Synthesis and properties of thermotropic liquid-crystalline polyesters containing 9,10-diphenylanthracene moiety in the main chain

Article abstract of DOI:10.1007/s11164-012-0658-y

We synthesized thermotropic liquid-crystalline polyesters in which 9,10-diphenylanthracene moieties are incorporated into the main chain type of polyester forming the chiral smectic C (Sm C*). The polymers were prepared by the isopropyltitanate-catalyzed reaction of biphenyldicarboxylic acid and the corresponding diols, with different ratios of diol of 9,10- diphenylanthracene moiety to the alkane diols (1, 5, and 10 mol %) under nitrogen atmosphere. The polymers exhibited thermotropic liquid crystals despite the presence of a bulky diphenylanthracene moiety in the main chain. The circular dichroism spectra revealed that a Sm C*phase was formed in the polymer with 1 mol % of anthracene moiety, although only an Sm A phase was formed in the other polymers. This is the first example of a Sm C*polyester containing a diphenylanthracene moiety in the main chain. Furthermore, we measured the optical properties of the polymers and found that they exhibited very high fluorescent efficiency. The fluorescence spectra of the thin film differed from that of a CH₂Cl₂ solution.

Full text of DOI:10.1007/s11164-012-0658-y

Res Chem Intermed (2013) 39:403–414
DOI 10.1007/s11164-012-0658-y
Synthesis and properties of thermotropic liquid-
crystalline polyesters containing 9,10-
diphenylanthracene moiety in the main chain
•
Makoto Uchimura Ryohei Ishige
•
•
Masayuki Shigeta Yuki Arakawa Yosuke Niko
•
•
•
Junji Watanabe Gen-Ichi Konishi
Received: 3 November 2011 / Accepted: 11 May 2012 / Published online: 15 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract We synthesized thermotropic liquid-crystalline polyesters in which
9,10-diphenylanthracene moieties are incorporated into the main chain type of
polyester forming the chiral smectic C (Sm C*). The polymers were prepared by the
isopropyltitanate-catalyzed reaction of biphenyldicarboxylic acid and the corre-
sponding diols, with different ratios of diol of 9,10-diphenylanthracene moiety to
the alkane diols (1, 5, and 10 mol %) under nitrogen atmosphere. The polymers
exhibited thermotropic liquid crystals despite the presence of a bulky diphenylan-
thracene moiety in the main chain. The circular dichroism spectra revealed that a
Sm C* phase was formed in the polymer with 1 mol % of anthracene moiety,
although only an Sm A phase was formed in the other polymers. This is the first
example of a Sm C* polyester containing a diphenylanthracene moiety in the main
chain. Furthermore, we measured the optical properties of the polymers and found
that they exhibited very high fluorescent efficiency. The fluorescence spectra of the
thin film differed from that of a CH2Cl2 solution.
Keywords Liquid crystal ꢀ Polyester ꢀ 9,10-Diphenylanthracene ꢀ
Fluorescence ꢀ Chiral polymer
Dedicated to Professor Kazuhiko Mizuno for his outstanding contribution to organic photochemistry.
M. Uchimura ꢀ R. Ishige ꢀ M. Shigeta ꢀ Y. Arakawa ꢀ Y. Niko ꢀ J. Watanabe ꢀ G.-I. Konishi (&)
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama,
Meguro-ku, Tokyo 152-8552, Japan
e-mail: konishi.g.aa@m.titech.ac.jp
G.-I. Konishi
PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan
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Introduction
Recently, much attention has been paid to organic light-emitting diodes (OLEDs)
such as organic electroluminescent devices and organic lasers [1–20]. It is essential
to develop a highly luminescent dye for use in these diodes. A large number of
luminescent dyes have been synthesized. In particular, 9,10-diphenylanthracene
exhibits excellent fluorescent properties (e.g., high quantum efficiency, pure blue
fluorescence, and thermal stability) [21–27]. A variety of polymers containing 9,10-
diphenylanthracene moieties have been synthesized and researched because of their
potential applications in optical devices [21–25]. The polymers are soluble in
organic solvents for easy processability, and the strength of the polymer films
enables flexible devices to be realized. In addition, by introducing a diphenylan-
thracene moiety into the polymer main chain, intermolecular quenching can be
prevented efficiently and high luminous efficiency can be expected in solid-state-
like films. Therefore, diphenylanthracene and its related polymers are expected to be
profitable for applications in various optical devices such as OLEDs [22–28].
