Liquid crystalline polythiophene bearing phenylnaphthalene side-chain

Article abstract of DOI:10.1021/ma202469g

Polythiophene bearing a 2-phenylnaphthalene side group at 3-position was synthesized from a 2,5-dibromothiophene monomer by two polymerization methods, i.e., Yamamoto dehalogenative polycondensation using Ni(cod)₂ and Ni-catalyzed chain-growth polymerization. Polymers prepared by the former method had good solubility for organic solvents, 6300-8400 g mol^-1 of number-average molecular weights, absorption bands at around 300 and 385 nm due to π-π* transitions at the phenylnaphthalene moiety and polythiophene backbone, a main fluorescence emission band at around 540 nm from the polythiophene backbone in solution and film state, and presence of enantiotropic liquid crystalline phases which enabled to construct an arrayed state. On the other hand, the latter polymer showed considerably red-shifted absorption and emission bands at around 444 and 585 nm in solution and 509 and 720 nm in film state respectively, but had poor solubility and unresolved mesophases.

Full text of DOI:10.1021/ma202469g

Article
pubs.acs.org/Macromolecules
Liquid Crystalline Polythiophene Bearing Phenylnaphthalene Side-
Chain
Mari Watanabe,†,‡ Kazuhiko Tsuchiya,§ Toshinobu Shinnai,§ and Masashi Kijima*,†,‡,∥
†Institute of Materials Science, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba,
Ibaraki 305-8573, Japan
‡Tsukuba Research Center for Interdisciplinary Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573,
Japan
§Central Research Laboratory, Technology & Development Division, Kanto Chemical Co., Inc., 1-7-1 Inari, Soka, Saitama 340-0003,
Japan
∥Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki
305-8573, Japan
S
* Supporting Information
ABSTRACT: Polythiophene bearing a 2-phenylnaphthalene
side group at 3-position was synthesized from a 2,5-
dibromothiophene monomer by two polymerization methods,
i.e., Yamamoto dehalogenative polycondensation using Ni(cod)2
and Ni-catalyzed chain-growth polymerization. Polymers pre-
pared by the former method had good solubility for organic
solvents, 6300−8400 g mol−1 of number-average molecular
weights, absorption bands at around 300 and 385 nm due to
π−π* transitions at the phenylnaphthalene moiety and
polythiophene backbone, a main fluorescence emission band at
around 540 nm from the polythiophene backbone in solution
and film state, and presence of enantiotropic liquid crystalline
phases which enabled to construct an arrayed state. On the other hand, the latter polymer showed considerably red-shifted
absorption and emission bands at around 444 and 585 nm in solution and 509 and 720 nm in film state respectively, but had
poor solubility and unresolved mesophases.
the excellent liquid crystalline polythiophenes, in which a fair
percentage of insulating parts occupied, have faced a dilemma
of sacrificing inherent electrical and optical properties of
polythiophene. Replacement of the insulating side chains by
semiconducting ones might be a solution and development
strategy. Therefore, several conjugated polymers bearing
semiconducting side chains have been synthesized and their
basic properties were surveyed.24
2-Phenylnapthalene derivatives have been received much
attention as a calamitic liquid crystalline photoconductor with
ambipolar and high carrier transport properties.25,26 It is
considered that calamitic thermotropic liquid crystals are
suitable as the side chain of polythiophene that should take a
lamellar structure. In this paper, 2,5-dibromothiophene
monomer having 2-phenylnaphthalene side chain at 3-position
was prepared by two methods. The former is conventional 2,5-
polymerization of 3-substituted polythiophenes using Ni(cod)2
known as Yamamoto method27 under different conditions to
INTRODUCTION
■
Conjugated polymers bearing a liquid crystalline side-chain
have been studied for the purpose of adding specific
functionalities of birefringence, self-orientation, and induced
anisotropy to semiconducting polymer backbones.1−3 They are
prospective candidate of elaborate functional materials as well
or better than main-chain type liquid crystalline conjugated
polymers,4,5 because their electrical and optical properties are
expected to be controlled by the molecular orientation of
various liquid crystalline side chains and furthermore macro-
scopic alignment of them can be progressed by an external
perturbation such as shear stress, electric or magnetic field.6−8
Polythiophene derivatives have been drawing much attention
for their wide-spreading potential of applications, such as
organic light emitting diodes,9−11 polymer solar cells,12−15 and
organic field effect transistors.16,17 Therefore, they have also
been investigated as π-conjugated backbone of the side-chain
