Polythiophenes: electrically conductive polymers

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129

Advances in Polymer Science

Springer Berlin Heidelberg New York Barcelona Budapest Hong Kong London Milan Paris Santa Clara Singapore

Tokyo

Polythiophenes Electrically

-

Conductive

Polymers By G. Schopf and G. Kol3mehl

With 23 Figures and 11 Tables

Springer

Dr. G. S c h o p f b r o c o l o r Lackfabrik HenschelstraBe 2, D - 4 8 5 9 9 G r o n a u / F R G Prof. G. K o B m e h l Freie Universit/~t Berlin Institut ~ r O r g a n i s c h e C h e m i e TakustraBe 3, D-14195 Berlin / F R G

This series presents critical reviews of the present position and future trends in modem polymer research. It is addressed to all polymer and material scientists in industry and the academic community who whish to keep abreast of advances in the topics covered. As a rule, contributions are specially commissioned. The editors and publishers Will, however, always be pleased to receive suggestions and supplementary information. Papers are accepted for "Advances in Polymer Science" in English. In references "Advances in Polymer Science" is abbreviated Adv. Polym. Sci. and is cited as ajoumal. Springer WWW homepage: http://www.springer.de

I S B N 3-540-61483-4 Springer-Verlag Berlin Heidelberg N e w Y o r k I S B N 0-387-61483-4 Springer-Verlag N e w Y o r k Berlin H e i d e l b e r g

This work is s ubject to copyright. All rights are reserved, whether the whole or part of the material is concemed, specificallythe fightsoftranslation, reprinting, re-useof illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. Duplication of this publication or parts thereof is only permitted under the provisions ofthe German Copyright Law of September 9,1965, in its current version, and a copyright fee must always be paid. © Springer-Verlag Berlin Heidelberg New York 1997 ISSN 0065-3195 Printed in Germany The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Macmillan IndiaLtd., Bangalore-25 SPIN: 10508880 02/3020 - 5 4 3 2 1 0 - Printed on acid-free paper

Editors

Prof. Akihiro Abe, Department of Industrial Chemistry, Tokyo Institute of Polytechnics, 1583 Iiyama, Atsugi 243-02, Japan Prof. Henri Benoit, CNRS, Centre de Recherches sur Ies Macromol6cules, 6, Rue Boussingault, 67083 Strasbourg Cedex, France Prof. Hans-Joachim Cantow, Freiburger Materialforschungszentrum, Stefan Meier-Str. 3 l a, D79104 Freiburg i. Br., FRG Prof. Paolo Corradini, Universith di Napoli, Dipartimento di Chimica, Via Mezzocannone 4, 80134Napoli, Italy Prof. Karel Du~ek, Institute of Macromolecular Chemistry, Czech Academy of Sciences, 16206 Prague 616, Czech Republic Prof. Sam Edwards, University of Cambridge, Department of Physics, Cavendish Laboratory, Madingley Road, Cambridge CB30HE, UK Prof. Hiroshi Fujita, 35 Shimotakedono-cho, Shichiku, Kita-ku, Kyoto 603 Japan Prof. Gottfried Gl6ckner, Technische Universit~t Dresden, Sektion Chemie, Mommsenstr. ! 3, D-01069 Dresden, FRG Prof. Dr. Hartwig H6cker, Lehrstuhl ftir Textilchemie und Makromolekulare Chemie, RWTH Aachen, Veltmanplatz 8, D-52062 Aachen, FRG Prof. Hans-Heinrich H6rhold, Friedrich-Schiller-Universit~it Jena, Institut f'tirOrganische und Makromolekulare Chemie, Lehrstuhl Organische Polymerchemie, Humboldtstr. 10, D-07743 Jena, FRG Prof. Hans-Henning Kausch, Laboratoire de Polym6res, Ecole Polytechnique F6d6rale de Lausanne, MX-D, CH- 1015 Lausanne, Switzerland Prof. Joseph P. Kennedy, Institute of Polymer Science, The University of Akron, Akron, Ohio 44 325, USA Prof. Jack L. Koenig, Department of Macromolecular Science, Case Western Reserve University, School of Engineering, Cleveland, OH 44106, USA Prof. Anthony Ledwith, Pilkington Brothers plc. R & D Laboratories, Lathom Ormskirk, Lancashire L40 SUF, UK Prof. J. E. McGrath, Polymer Materials and Interfaces Laboratory, Virginia Polytechnic and State University Blacksburg, Virginia 24061, USA Prof. Lucien Monnerie, Ecole Superieure de Physique et de Chimie Industrielles, Laboratoire de Physico-Chimie, Structurale et Macromol6culaire 10, rue Vauquelin, 75231 Paris Cedex 05, France Prof. Seizo Okamura, No. 24, Minamigoshi-Machi Okazaki, Sakyo-Ku, Kyoto 606, Japan Prof. Charles G. Overberger, Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109, USA Prof. Helmut Ringsdorf, Institut ftir Organische Chemie, Johannes-Gutenberg-Universit~it, J.-J.-Becher Weg 18-20, D-55128 Mainz, FRG Prof. Takeo Saegusa, KRI International, Inc. Kyoto Research Park 17, Chudoji Minamima-chi, Shimogyo-ku Kyoto 600 Japan Prof. J. C. Salamone, University of Lowell, Department of Chemistry, College of Pure and Applied Science, One University Avenue, Lowell, MA 01854, USA Prof. John L. Schrag, University of Wisconsin, Department of Chemistry, 1101 University Avenue. Madison, Wisconsin 53706, USA Prof. G. Wegner, Max-Planck-lnstitut ftir Polymerforschung, Ackermannweg 10, Postfach 3148, D-55128 Mainz, FRG

Preface

Polymers containing thiophene systems have been a subject of research for more than twenty five years. One of the first examples was the polymer with thiophene units and conjugated double bonds [{poly(2.5-thiophenediylvinylene)]. It has been studied intensively because of its good electrical conductivity, being a member of the family of synthetic metals or organic metals. Polythiophene has been synthesized chemically by polycondensation reactions of difunctionalized thio-phene derivatives (starting from 2.5-dihalothiophene) as well as by electropolymerization of thiophene itself. One may also start with bithiophene, terthiophene or higher oligomers of thiophene in order to prepare polymers with thiophene systems, but with differing properties, especially with regard to the electrochemical behaviour. Polythiophenes are not only interesting because of their electrical properties. On oxidation (p-doping), the electrical conductivity can be enhanced from about 10-7 S.cm" (pristine material) up to several thousand S.cm -~ in the oxidized form. This layers of polythiophenes are also of interest due to their large number of special electrophysical properties: Thermochromism, electrochromism, solvatochromism, ionochromism, color changes under pressure and effected by electricity. Substituted polythiophenes have been synthesized and studied; those polythiophenes with long alkyl side chains are soluble and suitable for the preparation of free-standing films or coatings on surfaces of a variety of materials. Optical, magnetic and liquid crystalline properties have also been studied as well as the changes of wettability resulting from the redox behavior of polythiophenes during reversible oxidation-reduction reactions. The properties of polythiophenes can be influenced by the structure and the substituents of the starting materials, the synthetic methods, physical treatment (e.g. stretching, annealing, pressing, synthesis on oriented surfaces) and chemical modification (oxidation and reduction reactions, doping, dedoping). Therefore materials with tailor-made properties can be designed for selective applications. Soluble 3.4-disubstituted polythiophenes have found application as antistaticcomponents for film materials and are on the market; electrochromic and electroluminescent devices are subject to intensive research and surely will be effective in the near future. Biosensor devices with functionalized polythiophene carrier systems for immobilizing enzymes are also applicable for the electro-analytical determination of analytes in micromolar concentrations.

VIII

Preface

Many different applications are being researched and it is becoming apparent that all the activities in basic and applied research which have been increasing during recent years will be successful in offering new materials helpful for technical applications. Through its good characteristics, its stability, ready availability at low costs and its lack of toxicity, in material science polythiophene is an excellent candidate in the group of materials named synthetic metals for serious applications.

Table of Contents

List of Symbols and Abhreviati¢ns . . . . . . . . . . . . . . . . . . . . . . lnlroducti~u . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 1.2 1.3

Polymeric Materials Described Several Times in this P a p e r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P o l y m e r s with C o m p l e x Structures C o n t a i n i n g T h i o p h e n e Systems . . . . . . . . . . . . . . . . . . . . . . . . . Oligomers a n d M o n o m e r s with C o m p l e x Structures . . . . . .

Structure ~f Poly(thiophene)s . . . . . . . . . . . . . . . . . . . . . . . 2.1

2.2 2.3

P r i m a r y Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Poly(3-alkylthiophene)s . . . . . . . . . . . . . . . . . . . 2.1.2 Poly(3-alkyloxythiophene)s. . . . . . . . . . . . . . . . . 2.1.3 Poly(thiophene)s with P e n d a n t Reactive Groups ............................ Secondary Structure . . . . . . . . . . . . . . . . . . . . . . . . . Tertiary Structure . . . . . . . . . . . . . . . . . . . . . . . . . .

Properties of Poly(thi~p[ene)s . . . . . . . . . . . . . . . . . . . . . . 3.1 3.2

3.3 3.4

3.5 3.6 3.7

P o l a r o n s a n d Bipolarons . . . . . . . . . . . . . . . . . . . . . . Doping ................................ 3.2.1 The D o p i n g Process . . . . . . . . . . . . . . . . . . . . 3.2.2 Poly(thiophene) as a Redcx System . . . . . . . . . . . 3.2.3 D o p a n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . C o n d u c t i n g Relaxation, a.c. a n d d.c. C o n d u c t i v i t y . . . . . . . Chromisms .............................. 3.4.1 T h e r m o c h r o m i s m . . . . . . . . . . . . . . . . . . . . . . 3.4.2 S o l v a t e c h i o m i s m . . . . . . . . . . . . . . . . . . . . . . 3.4.3 Electrochromism. . . . . . . . . . . . . . . . . . . . . . . 3.4.4 I o n o c h r o m i s m . . . . . . . . . . . . . . . . . . . . . . . . 3.4.5 C o l o r Changes Caused by Pressure a n d Electricity . . . . . . . . . . . . . . . . . . . . . . . . Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . M a g n e t i c Properties . . . . . . . . . . . . . . . . . . . . . . . . . L a n g m u i r - B l o d g e t t T h i n Films . . . . . . . . . . . . . . . . . .

26 33 36 36 38 41 42 43 48 51 51 54 54 60 61 66 67 67 69 70 71 71 71 76 76

X

Table of Contents 3.8 3.9 3.10 3.11

4

Influence on Properties 4.1

4.2 4.3

5

.......................... Influences of Structure on P r o p e r t i e s . . . . . . . . . . . . . . . 4.1.1 Influence of M o l e c u l a r W e i g h t . . . . . . . . . . . . . . 4.1.2 Influence of A l k y l Side C h a i n s . . . . . . . . . . . . . . 4.1.3 Influence of C o u p l i n g - R e g i o r e g u l a r P A T . . . . . . . 4.1.4 Influence of Stretching - C r y s t a l l i n i t y a n d Morphology ......................... Influence of S t a r t i n g M a t e r i a l s . . . . . . . . . . . . . . . . . . O t h e r Influences o n P r o p e r t i e s . . . . . . . . . . . . . . . . . .

Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1

5.2

5.3

6

B e h a v i o r of Interfaces, Bilayers . . . . . . . . . . . . . . . . . . Gels .................................. L i q u i d Crystalline C o m p o u n d s . . . . . . . . . . . . . . . . . . Wettability ..............................

C h e m i c a l Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 Synthesis U s i n g N i c k e l o r P a l l a d i u m C o m p l e x e s . . . . 5.1.2 R e a c t i o n with C o p p e r P e r c h l o r a t e , F e r r i c Perchlorate, Copper Tetrafluoroborate, or Thallium Trifluoroacetate ................. 5.1.3 R e a c t i o n with F e r r i c C h l o r i d e . . . . . . . . . . . . . . . 5.1.4 Synthesis U s i n g A1C13-CuC1 0 2 . . . . . . . . . . . . . 5.1.5 P o l y m e r s D e r i v e d from P r e c u r s o r P o l y m e r s , Oligo(thiophene)s, o r P o l y ( t h i o p h e n e ) D e r i v a t i v e s . . . 5.1.6 P l a s m a P o l y m e r i z a t i o n . . . . . . . . . . . . . . . . . . . 5.1.7 P r e p a r a t i o n of C o m p o s i t e s . . . . . . . . . . . . . . . . . 5.1.8 Synthesis of R e g i o r e g u l a r 3-substituted Poly(thiophene)s ....................... 5.1.9 F u r t h e r C h e m i c a l Synthesis M e t h o d s . . . . . . . . . . E l e c t r o c h e m i c a l Synthesis . . . . . . . . . . . . . . . . . . . . . 5.2.1 D e p o s i t i o n a n d G r o w t h . . . . . . . . . . . . . . . . . . 5.2.2 Synthesis C o n d i t i o n s . . . . . . . . . . . . . . . . . . . . 5.2.3 Special E l e c t r o p o l y m e r i z a t i o n M e t h o d s . . . . . . . . . C o m p a r i s o n of Selected C h e m i c a l a n d E l e c t r o c h e m i c a l Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

78 79 79 80 80 80 80 82 86 87 88 90 93 93 93

95 95 97 97 98 99 100 102 102 102 104 111 112

Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

113

6.1 6.2 6.3 6.4

113 114 114

6.5 6.6 6.7

E l e c t r o c h r o m i c Devices . . . . . . . . . . . . . . . . . . . . . . . E l e c t r o l u m i n e s c e n t Devices . . . . . . . . . . . . . . . . . . . . . S o l a r Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Resists, R e c o r d i n g M a t e r i a l s , F a b r i c a t i o n of P a t t e r n s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schottky Barrier Diodes ...................... Field-Effect T r a n s i s t o r s (FETs) . . . . . . . . . . . . . . . . . . Antistatic Coatings .........................

115 117 117

119

Table of Contents

XI

6.8 6.9 6.10 6.11 6.12

120 120 122 123 123

Junction Devices, Rectifying Bilayer Electrodes . . . . . . . . . Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shielding Materials . . . . . . . . . . . . . . . . . . . . . . . . . Other Applications . . . . . . . . . . . . . . . . . . . . . . . . .

7

S u m m a r y , Conclusion and Outlook . . . . . . . . . . . . . . . . . . .

125

8

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

126

9

Author Index Volumes 101-129 . . . . . . . . . . . . . . . . . . . . .

147

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

157

I0

The studies and results of poly(thiophene) and its derivatives published in the literature during 1990-1994 have been summarized in this review. Their primary, secondary and tertiary structures, the chemical and physical properties, the structure-properties relationships, the chemical and electrochemical syntheses and the applications of poly(thiophene)s and their derivatives are described. In order to give an overview about the described polymeric materials and their structures the references are ordered by the polymers in tables. The tables contain the polymer keywords, references and the pertinent chapter in this review.

Symbols and Abbreviations a,c,

D

d.c. DEFA dppp

Eg Eox Ered EMI EPR FET FT-IR g GPC HOMO HPA

AUpp HH HT

IR ITO L LB LDA LUMO MESFET MISFET MNDO MO MOSFET Ns NADH NOESY NMR nT PAT PBT PBuT PDDT PDST PDT PET

alternating current diffusion coefficient direct current differential evolutionary factor analysis 1,3-bis(diphenylphosphino)propane polaronic gap oxidation potential reduction potential electromagnetic interference electron paramagnetic resonance field-effect transistor Fourier-transform infrared g-factor (EPR) gel permeation chromatography highest occupied molecular orbital heteropolyanion peak-to-peak line width of the EPR spectrum head-to-head coupling head-to-tail coupling infrared indium tin oxide ligand Langmuir-Blodgett lithium diisopropylamide lowest unoccupied molecular orbital Schottky gated field-effect transistor metal-insulator-semiconductor field-effect transistor one of the quantum chemical semiempirical methods molecular orbital metal oxide semiconductor field-effect transistor spin concentration nicotinamide adenine dinucleotide nuclear Overhauser effect spectra nuclear magnetic resonance oligo(thiophene) with n thiophene units poly(3-alkylthiophene) poly(bithiophene) poly(3-butylthiophene) poly(3-dodecylthiophene) poly(3-docosylthiophene) poly(3-decylthiophene) poly(3-ethylthiophene)

G. Schopf and G. KoSmehl

PHDT PHeT PHT PITN PMT PNT PODT POT PPEP PPrT PPT PT PTDT PTT PUDT RB SOMO

STM TCNQ THF ToF-SIMS TOS TT TTF UHV UV vis wt% XPS

~rnax t3-+ + O'p

poly(3-hexadecylthiophene) poly(3-heptylthiophene) poly(3-hexylthiophene) poly(isothianaphthene) poly(3-methylthiophene) poly(3-nonylthiophene) poly(3-octadecylthiophene) poly(3-octylthiophene) potential-programmed electropolymerization poly(3-propylthiophene) poly(3-pentylthiophene) poly(thiophene) poly(3-tetradecylthiophene) poly(terthiophene) poly(3-undecytthiophene) rhodamine B singly occupied molecular orbital scanning tunneling microscopy 7,7,8,8-tetracyanoquinodimethane tetrahydrofuran time of flight secondary ion mass spectroscopy p-toluene sulphonate tail-to-tail coupling tetrathiafulvalene ultrahigh vacuum ultraviolet visible weight percent X-ray photoelectron spectroscopy absorption maximum Hammett constant Brown constant

Polythiophenes

Electrically Conductive Polymers

1 Introduction The growth in the intensive study of highly conducting polymers began in 1977 with the discovery of the change in the electrical conductivity of poly(acetylene) on doping with Br2, I2 and AsFs [1]. Other conjugated polymers which exhibit interesting electrical and electrochemical properties associated with their extended n-bonding system are now known (Fig. 1). The electrical conductivity of all these polymers can be increased through appropriate chemical or electrochemical oxidation (so-called p-type doping) and sometimes through reduction (so-called n-type doping). The archetype of conducting polymers, poly(acetylene), becomes very highly conducting on doping in comparison with other conjugated polymers, but its thermal and environmental instability, its insolubility, its infusibility, and its lack of processability are obstacles to technical applications. Polymers containing heterocyclic units in the backbone (Fig. 1) were found to have notable electrical conductivities and to offer increased stability and processability in both the doped and neutral states when compared with poly(acetylene)s. Among the many poly(heterocyclic)s, poly(thiophene) (PT) and its derivatives have aroused great interest. The synthesis, structure, electrochemical, electrical, and physicochemical properties and applications of PTs and their derivatives were reviewed in 1986 [2-4] and 1992 [51. Many questions, such as structure-properties relationships, increasing of stability and processability, were not answered completely. A large number of investigations were later carried out to solve these problems in the case of PTs, since these compounds exhibit a range of advantages: the applications of doped and neutral states, the reversibility of the transition between the doped and neutral states of PTs due to their conductivity, redox activity, and the possibilities of modification of their electronic and electrochemical properties by manipulation of the monomer

structure.

poly(acetylene)

--O--O--'O--O--O'--O-H N

H N

i

i

H

,-9-0-9-0-%©@-0-

~

i

_0_,,-0-,,_0 ,,-QFig. 1. Conjugated polymers

poly(phenylene) poly(pyrrole) poly(thiophene) poly(aniline) poly(phenylene vinylene)

4

G. Schopf and G. Kol3mehl

Based on the reviews [2-5] covering the developments of this field up to 1991 and the published results of investigations of synthesis, the characteristics, reaction mechanisms, and novel applications of PTs and their derivatives up to 1994 are discussed in this review.

1.1 Polymeric Materials Described Several Times in this Paper In order to provide an overview of the chemical structures of the described polymeric materials published between 1990 and 1994, a list of the materials is provided (Tables 1-9) with, under each polymer name, a list of key words with the relevant literature references and references to the relevant sections of this review. Table 1. Unsubstituted poly(2,5-thiophenediyl), poly(thiophene), (PT) In references marked with an asterisk (*), the definition of unsubstituted PT or a universal notation for substituted and unsubstituted PT is not clear.

Key words

Section in this review

Ref.

Ageing effect All-organic FET Antistatic agents Bilayer Calculated and experimental band gap Cathodes having a PT membrane Cathodes with PT Charge-transfer complexes with TCNQ

5.2.2 6.6 6.7 3.8 2.1 6.10 6.10 3.2.3 6.12 3.2.3 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 3.4.3 6.12 5.2.2 3.2.3 4.1.1 4.3 5.2.2 3.4.2 2.t 4.1.1

659 775 782, 783 446 81 811 806-809 119, 312-316 46, 266, 271, 272, 288, 576, 589, 599

Chemical synthesis

Colorimetric evaluation Composite with paper Conductance of growing layer Conductivity

Conformation Conjugation length

357 885 650 271, 272, 276, 470, 546, 659 *346 *83, 84-86 Contin.

Polythiophenes


5

Table 1. Continued Key words


Ref.

Copper powder as filler Corrosion protection Coupling of monomeric units Crystal Crystallinity

6.12 6.1 2.1 2.2 2.2 2.3 5.2.2 5.2.1 3.1 3.2.1 3.2.3

821 714 70 117-119 84, 131 658 210 86, 119, 193, 194, 198, 267, 301,306 244, 246

Deposition temperature Dopants, diffusion of Doping

Doping, p- and n-type

Glassy carbon electrode modified with PT Electrogeneration on steel Electroluminescent device Electron-beam and 7-irradiation Electron-beam resist Electropolymerization

3.2.1 3.2.3 3.2.1 3.3 6.1 6.1 6.9 6.10 6.9 5.2.2 6.2 4.3 6.4 5.2.2

Electropolymerization at low temperatures

5.2.2

Electropolymerization in aqueous solution Electropolymerization on Ag Electropolymerization on Au Electropolymerization on ITO glass Electrothermal imaging devices Ellipsometry Emitting polarized light End substituted

5.2.2 5.2.2 5.2.2 5.2.2 6.4 5.2.1 3.5 2.t 2.2 3.1 3.5 4.1.1 4.2 5.2.1 3.2.3 4.1.1 6.6

Doping process Electrical properties Electrochromic display Etectrochromic window Electrodes coated with PT

EPR linewidth Femtosecond spectroscopy FET Field-effect mobility Growth Herringbone arrangement

4.1.1 6.6 5.3 2.2

206 328, *329 712, 714 707 797 810 798 647, *648 716, 719 "551 741 264, 265, 576, 638-640, 653, 663-666, 673,676,692 *329, 659, 660 641,667 643 213, 642 641,644 752 264,265,631 *373 58-60, 120, 124, 165, 166, 480

289 478 772 896 474-476, 771 706 121-123 Contin.

6




Refi

Heterojunction devices Impedance study

6.8 3.2.1 3.8 4.12 3.8 5.2.1 3.5 3.7 6.12 4.1.1 3.5 6.4 3.2.1 5.1.4 6.12 6.12 6.11 3.5 4.1.4 5.2.3 5.3 2.3 5.2.1 5.2.2 3.2.3 3.2.3 4.1.1 6.12 6.6 5.1.3 3.2.1 6.12 5.2.2 6.4 3.5 3.5 3.5 5.1.6 5.3 3.1 3.2.1

784 *225 443 886 440 629 383

Incoherent-to-coherent optical converter Interface between A1 and PT Interfacial reactions LB-films Liquid crystal display device Luminescence Magnetic recording Mass changes during doping Mass production Memory device Metal particles coated with PT Microwave shield Model for optical properties Molecular orientation Morphology

Neutral islands NMR relaxation time and Overhauser shift Nonlinear susceptibility Nonlinear two-terminal device Operation of a FET Organic molecular beam deposition Overoxidation Passivation of the surface of n-GaAs Pattern polymerization Photocurrent Photoexcitation process Phototoxicity Plasma polymerization Polaron/bipolaron

Polymeric pattern Polymerization Polymerization in zeolite hosts Polymerization of silylated monomers Polymerization with F2 Potential cycling Potentiometric response Printing PT salts for LB-films PT with Nation Raman spectra

6,4 5.2.1 5.1.7 5.2.2 5.1.9 5.2.2 6.9 6.4 3.7 5.2.2 3.2.1

840-843 360, 369, 389 753 212,'214 *590 829, 830 820 819 409 513,700 146,*149, 150,633 291,293 284, 897 479 838, 839 477 588 219 837 691 394 379, 380 381 600, 601 11,162,163, 172, 176-179, 185-187 728,748 4 615 679,680 "621 657 800 *749 430 693 t95 Contin.

Polythiophenes




Ref.

Redox behavior Resist Scanning tunneling microscopy (STM) Schottky diodes

3.2.2 6.4 2.3 6.5

Seamless polymeric belts Secondary batteries Self-discharge in electrodes Sensors for pesticide detection Solar cells Solid electrolytic capacitor Solid state doping

6.4 6.10 6.10 6.9 6.3 6.12 3.1 3.2.1 3.2.3 2.1 3.2.1 3.2.3 4.1.1 4.3 2.1.1 2.2 6.1 6.10 5.2.2 2.1 5.2.2

264, 265 730 135 760, 761, *762, 763 751 813, 816 817 796 714 845 879 181 182

Solid state NMR Specular-reflectance FT-IR spectroscopy Stability Structure Terthiophene as side chain Tetrathiafulvalene-derivatized PT Theoretical IR and Raman spectra Ultrasonic irradiation

69 196 177, 271, '541 97, 128-130 711 672 73, 74 689

Table 2. Poly(3-alkyl-2,5-thiophenediyl), poly(3-alkylthiophene), PAT

Key words

Poly(3-methylthiophene) (PMT);

Antistatic agents Catalysts dispersed on PMT Cathodes with PMT Chemical synthesis

Conductance of growing layer Conducting system Contacts (PMT/metal) Crystal/doping


Ref.

6.7 6.12 6. I 0 5.1.1 5.1.2 5.1.4 5.3 5.2.2 3.1 3.1 2.2 3.2.3

783 823 802, 803 288, 576, 589

n = 1

650 159 160 132 Contin.

8

G. Schopf and G. Kof~mehl



Ref.

Cu(II)-containing PMT electrodes Differential evolutionary factor analysis Doping

6.9 3.2.1 3.2.1 3.2.3 3.2.1 3.2.3 5.2.2 4.1.1 4.1.2 4.3 5.2.1

794 197 29,159 198,255 203

Doping, p- and n-type Doping with heteropolyanions Electrical conductivity Electrochemical deposition Electrochromic devices Electrode rotation Electrodeposition process Electrodes coated with PMT

6.1 2.3 5.2.2 5.2.1 2.3 6.9 6.12

Electrophotographic photoreceptor Electropolymerization

6.4 5.2.2 5.3

Electropolymerization on ITO glass Exchange of BF4 by Fe(SPh)~FET Gas-phase conformation Growth mechanism

5.2.2 3.2.3 6.6 2.2 5.2.1 5.2.2 5.3 6.8 3.8 3.2.1 4.3 6.4 3.2.1 5.1.3 5.3 5.2.2 6.10 3.2.1 6.12 6.9 4.1.2 4.1.4 5.2.3

Heterojunction device Heterojunction of Si/PMT Impedance analysis Inclusion of copper and iron species Inks Ionic motions across PMT film Layer-by-layer deposition Layers Li batteries with PMT Mass changes Memory device Mercury deposited on PMT electrodes Methyl substituted sexithiophenes Molecular orientation Morphological control of electropolymerization Morphology/doping Nonaqueous batteries Overoxidation

5.2.2 2.3 3.2.1 6.10 3.2.1

696-699 384, 469,483, 546 623,624, 626 7O9 151 632 152,788, 791,792, 797,799, 824 732,733 576,653 682,683, 69O 644 304 768 116 625,628 630,634, 635,706 784 439 227-229 561,562 75O 191 587 662 804,805 216 827,828 795 501 514 7O0 646 153 812 219,220 Contin.

Polythiophenes - Electrically Conductive Polymers Table 2. Continued Key words


Ref.

Oxidation of PMT

3.1 3.2.1 3.8 3.5 6.2

169, 202,222, 223,238 395 721,723, 724,725 601

Photocurrent Photovoltaic cells Plasma polymerization PMT with 1,6-bis(3-thienyl)hexane

PMT with 1,8-bis(3-thienyl)octane

Polymerization in zeolite hosts Potential control technique Potential cycling Potentiometric response Projection matrix method Relaxation Role of water during electropolymerization Sensitivity towards anions in aqueous solutions Side chains, PMT as Solid electrolytic capacitor Solid state NMR Spectroelectrochemical study Spectroscopic behavior Stability Superthin membrane Switchable gate membranes

5.1.6 5.3 2.2 2.3 4.2 2.2 2.3 4.2 5.1.7 5.2.2 5.2.2 6.9 3.2.1 3.3 5.2.2

145

145

615 649 657 8OO 197 318,321,322 673

3.2.1 6.9 5.1.7 5.2.3 6.12 2.1 3.1 3.1 3.2.1 5.2.2 4.1.2 4.3 6.12 6.12

200 786,787 610

6.7 6.6 6.12

783 774 880

6.12

880

4.1.2 4.3 5.1.3 3.2.1 3.2.3

494

866,880 69 170 164 192 490 836 153

Poly(3-ethylthiophene) (PET); n = 2 Antistatic agents FET Solid electrolytic capacitor Poly(3-propylthiophene) (PPrT); n = 3 Solid electrolytic capacitor Poly(3-butylthiophene) (PBuT); n = 4 Charge mobilities Chemical synthesis Doping

286 221,305 Contin.

10

G. Schopf and G. KoBmehl



Ref.

Doping/structure

2.3 3.2.1 3.2.3 4.1.1 4.1.2 4.3 3.3 6.4 6.6 6.9 4.1.2 2.2 2.3 3.2.1 4.1.2 3.7 3.2.1 4.1.2 3.3 6.12 3.2.3 4.1.2 4.3 5.3 2.1.1 4.1.2 4.1.3 5.1.8 2.2 3.1 4.1.1 4.1.2 3.2.3 4.3 4.1.2

155, 275

Electrical conductivity

Electrical properties Electron-beam resist FET Field-effect mobility Influence of doping on crystal LB-films Learning effect Memory effect Solid electrolytic capacitor Stability

Structurally homogeneous PBuT

Structure/properties

Thermal analysis Thermal dedoping

469, 483, 500 323 741 774 493 143 415 232, 233 319,320 881 296, 489, 490 94, 95, 99, 275 84, 140, 188 298 491

Poly(3-pentylthiophene) (PPT); n = 5

Electrical conductivity Influence of ~-radiation PPT as side chains Radiation resist Structure/spectroscopy Synthesis/structure

3,2.3 4.3 5.1,7 5.2.3 6.4 -

-

5.2.2

273 555 610 746 898 651,652

Poly(3-hexylthiophene) (PHT); n = 6

Ageing effect Bimorph Capacitors Changing upon irradiation Charge mobilities

4.3 6.12 6.12 6.4 4.1.2 4.3

549 460, 884 882 726 494

Contin.

11



Ref.

Chemical synthesis

5.1.1 5.1.3 2.1 4.3 2.1 3.2.1 3.2.3 3.2.3 3.1 3.2.1 3.2.3 4.1.2 4.1.4 5.3 3.3 5.3 3.5 5.2.2 5.2.2 6.6 4.1.2 6.6 3.5 6.12 3.9 4.1.2 6.12 2.2 2.3 3.2.1 4.3 4.3 2.1 3.2.3 3.4.1 3.7 4.1.2 4.3 6.6 3.2.1 3.5 4.1.2 4.1.4 6.2 2.1.1 4.1.1 6.10 6.4 3.1 5.1.1 3.6 2.2 3.5

92, 266,286 87 324 82 221 307-310 184 302 244

Conjugation length Dedoping Determination of molecular weight Doping Doping with C60 Doping with NOPF6 Doping, p- and n-type Electrical conductivity Electrical properties Electroluminescence Electropolymerization Electropolymerization on ITO glass FET Field-effect mobility Fixation of CO2 Gel Influence of doping on crystal

Influence of pressure Irradiation LB-films

LB-films in FET Learning effect Luminescence

Model configurational triads Molecular weight/properties Nonaqueous batteries Nonlinear semiconductor devices Oligo(thiophene)s, hexyl substituted Optical and magnetic properties Orientation of molecules Phosphorescence

92, 481,483 492, 508,543 323,324 327 382 481 644 765,767 493, 769 367 368 454-457, 459~461 143

342 554 87 270,341 415,417-419, 423-427, 429,436 428 234 100 361 363, 367 96 464 812 747 180, 572,573 411-413 134 375 377 Contin.

12




Ref.

Photo-induced doping

4.3 6.4 4.3 6.4 3.4.2 6.4 4.1.2 6.5

558

Photodegradation Photoimaging PHT solution Polymeric pattern Redox potential Schottky barrier Spin concentration Stability

Structurally homogeneous PHT

Structure/charge transport Structure/doping Synthesis/structure Thermochromism Trication radicals Triplet polaronic excitation Type of coupling

Whiskers formation

4.1.2 3.2.3 4.1.2 4.3 5.3 2.1.1 4.1.2 4.1.3 5.1.8 2.2 3.2.3 4.1.2 5.2.2 3.4.1 3.1 3.1 2.1.1 2.2 4.1.3 5.1.8 2.2

556,557 727 350 728 481 757,759 764 481 92,296, 489,490 94,95, 99 275 118 268,269, 275 651,652 342 180 171 92 97 100 138

Poly(3-heptylthiophene) (PHeT); n = 7

Chemical synthesis

5.1.3

565

2.1 3.8 4.1.2 4.3 5.1.1 5.1.3

75 444 494

Poly(3-octylthiophene) (POT); n = 8

Anomalous X-ray scattering Bilayer films Charge mobilities Chemical synthesis Conformation Crystallinity Deep-UV and electron-beam resist Doping Doping, thermal dedoping

Electrical conductivity

3.4.2 2.2 6.4 3.1 3.2.1 3.2.3 3.2.1 3.2.3 4.1.2 4.3 3.2.3 3.3 4.1.1

55,92, 266, 286,510 199 141 743 183,199, 222,239 189,190, 491 92,273 325,472, 481,492, Contin.

13


Electrical properties Electrochromic material Electrochromic properties Electropolymerization Electropolymerization on ITO glass Field-effect mobility Gel Heat resistant POT Hydrodynamic radius Impedance study Influence of doping on crystal Interface between AI and POT Irradiation LB-films Learning effect Liquid crystal display device Magnetic properties Memory device Metal-polymer contacts Octyl substituted sexithiophenes Orientation of molecules Photoconductivity Potentiometric response Schottky diodes Solid electrolytic capacitor Stability

Structurally homogeneous POT

Structure/doping

Structure/properties Synchrotron radiation Thermal conductivity Using of electron-beam


Ref.

4.1.2 4.1.4 4.3 5.3 3.3 4.1.1 6.1 3.4.3 5.2.2 5.2.2 4.1.2 3.9 4.1.2 4.3 2.3 3.2.1 2.2 2.3 3.2.1 3.8 4.3 3.7 4.1.2 3.2.1 4.1.2 6.12 3.6 6.12 3.8 4.1.2 2.2 4.1.4 3.5 6.9 6.5 6.12 3.2.3 4.1.2 4.3 5.3 2.1.1 4.1.2 4.1.3 5.1.8 2.3 3.2.1 3.2.3 4.1.2 2.2 4.3

500, 543, 547, 548, 564 323, 465,471 713 351 481 644 493 452, 453, 457, 459 563 156 226 143 440 554 415 232, 233 841 411-413 825,826 438 501 134, 510,511 392 800, 801 755,756 881 92, 296, 489, 490, 495 94,95, 99, 275 140, 155, 268,275

6.4

140 550 899 742

3.5 6.4

385 738

-

Poly(3-nonylthiophene) ( P N T ) ; n = 9

Photooxidation Resist material

Contin.

14

G. Schopf and G. Kogmehl



Ref.

5.1.1 5.1.3 3.2.3 3.2. I 3.2.1 3.2.3 4.3 4.1.2 4.3 4.1.2 3.9 4.1.2 2.2 2.3 3.2.1 4.3 3.7 3.6 3.5 2.1 3.4.1

286, 565 286 208 189

Poly(3-decylthiophene) (PDT); n = 10

Chemical synthesis Doping Doping/orientation Doping/thermal dedoping

Electrical conductivity Field-effect mobility Gel Influence of doping on crystal Irradiation LB-films Magnetic properties Photoexcitation Structure Structure/temperature

492, 564 493 457 143 553,554 433 411-413 386 72 335

Poly(3-undecylthiophene) ( P U D T ); n = 11

Chemical synthesis

5.1.3

565

4.3 4.1.2 4.3 5.1.1 5.1.3 4.3 3.2.3 3.2.3 4.1.2 4.3 5.3 3.3 3.5 5.2.2 3.9 4.1.2 5.2.1 2.2 2.3 3.2.1 3.4.5 3.7 3.2.1 4.1.2 3.5 4.1.2 4.1.4 4.3 6.2

364 494

Poly(3-dodecylthiophene) ( P D D T ) ; n = 12

Band gap Charge mobilities Chemical synthesis Crystallinity Doping Electrical conductivity

Electrical properties Electroluminescence Elect ropolymerization Gel Growth Influence of doping on crystal Influence of pressure on properties LB-films Learning effect Luminescence

92,266, 274,286 364 266,274 92,273, 364,481, 500,542, 543,564 323 382 90,481 457 627 143 359 433 232, 233 362-364

Contin.

15



Ref.

NMR spectra Orientation of molecules Photoconduction Properties in solution Resin adhesives Solid electrolytic capacitor Spin concentration Stability

2.1.1 2.2 3.5 3.4.2 6.12 6.12 4.1.2 4.3 5.3 2.1.1 4.1.2 4.1.3 5.1.8 2.3 3.2.1 3.2.3 4.1.2 2.2 3.2.3 4.3 4.3 3.4.1 3.1

90,91 134 388 349 834,835 881 481 92, 485 94,95, 99,275, 505, 616,617 155, 268,275

3.4.2 2.2 3.2.1

199 141 199

3.2.3 3.7

273 432-435

3.2.3 4,1.2 6,2 5,2.2 2.2 2.3 3.2.1 3.8 6.8 3.7 4.1.2 6.12 4.1.2 5.3

307,310 481 717 481 143

4.1.2 3.5 4.1.2 6.2 6.12

488 363

Structurally homogeneous PDDT

Structure/doping

Structure/properties Thermal analysis Thermal relaxation Thermochromic properties Triplet polaronic excitation

140 298 545 331~332 171

Poly(3-tetradecylthiophene) ( P T D T ) ; n = 14

Conformation Crystallinity Doping process Poly(3-hexadecylthiophene) ( P H D T ) ; n = 16

Electrical conductivity LB-films Poly(3-octadecylthiophene) ( P O D T ) ; n = 18

Doping with C6o Electrical conductivity Electroluminescence devices Electropotymerization Influence of doping on crystal Junction device LB-films Resin adhesives Spin concentration Stability

449 415,417, 433 834 481 490

Poly(3-docosylthiophene) ( P D S T ) ; n = 22

Conjugation length Luminescence Solid electrolytic capacitor

881

16


Table 3. Other 3-substituted Poly(thiophene)s

Key words


Ref.

Chiral alkyl side chains Chiral amino acid side chains Oxygen atoms containing alkyl side chains Poly(3-cyclohexylthiophene)

2.1.1 3.4.2 3.7 2.1.1 2.2 2.3 4.2 5.1.3 2.1.1 3.2.1 3.2.3 5.2.2 4.2

101,102 348 420 93, 522

Poly(3-phenylthiophene)

Poly(3-styrylthiophene) Poly(3-fluoromethylthiophene) Poly[-3-(4-fluorophenyl)thiophene] Poly[3-(3-phenylpropyl)t hiophene] Poly[3-(4-propylphenyl)thiophene] Poly [3-(4-hexylphenyl)thiophene] Poly[3-(4-octylphenyl)thiophene]

Ester-functionalized PAT Poly(methyl thiophene-3-carboxylate) Poly(sodium thiophene-3-carboxylate) Poly(alkyloxythiophene) Poly(3-methoxythiophene)

Poly(3-ethoxythiophene) Poly(3-butoxythiophene) Poly(3-pentoxythiophene) Poly(3-heptyloxythiophene) Poly(3-octyloxythiophene) Poly[3-(2-hydroxyethyl)thiophene]

3.2.1 3.2.3 6.12 5.1.2 5.1.3 2.1.1 5.1.3 3.5 4.1.2 5.1.3 5.1.8 5.1.3 5.1.1 5.1.1 5.2.2 3.2.1 3.2.3 5.1.1 5.1.3 5.2.2 6.4 6.7 6.7 3.2.1 3.2.3 5.1.1 3£ 2.1 3.4.1 2.1 5.2.2 6.9

103,219, 243,246, 668 529 900 246, 844 575 103 58O 372,496, 580, 620 583, 584 566 566 694, 695 46, 247, 684, 686, 738,783

776, 777 247 399 104, 105 331,332 111, 112 Contin.

17



Ref.

Poly(3-methoxymethylthiophene) Poly[3-(2-methoxyethyl)thiophene]

5.2.2 2.1 5.2.2 6.9 5.2.2 5.1.3 4.2 3.2.1 3.2.3 5.1.1 2.1 3.5 2.1 5.1.3 4.2 4.2 4.2

661 111

Poly(3-ethoxymethylthiophene) Poly(3-hexanoyloxyethylthiophene) Poly(3-methoxyethoxythiophene) Poly[3-(2-methoxyethoxy)thiophene] Polyl-3-(2-butoxyethoxy)thiophene] Poly{ 3-[2-(3-methylbutoxy)ethyl]thiophene} Poly{ 3-[2-(2-methoxyethoxy)ethoxy]thiophene} Poly[3-(methoxyphenyl)thiophene] Poly [3-(octyloxyphenyl)thiophene] Polyl-3-(2,2,2-trifluoroethoxy)thiophene] Poly[3-(4,4,5,5,6,6,7,7,8,8,9,9,9,-tridecafluorononyl) thiophene] Poly[3-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecaftuorooctyloxy)thiophene] Poly(3-perfluoropropylthiophene) Poly(3-cyanothiophene) Poly(3-fluorothiophene) Poly(3-chlorothiophene) Poly(3-bromothiophene)

Poly(3-nitrothiophene) Poly(3-thienylacetic acid) Poly(3-thienylpropanesulfonic acid) Poly(3-thienyl-2-ethanesulfonate) Poly(heptadecyl-3-thienylacetate) Poly[2-(3-thienyl)ethyl acrylate] Poly(3-ethylmercaptothiophene) Poly[3-(3,6-dioxaheptyl)t hiophene] Poly{N-methyl-N'-[3-(3-thienyl)propyl]-4,4'bipyridinium salt} Di-(3,4-dioxaheptyl)sexithiophene Polymer of 4-cyano-4'-[8-(3-thienyl)octyloxy]biphenyl Poly(3-hexylthiophenediylacetylene)

661 585 534 247 104, 105 378 104, 105, 581 521 521 520

3.4.1 3.7 3.8

333, 421

4.2 5.1.1 5.1.1 6.7 4.2 5.2.2 6.7 4.2 5.2.2 4.2 5.1.6 5.2.2 6.7 6.8 6.7 3.2.1 3.7 5.2.1 3.2.1 3.4.3 6.4 35 3.7 4.3 3.5 3.2.1

520 567 266 783 527, 783

3.2.1 4.1.2 2.2

527 527, 602, 783, 785

783 196, 431, 636, 637 257-260 737 365 553 393 248 219 501 127 901

18


Table 4. Disubstituted Poly(thiophene)s

Key words


Ref.

Poly(3,4-dimethylthiophene) Poly(3,4-dibutylthiophene)

6.7 5.1.3 6.4 2.2 3.4.2 5.2.2 2.2 3.4.2 5.1.2 5.1.3 6.7 2.1 2.2 3.4.2 2.2 3.4.2 4.2 3.2.1 3.4.1 3.4.2

783 579, 746 126

3.4.1 3.7

333

Poly(3,4-dihexylthiophene) Poly(3,4-dimethoxythiophene) Poly(3,4-dibutoxythiophene)

Poly(3,4-dibromothiophene) Poly[3,4-bis(ethylmercapto)thiophene] Poly(3-octyl-4-methylthiophene) Poly(3-butoxy-4-methylthiophene) Poly(3-octyloxy-4-methylthiophene) Poly[3-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8pentadecafluorooctyloxy)-4-methylthiophene]

684 126, 575 783 81 126 126 199, 330-332, 334

Table 5. Poly(isothianaphthene), PITN

Key words


Ref.

Calculated and experimental band gap Doping, n- and p-type

2.1 3.2.1 3.2.3 6.1 6.1 6.12 3.4.3 3.2.3 2.1 2.1 -

81 253, 254 253 707 253 257 290 77, 78 79 902

Electrochromic display Electrochromic window Learning effect Optical switching Properties Structure Structure upon doping Poly(tetrafluoroisothianaphthene)

Polythiophenes

19


Table 6. Poly(bithiophene), Poly(terthiophene), and Poly(tetrathiophene) Key words


Ref.

