Synthesis and characterization of fluorene-carbazole and fluorene-phenothiazine copolymers with carbazole and oxadiazole pendants for organic light emitting diodes

Article abstract of DOI:10.1016/j.polymer.2010.10.061

We report the synthesis and characterization of new series of the fluorene based polymers with carbazole and oxadiazole pendants for the generation of the white emission out of the EL device. In the fluorene backbone, hole transporting units such as carba

Full text of DOI:10.1016/j.polymer.2010.10.061

Polymer 51 (2010) 6174e6181
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Polymer
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Synthesis and characterization of fluorene-carbazole and fluorene-phenothiazine
copolymers with carbazole and oxadiazole pendants for organic light emitting
diodes
,
g
,
,
,
Shuhei Yamada a 1, Seijung Park a 1, Suhee Song a 1, Mihee Heo b, Joo Young Shim a, Youngeup Jin c,
**
Il Kim d, Heesoo Lee e, Kyungmee Lee f, Kohji Yoshinaga g, Jin Young Kim b , Hongsuk Suh
,
a,
*
a Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, South Korea
b Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology, Ulsan 689-798, South Korea
c Department of Industrial Chemistry, Pukyong National University, Busan 608-739, South Korea
d The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of polymer Science and Engineering, Pusan National University, Busan 609-735, South Korea
e School of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea
f Department of Chemistry, Sungkyunkwan University, Gyeonggi-do, South Korea
g Department of Applied Chemistry, Kyushu Institute of Technology Faculty of Engineering, Fukuoka 804-8550, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis and characterization of new series of the fluorene based polymers with
carbazole and oxadiazole pendants for the generation of the white emission out of the EL device. In the
fluorene backbone, hole transporting units such as carbazole or phenothiazine were incorporated to
improve the EL brightness and efficiency. The PFCzOxd-co-PCzs and PFCzOxd-co-PPTZs in EL spectra
showed maximum peaks at around 430 nm and additional large peaks at around 530 and 500 nm,
respectively. In case of PFCzOxd-alt-PCz and PFCzCzPCz-co-PFOxdOxdPCz, the EL spectra of the poly-
mers showed two distinct peaks comprising the maximum at 427 nm, which corresponds to the EL
spectra of the conjugated backbone, and additional broad peaks at around 540 and 530 nm, respectively.
The CIE coordinates of the devices from PFCzOxd-alt-PCz and PFCzCzPCz-co-PFOxdOxdPCz were (0.28,
0.33) and (0.25, 0.32), respectively, approaching the value of the standard white of National Television
System Committee (NTSC) (0.33, 0.33).
Received 18 August 2010
Received in revised form
20 October 2010
Accepted 31 October 2010
Available online 4 November 2010
Keywords:
Conjugated polymer
Carbazole
Oxadiazole
Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
1. Introduction
multilayer devices show high efficiencies and long lifetimes, the
fabrication of the multilayers using spin-coating process is unsuit-
Conjugated polymers have been utilized as active materials in
several kinds of electronic devices such as thin film transistor [1],
photovoltaic cell [2,3], and organic light emitting diodes (OLEDs) [4],
including flexible displays [5]. OLEDs have attracted much attention
with several advantages over conventional devices including low
driving voltage, wide viewing angle, and a simple manufacturing
process [6e9]. Much effort has been devoted to the fabrication of
white-organic-light-emitting diodes (WOLEDs), because of possible
applications for low-cost room illumination. One of the methods of
the realization of WOLED is the multilayer device system with red,
green and blue light emitting materials [10e12]. Although the
able with polymeric light emitting materials which are soluble only
in organic solvents. Another method is using a single light emitting
layer with red, green, and blue light emitting polymer blends [13,14].
Even though using polymer blends is a promising process for the
generation of white light emission, phase separation, low efficiency,
and voltage dependence of emitting color limit their utilization.
Recently, several groups achieved the fabrication of WOLEDs from
polyfluorene-based polymers containing red and green light emit-
ting portions on the backbones [15e18]. In these cases, the color of
the emission was easily tuned by adjusting the composition of the
polymers to control the partial energy transfer to red, or green
emissive units.
We previously reported the results of white light emission from
poly(9-(6-(9H-carbazolyl)hexyl)-9-(6-(4-(5-phenyl-1,3,4-oxadiazolyl)-
phenoxy)hexyl)-9H-fluorene) (PFCzOxd), containing carbazole and
oxadiazole units as pendants, due to the peak at 426 nm and broad peak
around 540 nm which is attributed to the aggregations caused by the
* Corresponding author. Tel.: þ82 51 510 2203.
** Corresponding author.
E-mail addresses: jykim@unist.ac.kr (J.Y. Kim), hssuh@pusan.ac.kr (H. Suh).
1
These authors contributed equally to this work.
0032-3861/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.10.061

S. Yamada et al. / Polymer 51 (2010) 6174e6181
6175
carbazole and oxadiazole units [19]. The OLEDs fabricated with
PFCzOxd [19] generated electroluminescence (EL) with excellent
Commission Internationale d’Enclairage (CIE) coordinates (x ¼ 0.31,
y ¼ 0.33) for the white color. In this paper, we report the synthesis and
characterization of new series of the fluorene based polymers with
carbazole and oxadiazole pendants for the generation of the white
emission out of the EL device. In the fluorene backbone, hole trans-
porting units such as carbazole or phenothiazine were incorporated to
improve the EL properties since carbazole and phenothiazine units have
good hole transporting behavior caused by the electron lone pair on
nitrogen or sulfur atoms. The carbazole and phenothiazine were
incorporated into the backbone to perform as hole transporter to
improve hole injection and hole trapping site for efficient electron-hole
recombination to yield emitting excitons [20].
