequency in the system. For an unconstrained system this usually requires a time-step of less than lfs, which currently limits the total simulation time on workstations for systems consisting of a few thousand atoms to
5 2 . 4 Crystal Packing. Fihers atid Surface hiriirced Ortkr

339

for this method to be useful for the study of molecular packing, including temperature effects like thermal expansion or structural phase changes, it needs to be enhanced to allow for size and shape changes of the simulation box. Such an enhancement for anisotropic vo,1um_e changes is given by Parinello and Rahman [84, 85J, y h o iqtroduce vectors / I , , h-,, and hi spanning the simulation volume, i.e. V = h l - 1 1 x~ h3. which themselves are dynamic variables. The dynamics of the latter are governed by additional differential equations coupled to those for the An alternative approach, which is based on position scaling following each integration step, is discussed in [86]. Constant pressure molecular dynamics applied to the structure and dynamics of molecular crystals is discussed extensively by Klein and coworkers [87,83,88]. They discuss the application to the crystal structure of alkali cyanides, carbon tetrafluoride, sulfur hexafluoride, nitrogen. a nitrogen-argon alloy, ammonium bromide, lithium sulfate, hydrocarbons, and others. The dynamic modeling of polymer crystal structures is discussed in some detail in [77, 121 (and references therein). Additional recent references addressing the effect of temperature on the unit cell geometry, i.e. the calculation of the coefficients of thermal expansion, as well as the temperature dependence of the elastic moduli are [89,90] (for polyethylene) and [911 (for aromatic polyamides).

c.

5.2.4 Crystal Packing, Fibers and Surface Induced Order 5.2.4.1 Crystal Structure of Pentamethyl Ferrocene Prediction of the crystal structure of pentamethyl ferrocene [92,93], which utilizes an empirical force field as discussed above. The global search procedure is based on the (local) steepest-descent minimization of a large number of trial configurations for a given space group and molecular structure. Different plausible space groups are tested separately in identical fashion (Note that 87% of all molecular crystals belong to one of the most common 12 space groups.). Figure 3 illustrates the rather good agreement between the predicted crystal structure and the subsequently determined (!) experimental structure.

5.2.4.2 Fibrous and Globulan Proteins A rather complicated but successful application of the ‘build-up’ approach discussed above has been to fibrous and globular proteins. Figure 4 shows the calculated triple-stranded coiled-coil complex of poly (Gly-Pro-Pro) of lowest energy taken from reference [36]. Again the calculated structure is in agreement with X-ray coordinates to within a root-mean-square deviation of 0.3A based on all nonhydrogen atoms.

340

5.2 Packing Calculations Based on Empiricul Force Fields

Figure 3. Calculated (top) and experimental (bottom) crystal structure of pentamethyl ferrocene taken from reference [92].

5.2.4.3 Surface Induced Order These methods can be used to describe surface induced crystalline order at liquidsolid interfaces. In situ scanning tunneling microscopy (STM) at the interface between atomically flat solid surfaces and solutions containing alkyl chains or alkyl derivatives shows that the solute molecules often adsorb from solution to form dense crystalline monolayers at the liquid-solid interface. The structure of these layers depends on the nature of the substrate, and thus cannot be predicted from substrate independent packing considerations alone. As an example of a theoretical approach to this problem on the atomic level let us consider the calculation of the monolayer structure of dotriacontane on MoSez [94]. The calculation of

Figure 4. Calculated triple-stranded coiled-coil complex of poly (Gly-Pro-Pro) of lowest energy taken from reference [36].

5.2.4 Crystal Packing. Fibers nnd Surfi-rcr Iridirml Order

34 1

the total energy utilizes a phenomenological force field to describe the intraadsorbate interaction. In addition, the adsorbate-substrate interaction is described via the full surface potential discussed above. Here the non-bonded adsorbatesubstrate parameters are obtained by fitting the combined Lennard-Jones and Coulomb force field terms to the energies obtained via ub irzitio second order Mraller-Plesset perturbation calculations for small adsorbate-substrate clusters. Figure 5 (a) shows an example of the rib iizitio interaction of ethane with a substrate cluster together with the fitted empirical potential. The so obtained parameters are used to find the minimum energy monolayer structure of C3?Hh6on MoSel by systematically varying the adsorbate unit cell parameters. The final result agrees

6

4

8

10

z t /A

” -2.601 20

0

.I/.

Y

. . ... . ..... . .. .. . . . ..

I

I

30

40

a

I

I

50

60

I

I

70

a

I

80

I

90

Y/deg Figure 5. (top) Subtracted cluster energy LIE(:’) = Eloro,(:’)- E,,),,,(x)vs. :’ (cf. the dotted line in the insert). Circles: nh iiiirio result: crosses: Coulomb contribution; solid line: fit to the difference between the quantum result and the Coulomb contribution based on Lennard-Jones pair potentials between the atoms. The figure shows a result obtained by J . Burda [94]. (bottom) The potential energy of a periodic patch consisting of 31 C32H6hmolecules on the MoSe2 basal plane as a function of the tilt angle y between the lamellar axis and the long axis of the molecules within the lamella. The insert shown a section of the STM image of C32H6hon the MoSez (taken from Fig. 8 of reference [94]). The solid line is a spline fit through the calculated points indicated by the solid circles.

342

5.2 Packing Calculations Based on Empirical Force Fie1d.r

well with the observed structure. This is illustrated in Fig. 5 (b) which shows the potential energy (based on 32 C32H66molecules) as function of the tilt angle 7, describing the orientation of the long molecular axis with respect to the direction of the lamellae formed by the molecules on the substrate.

References 1. Ermer, 0. Aspekte von Kruf’tf‘eldrecAnungen;Bauer: Miinchen, 198 1. 2. Burkert, U., Allinger, N. L. Molecular Mechanics; American Chemical Society: Washington, D.C., 1982; Vol. 177. ~ Chemistry: Lipkowitz, Boyd, Ed.; VCH: Weinheim, 1990- 1995; Vol. 3. R e i ~ i min~ .Computational 1-6. 4. Corripirter Modelling qfFluids Polymers and Solids; Catlow, C. R. A,, Parker, S. C., Allen, M. P., Ed.; Kluwer: Dordrecht, 1990; Vol. NATO AS1 Series C - Vol. 293. 5. vancunsteren, W. F., Berendsen, H. J. C. Angew. Chem. 1990, 102, 1020. 6. Cornpufrr Siniulation in Materia1.s Science; Meyer, M., Pontikis, V., Ed.; Kluwer: Dordrecht, 1991; Vol. NATO AS1 Series E Vol. 205. 7. Cornpurer Sinidation in Chemical Physics; Allen, M. P., Tildesley, D. J., Ed.; Kluwer: Dordrecht, 1993; Vol. NATO AS1 Series C - Vol. 397. 8. Arhmc,es in Polymer Science: Atomistic Modeling qf‘ Ph a1 Properties, Monnerie, L., Suter, U. W., Ed.: Springer-Verlag: New York, 1994; Vol. 116. 9. Glaser, L. In Fundcinzcwtal Principles qf’Molecular Modeling; W. Gans, A. Amann and J. C. A. Boeyens, Ed.; Plenum Press: New York, 1996; p. 199-207. 10. Bowen, J. P., Allinger, N. L. In Reviews in Conzputational Chemistry;K. B. Lipkowitz and D. B. Boyd, Ed.; VCH Publishers: New York, 1991; Vol. 2; p. 81-98. 1 1 . Dinur, U., Hagler, A. T. In Revieivs in Computational Chetnistrj,; K. B. Lipkowitz and D. B. Boyd, Ed.; VCH Publishers: New York, 1991; Vol. 2; p. 99-164. 12. Gelin, B. R . Molecular Modeling q f Pol.vmer Structures and Properties; Hanser/Gardner: Cincinnati, 1994. 13. Landis, C. R., Root, D. M., Cleveland, T. In Reviews in Computational Chemistry; K. B. Lipkowitz and D. B. Boyd, Ed.; VCH Publishers: New York, 1995; Vol. 6; p. 73-148. 14. Israelachvili, J. Internzolec.ular & Surfuce Forces; 2 edn.; Academic Press: London, 1992. 15. Warshel, A,, Lifson, S. J . Chem. Phys. 1970, 53, 582. 16. Lifson, S., Hagler, A. T., Dauber, P. J . Am. Chem. Soc. 1979, 101, 51 1 I . 17. Hirschfelder, J. O., Curtiss, C. F., Bird, R. B. Molecular Theory of’ Gases and Liquid.7; John Wiley & Sons: New York, 1964. 18. Williams, D. E., Starr, T. L. Computers and Chemistry 1977, I , 173. 19. Barker, J. A. Phys. Rev. Lett. 1986, 57, 230. 20. Berendsen, H. J. C., Grigera, J. R., Straatsma, T. P. J . Phys. Chem. 1987, 91, 6269. . 1989, 131, 157. 21. Watanabe, K., Klein, M. L. C h ~ mPhys. 22. Sprik, M., Klein, M. L. J . Chem. Phj,s. 1988, 89, 7556. 23. Rick, S. W., Stuart, S. J., Berne, B. J. J . Chem. Phys. 1994, 101, 6141. 24. Weiner, S. J., Kollman, P. A., Case, D. A. et ul., J . Am. Chem. Soc. 1984, 106, 765. 25. Weiner, S., Kollman, P., Nguyen, D. T., Case, D. A. J . Comp. Chem. 1986, 7 , 230. J . Phys. Chem. 1992, 96, 6472. 26. Nemethy, G.. Gibson, K. D., Palmer, K. A. et a/., 27. Brooks, B. R., Bruccoleri, R. E., Olafson, B. D. r t a/., J . Comp. Chern. 1983, 4 , 187. 28. Mayo, S. L., Olafson, B. D., Goddard, W. A. J . Phys. Chem. 1990, 94, 8897. 29. Hagler, A. T., Huler, E., Lifson, S. J . Am. Chem. Soc. 1974, 96, 5319. 30. Jorgensen, W. L., Tirado-Rives, J. J . Am. Chem. Soc. 1988, 110, 1657. 3 I . Cornell, W. D., Cieplak, P., Bayly, C. I. et a/., J . Am. Chern. Soc. 1995, 117, 5 179. 32. Jeffrey, G. A., Saenger, W. Hydrogen Bonding in Biological Structures; Springer-Verlag: New York. 1991. -

Refererices

343

Steele, W. A. Surfuce Science 1973. 36. 317. Hentschke, R. Macroriiol. Tlieorr Sitnril. 1997, 6 , 287. Waldman, M.. Hagler. A. T. J . Conip. C/?enZ. 1993, 14, 1077. Scheraga, H. A. In Reviews in Conzputntionul Clietnistrj.; K. B. Lipkowitz and D. B. Boyd, Ed.; VCH Publishers: New York, 1992; Vol. 3: p. 73-14?, 37. Hopfinger, A. J., Pearlstein. R. A. J . Coriip. Clzetiz. 1984. 5 , 486. 38. Nilsson. L., Karplus. M. J . Coriip. Clieni. 1986, 7, 591. 39. Zielinski, P., Fouret. R., Foulon, M.. More, M. J . Chetii. Plrw. 1990. 93. 1948. 40. Williams. D. E. J . Clzern. PIzys. 1966. 45. 3770. 41. Williams. D. E. J . Ckeni. P h j ~ .1967. 47. 4680. 42. Williams. D. E. Actu. Cryst. 1974, A 30, 71. 43. Hagler, A. T.. Lifson. S. J . Atn. Clierii. Soc. 1974, 96. 5327. 44. Hagler, A. T., Dauber, P., Lifson, S. J . An?. Clieni. Soc. 1979, 101. 5131. Cheni. Soc. 1979. 101, 5122. 45. Hagler, A. T., Lifson. S.. Dauber, P. J . h??. 1977. 99. 8127. 46. Allinger, N. L. J . A m . Chenz. SOC. 47. Allinger, N . L., Yuh, Y. H., Lii, J.-H. J . h i . Chern. Soc. 1989. 111, 8551. 48. Battezzati, L., Pisani, C.. Ricca. F. J . C / i m . Soc. Fnrncinl, Trans. I / 1975. 71. 1629. 49. Sippl, M. J., Nkmethy, G.. Scheraga. H. A. J . P h j x C h n . 1984. 88. 6231. 50. Yashonath, S.. Price. S. L., McDonald, I. R. Mol. P/ij*s. 1988, 64. 361. 51. Lii, J.-H., Allinger, N. L. J . ,+?I. Chetn. So(,.1989, 111. 8576. 52. Hwang, M. J.. Stockfisch. T. P., Hagler, A. T. J . A m . C/iettz. Sot,. 1994. 116. 2515. 53. Hill, J. R., Sauer. J. J . P l i j ~ C/ietn. . 1994, 98, 1238. 54. Sun. H., Mumby. S. J., Maple, J. R., Hagler. A. T. J . A m . C%rtii.Soc. 1994, 116, 2978. 55. Discover 2.9.5194.0 USER GUIDE, Part 1; BIOSYM Technologies: San Diego. 1994. 56. Levine, I. N. Quuntirni Chernisti.j,; Prentice-Hall: Engelwood Cliffs. 1991. 57. Helfrich, J., Doktorarbeit, UniversitHt Mainz, 1995. 58. Osawa, E.. Lipkowitz, K. B. In Rei+n~s in Cotiip~rrrttiotirrIC / t e r i t i . q ~K.; B. Lipkowitz and D. B. Boyd, Ed.; VCH Publishers: New York, 1995; Vol. 6; p. 355-381. 59. Bayly, C. I., Cieplak. P.. Cornell. W. D., Kollman. P. J . Plrjx. C/iet?i. 1993, 97, 10269. 60. Williams. D. E. In Reviews in Conipututionnl C/wnistrj,:K . B. Lipkowitz and D. B. Boyd. Ed.; VCH Publishers: New York, 1991; Vol. 2; p. 219-271. 61. Besler, B. H.. Merz, K. M.. Kollman, P. A. J . Cortzp. C h n . 1990. 11. 431. 62. Gasteiger, J., Marsili, M. Tetrrrlierlrori 1980, 36, 3219. 63. Houser, J. J., Klopman, G. J . Cornp. Chenz. 1988. 9. 893. 64. Rappe, A. K., Goddard, W. A. J . Phys. C/zetii. 1991. Y.5, 3358. 65. Gregory, J. K., Clary. D. C.. Liu, K.. Brown, M. G., Saykally, R. J. Science 1997, -775. 814. 66. Tironi. I. G.. Sperb, R., Smith, P. E.. Gunsteren, W. F. v . J . C/ietii. P h j x 1995. 10-7. 5451. Clarendon Press: Oxford, 1990. 67. Allen. M. P.. Tildesley. D. J. Cornpurer Siniirlrttions of'Lipici..~; 68. Sperb, R. Mol. Siriiztlution 1994. 13, 189. 69. Ziman, J. M. Principlrs Of'tlze T/ieoy.of'Solid.~;Cambridge University Press: Cambridge, 1972. 70. Hunclbook of'Mutheriinticul Fwzctions; Abramowitz, M.; Stegun. I. A,. Ed.; Dover: New York. 1972. 71. Leach, A. R. In Reviews in Cotqmtutionul Chernistrr; K. B. Lipkowitz and D. B. Boyd, Ed.; VCH Publishers: New York. 1991: Vol. 2; p. 1-55. 72. Schlick. T. In R e i + w in Conipufutiorirrl Chiiistrj,; K. B. Lipkowitz and D. B. Boyd. Ed.: VCH Publishers: New York. 1992; Vol. 3; p. 1-71. 73. Kitaigorodsky. A. I. Molecirlctr Ct.ytcr1.s uriri hfolec~irl~~s: Academic Press: New York. 1973. 74. Press, W. H.. Flannery. B. P.. Teukolsky. S. A , , Vetterling. W. T. Nirriiericnl Rrcipes; Cambridge University Press: Cambridge, 1986. p. Chap. 10. 75. Gibson. K. D., Scheraga. H. A. J . Ph.vs. C/ietii. 1995. 99, 3752. 76. Gibson, K. D., Scheraga. H. A. J . P/ij.s. CIicwi. 1995, 99, 3765. 77. Boyd, R. H. In Adiwices in Poljwer Scicwce: .4tonii.stic~ Modeling of' Phj~sicuI proper tic,.^; L. Monnerie and U. W. Suter, Ed.; Springer-Verlag: New York, 1994; Vol. 116: p. 1 --25. 78. Williams, D. E. Actn. Crj~st.1980, A 36, 715. 79. Scheraga, H. A. In Tnlk presented at tlir 213th A m . C/wii. Soc. N u t / . MecJtitig: San Francisco. 1997. 33. 34. 35. 36.

344 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94.

5.2 Packing Calculations Based on Empirical Force Fields

Kostrowicki. J., Piela, L., Cherayil, B. J., Scheraga, H. A. J . Phys. Chem. 1991, 95, 4113. Wawak, R. J., Wimmer, M. M., Scheraga, H. A. J . Phys. Chem. 1992,96, 5138. Kostrowicki, J., Scheraga, H. A. J . Phys. Chem. 1992, 96, 7442. Klein, M . L. In Dynamics of Molecular Crystals. Proc. 41st Intl. Meet. Sociefi Francaise de Chemie, Grenoble, 1986; J . Lascombe, Ed.; Elsevier: Amsterdam, 1987; Vol. 46. Parinello, M . , Rahman, A. Phjis. Rev. Lett. 1980, 45, 1196. Parinello, M., Rahman, A. J . Appl. Phys. 1981, 52, 7182. Berendsen, H. J., Postma, J. P. M., Gunsteren, W. F. v., DiNola, A., Haak, J. R. J . Chem. Phys. 1984,81, 3684. Klein, M. L. Ann. Rev. Phys. Chem. 1985, 36, 525. Klein, M . J., Lewis, L. J. Chem. Rev. 1990, 90, 459. Lacks, D. J., Rutledge, G. C. J . Phys. Chem. 1994, Y8, 1222. Klunzinger, P. E., Eby, R. K. Polymer Preprints 1995, 36, 631. Lacks, D. J., Rutledge, G. C. Macromolecules 1994, 27, 7197. Schmidt, M. U. Kristallstrukturberechnungen metallorganischer Molekiilverbindungen (Doktorarbeit, Universitat Aachen); Verlag Shaker: Aachen, 1995. Schmidt, M . U., Englert, U. J . Chem. Soc., Dalton Trans. 1996, 2077. Cincotti, S., Burda, J., Hentschke, R., Rabe, J. P. Phys. Rev. E 1995, 51, 2090.

6 Structure and Optical Properties of Conjugated Oligomers from their Vibrational Spectra G. Zerbi, C. Castiglioni and M. Del Zoppo

6.1 Introduction Traditionally vibrational infrared and Raman spectra are used by synthetic chemists for the simple chemical characterization of materials. Most chemists just give a quick look at the spectrum to judge the success of their synthesis or the chemical purity of the sample. A smaller scientific community has focussed on the more detailed vibrational assignments based on symmetry arguments and/or on isotopic rules for the determination of the molecular shapes and sometimes for the actual measurement of the molecular geometrical parameters. An even smaller community has accounted for the vibrational spectra in terms of molecular dynamics supported by theoretical arguments and by numerical computations. A more modern approach to the understanding of the molecular vibrational spectra is the use of numerical simulations based on ‘molecular mechanics’ or semiempirical or ab initio quantum mechanical calculations. These calculations have reached a level of automation which we feel is seriously damaging to science when used in an uncritical way. We hope to show that the vibrational spectra of conjugated oligomers and polymers are a very rich source of data which are lost by quick and superficial analysis based on molecular correlations; are a unique source of specific information on the structure and properties of these materials if an attempt is made to go beyond simple diagnosis based on chemical correlations; and are a prototype for the simultaneous use of vibrational frequencies and intensities which carry the understanding of molecular properties to extreme limits. In the past vibrational spectra of conjugated materials have been dealt with both by ‘problem oriented’ material scientists and by ‘technique oriented’ molecular spectroscopists. The two approaches are different and the information derived differs in depth and detail. Our aim is to provide material scientists with the technical tools developed by specialized spectroscopists in order to derive unique and specific information on the molecular properties of such technologically relevant materials. First we briefly sketch the physics and the derived vibrational frequency and intensity spectroscopy of chain molecules considered first as ideal infinite and periodic systems and then reduced to realistic finite and possibly disordered molecular systems. The purpose is to provide the reader with the techniques used in spectroscopy for the study of chain molecules and the derived conclusions of general validity. Then we offer an overview of the electronic and optical properties of conjugated oligomers and polymers and present a detailed list of the particular

346

6 Structirrc. and Opticcrl P r o p r t i c . ~of Conjugated Oligomers

features of the vibrational spectra of these systems. The additional theoretical concepts which had to be developed in order to account for the specific observed features are introduced. With the use of all these tools the infrared and Raman spectra of oligomers and polymers with delocalized r-electrons (in the pristine and doped states) are interpreted in detail. The spectra of a few classes of polyconjugated materials are interpreted to derive structural characterizations useful for theoretical or experimental materials scientists.

6.2 Frequency and Intensity Spectroscopy No mention will be made in this chapter of vibrational analysis based on empirical vibrational correlations. The principles of such analysis and its applications in chemistry may be found in textbooks and excellent and comprehensive treatments are available [l-31. We think it necessary to emphasize that such empirical correlations may seriously fail and may lead to wrong conclusions when the vibrational spectra of conjugated oligomers and polymers are analyzed. A proof of this will be presented in this chapter. The justification of this statement is that in the many thousands of correlative spectroscopic works available in the literature the physics of delocalized 7r-electrons and the consequent optical phenomena have been overlooked.

6.2.1 Frequency Spectroscopy Vibrational frequencies are easily observed in spectra and are commonly used. Traditionally the 3N - 6 oscillations of the atoms in a N atomic molecule can be described, in the harmonic approximation, in terms of changes of the chemical internal coordinates Ri (where R represents vibrational displacements i.e. bond stretchings, angle bendings and torsions) [4-71. In the harmonic approximation the potential energy function, V ,can be written in the form: =R

2~

F~R

(1)

where FR is the potential energy matrix where each element

r;J

= (d2V/dRidRj)es

is evaluated at the equilibrium molecular structure. Correspondingly the kinetic energy, T , can be written as 2T

= R(G,)-'R

(3)

where GR is the kinetic energy matrix which is determined by the atomic masses and the molecular geometry.

The vibrational frequencies can be calculated by the solution of the eigenvalue equation

diagonal matrix of the 3N - 6 vibrational frequency parameters with where A1 is7 the 1 A, = 4n-c-v; where v, (cm-') is the vibrational frequency and UI, (s-') = v,c (c = velocity of light); LR is the matrix of the eigenvectors; the eigenvector L, describes the i-th normal mode in terms of the set of R coordinates. The quadratic force constants which appear in FR are the key factor in any molecular dynamical calculation and play a determining role in molecular dynamics of polyconjugated systems. The numerical values of h, cannot be easily reached and are still under research. Values of A, have been obtained for many classes of molecules by elaborated least squares refinements on the experimental data derived from the analysis of the vibrational spectra and from many additional experimental data directly or indirectly related to atomic oscillations [8]. More recently successful attempts have been made to calculate ub initio quadratic force constants. A whole new batch of force constants has been and is being presented by commercial computing programs which are based on semiempirical interatomic potentials whose validity is presently being assessed. While physically realiable sets of quadratic force constants are available for simple molecules (mostly o bonded) [9-121, the case of molecules with conjugated T bonds is still very uncertain since the phenomenon of delocalization is hard to be handled and is still poorly described even by ah initio methods (see later in this chapter). The determination of the vibrational potential has been researched for many years. On the basis of these studies, a set of quadratic force constants can be taken as physically reliable when in the least squares refinement the ratio (no. of experimental data/no. of force constants to be refined) is at least 5 : 1 and the statistical dispersion of the calculated force constants is very small. It follows that the fitting between experimental and calculated spectra obtained by merely empirically adjusting a few force constants chosen at will and with no statistical analysis cannot be considered acceptable. Any dynamic and/or structural conclusion derived in terms of such kind of force constants has no physical basis.

6.2.2 Intensity Spectroscopy The intensity of an absorption band is as easy to observe from spectra as the vibrational frequency datum. However, unfortunately for the development of spectroscopy, it has never attracted the attention of researchers, in spite of the fact that it can provide in depth essential information on the electronic properties of molecules. Since in the field of conjugated materials the meaningful targets are electronic effects (e.g. electron delocalization, charge displacements, charge transfer and inter or intramolecular polarizations) vibrational intensities become an essential probe.

348

6 Stritcture and Optical Propertie5 of Conlugated Oligomers

We first consider the case of infrared spectroscopy. The intensity of an infrared band related to the i-th normal mode is defined as [ 5 , 7, 13-15]: Ai = (1/C1)

1

ln(Zo/Zl)dv

band

(5)

where C is the concentration of the sample in mmol/cm3, I the pathlength in cm, du is in cm-* and A , is measured in darks (cm/mrnol). When the double harmonic approximation is adopted (forces and dipole moment changes linear with the atomic displacements) the integrated intensity A, is related to molecular properties by the following relationship A, = ( ~ - . s 1 / 3 ~ 2 ~ l ) 1 ~ M / ~ Q , 1 2

(6)

where N is Avogadro’s number, c the speed of light in vacuo, g, the degeneracy of the i-th normal mode Q, and M is the total molecular electric dipole moment. The direct relationship between vibrational intensities and the vectorial quantity M opens the way to use infrared spectra as a probe of molecular electronic properties. Indeed the instantaneous total molecular electric dipole moment M can be expressed in terms of effective electrical charges, qp, localized on the atoms.

M = c p qprp

(7)

where the sum extends over all p atoms. M varies with the i-th normal mode. The total dipole moment change can be expressed (on the basis of the Equilibrium Charge and Charge Flux model (ECCF) [16--191) as a function:

where q i measures the equilibrium atomic charge of atom p , dqp/dR, the charge flux on atom p when the t-th internal coordinate is activated and dRt/aQi = (L& is the component of the eigenvector (Eq. 4) which represents the dynamic term in the expression of the intensity. Atomic charges and charge fluxes have been measured for many molecules [ 16191 and the results have been rationalized in terms of the various inductive and mesomeric effects which are substantially related to displacement of charges which results in changes of polarization of chemical bonds. The case of Raman intensities is of particular importance in the spectroscopy of polyconjugated materials. Raman intensities are expressed as [5]:

where vL is the frequency of the exciting laser line; vk is the frequency of the Raman shifted line associated to the k-th mode; the +/- sign refers to anti-Stokes or Stokes lines respectively; auv(u,‘u = x,y , z ) are the elements of the polarizability derivative tensor (acu/dQk) associated to the transition from the vibrational ground state to the final vibrational state. The Raman spectra of conjugated compounds show peculiar and characteristic features whose interpretation have required the development of new theoretical

6.3 D~wriniicst i r i d Spectra qj' Orie-Dir,ier7sioriulLuffices

349

aspects which lie outside classical vibrational spectroscopy. The information which can be derived from the Raman spectra are essential and unique for the science of materials whose properties are mostly originated from the intramolecular delocalization of 7r-bonds. In order to account for the observed intensities of low band-gap materials containing highly polarizable electrons the term [auv]k of Eq. (9) needs further development which will be discussed later in this chapter.

6.3 Dynamics and Spectra of One-Dimensional Lattices The dynamics of chain molecules considered as one-dimensional crystals has been thoroughly developed with the main aim of understanding the dynamics and spectra of organic synthetic and natural polymers [20-251. The one-dimensionality of classical organic polymers is assured by the fact that intramolecular forces are much larger than intermolecular forces. For this reason the intramolecular vibrations of such systems can be treated as if the molecule were in wcim and only intramolecular effects were active. Possible weak intermolecular interactions act merely as weak perturbations which, in a first approximation, can be neglected. In the present discussion we consider a periodic one-dimensional repetition of a basic repeat unit made up by p atoms. For such a system 3p is the number of the degrees of freedom of the unit when translations and rotations are included. Simple dynamics establishes that such 3p vibrations when coupled in an infinite periodic chain generate 3p branches in a phonon spectrum. Phonon dispersion curves are generally plotted with phonon frequencies against the wave vector k or the phase shift cp. Each phonon branch spans a certain frequency range depending on the extent of intramolecular coupling between repeat units. The understanding of the role played by the various terms of the vibrational potential of chain molecules is of basic importance for the interpretation of the spectra of polyconjugated one-dimensional systems. The potential energy written in Eq. (1) for a simple molecule can be re-written for an infinite chain in the following way [20, 261:

where n and n' are the indices which identify the site of the repeat units along the chain and i.1 refer to fhe 3p internal coordinates R within the chemical repeat unit. In Eq. (10) (FR)F= ( F R ) l " . The periodicity of the chain requires that:

where s = In - n'J defines the distance of interaction.

350

6 Structure und Optical Properties of' Conjuguted Oligonier.,

If the phonon internal coordinate is defined as Rr' = RPL''~"(where cp is the phase shift between the displacements of two translationally equivalent internal coordinates) the final expression of the vibrational potential energy in terms of the phonon coordinates of the one dimensional polymer chain is given by: 2 v = F" + C, FSelr+ + C, FPSe-"+

(12)

where F", FS and FPs represent the matrices of the quadratic force constants of the units at sites 0, s and -s respectively. In Eq. (12) the first term gives the contribution to the intramolecular potential by the starting unit at the n-th site and the second and third terms collect the contributions of intramolecular (inter-unit) coupling of the n-th unit with the neighbouring units at distances s and -s. The challenge to spectroscopists and to theoretical and/or experimental material scientists is to determine the distance of interaction in conjugated materials. The kinetic energy matrix of the system can be written in an analogous way in terms of the momenta, P",conjugated to the coordinate, R", and in term of the kinetic energy matrix GR [5] analogous to what has been done in Eq. (3). The solution of the dynamical problem for an infinite one-dimensional crystal in terms of internal coordinates, R, and with the potential and kinetic energies as discussed above gives solutions of the type:

R:+'

= A , exp[(-i(wt - scp)]

(13) where the amplitude A , is independent of n, cp is the phase shift between two adjacent equivalent internal coordinates and w is the circular frequency. The phonon frequencies and the corresponding displacements can be calculated by the phase dependent eigenvalue equation: G(P)F(cp)L(cp)= L(cp)Ncp)

(14)

which is similar to that written in Eq. (4) for an isolated molecule, but where cp appears as variable in order to account for the periodicity of the lattice. Eq. (14) allows the phonon dispersion curves to be calculated for any one-dimensional crystal [26-291. A similar equation can be written by replacing the phase shift cp with the wave vector k remembering that:

k =p/d (15) where d is the repeat distance along the one-dimensional lattice. An example of phonon dispersion curves calculated for trans-polyacetylene with a given vibrational potential is reported in Fig. 1 [30]. Of the infinite phonons of the one-dimensional lattice only very few can interact with the electromagnetic radiation giving rise to absorption in infrared and/or Raman scattering. For finite molecules the principles of spectroscopic activity are stated in Eqs. (6) and (9), namely the i-th normal mode is infrared active if during the vibrational motion at least one of the components of the dipole moment derivatives is different from zero; likewise the k-th normal mode is Raman active if at least one of the six polarizability derivatives is different from zero. In the case of one or tridimensional lattices these conditions can be fulfilled only for phonons with k = 0. Recalling Eq. (15) it can be stated that the spectroscopically active phonons are

6 . 4 Froni Orze-Dir,zerisiorial Cq*strils to Finite Molecirlar Chairis

35 1

those in which equivalent atoms in translationally equivalent unit cells move in phase (i.e. p = 0). The selection rules stated by Eqs. (6) and (9) are consistent with the conclusions arrived at when group theory is applied to our systems which may possess some symmetry properties. The classification of vibrations (phonons) in the various symmetry species [25, 3 1, 321 allows a more detailed analysis of the observed spectra to be carried out in terms of molecular geometry and electronic configuration. Because of the very strict selection rules symmetry drastically simplifies the observed vibrational spectra (Fig. 2 ) , bringing into focus a few vibrational transitions which may be selectively relevant in the understanding of the dynamical and electronic properties of the systems. This is what happens when the vibrational spectra are used as a structural probe in the case of conjugated materials [33-38].

6.4 From One-Dimensional Crystals to Finite Molecular Chains The next step is to consider the fact that polymer molecules never form a perfect one-dimensional lattice [24, 391. The following intramolecular structural imperfections should be considered: (i)

(ii) (iii) (iv) (v)

molecular chains are finite and polydisperse (i.e. the material consists of a distribution of chain lengths); the linking of chemical units is generally never perfect (e.g. defects of the type head-to-head linking in an otherwise head-to-tail chain); perfect stereoregularity or regiospecificity are never attained; because of intra-chain defects or interchain disorder (amorphous materials), chain conformational periodicity is not perfect and; even for long perfect chain segments the process of crystallization necessarily induces some sort of chain folding which causes conformational distortions.

Various models have been presented to describe the reality of a polymeric material at the molecular level (e.g. fringed micelle model) [39]. Spectroscopists are then asked to account for the vibrational spectra of a material partly structurally ordered and partly disordered. For these systems translational periodicity and all symmetry properties are removed; the problem is reduced to account for the dynamics of a huge molecule. Theories and numerical algorithms for the treatment of the dynamics and spectra of disordered polymers are available [40, 4 13 and have been applied to the detailed understanding of the vibrational spectra of many technologically relevant organic polymers [27, 28, 411. The same methods have been represented and applied later to calculations of the electronic density of states in polymeric and biological systems with electronic defects [42-441. For the density of electronic states for polaron containing trans-polyacetylene see ref. 42.

352

6 Structure and Optical Properties of' Conjugated Oligomers

700 cm-'

cm-'

600 -

500 -

400 -

300 -

200 -

I

cm-'

190(

180C

L

13000

90

180

1700

6.4 From Otie-Dir?ietisiotinlCrj,stuls to Finite Molecirlur. Chairis

353

(e)

33001

I

Figure 1. Phonon dispersion curves of trrrns-polyacetylene as one-dimensional lattice calculated for a given vibrational force field. (R. Rabaioli, Thesis in Physics. to be published).

Since the object of this chapter is the discussion of the vibrational spectra of oligomeric conjugated molecules it is time to introduce a short outline of the theory of the vibrations of finite chain molecules. The theory is very clearly outlined in ref. [22] and we report here the points more relevant for our discussion. For clarity in our discussion let us consider a finite linear chain of N identical point masses of mass ni connected at distance c/ by a spring with force constant f . The chain has either free or fixed ends. The fixing of chain ends represents the extreme case of a heavy mass placed at either ends. We restrict the analysis to the N longitudinal vibrations. The dynamics of such a system leads to the following results: The N normal modes can be described as N standing waves (Fig. 3) with wavelength A,

=

[ 2 N / ( N- s ) ] d : A,

=

[2(N

+ 1 ) / ( N+ 1

-

s)]d

(16)

where s = 1 , 2 , 3 . .. N . The first equation refers to a chain with free ends and the second to a chain with fixed ends. Let the wave vector k be defined as

6 Structure uizcl Opticul Properties

of Conjuguted 0ligomer.r

CHrtretch

i

-

1519

1000 1500

1500

10009500

1000

Raman shift (cm-I )

Figure 2. (A) Infrared spectrum of trans-polyacetylene; (B) Raman spectra of (a) PA = (CH),, (b) ("CH), and (c) (CD), measured at 647.1 nm (top), 488.0nm (middle) and 350.7nm (bottom) (see ref. 58).

Substitution of Eq. (17) in Eq. (1 5) gives an expression for the phase shift between the displacements of two adjacent atoms for each mode labelled by the index s. 'p, =

k,d

= 2.ird/AS

(18)

using in Eq. (18) the expressions for A, (Eq. 16) simple expressions for the phase shifts are obtained for chains with free and fixed ends: p, = ? j / N ;

'p, =

.iri/N

+1

(19)

respectively wherej = 1 . . . N - 1 and i = 1 , 2 . . . N . When the equation of motions of the two chains are worked out the vibrational frequencies of the N longitudinal normal modes are given by

w,= 2(m/lf) sin(k,d/2)

= 2(rn/lf') sin(rd/A,)

(20)

t d +

Figure 3. (a) Longitudinal vibrations o f a linear chain with N = 8 point masses with free ends. The displacements of the masses are indicated by rotating the displacement vectors by 90‘: ( b ) for a chain with fixed ends (from ref. 22).

356

6 Structure and Optical Properties of Conjugated Oligomers

0 FREE BOUNDARY 0 FIXED BOUNDARY

Figure 4. Dispersion curve for the longitudinal vibrations of a monoatomic chain. The frequencies fixed ends (from ref. 22). for a chain of 8 point masses are indicated (Q free ends, (0)

Equation (20) shows that the dispersion curve (Fig. 4) is the same for free and fixed end chains and that the curve is independednt of chain length. The important conclusions which are extremely relevant in the study of the spectra of finite molecular chains (oligomers) are the following: The N discrete frequencies lie on the frequency branch of the dispersion curve which is that also calculated for an infinite one-dimensional lattice. Indeed by increasing N , the number of normal modes increases with frequencies all lying on the frequency branch calculated with Eq. (20). The kinetic effect of chain ends is that of defining different phase shifts which merely move the frequencies along the dispersion curve [22, 451. (ii) From Eq. (20) it becomes apparent that the frequency range spanned by the frequency branch depends on the value of the spring constantf which defines the intramolecular coupling due to the vibrational potential. (iii) By increasing N , the limiting phonon frequency approaches k = 0. This is the condition reached in the case of the one-dimensional lattices treated in section 3 . In this way the theory of perfect one-dimensional lattices with no boundaries and that of the finite chains coherently meet. (iv) As to the activity of the N modes in infrared, one has to calculate the values of the dipole derivatives required in Eq. (6). A simple inspection of the shape of the normal vibrations of Figs. 3(a) and 3(b) (with N even) shows that for a chain with free ends the total dipole moment changes are zero for s # N (since each dipole in one direction is cancelled by one in the opposite direction), while they take a non-zero value for s = N . In the case of the chain with fixed ends dipole changes are different from zero for s even and zero for s odd. In the case of fixed ends the intensity must be maximum for s = N and decreases quickly when s increases. Similar reasoning can be done for chain with N odd (see ref. 22). (i)

6.4 From Ow-Dimetisiorial crystal.^ to Finite Molecirlur Chains

357

One then expects in the vibrational spectrum of a chain with fixed ends a band progression with the strongest line for s = N and with intensity quickly decreasing when s increases. The band progression is spread over a frequency range directly related to the frequency range spanned by the frequency branch (see (ii) above). For N + cc the k = 0 mode is the strongest mode and all s # N modes are inactive. (v) The principles presented above in (i) through (iv) establish an important theoretical principle which makes vibrational spectroscopy of chain molecules an excellent experimental tool in molecular dynamics. It is known that the experimental phonon dispersion curves can be derived generally from neutron scattering experiments which are difficult and require extremely expensive instruments available in a very few locations in the world. For polymers generally the density of vibrational states is measured from neutron experiments [32, 46, 471. On the other hand, if oligomers with increasing chain length are available the simple examination of the vibrational infrared and Raman spectra locates the band progression; from the trend of the intensities the strongest mode with s = N can be identified and consequently the other modes with s # N . In this way the phonon dispersion curve can be directly derived from the infrared spectrum. (vi) The same concepts apply to realistic molecular chains made of N repeating units with p atoms per repeat unit. In such a case 3p frequency branches can be calculated with Eq. (14) from which band progressions with a strong band for s = N and weaker bands for s # N can be predicted. On the contrary, if the vibrational spectra of chain molecules are experimentally available experimental dispersion curves for the corresponding ‘infinite’ polymer can be directly derived. The best available prototype for this kind of analysis is that carried out by Schachtschneider and Snyder on a series of n-alkanes CH3-(CH2),-CH3 (with s = 1 to 35) as oligomeric model compounds of polyethylene i.e. on a serie of n-hydrocarbons in all-trans conformation [9]. The infrared spectrum of solid n-nonadecane (s= 17) and of polyethylene reported in Figs. 5 and 6 respectively are an example. (vii) The simple model assumed at the beginning of this section for the discussion of the normal modes of a molecular chain assumed identical point masses for all particles. In real molecules chain ends are chemical groups fixed by the chemistry of the system or chosen by the researcher. The contribution by the kinetic and potential energies of chain ends introduces perturbations in the vibrational modes (frequencies and shapes) of the whole chain. Depending on the extent of coupling some of the 3N vibrations may be strongly localized at either ends of the molecular chain (endgroup modes) or may become more and more collective by coupling with the vibrations of the units inside the chain. Endgroup modes are immediately identified because their frequencies remain constant when N increases and their relative intensity decreases when ratioed to the intensity of collective chain vibrations when N increases (Fig. 7).

6 Structure und OpticuI Proprrties of Cor!jugated Oligotners

P

ili

u.w "is

Ill

T

P

9nL

Figure J. inrrarea spectrum or n-nonaaecane progressions are indicated.

L ~ ~ - - \ L ~ ~ ~ , in , -rne L Isoiia I ~

stare.

N

rew oand

stretching

CH2 bending 1

CHZrocking

\

00

Figure 6. Infrared spectrum of polyethylene (CH2)"as an infinite one-dimensional lattice. Notice that the band progressions appearing in Fig. 5 have disappeared and only the k = 0 limiting modes are observed. Notice that while the ordinate in Fig. 5 is in % transmittance the spectrum shown here is plotted in absorbance units.

6.5

Siirwj,

qf the Electrotiic ntid Optical Properties of Conjugated Oligonier.7

359

Number of unit Figure 7. Identification of chain endgroup vibrations in the infrared spectra of oligo-p-phenylenes with the use of relative intensities (from ref. 37).

6.5 Survey of the Electronic and Optical Properties of Conjugated Oligomers and Polymers The discussion of the vibrational spectra of polyconjugated oligomers and polymers has first to consider some of the physical properties of these materials. (a) All the materials considered consist of chain molecules with C atoms in sp' state of hybridization and with a molecular geometry which favors electron hopping between adjacent pz orbitals [48, 491. Such hopping (delocalization, conjugation) is thought to occur along a limited domain of the molecular chain. The length of such domain (delocalization length or confinement length) is not ' u priori' known, but is the target of experimental and theoretical research as a structurally relevant parameter. (b) By increasing the number N of chemical repeat units involved in delocalization the HOMO-LUMO energy gap Eg generally decreases and reaches a limit for the polymer (Fig. 8). It is generally thought that Eg decreases linearly with 1/N, but this may not be always true when electron/phonon coupling and Peierl's distortion takes place. Along with the decrease of Eg the ionization potential decreases as well as the redox potential, thus opening the way to the possibility of doping for obtaining materials which become good electrical conductors.

360

6 Structure and Optical Properties of Conjuguted Oligomers

5.00

4.50

4.00

z

3.50

Y

Lu“ 3.00

/*

m

,’

i

/’

2.50

i 8.

- .‘-I’

2.00

*..4

,.’,*‘

I

1.50

-+++++a

n=

*

10 7

,

--5 4 ~

-

-3 - -

-_

2

I in

Figure 8. Band gap, Eg,as function of l / N for a series of conjugated oligomers. The measured Eg for a few polymers are also indicated. (a) oligoenes, ( 0 ) ohgo( p-pheny1enevinylene)s; (A) oligo( p-pheny1ene)s; (+) N-Boc protected oligopyrroles; (+) oligothiophenes; (*)oligofuranes. From Ref. (61).

(c) For technologically relevant materials Eg must be in the range -1 -3 eV. The molecules can then be classified as low-band gap systems whose vibronic spectrum occurs in the UV-VIS region. Necessarily when N increases the vibronic spectrum shows a red shift toghether with an increase of the intrinsic absorption coefficient [48-501. (d) When the oligomer or polymer chains allow conformational flexibility the conformationally dependent hopping integral plays a key role in affecting the intramolecular delocalization and the related vibronic spectra (Fig. 9) [5 11. (e) The location of the first excited electronic level (LUMO) which can be reached by a symmetry-allowed dipole transition from the totally symmetric ground HOMO state is generally well known experimentally [52, 531 and can be satisfactorily calculated by ‘ab initio’ or semiempirical methods. The location of the higher excited electronic states is not easy to determine, neither experimentally nor theoretically through quantum chemical calculations. The determination of the energy of higher excited states in large conjugated molecules cannot easily be done even with ‘ab initio’ calculations with large basis sets unless particular

6.5 Survej~qf the Electronic arid Optical Properties of Conjugated Oligomers

36 1

-8- H-Tz-H

+

H-T3-H

+

H-(PyL-H

-tH-(Py),-H

0

20

40

60

80

100

120

140

160

180

Torsion angles (deg)

Figure 9. MNDO calculated Eg as function of the torsional angle for dithiophene, terthiophene. dipyrrole and terpyrrole (from J. T. Lopez Navarrete, B. Tian and G. Zerbi, Synt. Met., 1990, 38, 299).

methods are applied. Calculations become time consuming and must necessarily be restricted to small size molecules. In the discussion which follows it will be shown that most of the optical properties of low band gap materials can be accounted for satisfactorily with theoretical models which consider the properties of the ground and first excited states. (f) By chemical doping, electrons are easily removed or added thus generating electronic defects which are variously described as spinless charged solitons, or spinless (2 /2-) bipolarons or +/- polarons (with spin 1/2) (see also Chapter 7.2) [54, 551. The system is brought by photoexcitation into a transient excited state which, because of the strong e/ph coupling quickly relaxes, trapping the electrons into a potential well, corresponding to some of the electronic defects generated by chemical doping. Some of the electronic levels are moved into the gap thus allowing additional new electronic transitions detectable in the UV-VIS electronic spectrum. The electronic defects can be identified from the pattern of the electronic spectrum [55].

+

362

6 Structure and Optical Properties of Conjugated Oligomers

+ POLARON

T

A 0

* *

CHARGEICH UNIT

--

* * * * * * * -- * * * * -- * * * 0.05

l ; ! i i i \ i

--

*it

I i Iw+l-+f-ttnlI:I

* *

:+i+i:I:I

t Figure 10. Lattice polarization (net charge per CH unit) for a positive soliton in the polyene molecule C4,,H4> from MNDO calculations. (From: R. Chance, D. S. Boudreaux, J. L. Bredas and R. Silbey, in ref. 5 5 , p. 825).

The point relevant to the present discussion is the electronic distribution which is generated upon chemical or photochemical doping. When charges are removed or added by the electron acceptor or donor the electronic distribution at the site of the doping is drastically modified. The charges are strongly displaced from where they were before doping and chemical bonds, originally apolar, become strongly polar (Fig. 10). For particular vibrational modes the dipole moment changes become extremely large thus generating very strong infrared bands (see Eq. (6)). The structural variable to be determined is the length of the molecular domain occupied by chemical defect. As an example the size of a positively charged soliton in Cqocalculated by quantum chemical methods is approximately of 20 CH units as shown in Fig. 10.

6.6 Survey of the Vibrational Spectra of Conjugated Molecules The spectroscopic features in the infrared and Raman spectra of oligomers and polymers need to be rationalized possibly in a comprehensive way (for detailed reviews see ref. 33 and 56). These observations are derived from the study of the spectra of a large number of conjugated oligomers and polymers which belong to the classes of polyenes or of polyaromatics. We neglect minor subtle spectroscopic observations which are left to the analysis of the specialist and focus on the most meaningful and striking features of the vibrational spectra of conjugated molecules which can provide useful structural information. The labelling of each observation will be used below in sections 7 and 8 for the theoretical discussion.

6.6 Survej. of’ the L‘ibrnrioilul Spectra

o/ Coiljugaten Molecirles

363

6.6.1 Infrared and Raman Spectra of Undoped (Pristine) Materials The infrared spectra of undoped oligomers and polymers show features which at the first sight can be interpreted in terms of traditional and classical group frequency correlations [ 1-31. The vibrational frequencies are practically independent from N and few bands are easily identified as endgroup modes which disappear with large N (or in polymers) (Fig. 11) (see section 6.4(vii)). Small frequency shifts with N are observed for a few series of oligomers. In some cases the infrared spectrum of stretch-oriented samples in polarized light shows dichroic behavior. For the simplest case of stretch-oriented polyacetylene (PA) [57] and poly( p-phenylene) (PPP) [58] the observed dichroic behavior is anomalous (Fig. 12). In spite of the large number of degrees of freedom, the Raman spectra (excited with various lines from visible to near IR) of even structurally complex oligomers and polymers are extremely simple and show very few and generally strong characteristic lines (Fig. 13) [33, 561. The Raman cross-sections are very large if compared with those of 0 bonded compounds (Table 1) [56, 591. In a number of cases frequency dispersion with N has been observed for the strongest Raman lines. Generally the frequency decreases with increasing N (softening) (Figs. 14 and 15) [60, 611. In a few other cases the strongest Raman line does not soften with increasing N (Fig. 16) [61].

6.6.2 Infrared and Raman Spectra of Doped (or Photoexcited) Conjugated Materials (vii) The vibrational spectra of doped (or photoexcited) materials drastically differ from the spectra of the pristine material [56, 62, 631. A new extremely strong group of bands with complex structure occurs generally in the range ~ 1 4 0 0 - 7 0 0 ~ m(Fig. - ~ 16 and 17). The Doping Induced Infrared Spectrum (DIIRS) is generally independent of the doping species. (viii) In a few series of oligomers the frequencies of the DIIRS and of photoinduced infrared spectrum (PIRS) softens with increasing N (Fig. 18). (ix) For some molecules (e.g. tram-polyacetylene) PIRS is shifted to lower frequencies if compared with the spectrum of DIIRS [65]. In other cases (e.g. poly( p-phenylenevinylene), polythiophene) the two spectra are fully superimposable (Fig. 19) [33]. (x) In some case the complexity of the spectral pattern of DIIRS is reduced and a ‘cleaner’ spectrum is obtained (Fig. 20) [66]. (xi) DIIRS or PIRS of some stretch-oriented materials in polarized light shows clear dichroic behavior [56, 67, 681 (Fig. 21). (xii) The Raman spectra of doped materials are extremely weak, broad and almost unobservable [56]. Some authors have reported Raman spectra of doped

364

6 Structure and Optical Properties of Conjugated Oligomers

2

1602

(a) I

I

I

1600

la00

1400

I

I

I

I

/

I

1100 I

\

I

1000 I

1576 1049

(b)

1

I

1000

1500

1

I

I

1400

1100

1000

6.6 Survey of the Vibratiorial Spectra of Conjugated Molecules

365

Figure 11. Raman spectra of oligopyrroles, (a) N = 3; (b) N = 5; (c) N = 7 ( N = number of repeat units) and of polypyrrole (d) showing the scattering due to endgroup modes (A,, = 514nm) (from ref. 99).

3hOO

2800

2600

2bOO 1sOO Wavenurnbers

2300

1QOO

li00

Boo

Figure 12. Infrared spectrum of stretch-oriented trans-polyacetylene (drawing ratio 7) recorded with light linearly polarized parallel (-) and perpendicular ( - - - ) to the drawing direction (from ref. 58).

materials obtained by chosing an exciting line in resonance with the electronic transition [69, 701. (xiii) Most of the times the Raman spectra of the doped materials show the features of the undoped species (Fig. 22) [56].

366

6 Structure and Optical Properties of Conjugated Oligomers

3.50

3.10

I

$!

2.70

E

N

s

$

2.30

Y

1.5

w h e r e X = t Bn

3

Scheme 1

6.7 The Amplitude Mode or the Effective Conjugation Coordinate Observations 6.6.l(iii) and (iv) are of fundamental importance and are the first which need justification. The claim that such experimental facts can be understood simply in terms of resonance Raman effects cannot be accepted since the same spectral behavior is observed (i) with excitation in the near IR at 1.064pm, at energies far below Eg and (ii) with short oligomers with Eg well above the energies of the commonly used optical lasers [33, 56, 591. Further evidence comes from the results of ab initio calculations which show that the calculated Raman scattering cross section of 0 bonded molecules (e.g. n-octane) is two orders of magnitude smaller than that calculated for octatetraene [56] (Table 1). It must be emphasized that since ab initio Raman cross-sections are calculated by application of a static electric field, ab initio computed Raman intensities are related to an ideal experiment

6.7 The Amplitude Mode or the Effective Conjugation Coordinate

367

Table 1. Raman intensities of the Raman active bands of 0 bonded octane and 7r bonded octatetraene calculated with STO 3G basis set. Octane uk

116 157 216 262 307 527 864 965 1082 1156 1208 1255 1270 1364 1449 1531 1555 1584 1682 1742 1756 1803 1808 1823 1839 1847 3566 3594 35697 3600 3720 3725 3736 3750 3751

[45(ak)ib + 4 ( Y h ) h a l

0 0 0 0 5 0 0 0 12 0 3 4 33 19 9 6 74 0 1 5 3 67 12 2 35 14 77 120

Octatetraene vh [45(cyk):~ f 4(Yk)kl 140 246 375 386 594 793 1070 1104 1144 1196 1251 1287 1374 1514 1541 1557 2017 2050 3656 3706 3714 3714 3815

1 3 3 2 4 13 83 26 12 1 4 71 548 4 137 34 129 3730 82 2 153 31 155

51 4 98 56 5 70 27

with the exciting laser frequency = 0. When frequency dependence is not explicitly introduced no resonance effects can be claimed. The above arguments lead to the suggestion that the observed particular features 6.6.1(iii) and (iv) must be regarded not as due to an accidental proximity of the laser energy to Eg which causes resonance or pre-resonance effects, but as due to an intrinsic property of conjugated oligomers and polymers. The general theories of Raman scattering off resonance and in resonance have been worked out by various authors [71]. As anticipated in section 5, for simplicity in this chapter we restrict our discussion to a two-state model for which only one strongly dipole-allowed electronic transition is relevant for the description of the Raman process. Among the various theoretical models we refer here to the work by Peticolas

368

6 Structure and Optical Properties of Cotvuguted Oligomers

A A

5 T:

h

I

d

. 7

3

8

A,

9

h

s P

A -

11

I

et al. [72] who have described the Raman process in terms of the time dependent perturbation theory. We restrict the Peticolas model to a system consisting of the ground state of A s mmetry and only one electronic excited state of B, species. g .y Under this assumption the Raman scattering cross section is written as:

da/dR

c(

(w2/c) exp( 1/wk)(R(-wI,w2,q)I2 with [(g'le2 ' clle')(e'l(aH/aQk)"le')(e'le,

R(-"JI~w2,wk) = a

*pk')l

*

2I(EegI2 + h 2 ~ P J 2 1 { r < e g- ( f i 4 2 1 " e g - (fiw2)2)-'

(21)

where w1 is the frequency (s-') of the exciting laser, w2= w1 - wk the Raman scattered frequency of the k-th normal mode, (dH/dQk)O the electron-phonon interaction operator, p the dipole moment operator, el and e2 the polarization vectors of the incident and scattered light respectively, g' and e' the purely electronic eigenfunctions of the ground and excited states respectively and Eeg the HOMOLUMO energy difference between excited and ground state.

369

6.7 Tlir At?iplitucie Mode or the Efrctive Conjiigution Coordiiiuir

1620 1600 1580 1560

r

8

1500 1480 1460 1440

4

!

'

i ,I'

k' +--------*--*-- - -c - ---- - - _ _ _ _ _ _ -* I ~-_

------+++--+-~n=

10

~. ~-

i~

7

5

4

~

3

iln

Figure 15. Chain length dependence (softening) of the strongest Raman line associated to the 'amplitude mode', or FI mode, (see text) for the series of oligomers and polymers of Fig. 8. (W) oligopolyenes, ( 0 )oligo( p-phenyleneviny1ene)s; (A) oligo( p-pheny1ene)s; (+) N-Boc protected oligopyrroles; (+) oligothiophenes; (*) oligofuranes. From. Ref. (61).

Three factors need to be identified in Eq. (21) [56, 591: (a) a frequency factor which accounts for the intensity enhancement in resonance conditions; (b) a term which depends on the value of the dipole transition between g and e; and (c) the electron-phonon coupling term. This last term, according to Hellman-Feynmann theorem, can be rewritten as: (e'I(dH/dQk)"Ie')

= (aEe/aQk)"

(22)

Equation (22) is the derivative of the energy of the electronically excited state with respect to the coordinate Qk evaluated at the equilibrium nuclear configuration of the ground state. It has been shown that in first approximation: (dEe/@k)O

M w,(AQk)'"

(23)

where wk is the vibrational frequency, (AQk)e,gis the displacement of the equilibrium position of the normal coordinate Qk in going from the ground to the excited state. From Eq. (21) it follows that only normal modes with a non-vanishing

310

6 Structure and Optical Properties of Conjugated Oligomers

4000

3280

2560 1840 WAVE NUMBE R

1120

400

Figure 16. Infrared spectrum of FeCI3 doped regiospecific poly(dihexy1trithiophene) (Scheme 3). The large intensity of the doping induced bands can be judged relative to the intensity of the C-H stretchings of the alkyl side groups near 3000cm-' which are generally medium-strong. /R

R\ R = rn-hexyl

Scheme 3

N

8 0 ;1$00

1570

l6VO

lalo

lie0

lb50

620

?a0

860

630

WAVENUMBER

Figure 17. Infrared difference spectrum (pump and probe) of a sample of sexithiophene (at liquid nitrogen temperature) illuminated by a laser light at 514nm (from ref. 68).

1585

1331

1277

1223

li69

lit5

WAVENUMBER

1661

lb07

653

899

Figure 18. Dispersion with chain length of the photoinduced infrared spectrum for a series of alkylsubstituted oligothiophenes (T,?with I I numbers of thiophene rings) illuminated by a laser light at 514nm (M. Veronelli and G. Zerbi. to be published).

projection along the direction A R (where A R describes the geometry difference between g and e ) have appreciable Raman intensity. The existence of the e/ph coupling term in Eq. (21) justifies the intensity distribution among the Raman active vibrations. The study of conjugated oligoenes and rrmis-polyacetylene has led to a better understanding of Eq. (31). From quantum mechanical calculations the largest geometry changes in these systems in going from the gound (A,) to the first excited electronic state (B,) consists in the lengthening of the double bonds and in the shrinking of the single bonds [ 7 3 ] . Such geometrical change can be described by defining a new vibrational coordinate, generally labelled as II. In terms of the commonly known vibrational internal coordinates R (section 6.2) 2 describes the direction of nuclear trajectory along which the minimum of the potential energy in the excited state is displaced with respect to the ground state minimum. It follows that II is the direction of the atomic displacements along which the e/ph coupling is most effective. The other 3 N - 7 internal coordinates can be chosen orthogonal to II, such that in this vibrational subspace e/ph coupling is negligably small or zero, thus reducing the intensity of all the other modes which do not contain a contribution by the II mode. These modes become negligably small compared with the large intensity of the modes which contain a contribution of II [ 5 6 ] . One can then write

( dEe/dQk)'

=

(d E e / d I I ) " L ~ k

(24)

Since the II coordinate is totally symmetric, the Raman active totally symmetric (TS) modes are predicted to be stronger than modes of other species. Of the TS modes those containing a large contribution by the II coordinate are preferentially strongly

312

6 Structure and Optical Properties qf Conjugated Oligomers

JJk 3

frequemwcm -'I Figure 19. Comparison of the doping induced and photoinduced infrared spectra in poly( p-phenylenevinylene): (a) pristine; (b) FeC13 doped; and (c) photoexcited. In each pair of spectra the top spectrum is calculated and the bottom spectrum is observed (from ref. 33).

6.7 The Amplitude Alorlr or the Eflixtiw Conjirgatiuii Coordiriritc

373

WAVE NUMB E R

Figure 20. Blown up detail of the spectrum of Fig. 16

enhanced because in Eq. (31) the dipole transition moments are large as well as the e/ph coupling term. The definition of the R coordinate for a few classes of molecules is given in Fig. 23. The tI coordinate for polyenes coincides with the 'amplitude mode' proposed by the school of Horovitz [74] when projected on the molecular axis. In the case of aromatic systems. the R coordinate describes the changes from the aromatic to the quinoid structure.

vv

VH

HV

HH

Figure 21. Dichroism in the photoinduced infrared spectrum of stretch-oriented poly(octy1thiophene)illuminated by a laser light at 514nm. Notice that the extremely weak spectra are the results of the superposition of 10000 scans. V: parallel to the stretching direction; H: perpendicular to the stretching direction (from M.Veronelli. M. C. Gallazzi and G. Zerbi. A m Po/jwier, 1994.15, 127).

mo

QOD

1400

(cm-')

1600

Figure 22. Comparison of the Raman spectra of pristine and doped trans-polyacetyleme showing the lack of scattering due to the electronic defect and the upward shift and changes in spectral shape in the region of Q associated to the change in relative concentration of chain segments which have become, on the average, shorter upon doping: (a) pristine; (b) I doped (2%); (c) C104 doped (7%); (d) highly doped with Li; (e) FeC1, doped (13%). (a), (b), (c) ,A,, = 458nm; T = 78 K; (d) ,A,, = 458 nm; T = 295 K; (e) ,A,, = 413 nm; T = 295 K. (For a discussion and references see ref. 56).

P O LY ( P A R A - P HE N YL EN E)

POLYPYRROLE

AND

( X = N H )

POLYTHIOPHENE

( X = S )

P O L Y ( P A R A - P H E N Y L E N E V I N Y L E N E)

Figure 23. Definition of the II modes for a few common conjugated molecules. R defines the stretching of the CC bonds.

376

6 Structure and Optical Properties of' Conjugated Oligorners

It is obvious that since the 5I coordinate describes the preferential path along which the e/ph coupling is maximized, the strongest Raman line has to be assigned to the motion which contains the largest contribution by 5I. The definition of II has allowed to propose the 'Effective Conjugation Coordinate' model (ECC) [56, 571 which has been widely used in the interpretation of the spectra of many polyconjugated systems. (For reviews see ref. 33 and 56). The introduction of II in the study of the dynamics of several polyconjugated systems allows one to rationalize the observed frequency dispersion (observation 6.6.l(v) and Figs. 14 and 15). When the II coordinate is directly introduced in the dynamical treatment the expression of the corresponding quadratic force constants FHin terms of the force constants F R (see section 2) the following expression is obtained: where k i and k: are the diagonal force constants associated to the C=C and C-C stretchings (R and r respectively) of the central unit (unit 0) taken as reference and f O refers to the interactions between CC bonds within the 0-th unit; f S are the interaction force constants between CC bonds at distance s along the chain. From quantum mechanics it has been shown [75] that the terms fRR andf,, are always negative whilefR, always positive; it is then apparent that the sum in Eq. (25) is always positive. It thus follows that the larger the sum the lower is the value of FR.This accounts for the softening of the frequency associated to the 5I mode when the conjugation length increases by increasing the number N of units in the chain [60] (observation 6.l.v). The distance of interaction s in Eq. (25) has the same meaning as the distance of interaction s in the potential energy function of a one dimensional lattice (Eq. (1 2)). For the discussion which follows Eq. (25) can be schematically re-written as: Fx

+

(K" + f " ) Csfq

(26) where the intra-unit contribution is represented by the terms in brackets and the contributions by the inter-unit interactions are collected under the sum extended to the s-th unit. An unequivocal definition of the effective conjugation length (ECL) can then be proposed as the threshold distance s at which the termsfs vanish (see also Chapters 7.1, 7.2 and 10). The value of FFIcan then be used as an effective parameter which describes the state of conjugation in a series of conjugated oligoiners with increasing chain length. =

6.8 Electron-Phonon Coupling, Confinement Length and Pinning Potential In conjugated molecules the distance of interaction (s in Eqs. (25) and (26)) has allowed definition of the effective conjugation length (ECL) and is one of the

6.8 Electron-Phoiion Coupling, Confinement Length and Pinning Potential

377

'T 0.8 -. 0.6 -.

2 le

0.4 -.

0.2 -0 , 0

1

I

I

1

2

3

1

I

4

5

S Figure 24. Changes of the values of interaction force constants along a chain of oligoenes (-) MNDO; (---) 631-G (from ref. 56).

most important unknown parameters to be determined. Quantum mechanical calculations using various basis sets have been carried out for the calculations of each term in Eq. (25) [35, 771. The relevant conclusion is that in all the cases studied by quantum chemistry the interactions decrease very quickly with increasing s as if delocalization were restricted to a very short molecular domain (say -2 double bonds at either side of the central unit taken as reference) (Fig. 24). If true this conclusion could be very relevant in the physics of conjugated oligomers and polymers. Indeed if ECL were to be as short as indicated by quantum chemical calculations the efforts by chemists to synthesize long oligomers or polymers should be vain since the relevant properties (bond order, bond length, effective charges etc.) seem to be reached already for very short oligomers [77]. At least for a few simpler systems we suggest that new calculations with electron correlations and/or other methods of calculations should be tested before reaching any final conclusion of physical relevance. The possible inadequacy of the quantum chemical methods so far applied is indicated by the fact that the values of the minimized geometrical parameters and of atomic charges calculated for oligomers with increasing N reach a plateau after too few units. In other words quantum chemical numerical calculations seem to define very limited conjugation lengths while in some cases interactions at much larger distances are required for the justification of the optical properties of oligomers and polymers (see section 14). At this stage of the discussion observations 6,6.l(v) and 6.6.l(vi) together with Figs. 8 and 15 need to be accounted for. Figure 15 shows in a comprehensive way the data collected on several series of oligomers and polymers [61] and shows that the softening is strong for oligoenes, medium-weak for oligofuranes, oligopyrroles

378

6 Structure and Optical Properties of Conjugated Oligonzers

and their N-BOC protected analogs. Little or no softening is observed for oligothiophenes, oligo( p-pheny1ene)s and oligo( p-phenyleneviny1ene)s. The relationship between FR and the e/ph coupling needs to be stressed as follows. Fx (Eq. 26) can be related to the adimensional parameter 5, introduced by the school of Horovitz in the theory of the ‘Amplitude Mode.’ This theory has focussed on PA and expresses the effective e/ph coupling in polyene chains of a given length and represents the changes of the potential from bare chains (only 0 bonds) to chains dressed with 7r bonds.The relation between FR and 5, is the following [56, 571:

x

Fi

-

-

FH = const(1 - 2)X

(27)

where F;f is the II force constant of the molecule taken as reference [57]. For a better understanding of the connections between various force constants and the e/ph coupling in terms of the Hiickel Model another relation can be written:

where 7rij are the bond-bond polarizability coefficients, d,B/dRi is the e/ph coupling parameters and /?the hopping integral [78]. According to Eqs. (25) and (26) the softening of the II mode is directly related to the changes of the electronic structure of the molecule when N increases. The fact that in a few series of oligomers ECL does not affect vR means that delocalization is strongly damped within a few units and necessarily remains constant when N increases. On the other hand in the case of oligoenes the e/ph is very large indicating strong interactions between 7r electrons and/or large interaction distances s. It has recently been shown that in conjugated oligomers containing aromatic or heteroaromatic building blocks intra-unit and inter-unit delocalization are two competing phenomena [6 11. Because of ‘aromaticity’ 7r-electrons tend to be strongly confined within each ring while inter-ring electron hopping tends to extend the delocalization along the molecular chain. The different slopes of the plots vR vs. 1/N of Fig. 16 can be interpreted in terms of the strength of a ‘pinning potential’ which tends to localize 7r-electrons, the stronger the pinning the smaller the slope. The interpretation of the data of Fig. 16 seems somewhat at odd with the values of the experimental transition energy Eg plotted in Fig. 8 for the same oligomers studied in Fig. 16. The Eg decreases almost linearly (with different slopes) for all the oligomers studied and extrapolate nicely to the Eg values for the polymers. The different dispersion of Eg with 1/N and vH can be rationalized by noticing that within Huckel’s model a non-zero and fixed value of the hopping integral between two adjacent rings is sufficient for predicting a dispersion of Eg with N since the number of levels increases with N . The dispersion of Eg can be accounted for without the need to include interactions at distances larger than the first neighbour. On the contrary FRchanges only if medium or long range interactions are introduced when N increases. It follows that the dispersion of vH is a more direct probe of the changes of the overall (collective) electronic structure when N increases.

6.9 The R Mode and the Infrared Specirimn of Doped Species

379

It has to be noticed that for systems where v2 does not soften with increasing N the e/ph coupling term cannot be zero but can be practically constants with N . This is required by the experimental observation that the Raman cross-section (see for instance Eq. (21)) is not zero. This observation implies that some delocalization takes place, but it is very restricted to short molecular domains.

6.9 The II Mode and the Infrared Spectrum of Doped Species Our analysis proceeds by considering observations 6.6.2(vii). (viii) and (ix). As discussed in section 6.5(f), upon doping electronic charges are displaced, bonds originally apolar become strongly polar because of charge displacements and the dipole changes for a few particular modes become very large, thus originating very strong infrared bands. This is indeed the case observed in all the spectra of doped oligomers or polymers. Again the 2 mode plays a determining role also in the spectroscopy of doped oligomers or polymers. First the dynamics should be considered. The charge transfer which occurs upon doping strongly perturbs the distribution of electrons thus changing the force constants which are sensitive to 7r electron delocalization. It follows that the value of the force constant FH changes drastically and decreases because the terms in the sum of Eqs. ( 2 5 ) and (26) are much larger and possibly extend further along the chain. It is thus expected that vH for doped oligomers or polymers has to occur at much lower frequencies as indeed is observed and calculated [33-38, 561. Moreover, the softening of vH has been seen to be dependent on chain length in doped oligoenes [79] and chain length independent in doped oligo-paraphenylene vinylenes [go]. In the framework of ECC theory these observations are simply accounted for in terms of the length s and strength of the interaction of the central doped units with the neighbouring units. Again the concept of weak or strong electronic pinning respectively can be introduced. The pinning of electrons accounts also for observation 6.6.2(ix). In traia-polyacetylene the pinning by the counterion is stronger than when the defect is photogenerated and DIIRS and PIRS are noticeably shifted one from the other [65]. On the contrary, in the case of oligothiophenes and oligo( p-phenylenevinylene related polymers (Scheme 4) the pinning is strong in both cases and the DIIRS and PIRS are almost identical and almost superimposable [33]. As discussed in section I 1 for trans-polyacetylene it has been shown that the polarization of the CC bonds induced by the dopant is not enough to account for the very large intensity of vx and of the modes coupled with it. It has been shown that during the dimerization oscillation (amplitude mode, II mode) a considerable fluctuation of charges takes place along the chain [81]. In the ECC model this is represented by very large charge fluxes (Eq. (8)) which

380

6 Structure and Opticul Properties of Conjugated Oligomers

Scheme 4

become relevant only during the normal modes which are coupled to through II [81].

7~

electrons

6.10 The Raman Spectra of Doped Species Observations 6.6.2(xii) and (xiii) are still matters of discussion. Let us take a pristine sample (with a well understood Raman spectrum) and proceed with a doping process. The doping level reaches a certain plateau which differs in different compounds. It is approximately 18% for trans-polyacetylene (PA) (when doped with alkali metals) and -30% for polythiophene. If we consider the material as fully doped it means that one dopant molecule is shared by -6 CH units in PA by 3 thiophene units respectively. This approach would consider that and doping is homogeneous tending towards a ‘polaron’ or ‘bipolaron’ lattice sometimes seemingly accompanied by phase transitions not always fully understood. Another approach, especially at doping levels below ‘saturation’, considers that doping be inhomogenous and that the material must be considered as a twophase model consisting of a doped part and an undoped (pristine) part. According to section 9, the existence of doped sites is proven by the DIIRS. The possible existence of sections of chains which are still in the undoped state is assured by the observation in the Raman spectra of the characteristic II mode whose frequency may give an estimate of the ECL of the undoped chains. If the sample consists of a collection of chains of different ECL one can derive a detailed description of the distribution of ECL using the fact that the Raman spectra of a collection of chains of different length (hence with a distribution of energy gaps) may show dispersion with the wavelength of the exciting laser radiation since each wavelength may approach the Eg of a certain species, thus originating its selective resonance Raman scattering. Our last target is to predict theoretically the appearance of the Raman spectra arising from the doped sections of the material. A discussion of the theoretical concepts has been first reported in ref. 56 where the results originate from ab initio calculations. It is shown that the Raman spectrum of the doped site should be extremely weak if ever observable. Another approach to the same problem may be suggested by examining the plot of the N

6.10 The Rariiari Spectra ojDoped Species

t

38 1

I

Figure 25. Polarizdbihty a: (+), hyperpolarizabilities (0)and y ( A ) as function of the reaction field F which modulates the dimerimtion parameter as sketched in the figure (from F Meyers et al.. Noillinear Optics. 1995. 9, 59).

molecular polarizabilities as function of the degree of bond alternation which will be discussed in more detail in section 14 of this chapter. While the slopes of Fig. 25 are either positive or negative when bond alternation has a non negligeable value, when bond alternation approaches zero (bond equalization) the derivative (&I/@) is zero. It follows that if the doping process produces domains where adjacent bonds tend to be equalized the corresponding Raman intensity should tend to zero. The decrease of the intensity and the broadening of the Raman spectra when the doping levels increases were first presented by Harada et al. [69]. More recently Sakamoto et al. have reported the Raman spectra of alkyl derivatives of oligo(ppheny1ene)s and of oligo( pphenyleneviny1ene)s (Scheme 5 ) [70]. These authors were able to enhance the weak intensity of the Raman lines by using the 1.06 pm

Scheme 5

382

6 Structure and Optical Properties of Conjugated Oligomers

laser line which happens to be in resonance with one of the intra band transitions of the systems studied. The intensity of the Raman lines as function of the degree of bond alternation (amplitude) will be an important ingredient in the study of the nonlinear optical response of oligomers as discussed later in section 14.

