Mixed alkylthiophene-based heterocyclic polymers containing oxadiazole units via electrochemical polymerisation: Spectroscopic, electrochemical and spectroelectrochemical properties

Article abstract of DOI:10.1039/b415587d

Symmetric alkylthiophene-based mixed heterocyclic trimer and pentamer. containing central oxadiazole units, have been prepared. Because of the electron-withdrawing properties of oxadiazole, the trimer cannot be electropolymerised and undergoes an oxidative-type destruction at high potentials. In contrast, the pentamer readily polymerises, giving a short chain polymer. Both trimer and pentamer exhibit strong photoluminescence with a maximum at 399 nm (13% quantum yield) and 467 nm (46% quantum yield), respectively. The polymer resulting from the electropolymerisation of the pentamer is also luminescent with the maximum of the excitation band at 528 nm (33% quantum yield). The polymer can be oxidatively doped as demonstrated by cyclic voltammetry, showing a clear anodic peak at 0.62 V versus Ag/Ag ⁺ and its cathodic counterpart at 0.56 V, associated with the undoping process. The significantly higher potential of the oxidative doping of the prepared mixed heterocyclic polymer, as compared to the poly(alkylthiophene) homopolymer of similar molecular weight, is caused by the presence of the oxadiazole unit, which lowers the electron density in the π-electron system of the oligothiophene subunit and makes its oxidation more difficult. The spectroelectrochemical investigation of the polymer is consistent with its voltammetric behaviour, exhibiting doping-induced bleaching of the band originating from the π-π* transition and simultaneous growth of the bipolaron bands. The observed clear and reversible spectroelectrochemical behaviour makes the developed polymer a promising candidate for applications in electrochromic devices or electrochemical sensors. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2005.

Full text of DOI:10.1039/b415587d

View Article Online / Journal Homepage / Table of Contents for this issue
P A P E R
Mixed alkylthiophene-based heterocyclic polymers containing
oxadiazole units via electrochemical polymerisation: spectroscopic,
electrochemical and spectroelectrochemical properties
Alexander S. Fisyuk,wa Renaud Demadrille,a Claudia Querner,a Malgorzata Zagorska,b
¨
Joel Bleuse and Adam Pron*
c
a
a
´
´
´
Laboratoire de Physique des Metaux Synthetiques, DRFMC (CEA-CNRS-Universite J.
Fourier Grenoble I UMR 5819 SPrAM), CEA Grenoble, 17 Rue des Martyrs, 38054 Grenoble
cedex 9, France. E-mail: pron@cea.fr; Fax: +33 438 785113; Tel: +33 438 784389
b Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00 664 Warszawa,
Poland
c Laboratoire Nanophysique des Semiconducteurs, DRFMC, SP2M, CEA Grenoble,
17 Rue des Martyrs, 38054 Grenoble cedex 9, France
Received (in Montpellier, France) 7th October 2004, Accepted 10th January 2005
First published as an Advance Article on the web 23rd March 2005
Symmetric alkylthiophene-based mixed heterocyclic trimer and pentamer, containing central oxadiazole
units, have been prepared. Because of the electron-withdrawing properties of oxadiazole, the trimer cannot
be electropolymerised and undergoes an oxidative-type destruction at high potentials. In contrast, the
pentamer readily polymerises, giving a short chain polymer. Both trimer and pentamer exhibit strong
photoluminescence with a maximum at 399 nm (13% quantum yield) and 467 nm (46% quantum yield),
respectively. The polymer resulting from the electropolymerisation of the pentamer is also luminescent with
the maximum of the excitation band at 528 nm (33% quantum yield). The polymer can be oxidatively doped
as demonstrated by cyclic voltammetry, showing a clear anodic peak at 0.62 V versus Ag/Ag+ and its
cathodic counterpart at 0.56 V, associated with the undoping process. The significantly higher potential of
the oxidative doping of the prepared mixed heterocyclic polymer, as compared to the poly(alkylthiophene)
homopolymer of similar molecular weight, is caused by the presence of the oxadiazole unit, which lowers the
electron density in the p-electron system of the oligothiophene subunit and makes its oxidation more
difficult. The spectroelectrochemical investigation of the polymer is consistent with its voltammetric
behaviour, exhibiting doping-induced bleaching of the band originating from the p-p* transition and
simultaneous growth of the bipolaron bands. The observed clear and reversible spectroelectrochemical
behaviour makes the developed polymer a promising candidate for applications in electrochromic devices or
electrochemical sensors.
electrochemical polymerisation.4 In this case, the electro-
Introduction
chemical and spectroelectrochemical properties of the resulting
polymers can be conveniently tuned by the presence of elec-
tron-donating or electron-withdrawing side groups that influ-
ence the electron density in the main conjugated backbone.5a
Another approach involves the synthesis of mixed heterocyclic
polymers in which electron-donating units coexist with elec-
tron-withdrawing ones in the polymer main chain.5b Typical
examples of such systems are copolymers containing thiophene
and oxadiazole units.6
Electropolymerisation of monomers in which the oxadiazole
ring is adjacent to the thiophene one may be difficult to achieve
because of the acceptor character of oxadiazole, which makes
the oxidative coupling of thiophene, via the a carbon, more
difficult. Therefore, polymers containing thiophene and oxa-
diazole are prepared by a polycondensation reaction leading to
a nonconjugated precursor polymer that is then transformed
into its conjugated analogue with simultaneous formation of
the oxadiazole rings. Moreover, the use of this method inevi-
tably leads to the incorporation of a phenylene ring into the
polymer backbone.6a–c
Conjugated oligomers and polymers have been extensively
studied since more than two decades for three main reasons.
