A novel mesogen-jacketed liquid crystalline electroluminescent polymer with both thiophene and oxadiazole in conjugated side chain

Article abstract of DOI:10.1002/pola.23908

Mesogen-jacketed liquid crystalline polymers (MJLCPs) with both electron-transport oxadiazole and holetransport thiophene in the side chain were reported for their promising electroluminescent property. Monomers of 2,5-bis{5- [(4-alkoxyphenyl)-1,3,4-oxadiazole]thiophen-2-yl}styrene (M-Cm, m is the number of the carbons in the alkoxy groups, m = 8,10) were synthesized and confirmed by 1H-NMR, mass spectrometry, and elemental analysis. The corresponding polymers were successfully obtained and characterized by thermal analysis, optical spectroscopy, cyclic voltammetry, electroluminescent analysis, polarized light microscopy (PLM), and wide-angle X-ray diffraction (WAXD). The polymers exhibited high decomposition temperatures reaching 382 °C and high T_g's reaching 184 °C. The absorption spectra indicated that both themonomers and polymers had little aggregation in film than that in solution, and the absorption spectra of the polymers showed an obvious blue-shift compared with those of the monomers. Both the monomers and the polymers had bluegreen emission, and the photoluminescence spectra of the polymers in film suggested the formation of excimer or exciplex. The polymers showed lower HOMO energy levels and LUMO energy levels than those of the MJLCPs containing oxadiazole unit reported before. Electroluminescence study with the device configuration of ITO/PEDOT/PVK/polymer/TPBI/Ca/Ag showed maximum brightness and current efficiency of 541 cd/m² and 0.10 cd/A, which proved that the introduction of directly connected electron- and hole-transport units could greatly improve the EL property of sidechain conjugated polymers. The phase structures of the polymers were confirmed to be smectic A phase through the results of PLM and WAXD. The annealed samples emitted polarized photoluminescence at room temperature, which indicated potential utility for practical applications in display.

Full text of DOI:10.1002/pola.23908

A Novel Mesogen-Jacketed Liquid Crystalline Electroluminescent Polymer
with Both Thiophene and Oxadiazole in Conjugated Side Chain
QIAN YANG, YIDING XU, HAO JIN, ZHIHAO SHEN, XIAOFANG CHEN, DECHUN ZOU, XINGHE FAN, QIFENG ZHOU
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Received 3 December 2009; accepted 18 December 2009
DOI: 10.1002/pola.23908
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Mesogen-jacketed
liquid crystalline polymers
mers in film suggested the formation of excimer or exciplex. The
polymers showed lower HOMO energy levels and LUMO energy
levels than those of the MJLCPs containing oxadiazole unit
reported before. Electroluminescence study with the device config-
uration of ITO/PEDOT/PVK/polymer/TPBI/Ca/Ag showed maximum
brightness and current efficiency of 541 cd/m2 and 0.10 cd/A, which
proved that the introduction of directly connected electron- and
hole-transport units could greatly improve the EL property of side-
chain conjugated polymers. The phase structures of the polymers
were confirmed to be smectic A phase through the results of PLM
and WAXD. The annealed samples emitted polarized photolumi-
nescence at room temperature, which indicated potential utility for
(MJLCPs) with both electron-transport oxadiazole and hole-
transport thiophene in the side chain were reported for their
promising electroluminescent property. Monomers of 2,5-bis{5-
[(4-alkoxyphenyl)-1,3,4-oxadiazole]thiophen-2-yl}styrene (M-Cm, m
is the number of the carbons in the alkoxy groups, m ¼ 8,10) were
synthesized and confirmed by 1H-NMR, mass spectrometry, and
elemental analysis. The corresponding polymers were successfully
obtained and characterized by thermal analysis, optical spectros-
copy, cyclic voltammetry, electroluminescent analysis, polarized
light microscopy (PLM), and wide-angle X-ray diffraction (WAXD).
The polymers exhibited high decomposition temperatures reach-
ing 382 ꢀC and high Tg’s reaching 184 ꢀC. The absorption spectra
indicated that both the monomers and polymers had little aggrega-
tion in film than that in solution, and the absorption spectra of the
polymers showed an obvious blue-shift compared with those of
the monomers. Both the monomers and the polymers had blue-
green emission, and the photoluminescence spectra of the poly-
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practical applications in display.
2010 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 48: 1502–1515, 2010
KEYWORDS: electroluminescence; mesogen-jacketed liquid crys-
talline polymers; oxadiazole; side-chain conjugated polymers;
thiophene
INTRODUCTION Liquid crystalline electroluminescent (EL)
polymers have aroused researchers’ intense interest as the
self-assembling nature of liquid crystals that gives improved
carrier mobility and polarized emission when uniformly
aligned,1,2 and, therefore, the advantage of providing
improved viewing in bright-light conditions, which is the
irreplaceable advantage over conventional liquid crystal dis-
play. Nematic and smectic luminescent liquid crystals has
the key advantages of high carrier mobility, polarized emis-
sion, and enhanced output-couping, and their self-assem-
bling properties and supramolecular structures can be regu-
lated to improve the performance of semiconductors.3–6 In
EL area, side-chain conjugated polymers are promising as
they have not only the advantages of polymers, such as
good film forming and easy processing property, but also
the advantages of low molecular weight compounds, such as
good solubility in common organic solvents, high purity, a
variety of possible pendant groups, well-defined polymer
architecture and a well-defined conjugation length as a
result of carrying isolated luminescent functional groups in
their side chains therefore the conjugation lengths can be
controlled.
The side chain of MJLCP (mesogen-jacketed liquid crystalline
polymer) is attached laterally to the main chain without or
with only short spacer,7–9 so the large hindrance of the bulky
side chain can avoid the intermolecular aggregation, which is
favorable for the high efficiency of the EL devices. The previ-
ous work in our group had attempted to introduce the elec-
tron-transport oxadiazole unit into the side chain of the
MJLCPs, and such MJLCPs had better electron-transport
property and the maximum brightness and external quantum
efficiency could reach 290.8 cd/m2 and 0.24%, respectively,
when multilayer devices were used.10
Although in the processing of multilayer devices of light-
emitting polymers, spin-coating is the most widely used
method, and the existence of solvent makes the two func-
tional layers permeated into each other, which makes the
functional layer less effective. So we consider to introduce
hole-transport thiophene group into the side chain further to
Correspondence to: X. H. Fan (E-mail: fanxh@pku.edu.cn) or Z. H. Shen (E-mail: zshen@pku.edu.cn) or Q.-F. Zhou (E-mail: qfzhou@pku.edu.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1502–1515 (2010) 2010 Wiley Periodicals, Inc.
