New conjugated polymerizable pyrrole and 2,5-dithienylpyrrole azobenzene dyes: Synthesis and spectroelectrochemical properties

Article abstract of DOI:10.1039/b210906a

We describe the first easy syntheses of new fully conjugated monomers and their derived polymers containing a photoisomerizable azobenzene group, along with their electrochemical properties. Two classes of compounds have been studied differing in the nature of the polymerizable unit: pyrrole or 2, 5-dithienylpyrrole. A rather effective electronic interaction between the pyrrole and azo moieties has been demonstrated, while the conjugation is much less pronounced in the case of the terheterocycle. This behavior has been ascribed to a twisted conformation of the 2,5-dithienylpyrrole compounds in the neutral form, as demonstrated by theoretical modeling. All these molecules are electropolymerizable monomers, although the 2,5-dithienylpyrrole compounds lead only to thin films of conducting polymers when compared to their pyrrole analogs.

Full text of DOI:10.1039/b210906a

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New conjugated polymerizable pyrrole and 2,5-dithienylpyrrole
azobenzene dyes: synthesis and spectroelectrochemical properties
Pierre Audebert,*a Sa¨ıd Sadki,a Fabien Miomandre,a Philippe Hapiotb and Kathleen Chane-Chingc
a
´
Laboratoire de Photophysique et Photochimie Supramoleculaires et Macromoleculaires
(CNRS UMR 8531), Ecole Normale Superieure de Cachan, 61 Av. du Pt Wilson, 94235,
´
´
Cachan, France. E-mail: audebert@ppsm.ens-cachan.fr
b
c
´
Laboratoire d’Electrochimie Moleculaire et Macromoleculaire, Synthese et Electrosynthese
Organiques (CNRS UMR 6510), Universite de Rennes 1, Bat. 10C, 35042,
´
`
`
´
Rennes cedex, France
Institut de Topologie et Dynamique des Systemes, Universite Denis Diderot (Paris 7),
`
1, rue Guy de la Brosse, 75005, Paris, France
´
Received (in Montpellier, France) 5th November 2002, Accepted 20th January 2003
First published as an Advance Article on the web 9th April 2003
We describe the first easy syntheses of new fully conjugated monomers and their derived polymers containing a
photoisomerizable azobenzene group, along with their electrochemical properties. Two classes of compounds
have been studied differing in the nature of the polymerizable unit: pyrrole or 2, 5-dithienylpyrrole. A rather
effective electronic interaction between the pyrrole and azo moieties has been demonstrated, while the
conjugation is much less pronounced in the case of the terheterocycle. This behavior has been ascribed to
a twisted conformation of the 2,5-dithienylpyrrole compounds in the neutral form, as demonstrated by
theoretical modeling. All these molecules are electropolymerizable monomers, although the 2,5-dithienylpyrrole
compounds lead only to thin films of conducting polymers when compared to their pyrrole analogs.
(1-pyrrolyl)azobenzene (3), 4-[(2,20-bis thienyl)-N-pyrrolyl]azo-
1. Introduction
benzene (4) and bis-4,40-[(2,20-bisthienyl)-N-pyrrolyl]azoben-
zene (5), which are all represented in Scheme 1. We describe
here the synthesis of the monomers and their derived polymers
prepared by electrooxidation, as well as their electrochemical
and spectroscopic properties.
The search for new superstructured conjugated organic poly-
mers1–7 continues to be of interest, in particular as these
systems are expected to show NLO properties,1,2 entrap
metals,4–6 as well as exhibit adjustable conductivities.7,8 With
regard to these properties, electrodeposited polymers with
photoisomerizable azobenzene moieties appear to be useful
candidates, in particular as concerns the electronic conjugation
between the chains and the functional group. This conjugation
may lead to mutual cooperativity such that the intrinsic prop-
erties of the polymer chain (like conductivity) could be tuned
by the state of the azo pendant group, for example, by UV
light irradiation. Conversely, the tuning of the functional
group properties through the redox state of the chain could
also be envisaged. Although electroactive polythiophenes and
polypyrroles bearing azo pendant groups have been previously
described,9–12 this is the first time that conjugated electroactive
polymers based on an azo dye are presented.
