Carbazole/fluorene copolymers with dimesitylboron pendants for blue light-emitting diodes

Article abstract of DOI:10.1016/j.polymer.2010.12.060

A series of random and alternating carbazole/fluorene copolymers with various dimesitylboron-containing carbazole derivative contents were synthesized by Suzuki polymerization for use as a light-emitting layer in blue light-emitting diodes. Two carbazole derivatives, CzPhB and CzPhThB consisted of a carbazoyl group as the donor and a dimesitylboron group as the acceptor group, separated by phenyl and phenyl-thiophene groups π-conjugated systems, respectively. The copolymers exhibited good thermal stability and blue emission in both solution and the solid state. Moreover, the CzPhB/fluorene and CzPhThB/fluorene copolymers exhibited a higher PL quantum efficiency than the fluorene-based homopolymer (POF). Higher brightness and larger current efficiency were observed for the CzPhB/fluorene and CzPhThB/fluorene copolymer-based devices compared to the POF-based device. Additionally, the CzPhThB/fluorene copolymer-based devices had better EL performances than the CzPhB/fluorene copolymer-based devices. The turn-on voltage, maximal brightness, and highest luminescence efficiency of the carbazole/fluorene copolymer-based devices were found to be 4.5-8.5 V, 436 cd/m², and 0.51 cd/A, respectively.

Full text of DOI:10.1016/j.polymer.2010.12.060

Polymer 52 (2011) 976e986
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Carbazole/fluorene copolymers with dimesitylboron pendants for blue
light-emitting diodes
,
c
Ying-Hsiao Chen a, Yu-Ying Lin a, Yung-Chung Chen b, Jiann T. Lin b, Rong-Ho Lee a , Wen-Jang Kuo ,
*
,
Ru-Jong Jeng a
*
a Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
b Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan
c Department of Applied Chemistry, National University of Kaohsiung, Kaohsiung 811, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of random and alternating carbazole/fluorene copolymers with various dimesitylboron-contain-
ing carbazole derivative contents were synthesized by Suzuki polymerization for use as a light-emitting
layer in blue light-emitting diodes. Two carbazole derivatives, CzPhB and CzPhThB consisted of a carbazoyl
group as the donor and a dimesitylboron group as the acceptor group, separated by phenyl and phenyl-
Received 25 September 2010
Received in revised form
16 December 2010
Accepted 24 December 2010
Available online 9 January 2011
thiophene groups
p-conjugated systems, respectively. The copolymers exhibited good thermal stability
and blue emission in both solution and the solid state. Moreover, the CzPhB/fluorene and CzPhThB/fluo-
rene copolymers exhibited a higher PL quantum efficiency than the fluorene-based homopolymer (POF).
Higher brightness and larger current efficiency were observed for the CzPhB/fluorene and CzPhThB/flu-
orene copolymer-based devices compared to the POF-based device. Additionally, the CzPhThB/fluorene
copolymer-based devices had better EL performances than the CzPhB/fluorene copolymer-based devices.
The turn-on voltage, maximal brightness, and highest luminescence efficiency of the carbazole/fluorene
copolymer-based devices were found to be 4.5e8.5 V, 436 cd/m2, and 0.51 cd/A, respectively.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Light-emitting polymer
Carbazole derivative
Dimesitylboron group
1. Introduction
Conjugated polyfluorenes (PFs) have evolved to be the most
promising candidates for blue-emitting materials for PLEDs
In the past decades, polymer light-emitting diodes (PLEDs) have
received attention because of their applications in full-color flat
panel displays and solid state lighting sources [1,2]. PLEDs have
a number of advantages over inorganic or organic small molecule-
based light-emitting diodes, including easy processing, low oper-
ating voltages, low cost fabrication, and high flexibility [3]. For
a full-color display, the need to develop more stable and highly
efficient three primary color (red, green, and blue) emitters is
important for allowing PLEDs to become commercial products [4].
Of particular interest are blue light-emitting materials, which can
serve as either blue light sources in a full-color display or as host
materials for lower energy fluorescent or phosphorescent dyes
[5e8]. Therefore, developing stable blue EL materials with high
efficiency and excellent Commission Internationale de L’Enclairage
(CIE) coordinates (y-coordinate value < 0.15) is essential to real-
izing such applications.
