Synthesis and optical properties of copolymers containing carbazole and 1,3,4-oxadiazole groups as blue host matrix and charge transport materials for polymer light emitting devices

Article abstract of DOI:10.1080/15421400601013445

Alternating copolymers with N-arylcarbazole as hole injection and transport moieties with blue emission and 1,3,4-oxadiazole as electron transporting and hole blocking moieties were synthesized by Pd-catalyzed polycondensation reaction, and named as poly[

Full text of DOI:10.1080/15421400601013445

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Synthesis and Optical
Properties of Copolymers
Containing Carbazole and
1,3,4-Oxadiazole Groups as
Blue Host Matrix and Charge
Transport Materials for Polymer
Light Emitting Devices
a
a
b
Hui Wang , Jeong-Tak Ryu , Yoon Soo Han , Dae-
b
b
Hwan Kim , Byeong Dae Choi & Younghwan Kwon
c
a
College of Information and Communication
Engineering, Daegu University Gyeongsan ,
Gyeongbuk, Korea
b
Department of Display & Nano Materials , Daegu
Gyeongbuk Institute of Science & Technology ,
Daegu, Korea
c
Department of Chemical Engineering , Daegu
University , Gyeongsan, Gyeongbuk, Korea
Published online: 12 Mar 2007.
To cite this article: Hui Wang , Jeong-Tak Ryu , Yoon Soo Han , Dae-Hwan Kim ,
Byeong Dae Choi & Younghwan Kwon (2007) Synthesis and Optical Properties of
Copolymers Containing Carbazole and 1,3,4-Oxadiazole Groups as Blue Host Matrix
and Charge Transport Materials for Polymer Light Emitting Devices, Molecular Crystals
and Liquid Crystals, 463:1, 3/[285]-13/[295], DOI: 10.1080/15421400601013445
To link to this article: http://dx.doi.org/10.1080/15421400601013445

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Mol. Cryst. Liq. Cryst., Vol. 463, pp. 3=[285]–13=[295], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400601013445
Synthesis and Optical Properties of Copolymers
Containing Carbazole and 1,3,4-Oxadiazole Groups
as Blue Host Matrix and Charge Transport Materials
for Polymer Light Emitting Devices
Hui Wang
Jeong-Tak Ryu
College of Information and Communication Engineering,
Daegu University Gyeongsan, Gyeongbuk, Korea
Yoon Soo Han
Dae-Hwan Kim
Byeong Dae Choi
Department of Display & Nano Materials, Daegu Gyeongbuk Institute
of Science & Technology, Daegu, Korea
Younghwan Kwon
Department of Chemical Engineering, Daegu University,
Gyeongsan, Gyeongbuk, Korea
Alternatingcopolymerswith N-arylcarbazoleasholeinjection andtransport moieties
with blue emission and 1,3,4-oxadiazole as electron transporting and hole blocking
moieties were synthesized by Pd-catalyzed polycondensation reaction, and named
as poly[N-(4-aminophenyl)-carbazole-alt-N-(2-ethylhexyl)-3,6-carbazole] (P1), poly
[N-(4-aminophenyl)-carbazole-alt-2,5-bis(4-phenyl)-1,3,4-oxadiazole] (P2) and poly
[N-(4-aminophenyl)-carbazole-alt-2,5-bis(4-methylenephenyl)-1,3,4-oxadiazole]
(P3). It was observed that UV-Vis absorption spectra showed two main peaks at
2
90ꢀ295 and 310ꢀ380 nm and maximumphotoluminescence peaks of the polymers
were all in the range of blue light emission from 418 to 453 nm. It appeared that
optical and electrochemical properties of polymers such as HOMO, LUMO levels
and band gap energy could be fine tuned by chemical modification of polymer
structures.
Keywords: 1,3,4-oxadiazole; blue emission; charge transport; N-arylcarbazole;
palladium-catalyzed polycondensation
This research was supported in part by Daegu University Research Grant, 2006.
Address correspondence to Younghwan Kwon, Department of Chemical Engineering,
Daegu University, Gyeongsan, Gyeongbuk 712-714, Korea. E-mail: y_kwon@daegu.ac.kr
3=[285]

