Synthesis and properties of polymers containing charge transport pendant group

Article abstract of DOI:10.1080/15421400903240464

Non-conjugated polymers containing charge transporting pendant groups were synthesized and characterized. Poly(N-(4-vinylphenyl)-carbazole) (PPCZ) with a hole transporting carbazole group and poly(2-methylphenyl-5-(4-vinylphenyl)-1,3, 4-oxadiazole) (PPOXD) with an electron transporting 1,3,4-oxadiazole group were synthesized by free radical polymerization of corresponding monomers, respectively, N-(4-vinylphenyl)-carbazole (VPCZ) and 2-methylphenyl-5-(4- vinylphenyl)-1,3,4-oxadiazole (VPOXD). Poly(vinyl carbazole) (PVK), well-known hole transporting material, was also prepared for comparison. These polymers with moderate molecular weight and molecular weight distribution were found to be all solution-processable. Thermal, optical and electrochemical properties of these polymers were investigated for an attempt to apply for charge transporting layer and/or host matrix in emitting layer in OLEDs. It appeared that photophysical properties such as ionization potential and electron affinity were dependent on respective pendant groups in polymers. High band gap energy levels (3.19eV ～ 3.44eV) could make polymers a promising candidate for host matrix in OLEDs.

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Synthesis and Properties of
Polymers Containing Charge
Transport Pendant Group
Hui Wang a , Jeong-Tak Ryu a , Young Hwan Shin b &
Younghwan Kwon b
a College of Information and Communication
Engineering , Daegu University , Gyeongsan,
Gyeongbuk, Korea
b Department of Chemical Engineering , Daegu
University , Gyeongsan, Gyeongbuk, Korea
Published online: 10 Nov 2009.
To cite this article: Hui Wang , Jeong-Tak Ryu , Young Hwan Shin & Younghwan Kwon
(2009) Synthesis and Properties of Polymers Containing Charge Transport Pendant
Group, Molecular Crystals and Liquid Crystals, 514:1, 158/[488]-170/[500], DOI:
10.1080/15421400903240464
To link to this article: http://dx.doi.org/10.1080/15421400903240464
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Mol. Cryst. Liq. Cryst., Vol. 514, pp. 158=[488]–170=[500], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903240464
Synthesis and Properties of Polymers Containing
Charge Transport Pendant Group
Hui Wang1, Jeong-Tak Ryu1, Young Hwan Shin2, and
Younghwan Kwon2
1College of Information and Communication Engineering, Daegu
University, Gyeongsan, Gyeongbuk, Korea
2Department of Chemical Engineering, Daegu University,
Gyeongsan, Gyeongbuk, Korea
Non-conjugated polymers containing charge transporting pendant groups were
synthesized and characterized. Poly(N-(4-vinylphenyl)-carbazole) (PPCZ) with a
hole transporting carbazole group and poly(2-methylphenyl-5-(4-vinylphenyl)-
1,3,4-oxadiazole) (PPOXD) with an electron transporting 1,3,4-oxadiazole group
were synthesized by free radical polymerization of corresponding monomers,
respectively, N-(4-vinylphenyl)-carbazole (VPCZ) and 2-methylphenyl-5-(4-
vinylphenyl)-1,3,4-oxadiazole (VPOXD). Poly(vinyl carbazole) (PVK), well-known
hole transporting material, was also prepared for comparison. These polymers
with moderate molecular weight and molecular weight distribution were found
to be all solution-processable. Thermal, optical and electrochemical properties of
these polymers were investigated for an attempt to apply for charge transporting
layer and=or host matrix in emitting layer in OLEDs. It appeared that photophy-
sical properties such as ionization potential and electron affinity were dependent
on respective pendant groups in polymers. High band gap energy levels (3.19 eV
ꢀ 3.44 eV) could make polymers a promising candidate for host matrix in OLEDs.
Keywords: 1,3,4-oxadiazole; carbazole; charge transport; non-conjugated polymer;
OLED; PVK
INTRODUCTION
Organic light-emitting diodes (OLEDs) have been paid tremendous
attention for flat panel displays (FPDs), due to their attractive advan-
tages of low driving voltage, wide viewing angle, ease of fabrication by
This work was supported by Daegu University Research Grant 2009.
Address correspondence to Younghwan Kwon, Department of Chemical Engineering,
Daegu University, Gyeongsan, Gyeongbuk 712-714, Korea. E-mail: y_kwon@daegu.ac.kr
158=[488]

