3,4-Ethylenedioxythiophene (EDOT) as a Versatile Building Block For

7/31/2019 3,4-Ethylenedioxythiophene (EDOT) as a Versatile Building Block For

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3,4-Ethylenedioxythiophene (EDOT) as a versatile building block foradvanced functional p-conjugated systems

Jean Roncali,* Philippe Blanchard and Pierre Frere

Received 5th October 2004, Accepted 24th December 2004

First published as an Advance Article on the web 31st January 2005DOI: 10.1039/b415481a

The potentialities offered by EDOT as a building block for the synthesis of functional

p-conjugated systems are reviewed. The first part underlines the specific advantages of the

EDOT unit for the design of precursors of electrogenerated functional conducting polymers

combining high reactivity and low polymerization potential. This topic is illustrated by some

recent examples of polymers with specific electrochemical properties and/or a reduced band gap.

The second part is focused on the interest of EDOT as a building block for the synthesis of

various classes of molecular functional p-conjugated systems. Examples of fluorophores, push

pull chromophores for nonlinear optics and extended p-donors are presented. Emphasis is placed

on the optical and crystallographic structure of these compounds, which shows that a major

advantage of the EDOT building block lies in an unique combination of strong electron donor

properties and self-structuring effects related to intramolecular non-covalent interactions between

oxygen and sulfur. Such intramolecular interactions also exert a determining influence on the

structure and electronic properties ofp-conjugated oligomers incorporating EDOT units which

represent some of the more recent uses of the EDOT building block. The structureproperty

relationships of various classes of EDOT-containing conjugated oligomers are discussed in

relation to their potential use as organic semi-conductors. In a brief last section, various synthetic

approaches devoted to the chemistry of EDOT itself, or to the modification of its chemical

structure, are discussed in relation with possible future research directions.

Introduction

During the past two decades, research on functional

p-conjugated systems has rapidly grown as a broad

Jean Roncali was born in Paris in 1949. He received hiseducation in chemistry at the Conservatoire National des Arts etMetiers. He received his PhD from the University of Paris 13

under the supervision of Francis Garnier. After successivepositions as an engineer and Charge de Recherche at CNRS,he is currently Directeur de Recherche at CNRS and head of theLinear Conjugated Systems group at the University of Angers.

His research interests encompass the development of organicmolecules and materials with tailored electronic properties in

view of applications in energy conversion, electronic and photonicapplications and nanodevices.

Philippe Blanchard was born in Vendee, France in 1967.He received his PhD in 1994 from the Universities ofNantes and Angers under the supervision of Professors

G . D ug u ay a n d A . G o rg u es ; h i s s u bj ec t c on ce r ne d tetrathiafulvalene-based molecular materials. He spent one

year as a postdoctoral fellow in the group of Jan Becher atthe University of Odense (Denmark) where he developed

macropolycyclic electroactive compounds. In 1995, hejoined the group of Jean Roncali in Angers as Charge de

Recherche at CNRS to develop thiophene-based pi-conjugatedoligomers and polymers. He obtained his Habilitation in

2001 . His current research int erests concern

the design of pi-conjugated systems for organicelectronic devices.

Pierre Frere was born in Laval, France in 1961.

He received his PhD in organic chem istryfrom the University of Angers in 1993, under thesupervision of Professor A. Gorgues. He then

became Matre de Conferences at the Universityof Angers, where he was promoted to Professorin 1999. His research interests cover various

classes of organic materials derived from tetrathiafulvalene analogues, oligothiophenes and

poly(thiophene)s for electronic applications andmolecular conductors.(Left to right) Philippe Blanchard, Jean Roncali and Pierre Frere

*[email protected]

FEATURE ARTICLE www.rsc.org/materials | Journal of Materials Chemistry

This journal is The Royal Society of Chemistry 2005 J. Mater. Chem., 2005, 15, 15891610 | 1589


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multidisciplinary field extending from theoretical chemistry to

organic chemistry, electrochemistry, photophysics, solid-state

physics and device implementation.1

The discovery of the metal-like conductivity in oxidized

poly(acetylene) or shortly after in polyaromatic systems

such as poly(pyrrole), poly(thiophene) or poly(aniline) in the

late seventies and early eighties,15 has progressively generated

a rich synthetic chemistry in the more general context offunctional p-conjugated systems.

As for conducting polymers in general, the synthetic

chemistry of linearly p-conjugated systems has developed

considerably in the past twenty years and been subject to great

diversification in terms of objectives and methods. During the

period 198090, corresponding to the early developments in

the field, conducting polymers were essentially considered for

bulk applications such as conducting materials for anti-static

coatings, EMI shielding or as electrode materials for energy

conversion and storage. However, after several years of

intensive research effort, it became clear that the insufficient

stability of the charged conducting state of conjugated

polymers represented a major obstacle for further industrialdevelopment.

In 1990, the realization of the first electroluminescent

devices, in which a p-conjugated polymer was used as lumino-

phore by Friend and coworkers, represents a turning point in

the field of p-conjugated systems.6,7 This discovery, together

with a parallel intensification of research on field-effect

transistors8,9 and photovoltaic cells based on p-conjugated

polymers and oligomers,1012 strongly contributed to build up

a quite different vision of linear p-conjugated systems.

With the emergence of the concept of plastic, soft or

flexible electronics p-conjugated systems are no longer viewed

as bulk materials associated with mass industrial production

but as organic semi-conductors synthesized and employed at amuch smaller scale. In this regard, the considerable develop-

ment of nanosciences and molecular electronics in the past few

years, and the emergence of problems related to the design and

synthesis of nano-objects such as molecular wires, switches or

dynamic devices, has further emphasized this molecular vision

of functional p-conjugated systems

In the historical context of the 198090 period, the chemistry

of conducting polymers can be roughly qualified as an

additive side chain approach, with as main goal the addition

of new properties such as solubility, hydrophilicity, or

molecular recognition to the inherent electronic, optical or

electrochemical properties of the conjugated backbone of

polymers such as poly(pyrrole) or poly(thiophene) by covalentfixation of functional side groups.15,13

The emergence of plastic electronics has generated a quite

different view of the chemistry of functional p-conjugated

systems and the control and manipulation of quantities such

as absorption and emission spectra, oxidation and reduction

potentials or luminescence efficiency became the new priority

targets.

At the begining of the 90s, chemists at the Bayer company

described a novel conducting polymer poly(3,4-ethylenedioxy-

thiophene) (PEDOT). Owing to several distinct advantages,

PEDOT rapidly acquired a prominent position among con-

ducting polymers. A unique combination of moderate band

gap and low oxidation potential confers on PEDOT an

exceptional stability to the oxidized charged state which

furthermore exhibits high conductivity and good optical

transparency in the visible spectral region.1416 Based on these

properties many applications of PEDOT have been rapidly

developed including anti-static coatings, electrode material

in supercapacitors or hole injection layer in OLEDs and

solar cells.