4=[286]
H. Wang et al.
INTRODUCTION
Polymer light emitting diodes (PLEDs), based on conjugated poly-
mers or polymers with conjugated segments with different chemical
structures, emitting different colors across entire visible spectrum
have been widely investigated [1]. In the field of research and indus-
try, there is still a great demand to expand blue light emitting poly-
mers for full color displays, because they can act as both active layer
and host materials in PLEDs. However, stable blue PLEDs with
balanced charge injection from two diodes have been still a challenge,
because of wide band gap between highest occupied molecular orbit
(
HOMO) and lowest unoccupied molecular orbit (LUMO) energy
levels [2]. Many blue PLEDs reported in literature have mainly used
poly(1,4-phenylene)s [3], derivatives of poly(1,4-phenylenevinylene)
[
4], poly(thiophene)s [5], and especially carbazole- and fluorene-
containing polymers [6].
Recently, increasing interest has been paid to carbazole-based poly-
mers because electron donating carbazole groups are known as blue
emission and hole transporting groups [7]. It was reported [8] that
polycarbazole had a wide band gap of 3.2 eV, and an entirely new class
of polycarbazole-based main chain copolymers with varieties of aro-
matic rings and heterocycles as a comonomer were synthesized by
Suzuki coupling reaction. Leclerc’s research group first reported a
series of homopolymers and copolymers derived from N-alkyl-2,7-
carbazoles [9]. Wong et al. prepared 9,9-dihehexylfluorene=9-aryl-
carbazole hybrid copolymers. They pointed out that incorporating
the electron-rich N-arylcarbazole hole-transporting groups into poly-
fluorene main chain could increase the HOMO levels as well as retain
their blue emission. They also noted that advantage of the robust
N-aryl groups increased the thermal stability and glass transition
temperature of the resulting polymers, relative to N-alkyl-substituted
congeners [10]. Some reports showed that soluble poly(1,4-phenylene-
1
,3,4-oxadiazole) were useful as a new type of blue electroluminescent
materials [3(c), (d)]. Another prominent electron transporting pro-
perty of oxadiazole-containing polymers has also been paid much
attention [11].
In this study, we present new series of thermally stable alternat-
ing copolymers with well-defined chemical structure. The purpose of
this study was to synthesize the alternating copolymers containing
electron-rich N-arlycarbazole with either carbazole or 1,3,4-oxadia-
zole group. Structural effect of the polymers on the optical and elec-
trochemical properties was also presented.

Copolymers Containing Carbazole and 1,3,4-Oxadiazole Groups
5=[287]
EXPERIMENTAL
Materials
-Ethylhexylbromide (97%), 3,6-dibromocarbazole (97%), p-toluic
hydrazide (99%), p-toluoly chloride (99%), phosphorus oxychloride
99%), polyphosphoric acid, copper (I) iodide (98%), trans-1,2-cyclo-
hexane-diamine (99%), tin (II) chloride (98%), sodium tert-butoxide
97%), tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba) ), and tri-
2
(
(
3
tert-butylphosphine (P(t-Bu) , 90%) were purchased from Aldrich
3
Chemical Co. Reagent grades of carbazole, 4-bromobenzoylhydrazine,
4
-bromobenzoylchloride, 4-iodonitrobenzene, potassium carbonate
anhydrous and potassium phosphate were obtained from Tokyo Kasei
Co. and used as received. Solvents were reagent grades and purified
prior to use.
Synthesis
N-(4-aminophenyl)-carbazole (1)
In the first step, a mixture of 4-iodonitrobenzene (20 mmole), carba-
zole (15.24 mmol) and K PO (42 mmol) were added to 100 ml of 1,4-diox-
3
4
ane at room temperature. After stirring for 30 min., CuI (0.4 mmol) and
trans-1,2-cyclohexanediamine (2.4 mmol) were added to the mixture.
The reaction mixture was refluxed for 24 h. After cooling down to room
temperature, theresulting mixture wasfiltered. 1, 4-Dioxane wasevapo-
rated and ethyl acetate was added. The mixture was washed with dis-
tilled water and dried with MgSO4. N-(4-Nitrophenyl)-carbazole
intermediate was obtained through column chromatography (hexane:
1
ethylacetate¼9:1) with 98% of yield. H-NMR (300MHz in CDCl ):
3
d 8.58 (1H, ArꢁH), 8.55 (1H, ArꢁH), 8.26 (1H, ArꢁH), 8.23 (1H, ArꢁH),
8
7
.01(1H, ArꢁH), 7.98 (1H, ArꢁH), 7.60(1H, ArꢁH), 7.58 (1H, ArꢁH),
13
.51ꢀ7.45 (2H, ArꢁH), 7.39ꢀ7.33 (2H, ArꢁH); C-NMR (300 MHz in
CDCl ): d 144.35, 140.70, 131.85, 127.94, 127.34, 126.36, 124.82,
3
1
21.91, 121.33, 110.65; GC-MS : m=z¼288 (mþ).
In the second step, a mixture of N-(4-Nitrophenyl)-carbazole
ꢂ
(
5 mmol), SnCl2 (30.4 mmol) and EtOH (15 ml) was heated at 70 C for
4
h. After the mixture was cooled to room temperature, 1 M NaOH
solution was added until the mixture became basic. After extraction
with ethyl acetate, the combined organic layers was washed with brine,
dried with MgSO and evaporated. The residue was recrystallized from
4
ethanol and the final compound (1) was obtained with 98% yield.
1
H-NMR (300 MHz in CDCl ): d 8.15 (1H, ArꢁH), 8.12 (1H, ArꢁH),
3
7
.42ꢀ7.22 (8H, ArꢁH) 6.87 (1H, ArꢁH), 7.84 (1H, ArꢁH), 3.81

