Synthesis and properties of light-emitting polythiophene derivatives bearing terphenyl mesogenic pendant

Article abstract of DOI:10.1080/15421400903568047

A series of polythiophenes containing terphenyl mesogenic side chain at the third position through ester bonding was synthesized by dehalogenative polycondensation. The thiophene monomers 2,5-bromo-3-{[(4-(4'-cyano) terphenyloxy)carbonyl]-methyl]}-thiophene [M(CN)] and 2,5-bromo-3-{[(4-(4'- methoxy)terphenyloxy)-carbonyl]methyl}-thiophene [M(OCH₃)] were prepared via Suzuki reation and esterification reactions, in sequence. The structures and properties of the monomers and polymers were characterized and evaluated with nuclear magnetic resonance, infrared spectroscopy, thermogravimetry, differential scanning calorimetry, polarized optical microscopy, ultraviolet spectroscopy, and photoluminescence. All of the polymers were stable, losing little of their weights when heated to 300°C. The polymers showed good solubility and could be dissolved in common solvents such as CHCl₃, THF, DMF, etc. The monomers exhibited enantiotropic SmA_d phases with a bilayer arrangement in the heating and cooling processes. Due to short spacer methylene ester between the bulk terphenyl mesogenic side chain and rigid polythiophene main chain, the polymers could not exhibit liquid crystallinty at elevated temperature. The existence of the chromophoric terphenyl core endows the polymers with high photoluminescence. When their CH₂Cl₂ solutions were photoexcited, the polymers emitted a strong UV light about 400nm.

Full text of DOI:10.1080/15421400903568047

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Synthesis and Properties of Light-
Emitting Polythiophene Derivatives
Bearing Terphenyl Mesogenic Pendant
Lie Chen a , Yiwang Chen a , Weihua Zhou a , Fan Li a , Xiaohui He a &
Ben Zhong Tang b
a Institute of Polymers/Institute for Advanced Study, Nanchang
University , Nanchang, China
b Department of Chemistry , The Hong Kong University of Science &
Technology (HKUST), Clear Water Bay , Kowloon, Hong Kong
Published online: 18 Mar 2010.
To cite this article: Lie Chen , Yiwang Chen , Weihua Zhou , Fan Li , Xiaohui He & Ben Zhong
Tang (2010) Synthesis and Properties of Light-Emitting Polythiophene Derivatives Bearing
Terphenyl Mesogenic Pendant, Molecular Crystals and Liquid Crystals, 518:1, 70-83, DOI:
10.1080/15421400903568047
To link to this article: http://dx.doi.org/10.1080/15421400903568047
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Mol. Cryst. Liq. Cryst., Vol. 518: pp. 70–83, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903568047
Synthesis and Properties of Light-Emitting
Polythiophene Derivatives Bearing Terphenyl
Mesogenic Pendant
LIE CHEN,1 YIWANG CHEN,1 WEIHUA ZHOU,1
FAN LI,1 XIAOHUI HE,1 AND BEN ZHONG TANG2
1Institute of Polymers=Institute for Advanced Study, Nanchang
University, Nanchang, China
2Department of Chemistry, The Hong Kong University of Science &
Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong
A series of polythiophenes containing terphenyl mesogenic side chain at the third
position through ester bonding was synthesized by dehalogenative polycondensation.
The thiophene monomers 2,5-bromo-3-{[(4-(40-cyano)terphenyloxy)carbonyl]-
methyl]}-thiophene [M(CN)] and 2,5-bromo-3-{[(4-(40-methoxy)terphenyloxy)-
carbonyl]methyl}-thiophene [M(OCH3)] were prepared via Suzuki reation and
esterification reactions, in sequence. The structures and properties of the monomers
and polymers were characterized and evaluated with nuclear magnetic resonance,
infrared spectroscopy, thermogravimetry, differential scanning calorimetry, polar-
ized optical microscopy, ultraviolet spectroscopy, and photoluminescence. All of
the polymers were stable, losing little of their weights when heated to ꢀ300ꢁC.
The polymers showed good solubility and could be dissolved in common solvents such
as CHCl3, THF, DMF, etc. The monomers exhibited enantiotropic SmAd phases
with a bilayer arrangement in the heating and cooling processes. Due to short spacer
methylene ester between the bulk terphenyl mesogenic side chain and rigid polythio-
phene main chain, the polymers could not exhibit liquid crystallinty at elevated
temperature. The existence of the chromophoric terphenyl core endows the polymers
with high photoluminescence. When their CH2Cl2 solutions were photoexcited, the
polymers emitted a strong UV light about 400 nm.
Keywords Liquid crystallinity; photoluminescence; polythiophene; terphenyl
Introduction
Conjugated polymers have been extensively studied for their potential application
in light-emitting diodes (LEDs), organic lasers, thin film transistors, and solar cells
[1–7]. Among various conjugated polymers, polythiophenes have received consider-
able attention due to their excellent electrical characteristics [8–10]. The polythio-
phene family is well known for its environmental stability, melt and solution
Address correspondence to Yiwang Chen, Institute of Polymers=Institute for Advanced
Study, Nanchang University, Xuefu Rd. 999, Nanchang 330031, China. E-mail: ywchen@
ncu.edu.cn
70