First, in their doped (charged) state, they behave as organic
conductors showing, in some cases, a very high electronic
conductivity of the metallic type.1 Second, in their undoped
(neutral) state conjugated oligomers or polymers are organic
semiconductors and, for this reason, they can serve as the main
constituents of active elements in organic electronic devices
such as field effect transistors (FETs), light-emitting diodes and
photovoltaic cells, to name a few.2 Third, conjugated mono-
mers and oligomers frequently electropolymerise, giving poly-
mers that can be electrochemically switched between the doped
and the undoped states. Since the switching involves extremely
pronounced and reversible changes in their physical properties,
such as the electrical conductivity and the spectroscopic and
magnetic properties among others, conjugated polymers can
serve as electrochemical sensors of various types.3
Heterocyclic monomers and oligomers, especially those from
the pyrrole and thiophene family, are very well suited for
To the best of our knowledge, no conjugated polymers
consisting solely of thiophene and oxadiazole units have ever
been reported to date. In this article, we demonstrate that it is
w Permanent address: Department of Chemistry, Omsk State Univer-
sity, Pr. Mira 55a, 644077 Omsk, Russia.
T h i s j o u r n a l i s & 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 a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 0 7 – 7 1 3
707

View Article Online
possible to obtain such polymers via simple electropolymeri-
sation of specially designed monomers.
Hz), 6.93 (d, 2H, J = 5.1 Hz), 2.57 (s, 6H), 8.66 (s, 2H); 13C
NMR (CDCl3, 200 MHz): d 15.8 (2C), 127.4 (2C), 128.2 (2C),
132.0 (2C), 143.1 (2C), 160.4 (2C); IR: n/cmꢀ1 3222 (m, br),
3110 (w), 2996 (w), 2924 (w), 2846 (w), 1678 (m), 1640 (w),
1618 (s), 1510 (m), 1502 (s), 1482 (w), 1468 (w), 1442 (w), 1412
(s), 1380 (m), 1368 (m), 1326 (w), 1278 (s), 1230 (w), 1218 (s),
1104 (m), 1060 (w), 1032 (m), 944 (w), 912 (m), 892 (w), 878
(w), 852 (w), 824 (s), 744 (m), 714 (s), 688 (m), 638 (m), 606 (s),
566 (m); anal. calcd for C12H12N2O2S2: C, 51.41%; H, 4.31%;
N, 9.99%; S, 22.87%; found: C, 51.35%; H, 4.36%; N,
10.06%; S, 23.10%.
Experimental
Materials and characterisation methods
All reagents were purchased from Aldrich and used as received.
The synthesised products were characterised by 1H and 13C
NMR spectroscopy, FT-IR spectroscopy, mass spectroscopy
and elemental analysis. NMR spectra were recorded on a
Bruker AC200 spectrometer. Chloroform-d, containing tetra-
methyl silane as internal standard, was used as solvent. Ele-
mental analyses were carried out by the analytical service of the
CNRS in Vernaison (France). FT-IR spectra were recorded on
a Perkin Elmer Paragon 500 spectrometer (wavenumber range:
4000–400 cmꢀ1, resolution 2 cmꢀ1) using the ATR technique.
2,5-Bis(3-methylthien-2-yl)-1,3,4-oxadiazole (3). A mixture
of 2.8 g (1 mmol) of N,N’-bis(3-methylthien-2-ylcarbonyl)
hydrazide (2) and 8 ml of POCl3 was boiled with constant
stirring for 1 h. The excess of POCl3 was then pumped off and
the reaction mixture was poured on ice, filtered and the residue
was washed with water. The raw product was purified by
recrystallisation from ethanol (2.1 g, 81.0% reaction yield).
M.p. 137–138 1C; 1H NMR (CDCl3, 200 MHz): d 7.41 (d, 2H,
J = 5.0 Hz), 6.98 (d, 2H, J = 5.0 Hz), 2.65 (s, 6H); 13C NMR
(CDCl3, 200 MHz): d 16.0 (2C), 119.4 (2C), 128.6 (2C), 131.8
(2C), 142.1 (2C), 160.3 (2C); IR: n/cmꢀ1 3100 (s), 2970 (w),
2940 (w), 2916 (m), 2848 (w), 2734 (w), 2616 (w), 1584 (m),
1564 (s), 1556 (m), 1538 (w), 1530 (m), 1514 (w), 1504 (m), 1494
(m), 1484 (w), 1470 (w), 1462 (w), 1444 (w), 1432 (w), 1426 (w),
1414; anal. calcd for C12H10N2OS2: C, 54.94%; H, 3.84%; N,
10.68%; S, 24.44%; found: C, 54.95%; H, 3.76%; N, 10.67%;
S, 24.71%.
