Polythienobenzothiophenes, a new family of electroactive polymers: Electrosynthesis, spectral characterization and modelling

Article abstract of DOI:10.1039/b308779b

New conducting polymers, including poly[thieno[3,2-b][1]benzothiophene] (poly-TBT) and poly [6-methoxythieno[3,2-b][1]benzothiophene] (poly-MeOTBT) were electrosynthesized by anodic oxidation of the corresponding monomers in 0.1 M LiClO₄ acetonitrile solution. Poly-TBT and poly-MeOTBT electroactive films were formed on platinum electrodes and characterized spectroscopically. FT-IR studies show that both polymers present couplings occurring on the thiophene moiety and the phenyl ring, with a step-like structure. MALDI-TOF mass spectrometry indicates that poly-TBT and poly-MeOTBT films are mainly constituted of short-chain oligomers. The results of molecular orbital (MO) calculations performed on the basis of a radical-cation electropolymerization mechanism are in good agreement with our spectral data.

Full text of DOI:10.1039/b308779b

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A R T I C L E
Polythienobenzothiophenes, a new family of electroactive
polymers: electrosynthesis, spectral characterization and
modelling
Irari Fouad,a Zouhair Mechbal,a Kathleen I. Chane-Ching,a Alain Adenier,a
Franc¸ois Maurel,a Jean-Jacques Aaron,*a Petr Vodicka,b Katerina Cernovska,b
Vaclav Kozmikb and Jiri Svobodab
aITODYS, Universite´ Paris 7-Denis Diderot, associe´ au CNRS (UMR 7086) 1,
rue Guy de la Brosse, 75005 Paris, France. E-mail: aaron@paris7.jussieu.fr
bDepartment of Organic Chemistry, Prague Institute of Chemical Technology,
Technicka 5, 16628 Prague 6, Czech Republic
Received 25th July 2003, Accepted 19th March 2004
First published as an Advance Article on the web 10th May 2004
New conducting polymers, including poly[thieno[3,2-b][1]benzothiophene] (poly-TBT) and poly
[6-methoxythieno[3,2-b][1]benzothiophene] (poly-MeOTBT) were electrosynthesized by anodic oxidation of the
corresponding monomers in 0.1 M LiClO4 acetonitrile solution. Poly-TBT and poly-MeOTBT electroactive
films were formed on platinum electrodes and characterized spectroscopically. FT-IR studies show that both
polymers present couplings occurring on the thiophene moiety and the phenyl ring, with a step-like structure.
MALDI-TOF mass spectrometry indicates that poly-TBT and poly-MeOTBT films are mainly constituted of
short-chain oligomers. The results of molecular orbital (MO) calculations performed on the basis of a radical-
cation electropolymerization mechanism are in good agreement with our spectral data.
temperature interval (about 100 uC) and, in addition, for
some of these molecules, a narrow cholesteric and an extra-
ordinary wide, blue phase.14
Introduction
A large number of studies have been devoted to conjugated,
conducting polymers and oligomers because of their potential
applications as materials for electronic components, non linear
optics, and electroluminescent diodes. Semiconducting, sub-
stituted oligothiophenes have been particularly investigated
due to their simple chemical structure, interesting physico-
chemical properties, stability in air and possibility of use as a
model for the evaluation of charge storage.1–5 For instance,
electrosynthesized thin films of poly(3,4-ethylenedioxy-
thiophene) and poly(3-methoxythiophene) have been shown
to present good optical, mechanical and anticorrosion proper-
ties, especially when electropolymerization was performed in
aqueous micellar media.3–5 Until now, most monomers used as
precursors for the preparation of these oligothiophenes were
generally simple, mono- or disubstituted thiophene derivatives.
In contrast, far fewer studies have concerned the use of fused
ring systems containing one or more thiophene units for the
preparation of conducting polymers. Several reports have
concerned the chemical and electrochemical synthesis of
polythieno[3,2-b]thiophenes and various, related derivatives
from the corresponding monomers, and their properties.2,6–13
Generally, the formation of films of polymeric materials at
metallic electrodes was observed. Because of the conjugated
structures of these monomers, the electropolymerization of
thieno[3,2-b]thiophenes occurred at lower oxidation potentials
than thiophene. Moreover, several potential, interesting
applications, including thin film transistors13 and photolumi-
nescent systems12 have been described. Recently, we have
demonstrated that fused benzothiophenes and other fused
heterocyclic derivatives possessed remarkable ferroelectric
liquid crystal and/or luminescence properties.14–16 For exam-
ple, it was discovered that the incorporation of a fused
heterocyclic core into organic molecules usually possessing
ferroelectric liquid crystal properties led to new compounds
forming a bright-yellow smectic phase in a very broad
Also, we have found that four thienobenzothiophene and
thienobenzoindole derivatives exhibited room-temperature
fluorescence emission spectra in the near UV (320–380 nm)
region with fluorescence quantum yields varying widely in the
range 0.0002–0.56, according to the monomer structure and the
polarity of the solvent.16 It was shown in the same study that
oligomers prepared from fused benzothiophenes were charac-
terized by a strong luminescence occurring in the visible, with
dramatic red-shifts of the emission maxima (Dl # 90–110 nm)
and an important increase of fluorescence intensity relative to
the corresponding monomers.16 All these features underline the
interest in obtaining polymer films with a well-defined struc-
ture, starting from fused benzothiophene derivatives as build-
ing blocks. In particular, such materials should have important,
potential applications to luminescent devices.
In this work, we describe in detail the electrosynthesis of two
new, electroactive conducting polymers prepared from thieno-
[3,2-b][1]benzothiophene (TBT) and 6-methoxythieno[3,2-b][1]-
benzothiophene (MeOTBT) (Scheme 1). We investigate the
electrochemical, optical and physicochemical properties of
these polymers and we characterize spectroscopically their
Scheme 1 Structure of TBT and MeOTBT.