Synthesis and Properties of Polymers
159/[489]
solution process with large area, and capability of tuning the color
emission by selecting proper emitting materials [1–3].
Conjugated polymers have been extensively developed, since elec-
troluminescence of poly(p-phenylene vinylene) (PPV) was first repor-
ted in 1990 [4]. Since then, PPV, polyphenylene (PPP), polyfluorene
(PF), polythiophene (PTh), conjugated polycarbazole (PCZ) and their
derivatives have been widely investigated as promising candidate
materials for OLEDs, because the modification of their chemical
architectures could be easily achieved via various polymerization tech-
niques to satisfy the requirement for improved performance of OLEDs.
In addition, conjugated polymers synthesized have been favored by
combining the nature of the monomers, tuning the physical, optical,
electrochemical properties and transferring the energy between differ-
ent band gaps of the monomers. To achieve stable and efficient OLEDs
for commercialization, molecular design and synthesis of conjugated
polymers have been still investigated.
Non-conjugated polymers have been also used as ancillary materi-
als to support the high performance of conjugated polymers in actual
devices. One of the most widely used of these materials was poly(N-
vinyl carbazole) (PVK), exhibiting interesting electrooptical and
charge transporting properties in OLEDs, solar cells, organic semi-
conductor, etc. Specifically, due to the carbazole side groups, PVK
showed good hole transporting property, which was important for
improving the performance of OLEDs [5–8]. In addition, phosphores-
cent OLEDs (PhOLEDs) fabricated with PVK as a host material for
a phosphorescent green Ir(ppy)3 dopant by simple solution process
were reported [9–11]. It revealed that non-conjugated PVK generally
showed higher electroluminescent performance than conjugated poly-
fluorene, due to lower triplet energy of conjugated polymers resulting
in the phosphorescent quenching.
Aromatic 1,3,4-oxadiazole compounds have demonstrated to facili-
tate electron transport and injection from the cathode, due to their
high electron affinity [12–14]. Small molecule, 2-(4-biphenyl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiazole (PBD), was used to balance hole=
electron transport by increasing the rate of electron transport in
OLED application [15]. Non-conjugated polymers containing aromatic
pendant could be expected to effectively function as electron transport
and prevention of recrystallization, when compared with small
molecular PBD derivatives.
In this work, design, synthesis and characterization of non-
conjugated polymers containing a hole transporting group and an
electron transporting group, respectively, as a pendant group were
reported. For the synthesis of polymers, two different types of

160/[490]
H. Wang et al.
monomers, N-(4-vinylphenyl)-carbazole (VPCZ) with a hole transport-
ing carbazole and 2-methylphenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole
(VPOXD) with an electron transporting 1,3,4-oxadiazole, were pre-
pared. Then, two non-conjugated copolymers with a hole transporting
carbazole for poly(N-(4-vinylphenyl)-carbazole) (PPCZ) and an elec-
tron transporting 1,3,4-oxadiazole for poly(2-methylphenyl-5-
(4-vinylphenyl)-1,3,4-oxadiazole) (PPOXD) were synthesized by free
radical polymerization of the corresponding monomers. Thermal, opti-
cal and electrochemical properties of these polymers were investigated
for an attempt to apply for charge transporting layer and=or host
matrix in emitting layer in OLEDs.
EXPERIMENTAL
Materials
4-Iodo-bromobenzene (96%), copper (I) iodine (CuI, 98%), trans-1,2-
cyclohexane- diamine (ligand) (99%), tributylvinyltin (97%),
tetrakis(triphenylphosphine)palladium (0) (Pd(PPh3)4, 99%), polypho-
sphoric acid (POCl3, 99%), N-vinyl carbazole (98%), benzoyl peroxide
(BPO, 75%) were purchased from Aldrich Chemical Co. Reagent grade
carbazole, 4-bromobenzoyl chloride, 4-methylbenzoyl hydrazide,
triethylamine, and potassium phosphate (K3PO4) were obtained from
Tokyo Kasei Co. and used as received. Solvents were reagent grades
and purified prior to use.
Monomer Synthesis
N-(4-vinylphenyl)-carbazole (VPCZ)
Scheme 1 presents the synthesis of N-(4-vinylphenyl)-carbazole
(VPCZ) in two steps. First, a mixture of 4-iodo-bromobenzene
(2 mmol), carbazole (2 mmmol) and K3PO4 (4 mmol) were added
SCHEME 1 Synthetic route to vinyl N-phenylcarbazole (VPCZ).