Melting points were measured on a Buchi 510 apparatus.
¨
Photoluminescence measurements of the monomers and the
polymer in solution were carried out on an AvaSpec-2048-2
spectrofluorometer using a blue 400 nm diode or a UV lamp
(254 nm, 25 W) for excitation. The photoluminescence quan-
tum yields were determined by comparison with rhodamine 6G
(Qf = 95% in ethanol) or quinine sulfate (Qf = 54% in 0.5 M
sulfuric acid) using a dedicated setup, the excitation light being
provided by a 10 mW Argon laser. The photoluminescence of
the solid polymer film was measured only qualitatively. The
excitation was performed with a 500 W deuterium lamp via a
Jobin–Yvon Gemini 180 double monochromator. The variable
wavelength excitation power output was in the range from 1 to
100 mW depending on the selected wavelength.
The molecular weight of polymer P5 was determined using
size exclusion chromatography (SEC) on a 1100HP Chemsta-
tion equipped with 300 ꢁ 7.5 mm2 PL gel mixed-D 5 mm/104 A
column. Detection was performed by a diode array UV-vis
detector and a refractive index detector. The column tempera-
2,5-Bis(5-bromo-3-methylthien-2-yl)-1,3,4-oxadiazole
(4).
2,5-Bis(3-methylthien-2-yl)-1,3,4-oxadiazole (3; 1.1 g, 4.9
mmol) was mixed with 1.56 g (8.77 mmol) of NBS in 10 ml
CF3COOH; the solution was stirred for 18 h and then poured
into water. The precipitate was filtered, dissolved in boiling
ethanol and filtered again. The filtrate was then placed in a
refrigerator for slow crystallisation of the reaction product. A
second recrystallisation from ethanol yielded 0.68 g of the
desired dibromo derivative (38.6% reaction yield). M.p. 129–
ture and the flow rate were fixed to 313 K and 1 ml minꢀ1
,
respectively. The calibration curve was built using ten poly-
styrene (PS) narrow standards (S-M-10* kit from Polymer
Labs). Twenty microlitres of ca. 2 mg mlꢀ1 polymer in THF
(HPLC grade) were injected and analysed using a UV-vis
detector at 355 nm.
1
130 1C; H NMR (CDCl3, 200 MHz): d 6.97 (s, 2H), 2.60 (s,
6H); 13C NMR (CDCl3, 200 MHz): d 15.9 (2C), 116.6 (2C),
120.5 (2C), 134.4 (2C), 142.5 (2C), 159.2 (2C); IR: n/cmꢀ1 3072
(w), 2920 (w), 2848 (w), 1564 (s), 1528 (m), 1518 (w), 1498 (s),
1462 (w), 1428 (m), 1410 (m), 1380 (w), 1354 (w), 1310 (w),
1250 (w), 1186 (w), 1092 (w), 1036 (m), 990 (w), 938 (w), 912
(m), 828 (m), 786 (w), 728 (m), 696 (w), 630 (w), 604 (w); anal.
calcd for C12H8Br2N2OS2: C, 34.31%; Br, 38.04%; H, 1.92%;
N, 6.67%; S, 15.26%.; found: C, 33.96%; Br, 38.17%; H,
1.92%; N, 6.55%; S, 14.96%.
Syntheses
3-Methylthiophene-2-carbonyl chloride (1). 3-Methylthio-
phene-2-carboxylic acid (14.2 g, 0.1 mol) was mixed with
60 ml of SOCl2 in a reaction flask. The reaction mixture was
boiled for approximately 2 h until the evolution of HCl was no
longer observed. The excess of SOCl2 was pumped off. The raw
product was then purified by vacuum distillation. The fraction
at 98 1C/10 mm Hg was collected (14.14 g, 88% reaction
yield).1H NMR (CDCl3, 200 MHz): d 7.63 (d, 1H, J = 5.1
Hz), 7.01 (d, 1H, J = 5.1 Hz), 2.53 (s, 3H); 13C NMR (CDCl3,
200 MHz): d 16.8, 131.9, 132.9, 135.8, 150.1, 158.8; IR: n/cmꢀ1
3102 (w), 2942 (w), 2924 (w), 1738 (s), 1512 (m), 1442 (w), 1390
(s), 1370 (s), 1256 (w), 1242 (w), 1198 (s), 1090 (w), 1030 (m),
950 (w), 854 (s), 800 (s), 742 (s), 666 (s), 630 (m), 596 (m).
