On the origin of optical activity in polythiophenes

Article abstract of DOI:10.1016/S0022-2860(99)00444-5

Optically active substituents attached as sidechain to π-conjugated polymers provide a particularly interesting and revealing tool to study the nanoscopic and mesoscopic supramolecular organization of π-conjugated polymers in general. Chiral substituents allow the properties of these polymers to be studied with circularly polarized light in both absorption and emission. Here we describe a comprehensive investigation on the structure and optical properties of poly{3,4-bis[(S)-2-methylbutoxy]thiophene} (PBMBT) as an example to elucidate the molecular origin of the chiroptical effects in chiral conjugated polymers. The chiral side chains of PBMBT induce very strong bisignate circular dichroism (CD) effects in the π-π* absorption when the polymer is in an aggregated phase. The anisotropy factor in absorption (g(abs)) is as high as 8 x 10^-2. The photoluminescence of PBMBT aggregates is characterized by a very small stokes shift (<90 meV) and exhibits circular polarization (CPL). The chiroptical properties in absorption and emission of PBMBT are associated with a high degree of intrachain and interchain order as they are present in aggregated phases but absent when the polymer chains are molecularly dissolved or in a melt. The observed CD and CPL effects are consistent with an exciton splitting between transition dipole moments in the excited state on different polymer chains. The strong optical activity associated with the π-π* transition of PBMBT is attributed to originate from a helical packing of predominantly planar chains, rather than from a helical intrachain conformation. As a result a detailed molecular description of the polythiophene morphology is obtained. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0022-2860(99)00444-5

MOLSTR 11237
Journal of Molecular Structure 521 (2000) 285–301
www.elsevier.nl/locate/molstruc
ଝ
On the origin of optical activity in polythiophenes
*
B.M.W. Langeveld-Voss, R.A.J. Janssen , E.W. Meijer
Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven,
The Netherlands
Received 24 March 1999; accepted 19 April 1999
Abstract
Optically active substituents attached as sidechain to p-conjugated polymers provide a particularly interesting and revealing
tool to study the nanoscopic and mesoscopic supramolecular organization of p-conjugated polymers in general. Chiral substi-
tuents allow the properties of these polymers to be studied with circularly polarized light in both absorption and emission. Here
we describe a comprehensive investigation on the structure and optical properties of poly{3,4-bis[(S)-2-methylbutoxy]thio-
phene} (PBMBT) as an example to elucidate the molecular origin of the chiroptical effects in chiral conjugated polymers. The
ء
chiral side chains of PBMBT induce very strong bisignate circular dichroism (CD) effects in the p–p absorption when the
Ϫ2
polymer is in an aggregated phase. The anisotropy factor in absorption (gabs) is as high as 8 × 10 : The photoluminescence of
PBMBT aggregates is characterized by a very small Stokes shift (Ͻ90 meV) and exhibits circular polarization (CPL). The
chiroptical properties in absorption and emission of PBMBT are associated with a high degree of intrachain and interchain order
as they are present in aggregated phases but absent when the polymer chains are molecularly dissolved or in a melt. The
observed CD and CPL effects are consistent with an exciton splitting between transition dipole moments in the excited state on
ء
different polymer chains. The strong optical activity associated with the p–p transition of PBMBT is attributed to originate
from a helical packing of predominantly planar chains, rather than from a helical intrachain conformation. As a result a detailed
molecular description of the polythiophene morphology is obtained. ᭧ 2000 Elsevier Science B.V. All rights reserved.
Keywords: Conjugated polymers; Polythiophenes; Chirality; Circular dichroism
1
. Introduction
on the nature and radiative properties of excited states
in p-conjugated polymers. Upon going from an isolated
polymer chain to a macroscopic material, optical prop-
erties will be modified as a result of the intermolecular
interactions that appear in solids. Strong interchain
interactions may result in delocalization of the excited
state over more than one chain and the formationof low-
energy non-radiative states [2–7]. Professor Zerbi and
coworkers showed vibrational spectroscopy as being a
very rich source of information on the structure and
properties of conjugated oligomers and polymers, if an
attempt is made to go beyond simple diagnosis based on
chemical correlation [8]. As a tribute to his pioneering
work we like to use chiroptical techniques to gain
The interesting electrical and optical properties of
semiconducting polymers critically depend on the
conformation of the p-conjugated chains in both the
ground and the excited state and on the interchain
orientation [1]. Several recent reports demonstrate
that chain packing effects exert a considerable effect
ଝ
In honour of Professor Giuseppe Zerbi on the occasion of his
6
*
2
5th birthday.
Corresponding author. Tel.: ϩ31-40-247-3597; fax: ϩ31-40-
45-1036.
E-mail address: r.a.j.janssen@tue.nl (R.A.J. Janssen).
0
022-2860/00/$ - see front matter ᭧ 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(99)00444-5

2
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B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
Fig. 1. Optically active regioirregular [10–17] and regioregular [24–27] polythiophenes.
detailed information on these materials at the meso-
scopic level to complement the insights gained from
vibrational and Raman spectroscopy.
chiroptical techniques such as optical rotatory disper-
sion (ORD), circular dichroism (CD) spectroscopy,
and circular polarization of luminescence (CPL).
These techniques are well accepted for probing the
conformation of ground and excited state, and have
been extensively used to obtain information on the
tertiary structure of biopolymers [57–60].
Fig. 1 shows examples of chiral polythiophenes that
have been reported in the literature up to date. In the
initial accounts on optically active regioirregular poly-
The synthesis, chiroptical properties, and perspec-
tives for application of optically active p-conjugated
polymers have recently been reviewed by Pu [9].
Chiral conjugated polymers have been investigated
for various applications, e.g. membranes, catalysis,
electroactive chiral surfaces, imprinting and chroma-
tography [10–23]. It is well established that p-conju-
gated polymers with enantiomerically pure side
chains or chiral dopants exhibit optical activity asso-
thiophenes specific optical rotations as high as ‰aŠ ˆ
D
3000 have been reported [11]. In 1994, however, a
comparison of the chiroptical properties of regio-
irregular and regioregular poly{3[2((S)-2-methyl-
butoxy)ethyl]thiophene} revealed ‰aŠ513 ˆ 5000 for
ء
ciated with the p–p absorption band [10–17,22–56].
By introducing chirality, the structural characteristics
of the polymer conformation can be assessed with
Fig. 2. Helical organization of polythiophenes. A: Helical transoid (large pitch). B: Helical cisoid (small pitch). C: Helical packing of
predominantly planar chains.