Bilayer

3.8

Bilayer electrodes Chemical synthesis

6.8 5.1.2 5.1.3 2.1 2.2 3.2.3 3.2.3 4.3 6.9 3.2.1 3.2.3 3.2.1 3.2.1 3.2.3 5.2.2 6.9 5.2.2

441,442, 445-448 785 288, 574, 577 70 132

Poly(2,2'-bithiophene-5,5'-diyl) Poly(bithiophene) (PBT)

Coupling of monomeric units Crystal/doping Decomposition Detector for ascorbic acid Doping Doping/dedoping Doping, n-type Electrical conductivity Electrodes coated with PBT Electropolymerization

Polaron/bipolaron Polymerization Polymerization in zeolite hosts Potentiometric response Redox potential Role of water during electropolymerization

2.2 5.2.1 5.1.7 5.1.3 5.3 3.2.1 2.3 4.1.4 5.2.2 3.2.1 3.2.3 3.1 5.2.1 5.1.7 6.9 4.2 5.2.2

Seamless polymeric belts Secondary batteries Stability

6.4 6.10 4.3

Thickness/rigidity Wettability

5.2.2 3.2.2 3.11 4.1.4 6.4

Film structure Growth Intercalative polymerization Layer-by-layer deposition Mass changes during doping Morphology Overoxidation

287, 288 790 250, 301 237 213, 250,251 659 790, 797 217, 261, 677,678 136 213 614 587 211,213, 215 146,217, 261,518 217 175,188 4 615 800 525 237, 673-675 751 815 217, 26t, 518 688 217, 261-263, 518 754 Contin.

2O




Ref.

Poly(4,4'-dialkyl-2,2'-bithiophene)

2.2 3.2.1 3.3 3.6 3.2.1 4.1.3 3.2.1 3.4.1 6.9 2.1.1 2.2 3.4.1 4.1.3 5.2.2 5.2.2 4.2 5.1.3 4.2 3.5 4.2 4.2 3.1 3.4.1 4.2

133, 224, 319, 320, 411,412

Poly(4,4'-dibutyl-2,2'-bit hiophene) Poly(4,4'-dihexyl-2,2'-bithiophene) Poly(3,Y-dihexyl-2,2'-bithiophene) Poly(4,4'-dioctyl-2,2'-bithiophene) Poly(4,4'-didecyl-2,2'-bit hiophene) Poly(3,3'-dimethoxy-2,2'-bit hiophene) Poly(4,4'-dimethoxy-2,2'-bit hiophene) Poly(3,3'-dibutoxy-2,2'-bithiophene) Poly(4,4'-dibutoxy-2,2'-bithiophene) Poly(4,4'-dipent oxy-2,2'-bithiophene) Poly(3,3'-dibromo-2,2'-bithiophene) Poly(3,3'-dichloro-2,2'-bithiophene) Asymmetric disubstituted poly(bithiophene)s Poly(3-butoxy-3'-decyl-2,2'-bithiophene) Poly(4-butoxy-4'-decyl-2,2'-bit hiophene) Poly(3-dodecyl-2,2'-bithiophene)

Poly(3-hexyl-2,2'-bithiophene) Poly(3 -oct yl-2,2'-bithiophene)

4.2 3.4.1 4.1.2 4.3 5.1.2 5.1.3 4.1.2 3.4.1 4.1.2

221, 506 221 331, 332 800 98, 335 506,507, 686 684-686 526,528, 586, 526, 528 405 525 525 173 528 330, 334, 526 526, 528 330, 485, 487, 575 485, 486 340, 495

Poly(2,2';5',2"-terthiophene-5,5"-diyl) Poly(terthiophene) (PTT)

Alkyl and aryl substituted PTT Chemical synthesis Coupling of monomeric units Morphology Polymerization in zeolite hosts Properties Poly(Y-octyl-2,2';5',2"-terthiophene) Poly(3'-perfluorohexyl-2,2';5',2"-terthiophene)

5.2.2 5.1.2 2.1 2.3 5.2.2 5.1.7 4.2 3.4.1 4.1.2 3.4.1 3.7

524, 681 577 70 146, 148 615 524 340, 495 333 Contin.

Polythiophenes

21




Ref.

Poly(3'-phenyl-2,2';5',2"-terthiophene) Poly[3'-(4-cyanophenyl)-2,2';5',2"-terthiophene] Poly[3'-(4-methoxyphenyl)-2,2';5',2"-terthiophene] Poly[3'-(4-pyridyl)-2,2';5',2"-terthiophene] Polyl-Y-(2-thienyl)-2,2';5',2"-terthiophene] Poly[-3'-(3-methyl-2-thienyl)-2,2';5',2"terthiophene] Poly(3,3"-dihexyl-2,2';5',2"-terthiophene) Poly(3',4'-dibutyl-2,2';5',2"-terthiophene)

4.2 4.2 4.2 4.2 4.2

524 524 524 524 524

4.2 5.1.8 2.1 5.1.8 3.4.1 3.5 5.1.8 5.1.8

524 618 88, 619 338 405, 618 618

2.1.1

89

Poly(3',4'-dihexyl-2,2';5',2"-terthiophene) Poly(4,4"-dipentoxy-2,2';5',2"-terthiophene) Poly(3,3"-dipentoxy-2,2';5',2"-terthiophene)

Poly(2,2';5',2"; 5",2'"-tetrathiophene-5,5'"-diyl) Poly(tetrathiophene)

~N Poly(3,3 '"-dimethyl-2,2';5',2"; 5",2" '-tetrathiophene)

Table 7. Conjugated Polymers Containing Thiophene Systems Key words


Ref.

Calculated and experimental band gap Chemical synthesis

2.1 5.1.1 5.1.3 5.1.5 5.1.9

Doping

3.2.1 3.2.3 3.2.3 4.1.4 3.4.3 6.2 5.2.2 6.6 3.5

81 14, 277-279, 353,512, 568,569, 591-598,622 230, 277-279,300 277-279, 512 353 718,720 655 593,765 387

Poly-(2,5-thiophenediyl vinylene) Poly(thienylene vinylene)

Electrical conductivity Electrochromism Electroluminescent device Electropolymerization FET Photoconductivity

Contin.

22


Table 7. Continued

Key words


Ref.

Polymeric pattern Polymerization Radiation resist Poly(alkylthienylene vinylene) Poly(3-methoxythienylene vinylene)

6.4 5.2.1 6.4 3.5 2.1 3.4.3 3.4.3 3.5 2.1 3.2.3

729 4 746 396-398,402 81, 352 352 399 106,107, 294,295

5.1.3 3.2.1 4.2 3.2.1 4.3 5.2.2 3.2.3 6.9 4.1.2 2.3 3.4.3 4.1.2 5.3 2.3 3.4.3 3.5 4.1.2 5.3 2.3 3.4.3 4.1.2 5.3

578 231, 519 245

3.2.1 4.2

231, 519

Poly(3-ethoxythienylene vinylene) Poly(3-ethoxythienylene vinylene) Poly(3,4-dibutoxyt hienylene vinylene)

Poly(2,2'-bithiophene-5,5'-diyl vinylene) Poly[1,2-di(2-thienyl)ethylene]

Chemical synthesis Doping Doping, n- and p-type Electropolymerization Reaction with bromine Solubility Poly[ 1,2-bis(3-methyl-2-thienyl)ethylene]

Poly[ 1,2-bis(3-butyl-2-thienyl)ethylene]

Poly[ 1,2-bis(3-octyl-2-thienyl)ethylene]

654 283 147 147

147, 396

147

Poly[ l,4-di(2-thienyl)-l,3-butadiene]

Doping

Contin.

Polythiophenes

23




Ref.

3.2.3 4.1.1 4.1.1

280

Poly(2,5-thiophenediyl ethynylene) Poly(2,5-thienylene ethynylene)

Electrical conductivity Ethyl substituted oligo(2,5-thienylene ethylene)s

467,468

Table 8. Copolymers containing thiophene systems In references marked with an asterisk (*), the composition of the copolymers is not well-defined Key words


Ref.

Conformation Thiophene/benzene copolymer

3.4.1 2.1 3.2.3 4.1.4 5.1.7 5.2.3 4.1.2 5.1.7 3.2.1 4.2 5.1.7 3.2.1 3.5 3.5 5.1.7 3.5 5.1.7 3.5 3.4.3 3.6 4.1.2 4.1.3 4.1.4 4.2 5.2.2 5.1.7 3.2.3 5.1.7 3.2.1 3.5 4.3 4.1.2 5.1.7 6.4

*344 69, 281,607

Thiophene/pyrrole copotymer Thiophene/furan copolymer Thiophene/pyridine copolymer Thiophene/ethynylene/pyridine copolymer Thiophene/quinoxaline copolymer Thiophene/cyclopentadienonecopolymer Thiophene/silol copolymer and cooligomer Thiophene/silanylene copolymer Thiophene/vinyl alcohol copolymer Thiophene/3-methylthiophene copolymer Thiophene/3-octylthiophene copolymer Thiophene/3-methoxyethyoxythiophene copolymer Thiophene/1,2-di-(2-thienyl)ethylene copolymer Thiophene/polystyrene copolymer Mono-, bi-, terthiophene/silane copolymer Poly(5-vinyl-2,2';5',2"-ter thiophene) 3-Methylthiophene/3-butylthiophene copolymer

491,608,903 252, 608 204,205 370 370 604 371 603 374 355 414 496, 504,516 534 670 605 303 242 286, 737 Contin.

24


Table 8. Continued

Key words


Ref.

3-Methylthiophene/3-hexylthiophenecopolymer

4.1.2 5.t.7 4.1.2 4.2 5.1.7 2.1.1 5.2.3 2.1 5.2.1 5.2.2

286

3-Methylthiophene/3-octylthiophenecopolymer 3-Methylthiophene/3-dodecylthiophenecopolymer 3-Methylthiophene/3-thienylaceticacid copolymer 3-Methylthiophene/3-(2-benzyloxyethyl)thiophene copolymer

496,497, 531 91, 701 108 110

2.1

112

2.1

112

2.1

112

2.1

112

2.1

112

2.1 2.1 6.9 2.1

112 110

2.1 5.2.3 3.5 4.2 5.2.2

113 705 407, 532,533 669

5.2.2 2.2 4.1.2 5.1.7 2.2 2.1 2.1 4.1.2 5.1.7 3.8 4.3 2.2

656 144, 286

3-Methylthiophene/3-[2-(dimethyl-tert-butylsilyloxy) ethyl]thiophene copolymer 3-Methylthiophene/3-(2-acetoxyethyl)thiophene copolymer 3-Methylthiophene/3-(2-benzoyloxyethyl)thiophene copolymer 3-Methylthiophene/3-(2-(p-nitrobenzyloxy)ethyl] thiophene copotymer 3-Methylthiophene/3-[2-(3,5-dinitrobenzoyloxy)ethyl] thiophene copolymer 3-Methylthiophene/methyl 3-thienylacetate copolymer 3-Methylthiophene/3-(2,4-dinitrostyryl)copolymer 3-Methylthiophene/3-(2,4,6-trinitrostyrylthiophene) copolymer 3-Methylthiophene/pyrrole copolymer 3-Methylthiophene/methylmethacrylate copolymer 3-Methylthiophene/2,6-naphthyridinecopolymer Copolymer from 3-methylthiophene and ruthenium complexes 3-Butylthiophene/3-octylthiophenecopolymer 3-Butylthiophene/3-dodecylthiophenecopolymer 3-Butylthiophene/3-thienylaceticacid copolymer 3-Butylthiophene/methyl3-thienylacetate copolymer 3-Hexylthiophene/3-octylthiophenecopolymer 3-Hexylthiophene/3-thienylmethanolcopolymer 3-Hexylthiophene/azobenzenecopolymer 3-Octylthiophene/3-dodecylthiophenecopolymer 3-Octylthiophene/4-N-(3-thienyl)amino-2-nitrophenol copolymer 3-Undecylthiophene/3-thienylmethanolcopolymer Bithiophene/pyrrole copolymer

Bithiophene/benzenecopolymer

3.5 3.8 6.9 2.3 3.2.1 5.2.3 6.12 5.2.2

113

144 110 110 286 450 559,560 144 408 451 157, 703,704, 822 582 Contin.

25



Ref.

5.1.7

583,584

5.2.2

656

4.2

530

3.7

422

Key words


Ref.

Poly(thiophene) and poly(acrylonitrile) Poly(thiophene) and polyester Poly(thiophene) and polystyrene Poly(thiophene) and poly(aniline)poly(sodium acrylate)

5.1.7 6.12 5.1.7

609 833 612

3.4.3 6.1 5.2.3

356

3.4.3 6.1 4.2

356

3.4.1 6.7

345 778 540

4.1.2

498

2.3 3.4.2 4.3 4.2 4.1.4 4.2

158 544,552

4.2

540

4.3 4.1.4 4.2 5.1.7 6.7 6.10 6.11 6.12 5.1.7 6.11 3.2.3

5O0 510,535, 611,779, 831

Ethyl 2-(3-thienyl)hexanoate/2-(3-thienyl)ethanol copolymer Copolymer from bithiophene and ruthenium complexes Poly(p-phenylene vinylene-co-2,5-thienylene vinylene) Poly[3-(bromooctyl)thiophene-co-3-(vinylhexyl)thiophene]

Table 9. Blends

Poly(thiophene) in inorganic matrix Poly(3-methylthiophene) and poly(aniline)poly(sodium acrylate) Poly(3-methylthiophene) and nitrilic rubber Poly(3-hexylthiophene) and poly(3-oct ylt hiophene) Poly(3-hexylthiophene) and polystyrene Poly(3-octylthiophene) and poly(carbonate) Polymer network of poly(3-octylthiophene) and polystyrene Poly(3-octylthiophene) and copoly(ethylene vinylacetate) Poly(3-octylthiophene) and poly(phenylene oxide) Poly(3-octylthiophene) and polyethylene Poly(3-octylthiophene) and poly(ethylene-co-butyl acrylate) Poly(3-octylthiophene) and poly(3-dodecylthiophene) Poly(3-octylthiophene) and nonconductive polymers

Poly(3-octylthiophene) and plastics Poly(3-octylthiophene) and polystyrene

702

536,537

540 5O9 538 540

613, 818 282 Contin,

G. Schopf and G. Ko8mehl

26 Table 9. Continued

Key words Poly(3-dodecylthiophene) and copoly(ethylene vinylacetate) Poly(3-dodecylthiophene) and polystyrene Poly(3-dodecylthiophene) and radiation sensitive agent Poly(3-dodecylthiophene) and poly(ethyleneterephthalate) Poly(3-dodecylthiophene) and nonconductive polymer Poly(3-docosylthiophene) and polystyrene Poly(3-methoxythiophene) and polystyrene Poly(3-dodecyloxythiophene) and poly(methyl methacrylate) Poly(3,4-ethylenedioxythiophene) and polystyrene sulfonic acid Poly(3-thienyl acetic acid) and polyethylene Poly(chlorothiophene) and poly(4-vinyl-N-methylpyridiniumiodide) Poly(bithiophene) and polystyrene Poly(bithiophene) and polymer sulfates

Poly(thienylene vinylene) and poly(p-phenylene vinylene) Poly(thienylene vinylene) and nonconductive polymer Terthiophene and polycarbonate


Ref.

2.3 3.4.2 3.2.3

158 282

6.4

740

6.12

883

6.12 3.2.3 6.4

831 282 739

6.4

744

3.4.3 6.7 6.12 6.1 6.9 6.10

44, 45

3.2.1 5.1.7 5.2.2 6.7 6.10 6.11

249 574,612 671

4.2 6.12 6.6

530 831 771

710

1.2 Polymers with Complex Structures Containing Thiophene Systems These compounds are not mentioned in detail in this review:

~ n

(a)

~

(b)

n

P oly( 2,2';5',2"-terthiophene- 5,5"-diyl-tr ans-vin ylene- 2,5-thiophenediyl) (a) P ••y( 2•2' ;5'•2''-terthi•phene- 5•5''-diy•-tr ans-vin y•ene- 2•2'-bithi•phene- 5•5'-diy•) (b) [6, 7]

Polythiophenes ElectricallyConductivePolymers

R-- CH= C H - ~ C H 2 ~ - ~ C H = C H - -

27

R

R: "-~CI-!=CH-~ (a) R: - ~

(b)

R:--~CH=CH-~

(e)

R: - ~ C H 3

(d)

R: ~

(e)

all-E-a,~o-bis{5-[4-(styryl)styryl]-2-thienyl}alkanes (see also Sect. 3.10) (a) [-8] a, co-bis[-5-(2-thienylvinyl)-2-thienyl]alkanes (b) 1-9] a, eo-bis{5-1-4-(2-thienylvinyl) styryl]-2-thienyl}alkanes (c) [9] a,eo-bis[5-(4-methylstyryl)-2-thienyl]alkanes (d) [9] a,eo-bis[5-(2-fluorenylvinyl)-2-thienyl]alkanes (e) 1-10]

R-- N= C H - - - ~ C H 2 ~ H = N - - R

R.'

~OC~ H9

a, co-bis{5- [N-(4'-butyloxybiphenyl-4-yl)imino] formyl-2-thienyl}alkanes [10]

~

e

Poly[2,5-thienylene vinylene (N-methylpyrrolylene)vinylene] [11]

28

G. Schopfand G. Kol3mehl

(o)

(b)

~S# ._-._.~. In

(c)

Poly(2,5-thiophenediyl- 1,3-butadiynediyl) (a) Poly(2,2'-bithiophene-5,5'-diyl- 1,3-butadiynediyl) (b) Poly(thieno[3,2-b]thiophene-2,5-diyl-l,3-butadiynediyl) (c) [12] (o) R=H n

(b) R= Me

Poly(thieno[3,2-b]thiophene-2,5-diyl) (a) Poly(3-methylthieno[3,2-b]-thiophene-2,5-diyl) (b) [4, 12]

-~CH2~H ---~

Poly[oligo, poly(thiophene-2-yl)ethylene] (see also Sect. 5.1.5) [13, 14]

Viologene-substituted poly(3-propylthiophene) [11]

Poly(2,2'-bithiophene-5,5'-diylmethylidyne-2,2'-bithiophene-5,5'-diylidene) [15-181

Polythiophenes- ElectricallyConductivePolymers

%

SO:,

0

29

0 __ CH3

&

Poly(thiophene-2,5-diylsulfonyl- 1,4-phenyleneoxy- 1,4-phenylene-2,2-propandiyl- 1,4-phenyleneoxy) [19]

rLs o Poly(2,5-thiophenediylthio-2,5-thiophenediyl) [4, 11] E!--~i LIR

(o) R=Me "

Jn

(b) R=Ph

Poly(2,5-thiophenediyldimethylsilanylene)(a) Poly(2,5-thiophenediyldiphenylsilanylene)(b) [20, 21]

S~--Si--J,~.~)~-Jr e Mle "OJn

(b) R = Ph (c) R=p-tolyl

Poly(2,5-thiophenediyltetramethyldisilanediyl)(a) (see also Sect. 6.4) Poly[2,5-thiophenediyl(1,2-dimethyl-1,2-diphenyl)disilanediyl] (b) Poly[2,5-thiophene(1,2-dimethyl-l,2-di-p-tolyl)disilanediyl(c) [20, 22-24]

Me

_I n

Poly(2,5-thiophenediyltetramethyldigermanediyl) [24]

~e

Sn

LMe

I n (o)

Nile (b)

Poly(2,5-thiophenediyldimethylgermanediyl)(a) Poly(2,5-thiophenediyldimethylstannyl) (b) [25]

30


Poly(cycloalkano[c]thiophene-2,5-diyl) [26] Ca)

(b)

=•sN"I•S,•s

x.--~

~.~L.N/~S ~/--~

S/-]- n

(c)

Poly{ [benzo(1,2-d:4,5-d')bisthiazole-2,6-diyl]2,5-thiophenediyl} (a) Poly{ [benzo(l,2-d:4,5-d')bisthiazole-2,6-diyl]2,2'-bithiophene-5,5'-diyl} (b) Poly{ rbenzo(1,2-d:4,5-d')bisthiazole-2,6-diyl]2,2':5',2"-terthiophene-5,5"-diyl} (c) [27] Me3Si~SiMe3 (,~l,j

(o)

Me3Si ---~~ S.~L--C. ~ S],L-.~S).~--SiMe3

M

e

"

S

3~ ~SS ~ ~S/~l ~S/~ ~S/J~ ~S/~ ~Sk SiNe3 (b)

M~si--Ks>~.s>,--C,s,~--~,s>'-Ks,~-~.s>~-KS.~---SiMe3

Spiro-fused thiophene trimer (a) and spiro-fused thiophene heptamer (see also Sect. 3.2.1) (b) [28, 29]

Polythiophenes ElectricallyConductive Polymers

S

~

~

31

S S S

n

S --

--

n

n

Polymers of dithieno[3,4-b;3',4'-d]thiophene [4, 3~32]

Poly(phenanthro[9,10-c]thiophene-2,11-diylidyne) (see also Sect. 3.4.3) [-33]

Poly(naptho [2,3-c]thiophene-2,9-diyl-2,2'-bithiophene-5,5'-diyl) [-34]

Poly(thieno [3,4-c] thiophene-2,6-diylidynemet hylidene-2,2'-bithiophene- 5,5'diylmethylidene) [-35] R R

Polymers of 4,4-dialkylcyclopenta[-2,1-b;3,4-b']bithiophene n = 1, 3, 4, 6, 8, 16) [,11, 36, 37]

s} (o)

(R=CnH2n+I,

5 (b)

Polymers of 4-(1,3-dithiol-2-ylidene)-4H-cyclopenta[2,1-b;3,4-b']bithiophene (a) and of 7-(1,3-dithiol-2-ylidene)-TH-cyclopenta[1,2-b;3,4-b']bithiophene (b) [38]

32

G. Schopfand G. Ko6mehl

(o)

(b)

Poly(cyclopenta[2,1-b;3,4-b'] bithiophene-4-one-2,6-diyl) (a) Poly(4,4-ethylenedioxy-4H-cyclopenta[2,1-b;3,4-b']bithiophene-2,6-diyl) (b) [39] 0

0

0

(o)

0

(b)

Poly(3,4-ethylenedioxythiophene-2,5-diyl) (a)(see also Sects. 3.4.3, 5.1.3, 6.1, 6.7 and 6.12) Poly(3,4-trimethylenedioxythiophene-2,5-diyl) (b) [40-46] :"O'h 0 0 co o~

c-o--,,

2,

t?o5

Crown ether functionalized poly(thiophene)s (see also Sects. 3.2.3, 3.4.4) [47-50] r~oH~ o/-E'Xo'-~3 o

°,4_,o~o t _

,.a3

Poly(thiophene) in pseudopolyrotaxane structure [51] Other organosilicon polymers: [52, 53] Polymerized N-E3-(2-ethyl)thiophene]-3,4-dichloromaleimide: [54]

33


1.30ligomers and Monomers with Complex Structures These c o m p o u n d s are not mentioned in detail in this review:

s'~CH3 CH3"-' C ~k,~,S

C H ~S/j ~ H S

~

H3 CH3

CH3CH3

c

N

cH3

CH3

3

CH3 CH3

~

CH 33

R.,,~R

CH3._ ~ ch

~@ CH3

R~R

R" "R R ' ' R R,,c,_,_,_,_,._~R R..h,_............._~RR%_._._._._._._._._~R

R= CH3,H Several methyl substituted oligo(thiophene)s (see also Sects. 2.1, 3.1, 5.2.1, 5.2.2) [58-60]

Me r

~e

Triheterocycles based on thiophene, furan and pyrrole [56, 57]

34

G. Schopfand G. KoSmehl

(a)

(c)

(b) ~

~

(d)

1,8-Di(2-thienyl)naphthalene (a) 1,8-Bis(2,2'-bithiophene-5-yl)naphthalene (b) 1,8-Bis(2,2';5',2"-terthiophene-5-yl)naphthalene (c) 1,8-Bis(2,2';5',2";5",2'"-quaterthiophene-5-yl)naphthalene (d) [61]

(o)S / ~

(c)

(5)

5,5'-BisF8-(2-thienyl)naphthalene-1-yl]-2,2'-bithiophene (a) 5,5"-Bis[8-(2-thienyl)naphthalene-1-yl]-2,2';5',2"-terthiophene (b) 5,5'"-Bis[8-(5,2'-bithiophene-2-yl)- 1-naphthyl]-2,2';5',2";5",2"'-quaterthiophene (c) [62]

Cycloalkane end-capped thiophenes and oligo(thiophene)s [63]

(a)

(b)

(c)

4H-Cyclopenta[2,1-b;3,4-b'] bithiophene (a) 4,5-Dihydrobenzo[-2,1-b;3,4-b'] bithiophene (b) 4H-5,6-Dihydrocyclohepta[-2,1-b;3,4-b']bithiophene (c) [64]

35

Polythiophenes- ElectricallyConductivePolymers R

I

Thiophene-based tricyclic systems [11]

d°:

or-,¥,'qo ~N~N~

Et

Thiophene-based macrocycles [65]

MeSx~S ~ = C H ~

0

CH=~/s~SMe

__S-.fSMe "S'~sMe MeSx~S. ~ ~. S>::CH--K. S>'---K, s>-CH=~ II MeS S~SMe

MeS/'-S

o

o

MeSx/_~Me

Neb MeS-w~S. f f - ~

__S~,/SMe

MeS~S~CH-~ $ ~ S>t'-CH=<S]IXSMe

Tetrathiafulvalene-bithiophene systems [66]

(o)

36

G.

(b)

Schopfand G.

KoBmehl

(c)

Bronzaphyrin (a) Oxobronzaphyrin (b) Thioozaphyrin (c) [67] Thiophene containing bisphenol (see Sect. 3.4.5) [68] F F

HO

F

OH

2 Structure of Poly(thiophene)s

2.1 Primary Structure The predominating structure of PT and its derivatives corresponds to the coupling ofmonomeric units in the 2,5 position, so-called a,0~' coupling, but PTs also have non-negligible amounts of o(,[3' coupling; the coupling of monomeric units in the 2,4 position is verified by IR and 13C-NMR spectroscopy [2, 5]. After chemical oxidation of methyl-substituted bithiophenes (cf. Sect. 1.2) with free [3 positions, tetrathiophenes with only a,~' coupling (41%) and sexithiophenes with ~,a' and a,[Y couplings (6%) are obtained [58, 60]. Oligo(thiophene)s (cf. Sect. 1.2) with 13,[3' coupling are not found. These facts demonstrate the lower reactivity of the 13 positions compared with the positions [60]. Different 13positions in end-substituted oligo(thiophene)s show a graduated reactivity; higher electron spin densities for the 3 and 3'" positions in relation to the 4 and 4'" positions in the EPR studies of 5,5'"-dimethyl2,2';5',2";5",2'"-quaterthiophene are observed [60]. The chemical structure of PT and PMT in the solid state can be established with high resolution solid-state 13C.NMR; the NMR studies of the thiophene/ benzene copolymers show that a random distribution of the repeated units exists [69]. In contrast to PT, which has a disordered cross-linked polymer structure, PBT and PTT are claimed to have a regular ~,a' coupling of the monomeric

36

G.

(b)

Schopfand G.

KoBmehl

(c)

Bronzaphyrin (a) Oxobronzaphyrin (b) Thioozaphyrin (c) [67] Thiophene containing bisphenol (see Sect. 3.4.5) [68] F F

HO

F

OH

2 Structure of Poly(thiophene)s

2.1 Primary Structure The predominating structure of PT and its derivatives corresponds to the coupling ofmonomeric units in the 2,5 position, so-called a,0~' coupling, but PTs also have non-negligible amounts of o(,[3' coupling; the coupling of monomeric units in the 2,4 position is verified by IR and 13C-NMR spectroscopy [2, 5]. After chemical oxidation of methyl-substituted bithiophenes (cf. Sect. 1.2) with free [3 positions, tetrathiophenes with only a,~' coupling (41%) and sexithiophenes with ~,a' and a,[Y couplings (6%) are obtained [58, 60]. Oligo(thiophene)s (cf. Sect. 1.2) with 13,[3' coupling are not found. These facts demonstrate the lower reactivity of the 13 positions compared with the positions [60]. Different 13positions in end-substituted oligo(thiophene)s show a graduated reactivity; higher electron spin densities for the 3 and 3'" positions in relation to the 4 and 4'" positions in the EPR studies of 5,5'"-dimethyl2,2';5',2";5",2'"-quaterthiophene are observed [60]. The chemical structure of PT and PMT in the solid state can be established with high resolution solid-state 13C.NMR; the NMR studies of the thiophene/ benzene copolymers show that a random distribution of the repeated units exists [69]. In contrast to PT, which has a disordered cross-linked polymer structure, PBT and PTT are claimed to have a regular ~,a' coupling of the monomeric

Polythiophenes


37

units [70]. Two 2,5-thienylene units of a neutral PT molecule have a distance of 0.85 nm [71] and of a low molecular weight PDT 0.7 nm [72]. It is possible to calculate the vibrational properties and theoretical IR and Raman spectra of oligo(thiophene)s and PTs by MNDO calculations and by a newly developed vibrational analysis method based on ab initio calculations [73, 74]. The structure of PT and its derivatives can be studied by X-ray diffraction and by anomalous X-ray scattering. The first anomalous X-ray scattering was performed at the sulfur K edge of a partially crystalline POT [75]. Polymers with heteroaromatic units such as PT and poly(pyrrole) and their derivatives are nondegenerate in their ground state and possess two possible types of structure: the aromatic and the quinoid structure (Fig. 2). Theoretical and experimental results demonstrate that poly(pyrrole) and PT have an aromatic ground state. In calculations it was found that the band gap decreases, but does not vanish, when passing from the aromatic to the quinoid form [76]. For poly(isothianaphthene) (PITN), on the other hand, it is calculated that the quinoid form for the ground state is slightly more stable than even the nonplanar aromatic form [77, 78], and the transition from a quinoid to an aromatic character of the PITN backbone with doping can be simulated [-79]. Geometries and vibrational force fields of the ground and excited electronic states of oligo(thiophene)s (nT, n = 1, 2, 3, 4) are obtained by a version of the semiempirical quantum consistent force field/~ electron method implemented to describe sulphur atoms and by ab initio Hartree-Fock and configuration interaction single methods [-80]. The predicted band gaps of several PT derivatives by the extended Hiickel band calculation method have average errors of about 10% compared to the experimental values [81]. The molecular weight of a PT (e.g. PHT) can be determined by gel permeation chromatography (GPC), Mark-Houwink constants are used to

A

Q PT

PITN

Fig. 2. Aromatic(A) and quinoid(Q) structuresof PT and PITN [77]

38

G. Schopfand G. KoBmehl

calibrate GPC columns. Number-average molecular weights of PHT determined with modified calibration curves agree well with those determined by an absolute method. Molecular weights estimated using unmodified polystyrene calibration procedures are significantly higher than true values [82]. The electrical conductivity of PT is determined by the effective mean conjugation length of PT which is determined by the stereoregularity of the cx,a' linkages in the polymer [5] and the main chain length. One possible method of measurement of the effective delocalization length is by photoinduced IR spectra and appropriate interpretation of these data [83]. Furthermore, with increasing length of oligo(thiophene)s, the absorption maximum shifts towards longer wavelength due to the increasing conjugation length [-84]. The conjugation length for doped PT of not much longer than 11 units is given in Ref. [85]. The conjugation length for electropolymerized PT ranges from 15 to 20 thiophene rings [86]. An average conjugation length of about four thiophene rings can be found for spin coated PHT films by resonance Raman scattering and appropriate interpretation. The value for PHT/arachidic acid LB-films is higher than for spin coated PHT [87]. A high molecular weight fraction (Mw = 9.1 x 103, determined by GPC) of poly(3',4'-dibutyl-2,2';5',2"terthiophene) has one of the longest n-conjugation lengths known for PAT [-88].

2.1.1 Poly(3-alkylthiophene)s Substitution in the 3 position of a thiophene ring by a donor group like an alkyl group leads to more regular polymeric chains [2]. The most studied PATs (see Table 2) are PMT, PHT, and POT. An intermediate between PT and PMT is poly(3,Y"-dimethyl-2,2',5',2";5",2'"-tetrathiophene) [89], which may be considered as the copolymer P(MT-T-T-MT). The standard synthetic methods used to make PATs generate a large number of defects due to the random couplings at the 2,5 position in the thiophene ring. The structures contain a large number of thiophene rings that are twisted far out of conjugation due to the steric interactions between alkyl groups. Possible couplings of thiophene rings are: a head-to-tail-head-to-tail coupling (HT-HT); a tail-to-tail-head-to-tail coupling (TT-HT); a head-to-tailhead-to-head coupling (HT-HH); a tail-to-tail-head-to-head coupling (TT-HH); (Fig. 3). With the aid of NMR spectroscopy (1H-IH NOESY) for PDDT and its copolymer with 3-methylthiophene in deuterated chloroform, the configurational structures were determined [90, 91]. The 3-dodecylthiophene monomer is proved to be attacked predominantly at the [3 position of the thiophene ring (head) with a probability of 82% during the electrochemical polymerization [90]. PHT prepared by using a zero-valent nickel complex contains a larger proportion of HH units than HT units [92]. Chemically polymerized poly(3-cyclohexylthiophene) contains HT and HH TT coupled components in the ratio of 7: 3 [93].

Polythiophenes ElectricallyConductivePolymers R

R

S/j

R

n

R

~Sh

R

~ /j

A Head-to.TaiI-Head-lo-Tail Coupling (HT-HT) R

Head-to.TaiI-Head-to-Head Coupling (HT-HH)

n

A Tail-to-TaiI-Head-to-Tail Coupling (TT-HT) R

n

39

B

S/~

n

A Tail-to-Tail-Head-to-Head Coupling (TT-HH)

Fig. 3. Couplingtypes of PATs [95]

McCullough et al. first described structurally homogeneous head-to-tail arrangements in PATs [94, 95]. Structural homogeneity is defined as a regiochemically well-defined polymer structure, which in this case contains almost exclusively head-to-tail couplings [953. By analysis of the NMR data, the content of H T - H T coupling is 93% for PBuT, 98% for PHT, 97% for POT, and 95% for PDDT. A comparison of the NMR spectra of a regioregular and a regiorandom PDDT and the assignments of the different types of coupling are shown in Fig. 4 [94]. The synthesis and the 1H- and 13C-NMR characterization of the fourmodel configurational triads (HT-HT; HT-TT; HT-HH; TT-HH) of PHT allow the unambiguous assignment of the regular structure of the polymer [96]. The absorption properties of these PATs are indicative of longer main chain conjugation length [95]. UV/vis absorption studies of polymers in solution indicate that steric crowding in HH coupling limits the degree of main chain coplanarity [97]. Molecular mechanics calculations show that the lowest energy conformations for structures like A (Fig. 5) have all rings nearly coplanar. In contrast, structures like B have lowest energy conformations that possess a large twisting of the rings out of coplanarity at the HH junction [95]. The chains of poly(4,4'-didecyl-2,2'-bithiophene), the HH and TT analog of PDT, are nonplanar [98]. In the solid state the polymers with HH coupling possess a higher degree of coplanarity and hence conjugation as in solution [97]. The HT-HT PATs undergo a solid-state macromolecular self-assembly and give self-oriented structures, as determined by X-ray diffraction studies. PDDT induces the formation of planar main chain structures and has the greatest effective conjugation length, followed by HT-HT POT, HT-HT PHT and HT-HT PBuT [99]. PHT with an HT coupling content of 80% is semicrysta!line as shown by X-ray diffraction. The distance between stacked chains is 3.8 A, which is similar

40

G. Schopf and G. KofSmehl R

R

R

R

R

R

HT-HT 8 6.g8

t-r-..HT 8 7.02

R

R

R

ST.OO

I

I

I

I

I

R

~ F

I

I

I

!

I

I

1

I

I

I

I

I

I

1

7.20 7.16 7.12 7.08 7.04 7.00 6.96 6.92 6.88 6.84 8.

I

I

1

I

1

I

f

7.20 7.16 7.12 7.08

I

1

t

1

l

!

I

l

7.04 7.00 6.96 6.92 8

I

1

I

1

1

6.88 6.84

Fig. 4. 1H-NMR spectra of regioregular (a) and regiorandom (b) PDDT 1941. The values are the assignment of the different coupling types in ref. [901, and the a r r o w s indicate the assignment in ref. [941

to the distance of excimer formation (Fig. 6). With decreasing HT coupling content the twisted character increases and the cofacial distance increases. X-ray diffraction studies of PHT with 50% HT coupling content show that it is virtually amorphous 1-97,100]. The following chiral conducting PATs and oligo(3-alkylthiophene)s are described: PTs with chiral 2'-methylbutyl, racemic 2'-methylbutyl, chiral

41

Polythiophenes - Electrically Conductive Polymers

I

I.2 fu.,o~..n. sileo!

/

,

,

__),

_>#

=o..

less conjugationo r overlap smaller'~N, smallerbano~idlhs larger bandgaps

/ower conductivities Fig. 5. Molecular mechanics calculations of the lowest energy conformations of different structures [-95]

3'-methylpentyl, and racemic 3'-methylpentyl side chains [-101]; 3,4',4"trimethyl-2,2';5',2"-terthiophene in the solid state [102]. Oxidation potential and localized states are changed markedly by the introduction of phenyl and phenylpropyl groups to the 3 positions of thiophene rings [103].

2.1.2 Poly(3-alkyloxythiophene)s Chemically synthesized poly(3-alkyloxythiophene)s (cf. Table 3) containing C7H150, poly(3-heptyloxythiophene); CgH9OCzHgO , poly[3-(2-butoxyethoxy)thiophene]; CH3OC2H4OC2H40, poly{3-[2-(2-methoxyethoxy)ethoxy]thiophene} are insoluble in organic solvents due to cross-linking by a, lY coupling between thiophene rings. The locations and number of oxygen atoms have

42

G. Schopfand G. Kof3mehl

Fig. 6. Simplifiedarrangement of coplanar conjugated segments (HT dyads) illustrating the cofacial stacking which facilitatesexcimerformation and intermolecular nonradiative decayof the excitedstate [-100]

significant effects on the optical and electrical properties of the substituted PTs due to the electron donation nature of the oxygen atoms [104, 105]. In poly(3,4dibutoxythienylene vinylene) the vinylene unit reduces steric interactions; substitution in both the 3 and 4 position is possible without loss of the electrical conductivity. The dibutoxy substitution yields advantages in processing, as the high molecular weight polymer dissolves easily in common organic solvents [106, 107].

2.1.3 Poly(thiophene)s with Pendant Reactive Groups 3-Methylthiophene and 3-butylthiophene can be electrocopolymerized with 3-thienylacetic acid and its methyl ester [108-110] (cf. Table 8). The COOH group is covalently bound to the copolymer structure: the carboxylic group does not take part in the electrochemical reaction. The dimerization tendency of carboxylic acids is possibly reduced by the electrolytic conditions (cf. Sect. 5.2.2). The inductive effect (-I) of the COOH group on the thiophene system does not prevent anodic oxidation of the thiophene system [108]. Esterification and the consequent reduction of 3-thienylacetic acid produces 3-(2-hydroxyethyl)thiophene, which must be protected before electropolymerization. The resulting thin polymer film, e.g., of poly[3-(2-methoxyethyl)thiophene], with protected OH groups, can be reacted to produce reactive hydroxy groups on its surface, such as poly[3-(2-hydroxyethyl)thiophene] [i 11] (cf. Table 3). Further thiophene derivatives with a protected OH group have been synthesized starting from 3-(2-hydroxyethylthiophene): 3-(2-benzyloxyethyl)thiophene, 3-[2-triphenylmethyloxy)ethyl]thiophene, 3-[2-(trimethylsilyloxy)-

Polytbiophenes - Electrically Conductive Polymers

43

ethyl]thiophene, 3-[2-(dimethyl-tert-butylsilyloxy)ethyl]thiophene, 3-(2-acetoxyethyl)thiophene, 3-(2-benzoyloxyethyl)thiophene, 3-[2-(p-nitrobenzyloxy)ethyl]thiophene, and 3-[2-(3,5-dinitrobenzoyloxy)ethyl]thiophene [112] (cf. Table 8). The electrochemical homopolymerization of these monomers is not possible under standard conditions, but the electrochemical copolymerization of most of these monomers with 3-methylthiophene is successful. No copolymerization is possible with 3-[2-(triphenylmethyloxy)ethyl]thiophene and 3-[2(trimethylsilyloxy)ethyl]thiophene[ 112]. The electrochemical copolymerization of 3-(2,4-dinitrostyryl) and 3-(2,4,6-trinitrostyrylthiophene) with 3-methylthiophene results in functionalized PT layers at the surfaces of electrodes suitable for NLO (non linear optical) layers, and, after reduction, for further reactions [113] (cf. Table 8). Alcohol dehydrogenase can be covalently bonded on the surface of a poly[3-(2-hydroxyethyl)thiophene] electrode by the cyanogen bromide method [111]; glucose oxidase is covalently immobilized at the surface of a copolymer electrode from 3-methylthiophene and methyl 3-thienylacetate by the azide method [110]; after activation of the carboxylic groups in the copolymer of 3-methylthiophene and 3-thienylacetic acid by dicyclohexylcarbodiimide, lactate oxidase can be covalently immobilized [109]. This "enzyme electrode" can be used for the electrochemical analysis of lactic acid (cf. Sect. 6.9).

2.2 Secondary Structure The bond between adjacent thienylene units in PT can adopt the "trans" or "cis" configuration by virtue of adjacent units being syn or anti. X-ray diffraction reveals an anti conformation (planar) leading to straight chains with an orthorhombic unit cell in the solid state [71, 97, 114, 115]. The syn conformation which can occur in alkyl substituted oligo- and poly(thiophene)s represents a deviation from the ideal all-anti conformation and introduces rotational defects into the polymer which interrupt or weaken the extent of conjugation [115]. Theoretical information on the gas-phase conformations in PMT in the neutral state can be obtained by the ab initio quantum-chemical calculations on bithiophene and methyl substituted bithiophenes [116]. Vacuum-evaporated or cast thin films of oligo(thiophene)s (e.g., quater- and sexithiophene) consist of layered structures in a monoclinic arrangement, with all-trans planar molecules on the substrate [117 120]. Oligo(thiophene)s show a typical herringbone arrangement [121-123]. The crystal structure with approximately planar sexithiophene molecules is shown in Fig. 7. Sexithiophene molecules with two bulky terminal groups (triisopropylsilyl groups) crystallize in a triclinic system and appear in staggered parallel arrays [124]. Molecules of dialkyl-substituted sexithiophenes have neighboring thiophene rings placed in an antiparallel arrangement, the molecules are planar and the alkyl side chains are in the planar zigzag conformation [125].

44


/

g l ) 0

)

a

(a)

a ¢dn13

~b

(b) Fig. 7. Crystal structure of a vacuum-evaporated sexithiophene film: a stereoscopic view of the unit cell b view along the c axis 1117]

Poly(3,4-dialkylthiophene)s (e.g. poly(3,4-dihexylthiophene) and poly(3methyl-4-octylthiophene)) have nonplanar conformations of the polymer backbone resulting from steric interactions between the substitutents and the thiophene rings. This nonplanar conformation reduces the effective conjugation length, which leads to a blue shift of the UV/vis absorption in comparison to PAT 1-126]. In contrast to poly(3,4-dialkylthiophene)s, poty(3-butoxy-4-methylthiophene) shows an absorption maximum (cf. Table 10) which can be related to a highly conjugated backbone. The minimized steric hindrance in the vicinity of the backbone is able to form a coplanar conformation (a rod-like structure) in the solid state. The conformation of poly(3-octyl-4-methylthiophene)in solution is that of a coil with smaller effective conjugation length and an absorption


45

Table 10. Physicalproperties of alkyl and alkyloxydisubstitutedPTs [126]. Polymer

km,x"(nm)

~-maxb (rim)

Conductivity(S cm- 1)

poly(3-octyl-4-methylthiophene) poly(3,4-dihexylthiophene) poly(3-butoxy-4-methylthiophene) poly(3,4-dibutoxylhiophene)

325 315 545 460

325 315 420 480

10- s < t0 -s 2 10 s

"Taken from neutral polymerfilmscast on quartz plates. bTaken from neutral polymersdissolvedin chloroform.

maximum at shorter wavelength than in the solid state. However, poly(3,4dibutoxythiophene) exhibits an absorption maximum at a shorter wavelength than poly(3-butoxy-4-methylthiophene), which indicates that steric interactions between the long substituents induce a twisted conformation of the backbone [126]. The electrochemical polymerization of 4-cyano-4'-[-8-(3-thienyl)octyloxy]biphenyl, a thiophene monomer containing a p-cyanobiphenyl group which is electronically and sterically decoupled from the thiophene ring by means of an octyl spacer, leads to a highly conducting and electroactive conjugated polymer [127]. The main chain of a vacuum-deposited and vapor-deposited PT film is preferably oriented perpendicular to the substrate surface [128-130]. The chain tilt angle to the normal to the surface decreases with increasing substrate temperature during vacuum deposition [128]. In general, PT films deposited on oriented graphite or gold surfaces have helical structures which assemble to form a crystalline array [131]. The doped P M T has a helical syn conformation and crystallizes in the hexagonal system, After dedoping, P M T molecules retain their helical syn conformation, in contrast to PBT, which changes its syn conformation to a rod-like anti conformation and crystallizes in an orthorhombic or monoclinic system [132], The PMT helical macromolecule behaves like a spiral spring, and on doping becomes axially compressed so that the unit cell volume becomes smaller. Chemical dopin~dedoping cycles lead to amorphous PBT and P M T in either doped or dedoped states [132]. Poly(4,4'-dialkyl-2,2'-bithiophene)s which are the "head-to-head" and "tail-to-tail" analogues of PATs are more twisted than PATs. During doping, they adopt a structure more similar to that of doped PATs [98, 133]. Crystallinity decreases with increasing length of oligomers. X-ray diffraction studies of oligo(thiophene)s with 11 thiophene units indicate a nearly amorphous compound [84]. Orientation of PAT macromolecules is achieved by tensile drawing of films and fibers at elevated temperatures [134]. The mostly amorphous state of PT or PAT originates from a noncontrolled polymerization reaction which leads to spatially disordered polymer chains involving a wide distribution of conjugated segments linked through chain defects. Defect-free molecules organize in well layered structures when deposited as thin films [118].

