Photochemical control of conductivity of polythiophenes with photochromic moieties

Article abstract of DOI:10.1039/a902596k

The synthesis, optical and electrical properties of azobenzene- substituted polythiophene derivatives at the 3-position are reported: one without flexible spacer (PT0) and the other with a hexamethylene spacer (PT6). Photoirradiation of PT6 at 366 nm caus

Full text of DOI:10.1039/a902596k

J O U R N A L O F
Photochemical control of conductivity of polythiophenes with
photochromic moieties
Hiroyuki Mochizuki,a Yuki Nabeshima,a Takashi Kitsunai,a Akihiko Kanazawa,a Takeshi Shiono,a
Tomiki Ikeda,*a Tamejiro Hiyama,a Tsukasa Maruyama,a Takakazu Yamamotoa and
Naoyuki Koideb
C H E M I S T R Y
aResearch Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta,
Midori-ku, Yokohama 226–8503, Japan. E-mail: tikeda@res.titech.ac.jp
bDepartment of Chemistry, Faculty of Science, Science University of Tokyo
Received 31st March 1999, Accepted 14th June 1999
The synthesis, optical and electrical properties of azobenzene-substituted polythiophene derivatives at the 3-position
are reported: one without flexible spacer (PT0) and the other with a hexamethylene spacer (PT6). Photoirradiation
of PT6 at 366 nm caused trans–cis isomerization of the azobenzene moiety and this photochromic reaction induced
a change in conductivity of the thin film of PT6: the conductivity was 1.1×10−7 S cm−1 before irradiation and
8.3×10−7 S cm−1 after photoirradiation. After annealing, the conductivity of the film returned to the initial value.
The conductivity of a compound without the azobenzene moiety, a-terthienyl (aT3), showed no change on
photoirradiation at 366 nm.
ophene derivatives azobenzene-substituted at the 3-position
on thiophene rings with different spacer lengths and evaluated
their optical and electrical properties as well as control of their
conductivity by photochemical reaction of the azobenzene
moieties. The photoinduced change in conductivity was dis-
cussed on the basis of the isomerization behavior of the
azobenzene introduced to the side chain. We tried to evaluate
Introduction
p-Conjugated polymers are a class of materials that exhibit
intrinsic conducting properties originating from the polymer
chains.1,2 It is well known that the conductivity of polythioph-
enes (PTs) can vary in response to minor perturbations in
chemical structure, oxidation state, and/or solid state ordering
of materials. In particular, chemical doping with I significantly
enhances the conductivity of PTs. Recent studies have shown
the change in conductivity of I -doped polythiophene deriva-
2
2
tives on photoirradiation. However, they absorbed light in the
that the conductivity of these materials is sensitive to the
regiospecificity of the side chain, which in turn indicates that
conformational changes provide large effects on the conduc-
tivity.3–7 Introduction of functional groups to PTs is therefore
expected to be favorable from the viewpoint of control of the
conductivity.
whole visible region and they have very large molar extinction
coefficients. Therefore, when we irradiated the I -doped poly-
2
thiophene derivatives, the films absorbed light entirely in the
surface region of less than 50 nm thickness and the photochem-
ical reaction could not take place completely across the film.
Thus, we used non-doped polythiophene samples in this study.
Photochromic compounds such as azobenzenes and
spiropyrans exhibit a reversible structural change due to
photochemical reaction on photoirradiation and simul-
taneously show changes in the spectra in the UV–VIS absorp-
tion band.8–13 Several studies have been performed so far on
the conformational change of PTs containing photochromic
moieties in the side chain.14,15 As an alternative to PTs,
tetrathiafulvalene derivatives with substituted azobenzenes
have been demonstrated to undergo trans–cis isomerization
upon photoirradiation.16
Experimental
Reagents
All reagents were purchased from Tokyo Kasei Co. Methylene
chloride and chloroform were purified under an argon atmos-
phere before use. Ferric chloride was dried under vacuum
immediately before use. a-Terthienyl was recrystallized from
hexane before use.
In these studies, however, the conductivity of PTs was not
controlled by means of the photochromic reaction. The optical
control of conductivity was achieved for the first time by Chen
and Liao.17,18 They used copolymers composed of thiophenes
with hexyl side chains and azobenzene moieties in the side
chains and showed a change in the conductivity on photoirradi-
ation. They concluded that the enhancement of conductivity
on photoirradiation is due to an increase in the number of
hopping sites due to the isomerization of azobenzene. The
spectra in absorption bands of both azobenzene and con-
ducting units, however, were unchanged before and after
photoirradiation. The mechanism of change of the conductivity
of polythiophene derivatives on photoirradiation has not been
clear. Detailed information on isomerization of the azoben-
zene affecting the conductivity could not be obtained by
absorption spectroscopy.