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obtain better EL properties by balancing electrons and holes
in molecular structure scale. Main-chain conjugated polymers
with electron- and hole-transport segments directly con-
nected in the main chain have been widely studied in the
area of PLEDs (polymeric light-emitting diodes). For
instance, Yu et al. synthesized light-emitting copolymers with
p-dopable bithiophene and n-dopable oxadiazole group
directly connected and alternated in the conjugated back-
bone, and the n-doping potential of the polymer was similar
with that of oxadiazole-containing polymers, whereas its
p-doping potential was comparable with that of the typical
p-dope type polymers like poly(p-phenylenevinylene) (PPV)
and polythiophenes (PT), which firstly presented a possible
approach to balance the rates of injection of electrons and
holes in PLEDs by designing the molecular structure so as to
control the intrinsic properties of light-emitting materials.11
Huang et al. devoted effort to the synthesis of oxadiazole-
containing light-emitting polymers with thiophene rings
directly connected with oxadiazole rings in the conjugated
backbone and the fabrication of single-layer device to inves-
tigate the diode character.12 Mochizuki et al. designed four
electroactive polymers with oxadiazole directly connected
with different arylamine moieties in the conjugated side
chains, and all of the polymers emitted strong blue fluores-
cence, with the maximum luminance of 30 cd/m2 for the EL
device consisting of the polymer containing oxadiazole and
triphenylamine groups.13 So in this work, the two kinds of
functional groups of thiophene and oxadiazole were designed
to be connected directly expecting to realize highly efficient
EL.
were recorded with a Bruker ARX400 spectrometer at room
temperature with deuterated chloroform (CDCl3) as the sol-
vent, and the chemical shifts were reported in ppm with tet-
ramethylsilane (TMS) as the internal standard. Mass spectra
were recorded on a Finnigan-MAT ZAB-HS spectrometer
when molecular weight was under 800 and on a Bruker Dal-
tonics Autoflex III MALDI-TOF spectrometer when molecular
weight was higher than 800. Elemental analysis was carried
out on an Elementar Vario EL instrument. The molecular
weight and molecular weight distribution of the polymers
were determined by gel permeation chromatography (GPC)
on a Waters 2410 instrument equipped with three Waters l-
Styragel columns (103, 104, and 105 Å) in series, with THF
as the eluent at a flow rate of 1.0 mL/min at 35 ꢀC. Mono-
disperse polystyrene standards were used for calibration.
The thermogravimetric analysis (TGA) of the polymers was
performed under nitrogen atmosphere and in the air both at
a heating rate of 10 ꢀC/min with a TA SDT Q600 analyzer.
Differential scanning calorimetry (DSC) examination was car-
ried out on a TA DSC Q100 calorimeter with a programmed
heating procedure in nitrogen. The sample size was about 5
mg and encapsulated in hermetically sealed aluminum pans,
whose weights were kept constant. The temperature and
heat flow scale at different cooling and heating rates were
calibrated using standard materials such as indium and ben-
zoic acid. The glass transition temperatures (Tg) were
obtained from the second heating curves.
Polarized light microscopy (PLM) observation was performed
on a Leitz Laborlux 12 microscope with a Leitz 350 hot
stage. The samples were cast from THF solution and slowly
dried at room temperature.
In this investigation, we report the synthesis of a novel class
of electroluminescent MJLCPs with thiophene and oxadiazole
directly connected in side chain with the aim of balancing
the injection and transportation of both the holes and elec-
trons. Their thermal, optical, electrochemical, and electrolu-
minescent properties are discussed. Furthermore, as previ-
ous research in our group discovered that when the side
chain was bulky enough, the polymers could form lamellar
SmA phase in liquid crystalline state.14,15 The polymers in
this work own the largest p-conjugated system among the
MJLCPs, so the liquid crystalline properties are studied in
detail, either, with polarized light microscopy (PLM), 1D
wide-angle X-ray diffraction (WAXD), and 2D WAXD used.
One-dimensional (1D) wide-angle X-ray diffraction (WAXD)
experiments were performed on a Philips X’Pert Pro diffrac-
tometer with an X’celerator detector and a 3 kW ceramic
tube as the X-ray source (Cu Ka). The reflection peak posi-
tions were calibrated with silicon powder (2y > 15ꢀ) and
silver behenate (2y < 10ꢀ). The sample stage of Philips
X’Pert Pro was set horizontally, with the samples protected
by nitrogen purge gas during the measurements, and a
temperature control unit (Paar Physica TCU 100) in con-
junction with the diffractometer was utilized to study the
phase structure evolution with the varied temperature. The
heating and cooling rates in the WAXD experiments were
5
ꢀC/min and the sample temperature was controlled
EXPERIMENTAL
within 61 ꢀC.
Materials and Measurements
THF was heated under reflux over sodium/benzophenone
and freshly distilled before use. Triethylamine was heated
over potassium hydroxide for 22 h and distilled. DMF was
dried over anhydrous MgSO4 for 24 h and distilled under
reduced pressure. CuBr was synthesized from CuSO4, NaBr,
and Na2SO3, and purified by washing in acetic acid and
methanol, and then dried under vacuum before use. All other
chemicals were purchased from Aldrich, Acros, and Alfa
Aesar, and used without further purification.
Two-dimensional (2D) WAXD pattern was obtained using
Bruker D8Discover diffractometer with GADDS as a 2D de-
tector. The 2D diffraction patterns were recorded in a trans-
mission mode at room temperature with oriented samples
prepared by mechanically shearing from the LC phase when
applicable. Silicon powder and silver behenate were used as
standards again. The sample was positioned on the sample
stage so that the mechanically shearing direction was per-
pendicular to the point-focused X-ray beam. For both the 1D
and 2D diffractions, the background scattering was recorded
and subtracted from the sample patterns.
All new compounds were identified by 1H-NMR, mass spec-
troscopy, and elemental analysis (EA). The 1H-NMR spectra
MJLCPS WITH BOTH THIOPHENE AND OXADIAZOLE, YANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Absorption spectra were measured with a PerkinElmer
Lambda 35 UV–vis spectrophotometer, and PL spectra were
obtained using a Hitachi F-4500 fluorescence spectrophotom-
eter. The cyclic voltammograms were recorded with a voltam-
metric analyzer (model CHI630C from Shanghai Chenhua
Instrument Co.) in solution of tetrabutylammonium perchlo-
rate (0.1M) in acetonitrile with a scanning rate of 100 mV/s at
room temperature under nitrogen atmosphere. The measuring
cell consisted of vitreous carbon electrode as the working elec-
trode, Ag/AgCl electrode as the reference electrode, and plati-
num wire electrode as the auxiliary electrode. The polymer
films were coated on the working electrode by casting and
then dried in air. The energy levels were calculated with the
ferrocene (FOC) value of ꢁ4.8 eV with respect to vacuum level
as a calibration reference. After the measurement, two drops
of FOC in acetonitrile (3 mM) were added and measured for
calibration. Therefore, the HOMO levels of the polymers could
be calculated by the equation EHOMO ¼ ꢁe (Eonset(ox) ꢁ E1/2,
FOC) ꢁ 4.8 eV, and the LUMO level could be estimated by the
equation ELUMO ¼ ꢁe (Eonset(red) ꢁ E1/2, FOC) ꢁ 4.8 eV, where
the E1/2, FOC standards for the half-wave potential of Fc/Fþc .
were depicted in Scheme 1. The experimental details were
described later taking M-C10 as an example.