46

G, Schopf and G. KoBmehl

Scanning tunneling microscopy (STM) images show that conducting polymer films gradually transform from an ordered crystalline array located at the electrode surface to an amorphous material in the bulk [-131,135]. Both neutral or p-doped PBTs comprise a dense inner region containing virtually no solvent and an extended diffuse outer region containing a considerable amount of solvent. The p-doped film is slightly more diffuse [136]. Self-assembly properties and highly ordered films can be generated by the bonding of two hexyl groups on both terminal ~ positions in sexithiophenes. The alkyl-alkyl recognition phenomena, based on hydrophobic-lipophilic interactions, are strong enough to ensure long range order [118, 120]. In contrary to sexithiophenes substituted by hexyl groups at the terminal ~ positions, sexithiophenes substituted by hexyl groups at the ]3 positions show only low electrical conductivity and mobility [120]. The amount of crystallinity in precipitated PATs with side chain length of n > 4 is high compared to ordinary solutions of cast samples, whereas the crystal structure is the same. The precipitation of PATs is accompanied by a color change from yellow to red, which is in accordance with the thermochromic and solvatochromatic shifts. The polymer mats are anisotropic with the alkyl chains oriented perpendicular to the mat surface [137]. PATs crystallize from dilute solution in poor solvents in the form of ribbonlike crystals, so-called whiskers [138]. Poly(3-cyclohexylthiophene) also forms a ribbon-like whisker structure in the neutral state [93]. The whisker widths in PATs of 15 nm appear to be invariant with the alkyl side chain length, while the thickness increases with increasing side chain length. The degree of order within a single whisker is very high and the packing of the PAT macromolecules is such that the polymer backbone is normal to the whisker length (Fig. 8) [138]. The shift of the absorption maximum upon heating (thermochromism) and by changing of solvents (solvatochromism) is caused the generation of twists (disruption of planarity) and subsequently results in a decrease of the conjugation length [-139,140]. It is possible to distinguish three types of twist [140]: (1) a localized twist where the twist occurs between two repeating units previously in the same plane as that of their neighbors in the same chain (2) a soft twist where the torsion between the two coplanar subchains is disturbed over several repeating units (3) wormlike chains in which no repeating unit is in the same plane as that of their neighbors in the same chain and which appears at high temperature. Various thermal analyses, spectroscopic methods, and conductivity measurements show that PATs consist of an ordered and a disordered phase. In unoriented PAT films with long side chains (n > 10, e.g. PDDT, PTDT), PAT forms are separated ordered phase resulting in hexagonal packing of the alkyl side chains between the main chain layers [140-142]. The effective distance of the side chain overlap is estimated to be 6.7 A for PTDT; for POT having no

Polythiophenes


I~

15nm

47

_1 a-axis c-axis

~-axis

Fig. 8. Schematic representation of the molecular arrangement within a PHT whisker [138]

side chain crystaUinity the distance of overlap is about 2.6 A [141]. The distance between the main chains along the alkyl side chain increases with increasing alkyl side chain length, whereas that between the neighboring r~-conjugation planes on the main chains increases slightly with an increasing alkyl chain length longer than the dodecyl group [143]. In the ordered phase of PATs [140], the side chains are nearly fully extended and packed to give a two-layer structure and a one-layer structure, the former being predominant (Fig. 9a and 9b). The coplanar subchains are stacked to form planes, and the spaces between two successive stacking planes are filled with aligned side chains, as shown in Fig. 9c. The disordered phase before melting is shown in Fig. 9d. As the temperature rises to the melting range of the side chains of PDDT, there are disordered side chains and ordered main chains (Fig. 9e). The increased sorption of hexane vapor by a PTDT film at the side chain melting temperature is ascribed to the disruption of the side chain order [141]. As the temperature further rises above the melting temperatures, the whole polymer is represented by the structure given in Fig. 9f [140]. X-ray diffraction results of copolymers of 3-alkylthiophenes (n = 4, 8, 12) indicate that the copolymers have an ordered layered structure as in the homopolymers (Fig. 9a and 9b), but the space between two planes formed by stacking of coplanar subchains is filled with aligned long-long, short-short and long-short side chains. The reduced coplanarity of the main chains is reflected by the increase in energy of the absorption maximum to a higher level than those of the corresponding homopolymers [144]. The hindrance to the

48


Cb)

(t) Fig. 9. Schematic diagram of the layer structure of a PAT: a ordered two-layer structure b ordered single-layer structure e planar packing d disordered structure before melting (in which a certain degree of chain alignment is still retained) e disordered side chain and ordered chain structure f melt structure (in which the conformation of the main chain is random coil-like) [140]

formation of a coplanar configuration of PMT is achieved by the addition of 1,6bis(3-thienyl)hexane and 1,8-bis(3-thienyl)octane [145].

2.3 T e r t i a r y S t r u c t u r e Scanning tunneling microscopy is one of the tools for studying the surface and morphology of conducting polymers. Structural information about morphology and molecular arrangement of PTs can be obtained. However, a satisfactory detailed characterization of such an amorphous system is not possible [135]. The morphology of PT films depends on several factors:

Fig. 10. Scanning electron microscopy (SEM) analysis of PT b PBT c PTT electrochemically synthesized in acetonitrile with tetrabutylammonium tetrafluoroborate at a current density of 10 mA cm -2 on Pt 1-146] a

I

5

50


1. The Structure of Monomers/Startin9 Materials. PT, PBT and PTT - polymers of repeating thiophene units - were electrochemically synthesized under the same conditions, but scanning electron microscopy shows different morphologies (Fig. 10) [-146]. PMT films in which the thiophene ring is substituted by a methyl group in the 3 position show "noodle"-like structures, in contrast to PTT films [2]. The surface of PMT is covered with particles of fibrous structure. Particles consisting of smaller subparticles are piled on the surface of PMT when 1,6-bis(3-thienyl)hexane is added [145]. 2. The Conditions Durin9 the Preparation of the Polymer Film. Electropolymerized PT films have a more compact morphology in contrast to chemically synthesized PT films [146]. Poly[1,2-bis(3-alkyl-2-thienyl)ethylene] prepared chemically is a bulk powder, in contrast to electrochemically prepared polymers which form homogeneous films (see also Sect. 5.3) [147]. The surface of electropolymerized PTT films is also influenced by the current density. PTT films prepared at a current density of 0.4 mA c m - 2 (7.5 min) have typically rough surfaces. PTT films prepared at a current density of 0.05 mA cm -z (60 min), with the same quantity of electricity, have a compact homogeneous surface [146, 148]. These characteristics are independent of the material of the electrodes. PTT films electrochemically prepared at room temperature have a more homogeneous and more compact and smooth surface than at - 5 °C, independently of the current density, with the same quantity of electricity [148]. A more homogeneous and compact surface of PT, PBT, and PTT films is achieved on Ni electrodes covered with Au [146]. The morphology of several conducting poly(heterolene) films synthesized galvanostatically on optically transparent SnOx electrodes shows substantial differences as compared with films formed on Pt surfaces [149]. Four films of PT prepared in the same electrochemical way show clearly different properties according to in situ IR spectroscopy during switching. This is because of the differences in the morphology of the films. The morphological properties depend in a subtile way on the concentration of adventitious nucleophiles in the solution, on the extent to which the electrolyte and solvent are incorporated into the film, and on the mechanism of charge neutralisation during switching. Attempts to control the properties of PT during electrochemical growth are likely to prove exceptionally difficult. Carefully controlled chemical synthesis is likely to prove the more effective way of generating polymers of closely controlled properties [-150]. The electrode rotation influences the morphology of PMT (cf. Sect. 5.2.2). Porous materials suitable for application in batteries and electrocatalysis can be prepared on a stationary or slowly rotating electrode, while much less permeable compact films (for analytical applications) can be prepared at high rotation rates [-151]. 3. The Thickness of the Film. Thin polymer films (about 102 to 2 x 103 ,~ thick) have a very homogeneous surface, but with increasing thickness of the film the surface is no longer homogeneous. A granular structure and defects appear


51

depending on the polymer and on the dopant [2, 152]. A thick PMT film shows poorer selective permeability behavior towards different anions than that of a thin PMT film (cf. Sect. 6.9) [152]. A thin film of PMT can be distinguished visually from a thick PMT film: the thin film is green with a metallic lustre and the thick film is somewhat darker [152]. A gradual change of the crystallinity within a polymer film [131] is described in Sect. 2.2. The morphology also depends on 4. The Nature of the Dopant [2,93, 153] (cf. Sect. 3.2.3) and 5. The Oxidation State [143, 153-155] (cf. Sect. 3.2.1). Forced Rayleigh scattering can be used to measure the z-average diffusion coefficient of a PAT in dilute solution. For POT in trichlorobenzene, the diffusion coefficient D = 8 x 10- 8 cm z s- 1 and hence the hydrodynamic radius of POT is about 130 ~, [ 156]. For copolymers of bithiophene and pyrrole, cyclic voltammetry (cf. Sect. 3.2.1) and UV/vis data support the formation of copolymers, which consist of three distinct oxidizable (dopable) units: (i) short blocks of bithiophene units, (ii) short blocks of pyrrole units, (iii) random and alternating groupings of bithiophene and pyrrole [157]. The degree of blending of neutral PAT and a flexible polymer matrix, poly(ethylene-co-vinylacetate), was investigated. Phase separation is observed in blends with a copolymer with a high vinylacetate content. A homogeneous morphology is obtained at low PAT content when the blending copolymer contains 20% vinlyacetate [158].

3 Properties of Poly(thiophene)s 3.1 Polarons and Bipolarons Organic semiconductors such as PTs show basic differences when compared with inorganic semiconductors such as Si. For example, conducting polymers are one-dimensional conducting systems and the inorganic semiconductors are three-dimensional conducting systems [159]. Also, the time constant of transient characteristics for PMT/metal contacts is much longer than that for conventional semiconductor/metal contacts [160]. Organic semiconductors are mostly molecular materials formed by assemblies of molecules held together by weak van der Waals forces. Polyaromatic conducting polymers as PTs have a non-degenerate ground state and two limiting mesomeric structures (Fig. 2, Sect. 2.1). The removal of an electron from the conjugated system provokes a local distortion of the chain and the appearance of two states in the gap corresponding to a polaron (or radical cation) with spin 1/2 [5]. A polaron is a quasi particle consisting of an electron and the appropriate polarization. Electrons moving over the crystal produce


51

depending on the polymer and on the dopant [2, 152]. A thick PMT film shows poorer selective permeability behavior towards different anions than that of a thin PMT film (cf. Sect. 6.9) [152]. A thin film of PMT can be distinguished visually from a thick PMT film: the thin film is green with a metallic lustre and the thick film is somewhat darker [152]. A gradual change of the crystallinity within a polymer film [131] is described in Sect. 2.2. The morphology also depends on 4. The Nature of the Dopant [2,93, 153] (cf. Sect. 3.2.3) and 5. The Oxidation State [143, 153-155] (cf. Sect. 3.2.1). Forced Rayleigh scattering can be used to measure the z-average diffusion coefficient of a PAT in dilute solution. For POT in trichlorobenzene, the diffusion coefficient D = 8 x 10- 8 cm z s- 1 and hence the hydrodynamic radius of POT is about 130 ~, [ 156]. For copolymers of bithiophene and pyrrole, cyclic voltammetry (cf. Sect. 3.2.1) and UV/vis data support the formation of copolymers, which consist of three distinct oxidizable (dopable) units: (i) short blocks of bithiophene units, (ii) short blocks of pyrrole units, (iii) random and alternating groupings of bithiophene and pyrrole [157]. The degree of blending of neutral PAT and a flexible polymer matrix, poly(ethylene-co-vinylacetate), was investigated. Phase separation is observed in blends with a copolymer with a high vinylacetate content. A homogeneous morphology is obtained at low PAT content when the blending copolymer contains 20% vinlyacetate [158].

3 Properties of Poly(thiophene)s 3.1 Polarons and Bipolarons Organic semiconductors such as PTs show basic differences when compared with inorganic semiconductors such as Si. For example, conducting polymers are one-dimensional conducting systems and the inorganic semiconductors are three-dimensional conducting systems [159]. Also, the time constant of transient characteristics for PMT/metal contacts is much longer than that for conventional semiconductor/metal contacts [160]. Organic semiconductors are mostly molecular materials formed by assemblies of molecules held together by weak van der Waals forces. Polyaromatic conducting polymers as PTs have a non-degenerate ground state and two limiting mesomeric structures (Fig. 2, Sect. 2.1). The removal of an electron from the conjugated system provokes a local distortion of the chain and the appearance of two states in the gap corresponding to a polaron (or radical cation) with spin 1/2 [5]. A polaron is a quasi particle consisting of an electron and the appropriate polarization. Electrons moving over the crystal produce

52


a polarization due to their electrical charge. The neighboring electrons are repelled and the atomic nuclei are attracted [161]. The interaction ofa polaron having a spin of 1/2 with identical species favors energetically the formation of a spinless double charged bipolaron [5]. The calculations of ground state, polaron, and bipolaron excitations in PT chains indicate that the formation of a double charged bipolaron is energetically more favorable than that of two polarons charged singly [162]. The structures of oligo(thiophene) dications (bipolarons) can be computed [163]. In situ derivative cyclic volt-absorptometric studies on PMT [164] show that the neutral form of PMT, absorbing at 490 nm (at less than 0.3 V vs. Ag), changes to the radical cation form which absorbs at 760 nm. Initially, the formation of the radical cation passes an isosbestic point, indicating that the conversion of the neutral to radical (polaron) form is reversible. However, upon increasing the electrode potential, the rate of radical formation indicated by an absorption at 760 nm starts to decrease, with the formation of another band at about 1250 nm, attributable to a bipolaron form. This trend begins at a potential higher than 0.6 V. This observation indicates that the electrochemical conversion of the neutral to radical form, followed by the quinoid form, is a slow process controlled by the diffusion of counter ions through the film [164]. The radical cation form of oligo(thiophene)s can be stabilized by substitution of electron donor groups at the ends of the chain, and a good fit is obtained when the formal redox potentials of the radical cation generation for end substituted terthiophenes are plotted against the Hammett constants cyp+ of the substituents [165]. It is also possible to correlate the redox potentials of endsubstituted quaterthiophenes with Brown cy+ coefficients of the substituents [166]. The role of the dopant potential on the stability and magnetic and optical properties of polarons and bipolarons in conducting polymers is shown with the aid of calculations of singlet and triplet states of a bipolaron [167] and by spectroelectrochemical and conductivity measurements [168-170]. The X-band optically detected magnetic resonance of PHT and PDDT shows that the distant intrachain polaron recombination is temperature-independent and identical in films and solutions. However, the triplet polaronic excitation decay is observable in films, but not in solutions [171]. Electrochemical in situ conductivity and EPR measurements of PT films were performed in several solutions [172]. The results indicate that polarons merely seem to initiate the electrical conductivity. The electronic delocalization of polarons is restricted to a relatively short chain length at low potentials. As the polaron concentration increases (spin density maximum), bipolarons are generated immediately (probably too fast for the detection of polarons by EPR). Thus the bipolarons prevail in the fully conducting polymer films and as a consequence should be mainly responsible of the intrinsic conductivity [172]. Asymmetrically disubstituted PBT display well-defined redox processes which are correlated to the consecutive formation of radical cations, dimerized radical cations, and dications [173].


53

Different spectroscopic characteristics of solid films and solutions of alkyt substituted oligo(thiophene)s are observed. The origin is related to the presence of molecular associates as charged species [174]. In PBT at very low doping levels ( _< 11%), the data of in situ IR spectroscopy can be interpreted in terms ofa polaron-hopping model. The polarons are located at areas of five to six monomer units, with a mobility of 3.7 x 10 -6 m 2 (Vs)-l, a diffusion coefficient of 9 x 10 -s m 2 s-1, and a residence time of 1.1 x 10 13 s. At higher doping levels, the results are consistent with the polymer behaving as a narrow band gap semiconductor [175]. In contrast to the electrochemical doping of oligo(thiophene)s, where the doping process occurs in one step [86], the chemical doping of sexithiophene (6T) with FeC13 occurs as a two-step process [-176]. The polarons (radical cations, 6T + ) are quantitatively generated in the first step according to 6T + 2FeC13 --* 6T+'FeCI~- + FeCI2.

(1)

The radical cation salt is precipitated as a highly stable, paramagnetic (g = 2.003, A H p p ~ 4 - 8 G) and conducting (10-1-10-2 S cm-1, at 300 K) black powder, with FeC12 as counter ions. The electrical conductivity of the 6T + salt is weakly temperature-dependent, with an activation energy of 32meV at > 100 K and a tendency to level off at lower temperatures [176-179]. In the second step of oxidation the polarons are converted into bipolarons (diamagnetic dications, 6T + +), as revealed by two isosbestic points on the optical spectra [177, 179]. The bipolaron (dication) of 12T is directly produced 1-179]. Hexyl substituted sexithiophenes doped with FeC13 also yield polarons (radical cations) during the first oxidation. However, during the second oxidation, trication radicals are formed instead of bipolarons (dications) [180]. The 6T + and 6T + + (unsubstituted sexithiophene) electronic structures can be compared to those of the PT hole polaron and bipolaron, respectively. The SOMO and L U M O of 6 T + (Fig. 11) would then correspond to the two polaronic gap states in PT. The comparison between doped 6T and doped PT reveals some inconsistencies. Thus, the polaronic gap of PT is given as Eg = 1.3-1.4 eV, while the SOMO to L U M O transition of 6T +' occurs at 0.84 eV and the energy gap between the two PT bipolaron states, expected to be of the order of 1.0 eV, is found to be extremely small in 6T ++ (0.12 eV). The main origin of these discrepancies is probably due to the highly disordered structure of PT and the 3-dimensional effects [176]. The solid-state doping of 6T with FeC13 leads to polaron formation up to a dopant level of one effective charge per 6T, and at higher dopant concentrations polaron bands rather than bipolarons are formed [181,182]. The formation of polarons and of bipolarons at higher doping levels are also observed in POX [183]. The existence of a polaron lattice at saturation doping of PHT with NOPF6 is described in [184]. Oligo(thiophene)s with different chain lengths (2T, 3T, 4T, 6T, 8T and 10T) which are methyl-protected at the terminal ~ positions were investigated by cyclic voltammetry and EPR [11,58,59,185]. Reversible

54


L~'~O

~'~"'""~'~ ""L ' ~ ~ LUMO" , , ~ . . . ~

SOMO,"TV'-F[" "" HOMO h----

6T

1~11 . . . . . . 6T+"

II ~

HOMO

6T++

Fig. 11. MO-diagramof 6T as a functionof its oxidationstate. Vertical arrows denotethe observed transitions (s = strong, w = weak) [176]

oxidation proceeds from a single one-electron step (4T) to a single two-electron step (8T and 10T) through two separate one-electron steps (6T). The redox cycles of 6T centers around 0.50 and 0.72 V. 8T and 10T show a single cycle centered around 0.50 and 0.45 V, respectively, corresponding to the involvement of two electrons per molecule [1 I, 186]. EPR indicates strong magnetic dimerization for the one-electron oxidized 6T. The close resemblance of the electrochemical and EPR behavior of 6T with that of PT suggests that oxidation of the latter occurs via 6T spin-dimerized polarons [11,185, 187]. The radical cations of sterically unhindered oligo(thiophene)s (2T, 3T,4T) which are methyl-protected at the terminal a positions are planar, and exist as mixtures of trans and cis conformers (e.g., radical cations of 5,5'-dimethyl-2,2'-bithiophene: trans: cis = 38:62). Sterically hindered oligo(thiophene) radical cations (radical cations of 3,3',3";4,4',4";5,5"-octamethyl-2,2';5',2"-terthiophene)are not planar and give uniform species [58, 59]. In poly(4,4'-dibutyl-2,2'-bithiophene), spin creation and annihilation occurs at more positive potentials and within a narrower potential range compared with PBuT, despite the same stoichiometry. Nevertheless, in both polymers the majority of the charge is stored in the form of bipolarons [188].

3.2 Doping 3.2.1 The Doping Process p-Type Doping. Modification of conjugated polymers leading to a change in the

electrical conductivity is called "the doping reaction" and involves chemical and electrochemical oxidation or reduction of the polymer backbone (Fig. 12) with simultaneous insertion of charge-compensation ions called "dopant ions" or

Polythiophenes

55

Electrically ConductivePolymers

neutralstate dopingJ /

~doping dedoping

reducedstate n-typedoped

oxidizedstate p-typedoped

Fig. 12. Schemeofdoping/dedopingprocesses

"counter ions". The application of PTs requires relatively high stability of the chemical structure and of the electrical conductivity in the doped state. Dedoping in PATs is a thermally activated process. The doped and dedoped states are separated by an energy barrier, the dedoped state having a lower energy than the doped state. The doped state is therefore thermally unstable [189, 190]. The degree of doping is expressed as the content of counter ions per monomeric unit. The ionic movements across the polymer films are detected by cyclic voltammetric measurements of effects of the electrolyte concentration [191]. The neutral (dedoped) film of electrochemically prepared PMT behaves as a p-type semiconductor and the oxidized (doped) form as a metallic conductor [-192]. In PMT the doped impurity is located in the spaces between the chains [159]. The properties of PT films are dependent on the type of dopant used and its method of introduction. Doping is usually performed in two ways [181]:

Electrochemical doping is carried out by the electrochemical synthesis of the polymer in a solvent containing the dopant. This allows an easy control of the doping rate and of the amount of dopant. After doping, the material still contains solvent molecules [181]. Chemical doping is carried out by exposure of the clean film material to the doping gases. The doping rate and the amount of dopant cannot easily be controlled. Also, a suspension of the solid polymeric material in a solvent can be doped by dissolved reagents. A new method is solid-state doping under UHV conditions [181,182]. The mechanism and kinetics of electrochemical reactions at electrodes covered with PT studied using the voltammetric behavior of PT films depend on the nature and concentration of mobile charge carriers within the polymer and therefore on the potential range [-193, 194]. The dependence of the processes during the electrochemical reduction and oxidation on the medium in contact with the PT film can be revealed by in situ Raman spectra [195]. The doped and neutral states of PMT and poly(3-thienylacetic acid) and the anion and cation dopants can be clearly identified by an improved specular reflectance IR spectroscopy. This method allows the use of the same electrode as that used in

56


the electrochemical experiments as the optical reflectance substrate and requires no further sampling [196]. Two new methods to study the doping-dedoping process of conducting polymers (e.g. PMT) have been developed [-197]: (i) differential evolutionary factor analysis (DEFA), which is sensitive to changes in the composition of the mixture during the evolution process, and (ii) the projection matrix method, which can be used to eliminate the influence of a known spectrum from a set of measured spectra in order to obtain information about the unknown components [197]. In a cyclic voltammogram of PMT and of oligo(thiophene)s in organic solvents (acetonitrile), the anodic branch shows a single anodic peak assigned to the doping process; the cathodic branch assigned to the dedoping process exhibits two cathodic steps [5, 86, 198], in contrast to the chemical doping of sexithiophene as a two-step-process (see Fig. 11) [176]. Cyclic voltammograms of POT, poly(3-tetradecylthiophene) (PTDT) and poly(3-methyl-4-octyloxythiophene) in solution and in precast films show at least two oxidation waves [199]. The voltammogram for PMT in aqueous solutions consists of two anodic peaks: the first corresponding to the doping process and the second, at more anodic potentials, to polymer degradation [.200]. The splitting of the anodic peak can also be detected at low temperature [,201]. When cyclic spectrovoltammerry is applied to thin films of PMT, two successive oxidation processes can be observed during the single oxidation peak in the cyclic voltammogram [202,203]. In copolymers of bithiophene and pyrrole, three anodic oxidation peaks are observed in cyclic voltammograms. Two of these match the oxidation potentials of homopolymeric bithiophene and pyrrole, and the third is intermediate [157]. The cyclic voltammogram of copolymers consisting of electronwithdrawing pyridine units and electron-donating thiophene units indicates two oxidation and reduction peaks related to the oxidation and reduction peak of thiophene units and furan units. An unusually large potential difference between the oxidation peak and the reduction peak of thiophene units suggests the occurrence of an intramolecular charge transfer after the oxidation of a thiophene ring [204, 205]. The electrochemical doping process of a PT film with C102 proceeds as follows [206]: When an ion penetrates into the polymer it promotes a movement of the closer polymer molecules, favoring penetration of further C102. The movement and ionic penetration proceed on the surface in a sigmoidal fashion, and the electrical conductivity of the film increases. So, films with the same structure and similar surface morphology must show similar behavior [206]. A computer simulation of the dopant migration in conducting polymers indicates that the ion migration depends on (i) the electrical field due to the applied potential difference, (ii) the redox potential gradient in the conducting polymer due to an uneven dopant distribution and (iii) the dopant gradient [207]. Some dopant molecules (e.g. FeC14) are located between the planes of polymer chains (e.g. PDT). Their presence in these regions induces easier reorientation of polymer chains during stretching than is the case with neutral samples [.208]. The dopant

Polythiophenes


57

counter ions act as tunneling bridges between neighboring chains [-209]. The diffusion of dopants into and out of PTs along with its dependence on temperatures and dopant concentrations can be studied by the galvanostatic pulse method [210]. Mass changes in PT or PBT observed by the quartz crystal microbalance [211 214] during p- and n-type doping and the dedoping process are close to but not identical to those anticipated if counter ions (anion/cation) are the only transferred species [213 215]. There is a salt and a solvent transfer. The salt and the solvent are transferred in opposite directions [213,215]. An NaClO4 electrolyte containing radioactive 24Na was used to determine the mass changes of the cation in a PMT film electrode during positive and negative potential scans [-216]. At higher overpotentials the oxidation process is more intensive and faster. When PT is oxidized at a much higher overpotential than the oxidation potential, both overoxidation and polymer degradation (loss of electrical conductivity) coexist with the reversible oxidation [,206,217]. The problem of the degradation of electronic properties by overoxidation resulting from nucleophilic attack of water at the dication (bipolaron) charge carriers [218] can be avoided by the use of extremely dry non-nucleophilic electrolyte solutions [219,220]. PMT films made inactive by overoxidation in the presence of C1can be reactivated both electrochemically and chemically to produce a partially chlorinated conducting polymer [220]. Although in the neutral state the absorption of poly(4,4'-dialkyl-2,2'-bithiophene) is blue shifted compared to PAT, the polaronic states included by doping are located at the same region for both families of polymers because of a significant change induced in poly(4,4'-dialkyl-2,2'-bithiophene) by doping [221]. The redox mechanism of PAT and poly(4,4'-dialkyl-2,2'-bithiophene) brings into effect two types of doping [222-224]. The fast doping mechanism, resulting in an important capacitive effect, takes place at the surface of small aggregates of compact chains and does not depend on the thickness or on the nature of the polymer. This capacitive doping is accompanied by the slower Faradaic charge which results from the displacement of the dopants into the aggregates of the compact polymer, their movements being governed by diffusion processes [169,222-224]. The separation of Faradaic and capacitive components of the oxidation-reduction process is possible by impedance spectroscopy [224-229]. Upon doping, the crystal and molecular structure of PAT [143, 154, 155] and also of poly(thienylene vinylene), poly[1,2-di(2-thienyl)ethylene] and poly[1,4-di(2-thienyl)-l,3-butadiene] [230, 231] changes. The distance between main chains of PAT increases and the distance between neighboring conjugation planes decreases slightly. These changes depend on the size of dopant ions and the alkyl side chain length (n = 4, 8, 18), and indicate an incorporation of dopant anions into the crystal region [143, 155]. The diffusion of some common solvents through the films depends on the dopant anions and on the oxidation state of the polymers due to significant morphological differences between the oxidized and reduced polymers [-153].

58

G. Schopf and G, KoBmehl

The diffusion coefficient of PAT increases with repetitive doping-dedoping cycles [232-234]. This enhancement of diffusion coefficient can be interpreted as a learning effect. The learning effect is influenced by the alkyl side chain length (see Sect. 4.1.2) and is controlled by the selection of solvent [233]. The role played by electrostatic interactions between the organic molecule and the dopant is emphasized by quantum-chemical calculations [235, 236]. During the transition cycle from the conducting to the insulating state (doping-dedoping), dopant anions stay in the polymer; electroneutrality is achieved by cations driving into the film [237]. In its neutral state, PMT layers lie flat on Pt; when doped, the first layer lies flat on the Pt surface while the others are randomly oriented, due to the intercalation of anions. The orientation of the polymeric chains switches from a "lying-down" to an "on-edge" configuration when a long alkyl chain is present on the ring, this alkyl chain being oriented perpendicular to the Pt surface [238]. The solubility of POT not only facilitates its processing, but allows the polymer to be doped in solution as an alternative method to the formation of a solid conducting material [239]. As the result of the electrochemical doping of PATs in solution, the doped conducting polymer film is deposited on the anode surface [240]. The thiophene heptamer segments in the spiro-fused methyl substituted thiophene heptamer with terminal ~-Si(CH3)3 (see Sect. 1.2) can be oxidized sequentially with FeC13 to produce the mono(radical cation), the bis(radical cation), the radical cation/dication and the bis(dication) [29]. EPR measurements on the one-electron oxidation products of spiro-fused oligomers indicate that the charge remains on a single oligomer unit and does not hop between two bridged oligomeric chains at temperatures up to 300 K [241]. In poly(5-vinyl-2,2';5',2"-terthiophene), in which terthiophene is a side group of a carbon-carbon chain polymer, the terthiophene side chains cross-link when the polymer is doped with FeC13 or SbC15 in solution or as thin film. The terthiophene radical cations, initially generated upon doping, couple with neighboring neutral species to yield the radical cations of sexithiophene. The dedoping of the polymer with triethylamine yields an insoluble, cross-linked material [242].

n-Type Doping. While the above-described doping processes are p-type doping and much work has been done on p-doping of PT, only a few studies have been devoted to n-type doping. The study of n-type doping is important for its practical use for an anode material in an electrolysis cell but little work has been carried out because of the greater difficulties compared with those associated with p-type doping [243]. Although successful n-type doping in PHT could not be anticipated from the electronic band scheme, n-type doping (Bu4N + as counter ions) in PHT was confirmed by cyclic voltammetry [244]. The Coulombic efficiency is about 30% and indicates a low degree of electrochemical reversibility, in contrast to the Coulombic efficiency of p-doping of PHT

Polythiophenes


59

being > 90%. The n-type dopants of PHT are unstable and tend to be spontaneously dedoped [244]. The spectral change in PMT and in PHT upon electrochemical n-type doping is similar to that of p-type doping. The n-type doping of PHT is more difficult than that of PT. However, the p-type dopant in PHT is more stable compared with that in PT [203, 244]. The cathodic doping is greatly affected by cations, probably due to the accompanying movement of cations in the film [203]. The radical anions produced in n-type doped poly[ 1,2-di(2-thienyl)ethylene] are moderately unstable towards disproportionation, whereas, in p-type doping, radical cations are stabilized [245]. In situ conductivity of n-doped polymer decreases as the size of the dopant cation increases, suggesting charge transport control by interchain hopping. The different conductivities of n- and p-doped polymers are due to the different sizes of the counter ions [245]. From the energy band scheme of poly(3-phenylthiophene), a successful n-type doping can be anticipated [243]. The n-type doping of poly(3-phenylthiophene) with LiCIO4 (Li + as counter ions) is very difficult, but doping with BugNBF4 (Bu4N + as counter ions) is successful, the Coulombic efficiency being > 90% [243]. A significant improvement in n-doping of poly[3-(4fluorophenyl)thiophene] relative to poly(3-phenylthiophene) and PT occurs as a result of intramolecular electron transfer from the PT backbone to the fluorophenyl substituent [246]. Poly(3-alkyloxythiophene) shows not only the usual p-type doping and dedoping behavior but also a unique n-type doping (sodium) and dedoping behavior due to the stabilization of the n-doped state by interaction between dopant cation and etheric oxygen [247]. A stable and fully reversible n-type doping for poly[3-(3,6-dioxaheptyl)thiophene] is shown by cyclic voltammetry [248]. A composite membrane containing poly(chlorothiophene) and poly(4-vinyl-N-methylpyridinium iodide) has a cation as fixed dopant [-249]. Successful n-type doping of PBT occurs with alkylammonium, potassium, and lithium salts. The results show that the n-type doping of PBT is strongly dependent both on the solvent properties and on the counter ion size [250]. The infrared spectra of n-doped PBT are qualitatively similar to those of p-doped PBT [251]. The cyclic voltammetry data give evidence of the following differences between n- and p-type doping of PBT [-250]: 1. 2. 3. 4.

The n-type doping level is less than that of p-type doping. The efficiency of n-type doping is lower than that of p-type doping. The activation phase of n-type doping is longer than that of p-type doping. The n-type doping process exhibits more kinetic limitation than the p-type doping process, presumably due to diffusion of counter ions in the polymer.

A theoretical study of a thiophene/furan copolymer shows that with increasing percentage of thiophene and with increasing block sizes for a given composition the dopability of the copolymer by electron acceptors and donors (p- and n-type doping) is increased [252]. Poly(isothianaphthene) (PITN) can be reversibly cation- and anion-doped (Bu4NBF4) without decomposition of the material

60

G. Schopfand G. Kogmehl

[253]. PITN with methyl, chloro, and fluoro substitution in the aromatic ring can be p- and n-type doped. In contrast with p-doping, the n-type doping is strongly dependent on the substituent. The presence of electron-withdrawing groups makes n-type doping easier to achieve [254]. Anodically and cathodically conducting samples of PMT have been studied by transient and steady-state electrochemical methods in the presence of different anions (C102, PF6) and cations (Bu4N +, Dec4N +). The neutral polymer is described as a mixture of two species with different conjugation lengths. The effective conjugation length of one species is twice that of the other. In the redox process these two species are oxidized (or reduced) to mono and diions respectively. Neutral species of lower conjugation length are more stable, and the segments of longer effective conjugation length are the first to be transformed in the cases of both anodic and cathodic doping [255,256].

The "Self-doping" Process. Whereas the ordinary doping process is dominated by large anion/cation migration, the "self-doping" process is not. Such doping is possible without applying any electrical potential. During the oxidation and reduction of n-conjugated polymers, small "cations" can hop off the polymer, leaving with the covalently bonded counter ion, as depicted in Fig. 13. Selfdoped polymers, for instance poly(3-thienylpropanesulfonic acid), exhibit a fast optical switching response (see Sect. 3.4.4) 1-257-260].

3.2.2 Poly(thiophene) as a Redox System The oxidized (doped) form of PBT can be reduced (dedoped) reversibly by controlling the potential of the polymer coated electrode. The redox process is accompanied by a drastic change in the properties of the polymer (e.g. PBT), such as electrical conductivity (cf. Sect. 3.2.3, Bu4NC104), stability (cf. Sect. 4.3, the doping/dedopping degree), color (cf. Sect. 3.4.3), optical properties (cf. Sect. 3.5) and wettability (see Sect. 3.11) 1-217,261-263]. The reversible redox behavior of PT depends on the monomer concentration [264,265]. PT films electrochemically formed in solutions of 0.4 M thiophene have reversible redox behavior, in contrast to PT films formed in 0.01 M solutions of thiophene. This is explained by overoxidation due to the limited transport of the monomer by

S031,4~

--75.~M SO3M

R,a.:,,. S03M

+

s0~ S03M S03M

nM÷

503"

Fig. 13. Self-dopingprocessbasedon smallcationmigration by cyclicvoltammetry(M = H +, Na +, etc.) [257]


61

diffusion. A second explanation is that the reduced monomer concentration increases the water/monomer ratio [264]. Poly(isothianaphthene) (PITN) can be reversibly cation- and anion-doped without decomposition of the material. PITN with these two reversible and stable redox states of different colors is a potential candidate for electrochromic displays. The reversible redox reaction of PITN and the existence of a relatively stable residual charge can be used in electronic devices, such as memories with learning effect (reading-writing device) [253].

3.2.3 Dopants A wide range of dopants are used for the doping of PTs:

Iodine, bromine. Iodine seems to form polyiodide anions such as [I-I-I ... I-I] whose length (1.7 nm) corresponds to four thienylene units (4T) in PT. The 120 wt% of iodine per PT corresponds to about a molar ratio of 5:8 between [I-I I... I - I ] - and 4T. The reversibility of the doping and dedoping processes in PT using iodine and hydrazine is very good in view of the chemical and crystal structure [266,267]. The structural evolution during iodine vapor doping of PHT, POT, and PDDT was studied by in situ X-ray diffraction [268,269]. Dopant ion diffusion into the polymer host matrix induces cooperative movements of both the thiophene backbone and alkyt side chains with variation in the lattice spacing perpendicular to the polymer chain axis. This evolution is sensitive to the alkyl side chain length (cf. Sect. 4.1.2) [268]. Gas-phase doping does not affect the molecular morphology of a mixed Langmuir-Blodgett film containing PHT, in contrast to doping from solution with NOPF6 and SbCls [270]. Iodine-doped PTs show an electrical conductivity of 6 S cm-* [271] (8 S cm-1 [272]). The highest electrical conductivity of PATs (n = 5, 8, 12, 16) is achieved at the ratio one iodine molecule per two monomer units (PDDT: 0.2 S cm-1). Additional increase of iodine content decreases the electrical conductivity. The free dopant is probably added to double bonds, and by this action the conjugated system may be disrupted [273]. However, solution cast polymer films after iodine doping exhibit an electrical conductivity of 12 Scm-1 [274]. An electrical conductivity of up to 1000 S cm -1 is reached for PAT containing almost exclusively head-to-tail couplings (cf. Sect. 4.1.2) [275]. The highest electrical conductivity for 5,5'"dimethylquaterthiophene is 0.1 Scm -1 at a doping level of 0.4 iodine atoms per 5,5'"-dimethylquaterthiophene molecule [276]. Poly(3-alkyloxythiophene)s themselves are insulators. Upon iodine-vapor doping, the electrical conductivity increases to 1.8 S cm- 1 for poly(3-methoxythiophene), 2.1 x 10 -2 S cm- 1 for poly(3-butoxythiophene), and 2.1 x 10- i S cm- i for poly[3-(2-methoxyethoxy)thiophene] [247]. I2-doped poly(thienylene vinylene) reaches a conductivity of 315 S cm- 1 [277]. The electrical conductivity of doped poly(thienylene vinylene)

62

G. Schopfand G. Ko6mehl

fibers increases with the draw ratio up to the maximum conductivity of 2000 S cm-1 [278,279]. The values of the electrical conductivity of iodinedoped poly(2,5-thienylene ethynylene) are independent of the molecular weight of the sample, but have a large dependence on the iodine doping content and can be as high as 3 x 10 - 4 S c m - 1 [ 2 8 0 ] . The electrical conductivity of a thiophene/benzene copolymer increases from 9.5 x 10 -a4 Scm -1 in the neutral state to 10 - 7 Scm -1 on iodine doping, in contrast to poly-p-phenylene which has no significant evolution of conductivity on doping. The crystalline structure of poly-p-phenylene and of a thiophene/ benzene copolymer is similar, but the copolymer has a lower degree of crystallinity which allows for easier penetration of iodine into the bulk material [281]. A blend of PAT (n = 8, 12, 22) and polystyrene doped with iodine vapor has an electrical conductivity of up to 10 S cm- ~ [282]. The greatest disadvantage of using iodine as a dopant is that it evaporates gradually from the polymer matrix, and therefore the environmental stability is low [273]. For I2-doped PATs (n = 8, 10), the decay time constant at room temperature was found to be a few hours [189]. The electrical conductivity of I2-doped PT remains almost constant after allowing the iodine adduct to stand at room temperature for 150 days in air [271]. The reaction of poly[1,2-di(2thienyl)ethylene] as a thin film with bromine occurs in two subsequent stages, i.e., doping to the conductive state and addition to an insulating material (see Sect. 6.9) [283].

HCI04, Bu4NCI04. The NMR relaxation time of ~H was determined for PT doped with perchlorate [284]. The calculated lattices of PT are found to exhibit polymorphism like that of BF2-doped PT [285]. The electrical conductivity of HC104 doped poly(3-phenylthiophene) at room temperature increases from 1.3 x 10 -a2 Scm -~ at the neutral state to about 6 Scm -1 at a dopant concentration of about 22.4mo1%, and the Coulombic efficiency is about 98%, indicating a high degree of reversibility [243]. The electrical conductivity of electrochemically synthesized PBT with Bu4NC104 increases from < 10 .4 S cm- ~ for the neutral form (at 0.2 V) to a maximum of 8 to 10 S cm- t (at 1.0 V): further oxidation gives rise to lower conductivities of 0.5 to 1 S cm-1 (at 1.6 V) [217]. Films of doped PDT reach an electrical conductivity of 0.1 S cm -~ [286]. During the thermal decomposition process of perchlorate doped PBT, the perchlorate anions undergo reactions resulting in both volatile chlorine species and chlorine that is covalently bonded to the polymer [287,288]. The EPR line width of PT heavily doped with C104 is a function of both temperature and frequency [289]. PITN is electrochemically doped with C104, and the electrochemical, optical, and magnetic properties as a function of doping potential and electronic states are described in ref. [290]. Organic Acids - Trifluoroacetic Acid. The electrical conductivity of PDDT doped with trifluoroacetic acid is about two orders of magnitude lower than that obtained with iodine at the same concentration. This fact can be explained by the protonation of the main chain by the acid that therefore by the disruption of

Polythiophenes


63

the conjugation. The environmental stability of trifluoroacetic acid-doped films is higher than in the case of iodine [-273].

Organic Acids, Sulfonic Acids and Propionic Acid. The electrical conductivity of PDDT is about four to five orders of magnitude lower than in the case of iodine, but the electrical conductivity of the films is environmentally more stable. In addition, the dopant 4-dodecylbenzenesulfonic acid can act as a plasticizer for the PT film [273]. 1-Naphthalenesulfonic, ethylenebenzenesulfonic, methylethylsulfonic, butanesulfonic, and pentafluoropropionic acids can be used for doping of POT and PDT 1-189]. FeCl3. PT doped with FeC13 studied by scanning tunneling microscopy (STM) indicates low doped and neutral domains surrounded by highly doped regions [-291]. This is confirmed by theoretical analysis [292, 293]. FeC13-doped PT is described as having an electrical conductivity of 0.5 S cm- 1 [272]. Films of doped PDT reach an electrical conductivity of 5 S cm- 1 [286]. The electrical conductivity of FeC13-doped poly(2,5-thienylene ethynylene) is 1 x 10- 3 S cm- 1 [280] and of FeC13-doped poly(thienylene vinylene) 110 S cm- l [277]. FeC13doped poly(3,4-dibutoxythienylene vinylene) films have an electrical conductivity of 1 S cm -1 and are nearly optically transparent [-106, 107, 294, 295]. Solutions of POT can be doped with FeC13 • 6H20, and films with conductivities up to 1 S cm-1 may be cast directly from the doped solutions [239]. FeC13doped POT and PDT are stable at room temperature for 10~100 years (by extrapolation) and the values of the electrical conductivity are more stable than those of NOPF6-doped PATs [189,296]. Photochemical dedoping of PATs doped with FeC13 is possible (cf. Sect. 4.3) [297]. Upon FeCI3 doping, the glass transition temperatures of PATs (n = 4, 12) increase and the melting points of PAT (n = 12) disappear. The dopant anions decompose in the range of about 150 to 230°C [298]. PMT and PBT containing CI- and FeCI2 have an identical helical syn conformation, but, after dedoping, PMT and PBT structures are different (see 2.2) [132]. The reversibility of the doping and dedoping processes of PT using FeC13 and hydrazine is less compared with that for PT doped with iodine and hydrazine [267]. In a new solid-state doping, the dopant FeC13 is cosublimed with sexithiophene (6T) under UHV-conditions. Subsequent gas exposure experiments indicate that FeC13 is not stable as a dopant in the presence of water and oxygen [181,182]. AuCl3. The oxidative doping of n-conjugated polymers with gold trichloride in acetonitrile yields electrically conductive polymers containing AuCI4 counter ions. The doped films are more stable in ambient atmosphere than polymers doped with conventional dopants (FeCIa). Oxidation of polymer films using solutions of AuC13 in nitromethane results in the formation of zero-valent gold (see Sect. 3.8) in addition to the AuCt2 doped polymer [297, 299].

SbC15, NOSbF6. In doped oligomers of poly(thienylene vinylene), stable bipolaron-like charged species are formed. In contrast to poly(p-phenylene vinylene) oligomers, polaronic-like charge states can also be formed by careful

64

G. Schopfand G. Kof~mehl

control of the oligomer-oxidant ratio [300]. Upon doping of a mixed Langmuir-Blodgett film containing PHT from solution, the original molecular organization of this multilayer film is disrupted. Gas-phase doping, on the other hand, could be accomplished without significantly altering the molecular morphology of the multilayer film [270]. An enhancement of the electrical conductivity can be achieved up to 9 × 10- 5 S cm- 1 for NOSbF6-doped PT and up to 4 x 10 -2 S cm -1 for NOSbF6-doped PBT. Doped PT and PBT are relatively stable against water [301].

AsFs. The electrical conductivity of a thiophene/benzene copolymer is significantly higher when it is doped with AsF5 (1.2 S cm-1) compared with doping with iodine (10 -7 S cm -1) [281]. NOPF6. The highest electrical conductivity for doped 5,5"'-dimethylquaterthiophene is 2.1 × 10-2S cm -1 [276]. Solutions of POT can be doped with NOPF6, and films with conductivities of up to 0.05 S cm- 1 may be cast directly from the doping solutions [-239]. During doping, the electrical conductivity increases to 2× 10 -5 S cm -1 for PT and to 7 x 10 -5 S cm -1 for PBT. The stability against water is lower than that for PT and PBT doped with NOSbF6 [301]. Upon doping of a mixed Langmuir-Blodgett film containing PHT from solution, the original molecular organization of this multilayer film is disrupted [270]. Upon NOPF6 doping of PHT, a shift of the Fermi level, and at saturation doping a finite density of states at the Fermi level, is observed with photoelectron spectroscopy. It can be interpreted in terms of the formation of a polaron lattice at a high doping level [184, 302]. For PF6-doped PATs (n = 8, 10) the decay time constant at room temperature was found to be a few days, and the values of the electrical conductivity arc less stable than those of FeCla-doped PATs [189, 296].