A liquid crystal (LC)-laser system is one such optical device. Cholesteric, chiral
smectic C (Sm C*), and some LCs are known to have a helical structure. When a
fluorescent dye is injected into these materials, the emission is enhanced because of
the helical structure and thus laser oscillation can be observed. Such LC-laser
systems are expected to have tunable wavelengths and very small size [29–35]. In
addition, in the case of such LC-laser systems, high-luminous-efficiency polymers
such as diphenylanthracene are very attractive materials, because they can use
energy more efficiently. However, Jin et al. [21] reported that the diphenylanthra-
cene moieties in the polymer main chain inhibit the arrangement of LC molecules,
and hence, these polymers cannot form a liquid-crystalline phase.
Here, we report the first example of a Sm C* polyester containing a 9,10-
diphenylanthracene moiety in the main chain, in which a helical structure is formed
along the average polymer chain axis [36–38], and investigate the effect of
diphenylanthracene moieties on the liquid-crystalline behavior. The partly intro-
duced diphenylanthracene group is expected to act as a laser dye for LC-lasers.
Results and discussion
Synthesis and characterization
A 9,10-diphenylanthracene monomer, which is a fluorescent moiety of the polymer,
was synthesized according to Scheme 1. First, a 4-methoxyphenyl group was
introduced to 9,10-dibromoanthracene by Suzuki–Miyaura coupling [39, 40]. Then,
after the phenol moiety was deprotected by BBr3, alkyl chains were introduced as a
spacer by Williamson ether synthesis. The overall yield was 45 %.
The polymerization was carried out according to the procedure in our previous
report [36]. The polymers were synthesized by melt transesterification of dimethyl
4,40-biphenyldicarboxylate and an appropriate mixture of 1,6-hexanediol (M1),
(S)-2-methyl-1,4-butanediol (M2), and anthracene derivative diol (M3) in the
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Synthesis and properties of thermotropic liquid-crystalline polyesters
405
Scheme 1 Synthesis of anthracene monomer
Table 1 Properties of polymers
1–3
Molar ratio
(M1 ? M2a:M3)
Yield (%)
Mnb
Mw/Mbn
Polymer 1
Polymer 2
Polymer 3
99:1
95:5
90:10
63
65
70
3,600
2,600
3,200
1.7
1.5
1.5
a
M1:M2 = 3:7
b
Determined by GPC (eluent:
THF, polystyrene standards)
presence of isopropyltitanate (Scheme 2). According to the previous study [38], BB-
4*/6(70/30) polyester was selected as a based polyester with M1 and M2 in molar
ratio of 3:7 since it formed a Sm C* having pitch of 400–450 nm. Three polymers
having different ratios of 9,10-diphenylanthracene-based M3 (1, 5, and 10 %) to the
sum of M1 and M2 were synthesized (Table 1).
The polymerization results are summarized in Table 1. The structures of the
1
obtained polymers were confirmed by H nuclear magnetic resonance (NMR) and
1
Fourier-transform infrared (FT-IR) spectra. Figure 1 shows the H NMR spectrum
of polymer 3. 1H NMR integration confirmed that the main chain contained
10 mol % of diphenylanthracene moieties, 27 and 44 mol % of M1 and M2,
respectively, and 19 mol % methoxy ester. The number-average molecular weight
1
(Mn) determined by H NMR was about 3,400. The amount of diphenylanthracene
1
moieties in polymer 1 was too little for them to be observed in the H NMR
spectrum. However, by using gel permeation chromatography (GPC) (detecting
wavelength changed from 254 to 350 nm), it was confirmed that the diphenylan-
thracene moiety existed in the main chains of polymers 1–3.