type polymer liquid crystals. For development of liquid
crystallinity of polythiophenes, selective 2,5-polymerization of
thiophene having mesogenic group at 3-position was crucial18
and several methods have successfully produced characteristic
liquid crystalline polythiophenes, respectively.3,7,19−23 However,
Received: November 8, 2011
Revised: January 27, 2012
Published: February 9, 2012
© 2012 American Chemical Society
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temperature for PTPhN1b and at 120 °C for PTPhN2. IR spectra
were recorded on a JASCO FT/IR 550 spectrometer. UV−vis
absorption and photoluminescence (PL) spectra were recorded on a
Shimadzu UV-3100PC and a Hitachi F-4500. The thermal properties
of compounds were analyzed by differential scanning calorimetry
(DSC) using SII EXSTAR 6000 at a heating/cooling rate of 10 °C
min−1. The optical textures of mesophases were observed by a Nikon
ECLIPSE E 600 POL and a Nikon ECLIPSE LV 100 polarizing optical
microscope (POM) with a Linkam TH-600PM thermocontroller. X-
ray diffraction (XRD) measurements were performed using a Rigaku
RINT2100 and a PANalytical X’pert diffractometer with Cu−Kα
radiation (λ= 1.5418 Å). Electrochemical measurements of drop-cast
films of the polymers on an electrode were performed by cyclic
voltammetry using a Hokuto Denko HB-305 function generator and a
HAL3001 potentiostat equipped with a Pt disk electrode as the
working electrode, a Pt plate as a counter electrode, and a saturated
calomel electrode (SCE) as a reference electrode. The measurements
were carried out at a scanning rate of 50 mV s−1 in acetonitrile (0.1
mol dm−3 Et4NBF4) under an Ar atmosphere. Ionization potentials (vs
vacuum) of the polymers were estimated from the onset of their first
oxidation peak (Eonset) in the cyclic voltammograms on the basis that
ferrocene/ferrrocenium is 4.8 eV below the vacuum level.30,31
give two polythiophenes with different degree of polymer-
ization (PTPhN1a and PTPhN1b) whose regioregularity of
head-to-tail is uncontrolled, and the latter is Ni-catalyzed chain-
growth method28,29 to provide a 3-substituted polythiophene
with high regioregularity of head-to-tail conformation,
PTPhN2, as summarized in Scheme 1. Furthermore, basic
Scheme 1. Polymerization Methods
Materials. Following compounds shown in Scheme 2 were
prepared according to methods described in a preceding report24
with a little modification and the experimental details were described
in Supporting Information; 2-(10-bromodecyloxy)tetrahydro-2H-
pyran (1), 3-(10-tetrahydro-2H-pyran-2-yloxydecyl)thiophene (2), 3-
thiophenedecanol (3), 2,5-dibromo-3-thiophenedecanol (4), 4-octyl-
phenylboronic acid (5), 2-Methoxy-6-(4-octylphenyl)naphthalene (6).
Other chemicals used in this study were purchased from Kanto
Chemical Co., Inc., Tokyo Chemical Industry Co., Ltd., Nacalai
Tesque Inc., Sigma-Aldrich Co., LLC., Merck & Co., Inc., or Acros
Organics of Thermo Fisher Scientifics, and used without further
purification unless stated. THF and DMF were purified by distillation
according to common methods.
Synthesis. 2-(4-Octylphenyl)-6-hydroxynaphthalene (7). Into
6 (2.66 g, 7.68 mmol) in dry CH2Cl2 (100 mL) boron
tribromide (15.3 mL, 1.0 mol dm−3 in CH2Cl2) was added
dropwise under N2 atmosphere at 0 °C. After stirring at room
temperature overnight, water was poured into the reaction
solution at 0 °C. The product was extracted with CH2Cl2,
properties of these polymers were investigated by comparing
their conformations, molecular weights, absorption−emission
characteristics, and thermotoropic liquid crystalline behaviors.
EXPERIMENTAL SECTION
General Data. Average molecular weights of the polymers were
■
determined by gel permeation chromatography (GPC) calibrated with
polystyrene standards using THF as an eluent. H and 13C nuclear
1
magnetic resonance (NMR) spectra were measured with a JEOL
1
JNM-ECS 400 at a resonance frequency of 400 MHz for H and 100
MHz for 13C in CDCl3 at room temperature. A Bruker AVANCE 600
was also used to measure NMR spectra at a resonance frequency of
1
600 MHz for H and 150 MHz for 13C in chlorobenzene-d5 at room
Scheme 2. Synthetic Routes of the Compounds
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washed with aqueous NaHCO3 and water, and dried over
anhydrous Na2SO4. Recrystallization from toluene gave 7 as a
solid product (2.01 g, 78.8% yield). H NMR (400 MHz,
CDCl3, δ ppm): 0.89 (t, 3H), 1.28 (br, 10H), 1.66 (m, 2H),
2.66 (t, 2H), 5.04 (s, 1H), 7.12 (dd, 1H), 7.17 (d, 1H), 7.28 (d,
2H), 7.61 (d, 2H), 7.68−7.75 (m, 2H), 7.80 (d, 1H), 7.96 (s,
1H). 13C NMR (100 MHz, CDCl3, δ ppm): 14.11, 22.67, 29.27,
29.38, 29.50, 31.53, 31.89, 35.63, 109.29, 118.09, 125.36,
126.25, 126.77, 127.02, 128.89, 129.18, 130.09, 133.61, 136.40,
138.42, 141.99, 153.39.