Moreover, it is interesting to try to prepare ‘‘ladder’’ poly-
mers, connecting the conducting chains with the functional
group as a ‘‘bar’’ between the chains. This way, enhanced
effects could be expected due to the tighter link between the
chains and the functional group. Up to now, almost all of
the ladder conjugated polymers described in the literature
exploit the ‘‘bar’’ of the ladder, focusing only on the conju-
gated or nonconjugated character. The aim here is to investi-
gate the influence on the polymer of the existence or absence
of additional electronic connectivity between the chains7,8
through the conjugated linker. Following these ideas, we found
it interesting to synthesize fully conjugated ‘‘one head’’ and
‘‘two head’’ monomers containing an azo moiety. Hence we
have prepared five new heterocyclic azo dyes: 4-(1-pyrrolyl)-
azobenzene (1), 4-(1-pyrrolyl)-40-nitroazobenzene (2), bis-4,40-
2. Experimental
2.1 Synthesis of the monomers
The pyrrole derived azobenzene compounds were prepared by
a classical Paal–Knorr procedure. Typically, a stoichiometric
mixture of the starting compounds is heated to reflux in acetic
acid, after which the heating is immediately stopped and the
solution is poured into iced water, neutralized with alkali,
and extracted with dichloromethane. Chromatography
(dichloromethane–petroleum ether 50:50) afforded the desired
product (first eluted) with 40–60% yields.
1
1. H NMR (300 MHz): 6.45, t, 4H (a-py); 7.20, t, 4H (b-
py); 7.60, m, azo; 7.95, m, azo; 8.05, m, azo. Anal. calcd: C:
77.71, N: 16.99, H: 5.30; found: C: 77.70, N: 16.88, H: 5.19%.
2: 1H NMR: 6.47, t, 4H (a-py); 7.22, t, 4H (b-py); 7.55, d(t),
azo; 8.34, d(t), azo. Anal. calcd: C: 65.75 N: 19.17, H: 4.11;
found: C: 65.66, N: 19.21, H: 4.14%.
3: 1H NMR: 6.42, t, 4H (a-py); 7.22, t, 4H (b-py); 7.55, d(t),
azo, 8.05; m, 8H, azo; 8.40, d(t), azo. Anal. calcd: C: 76.92, N:
17.95, H: 5.13 found: C: 76.70, N: 17.89, H: 5.10%.
IR (KBr) data for 1 and 3 (very similar spectra): 3065, 1600,
1330.
IR (KBr) data for 2 (very similar spectra): 3000, 1600, 1514,
1330.
798
New J. Chem., 2003, 27, 798–804
DOI: 10.1039/b210906a
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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Scheme 1
2,5-Dithienylpyrrole derivatives were synthesized according
to a modified Paal–Knorr procedure as shown in Scheme 2.
Standard synthetic procedures8 have been used for all the
steps, with only a modification of the purification procedure
in the last one. A typical procedure for the synthesis of 5 is
as follows: 4,40-azodianiline (1.06 g, 5 mmol) and bis(2-thie-
nyl)-1,4-butanedione (3 g, 10 mmol) are refluxed for 12 h in
anhydrous toluene (20 mL) with 200 mg of paratoluenesulfo-
nic acid. After reaction, excess toluene is evaporated, 5 mL
of dichloromethane are added, and the crude mixture is poured
on a silica gel column. After elution with 20% dichloromethane
and 80% petroleum ether, 5 is collected and recrystallized from
the elution solvent, with 60% yield.
5: 1H NMR: 8.25, d (4H, a azo); 7.44, d (4H b azo); 7.17, d
(4H, a thioph.); 6.90, t (4H, b pyr.); 6.58, s (m) (16H, b
thioph.). 13C NMR: 9 aromatic C, 147.3, 144.0, 133.8, 130.4,
129.6, 127.0, 125.6, 125.1, 124.2. Anal. calcd: C: 67.50, N:
8.75, H: 3.75; found: C: 67.32, N: 8.65, H: 3.62%.