Liquid crystallinity
The thermal behaviors of polymers 1–3 were investigated using differential scan-
ning calorimetry (DSC) and polarizing optical microscopy (POM). The curves in
Fig. 2a, b show the DSC thermograms of polymers 1 and 3, respectively. Polymer 1
showed a phase transition similar to that of the based BB-4*/6(70/30) polyester
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M. Uchimura et al.
Scheme 2 Synthesis of liquid-crystalline polymers
O
Ha
Hb
O
O
O
O
O
O
O
O
He
Hf
O
O
Hc
O
O
Ho
Hn
Hh
Hg
Hj
O
m
Hd
O
O
O
Hl
Hm
l
O
Hi
Hk
n
Hg
Hd
Hc
Hj
Hf
Hk, Hl, Hm, Hn
Ho MeOH
Hh, Hi
He
Ha, Hb
Fig. 1 1H NMR spectrum of polymer 3 (400 MHz, CDCl3)
[crystal $ Sm C* $ Sm A $ isotropic (Iso)] [38]. Although the Sm C*-Sm A
transition is hardly detected from the DSC curve because of its small enthalpy
change, it is clearly detected from the POM observation; very fine fan-shaped
textures of Sm A are developed at 220 °C on cooling from the isotropic phase, and
then at 160 °C, a fan-shaped Sm A texture transforms to a broken-fan-shaped
texture characteristic of Sm C* [38]. The POM images of respective textures for
polymer 1 at 170 and 80 °C are shown in Fig. 3a and b, respectively. On the other
hand, polymer 3 did not show the evident peaks in DSC thermogram (Fig. 2b).
However, crystal $ Sm A $ Iso transitions could be detected from the POM
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Synthesis and properties of thermotropic liquid-crystalline polyesters
407
Fig. 2 DSC thermograms of polymer 1 (a) and 3 (b)
Fig. 3 POM images of polymers 1 and 3. a Fan-shaped texture at 170 °C for polymer 1. b Broken fan-
shaped texture at 80 °C for polymer 1. c Fan-shaped texture at 160 °C for polymer 3
observation for polymer 3, in which the fan-shaped texture appeared at 160 °C
(Fig. 3c). Moreover, polymer 2 showed similar results to polymer 3.
For polymer 1, the Sm C* formation from the Sm A was also clarified from the
observation of the circular dichroism (CD) reflection spectrum which is due to the
formation of helical structure. The CD spectra observed for the homeotropically
aligned sample are shown in Fig. 4. The reflection band is clearly observed with the
peak maximum around 440 nm, although it disappears on heating to the Sm A
phase. The peak maximum position is nearly equal to that of the based BB-4*/6(70/
30) polyester. The above two results show that the Sm A–Sm C* transition is
around 160 °C.
The phase-transition temperatures of polymer 1–3 are summarized in Table 2.
The introduction of diphenylanthracene, as a foreign compound from the BB-6
polyester, decreased the phase-transition temperatures and then diminished the
Sm C* phase with content above 5 %, although the Sm A phase was still formed for
the high content (10 %).
The phase behaviors of polymer 1 and 3 were also detected by wide-angle X-ray
diffraction. The temperature dependence of the layer spacing and tilt angles are
shown in Fig. 5. In the case of polymer 1, the layer spacing increased with
temperature whereas the tilt angles decreased. These values became constant at
around 160 °C. This result indicates that the phase transition (Sm C* $ Sm A)
occurs at around 160 °C. In contrast, in the case of polymer 3, the layer spacing and
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M. Uchimura et al.
20
15
10
5
Fig. 4 CD spectra of polymer 1
at 180 °C (broken line) and
100 °C (solid line). Here, the
spectra is observed for the
homeotropic aligned sample that
is prepared by sharing the Sm A
phase between glasses [38]
180°C
100°C
0
400
500
600
700
wavelength/nm
Table 2 Phase-transition
temperatures for polymers 1–3
Transition temperaturea/°C
Polymer 1
Polymer 2
Polymer 3
Iso 224.8 $ Sm A 160 $ Sm C* 142.7 $ Tg 29.8
Iso 220.2 $ Sm A 140.6 $ Cr
Iso 200–220 $ Sm A 137.4 $ Cr
a
Polymers 1–3 were cooled at a
rate of 10 °C min-1
tilt angles did not change at all, indicating the formation of Sm A in the total
mesophase temperature region, as detected in POM observation.