2,5-Bis[5-(3-octylthien-2-yl)-3-methylthien-2-yl]-1,3,4-oxadia-
zole (5). 5,5-Dimethyl-2-(3-octylthien-2-yl)[1,3,2]dioxaborinane
(760 mg, 2.46 mmol) and 2,5-bis(5-bromo-3-methylthien-2-yl)-
1,3,4-oxadiazole (4; 470.5 mg, 1.12 mmol) were placed in
anhydrous DMF (15 ml). The mixture was stirred under argon
for 10 min and then 522 mg of K3PO4 (2.46 mmol) and 77.6 mg
of Pd(PPh3)4 (0.672 mmol) in 15 ml of DMF were added. The
mixture was kept for an additional period of 14 h at 90 1C with
constant stirring and then allowed to cool to room temperature.
After filtration, the solution was extracted with diethyl ether
(3 ꢁ 25 ml) and the organic layer was washed with brine (3 ꢁ
25 ml). The combined organic layers were then dried over
magnesium sulfate and filtered. The solvent was removed using
a rotary evaporator and the crude product was purified by
column chromatography over silica gel, eluting with mixtures of
solvents of increasing polarity (pentane–methylene chloride
100 : 0 to 60: 40). The blue luminescent waxy product was then
recrystallised from chloroform and methanol at ꢀ10 1C to give
50 mg of yellow crystals (10% reaction yield). 1H NMR (CDCl3,
N,N 0-Bis(3-methylthien-2-ylcarbonyl)hydrazide (2). Anhy-
drous pyridine (42 ml) and 2.2 ml of hydrazine monohydrate
were consecutively added to a round bottom flask equipped
with a dropping funnel and a back condenser. Then 13.6 g
(0.0847 mol) of 3-methylthiophene-2-carbonyl chloride (1)
were added dropwise to the reactor using the dropping funnel;
the reaction mixture was boiled for 20 min, then cooled to
room temperature, poured on ice, filtered and the filtered solid
was washed with water. The raw reaction product was recrys-
tallised from ethanol (8.02 g, 67.6% reaction yield). M.p. 175–
1
176 1C; H NMR (CDCl3, 200 MHz): d 7.36 (d, 2H, J = 5.1
708
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 0 7 – 7 1 3

View Article Online
200 MHz): d 7.23 (d, 2H, J = 5.24 Hz), 7.00 (s, 2H), 6.96 (d, 2H,
J = 5.24 Hz), 2.80 (t, 4H, J = 7.66 Hz), 2.66 (s and t, 6H), 1.60–
1.75 (m, 4H), 1.25–1.45 (m, 20H), 0.83–0.90 (m, 6H); 13C NMR
(CDCl3, 200 MHz): d 14.1 (2C), 16.1 (2C), 22.6 (2C), 29.2 (2C),
29.3 (2C), 29.4 (2C), 29.5 (2C), 30.6 (2C), 31.8 (2C), 118.5 (2C),
124.0 (2C), 129.4 (2C), 129.9 (2C), 130.3 (2C), 134.4 (2C), 139.5
(2C), 142.1 (2C), 160.3 (2C); IR: n/cmꢀ1 3060 (w), 2942 (w),
2920 (s), 2850 (s), 1566 (s), 1536 (s), 1518 (w), 1496 (s), 1460 (w),
1452 (w), 1434 (w), 1406 (w), 1378 (m), 1258 (m), 1082 (m), 1036
(s), 926 (w), 918 (w), 870 (w), 830 (s), 798 (m), 728 (s), 688 (m),
652 (m), 630 (s), 608 (m); MS: m/z = 651.1 (M+).
mediate, upon dehydration, gives the mixed heterocyclic trimer
with the oxadiazole central unit 3.
Synthesis of the pentamer 5 from the trimer requires two
additional steps (Scheme 1). The first step consists in the
bromination of 3. The use of classical procedures [treatment
with Br2 or N-bromosuccinimide (NBS) in classical solvents]
was unsuccessful. To achieve this reaction, the deactivated
aromatic compound 3 was treated with NBS in trifluoroacetic
acid solvent to give the corresponding dibromo aromatic
compound 4 in reasonable yields. The applied procedure was
quite similar to the procedure described in ref. 8. 5 is then
obtained via Suzuki coupling, that is, the reaction of 4 with
5,5-dimethyl-2-(3-octylthien-2-yl)[1,3,2]dioxaborinane, the lat-
ter being prepared according to the literature9. Suzuki or Stille
coupling are convenient methods not only for the preparation
of alternating conjugated copolymers10 but also for the synth-
esis of regiochemically well-defined oligomers, which can then
constitute building blocks for the preparation of polymers
with tunable electronic, spectroscopic and electrochemical
properties.11
In Fig. 1 the UV-vis absorption spectra of the trimer 3 and
the pentamer 5 are compared. As expected, the main band,
ascribed to the p-p* transition in the conjugated p system,
undergoes a significant bathochromic shift by ca. 55 nm when
going from 3 to 5. One must also note here that the p-p*
transition band in the alkyl substituted mixed pentamer 5
is located at a shorter wavelength (371 nm) when compared
to the same band (at 402 nm) for the end-capped unsubsti-
tuted analogue of 5 studied in ref. 7. This can be taken as a
spectroscopic evidence of the chain torsion induced by the
substituents.