Ethyl 2-thiophenecarboxylate 1
To a stirred solution of 2-thiophenecarboxylic acid (15.38 g,
0.12 mol), ethanol (5.52 g, 0.12 mol) and 4-dimethylamino-
pyridine (DMAP) (0.36 g, 3.00 mmol) in dried dichlorome-
thane (DCM) (270 mL), a solution of N,N0-dicyclohexylcarbo-
diimide (DCC) (24.76 g, 0.12 mol) in DCM (30 mL) was
added. The reaction mixture was stirred at room tempera-
ture, with or without the protection of dry argon, for 24 h.
The floating solid was then filtered and the solvent was
removed by distillation under reduced pressure. The crude
product was purified by column chromatography (silica gel)
with dichloromethane as eluant to give a pale yellow oil.
Yield: 76.3%. 1H-NMR (CDCl3, d, ppm): 7.79–7.80 (d, 1H,
ArAH), 7.53–7.54 (d, 1H, ArAH), 7.08–7.09 (d, 1H, ArAH),
4.32–4.39 (t, 2H, AOCH2A), 1.35–1.40 (t, 3H, ACH3).
Ethyl 5-(tri-n-butylstannyl)-2-thiophenecarboxylate 2
To a stirred solution of diisopropꢀylamine (1.01 g, 10.00
mmol) in dry THF (30 mL) at ꢁ78 C under an atmosphere
of dry argon, n-butyllithium (4 mL, 2.5M solution in hexane,
10.00 mmol) was added by syringe. The reaction mixture
was stirred for 1 h, and then a solution of compound 1
(1.56 g, 10.00 mmol) in dry THF (3 mLꢀ) was added by sy-
ringe, with the temperature kept at ꢁ78 C. After stirring for
another 1 h, a solution of tri-n-butylstannyl chloride (3.26 g,
10.00 mmol) in dry THF (4 mL) was added by syringe, with
the temperature still kept at ꢁ78 ꢀC. After the addition, the
reaction mixture was stirred at ꢁ78 ꢀC for 1 h, and then
allowed to reach room temperature and react overnight. It
was then carefully quenched with water (40 mL) and the or-
ganic layer was separated. The aqueous phase was washed
with dichloromethane (3 ꢂ 40 mL) and the combined or-
ganic extract was then dried over anhydrous MgSO4. The
crude product was purified by column chromatography
(silica gel) with dichloromethane/petrol ether 1:3 (v/v) as
eluant to give a colorless oil. Yield: 72.3%. 1H-NMR (CDCl3,
d, ppm): 7.88–7.89 (d, 1H, ArAH), 7.14–7.15 (d, 1H, ArAH),
4.31–4.38 (t, 2H, AOCH2A), 0.87–1.61 (m, 30H, ACH3,
ACH2CH2CH2CH3). MS (m/z): 446.
Fabrication of the PLED Devices
Multilayer light-emitting diodes with the configuration of
ITO/PEDOT/polymer/TPBI (15 nm)/Mg:Ag (10:1, wt %)/Ag
and ITO/PEDOT/PVK/polymer/TPBI (15 nm)/Ca (25 nm)/
Ag (80 nm) were fabricated for the investigation of optoelec-
tronic characteristics. The patterned indium tin oxide (ITO)
glass substrate was cleaned in turn with substrate-cleaning
detergent, deionized water, acetone, and ethanol for 15 min
under ultrasonic condition, and finally treated with UV-ozone
for about 25 min. Then a hole-injection layer of PEDOT was
spin-coated on it and dried by baking in air at 120 ꢀC for
10 h. PVK was spin-coated from 1,1,2,2-tetrachloroethane so-
lution (10 mg/mL) and dried by baking in air at 150 ꢀC for
2 h. Then the emitting layer was spin-cast onto the PVK
layer from toluene solution (10 mg/mL) through a 0.45 lm
Teflon filter. The thickness of the spin-coated film was con-
trolled by regulating the spinning speed. Finally, the coated
ITO was transferred into a deposition chamber with a base
pressure of 1 ꢂ 10ꢁ6 Torr, TPBI and metal layer was evapo-
rated onto the polymer layer. The emitting area was 2 mm ꢂ
2 mm. The EL spectra were measured with a spectrofluor-
ometer FP-6200 (JASCO). A source-measure unit R6145
(Advantest), multimeter 2000 (Keithley), and luminance me-
ter LS-110 (Minolta) were used for I-V-L measurements. Rel-
ative luminance was directly detected by using a multifunc-
tional optical meter 1835-C (Newport). All of the fabrication
and characterization of the light-emitting diode devices was
performed in air and at room temperature without protec-
tive encapsulation.
2,5-Bis[(5-ethoxycarbonyl)thiophen-2-yl]styrene 4
To a mixture of bis (triphenylphosphine) palladium (II) chlo-
ride (0.84 g, 1.20 mmol) and hydroquinone (0.08 g) in DMF
(70 mL) at 100 ꢀC under argon atmosphere, compound 2
(13.38 g, 30.00 mmol) and compound 3 (3.87 g, 15.00
mmol) in dry DMF (50 mL) were added by syringe in order,
and the reaction mixture was stirred for 24 h. Then the mix-
ture was washed with hydrochloric acid (6 vol %, 300 mL)
and dichloromethane to remove the high boiling DMF and
extract the product from the aqueous phase. Then the or-
ganic phase was washed by NaHCO3 saturated aqueous solu-
tion (200 mL) to be neutralized. After removing the solvent
under reduced pressure and drying over anhydrous MgSO4,
the residue was subjected to column chromatography (silica
gel) with dichloromethane/petrol ether 2:1 (v/v) as eluant
to get a pale yellow solid. Yield: 68.0%. 1H-NMR (400 MHz,
Synthesis of the Monomers
The synthetic routes of the monomers of 2,5-bis{5-[(4-alkoxy-
phenyl)-1,3,4-oxadiazole]thiophen-2-yl}styrene (M-Cm,
m is
the number of the carbons in the alkoxy groups, m ¼ 8,10)
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ARTICLE
SCHEME 1 Synthetic route of monomers and polymers.