From these results, it is considered that excess amount of anthracene moiety
inhibits the formation of Sm C*. It is well known that these helical structures of LCs
are very sensitive to aspects of the environment such as the nature of the chiral
dopant, temperature, mechanical stress, and field conditions [29–35, 41]. In this
case, a small amount of diphenylanthracene moiety may have changed the balance
of the environment, interrupting the formation of a helical structure, while even a
high content (10 mol %) in the main chain does not prevent formation of a liquid-
crystalline phase.
Optical properties
The normalized absorption and fluorescence spectra of M3, polymer 1, and polymer
3 are shown in Fig. 6, and their optical properties are summarized in Table 3. To
compare their fluorescence efficiencies, the relative fluorescence quantum yield
(UF) emitted from anthracene moiety in polymers 1 and 3 were calculated as the M3
standard (the calculation method is summarized in the ‘‘Experimental’’ section).
There exist no differences between the spectra of monomers and polymers.
However, the relative UF of all the polymers are lower than that of M3.
We measured the fluorescence spectrum of polymer 1 in a film state (Fig. 7). The
fluorescence peak was observed at 437 nm. The value was shifted by 10 nm from
that of the polymer solution. Remarkably, the fluorescence spectrum of the film
perfectly overlapped the CD spectrum of polymer 1 (Fig. 8). This indicates the
possibility of interaction between the fluorescence of our synthesized polymers and
123

Synthesis and properties of thermotropic liquid-crystalline polyesters
409
18
17
16
Fig. 5 Temperature
polymer 1
polymer 3
dependence of layer spacing and
tilt angles (filled triangles
polymer 1, filled circles polymer
3)
ο
20
15
10
5
0
80
100
120
140
160
180
200
Temperature (°C)
M1
polymer 1
polymer 3
M1
polymer 1
polymer 3
340
360
380
400
420
440
400
450
500
550
600
wavelength/nm
wavelength/nm
Fig. 6 Normalized absorption and fluorescence spectra of M3, polymer 1, and polymer 3 measured in
CH2Cl2
their helical structure [42], suggesting the possibility of laser oscillation. The
realization of laser oscillation is now in progress.
Conclusions
We synthesized new liquid-crystalline polyesters, polymer 1–3, containing a
9,10-diphenylanthracene moiety in the main chain. These polymers exhibited
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M. Uchimura et al.
Table 3 Optical properties of M3, polymer 1, and polymer 3
kabs (nm)a
kem (nm)a,b
UFa
M3
358, 375, 394
358, 373, 394
358, 373, 393
427
427
427
0.67c
0.50d
0.51d
Polymer 1
Polymer 3
a
Measured in de-aerated CH2Cl2
Excited at absorption maxima
Absolute quantum yield
b
c
d
Calculated by using M3 as a standard
Fig. 7 Fluorescence spectra of
polymer 1 in CH2Cl2 and film
state (film thickness: 25 lm)
CH2Cl2solution
film
400
450
500
550
600
wavelngth/nm
20
15
10
5
Fig. 8 Fluorescence and CD
spectra of polymer 1
CD (right)
fluorescence (left)
0
400
450
500
550
600
wavelength/nm
liquid-crystalline behavior despite the presence of a bulky fluorescence moiety in
the main chain. A Sm C* phase was observed in polymer 1 with 1 mol % of
anthracene moiety similarly to in the based BB–4*/6(70/30) polyester. This is the
first example of a Sm C* polyester containing a bulky fluorescent moiety in the
main chain. On the other hand, a Sm C* phase disappears in the polymers
containing 5 and 10 mol % of anthracene moiety, although the ability for formation
of Sm A is sustained. This indicates that an excessive amount of dye molecules
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Synthesis and properties of thermotropic liquid-crystalline polyesters
411
inhibits only the formation of the Sm C* phase. These polymers exhibit high
quantum yields (ca. UF = 0.51). In the case of polymer 1, overlap of the
fluorescence and CD spectra was observed, indicating the possibility of laser
oscillation in the helical field of Sm C*.
