Synthesis and optical properties of copolymers containing electron transporting 1,3,4-oxadiazole pendant groups

Article abstract of DOI:10.1080/15421400600930409

Well-defined copolymer, P(3,6-EHCZ-alt-MPAOXD), containing a triarylamine and N-alkylcarbazole groups (as hole transport moieties with blue emission) in the main chain and a 1,3,4-oxadiazole pendant group (as an electron transporting moiety) was synthesiz

Full text of DOI:10.1080/15421400600930409

Mol. Cryst. Liq. Cryst., Vol. 459, pp. 109=[389]–118=[398], 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600930409
Synthesis and Optical Properties of Copolymers
Containing Electron Transporting 1,3,4-Oxadiazole
Pendant Groups
Hui Wang
Jeong-Tak Ryu
College of Information and Communication Engineering, Daegu
University, Gyeongsan, Gyeongbuk, Korea
Yoon Soo Han
Department of Display & Nano Materials, Daegu Gyeongbuk Institute
of Science & Technology, Daegu, Korea
Youngjune Hur
Lee Soon Park
ADMRC, Department of Polymer Science, Kyungpook National
University, Daegu, Korea
Minhyeon Song
Younghwan Kwon
ADMRC, Department of Chemical Engineering, Daegu University,
Gyeongsan, Gyeongbuk, Korea
Well-defined copolymer, P(3,6-EHCZ-alt-MPAOXD), containing a triarylamine
and N-alkylcarbazole groups (as hole transport moieties with blue emission) in
the main chain and a 1,3,4-oxadiazole pendant group (as an electron transporting
moiety) was synthesized by Pd-catalyzed polycondensation of N-(2-ethylhexyl)-3,6-
dibromo carbazole with 2-methylphenyl-5-(4-aminophenyl)-1,3,4-oxadiazole. For
comparison, P(3,6-EHCZ-alt-AL) containing a triarylamine and a carbazole
groups in the main chain was also prepared. P(3,6-EHCZ-alt-AL) exhibited
k
max,UV at 309 nm and kmax,PL in the range of blue emission at 452 nm. However,
P(3,6-EHCZ-alt-MPAOXD) exhibited kmax,UV at 283 nm with a new peak at
59 nm, and red-shifted PL emission at 506 nm, possibly attributed to the extended
3
conjugation by 1,3,4-oxadiazole pendant group. The electrochemical results
revealed that incorporation of 1,3,4-oxadiazole group in the polymer side chain
This research was supported by the Daegu University Grant, 2006.
Address correspondence to Younghwan Kwon, ADMRC, Department of Chemical
Engineering, Daegu University, Gyeongsan, Gyeongbuk, 712-714, Korea. E-mail: y_kwon@
daegu.au.kr
109=[389]

1
10=[390]
H. Wang et al.
provided a closely matched HOMO energy levels with hole-injecting PEDOT layer,
and reduced the band gap energy level.
Keywords: hole transport; N-alkylcarbazole; 1,3,4-oxadiazole; triarylamine
INTRODUCTION
Polymer light emitting diodes (PLEDs) have been attracted attention
because of their potential to use in display, which are based on conju-
gated polymers [1,2]. As for the simple sandwiched diode, holes inject-
ing from the anode into the p orbital, while the electrons injecting from
ꢀ
the cathode into the p orbital of the polymer, to form exitons and give
light. To achieve high luminescence efficiency, it is very important
to keep the balance of hole and electron current [3–5]. Many research
groups have studied carbazole derivatives as hole-transporting
materials and small molecular oxadiazoles as electron-transporting
materials [6,7].
There are mainly two approaches to increase the efficiency of the
diodes. One is to blend the charge injection=transport molecules into
a polymer matrix and keep the single-layer structure by spinning coat-
ing. An available example showed that devices fabricated by blending
2
-(4-biphenyl)-5-(4-tert-butylhpenyl)-1,3,4-oxadiazole
(PBD)
into
poly(N-vinylcarbazole) (PVK) gave high efficiency and wide color tun-
ing [6–9]. In these systems, however, the crystallization and phase
separation are likely to form, which can make the defects and cause
devices failure. Another approach is covalently attaching the hole
and electron transporting moieties to the same polymer. Polymers con-
taining carbazole and oxadiazole units in either the main chain or side
group have been synthesized. Kido and his coworkers [10] synthesized
well-defined copolymers with triarylamine (hole-transporting) and
oxadiazole moieties in the backbone. Jiang et al. [11] incorporated car-
bazole and oxadiazole groups through free radical copolymerization of
N-vinylcarbazole with two oxadiazole-bearing monomers. A series of
polymers with poly(p-phenylene vinylene), PPV, and oxadiazole have
been also synthesized. It demonstrated that no matter placing oxadia-
zole group in main chains or side chains, the polymers both showed
great efficiencies [12–14].
In this paper, we present the synthesis of new copolymer, P(3,6-EHCZ-
alt-MPAOXD), consisting of a triarylamine and a carbazole groups (hole
transporting ability with blue emission) in the main chain as well as the
electron transporting 1,3,4-oxadiazole pendant moiety through pal-
ladium-catalyzed polycondensation. In addition, P(3,6-EHCZ-alt-AL)