CDCl3, d, ppm): 7.84–7.85 (d, 1H, phenyl-H), 7.78–7.80 (t,
2H, thiophene-H), 7.58–7.61 (t, 1H, phenyl-H), 7.47–7.50
(d, 1H, phenyl-H), 7.37–7.38 (d, 1H, thiophene-H), 7.10–7.11
(d, 1H, thiophene-H), 6.92–7.01 (t, 1H, ACH¼¼), 5.78–5.84 (d,
1H, ¼¼CH2), 5.38–5.43 (d, 1H, ¼¼CH2), 4.33–4.42 (t, 4H,
AOCH2A), 1.37–1.43 (t, 6H, ACH3). MS (m/z): 412.
(s, 2H, ACOOH), 7.97–7.98 (d, 1H, phenyl-H), 7.70–7.77 (m,
3H, thiophene-H), 7.57–7.60 (d, 1H, phenyl-H), 7.52–7.54 (d,
1H, phenyl-H), 7.24–7.25 (d, 1H, thiophene-H), 6.90–6.99 (t,
1H, ACH¼¼), 5.99–6.04 (d, 1H, ¼¼CH2), 5.45–5.49 (d, 1H,
¼¼CH2). MS (m/z): 356.
2,5-Bis(5-chlorocarbonylthiophen-2-yl)styrene 6
With 5 (1.06 g, 3.00 mmol) and oxalyl chloride (80 mL) well
mixed, several drops of dried DMF were added as catalyst.
The reaction mixture was stirred at room temperature for
quite a long time until the mixture turned to be homogene-
ous. The excess of oxalyl chloride was removed under
reduced pressure and then carried away completely with the
addition of petrol ether to afford a yellow solid. The product
was sealed and kept dry.
2,5-Bis(5-carboxythiophen-2-yl)styrene 5
The compound 4 (2.10 g, 5.00 mmol), hydroquinone (0.50
g), and methanol (200 mL) were put into a 500 mL flask, as
the compound 4 was reluctant to dissolve completely in
methanol, the mixture was heated to reflux. After the mix-
ture appeared homogeneous, KOH aqueous solution (20
wt %, 90 mL) was added. After 3 h, dilute hydrochloric acid
was added into the solution until the PH reached 2. The gen-
erated solid was then collected and washed with water and
methanol. The final product appeared to be dark yellow
powder. Yield: 97.6%. 1H-NMR (DMSO-d6, d, ppm): 13.23
p-Alkoxybenzhydrazide 7
This compound was synthesized according to the procedure
we reported before.14
MJLCPS WITH BOTH THIOPHENE AND OXADIAZOLE, YANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
2,5-Bis[5-(4-decyloxyphenyl-bishydrazide)thiophen-2-yl]
styrene 8
amine (Me6TREN) (2.30 mg, 0.01 mmol) in N-methyl pyrroli-
done (NMP) (0.80 g), M-C10 (0.87 g, 1.00 mmol), and NMP
(4.45 g) were placed in a well-dried glass tube with a mag-
netic stir bar in sequence. Then the reaction mixture was
purged with dry nitrogen and subjected to three freeze-
pump-thaw cycles and sealed under vacuum. The tube was
placed into an oil bath thermostatic at 110 ꢀC. After 4 days,
the tube was broken after the polymerization was termi-
nated by putting the tube into ice/water mixture. The reac-
tion mixture was diluted with THF, passed through a basic
alumina column to remove the copper complex and precipi-
tated in methanol. As the solubility of M-C10 in methanol
was not good, the precipitate was redissolved in THF and
reprecipitated twice by controlling the addition of methanol
to eliminate the unreacted monomers. Finally, the polymer
was dried under vacuum to a constant weight.
To the intensely stirred solution of 7 (3.31 g, 11.34 mmol)
in dry THF (50 mL), hydroquinone (0.10 g), and triethyl-
amine (10 mL) were added. The solution of 6 (2.23 g, 5.67
mmol) in dry THF (80 mL) was slowly dropped at the tem-
perature of ice bath. A great deal of suspended white solid
generated immediately and its color turned darker when the
reaction proceeded further, and the reaction mixture was
stirred at room temperature for another 24 h after the addi-
tion. Then the solvent was removed under reduced pressure,
and the residue was dispersed with water and neutralized to
gain dark yellow solid. As the product could hardly dissolved
in any common organic solvent, no further purification was
performed.
2,5-Bis{5-[(4-decyloxyphenyl)-1,3,4-oxadiazole]thiophen-2-
yl}styrene 9 (M-C10)
RESULTS AND DISCUSSION
The aforementioned 8 (14.37 g), hydroquinone (1.47 g), and
phosphorus oxychloride (350 mL) were mixed and heated to
reflux for 18 h. The cooled mixture was poured slowly into
excessive ice water with intense stirring, and a great deal of
solid generated rapidly. Then sodium hydroxide aqueous so-
lution was added to neutralize the mixture, and the solid
was filtered, washed with water and dried under vacuum.
The crude product was purified firstly by column chromatog-
raphy (silica gel) with dichloromethane/ethyl acetate 10:1
(v/v), and then by recrystallization in THF to yield a bright
Synthesis and Characterization of Monomers and
Polymers
As shown in Scheme 1, the monomers were mainly synthe-
sized in seven steps. In the synthesis of compound 1, we
also had a try that the reaction was performed in argon
atmosphere, and it was found that the yield was 10% higher
than that in the common condition. The synthesis of com-
pound 4 was the crucial step in the entire synthesis of the
monomers, which directly influenced the purity of the next
products. Stille cross-coupling was used, which contained
two important steps: synthesis of organotin reagents and
process of cross-coupling.16 The synthesis of organotin
reagents needed to be performed in absolutely anhydrous
and inert atmosphere at ꢁ78 ꢀC, and diisopropylamine was
added to stabilize the lithium complex. The color change was
fairly obvious and was used to judge whether the reaction
was successful or not: After the solution of compound 1 was
added to the system, the color of the mixture changed to yel-
low-green and became darker as the addition continued, and
finally became black completely. When tri-n-butylstannyl
chloride was added, the color of the mixture turned yellow,
and when the temperature rise to room temperature, the
color of the mixture turned darker again, and became dark
red in the end. In the cross-coupling step, it was unavoidable
that homo-coupled product generated, which was in agree-
ment with the statement in the literature.17 In our synthesis,
the homo-coupled product was removed by careful separa-
tion through column chromatography. Because vinyl group
was unstable in high temperature as thermal polymerization
might occur, hydroquinone was always added in the reaction
mixture as polymerization inhibitor. In the synthesis of com-
pound 6, oxalyl chloride was used in stead of thionyl chlo-
ride, as the reaction could be performed under room tem-
perature in stead of under reflux. Compound 8 had very
high polarity and rather poor solubility in common solvents,
so no further purification was carried out. The solubility of
compound 9 was rather poor, too, which may be due to the
large rigidity of the molecule, so dry method was used in
column packing in stead of wet method to get better
1
yellow powder (2.34 g, 17.0%). H-NMR (400 MHz, CDCl3, d,
ppm): 8.02–8.05 (d, 4H, phenyl-H), 7.84 (s, 1H, phenyl-H),
7.76–7.77 (d, 2H, thiophene-H), 7.59–7.60 (d, 1H, phenyl-H),
7.52–7.53 (d, 1H, phenyl-H), 7.43–7.44 (d, 1H, thiophene-H),
7.18–7.19 (d, 1H, thiophene-H), 6.99–7.02 (p, 5H, phenyl-H,
ACH¼¼), 5.82–5.87 (d, 1H, ¼¼CH2), 5.44–5.47 (d, 1H, ¼¼CH2),
4.00–4.04 (t, 4H, AOCH2A), 1.78–1.85 (p, 4H, AOCH2CH2A),
1.44–1.51 (p, 4H, ACH2CH3), 1.28–1.33 (d, 24H,
ACH2CH2CH2A), 0.87–0.90 (t, 6H, ACH3). Anal. Calcd. for
C
52H60N4O4S2: C, 71.86; H, 6.96; N, 6.45. Found: C, 71.69; H,
7.01; N, 6.41. MS (MALDI) m/z: 869.6 (calcd. for
52H60N4O4S2, 869.2).