6=[288]
H. Wang et al.
1
3
(
1
2H, ArꢁNH ); C-NMR (300 MHz in CDCl ): d 145.89, 141.45, 128.48,
2
3
25.70, 122.92, 120.16, 119.39, 115.89, 109.75; GC-MS : m=z¼258 (mþ).
2,5-(4-Bromophenyl)-1,3,4-oxadiazole (3)
In the first step, 4-bromobenzoyl chloride (0.1 mol) was added drop-
wise to a solution of 4-bromobenzoyl hydrazide (0.1 mol), triethylamine
0.1 mol) and DMF (150 ml) at room temperature. The resulting mix-
(
ture was stirred for 2 h and then filtered. The solid collected was
washed with water and methanol to give 2,5-bis(4-bromophenyl)-
1
dihydrazide with 98% yield. H-NMR (300 MHz in CDCl ): d 10.66
3
(
7
1
2H, ꢁNHNHꢁ), 7.87(2H, ArꢁH), 7.84(2H, ArꢁH), 7.77(2H, ArꢁH),
1
3
.74(2H, ArꢁH); C-NMR (300 MHz in CDCl ): d 164.96, 131.69,
3
31.60, 129.58, 125.79.
In the second step, a mixture of 2,5-bis(4-bromophenyl)-dihydrazide
(
5 mmol) and polyphosphoric acid (0.2 mol) was heated for the cycliza-
ꢂ
tion at 200 C for 2 h. After completion of reaction, the mixture was
precipitated into 75 ml of deionized water. The precipitate was recrys-
tallized from ethanol, and dried to give the product (3) with 89% yield.
1
H-NMR (300 MHz in CDCl ): d 8.02 (2H, ArꢁH), 7.99 (2H, ArꢁH), 7.70
3
1
3
(
2H, ArꢁH), 7.68(2H, ArꢁH); C-NMR (300 MHz in CDCl ): d 164.05,
3
1
32.50, 128.34, 126.64, 122.59.
Polymerization
All the polymers were prepared using Pd-catalyzed polycondensation.
A mixture of monomer 1 (1.30 mmol) and monomer 2 (monomer 3 or 4)
(
1.30 mmol) was dissolved in toluene=DMF solvent (15 ml). NaO-t-Bu
(
3.9 mmol), Pd (dba)3 (0.033 mmol) and P(t-Bu)3 (0.2 mmol) were
2
added to the solution at room temperature. The reaction mixture
ꢂ
was stirred at 100 C for 48 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 concen-
3
trated and reprecipated from CHCl =methonal several times. Then,
3
the products were filtered and dried in vacuum.
Characterization
NMR spectra were recorded on a Varian Unity Plus 300 with solvent of
CDCl . GC-Mass spectra were recorded on a QP 5050 mass spec-
3
trometer. Number and weight average molecular weights were mea-
sured by using Waters gel permeation chromatography (GPC)
equipped with Styragel HR 5E column using THF as an eluent against
polystyrene standards at room temperature. Thermal analysis was