Synthesis and Optical Properties of Copolymers
111=[391]
with a triarylamine and a carbazole groups in the main chain was also
synthesized. Structural effect of the polymers on the optical and electro-
chemical properties was investigated.
EXPERIMENTAL
Materials
2
-Ethylhexylbromide (97%), 3,6-dibromocarbazole (97%), p-toluic
hydrazide (99%), phosphorus oxychloride (POCl , 99%), tin(II) chloride
3
(
SnCl , 98%), sodium tert-butoxide (NaO-t-Bu, 97%), tri(dibenzylidenea-
2
cetone)dipalladium(0) (Pd (dba) ), tri-tert-butylphosphine (P(t-Bu) ,
2
3
3
9
0%) and aniline (99%) were purchased from Aldrich Chemical Co.
Reagent grades of potassium carbonate anhydrous (K CO ) and 4-nitro-
2
3
benzoyl chloride were obtained from Tokyo Kasei Co. and used as
received. Solvents were reagent grades and purified prior to use.
Synthesis of Monomers
N-(2-Ethyhexyl)-3,6-dibromocarbazole (3,6-DBEHCZ) was synthesized
according to the reference [15].
2-Methylphenyl-5-(4-aminophenyl)-1, 3, 4-Oxadiazole (MPAOXD)
The synthetic scheme for MPAOXD is shown in Scheme 1. In the
first step, 4-nitrobenzoyl chloride (0.1 mol) was added dropwise to a sol-
ution of p-toluic hydrazide (0.1 mol), triethylamine (0.1 mol), and
chloroform (150 ml) at room temperature. The resulting mixture was
stirred for 2 h and then filtered. The solid collected was washed with
water and methanol to give the product, 2-methylphenyl-5-(4-nitrophe-
1
nyl)-dihydrazide (MPNDHA), with the yield of 98%. H NMR (300 MHz
in CDCl ): d 10.88 (1H, ꢁNHꢁ), 10.62 (1H, ꢁNHꢁ), 8.45 (2H, ArꢁH),
3
8
.21 (2H, ArꢁH), 7.88 (2H, ArꢁH), 7.40 (2H, ArꢁH), 2.36 (3H,
13
ArꢁCH ); C NMR (300 MHz in CDCl ): d 166.5, 165.2, 150.3, 142.9,
3
3
1
39.1, 130.4, 129.9, 129.8, 128.4, 124.7, 21.9. GC-MS m=z ¼ 299 (mþ).
SCHEME 1 Synthetic route to MPAOXD.