Polythiophene Bearing Terphen Mesogen
71
processability, and versatile synthesis, which allow us to have a significant control
over optical and electronic properties [11].
Conjugated polymers with liquid-crystalline (LC) groups in their side chains are
currently drawing interest from the viewpoint of multifunctional electrical and
optical materials [12–25]. In side chain type of LC-conjugated polymer, the main
chain can be aligned by virtue of spontaneous orientation of the LC side chain.
Besides, macroscopic alignment of LC domains is achieved by an external force such
as shear stress or electric or magnetic field [26]. Particularly, LC polythiophene
derivative is one of the most intriguing polymers, because its electrical and optical
properties are expected to be controlled using molecular orientation of the LC side
chain. A variety of polythiophenes containing liquid-crystalline mesogens and light-
emitting chromophores have been prepared [27–37].
Terphenyls are chemically, electrochemically, and photochemically very stable
[38]. Because the strong p-p interaction among phenyl rings favors the self-
organization of the calamitic units, they have been widely used to generate liquid
crystals with high birefringence [39].
Previously, we have synthesized a series of polythiophene derivatives with the
mesogenic units directly attached to the main chain without spacers [40]. In this
study, polythiophene derivatives by introducting terphenyl-containing mesogen into
the 3-position of the thiophene ring through ester bonding have been synthesized.
The influence of the chromophoric terphenyl core on the structures and properties
of the monomers and polymers has also been investigated.
Experimental
Materials
Trimethyl borate, n-butyllithium, thiophene-3-acetic acid, trimethyl borate, 4-(4-
bromophenyl)phenol, 4-bromobenzonitrile, 1,3-dicyclohexylcarbodiimine (DCC), 4-
(dimethylamino)pyridine (DMAP), and tetrakis(triphenylphosphine)palladium were
purchased from Alfa Aesar and used as received without any further purification.
Tetrahydrofuran (THF) and CH2Cl2 were dried over sodium. Other chemicals were
obtained from Shanghai Reagent Co., Ltd. (Shanghai, China), and used as received.
Techniques
The nuclear magnetic resonance (NMR) spectra were collected on a Bruker ARX
400 NMR spectrometer (Switzerland) with deuterated chloroform or THF as the
solvent and with tetramethylsilane (d ¼ 0) as the internal standard. The infrared
(IR) spectra were recorded on a Shimadzu IRPrestige-21 Fourier transform infra-
red (FTIR) spectrophotometer (Japan) by drop-casting sample solution on KBr
substrates. The ultraviolet-visible (UV) spectra of the samples were recorded on a
Hitachi UV-2300 spectrophotometer (Japan). Fluorescence measurement for photo-
luminescence (PL) of the polymers was carried out on a Shimadzu RF-5301 PC
spectrofluorophotometer (Japan) with a xenon lamp as the light source. The gel
permeation chromatography (GPC), so-called size-exclusion chromatography
(SEC) analysis, was conducted with a Breeze Waters system (US) equipped with
a Rheodyne injector, a 1515 Isocratic pump, and a Waters 2414 differential refrac-
tometer using polystyrenes as the standard and THF as the eluent at a flow rate of