NOBF4, LiBF4, Bu4NBF4. A doped 5,5"'-dimethylquaterthiophene reaches an electrical conductivity of 4.4 x 10 - 2 S cm-1 [276]. The electrical conductivity of NOBF4-doped copolymers with alternating silylene and thienylene units is _< 10 - 1 S c m - 1 [303]. The electrical conductivities increase to 2x 10 -4 S cm- x for PT and to 7 x 10 - 3 S cm- 1 for PBT during doping. The stability against water is lower than for PT and PBT doped with NOSbF6 [301]. The calculated lattices of PT are found to exhibit polymorphism like that of PT containing ClOg [285]. The exchange of BF,~ in PMT by Fe(SPh) 2- is described [304]. PITN can be reversibly cation- and anion-doped with Bu4NBF4 [253].

Trifluoromethanesulfonic Acid (Triflate). The NMR relaxation time of 19F has been determined for triflate-doped PT [284]. Heteropolyanions. PBuT doped with the "Keggin-type" heteropolyanions 12molybdophosphoric acid (H3[PMo1204o]) has an electrical conductivity of 0.2 S cm- 1. The heteropolyanion doping-induced spectroscopic changes are the same as in the case of classical, small monovalent anion doping. Doped


65

polymers exhibit a weak temperature dependence of the electrical conductivity (activation energy 31 meV) and show high stability under ambient conditions [-305]. The chemical oxidation of sexithiophene by a "Keggin-type" heteropolyanion generates a [(6T+)4/PMo120]o] charge-transfer complex. These large inorganic anions confer new properties on polymers, for example, multiple redox exchanges, paramagnetism, electrochromism, and catalytic activity [306].

Buckminsterfullerene (C60 , C70 ). The slight enhancement of electrical conductivity and the change of EPR by C6o-doping of PAT is less significant compared to that caused by conventional strong dopants [307]. The electrical conductivity of PAT increases upon doping, and, after indicating a relatively low maximum value, the conductivity decreases again at higher C6o concentrations. The electrical conductivity increases with increasing temperature, and, after reaching about 60 °C, it again decreases at higher temperatures. EPR line width decreases from 7.1 G to 1.9 G upon doping, and spin density is enhanced [308]. The absorption spectrum and cyclic voltammogram for PHT (n = 6) change upon doping, but those for PODT (n = 18) do not [308-310]. The electron in the valence band of PHT is transferred to the L U M O of C6o. The top of the valence band may consequently be lowered in energy with increasing dopant concentration. This can be observed in the cyclic voltammogram in contrast to PODT, where the lowering of the valence band top cannot be observed. The charge transfer to C6o may be favorable in PATs with shorter alkyl side chains in accordance with the results of photoemission spectroscopy. Upon doping, the quenching of photoluminescence and a blue shift of the emission peak is observed [307, 309, 310]. The photoconductivity of PAT is enhanced and the response time becomes shorter on C6o doping [307, 311]. Small enhancements of electrical conductivity and quenching of photoluminescence have also been observed upon Cvo doping, but the change of optical absorption is less remarkable [307]. 7,7,8,8-Tetracyanoquinodimethane (TCNQ). Charge-transfer complexes based on oligo(thiophene) as electron donor and TCNQ indicate a charge-transfer band at 800nm in the UV/vis spectra [-312]. Electrically conductive complexes of oligo(thiophene)s and TCNQ are chemically synthesized and press molded forming a l-ram sheet; they have electrical conductivity values of > 10 - 4 S cm-1 in the thickness direction [313]. Charge-transfer complexes of PT and TCNQ are also described in [314] as electrolytes for an electrolytic capacitor (see Sect. 6.12) [315]. Thin films of oligo(thiophene)s can also be doped with 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane [119, 316]. n-Type Dopants. Tetramethylarnmonium trifluoromethanesulfonate, Me4NCF3SO3, as a source of Me4N + counter ions is used for poly[3-(4-fluorophenyl)thiophene] [246]. With Bu4N + as counter ions, tetrahutylammonium tetrafluoroborate, Bu4NBF4 can be used for doping of PMT [203], PHT, PT [244],

66


poly(3-phenylthiophene) [243], and PITN [253], tetrabutylammonium hexafluorophosphate, Bu4NPF6, for doping of PBT [250] and PMT [255], and tetrabutylammonium perchlorate, BugNC104, for doping of PMT [255]. Tetradecylammonium hexafluorophosphate, Dec4NPF6, and tetradecylammonium bromide, Dec4NBr, (Dec4N + as counter ions) are used for the reduction of PMT 1-255]. Sodium dihydronaphthylide (Na + as counter ion) is used for poly(3methoxythiophene) and poly[3-(2-methoxyethoxy)thiophene] [247]; also metallic sodium (Na + as counter ion) in a tetrahydrofuran solution is used for poly(3,6,9,12,15-pentaoxa- 19-thiabicyclo [ 15.3.0] -eico sa- 1(20), 17-diene- 1(20)- 17diyl), a crown ether containing PT (structure see Sect. 1.2) [48]. Potassium hexafluorophosphate, KPF6, (K + as counter ion) is a possible n-type dopant for PBT [250]. Doping of PBT with lithium perchlorate, LiC104, (Li + as counter ion) is ineffective in acetonitrile and propylene carbonate, but it is successful when the effective radius of the lithium ion is increased by using hexamethylphosphoric acid triamide as a solvent with high solvating power [250].

3.3 Conducting Relaxation, a.c. and d.c. Conductivity The first cyclic voltammogram obtained after the film has been held in its neutral state differs in shape and peak position from the subsequent ones [317, 318]. This shift in oxidation peak position induced by the so-called waiting time - the memory effect - is more pronounced in poly(4,4'-dialkyl-2,2'-bithiophene)s than in PATs (n = 4). The magnitude of this effect is associated with the difficulty of the dedoping and the slow relaxation process of the reduced flexible polymer matrix. The reduction part of the cyclic voltammetry of poly(4,4'-dialkyl-2,2'-bithiophene)s is similar to that in PAT [319, 320]. PMT exhibits a relaxation effect when maintained in its neutral state. Both peak position and peak height versus waiting time obey a logarithmic law. The waiting time behavior of this relaxation effect is in agreement with the results found with poly(aniline). Use of the three electrolytes LiC104, TEAC104 and TBAPF6 leads to the same relaxation behavior of PT. Thus, the formation of ion pairs as the origin of the relaxation phenomenon seems unlikely. In addition, the film relaxes in the same way in open circuit conditions and with imposed potential 1-321,322]. Orientation of a polymer in an electric field involves a large expenditure of time and efforts in the relaxation process [161]. The conductivity relaxation in conjugated polymers arising from carrier hopping is different from the dielectric relaxation arising from permanent dipole reorientation in conventional polymers. The relaxation of an electric field in a charge carrier system results from the charge hopping of mobile carriers over potential barriers, which can lead to short-range (or local) a.c. conductivity and long-range d.c. conductivity [323]. The conductivity relaxation behavior of neutral PAT (n = 4, 6, 8 and 12) is


67

investigated on the basis of dielectric relaxation measurements at - 100 to + 180°C and 0.4 to 105 Hz after conversion to complex electric modulus formalism [323 325]. The conductivity relaxation time can be represented by the non-exponential decay function. The variation of relaxation time distribution with temperature is found to be highly related to chain motions. The side chain motion has no appreciable effect on the charge transportation. As an electric field is applied, charges which are delocalized along coplanar subchains hop over localized conformons (conformational defects) to the neighboring sites. The contribution to the conductivity due to the charge hopping across the side chains is insignificant [323, 325 327]. Moreover, the measurements indicate that, in the glass transition and rubbery regions, the relaxation time distribution becomes broadened with temperature and, as the coplanar subchains in the order region melt, the conjugation length decreases, and its distribution (and therefore relaxation time distribution) becomes more narrow [325]. The behavior of the a.c. conductivity of a PT film is correlated with the inhomogeneous structure of the film, whose behavior can be described by a model in which there is serial segregation into conducting and semiconducting regions [-328]. The a.c. conductivities of four kinds of PT prepared at low temperature ( - 20 to + 10 °C) in the plane direction are almost independent of the frequency (10 kHz to 1 MHz). But the conductivity in the vertical direction and the conductivity in the direction of the plane for PTs prepared at comparatively high temperatures is slightly dependent on the temperature ( - 140 to + 120 °C) at I M Hz. The d.c. conductivity in the direction of the plane is higher than that in the vertical direction by 4 to 7 orders of magnitude [329].

3.4 Chromisms 3.4.1 Thermochromism Two different types of thermochromism, which are correlated to the substitution pattern of the polymers, may be distinguished [330]: 1. A two-phase thermochromic behavior, as in PDDT and poly(3-methyl-4octyloxythiophene), is related to the formation of delocalized conformational defects upon heating. These defects are possible due to the presence of sterically demanding substituents between each consecutive repeating unit [330]. In the solid state at room temperature, PDDT and poly(3-octyloxy-4-methylthiophene) have a coplanar conformation for the main chain. Heating (25 to 150°C) increases the repulsive intrachain steric interactions and introduces some conformational disorder in the side groups, forcing the polymer backbone to adopt a nonplanar conformation [331,332]. Temperature dependent UV/vis absorption measurements of fluorinated PTs, e.g., poly(3'-perfluorohexyl2,2';5',2"-terthiophene), poly[3-(pentadecafluorooctyloxy)-4-methylthiophene] and poly[3-(tridecafluorononyl)thiophene], show a blue shift of the maximum

68


upon heating, with presence of an isosbestic point. This behavior can also be explained in terms of the conformational transition of the polymer backbone from a relatively planar structure at low temperatures to a more twisted form at high temperatures [333]. 2. In contrast to polymers having two distinct forms, poly(3-butoxy-3'-decyl2,2'-bithiophene) and poly(3-dodecyl-2,2'-bithiophene) have no isosbestic point in the temperature-dependent UV/vis spectra. The absorption maximum shows a continuous and monotonic blue shift on heating [-330, 334]. This phenomenon can be described as a continuous modification of the polymer backbone, where the rotation of a first thiophene unit will induce the twisting of the following units [334]. These polymers allow only the formation of localized conformational defects [-330]. If the steric interactions are too large [e.g., poly(3,3'-dihexyl-2,2'-bithiophene), poly(4,4'-didecyl-2,2'-bithiophene)], no coplanar conformation is accessible at room temperature. A structural transformation of PDT and poly(4,4'-didecyl-2,2'-bithiophene) can occur between 300 and 200 K as a result of the lowering of the mean intermolecular distance in the polymer. This effect is fully reversible with temperature for doped poly(4,4'-didecyl-2,2'-bithiophene). Conversely, in the absence of significant steric interactions, the polymer [e.g. poly(3-octyloxythiophene)] can maintain nearly coplanar conformations and hence a highly conjugated structure even at high temperatures. A thermochromic effect cannot be observed in these cases [98, 331,332, 335]. PATs with HT coupling have a crystalline state, while HH-coupled compounds do not crystallize and form a glass at low temperature. Steric hindrance and the barrier against planarity are maximized in HH coupled compounds and should weaken intermolecular interactions [336, 337]. Poly(Y,4'-dihexyl-2,2';5',2"-terthiophene) has a highly conjugated structure at low temperatures with an absorption maximum at 494 nm; above 100 °C a blue shift of the absorption maximum is observed due to the formation of localized twisting of the backbone induced by an increase in the steric hindrance of the side chains on heating [338]. The temperature dependence of the structure of PAT can be studied with the aid of ultraviolet photoemission spectroscopy [339]. Thermochromism studies of mono- and multilayers of Langmuir-Blodgett (LB) films of PATs, poly(3-octyl-2,2'-bithiophene), and poly(3'-octyl-2,2';5',2"terthiophene) show that in sparsely alkylated PTs the magnitude of the thermochromic shift is smaller than in PAT and roughly proportional to the side chain concentration [340]. The absorption maximum of a dedoped LB film of PHT/arachidic acid shifts to higher energies with increasing temperature. In complementary measurements of spin-coated thin films the shift occurs at higher temperature than in the LB films [341]. Thermochromic transitions are completely inhibited at pressures as low as 14 kbar [-342]. At the phase transition of PAT during heating, absorption spectra change rapidly. These phenomena are also discussed in terms of the increase of the energy gap in the liquid state due to the decrease of coplanarity of thiophene rings accompanied


69

by remarkable conformational changes [-343]. The planar to nonplanar transition is driven by a balance between repulsive intrachain steric interaction and attractive interchain interactions [331]. In addition to the melting transition, transitions due to main chain and side chain relaxations are found in the thermal and spectroscopic analysis of PAT. These two additional transitions resulting from conformational defects give changes in the energy of the absorption maximum of about 0.1 eV and 0.015 eV, respectively. In a copolymer, the conformational defects are more pronounced owing to the presence of different lengths of side chains along the main chains, which disturbs the stacking of the polymer chains to form layered structure [344]. A mixture of PHT and POT is a single phase material which is structurally intermediate between PHT and POT. The thermochromic transition temperature and the longest interchain distance in the crystalline part of the polymer change non-linearly with composition, but in such a way that the thermochromic transition temperature and the interchain distance are in an approximately linear relationship to each other. This indicates that the side chains act primarily as spacers between main chains, rather than being directly involved in the thermochromic transition [345].

3.4.2 Solvatochromism Oligo(thiophene)s, POT, PTDT, poly(3,4-dibutoxythiophene), poly(3-butoxy-4methylthiophene), and poly(3-methyl-4-octyloxythiophene) in the solid state and in solution have different conformational properties [-126, 199, 346]. These polymers have a nonplanar conformation in solution and a coplanar structure in the solid state. However, the conformational differences cannot be detected in their cyclic voltammograms. It is therefore assumed that the dissolved polymers adopt a coplanar conformation in the vicinity of the electrode [199]. Different cyclic voltammetric results can be observed for oligo(phenylene vinylene) in the solid state and in solution [347]. In contrast to PATs, poly(3,4-dialkylthiophene)s, e.g., poly(3,4-dihexylthiophene) and poly(3-methyl-4-octylthiophene), do not show any solvatochromic properties due to the nonplanar conformation of the polymer backbone resulting from steric interactions between the substituents and the thiophene rings. Poly(3,4-dialkylthiophene)s exhibit similar nonplanar conformations in solution and in the solid state [-126]. The conformation of PAT in several solvents can be different (solvatochromism). The regiospecific HH coupled PATs do not present solvatochromism, which implies a rigid backbone structure and which is consistent with a high torsion barrier [336, 337]. A P T with chiral amino acid side chains shows a blue shift from methanol to water in the UV/vis spectra. In water or in a film on glass, the polymer is nonplanar with small helical fragments and shows high specific rotation, which is rationalized as being due to the orientation of adjacent side groups in the same direction (syn conformation). Other solvents induce more straight chain

70


conformations with a large ~-electron overlap between the thiophene rings [348]. Solvatochromism is shown in the anomalous voltage-dependent viscosity of the solution of PAT in anisole, in contrast to the solution of PAT in chloroform [349]. In the temperature range 1 0 4 0 °C the viscosity increases with increasing applied voltage, but at temperatures of about 60 °C it decreases with increasing applied voltage, with a tendency to saturation. The absorption spectrum of PAT in anisole exhibits a red shift of about 70 nm above the critical temperature. These results are interpreted by assuming a rodlike structure of PAT in anisole at room temperature with a high effective conjugation length, and a coiled structure with a low effective conjugation length at high temperatures and in chloroform [349, 350]. A chromatic transition occurs in a solvatochromic fashion in the compatibility region of the blend of PATs (e.g. POT, PDDT) with a flexible matrix polymer. Thus it occurs as a function of concentration when diluting PAT in the matrix polymer [158].

3.4.3 Electrochromism Moreover, changes in the visible absorption spectra can be caused by electrochemical redox processes (doping/dedoping process). For instance, a POT film is dark red in the insulating state and light blue in the oxidized state, and gives a good contrast with poly(aniline) as the counter electrode [-351]. Methoxy or ethoxy substituted poly(thienylene vinylene)s are semiconductors with their absorption edge in the near IR: in the heavily doped state, these polymers become transparent conductors [352]. A disubstituted poly(thienylene vinylene) film is blue in the insulating state, while the polymer in the oxidized state is transparent. The absorption maximum is shifted to a wavelength higher than 800 nm, but the doping process is not completely reversible [353]. The same electrochromism can be observed in poly[1,2-bis(3-alkyl-2-thienyl)ethylene] films, which are blue in the insulating state and transparent in the doped state [147]. A reversible process and a fast-switching response of the electrochromic phenomenon is observed in self-doped PTs. The optical switching response of poly(3-thienylpropanesulfonic acid) is considerably faster (saturated within about 50 ms) than that of PITN (saturated within about 500 ms) and is stable for more than 10000 cycles for a given electrolytic condition [257]. A novel lightly colored conducting polymer is poly(phenanthro[9,10]thiophene-2,11diylidyne) (cf. Sect. 1.2) which exists in five distinct oxidation states producing colors ranging from white, yellow, red, and brown to black. The most oxidized red form reaches an electrical conductivity of up to 105 S cm- 1 [33]. The change of optical absorption in the visible region of poly(3,4ethylenedioxythiophene-2,5-diyl) (structure, cf. Sect. 1.2) is appropriate for a smart window, and the required applied voltage is small. The switching time at room temperature from fully colored to fully bleached is about 4 s, and the stability on repeated switching is very good [43]. The blend of poly(3,4-


71

ethylenedioxythiophene-2,5-diyl) and polystyrene sulfonic acid in the doped state is transparent light blue, and, after dedoping, dark blue [44]. Conducting composites are one way to improve electrochromic properties and are also useful to improve physical properties [351,354]. An oligo(thiophene)/poly(vinyl alcohol) film is reddish-brown in the neutral state and changes to deep blue-black on doping [355]. The combined films containing the poly(aniline)-poly(sodium acrylate) composite films and PT and PMT show overlapping absorption spectra of each component polymer during electrochemical oxidation. The individual electrochromic processes of the components do not influence each other [356]. Inorganic electrochromic materials can also be combined with conducting polymers to extend the expected color change and reduce the operational potential [354]. An application of colorimetric evaluation to electroactive conducting polymers (e.g. PT) to explore the electrochromic properties and electrochemical doping process is described in Refs. [354, 357].

3.4.4 Ionochromism An ionochromic effect is observed in solutions of crown ether containing PTs (structure, cf. Sect. 1.2) or PATs. The polymers show an absorption maximum shift up to 91 nm with alkali metal ions (K +, Na +, Li +) [47, 48, 50, 358].

3.4.5 Color Changes Caused by Pressure and Electricity The absorption edge energies of PHT and PDDT decrease with increasing pressure (up to 3 GPa) and then show leveling off and subsequent increase. The decrease is probably due to an increased conjugation length of the thiophene main chain caused by packing of side chains. The increase in the edge energy is attributed to a distortion of a planar polymer structure caused by the steric hindrance between neighboring polymer chains [359]. A thiophene-containing molecule which can exist in three different forms is shown in Fig. 14. The solutions of the three molecular forms have different colors. Compound 1 is colorless, compound 2 is blue and compound 3 is violet. All similar switchable molecules developed so far have been activated either by light or by electrical signals. This molecule can be switched in both ways: by electricity and by light ~68].

3.5 Optical Properties Photoluminescence and electroluminescence are observed in PTs. The fluorescence lifetime and the fluorescence quantum yield of oligo(thiophene)s increase with increasing ring number (see Sect. 4.1.1) [360]. In quasi-one-dimensional

72

G. Schopf and G. Kof3mehl

Compound I

HO

OH

vis light

T

Compound 2

Active electrically and optically

light

F F .

F,~ F

f

~

.. F F

HO

OH

electrical

Compound 3

Optically inert

o

;

F F

F

o

Fig. 14. Threemolecularformsof a thiophene-containingmolecule[68] oligo(thiophene)s and thiophene-based oligomers, the luminescence yield, luminescence lifetime, and luminescence wavelength can be controlled by changing the conjugation length [361]. The quantum yield of photoluminescence of

Polythiophenes


73

PATs (n = 6, 12) also increases with increasing alkyl chain length (cf. Sect. 4.1.2) [362], and the quantum yield of amorphous PAT films is higher than that of semicrystalline films (cf. Sect. 4.1.4) [362]. The nitro group at the end of an oligo(thiophene) chain induces a significant increase in fluorescence quantum yield and in lifetime of the excited state [165]. Increasing temperature increases the luminescence intensity of PATs (n = 6, 12, 22) in the solid phase; this temperature dependence is enhanced with increasing number of alkyl side chains (cf Sect. 4.1.2) [363]. The intensity of photoluminescence of PDDT and the band gap decreases at room temperature after heat treatment [364]. The wavelength of the emission maximum of the Langmuir-Blodgett film is longer than that of the solution of poly(heptadecyl 3-thienylacetate) [365]. The photoluminescence intensity of PHT decreases markedly with pressure [342] and is much enhanced in the expanded volume of PAT (as gel) [366]. Photoluminescence spectra of PHT are quenched by CO2 gas and phenol molecules, suggesting the occurrence of photochemical electron-transfer reactions between photoexcited PHT and these substrates. PHT can be used for photocatalytic fixation of CO2 (cf. Sect. 6.12) [367, 368]. The low-temperature (4.2 K) photoluminescence spectrum of sexithiophene shows good correspondence with similar spectra of chemically synthesized PT of good crystalline quality [369]. Polyethylene with terthiophene side chains shows a strong blue-green photoluminescence [242]. Absorption spectra of copolymers consisting of electron-withdrawing pyridine units and electron-donating thiophene units show a red shift of the absorption maxima (490 nm) compared with those of PT (420 to 480 nm) and poly(pyridine) (370nm). Thienylpyridines and thienylethynylenepyridines, the precursors for the thiophene/pyridine and thiophene/ethynylene/pyridine copolymers, indicate a considerable interaction between the ~-orbitals of thiophene, pyridine and ethynylene moieties. These copolymers give rise to strong fluorescence both in formic acid and in the solid state. The fluorescence peak in dilute formic acid is found at a longer wavelength than those of poly(pyridine) and 2-(2'-thienyl)pyridine, indicating that the fluorescence centre is delocalized along the polymer chain, and the interaction between the thiophene ring and the pyridine ring causes a change in the fluorescence energy from that of poly(pyridine) [204,205, 370]. A bathochromic shift of the absorption maximum can also be observed for the cooligomer of thiophene and cyclopentadienone in comparison to the homooligomer of thiophene

[371]. The poly[3-(4-octylphenyl)thiophene] film exists in two forms, giving widely different optical absorption, as well as photoluminescence and electroluminescence spectra. In the low-band gap form a high emission intensity is observed centered at 800 nm (1.55 eV), well into the infrared, while the high-band gap form shows a maximum at 670 nm (1.85 eV). The conversion from the high-band gap form to the low-band gap form can be achieved upon thermal treatment of polymeric light-emitting diodes [372]. Stretched PT can be used in lightemitting diodes emitting polarized light [373].

74


Multi-block copolymers of poly[(silanylene)thiophene]s consisting of conjugated and non-conjugated sequences show photo- and electroluminescence. The re-electron density is delocalized along the oligo(thiophene) block. The band gap strongly depends on the delocalization length, as can be inferred from the blue shift of the absorption maxima that is found when the number of thiophene rings is decreased. A change in the silanylene block length is found not to have a large effect on the absorption, indicating that only weak coupling occurs between the silanylene and the oligo(thiophene) blocks [374]. Thin films of PHT show a long-lived photoluminescence at 826 nm, which is attributed to a radiative, spin-forbidden, TI ~ So transition, i.e., phosphorescence. This is the first observation of phosphorescence from r~-conjugated polymers [375-377]. A metastable triplet state photoexcitation of poly{3-[2-(3methylbutoxy)ethyl]thiophene} can be observed in solution. Photoexcitation of solutions of this polymer containing C60 results in an efficient energy charge transfer reaction [378]. Metastable triplet state photoexcitation can also be observed for oligo(thiophene)s (nT, n = 6, 7, 9, 11). This photoexcitation is formed via intersystem crossing from the photoexcited singlet state [379]. Immediately formed excited triplet states of oligo(thiophene)s [3(nT)*, n = 3 to 6] show a lifetime in the order of few tens of microseconds. The radical cations of these oligo(thiophene)s (nT +') are obtained either by electron transfer from 3(nT)* to an electron acceptor or by direct photogeneration from nT using high excitation energy [380]. The singlet-to-triplet intersystem crossing quantum yield of terthiophenes was determined in 95% ethanol solution. The variation in the phototoxicity of these compounds towards some insect and mite species is large relative to the changes in the quantum yield of triplet state formation [381]. A comparison of the electroluminescence in PHT and PDDT using calcium and aluminum as electron-injecting contacts indicates significantly higher efficiencies than those reported elsewhere; they achieve value of 0.2% photons per electron in PDDT [382]. In LB films of 9,10-diphenylanthracene and sexithiophene with arachidic acid, the sexithiophene can be used as a quencher for the 9,10-diphenylanthracene donor fluorescence. The efficiency of the energy transfer from this donor to the sexithiophene acceptor decreases with the square of the distance separating them [383]. Bipolarons can be induced by doping and by photoexcitation [384]. When a PNT film is irradiated with light of wavelength longer than 466 nm, electrons are transferred from the polymer to oxygen, indicated by IR experiments. The photooxidation process results in the formation of positive polarons on the polymer chains. In the dark, electrons are transferred back to the polymer chains which therefore return to their neutral state [385]. Irradiation with light of wavelength shorter than 466 nm induces IR absorptions due to both polarons and sulfonyl groups, as well as conjugation-breaking reactions [385]. From the studies of the photoinduced absorption of PDT in solution, it is concluded that the interchain transfer of the charge cannot be the dominant process leading to the creation of photoexcited states in solution [386]. Considerable photoconductivity is found when poly(thienylene vinylene) films are irradiated with


75

near-IR light [387]. The photoconduction in PDDT was investigated by means of time of flight studies [388]. The results are interpreted in terms of a hopping motion within an energetically random array of hopping sites formed by ordered chain segments. The polaron contribution towards the activation energy for charge carrier motion is found to be negligible relative to the disorder contribution [388]. Disordered sexithiophene favors non-radiative decay of intramolecular singlet excitations into an intermolecular electron hole pair, and the layer-like structure prevents rapid geminate pair recombination and ensures that virtually all pairs dissociated into free carriers [389]. The photoconductivity action spectrum of sexithiophene is complementary to the luminescence excitation spectrum, indicating that electron hole pair creation is the dominant channel of non-radiative decay [-369]. In photoexcitation dynamics, there is a fast component due to the formation of neutral bipolarons from free electron hole pairs and a slow component due to the non-radiative decay of bipolarons [390]. On C60 doping, the photoconductivity of PATs (e.g., POT) is enhanced and the decay time of photoconduction becomes shorter [391,392]. The photocurrent response of poly(3-ethylmercaptothiophene) is quite different from the behavior of similar conducting polymers. The long-living decay of the photocurrent makes this a potentially useful substance for storing light-induced charges [393]. The photocurrent of sexithiophene [394] and PMT [395] depends on the light intensity and the wavelength. Bleaching decay in several PT derivatives is observed by picosecond and femtosecond pump-and-probe experiments. The transient absorption data in poly(3-alkylthienylene vinylene)s and alkyl-substituted poly[1,2-di(2thienyl)ethylene]s seem to be consistent with the formation and decay of self-trapped excitons [396-398]. The initial part of the decay in poly(3pentoxythiophene) and poly(3-pentoxythienylene vinylene) is consistent with self-trapped exciton relaxation processes [399, 400]. The bleaching decays are independent of the size and location of the side groups, while they are sensitive to backbone rigidity [396, 397]. Disorders in the films of PTs which take the form of distributions of conjugation length, the nature of the photoexcited species, and their degrees of confinement must play important roles in photoinduced absorption dynamics [-401]. A complete and fast absorption recovery after photoexcitation of poly(3,4-dibutylthienylene vinylene) occurs in solution, while a long-lived component is observed in the film [402]. Nonlinear optical properties of PTs which exhibit ultrafast responses and large nonlinearities attributed to one-dimensionality and delocalization of reelectrons along the polymer chains are also described [403, 404]. Poly(4,4'dipentoxy-2,2'-bithiophene) and poly(4,4"-dipentoxy-2,2';5',2"-terthiophene) show a fast and high third-order nonlinearity [405]. Third-order nonlinearities depend on the nature of the polymer backbone and only slightly on the substituents [406]. The optical transparency and the third-order optical nonlinearities can be tailored in random copolymers of 3-methylthiophene and methyl methacrylate [407]. A solution-processable thiophene copolymer with a side

76

G. Schopf and G, Kogmehl

chain nonlinear optical chromophore (3-octylthiophene/N-(3-thienyl)-4-amino2-nitrophenol has been chemically synthesized. The copolymer was further functionalized with a photo-cross-linkable cinnamoyl group and was doped with a photo-cross-linkable nonlinear optical dye. The poled, photo-crosslinked, dye-doped copolymer exhibits relatively large second-order nonlinear optical effects [408]. A simple model describes the linear and nonlinear optical properties of conjugated polymers [e.g., oligo(thiophene)s] found from the measurements of the optical band gap of two oligomers of different length, without the need for supercomputing power [409]. Photocatalytic CO2 fixation is observed in photoirradiation experiments on a PAT film immersed in ethanol solution containing 4-butylphenol in a CO2 atmosphere [-367, 368]. An electric field induces changes in the optical properties of PATs and poly(thienylene vinylene)s. The induced absorption is proportional to the square of the applied electric field and its line shape is field invariant [410].

3.6 Magnetic Properties The magnetic properties of FeC13-doped PATs (n = 6, 8, 10) and their "head-tohead" and "tail-to-tail" coupled analogues [ poly(4,4'-dialkyl-2,2'-bithiophene)sl were investigated over a wide temperature range (4.2 273 K). A classical dependence of the static magnetic susceptibility on temperature can be found at high temperatures. On the other hand, a different magnetic effect appears at low temperatures [-411,412]. Because of the higher mobility of the charge carriers at high temperatures, the doped polymer behaves like a paramagnetic material. Transition from a paramagnetic phase to an ordered phase occurs as the temperature is lowered [413]. Copolymers of thiophene/3-methylthiophene and of poly(pyrrole)/phenylene oxide doped with FeC13 are the first examples of conducting polymers with magnetic ordering at room temperature [414].

3.7 Langmuir-Blodgett Thin Films The Langmuir-Blodgett (LB) technique is one of the few means of preparation of highly ordered systems with molecular architecture and thickness that are controllable at the molecular level [415, 416]. It is possible to produce highquality multilayer LB films from a variety of soluble PATs (n = 4, 6, 8, 18) by simply dispersing the polymer with suitable proportions of stearic acids [415,417~19]. Oxidation of the multilayers creates electrically conductive LB films (up to 2 S cm- 1), the level of conductivity being strongly influenced by the type of dopant used and the manner in which it is introduced into the film (cf. Sects. 3.2.3 and 4.3) [270, 417, 418], the PAT content of the film [415], and the alkyl side chain length of the PT backbone (cf. Sect. 4.1.2) [415]. The


77

modification of the side chain of PATs by introducing an oxygen atom in different positions on the alkyl side chain has the advantage that the backbone is unperturbed by the change in chemical structure, and the polymers obtained are useful for the preparation of multilayers [420]. In contrast to poly[3-(pentadecafluorooctyloxy)-4-methylthiophene] and poly(3'-perfluorohexyl-2,2';5',2"-terthiophene), poly[(4,4,5,5,6,6,7,7,8,8,9,9,9tridecafluorononyt)thiophene] is soluble in octafluorotoluene and, therefore, is available for LB experiments. Stable monolayers of poly[3(4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononyl)thiophene] are obtained at an air-water interface. The presence of fluorinated substituents improves the stability of these monolayers compared with that of their adequate alkyl analogues [333,421]. An LB film of poly[3-(bromooctyl)thiophene-co-3-(vinylhexyl)thiophene] and stearic acid reaches an electrical conductivity of 2 S cm- 1 on doping with NOPF6 an FeC13 [422]. In neutral LB thin films of PHT, the electrical conductivity is best described by variable-range hopping, whereas for the NOPF6 doped and dedoped states the model of charging energy-limited tunneling between conducting islands is applicable [423]. LB multilayer thin films can also be formed from nickel-tetra-tert-butylphthalocyanine, iron-tetra-tert-butylphthalocyanine, and mixtures of these phthalocyanines with PHT. The phthalocyanine molecules serve as both a surface active agent to promote spreading and an electroactive material [424, 425]. Another LB multilayer thin film is produced from mixed monolayers containing PHT and 3-octadecanoylpyrrole [426, 427]. PHT/arachidic acid and quinquithiophene/arachidic acid LB films are used in thin film field-effect transistors (cf. Sect. 6.6) [428]. Some excellent LB films contain sexithiophene and 9,10-diphenylanthracene with arachidic acid [383]. Molecular structure devices can be composed of LB films of PHT and poly(pyrrole) [429]. Another type of conducting LB film contains PT salts [431], polyion complexes of acidfunctionalized PT, and sulfonated poly(aniline) [431] or poly(heptadecyl 3thienylacetate) [365]. Gases like CO2, CO, and NO2 increase and NO decreases the electrical conductivity of LB films of PHDT and stearic acid [432]. LB films can also be formed by PATs (n = 10, 12, 16, 18) and docosanoic acid. The electrical conductivity of a PDT/docosanoic acid LB film increases dramatically on exposure to nitrogen oxides, but subsequent exposure to air results in decreasing the electrical conductivity to almost the initial value. However, this effect is only partially reversible. The action of nitrogen oxides also changes the absorption spectra. The exposure of the film to SO2 affects the electrical conductivity only insignificantly [433]. Both heat and v-irradiation lead to some structural surface layer changes [434,435]. The temperature-dependent structure of LB films fabricated from mixtures of PHT and stearic acid is also described [436]. A mixed film of 3-thienylpentadecanoic acid and distearylviologene prepared by the LB technique forms stable monolayers at the liquid-gas interface [437].

78


3.8 Behavior of Interfaces, Bilayers Rectifying contacts between aluminum, indium, or titanium and a solution- or a melt-processabte conducting polymer (e.g. POT) have been formed by melt processing. Comparison of the current voltage and capacitance-voltage characteristics of a melt-processed device and a device manufactured by metal sputtering shows no significant differences between these structures [438]. The interface of a p-n heterojunction consisting of PMT and n-type Si substrates has been studied. The rectifying behavior is induced by covalent bond formation between PMT and n-Si; the number of covalent bonds is enhanced by sandblasting the Si substrate [439]. Aluminum interacts strongly with PT chains in an interface between aluminum and PT derivatives (cx-sexithiophene and POT). The A1 atoms in the surface bind covalently to certain cx carbon atoms of thiophene systems and modify the electron density on these atoms and on the neighboring sulfur atoms. The electron transfer from A1 to the PT chains is fundamentally different from the classical doping process. The metal/conjugated polymer interactions depend not only on the nature of the metal (metallization contrasts with the doping of a conjugated polymer with an alkali metal), but also on the nature of the conjugated polymer, e.g., the aluminum/PT interactions are different from the aluminum/poly(acetylene) interactions [440]. The orientation of the polymer layers of metal/PMT interfaces changes during the doping process (cf. Sect. 3.2.1) [238]. Oxidation of conjugated polymer films using solutions of AuC13 in nitromethane results in electroless deposition of zerovalent gold in addition to the formation of AuC13 doped polymer (cf. Sect. 3.2.3). Thick films afford growth of a homogeneous and continuous metallic layer on top of the polymer film. The conductivities of the metal/polymer bilayers are as high as 11000 S cm- 1 [299]. PBT films are electrochemically deposited on n-titanium dioxide prepared by electrochemical oxidation of titanium. The rectifying behavior of this bilayer is studied by means of cyclic voltammetry, the electrochemical quartz crystal microbalance, and photoelectrochemical investigations to observe the photovoltaic effect [441,442]. The electrochemical behavior of metal/PT/electrolyte (wet case), of metal/PT/metal (dry case) and of a POT/poly(pyrrole) bilayer can be studied by cyclic voltammetry and impedance spectroscopy [443, 444]. The charge trapping under transient conditions in PBT/polyxylylviologene and PT/polyxylylviologene bilayers can be studied by in situ visible transmission cyclic voltammetry [445, 446]. Electrochemically driven charge trapping and untrapping reactions, current/potential characteristics, and a schematic representation of energy levels for a PBT/poly(xytylviologene)bilayers are described in [447, 448]. In a PODT/C6o junction device, a photo-induced charge transfer between PODT and C6o is observed. The charge transfer influences the currentvoltage characteristics, which depend on the wavelength of the illumination light [449]. Biotinylated copolymers of 3-hexylthiophene/3-thienylmethanol and of 3undecylthiophene/3-thienylmethanol form stable monolayers at the air-water


79

interface owing to the polar groups along the polymer backbone, and they can be used in opto-electronic signal transduction for optical displays, color mimicking and biosensors (see Sect. 6.9) [450, 451]. Stable monolayers of polyE3-(4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononyl)thiophene] at an air-water interface [-333, 421] are described in Sect. 3.7.

3.9 Gels Many derivatives of PT are soluble and fusible, but several derivatives (e.g. POT) are not always 100% soluble in organic solvents such as chloroform, THF and anisole. The solubility decreases as a function of time. The insoluble part of the polymer forms a gel which swells in different solvents [452]. Conducting gels of POT are prepared by thermoreversible gelation from semidilute solutions in decahydronaphthalene (decalin) and subsequent doping with iodine [453]. Soluble PATs can be transformed into a conducting polymer gel by contacting the polymer with solvents which cannot dissolve the polymer (chloroform) [454], by using a radical initiator (benzoyl peroxide) [455, 456], or by y-radiation [457] and subsequent iodine doping of the gel. The volume of the sample in chloroform shrinks drastically on introduction of ethanol, on changing temperature and also on doping [455-459]. The effect of doping on volume shrinkage is explained in terms of enhancement of the interchain interaction by dopants [460]. The shrinkage ratio depends on the preparation conditions of the sample (e.g., concentration of benzoyl peroxide). The time for the volume change of a PAT gel depends on the solvent and on the size of the PAT [459]. The characteristic time for expanding from the shrunken (dry) state increases with increasing thickness and length. The expansion characteristic occurs in two steps: (i) The solvent diffuses into the gel from the surface. (ii) The gel network diffuses into the solution and the gel expands [455458, 461]. The visible spectrum of the gel changes reversibly as the volume varies [455, 460]: the photoluminescence of a PAT gel is enhanced compared with that in the shrunken state [3661. These characteristics are interpreted in terms of change of the interchain interaction due to the change of planarity of thiophene rings triggered by the trans-gauche conformation transition of alkyl side chain and resulting in a steric hindrance effect between the sulfur atom and the alkyl side chain [460]. An electrically controllable gel can have various uses, e.g. in actuators, artificial muscles, bimorphs (cf. Sect. 6.12) and optical recording materials [460, 461].

3.10 Liquid Crystalline Compounds Compounds with two mesogenic groups with and without terminal alkyl chains containing thiophene rings (cf. Sect. 1.2) are described [8-10, 462]. The liquid crystalline behavior of the various compounds is dependent on the length of the

80


flexible inner chain [9]. For example, all E-~,o~-bis{5-[4-(4-styryl)styryl]-2thienyl}alkanes (cf. Sect. 1.2) show increasingly highly ordered mesophases with increasing inner chain length [8]. Furthermore, the liquid crystallinity of thiophene compounds is promoted by attaching a dipole or polarizing group at the end thiophene moieties or by connecting two aromatic units through an alkylene spacer to give a compound with two identical mesogenic units [462]. Liquid crystalline compounds based on 2-arylthiophene and 2-(biphenyl-4-yl)thiophene are described in the literature [463].

3.11 Wettability During the doping process (the oxidation reaction), the chemical structure of a PBT layer is changed. Neutral PBT is a typical non-polar aromatic material and has a hydrophobic surface which is wettable by aromatic solvents but not by water. Doped (oxidized) PBT is polar and has a hydrophilic surface which is wettable by water. These wettability changes may be performed reversibly between the two forms of a PBT layer several times (up to 400) and may be used in imaging systems (cf. Sect. 6.4) [262, 263]. The wettability can be studied by measuring the contact angle. The hydrophilic oxidized form of PBT has a contact angle with water as low as 0°; the non-polar hydrophobic neutral form has a contact angle with water of up to ca. 180°. Intermediate states of oxidation are available during the electrochemically conducted redox process resulting in materials with contact angles between 0 ° and 180° [217, 261]. It is possible to change the degree of oxidation by controlling the potential of the polymer electrode [217]. The morphology has also an important effect on the wettability and is affected by the current density (cf. Sect. 5.2.2), concentration (cf. Sect. 5.2.2) and thickness (cf. Sect. 4.3) of the PBT layer. Smooth films have less distinct changes in wettability. The optimum range (for a contact angle change 180°/0 °) in current density is 0.5-2 mA cm -2 if the monomer concentration is 0.1 M [261]. The change of wettability increases with increasing film thickness [261]. A rough film surface is essential for an optimum change in wettability: rough platinated platinum electrodes with thin PBT films have similar contact angles to those of thick films of PBT on smooth platinum electrodes [189, 261].

4 Influence on Properties

4.1 Influences of Structure on Properties 4.1.1 Influence of Molecular Weight The conjugation length of oligo(thiophene) increases with the length of the re-electron system [84]. The absorption maxima of oligo(thiophene)s in CHC13

80


flexible inner chain [9]. For example, all E-~,o~-bis{5-[4-(4-styryl)styryl]-2thienyl}alkanes (cf. Sect. 1.2) show increasingly highly ordered mesophases with increasing inner chain length [8]. Furthermore, the liquid crystallinity of thiophene compounds is promoted by attaching a dipole or polarizing group at the end thiophene moieties or by connecting two aromatic units through an alkylene spacer to give a compound with two identical mesogenic units [462]. Liquid crystalline compounds based on 2-arylthiophene and 2-(biphenyl-4-yl)thiophene are described in the literature [463].

3.11 Wettability During the doping process (the oxidation reaction), the chemical structure of a PBT layer is changed. Neutral PBT is a typical non-polar aromatic material and has a hydrophobic surface which is wettable by aromatic solvents but not by water. Doped (oxidized) PBT is polar and has a hydrophilic surface which is wettable by water. These wettability changes may be performed reversibly between the two forms of a PBT layer several times (up to 400) and may be used in imaging systems (cf. Sect. 6.4) [262, 263]. The wettability can be studied by measuring the contact angle. The hydrophilic oxidized form of PBT has a contact angle with water as low as 0°; the non-polar hydrophobic neutral form has a contact angle with water of up to ca. 180°. Intermediate states of oxidation are available during the electrochemically conducted redox process resulting in materials with contact angles between 0 ° and 180° [217, 261]. It is possible to change the degree of oxidation by controlling the potential of the polymer electrode [217]. The morphology has also an important effect on the wettability and is affected by the current density (cf. Sect. 5.2.2), concentration (cf. Sect. 5.2.2) and thickness (cf. Sect. 4.3) of the PBT layer. Smooth films have less distinct changes in wettability. The optimum range (for a contact angle change 180°/0 °) in current density is 0.5-2 mA cm -2 if the monomer concentration is 0.1 M [261]. The change of wettability increases with increasing film thickness [261]. A rough film surface is essential for an optimum change in wettability: rough platinated platinum electrodes with thin PBT films have similar contact angles to those of thick films of PBT on smooth platinum electrodes [189, 261].