6.11 Evidence of Large Charge Fluxes from Oriented Samples in Polarized Light From the theoretical viewpoint the existence of charge flux during any normal mode Qk can be expressed as [56, 821. ( d P u / d Q k ) c h a r g e f l u x = CeP~(e'IdH/dQkIg')/V~, (U = X, Y) Z) (29) for the definition of the symbols see Eq. (21), where dH/dQk is the e/ph coupling operator. Eq. (29) indicates that: (i) the charge flux increases with increasing conjugation length since vge decreases with increasing conjugation length and increases and (ii) the charge flux must be larger in 7r-systems than in 0-systems since the band gaps are much smaller in 7r-conjugated molecules. From the experimental viewpoint we refer first to the case of stretch-oriented samples of pristine trans-PA (drawing ratio 7) whose infrared spectra are recorded with polarized light with the electrical vector parallel or orthogonal to the stretching direction which coincides with the direction along which molecular chains are aligned [57]. After correction for chain misalignment the transition moment of the C-H stretching vibration (in which the H atom move practically orthogonal to the chain axis) shows an unexpected strong parallel component (Fig. 12). If PA chains were undimerized (with no bond alternation, i.e. metallic) the transition moment for the C-H stretching mode would lie exactly along the C H bond and orthogonal to the stretching direction since on stretching the CH bond the charge flux coming from all equal CC bonds on either side of the CH bond must be the same. If the chain is dimerized the charge flux from the C-C side differs from that of the C=C side. Hence the transition moment necessarily cannot lie along the C H bond, but makes an angle with respect to the chain axis, thus generating a parallel component in the polarized spectrum. This is clear spectroscopic evidence from intensity spectroscopy that: (i) the backbone chain is dimerized and (ii) sizeable charge fluxes take place along the C C skeleton. The angle of the transition moment with respect to the chain axis is approximately 42". The charge flux along the chain axis during C-H stretching can be expressed as charge fluctuation along C=C and C-C bonds adjfcent to the vibrating C-H unit: A = dqcZc/dd - dqc-c/dd. A value of A = 0.1 e A-' has been obtained from the analysis of the experimental relative intensities in the polarized spectrum of stretchoriented trans-polyacetylene [57]. We then turn to the huge intensity of DIIRS of doped molecules. As already discussed in section 6.9 the DIIRS is associated to the activation in IR of the H

,$

6.12 What do we Learri from Vibrational Spectra:)

383

mode because of the lowering of the symmetry caused by the large c$arge displacement at the site where doping takes place. A charge flux of -3.5eA-' along the doped chain during the II vibration has been obtained from the experimental intensities of DIIRS.

6.12 What do we Learn from Vibrational Spectra? The conclusions briefly given in this section have been reached, mostly in the Milano laboratory, from the analysis of the spectra of oligomers and polymers based on the methods described in this chapter. The concepts outlined in the previous sections find textbook examples in the applications presented below. The reader should look at the quoted references for a more detailed analysis.

6.12.1 All Trans-Oligoenes and Trans-Polyacetylene From Fig. 8 the dependency of the Eg on 1/N is not fully linear, but in a first approximation Eg of PA can be obtained by extrapolation giving a value of 1.5- 1.6 eV. According to theory the totally symmetric modes are those which first occur in the vibronic spectrum of a polyconjugated molecule [83]. The spacing of the vibronic components in the observed vibronic spectrum turns out to be determined mostly by the frequency of longitudinal II mode. The infrared frequency and intensity spectra of polyenes and of pristine (PA) give unquestionable experimental evidence that the equilibrium structure is dimerized (i.e. with bond alternation) and that a large charge flux occurs along the chain [57]. The values of the parameters derived from infrared intensities according to the ECCF theory (Eq. (8)) are the folJowing: q i = 0.134e; q: = -0.134e; d q H / d d = -0.207eA-'; dqc/dd = -0.242eA-' [84]. The value of q& is in good agreement with that calculated from ab initio methods on some oligoenes ( e g = 0.127e) from a Mulliken population analysis with a 6.31G' basis set [85]. For a comparison between Mulliken's charges and infrared charges see ref. 86. For the one-dimensional lattice of PA, p = 4 is the number of atoms which generate 2 x 4 = 8 frequency branches in the phonon spectrum for the vibrations in the molecular plane. In-plane motions have attracted the attention of many workers, while not much has been done for the out-of-plane deformation. Let vi (i = 1 , 2 . . .8) label the frequency branches of the phonon spectrum of the inplane motions (the labelling of the frequency branches and of the corresponding normal modes is rather confused in the literature). The softening of the strong Raman active totally symmetric modes for oligoenes is given in Fig. 14. Let v l , v3, u4 and v6 label the k = 0 Raman active modes of PA which also represent the lines observed for oligoenes. y and u6 correspond to two extremely strong longitudinal modes which contain a large contribution of the coordinate II. v4 N

384

6 Structure and Optical Properties of Conjugated Oligomers

corresponds to the in-phase stretching of C=C and C-C bonds which move C atoms almost, but not fully, orthogonal to the chain axis, hence does not contain contribution from the II mode [56]. The very small contribution by II predicts that the corresponding Raman line should be very weak, as indeed observed in oligoenes and in PA. The dispersion of vR with ECL has been the subject of extensive studies by several groups for the understanding of the heterogeneity in polymer length of real samples of trans-PA [87]. The dispersion of the Raman spectrum of a real sample with the excitation wavelength provides information on the ECL of the material. Various theoretical models have been proposed which yield various kinds of multimodal distributions of ECL [87]. We think necessary to point out that, because of Eq. (21), the Raman cross section becomes stronger the longer ECL. Thus the relative intensities of the Raman lines cannot be directly related to the relative concentrations of various oligoenes or of various chain sections in the polymer. Not all the models proposed have taken these aspects into account. The Raman spectral pattern observed in oligoenes is re-encountered in many classes of molecules which contain oligoene chains. The class of carotenoids existing in many natural materials has been studied in detail in terms of ECC and has provided structural information of biochemical relevance [88]. Moreover the same spectral pattern has been observed in resonance Raman scattering of retinal with 'in situ', 'in vivo' and time-dependent experiments related to the mechanism of vision [89]. More recently the II mode of retinal and bacteriorhodopsin has become the center of interest in the study of nonlinear optics (NLO) [90]. Assuming a linear dependence of vR on l / N the k = 0 phonon frequency for an infinite one-dimensional lattice of trans-PA is obtained from Fig. 15 in the range 1460-1440 cm-' , The dispersion curves of the in-plane phonons have been calculated and the role of the e/ph coupling through the force constant FR has been clarified. Increasing ECL by adding chemical units softens FXwith the consequent softening of vR. The softening occurs mostly near cp = 0 of the frequency branch v3 which mostly involves the II coordinate (Fig. 26): the frequency branches of the full phonon spectrum are strongly dispersed thus indicating the existence of large and long range interactions (i.e. large values of s in Eqs (25) and (26)). The frequency branches can be constructed from the k # 0 normal modes of oligomers. This has been done at least with theoretical calculations confirming the bending of v3 near cp = 70" and is at present being studied experimentally on polar oligoenes [91]. Also " 6 branch softens slightly at p = 7~ [91]. The dispersion of the out-of plane phonons has never been treated in great detail. Only recently the phonon dispersion has been constructed experimentally from the k # 0 combination lines observed as clear band progressions in the infrared spectra in the 1950-1700cm-' range of several oligoenes [92]. In this way it has been possible to account for the puzzling broad and medium strong absorption in infrared observed in the same range which lies outside the traditional absorption interval which can be accounted for by chemical correlations. The infrared spectrum of doped and photoexcited trans-PA has been studied during the development of the science of polyconjugated materials in the 1980s. Following the ECC theory, the vibrational infrared and Raman spectra are well

6.12 Wliat do n>eLearn fj.oi)i Vibrational Spectra?

385

cm'

1901

180(

- c10 170C

--- C 8 c 22 - - c12 I

I

I

20

40

60

I

I

I

I

I

I

80 100 120 140 160 180

Figure 26. Softening of the k = 0 mode of trails-polyacetylene due to the softening of FR force constant thus showing the effect of the increase of electron-phonon coupling (see text).The values of FA have been obtained by ah initio calculations on oligoenes with increasing chain length CxHlo,CloH12,C l z H l j and C,zH24. (From ref. 91.)

accounted for in term of the 2 mode, originally strong in the Raman spectrum of the pristine material, but which become very strongly infrared active upon doping because of a new asymmetrical electronic situation at the site of doping which causes the lowering of the symmetry. The strong intensity is not only due to symmetry lowering, but on the basis of the ECC model, very large charge fluctuations contribute to the intensity enhancement. The DIIRS of several potassium-doped oligoenes (with N double bonds, where N = 4, 6, 9 and 17) have been recorded and used for the estimate of the length of the charge carrier in doped polyenes [93]. Careful doping in an oxygen free environment has allowed to produce materials whose DIIRS shows dispersion with chain length. The frequency of the vfl mode occurs at 1550cm-l for N = 4 and shifts to 1453cm-' for N = 17, but does not reach the value of -1400cm-l observed for doped PA. It follows directly that the average length of the perturbed domain in PA must be larger than at least 20 C=C. Analogous conclusions can be drawn of the doping induced v6 mode which grows in intensity and softens with increasing N . It is remarkable to note that the behavior is analogous to that observed in the Raman s ectra of the pristine polyenes. In that case vx = 1611 cm-' ( N = 4); 1568cm- ( N = 6); 1511 cm-' ( N = 9); 1488cm-'(N = 17) and x1450cm-' for PA. These experiments show that ECL in pristine PA must be 2x20 CC bonds.

P

386

6 Structure and Optical Properties qf Conjuguted O/igomers

6.12.2 Oligomers and Polymers of Heteroaromatic Building Blocks The class of oligomers and polymers of heteroaromatic molecules is studied for their relevance for the possible development of devices based on their electrical conductivity if doped or on NLO responses in the pristine state. The experimental estimate of the effective atomic charges and of a few structural parameters can be made directly by using concepts of combined intensity and frequency spectroscopy applied to the stretching of the C-H bonds. Following the methods first proposed by the school of McKean [94], and further developed by Gussoni et al. in Milan [19] relations of the type frequency/bond length, v / d , and frequency/effective atomic charge, v / q p ,have been worked out and can also be made in the case of the molecules discussed in this section. Since C H stretching frequencies of aromatic and heteroaromatic systems can be considered as ‘local modes’ (i.e. as isolated oscillators decoupled from the neighbouring oscillators) we have applied McKean correlations as shown in Fig. 27. It is very pleasing that v / d is also linear for heteroaromatic molecules. Moreover it turns out that the C-H stretching frequency is extremely sena;itive to changes in bond length (from Fig. 27 a bond length change of 0.007A corresponds to a change in the C H stretching frequency of -100cm - l . It also shows the fine details consisting in the fact that the C-H groups in a! and p positions with

3160

T

I

3080 ! 1.074

0

Pya

I

I I

,

1.076

1.078

1.08

rCH

I 1.082

(A)

Figure 27. C-H stretching frequencies (considered as local modes) vs. experimental C-H bond length in furane (Fu), thiophene (Th) and pyrrole (Py). cy and /3 label the position of the C-H with respect to the heteroatom.

respect to the heteroatom are clearly separated. It is thus possible to estimate C-H bond lengths. From Gussoni's correlations u / q p it is also possible to estimate the effective atomic charges on the various hydrogen atoms of monomers and of a few oligomers. Next we have to account for the fact that the strongest II mode observed in the Raman spectrum of the systems generally described as polyfurane, polypyrrole and polythiophene have different frequencies, namely 1597, 1562, I460 c m p l respectively. The heteroatoms can affect ring stretching frequencies through their masses (kinetic energy, Eq. (3)) or electronegativities (potential energy, Eq. (1) or (10)). The problem of the electronic effects of the heteroatoms has been discussed in detail by Coulson [49] and later by Streitwieser [95]; the latter has introduced in his calculations a parameter, 11, which represents the Coulomb integral somehow related to the electronegativity of 0, N and S. / I takes the values 2, 1.5 and ~0 for 0, N(H) and S respectively. The plot of the strongest Raman active II mode against h gives a remarkable straight line which can also be extrapolated to the frequency of polyselenophene (1440 cm-') yielding a value 11 -0.40. It can then be concluded that the observed trend in the Raman active usris consistent with the fact that the heteroatoms (0,N H and S) affect the dynamics of the corresponding heteroaromatic rings mostly through the vibrational potential. i.e. the electron distribution (related to the degree of aromaticity of the systems and the electronegativity of the heteroatoms); mass effects are negligible, at least for the tI mode. N

6.12.2.1 Oligo- and Polypyrroles

Polypyrrole (PPy) in the doped state is a sufficiently stable material which has been and is being considered for technological applications [96]. In spite of its stability in the doped state its chemical structure and its structural properties are not yet fully understood. It is claimed that chemical defects such as cross-linking and cyclization occur in such a system (see Chapter 3) [97]. Attempts of de-doping of PPy to reach the pristine polymer have been made, but the chemical nature of these samples is obscure since the infrared spectra of samples obtained in different laboratories show drastic differences [33, 341. Moreover pristine PPy is very unstable. On the contrary, probably because of the overwhelming enhancement of the bands associated to the electronic defect, the DIIRS of all the doped samples are practically identical. An improvement in the understanding of the structure and properties of pristine and doped PPy has been made possible by the careful synthetic work by Martina rt al. [98]who were able to synthesize a long series of N-Boc- protected oligopyrroles (very stable), where Boc protection means that all nitrogen atoms carry a t-butoxycarbonyl protecting group. These molecules were then de-protected and samples of very unstable oligopyrroles were obtained and carefully kept in sealed tubes in oxygen and water free environment. Careful analysis have been carried out on (i) the Raman spectra of N-Boc-protected oligomers ( N = 3 , 5, 7, 9) and polymers (degree of polymerization, D P FZ 20) [99] and (ii) oligopyrroles ( N = 3. 5, 7, 9, 11) and polypyrrole (DP = 20) [loo].

388

6 Structure and O p t i d Properties of' Conjugated Oligomers

The crystal and molecular structures of N-Boc-protected oligopyrroles are known [101, 1021; all protected oligomers turn out to be strongly conformationally distorted forming a helical structure. Let 8 be the torsional angle defined by the sequence of bonds X-C-C-X (X = 0, N or S) connecting two heteroaromatic rings (0 = 180" or = 0" for configurations anti and syn respectively). For the crystalline trimer and pentamer the configuration is syn and 0 = 70". The Raman spectra observed for both protected and de-protected oligomers and polymers are in full agreement and make up a textbook case for all the observations discussed in the previous sections of this chapter. The softening of the tI mode (Fig. 15) is large (Av = 45 cm-') in deprotected oligo and polypyrrole, but is still sizeable (Av = 21 cm-') for the corresponding (N-Boc)-protected molecules. For deprotected oligopyrroles and PPy the large softening indicates the existence of a large e/ph coupling, i.e. the intra-ring pinning is relatively weak and 7r electrons can be easily delocalized along the chain such that a large ECL ( ~ 2 units) 0 can be foreseen. In the protected systems, because of the relatively weak pinning, the conformationally dependent hopping integral is still sizeable to allow delocalization along the chain in spite of the large value of the conformational angle 0. Endgroup modes have been identified both in IR and Raman and their relative intensities can be used for estimating the chain length of the polymers [99, 1001. Dynamic calculations have been performed on PPy pristine and doped for the understanding of the Raman spectra in terms of the ECC model [34]. Frequency and intensity dispersion with FH have been calculated and the observed Raman spectrum has been accounted for for both FH= 5.1 mdyn/A and = 3.5 mdyn/A for pristine and doped materials respectively. A first guess of the vibrational force field has been obtained by semiempirical quantum methods (MNDO) on the dimer, trimer and tetramer; after suitable scaling the calculated set of force constants has been used for subsequent least squares calculations which required the fitting of FH and of a very few other modes to the totally symmetric Raman active A, frequencies of PPy, deuterated PPy and I3C-PPy [103]. It is remarkable that the A,, k = 0 phonon frequencies calculated and observed for PPy (1569, 1452, 1307, 1051, 1000 and 387cm-' coincide with several of the Raman lines observed for N-Boc-protected polypyrrole (1 579, 1457, 1306, 1020, 856, 761, 510cm-') This observation proves that the vibrations of the protected molecule arise mainly and selectively from the vibrations of the pyrrole backbone. The general rule derived from the ECC model can then be re-stated namely that because of the intensity enhancement of the tI containing backbone vibrations generally the Raman spectrum of polyconjugated polymers very rarely shows the scattering arising from side chains.

6.12.2.2 Oligo- and Polythiophenes The class of oligo- and polythiophene derivatives studied by organic chemists, physicists and technologists is very large. Many oligomers and polymers have been prepared and their structural and optical properties have been studied. We focus here on a few spectroscopic results as examples of the relevance of the spectroscopic data in material science (see Chapter 2.1).

6.12 Wlmt do it'e Learn j k o l n LJibr.ational Spectra?

389

iii)

Figure 28. Molecular structure of model thiophene molecules for which effective atomic charges qH and C-H bond length have been derived from C-H stretching frequencies (see text).

The application of the concepts presented in section 6.12.2 provides the way to measure the effective atomic charges qH on the H atoms in thiophene oligomers [ 1041. Attention was focussed on thiophene, terthiophene and polythiophenes whose chemical formulae are sketched in Fig. 28. For strictly planar molecules in thiophene two symmetry inequivalent sets of C-H bonds are found in cy and ,6' positions with respect to the S atom. In terthiophene one can distinguish C-H bonds in the following four symmetry inequivalent positions: a?, P3, p4and /3,. In polythiophene only one translationally equivalent set of C-H of type ,O occur. The k = 0 limiting C-H stretching frequencies of types 4 and 7 absorb in polythiophene in the infrared at 3064cm-I; for the oligomers the stretching of C-H of type 2 ( a ) and of type 3 (p) absorb at 3099 and 3072cm-l respectively. It can then be concluded that qH(2) > qH(3) > qH(4 or 7). The values of the charges vary around $0. le. The measured bond distances vary around 1.080 A. The existence of weak absorption at 3072 and 3099cm-' in the spectrum of the polymer gives again an estimate of the relatively small degree of polymerization of the material. The lack of frequency changes from the oligomers to the 'short polymer' indicates that no dramatic changes in the electronic structure occur when chain length increases, in agreement with the existence of a short ECL as suggested from other experimental data. From Fig. 8 the value of Eg cannot easily be extrapolated since the line bends for large value of N . The same plot casts some doubt on the value of Eg for the sample of polythiophene taken from the literature and reported in ref. 61. Such values seem to indicate, again, that the average length of the molecules in that

390

6 Structure und Optical Proppertie.\ of' Conjugated Oligomrrs

sample is ~ 1 2 The . electronic spectrum unquestionably shows a red shift with increasing N (see Fig. 29) as if delocalization could easily occur along the chain just as in the case of protected oligopyrroles (Fig. 8). The information from the vibronic spectrum contrasts with that derived from the analysis of the II mode in a very large family of oligothiophenes. The puzzle consists in the fact that the very strong Raman line has practically constant frequency vR independent from N . This would mean that the pinning potential within the thiophene ring is strong and strongly counteracts the inter-ring delocalization such that ECL turns out to be small [61]. The issue of the softening of vH in oligo and polythiophenes has been confused for a few years also because of a wrong interpretation proposed by our group [35]. In the Raman spectra of oligothiophenes at the higher frequency side of the strong Raman line presently assigned to II a characteristic satellite line is always observed. Such a line shows unquestionable softening and decreases its relative intensity when N increases as hown in Fig. 30. After the work by Negri et a/. [lo51 we have reanalyzed the vibrational assignment based on a very large number of oligomers (-50) and on the basis of the concepts discussed in section 6.4 we concluded that the satellite line must be associated to ring vibrations of the endgroups whose 7r electrons are conjugated with the adjacent thiophene units (at one side) with a distance of interaction decreasing along the chain (in agreement with ref. 105, 106) and extending with s = 2-3 thiophene units. While the R mode for thiophene units within the chain does not soften the II mode at either ends shifts because of dynamical and effects. The conformational dependence of spectroscopic quantities in oligo- and polythiophenes is an interesting problem which has never been specifically faced in the literature. X-ray diffraction studies of several unsubstituted oligomers have

6 I

4.5 -

I

I

4.0

-

3.5 -

?0 \ \

2

4

6

8

1 0 1 2

Polymer

No. of Thiophene u n i t s Figure 29. Energy gap, E, (eV), against number of thiophene units for unsubstituted (0solution, * solid) and alkylsubstituted (A solution, 0 solid) oligo and polythiophenes. (From ref. 104).

6.12 What do

HY

Learti fioriz Vibrational Spectra?

39 1

I 1500

1510

1520

1530

1540

V R (ern-') Figure 30. Energy gap, Eg against satellite uI1for oligo- and polythiophenes: ( * unsubstituted. 0 alkyl substituted (solid), A in chloroform solution). Symbols as in Fig. 29. The general formula T,,R (j, k. . . .) has the following meaning: T,, indicates the number of thiophene rings; R is the alkyl substituent ( b = butyl, h = hexyl. d = dodecyl) and ( j , k. . . .) represents the numbering of the ring to which the substituent is attached.

clearly established that thiophene units are practically co-planar with inter ring torsions 0 # 0 only of a few degrees. ( N = 2: B = 0 at -140°C [107], N = 3: 19E 6-9' [log], polymer: 0 0" [109]). In this conformation the inter-ring electron hopping should be maximum but, according to what was discussed above, is balanced by a strong intraring pinning potential. The experimental evidence of conformational distortion from spectroscopic quantities is neither unequivocal nor overwhelming. When the materials are dissolved in various solvents it is generally believed that the removal of packing forces allows intramolecular interactions to be active, thus reaching a new equilibrium conformation which is the result of a balance between steric interactions and IT electron delocalization. One would expect larger values of B which cause the decrease of the inter-ring hopping integral and possibly the e/ph coupling. The experimental vibrational optical data possibly related to conformational changes are the following: (i) in going from the solid to solution the observed blue shift for the oligomers examined (with or without alkyl substituents) is of approximately 0.2-0.3 eV, almost constant for all oligomers and polymers (Fig. 29); (ii) for a few systems which were studied vR shifts upward of ~ 1 cm-' 0 [ I lo]. We think that the issue on conformational dependence of frequency and intensity vibrational spectroscopy compared with vibronic spectra is not yet settled. A thorough study is required for disentangling solvent effects and changes due to conformational distortions; moreover an experimental and theoretical study of the conformational dependence of the e/ph coupling is needed. Intensity spectroscopy is a useful probe of the inductive and mesomeric effects due to the type and position of the substituents in a polythiophene chain. Spectroscopic

-

392

6 Structure and Optical Properties of' Conjugated Oligomers

Scheme 6

work has been carried out on a series of oligoalkoxythiophenes specifically synthesized with the purpose to clarify the role played by the alkoxy group substituted in various position. The aim of this work was to find the origin of the great stability of polyalkoxythiophenes in the pristine as well as in the doped states. Because of Eqs. (6) and (8) the displacements of electrons along the bonds have been monitored by intensity measurements. The series of oligomers includes several regiospecific oligoalkoxythiophenes suitably synthesized. Through bond and through space interactions between the 0 and S atoms have been revealed and the distance of interaction through T electron delocalization has been determined [ 1lo].

6.12.2.3 Oligo- and Poly( p-phenylenevinylene) From Fig. 16 oligo( ppheny1enevinylene)s carrying r-butyl groups at the 3,5-positions of the terminal phenyl rings show a chain length independent 5l mode as if r electrons were strongly pinned and e/ph coupling, if active, were strongly localized within a short portion of the chain (see Chapter l). The e/ph coupling is non zero as shown by the measurement of vibrational hyperpolarizabilities (see section 6.13). If ECL is rather short the size of the electronic defect generated upon doping should be relatively small, i.e. the defect should be strongly localized. The infrared spectra of the potassium doped derivatives of the above oligomers, suitably synthesized [ 1 1 11, were studied. Special techniques were developed for the preparation and handling of chemically very unstable species. Spectroscopic evidence has been collected [112] on the fact that (i) phonons and 7r electrons are largely confined within either the -CH=CH- group or the benzene ring; (ii) the site of doping in PPV is preferentially the benzene ring and the size of the doped species is limited approximately to no more that two benzene rings on either side of the central ring. The approximate size of the polaron is thus ~ 4 - 5-C6H5-XH=XH- units and (iii) in the polaron the existence of a quinoid structure is confirmed (Scheme 6).

6.13 Nonlinear Optical Responses with Intensity Spectroscopy In sections 6.7 and 6.8, we have discussed how the frequency dispersion of the 5l mode observed for conjugated oligomers of increasing chain length can be

6.13 Notilitieur. Opticul Rt7.spotise.s with Ititetisitjs Spectrosropj,

393

interpreted and how it is possible to obtain estimates of relevant physical properties from such observations. In particular ECL and T electron mobility along the backbone chain or within the conjugated units making up the molecular skeleton can be determined. Here we wish to analyze how the intensity dispersion shown by these oligomers is related to other physical properties of great relevance in technology. This discussion is based on very recent studies in the field. The general features of the infrared and Raman spectra of pristine conjugated systems have been already described in section 6.6.1. Particularly relevant for what follows is the very simple Raman spectral pattern which is observed independently from the structural complexity of the compound examined. Only few normal modes show up in the spectrum but their absolute intensities are found to be anomalously large. Moreover the intensity evolution within a homologous series shows a superadditive enhancement of the absolute intensity as N increases. This nonlinear dependence of the Raman intensity from N clearly reveals the existence of a cooperative effect which involves the whole molecular electronic cloud. From Eq. (21), the superlinear increase of the Raman intensity must be reflected into the behavior of the three terms appearing on the right hand side of this equation. In section 6.7 it has been shown that the electron/phonon coupling term selects which normal modes are to appear in the actual spectrum and hence this factor is of capital importance in understanding the spectral pattern. However, this term alone cannot justify the superlinear increase of the intensity. Indeed the equilibrium geometry variation, related to this term (Eq. (23)), is not large enough. Also the frequency factor cannot be responsible for the peculiar intensity behavior since the energy variation, when the electronic excited states are modified by increasing N , is not enough to account for the observed enhancement. Hence it must be the transition dipole moment factor that determines the large measured values. Eq. (21) also shows how the Raman intensity is directly related to the electronic charge distribution in the ground and excited electronic states. This means that the vibrational Raman intensity must depend from the nature of the electronic cloud. The obvious consequence is that it must be possible to extract from the Raman intensities information on the molecular electronic properties. The cooperative phenomena which determine the intensity behavior can be understood considering the delocalized nature of the 7r electrons which makes the addition of one conjugated unit be felt by various other units in a mutually reinforcing way. The key question is to what extent this phenomenon takes place, i.e. how many units are able to feel one another. Experimentally a phenomenon of saturation is observed and should be related to the distance of electrical interaction. Depending on the kind of compounds considered, there will be a maximum average number of units that can feel each other. Intensity saturation is observed for all aromatic and heteroaromatic compounds. Only in the case of polyenes saturation seems to be reached with much larger values of N . This suggests that the intensity dispersion can be taken as a qualitative measure of the cooperativity of the electronic excitation. Opposite to cooperativity is the possibility that the electronic cloud be pinned onto the aromatic unit. The

394

6 Structure and Optical Propcrties of Conpguted Olrgomers

experimental correlation found seems to be that the stronger the aromaticity the smaller ECL. Similar conclusions were reached in section 6.8 in the analysis of the frequency dispersion of the tI mode of several conjugated systems. The only apparent contradiction is that no frequency dispersion could be detected for some systems such as poly( p-phenylene), poly( p-phenylenevinylene) and polythiophene while some intensity enhancement is always observed for the same systems. It must then be concluded that intensity is a much more sensitive probe than frequency. Structure and dynamical properties are much more localized than the overall electronic properties such as intensities. The peculiar intensity behavior which is a consequence of the peculiar electronic charge distribution, heavily affects other physical properties as shown in what follows. Very recently it has been pointed out [ 1 131 that organic conjugated molecules may be very relevant in the field of photonics since they show strong nonlinear optical responses which may turn out to be more useful to technology than the traditional inorganic systems. We have recently shown [ 114, 1 151 that it is possible to estimate the molecular nonlinear optical response (hyperpolarizability) of organic conjugated molecules on the basis of absolute Raman and infrared intensities. Using a semiclassical model we have obtained analytic expressions which enable to evaluate the vibrational contribution p' and yr to molecular hyperpolarizabilities.

+

(2) (%)+(%)(%)I

The quantities dpn/dQk, acxn,/aQk, apn,,/aQk, where n, m, s indicate the Cartesian components, can be obtained from infrared intensities, Raman and Hyper Raman cross-sections, respectively. vk is the vibrational frequency of the k-th normal mode Qk. Equations (30) and (31) are derived under the hypothesis of both mechanical and electrical harmonicity and in the presence of an applied static field. Similar expressions have been obtained by other authors with quantum chemical theories [116]. What is new in our approach is the application of the method to the study of low band gap conjugated systems with large electron/phonon coupling. In this case the striking consequence, which is essentially due to the peculiar electronic nature of these systems, is that the vibrational hyperpolarizabilities turn out to be not a partial and negligible contribution to

Table 2. Comparison between J and >drvalues for some selected organic molecules obtained both theoretically (with rib irzitio 3-31 G basis set) and experimentally (see Ref. 117). Compounds are defined in Scheme 4. ~

(3-21 G)

Compound

I I1 I11 IV

v

10.67 28.24 8.37

3'' (3-21 G )

4 (exp)

J" (exp)

9.55 30.96 11.08

12.6 -

10 (a) 24 (a)

-

I1 =

1

n=3 I7 4 1

11.6 34.7

10.2 32.0

-

-

50 10.2

34.6 50.9

-

46 (c) 3.8 ( d ) 39.8 (d) 42.1 (d)

11 values are in units of 10-j" esu; ( a ) from EFISH experiments; (b) 3yyy:(c) from HLS experiments; (d) from EFISH experiments.

NLO response, but can be related directly to the response by the electrons in the molecule. In the case of alternated conjugated oligomeric and polymeric systems, which we have seen have an enormous Raman activity, our method can be further simplified;

NO,

NH,

NH2-Q-No2 NO,

NH,

(V) Scheme 7

loooi

396

6 Structure and Opticul Properties

of

Conjugated Oligorriers

* 0

*

A

P

A

A

A

0

100

0

2

4

6

8

10

12

n

Figure 31. Comparison among y values obtained from different methods for oligoene systems of increasing chain length (see Ref. 117): (0)yr from calculated (ah initio 6-31G) Raman intensities; (A) y e from ah initio calculations (6-31G); ( 0 )yr from experimental Raman cross sections; (*) y e from THG measurements; (0)yr from experimental Raman cross sections of polyenovanillines.

indeed it has been shown [116] that for these systems yr can be approximated with high accuracy with only the Raman terms in Eq. (31). A large number of different molecules have been studied and experimental and/or calculated infrared and Raman intensities have been used together with Eqs. (30) and (31) to evaluate Pr and yr. The obtained values have been compared with independent deteminations of pe and ye obtained with the traditional methods (e.g. Electric Field Induced Second Harmonic Generation and Third Harmonic Generation). The data collected in Table 2 (Scheme 7) and Figs. 31 and 32 clearly show how, for low band gap compounds with large electron/phonon coupling p‘ ”- pe and yr E ye. These experimental findings, supported also by quantum mechanical (‘ab initio’ and semiempirical) calculations find their theoretical justification in the fact that, as a consequence of the strong electron/phonon coupling, it is impossible to carry out a complete separation of the electronic from the vibrational spaces. The existence of a preferential direction in the vibrational space (H),along which electron/phonon coupling is maximum, makes it possible to probe with an

6.13 Nonlinear Optical Responses with Intensit), Spectroscopy

397

loo( 0

0

0

0

A

1oc

40

0

A

d

3 P)

3

10

n

9

0

1 0

- - - - - - - - - ~ $ - l 0

2

4

6 n

0

10

12

Figure 32. Comparison between yr values obtained from experimental (0) and computed ( A ) Raman cross sections and y e values ( 0 )calculated with SOS methods for oligothiophenes with increasing chain length (see Ref. 117).

oscillation of the nuclei along this path, the same state of molecular polarization which can be directly obtained with an electronic excitation. The intensity dispersion discussed above has obviously a direct effect on the NLO response which turns out to saturate after a threshold value N , different for the various systems considered. Since N , is ruled by ECL the concept of ‘pinning potential’ previously introduced has a marked influence also on the NLO behavior. As already discussed, in this respect the most interesting systems are polyenes. Indeed third harmonic generation (THG) experiments by Samuel et al. [118] have shown that in this case the saturation onset is for N % 140. Another example of the role played by aromaticity, pinning and delocalization is given by the comparison of the experimental data reported in Fig. 33. Here we compare data relative to paraphenylenes and perylenes. What is interesting is that we are dealing with the same constitutive units (benzene rings) which have different topologies. The NLO behavior is markedly different as are their Raman spectra. In the first case we have a strong pinning as could be expected from the lack of

398

6 Structure and Optical Properties

of' Conjugrited Oligomers

1 WE-33

- -

- _

_

_

1 WE-36

0

2

4

6

8

n Figure 33. Comparison between yr values obtained from experimental Raman cross sections for oligo-p-phenylenes ( 0 )and oligoperylenes (A) (Scheme 1).

frequency dispersion of the II mode and a relatively rapid onset of intensity saturation, moreover also the absolute yr values are smaller. On the contrary, in the second case, a much steeper increase of yr is observed together with a much more selective Raman spectrum which shows a large dispersion of the II mode. These observations suggest that in the design of new molecules to be used in photonic applications, not only the chemical nature of the units involved but also their geometrical distribution in space must be taken into account. The few and selected examples discussed here, have the purpose of pointing out the role that vibrational intensity spectroscopy, if appropiately exploited, can play in offering to the attention of researchers guidelines in the development of new fields and new materials of scientific and technological interest.

Acknowledgment We thank Dr. P. Zuliani for fruitful scientific discussions and for her help in the preparation of the manuscript. The works presented in this chapter have been supported by the National Research Council of Italy (Progetto Finalizzato Nuovi Materiali and Progetto Finalizzato Fotonica per Telecomunicazioni) and by the funds of the Italian Ministry of Scientific Research (MURST).

References

399

References 1 . R. N. Jones and C. Sandorfy, in C / i ~ w i c Applicatii~n.~ d 01 Sp~'ctroscopj..Tt,chniqirrs q/'Organi(, Cliiwiistry. (Ed. A. Weissenberger) Interscience, New York, USA, 1956, vol. IX. 2. L. Bellamy. The It!fiared Spectra of Coniples Molecules, Wiley. New York, USA, 1958. 3. K . Nakamoto, Infrared Specira oflnorganic and Coorciination Conipoioic/s, Wiley, New York,

USA, 1963. 4. G. Herzberg, Infinred and Ramtrn Spectrri of'Polj~rtoniicMolecules, Van Nostrand, Princeton. USA, 1959. 5 . E. B. Wilson. J. C. Decius and P. C. Cross, Molecular b'ilmtions, McGraw Hill, New York, 1955. 6. M. V. Volkenstein, M. A. Eliashevich and B. Stepanov. Kolehariij,a MolekulII. Moscow, 1949. 7. S. Califano. Vibrational Statcx Wiley, New York. USA. 1976. 8. For a discussion of the problem see: G. Zerbi in A d i m c e s in Infrnre~ltinriRanian Speciroscopj~ (Ed. R. J. H. Clark and R. E. Hester) Wiley, Heyden. New York, USA. 1984, p. 301. 9. J. H. Schachtschneider and R. G. Snyder, Specirodiini Acta. 1961. IY. 17. 10. R . G. Snyder. J . Clieni. Plijx. 1965. 42. 1744. 11. See the series of papers such as: T. Shimanouchi. Tables of Molecular Frequencies, Part 7, J . Phjls. Clieni. Reference Daia. 1973, 2. 225. 12. R. G. Snyder and G . Zerbi, Specirochini. Acta. 1967. 23A, 391. 13. For a thorough discussion see: W. Person and G. Zerbi, Vibrational Intensities in It!fiarer/and Rarnari Spectroscopv, Elsevier. Amsterdam, The Netherland, 1984. 14. M. Gussoni, in Adi~ancesin Infrared trnd Ranian Spectroscop~(Ed. R . J. H. Clark and R. H. Hester) Heyden, London. England. 1979, vol. 6. p. 96. 15. L. A. Gribov. Interisiij, Theor?. f b r lt~frart~ti Spectra of Polj~atoniic Molecules. Consulting Bureau, New York, USA, 1964. 16. M. Gussoni, C. Castiglioni and G. Zerbi, J . Molec. Struct., 1989, 198, 475. 17. M. Gussoni. C. Castiglioni, M. N. Ramos, M. Rui and G. Zerbi, J . Molec. Struct., 1990,224.445. 18. J. C. Decius, J . Mol. Spectrosc.. 1975. 57. 384: A. J. Van Straten and W. Smit, J . Molec. Specirosc., 1976. 62. 297. 19. M. Gussoni. C. Castiglioni and G. Zerbi, J . Ph1a. Clieni., 1984. 88. 600. 20. L. Piseri and G . Zerbi. J . Mol. Spectrusc.. 1968, 26. 254. 21. G . Zerbi, F. Ciampelli and V. Zamboni. J . Poljwi. Sci. part C, 1964. 141. 22. For a general discussion of the dynamics of simple and simplified chain molecules see: R. Zbinden. Infrared Spectroscopy of High Polymers, Academic, New York, USA, 1964. 23. G. Zerbi, in Vibrational Spectra o f H i g h Polyniers. (Ed. E. G. Brame) Applied Spectroscopy Reviews, Dekker. New York, USA, 1963. vol. 2. p. 193. 24. G. Zerbi, in Atfrance.~in Cliemisiry Series. American Chemical Society. 1983. 203. 487. . 25. P. C. Painter, M. M. Coleman and J. L. Koenig. The T / i ~ ~ofr jVihra~ionalSpectroscop~~atic~its Applications io Polymeric Maierials, Wiley, New York. USA, 1982. 26. L. Piseri and G. Zerbi, J . Cheni. P h j x , 1968. 48, 3561; G. Zerbi and L. Piseri, J . C/zetti. P/ij>s.. 1968, 49, 3840. 27. A. Rubcic and G. Zerbi, Macroniolecules, 1974. 7, 754. 28. G. Zerbi and M . Sacchi. Macroniolecules. 1973. 6. 692. 29. B. Orel. R. Tubino and G. Zerbi, Mol. Pkjss., 1975, 30. 37. 30. R. Rabaioli, Thesis in Physics. University of Milano, 1990. 31. V. Heine, Group Theory in Qiraiitiim Mechatiics, Pergamon Press, New York, USA. 1959. 32. A. A. Maradudin, E. W. Montroll and G. H. Weiss, Solid State Phys.. Siippl. 1963, 3. I . 33. For a review see: G. Zerbi. in Cot2jugated Poljnicrs (Ed. J. L. Brkdas and R. Silbey) Kluver. Amsterdam. The Netherlands, 1991. p. 435; M . Del Zoppo, C . Castiglioni, P. Zuliani and G. Zerbi in Handbook of Conducting Polymers, Ilnd Edition (Eds. T. Skotheim. R. L. Elsenbaumer, J. R. Reynolds), Dekker. New York, 1998, p. 765. , 92, 3886, ibid. p. 3982. 34. B. Tian and G. Zerbi, J . Client. P l i j ~ . 1990, 35. J. T. Lopez Navarrete and G. Zerbi, J . C / i e n ~P / J J ~ S1991. . , 94, 957; ibid. p. 965. 36. G. Zerbi. B. Chierichetti and 0 . Inganas, J . Cliem. PIiys., 1991, 94, 46379.

400

6 Structure und Opticd Properties of Conjqared Oligomers

37. B. Tian, G. Zerbi, R. Schenk and K. Miillen, J . Chem. Phys., 1991,95,3191; B. Tian, G. Zerbi and K. Miillen, J . Chmz. Phys., 1991, 95, 3198. 38. (a) H. Ohtsuka, Y. Furukawa and M. Tasumi, Spectrochirn, Acta, 1993, 4YA, 431; Y. Furukawa, H. Ohta, A. Sakamoto and M. Tasumi, Spectrochim. Acta, 1991, 47A, 1367; (b) C. Castiglioni, M. Gussoni and G. Zerbi, Synth. Met., 1989, 29, E l . 39. B. Wunderlich, Mucromolecular Physics, Academic Press, New York, USA, 1976, vols. 1 and 2. 40. G. Zerbi, in Luttice Dvnaniics and Intermolecular Forces, (Ed. S. Califano) Academic Press, New York, USA, 1975, p. 384; G. Zerbi in Advances in Infrared and Raman Spectroscopy (Eds. R. J. H. Clark, R . E. Hester) Heyden, London, 1984, p. 301. 41. (a) P. Dean, Rev. Mod. Phys., 1972,44. 127; (b) M. Tasumi and G. Zerbi, J. Chem. Phyx., 1968, 48, 3813. 42. G. Zannoni and G . Zerbi, Solid State Comm., 1984, 50, 55. 43. F. Martino, in Quuntum Chemistry of Polymers: Solid Stuie Aspccrs (Ed. J. Ladik and J. M. Andre) Reidel, Dordrecht (NL), 1984, p. 279. 44. R. S. Day and F. Martino, J . Phys. C., 1980, 14, 4247. 45. T. Uno and K. Machida, Spectrochim. Actu, 1968, 24A, 1741; T. Uno, K. Machida and K. Miyajima, ;bid. p. 1749; K.Machida, S . Kojima and T. Uno, Spectrochim Actu, 1972, 28A, 235. 46. L. V. Tarasov, Soviet Physics-Solid Stute Engli.sh Transl., 1961, 3, 193. 47. S. Trevino and H. Boutin, J . Chem. P l z j ~ . ,1966, 45, 2700. 48. L. Salem, Molecular Orbital Theorji ufConjuguterl Systems, Benjamin, New York, USA. 1961. 49. C. A. Coulson, Vulence,Oxford University Press, 2nd edition, 1961. 50. (a) A. E. Gillam and E. S. Stern, An Introduction to Electronic Absorption Spectroscopy in Organic Chmistry, Arnold, London (UK), 1955; (b) B. Kohler, in Conjugated Polymers (Ed. J. L. BrCdas and R . Silbey), Kluwer, Dordrecht (NL), 1991, p. 403. 51. The case of biphenyl is prototypical, see: H.Suzuki, Electronic Absorption Spectra and Geometry of Organic Molecules, Academic Press, London, England, 1967. 52. M. F. Granville, B. E. Kohler and J. B. Snow, J . Chem. Phys., 1981, 75, 3765. 53. D. Brinbaum, D. Fichou and B. E. Kohler, J . Chem. Phys., 1992, Y6, 165 and ref. herein. 54. W. P. Hsu, J. R. Schrieffer and A. J. Heeger, Phys. Rev., 1980, B22, 2099. 5 5 . For a general discussion see: J. L. BrCdas, in Handbook of’Conducting Polymers (Ed. T. A. Skotheim) Dekker, New York, USA, 1986, vol. 2, p. 825 and 859. 56. For a review see: M. Gussoni, C. Castiglioni and G. Zerbi, in Spectroscopy of Advanced Materials (Ed. R. J. H. Clark and R. E. Hester) J. Wiley, New York, USA, 1991. 57. C . Castiglioni, G . Zerbi and M. Gussoni, Solid Srate Commun., 1985, 56, 863.

58. D. C. Bradley, R. H. Friend, T. Hartmann, E. A. Marseglia, M. M . Sokolowski and P. D. Townsend, Proceedings qf the I n fernutional Confkrence on Science and Technology of Synthetic Metals, Kyoto, paper 4A-14, 1986. 59. C. Castiglioni, M. Del Zoppo and G. Zerbi, J . Ruman Spectr., 1993, 24, 485. 60. H. E. Shaffer, R. R. Chance, R. J. Silbey, K. Knoll and R. R. Schrock, J . Chem. Phys., 1991, 94, 4161. 61. V. Hernandez, C. Castiglioni, M. Del Zoppo and G. Zerbi, Phy.r. Rev. B., 1994, 50, 9815. 62. A. Heeger, in Handbook of Conducting Po1ymrr.r (Ed. T. A. Skotheim) Dekker, New York, USA, 1986, vol. 2. 63. H. Bleier, in Organic Material f o r Photonics: Science and Technology (Ed. G. Zerbi) Elsevier, Amsterdam, The Netherlands, 1993. 64. M . Veronelli and G. Zerbi, to be published. 65. J. Orenstein, in Handbook qf Conducting Polyniers (Ed. T. A. Skotheim) Dekker, New York, USA, 1986, vol. 2. 66. E. Agosti and G. Zerbi, Synth. Met., 1996, 79, 107. 67. H. Hotta, M. Soga, N. Sonoda, J . Phys. Chem., 1989,93,4994. 68. M. Veronelli, M. C. Gallazzi and G . Zerbi, Acta Polymericu, 1994, 45, 127. 69. I. Harada, Y. Furukawa, M. Tasumi, H. Shirakawa and S. Ikeda, J . Chem. Phys., 1980, 73,4746. 70. A. Sakamoto, Y. Furukawa and M. Tasumi, J . Phys. Chem., 1994, 98, 4365; A . Sakamoto, Y. Furukawa and M. Tasumi, Synrh. Met., 1993, 55-57, 593; A. Sakamoto, Y . Furukawa and M. Tasumi, J . Phys. Cliem., 1992, Y3, 3870.

71. J. Tang and A. C. Albrecht, in Raman Spectroscopy (Ed. H. Szymanski) vol. 2, Plenum Press, New York, 1970. 72. W. L. Peticolas, L. Nafie, P. Stein and B. Fanconi, J . Clzeni. Phys., 1970. 52, 1576; W. L. Peticolas and C. Blazej, Chem. Phys. Lett., 1979, 63. 604. 73. F. Negri, G. Orlandi, F. Zerbetto and M. Z. Zgierski, 1. Chenz. Phys., 1989, 91, 6215. 74. B. Horovitz, Solid Stute Conzm., 1982, 41, 729; B. Horovitz, Phys. Rev. Lett., 1982, 47, 1491; E. Ehrenfreund, Z. Vardeny, 0. Brafman and B. Horovitz, Phy. Rev., 1987, B36, 1535. 75. C. Castiglioni, M. Gussoni and G. Zerbi, Solid Stute Conzm., 1985, 56, 863. 76. T. Kakitani, Progr. Theor. Phys., 1973, 50, 17. 77. Ab initio: 6-31; G. Y. Furukawa, H. Takeuchi, I. Harada and M. Tasumi, J . Mol. Struct., 1983, 100, 341; H. Yoshida and M. Tasumi, J . Chem. Phys., 1988, 88, 2803; M. Dupiiis and E. Clementi, private communication; MNDO: J. T. Lopez Navarrete and G. Zerbi, Syntlz. Met., 1989, 32, 151; QCFFIPI F. Zerbetto, M. Z. Zgierski, F. Negri and G. Orlandi, J. Chem. Phys., 1988, 89, 3681; C. Rumi and G. Zerbi, to be published; Y. Mori and S. Kurihara, Solid State Commun., 1987, 60, 201; Y. Mori and S. Kurihara, Synth. Met., 1988, 24, 357; Y. Mori, H. Tabei and F. Ebisawa, Sjwrl7. Met., 1987, 17, 447; H. 0. Villar, M. Dupuis and E. Clementi, PhJs. Rev., 1988, B37, 2520. 78. E. J. Mele and M. Rice, Solid State Conim., 1980, 34, 339; L. Piseri, R. Tubino, R. Paltrinieri and G. Dellepiane. Solid Stute Comm., 1983, 46, 183. 79. (a) P. Piaggio, G. Dellepiane, E. Mulazzi and R. Tubino, Polymer. 1987,28, 563; (b) C. Rumi, A. Kiehl and G. Zerbi, Chem. Phj5.r.. Letts, 1994. 231, 70. 80. G. Zerbi, C. Castiglioni. M. Del Zoppo, R. Schenk and K. Mullen, to be published. 81. G. Zerbi, M. Gussoni and C. Castiglioni. in Electronic Properties of’ Polrmers and Relatcd Conipounds (Ed. H. Kuzmany, M. Mehring and S. Roth) Springer, Herbelberg (D), 1985, p. 156. 82. U. Dinur, Chem. Phys. Leu., 1982, 93, 253; see also ref. 72. 83. M. Gussoni, C. Castiglioni, M. Del Zoppo and G. Zerbi, in Organic Materiuls,fiw Photonics, Science and Technologj., North Holland, Amsterdam, The Netherlands, 1993, p. 27. 84. M . Gussoni, C . Castiglioni, M. Miragoli, G. Lugli and G . Zerbi, Spectrochim. Acta. 1983,41A, 371. 85. S . Marriott and R. Thopson, J . Mol. Srrtrct., 1982, 89, 83. 86. M. N. Ramos, M. Gussoni, C. Castiglioni and G. Zerbi, Chem. Phys. Letts, 1988, 181, 397; M. Gussoni. M. N. Ramos, C. Castiglioni and G. Zerbi. Chcvn. Phys. Letts, 1989, 160, 200; M. N. Ramos, M. Gussoni, C. Castiglioni and G. Zerbi, Croutica Chimica Acfri, 1989,62, 595. 87. Z. Vardeny, E. Ehrenfreund and 0.Brafman, Phys. Rev. Lett. 1983,30, 876; Chem. Phjx. Lett, 1983, 95. 555; H. Kuzmany, J . Pl7y.s. (Paris). 1983, 44, C3, 255. 88. M. Veronelli, G. Zerbi and R. Stradi, J . Raman Spectrj., 1995, 26, 683. 89. M. E. Heyde, D. Gill, R. G. Kilponen and L. Rimai, J . Am. Chenz. Soc., 1991,93,6776; T. G. S. Spiro, in Chenzicd und Biochemical Applications cfLn.ser.s(Ed. C. Bradley Moore), Academic, New York, 1994, p. 29. 90. P. Zuliani and G . Zerbi, to be published. 91. R. Rabaioli. Thesis in Physics, Universifj’ of’ Milano, 1990; R. Rabaioli, M. Gussoni, C. Castiglioni and G . Zerbi, to be published. 92. M. Rumi and G. Zerbi, Chenz. Phys. Lett., 1995, 242, 639. . 1994, 231, 70. 93. M. Rumi, A. Kiehl and G. Zerbi, Chern. P h j ~ Lett.. 94. D. C . McKean, Chem. Soc. Rev., 1978, 7, 399; J . Mol. Struct., 1984, 113, 251. 95. A. Streitwieser, Molecular Orbital Theory for Organic Chemists,Wiley, New York, USA, 1961. 96. H. Munstedt, in Elektrisch Leitende Ktmststoffi~, Munich, Germany, 1986, p. 207. 97. H. Naarman, in Applications qf’Conducring Po/ynier.s (Ed. W. R. Salaneck, D. T. Clark and E. J. Samuelsen). Adam Hilger, Bristol (UK), 1991. 98. S. Martina, V. Enkelman, A. D. Schluter. G. Wegner and G. Zerbi, Synth. Met., 1993, 55-57, 1096. 99. G. Zerbi, M. Veronelli, S. Martina, A. D. Schluter and G. Wegner, Advanced Materiuls, 1994, 6, 385. 100. G. Zerbi, M. Veronelli. S. Martina. A. D. Schluter and G. Wegner. J . Chrm. Phys., 1994, 100, 987.

402

6 Structure and Optical Properties of Conjugated Oligorners

101. S. Martina, V. Enkelmann, G. Wegner and A. D. Schliiter, Synt. Met., 1992, 51, 299. 102. The X-ray diffraction data on Py2 and Py3 were kindly provided by Dr. G. B. Street as private communication. See also: G. B. Street, Handbook ofconducting Polymers, Dekker, New York, USA, 1986, vol. 1, p. 256; the structure of Py3 has also been solved by the Mainz Group, see ref. 101. 103. Y. Furukawa, S. Tazawa, Y. Fuji and I. Harada, Synth. Met., 1988, 24, 329. 104. G. Zerbi, in New Per.ypective.7 on Yibrationul Spectroscopy in Muterial Science (Ed. M. W. McKenzie) Wiley, New York, USA, 1988, ch. 6, p. 247. 105. F. Negri and M. Z. Zgierski, J . Chem. Phys., 1994, 100, 2571. 106. E. Agosti, M. L. Rivola, V. Hernandez and G. Zerbi, to be published. 107. G. J. Visser, G. J. Heeres, J. Wolters and A. Vos, Acta Cryst., 1968, B24, 467. 108. F. Van Bolhuis, H. Wynberg, E. E. Havinga, E. W. Meijer and E. G. J. Stirling, Synth. Met., 1989, 30, 381. 109. S. Briickner, W. Porzio, Makromol. Chem., 1988, 189,961. 110. E. Villa, E. Agosti, C. Castiglioni, M. C . Gallazzi and G . Zerbi, J . Chem. Phys., 1996, 105, 946 1. 11 1. R. Schenk, H. Gregorius, K. Meerholz, J. Heinze and K. Miillen, J . Am. Chem. Soc., 1991, 113, 2643; R. Schenk, M. Ehrenfreund, W. Huber and K. Miillen, Adv. Mat., 1991, 3, 492. 112. G. Zerbi, E. Galbiati, M. C. Gallazzi, C. Castiglioni, M . Del Zoppo, K. Miillen, J . Chem. Phys., accepted. 113. (a) D. S. Chemla and J. Zyss, Eds., Nonlineur Optical Properties of’ Organic Molecules und Cry7tal.s; Academic: New York, 1987, Vols. 1 and 2; (b) D. J. Williams, Angeiv. Chem. Int. Ed. Engl.; 1984, 690; (c) P. N. Prasad and D. J. Williams, Introduction to Nonlintwr Optical &fects in Molecules and Polymers; Wiley: New York, 1991; (d) S. R. Marder, J. E. Sohn and G . D. Stucky, Eds., Material.r,for Nonlinear Optics; American Chemical Society: Washington, 1991; (e) R. A. Hann and D. Bloor, Eds., Organic Materialsfor Nonlinear Optics;Royal Society of Chemistry: London, 1989. 114. C. Castiglioni, M. Gussoni, M. Del Zoppo and G . Zerbi, Solid State Comm., 1992, 62, 343. 115. C. Castiglioni, M. Del Zoppo and G. Zerbi, Phys. Rev. B, 1996, 53, 13319. 116. (a) D. M. Bishop, Rev. Mod. Phys., 1990,62, 343; (b) C. Flytzanis, Phys. Rev., 1972, B6, 1264. 117. C. Castiglioni, M. Del Zoppo, P. Zuliani and G. Zerbi, Synth. Met., 1995, 7 4 , 171. 118. I. D. W. Samuel, I. Ledoux, C. Dhenaut, J. Zyss, H. H. Fox, R. R. Schrock and R. J. Silbey, Science, 1994, 256, 1070.

7 Electronic Excitation 7.1 Electronic Excitations of Conjugated Oligomers Heinz Bassler 7.1.1 Introduction Solid state physicists found it fascinating to consider a conjugated polymer as a model for an infinite one-dimensional system [ 1-31. The simplest system in this respect is polyacetylene (PA). Its structural formula suggests that it would be metallic if there was no bond alternation. This led Heeger et al. [4] to set up their famous Hamiltonian that ignores Coulomb as well as electron-electron correlation effects but invokes strong electron-phonon coupling. Within this formalism the existence of a gap in the absorption spectrum of (CH), is solely attributed to Peierls’ distortion leading to bond alternation. Neglect of Coulomb effects implies absence of exciton effects and is equivalent to considering the system as a 1 D-semiconductor, tractable within the framework of one electron theory. The above concept for the excited states of a conjugated polymer is very different from theoretical models developed to describe excited states of molecules. On-site electron repulsion as well as long range potentials have been recognized as being of central importance. It is, therefore, legitimate to ask whether or not there is a fundamental change concerning the relative importance of the various interactions - electron-electron and electron-phonon, respectively - when going from a 7rconjugated molecule to a 7r-conjugated polymer, in particular since another class of conjugated polymers. the polydiacetylenes, have long been known to resemble oligomers concerning their spectroscopic properties [ 5 ] . One purpose of this chapter is to collect relevant spectroscopic information needed to clarify this issue from an experimental point of view, the emphasis being on the presentation of prototypical results rather than on a full coverage of available data. The evolution of optical absorption and fluorescence spectra of various oligomer systems with increasing chain length will demonstrate that there is, in fact, no fundamental difference between a short and a long 7r-conjugated chain as far as the (linear) optical properties are concerned. Apart from the above fundamental question, spectroscopic characterization of oligomers is a field of interest in its own right. Due to their well-defined structure their spectroscopic properties are similarly well defined and their knowledge allows electro-optic devices such as electroluminescent diodes to be tailored.

7.1.2 Concepts Optical transitions in organic molecules occur between the highest occupied

404

7 Electronic Excitation

molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) under the premise of the Franck-Condon principle. Excitations are accompanied by a change in the electron distribution within the molecule with concomitant change in the nuclear coordinates. If there were no readjustment of the bond lengths, i.e. no displacement of the potential energy curves along the configurational coordinate, only a single absorption line corresponding to the S, + SOO-0 transition would be allowed because the higher vibronic transitions would be forbidden by virtue of the orthogonality of the vibrational wavefunctions. In the case of coupling to a single harmonic oscillator of reduced mass A4 and angular frequency w the strength of coupling is described by the Huang-Rhys factor:

AQ being the displacement of the minima of the potential energy curve along the configurational axis upon excitation. The absorption spectrum consists of an electronic origin, the S, t SO 0 + 0 line, followed by a vibronic replica, S , + So n c- 0, whose intensity distribution, Z,,,is a Poissonian mapping the overlap between the vibrational wavefunctions I,, = S”e-“/n!

It has a maximum at an energy Shw above the electronic origin. For large values of S , Z,,approaches Gaussian with variance hwS’”. S is thus a crude measure of the number of vibrations generated when the excited molecule relaxes from the ground state configuration to the new equilibrium configuration in the excited state and Shw is the relaxation energy. The fractional intensity of the 0 t 0 transition is

In reality, w is different in the ground and the excited states and potentials are not exactly parabolic. For large molecules the concomitant modifications in the above scheme are small, though. For spectral analysis it is of importance, however, that there is a large number of molecular modes that can couple to an electronic transition. Eq (1) has then to be replaced by a sum over the displacements associated with the individual modes i of angular frequency w,,each individual oscillator being characterized by a fractional Huang-Rhys factor S,. The average relaxation energy of the molecule after excitation is then LIE,.,, = C hw,S, and the fractional intensity of the S 1 t So 0-0 origin band is a measure of the average Huang-Rhys factor S = C S,. A fractional intensity of 0.13 corresponds to S = 2. If a particular vibronic line with vibrational quantum number n = 1 carries the same intensity as the 0-0 line, the fractional Huang-Rhys factor of that mode would be 1. It is important to note that observing the 0-0 line in an absorption spectrum is all but a signature of no configurational relaxation occurring. The mean relaxation energies must, in fact, be comparable to the energy of the coupling mode itself. Within the context of theoretical work on linear conjugated polymers the strength of electron phonon coupling is often expressed in terms of the change of the elastic energy aAx

upon generation of an excitation on a bond. It translates into the Huang Rhys factor via S = aA.x/fiw,w being the energy of the dominant Yibrational mode of that bond and Ax the change in ban$ length. For a = 4eViA [6] and fiw = 0.1 eV, S = 1 corresponds to Ax = 0.025 A. In the case of a dipole allowed transition the fluorescence spectrum of an excited molecule in the gas phase is the mirror image of the absorption spectrum, the S1 + SoO-O transition being resonant with absorption. For S > 1 the vibrational components carry more intensity than the electronic origin band. In that case the energetic displacement between the maxima of absorption and emission spectra is approximately 2Sfiw. The situation becomes more complicated for chromophores in liquid solution or in a non-crystalline (random) solid. In the former case there is solvation of the excited state, usually occurring on a ps or sub-ps time scale. Absorption and eniission are then no longer resonant. Instead, there is a finite displacement between their origins equal to twice the solvation energy. In addition, there is spectral broadening of homogeneous as well as inhomogeneous origin. Absorption and emission spectra of chromophores embedded in glassy matrices are dominated by inhomogeneous broadening typical linewidths (fwhm) being of order of several hundred cm-] [7,8] translating into a variance of (Gaussian shaped) profiles via fwhm = 2 c r m . It reflects the local variation of the van der Waals interaction energy with the surrounding molecules. Since the spectral width can exceed the energy of the low energy molecular vibrations the lowl’high energy absorption/emission-bands need no longer be pure S, SoO-O bands. For this reason the fractional intensity of ‘origin’ bands can only be a crude measure of the true S-value(s). Line narrowing techniques for eliminating inhomogeneous broadening are hole burning [9] as well as site selective fluorescence spectroscopy (SSF) [7,8] of which only the latter technique will be considered in this chapter. It involves the use of a spectrally narrow laser, which makes it possible to excite selected chromophores from amongst a large ensembles contributing to an inhomogeneous broadened absorption. Only those whose transition energy is accidentally resonant with the laser are excited and provided that excitation is into the S1 + SoO-0 line, the resulting emission spectrum is a homogeneously broadened emission as long as any inter-chromophore interaction is vanishingly small. SSF spectra provide a means to determine the frequencies of the vibrational modes that couple to a transition as well as the fractional strength of the various vibronic lines. One problem, however, is that stray light effects preclude measuring the S , + SoO-0 transition which would, by definition, be resonant which the laser unless S >> 1. It is, therefore, not generally possible to determine the exact value of the electron phonon coupling constant. If there is an intensive and spectrally well separated vibronic transition it may be possible to excite into a S I t So 1-0 band and observe the entire S1 + So spectrum without loosing site-selectivity completely. A well resolved pattern of sharp vibronic S l + So0 3 1 modes in resonantly excited SSF spectra does, however, indicate that S must be in the range 1 . . . 2 which is typical for rigid 7r-electron systems. Energy transfer (ET) among the chromophores erodes site-selectivity. This is usually a problem in more concentrated solutions and, notably, in bulk systems. t)

406

7 Electronic' E.witu t ion

ET among chromophores that differ in excitation energy leads to spectral relaxation if the width of the distribution of excited states (DOS) is > k T . This process has been described in detail in previous work [lo, 1 I]. Suffice it to mention here that spectral relaxation follows non-exponential kinetics. Since excitations tend to settle within the tail states of the DOS it leads to a dynamic Stokes shift between the origin bands of absorption and emission that amounts to approximately twice the variance (T of the inhomogeneously broadened DOS profile [12]. Emission coming from the tail states of the DOS it is also inhomogeneously broadened. As a rule of thumb the variance of the high energy tail of the emission band is half the width of the inhomogeneously broadened S , +- So 0-0 transition. It is important to recognize that this type of Stokes shift is solely a consequence of energetic disorder rather than a reflection of structural relaxation a chromophore might suffer after excitation. SSF spectra recorded upon scanning the exciting laser across the low energy tail of the inhomogeneous S , t So 0-0 band provide a simple way to overcome spectral relaxation via energy migration and thus to separate transport controlled spectral relaxation from a Stokes shift originating from structural relaxation of the chromophore. Since energy transfer depends on the availability of an energy acceptor, it must be eliminated below a certain threshold energy &loc. SSF spectrum taken at excitation energies E~~~ < E~~~ must, therefore, reflect the emission spectra of resonantly excited chromophores unaffected by spectral relaxation.

7.1.3 Experimental Results 7.1.3.1 Polyenes

Oligoenes(0E) are the simplest olefinic molecules and the oligomeric model compounds of polyacetylene( PA). Their prototypical character generated an intensive effort to elucidate their spectroscopic properties, both experimentally and theoretically key issues being the evolution of absorption spectra with chain length and the excited state ordering. It has long been known that the absorption spectra of Q,Wdiphenyloligoenes (DPOE) retain their character upon increasing the length of the polyene moiety including the intensity distribution within the vibronic replica which is dominated by the C-C and C=C stretch modes. There is only a bathochromic shift as the chain gets longer while S-values are virtually independent of chain length [I31 (see Scheme I ) . Extensive experimental work by Kohler [ 14,151 and his group substantiated this reasoning. Linear oligoenes can be incorporated in low temperature n-alkane crystals. Since it is known from X-ray crystallography that in an alkane crystal the molecules pack as planar all-anti chains this must also be the conformation a linear oligoene adopts in such a structure if it substitutes for a host molecule. Since the crystalline environment more or less eliminates inhomogenity effects this technique offers a unique opportunity to study the spectroscopy of oligoenes in alltr'rrris configuration unaffected by disorder. Plotting the l ' B L ,t l'A, transition energies as a function of the reciprocal number of double bonds reveals a straight line extrapolating to a finite ordinate

7 .I Electronic E.rcitrrtions of' Conjugcited O l i g o m m

407

OE:R=H " W R DPOE:R=Ph Scheme 1

intercept (Fig. 1). Its exact value is subject to a considerable uncertainty implied by the extrapolation. In ref [22] Kohler quotes a value of 16,200cmp' as opposed to 14,20Ocm-' in ref. [15]. If one includes data on the end-capped diphenylpolyenes 2 and counts both phenyl rings as three double bonds one arrives at AE(n + co)= 14,000 f 500cm-I. A simple Hiickel calculation with alternating resonance integral due to bond alternation is able to reproduce the chain length dependence in a quantitative fashion [16]. It has been shown that the vibronic development of the 1 IB,, +- 1' A, absorption can be fitted by treating the vibrational coordinates as independent harmonic modes assuming the vibrational normal coordinates are the same for ground and excited states. Such an analysis gives a precise determination of the difference between normal coordinate equilibrium values for ground and excited states, respectively, and concomitantly, allows one to derive the difference between vertical und 0-0 transition energies, i.e. the configurational relaxation energy. The latter comes out to be of order 2000 c m p ' . Comparison with the C-C and C=C stretch mode energies (-1230cmp1 and %1640cmpI , respectively, with slight dependence on chain length) indicates that the Huang-Rhys factor is slightly bigger than 1 and changes very little with oligomer length. Of crucial importance for the understanding of oligoene as well as polyacetylene spectra has been the recognition that the lowest excited state is of 2'A, rather than of 1 'B, character [ 171. Experimental proof comes from fluorescence excitation spectroscopy. Figure 2 portrays the fluorescence as well as fluorescence excitation 40,

r

0

I

I 02

01

03

l/n

-

Figure 1. Experimental optical transiton ener ies for oligoenes (OE) (O,+) (from ref. 22) and ]'A, transitions (from ref. 24) as a ftinction calculated transition dipole moments for the I B, of reciprocal number of C=C double bonds. In the case of the a,u-diphenyloligofenes (DPOE) (0) the two phenyl rings are counted as being equivalent to 3 double bonds.

F

7 Electronic. E.ycitatiotz

I

I

19000

20000

Wavenumbers

21000

22000

23000

i n l/cm

10

I

16000

16500

17500

17000

Wavenumbers

in

ia000

l/cm

Figure 2. Fluorescence (bottom) and fluorescence excitation (top) spectra of 2,4,6,8,10,12,14.16hexadecaoctaene in n-decane. Peak labelled 1 is the 2 ' A , t I'A, origin - weakly allowed due to site-s ecific deviations from centrosymmetry - onto which a vibronic replica is built. Peak 19 is the 1 B,, t 1 'A, origin (from ref [15]).

P

spectra of the all-trans oligoene octadecaoctaene in an n-hexadecane matrix reported by Kohler et uf. [15]. The common origin is at 17,87Ocm-', off-set from the origin of the IB,, t 1 ' A , band by 4900cmp'. Although parity-forbidden, the 2'A, t I'A, transition acquires some intensity because of small changes in the local environment of the oligoene molecule that causes partial symmetry-breaking. The same fluorescence spectrum is obtained upon exciting the I'B, state. It decays non-radiatively to the 2'A, state on a ps-time scale. Two photon fluorescence

7.I Electronic E.ucitutions of Conjugufrd Oiigomers

409

excitation spectroscopy on cis-cis octatetraene in a 1 OK n-octane matrix confirmed this interpretation [ 181. Both sites the polyene molecule can occupy in the matrix are exactly centrosymmetric. Therefore the 2IA, c 1 'A, transition is strictly forbidden in one photon spectroscopy but is two photon allowed. Fluorescence can only occur if promoted by a vibrational mode. To explain the inverse level ordering in oligoenes as opposed to conventional 7relectron systems requires explicit consideration of electron-electron Coulomb interaction [14, 19-21] not included in single electron models such as the SuSchrieffer-Heeger model. The 2'A, state (S, state) is described as a mixture of singly and doubly excited configurations. To the extent that S I is doubly excited with respect to the ground state - resembling a pair of triplet excitations with opposite spin - the S I + So transition is dipole forbidden. Extrapolating the 2'Ag energies to infinite chain length yields 7370cm-' [14] which is about 7000cm-' below the absorption edge of trans-PA. Since the non-radiative decay rate increases exponentially with decreasing energy gap this provides a straightforward explanation why trans-PA does not fluoresce. Although the all-trans configuration is the stable configuration of polyenes there is conformational disorder unless the molecules are arrested in an all-trans conformation imposed by packing contraints in, e.g. a crystalline environment. The reason is that the energy difference between trans-trans, trans-cis, and cis-cis are only 6 and 8 J/mole, respectively, and the activation energy for isomerization is close to zero in the 2IA, state [22]. Oligoenes in liquid solution or in a random solid solution as well as PA are, therefore, likely to exist in the form of an array of conjugated subunits separated by topological faults, such as kinks or twists about single bonds that interrupt 7r-conjugation. The length of the interrupted segments is called the effective conjugation length (see Chapters 6 and 7.2). One has to keep in mind, though, that its value is not neccessarily identical with the physical length of the subunits. It is so only if the interruption of the 7r-conjugation is complete and the segments are perfectly ordered. Despite this uncertanity the concept of the effective conjugation length is useful for system characterization. Polyenes with up to 240 double bonds have recently been synthesized by Samuel et al. [23]. Their absorption spectrum in T H F solution are almost featureless and shifted to higher energies compared to what one would expect in the basis of Fig. 3 [24]. Kohler and Woehl [25] have recently analysed these spectra in order to determine the distribution of effective conjugation lengths. For such a procedure one needs to know the absorption spectra of the individual polyenes in all-trans configuration, the inherent spectral broadening in solution as well as the variation of the transition dipole moment with chain length. While the former informations are available, that latter is not. However, adopting the Hiickel treatment, known to reproduce I ' B t I'A, transition energies with remarkable accuracy, it is possible to calculate (1 Ag/kll l B u ) .Data are included in Fig. 1. T o calculate the absorption spectrum that corresponds to a given distribution of chain lengths, the band profile of a known polyene in solution is shifted to the 1' B, t I'A, transition energy for a chain with n double bonds in conjugation, weighted by the probability for that chain length times the squared transition dipole, and accumulated. The probability amplitudes are adjusted to minimize the RMS deviation between the calculated and

r

410

7 Electronic, E.uc~itutiori

Figure 3. Fluorescence (left) and fluorescence excitation spectra for terthiophene H-TJ-H in ndecane at 10 K (from ref. [27]).

measured spectra. The result of this procedure is that the spectra can be reproduced by the sum of a narrower and a broader Gaussians, both centered at n = 2. A statistical treatment based upon the notion that the energy needed to create a conjugation-break is independent of where it occurs is able to recover the result of the spectral analysis, the essential message being that for long chains the distribution is dominated by short conjugated fragments. It is straightforward to conjecture that the absorption spectrum of PA can be explained on the same premise. The fact that the absorption peak of trans-PA is close to the value of the oligoene absorption extrapolated to n -+ 00 in conjunction with the finding that improved interchain ordering gives rise to a bathochromic shift indicates that the polymer absorption is dominated by segments with long conjugation lengths. Interchain ordering obviously reduces the probability for intra-chain twists. A cautionary note is of order concerning the spectral red-shift one expects for a oligoene embedded in a PA environment, as opposed to an alkane matrix. The larger n-electron polarizability in conjunction with short intermolecular distances tends to increase the van der Waals interaction energies and , concomitantly, the gas to solid shift of the spectra. 7.1.3.2 Oligothiophenes

High resolution fluorescence as well as fluorescence excitation spectroscopy in oligothiophenes (H-Tn-H) (see Scheme 2) with two [26], three [27] and four [28] rings embedded in n-hexane at low temperatures (4.2K) have been published by Kohler and coworkers and analysed with respect to the energies of the vibrations

7.1 Elec,tronic E.ur.itations of' Conjugated Oligomrr.,

41 1

H-Tn-H Scheme 2

coupling to the ground state and the excited state, respectively. Figure 3 shows 10K fluorescence excitation spectra of terthiophene (H-T,-H) in n-decane matrix while Fig. 4 portrays the emission spectrum upon site selective excitation. The fundamental vibrations coupling to the ground state are 209 cm-I; 342 cm-I; 695 cm-l; 1470cm-I and 1540cm-l [27]. 0-0 transitions in absorption and emission are resonant and the intensity distribution within the vibronic manifold is characteristic of a rigid aromatic molecule with a Huang-Rhys factor of order unity. Plotting the electronic transition energies as a function of the reciprocal number of thiophene rings bears out a linear relation in analogy to the case of oligoenes. The n + x intercept is AE,,, = 15,00Ocrn-' (Fig. 5 ) close to the value obtained with oligoenes. Combined with the fact that the spectra of 2.2'-thienylpyrrole and 2.2' bi-pyrrole in n-hexane matrices at 4.2K have nearly the same 0-0 transition energies and remarkably similar vibronic development as bithiophene (H-T2-H) [26] this indicates that the hetero-atom participates little in the delocalization of the excited state. Room temperature absorption spectra of oligothiophenes (H-T,-H) in dioxane solution are almost featureless [29] (Fig. 6). They carry a low energy shoulder which can be assigned to the solution broadened Sl + SoO-0 transition. Taking the position of that shoulder as the maximum of that transition reveals a dependence of the electronic transition energy on number of rings that parallels that of the molecule in a crystalline n-hexane matrix. Fluorescence spectra consist of a well resolved S1 SoO-0 band and a vibronic replica dominated by a superposition of the ~ 1 5 0 cm-I 0 ring modes. Plotting the energy of the fluorescence origin band as a function of l / n bears out a straight line off-set from the absorption line by IOOOcrn-' yet coinciding with the low temperature data in n-hexane (Fig. 5). It is --f

12

23000

23500

24000

24500

Wavenumbers (cm-l)

Figure 4. Site-selected fluorescence spectrum for terthiophene H-T3-H in n-decane at 4.2 K . The spectrum is produced by selectively exciting the S , t So 0-0 origin of H-TJ-H occupying one of four possible sites in the n-decane matrix (from ref. [ 2 7 ] ) .

n

654 3

l/n

Figure 5. I'B,, @ 1 'A, transition energies in oligothiophenes as a function of the reciprocal number of thiophene rings. (Data for H-T,-H in n-hexane are from ref. [26-281, room temperature solution are from ref. [29]).

straightforward to attribute the Stokes shift of l000cm-' to a combination of solvent and molecular relaxation. (Contrary to this work the authors of ref. [29] define the Stokes shift as the energy difference between the absolute maxima of the vibronic absorption and emission replica). This would place the adiabatic S, H So 0-0 transition 500cm-' above that in solid hexane matrix. The most likely explanation is that, owing to the rather flat torsional potential [30,31] H-T,-H-molecules in liquid solution are, on average, subject to some torsional displacement that increases the transition energy. Support for the importance of torsional displacement comes from Raman studies [32] as well as from the different intensity distribution in absorption and emission. While the emission spectrum more or less reflects the solution broadened profile characteristic of the all-anticonformation in solid n-hexane matrix, the absorption spectrum in liquid dioxane is much broader and the higher energy portion is more intense. In the light of the analysis of oligoene spectra in liquid solution (see section 7.3.1) this is equivalent to a distribution of effective conjugation length of the H-T,-H-molecules. Oligothiophenes differ from polyenes, however, as far as the distribution off effective are chain lengths is concerned. A significant fraction of H-T,-H-molecules almost planar and tends to relax to the fully planar structure after optical excitation. Chrosrovian et a!. 1291 measured both fluorescence yield, decay time of H-T,-H (n = 2 . . . 6 ) and the molar extinction coefficient in dioxane solution and determined ~ ) constants (Table 1). With increasradiative (kT)as well as non-radiative ( l ~ , ,decay increases significantly indicating that non-radiative decay ing number of rings aPF becomes less efficient. This is surprising since, according to the gap law, internal conversion becomes, in general, more efficient as the energy to be released into vibrational modes decreases. The authors argue that this effect is overcompensated by increasing structural stability of unspecified origin, though. An alternative explanation would be that with increasing excitation energy, i.e. smaller iz, an increasing number of unidentified inadvertant impurities can act as quenchers. If

7.1 Electronic Eucitutions of' Conjugated Oligoiners

(a)

4 13

absorption coefficient [lo' I/mol*cm]

6

1.000 800

'-

--,, 5T

600 -

4T

,

!'\

400

-

200

-

3T

08

'*

''

I

2T

- - _ _- - _ _

x

Inml

Figure 6. Absorption (a) and fluorescence (b) spectra of oligothiophenes in dioxane solution (from ref. [29]).

so, the variation of Q f l with n would be an extrinsic effect. It would, however, be difficult to explain why Qfl values are reproducible [33]. The high fluorescent yield of H-T,-H in dilute solution is an unambiguous signature of the 2'A, statelying above the 1' B, state indicating that electron correlation is less important in the oligothiophenes than in the linear oligoenes. Employing two-photon spectroscopy Birnbaum and Kohler located the 2'A, state of bithiophene at an energy 36173 cm-' , i.e. 6570 cm-' above the 1 B, state. Drawing

414

7 Electronic Excitation

Table 1 Molar extinction coefficient, fluorescence yield, radiative and nonradiative singlet decay constants of oligothiophenes H-Tn-H (from ref. 29). Data marked with an asterix are taken from ref 33.