72
L. Chen et al.
1.0 mL=min and 40ꢀC through a Styragel column set, Styragel HT3 and HT4
3
4
˚
(19 mm ꢁ 300 mm, 10 þ 10 A) to separate molecular weight (MW) ranging from
102 to 106. Thermogravimetric analysis (TGA) was performed on a PerkinElmer
TGA 7 (US) for thermogravimetry at a heating rate of 20ꢀC=min under nitrogen
with a sample size of 8–10 mg. Phase-transition temperatures were determined with
a Perkin-Elmer DSC 7 differential scanning calorimeter (US) with a constant
heating=cooling rate of 10ꢀC=min. Texture observations were made with a Nikon
E600POL polarizing optical microscope (Japan) equipped with an Instec HS 400
heating and cooling stage. The X-ray diffraction (XRD) study of the samples was
carried out on a Bruker D8 Focus X-ray diffractometer (Switzerland) operating at
ꢀ
˚
30 kV and 20 mA with a copper target (k ¼ 1.54 A) and at a scanning rate of 1 =min.
Synthesis of the Monomers
The synthesis and structures of the monomers are outlined in Scheme 1. All the
reactions and manipulations were carried out under a nitrogen atmosphere.
2,5-Dibromothiophene-3-acetic Acid (1). Thiophene-3-acetic acid (10 g, 70.3 mmol)
was very slowly added to a solution of N-bromosuccinimide, NBS (30.2 g,
172 mmol), in 50 mL of DMF by a dropping funnel. After the addition, 10 mL of
DMF was further added to the reaction mixture and refluxed at 50ꢀC for 20 h
under an argon atmosphere. The reaction vessel was wrapped by aluminum foil to
shield the reaction from light. When the reaction finished, the reaction mixture
was allowed to warm to room temperature. The solution was poured into a large
Scheme 1. Illustration of procedures for synthesis of the monomers and their polymerization.

Polythiophene Bearing Terphen Mesogen
73
amount of saturated sodium sulfate cooled by ice water. The yellow precipitate was
filtered off and recrystallized from an ethanol: water mixture to yield product as
white, needlelike crystals. Yield: 63.0%. IR (cmꢂ1): 3101, 2918, 1707, 1415, 1327,
1
1232, 1016, 829, 735, 632, 469. H NMR (CDCl3, d from TMS, ppm): 3.63 (s, 2H,
CH2), 6.95 (s, 1ArH, BrCCHC).
4-Cyanobenzeneboronic Acid [2(CN)]. A solution of n-butyllithium (30 mL, 2.87 M
in hexane, 0.086 mol) was added dropwise to a stirred, cooled (ꢂ110ꢁC) solution of
4-bromobenzonitrile (15 g, 0.082 mol) in dry THF (180 mL) under dry nitrogen. The
solution was stirred at below ꢂ100ꢁC for 1 h and a solution of trimethyl borate
20.8 mL in dry THF (60 mL) was added at below ꢂ100ꢁC. The solution was
allowed to warm to room temperature overnight. Ten percent hydrochloric acid
was added and the solution was stirred for 1 h at room temperature. The product
was extracted into ether and the organic layer was washed with water and dried
with MgSO4. The solvent was removed in vacuo and the crude product dissolved
in THF and precipitated with n-hexane to give a yellow solid with yield of 70%
[2(OCH3)], white solid, the synthetic route is the same to [2(CN)].
4-Hydroxy-40-cyanoterphenyl [3(CN)]. Under a dry nitrogen atmosphere a solution
of 2.00 g of 4-cyanobenzeneboronic acid (13.6 mmol) in 10 mL of ethanol was added
to a solution of 2.75 g of 4-(4-bromophenyl)phenol (97%, 11.02 mmol) and 0.42 g of
tetrakis(triphenylphosphine)palladium(0) (99%, 0.36 mmol) in 20 mL of benzene and
20 mL of aqueous Na2CO3 (2 M). The reaction was conducted under reflux
overnight. The reaction mixture was then shaken with ethyl acetate and the
insoluble parts were filtered off. The organic layer was dried with anhydrous
MgSO4, and the solvent was removed by evaporation in vacuo. The crude product
was recrystallized from acetone to provide a yellow powder, 65% yield.
1
3(CN), IR (KBr, cmꢂ1): 2215 (CꢃN), 3351 (ꢂOH). H NMR (ppm, CDCl3):
7.73–7.65 (four d, aromatic, 8H), 7.53 (d, aromatic, 2H ortho to cyano), 6.93 (d,
aromatic, 2H ortho to hydroxyl), 4.91 (s, 1H, ꢂOH).
3(OCH3), colorless crystal, IR (KBr, cmꢂ1): 3393, 2956, 2835, 1608, 1491, 1251, 1031,
815, 644. 1H NMR (ppm, CDCl3): 7.60–7.51 (m, aromatic, 8H), 7.00–6.99 (d, aromatic,
2H ortho to hydroxyl), 6.90, 6.93 (d, aromatic, 2H ortho to OCH3), 5.52 (s, 1H, ꢂOH).
2,5-Bromo-3-{[(4-(40-cyano)terphenyloxy)carbonyl]methyl}-thiophene [M(CN)].
2,5-Dibromothiophene-3-acetic acid (3.43 g, 12 mmol) was added to a mixture of
4-hydroxy-40-cyanoterphenyl (2.71 g, 10 mmol), (dimethylamino)pyridine, DMAP
1.47 g (12 mmol), and dicyclohexylcarbodiimide, DCC (2.46 g, 12 mmol), in 30 mL
of absolute THF and further stirred for 24 h at room temperature under an argon
atmosphere. Then the solution was filtered to remove the urea crystals, and the
solvent was removed by evaporation. The crude product was purified by column
chromatography (n-hexane=CHCl3 ¼ 2=1) to afford M(CN) as white crystal.
Yield ¼ 89%.
1
M(CN), IR (KBr, cmꢂ1): 2223, 1756, 810, 665, 475. H NMR (ppm, CDCl3):
7.76–7.67 (m, aromatic, 8H), 7.64, 7.62 (d, aromatic, 2H), 7.25, 7.22 (d, aromatic,
2H ortho to ꢂOꢂ), 7.05 (s, aromatic, 1H, BrCCHC), 3.85 (s, 2H, ꢂCH2ꢂCOOꢂ).
M(OCH3), white crystal, IR (KBr, cmꢂ1): 2939, 2861, 1745, 806, 659, 473.
1H NMR (ppm, CDCl3): 7.63–7.51 (m, aromatic, 8H), 7.19, 7.17 (d, aromatic, 2H
ortho to ꢂOOCꢂ), 7.00, 6.98 (d, aromatic, 2H ortho to ꢂOCH3), 7.05 (s, aromatic,
1H, BrCCHC), 3.86 (s, 3H, ꢂOCH3), 3.69 (s, 2H, ꢂCH2ꢂCOOꢂ).