1
12=[392]
H. Wang et al.
In the second step, a mixture of MPNDHA (0.1 mol) and POCl3
50 ml) was refluxed for 6 h. The excess POCl was evaporated out
(
3
and the residue was poured into water. The crude solid product was
collected by filtration, and purified by recrystallization from chloro-
form=methanol to give the product, 2-methylphenyl-5-(4-nitrophe-
1
nyl)-1,3,4-oxadiazole (MPNOXD), with the yield of 90%. H NMR
(
300 MHz in CDCl ): d 8.41 (2H, ArꢁH), 8.32 (2H, ArꢁH), 8.05 (2H,
3
13
ArꢁH), 7.35 (2H, ArꢁH), 2.46 (3H, ArꢁCH ); C NMR (300 MHz in
3
CDCl ): d 166.1, 163.0, 149.9, 143.4, 130.3, 129.9, 128.1, 127.5, 124.8,
3
1
20.9, 22.1. GC-MS m=z ¼ 281 (m þ).
A mixture of MPNOXD (10 mmol), SnCl (60.7 mmol) and ethanol
2
ꢂ
(
30 ml) was heated at 70 C for 4 h. After the mixture was cooled to room
temperature, 1 M of NaOH solution was added until the mixture became
alkaline. After extraction with ethyl acetate, the combined organic
layers were washed with brine, dried with MgSO and concentrated.
4
The residue was recrystallized from ethanol and final compound,
2
-methylphenyl-5-(4-aminophenyl)-1,3,4-oxadiazole (MPAOXD), was
1
obtained with the yield of 90%. H NMR (300 MHz in CDCl ): d 7.98
3
(
2H, ArꢁH), 7.77 (2H, ArꢁH), 7.32 (2H, ArꢁH), 6.74 (2H, ArꢁH), 4.50
13
(
2H, ꢁNH ), 2.46 (3H, ArꢁCH ); C NMR (300 MHz in CDCl ): d
2
3
3
1
2
64.7, 163.9, 149.6, 141.8, 129.7, 128.6, 126.7, 121.5, 114.7, 113.8,
1.6. GC-MS m=z ¼ 251 (m þ).
Polymerization
A mixture of 3,6-EHDBCZ (1.30 mmol) (or MPAOXD) and aniline was
dissolved in toluene (15 ml). NaO-t-Bu (3.9 mmol), Pd (dba)3
2
(
0.033 mmol) and P(t-Bu) (0.2 mmol) were added to the solution at
3
ꢂ
room temperature. The reaction mixture was stirred at 100 C for
4
8 h. After cooling to room temperature, the mixture was quenched
by adding aqueous ammonia (25 ml) and the product was extracted
with CHCl . The organic fraction was concentrated and reprecipitated
3
from CHCl =methonal several times. Then, the products were filtered
3
and dried in vacuum.
Characterization
NMR spectra were recorded on a Varian Unity Plus 300 with CDCl3
solvent. GC Mass (GC-MS) spectra were recorded on a QP 5050 mass
spectrometer. Molecular weights and molecular weight distributions
were measured by Waters gel permeation chromatograph (GPC)
equipped with Styragel HR 5E column using THF as an eluent against
polystyrene standards at room temperature. Thermal analysis was

Synthesis and Optical Properties of Copolymers
113=[393]
performed on a Seiko EXSTAR 6000 TG=DTA 6300 thermal analyzer
ꢂ
at a heating of 20 C=min for thermogravimetric analyzer (TGA) and at
ꢂ
a heating of 10 C=min for differential scanning calorimetry (DSC). UV-
Visible absorption spectra were taken by Shimadzu UV-2100. The
photoluminescence (PL) spectra excited by He-Cd laser at 325 nm were
monitored by Optical Multichannel Analyzer (Laser Photonics, OMA
system). The ionization potential (IP) was measured by a photoelec-
tron spectroscopy (Riken Keiki AC-2).
RESULTS AND DISCUSSION
Conjugated copolymers such as P(3,6-EHCZ-alt-AL) and P(3,6-EHCZ-
alt-MPAOXD) were prepared by Pd-catalyzed polycondensation, as
depicted in Scheme 2. P(3,6-EHCZ-alt-AL) was designed to have a
triarylamine and a carbazole groups (hole transporting ability with
blue emission) in the main chain. In addition to these functional groups
of P(3,6-EHCZ-alt-AL), the electron transporting 1,3,4-oxadiazole moi-
ety was incorporated into the side chain of P(3,6-EHCZ-alt-MPAOXD).
The molecular weights and molecular weight distributions were
measured by using GPC, and summarized in Table 1. It appeared that
the molecular weight of P(3,6-EHCZ-alt-MPAOXD) was lower than
that of P(3,6-EHCZ-alt-AL), which was likely to be explained by the
poor solubility of P(3,6-EHCZ-alt-MPAOXD). Though the solubility
of P(3, 6-EHCZ-alt-MPAOXD) is not favorable, it is still readily soluble
in chloroform, DMF and THF. Figure 1 represents thermal stability of
the polymers measured by TGA. Based on the decomposition tempera-
ture (T ) at 5 wt% loss of the initial weight, two polymers exhibited
d
ꢂ
ꢂ
relatively high Td around 440 C. The weight loss at 800 C was
measured to be around 37% and 46% for P(3,6-EHCZ-alt-AL) and
P(3,6-EHCZ-alt-MPAOXD), respectively. More weight loss of P(3,6-
EHCZ-alt-MPAOXD) could be possibly assigned to the decomposition
of 1,3,4-oxadiazole pendant groups. While DSC curves of two polymers
showed neither glass transition temperature nor melting peak.
UV-Vis absorption and PL emission spectra of P(3,6-EHCZ-alt-AL)
and P(3,6-EHCZ-alt-MPAOXD) are presented in Figures 2 and 3,
respectively. In Figure 2, maximum absorption of P(3,6-EHCZ-alt-AL)
was observed at 309nm, however, P(3,6-EHCZ-alt-MPAOXD) exhibited
ꢀ
kmax,UV at 359nm, attributed to the p-p electronic transitions of
N-alkylcarbazole and 1,3,4-oxadiazole moieties [16]. PL emission spectra
were obtained after excitation at their kmax,UV of two copolymers. As can
be seen in Figure 3, PL emission of P(3,6-EHCZ-alt-AL) was in the blue
range at 452 nm, while P(3,6-EHCZ-alt-MPAOXD) exhibited kmax,PL at
5
06 nm. These optical properties such as red-shifted UV-Vis absorption