Method II: Into the monomer 8 (252 mg, 0.35 mmol) and LiCl (18
mg, 0.4 mmol) in dry THF (1.5 mL), was added iPrMgCl (2.0 mol
dm−3 in THF, 180 μL) at 0 °C, which was stirred at 0 °C for 1 h. After
addition of dry THF (2.0 mL), the part of the reaction mixture (0.5
mL) was sampled for analysis of metalated intermediates.32 Then [1.3-
bis(diphenylphosphino)propane]dichloronickel(II) (Ni(dppp)Cl2) (4
mg, 0.008 mmol) was added to the reaction mixture, which was stirred
at room temperature overnight. After addition of an aqueous HCl
solution, the reaction mixture was extracted with chloroform for
several times to obtain a polymer-dispersed organic layer, which was
washed with water and dried over CaCl2. The product dissolved in hot
o-dichlorobenzene was reprecipitated from MeOH, acetone, and
chloroform/acetone, affording PTPhN2 as a purple solid (104 mg,
1
2,5-Dibromo-3-{10-[6-(4-octylphenyl)naphthalene-2-yloxy]-
decyl}thiophene (8). To a solution of 4 (1.06 g, 2.66 mmol), 7 (0.86
g, 2.59 mmol), and triphenylphosphine (0.85 g, 3.24 mmol) in dry
THF (20 mL), diethyl azodicarboxylate (DEAD) (1.5 mL, 40% in
toluene) was added at 0 °C, which was stirred at room temperature for
3 days. The product was purified by column chromatography on silica
gel (CH2Cl2/hexane, 1:1) and recrystallization from acetone, which
1
62% yield). H NMR (600 MHz, chlorobenzene-d5, δ ppm): 0.87 (t,
3H), 1.33 (br, 20H), 1.47 (m, 4H), 1.66 (br, 2H), 1.77 (br, 2H), 2.54
(m), 2.61 (br, 2H), 2.88 (m), 3.99 (br, 2H), 6.96−7.25 (m, 5H), 7.55
(br, 2H), 7.57−7.68 (m, 3H), 7.87 (br, 1H). 13C NMR (150 MHz,
chlorobenzene-d5, δ ppm): 13.94, 22.81, 26.57, 29.48, 29.68, 29.75,
29.87, 29.96, 30.65, 30.82, 31.44, 32.13, 35.93, 68.81, 108.07, 119.59,
125.57, 125.80, 126.12, 126.24, 127.35, 127.48, 128.00, 128.41, 128.50,
128.58, 128.69, 128.77, 128.83, 128.96, 129.10, 129.21, 129.27, 129.43,
129.54, 129.62, 129.83, 130.05, 134.42, 135.50, 134.85, 136.859,
139.19, 141.99, 157.96.
Estimation of Regioregularity and Degree of Polymerization by
1H NMR. Regioregularity (rr) of the 3-substituted polythiophenes
(content of head-to-tail linkages in the polymer chains) was estimated
from integration values of α-methylene proton signals of a head-to-tail
linkage at about 2.8 ppm (Iht) and a head-to-head linkage at about 2.58
ppm (Ihh) using an equation, rr = Iht/(Iht + Ihh), by 1H NMR analysis.33
Practically, in the cases of PTPHN1a and PTPHN1b measured in
CDCl3 with the JEOL JNM-ECS 400, Ihh was roughly determined by
subtracting overlapping integration of α-methylene protons (2H) at
2.61 ppm of the (4-octylphenyl)naphthalene moiety from total
integration of signals in the region of 2.47−2.65 ppm. In the cases
of 1H NMR analysis of PTPHN1b and PTPHN2 in chrorobenzene-d5
measured with the Bruker AVANCE 600, rr was more properly
calculated using eq 1 with I′ht (2.95−2.77 ppm), I′hh (2.77−2.65 ppm),
and an integration value of α-methylene protons of polymer terminals
(I′term) (2.58−2.40 ppm).34,35 Similarly, number-average degree of
polymerizations (dp) of PTPHN1b and PTPHN2 were estimated
using eq 2 with these integration values.