emissive polymer film was obtained by spin casting chlorobenzene
solution of the polymer. The emissive polymer thin film prepared
had a uniform surface with a thickness of around 110 nm. The
emissive film was dried in vacuum, and calcium (20 nm) and
aluminum (100 nm) electrodes were deposited on the top of the
polymer films through a mask by vacuum evaporation at pressures
below 10ꢁ7 Torr, yielding active areas of 4 mm2. For the determi-
nation of device characteristics, current densityevoltage (JeV) and
luminanceevoltage (LeV) characteristics of the devices were
measured using a Keithley 2400 Source Measure Unit equipped
with a calibrated photo-multiplier tube. EL device was totally
fabricated and measured in glove box. The CIE coordinates numbers
are calculated automatically by the Keithley 2400 Source Measure
Unit, for the EL measurement.
2. Experimental
2.3. Synthesis of 2,7-dibromo-9,9-bis(6-(4-(5-phenyl-1,3,4-
oxadiazolyl)phenoxy)hexyl)-9H-fluorene (2)
2.1. Materials and instruments
A solution of 2,7-dibromo-9H-fluorene (5 g, 15.4 mmol), 2-(4-
(6-bromohexyloxy)phenyl)-5-phenyl-1,3,4-oxadiazole (1) (12.4 g,
30.8 mmol), and catalytic amounts of triethylbenzyl ammonium
chloride in 200 mL of DMSO was stirred at 60 ꢀC for 1 h under
argon. The reaction mixture was treated with 4 mL of 50% NaOH
aqueous solution at room temperature and then stirred for 5 h. An
excess amount of ethyl acetate was added to the resulting mixture
to generate a precipitation of NaOH. After collection of NaOH by
filtration, the organic phase was washed with water, dried over
MgSO4 and then concentrated under reduced pressure. The crude
product was purified by flash column chromatography (ethyl
acetate/hexane ¼ 1/2 as eluent) to get 5.60 g (37%) of the target
The materials, 2-(4-(6-bromohexyloxy)phenyl)-5-phenyl-1,3,4-
oxadiazole (1), 2,7-dibromo-9-(6-(9H-carbazolyl)hexyl)-9-(6-(4-(5-
phenyl-1,3,4-oxadiazolyl)phenoxy)hexyl)-9H-fluorene (3), 2,7-di-
bromo-9,9-bis(6-(9H-carbazolyl)hexyl)-9H-fluorene (4), 9-(hepta-
decan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-
carbazole (5), 2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole (6),
and 3,7-dibromo-10-octyl-10H-phenothiazine (7) were synthesized
according to the procedure outlined in the literature [19e24]. The
hole-injection-transport material, PEDOT:PSS, was purchased from
H.C. Starck with Clevios PH grade. All used reagents for organic
synthesis were purchased from Aldrich and used without further
purification. 1H and 13C NMR spectra were recorded with a Varian
Gemini-300 (300 MHz) spectrometer and chemical shift were recor-
ded in ppm units with TMS as the internal standard. Flash column
chromatography was performed with Merck silica gel 60 (particle size
230e400 mesh ASTM). Analytical thin layer chromatography (TLC)
was conducted using Merck 0.25 mm silica gel 60F precoated
aluminum plates with fluorescent indicator UV254. Molecular weight
and polydispersities of the polymer were determined by gel perme-
ation chromatography (GPC) with a polystyrene standard calibration.
The differential scanning calorimetry analysis was performed under
a nitrogen atmosphere (50 mL/min) on a DSC 822 at heating rates of
10 ꢀC/min. Thermogravimetric analysis was performed with a Dupont
951 TGA instrument inanitrogen atmosphere at aheating rateof 10ꢀC/
min to 600 ꢀC. Fast atom bombardment (FAB) mass spectra were
determined using at Korea Basic Science Institute Seoul Branch and
Korea Basic Science Institute Daegu Branch. Cyclic voltammetric waves
were produced by using an EG&G Parc model 273 potentiostat/gal-
vanostat at a constant scan rate of 100 mV/s. UV spectra were recorded
with Varian CARY-5E UV/vis spectrophotometer. The PL and EL spectra
were measured using Oriel InstaSpec IV CCD detection systems. For PL
spectrum measurement, a xenon lamp was used as the excitation
source and incident beam took the maximum absorption peak of the
polymers. For the determination of device characteristics, cur-
rentevoltage (IeV) characteristics were measured using a Keithley
2400 source measure unit.
product as a white solid. 1H NMR (300 MHz, CDCl3)
d (ppm):
8.15e8.10 (m, 4H), 8.05 (d, 4H, J ¼ 9.0 Hz), 7.56e7.44 (m, 12H), 6.99
(d, 4H, J ¼ 9.0 Hz), 3.93 (t, 4H, J ¼ 6.3 Hz), 2.00e1.93 (m, 4H),
1.67e1.62 (m, 4H), 1.29e1.16 (m, 8H), 0.90e0.62 (m, 4H). 13C NMR
(75 MHz, CDCl3)
d (ppm): 164.82, 164.31, 162.08, 152.48, 139.32,
131.75, 130.55, 129.27, 128.88, 127.04, 126.33, 124.32, 121.80, 121.50,
116.36, 115.15, 68.28, 55.84, 40.37, 29.78, 29.19, 25.88, 23.84. HRMS
(m/z FABþ) Calcd for C53H48Br2N2O2 964.2022, found 965.2215.
