Synthesis and emission behavior of liquid-crystalline main-chain polyesters containing carbazole and oxadiazole moieties

Article abstract of DOI:10.1080/15421406.2012.688625

Organic light-emitting diodes (OLEDs) based on chromophore-containing liquid crystals (LCs) can emit polarized light due to self-assembly of mesogenic chromophores. The stability of LC phases in LC polymers with a mesogenic group in the main chain is higher than that in the polymers containing a mesogenic group in the side chain. In this study, a novel family of polyesters containing a carbazole moiety as a hole-transporting unit and an oxadiazole (OXD) moiety as an electron-transporting unit in the main chain, P10-DOXDCz and P16-DOXDCz, were designed and synthesized. These polyesters were prepared by transesterification of N-hexyl-3,6-di[2-([1,3,4]oxadiazol-5-yl)-4- methylbenzoate]carbazole and 1,10-decanediol or 1,16-hexadecanediol at 220C for 20h. They showed a nematic phase. Both P10-DOXDCz and P16-DOXDCz emitted intense blue fluorescence with a maximum at 439nm and 450nm in thin films, respectively, and their fluorescence quantum yields were 0.74 and 0.81 in solution, respectively. Both compounds exhibited polarized emission in a LC phases, and the order parameters estimated from the polarized emission spectra were 0.10 for P10-DOXDCz and 0.14 for P16-DOXDCz. Fabricated devices, ITO/PEDOT:PSS/P10-DOXDCz and P16-DOXDCz/MgAg, emitted blue light when a positive voltage was applied to the ITO electrode. Taylor and Francis Group, LLC.

Full text of DOI:10.1080/15421406.2012.688625

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Synthesis and Emission Behavior of
Liquid-Crystalline Main-Chain Polyesters
Containing Carbazole and Oxadiazole
Moieties
Motoi Kinoshita a , Kenji Takamura a , Masuki Kawamoto a , Atsushi
Shishido a , Takeshi Shiono a & Tomiki Ikeda a
a Chemical Resources Laboratory, Tokyo Institute of Technology,
R1-12, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan
To cite this article: Motoi Kinoshita , Kenji Takamura , Masuki Kawamoto , Atsushi Shishido , Takeshi
Shiono & Tomiki Ikeda (2012) Synthesis and Emission Behavior of Liquid-Crystalline Main-Chain
Polyesters Containing Carbazole and Oxadiazole Moieties, Molecular Crystals and Liquid Crystals,
563:1, 92-100, DOI: 10.1080/15421406.2012.688625
To link to this article: http://dx.doi.org/10.1080/15421406.2012.688625
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Mol. Cryst. Liq. Cryst., Vol. 563: pp. 92–100, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.688625
Synthesis and Emission Behavior
of Liquid-Crystalline Main-Chain Polyesters
Containing Carbazole and Oxadiazole Moieties
MOTOI KINOSHITA,∗ KENJI TAKAMURA, MASUKI
KAWAMOTO, ATSUSHI SHISHIDO, TAKESHI SHIONO,
AND TOMIKI IKEDA
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-12,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Organic light-emitting diodes (OLEDs) based on chromophore-containing liquid crys-
tals (LCs) can emit polarized light due to self-assembly of mesogenic chromophores.
The stability of LC phases in LC polymers with a mesogenic group in the main chain is
higher than that in the polymers containing a mesogenic group in the side chain.
In this study, a novel family of polyesters containing a carbazole moiety as a hole-
transporting unit and an oxadiazole (OXD) moiety as an electron-transporting unit in the
main chain, P10-DOXDCz and P16-DOXDCz, were designed and synthesized. These
polyesters were prepared by transesterification ofN-hexyl-3,6-di[2-([1,3,4]oxadiazol-5-
yl)-4-methylbenzoate]carbazole and 1,10-decanediol or 1,16-hexadecanediol at 220 ◦C
for 20 h. They showed a nematic phase. Both P10-DOXDCz and P16-DOXDCz emit-
ted intense blue fluorescence with a maximum at 439 nm and 450 nm in thin films,
respectively, and their fluorescence quantum yields were 0.74 and 0.81 in solution, re-
spectively. Both compounds exhibited polarized emission in a LC phases, and the order
parameters estimated from the polarized emission spectra were 0.10 for P10-DOXDCz
and 0.14 for P16-DOXDCz. Fabricated devices, ITO/PEDOT:PSS/P10-DOXDCz and
P16-DOXDCz/MgAg, emitted blue light when a positive voltage was applied to the ITO
electrode.
Keywords Electroluminescence; polarized emission; carbazole; oxadiazole; main-
chain liquid-crystalline polymer; bipolar
Introduction
Recently, much attention has been devoted to organic light-emitting diodes (OLEDs), which
consist of electron-transporting, hole-transporting and emitting materials [1–4]. Among
various materials, polymers are generally expected to be suitable for OLEDs because of
their good processability and simple fabrication by solution processes, even though the
efficiency of single-layer OLEDs is lower than that of multi-layer OLEDs. One useful
approach for the fabrication of simple and efficient single-layer devices is to employ
∗Address correspondence to Dr. Motoi Kinoshita, Chemical Resources Laboratory, Tokyo In-
stitute of Technology, R1-12, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. Tel: (+81)
42-924-5241. Fax: (+81) 42-924-5275. E-mail: mkinoshita@res.titech.ac.jp
92