C
1
Basic characterizations of M-C8 are as follows: H-NMR (400
MHz, CDCl3, d, ppm): 8.05–8.07 (d, 4H, phenyl-H), 7.88 (s,
1H, phenyl-H), 7.80–7.81 (d, 2H, thiophene-H), 7.63–7.66 (d,
1H, phenyl-H), 7.54–7.57 (d, 1H, phenyl-H), 7.46–7.47 (d, 1H,
thiophene-H), 7.20–7.22 (d, 1H, thiophene-H), 6.99–7.08 (p,
5H, phenyl-H, ACH¼¼), 5.83–5.89 (d, 1H, ¼¼CH2), 5.44–5.48
(d, 1H, ¼¼CH2), 4.02–4.06 (t, 4H, AOCH2A), 1.78–1.87 (p, 4H,
AOCH2CH2A), 1.48–1.49 (d, 4H, ACH2CH3), 1.25–1.33 (d,
16H, ACH2CH2CH2A), 0.88–0.90 (t, 6H, ACH3). Anal. Calcd.
for C48H52N4O4S2: C, 70.90; H, 6.45; N, 6.89. Found: C, 70.34;
H, 6.75; N, 6.55. MS (MALDI) m/z: 813.3 (calcd. for
C48H52N4O4S2, 813.1).
Polymerization
Atom transfer radical polymerization (ATRP) was used to
obtain the polymers. Typically, CuBr (1.44 mg, 0.01 mmol), a
solution of ethyl 2-bromo-2-methylpropionate (EBP) (1.95
mg, 0.01 mmol) and 1,1,4,7,10,10-hexamethyltriethylenetetr-
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FIGURE 1 1H-NMR spectra of (a) the monomer M-C10 and (b) the polymer P-C8 in CDCl3.
separation efficiency, and pure monomers were obtained af-
ter recrystallization in THF. The uncertainty in the last step
of cyclization and the poor solubility induced the yield of
pure monomers largely decreased. However, the successful
synthesis and accumulation of sufficient amount of the
monomers to polymerize was exciting. The structures of the
monomers had been confirmed by H-NMR, mass spectrome-
try, and elemental analysis.
1
ATRP method was used to get the corresponding polymers.
As the oxadiazole rings could coordinate with the catalyst of
MJLCPS WITH BOTH THIOPHENE AND OXADIAZOLE, YANG ET AL.
1507

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Polymerization Results and Thermal Properties of the Polymers
Polymer
Mn (ꢂ104)a
Mw (ꢂ104)a
PDIa
Td (ꢀC)b
Td (ꢀC)c
Tg (ꢀC)d
P-C8
P-C10
a
0.8
1.2
0.9
1.8
1.22
1.55
c
378
382
359
350
184
174
Mn, Mw, and PDI of the polymers were determined by GPC using poly-
Temperature at 5 wt % loss in air determined by TGA at a heating rate
of 10 ꢀC/min.
styrene standards in THF solutions.
b
d
Temperature at 5 wt % loss under nitrogen atmosphere determined
Glass-transition temperature determined by DSC in nitrogen in the
by TGA at a heating rate of 10 ꢀC/min.
second heating curve at a heating rate of 20 ꢀC/min.
CuBr, Me6TREN was used as a stronger ligand. NMP was
used as solvent in polymerization instead of the mostly used
chlorobenzene, as the monomers with such a long conjuga-
tion had better solubility in elevated temperatures than that
in other familiar solvents. The polymers were then success-
fully obtained, as the spectra of 1H-NMR shown, the signals
of the vinyl group at 5.4, 5.8, and 7.0 ppm in part (a) of Fig-
ure 1 completely disappeared in part (b), indicating the suc-
cessful polymerization, and the broad peaks in part (b) of
Figure 1 were consistent with the expected polymer struc-
ture. The low polymerization reactivity of the monomers
induced longer reaction time (several days) and slightly
higher polydispersity than what was observed for simple
monomers such as styrene, due to the large delocalization of
the radicals.18 The polymers were completely soluble in the
common organic solvents such as chloroform, THF, chloro-
benzene, and dichloromethane, and they showed much bet-
ter solubility than the corresponding monomers. The charac-
terization results of the polymers were listed in Table 1.
sition temperatures even in air, with the temperatures at
5 wt % loss at 359 ꢀC and 350 ꢀC for P-C8 and P-C10, respec-
tively, indicating that the polymers were quite stable at ele-
vated temperatures, which was valuable in the application as
optoelectronic devices and in heat treating in the study of liq-
uid crystalline phase structures. DSC was performed to mea-
sure glass transition temperatures (Tg) of the polymers. The
polymers had Tg’s at about 184 ꢀC and 174 ꢀC, respectively,
which were much higher than that of the polystyrene back-
bone and the MJLCPs reported before,14,15 which indicated
that the introduction of the pendant large waist-attaching side
chain made the polymers more rigid. In general, high decom-
position temperatures and high Tg’s were advantageous for
the polymers to be used as emissive materials in PLEDs.19
Liquid Crystalline Properties
To investigate the phase structure of the polymers, PLM, 1D
WAXD, and 2D WAXD experiments of P-C10 were performed.