1
gave a colorless solid (1.18 g, 62.1% yield). H NMR (400 MHz,
CDCl3, δ ppm): 0.88 (t, 3H), 1.31 (br, 20H), 1.46−1.57 (m, 4H), 1.66
(m, 2H), 1.85 (m, 2H), 2.50 (t, 2H), 2.65 (t, 2H), 4.08 (t, 2H), 6.77
(s, 1H), 7.13−7.17 (m, 2H), 7.27 (d, 2H), 7.61 (d, 2H), 7.69 (dd,
1H), 7.75−7.78 (m, 2H), 7.94 (d, 1H). 13C NMR (100 MHz, CDCl3,
δ ppm): 14.10, 22.67, 26.09, 29.06, 29.24, 29.27, 29.32, 29.37, 29.43,
29.45, 29.50, 29.52, 29.54, 31.53, 31.89, 35.63, 68.05, 106.39, 107.92,
110.29, 119.36, 125.25, 125.93, 127.00, 127.09, 128.87, 129.12, 129.55,
130.95, 133.65, 136.22, 138.51, 141.88, 142.97, 157.14. Anal. Calcd for
C38H48OSBr2: C 64.04, H 6.79, N 0.00. Found: C 63.89, H 6.52, N
0.06.
2-(4-Octylphenyl)-6-decyloxynaphthalene (PhN). 1-Bromodecane
(95.0 μL, 0.46 mmol) and 7 (152 mg, 0.46 mmol) in acetone (4 mL)
were refluxed for 3 days in the presence of K2CO3 (94 mg, 0.68
mmol). The mixture was washed with water/CH2Cl2 and the organic
layer was dried over anhydrous Na2SO4. The product was purified by
column chromatography on silica gel (CH2Cl2/hexane, 1:1) and
recrystallization from hexane, yielding a colorless solid (92 mg, 42.6%
1
yield). H NMR (400 MHz, CDCl3, δ ppm): 0.88 (t, 3H), 1.28 (br,
24H), 1.50 (m, 2H), 1.66 (m, 2H), 1.85 (m, 2H), 2.65 (t, 2H), 4.07 (t,
2H), 7.13−7.18 (m, 2H), 7.27 (d, 2H), 7.61 (d, 2H), 7.69 (dd, 1H),
7.76 (d, 2H), 7.94 (d, 1H). 13C NMR (100 MHz, CDCl3, δ ppm):
14.11, 22.67, 26.10, 29.25, 29.27, 29.33, 29.38, 29.42, 29.50, 29.57,
29.59, 31.54, 31.89, 35.62, 68.05, 106.33, 119.36, 125.25, 125.91,
126.99, 127.09, 128.86, 129.09, 129.54, 133.64, 136.18, 138.50, 141.86,
157.14.
rr = I′ /(I′ + I′ + I′
ht ht hh term
)
(1)
(2)
Poly(3-{10-[6-(4-octylphenyl)naphthalene-2-yloxy]decyl}-
thiophene-2,5-diyl) (PTPhN). Method I: Nickel bis(cycloocadiene)
(Ni(cod)2) (235 mg, 0.85 mmol), 2,2′-bipyridine (bpy) (135 mg, 0.86
mmol), and 1,5-cyclooctadiene (cod) (96 μL, 0.78 mmol) in dry DMF
(2 mL) were stirred at room temperature under N2 atmosphere for 30
min. Then the monomer 8 (297 mg, 0.42 mmol) in dry THF (2 mL)
was added into the catalyst solution, which was stirred at 80 °C for 3
days. The reaction mixture was poured into the solution of MeOH
containing aqueous HCl, and the resultant precipitate was successively
purified by reprecipitation from MeOH and acetone, respectively.
dp = (I′ + I′ + I′ )/I′
ht hh term term
RESULTS AND DISCUSSION
■
Synthesis. Synthetic routes for the monomer 8, model
liquid crystal (PhN), and polymers (PTPhN) are summarized
in Scheme 2. 2,5-Dibromothiophene derivative 4 was
synthesized from 3-bromothiophene and the THP protected
10-bromodecanol 1 by Kumada−Tamao−Corriu cross cou-
pling,36 followed by dibromination with NBS. On the other
hand, phenylnaphthalene derivative 7 was prepared from 2-
bromo-6-methoxynaphthalene and 5 by Suzuki−Miyaura cross
coupling37 and successive ether cleavage reaction. The
monomer 8 was synthesized from the thiophene moiety 4
and phenylnaphthalene moiety 7 by Mitsunobu reaction.38 The
model liquid crystal, 2-phenylnaphthalene derivative PhN, was
also synthesized by Williamson reaction of 1-bromodecane and
7 for the purpose of comparison with the polymers. PTPhN1a
and PTPhN1b were obtained by polymerization of the
monomer 8 by Yamamoto method27 (method I) in DMF-
THF and in DMF, respectively. PTPhN2 was obtained from
the same monomer 8 by Ni-catalyzed chain-growth polymer-
ization method28,29 (method II).