2.4. Synthesis of alternating copolymers via Suzuki coupling
polymerization
Carefully purified monomer 3 (or 4 or (2 þ 4)), monomer 5, and
tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) as the cata-
lyst dissolved in a mixture of toluene (3 mL) and aqueous 2 M
K2CO3 (2.5 mL). The mixture was refluxed with vigorous stirring for
3 days under argon atmosphere. After 72 h, phenylboronic acid was
added to the reaction then 12 h later, bromobenzene was added and
the reaction mixture refluxed overnight to complete the end-
capping reaction. After the mixture was cooled to room tempera-
ture, it was poured into methanol. The precipitated material was
recovered by filtration. The resulting solid material was reprecipi-
tated using 100 mL of THF/1.0 L of methanol several times to
remove catalyst residues and to generate PFCzOxd-alt-PCz (with
monomer 3), PFCzCz-alt-PCz (with monomer 4), and PFCzCzPCz-
co-PFOxdOxdPCz (with monomer 2D4). The yields of the poly-
mers ranged from 40 to 60%. The resulting polymers were soluble in
THF, CHCl3, ODCB and toluene.
2.2. EL device fabrication and measurements
For the EL experiment, poly(3,4-ethylenedioxythiophene)
(PEDOT) doped with poly(styrenesulfonate) (PSS), as the hole-
injection-transport layer, was introduced between emissive layer
and ITO glass substrate cleaned by successive ultrasonic treat-
ments. The solution of the PEDOT:PSS in aqueous isopropyl alcohol
was spin-coated on the surface-treated ITO substrate and dried on
a hot plate for 30 min at 110 ꢀC. On top of the PEDOT layer, the
PFCzOxd-alt-PCz: 8.18 (s), 8.12e7.99 (m), 7.87 (s), 7.73 (s),
7.51e7.50 (m), 7.40e7.14 (m), 6.94 (d), 4.17 (s), 3.90 (s), 2.40 (s), 2.15
(s), 1.99 (s), 1.67 (m), 1.27e1.15 (m), 0.89e0.78 (m).
PFCzCz-alt-PCz: 8.02 (s), 8.06 (d), 7.87 (s), 7.71 (m), 7.55e7.41
(m), 7.38e7.15 (m), 4.68 (s), 4.17e4.15 (m), 2.38 (s), 2.19 (s), 2.11 (s),
1.96 (s), 1.82 (s), 1.72 (s), 1.27e1.14 (m), 0.83e0.78 (m).

6176
S. Yamada et al. / Polymer 51 (2010) 6174e6181
Oxd
Oxd
50% NaOH (aq)
DMSO
O
Br
Br
Br
O
Br
Br
N
N
1
2
Cz
Oxd
Cz
Oxd
C8H17
C8H17
C8H17
C8H17
N
N
1. Pd(PPh3)4 / 2M K2CO3
O
O
O
O
Toluene
Br
Br
B
B
n
2. phenylbronic acid
3. bromobenzene
5
3
PFCzOXd-alt-PCz
Cz
Cz
Cz
Cz
C8H17
C8H17
C8H17
C8H17
N
N
1. Pd(PPh3)4 / 2M K2CO3
Toluene
O
O
O
O
Br
Br
B
B
n
2. phenylbronic acid
3. bromobenzene
5
4
PFCzCz-alt-PCz
Cz
Cz
Oxd
Oxd
C8H17
C8H17
C8H17
N
C8H17
N
1. Pd(PPh3)4 / 2M K2CO3
Toluene
2
+
4
+
5
2. phenylbronic acid
3. bromobenzene
m
n
m:n = 1:1 PFCzCzPCz-co-PFOxdOxdPCz
N
O
O
Oxd
=
Cz
=
N
N
Scheme 1. Synthetic routes for the monomer and the polymers.
PFCzCzPCz-co-PFOxdOxdPCz: 8.16 (s), 8.11e8.03 (m), 7.99 (s),
7.87e7.69 (m), 7.60e7.39 (m), 7.36e7.13 (m), 6.97 (d), 4.69 (s), 4.15
(s), 3.89 (d), 2.38 (s), 1.98e1.59 (m), 1.25 (s), 1.14e0.77 (m).
The comonomers feed ratios of 3 to 6 are 99:1, 97:3, 95:5 and 90:10,
and the corresponding copolymers were named as PFCzOxd-co-
PCz1, PFCzOxd-co-PCz3, PFCzOxd-co-PCz5, and PFCzOxd-co-
PCz10, respectively. The comonomers feed ratios of 3e7 are 99:1,
97:3, 95:5 and 90:10, and the corresponding copolymers were
named as PFCzOxd-co-PPTZ1, PFCzOxd-co-PPTZ3, PFCzOxd-co-
PPTZ5, and PFCzOxd-co-PPTZ10, respectively. PFCzOxd-co-PCz1:
8.05e7.93 (m), 7.77e7.47 (m), 7.39e7.10 (m), 6.88 (d), 4.07 (s), 3.78
(s), 2.05 (s), 1.59e1.12 (m), 0.88e0.82 (m).
2.5. Synthesis of random copolymers via Yamamoto polymerization
In a three neck flask was placed bis(1,5-cyclooctadiene)nickel(0)
(Ni(COD)2) (350 mg, 1.27 mmol), 2,20-dipyridyl (200 mg, 1.27 mmol),
1,5-cyclooctadiene (0.16 mL, 1.27 mmol), and N,N-Dimethylforma-
mide (DMF) (8 mL). After three freezing-thaw cycles, the catalyst was
heated to 80 ꢀC for 30 min to form the purple complex. The reaction
mixture was treated with monomer 3 and monomer 6 (or monomer
7) in 5 mL of toluene and heated at 80 ꢀC for 4 days. After cooling to
room temperature, the reaction mixture was poured into the
mixture of 1 N HCl solution (100 mL), acetone (100 mL), and
methanol (200 mL), and stirred for 6 h. The precipitate was filtered,
redissolved in chloroform and precipitated again with large amount
methanol. The yields of the polymers ranged from 40 to 50%. The
resulting polymers were soluble in THF, CHCl3, ODCB and toluene.