B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
287
Scheme 1.
the regioirregular polymer, while the regioregular
lamellae of straight chains [64,65]. Hence, in trying to
rationalize the high optical activity of chiral p-conju-
gated polymers in terms of molecular structure, the
chiral organization of stiff, predominantly planar, poly-
thiophene chains as is shown in Fig. 2C should be
considered in addition to the helical intrachain organi-
zations. Such a helical organization of polymer chains
bears a close resemblance to cholesteric liquid crystals.
Here, the structural and (chir)optical properties of
optically active poly{3,4-bis[(S)-2-methylbutoxy]-
thiophene} (PBMBT, Scheme 1) are described. The
aggregated phase of this polymer is optically active
and highly ordered as a result of the stereoregular
structure of the side chains. With CD direct spectro-
scopic evidence is obtained for a splitting of the
excited state of PBMBT in two exciton levels when
the polymer is in an aggregated phase. This splitting,
which is not detected in absorption or emission spec-
troscopy, gives strong support for an interchain origin
of the optical activity.
isomer exhibited ‰aŠ5 ˆ 140 000 [24]. Apparently,
13
a high degree of intrachain order gives rise to an
enhancement of the optical activity by more than
one order of magnitude. It is important to note that
ء
the optical activity in the p–p transition is not
observed in good solvents or at high temperatures,
but only in poor solvents or in solid films at ambient
temperatures. Apparently, the presence of strong
optical activity is associated with an aggregated,
possibly well-ordered, state.
Although very strong chiroptical effects have been
observed for chiral conjugated polymers, their precise
nature and molecular origin have been less clear and
remain matter of discussion. The possibility that poly-
thiophenes adopt a helical conformation has been
raised on several occasions. Theoretical calculations
by Cui and Kertesz [61] predict that for methyl-substi-
tuted polythiophenes a helical conformation is most
stable, with only marginal energy differences between
the transoid and the cisoid helix (Fig. 2A and B).
Experimental evidence to support this theoretical
prediction is, however, scarce. Evidence for helical
polymers has been claimed by Garnier from electron
diffraction data on doped substituted polythiophenes
in the solid state [62]. Scanning tunneling microscopy
2
. Results and discussion
2.1. Synthesis
(
STM) studies on individual, single chains of substi-
The detailed synthesis of PBMBT is outlined in
Scheme 1. Thiophene is fully brominated using
bromine [66,67]. Tetrabromothiophene is selectively
debrominated at the a-positions with n-BuLi in
diethyl ether at 0ЊC, yielding 3,4-dibromothiophene.
The remaining bromines are exchanged for methoxy
groups in a methanol/sodium methoxide medium,
assisted by CuO and a catalytic amount of KI in
tuted polythiophenes on graphite substrates have been
performed and interpreted as resulting from a variety
of chain conformations differing from straight and
flat, to curled, and even helical [63]. B a¨ uerle used
STM to study the organization of sexidecithiophene
with eight dodecyl b-substituents on graphite surfaces
and reported ensembles of molecules, as ordered

2
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B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
described for poly(3-alkylthiophene)s [71]. The
mixture is stirred for 4–16 h at room temperature.
The polymer is obtained in the doped state. Dedoping
proceeds by precipitation of the reaction mixture in
methanol/25% ammonia (v/v, 4/1). Salts and short
oligomers are washed out by Soxhlet extractions
with methanol/25% ammonia and hexane, respec-
tively. The polymer is isolated by Soxhlet extraction
with chloroform. PBMBT typically exhibits a mole-
cular weight Mw of 15.4 kg/mol (polydispersity
1
13
D ˆ M =M ˆ 3:5). H- and C-NMR spectroscopy
w
n
reveal the desired 2,5-polymerization without any
observable irregularities. The purification procedure
of the poly(3,4-dialkoxythiophene)s with a series of
Soxhlet extractions is a tool to separate the polymer-
ization mixture in a number of fractions that differ in
molecular weight and have a reasonable polydisper-
sity. Since the flexible substituents are responsible for
the solubility of the polythiophenes, the specifications
of e.g. the chloroform Soxhlet extract differ signifi-
cantly per side chain. Moreover, the polymerization of
3,4-bis(hexyloxy)thiophene or 3,4-bis(decyloxy)thio-
phene, i.e. thiophenes with long and/or linear alkoxy
chains, yields polymers with a strong tendency to
aggregate. This implies that yield and structural para-
meters of these polymers are difficult to compare.
Fig. 3. Cyclic voltammogram of PBMBT on a platinum electrode,
6
recorded at different rates in acetonitrile containing 0.1 M NBu PF .
4
81% yield [68]. 3,4-Dimethoxythiophene is a versatile
synthon to prepare a variety of monomeric 3,4-
dialkoxythiophenes [69] and 3,4-bis(alkylthio)thio-
phenes [70]. Treatment of this intermediate with
2.2 equivalents of (S)-(Ϫ)-2-methylbutanol under
slightly acidic conditions ( p-toluenesulfonic acid) in
toluene at 80ЊC under removal of methanol gives 3,4-
bis[(S)-2-methylbutoxy]thiophene in 79% yield after
distillation [69]. This transetherification reaction is
restricted to primary alcohols or thiols. Apart from
linear and branched monoalcohols (or monothiols),
diols, e.g. ethyleneglycol and 2,2-dimethylpropane-
diol, can be introduced.
2
.2. Redox properties
The cyclic voltammogram of 3,4-bis[(S)-2-methyl-
butoxy]thiophene in dichloromethane exhibits an irre-
versible oxidation at 1.44 V vs. SCE, similar to the
value reported for 3,4-ethylenedioxythiophene
(1.51 V vs. SCE) [72]. Fig. 3 shows the cyclic volt-
ammogram of a thin film of PBMBT on a platinum
electrode. The maximum of the oxidation wave is
found at 0.59 V vs. SCE, the back reduction at
The introduction of two identical b-substituents on
thiophene results in monomers with a C -symmetry
2
axis, eliminating the asymmetry of 3-substituted thio-
phenes. As a result, regio-regular polymers can be
obtained by oxidative coupling with iron(III) chloride.
In a typical polymerization reaction a concentrated
solution of 3,4-dialkoxythiophene monomer is added
to a suspension of 1–4 equivalents of iron(III)
0
.50 V. The value for poly(3,4-ethylenedioxythio-
phene) (ϩ0.04 V vs. SCE) [72] is substantially
below the value for PBMBT. This difference may be
explained by an out-of-plane orientation of the 2-
methylbutoxy side chains in PBMBT as suggested
from the X-ray crystallographic structure of TMB-
BT (see Section 2.5).
1
chloride in chloroform in an argon atmosphere, as
1
2.3. Thermal analysis: TGA and DSC
3
Iron(III) chloride (FeCl ) is very hygroscopic, therefore anhy-
drous FeCl
3
is used which is stored and weighed in an inert-atmo-
Thermal gravimetric analysis (TGA) and differen-
tial scanning calorimetry (DSC) were used to
sphere glovebox to improve on the reproduciblity of the
polymerization reactions.