1
PTPhN1a was obtained as a yellow solid (220 mg, 95% yield). H
NMR (400 MHz, CDCl3, δ ppm): 0.88 (br, 3H), 1.15−1.87 (m, 28H),
2.47−2.80 (m, 4H), 3.99 (br, 2H), 6.95−7.20 (m, 3H), 7.20−7.29 (br,
2H), 7.48−7.79 (m, 5H), 7.89 (br, 1H). 13C NMR (100 MHz, CDCl3,
δ ppm): 14.11, 22.67, 26.12, 29.28, 29.40, 29.50, 29.59, 30.61, 31.53,
31.89, 35.61, 67.98, 106.26, 119.31, 125.18, 125.84, 126.94, 127.09,
128.82, 129.06, 129.51, 133.63, 136.10, 138.43, 141.80, 157.10.
Similarly, the monomer 8 (205 mg, 0.29 mmol) in dry DMF (3.5
mL) was added into the solution of Ni(cod)2 (252 mg, 0.92 mmol),
2,2′-bipyridine (149 mg, 0.95 mmol), and 1,5-cyclooctadiene (80 μL,
0.58 mmol) in dry DMF (0.7 mL), and the reaction mixture was
stirred at 80 °C for 3 days under N2 atmosphere. The reaction mixture
was precipitated from the MeOH containing aqueous HCl solution,
and reprecipitated from MeOH and acetone, respectively, affording
PTPhN1b as a yellow solid (151 mg, 95% yield). NMR spectra of
PTPhN1b were identical with those of PTPhN1a.
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General Properties. The polymerization results and
average molecular weights and rr of PTPhN1 and PTPhN2
are summarized in Table 1. PTPhN1a and PTPhN1b exhibited
Table 1. Polymerization Results and Average Molecular
Weights and Regioregularity of the Polymers
a
b
polymer
yield/% Mn/g mol−1 (Mw/Mn)
dp
rr/%
(30)
c
c
PTPhN1a
PTPhN1b
PTPhN2
95
95
62
6300 (1.24)
11.4
c
d
c
d
8400 (1.62), 4650
15.2, 8.4
29.9 (30)
76.3
d
d
4090
7.4
a
b
Number-average degree of polymerization. Regioregularity deter-
c
mined by 600 MHz (and 400 MHz) 1H NMR. Determined by GPC.
d
1
Determined by H NMR.
good solubility for organic solvents such as chloroform and
THF at room temperature, while PTPhN2 was insoluble in
such solvents, but soluble in hot chlorobenzene or hot o-
dichlorobenzene. The GPC analysis showed that the number-
average molecular weight (Mn) and polydispersity (Mw/Mn)
were 6300 g mol−1 and 1.24 for PTPhN1a and 8400 g mol−1
and 1.62 for PTPhN1b, respectively. GPC analysis of PTPhN2
was not carried out because of the insolubility for the THF
eluent. Head-to-tail content (rr) of PTPhN1 could be roughly
estimated to be about 30% from 1H NMR spectra at 400 MHz
in CDCl3 in the region of 2.2−3.2 ppm for α-methylene
protons of 3-substituted polythiophene. In order to compare rr
between PTPHN1 and PTPHN2, it was precisely examined by
1
600 MHz H NMR in chlorobenzene-d5 at room temperature
for PTPhN1b and at 120 °C for PTPhN2 (Figure 1). A sharp
and strong peak at about 2.61 ppm is assigned to α-methylene
protons of the 4-octylphenylnaphthalene moiety (B), and the
other signals are due to α-methylene protons of the 3-
substituent moiety of various thiophene units (A). Signals
observed between 2.95 and 2.76 ppm (A1), 2.76−2.65 ppm
(A2), and 2.58−2.40 ppm (A3) are assigned to regioregular
units, regioirregular ones and chain-end ones, respectively.33−35
From these integration areas, rr of PTPhN1b and PTPhN2
could be estimated from eq 1 (see Experimental Section) to be
29.9% and 76.3% . On the basis of this NMR analysis, number-
average degree of polymerization (dp) and Mn of PTPhN1b
and PTPhN2 can be estimated using eq 2 with these
integration values (see Experimantal Section). Interestingly,
Mn values of two polymers estimated by NMR are in the range
of 4000−5000 g mol−1 and are not so much different.
Considerable difference between Mn for PTPhN1b obtained
by respective two methods is due to overestimation of Mn
usually observed in rigid poly(3-alkylthiophene)s by GPC.39
Consequently, the characteristic poor solubility of PTPhN2
compared to PTPhN1 was suggested to be caused by the
higher rr, probably which caused strong intermolecular
interaction between the main chains and side chains. Other
than the NMR investigation, the good rr for PTPhN2 is also
supported by the fact that an intermediate, 3-substituted 2-
bromo-5-chloromagnesiothiophene, produced 4 times larger
than a 5-bromo-2-chloromagnesio derivative in the initial
1
Figure 1. H NMR (600 MHz) spectra of PTPhN1b (a) at room
temperature and PTPhN2 (b) at 120 °C in chlorobenzene-d5 in the
range 2.2−3.2 ppm.