Synthesis and Emission Behavior of LC Polyesters
93
polymers containing oxadiazole (OXD) as an electron-transporting moiety and carbazole
as a hole-transporting moiety [5–11].
Liquid crystals (LCs) are ordered fluids, exhibiting anisotropic mechanical, electri-
cal, magnetic and optical properties. Most of practical applications using LCs require
control of molecular alignment on a substrate. Several low-molecular-weight LCs have
been synthesized and investigated as electroluminescent (EL) materials. OLEDs us-
ing 2-(p-hexyloxybiphenyl)-5-(4-hexyloxyphenyl)-1,3,4-oxadiazole reported by Tsutsui
et al. emitted light with a brightness of 0.8 cd/m2 over a driving voltage of 280 V
[12]. In the previous study, we evaluated the EL behavior of a cell containing 1,4-
bis(N,N-diphenylaminophenyl-1,3,4-oxadiazolyl)benzene in a low-molecular-weight LC,
4-octyloxy-4ꢀ-cyanobiphenyl [13]. EL was observed at the LC state at a driving voltage of
135V [14, 15]. Poly(arylenevinylene) and polyfluorene (PF) derivatives reported by Wen-
dorff et al. were explored in OLEDs by use of LC polymers with conjugated rigid rods and
flexible alkyl spacers. The alkyl spacer allows easier molecular alignment in LC phases.
For these polymers, maximum order parameters achieved in aligned layers were below 0.7.
We have previously synthesized a novel family of side-chain LC polymers containing
an OXD and an amine or a carbazole moiety for OLEDs. These side-chain LC polymers
showed bipolar character and functioned as polarized blue-emitting materials in OLEDs [11,
12]. In this study, we designed and synthesized a novel family of main-chain LC polymers
with bipolar character, P10-DOXDCz and P16-DOXDCz, to enhance the stability of LC
phases with a mesogenic group in the main chain.
Experimental
The methyl ester containing the OXD and carbazole moieties, N-hexyl-3,6-di[2-
([1,3,4]oxadiazol-5-yl)-4-methylbenzoate]carbazole, was prepared starting from carbazole
by the synthetic route shown in Scheme 1. All reagents were purchased from Tokyo Kasei
Co. Ltd. Kanto Kagaku Co. Ltd., and Aldrich Co. Ltd.. The compounds were identified
1
by H NMR (JEOL, GX300 (300 MHz)), elemental analysis and mass spectra (JMS-700
spectrometer with fast atom bombardment (FAB)). Molecular weight was determined by
gel permeation chromatography (GPC; JASCO, model DG-980-50; column, Shodex GPC
K802 + K804 + K805; eluent, chloroform) calibrated with standard polystyrenes. Ab-
sorption spectra were measured using a JASCO model U-550. Photoluminescence (PL)
and EL spectra were obtained using a Hitachi F-4010 luminescence spectrometer. Fluo-
rescence quantum yields of the polymers were determined by using quinine sulfate as a
standard compound. LC and phase-transition behavior was examined on an Olympus BH-2
polarizing microscope equipped with Mettler hot-stage (FP-90 and FP-82). Thermotropic
properties of LCs were determined with a differential scanning calorimeter (Seiko I&E,
◦
SSC-5200 and DSC220C) at a heating/cooling rate of 10 C/min. The EL behavior was
evaluated with an electrometer (Advantest R6243).
Synthesis
N-Hexylcarbazole
A mixture of carbazole (75.0 g, 450 mmol) and potassium carbonate (75.4 g, 545 mmol) in
DMF (250 ml) was added into a 1-bromohexane (74.3 g, 450 mmol) followed by refluxing
for 12 h. The crude product was extracted with chloroform and washed water. The product