B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
289
Fig. 4. TGA (a) and DSC (b) of PBMBT.
characterize the weight-loss and phase behavior of
PBMBT as a function of temperature (Fig. 4). TGA
performed in air atmosphere, recorded with 20ЊC/min,
reveals that PBMBT is stable up to ca 225ЊC. Above
between 130 and 108ЊC upon cooling. The position of
the endothermic peak is hardly dependent on the
heating rate. The exothermic transition, however, is
observed between 134 and 118ЊC for a cooling rate of
10ЊC/min and between 120 and 91ЊC for a cooling rate
of 50ЊC/min.
225ЊC, PBMBT decomposes in two stages by an
initial loss of the alkyl substituents (ca. 350ЊC).
DSC cycles between 20 and 190ЊC have been repeated
at different rates. In Fig. 4b the second heating/cooling
cycle is shown, measured with 20ЊC/min. The DSC
curve exhibits an endothermic melting peak between
2
.4. Transmission electron microscopy
The microstructure of PBMBT aggregates has been
143 and 163ЊC upon heating, and an exothermic peak
studied with transmission electron microscopy
Fig. 5. TEM pictures of aggregates of PBMBT (a) 10 800 × (b) 28 800 × .

2
90
B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
When the first adopts conformation C, the second
cannot take up conformation D because of the close
contacts that would occur. Disorder in crystals is a
well-known phenomenon and often observed. Espe-
cially the sec-butyl group proves to be susceptible for
this disorder as shown in crystals of isoleucine and
sec-butyl phthalamide [74–76].
The molecular structure of TMB-BT is character-
ized by a planar, s-trans, bithiophene unit that appears
to be conjugated (the C–C bond lengths along the
molecule are 1.355(6)-1.432(6)-1.368(6)-1.461(6)-
˚
Fig. 6. Molecular structure of TMB-BT showing the conformational
disorder in the 3- and 3 -2-methylbutoxy substituents.
1.361(6)-1.428(6)-1.338(6) A). These characteristics
0
corroborate with recently determined X-ray structures
of 2,2 -bithiophene [77], and more specifically with
the 3,3 - and 4,4 -isomers of dimethoxy-2,2 -bithio-
phene [78,79] and bispentyloxy-2,2 -bithiophene
[80], and 3,3 -bis(octyloxy)-2,2 -bithiophene [81]. A
remarkable feature of TMB-BT is the orientation of
the side-chains with respect to the bithiophene unit.
The C(1) carbon of the side chains in 4- and 4 -posi-
tions is almost in the plane of the molecule (dihedral
0
0
0
0
(TEM). Samples have been prepared by evaporating
0
chloroform, chloroform/methanol or 1-decanol solu-
tions of PBMBT (Fig. 5). The whiskers at the edges of
the associates have also been observed for poly(3-
alkylthiophene)s [73]. The size of the aggregates of
PBMBT varies between experiments, showing that
the morphology of PBMBT is rather sensitive towards
sample preparation.
0
0
0
0
0
angles 5–4–O–C(1): Ϫ6.0(6)Њ and 5 –4 –O–C(1):
ϩ6.9(6)Њ, respectively), while the side chains at the
0
3
- and 3 -positions are in an almost perpendicular
2
.5. X-ray structure of TMB-BT
orientation (dihedral angles of ϩ108.4(4) and
Ϫ108.1(5)Њ, respectively). Previous studies invariably
yielded planar orientations of the alkoxy side chains
[78–81].
In order to assess the geometry of PBMBT in more
0
0
detail, the crystallographic structure of 3,4,3 ,4 -tetra-
kis[(S)-2-methylbutoxy]-2,2 -bithiophene (TMB-BT)
0
as a model compound has been determined by X-ray
diffraction at 150 K using Mo Ka radiation. TMB-BT
2
.6. Wide angle X-ray diffraction of PBMBT
crystallizes in a monoclinic space group P2 (no. 4)
1
with cell parameters a ˆ 10:9146ꢀ15†; b ˆ
Fig. 7 shows the X-ray diffraction pattern recorded
ꢀ
using Cu Ka radiation of a powdered sample of
PBMBT, which was prepared by precipitation from a
1-decanol solution by slow cooling. The diffraction
pattern reveals a high degree of crystallinity. The inter-
pretation of the various reflections, representing
1
0:0487ꢀ15†; c ˆ 13:326ꢀ2† A; b ˆ 91:915ꢀ8†Њ; V ˆ
3
ꢀ
1
ꢀ
460:7ꢀ4† A and two molecules in the unit cell
2
Z ˆ 2†. The TMB-BT molecule has no internal
crystallographic symmetry (Fig. 6).
The two 2-methylbutoxy side chains at the 3- and
-positions show conformational disorder in the
0
˚
3
distances of 9.78, 6.19, 4.90, 4.36, and 3.84 A in terms
˚
crystal, which is not coupled intramolecularly.
When the conformations are labeled with A and C
for the side chain at the 3-position, and B and D for
of crystalline structure isnot trivial. The 3.84 A distance
˚
is closely related to the value of around 3.85 A found in
many poly(3-alkylthiophene)s (P3ATs) and assigned to
the p-stacking distance between the polymer main
chains, in literature denoted as the b-axis [82]. P3ATs
usually exhibit three low-angular sharp h00 reflections
(i.e. corresponding to the crystallographic a-axis), of
which the 100 reflection is most intense. The a-direction
is found to vary strongly with the length of the alkyl side
chain, indicating that the extension of the side chains is
0
the side chain at the 3 -position, then in addition to the
conformations A–B and C–D as shown in Fig. 6, the
combinations A–D and B–C exist. Intermolecularly,
there are restrictions when the side chains at the 3- and
0
3
-positions of different molecules come into contact.
2
wR2 ˆ 0:139; R1 ˆ 0:055; S ˆ 1:04.