74
L. Chen et al.
Polymerization
All the polymerization reactions and manipulations were carried out under nitrogen
using Schlenk techniques in a vacuum line system or in an inert-atmosphere glove-
box (Vacuum Atmospheres), except for the purification of the polymers, which
was done in an open atmosphere. A typical experimental procedure for the polymer-
ization of P(CN) is given below.
2,5-Bromo-3-{[(4-(40-cyano)terphenyloxy)carbonyl]methyl}-thiophene [M(CN)]
(0.8 g, 1.3 mmol), 0.346 g (5.4 mmol) of Cu powder, and 5 mL of dry DMF were
put into a 100-mL one-necked flask in a dry box. The flask was removed from the
dry box and equipped with a magnetic stirring bar and an air-cooled condenser
capped with a drying tube. The polymerization was carried out at 145ꢁC for 7 days.
The mixture then was cooled, diluted with CHCl3 to about 100 mL, and filtered to
remove excess Cu powder. The organic phase was washed with water and dried over
MgSO4. The CHCl3 was removed with a rotary evaporator and dried in a vacuum
oven to a constant weight.
P(CN), brown powder, IR (KBr, cmꢂ1): 2221, 1749, 802. 1H NMR (ppm, CDCl3):
7.73–7.62 (m, aromatic, 8H), 7.54–7.52 (m, aromatic, 2H), 7.20–7.18 (m, aromatic, 2H
ortho to ꢂOꢂ), 6.99–6.92 (m, aromatic, 1H, BrCCHC), 3.49 (m, 2H, ꢂCH2ꢂCOOꢂ).
1
P(OCH3), brown powder, IR (KBr, cmꢂ1): 2930, 2851, 1745, 803. H NMR
(ppm, CDCl3): 7.63–7.52 (m, aromatic, 8H), 7.19–6.77 (m, aromatic, 5H, 2H ortho
to –OOC–, 2H ortho to –OCH3, 1H of BrCCHC), 3.86–3.78 (m, 3H, ꢂOCH3),
3.49–3.41 (m, 2H, ꢂCH2ꢂCOOꢂ).
Results and Discussion
Synthesis of the Monomers and Polymers
The synthetic routes of the monomers are shown in Scheme 1. The compound of
[3(R)] was synthesized through Suzuki reaction between the compound of 2(R)
and 4-(4-bromophenyl) phenol using tetrakis(triphenylphosphine)palladium(0) as
the catalyst. The monomers were synthesized through an esterification reaction route
in the presence of 1,3-dicyclohexylcarbodimine (DCC) and 4-(dimethylamino)
pyridine (DMAP). All the intermediate and final products were thoroughly purified
and fully characterized, and satisfactory analysis data were obtained (detailed
spectroscopic data for the key intermediates and for all the monomers being given
in the Experimental Section).
The polymerizations of 2,5-dibrominated thiophenes monomers were carried out via
dehalogenative polycondensation, giving poly-(3-position substituted thiophene) deriva-
tives, P(CN), and P(OCH3). All polymers synthesized were fusible and soluble in com-
mon organic solvents including THF, CHCl3, DMF, etc. Number-average (Mn) and
weight-average (Mw) molecular weights of the polymers are summarized in Table 1.
The chemical structures of the polymers were confirmed by FTIR and 1H NMR.
Structural Characterization
The monomers and the polymeric products were characterized by spectroscopic
methods and all the products gave satisfactory data corresponding to their expected
molecular structures (see Experimental Section for details). An example of the IR
spectrum of P(CN) is shown in Fig. 1. For comparison, the spectrum of its monomer