94
M. Kinoshita et al.
Scheme 1. Synthetic route of the polymers used in this study.
was purified by silica gel column chromatography with an eluent of ethyl acetate/n-hexane.
Yield: 62.1 g, 55%. 1H NMR (δ, CDCl3): 0.74–0.81 (t, 3H), 1.16–1.29 (m, 6H), 1.74–1.79
(m, 2H), 4.13–4.18 (t, 2H), 7.09–7.38 (m, 6H), 7.98–8.01 (d, 2H), 8.32–8.35 (s, 2H). Anal.
Calcd. for C18H21N: C, 86.01%; H, 8.42%; N, 5.57%. Found. C, 86.13%; H, 8.51%; N,
5.67%.
N-Hexyl-3,6-diformylcarbazole
Phosphoryl chloride (91.2 g, 595 mmol) was added slowly into DMF (43.5 g, 595 mmol),
◦
which was purged with nitrogen and cooled to 0 C. T◦he reactant was warmed to room
temperature and stirred for 1 h and cooled again to 0 C. This mixture was added to a
solution of N-hexylcarbazole (30.0 g, 119 mmol) in 1,2-dichloroethane (100 ml) and the
resulting mixture was kept at 90 ◦C for 48 h. The crude product was poured into ice water
and extracted with chloroform and washed water. The product was purified by silica gel
column chromatography with an eluent of ethyl acetate/n-hexane and recrystallized from
methanol. Yield: 2.17 g, 22%. 1H NMR (δ, CDCl3): 0.82–0.87 (t, 3H), 1.24–1.53 (m, 6H),
1.85–1.95 (m, 2H), 4.35–4.37 (t, 2H), 7.52–7.55 (d, 2H), 8.06–8.09 (d, 2H), 8.66 (s, 2H),