because of their highly efficient blue emission in photo-
luminescence (PL) and electroluminescence (EL), excellent thermal
and chemical stability, and good solubility in common organic
solvents [9e13]. However, an undesired emission appearing in the
long-wavelength division (from 500 to 600 nm) of the emission
spectra of PF homopolymers not only hampers the EL efficiency,
but impairs the color purity. Either the formation of aggregates/
excimers or degradation of the polymers during operation of the
PF-based PLEDs has been proposed as the source of this problem
[14e16]. The long-wavelength emission could be curtailed by
introducing bulky substituents or long alkyl chains at the C-9
position of the fluorene unit through copolymerization with
appropriate co-monomers or the attachment of bulky end-capping
groups, among others [17e24]. Additionally, most PF-type poly-
mers with low highest occupied molecular orbital (HOMO) levels
have a high energy barrier to hole-injection from the anode,
resulting in imbalanced charge mobility and subsequent low
quantum efficiency [25,26]. Chemical structure modification of PFs
by incorporating electron-donor moieties, such as triarylamine,
carbazole, and thiophene groups, seems to improve the deficiency
in hole-injection properties [20,27,28]. It is well-known that
* Corresponding authors. Tel.: þ886 4 22854724; fax: þ886 4 22854734.
E-mail addresses: rhl@dragon.nchu.edu.tw (R.-H. Lee), rjjeng@dragon.nchu.edu.
tw (R.-J. Jeng).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.12.060

Y.-H. Chen et al. / Polymer 52 (2011) 976e986
977
attaching a carbazole moiety to the molecular scaffold can signifi-
cantly enhance the thermal stability and HOMO energy level of
light-emitting polymers (LEPs). Moreover, the 3-, 6- or 9-position of
the carbazole moiety can be easily functionalized, and thus the
photo-physical properties of the polymers can be tuned [29e32].
N-arylated carbazoles, in which a phenyl or napthyl group is
attached at the 9-position of the carbazole, have shown excellent
thermal stability and good electro-optical properties in small-
molecule OLEDs [33]. As far as the conjugated polymer is con-
cerned, the imbalanced transport properties between the holes and
electrons are another crucial factor deciding the efficiency of PLEDs.
Due the poor electron-mobility of LEPs, the attachment of electron-
deficient groups (such as pyridine, benzothiadiazole, quinoxalines,
and oxadiazole, etc.) onto a polymer chain has proven to be an
effective methodology to promote its electron transport capabil-
ities [34e37]. Several studies have also demonstrated that borane-
derivatives are potential luminescent and electron-transporting
materials [38e40]. The air and moisture stabilities of electron-
deficient arylboranes can be improved by incorporating the non-
coplanar dimesityl group into the molecules [41e43]. Moreover, the
non-coplanar structure of dimesitylborane could hinder the
molecular close packing in the solid state. A stable amorphous film
was formed for a dimesitylborane moiety containing carbazole
derivative [40]. Although arylborane-containing fluorophores
have been successfully developed as light emitters, only a few
arylborane moieties containing LEPs have been proposed for PLED
applications [44e47]. Chujo et al. synthesized a series of alternating
copolymers by the hydroboration polymerization of aromatic
diynes and mesitylborane [44]. Blue emission was observed for the
copolymers in dilute solution state. The copolymers containing
boron atoms in the main-chains are expected to act as electron-
derivatives, CzPhB and CzPhThB consisted of a carbazoyl group as
the donor and a dimesitylboron group as the acceptor group,
separated by phenyl and phenyl-thiophene groups
p-conjugated
systems, respectively. Incorporating the bipolar carbazole deriva-
tive as the pendant of the conjugated polymer was favorable for
improvement of the charge-injection/transporting characteristics
of the PF. Excellent EL properties were expected for these carbazole/
fluorene copolymer-based PLEDs. The thermal stability, electro-
chemical properties, photo-physical behavior, and EL performances
of the carbazole/fluorene copolymer-based devices are discussed in
detail as the chemical structures and carbazole derivative content
of the copolymers are taken into account.
2. Experimental
2.1. Materials
All reactions and manipulations were performed in a nitrogen
atmosphere using standard Schlenk techniques. All chromato-
graphic separations were carried out on silica gel. 9H-carbazole,
1,4-dibromobenzene, N-bromosuccinimide (NBS), copper powder
and 18-crown-6 were purchased from Acros Co. Potassium
carbonate (K2CO3) was bought from Fisher Scientific. Tetrakis-
triphenylphosphine palladium(0) [Pd(PPh3)4] was purchased from
Strem Chemicals. 2.7-Dibromo-9,9-dioctylfluorene (compound 3),
n-butyl lithium and dimesitylboron fluoride were obtained from
Aldrich Co. o-Dichlorobenzene (DCB) was bought from TEDIA.