Both 3 and 5 are photoluminescent (Fig. 2). The measured
photoluminescence quantum yield for the trimer and the
pentamer are 13% and 46%, respectively (see Table 1). In
the latter case, it approaches that reported for the unsubsti-
tuted pentamer containing the oxadiazole central ring.7
Attempts to electropolymerise 3 were unsuccessful. Fig. 3
shows a voltammetric scan obtained in the Bu4NBF4/acetoni-
trile electrolyte containing 3. For this scan only, a strong,
irreversible anodic peak can be observed with a maximum at
E = 1.37 V, that is, in the potential range where the thiophene-
based polyconjugated systems undergo oxidative degrada-
tion.12 This degradation product passivates the electrode since,
in the consecutive scan (not shown in Fig. 3), the intensity of
the anodic peak decreases by ca. one order of magnitude. Such
behaviour is a direct consequence of the electron-withdrawing
character of the oxadiazole ring, which, via mesomeric and
inductive effects, lowers the electron density in adjacent thio-
phene rings. As a consequence, the formation of a radical
cation, which initiates the oxidative polymerisation of the
trimer, is more difficult. The oxidation starts at very high
potentials, leading to an unstable radical cation that undergoes
several consecutive degradation reactions at the rate exceeding
the desired polymerisation via Ca–Ca.
Electropolymerisation, electrochemical and
spectroelectrochemical studies
Electropolymerisation experiments were performed in a one-
compartment electrochemical cell using a platinum disc work-
ing electrode, a platinum counter electrode and an Ag/0.1 M
AgNO3/acetonitrile reference electrode. The latter was cali-
brated versus ferrocene redox couple measured in the same
electrolytic solution. The electrolytic solution used in the
electropolymerisation consisted of 0.1 M tetrabutylammonium
tetrafluoroborate (Bu4NBF4) in a 2 : 3 v/v mixture of methy-
lene chloride and acetonitrile, to which an appropriate amount
of the monomer was added to give a concentration of 1 ꢁ 10ꢀ2
M in the case of 3, and 5 ꢁ 10ꢀ3 M in the case of 5. The same
electrochemical cell was used for cyclic voltammetry investiga-
tions of the polymer, the electrolyte consisting of 0.1 M
Bu4NBF4 in acetonitrile. For spectroelectrochemical investiga-
tions, the polymer was deposited on an ITO transparent
electrode using the same analytical conditions. The corre-
sponding UV-vis-NIR spectra were obtained in a specially
designed rectangular spectroelectrochemical cell and recorded
in the increasing electrode potential mode, using a Lambda 2
Perkin Elmer spectrometer (wavelength range 280–1100 nm).
Results and discussion
Mixed heterocyclic oligomers containing an oxadiazole central
ring have previously been prepared by Mitschke et al.7 in a
search for new materials with tunable luminescent and electro-
chemical properties. These authors prepared end-capped thio-
phene-based pentamers with an oxadiazole central ring. The
blocking of the Ca and the Cb positions in the terminal
thiophene rings was intended to prevent electropolymerisation
of the synthesised oligomers. To the contrary, the goal of our
research was to obtain electropolymerisable thiophene-oxadia-
zole oligomers.
In the preparation of the mixed heterocyclic trimer 3, we
have applied a very similar synthetic pathway to that used in
ref. 7 for the preparation of the end-capped pentamers. Thus, 3
is obtained from 3-methylthiophene-2-carboxylic acid in a
three-step procedure that involves the formation of the corres-
ponding carbonyl chloride 1, followed by reaction with hydra-
zine to give the N,N’-diacylhydrazine derivative 2. This inter-
In the case of pentamer 5, the effect of the electron-with-
drawing properties of the central oxadiazole ring on the
Scheme 1 Synthetic pathway for the preparation of the mixed heterocyclic monomers 3 and 5: (i) SOCl2, 2 h, reflux, 88% yield; (ii) pyridine,
H2NNH2, 20 min, reflux, 68% yield; (iii) POCl3, 1 h, reflux, 81% yield; (iv) NBS, CF3COOH, 18 h, r.t., 39% yield; (v) 5,5-dimethyl-2-(3-octylthien-
2-yl)[1,3,2]dioxaborinane, Pd(PPh3)4 (cat), K3PO4, DMF, 14 h, 90 1C, 10% yield.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 0 7 – 7 1 3
709

View Article Online
Table 1 Optical properties of 3, 5 and P5
max, abs/nma
l
max, PL/nmb
Qf
c
Compound
l
3
316
371
431
399
467
528
651
0.13 ꢂ 0.03
0.46 ꢂ 0.02
0.33 ꢂ 0.02
—
5
P5
P5 (thin film)
545 (0 - 0)
504 (0 - 1)
472 (0 - 2)
a
Absorption maxima in the UV-vis spectra from chloroform solution
b
or thin film (as indicated). Photoluminescence maxima from chloro-
c
form solution or thin film. Quantum yield.
couple with each consecutive scan. In the subsequent text, the
polymer obtained by voltammetric polymerisation of 5 will be
denoted as P5.