4 Influence on Properties

4.1 Influences of Structure on Properties 4.1.1 Influence of Molecular Weight The conjugation length of oligo(thiophene) increases with the length of the re-electron system [84]. The absorption maxima of oligo(thiophene)s in CHC13


81

and of PHT and POT films are shifted to higher values of the wavelength [85, 464, 465], and an inverse correlation is obtained between the transition energies and the chain length; this reflects increasing electronic delocalization with increasing number of thiophene rings [466]. The shift is larger in PHT films with relatively low molecular weight ( < 25 000) than in films with high molecular weight [464]. The absorption maxima of ethyl-substituted oligo(2,5-thienylene ethynylene)s increase with increasing number of units in the oligomer, the shift being larger for oligomers with a small number of units (4-7) than for oligomers with a large number of units (10-16) [467, 468]. The increasing conjugation length with increasing molecular weight results in a decrease of the oxidation potential, whereas the reactivity of the chemically formed radical cations decreases because of stabilization by resonance [60, 464]. Cyclic voltammograms of PHT films show that the oxidation potential and the reduction potential decrease with increasing molecular weight. The decrease is more prominent for PHT with relatively low molecular weight ( < 25 000) [464]. The electrical conductivity of PMT and PBuT as well as the redox potential decrease because of an increase in the main chain conjugation length [469]. The dopant concentration increases with decreasing oxidation potential and therefore with increasing molecular weight [464]. The electrical conductivity increases with the conjugation length [85, 470]. For PHT films with molecular weight < 25 000 and constant dopant concentration, the electrical conductivities increases from 10-~ to 101 S cm-1. For molecular weight > 25 000, the increase in conductivity is minor [464]. The increasing electrical conductivity (from 1.9 to 10.4 Scm -~) with increasing molecular weight (from 22000 to 130 000) of a doped POT film is interpreted in terms of an increase in the radius of a localized wave function with the molecular weight of POT [465]. The electrical conductivity increases with the square of the elongation ratio for high molecular weight POT [471]. The electrical conductivities of iodine-doped poly(2,5-thienylene ethynylene) are nearly independent of the molecular weight of the sample. This result is consistent with the results obtained for poly(acetylene) [280]. The electrical conductivity of an iodine doped undecathiophene (11 thiophene units) with three substituted alkyl group is of the same order of magnitude as that for different samples of doped PTs. Hence, the effective conjugation length of doped PTs is not much greater than 11 units [85]. The relative stabilities of the doped oligo(thiophene)s depend on the oxidation level (i.e. the doping level) [177]. Wide-angle X-ray analyses of PHT films show that the increase in molecular weight results in an increase in the regularity of the supermolecular structure, which enhances the development of a n-conjugated system [464]. A series of fractionated samples of POT has no significant molecular weight dependence of the electrical conductivity within the range 30000 _< molecular weight _< 400000, indicating that interchain transport does not limit the macroscopic conductivity of these samples. Small differences in the degree of crystallinity, however, have an effect on the electrical conductivity [472]. For oligo- and poly(phenylene)s it was found that at low doping levels the conductivity of

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oligo(phenylene) is better than that of poly(phenylene). However, at high doping level, the situation is reversed. This may be explained in terms of two different conductivity mechanisms, hopping between chains and chain conductivity respectively [473]. A strong correlation was found between field-effect mobility in thin-film transistors and chain length of the oligo(thiophene)s used (number of thiophene units between 3 and 8). The increase in mobility with increasing chain length (up to sexithiophene) is explained by the longer lifetime of the oxidized (doped) form of longer oligomers; the decrease in mobility in the case of octithiophene is attributed to an increase in structural defects [474-477]. The melting points increase with increasing length of conjugated oligo(thiophene)s. The solubility in CHC13 follows the opposite pattern. These trends are explained by the increase in polarizability with increasing length [84]. Oligo(thiophene)s with 2, 3 and 4 thiophene units in solution were studied by femtosecond spectroscopy. A fast emergence and red shift of a transient absorption band within one picosecond was observed. The band position is sizedependent, but band shift and broadening seem to be size independent due to interactions of the excited molecules with the solvent [-478]. The two-photon absorption coefficient rises with oligo(thiophene) chain length and the real part of third-order optical nonlinear susceptibility increases with increasing chain length [-479]. The fluorescence lifetime and the fluorescence quantum yield of oligo(thiophene)s (2 to 6 thienyl units) increase with increasing ring number, mainly caused by a decrease of nonradiative decay [360]. The effect of end substituents of oligo(thiophene)s (2-6 thiophene units) on electrochemical and optical properties is limited to short oligomeric chains [165, 480].

4.1.2 Influence of Alkyl Side Chains The effect of the alkyl side chain length on the physical and chemical properties of the PATs has been studied intensively by many authors and the results can be summarized as follows. With increasing alkyl side chain length: 1. The oxidation potential (Eox) of PAT films increases (Table 11) [481], 2. The reduction potential (Ered) of PAT films also increases (Table 11) [481], and 3. The values for the electrical conductivity of the PAT films (n = 1, 4, 6, 8, 12, 18) decrease (Table 11) [481]. These characteristics are similar to those of the N-alkyl substituted poly(pyrrole)s [-482]. The reason for these changes is the steric effect of the side chain. This leads to the changes in the interaction, the variation of the symmetry of the polymer chain, and the disturbance of the ring-ring planarity along the polymer chain [481]. The decrease in electrical conductivity with increasing interchain distance can also be observed by reducing the pressure [483, 484].

Polythiophenes

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Table ll. Oxidation potential (Eox), reduction potential (Er,o), value of the electrical conductivity, spin concentration (N~), and peak-to-peak linewidth (AHpp) of several PATs [481]. PAT (n)

Eox (V)

Ered (V)

Conductivity (S cm- 1)

N~ (g-1)

AHpv (Gauss)

6 8 12 18

3.97 4.00 4.03 4.15

3.89 3.92 3.98 4.00

29.4 26.9 18.5 6.6

2.09 × 1019

7.34

9.61 x 1018 8.77 x 1018

8.23 8.45

However, electrochemically polymerized 3-alkyl-2,2'-bithiophenes (alkyl = hexyl, dodecyl) give conducting films whose electrical conductivities are independent of the length of the alkyl chains [485-487]. With increasing alkyl side chain length: 4. The PAT films are stable at higher polymerization potential [481], 5. The values for the electrical conductivity of PAT film (n = 18) are lower for lower polymerization temperatures, in contrast to PAT films with shorter alkyl chain lengths (n = 6, 8, 12) where the conductivities increase as the polymerization temperature decreases [481], 6. The spin concentration (Ns) of the PAT films decreases (Table 11) [481], 7. The peak-to-peak line width (AHvv) of the PAT films increases (Table 11) [481]. Results 6 and 7 are in agreement with result 3, suggesting that the effective conjugation length in PAT decreases with increasing alkyl chain length [481]. In contrast to this result, the literature [488] describes an increase in the conjugation length of the thiophene skeletal chain as the alkyl side chain length increases (n = 0-12) due to enhanced intermolecular interactions between the longer alkyl groups. The thermochromic transition occurs more cooperatively in a lower and narrower temperature range for the longer alkyl groups. When the alkyl side chain length increases further up to n = 22, the conjugation length at low temperature becomes as short as those of the high temperature phases of the other members. In the temperature range close to the melting point, the conjugation length increases to some extent [488]. With increasing alkyl side chain length: 8. The environmental and thermal stability of the PATs decrease (n = 1, 4, 6, 8) [296, 489, 490], the thermal dedoping process of doped PAT (n = 4, 8) is faster [491], the generation of charge carriers in PAT (n = 6, 8, 10) is more difficult [492], and the field-effect mobilities in PATs (n = 4, 6, 8, 10, 12) [493, 494] decrease. No simple reaction kinetic was found to describe the thermal dedoping process. It can be described using a master-plot technique. This master plot can be fitted with a Williams Landell-Ferry type equation [491]. A reduction of the number

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of the alkyl side chains leads to more space for the counter ions on doping, a planar re-system and rigid polymer backbone. For instance, poly(3'-octyl2,2';5',2"-terthiophene) and poly(3-octyl-2,2'-bithiophene) show better stability towards thermal dedoping than POT [495], and poly(3-dodecyl-2,2'-bithiophene) is more stable in doped states than PDDT [485]. In order to suppress the interactions due to steric hindrance caused by the long flexible side chains which twist the conjugated main chain and eject the dopants, it is necessary to separate the side chains from each other [496], to change the geometry and chemistry of the side chains [491], to restrict the polymer chain mobility via cross-linking of the side groups [490], or to modify the main chain in such a way that the torsion of the main chain is less deleterious to the doping state. This implies, most probably, a lower redox potentiai of PAT. For instance, copolymers of thiophene and pyrrole [491], random copolymers of 3-methylthiophene and 3-octylthiophene, regular copolymers of thiophene and 3-octylthiophene and poly[3-(4octylphenyl)thiophene] [496, 497] or a semiinterpenetrating polymer network of POT, polystyrene and a cross-linking agent [498] would be attractive for reducing the redox potential. The generation of charge carriers seems to be most easy in PAT (n = 6, 8, 10) with short side chains [492]. With increasing alkyl side chain length: 9. The PATs (n = 6, 12, 22) show stronger luminescence, an increasing quantum yield, and enhanced temperature dependence of the luminescence intensity [362, 363, 499]. These characteristics are explained in terms of trans-gauche conformational changes resulting in increased effective conjugation length and larger band gaps: the decreased interchain transfer probability of excited species in PAT with longer alkyl side chain can then explain the stronger luminescence [363]. The luminescence intensity in poly(3-alkylfuran) is not strongly dependent on the alkyl side chain length. It is explained in terms of the small steric hindrance between the O atom and the alkyl side chain because the size of an O atom is relatively small compared to that of the S atom [363]. With increasing alkyl side chain length and/or increasing number of alkyl side chains: 10. The melting temperature of PATs decreases [84], 11. The solubility of PATs increases [84]. Similar solubility properties are also observed in poly[1,2-bis(3-alkyl-2thienyl)ethylene] films, the unsubstituted and the methyl substituted polymers being insoluble. The introduction of longer alkyl side chains (n = 4, 8) imparts solubility to the polymer [147]. With increasing alkyl side chain length and/or increasing number of alkyl side chains: 12. The temperature at which the electrical conductivity reaches a maximum decreases [500],


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13. The learning effect (see Sect. 3.2.1) occurs more slowly [233], 14. The gel formation ratio due to radiation increases [-457, 461], and 15. The molecular weight of a PAT/PAT (e.g. PMT/PBuT, PMT/PHT, PBuT/ PHT, PHT/POT) copolymer increases [-286]. With increasing alkyl or oxyalkyl side chain length: 16. Conjugated ~-sexithiophenes show differences in electrochemical behavior, although the electronic inductive effects of these substituents are comparable. Long alkyl or oxyalkyl side chains lead to an enhancement of electrochemical reversibility and stabilization of the cation radical state [501]. 17. The molecular organization and the electrical and optical properties of Langmuir Blodgett multilayer thin films fabricated from mixed monolayers containing stearic acid and various PATs depend on the alkyl side chain length of the PAT backbone [415]. Monolayers from PAT with short side chains (n < 8) undergo a phase transition at elevated surface pressures in which the polymer molecules are rejected from the mixed two-phase monolayer, and the stearate molecules thereby achieve their optimum packing geometry. The long alkyl chains of PODT (n = 18) encourage stronger interactions between the two phases and promote a higher level of molecular mixing in the monolayer [415]. 18. The structural evolution during iodine vapor doping of PAT is sensitive to the alkyl side chain length. PATs with n = 6 and 8 undergo continuous changes within a single "homogeneously doped" structure, while PAT with n = 12 exhibits a region of two-phase coexistence [-268]. These results exhibit a strong dependency of properties on the alkyl side chain length, which can be explained by variation of the electronic band structure and interaction between the side chains influenced by the dimensions and coplanarity of the conjugated main chain, depending on the alkyl chain length and its conformation. In particular, an unique doping effect of C6o in PAT is observed [502, 503]. In contrast to some of the above-mentioned alkyl side chain length-properties relationship, the properties of regioregular H T - H T PATs show dependencies on the alkyl side chain different from those of regiorandom PATs. The electrical conductivity increases with increasing alkyl side chain length (n = 4, 6, 8, 12) when the PATs contain only H T - H T coupling [-99]. Values of electrical conductivity determined for I2 doped thin films with H T - H T coupling were as follows at room temperature [-275]: PDDT, 1000Scm-1; POT, 200 S cm- 1; PHT 60 S cm- 1. H T - H T PAT with long alkyl side chains (n = 12) exhibits the largest effective conjugation length due to the formation of a planar main chain. In regiorandom PAT, steric repulsions between the alkyl chains will be at a maximum when the alkyl chain is the longest, thereby decreasing conjugation between thiophene rings [95]. Furthermore, PDDT exhibits reversible

86


electrochemical behavior and the lowest oxidation potential in the alkyl series, followed by POT and PHT [99]. It is expected as the adjacent thiophene rings move towards coplanarity that the increase in molecular orbital overlap will help to stabilize both the polaronic and bipolaronic states, thus lowering the oxidation potentials [336]. Steric constraints in the packing of substituted oligo(thiophene)s are an another important factor. An all a,a'-linked undecathiophene (11 thiophene units,) with three tert-butyl groups at the thiophene units 4, 6 and 8 shows a greater influence of the butyl groups on the melting temperature and solubility than the corresponding undecathiophene with tert-butyl groups at the thiophene units 2, 6 and 10. An explanation is that next-nearest neighboring chains cause additional twisting of the main chain, tert-Butyl end groups seem to have only a minor influence [84]. 4.1.3 Influence of Coupling Type-Regioregular PAT The properties of PAT correlate with the degree of regioregularity and the type of coupling between substituted thiophene rings. Polymers with the largest percentage of H T - H T coupling exhibit 1. A higher degree of long-range order, 2. Higher electrical conductivities, and 3. Longer wavelengths of maximum absorbance [97, 99, 100, 275]. The shift to lower energy of the absorption maximum of H T - H T coupled PATs is up to 14 nm in solution, 46 nm in the solid state, and other intensive lower energy peaks with shifts of up to 129 nm (609 nm) from the normal PATs [95]. A blue shift of the absorption maximum with increasing coplanar block length is observed in copolymers of thiophene and HH coupled 3-octylthiophene, which interrupts the coplanarity of the polymer backbone [504]. In PHT and in copolymers of thiophene/HH coupled 3-octylthiophene it was found that, with increasing HT HT coupling content, 4. The maximum emission is shifted to longer wavelengths [-100, 504]. The repulsive forces due to the steric hindrance in HH coupling are partially relieved in the excited state. With increasing H T - H T coupling content in PHT: 5. The Stokes shifts decrease [100]. 6. The quantum yields of fluorescence from PHT solutions increase [100] owing to a larger conjugation length, higher rigidity, and consequently a decrease in non-radiative, torsional mode shunt processes (intramolecular decay manifolds) [100]. With increasing H T - H T coupling content in PHT: 7. The quantum yields of thin films of PHT decrease, in contrast to solutions (result 6) [100].


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The low quantum yields in planar conjugated PHT can be explained by classical concentration quenching effects which arise from non-emissive excimer complexes (intermolecular decay channels) [100]. With high HT-HT-coupling content: 8. Polymers have the smallest band gap (1.7 eV), which is 0.3-0.5 eV lower than that of regiorandom PAT [505], and 9. Polymers have the highest intrinsic conductivity (10 -6 Scm-1 for neutral PAT), which is three orders of magnitude higher than that of regiorandom PAT [505]. A bipolaron gap state is generated in an I2-doped regioregular (HT-HT coupling) PAT film by chemical oxidation, but fails to be generated for the regiorandom PAT [505]. The influence of side group distribution on the properties of substituted PBTs has been studied [506, 507]. Poly(4,4'-dibutyl-2,2'-bithiophene) and poly(3,3'-dimethoxy-2,2'-bithiophene) correspond in their chain geometry to head-to-head and tail-to-tail coupled 3-substituted PTs. In cyclic voltammetry both polymers exhibit much narrower oxidation peaks as compared to the corresponding head-to-tail coupled PATs and poly(3-alkyloxythiophenes). Poly(dialkyloxy-2,2'-bithiophene)s seem to be more stereoregular compared to poly(3-alkyloxythiophene)s [506, 507].

4.1.4 Influence of Stretching - Crystallinity and Morphology The influence of the main chain length, of substitution by alkyl groups on the thiophene rings, and of coupling type between the thiophene units on the crystallinity and morphology are described in Sects. 2.2 and 2.3, respectively. Different cyrstallinities and morphologies affect different properties. No significant molecular weight dependence of the electrical conductivity can be found for POT, but small differences in the degree of crystallinity have a profound effect on the electrical conductivities 1-472]. PHT initially produced as amorphous material and PHT blends with POT exhibit an increase in the electrical conductivity in the stretch direction after alignment of the polymer chains upon stretching [508, 509]. Drawing of POT (unstretched 5 S cm -a) yields partially oriented films and fibers reaching an electrical conductivity of 20Scm -1 (doped with FeCI3) [510] and of 180Scm -1 (doped with FeCla'6HzO) [511], and a significant enhancement of the stiffness during I2-doping is found [511 ]. The electrical conductivity of undrawn poly(thienylene vinylene) fibers is 80 S cm- 1 and increases with the draw ratio up to a maximum conductivity of 2000 S cm -1 (doped with I2) [278, 279, 512]. The electrical conductivity of the neutral drawn films is different from that of the doped drawn film [513]: The electrical conductivity of iodine-doped PT films increases along the drawing direction, and decreases perpendicular to the drawing direction.

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In neutral films the electrical conductivity increases with increasing drawing ratio for both directions [513]. PMT fibrils in which the polymer chains in the narrowest template-synthesized fibrils are preferentially oriented parallel to the axes of these fibrils show higher electrical conductivities (as high as 6600 S cm -1) than the conventional versions [514, 515]. Poly(thiophene-cooctylthiophene) shows an increase in the electrical conductivity when polymerized in microporous membranes owing to the increased ordering [516]. Furthermore, an amorphous polymer is generally easier to dope than a densely packed crystalline polymer, and the degree of crystallinity decreases as the doping level increases [517]. The quantum yield of photoluminescence of amorphous PAT (n = 6, 12) films is higher than that of semicrystalline films [362]. A copolymer from thiophene and benzene has a similar crystalline structure to that of poly-p-phenylene, but the lower crystallinity ratio of the copolymer allows an easier penetration of iodine into the bulk material. Therefore, the increase of the electrical conductivity after iodine doping (cf. Sect. 3.2.3) is higher for the copolymer [281]. The morphology of a PBT layer has also an important effect on the wettability (cf. Sect. 3.11) [217, 261,518].

4.2 Influence of Starting Materials The properties of the products depend on the starting materials used and on the composition of copolymers and blends (cf. also Sect. 4.1). Doped poly[1,2-di(2-thienyl)ethylene] and poly[1,4-di(2-thienyl)-l,3-butadiene] are more stable than doped PT [231,519]. The polymerization of monomers with a polyfluoroalkyloxy substituent in position 3 leads in the case of a short chain (poly[3-(2,2,2-trifluoroethoxy)thiophene]) to a soluble material with an electrical conductivity of 2 x 10-1 to 10- 2 S cm- 1, and, in the case of a long chain (poly[3-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyloxy)thiophene]), to a polymer with high doping level and stability and an electrical conductivity of 1 to 10 S cm -1 [520]. In poly[3-(alkyloxyphenyl)thiophene] derivatives, the chemical structure, the nature, and the position of the alkyloxy chain on the phenyl ring have a strong influence on the linear optical properties and on the solubility [521]. The electrical conductivity of poly(3-cyclohexylthiophene) (1.7x 10 -3 Scm-1/C102 and 1.0x 10 . 3 Scm-1/PF6) is about three orders of magnitude lower than the electrical conductivity of PHT, and the stability of poly(3-cyclohexylthiophene) is reduced because of the steric hindrance produced by the cyclohexyl group [93, 522]. A series of terthiophenes substituted with an aryl group at the central thiophene ring (3' position) were polymerized, and the properties of the monomers and the polymers were studied [523, 524]. An improvement of the properties compared with the unsubstituted PTT is achieved. The best results are obtained with electron-withdrawing substituents, especially with a 4-cyanophenyl substituent [524]. The redox potentials of PBT derivatives decrease in the order poly(3,3'-dibromo-2,2'-bithiophene) >poly(3,3'-dichloro-2,2'-


89

bithiophene) > PBT, in accordance with the inductive effect of the halogen substituents [525]. The oxidation potential of poly(4,4'-dibutoxy-2,2'-bithiophene) ( + 0.05 V vs SCE), one of the lowest values for a PT derivative, allows the formation of a stable conducting state in the polymer [526]. The doped state of poly(3-butoxy-4-methylthiophene) is also more stable than that of PATs resulting from the decrease of the oxidation potential due to the electron-donating alkyloxy group [126]. End-substituted terthiophenes and quaterthiophenes show a correlation between the redox potential and the Hammett-Brown constants of the substituents [165, 166]. Poly(3halothiophene) does not follow the correlation between the oxidation potential and the Hammett-Brown constants of substituents [527]. Poly(3-fluorothiophene) differs considerably in this respect from its halogen analogs [527]. Poly(4,4'-dibutoxy-2,2'-bithiophene) and poly(3,3'-dibutoxy-2-2'-bithiophene) are only partially soluble in common organic solvents, in contrast to asymmetrically disubstituted PBTs like poly(3-butoxy-3'-decyl-2,2'-bithiophene) and poly(4-butoxy-4'-decyl-2,2'-bithiophene), which are completely soluble in chloroform. The properties of these asymmetrically disubstituted PBTs are intermediate between those of PAT and poly(dialkyloxy-2,2'-bithiophene) [526, 528]. Poly(3-styrylthiophene) films deposited on the anode surface by electropolymerization have conductivites of the order of 10-6 S cm- 1 and are redox inactive [529]. Periodic copolymers of thiophene and furan have electronic properties intermediate between those of PT and poly(furan). Random copolymerization is expected to lead rather quickly to the saturated electronic properties characteristic of the lower band gap component of PT and largely independent of the larger gap component poly(furan). A higher percentage of thiophene is necessary for not only better intrinsic electrical conductivity of the copolymer, but also for achieving effective n-type and p-type doping of the copolymer (cf. also Sect. 3.2.1) [252]. The electrical conductivity of the copolymer poly(p-phenylene vinylene-co-2,5-thienylene vinylene) is dependent on the size of the block of contiguous poly(thienylene vinylene) repeating units, whereas the electrical conductivity of a poly(p-phenylene vinylene)/poly(thienylene vinylene) blend is linearly dependent on the thienylene vinylene content above a low concentration of thienylene vinylene. Below this threshold, the electrical conductivity is independent of the thienylene vinylene content [530]. In copolymers of 3-octylthiophene and 3-methylthiophene, the electrical conductivity of iodine-doped films increases from 13 S cm-1 up to 26 S cm-a with increasing 3-methylthiophene content, and shows a maximum at around 25 tool% 3-methylthiophene content [531]. A doped copolymer of 3-methylthiophene and methyl methacrylate exhibits an electrical conductivity of 6.5 Scm-a depending on the amount of incorporated conjugated 3-methylthiophene blocks. These copolymers are soluble, unlike the homopolymer of 3-methylthiophene which is an insoluble material [532, 533]. The electrochemcial polymerization product of 3-methoxyethoxythiophene is soluble in organic

90


solvents, but the solution is not stable in air. When a copolymerization is performed with thiophene, the copolymer is soluble and stable in air [534]. A blend of POT and a nonconductive polymer matrix material exhibits conductivities between those of the insulating matrix and 1 x 10 -2 S cm-1, as a function of polymer matrix and dopant concentration [535]. A composite of PMT and a nitrilic rubber has an electrical conductivity of one order of magnitude lower than that of the pure PMT, the cyclability being higher and the redox behavior being comparable to that of pure PMT [536, 537]. Polymer blends (e.g. POT/ultra-high molecular weight polyethylene) are improved with respect to the mechanical properties of the thiophene polymer and show decreased electrical conductivity [509, 538, 539]. Doped blends of POT and poly(phenylene oxide) show conductivities of up to 3 S cm-~. Addition of polyethyleneglycol to POT/polyethylene and to POT/poly(ethylene-co-butyl acrylate) decreases the size of POT domains and causes the electrical conductivity to increase by several orders of magnitude, indicating that polyethyleneglycol has a compatibilizing effect [540]. The addition of 1,6-bis(3-thienyl)hexane and 1,8-bis-(3-thienyl)octane to PMT decreases the electrical conductivity and increases the tensile strength [145].

4.3 Other Influences on Properties The properties depend on the following: 1. The dopant (cf. also Sect. 3.2.3). The electrical conductivity of FeC13-doped PAT is more stable than that of PAT doped with NOPF6, I2, and organic acids [189, 296, 490]. For FeC13 and acids the dedoping process is irreversible and degeneration of PATs occurs [ 189, 190]. The level of conductivity of multialyer Langmuir-Blodgett films is strongly influenced by the type of dopant used and the manner in which it is introduced into the film: doping with NOPF6, in comparison to SbC15 and Ia, [-418] and doping from solution phase [417] produce the most highly conducting thin films. Increasing the size of the counter anion leads to an increase in interchain distance in PATs and therefore to a decrease in the electrical conductivity [484]. In situ conductivity of n-type doped poly[1,2-di(2-thienyl)ethylene] decreases as the size of the dopant cation increases [245]. 2. The degree of doping/dedoping. The relative stabilities of the oxidized oligo(thiophene)s and PTs depend on the oxidation state (i.e. the doping level) [177, 541]. The dependence is weakest for FeC13-doped PAT above 10-3 S cm-1. Below that value the dedoping rate decreases with decreasing electrical conductivity, as is the case for organic acid anions. For the dopant Iz and the dopant anion PF6 the decay rate is proportional to the electrical conductivity [189]. The deterioration of the electrical conductivity with time depending on the doping level is also found for PAT with n = 12 [542]. PBT in the neutral (dedoped) form is stable up to 300 °C and

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91

can be stored over a long period of time, while increasing degree of oxidation (doping) lowers the thermal stability [217, 261,518]. 3. The temperature [324, 139]. The temperature dependence of the dedoping process follows the Arrhenius law [189]. The thermal dedoping process of doped PAT appears to be driven by a conformational change of the polymer backbone [490]. The electrical conductivities of PATs increase with increasing temperature and after reaching a maximum temperature (135 °C [298] and 104 °C [500] for n = 4; 72 °C [500] for n = 8; 28 °C [298] and 39 °C [500] for n = 12), they decrease sharply at higher temperatures [298, 343, 500]. In heavily doped PATs (n = 6, 8, 12) the d.c. conductivity reaches a maximum between 300 and 370 K (27 and 97 °C) [543]. For a blend of POT (n = 8) and PDDT (n = 12), two local conductivity maxima at 79 °C and 31 °C are observed [500]. The d.c. conductivity behaviour for a blend of POT and poly(ethylene-co-vinylacetate) is stable up to the melting point of poly(ethylene-co-vinylacetate); a spontaneous conductivity above the melting point is interpreted to be related to additional doping by dopants trapped inside the network of partially cross-linked POT [544]. After one cycle of heating of 150 °C and then cooling to room temperature, perchlorate-doped PBT does not undergo significant changes in the doping level, and the value of the electrical conductivity is about half or more of the original value. After 24 h exposure to 150 °C in air, PBT is partly decomposed [287]. The electrical conductivity, the intensity of photoluminescence, and the band gap of PDDT decrease and the crystallinity and the size of crystallites of PDDT increase at room temperature after heat treatment [364]. The electrical conductivity of PDDT, which decreases after a heating-cooling cycle, again increases to the original value in a period of several days, while EPR and the optical properties show no marked relaxation behavior. The increase in the electrical conductivity during the relaxation can be interpreted in terms of the increase in the probability of interchain hopping of charge carriers [545]. The low temperature results indicate that a metal-insulator (or semiconductor) transition takes place at 30 K for PT and 10 K for PMT [546]. The temperature dependence of the electrical conductivity corresponds to the threedimensional variable range hopping mechanism [485]. The temperature dependence of charge mobilities in PATs (n = 4, 6, 8, 12)exhibits a local maximum immediately after the end of the glass transition region, which can be attributed to the transition of soft conformons (conformational defects) in the disordered phase to localized conformons [-494]. 4. The sample thickness (sometimes). For the dopant Is and the dopant anion PF6 the decay rate is inversely proportional to the square of the sample thickness. No thickness dependence can be found in FeC13-doped PAT (50-300 pm thick pressed films) [189]. The change of wettability of a PBT film (cf. Sect. 3.11) increases with increasing film thickness, and depends on the flow of charge during the electropolymerization. The maximum change is reached for PBT layers formed by > 300 mC/cm 2 [217, 261].

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5. The humidity of the atmosphere. Increasing humidity of the surrounding atmosphere induces a dedoping reaction in POT, probably because of a nucleophilic attack of water molecules, and therefore a decrease of radical cationic centers; the electrical conductivity increases in dry air [-324, 547, 548]. 6. Ageing. The decay rate of the electrical conductivity of FeC13-doped PHT on ageing follows the Arrhenius law. Ageing enhances the temperature dependence of the electrical conductivity. Negative magnetoresistance, at weak fields, increases slightly on ageing, but, at strong fields, positive magnetoresistance remains unchanged [-549]. 7. Irradiation. A POT film (electrical conductivity of 1 S cm- 1) has an electrical conductivity of 10-s S cm-1 after irradiation with synchrotron radiation [-550]. The electrical conductivity of PT pellets decreases with increasing irradiation dose of both electron-beam and ,/-irradiation in vacuo. On the other hand, in the case of irradiation under a dopant atmosphere, the conductivity of the pellets is increased [551]. The conductivity mechanism of an FeC13-doped blend of POT and poly(ethylene-co-vinylacetate) changes when the material is cross-linked by electron irradiation [544, 552]. The oxidation wave of PDT shifts to higher potentials on electron beam irradiation; with polymers of acrylated thiophenes a similar but larger change occurs. It is caused by cross-links formed between the acrylate substituents which fix the main chain parts in twisted states and reduce the conjugation length [-553]. Pulsed irradiation with high energy electrons leads to long-lived conductivity transients in PATs (n = 6, 8, 10) [554]. The influence of ,/-radiation on iodine-doped PPT was studied. Doping accompanied with ,/-radiation decreases the iodine release rate [-555]. PHT degrades in non-aqueous solvents containing dissolved molecular oxygen after irradiation with UV or visible light. The degradation takes the form of both reduced re-conjugation and chain scission. The quantum yields of chain scission decrease for the following wave length in the order 313 > 366 > 436 nm; the rate of chain scission is reduced when oxygen is absent, and is dependent on the nature of solvent [556, 557]. Films of poly(5-vinyl-2,2';5',2"terthiophene), a polyethylene chain with terthiophenes as side chains, upon cross-linking by UV radiation, change their absorption spectrum and become insoluble. The polymer films, on irradiation in an argon atmosphere, remain soluble, and the changes in the UV/vis spectrum are minimal [242]. The electrical conductivity of a PHT film containing Ph2IAsF6 is enhanced remarkably and the solubility of the film decreases upon light irradiation. This photo-induced doping is confirmed by the spectral change on light irradiation; PhzIAsF 6 dissociates into PhzI + and AsF6 upon light, electron beam, and 7-irradiation, and AsF6 effects the doping. This phenomenon can be successfully used for optical recording (cf. Sect. 6.4) [588]. Light irradiation of PATs with FeC14 as the counter ion results in a loss of electrical conductivity from

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6 Scm -1 to 10 -5 Scm -1. The mechanism proceeds via (i) direct photolysis of FeC14 and (ii) photoreduction of FeC14 to FeC142- followed by (iii) electron transfer to bipolaronic residues. On prolonged photolysis the products are neutral, a non-conductive polymer and octahedral iron(III) complexes being formed. Photochemical dedoping occurs in vacuo but is greatly accelerated by moisture. The reaction is sufficiently facile to occur in ambient light. Replacement of FeCff by the less photolabile counterion AuCI£ results in an increase in the longevity of the irradiated conducting polymer by several orders of magnitude [297]. On UV, visible, and white light irradiation, the conductivities of a film of the copolymer of 3-hexylthiophene modified with an azobenzene moiety increase by a factor of 0.5 to 1.7 within 10 s to reach maxima and, on turning off the light, return to their original levels within the same period of time. This phenomenon can be mainly attributed to the generation of photoexcited hopping sites in the azobenzene moiety on irradiation with light [-559, 560]. 8. The conditions during the polymer synthesis (cf. also Sect. 5.3) [324, 490]. 9. The inclusion of metallic particles in the polymer, The electrochemical inclusion of copper and iron particles in PMT leads to an increase of the ex situ macroscopic conductivity due to comptexation with the sulfur atoms of the polymer backbone. The mechanism and kinetics of the complexation processes are specific to the metallic particles incorporated in the polymer [561,562]. Polymerized 3-octylthiophene containing < 0.4% iron is described as a heat-resistant polymer [563]. 10. Pressure (cf. also Sect. 3.4.5). Although changes with pressure (up to 80 kbar) in the chain conformation and in the electronic polarizability are observed, these are not accompanied by changes in the interchain transfer integral [342]. With increasing pressure, the values of the electrical conductivity of PATs increase due to a decrease of interchain and intrachain distances [483, 484]. Similar effects are qualitatively obtained by increasing alkyl side chain length [-484]. There is a maximum in the pressure dependence in the low conductivity region. The pressure dependence increases with increasing side chain length (n = 8, 10, t2) [564].

5 Synthesis 5.1 Chemical Synthesis 5.1.1 Synthesis Using Nickel or Palladium Complexes One of the most useful methods of chemical synthesis is the Ni-catalyzed dehalogenation polycondensation of dihaloaromatic compounds [271]:

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6 Scm -1 to 10 -5 Scm -1. The mechanism proceeds via (i) direct photolysis of FeC14 and (ii) photoreduction of FeC14 to FeC142- followed by (iii) electron transfer to bipolaronic residues. On prolonged photolysis the products are neutral, a non-conductive polymer and octahedral iron(III) complexes being formed. Photochemical dedoping occurs in vacuo but is greatly accelerated by moisture. The reaction is sufficiently facile to occur in ambient light. Replacement of FeCff by the less photolabile counterion AuCI£ results in an increase in the longevity of the irradiated conducting polymer by several orders of magnitude [297]. On UV, visible, and white light irradiation, the conductivities of a film of the copolymer of 3-hexylthiophene modified with an azobenzene moiety increase by a factor of 0.5 to 1.7 within 10 s to reach maxima and, on turning off the light, return to their original levels within the same period of time. This phenomenon can be mainly attributed to the generation of photoexcited hopping sites in the azobenzene moiety on irradiation with light [-559, 560]. 8. The conditions during the polymer synthesis (cf. also Sect. 5.3) [324, 490]. 9. The inclusion of metallic particles in the polymer, The electrochemical inclusion of copper and iron particles in PMT leads to an increase of the ex situ macroscopic conductivity due to comptexation with the sulfur atoms of the polymer backbone. The mechanism and kinetics of the complexation processes are specific to the metallic particles incorporated in the polymer [561,562]. Polymerized 3-octylthiophene containing < 0.4% iron is described as a heat-resistant polymer [563]. 10. Pressure (cf. also Sect. 3.4.5). Although changes with pressure (up to 80 kbar) in the chain conformation and in the electronic polarizability are observed, these are not accompanied by changes in the interchain transfer integral [342]. With increasing pressure, the values of the electrical conductivity of PATs increase due to a decrease of interchain and intrachain distances [483, 484]. Similar effects are qualitatively obtained by increasing alkyl side chain length [-484]. There is a maximum in the pressure dependence in the low conductivity region. The pressure dependence increases with increasing side chain length (n = 8, 10, t2) [564].

5 Synthesis 5.1 Chemical Synthesis 5.1.1 Synthesis Using Nickel or Palladium Complexes One of the most useful methods of chemical synthesis is the Ni-catalyzed dehalogenation polycondensation of dihaloaromatic compounds [271]:

94


X-Ar-X + Mg --, [X-Ar-MgX]

Ni-complex

, (-Ar-)~

(2)

PATs (n = 10) prepared using NiC12[1,3-bis(diphenylphosphino)propane], NiCl2(dppp), as a catalyst can be highly regular compared with those prepared with FeC13 [565]. The polymerized POT is very pure and is stable up to 260 °C in oxygen and up to 400 °C in nitrogen [510]. The 1H-NMR spectral features of PDDT suggest the coexistence of head-to-head and head-to-tail coupling of thienyl units (Fig. 3, Sect. 2.1.1), but head-to-tail coupling predominates [274]. The polymerization of methyl 2,5-dichlorothiophene-3-carboxylate with nickel(II) bromide, zinc, and triphenylphosphine forms poly(methyl thiophene3-carboxylate); poly(sodium thiophene-3-carboxylate) is obtained after hydrolysis with sodium hydroxide [566]:

/)__~CO2CH3~CH~

Na

NaOH

c~__~,s.~__cI __ ~ ~o~ .~ JR

(3) Fluoroalkylated heteroaromatic polymers can be prepared from 2,5-dibromo-3perfluoropropylthiophene with NiC12 [567]. The cross coupling reaction of dihaloheteroaromatic compounds and 1,2-bis(tributylstannyl)ethylene catalyzed with palladium complexes leads in a single step to poly(thienylene vinylene) [353, 568, 569]. The synthesis of structurally homogeneous PATs is described in Sect. 5.1.8.

Zero-Valent Nickel Complexes. A new polycondensation reaction uses the zero-valent nickel complex itself as the dehalogenation reagent [271,570, 571]: n X-Ar-X + n Ni(O)Lm --+(-Ar-)n + n NiX2Lm

(4)

As zero-valent nickel complexes, mixtures of bis(1,5-cyclooctadiene)nickel and neutral ligands L have been employed. This polycondensation affords PT and PAT with relatively welt-defined linkages between the monomer units in the polymer chains in high yields [92, 266, 271,272, 571]. Vacuum deposition of PT gives crystalline films with their main chains essentially perpendicular to the surface of substrates such as carbon and gold [266]. The iodine-doped PT reaches an electrical conductivity of 8 S c m - i and the FeC13-doped PT an electrical conductivity of 0.5 S cm- 1 [272]. For the preparation of PAT (n = 1, 6, 8, 12) and of poly(3-cyanothiophene) with high yields, a zero-valent nickel complex is also used [266]. Hexyl substituted oligo(thiophene)s (number of thiophene rings: 6, 9, 12, 15) can be prepared by the Ni(O)-catalyzed coupling reaction of a 5,5"-dibromo-3,3"-dihexyl2,2';5',2"-terthiophene) [572, 573]. PT derivatives having alkyloxy substituents at 3 positions are synthesized by dehalogenation polycondensation of the corresponding 2,5-dibromothiophene derivatives with zero-valent nickel complexes [247].

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5.1.2 Reaction with Copper Perchlorate, Ferric Perchlorate, Copper Tetrafluoroborate, or Thallium Trifluoroacetate Neither copper perchlorate nor ferric perchlorate reacts with thiophene to yield a conducting polymer. However, electrically conductive polymers are synthesized by the reaction of 3-methylthiophene or bithiophene with ferric perchlorate. With copper perchlorate, only bithiophene undergoes a simultaneous polymerization and oxidation reaction. X-ray photoelectron spectroscopy of the PT derivatives with perchlorate as counter ion indicates that a significant amount of the chlorine may be covalently bonded to the polymer [--288,574]. The electrical conductivity of polymerized bithiophene reaches values as high as 4.5 S cm- 1 [574]. 3-Dodecyl-2,2'-bithiophene, 3-(3-phenylpropyl)thiophene and 3,4-dibutoxythiophene can be polymerized oxidatively using either copper perchlorate, copper tetrafluoroborate, or ferric perchlorate [575]. A new method of preparation of electrically conductive PT and PAT (e.g. n = 1, PMT) is the direct oxidation of thiophene or 3-alkylthiophene in the presence of a small amount of bithiophene with ferric perchlorate. The obtained PMT has higher electrical conductivity (about 30 S cm- 1) than those prepared by the conventional method of polycondensation of 2,5-dihalo-3-methylthiophene [576]. A simultaneous polymerization and doping of bithiophene and terthiophene can be carried out with thallium(III) trifluoroacetate in trifluoroacetic acid [577].

5.1.3 Reaction with Ferric Chloride A one-step oxidative coupling reaction of bithiophene with ferric chloride and ferric chloride hydrate produces a doped polymer [574]. The direct oxidation of 3-octylthiophene using FeC13 as the oxidant is easier to carry out than the method described in 5.1.1, but the molecular weight obtained is higher as well as the amount of impurities (Fe). The high molecular weight polymer dissolves poorly, and it is difficult to melt-process and mix the polymeric material obtained with other polymers in the molten state. Owing the impurities, POT degrades completely at 260°C in oxygen [55, 510]. Poly[-1,2-di(2-thienyl)ethylene] and its derivatives are prepared from the appropriate 1,2-di(2-thienyl)ethylene in the presence of an oxidant such as FeC13 [578]. The polymerization of 3-alkylthiophenes (n = 7, 11) with FeC13 is also possible, but the polymers obtained are more irregular than those prepared with a nickel complex as catalyst [565]. PATs (n = 4, 6, 8, 10, 12) can be prepared in chloroform with anhydrous FeC13 while bubbling dry air through the reaction mixture. The use of air gives a somewhat higher molecular weight [286]. 3,4-Dibutylthiophene is polymerized with FeC13 in Et20 under reduced pressure (20 mm Hg) and gives a polymer film having an electrical conductivity of 0.47 S cm -1 [579]. 3-Dodecyl-2,2'-bithiophene, 3-(3phenylpropyl)thiophene and 3.4-dibutoxythiophene can be polymerized oxidatively using ferric chloride [575].

96


Thiophene monomers with a benzene ring as substituent containing a hexyl or octyl chain in the para position of the aromatic ring [580] and 3-[2-(2methoxyethoxy)ethoxy]thiophene [581] can be polymerized in chloroform using ferric chloride as an oxidant. The polymerisation of 1,4-di-2-thienylbenzene and of its disubstituted derivatives shown in Fig. 15 leads to fully conjugated polymers linked exclusively at the a-thienyl sites. The polymers are stable up to about 400 °C; in their doped state they have electrical conductivities of the order of 1 S cm- 1 [582]. The monomer 2-(3-thienyl)ethyl hexanoate is polymerized by chemical oxidation in anhydrous nitromethane under argon using anhydrous FeC13 and gives a soluble ester-functionalized PAT with an electrical conductivity in the doped state of 1.1 x 10 - 3 S cm -1 1-583,584]. Polymerized 2-(3thienyl)ethanol reaches an electrical conductivity of 8.6 x 1 0 - 6 S cm -1 1,-584]. Poly(3-hexanoyloxyethylthiophene) is polymerized with FeC13 in both nitromethane and chloroform. A completely homopolymeric structure is obtained in chloroform [585]. Although anodic polymerization is ineffective, it is possible to polymerize 3-cyclohexylthiophene, a sterically hindered thiophene derivative to yield poly(3-cyclohexylthiophene) with FeC13 [93, 522]. Poly(3,3'-dibutoxy-2,2'-bithiophene) is obtained via chemical oxidation of 3,3'-dibutoxy-2,2'-bithiophene using FeC13. The doped polymer reaches an electrical conductivity of up to 2 S cm- 1 [586]. The reaction of alkylenedioxythiophenes with anhydrous FeC13 in acetonitrile yields black powders with conductivities in the range 10 -2 to 1 S c m - k If 3,4-ethylenedioxythiophene is reacted in boiling benzonitrile (188°C) with FeC13 for 2 h, the resulting poly(3,4-ethylenedioxythiophene2,5-diyl) (cf. Sect. 1.2) has an electrical conductivity of 19 S cm- 1, and after a reaction time of 6 h the electrical conductivity is 31 S cm- 1 [46]. Under these conditions polymers from other monomers, e.g., thiophene, 3-methoxythiophene, show very low conductivities or the polymer is destroyed [46]. For a synthesis giving a high percentage of HT (head-to-tail) coupling in PAT see Sect. 5.1.8.

The Layer-by-Layer Deposition Method [587]. This method, based on the chemical oxidative coupling polymerization reaction, enables the simultaneous synthesis and deposition of a thin film of different insoluble conducting polymers in the conducting state such as PMT and PBT. Films (40-1000 nm thick) R

1. 3

FoCl3 / RT

R

2. NH4OH R

R

2.,a: R,,H 2b: R=Mo 2c: R ,, OMo

Fig. 15. The polymerization of 1,4-di(2-thienyl)benzeneand its disubstituted derivaties [582]

Polythiophenes


97

are obtained layer-by-layer on a substrate by successive alternating substrate immersions in a monomer solution and in an oxidant solution (FeC13). There are three growth mechanisms for the electrochemical synthesis: (a) deposition of chains, (b) deposition of clusters of chains, (c) extension of already deposited chains. The growth rate and the physical properties of these films are strongly dependent on the operating conditions (cf. Sect. 5.3) [587].

Organic Molecular Beam Deposition [588]. A molecularly oriented thin film of sexithiophenes can be produced by the organic molecular beam deposition technique. The sexithiophene powder synthesized by the oxidative coupling reaction of terthiophene using ferric chloride is heated in a Knudsen cell and a molecular beam is generated. Fused silica glass plates are used as substrates. The sexithiophene molecules are aligned almost exactly parallel to the surface in a thin film deposited under the ultra-high vacuum at low deposition rate. The molecular orientation of the sexithiophene deposited films can be controlled by the substrate temperature as well as by the deposition rate and/or the pressure during deposition [588].

5.1.4 Synthesis Using A1C13-CuC1-Oz PT and its derivatives, e.g. PMT, are prepared with oxygen in organic solvents in the presence of A1C13 and CuC1 with a significantly higher yield than with FeC13. The PT obtained has an electrical conductivity of 2.2x 10 ~2S cm -1 and a decomposition temperature of 302°C [589]. Mass production and an improvement in processability of PT and its derivatives is possible when A1C13-CuCI-O2 is used as a catalyst [590].