~

2T

3T

4T

5T

6T

1.6 1.2'

2.4 2.5* 0.07 0.35 4.65 0.2

4.2 4.6' 0.2 0.40 1.60 0.5

4.7 5.5' 0.28 0.33 0.85 0 85

-6"

~

molar extinction coefficient ( 1 o4 IjmoIe cm) @/I

kr (109s-') k,, (lo's-') (kr + knr1-I (ns) oscillator strength

0.42 0.48 0.66 0.88 1.02

upon the analogy with oligoenes, for which the energy of the 2'A, state has been observed to drop more rapidly with increasing chain length than that of the l'B, state, one might expect that a level crossing might occur for n 6. In fact, Periasamy et al. [34] located the 2IA, state of a H-T6-H-film a t 18,350cm-' via two-photon fluorescence excitation. This is slightly below the extrapolated I'B, t 1' A, transition of the matrix isolated molecule (Fig. 5). The low luminescence efficiency of H-T6-H-filmS is, in fact, suggestive of a reversal of rhe level ordering. From the fact that the fluorescence efficiency of the isolated H-T6-Hmolecule is 42% (Table 1) one has to conclude, however, that this is a solid state effect [35,36]. Further discussion of this problem is beyond the scope of this chapter. The existence of a triplet state below the singlet state has been verified experimentally yet the information is even less complete than for the states of the singlet manifold. For H-T3-H the T I level has been located 13,800cmp' above the ground state [37]. From the fact that the triplet state of oligomers ranging in size from 6 to 11 thiophene rings can be quenched via enery transfer to C60[38] whose triplet level is at 12,70Ocm-'[39] one has to conclude that the TI + So gap decreases less with increasing molecular length than the S I + So gap. This is due to the stronger confinement of the T I state with respect to the S 1 state as evidenced by ODMR measurements [40]. 7.1.3.3 Oligoarylenevinylenes

The desire to vary the optical properties of polyphenylenevinylene (PPV) in a controlled fashion, e.g. via copolymerization, has led to a systematic study of the corresponding oligomers (OPV) (see Scheme 3). Because of the decreasing solubility with increasing lengths work on unsubstituted OPV has been restricted to oligomers with n 5 3. Recently the series n = 1 to n = 5 carrying solubilizing t-butyl substituents at the terminal phenyl rings was synthesized by Miillen and coworkers [41,42] and characterized by absorption, fluorescence and photoelectron spectroscopy [43]. Figure 7 shows a series of absorption and fluorescence spectra of the series tbutyl-substituted R-OPV (n = 1) to ( n = 5 ) (Scheme 4) present in the form of thin solid films which had been deposited from a Knudsen cell onto sapphire substrates. Except for a low energy shoulder absorption spectra are almost featureless and

7.1 Electronic E-witations of Conjugated Oligomrrs

415

oligo-p-phenylenevinylene(OPV) Scheme 3

q..; .....

3rn g

.-

-5

0

c

%

..........

V

I

: .'".. .. ..

)..."

: ,

_.....

S

a

3

.........

-

..' '._,

2 ;

... ............

1 '

,,."

2.0

s

-8

2.5

........

3.0

3.5 4.0

energy [eV]

Figure 7. Absorption and fluorescence (do1 :d) spectra of thin solid films of t-butyl substituted oligo-p-phenyenevinylenes (R-OPV) (see scheme 4) measured at room temperature (from ref. [43]).

exhibit a bathochromic shift upon increasing n. By and large the spectra are equivalent to those reported earlier [44] for OPV incorporated in KBr pellets. If one identifies the shoulder with the peak of the inhomogeneously broadened S1 c SoO-O - transition - high resolution optical spectroscopy will support this assignment - and plots the transition energy versus reciprocal chain length a linear relation is recovered in analogy to what has been observed with polyenes and oligothiophenes (Fig. 8). In Fig. 8 the chain length is expressed in terms of the number of carbon atoms ( N ) in the shortest path between the ends of the molecule, L = 6 N + 10. When attempting to fit the low energy shoulder by a Gaussian one ends up with a Gaussian width of that band of order 0.1 eV.

t-butyl substituted oligo-p-phenylenevinylene(R-OPV) Scheme 4

416

7 Electronic Excitution

1I1

Figure 8. Peak energies of the SI + SO 0-0 absorption band in oligo-p-phenylenevinylenes (OPV) as a function of the reciprocal number of carbon atoms in the shortest path between the ends of the molecule ( L = 6 N + 10, for the definition of N see text). The transition energies of ordinary and SOO-0 emission peak of improved PPV is improved PPV as well as the position of the S, indicated for comparison (from refs. [43] and [46]). The energies for the T2 + TI transition is also included (from ref. [44]).

-

Fluorescence spectra (Fig. 7) are considerably narrower and show vibronic structure. They bear out a Stokes shift, defined as the difference between the low energy absorption and high energy emission feature, that decreases from 0.29 eV for OPV(n = 1) to 0.16eV for OPV(n = 5). This is a signature of the presence of disorder in the film leading to spectral diffusion due to energy migration as will be shown below by drawing upon site-selectively recorded spectra. The SSF spectra of unsubstituted OPV(n = 1) and OPV(n = 2) in a low temperature (6K) M T H F glass reveal a well resolved vibronic structure [45,46] (Fig. 9). Zero phonon lines can be identified for vibrational energies of 160cm-'(s), 320cm-'(w), 1170cm-' (s), 1340cm-' (3), 1500cm-' (w), 1570cm-' (w), 1620cm-' (3), 1670cm-I (s) and 1790cm-' (s), (s) and (w) standing, respectively, for strong and weaker transitions. The spectral origin is set by the laser line implying the absence of a Stokes shift and, sadly, the indeterminability of the S, SoO-O origin on account of stray light effects. There is, however, a n indirect indication that the origin transition is, in fact, somewhat weaker than the 160 cm-' vibronic satellites because in non-resonantly excited spectra the energy separation between the high-energy band and the convolution of the dominant vibronic band appears to be the difference between the (average) vibrational energies and that of the 160cm--' mode which is assigned to a vinyl bending mode, present also in the fluorescence spectrum of trans-P-methyl styrene [47] which is the smallest molecule resembling the OPV repeat unit. Since the fractional intensity of the SI + SoO-O line is not known the exact value of the Huang-Rhys factor cannot be determined. However, the fact that the 0-1 fundamental vibrations rather than overtones dominate the spectrum indicates that 1.5 < S < 2. --f

7.1 Electronic E.witution5 of Conjugated Oligoniers 60t

417

I

AV

(10*crn-’)

Figure 9. Comparison of resonantly excited fluorescence spectra of an intra-chain ordered PPV film which the spectra of poly-p-phenylphenylenevinylene and oligo-p-phenylenevinylenes (OPV) in MTHF-glasses. The abscisse scale is normlized to the laser excitation energy (from ref. [46]). In that work the OPVs have been labelled differently. The symbol (3) there corresponds to n = 2 in this work).

It has been of particular importance to recognize that, except for some residual broadening of so far unidentified origin, the fluorescence spectrum of the polymer is virtually the same as that of the oligomer [45].This indicates that molecular vibrational modes are essentially unaffected by chain elongation, a conclusion in complete agreement with the results of resonance Raman studies and theoretical calculations [48-501. It is further noteworthy, that the intensity distribution is also very similar indicating that the S-value and, hence, the relaxation energy in the excited state is also the same for oligoiner and polymer. Taking S 2 1.5 and ttw = 0.2eV as an average vibrational energy yields as a rough estimate Ere’E 0.3 eV. This supports the notion that the spectroscopic properties of PPV reflect those of an ensemble of oligomers with statistically varying effective conas well as its variance depend on jugation length &. The magnitude of (LefF) sample perfection which, in turn, depends on the sample preparation, e.g. stretch orientation, and interchain packing constrainsts imposed, for instance, by substitution [&]. Given the fact that absorption and fluorescence spectra should be mirror symmetric because S-values are basically the same, a’ comparison between SSF spectra and film absorption spectra not only testifies on substantial disorder in the film but also in the occurrence of molecules with shorter effective conjugation length. This accounts for the blue-shift of the maxima in the absorption spectra of oligomer films (Fig. 7). Obviously vapor deposition does not allow that, on average, the molecules adopt their configuration of minimum energy. In a bulk system those chromophores rapidly transfer excitation energy to more perfectly ordered molecules with lower transition energy. This explains both the spectral narrowing as well as the occurrence of a Stokes shift that does not result from molecular relaxation.

418

7 Electronic E.ucitution

r PDPV

coly(4,4'-biphenylene-(1,2-diphenylvinylene)) (PDPV)

~

C =CH+ PDMPVH n

poly(2.2'-dimethyl-l,l'-jiohenylene

-CH=CH-+

-4.4I-vinylene)

n

(PDMPV)

PFV

poly(2,7-fluorenylenevinylene ( P F V ) Scheme 5

There are cases, though, in which a finite Stokes shift is the manifestation of substantial molecular relaxation after excitation. There are PPV-like systems in which the phenylene ring is replaced by a biphenylene moiety, if modified by substitution (see Scheme 5). Although the spectroscopy has been done on polymers the results shall be quoted within the present context because, given the analogy between oligomer and polymer, they are relevant for oligomers, too. The systems are poly(4.4'-biphenylene-(l,2-diphenylvinylene)) (PDPV), poly(2.2'-dimethyl- 1,l'biphenylene-4,4'-vinylene) (PDMPV) and poly(2.7-fluorenylenevinylene) (PFV) [51]. Site-selective fluorescence spectra recorded under the premise that the emission energy shifted linearly, though non-resonantly, with the excitation energy are portrayed in Fig. 10. The appearance of a Stokes shift 6, decreasing in the series PDPV, PDMPV, PFV and PPV is obvious. Values for 5 , S and the energy of the coupling mode are listed in Table 2. That PDPV and PDMPV exhibit both the largest S values and the largest phonon energies proves that 6 is related to coupling of the excited state to a torsional mode of the chain. In the ground state the structure of the biphenyl is tilted, the angle between the planes of the rings being about 20°, while in the excited state a planar geometry is favoured. This is the result of a tradeoff between steric repulsion and conjugation. Upon excitation the conformation of PDPV relaxes towards a new, planar, equilibrium position, equivalent to strong

7 .I Electronic E.witations

of' Conjugated Oligornrrs

419

Figure 10. Fluorescence spectra of biphenylenevinylene derivatives obtained under site-selective excitation conditions (from ref. [5 I]). (Compounds see Scheme 5).

coupling to a torsional mode. This assignment is supported by the decrease of hwph due to the combined effect of the increase of the moment of inertia upon adding a methyl group in meta position and their mutual steric repulsion. The absolute is comparable to the energies of twist modes in related molecules magnitude of hph in the gas phase, such as biphenyl (hw,,, = 70cm-I [52]), benzaldehyde (1 IOcm-l (50)) and p-methylbenzaldehyde (85 cm-' [53]). Locking phenyl group motion by covalent bridging in PFV reduces 5 to 200 cm-' . Oligoanthrylenevinylenes (Scheme 6) are another class of arylenevinylene derivatives in which steric effects have a major impact on the spectroscopic properties. Oligo-9,1O-anthryfenevinylenes (OAV) have been synthesized by Mullen and coworkers [54] and spectroscopically characterized employing the SSF technique [55]. Absorption spectra, shown in Fig. 1 1 display a high energy wing with weakly developed vibronic structure virtually independent of the number of anthrylene units ( n )yet coinciding with the positions of the main vibronic transitions of anthracene. It is followed by a broad low-energy tail that extends further to the Table 2 Stokes shift, h, Huang-Rhys factor S, and phonon energy, fiwpl,forply-biphenylenevinylenes PDMPV (Scheme 5 ) h (cni-'

PDPV PDMPV PFV

1700 770 E200

S

trw,,, (cn1-I)

11 9 =3

155 85 E33

420

7 Elec,troriic Ex-citution

Scheme 6

red the larger n is. This observation is in accord with the known all-trans conformation of OAVs derived from ' H and I'C-NMR spectra. The intramolecular twist [56,57] prevents unhindered n-electron delocalization, implying that the average effective conjugation length is less than the molecular length. Absence of vibronic structure in the low-energy tail, on the other hand, is indicative of large structural disorder leading to inhomogeneous band broadening that overrides vibronic splitting. It is straightforward to identify the dominant source of disorder with

(103cm")

Figure 11. Room temperature absorption spectra of oligoanthrylenevinylenes (OAV). The number indicates the number of anthrylene units (from ref. 1551).

7 .I Electronic E,xcitutions of' Conjugated Oligomm

42 1

i

22 20 18 16 Wovenumber ilO3crn-'i

Figure 12. A series of low temperature fluorescence spectra of matrix-isolated OAV ( n = 5 ) parametric in excitation energy. The latter is marked by the high energy spike (from ref. [ 5 5 ] ) .

fluctuations of the intramolecular twist angle. The variations of the transition energy due to different positions of the solubilizing substituent at the anthrylene group are of minor importance. In the extreme case it leads to complete decoupling of the anthrylene moieties manifest in the appearance of the anthracene type absorption spectrium at the high-energy side. SSF spectra of OAVs excited at the low-energy tail of the absorption spectrum shift linearly with excitation vex featuring a Stokes shift of 800cm-', between the laser line and the peak of the high energy emission independent of vexand remain broad down to the lowest excitation energies. The half-widths at half maximum of the high energy peak is about 450 cm-' (Fig. 12). A band profile analysis of the premise of Eq. 2 yields S = 3 . . . .4 and a phonon energy hwph = 115 f 15 cm-'. The obvious candidate is a torsional vibration of the anthylene moiety about the long molecular axis. The reason for the larger electron-phonon coupling in OAVs as compared to PPV is the steric hindrance of the anthrylene and vinylene groups in the ground state [56,57]. In the excited state the molecule is likely to relax to a conformation with improved planarity favoring n-electron delocalization. By comparing the SSF spectra of OPVs and OAVs of the same chain length upon excitation at the very absorption tail where the most elongated conformers absorb one estimates that replacement of the phenyl by an anthryl group lowers the excitation energy by 3000 cm-' . However, because of the larger degree of disorder due to steric repulsion only very few molecules exist in a confirmation in which the effective conjugation length equals the molecular length. Most of the molecules absorb at higher energies because intramolecular twist reduces Leff. For application of conjugated polymers and their oligomeric counterparts in electro-optic devices such as light emitting diodes (LED) the energetic positions of electron donating and electron accepting levels are important because they determine the energetic barrier for hole and electron injection from contacts. The former can be determined by photoelectron spectroscopy (UPS or ESCA) while the latter can only be inferred from the sum of the oxidation and reduction potential

422

7 Electronic Escitrition

I"

=

5 4 3 2 1

a 7 6 binding energy [eV]

Figure 13. Normalized photoelectron spectra of thin solid films of t-butyl substituted oligo-pphenylenevinylenes (R-OPV) in the region between 5.2 and 8.5 eV binding energy (vacuum) (from ref. [43]).

measurement employing cyclic voltammetry once the absolute position of the ionization potential is known. One should keep in mind, though, that the electrochemical method notoriously underestimates the gap between electron donating and electron accepting states because those measurements are usually performed in polar solvents in which the stabilization energy of the ions exceeds that in a solid system. Solvation by the mobile dipoles of the solvent thus adds to to electronic stabilization of the ion via van der Waals coupling with the environment. UPS spectra for vapor deposited films of t-butyl-substituted oligo-phenylenevinylenes (R-OPV) measured by Schmidt et al. [43] are presented in Fig. 13. Assuming an energy-independent matrix element for excitation they represent the joint density of states involving occupied valence states and unoccupied states above the vacuum level. To first order the density of unbound states can be approximated by a step function and the UPS spectrum can be considered to reflect the density of occupied valence states only. Onset of the UPS spectrum is then a measure of the minimum ionization potential (IP) while the peaks indicate the energetic position of the center of the highest filled molecular orbital (HOMO) and consecutive filled orbitals, respectively. The shift of the spectrum indicates a reduction of IP with increasing oligomer length. With increasing M the separation between the features decreases and the spectrum converges to that of the polymer. In Fig. 14 the variation of IP and the location of the center of HOMO is plotted versus L-' . The difference between IP and the average HOMO energy reflects both inhomogeneous broadening in analogy to optical spectra and solid state broadening. Extrapolating the variations of 1P and the HOMO-energy with L towards L -+ 00 allows the corresponding values for the polymer to be estimated since the effective conjugation can be inferred from the optical spectra. For conventional PPV (Leff= 70, equivalent to ( n ) = lo), strech oriented PPV (Leff= 106, equivalent to ( ( n )= 16) and poly-phenylphenylenevinylene (PPPV) (Leff= 28, equivalent to ( M ) = 3), IP = 5.25 eV, 5.20 eV and 5.45 eV, respectively, while the HOMO positions are at -5.64eV, -5.09 eV and -5.85 eV., respectively. As far as the hole injection from a metal contact is concerned the IP value rather than the HOMO (i.e. the

7.1 Elmronic Excitations of Conjugated Oligonier.7

423

Figure 14. Energetic location of the HOMO, derived from the center of the associated photoelectron feature, relative to the vacuum level, versus reciprocal number of carbon atoms in oligop-phenylenevinylenes (R-OPV). (from Fig. 13). Onset of the photoelectron feature associated with the HOMO defines the ionization potential IP. Position of the LUMO is obtained by adding the S , +- So 0-0 transition energy to the ionization energy (IP). Note that this LUMO energy refers to a neutral excited molecule rather than to a radical anion formed upon adding an extra electron to a previously neutral molecule.

band maximum) position should be relevant for estimating the injection barrier since injection involves a trade-off between density of acceptor states and injection barrier. It is gratifying to note that for hole injection from I T 0 into PPPV a barrier height of 0.65 eV has been inferred from the current voltage curve [58]consistent with the above IP value and a workfunction of I T 0 of 4.8 eV. Figure 14 also indicates the energetic postion of the LUMO level to which an electron is put upon optical excitation. It varies only little with n. Note that the binding energy of that level would be equal to the electron affinity, i.e. the energy gained upon adding an extra electron to a neutral molecule, only if Coulomb effects were negligible or, in other words, if the excitation binding energy AE,,, to the fully dissociated state were zero. Thus the LUMO of a neutral molecule is shifted by an energy AE,,, towards higher energies. AE,,, is difficult to estimate. The extreme values offered in the literature range from kT = ? [59] to 0.9 [60] eV. Realistic values appear to be 0.4, 0.5eV as inferred from the fluorescence quenching data on PPPV [61] as well as from the temperature and field dependence of photoconduction in a film of tristilbeneamine dispersed in polycarbonate [62]. The same values for AE,,, have previously been reported for polydiacetylenes [63] and confirmed by theory [64,65]. Information concerning the triplet state of OPV is sparse. Triplet states in organic molecules are produced via intersystem crossing from the singlet manifold or via charge carrier recombination. The most direct way of detection is via phosphorescence spectroscopy, usually hampered by the difficulty to separate it from prompt bulk or defect fluorescence. One way to circumvent this problem is to use delayed detection techniques, e.g. by using a phosphoroscope. Another problem is the small radiative yield because the triplet lifetime is usually determined by

424

7 Electronic. Exc.itntion

Energy (eV)

Figure 15. Photo-induced T2 + T, absorption spectra of oligo-p-phenylenevinylenes (OPT) (from ref. [44]).

non-radiative channels. The only reliable value for the T , c So gap available to date appears to be that of trans-stilbene (2.0 eV). It implies a SI-TI gap of 1.7 eV. More information is available on the T2 t T, transition. Spectra have been measured in the form of transient absorption spectra following excitation into the singlet manifold. In Fig. 15 a series of T2 t TI spectra is portrayed for OPV ( n = 3 to 7). The T2-TI-gap energy decreases with increasing chain length, approximately, following a AE(T2 t T I )= A - B/L law, (see Fig. 8) with the same slope parameter as found for the SI t Sotransition. The polymer data fit into this scheme ~71. 7.1.3.4 Oligo-p-phenylenes The absorption and fluorescence spectra of unsubstituted oligo-p-phenylenes (OP) display only a weak bathochromic shift upon increasing the chain length [ 18,68,69]. In conjunction with the absence of vibronic structure and a large Stokes shift this testifies to weak 7r-conjugation among the rings due to the twisted structure. Also blue electrolumincscence has been observed with a poly-p-phenylene-based LED

oligo-p-phenylene (OP) Scheme 7

7.1 EIecimiic E.xcitniions of Conjugated Oligomers

425

[79]. The application of the unsubstituted polymer is hampered by its insolubility and by the occurrence of structural relaxation upon excitation. The latter is a particular handicap in LED applications since the HOMO-LUMO gap in absorption determines the sum of the energy barriers at the contacts that have to be overcome for charge injection to occur. A significant Stokes shift due to structural relaxation is, therefore, a handicap as far as the desire to minimize injection barriers is concerned. Significant progress on the material side was made by the work of Scherf et al. [7 1,721 who succeeded in synthesizing ladder-type oligo- and poly-p-phenylenes (LOP and LPP) in which covalent bridging among adjacent phenylene rings both planarizes the structure and prevents structural relaxation after excitation (see Scheme 8). Appropriate substitution renders the material soluble.

LOP (n = 3)

LOP (n = 5)

ladder-type oligo-p-phenylene (LOP) Scheme 8

426

7 Electronic Excitation

I

24

23

22

21

20

Wavenumber (103cm-') Figure 16. A series of fluorescence spectra of the trimeric ladder-type oligo-p-phenylene (LOP(n = 7)) parametric in excitation energy. The system was a M solution in MTHF at appr. to 6 K.

Absorption and fluorescence spectra of the polymer (LPP) testify to the rigidity of the backbone and the improved 7r-conjugation as compared to conventional PPP [73] More insight into the spectroscopy of LOP is provided by site selectively recorded fluorescence spectra [73]. Figure 16 shows a family of SSF spectra, parametric in excitation energy, on LOP (n = 3) embedded in a 6 K MTHF matrix at a concentration of mole/mole. Figure 17 presents spectra of the same oligomer without alkyl substitutuents at the methylene bridges. The difference of SSF spectroscopy on matrix isolated molecules as opposed to a bulk polymer film or a matrix isolated polymer whose physical length is a multiple of the effective conjugation length is that in former case energy transfer is absent. Therefore absorber and emitter molecules are identical irrespective of the spectral position within the inhomogeneously broadened absorption band at which excitation occurs. Nevertheless, the resonance effect becomes eroded if excitation occurs at higher energies. Then different electronic origins can be populated via excitating into (i) the phonon wing that accompanies any vibronic line because of coupling to low energy modes of the glassy environment and (ii) into the vibronic

=

7.I Electronic Excitations of’ Conjugated Oligomrrs

h

5.5

-

5.0

-

4.5

-

4.0

-

.-3

5

427

eca 3.5 v

.-0 u)

3.0

-

=B8

2.5

-

5V

2.0

-

2

1.5

-

Ir

1.0

-

0.5

-

0.0

L

C

u)

9

30

IY

W L I

I

I

I

29

20

27

26

Figure 17. A series of resonantly excited fluorescence spectra of a ladder-type oligo-p-phenylene (LOP(n = 3)) without alkyl substituents at the methylene bridges.

replica. Therefore emission spectra become independent of excitation energy above a certain threshold energy. Phenomenologically this is in analogy to the localization effect observed in bulk systems (see section 2) but has a different physical origin. Comparing the spectra of Figs. 16 and 17 leads to several conclusions. (i)

Superimposed onto the emission spectrum of LOP (n = 7) with sharp vibronic features there is a broad band near 23,200 cm-’. It vanishes upon dilution from to lop6 mole/mole and is lost upon excitation at the tail of the absorption spectrum. The concentration dependence rules out a single molecule property and suggests assignment to an adduct instead. Dimerization to aggregates is known to split the molecular levels into doublets. Since interaction in the excited states is enhanced upon closer molecular approach there is relaxation in the excited state, the extreme case being realized in excimers. The aggregate band is therefore broad and associated with a genuine Stokes shift. The surprising fact is that it requires dilution to mole/mole to eliminate it. This illustrates the strong tendency of the molecule in aggregate and explains why aggregates play a key role in the fluorescence of bulk LPP-films (74).

428

7 Electronic E.xcitation

(ii) The spectrum of LOP ( n = 3) consists of a series of sharp vibronic zero phonon features each carrying a phonon wing as usually found with rigid molecules in a glassy matrix. The energies of the dominant vibrations are 185 cm-' (s); 240cm-' (w); 720cm-' (w); 780 (cm-') (s); 1370 (cm-') (s) and 1640 (cm-') (s). (iii) Presence of a -C6Hl3group suppresses the 720 cm-' and 780 cm-' modes which is a strong feature in the LOP ( n = 3) spectrum but absent in the LOP ( n = 7) spectrum. Since the furane molecule has a bending mode at 725cm-' [75] it is straightforward to assign the 708cm-' mode to a bending mode of the unsubstituted five-membered carbon ring. (iv) The rigidity of the LOP backbone is reflected in the facts that the vibronic features carry well-resolved zero phonon lines followed by phonon wings and that vibronic overtones are very weak. This indicates weak vibrational coupling, i.e. an S value close to unity. (v) The energies of the inhomogeneously broadened Sl + So 0-0 band, measured upon non-resonant excitation are 29,500 cm-' (LOP (n = 3)), 25,700 cm-' (LOP (n = 5 ) ) and 23,800cm-' (LOP (n = 7)). When plotted versus the reciprocal number of phenylene rings the data bear out an exact linear law extrapolating to 19,600cm-' for the infinite chain. The effective conjugation length of the polymer, whose S1 t SoO-O transition is at 22,100cm-I, corresponds to 12 phenylene units coupled together. This concurs with the conclusions of Grimme et al. [76].

7.1.4 Conclusions A systematic comparison of the absorption and emission spectra of n-conjugated oligomers with different chemical structure indicates that, except for a bathochromic shift that obeys a A E = AE, const./l law, the basic spectral features are retained upon increasing the molecular length L. The associated polymer can be considered as an array of oligomers the main difference between polymer and oligomer being that in the former the length is morphology-controlled and, concomitantly, is a statistically varying quantity. Even in the case of a perfectly ordered, fully elongated conjugated chain, realized with crystalline polydiacetylenes, the analogy between oligomer and polymer spectra is retained [77] indicating that neither the strength of electron phonon coupling nor the strength of electronelectron interaction changes significantly with increasing chain length. It is, however, important to note that the 2'A, t l'A, transition energy drops more rapidly with chain length than does the I'B, t I'A, transition energy. In the oligoenes the level ordering is reversed already in the shorter chains. The effective conjugation length, defined as the length of a perfectly aligned oligomer having the same transition energy, of either oligomer or polymer depends on morphology. If the torsional potentials are flat as they are in the polyenes and to a lesser extent in the thiophenes, there is a broad distribution with shorter chains having higher probabilities. This causes both spectral broadening and a hypsochrornic shift of the absorption maximum in disordered systems which must not be confused with

+

Reference.y

429

a band shift due to strong coupling to molecular vibrations. By the same token presence of conformational disorder gives rise to energy transfer from shorter and larger conjugated segments in condensed bulk systems of oligomers. Intra-chain energy transfer also occurs among sub-units of a polymer chain, even if matrixisolated, which is manifest in spectral relaxation. The associated Stokes-shift reflects the variance of the transition energies. A genuine Stokes shift due to conformational relaxation can but need not be superimposed. With few exceptions the Huang-Rhys factor for conjugated oligomers is of order unity, no different from other rigid aromatic molecules.

Acknowledgement The contribution of S. Heun, R. F. Mahrt and T. Pauck is gratefully acknowledged. The work carried out in the author’s laboratory was supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 383).

References I. 2. 3. 4. 5. 6. 7.

8. 9. 10. 1I . 12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

See e. g . Organic Conductors, J. P. Farges, (ed.), Marcel Dekker, New York, 1994. J. H. Burroughes, D. D. C. Bradley, A. R. Brown, et a/., Nature 1990, 347, 539. D. D. C. Bradley, Synth. Met. 1993, 54, 401. A. J. Heeger, S. Kivelson, R. J. Schrieffer,and W. P. Su, Rev.Mod. Phys. 1988, 60, 782. M. Schott und G. Wegner, in Nonlinear Optical Properties of Organic Molecules and Crystals, Vol. 2 (D. S. Chemla and J. Zyss, eds.) Academic Press, Orlando, 1987, p, 1 . K. Fesser, A. R. Bishop and D.K. Campbell, P h p . Rev. 13. 1983,27,4804. R. I. Personov, in Spectroscopy and Escitation Dynamics in Condensed Molecular Systems (V. M. Agranovich and R. M. Hochstrasser, eds.) North Holland, Amsterdam, 1983, p. 555. H. Bassler, in Optical Techniques to Characterize Polvmer Systems (H. Bassler ed.) Elsevier, Amsterdam, 1989, p. 181. See e.g. Persistent Spectral Hole Burning: Science and Applications (W. E. Moerner, ed.) Springer Verlag, Berlin 1988. H. Blssler, in Disorder Efects on Rela.xationa1 Processes (R. Richert and A. Blumen, eds.) Springer Verlag 1994, p. 485. R. Richert, H. Bassler, B. Ries, B. Movaghar and M. Grunewald, Phil. Mag. Lett. 1989,59,95. A. Elschner, R. F. Mahrt, L. Pautmeier, H. Bassler, M. Stolka, and K. McGrane, Chem. Phys. 1991, 150, 81. H. A. Staab, Einfuhtng in die Theoretische Chemie, Verlag Chemie 1966. B. S. Hudson, B. E. Kohler, and K. Schulten, Excitedstates 1982, 5, 1. B. E. Kohler. C. Spangler, and C. Westerfield, J . Cham. Phys. 1988, 89, 5422. 1990, 93, 5838. B. E. Kohler, J . Chem. P/~J>S., I. Ohmine, M. Karplus, and K. Schulten. J . Chern. PIzys. 1978, 68, 2298. B. E. Kohler and B. West. J . Chem. Phys. 1983, 79, 583. S. N. Dixit and S. Mazumdar, Phys. Rev. 5. 1984, 29, 1824. Z. Soos and S. Ramaseha, Phys. Rev. 5. 1984,29,5410. D. Baeriswyl and K. Maki, Phys. Rev. B. 1988, 31, 6633. B. E. Kohler, J . Chem. Phys. 1988, 88, 2789. I. D. W. Samuel, 1. Ledoux, C. Dhenaut, et al., Science 1994, 265, 1070.

430 24. 25. 26. 27. 28. 29. 30. 31. 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.

7 Electronic Excitation

B. E. Kohler, J . Chem. Phys. 1995, 103, 000. B. E. Kohler and J. C. Woehl, J . Chem. Phys. 1995, 103, 000. D. Birnbaum and B. E. Kohler, J . Chem. Phys. 1991, 95, 4783. D. Birnbaum and B. E. Kohler, J . Chem. Phys. 1989, YO, 3506. D. Birnbaum, D. Fichou and B. E. Kohler, J . Chem. Phys. 1992,96, 165. H. Chosrovian, S. Rentsch, D. Grebner, D. U. Dahm, E. Birckner and H. Naarmann, Synth. Met. 1993, 60, 23. J. L. Bredas, G. B. Street, B. Themans and J. M. Andre, J . Chem. Phys. 1985, 83, 1323. C. Quattrocchi, R. Lazzaroni and J. L. Bredas, Chem. Phys. Lett. 1993, 208, 120. G. Zerbi, B. Chierichetti, and 0. Inganas, J . Chem. Phys. 1991, 94,4637. F. Chana, D. Fichou, J. M. Nunzi, and N. PfeifTer, Chem. Phys. Lett. 1992, 192, 566. N. Periansamy, R. Danieli, G. Ruani, R. Zamboni and C. Taliani, Phys. Rev. Lett. 1992, 68, 919. H.-J. Egelhaaf and D. Oelkrug. F. Deloffre, F. Garnier, P. Srivastava, A. Yassar, and J. L. Fave, Synth. Met. 1994, 67, 223. J. C. Sciano, R. W. Redmond, B. Mehta, and J. T. Arnason, Photochem. Photobiol. 1990, 52, 655. R. A. J. Janssen, D. Moses, and N. S. Sariciftci, J . Chem. Phys. 1995, 101, 9519. X. Wei, Z. V. Vardeny, D. Moses, V. I. Srodanov, and F. Wudl, Spnth. Met. 1993, 54, 273. M. Bennati, A. Grupp, P. Bauerle and M. Mehring, Mol. Cryst. Liq. Cryst. 1994, 256, 751. J. Heinze, I . Mortensen, K. Miillen and R. Schenk, J . Chem. SOC. 1987, 701. R. Schenk, H. Gregorius, K. Meerholz, J. Heinze, and K. Mullen, J . Am. Chem. SOC. 1991,113, 2634. A. Schmidt, M. L. Anderson, D. Dunphy, Th. Wehrmeister, K. Miillen, and N. R. Armstrong, Adv. Mut. 1995, 7 , 722. H. S. Woo, 0. Lhost, S. C. Graham, et al., Synth. Met. 1993, 59, 13. R. F. Mahrt, J. Yang, A. Greiner, H. Bassler, and D. D. C. Bradley, Macromol. Chem. Rapid Commun. 1990, 11,415. S. Heun, R. F. Mahrt, A. Greiner, et al., J . Phj:~.Condens. Matter 1993, 5, 247. Y. Haas, S. Kendler, E. Zingher, H. Zuckermann, and S. Zilberg, J . Chem. Phys. 1995, 103, 37. S. Lefrant, E. Perrin, J. P. Buisson, H. Eckhardt, and C. C. Han, Synth. Met. 1989, 29, E91. D. Rakovic, R. Kostic, L. A. Gribov, and I. E. Davidova, Phys. Rev. B. 1990, 41, 10744. B. Tian, G. Zerbi, and K. Miillen, J . Chem. Phys. 1991, 95, 3198. R. F. Mahrt, and H. BBssler, Synrh. Met. 1991, 45, 107. H. A. Stuart, Molekiilstruktur, Springer Verlag, Berlin 1967. K. D. Moller, and W. G. Rothschild, Far lnSaredSpectroscop~,Wiley, New York 1971, p. 300. H. P. Weitzel, A, Bohnen, and K. Miillen, Mukromol. Chen?. 1990, 191, 2815. S. Heun, H. Bassler, U. Miiller, and K. Miillen, J . Phys. Chem. 1994, 98, 7355. H. D. Becker, L. M. Engelhardt, L. Hansen, V. A. Patrick and A. H. White, Austr. J . Chem. 1984, 37, 1329. H. D. Becker, Chem. Rev. 1993, 93, 145. H . Vestweber, J. Pommerehne, R. Sander, et ul., Synth. Met. 1995, 68, 263. K. Pakbaz, C. H. Lee, A. J. Heeger, T. W. Hagler, and D. McBranch, Synth. Met. 1994,64,295. M. Chandross, S. Mazumdar, S. Jeglinski, et al., Phys. Rev. B. 1994, 50, 14702. M. Deussen, M. Scheidler, and H. Bassler, Synth. Met. 1995, 73, 123. H. Y . Tak, unpublished data, cited in H. Bassler, Macromolecular Sjimposia, 1996, 104, 269. G. Weiser, Phys. Rev. B. 1992-11 45, 14076. P. Gomes da Costa and E. M. Conwell, Phys. Rev. B. 1993, 48, 1993. Z. Shuai, J. L. Bredas, and W. P. Su, Chem. Phys. Lett. 1994, 228, 301. T. Ikeyama and T. Azumi, J . Phys. Chem. 1985,89,5332. K. Pichler, D. A. Halliday, D. D. C. Bradley, P. L. Burn, R. H. Friend, and A. B. Holmes, J . Phys. Condens. Mutter 1993, 5 , 7155. W. Werncke, M. Pfeiffer, T. fohr, A. Lau, and H.-J. Jiipner, Chern. Phys. 1995, 199,65. K. Migashita and M. Kaneto, Macromol. Chem. Rapid Commun. 1994, 15, 511. G. Grem and G. Leising, Synth. Met. 1993, 57, 4105. U . Scherf and K. Miillen, Macromol. C h ~ mRapid . Commun. 1991, 12, 489.

References

72. 73. 74. 75.

43 1

U. Scherf and K. Mullen, Synthesis 1992, 23, 23. T . Pauck, Dipi'oma thesis, Marburg 1995, to be published. T. Pauck, R. Hennig, M. Perner, et al., Chem. Phys. Lett., 1995, 244, 171. H. M. Randall, R. G. Fowler, N. Fuson and J. R. Dangel, Infrared Determination of Organic Structures, D. van Nostrand Co, Princeton, 1961. 76. H. Sixl, in Polydiacetylenes (D. Bloor and R. R. Chance eds.), NATO AS1 Series E. Applied Science No. 102, Martinus Nijhoff Publ., Dordrecht 1985. 77. J. Grimme, M. Kreyenschmidt, F. Uckert, K. Muflen and U. Scherf, A h . Mat. 1995, 7, 292.

7.2 A Quantum Chemical Approach to Conjugated Oligomers: The Case of Oligothiophenes J. Cornil, D. Beljonne, and J. L. Bredas

7.2.1 Introduction In this chapter, we focus on a widely studied class of conjugated oligomers, the oligothiophenes (see Chapter 2. l), and review our recent investigations on the evolution with chain length of the geometric and electronic structure and optical properties of neutral and charged molecules; particular attention is paid to the extent to which the calculated properties can be extrapolated to the scale of a very long chain. Our choice of thiophene compounds is explained by their significant contributions to the development of electronic or electro-optical devices [ 1-31: the design of polythiophene-based light emitting diodes (LEDs) with tunable emission color has recently been achieved [4] and oligothiophenes themselves can serve as active materials in devices such as organic transistors [ 5 ] , LEDs [6], or spatial light modulators [7]. We first provide an overall description of the nature of the lowest excited states of neutral oligothiophenes in both the singlet and triplet manifolds; these results are important in comprehending the performances of LEDs based on polythiophene and its oligomers. We focus on: (i) the lowest singlet excited state (S,) responsible for electroluminescence following radiative decay of the polaron-excitons; (ii) the lowest excited triplet state (TI)that can be reached either by intersystem crossing or by triplet recombination of the injected electrons and holes; and (iii) the higher-lying triplet state (T,) giving rise to a strong, long-lived feature in photoinduced absorption experiments. We analyze the evolution with chain length of the So -+ S , and T I -+ T, excitation energies as well as of the energy difference between the So and TI states. We also discuss how singlet-to-triplet intersystem crossing is affected by chain length. Finally, we investigate the strength of the lattice relaxations occurring in the S , and TI states and show the stronger confinement of the triplet with respect to the singlet. We then characterize the changes occurring in the properties of oligothiophenes upon doping in order to rationalize the discrepancies found in the literature regarding the interpretation of the optical absorption spectra; we demonstrate the importance of considering the selection rules imposed by the symmetry of the systems. We also revisit on that basis the assignment of optical transitions in conjugated polymers that are oxidized or reduced [8]. Thereby, we shall also make an attempt to close the gap between chemical terminology and the matter of condensed-matter physics.

7.2.2 Theoreiicul Appronch

433

7.2.2 Theoretical Approach At first, it is useful to briefly depict the main lines of our methodology. We have investigated unsubstituted oligothiophenes (H-T,-H) ranging in size from IZ = 2 to 11 rings; we sketch in Fig. I the chemical structure of the tetramer (H-T,-H) as well as the atom labelling adopted when introducing the theoretical results. Note that compounds with an even number of thiophene units are characterized by C2h symmetry while oligomers with an odd number of rings present C2" symmetry. The optimal geometrics of the oligomers are determined by means of NDDO (Neglect of Differential Diatomic Overlap)-based semiempirical HartreeFock methods such as AM1 (Austin Model 1) [9] or M N D O (Modified Neglect of Differential Overlap) [lo]. The MNDO formalism has been shown to provide bond-length values in the ground state that are in good agreement with X-ray diffraction data [ll]. We stress that a good description of the C-C bond-length alternation is required since theoretical studies have established that the So 4 S1 transition energy decreases when bond alternation is reduced [ 121. On the basis of the ground-state geometries, we make use of the semiempirical Hartree-Fock Intermediate Neglect of Differential Overlap (INDO) Hamiltonian [ 131 coupled to a MultiReference Double-Configuration Interaction (MRD-CI) scheme [I41 to describe the lowest singlet and triplet excited states in oligomers up to the hexamer. The CI expansion is built with configurations corresponding to single and double excitations from the 6 highest occupied levels to the 4 lowest unoccupied levels with respect to two reference determinants that are: (i) the SelfConsistent-Field (SCF) determinant itself; and (ii) the configuration obtained by promoting one electron from the HOMO level to the LUMO level. Note that it is of prime importance to incorporate doubly excited configurations in the CI expansion in order to provide a reliable description of the nature of the lowest excited states; the mere consideration of single excitations leads for instance to oligoenes to wrong ordering of the lowest excited states, the lowest two-photon excited state (2A,) appearing above the lowest one-photon excited state ( 1 Bu), in contrast to the experimental data [12]. To estimate the extent of the lattice (geometry and energy) relaxations in the excited states, the geometries in both the S I and T I states are optimized at the M N D O level, on the basis of calculations performed with a level occupancy characterized by the transfer of one electron from HOMO to LUMO (one spin being flipped for the triplet). Combining the ground-state deformation energy when going from the So to the S , ( T I ) geometries with the INDO/MRD-CI So -+ S 1

Figure 1. Chemical structure and atom labelling of unsubstituted quaterthiophene H-T4-H.

434

7.2 A Quantuni Chetnical Approach to Conjugated Oligotners

(So -+ T , ) vertical energy differences, then allows us to evaluate the relaxation energy in the lowest singlet (triplet) excited state. The geometry of the singly and doubly oxidized conjugated oligomers are optimized by means of the AM1 method, the radical-cations being treated within the Restricted Open-Shell Hartree-Fock (ROHF) formalism; note that we have neglected to impact of the presence of the counter-ions in the present calculations. The singlet transition energies of the oxidized compounds and relative intensities are then estimated with the help of the nonempirical Valence Effective Hamiltonian (VEH) method [15], which is known for its ability to provide reasonable locations of the defect levels inside the gap. The use of a one-electron picture is validated by earlier calculations conducted at the correlated level, showing that the sub-gap optical transitions appearing in the absorption spectra of single and double charged oligothiophenes are characterized by a single dominant configuration [ 161.

7.2.3 Neutral Oligomers 7.2.3.1 Chain-length Evolution of the Lowest Excited States On the basis of the MNDO-optimized geometries of the neutral oligomers [ 171, we have investigated the evolution with chain length of the transition energies between the ground state So and the lowest singlet excited state S , , This excited state mainly originates from an electron transition between HOMO and LUMO. The theoretical results are found to compare very well with the experimental values extracted from optical absorption measurements in solution [18-211, as shown in Fig. 2. Note that a complete theoretical treatment of absorption in solution would require taking account of the possible solvation effects; such an approach, however, is beyond our current capabilities. A linear dependence, typical of conjugated compounds, appears between the transition energies and the inverse number of repeat units, (see Fig. 2); there indeed occurs a significant red-shift of the lowest electronic transition as the chain grows, due to the progressive extension of the n-delocalized system. Figure 2 also includes the energy difference between the ground state So and the lowest triplet excited state T I ,as calculated at the INDO/MRD-CI level on the basis of the So geometry. We find the evolution with chain size of the So 4 T I transition energies to be much slower than for the So -+ SI excitation: the singlet-triplet excitation is only lowered by ~ 0 . 2 e Vwhen going from the dimer to the hexamer while a bathochromic shift of -1.4 eV is observed for the singlet-singlet transition. Such behavior actually reflects the stronger confinement of the triplet exciton with respect to the singlet; these trends are consistent with Optically-Detected Magnetic Resonance (ODMR) experiments on polythiophene that indicate that the T I triplet state hardly extends over more than a single thiophene unit [22]. Optical absorption spectra measured in a solvent containing heavy atoms have located the position of TI state in terthiophene (H-T3-H) [23]; in this case, a non-vanishing intensity is observed for the So + TI transition due to spin-orbit coupling related to

7.2.3 Neutrrrl 0ligomcv.r

435

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P

3.0

2.0 1.5

0.0

0.1

0.2

0.3

0.4

0.5

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Figure 2. Evolution of the INDO/MRDCI-calculated So iS , and So 4 T, transition energies (full circles) as a function of the inverse number of repeat units ( l / n ) . The experimental values are represented by open squares and are extracted from Refs. 18, 19, and 21 for the singlet-singlet transition and from Ref. 23 for the singlet-triplet excitation.

‘heavy-atom effects’ induced by the solvent; this leads to the appearance of a broad and weak absorption band around 1.7 I eV, in very good agreement with the I .68 eV calculated value. Janssen and co-workers have also recently reported that addition of buckminsterfullerene C60to solutions of oligothiophenes ranging in size from the hexamer to the undecamer results in a quenching of the triplet state of the oligothiophenes to produce a C60 triplet state via energy transfer [19]; the TI state in these oligothiophenes is therefore expected to lie between the energies of the (260 triplet state and of the T, state in the trimer, which are estimated at 1.57eV [24] and 1.71 eV [23], respectively. Finally, we also mention that Xu and Holdcroft have detected a phosphorescence peak at -1.5 eV in polythiophene [25], a value close to that obtained by extrapolating the So 4T, excitation energies at the scale of an infinite polymer chain. Singlet excitons formed upon photoexcitation or electron-hole recombination can decay nonradiatively via intersystem crossings to lead to the creation of metastable triplet excitons; such species give rise to a single, intense and longlived triplet-triplet T I + T,, electronic transition seen in photoinduced absorption experiments on oligothiophenes in solution [ 19,261. These observations are consistent with the results of MRD-CI calculations where we find the lowest-energy triplet state to be strongly coupled to just a single higher-lying triplet excited state whose wavefunction corresponds to a complex mixing of singly and doubly excited configurations. The evolution with chain length of the theoretical T, ---+ T,, transition energies are reported in Fig. 3 together with the experimental values derived from

436

7.2 A Qucintum Clremicul Approuclr to Corrjugatecl Oligomers

4.0

% v

Is

z s

' 9

7j

3.0

2.0

3 I1.o

0.0

0.0

0.1

0.2

0.3

0.4

0.5

1In

Figure 3. Evolution of the INDO/MRDCl-calculated (full circles) and experimental (open squares) T, -+ T,, excitation energies versus the inverse number of thiophene units (1,'n). The experimental data are taken from Ref. 19.

photoinduced absorption experiments. Despite a systematic overestimation of the theoretical values, a similar chain-length dependence of the calculated and experimental transitions is obtained and points to a significant red-shift of the T, + T,, transition as the chain grows. It thus appears that the higher-lying triplet state T, has a less confined wavefunction than the T I state, which is consistent with its smaller binding energy.

7.2.3.2 Intersystem Crossing In LED devices, the emission of light resulting from radiative decay of the singlet excitons, formed upon recombination of the injected electrons and holes, competes with numerous nonradiative decay routes [27]; these processes include interchain effects related for instance to the formation of excimers, quenching of the singlet excitons by extrinsic or conformational defects as well as by low-lying twophoton states [28], singlet fission into two triplets [29], and intersystem crossing (ISC) from the singlet to the triplet manifolds [30]. Note that the possibility of quenching of the excitons by a lower-lying two-photon state is ruled out in oligothiophenes up to the hexamer since both experimental [3 11 and theoretical [32] studies have located the 2A state above the 1B state. Recent time-resolved fluorescence measurements on unsubstituted thiophene oligomers in solution indicate a sharp increase of the fluorescence quantum yield

aFwhen the number of thiophene units is increased from two to seven [ 18,331; in such experiments, we expect the migration of the excitons towards trapping centers to be minimized due to the finite size of the involved systems and the interchain effects to be less efficient than in the solid state. The evolution of Q F with chain size has been related to a decrease in nonradiative decay rate kNR,since the radiative decay constant k R is observed to be almost unaffected when going from one compound to the next [ 3 3 ] . Among the various nonradiative processes, the singletto-triplet intersystem crossing has been found to provide the most significant contribution to kNR [ 3 3 ] . We have tried to provide a coherent picture of the ISC processes in oligothiophenes in order to rationalize the trends observed experimentally [17]. As first suggested by Rossi et 01. in the case of terthiophene (H-T,-H), k~~ can be expressed as a sum of two contributions, k l and k2:

-1

kT

Here, k l includes various nonactivated nonradiative phenomena while k2 corresponds to an activated intersystem crossing process. The amplitude of k2 depends both on the spin-orbit interaction through the pre-exponential A 2 factor and on the singlet-triplet energy difference through the AElsc value. Although a precise description of the ISC processes would require taking into account the spin-orbit coupling interaction, it is unlikely that the strong evolution with chain length that is seen, be based on significant modifications in the strength of this coupling. We thus relate the observed decrease with chain length in the k N Rdecay rate to the evolution of the energy difference AElsc between the singlet and triplet states involved in the crossing. The S , - TI energy differences calculated at the INDOIMRD-CI level in oligothiophenes are much too large to lead to an efficient singlet-triplet overlap, and hence to a significant probability of intersystem crossing. However, the calculations indicate that one higher-lying triplet excited state (T4) lies within the same energy range as S , . We plot in Fig. 4 the evolution of the So + S , and So 4 T4 excitation energies versus the inverse number of thiophene rings. Starting in bithiophene from a situation where the triplet T4 is located below the singlet S , , we observe a reversal of the ordering of these two states as the chain size is increased: the crossing between the evolution of the S I and T4 states occurs for a chain length corresponding to the trimer. Note that the position of the T4 excited state is overestimated for the trimer due to the absence in the calculations of spin-coupling interactions (these are expected to mix efficiently the singlet and triplet wavefunctions). The trends derived from the consideration of the T4 triplet excited state as the essential state leading to intersystem crossing in oligothiophenes, are consistent with the experimental measurements. In bithiophene, the fact that the T4 state lies below the S, state gives rise to a nonactivated and very efficient ISC process, and hence to a very low fluorescence yield. The activation energy, however, increases when the chain elongates, making the probability of ISC processes lower and the fluorescence yield @F substantially higher.

438

7.2 A Quunluin Chetiiicnl Approtidi t o Conjugated Oligorners

4.5

4.0

1z v

E?

5

3.5

c

.I

2 L

t-

3.0

2.5 0.1

0.2

0.3

0.4

0.5

1In

Figure 4. Evolution of the INDO/MRDCI-calculated So 4 S , (open squares) and So 4 T4 (full circles) excitation energies as a function of the inverse number of thiophene rings (l/n).

7.2.3.3 Relaxation Phenomena in the Lowest Excited States The MNDO equilibrium geometries in the S, and T I states of the dimer, trimer, and tetramer are reported in Table 1, together with the ground-state geometry and the relaxation energies with respect to vertical transitions from the ground state. The evolution with site position i of the bond-length alternation (defined as the difference between the lengths of the (i, i + 1) and (i, i - 1) C-C bonds) is illustrated in Fig. 5 for the So, S1 and TI states of the tetranier. Analysis of the geometry deformations occurring in the S1 state reveals lattice distorsions that are small and characterized by the appearance of a quinoidic character within the central rings; the C-C bondlength altern!tion is found to drop from 0.04-0.05 A in the external rings to a value close to 0.02 A in the central part of the system. Such geometry changes are reminiscent of those associated to the creation of a soliton-antisoliton pair in polyacetylene [34], except that the electron-hole pairs in oligothiophenes are bound due to both the Coulombic attraction between the charge carriers and the nondegenerate ground state nature of polythiophene. On the basis of the MNDO-calculated bond-length deformations and additional theoretical calculations [ 171, we estimate the singlet exciton to extend over 3-4 repeat units. In contrast, milch more pronounced lattice distortions are observed for the lowest triplet excited state, as illustrated by the amplitude of the relaxation energy that is almost twice as big as in the singlet state (0.33eV vs. 0.16eV). The formation of a (bound) soliton-antisoliton pair clearly emerges when looking at the geometric deformations along the chain axis; indeed, going from the end towards the center of the chain, the C-C bond-length alternation first decreases (as a consequence

7.2.3 N e u f ~ u0ligoiwr.s l

439

Table 1 C-C and C-S bond lengths (in A) in: (i) the ground state So; the lowest singlet excited state S , ; and (iii) the lowest triplet excited state TI of the dimer H-T2-H (T2). trimer H-T,-H (T3) and tetramer H-Td-H (T4) of oligothiophenes. as optimized at the MNDO level. We include the relaxation energies (Ere,,in eV) with respect to vertical excitations. The atoms are labelled according to Fig. I ; we prevent any redundancies by taking explicit account of the symmetry of the systems.

Bond

TI

SI

SO

T2

T3

T4

T2

T3

T4 ~

1-2 2-3 3-4 I-SI 4-SI 4-5 5-6 6-7 5-S2 7-8 8-S2 8-9

1.374 1.447 1.388 1.674 1.694 1.447

1.374 1.447 1.388 1.674 1.694 1.447 1.388 1.442 1.689

1.380 1.447 1.388 1.674 1.694 1.447 1.388 1.441 1.689 1.388 1.689 1.446

1.396 1.421 1.427 1.671 1.701 1.415

0.25

Ere1

1.387 1.431 1.414 1.670 1.692 1.420 1.424 1.410 1.696

1.384 1.437 1.405 1.670 1.690 1.425 1.421 1.406 1.692 1.428 1.696 1.415 0.16

0.15

T2 ~

T3

T4

1.382 1.434 1.416 1.673 1.692 1.405 1.457 1.376 1.687

1.378 1.441 1.397 1.670 1.689 1.431 1.429 1.394

~~

1.397 1.408 1.457 1.677 1.699 1.382 1.697

0.39

0.34

1.451 1.699 1.383 0.33

of the shortening of the single bonds and of the elongation of the double bonds), then vanishes at the connection between the second and third rings (the C-C bond-lengths are there equal), and finally becomes negative and peaks in absolute value at the center of the oligomer (the single-double C-C bond pattern is reversed 0.08 0 0.06 0.04

s

0.02

E 3%

2

*

P -

0.00

Q)

-0.02

0

m -0.04 -0.06

v

0

-0.08

,

I

I

I

I

I

I

2

4

6

8

10

12

14

16

Site

Figure 5. Evolution, with site position i, of the bond-length alternation in quaterthiophene. H-T4-H, Ar (calculated at the MNDO level as the difference between the lengths of the (i,i + I ) and (i, i - 1) carbon-carbon bonds). in the So (solid line). Sl (dashed line) and T, (dotted line) states.

440

7.2 A Qimntrrin Chertiical Approach to Conjugated Oligomers

and the absolute value of the bond-length alternation is recovered). Due to the exchange potential term, a stronger confinement is obtained for the triplet with respect to the singlet; the results indicate that 1 or 2 thiophene rings are needed for a proper accommodation of the triplet defect, as also suggested by ODMR data [22]. The theoretical insight that we are gaining into the intrinsic electronic properties of polythiophene and corresponding oligomers can prove very useful in order to set up new strategies aimed at the achievement of improved efficiencies in LED devices. For instance, the knowledge of the relative locations of the lowest singlet and triplet excited states is valuable information that can stimulate the design of novel materials in which nonradiative decay processes such as intersystem crossing would be prevented at best.

7.2.4 Charged Oligomers In this section, our goal is to investigate the optical properties of charged (oxidized) oligomers and to establish whether they are consistent with those of the corresponding polymers. The polymer optical properties have often been treated in the framework of one-electron band-structure models [8]. In this context, the conjugated polymers are characterized in the ground state by a filled 7r-band and an empty 7r*-band (i.e. the valence and conduction bands, respectively) separated by a band gap that governs the electrical properties of the pristine system. Upon doping (i.e. oxidation or reduction), charges removed from or iiijected to the conjugated backbone give rise to the appearance of spatially localized geometric defects, as a result of the strong electron-phonon coupling characteristic of conjugated chains [35]. In terms of condensed-matter physics, such charges coupled to a local lattice distorsion of the backbone are described as positive (negative) polarons or bipolarons upon single or double oxidation (reduction) processes; note that in the following, we will also make use of the chemical terminology by referring to radical-cations and di-cations (anions) for singly and doubly oxidized (reduced) chains, respectively. The formation of polarons and bipolarons is also accompanied by a strong deformation of the electronic structure of the polymer: two new localized one-electron levels appear within the original forbidden gap, as shown in Fig. 6. This figure also illustrates that in the case of oxidation, the lowest polaronic level is singly occupied (which leads to the magnetic signature of this radical-ion species) whereas the bipolaronic levels are both empty. Furthermore, according to the one-electron band-structure model developed by Fesser et al. [8], three new sub-gap optical transitions are expected following the formation of polarons while only two are predicted in the presence of bipolarons (see Fig. 6). It is worth stressing that this model appears to be consistent with most of the experimental absorption spectra of doped polymers reported to date. We now turn to the discussion of the results obtained from the calculations performed on conjugated oligothiophenes. In this context, the AM 1-optimized

7.2.4 Charged Oligorners

a)

b)

44 1

C)

Figure 6. Sketch of the one-electron band-structure model for polythiophene in: (a) the neutral state; (b) in the presence of a positively charged polaron; and (c) in the presence of a positively charged bipolaron. The new sub-gap optical transitions induced upon doping are also represented.

geometries of the doubly oxidized oligomers show that the charged species (bipolaron) is localized at the center of the molecule and is characterized by a reversal of the single and double character of the C-C bonds, while the C-S bonds are almost unaffected; the formation of such bipolarons thus induces the appearance of a strong quinoidic character within the rings [16]. In contrast, the geometries of the radical-cations exhibit weaker structural deformations; the AM 1 results provide C-C bond lengths intermediate between those obtained for the neutral and doubly oxidized systems, and thus show that the formation of polarons leads to the appearance of a semiquinoidic character along the chain [16]. As the size of the oligomer is increased, the amplitude of the geometric deformations is found to diminish when going from the center to the end of the molecule. We describe in Fig. 7 the typical evolution of the C-C bond lengths upon oxidation; this plot indicates that the bipolarons extend over nine repeat units, as also suggested by the theoretical calculations of Ehrendorfer and Karpfen [36], while a weaker spatial extension of five rings is expected for the polarons. However, these estimates have to be considered as upper limits due to the fact that the influence of counter-ions is neglected in the present calculations. As mentioned above, the geometric relaxations taking place upon oxidation are accompanied by a strong modification of the one-electron structure of the oligomers. Two molecular orbitals move inside the original gap almost symmetrically to give rise to new sub-gap features in the optical absorption spectra [35]. In the case of radical-cations, the VEH calculations show the appearance of two new sub-gap features that, in ascending order, originate from an electron transition between the HOMO level and the lower polaronic level (H + POL1) and between the two polaron levels (POL1 -+ POL2); the transition energies and intensities are reported in Table 2. The VEH-calculated transitions are found to be in very good agreement with experimental data measured upon oxidation [20,21,37,38], photoinduced absorption [39,40] or voltage-modulation spectroscopy [41]. Moreover, both theoretical and experimental polaron transition energies are found to evolve linearly with the inverse number of rings. The absence of any transition between the HOMO level and the upper polaronic level is actually directly related to the selection rules imposed by the symmetry of the

442

7.2 A Quantum Chemical Approach to Conjugated Oligomers

2

4

6

8

10

12

14

16

20

18

22

Bond number

Figure 7. AM 1 -optimized C-C bond-lengths (in A) for half the 1 I-ring oligomer in the neutral (full circles), singly oxidized (open squares), and doubly oxidized (open triangles) states. Bond 1 is located at the center of the carbon-carbon path while bond 22 lies at the end of the chain.

oligomers. Taking for instance account of Czvsymmetry, we find the strength of this transition to be strongly limited since the two levels that are involved, HOMO and POL2, belong to the same a2 irreducible representation (POL1 has bl symmetry); this therefore leads to an excitation which is polarized in a direction transverse to the chain axis (the symmetry constraints are even more drastic when dealing with CZhsymmetry since the same HOMO -+ POL2 transition then becomes forbidden). On the other hand, electron transitions from a2 to b l levels (or vice versa) give rise to excitations that are polarized along the chain axis and can thus present significant intensities, such as those observed in the spectra of charged oligothiophenes and related to HOMO --t POLl and POLl --+ POL2 transitions. We note that a third symmetry-allowed transition is given by our calculations [ 161; this feature, which Table 2 VEH-calculated polaron transition energies (between the HOMO level and the lower polaronic level (H + POLI) and the two polaron levels (POLI + POL2) and bipolaron transition energies (between the HOMO level and the lower bipolaronic level (H + BIPl)) in oligothiophenes H-T,-H containing 3, 5 , 7, and 9 rings. The relative intensities of the transitions (in arbitrary units) are reported between parentheses. Number of rings

POL1

3 5 7 9

2.72 (30.1) 1.64 (50.8) 1.39 (65.9) 1.26 (77.2)

+

POL2

H

-+

POL1

1.58 (37.0) 1.08 (58.7) 0.82 (74.6) 0.64 (86.7)

H

-+

BIPl

2.01 (38.4) 1.30 (63.0) 0.99 (83.5) 0.81 (101.4)

7.2.4 Charged Oligomers

0.5

1 .o

1.5

2.0

443

2.5

Energy (eV)

Figure 8. VEH-calculated absorption spectra of septithiophene H-T,-H in the neutral state (solid line), singly oxidized state (dotted line) and doubly oxidized state (dashed line). The spectra are simulated by convolution with Gaussians whose full width at half-maximum is 0.2 eV.

is weak, is dominantly described by electron transitions between the lower polaronic level and the LUMO + 1 level (POL1 -+ L + 1) and between the HOMO-1 level and the upper polaronic level (H-1 --+ POL2); this third absorption peak is calculated to lie within the same energy range as the first excitation of the neutral system (it thus tends to be overshadowed by the presence of remaining neutral molecules). The formation of positive bipolarons leads to the appearance of a single sub-gap absorption peak that originates from an electron transition between the HOMO level and the lower bipolaronic level; this transition also red-shifts with increasing chain length, as shown in Table 2. The symmetry considerations invoked above result in a vanishing intensity for the transition between the HOMO level and the upper bipolaronic level. There is once again an excellent agreement between the theoretical values and the spectroscopic data [20,21,37,38]; both demonstrate the existence of a linear relationship between the bipolaron transition energies and the inverse number of rings. We present in Fig. 8 the VEH-simulated spectra of the seven-ring oligomer in various oxidation states (it should be noted that the relative intensities of the two polaronic transitions, as obtained within the framework of the VEH one-electron picture, are significantly lowered when conducting similar calculations at the correlated level [ 161). These theoretical results allow us to rationalize a wide range of experimental observations. However, an additional type of charged species, referred to as 7rdimers, has been recently isolated; the 7r-dimers correspond to complexes formed upon interaction of two polaron-carrying oligomers [42,43]. Such defects, which are spinless, are invoked when two sub-gap absorption features are observed in

444

7.2 A

Qirriri firin

Clirniical Approtrcli to Conjugated Oligonirrs

the optical absorption spectra of lightly doped oligomers where no paramagnetic signal is detected, thus excluding the formation of polarons; the differentiation with respect to isolated bipolarons is straightforward since the latter would lead to a single sub-gap feature. In the case of the oxidized dodecamer (H-TIZ-H), spinless species are generated and two strong sub-gap absorption peaks are observed (these were initially assigned to bipolaronic transitions) [44]. Several models could be consistent with such experimental data: (i) A first one considers on the interaction of two bipolarons present on the same molecule. Indeed, we have recently shown, via calculations performed at the correlated level, that the formation of interacting bipolarons results in the appearance of four molecular levels inside the gap (due to the splitting of the defect levels related to a single bipolaron) and gives rise to the appearance of two new intense sub-gap peaks in the spectra [16]. However, in this dodecamer case, such an assumption appears not to be consistent with the degree of doping reached in the experiment, namely two charges per molecule. (ii) Another interpretation, based on spectrovoltammetry measurements [45], relies on the formation of a four-fold oxidized entity corresponding to a double T dimer. (iii) A final model that has been recently proposed suggests that the two sub-gap features originate from the formation of two polarons [46,47]. The latter model illustrates the very fine balance that exists, in the case of doubly oxidized compounds, between the energetics of formation of two polarons (whose wavefunctions can interact) and that of a bipolaron [48]. We sketch in Fig. 9 the amplitudes of geometry deformations along the oligomer chain that correspond to these two cases. It is very difficult to model accurately the two situations at the quantum-chemical level: not only are highly correlated calculations requested but explicit consideration of the medium (solvent) effects is also required. The fact that a single sub-gap absorption feature can be observed in the optical absorption spectra of doubly charged oligomers is in marked contrast to the typical

Site

Figure 9. Sketch of the amplitude of geometry deformations taking place on a long oligomer chain in the case of two interacting polarons (solid line) and a bipolaron (dashed line).

7.2.5 Conclusion

445

optical properties of conjugated polymers supporting charged bipolarons: two intense sub-gap peaks are then observed. A first reason that can be invoked to try and rationalize this discrepancy, is that the symmetry rules governing the nature of the optical transitions in the oligomers, d o not have to hold true at the scale of a long disordered polymer chain; however, calculations where we impose the bipolarons to be localized near one end of the chain reveals that such a breakdown of the symmetrical relaxation of the defects does not give any significant changes in the aspect of the spectra [ 161. As a consequence, the spectra of polymers supporting isolated bipolarons should be characterized by a single dominant sub-gap feature at low doping level. As we stressed above, an important aspect is, however, that the lineshape of the spectra is strongly affected as soon as interaction between the charged defects takes place; this then leads to the appearance of two intense subgap features such as those observed in the spectra of conjugated polymers at high doping level [49]. The signature of isolated defects should in principle be detected in photoinduced absorption experiments, since the concentration of the photogenerated charged species is expected to be very weak. The two long-lived sub-gap absorption features observed in such polymer photoinduced absorption spectra have been assigned as bipolaronic transitions [50]. This interpretation is thus in contrast with the results of our calculations on isolated bipolarons. This discrepancy can be explained in several ways: (i) recent optical modulation experiments have provided evidence that the two sub-gap features observed in the photoinduced spectra correspond to the optical signature of polarons [51]; (ii) the long-lived bipolarons might actually be trapped in more ordered regions where they could interact; or (iii) .ir-dimers are formed. The analysis we have presented in this section is applicable to any conjugated system possessing some symmetry in its geometric structure. We have indeed shown that the same trends prevail when looking at the spectra of oxidized or reduced oligopyrroles [52] (see Chapter 3), oligophenylenes [53], or oligo(pheny1eneviny1ene)s [54] (see Chapter 1). In the case of short oligoenes where charge storage occurs through generation of soliton pairs, a single sub-gap peak is also induced upon double ionization [55]. A particular feature of these compounds is that the formation of polarons is accompanied by the appearance of a single dominant feature: in this specific case, the one-electron picture does not hold true because further symmetrical considerations have to be addressed due to the fact that the two possible polaron transitions possess nearly the same energy [56].

7.2.5 Conclusion Besides their technological potential, conjugated oligomers also constitute attractive model systems for the corresponding polymers. In this context, the actual trend is to

446

7.2 A Quantum Chemical Approach to Conjugated Oligomers

extrapolate to very long chains the precise description obtained either from carefkl measurements or from sophisticated quantum-chemical calculations performed on well-defined molecules. Gaining a refined description of conjugated oligomers is thus without any doubt a very challenging task that could open the way to the design of novel advanced materials.

Acknowledgements The work on conjugated oligomers and polymers in Mons is partly supported by the Belgian Prime Minister Office of Science Policy ‘PBle d’Attraction Interuniversitaire en Chimie Supramoleculaire et Catalyse’, FNRS/FRFC; the European Commission (ESPRIT project LEDFOS-8013 and T M R Network Seloa); Ministere de la Region Wallonne; and an IBM Academic Joint Study. JC is Aspirant and DB Charge de Recherches of the Belgian National Fund for Scientific Research (FNRS).

References 1. Conjugated Polymers: The Novel Science and Technology of’ Highly Conducting and Nonlinear

2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18.

Opticully Active Materials, edited by J. L. Bredas and R. Silbey (Kluwer, Dordrecht, 1991); Conjugated Polymers and Related Materials, edited by W. R. Salaneck, I. Lundstrom, and B. Ranby (Oxford University, New York, 1993); Intrinsically Conducting Polymers: An Emerging Technology, edited by M. Aldissi (Kluwer, Dordrecht, 1992). J. H. Burroughes, D. D. C. Bradley, A. R. Brown, et al. Nature 1995, 347, 539. J. Roncali, Chem. Rev. 1992, 92, 711. M. Berggren, 0. Inganas, G. Gustafsson, et al. Nature 1994, 372, 444. F. Garnier, G. Horowitz, X. Peng, and D. Fichou, Adv. Muter. 1990, 2, 592; F. Garnier, R. Hajlaoui, A. Yassar, and P. Srivastava, Science 1994, 265, 1684. F. Geiger, M. Stoldt, H. Schweizer, P. Bauerle, and E. Umbach, Adv. Mater. 1993, 5, 922; G . Horowitz, P. Delannoy, H. Bouchriha, et al. Adv. Muter. 1994, 6 , 752. D. Fichou, J. M. Nunzi, F. Charra, and N. Pfeffer, Adv. Muter. 1994, 6 , 64; D. D. C. Bradley, Physics World 1994 April, p. 29. K. Fesser, A. R. Bishop, and D. K. Campbell, Phys. Rev. B 1983, 27, 4804. M. J. S. Dewar, E. G. Zoebisch, E. F. Healy, and J. J. P. Stewart, J . Am. Chem. Soc. 1985, 107, 3902. M. J. S. Dewar and W. Thiel, J . A m . Chem. Soc. 1977, 99, 4899. C. Adant, D. Beljonne, and J. L. Bredas, J . Chem. Phys. 1994, 101, 8048. P. Tavan and K. Schulten, J . Chem. Phys. 1986,85,6602. J. Ridley and M. Zerner, Theoref. Chim. Acta 1973, 32, 11 I . R. J. Buenker and S. D. Peyerimhoff, Theoret. Chim. Acta 1974, 35, 33. G. Nicolas and Ph. Durand, J . Chem. Phys. 1980,72,453;J. L. Bredas, R. R. Chance, R. Silbey, G. Nicolas, and Ph. Durand, J . Chem. Phys. 1981, 75, 255. J. Cornil, D. Beljonne, and J. L. Bredas, J . Chem. Phys. 1995, 103, 834; 1995, 103, 842. D. Beljonne, J. Cornil, J. L. Bredas, R. H. Friend, and R. A. J. Janssen, J . Am. Chem. Soc. 1996, 118, 6453. H. Chosrovian, S. Rentsch, D. Grebner, D. U. Dahm, and E. Birckner, Synth. Met. 1993, 60, 23.