Photoabsorption and Fluorescence Properties. UV−
vis absorption spectra of the polymers in solution and in thin
solid film state are shown in Figure 2 and Figure 3, and their
Figure 2. Normalized UV−vis spectra of chloroform solutions of
PTPhN1a (black solid line), PTPhN1b (black dashed line), PhN
(black dotted line), and an o-dichlorobenzene solution of PTPhN2
(red solid line).
optical data of absorption and PL are summarized in Table 2.
Absorption spectra of PTPhN1a and PTPhN1b were basically
same in solution and in film state, and showed two
characteristic absorption bands due to π−π* transition of the
phenylnaphthalene moiety in the ultraviolet region around 300
nm and polythiophene backbones in the visible light region
around 385 nm. The assignment is considered reasonable in
comparison with spectrum of PhN shown in Figure 2. The
stage.32 which was confirmed by H NMR analysis of the
1
sampling according to the procedure described in the
Experimental Section. The poor solubility of PTPhN2 brought
about precipitation of polymeric products in an early stage of
polymerization, which rather lowered dp than expected from
the molar ratio of the monomer/Ni catalyst about 44 and at the
same time lowered rr.
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Figure 3. UV−vis spectra of a pristine film (black solid line) and an
annealed film heat treated at 140 °C (black dashed line) of PTPhN1b
and pristine film of PTPhN2 (red solid line, y axis is Abs. (arbitrary
unit)) on a quartz glass plate.
Figure 4. Normalized PL spectra of chloroform solutions of
PTPhN1b excited at 256 nm (black solid line), 296 nm (red solid
line), 387 nm (blue solid line), PhN excited at 296 nm (black dotted
line), and o-dichlorobenzene solution of PTPhN2 excited at 444 nm
(black dashed line).
latter band observed in the region rather blue-shifted than usual
3-substituted polythiophenes and little difference between
absorption λmax in solution and film state suggest that π-
conjugation of the polythiophene backbones is significantly
restricted by the large side chains and their rr. A little
bathochromic and hyperchromic shift of the latter band
observed for PTPhN1b by comparison with PTPhN1a is due
to longer π-conjugation of the higher molecular weight
polymer. These results suggest that polymerization performed
successfully. In contrast, the latter band of PTPhN2 was
considerably bathochromic shifted compared to PTPhN1
especially in the film states. The absorption λmax of PTPhN2
in CHCl3 (444 nm) suggests that the π−π* transition energy is
lower than those of regioirregular poly(3-alkylthiophene)s (λmax
= 428 nm)33 and a regioregular octamer of 3-octylthiophene
(λmax = 422 nm)40 and higher than that of regioregular poly(3-
hexylthiophene (λmax = 456 nm),33 Considering this, the optical
characteristic of PTPhN2 is basically corresponding to the
estimated rr (76.3%) and dp (7.4). Optical bandgaps (Eg)
between energy levels of highest occupied molecular orbital
can be confirmed by comparing with PL spectra of poly(3-
hexylthiophene) polymerized under the same conditions (λmax
= 550 nm in chloroform) and PhN (λmax = 369 nm in
chloroform). This result suggests that Forster resonance energy
̈
transfer41 efficiently occurs from the phenylnaphthalene side
group to the polythiophene backbone in the solution and film
states taking into consideration that the PL band of the
phenylnaphthalene side group overlaps with the absorption
band of the polythiophene main chain. On the other hand,
PTPhN2 showed an emission peak from polythiophene
backbones at 585 nm in solution and 720 nm in film state,
which are almost same with those of regioregular poly(3-
hexylthiophene).33
Liquid Crystalline Behavior. Thermal behavior of PTPhN
and PhN was investigated by temperature-controlled POM and
DSC, and the DSC thermograms are shown in Figure 5 and
phase transition temperatures and changes of enthalpy (ΔH) of
the polymers and PhN are summarized in Table 3. The DSC
analysis showed that the phase transitions for PTPhN1a and
PTPhN1b were enantiotropic. The POM analysis revealed that
all mesophases of PTPhN1 showed fan-shaped textures typical
of smectic phases (Sm), and sizes of domains were respectively
large about several tens of μm regardless of their different
average size of molecular weights (Figure 6a,b). PTPhN1a
made a fan-shaped texture at around 115 °C under heating
process, and it was changed into a broken one at about 122 °C.
Further heating to 149 °C led to disappearance of the
birefringence, which corresponds to transition into isotropic
liquid phase (Iso). On the other hand, PTPhN1b showed one
distinct smectic liquid crystalline phase, and the phase
transition temperatures of glass phase (G)-Sm and Sm-Iso
were higher than those of PTPhN1a. These results suggest that
the higher molecular weight heightens phase transition
temperatures of PTPhN1, while the higher polydispersity
(EHOMO) and the lowest unoccupied molecular orbital (ELUMO
)
of PTPhN1 and PTPhN2 estimated from absorption edge in
the film state were 2.25 and 1.86 eV, respectively. EHOMO of
PTPhN1a and PTPhN1b were estimated at about −5.50 eV
from onset oxidation potential of their cyclic voltammograms,
and consequently, ELUMO were calculated being about −3.25 eV
from the values of Eg and EHOMO. In the same way, EHOMO and
ELUMO of PTPhN2 were estimated to be −5.34 and −3.46 eV.