IR (KBr) data for 4 and 5 (very similar spectra): 2980, 1590,
1505, 1405.
Although the intermediate 4,40-azodianiline is a commercial
compound, because of its high price we found it more conve-
nient to prepare it by reduction of the very classical ‘‘DO3’’
dye: reduction is achieved by using ammonium formiate in
the presence of Pt/C (70% yield), according to an elegant
method recently described.13
1
4: H NMR: 8.0, m (4H, a azo); 7.55, m (3H azo); 7.48, d
(2H, azo); 7.12, d (2H, a thioph.); 6.85, t (2H, b pyr.); 6.58,
s(m) (4H, b thioph.). Anal. calcd: C: 70.05, N: 10.21, H:
4.16; found: C: 69.78, N: 9.86, H: 4.11%.
2.2 Electrochemistry and spectroscopic measurements
The electropolymerization and subsequent study of the poly-
mer films were performed using an EG&G PAR 273 potentio-
stat, interfaced to a PC computer. The high speed cyclic
voltammetry experiments were performed using a homemade
potentiostat,14 equipped with an ohmic drop compensation
system.
The reference electrode used is an Ag+/Ag electrode filled
with 0.01 M AgNO3 . This reference electrode was checked
vs. ferrocene as recommended by IUPAC. In our case,
Eꢀ(Fc+/Fc) ¼ 0.114 V in toluene–ACN with 0.1 M TEAP,
and 0.045 V in acetonitrile with 0.1 M TEAP. The reference
electrode used for the spectroelectrochemistry is an Ag wire.
In our conditions, we measured Eꢀ(Fc+/Fc) ¼ ꢁ0.423 V in
toluene–acetonitrile with 0.1 M TEAP. The working electrode
is either a platinum disc with 0.785 mm2 area, or an ITO
coated glass electrode with ca. 2 mm2 immersed area, as stated
in the text.
Tetrabutylammonium perchlorate was purchased from
Fluka (puriss). Acetonitrile (Aldrich, 99.8%), dichloromethane
(SDS, 99.9%) and toluene (Aldrich, 99.5%) were used as
received. All solutions were deaerated by bubbling through
argon gas for a few minutes prior to electropolymerization
and electrochemical measurements.
UV-visible characterization of monomers was performed
using a quartz cell (1 cm optical path) and a Cary500 (Varian)
spectrophotometer. Spectra were recorded between 200 and
800 nm. UV-visible spectroelectrochemistry of polymers was
performed using a homemade cell: the working electrode was
made of ITO covered glass. Platinum and silver wires were
used respectively as counter and reference electrodes and
located on the cell sides in order not to disturb the beam. A
double beam UVIKON 923 (Bio-Tek Kentron) spectrophoto-
meter was used for spectra acquisition: a spectrum was
recorded every 100 mV, after which the film was equilibrated
in the given redox state for about 2 min.
Scheme 2
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Table 1 Spectroscopic features of azo compounds 1–5 in dichloro-
)
2.3 Theoretical modeling
methane (conc. 2.5 ꢃ 10ꢁ5 mol Lꢁ1
The calculations were performed using the Gaussian 98 pack-
age15 for density functional theory. Gas phase geometries and
electronic energies were calculated by full optimization without
imposed symmetry of the conformations, using the B3LYP16
density functional with the 6-31G* basis set.17,18 Spin contam-
ination was low for all calculations (s2 lower than 0.76).
Compound
lmax/nm
e/104 molꢁ1ꢂLꢂcmꢁ1
1
2
3
4
5
353
382
325
328
304
2.07
1.37
2.73
3.77
4.02
3. Results and discussion
3.1 Synthesis and spectroscopic features of the monomers
molecular conformation between fundamental and excited
states.