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1
1 7 1 1

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molecular structure. MO calculations are also performed on
the basis of a radical-cation electropolymerization mechanism
and compared with the experimental spectral data.
deposited onto the electrode simultaneously to the oxidation of
the monomer (first peak), as commonly reported.24,25
Repetitive voltammetric cycles in oxidation of 5 6 1022
M
TBT or MeOTBT and 0.1 M LiClO4 acetonitrile solutions
exhibited a similar behaviour for both monomers. Electro-
active polymer coatings were generated onto the electrode
when cycling from 0 V up to 1.3 and 1.2 V for TBT and
MeOTBT, respectively. However, the shape of the resulting
voltammograms strongly depended on the monomer structure.
A broad redox signal ranging from 0.2 to 1.2 V was observed
for the growth of poly-TBT while poly-MeOTBT presented
very narrow redox waves centered at about 1.0 and 0.55 V for
the oxidation and reduction processes, respectively (Fig. 1A
and 1B).
This may indicate that more or less long oligomer chains are
obtained when TBT is oxidized, whereas shorter species are
generated by MeOTBT oxidation. Both polymers can also be
electrosynthesized either in the galvanostatic or the potentio-
static mode. In the galvanostatic mode, a current density
minimum value of 0.25 mA cm22 was required to form polymer
deposits onto the platinum electrode, which corresponded to
the deposition potential of the polymers. When the current
density values ranged from 0.25 to 1 mA cm22, black, adherent
and homogeneous films were generated at a potential value
near 1.2 and 1.1 V for poly-TBT and poly-MeOTBT,
respectively. In both cases, the thickness of the film could be
easily controlled by the timescale of the electrosynthesis.
Moreover, films of poly-TBT and poly-MeOTBT can also be
obtained in the potentiostatic mode, by applying a minimum
potential of 1.2 and 1.05 V for poly-TBT and poly-MeOTBT,
respectively.
Results
Monomer synthesis
Thieno[3,2-b][1]benzothiophene (TBT) 1 has been obtained by
various multistep procedures17–21 in variable yields. Based on
the method of Ricci et al.,18 a new and efficient preparative
synthesis of 1 and its 6-methoxy derivative 2 (MeOTBT) was
elaborated. The starting thiosalicylic acid and 4-methoxy-
thiosalicylic acid,22 respectively, were alkylated with chloro-
acetic acid to afford diacids 3 and 4 (Scheme 2). Cyclization23
of these diacids by means of N,N-dimethylformamide and
phosphorus oxide chloride was followed by a subsequent
formylation and chlorination leading to chloro aldehydes 5 and
6 in high yield. Annelation of the second thiophene ring was
accomplished with methyl thioglycolate in basic media and
the corresponding methyl esters 7 and 8, respectively, were
obtained. Base catalyzed hydrolysis of esters 7 and 8 gave rise
to acids 9 and 10, which were decarboxylated by a standard
method with copper bronze in quinoline to the required
heterocycles 1 (TBT) and 2 (MeOTBT).
Electrochemical studies
First we investigated the cyclic voltammetry (CV) behaviour of
TBT and MeOTBT in acetonitrile solutions containing 1023
M
Whatever the mode of polymerization chosen, the obtained
films exhibited a stable electroactivity (Fig. 2). They were
reversibly driven between the oxidized and neutral states, at
various scanning rates, and the ipa or ipc values varied linearly
with the sweep rate. This is consistent with the reaction of
surface-localized species, generally encountered for conducting
polymers. When submitted to a great number of redox cycles
(n ~ 50), the poly-TBT films exhibited a moderate loss of their
electroactivity (about 24%). These electrochemical properties
were accompanied by color changes of the films which turned
from black to yellowish upon reduction. Therefore, our elec-
trochemical study shows that, under various electrosynthesis
conditions, it is possible to form electroactive, relatively stable,
monomer and 0.1 M LiClO4. Potential scanning was performed
between 0.7 and 1.8 or 2.0 V/SCE at scan rates ranging from 20
to 500 mV s21. The first onset of significant oxidation appeared
near 1.4 and 1.2 V/SCE, respectively, for TBT and MeOTBT.
This oxidation potential difference between both monomers
can be attributed to the electron-donating effect of the methoxy
group in MeOTBT that contributes to lowering the oxidation
potential of the molecule.
In contrast, a second, broad oxidation wave, occurring near
1.8 V, was found to be practically independent of the monomer
structure, whereas no reduction wave appeared during the
reverse sweep. This second wave can be attributed to the over-
oxidation of oligomeric species (dimers, trimers or more),
Scheme 2 Synthetic route to TBT and MeOTBT.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1
1 7 1 2

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Fig. 1 Cyclic voltammograms of the electrodeposition of poly-TBT (A), and poly-MeOTBT (B) in acetonitrile solutions containing 5 6 1022 M of
the monomer 1 0.1 M LiClO4; scan rate: 100 mV s21
.
conducting films of poly-TBT or poly-MeOTBT, from fused,
heterocyclic monomers.
Scheme 1. A careful inspection of the listed assignments shows
that most predicted vibrations are not pure and involve a
coupling of various modes of vibration. For characterization
purposes, it is useful to distinguish the various modes, as a
function of their weight (strong or weak contribution) in the
overall vibration.
Owing to the structure of both monomers, two different
coupling sites might be involved in the formation of polymer
chains. Indeed, on the one hand, the thiophene moiety
possesses two available, oxidizable sites in the case of TBT
and MeOTBT. On the other hand, coupling might also occur
on the phenyl ring in both compounds. One way to discri-
minate between the possible polymer structures is to compare
the IR spectra of the monomers with those of the resulting
polymers.