These results indicate that PTPhN2 has not only the narrower
Eg but also larger electron accepting ability and lower ionization
ability than PTPhN1.
When PTPhN1 are excited with light energies at 256, 296,
and 387 nm, they showed a characteristic emission band around
540 nm in chloroform (Figure 4) and 550 nm in the film state.
The PL band is due to emission from the polythiophene
backbone and not from the phenylnaphthalene moiety, which
Table 2. Optical Properties and Energy Levels of the Polymers
UV−vis λmax/nm
solution film
386
PL λmax/nm
c
c
d
e
f
polymer
solution
film
Eg/eV
EHOMO/eV
ELUMO/eV
a
a
b
a
PTPhN1a
PTPhN1b
PTPhN2
385
387
444
534
550
559
720
2.25
2.25
1.86
−5.51
−5.50
−5.34
−3.26
−3.26
−3.46
a
387
509
541
b
585
a
b
c
d
e
In chloroform. In o-dichlorobenzene. Drop-cast films on a quartz plate. Calculated from absorption edge of the films. Estimated from onset of
f
the oxidation peak from cyclic voltammerty. Calculated from HOMO level and the optical bandgap.
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Figure 5. DSC thermograms of PTPhN1a (a), PTPhN1b (b),
PTPhN2 (c), and PhN (d) in the second sweeps with a scan rate of 10
°C min−1.
Table 3. Phase Transition Temperatures and Changes of
a,b
Enthalpy of Compounds
c
phase transition temperature/°C (ΔH /J g−1
heating process cooling process
)
compound
PTPhN1a G 115 (6.7) SmE 122 (10.4)
Iso 146 (−19.4) SmA
118 (−8.2) SmE
111 (−5.9) G
SmA 149 (19.9) Iso
PTPhN1b G 143 (19.9) Sm 157 (16.7)
Iso 151 (−17.4) Sm
Iso
132 (−16.3) G
d
d
d
d
PTPhN2
PhN
G 124 Sm 192 Iso
Iso 171 Sm 115 G
K 76 (16.1) SmE 83 (7.9)
SmB 106 (9.8) SmA
127 (21.4) Iso
Iso 123 (−21.5) SmA
103 (−8.4) SmB 79 (−10.1)
SmE 69 (−15.6) K
a
Abbreviations: K, crystal; G, glass state; SmA, B, E, defined smectic
phases as A, B, E; Sm, unresolved smectic phases; Iso, isotropic liquid.
b
Liquid crystalline phases were determined by DSC, POM, and XRD.
c
Determined from second DSC traces at a scan rate of 10 °C min−1.
Figure 6. Polarized optical micrographs of PTPhN1a at 128 °C in the
heating process (a), PTPhN1b at room temperature after isotropic
phase transition, annealing at 140 °C, and successive rapid cooling (b),
and PTPhN2 at room temperature after isotropic phase transition,
annealing at 150 °C, and rapid cooling (c).
d
Approximate transition temperatures of undefined mesophases.
makes phase transitions indistinct. Meanwhile, PTPhN2 did
not show any distinct mesophases both in DSC and POM
analyses, and the observed POM texture was a polygonal
(Figure 6c). This is ascribed to the strengthened interactions
between main chains as well as side mesogenic groups caused
by the high rr.
The phase transition behaviors of PTPhN1 thought to be
dominantly affected by liquid crystalline properties of the
phenylnaphthalene side chain. As listed in Table 3, PhN
showed three enantiotropic smectic phases (SmE, SmB, SmA)
between crystal phase (K) and Iso, whose behavior is similar to
those of series of phenylnaphthalene liquid crystals reported
previously.42 The ΔH of phase transition from Sm to Iso was
within 17−20 J g−1 for PTPhN1 and 21 J g−1 for PhN, which
approximately equated to 10 kJ mol−1. Although PTPhN1a and
PTPhN1b had different phase transition temperatures and
different number of distinct mesophases, their total ΔH
between G and Iso are almost equal to 37 J g−1 in the heating
process and −34 J g−1 in the cooling one. These results suggest
that similar change of molecular arrangement occurs at the side
chain moieties in PTPhN1 between the states of G and Iso.