5.1.5 Polymers Derived from Precursor Polymers, Oligo(thiophene)s, or Poly(thiophene) Derivatives Poly(thienylene vinylene) is prepared as a high molecular weight, flee-standing, very stable film. I2- and FeC13-doped polymers reach an electrical conductivity of 315 S cm -1 and 110 S cm -1 respectively. The synthesis is derived from a pyridine-stabilized new precursor polymer (Fig. 16) [277]. Another synthesis of poly(thienylene vinylene) is the reaction via the alkyloxy pendant precursor (Fig. 16) containing 10% of pendant hydroxy groups. The iodine doping of this polymer increases the electrical conductivity to 100 S cm- t, the electrical conductivities of doped films being stable even in air; a stretched film reaches an electrical conductivity of up to 1100 S cm-1 [278, 279, 591-595]. A thermally stable, gel-free polyelectrolyte precursor of poly(thienylene vinylene) starts from 2,5-bis(dimethylsulfoniummethyl)thiophene dichloride [596]. Oriented poly(thienylene vinylene) films are prepared from the tractable precursor polymer poly[2,5-thienylene(1-methoxyethylene)] via two different

98


ncl_

(a)

O~CH3 (b)

Fig. 16. Pyridine-stabilized precursor polymer (a) [277] and alkoxy precursor polymer (b) [592] for the synthesis of poly(2,5thienylene vinylene)

processing routes, i.e., the more common simultaneous tensile deformation and conversion method and the sequential conversion and drawing method. Both techniques yield materials of identical electrical conductivity (up to 1000 S cm-1) and mechanical properties, although the sequential conversion and drawing process is more effective [597]. Poly(thienylene vinylene) can be prepared by contacting poly[oligo, poly(thiophene-2-yl)ethylene] (cf. Sect. 1.2) with dihydrogenation agents in the presence of solvents [14]. The reaction of the alternating copolymer of ethylene and carbon monoxide with Lawesson's reagent [2,4-bis(4-methoxyphenyl)-2,4dithioxo-l,3,2,4-dithiadiphosphetan] results in a conversion of 75% of the carbonyl groups to thiophene units and to the formation of poly(thienylene vinylene) by oxidation. An evolution of hydrogen sulfide is observed during this reaction. Thioketone and unreacted carbonyl groups can be observed by IR spectroscopy [598]. It is possible to prepare PT by heating a-sexithienyl. Under carefully controlled reaction conditions (temperature, time, and amount of oxygen) the PT obtained exhibits a quality comparable with that of PTs synthesized from 2,5-diiodothiophene [599].

5.1.6 Plasma Polymerization Plasma polymerization is a typical dry process for the preparation of thin solid films. In the plasma polymerization starting from thiophene and 3-methylthiophene, the glow discharge was carried out at 7.3 kHz audio frequency (AF) and 13.56 MHz radio frequency (RF) and Ar was employed as the carrier gas [600, 601]. The electrical conductivity of iodine-doped AF-PT and RF-PT is 2.2 x 10- 3 S cm - 1 and 4.3 x 10- 4 S cm- 1 (1.8 x 10- 4 S cm- 1 [600] ), respectively, and of iodine-doped AF-PMT and RF-PMT is 1.0 x 10 .3 S cm -~ and 2.8 x 10 -4 S cm -1, respectively [601]. Plasma-polymerized poly(3-bromothiophene) shows high electrical conductivity [602].


99

5.1.7 Preparation of Composites Co-oligomers and copolymers from thiophene and silol are synthesized by palladium-catalyzed cross coupling reactions [603]. Copolymers from 3-octylthiophene and 3-methylthiophene are prepared by oxidative copolymerization using FeC13. The compositions of the copolymers are consistent with the monomer ratio in the feed; the molecular weights of the copolymers decrease with increasing 3-methylthiophene content [531]. 3-Methylthiophene/3-butylthiophene, 3-methylthiophene/3-hexylthiophene, 3-butylthiophene/3-octylthiophene and 3-hexylthiophene/3-octylthiophene are copolymerized with FeC13 in chloroform by bubbling air through the reaction mixture. The molecular weights of the copolymers obtained depend on the alkyl side chain length (cf. Sect. 4.1.2) [286]. Polycondensation of dilithiated derivatives of mono-, bi- or terthiophene with chlorosiIanes or aryl bromides has allowed the preparation of polymers with alternating silylene and thienylene units. The electrical conductivity of NOBFg-doped polymers is < 10-1 S cm-1 [303]. Palladium-catalyzed polycondensation between 2,5-bis(trimethylstannyl)thiophene and 5,8-dibromoquinoxaline derivatives gives a soluble copolymer of electron-donating thiophene and electron-withdrawing quinoxaline. The absorption at 575 nm is assignable to the intramolecular charge transfer band [604]. Soluble block and graft polystyrene/PT copolymers are prepared by polymerization of thiophene or 2-bromothiophene on thiophene groups attached at the end of polystyrene chains. The length of PT sequences increases with the length of the polystyrene blocks. Copolymer films can be converted into pure PT films by heating at 380°C in vacuo or argon on depolymerization of the polystyrene sequences. These PT films doped with FeCI3 reach electrical conductivities of up to 60 S cm- ~ [605]. PAT films are susceptible of near-UV light-induced graft copolymerization with hydrophilic functional polymers. The density of grafting is enhanced by Ar plasma or 03 pretreatment. The surface/interface grafts enable further functionalization of the electroactive polymer surfaces, such as self-doping and enzyme/protein immobilization via covalent bonding [606]. A polymer containing thiophene-benzene-thiophene units in the polymer chain is chemically or electrochemically prepared from 1,4-bis(2-thienyl)benzene with H, Me, or OMe groups in the 2,5-phenylene positions [607]. Cyclization of 1,4-dithienyl-l,4dioxobutane by acid catalysts leads to furan-containing oligo(thiophene)s, whereas the condensation with ammonium acetate provides pyrrole-containing oligo(thiophene)s [608]. Transparent poly(acrylonitrile)/PT composite films are prepared by polymerizing thiophene vapor in poly(acrylonitrile) films impregnated with sulfuric acid or aluminum chloride as an initiator for the polymerization reaction. The electrical conductivities of these composites are in the range of 10- 7 to 10- 3 S cm- 1, depending on the initiator and its concentration [609]. An electrically conducting polymer (0.3 to 40 S cm-X) with a polymethacrylate backbone and electroactive PAT side chains, e.g., PMT and PPT, is

100


prepared from poly(2,2'-bithienylmethyl methacrylate) which is introduced on both electrochemical and chemical oxidative polymerization of 3-alkylthiophene E610]. In a method for preparing a conductive polymer product by doping a conductor polymer, a mixture of POT and a matrix polymer (thermoplastics or elastomers) is brought in a molten state into contact with a doping agent. The doped polymer blend achieves an electrical conductivity of 1 S cm- 1 [510, 611]. A conducting polymer composite based on highly porous cross-linked polystyrene as host polymer and polymerized bithiophene is prepared by imbibing the host polymer with the monomer solution, partially drying the saturated host polymer, and imbibing again with an oxidant solution for polymerization to occur. The composites, prepared using ferric chloride as initiator and dopant, are environmentally stable and reach electrical conductivities as high as 4.8 S cm- 1 and an electrical conductivity of 3.63 S cm- 1 for PT composites. The electrical conductivity is influenced by the type of oxidant and the initial concentrations of oxidant and monomer [574, 612]. A conductive plastic composite is prepared when the conductive polymer (e.g. POT) is dissolved or dispersed in a matrix monomer or a matrix monomer mixture and the monomer or the mixture is polymerized to form the matrix polymer [613]. Intercalation polymerization of bithiophene in V205 xerogels results in an electrically conducting material composed of alternating monolayers of VzO5 and conductive polymers. The electrical conductivity is a function of the polymer/V205 xerogel ratio [614]. A 1 : 1 molar mixture of monomers 2-(3-thienyl)ethyl hexanoate and 2-(3thienyl)ethanol can be polymerized by chemical oxidation with FeC13 and gives a soluble ester-functionalized PAT with an electrical conductivity in the doped state of 4.9 x 10 .5 S cm -x [583, 584]. Thiophene, 3-methylthiophene, bithiophene and terthiophene are polymerized in the channels of molecular sieve zeolite hosts. Conducting polymers can be isolated after dissolution of the zeolite host in HF [615].

5.1.8 Synthesis of Regioregular 3-substituted Poly(thiophene)s The standard synthetic methods lead to coupling products in the 2,5 positions with random regiospecificity. These polymers contain a number of defects, which in turn reduce the electrical conductivity of the polymers. Pure isomers of mono-Grignard reagents of diiodinated alkylthiophenes cannot be achieved by the simple reaction with magnesium, as this leads to a variety of coupling modes (52% to 63% HT linkages) [97]. PHT synthesized by oxidative coupling using ferric chloride contains up to 80% HT linkages [97]. PATs (n = 4, 6, 8, 12) containing almost exclusively head-to-tail (HT) couplings are synthesized from 2-bromo-3-alkylthiophene in a one-pot reaction as shown in Fig. 17 [94, 95, 99, 275]. HT-coupled PHT can also be obtained by self-coupling of a mixture containing 2-bromo-5-(bromozincio)-3-hexylthiophene and a nickel

Polythiophenes ElectricallyConductive Polymers R

LBr'

101 R

R

--- Li

I

Br

BrMg~Br 3

2

/

n

R Methyl Butyl Hexyl Octyl Dodecyl

Yield

(%)

R

R 4

69 20 36 33

91% HT-HT C o u p l i n g s , w h e n R = CI~H25

Fig. 17. One-pot reaction to prepare PATs containing almost exclusivelyhead-to-tail (HT) couplings (conditions: i, LDA, THF, -40~'C; ii, MgBraOEt2, -60°C; iii, -40°C; iv, -10°C; v, NiCl2(dppp), 5 :~C 25 ~C; LDA = lithium diisopropylamide;THF = tetrahydrofuran; dppp = 1,3-bis(diphenylphosphino)propane) [94]

complex [99]. The isomerically pure bromo(dodecyl) thiophene is the key intermediate for ensuring the synthesis of rigidly defined higher oligomers, for instance isomerically pure dialkylsexithiophenes [-616, 617]. Regioregularly substituted PT derivatives are prepared by polymerization of suitably substituted terthiophene derivatives both with alkyl and alkyloxy groups. The reactivity of the alkyloxy substituted terthiophene derivatives toward polymerization depends on the position of the substituents in the ring [618, 619]. The poly(3',4'dibutyl-2,2';5',2"-terthiophene) obtained contains two soluble fractions of different molecular weight. The high molecular weight fraction has one of the longest re-conjugation lengths known for PATs and high electrical conductivity. The low molecular weight fraction has a smaller conjugation length and an electrical conductivity (10-2 S cm -1) at room temperature smaller by two orders of magnitude [88]. It is possible to polymerize regioselectively 3-(4-octylphenyl)thiophene with FeC13. Adding FeC13 slowly to the monomer leads to a soft and therefore regioselective polymerization. The HT content in poly[-3-(4-octylphenyl)thiophene] is 94% (by 1H-NMR) [620]. Free-standing films have an electrical conductivity of 4 S cm- 1 [620], which is 100 times higher than that of the poly[3-(4-octylphenyl)thiophene] prepared earlier (cf. Sect. 5.1.3, [580]).

102

G. Schopf and G. KolSmehl

5.1.9 Further Chemical Synthesis Methods Thiophene is allowed to react with a mixture of 5-20 vol% F2 and 80-95 vol% N2 at - 8 0 ° C to 0°C to give an eIectrically conductive polymer [-6213. Poly(thienylene vinylene) and other conducting polymers can be obtained from 5-membered 5-methyl-2-heteroarylcarbaldehydes and their Schiff bases [622].

5.2 E l e c t r o c h e m i c a l S y n t h e s i s 5.2.1 Deposition and Growth The electrodeposition process investigated for the deposition of PMT occurs in several steps [623-626]: (1) nucleation, (2) first monolayer, (3) fibrous film (layer by layer), (4) granular structure. The formation of isolated nuclei during the initial stages has also been demonstrated for PBT [213]. The dynamics of the polymer growth, e.g., PT and PAT, is explained as follows [4, 58, 627, 628]: In the initial stage (Fig. 18) of polymerization, one electron is removed from the monomer molecule (educt), i.e., the monomer is oxidized. The resulting radical cation (polaron) encounters another radical monomer or oligomer (reaction a, Fig. 18). Alternatively, the radical cation reacts with a neutral monomer molecule (reaction b, Fig. 18) and two a-hydrogen atoms are then split off as protons. This process is repeated. At the same time, the polymer chains are also oxidized and obtain electroneutrality

reaction

"-I~

b

+ ~

-e

" reaction

a

-7 2. -2H +

educt

I-H + -H + +

-q

--1. -e

Fig. 18. Initial stage of the electropolymerization of thiophene 1-58]

$


103

through the counter ion, e.g., BF2 from the electrolyte. The polymer has good electrical conductivity and acts as a new electrode. The polymerization reaction, i.e., the reaction of two radical cations or the reaction of a radical cation with a neutral monomer molecule, depends on the conditions during electrochemical polymerization [58, 627, 629, 630]. During the electrocopolymerization of 3-methylthiophene and 3-thienylacetic acid, a radical cation (of 3-methylthiophene) attacks at a neutral monomer (3-thienylacetic acid). It is possible to produce the copolymcr at a potential at which only one of the monomer species can be oxidized [108]. Fig. 19 shows a partial model of interracial reactions taking place during the electrogeneration of PT or poly(pyrrole) from acetonitrile solutions containing the electrolyte LiC104. The relative influence of each of these reactions depends on the chemical and electrical conditions of synthesis [629]: Reaction (1) recovers the metal oxide at lower potential than the monomer oxidation. Reaction (2) is the direct oxidation of the educt and subsequent polycondensation of radicals and the release of protons. Reaction (3) represents two acid-catalyzed ways to produce electroactive (3a) and non-electroactive (3b) polymers. The non-electroactive polymer can interact with the electrode or diffuse into the solution. Reaction (4) leads to a marked effect of the perchlorate on the empirical kinetics by the discharge of perchlorate on the polymer. The radical is immediately transferred to a monomer molecule. Furthermore, perchlorate may act on the metal oxide production, both by stabilization of the monomeric radicalcation and through polymer oxidation. Reaction (6) represents polymer degradation through a nucleophilic attack of the polymer by a water molecule, forming an oxidation product [629]. Ellipsometry can be used to study the adsorption processes, the growth, and the electrodeposition [264, 265, 631]. Atomic force microscopy can lead to an explanation of the optical spectra of PMT layers at several stages of the electrodeposition process [632]. The morphology of PT films grown electrochemically can be explored by IR dichroic measurements. The early stages of growth, giving long, well-ordered chains, is succeeded by at least two different growth mechanisms involving either relatively long, flexible chains or short, stiff, inter-linked structures, depending on the details of the electrochemical growth protocol [633]. Comparison of the growth of thin films of PMT deposited on platinum under both the galvanostatic regime and the potentiostatic regime by ellipsometry indicates that the electronic properties of the films differ [634]. A chemical pretreatment of the platinum electrode influences the growth rate of PMT [635].

Layer-by-Layer Deposition. Studies of the kinetics and mechanism of poly-(3thienylacetic acid) formation show that electrochemical deposition proceeds favourably through 2-D layer-by-layer nucleation and growth, following the first monolayer deposited through oxidative adsorption of 3-thienylacetic acid

104

G. Schopf and G. Kol3mehl SOLUTION

METAL

H~O (I)

(3)

HO" + H"

'~'~ " Z"

" ~'~ "Z"

/ -,qmmmm/(3a)

/

(3b~ Acid-catalyzed " "I polymerization

C104

ATION /

POLYMER

H --- NCCH~ H --- NCCH~

(710,"

(5)

-I HO+H"

•, ' / / / / ~ . ~ ,

-

\

-

/

DEOR~O,~TION \

/

Fig. 19. Partial model of interracial reactions for PT and poly(pyrrole) electrogeneration on a platinum electrode using LiCIO4 as electrolyte salt [629]

on a bare Pt surface [636, 637]. This indicates the possibility of preparing ultrathin, compact, conducting polymer films. 5.2.2 Synthesis Conditions Electrode Material. Photocurrent spectroscopy has proved to be a powerful technique for the study of the dependence of initiation and growth of PMT


105

on various electrode surfaces [630]. The electropolymerization of thiophene is possible on platinum in LiC1Og/propylene carbonate [638], in BugN + or Et4N + salts/organic solvents [264, 265, 639], in other quaternary ammonium salts/dichloroethane [640], and in aqueous perchloric acid [641]. A chemical pretreatment of the platinum electrode by sulfides and by bipolar molecules influences the growth rate of PMT [635]. PMT [192], oligo(thiophene)s, and PT [213, 642] films can be formed on gold electrodes. The thiophene adsorbed on a roughened gold electrode is oxidized at a relatively low electrode potential to produce oligomers and polymers [642]. Thiophene derivatives are polymerized on roughened Ag electrodes and are oxidized only on the non-totally reduced surface as obtained during oxidative reductive pretreatment under daylight conditions [643]. Poly(2,2';5',2"-terthiophene) can be electrochemically prepared on Pt, Au, Ni and Ni electrodes sputtered with Au [148]. The most homogeneous and compact surface of PT, PBT and PTT films is achieved using Ni electrodes covered with Au [146, 148], in contrast to the above-mentioned electrode materials. The morphologies of several conducting poly(heteroarylene) films synthesized galvanostatically on optically transparent SnOx, electrodes are substantially different from those of the films formed on Pt surfaces [149]. Electrochemical polymerization of thiophene and 3-alkylthiophenes (n = 1, 6, 8) at elevated voltages is a suitable technique to produce smooth, flexible, electroactive, free standing films or thin layers of polymers on indium tin oxide (ITO) anodes. The samples of PT obtained are less regular than their chemically or low-voltage electrochemically prepared counterparts [644]. An ITO electrode can also be used for the electropolymerization of thiophene in aqueous perchloric acid [641]. 3-Fluorothiophene, 3-chlorothiophene, and 3bromothiophene are polymerized on ITO electrodes. The homogeneous freestanding film of poly(3-fluorothiophene) obtained has an electrical conductivity of 5 S cm- 1 [527]. Powdered heteroaromatic polymers are anodically deposited on specially pretreated A1 electrodes from HNO3 solution of the monomers [645]. Braided and helical fibrils are formed by electropolymerization of 3-methylthiophene on a Nucleopore-covered electrode in a tetrabutylammonium hexafluorophosphate/propylene carbonate electrolyte. When the electrodes are arranged horizontally with the working electrode on top, better morphological control is achieved than with vertical electrodes [646]. The electrogeneration of thick PT films on stainless steel is described [647]. Thin films (10 [am) of PT deposited on steel have a higher tensile strength than an aluminum film of the same thickness. PT films deposited on other electrode materials (platinum, aluminium, tin or silicon) have less good mechanical properties [648]. The influence of electrode rotation (platinum rotating-disc electrode) on the rate of electrochemical deposition of PMT and the morphology of the resulting films demonstrate a significant role of soluble intermediates (cf. Sect. 2.3) [151].

Current Density. PT films synthesized galvanostatically at relatively high current densities ( > 1 mA cm -z) show an increase in the Coulombic capacity followed by a slight decrease with increasing cycle number. About 90% of the

106


initial Coulombic capacity is obtained after 200 cycles [638]. The morphology of a PBT film is affected by the current density. Current densities between 0.5 and 2 mA cm -2 create uniformly structured PBT layers with rough surfaces. Current densities of 0.5 mA cm -2 create smoother films. Current densities of > 2 mA cm-2 reduce the adhesion of the PBT layer on the electrode [217, 261]. PTT coatings with a rough surface structure are formed at higher current densities, whereas low current densities build up a homogeneous, compact, and smooth structure [148].

Applied Potential. The polymerization rate of 3-alkylthiophenes (n = 6, 8, 12, 18) increases with increasing applied potential and the electrical conductivities increase with increasing polymerization potential up to 10.0 V [481]. PMT prepared by the oxidation potential control technique exhibits an electrical conductivity as high as 170 S cm- 1 [649]. The conductance of PT and PATs such as PMT, which is linearly related to the electrical conductivity, shows a strong decline after a prolonged electropolymerization, presumably as a result of the "poly(thiophene) paradox", which refers to the fact that PTs are produced under conditions which are capable of destroying the polymer. Most probably, the high potential leads to cross-linking of the polymer chains and/or side reactions [650]. The molecular weight of PPT and PHT synthesized using the potentiostatic technique increases as the applied potential is increased up to about 1.7 V vs standard calomel electrode (SCE). It then levels off and decreases because of chain degradation [651,652]. The introduction of bithiophene and terthiophene to the polymerization system results in a lowering of the required applied potential (cf. "Additives", below) [576, 653]. It is possible to produce the copolymer from 3-methylthiophene and 3-thienylacetic acid at a potential at which only one of the monomer species can be oxidized [108]. The structure of poly[1,2-di(2-thienyl)ethylene] depends on the height of the oxidation potential E654]. The electropolymerization of 2-vinylthiophene at constant potential is described [655]. Copolymers of ruthenium complexes with 3-methylthiophene or bithiophene are prepared under potentiostatic or potential cycling conditions. Better reproducibility and more uniform coverage of the electrode are obtained with potential cycling. The film thickness can be controlled by the polymerization time and/or by the number of cycles [656]. Polymer films from thiophene, 3-methylthiophene and several unsubstituted and 3-methylsubstituted oligo(thiophene)s formed by potential cycling are electrochemically unstable compared to those formed by constant potential oxidation [657].

Temperature. The deposition temperature influences the structure, orientation, and morphology of vacuum-evaporated sexithiophene films. A high degree of orientation can be achieved even in films several micrometers thick deposited above 190 °C. The field effect mobility is enhanced for deposition temperatures close to the melting temperature (290 °C), which is associated with a suitable orientation, sometimes a favorable crystalline structure, and coalescent lamellae morphology [658].


107

The electrical conductivity and mechanical properties of polymerized thiophene are improved when the synthesis is carried out at a low temperature (5 °C) [659]. The current efficiency of thiophene polymerization and the charge storage efficiency increase when the temperature of polymerization decreases from 60°C to - 12 °C [660]. For PTs prepared at low temperatures ( - 20°C to + 10 °C), the d.c. conductivity in the planar direction is almost independent of the frequency [329]. The values of the electrical conductivity of PATs (n = 6, 8, 12) increase with decreasing polymerization temperature, but the electrical conductivity of a PAT with n = 18 decreases at a polymerization temperature of 5 °C [481]. A polymerization at - 20 °C is described for poly-(3alkyloxymethylthiophene) [661]. The polymerization of terthiophene on Ni working electrodes at room temperature gives a more homogeneous, more compact, and smoother surface than at - 5 °C [148].

Solvent/Electrolyte. The electrochemical polymerization of thiophene or of 3-alkylthiophene is often carried out in acetonitrile using several electrolytes, e.g., tetraalkylammonium perchlorate, iodide or fluoroborate salts [662-665] in nitrobenzene, Bu4N ÷ salts [666] in acetonitrile, or Bu4N + [265] or Et4N + salts [-264]. The anions (e.g. BF~, ClOg, PF6, SOaCF~) have no influence on the I-E curves, in contrast to the case of poly(pyrrole) [264]. The use of aqueous HC104 solution for the electropolymerization of thiophene increases the solubility of thiophene and lowers the oxidation potential of the monomer to about 0.9 V vs. SCE instead of 1.6 V found in acetonitrile [641, 667]. Poly(3-phenylthiophene) was synthesized in the presence of several electrolytes salts (MC104, M + = Li +, Na +, Bu4N+). An increase in the oxidation potential of 3-phenylthiophene is observed with an increase in the radius of the cation of the electrolyte salt. An analysis of the poly(3-phenylthiophene) reduction cyclic voltammograms shows that, during the electropolymerization, the nature of the electrolyte cation affects the electrochemical properties of the resulting films and the reduction potential of poly(3-phenylthiophene) [668]. A copolymer from 3-methylthiophene and 2,6-naphthyridine (electrical conductivity of 2500 S cm 1) is electrochemically obtained in nitrobenzene with Et4NC104 [669]. A copolymer from thiophene and 1,2-di-(2-thienyl)ethylene (electrical conductivity of 20 230 S cm- 1) is prepared with Et4NC104 in propylene carbonate solution [670]. Copolymers from ruthenium complexes and 3-methylthiophene or bithiophene are prepared in acetonitrile, propylene carbonate nitromethane, and dichloromethane. Acetonitrile is generally the best solvent for the incorporation of the ruthenium complex into the polymer film, the ruthenium complex content in films being higher than in propylene carbonate, nitromethane, and dichloromethane [656]. Propylene carbonate containing BugNPF6 is used for the electrochemical polymerization of 3-alkyloxymethylthiophene [661]. The polymerization of precursors in aqueous, organic or aqueous-organic solvents is possible for the preparation of a composition with PBT and polymeric sulfates [671]. Thiophene derivatives containing a tetrathiafulvalene (TTF) moiety covalently attached to the thiophene ring via a linear octadecyl

108


spacer cannot be polymerized in acetonitrile, but can be polymerized successfully in nitrobenzene. The formation of a donor-acceptor complex between nitrobenzene and the TTF moiety decreases its reactivity toward thiophene cation radicals [672]. The electrical conductivity of PAT (n = 6, 8, 12, 18) films increases with the electrolyte concentration up to 0.04 M where it reduces a saturation value [481].

Role of Water. With increasing water content in acetonitrile the current efficiency for the galvanostatic electropolymerization of thiophene decreases rapidly, but much more slowly for the polymerization of 3-methylthiophene and bithiophene. The competition for radical cations generated from the monomer in the initial electrochemical step between the main reaction (the electropolymerization) and the side reaction (nucleophilic attack of the radical cation by water) explains this decrease in current efficiency [673]. A decrease in the monomer concentration increases the water/monomer ratio and impairs the electrical properties of PT. The presence of water in the electropolymerization process leads to more nonconducting and passivated films [264]. The deposition of copolymers of ruthenium complexes with 3-methylthiophene or bithiophene is inhibited by residual water in the polymerization solution [656]. The surface chemistry of PBT studied by parallel time-of-flight secondary ion mass spectrometry/X-ray photoelectron spectroscopy (ToFSIMS/XPS) is strictly correlated to the amount of water in the cycling solution [237, 674, 675]. PT films prepared by electrochemical polymerization in aqueous HC104 have about 8-10 mot% of ClOg at the surface, but not in the bulk. Carbonyl groups are formed due to the participation of water in the polymerization of thiophene [676]. Ageing of the Solution. The ageing of the solution does not appear to have any effect on the quality of a PT film [659]. Type and Concentration of Monomer. At the lower monomer concentration, not only are longer polymerization times needed to prepare the same PAT (n = 6, 8, 12, 18) film thickness, but also low electrical conductivities are obtained [481]. Films of PT formed by anodic oxidation in 0.4 M solutions of thiophene in acetonitrile have a large capacity for storing charge, in contrast to PT films formed from 0.01 M solutions of thiophene [-264, 265]. The effect of the bithiophene monomer concentration on the electrodeposition of the monomer is insignificant over a wide range. However, very small monomer concentrations ( < 0.01 M) lead to less uniform PBT depositions, and monomer concentrations of > 1 M reduce the adhesion of the PBT to the electrode [217, 261]. During the electrocopolymerization of 3-methylthiophene and 3-thienylacetic acid, the dimerization tendency of carboxylic acids can be reduced when redox active polymer layers are formed even at monomer concentrations of 10-a M [108]. Esterification and the consequent reduction of 3-thienylacetic acid produces 3-(2-hydroxyethyl)thiophene, which must be protected before electropolymerization [111, 112]. In copolymers of ruthenium complexes with

Polythiophenes


109

3-methylthiophene, the reproducibility and homogeneity of electrode coverage are enhanced by increasing the 3-methylthiophene concentration, but the increase of the ruthenium complex concentration generally decreases the reproducibility of copolymerization. This concentration dependence is important when considering the content of water in the polymerization system (cf. also "Role of water", above) [656]. Increase in the conjugation of the monomer leads to a decrease in the polymerization length and low conductivities. Thus, the conductivities of PBTs are inferior to those of PTs [659]. However, the polymerization of bithiophenes proceeds at a lower oxidation potential and lower monomer concentration and gives polymers with higher yield and better regularity than the polymerization of thiophenes [677]. The polymerization of bithiophenes occurs predominantly without branching [678]. The electrochemical polymerization of 2,5-bis(trimethylsilyl)thiophene, 5,5'-bis(trimethylsilyl)-2,2'-bithiophene, 5,5"-bis(trimethylsilyl)-2,2';5',2"-terthiophene, 2,4-bis(trimethylsilyl)thiophene, and 3-trimethylsilylthiophene produces electrically conducting PT fihns [679]. The polymerization occurs through a complete electrodesilylation of the monomers with silyl groups at the a position of the thiophene ring. The polymers obtained appear highly structured, with higher mean conjugation lengths and lower amounts of defects compared to polymers from nonsilylated monomers. Only partial desilylation occurs upon electropolymerization of monomers with the silyl group in the 13position of the thiophene ring (2,4-bis(trimethylsilyl)thiophene, 3-trimethylsilylthiophene) [680]. Several 2,2';5',2"-terthiophene (3T) derivatives substituted at the central thiophene ring with alkyl groups or with an aryl group were synthesized and their electrochemical polymerization has been compared with that of unsubstituted 3T. The electrooxidation process of the substituted 3T derivatives depends on the initial substrate concentration, and the nature of the attached substituent affects the electropolymerization process and the structure of the resulting material [524, 681]. Regiospecifically methyl-substituted oligo(thiophene)s have been synthesized and used as starting materials for electropolymerization. The obtained PMTs show spectroelectrochemical properties which depend on the size and substitution pattern of the starting oligomer [682]. Methyl groups in the [3position in the thiophene ring increase the stability of the radical cations of the oligo(thiophene)s [58]. A comparison of the polymerization of 3-thienylmethanol and 3-methylthiophene and the electrochemical properties of the corresponding polymers shows significant differences. The determining role is played by the nucleophiticity of the CHzOH substituent [683]. The electrochemical polymerization of 1,4-di-2-thienyl-2,5-disubstituted benzene derivatives leads to fully conjugated polymers linked exclusively at the a-thienyl sites and is more regular than that of 3-substituted heterocycles. The appropriate doped polymers have electrical conductivities of the order of 1 S cm- 1 and are stable up to 400 °C [5821. The electrochemical polymerization of several methoxythiophenes (e.g.,

110


3-methoxythiophene, 3,4-dimethoxythiophene, 3,Y-dimethoxy-2,2'-bithiophene, 4,4'-dimethoxy-2,2'-bithiophene) depends on the position of the substituent [684-686]. Different results of anodic coupling of monomers composed of two bithienyl or terthienyl moieties end bridged with one ethylene fragment are obtained by varying the number of the thiophene rings in the monomeric unit [687]. Thickness of Films. The electrical conductivity of PT films formed galvanostatically is weakly dependent on the thickness, but no significant variation in the electrical conductivity of the films can be observed when the films are prepared under potentiostatic conditions [659]. The rigidity of potentiostatically obtained PBT films is dependent on their thickness [688]. Irradiation. Ultrasonic irradiation during electrochemical polymerization of thiophene results in an improvement of polymer yield and in a decrease in the anode potential. The effects are especially marked when the polymerization is carried out at low temperature, low monomer concentration, and high current density. It is explained by the formation of a diffusion layer in the vicinity of the electrode during electrochemical reactions [689]. Additives. The presence of bithiophene and terthiophene in the polymerization system enhances the regularity of the structure of the polymer chains (PAT with n = 5, 6). The molecular weight becomes lower with an increasing amount of additives [651,652]. A significant increase in the rate of polymerization of thiophene and 3-alkylthiophene (n = 1), and a decrease in the required applied potential and of the number of nucleation sites of the polymers on the surface of the electrode is observed. Since the oxidation potential of bithiophene is lower than that of thiophene monomer, the bithiophene in the polymerization system should be oxidized first, leading to nucleation species on which the polymer grows. There is no apparent structural difference between the polymers produced in the absence and in the presence of these additives [576, 653, 690]. The electrochemical polymerization of 3-alkylthiophene is also significantly facilitated by the presence of a small amount of indole, 5-methoxyindole, or 2,2'bipyrrole [690]. The matrix polymerization of thiophene is successfully performed in an organopolysilane film as a result of the difference in solubility properties between the UV-exposed and the UV-unexposed areas of the film (cf. Sect. 6.4) [691]. Electrically conductive PTs are prepared by polymerization in an electrolyte in the presence of polymeric compounds with sulfonic acid groups 1-692]. The electrochemical polymerization of thiophene and Nation gives a self-supporting film showing a uniform distribution of sulfur across the film and a reversible voltammogram [693]. Alkoxy substituted thiophene derivatives are electrochemically polymerized in a solvent in the presence of a proton source in the form of a Br6nsted acid [694]. The electrochemical preparation of poly(3-alkyloxythiophene) without a Br6nsted acid has also been studied [695].

Polythiophenes


111

Immobilization of Keggin-type and Dawson-type heteropolyanions (HPA): PMo12043o, SiW12044o and P2W18062 in PMT is performed by electropolymerization of 3-methylthiophene in aqueous and nonaqueous solution in the presence of corresponding heteropolyacids and salts. It is possible to obtain electrodes modified with HPA molecules. These are stable in acidic solutions

[696-699].

5.2.3 Special Electropolymerization Methods

Oriented Polymers. In the preparation of an electrically conducting polymer, e.g., PT or PMT, the two ends of a polymer sample, electrochemically polymerized and still containing solvent, are fixed. The polymer is then dried and heated to cause it to contract, so that molecular orientation is produced in the longitudinal direction. This process can give a 2 to 4 fold increase in the electrical conductivity of the polymer [700].

Copolymerization, Composites. The electrochemical copolymerization of 3-dodecylthiophene and 3-methylthiophene is carried out galvanostatically. A molar ratio of 1 : 1 gives a copolymer with an electrical conductivity of 260 S cm- 1 [701]. A polymer containing a thiophene-benzene-thiophene unit is chemically or electrochemically prepared from the monomer 1,4-di(2-thienyl)benzene with H, Me or OMe groups in 2,5 position of the phenylene ring [607]. An electrically conducting polymer (0.3 to 40 S cm- 1) with polymethacrylate backbone and electroactive PAT side chains, e.g. PMT and PPT, is prepared from poly(2,2'-bithienylmethyl methacrylate) which is introduced to both the electrochemical and chemical oxidative polymerization of 3-alkylthiophene

[610]. Silica gel-containing thiophene units can be obtained from molecular organosilicon precursors. The electrochemical oxidation of thiophenylene bridged gels leads to the polymerization of the thiophene units. The formation of PT within the inorganic matrix can be established by Raman spectroscopy. The polymerization which occurs in the solid state gives a composite material containing PT chains within a silica network [702].

Potential-Programmed Electropolymerization (PPEP). This technique allows compositional modulation over distances of the order of 100 A. Mesoscopic layered structures with high lateral quality are produced by the PPEP method. An improvement of the flatness and uniformity of the layered structure is achieved by careful choice of an appropriate working electrode (silicon single-crystal wafer), monomer (pyrrole/bithiophene or pyrrole/3-methylthiophene) and solvent (propylene carbonate). The PPEP method can produce materials having desirable properties tailored by the quantum size effect [703-705].

112


5.3 Comparison of Selected Chemical and Electrochemical Synthesis The differences in structure of chemically and electrochemically prepared PAT which arise from the synthetic method can be studied with ultraviolet photoemission spectroscopy [339]. The electrochemical method of synthesizing conducting polymers is rapid, and a film can be obtained in a few seconds of polarization. Each electropolymerization system formed by a monomer, a solvent, a salt, and an electrode material simultaneously develops, at least, electrochemical, chemical and degradation processes during the electrogeneration. The final result is a polymer film formed by a mixed material: an electroactive part and passive isles [629]. The common feature of the electropolymerization is the fact that the resulting material, for example PT and PMT, is not in the equilibrium state. Consequently, spontaneous relaxation processes take place during and after the growth until the PT reaches a stable low-energy state [706]. The IR spectrum of PMT chemically prepared with addition of bithiophene (cf. Sect. 5.1.2) is very similar to that of PMT prepared electrochemically with addition of bithiophene [576]. Chemically doped (I2 and FeC13) and electrochemically doped PATs (n = 6, 8, 12) exhibit similar electrical conductivity [92]. PATs (n = 4, 6, 8, 12, 18) synthesized via a chemical polymerization route are in general slightly more stable than those prepared via an electrochemical route [92, 490]. For instance, the electrical conductivity of PHT synthesized via oxidative coupling is stable up to higher temperatures than PHT obtained by electropolymerization [92, 324]. PMT and PBT films prepared according to the layer-by-layer deposition method (cf. Sect. 5.1.3) shows in comparison to electrochemically synthesized films: (i) less structural disorder, (ii) a higher degree of crystallinity, (iii) a homogeneous space filling, and (iv) a similar degree of polymerization and similar conjugation length [-587]. The plasma-polymerized (cf. Sect. 5.1.6) films (e.g. PT and PMT films) have a higher content of hydrogen than that in electrochemically polymerized films, and the electrical conductivities of the doped films are lower than those of electrochemically polymerized films [601]. The surface morphology of the PT films deposited away from the high radio frequency flux-density region is similar to that of films prepared by electrochemical methods [600]. Electropolymerized PT films have more compact morphology compared to that of chemically synthesized PT films [146]. Poly[1,2-bis(3-alkyl-2-thienyl)ethylene] is obtained by either chemical or electrochemical polymerization of 1,2-bis(3-alkyl-2-thienyl)ethylene. The sample morphology of chemically and electrochemically prepared polymers is quite different. A bulk powder is obtained by the chemical route, while homogeneous films are produced by electropolymerization. Chemical synthesis would seem to be more convenient to prepare polymers because the oxidation with FeC13 gives standard quality polymers in good yield. Electropolymerization is more sensitive to the synthesis parameters. Electrochemically prepared films are more sensitive to photooxidation [147].


113

6 Applications PT and its derivatives possess many unusual physical properties which are of great potential interest in the area of material science. PT as an electrically conductive material can only be used in such fields where other parameters such as long term stability of the electrical conductivity, general chemical and thermal stability, and stability of the solid state morphology (the content and size of crystalline domains) do not influence or change the electrophysical properties. The application of PT as a conductive material has proved to be rather problematical. So far, only one polymer has been found to be applicable as an antistatic coating material (cf. Sect. 6.7) and has achieved commercial success. Other electrophysical properties reviewed in Sects. 6.1 to 6.12 are claimed in scientific papers and patents to be of use in special applications.

6.1 Electrochromic Devices The reversible changes to the visible absorption spectra caused by electrochemical redox processes, so-called "electrochromism" (cf. Sect. 3.4.3) can be used for electrochromic displays and windows [356, 707-713]. Figure 20 shows a schematic drawing of electro-optical cell on the basis of a conducting polymer [354]. When a multicolor electrochromic device is constructed by combination of different species of electrochromic conducting polymers, an electrochromic conducting polymer is often combined with a second electrochromic polymer showing a different specific color at a quite different operation potential. Poly(aniline)-poly(sodium acrylate)-PT and poly(aniline)-poly(sodium acrylate)P M T combined films are constructed by combining poly(aniline)-poly(sodium acrylate), which shows its specific color in the oxidized state, with PT and PMT, which show their specific color in the reduced state [356]. An electroactive polymer laminate for use in an electrochromic display device comprises a conductive substrate, a first layer of an electroactive polymer, and a second layer of an electroactive polymer prepared from a second monomer having an oxidation potential higher than that of the first monomer and adhering to the first layer [712]. By combining an electrically conductive polymer (e.g. POT) prepared by spin coating from solution with a solid polymer electrolyte and a metal oxide, a solid state electrochromic device is constructed [713]. Substrates coated with PT can be used in electrochromic displays, in solar cells (cf. Sect. 6.3), and for corrosion protection [714]. Poly(3,4-ethylenedioxythiophene-2,5-diyl), which has good electrochromic properties, (for structure cf. Sect. 1.2) can used as an electrode in a solid state electrochromic cell (cf. Sect. 3.4.3) [43]. PITN can be reversibly cation- and anion-doped without decomposition. This polymer, with

114


electrolyte

m :~....~;-_~ ~'" "i :~:er

! :--:back i ,ass#ate i

,

1

polymer"Tl" ~ ~ - . ~ I f"m U ` nesa

:-:,n~gauze ' IITO) gl;uss

Fig. 20. Schematic drawing of an electro-optical cell using conductingpolymers [354]

two reversible and stable redox states of different color, is a potential candidate for electrochromic displays [253].

6.2 Electroluminescent Devices Visible red-orange PAT and blue poly(alkylfluorene) electroluminescent diodes can be successfully fabricated. The diodes consist of thin polymer films sandwiched between ITO and Mg alloy electrodes. The emission intensity of PAT diodes is stronger with longer alkyl side chain lengths (cf. Sect. 4.1.2) [363, 499, 715]. A light-emitting diode using poly(2,3-diphenylquinoxaline-5,8diyl) as a light-emitting material (ITO/poly(2,3-diphenylquinoxaline-5,8-diyl) /MgAg) emits blue-green light. Introduction of a hole-transporting layer (e.g. PT) between ITO and the light-emitting material in the diode enhances the electroluminescence efficiency by about two orders of magnitude [716]. The pulse response of emission from a PAT, e.g., PODT, consists of two independent parts: a fast and a slow transition part. The fast response corresponds to carrier transit between electrodes, and the anomalous slow response, which becomes significant at higher current, is explained by heating at the junction due to the injection current [-717]. The use of poly(thienylene vinylene) thin film as a buffer layer between ITO and poly(2,5-dialkyloxy-p-phenylene vinylene) results in increasing the breakdown voltage and increasing luminescence [718]. Further organic electroluminescence devices are described in literature [719, 720].

6.3 Solar Cells In a two-layer organic photovoltaic cell based on PMT/vacuum-evaporated rhodamine B (RB), the dedoping of PMT improves the open-circuit photovolt-

Polythiophenes


1 15

age and short-circuit photocurrent of a PMT/RB two-layer organic photovoltaic cell together with its durability [-721]. Solar cells containing layers of PMT having high energy-conversion efficiency and suitable for driving a liquid crystal display have been described [722-725]. Substrates coated with PT can be used in solar cells, electrochromic displays (cf. Sect. 6.1) and for corrosion protection [714].

6.4 Resists, Recording Materials, Fabrication of Patterns Insoluble PAT (n = 6) can be solubilized by light irradiation (Ar-ion laser and a halogen lamp as light source) in organic solvents such as chloroform in the presence of oxygen. The solubilization occurs on irradiation with light of a photon energy higher than the band gap energy of the PHT, i.e., ca. 2.2eV (560 nm). Using this phenomenon, it is possible to etch and cut the conducting polymers as well as record optical images on the conducting polymer [726]. In contrast to this, thin films of soluble PHT undergo cross-linking and therefore insolubilization on irradiation with UV/visible light [727, 728]. Irradiation of thin polymer films through a photomask and subsequent development with solvent leaves a polymer image of the mask. The resulting polymer pattern can be rendered electronically conducting by chemical oxidation. Thus, electronically conducting, organic "channels" can be fabricated using conventional photolithographic techniques [727, 728]. The electrochemical polymerization of thiophene is successfully performed in an organopolysilane film on an indium tin oxide (ITO) glass electrode as patterned by UV light irradiation. The UV-exposed polysilane is dissolved in a polar solvent such as propylene carbonate used as an electrolytic solvent and electrochemical polymerization proceeds on the ITO electrode masked with the polysilane not exposed to UV [691]. PHT films containing PhzIAsF6 are used for optical recording by light irradiation followed by washing with chloroform (cf. Sect. 4.3) [-558]. Poly(thienylene vinylene) patterns showing an electrical conductivity of 0.1 Scm -~ on I2-doping are prepared upon exposure of the polymer layer to patterned UV and subsequent development [729]. Microlithographic patterning of micrometer scale structures in PT films can be achieved by two methods [730]: (i) Trenches in oxidized Si wafers are created and filled with PT using flowing afterglow synthesis; (ii) PT films are coated with a lowtemperature spin-on glass. A photoresist is then applied, exposed, and developed. A CF4: 02 plasma transfers the lithographic pattern to the glass layer. A final O2 plasma etch processes the film and removes the photoresist at the same time [730]. A photoreceptor for electrophotography has an interlayer containing a PAT (e.g. PMT) between an electrically conductive substrate and a light-sensitive layer [731-733]. Electrophotographic photoreceptors with different thiophene derivatives are also described in the literature [734-736]. In electron beam lithography, resist charging during electron-beam exposure can lead to image distortion and pattern placement error. A PAT (e.g.,

116


a copolymer of 3-methylthiophene and 3-butylthiophene) and water-soluble poly(3-thienyl-2-ethanesulfonate) as discharge interlayer and top layer eliminates the charging problem in the electron beam exposure ]-737]. Resist materials consist of electrically conductive and UV-absorbing polymers or of organic materials containing polymers such as poly(3-methoxythiophene). When such a resist is used as the lower layer of bilevel resists, deformation of pattern and decrease of resolution are suppressed when the material is exposed to U ¥ or to an ion beam [738]. A polystyrene composite with heterocyclic polymers, e.g., poly(3-methoxythiophene), is used as the lower layer of bilevel electron beam resists to prevent charge accumulation and to achieve accurate patterning [739]. A resist composition containing a soluble conductive polymer (PDDT) and a radiation sensitive (UV, deep UV, electron beam) acid- or base-generating agent shows high sensitivity and resolution [740]. Electrically conductive structures such as PT and PBuT are used as electron beam resists [741]. A novel method of producing a negative electron beam resist uses an electron beam to both functionalize and cross-link POT in a single step to produce submicron scale polymer structures carrying functionalized groups [742,743]. A composition of POT with the cross-linker ethylene [1,2-bis(4-azido-2,3,5,6-tetrafluorobenzoate)] gives a negative deep UV resist [743]. A grid pattern is reproduced without distortion on an electrically conductive electron beam resist containing 3-substituted PT, e.g. poly(3-dodecyloxythiophene) [744,745]. Radiation resists containing novolac and conductive polymers (e.g. poly(thienylene vinylene), poly(3,4-dibutylthiophene), PPT) decomposed by irradiation with charged beams can produce fine patterns [746]. Electrically conductive materials useful for photoresists and nonlinear optical materials can contain poly(2,5-thiophenediyltetramethyldisilanylene)(cf. Sect. 1.2) [22] or PHT as a semiconducting material dispersed in an insulating polymer [747]. An electrically conductive pattern film is formed by electrochemical polymerization on a gold film which is also acting as an electrode in a desired pattern on a substrate: the gold is then dissolved by electrochemical oxidation to obtain the pattern film [748]. A method for printing on insulating material involves printing with a lipophilic ink on an insulating material covered with a layer of electrically conductive polymers [749]. Electrically conductive powdered inks for color electrothermal-transfer printing comprise ink nuclei and electrically conductive polymers (e.g. PMT) on the surface [750]. Seamless polymeric belts comprise a laminate of a host polymer layer and a laminate of a conductive polymer layer, PT, PBT. This laminate is used for an imaging process [751]. An electrothermal imaging device comprises an array of pyroelectrical sensor elements supported by a pillar of a semiconductor, e.g., PT [-752]. A material used in magnetic recording comprises PT incorporated with a homogeneous dispersion of permanent magnetic particles linked either chemically or physicochemically to the polymer or as a dopant [753]. PBT layers can be used as imaging systems for the offset printing process, as this process is based on the different wettability of printing and nonprinting areas. The neutral hydrophobic polymer


117

can be oxidized electrochemically in an electrolyte solution (Bu4NCIO4/ acetonitrile) to form the doped (oxidized) hydrophilic form of the polymer. This reaction can also be performed in reverse. Many thousand cycles of hydrophilization/hydrophobization are possible [261-263, 518, 754].