Synthesis and Properties of Polymers
161/[491]
to1,4-dioxane (10 ml) at room temperature. After stirring for 30 min,
CuI (0.06 mmol) and trans-1,2-cyclohexanediamine (0.024 mmol) were
added to the mixture. The reaction mixture was refluxed for 24 h.
After cooling down to room temperature, the reaction mixture was
filtered. 1, 4-Dioxane was evaporated, and ethyl acetate was added.
The mixture was washed with distilled water, and dried over MgSO4.
The final product (compound 1) was obtained through column
chromatography (eluent:hexane) with the yield of 90%.
In the second step, a mixture of compound 1 (4.7 mmol), tributylvi-
nyltin (4.7 mmol) and Pd(PPh3)4 (0.1 mmol) was dissolved in toluene
(2 ml) and heated at 70ꢁC for 24 h. After the mixture was cooled to
room temperature, the solid was filtered out. After extraction with
ethyl acetate three times, the combined organic layers was washed
with distilled water, dried over MgSO4 and evaporated. The final pro-
duct, N-(4-vinylphenyl)-carbazole, was obtained through column chro-
matography (eluent: hexane) with the yield of 60%. 1H NMR (300 MHz
in CDCl3): d 8.13–8.16 (d, 2 H, Ar-H), 7.23–7.78 (m, 10 H, Ar-H),
6.77–6.87 (t, 1 H, olefinic H), 5.82–5.88 (d, 1 H, olefinic H), 5.34–5.37
(d, 1 H, olefinic H). 13C NMR (300 MHz in CDCl3): d 140.7, 135.9,
133.1, 128.7, 127.5, 125.9, 123.3, 120.4, 119.9, 114.7, 109.8, 109.5.
2-Methylphenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole (VPOXD)
As shown in Scheme 2, 2-methylphenyl-5-(4-vinylphenyl)-1,3,4-
oxadiazole (VPOXD) was prepared in three steps. 4-Bromobenzoyl
chloride (10 mmol) was added dropwise to a solution of 4-methylben-
zoyl hydrazide (10 mmol), triethylamine (10 mmol), and chloroform
(150 ml) at room temperature. The resulting mixture was stirred for
4 h and then filtered. The solid collected was washed with water and
methanol to give compound 2 with the yield of 95%. 1H NMR
SCHEME 2 Synthetic route to vinyl 2-methylphenyl-5-phenyl-1,3,4-oxadia-
zole (VPOXD).