Synthesis and Emission Behavior of LC Polyesters
95
10.12 (s, 2H). Anal. Calcd. for C20H21NO2: C, 78.15%; H, 6.89%; N, 4.56%. Found. C,
78.08%; H, 7.14%; N, 4.54%.
N-Hexyl-3,6-dicyanocarbazole
A mixture of N-hexyl-3,6-diformylcarbazole (2.01 g, 6.51 mmol), hydroxylamine hy-
drochloride (2.10 g, 32.5 mmol), acetic anhydride (3.32 g, 32.5 mmol), pyridine (2.57g,
32.5 mmol) and DMF (50ml) was stirred and heated at 140 ◦C for 8 h. The crude product
was extracted with chloroform and washed water. The product was purified by silica gel col-
umn chromatography with an eluent of chloroform and recrystallized from hexane/ethanol.
Yield: 1.61 g, 83%. 1H NMR (δ, CDCl3): 0.82–0.95 (t, 3H), 1.20–1.56 (m, 6H), 1.81–1.88
(m, 2H), 4.31–4.36 (t, 2H), 7.49–7.56 (d, 2H), 7.75–7.8 (d, 2H), 8.32–8.35 (s, 2H). Anal.
Calcd. for C20H19N3: C, 79.70%; H, 6.35%; N, 13.94%. Found. C, 79.25%; H, 6.80%; N,
13.60%.
N-Hexyl-3,6-ditetrazolylcarbazole
A mixture of N-hexyl-3,6-dicyanocarbazole (1.3 g, 4.31 mmol), sodium azide (0.84 g,
12.9mmol), ammoniumchloride(0.7g, 12.9 mmol) and DMF(20ml) was stirredand heated
at 140 ◦C for 10 h. The cooled solution was slowly poured into water. Recrystallization from
chloroform/methanol gave the desired product. Yield: 1.67 g, 75%. 1H NMR (δ, DMSO):
0.77–0.87 (t, 3H), 1.15–1.33 (m, 6H), 1.76–1.86 (m, 2H), 4.21–4.31 (t, 2H), 7.33–7.45
(d, 2H), 7.73–7.74 (d, 2H), 8.32–8.35 (s, 2H). Anal. Calcd. for C20H21N9: C, 62.0%; H,
5.46%; N, 32.54%. Found. C, 62.50%; H, 5.70%; N, 32.22%.
N-Hexyl-3,6-di[2-([1,3,4]oxadiazol-5-yl)-4-methylbenzoate]carbazole
A mixture of N-hexyl-3,6-ditetrazolylcarbazole (1.6 g, 4.13 mmol), terephthalic acid
monomethyl ester chloride (8.2 g, 41.3 mmol) and pyridine (20 ml) was stirred and heated
at 140 ◦C for 24 h. The cooled solution was slowly poured into dilute hydrochloric acid. The
crude product was extracted with chloroform and washed water. The product was purified
by silica gel column chromatography with an eluent of chloroform/THF and recrystallized
1
from chloroform/methanol. Yield: 1.3 g, 46%. H NMR (δ, CDCl3): 0.84–0.88 (t, 3H),
1.20–1.38 (m, 8H), 1.88–1.92 (m, 2H), 3.95–3.96 (s, 6H) 4.19–4.34 (t, 2H), 7.47–7.49(d,
2H), 8.15–8.22 (d, 10H), 8.80–8.82 (s, 2H). MS (FAB): 656 (MH+). Anal. Calcd. for
C38H33N5O6: C, 69.61%; H, 5.07%; N, 10.68%; O, 14.64%. Found. C, 69.70%; H, 5.34%;
N, 10.66%; O, 14.57%.
P10-DOXDCz or P16-DOXDCz
A mixture of N-hexyl-3,6-di[2-([1,3,4]oxadiazol-5-yl)-4-methylbenzoate]carbazole (1.0 g,
1.53 mmol), 1,10-decanediol (0.32 g, 1.83 mmol) or 1,16-hexadecanediol (0◦.47 g,
1.83 mmol) and di-n-butyltin oxide as a catalyst was stirred and heated at 200 C for
2 h under nitrogen atmosphere. Then, the temperature of the polymerization mixture was
◦
slowly raised to 220 C and then kept for 48 h under vacuum (5 mmHg). The obtained
polymer was purified by reprecipitation from chloroform and methanol three times [16,17].
Yield: P10-DOXDCz, 0.6 g (51%); P16-DOXDCz, Yield: 0.4 g (31%).