It should be noticed that the synthesis of the monomers is quite
easy with the Paal–Knorr procedure (or its modified version
for the substituted thiophene rings, see Scheme 2), despite
the usually alleged sensitivity of the azobenzene group to acids.
In addition, either in the case of the pyrrole or the 2,5-dithi-
enylpyrrole derivatives, the synthesis is straightforward in
one or two steps, with overall yields in the 50% range, which
is quite significant in view of further applications, especially
with regard to the ever increasing complexity of the syntheses
of recent molecular materials. This allows gram quantities to
be obtained from the relatively cheap precursors dimethoxy-
tetrahydrofuran, thiophene, succinyl chloride and classical
‘‘DO’’ type dyes.
All the monomers are new dyes. Their UV-vis spectra are
represented in Fig. 1 and the parameters listed in Table 1. It
can be seen that all the absorption coefficients at the maximum
wavelength are at the 104 molꢁ1ꢂLꢂcmꢁ1 level, which is charac-
teristic of p-p* transitions in the azo chromophore. A batho-
chromic shift of the absorption maximum is observed for
monosubstituted azo compounds (1, 2, 4) relative to azoben-
zene (for the latter lmax ¼ 313 nm19): this can be explained
by an extended conjugation with the polymerizable moiety;
the shift is very significant in the case of 2, which must exhibit
a strong ‘‘push-pull’’ character. Another noticeable effect is
evidenced by the hypsochromic shift in the absorption maxima
of symmetric compounds relative to their asymmetric counter-
parts (1 and 4 vs. 3 and 5). This tends to demonstrate a lower
conjugation for symmetrically substituted azobenzene deriva-
tives. Since the donor effects of the substituents should lead
to the inverse behavior, one must invoke geometrical aspects
like a twisted conformation of the molecule in the fundamental
state to explain this absorption shift. In agreement with this,
the absorption coefficients are all smaller for the symmetrically
substituted azobenzenes, probably due to differences in the
3.2 Electrochemical behavior of the pyrrole derivatives
The electrochemistry of pyrrole compounds, substituted on the
nitrogen atom by phenyl rings, themselves bearing electron-
donor or -attracting groups, has been described a long time
ago by Diaz, Salmon and coworkers20 The authors found
some correlation between the oxidation potential and the sub-
stituent nature, and in addition polymerization was quite less
efficient when an attracting group was located at the para
position of the phenyl ring. In the case of our molecules, the
electron-attracting power of the azo group, although not very
high, is however quite obvious, with unavoidable consequences
on the polymerization ability of compounds 1–3.
In fact, 1 and 2 give rise to conducting polymers upon elec-
trooxidation, while 3 does not polymerize when alone in solu-
tion. Electropolymerization of 2 is difficult and requires high
anodic potential (ca. 2 V vs. Ag), due to the strong withdraw-
ing power of the nitro substituent. The reason why 3 does not
polymerize seems more obscure at first sight, even if a strong
stabilization of the radical cation is likely to be achieved
through extended conjugation (see x3.4 below). Copolymeriza-
tion with pyrrole is actually possible, but the incorporation of
azobenzene is difficult to check by UV spectroscopy or related
methods. Fig. 2(a) features the electrochemical behavior of 1 in
dichloromethane. It is clear that in this case a classical con-
ducting polymer is obtained, whose cyclic voltammogram in
a solution free of monomer is shown in Fig. 2(b). The standard
potential of the polymer is found at ꢁ0.13 V vs. Ag+/Ag,
which is about 0.1 V higher than for classical polypyrrole
due to the slight electron-attracting power of the azo linker.
