Synthesis and electrooptical properties of novel PPV derivatives containing 1,3,4-oxadiazole

Article abstract of DOI:10.1080/15421400390263884

Poly[2-{5′-(4″-t-butylphenyl)-1′, 3′, 4′-oxadiazole}-1,4-phenylenevinylene (BPOD-PPV)] containing 4-t-butylphenyl-1,3,4-oxadiazole pendant was synthesized by Gilch reaction. The obtained polymer shows a strong absorption centered around 300nm and broader peak at about 350-520 nm. The efficiency of ITO/ PPV/polymer/Al device is greater than by more than 2 orders of magnitude when compared with the ITO/PPV/Al device.

Full text of DOI:10.1080/15421400390263884

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SYNTHESIS AND
ELECTROOPTICAL PROPERTIES
OF NOVEL PPV DERIVATIVES
CONTAINING 1,3,4-OXADIAZOLE
Yun-Hi Kim a , Dong-Cheol Shin a , Sung-Ouk Jung a ,
Soon-Ki Kwon a & Do-Hoon Hwang b
a Department of Polymer Science & Engineering and
Engineering Research Institute, Gyeongsang National
University, Chinju 660-701, Korea
b Kumoh National University of Technology, Kumi
730-701
Published online: 15 Jul 2010.
To cite this article: Yun-Hi Kim , Dong-Cheol Shin , Sung-Ouk Jung , Soon-Ki Kwon &
Do-Hoon Hwang (2003) SYNTHESIS AND ELECTROOPTICAL PROPERTIES OF NOVEL PPV
DERIVATIVES CONTAINING 1,3,4-OXADIAZOLE, Molecular Crystals and Liquid Crystals,
405:1, 27-32, DOI: 10.1080/15421400390263884
To link to this article: http://dx.doi.org/10.1080/15421400390263884
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Mol. Cryst. Liq. Cryst., Vol. 405, pp. 27–32, 2003
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400390263884
SYNTHESIS AND ELECTROOPTICAL PROPERTIES
OF NOVEL PPV DERIVATIVES CONTAINING
1,3,4-OXADIAZOLE
Yun-Hi Kim, Dong-Cheol Shin, Sung-Ouk Jung,
and Soon-Ki Kwon*
Department of Polymer Science & Engineering and
Engineering Research Institute, Gyeongsang National
University, Chinju 660-701, Korea
Do-Hoon Hwang
Kumoh National University of Technology,
Kumi 730-701
Poly[2-{50-(400-t-butylphenyl)-10,30,40-oxadiazole}-1,4-phenylenevinylene (BPOD-
PPV)] containing 4-t-butylphenyl-1,3,4-oxadiazole pendant was synthesized by
Gilch reaction. The obtained polymer shows a strong absorption centered
around 300 nm and broader peak at about 350–520 nm. The efficiency of ITO/
PPV/polymer/Al device is greater than by more than 2 orders of magnitude
when compared with the ITO/PPV/Al device.
Keywords: 4-t-butylphenyl-1,3,4-oxadiazole; efficiency; electron transfer; PPV
INTRODUCTION
Luminescence properties of poly(p-phenylenevinylene), PPV, and other
conjugated polymers have been attracting a great deal of interests since the
first light emitting diodes (LEDs) based on PPV were reported a decade
ago by the Cambridge group [1–2]. Electroluminescence (EL) efficiency of
the devices, however, was far from satisfactory. And it soon was found that
chemical modifications of PPV and use of different electrodes together with
utilization of electron- and/or hole- transporting layers can improve the
device efficiency to impressive extents.
This work was supported by Korea Research Foundation Grant (KRF-2001-005-D00005)
and the Basic Research Program of the Korea Science & Engineering Foundation (R05-
2000-00016).
*Corresponding author. E-mail: skwon@nongae.gsnu.ac.kr
27

28
Y.-H. Kim et al.
It is understood that in LED devices electrons and holes are separately
injected from an anode and a cathode, respectively, under a bias voltage
into the light emitting polymer layer where the injected negative and
positive carriers from excitons. The excitons can disappear via various
mechanisms; one of them is luminescence decay or radiative decay. To
improve devices efficiency of LEDs there have been many attempts to
balance the injection of carriers from electrodes and also their mobility in
the emitting polymer layer. Unfortunately, the hole mobility in PPV and its
simple derivatives is typically higher than that of electron mobility. This is
one of the reasons why the LED devices fabricated using these polymers
exhibit rather unsatisfactory efficiencies. Utilization of low work function
metals as cathodes makes the electron injection easier. Calcium and lithium
are representative examples. Another approach for improving electron
transporting is to use of additives of electron deficient compounds such as
2-(4-biphenylyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole (PBD). But their
efficiency can be limited due to their crystallization and aggregation. This
problem is mitigated by incorporating the electron transporting units into
the main chain or as pendants attached to the backbone of a polymer. Or
they can be included both in the main chain and in the side group [3–6].
In this article, we report synthesis, structural analysis, and spectral and
luminescence properties of poly[2-{50-(400-t-butylphenyl)-10,30,40-oxadia-
zole}-1,4-phenylenevinylene (BPOD-PPV)] that bears 4-t-butylphenyl-
1,3,4-oxadiazole (BPOD) pendants directly attached to the PPV backbone.
LED devices were fabricated using the polymer as emitting material or
electron transfer material, and devices characteristics also are discussed.
EXPERIMENTAL
Synthesis of 2-(40-t-butylphenyl)-5-(200,500-dibromomethyl-
phenyl)-1,3,4-oxadiazole
In the solution of 150 mL of toluene and 30 mL of methanol, 16.3 g
(302.5 mmol) of hydrazine monohydrate was added and stirred. In the
mixture, 19.9 g (103.5 mmol) of methyl-t-butylbenzoate was dropped. After
reflux for 6, the solvent was removed. The 4-t-butylbenzoic hydrizide was
recrystallized in 10% aqueous methanol. In the solution of obtained 30 g
(156 mmol) of t-butylbenzoyl hydrazine in 60 mL of pyridine and 100 mL of
toluene, 26.3 g (156 mmol) of 2,5-dimethylbenzoyl chloride was dropped.
After stirring for 6 h in 60ꢀC, the reaction was terminated. After the mix-
ture was extracted by ethylacetate, the 1-(40-t-butylbenzoyl)-2-(200,500-
dimethylbenzoyl)hydrazine was recrystallized in toluene. In the solution
of synthesized 30 g (92.6 mmol) of 1-(40-t-butylbenzoyl)-2-(200,500-di-
methylbenzoyl) hydrazine, 50 g (420 mmol) of thionyl chloride was slowly