Polythiophene Bearing Terphen Mesogen
Table 1. Molecular weights of the polymers
75
Polymer
Mn
Mw
MWDa
DPb
P(CN)
P(OCH3)
4700
7400
7800
17,600
1.66
2.38
17
27
aMWD ¼ molecular weight distribution.
bDP ¼ degree of polymerization calculated by Mn=mru (mru: molecular weight of molecular
repeat unit).
M(CN) is also given. The monomer absorbs at 2230, 665–475 cmꢂ1, due respectively
to the CꢃN stretching and C-Br bending vibrations. The C-Br bending vibrations
absorption bands (665–475 cmꢂ1) disappeared in the spectrum of its polymer, indi-
cating that the monomer has been transformed to the polymer via dehalogenative
1
polycondensation. The H NMR spectra of polymers and monomers are shown in
1
Fig. 2. The resonance peaks in the H NMR spectra of polymers are broader than
that of monomers and the protons of polymers are upfield-shifted after the mono-
mers. Except for the peaks of solvent and water remained in the spectra, no unexpec-
ted signals are observed in the spectra and all the resonance peaks can be assigned to
appropriate protons as marked in Fig. 2. This shows that the monomers were poly-
1
merized successfully. In H NMR of polymers, the integration ratio of thiophen
proton and CH2COO (refer to signals a and b in spectrum) seems to be relatively
low in comparison to other protons due to steric hindrance. However, almost
reasonable ratio of integration indicates a correct chemical structure.
Thermal Stability and Liquid Crystallinity
The polythiophenes lose little weight at a temperatures over 300ꢁC (shown in Fig. 3).
The thermal stability of the polymers is further substantiated by the differential
Figure 1. FTIR spectra of the monomer M(CN) and polymer P(CN).

76
L. Chen et al.
1
Figure 2. H NMR spectra of monomers and polymers (symbol : CDCl3 as the solvent).
ꢄ
scanning calorimetry (DSC) analysis: no irreversible peaks suspiciously associated
with polymer degradation are observed at the high temperatures during the cycles
of repeated heating-cooling scans.
All the monomers are white crystals at room temperature and exhibit liquid crys-
tallinity at elevated temperatures. The thermal transition behaviors of the monomers
and polymers are examined by DSC and polarized optical microscopy (POM). The
POM textures of the monomers are displayed in Fig. 4. When the monomers were
cooled from their isotropic state, a large number of anisotropic entities appeared from
the dark background of the isotropic liquid, the focal conic fan texture associating to
the mesophasic nature of a SmAd phase. Reheating the monomers regenerates the

Polythiophene Bearing Terphen Mesogen
77
Figure 3. TGA thermograms of polymers under nitrogen at a heating rate of 20ꢁC=min.
Figure 4. Mesomorphic textures observed on cooling from the products isotropic states at a
cooling rate of 1ꢁC=min.