PFCzOxd-co-PCz3:
8.062e7.949(m),
7.671e7.491(m),
7.338e7.138(m), 6.861(d), 4.084(s), 3.793(s), 2.012(s), 1.691e1.135
(m), 0.974e0.888(m).
PFCzOxd-co-PCz5: 8.06e7.95 (m), 7.78 (s), 7.67 (s), 7.49 (s),
7.36e7.14 (m), 6.86 (d), 4.09 (s), 3.79 (s), 2.08 (s), 1.29e1.14 (m),
0.89e0.79 (m).
PFCzOxd-co-PCz10: 8.05e7.94 (m), 7.66e7.47 (m), 7.39e7.12
(m), 6.85 (d), 4.07 (s), 3.78 (s), 2.02 (s),1.25e1.13 (m), 0.88e0.78 (m).
PFCzOxd-co-PPTZ1: 8.07e7.99 (m), 7.66e7.47 (m), 7.26e7.14
(m), 6.86 (s), 4.07 (s), 3.78 (s), 2.06 (s), 1.57e1.13(m), 0.87e0.81 (m).

S. Yamada et al. / Polymer 51 (2010) 6174e6181
6177
Table 1
PFCzOxd-co-PPTZ3: 8.06e7.95 (m), 7.78e7.31 (m), 7.27e7.14 (m),
Characterization of the synthesized polymers.
6.87 (d), 4.08 (s), 3.79 (s), 2.05 (s), 1.59e1.13 (m), 0.88e0.83 (m).
PFCzOxd-co-PPTZ5: 8.05e7.94 (m), 7.78e7.40 (m), 7.33e7.11 (m),
6.86 (d), 4.08 (s), 3.79 (s), 2.06 (s), 1.65e1.13 (m), 0.88e0.75 (m).
PFCzOxd-co-PPTZ10: 8.06e7.94 (m), 7.80e7.40 (m), 7.38e7.14
(m), 6.87 (d), 4.08 (s),0 3.79 (s), 2.06 (s),1.71e1.13 (m), 0.89e0.80 (m).
PDIa DSC(Tg)b TGA(Td)c
a
a
Polymers
Mn
Mw
PFCzOxd-alt-PCz
PFCzCz-alt-PCz
PFCzCzPCz-co-POxdOxdPCz 12 000
12 000
16 000
47 000 3.92
58 000 3.63
46 000 3.83
74
84
83
375
407
394
364
367
341
361
362
371
343
371
PFCzOxd-co-PCz1
PFCzOxd-co-PCz3
PFCzOxd-co-PCz5
PFCzOxd-co-PCz10
PFCzOxd-co-PPTZ1
PFCzOxd-co-PPTZ3
PFCzOxd-co-PPTZ5
PFCzOxd-co-PPTZ10
34 000 1 37 000 4.03 108
36 000 1 31 000 3.64 101
35 000 1 17 000 3.34 104
43 000 1 43 000 3.32 100
30 000 1 08 000 3.60 107
38 000 1 16 000 3.05 106
52 000 1 65 000 3.17 109
32 000 1 00 000 3.13 108
3. Results and discussion
3.1. Synthesis and characterization
The general synthetic routes toward the monomer and poly-
mers are outlined in Scheme 1 and Scheme 2. The monomers (3, 4,
5, 6, and 7) used for the polymerization were synthesized and
purified carefully according to the procedure outlined in the liter-
ature [19e23]. The monomer 2 was synthesized by the alkylation of
2,7-dibromo-9H-fluorene with 2-(4-(6-bromohexyloxy)phenyl)-5-
phenyl-1,3,4-oxadiazole (1) in the presence of catalytic amounts of
triethylbenzyl ammonium chloride and 50% NaOH aqueous solu-
tion. The polymerization was affected by the Pd(0)-catalyzed
Suzuki coupling polymerization. In case of poly(9-(6-(9H-carba-
zolyl)hexyl)-9-(6-(4-(5-phenyl-1,3,4-oxadiazolyl)phenoxy)hexyl)-
9H-fluorene-alt-9-(heptadecan-9-yl)-9H-carbazole) (PFCzOxd-alt-
PCz) (or poly(9,9-bis(6-(9H-carbazolyl)hexyl)-9H-fluorene-alt-
9-(heptadecan-9-yl)-9H-carbazole) (PFCzCz-alt-PCz)), the molar
feed ratio of monomer 3 (or 4) and diboromic ester monomer 5 was
1:1. In case of poly(2-(9-(heptadecan-9-yl)-9H-carbazol-2-yl)-9,9-
bis(6-(9H-carbazolyl)hexyl)-9H-fluorene-co-2-(9-(heptadecan-9-yl)-
9H-carbazol-2-yl)-9,9-bis(6-(4-(5-phenyl-1,3,4-oxadiazolyl)phenoxy)-
hexyl)-9H-fluorene) (PFCzCzPCz-co-PFOxdOxdPCz), the molar feed
ratio of monomer 2, 4 and 5 was 1:1:2. Monomer 3 was copoly-
merized with monomer 6 or 7 by using nickel-catalyzed Yamamoto
coupling methods, using bis(1,5-cyclooctadiene)nickel(0) (Ni(COD)2),
2,20-dipyridyl, and 1,5-cyclooctadiene. To generate the copolymers,
poly(9-(6-(9H-carbazolyl)hexyl)-9-(6-(4-(5-phenyl-1,3,4-oxadiazolyl)-
phenoxy)hexyl)-9H-fluorene-co-9-(heptadecan-9-yl)-9H-carbazole)
a
Molecular weight and Polydispersity (PDI) of the polymers were determined by
gel permeation chromatography (GPC) in THF using polystyrene standards.
b
Glass temperature was measured by DSC under N2.