The IR spectra of TBT (curve a) and poly-TBT (curve b) are
presented in Fig. 3. In good agreement with previously
reported literature IR data for simple thiophene derivatives
and oligomers,26,27 we found that the bands at 703 and
853 cm21 could be attributed to simultaneous C–H out-of-
plane bending vibrations (c-type) of the two hydrogen atoms
located on the thiophene ring (Table 1); both vibrations are
coupled, involving, respectively, a stronger contribution of the
Ca–H bending mode (on the C-2 position) for the 703 cm21
band, and of the Cb–H bending mode (on the C-3 position) for
the 853 cm21 band. The very strong band at 752 cm21 is
characteristic of the in-phase cC–H bending vibrations of the
four adjacent hydrogens on the phenyl ring, and the weaker
one at 725 cm21 corresponds to the cC–H vibration on the
overall extended fused ring system. Finally, the band appearing
at 906 cm21 is due to a dCa–H bending vibration and moderate
nC–C and nC–S stretching vibrations, which constitutes a
convenient means to probe a coupling occurrence at the C-2
position during polymerization. The bands at 3093 and
3078 cm21 correspond to nC–H stretching vibrations on the
thiophene ring, respectively at the C-2 and C-3 positions,
whereas the three other bands located at 3054, 3034 and
3006 cm21 can be attributed to nC–H stretching vibrations of
Poly-TBT and poly-MeOTBT structural characterization
In order to elucidate the type of coupling and molecular com-
position of the poly-TBT and poly-MeOTBT films, we investi-
gated their FT-IR and MALDI-TOF mass spectra. For
spectral analysis, the poly-TBT films were electrosynthesized
galvanostatically (j ~ 0.5 mA cm22), whereas the poly-
MeOTBT films were obtained potentiostatically (E ~ 1.05 V/
SCE) in 0.1 M LiClO4 acetonitrile. In addition, we modelled
the coupling reaction and reactive sites by using molecular
orbital (MO) calculations, on the basis of the radical-cation
electropolymerization mechanism.
a) FT-IR spectroscopy. Typical IR data (including the
experimental and calculated vibration wavenumbers, intensi-
ties and assignment of the main IR bands) for TBT, poly-TBT,
MeOTBT and poly-MeOTBT are presented in Table 1. The
atom numbering refers to their position, as indicated in
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1
1 7 1 3

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Fig. 2 Electroactivity in a 0.1 M LiClO4 acetonitrile solution of poly-TBT (A) and poly-MeOTBT (B) films, electrosynthesized galvanostatically
(A) and potentiostatically (B) on a Pt electrode. Voltammograms were obtained at various scan rates.
the four adjacent H on the phenyl ring. Important changes take
place in the IR spectrum of poly-TBT (Fig. 3, curve b and
Table 1). First, in the fingerprint region, the two bands at 703
and 853 cm21 vanish, which indicates that a coupling occurs
during polymerization at the C-2 and C-3 thiophene positions;
also, the 906 cm21 band disappears, which reveals a greater
efficiency of the coupling at the C-2 position. Moreover, an
unusually wide and dissymmetric peak at 746 cm21 and a less
intense one at 719 cm21 appear. As a result from the best-peak
fitting treatment, we found that the 745 cm21 band could be
deconvoluted in narrower bands at 752, 746 and 740 cm21, and
the smaller one at 719 cm21 in two components at 717 and
721 cm21. We decided to assign the 752 and 721 cm21
components to the in-phase cC–H bending vibrations of the
four adjacent hydrogens on the phenyl ring, the 746 and
740 cm21 components being considered as inherent to poly-
TBT. As indicated by the presence of the 752 and 721 cm21
bands, some phenyl rings could remain untouched, probably
located at the end of polymer chains, suggesting that the phenyl
ring is partially involved in the polymerization process and that
poly-TBT is probably constituted of short chain oligomers. On
the basis of our calculations, we estimated that the small bands
at 812 and 885 cm21 could be reasonably ascribed, respectively
to cC–H bending vibrations at C-6 (a single H) and C-8 (two
adjacent H) phenyl ring positions, suggesting the occurrence of
a Ca (thiophene)–C6 (phenyl) coupling between repeat units in
the polymer. Accordingly, in the stretching region of the poly-
TBT spectrum, the nCa–H thiophene band at 3093 cm21
disappears, while the shoulder at 3064 cm21 overlaps a small
component at around 3078 cm21 which can be assigned to the
other nCb–H thiophene band (Fig. 3).
and 835 cm21 arise from cC–H bending vibrations on C-5, C-7
and C-8 positions of the phenyl ring. Finally, the band at
868 cm21 can be attributed to nS4–C4a and dC–C benzene
vibrations; moreover, this relatively strong band overlaps a
weaker component located at around 868 cm21 probably due
to the cCb–H bending vibration of the thiophene ring. A
number of important differences can be noted between the
poly-MeOTBT and MeOTBT spectra: several bands located at
732, 902 and 1182 cm21 in the monomer—attributed mainly to
bending vibration modes of thiophene—disappear completely
in the polymer, which indicates a coupling at the thiophene ring
Ca and/or Cb positions (Fig. 4, curves a and b). On the other
hand, only slight shifts are observed for bands occurring, in the
monomer at 793 (complex coupling, see Table 1), 819 and
835 cm21 (phenyl cC–H), and 868 (see Table 1), 1029, 1056,
1230 and 1265 cm21(phenyl cC–H bending vibrations) and, in
the polymer, respectively, at 789, 802 and 827 cm21, and 870,
1033, 1058, 1228 and 1270 cm21 (see Table 1 for detailed
assignments). These moderate shifts between the polymer and
monomer in the phenyl C–H bending vibration bands region
suggest that the phenyl ring is probably not very much involved
in the polymerization process. Therefore, these spectral results
demonstrate that coupling in poly-MeOTBT takes place
essentially at the thiophene ring Ca and Cb positions.
b) MALDI-TOF mass spectrometry. The poly-TBT and
poly-MeOTBT films were analyzed by MALDI-TOF mass
spectrometry. Fig. 5 shows the MALDI-TOF spectrum of a
neutral poly-TBT film electrosynthesized galvanostatically as
indicated previously. As can be seen, this spectrum displays six
peaks at m/z ~ 564, 754, 943, 1123, 1321 and 1521, corres-
ponding, respectively to the trimer (44%), the tetramer (17%),
the pentamer (25%), the hexamer (8%), the heptamer (3%) and
the octamer (3%). Therefore, poly-TBT is constituted mainly of
trimer, tetramer and pentamer.