3.3 Electrochemical behavior of the 2,5-dithienylpyrrole
derivatives
Both 4 and 5 are very soluble in dichloromethane but in this
solvent, the polymer electrogenerated from 5 is unfortunately
quite soluble. Therefore, we have chosen for this compound
a mixture of toluene–acetonitrile (50:50, v/v) with 0.1 M
TBAP as the electropolymerization medium. It had been pre-
viously shown to be very convenient for the electrochemical
study of thiophene oligomers.21 The oxidation of monomers
4 and 5 occurs at around +0.6 V (vs. Ag/Ag+) on a platinum
disc electrode (Fig. 3). Electrochemical polymer growth can be
obtained by repeated cycling and an adherent film is deposited
on the working electrode after ten cycles. Nevertheless, it
seems difficult to obtain thick films, since the growth nearly
stops after a few cycles. Polymers can also be potentiostatically
synthesized in the same electrolyte. The optimum polymeri-
zation potential of 4 is +0.65 V (vs. Ag/Ag+), either on a
platinum disc or an ITO glass electrode.
Fast scan rate electrochemistry of 4 and 5 has been per-
formed under the same conditions as stated above and the
resulting voltammograms are given in Fig. 4. It is clear, espe-
cially in the case of 5, that radical cation formation is chemi-
cally irreversible up to 20
reversible above 50 V sꢁ1, showing that this radical cation is
Fig. 1 Absorption spectra of 1–5 in dichloromethane (conc. 2.5 ꢃ
10ꢁ5 mol Lꢁ1). Maximum absorption wavelengths and corresponding
molar absorption coefficients are listed in Table 1.
V
sꢁ1 and becomes rather
800
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Fig. 2 (a) Electrochemical polymerization of 1 by cyclic voltammetry
(v ¼ 50 mV sꢁ1) on a platinum working electrode (Ag/10ꢁ2 M Ag+
reference). First and tenth cycles in 0.1 M TEAP in dichlomethane.
(b) Cyclic voltammogram of poly(1) in 0.1 M TEAP in ACN at 10
mV sꢁ1 (Ag/10ꢁ2 M Ag+ reference).
Fig. 4 Fast scan rate electrochemistry of (a) 4 and (b) 5 in 0.1 M
TBAP, acetonitrile–toluene (v/v, 50:50), on a platinum disc electrode
(reference Ag/10ꢁ2 M Ag+) at the following scan rates: (a) 100, 200
and 500 V sꢁ1 for curves a–c, respectively; (b) 10, 20, 50 and 100 V
sꢁ1. for curves a–d, respectively.
relatively stable (lifetime > 10 ms). This stability is actually
similar to the one of unsubstituted 2,5-dithienylpyrrole and
2,5-dithienyl-N-methylpyrrole22 radical cations, which seems
to account for a weak stabilizing role through extended conju-
gation played by the azo moiety, which therefore increases the
radical cation lifetime.
Comparison with ferrocene allows us to measure the stan-
dard potential of the radical cation/monomer redox couple,
along with the number of exchanged electrons. Standard
potentials of +350 and +390 mV vs. Fc+/Fc are obtained
respectively for 4 and 5 (compared to +290 mV for unsubsti-
tuted 2,5-dithienylpyrrole under the same conditions), while
the numbers of exchanged electrons are respectively 0.9 and
1.8. These values confirm that there is no significant stability
induced by the possible conjugation of the terheterocycle with
the azo group and that the first oxidation process in 5 actually
involves both polymerizable moieties.This latter point indi-
cates that a ‘‘ladder’’ structure is likely to be obtained in the
case of poly(5).
3.4 Theoretical modeling on pyrrole and 2,5-dithienylpyrrole
derivatives
In order to explain the electrochemical results, we performed
geometry optimizations using the density functional B3LYP
for molecules 1–5 and their corresponding radical cations
for 1–4. This method was chosen as a compromise between
acceptable calculation times and good accuracy.
For neutral pyrrole molecules (1–3), the ‘‘diphenyl azo’’
cores were found to be planar with a small dihedral angle with
the pyrrole ring (y ¼ 32.5ꢀ). In the radical cation, the dihedral
Fig. 3 Electrochemical polymerization of (a) 4 in 0.1 M TEAP in
dichloromethane and of (b) 5 in 0.1 M TEAP in acetonitrile–toluene
(v/v 50:50), on a platinum disc working electrode (reference Ag/
10ꢁ2 M Ag+); v ¼ 100 mV sꢁ1. First and tenth cycles in each case.