Copolymers Containing Carbazole and 1,3,4-Oxadiazole Groups
7=[289]
SCHEME 1 Synthetic route to monomer 1.
performed on a Seiko EXSTAR 6000 TG=DTA 6300 at a heating of
ꢂ
ꢂ
2
1
0 C=min for thermogravimetric analyzer (TGA) and at a heating of
0 C=min for differential scanning calorimetry (DSC). UV-Visible
absorption spectra were obtained by Shimadzu UV-2100. The photolu-
minescence (PL) spectra excited by He-Cd laser at 325 nm were mon-
itored 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
Synthesis and Characterization of Polymers
The synthetic procedures used to prepare the monomers 1 and 3
are outlined in Schemes 1 and 2, respectively. In Scheme 1, N-(4-nitro-
phenyl)-carbazole was first synthesized via Ullmann coupling of 4-
iodonitrobenzene and carbazole with a high yield. The combination
of air stable CuI and racemic trans-1,2-cyclohexanediamine in the
presence of K PO was employed as an efficient catalyst for the
3
4
SCHEME 2 Synthetic route to monomer 3.

8=[290]
H. Wang et al.
N-arylation of a number of heterocycles [12]. Then, the reduction of
N-(4-nitrophenyl)-carbazole with a SnCl catalyst in methanol gave
2
the momoner 1. As can be seen in Scheme 2, the monomer 3 was pre-
pared by oxadiazole ring cyclization reaction with polyphosphoric acid
ꢂ
at 200 C. Two monomers N-(2-ethyhexyl)-3,6-dibromocarbazole (2) [13]
and 2,5-bis(4-bromomethylenephenyl)-1,3,4-oxadiazole) (4) [11(a)]were
synthesized according to the reference, respectively.
The synthesis of alternating copolymers such as poly[N-(4-amino-
phenyl)-carbazole-alt-N-(2-ethylhexyl)-3,6-carbazole] (P1), poly[N-(4-
aminophenyl)-carbazole-alt-2,5-bis(4-phenyl)-1,3,4-oxadiazole] (P2)
SCHEME 3 Synthesis of conjugated copolymers.

Copolymers Containing Carbazole and 1,3,4-Oxadiazole Groups
9=[291]
TABLE 1 Physical Properties of Polymers
ꢂ
a
ꢂ
Polymers
Mn (g=mole)
PDI
T
g
( C)
T ( C)
d
P1
P2
P3
5700
2800
2400
1.54
1.12
1.41
—
465
461
372
121
—
a
Temperature resulting in 5 wt% loss of the initial weight.
and poly[N-(4-aminophenyl)-carbazole-alt-2,5-bis(4-methylenephenyl)-
,3,4-oxadiazole] (P3) is based on Pd-catalyzed polycondensation, as
1
depicted in Scheme 3. All synthesized copolymers are designed to have
a triarylamine and carbazole moieties (hole transporting ability with
blue emission) as well as 1,3,4-oxadiazole group (electron transporting
and hole blocking properties) in order to study how the chemical struc-
ture affects on optical and electrochemical properties of the polymers.
The molecular weights of the copolymers were measured by using
GPC, and summarized in Table 1. During the polymerization, prema-
ture precipitation of the polymers were observed, which might be
explained as the low solubility of the polymers in toluene or toluene=
DMF mixture, resulting in relatively low molecular weights of the
synthesized P1-P3. The thermal stability of the polymers measured
by TGA was compared in Figure 1. Based on the decomposition tem-
perature (T ) at 5 wt% loss of the initial weight, P1 and P2 exhibited
d
ꢂ
ꢂ
higher T around 460 C. P3 exhibited lower T around 370 C, due to
d
d
the C-N linkages in the main chain. However, these data revealed that
FIGURE 1 TGA curves of the polymers; (a) P1, (b) P2, and (c) P3.