12,1720

Although PEDOT was initially viewed as a novel conduct-

ing polymer, it became rapidly clear that the EDOT molecule

itself presented some specific chemical properties which made

it an interesting building block for the synthesis of functional

p-conjugated systems. In fact, in addition to their strong

electron donor effect, the ether groups at the b,b9 positions of

thiophene ring prevent the formation of parasitic ab9 linkages

during polymerization while conferring a high reactivity to

the free a,a9 positions. During the past decade, these properties

have been widely used to synthesize various classes of

electrogenerated polymers combining some of the specific

properties of PEDOT to original electrochemical or optical

properties. These various aspects of the electro-optical andelectrochemical properties of EDOT-based polymers have

already been subject to two review articles.19,20 However,

beyond the synthesis of functional conducting polymers,

EDOT also represents a unique building block for the design

of various classes of molecular p-conjugated systems with

electronic and optical properties specifically tailored for

applications in light-emitting devices, chromophores for non-

linear optics, low energy gap systems or more recently organic

semi-conductors.

This increasing attention for EDOT-based systems has in

turn generated a strong interest in the chemistry of the EDOT

molecule itself and/or in the modification of its structure.

In this general context, the aim of this short review is not topresent a comprehensive survey of the synthesis and applica-

tions of PEDOT and its derivatives, but rather to emphasize

the opportunities offered by the EDOT building block for the

design and synthesis of new classes of functional p-conjugated

systems.

1. EDOT as a building block for the design of

precursors of functional p-conjugated polymers

The electropolymerization of substituted precursors represents

a simple and straightforward method for the elaboration of

modified electrodes in which the inherent electrochemical

and electronic properties of the p-conjugated backbone areassociated with specific properties brought by the covalent

fixation of a functional group on the monomer.21,22 During the

past two decades modified electrodes based on this concept

have represented a major topic in the chemistry of conducting

polymers. Initially focused on the conversion and storage

of electrical energy23,24 or electrochromic devices,25,26 the

field has progressively evolved towards more sophisticated

systems such as electrode materials for electrocatalysis and

(bio)electrochemical sensors.21,22,27

Despite its conceptual simplicity, the synthesis of functional

electrogenerated conducting polymers poses several complex

problems related to the direct and indirect effects of the

1590 | J. Mater. Chem., 2005, 15, 15891610 This journal is The Royal Society of Chemistry 2005


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attached functional group on the polymerization reaction

and on the structure and properties of the resulting polymer.

Thanks to the intensive research effort invested in this

area during the 80s, rules for the molecular engineering of

functional conjugated polymers have progressively emerged

with, in particular, the need to introduce a spacer between

the functional group and the p-conjugated chain in order to

minimize electronic and steric effects of the substituents onthe polymerization process, and on the effective conjugation

length of the resulting polymer.13,21

Besides pyrrole-based systems, which will not be discussed

here,27 most of the functional conducting polymers of the first

generation have been obtained by electropolymerization of

precursors in which a functional group is attached at the

3-position of a thiophene ring through an appropriate spacer

group. In general, a spacer involving at least two carbons

allows the neutralization of the electronic effects of most

electron-releasing or -withdrawing groups on the reactivity

of the thiophene ring.13 However, in the case of bulky sub-

stituents longer spacers may be needed to reduce steric

interactions between substituents and to preserve the planarityof the p-conjugated backbone.21

Although widely applied, the scope of this approach is

limited by the highly positive potential required to electro-

oxidize 3-alkythiophenes or functionalized monomers of

related structure. In fact, when the oxidation potential of

the functional group is significantly lower than that of the

thiophene, various kinds of deleterious effects can be expected,

namely, (i) a large part of the anodic current will be consumed

in the oxidation of the side substituent leading to its eventual

irreversible degradation, and (ii) the attached functional group

can act as a scavenger for thiophene cation radicals and thus

inhibit the electropolymerization process.28,29

A first possible way to decrease the oxidation potential of

the precursor consists of replacing thiophene by a longer

oligomer with lower oxidation potential. However, the pro-

gressive decrease in the reactivity of the cation radical as chain

length increases13,30 severely limits the scope of this approach.

In fact, as exemplified in the case of the poly(thiophenes)

functionalized by crown ethers, synthesized by Bauerle and

coworkers,

31

bithiophenic systems seem to represent the besttrade-off between the decrease of the polymerization potential

and the preservation of a sufficient reactivity of the cation

radical.

In this context, the specific electronic properties of EDOT,

in terms of reactivity and donor effect, represent interesting

tools for the design of precursors combining the low electro-

polymerization potential and high reactivity of the cation

radical.

A widely-used approach, initially developed by Reynolds

and coworkers, involves the synthesis of hybrid tricyclic

precursors in which a functional block, eventually possessing

specific electronic, chemical or electrochemical properties, is

inserted between two EDOT groups (Scheme 1).32

In thepast decade this in chain approach has been widely applied

to the synthesis of many precursors with a median block

extending from simple moieties, i.e. double bonds (1),33

thiophenes (2), furans (3), benzenes (4),32 substituted thio-

phenes,34 or pyridines (5),35 to larger systems such as biphenyls

(6),32 bipyridines (7),36 fluorenes (8),37 carbazoles (9),38 N,N9-

ethylene-bis-(salicylideimine) metal complexes (10),39,40 or

tetrathiafulvalenes (11).41

A major advantage of this approach is that the high

reactivity of the lateral EDOT groups makes it possible to

electropolymerize precursors in which the central block limits

or even interrupts electron delocalization along the polymer

Scheme 1 Examples of in chain functionalized EDOT-based conjugated polymers.



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hydrophilic polymers possess the unique ability to undergo

fast solid-state ionochromism when immersed in solutions of

various metal ions, or immediate contraction of band gap

when immersed in water. Finally, a comparative analysis of

the cyclability of the polymers showed that the polymers

synthesized from the two-site precursors 18 are considerably

more stable under long-term repetitive redox cycling than

their analogs derived from the singly substituted-EDOTmonomer 17.47

In addition to their rather complex synthesis, functionalized

monomeric EDOT precursors exhibit an oxidation potential

similar to that of EDOT (ca. 1.70 V vs. SCE), which still

represents a major obstacle for the electropolymerization

of precursors containing sensitive functional groups. For

example, in the specific case of compound 14, the considerably

lower oxidation potential of ferrocene relative to that of

EDOT renders the electropolymerization very difficult. Con-

sequently, stable ferrocene-derivatized polymers were only

obtained by co-polymerization with other EDOT derivatives.45

In order to define an optimal trade-off between reactivity

and the lowered polymerization potential we have recentlydeveloped a new class of precursors based on the EDOT

thiophene structure in which the functional group is attached

by formation of an electron releasing sulfide or ether group.

Substituted thiophene monomers based on a 3-alkylthio-

phene structure present an oxidation potential of ca. 1.80 V vs.