After electropolymerisation, the deposited polymer layer
was repeatedly rinsed with acetonitrile, with the goal to remove
any pentamer 5 possibly adsorbed on the polymer surface, then
reduced at the potential E = 0.25 V to give fully undoped
(neutral) P5.
SEC investigations of neutral P5 showed that the obtained
product is a mixture of oligomers of 5 rather than a true
polymer. The chromatogram shows a trimodal distribution
with a dominant peak corresponding to 1337 Da, a second
peak corresponding to 2450 Da and a third broad peak of very
low intensity corresponding to higher molecular weights. Tak-
ing into account correction factors for the determination of
molecular weights of polyconjugated systems by SEC with the
use of polystyrene standards,13 we can conclude that P5 is a
mixture of dimers and higher oligomers of 5.
In Fig. 6, the chloroform solution absorption and lumines-
cence spectra of P5 are shown. As expected, coupling via
electropolymerisation induces a strong (60 nm) bathochromic
shift in both the absorption and the photoluminescence bands
(compare Figs. 2 and 6 and Table 1). The chloroform solution
of 5 emits blue radiation whereas that of P5 emits green
radiation (33% quantum yield).
Fig. 1 UV-Vis absorption spectra of 3 (n) and 5 (K) registered in
chloroform solution.
reactivity of the terminal Ca carbons is much less pronounced
and, as a result, the pentamer readily electropolymerises. Fig. 4
shows voltammetric behaviour of 5 dissolved in the Bu4NBF4/
acetonitrile electrolyte. Three independent scans were regis-
tered with increasing reverse potential: 1, 1.2 and 1.5 V,
respectively. As clearly seen, the oxidation of 5 starts at E o
0.9 V with the formation of a polymer layer, which gives a clear
cathodic peak on the reverse scan. This peak, attributed to the
undoping of the polymer formed, is observed for scans limited
to 1 and 1.2 V. The extension of the limiting potential to 1.5 V
results in the oxidative destruction of the deposited polymer,
which, in this case, shows no electrochemical activity in the
reverse scan. Since the polymerisation anodic peak partially
overlaps with the oxidative destruction peak, it is worthwhile
to carry out the polymerisation at the lowest possible potential.
Therefore, in further cyclic voltammetry polymerisations we
have limited the reverse potential to E = 0.9 V. These condi-
tions assure smooth deposition of the polymer on the electrode
as seen in Fig. 5. The polymer growth is manifested by an
increase in the intensity of the polymer doping/undoping redox
Solid state absorption and photoluminescence spectra of P5
are shown in Fig. 7. The absorption spectrum shows a clear
vibrational structure. The exact positions of the individual
peaks originating from the fine structure, obtained through
the second derivative analysis, are listed in Table 1. Vibrational
structure in the solid state spectra of polyconjugated systems is
Fig. 2 Photoluminescence spectra of 3 and 5 registered in chloroform
solution with excitation at 254 nm for 3 and at 400 nm for 5.
Fig. 3 Cyclic voltammogram of 3 in 0.1 M Bu4NBF4 in a 2 : 3 v/v
mixture of methylene chloride and acetonitrile at 50 mV sꢀ1 scan rate.
710
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 0 7 – 7 1 3

View Article Online
Fig. 6 UV-Vis absorption and photoluminescence spectra of P5
recorded in chloroform solution with an excitation wavelength of
400 nm.
Fig. 4 Cyclic voltammetry electropolymerisation of 5 in 0.1 M
Bu4NBF4 in a 2 : 3 v/v mixture of methylene chloride and acetonitrile
with different limiting potentials E ¼ 0.9 V (solid line), 1.2 V (dashed
line), and 1.5 V (dotted line); scan rate of 50 mV sꢀ1
.
active mode of C C antisymmetric stretching in the alkyl-
Q
substituted 2,5-thienylene ring.
a typical manifestation of electron lattice coupling. The 0 - 0
transition, that is, the transition from the ground state to the
relaxed excited state, is roughly inversely proportional to the
conjugation length. It should be noted that in P5 the band
corresponding to this transition is significantly red-shifted as
compared to the corresponding band in the solid state spec-
trum of the acetone fraction of regioregular poly(3-hexylthio-
phene), which shows a comparable molecular weight and,
consequently, comparable chain length.14 This means that
neither the insertion of the oxadiazole ring nor the presence
of the alkyl substituents in the head-to-tail, tail-to-tail coupling
sequence (HT–TT, see inset in Fig. 5) perturb, to a significant
extent, the conjugation in P5. The energy difference between
the first and the second shoulders of the vibrational peak
structure is 1540 cmꢀ1, which corresponds well to the Raman
In both its absorption and emission spectra, P5 is solvato-
chromic. Its solid state photoluminescence band is red-shifted
by 123 nm with respect to the corresponding band recorded for
the chloroform solution (see Table 1). This large shift can be
considered as indicative of p-stacking phenomena in the solid
state, which, in turn, may negatively influence the solid state
photoluminescence quantum yield. In any case, all data con-
cerning the solid state spectra presented here must be consid-
ered as preliminary since they are not quantitative.