Synthesis and Electrooptical Properties
29
dropped. After the reaction mixture was refluxed during 24 h, the solvent
was removed by simple distillation. The crude 2-(40-t-butylphenyl)-5-
(200,500-dimethylphenyl)-1,3,4-oxadiazole was recrystallized in methanol.
After the obtained 10 g (32.7 mmol) of 2-(40-t-butylphenyl)-5-(200,500-
dimethylphenyl)-1,3,4-oxadiazole was dissolved in CCl4, 12.8 g (72 mmol)
of N-bromosuccinimide and BPO was added. After cooling of the
mixture, succinimide was filtered and the 2-(40-t-butylphenyl)-5-(200,500-
dibromomethylphenyl)-1,3,4-oxadiazole was recrystallized in n-hexane:
ethylacetate (1:1). mp: 128ꢀC. Yield: 43%. 1H NMR (500 MHz, CDCl3)
(ppm): aromatic(C-H), d 8.4 (m, 4H), 7.8(m, 3H), aliphatic(C-H) d 5.3(s,
2H), 4.8(s, 2H), 1.3(s, 9H) FT-IR (KBr) (cmꢁ1) :3047(aromatic C-H),
2946(aliphatic CH3), 1025(C¼N).
Synthesis of 2-(50-(400-t-butylphenyl)-10,30,40-oxadiazole-1,4-
phenylenevinylene (BPOD-PPV)
In the ice-bath, 0.5 g of 2-(40-t-butylphenyl)-5-(200,500-dibromomethylphe-
nyl)-1,3,4-oxadiazole and catalytic amount of 18-crown ether were
dissolved in 30 mL of THF, and 0.24 g of potassium t-butoxide was slowly
dropped. After the polymerization was terminated by methanol, the
FIGURE 1 Synthetic scheme of ETS-PPV.

30
Y.-H. Kim et al.
product was dissolved in chloroform and the solution was precipitated in
methanol. Yield: 65%.
RESULT AND DISCUSSION
The synthetic scheme of the monomer and polymer is outlined in Figure 1.
2-(40-t-butylphenyl)-5-(200,500-dibromomethylphenyl)-
The
monomer,
1,3,4-oxadiazole, was prepared via cyclization and NBS reaction. The
polymer, poly[2-(50-(400-t-butylphenyl)-10,30,40-oxadiazole-1,4-phenylenevi-
nylene)(BPOD-PPV), was obtained through the typical GILCH method.
1H-NMR and FT-IR spectra agree with the proposed structures of various
compounds and polymer. In the 1H-NMR spectrum, the vinyl protons
appeared at the range of 7.8–6.6 ppm with aromatic protons, instead of
disappearing of methylene protons at around 4.5 ppm and 5.3 ppm. The
molecular weight of polymer determined by gel permeation chromato-
graphy against polystyrene standards is Mn ¼ 5600 with a polydispersity
index of 1.4.
FIGURE 2 I-V characteristics of devices.

Synthesis and Electrooptical Properties
31
FIGURE 3 Comparison of current density-external quantum efficiency curves for
devices.
When the UV-vis spectra of obtained polymer and PPV, which was
prepared via the Wessling-Zimmerman water soluble precursor route and
thermally treated at 270ꢀC in vacuo for 12 h, were compared, the obtained
polymer shows a strong absorption centered around 300 nm and broader
peak at about 350–520 nm. In contrast, PPV absorbs broadly over the
wavelength of 290–570 nm. It is suggested that the absorption in the
300 nm is attributed to the oxadiazole pendant and that in the longer
wavelength region is attributed to the p–p* transition of the main chain. In
the photoluminescence spectrum of polymer, the emission maximum was
observed at 505 nm.
LED devices were fabricated using the polymer as emitting material or
electron transfer material. Figure 2 shows the I-V characteristics of the
devices. The PPV/polymer devices showed lower turn on voltage compared
with that of PPV. The result suggested that electron injection is generally
more difficult than hole injection in the PPV derivatives, but the electron
transfer can be ease by the introducing of oxadiazole pendant in PPV.
The current density-quantum efficiency of the devices constructed are
compared in Figure 3. The efficiency of ITO/PPV/polymer/Al device is
greater than by more than 2 orders of magnitude when compared with the
ITO/PPV/Al device.

32
Y.-H. Kim et al.
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