SCE and require a minimal concentration of 0.10 M in order

to produce electrodeposited polymer films of good quality.13,21

As already indicated, replacing thiophene with bithiophene

allows the reduction of the polymerization potential to 1.30 V

vs. SCE and the minimal concentration to about 1022 M.31,48

A further improvement was achieved in 1997, when we showed

that replacement of thiophene by EDOT in bithiophenic

precursors permitted a further decrease of the electropoly-merization potential to 1.15 V. This kind of precursor proved

to be particularly effective for the electrosynthesis of tetra-

thiafulvalene-derivatized poly(thiophenes).48,49 However, like

many of the 3-substituted functionalized thiophenic pre-

cursors, these improved bithiophenic systems were still based

on the 3-(v-halogenoalkyl)-thiophenes prepared according

to the method described by Bauerle.50 Despite its interest,

this method presents several important limitations, namely

(i) it requires multiple synthetic steps, (ii) final deprotection of

the halide group requires drastic conditions which are not

applicable to longer oligomers or to polymers and (iii) it is not

valid for alkyl chains containing less than four carbons.

In an attempt to solve some of these problems, we havedeveloped alternative methods of functionalization based on

the formation of sulfide or ether groups. In these cases,

functionalization is realized by reacting a functional group

bearing a terminal halogenomethyl moiety onto a 3-thiophene-

thiolate or 3-thiophene-alcoolate. These intermediate

compounds are produced by cleavage of 3-(2-cyanoethyl-

sulfanyl)thiophene or 3-(2-cyanoethyloxy)thiophene under

mild conditions.51,52 These two compounds are easily obtained

from 3-bromothiophene or 3-methoxythiophene respectively

(Scheme 2). An advantage of the cyanoethyl protected group is

that it can be engaged in further chemistry to synthesize hybrid

EDOT-based precursors. Furthermore, in addition to an easy

and rapid method of functionalization, the strong electron-

donor effect of the thus formed sulfide or ether group produces

a further decrease in the oxidation potential of the bithiophe-

nic precursor.With these new hybrid EDOT-based bithiophenic systems it

is now possible to efficiently electropolymerize functionalized

precursors at potentials lower than 0.90 V vs. SCE and sub-

millimolar concentrations.52 This recent progress in the design

of precursors enables researchers to envision the synthesis

of more elaborated functional polymers or to reconsider the

design of polymers which proved to pose specific problems in

the past.

Thus, although several groups have reported attempts to

synthesize poly(thiophenes) containing bipyridyl ligands and

some of their metal complexes, conclusive evidence for the

efficient electrosynthesis of well-defined stable and extensively

conjugated polymers has not been reported until now.21

Therefore, one of the first applications of the newly designed

low oxidation potential bithiophenic precursors has involved

the synthesis of bipyridine-derivatized conjugated polymers.

To this end, precursors 19 and 20 in which two EDOT-based

hybrid bithiophene groups are attached on a bipyridine have

been synthesized. As shown in Fig. 1, the combination of these

low oxidation potential precursors with the already discussed

advantages of the multi-site approach allows a straight-

forward electropolymerization to form stable and extensively

p-conjugated polymers containing bipyridine ligands.52

In the next step, various transition-metal complexes contain-

ing two to six hybrid EDOTthiophene groups (2123) were

Scheme 2 Synthesis of low polymerization potential precursors.



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synthesized and electropolymerized. Thus, the cyclic voltam-

mogram of poly(23) shows a first reversible redox system

around 0.60 V characteristic of an extensively conjugated

poly(thiophene) backbone followed by a reversible anodic

wave at 1.00 V typical of the [Fe(Bipy)3]2+/3+ redox couple.

In the negative potential region, the CV shows the two

successive redox systems associated with the [Fe(Bipy)3]2+/+

and [Fe(Bipy)3]

+/0

redox couples (Fig. 2).

52

Another recent application of the same strategy has

involved the synthesis of poly(thiophenes) containing pendant

C60-fullerene groups. Such materials have been essentially

developed in view of their potential use as active materials

in organic solar cells.53 Precursors 24 and 25, containing one

and two polymerizable groups respectively, have been synthe-

sized using successively thiophenethiolate chemistry and the

Bingel reaction.54,55

Comparison of the cyclic voltammograms corresponding to

the potentiodynamic electropolymerization of compounds 24

and 25, shows that the two site-precursor 25 leads to a faster

increase of the less positive anodic wave around +0.30 V,

which is indicative of the growth of a more conjugated polymer(Fig. 3). This conclusion has been confirmed by the analysis of

the electrochemical and optical properties of the two polymers.

Furthermore, chronoamperometric experiments carried out on

the two polymers have shown that for the same film thickness,

poly(25) exhibits a much faster response to a potential step, in

agreement with the expected more porous structure.55

Fig. 1 Potentiodynamic electropolymerization of compound 19 (top)

and 20 (bottom), 1 mM in 0.10 M Bu4NPF6CH2Cl2, 100 mV s21, Pt

electrodes. (From ref. 52, reproduced by permission of The Royal

Society of Chemistry.)



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Preliminary tests of the photoelectrochemical response of afilm of poly(25) on a platinum electrode (Fig. 4) showed that

the photogenerated current is ca. three times larger than that

obtained under the same conditions with a reference parent

polymer in which the C60 group of poly(25) is replaced by an

hexyl chain linking the two sulfide groups. This result thus

underlines the potentiality of this new class of functional

conjugated polymers for photovoltaic conversion.55,56

Compounds 26 and 27 represent the first members of

another interesting class of compounds combining EDOT and

fullerene C60. These compounds present the particularity to

form self-assembled monolayers (SAMs) on gold surfaces.57

The resulting SAMs are quite stable and exhibit unchanged

CV after several weeks storage under ambient atmosphere. The

cyclic voltammogram of a SAM of compound 27 at various

scan rates shows two successive waves corresponding to the

reduction of C60 into its anion radical and dianion (Fig. 5).

The linearity of the plot of the current peak with scan rate

confirmed that the reduction corresponds to a surface-

confined electrochemical reaction.

No change in the CV is observed under repetitive cycling in

the negative potential region (down to 21.80 V vs. Ag/AgCl).However, a particularly interesting property of these SAMs

is that they can be electrochemically desorbed from the

electrode surface by anodic oxidation of the attached EDOT

or bi-EDOT group.57

2. EDOT as a tool for molecular engineering of the

energy gap ofp-conjugated systems

Most of the relevant electronic properties of p-conjugated

systems depend on the energy level of the frontier orbitals and

the difference between them. Consequently, the control of the

HOMOLUMO gap (DE) of conjugated systems and of the

Fig. 2 Cyclic voltammogram of poly(23) in 0.10 M Bu4NPF6

CH3CN, scan rate 100 mV s21. (Reprinted from ref. 52, reproduced

by permission of The Royal Society of Chemistry.)

Fig. 3 Potentiodynamic electropolymerization of compound 24 (top),

and 25 (bottom) 1 mM in 0.10 M Bu4NPF61 : 4 CH3CNCH2Cl2,

100 mV s21. (Reprinted from ref. 55, Copyright 2003, American

Chemical Society.)

Fig. 4 Variation of the photocurrent under polychromatic irradia-

tion of polymer films electrodeposited on platinum electrodes, (black)

reference polymer (see text) (red) poly(25). Polarization 20.10 V vs.

Ag/AgCl) in 0.10 M Bu4NPF6CH3CN. (Reprinted from ref. 55,

Copyright 2003, American Chemical Society.)