Tetrahydrofuran (THF) and toluene were purified by distillation
from sodium in the presence of benzophenone. Detailed synthetic
procedures for the carbazole derivatives (CzPhB and CzPhThB) and
carbazole/fluorene copolymers are report hereafter and are shown
in Schemes 1 and 2.
deficient
p-conjugated systems, where p-conjugation length is
extended via the vacant p-orbital of the boron atom [44]. However,
the EL properties of the main chain organoboron polymer-based
emitting layers have not been investigated. In addition, Yamaguchi
2.2. Synthesis of monomers
and coworkers reported
a
series of highly emissive dibor-
2.2.1. Synthesis of 9-(4-bromophenyl)-9H-carbazole (compound 1)
[48]
ylphenylene containing poly(arylenethyynylene)s [45,46]. The
presence of bulky diarylboryl substituents not only acts as a good
electron-accepting unit, but also prevents the interaction and
aggregation between the polymer backbones. However, these poly
(arylenethyynylene)s exhibited sky-blue to green emission as thin
films due to the strongly intramolecular charge transfer (ICT)
A stirred mixture of 9H-carbazole (5.02 g, 30 mmol), 1,4-dibro-
mobenzene (14.13 g, 60 mmol), K2CO3 (16.56 g, 120 mmol), Cu
powder (1.94 g, 30 mmol) and 18-crown-6 (3.71 g, 15 mmol) in DCB
(120 ml) was degassed with nitrogen for 1 h. The reaction mixture
was then refluxed under a nitrogen atmosphere for 16 h. The crude
mixture was filtered, and the residue was washed with dichloro-
methane. The combined filtrates were then evaporated to dryness.
The product was purified by flash column chromatography (silica
gel, hexane) to give compound 1 (7.0 g, yield ¼ 73%) as a white solid.
transition from the
p-conjugated backbone to the dibor-
ylphenylene unit. EL properties of the poly(arylenethyynylene)s
were not discussed even through the fact that highly absolute
quantum yields were observed for these polymer-based thin films
[46]. More recently, two N-p-(diarylboryl)phenyl-substituted pol-
ycarbazoles were reported by Lambert et al. [47]. Although the
authors demonstrated a light-emitting device based on N-p-(dia-
rylboryl)phenyl-substituted 3,6-linked polycarbazoles, but only the
EL spectrum was shown in the literature.
Base on the above, highly blue emissive conjugated polymers
could be obtained by the incorporation of the electron-deficient
arylborane unit into the conjugated polymers [44e47]. It is
important to note that EL properties have never been discussed for
these arylborane unit containing conjugated polymers. In addition
to the bipolar structure in the polymer, the conjugation length in
both side-chain moiety and polymer backbone should be taken into
account for pursuing a polymer-based device with excellent EL
performances. In this study, a series of random and alternating
carbazole/fluorene copolymers with various contents of dimesi-
tylboron-containing carbazole derivatives were designed and
synthesized for use as blue emitters in PLEDs because the bipolar
1H NMR (600 MHz, CDCl3):
d
[ppm]: 7.26e7.31 (m, 2H), 7.35e7.38
(m, 3H), 7.40e7.45 (m, 3H), 7.72 (d, J ¼ 8.7 Hz, 2H), 8.13 (d, J ¼ 7.8 Hz,
2H). 13C NMR (75 MHz, CDCl3):
[ppm]: 109.53, 120.20, 120.37,
d
120.87, 123.47, 126.07, 128.71, 133.10, 136.79, 140.60. HRMS (m/z):
calcd for C18H12BrN: 321.0153. Found: 321.0162. Anal. calcd for
C18H12BrN: C, 67.10; H, 3.75; N, 4.35. Found: C, 67.45; H, 3.73; N, 4.21.
2.2.2. Synthesis of 9-(4-(thiophen-2-yl)phenyl)-9H-carbazole
(compound 2)
50 mL of anhydrous toluene was added to a mixture of compound
1 (3.22 g, 10 mmol) and 2-thiopheneboronic acid (2.77 g, 10 mmol).
After 10 mL of 2 M aqueous K2CO3 was added to the stirred mixture,
the reaction mixture was degassed with nitrogen for 1 h. Then Pd
(PPh3)4 (0.03 g, 0.05 mmol) was added to the mixture. The solution
was further refluxed under a nitrogen atmosphere for 48 h. After the
reaction solution was cooled to room temperature, the whole
mixture was poured into water, and the organic layer was separated
and washed with water. The organic extracts were dried over MgSO4
and concentrated by rotary evaporation. After the solvent was
carbazole-p-dimesitylboron unit exhibits a high PL quantum effi-
ciency in both solution and the solid state [40]. Two carbazole

978
Y.-H. Chen et al. / Polymer 52 (2011) 976e986
Scheme 1. Synthetic routes of monomers.
evaporated, the crude product was further purified by column
chromatography on silica gel using hexane as the eluent to yield
compound 2 (2.34 g, yield ¼ 72%) as a white solid.1H NMR (600 MHz,
Found: 325.0923. Anal. calcd for C22H15NS: C, 81.20; H, 4.65; N, 4.30.
Found: C, 81.25; H, 4.30; N, 4.28.