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Table 2 Bond lengths in the neutral and oxidized forms of compounds 1–5 from B3LYP/6-31G* calculationsa
=
C1 C2
=
C4 C5
=
N2 N3
=
C8 C9
Compound
N1–C3
C6–N2
N3–C7
1
1.372
1.359
1.372
1.358
1.373
1.361
1.388
1.428
1.387
1.379b
1.412
1.375
1.407
1.373
1.412
1.379
1.242
1.256
1.428
1.465c
1.386
1.373
1.389
1.371
1.386
1.372
1.393
1.389
1.391
1.397d
1.415
1.361
1.408
1.360
1.413
1.372
1.419
1.420
1.416
1.465c
1.263
1.253
1.264
1.252
1.264
1.292
1.262
1.264
1.262
1.246e
1.417
1.381
1.417
1.389
1.412
1.371
1.417
1.408
1.416
1.465c
1.393
1.383
1.387
1.389
1.390
1.375
1.389
1.393
1.388
1.397d
1+
2
2+
3
3+
4
4+
5
Reference
a
b
c
C in pyrrole. N–C in H2N–CH3 .
d
e
=
=
= =
N N in HN NH.
See structure above for atom numbering.
C
C
C in benzene.
angle decreases (y ¼ 15.8ꢀ), but the most considerable chan-
ges are located in the azo core, which becomes highly distorted.
Similar behavior was observed for the radical cation of 2, while
the deformations of the azo core were much smaller in the
radical cation of 3. To get more quantitative data on the geo-
metry changes, it is interesting to compare the bond length
changes from the neutral to the cation state in the case of a
few characteristic bond lengths (see Table 2). The results of
the calculations demonstrate that bond lengths tend to equal-
ize in the radical cation, with the largest modifications occur-
ring in the ‘‘diphenyl azo’’ group, indicating a high degree
of conjugation in all the molecules. Similarly, the spin densities
in the radical cations are also consistent with extensive deloca-
lization of the unpaired electron over the whole molecule with
maxima around the azo moiety (Table 3). This delocalization
of the unpaired electron seems to be better in the bispyrrole
molecule 3 than in the two other substituted pyrroles, explain-
ing the difficulties to polymerize it. Thus, we can conclude that
electronic interactions actually exist between the pyrrole rings
and the azo group in the neutral and even more in the radical
cation forms, although remaining globally less important that
what could have been expected.
The situation is completely different with the terheterocycles.
In the neutral and radical cation forms of 4 and 5, the phenyl
group is too large to be planar with the heterocycle. Due to the
steric hindrance, the ‘‘diphenyl azo’’ core is almost perpendi-
cular to the terheterocycle head (see Fig. 5). This results in a
very weak conjugation between the polymerizable moiety
and the azo group. This is confirmed by looking at the bond
changes in the radical cation form of compound 4: the bond
lengths in the azo core remain almost unchanged when passing
from the neutral to the cation state. Similarly, the spin density
in the oxidized form of 4 is close to zero in the azo group.
Because of limited calculation times, we calculated only the
neutral form of 5, but a similar nonplanar geometry was
observed, indicating a very low electronic interaction between
the two heterocycle moieties through the azo linker. All these
results tend to evidence a much weaker conjugation for the
2,5-dithienylpyrrole derivatives than for the pyrroles, due to
larger steric hindrance between the polymerizable moiety and
the azo chromophore.