78
L. Chen et al.
birefringent texture; that is, the mesomorphism is enantiotropic. However, in sharp
contrast to the fact of the monomers, the polymeric products could not exhibit any
optical anisotropy when heated or cooled. The results suggest that the thiophene
monomers were liquid-crystalline but the polymers were not. It was expected that a
liquid-crystalline monomer would result in a polymer with at least one liquid-
crystalline phase; however, the expectation was demolished. Because the mesogenic
pendant is closely coupled with the rigid backbone, it destroyed the packing arrange-
ments of the mesogens and inhibit the mesogen to move together to pack in a regular
fashion.
To learn more about the thermal transitions of the monomers and polymer, we
measured their thermograms under nitrogen on a differential scanning calorimeter
and the DSC curves are shown in Fig. 5. In the case of monomer M(OCH3), upon
cooling from its isotropic melt, a SmAd phase with a focal-conic fan texture was
observed at 197.3ꢁC. Finally, monomer M(OCH3) changed into crystal at
105.7ꢁC. The DSC thermogram recorded in the second heating scan also exhibits
two peaks at 183.6ꢁC and 122.5ꢁC, a focal-conic fan texture corresponding to a
SmAd phase emerges. M(CN) enters SmAd mesophase from its isotropic state at
139.5ꢁC. The mesophase is stable in a temperature range over 67.8ꢁC before
M(CN) finally solidifies at 71.7ꢁC. The associated k–SmAd and SmAd–i transition
profiles of M(CN) can be observed at 95.3ꢁC and 156.0ꢁC. The SmAd phase of
the monomers is thus enantiotropic. Different from the monomers, the two resulting
polymers show no peaks in the DSC thermograms during the cooling and heating
scan, indicating that the polymers are not liquid-crystalline.
With the aid of XRD measurements, the mesomorphic textures of monomers
and polymer were identified (Fig. 6). WAXD patterns of all the polymers were
obtained at room temperature after the samples had been quenched from liquid-
crystalline states with liquid nitrogen. The WAXD pattern of M(OCH3) shows a
sharp reflection at 2h ¼ 3.466ꢁ (d-spacing d ¼ 25.47 A). The calculated length of
˚
˚
one repeat unit of M(OCH3) is 19.80 A . Because the d=l ratio is ꢅ1.30, which
confirms the SmAd nature of the mesophase and suggests that the mesogens are
interdigitated in an antiparallel fashion with a bilayer structure. The diffractogram
of M(CN) showed a sharp reflection at 2h ¼ 2.81ꢁ, from which d-spacings of 31.32
Figure 5. DSC thermograms of the monomers and polymers recorded under nitrogen during
the (a) first cooling and (b) second heating scans at a scan rate of 10ꢁC=min.

Polythiophene Bearing Terphen Mesogen
79
Figure 6. X-ray diffraction patterns of the monomers and polymers quenched from their
liquid-crystalline states.
were derived. The d-spacing derived from the low-angle peak was considerably in
excess of than the calculated molecule length of M(CN) at its most extended confor-
˚
mation (19.26 A), thus also providing SmAd mesophase in a bilayer arrangement,
in which the mesogens also arrange in an antiparallel overlapping interdigitated
manner. The mesogens of the monomers are packing so well that the high-order sec-
ondary reflection at a middle angle at a middle angle at 5–6ꢁ were readily detected by
˚
the diffractometer, and the d-spacing is about 15.00–16.00 A, which associated to the
distance of the terphenyl mesogen. The diffractograms of the polymers show no
peaks, confirming that the polymers are not liquid-crystalline. The short methylene
ester spacer between the mesogenic pendant and the polythiophene backbone limited
Figure 7. UV spectra of CH2Cl2 solution of the monomers and polymers. The concentration
was 0.125 Mm.