The MeOTBT IR spectrum exhibits a set of narrow bands of
relatively strong intensities at 902, 868, 835, 819, 793, 732 and
709 cm21 (Fig. 4, curve a). The bands at 902, 793, 732 and
709 cm21 can be assigned similarly to the TBT bands at 906,
796, 703 and 631 cm21, respectively (Table 1). The bands at 819
In the case of poly-MeOTBT, the oligomeric composition of
1 7 1 4
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1

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Table 1 IR frequencies (cm21) and assignments of the main bands of TBT, MeOTBT, and thin films of poly-TBT and poly-MeOTBT deposited on
a platinum electrode in 0.1 M LiClO4 acetonitrile
Band position/cm21
TBT
Band position/cm21
Poly-TBT
Calculated/cm21
Assignment
631 s
703 s
602
666
cC-H2 1 cC-H3 1 cC-C (thio 1 benz)
cC-H2 1 cC-H3 1 cC-H (benz)
e
e
719 s
746 s
725 w
752 vs
797 w
853 s
704
735
777
823
825
cC-H8 1 cC-H7 1 cC-H6 1 cC-H2 1 cC-H3
cC-H5 1 cC-H6 1 cC-H7 1 cC-H8
nS1-C8b 1 nS1-C2 1 dC-H3 1 nS4-C3a
cC-H3 2 cC-H2 1 cC-H (benz)
cC-H3 1 cC-H5 2 cC-H7 2 cC-H8
e
e
867 w
870 w
885 w
928 w,br
906 s
978 w
885
958
dC-H2 1 nS4-C3a 1 nC-C thio 1 dC-C (benz)
nS1-C8b 1 dC-C (benz)
dC-H5 2 dC-H8 1 nC7-C6 1 nC5-C6
dC-H5 2 dC-H8 1 nS4-C4a 1 dC-C (benz)
dC-H2 2 dC-H3
dC-H8 2 dC-H7 2 dC-H6 1 nC6-C5 1 nC7-C8
dC-H5 1 dC-H7 2 dC-H6
dC-H2 1 dC-H3 1 nS4-C3a 1 nC-C (thio)
dC-H5 1 dC-H8
dC-H5 1 nC8a-C8b (thio)
1017 w
1064 w
1082 w
1130 w
1158 w
1189 w
1252 w
1297 w
1321 w
1352 s
1432 s
1023
1047
1085
1128
1160
1174
1249
1313
1334
1370
1444
1019 w
1065 w
e
1157 w
1190 w
1252 w
1301 w
1324 w
dC-H2 1 dC-H3 1 dC-H5 1 nC-C (benz 1 thio)
dC-H7 1 dC-H8 1 dC-H2 1 dC-H3 1 nC-C (benz, thio)
dC-H5 2 dC-H8 1 nC-C (benz, thio)
1450 s
1465 s
3006 w
3033 w
1474
3111
3121
dC-H6 1 dC-H7 1 n3a-C3 1 nC8a-C8b
nC-H5 1 nC-H8 2 nC-H7 2 nC-H6
e
e
nC-H5 1 nC-H6 2 nC-H8 2 nC-H7
3048 m,br
3055 m,sh
3064 m,br
3078 w,sh
e
nC-H6 1 nC-H7 1 nC-H8 1 nC-H5
3054 w
3130
e
nC-H3 2 nC-H2
3076 s
3093 vs
3155
3189
e
nC-H2 1 nC-H3
PolyMeOTBT
698 m,b
MeOTBT
709 s
732 s
793 s
819 s
835 s
865 w,sh
868 s
902 s
599
656
773
786
796
820
854
885
cC-H2 1 cC-H3 1 cC-C (thio, benz)
cC-H2 1 cC-H3 1 cC-H (benz)
e
e
e
nS1-C8b 1 nS1-C2 1 dC-H3 1 nS4-C3a
789 s
802 s
827s
cC-H5 2 cC-H7 2 cC-H8
cC-H5 1 cC-H7 1 cC-H8
cC-H3 2 cC-H2 1 cC-H (benz)
e
870 w
935 m
nS4-C4a 1 dC-C (benz)
dC-H2 1 nS4-C3a 1 nC-C (thio) 1 dC-C (thio)
975 w
1029 s
1056 s
1090 w
1182 w
1230 s
1265 s
958
nS1-C8b 1 dC-C (benz)
dC-H5 2 dC-H7 2 dC-H8 1 nC-C (benz) 1 nO-CH3
e
1034
1046
1085
1171
1225
1269
1033 m
1058 w
dC-H5 1 nS4-C4a 1 dC-C (benz) 1nO-CH3
dC-H2 2 dC-H3
dC-H2 1 dC-H3 1 nS4-C3a 1 nC-C (thio)
e
1229 m
dC-H8 2 dC-H5 1 nC6-OCH3
dC-H8 1 dC-H7 1 dC-H5 1 nC6-O(CH3) 1 CH2 wag
1270 m, b
1300 sh
1308 m,b
1303 m
1317 w
1351 w
1418 w
1435 m
1458 w
1478 s
1553 m
1599 m
2835 w
2891 w
2909 w
2937 w
2968 w
3015 w
3063 w
3079 w
3095 w
1315
1328
1370
1400
1443
1470
1489
1558
1608
2937
2997
dC-H7 1 dC-C (benz) 1 nS1-C8b
dC-H2 1 dC-H3 1 dC-H5 1 nC-C (thio 1 benz)
dC-H7 1 dC-H8 1 dC-H2 1 dC-H3 1 nC-C (thio 1 benz)
dC-H7 1 dC-H5 1 nC7-C8 1 nC5-C4a 1 n3a-C31 n8a-8b
d(CH3)sym 1 dC-H5
d(CH3)asym
d(CH2) scissor 1 nC2-C3
nC7-6 1 nC8a-C4a 2 nC6-5 2 nC8a-C8
nC7-C8 1 nC5-C4a 1 nC8a-C8b
nC-Hsym (CH3O)
1435 w
1460 w
1482 w
1553 w
1599 m,b
nC-Hasym (CH3O)
3080
3112
3137
3142
3155
3189
nC-H (CH3O)
nC-H8 2 nC-H7
e
e
nC-H7 1 nC-H8
nC-H5
nC-H3 2 nC-H2
e
e
nC-H2 1 nC-H3
a w: weak; m: medium; s: strong; v.s: very strong; b: broad; sh: shoulder; n: stretching vibration (sym: symmetric; asym: antisymmetric); d: in
plane bending vibration; c: out-of-plane bending vibration; e: a minor contribution of the considered vibration; wag: wagging vibration; scissor:
scissoring vibration; thio: thiophene ring; benz: phenyl ring; 1: in-phase vibration; 2: out-of-phase vibration.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1
1 7 1 5

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Fig. 3 FT-IR spectra of (a) TBT in a KBr pellet and (b) poly-TBT film (reduced form).