CDCl3):
d
[ppm]: 7.12 (t, J ¼ 3.9 Hz,1H), 7.28e7.35 (m, 3H), 7.39e7.44
2.2.3. Synthesis of 9-(4-(dimesitylboryl)phenyl)-9H-carbazole
(compound 3)
Under a nitrogen atmosphere, n-butyl lithium (4.4 ml, 11 mmol,
2.5 M in hexane) was added to a solution of compound 1
(3.2 g, 10 mmol) in dry THF (50 mL) at ꢀ78 ꢁC. After 1 h of stirring,
(m, 5H), 7.56 (d, J ¼ 8.4 Hz, 2H), 7.70 (d, J ¼ 8.7 Hz, 2H), 8.14
(d, J ¼ 7.8 Hz, 2H). 13C NMR (75 MHz, CDCl3):
d
[ppm]: 109.76,120.01,
120.31, 123.42, 123.57, 125.30, 125.97, 127.22, 127.41, 128.22, 133.51,
136.77, 140.74, 143.39. HRMS (m/z): calcd for C22H15NS: 325.0925.
Scheme 2. Synthetic routes of the carbazole/fluorene copolymers.

Y.-H. Chen et al. / Polymer 52 (2011) 976e986
979
dimesitylboron fluoride (3.0 g, 11 mmol) in 15 mL of THF was
2.3. Synthesis of carbazole/fluorene copolymers
added slowly to the reaction mixture, which was stirred for
a further 12 h. The reaction mixture was then cooled to room
temperature and quenched with 2 N HCl. The solution was
extracted with ethyl acetate and the organic layer was dried over
MgSO4, filtered, and evaporated to yield a crude solid. The crude
product was purified by column chromatography to afford a white
solid of compound 3 (3.42 g, yield ¼ 70%). 1H NMR (600 MHz,
2.3.1. Synthesis of PFCzPhB10
As shown in Scheme 2, the carbazole/fluorene copolymers were
prepared via the standard Suzuki polycondensation reaction, as
previously reported [49]. A stirred mixture of compound 5 (0.13 g,
0.2 mmol), compound 7 (0.44, 0.8 mmol), compound 8 (0.64 g,
1 mmol), Pd(PPh3)4 (0.035 g, 3 mol %), an aqueous solution of K2CO3
(2 M, 12 mL), and anhydrous toluene (30 mL) was degassed with
nitrogen for 1 h. The mixture was heated to 90 ꢁC and stirred under
a nitrogen atmosphere for 48 h. After cooling to room temperature,
the whole mixture was poured into methanol, and the precipitated
polymer was filtered out. The crude product was then dissolved in
a small amount of THF and precipitated several times with meth-
anol. The polymer was further purified by a Soxhlet extraction in
acetone for 24 h. A pale yellow solid product PFCzPhB10 was
CDCl3):
d [ppm]: 2.08 (s, 12H), 2.31 (s, 6H), 6.85 (s, 4H), 7.28
(t, J ¼ 8.1 Hz, 2H), 7.40 (t, J ¼ 8.1 Hz, 2H), 7.49 (d, J ¼ 7.8 Hz, 2H),
7.56 (d, J ¼ 8.4 Hz, 2H), 7.73 (d, J ¼ 8.1 Hz, 2H), 8.12 (d, J ¼ 7.2
Hz, 2H). 13C NMR (75 MHz, CDCl3):
d [ppm]: 21.24, 23.51, 109.94,
120.22, 120.34, 123.66, 125.89, 125.98, 126.89, 127.2, 128.309,
137.95, 138.89, 140.37, 140.83. FABMS (m/z): calcd for C36H34BN:
491.2784. Found: 491.2786. Anal. calcd for C36H34BN: C, 87.98; H,
6.97; N, 2.85. Found: C, 87.57; H, 7.11; N, 2.38.
obtained in 61% yield (0.49 g). 1H NMR (600 MHz, CDCl3):
d [ppm]:
2.2.4. Synthesis of 9-(4-(5-(dimesitylboryl)thiophen-2-yl)phenyl)-
9H-carbazole (compound 4)
0.78e0.86 (br, 54H), 0.90e1.35 (br, 216H), 2.06e2.18 (br, 48H), 2.34
(s, 6H), 6.88 (s, 4H), 7.58e7.90 (m, 62H), 8.35e8.52 (br, 2H). Anal.
Compound 2 (3.25 g, 10 mmol), n-butyl lithium (4.4 mL,
11 mmol, 2.5 M in hexane) and dimesitylboron fluoride (3.0 g,
11 mmol) were allowed to react according to the procedure for
compound 3. A pale yellow solid of compound 4 was obtained
Found (%) for PFCzPhB10 [(C36H32NB)0.1(C29H40)0.9]: C, 89.55;
H, 9.85; N, 0.35. Found: C, 88.99; H, 10.84; N, 0.38.