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band gap (Eg) of the resulting molecular or polymeric

materials has progressively become a major topic for the

chemistry of functional p-conjugated systems.58 The HOMO

LUMO gap of a linear p-conjugated system depends on

various structural factors such as chain length, bond length

alternation, planarity, the presence of electron-acceptor or

electron donor substituents and the resonance stabilization

energy of the aromatic cycles. Although not fully independent,

these various factors represent possible synthetic approaches

for gap engineering such as the grafting of donor and/or

acceptors groups on the conjugated backbone, the searchfor systems with a quinoid ground state, or the rigidification

of the conjugated structure in order to improve planarity and

to reduce bond length alternation.58

In this context, the strong electron donor properties

of EDOT represent an interesting tool for the molecular

engineering of the band gap of p-conjugated systems. One

of the simplest ways to take advantage of these properties

is to associate EDOT with electron withdrawing groups by

synthesizing copolymers. Thus Huang and Pickup have

reported that electrochemical copolymerization of EDOT and

4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b9]dithiophene

(28) led to a copolymer for which a very low band gap

was claimed.59

More recently, Wudl and coworkers havesynthesized another low band gap copolymer using a Stille

reaction between di-stannyl-EDOT and the dibromo

derivative of thienyl-benzo[c]thiophene-N-20-ethylhexyl-4,5-

dicarboximide (29) (Scheme 3). The copolymer showed

electrochromic properties in the IR region.60

The possibility to tune the gap, and hence the optical pro-

perties, of EDOT-containing block co-polymers was initially

investigated by Reynolds and coworkers, who reported many

examples of in chain functionalized co-polymers obtained

by electropolymerization of tricyclic precursors. In this case,

gap modulation was achieved by changing the chemical nature

of the median group (Scheme 1).19,20

More drastic reduction of the band gap of poly(thiophene)

can be achieved by introduction of electron acceptor groups

on the conjugated backbone.

21,58

As shown already forpoly(p-phenylene),61,62 and thiophene-based systems,63,64 the

grafting of cyano groups at the ethylene linkage connecting

two phenyl or thiophene rings represents a simple and efficient

method for gap reduction. Furthermore cyano groups can

be easily introduced at a vinylene linkage by Knoevenagel

condensation.6165 Thus, precursors 30 and 31 have been

prepared by condensation of 2-formyl-EDOT with the appro-

priate thiophene-acetonitrile. Electropolymerization of these

compounds led to polymers with band gaps smaller than

1.50 eV.65,66

Until now, p-conjugated polymers with the smallest band

gaps have been obtained by electropolymerization of tricyclic

systems in which two donor side groups such as pyrrole orthiophene are attached to a median fused ring system which is

as the same time a strong electron acceptor and a proquinoid

system. This strategy has been extensively developed by

Yamashita and coworkers who used many examples of pro-

quinoid acceptor groups.58,67,68

More recently, our group has extended this approach to

several tri-block systems in which EDOT serves as side donor

group. Thus tricyclic systems based on thieno[3,4-c]-pyrazine

(32),6971 benzo[3,4-c]thiophene (33),69,72 benzothiadiazole

(34)72 or thienothiadiazole (35)69 have been synthesized as

precursors of low band gap polymers. As shown by the X-ray

structure of compound 32, the molecule adopts a fully planar

geometry (Fig. 6). Furthermore, examination of the non-bonded distances between sulfur and oxygen or nitrogen and

oxygen atoms of the adjacent cycles shows that the sulfur

nitrogen and sulfuroxygen distances are markedly smaller

than the sum of the van der Waals radii of the individual

atoms, thus demonstrating the existence of non-covalent

intramolecular interactions which contribute to planarize the

system. Thus, in addition to the electronic effects already

observed in the tricyclic systems synthesized by Yamashita and

coworkers, EDOT introduces a self-structuration effect which

contributes to a further reduction of the gap. Comparison of

the electronic absorption spectra of compounds 3235 to those

of terthienyl reveal considerable red shifts of the absorption

Fig. 5 Cyclic voltammograms of a SAM of 27 in 0.05 M Bu4NPF6 in

o-dichlorobenzene, scan rate 100 to 1000 mV s21. (From ref. 57,

reproduced by permission of The Royal Society of Chemistry.)

Scheme 3 Synthesis of low band gap copolymers.



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maximum indicative of the reduction of the HOMOLUMO

gap. Electropolymerization of tricyclic compounds 3235 leads

to polymers with band gaps in the range of 1.101.30 eV. 6973

Lee et al. recently reported the synthesis and electropoly-

merization of a related system involving a median 3,4-

diphenylsilole (36). The optical spectrum of the obtained

polymer indicated an estimated band gap of 1.301.40 eV.74

Wudl and coworkers have described the synthesis of 1,3-bis(29-[39,49ethylenedioxy]thienyl-benzo[c]thiophene-N-20-ethylhexyl-

4,5-dicarboximide (37). The resulting electrogenerated polymer

presented an optical band gap of 1.10 eV and was reported

to be very stable in both the oxidized and reduced forms. 75

Cyclopenta[2,1-b;3,4-b9]dithiophene-4-one and 4-dicyano-

methylene-4H-cyclopenta[2,1-b;3,4-b9]dithiophene are well-

known precursors of low band gap polymers.7578 Berlin et al.

have recently synthesized compounds 38 and 39 in which

EDOT groups are connected at both ends of these electron

acceptor bithiophenic systems. Electrochemical and optical

data provided consistent results showing that these polymers

present band gaps of 0.801.30 eV.79

Although these various results confirm the interest of the

association of EDOT with proquinoid acceptors, the reported

values of the band gap remain in the whole rather large. For

Fig. 6 Crystallographic structure of compound 32 (from ref. 69).



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example the band gap of poly(33) is still comparable to that of

the homopolymer of benzo[c]thiophene initially described by

Wudl et al.80 This result can be attributed to two causes. On

the one hand, polymerization of tricyclic precursors leads to

bis-EDOT sequence alternating with acceptor group and it is

likely that optimal gap reduction requires a regular alternation

of donor and acceptor groups.58,81 On the other hand, and as

already discussed, oligomeric precursors are in general moredifficult to polymerize efficiently due to the limited reactivity

of their cation radicals.13,30 Indeed, conclusive evidences for

incomplete electropolymerization have been obtained in the

case of poly(33), for which the CV recorded in a monomer-free

electrolytic medium reveals a negative shift of the anodic peak

upon repetitive cycling, a phenomenon characteristic for the

coupling of unreacted precursor molecules trapped in the

polymer during electrodeposition.82

In this regard, the use of di-block systems in which EDOT is

associated with a proquinoid thiophenic acceptor present

distinct advantages related to (i) the higher reactivity of

bithiophenic precursors and (ii) the difference in reactivity of

the terminal a-positions of EDOT and thiophene which allowsthe formation of a regular alternation of donor and acceptor

blocks after an initial EDOTEDOT coupling. These effects,

associated with the non-covalent self-rigidification observed in

the model compound 32, could explain the very small band

gap of the polymer obtained by electropolymerization of the

EDOT-thieno[3,4-b]pyrazine precursor (40) which reaches one

of the smallest values know to date (Eg, 0.40 eV) and exhibits

an exceptional stability under repetitive n-doping cycling.83

This considerable decrease of Eg compared to poly(32)

(1.10 eV) can be attributed to the combined effects of regular

alternation of donor and acceptor groups, to the already

mentioned incomplete polymerization of tricyclic systems such

as 32 or 33 and to the self-rigidification of the structure by non

covalent intramolecular interactions. On the other hand, the

unusual stability under long-term n-doping cycling suggeststhat a dense packing of the polymer chains may limit oxygen

permeation in the polymer bulk. Such a tight packing, which

also contributes to reduce the band gap, can also explain the

complete insolubility of the polymer despite the presence of

two hexyl chains on the pyrazine ring.