the films was found to depend on the electrosynthesis mode.
Indeed, for a poly-MeOTBT film prepared potentiostatically
(3 min), then electrochemically reduced (10 min), the MALDI-
TOF spectrum presented two peaks at m/z ~ 438 (strong,
dimer) and 654 (very weak, trimer), indicating that MeOTBT
dimerization was the main process in these conditions. In
contrast, when the poly-MeOTBT film was deposited poten-
tiostatically during 3 min, then submitted to successive scans
for 30 min and electrochemically reduced (10 min) the
MALDI-TOF spectrum exhibited a strong peak at m/z ~
438 (dimer) and five other, weaker peaks at m/z ~ 656 (trimer),
874 (tetramer), 1092 (pentamer), 1307 (hexamer) and 1529
(heptamer), suggesting that the electrolysis time and condi-
tions play an important role in the formation of short-chain
oligomers.
films deposited during a very short time (3 min), very sharp
oxidation and reduction peaks are observed in the CV curves,
which corresponds to the formation of dimers. In contrast, for
films submitted to successive scans during 30 min, the CV
curves show the presence of very wide anodic and cathodic
peaks, which reflects the formation of oligomers of various
lengths.
More generally, the MALDI-TOF spectra reveal that the
poly-TBT films are formed of oligomers of chains longer than
those of the poly-MeOTBT films. Steric and electrostatic
interactions of the methoxy group, inhibiting the coupling
process of MeOTBT species, might explain this difference of
behaviour between both oligomers.
c) Molecular orbital calculations and electropolymerization
mechanism. In order to verify the consistency of our spectral
results concerning the poly-TBT and poly-MeOTBT molecular
These spectral results are in good agreement with the
electrochemical data. Indeed, in the case of poly-MeOTBT
1 7 1 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1

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Fig. 4 FT-IR spectra of (a) MeOTBT in a KBr pellet and (b) poly-MeOTBT film (reduced form).
structure with the radical-cation electropolymerization mecha-
nism theory, we applied some MO calculations. For this pur-
pose, we determined as a first approximation the intrinsic
reactivity of both monomers (TBT and MeOTBT) and the
most probable linkage sites between the monomer units. We
assumed that the majority of species present in solution were
those appearing first during the reaction (kinetic control). Also,
to establish the most probable polymer structure, we con-
sidered only the propagation step within the radical-cation
mechanism. Indeed, the generally-accepted mechanism for
the electropolymerization reaction of thiophene derivatives
includes three steps. The first one is a one-electron oxidation of
the monomer to form the corresponding radical-cation; its
probability can be evaluated by means of the molecule
ionization potential. The second step consists in a coupling
reaction of two radical-cations to yield a dihydro-dimer
dication. In a third step, the elimination of two protons
leads to dimer formation. It is generally assumed that the
radical-cation coupling (second) step is rate determining for the
overall electropolymerization process.
Although a rigorous modelling of the radical-cation coupling
step needs to localize and characterize the transition state
involved in this step, the reactivity can be estimated by using
two different, simple theoretical approaches. Both approaches
were applied in order to compare which model was more
adapted to predict the coupling sites in the electropolymeriza-
tion reaction of TBT and MeOTBT.
The first approach is based on the correlation between the
reactivity and the unpaired electron density. Indeed, since
the radical-cation coupling is expected to occur by pairing off
the electronic spins of each radical-cation to form a bond, the
reactivity of this step should be closely related to the unpaired
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1
1 7 1 7

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Fig. 6 Highest singly occupied molecular orbitals (SOMOs) of
radical-cations of (a) TBT and (b) MeOTBT.
Fig. 5 MALDI-TOF mass spectrum of a poly-TBT film (reduced
form) electrosynthesized galvanostatically in 0.1 M LiClO4 acetonitrile.
coupling step reactivity is based on the frontier orbital model.
In the frame of this theory, it is expected that, for both species,
the strongest stabilization involves the two singly molecular
occupied orbitals (SOMO) of the radical-cations. Since this
interaction is proportional to the overlapping of SOMOs, the
increase of SOMO coefficient values should result in stronger
interactions. However, the SOMO coefficients show different
patterns in TBT and MeOTBT, and it is clear that the atoms
have different degrees of participation in these orbitals. We
have displayed in Fig. 6 the 3-D shapes of the SOMOs for TBT
and MeOTBT radical-cations. Table 2 also lists the SOMO
coefficients. In the radical-cation/radical-cation coupling mechan-
ism, it is expected that the strongest interaction between mole-
cular orbitals involves both SOMOs. These orbitals are of p
type, and are delocalized along the three heteroaromatic and
aromatic rings.