Characterization of monomers and polymers
1H-NMR spectra were recorded with a JEOL-400 (400 MHz)
spectrometer. Molecular weight was determined by GPC
(Tosoh HLC-802; column, GMH6×2+G4000H8; eluent,
chloroform) calibrated with standard polystyrenes. Glass
transition temperature (T ) was determined with a differential
g
scanning calorimeter (DSC; Seiko I&E SSC-5000) operated at
a heating rate of 10 °C min−1. Melting points were determined
with an Olympus Model BHSP polarizing microscope
equipped with Mettler hot stage Models FP-80 and FP-82. IR
spectra were recorded with a JEOL FT/IR-3 spectrometer.
Preparation of thiophene derivatives
Azobenzene-substituted polythiophene derivatives at the
3-position were synthesized following the synthetic route
shown in Scheme 1 and Scheme 2.
To obtain a deeper understanding of the mechanism of such
photoinduced change in the conductivity, we prepared polythi-
J. Mater. Chem., 1999, 9, 2215–2219
2215

pound 1 as a dark yellow solid (plates, yield 62%): mp 110 °C;
1H-NMR (400 MHz, CDCl ) d 0.95 (3H, t, J=7.0 Hz, -CH ),
3
3
1.39–1.54 (2H, m, -CH CH ), 1.63–1.82 (2H, m,
2
3
-CH CH CH ), 4.03 (2H, t, J=7.7 Hz, -OCH CH CH CH ),
2
2
3
2
2
2
3
5.29 (1H, br, -OH), 6.94 (2H, d, J=8.8 Hz, 3-H, 5-H), 7.01
(2H, d, J=8.4 Hz, 3∞-H, 5∞-H), 7.82 (2H, d, J=8.4 Hz, 2∞-H,
6∞-H), 7.84 (2H, d, J=8.8 Hz, 2-H, 6-H). IR (KBr, cm−1)
3480, 1600, 1580, 1500, 1250, 850, 820. Anal. Calcd. for
C H N O : C, 71.11; H, 6.67; N, 10.37. Found: C, 71.10; H,
16 18 2 2
6.83; N, 10.16%.
4-(4-Butoxyphenyldiazenyl)phenyl
(3-thienyl)ethanoate
(MT0).20 Under an argon atmosphere, 6.12 g (30.0 mmol) of
N,N∞(DCC) in 100 ml of CH Cl were added to a solution
2
2
of 1 (3.27 g, 11.0 mmol), (3-thienyl)ethanoic acid (1.56 g,
11.0 mmol) and 4-(dimethylamino)pyridine (DMAP; 0.14 g,
1.1 mmol). The mixture was stirred for 20 h at room tempera-
ture, and the reaction was monitored by thin layer chromatog-
raphy. The precipitated salt was filtered off and the solvent
was removed under reduced pressure, and the residue was
purified by column chromatography on silica gel (eluent,
CHCl ). Recrystallization of crude product from ethanol–ace-
3
tone (751) gave 3.64 g of MT0 as an orange solid (powder,
yield 84%): mp 129 °C; 1H-NMR (400 MHz, CDCl ) d 0.99
3
(3H, t, J=6.9 Hz, -CH ), 1.48–1.56 (2H, m, -CH CH ),
3
2
3
1.78–1.86 (2H, m, -CH CH CH ), 3.94 (2H, s, Th-CH COO-),
2
2
3
2
4.08 (2H, t, J=7.7 Hz, -OCH CH CH CH ), 7.00 (2H, d, J=
2
2
2
3
8.5 Hz, 3-H, 5-H), 7.16 (1H, d, J=5.7 Hz, Th-4-H), 7.22 (2H,
d, J=8.9 Hz, 3∞-H, 5∞-H), 7.29 (1H, s, Th-2-H), 7.35 (1H, d,
J=5.7 Hz, Th-5-H), 7.87 (2H, d, J=8.5 Hz, 2-H, 6-H), 7.89
(2H, d, J=8.9 Hz, 2∞-H, 6∞-H). IR (KBr, cm−1) 2960, 2940,
2880, 1750, 1600, 1580, 1490, 1230, 840, 730. Anal. Calcd for
Scheme 1 Synthetic route to PT0.