In order to improve the solubility of this type of structure we

have recently synthesized a parent precursor in which EDOT is

substituted by an n-decyloxy chain grafted at the ethylenedioxy

bridge (41). Comparison of the electrochemical and optical

properties of the resulting polymer to those of poly(40) reveals

a 0.35 V negative shift of the peak potential corresponding to

the n-doping and a considerable decrease of stability under

reductive cycling. Furthermore, comparison of the optical

spectra of the neutral polymers shows that lmax shifts

hypsochromically from 1460 nm for poly(40) to 1070 nm for

poly(41), which corresponds to an increase of the band gap

from 0.40 to ca. 0.80 eV (Fig. 7).84 These unexpected results

are in fact the consequences of the introduction of the decyl

substituent at an sp3 carbon of the ethylenedioxy bridge. This

implies that the location of the solubilizing alkyl chain above

or below the plane of the p-conjugated polymer backbone

significantly increases interchain distance and hence decreases

p-stacking interactions. This situation is in total contrast with

the case of poly(3-alkylthiophenes) for which p-stackinginteractions between conjugated chains are possible since the

alkyl chains are attached at a sp2 carbon of the thiophene ring.

Thus, in poly(41) the grafting of the decyl chain contributes

at the same time to increase the band gap by hindering

intermolecular p-stacking interactions and to decrease the

stability of the reduced form of the polymer due to an easier

oxygen permeation in the bulk material.

These results show that despite the high interest of EDOT

for band gap engineering, its solubilization poses specific

problems which require the definition of appropriate synthetic

strategies.

3. EDOT-based functional p-conjugated systems forelectro-optical applications

The combination of the strong electron-donor and self-

structuring effects of EDOT also represent powerful tools for

the design and synthesis of molecular functional p-conjugated

systems for various optical and electro-optical applications.

Thus, in addition to their use as precursors of low band gap

polymers, tricyclic compounds such as 33 and 34 present a

unique combination of electronic properties which make

them potentially interesting as fluorophores for light-emitting

devices. As shown in Table 1, compared to a terthienyl

reference (3T) these compounds exhibit lower oxidation

Fig. 7 Spectroelectrochemistry of a poly(41) in 0.10 M Bu4NPF6

CH3CN. Electrodeposited on a 5 mm diameter Pt disk. (From ref. 84,

reproduced by permission of The Royal Society of Chemistry.)



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potentials, positively shifted reduction potentials and very high

fluorescence quantum yields.72

More recently, other EDOT-based tricyclic compounds

based on median groups such as substituted thiophenes (42),substituted phenyls (43), thiazoles (44) or oxadiazoles (45)

have been proposed as photoluminescent materials.34,8587

However, as shown in Table 1, the reported luminescence

quantum yields remain in general low and comparable to that

of terthiophene.

From a different viewpoint, compound 47 has been recently

shown to exhibit a dramatic enhancement of fluorescence in

the presence of pyrophosphate anions.88

Thiophene-based systems have been widely used as

p-conjugating spacers in pushpull chromophores designed

for 2nd order nonlinear optics.89 In this context, we have

synthesized various series of pushpull NLO-phores in which

thiophene (48) or bithiophene (50) have been replaced by

EDOT (49) or bis-EDOT (5153) as conjugating spacers.90,91

The optical data in Table 2 show that, as expected, the increase

of the acceptor strength in the 5153 series leads to a

bathochromic shift of lmax accompanied by an increase of

the quadratic hyperpolarisability (mb). Comparison of the datafor compound 48 and 49 shows that despite a 78 nm red shift

oflmax, replacement of thiophene by EDOT produces in fact a

decrease ofmbwhich is partly due to a decrease of the dipole

moment of the pushpull system.90

On the other hand, comparison of the efficiency of

compounds 5153 to that of other systems based on the same

donoracceptor couple but containing different conjugating

spacers shows that the bis-EDOT spacer leads to an efficiency

comparable to that of dithienylethylene (DTE) but inferior to

that of bridged DTE.91,92

However, comparison of the data for compounds 50 and 53

which contain the same donoracceptor couple clearly shows

that replacement of the bithiophene by the bis-EDOT con-jugating spacer produces a 118 nm red shift of lmax and

more than a twofold increase of mb. Contrary to the case of

compounds 48 and 49, replacement of bithiophene by bis-

EDOT does not produce a decrease of the dipole moment, but

on the contrary a slight increase. However since this small

increase of m alone cannot explain the observed strong

enhancement of mb, these results confirm that bis-EDOT is a

more efficient p-conjugating spacer than bithiophene.

The analysis of the crystallographic structure of a single

crystal of the EDOT dimer90,93 provides some interesting

information about the origin of the enhanced p-electron

delocalization observed on pushpull chromophores based

on bis-EDOT. Examination of the non-bonded distancesbetween sulfur and oxygen (Fig. 8) shows that these distances

(2.92 A) are significantly shorter than the sum of the van der

Waals radii of sulfur and oxygen (3.25 A). These short

distances confirm the occurrence of the already discussed

strong intramolecular non covalent interactions which rigidify

the p-conjugated structure in a fully planar anticonformation.

This self-rigidification of the bis-EDOT structure thus signifi-

cantly contributes to improve the electron transmitting

properties of bis-EDOT compared to bithiophene.

In addition to pushpull chromophores, EDOT-based

p-conjugated systems have also been incorporated in various

kinds of donor or acceptor compounds potentially useful for

Table 1 Electrochemicala and optical properties of EDOT basedfluorophores

Compound Epa/V Epr/V lmax abs/nm lmax em/nm wem

3T 1.10 22.00 350 430 0.066b

33 0.56 21.80 450 613 0.920c

34 0.92 21.40 481 630 0.750c

42a 361 441 0.034d

42b 361 441 0.032d

43a 326 413 0.001d

43b 339 397 0.096d

43c 338 440 0.025d

43d 336 410 0.103d

44 377 452 0.054d

45 ,320 413 0.090d

46a 595 629 Nr46b 614 657 Nra In 0.10 M Bu4NPF6MeCN, ref. SCE.

b In dioxane, ref. 128.c Ref. 72. d Quinine sulfate as standard, ref. 86.