References

441

19. R. A. J. Janssen, L. Smilowitz, N. S. Sariciftci, and D. Moses, J . Chem. Phys. 1994, 101, 1787; R. A. J. Janssen, D. Moses, and N. S. Sariciftci, J . Chern. Phjx. 1994, 101, 9519. 20. J. Guay, P. Kasai, A. Diaz, R. Wu, J. M. Tour, and L. H. Dao, Chem. Matter. 1992, 4, 107. 21. G. Horowitz, A. Yassar, and H. J. von Bardeleben, Synth. Met. 1994, 62, 245. 22. L. S. Swanson, J. Shinar, and K. Yoshino, Phys. Rev. Lett. 1990, 65, 1140. 23. J. C. Scaiano, R. W. Redmond, B. Mehta, and J. T. Arnason, Photochenz. Photobiol, 1990,52, 655. 24. Y. Zeng, L. Biczok, and H. Linschitz, J . Phjls. Chem. 1992, 96, 5237. 25. B. Xu and S. Holdcroft, J . Am. Chem. Soc. 1993, 115, 8447. 26. J. P. Reyftmann, J. Kagan, R. Santris, and P. Morliere, Photochem. Photobiol., 1985, 41, 1. 27. N. C. Greenham, I. D. W. Samuel, G. R. Hayes, et ul. Chem. Phys. Lett. 1995, 241, 89. 28. B. E. Kohler, Chem. Rev. 1993, 93, 41. 29. R. H. Austin, G. L. Baker, S. Etemad, and R. Thompson, J . Chem. Phys. 1989, 90, 6642. 30. R. Rossi, M. Ciofalo, A. Carpita, and G. Ponterini, J . Photochem. Photobiol. A : Chem. 1993, 70, 59. 31. N. Periasamy, R. Danieli, G. Ruani, R. Zamboni, and C. Taliani, Phys. Rev. Lett. 1992,68,919. 32. Z. G. Soos, D. S. Galvao, and S. Etemad, Adv. Muter. 1994, 6, 280. 33. R. S. Becker, J. S. de Melo, A. L. Maqanita, and F. Elisei, Pure & Appl. Chem. 1995, 67, 9. 34. W. P. Su, J. R. Schrieffer, and A. J. Heeger, Phys. Rev. B. 1979, 22, 2099. 35. G. B. Street and J. L. Brkdas, Acc. Chem. Res. 1985, 18, 309. 36. C. Ehrendorfer and A. Karpfen, J . Phys. Chem. 1994, 98, 7492. 37. D. Fichou, G. Horowitz, B. Xu, and F. Garnier, Synth. Met. 1990, 39, 243. 38. S. Hotta and K. Waragai, J . Phys. Chem. 1993, 97, 7427. 39. G. Lanzani, L. Rossi, A. Piaggi, A. J. Pal, and C. Taliani, Chem. Phys. Lett. 1994, 226, 547. 40. J. Poplawski, E. Ehrenfreund, J. Cornil, et al. Mol. Cryst. Liq. Cryst. 1994, 256, 407. 41. M. G . Harrison, R. H. Friend, F. Garnier, and A. Yassar, Synth. Met. 1994, 67, 215. 42. P. Bauerle, U. Segelbacher, A. Maier, and M. Mehring, J . A m . Chem. Soc. 1993, 115, 10217. 43. M. G. Hill, J. F. Penneau, B. Zinger, K. R. Mann, and L. L. Miller, Chem. Muter. 1992,4, 1106. 44. A. Yassar, D. Delabouglise, M. Hmyene, B. Nessakh, G . Horowitz, and F. Garnier, Adv. Muter. 1992, 4, 490. 45. B. Nessakh, G. Horowitz, F. Garnier, F. Deloffre, P. Srivastava, and A. Yassar, J . Electround Chem. 1995,399,97. 46. J. A. E. H. van Haane, E. E. Havinga, J. L. J. van Dongen et al., submitted for publication. 47. A. J. W. Tol, Cliem. Phys. 1996, 208, 73. 48. A. 0. Patil, A. J. Heeger, and F. Wudl, Chem. Rev. 1988, 88, 183. 49. T. C. Chung, J. H. Kaufman, A. J. Heeger, and F. Wudl, Phys. Rev. B 1984,30, 702. 50. Z. Vardeny, E. Ehrenfreund, 0. Brafman, et ul. Phys. Rev. Lett. 1986, 56, 671. 51, P. A. Lane, X. Wei, and Z. V. Vardeny, Phys. Rev. Lett. 1996, 77, 1544. 52. G. Zotti, S. Martha, G. Wegner, and A. D. Schliiter, Adv. Muter. 1992, 4, 798. 53. R. K. Khanna, Y. M. Jiang, B. Srinivas, C. B. Smithhart, and D. L. Wertz, Chem. Muter. 1993, 5, 1792; H. Gregorius, W. Heitz, and K. Miillen, Adv. Muter. 1993, 5, 279. 54. R. Schenk, H. Gregorius, and K. Miillen, A h . Muter. 1991, 3,492. 55. E. Ehrenfreund, D. Moses, A. J. Heeger, J. Cornil, and J. L. Bredas, Chem. Phys. Lett. 1992, 196, 84; M. Logdlund, P. Dannetun, S. Stafstrom, et al. Phys. Rev. Lett. 1993, 70, 970. 56. T. Bally, K. Roth, W. Tang, R. R. Schrock, K. Knoll, and L.Y Park, J . A m . Chem. Soc. 1992, 114, 2440.

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8 Nonlinear Optical Properties of Oligomers Christoph Bubeck

8.1 Introduction The development of materials with large optical nonlinearities is a key to controlling the propagation of light beams by optical means. The control of light by light requires that photons can strongly interact. This is only possible in media where the optical properties of the materials, such as the refractive index, depend on the light intensity. The availability of appropriate materials for this purpose could revolutionize information technology again in a similar manner to the development of materials for semiconductor electronics. Optical materials with a sufficiently large intensity-dependent refractive index, which could play a similar role to silicon in electronics, have not yet been identified. This field is still at the stage of basic research, where strong efforts are devoted to an understanding of the fundamental relations between structure and optical nonlinearities. It is clear, however, that materials with a highly polarizable electron system are interesting candidates for achieving strong polarizations of the medium, which follow the electric field of the lightwave in a nonlinear manner. Therefore, organic materials with a delocalized n-electron system have found much interest and large nonlinearities of one-dimensional (1 D) conjugated polymers have been reported [l-111. As the optical properties of conjugated polymers are determined largely by the extent of electron delocalization, the corresponding conjugated oligomers have a key role in the study of the scaling of the linear and nonlinear optical properties with the size of the system. It will be seen that the size of the delocalized electron systems and electron correlation effects primarily determine the optical nonlinearities of oligomers. The emphasis of this chapter is on third-order nonlinearities, because they can lead to an intensity-dependent refractive index. The large potential of oligomers, such as to vary their chain lengths systematically, can be used to study third-order phenomena, especially to elucidate characteristic structure-property relationships. Second-order phenomena are the basis for frequency doubling and electro-optical processes. However, they require other chemical systems such as a combination of electron donors and acceptors in noncentrosymmetric structures, which will not be treated here.

8.2 Nonlinear Optical Phenomena 8.2.1 Physical Background At very high electrical fields E the macroscopic polarization P of a medium no

450

8 Nonlinear Optical Properties qf Oligomers

longer follows the electric field linearly. This is due to anharmonic motion and response of the bound electrons. Therefore. P is usually written as a power series expansion of the electric field amplitude: p = EO(Z(')E+ x ( ~ ) + E ,y(3)E3 ~ + . . .)

(1) x(") are the optical susceptibilities of order tz and E~ is the dielectric permittivity of free space. In the SI system of units, X ( n ) have the dimensions (m/V)"-'. In the earlier literature, however, it was common to use the cgs/esu system of units, which is still preserved in most present publications. The conversion relations are given by [12]: x'"'[S1]/,y('i)[esu]= 4n/( lop4c ) " I~

(2) with c = 3 x 10'. Taking into account the vectorial nature of P, E and the tensor character of x@), the components of P can be written in the form 1

- P I( w ) = x:;)(-w; EO

w)E,( w )

+ K(')x$) ( - w ; w,, w2)E,(w , )Ek (wz)

The indices i, j , k, 1 indicate the Cartesian coordinates x,y , z.Here the convention is used that repeated subscripts on the right-hand side of Eq. (3) are understood to be summed over x,y and z.In the most general case the incident optical waves have different frequencies w I ,w2, w j and wavevectors k l , k2 and k3. Energy conservation yields the new frequencies w after the nonlinear interaction: w = w1+ w 2 or w = w I + w2 w 3 , respectively. The convention of writing the frequencies with positive and negative signs is practical, to indicate the type of electronic interaction and the conservation of momentum of the wave vectors. The frequency before the semicolon is written with a negative sign. I t symbolizes that the generated wave with the frequency w can leave the system after the nonlinear optical process. The numerical factors d2) and Id3)arise from degeneracies in the number of distinguishable permutations of the frequencies [12]. They are shown in Table 1 for several important nonlinear optical processes. If the numerical factors are not written explicitly as in Eq. (3), these factors are included in the definition of X ( n ) , which happens frequently in the literature of nonlinear optics. Only if these factors are written explicitly is the definition of x(")not dependent on the process.

+

Table 1. Selection of some nonlinear optical processes, common abbreviations, frequency arguments of $'I) and their numerical factors (from [12]). Process Second harmonic generation Electric field induced second harmonic generation Third harmonic generation General four wave mixing Degenerate four wave mixing

(SHG) (EFISH) (THG)

Susceptibility

K

p - 2 w ; w,w) x ( ~ -2w; ) ( w , w,0 ) $3) ( - 3w; w , w , w )

1/ 2 312 114

x ( ~ ) ( - - w ~ : w ~ , w ~ , 3w/ 2~ )

(DFWM)

~(~'(-w;w,w,-w)

314

8.2 Nonlinear Optical Phenomena

45 1

The macroscopic optical susceptibilities x(”)are related to microscopic optical w I,w 2 ,q )via polarizabilities a( -w; L J ) , 13( -w: wI, w 2 ) and y( -0; ,y(I’(-w: w ) = N f(w)ck( --w; w )

p - w ; w , w2) = N f(u)f(w1) f(wz):T(-w; w1, w2) x(3)(--w;wl,w2,w3) = Nf(w) f(w1)

f(4 f(LJ3)Y(--W;Wl:W2,W3)

(4a) (4b) (4c)

where N is the number of molecules per unit volume and f(wj) are the dimensionless Lorentz local field factors, which depend on the refractive index n(wi)as [13]:

+

f(q) = [ ~ ( w , ) ’ 2]/3

(5)

If the order of the nonlinear optical process is indicated, confusion can arise because !, and y are also called ‘first’ and ‘second hyperthe microscopic polarizabilities ? polarizability’, respectively. However, they are related to second- and third-order phenomena as expressed in Eqs. (4b) and (4c). The microscopic polarizabilities a, ,!? and y can be calculated by means of the ground and excitations states G, A, B, C,. . . of a molecular system using time dependent perturbation theory and the density matrix formalism. Following basic work of Bloembergen [I41 and others [15, 161 the microscopic polarizabilities can be expressed in the simplified forms [3]:

a( -LJ; w )

Here p is the operator for the dipole transitions between the quantum states. The symbol Pindicates that a summation over all permutations of states and frequencies must be performed. Here only terms which dominate in case of resonances are displayed. This happens if either one, a combination of two or three incident field-frequencies coincide with internal transition frequencies R of the system. Therefore, these resonances are called 1-, 2- or 3-photon resonances. At resonance, the polarizabilities are complex due to the remaining damping coefficients I?. Therefore, y(”) are complex uantities and can be described by a modulus and a phase can be expressed as angle 0. For example

s3)

x(3)= ~ x I(exp(i4) ~ ) = 1,yi3)1 (cos 0 + i sin 4)

(7)

452

8 Nonlinenr Opticul Properties of Oligorners

Note that in general y and x ( ~are ) tensors of rank 4. For a homogenous liquid, the average value (y) is related to the tensor components of y in the molecular reference frame as [ 171 1

(7) = 5 [ Y Y S X U

+ y,v.r;vy+

YZ3X

+ 2(Y.,..\-.ly+ "i\-.\-z: + ? + ) I

'

(8)

For example, in the case of chains with one-dimensional electron delocalization along the chains (x) axis, the tensor component yvIY\.is much larger than all others.

8.2.2 Third-Order Phenomena and Measurement Techniques Many phenomena of nonlinear optics can be derived from the power series expansion of P, if the expression E = Eo cos(wt - kz) for a traveling wave is inserted into Eq. (1). By use of appropriate trigonometric identities we obtain

+ x ( ~ ) E ~ [ $ c o-s (kwz )~+ icos(3wt - 3kz)]

(9)

The prefactors from the trigonometric identities are the same as the Idn)factors shown in Table I . In Eq. (9) we recognize contributions to P at new frequencies 2w and 3w respectively. This is called second- and third-harmonic generation (SHG and THG), respectively. In addition, there is a frequency independent contribution which is referred to as optical rect@ation. The contribution x(3)Ei:cos(wt - kz) leads to an intensity dependent index qf refmctian, which is commonly written in the form n

= no

+ n2Z

(10)

where no is the linear index of refraction and Zis the light intensity. The derivation of the linear relation between n2 and x(3)is presented elsewhere [12, 181. Since Z is usually expressed in units of W/m2, the relation between n2 and x ( ~is)[12]: n2 [m2/w] = 3.9 x 1 0 - % ; ~ ~ ( ~ ) (w- ,~w;, -w) [esu]

(11) Note that x(3)is the nonlinear optical susceptibility of lowest order which contributes to an intensity dependent refractive index. The frequency arguments of ~ ( ~ ) ( - - ww ,;w , -w) indicate that this susceptibility corresponds to degenerate four wave mixing (DFWM). There are several measurement techniques of x ( ~ whose ) , detailed explanation is beyond the scope of this article. These techniques are well described in references for T H G [18-211, DFWM [18, 22-24] and EFISH (electric field induced second harmonic generation) [25]. Several problems and sources of error sometimes make it difficult to compare results from different laboratories which use different techniques for the following reasons:

(i)

Single- or multi-photon resonances of the laser frequency with electronic states of the molecules lead to an increase of Ix(')l by several orders of magnitude as

8.3 Esperimenlal Results

1

2

3

4a

4b

5

453

6

Figure 1. Third-harmonic generation (THG) and degcnerate four wave mixing (DFWM) under nonresonant ( I , 4a, 4b) and resonant conditions (2, 3, 5 , 6) with molecular levels. Here g is the ground state of even parity, u and g’ are excited states of odd and even parities. Dashed lines indicate virtual levels.

compared to the nonresonant situation [26]. The type of resonances in THG and DFWM are shown in Fig. 1. Therefore, values can only be compared if they are measured at similar resonant or nonresonent conditions. Where x ( 3 ) data are presented, it is crucial that the laser wavelength XL (or frequency) and the measurement technique are specified. (ii) The numerical factors K shown in Table 1 can differ up to a factor of 6. These factors must be taken into account, if results of different measurements are compared. ). (iii) It is a severe experimental problem to obtain ahsolure values of x ( ~ Usually, the values are evaluated relative to reference materials such as fused silica, CS2 or others. Frequently ~ ( ~ ) ( - 3w, ww, ; d) = 3.1 1 x esu at XL = 1064 nm is used for fused silica [20]. In many cases, however, it happens that different values for the same reference material are used by different authors. Additionally, it is often not clear whether the x ( ~values ) of standards contain the K-factors implicitly or not. Because of these problems, it is no surprise that the x ( ~and ) y values of the same compounds can differ by an order of magnitude, if they result from studies performed in different laboratories.

8.3 Experimental Results In studies of third-order polarizabilities of long-chain molecules by Hermann and Ducuing [27], a striking difference between saturated and unsaturated hydrocarbons

was found. For alkanes (C,,HZn+?), y increases linearly with the number of carbon atoms and the total hyperpolarizability of the chain is just the sum of the contributions of the a-bonds. The hyperpolarizability y per CH2 unit was found in the order of 3.4 x esu [27]. The bond additivity of y for saturated compounds has been studied in more detail by Kajzar and Messier [28] and by Meredith et al. [29]. On the other hand, unsaturated hydrocarbons like oligoenes (CnHn+Z)show a strong superlinear increase of y with the chain length. This behavior is attributed to the delocalized .ir-electron cloud. It was further noted that the increase of y follows a power law dependent on the size of the conjugated system. Therefore, the study of oligomers with extended 7r-electron conjugation gives a means to elucidate the influence of the number n of repeat units on the electronic states and on the magnitude of y. A survey of representative chemical structures of molecules, oligomers and polymers with 7r-electron conjugation is shown in Fig. 2. Selected physical properties and

PPV

* H

OPV-n

OTn

OPn

en

PPT Figure 2. Chemical structures of oligomers, polymers and several dyes (for R see Table 4).

8.3 E.uperimentul Results

CY

c I-

M3

ORy-2

ORy-3

ORy-4

A

Figure 2. (Cont.)

M=Ru

455

456

8 Nonlincwr Opticol Propertic,.s of‘ 0ligoiner.s

Table 2. Experimental data of short oligomers. (For names and symbols see text and Fig. 2, where H-P,,-H and H-T,,-H correspond to OPn and OTn, respectively.) 11 is the number of double bonds or oligoenes and cyanines, or otherwise the number of repeat units, and L is the estimated chain length. The orientation average of the hyperpolarizability of the whole molecule is denoted by ( 2 ) .The data are from diluted solutions (liquid or solid) if not specified otherwise (*: gas phase exoerimentsl Name

0Irgoc~fle5 E OE2 DMP2 TMP2 OE3 OE3 TMP3

w

I

3C

2 2 2 3 3 3 3 4 1I II

iK

11

D K

19

A0

TMP4 3C 6

L i 10%

A,,,

I(*/)l

lnml

lnml

hul

0.135 0.36 0.36 0.63 0.61 0.61 0.86 0.86 1.10 2.8 2.8 2.8 4.7

I62 21 I 226 242 25 I 258 262 276 309 452 452 452 552

(7.58 5 0 . 1 7 ) 10-j’ (2.34Z0.13) x I0 3h (4.9 f 0.1) x ( 7 . 4 f 0 . 1 ) x lo-’‘ (7.53 4Z 0.7) x (9.1 &0.4) x 10 j h (3.8 5 0.05) x (9.7 f 1.7) x (4 i 0.5) 1 0 - 3 ~ (8 + 4 ) x lo-” ( I . 1 i 0.25) x (9.2 50.21) ( I .7 f 0.6) x lo-’’

EFISH’, 694 nm EFISH*, 694 nm THG, 1908nm THG, 1908nm EFISH’, 694iim THG, 1908 nm T H G , 1908nm THG, 1890 nm THG, 1908nm THG, 1890nm THG, 1908 nm THG, 1908nm THG, 1890nm

312 416 519 506 504

(2 5 0.15,) x (5.2 f 0.1) x lo-’’ (5.1 5 0 . 5 x 2 . 2 10~ $4 2.0 x 10-”

THG, THG, THG, THG, THG,

203.7 203.7 246.8 275.8 331.4

(2.06 i 0.05) x (6.4 i- 0.8) x lo--” (2.9 f 0.45) x (8.5 5 1.5) x (2.1 4Z0.17) x

EFISH’, 694nm DFWM, 602nm DFWM, 602 nm DFWM, 602 nm DFWM, 602 nm

231 301.8 350.2 340 390.6 399 412.0 415 429.2 444 449 451

4.1 x 2.3 x 1.6 x 10- j 4 9.9 x 8.0 2.2 x 10.~33 2.6 x 1.07 x lo-” 1.0 x lo-” 3.6 x lo-.’? 3.7 x 4.6 x 10-.”

DFWM, 602nm DFWM, 602 nm DFWM, 602 nm EFISH, I064 nm DFWM. 602 nm EFISH, 1064 nm DFWM, 602nm EFISH, 1064nm DFWM, 602 nm EFISH, 1064nm EFISH, 1064nm EFISH, 1064nm

Cjmines r i n d related L/~V,.S cy2 2 0.49 cy3 3 0.74 cy4 4 1 .o MY‘) I .o M3lIl’ (i: in C H 3 N 0 2 ,ii: in ChH6) 0li~oplle~lj~l~nc.s B 1 0.28 B I 0.28 H-P2-H 2 0.71 HPP3-H 3 1.14 H-P5-H 5 2.0

Oligotliioplirnr~s 1 2 3

T H-T? - H H-Tj-H H-T3-H H-Td-H H-T4-H H-TS-H H --T6-H H--T,-H H-Tg-H H-Tc)-H H-TT1-H

3 4 4 5 5 6 7 9 I1

0.29 0.73 1.17 1.17 1.61 1.61 2.04 2.04 2.47 2.9 3.76 4.62

Method, AL

1908nm I908 nm 1906nm 1908nm 1908nm

Reference

[29] [3 I] [29] (34, 351 [34, 351

[30] [36] [36] [36] [36]

N .3 Experirnen tril

Ri~iili.~

457

Table 3. Experimental data of long chains with 7r-electron conjugation. (7r) denotes the orientation average of per repeat unit. n(

~~

~

Pol~'pi1rn~lric~et~~ler2r.s (suhstituti4) PPA- 1 330 (1.26 2~ 0.4) x PPA-2 350 (4.0 & 1.0) x 10-j' PPA-3 414 (1.0 0.2) x PPA-4 436 (1.8 i 0.4) x lo-'' PPA-5 429 (2.2 k 0.4) x 10PPA-6 438 (2.0 5 0.4) x PPA-7 462 (1.1 i 0.4) x PPA-8 515 (4.8 0.8) x

THG, THG, THG, THG, THG, THG, THG. THG,

1064nm 1064nm 1064nm 1064nm 1064 nm 1064nm 1064nm l064nm

Polyene oligomi~,-s(.suh.rtitutid) OE28 466 OE39 486 OE50 516 OE68 530 OE88 538 OE152 550 OE240 552

THG, THG, THG, THG, THG, THG. THG.

1910nm 1910nm 1910nm 1910nm 1910nm 1910nm 1910nm

*

''

*

(2.89 f 0.54) lorJ4 (3.62 f 0.51) x 10-j' (4.94 0.4) (8.13 0.74) x (1.165 It0.17) x (1.797 410.2) x lo-'' (1.581 k0.21) x

+

estimated physical lengths L of the conjugated systems are collected in Tables 2, 3 and 4 together with an indication of the measurement techniques for y or x ( ~ [30-521. The y and x ( ~values ) are those given in the references. Attempts to correct them with respect to standardized reference materials have not been made. Therefore, the y values of the same compounds can differ considerably because of the problems described at the end of section 8.2. The third-order nonlinearities of ethylene (E) and the following oligoene derivatives have been studied: butadiene (OE2), 2,3-dimethyl-l.3-butadiene(DMP2), 2,5dimethyl-2,4-hexadiene (TMP2), hexatriene (OE3), 2,7-dimethyl-2,4,6-octatriene (TMP3), allo-ocimene (AO), 2,9-dimethyl-2,4,6,8-deca-teraene (TMP4), trans-/% carotene (BC) and dodecapreno-P-carotene (DPC). Inspection of Table 2 shows that y increases over 4 orders of magnitude in going from n = 2 to n = 19. The poor solubility of longer oligoenes makes it necessary to use an appropriate substitution to obtain materials which are soluble in common organic solvents. This was accomplished in the series of long chain oligoenes (OEn) with n = 28 to 240 [40] and poly-phenylacetylenes (PPA-I?) [39]. The numbers which are added to PPA indicate in this case different substituents, R, and synthetic routes (see Table 4). The experimental data of the hyperpolarizability per repeat unit yr of OEn and PPA-n are shown in Table 3. Because all-trans polyacetylene (tPA) is not soluble, its ) be derived from studies of thin films. These data macroscopic susceptibility x ( ~can are given in Table 4 for comparison with other thin film data of conjugated systems. The study of cyanines (Cy2, Cy3 and Cy4) and polymethine dyes such as compound M3 by Marder et uf. [34, 351 yield an interesting comparison with oligoenes, because the n-bond order alternation of the former one is much reduced. The y

)

458

8 Nonlinear Optical Properties of Oligomers

Table 4. Thin film data of ( x ( ~-3w; ) ( w,w,w)) and phase angles 4, measured with THG at the fundamental laser wavelength AL relative to fused silica with ~(')(-3w;w,w,w)= 3.1 1 x 10~-'4esu.(x,,, denotes the intensity absorption coefficient of the thin films at the wavelength , ,A, of the lowenergy absorption maximum. Name 0ligomer.s und dyes OPV-3

% I , ,

~,,,

[104cm-']

[nml

XI[nm]

20

383

1064 1155

OPV-4

21

394

I440 1064 1155

OPV-5

24

406

ORy-2 ORy-3 ORy-4 OPC R6G PCd in polystyrene

12.0 14.5 15.0 27 1.05

457 574 689 656 510 469

0.56

412

We

15

in polystyrene Polymers PPV

34

458

t PA

9.2 30

463 652

cPA PPA-0, R == H PPA-I, R = H PPA-2, R = H PPA-3, R == CH1 PPA-4, R = CHI PPA-5, R CzHj PPA-6, R = CZHS PPA-7, R = C8Hi7 PPA-8, R = Si(CH3)' PT-I, R = CIOH2I PT-2, R = CioH2l PT-3, R CIOHZ1

7.0 7.07 6.95 6.18 9.02 6.24 7.05 8.70 6.14 9.7 9.5 10.6

536 388 330 350 414 436 429 438 462 515 455 485 505

PPT, R = C12HZj

1

1

1485 1064 1222 1485 1064 I064 1064 1064 I064 1064 1420 1670 1430 1635

1064 1336 1512 1064 1064 2070 1722 1064 1064 I064 1064 1064 1064 1064 I064 1064 1064 1064 1064

I(P)l

4

[10-12esu]

[I

16.1 i 2 15.2 f 2 3.2f I I752 19.1 f 2 4 51 17.2 & 2 36f4 8.1 & 2 0.48 0.81 1.57 3 . 7 f 1.5 3.4 1.6 7.2 2.1 1.5 0.73

102 f 15 93 f 15 23* 15 101 f 15 9 5 f 15 1 0 f 15 I23 f 15 8 9 i 15 23& 15 263 153 5 155 f 10 24 1 214 127 41

83.6 f 7 I60 f 17 32.1 f 7 21.3 f 3 100 5600

227 & I5 115 615 43* 15 220 f 20

1000 2.1 f 0.3 1.0 & 0.5 2.5 f 0.5 4.9 & 0.2 9.3 f 1.5 5.4 f 0.8 7.1 &0.6 6 . 4 5 1.0 13.0 & 2.0 8.9 & 1.5 9.5 f 1.5 11.8f-1.5

153f5 131 & 10 170 f 10 200 & 10 227 f 10 218 f 10 223 3z 10 2 4 0 5 10 2 8 0 k 10

239 k 20 276 f 20 295 & 20

Reference

8.3 E.xperirnental Resulrs

459

values of M3 depend strongly on the polarity of the solvents, but the order of magnitude of these y values is similar to oligoenes of comparable lengths. The relative influences of the size of the conjugated systems and n--bond order alternation will be discussed later (see section 8.5.1). The data of benzene (B) and thiophene (T) and their corresponding oligomers oligo( p-pheny1ene)s (H-P,-H) and oligothiophenes (H-T,-H) show also a strong increase of y with chain length as we have noted already in the case of oligoenes (see Table 2).

180

,

~

160

-

I

I

(b)

OPV-5

7

-

120 c

100

-

0,

a,

9

80-

8 60

-

40

-

20

-

0

I

I

900

1200

h,

1500

[nml

Figure 3. THG investigations of thin films of OPV-5, performed with variable laser wavelengths XL (from Mathyetul. [41]). The modulus oftheorientation average o f ~ ( ~ ) ( - 3 i ~ ; ~(data . d , ipoints) ~ ) is plotted in part (a) for comparison with the absorption spectrum .(A) (full line). Part (b) shows the phase angle q5 of x i 3 ) ( - 3 w : ~ . d J . d ) .

460

H Nonlinear Optictrl Propertir.c. (if Oligonii~rs

The comparison of polymers with their corresponding oligomers requires the study of thin films in many cases because of solubility problems. Several polymers are available only via thin film preparations of precursor polymers followed by a thermal conversion reaction. Examples are cis- and zrans-polyacetylene (cPA and tPA), poly(p-phenylenevinylene) (PPV) and polyphenothiazinobisthiazole (PPT). The optical data of several compounds are presented in Table 4: oligo(p-phenyleneviny1enes)s (OPV-n), oligorylenes (ORy-n), an oligomeric bridged phthalocyaninato ruthenium complex (OPc), rhodamine 6G (R6G) and several other polymers which are introduced above. The chemical structures of these compounds are also shown in Fig. 2. The experimental data shown in Table 4 are all measured with THG, which is a rather accurate and sensitive method even for small nonlinearities. The experimental procedures and precautions for the evaluation of ~ ( ~ ) ( - 3 w,w,w) w; of thin films have been described elsewhere [18, 211. If ~ ( ~ ) ( - 3 w, w w, ; w ) data are used to reveal structure-property relations, it must be kept in mind, that x ( ~-3w; ) ( w,w , w)can strongly depend on the laser frequency ~ j which , is related to the laser wavelength XL.An example of a three-photon resonance in investigations of thin films of an oligo(p-phenylenevinylene) (OPV-5) is -3w; w,w , w)is observed displayed in Fig. 3. A single resonance maximum of at XL x 1200 nm. This corresponds to a three-photon resonance with the absorption maximum at, ,A, = 406 nm. A phase angle (5 = 90" is found at AL = 3A,, which is in accordance with Eq. (6c) and (7). The single three-photon resonance seen with OVP-5 [41] appears very similar to the T H G spectra reported for trans /3-carotene (PC) [46,471. This set of experimental data will be used in section 8.5 to discuss characteristic structure-property relations: the influences of dimensionality of the .ir-electron delocalization and of the shape of the conjugated systems, the influences of heteroatoms and the comparison of oligomers and polymers.

8.4 Survey of Theories The explanation of the superlinear increase of y with the size of the conjugated system is a major challenge to theory. The problems with the calculations of y for conjugated chains have been described in recent reviews [53-561. A survey of calculation methods, results and references is given in Table 5, which summarizes their keywords and major results. Details of these theoretical methods may be found elsewhere [57-711. Basically all different theoretical approaches yield power laws of the form

y n" (12) for the limit of very small conjugated systems, which have a physical size of less than approximately 2 nm. In the various theories, n denotes the number of repeat units or the number of atomic or molecular sites. Although the complexity and accuracy of these theories differ considerably, they accordingly lead to exponents p around 4 to 5 in the limit of short chains. N

8.4 Survc>j,of' Theories

46 1

Table 5. Theoretical power laws y = ni' for chains with delocalized 7r-electrons, where n is the number of repeat units or atomic sites [(c) and (t) denote p for cis- and trrtns-oligoenes]. Single values of are for the short chain limit only; p = f(n) indicates that this theory yields a variable and decreasing 11 for longer chains. Model and method o f calculation

P

Free electron in a box Huckel Hiickel Pariser-Parr-Pople Self-consistent-field configuration interaction rib inifio Hartree-Fock Pariser--Pan-Pople INDO combined with single- and double-excitation configuration interaction One-electron, sum-over-states Self-consistent-field configuration interaction Electron-hole anharmonic oscillator (exciton) model Confined electrons in coupled quantum oscillators Sum-over-states, Huckel Sum-over-states, Hartree-Fock, ub inirio

5 5

Reference

f(n)

4:25 4.7'c', 5.4(') 4.0 4.0 4.3 4.32 3.9"). 4.0(')

The influence of the conformation of cis- and rrans-oligoenes on the power law (12) was studied in detail by Heflin and Garito et ul. [33, 621. They calculated the hyperpolarizabilities y.y.,.yy( -3w: w , w,w ) and plotted them on a log-log scale against the actual chain length L , which is defined as the distance in the x direction (along the chain axis) between the two end carbon sites. The calculated values for both the cis- and truns-oligoenes are very well fit by a single line. This plot unifies the calculated values for the two conformations and yields

-/i

L/" (13) In the initial work = 4.6 Z!I 0.2 was found [62]. This exponent was correction later to p' = 3.5 [ 3 3 ] .These interesting results may indicate that the physical size L of the delocalized 7r-electron system is the crucial quantity, which primarily determines the magnitude of the hyperpolarizability y of short chains. This is a quantum size effect and is discussed in more detail in section 8.5.2. Clearly, the increase of y according to the power laws (12, 13) cannot go on unlimited. At a characteristic, critical length, the .ir-electrons are no longer correlated. Loosely speaking, they start to respond independently to the external optical fields and, consequently, the microscopic polarizabilities ~ ( ww ,) and y increase linearly with the size of the system. This situation is commonly called saturation of the polarizabilities. The transition between the regimes of the power law dependence (12, 13) and the linear increase of y with L is treated by several theories, which are listed also in Table 5. They yield a size dependent exponent p, = f(n), where p converges to 1, if y1 grows to infinity. There is no general agreement on the functional dependence of y on n in this transition regime. y\..u\-\-(-3w; w.w , w)

8.5 Structure-Property Relations 8.5.1 Size Dependence of the Hyperpolarizability y The set of experimental data and the power laws (12) and (1 3) indicate that the size of the conjugated system is a very important quantity which has strong influence on the magnitude of the hyperpolarizability y.Therefore, the experimental data of (7) are displayed in Fig. 4 on a double logarithmic scale versus the chain length L of short oligomers. This plot visualizes the problems (i)-(iii) which are described at the end of section 8.2.2. For example, the large differences of the data for a single compound such as /jC or between the series of oligothiophenes (H-T,-H) from Zhao et ul. [37] and Thienpont et ul. [38] show how difficult it is to compare the results of different laboratories. Therefore, it is only possible to evaluate studies of homologous series of oligomers, if the results are from the same laboratory. They are identified by the various lines, which connect related data points in Fig. 4. It would be highly desirable to calibrate individual experiments with respect to a generally accepted reference material. In spite of these experimental problems, Fig. 4 can be used to elucidate some significant structure-property relations.

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8.5 Structure-Property Relations

463

The data of oligothiophenes (H-T,-H) from Thienpont et al. [38] show the characteristic power law (12) and the onset of the saturation, where y increases linearly with L . Obviously, this transition regime is located for H-T,-H in the range around L = 3 nm. The data of Zhao rt a/. [37] for similar, but shorter H-T,-H seem to deviate from the power law (12). Presumably, this deviation occurs because the absorption maxima of the longest oligothiophenes come closer to resonance with the fixed laser wavelength AL = 602nm. Therefore, their (y) values have a much larger resonance enhancement than the shortest H-T,-H. The experimental data of oligophenylenes (H-P,-H) show a much smaller increase of (7) with L as compared to oligothiophenes. The (7) values of cyanines (Cy), on the other hand, show the strongest increase with L of the different materials which are considered here. Obviously, a single exponent x does not exist, which could describe the dependence of y on L. Commonly, it is argued that the different extent of nelectron delocalization accounts for these differences. In the case of H-P,-H, for example, the individual phenylene rings can twist around the chain axis and, consequently, the n-electron overlap is hindered between neighboring rings. The opposite extreme case is observed for cyanines, where the n-electrons of the carbon chain are completely delocalized such that the bond order alternation (BOA) is minimal. This has led to the expectation [34] that BOA is the key quantity which determines the polarizabilities. However, this cannot be used as a universal concept to explain the structure-property relations, because the dye M3 or cyanines (which show small BOA) have (y) values which are similarly large as those of oligomers with large BOA and comparable chain length like oligothiophenes or oligoenes. Clearly, the chain length L is only one of several parameters which have strong influence on y and the question arises, whether there exists a characteristic length of the conjugated n-electron systems which accounts for the magnitude of y.Different attempts to establish this length are described below.

8.5.2 Electronic Excitations and Characteristic Lengths Before we introduce several characteristic lengths of conjugated systems, we or energy Em,, of the consider first the size dependence of the wavelength A,,, absorption maximum for several model compounds. This is shown in Fig. 5. The with L is very different for the case of cyanines, oligorylenes and increase of A,, oligomers with one-dimensional (1D) delocalized n-electron systems such as oligoenes, oligothiophenes or oligophenylenevinylenes. These differences are treated in many textbooks on theoretical organic chemistry or physical chemistry [72, 75781. For cyanines, they are attributed to the positive charge, which is delocalized between the nitrogen atoms. In the case of oligorylenes, the delocalization of the n-electrons occurs in two directions of space. These effects lead to the significant red-shift of the optical absorptions of cyanines and oligorylenes as compared to to the the 1D conjugated oligomers of similar lengths. The convergence of A,, values of the corresponding polymers can be seen in the case of ID conjugated oligomers. However, this convergence is not as clear for cyanines and oligorylenes, because longer units are either chemically unstable or not available.

464

E c

8 Nonlineur 0 p t i c . d Properties of 0ligomer.s

8

Cyanines

900

EE

Oligorylenes

800

o Oligoenes

700

-

600

-

500

-

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Oligo-p-phenylenevinylenes @

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Figure 5. Wavelength of the low-energy absorption maximum of various model oligomers as a function of chain length. References for the ,,A;, data: cyanines [72], oligorylenes [42,43], oligoenes [73], oligothiophenes [37, 38, 741, oligo(ppheny1enevinylene)s [41], stilbene [72].

The relationship between the length L or the number n of the repeat units and Em,, can be calculated by means of the free-electron-in-the-box model by Kuhn [79] or by one-electron Huckel models (see e.g. ref. [75]). These models lead to the relation

Em,,(n)

=A

+ B/H

(14) where A and B are constants to fit the experimental data. Although Eq. (14) is generally accepted as a good description of the optical excitations of oligomers, this relation describes the experimental results nof accurately. For many homologous series of oligomers, always the same systematic deviations from Eq. (14) can be seen, if the energies of the optical transitions are plotted versus l / n as in Fig. 6, for example. Even the experimental data of Kohler [ 8 0 ] , which can be considered as the most accurate values for oligoenes presently available, show these deviations, which are visible similarly, if the plots of other groups are inspected critically (see for example ref. [8 1, 821). The inaccuracy of (14) can be interpreted by the fact that it is the result of oneelectron models, which do not take into account electron correlation effects and, therefore, do not do an adequate job of describing the excited state energies and transition moments of these molecules, even though it may give the trends in a gross qualitative manner [ 5 5 ] .

8.5 Structure-- Proper!,* Rrlutioii.r

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(b) Oligoenes, data of Kohler 1990

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1/ n Figure 6. Excitation energies of oligoenes and ethylene versus the reciprocal number of double bonds. The straight lines are drawn to visualize the systematic deviations of the experimental data from equation (14). Plot (a) shows the energies of the absorption maxima (data from refs. [30,73]). Plot (b) shows the energies of the zero-vibrational transition to the 1 ‘B, electronic state. which have an experimental error less than the symbol size [go].

466

K Nonlinear Optic (11 Propertiec. of Oligomers

70000

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Oligoenes, Ethylene (a)

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30000

r

r

“8 28000 Lu 26000 24000 3

5

4

6

7

n Figure 7. Excitation energies of oligoenes and ethylene as a function of the number of double bonds in a double logarithmic scale. The experimental data are the same as in Fig. 6. The solid lines represent a fit to the experimental data.

8.5 Structure-Propert). Rr1ufioii.r

467

The same experimental data, as shown in Fig. 6, can be plotted on a log-log scale versus n. This is performed in Fig. 7, which shows that the limiting case of very short oligoenes can be fitted much better by an empirical relation for small n

Emax(n)= Eln-” (15) where El = Em,, ( n = 1 ) and v are constants, which are typical for the chemical structure of the oligomers. It is interesting to note that the experimental value for ethylene ( E , = 61730cm-’) fits quite well to the data of oligoenes, which is not the case in relation (14). For oligoenes, the exponents v = 0.47 and 0.53 are close to the value 1/2, which was already recognized in the early investigations of oligoenes [83] and explained by means of models that treat the repeat units as coupled oscillators [84, 851. Although very short oligomers are quite well described by relation (15), longer oligomers deviate from this power law and converge to the absorption maxima of the polymer. This situation is shown in Fig. 8. The deviations from the power law (15) occur for L > 1.5-2nm and depend on the chemical structures of the oligomers. An alternative starting point to describe optical properties of conjugated oligomers is to consider first an undisturbed polymer chain with extended 7r-electron conjugation, for example a nearly perfect single crystal of polydiacetylene (PDA). There is general agreement that the main optical absorption of polydiacetylenes at 2eV is caused by an exciton [86]. An exciton is a neutral excitation state, which consists of an excited electron coupled to the defect-electron (or hole) by Coulomb

500

400

-E S

Y

X

2 x

300

200

P

I 0,3

0,4

0,5 0,6

0,8

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2

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L [nml Figure 8. Length dependence of ,,A,, for several ID conjugated oligomers. The experimental data are from diluted solutions (from ref. [41]).

468

8 Nonlineur Optical Properties of Oligoniers

interaction. The spatial extension of this electronic excitation state on a I D chain is referred to L,. In the case of polydiacetylenes, L, is in the order of 2-3nm as concluded consistently by several authors [87, 881. There is strong and growing evidence that the main optical absorptions of other 1D conjugated polymers (such as polythiophene, polyacetylene and poly(ppheny1enevinylene)) are also caused by an exciton [41, 89, 901. If L, is limited by the length L of the oligomer, quantum confinement of the electronic wavefunctions occurs. Clearly, this has a strong influence on both the hyperpolarizability y and also on the energies of the electronic excitation states. It is an important experimental fact that all oligomers, with an increasing number of repeat units and conjugated 7r-electrons, show a significant increase of y and also a lowering of their electronic excitation energies. The extent of this general trend can vary depending on the chemical nature of the repeat units. There is an interesting analogy to semiconductor quantum structures, where similar quantum size effects are well established [91]. Significant blue-shifts of the optical absorptions occur, if the size of the semiconductor quantum structures (such as quantum wells, wires or dots) becomes comparable to or smaller than the exciton size. Similar blue-shifts of the main optical absorptions occur in conjugated polymers, if the length of the undisturbed chain segments approaches the exciton size. Consequently, the conjugated oligomers can be viewed as natural quantum confined systems, where all three spatial extensions are limited as in a semiconductor quantum dot. The extended conjugated polymer chain would then correspond to a quantum wire. The experimental results shown in Fig. 8 can be interpreted in the context of a quantum size effect. It is straightforward to expect that the optical absorption will also be blue-shifted, if the length L of the oligomer becomes comparable to or smaller than L,. For L < 2-5nm, this remarkable blue-shift of , ,A, is clearly visible. This indicates that L, of polythiophene and poly( p-phenylenevinylene) is in the order of 2-5nm. It is remarkable that this is the same length scale, where the so-called saturation of y is observed (see section 8.4 and Fig. 4). Obviously, the linear and nonlinear optical properties of oligomers are governed by the size of their .ir-electron system. The problem is to find the relevant description of its characteristic length. We have introduced above the length L,, which is the spatial extension of the electronic excitation state. A possible way to estimate L, for a polymer would be to extrapolate the corresponding number n of repeat units, if Em,, of the polymer is used in Eq. ( I 5). Frequently, in this context, the term ‘effective conjugation length’ appears in the literature. But it is applied sometimes without any definition or very often with different meanings. In order to identify this problem, we refer to various definitions of conjugation lengths that have appeared. The most popular meaning of the effective conjugation length is the length of a planar, undisturbed segment of the polymer chain. Within this segment, the delocalized .ir-electrons result from p,-orbitals of neighboring carbon atoms which are oriented parallel to each other [81,92-941. This meaning of the effective conjugation length is denoted here by L,. Clearly, L, is limited by conformational defects. If they are numerous, L, can be very short and can follow a certain distribution function

[94]. A problem with this definition of L, arises, if very well ordered conjugated polymers are considered, such as polydiacetylene single crystals. In this case, L, can be much larger than L, and, therefore, L, becomes meaningless for the description of the optical properties of well-ordered conjugated polymers. Another conformational conjugation length is introduced by Rossi er d.[95]. They consider torsional motions of polymer backbone segments around an angle 8, which also leads to a deviation from the coplanarity of the polymer backbone and to a limitation of the -ir-electron delocalization to an effective length, which is denoted by L H . The onset of saturation in the scaling of the polarizabilities o and 7 with chain length L of oligomers can also be used to define a conjugation length. This was considered by Silbey [53], who introduced in this way a conjugation length for the linear polarizability a and for the hyperpolarizability 7,which are denoted here by L,, and L?, respectively. The strong influences of the .rr-electron delocalization on the linear and nonlinear optical properties of ID conjugated chains have been calculated by Flytzanis c f ul. in the framework of a one-electron Huckel approximation [%I. They introduced the .ir-electron delocalization length Ld which depends on the resonance energies for the bond alternation. General scaling laws result from this theory. They relate L,, to x"' and x ( ~ This ) . will be discussed and compared with experimental results in more detail in section 8.5.4. This compilation of characteristic lengths may still be incomplete. We see that it is a considerable challenge to future theoretical and experimental work. to clarify the relevance and applicability of the different meanings of L,, L , , Lo, L,,, L- and Ld. The major open questions are: How can we measure these quantities with suffcient accuracy in order to allow their comparison? How are they related to each other'!

8.5.3 Comparison of Polymers, Oligomers and Dyes To reveal characteristic structure-property relations and a comparison ofpolymers, oligomers and dyes, their y or x(31values are plotted versus A,,; in a doublelogarithmic scale, see Figs. 9 and 10. The wavelength of the low-energy absorption maximum , ,A, is a very valuable parameter, because it is related to the characteristic length L, in a systematic manner, which can differ, however, for the various classes of compounds. These plots can be used also as masterplots to judge the utility of a material for potential applications, such as nonlinear waveguide devices [26]. This type of application requires a large nonresonant xO' value, which is proportional to nl according to Eq. (1 l ) , and negligible absor tion losses of the materials. Therefore, the best materials should possess large yiR combined with small A,; values. Figure 9 shows ( y ) / Ldata of oligomers from Table 2 which can be compared with the (?,.) values (the orientation averages of 7 per lengths of the repeat units) of polyphenylacetylenes (PPAs) and polyene oligomers (OE28-OE240) from Table 3.

0

Oligoenes

'9

Polyphenylacetylenes A Oligophenylenes

0 Oligothiophenes \

8

3

0

Cyanines

0

0

0

cn

a,

U

A

0 8

0

8

200

400

300

A,,

500

600

[nml

Figure 9. Comparison of the hyperpolarizabilities of various one-dimensional conjugated systems. The orientation average of y per length of the -ir-electron systems is plotted on a double-logarithmic sc;ile versus the wavelength of the absorption maximum A,,;,,. The references for the data are given in Tables 2 and 3 (the circles with dot represent the substituted oligoenes of Samuel et a/. [40], (a) and (b) are from references [32] and [31], respectively). The slope of 10 is indicated for comparison with Fig. 10(b).

Although there is some scatter in the data which is caused by the problems (i)-(iii) (see section 8.2.2) some significant trends can still be derived from Fig. 9. The data points of conjugated oligomers and polymers appear at very similar positions in this values very close to plot. The polyene oligomers OE28-OE240 have ( y ) / Land ,,A, the data of ,K and DOC. Therefore, their effective conjugation lengths L, are in the range of the chain lengths of [IC or D K . Based on a detailed analysis of their absorption spectra, a similar conclusion was also derived recently by Kohler [96]. ~

Figure 10. Masterplots of ID Conjugated polymers and oligomers in a double logarithmic scale to visualize difrerent scaling laws with exponents given by the slopes of the full, dotted and dashed lines, respectively (adapted from ref. 1411). The individual data points represent the orientation w ;w.w ) for various compounds with ID and 2D electron delocalization (for averages of ~ ( ~ ' ( - 3 w. assignments see insets and text). which are measured at the laser wavelength XI. = 1064nm. The upand down-triangles show their resonant (A, % 3X,,) and low-resonant (A, ? 3Xo) xf3)data, respectively. The vertical bars between them indicate the ranges of );13) values, which are obtained at variable XI,. Plot (a) shows that the xI3)data can scatter significantly due to different concentrations of chroinophores. In plot (b) this dilution effect is compensated by ihe ratio X ( 3 ) / a m a x ,where o,ll:,y denotes the absorption coefficient of the thin fihns at ,,,A, and the unique scaling behavior of the ID conjugated systems can be seen.

A remarkable, superlincar increase of ( y ) / Lin function of ,,A,, of related compounds can also be seen in Fig. 9. Because the experimental and systematic uncertainties of these data are quite large, a general, functional dependence of ( y ) / Lon Anlax is not clearly visible. Nevertheless, the positions of the data points of cyanines and the related dye M 3 in Fig. 9 indicate that their optical properties are not as useful for applications a s compared to neutral, 1D conjugated systems, because the latter have much larger y values at similar .,A, The results of THG investigations of thin films shown in Table 4 are displayed in Fig. 10 to visualize general structure-property relations. Figure 3 shows that large ) occur, if w is varied. Therefore, it is necessary to changes of ~ ( ~ ) ( - 3 w ; w , w , wcan ) for a comparison of different materials. The followchoose representative x ( ~data ing ,y(3) values are extracted from the wavelength dependent THG experiments at three characteristic laser wavelengths: (1) XL = 3X,;,, at the three photon resonance with the linear absorption maximum and (2) at the laser wavelength XL = 3x0 where A. denotes the optical absorption edge, which can be obtained by a linear extrapolation of the long wavelength flank of the main absorption band to zero absorption. (3) The fundamental wavelength of the Nd:YAG laser at XL = 1064nm is used only because most THG studies are performed at this wavelength. The choice of XL = 3X,,1,, and XL = 3X0 helps to obtain comparable ~ ( ~ ) ( - 3 w ; w , w , data w ) of different compounds, because they have the same resonance contributions which are symbolized in Fig. 10 by vertical bars between triangles pointing up and down. Because the ~ ( " ( - 3 w : w,w ,w)value at XL = 3X0 was not available for tPA, the lower data point represents in this case the smallest ,y(3) value, which was found at XL = 1064 nm [49]. On a first view of Fig. 1 Oa, the x ( ~ -3w; ) ( ~ j~, jw) , data seem to scatter too much to derive reasonable conclusions. However, closer inspection and comparison with the chemical structures of the different materials does indeed reveal a systematic behavior. First, it must be taken into account that the chromophores of the materials can be very diluted. This happens in the case of /K dissolved in polystyrene and in the case of the polymers PT, PPT and PPA, which have long alkyl substituents. The different number of n-electrons per unit volume influences also the absorption coeficient ( Y , ~ ~of~ ~the thin films. The easiest way to compensate this dilution effect is to plot the ratio x ( ~ ) ( - ~ w I ~ , w , w ) / ( Y , , versus ,, ,,A,, as shown in Fig. lob. In this normalized masterplot, we recognize that the nonlinearities of PPV, PPT and ,OC are indeed very similar, which is not clear in Fig. 10a. ) measured at XL = 1064 nm also fit to the masterplot in a systematic The ~ ( jdata manner. For materials which have A,, close to 1/3 of XL = 1064nm the threephoton resonant ,y(" values are obtained; see for example OPV-3 and OPV-4. For materials with X,:, in the range between 450 nm and approximately 500 nm, the x(')(-3w: LJ, w ,w) values which are measured at 1064 nm and 3Xo have a similar is larger than 500 nm, the y ( j ) (-3w;w, w,w)value measured at magnitude. If,,A, 1064nm can be much smaller than at XL = 3X0, because the harmonic wavelength falls into the spectral window of low absorbance which is usually observed between the strong bands in the visible and UV spectral ranges. This is the case for tPA. The observation that the oligomers OPV-n and PC fit closely to the nonlinear optical properties of the 1D conjugated polymers demonstrates that relatively

short chain segments of the polymers are responsible for their optical properties and not their entire chain length. As such, conjugated polymers can be considered as arrays of chromophores originating from subunits of the polymer chain [41, 971. High degrees of polymerization, therefore, are not necessary to achieve large ~ ( values. Conjugated oligomers with a chain length in the order of 10nm should have similarly large x(') values. It can be further concluded from Fig. 10b that materials with heteroatomic composition such as PT, PPT or OPc d o not offer ) ( w,w ,w) values than pure hydrocarbon compounds like PPA or larger x ( ~-3w; PPV. The dyes ORy-n and OPc have a .n;-electron system which is distributed in two directions of space. It is obvious from Fig. 10 that the nonlinearities of these 2D dyes deviate strongly from the ID systems. This difference can be explained with the model of an electron in a 1D and 2D potential well [78]. Owing to additional degeneracies, the energy levels in a 2D well are more closely arranged than in the 1D well. With this argument, it can be qualitatively understood that a 2D well of the same maximum lateral length L as the ID well should have a larger A,, value. Thus, the 2D dyes appear at a very different position in the masterplot as the 1D conjugated systems. Using the same argument as above for cyanines, we can conclude from Fig. 10b that systems with 1D electron delocalization are much better suited for applications of the nonresonant third-order nonlinearities than systems with 2D electron delocalization.

8.5.4 Scaling Laws for One-Dimensional Conjugated Systems For polymers with a 1 D conjugated .n;-electron system, Flytzanis et al. used the oneelectron Hiickel approximation to derive a universal behavior between and the 7r-electron delocalization length Ld [%I. Their theory yields the simple but general relations

and

As they calculated that Ld ,,A, for 1D conjugated chains, the scaling law for the nonresonant x(3)values can be expressed by N

p

(17) Although this scaling law became very popular and was frequently used to interpret the third-order nonlinearities of conjugated systems, a final experimental test of its validity has been performed only recently [41]. This is possible by means of the normalized masterplot shown in Fig. lob. The dashed lines show that the experimental data of ID conjugated systems follow an empirical relationship ~6 m'M

~

1

with a n exponent s = 10 f 1. As multi-photon resonances lead to large variations of ~ ( ~ ) ( - 3w, w w. ; w),relationship (18) is valid only for those experimental data which are measured at comparable resonant, low-resonant or non-resonant conditions. The 1D conjugated systems, which have a similar length dependence of, , , A, indeed follow a general scaling of their third-order nonlinearities with respect to their linear optical properties. However, the exponent x = 10 differs significantly from the exponent 6 given in the scaling law (17). A possible reason for this discrepancy was described recently [411. Presumably, the Huckel approach in the theory of Flytzanis does not appropriately describe the relationship between, , ,A and the n-electron delocalization length Ld of the 1D conjugated systems. This which obviously is in disagreement with the results for theory yields Ld A, short 1D conjugated oligomers as we can conclude from Fig. 8. The experimental value .-c = 10 i 1 in relationship (18) can be interpreted in a speculative manner for short chains: If we take into account that the extinction coefficient of short oligoenes increases nearly linearly with L [72], we have to set o,,,~,~ L”. For oligophenylenevinylenes, the value of n is found in the range 0 < h: < I . Using Eq. (15) and n L for oligomers we obtain, , A, L”. By use of Eqs. (4c) and (12) the exponent s in (18) can be interpreted by x = (1- K ) / V . With p = 5 and a typical value 0.3 < v < 0.5 a rough agreement with the experimental value of .Y = 10 is obtained.

-

N

N

-

8.6 Conclusions The fundamental understanding of the optical nonlinearities of conjugated oligomers is closely related to the understanding of the physical nature of their electronic ground and excitation states. Critical inspection of the chain length dependence of their excitation energies reveals that the popular and often claimed l / n dependence is not an appropriate description. In most cases, the experimental data deviate significantly and always similarly from this 1 / n dependence. These deviations are especially visible for small numbers of repeat units, e.g. n = I , 2 , 3 , . . . The excitation energies of short chains are much better described by an empirical power law dependence n-”, where v is smaller than 1 and depends strongly on the chemical structures of the repeat units. It is expected that an appropriate consideration of electron correlation effects such as exciton-type states and their scaling with the size of conjugated oligomers will lead to improved theoretical understanding of their optical properties. Quantum size effects of electronic states are in the center of actual trends in modern solid state physics. New physical effects in low-dimensional and mesoscopic systems become visible, if the structure-size approaches the typical extension of the corresponding electronic wavefunction. Although modern lithographic methods yield quantum semiconductor structures with increasing perfection [9 I], the ultimate precision of such quantum structures is reached only if every atom can be placed at a specific position and can be fixed there by chemical bonds. These most precise ~

low-dimensional quantum structures exist already and the chemists have them in their hands! Oligomers with strongly delocalized electrons could fulfill an important role in the improved understanding and development of quantum semiconductor structures, because they can represent such structures with ultimate atomic precision. The linear and nonlinear optical properties of conjugated oligomers show an intriguing size dependence in the range up to approximately 5-10nm. Only minor changes of the optical properties occur, if the chains are made longer and polymers are formed. This means that macromolecules are not really necessary to achieve large optical nonlinearities. The optical properties of conjugated polymers are determined by relatively short segments. Their characteristic lengths are frequently called ‘effective conjugation lengths’ in the literature, however, with sometimes poor definitions and different meanings. This chapter presents an attempt to discriminate between the different means of ‘conjugation lengths’. The preferred interpretation of this characteristic length is the spatial extension of an electronic excitation state which probably has excitonic character. General structure-property relations of conjugated .Ir-electron systems can be made visible in masterplots of the third-order nonlinear optical quantities y or x(’) versus the wavelength , , ,A of the low-energy absorption maximum. A unique scaling law of the form ~ ( ~ ) ( - 3 w ; w , w , w ) / ( ~ (Amax)” ,*~~ is found for neutral ID conjugated oligomers and polymers and compared with theory. These masterplots can be further used to estimate the utility of organic materials for applications of their third-order nonlinearities. It is concluded that 1 D conjugated systems in their neutral state are superior to 2D or charged systems like cyanine dyes for such applications, which require large optical nonlinearities combined with small absorption losses.

-

Acknowledgments I want to thank my former and present coworkers for their contributions and intensive interactions: Dr. D. Neher, Dr. A. Kaltbeitzel, Dr. R. Schwarz, Dr. A. Mathy, Dr. A. Grund, Dr. M. Baumann, K. Ueberhofen, U. Baier, M. Loddoch and H. Menges. The fruitful cooperation and helpful discussions with Prof. G. Wegner, Prof. K. Mullen and their coworkers are gratefully acknowledged. Financial support to this work was given in part by the Bundesministerium fur Forschung und Technologie and the Volkswagen-Stiftung.

References I . C. Sauteret. J.-P. Hermann, R. Frey P I trl. P h p . Re],. L ~ i t1976, , 36, 956. 2. D. J. Williams (Ed.), Nonlinear Optical Properties of’ Organic and Polymeric, Mutc~riuls,ACS Symp. Ser. 233, Am. Chem. Soc., Washington, 1983. 3. D. S. Chemla. J. Zyss (Eds.), Nonlineur Optical Propt.rties of’ Orgunic Molwules r n i d Cr.~.strrO. Vol. I and 11. Academic Press, Orlando. 1987.

4. P. N. Prasad, D. R. Ulrich (Eds.), Noriliricw 0pric.d and Elwtrouctiw Polyr?icr.r, Plenum Press, New York, 1988. 5 . A. J . Heeger. .I. Orenstein. D. R. Ulrich (Eds.), Nodinetrr Opticd Properties of Polwzrrs, Materi2ils Research Soc., Pittsburgh. 1988. 6. J. Messier, F. Kajzar. P. Prasad. D . Ulrich (Eds.). Nonli/ieur Opticul Efli,cts in Organic Po/),i i i e r s , N A T O AS1 E 162. K ~ L I w Dordrecht, ~~, 1989. 7. J . L. Bredas, R. R. Chance (Eds.), Coniugtrted Polyriieric Mutrrirrls: Opportunitie.7 in Electronics, Optoe/c,c.tronic,.sr i n d Mol~~ciilctr Elec~troriics,NATO AS1 E 182, Kluwer, Dordrecht, 1990. 8 . J. Messier. F. Kajzar, P. Prasad (Eds.), Orgtrnic MoiecirIe,s,fi~rNonlineur Optics nnd Plzofonics, NATO AS1 E 194. Kluwer. Dordrecht, 1991. 9. J. L. Bredas. R. Silbey (Eds.), Conjirgaterl P0/~1?71’I’.S. Kluwer, Dordrecht. 1991. 10. G. Zcrbi (Ed.). Orgnriic, Mtrtei.ial.v,fbr Photonic.s, North-Holland, Amsterdam, 1993. 1 I . W. R. Salaneck, I. Lundstrom, B. Rinby (Eds.), Co~jugcr/i~cl Polymers and Rrluied Muteriuls: Thi, I/itc~rc~on,iectiolr of C‘hiwic~crlund Electronic, Structure, Oxford Univ. Press, Oxford, 1993. 12. P. N. Butcher, D. Cotter, The Elernc,rrt.y of’ Nonlincwr Optics, Cambridge Univ. Press, Cambridge, 1990. 13. R. W. Boyd, Noriliriiwr Optii.,~,Academic Press. Boston, 1992. 14. J. A. Armstrong. N . Bloenibergen, J. Ductling, P. S. Pershan, P1z.v.T. REV. 1962, 127, 1918. 15. J . F. Ward, Rci,. Mot/. P h ~ x .1965, 37, I . 16. T. K. Yee. T. K. Gustafson. Phv.7. R i v . A 1979, 18, 1597. 17. A. D. Buckinghani, B. J . Orr, Q. Rep. Chcni. Soc. 1967, 21, 195. 18. C. Bubeck in ref. [ 101 and references therein. 19. S. Singh in M. J . Webcr (Ed.), H o n t h k of.Lrisrr Sciewe ill, I , C R C Press, Boca Raton, 1986. 20. F. Kajzar, J. Messier, P/JJ..S.Rci,. A 1985. 32, 2352. 21. D . Neher, A. Wolf. C. Bubcck, G. Wegner. Chon. P/IJ.S.Lrrt. 1989, 163, 116. 22. G. M. Carter, J . V. Hryniewicz, M. K . Thakur. Y. J. Chen, S. E. Meyer, Appl. Phys. Lett. 1986, 4Y. 16. 23. G . M . Carter. J . Opt. Soc. Am. 1987. B4, 1018. 24. C. Bubeck, A. Kaltbeitzel, A. Grund, M . LeClcrc, Cheni. PI7j3. 1991, 154, 343. 25. P. N. Prasad, D. J. Willinins, Introtlrrcfion to N o n l i n t w Opticd Efl;crs in Molecules und Polvnii’r.v. Wiley. New York. 1991. 26. C. Bubeck in F. Kajzar, J . D . Swalen (Eds.). Sc,ieweuntl Tcdinologjj of’Ovgunic Thin Filnis for U’ui~~gr~itli~ig Norz/iriecrr Op/ic,s,Gordon & Breach Sci. Publ., Amsterdam, 1996, p. 137. 27. J . P. Herniann, J . Ducuing, J . App/, P/i.vs. 1974, 35, 5100. 28. F. Kajzar, J. Messier, J . Opt. Soc. A m . 1987, B4. 1040. 29. G. R. Meredith. S. H . Stevenson in ref, [6], p. 105. 30. J. F. Ward. D . S. Elliott, J . Cheni. P h j ~ s .1978, 69, 5438. 31. S. H. Stevenson, D . S. Donald, G. R. Meredith in ref. [ S ] , p. 103. 32. J. P. Hermann, D. Ricard. J. Ducuing, A p p l . Ph~.s.Lett. 1973, 23, 178. 33. J. R. Heflin, A. F. Garito, L. A. Hornak (Eds.). PolJviersjbr Lightiiuve unil Intcyyuted Optics, Marcel Dekker, New York, 1992, p. 501. 34. S. R. Marder, C. B. Gornian, F. Meyers et ( I / , Science 1994, 265, 632. 35. G. Bourhill, J. L. Bredas, L.-T. Cheng et a/. J . Am. C.‘lzenz.Soc. 1994, 116. 2619. P. N . Prasad, J . P h ~ s C/zwz. . 1989, 93, 7916. 36. M.-T. Zhao, M. Sanioc, B. P. S d. J . Phjs. Chern. 1988, 89, 5535. 37. M.-T. Zhao, B. P. Singh. P. N. 38. H . Thienpont, G. L. J . A. Rikken. E. W. Meijer, W. ten Hoeve, H . Wynberg, P/i,rs. R ~ PLett. . 1990, 65. 2141. 39. D. Neher, A. Kaltbeitzel. A. Wolf, C. Bubeck, G. Wegner. J . P h j s . D: Applictl Phys. 1991, 24, 1193. 40. I. D. W. Samuel, 1. Ledoux, C. Dhenaut ot t i / . Sc,irric,e 1994, 265, 1070. 41. A. Mathy, K. Ueberhofen, R. Schenk et rrl. Phys. Rev. B 1996, 53, 4367. 42. S. Schrader, K. H. Koch. A. Mathy. C. Bubeck, K. Mullen, G. Wegner, Sjwth. M e t . 1991,4143, 3223. 43. S. Schrader, K. H . Koch. A. Mathy, C. Bubeck, K. Mullen, G. Wegner, Progr. Colloid P o l w . Sci. 1991, 85, 143. 44. A. Grund, A. Kaltbeitzel, A. Mathy rt td. J . Cheni. Phy.s. 1992, Y6, 7450.

45. A. Grund, A. Mathy, A. Kaltbeitzel, D. Neher, C . Bubeck, G. Wegner in R. A. Hann, D. Bloor (Eds.), Orgunir Mareriuls fbr Nonlineur 0pfic.s 11, Royal Soc. Chem., Cambridge, 1991, p. 288. 46. S. Aramaki, W. Torruellas, R. Zanoni, G. I. Stegeman, Opt. Cornnuin. 1991, 85, 527. 47. J. B. van Beek, F. Kajzar, A. C . Albrecht, J. Cheni. Phys. 1991, 95, 6400. 48. A. Kistenmacher, T. Soczka. U. Baier, K. Ueberhofen, C . Bubeck, K . Mullen. Artu Po1yrnr.r. 1994. 45, 228. 49. F. Kajzar, S. Etemad, G . L. Baker, J. Messier, Synth. Met. 1987, 17, 563. 50. W. S. Fan. S. Benson, J. M. J. Madey, S. Etemad, G. L. Baker. F. Kajzar, Phj,.y. Rev. Lett. 1989, 62, 1492. 51. C . Halvorson, T. W. Hagler. D. Moses, Y. Cao, A. .I.Heeger. Cheni. Phj1.s. Lett. 1992, 200, 364. 52. D . Neher, A. Wolf, M. LeClerc, A. Kaltbeitzel, C . Bubeck. G . Wegner, S j ~ i t hM . e t . 1990. 37, 249. 53. R. Silbey in ref. [7], p. I . 54. S. Etemad, Z. G . Soos in R. J. H. Clark, R. E. Hester (Eds.), Spectroscopy of' Advunced Muteriuls, vol. 19, J. Wiley, New York, 1991, p. 87. 55. R. Silbey in ref. [ I I], p. 229. 56. J. L. Bredas, C . Adant, P. Tackx, A. Persoons, B. M. Pierce, Chern. Rev. 1994, 94, 243. 57. K. C. Rustagi, J. Ducuing, Opt. Commuti. 1974, 10, 258. 58. (a) G. P. Agrawal, C. Cojan, C. Flytzanis, P/ij,s. Rev. B, 1978, 17, 776. (b) C. Flytzanis in ref. [3] vol 11, p. 121. 59. D. N. Beratan, J. N. Onuchic, J. W. Perry, J . Phj3.y. Chern. 1987, 91, 2696. 60. C . P. DeMelo, R. Silbey, Clzenz. Plzj,s. Le//. 1987, 140, 537. 61. C. P. DeMelo, R. Silbey. J . Chern. PhJs., 1988. 88, 2567. 62. J. R. Heflin, K . Y. Wong. 0. Zamani-Kamiri, A. F. Garito, Pliys. Rev. B 1988, 38, 1573. 63. G . J. B. Hurst, M. Dupuis, E. Clementi. J . Cheni. Phj,.~.1988, 89, 385. 64. Z. G. Soos, S. Ramasesha, J . Chern. P/7.i,.~.1989, YO, 1067. 65. B. M. Pierce, J . Chem. Phys. 1989, Y l , 791. 66. Z. Shuai, J. L. Bredas, Phys. Reb'. B 1991, 44. 5962. 67. S. Mukamel, H. X. Wang, Phys. Rev. L d t . 1992, 69, 65. 68. Y . Verbandt, H . Thienpont, I. Veretennicoff, Phj*.s. Rev. B 1993, 48, 8651. 69. Y. Verbandt, H. Thienpont, I. Veretennicoff, P. Geerlings, G . L. J. A. Rikken, Nonlin. Opt. 1995, 12, 75. 70. F. C. Spano, Z. G . Soos. J . Chem. Phys. 1993, 99, 9265. 71. D. Beljonne, Z. Shuai, J. L. Bredas, J . Cheni. Phys. 1993, Y8, 8819. 72. H. A. Staab, E i n f i h x n g in die throretische organische Chernie, Verlag Chemie, Weinheim, 1975. 73. G . J. Exarhos, W. M. Risen, R. H. Baughman, J . A m . Clzem. So<,.1976, 98, 481, Table I and references therein. 74. P. Bauerle, T. Fischer, B. Bidlingmeier, A. Stabel. J. P. Rabe, Angrw. Cheni. 1995, 34, 303. 75. W. Kutzelnigg, Einjihrurig in die Theoretische Chemie. Band 2: Die rhernische Binriung, Verlag Chemie, Weinheim, 1978. 76. J. Fabian, H. Hartmann, Light Absorption ?/Organic Colorants, Springer, Berlin, 1980. 77. I. N . Levine, Quuntuni Chemistry, Allyn and Bacon, Boston, 1983. 78. R. S. Berry, S. A. Rice, J. Ross, Phy.sical C/iemistry. Wiley. New York, 1980. 79. H. Kuhn, J . Chem. Phys. 1949. 17, 1198. 80. B. E. Kohler, J . Chern. Phys. 1990, 93, 5838. 81. H. E. Schaffer, R. R. Chance, K . Knoll, R. R. Schrock, R. Silbey in ref. [7], p. 365. 82. G. S. W. Craig, R. E. Cohen, R. R. Schrock et d . J . A m . Chem. Soc. 1993, 115, 860. 83. K. W. Hausser, R. Kuhn er a/. Z . phvsik Chem. B 1935, ZY, 363, 371, 378, 384, 391, 417. 84. G . N. Lewis, M. Calvin, Cliern. Rev. 1939. 25. 273. 85. W. Kuhn, Hrlv. Chini. Acru 1948, 31, 1780. 86. D. Bloor, R. R. Chance (Eds.), Polydiucetylenes, NATO AS1 E 102, Martinus Nijhoff, Dordrecht, 1985. 87. (a) M. Schott, G. Wegner in ref. [3] Vol. 11, p. 3. (b) G . Weiser, Phys. Rev. B 1992, 45, 14076. 88. B. I. Greene. J. Orenstein, R. R. Millard, L. R. Williams, Phys, Rev. Lett. 1987, 58, 2750.

89. D. J. Sandman, Trench Polym. Sri. 1994, 2, 44. 90. S. Heun, R. F. Mahrt, A. Greiner et ul. J . P h p . : Contlens. M u i t c ~1993, 5. 247. 9 I . (a) C. Weisbuch, B. Vinter, Quenutum ScmiconN‘uc.torStructures, Academic Press, Boston, 1991, (b) H. Haug, S. W. Koch, Quuntirm Tlwory qf’tlrr Optic,al iind Electronic Properties of’Serwicorrrlurtors, World Scientific, Singapore, 1993. 92. M. L. Shand, R. R. Chance. M. LePostollec, M. Schott, Phjv. Rev.B 1982, 25, 4431. 93. H. Kuzmany, Pure Appl. Clwni. 1985, 57, 235. 94. B. E. Kohler, J. C. Woehl, J . Chem Phys. 1995. 1113, 6253. 95. G. Rossi, R. R. Chance, R. Silbey, J . Chem. P h y . 1989, YO, 7594. 96. B. E. Kohler, 1. D. W. Samuel, J . Clwni. Pliys. 1995, 103, 6248. 97. U. Rauscher, H. Blssler, D. D. C. Bradley, M. Hennecke, Phyy. Rev. B 1990, 42, 9830.

9 Electrochemical Properties Jiirgen Heinze, Peter Tschuncky

9.1 Introduction Systematic electrochemical research on the redox properties of conjugative 7rsystems has been carried out since the mid-1950s beginning with the pioneering work of Hoijtink [l]. He showed that the cathodic reduction of olefinic and aromatic 7r-systems is energetically facilitated when the 7r-system is extended by introducing additional aromatic units or conjugated double bonds. Very reliable linear correlations between the reduction potentials and molecular orbital energies of a series of aromatic hydrocarbons and diphenylpolyenes were one of the first successful applications of the Huckel molecular orbital theory (HMO) and allowed the development of a coherent picture of redox processes. Hoijtink et al. had already noticed that most aromatics and polyenes can be further reduced to their respective dianions. Although at the time all the measured dianion potentials were influenced by follow-up processes, it became obvious that their positions also shift to less negative values for larger 7r-systems. Moreover, in the case of 1,n-diarylpolyenes it was observed that the separation between the first and the second redox step diminished as a function of the polyenic chain length [2, 3 , 41. Later on, progress in electrochemical instrumentation and techniques [5, 61 facilitated the generation and study of multiple-charged anionic and cationic species. Thus, Hammerich and Parker [7,8] were able to generate persistent diions of numerous aromatic systems in cyclic voltammetric experiments. Hunig and coworkers [9] showed that chain-like oligovinylene systems with electroactive end groups, may, in principle, undergo reversible two step redox processes, and that the separation between the two redox potentials diminishes drastically with an increasing number of vinylene groups. The discovery of the redox properties of conducting polymers in 1979 gave new impetus to the electrochemistry of conjugated organic systems, initiating extended studies on the charge storage characteristics of these species. However, at the beginning of this research electrochemists preferred to focus attention on polymeric materials, largely neglecting all the experience gained in the characterization of monomeric or oligomeric species. Only two independent studies by Diaz [lo] and Bredas [ I l l pointed out that, in agreement with the previous results of Hoijtink [2],the oxidation and reduction onsets in chain-like conjugated oligomers are dependent on the chain length and shift linearly to lower energies with the inverse number of aromatic units or double bonds. For the rest, the interpretation of electrochemical data was determinated by experimental results and theories obtained from solid state physics and macromolecular chemistry. As a consequence of this view the bipolaron model was applied to conducting polymers [12, 131. It ideally assumes during the charging process the formation of

480

9 EIPCtrocliem icri 1 Properties

multiple thermodynamically stable diionic states, the so-called bipolarons, associated with local geometric distortions of the chain. Regarding the formation of conducting polymers, the mechanism of electropolymerization was described in terms of a classical chain propagation process in which monomers are attached step by step to a growing chain [14, 151. It took more than eight years, until 1987, before the first synthesis of monodisperse oligomeric systems of the phenylenevinylene type, and their electrochemical characterization by voltammetric measurements, opened the way for a reinterpretation of electrochemical findings on conducting polymers [ 161.