B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
291
recently been identified for regioregular poly(3-
octylthiophene) and poly(3-dodecylthiophene) [83].
2
2
.7. Optical spectroscopy
.7.1. Solid phase
Fig. 8a shows the UV/Vis spectrum of a solid film
of PBMBT, as spin-cast from chloroform solution.
Three major bands are observed at 505, 541, and
5
89 nm. These bands are assigned to the vibronic
ء
bands of the first p–p transition [84]. The peak
separation (1320 and 1510 cm , respectively) corre-
Ϫ1
sponds to the CyC vibration of thiophene [84]. The
absorption in the solid phase is assigned to an aggre-
gated, almost coplanar state, of the thiophene moieties
ء
in the polymer. In Fig. 8b optical activity of the p–p
transition is revealed by the appearance of a bisignate
Cotton effect, which exhibits vibronic fine structure.
Fig. 7. X-ray diffraction pattern of PBMBT recorded with Cu Ka.
The degree of circular polarization in absorption g
abs
Ϫ3
essentially along the a-axis. Based on these considera-
tions, indexing the 9.78 and 4.90 A reflections of
is ϩ7 × 10 at l ˆ 597 nm (g ˆ D1=1; where e is
abs
˚
the extinction coefficient and De the difference of the
extinction coefficients of left and right circularly
polarized light).
PBMBT as 100 and 200, respectively, seems straight-
forward. However, this leads to the conclusion that the
˚
length of the a-axis amounts 9.78 A, i.e. considerably
To obtain more information about the orientation of
the dipole transition moment, PBMBT chains have
been stretch-oriented in films of UHMW-PE [73,85].
Fig. 9 shows the linearly polarized absorptions of
PBMBT parallel and perpendicular to the stretching
direction of UHMW-PE with a stretching ratio of 20–
30. The results demonstrate that the absorption occurs
primarily parallel to the stretching direction, which is
expected for a polythiophene in an all-trans confor-
mation. Hence, a cisoid helical orientation as shown
schematically in Fig. 2B is very unlikely for PBMBT.
From the ratio of the parallel and perpendicular
absorption it is possible to determine the angle
between the orientation of the transition dipole
moment and the direction in which the film was
˚
less than the values of 12.4–13.2 A reported for poly(3-
3
butylthiophene), carrying a shorter side chain [82].
Such a low value can only be explained by assuming
that the alkoxy side chains are strongly tilted with
respect to the plane of the main chain. This suggestion
issupportedbytheorientationofthealkoxychainsatthe
0
3
- and 3 -positions of TMB-BT (Section 2.5). The
˚
reflections corresponding to 6.19 and 4.36 A remain
unaccounted for. It is not likely that these are related
tothecrystallographicc-axis.Inearlierworks,thec-axis
has not been obtained from a direct observation of
diffraction peaks but was taken along the assumed
˚
straight polymer chains and calculated to be 7.78 A
from the dimension of 2,2 -bithiophene. A tentative
0
4
explanation for the remaining reflections is the possibi-
lity that a mixture of two different crystal structures of
PBMBT is present. Multiple crystal structures have
stretched. The minimal value of OD =OD was
0
Ќ
k
.11 at 604 nm. This results in an angle of 9.7Њ. As
the polymer might not be completely oriented in the
stretching direction, this angle puts an upper limit to
the angle between the transition dipole moment and
the polymer axis, because of a spread in orientations
and non-fully trans chains. It is important to note that
also in the blends PBMBT is not present as a
3
An independent study of the wide angle X-ray diffraction of
poly{3,4-bis(decyloxy) thiophene} gave three consecutive h00
˚
reflections at 31.5, 16.2, and 10.8 A, which would give a ˆ
ꢀ
˚
3
1:5 A; i.e. longer than the values of 23.4–27.0 A reported for
4
poly(dodecylthiophene), see Ref. [82].
a ˆ arctan‰ꢀp=2† × ꢀOD =OD †Š.
Ќ
k

2
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B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
Fig. 8. UV/Vis absorption (a) and CD (b) spectrum of a PBMBT film, spin-cast from chloroform solution.
molecularly dissolved chain, but rather as microcrys-
tallites or aggregates.
by unfolding at lower temperatures, giving a red-
shifted absorption. Second, in concert with the
unfolding, an aggregation process, which gives rise
to new vibronic bands.
The CD spectra (Fig. 10b) show that the ordered
phase, which develops upon cooling, is optically
active. The thermochromic behavior is accompanied
by the appearance of a very strong, bisignate Cotton
effect with pronounced vibronic structure. The degree
2.7.2. Thermochromism and solvatochromism
The temperature dependence of the absorption of
PBMBT in 1-decanol (Fig. 10a) reveals that the well-
known thermochromism of P3ATs [84,86,87] is also
present in PBMBT. At 393 K, conformationally disor-
dered, molecularly dissolved polymer chains are
present in 1-decanol, with lmax ˆ 438 nm: Cooling
to 293 K affords a highly ordered form of PBMBT
as evidenced from the significant red-shift of the
onset of the absorption and the appearance of well-
resolved vibronic transitions at l ˆ 497; 531 and
581 nm. This ordered phase of substituted polythio-
phenes is usually associated with the formation of
small aggregates or microcrystallites with interchain
5
interactions [86–89]. Under the conditions of the
thermochromic experiment the aggregates have
become rather large and some scattering can be
observed in Fig. 10a. The scattering slightly distorts
the absorption spectrum of PBMBT and prevents the
observation of a clear isosbestic point. The absence of
a clear isosbestic point can also be explained by the
possibility that two different processes occur simulta-
neously. First a planarization of the polymer backbone
5
The discussions presented here do not distinguish interchain
interactions between different polymer chains from those between
two parts of a single, but folded, polymer chain.
Fig. 9. UV/Vis absorption spectra of PBMBT in stretched films of
UHMW–PE.

B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
293
Fig. 10. UV/Vis absorption (a) and CD (b) spectra of PBMBT in 1-decanol at different temperatures.
of circular polarization in absorption, as measured
from the anisotropy factor, is unsurpassed high for a
seem to intersect at a single point is consistent with
the proposition that a combination of planarization
and aggregation occurs, since planarization is not
expected to result in a CD effect.
ء
Ϫ2
p–p transition; typically gabs ˆ ϩ2 × 10 : The
actual value of gabs critically depends on the sample
(
molecular weight and polydispersity) and the sample
A similar transition from a well-dissolved and
conformationally disordered polymer to an ordered
form can be accomplished by varying the nature of
the solvent (solvatochromism). Fig. 11 shows the
preparation: medium, temperature, cooling rate and
aggregation time. The largest gabs value recorded for
PBMBT is ϩ8 × 10 : The fact that all CD spectra
Ϫ2
3
Fig. 11. UV/Vis absorption and CD spectra of PBMBT in various CHCl /MeOH mixtures at room temperature.