C H SN O : C, 67.00; H, 5.58; S, 8.11; N, 7.11. Found: C,
22 22 2 3
66.77; H, 5.63; S, 8.22; N, 7.21%.
Poly[4-(4-butoxyphenyldiazenyl)phenyl (3-thienyl)ethano-
ate] (PT0).21 To the mixture of MT0 (1.18 g, 3.01 mmol)
and FeCl (1.95 g, 12.0 mmol) was added slowly anhydrous
3
CHCl (20 ml) and the resulting solution was stirred at room
3
temperature for 10 h under vacuum. The reaction mixture was
poured into 900 ml of acetone, and the precipitate formed was
filtered off, washed with acetone and with methanol several
times, and dried under vacuum. Ionic impurities were removed
completely from the polymer by successive washing with
concentrated ammonia (5%, 10 ml) and 1 M HCl (10 ml).
This procedure was repeated five times. Pure polymer was
dried under vacuum to give 0.127 g of PT0 as a brown solid
(yield 10.7%), which was insoluble in common solvents: T
g
30 °C. Anal. Calcd for (C H SN O ) : C, 67.34; H, 5.10; S,
22 20 2 3 n
8.16; N, 7.14. Found: C, 67.54; H, 5.38; S, 8.14; N, 6.83%.
Because of insolubility, no stereoregularity of the thiophene
units could be determined.
4-Butoxy-4∞-(6-hydroxyhexyloxy)azobenzene (2). A small
amount of KI, 1 (3.27 g, 11.0 mmol), 6-chlorohexan-1-ol
(1.64 g, 12.0 mmol) and K CO (1.40 g, 11.0 mmol) were
2
3
dissolved in dimethylformamide (150 ml) and allowed to stir
for 10 h at 120 °C. The reaction was monitored by thin layer
chromatography, and upon adding water (500 ml), a pale
yellow solid was precipitated. The residue was dissolved in
CHCl and the organic layer was dried (MgSO ) and concen-
Scheme 2 Synthetic route to PT6.
3
4
trated under reduced pressure. The crude product was purified
by recrystallization from hexane to give 3.51 g of 2 as a pale
yellow solid (powder, yield 86%): mp 119 °C; 1H-NMR
(400 MHz, CDCl ) d 0.99 (3H, t, J=7.3 Hz, -CH ), 1.25 (1H,
4-Butoxy-4∞-hydroxyazobenzene (1)19. Butoxyaniline (6.91 g,
41.9 mmol) was dissolved in acidified water (15 ml HCl–75 ml
water) and diazotized with sodium nitrite (2.89 g, 41.9 mmol)
in an ice bath. Aqueous phenol (3.94 g, 41.9 mmol) was added
to the resulting diazonium salt solution while stirring for 2 h
at 0 °C. The crude product was collected by filtration and
purified by column chromatography on silica gel (eluent,
3
3
br, -OH), 1.44–1.61 (6H, m, -CH CH , -OCH∞ CH∞ (CH∞ ) ),
2 2
1.64–1.70 (2H, m, -CH∞ CH∞ OH), 1.77–1.88 (4H, m,
2
2
3
2
2
2
-OCH CH , -OCH∞ CH∞ ), 3.67 (2H, br, J=7.5 Hz,
-CH∞ CH∞ OH), 4.04 (4H, t, J=7.7 Hz, -OCH CH ,
-OCH◊ CH∞ ), 6.98 (4H, d, J=8.5 Hz, 3-H, 5-H, 3∞-H, 5∞-H),
2
2
2
2
2
2
2
2
CHCl ). Recrystallization from hexane gave 7.04 g of com-
3
2
2
2216
J. Mater. Chem., 1999, 9, 2215–2219

7.86 (4H, d, J=8.5 Hz, 2-H, 6-H, 2∞-H, 6∞-H). IR (KBr, cm−1)
chloroform onto a glass substrate and by drying completely
under reduced pressure. The samples were prepared at a size
of 1×1 cm2. The size of the sample films was measured with
a micrometer three times and a mean value was taken. The
thickness of the films was estimated by UV absorption spec-
troscopy. The conductivity of the films was measured with an
Advantest R-8340A resistance meter using the two-probe
method.