9.2 Charge Storage Mechanism of Conjugated Oligomeric Systems 9.2.1 Redox Behavior in Solution The method of choice for the characterization of redox processes of oligomeric systems is cyclic voltammetry (CV) [5, 171. It provides information on both thermodynamic and kinetic data of electron transfer reactions and usually allows a precise analysis of multistep redox mechanisms. Thus, for oligomeric systems cyclic voltammetry gives details about 0 0 0

the number, stability and potential of redox pairs, Coulombic repulsions of excess charges, the HOMO-LUMO gap.

First systematic studies of redox mechanisms of oligomers have been carried out with oligo-(aryleneviny1ene)s. 9.2.1.1 Oligo-(aryleneviny1ene)s The correlation of these data with structural properties of the oligomers provides information about the redox behavior as a function of chain-length and constitutes the basis of the oligomeric approach leading by extrapolation to a description of corresponding polymers. Some representative structures of this class of oligomers are shown in 1-3. Using cyclic voltammetry, Heinze et al. [16] studied the reduction behavior of several structurally defined soluble oligomers from the series of oligo-( p-phenyleneviny1ene)s (Fig. l), and deduced rules which describe the redox properties of such conjugated systems as a function of chain length. These rules, which complement theoretical and experimental results already presented in the older literature [2, 10, 1 I], are still valid. The following trends have been established: With increasing chain length of an oligomer the conjugated 7r-system increases. This implies that:

9.2 Cliargc. Storage Mec~hmiisnio f Conjugated Oligonirric Sj~strn7.~

2

48 1

3

(i) the number of possible redox states increases; (ii) redox states of identical charge (e.g. mono- or diion) shift towards lower energies (to more positive potentials upon n-doping, towards more negative potentials upon p-doping). For long chain lengths, the low redox potentials converge towards a limiting potential characteristic for the investigated conjugated polymer. Plots of EY versus the inverse chain length (1/n) show a linear dependence [lo, 1 I , 161. The limit 1 / t i = 0 gives an estimate of the first redox potential of a hypothetical polymer consisting of infinitely long chains in solutions (I&). (iii) the potential difference between subsequent redox states decreases, because the Coulombic repulsion between excess charges is reduced. At a certain chain length with about 8 repeating units, the potential difference between the first two redox states AE,,* vanishes. One can deduce that in a corresponding

,

-2.0 E [vs. AgIAgCIIIV

-3.0 -2.5

Figure 1. Cyclic voltammograms for the reduction of oligo-p(phenyleneviny1ene) oligonicrs 1 containing IZ = 1-5 repeating units. (THF:"aBPh4) [16].

482

9 Elwtroch~wiculProperties

polymer with a degree of polymerization n, r1/4 charges can be stored which interact only by spin pairing. In the past, this fact has often been interpreted as the minimum chain length for the formation of bipolarons. Yet no extra stabilization of the diionic state relative to monoionic one is observed in solution, as predicted by the bipolaron model [ 131. Moreover, all experimental data reveal, in excellent agreement with quantummechanical calculations, that the generation of higher redox states requires essential more energy than the formation of the monoionic (polaron) state. This is not in agreement with predictions of the bipolaron model. (iv) redox steps, leading to charging levels larger than y = 0.5, shift to higher energies compared with identical charging levels of shorter oligomers. Often, they can no longer be observed in the experimental accessible potential range. Generally, it can be stated that in a oligomeric (polymeric) chain with excess charges >n/4 every further redox step will be energetically separated from the proceeding one by at least the amount of the Coulombic repulsion energy created by the new charge. (v) the chemical stability of the charged species increases and, therefore, the tendency for follow-up processes, e.g. polymerization, decreases. This is a consequence of rule (ii). Quite a large number of publications that have appeared since that time support these findings, but have also introduced new aspects that show the complexity of redox mechanisms in such systems. Very systematic studies have been carried out by Mullens group who have varied in chain-like oligomers the type and coupling position of the electroactive monomeric building blocks and the modes of linkage, using both saturated and unsaturated species with different lengths [18, 191. Their results clearly show that the size of the aromatic subunit, the overall number of 7r-electrons, the 7r-topology as well as steric effects are important factors for the redox behavior of all these species. Thus in the series of oligoarylenevinylenes, replacement of the phenylene unit by larger arylene units enlarges the charge capacity of the respective systems. While the p-oligophenylenevinylene 1 ( n = 1) with three phenylene units can be electrochemically reduced up to a dianion, the corresponding naphthalene derivative 2 reaches a trianion level and the anthracene derivative 3 even a hexa-anion state [ 18, 20-221. This is an expansion of rule (i), showing that the number of accessible redox states in chain-like oligomers is a function of both the length of the conjugated chain system and the size of the arylene building block. ( I t should be noted that the chemically accessible charge number via alkali metal reduction may significantly exceed the number of observed electrochemical redox steps.) An important reason for the latter phenomenon is the fact that the better the excess charges in condensed aromatic units are stabilized, the larger the 7r-structure is. Of course, the energetic stabilization of an excess charge in a large aromatic unit diminishes the trend for its delocalization and Coulombic repulsion effects along the chain. Therefore, the shift of the first redox potential in dependence on the chain length is less pronounced for the naphthalene and anthracene derivatives than for the phenylene system or the pure oligoene chain and, moreover, the separation

9.2 Clitrrgr Siorage Mei~hatiisrnof’ CotIjugated Oligomeric Sv.sterns

483

between successive redox steps becomes substantially smaller as the number of T subunits increases. A further influence on the redox properties results from the coupling pattern between the vinylene and the arylene units. Measurements on para- meta- and ortho-coupled phenylenevinylenes reveal that it is more difficult to charge meta- and ortho-homologs than the corresponding para-homologs. However, the conjugative uncoupling of two meta-groups in a phenylene ring diminishes Coulombic repulsion and, therefore, the energetic separation between successive redox steps decreases [22]. Recently, cyclic voltammetric experiments of the oxidation of 1 ( n = 2.3) indicated strong changes of the voltammeric response in dependence on temperature, concentration and scan rates. A detailed evaluation of all data gave clear evidence that the formation of the radical ions is followed by a rapid reversible dimerization between oligomer chains accompanied by the formation of a 0-bond [23]. In the case of 1 ( n = 2), the reduction of the dimeric dication occurs at a potential which lies more than 0.6 V negative to the oxidation potential of the neutral starting species indicating a stabilization of this dimer of more than 60 kJ mol-’.

9.2.1.2 Oligoenes The simplest vinylene polymer is polyacetylene (PA). The discovery of its ‘metallic’ properties by Heeger and McDiarmid [24] at the end of the 1970s stimulated worldwide efforts to develop new materials with unconventional properties. Although the stability of PA is poor, due to the structural simplicity of its polyene chain, it is still the subject of much basic research. Despite the great interest in PA, there are only a few electrochemical studies of monodisperse oligoene systems. The reason is the high reactivity of doped alkyl substituted oligoenes in the presence of nucleophiles or electrophiles. Normally, these oligomers consist of a carbon chain with alternating single and double bonds and two terminating groups, which are equal in most cases. Thus, a t-butyl group [25-27] or, in the case of a- and P-carotenoids, a cyclohexenyl group [28], has been used, while phenyl or other aromatic substituents have been used as end-groups in the so-called arylpolyenes [4, 291. The chain length n (12 = number of double bonds in the conjugated system) again determines the electronic properties of the oligomers. In accordance with the findings for PA, the oligoenes can be both reduced and oxidized, which allows a simple electrochemical determination of the bandgap and the overall charge-storage properties of such systems. Their redox potentials shift to lower energies and additional redox states become accessible as a function of increasing n. In summary, all the results from theoretical calculations [30] and electrochemical [3 I] and spectroscopic [22, 261 investigations support the rules presented in section 9.2.1.1. A series of carotenes [3 11 4a-h may serve as an illustration of this redox behavior. Cyclic voltammograms for the different species are shown in Fig. 2. As can be seen, the reduction for the oligomer with n = 5 starts with two well-separated oneelectron redox steps, whereas its oxidation already involves a two-electron transfer. With increasing chain length, additional weakly separated redox pairs appear (EOx,? and Eox,4for oxidation, EKed and ERed,4 for reduction). The potential gaps in the

4x4

9 Elec,trocheniical Propertie.\

4c

n = 1: 4d n = 2; 4f n = 3: 4g n = 4:4h

single pairs and between them decreases according to rule (iii). Thus, two-electron transfer steps are most likely to occur for the longer polyenes (n 2 19). A strictly linear dependence of the redox potentials vs. the chain length of the oligomers is observed for all related redox states in the series (Fig. 3). Extrapolating from the first oxidation and reduction potentials vs. l / n , the bandgap for infinitely long chains can be determined as 1.25eV, which is less than the

Figure 2. Cyclic voltammograms for (a) reduction (d~methylamine/O.1 M TBABr) and (b) oxidation (SOz/O.l M TBAPF6) of oligomeric P-carotenc homologs 4. n indicates the number of double bonds in the molecule including the double bonds in the terminating cyclohexene caps. T = -60°C [23a].

9.2 Charge Storrige Mechanism of Conjugated Oligorneric Systenzs

485

Figure 3. Plot of redox potentials vs. inverse chain length for a series of carotenes 4. n indicates the number of double bonds in the molecule including the double bonds in the terminating cyclohexene caps [23a].

experimental bandgap of PA [32]. It is interesting to note that the potential separation between successive electron-transfer steps is significantly higher in the case of reduction than in the case of oxidation. Obviously, specific ion pairing and solvent effects cause these differences. Similarly to the arylenevinylenes 1, the anodic oxidation of the diphenyloligoenes 5 and 6 results in the reversible formation of the respective dimeric dications. Electrochemical, spectroscopic and theoretical studies give clear evidence for the formation of intermolecular a-bonds between the radical cations of 5 and 6 [33]. In the case of 4,4'-dimethoxystilbene 7, the smallest unit of the structurally related diphenylpolyenes and phenylenevinylenes, whose radical cations also undergo reversible dimerization on a benzylic position, the product of coupling after the addition of water could be isolated as a derivative of tetrahydrofurane [34]. The most important point of these observations is that such dimeric dications are stabilized by more than 60 kJ mol-' in comparison to the corresponding monomeric radical cations and that this energetic stabilization does not result from a geometric distortion within a conjugated chain but from the intermolecular interaction between two charged chains. Applying these results to the properties of conducting polymers opens up new perspectives for interpreting charge storage.

9.2.1.3 Linear Oligoarylenes and Double-Stranded .n-Systems In principle, the redox behavior of linear oligoarylenes and double-stranded oligomers is similar to that of all other oligomeric systems, the properties of which have

5

6

7

been discussed before. However, steric factors, the mode of linkage, and the 7rtopology can cause complications that may significantly change the redox properties of such oligomers. The simplest systems are the oligophenylenes, followed by the oligonaphthylenes and the oligoanthrylenes. Rules (i) to (v) apply to unsubstituted oligophenylenes and the same compounds with t-butyl-substituents in the terminal rings [18, 35-37]. The redox properties of oligo( p-pheny1ene)s change when sterically relevant methyl groups exist in the central rings. Thus, in comparison with the unsubstituted oligomers in methyl-substituted homologs 8a-e with four or more phenylene units, the first reductive redox step is shifted to a more negative potential and a twoelectron wave appears in the voltammetric response [37]. This can be interpreted by assuming that, due to steric hindrance, additional energy is needed to planarize the phenylene chain for the first electron transfer, and that the second electron is able to enter the now flattened system at the same or even a more positive potential. When benzene building blocks are replaced by larger arene units, steric influence further increase. In the case of oligo( 1,4,-naphthylene)~9 and oligo( 1,5-naphthylene)s 10, the naphthalene peri-protons create additional steric hindrance of the 7r-conjugation [18,38]. Studies of the geometry of the parent systems 1,l’-binaphthyl show that the potential curve for rotation about the inter-ring bond is rather shallow

9.2 Charge Storage Meclianim of Conjugated Oligonzeric Systems

487

b

d

with two minima at 50°C and 130°C. Unlike the results with the phenylene series, the first oxidation potential of the oligo( 1,4-naphthylene)s at 1.31 V is independent of chain length [18, 391. From the pentamer 9 (n 2 3) onwards, the single steps corresponding to monoand dication formation are no longer resolved, and even the potential separation between the tri- and tetracation formation slowly approaches a minimum. Similar results are obtained for the oligo( 1,5-naphthylene)s 10. Because the oxidation potential of 1,l-binaphthyl as starting entity in both series is about 200mV lower than that of naphthalene itself, it can be concluded that the oligonaphthylenes behave as though built up of essentially independent binaphthyl units. If the naphthylene is replaced by an anthrylene unit, the electronic interaction between the arene moieties becomes extremely small, and both the first reduction and oxidation potential are almost independent of the chain length of the system [22,40]. Once again, rules (i) to (v) apply for higher redox states. The reason for the very small shift of the first redox step with increasing chain length is that the larger the single arene building block, the better the excess charges in condensed aromatic units are stabilized. Therefore, in combination with the strong steric hindrance, the result is a low conjugative aryl-aryl interaction.

mR

R

R n=0-4

9

n = 0-2

I0

A drastic change in redox properties occurs when the oligo( 1,4-naphthylenes) 9 are cyclized to a planar, double-stranded structure, as is the case with the oligorylenes 11 [lX, 411. In principle, the redox data are still in good agreement with rules (i) to (v). However, due to the excellent conjugation over the ladder-type 7r-system, the electronic effects are more pronounced than in their linear analogs. Thus, on the one hand, the onset of the reduction and oxidation processes are much lower than those for chainlike 7r-oligomers. On the other hand, the Coulombic repulsion between excess charges increases strongly, leading to large gaps between the lowest and highest redox states, e.g. the quaterrylene l l c ( n = 2) is oxidized at 0.23 V (vs. SCE) and reaches the tetracation state within a potential range of 2.0V, while the isoelectronic biperylenyl 12 is oxidized at 0.77 V (vs. SCE) and its tetracation is formed at 1.69 V, a difference of 0.92V between it and the monocation. In the case of the third isoelectronic system, quater( 1,4-naphthylene) 9 ( n = 2), oxidation begins at 1.3 V (vs. SCE) and the trication state follows within a potential range of 0.41 V.

n = 0-3 11 a-d

12

Taken together, all these results clearly constitute a further rule, which complements the criteria discussed above for conjugated 7r-systems: the planarization of chain-like aromatic 7r-systems via the introduction of additional bonds between the building blocks considerably facilitates the first redox steps of both reduction and oxidation. On the other hand, the Coulombic repulsion between additional excess charges increases drastically, which may lower the overall charge capacity. 9.2.1.4 Oligothiophenes It has been convincingly shown that studies on defined oligomers, which have the obvious advantage of perfect, defect-free structures, improve our understanding of the corresponding polymers. The oligomeric approach for analyzing the properties of the corresponding polymers has been applied extensively to polythiophene.

9.2 Chnrge Storage Mecliunisni qf’ Conjugated Oligomeric Systems

489

To appreciate this strategy, one must keep in mind that the polymer derived from polythiophene, e.g. via electropolymerization, does not consist of infinitely long chains. Using bithiophene as starting material, for instance, polymerization degrees P, of between only 12 and 35 are achieved [38, 42l.This means that the object of interest, the polymer itself, is nothing but a ‘longer’ oligomer. The exact value of PN and the electrochemical and physical properties of the polymer depend on the solvent used in electropolymerization experiments as well as on the concentration of the monomer [43, 441. Early electrochemical studies on unsubstituted oligothiophenes of between 3 and 6 units suffered from the problem that upon oxidation fast follow-up reactions occurred (see Chapter 2.1). Only the oxidized pentamer and hexamer were stable enough to be adequately characterized. A discussion of the reactions of the radical cations is presented in section 9.3.1. In order to investigate the pure redox behavior without interference from chemical side and follow-up reactions, model compounds with substituted terminal rings have been synthesized. Normally, this means that at one or both ends of the oligomer chain the a-carbon atom of an outer thiophene ring is substituted. Even better results are achieved by blocking the N- and the P-positions of the outer rings, which has been realized in the so-called end-capped oligothiophenes 13 and 14 (see Chapter 2.1). Other a,@”-disubstituted oligothiophenes of the general structure type R-T,-R comprise methyl [45, 461 , hexyl [47], methylthio [48] and methoxy [49] groups, but the a,cu’-trimethylsilyl-group [50] and a,a’-dibromo group [52] have also been described in the literature, as have some shorter aryl-substituted oligomers [5 I ] and a tert-butyl substituted heptamer [52]. The series of tetrahydrobenzo[b] a,P,a’P’-blocked oligomers 14 have been successfully employed to suppress follow-up reactions [53]. With methyl groups in one of these positions, one can study the reactivity of single carbon centers by leaving one site ‘unprotected’ [54].

13 14

(n=0-4)

Using the end-capped thiophenes 13 and 14, it is possible to generate radical cations that appear to be stable within the time scale of slow voltammetric experiments (l00m Vs-I) for chain lengths 2 3 . Cyclic voltammetry on smaller oligomers indicates follow-up reactions. However, starting with an oligomer containing 4 thiophene units, even dications could be reversibly generated [56, 581. As can be seen in Table 1 the redox potentials of the different oligomers change in accordance with rules (i) to (v). Although the a,@-substituted radical cations derived from 14 appear to be protected against further chemical reactions, these species are not stable. In contrast to electrochemical data, UV measurements clearly reveal that the radical cations

490

9 Electrochemicrrl Pvopc'rtie.r

Table 1. Redox potentials for the oxidation of end-capped thiophenes 13 and 14. All values are taken from cyclic voltammetry in CH2Cl2/TBAHFP[53] and refer to Fc/Fc+. Compound

E: [VI

E: [VI

13

1.Ola 0.53d 0.38 0.32 0.26 0.22

-

14n=O 14n= 1 14n=2 14n=3 14n=4

I .20" 0.79 0.66 0.55 0.41

(a) irreversible process potential taken at i = 0.8552',.

of substituted thiophene oligomers reversibly dimerize, forming 7r-complexes [48,49,53,55].ESR-spectroscopic studies provide further evidence of reversible spin-coupling processes [55b, 561. Of these. the so called 7r-dimer formation has been discussed most. The reaction enthalpy varies between -40 to -70 kJ(mo1-') and the reaction entropy has a value of about -lOOJ(Kmol)-' [42, 491. The tendency towards 7rdimerization increases with increasing chain length of the oligomers [55b] and with lower temperatures. Simultaneously, the rate constants of all other competing reactions decreases [57]. This means that the formation of 7r-dimers is favored upon charging of long-chain oligomers and polymers. However, it is still unclear whether the 7r-dimerization for long-chain oligomers occur at the level of the radical cation or at a higher oxidation level. Recently, additional aspects of reactivities and redox properties of oligomeric thiophenes were discovered during studies of tailor-made bithiophenes in which two of the a- and P-positions in each thiophene unit were substituted with methyl or methoxy groups [58, 591.It was found that 3,3',5,5'-tetramethyl-2,2'-bithiophene 15 undergoes a reversible dimerization reaction with an equilibrium constant of K = 17100 and a kf value of 33200 M-' s-' (Fig. 4). Simulated and experimental curves for the dimerization reaction are shown in Fig. 4. The simulation parameters were set as described above and fit the experimental data even for different scan rates nicely [58]. The characteristic changes in the voltammograms result from the following processes. At low scan rates (v = 20 m Vs-') the dimerization process linked to the charge transfer achieves almost thermodynamic equilibrium. Therefore, an almost reversible redox reaction can be seen in the voltammogram. As the scan rate increases, the influence of the slow backward reaction decreases, and the radical cation ceases to be regenerated in the experimental time scale. Consistent with this, in the voltammetric response, the signal for the reduction of the radical cation diminishes, and a new wave for the reduction of the dimer appears. Finally, at high scan rates the time scale becomes so short that the radical cation can no longer react within it and in the cyclic voltammogram the waves for the radical cation couple approach those of pure redox processes. The highly negative reaction enthalpy AHo = -60 kJ mol-' for the formation of the dimer and the extremely low rate constant for the cleavage of the dimeric

9.2 Charge Storage Merhrmism of Conjugcited Oligorneric Sj1stem.s

15

49 1

16

OCH, /

CH30

17

18

dication are evidence for a covalent interaction between the two cationic moieties. PM3 calculations support the view that a n-bond is formed between the two charged monomers (Fig. 5). Since then, further examples for the reversible formation of 0-bonded dimers have been discovered, e.g. in the case of the substituted bithiophenes 16 and 17 [59]. It is interesting to note that the methoxy substituted bithiophene 18 does not dimerize but forms a stable radical cation [48e, 591. Obviously, the position of the methoxy group influences the reactivity of the radical cation. While 18 with the two methoxy groups at the inner P-position is stable the bithiophene 17 with its methoxy groups at the outer /3-position dimerizes to form a 0-bonded tetrameric dication. Because the a-positions are blocked with methyl groups no proton elimination can take place. The reason for this behaviour lies in the electronic properties of the methoxy group, which produces in the case of compound 17 a high spin density at the outer a-positions. Additional details of these different properties of 17 and 18 are presented in Section 9.3.1. Although we know a fairly large number of a-bonded ‘dimers’, it is still unclear whether the radical n-coupling will substitute the .ir-merization hypothesis or constitute a second reaction path for conjugated oligomers. In 4,5-methyl-substituted oligothiophenes steric hindrance causes a strong distortion, producing non-planar conformations that subvert the simple correlation between the inverse chain length and typical redox properties, e.g. the first oxidation potential, or spectroscopic data [47,54, 55b, 601. The resulting effects resemble those of the methyl-substituted oligophenylenes 5 . This is clearly demonstrated by voltammetric measurements using methyl-substituted quarterthienyls 19, 20, 21.

492

9 Electrochemical Properties

.

%

.lo. -

160 mV/s

0. 1

I

4

0

0.5

1

0

0.5

1

.

.

0

0.5

1

. I

4

.

.

0.5 1 E (vs.AgIAgCI) I V 0

-Experiment -c-

Simulation

0 0.5 1 E (vs. AgIAgCI) I V Figure 4. Cyclic voltammograms (solid line) and simulations (dots) for the oxidation of 15 (c = 1.1 10 M). Experimental conditions: acetonitrile/O.l M TBAPF6), T = 298 K. Fitting parameters: equilibrium K = l!OSO, forward reaction k f = 33263 1mol-' s-', backward reaction kb = 1.955s-', D = 1 . 10-5cm's-l, E:,, = 0.96V, E!lm = 0.095V [58].

'

As can be seen in Fig. 6, the first oxidation step shifts to more positive potentials with increasing number of methyl substituents in the 4 and 5 positions of the available thiophene units. In the case of the permethylated species 21, where there is strong steric hindrance among the electrophores, even a two-electron transfer is observed where the second redox step is energetically favorable. This finding can be interpreted by assuming that the first electron transfer induces a planarization of the system, which requires a considerable amount of additional energy, while the second step occurs in the now flattened system at the same or a lower potential. -0,127

-0,086

-0,093

-0,09 -0,119

Figure 5. Structure of 15'. dimer according to PM3 calculations and local charge distributions, C C distance between bithiophene units A d = 1.549A.

9.2 Charge Storiige Mec,liiini.sn?of Conjugated Oligomeric Systemc

CH3

CH3

19

CH,

CH3

CH,

493

20

CH,

21 CH3

CH3

CH,

CH3

9.2.1.5 Oligopyrroles

The first successful electropolymerization to form electronically conducting materials, using monomeric pyrrole as starting system, was reported by Diaz et ul. in 1979 [61]. At that time, pyrrole was thought to be one of the most promising candidates in the field of conducting polymers. Subsequently, interest decreased, largely owing to the difficulties of synthesizing substituted pyrroles and defined oligomeric derivatives.

0

1

E (vs. Ag/AgCI)IV Figure 6. Cyclic voltammograms of methyl-substituted quaterthiophenes 19-21, Experimental conditions: Acetonitrile/TEABF4, room temperature.

The mechanism for electropolymerizing pyrrole has been widely discussed [62651. Although there is consensus that precipitation of insoluble oligomers is one of the most important features during the formation process, little is known about these oligomers themselves. Nevertheless, there have been many theoretical attempts to gain information about pyrrole oligomers. Quantum chemical calculations [66-681 predicted bandgap values, correlated spectroscopic properties, and details of the favorable anti-planar conformation. One interesting feature of these theoretical results is that the spin densities in the radical cations of pyrrole oligomers decrease in the tr-position as the chain length, and consequently the delocalization increases. At the same time, the spin density in $position is predicted to increase. This means that so-called misslinking reactions in the P-position of the pyrrole rings become more and more dominant with increasing polymerization [69]. The first synthesis of 2,2’-bipyrrole was reported by Rapoport et a/. in 1962 [70]. It was a long time before the controlled chemical synthesis of the higher members of the family was described. In recent studies, Pugh et a/. [71] have tried to tackle this problem via electro-copolymerization and successive milipore filtering. Very recently, further progress in the synthesis has been reported [72]. Applying a Pdcatalyzed polycondensation reaction, while simultaneously protecting the nitrogen with the BOC (t-butoxy-carbonyl) group, resulted in the formation of a mixture of oligomers with a polymerization degree P,, of between 2 and 36 (see Chapter 3). Many oligomers could be isolated by, e.g. column chromatography. Thus, defined oligomers of type 22 with n = 3,5,7, -20 could be isolated and have been applied to various spectroscopic and electrochemical investigations. It turned out that the mean length of delocalization of a positive charge determined from measurements of the vibrational spectra of the compounds [79] consists of about 7 monomer units. Electrochemical and UVjVIS studies on the compounds [74] provide a detailed picture of the thermodynamic properties and reactivities. As expected, there is a linear correlation of redox potentials and absorption maxima vs. the inverse chain length. Extrapolating from linear plots to infinite chain length, the oligomer consisting of approximately 20 units behaved like the ideal defect-free polypyrrole. The potential values are listed in Table 2 together with the UVjVIS absorption maxima. The reactivity of the oligomers decreases with increasing chain length. While a reversible oxidation of monomeric pyrrole is only possible under fast-scan conditions

22 (R = H, n

3 , 5 , 7 , =20)

23 (R = phenyl, n = 1-4,6)

9.2 Churge Storage Mechanism of' Conjugated Oligomeric Systems

495

Table 2. Redox potentials (vs. Ag/O.l M Ag' = 0.34V vs. SCE) and absorption maxima for a series of unsubstituted pyrroles 22 (H-P-H) [74a]. n

E: [VI

Ezo [VI

~ m a x[nml

1

0.97 0.23 -0.12 -0.28 -0.35

-

208 216 317 367 38 1

2 3 5

I

-

-0.08 -0.25

at scan rates of 18000 V s-' [75a], the dimer can be oxidized completely reversible at -30°C and scan rates of 500-1000Vs-' [74a]. Above the trimer, the follow-up reaction can be suppressed even at room temperature and slow scan rates (>1 Vs-I). The pentapyrrole 22 ( n = 5 ) is the shortest oligomer for which one can observe two reversible one-electron oxidation processes. At higher potentials (Epa= 0.75 V) yet a third oxidation process was found [74a]. Recently, Hapiot et al. [75b] reported that cyclic voltammetry of the oxidation of quaterpyrrole 22 (n = 4) displays two reversible waves even at low scan rates. Janssen rf al. reported on the redox properties of a series of a,a'-phenyl substituted oligopyrroles 23 (n = 1-4) [76]. As usual, the redox properties of this series can be described in terms of a linear relationship of the inverse chain length versus the oxidation potentials (Table 3). Although there was no electrochemical evidence for a 7r-dimerization, the authors discuss this phenomenon on the basis of UVjVIS and ESR-spectroscopic measurements and conclude that similar to the oligothiophene cation radicals 7r-interaction takes place with reaction enthalpies about 50 kJ mol-' . In the case of mixed thiophene/pyrrole oligomers 24 investigated by the same group, the situation is more complex. Because the relative proportion of thiophene increases, the first oxidation potential shifts to positive values, although the number of aromatic units increases [77]. The synthesis and electrochemical characterization of a series of oligo-N-methylpyrroles 25 with chain lengths of n = 1-4 and 6 has been described in the literature [78]. As expected, there is a linear correlation between 1/n and the first oxidation Table 3. Redox properties of some a,iJ-phenyl substituted oligopyrroles 23 (n),potentials vs. SCE, oligomer concentration c = 5-10mM. measured in CH,CL/ TBAPF6 [77].

~~

1 2 3 4

i 0.42 0.16 0.06

-

-

0.33 0.1 -0.04

1.09 0.64 0.43

~

i: irreversible oxidation. Eg = bandgap from absorption spectroscopy in ACN solution. E,, = peak potentials.

-

1 .00

0.57 0.32

3.44 3.05 2.89 2.78

496

9 Electrochemical Proprvties

24 (n = 1-3)

25 (n= 1-6)

potential. The same authors also studied a series of oligomers composed of Nmethyl-pyrrole and aromatic systems, e.g. carbazole [79]. Similarly to the o,o'-blocked oligothiophenes, the cyclic voltammetric response of a,a'-substituted oligopyrroles also changes in dependence of the scan rate [75b]. Thus, at 1 V s-' voltammograms appear as reversible, at higher scan rates the reversibility decreases and reappears for scan rates higher than 2000 V s-' . These observations indicate that oligopyrroles also dimerize. 9.2.1.6 Oligoanilines Electrochemical measurements on polyaniline (PANI) produce a picture of a charge-storage mechanism that differs fundamentally from that obtained using polythiophene (PT) or polypyrrole (PP). In cyclic voltammetric experiments one observes two reversible waves upon oxidation. During the first redox step, the polymer is transformed from the insulating into the conducting state. At potentials in the range of the second peak, the polymer becomes insulating again [80]. Current plateau and overoxidation effects, as with PP and PT, are less pronounced [81-831. The striking differences between the redox behavior of PAN1 and that of PP and PT are largely explained by the influence of the electron-donating nitrogen atoms, which stabilize a considerable part of the positive charge, thereby eliminating most of the Coulombic interaction between the charged centers. Although it is usually assumed that PANI has a chain structure (emeraldine) with head-tail connections between the aniline units [84], the existence of cyclic structures has also been postulated in the literature [85]. It is generally accepted that there are different, possibly coexisting forms of PANI, including benzoid and quinoid rings, free amines (NH), imines (=N), and protonated amines and imines [86]. For a long time, only a few model oligomers have been known [87]. Generally, two different groups of oligomeric model systems have been used. First, oligomers with phenyl rings at both ends of the system, so-called end-capped oligomers, are suitable candidates for analyzing the charge-storage mechanism, because their structure rules out further polymerization. Second, oligomers with a -NH2 end-group can be synthesized to study the chain length dependent reactivities of the oligomers. However, owing to interference from chemical reactions, there are no data on the pure redox properties of the latter class of materials. Different possible model systems are outlined by Structures 26-28. The number of nitrogen atoms per molecule defines the degree of oligomerization. To put the

26 (R = H, n

= 2,4,6,8)

27 (R = CH?, n = 2,4,6) 28 (R = phenyl, n = 2,3.6)

problems investigated with oligomeric model compounds in context, the following section provides a short review of the state of knowledge on the charge-storage mechanism of PANI itself. Several models have been proposed for the charge-storage mechanism in PANI [80, 881. Depending on the pH-value of the solution, different protonated structures are formed. In the simplest case [89], it is assumed in agreement with quantum mechanical calculations that poly-radical cationic states (polaron) are formed in the first step. In the second step, which correlates with the second oxidation peak in the CV diagram, four protons and two-electrons are transferred. This corresponds with MacDiarmids observations [88], which show that the second redox step is strongly pH-dependent. MacDiarmid further improved his redox model by taking into account that pure leucoemeraldine with its amine-N is already protonated at pH values 2 2 , and that the totally oxidized pernigraniline with its less basic imine-N can also be protonated. Therefore, all data of such aniline systems include pH-dependent phenomena. A simple version of the charge storage mechanism of PAN1 is summarized in Scheme 1 . Investigations of aniline oligomers in neutral solution suffered from the fact that, upon charging, deprotonation reactions occur, forming emeraldine bases. The products of these reactions are imines, which react strongly to bases. Thus, irreversible electron-transfer reactions occur during redox charging. Substitution of nitrogen sites with alkyl or phenyl-groups [90, 911 opens the way to overcoming the problem of protonation processes. N-methyl and N-phenyl substituted oligomers have the advantage that deprotonation reactions cannot occur in the emeraldine state. Furthermore, measurements using protonated leucoemeraldine as starting material can be carried out in acidic media. In spite of these findings, many authors interpret the results obtained in acidic media in terms of the nonprotonated, neutral form of the oligomers [92, 931. The first electrochemical data on phenyl end-capped oligoanilines 26 with chain lengths of I I = 2, 3 , 4 and 6 ( n = number of nitrogens in the oligomer) were reported by Honzl et al. [87], who described polarographic half-wave potentials and conductivities of the iodinedoped couples. In cyclic voltammetric experiments in acidic media the phenyl end-capped dimer and tetramer exhibit two reversible electron transfer reactions. According to rule (ii), the first redox peak shifts towards less positive potentials with increasing

498

9 El.ctroclzt,niit.al

Properties

= 12- f

c

0

+

+

I

c 0 P 3 0

c ._ L 3 V

0

Table 4. Redox potentials of phenyl end-capped N-H aniline oligomers 26 obtained via cyclic voltammetry in acidic solution (ACN/TBAPF,!HCIO,). Potential vs. Ag/AgCI reference [91b, 951.

2 4 6

500 500' 640'

800 900' 730''

1 140"

(a) two-electron transfer.

chain length. However, the potential gap between the two sets of redox processes increases from the dimer to the tetramer. Shacklette et al. showed in very thorough studies of the tetramer (ti = 4) [92] and dimer ( n = 2) [93] of 26 in acidic (lo-' M HC104) aqueous solution that the first oxidation step involves a oneelectron transfer for the dimer, while a two-electron transfer was observed for the tetramer, both forming emeraldine salts in which 50% of the nitrogens are oxidized. The second oxidation wave also indicates a one-electron and a two-electron transfer respectively. Its potential varies with pH at a rate of approximately 120mV/pH, which suggests a deprotonation of 2 protons per electron for both species. In this case the resulting species should be the pure imine form. Heinze et al. [9 1 b] observed the same behavior in acetonitrile (ACN)/perchloric acid. Additionally, they observed that under these conditions the end-capped hexamer can be oxidized to its hexacation in three distinct two-electron steps. Wudl, Heeger et al. [94] studied the end-capped N-H octamer 26 ( n = 8) under strongly acidic conditions (1 M HC1). Using cyclic voltammetry, they investigated thin films cast onto I T 0 glass from DMF solution and stated that the octamer exhibits the same features as a PAN1 film prepared under the same conditions. The redox potentials for the phenyl end-capped oligoanilines measured by cyclic voltammetry in acidic solution are listed in Table 4. The electrochemistry of N-methyl substituted oligomers 27 of chain lengths I I = 2, 4 and 6 has been performed by Heinze el a/. [98b]. As outlined before, deprotonation reactions of the emeraldine to its corresponding base are not possible in this case. Cyclic voltammetry in neutral solution indicates two reversible one-electron transfer reactions for the dimer. In contrast to the unsubstituted compound, which could only be investigated in acidic solution, the cyclic voltammogram for the tetramer exhibits two one-electron and one two-electron transfer reactions upon oxidation. Due to the poor solubility of the compound, the interpretation of the cyclic voltammogram for the hexamer is somewhat difficult. In a mixture of ACN and CH2C12, three electron-transfer steps could be observed. The corresponding potentials are listed in Table 5. N-phenyl substituted end-capped systems 28 also demonstrate the influence of the N-substituent. Again, one-electron steps are observed even for the higher homologs. Compared to the methyl-substituted oligomers, the potentials have shifted slightly to positive values, due to the steric hindrance of the N-phenyl rings (Table 6). In the case of the hexamer, a tetracation is formed in four successive one-electron redox steps. The second redox process splits into two one-electron transfer steps.

500

9 Electrochernicnl Properties

Table 5. Redox potentials of phenyl end-capped N-CH, substituted aniline oligomers 27 obtained via cyclic voltammetry in neutral solution (ACN/TBAPF,). Potential vs. Ag/AgCl reference [91b, 951. I1

2 4 6

400 260 100"

900 550 290*

990' 540a

(a) two-electron transfer

Table 6. Redox potentials of phenyl end-capped N-phenyl substituted aniline oligomers 28 obtained via cyclic voltammetry in neutral solution (ACN/TBAPF,). Potential vs. Ag/AgCl reference [91b, 951.

n

2 3 6

480 330 300

1000 690 560

1420 1030

1190

n=3

n=4

fl

P

--0.5

1.0

1.5

E (vs. Ag/AgCI)N

0.0

1.o

E (vs. AgIAgCI)N

0.5

1.o

E (vs. Ag/AgCI)N

Figure 7. Cyclic voltammograms of different PAN1 oligomers. 26-28, (a) experiments under acidic conditions for 26 (ACN/TBAPF6/HClO4).(b) in neutral solution (ACN/TBAPF,) for 27 and (c) 28 (room temperature) [95].

Studies of the redox behavior ofN-H oligomers in neutral butyronilrile/’tetrabutylammoniumhexafluorophosphate reveal similar splitting of the two-electron transfer waves into multiple one-electron transfers [95]. This means that the two-electron transitions only occur in the protonated state, while multiple successive one-electron steps dominate in neutral solutions. Figure 7 shows cyclic voltammograms that correspond to the values listed in the tables above (acidic conditions (ACN:HCI04) for N-H and neutral solvent ACNITBAPF, for N -R).

9.2.2 Solid-state Measurements on ‘Short Chain’ Oligomers U p to now, most voltammetric studies on conjugated oligomers have been carried out in solution. Although they have provided valuable information about the redox properties of the respective polymers. they have not disclosed characteristic solidstate properties of the polymers. Cyclic voltammetric measurements of redox-active layers differ markedly from solution experiments. Ideally, voltammograms of electroactive films should show completely symmetrical and mirror-image cathodic and anodic waves with half-widths of 90 mV and identical peak potentials and current levels. Another typical property is that faradaic current and scan rate are proportional to each other [96]. Electrochemical solid-state studies on typical redox polymers with ‘i\olated’ redoxactive building blocks are in good agreement with these theoretical predictions [97]. By contrast, solid state voltammograms of conducting polymers very often exhibit quite unusual behavior, which involves a strong hysteresis between the cathodic and anodic waves at the start of a charging/discharging cycle, followed by a broad flat current plateau as potential increases [98]. Although there are several models to explain this behavior [99- 1011, the lack of data on monodisperse conjugated oligomers has long prevented a conclusive and unambiguous interpretation of the redox properties of conjugated polymers. At the early stage of the oligomeric approach, all attempts to measure redox properties under solid state conditions failed due to the solubility of short chain oligomers. In 1988 Heinze et 111. took advantage of the poor solubility of longer oligomers and performed electrochemical solid-state studies on oligomers of the p-phenylene series for the first time [102]. Later on, Zotti et cil. published similar experiments using oligothiophenes [42, 1031. In the meantime, further studies on oligophenylenevinylenes and other homo- and heterooligoarylenes [ 104, 1051 have changed the understanding of the redox properties of conjugated polymers. A general problem of such solid-state oligomers with medium chain lengths is that they are insoluble in the neutral state, but to some extent soluble in the ionic state, so that they dissolve from the electrode during the electrochemical measurement. Consequently, most of the investigations have been carried out at low temperatures. Usually, the solid-state behavior of all conjugated short-chain oligomers is very similar, irrespective of whether they are reduced or oxidized. Figure 8 gives typical voltammograms for the solid-state reduction of a series of oligophenylenevinylenes 1. As can be seen, rules (i) to (vi) (see Chap. 9.2.1.1) derived from experiments in

n=3 n=4

n=5

n=6 polymer

-1.0 E (vs. Ag/AgCI)N

-3.0 -2.0

Figure 8. Voltaiiimograms for the solid-state reduction of a series of oligophenylenevinylenes 1 3 6 ) . Experimental conditions: dimethylamine/O. I M TBABr, T = -65'C [ 1051.

(/I =

solution still hold. However, there are additional solid-state characteristics which can be summarized as follows.

(vii) The voltammograms exhibit several sharp waves between which the current drops significantly. Coulometry indicates that every single wave involves the transfer of complete charge equivalents. The charging process in the solid state always starts with the transfer of two-electrons. The two-electron step is followed by further one-electron steps in accordance with rules (i), (ii) and (iii). (viii) All solid-state voltammograms for a charging/discharging cycle of oligomeric systems show strong hysteresis effects, in particular for the first step. Upon discharging, all reversal peaks shift towards lower energies compared to the corresponding original peaks upon charging (that means positively shifted upon n-doping, negatively upon p-doping in solid-state experiments). This is not observed in solution. The hysteresis (i.e. the peak-potential separation between corresponding anodic and cathodic waves) is particularly strong for the first two-electron charging step. It becomes less pronounced with increasing charging level. With increasing chain length in an oligomeric series, the discharging peak potentials appear at lower energies. By analogy to rule (ii), this shift probably reflects the increasing 7r-system. By contrast, the shift of the charging peak potentials cannot be generalized. Whereas solid-state charging is easier for longer oligo-p-phenylenes, charging requires more energy for longer oligo-a-thiophenes and oligo-p-phenylenevinylenes 1 than for short ones.

At first sight, the solid-state redox behavior of well-defined conjugated oligomers and the corresponding polymers differ significantly. Voltammograins of shortchain oligomers exhibit sharp waves with almost no current between them, whereas polymers show a high wave at the onset of doping. followed by a broad current plateau. The behavior of long-chain oligomers (n = 12, 16). e.g. oligo-p-phenylenes or oligo-a-thiophenes, lies in between these two limiting cases, i.e. the voltammograms exhibit current waves that are a little broader than those for short-chain oligomers, superimposed on a plateau that is smaller than in polymers. Obviously, with increasing degree of polymerization, the plateau increases at the expense of the peaks; it is almost undetectable for short-chain oligomers and dominates in polymers. This behavior is observed for n-doping as well as p-doping (Fig. 8). In the past, the current plateau has been a subject of controversy. I t was postulated that capacitive charging processes take place in the potential region of the current plateau [ 1061. However, data of the polymerization of defined oligomers show that the amount of charge involved in the pure chargingjdischarging process in a given potential range remains in the same order of magnitude for all the materials in a series of long-chain oligomers, regardless of the degree of polymerization [ 1041. Slight differences result from the shift of redox potentials during polymerization. The current plateau can, therefore, be unambiguously attributed to Faradaic redox processes. Capacitive effects, by contrast, play only a negligible role. However, the origin of the strong hysteresis has not been satisfactory explained yet. Previously, it was thought that more energy was required for the doping process in the solid state because the molecules were initially in a twisted, benzoid conformation. In this conformation the n--conjugation is not at its maximum. Therefore, the molecules are charged at potentials of higher energy in the solid state than in solution, where free rotation of the monomer units in the molecule is possible [ 102, 1071. In the charged state the molecules stabilize themselves from the originally twisted structure into a more planar quinoid-like one with better sr-conjugation, which, therefore, is discharged at potentials of lower energy. On the other hand, electrochemical solid-state investigations on rigid 7r-systems [ 108-1 101 like fullerenes and oligorylenes 1 I have observed similar phenomena. Hence, intermolecular processes must be taken into consideration. Investigations on end-capped oligothiophenes in solution demonstrate oligomeric equilibria between the monomeric radical cations and sr-dimers [48, 551. Recently, the formation of such n--dimers in the solid state has also been postulated [42, l l l]. It can be assumed that the spatial proximity and favorable geometry of the molecules facilitate such intermolecular interactions in the solid state. This leads to the conclusion that intermolecular stabilization through interactions between 7r-electron clouds of neighboring charged segments takes place in addition to the planarization. Very recent results provide clear evidence for the existence of 0-bonded dimers generated during the charging of oligoenes and oligothiophenes [23, 33. 58, 591. As a consequence, the effect of hysteresis in such conjugated oligomers can be understood as the oxidation of noninteracting chains and the reduction of a thermodynamically stabilized cr-dimer. Applying these results to the properties of conducting polymers opens new perspectives for interpreting charge storage and conductivity. Contrary to the predictions of the bipolaron model, the energetic

stabilization of such systems depends not on lattice relaxation after distortion of the geometry of the chain segment by a polaron or bipolaron, but results from the intermolecular coupling of two n-radical centers to form a a-bond.

9.3 Electropolymerization 9.3.1 Oligomerization in Solution The mechanism of the electropolymerization reaction has frequently been discussed in the literature. In contrast to the established chain-mechanisms found in, for instance, anionic or radical polymerization, the reaction pathway involves a sequence in which each step has to be activated by oxidation of two species [112]. In a first step, the starting monomer is oxidized to its corresponding radical cation, which subsequently undergoes a fast radical-radical coupling reaction to form a dimeric dication. Upon deprotonation, this species regains its aromatic character. The rate determining process in this sequence is the radical-radical coupling step. The dimer itself exhibits an enlarged n-system and, therefore, has a lower oxidation potential compared to the monomer. Consequently, as soon as the dimer is formed it will be oxidized again to its corresponding radical cation, which then ‘dimerizes’ to a tetramer at a lower rate than in the case of the monomer. Quantitative investigations of the kinetics of these a-coupling steps suffered because rate constants were beyond the timescale of voltammetric experiments, until ultramicroelectrodes and improved electrochemical equipment made possible a new transient method called ‘fast scan voltammetry’ [113]. With this technique, cyclic voltammetric experiments up to scan rates of 1 MVs-’ are possible and species with life times in the nanosecond scale can be observed. Using this technique, P. Hapiot rt (11. [114] were the first to obtain data on the lifetimes of the electrogenerated pyrrole radical cation 22 and substituted derivatives 29-32. The authors’ values for the ‘lifetimes’ of the radical cations are somewhat difficult to interpret in terms of rate constants, because the follow-up oxidation is a dimerization and, hence, a second order process. Nevertheless, the voltammograms clearly revealed the pronounced effect of substitution on the kinetics (Fig. 9). However, this investigation brought no insight into the course of the oligomerization reaction after the first coupling step had been terminated. Later on, Heinze et al. [ 1 151 studied the oxidation of a homologous series of methoxythiophenes, including the monomeric 3-methoxythiophene 33 and the isomeric dimers 3,3’-dimethoxy2,2’-bithiophene 34 and 4,4‘dimethoxy-2,2‘-bithiophene 36. 3-Methoxythiophene 33 was found to undergo a very fast coupling reaction, the kinetics of which could not be quantified even at high scan rates. From analogous measurements with other systems it has been concluded that the rate constant for the dimerization ofthe monomeric radical cations is about lo9 M-’ s-’ . Nevertheless, experiments on the formation of polymers using this starting compound produced poor results. On account of the 3-position of the methoxy substituent in the thiophene ring, three

2 +*

+* I

\

N

t

$3

a,

5

% I

I \

ln

a a,

w

ln

._ -L

a 3

0

V 4-

tl ._ LL

rri

506

a.

C.

e.

1.0 1.5

E (vs.AgIAgCI) / V

I

I

1.0 1.5

E (vs.AglAgCI) / V

Figure 9, Fast scan cyclic voltanitnograms for the oxidation of pyrrole 22 and substituted derivatives 29-32, inicroelectrodes with diameters of 5 to 17pm. Scan rates: (a) (22) 18 kVs-l; (b) (22) I . 6 k V s - l ; (c) (29) 2.8kVs-I: (d) (30) 3 . 6 k V s - ' ; (e) (31) 6kVs-I; ( f ) (32) 1.8kVs-'. All measurements performed in ACN/Et,NCIO, at room temperature [49].

isomeric dimers can be formed. The main reaction path can be deduced from the mesomeric forms of the corresponding 3-methoxythiophene radical cation. The two most important mesomeric structures are those with the unpaired electron in the a-position. The structure with the positive charge next to the oxygen of the substituent is preferred, because of the stabilizing +M-effect of the methoxy substituent. Therefore, dimer 34 is the essential product of the radical radical coupling process. Dimers 35 and 36 are minor side products of the dimerization (Scheme 3 ) . The main product initially formed is 34 which will undergo further slow coupling steps. At the given potential, dimer A will immediately be oxidized to the corresponding mono-radical cation. Again the mono-radical cation can be found in different mesomeric structures. The most reasonable notation has the positive charge next to the oxygen of a substituent and the unpaired electron at the blocked inner tr-position (34+ ).

9.3 El~ctropol~merircltiolr

6

R.fi fast-

34

'2

Ro f.;

s

33

-2H'

yw

\

R'

35

R

S

Ra. -

RQQ

/ '2

S

501

2 H'

slow

36

\ I R

R = -0CH3 Scheme 3. Possible reaction pathways for the anodic coupling of 3-rnethoxy-substituted thiophene.

34+

36'

Therefore, the reactivity of this species is low, with a rate constant for the dimerization of lo6 M p l s-' [115].The tetrameric product of the subsequent coupling is even more stable than the dimer, which means that the 'polymerization' process stops at this level, or becomes very slow. Figure 10 shows cyclic voltammograms for the oxidation of 34 sampled at increasing scan rates and, consequently, under conditions of increasing reversibility. The voltammograms show reversible waves for the first and second oxidation of the tetramer at low scan rates, which is evidence that this species has a low reactivity. Under favorable experimental conditions, one further step between the tetramer and the starting monomer or its dimer may lead to a pentameric or hexameric species [116]. In any case, these oligomers are soluble and no deposition process occurs. If 36 is used as starting material, a completely different situation evolves. Voltammograms indicate fast growth of a conducting oligomer film. The kinetics of the coupling reaction are very fast. Even at high scan rates (>lOkVsp') the oxidation of the 'monomer' remains irreversible. The reason for this effect is that the most important mesomeric structure 36' has the unpaired electron at a non-blocked site. The coupling of the radicals is very fast and the products of the coupling steps also have substituents in the outer R-positions of the chain, which, again, makes them very reactive. From these findings, all members of a family of thiophene derivatives with a structure that has the methoxy or other electron donating substituents in the 'head-tail' position are promising candidates for the synthesis of conducting polymers [ I 171. Thus, studies of ter- and quaterthiophenes with methoxy- [118, 1191 or methylthio

100 Vls

I

015 1l.1 E(vs. AglAgCI) I V

-0.1

Figure 10. Cyclic voltammograms for the oxidation of 34 at different scan rates. All measurements have been performed in ACN/O. I M Et4NCIO4 at room temperature.

groups [I201 in the outer p-positions of the oligomers have revealed that even the longer oligomers exhibit good film-forming properties and could be electropolymerized to high-quality polythiophenes. It should be noted that the strong influence of the spin density of the radical cations on the coupling patterns is further evidence for the radical-ion coupling mechanism. Moreover, in the case of oligomers with methyl substituents in the outer a-positions such as 17 anodic oxidation leads to the reversible dimerization of 17 generating a tetrameric dication with a new gbond at the 2,5'-position between both bithiophene moieties (see Fig. 5). Recently, Zotti rt 01. [45] presented kinetic data on the further oligomerization of a series of single end-capped thiophenes with 3-5 units. As expected the rate constants, ranging from 106 M-' s-' for the trimer to 19 M-' s-' for the pentamer, decrease as oligomer length increases. The free energies of activation are high, indicating excellent stabilization of the radical cation for the longer oligomers. In summary, two simple rules can be deduced: (ix) the rate constants for oxidative dimerization reactions decrease with increasing chain length of the radical cation; and (x) the electronic nature and position of the substituents have a pronounced effect on the kinetics and the type of coupling processes. It would be an oversimplification to express the reaction scheme as a simple sequence of radical coupling steps. Parallel reactions such as homogeneous disproportionations (D" + M + M+ + D' ) autocatalytically produced radical cations of the lower oligomers also occur [98]. In cyclic voltammetry, they may be detected through crossing effects in the early stage of electropolymerization experiments. Other possible side reactions are e.g. reversible dimerizations, [%linkage reactions, and, especially for longer oligomers with drastically decreased rate-constants of

9.3 Elrctrc~ppol~nieri-atiori

509

a-coupling, the formation of complexes under spin coupling of the unpaired electrons. For long-chain oligomers and thus for the polymer, the 7r-dimerization or the reversible cr-dimerization are more and more probable since the dimerization rate constant decreases with increasing chain length of the oligomers. Scheme 4 provides a corresponding representation of the oligomerization reaction.

n-merization, reversible

0

.-0 k cu

a,a -coupling

D+'+$= D2' I

D"

+

- ..v

+-,

8

- e-

I

-

A''

spin-spin-coupling n-meeation, reversible

dimehzation

I

oligomer

1

Scheme 4. Reaction scheme for the anodic oligomerization of conducting polymers

510

9 Elt~ctrocliernic~riI Properties

9.3.2 Solid-state Electropolymerization of Oligomers A few years ago, solid-state voltammetric measurements on sexiphenylene layers revealed for the first time that the material ‘polymerizes’ upon p-doping in the solid-state on the electrode [102]. It was shown that sexiphenylene dimerizes at low oxidation potentials, while at high potentials long chains are produced and crosslinking steps become more and more predominant. Since then, further solidstate experiments with monodisperse oligo-thiophenes have confirmed these early results and provided new insight into the general polymerization mechanism of conducting polymers [104]. Thus, applying low-temperature voltammetry to e.g. octathiophene (H-T,-H) allows the reversible generation of trications or even tetracations. This stability disappears when the temperature is raised. If in the case of H-T8-H the switching potential is set in the ascent of the anodic trication wave, two new waves appear and gradually increase, whilst the original signals for redox processes of the starting material decrease (Fig. 11). The resulting isopotential point confirms that H-T8-H reacts to give a new electroactive species without side reactions. The optical absorption of the electrochemically generated product is red-shifted and its cathodic peak potentials upon discharging (reduction) lies negative to those of the educt, indicating that the product consists of larger molecules with a more extended redox system. If experiments are carried

0.0

0.5

1.0

E (vs. Ag/AgCI) / V Figure 11. Solid-state oligomerization of octathiophene, (a) H-T8-H, (b) oligomerization, (c) H-TI6--H; Experimental conditions: CH2CI2/TRAPF6, T = - 5 T , Pt working electrode ( r = 0.5mm) [104].

out at higher sweep rates ( u > lOOmVs-'), broad waves are observed during the cathodic reverse scan at potentials around 0 V. This is typical for the discharging of protons formed during the process. The Coulometric analysis of the voltammograms show that one charge is lost per molecule (by proton cleavage) in the condensation reaction. The average functionality of a monomer-unit, ,f, has been calculated from coulometric data. The resulting values of,f < 2, together with all other observations, give clear evidence that the short-chain H-T8-H dimerizes quantitatively in this solid-state reaction, forming an isomer of sedecimthiophene. Analogous reactions have been observed for other short-chain oligomers such as sexithiophene or sedecimphenylene, leading to dodecathiophene and -phenylene. All these materials can be polymerized further at higher formation potentials. The number of coupling steps strongly depends on the applied potential. At the end of such processes the voltammograms have the typical shape of those of conducting polymers in general, i.e. they exhibit the characteristic current plateau and high current waves at the onset of the charging and the end of discharging. Under these conditions, the average functionality may be larger than the limiting value for infinite chains ( f = 2). From this, however, it must be concluded that chain lengthening steps as well as coupling reactions between the chains take place, leading to a network with an intact .ir-system. Thus, the oligomeric approach clearly shows that the final steps of electropolymerization are typical solid-state reactions in which oligomers with chain length between 6 and 12 after their deposition on a electrode form the 'polymeric' material by radical-radical coupling.

References I. G . J. Hoijtink, Rec. Trav. Chirn. Pays-Bas, 1955, 74, 1525. and Electrochemical Engineering Vol. 7, (Ed. 2. G . J. Hoijtink, Advances in Electrocberni.~tr~ P. Delahay), Wiley-Interscience, New York, 1970. 3. G. J. Hoijtink, J. van Shooteny, E. De Boer, W. Aalbersberg, Rec. Trail. Cbim. Pays-Bas, 1954, 73. 355. 4. G . J. Hoijtink, P. H. van der Meij, Z . Phjx. Chcm. N . F., 1959, 20, 1. 5. E. R. Brown, R. F. Large, Pb)~sicalM ~ t b o d sof'Chemistry,Purr I I A , Techniques qf'C11emistry Vol I (Ed. A. Weissberger, B. W. Rossiter), Wiley, Interscience, New York, 1971, 423. 6. R. N. Adams. Electrochemistry at Solid Electrodes. M. Dekker. New York, 1969. 7. 0. Hammerich, V. D. Parker, J . h i . Cliem. Soc.. 1973, 18. 537. 8. B. S. Jensen, V. D. Parker, J . h i . Chem. Soc,., 1975, Y7, 521 1. 9. K. Deuchert, S. Hiinig. Angew. Clitwi.. 1978, 90, 927-938; Angew. Cbem. I n t . Ed. Engl., 1978, 17. 875. 10. A. F. Diaz. J. Crowley, J. Bargon. G. P. Gardini, F. B. Torrance, J . Electrounal. Chwn., 1981, 121. 355. 11. J. L. Bredas, R. Silbey, D. S. Bourdreaux, R. R. Chance, J . Am. Chem. Soc., 1983, 105, 6555. 12. J. L. Bredas, G. B. Street, Acc. Clienz. Res., 1985, 18, 308. 13. J. L. Bredas. A. J. Heeger, Macromolr,cules, 1990, 23, 1150. 14. A. F. Diaz. J. 1. Castello, J. A. Logan, W.-Y. Lee, J . Ekec~orinal.C h ~ m . 1982, , 129, 115. 15. E. M. Genies, G. Bidan. A. F. Diaz. J . Electroanal. C h m . , 1983, 149, 101. 16. J . Heinze, J. Mortensen. K . Miillen, R. Schenk, J . Clzmi. Soi,.,Chem. Cornnzun., 1987, 701. 17. J. Heinze, Angew. C h e m , 1984, 86, 823: Angew. Chm7. I n t . E d Engl., 1984, 23, 831.

512

9 Electrochemical Propertie.,

18. A. Bohnen, H. J. RHder, K. Miillen, Sjnth. M e t . , 1992, 42, 37. 19. M. Baumgarten, K. Miillen, Top. Curr. Cem., 1992, 169, I . 20. A. Ohlemacher, R. Schenk, H.-P. Weitzel, N. Tyutyulkov, M. Tesseva, K. Miillen. Macromol. Chem., 1992, 193, 81. 21. H. P. Weitzel, A. Bohnen, K. Miillen, Makromol. Chem., 1990, 191, 2815. 22. A. Bohnen, Dissertarion, Mainz, 1992. 23. (a) J. Heinze, P. Tschuncky, A. Smie, J . SolidState Electrochem., 1998,2, in press; (b) A. Smie, Dissertation, Freihurg, 1991. 24. ( a ) H. Shirakawa, E. J. Louis, A. G. McDiarmid, C. K. Chiang, A. F. Heeger, J . Chem. Soc., Chem. Commun., 1977, 578; (b) P. J. Nigrey, A. G. McDiarmid, A. J. Heeger, J . Chem Soc., Chem. Commun., 1979, 594. 25. A. Kiehl, A. Eberhardt, M. Adam, V. Enkelmann, K. Miillen, Angew. Chem., 1992,104, 1623; Angew. Chem. Int. Ed., 1992, 31, 1588. 26. T. Bally, K. Roth, W. Tang, R. R. Schrock, K. Knoll, L. Y. Park, J . A m . Chenz. Soc., 1992, 114, 2440. 27. K. Knoll, R. Schrock, J . A m . Chem. Soc., 1989, I l l , 7989. 28. (a) E. Ehrenfreud, D. Moses, A. J. Heeger, J. Cornil, J. L. Bredas, Chem. Phys. Lett., 1992, 196, 84; (b) A. Jeevarajan, M. Khaled, L. D. Kispert, J . Phys. Chem., 1994, 98, 7777. 29. L. M. Tolbert, Acc. Chem. Res., 1992, 25, 561. 30. C. X. Cui, M. Kertesz, Y. Jiang, J . Phys. Chern., 1990, 94, 5172. 31. G. Broszeit, F. Diepenbrock, 0 . Graf et al., Liehigs Ann., 1997, 2205. 32. R. H. Baughman, J. L. Bredas, R. R. Chance, R. L. Elsenbaumer, L.W. Shacklette, Chem. Rev., 1982, 82, 209. 33. A. Smie, J. Heinze, Angew. Chem., 1997, 109, 315; Angew. Chem. Int. Ed. Engl., 1997,36, 363. 34. (a) G. Burgbacher, H. J. Schafer, D. C. Roe, J . Am. Chem. Soc., 1979,101,7590; (b) J. Heinze, H. J. Schafer, P. Hauser, unpublished results. 35. J. Heinze, M. Storzbach, J. Mortensen, Ber. Bunsenges. Phys. Chem., 1987, 91, 960. 36. K. Meerholz, J. Heinze, J . A m . Chem. Soc., 1989, 111, 2325. 37. A. Bohnen, W. Heitz, K. Miillen, H. J. Rader, R. Schenk, Makromol. Clzem., 1991, 192, 1679. 38. M. Akimoto, Y. Furukawa, I. Harada, Synth. Met., 1986, 15, 353. 39. A. Bohnen, K. H. Koch, W. Liittke, K. Miillen, Angew. Cheni., 1990, 102, 548; Angew. Chem. Int. Ed. Engl., 1990, 29, 525. 40. M. Baumgarten, U . Muller, A. Bohnen, K. Miillen, Angebv. Chem., 1992, 104, 482; Angew. Chem. Int. Ed., 1992. 31, 448. 41. A. Bohnen, K.-H. Koch, W. Liittke, K. Miillen, Angew. Ckem., 1990, 102, 548; Angew. Chem. Int. Ed. Engl., 1990, 29, 525. 42. G. Zotti, G. Schiavon, A. Berlin, G. Pagani, Chem. Muter., 1993, 5, 620. 43. B. Krische, M. Zagorska, Synth. Met., 1989, 33, 257. 44. K. Tanaka, T . Shichri, S. Wang, T. Yamabe, Synrh. Met., 1988, 24, 203. 45. (a) G. Zotti, G. Schiavon, A. Berlin, G. Pagani, Chem. Mater., 1993, 5, 430; (b) G . Zotti, G. Schiavon, A. Berlin, G. Pagani, Chem. Mater., 1993, 5, 620; (c) G. Zotti, G. Schiavon, A. Berlin, G. Pagani, Adv. Mater., 1993, 5, 551. 46. S. Hotta, K. Waragai, J . Phys. Cheni., 1993, 97, 7427. 47. G. Horowitz, A. Yassar, T. H. Bardeleben, Synrh. Met., 1994, 62, 245. 48. (a) M . Hill, K . R. Mann, L. Miller, J.-F. Penneau, J . Am. Chem. Soc., 1992, 114, 2728; (b) M. Hill, J.-F. Penneau, B. Zinger, K. R. Mann, L. Miller, Chem. Muter., 1992, 4 , 1106; (c) B. Zinger, K. R. Mann, M. Hill, L. Miller, Chem. Mater., 1992, 4 , 11 13; (d) L. L. Miller, K. R. Mann, Acc. Chem. Res., 1996, 29, 417; (e) Y. Yu, E. Gunic, B. Zinger, L. L. Miller, J . Am. Chem. Soc., 1996, 118, 1013. 49. (a) P. Hapiot, P. Audebert, K. Monnier, J.-M. Pernaut, P. Garcia, Chem. Muter., 1994, 6, 1549; (b) P. Garcia, J. -M. Pernaut, P. Hapiot et al., J . Phys. Chem., 1993, 97, 513. 50. (a) J. Guay, A . Diaz, R. Wu, J. Tour, L. Dao, Chem. Muter., 1992,4,254;(b) J. Guay, P. Kasai, A. Diaz, R. Wu, J. Tour, L. Dao, Chem. Muter., 1992, 4 , 1097. 51. (a) R. Nakajima, H. Iida, T. Hara, Bull. Chem. Soc. Jpn., 1990, 63, 636; (b) K. Takahashi, T. Suzuki, J . Am. Chem. Soc., 1989, I l l , 5483; (c) K. E. Schulte, A. Kreuzberger, G. Bohn, Chem. Ber., 1964, 97, 3263.

Rrfirences

513

52. (a) W. ten Hoeve, H. Wynberg. E. Havinga, E. Meijer, J . A m . Chem. Soc,., 1991, 113. 5887; (b) E. Havinga, I. Rotte. E. Meijer, W. ten Hoeve, H. Wynberg, S y i t h . Met., 1991, 4/, 473. 53. P. BLuerle, A t h . Muter.. 1992, 4. 102. 54. (a) G. Engelmann. G. Kossmehl, J. Heinze. P. Tschuncky, W. Jugelt, H. Welzel, J , C/7t>m.Soc. Perkin TrLmsact. 2 , in press; (b) G. Engelmann, Dissertation, Berlin, 1996. . 1993. 57, 55. (a) U. Segelbacher, N . Sariciftci, A. Grupp, P. BBuerle, M. Mehring, S ~ t l 7 Met., 4728; (b) P. BLuerle, U. Segelbacher, A. Maier, M. Mehring. J . Am. Chen?. Soc., 1993, 115, 10217; (c) P. Biuerle, U . Segelbacher, K.-U. Gaudl. D. Huttenlocher, M. Mehring, Angei~.. Chem. Int. Ed. Engl., 1993, 32, 76. 56. G. Zotti, G. Schiavon. A. Berlin, G. Pagani, Sywt/7. Met,, 1993, 61, 81. 57. (a) A. F. Diaz, J. Crowley. Y. Bargon, G. P. Gardini, J. B. Torrance. J . A n d . Clzem., 1981, I21, 355; (b) Z. G. Xu, D. Fichou, G. Horrowitz, F. Garnier, J . Electrounul. Cheni.. 1989, 267, 339. 58. J. Heinze. P. Tschuncky, G. Kossmehl. A. Smie, J. Engelmann, J . Electrouncrl. Chem., 1997, 433, 223. 59. J. Heinze, M. Dietrich, H. John, A. Smie, to be published. 60. L. Laguren-Davidson, C. van Pham, H. Zimmer, H. B. Mark, D. J. Ondrus, J . Ekecrrocheni. Soc., 1988, 135, 1406. 61. A. F. Diaz, K . K. Kanazawa, G. P. Gardini, J . Chem. Soc. Cl7em. Commun., 1979, 635. 62. (a) C. K. Baker. J. R. Reynolds, J . Electrounul. Cheni.. 1988. 251. 307; (b) C . K. Baker, J. R. Reynolds, S y t h . Met., 1989, 28, C2 I . 63. (a) B. R. Scharifker, D. J. Fermin, J . Electrounnl. Cheni., 1994,365,35; (b) D. J. Fermin, B. R. Scharifker, J . Ekectroanul. Chem., 1993, 357, 273; (c) B. R. Scharifker, E. Garcia-Pastoriza, W. Marino, J . Elecrrounul. Chem., 1991, 300, 85. 64. D. E. Raymond, D. J. Harrison, J . Elc~ctrouncil.Cham., 1993, 355, 115. 65. R. John, G. G . Wallace, J . Elwtroanul. Chem.. 1991, 3/16. 157. 66. J. L. Bredas, R. Silbey, D. S. Bourdreaux. R. R. Chance, J . A m . Chem. Soc., 1983, 105, 6555. 67. M . Karelson, M. C . Zerner, C/7em. P/7ys. Lett., 1994, 224, 213. 68. M. Kofranek, T. Kovr, A. Karpfen, H. Lischka. 1. Chem. Phys., 1992, Y6, 4464. 69. R. J. Waltman, J. Bargon, Tetruherlron, 1984, 40, 3963. 70. H. Rapoport, N. Castagnoli, J . A m . Chem. Soc., 1962, 84, 2178. 71. S. Pugh, D. Bloor, S p t h . Met., 1989, 28, C187. 72. (a) S. Martina, V. Enkelmann, A. D. Schluter, G. Wegner. Sjvtli. Mut., 1991, 41, 403; (b) S. Martina, V. Enkelmann, G . Wegner, A. D. Schluter, Synth. Met., 1992, 51, 299. 73. G. Zerbi, M . Veronelli, S. Martina, A. D. Schluter, G . Wegner, J . Chem. Phys., 1994, 100,978, 74. (a) G . Zotti, S. Martina, G. Wegner, A. D. Schluter. Adv. Muter., 1992,4, 798; (b) S. Martina, V. Enkelmann, A. D. Schluter, G. Wegner. G . Zotti, G. Zerbi, Synth. Met., 1992, 55, 1096. 75. (a)C. P. Andrieux, P. Audebert, P. Hapiot, J.-M. Saveant, J . A m . Chem. Soc., 1990,112,2439; (b) C . P. Andrieux, P. Hapiot, P. Audebert et ml., Chem. Muter., 1997, 9, 723. 76. J. A. E. H. van Haare, L. Groenendaal, E. E. Havinga, R. A. Janssen, E. W. Mejer, Angen.. Chem., 1996, 108, 696; Angew. Chem. Int. E d , 1996, 35, 638. 77. J. A. E. H. van Haare, L. Groenendaal, H. W. 1. Peerlings et u/., C/iein. A4aicv.. 1995, 7, 1984. 78. N. Rohde, M. Eh. U. GeiOler, M. L. Hallensleben. B. Voigt, M. Voigt, Adv. Muter., 1995, 7, 401. 79. U. Geissler, M. Hallensleben, L. Toppare, Sjvdi. A4et.. 1993, 35, 1662. 80. (a) E. M. Genies, M. Lapkowski, J . Electrocma/. Cheni., 1987, 220, 67; (b) E. M. Genies, M. Lapkowski, in: H. Kuzmany M. Mehring, S. Roth (Eds.) Electronic properties of'conjugutecl po/~w7er3,Springer, Berlin Heidelberg New York, 1987, p.223; (c) E. M. Genies, M. Lapkowski, Synth. M e t . , 1987, 21, 117. 81. A. Kitani, J. Izumi. J. Yano, J. Hiromoto, K. Susaki, Bull. Chem. Soc. Jpn., 1984, 57, 2254. 82. W . 4 . Huang, B. D. Humphrey, A. G. MacDiarmid, J . Chem. Soc., Furucfi, Truns., 1986, 182, 2385. 83. J. Heinze, J. Mortensen, J. Hinkelmann, Synth. Mer.. 1987, 21. 209. 84. F. Wudl, R. 0. Jr. Angus, F. L. Lu et al., J . A m . Chem. Soc., 1987, 109, 3677. 85. L. Dunsch, J . Ekectroanul. Clieni.. 1975, 61, 61; J . prukt. Chern., 317, 409. 86. J. C. Chiang, A. G. MacDiarmid, S p t l i . Met., 1986, 13, 193. 87. J. Honzl. M. Tlustikovi, J . Polym. Sci. Port C, 1968, No. 22, 451.