Birefringence of the polymers were observed with PLM. The
samples were cast from THF solution and slowly dried at
room temperature. The birefringence appeared when the
sample P-C10 was heated to 180 ꢀC, which was a little
higher than Tg (Fig. 3). When cooled to room temperature,
the birefringence remained. So it was implied that the or-
dered structure formed at high temperature kept unchanged
upon cooling, which was similar to some MJLCPs synthesized
in our group before.15,20,21
Thermal Properties
Thermal properties of the polymers were evaluated by TGA
and DSC, and the results were summarized in Table 1. The
thermal stability of the polymers was investigated with TGA
in both nitrogeꢀn and air (Fig. 2). The temperatures at 5 wt %
ꢀ
loss were 378 C and 382 C for P-C8 and P-C10 in nitrogen,
respectively. This series of polymers had fairly high decompo-
FIGURE 2 TGA plots of the polymers with a heating rate of 10
ꢀC/min under nitrogen atmosphere and in air. [Color figure can
be viewed in the online issue, which is available at
www.interscience.wiley.com.]
FIGURE 3 PLM image of P-C10 taken at 240 ꢀC. [Color figure
can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
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ARTICLE
FIGURE 4 1D WAXD patterns of sample P-C10 obtained during the first heating (a) and partial pattern of 2h from 4ꢀ to 30ꢀ (b) of
the as-cast film. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
1D WAXD was used to investigate the phase transition and
phase structure further. About 50 mg of polymer was dis-
solved in THF and cast onto a copper substrate, and then
the solvent was allowed to evaporate slowly at room temper-
ature. Figure 4 showed the 1D WAXD patterns of P-C10 from
40 to 260 ꢀC at programmed temperatures. When the tem-
perature reached 180 ꢀC, a diffraction peak appeared, and
the intensity of the peak increased when the heating was
continued. At the same time, a higher-order diffraction peak
ꢀ
was observed, too. At 230 C, the position of the two diffrac-
tion peaks had 2y values of 2.26ꢀ and 4.38ꢀ. The ratio of
scattering vectors of these two peaks was ꢃ1:2, indicating a
smectic structure of the sample. When the sample was
cooled, the smectic phase was unchanged. The maximum d-
spacing value was 3.90 nm, and it was a little less than 4.7
nm of the calculated length of the monomers with their
alkoxy tails in stretched conformation. That was because the
length of the flexible alkoxy chain was calculated as rigid
conformation, and the flexible alkoxy groups also had the
trend of interpenetrating with each other after the polymer
entered liquid crystalline phase.21 There were always
FIGURE 5 2D WAXD pattern of annealed P-C10 film recorded
at room temperature. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
MJLCPS WITH BOTH THIOPHENE AND OXADIAZOLE, YANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Optical Properties of Monomers and Polymers in Chloroform Solutions and Solid Films
Solutiona
Filmb
Absorption kmax (nm)
d
PL
Sample
Absorption kmax (nm)
PL kmax (nm)c
/
PL kmax (nm)c
M-C8
P-C8
371
358
372
358
437, 464
446, 462
437, 461
450, 464
0.48
0.38
0.52
0.29
388
360
375
359
495, 511
519
M-C10
P-C10
521
498
a
c
In chloroform (1
ꢂ
10ꢁ5M for monomers and
1
mg/10 mL for
Excitation wavelength was the maximum absorption wavelength.
Fluorescence quantum efficiencies were measured in chloroform rela-
d
polymers).
b
Films were spin-cast from toluene solutions.
tive to quinine sulfate (/PL ¼ 0.55).
scattering halos at the 2h of around 20ꢀ corresponding to
the distance of 0.44 nm, which arise from the p–p stacking
of the aromatic rings in the side chain, and was attributed to
the lateral distance between the side chains within the layer.
The positions of the halos slightly shifted toward lower
angles upon the heating as a result of thermal expansion.
quartz plate. The absorption maximum of M-C8 and M-C10
in solution was at 371 and 372 nm, whereas at 388 and 375
nm in film, respectively. The absorption maximum of M-C8
in film red-shifted 17 nm than that in solution, which was
much larger than the 3 nm red shift of M-C10, which indi-
cated that M-C8 was easier to aggregate than M-C10, due to
the less bulky tails. The absorption maximum of P-C8 and P-
C10 in solution were both at 358 nm and were at 360 and
359 nm in film, respectively, which suggested that the molec-
ular arrangement of polymer chains was almost the same
both in solution and in thin film, indicating that the poly-
mers lacked order even in the solid state, and the aggrega-
tion of the polymers in film was not very great, either, from
which it was inferred that the polymer backbone limited the
motions of the side chains. It was worth noticing that the
absorption spectra of the polymers showed an obvious blue-
shift compared with those of the corresponding monomers,
which was ascribed to the loss of conjugation of the vinyl
group with the directly connected aromatic rings.23
As for the phase structure would be smectic A or smectic
C, 2D WAXD measurements were performed for annealed
samples to confirm the phase structure. As the pattern in
Figure 5 showed, when X-ray incident beam was incident, a
pair of sharp diffraction arcs appeared on the meridian
direction at 2h ¼ 2.20ꢀ (d-spacing is 4.02 nm), and the sec-
ond-order diffraction could be observed clearly at 2h ¼
4.30ꢀ (d-spacing is 2.06 nm), which indicated that the or-
dered structure was developed on the nanometer scale.
Meanwhile, two scattering halos in the high 2h angle aris-
ing from the p–p stacking of the aromatic rings in the side
chains were more or less concentrated on the equator
direction with rather broad azimuthal distributions. This
revealed that only the short-range orders exist.22 Similar
results were obtained from P-C8, and the position of two
diffraction peaks had 2h values of 2.39ꢀ and 4.67ꢀ corre-
sponding to the maximum d-spacing value of 3.69 nm, with
the calculated length of 4.39 nm.
The photoluminescence spectra of monomers and polymers
were recorded both in chloroform solution and in film, too.
They were both blue-emitters. In solution, the emission
bands were at 437 and 464 nm for M-C8 and 437 and 461
nm for M-C10, and at 446 and 462 nm for P-C8 and 450 and
464 nm for P-C10, respectively. Although in film state, the
emission bands of the two monomers were at 495 and 511
nm and 521 nm, respectively, and those of the two polymers
were at 519 and 498 nm, respectively. The emission of the
polymers in film had a great red-shift of about 50 nm rela-
tive to that in solution, and the red shift of the emission of
monomers in film were even larger than those of the poly-
mers, to be 60–80 nm, which was ascribed to the strong
intra- or inter-chain interaction and the generation of exci-
mer or exiplex that provided nonradiation decay channels
for the excited states.24–28 The maximum emission of M-C10
was 26 nm red-shifted than that of M-C8, which was attrib-
uted to its greater tendency of recrystallization and subse-
quent energy dissipation in nonradiative transition, which
caused the emission red-shift, due to its longer tails, there-
fore, better mobility.29 The maximum emission of P-C8 was
situated at 519 nm and was 21 nm red-shifted than that of
P-C10, which was ascribed to the great intermolecular inter-
action besides the intramolecular interaction as a result of
the shorter alkoxy tails.