Onset decomposition temperature (5% weight loss) was measured by TGA
c
under N2.
(PFCzOxd-co-PCz), different ratios of monomer 3 and 6 were used.
The feed ratios of fluorene unit with carbazole and oxadiazole
pendants and carbazole unit fragments were 99:1, 97:3, 95:5,
and 90:10 for the syntheses of PFCzOxd-co-PCz1, PFCzOxd-co-
PCz3, PFCzOxd-co-PCz5, and PFCzOxd-co-PCz10, respectively. To
generate the copolymers, poly(9-(6-(9H-carbazolyl)hexyl)-9-(6-(4-
(5-phenyl-1,3,4-oxadiazolyl)phenoxy)hexyl)-9H-fluorene-co-10-octyl-
10H-phenothiazine) (PFCzOxd-co-PPTZ), different ratios of monomer 3
and 7 were also used. The comonomer feed ratios of monomer 3
to phenothiazine unit are 99:1, 97:3, 95:5, and 90:10 and the corre-
sponding polymers were named as PFCzOxd-co-PPTZ1, PFCzOxd-co-
PPTZ3,PFCzOxd-co-PPTZ5,andPFCzOxd-co-PPTZ10, respectively. The
carbazole or phenothiazine unit in the polymer backbones was intro-
duced to improve the device performance. All the resulting polymers
were soluble in organic solvents such as chloroform, chlorobenzene,
THF, dichloromethane and ODCB.
The polymerization results including molecular weight, poly-
dispersity (PDI), and thermal stability of the polymers were
Cz
Oxd
Cz
Oxd
C8H17
C8H17
C8H17
C8H17
1. Ni(COD)2 / COD
2,2'-dipyridyl
N
N
DMF / Toluene
Br
Br
Br
Br
2. bromobenzene
n
m
6
3
m:n = 99:1 PFCzOxd-co-PCz1
m:n = 97:3 PFCzOxd-co-PCz3
m:n = 95:5 PFCzOxd-co-PCz5
m:n =90:10 PFCzOxd-co-PCz10
Cz
Oxd
Cz
Oxd
C8H17
N
C8H17
N
1. Ni(COD)2 / COD
2,2'-dipyridyl
DMF / Toluene
Br
Br
S
Br
S
Br
2. bromobenzene
n
m
7
3
m:n = 99:1 PFCzOxd-co-PPTZ1
m:n = 97:3 PFCzOxd-co-PPTZ3
m:n = 95:5 PFCzOxd-co-PPTZ5
m:n =90:10 PFCzOxd-co-PPTZ10
Scheme 2. Synthetic routes for the copolymers by Yamamoto polymerization.

6178
S. Yamada et al. / Polymer 51 (2010) 6174e6181
Table 2
summarized in Table 1. In the case of the copolymers via Suzuki
Characteristics of the UV-vis absorption and photoluminescence.
coupling polymerization, the number-average molecular weights
(Mn) and weight-average molecular weights (Mw) were
12 000e16 000 and 46 000e58 000, respectively. In the case of the
random copolymer via Yamamoto polymerization, the number-
average molecular weights (Mn) and weight-average molecular
weights (Mw) were 30 000e52 000 and 1 00 000e1 65 000,
respectively. The thermal properties of the polymers were charac-
terized by both differential scanning calorimetry (DSC) and thermal
gravimetric analysis (TGA) as shown in Table 1. DSC analysis was
performed under a nitrogen atmosphere (50 mL/min) on a DSC 822
at heating rates of 10 ꢀC/min. TGA was performed with a Dupont
951 TGA instrument in a nitrogen atmosphere at a heating rate of
10 ꢀC/min to 600 ꢀC. TGA showed that copolymers are thermally
stable with only about 5% weight loss at temperatures of
341e407 ꢀC, which could afford the advantages for the PLED device
fabrication. The polymers showed good thermal stability with
Polymers
Solution
Abs PL
lmax lmax of PL
(nm) (nm)
Film
EL lmax
(nm)
Fwhma Abs PL
Fwhma
of PL
lmax lmax
(nm) (nm)
PFCzOxd-alt-PCz
PFCzCz-alt-PCz
PFCzCzPCz-co-POxd 383 418 39
383 418 39
386 418 39
383 427
383 428
383 428
43
45
47
427, 541
425
425, 529
OxdPCz
PFCzOxd-co-PCz1
PFCzOxd-co-PCz3
PFCzOxd-co-PCz5
PFCzOxd-co-PCz10 388 419 40
PFCzOxd-co-PPTZ1 384 418 41
PFCzOxd-co-PPTZ3 385 418 41
PFCzOxd-co-PPTZ5 386 418 42
PFCzOxd-co-PPTZ10 384 418 42
388 419 40
388 419 40
387 419 40
384 430, 537 117
385 428, 534 100
383 429, 537 118
383 429, 539 144
383 430, 480 109
384 484
383 487
383 490
427, 543
428, 547
426, 541
428, 550
429, 488
487
111
111
86
492
490
a
Full width at half-maximum of PL spectra.
1.2
a
PFCzOxd-alt-PCz (sol.)