Table 2 Absorption maximum,a, quadratic hyperpolarisabilityb, cal-culated dipole moment (m)c, and decomposition temperatured ofchromophores 4853

Compound lmax/nm mb/10248 esu m/D Td/uC

48e 690 6100 9.4 49 768 4600 (1300) 8.0 23050 712 5000 (1900) 15.9 21551 588 2120 (1200) 30852 649 2000 (950) 30053 830 11600 (2400) 17.1 206a In CH2Cl2.

b Measured in CHCl3 at 1.9 mm by the electric field-induced second harmonic generation (EFISH), values in parenthesesrepresent the zero frequency hyperpolarisability product mb0.c Calculated using Gaussian 98 after optimization of the geometries.d Determined in atmospheric conditions by DCS at a rate of10 uC min21. e Ref. 129.



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IR region which were assigned to oligothiophene to Fe(III)charge-transfer transitions.99

4. EDOT-based p-conjugated oligomers

In recent years, monodisperse p-conjugated oligomers have

acquired a growing importance in materials science.100,101

Initially considered as models of the corresponding poly-

disperse polymers, conjugated oligomers have progressively

emerged as a new class of materials with electronic properties

often equalling or even surpassing those of the corresponding

polymers. This point has been illustrated in particular by the

large number of publications in which oligomers are used

as organic semi-conductors for the realization of devicessuch as field-effect transistors or light-emitting diodes.7,9

Consequently, control of the electronic properties of con-

jugated oligomers in view of these advanced technological

applications has become a focus of intensive research effort. In

this context, it is clear that the combined electron donor and

self-structuring properties of EDOT represent an invaluable

tool for the design of new oligomeric structures with tailored

electronic properties.

Because of the high reactivity of the terminal a-positions of

EDOT, the length of pure EDOT oligomers has been until now

limited to rather short-chain systems. The EDOT dimer (61)

was initially synthesized as a low oxidation potential precursor

for electropolymerization.93

Almost simultaneously it wasreported that bis-EDOT end-capped with trimethylsilyl

groups102 could be electropolymerized, as previously estab-

lished for various other thiophenic precursors.103

The EDOT trimer (62) was first synthesized by Reynolds

and coworkers as a precursor for electropolymerization.32

However this compound was described as very unstable. This

instability, attributed to the high reactivity of the terminal

a-positions of ter-EDOT has led several groups to synthesize

end-capped EDOT oligomers.104106

Hicks and Nodwell first reported the synthesis of di-and ter-

EDOT end-capped with mesitylthio groups (63, 64) and

analyzed their optical and electrochemical properties. As

expected, the combined donor effects of the EDOT andbis(arylthio) groups produced a significant decrease in the

oxidation potential of the conjugated chain and a red shift of

the absorption maximum compared to oligothiophenes.104

Janssen and coworkers have reported a comparative analysis

of the optical and redox properties of a series of EDOT

oligomers containing one to four EDOT units end-capped with

phenyl groups (65) and of their thiophene based analogs.105

Absorption and fluorescence emission spectra revealed a

markedly higher degree of intra-chain order in EDOT

oligomers. Electrochemical data confirmed the lower oxidation

potential of EDOT oligomers compared to oligothiophenes.

Linear plots of the oxidation potential vs. the reciprocal chain



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length revealed significantly steeper slopes for oligo-EDOTs,

suggesting a more effective conjugation for a given chain

length. Extrapolation of the first and second oxidation

potentials suggests that coalescence of the two oxidation steps

should occur at a shorter chain length for EDOT oligomers

than for oligothiophenes.105

More recently, our group has described a series of EDOT

oligomers end-capped with n-hexyl chains and containing one

to four EDOT units (66).106 As shown in Fig. 9, the electronic

absorption spectra of these oligomers in dichloromethanesolutions present a well-resolved vibronic fine structure

consistent with a planar and rigid conjugated system.

As already discussed at several instances, this self-structur-

ing effect results from intramolecular interactions between

sulfur and oxygen atoms of adjacent EDOT units. Again these

interactions are clearly apparent in the crystallographic struc-

ture of the 66 series trimers.106

The cyclic voltammograms of the 66 series oligomers show

that the trimer and tetramer can be reversibly oxidized into

their cation radical and dication state. Linear plots of the two

oxidation potentials vs. the reciprocal chain length indicate

that coalescence of the two oxidation steps should occur for an

infinite chain length. This conclusion, which contradicts that

drawn by Janssen and co-workers for their phenyl-cappedEDOT oligomers,105 suggests that in this latter case, the

terminal phenyl groups contribute to the charge delocalization

which is, of course, not the case for our n-hexyl end-capped

oligomers.

Hybrid oligomers

Because of the strong electron donor properties of EDOT, the

maximum chain length of oligo-EDOTs has remained so far

limited to the tetramer, whatever the nature of the terminal

blocking group.104106 A possible way to circumvent this

obstacle can consist of the combination of EDOT with other

building blocks in order to reach more extended conjugatedchain lengths.

The first EDOT-containing conjugated oligomers were

reported in 1999 by Cava and coworkers who described the

synthesis of hybrid systems based on various combinations of

EDOT and vinylene linkages (6769) and the hybrid EDOT

thiophene tetramers 72 and 73.107 Shortly after we described

the synthesis and electrochemical and optical properties of

the thienylenevinylene hybrid systems 70 and 71, and a new

synthesis of tetramers 72 and 73.108

The analysis of the electrochemical properties of compounds

7073 shows that when the two EDOT groups occupy the

inner positions of the molecule, the cyclic voltammogram

Fig. 9 Electronic absorption spectra of EDOT oligomers 66 (n = 2, 3,

4) in CH2Cl2. Short dashed line, n 5 1; long dashed line, n 5 2; solid

line, n 5 3. (Reprinted from ref. 106, Copyright 2003, American

Chemical Society.)



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MesS groups which cause charge density in the cation radical

to concentrate at the chain ends thus lowering the Coulombic

barrier to the introduction of a second charge.104

Recently, we have carried out a systematic analysis of

the structureelectronic property relationships in three series

of hybrid oligomers end-capped with n-hexyl chains and

based on various combinations of thiophene and EDOT

groups (7785).

109

The analysis of the crystallographic structure of two hybrid

quaterthiophenes 77 and 78 by X-ray diffraction shows that

when the EDOT group occupy the outer positions, the two

inner thiophenes adopt a syn conformation with a 13u dihedral

angle. In contrast insertion of a bis-EDOT block in the middle

of the molecule leads to an all-anti fully planar conformation

stabilized by strong intramolecular SO interactions (Fig. 11).

UV-Vis absorption spectra show that increasing the number

of EDOT groups in the structure leads to a red shift of the

absorption maximum with an exaltation of the vibronic fine

structure. This latter effect, indicative of a rigid p-conjugated

structure, strongly depends on the position of the EDOT

groups and becomes particularly intense when adjacent EDOTgroups are inserted in the middle of the molecule. Nevertheless

the vibronic fine structure persists in alternated oligomers and

the fact that it is still discernible in the heptamer indicates that

the coherence length of the intramolecular self-rigidification

associated with the non-covalent SO interactions is at least

equal to seven thiophene units (Fig. 12).