2
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B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
Fig. 12. The ORD (solid line) and first derivative of the CD spec-
trum (dashed line, arbitrary units) of PBMBT in 1-decanol at room
temperature.
Fig. 13. Emission and excitation spectra of PBMBT in 1-decanol at
2
3
93 K (solid lines, right axis, lex ˆ 540 nm; l ˆ 660 nm) and at
em
83 K (dashed lines, left axis, lex ˆ 450 nm; l ˆ 580 nm). The
em
inset shows the circularly polarized luminescence spectrum (B) at
evolution of the absorption and CD upon addition of
methanol as a poor solvent to a solution of PBMBT in
a good solvent such as chloroform (lmax ˆ 484 nm).
The increase of the polarity of the solvent results in a
significant solvatochromism featuring well-resolved
and optically active vibronic bands at l ˆ 503; 538,
and 589 nm. In contrast to the thermochromism the
solvatochromic experiment does reveal a clear
isosbestic point. This is consistent with the model
proposed for the thermochromic experiments, because
the first step, i.e. planarization is less likely to occur in
a solvatochromic experiment at constant temperature.
Contrary to earlier reports on poly(3,4-dialkoxythio-
phene)s, these results, together with the X-ray data
presented in Section 2.6, demonstrate that the two
alkoxy substituents in PBMBT do not prevent the
formation of a well-ordered phase [88].
2
93 K of PBMBT recorded with excitation at 540 nm.
‰aŠ ˆ 98 000; whereas at the sodium D-line
530
‰aŠ5 ˆ Ϫ3000 only.
89
2.7.4. Photoluminescence
The formation of an ordered phase of PBMBT also
results in significant changes in photoluminescence
(Fig. 13). The emission of the free chains (1-decanol,
383 K, excitation at lex ˆ 450 nm† is strong, with
lmax ˆ 558 nm: The excitation spectrum (emission
recorded at l ˆ 580 nm† closely follows the
em
absorption spectrum of free PBMBT chains. The
emission of the aggregated phase, recorded at
293 K, is weaker by one order of magnitude, but exhi-
bits a well-resolved vibronically structured emission
at l ˆ 606; 660, and ca. 730 nm when using excita-
tion light with l Ͼ 530 nm: When the sample is
ex
excited at lex ˆ 450 nm; some remaining emission
of the disordered chains is observed at shorter wave-
lengths (lem ˆ 553 nm). The excitation spectrum
(lem ˆ 660 nm) of the structured long-wavelength
emission resembles the absorption spectrum of the
aggregates with peaks at ca. 500, 532, and 580 nm.
2
.7.3. Optical rotatory dispersion
The ORD spectrum of PBMBT in 1-decanol is
shown in Fig. 12. Like the g -value (as derived
abs
from CD spectroscopy), the optical rotation is
strongly wavelength dependent. The shape of the
ORD is similar to the first derivative of the CD spec-
trum [58]. Maximum rotation is found at l ˆ 530 nm;
The Stokes shift (E ) between 0–0 transitions of
S
absorption and emission in 1-decanol is less than

B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
295
2
ꢀI Ϫ I †=ꢀI ϩ I †; shown in Fig. 13, has the same
L
R
L
R
sign as g on the high wavelength side. The magni-
abs
Ϫ3
tude of glum is on the order of ꢀϩ5:0 ^ 1:0† × 10 ; a
factor of four smaller than in absorption. The cause of
this difference is at present unclear, but might be
related to small conformational changes (quinoid
form) upon photoexcitation or to emission from
polymer chains other than those excited initially as a
8
result of energy migration in the aggregates.
2
.8. Davydov splitting
In Section 2.7 a strong optical activity has been
established for PBMBT in a solid film or as aggregates
in solution. The molecular origin of the optical
ء
activity associated with the p–p transition has not
been addressed until now. In Fig. 2, the three basically
different types of helical organizations of polythio-
phenes that could be present, have been drawn sche-
matically. The experimental results on PBMBT in
stretch oriented UHMW-PE (Fig. 9) rule out that
PBMBT adopts a cisoid helical conformation. For a
transoid helical conformation, the CD spectrum
should theoretically consist of a positive or a negative
Fig. 14. Normalized absorption (dashed line, left axis), first deriva-
tive of the absorption (dotted line, left axis), and CD spectrum (solid
line, right axis) of PBMBT in dichloromethane at 243 K.
90 meV, indicating a small structural reorganization
in the excited state. The small Stokes shift is close to
the ones observed for other highly ordered polymers
ء
0
CD effect at the p–p transition, but not a bisignate
such as gel-drawn blends of poly[2-methoxy-5-(2 -
CD effect as is seen in Figs. 10 and 11 [58]. Such
bisignate CD spectra originate from two close-lying
states having rotational strengths of opposite sign and
approximately equal amplitudes. Such typical CD
effects are often observed when exciton-coupled
planar chromophores are present in a chiral orienta-
tion and can be rationalized qualitatively by a clas-
sical coupled oscillator mechanism [91,92]. Theory
ethylhexyloxy)-1,4-phenylenevinylene] in UHMW–
PE and ordered poly( p-phenylene vinylene) films
6
where E ranges from 50 to 70 meV [85,90]. The
S
linear polarization of the fluorescence in the aggre-
7
gated phase is high, plum Ն ϩ0:343; which indicates
that the transition dipole moments in the long-wave-
length absorption and emission are to a large extent
parallel.
In addition to optical activity in absorption, the
photoluminescence of PBMBT in the associated
phase is circularly polarized. The degree of circular
ء
on exciton coupling of p–p states predicts that the
two split levels have identical rotational strengths of
opposite sign but that their dipole strengths differ,
depending on the relative orientation [93]. A helical
packing of predominantly planar chains as shown in
Fig. 2 is qualitatively consistent with the observed CD
spectra and corroborates with the experimental result
that the CD spectra are only observed under condi-
tions where chains assemble into aggregates. The CD
spectra give, however, no indication of the magnitude
polarization in emission, defined as glum
ˆ
6
The confinement parameter g as determined by the ratio of the
energies of the 0–0 transitions in emission and absorption, ꢀv =D †;
p
0
0
Ϫ1
2
is given by g ˆ ꢀv =D † sin ꢀv =D †= 1 Ϫ ꢀv =D † : For PBMBT
0
0
0
0
0
0
g ˆ 4:3; a value larger than previous estimates of g Ϸ 1 reported for
8
0% head-to-tail poly(3-alkylthiophene)s. (B. Xu, S. Holdcroft,
Macromolecules, 26 (1993) 4457.)
7
8
Plum ˆ ꢀI Ϫ Ihor†=ꢀI ϩ Ihor†: Linearly polarized lumines-
For poly{3[2((S)–2–methylbutoxy)ethyl]thiophene}, a chiral
regioregular poly(3-alkylthiophene) [24], there is a good agreement
vert
vert
cence has been measured by the photoselection method, i.e. excita-
tion takes place with a vertically polarized beam, perpendicular to
between circular polarization in absorption and emission: g
ˆ
abs
Ϫ3
Ϫ3
the emission detection. Theoretically Ϫ1/3 (absorption
emission) Ͻ Plum Ͻ ϩ1/2 (absorption k emission).
Ќ
Ϫ10 × 10
and glum ˆ Ϫꢀ8:7 ^ 0:6† × 10
(lex ˆ 570 nm;
l
em
ˆ 660 nm) in 1-decanol at 20ЊC.