PFCzOxd-alt-PCz (film)
PFCzCz-alt-PCz (sol.)
PFCzCz-alt-PCz (film)
1.0
0.8
0.6
0.4
0.2
0.0
PFCzCzPCz-co-POxdOxdPCz (sol.)
PFCzCzPCz-co-POxdOxdPCz (film)
PFCzOxd (sol.)
PFCzOxd (film)
300
400
500
600
Wavelength (nm)
1.2
1.0
0.8
0.6
0.4
b
PFCzOxd-co-PCz1 (sol.)
PFCzOxd-co-PCz1 (film)
PFCzOxd-co-PCz3 (sol.)
PFCzOxd-co-PCz3 (film)
PFCzOxd-co-PCz5 (sol.)
PFCzOxd-co-PCz5 (film)
PFCzOxd-co-PCz10 (sol.)
PFCzOxd-co-PCz10 (film)
0.2
0.0
300
400
500
600
Wavelength (nm)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
c
PFCzOxd-co-PPTZ1 (sol.)
PFCzOxd-co-PPTZ1 (film)
PFCzOxd-co-PPTZ3 (sol.)
PFCzOxd-co-PPTZ3 (film)
PFCzOxd-co-PPTZ5 (sol.)
PFCzOxd-co-PPTZ5 (film)
PFCzOxd-co-PPTZ10 (sol.)
PFCzOxd-co-PPTZ10 (film)
300
400
500
600
Wavelength (nm)
Fig. 1. UVevis absorption spectra of the polymer: alternating copolymer (a), random
Fig. 2. PL emission spectra of the polymer: alternating copolymer (a), random
copolymer with PCz units (b) and random copolymer with PPTZ units (c).
copolymer with PCz units (b) and random copolymer with PPTZ units (c).

S. Yamada et al. / Polymer 51 (2010) 6174e6181
6179
a glass transition temperature (Tg) of 74e109 ꢀC, using DSC per-
formed at a temperature range of 30e250 ꢀC.
Table 3
Electrochemical potentials and energy levels of the polymers.
Eonset (V) HOMOb (eV) LUMOc (eV) Egd (eV)
a
Polymers
3.2. Optical properties
PFCzOxd-alt-PCz
PFCzCz-alt-PCz
PFCzCzPCz-co-POxdOxdPCz 0.88
0.87
0.86
ꢁ5.67
ꢁ5.66
ꢁ5.68
ꢁ5.68
ꢁ5.68
ꢁ5.69
ꢁ5.69
ꢁ5.68
ꢁ5.68
ꢁ5.67
ꢁ5.67
ꢁ2.78
ꢁ2.76
ꢁ2.80
ꢁ2.79
ꢁ2.78
ꢁ2.78
ꢁ2.79
ꢁ2.78
ꢁ2.78
ꢁ2.77
ꢁ2.79
2.89
2.90
2.88
2.89
2.90
2.91
2.90
2.90
2.90
2.90
2.88
The absorption and photoluminescence (PL) properties of the
synthesized polymers were investigated in chloroform solutions
and the thin films by spin-coating on quartz plates from the solu-
tions in ODCB, as summarized in Table 2. As shown in Fig. 1, the
PFCzOxd-co-PCz1
PFCzOxd-co-PCz3
PFCzOxd-co-PCz5
PFCzOxd-co-PCz10
PFCzOxd-co-PPTZ1
PFCzOxd-co-PPTZ3
PFCzOxd-co-PPTZ5
PFCzOxd-co-PPTZ10
0.88
0.88
0.89
0.89
0.88
0.88
0.87
0.87
adsorption peaks, corresponding to
pe
p* transition of the conju-
gated backbones, appear at around 380e390 nm. The absorption
spectra of the films are very similar to those of the solutions with all
of the polymers. The absorption onset wavelengths of the polymers
in the film states were around 430 nm, which corresponds to the
band gap of 2.88 eV. The edge of the absorption spectra are red-
shifted caused by interactions between conjugated polymer chains
in film, which results in a higher degree of excitoneexciton anni-
hilation [25].
a
Onset oxidation and reduction potential measured by cyclic voltammetry.
Calculated from the oxidation potentials.
Calculated from the HOMO energy levels and optical band gap.
Optical energy band gap was estimated from the onset wavelength of the optical
b
c
d
absorption.
PFCzOxd-alt-PCz
PFCzCz-alt-PCz
a
PFCzCzPCz-
co-POxdOxdPCz
PFCzOxd-co-PCz1
150000
PFCzOxd-co-PCz3
PFCzOxd-co-PCz5
PFCzOxd-co-PCz10
PFCzOxd-co-PPTZ1
PFCzOxd-co-PPTZ3
1.2
1.0
0.8
0.6
0.4
0.2
0.0
a
PFCzOxd-alt-PCz
PFCzCz-alt-PCz
PFCzOxd-co-PPTZ5
100000
PFCzOxd-co-PPTZ10
PFCzCzPCz-co-POxdOxdPCz
PFCzOxd
50000
0
0
5
10
15
Voltage (V)
PFCzOxd-alt-PCz
PFCzCz-alt-PCz
PFCzCzPCz-
400
500
600
700
b
co-POxdOxdPCz
PFCzOxd-co-PCz1
PFCzOxd-co-PCz3
PFCzOxd-co-PCz5
PFCzOxd-co-PCz10
PFCzOxd-co-PPTZ1
PFCzOxd-co-PPTZ3
PFCzOxd-co-PPTZ5
PFCzOxd-co-PPTZ10
Wavelength (nm)
1500
1.2
1.0
0.8
0.6
0.4
0.2
0.0
b
PFCzOxd-co-PCz1
PFCzOxd-co-PCz3
PFCzOxd-co-PCz5
PFCzOxd-co-PCz10
1000
500
0
5
10
15
Voltage (V)
400
500
600
700
c
Wavelength (nm)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.1
0.01
1E-3
c
PFCzOxd-co-PPTZ1
PFCzOxd-co-PPTZ3
PFCzOxd-co-PPTZ5
PFCzOxd-co-PPTZ10
PFCzOxd-alt-PCz
PFCzCz-alt-PCz
PFCzCzPCz-co-POxdOxdPCz
PFCzOxd-co-PCz1
PFCzOxd-co-PCz3
PFCzOxd-co-PCz5
PFCzOxd-co-PCz10
PFCzOxd-co-PPTZ1
PFCzOxd-co-PPTZ3
PFCzOxd-co-PPTZ5
PFCzOxd-co-PPTZ10
0
30000
60000
90000
2
Current Density
(
mA/cm
)
400
500
600
700
Fig. 4. Current densityevoltageeluminescence (JeVeL) characteristics and efficiency
of OLED with the configuration of ITO/PEDOT:PSS/Polymer/Ca/Al. Current densi-
tyevoltage characteristics (a), luminescenceevoltage characteristics (b), and efficiency
(c).