In the case of TBT radical-cation the most important con-
tributions clearly come from the sulfur atoms, whereas for the
electropolymerization process, the reactive sites would range
according to the following, decreasing order of the carbon
atom positions: C-3 w C-5 w C-8 w C-2. On the basis of this
analysis, it can be expected that the coupling process
(dimerization) would take place mainly at the C-3 and C-5
positions, whereas the C-8 and C-2 positions would be less
involved in this process. This apparent discrepancy with the
results of the first theoretical approach is probably due to the
fact that the SOMO treatment is based on a more simplified
MO model. It should be noted that the frontier orbital model
exaggerates the influence of molecular orbital interactions, but
completely neglects the electrostatic interactions. This suppres-
sion of the electrostatic interactions can be considered as
equivalent to the use of an imaginary solvent with maximum
screening effects.
electron density and the steric effects between both radical-
cations. Table 2 lists the calculated atomic electron spin
densities of the radical-cations corresponding to the TBT
and MeOTBT monomers. The spin density values increase with
the probability of bond formation through radical-cation
coupling, the maximum values indicating that the electronic
orbitals of neighbouring radical-cations should begin to over-
lap. As can be seen, the radical-cations of both monomers show
rather similar spin density distributions at the various cyclic
positions. For both species, the highest spin density values are
observed at the C-2 position on the thiophene ring. TBT
radical-cation shows also a significant spin density distribution
on the phenyl ring, the largest spin density values being found
at the C-6 and C-8 atoms. The electron-donating effect of the
methoxy substituent induces a slightly different distribution
with a decrease of spin density on the benzene ring in the
MeOTBT radical-cation. Indeed, in contrast with TBT, the
MeOTBT radical-cation spin density at the C-8 position is
found to be practically negligible. This significant asymmetric
distribution in the MeOTBT radical-cation spin density
produces a strong directive power for the reactivity of this
species. As a consequence, the main regioselective products
of the propagation reaction can be expected to result from
coupling at the C-2 and C-6 reactive positions for TBT and at
the C-2 position for MeOTBT, respectively. In addition, it
can be noted that steric hindrance interactions between the
methoxy group and the aromatic or the thiophene ring should
also play an important, destabilizing role during the coupling
step of MeOTBT electropolymerisation. These steric interactions
might partly explain the lower probability of the C-8/C-8 and
C-8/C-2 coupling.
In the case of the MeOTBT radical-cation, the methoxy
electron-donating group, causes a significant change in the
highest SOMOs. The main contribution in this orbital now
comes from the C-2, C-7 and C-8 atoms and to a less extent
from the C-5 atom. Thus, for MeOTBT, according to this
approach, the dimerization process should take place mainly at
the C-2 and C-7 positions.
The second theoretical approach used to investigate the
Table 2 Total atomic spin density distribution (atom numbering is
displayed in Scheme 1) and SOMO coefficients for TBT and MeOTBT
radical cationsa
C-2
C-3
C-5
C-6
C-7
C-8
The better agreement to the experimental results of the
approach based on the unpaired spin density calculations,
relative to the frontier orbital model, can be attributed to the
fact that the electrostatic interactions—which are practically
not taken into account in the latter model—play an important
role in the electropolymerization process, because of the
occurrence of a radical-cation mechanism and of the utilisation
of a polar solvent.
Spin density
TBT
MeOTBT 0.020 20.005
0.023 20.008 20.001
0.018 20.008 0.018
0.011 20.001 0.003
0.003
SOMO coefficients
TBT
MeOTBT 0.207
0.064 20.094 20.144 20.073
0.103 0.160
0.063 20.146 20.159 20.078 0.103
a Values in bold indicate the highest probability of coupling.
1 7 1 8
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1

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Film morphology
be attributed to the fused aromatic and thiophene ring local
p A p* transition. The other, strongly red-shifted, weaker
peaks, located at increasing wavelengths, result from the pro-
gressive delocalization of p electrons, related to the increasing
number of conjugated segments in the oligomeric chains.
Therefore, these peaks can be ascribed, respectively, to
oligomers of various lengths containing an increasing
number of repeat units. Indeed, we found that a linear
relationship was established between the electronic transition
energy (E, in eV) for each peak and 1/N (N, number of repeat
units in the oligomer). This relationship obeys the following
equation (correlation coefficient: r ~ 0.995):
The morphology and thickness of poly-TBT and poly-MeOTBT
films were investigated by scanning electron microscopy
(SEM). These films were electrosynthesized galvanostatically
(j ~ 0.5 A cm22) in 0.1 M LiClO4 acetonitrile solutions and
investigated either in the oxidised or the electrochemically
reduced state.
Poly-TBT films present a relatively homogeneous smooth
surface, on which aggregates of various shape and size are
irregularly spread about. In contrast, poly-MeOTBT films are
characterized by a very rough surface, showing the growth of
an irregular relief and an augmentation of porosity, upon
increasing the electrolysis time. These morphological differ-
ences can be related to distinct microstructures of both
polymers.
The thickness of poly-TBT and poly-MeOTBT films was
also measured using SEM micrographs. As expected, the film
thickness was found to depend linearly on the amount of
anodic charge used during electrolysis time. A growth of about
one micrometer every 240 and 171 C cm22 was measured for
TBT and MeOTBT, respectively. These results allow to predict
the thickness of poly-TBT or poly-MeOTBT films for a given
value of anodic charge.
E ~ 2.67/N 1 2.43
Similar results were obtained in the case of the UV-visible
absorption spectra of polythiophenes28,29 and polypyrroles30
with oligomers of different lengths.
Our interpretation of the poly-TBT absorption spectrum is
in good agreement with the MALDI-TOF mass spectrometry
results, which show also the presence of various oligomers.
In contrast, the absorption spectrum of poly-MeOTBT
exhibits only two maxima at 285 nm (E ~ 4.35 eV) and 390 nm
(E ~ 3.18 eV). The latter one is strongly red-shifted relative to
the spectrum of the monomer and can be attributed to the
delocalized p electronic system due to the presence of dimers
and/or trimers, as demonstrated by our MALDI-TOF mass
spectral data.