162/[492]
H. Wang et al.
(300 MHz in CDCl3): d 10.54 (s, 2 H, NH), 7.74–7.87 (m, 6 H, Ar-H),
7.31–7.34 (m, 2 H, Ar-H), 2.74 (s, 3 H, CH3). 13C NMR (300 MHz in
CDCl3): d 165.75, 165.06, 142.02, 131.71, 129.62, 129.57, 129.14,
129.13, 127.55, 125.78, 21.13.
Second, a mixture of compound 2 (3.6 g) and polyphosphoric acid
(50 ml) was heated to reflux for 5 h. After completion of reaction, the
mixture was precipitated into deionized water. The precipitate was
recrystallized from ethanol and dried to give the compound 3 with
1
the yield of 90%. H NMR (300 MHz in CDCl3): d 7.99–8.04 (m, 4 H,
Ar-H), 7.66–7.71 (m, 2 H, Ar-H), 7.33–7.36 (m, 2 H, Ar-H), 2.45 (s,
3 H, CH3). 13C NMR (300 MHz in CDCl3): d 164.88, 163.59, 142.49,
132.37, 129.80, 128.24, 126.88, 126.26, 122.85, 120.84, 21.68.
In the third step, a mixture of compound 3 (6 mmol), tributylvinyl-
tin (6 mmol) and Pd(PPh3)4 (0.12 mmol) was dissolved in DMF (80 ml)
and heated at 70ꢁC for 24 h. After cooling down to room temperature,
the mixture was poured into distilled water. After extraction with
ethyl acetate, the combined organic layers was washed with distilled
water, dried over MgSO4 and evaporated. The final product, 2-methyl-
phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole, was obtained through
1
column chromatography (eluent: hexane) with the yield of 55%. H
NMR (300 MHz in CDCl3): d 7.93–8.16 (m, 4 H, Ar-H), 7.61–7.64 (d,
2 H, Ar-H), 7.27–7.36 (d, 2 H, Ar-H), 6.73–6.83 (t, 1 H, olefinic H),
5.87–5.92 (d, 1 H, olefinic H), 5.38–5.42 (d, 1 H, olefinic H), 2.45–2.60
(s, 3 H, ꢂCH3). 13C NMR (300 MHz in CDCl3): d 164.9, 163.6, 142.5,
140.7, 135.9, 132.4, 129.8, 128.3, 127.1, 122.9, 120.9, 116.2, 21.7.
Polymer Synthesis
All the polymers were prepared with free radical polymerization by
using BPO as an initiator, as presented in Scheme 3. For the synthesis
of poly(N-vinyl carbazole) (PVK), a mixture of N-vinyl carbazole was
dissolved in toluene (10 ml). After the mixture was stirred for 30 min
in nitrogen atmosphere, the reaction mixture was heated to 80ꢁC.
Then, BPO in toluene was added dropwise. The mixture was kept at
80ꢁC for 12 h. After cooling to room temperature, the mixture was pre-
cipitated in methanol. The polymer, PVK, was obtained by filtration
and drying in vacuum at 40ꢁC. Two other polymers, PPCZ and PPOXD,
were prepared with the same procedure mentioned previously.
Characterization
NMR spectra were recorded on a Varian Unity Plus 300 with CDCl3 as
a solvent. Molecular weights and molecular weight distributions of

Synthesis and Properties of Polymers
163/[493]
SCHEME 3 Synthesis of polymers, (a) PVK, (b) PPCZ, and (c) PPOXD, by free
radical polymerization.
polymers were measured by using Waters gel permeation chromato-
graph (GPC) equipped with Styragel HR 5E column with tetrahydro-
furan (THF) as an eluent against polystyrene standards at room
temperature. Thermal analysis was performed on a Seiko EXSTAR
6000 TG=DTA 6300 thermal analyzer at a heating of 20ꢁC=min for
thermogravimetry analyzer (TGA). UV-Visible absorption spectra
were taken by Shimadzu UV-2100. Photoluminescence (PL) spectra
excited by He-Cd laser at 325 nm were monitored by Optical

164/[494]
H. Wang et al.
Multichannel Analyzer (Laser Photonics, OMA system). The ioniza-
tion potential (IP) was measured by a photoelectron spectroscopy
(Riken Keiki AC-2).
RESULTS AND DISCUSSION
Synthesis and Structural Characterization
For the synthesis of polymers, two monomers, VPCZ and VPOXD,
were synthesized as outlined in Schemes 1 and 2, respectively. The
structures of intermediates and monomers were identified by using
1
1H NMR and 13C NMR measurements. From H NMR spectrum of
VPCZ in Figure 1(a), quantitative assignment of VPCZ was confirmed
by comparing the signal intensity ratio of olefinic hydrogen (1 or 2) to
aromatic hydrogen (9) peaks. Structure of VPOXD was also identified
1
FIGURE 1 (a) H NMR and (b) 13C NMR spectra of vinyl N-phenylcarbazole
(VPCZ).