Experimental
Instruments
Infrared (IR) spectra were recorded ona JASCO FT-IR460 plus spectrometer. Nuclear
magnetic resonance (NMR) spectra were measured in CDCl3 on a JEOL LNM-EX 400
at room temperature using tetramethylsilane as internal standard. GPC was carried out
by a JASCO UV-2070 detector and a JASCO RI-2031 detector (TOSOH TSKgel
G3000HXL column) using tetrahydrofuran (THF) as eluent after calibration with
polystyrene standards. The thermal behavior was investigated by POM (Leica
DM2500P microscopy with a Mettler FP90 hot stage) and differential scanning
calorimetry (Perkin Elmer DSC7) with heating and cooling scans performed at 10 °C
min-1. X-ray diffraction photographs were taken at different temperatures by using
Ni-filtered Cu Ka radiation. The temperature was measured and regulated by using a
Mettler FP90 heater. The sample-to-specimen distance was determined by calibration
with silicon powder. The reflection spectra of light were measured with Jasco model J-
20 and Hitachi model 330 spectrometers. UV–Vis spectra were recorded by a
Beckman Coulter DU800 UV–Vis spectrophotometer. Fluorescence spectra were
recorded by a JASCO FP-6500 spectrofluorometer. The relative quantum yield was
calculated according to Eq. (1), where UF(ref) = 1.00 (relative quantum yield in
CH2Cl2 solution of M3), UF(A) are relative UF in CH2Cl2 solution of synthesized
polymer, PLA and PLref are the integrated fluorescence intensityes of the sample and
the standard, respectively, UVA and UVref are the absorbances of the sample and the
standard at the excitation wavelength, respectively, and gA and gref are the refractive
indices of the corresponding solutions.
ꢀ
ꢁ
2
PLA UVref
ꢁ
gA
/
¼ /FðrefÞ ꢁ
ꢁ
:
ð1Þ
FðAÞ
UVA PLref
gref
Materials
Unless otherwise noted, all chemicals were commercially available and used as
received. Dibromoanthracene, 3,5-dimethoxyphenylbronic acid, 6-bromohexanol,
dimethyl 4,40-biphenyldicarboxylate, 1,6-hexanediol, (S)-2-methyl-1,4-butanediol,
and Pd(PPh3)4 were purchased from TCI, Japan.
9,10-Bis(4-methoxyphenyl)anthracene (1)
A solution of 1 M Na2CO3 aq (10 mL), toluene (40 mL), 3,5-dimethoxyphenylb-
ronic acid (12 mmol, 1.82 g), dibromoanthracene (3 mmol, 1.00 g), and Pd(PPh3)4
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M. Uchimura et al.
(0.15 mmol, 0.17 g) was stirred and heated at 80 °C under argon atmosphere for
2 days. The progress of the reaction was checked by thin-layer chromatography.
The reaction mixture was extracted with toluene, and the organic layer was dried
over MgSO4. After removal of solvent under reduced pressure, the residue was
washed with THF and recrystallized with chloroform. The objective material, 1, was
obtained as solids (1.1 g, 2.82 mmol, 94 %). 1H NMR (400 MHz, CDCl3): d
7.72–7.75 (m, Ar–H, 4H), 7.39 (d, J = 8.1 Hz, Ar–H, 4H), 7.26–7.34 (m, Ar–H,
4H), 7.14 (d, J = 8.3, Ar–H, 4H), 3.97 (s, –OCH3, 6H) ppm.