3.5 Spectroelectrochemical characterization of the polymers
Fig. 6 shows cyclic voltammograms of poly(4) in 0.1 M TEAP/
acetonitrile. The polymer film exhibits the classical features
observed with conducting polymers, especially the peak cur-
rents vary linearly with the scan rate. The oxidation peak
Epa of poly(4) is observed at 0.30 V and the reduction peak
Epc is at 0.22 V, corresponding to a standard potential equal
to 0.26 V. Poly(5) shows analogous behavior, with the oxida-
tion peak Epa at 0.36 V and the reduction peak Epc at 0.28
V, leading to a standard potential equal to 0.32 V. One can
notice a minor shift between the standard potentials of the
Table 3 Mulliken spin densities in the radical cation forms of
compounds 1–4 from B3LYP/6-31G* calculations
Atoma
1+
0.06
2+
0.07
3+
0.07
4+
0.06
C1
C2
N1
C3
C4
C5
C6
N3
N4
C7
C8
C9
C10
ꢁ0.02
0.09
0.07
0.02
0.02
0.09
0.19
0.25
0.00
0.07
ꢁ0.04
0.11
ꢁ0.03
0.11
0.06
ꢁ0.03
0.105
0.10
0.02
0.02
0.11
0.03
0.03
0.11
0.02
0.02
0.10
0.22
ꢁ0.07
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.00
0.12
0.17
0.25
ꢁ0.02
0.07
ꢁ0.04
0.10
Fig. 5 Optimized geometry of the neutral form of 4 calculated at the
B3LYP/6-31G* level, showing the position of the azo group perpendi-
cular to the (2,5-dithienylpyrrole) plane.
a
See structure above for atom numbering.
802
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which is attributed mainly to the azo chromophore. The p-
p* transition of the conjugated polymer is hidden behind the
azo chromophore absorption, making it difficult to evaluate
the band gap of the polymer. One can see clearly in Fig. 7
the strong increase of absorption in the red part of the spec-
trum when the applied potential becomes more anodic: this
corresponds to the generation of charge carriers (polarons
and bipolarons) in the polymer chains.23 The analysis of the
spectra for poly(4) tends to show a maximum contribution
of radical cation species (polarons, lmax ꢄ 640 nm) at 0.6 V,
while at more anodic potentials, these species are converted
into dications (bipolarons), as evidenced by the gradual
absorption increase beyond 800 nm along with the stagnation
of the absorption around 640 nm. In the case of poly(5), the
amount of polarons increases regularly, while the contribution
of bipolarons appears significantly only at 1.2 V. The dif-
ference between both polymer spectra is consistent with the
participation of both 2,5-dithienylpyrrole moieties in the for-
mation of polarons in poly(5), leading to a higher amount of
polarons for approximately the same doping level.
Fig. 6 Cyclic voltammograms of poly(4) in 0.1 M TEAP in aceto-
nitrile on a platinum disc electrode (Ag/10ꢁ2 M Ag+ reference) at scan
rates of 20, 40, 60 and 100 mV sꢁ1 for curves a–d, respectively.
monomers (deduced from high scan rate measurements) and
those of their corresponding polymers: this is indicative of a
low degree of polymerization or of short conjugation lengths
due to a high number of structural defects.
4. Conclusion
Poly(4) and poly(5) films were deposited potentiostatically
on ITO-coated glass plates, in their respective polymerization
media, with 0.01 M monomer and 0.1 M TEAP, in order to
perform spectroelectrochemical studies. The spectra were initi-
ally recorded in the fully reduced state of the polymers. For
this state, an absorption peak is observed at lmax ¼ 420 nm,
We have presented the synthesis and electrochemical study of
new conjugated polymers with azo groups directly connected
to the polymer chain. The synthesis procedure is quite straight-
forward and affords the target molecules with rather good
yields. The first results obtained from spectroscopic and elec-
trochemical measurements, as well as the theoretical calcula-
tions, have demonstrated an extended conjugation brought
about by the azo moiety in the pyrrole derivatives. In the case
of 2,5-dithienylpyrrole derivatives, no stabilizing effect through
the conjugation with the azo group has been observed, leading
to oxidation localized on the polymerizable units. Therefore, it
seems unfortunately unlikely that in conjugated polymers of
this kind, a high conjugation may be achieved between
the main chain and the functional group. However, this feature
remains to be confirmed by further studies.
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