Synthesis and characterization of a light-emitting poly(p- phenylenevinylene)-based copolymer containing 1,3,4-oxadiazole functionality

Article abstract of DOI:10.1080/15421400601010813

A new class of high brightness and luminance efficiency of poly[2-{4-[5-(3-(3,7-dimethyloctyloxy)phenyl)-1,3,4-oxadiazole-2-yl]phenyl}-1, 4-phenylenevinylene] (C10PhOxa-PPV) was synthesized by Gilch polymerization. The weight average molecular weight (Mw) and the polydispersity of C10PhOxa-PPV were found to be 2.7104 and 2.6, respectively. Double-layer light-emitting device with an ITO/PEDOT/C10PhOxa-PPV/Al was fabricated and characterized. The maximum brightness, and luminance efficiency of C10PhOxa-PPV showed values of up to 8.3cd/A and 4,426cd/m2, respectively.

Full text of DOI:10.1080/15421400601010813

Mol. Cryst. Liq. Cryst., Vol. 462, pp. 103–109, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400601010813
Synthesis and Characterization of a Light-Emitting
Poly(p-phenylenevinylene)-Based Copolymer Containing
1,3,4-Oxadiazole Functionality
Sung-Ho Jin
Sung-Chul Kim
Jong-Min Shim
Sung-Min Park
Eun-Jae Lee
Won Suk Shin
Department of Chemistry Education and Interdisciplinary Program of
Advanced Information and Display Materials, Pusan National
University, Busan, Korea
Jae-Wook Lee
Department of Chemistry, Dong-A University, Busan, Korea
Yeong-Soon Gal
Polymer Chemistry Laboratory, College of Engineering, Kyungil
University, Kyungsan, Korea
A new class of high brightness and luminance efficiency of poly[2-{4-[5-(3-(3,
7-dimethyloctyloxy)phenyl)-1,3,4-oxadiazole-2-yl]phenyl}-1,4-phenylenevinylene]
(C10PhOxa-PPV) was synthesized by Gilch polymerization. The weight average
molecular weight (Mw) and the polydispersity of C10PhOxa-PPV were found to
be 2.7 ꢀ 104 and 2.6, respectively. Double-layer light-emitting device with an ITO=
PEDOT=C10PhOxa-PPV=Al was fabricated and characterized. The maximum
brightness, and luminance efficiency of C10PhOxa-PPV showed values of up to
8.3 cd=A and 4,426 cd=m2, respectively.
Keywords: brightness; efficiency; electroluminescence; photoluminescence; PLED;
PPV
This work was supported by Pusan National University Research Grant.
Address correspondence to Sung-Ho Jin, Department of Chemistry Education and
Center for Plastic Information System, Pusan National University, Busan 609-735,
Korea. E-mail: shjin@pusan.ac.kr
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INTRODUCTION
Since the first reports of light-emitting diodes (LEDs) based on either
small molecule fluorescent dye [1] or p-conjugated polymer [2] emit-
ting layers, progressive efforts have been devoted to synthesizing
highly efficient emitting materials for LED applications. Of these
two types of emitting materials, p-conjugated polymer-based LEDs
(PLEDS) appear to be the most promising for the development of
low-cost, full-color, large area flexible displays. However, these mate-
rials have low electron injection and=or transport rates compared to
the number of injected holes within the emitting layer, resulting in
an imbalance in the injected charge rates and a concomitant reduction
in device performance [3]. Oxadiazole-containing polymers have been
investigated since the 1960s and only a few reports on using oxadia-
zole-containing polymer, poly(1,4-phenylene-1,3,4-oxadiazole), as a
charge transporting materials into the electronic devices [4]. The most
important low molecular weight oxadiazole compound, 2-biphenyl-4-
yl-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), was first utilized by
Tsutsui et al. in vapor deposited organic LEDs [5]. Since then, many
research groups have developed various oxadiazole-containing materi-
als, which have been widely used as electron transporting materials
[6–10]. Previously, we described the synthesis of various poly(p-pheny-
lenevinylene)s (PPVs) [11–13] and poly(9,9-dialkylfluorenyl-2,7-viny-
lenes) (PFVs) [14–16], which were found to exhibit high performance
as PLEDs. In this paper, we report the synthesis and characterization
of PPV derivative with oxadiazole pendant group by Gilch polymeriza-
tion method, and highlight its ability to achieve strong brightness and
high luminous efficiency.
EXPERIMENTAL
Emission spectra of a dilute (ꢁ10ꢂ6 M) solution of the polymer were
recorded on a Perkin-Elmer LS-50 fluorometer utilizing a lock-in
amplifier system with a chopping frequency of 150 Hz. Solid-state
emission measurements were achieved using films supported on a
glass substrate and mounted with front-face excitation at an angle
of <45ꢃ. The polymer film was excited with several portions of visible
light from a xenon lamp. To measure EL, a polymer LED was con-
structed as follows: The glass substrate, coated with a transparent
ITO electrode, was thoroughly cleaned by successive ultrasonic treat-
ments in acetone, isopropyl alcohol, and distilled water, and heated
under nitrogen gas for drying. The polymer film was prepared by spin
casting a chlorobenzene or 1,2-dichlorobenzene solution (0.5–1.5 wt%).