Combining the 1D WAXD results and the 2D WAXD pattern
style, it was concluded that such polymers as P-C10 and P-
C8 formed SmA phase after the liquid crystalline phase tran-
sition (Fig. 6).
Photophysical Properties
The photophysical characteristics of the polymers were
investigated both in chloroform and as films cast from tolu-
ene solutions. The photophysical properties of the monomers
were also discussed as the molecular structures of mono-
mers reflected those of the repeat units of the polymers.
Their absorption and emission spectral data are summarized
in Table 2. Figures 7 and 8 display the absorption and pho-
toluminescence (PL) spectra both in solution and film state.
The UV–vis absorption spectra (Fig. 7) of the monomers and
polymers in solution were recorded at room temperature in
chloroform, which was a relatively nonpolar solvent to make
sure the interaction between the solute molecules and the
solvent was little. The absorption spectra in film were per-
formed after the toluene solutions were spin-coated on a
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ARTICLE
tions resulted from the ordered structure in the annealed
samples. Such tendency of forming ordered structure was
expected to be favorable to the transportation of charge car-
riers and the potential utility for practical applications in the
area of display. Similar considerations of introducing self-
organized structures such as liquid-crystalline polymers have
also been reported, and the improvement of the EL proper-
ties was ascribed to the bipolar carrier-transporting abilities
of the LC molecules.30,31 Moreover, high ordered phase struc-
ture was expected to replace N phase to facilitate the charge
carrier transport,32 which was realized in this investigation.
Electrochemical Properties
The electrochemical cyclic voltammetry was performed for
determining the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) energy
levels of the polymers, which correspond to ionization poten-
tial (IP) and electron affinity (EA), respectively. Figure 10
shows the cyclic voltammograms of the polymer films, and
all related electrochemical data are summarized in Table 3.
The onset oxidation potentials were situated at 1.66 V for P-
C8 and 1.77 V for P-C10, from which their HOMO levels
were estimated to be ꢁ6.07 and ꢁ6.18 eV, respectively,
according to the equation IP ¼ ꢁe(Eonset(ox) ꢁ E1/2, FOC) ꢁ
4.8 eV. The onset reduction potentials were situated at
ꢁ1.19 V for P-C8 and ꢁ1.36 V for P-C10, with their LUMO
levels estimated to be ꢁ3.22 and ꢁ3.05 eV according to the
equation EA ¼ ꢁe(Eonset(red) ꢁ E1/2, FOC) ꢁ 4.8 eV. From the
HOMO and LUMO levels, the band gaps of the P-C8 and P-
C10 were estimated to be 2.85 and 3.13 eV, respectively.
FIGURE 6 Schematic drawing of the SmA layer structure of the
polymers.
The quantum efficiencies of the monomers and polymers
were measured in chloroform with quinine sulfate in dilute
sulfuric acid as reference. The quantum efficiencies were
0.48 and 0.52 for M-C8 and M-C10, and those of the corre-
sponding polymers were 0.38 and 0.29. The polymers had
lower fluorescence quantum efficiencies than corresponding
monomers, due to the concentration effect of the close side
chains attached to the backbones.
Polarized Photoluminescence
To make use of the liquid crystalline self-assembling proper-
ties of the polymers, we demonstrated the polarized photolu-
minescence spectra of P-C10 film in Figure 9. The film was
annealed at 220 ꢀC for 6 h, and the polarized light entered
the film in two perpendicular directions, the fluorescence in-
tensity changed about 10 times. The thermal treatment was
the same as that in 2D WAXD measurements, so such large
difference was ascribed to the anisotropy in different direc-
The LUMO energy levels were much lower than those of the
MJLCPs containing electron-transport oxadiazole in the side
chains reported before,10 which indicated that the electron
injection and transportation was improved further. The
HOMO energy levels decreased, too, which was consistent
with the results that the LUMO and HOMO energy levels
both lower when thiophene was introduced into the conju-
gated side chain,33 as a result of the increased electron-
FIGURE 7 Normalized UV–vis absorption spectra of monomers
(1 ꢂ 10ꢁ5M) and polymers (1 mg/10 mL) in chloroform solu-
tions and film state measured at room temperature.
FIGURE 8 Normalized PL spectra of monomers (1 ꢂ 10ꢁ5M)
and polymers (1 mg/10 mL) in chloroform solutions and film
state measured at room temperature.
MJLCPS WITH BOTH THIOPHENE AND OXADIAZOLE, YANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 9 Polarized photoluminescence spectra of P-C10 film.
I// and I denoted the fluorescence polarized in two perpendicu-
?
lar directions. [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]
withdrawing characteristic of the pendant side chains. The
electrochemical band gaps were consistent with the optical
band gaps on the whole, and larger in a certain extent,
because of the interface barrier for charge injection, which
was in agreement with those observed in main-chain conju-
gated polythiophenes.34
Electroluminescent Properties
EL devices of P-C10 with different configurations were fabri-
cated and taken as examples to investigate the optoelectronic
characteristics of such kinds of MJLCPs, and the data were
summarized in Table 4. At first, device a with the configura-
tion of ITO/PEDOT/polymer/TPBI (15 nm)/Mg:Ag (10:1, wt
%)/Ag was fabricated. The turn-on voltage was 8.7 V, and
the maximum brightness, maximum current efficiency and
FIGURE 11 Energy level diagrams of device a (a) and device b
(b). [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
luminescent efficiency were 142 cd/m2 at 18.5 V, 0.06 cd/A
and 0.019 lm/W at 10.2 V, respectively. When compared
with the requirements in the progress of PLEDs, the
FIGURE 10 Cyclic voltammograms of films on a vitreous car-
bon electrode in 0.1 mol/L Bu4NClO4, acetonitrile solution.
[Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
FIGURE 12 Normalized EL spectra of the device (ITO/PEDOT/
PVK/polymer/TPBI/Ca/Ag) at different bias voltages. [Color fig-
ure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
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ARTICLE
FIGURE 14 Current efficiency-current density and luminescent effi-
ciency-current density characteristics of the EL device (ITO/PEDOT/
PVK/polymer/TPBI/Ca/Ag) of P-C10. [Color figure can be viewed in
the online issue, which is available at www.interscience.wiley.com.]