9,10-Bis(4-hydroxyphenyl)anthracene (2)
Anhydrated CH2Cl2 (30 mL) was added to 1 (2.5 mmol, 0.98 g) under argon
atmosphere. Then, BBr3 in hexane (12.5 mmol, 12.5 mL) was added to the resulting
solution at 0 °C. After stirring overnight at room temperature, the mixture was
quenched with water. The reaction mixture was extracted with AcOEt, and the
organic layer was dried over MgSO4. After removal of solvent under reduced
pressure, the solid was recrystallized from acetonitrile. The objective material, 2,
1
was obtained as solids (0.66 g, 1.83 mmol, 73 %). H NMR (400 MHz, CDCl3): d
7.72–7.74 (m, Ar–H, 4H), 7.32–7.35 (m, Ar–H, 8H), 7.07 (d, J = 7.8, Hz Ar–H,
4H), 4.88 (s, Ar–OH, 2H) ppm.; FT-IR (KBr): m 3,421, 3,063, 1,605, 1,512, 1,393,
1,246, 1,167, 822, 772 cm-1
.
9,10-Bis(4-(4-hydroxyhexyl)oxyphenyl)anthracene (M3)
2 (1.40 mmol, 0.51 g), K2CO3 (5.00 mmol, 0.69 g), 6-bromohexanol (6.00 mmol,
0.82 mL), and small amount of KI were mixed in acetonitrile (30 mL). The
resulting mixture was refluxed for 4 days. Then, the mixture was extracted with
CHCl3, washed with water, and dried over MgSO4. After removal of solvent
under reduced pressure, the solid was recrystallized from MeOH. The objective
material, M3, was obtained as solids (0.52 g, 0.92 mmol, 66 %). 1H NMR
(400 MHz, CDCl3): d 7.74 (dd, J = 3.4, 6.8 Hz, Ar–H, 4H), 7.37 (d, J = 8.3 Hz,
Ar–H, 4H), 7.32 (dd, J = 3.4, 6.8 Hz, Ar–H, 4H), 7.12 (d, J = 8.3 Hz, Ar–H,
4H), 4.12 (t, J = 6.4 Hz, Ar–OCH2, 4H), 3.71 (brdt, J = 4.9, 6.4 Hz, -OCH2,
4H), 1.92 (tt, J = 6.4, 7.3 Hz, 4H), 1.71–1.49 (m, 12H), 1.30 (brt, J = 4.9 Hz,
-OH, 2H) ppm.; FT-IR (KBr): m 3,422, 2,933, 2,859, 1,605, 1,510, 1,389, 1,241,
1,175, 769 cm-1
.
General procedure for polymerization
Dimethyl 4,40-biphenyldicarboxylate and an appropriate mixture of M1, M2, M3,
and small amount of isopropyltitanate were placed in polymerization tube with
bubbled nitrogen, and stirred at 230 °C. After polymerization, obtained polymer
was dissolved into the minimum volume of chloroform, and reprecipitated in
methanol twice. After filtering, the objective polymer was obtained.
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Synthesis and properties of thermotropic liquid-crystalline polyesters
413
Polymer 1
According to the general procedure, 4,40-biphenyldicarboxylate (270 mg, 1 mmol),
M1 (35 mg, 0.30 mmol), M2 (72 mg, 0.69 mmol), M3 (5.6 mg, 0.01 mmol).
Yield: 63 %. 1H NMR (400 MHz, CDCl3): d 8.12, 7.65, 4.43–4.54, 4.37, 4.30, 3.94,
2.24, 2.05, 1.75–1.84, 1.16 ppm.; FT-IR (KBr): m 3,421, 2,955, 2,888, 1,715, 1,607,
1,510, 1,396, 1,274, 1,175, 756 cm-1
.
Polymer 2
According to the general procedure, 4,40-biphenyldicarboxylate (500 mg, 1.85
mmol), M1 (60 mg, 0.50 mmol), M2 (120 mg, 1.17 mmol), M3 (104 mg, 0.185
1
mmol). Yield: 70 %. H NMR (400 MHz, CDCl3): d 8.12, 7.73–7.75, 7.65–7.68,
7.37, 7.30–7.32, 7.12, 4.45–4.54, 4.36–4.43, 4.29–4.31, 4.12–4.15, 3.95, 2.22–2.25,
2.03–2.07, 1.77–1.96, 1.16 ppm.; FT-IR (KBr): m 3,421, 2,947, 2,864, 1,715, 1,607,
1,512, 1,395, 1,274, 1,175, 1,105, 756 cm-1
.
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