2
96
B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
Fig. 15. UV/Vis absorption (a) and CD (b) of PBMBT in 2-MeTHF at different temperatures.
of the exciton coupling, also referred to as the
Davydov splitting. In an attempt to determine the
strength of the exciton coupling, PBMBT was studied
in dichloromethane and 2-methyltetrahydrofuran (2-
MeTHF) at low temperatures, where spectra are
generally better resolved.
width-at-half-maximum of the first vibronic transition
at 243 K (ca 70 meV, Fig. 14) serves as an upper limit
to the energy separation between these exciton levels.
The oscillator strengths of the optical transitions to
these exciton levels strongly depend on the mutual
orientation of the transition dipole moments; for a
parallel orientation, excitation to (and emission
The UV/Vis and CD spectra of PBMBT in dichloro-
methane at 293 K reveal conformationally disordered
1
0
from) the lower level is formally forbidden.
ء
polymer chains with no optical activity in the p–p
Higher resolution was expected in 2-MeTHF,
which forms a rigid glass between 70 and 137 K. In
Fig. 15a the thermochromic behavior of PBMBT in 2-
MeTHF is shown. At 300 K PBMBT is molecularly
transition ꢀl
ˆ 438 nm†: Cooling to 243 K affords
max
the highly ordered phase (well-resolved vibronic tran-
sitions at lmax ˆ 508; 544, and 592 nm, Fig. 14)
which is accompanied by the appearance of a strong,
bisignate, and vibronically structured Cotton effect in
dissolved ꢀl
ˆ 462 nm†: Between 300 and 260 K,
max
l
shifts to lower energies, indicating an increase in
max
9
the CD spectrum. It is found that the CD spectrum
conjugation length via the coplanarization of the thio-
phene rings. Only upon cooling below 260 K aggre-
gation occurs. In the UV/Vis absorption spectrum at
100 K the three principal vibronic bands are split: 493
and 506 (64 meV), 531 and 545 (60 meV), and 575
and 589 (52 meV) (nm). As soon as aggregation starts
at 260 K, a CD effect (Fig. 15b) is observed.
The split vibronic transitions in UV/Vis at 100 K,
however, do not appear as bisignate signals in CD.
Hence, the 50–60 meV splitting cannot be attributed
to the Davydov splitting. The exact nature of this
splitting is unknown, but similar splittings have
previously been assigned to ring deformation modes
[94].
follows the first derivative of the absorption spectrum
(Fig. 14).
Although the Davydov splitting is not resolved, the
resemblance of the bisignate CD spectrum with the
first derivative of the absorption spectrum is in full
agreement with a splitting of the excited state into two
1
0
exciton levels via a Davydov interaction. The full-
9
The CD spectrum is found to be independent of the lateral or
angular orientation with respect to the photomultiplier detector.
1
0
For randomly ordered transition dipole moments, on average the
lower lying level will have lower oscillator strength: R.S. Knox, J.
Phys. Chem., 98 (1994) 7270.

B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
297
3
. Conclusion
nescence is the presence of the exciton coupling as
observed in the CD spectra which results in the forma-
tion of a non-emissive low-energy state. For two
parallel chains with a Davydov interaction the transi-
tions between ground state and the low-energy level
are formally dipole-forbidden.
The results suggest that understanding interchain
interactions is important in the design of highly lumi-
nescent conjugated polymers. The use of chiral semi-
conducting polymers in combination with chiroptical
spectroscopy may aid in developing guidelines for
understanding photoluminescence in the solid state
and for future design of polymer materials for electro-
luminescent devices.
In conclusion, it has been demonstrated that
PBMBT can be obtained in a highly ordered aggre-
gated phase in solution. The optically active ordered
phase of this chiral semiconducting polymer has been
studied in detail using CD and CPL spectroscopy and
was shown to exhibit circularly polarized absorption
(vibronically resolved) and luminescence due to split-
ting of the excited state into two exciton levels.
Despite the fact that it has not been possible to
determine the strength of the Davydov splitting
experimentally, the molecular origin of the strong
CD effect, and especially the large gabs value, is
ascribed to a highly ordered packing of the polythio-
phene chains in a chiral aggregate. Classical theory on
exciton coupling (using a point dipole approximation)
cannot explain high gabs values as are presently found
for PBMBT with long conjugated segments [93].
Therefore, it seems necessary to include either the
effect of a helical main chain conformation or a delo-
calization over more than two chains or chain-frag-
ments into the theory.
The circular polarization in the luminescence of the
aggregated form proves that the chirality in conforma-
tion or packing is not destroyed by photoexcitation.
The relatively weak emission observed from aggre-
gates, as compared to the optically inactive emission
of free chains, is the result of a more efficient radia-
tionless decay in the ordered phase. This is usually
explained by an enhanced mobility of excitations in
the ordered phase, resulting in efficient quenching of
photoluminescence at impurity sites. However, an
alternative explanation for the decreased photolumi-
4
. Experimental procedures
1
1
4.1. General procedures
1
13
The H and C NMR spectra were recorded on a
Varian Gemini 300 spectrometer. Chemical shifts are
reported in ppm downfield from TMS. Elemental
analyses were obtained using a Perkin–Elmer 2400.
Optical spectra were recorded using a Perkin–Elmer
Lambda 900 spectrophotometer for UV/Vis, a Jasco J-
6
00 Spectropolarimeter for CD, a Perkin Elmer
LS50B luminescence spectrometer for fluorescence,
and a home-built instrument for circularly polarized
1
2
luminescence. CPL spectra were measured with the
emission detector in a direction perpendicular to the
excitation beam. To avoid artifacts due to photoselec-
tion, the excitation light was linearly polarized in the
plane spanned by excitation and emission beams. Low
temperature spectra were recorded using an Oxford
Optistat continuous flow cryostat; during measure-
ments the temperature was kept constant within
1
1
ء
Classical dipole–dipole theory on exciton coupling of p–p
states shows that the two split levels have identical rotational
strengths of opposite sign [92]. Within this model, the magnitudes
of the rotational and dipole strengths are determined by the intrinsic
dipole strength of the isolated chromophores and their orientation:
0
.1 K. Spectra at different temperatures are not
corrected for volume changes of the solvent. The X-
ray diffraction pattern was taken using Cu Ka radia-
tion and a multi-wire area detector X-1000 coupled
with a graphite monochromator. Electrochemical
measurements were carried out with 0.1 M tetrabutyl-
ammonium hexafluorophosphate as supporting
^
1
^
1
2
2
R
ˆ ^ꢀ †ps R ·ꢀm × m †; D ˆ ꢀ †ꢀm ^ mb0n† ; where
2
0
ab
a0n
b0n
a0n
s
0
and m0n are the excitation energy and electric transition moment
of transition ͉0͘ ! ͉n͘; and Rab is the distance vector of the chromo-
Ϫ3
phores a and b. The exciton interaction energy scales with R
:
ab
Using this theory, the maximum possible value of gabs ˆ 4R=D ˆ
ps R is independent of the value of the transition dipole moment.
0
ab
Ϫ2
12
A value of gabs ˆ 8 × 10 ; measured at 598 nm, is irreconcilable
For a description of the instrument, including a detailed analysis
ꢀ
with this exciton model as it would lead to R Ϸ 150 A and, hence,
of the possible artifacts in the measuring technique see: R.B.
Rexwinkel, P. Schakel, S.C.J. Meskers, H.P.J.M. Dekkers, Appl.
Spectrosc. 47 (1993) 731.
ab
a vanishing small exciton interaction energy, resulting in a zero net
CD effect.