Optical properties
The UV-Vis absorption spectra were recorded for poly-TBT
and its monomer in dimethyl formamide solution, and for poly-
MeOTBT and the corresponding monomer in tetrahydrofuran.
The films electrodeposited on platinum were dissolved in the
above-mentioned solvents.
Conclusion
We have shown in this work, for the first time, that the
electropolymerization of fused thieno[3,2-b][1]benzothiophene
and its 6-methoxy derivative leads to the formation of new
materials, constituted of electroactive, conductive polymer
films deposited on platinum electrode. Our FT-IR and
MALDI-TOF mass spectral analysis suggest that both poly-
mers present inter-ring bonding on the thiophene and phenyl
cycles and include mainly short-chain oligomers. Our spectro-
scopic results are also confirmed by applying simple theoretical
MO approaches. These treatments, used on the basis of a
radical-cation electropolymerization mechanism, confirm the
type of coupling observed and the respective positions of
the reactive sites on both thienobenzothiophenes. Moreover,
the remarkable optical properties of these polythienobenzo-
thiophenes, including the absorption spectra as well as the fluo-
rescence data which have been discussed in a previous paper,16
suggest that they possess a well delocalized p-electronic system.
Such optical properties allow to predict a very planar micro-
structure for the derivatives belonging to this new family of
electroactive, fused heterocyclic polymers.
The absorption spectrum of TBT (1026 M) presents a
maximum at 260 nm (electronic transition energy, E ~ 4.77 eV;
e ~ 3.77 6 105 L mol21 cm21) and secondary peaks at 262 nm
(E ~ 4.73 eV; e ~ 2.85 6 105 L mol21 cm21) and 265 nm (E ~
4.68 eV; e ~ 2.16 6 105 L mol21 cm21), attributed to the p A
p* transitions of the conjugated, fused aromatic and hetero-
aromatic system. In the case of poly-TBT (2 6 1025 r. u. L21
)
in the reduced form, the spectrum possesses a strong maximum
at 273 nm (E ~ 4.54 eV) ; and weaker peaks located,
respectively at 330 nm (E ~ 3.75 eV) ; 373 nm (E ~ 3.32 eV),
397 nm (E ~ 3.12 eV); and 423 nm (E ~ 2.93 eV) (Fig. 7). The
273 nm band of the poly-TBT spectrum, which is only slightly
shifted relative to the maximum of the monomer spectrum, can
Experimental
General methods
The electrochemical studies were carried out in a single-
compartment three-electrode cell using an EG&G PAR 362
potentiostat. Platinum disk (disk area ~ 19.6 6 1022 cm2) and
platinum coated glass (plate area ~ 2 to 3 cm2) were used as
anodes. The counter-electrode was in all cases a platinum grid.
All potentials were measured with respect to a saturated
calomel electrode (SCE). The electrolyte solution consisted of
acetonitrile (CH3CN) (Prolabo Chromanorm2 HPLC grade,
used without further purification) containing 0.1 M lithium
perchlorate (Acros) as supporting salt.
For poly-TBT and poly-MeOTBT characterization, FT-IR
spectra were recorded on a Magna-IR 860 spectrometer
(Nicolet Instrument Corp. Madison, WI, USA) at 4 cm21
spectral resolution. Transmittance absorption spectroscopy of
Fig. 7 UV-Visible absorption spectra of a 2 6 1025 (r.u.) L21 poly-
TBT dimethylformamide solution at 25 uC. (a) Oxidized form and
(b) reduced form.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1
1 7 1 9

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TBT and MeOTBT monomers was performed in KBr pellets
using a standard DTGS detector and by collecting 50 scans.
Plates of electrodeposited polymers were analyzed by Infra Red
Rasing Angle Spectroscopy (IRRAS). Each IRRAS spectrum
was recorded using a 80u incidence rasing angle accessory and a
high sensitivity Mercury Cadmium Telluride (MCT-A/4000-
625 cm21) detector, by adding 200 scans. All spectra were
baseline corrected using OMNIC internal software and were
presented unsmoothed. The Aldrich FT-IR condensed edition
II spectral library was used to compare our spectra with those
of thiophene and some substituted derivatives.
The matrix-assisted light desorption ionization–time of flight
(MALDI-TOF) mass spectra of poly-TBT and poly-MeOTBT
were obtained on a Voyager Elite (Perseptive Biosystems,
Framington, MA, USA) mass spectrometer. 2,5-Dihydroxy-
benzoic acid was used as a matrix.
3.08% H, 15.33% Cl, 14.12% S. IR/cm21: 1718 (CLO). 1H
NMR: d 3.94s, 3 H, (OCH3); 7.05d, 1 H, J ~ 8.8 (H-4); 7.14s,
1 H, (H-7); 7.97d, 1 H, J ~ 8.8 (H-5); 10.15s, 1 H, (CHO).
Methylthieno[3,2-b][1]benzothiophene-2-carboxylates 7, 8.
To a solution of sodium methoxide (7.72 g; 0.143 mol) in
dry methanol (300 ml) was added dropwise methyl thiogly-
colate (17.0 g; 0.16 mmol). After 5 min stirring, aldehyde 5, 6
(66.5 mmol) was added in one portion and the mixture was
stirred and heated to reflux for 2 h. After cooling, the pre-
cipitate was filtered off, washed thoroughly with methanol,
water, and dried in vacuo to afford esters.
Methylthieno[3,2-b][1]benzothiophene-2-carboxylate (7).
Mp 137.5–138.5 uC. For C12H8O2S2 (248.3) calculated:
58.04% C, 3.25% H, 25.82% S; found: 57.85% C, 3.09% H,
25.47% S. IR/cm21: 1709 (CLO). 1H NMR: d 3.96s, 3 H
(OCH3); 7.46m, 2 H (H-6 and H-7); 7.87m, 2 H (H-5 and H-8);
8.03s, 1 H (H-3).