80
L. Chen et al.
the mesogens to undergo thermal transitions in a relatively independent fashion. The
results are as well as identified with the POM and DSC.
Electronic Absorption and PL
The ultraviolet-visible (UV) and PL spectra of M(OCH3), M(CN), P(OCH3), and
P(CN) in CH2Cl2 are given in Figs. 7 and 8, respectively. The monomers and
polymers absorb photons at about 300 nm. The absorptions of the monomers
and polymers were assignable to the p–pà bands of the terphenyl mesogenic pendant
because the monomers and polymers had similar absorption wavelengths. Besides,
the absorptions of the polymers are stronger than that of its corresponding
monomers and the molar absorption coefficient of the polymer P(CN) and
P(OCH3) are 1.83 Â 105 and 1.48 Â 105 molÀ1 L cmÀ1, respectively. Compared with
the high absorption of the the terphenyl mesogenic pendants, the low absorptions
of the backbone (e < 1.58 Â 104 molÀ1 L cmÀ1) observed at wavelengths longer than
350 nm may have been due to the reduction of the effective conjugation lengths along
the polymer backbone by the bulky pendant groups.
A polymer with both liquid crystal mesogens and light-emitting properties may
find unique technological applications [41], we thus investigated the fluorescence
properties of the polymers in dilute CH2Cl2 solutions. When the polymers were
photo-excited at 300 nm, the two strong light-emitting bands at 350 and 415 nm
observed for a CH2Cl2 solution of P(OCH3) are assigned to the emitting center of
the terphenyl mesogenic core and that of the conjugated polythiophene main chain,
respectively. On the other hand, energy transfer from terphenyl mesogenic core to
the backbone favors stronger emission in the photoluminescence of polythiophene.
The two light-emitting bands of polymer P(CN) was slightly red-shifted to 380
and 425 nm, respectively, due to the favor of cyano auxochrome group linked
directly to the terphenyl mesogenic core. The photoluminescence efficiency UF of
the polymers was also measured using quinine sulfate as the standard. Thanks to
Figure 8. Photoluminescence spectra of CH2Cl2 solutions of the polymers. The concentration
was 0.125 Mm. Excitation wavelengths: 300 nm.

Polythiophene Bearing Terphen Mesogen
81
terphenyl mesogenic favoring the photoluminescence, both of the polymers
possessed high quantum efficiency, and UF of polymer P(CN) (82%) was higher than
that of P(OCH3) (74%), which is in agreement with the Tang’s observations [42] that
the polymers with donor–acceptor pairs luminesce more strongly than those without
such push–pull pairs. Most conjugated polymers emit intensely in solution but
become weak emitters when fabricated into films [43,44]. This is mainly caused by
strong interchain interaction. In the solid state, the polymer strands aggregate
together to form less luminescent species such as excimers, leading to red-shifted
emissions with low efficiencies [45]. We thus fabricated the polymers into film to
check their fluorescence properties. Although we have technical difficulty in measur-
ing the quantum efficiency of the polymers, both of the polymers maintained their
strong light-emitting properties and no significant shifts in the peak maximum are
observed in comparison to those in the solutions. This is suggestive of little excimer
absorption and emission, and the segregation of the backbone effectively by the
bulky pendants decreases the strong interchain interaction.
Conclusion
Polythiophenes with terphenyl chromophoric mesogenic side chain were successfully
synthesized via dehalogenative polycondensation. The effects of the structural varia-
tions on the chemical and physical properties of the monomers and polymers were
investigated. The monomers are liquid-crystalline and exhibit enantiotropic smectic
A with focal-conic fan textures, but the polymers are not. The short spacers between
the mesogenic pendant and the polythiophene backbone limited the mesogens to
undergo thermal transitions in a relatively independent fashion, with a consequent
loss of liquid crystallinity. On the other hand, the terphenyl chromophoric mesogenic
core endows the monomers and polymers with the high absorptivity of UV light and
efficient light emission. The polymer P(CN) can emit UV light of 425 nm, which is
longer and stronger than that of the polymer P(OCH3).
Acknowledgment
Financial support for this work was provided by the National Natural Science Foun-
dation of China (50773029), the Natural Science Foundation of Jiangxi Province
(2007GZC1727 and 2008GQH0046), Jiangxi Provincial Department of Education,
the Program for New Century Excellent Talents in University (NCET-06-0574),
the Program for Innovative Research Team of Nanchang University, Program for
Innovative Research Team in University of Jiangxi Province, and Program for
Changjiang Scholars and Innovative Research Team in University (IRT0730).
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