6.5 S c h o t t k y B a r r i e r D i o d e s In Schottky diodes of aluminum/POT/indium-tin oxide (ITO) with large areas prepared by a new casting technique, the rectifying behavior and junction characteristics are dependent on whether or not the POT is doped and on the storage time, but independent of the thickness of the POT layer [755, 756]. Schottky diodes with a large temperature dependence and with gas-sensitive characteristics are fabricated using aluminum and gold-tin electrodes vacuumevaporated onto PAT [757]. Schottky diodes can also be fabricated using aluminum and gold electrodes and PAT [758]. PHT, in the highly conducting state, when doped with FeC13, is an excellent semiconductor in two-terminal electronic devices (Schottky and metal-insulator-semiconductor diodes). The high electrical conductivity can lead to the observed high rectification ratios [759]. Two types of metal-insulator-semiconductor devices with sexithiophene (nT, n = 6) as semiconductor were realized: (i) with a polymeric insulator, and (ii) with a mineral insulator [760]. Metal-insulator-semiconductor Schottkytype diodes were made from dodecathiophene (12T) which is substituted with dodecyl side chains [761]. In Schottky diodes with an organic conductor (PT)/inorganic semiconductor (n-Si, n-GaAs)/metal, the metallic polymer (PT) provides a good rectifying contact to n-Si and n-GaAs semiconductors [762, 763]. Thin films of PHT and poly(pyrrole) on highly oriented pyrolytic graphite have a crystalline order in the form of micro-islands and parallel strands of polymer, and show a new effect. The corrugation measured perpendicular to the strands in constant current mode is different for positive and negative bias voltages. This difference can be attributed to the formation of a Schottky barrier between the metallic tip and the semiconductor polymer [764].

6.6 Field-Effect T r a n s i s t o r s ( F E T s ) Electrically conducting polymers can be used as the active element in metalinsulator-semiconductor field-effect transistors (MISFET). MISFETs are generally produced by spin-coating a solution of a polymer onto the surface of oxidized silicon onto which metal electrodes have previously been deposited to form source and drain contacts (Fig. 21) [765]. The operation of a field-effect transistor is described in detail in the literature [477]. A field-effect transistor can use a vacuum-deposited thin layer film of PT prepared by the organometallic method [766]. A thin film field-effect transistor

118


C6Hm PTV

PHT

Source~'::': ::.',.:"~ Oroin ~ " C N''~~ ~ " : : ~ " ~ " ~.~.="-"-1

.t. IS A~

sio,

~

\

n-Si

1 ,-.1.-Vo,

lJ

Fig. 21. Schematic cross-sectional view of a FET device [765]

prepared from PHT and poly(thienylene vinylene) by using spin-coating techniques has been described [-765, 767]. The device with a doped PHT film shows enhanced-type FET characteristics, although they are unstable; devices with dedoped poly(thienylene vinylene) films show excellent FET characteristics, are very stable, and have a carrier mobility of the same level as that of amorphous silicon transistors [593, 765]. A thin film of PMT grown electrochemically onto performed source and drain electrodes can be the active layer in a field-effect device. After electrochemical dedoping and thermal annealing in vacuo, the characteristics of the devices are similar to those made by spin-coating [768]. The performance of a FET depends on polymer thickness. The I/V characteristics of a PHT film with a thickness of 700-1000 A, are similar to those of an inorganic device. The mobility and electrical conductivity of polymer transistors decrease more rapidly for samples stored in air than for those stored in vacuo. The mobility and electrical conductivity also decrease on exposure to NH3 gas. This decrease is reversible over short periods of time [769]. The field-effect mobility and electrical conductivity of PATs increase with increasing pressure up to 2 kbar, and above 2 kbar reach saturation values of about 1.5 to 2.0 times ambient pressure [770]. The calculated field-effect mobility for pure ~x-terthiophene is in agreement with the extrapolated value inferred from field-effect mobilities in longer chain oligo(thiophene)s, and the hole mobility of cx-terthiophene increases with applied electrical field [771]. A field-effect transistor can be produced by using two different types of conducting polymers, PT and poly(pyrrole) [772]. PT acts as a semiconductor and poly(pyrrole) as a source and drain electrode. The barrier formation near the interface between source and semiconductor depresses the off-current. The introduction of the barrier against the major carrier in an accumulation-type FET is one way of obtaining a large modulation ratio of the channel current [772]. Thin film field-effect transistors are prepared by using LB films with thicknesses ranging from a monolayer to some ten monolayers as the active material (cf. Sect. 3.7) [428]. Metal oxide semiconductor field-effect transistor (MOSFET) devices work through the modulation of an accumulation layer at the

Polythiophenes


119

semiconductor - insulator interface [758]. A Schottky gated field-effect transistor uses PAT as an active material [715, 758, 773, 774], e.g., gas-sensitive and temperature-dependent Schottky gated field-effect transistors (cf. Sect. also 6.9). The temperature dependence of PET shows an increase in the source-drain current with increasing temperature which is mainly due to the increase in carrier mobility [715, 774]. Wholly organic field-effect transistors are based on various oligo(thiophene)s and insulators. These devices generally work in the accumulation mode, but can also be operated in the depletion mode. The performance of organic FETs is strongly dependent on the nature of both the semiconducting oligomer/polymer and the insulator. A general trend is the increase of field-effect mobility with increasing chain length (cf. Sect. 4.1.1). An a-substitution with alkyl chains also increases the mobility, whereas ]3-substitution leads to an almost complete disappearance of the field effect. The absence of any field-effect mobility in the distorted chains of sexiphenylene confirms that the extent of conjugation and absence of structural defects are the most significant parameters for achieving high carrier mobility [476]. The use of an organic insulator with high dielectric constant results in an increase of field-effect mobility [775].

6.7 Antistatic Coatings The prevention of static charge is required in many applications, for example for packaging materials for electronic parts or in photographic films. To eliminate these problems, the materials need to be made antistatic by coating or filling them with electrically conductive materials [45]. Soluble electrically conducting poly(3alkyloxythiophene), e.g. poly(3-ethoxythiophene), and poly(dialkyloxythiophene), e.g., poly(3,4-ethylenedioxythiophene-2,5-diyl) (cf. Sect. 1.2), or a blend of poly(3,4ethylenedioxythiophene-2,5-diyl) and polystyrenesulfonic acid are used to produce transparent, abrasion-resistant, non-corrosive coatings for films with controlled antistatic properties [40, 41, 44-46, 776, 777]. The blend of poly(3,4ethylenedioxythiophene-2,5-diyl) and polystyrenesulfonic acid is already being used on a commercial scale to produce antistatic photographic films. This blend has good electrical conductivity and environmental resistance properties. The photographic development process has no adverse effect on its conductivity [45]. This blend can be used for antistatic coatings on plastics such as transparencies for overhead projectors, packaging materials, and glass, e.g., screens [44]. Another antistatic composition comprises a 10: 90 blend of PHT and polystyrene [778] or polyheteroaromatic compounds, e.g. PBT, and polymeric sulfate [671], or an electrically non-conductive polymer matrix and POT [779]. Silver halide photographic materials containing electrically conductive polymers, e.g. PT, PMT, PET, poly(3-methoxythiophene), poly(3-cyanothiophene), poly(3-fluorothiophene), poly(3-nitrothiophene), poly(3-bromothiophene), poly(3,4-dibromothiophene), poly(3,4-dimethylthiophene), are claimed to have improved antistatic properties [780 783].

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6.8 Junction Devices, Rectifying Bilayer Electrodes A p-n-heterojunction type device can be fabricated on the basis of the junction formed between poly(pyrrole) and PT derivatives (e.g. PT or PMT) [784]. A PODT/C6o junction device with a photo-induced charge transfer between PODT and C6o is shown in Fig. 22 [449]. Rectifying bilayer electrodes with sequential bilayer structures are prepared from any pair of PBT/poly(pyrrole) and poly(3-bromothiophene)/poly(pyrrole) by anodic electropolymerization on platinum electrodes [785].

6.9 Sensors Electrochemically prepared PMT films on Pt electrodes show an enhanced anodic current response for cyclic voltammetry in nitrate and perchlorate solutions compared to the responses in other solutions owing to the oxidizing power of the nitrate and perchlorate ions. This effect may suggest the possibility of applying PMT as a sensor for oxidizing ions in aqueous solution 1-786, 787]. Gas-sensitive and temperature-dependent Schottky gated field-effect transistors contain PATs (see also 6.6). Gas sensitivity of PBuT has been investigated for air, water vapor, ethanol, and chloroform gases. These gases show enhancement of source-drain current compared to that in vacuo [715, 774]. A PMT filmmodified glassy carbon electrode with selective response to anions with different hydrated anion radius (NO;-, BF,~, ClOg, F - , H2POg, SO ] - , TOS-) has been achieved. The selectivity depends on the thickness of PMT (cf. Sect. 2.3) [152]. Biotinylated copolymers of 3-undecylthiophene and 3-thienylmethanol using streptavidin as a cross-linker protein is an electroactive matrix for the attachment of a photoactive protein, phycoerythrin. The biotinylation of the copolymer improves the film forming properties and results in a stable monolayer. The phycoerythrin binding to the biotinylated copolymer monolayer can be monitored through fluorescence microscopy at the air-water interface [451]. The covalent immobilization of an enzyme such as alcohol dehydrogenase on the surface of poly[3-(2-hydroxyethyl)thiophene] (cf. Sect. 2.1.3) is an interesting way to produce enzyme electrodes [111, 112]. Glucose oxidase is covalently immobilized at the surface of a copolymer from 3-methylthiophene

Fig. 22. Layer structure of PAT/C6ojunction [449]


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and the methyl ester of 3-thienylacetic acid [110]. A lactate sensor for the determination of lactate in micromolar concentrations is formed by activation of the carboxylic group in the copolymer made of 3-methylthiophene and 3-thienylacetic acid with dicyclohexylcarbodiimide and reaction of lactate oxidase [109]. Electrodes modified by the electrodeposition of PMT are used as chemical sensors for some organic and biological molecules. The modified surface catalyzes the oxidation of several compounds (ferri/ferrocyanide, catechol, ascorbic acid, hydroquinone, dopamine, epinephrine, acetaminophene, paminophenol, and NADH). Binary and ternary mixtures can also be analyzed, and the polymer-coated electrode is used in an amperometric detector for flow injection analysis. The responses of the polymer electrode are 4-10 times as sensitive as those of platinum [788, 789]. A chemical detector for ascorbic acid is a PBT-modified indium tin oxide (ITO) glass electrode. Reduction of PBT occurs when doped polymer films are brought into contact with an aqueous solution of ascorbic acid. This redox reaction is accurately indicated by the change in the chromatographic property of the film [790]. Platinum electrodes coated with PT and PMT and the mechanism of the anodic oxidation of N-phenyl-p-phenylenediamine on these is described in refs. [791,793]. An electrode consisting of a conducting polymer and an organic compound is useful for sensors, batteries (cf. Sect. 6.10), and electrochromic displays (cf. Sect. 6.1) [710]. Chemically synthesized Cu(II)-containing PMT is used as the working electrode of a 3-electrode system in a thin layer amperometric cell unit to detect ionic analytes in an aqueous stream by flow injection analyses. The electrode response is linearly dependent on the applied voltage. The electrode possesses favorable sensitivity and stability in comparison to other metallic electrodes such as steel and platinum [794]. The incorporation of mercury into PMT results in an effective electrode for the analyses for lead(II) ions in aqueous media (detection limit: 0.05 ppm). The mercury "films" are deposited electrochemically after the electropolymerization step [795]. The two-stage reversible reaction of bromine with poly[1,2-di(2thienyl)ethylene] can be used in a gas sensor (cf. Sect. 3.2.3) [283]. A solid support containing PT is used for the detection of pesticide residues in food and soil samples as well as in ground water [796]. Polymeric layers for polymercoated microelectrodes are fabricated by the electropolymerization of thiophene, bithiophene, 3-methylthiophene. An extension of this invention allows for the production, at low potential, of polymers with low counter ion content or with low affinity ions, both of which may be readily exchanged by ion-exchange techniques for useful agents such as proteins, antibodies, antigens, and drugs in one or several monolayers [797]. A PT-modified vitreous carbon electrode can be used as a universal response potentiometric sensor. This all-solid-state electrode is readily prepared, responds rapidly, and has good stability and reproducibility [798]. A potentiometric iodide sensor based on a PMT film electrode has been developed. This

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electrode is suitable for the measurement of iodide concentrations down to 1 x 10 - 6 M [799], PT, PBT, PMT, POT, and poly(4,4'-dioctyl-2,2'-bithiophene) are electrochemically deposited on platinum. These indicator electrodes give a cationic response to monovalent cations such as H +, Li +, Na +, K + and NH~, and show some sensitivity to divalent cations such as Mg 2+ and Ca 2 + [800]. All-solid-state ion-selective electrodes use POT as solid contact material. The electrochemically prepared POT deposited on a solid substrate (platinum, gold, or vitreous carbon) is coated with an ion-selective membrane to produce a solid-contact ion-selective electrode for several ions (Li +, Ca 2+, Cl-) [801].

6.10 Batteries Rechargeable lithium/SO2 cells generally have Li(SOz)3A1C14 as the electrolyte and porous carbon cathodes. One alternative to carbon is to use electrically conducting polymers, PMT, produced as a thin films with high surface area. With PMT films, a high discharge capacity is obtained and recharge is achieved at potentials below 3.9 V, which should avoid the corrosive effects of any chlorine formed [802-804]. A rechargeable lithium cell with a PMT cathode is produced with lithium hexafluoroarsenate-dimethylcarbonate as the electrolyte [805]. Chemically prepared PT is also used as cathode material in secondary batteries [806-809]. A lithium cell can be fabricated using an electrochemically prepared PT-coated Fe cathode and a lithium anode [810]. Cathodes having a PT membrane on a carbon-fiber current collector are described in the literature [811]. Lithium batteries using cathodes composed of a composite of electrically conducting polymers, e.g. PMT or PHT, and graphite, they possess a high capacity and long cycle life [812]. A composite of polyheteroaromatic compounds, PBT, and a polymeric sulfate [671] or of an electrically nonconductive polymer matrix and POT [779] may also be used as cathodes in batteries. Secondary batteries with a long life cycle can use anodes coated with a first protective layer (salts, oxides, hydroxides) and an elastomeric conducting polymer, e.g., PT [813]. A composite electrode consisting of an electrically conducting polymer and an organic compound can be used in batteries, sensors (cf. Sect. 6.9), and electrochromic displays (cf. Sect. 6.1) [710, 814]. Secondary batteries with excellent discharge properties and electrochromic display devices (cf. Sect. 6.1) uses poly(vinyl-2,2';5',2"-terthiophene)as an organic semiconductor [711]. Poly(thienylene vinylene) shows reversible doping-dedoping behavior and excellent charge-discharge characteristics such as a Coulombic efficiency higher than 99% and a stable electrode potential even under a high chargedischarge current, and can be used in secondary batteries [815]. Batteries with PT between the electrodes for the control of charging have been described [816]. Self-discharge in electrically conductive polymer electrodes is one of the obstacles to the practical use of polymer batteries. A conceivable mechanism is the decomposition of the electrolyte solution on the polymer surface followed by


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dedoping of the polymer. A possible way to decrease self-discharge may be to increase the density of the polymer on the electrode [817].

6.11 Shielding Materials Electrically conducting polymers can be used as shielding materials against electromagnetic interference (EMI). Measuring the shielding efficiency against electromagnetic interference on the other hand gives important data regarding the homogeneity and stability of the electrical conductivity in the polymer [818]. Blends with 20% or less POT are not readily applicable as a shielding material owing to the low electrical conductivity and the unintentional "holes" and "slits" in the material, which contains an insulating matrix polymer, e.g. polystyrene, polyvinyl chloride, or poly(ethyl vinyl acetate). The use of unblended POT would increase the electrical conductivity and improve the situation [818]. Electromagnetic shields containing compositions with polyheteroaromatic compounds, PBT, and a polymeric sulfate [671], or with an electrically non-conductive matrix polymer and POT [779] are described. A microwave shield comprising one or several layers of a doped electrically conductive polymer, PT, PBT, and an interpenetrating organic insulating polymer has been described [819].

6.12 Other Applications Fine metal particles coated with PT are protected from oxidation and have improved handling properties [820]. Electrical conductors with good heat and moisture resistance (no discoloration) contain copper powder and 0.5% PT powder [821]. The electrochemical oxidation of hydrogen, formic acid, and methanol is possible on an active electrode with a small amount of platinum dispersed into a pyrrole/bithiophene copolymer as matrix [822] or on Pt-Sn catalysts electrodeposited on a PMT [823]. The reduction of tetracyanoquinodimethane and chloranil at a PMT coated glassy carbon electrode is possible [824]. CO2 molecules can be converted to salicylic acid derivatives by photoirradiation in the presence of phenol derivatives and PHT in ethanol. PHT acts as a photocatalyst on irradiation with visible light, and the luminescence is quenched by both COg gas and phenol molecules (cf. Sect. 3.5) [367]. An electrochemical memory device can be constructed using chemically prepared PAT (n = 1, 8) [825-827]. The PMT is used as a memory channel whose electrical conductivity is varied by 3 to 4 orders of magnitude by electrochemical doping. The writing and erasing times of the memory states between minimum and maximum conductivities are 4-5 s and 1(~20 s, respectively [827, 828]. The memory states or the channel conductivities are controlled by the number of applied writing and erasing pulses [825-827]. An electrical controlling device having PT as electron-conjugated semiconductor is useful

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G, Schopf and G. KoBmehl

for memory devices [829, 830]. The reversible redox reaction of PITN may be a candidate for applications in electronic devices such as memories with learning effect. The residual charge (positive and negative) is relatively stable, and this polymer can be applied to a reading-writing device [253]. Shaped articles, e.g. rods, fibers, and films, which are electrically conductive can be manufactured from composites containing a nonconductive flexible chain-carrier polymer and a conjugated polymer (e.g., POT, PDDT, poly(thienylene vinylene)) [831,832]. Polyester blends containing PT can be used as synthetic fibers [833]. Electrically conductive resin adhesives, heat, stock, and moisture resistant with bifunctionality as adhesives and solders can consist of epoxy methacrylates and conducting polymers, e.g., PDDT, PODT [834, 835]. The IR-absorbing, transparent films of a blend of poly(3,4-ethylenedioxythiophene-2,5-diyl) (cf. Sect. 1.2) and polystyrene-sulfonic acid can be used for the fabrication of greenhouses and building glass [44]. This blend is transparent to visible light and has a high absorption at wavelengths higher than 800 nm [44, 45]. A superthin membrane can consist of a heterolayer polymer, PMT, and poly(pyrrole) [836]. Organic electrically conducting polymers such as PMT synthesized on porous polycarbonate microfilter membranes can be used for the separation of neutral solutes [153]. Passivation of the surface of n-GaAs is possible with thin films of plasmapolymerized thiophene [837]. A composition containing an electrically nonconductive polymer matrix and POT can also be used in paraboloid antennas, reflectors for radar, heating systems, photoelectric devices, and electric circuits and apparatus [779]. PTs are used for manufacturing a nonlinear two-terminal device. This device is not asymmetrical, gives stable electrical characteristics, and is useful as a display device [838, 839]. Liquid crystal display devices contain an electrically conductive polymer, e.g., PT or POT as oriented film [840-842]. PTs are also used for the production of color filters for liquid crystal displays [843]. Electrically conducting polymers represent an interesting class of materials for use in electrochemical capacitors owing to the combination of high capacitive energy density and low material cost [246, 844]. Impedance studies for both n- and p-type doped poly[3-(4-fluorophenyt)-thiophene] suggest that high power densities can be obtained in electrochemical capacitors [246, 844]. The following PT derivatives are used as solid electrolytes in solid electrolytic capacitors: PT [314, 845-879], PMT [866, 880], PET [880], PPrT [880], PBuT [881], PHT [882], POT [881], PDDT [881], PDST [881] or poly(3,4ethylenedioxythiophene-2,5-diyl)(cf. Sect. 1.2) [41, 45, 46]. A composite film of PDDT and poly(ethyleneterephthalate)is resistant to migration and can be used in a film capacitor [883]. Electrically conducting polymer gels (cf. Sect. 3.9) can be combined with other materials to establish bimorphs. The bimorph of the structure shown in Fig. 23 is prepared by successive electrochemical polymerization of pyrrole and then 3-hexylthiophene. This bimorph can be curled as shown in Fig. 23b when it


(o)

(b)

125

Fig. 23. Bimorphconstructed with polypyrrole(11)and PAT gel (IS) at a neutral state and h curled state [460]

is placed in chloroform, in which only PAT expands. The curled bimorph again becomes straight on placing it in ethanol, shrinking the PAT. The curvature can be controlled by changing the doping level [460, 884]. The response time can also be controlled by the doping potential and the electrolyte concentration [884]. A composite comprising paper and a conducting polymer can be prepared by impregnation of paper with a solution of a precursor polymer and heat treating the coated paper. The electrically conducting polymer is located on the surface or between the fibers of the paper [885]. Sexithiophene and other oligo(thiophene)s are ideal photochromic materials for efficient and ultrafast incoherent-to-coherent optical converters [886].

7 Summary, Conclusion, and Outlook Studies and experimental results in the field of PTs and their derivatives published in the literature between 1990 and 1994 are summarized in this review. In these five years, an enormous number of papers have been published. New results on structure, properties, applications and novel PT derivatives presented month by month in the literature reflect the continuing interest in PT and its derivatives. Reviews of partial results relating to PT and other electrically conducting polymers [-73, 571,887-894] and an excellent book about the physics of PT and other electrically conducting polymers [895] have also been published. Manipulations of the properties of PTs are possible by changing the primary structure (e.g., choice of monomer), the secondary structure (e.g., type of coupling between monomeric units), and the tertiary structure (e.g., surface morphology). Different PT derivatives can therefore have very different properties, such as differences in solubility, stability, absorption behavior, electrical conductivity, and etectrophysical properties. However, the systematic changing of properties by systematic changing of the structure is problematical owing to the complicated structure of the polymeric material. The investigations into the relationship between the properties and the alkyl side chain length in PATs, and between the properties and the type of coupling between monomeric units show promising results. Another problem is the systematic synthesis of polymers with desirable structures. The synthesis of structurally homogeneous PATs has achieved


(o)

(b)

125

Fig. 23. Bimorphconstructed with polypyrrole(11)and PAT gel (IS) at a neutral state and h curled state [460]

is placed in chloroform, in which only PAT expands. The curled bimorph again becomes straight on placing it in ethanol, shrinking the PAT. The curvature can be controlled by changing the doping level [460, 884]. The response time can also be controlled by the doping potential and the electrolyte concentration [884]. A composite comprising paper and a conducting polymer can be prepared by impregnation of paper with a solution of a precursor polymer and heat treating the coated paper. The electrically conducting polymer is located on the surface or between the fibers of the paper [885]. Sexithiophene and other oligo(thiophene)s are ideal photochromic materials for efficient and ultrafast incoherent-to-coherent optical converters [886].

7 Summary, Conclusion, and Outlook Studies and experimental results in the field of PTs and their derivatives published in the literature between 1990 and 1994 are summarized in this review. In these five years, an enormous number of papers have been published. New results on structure, properties, applications and novel PT derivatives presented month by month in the literature reflect the continuing interest in PT and its derivatives. Reviews of partial results relating to PT and other electrically conducting polymers [-73, 571,887-894] and an excellent book about the physics of PT and other electrically conducting polymers [895] have also been published. Manipulations of the properties of PTs are possible by changing the primary structure (e.g., choice of monomer), the secondary structure (e.g., type of coupling between monomeric units), and the tertiary structure (e.g., surface morphology). Different PT derivatives can therefore have very different properties, such as differences in solubility, stability, absorption behavior, electrical conductivity, and etectrophysical properties. However, the systematic changing of properties by systematic changing of the structure is problematical owing to the complicated structure of the polymeric material. The investigations into the relationship between the properties and the alkyl side chain length in PATs, and between the properties and the type of coupling between monomeric units show promising results. Another problem is the systematic synthesis of polymers with desirable structures. The synthesis of structurally homogeneous PATs has achieved

126


noteworthy progress over the last 5 years. This synthesis was one of the prerequisites for investigating the relationship between the type of coupling between monomeric units and the properties of the polymer. Considering the various properties and the various syntheses of the PT derivatives, it is not surprising that their applications are so abundant. Their electrical and electrochemical properties are employed in several devices such as batteries, field-effect transistors, Schottky diodes, sensors, capacitors, shielding materials, solar cells, antistatic coatings, and catalysts. The optical properties are also employed in electrochromic and electroluminescent devices and in solar cells. Other properties such as the changes in the volume of a polymer gel depending on the solution in which it is immersed may also be used (e.g., in biomorphs). These conducting polymers, so-called "organic metals" or "synthetic metals", will play an interesting fundamental role in the basic research of chemistry, physics, material sciences, and applied sciences in the future. Further knowledge of synthesis-structure-properties relationships will improve the route that starts with the identification of a need and finally arrives at the identification and synthesis of the new polymer structures required. The actual and potential applications of PT and its derivatives indicate that they will play an important role in daily life in the near future. We hope that our review of PT and its derivatives taken from the literature published between 1990 and 1994 is complete and comprehensible. We offer our apologies to any author whose paper we may have inadvertently overlooked.

8 References

1. Chiang CK, Fincher CR, jr., Park YW, Heeger AJ, Shirakawa H, Louis EJ, Gau SC, MacDiarmid AG (1977) Phys Rev Lett 39:1098 2. TouriUon G (1986) Polythiophene and its Derivatives. In: Skotheim TA (ed) Handbook of Conducting Polymers. Marcel Dekker, New York, p 293 3. KoBmehl GA (1986) Semiconducting and Conducting Polymers with Aromatic and Heteroaromatic Units. In: Skotheim TA (ed) Handbook of Conducting Polymers. Marcel Dekker, New York, p 351 4. Kol3rnehl G (1986) Makromol Chem, Macromol Symp 4:45 5. Roncali J (1992) Chem Rev 92:711 6. Onoda M, Iwasa T, Kawai T, Nakayama J, Nakahara H, Yoshino K (1993) J Electrochem Soc 140:397 7. Iwasa T, Kawai T, Onoda M, Nakayama J, Nakahara H, Yoshino K (1992) J Phys Soc Jpn 61: 666 8. KoBmehl G, Hoppe FD (1993) Z Naturforsch 48b: 1807 9. KoSmehl G, Hoppe FD (1994) Mol Cryst Liq Cryst 257:169 10. Kol~mehl G, Hoppe FD, Hirsch B (1993) Z Naturforsch 48b: 826 11. Pagani G (1994) Heterocycles 37:2069 12. Rutherford DR, Stille JK, Elliott CM, Reichert VR (1992) Macromolecules 25:2294 13. Nawa K, Shirota Y (1992) Jpn Kokai Tokkyo Koho, JP 04108784, 6 pp 14. Iwatsuki A, Kubo M (1990) Jpn Kokai Tokkyo Koho, JP 02229825, 3 pp 15. Braeunling H, Bloechl G, Becker R (1991) Synth Met 41:487 16. Braeunling H, Becket R, Bloechl G (1991) Synth Met 42:1539

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noteworthy progress over the last 5 years. This synthesis was one of the prerequisites for investigating the relationship between the type of coupling between monomeric units and the properties of the polymer. Considering the various properties and the various syntheses of the PT derivatives, it is not surprising that their applications are so abundant. Their electrical and electrochemical properties are employed in several devices such as batteries, field-effect transistors, Schottky diodes, sensors, capacitors, shielding materials, solar cells, antistatic coatings, and catalysts. The optical properties are also employed in electrochromic and electroluminescent devices and in solar cells. Other properties such as the changes in the volume of a polymer gel depending on the solution in which it is immersed may also be used (e.g., in biomorphs). These conducting polymers, so-called "organic metals" or "synthetic metals", will play an interesting fundamental role in the basic research of chemistry, physics, material sciences, and applied sciences in the future. Further knowledge of synthesis-structure-properties relationships will improve the route that starts with the identification of a need and finally arrives at the identification and synthesis of the new polymer structures required. The actual and potential applications of PT and its derivatives indicate that they will play an important role in daily life in the near future. We hope that our review of PT and its derivatives taken from the literature published between 1990 and 1994 is complete and comprehensible. We offer our apologies to any author whose paper we may have inadvertently overlooked.

8 References

1. Chiang CK, Fincher CR, jr., Park YW, Heeger AJ, Shirakawa H, Louis EJ, Gau SC, MacDiarmid AG (1977) Phys Rev Lett 39:1098 2. TouriUon G (1986) Polythiophene and its Derivatives. In: Skotheim TA (ed) Handbook of Conducting Polymers. Marcel Dekker, New York, p 293 3. KoBmehl GA (1986) Semiconducting and Conducting Polymers with Aromatic and Heteroaromatic Units. In: Skotheim TA (ed) Handbook of Conducting Polymers. Marcel Dekker, New York, p 351 4. Kol3rnehl G (1986) Makromol Chem, Macromol Symp 4:45 5. Roncali J (1992) Chem Rev 92:711 6. Onoda M, Iwasa T, Kawai T, Nakayama J, Nakahara H, Yoshino K (1993) J Electrochem Soc 140:397 7. Iwasa T, Kawai T, Onoda M, Nakayama J, Nakahara H, Yoshino K (1992) J Phys Soc Jpn 61: 666 8. KoBmehl G, Hoppe FD (1993) Z Naturforsch 48b: 1807 9. KoSmehl G, Hoppe FD (1994) Mol Cryst Liq Cryst 257:169 10. Kol~mehl G, Hoppe FD, Hirsch B (1993) Z Naturforsch 48b: 826 11. Pagani G (1994) Heterocycles 37:2069 12. Rutherford DR, Stille JK, Elliott CM, Reichert VR (1992) Macromolecules 25:2294 13. Nawa K, Shirota Y (1992) Jpn Kokai Tokkyo Koho, JP 04108784, 6 pp 14. Iwatsuki A, Kubo M (1990) Jpn Kokai Tokkyo Koho, JP 02229825, 3 pp 15. Braeunling H, Bloechl G, Becker R (1991) Synth Met 41:487 16. Braeunling H, Becket R, Bloechl G (1991) Synth Met 42:1539

Polythiophenes 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63.


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Author Index Volumes 101-129 Author Index Vols. 1-I00 see Vol. 100

Adolf D. B. see Ediger, M. D.: Vol. 116, pp. 73-110. Aharoni, S. M. and Edwards, S. F. : Rigid Polymer Networks. Vol. 118, pp. 1-231. Amdduri, B., Boutevin, B. and Gramain, P. : Synthesis of Block Copolymers by Radical Polymerization and Telomerization. Vol. 127, pp. 87-142. Amdduri, B. and Boutevin, B.: Synthesis and Properties of Fluorinated Telechelic Monodispersed Compounds. Vol. 102, pp. 133-170. Amselem, S. see Domb, A. J.: Vol. 107, pp. 93-142. Andrady, A. L.: Wavelenght Sensitivity in Polymer Photodegradation. Vol. 128, pp. 47-94. Andreis, M. and Koenig, a~ L. : Application of Nitrogen- 15 NMR to Polymers. Vol. 124, pp. 191-238. Angiolini, L. see Carlini, C.: Vol. 123, pp. 127-214. Anseth, K. S., Newman, S. M. and Bowman, C. N.: Polymeric Dental Composites: Properties and Reaction Behavior of Multimethacrylate Dental Restorations. Vol. 122, pp. 177-218. Armitage, B. A. see O'Brien, D. F.: Vol. 126, pp. 53-58. Arndt, M. see Kaminski, W.: Vol. 127, pp. 143-187. Arnold Jr., F. E. and Arnold, F. E. : Rigid-Rod Polymers and Molecular Composites. Vol. 117, pp. 257-296. Arshady, R. : Polymer Synthesis via Activated Esters: A New Dimension of Creativity in Macromolecular Chemistry. Vol. 111, pp. 1-42. Bahar, 1.. Erman, B. and Monnerie, L.: Effect of Molecular Structure on Local Chain Dynamics: Analytical Approaches and Computational Methods. Vol. 116, pp. 145-206. Balt6-Calleja, F. J, Gonz~lez Arehe, A., Ezquerra, T. A., Santa Cruz, C., Batall6n, F., Frick, B. and L6pez Cabarcos, E.: Structure and Properties of Ferroelectric Copolymers of Poly(vinylidene) Fluoride. Vol. 108, pp. 1-48. Barshtein, G. R. and Sabsai, O. E: Compositions with Mineralorganic Fillers. Vol. 101, pp.t-28. Batall6n, F. see Balt~-Calleja, F. J.: Vol. 108, pp. 1-48. Barton, J. see Hunkeler, D.: Vol. 112, pp. 115-134. Bell, C L. and Peppas, N. A.: Biomedical Membranes from Hydrogels and Interpolymer Complexes. Vol. 122, pp. 125-176. Bennett, D. E. see O'Brien, D. F.: Vol. 126, pp. 53-84. Berry, G.C: Static and Dynamic Light Scattering on Moderately Concentraded Solutions: Isotropic Solutions of Flexible and Rodlike Chains and Nematic Solutions of Rodlike Chains. Vol. 114, pp. 233-290. Bershtein, V. A. and Ryzhov, V. A. : Far Infrared Spectroscopy of Polymers. Vol. 114, pp. 43-122. Bigg, D. M.: Thermal Conductivity of Heterophase Polymer Compositions. Vol. 119, pp. 1-30.

148

Author Index Volumes 101-129

Binder, K.: Phase Transitions in Polymer Blends and Block Copolymer Melts: Some Recent Developments. Vol. 1 I2, pp. 115-134. Bird, R. B. see Curtiss, C. F.: Vol. 125, pp. 1-102. Biswas, M. and Mukherjee, A.: Synthesis and Evaluation of Metal-Containing Polymers. Vol. 115, pp. 89-124. Boutevin, B. and Robin, J. J.: Synthesis and Properties of Fluorinated Diols. Vol. 102. pp. 105-132. Boutevin, B. see Am6douri, B.: Vol. 102, pp. 133-170. Boutevin, B. see Am6duri, B.: Vol. 127, pp. 87-142. Bowman, C. N. see Anseth, K. S.: Vol. 122, pp. 177-218. Boyd, R. H.: Prediction of Polymer Crystal Structures and Properties. Vol. 116, pp. 1-26. Bronnikov, S. V., Vettegren, K 1. and Frenkel, S. Y.: Kinetics of Deformation and Relaxation in Highly Oriented Polymers. Vol. 125, pp. 103-146. Bruza, K. J see Kirchhoff, R. A.: Vol. 117, pp. 1-66. Burban, J. 1-1.see Cussler, E. L.: Vol. 110, pp. 67-80. Cameron, N. R. and Sherrington, D. C.: High Internal Phase Emulsions (HIPEs)-Structure, Properties and Use in Polymer Preparation. Vol. 126, pp. 163-214. Candau, F. see Hunkeler, D.: Vol. 112, pp. 115-134. Capek, I.: Kinetics of the Free-Radical Emulsion Polymerization of Vinyl Chloride. Vol. 120, pp. 135-206. CarlinL C. and Angiolini, L.: Polymers as Free Radical Photoinitiators. Vol. 123, pp. 127214. Casas-Vazquez, J. see Jou, D.: Vol. 120, pp. 207-266. Chen, P. see Jaffe, M.: Vol. 117, pp. 297-328. Choe, E.-W. see Jaffe, M.: Vol. 117, pp. 297-328. Chow, T. S.: Glassy State Relaxation and Deformation in Polymers. Vol. 103, pp. 149-190. Chung, T.-S. see Jaffe, M.: Vol. 117, pp. 297-328. Connell, J. W. see Hergenrother, P. M.: Vol. 117, pp. 67-110. Criado-Sancho, M. see Jou, D.: Vol. 120, pp. 207-266. Curro, o~G. see Schweizer, K.S.: Vol. 116, pp. 319-378. Curtiss, C. F. and Bird, R. B.: Statistical Mechanics of Transport Phenomena: Polymeric Liquid Mixtures. Vol. 125, pp. 1-102. Cussler, E. L., Wang, K. L. and Burban, J. H. : Hydrogels as Separation Agents. Vol. 110, pp. 67-80. Dimonie, M. K see Hunkeler, D.: Vol. 112, pp. 115-134. Dodd, L. R. and Theodorou,/3. N.: Atomistic Monte Carlo Simulation and Continuum Mean Field Theory of the Structure and Equation of State Properties of Alkane and Polymer Melts. Vol. 116, pp. 249-282. Doelker, E.: Cellulose Derivatives. Vol. 107, pp. 199-266. Domb, A. J., Amselem, S., Shah, J. and Maniar, M.: Polyanhydrides: Synthesis and Characterization. Vol. 107, pp. 93-142. Dubrovskii, S. A. see Kazanskii, K. S.: Vol. 104, pp. 97-134. Dunkin, I. R. see Steinke, J.: Vol. 123, pp. 81-126. Economy, ,i.. and Goranov, K.: Thermotropic Liquid Crystalline Polymers for High Performance Applications. Vol. 117, pp. 221-256.


149

Ediger M. D. and Adolf D. B.: Brownian Dynamics Simulations of Local Polymer Dynamics. Vol. 116, pp. 73-110. Edwards, S. F. see Aharoni, S. M.: Vol. 118, pp. 1-231. Endo, T. see Yagci, Y.: Vol. 127, pp. 59-86. Erman, B. see Bahar, I.: Vol. 116, pp. 145-206. Ezquerra, T. A. see Balt~-Calleja, F. J.: Vol. 108, pp. 1-48. Fendler, ~H.: Membrane-Mimetic Approach to Advanced Materials. Vol. 113, pp. 1-209. Fetters, L. J. see Xu, Z.: Vol. 120, pp. 1-50. FOrster, S. and Schmidt, M.: Polyelectrolytes in Solution. Vol. 120, pp. 51-134. Frenkel, S. Y. see Bronnikov, S. V.: Vol. 125, pp. 103-146. Frick, B. see Balt~.-Calleja, F. J.: Vol. 108, pp. 1-48. Fridman, M. L.: see Terent'eva, J. P.: Vol. 101, pp. 29-64. Ganesh, K. see Kishore, K.: Vol. 121, pp. 81-122. Geckeler, K. E. see Rivas, B.: Vol. 102, pp. 171-188. Geckeler, K. E.: Soluble Polymer Supports for Liquid-Phase Synthesis. Vol. 121, pp. 31-80. Gehrke, S. H.: Synthesis, Equilibrium Swelling, Kinetics Permeability and Applications of Environmentally Responsive Gels. Vol. 110, pp. 81-144. Godovsky, D. Y.: Electron Behavior and Magnetic Properties Polymer-Nanocomposites. Vol. 119, pp. 79-122. GonzdlezArche, A. see Balt~.-Calleja, F. J.: Vol. 108, pp. 1-48. Goranov, K. see Economy, J.: Vol. 117, pp. 221-256. Gramain, P. see Am6duri, B.: Vol. 127, pp. 87-142. Grosberg, A. and Nechaev, S.: Polymer Topology. Vol. 106, pp. 1-30. Grubbs, R., Risse, VK and Novac, B.: The Development of Well-defined Catalysts for Ring-Opening Olefin Metathesis. Vol. 102, pp. 47-72. van Gunsteren, W. F. see Gusev, A. A.: Vol. 116, pp. 207-248. Gusev, A. A., Mfdler-Plathe, F., van Gunsteren, W. F. and Suter, U. W.: Dynamics of Small Molecules in Bulk Polymers. Vol. 116, pp. 207-248. Guillot, J. see Hunkeler, D.: Vol. 112, pp. 115-134. Guyot, A. and Tauer, K.: Reactive Surfactants in Emulsion Polymerization. Vol. 111, pp. 43 -66. Hadjichristidis, N. see Xu, Z.: Vol. 120, pp. 1-50. Hall H. K. see Penelle, J.: Vol. 102, pp. 73-104. Hammouda, B.'. SANS from Homogeneous Polymer Mixtures: A Unified Overview. Vol. 106, pp. 87-134. Hedrick, ,~ L. see Hergenrother, P. M.: Vol. 117, pp. 67-110. Heller, J.: Poly (Ortho Esters). Vol. 107, pp. 41-92. Hemielec, A. A. see Hunkeler, D.: Vol. 112, pp. 115-134. Hergenrother, P. M, Connell, J. W., Labadie, J. W. and Hedrick, J. L. : Poly(arylene ether)s Containing Heterocyclic Units. Vol. 117, pp. 67-110. Hiramatsu, N. see Matsushige, M.: Vol. 125, pp. 147-186. Hirasa, O. see Suzuki, M.: Vol. 110, pp. 241-262. Hirotsu, S. : Coexistence of Phases and the Nature of First-Order Transition in Poly-Nisopropylacrylamide Gels. Vol. 110, pp. 1-26. Hunkeler, D., Candau, F., Pichot, C., Hemielec, A. E., Xie, T. Y., Barton, J., Vaskova, V., Guillot, J., Dimonie, M. V., Reiehert, K. H.: Heterophase Polymerization: A Physical and Kinetic Comparision and Categorization. Vol. 112, pp. 115-134.

150


Ichikawa, T. see Yoshida, H.: Vol. 105, pp. 3-36. Ilavsky, M. : Effect on Phase Transition on Swelling and Mechanical Behavior of Synthetic Hydrogels. Vol. 109, pp. 173-206. lnomata, H. see Saito, S.: Vol. 106, pp. 207-232. lrie, M.: Stimuli-Responsive Poly(N-isopropylacrylamide), Photo- and Chemical-Induced Phase Transitions. Vol. 110, pp. 49-66. Ise, N. see Matsuoka, H.: Vol. 114, pp. 187-232. lvanov, A. E. see Zubov, V. P.: Vol. 104, pp. 135-176. Jaffe, M., Chen, P., Choe, E.-W., Chung, T.-S. and Makhija, S.: High Performance Polymer Blends. Vol. 117, pp. 297-328. Jou, D., Casas-Vazquez, J. and Criado-Sancho, M.: Thermodynamics of Polymer Solutions under Flow: Phase Separation and Polymer Degradation. Vol. 120, pp. 207-266. Kaetsu, L: Radiation Synthesis of Polymeric Materials for Biomedical and Biochemical Applications. Vol. 105, pp. 81-98. Kaminski, W. and Arndt,/14.: Metallocenes for Polymer Catalysis. Vol. 127, pp. 143-187. Kammer, H. W., Kressler, H. and Kummerloewe, C. : Phase Behavior of Polymer Blends Effects of Thermodynamics and Rheology. Vol. 106, pp. 31-86. Kandyrin, L. B. and Kuleznev, V. N.: The Dependence of Viscosity on the Composition of Concentrated Dispersions and the Free Volume Concept of Disperse Systems. Vol. 103, pp. 103-148. Kaneko, M. see Ramaraj, R.: Vol. 123, pp. 215-242. Kang, E. T., Neoh, K. G. and Tan, K. L.: X-Ray Photoelectron Spectroscopic Studies of Electroactive Polymers. Vol. 106, pp. 135-190. KazanskiL K. S. and Dubrovskii, S. A. : Chemistry and Physics of,,Agricultural" Hydrogels. Vol. 104, pp. 97-134. Kennedy, J. P. see Majoros, I.: Vol. 112, pp. 1-113. Khokhlov, A., Starodybtzev, S. and Vasilevskaya, V.: Conformational Transitions of Polymer Gels: Theory and Experiment. Vol. 109, pp. 121-172. Kilian, 14. G. and Pieper, T.: Packing of Chain Segments. A Method for Describing X-Ray Patterns of Crystalline, Liquid Crystalline and Non-Crystalline Polymers. Vol. 108, pp. 49-90. Kishore, K. and Ganesh, K.: Polymers Containing Disulfide, Tetrasulfide, Diselenide and Ditelluride Linkages in the Main Chain. Vol. 121, pp. 81-122. Klier, J. see Scranton, A. B.: Vol. 122, pp. 1-54. KobayashL S., Shoda, S. and Uyama, H.: Enzymatic Polymerization and Oligomerization. Vol. 121, pp. 1-30. Koenig, J. L. see Andreis, M.: Vol. 124, pp. 191-238. Kokufuta, E.: Novel Applications for Stimulus-Sensitive Polymer Gels in the Preparation of Functional Immobilized Biocatalysts. Vol. 110, pp. 157-178. Konno, M. see Saito, S.: Vol. 109, pp. 207-232. Kopecek~ J. see Putnam, D.: Vol. 122, pp. 55-124. Koflmehl, G. see Schopf, G.: Vol. 129, pp. 1-145. Kressler, J. see Kammer, H. W.: Vol. 106, pp. 31-86. Kirchhoff R. A. and Bruza, 1£. J.: Polymers from Benzocyclobutenes. Vol. 117, pp. 1-66. Kuchanov, S. I.: Modern Aspects of Quantitative Theory of Free-Radical Copolymerization. Vol. 103, pp. 1-102. Kuleznev, V. N. see Kandyrin, L. B.: Vol. 103, pp. 103-148.