Fig. 8 shows cyclic voltammograms of P5 recorded at
different scan rates in Bu4NBF4/acetonitrile electrolyte. The
oxidative doping/reductive undoping redox couple is clearly
visible with anodic and cathodic peak maxima at 0.62 and
0.56 V, respectively, at a scan rate of 20 mV sꢀ1. The positions
of both peaks are only slightly dependent on scan rate. The
capacitive current plateau at potentials exceeding the anodic
peak is indicative of efficient p-type doping and high polymer
conductivity in the doped state.15
Fig. 5 Cyclic voltammetry electropolymerisation of 5 to give P5 at the
limiting potential E = 0.9 V in 0.1 M Bu4NBF4 in a 2 : 3 v/v mixture of
methylene chloride and acetonitrile; scan rate of 50 mV sꢀ1. Inset:
chemical structure of polymer P5.
Fig. 7 UV-Vis absorption and photoluminescence spectra of a thin
film of P5 deposited on ITO at the excitation wavelength of 365 nm.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 0 7 – 7 1 3
711

View Article Online
completely bleached, indicating the maximum doping level,
which is confirmed by a high value of the capacitive current
observed at this potential in the cyclic voltammogram of P5
(compare Figs. 8 and 9). The bipolaron band, ascribed to the
transition from the valence band to the second bipolaron level,
appears at relatively short wavelengths (lmax = 644 nm). This
could be interpreted as a consequence of the rather low
molecular weight of P5. For conjugated polymers of low
molecular weight, the band gap is dependent on the chain
length, increasing as the length decreases. As a consequence,
the band associated with the transition from the valence band
to the second bipolaron level appears at higher energies for
short chain polymers. This was demonstrated in the case of
fractionated regioregular poly(3-alkylthiophene),14 for which
the higher energy bipolaron band continuously shifted towards
higher wavelengths with increasing Mn, in the molecular
weight range 2000–5400 Da. For higher Mn values, its position
stabilised.
Fig. 8 Cyclic voltammograms of P5 in 0.1 M Bu4NBF4/acetonitrile at
different scan rates between 10 and 100 mV sꢀ1
Conclusion
.
To summarise, we have prepared a mixed heterocyclic penta-
mer consisting of a central oxadiazole unit and of four
alkylthiophene rings. The pentamer shows strong photolumi-
nescence and electrochemical activity. Electrochemical oxida-
tive coupling of the pentamer leads to a short chain polymer
emitting red light. Its electrochemical and spectroelectrochem-
ical properties are modified as compared to poly(alkylthio-
phene) homopolymers of comparable molecular weight, as a
consequence of the electron-withdrawing properties of the
oxadiazole unit.
The effect of the presence of the oxadiazole unit in the chain
of P5 is manifested here by a significantly higher potential of
the anodic peak corresponding to the oxidative doping, as
compared to poly(alkylthiophene) homopolymers of similar
chain length and the same sequence of substituent distribution.
Evidently, the electron-withdrawing properties of oxadiazole
make the oxidation of the adjacent 2,5-thienylene units first to
radical cations (polarons) and then to dications (bipolarons),
more difficult.
Electrochemical oxidative doping of conjugated polymers
results in a gradual bleaching of the band ascribed to the p-p*
transition with simultaneous growth of two new bands at
higher wavelengths, associated with the formation of bipolar-
ons (dications). For this reason, it is convenient to follow the
electrochemical doping by spectroelectrochemistry. Doping-
induced spectral changes of a thin layer of P5, deposited on
an ITO electrode, are presented in Fig. 9.
Acknowledgements
The authors thank Dr Patrice Rannou for performing the SEC
measurements. One of the authors (A.F.) wants to acknowl-
edge the financial support of Joseph Fourier University in
Grenoble in the form of a visiting professor fellowship.
Since the spectral range of the spectrometer used is limited to
1100 nm, only the higher energy bipolaron band and the onset
of the lower energy one are seen. Spectral changes induced by
the oxidative doping start to appear at E = 0.55 V, which is
consistent with the cyclic voltammetry investigations, which
show a small but nonzero current at this potential. At E =
0.875 V, the band originating from the p-p* transition is
References
1
See for example: (a) Handbook of Conducting polymers, eds. T. A.
Skotheim, R. L. Elsenbaumer and J. R. Reynolds, Marcel Dekker,
1998; (b) Handbook of Organic Conductive Molecules and Poly-
mers, ed. H. S. Nalva, John Wiley, 1997.
2
(a) Z. Bao, Adv. Mater., 2000, 12, 227–230; (b) A. C. Kraft, A. C.
Grimsdale and A. B. Holmes, Angew. Chem., Int. Ed., 1998, 37,
402–428; (c) G. Yu, J. C. Hummelen, F. Wudl and A. J. Heeger,
Science, 1995, 270, 1789–1791.