Wavelength (nm)
Fig. 3. EL spectra of the polymer: alternating copolymer (a), random copolymer with
PCz units (b) and random copolymer with PPTZ units (c).

6180
S. Yamada et al. / Polymer 51 (2010) 6174e6181
Table 4
Device performance characteristics of devices with the configuration of ITO/PEDOT/polymer/Ca/Al.
Polymers
Turn-on voltagea (V)
Voltageb
Current densityb (mA/cm2)
Luminanceb (cd/m2)
LEmaxc (cd/A)
CIE (x, y)d
PFCzOxd-alt-PCz
PFCzCz-alt-PCz
7.5
3.5
4.5
7.5
7.0
4.0
10
4.0
4.0
6.5
4.5
11.5
6.5
7.0
11.0
11.0
7.0
13.5
7.0
48600
87400
64400
35300
72700
60200
37400
80800
55800
45400
64200
760
440
1100
580
1420
980
960
1260
1000
300
0.15
0.03
0.10
0.11
0.11
0.15
0.17
0.13
0.16
0.12
0.07
(0.28, 0.33)
(0.18, 0.17)
(0.25, 0.32)
(0.33, 0.42)
(0.36, 0.46)
(0.33, 0.44)
(0.41, 0.52)
(0.21, 0.34)
(0.20, 0.37)
(0.24, 0.42)
(0.22, 0.41)
PFCzCzPCz-co-POxdOxdPCz
PFCzOxd-co-PCz1
PFCzOxd-co-PCz3
PFCzOxd-co-PCz5
PFCzOxd-co-PCz10
PFCzOxd-co-PPTZ1
PFCzOxd-co-PPTZ3
PFCzOxd-co-PPTZ5
PFCzOxd-co-PPTZ10
8.0
12.0
8.0
610
a
Voltages required to achieve a brightness of 1 cd/m2.
Measured under the condition of maximum brightness.
Maximum luminescence efficiency.
b
c
d
Calculated from the EL spectrum.
Fig. 2 shows the PL spectra of the polymers with the excitation of
Table 3. HOMO levels were calculated according to the empirical
ox
380 nm in both chloroform solution and thin film. As shown in
Fig. 2 (a), the solutions of the polymers (PFCzOxd-alt-PCz, PFCzCz-
alt-PCz, PFCzCzPCz-co-PFOxdOxdPCz) showed PL emission peaks
with a maximum at 418 nm, and a shoulder at 440 nm. The PL
spectra of their thin films consist of typical vibronically structured
bands comprising a maximum at 427 nm, a shoulder at 453 nm, and
a broad tail at 490 nm. All of PL emission peaks in the film states
were red-shifted about 10e20 nm as compared to those of the
solutions, which can be attributed to increased interactions
between conjugated polymer chains in film, which results in
a higher degree of excitoneexciton annihilation [25]. In case of the
random copolymers containing carbazole units in the polymer
backbone (Fig. 2 (b)), the PL spectra of the solutions were quite
similar as compared to the case of polyfluorene. In contrast the PL
spectra of the thin films showed peaks at around 540 nm, caused by
the aggregation due to the oxadiazole unit with electron with-
drawing effect and carbazole unit with electron donating effect. As
compared to the homopolymer, PFCzOxd reported previously by us
[19] , the intensity of the PL emission peak at long wavelength was
enhanced by the incorporation of carbazole units in the backbone
as low as 1% on the backbones. The PL emission of the random
copolymers containing phenothiazine units was investigated as
shown in Fig. 2 (c). The PL emission peaks in the solutions were also
quite similar to the case of fluorene. In the film state, all of the
polymers showed two PL emission peaks at around 430 nm and
480 nm. With increasing amounts of phenothiazine contents, the
PL spectra exhibit decreased peaks at 430 nm and increased peaks
at 480 nm. Consequently, the PL emission peak at 430 nm was
nearly quenched with phenothiazine contents of 10% in the
copolymers.
formula (EHOMO ¼ ꢁ ([Eonset
]
þ 4.8) eV). The polymers exhibited
the absorption onset wavelengths of 426e430 nm in solid thin films,
which correspond to the band gaps of 2.88e2.91 eV. The polymers
exhibit irreversible processes in an oxidation scan. The oxidation
onsets of the polymers were estimated to be 0.86e0.89 V, which
correspond to HOMO energy levels of ꢁ5.66 ∼ ꢁ5.69 eV. The LUMO
energy levels of the polymers can be calculated with the HOMO
energy levels and optical band gaps. The LUMO energy levels of the
polymers were thus determined to be ꢁ2.76 ∼ ꢁ2.80 eV.