151

Kulichkhin, S. G. see Malkin, A. Y.: Vol. 101, pp. 217-258. Kummerloewe, C. see Kammer, H. W.: Vol. 106, pp. 31-86. Kuznetsova, N. P. see Samsonov, G. V.: Vot. 104, pp. 1-50.Labadie, J. W. see Hergenrother, P. M.: Vol. 117, pp. 67-110.

Lamparski, 11. G. see O'Brien, D. F.: Vol. 126, pp. 53-84. Laschewsky, A.: Molecular Concepts, Self-Organisation and Properties of Polysoaps. Vol. 124, pp. 1-86.

Laso, M. see Leontidis, E.: Vol. 116, pp. 283-318. Lazdtr, ~kL and Rychlf~, R : Oxidation of Hydrocarbon Polymers. Vol. 102, pp. 189-222. Lenz, R. IV.: Biodegradable Polymers. Vol. 107, pp. 1-40. Leontidis, E., de Pablo, I J., Laso, M. and Suter, U 144: A Critical Evaluation of Novel Algorithms for the Off-Lattice Monte Carlo Simulation of Condensed Polymer Phases. Vol. 116, pp. 283-318. Lesec, J. see Viovy, J.-L.: Vol. t 14, pp. 1-42. Liang, G. L. see Sumpter, B. G.: Vol. 116, pp. 27-72. Lin, J. and Sherrington, D. C.: Recent Developments in the Synthesis, Thermostability and Liquid Crystal Properties of Aromatic Polyamides. Vol. 111, pp. 177-220. L6pez Cabarcos, E. see Balta-Calleja, F. J.: Vol. 108, pp. 1-48.

Majoros, L, Nagy, A. and Kennedy, J. P.: Conventional and Living Carbocationic Polymerizations United. I. A Comprehensive Model and New Diagnostic Method to Probe the Mechanism of Homopolymerizations. Vol. 112, pp. 1-113. Makhija, S. see Jaffe, M.: Vol. 117, pp. 297-328. Malkin, A. Y. and Kulichkhin, S. G.: Rheokinetics of Curing. Vol. 101, pp. 217-258. Maniar, lff see Domb, A. J.: Vol. 107, pp. 93-142. Matsumoto, A. : Free-Radical Crosslinking Polymerization and Copolymerization of Multivinyl Compounds. Vol. 123, pp. 41-80. Matsuoka, H. and Ise, N. : Small-Angle and Ultra-Small Angle Scattering Study of the Ordered Structure in Polyelectrolyte Solutions and Colloidal Dispersions. Vol. 114, pp. 187-232. Matsushige, K., Hiramatsu, N.and Okabe, 11.: Ultrasonic Spectroscopy for Polymeric Materials. Vol. 125, pp. 147-186. Mays, 144.see Xu, Z.: Vol. 120, pp. 1-50. Mikos, A. G. see Thomson, R. C.: Vol. 122, pp. 245-274. Miyasaka, K. : PVA-Iodine Complexes: Formation, Structure and Properties. Vol. 108. pp. 91-130. Monnerie, L. see Bahar, I.: Vol. 116, pp. 145-206. Morishima, E : Photoinduced Electron Transfer in Amphiphilic Polyelectrolyte Systems. Vol. 104, pp. 51-96. Miillen, K. see Scherf, U.: Vol. 123, pp. 1-40. Miiller-Plathe, 1=. see Gusev, A. A.: Vol. 116, pp. 207-248. Mukerherjee, A. see Biswas, M.: Vol. 115, pp. 89-124. Mylnikov, V.: Photoconducting Polymers. Vol. 115, pp. 1-88.

Nagy, A. see Majoros, I.: Vol. 112, pp. 1-11. Narasinham, B., Peppas, N. A.: The Physics of Polymer Dissolution: Modeling Approaches and Experimental Behavior. Vol. 128, pp. 157-208.

Nechaev, S. see Grosberg, A.: Vol. 106, pp. 1-30. Neoh, K. G. see Kang, E. T.: Vol. 106, pp. 135-190.

152


Newman, S. M. see Anseth, K. S.: Vol. 122, pp. 177-218. Noid, D. W. see Sumpter, B. G.: Vol. 116, pp. 27-72. Novac, B. see Grubbs, R.: Vol. 102, pp. 47-72. Novikov, V. V. see Privalko, V. P.: Vol. 119, pp. 31-78. O'Brien, D. F., Armitage, B. A., Bennett, D. E. and LamparskL H. G.: Polymerization and Domain Formation in Lipid Assemblies. Vol. 126, pp. 53-84 Ogasawara, M. : Application of Pulse Radiolysis to the Study of Polymers and Polymerizations. Vol.105, pp.37-80. Okabe, 1-1.see Matsushige, K.: Vol. 125, pp. 147-186. Okada, M.: Ring-Opening Polymerization of Bicyclic and Spiro Compounds. Reactivities and Polymerization Mechanisms. Vol. 102, pp. 1-46. Okano, T.: Molecular Design of Temperature-Responsive Polymers as Intelligent Materials. Vol. 110, pp. 179-198. Onuki, A.: Theory of Phase Transition in Polymer Gels. Vol. 109, pp. 63-120. Osad'ko, I.S.: Selective Spectroscopy of Chromophore Doped Polymers and Glasses. Vol. 114, pp. 123-186. de Pablo, ,1. ,1. see Leontidis, E.: Vol. 116, pp. 283-318. Padias, A. B. see Penelle, J.: Vol. 102, pp. 73-104. Pascault, J.-P. see Williams, R. J. J.: Vol. 128, pp. 95-156. Pasch, H.: Analysis of Complex Polymers by Interaction Chromatography. Vol. 128, pp. 1-46.

Penelle, J., Hall, H. K., Padias, A. B. and Tanaka, H.: Captodative Olefins in Polymer Chemistry. Vol. 102, pp. 73-104. Peppas, N. A. see Bell, C. L.: Vol. 122, pp. 125-176. Peppas, N. A. see Narasimhan, B.: Vol. 128, pp. 157-208. Pichot, C. see Hunkeler, D.: Vol. 112, pp. 115-134. Pieper, T. see Kilian, H. G.: Vol. 108, pp. 49-90. Posplgil, J.: Functionalized Oligomers and Polymers as Stabilizers for Conventional Polymers. Vol. 101, pp. 65-168. Posplgil, ,i.: Aromatic and Heterocyclic Amines in Polymer Stabilization. Vol. 124, pp. 87-190. Priddy, D. B. : Recent Advances in Styrene Polymerization. Vol. 111, pp. 67-114. Priddy, D. B. : Thermal Discoloration Chemistry of Styrene-co-Acrylonitrile. Vol. 121, pp. 123-154. Privalko, V. P. and Novikov, V. V.: Model Treatments of the Heat Conductivity of Heterogeneous Polymers. Vol. 119, pp 3 !-78. Putnam, D. and Kopecek, J.: Polymer Conjugates with Anticancer Acitivity. Vol. 122, pp. 55-124. Ramaraj, R. and Kaneko, M.: Metal Complex in Polymer Membrane as a Model for Photosynthetic Oxygen Evolving Center. Vol. 123, pp. 215-242. Rangarajan, B. see Scranton, A. B.: Vol. 122, pp. 1-54. Reichert, K. H. see Hunkeler, D.: Vol. 112, pp. 115-134. Risse, W. see Grubbs, R.: Vol. 102, pp. 47-72. Rivas, B. L. and Geckeler, K. E.: Synthesis and Metal Complexation of Poly(ethyleneimine) and Derivatives. Vol. 102, pp. 171-188. Robin, J. J. see Boutevin, B.: Vol. 102, pp. 105-132.


153

Roe, R.-J.: MD Simulation Study of Glass Transition and Short Time Dynamics in Polymer Liquids. Vol. 116, pp. 111-114. Rozenberg, B. A. see Williams, R. J. J.: Voh 128, pp. 95-156. Ruckenstein, E.: Concentrated Emulsion Polymerization. Voh 127, pp. 1-58. Rusanov, A. L.: Novel Bis (Naphtalic Anhydrides) and Their Polyheteroarylenes with Improved Processability. Vol. I 11, pp. 115-176. Rychlf4 J. see Laz~tr, M.: Vol. 102, pp. 189-222. Ryzhov, 1~ A. see Bershtein, V. A.: Vol. 114, pp. 43-122. Sabsai, O. Y. see Barshtein, G. R.: Vol. 101, pp. 1-28. Saburov, V. V. see Zubov, V. P.: Vol. 104, pp. 135-176. Saito, S., Konno, M. and Inomata, H.: Volume Phase Transition of N-Alkylacrylamide Gels. Vol. 109, pp. 207-232. Samsonov, G. V. and Kuznetsova, N. P.: Crosslinked Polyeleetrolytes in Biology. Vol. 104, pp. 1-50. Santa Cruz, C. see Balt~-Calleja, F. J.: Vol. 108, pp. 1-48. Sato, T. and Teramoto, A.: Concentrated Solutions of Liquid-Christalline Polymers. Vol. 126, pp. 85-162. Scherf U. and MiHlen, K.: The Synthesis of Ladder Polymers. Vol. 123, pp. 1-40. Schmidt, M. see F0rster, S.: Voh 120, pp. 51-134. Sehopf G. and Koflmehl, G.: Polythiophenes - Electrically Conductive Polymers. Voh 129, pp. 1-145. Sehweizer, K. S.: Prism Theory of the Structure, Thermodynamics, and Phase Transitions of Polymer Liquids and Alloys. Voh 116, pp. 319-378. Scranton, A. B., Rangarajan, B. and Klier, J.: Biomedical Applications of Polyelectrolytes. Vol. 122, pp. 1-54. Sefion, M~ lA and Stevenson, W. Z K.: Microencapsulation of Live Animal Cells Using Polycrylates. Vol. 107, pp. 143-198. Shamanin, V. V.: Bases of the Axiomatic Theory of Addition Polymerization. Vol. 112, pp. 135-180. Sherrington, D. C. see Cameron, N. R., Vol. 126, pp. 163-214. Sherrington, D. C. see Lin, J.: Vol. 111, pp. 177-220. Sherrington, D. C. see Steinke, J.: Voh 123, pp. 81-126. Shibayama, M. see Tanaka, T.: Voh 109, pp. 1-62. Shoda, S. see Kobayashi, S.: Voh 121, pp. 1-30. Siegel, R. A.: Hydrophobic Weak Polyelectrolyte Gels: Studies of Swelling Equilibria and Kinetics. Voh 109, pp. 233-268. Singh, R. P. see Sivaram, S.: Voh 101, pp. 169-216. Sivaram, S. and Singh, R. P.: Degradation and Stabilization ofEthylene-Propylene Copolymers and Their Blends: A Critical Review. Voh 101, pp. 169-216. Starodybtzev, S. see Khokhlov, A.: Vol. 109, pp. 121-172. Steinke, J., Sherrington, D. C. and Dunkin, I. R. : Imprinting of Synthetic Polymers Using Molecular Templates. Vol. 123, pp. 81-126. Stenzenberger, H. D. : Addition Polyimides. Vol. 1 t7, pp. 165-220. Stevenson, V£ T. K. see Sefton, M. V.: Voh 107, pp. 143-198. Sumpter, B. G., Noid, D. W., Liang, G. L. and Wunderlieh, B. : Atomistic Dynamics of Macromolecular Crystals. Vol. 116, pp. 27-72. Suter, U W. see Gusev, A. A.: Vol. 116, pp. 207-248. Suter, U. W. see Leontidis, E.: Vol. 116, pp. 283-318.

154


Suzuki, A.: Phase Transition in Gels of Sub-Millimeter Size Induced by Interaction with Stimuli. Vol. 110, pp. 199-240.

Suzuki, A. and Hirasa, O. : An Approach to Artifical Muscle by Polymer Gels due to MicroPhase Separation. Vol. 110, pp. 241-262.

Tagawa, S.: Radiation Effects on Ion Beams on Polymers. Vol. 105, pp. 99-116. Tan, K. L. see Kang, E. T.: Vol. 106, pp. 135-190. Tanaka, T. see Penelle, J.: Vol. 102, pp. 73-104. Tanaka, H. and Shibayama, M.: Phase Transition and Related Phenomena of Polymer Gels. Vol. 109, pp. 1-62.

Tauer, K. see Guyot, A.: Vol. 111, pp. 43-66. Teramoto, A. see Sato, T.: Vol. 126, pp. 85-162. Terent'eva, J. P. and Fridman, M. L.: Compositions Based on Aminoresins. Vol. 101, pp. 29-64.

Theodorou, D. N. see Dodd, L. R.: Vol. 116, pp. 249-282. Thomson, R. C., Wake, M. C., YaszemskL M. J. and Mikos, A. G.: Biodegradable Polymer Scaffolds to Regenerate Organs. Vol. 122, pp. 245-274.

Tokita, M.: Friction Between Polymer Networks of Gels and Solvent. Vol. 110, pp. 27-48. Tsuruta, T.: Contemporary Topics in Polymeric Materials for Biomedical Applications. Vol. 126, pp. 1-52.

Uyama, 1-1.see Kobayashi, S.: Vol. 121, pp. 1-30. Vasilevskaya, V. see Khokhlov, A.: Vol. 109, pp. 121-172. Vaskova, V. see Hunkeler, D.: Vol.:112, pp. 115-134. Verdugo, P.: Polymer Gel Phase Transition in Condensation-Decondensation of Secretory Products. Vol. 110, pp. 145-156.

Vettegren, V. I.: see Bronnikov, S. V.: Vol. 125, pp. 103-146. Viovy, J.-L. and Lesec, J.: Separation of Macromolecules in Gels: Permeation Chromatography and Electrophoresis. Vol. 114, pp. 1-42.

Volksen, W: Condensation Polyimides: Synthesis, Solution Behavior, and Imidization Characteristics. Vol. 117, pp. 111-164.

Wake, M. C. see Thomson, R. C.: Vol. 122, pp. 245-274. Wang, K. L. see Cussler, E. L.: Vol. 110, pp. 67-80. Williams, R. J. J., Rozenberg, B. A., Pascault, J.-P. : Reaction Induced Phase Separation in Modified Thermosetting Polymers. Vol. 128, pp. 95-156.

Wunderlich, B. see Sumpter, B. G.: Vol. 116, pp. 27-72. Xie, T. E see Hunkeler, D.: Vol. 112, pp. 115-134. Xu, Z, Hadjichristidis, N., Fetters, L. J. and Mays, J. W.: Structure/Chain-Flexibility Relationships of Polymers. Vol. 120, pp. 1-50.

Yagci, Y. and Endo, T.: N-Benzyl and N-Alkoxy Pyridium Salts as Thermal and Photochemical Initiators for Cationic Polymerization. Vol. 127, pp. 59-86.

Yannas, I. V.: Tissue Regeneration Templates Based on Collagen-Glycosaminoglycan Copolymers. Vol. 122, pp. 219-244.

Yamaoka, H.: Polymer Materials for Fusion Reactors. Vol. 105, pp. 117-144. YaszemskL M. J. see Thomson, R. C.: Vol. 122, pp. 245-274.


155

Yoshida, 11. and Ichikawa, T.: Electron Spin Studies of Free Radicals in Irradiated Polymers. Vol. 105, pp. 3-36. Zubov, V. P., Ivanov, A. E. and Saburov, V. V.: Polymer-Coated Adsorbents for the Separation of Biopolymers and Particles. Vol. 104, pp. 135-176.

Subject Index See also key words and polymers in Tab. 1 to 9 and - not included in this index - polymers, oligomers and monomers with complex structures in Sect. 1.2 and 1.3. Information about a group of substances is given under the name of the basis material (for example substituted oligothiophenes see oligothiophenes). Abbreviatons see List of Symbols and Abbreviations, p. 1.

Absorption spectra 45 Acetaminophene-sensor 121 Actuators 79 Adhesives 124 Additives 110 Alcohol dehydrogenase 43, 120 3-Alkylbithiophenes 83 3-Alkylthiophenes 111 Alkyl side chain 82, 83, 84, 85 Aluminum electrodes 105 4-Aminophenol-sensor 121 Amorphous polymers 88 Amperometric detector 121 Antimony pentachloride 61, 63 Antistatic coatings 119 Antistatic compositions 11 Antistatic photographic films 119 Applications 113 Arsenic pentafluoride 64, 90 Artifical muscles 79 Ascorbic acid-sensor 121 Atomic force microscopy 103

Band gap 37 Batteries 121,122 Bilayer electrodes 120 Bilayers 78, 120 - , electrical conductivity 78 Bimorphs 79, 124, 125 Biosensors 79 Bipolarons 51, 52, 53, 75, 86, 87, 93 Bithiophene 43, 106, 110 - , as additive 112 -, oxidation potential 110 Blends -, with polythiophenes 25, 26 -, with poly(3-alkylthiophene)s 51, 62 Bromine as dopant 61

Capacitors, electrochemical/electrolytic 65, 124 Carrier mobility 118, 119 Catalytic activity 65 Catechol-sensor 121 Chain length 81, 82 Charge-discharge characteristic 122 Charge mobility 91 Charge storage 107, 108 Charge transportation 67 Chemical oxidation 3 Chemical reduction 3 Chiral amino acids 69 Chromatic transition 70 cis/trans Configuration 43 Coated electrodes 121 Coating materials 123 Coiled structure 70 Color changes 71 Color filters 124 Color mimicking 79 Composites 99, 100, 111 -, electrical conductive 99 Conducting gels 79 Conducting films 103 Conducting materials 58 Conducting polymer gels 79 Conducting polymers 3, 51 - , electroactive 712 Conductivity 66 -, a. c. conductivity 66, 67 -, d. c. conductivity 66 Configuration 37, 38 Conformation 41, 43, 45, 63, 91 -, coplanar 44, 69 -, nonplanar 44, 69 -, trans-gauche 79 Conformers 54 Conformons 67, 91

158 Conjugated polymers 51, 78 Conjugation length 38, 39, 44, 60, 67, 70, 71, 72, 75, 80, 81, 83, 84, 85, 86, 92, 101, 112 Contact angle 80 Coplanarity 39 Copolymers ofalkylthiophenes 47 -, ordered structure 47 Copolymers from bithiophene/ methylthiophene 106 Copolymers from bithiophene/pyrrole 111 -, as catalyst 123 Copolymers from bithiophene/ruthenium complexes 107, t08 Copolymers from butylthiophene/ methylthiophene 99, 116 Copolymers from butylthiophene/ octylthiophene 99 Copolymers from dodecylthiophene/ methylthiophene 111 Copolymer from hexylthiophene/ azobenzene derivatives 93 Copolymers from hexylthiophene/ methylthiophene 99 Copolymers from hexylthiophene/ octylthiophene 99 Copolymers from hexylthiophene/pyrrole 124 Coplymers from hexylthiophene/3thienylmethanol, biotinylated 78 Copolymers from methylthiophene/methyl thienylacetate 43, 89 Copolymers from methylthiophene/methyl methacrylate 75, 111 Copolymer from methylthiophene/ naphthyridine 107 Copolymers from methylthiophene/ octylthiophene 84, 89,99 Copolymers from methylthiophene/pyrrole 111 Copolymers from methylthiophene/ ruthenium complexes 107, 108, 109 Copolymers from methylthiophene/3thienylacetic acid 103, 106, 108 -, immobilized with glucose oxidase 120, 121 -, immobilized with lactate oxidase (lactate sensor) 121 Copolymers from pentylthiophene/methyl methacrylate 111

Subject Index Copolymers from 3-thienylmethanol/ undecylthiophene, biotinylated 120 Copolymers from thiophene/benzene 62, 64, 88 Copolymers from thiophene/furan 89 Copolymers from thiophene / methylthiophene 7 Copolymers from thiophene/octylthiophene 84 Copolymers from thiophene/silols 99 Copolymers with thiophene systems 23, 24, 25, 99, 100 Copolymers with substituted thiophene systems 99 Copper powder, coated with PT 123 Copper salts as dopants 95, 121 Corrosion protection 113, 115 Counter ions 103, 107 a,a-Coupling 36, 38 a,b-Coupling 36 Coupling types 39 Cross-linked polymeric structure 36, 58 Crown ether units 71 Crystal structure 47 -, PHT 47 Crystallinity 45, 87 Crystallites 91 Crystallization 46 Current density 105 Cyclic voltammetry 53, 55, 56, 59, 60, 66, 78, 81, 87

Daws0n-type polyanions 111 Decithiophene 54 Dedoped state 55 Dedoping 55, 83, 90, 91, 92, 93, 118 -, degree 90 -, electrochemical 118 -, influence of temperature 91 -, photochemical 63 -, thermal 84, 95 Delocalization 75 Delocalization length 38, 74 Deposition 102 -, electrochemical 102, 103 Dielectric relaxation 67 Diffusion coefficient 51 Diodes 117 Discharge capacity 122

Subject Index Docosanoic acid 77 Dodecathiophene 53, 117 4-Dodecylbenzenesulfonic acid 63 Dopamine-sensor 121 Dopants 54, 55, 61, 62, 63, 65, 90, 91 -, concentration 81 Doped state 3, 55 Doping 3, 45, 46, 53, 54, 55, 56, 57, 58, 59, 60, 61, 70, 78, 79, 80, 81, 87, 90, 91,92, 106, 107, 108, 112 -, chemical 55, 112 -, degree 90, 125 -, electrochemical 56, 58, 71, 112 -, photoinduced 92 -, n-type doping 3, 55, 58 -, p-type doping 3, 54, 55

Electrical conductive adhesives 124 Electrical conductivity 3, 45, 54, 55, 61, 62, 66, 70, 77, 81, 83, 87, 88, 89, 90, 92, 95 -, ageing 92 -, bilayers 78 -, blends 62 -, composites with polystyrene 100 -, conjugation length 81 -, crystallinity 81 -, influence of the humidity 92 -, LB films 77 -, mechanism 92 -, PAT 61, 62, 82, 83, 84, 85, 90, 91, 92, 101,106, 107, 108 -, PBT 62, 64, 95 -, PBuT 81 -, Poly(dodecylthiophene -comethylthiophene) 111 -, Poly(3-fluorothiophene) 105 -, PDT 62, 63 -, PDDT 85 -, Poly(3,4-ethylenedioxythiophene) 96 -, PHT 85, 92 -, PITN 62 - , P M T 81,87, 106, 111 -, Poly(methylthiophene-co-naphthyridine) 107 -, Poly[3-(4-octylphenyl)thiophene] 101 -, POT 63, 64, 85, 87, 92, 100 -, PPT, modified 111 -, PT 55, 56, 61, 62, 63, 64, 91, 94, 106,

159 107, 108, 112 -, PT, plasma-polymerized 98 -, poly(2,5-thienylene ethynylene) 63 -, poly(2,5-thienylene vinylene) 63, 87, 97 -, poly(thiophene co 3-octylthiophene) 88 -, sexithiophene 53 Electroactive polymers 105, 113 Electrochemical memory devices 123 Electrochemical oxidation (doping) 3, 55 Electrochemical redox processes 113 Electrochemical reduction (dedoping) 3 Electrochromic devices 113, 114, 122 Electrochromic displays 61,113, 114, 115, 121 Electrochromism 61, 65, 70, 71, 113 -, switching time 70 Electrocopolymerization 43, 103, 104, 108, 111 Electrode materials 104, 105, 111, 114, 116, 117, 122 Electrodeposition 102, 103 Electrodes, modified 121 Electroluminescence 71, 73, 74, 114 -, efficiency 114 Electroluminescent diodes 114 Electrolytes 107, 108 Electromagnetic shields 122, 123 Electron beam lithography 115 Electron beam resisit 116 Electron hole pair creation 75 p-Electron system 80 Electrophotography 115 Electropolymerization 42, 43, 62, 78, 102, 103, 104, 105, 106, 107, 109, 110, 111, 112, 115 -, mechanism 102 -, role of water 108, 109 -, special methods l 11 Ellipsometry 103 Enzyme electrodes 43, 120 Enzyme immobilization 120 Epinephrine-sensor 121 Excimer complexes 87 Excited state 73 Exciton 75 -, relaxation process 75 Extended p-bonding system 3

Femtosecond spectroscopy 82

160 Ferric chloride as dopant 63, 76, 77, 87, 90, 91, 92, 94, 95, 96, 97, 100, 112, 117 Ferric perchlorate as dopant 95 Fibers of electrical conducting polymers 62 Fibrills 88 Field-effect mobility 82, 83, 106, 118, 119 Field-effect transistors (FET) 77, 117, i 18, 119 -, thin film 77 Flow injection analysis 121 Fluorescence 71, 73, 82, 86 Free standing films 97, 105 Fullerene as dopant 65, 74, 75, 78, 85, 120

Galvanostatic conditions 110 Galvanostatic deposition 103, 105, 108, 111 Galvanostatic pulse method 57 Gas-phase doping 61 Gas sensors 117, 120, 121 Gels 79, 85, 124 -, electrical conductive 124 Glass transition 67 Glassy carbon as electrode material 120, 122 Glucose oxidase 43, 120 Gold electrodes 105, 116, 12 Gold trichloride 63 Graphite 117, 122 Growth 102

Subject Index Imaging system 80, 116 Immobilization 43, 120 -, alcohol dehydrogenase 120 -, antibodies 121 -, antigens 121 -, drugs 121 -, proteins 121 Impedance spectroscopy 78 Indium tin oxide (ITO) electrodes 105, 114, 115 Interchain hopping 59, 91 Interchain interactions 69, 79 Interchain transport 81 Interfacial polymerization 104 Intersystem crossing 74 Intermolecular interaction 83 Intrachain interaction 69 Intrinsing conductivity 87 Iodine as dopant 61, 79, 87, 88, 90, 91, 94, 97, 98, 112 lonochromism 71 IR absorbers 12 Iron particles as fillers 93 Irradiation 110

Junction devices 120

Keggin-type polyanions 111

Head-to-head coupling 68, 69, 86, 87, 94 Head-to-tail coupling 68, 87, 94, 100, 101 Head-to-tail-head-to-head coupling 38 Head-to-tail-head-to-tail coupling 38, 39, 85, 86, 87 Head-to-tail linkages 100, 101 Helix structure 45, 63, 69, 105 Heterojunetion 78 Heteropolyanions 64, 111 Hole mobility 118 Hopping 75, 77, 82, 91 -, photoexcited 93 Humidity 92 Hydroquinone-sensor 121

Lactate oxidase 43, 120 Lactate sensor 121 Langmuir Blodgett films 61, 64, 68, 73, 74, 76, 77, 90, 118 -, electrical conductive 76, 77 -, multilayers 90 Layer-by-layer deposition 96, 103, 112 Layer structures 48 Learning effect 58, 61, 85 Light emitting diodes 114 Linear optical properties 76 Liquid crystal display devices 124 Liquid crystalline compounds 79, 80 Lithium salts 64, 66 Luminescence 72, 73, 82, 8 Luminescence action spectrum 75

Imaging process 116

Magnesium alloy electrodes 114

Subject Index Magnetic dimerization 54 Magnetic properties 76 Magnetic recording 116 Magnetoresistance 92 Mechanism 103 Melting temperature 84 Memory devices 123, 124 Memory effect 66 Mercury 121 Metallic particles as fillers 93 Microlithographic devices 115 Microwave shields 123 MO-diagram 54 Modified electrodes 121 Molecular weight 37, 38, 80, 81, 85, 95, 106 -, copolymers of alkylated PTs 85, 99 -, influence of additives 110 Monolayers 77, 78, 85, 103 Monomer concentration 107 Morphology 49, 50, 51, 56, 61, 80, 87, 88, 103, 105, 106, 112 Multilayers (LB) 76, 77

NADH-sensor 121 Neutral state 3 Nickel electrodes 105, 107 Nitrosonium hexafluoroantimonate 63 Nitrosonium hexafluorophosphate 61, 64, 77, 90 Nitrosonium tetrafluoroborate 64, 99 Non linear optics (NLO) 43, 76, 116 Nucleation 102

Octithiophene 54, 82 Offset printing process 116 Oligo(phenylene)s 81, 82 Oligo(phenylene vinylene)s 69 Oligo(2,5-thienylene ethynylene)s 81 Oligo(thiophene)s 37, 38, 43, 45, 52, 53, 54, 56, 65, 69, 71, 73, 74, 76, 80, 81, 82, 86, 90, 94, 97, 105, 106, 109, 118, 119, 125 -, blends with polyvinylalcohol 71 One-dimensional conductors 51 One-dimensionality 75 Optical converters 125 Optical properties 71

161 Optical recording devices 115 Optical recording materials 79 Opto-electronic signal transduction 79 Ordered phases 46, 47 Organic molecular beam deposition method 97 Organic semiconductors 51 Organic metals 126 Orientation 45, 106 Oriented films 45, 97 Oriented polymers 111 Overoxidation 57, 60 Oxidants for polymer formation 95 Oxidation, electrochemical 71 Oxidation potential 81, 82, 83, 84, 85, 86, 89, 107, I10, 113 -,control technique 106 Oxidation state 51, 55, 70, 90 Oxidative coupling 97, 98

Packaging materials 119 Paraboloid antennas 124 Paramagnetic materials 76 Paramagnetism 65 Perchloric acid 62 Permeability 51 Phosphorescence 74 Photocatalysis 123 Photocatalytic activity 76 Photocatalytic fixation of CO2 73, 76 Photochemical dedoping 63 Photochromic materials 125 Photoconductivity 65, 74, 75 Photoconductivity action spectrum 75 Photocurrent 75 Photocurrent spectroscopy 104 Photoelectric devices 124 Photoexcitation 74, 75 Photographic films, antistatic coatings for 119 Photo-induced charge transfer 120 Photolithographic techniques 115 Photoluminescence 65, 71, 72, 73, 74, 79, 88, 91 Photolysis 93 Photooxidation 74, 112 Photoreceptors 115 Photoreduction 93 Photoresists 115, 116

162 Phototoxicity 74 Photovoltaic cells 114, 115 Photovoltaic effect 78 Phycoerythrin 120 Planar p-systems 84 Plasma polymerization 98, 112, 124 Platinum electrodes 1(~5,120, 121,122 Polarons 51, 52, 53, .' ~, 57, 75, 86, 102 -, excitation 52 Polaron-hopping model 53 Poly(acetylene) 3, 78, 81 Poly(alkylfluorene)s 114 Poly(3-alkylfuran) 84 Poly(3-alkyloxymethylthiophene)s 107 Poly(3-alkyloxytbiophene)s 41, 42, 59, 61, 87, 110, 119 Poly[3-(alkyloxyphenyl)thiophene]s 88 Poly(3-alkylthiophene)s, PAT 7, 38, 39, 40, 44-48, 51, 57, 58, 61, 62, 63, 64, 65, 66, 68, 69, 70, 73, 75, 76, 77, 79, 82- 92, 94, 95, 99, 100, 101,102, 105, 106, 107, 108, 110, 111, 112, 114, 115, 117-120, 123 -, blends/blending 51, 62 -, functionalized 96, 99, 100 Poly(3-alkylthienylene vinylene)s 75 Poly(aniline) 3, 70, 77, 113 -, blend with poly(sodium acrylate) 71 Poly[l,2-bis(3-alkyl-2-thienyl)ethylene]s 84, 112 Poly(bithienylmethyl methacrylate) 111 Poly(bithiophene), PBT 19, 20, 36, 45, 46, 48, 52, 57, 59, 60, 62, 63, 64, 66, 78, 80, 87, 88, 90, 91, 95, 96, 100, 102, 105, 106, 107, 108, 109, 110, 112, 116, 119, 120, 121,122, 123 -, composites with porous polystyrene 100 -, composites with polymeric sulfates 107 Poly[3-(bromooctyl)thiophene-co-3(vinylbexyl)tbiopbene] 77 Poly(3-bromothiophene) 98, 105, 119, 120 Poly(3-butoxy-3"-decylbithiophene) 68, 89 Poly(4-butoxy-4"-decylbithiophene) 89 Poly(3-butoxythiophene) 61 Poly(3-butoxy-4-methylthiophene) 44, 69, 89 Poly(3-butylthiophene), PBuT 9, 39, 42, 62, 64, 81, 85, 116, 120 Poly(3-chlorothiophene) 105 Poly(3-cyanothiophene) 119

Subject Index Poly(3-cyclohexylthiophene) 88, 96 Poly(3-decylthiophene),PDT 13, 56, 62, 63, 68, 74, 77, 92 Poly(4,4"-dialkylbithiophene)s 57, 66, 76, 87 Poly(dialkyloxybithiophene)s 89 Poly(dialkyloxyphenylenevinylene)s 114 Poly(3,4-dialkyloxythiophene)s 45, 119 Poly(3.4-dialkylthiophene)s 44, 45, 69 Poly(3,3"-dibromobithiophene) 88 Poly(3,4-dibromothiophene) 119 Poly(3,3"-dibutoxybithiophene) 89, 96 Poly(4,4"-dibutoxybithiophene) 89 Poly(3,4-dibutoxythienylenevinylene) 63 Poly(3,4-dibutoxythiophene) 95 Poly(4,4"-dibutylbithiophene) 87 Poly(3",4"-dibutylterthiophene) 38, 101 Poly(3,4-dibutylthienylenevinylene) 75 Poly(3,4-dibutylthiophene) 95, 116 Poly(3,3"-dichlorobithiophene) 88 Poly(4,4"-didecylbithiophene) 68 Poly(3,3"-dihexylbithiophene) 68 Poly(3",4"-dihexylterthiophene) 68 Poly(3.4-dihexylthiophene) 44, 69 Poly(3,3"-dimethoxybithiophene) 87, 110 Poly(4,4"-dimetboxybithiophene) 110 Poly(3,4-dimethoxythiophene) 110 Poly(3,4-dimethylthiophene) 119 Poly(4,4"-dioctylbithiophene) 122 Poly[3-(3,6-dioxaheptyl)thiophene] 59 Poly(4,4"-dipentoxybithiophene) 75 Poly(4,4"-dipentoxyterthiophene) 75 Poly(2,3-diphenylquinoxaline) 114 Poly[l,4-di(2-thienyl)benzene] 96, 109, 111 Poly[ 1.2-di(2-thienyl)-1.3-butadiene] 22, 57, 88 Poly[1.2-di(2-tbienyl)ethylene] 22, 57, 59, 62, 70, 75, 88, 90, 95, 106, 121 Poly(3-docosylthiopbene), PDST 15 Poly(3-dodecylbithiophene) 68, 84, 95 Poly(dodecyloxythiophene) 116 Poly(3-dodecylthiophene),PDDT 14, 38, 39, 52, 61, 62, 63, 67, 70, 71, 73, 74, 75, 84, 85, 91, 94, 116, 124 -, blends 91 -, composites with poly(ethylene terephthalate) as film capacitor 124 -, crystallites 91 Poly(3-ethyloxythiophene) 119

Su~ectlndex Poly(3,4-ethylenedioxythiophene) 70, 96, 113, 119, 124 -, blends with polystyrenesulfonic acid 71, 119 Poly(3-ethylmercaptothiophene) 75 Poly(3-ethylthiophene), PET 9, 119 Poly[3-(4-fluorophenyl)thiophene] 65, 124 Poly(3-fluorothiophene) 89, 105, 119 Poly(furan) 89 Poly(3-halothiophene)s 89 Poly(heptadecyl 3-thienylacetate) 73, 77 Poly(3-heptylthiophene), PHET 12 Polyheterocyclics 3 Poly(3-hexadecylthiophene), PHDT 15, 77 Poly(3-hexylthiophene), PHT 10, 37, 38, 39, 40, 52, 53, 58, 59, 61, 64, 65, 69, 71, 73, 74, 77, 81, 85, 86, 87, 92, 100, 106, 112, 115, 116, 117, 118, 119, 122, 123 -, blends with POT 87 -, cross-linking 115 Poly[3-(2-hydroxyethyl)thiophene] 42, 43, 96, 100, 120 -, hexanoate 96, 100 Poly[3-(hydroxymethyl)thiophene] 109 Poly(isothianaphthene), PITN 18, 37, 59, 60, 61, 62, 66, 70, 113, 114, 124 Polymer chain mobility 84 Polymer coated electrodes 121 Polymerization, electrochemical 103 Polymeruzation in zeolite 100 Poly[3-(methoxyethoxy)thiophene] 61, 66, 89 Poly{3-[2-(2methoxyethoxy)ethoxy]thiophene} 96 Poly(3-methoxythiophene) 61, 66, 110, 116, 119 -, composites 116 Poty(3-methylthiophene), PMT 7, 38, 42, 43, 45, 50, 51, 52, 55, 56-60, 63, 64, 65, 66, 71, 75, 78, 81, 85, 88, 91, 93, 95, 96, 97, 100, 102, 103, 104, 105, 108, 109, 111,112, 113, 114, 115, 116, 118, 119, 120, 121,122, 123, 124 -, composites 90, 99 -, structure, ,,noodle"-like 50 Poly(3-methyl-4-octyloxythiophene) 69 Poly(3-methyl-4-octylthiophene) 44, 56, 67, 69 Poly(3-nitrothiophene) 119

163 Poly(3-nonylthiophene), PNT 13, 67, 74 Poly(3-octadecylthiophene), PODT 15, 65, 78, 85, 114, 120, 124 -, adhesives 124 -, doped with fullerene 120 Poly(3-octylbithiophene) 68, 84 Poly(3-octyloxythiophene) 68 Poly(3"-octylterthiophene) 68, 84 Poly[3-(4-octylphenyl)thiophene] 73, 84, 101 -, free-standing films 101 Poly(3-octylthiophene), POT 12, 38, 39, 51, 58, 61, 63, 64, 69, 70, 75, 78, 79, 81, 84, 85, 86, 87, 90, 91, 92, 93, 94, 95, 100, 113, 116, 117, 119. 122, 124 -, blends 90, 91,100 -, composites 100, 116 Poly[3(pentadecafluorooctyloxy)thiophene] 88 Poly[3-(pentafluorooctyloxy)-4methylthiophene] 77 Poly(3-pentoxythienylene vinylene) 75 Poly(3-pentoxythiophene) 75 Poly(3-pentylthiophene), PPT 10, 92, 106, 116 -, composites 99 Poly(3 "-perfluorohexylterthiophene) 77 Poly(phenanthrothiophene) 70 Poly(phenylene) 3, 62, 81, 82, 88 Poly(phenylene vinylene) 3, 63 -, blends 89 Poly(phenylene vinylene co-2,5-thienylene vinylene) 89 Poly[3-(3-phenylpropyl)thiophene] 95 Poly(3-phenylthiophene) 59, 62, 66, 107 Poly(3-propylthiophene), PPrT 9 Poly(pyridine) 73 Poly(pyrrole) 3, 37, 78, 82, 103, t04, 107, 118, 120, 124 Poly(3-styrylthiophene) 89 Poly(terthiophene), PTT 20, 21, 36, 48, 50, 67, 88, 105, 106 Poly(3-tetradecylthiophene),PTDT 15, 56, 69 Poly(tetrathiophene) 21 Poly(3-thienylacetic acid) 42, 55, 103 -, methylester 42 Poly(2.5-thienylene ethynylene) 23, 62, 63, 81

164 Poly(2,5-thienylene vinylene) 21, 57, 61, 63, 70, 76, 87, 94, 97, 98, 114, 115, 116, 118, 122, 124 -, blends 89 Poly(3-thienylethanesulfonate) 116 Poly(3-thienylpropanesulfonic acid) 60, 70, 74 Poly(thiophene), PT 3, 4,37, 38, 43, 45, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 62, 63, 64, 65, 66, 67, 70, 71, 73, 75, 77, 78, 79, 81, 87, 89, 90, 91, 92, 94, 95, 97, 98, 100, 101,102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 115, 116, 117, 118, 119, 120, 121,122, 123, 124 -, blends with polyester 124 -, chiral amino acids containing 69 -, composites 100 -, cross-linking 106 -, disubstituted 18 -, irradiation 92 -, properties 51 -, reactive groups, with (functionalized) 42, 43 -, structures 36 -, 3-substituted 17,18 Polythiophene paradox 106 Poly(3-thiophenecarboxylic acid) 94 -, methyl ester 94 -, sodium salt 94 Poly(thiophene-co-3-octylthiophene) 88 -, electrical conductivity 88 Poly [(tridecafluorononyl)thiophene] 77, 79 Poly[3-(trifluoroethoxy)thiophene] 88 Poly(3-undecylthiophene), PUDT 14 Poly(5-vinylterthiophene) 92, 122 Polyxylylviologene 78 Potassium hexafluorophosphate 66 Potential 106 Potential cyclic conditions 106 Potential-programmed electropolymerization (PPEP) 111 Potentiometric deposition 103 Potentiometrie sensors 121 Potentiometric conditions 110 Potentiostatic technique 106 Precursor polymers 97 Pressure, influenc of 71, 93 Printing devices 116

Subject Index Protein/enzyme binding 99 Pyroelectrical sensors 116

Quantum chemical calculation 43 Quartz crystal microbalance 57, 78 Quaterthiophene 43, 52, 64, 89 Quenchers 74 Quenching effect 87 Quinquithiophene 77

Radical cations 102, 103, 108 Reactive groups 42, 43 Reading-writing devices 124 Rechargeable batteries 122 Recording materials 115 Rectifying bilayer electrodes 120 Redox activity 3 Redox process, electrochemical 70, 80 Redox potential 81, 88, 89 Redox states 61 Redox system 60 Reduced state 55 Reduction potential 81, 82, 83, 84 Reflectors for radar 124 Regioregular structures 39, 100 Regular structures 36, 100, 101 Relaxation effect 66 Resists 115, 116 Reviews 4, 125 Rotating disc electrode 55, 105

Scanning electron microscopy (SEM) 48 Scanning tunneling microscopy (STM) 46, 63 Schottky barrier diodes 117 Second-order nonlinear optical effect 76 Self-assembly in the solid state 39, 46 Self-discharge 122 Self-doping 60, 70, 99 Self-oriented structures 39 Semiinterpenetrating networks 84 Sensors 120, 121,122 -, potentiometric 121 Sexiphenylene 119 Sexithiophene 43, 46, 54, 63, 65,. 73, 74, 75, 78, 82, 85, 97, 98, 106, 117, 125 -, crystal structure 44

Subject Index

-, doping 53, 56 -, electrical conductivity 53 -, LB films 77 -, MO-diagram 54 -, morphology 106 -, oxidative coupling 97, 98 -, photocurrent 75 -, quenchers 74 -, shaped particles 124 Shielding materials 123 -, against electromagnetic interference (EMI) 123 Shrinkage 79 Silica networks 111 Silicon electrodes 105, 111 Silver electrodes 105 Sodium dihydronaphthalide 66 Solarcells 113, 114, 115 Solder materials 124 Solid electrolytes 113, 124 Solid state electrochromic cells 113 Solid state self-assembly 39 Solubility 84 Solubilization by irradiation 115 Solvatochromism 46, 69, 70 Solvents 107 Spin coating 113 Spin concentration 83 Starting materials 88 Stearic acid 77 -, concentration 108 Steel electrodes 105 Stretching 87 Structural homogeneity 39 Structure -, fibrous films t02 -, granular 102 -, helical 45 -, interlinked 103 -, ,,noodle"-like 50 -, primary 36 -, secondary 43 -, stereochemically x~ell-defined 39 Surface analysis 108 Surfaces, hydrophilic 80 Surfaces, hydrophobic 80 Synchrotron irradiation 92 Synthesis 93, 94, 95, 96, 97, 98, 112 -, chemical synthesis 93, 112 -, conditions 104

165 -, electrochemical synthesis 97, 102, 112 -, via precursor polymers 97 Synthetic metals 126

Tail-to-tail coupling 87 Tail-to-tail-head-to-head coupling 38 Tail-to-tait-head-to-tail coupling 38 Temperature influence 107 Terthiophene 52, 74, 88, 89, 106, 107, 109, 110, 118 Tetraalkylammonium salts 107 Tetrabutylammonium salts 62, 64, 65, 66, 107 Tetracyanoquinodimethane 65 Tetradecylammonium salts 66 Tetraethylammonium salts 107 Tetrathiafulvalene (TTF) moieties in PT 107 Thallium trifluoroacetate as oxidant 95 Thermal stability 83, 90, 91 Thermochromism 46, 67, 68, 69, 83 Thermoreversible gelation 79 3-Thienylpentadecanoic acid 77 2-(2"-Thienyl)pyridine 73 Thiophene 105 -, adsorption on gold 105 -, electropolymerization 105 Third-order nonlinearity 75 Third-order optical nonlinear susceptibility 82 Time-of-flight secondary ion mass spectroscopy/X-ray photoelectron spectroscopy 108 Tin dioxide electrodes 105 Tin electrodes 105 Transistors 82, 117, 118, 119, 120 Transparencies for overhead projectors 119 Trifluoroacetic acid 62 Trifluoromethanesulfonic acid 64 Tunneling 57, 77 Twisted states 92 Two-photon absorption 82 Ultrasonic irradiation 110 Ultraviolet photoemission spectroscopy 68, 112 Undecathiophene 81, 86 UV resists 116

166 Vapor doping 61 Vinylthiophene 106

Water in electropolymerization 108 Wettability 60, 80, 88, 91

Subject Index X-ray diffraction 37, 39, 40, 43, 45, 61 -, wide angle 81 X-ray photoelectron spectroscopy 95 X-ray structure 45

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