3
4
(a) B. Adhikari and S. Majumdar, Prog. Polym. Sci., 2004, 29,
699–766; (b) M. Schaferling and P. Bauerle, J. Mater. Chem.,
2004, 14, 1132–1141.
(a) K. Kaeriyama, M. Sato and S. Tanaka, Synth. Met., 1987, 18,
233–236; (b) S. Hotta, Synth. Met., 1987, 22, 103–113; (c) M.
Bouachrine, J. P. Lere-Porte, J. J. E. Moreau and M. Wong Chi
Man, J. Mater. Chem., 1995, 5, 797–799; (d) Y. Zhu and M. O.
Wolf, Chem. Mater., 1999, 10, 2995–3001; (e) A. Naji, M. Cretin,
M. Persin and J. Sarrazin, J. Appl. Polym. Sci., 2004, 91,
3947–3958.
¨
¨
5
6
(a) N. H. S. Lee, Z. K. Chen, S. J. Chua, Y. H. Lai and W. Huang,
Thin Solid Films, 2000, 363, 106–109; (b) S. Y. Song, T. Ahn, H. K.
Shim, I. S. Song and W. H. Kim, Polymer, 2001, 42, 4803–4811.
(a) W. Huang, H. Meng, W. L. Yu, J. Gao and A. Heeger, Adv.
Mater., 1998, 10, 593–596; (b) H. Meng and W. Huang, J. Org.
Chem., 2000, 65, 3894–3901; (c) W. Huang, W. L. Yu, H. Meng,
J. Pei and S. F. Y. Li, Chem. Mater., 1998, 10, 3340–3345; (d)
Y. Chen, Y. Y. Huang and T. Z. Wu, J. Polym. Sci., Part A:
Polym. Chem., 2002, 40, 2927–2936.
7
8
(a) U. Mitschke, T. Debaerdemaeker and P. Bauerle, Eur. J. Org.
¨
Chem., 2000, 425–437; (b) U. Mitschke, E. Mena Osteritz,
T. Debaerdemaeker, M. Sokolowski and P. Bauerle, Chem.-Eur.
¨
Fig. 9 UV-Vis-NIR spectra of P5 recorded at increasing electrode
potential E versus Ag/Ag+ with 0.1 M Bu4NBF4/acetonitrile electro-
lytic solution.
J., 1998, 4, 2211–2224.
D. J. Duan, L. H. Zhang and W. R. Dolbier, Synlett., 1999, 8,
1245–1246.
712
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 0 7 – 7 1 3

View Article Online
9
´
G. Bidan, A. De Nicola, V. Enee and S. Guillerez, Chem. Mater.,
1998, 10, 1052–1058.
W. L. Yu, Y. H. Lai and W. Huang, Macromolecules, 2000, 33,
8945–8952.
10 (a) M. Bouachrine, J. P. Lere-Porte, J. J. E. Moreau, F. Serein
Spirau, R. A. Da Silva, K. Lmimouni, L. Ouchani and C. Dufour,
Synth. Met., 2002, 126, 241–244; (b) J. P. Lere-Porte, J. J. E.
Moreau, F. Serein Spirau, C. Torreilles, A. Righi, J. L. Sauvajol
and M. Brunet, J. Mater. Chem., 2000, 10, 927–932; (c) F. Embert,
J. P. Lere-Porte, J. J. E. Moreau, F. Serein Spirau, A. Righi and J.
L. Sauvajol, J. Mater. Chem., 2001, 11, 718–722; (d) M. Jayakan-
nan, J. L. J. van Dongen and R. A. J. Janssen, Macromolecules,
11 (a) W. L. Huang and Y. H. Lai, Thin Solid Films, 2002, 417,
211–214; (b) M. Jayakannan, P. A. van Hal and R. A. J. Janssen,
J. Polym. Sci., Part A: Polym. Chem., 2002, 40, 251–261; (c)
J. P. Sauvage, J. M. Kern, G. Bidan, B. Divisia-Blohorn and
P. L. Vidal, New J. Chem., 2002, 26, 1287–1290.
12 B. Krische and M. Zagorska, Synth. Met., 1989, 28, 263–268.
13 R. Demadrille, P. Rannou, J. Bleuse, J.-L. Oddou, A. Pron and
M. Zagorska, Macromolecules, 2003, 36, 7045–7054.
2001, 34, 5386–5393; (e) U. Asawapirom, R. Guntner, M. Forster,
14 M. Trznadel, A. Pron, M. Zagorska, R. Chrzaszcz and J. Pieli-
chowski, Macromolecules, 1998, 31, 5051–5058.
15 J. Tanguy, A. Pron, M. Zagorska and I. Kulszewicz-Bajer, Synth.
´ ´
¨
T. Farrell and U. Scherf, Synthesis, 2002, 9, 1136–1142;
(f) J. Huang, Y. Niu, W. Yang, Y. Mo, M. Yuan and
Y. Cao, Macromolecules, 2002, 35, 6080–6082; (g) B. Liu,
Met., 1991, 45, 81–105.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 7 0 7 – 7 1 3
713