Light-Emitting PPV Based Copolymer Containing 1,3,4-Oxadiazole
105
Uniform and pinhole free films with a thickness around 100 nm were
easily obtained from the resulting polymer solution. Aluminum metal
was deposited on the top of polymer film through a mask by vacuum
evaporation at pressure below 1ꢀ10ꢂ6 torr, yielding active areas of
4 mm2. For the measurements of device characteristics, current
density-voltage-luminescence (J-V-L) changes were measured using
a current=voltage source (Keithley 238), an optical power meter
(CS-1000, LS-100). All processes and measurements mentioned above
were carried out in the open air at room temperature. To examine
electrochemical properties, polymer solution was prepared and the
electrochemical measurements were performed under 0.1 M tetrabuty-
lammonium tetrafluoroborate solution containing acetonitrile. ITO,
Ag=AgNO3 and platinum wire were used as a working, reference
and counter electrode, respectively.
RESULTS AND DISCUSSION
Scheme 1 outlines the synthetic route of the monomer and polymer. To
introduce the 4-{5-[3-(3,7-dimethyloctyloxy)phenyl]-1,3,4-oxadiazoyl-
2-yl}phenyl substitutent into the p-xylene backbone by coupling
SCHEME 1 Synthesis scheme.

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reaction, 1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,5-dimethyl
benzene (1) was first synthesized via the reaction of 2-bromo-p-xylene
with 2-isopropoxy-4,4,5,5,-tetramethyl-1,3,2-dioxaborolane in dry
THF at ꢂ78ꢃC (Scheme 1). 4-Bromobenzonitrile was then introduced
into the p-xylene derivative via Suzuki coupling to produce 4-(4-cyano-
phenyl)-2,5-dimethylbenzene (2). The third step involves the forma-
tion of 5-[4-(2,5-dimethylphenyl)phenyl]tetrazole (3) by the reaction
between 4-(4-cyanophenyl)-2,5-dimethylbenzene and sodium azide,
in the presence of an equivalent amount of NH4Cl in dry DMF. The
key reaction between 3 and 3-(3,7-dimethyloctyloxy)benzoyl chloride
proceeds via an intramolecular ring transformation to give 2-[3-(3,7-
dimethyloctyloxy)phenyl]-5-[4-(2,5-dimethylphenyl)phenyl]-1,3,4-oxa-
diazole (4) in high yield (caꢄ 70%) and excellent purity.
The oxadiazole pendant group is typically linked through a rigid
biphenyl moiety, which often causes poor solubility. Consequently,
the bulky 3,7-dimethyloctyloxy groups were introduced into the meta-
position of the phenyl side group in order to increase the overall solu-
bility of the final monomer and corresponding polymer. The target
2-{3-[2,5-bis(bromomethyl)phenyl]phenyl-5-[4-(3,7-dimethyloctyloxy)-
phenyl]-1,3,4-oxadiazole (5) monomer was obtained via bromination of
compound (4) in the presence of benzoyl peroxide (BPO), and purified
by column chromatography using hexane as an eluent. The synthesis
of the oxadiazole unit through the use of the tetrazole intermediate
exhibits several advantages compared to the ring closure procedure
using dehydrating agents [17,18]. Ring closure via the tetrazole path-
way typically proceeds to completion within 2 h, whereas heating with
phosphorus oxychloride can take up to 1–3 days. Moreover, purifi-
cation of the tetrazole intermediate (3) requires only extraction from
methanol. The formation of the target polymer, poly[2-{4-[5-(3-(3,
7-dimethyloctyloxy)phenyl)-1,3,4-oxadiazole-2-yl]phenyl}-1,4-phenyle-
nevinylene] (C10PhOxa-PPV), was synthesized by Gilch polymeriza-
tion, affording a high molecular weight polymer with narrow
polydispersity, and good thermal stability. Under a N2 atmosphere,
monomer (5) was polymerized in dry toluene with an excess of potass-
ium tert-butoxide for 3 h. During polymerization, the viscosity of the
reaction mixture was increased through irradiation with intense fluor-
escent light. The resulting polymer was obtained as fragile orange
fibers after several purification steps [15]. Interestingly, the bent 3-
(3,7-dimethyloctyloxy)phenyl substitutent in the oxadiazole moiety
enhances the solubility of C10PhOxa-PPV in common organic solvents.
The structure and purity of the intermediates, monomer, and final
C10PhOxa-PPV polymer were confirmed by NMR spectroscopy and
elemental analysis, respectively. The number average molecular