Several techniques were also used to characterize the various
products obtained during the synthesis of monomers. Melting
points were determined on a Leica VM TG block and were
uncorrected. Elemental analyses were carried out on a Perkin-
Elmer 2400. NMR spectra were measured on a Varian Gemini
1
300 HC (300 MHz for H). Deuteriochloroform or DMSO-d6
Methyl 6-methoxythieno[3,2-b][1]benzothiophene-2-carboxy-
late (8). Mp 165–167 uC (toluene). For C13H10O3S2 (278.4)
calculated: 56.10% C, 3.62% H, 23.04% S; found: 55.88% C,
were used as solvents and the signals of solvents served as
internal standards. IR spectra were recorded on a Nicolet 740
FT-IR spectrometer in chloroform or KBr.
1
3.84% H, 23.01% S. IR/cm21: 1708 (CLO). H NMR: d 3.94s,
3 H (OCH3); 3.99s, 3 H (OCH3); 7.02d, 1 H, J ~ 8.8 (H-7);
7.38s, 1 H (H-5); 7.82d, 1 H, J ~ 8.8 (H-8); 8.00s, 1 H (H-3).
Computational methods
The TBT and MeOTBT monomers were studied theoretically
in the framework of the Density Functional Theory (DFT)
method. The geometries of neutral species and radical-cations
were fully optimised without symmetry constraint (conver-
gence criteria ~ 1024) using the three-parameter functionals of
Becke (B3LYP)31 and the 6-31G(d)32 basis set. Radical-cations
were treated as open shell systems (uB3LYP). All DFT calcula-
tions were performed with the Gaussian 98 suite of programs.33
The optimised geometries, infrared frequencies and infrared
intensities were also calculated using BP86 functional pro-
gram34,35 which provides generally excellent agreement with
experimental data; no scaling factor was applied to match the
calculated infrared frequencies with experimental ones.
Thieno[3,2-b][1]benzothiophene-2-carboxylic acids 9, 10. A
mixture of ester (15 mmol), sodium hydroxide (1.8 g; 45 mmol),
methanol (70 ml) and water (30 ml) was stirred and heated to
reflux for 3 h and then poured onto a mixture of ice and
hydrochloric acid (10 ml). The precipitate was filtered off,
washed with water and dried to afford the acids.
Thieno[3,2-b][1]benzothiophene-2-carboxylic acid (9). Mp
258–260 uC, lit.18 259 uC.
6-Methoxythieno[3,2-b][1]benzothiophene-2-carboxylic acid
(10). Mp 270–273 uC. For C12H8O3S2 (264.3) calculated:
54.53% C, 3.05% H, 24.26% S; found: 54.33% C, 2.99% H,
24.07% S. IR/cm21: 3400 (OH), 3000–2250 (OH), 1664 (CLO).
1H NMR: d (DMSO-d6): 3.88 s, 3 H (OCH3), 7.04d, 1 H,
J ~ 8.8 (H-7); 7.41s, 1 H (H-5); 7.93d, 1 H (H-8); 8.01 s, 1 H
(H-3).
Chemicals and synthesis procedures
Thiosalicylic acid was purchased from Sigma-Aldrich.
4-Methoxythiosalicylic acid,22 2-[(carboxymethyl)thio]benzoic
acid36 and 2-[(carboxymethyl)thio]-4-methoxybenzoic acid37
were prepared by known procedures.
Thieno[3,2-b][1]benzothiophenes 1, 2. A mixture of acid 9,
resp. 10 (20 mmol), copper bronze (1.90 g; 30.1 mmol) and
freshly distilled quinoline (50 ml) was heated at 190–200 uC
under stirring for 1 h. After cooling this was decomposed by
pouring onto a mixture of ice water (250 ml) and hydrochloric
acid (50 ml), and extracted with toluene (2 6 100 ml). The
toluene solution was successively washed with water, brine, and
dried with anhydrous magnesium sulfate. The residue after
evaporation was purified by column chromatography (silica
gel, eluent hexane : toluene 2 : 1) to afford the desired
heterocycle.
3-Chloro[1]benzothiophene-2-carbaldehydes 5, 6 (see Scheme 2).
Phosphorus oxide chloride (200 g; 1.30 mol) was added
dropwise over a period of 1.5 h to N,N-dimethylformamide
(400 g; 5.47 mmol) under stirring and cooling to 0 uC. Diacid 3
or 4 (0.3 mol) was added in 5 portions at this temperature and
the mixture was stirred for 30 min. Then the cooling bath was
removed, at approx. 20 uC an exothermic reaction started and
the temperature increased to 45 uC. When the reaction
subsided, the reaction mixture was heated to 80 uC for 1 h
and decomposed by pouring on ice. The precipitated product
was filtered off, washed successively with aqueous sodium
hydrogencarbonate solution, water, and dried in vacuo to
afford the product.
Thieno[3,2-b][1]benzothiophene (1). Yield 81%, mp 87–
88 uC, lit.18 84.5–86 uC.
6-Methoxythieno[3,2-b][1]benzothiophene (2). Yield 52%,
mp 61.5–62.5 uC. For C11H8OS2 (220.3) calculated: 59.97% C,
3.66% H, 29.11% S; found: 59.66% C, 3.47% H, 28.84% S.
IR/cm21: 3011, 1613, 1491, 1440, 1030. 1H NMR: d 3.88s, 3 H
(OCH3); 7.01d, 1 H, J ~ 8.8 (H-7); 7.30d, 1 H, J ~ 5.5 (H-3);
7.34s, 1 H (H-5);7.48d, 1 H (H-2); 7.72d, 1 H (H-8).
3-Chloro[1]benzothiophene-2-carbaldehyde (5). Mp 103–
104 uC, lit.23 103 uC.
3-Chloro-6-methoxy[1]benzothiophene-2-carbaldehyde (6).
Mp 159–160 uC (ethanol). For C10H7ClO2S (240.7) calculated:
52.99% C, 3.11% H, 15.64% Cl, 14.14% S; found: 52.80% C,
1 7 2 0
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 1 1 – 1 7 2 1

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Acknowledgements
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