2
98
B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
1
3
electrolyte using a Potentioscan Wenking POS73
3.86 (s, 6H, OCH3). C NMR (CDCl ): d 147.8
3
potentiostat. The working electrode was a Pt disc
(ThC ); 96.3 (ThC ); 57.5 (OCH ).
3,4-Bis[(S)-2-methylbutoxy]thiophene [69].
3
2
3
2
(
0.2 cm ), the counter electrode was a Pt plate
A
2
(0.5 cm ), and a saturated calomel electrode was
mixture of 1.5 g (10.4 mmol) 3,4-dimethoxythio-
phene, 2.1 g (24 mmol) (S)-(Ϫ)-2-methylbutanol,
0.2 g p-toluenesulfonic acid monohydrate in 30 ml
toluene was heated at 80ЊC during 16 h in an open
flask with a flow of nitrogen. After a toluene/water
extraction, the organic layer was dried (MgSO4),
concentrated and distilled at 88–92ЊC/0.8 mbar or
purified by column chromatography (Al O , hexane).
used as reference electrode, calibrated against a Fc/
ϩ
Fc couple. Transmission electron microscopy was
measured with a Philips TEM 201 microscope
(
60 kV). Samples were prepared from 1-decanol and
shaded with Pt.
2
3
4.2. Synthetic procedures
1
Yield 2.3 g (87%). H NMR (CDCl ): d 6.16 (s, 2H,
3
2
2
Tetrabromothiophene [66,67]. To 84 g (1 mol)
thiophene in 30 ml chloroform was added drop wise,
under ice-cooling, 220 ml (4 mol) bromine, during
ThH
OCH
OCH
2H, CH
2
); 3.85 (dd, J ˆ 9:0 Hz; J ˆ 6:5 Hz; 2H,
2
2
); 3.75 (dd, J ˆ 9:0 Hz; J ˆ 6:5 Hz; 2H,
); 1.95–1.86 (m, 2H, CHCH ); 1.60–1.49 (m,
CH ); 1.43–1.19 (m, 2H, CH
7:0 Hz; 6H, CHCH
2
2
3
3
3
h. The last 65 ml bromine were added without
external cooling. The mixture was then refluxed for
.5 h, after which 150 ml 2 N NaOH solution was
2
3
2
); 1.00 (d, J ˆ
3
); 0.93 (t, J ˆ 7:5 Hz; 6H,
). C NMR (CDCl ): d 148.5 (ThC ); 97.9
); 75.9 (OCH CH); 35.1 (CHCH ); 28.7
CH ); 17.1 (CHCH ); 11.9 (CH CH ). MS:
11): 186; (M Ϫ 2C
3
1
3
3
CH
(ThC
(CH
CH
2
3
3
3
added cautiously and refluxing was continued with
vigorous stirring for another 30 min. The mixture
was transferred to a large beaker and allowed to
cool to room temperature. The crystalline mass was
filtered off and washed copiously with water. After
recrystallization from hot chloroform 311 g (78%)
tetrabromothiophene was obtained as pale yellow
2
2
3
2
3
3
2
ϩ
3
ϩ
ϩ
(M ): 256; (M Ϫ C
H
5
H11):
5
116. ‰aŠ5 ˆ ϩ13:18 (neat).
89
Poly{3,4-bis[(S)-2-methylbutoxy]thiophene} [71].
1.5 g (6 mmol) 3,4-bis[(S)-2-methylbutoxy]thiophene
was added, in an argon atmosphere, to 24 mmol
iron(III) chloride in 40 ml chloroform, and stirred
for 8 h at room temperature. The mixture was preci-
pitated into methanol/25% ammonia (v/v, 4/1). The
precipitate was purified from salts and oligomers by
Soxhlet extractions using methanol/25% ammonia
and hexane respectively. The polymer was isolated
1
3
crystals. C NMR (CDCl ): d116.9 (ThC ); 110.3
3
3
(
ThC ). m.p. 115ЊC (lit. 115.5–116ЊC).
2
3
,4-Dibromothiophene [66,67]. 100 ml 2.5 M n-
BuLi in hexane was added dropwise, under ice-
cooling, to 50 g (0.125 mol) tetrabromothiophene in
2
00 ml dry diethyl ether. The resulting suspension
was poured into ice-water, extracted with diethyl
by Soxhlet extraction with chloroform. Yield 0.47 g
1
ether, dried (MgSO ) and concentrated. The residue
(31%). H NMR (CDCl
3
): d 4.00–3.80 (m, 4H,
); 1.65–1.50 (m,
); 1.30–1.15 (m, 2H, CH CH ); 1.03 (d,
); 0.92 (t, J ˆ 7:5 Hz; 6H,
). C NMR (CDCl ): d 145.6 (ThC ); 115.7
); 78.1 (OCH CH); 35.5 (CHCH ); 26.2
); 16.6 (CHCH ); 11.3 (CH CH ).
4
was distilled (55–56ЊC/1 mbar; lit. 96.5–99ЊC/
OCH
2
); 2.05–1.90 (m, 2H, CHCH
CH
J ˆ 6:6 Hz; 6H, CHCH
3
1
1 mmHg) yielding 19.7 g (65%) 3,4-dibromothio-
2H, CH
2
3
2
3
1
13
3
3
phene. H NMR (CDCl ): d 7.31 (s, 2H, ThH ).
C
3
3
2
1
3
NMR (CDCl ): d 123.8 (ThC ); 114.0 (ThC ).
CH
(ThC
(CH
CH
2
3
3
3
3
3
2
3,4-Dimethoxythiophene [68]. A concentrated solu-
2
2
3
tion of sodium methoxide in methanol was prepared
by adding 16.1 g (0.7 mmol) sodium in small portions
to 225 ml absolute methanol. After addition of
2
CH
3
3
2
3
0
0
0
3,4,3 ,4 -Tetrakis[(S)-2-methylbutoxy]-2,2 -bithio-
phene. To 3.0 g (12 mmol) 3,4-bis[(S)-2-methylbu-
toxy]thiophene in 25 ml THF was added 8 ml
(1.6 M in hexane) n-BuLi at Ϫ70ЊC. After stirring
250 mg KI, 33.9 g (140 mmol) 3,4-dibromothiophene
and 11.2 g (140 mmol) CuO, the mixture was refluxed
for 100 h. The cooled mixture was filtered, diluted
with two volumes of water, extracted with diethyl
ether, dried (MgSO4) and distilled (58–62ЊC/
for 1 h, 1.6 g dry CuCl was added and the mixture
2
was left overnight. The THF was evaporated, the
crude product was isolated after a dichloromethane/
0
.7 mbar). Yield 16.3 g (81%) 3,4-dimethoxythio-
water extraction, drying of the organic layer (MgSO
and concentrating. Column chromatography (SiO ,
2
4
)
1
phene. H NMR (CDCl ): d 6.20 (s, 2H, ThH );
3
2