Light-Emitting PPV Based Copolymer Containing 1,3,4-Oxadiazole
107
weight (Mn) and polydispersity of C10PhOxa-PPV were determined as
2.7 ꢀ 104 and 2.6, respectively. The thermal stability at 5% weight loss
(385ꢃC) and high glass transition temperature (Tg¼147ꢃC) of
C10PhOxa-PPV are sufficient to prevent deformation of, or morpho-
logical changes in the emitting layer during the operation of PLEDs.
Figure 1 shows the optical absorption and photoluminescence (PL)
spectra of C10PhOxa-PPV in the thin film state, together with the
EL spectrum of a ITO=PEDOT=Polymer=Al device, fabricated in order
to investigate the current density-voltage-luminescence (J-V-L) char-
acteristics of C10PhOxa-PPV. UV-visible absorption of the
C10PhOxa-PPV in both solution and film almost identically show two
peaks at about 300 and 429 nm. The longer wavelength (380–
500 nm) is attributed to the p-pꢅ absorption of the conjugated polymer
chain, whereas the shorter wavelength (297–303 nm) originates from
the aromatic oxadiazole side chain. The C10PhOxa-PPV band gap
was estimated from the absorption edge to be 2.47 eV. The maximum
emission peak of C10PhOxa-PPV was 561 nm. The external PL quan-
tum efficiencies (UPL) of C10PhOxa-PPV and MEH-PPV for compari-
son, in the solid state, were measured and compared using the
integrating sphere method [19]. The UPL values were determined to
be approximately 37ꢆ2% and 10ꢆ2%, respectively, with the
C10PhOxa-PPV UPL being almost 4 times higher than that of MEH-
PPV. The higher external PL quantum efficiency is attributed to the
FIGURE 1 UV-Visible, PL, and EL spectra of the C10PhOxa-PPV thin film.

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bent configuration of the 4-{5-[3-(3,7-dimethyloctyloxy)phenyl]-1,3,4-
oxadiazoyl-2-yl}phenyl substitutents in the p-conjugated polymer
backbone, which are believed to increase the free volume effect and
inhibit the intermolecular interaction between the polymer backbones.
From cyclic voltammetry (CV) and optical absorption edge studies, the
HOMO and LUMO energy levels of C10PhOxa-PPV were observed at
5.47 and 3.00 eV, respectively. As shown in Figure 1, the EL and PL
spectra of C10PhOxa-PPV were identical, revealing maximum peaks
at 547 nm with green color. The current density-voltage-luminescence
(J-V-L) characteristic of C10PhOxa-PPV is shown in Figure 2. The
turn-on voltage of C10PhOxa-PPV was observed at about 4.0 V. The
linear dependence of the luminescence intensity on the voltage is indi-
cated that charge carriers are easily injected from the both electrode
by increasing the voltage or efficiency of the current density. The
maximum brightness of C10PhOxa-PPV was determined to be
4,426 cd=m2 at 28 V. The luminance efficiency of C10PhOxa-PPV,
shown as a function of current in the inset in Figure 2, was determined
to be 8.3 cd=A at 28.2 mA=cm2 and 2,234 cd=m2, based on double-layer
PLEDs using an Al cathode.
The higher luminance efficiency of C10PhOxa-PPV is attributed to the
oxadiazole pendant group, which is believed to form electron transport
channels through orbital interactions between the oxadiazole units.
In summary, we have synthesized a highly luminescent PPV deriva-
tive containing an electron transporting oxadiazole functionality. The
maximum brightness and luminance efficiency of C10PhOxa-PPV
FIGURE 2 Current density-voltage, luminescence-voltage, and luminance
efficiency-current characteristics of C10PhOxa-PPV.

Light-Emitting PPV Based Copolymer Containing 1,3,4-Oxadiazole
109
showed values of up to 8.3 cd=A and 4,426 cd=m2, respectively. Further
optimization of the molecular structure by copolymerization with
various monomers, together with a more detailed study of the EL
properties of C10PhOxa-PPV is currently in progress.
CONCLUSIONS
This study focused on the design and synthesis of multifunctional
light-emitting polymer through Gilch polymerization. The introduc-
tion of electron-deficient 1,3,4-oxadiazole units as side chains on the
PPV backbones enhances the polymers’ electron affinity and increases
their LUMO energy levels, both of which produce improvements in
device performance. The turn-on voltage, maximum brightness and
luminance efficiency of the resulting polymer was 4.0 V, 4,426 cd=m2
at 28 V, and 8.3 cd=A at 2,234 cd=m2.
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