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14=[394]
H. Wang et al.
SCHEME 2 Polymerizations via palladium catalyzed polycondensation;
ꢂ
Reaction condition (I): Toluene, NaO-t-Bu, Pd (dba) , P(t-Bu) , 100 C.
2
3
3
TABLE 1 Physical Properties of Polymers
ꢂ
a
Copolymers
M
n
(g=mol)
M
w
=M
n
T ( C)
d
P(3,6-EHCZ-alt-AL)
P(3,6-EHCZ-alt-MPAOXD)
7,300
3,400
1.3
1.2
442
443
a
Temperature measured at 5% weight loss based on initial weight.

Synthesis and Optical Properties of Copolymers
115=[395]
FIGURE 1 TGA curves of the polymers; (a) P(3,6-EHCZ-alt-AL) and
b) P(3,6-EHCZ-alt-MPAOXD).
(
and PL emission of P(3,6-EHCZ-alt-MPAOXD) might be originated from
the extended conjugation by the presence of 1,3,4-oxadiazole group in the
polymer side chain.
The electrochemical properties of the copolymers are summarized in
Table 2. The copolymers showed HOMO energy levels of ꢁ5.19 eV
for P(3,6-EHCZ-alt-AL) and ꢁ5.16 eV for P(3,6-EHCZ-alt-MPAOXD),
respectively. LUMO energy levels of the copolymers were also calcu-
lated from HOMO energy and band gap energy levels to be ꢁ2.28 eV
for P(3,6-EHCZ-alt-AL) and ꢁ2.30 eV for P(3,6-EHCZ-alt-MPAOXD).
Based on the above information, schematic fabrication of a device is
shown in Figure 4. These results revealed that such HOMO energy
FIGURE 2 UV-Vis absorption spectra of the polymers; (a) P(3,6-EHCZ-alt-
AL) and (b) P(3,6-EHCZ-alt-MPAOXD).

1
16=[396]
H. Wang et al.
FIGURE 3 Photoluminescence spectra of the polymers; (a) P(3,6-EHCZ-alt-
AL) and (b) P(3,6-EHCZ-alt-MPAOXD).
TABLE 2 Electronic Properties of Polymers
k
max,UV
k
max, PL
Band
gap (eV)
HOMO
(eV)
LUMO
(eV)
a
b
c
Copolymers
(nm)
(nm)
P(3,6-EHCZ-alt-AL)
P(3,6-EHCZ-alt-MPAOXD)
309
359
452
506
2.91
2.86
ꢁ5.19
ꢁ5.16
ꢁ2.28
ꢁ2.30
a
Calculated from the crosspoint of UV-Vis and PL spectrum.
Measured by a RIKEN Keiki AC-2.
b
c
Estimated from the HOMO and band gap.
FIGURE 4 Schematic view of energy levels of the polymers; P1: P(3,6-EHCZ-
alt-AL) and P2: P(3,6-EHCZ-alt-MPAOXD).

Synthesis and Optical Properties of Copolymers
117=[397]
levels of the copolymers might provide a close match to the function of
hole-injecting PEDOT layer, especially when they were used as hole-
transporting materials in the PLEDs. It was also found that the incor-
poration of the 1,3,4-oxadiazole group in the polymer side chain
reduced the electronic band gap energy level of the copolymer, which
might be explained by more conjugation formed in the molecular chain
by introducing 1,3,4-oxadiazole pendant groups in the side chain.
CONCLUSIONS
Conjugated copolymer, P(3,6-EHCZ-alt-MPAOXD), consisting of a
triarylamine and a carbazole groups (hole transporting ability with
blue emission) in the main chain as well as the electron transporting
1
,3,4-oxadiazole pendant moiety was synthesized through palladium-
catalyzed polycondensation. For comparison, P(3,6-EHCZ-alt-AL) with
a triarylamine and a carbazole groups in the main chain was also
synthesized and characterized. The optical and electrochemical results
revealed that the incorporation of the 1,3,4-oxadiazole group in the
polymer side chain reduced the electronic band gap energy level of
the copolymer. It was also found that the copolymers could be used
as hole-transporting materials in the PLEDs, due to closely matched
HOMO energy levels between the copolymers and hole-injecting
PEDOT layer.
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