FIGURE 13 Current density-voltage and brightness-voltage
characteristics of the EL device (ITO/PEDOT/PVK/polymer/TPBI/
Ca/Ag) of P-C10. [Color figure can be viewed in the online
issue, which is available at www.interscience.wiley.com.]
maximum brightness and efficiencies were too low to be
used in actual application. According to electrochemical
properties of P-C10, its HOMO energy level was ꢁ6.18 eV,
which was much lower than that of PEDOT (ꢁ5.0 eV), so the
energy barrier at the anode was about 1.18 eV, which might
be difficult for the hole injection between the interface of
PEDOT and polymer layers for device a. So poly(N-vinylcar-
bazole) (PVK), a well-known hole-transporting material in
use for PLEDs, with HOMO and LUMO energy levels at ꢁ5.5
eV and ꢁ2.3 eV was introduced between PEDOT and P-C10
layers. Ca (work function ¼ 2.9 eV) was better matched elec-
trode than Mg (work function ¼ 3.7 eV), and was used
instead of Mg:Ag. So device b with the configuration of ITO/
PEDOT/PVK/polymer/TPBI (15 nm)/Ca (25 nm)/Ag (80
nm) was fabricated to obtain better device performance. The
energy level diagrams of the different layers of device a and
device b were depicted in Figure 11. As shown in Figure 12,
the EL spectra of device b had only one intense peak at 498
nm in the blue-green region, and there was almost no red
shift with respect to the PL maximum emission in film state.
The spectra exhibited good stability when the bias voltage
was increased. The turn-on voltage was 11.4 V. Figures 13
and 14 showed the spectra of brightness and current density
versus bias voltage and the spectra of current efficiency and
luminescence efficiency versus current density of device b.
The maximum brightness of device b was 541 cd/m2. The
maximum current efficiency and luminescent efficiency were
0.10 cd/A and 0.020 lm/W, respectively. It was concluded
that the EL properties of device b was better than that of de-
vice a, as the brightness of device b was four times as that
of device a, and the current efficiency as well as luminescent
efficiency were both higher than those of device a, too. The
addition of PVK layer facilitated the injection and transporta-
tion of holes from the anode, and the quantities of the elec-
trons and holes increased largely, which caused the maxi-
mum brightness largely improved. The PVK layer may act
the electron-blocking function, and then improve the balance
of the electron and hole transporting, which enhanced the
EL efficiencies of the device in a certain extent. The EL
properties of device b were also much better than that of
the MJLCPs containing electron-transport oxadiazole in
side chains,10 which indicated that the introduction of thio-
phene as hole-transport unit into the side chian of MJLCP
and its directly connected way with the electron-transport
oxadiazole was an efficient approach to impove the EL
properties.
CONCLUSIONS
A series of electroluminescent MJLCPs with electron- and
hole-transport units directly connected in the side chain
were designed and synthesized for the first time, and their
TABLE 3 Electrochemical Properties of the Polymers
onset
onset
kedge
(nm)a
Optical
Eg (eV)b
E
vs.
E
vs.
EHOMO
ELUMO
Electrochemical
Eg (eV)c
ox
red
Polymer
Ag/AgCl (V)
Ag/AgCl (V)
(eV)
(eV)
P-C8
P-C10
a
446
427
2.78
2.90
1.66
1.77
ꢁ1.19
ꢁ1.36
c
ꢁ6.07
ꢁ6.18
onset
ꢁ3.22
ꢁ3.05
2.85
3.13
onset
onset
red
kedge was the onset value of absorption spectrum in long wavelength
Electrochemical Eg ¼ E
ꢁ E
eV, EHOMO ¼ ꢁe (E
ꢁ E1/2,
)
FOC
ox
ox
range.
ꢁ 4.8 eV, ELUMO¼ ꢁe (Eorendset ꢁ E1/2, FOC) ꢁ 4.8 eV
b
Optical band gap was obtained from the empirical formula Eg ¼ 1240/
kedge eV.
MJLCPS WITH BOTH THIOPHENE AND OXADIAZOLE, YANG ET AL.
1513

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 4 Electroluminescent Properties of the Devices
Lmax
(cd/m2)b
Emission
wavelength (kem, nm)c
Current
efficiency (cd/A)d
Luminescent
efficiency (lm/W)e
Device
Von (V)a
(x,y)f
EQE (%)
a
b
a
8.7
142
541
498
498
0.06
0.10
d
0.019
0.020
(0.270,0.445)
(0.270,0.445)
0.034
0.076
11.4
Turn-on voltage at 1 cd/m2.
Maximum luminance.
Maximum current efficiency.
b
c
e
f
Maximum luminescent efficiency.
1931 CIE coordinate.
Emission wavelength at the bias voltage of 12 V.
3 O’Neill, M.; Kelly, S. M. Adv Mater 2003, 15, 1135–1146.
electroluminescent property and liquid crystalline property
were discussed. The monomers were synthesized via seven
steps and confirmed by various characterization techniques,
and effective polymerization was realized. The polymers
were characterized by thermal analysis, optical spectroscopy,
cyclic voltammetry, electroluminescent analysis, PLM, 1D
WAXD, 2D WAXD, and polarized photoluminescence. The
resulting polymers possessed good thermal stability and
high glass transition temperatures. The absorption spectra of
both monomers and polymers had little red-shift in film
than that in solution, indicating the aggregation was little.
The photoluminescence spectra of the polymers and mono-
mers in film had great red shift, suggesting the formation of
excimer or exciplex. The polymers showed lower HOMO
energy levels and LUMO energy levels than those of the
MJLCPs containing oxadiazole unit reported before, which
illuminated that the introduction of thiophene induced the
electron transportation improved further and the HOMO
energy level lower. Electroluminescence was studied with
the optimized device configuration of ITO/PEDOT/PVK/poly-
mer/TPBI/Ca/Ag. The maximum brightness and current effi-
ciency reached 541 cd/m2 and 0.10 cd/A, which proved that
the introduction of thiophene and its directly connected way
with electron-transport units could improve the EL property
of side-chain conjugated polymers. The phase structures of
the polymers were investigated with PLM, 1D WAXD, and 2D
WAXD, and it was confirmed that the polymers formed smec-
tic A phase after they entered liquid crystalline phase. Polar-
ized photoluminescence measurements showed that the fluo-
rescence intensity changed about 10 times when the
polarized light entered in two perpendicular directions,
which showed the potential utility for practical applications
in display.
4 Zhao, N.; Botton, G. A.; Zhu, S.; Duft, A.; Ong, B. S.; Wu, Y.
L.; Liu, P. Macromolecules 2004, 37, 8307–8312.
5 McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Mac-
Donald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.;
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