A typical molecular arrangement of PTPhN1 was predicted
from X-ray diffraction (XRD) observations of a pristine drop-
cast film and an annealed sample of PTPhN1b on a Si
substrate. The annealed sample was prepared by heat-treating
at 175 °C (Iso) of the pristine film, cooling down and annealing
at 140 °C (Sm) for several minutes, and rapidly cooling down
to room temperature in a G state. As shown in Figure 7, the
polymer film samples showed two characteristic XRD signal
patterns, and the annealing sharpened and intensified both
signals with enhancement of molecular arrangements. The
former is a series of sharp peaks at 2θ = 2.5° (c ≈ 35 Å), 4.8 and
7.4 due to a layered distance of the phenylnaphthalene side
chains, which is assigned to Miller indices, (001)−(003). The
layer spacing of ca. 35 Å is almost corresponding to a length of
the side chain moiety in the most extended conformation. As a
result, the layered distance should be equivalent to interchain
distance of the polythiophene backbones. The latter is distinct
peaks at 2θ = 19.3° and 20.7° on broad multiple signals around
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phase was examined by XRD and UV−vis spectroscopies with a
cooled sample in glassy state. The patterns of XRD signals and
the anisotropy of UV−vis absorbance suggested that the
phenylnaphthalene side chains vertically oriented against to the
polythiophene backbones to construct a polymer array. The
observed distance of smectic layer (c ≈ 35 Å) of the side chains
is thought to correspond to the distance between the
polythiophene backbones. In addition, a lamellar structure
made by sheets of the rigid polymer backbones and
interdigitated flexible side chains are suggested to take a SmE
arrangement. Consequently, the arrayed PTPhN1 in the
smectic states are expected to be reflected by the semi-
conducting characteristics of phenylnaphthalene liquid crys-
tals,25,26,42 because the mesophases observed for PTPhN are
basically caused by aggregation of the phenylnaphthalene side
chains. On the other hand, PTPhN2 prepared by the latter
method is expected to have good photoabsorption and
semiconducting characteristics due to the fairly high regior-
egularity of polythiophene backbones. However, this attempt
brought about production of the extremely low soluble polymer
and its phase transition behaviors were obscure as observed in
DSC and POM in spite of having Mn comparable to PTPhN1,
which is a dilemma should be overcome by considering
intermolecular interactions between polymer main chains as
well as side groups.
Figure 7. XRD spectra of pristine (red) and annealed (black) films of
PTPhN1b and a suggested molecular arrangement.
15−30°, which is sum of the signals due to distances of
phenylnaphthalene moieties taking SmE domains involving
reflections such as (200), (020), (110), (220) with lattice
constants of a = 9.2 Å and b = 8.6 Å. Therefore, a layered π-
stacked distance between polythiophene sheets should come to
be 4.3 Å. PTPhN1a and PTPhN2 in G state basically showed
same XRD signal pattern of the pristine film of PTPhN1b. In
addition, heat treatment of PTPhN1a at 125 °C changed the
pattern to typical of SmA showing a layer distance (36 Å) read
from a sharp peak at 2θ = 4.8° and an intermolecular distance
(4.6 Å) between the mesogenic side chains read from a broad
signal at 2θ = 19.3°, while the pattern was almost invariant
against thermal annealing for PTPhN2.
Furthermore, comparison of the UV−vis absorption spectra
of the pristine and annealed films of PTPhN1b (Figure 3)
showed that the annealed film had a stronger absorbance at
around 400 nm due to the polythiophene backbone and weaker
absorbances at around 250 and 300 nm due to the
phenylnaphthalene moiety relative to those of the pristine
film. These results also support the molecular orientation of
PTPhN1 illustrated in Figure 7, because the perpendicular
oriented phenylnaphthalene moiety on a substrate hard to
absorb perpendicular incident radiation and, in contrast, the
parallel oriented polythiopehene backbones with torsion angles
in some degree are favorable for the light absorption.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures of compounds (1−6) and 1H NMR
spectra of the polymers. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax: +81-29-853-4490. Telephone: +81-29-853-6905. E-mail:
kijima@ims.tsukuba.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Dr. H. Goto, University of Tsukuba, for the
use of GPC and polarizing microscopes, and Chemical Analysis
Division, Research Facility Center for Science and Technology,
University of Tsukuba, for facilities of the NMR, elemental
analysis, DSC, UV−vis, and PL measurements.
CONCLUSIONS
■
2-Phenylnaphthalene liquid crystals that are ambipolar carrier
transporting semiconductor were introduced into 3-position of
thiophene as the side chain functionality. The synthesized
thiophene monomer was polymerized by Yamamoto method
using Ni(cod)2,27 and also polymerized by chain-growth
method using Ni(dppp)Cl2.28,29 PTPhN1a and PTPhN1b
prepared by the former method under the different conditions
were well soluble in organic solvents, and the observed general
characteristics of Mn (6000−9000 g mol−1), absorption λmax
(ca. 385 nm), and PL emission λmax (530−560 nm) were
ordinary for 3-substituted polythiophenes obtained by this
method. PL spectral analysis of PTPhN1 in solution and solid
states suggested that energy transfer from the phenyl-
naphthalene side-chain to the polythiophene backbone
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