96
M. Kinoshita et al.
Results and Discussion
LC Behabior
Both polymers exhibited a nematic phase. Figure 1 shows polarizing microphotographs of
P10-DOXDCz and P16-DOXDCz. Upon heating, the baseline shift due to glass transition
temperature (Tg) of P10-DOXDCz and P16-DOXDCz was observed at around 137 ◦C and
87 ◦C, respectivel◦y, and endothermic peaks for the LC-isotropic phase transition appeared
◦
at around at 182 C and 157 C, respectively. Tgs and phase transition temperatures are
summarized in Table 1.
Optical Properties
Figure 2 shows the absorption and fluorescence spectra of P10-DOXDCz and P16-OXDCz
in thin films. P10-DOXDCz and P16-DOXDCz exhibited maximum absorption wavelength
at 358 nm and 353 nm and blue fluorescence with an emission peak at 439 nm and 450 nm,
respectively.
The fluorescence quantum yield is an important factor to evaluate the suitability of
compounds for OLEDs. The fluorescence quantum yields of P10-DOXDCz and P16-
DOXDCz in chloroform were found to be 0.74 and 0.81, respectively.
Polarized Emission
The aligned thin films of P10-DOXDCz and P16-DOXDCz were prepared by a rubbing
technique. A preferential alignment was induced in the polyesters coated onto a rubbed
glass substrate. The films were annealed for 1 h at the temperature where the polymers
showed a nematic phase and cooled to glassy state. Polarized absorption and PL spectra
were measured to estimate the degree of alignment of the main chain as shown in Figure 3.
The order parameter can be determined by the following equation:
R − 1
S =
(1)
R + 2
where R = A /A is a dichroic ratio with A and A being the values of the polarized
ꢁ
⊥
ꢁ
⊥
absorption and PL parallel and perpendicular to the rubbing direction. The order parameters
(S) and dichroic ratio (R) of P10-DOXDCz and P16-DOXDCz were S = 0.10 and 0.14,
R = 1.33 and 1.47, respectively. Thus, these polymers were found to show dichroism.
Table 1. Phase transition temperatures and molecular weights of the LC polymers used in
this studya
Polymer
Mn
Mw/Mn
Phase transition temperature (◦C)
P10-DOXDCz
P10-DOXDCz
20,000
4,000
24.6
3.04
G 137 N 182 I
G 87 N 157 I
aAbbreviations: G, glassy; N, nematic; I, isotropic; Mn, number-average molecular weight; Mw,
weight- average molecular weight.

Synthesis and Emission Behavior of LC Polyesters
97
◦
Figure 1. Polarizing optical micrographs of P10-DOXDCz at 160 C (A) and P16-DOXDCz at
140 ◦C (B).

98
M. Kinoshita et al.
Figure 2. Absorption and PL spectra of P10-DOXDCz (a) and P16-DOXDCz (b) in thin films.
Figure 3. Polarized absorption and PL spectra of P10-DOXDCz (a) and P16-DOXDCz (b) aligned
on a rubbed glass substrate. (ꢁ) Parallel to the rubbing direction; (⊥) perpendicular to the rubbing
direction.

Synthesis and Emission Behavior of LC Polyesters
99
Figure 4. EL spectra of ITO/PEDOT:PSS/P10-DOXDCz/MgAg (a) and ITO/PEDOT:PSS /P16-
DOXDCz/MgAg (b).
Electroluminescence
We also fabricated devices using P10-DOXDCz or P16-DOXDCz as an emitter,
ITO/PEDOT:PSS/P10-DOXDCz/MgAg and ITO/PEDOT:PSS/P16-DOXDCz/MgAg. To
accelerate the hole-injection from an anode into the emitter, PEDOT:PSS was spin-coated
onto an ITO-coated glass substrate. Then, the polymer film was spin-coated from a 1.5 wt%
chloroform solution onto the PEDOT:PSS film and an alloy of Mg and Ag (volume rate
10:1) as a cathode was deposited onto the polymer film. The fabricated devices with both
polymers emitted blue light when a positive voltage was applied to the ITO electrode.
Figure 4 shows the EL spectra of the devices. P10-DOXDCz and P16-DOXDCz showed
a blue emission peak at 470 nm and 468 nm, respectively. Hence the polymers functioned
as blue emitters in OLEDs.
Conclusion
Novel liquid-crystalline polyesters containing a carbazole moiety as a hole-transporting
unit and an oxadiazole moiety as an electron-transporting unit in the main chain, P10-
DOXDCz and P16-DOXDCz, were designed and synthesized, and they showed a nematic
LC phase. Both polyesters emitted intense blue fluorescence with a maximum at 439 nm and
450 nm in thin films, and their fluorescence quantum yields were 0.74 and 0.81 in solution,
respectively. Polarized fluorescence was observed in a LC phase, and order parameters

100
M. Kinoshita et al.
estimated by the polarized emission were 0.10 and 0.14 for P10-DOXDCz and P16-
DOXDCz, respectively. Double-layer device using P10-DOXDCz and P16-DOXDCz as
emitters, ITO/PEDOT:PSS/P10-DOXDCz or P16-DOXDCz/MgAg, were fabricated. The
fabricated devices emitted blue light.
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