As expected, cyclic voltammetric data show that the first

oxidation potential decreases when increasing the number of

EDOT groups in the structure. However, the position of the

EDOT groups considerably affects the potential difference

(DEp) between the first and second oxidation steps; moving the

EDOT groups from the ends to the middle of the conjugatedsystem produces an increase of DEp. As confirmed by

theoretical calculations, this phenomenon reflects an increase

of the on-site Coulombic repulsion between positive charges in

the dication, since these charges tend to localize in the vicinity

of the electron donor EDOT groups.

In order to obtain a first evaluation of the potentialities of

these hybrid oligomers as organic semi-conductors, a thin

film field-effect transistor has been fabricated by vacuum

sublimation of pentamer 81 with alternated thiopheneEDOT

structure. The OFET was built on an n-doped silicon gate

with thermally grown SiO2 as dielectric. Gold source and

drain electrodes were deposited by sublimation on top of

the organic film through a mask. A hole mobility of 0.6 61023 cm2 V21 s21 has been determined.109 Although modest,

this first result obtained on a single unoptimized device shows

that EDOT-containing conjugated oligomers can be indeed

behave as organic semi-conductors. These preliminary results



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should provide a strong incitement to develop further research

aimed at a better understanding of the structure-electronic

properties of these hybrid oligomers.

Curtis and coworkers have reported the synthesis and

characterization of bis-EDOT-4,49-diakyl-2,29-bithiazole) co-

oligomers (8688) as potential organic semi-conductors.110

The UV-Vis spectrum of the shortest system 87 shows an

absorption maximum at 410 nm, while the spectra of com-

pounds 86 and 88 show the same absorption maximum at

466 nm. The spectra of solution-cast thin films revealed a

bathochromic shift of the absorption band with the appear-

ance of a fine structure which was attributed to the effects of

p-stacking, a point subsequently confirmed by the crystal-

lographic structure.111 Although all oligomers could be

electrochemically oxidized and reduced, the position of the

EDOT groups exerts a marked influence on the cyclic voltam-

mogram. Thus whereas the CV of compound 86 exhibits

a quasi-reversible oxidation wave, the oxidation process of

compounds 87 and 88 is irreversible, presumably because of

the subsequent chemical coupling of the electrogenerated

cation radicals.110 Chemical oxidation of these compounds led

to the sequential formation of the cation radical and dication.

The cation radical was found to form diamagnetic p-dimers.

The allowed pp electronic transition was interpreted in terms

of molecular exciton theory leading to a nearest-neighbor

exciton coupling constant of 0.2 eV.112

5. Modification of the chemical structure of EDOT

As the above-discussed multiple utilizations of the EDOT build-

ing block in functional p-conjugated systems progressively

Fig. 11 Crystallographic structure of 78 (left) and 77 (right). Top: molecular structure; bottom: packing mode (SO and SS intra and inter-

molecular interactions are represented by dotted lines) (from ref. 109).

Fig. 12 UV-Vis absorption spectra of alternated oligomers 79, 83 and

85 in CH2Cl2, from left to right (from ref. 109).



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developed, several groups began to investigate in more detail

the chemistry of EDOT or undertook the modification of

the chemical structure of the EDOT system itself in order to

manipulate its chemo-physical and electronic properties.

Hellberg and coworkers have recently reported a synthesis

of EDOT based on a novel one-step synthesis of 3,4-

dimethoxythiophene 89 by ring closure of 2,3-dimethoxy-1,3-

butadiene (Scheme 4).113 EDOT was then obtained by

transetherification of 89114 using ethylene glycol.113

EDOT and other 3,4-alkylenedioxy-thiophenes have been

synthesized by the double Mitsunobu reaction.115,116 Chiral

EDOT derivatives were obtained by the same method usingenantiomerically pure 1,2-propanediols.116

Pure chiral disubstituted EDOTs have been prepared by

transetherification of 3,4-dimethoxythiophene and electro-

polymerized into stereo- and regio-regular polymers.117

Probably one of the simplest modifications of the EDOT

structure involves the substitution of the ethylenedioxy bridge

in order to improve the solubility of the resulting polymer.

Thus, various examples of soluble polymers derived from

EDOT bearing alkyl, oligooxyethylene or alkylsulfonate

chains have been synthesized.15,19,20,42,43,47,118 As revealed by

spectroelectrochemical studies, the polymers derived from

alkyl-substituted EDOT or 3,4-propylenedioxythiophene

(ProDOT) show enhanced optical contrast in electrochromicdevices.20 On this basis, processable soluble polymers (9093)

have been recently developed around the ProDOT structure.119

Although the introduction of long alkyl chains represents

the simplest method to synthesize soluble conjugated poly-

mers. The mode of hybridization of the carbon serving as

anchoring point for the solubilizing group introduces a major

difference between poly(3-alkylthiophenes) (PATs) and the

polymers derived from alky-substituted EDOT and ProDOT.

For PATs, substitution of an sp2 carbon at the 3-position of

the thiophene cycle allows the side chain to remain coplanar

with the p-conjugated PT backbone. In contrast, for EDOT,

fixation of the alkyl chain at an sp3 carbon of the ethyl-

enedioxy bridge has two undesirable consequences. First, such

substitution creates a center of chirality thus giving rise to aconsiderable number of possible regio- and stereoisomers in

the resulting polymer. Second and more importantly, the fact

that the alkyl chain can no longer remain coplanar with the

p-conjugated system produces an increase of the distance

between the conjugated chains and hence an increase of the

band gap and a drop of the charge-carriers mobility.

A clear illustration of this effect is provided by the com-

parison of the UV-Vis spectra of a soluble polymer obtained

by oxidative chemical polymerization using FeCl3 of an EDOT

monomer derivatized with a decyl chain (94).

In chloroform solution, the spectrum of poly(94) exhibits

a well resolved fine structure with a lmax at 595 nm and a

00 transition at 654 nm (Fig. 13). Surprisingly, when the

polymer is processed into a solid film the optical spectrum

remains practically identical with however a small (ca. 5 nm)

Scheme 4 Synthesis of 3,4-dimethoxythiophene from 2,3-dimethoxy-

1,3-butadiene.

Fig. 13 UV-Vis absorption spectra of poly(94). Thin line in CH2Cl2,

thick line film spin-coated on glass (from ref. 120).