1
0=[292]
H. Wang et al.
the copolymers had excellent thermal stability under nitrogen atmos-
phere. While DSC curves of P1-P3 showed neither crystallization nor
ꢂ
melting peaks, only P2 showed relatively high T at 121 C. No obvious
g
glass transition temperatures of P1 and P3 were observed below
ꢂ
3
00 C.
Optical and Electrochemical Properties
UV-Vis absorption and PL emission spectral data for the polymers in
THF solution are shown in Figures 2 and 3, respectively. In Figure 2,
maximum UV absorption (kmax,UV) of P1 was observed at 292 nm with
a small shoulder at 312 nm. However, P2 showed kmax,UV at 292 nm
ꢃ
with a new broad peak at 379 nm, attributed to the p-p electronic
transitions of N-arylcarbazole and oxadiazole moieties [14]. This was
also supported from the fact that absorption peaks of P3 was mainly
observed at 293 nm, however, greatly decreased at 343 nm, implying
the interruption of the electronic transitions by the methylene groups
between N-arylcarbazole and oxadiazole groups.
When exited at their UV-Vis absorption maximum peaks, the poly-
mers exhibited blue emission in the range of 418 nm–453 nm. Com-
pared with kmax,PL of P1, kmax,PL of P2 were blue-shifted to shorter
wavelength, possibly due to the presence of an electron withdrawing
oxadiazole group in the polymer main chain. Moreover, kmax,PL of P3
appeared at 418 nm, demonstrating 12 nm more blue shift than that
of P2. This can be correlated with the fact that the interruption by
FIGURE 2 UV-Vis absorption spectra of the polymers; (a) P1, (b) P2 and (c) P3.

Copolymers Containing Carbazole and 1,3,4-Oxadiazole Groups
11=[293]
FIGURE 3 Photoluminescence spectra of the polymers; (a) P1, (b) P2 and (c) P3.
methylene groups in the polymer main chain decreases the conju-
gation length of P3.
The electrochemical properties of the polymers are listed in Table 2.
The polymers showed HOMO energy levels of ꢁ5.01 eV for P1,
ꢁ
5.46 eV for P2 and ꢁ5.54 eV for P3. LUMO energy levels of the poly-
mers were also calculated to be ꢁ2.04 eV for P1, ꢁ2.45 eV for P2
and ꢁ2.31 eV for P3. These data revealed that P1 with two electron-
donating carbazole units increased HOMO energy level as well as P2
and P3 with electron-withdrawing oxadiazole groups increased LUMO
energy level in our polymers. This implied not only better hole injection
ability of P1 due to a closer match to the work function of ITO (work
function ¼ ꢁ4.8 eV), but also good electron transporting and hole block-
ing properties of P2 and P3. The band gap energy of the polymers was
TABLE 2 Electrochemical Properties of Polymers
a
b
c
Polymers
kmax,UV (nm)
kmax,PL (nm) Band gap (eV)
HOMO (eV)
LUMO (eV)
P1
P2
P3
292, 312
293, 379
293, 342
453
430
418
2.97
3.01
3.23
ꢁ5.01
ꢁ5.46
ꢁ5.54
ꢁ2.04
ꢁ2.45
ꢁ2.31
a
Calculated from the crosspoint of UV-Vis absorption and PL emission spectra.
Measured by a RIKENKeiki AC-2.
b
c
Estimated from the HOMO and band gap.

1
2=[294]
H. Wang et al.
estimated from the wavelength at the junction of UV-Vis aborption and
PL spectra. It appeared that band gap energy of P3 was larger than
those of P1 and P2. This might be originated from short conjugation
length of P3 by introducing methylene groups in polymer chain.
CONCLUSIONS
This study reports the synthesis and characterization of N-arylcarba-
zole-based alternating copolymers that exhibit blue emissions in PL
spectra. The polymers exhibit good thermal stability and stable optical
properties. Moreover, possibility of tuning of charge transport properties
and band gap energy levels apparently indicate that N-arylcarbazole-
based polymers are promising materials as hole transporting and blue
emitting layers or as host matrix materials for phosphorescent dopants
in light-emitting devices.
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