2.3.2. Synthesis of PFCzPhB30
(3.44 g, Yield ¼ 60%). 1H NMR (600 MHz, CDCl3):
d
[ppm]: 8.15
With the same procedure as that described for the synthesis of
copolymer PFCzPhB10, compound 5 (0.39 g, 0.6 mmol), compound
7 (0.22 g, 0.4 mmol) and compound 8 (0.64 g, 1 mmol) were used to
prepare copolymer PFCzPhB30. A pale yellow solid was obtained in
(d, J ¼ 7.8 Hz, 2H), 7.90 (d, J ¼ 8.7 Hz, 2H), 7.54e7.59 (m, 3H),
7.47e7.49 (m, 5H), 7.41e7.45 (m, 2H), 6.86 (s, 4H), 2.32 (s, 6H), 2.18
(s, 12H). 13C NMR (75 MHz, CDCl3):
d [ppm]: 21.23, 23.48, 109.73,
120.12, 120.34, 123.49, 125.75, 126.01, 127.32, 127.52, 128.20, 133.11,
137.66, 138.61, 140.62, 140.86, 141.70, 155.73. HRMS (m/z): calcd for
C40H36BNS: 573.2662. Found: 573.2658. Anal. calcd for C40H36BNS:
C, 83.76; H, 6.33; N, 2.44. Found: C, 83.59; H, 6.77; N, 2.07.
63% yield (0.53 g). 1H NMR (600 MHz, CDCl3):
d [ppm]: 0.60e0.86
(br, 14H), 0.90e1.30 (br, 56H), 2.05e2.15 (br, 21H), 2.34 (s, 6H), 6.86
(s, 4H), 7.47e7.98 (br, 22H), 8.35e8.52 (br, 2H). Anal. Found (%) for
PFCzPhB30 [(C36H32NB)0.3(C29H40 0.7]: C, 89.3; H, 9.0; N, 1.0. Found:
)
C, 88.5; H, 9.56; N, 0.12.
2.2.5. Synthesis of 3,6-dibromo-9-(4-(dimesitylboryl)phenyl)-9H-
carbazole (compound 5, CzPhB)
2.3.3. Synthesis of PFCzPhB50
N-Bromosuccinimide (3.92 g, 22 mmol) was added portion wise
to a solution of compound 3 (4.91 g, 10.00 mmol) in THF (80 mL).
The mixture was stirred at room temperature under a nitrogen
atmosphere for 16 h. The whole mixture was then poured into
water, and the organic layer was separated and washed with water.
The organic extracts were dried over anhydrous Na2SO4, filtered,
and evaporated to yield a crude solid. The crude product was
further purified by column chromatography using silica gel and
eluting with hexane to afford a white solid, compound 5 (4.01 g,
With the same procedure as that described for the synthesis of
copolymer PFCzPhB10, compound 5 (0.65 g,1 mmol) and compound
8 (0.64 g, 1 mmol) were used to prepare copolymer PFCzPhB50. The
PFCzPhB50 polymer was obtained as a pale yellow solid after drying
under vacuum at 60 ꢁC overnight (0.45 g, yield ¼ 51%). 1H NMR
(600 MHz, CDCl3):
d [ppm]: 0.74e0.88 (br, 6H), 1.0e1.2 (br, 24H),
1.80e2.10 (br, 16H), 2.31e2.40 (d, 6H), 6.86 (s, 4H), 7.50e8.10
(m, 14H), 8.50e8.62 (br, 2H). Anal. Found (%) for PFCzPhB50
[C65H72NB]: C, 88.96; H, 8.2; N, 1.6. Found: C, 88.12; H, 9.52; N, 1.48.
yield ¼ 62%). 1H NMR (600 MHz, CDCl3):
d [ppm]: 2.08 (s, 12H), 2.32
(s, 6H), 6.86 (s, 4H), 7.32 (d, J ¼ 9.0 Hz, 2H), 7.31e7.48 (m, 4H), 7.74
2.3.4. Synthesis of PFCzPhThB10
(d, J ¼ 8.4 Hz, 2H), 8.15 (s, 2H). 13C NMR (75 MHz, CDCl3):
d
[ppm]:
With the same procedure as that described for the synthesis of
copolymer PFCzPhB10, compound 6 (0.15 g, 0.2 mmol), compound 7
(0.44 g, 0.8 mmol) and compound 8 (0.64 g, 1 mmol) were used to
prepare copolymer PFCzPhThB10. A paleyellowsolid wasobtainedin
21.26, 23.53, 111.65, 113.32, 123.24, 124.18, 125.70, 128.34, 129.42,
138.02, 139.04, 139.33, 140.78.FABMS (m/z): calcd for C36H32BBr2N:
647.0995. Found: 647.1002. Anal. calcd for C36H32BBr2N: C, 66.60; H,
4.97; N, 2.16. Found: C, 66.48; H, 5.17; N, 1.92.