Synthesis and Properties of Polymers
165/[495]
quantitatively by comparing the signal intensity ratio of olefinic
hydrogen (1 or 2) to methyl (8) peaks.
As shown in Scheme 3, the polymers were prepared by free radical
polymerization of corresponding monomers using BPO as the initiator
and toluene as the solvent. All the polymers exhibited high solubility
in common organic solvents such as chloroform, benzene, toluene,
1,1,2,2-tetrachloroethane and tetrahydrofuran, and showed good film
formation by spin-coating process. Number-average molecular weight
ðMnÞ, molecular weight distribution ðMw=MnÞ, and thermal stability
data of the polymers are summarized in Table 1. The number-average
molecular weight of polymers was measured to be 8,000 g=mole,
21,500 g=mole, and 23,300 g=mole with molecular weight distributions
in the range of 1.35 ꢀ 1.47 for PVK, PPCZ and PPOXD, respectively.
Thermal stability of the polymers was characterized by using TGA.
TGA thermogram in Figure 3 showed that thermal decomposition tem-
perature (Td) at 5 wt% loss based on the initial weight of polymers
were 296ꢁC for PVK, 292ꢁC for PPCZ, and 275ꢁC for PPOXD, respec-
tively, suggesting relatively good thermal stability of the polymers.
PPOXD with 1,3,4-oxadiazole group exhibited less thermal stability
than PVK and PPCZ having carbazole group.
Optical and Electrochemical Properties
Figure 4(a) and (b) present UV-Visible absorption and PL emission
spectra of the polymers in solution. The related optical data of the
polymers are listed in Table 1. As can be seen in Figure 4(a), PVK
exhibited UV-Visible absorption maximum peak (kmax,UV) at 296 nm
with the shoulder peaks at 330 nm and 343 nm in solution. Similarly,
TABLE 1 Physical, Optical and Electrochemical Properties of Polymers
Band
Mn
ðMw= Td
kmax,UV
(nm)
kmax,PL
(nm)
gap HOMO LUMO
UF (eV)d (eV)e (eV)f
Polymers (g=mole)a MnÞ (ꢁC)b
c
PVK
PPCZ
PPOXD
8,000
21,500
23,300
1.35 296 296, 330, 343
373
0.11 3.41 ꢂ5.70 ꢂ2.29
1.47 292 290, 325, 336 351, 359 0.17 3.19 ꢂ5.30 ꢂ2.11
1.35 275 292
344, 354 0.24 3.44 ꢂ5.52 ꢂ2.08
aMeasured by using GPC with polystyrene standard calibration.
bTemperature at 5 wt% loss based on initial weight.
cQuantum yield in THF.
dCalculated from the crosspoint of UV-Vis and PL spectra.
eMeasured by a RIKEN Keiki AC-2.
fEstimated from the HOMO and band gap.

166/[496]
H. Wang et al.
1
FIGURE 2 (a) H NMR and (b) 13C NMR spectra of vinyl 2-methylphenyl-5-
phenyl-1,3,4-oxadiazole (VPOXD).
PPCZ also displayed main UV-Visible absorption peak centered at
290 nm with shoulder at 325 nm and 336 nm in solution. These peaks
were considered to be attributed to p-pꢃ transition along carbazole
groups [16]. UV-Visible absorption of PPOXD was observed at 292 nm
in solution, originating from p-pꢃ transition along 1,3,4-oxadiazole
groups [16].
With excitation at their kmax,UV in solution, PL emission spectra of
polymers are shown in Figure 4(b). The PL emission data are also
summarized in Table 1. PL emission maxima (kmax,PL) of PPCZ and
PPOXD in solution were measured at 351 nm and 354 nm, respec-
tively, with quantum yields of 0.17 and 0.24. PVK as a reference
showed 373 nm of kmax,PL with 0.11 of quantum yield. It was generally
¨
reported that the possibility of Forster energy transfer from a host
matrix to a phosphorescent dopant increased with increasing the