B.M.W. Langeveld-Voss et al. / Journal of Molecular Structure 521 (2000) 285–301
299
dichloromethane/hexane, v/v, 1/2) and several crys-
tallizations from methanol and ethanol yielded
References
0
0
2
2
51 mg (8%) 3,4,3 ,4 -tetrakis[(S)-2-methylbutoxy]-
[1] For example, see Proceedings of the International Conference
on Science and Technology of Synthetic Metals: (a) Synth.
Met. 69-71 (1995). (b) Synth. Met. 84-86 (1997).
0
,2 -bithiophene as crystals suitable for X-ray crystal-
1
lography. H NMR (CDCl ): d 6.05 (s, 2H, ThH );
3
5
2
3
[2] S.A. Jehneke, J.A. Osaheni, Science 265 (1994) 765.
3
3
3
3
2
.97 (dd, J ˆ 9:0 Hz; J ˆ 6:0 Hz; 2H, OCH );
2
[
3] G. Rumbles, I.D.W. Samuel, L. Magnani, K.A. Murray, A.J.
DeMello, B. Crystall, S.C. Moratti, B.M. Stone, A.B. Holmes,
R.H. Friend, Synth. Met. 76 (1996) 47.
2
3
.87 (dd, J ˆ 9:0 Hz; J ˆ 6:9 Hz; 2H, OCH );
2
2
3
.82 (dd, J ˆ 9:0 Hz; J ˆ 6:0 Hz; 2H, OCH );
2
2
3
.75 (dd, J ˆ 9:0 Hz; J ˆ 6:6 Hz; 2H, OCH );
[4] A. Yassar, G. Horowitz, P. Valat, V. Wintgens, M. Hmyene, F.
Deloffre, P. Srivastava, P. Lang, F. Garnier, J. Phys. Chem. 99
2
.00–1.83 (m, 4H, CHCH ); 1.68–1.50 (m, 4H,
3
(1995) 9155.
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2
3
2
[5] I.D.W. Samuel, G. Rumbles, C. Collison, J. Phys. Rev. B 52
1
3
2
1
4H, 8 CH3). C NMR (CDCl ): d 149.7 (ThC );
3 4
(1995) R11573.
42.2 (ThC ); 117.7 (ThC ); 94.4 (ThC ); 74.6
[6] U. Lemmer, S. Heun, R.F. Mahrt, U. Scherf, M. Hopmeier, U.
Siegner, E.O. G o¨ bel, K. M u¨ llen, H. B a¨ ssler, Chem. Phys. Lett.
3
2
5
(
OCH CH); 35.6 (CHCH ); 34.8 (CHCH ); 26.2
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2
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(
CH CH ); 16.6 (CHCH ); 11.3 (CH CH ). mp
2 3 3 2 3
[
[
9
3.0–93.7ЊC. Anal: Calcd for C H S O : C, 65.84;
28 46 2 4
2
ϩ
H, 9.08. Found: C, 65.78; H, 8.77. MS: (M ):
ϩ
ϩ
510; (M Ϫ 2C H ): 370; (M Ϫ 3C H ): 300;
5 11 5 11
ϩ
(M Ϫ 4C H ): 230. UV/Vis absorption: lmax
5 11
[9] L. Pu, Acta Polym. 48 (1997) 116.
(
3
h2 e3 x na nm e.) ˆ 310 nm, shoulders at 282, 298, and
[
10] M. Lemaire, D. Delabouglise, R. Garreau, J. Roncali, in: S.
Torii (Ed.), Recent Advances in Electroorganic Synthesis,
Elsevier, Amsterdam, 1987, p. 385.
4.3. UHMW–PE drawn blend with PBMBT [85]
[11] M. Lemaire, D. Delabouglise, R. Garreau, A. Guy, J. Roncali,
J. Chem. Soc., Chem. Commun. (1988) 658.
In a flask equipped with mechanic stirrer PBMBT
[12] D. Kotkar, V. Joshi, P.K. Ghosh, J. Chem. Soc., Chem.
Commun. (1988) 917.
(10 mg) was dissolved in xylenes (200 ml). UHMW-
PE (1 g) was added. The mixture was stirred slowly
and placed in an oil bath (120ЊC) until a gel formed.
The hot gel was poured out in an aluminum box. The
box was covered with aluminum foil until the gel was
cold. The foil was removed and some solvent was
allowed to evaporate overnight. Small parts were cut
out the film and were stretched at ca 100ЊC (20–30
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