66% yield (0.54 g).1H NMR (600 MHz, CDCl3):
d [ppm]: 0.77e0.88 (br,
54H), 0.95e1.27 (br, 216H), 2.05e2.18 (br, 48H), 2.38 (s, 6H), 6.83
(s, 4H), 7.31e7.48 (m, 2H), 7.57e7.88 (m, 62H), 8.30e8.60 (d, 2H).
2.2.6. Synthesis of 3,6-dibromo-9-(4-(5-(dimesitylboryl)thiophen-
2-yl)phenyl)-9H-carbazole (compound 6, CzPhThB)
Anal. Found (%) for PFCzPhThB10 [(C40H34NBS)0.1(C29H40 0.9]:
)
N-Bromosuccinimide (3.92 g, 22.0 mmol) and compound
4 (5.73 g, 10.0 mmol) were allowed to react according to the
procedure for compound 5. A pale yellow solid, compound
6 (CzPhThB) was obtained (4.08 g, Yield ¼ 56%). 1H NMR (600 MHz,
C, 88.92; H, 9.70; N, 0.34; S, 0.79. Found: C, 88.17; H, 9.69; N, 0.30;
S, 0.74.
2.3.5. Synthesis of PFCzPhThB30
CDCl3):
d
[ppm]: 2.17 (s, 12H), 2.32 (s, 6H), 6.86 (s, 4H), 7.24
With the same procedure as that described for the synthesis of
copolymer PFCzPhB10, compound 6 (0.44 g, 0.6 mmol), compound 7
(0.22 g, 0.4 mmol) and compound 8 (0.64 g, 1 mmol) were used to
prepare copolymer PFCzPhThB30. A pale yellow solid was obtained
(d, J ¼ 8.7 Hz, 2H), 7.44e7.48 (m, 4H), 7.50e7.54 (m, 3H), 7.86
(d, J ¼ 8.4 Hz, 2H), 8.16 (d, J ¼ 1.8 Hz, 2H). 13C NMR (75 MHz, CDCl3):
d
[ppm]: 21.24, 23.48, 111.42, 113.25, 123.24, 124.01, 125.95, 127.16,
127.68, 128.22, 129.47, 133.79, 136.55, 138.67, 139.57, 140.83, 141.05,
141.64, 150.29, 155.18. FABMS (m/z): calcd for C40H34BBr2NS:
729.0872. Found: 729.0887. Anal. calcd for C40H34BBr2NS: C, 65.69;
H, 4.69; N, 1.92. Found: C, 65.63; H, 5.08; N, 1.39.
in 65% yield (0.58 g). 1H NMR (600 MHz, CDCl3):
d [ppm]: 0.60e0.92
(br, 14H), 0.95e1.20 (br, 56H), 1.80e2.18 (br, 21H), 2.36 (s, 6H), 6.82
(s, 4H), 7.34e7.50 (m, 2H), 7.55e7.97 (m, 22H), 8.30e8.60 (br, 2H).
Anal. Found (%) for PFCzPhThB30 [(C40H34NBS)0.3(C29H40 0.7]:
)

980
Y.-H. Chen et al. / Polymer 52 (2011) 976e986
C, 87.53; H, 8.63; N, 0.95; S, 0.72. Found: C, 87.45; H, 8.60; N, 0.89; S,
0.68.
characteristics of the devices were measured on a programmable
electrometer with current and voltage sources (Keithley 2400) and
a Newport Optics 1835C luminance spectrophotometer.
2.3.6. Synthesis of PFCzPhThB50
With the same procedure as that described for the synthesis of
copolymer PFCzPhB10, compound 6 (0.73 g,1 mmol) and compound
8 (0.64 g, 1 mmol) were used to prepare copolymer PFCzPhThB50.