5 14

9 Electroclirtwir LII Propcrfrr,

88. W. S. Huang, B. D. Humphrey, A.G. MacDiarmid, J . Chem. Soc., F u r u d q Truns. I., 1986,NZ, 2385. l. 1973, 43, 267. 89. M. Breitenbach, K. H. Heckner, J . E l r ~ w ~ u n nChmi., 90. D. S. Boudreaux, R. R. Chance, J. F. Wolf et ul., J . Chem. Phys., 1986, 85, 4585. 91. (a) P. Strohriegel, G. Jesberger, J. Heinze, T. Moll, Mukromol. Chem., 1992, 193, 909; (b) T. Moll, J. Heinze, Synth. Met., 1993, 55, 1521. 92. L. W. Shacklette, F. E. Wolf, A. Could, R. H. Baughman, J . Chrm. Phys., 1988, 88. 3955. 93. J . F. Wolf, C. E. Forbes, L. W. Shacklette, J . Elecrrochem. Soc., 1989, 136, 2887. 94. (a) F.-L. Lu, F. Wudl, M. Nowak, A. J. Heeger, J . Am. Cliern. Soc., 1986, I f M , 8311; (b) F. Wudl, R. 0. Angus, F. L. Lu et ul.. J . Am. Clietn. Soc., 1987, 109, 3617. 95. T. Moll, J. Heinze, unpublished results. 96. A. T. Hubbard, F. C. Anson, J . Electround. Chem., 1970, 4, 129. 97. J . B. Flanagan, S. Margel, A. J. Bard, F. C . Anson, J . A m . Chem. Soc., 1978, 100, 4248. 98. (a) J. Heinze, Top. C'urr. Chem., 1990, 152, I (b) J. Heinze, K. Hinkelmann, M. Dietrich, J. Mortensen, Ber. Blmsenges. Ohys. Chem., 1985, 89, 1225. 99. A. F. Diaz, J. 1. Castillo, J . A. Logan, W. Y. Lee, J . Electroanul. Chem., 1982, 129, 1 15. 100. H.-H. Horhold, M. Helbig, D. Raabe e f a/., Z . Chem., 1987, 27, 126. 101. J . L. Bredas, G . B. Street, Acc. Chem. Rus., 1985, 18, 308. 102. K. Meerholz, J. Heinze, Angew. Clzern. 1nt. Ed. Engl., 1990, 29, 692; A n g e ~ Chrm.. . 1990, 102. 655. 103. G. Zotti, G. Schiavon, A. Berlin, G. Pagani, Adv. Muter.. 1993, 5 , 551. 104. K. Meerholz, J. Heinze, Electrochim. A m , 1996, 41. 1839. 105. K. Meerholz, J. Heinze, Adv. MLitw., 1994, 6, 671. 106. (a) S. W. Feldberg, J . Am. Chem. Soc., 1984, 106.4671; (b) E. Vieil, J . F. Oudard, S. Servagent, Synfh.Met., 1988,28, (2598; (c) J. Tanguy, M. Slama, M. Hoclet, J. L. Baudouin, Synth. Met.. 1989, 28, C145. 107. K. Meerholz, J. Heinze, Syntli. Met., 1991, 41, 2871. 108. T. P. Henning, A. J. Bard, J . Elcctrochern. Soc., 1983, 130, 613. 109. C. Jehoulet, A. J. Bard, F. Wudl, J . A m . Clietn. Soc.. 1981, 113, 5456. 110. K. Meerholz, A. Bohnen, K. Mullen, J. Heinze, unpublished results. 1 1 I . G. Zotti, A. Berlin, G. Pagani, G . Schiavon, S. Zecchin, Adv. Muter., 1994, 6, 231. 112. J . Heinze, Syntli. Met., 1991, 41, 2805. 113. J. Heinze, Angew. C h m . , 1993, 105, 1327; Angew. chem. Inf. Ed. Engl., 1993, 32, 1268. 114. C. P. Andrieux, P. Audebert, P. Hapiot, J.-M. Saveant, J . A m . Clrcwi. Soc., 1990, 112, 2439. I 15. P. Tschuncky, J. Heinze, Syntli. Mcr., 1993, 55, 1603. 116. M. Feldhues, G . Kiimpf, H. Litterer, T. Mecklenburg, P. Wegener, Syirh. Met., 1989, 22, C487. 117. M. Dietrich, J . Heinze, SynfIi. Met., 1991, 41-43, 503. 118. G. Zotti, M. Galazzi, G. Zerbi, S.V. Meille, S y i h . Met., 1995, 73, 217. 119. J. Heinze, M. Dietrich, DECHEMA-Monogruphie, 1990, 121, 125. 120. (a) P. Bauerle, G. Gotz, A. Synowczyk, J. Heinze, Liebigs Ann., 1996, 279; (b) A. Smie, A. Synowczyk, J. Heinze et ul., J . Ekectrounul. Chem., in press.

10 Optical Applications Mark G. Harrison and Richard H. Friend

10.1 Overview In this chapter, a number of optical and opto-electronic device applications are discussed such as light-emitting diodes (LEDs), photovoltaic and photoconductive devices, field-effect optical modulator devices and all-optical modulator devices. Following an introduction to the basic operating principles of each device, progress in the development of each type of device is assessed and the underlying physics of semiconductors is discussed. The field of organic semiconductors, has existed for several decades. Molecular crystals of acenes (Fig. 1a), phthalocyanines, small molecules and metalorganic complexes such as Alq, (Fig. lb) were studied because of their photoconductive [ I , 21 and semiconducting [3, 41 properties and also as an approach to probe the opto-electronic properties of biological membranes. Small organic semiconductor molecules blended in polymer hosts have already found a major application in xerography [ 5 ] . Recently, there has been renewed commercial interest and research activity into organic semiconductors with the development of conducting and semiconducting conjugated polymers. These offer scope for preparing large area conducting films for lightweight conductors, electromagnetic shielding and large area semiconducting films for displays, solar cells and transistor arrays. In this chapter we will not dwell on the more established small molecular organic semiconductors but will focus on oligomers which can be viewed as finite model systems of recognizable conjugated polymers. We discuss oligomers of polythiophene (PT), because of their high field-effect mobilities. Structures of some oligothiophenes are shown in Fig. 2. Poly( p-phenylenevinylene) (PPV) shows highly efficient yellow-green emission and this family of polymers is used extensively in polymer LEDs. We therefore discuss its oligomers, such as stilbene and distyrl benzene (Fig. 3a), which are also highly fluorescent. Thirdly, we study oligomers of poly(p-phenylene) (PPP), a polymer similar to PPV but with a larger semiconductor gap, leading to blue emission. While the parent polymer is rather insoluble, its oligomers, such as p-sexiphenyl H-P6-H (Fig. 3b) and ladder oligophenylenes (Fig. 3c) are rather more amenable to the formation of thin films. Figures 1-3 show many of the oligomers used in optical device applications, which are discussed in this chapter. There are two main reasons for studying oligomers of conjugated polymers: Firstly, oligomers represent model systems for understanding the fundamental electronic properties of the corresponding polymer. Oligomers can be synthesized with a well-defined molecular length, as shown in the structural formula of the extensively studied oligomer, sexithiophene H-T6-H (see Chapter 2.1) (Fig. 2a)

516

10 Opticul Applicutions

Anthracene Tetracene Pentacene

Perylene

a)

Coronene

Figure 1. Structural formulae: (a) The acene oligomers: anthracene, tetracene, pentacene, perylene, coronene; (b) the metal-organic Alq,.

or sexiphenyl H-P6-H (see Chapter 1) (Fig. 3b). Oligomers have therefore been recognized for some time as model systems for theoretical [6] and experimental [7] investigations aimed at extrapolating physical properties of finite oligomers to the corresponding ideal polymer of infinite length. In marked contrast, real conjugated polymers exhibit a distribution of lengths, along which 7r-conjugation is effective. The coherent conjugated segments of the polymer chain are interrupted by defects, which may be of a conformational nature (e.g. twisting of the chain so that it is no longer planar) or of a chemical nature, such as a saturated sp3hybridized carbon atom located somewhere along the chain. Extrapolations of quantitative characteristics from studies of oligomers can therefore also yield estimates of the effective conjugation length in real polymers (see Chapters 6, 7.1 and 7.2). Oligomers are well-defined systems of monodisperse (uniform-length) molecules, with greatly reduced occurrence of defects within the molecular chains, in comparison with polymers. They therefore offer the possibility of better ordering of the molecules and consequently more well-defined optical properties. This renders them particularly appealing for both theoretical and experimental investigations into a number of issues, which cannot be so readily assessed in polymeric systems.

10.1 Overview

517

a)

ClOH21

C)

Figure 2. Structural formulae of (a) a-sexithiophene H-T6-H; (b) end-capped sexithiophene (EC6T); (c) regiorandom &substituted didodecyl sexithiophene; (d) derivative of H-T6-H with bulky triisopropylsilyl endgroups TIPS-T,-TIPS. A

Stilbene

a)

Figure 3. (a) Oligophenylenevinylenes: stilbene and distyrylbenzene; (b) sexiphenyl H-P,-H; ladder-type oligophenylene chains.

(c)

These include the following: (i) dependence of the energies and equilibria of neutral and charged excitations as a function of the coherence (molecular) length. (ii) substitution of oligomers, either with electro-active groups or with the aim of inducing order or disorder; (iii) the r61e of intermolecular processes. When trying to understand the behavior of oligomers, it is helpful to consider concepts employed for conjugated polymers and also those from the more established field of molecular semiconductors and charge-transfer salts, since interchain processes can be more easily observed in thin films of oligomers. Secondly, in some cases, oligomers have already been shown to exhibit characteristics superior to those currently found in many conjugated polymers. In the first part of this chapter, the high field-effect mobilities for thin film transistors employing oligothiophenes are shown to be due to very effective intermolecular charge transport. In this part of the chapter, we focus mainly on electroluminescence. Since the discovery of blue electroluminescence from anthracene [8,9], there has been interest in using short oligomers, particularly to achieve the blue emission [ 10- 171 required for full-color displays. The energies of optical transitions of oligomers often vary linearly with the reciprocal of the oligomer length [6,7], since the length of the molecule confines the spatial extent of many of the charged and neutral excitations of the oligomer. Therefore, in short oligomers the lowest excited state of the singlet exciton is more confined than in long polymers, so higher excitation energies can be achieved, leading to blue emission. We also include discussion of an all-optical spatial light modulator [ 181 prototype, which could have major applications in rapid image-processing.

10.2 Preparation of Thin Film Devices Thin film opto-electronic devices, such as LEDs, modulators and photocells all require the deposition of one or more thin semiconductor layers onto a substrate. Such films are usually of thickness in the range between 50 nm and 10 pm. A perceived advantage of organic semiconductors over traditional, crystalline semiconductor materials is that thin films can be deposited over large areas on a variety of substrates, including glass and flexible substrates [19-211, such as Mylar '' films, without the concerns about crystalline defects and matching of lattice periodicities during film growth. Thin films of organic semiconductors have been deposited onto substrates either from solution or from the vapor phase. The method of choice depends mainly on the molecular weight and solubility of the material. Many small molecular semiconductors and unsubstituted oligomers take the form of rather rigid, planar molecules, which are generally insoluble in many organic solvents. Thin films of these low weight materials are generally achieved by sublimation. Conversely, films of soluble conjugated polymers or soluble partially-conjugated precursor polymers can be deposited from solution by casting and spin-coating.

10.2 Prepmition of Thin Film Devices

519

However, most unsubstituted oligomers lack the advantage of solution-processing which can be achieved with polymers, since they are synthesized as fully conjugated rigid-rod molecules. These show a strong tendency to aggregate in solution, particularly at the high molecular concentrations required for film deposition. Unsubstituted oligomers are therefore usually deposited from the vapor phase, by sublimation under vacuum. In order to transfer the attractive solution-processing properties of polymers to oligomers, the smaller oligomers can be rendered soluble either by suitable chemical modifications, such as addition of alkyl side-chains [22-251 (see Fig. 2b,c) or else they can be blended within a soluble polymer [ 10- 12, 14, 151 or chemically grafted as pendent side-chains on a polymer backbone [16, 171, as shown in Fig. 14, below. The various deposition methods applied to oligomers are discussed below in greater detail.

10.2.1 Sublimation Sublimation is one of the more attractive deposition methods for oligomers. As discussed in Chapter 11, the morphology of films can be controlled by varying the rate of sublimation and also the substrate temperature. Low sublimation rates and high substrate temperatures both result in formation of larger microcrystallites within the film, up to several microns in size. In some cases, this enables the preparation of well-ordered polycrystalline films. This is often an advantage in the operation of the device concerned, for example, for transport of electric charge in field-effect transistors, resulting in relatively high electrical mobilities for FET applications [26], potentially faster switching speeds for electro-optical devices and enhanced chargecarrier separation and lower internal resistance [27] in photo-voltaic applications. Further improvements can be achieved by using low sublimation rates onto substrates coated with a thin layer or an oriented film, which acts as a template for crystalline growth. Such ‘oriented template’ films can be produced either by repeated rubbing of a PTFE rod in one direction along a heated substrate [28] or by rubbing of a thin sublimed oligomer layer [29]. Although alignment of conjugated polymers has been achieved using rubbed PTFE templates, relatively short oligomers are far more amenable to this form of ‘self-assembly’ [30] and may enable fabrication of effectively low-dimensional organic semiconductor devices for electrical transport or polarized light emission [29,31] in a more practical way than can be achieved with stretch-aligned polymer films cast from solution. Although charge transport is important, high intermolecular mobility [32] may not be advantageous for all devices. For example, electroluminescent devices prepared with relatively disordered films tend to show higher electroluminescence quantum efficiencies, perhaps because aggregation and subsequent charge separation between chains (competing non-radiative decay mechanisms) are reduced.

10.2.2 Solution-Processing Much of the commercial interest in conjugated polymers for device applications

is due to the fact that thin semiconducting films can be deposited onto various substrates from solution, often by spin-coating. Conjugated polymers are usually rendered soluble in one of two ways: 10.2.2.1 Substitution with Side-Chains These are generally long flexible alkyl chains, which give rise to entropic stabilization of the polymer chain in solution. Soluble derivatives of oligomers have been synthesized, so that they can be deposited from the solution phase by spin-coating or dip-coating. Flexible side-chains [24, 251 and cycloalkane end caps [33, 341 have been used, as shown in Fig. 2b,c, though solubility is sometimes achieved at the expense of the electrical transport in the films [35]. Cycloalkane end-caps have the additional advantage of inhibiting further polymerization of the oligomers, by blocking the reactive cw-carbons of the outermost thiophene rings. Substitution with side-chains can have its disadvantages if the structural regioregularity of the oligomer or polymer is not carefully controlled by the chemical synthesis. In the solid state, crystallographic studies [36-39] and images obtained by scanning tunneling microscopy (STM) [40] indicate that alkyl-substituted oligothiophenes and polythiophenes (see Chapter 2.1) have a tendency to aggregate or self-assemble in a stacked interlocking comb-like structure as shown in Fig. 4; the aromatic backbones rich in 7r-electron density tends to stack cofacially, while the alkyl side chains align perpendicular to the main chain and are attracted to alkyl substituents on adjacent oligomers lying above or below. In alkyl-substituted oligothiophenes, the a-carbons nearest to the sulfur atom are involved in the bonding of the oligomer backbone, leaving either one of the @carbons (two positions away from the sulfur atom) available for substitution. In Fig. 5a, we show the structural

Figure 4. Schematic view of intermeshed stacks of alkylthiophene chains.

a)

-

Head-to-head

Tall-to-tail

. I . -

4

Head-to-tail

Figure 5. (a) Regiorandom chain, showing head-to-head and tail-to-tail interactions resulting in twisting of the backbone; (b) regioregular alkylthiophene chain, showing exclusively head-to-tail interactions, resulting in a planar backbone.

formula of poly(3-alkylthiophene), in which side-chains are attached to either one of the P-carbons (furthest from the sulfur atom), giving head-to-head, head-to-tail and tail-to-tail interactions between adjacent thiophene rings. Regioregular alkyl-substituted polythiophenes [41, 421 and oligothiophenes [43] have also been synthesized, in which the alkyl chains are always substituted to the same type of @-carbon atom, thus producing predominantly head-to-tail interactions, as shown in Fig. 5b, and therefore reducing steric interactions between the alkyl chains. In those alkyl-substituted oligothiophenes and poly(alkylthiophene)s, where the synthesis is not controlled to yield regioregular substitution, head-to-head and tailto-tail interactions give rise to a twisted backbone, in which the thiophene rings are not coplanar. This results in a reduction of the intramolecular 7r-conjugation along the chain and also a slight increase in the separation between 7r-conjugated chains when stacked in the solid state, leading to reduced intermolecular -ir-stacking. Additionally, regiorandom a-substitution can cause spatial disorder in the wavefunction

522

10 Opticul Applic,ution.y

NaOH

vacuum

220'c

\

Figure 6. The tetrahydrothiophene precursor route to PPV. The bold arrows indicate the final thermal conversion performed after deposition of films of the partially-conjugated precursor polymer.

overlap of aromatic rings involved in cofacial -ir-stacking, which is detrimental to aggregation and intermolecular electrical conduction mediated by 7r-stacking. Regioregularity or lack of it also has implications for the quantum efficiencies of fluorescence since torsional oscillations are considered to be involved in some nonradiative decay channels [44].

10.2.2.2 Using a Soluble Partially-Conjugated Precursor Polymer This is subsequently converted into the fully conjugated material after film deposition, usually by heating under vacuum or in acidic vapor. An example of this, (Fig. 6) is the tetrahydrothiophene precursor route [45] to poly(pphenyleneviny1ene) (PPV); note that the final stage of the reaction (indicated by the bold arrows) is carried out once the thin film of the precursor polymer has been deposited. The precursor route strategy has not been extensively applied to oligomers, although very recently, Miillen and co-workers have developed precursor routes for oligoacenes [46], opening up the possibility of preparing thin films of pentacene and longer oligoacenes by solution-coating methods. Pentacene prepared from such a precursor route has been used in FETs, for which mobilities as high as 10-2 cm2 v-I s- I have been measured [47].

10.2.3 Blends within Polymers Whereas polymers are generally immiscible unless there is a specific advantageous chemical interaction, it is possible to incorporate small molecular semiconductors and oligomers within soluble polymeric hosts [lo-12, 14, 151, such as polystyrene

10.3 Elwtrotzic Escitatiotis

523

and poly(9-vinylcarbazole) (PVK). A further development of this technique is to chemically graft the oligomers as pendent side-chains of a polymer [16, 171. These methods retain the advantages of solution-processing properties of polymers and are discussed later in more detail in the section on LED applications.

10.2.4 Langmuir-Blodget t Technique Although the Langmuir-Blodgett (LB) technique has been used to deposit very thin films consisting of a few monolayers of oriented oligomers [48-501, it is generally considered not to be viable for device manufacture on a large scale. However, strategies of self-assembly used in LB techniques, such as the tendency of saturated hydrocarbon chains and electron-rich 7r-aromatic segments to segregate together separately, can be applied to self-assembly of sublimed films of suitably modified oligomers [35].

10.3 Electronic Excitations In oligomers, neutral excited states, or excitons, can be produced by photo-excitation or charge recombination (capture of electrons and holes in LEDs). These can either decay radiatively, as desired for light-emitting diodes or non-radiatively, with the possibility of yielding mobile charge carriers, for photoconductive and photovoltaic cells. We discuss here some of the physical issues involved in both oligomeric (and generally organic) LEDs and photocells, so that with this background, we can better appreciate the technological strategies for optimizing device performance in the later sections on LEDs and photocells. In the following discussion, we consider first the intramolecular non-radiative decay channels, which can occur for isolated oligomers, then intermolecular nonradiative decay channels, which may also operate in solid state thin films, where the oligomers are densely packed. We also consider the effects of inter-ring torsion and coplanarity of the 7r-conjugated chains, which give rise to both intramolecular and intermolecular effects.

10.3.1 Intra-Molecular Non-Radiative Decay Channels Figure 7 provides a schematic overview of the intramolecular decay processes which include internal conversion, intersystem crossing, and fission of singlet excitons. In the following sections, we give a brief introduction to each of the processes and also refer to predictions made on the basis of recent quantum chemical calculations on the geometries and energies of the singlet and triplet excited states of isolated

524

-

I0 Opticul Applicutions TRIPLET MANIFOLD

SINGLET MANIFOLD P

Internal conversion\

I

electron+hole

I

T2 Internal

Figure 7. Schematic energy level diagram showing singlet and triplet manifolds and intramolecular decay channels (internal conversion, intersystem crossing, singlet fission, etc.).

thiophene oligomers by Beljonne et al. [51] (see Chapter 7.2). Assuming planar molecules, they used Hartree-Fock semi-empirical modified neglect of differential overlap (MNDO) calculations to optimize the geometry, followed by intermediate neglect of differential overlap/multi-reference double-configuration interaction (INDO/MRD-CI) calculations to describe the singlet and triplet excited states. Their calculations indicate that the lowest triplet state (T,) is strongly confined and extends over approximately one thiophene ring, while the lowest singlet excited state (S,) is much more extended, the So ++ S1transition showing a much larger redshift with increasing chain length. The calculated energy (1.57eV) of the So ++ T, transition for terthiophene H-T3-H is in good agreement with experimental values determined by optical absorption in a solvent containing heavy bromine atoms [52] and by energy transfer from C60 [53,54]. Extrapolation to infinite chain length gives a value of the So ++ TI transition of polythiophene as 1.49eV, in very good agreement with the energy of the phosphorescence peak [ 5 5 ] in polythiophene (1.5 eV). Time-resolved photoluminescence (PL) measurements on dilute solutions of oligothiophenes [56-591 indicate that as the length increases, the radiative decay rate does not change significantly, although the quantum yield increases. This indicates a decrease in the non-radiative decay rate as the length of the oligomers increases [56, 57, 591. Referring to the calculations of Beljonne et al. [51], we include a discussion of how the rates of each of the intramolecular non-radiative decay processes, (internal conversion, intersystem crossing and singlet fission) depend on oligomer length.

10.3 Electronic Escitations

525

10.3.1.1 Internal Conversion Internal conversion refers to relaxation from a highly excited state to a state of lower energy, within the same manifold, either singlet or triplet by emission of phonons. This process can be rapid, on the femtosecond timescale. The excess energy is usually lost as phonons. Internal conversion can provide a non-radiative decay channel if the transition from the relaxed state to the ground state is forbidden by the symmetry of the wavefunction of the relaxed state. The 2A, and lB, states have been located in low-temperature n-alkane solutions of a,d-dithienyloligoenes [60] with up to four double bonds between the terminal thiophene rings. In these oligomers, the lowest singlet excited state is the 2A, singlet state, from which radiative transitions to the ground state are symmetry forbidden, but which can be coupled to the ground state by phonon emission. The difference in energy between the 2A, and lB, singlet states appears to increase as the oligomer length increases from a,w-dithienylbutadiene to a,w-dithienyloctatetraene. The question of whether or not the A, state falls below the B, state as the oligomer length increases towards an infinite polyene is still the subject of debate. A single-particle description, such as the Huckel and Hartree-Fock theories, predicts that the 2A, state lies above the lowest optically allowed lB, state [61] and therefore does not inhibit fluorescence of the molecule. Conversely, if many-particle electronelectron interactions are important, the 2A, state would lie below the 1 B, state and fluorescence transition would be forbidden by symmetry considerations. In sexithiophene H-T,-H, the 2A, singlet state has been located by two-photon spectroscopy [56] as being 0.1 eV higher in energy than the lowest (allowed) singlet excited state (1 B,). Therefore, internal conversion to a 2A, state does not represent a non-radiative decay channel for sexithiophene. However, the separation between 2A, and lB, states is less than the vibrational energy of the C=C stretch mode, so some coupling may be possible. On the basis of a l / r i extrapolation of the energies of the 2A, state and 1 B, states of bithiophene H-T2-H and sexithiophene H-T,-H, it had been suggested [62] that the 2A, state would lie below the lB, state for oligothiophenes with more than six rings. More recent photophysical measurements on oligomers with up to seven rings [59] show that this is not the case and estimate the crossover to be nearer nine rings. It has been argued [63] that extrapolations from oligomers of finite length to infinite polyenes based solely on a l / n dependence on the conjugation length, i.e. a 1-dimensional particle-in-a-box model, are unreliable and that for the higher energy excited states, the energy may depend nonlinearly on the reciprocal conjugation length, l / n , ~ = ~ + ~ ( i / n ) + ~ ( i / 2 )

(1)

For short oligomers, the contribution from nonlinear terms could be rather large, so that predictions of convergence or crossover of the A, and B, states based on l/n-type extrapolations from short oligomers should be treated with caution. The theoretical work of Mazumdar et al. [63] and also experimental studies on carotenes [64, 651 suggest that the 2A, state may only be weakly coupled to the 1Bu state and

that a higher A, state is responsible for two-photon absorption. The higher A, state is considered to be above the B, state, although the energy separation these states decreases with increasing conjugation length. 10.3.1.2 Intersystem Crossing

This refers to efficient energy transfer between the singlet and triplet manifolds. Subsequent internal conversion usually leads to the lowest triplet state (TI), which does not fluoresce. The rate of intersystem crossing is governed by the spin-orbit coupling term, which is enhanced by heavy atoms, such as those of the sulfur atoms in oligothiophenes. According to the calculations of Beljonne rt a/. [51], the energy difference between S , and T I is too large t o give efficient singlet-triplet overlap. However, the INDO/ MRD-CI calculations predict that there is one triplet excited state (T4) with energy close to that of the lowest singlet excited state (S,). For bithiophene H-T2-H, T4 lies below S , , so intersystem crossing occurs readily, resulting in a very low fluorescence yield for short chains. In Fig. 8a, we see that as the oligomer length increases, S I falls below T4,so intersystem crossing becomes increasingly unlikely, resulting in higher PL quantum efficiencies for longer oligomers. 10.3.1.3 Singlet Fission Singlet fission describes a process in which two triplet excitons are produced from the fission of one singlet excited state. Such a process has been observed in tetracene, anthracene [66, 671 and polydiacetylene [68]. The excited singlet state may be the lowest excited singlet state (denoted S , ) or in the studies cited above, a higher excited singlet state, produced by singlet-singlet fusion under intense irradiation. In the following discussion, we consider only the possible fission of the S1 state, which may play a r6le in the non-radiative decay of oligomers in light-emitting diodes. If singlet fission is to be an exothermic (energetically allowed) process, the energy of the singlet excited state undergoing fission must be at least twice as large as the energy of the lowest triplet excited state, in this case, E(S, H S , ) 2 2E(So H T I ) . As shown in Fig. Sb, the calculations of Beljonne rt al. indicate that this requirement is satisfied for short oligomers, although for longer oligomers the S1 level falls below twice the T I energy, so singlet fission can no longer contribute to non-radiative decay for long oligomers.

10.3.2 Intermolecular Non-Radiative Decay Channels in Thin Films Photoluminescence efficiencies in solid films of oligomers are generally much lower than those of isolated oligomers in solution [44]. Also, in the solid state, the substituted oligomers show higher photoluminescence yield than the unsubstituted oligomers. Below, we discuss the effects of aggregation and formation of charge-transfer excitons which constitute additional non-radiative decay channels when oligomers are densely packed in the condensed phase.

10.3 Electroiiic Escitutions

527

1In

2T

4.0

Singlet fission requires no thermal activation

2.0 -

-.-.-----T-------

---T-~---------.---------=

1.0 -

0.0 " " " " " " " " " " " " " 0.15 0.20 0.25 0.30 0.35 0.40

" "

0.45

0.50

0.55

lln

Figure 8. A comparison of the calculated energies of the (SOPS,)and (SO--T4)transitions of oligothiophenes, as a function of the number of rings, n . The rate of intersystem crossing is reduced for the longer oligomers. (b) A comparison of the calculated energies of the (SO-Sl) and (SOPTI)transitions of oligothiophenes, as a function of the number of rings. 11. The probability of singlet fission is reduced for longer oligomers. Adapted from Beljonne er ul. [51].

10.3.2.1 Aggregation and Davydov Splitting In polycrystalline thin solid films, the close packing of the oligomers often gives rise to aggregates, in which the exciton wavefunction is delocalized over two or more adjacent oligomers in close proximity, leading to a broad emission spectrum, shifted to lower energies than the fluorescence spectrum of the isolated oligomers, often accompanied by a reduction in the PL quantum efficiency. Many conjugated oligomers crystallize with a herringbone structure [69-731, in which there are two translationally inequivalent molecules per unit cell, as depicted in Fig. 9a. The coupling of the dipole moments between such molecules gives rise to Davydov splitting; in the crystal, the excited states of the single molecule are split into pairs of levels. The optical transition between the ground state and the lower

528

I 0 Optical Applicritions

Isolated

Crystal

Oligomer

Figure 9. (a) Herringbone arrangement of oligomers found within close-packed layers; (b) the Davydov splitting which results from having two translationally inequivalent molecules per unit cell.

crystalline excited level is completely forbidden, as indicated in Fig. 9b. The magnitude of the Davydov splitting decreases as r p 3 , where r is the separation between molecular centres. The dipole selection rules apply for an ideally infinite single crystal and are relaxed when the crystallite size is reduced or when introducing disorder. 10.3.2.2 Charge-Transfer Excitons In addition to intramolecular Frenkel excitons which are confined to one oligomer unit and give rise to the photoluminescence of isolated oligomers in dilute solution, charge-transfer excitons can also be generated by photo-excitation of oligomers in the condensed phase. Charge-transfer excitons consist of a weakly bound electron-hole pair, which is localized over two or more adjacent oligomers. Chargetransfer excitons are not readily observed in the linear absorption spectrum because there is usually poor overlap between the wavefunctions of the ground state and excited state, since the ground state is localized on a single oligomer, while the excited state is distributed over two or more adjacent oligomers. However, they can be detected in electroabsorption, since the excited state has a large dipole moment and results in an electroabsorption signal which follows the second derivative of the absorption spectrum. The energy separation of intramolecular Frenkel

10.3 Electronic E.ycitrition.s

529

excitons and charge-transfer excitons depends on the first ionization energy, the electron affinity and the intermolecular distance. As the length of an oligomer increases, so does the spatial extent of its delocalized electronic n-system, leading to stronger T-n van der Waals forces and lower intermolecular distances, as well as lower first ionization potentials and larger electron affinities. Therefore, as the length of an oligomer increases, all of these factors tend to reduce the energy separation of the intermolecular charge-transfer excitons and intramolecular Frenkel excitons and open up a competing non-radiative decay channel, lowering quantum yields for fluorescence in the solid state, while increasing quantum efficiencies for photoconductivity. However, charge-transfer excitons are only an intermediate species to photocarrier generation. If the weakly bound electron-hole pair can recombine sufficiently rapidly, they can form the intramolecular singlet exciton before charge separation takes place. For thin films of sexithiophene H-T6-H crystallographic studies have determined that the oligomers are arranged in a stacked layer structure, in which conductivity parallel to the stacks (perpendicular to the long oligomer axis) is much greater than that between layers [35]. Dippel et 01. [74] therefore explained the photoconductivity as resulting from rapid charge separation of longitudinal charge-transfer excitons which extend across stacks. Therefore, close packing of oligomers within the layers should favor efficient charge separation and photoconductivity rather than fluorescence. In order to favor fluorescence, efficient charge transport can be suppressed by reducing the degree of order within the film, either by using polycrystalline films with small crystallite size or by increasing the separation of oligomers by the addition of bulky substituent groups.

10.3.3 Effects of Inter-Ring Torsion and Coplanarity of Oligomers The issue of planarity and inter-ring torsion is relevant to oligomers in which the rings are linked by carbon-carbon single bonds, such as the oligothiophenes and oligo( p-pheny1ene)s. It is especially relevant to oligomers substituted with alkyl side-chains for solubility and also to rigid bridged ladder-type oligophenylenes synthesized for blue electroluminescence. It is perhaps worth emphasizing from the beginning, that the trends observed in dilute solution are in marked contrast with those i n the solid state, relevant to opto-electronic device applications. We therefore decided to discuss this topic after consideration of purely intramolecular and intermolecular decay mechanisms. While planarity may favor higher PL efficiencies in isolated molecules or dilute, welldispersed blends, planarity also favors aggregation and hence lower PL efficiencies in the solid state.

10.3.3.1 Solution Non-radiative decay channels are influenced by low frequency inter-ring torsional oscillations of the oligomer backbone. Berlman [44] noted that rigidity in the first

530

I0 Optical Applications

excited state was important for fluorescence. Nijegorodov et al. [75] have studied photophysics of dilute solutions of several series of oligomers (e.g. biphenyl, terphenyl) and have observed a systematic decrease in the intersystem crossing rate which decreases rapidly as the rigidity and planarity increase and conclude that high PL efficiencies can be achieved in solution if the ground state is nonplanar and of low symmetry, while the excited state should be approximately planar and of higher symmetry, as is the case for many oligothiophenes and oligophenylenes upon formation of the quinoid geometry in the singlet excite state. Becker e f a/. [59] have suggested that torsional oscillations may give rise to highly efficient non-radiative decay from the lowest triplet excited state to the ground state. In dilute solution or the gas phase, the ground state of oligothiophenes is often more twisted than the relaxed singlet excited state. This is particularly true at higher temperatures [76] and thermochromism and solvatochromism in poly(alky1thiophene)s is well known. At low temperatures, a red-shift of the absorption spectrum is observed [58, 591, while the fluorescence spectrum remains essentially unchanged. This indicates that at low temperatures, the ground state is more planar. Photoluminescence efficiencies in solution are generally much higher for planar molecules because the geometries in the ground and excited states are similarly coplanar and d o not allow torsional oscillations to couple so effectively [44]. Xu and Holdcroft [77] reported that in the case of polythiophenes substituted with alkyl side-chains, head-to-head and tail-to-tail interactions (see Fig. 5a) result in increased twisting of the ring and lower PL efficiencies in solution, while regioregular poly(alky1thiophene)s which have predominantly head-to-tail interactions exhibit higher PL efficiencies in solution. Time-resolved fluorescence studies [58,59,78] of dilute solutions of alkyl-substituted oligomers up to seven thiophene rings in length show that as the length of the oligomer increases, the extent of the T conjugation also increases and is accompanied by increased PL quantum yields, due primarily to a rapid decrease in the non-radiative decay rate and a decrease in the yield of triplets [59].

10.3.3.2 Solid State However, in the solid state, quantum yields of fluorescence are generally lower than in solution by one or two orders of magnitude. This has been attributed to quenching of fluorescence by aggregation effects and is more pronounced for planar molecules [44]. Moreover, in the solid state, the twisted alkylthiophene chains tend to show higher PL efficiencies. X-ray diffraction studies [37] of regiorandom alkylthiophenes (500/0 head-to-tail) show very little long-range crystalline order, while films of regioregular alkylthiophenes with 80% head-to-tail content are semicrysLalline with cofacial packing of the more planar aromatic chains, stacked with 3.8 A separation between .ir-conjugated backbones. Although the chains of regiorandom poly(hexy1thiophene) (with 50% head-to-tail interactions) are insufficiently planar for aggregation effects to be particularly strong, the decreased PL efficiency in the solid state is attributed to limited excimer formation in the excited state, which is usually more planar than the ground state.

10.3.4 Concluding Remarks We have discussed a number of the many non-radiative decay mechanisms considered to be active in oligomers. In the solid state environment which applies to opto-electronic devices, the interoligomer separation is reduced and the torsion of the oligomer backbone is also reduced, compared to isolated oligomers in dilute solution or the gas phase. Both of these factors favor aggregation of oligomers. This generally results in lower efficiencies for electroluminescence (EL) and improved charge transport and charge-separation efficiencies. Trends in solution are sometimes reversed in the solid state and there are clearly many factors and compromises which must be borne in mind when formulating strategies for synthesis of new oligomers optimized for both ease of fabrication (film deposition) and efficient operation in devices. However, as a general rule of thumb, it may be advantageous to employ relatively disordered films within EL devices, while better ordered films are more suitable for FETs and photovoltaic applications.

10.4 Electroluminescent Devices In electroluminescent devices, the semiconductor layer is sandwiched between two electrodes, as shown in Fig. 10a. One electrode, such as gold (Au) or indium tin

0

Anode with high worMunction (e g ITO. Au)

:a I

IT0

. . Injection of charges, holes from the cathode, electrons from the anode

. .

(3) .,

Electron-hole capture t o form excitons, both singlet and triplet

Radiative recombination of singlet excitons:

S,

f4) , , No fluorescence from triplet excitons

+,so hw +

Figure 10. (a) Schematic structure of a single-layer organic LED; (b) operation of a single-layer organic LED.

532

10 Opticd Applications Emissive layer Electrontransport

PPV

PBD

<-

in PMMA

Figure 11. Schematic energy level diagram of a double-layer polymer LED in which the thin PBD blend layer transports electrons. while blocking holes, thus enhancing radiative recombination in the PPV layer. Adapted from Brown c’t ul. [79].

oxide (ITO), is chosen to have a high work function, for injection of positive charges (holes). The other electrode, often aluminum (Al), calcium (Ca) or magnesium (Mg), is chosen to have a low work function for injection of negatively charged electrons. The operation of a typical single-layer organic electroluminescent device is summarized in Fig. lob. Once the charges are injected, electrons and holes combine to form excitons, in which the spins may either be opposed (singlet) or aligned (triplet). Electroluminescence is due to radiative decay of singlet excitons, from the first excited electronic state ( S , ) to the ground state (So). However, as discussed in the previous section, several non-radiative mechanisms also exist for de-excitation of singlet excitons, which have the effect of reducing the quantum efficiency for electroluminescence (EL) and photoluminescence (PL), by competing against the radiative decay of singlet excitons. Additional semiconductor layers (charge-transport layers) may be included between the emissive layer and the electrode to facilitate transport of charges of one polarity, while impeding charges of the opposite polarity, thereby encouraging radiative recombination within the emissive layer, as shown in Fig. 11, for a polymer heterojunction device [79].

10.4.1 Historical Survey of Oligomeric LEDs 10.4.1.1 LEDs Based on Molecular Semiconductors In the mid- 1950s, electroluminescence was reported from cellulose films doped with organic dye molecules [80, 811. Ten years later, the first report of electroluminescence from organic semiconductors was the observation of blue emission from

molecular crystals of anthracene. Pope er id. [8] used crystals 10-20 pin thick, prepared by sublimation and from solution. As electrodes, they used either silver paste electrodes of unequal area on opposite sides of the crystal or else liquid electrodes with aqueous sodium chloride solution as electrolyte. Large voltages, typically 400V were required and the solid state devices could be operated under either DC or AC voltage, while the LEDs with solution electrodes emitted light only under square-wave AC operation. during the parts of the square-wave cycle where the voltage was changing most rapidly. Helfrich et a/. [9] used glass tubes filled with liquid electrodes, which were cemented to either side of a single crystal, typically 5 m m thick. Large operating voltages, of the order of several kV were required, though the applied electric fields were in fact comparable with those currently employed in polymer LEDs (-10' V n - ' ) . However, the large drive voltages and use of liquid electrodes (usually solutions of anthracene cations and anions) made the devices commercially unattractive. One motivation for investigating oligomers and small molecules of relatively short conjugation lengths and hence high r-r*-energy gaps is to achieve blue electroluminescence. particularly since there are few blue phosphors to be found amongst inorganic semiconductors, unless multiple-quantutn-well structures are employed. For a review of the early work on organic electroluminescence, see also the review by Kalinowski [82]. By using evaporated thin films rather than single crystals of anthracene and perylene, Vincett et al. [83] reported drive voltages as low as 12 V, using solid state device structures in which the film is sublimcd onto an oxidized aluminum electrode, followed by a semi-transparent gold electrode. External quantum efficiencies in the range 0.03-0.06%1 were calculated for sublimed film anthracene devices. Since then, significant progress has been achieved, first by researchers at Kodak laboratories [84, 851, then in Japan [86--891, using small molecules, such as Alq, and oxadiazoles, both as charge-transporting layers and as emissive layers, either blended within a polymeric host or deposited as sublimed films. Tang er a/. [84] reported external quantum efficiencies up to 1 % for a bi-layer LED of the structure: GlasslITOi'Diamine,'Alq,/Mg:Ag, with luminous efficiencies of 1.5 lumen/Watt and brightness exceeding 1000cd inp2 at operating voltages less than IOV. The aromatic diamine layer served a s a hole-transporting layer, while electroluminescence originated from the Alq3 layer. Improved quantum efficiencies (2.5% photonslelectrons) were reported [85]for a multilayer device of the Alq3/Alq3/Mg:Ag.The amorphous aromatic form Glass/ITO/dianiine!'Alq~~doped diamine layer served to transport holes, while the undoped Alq, layers transported electrons. The doped Alq, layer contained fluorescent dye molecules, such as DCM and coumarin, allowing energy transfer from the excited Alq, host molecules to the fluorescent dye guest molecules. It was determined that a narrow recoombination zone was located at the diamine/Alq, interface, extending up to 50A into the Alq, layer. In undoped Alq,. a broad emis$on zone was observed, due to difyusion of excitons on a lengthscale of around 200 A. In the LEDs fabricated with the doped Alq, layer close to the interface, eficient energy transfer to the fluorescent dye guest molecules resulted in a much narrower emission zone.

Adachi et al. (871 also reported a three-layer LED of the structure Au/TPD/ Acene/PV/Mg, where T P D is an amorphous aromatic diamine for hole transport, PV is an amorphous layer of a perylene tetracarboxylic acid derivative and the polycrystalline acene layer consisted of either anthracene, perylene and coronene. Much higher efficiencies (up to 0.04% photons/electron external quantum efficiency, 0.001 YOWjW power conversion efficiencies) were obtained [86] by using a similar structure but a 12-phthalo perinone derivative rather than the acenes. By using a bi-layer device structure using an oxadiazole (PBD) as an electron-transporting layer, Adachi et a / . [88] obtained electroluminescence from a hole-transporting material, in this case a triphenylamine derivative, NSD. Green emission was obtained, with luminescence efficiencies of 0.2 lumen/Watt and a turn-on voltage as low as 4.0V. Intensities up to IOOOcdm-’ were reported for drive voltages of 16V and current densities of 100mAcmp’. Adachi et al. [89] also fabricated a triple-layer device of the structure ITO/TPD/ NSD/PBD/Mg:Ag, which showed luminance efficiencies as high as for the previous ITO/NSD/PBD/Mg:Ag bi-layer device even when the thickness of the emitter layer (NSD) was as low as 50 A. This clearly demonstrated confinement of excitons within the emitter layer, by insertion between two materials of larger X-T* energy gaps which allowed only unipolar charge transport into the emissive layer. Adachi rt 01. also emphasized the important role played by the charge-transport layers in providing a barrier layer between the emissive layer and the metallic electrodes, at which excitons are considered to be effectively quenched [90, 911. 10.4.1.2 LEDs Based on Oligothiophenes

As a result of the high field-effect mobilities reported for sexithiophene, oligothiophenes are probably one of the most intensively investigated families of oligomers. Recently, LEDs have been fabricated with oligothiophenes and oligophenylenes, as single-layer devices and in bilayer and multilayer devices (see Chapter 2.1). However, electroluminescence efficiencies generally remain much lower than those of polymeric devices. The first reported single-layer thin-film oligothiophene LEDs were fabricated by Geiger et (11. [34] using spin-coated films of cycloalkane end-capped oligothiophenes [34] (ECnT), as shown in Fig. 2b, in a structure ITO/ECnT/AI, where iz is the number of tr-conjugated thiophene rings per oligomer. The efficiency of these LEDs was low (10p2-10-’o/~)and decreased for longer chain lengths. EC5T gave devices with the highest efficiencies and this appears to be correlated with the presence of smaller microcrystals within the film and hence increased disorder/ reduced aggregation effects. Uchiyama ef ul. [92] later reported electroluminescence from single layer devices with highly crystalline films of o,w-dimethylsexithiophene Me-T6-Me deposited under ultra-high vacuum in the structure ITO/Me-T6-Me/AI. Although high rectification ratios were obtained (1500 at 5 l O V ) and onset voltages for EL were low (4V), very low quantum efficiencies ( - 3 x lO-’%) were obtained, presumably as a result of the high crystallinity resulting in the transition from the lowest excited state to the ground state being dipole-forbidden in the crystal.

Uchiyama et a/. [93] have reported an increase of the EL efficiency by a factor of up to 1000 by fabricating a bilayer device in which a shorter oligomer of higher energy gap is inserted between the T6 oligomer and the aluminum electrode in order to block holes and enhance recombination at the heterojunction, following the strategy of Adachi et a/. [88] This strategy has also been usedoby Muccini et @. [94] in a multilayer oligomeric LED of the structure IT0/400A H-T4-H/500A H-T6-H/100A H-T4-H/AI. Horowitz et al. have prepared bilayer LEDs [95] using unsubstituted sexithiophene H-T6-H and substituted derivatives, either substituted with two decyl side-chains Dec-T6-Dec as shown in Fig. 2c or with triisopropylsilyl end-groups TIPS-T6-TIPS as in Fig. 2d. Compared to single-layer ITO/H-T6-H/AI devices, the quantum yield of bilayer LEDs can be increased by three to four orders of magnitude, from 4 x lop6% for ITO/ H-T6-H/AI to 2 x lo-'% for the bilayer device ITO/H-T6-H/Dec-T6-Dec/Al and 1.5 x lop'% for the bilayer device ITO/ H-T6-H/TIPS-T6-TIPS/Al. There are two main effects which contribute to this improvement: Disorder and increased separation between oligomers Substitution with bulky tri-isopropyl end-groups is considered to increase separation of chains, as evidenced by X-ray diffraction [96]. This disorder and increased separation applies both longitudinally and laterally, and thus reduces the occurrence of charge-transfer exciton formation (considered to provide a non-radiative decay channel involved in photoconduction) and also lateral aggregation of chains ( T stacking), which can result in Davydov splitting, so that in the perfect crystal, the optical transition from the lowest lying excited state to the ground state is dipoleforbidden. The bulky side-groups therefore render the fluorescence partially allowed. More efficient electron injection The conductivity of films of the randomly &substituted oligomers is usually lower than that of unsubstituted H-T6-H [35]. Many conjugated polymers and oligomers are p-type materials, in which the holes have a much higher mobility than the electrons. Also, since the electron injection barrier from cathode to the LUMO level is usually much greater than the hole injection barrier from I T 0 to the HOMO level, EL quantum efficiency is usually limited by electron-injection from the cathode. This appears also to be the case for H-T6-H and its derivatives; a device of the shows increased efficiency compared to a form ITO/H-T6-H/TIP-6-TIPs/Al single-layer device, while exchanging the order of the oligomer films (i.e. constructing rTo/TIPs-T6-TIPs/H-T6-H/AI) reduces the efficiency. An explanation for the higher efficiency in the ITo/H-T6-H/TrPs/H-T6-H/A1 device is that the voltage is not dropped uniformly across both layers but that the field is much higher in the more resistive substituted DPS6T layer, thus lowering the tunneling barrier for injection of electrons, while holes are still transported relatively easily through both layers, as shown schematically in Fig. 12. Oligomers have also found applications as charge-transport and charge-blocking layers. Hosokawa et a/. [97] have fabricated multilayer oligomeric LEDs of the

536

I 0 Optical A pplico t ions

>

IT(

ITC H-T,-H

TIPS-T6 -TIPS

TI PS-T,j -TIPS H-T6-H

Higher EL efficiency due to narrower tunnelling barrier for electrons

Figure 12. Schematic diagram showing enhanced clectron injection (reduced width of barrier) in double layer devices when the highly resistive TIPS-T6-TIPS layer is inserted between the H-T6-H layer and cathode. Adapted from Horowitz 6" ~ J I [Y5]. .

structure ITO/H-T6- H/H-P,-H/Alq/Mg:Ag, where the emissive layer is Alq. TBS, a sexiphenyl H-P6--H derivative is used for electron confinement in the Alq emissive layer. H-T6-H is used as a hole-transporting layer, rather than triphenylenediamine derivatives previously used in bright organic LEDs. Bright green emission of 2300 cd mp2 was obtained at a voltage of 13 V and current density of 313mAcm-'. 10.4.1.3 LEDs Based on Oligomers Blended with Polymer Matrices

Electroluminescent devices have also been realized in hybrid polymer/oligomer systems, using blends of oligomers as dye molecules within soluble polymeric hosts. This offers several advantages, including solution-processing of polymer blends to form high quality thin films, without the need for subsequent thermal processing. Small oligomers are ideally suited for achieving emission in the blue region of the visible spectrum, although it is very important to avoid aggregation or re-crystallization of the included oligomers, which tends to shift the emission color to lower energy, as well as reducing the EL efficiency and ultimately leading to failure of the devices. The choice of host polymer is also critical to the successful operation of the EL diode. Frederiksen et al. [ 101 successfully blended oligomers of PPV and derivatives of bisanthracene with polystyrene. Single-layer devices were prepared, in which the polymer blend, containing 20% by weight of oligomeric dye molecules was spincoated on an etched ITO/glass substrate, upon which was subsequently evaporated calcium cathodes for electron-injection, capped by an aluminum layer for protection against oxidation or corrosion. From the trimer derivatives of PPV, blue-green

10.4 Eli~c,trolumiiiesceritDevic,e.s

537

emission was obtained, peaked in the range 460 -490 nm. The EL spectrum is similar to the PL spectrum, while the PL excitation spectrum is similar to the absorption of the oligomeric dye molecules. This indicates that the EL is originating from the oligomers. The absorption spectrum of the film of polymer blend, loaded to 20% by weight with oligomers matches the absorption spectrum of the oligomers in dilute solution, indicating that the oligomers are well dispersed and that no strongly bound aggregates are formed. These diodes are rectifying, showing appreciable current when the diode is forward biased (calcium cathode negative with respect to positive I T 0 anode) and the onset of EL occurs around 30 V forward bias. The diodes can be operated in air at a current of 50 mA, with a half-life of 90 min and stored for weeks under dry argon in their off state. The drive voltage was found to be proportional to the film thickness, with a critical field for EL of 1.7 x 10*VmP', indicating that the diodes function as tunnel diodes rather than Schottky diodes. In the absence of oligomer dye molecules, the polymeric films are insulating. The diodes are rectifying for doping greater than 10% by weight with oligomers. A steep rise in current is observed for loading greater than 15%, corresponding to the formatioon of percolation pathways and hopping between oligomers separated by around 8 A. The choice of polymeric host is also very important. Although poly(methylmethacry1ate) (PMMA) and poly(viny1 chloride) (PVC) will host dye molecules and form high quality films, these films show negligible electroluminescence. Frederiksen et a/. attribute the improved EL performance of the polystyrene host to its aromatic nature and relatively high -ir-density, which saturated host polymers such as PMMA lack; aromatic host polymers with high 7r-electron density present a lower barrier to tunneling between oligomers. Blue electroluminescence has also been reported [ 151 for a single-layer device using as an emissive layer spin-coated films distyrlbenzene (see Fig. 3) (the trimer of PPV) blended within a PMMA matrix, sandwiched between an I T 0 anode and a strontium/indium cathode. The short conjugation length resulted in blue-violet emission in the range 465nm. However, the quantum efficiencies were low (around 0.03%) and the devices showed poor stability (a 24-hour half-life of quantum efficiency when operated in air). Tachelet e f al. [12] have used alkoxy-substituted distyrylbenzenes blended within a polystyrene matrix to achieve blue electroluminescence, peaked at 450 nm (see Fig. 13), with a relatively high internal quantum efficiency of around 1%. The choice of host polymer is very important. The authors had also tried using polycarbonate as a host matrix, although the efficiency dropped by a factor of ten. The poor efficiency in polycarbonate is attributed to the carbonyl groups, which act as quenching centres for singlet excitons. Also, at high concentrations of the oligophenylenevinylenes within polycarbonate, phase separation occurred, forming aggregated clusters of oligomers and resulting in a ten-fold decrease of the lifetime of excited singlet excitons. Kido et a/. [14] have used oligomeric dye molecules within a polymeric host to achieve a single-layer highly intense organic LED emitting white light, which may be suitable for backlighting of LCD displays. Kido et a/. emphasize that the key to obtaining white emission is to achieve successful bright blue emission and

1.2 n

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+J .-

1.o

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700

750

Wavelength (nm) Figure 13. Electroluminescence spectrum obtained from a distyrylhenzene derivative dispersed within a polymer blend. Adapted from Tachelet c't ctl. [12].

avoid aggregation and exciplex formation. For this reason, they selected poly(viny1carbazole) (PVK) as the host, which serves as a hole-transporting polymer and emits in the blue-purple region. Electron-transporting materials, such as PBD with high energy gaps were selected, while further fluorescent dye molecules, such as TPB (a phenyl-substituted butadiene oligomer) were used as emitting centres. Films consisting of 3mol% TPB and 30 weight % PBD in PVK, sandwiched between I T 0 anodes and Magnesium/Silver anodes resulted in pure blue light, with the peak in emission at 440nm. High luminance (450candelamp2) was achieved at 18 V. The EL spectrum corresponded to emission from the TPB dye molecules and suggests that there is no aggregation of dopants. By adding further dye molecules, such as Coumarin 6 (PL at 490nm), DCMl(520 nm) and Nile Red (580nm), emission was achieved across the whole of the visible spectrum. Excitation mechanisms of the dye molecules include Forster-type resonance energy transfer from the chargetransporting groups (i.e. carbazole and PBD), as well as charge-trapping mechanisms, in which the dye molecules accept electrons and holes from the charge-transport groups with high energy gaps and provide sites for radiative recombination. Oligophenylenes are also candidates as blue-emitting materials. Although poly( p phenylene) emits in the blue region, it is not readily solution-processible. Scherf et al. [98, 991 have synthesized soluble ladder-type oligo(p-pheny1ene)s (LPPPs) with rigid, planar, regioregular structures. Gruner rt a[. [I I ] report that in dichloromethane solution, these oligomers show intense blue fluorescence, with PL quantum yields up to 60%. However, in the solid state, some LPPPs show additionally strong yellow photoluminescence, attributed to excimer formation due to short-range

ordering of the molecules. In EL, the broad yellow contribution is even more pronounced than in PL and moves to even lower energies with increasing operation time of the device. The yellow emission in PL can be strongly suppressed by diluting the LPPPs within a polystyrene matrix. For light-emitting diodes, poly(9-vinylcarbazole) (PVK) is instead used as a hole-transporting matrix, resulting in a marked decrease of the broad yellow emission. Quantum efficiencies greater than 0.1 YOhave been achieved. However, even with the LPPPs dispersed within a PVK matrix, the blue emission still bleaches to white after the device has been operating for tens of minutes, indicating that excimer sites can still form and that the presence of even a relatively small number of them is sufficient to give rise to strong yellow emission, thereby reducing the spectral purity of the blue-emitting material. Grem et ul. [13] also reported blue emission from sublimed films of p-sexiphenyl H-P6-H and also spin-coated films of LPPPs, noting that excimer emission is less pronounced for sexiphenyl, in which there is considerably more torsion between the phenylene rings, which would be expected to impede aggregation.

10.4.1.4 LEDs Based on Polymers with Pendent Oligomeric Side-Chains As we have just discussed, oligomers have been successfully blended with polymeric hosts to serve as both charge-transport materials and emissive materials, there are problems with long-term stability, in terms of the tendency of small molecules to diffuse through the polymer with time and particularly under high electric fields and elevated temperatures present under device operation. This usually results in aggregation and re-crystallization, which lowers the quantum efficiency and shifts the emission to lower energy than the blue emission which is often sought. One strategy for circumventing the problem of re-crystallization is to covalently bond the oligomers to the backbone of a polymer, such as PMMA or polystyrene, in the form of pendent side-chains. This has the effect of greatly reducing aggregate formation, since the bulky rigid oligomer side-chains actually increase the glass transition temperature of the backbone polymer, rather than lowering it, as is often the case for blends. High glass transition temperatures correspond to amorphous materials in which recrystallization and aggregate formation is difficult. Hesemann et al. [17] reported LEDs in which distyrylbenzene chromophores are attached as pendent side-chains to a polystyrene backbone to obtain an amorphous, glassy polymer with a glass transition temperature of 165°C. Blue-green emission was obtained, peaked at 500 nm. Li et 01. [ 161 have synthesized methacrylate polymers with pendent oligomeric oxadiazole side-chains, which function as electron-transporting material in LEDs [16, 1001. These are glassy, amorphous polymers, which are soluble in chloroform, toluene and tetrahydrofuran. Compared with unsubstituted PMMA (105'C), the polymers have relatively high glass transition temperatures (- 165 "C), which are attributed to the bulky nature of the side-chains, which reduce flexibility of the polymer backbone. However, the polymers are soluble in chloroform, toluene and tetrahydrofuran, though not in hexane or methanol. The polymers show purpleblue fluorescence in solution and in the solid state, with absorption maxima around 300 nm. When these polymers were used as an electron-transporting layer in a diode

Figure 14. Structural forriiula of a copolymer containing pendent oxidiazole segments for chargetransport and distyrylbenzene as blue emitters. Adapted from Li c’f d.[16].

of the structure ITO/PPV/oxadiazole-substituted-polymethacrylate/Ca, the internal quantum efficiency for EL increased by a factor of 4 at a current density of 0.5 mA c n p 2 , compared with diodes without the electron-transporting layer. In both cases, the yellow-green emission was identified as originating from the PPV layer. The electron-transporting polymer also significantly lowered the turn-on voltage for EL. A further development was the synthesis of a copolymer containing sections with pendent oxadiazole chains as electron-transporting material, and additionally, sections with pendent distyrlbenzene derivatives as emissive centres, as shown in Fig. 14. These are also amorphous, glassy polymers with glass transition temperatures higher than 130 C. When the copolymer was tested in single-layer diodes between I T 0 and calcium electrodes, low efficiencies and short lifetimes were obtained. Better results were obtained by blending the copolymer with the electron-transporting oxadiazole-substituted PMMA homopolymer in a double-layer diode structure, employing PPV as a hole-transporting layer (i.e. ITO/PPV/Blueemitting copolymer blended with oxadiazole-substituted PMMA/Ca). Good blue emission (&,,,(EL) = 457 nm, with a yellow-green tail) has been achieved using a similar double-layer structure with aluminum cathodes, yielding an internal quantum efficiency of 0.037%. Use of polymethacrylates with pendent oligomeric side-chains are therefore useful (i) as electron-transporting layers, sandwiched between the cathode and emissive layer, (ii) as electron-transporting blends with emissive polymers and (iii) as copolymers incorporating oligomeric side-chains for both charge-transport and fluorescence. 10.4.1.5 Polarized Electroluminescence from Oriented Oligomers

In addition to their potential use as blue-emitting materials, polarized electroluminescence [29, 311 has very recently been achieved in a multilayer LED in which oriented sexiphenyl H-P6-H oligoiners form the emissive layer and also in LEDs fabricated with oriented sublimed films of sexithiophene H-T6-H. Era

10.5 Pliotoc~~rirhtrtive and Photovoltaic Devices

54 1

et ul. [29] oriented the sexiphenyl oligomers by sublimation onto a template consisting of a thin film of sexiphenyl oligomers, which had been rubbed in one direction, in order to induce alignment parallel to the substrate. Marks er ul. [3 I ] reported polarized emission from sublimed films in which sexithiophene oligomers stand almost perpendicular to the substrate. The anisotropy was found to increase as the size of the crystallite grains was increased and the proportion of grain boundaries was reduced. The relative ease of inducing alignment of discrete oligomer chains, rather than their polymeric counterparts may yet render the use of oligomer films particularly advantageous in certain applications, such as waveguided or microcavity electroluminescent devices.

10.5 Photoconductive and Photovoltaic Devices Photoconductivity was first observed in anthracene by Pochettino [ 11 and Volmer [2] almost a century ago. Interest in using organic materials for xerography began in the 1950s and 1960s. Now small organic semiconductors have found widespread applications in xerography and laser printing. The principal advantages of using organic materials are that they can be prepared as flexible layers, suitable for coating cylindrical drums and belts, at relatively low cost, with spectral sensitivity throughout the visible region and into the near infra-red by the use of a mixture of photoreceptors of different semiconductor gaps, and with lower toxicity than the chalcogenide alloys, which they have largely replaced. Flexible layers for photogeneration and charge transport are prepared by dispersing the small organic semiconductors within a host polymer (blending) and depositing layers from solution. Because thermal charge generation rates are low, organic photoconductors can withstand the high voltages used in xerography. The disadvantages are that organic materials typically are not as resistant to abrasion as inorganic materials and have low mobilities for charge transport. Organic photoconductivity is generally explained in terms of photogeneration not of free electrons and holes, but rather of bound electron-hole pairs (excitons) and their subsequent dissociation into charge carriers which must then be transported to the electrodes: (i)

Absorption of light produces excitons, which will not readily ionize because their binding energies are greater than the energy available from thermal fluctuations, k , T . (ii) Exciton ionization and subsequent dissociation into charge carriers rarely occurs within the bulk of the film, but more usually when excitons interact with interfaces (with electrodes and other organic layers), trapped charges and regions of high electric field or suitably arranged discontinuities between respective HOMO and LUMO levels across a heterojunction. Hence, the heterojunction structure of Tang [I011 results in efficient dissociation, though only of those excitons generated within a diffusion length of the heterojunction. Oxygen is

also considered to play an important r61e in the dissociation of excitons at surfaces, by acting as deep traps for electrons [102, 1031. (iii) When charge pairs are generated, the less mobile carrier can be deeply trapped within the bulk of the film, leading to a build-up of space charge and hindered charge separation. Most organic photoconductors support unipolar charge transport, in which one polarity of charge carrier is much more mobile than the other. It is therefore important to carefully select materials with high carrier mobilities for (separately) electrons and holes. The use of a bi-layer device can circumvent this problem. In xerography, a thin layer for charge generation and a relatively thick layer for charge transport. Under applied fields, this has yielded very high xerographic gains (charges generated per incident photon), approaching unity and avoids potential problems of changes in sensitivity with time. In solar cells, the use of a p-n heterojunction device efficiently transports the electrons and holes away from the interface, so that they do not hinder further charge separation. For a much more detailed review of the organic photoconductivity, the interested reader is referred to the excellent books by Kao and Hwang [91] and (particularly for xerographic applications) Borsenberger and Weiss [5] and references therein. Although the small organic semiconductors used in xerography can often be viewed as oligomers, e.g. tetracene, pentacene, derivatives of distyrlbenzene (see Fig. 3), there have to date been relatively few reported investigations of the photoconductive and photovoltaic properties of oligomers of conjugated polymers. Having said that, the mechanism of photoconductivity in sexithiophene has been intensively studied, and is described in the following section.

10.5.1 Mechanism of Photoconductivity in Sexithiophene The photoconductivity action spectrum of sexithiophene is observed to be antibatic (complementary) with the photoluminescence excitation spectrum; at excitation energies higher than 2.2 eV, the fluorescence yield of sexithiophene thin films decreases rapidly, just as the material starts to become photoconductive. This indicates a new non-radiative decay channel opens up at higher excitation energies than that of the 1 B,, exciton which is considered to be responsible for photoluminescence. Zamboni et crl. [ 1041 noted that the crossover between photoluminescence and photoconductivity occurs close to the energy of the 22ARexciton band, determined by two-photon excitation spectroscopy [56], which might then result in singlet fission into two long-lived triplets or else offer an efficient crossing to extended ionized states, such as charge-transfer excitons in which a weakly bound electronhole pair is localized over two or more adjacent oligomers. Crystallographic studies of sexithiophene thin films [72, 1051 conclude that oligomers are arranged in stacked layers. Within the layers, the oligomers are arranged with their long molecular axes oriented at 32" to the plane normal, with their broad planes arranged in a herringbone pattern, in common with many molecular crystals. Conductivity is highly anisotropic; charge carriers are much more mobile

'

within layers than between stacks. Dippel et al. [74] proposed that in films of sexithiophene, the stacked layer structure is favorable for photoconductivity. At energies higher than the lowest singlet excited state ( 1 ' B , , ) ,a charge-transfer exciton is formed in which the weakly bound electron-hole pair is distributed over two or more adjacent molecules. Of those charge-transfer excitons which are generated longitudinally, i.e. along the long oligomer axis and perpendicular to the planes, the carrier mobility within the planes is sufficiently high to allow effective charge separation before the geminate charges recombine. Conversely, charge-transfer excitons generated parallel to the planes undergo rapid recombination of geminate electron-hole pairs and therefore d o not contribute to the photoconductivity. Therefore, on a nanometre scale, the intrinsic layered structure of close-packed monolayers in sublimed films of H-T,-H has advantages for charge generation, in a way somewhat analogous to the use of several distinct microscopic physical layers to enhance interfacial charge separation in photocells. It is also worth noting that evolution has resulted in the use of layers of close-packed antennae as photoreceptors in nature.

10.5.2 Photovoltaic Applications (Solar Cells) A related application is in the use of organic semiconductors as solar cells. Once again, the advantages of organic semiconductors are the possibility of depositing large area films from solution at low cost, with the possibility of selecting molecules of a variety of energy gaps and chemical modifications in order to make most efficient use of the solar spectrum. For a comprehensive review of organic solar cells, the reader is referred to Chamberlain [ 1061. Important parameters used to assess the quality of the device are the short-circuit current, Jsc, open-circuit voltage, V,, and fill factor, FF, the power conversion efficiency, 7). Figure 15 shows a schematic I- V curve, similar to that published by

---j-_-._ J o :ic sc

Figure 15. Schematic photocurrent-voltage curve, indicating important parameters, such as open circuit voltage Vo,, short-circuit current, J*,. The fill factor, FF. is the ratio of the shaded area divided by the product V,,J,,. Adapted from Tang et ul. [84]

544

10 Opticul Applicritioris

Tang. The fill factor is a measure of the ability of the cell to do useful work. It is defined as the ratio of the maximum electrical power which could be obtained (the shaded rectangle within the I-V curve in Fig. 15) to the product of J,cVc,,. The power conversion efficiency, rl, is the ratio of the maximum electrical power (shaded area) to the power of the radiation which produced it. A major breakthrough in the use of organic semiconductors in solar cells was the report by Tang [loll and Panayotatos [107, 1081 of efficient two-layer photovoltaic cells, in which charge generation occurs primarily at the interface between the two organic layers rather than at the Schottky barrier or interface with the electrodes, a,s is often the case for single-layer devices. Using 4 device of the form Glass/lT0/300 A sublimed copper phthalocyanine (CuPc)/500 A sublimed perylene tetracarboxylic derivative (PV)/Ag, power conversion efficiencies up to 1 O h were achieved, with the charge-generation efficiency relatively independent of bias. The open circuit voltage ( Vo,= 0.4 V) was found to be relatively insensitive to the choice of metal as the uppermost electrode (instead of Ag). The double-layer structure also drastically lowers the series resistance of the cells, resulting in fill factors up to 0.65, much higher than that which had been achieved with single-layer cells. By comparison with the absorption spectra, both layers were found to contribute to the spectral response. The operation of the device was attributed to the absorption of light in both layers, resulting in the creation of excitons which can diffuse to the interface between the two layers. At the interface, the excitons are efficiently split because the offset between the corresponding LUMO levels and HOMO levels across the interface (‘band offsets’) exceed the exciton binding energy, or for a less abrupt interface, because the local electric field in this region is expected to be particularly high. The holes are then transported through the p-type CuPc layer, to the IT0 electrode, while electrons are transported through the n-type perylene derivative layer, to the Ag electrode. The stability of the device was also impressive, with both the open circuit voltage and short circuit current decreasing by less than 2 % after five days of continuous operation in ambient atmosphere under illumination from a tungsten white light source (75 mW cmp2)and under short-circuit conditions (2 mA cm-2), although the fill factor decreased by about 30%. Very recently, Nonia et al. [I091 have used a similar bi-layer strategy with the place of copper ophthalocyanine, in a oligomer octithiophene H-T8-H used device of the structure: Glass/IT0/400 A sublimed PV/300 A sublimed H-T8-H/ Au. Under white light illumination at 105mW cm’, a fill factor of 0.5 and a conversion efficiency of ca. 0.6% were achieved, comparable with the figures of merit reported by Tang. The open-circuit voltage (V,, = 0.42 V) and short-circuit current (Jsc = 2.9 mA cm’) were also similar. The short-circuit photocurrent was linear with light intensity, indicating negligible recombination of photogenerated electrons and holes. The fill factors and conversion efficiencies also remained approximately independent of light intensity up to 200 mW cm2. Since octithiophene absorbs in the region 300-500 nm, while the PV perylene derivative absorbs in the 500800nm region, the spectral response of the oligomer-based cell responds to a wider spectral range than that of Tang. In an earlier study, Kuwabara rt ul. [110] reported much lower fill factors (0.29) and conversion efficiencies (0.02%) for a similar bi-layer cell using quinquethiophene

H-T,-H. The high conversion efficiency when H-T8-H is used rather than H-Ts-H is attributed to higher dark conductivity, higher charge-carrier mobility (pFET), improved film quality and absorption at longer wavelengths, where there is a greater proportion of solar radiation to be harvested. Recent photocurrent measurements on ladder-type oligophenylenes (LOP) by Kohler et NI. [ I 1 I ] (see Fig. 3 ) indicate that absorption into aggregate states may be highly efficient for the generation of photocurrents, leading to increased quantum yields by two orders of magnitude, compaJed with intramolecular excitation. For a device of the structure Glass/IT0/2500 A LPPP/AI, under illumination through the IT0 electrode, a broad structureless photocurrent response is observed in the action spectrum at 2.25eV. close to the energy of the yellow emission associated with aggregates. Under forward bias (IT0 positive) of 0.5V, a quantum yield of 0.7% was measured for the yellow aggregate peak, compared with 0.002% for the blue intramolecular excitation. A very weak peak ( a = 25 i 20cm-') is present in the absorption spectrum at this energy. These results suggest that in the LOPS, the yellow absorption and photocurrent signal are due to physical dimers or aggregates, stable in the ground state, rather than excimers which are stable only in the excited state. The deliberate use of materials which undergo aggregation may therefore offer an important strategy for improving organic solar cells. Furthermore, as emphasized by Karl er 01. [27], the power conversion of organic solar cells is often very low, due to a high internal cell resistance, because the mobility of many organic semiconductors is low. To solve the problem, they advocate the use of more crystalline materials with strong intermolecular .ir-electron interactions and high mobilities, as might yet be achieved with suitably oriented sublimed films of conjugated oligomers.

10.6 Field-Effect Devices Chapter 1 1 discusses in some detail the electrical characteristics of metal-insulatorsemiconductor (MIS) sandwich structure devices, particularly field-effect transistors (FETs). Here, we discuss the principles of the MIS diode and show how this can be used in electro-optical modulator devices, as application prototypes and also as research tools for optical spectroscopy. The MIS diode is a simple two-layer sandwich structure, as shown schematically in Fig. 16a, consisting of an insulator layer in contact with a semiconductor layer, the pair of layers being sandwiched between two metal electrodes. The electrode in contact with the insulator layer is called the gate electrode because in a MIS fieldeffect transistor, the voltage applied to the gate electrode controls the charge density within the semiconductor layer and hence the current which the transistor allows to flow between its source and drain electrodes. In the MIS diode, the source and drain electrodes are replaced by a continuous electrode making ohmic contact with the semiconductor layer, as can be seen by comparison with the FET structure shown in Fig. 16b. In the following discussion and the accompanying schematic diagrams

n

OHMIC ELECTRODE

’

‘GATE

ELECTRODE

vgate

(b) &I€Ukld-effecttruuiskx Source-Drain current

\

INSULATOR

I 4h ELECTRODE ’GATE

Vgate

Figure 16. (a) Cross-section through a MIS diode; (b) cross-section through a MIS field-effect transistor (MlSFET).

of Fig. 17, we assume the semiconductor to be p-type (i.e. the majority carriers are positive holes), since many conjugated polymers and oligomers, including the H-T6-H, which we discuss later, are p-type materials. Applying a negative DC gate voltage to the gate electrode induces injection of positive charges from the ohmic contact and also attracts the positive charges within the semiconductor to the interface between the oligomer and insulator layers. This is known as ACCUMULATION and results in a thin interfacial layer where there is a high charge density per oligomer. In the FETs discussed in Chapter 1 I , the accumulation layer gives rise to the conduction channel between source and drain electrodes when the FET is in the ‘on’ (conducting) state. Conversely, applying a positive DC gate voltage to the gate electrode repels the positive charges from the interface region, forming a DEPLETION layer, which is highly insulating. In an FET, the highly insulating depleted semiconductor layer cannot provide a conduction channel between source and drain electrodes and therefore corresponds to the case when the transistor is in the ‘off’ (nonconducting) state.

10.6.1 Electro-Optical Modulation As discussed in Chapters 6, 7.1 and 7.2, charging of oligomers is accompanied by a distortion of the molecular geometry, giving rise to new energy levels within the

C (probed in accumulatio

Series combination of

Range of validity of depletion approximation

0 - 4

ACCUMULATlON

METAL

INS

SEMICONDUCTOR

Positive chages attracted to interface =$ACCUMULATIONLAYER

DEPLETION

4

METAL

INS

v

*

*

SEMICONDUCTOR

Positive chages repelled from interface jDEPLETION LAYER

Figure 17. Operation of a MIS tield-effect diode: (a) in accumulation (high surface charge density, FET *on'): (b) in depletion (low charge density of bulk. F E T 'OF).

former HOMO-LUMO gap, as shown schematically in Fig. 18 for the formation of the radical cation (polaron) and dication (intramolecular bipolaron) of an isolated H-T6-H oligomer. Theoretically. although simple one-electron Huckel models can often qualitatively predict the position of new molecular levels within the gap

LUMO+l LUMO

LUMO+l

LUM0+2

LUMO+l

+ -&

HOMO HOMO-1

Neutral 6T [H-T6-HI

HOMO

Radical Monocation [H-T, -HI*+

HOMO

Dication [H-T6-H]*+

Figure 18. Simplified energy diagram for the charged excitations of sexithiophene, showing the new levels within the energy gap and new optical transitions associated with t$e radical cation (monopolaron) [H-Tn-HI" and dication (intramolecular bipolaron) [H-T,-H]-'.

associated with charged excitations, electron -electron interactions are also important and have been successfully included in valence effective Hamiltonian methods of Bredas ot al. Quantum chemical calculations are discussed in detail in Chapter 7.2. Furthermore, in the solid-state, the eflects of aggregation and a generally more coplanar structure result in further modifications to the energy levels, compared to those of isolated oligomers. Returning to the schematic diagram of Fig. 18, we now discuss the new optical transitions expected for the charged oligomers. When an electron is removed from a neutral oligomer, the distortion of molecular geometry (lattice relaxation) results in a raising of the former HOMO level [now denoted SOMO (singly occupied molecular orbital)] and lowering of the LUMO level. The level below the former HOMO level (HOMO-I) forms the HOMO of the radical cation and dication. Two new optical transitions are allowed for the radical cation (polaron), from the new HOMO to SOMO and SOMO to LUMO. Upon further oxidation to form the dication, the unpaired electron is removed from the SOMO and the former SOMO and LUMO move further towards the centre of the gap as the geometry distortion becomes even stronger. Now, one new optical transition is expected for the dication, from the HOMO to new LUMO (former SOMO), at a higher energy than the lower energy transition of the radical cation. In the solid state and aggregates, there is evidence from electron spin resonance [ 1 12- 1 171 and optical spectroscopy [ 1 12, 1 1 81 that the radical cations can aggregate and that the energy levels are subject to Davydov splitting. Aggregated cations are often referred to as charged 7r-dimers or 7r-stacks (see Chapter 9). Simultaneous measurement of the electron spin resonance signal during cyclic voltammetry of an oligomer film in an electrochemical cell show that the number of spins (radicals) is observed to decrease long before the dication (second oxidation peak) is reached, indicating possible pairing of spins in aggregates. This effect is particularly marked at low temperatures. This may be due to increased screening by solvent molecules, as the dielectric constant rises, and perhaps also due to lower torsion and hence closer intermolecular packing at low temperatures. Hence, charging of oligomers gives rise

10.6 Field-EffectDrvices

549

to a rich variety of new optical transitions below the fundamental absorption, indicative of the charged species concerned. The equilibria between the different charged species is a function of both the charge density and the structural order within the material . Probing changes in the optical absorption or reflection of an MIS device is very useful as a spectroscopic tool to determine the charged excitations in the solid state. The technique has already been used for investigation of conjugated polymers [ 119, 1201 and demonstrated as the basis of an electro-optic modulator device [121]. When light is passed perpendicularly through an MIS diode, the modulation of the transmission is very low (typically 0.001-0.0lY0). This is quite adequate for spectroscopic research, since lock-in amplification techniques can be used to detect such small changes. However, for a viable modulator device, a much larger modulation is required. This requires that the light to be modulated is coupled much more effectively to the region where the charge density is being modulated. This has been achieved for a conjugated polymer [I211 by employing a waveguide geometry within a FET structure. The light is effectively passed along the sourcedrain channel of the FET and hence a piuch longer modulation path can be used, typically 2mm, rather than the l000A thickness of the oligomer layer. Consequently, modulation up to 20% can be achieved, although modulation frequency is limited by the mobility of the conjugated semiconductor. Recently, a very similar device has been reported [ 1221, using an oligothiophene layer, presumably with the hope of increasing the modulation frequency, although to date the operational range reported for the oligomeric device is also limited to frequencies lower than a few kHz. Returning now to the MIS diode and optical probing of the charged states in oligomers, Fig. 19 shows a schematic view of the device structure and experimental

Figure 19. Schematic diagram of the experimental configuration for optical probing of field-induced charge within semi-transparent MIS diodes based on oligomers.