Synthesis and Properties of Polymers
167/[497]
FIGURE 3 TGA thermograms of polymers.
degree of spectral overlap between PL emission spectrum of the host
matrix and UV-Visible absorption spectrum of the phosphorescent
dopant. A green phosphorescent Ir(ppy)3 film was reported to show
strong absorption at 290 nm originated from the ligand-centered p-pꢃ
transition with weaker absorptions at 340–380 nm and 460 nm attrib-
uted to the singlet metal-to-ligand charge transfer (1MLCT) and tri-
plet metal-to-ligand charge transfer (3MLCT) transition, respectively
[17]. The PL emission band (300 nm ꢀ 500 nm) of polymers could be
expected to overlap with the absorption band of the MLCT transitions
in Ir(ppy)3, implying an expected energy transfer from polymers to
Ir(ppy)3. Similar result was reported with non-conjugated random
copolymers containing carbazole and 1,3,4-oxadiazole pendant
groups [18]. Stokes shifts of polymers in solution were calculated to
be 77 nm for PVK, 61 nm for PPCZ, and 62 nm for PPOXD, indicating
structural differences between the ground and excited states of the
polymers.
Table 1 presents the electrochemical data of the polymers. Highest
occupied molecular orbital (HOMO) levels were obtained from ioniza-
tion potential values measured by using a RIKEN Keiki AC-2 instru-
ment. HOMO energy levels of polymers were measured to be ꢂ5.70 eV
for PVK, ꢂ5.30 eV for PPCZ, and ꢂ5.52 eV for PPOXD. Lowest unoccu-
pied molecular orbital (LUMO) levels were estimated from HOMO and
band gap energy levels of the polymers. LUMO energy levels of PVK,
PPCZ and PPOXD were estimated to be ꢂ2.29 eV, ꢂ2.11 eV, and
ꢂ2.08 eV, respectively. From LUMO energy level of the polymers,
PPOXD showed enhanced electron affinity, compared to PVK and

168/[498]
H. Wang et al.
FIGURE 4 (a) UV-Visible absorption and (b) PL emission spectra of polymers.
PPCZ, presumably due to the incorporation of electron-affinitive aro-
matic 1,3,4-oxadiazole side groups in PPOXD. The band gap energy
of the polymers was estimated from the wavelength at the crossover
point in UV-Visible absorption and PL emission spectra. Large band
gap energy levels were found to be 3.41 eV for PVK, 3.19 eV for PPCZ,
and 3.44 eV for PPOXD, respectively. It appeared that the band gap
energy of PPCZ was lower than PVK. This small reduction of band
gap energy in PPCZ in comparison with PVK could be caused by incor-
poration of an additional aromatic phenyl side groups extending
p-system in PPCZ.

Synthesis and Properties of Polymers
169/[499]
The method for fabricating multi-layered polymer light-emitting
diode (PLED) involved solution processes such as solution-casting,
inkjet printing, and spin-coating. The problem of this method was
dissolution of the former deposited film by spin-coating on top of the
polymer film, when organic solvent was used continuously. It was
reported that polymeric thin films containing carbazole pendant
groups could be linked by electropolymerization or chemical oxidation
at all 3,6-positions of carbazole units, resulting in inter- or intramole-
cular insoluble crosslinked thin films [19,20]. Therefore, the synthe-
sized polymers with large band gap energy and charge transporting
groups could exhibit a potential to be used as charge transporting as
well as host matrix in emitting layer in PLED, coupled with multiple
sing-coating process after post crosslinking of charge transporting
pendant groups.
CONCLUSIONS
We have successfully synthesized and characterized new non-
conjugated polymers, PPCZ and PPOXD, containing the hole trans-
porting carbazole and the electron transporting 1,3,4-oxadiazole
pendant groups, respectively, from corresponding monomers, VPCZ
and VPOXD. Thermal and optical properties of these polymers were
characterized for an attempt to apply for PLEDs, and compared with
those of PVK. The polymers showed number average molecular
weights in the range of 8,000 ꢀ 23,300 g=mole and 1.35 ꢀ 1.47 of mole-
cular weight distributions. They were all solution-processable, due to
their good solubility in common organic solvents. The decomposition
temperature of PPCZ under inert atmosphere was higher than
PPOXD. UV-Visible absorption and PL emission peaks of polymers
showed characteristic dependence on respective pendant groups.
PPCZ exhibited lower ionization potential and PPOXD showed higher
electron affinity, due to their opposite pendant properties. Further
performance study of phosphorescent polymer light-emitting devices
can be done by applying these polymers as charge transporting and=or
host matrix in the emitting layer.
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