3510, 2910, 1600, 1580, 1500, 1220, 840. Anal. Calcd. for
C H N O : C, 71.35; H, 8.11; N, 7.57. Found: C, 71.23; H,
22 30 2 3
8.12; N, 7.77%.
6-[4-(4-Butoxyphenyldiazenyl)phenoxy]hexyl
(3-thienyl)
ethanoate (MT6).20 MT6 was synthesized following the pro-
cedure described for MT0, using 3.36 g (9.01 mmol) of 2 and
1.27 g (9.01 mmol) of (3-thienyl)ethanoic acid. The crude
product was purified by recrystallization from ethanol–acetone
(551) to give 3.41 g of MT6 as an orange solid (powder, yield
Results and discussion
Characterization of polymers
77%): mp 104 °C; 1H-NMR (400 MHz, CDCl ) d 1.00 (3H, t,
3
J=6.9 Hz, -CH ), 1.40–1.47 (2H, m, -CH CH ), 1.49–1.56
It was found that PT0 is insoluble in common organic solvents.
The low solubility of PT0 may be explained in terms of a
compact morphology, which results from the absence of any
degrees of freedom between the polymer chain and the azoben-
zene moiety. In contrast, PT6 is soluble. The number-average
molecular weight was 23000. 1H-NMR spectra provide useful
information on the substitution pattern in the polymer back-
bone. Previous reports have shown that a-methylene protons
directly attached to the thiophene ring can be resolved into
two different diads: head-to-tail (H–T) and head-to-head
(H–H).22–27 The two peaks around 3.7 ppm come from the
methylene group between the ester group and the thiophene
ring. The expanded 1H-NMR spectrum of PT6 (Fig. 2) shows
that PT6 has a stereorandom chain structure with equal
distribution of H–T and H–H linkages.
3
2
3
(4H, m, -CH CH CH ), 1.63–1.72 (2H, m, -COO-CH∞ CH∞ ),
1.75–1.85 (4H, m, -OCH CH ), 3.7 (2H, s, Th-CH COO),
4.04 (4H, t, J=7.6 Hz, -OCH CH , -OCH∞ CH∞ ), 4.13 (2H,
t, J=7.2 Hz, -COOCH∞ CH∞ CH ), 6.98 (4H, d, J=8.7 Hz,
3-H, 5-H, 3∞-H, 5∞-H), 7.05 (1H, d, J=5.7 Hz, Th-4-H), 7.16
(1H, s, Th-2-H), 7.28 (1H, d, J=5.7 Hz, Th-5-H), 7.89 (4H,
d, J=8.7 Hz, Be-2-H, 6-H, 2∞-H, 6∞-H). IR (KBr, cm−1) 2980,
2940, 2880, 1730, 1600, 1580, 1490, 1250, 1070, 1030. Anal.
2
2
3
2
2
2
2
2
2
2
2
2
2
2
2
Calcd. for C H SN O : C, 68.01; H, 6.88; S, 6.48; N, 5.67.
28 34 2 4
Found: C, 68.25; H, 6.77; S, 6.46; N, 5.77%.
Poly[6-{4-(4-butoxyphenyldiazenyl)phenoxy}hexyl (3-thien-
yl)ethanoate] (PT6). PT6 was synthesized following the pro-
cedure described for PT0 using 1.00 g (2.01 mmol) of 2 and
1.29 g (8.01 mmol) of FeCl . Pure PT6 was obtained as a
It is well known that when thiophenes are polymerized by
3
brown solid (0.057 g, 5.7% yield). 1H-NMR (400 MHz,
the chemical oxidation coupling method with FeCl , polymers
3
CDCl ) d 0.94 (3H, t, J=6.9 Hz, -CH ), 1.20–1.80 (12H, m, -
obtained contain both regular (H–H linkage) and irregular
3
2 6
3
(CH ) CH ), 3.60 (1H, br, Th-CH COO, for Head-to-Head),
(H–T linkage) polymers.28–30 The irregular parts are more
soluble than the regular ones, and the irregular parts are
removed during work-up. Yamamoto and co-workers have
demonstrated that 2,5-disubstituted-3-alkylpolythiophenes
have characteristic IR absorption peaks due to aromatic C–H
out-of-plane vibrations around 820 cm−1.31 In this study, IR
spectra of the polymers have also demonstrated that PT6 is
linked at the 2,5-positions of the thiophene rings.