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hypsochromic shift of the absorption maxima.120 In the case of

poly(alkylthiophenes) the same experiment leads to a red shift

of the absorption maximum from 430440 nm in solution to

ca. 500 nm in the solid state due to the combined effects of

planarization and strong p-stacking interactions between the

conjugated chains in the aggregated phase.13 The absence of

such effects for alkyl-substituted PEDOT shows that the large

interchain distances imposed by the substituent attached at ansp3 carbon leads to a considerable decrease of p-stacking

interactions in the solid-state. As already discussed the

suppression of these interactions significantly contributes to

increase the band gap of the polymers.58,84

A possible solution to this problem consists of the

modification of the EDOT structure in order to attach the

solubilizing side chain at an sp2 carbon. To this end, we and

others have synthesized 3,4-phenylenedioxythiophene

(PheDOT) (95).121123

In order to obtain soluble polymer compounds, 96 and 97

were synthesized by transetherification of 3,4-dimethoxy-

thiophene using the appropriate catechols.122 Although

electropolymerization could be achieved, it was considerably

more difficult than that of conventional EDOT derivatives. In

fact, as indicated by high scan rate cyclic voltammetry,

replacement of the ethylenedioxy bridge by with a phenyl-

enedioxy moeity leads to a strong stabilization of the cationradical. As shown by ab initio computations at the hybrid

density functional theory level, the HOMO level of PheDOT is

slightly higher than that of EDOT. However whereas the

SOMO of the EDOT radical has high coefficients on the 2,5-

positions, in agreement with its straightforward electropoly-

merization, for the PheDOT cation radical SOMO coefficients

are essentially localized on the dioxin ring and null on the

2,5-positions of the thiophene ring (Fig. 14). These results are

in full agreement with the experiment showing that the high

stability of the PheDOT cation radical is not compatible with

efficient polymerization.122

Despite its difficult electropolymerization PheDOT repre-

sents an interesting platform for the development of new

classes of p-conjugated systems in particular oligomers, in

which the strong donor properties of EDOT analogues will be

associated with a solubilization by substitution of sp2 carbons

compatible with compact p-stacked organization in the solidstate.

3,4-ethylenedioxypyrrole (96) can be viewed as another deep

modification of the EDOT system. Whereas the monomer

shows a lower oxidation potential than EDOT, the resulting

polymer presents a larger band gap and a lower conduc-

tivity.20,124 Cava and coworkers have described the multistep

synthesis of 3,4-ethylenedioxyselenophene (97).125 The cyclic

voltammogram showed that this compound oxidizes at a

potential 0.26 V lower than that of EDOT under the same

experimental conditions. The CV of the polymer exhibits a

broad redox system extending from 21.0 to +1.0 V vs. SCE,

while an acetonitrile solution of the neutral polymer prepared

chemically using FeCl3 as oxidant shows an absorption maxi-mum at 594 nm.125

The polymerization of the disulfur analog of EDOT, namely

ethylenedithiathiophene (98), was reported by Kanatzidis and

coworkers in 1995.126 Although this compound oxidizes at a

potential 0.18 V lower than that of EDOT, the resulting

polymer shows a much higher oxidation potential and larger

band gap than PEDOT. This shortening of the effectiveconjugation length can be attributed to the distortion imposed

on the conjugated backbone by the steric interactions between

adjacent monomers.

As an intermediate case, we have synthesized thieno[3,4-b]-

1,4-oxathiane (99), in which one of the oxygen atoms of EDOT

has been replaced by sulfur.127 This compound could be

straightforwardly electropolymerized into a stable polymer

and, as expected, the electrochemical and optical properties

of the resulting polymer were intermediate between those of

PEDOT and of poly(98). Thus, the anodic peak potential was

found at +0.40 V vs. Ag/AgCl and the absorption maximum of

the undoped polymer (532 nm) was between that of PEDOT

(590 nm) and poly(98) (448 nm). A particularly interesting

property of compound 99 is that the difference in the electronic

effect of alkoxy and alkylsulfanyl groups introduces a dis-

symmetry in the electronic density at the 2 and 5 positions of

the thiophene ring, thus inducing a difference in reactivity

useful for regioselective substitution.127

Conclusion

Almost 15 years after its synthesis, PEDOT occupies a

prominent position among conducting polymers due, among

other things, to the multiple well-established technological

applications of its various conducting forms. On the other

Fig. 14 SOMO of the cation radicals, Left: EDOT, Right: PheDOT

(95). (From ref. 122, reproduced by permission of The Royal Society of

Chemistry.)



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hand, the prevention of undesired ab9 couplings, the large

decrease of the oxidation potential and the preservation of a

high reactivity at the terminal a-positions represent decisive

advantages which have been widely used for the synthesis of

electrogenerated functional conducting polymers as electrode

materials. Whereas direct substitution of EDOT presents only

limited advantages over 3-substituted thiophenes in terms of

polymerization potential, the incorporation of the EDOT unitin the structure of precursors of electrogenerated conducting

polymers represents a very efficient approach which has led to

the development of a huge number of functional polymers with

a wide scope of potential applications.

While many of these polymers have been synthesized from

tricyclic precursor structures with a functional median group

inserted between two lateral EDOT groups, recent work

has shown that the association of EDOTthiophene based

precursors, multi-site approaches and the application of

thiolate or alcoolate chemistry for functionalization allows

the synthesis of stable electroactive polymers derivatized with

functional groups sometimes difficult to incorporate in electro-

generated p-conjugated polymers.One of the most salient features of the recent chemistry

of EDOT concerns its progressive emergence as a unique

building block for the synthesis of different classes of

molecular functional p-conjugated systems. The discovery

that, in addition to a strong electron donor effect, EDOT

gives rise to self-rigidification of linearly p-conjugated

structures by means of intramolecular non-covalent interac-

tions between oxygen, sulfur and eventually other hetero-

atoms, undoubtely represents an important result for the

synthetic chemistry of functional p-conjugated systems. In

recent years, this self-structuring effect has been successfully

used for the optimization of the (opto)electronic properties

of various classes of molecular functional p-conjugatedsystems.

The synthesis of monodisperse extended conjugated oligo-

mers represents one of the most recent developments of the

EDOT chemistry. As shown by recent work, in this case too

self-structuration plays a major role leading to a significant

enhancement of the effective conjugation. On the other hand,

the strong electron donor effects of the EDOT unit, which

constitute a major tool for the modulation of the electronic

properties of extended oligomers, also render the analysis of the

relationships between the molecular structure of the oligomer

and the structure and electronic properties of the resulting

molecular material considerably more complex. Although the

realization of the first example of a field-effect transistorbased on an hybrid EDOT-based conjugated oligomer

illustrates the potentiality of this class of oligomers as organic

semi-conductors, this preliminary result confirms the need

to concentrate much research effort on the elucidation of

the structureproperty relationships in these new classes of

conjugated oligomers.

Solubilization represents another specific problem posed

by the structure of EDOT. While phenylenedioxythiophene

represents a first attempt in that direction, it is clear that

further work is needed to develop soluble EDOT-based con-

jugated systems compatible with compact p-stacking interac-

tions in the solid state.

The modification of the chemical structure of EDOT itself,

by replacement of some of its constitutive atoms, is another

quite recent topic which, beyond its immediate fundamental

interest, could also lead to major advances in the future. Here

also, intense effort to develop creative synthetic chemistry

seems more than ever necessary.

AcknowledgementsThe authors wish to thank their co-workers, post-doctoral

fellows and PhD students named in some of the cited

references for their invaluable contribution to some of the

work covered in this review.

Jean Roncali,* Philippe Blanchard and Pierre FrereGroupe Systemes Conjugues Lineaires, CIMMA, UMR CNRS 6200,Universite dAngers, 2 Bd Lavoisier, Angers, France.E-mail: [email protected]

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