A pale yellow solid was obtained in 44% yield (0.42 g). 1H NMR
3. Results and discussion
3.1. Synthesis and Characterization
(600 MHz, CDCl3):
d
[ppm]: 0.60e0.92 (br, 6H), 0.92e1.25 (br, 24H),
Scheme 1 illustrates the synthetic routes of the monomer and
carbazole/fluorene copolymers. Compound 1 was synthesized from
carbazole and 1,4-dibromobenzene through an Ullmann coupling
reaction [48] and then further reacted with thiophen-2-ylboronic
acid via Suzuki coupling to obtain compound 2. Compound 1 and
compound 2 were then further reacted with n-BuLi in THF at
ꢀ78 ꢁC, followed by the addition of dimesitylboron fluoride to give
compounds 3 and 4, respectively. Compounds 5 (CzPhB) and 6
(CzPhThB) were obtained through bromination at the 3- and 6-
positions of carbazole compounds 3 and 4 with NBS in THF at room
temperature, respectively. A series of CzPhB/fluorene and CzPhThB/
fluorene copolymers (PFCzPhB and PFCzPhThB) were synthesized
via Suzuki coupling reaction of the appropriate diboronates and
dibromo compounds in toluene at 90 ꢁC for 48 h in the presence of
Pd(PPh3)4 and 2 M K2CO3. The chemical structures of the conju-
gated polymers were verified by 1H NMR spectroscopy, and the
spectra of PFCzPhB50 and PFCzPhThB50 are shown in Fig. 1. The
signals in the ranges of 0.79e2.27 ppm were assigned to the dihexyl
chains, whereas the peaks at 6.38e8.52 ppm were attributed to the
protons of the fluorene and carbazole rings. The absorption peak of
proton i was observed at 2.2e2.4 ppm, which corresponds to the
protons of methyl groups in the dimesitylboryl moiety. The char-
acteristic peak at around 0.6e0.9 ppm was assigned to the protons
of the methyl group in dihexyl chains of the fluorene unit.
According to the 1H NMR spectrum of PFCzPhB50, the integral
values of protons i and a are about 8.13 and 8.43, respectively. This
demonstrates that the molar percentage of the CzPhB unit was
about 50% for PFCzPhB50. In the case of PFCzPhThB50, the
integration values of peaks i and a are about 7.82 and 7.53,
respectively, meaning the amount of the CzPhThB unit was about
50% for PFCzPhThB50. The molar ratio of the carbazole derivative
and 9,9-dioctylfluorene in the copolymers according to 1H NMR
are summarized in Table 1. The chemical shifts and relative inten-
sities of the signals are in agreement with the proposed structures
for the carbazole/fluorene copolymers. The molar ratio of the
carbazole derivative and 9,9-dioctylfluorene in the copolymers
according to 1H NMR are summarized in Table 1. The chemical
shifts and relative intensities of the signals are in agreement with
the proposed structures for the carbazole/fluorene copolymers.
The number- and weight-average molecular weights (Mn and
Mw) of the carbazole/fluorene copolymers are summarized in Table
2. The Mn and Mw of copolymers PFCzPhB10-PFCzPhB50 were in
the range of 11.7e16.3 and 26.3e38.5 kg/mol, respectively, while
the Mn of the poly(9,9-dioctylfluorene) (POF) was 19.2 kg/mol and
the Mw for POF was 38.1 kg/mol. For copolymers PFCzPhThB10-
PFCzPhThB50, the Mn and Mw were in the range of 8.4e14.6 and
18.2e36.5 kg/mol, respectively. Generally, the thermal stability of
conjugated polymers plays an important role in the operating
stability of a PLED. Operational lifetime of a PLED is directly related
to the thermal stability of the LEP. Therefore, high Tg and Td are
important requisites for application of an LEP in outdoor displays.
TGA and DSC thermograms of the copolymers are shown in Fig. 2.
The temperatures at which five percent weight loss occurred for the
copolymers are summarized in Table 1. The Tds of PFCzPhB10-
PFCzPhB50 were found to be approximately 408e412 ꢁC, while the
copolymers PFCzPhThB10-PFCzPhThB50 had a Tds in the range of
340e425 ꢁC. Thermal stability is affected by the molecular weight
1.80e2.10 (br, 16H), 2.25e2.38 (br, 6H), 6.82 (s, 4H), 7.34e7.50 (br,
2H), 7.50e7.98 (m, 14H), 8.30e8.60 (br, 2H). Anal. Found (%) for
PFCzPhThB50 [C69H74NBS]: C, 86.36; H, 7.72; N, 1.46; S, 0.34. Found:
C, 86.31; H, 7.94; N, 1.36; S, 0.32.
2.4. Instruments
1H NMR spectra were recorded on a Bruker AMX-600 MHz spec-
trometer, and elemental analysis was carried out on an elemental
analyzer (Elementar Vario EL III). Mass spectra were carried out on
a Finnigan/Thermo Quest MAT mass spectrometer. Gel permeation
chromatography (GPC) measurements were performed on a Waters
chromatograph (Waters 1515 plus Autosampler) using two Waters
Styragel linear columns with polystyrene as a standard and THFas the
eluent. Glass transition temperatures (Tgs) were measured by differ-
ential scanning calorimetry (TechMax Instruments DSC 6220) in
a nitrogen atmosphere at a heating rate of 10 ꢁC/min. Thermo-
gravimetric analysis (TGA) of the polymers was measured under
a nitrogen atmosphere at a heating rate of 20 ꢁC/min by using
a Thermo-gravimetric analyzer (VersaTherm thermo-gravimetric
analyzer). The UVevis absorption and PL spectra were recorded on
a Shimadzu UV-1240 spectrophotometer and an Acton Research
Spectra Pro-150 spectrometer, respectively. Cyclic voltammetric (CV)
measurements were conducted on a CHI model611D with the use of
a three-electrode cell, in which an ITO sheet, a platinum wire and
silver/silver nitrate (Ag/Agþ) were used as the working electrode,
counter electrode and reference electrode, respectively. All electro-
chemical experiments were performed in deoxygenated acetonitrile
(CH3CN) solution with 0.1 M tetrabutylammonium perchlorate
(Bu4NCl6) as the electrolyte.