550

I 0 Optical Applications

configuration used in the following case study of sexithiophene, H-T6-H. The MIS diode is held under vacuum (- 2 x loe6 torr) in an optical access cryostat, aligned so that a monochromatic probe beam passes through the device and is detected by an appropriate optical detector. By superimposing a small oscillatory (AC) voltage on the DC bias, it is possible to modulate the charge density within the device, either to probe the accumulation region, for negative DC bias or the edge of the depletion layer (i.e. the bulk of the oligomer layer) for positive DC bias. The optical transitions of the charged species, are detected as a modulation of the probe beam at the same frequency as the AC modulation voltage, which is detected using a lock-in amplifier. It should be noted that this method provides a very direct means of probing the charged excitations within an opto-electronic device environment, in which charge is induced by purely electrostatic means, rather than by charge-transfer doping or photo-excitation.

10.6.2 Optical Probing of Field-Induced Charge in Sexithiophene Because oligomers can be synthesized and purified as monodisperse molecules and prepared as thin films in which the degree of order can be systematically controlled, we can expect better resolution of the optical properties. The technique has therefore been used to study charged excitations in oligothiophenes [123, 1241, with the aim of investigating which charged excitations were involved in charge-transport within the high mobility organic FETs. Semi-transparent metal-insulator-semiconductor (MIS) diodes were fabricated, in which a thin film of H-T6-H of thickness around l00nm forms the semiconductor layer. The MIS diode can indeed be viewed as an inverted thin-film transistor, with the source and drain electrodes shorted together, or rather replaced by a continuous indium tin oxide (ITO) electrode, while a semi-transparent gold film serves as the gate electrode, separated from !he H-T6-H layer by a sublimed film of silicon monoxide, also around l000A thick. Figure 20 shows the optical spectrum of modulated charge, taken at 298 K, with (a) 0 V D C bias (onset of depletion) and (b) -8 V DC bias (accumulation). The spectrum at OV shows two new optical absorptions, at 0.83 eV and 1.70eV, similar to the signature of radical cations produced by charge-transfer doping in dilute solution [125-1271 or photo-excitation [128, 1291. In the spectrum at -8 V bias, an additional spectral feature emerges at 1.34eV. This continues to increase in strong accumulation. This transition is attributed to a spinless dication. Figure 21 shows spectra taken with 0 V DC bias at (a) 298 K and (b) 170 K. The spectrum at room temperature shows the optical absorptions, at 0.70-0.83 eV and 1.70 eV, while the low temperature spectrum shows additional transitions at 1.01eV and 2.05eV. Similar transitions have also been observed in aggregates of the oligomers in poor solvents, at low temperatures or high concentrations [112, 1181. A particularly narrow feature at 0.60 eV is attributed to the additional transitions

10.6 Field-Effect De~rce.r

55 1

a) Gate Voltage: OVf2V (ac) Onset of depletion

t

L

Energy (eV) b)

Gate Voltage: 4 V i 2 V ( a d Accumulation

Energy (eV) Figure 20. Optical spectra of field-induced charge in sexithiophene H-T6-H: (a) at the onset of depletion (0 V gate voltage); (b) in accumulation (-4 V gate voltage).

of aggregated states of radical cations (also known as 7i.-stacks or -ir-dimers), corresponding to the charge-transfer band. These transitions were not observed in the amorphous films of spin-coated regiorandom side-chain substituted H-T6-H [130]. From the bias dependence spectra of Fig. 22, it can be seen that typically three charged excitations may be simultaneously present within an oligomer film. Dications

552

10 Optical Applications

a) Gate Voltage: OVf2V (ac) Temperature: 300K 0.5

.

0.0 .’’.,.

I

-..

.._::..

......

I

I

.........

.......

0.83eV

-2.5 0.5

I

1.o

I

1.5

2.0

2.5

Energy (eV) b) Gate voltage: OVi2V (ac) Temperature: 170K

Figure 21. Optical spectra of field-induced charge in sexithiophene H-T,-H: (a) at room temperature (300 K); (b) at low temperature (1 70 K).

are formed at high charge density, in the accumulation layer, while the feature at 2.05 eV is strongest in depletion, indicating that n-stacking plays an important r61e in intermolecular charge transport in the relatively well-ordered bulk of the film, though not at the more disordered interface probed in accumulation. Radical cations are usually present under all bias conditions, even in films which have not been intentionally doped.

10.7 All-Opticul Modulator Devices

553

YIR (4v) Accumulation

. . . .ym

1.34eV

+a

YIR +4v) Depletion

4 -

t I-

d

v)

0 7

-4 -6 "

-

1.2

1.4

1.6

1.8

2.0

2.2

2.4

Energy (eV) Figure 22. Bias-dependence of the optical spectra of field-induced charge in sexithiophene H-T6-H, showing evidence for three species of charged excitations.

10.7 All-Optical Modulator Devices A promising optical application of oligomers is their potential use in opticallyaddressed spatial light modulators (OASLM). OASLMs could be used for real-time parallel processing of images and switching of light beams for telecommunications. Unlike most of the optical devices which we have discussed so far, all-optical SLMs do not need pixelated electrodes and therefore do not have the problems of interconnections and addressing of pixels, which are associated with flat-panel displays. Therefore, SLMs can use very small pixel dimensions, limited by optical resolution to dimensions of around 1 pm and can therefore handle several channels of data in parallel, including graphic images. SLMs are required as input stages of optical computers and for image-processing (e.g. spatial filtering to eliminate periodic patterns from raster scanning or to enhance contrast) because the subsequent Fourier optics processing requires the use of coherent monochromatic light. An incoherent to coherent OASLM operates in two steps, as shown schematically in Fig. 23. Firstly, an image of an object is formed within a photosensitive film by exposure to light, which could even be incoherent and of a rather broad spectral range (e.g. white light). The image is usually stored as a change in absorption or refractive index within the film. Secondly, the stored image thus formed serves as a template, so that when the OASLM is later probed by a coherent probe beam (e.g. laser), a coherent image can be generated. Previous SLMs have used photoconductors coupled to liquid crystals [ 13I , 1321, multiple-quantum-well semiconductors [ 1331 and silver halide films [134] to exploit

Incoherent

Laser

Diode 800nm

light

'""T

Obe j ct*'

I

!

INCOHEREM TO COHERENT OPTICAL CONVERTER

(Signal)

7 +signal

I

SiO,

I

a-GT

!

\

Coherent image (screen),,'

,* '\ ,'

,,"30a~ plane

Figure 23. Diagram showing the operation of an all-optical spatial light modulator acting as an incoherent-to-coherent optical converter. Adapted from Fichou c v trl. [ 181.

such effects as photorefraction [135], photochromism [136, 1371 and saturable absorption, etc. The nonlinear optical properties of oligothiophenes make them promising candidates for active photochromic materials in optically-addressed spatial light modulator devices. In particular, speed and spatial resolution can be improved by two orders of magnitude, compared [I381 with devices based on liquid crystals or semiconductor heterostructures. Fichou et uf. [ 181 have reported such a device, based on sublimed films of sexithiophene H-T6-H. The optical switching mechanism is due to absorption by a longlived (7 = 5 ns) triplet state formed only 10 picoseconds [139] after photo-excitation in the absorption band of sexithiophene, as a result of rapid intersystem crossing from the lowest singlet excited state. In the areas initially exposed to the incoherent white light 'write' pattern, triplet excitons are thus formed. The probe beam can then detect an induced T I T,, absorption, which has a remarkably narrow linewidth (FWHM = 0.1 eV) compared to the linear absorption band (FWHM z 1.0 eV) and shows none of the vibronic replicas which are present in the linear absorption spectrum. The particularly narrow linewidth of the triplet-triplet absorption is attributed principally to three causes [ 1381: ---f

(i) damping of the vibronic side-bands; (ii) conservation of oscillator strength; (iii) homogeneity of the molecules.

The need for homogeneity of the molecules is satisfied particularly well by conjugated oligomers of uniform length and is obviously not satisfied by the corresponding conjugated polymers, for which there is always a distribution of conjugation lengths.

References

555

Similar photo-induced transient spectra are observed in dilute solutions of the oligomers, indicating that the fast optical transition is intramolecular. OASLMs based on oligothiophenes are therefore not limited either by slow collective effects, as for liquid crystals or polarization effects in photorefractive materials and multiple quantum wells, nor by RC time constants, as for hybrid photoconductor/liquidcrystal devices. Fichou has suggested [18] that by dispersing H-T6-H within an outgassed polymer matrix, the triplet lifetime could be increased from around 5 ns in the evaporated film to the microsecond regime. Although this would lower the operating speed from around 200 MHz to the 1 MHz regime, the ‘written’ image would persist for longer, therefore reducing the energy required for exposing (‘writing’) the image to a reasonable value of 1 kW cm-2. It is estimated [I401 that the isolated oligomers can withstand around lo6 excitation-relaxation cycles. Since the all-optical OASLM does not require any electrical connections, it would be possible to prepare uniform films of H-T6-H on continuous tape, so that after several exposures, the tape can be wound on, much like a photographic film, when the film degrades and needs to be replaced.

References 1. A. Pochettino. Acad. Liricei Rediconti 1906, 15, 355. M. Volmer, Ann. P/i.v.sik 1913, 40, 775. D. D. Eley, Nuture 1948, 162, 819. A. T. Vartanyan, Zh. Fiz. Khitn. 1948, 22, 769. P. M. Borsenberger. D. S. Weiss, Organic Receprors,fOr Imuging Systems, Vol. 39, Marcel Dekker, New York 1993. 6. A. S. Davydov, Zliur. Eh-spptl. i Teoret. Fiz. 1948, 18, 515. 7. A. E. Gillam, D. H. Hey. J . Chem. Soc. 1939, 1170. 8. M. Pope, H. P. Kallmann, P. Magnante, J . Chem. Phys. 1963, 38, 2042. 9. W. Helfrich, W. G. Schneider, Phys. Rev. Lett. 1965, 14, 229. 10. P. Frederiksen, T. Bjornholm, H. G. Madsen, K. Bechgaard, J . Muter. Chem. 1994, 4 , 675. I I . J. Griiner, H. F. Wittmann, P. J. Hamer, r t ul., Syrrrh. Mer. 1994, 67, 181. 12. W . Tachelet, S. Jacobs, H. Ndayikengurukiye, H. J. Geise, J. Griiner, Appl. Phys. Lett. 1994, 64. 2364. 13. G. Grem, V. Martin. F. Meghdadi, et ul., Syntli. M e t . 1995, 71, 2193. 14. J. Kido, H. Shionoya, K. Nagai, Appl. Phys. Lett. 1995, 67, 2281. 15. H. S. Woo, J. G . Lee, H. K. Min, et a/., Sjsnth. M e t . 1995, 71, 2173. 16. X . C. Li, F. Cacialli, M. Giles, et d.,A h . Muter. 1995, 7 , 898. 17. P . Hesemann, H. Vestweber, J. Pommerehne, R. F. Mahrt, A . Greiner, Adv. Muter. 1995, 7,

2. 3. 4. 5.

388. 18. D. Fichou. J. M. Nunzi. F. Charra, N. Pfeffer, Adv. Muter. 1994, 6 , 64. 19. F. Garnier, G. Horowitz, X. H. Peng, D. Fichou, A t h . Muter. 1990, 2, 592. 20. G. Gustaffson. Y. Cao. C. M. Treacy, F. Klavetter, N. Colaneri, A. J. Heeger, Nuture 1992, 357, 477. 21. F. Garnier, R. Hajlaoui. A . Yassar, P. Srivastava, Scierice 1994, 265, 1684. 22. K. Y. Jen, R. Oboodi, R. L. Elsenbaumer, Po(iwi. Murer. Sci. Eng. 1985, 53, 79. 23. R. L. Elsenbaumer, K. Y. Jen, R. Oboodi, Syntli. Met. 1986, 15, 169. 24. E. E. Havinga, I . Rotte, E. W. Meijer, W . Tenhoeve, H. Wynberg, Synth. Mer. 1991, 41, 473. 25. D. Delabouglise. M. Hymene. G. Horowitz, A . Yassar, F. Garnier, Adv. Muter. 1992, 4 , 107.

556

10 Optical Applications

26. B. Servet, G. Horowitz, S. Ries, et ul., Chetn. Muter. 1994, 6, 1809. 27. N. Karl, A. Bauer, J. Holzapfel, J. Marktanner, M. Mobus, F. Stolzle, Mol. Cryst. Liq. Cryst. 1994, 252, 243. 28. J. C. Wittmann, P. Smith, Nature 1991, 352, 414. 29. M. Era, T. Tsutsui, S. Saito, Appl. Phys. Lett. 1995, 67, 2436. 30. P. Lang, P. Valat, G. Horowitz, et ul., J . de Chim. Phys. et de Phys-Chim. Biol. 1995, 92, 963. 31. R. N. Marks, F. Biscarini, R. Zamboni, C. Taliani, Europhys. L e f t . 1995, 32, 523. 32. F. Garnier, F. Deloffre, G. Horowitz, R. Hajlaoui, Synfh.Met. 1993, 57, 4747. 33. P. Bauerle, Adv. Muter. 1992, 4 , 102. 34. F. Geiger, M. Stoldt, H. Schweizer, P. Bauerle, E. Umbach, Adv. Muter. 1993, 5, 922. 35. F. Garnier, A. Yassar, R. Hajlaoui, el ul., J . Am. Chem. Soc. 1993, 115, 8716. 36. J. K. Herrema, J. Wildeman, F. Vanbolhuis, G. Hadziioannou, Synth. M e t . 1993, 60, 239. 37. H. Mao, B. Xu, S. Holdcroft, Mucrotnolecules 1993, 26, 1163. 38. M. Leclerc, F. M. Diaz, G. Wegner, Makromol. Chem. 1989, 190, 3105. 39. M. J. Winokur, D. Spiegel, Y. Kim, S. Hotta, A. J. Heeger, Synth. Met. 1989, 28, C419. 40. A. Stabel, J. P. Rabe, Synth. Met. 1994, 67, 47. 41. R. D. McCullough, R. D. Lowe, J . Chem. Soc., Chem. Commun. 1992, 70. 42. R. D. McCullough, R. D. Lowe, M. Jayaraman, D. L. Anderson, J . Org. Chenr. 1993,58,904. 43. P. Biiuerle, F. Pfau, H. Schlupp, et a/., J . Chem. Soc. Perkin Trans. I I 1993, 489. 44. I. B. Berlman, J . Phys. Chem. 1970, 74, 3085. 45. P. L. Burn, D. D. C . Bradley, R. H. Friend, et al., J . Chem. Soc. Perkin Trans. I1992, 3225. 46. T. Horn, S. Wegener, K. Miillen, Mucromoleculur Chem. & Phys. 1995, 196, 2463. 47. A. R. Brown, A. Pomp, C . M. Hart, D. M. Deleeuw, Science 1995, 270, 972. 48. U . Schoeler, K. H. Tews, H. Kuhn, J . Chenr. Phys. 1974,61, 5009. 49. H. Nakahara, J . Nakayama, M. Hoshino, K. Fukuda, Thin Solid F i h s 1988, 160, 87. 50. A. K. Dutta, A. J. Pal, T. N. Misra, Bull. Chem. Soc. Jap. 1993, 66, 3576. 51. D. Beljonne, J. Cornil. R. H. Friend, R. A. J. Janssen, J. L. Bredas, J . Am. Chem. Soc. 1996, 118, 6453. 52. J. C . Scaiano, R. W. Redmond, B. Mehta, J. T. Arnason, Photochem. Photobiol. 1990,52,655. 53. R. A. J. Janssen, D. Moses, N . S. Sariciftci, J . Chem. Phys. 1994, 101, 9519. 54. R. A. J. Janssen, M. P. T. Christiaans, K. Pakbaz, D. Moses, J. C. Hummelen, N. S. Sariciftci, J . Chem. Phys. 1995, 102, 2628. 55. B. Xu, S. Holdcroft, J . A m . Chem. Soc. 1993, 115, 8447. 56. N. Periasamy, R. Danieli, G. Ruani, R. Zamboni, C. Taliani, Phys. Rev. Lett. 1992, 68, 919. 57. R. Rossi, M. Ciofalo, A. Carpita, G. Ponterini, J . Photochem. Photobiol. A : Chem. 1993, 70, 59. 58. H. Chosrovian, S. Rentsch, D. Grebner, D. U. Dahm, E. Birckner, H. Naarmann, Synth. Met. 1993, 60, 23. 59. R. S. Becker, J. S. de Melo, A. L. Mapnita, F. Elisei, Pure & Appl. Chem. 1995, 67, 9. 60. D. Birnbaum, B. E. Kohler, C. W. Spangler, J . Chem. Phys. 1991,94, 1684. 61. K. Schulten, I. Ohmine, M. Karplus, J . Chem. Phys. 1976, 64, 4422. 62. D. Birnbaum, B. E. Kohler, J . Chem. Phys. 1992, 96, 2492. 63. S. Mazumdar, D. Guo, S. N . Dixit, J . Chen?. Phys. 1992, 96, 6862. 64. J. B. Vanbeek, F. Kajzar, A. C . Albrecht, J . C k m . Phys. 1991, 95, 6400. 65. J. B. Vanbeek, A. C. Albrecht, Chem. Phys. Lett. 1991, 187, 269. 66. G. Klein, R. Voltz, M. Schott, Chem. Phys. Lett. 1973, 19, 391. 67. R. Katoh, M. Kotani, Chem. Phys. Lett. 1992, 196, 108. 68. R. H. Austin, G. L. Baker, S. Etemad, R. Thompson, J . Chem. Phys. 1989, 90, 6642. 69. D. P. Craig, S. H. Walmsley, Excitons in Molecular Crystals, Benjamin, New York 1968. 70. M. Pope, C . E. Swenberg, Electronic Processes in Orgunic Crystals, Oxford University Press, New York 1982. 71. J. Riihe, N. F. Colaneri, D. D. C. Bradley, R. H. Friend, G. Wegner, J . Phys. Cond. Mutter. 1990, 2, 5495. 72. W. Porzio, S. Destri, M. Mascherpa, S. Rossini, S. Bruckner, Synth. Met. 1993, 55, 408. 73. S. Hotta, K. Waragai, Adv. Muter. 1993, 5 , 896. 74. 0. Dippel, V. Brandl, H. Bassler, R. Danieli, R. Zamboni, C . Taliani, Chem. Phys. Lett. 1993, 216. 418.

Rqfhrences

557

N. I. Nijegorodov, W. S. Downey, J . Pl7y.v. Chem. 1994, Y8, 5639. R. A. J. Janssen, L. Smilowitz, N . S. Sariciftci, D. Moses, J . Cliem. Plijs. 1994, 101, 1787. B. Xu, S. Holdcroft, Macromolecules 1993, 26, 4457. M. Belletite, L. Mazerolle. N. Desrosiers, M. Leclerc, G . Durocher, Macromolecule,r 1995, 28, 8587. 79. A. R. Brown, D. D. C. Bradley, J. H. Burroughes, c’r a / . , A p p l . Phys. Lett. 1992, 61, 2793. 80. A. Bernanose, J . de Chim. Pliys. 1955, 52, 396. 81. A. Bernanose, P. Vouaux, J . de Chim. Plij*.s. 1955, 52, 509. 82. J. Kalinowski, Mater. Sci. 1981, 7, 44. 83. P. S. Vincett, W. A. Barlow, R. A. Hann, G. G . Roberts, Thin Solid F i l m 1982, Y4, 171, . 1987, 51. 913. 84. C. W. Tang, S. A. Van Slyke, Appl. P l i y ~Lett. . 65, 3610. 85. C. W. Tang, S. A. Van Slyke, C. H. Chen, J . Appl. P h j ~ 1989, 86. C. Adachi, S. Tokito, T. Tsutsui, S. Saito, Jup. J . Appl. Pliys. Pr. 2 Letters 1988, 27, L269. 87. C . Adachi, S. Tokito, T. Tsutsui, S. Saito, Jup. J . Appl. Phys. Pt. 2 Letters 1988, 27, L713. 88. C. Adachi, T. Tsutsui. S. Saito, Appl. Phys. Lett. 1989, 55, 1489. 89. C. Adachi, T. Tsutsui, S. Saito, .4ppl. Phys. Lett. 1990, 57, 531. YO. H. Kurczewski, H. Biissler, J . Lumin. 1977, 15, 261. 9 I . K. C. Kao, W. Hwang, Electricul Transport in Solids: With Purticulur Refirence to Orgunic Sen7icunductors, Vol. 14, Pergamon, Oxford 198I . 92. K. Uchiyama, H. Akimichi, S. Hotta, H. Noge, H. Sakaki, Sjwth. Met. 1994, 63, 57. 93. K. Uchiyama, H. Akimichi, S. Hotta, H. Noge, H. Sakaki, Mat. Res. Soc. Symp. Proc. 1994, 328, 389. 94. M. Muccini, R. F. Mahrt, U. Lemmer, et d.? Clirm. Pliys. Lett. 1995, 242. 207. 95. G. Horowitz, P. Delannoy, H. Bouchriha, et ul., Adv. Muter. 1994, 6, 752. 96. A. Yassar, F. Garnier, F. Deloffre, G. Horowitz, L. Ricard, Adv. Muter. 1994, 6, 660. 97. C. Hosokawa, H. Higashi, T. Kusumoto, Appl. Phys. Lett. 1993, 62, 3238. 98. ff. Scherf, K. Miillen, Mukromol. Cheni., Rapid Commun. 1991, 12, 489. 99. U. Scherf. K. Mullen, Synthesis 1992, 23. 100. F. Cacialli, X.-C. Li, R. H. Friend, S. C. Moratti, A. B. Holmes, Synth. Met. 1995, 75, 161. 101. C . Tang, Appl. Pliys. Left. 1986, 48, 183. 102. R. F. Chaiken, D. R. Kearns, J. Cheni. Pliys. 1966, 45, 3966. 103. G. R. Johnston, L. E. Lyons, Aust. J . Chem. 1970, 23, 1571. 104. R. Zamboni, N. Periasamy, G. Ruani, C. Taliani, Synrh. Met. 1993, 54, 57. 105. P. Ostoja, S. Guerri, S. Rossini, M. Servidori, C. Taliani, R. Zamboni, Synrh. Met. 1993, 54, 447. 106. G. A. Chamberlain, Solar Cells 1983. 8, 47. 107. P. Panayotatos, D. Parikh, R. Sauers, G. Bird, A. Piechowski, S. Husain, Solar Ce1l.v 1986, 18, 71. 108. P. Panayotatos, G . Bird, R. Sauers, A. Piechowski, S. Husain, Solar Cells 1987, 21, 301. 109. N. Noma. T. Tsuzuki, Y. Shirota, Adv. Mater. 1995, 7, 647. 110. K. Kuwabara, K. Miyawaki. K. Nawa. N . Noma, Y. Shiroto, Nippon Kugukukuislii 1992, 1168. 11 I . A. Kohler, J. Gruner, R. H. Friend, K. Mullen, U. Scherf, Cheni, Phys. Lett. 1995, 243. 456. 112. P. Biiuerle, U. Segelbacher, A. Maier, M. Mehring, J . A m . Chem. Sac. 1993, 115, 10217. 113. M. G . Hill, J. F. Penneau, B. Zinger. K. R. Mann, L. L. Miller, Chem. Muter. 1992, 4. 1106. 114. M. G. Hill, K. R. Mann, L. L. Miller, J. F. Penneau, J . Am. Chen7. S i c . 1992, 114, 2728. 115. G. Zotti, G. Schiavon, A. Berlin, G. Pagani, Clienl. Muter. 1993, 5 , 620. 116. G. Zotti, G. Schiavon, A. Berlin. G. Pagani. Ach. Muter. 1993, 5, 551. 117. G. Zotti, G. Schiavon, A. Berlin, G. Pagani, S w t h . Met. 1993. 61, 81. 118. U. Segelbacher, N. S. Sariciftci, A. Grupp, P. Biiuerle, M. Mehring, Synth. Met. 1993, 57, 4728. 119. K. E. Ziemelis, A. T. Hussain, D. D. C. Bradley, R. H. Friend, J. Ruhe, G. Wegner, Phys. Rev. Lett. 199I , 66. 223 1. 120. M. G. Harrison, K. E. Ziemelis. R. H. Friend, P. L. Burn. A. B. Holmes, SJxtli.Met. 1993,55, 218. 75. 76. 77. 78.

~

~

558

10 Opticd Applicritions

121. I. D. Parker, R. W. Gymer, M. G. Harrison, R. H. Friend, H. Ahmed, Appl. Phgs. Lett. 1993, 62, 1519. 122. T. Kurata, C . Fukada, H. Fuchigami, K. Hamano, S. Tsunoda, Jpn. J . Appl. Phys. Pi. 2 Lett. 1995, 34, L1464. 123. M. G. Harrison, R. H. Friend, F. Garnier, A. Yassar, molt^. Crysf. Liy. Cryst. Sci. Tdinology Scction A 1994, 252, 165. 124. M. G. Harrison, R. H. Friend, F. Garnier, A. Yassar, Synth. Met. 1994, 67, 215. 125. D. Fichou, G. Horowitz, F. Garnier, Sq'nth. Mrt. 1990, 39, 125. 126. D. Fichou, G. Horowitz, B. Xu, F. Garnier, Synth. Met. 1990, 39, 243. 127. D. Fichou, B. Xu, G . Horowitz, F. Garnier, Svnth. Met. 1991, 41, 463. 128. G. Lanzani, L. Rossi, A. Piaggi, R. Zamboni, A. J. Pal, C . Taliani, Cliern. Phys. Lett. 1994, 226, 547. 129. J. Poplawski, E. Ehrenfreund, J. Cornil, et ml., Molec. Cryst. Liq. Crg.s/. 1994, 256, 407. 130. M. G. Harrison, Ph.D. Thesis Cavendish Laboratory, University of Cambridge, 1994, p. 147. 131. J. Grinberg, A. Jacobson, W. Bleha, et ul., Opt. Eng. 1975, 14, 217. 132. K. Johnson, Phys. World 1992, 5 , 3 1 . 133. B. G. Sfez, E. V. K. Rao, Y. I. Nissim, J. L. Oudar, Appl. Phgs. Lett. 1992, 60, 607. 134. K. Biedermann, in Hologrcipliic Recording Matcv%ds (Ed.: H. M. Smit), Springer, Berlin, 1977, p. 21. 135. J. W. Yu, D. Psaltis, A. Marrakchi, A. R. J. Tanguay, R. V. Johnson, in Phoiorefrrrctivc~ Muteriuls und Their Applicritions (Ed.: P. Gunter, J. P. Huignard), Springer, Berlin, 1989. p. 275. 136. C. J. G. Kirkby, I. Bennion, IEE Proc. 1986, 133, 98. 137. T. Moriyama, J. Kajita. Y. Takanishi, K. Ishikawa, H. Takezoe, A. Fukuda, Jpn. J . Appl. Phgs. 1993, 32, L589. 138. J.-M. Nunzi, F. Charra, N . Pfeffer, J . r k . Phj,.s. I11 Frunce 1993, 3, 1401. 139. J. M. Nunzi, N. Pfeffer, F. Charra, D. Fichou. Chem. Pligs. Lett. 1993, 215, 114. 140. R. Raue, H. Harnisch, Hc~terocyc1r.s 1984, 21, 167.

11 Field-Effect Transistors Based on Conjugated Materials Francis Garnier

11.1 Introduction First work on organic semiconductors, such as polyacenes, started in the early 1950s and were mainly devoted to the fundamental aspect of charge transport [I]. Later, in the early 1970s, energy problems highlighted the potential interests of organic-based devices. The ease of processing organic materials held out the hope of developing low cost and large area photovoltaic devices, which stimulated active research in the field of organic semiconductors, mainly on dye molecules, merocyanines and phthalocyanines [2]. However, after a decade of significant efforts, the efficiency of organic-based photovoltaic cells did not exceed some 1 % under AM 1 solar illumination [ 3 , 41. Although this value was an order of magnitude too low for practical applications, the broad knowledge acquired through this research on organic semiconductors has been a key factor for the further development of photoconductors in the 1980s. Substantial work on organic photovoltaic cells is still carried out, mainly on dye molecules and organometallic complexes [5, 61. In the early 1980s, conjugated polymers emerged in the literature as a new class of organic materials with promising electrical properties. As shown by their prototype polyacetylene (PA), these polymers exist in two states. A non-intentionally doped state, with conductivities in the range of to loph Scm-I, is generally p-type semiconducting, although ion (Li') implantation provides n-type semiconduction. The second state, obtained by chemical or electrochemical oxidation, shows intrinsic electronic conduction. with conductivities up to lo5 Scm-', thus close to that of metals [7]. The major drawbacks shown by the first synthesized polyacetylene, i.e. low stability in air and very poor processability, were soon overcome by the development of new classes of environmentally stable conjugated polymers, such as polythiophenes [S] and polyphenylenevinylene [9], and also by the tailoring of their chemical structure toward easier processing, by using for instance soluble polymer precursors, or by grafting solubilizing groups. Intensive research, devoted both to the conducting and to the semiconducting states of these conjugated materials, aimed at the understanding of their chargetransport properties, in terms of chemical structure and structural organization. However, it soon appeared that these polymers were not well suited for rationalizing structure effects on charge transport, owing to the very low control of the polymerization reaction, which mainly leads to amorphous materials, with large distribution of conjugation lengths, high concentration of chemical impurities and structural defects. A new class of better-defined materials, conjugated oligomers, was later proposed, in the mid 1980s [lo], which have continued to attract interest, both as model

560

11 Field-Effect Trunsistors Bir.wil on Conjugritrcl Mntrrinls

compounds for their parent polymers, and also for their particular characteristics. Organic conjugated polymers and oligomers have thus been the subject of much work concerning their semiconducting properties, and various devices have been used for their characterization. Field-effect transistors (FETs) [I 1-29] and, more recently, light emitting diodes (LEDs) [30, 311 have been studied, with a longterm aim to develop a new area of organic electronics. Such applications require that the characteristics of organic devices reach those of their inorganic counterparts, and, what was considered unrealistic even some five years ago, now appears possible. Thus efficiency and extent of colors of organic LEDs are of the order of inorganic-based diodes [31, 321, and, in the case of FETs, oligothiophenes have proved recently to reach characteristics close to those of hydrogenated amorphous silicon [25, 281. These achievements open the perspectives of low cost and large area devices. Moreover, chemists know how to tailor the electronic properties of organic materials, by subtle chemical modifications of the basic molecules, which should allow the molecular engineering of their energy band levels [33]. Finally, many organic semiconductors are made from molecular materials, and without arguing about the relevance of the field named ‘molecular electronics’, the essentially molecular nature of organic semiconductors will also open the way for a large scaling down of the device geometries. We will describe in more detail the area of FETs based on organic materials. The fabrication and operating mode of these devices will be described, and the main conjugated materials, polymers and oligomers used as active layers, will be displayed. An overview of the literature results will be presented, and discussed in terms of classes of materials. i.e. oligomers as compared to their polymer counterparts. In particular, it will be shown that the electrical properties of organic semiconductors are essentially controlled by the structural organization of these materials.

11.2 Fabrication and Mode of Operation of FETs Basically, a metal-insulator-semiconductor (MIS) structure operates as a capacitor, in which a thin semiconducting film acts as a third electrode [34]. When considering for instance a p-type semiconductor, majority carriers consist of holes, h’. When polarizing the metal negatively, an excess of holes in the semiconducting film will be attracted to the insulator-semiconductor interface, where they will be distributed over a certain thickness and form a conducting channel. On the other hand, when applying a positive bias to the metal, a depletion of charge carriers occurs, which may extend over the whole thickness of the semiconducting layer. Thus, by applying a voltage on the metal (gate), one is able to generate a field across the insulating layer and either accumulate charge carriers at the semiconductor-insulator interface, or deplete this interface. The formation of such a conducting channel can be detected by two other electrodes at both ends of this channel, through which current can flow when polarized. A FET is thus a three-electrode device in which the conductance

of a thin channel at the seiiiiconductor-insulator interface can be monitored through the gate bias. Various gm?ietries have been described in the literature for the construction of FETs [35]. Owing to the thermal and mechanical fragility of organic semiconductors, the most widely used structures for organic-based FETs are inverted staggered (Fig. I a ) or inverted coplanar (Fig. 1 b). Various rorrte,P mid r?irrtcvicils have been used for fabricating FETs. The first devices involved conventional silicon processing together with polymer film deposition technique [12- 181. The substrate was made from a highly doped silicon chip. acting also as the gate electrode. An oxide layer SiO, was therinally grown on silicon, forming the insulator. Other classical insulating layers have been used, such as silicon nitride Si3NI, or aluminum nitride AIN. Other materials have been used as substrate. such as glass, on which a gate electrode (Ag, Al) has been vacuum evaporated. The source, S, and drain, D, electrodes were then deposited on the gate oxide, using either conventional lithography techniques. with chemical etching or lift-off techniques, or by evaporation through a mask. These electrodes are made out of gold. in order to build an Ohmic contact with the organic semiconductor. which was deposited in a last step, leading to an 'inverted coplanar' device structure. Due to the low processability of most conjugated polymers. various ways have been followed for the deposition of the thin film of semiconducting polymers, overcoming their infusibility and insolubility. A polymer film may be grown electrochemically from a monomer solution, on the source and drain electrodes. The electrodes were connected together. allowing the electropolymerized polymer to form a continuous film between them, as described in the case of polythiophenebased FETs [ 131. The oxidized polymer obtained is then electrochemically reduced to its neutral semiconducting state, either completely or only partially, allowing control of a remaining doping level. Soluble polymer precursors have also been used. in the case of polyacetylene or polythienylenevinylene [ 14, 231 (see Scheme 1 below). Soluble conjugated polymers, obtained by grafting of long alkyl chains, as shown in the case of poly(3-alkyl)thiophenes, may be spin-cast on the device structure [15, 18-20, 261. These routes of film deposition suffer from the relatively large concentration of chemical impurities and the poorly defined semiconducting film, which strongly affects the device characteristics. Another interesting route for the construction of FET has been based on the use of Langmuir-Blodgett technique [ 17. 291. Finally. one of the most convenient and rational routes toward pure and defect-free semiconducting film involves vacuum deposition, under low pressure, ranging typically between 10-' and IO-'Pa, from a semiconductor powder in a tungsten boat heated to the semiconductor melting point [16. 211. This technique has been successfully applied to fusible molecular semiconductors. such as phthalocyanines and conjugated oligomers. Although the concept of 'all-polymer' devices does not yet seem to be well understood [36], the potentially most interesting feature of an or3mic riei,icr originates from the fact that all its elements, substrate, insulator, semiconductor. and even electrodes can be realized out of organic materials. which require only low processing temperatures [37]. Thus, the substrate can be made from any polymeric material, such as polyethylene, polycarbonate or polyimide. The insulator, requiring

562

source

L

1"

drain

vd

-

e

Figure 2. Schematic view of an organic FET. W and L vre the channel length and width. respectively.

high resistivity and high dielectric constant, can also be made from polymer. such ;is polymethylmethacrylate (PMMA) or polyimide, which can be easily spin-coated on the substrate. An example has even been given, in which the electrodes were also made from a graphite-based polymeric ink, leading to the first all-organic device [38]. These features. specific to organic semiconductors, open interesting new perspectives for organic devices. As a matter of fact, contrary to the case ofconventional devices based on stiff. heavy and light-absorbing inorganic materials. devices based on oligomers and polymers offer the possibility of controlling their niechanical as well as their optical properties, ;is known from the chemistry of these materials. Flexible and/or transparent polymers and oligomers can be achieved, which should bring a new dimension to organic-based devices. The riiorlr q f o p c ~ ~ i t i oofn these insulated FETs has already been analyzed in the case of crystalline semiconductors [34]. and generalized to amorphous and polymerbased semiconductors [35].The source electrode is grounded, and the insulated gate electrode is polarized negatively in the case of p-type semiconductors (Fig. 7 ) . The positive charge carriers, majority carriers. are then attracted to the semiconductor insulator interface. and, due to the low conductivity of the semiconductor, distribute over a certain thickness, forming a conducting channel. through which current can flow between source and drain, Insulated-gate FETs can also be considered a s unipolar devices, which operate as variable resistors (Fig. 3). At low drain voltage. the druiii cnrrrnit increases proportionally to the drain the voltage between drain voltage, Vd, and when 1; reaches the gate voltage. and gate approaches 0. resulting in the pinch-off of the channel, which causes the saturation of the drain current. The critical parameters of such FETs are the channel width, W,the channel length. L. and the capacitance per unit area of the insulator layer of thickness h, C; = E E , / ~ .Two main regimes are observed when plotting the drain current a s function of drain voltage, at constant gate voltage (Fig. 3). A first linear regime, observed for low drain voltage. followed by a saturation one when the

Gatevoltage V’ (V) -2C

-It -20 -18

-12

-16

-8

-14

-4

-12 -10 -8

0 0

-1s Drain voltage Vd ( V )

-5

-10

-20

-2s Gate Voltage V, (V) 0

-1.2

0.96

0.72

+2

0.48

+4 +6

+8

0.24

+lo +20

n 0

-5

-10 -15 Drain voltage vd (V)

-20

-2s

Figure 3. Experimental output Id = f( V,) obtained with an organic FET, realized on glass substrate, with PMMA insulating layer (C, = IOnF), sexithiophene semiconducting layer (25 nm thick) and gold source and drain electrodes (W = 5 mm; L = 50 pm): (a) in accumulation regime (negative gate bias); (b) in depletion regime (positive gate bias).

11.2 Fubricatioii criid Mode oj'Opercirion of FETs

565

drain voltage exceeds the gate voltage. The basic equation relating the drain current, to the applied gate voltage Vgand the drain voltage Vd for the linear regime is

Id,

Id = ( W / L ) P C i [Vg ( - V#

-

$ v:]

(1)

where p is the field-effect carrier mobility, and VTis the threshold voltage, given by = (qdpo)/C,,where q is the absolute electron charge,po is the hole density at zero voltage, and d the semiconducting film thickness. An important parameter of such a device concerns the transconductance, gm,which is used to calculate the field-effect mobility, p, generally carried out at low v d value: gm = (dId/dvg) ;L

=,st

For higher values of Idmt.

= ( W/L)pCi Vd vd.

= (W/2L)PCi( Vg

(2)

the saturation regime is described by

-

VT)'

(3)

Whereas the accumulation regime, described by Eqs. (1)-(3), is generally observed under positive gate bias for p-type organic semiconductors, the occurrence of a depletion regime has seldom been characterized. The thickness of this depletion layer, W,,, varies following Eq. (4):

w,,= (&/C,)[(l+ 2C?(Vg

-

V f b ) / ( q N &p 11

(4)

where E, is the dielectric constant of the semiconductor, V , the flat band potential and N the dopant concentration. It must be mentioned that the thickness of the depletion layer, W,,, increases up to the total thickness of the semiconductor, d, which is reached at the 'pinch-off voltage Vp given by

vp = (qNd'/2&oes)(l+ 2CJCi)

(5)

where C, is the dielectric capacitance of the semiconducting layer. Equation ( 5 ) is particularly interesting, as it shows that the dopant concentration, N , can be obtained by the determination of the pinch-off voltage V p . The drain current observed under the depletion regime is given by Id =

( W d q p / L ) [ l- ( V g -

Vt%)/Vplvd

(6)

Experimental output I d = f ( v d ) , obtained with sexithiophene (H-T,j-H)-based FET in the accumulation and depletion regime, are presented in Figs. 3a and b respectively [27]. These as-obtained experimental curves, shown without any correction, confirm the relevance of organic-based FETs. Besides the obtained drain current, up to some tenths FA, a very important characteristic of such a device concerns the dynamic range, or Z o n / l o ~ratio, which must be as high as possible, and exceed some lo7 for practical applications. Classical FET structures involve, as source and drain contacts, back-to-back barriers, formed either by Schottky barriers or by n+-p junctions. Under these conditions, a very low current is expected at zero gate voltage. On the other hand, in the case of organicbased FETs, source and drain electrodes are generally Ohmic contacts, made from gold. In these organic devices, a large gate voltage is needed for creating an accumulation layer at the semiconductor-insulator interface. However, due to the bulk

conductivity of the semiconductor, an Ohmic contribution to the FET drain current has to be taken into account [40]. This current, which flows in the bulk of the semiconductor parallel to the channel, is not blocked by the Ohmic electrodes, and forms a leakage current which can mask the saturation of current at high drain voltages. These considerations allow one to point out that high field-effect mobility is not the only parameter which defines the relevance of a semiconductor. High mobility is indeed necessary for high current output for the device in its 'on' position, from Eqs. (1)-(3), as well as for short switching time, but low conductivity is at the same time a prerequisite for ensuring low loct.currents and thus high dynamic ratio lon/loff for the device. In fact, when considering the characteristics of amorphous hydrogenated silicon as the goal for such organic semiconductors, it must be remembered that its mobility reaches I cm' V-' s C ' , but that its conductivity is very low, of some loCxScm-', which ensures the high dynamic l o , , / I , , ~ratio, of the order of lo7 to lo', for a-Si:H based devices. From such experimental data, Eqs. (l)-(6) allow the j?field-eJhct mobility, p,, together with the doping level N to be calculated. The obtained field-effect mobility differs, however, from the intrinsic or microscopic mobility of the semiconductor, l ~ ~mainly ), due to the presence of surface states and of traps in the bulk of the semiconductor, in which field-induced charges are distributed with a temperature dependent profile. The presence of such traps leads to effective mobility values, which are much lower than the microscopic one, po. An analysis has been carried out, taking account of the presence of a shallow trap level, at an energy depth AE,, and at a concentration N , [41]. Under such conditions, a thermal equilibrium exists, relating the free charges to the trapped charges, HO = (N,./N,)exp(-AE,/kT) (7) where N , is the density of states near the top of the valence band. The current being dependent on the density of free charges, the consideration of such distribution of charges represented by Eq. (7) leads to a modification of Eq. (3) representing the saturation current, where the mobility becomes poBo. This situation holds for low gate voltages, but, as Vg is increased, the density of field-induced charges may exceed the trap density N , . All traps being filled, any additional field-induced charge can be considered as free, and the carrier mobility observed under such conditions becomes close to the microscopic one, po.This analysis of trapping effect has been formalized in terms of a multiple trapping and release mechanism for describing the charge transport in conjugated materials [41].

11.3 Conjugated Materials Used in Organic-Based FETs Many conjugotcri niatcriuls have been analyzed concerning their semiconducting properties, through the characterization of FETs made from these materials. Two main classes can be distinguished among these materials, macromolrcular ones, i.e. conjugated polymers, and tnolmdcir ones, i.e. conjugated oligomers and other 7r-electron rich molecules. The first organic FETs were based on trans-polyacetylene

(PA) [ I 11, and later, on polythiophene (PT) [13]. The low processability of these materials was partly overcome by the use of soluble polymer, poly(3-alkyl thiophene) (PAT) [15, 18-20], with various alkyl chain lengths, and recently polyaniline [29], and also by the use of soluble precursors. in the case of PA [14], and polythienylenevinylene (PTV) [23] (see Scheme 1). Interestingly, solubility of PATS allowed the realization of Langmuir-Blodgett films for building FET structures [ 171. The most-studied conjugated polymers are the following:

Mn

Polyacetylene (PA)

Polythiophene (PT)

Poly(3-alkylthiophene)

Pdy(thieny1ene vinylene)

( PATI

(PTY

Scheme 1. Conjugated polymers used in organic-based FETs

Conjirgrrted oligormrs have been proposed as active layers in FETs, and those most studied are the thiophene oligomers, either unsubstituted or alkyl substituted, as the pendent group in the 13-position or as end groups in the a,cu’-position [16,21, 25,441 (Fig. 4). Unsubstituted oligothiophenes. from terthiophene H-T3-H to octithiophene H-T8-H, as well as an’-dialkyl substituted oligothiophenes R-T,-R ( R = alkyl), from dialkylterthiophene to dialkyloctithiophene, have been deposited by vacuum evaporation, owing to their very low solubility. When substituted in the []-position, these oligomers become highly soluble, such as /j-didecylsexithiophene and &tetradecyldodecithiophene, and have thus been deposited by spin coating [45] (see Scheme 2). Other classes of 7r-rlecti~mrich i~ioleczrleshave been studied in the last ten years, such as scandium. lutetium and thulium diphthalocyanines (ScPc?, LuPc: and

a b

C

Scheme 2. Conjugated oligomers derived from (a) sexithiophene, (b) tr,o’-dialkylsexithiophene, and (c) J, j’-dialkylsexithiophene, used as active semiconducting layers in FETs

TinPC?) [46, 471, nickel and zinc phthalocyanines (NiPc and ZnPc) [48], fullerene (C(,") [49], tetracyanoquinodimethane (TCNQ) [50], and a charge transfer type salt, (N-octa)'-Ni(dn1it)2 [ 5 I]. These materials were deposited by vacuum evaporation, and Langmuir-Blodgett technique for the latter. Before analyzing the experimental results, it is worth comparing the particular properties of molecular semiconductors with covalent scwiiconductors, represented by silicon. Inorganic semiconductors, such as silicon, present a three-dimensional architecture, in which atoms are held by strong covalent bonds, with energies of the order of 78 kcalmol-' in the case of Si-Si bonds. Semiconductivity appears then as a collective property, which develops with the constitution of the 3D material. Owing to the strong interatomic bonds, the width of the conduction and valence bands is large, from which one may expect large carrier mobility values. These materials are very sensitive to chemical impurities, which constitute the principle of doping of these materials, and are also marked by the presence of free bonds at their surface, leading to high sensitivity of their electrical properties to surface states. On the other hand, organic molecular semiconductors are made out of molecules, which are only held together by weak van der Waals forces, of some 10 kcal mol-' . The electronic properties of the solid are already present in the individual molecules, as shown for instance by the closeness of the absorption spectra of individual molecules and of the solid. These features indicate that charge transport in molecular solids mainly operates through individual states, and furthermore that the width of the valence and conduction bands (when assuming validity of such representation) is small, and hence that carrier mobility is much lower in these solids. Finally, these organic molecular materials are less sensitive to chemical impurities, undergoing many fewer substitutions, and the quasi absence of dangling bonds prevents them from high sensitivity to surface states. Thus, these considerations show that organic molecular materials cannot be expected to reach as high mobilities as those of their inorganic counterparts, which range from some 10' cm2 V-' s-' for monocrystalline silicon to some lo-' to 1 cm2 V-' s-l for amorphous hydrogenated silicon, a-Si:H. However, it has already been shown that the carrier mobility of monocr stalline condensed aromatic hydrocarbons reaches values in the range of 1 to 10 cm- V - 1 s- I , at room temperature [I]. Thus, owing to the ease of realizing highly structured films of organic molecular compounds, one can reasonably hope that organic semiconducting films will be able to reach mobility values close to those shown by a-Si:H, and open a potentially interesting field of organic-based devices challenging some applications of amorphous inorganic semiconductors.

Y

11.4 Device Characteristics FET c.1irrraL.teristic.s will be analyzed for macromolecular and molecular materials, for which the relevant parameter regarding their electrical properties appears t o be their structural organization.

569

11.4 Device Cliurricteristics

1 1.4.1 Conjugated Polymers and Amorphous Materials The main data obtained with conjugated polwzers and other aniorphoiis materials are gathered in Table 1. Ranges of values are given for some compounds, which indicates attempts to increase the electrical properties by various treatments. The lowest value is associated to as-prepared films. Unless quoted, these materials behave as p-type semiconductors. The data indicate that as-prepared organic semiconducting films possess very low mobilities, in the range of lo-' cm2 V-' s-' to cm' V-' SKI, which have been interpreted in terms of poor efficiency of charge hopping in highly disordered materials, originating from self localization and defects. Various attempts have been performed in order to improve the$field-efiect mobility of these organic semiconductors. Thus their doping level has been intentionally increased, either electrochemically, in the case of electropolymerized polythiophene [ 131, or chemically, in the case of poly(3-alkylthiophene) [I 5, 18-20], poly(DOT)3 [26], Cb0[49] and TCNQ [50]. Some other attempts have been based on the 'annealing' of the semiconducting film, often realized under oxygen atmosphere [46, 471. Results in Table 1 confirm that a significant increase of field-effect mobility has been obtained by such doping, by one or two orders of magnitude, and many authors have underlined the existence of a relationship between intentional doping and carrier mobility [13, 15, 19,26,47,52,53]. A 'universal' relationship has recently been proposed [26], based on the observation that conductivity varies with doping level according to a CJ N ' relationship. The conductivity being given by the relation = Nfqm, where N f is the density of free carriers related to the total dopant concentration N by Nf= AN, it follows that mobility varies as ,LL N 7 - ' . As a matter of fact, the hopping mechanism associated with amorphous organic semiconductors implies a strong dependence of charge transport to the N

N

Table 1. Electrical characteristics of conjugated polymers and amorphous materials Material

Deposition technique

Conductivity (scm-')

Polyacet ylene Polyacetylene Polythiophene Pol yhexylthiophene Pol yalkylthiophene Poly(DOT)i Polythienylenevinylene Polyphenylenevinylene Polyaniline (n-type) Lu diphthalocyanine Sc diphthalocyanine Tm diphthalocyanine Fullerene (n-type) TCNQ (n-type) (N-octa')-Ni(dmit),

Polymerization Precursor Electropolymer Langmuir-Blodgett Spin coating Spin coating Precursor Precursor Dipping Vacuum evaporation Vacuum evaporation Vacuum evaporation Vacuum evaporation Vacuum evaporation Langmuir-Blodgett

10-5

IO-~ I O - ~ to 4x 10-8 to lo-* to 10-5 10-8 to lo-? I O - ~ to lo-' 5 x 10-6 10-4 to I O - ~ to 10-'O to 1

Mobility (cm' v-' s-')

Ref.

hopping distance, R, in the form of (T = 2q2R2vPhN(EF) exp(-2nR)(- W / k T ) , where R is the hopping distance, N ( E , ) the density of states at the Fermi level, N the electronic wavefunction overlap and W the energy difference between initial and final electronic states. The incveuse of doping level leads to the increase of N(EF),and to the decrease of the hopping distance, R, which should hence increase the efficiency of charge transport. The mobilities achieved by increase of the doping level N, up to p = lop2cm2 V-' s-I, are quite interesting values indeed. However, as shown by these equations, intentional doping also leads to a simultaneous increase of the conductivity, up to values of the order of 10-'Scm -'. Large Ohmic currents are therefore currents and dramatically reduce the dynamic ratio expected, which increase loff of the corresponding devices. The simultaneous increase of mobility and conductivity thus invalidates this approach for the improvement of charge-transport efficiency in conjugated materials. In conclusion, conjugated polymers and disordered materials, in which charge transport is best described by a hopping mechanism, do not meet the requirements for building efficient FET devices. The field-effect mobility of as-prepared films has been generally found much too low, and the attempts to increase it by chemical or thermal treatments cannot be considered as successful, as the conductivity rises simultaneously.

11.4.2 Conjugated Oligomers, Role of Structural Organization On the other hand, early work on conjuguted oligornevs has revealed a significant increase in field-effect mobility, the first results on sexithiophene showing values of 2 x cm2 V-' s p ' [16]. The first results obtained with conjugated oligomers stimulated a large interest, all the more since these materials offer the unique advantage of high purity, well defined structure and sharp effective conjugation length. These feature opened the way for the rationalization of structure effects on electrical properties of thin films of conjugated materials [25, 27, 561. The first work in this direction has been devoted to the vole of effectiveconjugution length of molecules on their electrical properties (see Chapters 6,7.1 and 7.2). In this regard, thiophene oligomers ranging from the trimer, terthiophene, H-T3-H, to the octamer, octithiophene, H-Tx-H, have been characterized concerning their conductivity and field-effect mobility [43]. Conductivity has been measured both parallel to the substrate surface, oil, and perpendicular to the substrate, i.e. across the film thickness in a sandwich-type structure, al.Mobility p has been measured from FET characteristics. The data obtained (Table 2) show an increase of conductivity with conjugation length, as generally observed on conjugated materials. The field-effect mobility increases much faster, up to the hexamer sexithiophene, H-T6-H, and then decreases slightly. It must be noticed that the decrease from H-T6-H to H-Ts-H may not be significant, as the chemical purity of H-Tx-H is much more difficult to achieve, owing to the extreme solubility of this compound. The results show that, on the conjugation length basis, one of the most interesting candidates for FET devices is the hexamer, sexithiophene H-T6-H, on which further work has been focused.

11.4 Device Clinracteristics

57 1

Table 2. Electrical characteristics of thin films of unsubstituted oligothiophenes. Oligothiophene

Conductivity ( S c m - ' ) perpendicular (mL)

Terthiophene H-T3-H Quaterthiophene H-T4-H Quinquethiophene H-T5-H Sexithiophene H-T6-H Octithiophene H-T8-H

Mobility (cm' V-' parallel (oil)

(PI

-

40-7 2 lo-' 2.5 2 x 10-3 2 x 10-'l

IO-~ 2 lo-' lo-'

1o-6 ~

SC')

The large increase in mobility from polythiophene to sexithiophene was soon attributed to the higher purity and better structural organization of the short conjugated oligomer. In fact X-ray characterization of vacuum evaporated films of sexithiophene revealed well structured layers, which were also confirmed in the case of alkyl-substituted short oligothiophenes [ 5 5 ] . This increase in chargetransport efficiency can be analyzed by considering the features known for highly conducting charge-transfer complexes, such as TTF-TCNQ [56]. These complexes are made of regular stacks of donor and acceptor molecules, with short intermolecular distances allowing important overlap of the 7r orbitals of neighboring molecules. The charge propagates preferentially along the stacking axis of donor (or acceptor) molecules, through the overlapping 7r orbitals, and it has been largely shown that structural organization of molecules in the solid plays a major role. Following this model, charge-transport efficiency in conjugated oligothiophenes would require the molecules to remain fully planar and parallel to each other, in the closest possible packing, with the longest possible range order, for avoiding grain boundaries known to be very efficient traps for charges. Various routes can be followed for an apriori control ojtlie structural organization of molecules in the semiconducting film, considered as a molecular assembly. Among them, one can use a physical approach, involving either the modification of the experimental conditions used for film deposition, or a film treatment. Another route involves a chemical approach, by tailoring the molecules in order to induce self-assembly properties. The final step concerns the growth of a single crystal of the organic semiconductor, whose properties can be considered as the achievable limits for charge transport.

11.4.3 Experimental Conditions of Film Deposition The structural organization of organic materials deposited as thin films on a substrate can be controlled by the teniperatiire of the substrate. as well as by the rate of evaporation. Thus it has been reported that the crystallinity and molecular orientation of vacuum-evaporated polythiophene are substantially improved by raising the substrate temperature from 20 to 15OT [57]. In the case of oligothiophenes, Biiuerle et 01. have shown, from UV-VIS-IR and fluorescence-polarized light spectroscopy measurements, that ultra-thin films in the monolayer range,

572

I 1 Field-Ejfi,ctTrunsistors Bused on Conjuguted Muterhls

vapor-deposited at room temperature, are strongly oriented, with the long molecular axes perpendicular to the substrate plane, but that this orientation is lost on thicker films [58]. A more detailed study has been carried out on sexithiophene, H-T6-H, which was deposited as 2-3pm thick films on Si substrates, held at temperatures varying from 7 7 K to 260"q [27]. Film deposition was performed eitber at low rate, ranging from 1 to SAs-', and also at high rate, of some 100Asp', for a sample at room temperature. Structure and morphology of these films were analyzed by X-ray diffraction, in 0-20 scanning symmetrical reflection mode, by polarized UV-VIS spectroscopy and also by SEM. Experimental results evidenced the polymorphism of H-T6-H, depending on the experimental conditions of film deposition. When the substrate is held at 77K, molecular motions are frozen, and the first deposited H-T6-H molecules nucleate and crystallize preferentially with the long axis lying parallel to the substrate plane. At room temperature and low deposition rate, a new peak at low angle, corresponding to 001 reflection, indicate that H-T6-H molecules adopt a different orientation, with crystallites having their c axis perpendicular to the substrate plane. At room temperature and high deposition rate, some H-T6-H molecules also remain frozen on the substrate, and the H-T6-H film involves the two preceding populations, with crystallites having their c long axis either parallel or perpendicular to the substrate plane, corresponding respectively to the kinetically favored or thermodynamically favored orientation of the crystallites. When deposited at 190°C, films of H-T6-H exclusively show several orders of meridional 001 reflections, which means that a great majority of crystals are grown with their c axis perpendicular to the substrate plane. The adsorbed molecules stand up and crystallize in a self-organized way with a single preferential 002 orientation. When further heating the substrate to 260°C, the XRD spectrum only reveals low angle 001 reflections, indicating that H-T6H films are entirely crystallized, with the crystallite (a,b) face parallel, and c axis perpendicular to the substrate plane respectively. Polarized UV-VIS spectroscopy was carried out on these films deposited at various substrate temperatures [27]. Absorption spectra were recorded at an incidence angle of 80°, using s-polarized light, with electrical field oscillating parallel to the substrate plane, and p-polarized light, containing both parallel and perpendicular components to substrate plane. Owing to the rigid all-trans planar conformation of the H-T6-H molecule in solid state, one can infer it T-X* electronic transition to be polarized along the long axis of this molecule. When increasing the substrate temperature from 77 K to 260°C, the absorption in the visible range under s-polarized light decreases strongly, with a simultaneous increase of the dichroic ratio. The spectra obtained at 260°C closely resemble those obtained by Egelhaaf et ul. on monolayer films [58]. These results thus confirm the very high orientational order obtained when depositing H-T6-H films on substrates held at elevated temperature. Finally, the morphology of these films has been analyzed by the use of scanning electron microscopy [27]. Results show that films deposited at 7 7 K present a uniform surface consisting of small crystallites (10-30 nm). When deposited at room temperature, the size of the crystallites increases to some SO nm diameter, with an isotropic distribution. When the substrate is held a t 190"C, the crystallites show an elongated

573

11.4 Device Characteristics

Table 3. Electrical characteristics of sexithiophene H-T6-H films deposited on substrates at various temperatures (from Ref. [27]). Substrate temperature

77 K RT 190 C 260°C

Morphology

Small plate: 10 x 30nmIsotropic grains 50 nm diameter Long grains 300 x 200 nm’ Connected long grains 50 x 400 nm’

Molecular orientation Parallel substrate Parallel and perpendicular substrate Perpendicular substrate Perpendicular substrate

Conductivity ( S cm-l)

~

2

lo-’

4

u/l

w

6x

6x

1.5

2

9

~

IO-~

Mobility (cm’V-I s-l)

1.2

lo-’ lo-’

lor3

9

2.5

lo-?

shape, with larger dimensions reaching 30 x 200 nm2, with a close packing arrangement. On the last sample realized at 260°C, the H-T6-H layer shows discontinuous surface, arising from a possible cellular growth. Long lamellae, 50nm wide, are observed, most of them being interconnected, giving rise to a network over the film surface. These results confirm that the control of substrate temperature allows monitoring of the grain size and shape, together with homogeneity of structural organization. In particular, the development of lamellar crystals, with possible interconnection resulting from their coalescence, provides highly organized molecular layers, with lower concentration of grain boundaries. More recently, another way has been described for improving the structural organization, which involves the annealing of a ,film of conjugated material. As sublimed films of H-T6-H are polycrystalline, with isotropic grain size of about 50nm. annealing has been performed, by heating the film for a short time, 1 s, at the melting temperature of H-T6-H, 310°C. The crystallite size increases up to some microns [28]. In conclusion, these structural characterizations confirm that the control of substrate temperature and evaporation rate for film deposition, together with the annealing of the film, allow control of the structural organization of oligothiophene molecules in their solid state. A large improvement of the charge-transport properties of these materials can be expected, as described in the following. These H-T6-H films, with various structural organization, have been characterized concerning their electrical properties, i.e. parallel and perpendicular electrical conductivity, together with field-effect mobility (Table 3). The conductivify parallel to the substrate surface, 011. has been measured using a four probe technique, and the perpendicular conductivity, o ~from , a sandwich type structure, with two gold contacts, Au-T6-Au. Parallel conductivity decreases when increasing the temperature, which can be ascribed to the desorption of impurities on a heated substrate. On the other hand, the very large conductivity shown by H-T6-H film deposited on a substrate maintained at 77K can be attributed to the pollution of the film during vacuum

deposition on a cold substrate. The perpendicular conductivity also decreases when increasing the substrate temperature, which shows that the conditions used for film deposition allow a further purification of the molecular material, which should significantly improve its electrical properties. Importantly, the anisotropy qfconu'trctivity increases with substrate temperature, in full agreement with the proposed scheme of charge-transport process occurring along the stacking axis of the HT6-H molecules, i.e. parallel to the substrate plane. The increase of anisotropy of conductivity rril/crl follows the increase in molecular ordering perpendicular to the substrate surface, as previously shown from structural characterizations. The field-ejfcjct mobility has been measured from FETs realized on glass substrate, using a thin (0.5 pm) PMMA insulating layer. two gold source and drain electrodes, constructed according to an inverted planar device structure (Fig. 1 b). Large channel geometries were used, W = 5 mm and L = 50 pm, owing to the conventional masking technique employed for FET fabrication. Field-effect mobility was calculated in the linear regime of Id-Vd curves, from Eq. (2). The lowest value is obtained for a film deposited at room temperature, which has been shown to be the less organized one, with two orientations of crystallites, one parallel to the surface, and the other perpendicular to the surface. The increase observed when cooling the substrate at 7 7 K can be attributed to the higher homogeneity of the film, with the main population of molecules lying parallel to the substrate plane. Although the stacking axis of molecules in this last film is not oriented parallel to the substrate plane, along the source-drain electrode axis, the higher observed mobility, as compared to room temperature H-T6-H, allows the extreme importance of grain boundaries in charge transport to be shown. Room temperature evaporated H-T6-H films, which show multiple crystallite orientation and highest density of grain boundaries, possess the lowest mobility. When films are deposited on heated substrate, the observed field-effect mobility increases rapidly, up one order of magnitude, reaching 2.5 x 10P2cm2V-' sP1. This value has been recently confirmed on FET devices with micron-sized channel, L = 12pm and W = 100pm [28]. These results bring a clear confirmation of the predominant role played by structural organization of the semiconducting film, and also on the control which can be exerted by the experimental conditions used for the film deposition.

11.4.4 Chemical Engineering of Molecules Chemical engineering of these oligothiophenes may also represent an interesting alternative route for controlling the molecular organization in the film. As a matter of fact, micellar chemistry has already shown the large potential of various chemical substitutions, using for instance alkyl or alkoxyl groups, for inducing a mesoscopic organization of molecules. Following this concept, oligothiophene molecules have been substituted with alkyl chains, both as pendent groups in the P-position [45], and as end group in cr,n'-positions [24, 441. These substitutions have been carried out on short oligomers, using methyl and ethyl groups, and also on longer oligothiophenes with six and twelve repeat units (Scheme 2).

11.4 Device Clrcirricteristics

575

Table 4. Electrical characteristics of films of oligothiophenes substituted by decyl pendent groups in the ;+position. Oligothiophene

;Ad’-Didecylsexithiophene P,~’,~”,d’’’-Tetradecyldodecithiophene

Conductivity (Scni-‘)

Mobility (cm’ V-’

lo-13

40-7 5 x 10-6

10-9

SC’ )

These oligomers have been deposited as thin films at room temperature on various substrates, and their structure has been analyzed by the use of X-ray diffraction and STM. Results have shown the existence of two classes of molecular organization, depending on the type of alkyl substitution, either in the @-position or in the cy,a’-position. P,P’-Dialkylsubstituted sexithiophenes have been analyzed by Xray diffraction spectra, carried out on monocrystals, and also by STM, performed on monolayers deposited on highly oriented pyrolytic graphite (HOPG) [59]. Results have shown that the molecules remain almost planar, the torsional angles between rings varying from 5 to 11 . The alkyl side chains are in the planar zigzag form, parallel to each other, almost confined to the plane of the oligothiophene backbone, and in all-trmzs conformation. This particular conformation leads to a staircase-like ordering of these molecules, with a large intermolecular spacing between conjugated chains, imposed by the length of the pendent alkyl chains. The large increase in intermolecular distance, and hence in the distance for charge hopping, can be expected to lead to a dramatic lowering of the charge transport in /3-substituted oligothiophenes. Indeed, this has been observed in the electrical in the electrical characterizations [25], as shown in Table 4,where the conductivity and field-effect mobility values are summarized for two oligothiophenes substituted in the Y-position by decyl chains. These results confirm that alkyl substitution in the p-position leads to almost insulating materials, as due to the large increase in the spacing between the conjugated sexithiophene backbones. Only in the case of the very long conjugated dodecithiophene, tetrasubstituted with decyl chains, are some significant conductivity and field-effect mobility restored. Various oligothiophenes substituted at their u,cy’-end positions have been described in the literature. Alkyl groups have been used, such as methyl and ethyl on terthiophene to sexithiophene, or hexyl groups on sexithiophene and octithiophene [25, 441. End-capped oligothiophenes, in which the terminal a,P-positions are blocked with a trimethylene bridging unit, have also been studied [60]. Among these compounds, alkyl substituted oligothiophenes have been the subject of the most intense structural characterizations, and the relevance of crystal structure to charge transport has been discussed at length in an interesting review article [56]. Crystal structure has been determined by X-ray diffraction on short substituted oligomers R-T,-R (dimethylterthiophene and dimethylquaterthiophene), whereas dihexylsexithiophene and dihexyloctithiophene, which d o not crystallize, have been characterized as thin films by 8-28 scanning XRD. Detailed spectroscopic studies under polarized light have also been used for analyzing the structural

576

I 1 Field-Effect Transistors Based on Conjugated Materials

Substrate

Figure 4. Schematic representation of a,[>’-dihexylsexithiophene film.

organization of films. X R D and spectral data have confirmed that, even when deposited on substrate at room temperature, films of dihexylsexithiophene are highly structured. Numerous high-order 001 reflections are observed in the X R D spectrum, up to the 34th order, in agreement with results obtained in dimethylquaterthiophene, which indicates that the crystallites, with monoclinic unit cell, have their long c axis perpendicular to the substrate plane. Furthermore, structural organization at the mesoscopic level has been obtained from X-ray pole figures, which confirmed the existence of almost one single population of molecules, standing up on the substrate plane, with their (a,b) face as contact plane. The analysis of the molecular organization in these films has led to the schematic representation shown in Fig. 4 [25].The almost complete structuration of molecular layers, realized on a substrate at room temperature, must be associated with the stacking properties brought by the terminal alkyl groups, which are already known for inducing long range ordering and mesophases. These films can be described by a liquid crystallike superstructure, imposed by the terminal alkyl groups for the whole molecular assembly. Alkyl-alkyl recognition, based on lipophilic-hydrophobic interactions, bring a strong driving force for a close packing of the conjugated sexithiophene backbones, and also, most importantly, for a long-range molecular ordering. This effect can be expected to depend on the chain length of the alkyl group, and work in the literature concerning for instance alkyl substituted phthalocyanines have shown that long range organization is optimized with CI2 to C16 alkyl chains. Too long alkyl chains may be expected on the other hand to dilute the T conjugated systems in a highly insulating paraffinic environment, which suggests that C6 to C9 alkyl chains represent a good compromise for ensuring structuration of molecular layers together with enhanced charge transport properties. The most remarkable feature realized through a,a’-dialkylsubstitution concerns thus the

577

11.4 Device Clmacter.istics

Table 5. Electrical characteristics of films of oligothiophenes, end-substituted with alkyl groups. Conductivity ( S cm-’)

Oligothiophene

Mobility (cm’ V-’

Ref. SC’)

perpendicular crL parallel cr11 n,cY‘-Diethylterthiophene a,o’-Diethylquaterthiophene 0.0’-Diethylquinquethiophene oi,a’-Diethylsexithiophene

0.0’-Dihexylsexithiophene 0.0’-dihexyloctithiophene

Eth-T,-Eth Eth-T,-Eth Eth-TS-Eth Eth-Th-Eth Hex-T6-Hex Hex-T8-Hex

~---

5x 5x

~

~

~

~

6 x lo-’ 4x

2 x lo-’ 5 x lo-’ 9x 1 x lo-? 8 x lo-’ I x lo-?

[44 [44 [44 [44

[25 [25

obtained mesophase-type structure of the film. Lower concentration of grain boundaries in these films can be expected to meet the requirements for improved charge transport properties. The electrical characteristics of a,a’-dialkylsubstituted oligothiophenes are summarized in Table 5. As compared to unsubstituted oligothiophenes, results show that cY,a’-dialkyloligothiophenes present an increased parallel conductivity, ~ 1 1 , which reflects the longer range order and fewer defects existing in layers of these last conjugated materials. The anisotropy ratio of conductivity also increases greatly, but it must be remembered that insulating layers, formed by the two sublayers of hexyl groups belonging to two adjacent molecules, decrease the perpendicular conductivity c ~ . More significantly, the field-effect mobility increases greatly, the highest value being obtained for a,a’-dihexylsexithiophene, which reaches p = 8 x 10-’cm’Vp’sp’. This value, close to that of a-Si:H gives reasonable hope for real applications of these materials as active layers in FETs. The most important conclusion concerns the potential interest of the chemical approach for controlling the structural organization of oligothiophene films, considered as molecular assemblies. Chemical engineering of oligothiophene films, realized by end-substitution with alkyl groups, is an elegant and powerful way for inducing self-assembly properties to the oligothiophene molecules toward highly structured molecular layers.

11.4.5 Single Crystals Single crystals represent the ultimate molecular organization, and should allow setting of the limit of achievable electrical properties. Unsubstituted or a,a’-dialkylsubstituted terthiophene and quaterthiophene have been described in the literature, but their low conjugation length together with experimental problems concerning their low melting temperature restricted their interest for devices. On the other hand, our group succeeded recently in growing single crystals of unsubstituted sexithiophene, whose size and shape are compatible for the fabrication of FETs. A complete crystallographic study has been carried out on H-T6-H single crystals [61], which showed that the unit cell is monoclinic and contains four molecules closely packed in a herringbone structure, as shown in Fig. 5.

518

f 1 Field-Efect Trurisistors Bused

011

Conjugcited Muteriuls

Figure 5. Crystal structure of sexithiophene

H-T6-H single crystals appear as small plates, with dimensions of some 3 x 3 mm2 surfaces and 5-10 pm thickness. The long axis of the monoclinic unit cell is perpendicular to the crystal plane, which means that the stacking axis of the H-T6-H molecules runs parallel to the large surface of the crystal. The most significant feature of the crystal structure concerns the complete planarity of the molecules, which is even more planar than that observed in terthiophene single crystals. Furthermore, H-T6-H molecules lie strictly parallel one to each other, ensuring a very large overlap of their K molecular orbitals. Electrical characterizations of H-T6-H single cr stals have shown that the conductivity is very low, with an upper limit of IO-"Scm-', which must be related to a very low doping level. Field-effect transistors have also been fabricated from H-T6-H single crystals [62], but technological problems only allowed realization of a staggered FET geometry (Fig. la) with source and drain electrodes on top of the crystal, which is not well suited for device characterization. As a matter of fact, as the conducting channel of the FET device lies at the semiconductor-insulator interface, the drain current involves a large Ohmic contribution, originating from current flow through the crystal thickness, of some 5-10 pm. This feature led to a typical inversely curved shape of output curve Zd-Vd, which does not allow an accurate calculation of field-effect mobility. Due to this experimental problem, the obtained value, p = 1.5 x 10-'cm2 V-' s-' must be considered as an inferior limit for field-effect mobility in a single crystal of H-T6-H [62]. Work is actually in progress for building a coplanar FET geometry, and also time-of-flight measurement of mobility in these oligothiophene single crystals. Interestingly, the amplification characteristics of H-T6-H single crystal based FETs have been used for the calculation of the dopant concentration, using Eq. (5). The observed pinch-off voltage of +30 V corresponds to the completely depleted semiconductor, when depletion width W,, reaches the thickness of the semiconducting crystal. The dopant concentration calculated from Eq. (9,N = 3 x lok4~ m - ~ , which corresponds to 0.2 ppm impurity in the H-T6-H single crystal, is much lower than that enerally observed in non-intentionally doped conjugated materials, of about 10' ;g-10'*cm-3. This very low concentration points out the high purity of the crystallized material. Furthermore, Eq. (5) explains some behavior observed in the literature for FETs based on sexithiophene, for which it has been proposed to use very low thicknesses, of the order of some tenths of nm, and very pure material for decreasing the drain current observed at zero gate voltage [28]. As a

matter of fact, the pinch-off potential in the depletion regime, V,,, varies with the dopant concentration, N , and with the square of semiconductor layer thickness. ‘1’. Thus simultaneous decrease of N and of d leads to a large decrease of V,,, which may become negligible, allowing the FET device to work only in an accumulation regime, with low Zoff current at zero gate voltage.