3
2
3.70 (1H, br, Th-CH COO, for Head-to-Tail), 3.86 (2H, br, -
2
OCH∞ CH∞ ), 3.97 (2H, br, -OCH CH ), 4.13 (2H, br,
2
2
2
2
7.2 Hz, -COO-CH∞ CH ), 6.84 (2H, d, J=8.5 Hz, Be-3∞-H, 5∞-
2
2
H), 6.98 (2H, d, J=8.7 Hz, Be-3-H, 5-H), 7.16 (1H, br, Th-
4-H), 7.82 (4H, d, J=8.5 Hz, Be-2-H, 6-H, 2∞-H, 6∞-H). IR
(film) 3090, 3050, 2920, 1750, 1625, 1600, 1540, 1250, 1150,
820. Anal. Calcd for (C H SN O ) : C, 68.29; H, 6.50; S,
28 32 2 4 n
6.50; N, 5.69. Found: C, 68.05; H, 6.80; S, 6.76; N, 5.97%. T
g
DSC measurements revealed that the glass transition
<−100 °C; M =2.3×104 (CHCl ; polystyrene standards);
temperature (T ) of PT6 is below −100 °C, and is lower than
n
3
g
M /M =5.1.
that for PT0 (30 °C). It has been reported that T decreased
w
n
g
when the 3-position of the thiophene ring was substituted with
a long alkoxyphenyl group.32,33 In general, the T of polymers
Measurements of photoisomerization and conductivity
g
with longer conjugation is expected to be higher than those
The sample solution was irradiated with light at 366 nm, which
was isolated from a 500 W high-pressure mercury lamp by the
use of glass filters (Toshiba UV-35+UV-D36A). The photo-
isomerization was followed spectroscopically with a Hitachi
UV-320 spectrometer and a Shimadzu UV-200 spectrometer.
Fig. 1 shows the experimental setup used for the observation
of change in conductivity induced by photoirradiation. The
sample film was prepared by casting the polymer solution in
with shorter conjugation because of the increased stiffness of
the polymer backbone. From this argument, the very low
value of T of PT6 may be due to six methylene spacers
g
incorporated in the side chain which decrease rigidity of
thiophene units. In other words, PT6 may be a highly
amorphous material.
Photoisomerization of PT6 in solution
Fig. 3 shows the change in absorption spectra of PT6 in CHCl
(3.3×10−5 M) after photoirradiation at 366 nm at room tem-
3
Fig. 1 Schematic representation of the apparatus used for the
conductivity measurement of PT6 on photoirradiation. The resistance
meter was linked so that the area of the thin film (1 cm×1 cm square)
which was irradiated at 366 nm could be in conduction.
Fig. 2 1H-NMR spectrum of PT6 in the region of the methylene
protons directly attached to the thiophene ring.
J. Mater. Chem., 1999, 9, 2215–2219
2217

Fig. 4 Absorption spectra of thin film of PT6 (A), before irradiation;
(B), after irradiation at 366 nm; (C), after annealing at 70 °C for 5 h;
(D), after annealing at 70 °C for 9 h. The conductivity of PT6 was
measured in the dark at room temperature.
Fig. 3 Change in absorption spectrum of PT6 in chloroform. The
sample solution was left in the dark at room temperature after
photoirradiation at 366 nm. [PT6]=3.3×10−5 M (based on
monomer unit).
PT6 is due to the isomerization of the azobenzene moieties,
the same measurement was performed using a-terthienyl (aT3)
without the azobenzene moiety. As shown in Fig. 5, photo-
irradiation at 366 nm caused no change in the spectrum nor
in the conductivity (8.3×10−7 S cm−1, 120 GV) of a thin film
of aT3 (thickness, 400 nm). Therefore, the change of the
conductivity in PT6 resulted from the isomerization of the
azobenzene moieties.
It might be possible that the change in the conductivity is
due to release of strain in the sample film associated with the
change in molecular shape of the azobenzene moieties on
photoisomerization. To see if this is the case, the thin film of
PT6 was annealed at 70 °C for 10 h in order to release the
strain before photoirradiation (Fig. 6). Annealing before
irradiation caused no change in the absorption spectra and
the conductivity within the experimental error. When this
annealed film was irradiated at 366 nm, the conductivity
increased in a similar manner to Fig. 4, although the absolute
value of conductivity after irradiation was smaller than that
in Fig. 4, and when the film was left in the dark at the same
temperature for 6 h, the conductivity was restored to the initial
value. From these results, it is clear that the change in
conductivity of PT6 was induced by isomerization of the
azobenzene moieties, not by the release of strain of the
polymer film.