3.4. Electroluminescence properties
The investigations of electroluminescence properties of the
copolymers were conducted by fabricating the devices with the
configuration of ITO/PEDOT (40 nm)/polymer (80 nm)/Ca (10 nm)/
Al (100 nm). The electroluminescence (EL) spectra of the devices
are shown in Fig. 3. In case of PFCzOxd-alt-PCz and PFCzCzPCz-co-
PFOxdOxdPCz, the EL spectra of the polymers showed two distinct
peaks comprising the maximum at 427 nm, which corresponds to
the peak of the polyfluorene, and additional large one at around
540 and 530 nm, respectively. As compared to homopolymer,
PFCzOxd [19], the intensities of the long wavelength EL peaks at
540 and 530 nm were obviously enhanced by the incorporation of
carbazole units at the polymer backbones. The CIE coordinates of
the devices with PFCzOxd-alt-PCz and PFCzCzPCz-co-PFOx-
dOxdPCz were (0.28, 0.33) and (0.25, 0.32), respectively, which are
close to that of the standard white of National Television System
Committee (NTSC) (0.33, 0.33). On the other hand, the EL spectrum
of PFCzCz-alt-PCz exhibited maximum peak at 427 nm and CIE
coordinates of (0.18, 0.17), corresponding to blue light emission. In
case of PFCzCz-alt-PCz, there is no oxadiazole unit in any part of
the structure. It suggests that the oxadiazole units played an
important role in the generation of the long-wavelength peak. The
long-wavelength peaks can be attributed to the aggregation caused
by the oxadiazole unit with electron withdrawing effect and
carbazole unit with electron donating effect. As shown in Fig. 3 (b)
and (c), the effects of the carbazole and phenothiazine units on the
EL spectra were also investigated. Typical EL spectra of PFCzOxd-
co-PCzs and PFCzOxd-co-PPTZs were nearly same as compared to
the PL of the solid films of the polymers, indicating that the EL and
PL phenomena originated from the same excited state. The
PFCzOxd-co-PCzs and PFCzOxd-co-PPTZs showed additional large
peak at around 530 and 500 nm, respectively, caused by the
aggregation and the excimer formation. The oxadiazole and
carbazole pendents in the fluorine unit do not affect the effective
conjugation length of the polymers directly, since the oxadiazole
3.3. Electrochemical properties
The electrochemical properties of the polymer were determined
from the band gap estimated from the absorption onset wavelength,
and the HOMO energy level which was estimated from the cyclic
voltammetry (CV). The CV was performed with a solution of tetra-
butylammonium tetrafluoroborate (Bu4NBF4) (0.10 M) in acetoni-
trile at a scan rate of 100 mV/s at room temperature under argon
atmosphere. A platinum electrode (∼0.05 cm2) coated with a thin
polymer film was used as the working electrode. Pt wire and
Ag/AgNO3 electrode were used as the counter electrode and refer-
ence electrode, respectively. The energy level of the Ag/AgNO3
reference electrode (calibrated by the Fc/Fcþ redox system) was
4.8 eV below the vacuum level. The oxidation potentials derived
from the onsets of electrochemical p-doping are summarized in

S. Yamada et al. / Polymer 51 (2010) 6174e6181
6181
and carbazole moieties were introduced as pendants using non-
conjugated chain. However, CIE coordinates of synthesized copol-
ymers for white light is not better than that of PFCzOxd caused by
more aggregation between pendant unit and carbazole or pheno-
thiazine units in the backbone. The aggregate is formed by aggre-
gation of several neighboring polymer segments at the ground state
[26]. The long-wavelength peaks of EL spectra are enhanced by
aggregate at ground state, excimer or exciplex at excited state, and
electric-field-induced complex (electromer or electroplex) [25].
The current densityevoltage (JeV), luminescenceevoltage
(LeV), and efficiency characteristics of ITO/PEDOT/polymer/Ca/Al
devices are shown in Fig. 4 and summarized in Table 4. The
maximum brightnesses of the devices were in the range of
300e1420 cd/m2. The luminescence efficiency of the polymers are
higher than 0.1 cd/A, except the cases of PFCzCz-alt-PCz (0.03 cd/A)
and PFCzOxd-co-PPTZ10 (0.07 cd/A). Among all the devices, the
device fabricated from PFCzOxd-co-PCz10 showed the best
performance with the efficiency of 0.17 cd/A, caused by the hole
transporting ability of the carbazole unit in the polymer backbone.
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (2010-
0007431).
Appendix. Supplementary material
Supplementary material related to this article can be found
online at doi:10.1016/j.polymer.2010.10.061.
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4. Conclusions
We report here the synthesis, characterization and EL properties
of novel conjugated copolymers based on fluorene with carbazole
and oxadiazole pendants for the generation of the white emission.
The polymers exhibited the absorption peak at around
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of PFCzOxd-co-PCzs showed PL emission peak at around 430 nm
and 530 nm. The peak at 530 was increasing with higher contents
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peaks comprising the maximum peak at 427 nm and additional one
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devices with PFCzCz-alt-PCz and PFCzCzPCz-co-PFOxdOxdPCz
were (0.28, 0.33) and (0.25, 0.32), respectively, approaching the
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Acknowledgements
This work was supported by National Research Foundation of
Korea Grant funded by the Korean Government (2009-0087143)
and by the Korea Government (MEST) (2010-0015069). This
research at Ulsan was supported by Basic Science Research Program