11.5 Charge Transport in Conjugated Materials Even if practical applications of organic-based devices in electronics are considered as potentially valuable, one of the most interesting goals of the electrical characterizations of conjugated polymers and oligomers concerns the analysis of the chargetransport mechanism in these materials. With this aim, studies have been performed on temperature effects on conductivity and on field-effect mobility [17, 23, 44, 63. 641. Analysis carried out on thin films of fullerene, C60. revealed that field-effect mobility is thermally activated at fixed gate voltage, and that the activation energy decreases for increasing gate voltage [63]. The large dependence of mobility on gate voltage has been attributed to an exponential distribution of traps in the gap, as reported for amorphous hydrogenated silicon. More recently, such analysis has been performed on H-T6-H and on cu,tr’-dihexylsexithiophene,Hex-T6-Hex, from 100 to 300 K [64], which revealed two temperature dependent regimes. At temperature higher than 150 K, the conductivity is thermally activated, with an activation energy of 0.22eV for Hex-T6-Hex and of 0.26V for H-T6-H. At lower temperature, a change of the slope is observed in the Arrhenius plot of conductivity. The field-effect mobility also shows a strong gate-bias dependent activation energy, and it tends to saturate at both high gate bias and high temperature. These data were analyzed within the frame of a multiple trapping and release model. and shown to fit with a double exponential distribution, associated to the presence of deep and tail states near the transport level, which can be compared to the case of a-Si:H. Importantly, the microscopic mobility po of both H-T6-H and Hex-T6-Hex are found to be comparable, the lower effective mobility observed in the case of H-T6-H being attributed to the higher density of deep traps. Following the results obtained in the structural analysis of conjugated oligomers. these traps can be associated to grain boundaries, which have been shown in higher concentration for H-T6-H films. When temperature decreases, the probability of thermal release from localized traps diminishes. and a transition from trapping to thermally activated hopping has been proposed as the dominant transport mechanism, in agreement with the significant change of slope of the Arrhenius plot. Furthermore. a back transition from hopping to trapping can be observed at low temperature, when increasing the gate voltage. Under increasing gate bias, traps become filled with injected charges, and, all deep traps being filled. charge transport switches back to a multiple trapping and release mechanism, in agreement with a fast increase of saturation current with gate voltage. Eventually. all traps can become filled, which means that any additional charge will then move freely with the microscopic mobility.

580

1 I Field-Efect Trnnsistors Based on Conjiiguted Muteriuls

The proposed multiple trapping and release mechanism appears to bring a satisfying picture of the charge-transport process in conjugated oligomers, pointing out the determining role played by traps (grain boundaries, chemical impurities). This model accounts for the large increase in field-effect mobility observed when depositing H-T6-H on heated substrate, which results in the increase of the crystalline size, and also when using superstructured Hex-T6-Hex, shown to present a highly homogeneous, liquid crystal-like structure. Thus, in the case of conjugated oligomers, the proposed trapping mechanism explains how field-effect mobility of thin films can be increased almost to the limit corresponding to a single crystal. It ratio for the drain current can be achieved, through also explains that a large lnn/lnff the decrease of dopant concentration. On the other hand, in the case of conjugated polymers and other amorphous materials, it has been shown that charge transport occurs through a hopping mechunism, the efficiency of which is highly dependent on the hopping distance, and hence on the doping level. Increase of doping level may enhance the carrier mobility, but also simultaneously the conductivity, which excludes any reasonable Znn/Znff ratio.

11.6 State of the Art of Organic FETs Main results from the literature show that organic FETs can be realized by simple processes, on various types of substrates, silicon, glass or polymer. Masking techniques have been used, leading to oversized device geometries, with channel length and width of the order of tenths of microns, and of millimeters respectively [25]. Micron-sized devices have been achieved, by the use of conventional microlithographic techniques for realizing the gate, source and drain electrodes, leading to conventional channel length of some microns, and channel width of some hundred microns [25, 281. The organic semiconducting layer is then simply evaporated, in a last step, on top of this structure, at room temperature. Characterizations of organic FETs have been carried out on devices based both on p-type and on n-type organic semiconductors. The most interesting characteristics to date have been obtained with sexithiophene H-T6-H, and a,ai'dihexylsexithiophene, Hex-T6-Hex. Field-effect mobilities in the range of lop2 to lo-' cm2V-' s-' have been obtained, the highest value being observed with Hex-T6-Hex deposited at room temperature, close to that observed with a single crystal of H-T6-H [25, 28, 44, 611. Switching times of the order of l o p s have been achieved, and appear to be limited by resistance-capacitance time constants [25, 281. The characteristics of the dynamic range, Znn/loff, are highly dependent on the purity and on the thickness of the film, values higher than lo6 being obtained. Innvalues typically reach some tenths of PA, and Znff values of the order of some tenths of pA can be obtained at OV gate voltage for very thin films of ultrapure H-T6-H, and appear to be only limited by current leakages through the insulating layer [25, 281. Long-term stability tests, carried out under ambient conditions on non-encapsulated devices, have shown stabilities exceeding 1O4 h under constant

11.7 Coriclusioii

58 1

operation, without any modification of the characteristics [28]. These values appear already to meet the requirements for various applications in electronic circuits. FETs based on n-type semiconductors have been more recently described, based on TCNQ [50], perylenes [65] and C60 [49]. The mobilities obtained are of the order of lo-' to 10-'cm2 V-' s-I, and the applicability of heterojunction concepts to organic semiconductors will soon be demonstrated. However, it must be remembered that, contrary to the case of p-doped organic materials, n-type doped ones do not show long-term stability under ambient conditions, particularly in the case of fullerene.

11.7 Conclusion Field-effect transistors have been made with thin films of a number of organic semiconductors, including conjugated polymers and oligomers. Two categories of behavior can be differentiated. First, in most of the conjugated polymers and in a great number of 'amorphous' molecular materials, conduction is governed by a hopping mechanism. Structural disorder and grain boundaries, together with a large density of chemical impurities, impose a very low efficiency for charge transport, with carrier mobilities p of about 10-5cm- V-' s-I. In agreement with the hopping mechanism, field-effect mobility depends on the doping level N , and can thus be improved by increasing N . However, conductivity also increases simultaneously, and FETs made with these materials present an inherently very poor Zon/Zoff dynamic ratio. The second category corresponds to molecular materials, such as short conjugated oligomers. In these materials, charge transport obeys a multiple thermal trapping mechanism, and is hence only dependent on the density of traps, whereas conductivity depends on the doping level. Highly ordered and very pure materials allow one to reach both a high mobility, of the order of 10-'cm2 V-' s-', and also a low conductivity, of the order of lo-' Scm-I, which meets the requirements for efficient FET devices. Highly crystallized films can be realized by adjusting the experimental conditions for film deposition. Long-range molecular ordering in the semiconducting film can also be easily achieved by an elegant chemical route, involving the substitution, at both ends of the conjugated molecule, of alkyl groups, which bring self-assembly properties to these molecules. Practical applications of organic FETs can be imagined in fields where conventional inorganic devices do not meet required characteristics. As compared to their inorganic counterparts, organic materials present many advantages of low cost, light weight and room temperature processing, which should open the field of low cost and large area electronics. Besides, organic materials present the unique possibility of tuning of their electronic, optical, thermal and mechanical properties, through subtle chemical modifications of their chemical structure, The upriori control of mechanical and optical properties also opens the field of flexible, and even transparent, electronics. In fact, a parallel can be made with the field of conventional polymers, whose spectacular development in our everyday life is

linked to the possible chemical design of their properties. Although at a very early stage, organic electronics possesses all the bases for an d la carte electronics, and thus promises a very interesting future.

References I . M. Pope, C. E. Swenberg, Electronic, Proces.ses in Organic Crystals, Oxford University Press, New York, 1982. 2. G. A. Chamberlain, Solur Cells, 1983, 8. 47. 3. A. K. Gosh, T. Feng. J . Appl. Phys., 1978, 49, 5982. 4. R. 0. Loutfy. J. H. Sharp, C. K. Hsiao, R. Ho, J . Appl. Phys., 1981, 52, 5218. 5. C. W. Tang, Appl. Phys. Lrrr.. 1986, 48, 183. 6. H. Antoniadis, B. R. Hsie, M. A . Abkowitz, S. A. Jenekhe, M. Stolka, Synth. Met., 1994, 62, 265. 7. N. Basescu, Z. X. Liu, D. Moses, A. J. Heeger, H. Naarmann, N. Theophilou, Nature, 1987, 327, 403. 8. G. Tourillon. F. Garnier, J . Electroanal. Clieni., 1982, 135, 173, 9. A . G . MacDiarmid, J. C. Chiang, S. L. Mu, N. L. D. Somasini, W. Wu, Mol. Cryst. Liq. Cryst., 1985, 121, 187. 10. D. Fichou, G. Horowitz, Y. Nishikitani, F. Garnier, Chernitronics, 1988, 3, 176. 1 1 . F. Ebisawa, T. Kurosawa, S. Nara, J . Appl. Phys., 1983, 54, 3255. 12. K. Pichler, C. P. Jarret, R. H. Friend, B. Ratier, A. Moliton, J . Appl. Phys., 1995, 77, 3523. 13. A. Tsumura, H. Koezuka, Y. Ando, Synth. Met., 1988, 25, 11. 14. J. H. Burroughes, C. A. Jones, R. H. Friend, Nature, 1988, 335, 137. 15. A. Assaqi, C. Svensson, M. Wilander, 0. Ingangs, Appl. Phy3. Lett., 1988, 53, 195. 16. G. Horowitz, D. Fichou, X. Z. Peng, Z. Xu, F. Garnier, Solid State Chem., 1989, 72, 381. 17. J . Paloheimo, P. Kuivaleinen, H. Stubb, E. Vuorimaa, P. Yli-Lahti, Appl. Phys. Left., 1990,56, 1157. 18. J. Paloheimo, E. Punkka, P. Kuivalainen, H . Stubb, P. Yli-Lahti, Acta Polytech. Scandin., El. Eng. Ser., 1989, 64, 178. 19. A. Tsumura, H. Fuchigami, H. Koezuka, Synrh. Met., 1991, 41, 1181. 20. K. F. Voss, D. Braun, A. J. Heeger, Sjnth. M e t . , 1991, 41, 1185. 21. H. Akamichi. K . Waragai, S. Hotta, H. Kano, H. Sakati, Appl. Phys. Lett., 1991, 58, 1500. 22. Z. Xie, M. S. A. Abdou, X. Lu, M. J. Deen, S. Holdcroft, Can. J . Phys., 1992, 70, 1171. 23. H. Fuchigami, A. Tsumura, H. Koezuka, Appl. Phys. Lett., 1993, 63, 1372. 24. M. S. A. Abdou, X. T. Lu, Z. W. Xie, F. Orfino, M. J. Deen, S. Holdcroft, Chem. Muter., 1995, 7, 63 1. 25. F. Garnier, A. Yaasar, R. Hajlaoui, et nl., J . Amer. Cheni. Soc., 1993, 115, 8716. 26. A . R. Brown, D. M. Deleeuw, E. E. Havinga, A. Pomp, Synth. Met., 1994, 68, 65. 27. B. Servet, G. Horowitz. S. Ries, c’t al., Chem. Muter., 1994, 6 , 1809. 28. A. Dodabalapour, L. Torsi, H. E. Katz, Scirnc,c,, 1995, 268, 270. 29. J. Paloheimo. K. Laakso, H. Isotalo, H . Stubb, SyntI7. Met., 1995, 68, 249. 30. J. H. Burroughes, D. C. C. Bradley, A. R. Brown, et al., Nature, 1990, 341, 539. 31. D. Braun, A. J. Heeger, Appl. Phys. Lett., 1991, 58, 1982. 32. M. Berggren, 0. Inganis, G. Gustafsson, et al., Nature, 1994, 372, 444. 33. P. Garnier, G. Horowitz, Siwth. Met., 1987, 18, 693. 34. S. M. Sze, P/zy.sics of Semicontlucror D e ~ i c i ~Wiley, . ~ , New York, 1981. of’ Thin Films, Academic, New York, 1964, p. 147. 35. P. K. Weimer, Pli~~sic.s 36. S. Roth, One Dirnmsionol metal.^, VCH, Weinheim, 1995, p. 210. 37. F. Garnier, G. Horowitz, X. Z. Peng, D. Fichou, A&. Mnter., 1990. 2, 592. 38. F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science, 1994, 265, 1684. 39. G. Horowitz, X. Z. Peng, D. Fichou, F. Garnier, J . Appl. Phys., 1990, 67, 528.

Rqfererices

583

F. Garnier, X. Z. Peng, G. Horowitz, D. Fichou, Molec. Bigirieer.. 1991, 1. 131. G. Horowitz, P. Delannoy, J. Appl. P / I J X .1991, . 70. 469. J. Paloheimo. H. Stubb, P. Yli-Lahti, P. Kuivalainen. Synth. Met., 1991. 41. 563. G. Horowitz. F. Deloffre. F. Garnier, R. Hajlaoui, M. Hmyene. A. Yassar, Syzth. Met.. 1993, 54, 435. 44. K. Waragai, H. Akimichi, S. Hotta. H. Kano. H. Sakaki, Syiith. Met.. 1993, 55-57. 4053. 45. D. Delabouglise, M. Hmyene. C . Horowitz, A. Yassar, F. Garnier, Acfv. Mare r . . 1992. 4, 107. 46. C. Clarisse, M. T. Riou, M. Ganeau. M. Le Contellec, EIectrori. Lett., 1988. 24, 674. . 1990, 167, 47. G. Guillaud, M . Al Sadoun, M. Maitrot, J. Simon, M. Bouvet, Clieni. P h y ~ Lett.. 503. 48. G. Guillaud, J. Simon, Cheni. P l i j x Letr.. 1994, 219, 123. 49. K. Hoshimono, S. Fujimori, S. Fujita. S. Fujita. Jpri. J . Appl. Plij:~.,Port 2. 1993, 32, L1070. 50. A. R. Brown, D. M. Deleeuw. E. J. Lous. E. E. Havinga, Sjwth. Mei., 1994, 66, 257. 51. C. Pearson. J. E. Gibson, A. J. Moore, M. R. Bryce, M. C. Petty, Elecirori. Lett., 1993.2Y, 1377. 52. E. R. Holland. D. Bloor, A. P. Monkman. et nl., J . Appl. Phys., 1994, 75. 7954. 53. E. Punkka, M. F. Rubner, J. D. Hettinger, J. S. Brooks, S. T. Hannahs, Pliys. Rev. B. 1991.43. 9076. 54. S. Hotta. K. Waragai, Adv. Murer., 1993. 5 . 826. 55. S. Hotta, K. Waragai. J . Muter. C/iern,, 1991. 1 , 835. 56. J. B. Torrance, in Mokcitlur M e i d s (W. E. Hatfield, Ed.), Plenum Press, New York. 1979, p. 7. 57. T. Yamamoto, T. Kanbara, C. Mori, S w t h . Met., 1990. 38. 399. 58. H. Egelhaaf. P. Biuerle. K . Bauer. V. Hoffmann, D. Oelkrug, J. Mol. Srri~r..1993, 2Y3, 249. 59. A. Stabel, J. P. Rabe, S~viih.M e t . . 1994. 67. 47. 60. P. Bauerle. Ad,!. Muter., 1992. 4 , 102. 61. G. Horowitz, B. Bachet, A. Yassar, e r a / . , Cherii. Moier., 1995. 7 , 1337. 62. G. Horowitz, F. Garnier, A. Yassar, R. Hajlaoui, F. Kowki, A d > .Maier., 1996, 8, 52. . 1993, 56, 3185. 63. J. Paloheimo, H. Isotalo, J. Kastner, H. Kutzmani, S ~ w t hMei., 64. G. Horowitz, R. Hajlaoui, P. Delannoy, J . P/ij.s. 111 Frunce, 1995, 5 , 355. 65. G. Horowitz, F. Kowki. P. Spearman, D. Fichou. C. Nogwes. X. Pan, F. Garnier. Ad,'. Muter., 1996, 8. 242. 40. 41. 42. 43.

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Index

A h itiitio calculations 347, 376-7, 380, 396 Absorption spectra - S P P also longest wavelength absorption; wavelength absorption - cY,a’-substituted oligothiophenes 160 - cY,$-substituted oligothiophenes 169-70 - oligo(pphenyleneviny1ene)s 68 regioregular oligothiophenes 146, 148-52 Accumulation regime 565 Acene oligomers 49-52 Acenes, structure 2 Acetylenes - Cadiot-Chodkiewicz coupling 13 - Eglington coupling 12 - Glaser coupling 12 Acyclic diene metathesis (ADMET) reaction 93-5 olefinic-aromatic structures 58-60 - oligoarylenevinylenes 58, 60, 62, 64, 86 Acyclic precursors, ring closure reactions 120-7 ADMET reaction see acyclic diene metathesis reaction Aggregation 537-8. 539, 545 Aldol condensation, oligoenes 4 Alkanes, chain hyperpolarizability 454 Alkyl side chains solubility 161 - substituted oligothiophenes 141-3, 155, 520- 1 Alkyl-substituted oligothiophenes 520, 521 Alkyl-substituted polythiophenes 521 8-Alkylated undecithiophene 171 Alkynes. bridging heterocycles 284 All trans-oligoenes, vibrational spectra 383-6 All-optical modulator devices 553-5 All-rratzs oligoenes - t-butyl-capped 6 I3C data 7 ‘H-NMRdata 7 All-trans polyacetylene, fluorescence excitation spectroscopy 409 All-/runs polyenynes, synthesis 19 Alpha-coupling steps SO4 ~

~

~

Alpha-linked quaterpyrrole 238 AM1 see Austin Model 1 p-Aminodiphenylamine 263-4 Amorphous materials, FETs 569-70 Amphiphiles - oligothiophenes 186-7 - shape-resistant macrocyclic 93-4 Amplitude mode (H) 366-76.379.380.383-5. 387. 390 Amplitude mode theory (Horovitz) 378 Angular frequency w 404-5 Anharmonicity 330 Aniline 235 Anthracene - electroluminescence 518, 533 photoconductivity 541 - 7r conjugated chains 83 - singlet fission 526 Anthrylene oligomers. structure 47-8 Arene/arene-coupling methods cross-coupling 1 17 oxidative coupling 109-13 Aromatic structures Clar type polycyclic 53-8 heteroaromatic oligomersl polymers 386-92 olefinic-aromatic structures 58-97 oligoarylenes 25-48 - quinoidal resonance 128 - softening 378 stretching modes 386 tetrahydrothiophenes 120-1 Aryl aldehyde 250 Aryl-aryl coupling, oligo(n7-phenylene)~ 40 Arylenevinylenes 300 Arylpolyenes 483 Au/TPD/Acene/PV/Mg 534 Austin Model I 433,434, 440-1 Autocondensation. crotonaldehyde 3 ~

~

~

~

~

~

~

~

~

~

~

Band gap (E,) 360-1 Band progression. vibrational spectra 357 /?$’-dialkylsubstituted sexithiophenes 575 BDT-TTP 215 Bechgaard salts 198

Belt molecules, his-tetrathiafulvalenes 2 17, 222 Benzo-fused TTF-systems 213 Benzodithiolyltriphenyl-phosphonium tetrafluoroborate 2 I Beta-beta-linked oligomers 242 Beta-substituted oligomers 535, 575 Beta-tetradecyldodecithiophene 567 I , 1’-binaphthyl 486, 487 Biphenyl 530 Bipolaron model 479-80 2,2’-Bipyridine 277, 281 2,2’-Bipyridine domains, heterocycles 28 1-5 2,2’-Bipyrrole 237, 494 /rans-1,2-Bis(2-naphthyl)ethene 300 (trans- 1,2-Bis(2-naphthyl)ethene)SbF6 300 a,rr’-Bis(aminomethyI)-functionalized oligothiophenes 178 Bisanthracene 536 Bis-BEDT-TTF 2 12- 13 Bis(butadiyny1)methanofullerene 15, 16, 18 Bis- I .3-dithiol-2-chalcogenones 225 n.w-Bis(ferroceny1)oligomethyne cations. synthesis 24, 29 Bis-tetrathiafulvalenes 199, 204- 12, 225 Bithiophenes 489-9 I , 525-6 - amphiphilic 186 - donor/acceptor-substituted 182-4 - quaterthiophenes synthesis 176, 180-1 - regioisomeric di(methylthi0)-substituted 176 - regioregular 145-7 - soluble sexithiophene 176 Blue-shifts, semiconductor quantum structures 468 BOC group, oligomers 494 N-BOC oligo(pyrrole-2,5-diyl)s 243 N-BOC pyrroles 251, 263 N-BOC substituted oligo(pyrrole-2,5-diyl)s 240- 1 N-BOC-protected oligopyrroles 387-8 Bond alternation, hyperpolarizabilities 38 1-2 Bond lengths, trun.s-polyacetylene 9- 10 Born-Oppenheimer approximation 329 Bpy SOP 2,2‘-bipyridine Bridged oligothiophenes 153-5 Bridging heterocycles - bpy domains 281-5 - tpy domains 285-8 Bridging ligands 275-88 Bromination cY,a‘-substituted oligothiophenes 156-7, 159-60 oligothiophenes 138, 140-1 Buckininsterfullerene, oligothiophene addition 435 ~

~

‘Build-up’ approach 336, 339 Butadiynyl-linked ‘dimeric’ methanofullerine, Hay coupling 15, 16, 18 N-tcrt-Butoxycarbonyl (BOC) group 238-40 t-Butyl-capped all-trans-oligoenes, random synthesis 6 13

C data, all-trans-oligoenes 7, 9 ‘ k - N M R spectroscopy, ladder oligomers 30 Cadiot-Chodkiewicz coupling, silylated diynes 13 Carboxylate ligands, oligomeric metal complexes 275-6 Carbynes, structure/synthesis 11-12, 15 /?-Carotene, Wittig reaction 4 Carotenes 483-4, 525 Carotenoids, Wittig reaction 4-5 Cascade molecules 222 Chain length characteristic 463-9 conjugated systems 463-9 - effective conjugation length 376-9 cvolution, oligomers 434-6 -- hyperpolarizability 454, 462-4 - oligo-(arylenviny1ene)s 480- 1 photoinduced infrared spectra 371 polyenes spectroscopy 407, 409 polymers 105-6 Raman spectra 368, 369, 388, 390 substituted oligothiophenes 141-4 - vibrational spectra 351-8 Charge fluxes 348, 382-3 Charge recombination 523 Charge storage mechanisms 480-504 Charge transport 532, 579-80 Charge-transfer complexes, his-tetrathiafulvalenes 204 - excitons 528-9, 543 Charged oligomers 440-5 Chemical doping .cer also doped materials electronic defects 361-2 Chemical engineering 574-7 Chlorocarbonyl-TTF 227 Chromophores, spectral analysis 404-6 cis-polyacetylene (cPA), thin films 460 Clar type polycyclic aromatic hydrocarbons 50, 53-8 C104 groups 211-12,213 Cluster build-up technique 336 ‘Complexes as metals, complexes as ligands strategy’ 284, 285-6 Conducting polymers 3 16- 18, 479-80 Conductivity, oligothiophenes 156, 158 Confinement length 376-9 Conjugated cyclophanes, synthesis 82 ~

~

-

~

~

~

~

~

~

~

~

Conjugation see Effective Conjugation Coordinate Conjugation length polymers 105-6 substituted oligothiophenes 134-7. 142 unsubstituted oligothiophenes 128 Cooling schedule 336 Coplanarity 236, 523. 529-30 Copper catalysis s w Hay coupling COT see cyclooctatetraene Coulomb case 334-5 Coupling methods oxidative coupling 112-13 ring closure reactions 130-7 transition metal catalyzed 113-20 Covalent linkers. bis-tetrathiafulvalenes 209 Covalent semiconductors 568 CPA .we cis-polyacetylene Criss-cross overlapped tetrathiafulvalenophanes 216-1 7 Cross-coupling reactions nickel-catalyzed 114- 18 palladium-catalyzed 118-20 Crotonaldehyde. autocondensation 3 Crystal packing calculations 329-44 Crystal structures 8. 15. 395-344 Cut off radius 333-4 Cyanines. hyperpolarizability 456, 463 Cyanoethylthio TTFs 228,229 Cyclic his-tetrathiafulvalenes 21 6-22 Cyclic oligo(r,i-phenylene)s 40- 1 Cyclic oligo(naphthyleneviny1ene)s 84 Cyclic oligothiophenes 132-4 Cyclic voltamrnetry carotenes 483-4 conjugated oligomers 480 - oligoenes 11 - oligomers/oligopyrroles 257 -- oligothiophenes 489 - polyaniline 496 short chain oligomers 500. 503-3 Cyclization 1.4-diketones 12 1-3 diacetylenes 123-5 - preformed tetrathiafulvalenes 326-9 thienyl-substituted diynes 125 Cycloalkane end-caps 520. 534 Cycloarenes 5 1-3 Cyclododecakisbenzene .we Kekulene Cycloinetallation sites 286-7 Cyclooctatetraene (COT). phenylene units replacement 85-6 Cyclopenta[2.1 -b;3.4-b’]dithiophene-4-one 154-5 Cyclopenta-rwphenylene 40- 1

Cyclopentadithiophenes, polymerization 153-5 Cyclophanes 82-3, 216 Cyclo(phenyleneethynylene), structure 92 Cyclopolyarenes, oxidative coupling 1 I 1

~

~

~

~

~

~

~

~

~

~

~

~

~

~

Davidov blue shift 165 Davydov splitting 527-8, 535. 548 Dechlorination. poly( p-phenyleneviny1ene)s 65 rwDeciphenyl 299 Degenerate four wave mixing ( D F W M ) 450, 452-3.456 Delocalized 7r electrons frequency, intensity spectra 346 heteroaroniatic oligomers 378-9 ladder-type oligoniers 32-3 nonlinear optical properties 449, 454, 457. 46 1. 463. 469 nonlinear optical responses 393 oligoenes 409 II mode 379-80 Dcndritic systems 222-3. 230, 282 Depletion layer thickness 565 Deprotonation, 1.3-dithiolylium salts 225 DFWM S P degenerate ~ four wave mixing trrrris-Di-t-butyl-dodecahexaeiie 305-6 Dincetylenes cyclization of 123-5 isophthalic acid derivatives-containing 19. 20 oligoyne synthesis I5 Diagonal force fields 330 Dialkoxy-substituted oligo(phenylviny1ene)s. structure 71 ci-Dialkylamino-,j-stilbene76 Dialkyloctithiophene 567 rr.rr’-Dialkylsubstituted oligothiophenes 567. 577 t i ,r,’-Dialkylsubstituted quaterthiophene 577 r~.r,’-Dialkylsubstitutedterthiophene 577 Dialkylterthiophene 567 3,4-Dialkynyl-3-cyclobutene-l,2-dione 15, 17 ‘,S-Diaminoterephthalic acid 266 1.ti-Diarylpolyenes 479 Dibenzotetrathiafulvalene 198. 215 Dichroisni, poly(octy1thiophene) 373 3.4-Dicyanopyrrole 243 .l-Didecylsexithiophene 567 Diethynylmethanofullerene. Hay coupling 15. 16. 18 Diffusion equation method 337 Dihexyloctithiophene 575 Dihexylsexithiophene 575-6 r1.n’-Dihexylsexithiophene 577, 579, 580 1.4-Dihydroterephthalic acid diethyl ester 266 ~

~

~

~

~

~~

-

~

~

588

Index

DIIRS see Doping Induced Infrared Spectrum 1,4-Diketones 121-3, 249-50, 252 Diketones, polycondensation 60 Dilithio TTFs 226 Dimerization, conductive polymer salts 3 17 Dimerization enthalpy, end-capped oligothiophenes 164 3,3/-Dimethoxy-2,2/-bithiophene 504 4,4/-Dimethoxy-2,2/-bithiophene 504 4,4/-Dimethoxystilbene 485 5,5”’-Dimethyl-2,2’:5/’,2/’-quaterthiophene 310-11 tr,w-Dimethylsexithiophene 534 Dioxane solution, absorption spectra 411-13 4,4/-Dipentoxy-2,2/-bithiophene 172-3 Diphenylamine 268 Irans-Diphenyloctatetraene 305-6 Diphenyloligoenes 485 a,w-Diphenyloligoenes (DPOE) 406 N,N‘-Diphenyl- 1,4-phenylenediamine 268 Dipole moments, OPV 76-7 Dispersion curves 356, 384 Distribution of excited states (DOS) 406 Distyrylbenzene 515, 517, 537, 539 2,2/-DistyrylbiphenyI, photodimerization 80 Disubstituted oligothiophenes 179-80, 182, 489 Dithia-tetrathiafulvalene 2 12 1,4-Dithiins, ring closure reactions 126-7 1,3-Dithiol-2-thiones, trithioorthooxalates coupling 225 1,3-Dithiolylium salts 225 Divinyl sulfone (DVS) 250 Dodecahexaene, crystal structure 8, 10 Dodecithiophene 575 Donor/acceptor substituted compounds - a,w-donor/acceptor substituted oligoenes 2 I -3 - a$-donor/acceptor-substituted stilbene 76 - oligo(o-phenyleneethyny1ene)s 9 1 - oligothiophenes 172-86 - conjugated triads 184 solvatochromatic properties 182, 183 - phenylenvinylene oligomers 77 Doped materials - charged oligomers 440 - conductive polymer salts 3 15- I7 - DIIRS 379-80, 382-3,385 - doping level 566, 569-70, 581 - electronic defects 361 -2 - infrared/Raman spectra 363 - oligothiophene films 158 - poly[(3,4-dioxyenthylen)thiophen] 172 - poly( p-pheny lenevinylene) infrared spectra 372

-

II mode infrared spectrum 379-80

Raman spectra 374, 380-2 short chain oligomers 503 -~ trcirzs-polyacetylene 374 Doping Induced Infrared Spectrum (DIIRS) 363, 379, 380, 382-3, 385 DOS see distribution of excited states Dotriacontane on MoSez 340-1 Double bonds oligoenes 465, 466 rrans-polyacetylene 9- 10 Double-stranded .rr-systems 485-8 DPOE see n,w-diphenyloligoenes Drain current 563, 565 Drain voltage 563, 565, 566 Duodecithiophene, synthesis of 142-3 Dyes structure-optical properties relationship 469-73 - structures 454-5 - thin film data 458 Dynamic range (current on/off ratio) 565,566, 580, 581 Dynamics, one-dimensional lattices 349-5 1 -

-

-

-

E see electrical fields ECCF see Equilibrium Charge and Charge Flux Model ECL see effective conjugation length Effective Conjugation Coordinate (ECC) model 376 Effective conjugation coordinate (2) 366-76, 379-80, 383-5, 387, 390, 398 Effective conjugation length (ECL) 33-4, 376-8, 380, 409 EFISH see electric field induced second harmonic generation Eg see band gap Eglington coupling, acetylenes 12 Eigenvalue equation 347 Electric field induced second-harmonic generation (EFISH) 450,452, 456 donor/acceptor substituted oligoenes 22 nonlinear optical responses 395-6 Electrical fields (E), NLO phenomena 449-51 Electro-optical modulation 546-50 Electrochemical properties 479-5 14 Electrocrystallization, bis-tetrathiafulvalenes 21 5 Electroluminescence 166-7, 51 8- 19, 53 1-41 Electron diffraction, conjugated polymers 302 Electron injection 535 Electron-phonon coupling 376-9, 393 Electronic defects, chemical doping 361 Electronic excitation 403-3 1 Electronic properties 359-62 ~

~~

Electronic structure. oligothiophenes

128-9,

Fullerenes 12. 14-17,503.579 Functionalized oligothiophenes 171-86 Furans 250 Fused systems. his-tetrathiafulvalenes 213- I6

132 Electropolymerization 107-8.480.489, 493-4.504-I 1 Electrospray ionization techniques 289 Emeraldine 264,270.3 16 Empirical force fields, packing calculations 329-44 End-capped oligothiophenes 163-7,503.575 End-capped thiophenes 489.508 Endgroups 296-301,357,359 Energy gap Eg.oligothiophenes 390-1 Energy transfer (ET), chromophores 405-6 EPR spectra oligothiophenes 164-5 sexithiopene 139-40 Equilibrium Charge and Charge Flux Model (ECCF) 348 Excitation. electronic 403-31.465-7

Gamma s(v hyperpolarizability Gate voltage 563.565.579 Giant-TTF 216 Glaser coupling, acetylenes 12 Global optimization 336 Global search procedure 339 Globulan proteins 339 Gold clusters 93-7 Green’s method 226 Grignard reagent 114-18

~

~

‘H-NMR data all-trrrr~s-oligoenes 7. 9 - ladder oligomers 30 - oligo( p-pheny1eneethynylene)s 90 Hagihara coupling oligoaryleneethynylenes 87-8 oligophenyleneethynylenes 93 Hairy rod polymers 30I . 309.3 18-22 Hamiltonian methods 548 Hartrec-Fock methods 433.524-5 Hay coupling. fullerenes 15. 16.1X Heavy-atom effects 435 Heck reaction olefinic-aromatic structures 60.64,74.78, ~

FAB methods 289 Facial isomers 285 Fast scan voltammetry 504 FETs see field-effect transistors Fibrous proteins 339 Field-effect devices 545-52 Field-effect mobility 565-6.569-70,574.577,

~

~

579,580-1 Field-effect transistors (FETs) 545.546.549 applications 519 - conjugated materials 559-83 - pentacene 522 Field-induced charges. sexithiophene 550-2 Fill factor 544 Finite molecular chains, vibrational spectra 351 -8 Flexible side-chains 520 Fluorene 36 Fluorescence excitation spectroscopy 0.8-substituted oligothiophenes 165 chromophores 405 - oligo-p-phenylenes 424-5 - oligoanthrylenevinylenes 419-21 ~- oligoarylenevinylenes 414-24 oligothiophenes 410-14 - poly/oligo(phenylenevinylene)s 67,416 polyenes 407-10 - sexithiophene 136 - star-shape hydrocarbon oligomers 38-40 terthiophene 410-11 - unsubstituted oligothiophenes 128-31 - unsubstituted OPV 416 Force fields, packing calculations 329-44 Frenkel excitons 528-9 Frequency spectroscopy 346-7 Friedel-Crafts method 123 ~

~

80 oligoarylenevinylenes 58-9 oligo(o-phenyleneviny1ene)s 78 oligo(phenyleneviny1ene)s 74.78 poly( 1.4-phenylene-vinylene-2,2’biphenylenrvinylene) 80 Hellnian-Feynmann theorem 369 Heptacene. synthesis 50-1 Herringbone structure 301-3, 307.309. ~

~

~

~

527

~

Heteroaromatic oligomersipolymers

~

~

~

~

349-63.

386-92 Heteroatoms. bis-tetrathiafulvalenes 209 Heterocycles 277-9 - bpy domains 281-5 tpydomains 285-8 Hexaacene. synthesis 50 Hexaalkyl-substituted hexabenzocoronene 53. 55-6 Hexabenzocoronene 53. 55-6 structure 2 2,4.6.8.10.12.14,16-Hexadecaoctaene 408 Hexadodecyl hexabenzocoronene 53. 55-6 Hexakis(quaterphen yl)benzene, synthesis 37-9 Hexakis(terpheny1)benzene.synthesis 37-9 ~

~

Highest occupied molecular orbital-lowest occupied molecular orbital see HOMO-LUMO o energy HOMO energy. s c c ~ h HOMO-LUMO gap 209. 422-3, 433, 434, 441-3, 548 HOMO-LUMO energy gap 359-60, 368, 403-4 - oligo-p-phenylenes 425 - oligoenes 10 rigidification 154-5 Homopolymers 236 Hopping mechanism 569-70, 580, 581 Horovitz amplitude mode theory 373, 378 HPLC, oligomers 241 Huang-Rhys factor 404, 407, 411, 416, 419 Hiickel Model 378,479, 525, 547 2-Hydroxycthylthio-TTF 227 Hydroxymethyl-TTF 227 Hyper Raleigh scattering 255 Hyperpolarizability (gamma) bond alternation 381-2 - conjugated systems 454-7, 460-3 o,d-donor,’acceptor substituted oligoenes 22-3 - nonlinear optical responses 396 - one-dimensional conjugated systems 470- I - size dependence 462-3 Hysteresis 503

lodoterthiophene 172 Ionenes 301 Ionization potential (IP) 422 IP SCP ionization potential ~ crossing ISC S P intersystem Isoniorphous replacement 299 lsophthalic acid derivatives, diacetylene ITO/ECnT/Al 534 ITO/TPD/NSD/PBD/Mg:Ag 534

19.20

~

I N DO s w Intermediate Neglect of Differential Overlap Infrared spectroscopy 348, 363, 365 - dilrerence spectrum (pump and probe), sexithiophene 370 doped materials, II mode 372. 379-80 ~ n o n a d e c a n e 358 - poly(dihexy1trithiophene) 370 - trc/,7.r-polyacetylene 354 Insulated-gate FETs 563 Intensity spectroscopy 347-9, 392-9 Inter-ring torsion 523, 529-30 Interaction force constants, oligoenes 377 Intermediate Neglect of Differential overlap (INDO) 433. 434. 437, 524, 526 Intermolecular non-radioactive decay channels 523, 526-9 Internal conversion 523, 525-6 Intersystem crossing (ISC) 436-7, 523, 526, 530 Intramolecular electronic interaction 209 Intramolecular non-radioactive decay channels 523-6 I nverse chain length, end-capped oligothiophenes 163 Inverted coplanar device structure 561

Kekulene 2, 52-3 Knoevenagel polycondensation reaction 59, 64 Kovacic reaction, rhombus Clar type PAHs synthesis 54, 58 Kumada coupling reaction a,B-substituted oligothiophenes 171 -- oligoenynes 19 oligothiophenes 1 14- 18, 180- I quaterthiophenes bromination 140 regioregular oligothiophenes 145

~

-

Ladder oligophenylenes (LOP) 5 15, 545 Ladder poly(p-phenylene) (LPPP) 28 Ladder-type oligomers 28-35 oligo-p-phenylene 425-8 oligo[n]acenes 48-51 Yamamoto coupling 3 1-2 Langmuir-Blodgett (LB) technique 186, 523, 56 I Laser printing 541 Lattice relaxation 432, 433, 438-40, 548 Lawesson’s reagent, ring closure reactions I22 LB s e e Langmuir-Blodgett technique LEB see leucoemeraldine base LEDs .see light emitting diodes Lennard-Jones pair interactions 33 I , 332 Leucoemeraldine base (LEB) 3 16 Light emitting diodes (LEDs) 432, 436, 518, 523, 560 based on oligothiophenes 534-5 conjugated materials 166-7 ~- hexakis(terpheny1)benzene 38, 40 - molecular semiconductors 532-4 oligomers blended with polymer matrices 536-9 oligothiophene-based 159 poly-p-phenylene-based 424-5 polymers with pendent oligomeric side-chains 539-40 Light intensity absorption coefficients, oligomersldyes 458 Linear oligoarylenes 485-8 Linking. tetrathiafulvalenes 204- 16, 226-9 Lithiation 226-7, 238 -

~

~

~

-

~

~

Ill tk..\-

Local optimization 336 Long wavelength absorption - donor/acceptor-substituted oligothiophenes 183 oligomers 18 OPV 72-6 - star-shape hydrocarbon oligomers 39 - terthiophenes 179 Longitudinal vibrations. linear chains 355 LPPP see ladder poly( p-phenylene) LUMO energy. see d s o HOMO-LUMO energy gap 360. 423,433, 434.443. 548 -

M see molecular electric dipole moment uz-phenylene systems 77-80 McMurry reaction 126-7 oligo(o-phenyleneviny1ene)s 78 PPVS 58-9. 61 - soluble oligo(naphthaleneviny1ene)s 83 Macrocyclic amphiphiles. shape-persistent 93-4 Macrocyclic oligothiophenes 11 1-12 Macromolecular conjugated materials, organic-based FETs 566 Macroscopic polarization (P). nonlinear optical phenomena 449-5 1 MALDI-TOF see Matrix Assisted Laser Desorption Ionisation - Time of Flight Mass spectrometry, oligomeric metal complexes 289 Matrix Assisted Laser Desorption Ionisation Time of Flight (MALDI-TOF) 289 Melting point - substituted oligothiophenes 143 unsubstituted oligothiophenes 128. 13 I Membranes, molecular wires 23 Meridional isomers 285 Merocyanines 559 Metal centers, a.d-donor/acceptor substituted oligoenes 13-4 Metal complexes, oligothiophenes 187 Metal-insulator-semiconductor (MIS) devices 545. 549-50, 560 Metallodendrimers 289, 291 Metallomacrocyclic systems 284 Methacrylate polymers 539-40 Methoxy groups. oligothiophenes 173-4 Methoxy substituted bithiophene 491 3’-Methoxy-1,2’:5’.3‘’-terthiophene172 Methoxythiophenes 504, 506 N-Methyl oligo(pyrrole-2,5-diyl)s 243 N-Methyl substituted oligoanilines 267 N-Methyl substituted oligomers 499 4,s-Methyl substituted oligothiophenes 49 1 Methyl-substituted quaterthienyls 491 Methylenedithio bridge 21 I -

-

-

59 1

N-Methylpyrrole 238 Microscopic optical polarizabilities 45 1-9 MIS .see metal-insulator-semiconductor (MIS) devices Mixed oligomers. pyrrole and (1ietero)aromatics 249-63 Modified Neglect of Diffrrential Overlap ( M N D O ) 361-2.433-4.138, 524 Modular approach 2. 28-9 Modulators 518 Molecular chains. spectroscopic studies 35 1-8 Molecular conjugated materials. organic-based FETs 566 Molecular crystal packing 301. 329-44 Molecular dipole moments, oligo(phenyleneviny1ene)s 77 Molecular dynamics 329. 338-9 Molecular electric dipole moment ( M ) 348 Molecular electronics 560 Molecular mechanics 329 Molecular packing 301. 329-44 Molecular semiconductors 568 Molecular wires conducting polymers 188-9 - rb,d-donor acceptor substituted oligoenes 23 oligoaryleneethynylenes. synthesis 86-97 Molybdenum. bridging ligands 279 Mono-TTF macrocycles 225 Monoalkylated terthiophenes 160- 1 Monolayer structures. polydiacetylene 19-20 Monothiol oligophenylkeneethynylene. self-assembled 95 Monte Carlo search 336 MRD-CI .si’e Multireference Double-Configuration Interaction Multidentate metal-binding domains 275 Multiple trapping and release mechanism 580 Multirefereiice Double-Configuration Interaction (MRD-CI) 433. 434. 437. 524. 526 -

-

N-BOC oligo(pyrro1e-2.5-diyl)s 243 N-BOC pyrroles 257, 263 N-BOC substituted oligo(pyrro1e-2.5-diyl)s 240- 1 N-BOC-protected oligopyrroles 387-8 N-trr.r-butoxycarbonyl (BOC) group 238-40 n-type scniiconductors 581 Naphthalene. conjugated cyclophanes 83 NBS,’DHF brominating system 156-7 Neglect of Differential Diatomic Overlap ( N D D O ) 433 Neutral oligomers 434-40 Nickel catalysis. s w t r h Yaniamoto coupling Nickel-catalysis. cross-coupling I 14- I8

592

Index

Nitrogen-containing oligomers 235-71 NLO see nonlinear optical properties Non-methoxylated polythiophenes 174 Non-programmed assembly, oligomeric metal complexes 275-9 Nonacene, synthesis 50 n-Nonadecane, infrared spectrum 358 Nonlinear optical properties intensity spectroscopy 393-9 oligomers 449-78 - physical aspects 449-52 - third-order phenomena 452-3 -

OAV see oligoanthrylenevinylenes Oblique structure 301-2 Octacene, synthesis 50 Octane, Raman spectra 367 trans-l,3,5,7-0ctatetraene 304, 305 Octatetraene, Raman spectra 367 Octathiophene 510 Octithiophene 544 organic-based FETs 567 synthesisof 142-3 Octyl-substituted hexamer 141 Olefinic structures 3-24 oligoenes 3-11 oligoenynes 11-24 oligoynes 11-24 Olefinic-aromatic structures 58-97 Heck reaction 60 Oligo(9,IO-anthrylene)~. structure 47-8 Oligo(9,IO-anthrylenevinylene)s,structure 85 Oligo-p-phenylenes (OP), spectroscopic analysis 424-8 Oligo-pyrroles, N-BOC protected 387-8 Oligo-tetrahydropyrenes 34 Oligoacenes 49-52, 522 Oligoanilines 263-70, 496-500 Oligoanthrylenes 47-8, 486 Oligoanthrylenevinylenes (OAV) 85, 419-21 Oligoaryleneethynylenes 86-97 Oligoarylenes 25-48, 36, 48, 86,485-8, 503 Oligo(aryleneviny1ene)s 58-86,414-24,480-3 Oligo(cyc1ooctatetraenylenevinylene)s 85-6 Oligoenes 3-11, 483-5 chain length dependent Raman spectra 368, 396 - cyclovoltammetric data 11 -- a,w-donor/acceptor substituted 21-3 - excitation energies 465-7 hyperpolarizability 456 interaction force constants 377 - spectroscopic analysis 406-10 - Stille coupling 8-9 - structure 2 - UV-VISdata 8-9 -

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Oligoenynes 2, 11-23, 19 Oligofluorenes 32 Oligo(m-phenylene)s (OMPs), synthesis 40,42 Oligomeric LEDs 532-41 Oligomeric metal complexes 273-94 Oligomerization 224-5, 504-9 Oligo-N-methylpyrroles 238, 495 Oligo[n]acenes, synthesis 48-51 Oligo(naphthy1ene)s 42-3, 486-8 Oligo(naphthyleneviny1ene)s 83-4 Oligo( 1,4-napthylene)s 486-8 Oligo(l,5-napthylene)s 486 Oligo(o-phenyleneethyny1ene)s 90- 1 Oligo(o-pheny1ene)s 40, 42 Oligo(o-phenyleneviny1ene)s 78-9 Oligo(oxymethy1enes) 295 Oligo(p-phenyleneethynylene)s, synthesis 88 Oligo(p-phenylene)s 27-8, 33-4, 381, 398 Oligo( p-phenyleneviny1ene)s absorption spectra 66-8 - addition reactions 81 - dipole moments 76-7 - doped Raman spectra 381 HOMO-LUMO 423-4 - soluble, terminal alkyl substitution 70 spectroscopic studies 68-9, 76, 414-17, 423-4 structure/optical properties 392 Wittig reaction 65 Oligoperylenes 398 Oligophenylene rods (telechelics) 28-30, 88 Oligo(phenyleneethynyl)benzenethiols 93-6 Oligophenyleneethynylenes 2, 86-8, 93 Oligophenylenes 2, 456, 463, 486, 534, 538 Oligo-rn-phenylenes 241, 299 Oligo-p-phenylenes 486, 502-3, 529 Oligo-p-phenylenevinylenes 480, 482, 502 Oligo(phenyleneviny1ene)s (OPV) see ulso oligo(o-phenyleneviny1ene)s; ohgo( p-phenyleneviny1ene)s fluorescence 67 - hyperpolarizability 470- I long-wavelength absorption 76 - olefinic-aromatic structures 61 - stepwise synthesis 74 - structure 2 - synthesis 58, 61, 71-3 thin film data 458-60 - trans isomers 72 - UV-VIS absorption 68, 72 Oligophenyls 304, 307-8, 317, 318 Oligo(pyrrole-2,4-diyl)s 237-49 Oligopyrroles 235, 237-49,493-6 - air sensitivity 237 (2,5-pyrroles) 31 1 - Raman spectra 365 -

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hicks

Oligorylenes 2, 42-6 Oligo(tetrahydr0pyrene)s 30- 1, 34-5 Oligo(tetrathiafulva1ene)s 198-233 Oligothiophenes 105-89, 309, 5 15, 529 - amphiphilic 186-7 - bridged 153-5 - chemical engineering 574-5 - a-conjugated 106-8. 127-34. 502-3 - coplanarity 530 - cu.cY‘-disubstituted 179-80 - donor/acceptor-substituted 180, 183, 184 - end-capped 163-7 - FETs 560, 567, 571 - fluorescence excitation spectroscopy 410- 14 - functionalized I7 1-86 - hyperpolarizability 456. 462-3 - LEDs 534-5 - nonlinear optical responses 397 - OASLMS 554-5 - orthogonally fused 170- 1 - photoluminescence 524 - porphyrin combinations 184-5 pyrrolidino group substituted 177-8 - quantum chemical approach 432-47 - redox behaviour 488-92 - structural/optical properties 371, 388-92 - a,a’-substituted 155-61 - P,P-substituted 134-55 u,P-substituted oligothiophenes 161-7 I synthesis of 109-87 transition metal complexes 187 - unsubstituted 109-34 Oligovinylene 479 Oligoynes 2, 11-23 OMPs see oligo(m-pheny1ene)s One-dimensional conjugated systems 463, 470- 1,473-4 One-dimensional lattices, dynamics/spectra 349-51 One-dimensional particle-in-a-box model 525 O P see oligo-p-phenylenes Optical absorption see long wavelength absorption Optical properties - absorption energies, p-phenylene-type structures 36 - applications 515-58 - conjugated oligomers 359-62 - n.w-donor/acceptor substituted oligoenes 21 - donor/acceptor-substituted oligothiophenes 183 - nonlinear 393-9, 449-78 polydiacetylenes 15

polymers/dyes/oligomers comparison 469-73 probing, field-induced charge 550-2 a,cu’-substituted oligothiophenes 182 transitions 432 vibrational spectra 345-402 Optically-addressed spatial light modulators (OASLM) 553-5 Optically-Detected Magnetic Resonance (ODMR) 434, 440 Optimization 336 OPV see oligo(pheny1enevinylene)s Organic FETs 566-8. 580-1 Organic semiconductors 515, 518, 541-5. 559, 560. 568. 569 Organoboranates 124-5 Organoboranes 1 12- 13 Organolithium compounds 109- 10 Organometallic complexes 13, 115-16, 120, 253 Organonickel complexes 115-1 6 Organozinc derivatives, Pd-catalysis 120 Oriented oligomers. polarized electroluminescence 540- 1 Oriented template films 519 Orthogonally fused oligothiophenes 170-1 Osmium dendrimers 292 Oxadiazoles 533, 534 Oxidation potential regioregular oligothiophenes 146-52 substituted oilgothiophenes 144 unsubstituted oligothiophenes 128-32 Oxidative coupling - chemical/electrochemical 1 12 copper(I1)-promoted 109-12 organoboranes 1 12- 13 - substituted oligothiophenes 144 ~

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P see macroscopic polarization Packing calculations 329-44 PAH see polycyclic aromatic hydrocarbons Palladium catalysis see ulso Suzuki coupling. 60 - bithiophenes 176-7 - cross-coupling 114, 118-20 trialkylstannyltetrathiafulvalenes 227 Palladium complexes 1 16 Paracyclophanes 82 PBD 538 PDMPV srr poly(2.2’-diniethyl-l. 1’biphenylene-4,4’-vinylene) PDPV see poly(4,4’-biphenylene-( 1,2diphenylvinylene)) 41 8 Pentacene 49, 51, 522 Pentakis-TTFs 222-3 ~

~

Pentainethyl ferrocene 339, 340 Pentapyrrole 495 3-Pentoxythiophene, synthesis 172 Pernigraniline base (PBN) 3 16 Peticolas Raman scattering model 367 ~ PFV S E poly(2.7-fluorenylenevinylene) Phenyl end capped oligomers 3 I6 N-Phenyl substituted end-capped systems 499 N-Phenyl substituted oligoanilines 267, 268 tr,c\’-Phenyl substituted oligopyrroles 495 Phcnyl-blocked oligopyrrole-thiophenes 257 Phenyl-blocked oligopyrroles 2.57, 263 Phenyl-blocked-,~-unsubstituted oligoanilines 266 Phenylacetylenes 18. 87 p-Phenylene 36, 501 rwPhenylene systems 77-80 Phenyleneethynylene chains 86-97 Phenylenevinylene oligomers, donor/acceptor-substituted 77 7-phenylheptatrien(2,4,6)-al 299 N-Phenyl-p-quinonediimine 264 Phonons 357, 384 electron-phonon coupling 376-9 one-dimensional lattices 349-51 - t,.rrri,s-polyacetylene 350, 353 Phosphate mediated coupling 225 Photocells SIX, 523 Photoconductivity 541 -3 Photodimerization 2,2’-distyrylbiphenyI 80 pol y( I ,4-phenylene-vinylene-2,2’biphenylenevinylene) 8 1 Photoexcitation 523 Photoexcited materials .sci~doped materials Photoinduced infrared spectrum (PIRS) 363, 371-2, 379 Photoluminescence 532 oligomers 526, 528 oligothiophenes 524 planar molecules 530 solid state 530 Phototoxic activity, o-oligothiophenes 107 Photovoltaic devices 519, 543-5, 559 Phthalocyanincs 559, 561 Pi electrons chain hyperpolarizability 454, 457, 461 characteristic polymer chain lengths 469 donors, tetrathiafulvalenes 198, 199 heteroaromatic oligomers 378-9 ladder-type oligomers 32-3 - linkers. his-tetrathiafulvalenes 2 13- I6 - nonlinear optical properties 393, 449 oligoenes 409 power laws 460-1 -

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Rmode 379-80 rich molecules, FETs 567-8 Pinch-off voltage 565 Pinning potential 376-9, 388 PIRS see photoinduced infrared spectrum Planarity, oligophenyls 308 Platinum centres 279 PMMA see poly(methy1methacrylate) POL1 441-3 POL2 441-3 Polarizability 38 1 microscopic optical 451 -9 Polarization, macroscopic (P) 449-51 Polarized electroluminescence, oriented oligomers 540-1 Polarized light, large charge fluxes 382-3 Poly( I ,4-phenylene-vinylene-2,2’biphenylenevinylene) 80- 1 Poly(2.2’-dimethyl- I , 1’-biphenylene4,4’-vinylene) (PDMPV) 41 8- 19 Poly(2.7-fluorenylenevinylene)(PFV) 41 8- I9 Poly(4.4’-biphenylene-( 1,2-diphenylvinylene)) (PDPV) 418 Polyacenes 559 Poly(acety1ene)s 295, 303-4, 483 carbyne hybrid structures 15 ci.Y-poly(acetylene) 303 FETs 561 fluorescence excitation spectroscopy 407-10 - oligoenes comparison 10 - Hmode 375 .- states 559 - trans-poly(acety1ene) 7- 10, 303-4, 566 Poly(alky1thiophene)s 520, 521, 530, 561, 567 Polyalkynes 284 Polyaniline 235-6, 316, 496, 567 Polyaromatics, vibrational spectra 362-6 Polyaryleneethynylene 89 Polyarylenes 24-48 Polycarbonate, as host matrix 537 Polycondensation reactions, olefinic-aromatic structures 58-61 Polyconjugated polymers 295 Polycyclic aromatic hydrocarbons (PAH) 51-8 Polydiacetylenes (PDA) 15, 18-20, 467-8, 526 Poly(dihexyltriothiophene), doped infrared spectrum 370 Polyenes 362-6, 406-10 Polyenynes 19 All-trans-polyenynes, synthesis 19 Polyethylene 358 Poly(hexy1thiophene) 530 Poly(nz-phenylenes) 42 .-

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Polymer structural models, oligomers 295-328 Polymerization. oligothiophenes 144-53 Poly(methylmethacry1ate) (PMMA) 537, 539 Poly(octy1thiophene) 373 Poly(p-phenylene) (PPP) I , 25-6, 33-4. 37, 304-9, 322. 375. 51 5 Poly( p-phenyleneethynylene) 302 Poly( p-phenylenevinylene) 375 dechlorination synthesis 65 - dopediphotoinduced infrared spectra 372 - fluorescence 67 - Heck reaction 59-60 - hyperpolarizability (gamma) 47 1 - molecular structure 313-16 - optical applications 515 - polystyrene blends 536 properties I structureloptical properties 392 structure/synthesis 58-9 tetrahydrothiophene precursor route 522 thin films 460 Poly(parapheny1ene imine) 3 I6 Polyphenothiazinobisthiazole (PPT) 460 Poly(phenyleneviny1ene) (PPV). see also poly(p-phenylenevinylene). 414, 41 7, 559 Polypyrroles (PPy) 235-6, 309-12, 375, 387-8 Polystyrene 522, 536, 539 Poly(tetrafluoroethy1ene) (PTFE) 12 519 - thin film devices preparation Poly(tetrahydr0pyrene)s. transition energies 34 Polythienylenevinylene. FETs 561, 567 Polythiophenes 309,312.437.435.488-9,520. 524. 530, 559 electronic properties 107-8 FETs 561. 571 nickel-catalyzed reactions 117-18 non-methoxylated I74 ODMR 434 - oligomers 515 - organic-based FETs 567 oxidative coupling 112 % m o d e 375 - rigidity 153-5 structural/optical properties 105-6, 388-92 Polytriacetylenes 16, 18 Poly(viny1 chloride) 537 Poly(9-vinylcarbazole) (PVK) 523, 539 Poly(vinylcarbazo1e) (PVK), LEDs 538 Porphyrins, oligothiophene combinations 184-5 Powder diffraction. conjugated polymers 302-3, 312 Power conversion efficiency 544 ~

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Power laws delocalized 7r electron chains 460- 1 one-dimensional conjugated systems 473-4 PPA. hyperpolarizability (gamma) 471 PPP see poly( p-phenylene) PPP-type structures 25-6, 33-4 - inter-ring angles of torsion 37 PPT, hyperpolarizability (gamma) 47 1 PPV see poly(phenyleneviny1ene) PPy see polypyrroles Precursor polymers 522 Pristine materials, Raman spectra 363-5 Processability. conjugated oligomers I Programmed assembly, oligonieric metal complexes 280-8. 289 N-Protected-2.5-dibromopyrrole 235 Protection/deprotection. functionalized TTF-derivatives 228-9 Proteins - fibrous 339 -- globulan 339 Protoemeraldine 270 Protonic acid-doping 236 (Pseudo)hexagonal packing 30 I PTFE see poly(tetrafluoroethy1ene) 'Pump and probe' see infrared difference spectrum 2-Pyridinecarboxylate 276 Pyridylthiophenes I8 1 Pyrrole 235,249-63 Pyrrolidino groups, oligothiophenes 177-8 ~

~

QP3(QP)SbF6 318. 319 Quantum structures 376-7. 432-7. 468 Quaterphenyl 307 Quaterpyrrole 495 Quaterrylene tetracarboxdiimide 43-4 Quaterthienyl 309 Quaterthiophenes 176. 18 1-2. 507 bromination of 140-1 regioregular see regioregular quaterthiophenes Quinoidal resonance structures 18 1 /pi-quinquephenyl 299 Quinquethienyl 309 Quinquethiophenes 151, 544 -

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Radical-radical coupling step 504, 5 1 1 Radioactive decay channels 523. 526-9 Raman spectra amplitude mode coordinate 366-76 chain length dependent 368-9 - conjugated oligomers 348-9. 362-6 - doped species 374. 380-2 - doped rrrrm-polyacetylene 374 N-BOC-protected oligomers 387-8 ~

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- nonlinear responses 393-9 oligothiophenes 390 - tetra r-butyl-pentarylene 368 - trrrns-polyacetyleiie 354 Random synthesis -- oligoarylenevinylenes synthesis 86 - oligoenes 3, 5-6, 8 - oligo(m-phenylene)s 40 - oligophenyleneethynylenes 93 Re-crystallization 539 Redox properties 198-204, 479 - cu,a’-snbstituted oligothiophenes 160- 1 - his-tetrathiafulvalenes 204, 209, 21 I , 212, 213, 215 - conjugated oligomers 480-500 - donor-substituted oligothiophenes 173 - oligoenes 10-11 - oligoynes 14 Reduction potentials, oligomers 18 Refractive index, light intensity-dependent 449 Regiochemistry - polymer properties 172 - regioisomeric di(methylthi0)-substituted bithiophenes 176 - regioisomeric oligothiophenes 117, 132-40 - regioregularity 145-53, 521-2 - substituted oligothiophenes 138-9 Relaxation phenomena 438-40 Resonance structures, conjugated chains 128 Restricted Open-Shell Hartree-Fock (ROHF) 434 Rhombus (Clar type PAHs). synthesis 53-5 Rigid rod polymers 88 Rigidity, bridged oligothiophenes 153-5 Ring closure reactions 120-7, 251, 252 - cyclization of 1,4-diketones 121-3 - cyclization of diacetylenes 123-5 - 1.4-dithiins 126-7 tetrahydrothiopenes aromatization 120- 1 Rods see oligophenylene rods R O H F see Restricted Open-Shell Hartree-Fock Ruthenium (II), oligomeric metal complexes 287, 291, 292 Ruthenium (11,111) benzoate 275 Rylene oligomers, synthesis/absorption spectra 42-6 ~

SAM .xe self-assembled monolayers Saturation regime 565 Scaling laws, one-dimensional conjugated systems 473-4 Scanning tunnelling microscopy (STM) 255, 340, 520 hexadodecyl hexabenzocoronene 56 ~

oligothiophenes 167-8 polydiacetylene 19-20 - rhombus Clar type PAHs 55 Second-harmonic generation (SHG) 89. 450, 452 Sccondary structure 298 Sedecithiophene, synthesis of 142-3 Self-assembled monolayers (SAM) 186 Self-assembly 93-7, 186, 519, 520 Self-Consistent-Field (SCF) determinant 433 Semiconductor layers, thin film devices preparation 5 18-23 Semiconductor quantum structures, blue-shifts 468 Septiphenyl 307 Sexiphenyl 307, 515 p-Sexiphenyl 5 15 Sexiphenylene 5 10 Sexithienyl 309, 310 Sexithiophenes 515 - absorption spectra/redox potentials 160- 1 bilayer LEDs 535 crystal structure 529 FETs 565, 570-1, 572, 578 field-induced charge 550-2 infrared difference spectrum (pump and probe) 370 - internal conversion 525 LEDs 540 - MISdiode 550 nionoalkylated 159-60 OASLMS 554 organic FETs 580 photoconductivity 542-3 regioregular see regioregular sexithiophenes soluble 176 - synthesis of 136-40 transition characteristics 106 Shape-persistent macrocyclic amphiphiles 93-4 SHG see second-harmonic generation ‘Short chain’ oligomers 500-4 Side-chain substitution, thin film devices preparation 520-2 Siegrist method 58-9, 70, 72 Sigma-bonded dimers 491, 503 Silicon, semiconductors 568 Silylated diynes, Cadiot-Chodkiewicz coupling 13 Singlet fission 523, 526 Singlet-to-triplet intersystem crossing 432 Singly occupied molecular orbital (SOMO) 548 Site selective fluorescence spectroscopy (SSF) 405-6 -

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oligoanthrylenevinylenes 419-2 1 terthiophene 410-1 1 unsubstituted OPV 416 Size dependence, hyperpolarizability 462-3 Softening see chain length dependence Solar cells 543-5 Solid state 530. 531 cr.cu’-substituted oligothiophenes 158 electropolymerization. oligomers 5 10- 1 1 end-capped oligothiophenes 165 - oligothiophenes 129 - short chain oligomers 500 Solubility - 0~3-substitutedoligothiophenes 161 - OPVjPPV 65 - substituted oligothiophenes 134-7, 141 Soluble ladder-type oligomers 31 Soluble oligo(p-pheny1ene)s 25-8 Soluble ohgo( p-phenylenevinylene)s, terminal alkyl substitution 69 Soluble oligorylenes, UV-VIS absorption spectra 44. 46 Soluble quaterthiophenes 176 Soluble sexithiophenes 176 Solution-processing 5 19-22 Solution-Spray-Flash-Vacuum-Pyrolysis (SS-FVP) 15, 17 Solvatochromatic properties 182, 183, 530 Solvent effects, oligothiophenes 391-2 Spatial light modulators (SLMs) 553-5 Spectroelectrochemistry 257 Spectroscopic studies - see also infrared spectra; long wavelength absorption; Raman spectra; UV-VIS 403-31 - frequency spectra 346-7 - intensity spectra 347-9 intensity spectroscopy 393-9 ladder oligomers 30 oligo-p-phenylenes 424-8 oligoarylenevinylenes 414-24 - oligoenes 7, 9 - oligo(o-phenyleneethyny1ene)s 90 - oligo( p-phenyleneviny1ene)s 69 - oligo(tetrahydr0pyrene)s 35 - oligothiophenes 410-14 - oligoynes 14 one-dimensional lattices 349-51 polyenes 406-10 soluble oligorylenes 44, 46 vibrational spectra 345-402 Spiro compound 212 Spontaneous assembly. oligomeric metal complexes 275-9 SS-FVP see Solution-Spray-Flash-VacuumPyrolysis -

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597

SSF see site selective fluorescence spectroscopy Staging 317 Stannylation, co-oligomer preparation 255 Star-shape oligomers, synthesis 37-40 Starburst polymers 222 Step-by-step synthesis Hagihara method 87 hydrocarbon oligomers 2 oligoarylenevinylenes synthesis 86 oligoenes 7-8 oligo(rn-phenylene)~ 40 oligomeric metal complexes 280-8 - substituted oligo(pheny1enevinylene)s 74 WittigjWittig-Horner reactions 86 Stereoelectronics, hydrocarbon oligomers 36 Steric hindrance 491, 492 Stetter reaction 135. 249-50. 252 Stilbene 58-9. 75-6. 316, 515 Stille coupling reaction 118-19. 227, 238-9, 240 co-oligomer preparation 253-5 - mixed aryl-aryl couplings 249 - octithiophene synthesis 168 - oligoenes 8-9 - oligo(o-phenyleneviny1ene)s 79 - orthogonally fused terthiophene 170 - regioregular terthiophene 147 - a,a’-substituted oligothiophenes 182 STM see scanning tunnelling microscopy Stokes shift 406.412, 417-19 Stretching modes - see also effective conjugation coordinate R charge fluxes in polarised light 382-3 heteroaromatic oligomers/polymers 386 oligothiophenes 389 - polyenes spectroscopy 406-7 Structural models, polymers 295-328 Structure-optical property relations, oligomers 462-9 Sublimation, thin film devices preparation 519 Substituted bithiophenes 491 0-Substituted oligomers 535, 575 Substituted oligo(phenylenevinylene)s, step-by-step synthesis 74 cu,cu‘-Substituted oligopyrroles 496 Substituted oligothiophenes 134-87 cu,cu’-Substituted oligothiophenes 155-61 cu,,Mubstituted oligothiophenes 161-71 /j.b’-Substituted oligothiophenes 135-71 /j,b-Substituted oligothiophenes, polymerization 144-53 a,a’-Substituted oligothiophenes. solvatochromatic/optical properties 182 ;?,/3’-Substituted oligothiophenes. structural peculiarities 135-44 -

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598

Index

Substituted poly(p-phenylenes) 308-9 Substituted polythiophenes 309 N-Substituted pyrroles 251 Sulfur-containing oligomers 105-89,

105-233 ‘Superbenzene’ 53,55-6 ‘Supernaphthalene’ 55 Surface induced order 340-2 Suzuki coupling 119-20,238-9 hydrocarbon oligomers synthesis 26,28-9 open chain oligo(naphthy1ene)s 42 regioregular oligothiophenes synthesis 153 terthiophene synthesis 168 Symmetry oligothiophenes 432,442,445 unsubstituted oligothiophenes 433 -

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polymers 223-4 TCNQ complexes 571 thiolate 228 Tetrathiafulvalenophanes 216-22 Thermochromism 530 T H C see third-harmonic generation Thiapentalene derivatives 215 Thin films - deposition 571-4 - devices preparation 518-23 intermolecular non-radiative decay channels 526-9 third-harmonic generation 458-60,472 Thiopenes 106,107,110 Thiophene/pyrrole oligomers 495 Thiophenes 250-1 FETs 570 mixed oligomers 249 oligothiophenes 177-80,184-7 - organic-based FETs 567 Third-harmonic generation (THG) nonlinear optical responses 396-7 - nonlinearly optically-active oligomers 450, -

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Tail corrections 334 TCNQ see tetracyanoquinodimethane Telechelics .see oligophenylene rods Temperature conductivity effects 579 - field-effect mobility 579 greater than zero 337-9 thin film deposition 571-2,573 zero 335-7 Terphenyl 38,530 2,2’,5’,2’’-terpyrrole 237 Terrylene tetracarboxdiimide, synthesis 45 Terthiophenes 434,437,507,524 - amphiphilic 186 - donor-substitution 179. 182 - fluorescence spectra 410-11 - monoalkylated 159-60 - monomethylated 174 organic-based FETs 567 - regioregular see regioregular terthiophenes synthesis 168 Tetra t-butyl-pentarylene, Raman spectrum 368 Tetracarboximide rylene oligomers 43 Tetracene 526 Tetracyanoquinodimethane (TCNQ) 198,212 1,3,5,7,9,11 , I 3-Tetradecaheptaene 3-4 8-Tetradecyldodecithiophene 567 2,3,3,3-Tetrafluorosuccinatospecies 276 4,5,6,7-Tetrahydrobenzo[b]thiophene 161-2 Tetrahydrothiopenes, aromatization of 120-1 -

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452-3,456-7

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Tetrakis(hydroxyethyIthi0) 222 3,3’,5,5’-TetramethyL2,2’-bithiophene490 Tetraniline 265 Tetrathiafulvalenes (TTF) 198-233 - carbonyl derivatives 227 - dendrimers 222-3 - extended analog 178 monothiolate 327

polymers/dyes/oligomers comparison 472 thin films 458-60,472 Third-order nonlinearities 449-78,452-60 Tilt angle 298,307 Titanium src McMurry coupling ( p-Toluenesulfonyl)methyl isocyanide (TOSMIC) 242 Topochemical polymerisation 16,I9 Topochemical reactions, 2,2’-distyrylbiphenyI photodimerization 80 Torsion potentials 330 N-Tosylation 242 TPB 538 Tpy domains, heterocyles 285-8 Training set 332 trans-membrane molecular wires 23 truns-polyacet ylene - crystal structure 7-10 doped/prostine Raman spectra 374,379-80 -- infrared spectra 354 - infrared spectrum 365 - vibrational spectra 351-4,383-6 Transconductance 565 Transition energies end-capped oligothiophenes 162-5 poly(tetrahydr0pyrene)s 34 Transition metal catalyzed coupling methods -

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1 13-20

substituted oligothiophenes 168 Transition metal complexes, olirothiouhenes 187 Tri-is>prop;l end-groups 535

hider

Triads, donor/acceptor-substituted oligothiophenes 184 Trialkylphosphite-mediated coupling, bis-1.3-dithiol-2-chalcogenones 225 Trialkylstannyltetrathiafulvalenes. Pd-mediated coupling 227

3.4’,”’-Trihexyl-2,2’:5’.2’’-terthiophene 147 Trilithio TTFs 226 Tris-tetrathiafulvalenes 209, 222-3 Trithioorthooxalates. coupling with 1,3-dithiol-?-thiones 225 T T F see tetrathiafulvalenes T T F donor/acceptor complexes 179 Tungsten catalysis see acyclic diene metathesis (ADMET) reaction

Vibrational spectra all rums-oligoenes 383-5 conjugated oligomers 345-402 coordinates 371-80 heteroaromatic oligomers 386-92 Vinylogs 215-16 -

Wavelength absorption regioregular oligothiophenes 146-52 substituted oligothiophenes 142-3 - unsubstituted oligothiophenes 128-31 Wessling-Zimmermann method 62-3 Wittig reaction 227 - 13-carotene 4 - 1,3-dithiolylium salts coupling 225 a,odonor/acceptor substituted oligoenes 21 - ladder-type oligomers 31 oligoarylenevinylenes 86 - oligoenes 4-5 - oligo(naphthaleneviny1ene)s 83 oligo(o-phenyleneviny1ene)s 78 oligo(p-phenyleneviny1ene)s 59, 64, 65 paracyclophanes 82 Wittig-Horner reaction 242 - 1,3-dithiolylium salts coupling 225 - cr.d-donor/acceptor substituted oligoenes 21 oligoarylenevinylenes 58-9. 86 - oligophenylenevinylenes 66. 69-70. 74 polyphenylenevinylenes 59. 64 Wudl’s bis-TTF 227 Wurtz-type polymerization 59, 64 -

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Ullmann coupling 240-2 The UIlriiutiii reuctiori 1 13 ni-Undeciphenyl 299 N-Unsubstituted mixed oligomers 257 Unsubstituted oligophenylenes 486 N-Unsubstituted oligopyrroles 242-3 Unsubstituted oligothiophenes 109-34, 433 - arene/arene-coupling methods 109- 13 physical properties 127-34 - ring closure reactions 120-7 transition metal catalyzed coupling methods 113-20 N-Unsubstituted pyrroles 251 UV-VIS absorption spectra ladder-type oligo( p-phenylene)s 32 - oligoenes 8, 9 oligo(o-phenylene)s 40 oligophenylenevinylenes 68, 72, 74 oligo(tetrahydropyrene)s 35 oligoynes 14 polyphenylenevinylenes 66 soluble oligorylenes 44. 46 -

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X-ray scattering. conjugated polymers 302 Xerography 541-2

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Yamamoto coupling ladder-type oligomers 3 1-2 oligorylenes 43 -

Vacuum deposition 561 Valence Effective Hamiltonian (VEH) method 434, 44 1.443 Vibrational coordinate II 371-80

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Zero temperature 335-7 Zinc benzoate 275

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