perature (24 °C). The absorption at 370 nm is due to the p–p*
transition of the azobenzene moieties in PT6. Taking into
account the literature data,34,35 the broad band around 450 nm
can be assigned to the sum of the absorption due to the n–p*
transition of the azobenzene moiety and the p–p* transition
of the p-conjugation of the polythiophene backbone, which
suggests a short p-conjugation length in PT6. When PT6 was
irradiated at 366 nm in CHCl , the absorbance at 370 nm
3
decreased and that at 470 nm increased. This means that
trans–cis isomerization of the azobenzene moieties was induced
on photoirradiation. After photoirradiation, the sample solu-
tion was kept in the dark at room temperature. It was found
that the absorbance at 370 nm increased and that at 470 nm
decreased as shown in Fig. 3, indicating that cis–trans isomeriz-
ation took place thermally, and after 6 h the absorption
spectrum returned to the initial one.
Effect of photoirradiation on conductivity of PT6
Since the extinction coefficient of the p–p* transition of the
azobenzene chromophore at 366 nm is very large (>3×104,
Fig. 3), photons are absorbed entirely in the surface region of
less than 1 mm. This means that thin films with thickness less
than 1 mm are needed to bring about the photochemical
reaction completely across the films. Using the setup shown
in Fig. 1, a thin film of PT6 (thickness, 300 nm) which was
connected to the resistance meter, was irradiated at 366 nm,
and the resistance was measured. The conductivity (s/S cm−1)
was calculated by the following equation [eqn. (1)],
Mechanism of photoinduced change in conduction of PT6
Conductivity (s) is expressed by eqn. (2)
s=nem
(2)
s=(1/R)×(l/U)
(1)
where R is resistance/V, l is length of the film, and U is the
conducting area (3.0×10−5 cm2). Fig. 4 shows the absorption
spectra of a thin film of PT6 annealed at 70 °C after photoirrad-
iation, as well as the values of conductivity. Photoirradiation
at 366 nm brought about a decrease of the absorbance at
380 nm and an increase at 550 nm, while the conductivity of
the thin film was 1.1×10−7 S cm−1 (300 GV) before
irradiation and 8.3×10−7 S cm−1 (40 GV) after photoirradi-
ation at 366 nm (Fig. 4B). After annealing at 70 °C for 5 h,
the conductivity of the thin film changed to 3.3×10−7 S cm−1
(120 GV) along with the increase of the absorbance in the UV
region and the decrease in the longer wavelength region
(Fig. 4C). After annealing at the same temperature for 9 h,
the conductivity returned to its initial value (Fig. 4D). These
results indicate that the conductivity of the polythiophene can
be controlled reversibly by the isomerization of the azobenzene
chromophores covalently attached to the thiophene rings.
In order to confirm that the change in the conductivity of
Fig. 5 Absorption spectra of a thin film of aT3 (A), before irradiation;
(B), after irradiation at 366 nm.
2218
J. Mater. Chem., 1999, 9, 2215–2219

but also on the formation of the charge transfer complex.
From the results obtained in the present study, photorespons-
ive polythiophene derivatives are promising materials in view
of the reversible control of electrical properties by light.
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Fig. 6 Absorption spectra of a thin film of PT6 (A), before irradiation;
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where n is the number of carriers, e is the charge of carriers,
and m is the mobility of carriers. PT6 is a very amorphous
material as discussed above, and in such amorphous materials,
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complex, was observed before and after photoirradiation. On
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increase in absorbance around 530 nm. This result means the
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number and the mobility of carriers.
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Conclusions
We propose here two hypotheses to interpret the observed
phenomenon. One is the conformational change of the polymer
backbone. The properties of cis-azobenzene are very different
from those of trans-azobenzene, e.g., the polarity of cis-
azobenzene is much higher than that of trans-azobenzene.
When properties of a polymer matrix such as density are
changed due to trans–cis photoisomerization, the coplanarity
of the thiophene rings may be increased. The other hypothesis
is the formation of a charge transfer complex. It may be
possible that a charge transfer complex is formed between cis-
azobenzene as an electron-withdrawing group and the thio-
phene ring as an electron-donating group, which results in a
similar effect to doping and enhances the conductivity with an
increase in the concentration of the cis form. Although it is
not clear at the present stage of research which hypothesis is
correct, our focus is not only on the conformational change
Paper 9/02596K
J. Mater. Chem., 1999, 9, 2215–2219
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