2.5. EL device fabrication and electro-optical characterization
The PLED structure in this study was ITO glass/hole-transporting
material (HTM)/copolymer/TPBI/LiF/Al. The ITO-coated glass, with
a sheet resistance of 8 U/sq, was purchased from Applied Film Corp.
Glass substrates with patterned ITO electrodes were washed well
and cleaned with an O2 plasma treatment. A thin film (60 nm) of
HTM poly(3,4-ethylenedioxythiophene) doped with poly(styr-
enesulfonate) (PEDOT:PSS, CH8000, Bayer) was formed on the ITO
layer of a glass substrate by the spin-casting method and dried at
130 ꢁC for 1 h. Light-emitting layers were then spin-coated
(1500 rpm) from the LEP solutions (10 mg/mL) onto the HTM layer
and dried at 80 ꢁC for 1 h in a glove box. LEP solutions were
prepared with 1,2-dichloroethane (DCE). The hole-blocking and
electron-transporting material, TPBI, was evaporated to a thickness
of 30 nm in a vacuum chamber. Finally, a thin LiF (1 nm) cathode
was thermally deposited onto the light-emitting layer, followed by
the deposition of Al (120 nm) metal as the top layer, in a high-
vacuum chamber. After the electrode deposition, the PLED was
transferred from the evaporation chamber to a glove box purged
with high-purity nitrogen gas to keep oxygen and moisture levels
below 1 ppm. The device was then encapsulated with glass covers
sealed with UV-cured epoxy glue in the glove box. The deposition
rate of the cathode was operated with a quartz thickness monitor
(FU-12CR). The EL spectra, luminescence, and current-voltage

Y.-H. Chen et al. / Polymer 52 (2011) 976e986
981
Fig. 1. 1H NMR spectra of (a) PFCzPhB50 and (b) PFCzPhThB50.
and the chemical structure of the conjugated polymer backbone.
The conjugated polymers with low molecular weight possess low
thermal stability. For example, PFCzPhThB50 has the lowest
molecular weight and showed the lowest Td of all of the polymers.
The Tg of the POF was about 100 ꢁC, while the Tgs values of CzPhB/
fluorene copolymers were around 83e92 ꢁC. For the CzPhThB/flu-
orene copolymers, the Tgs values were in the range of 105e117 ꢁC.
The results indicated that the CzPhThB/fluorene copolymers
showed a slightly higher Tg than the CzPhB/fluorene copolymers.
This can presumably be attributed to the more rigid molecular
segment of the CzPhThB/fluorene copolymers compared to those of
the CzPhB/fluorene copolymers. In addition, the solubilities of the
conjugated polymers in organic solvents are summarized in Table 2.
The carbazole/fluorene copolymers were completely soluble in
most organic solvent at room temperature, including toluene, THF,
DCE, chloroform (CHCl3), and chlorobenzene (CB). However, the
copolymers were only partially soluble in 1,4-dioxane and
dimethylacetamide at room temperature, but completely soluble
after a mild heating procedure.
3.2. Photo-physical properties
UVevis absorption and PL spectra of the carbazole/fluorene
copolymers in DCE (1 ꢂ 10ꢀ5 M) solution and thin films are shown
in Fig. 3. The photo-physical properties of the copolymers are
itemized in Table 3. All polymers exhibited only a distinct absorp-
tion band, which was attributed to the
of the conjugated polymer backbones. In dilute DCE solution, the
absorption maximum wavelengths lmax of the PFCzPhB10,
pep
* electronic transition
(
)
PFCzPhB30, and PFCzPhB50 were 376, 365 and 344 nm, respec-
tively, while PFCzPhThB10, PFCzPhThB30, and PFCzPhThB50 were
located at 378, 366, and 356 nm, respectively. Notably, with an
increase of carbazole-p-boron units in the polymer main chain, the
absorbance spectra of CzPhB/fluorene and the CzPhThB/fluorene

982
Y.-H. Chen et al. / Polymer 52 (2011) 976e986
Table 1
Table 2
Compositions, molecular weights, and thermal properties of the carbazole/fluorene
Solubility of the carbazole/fluorene copolymers in the organic solvents.
copolymers.
Toluene THFa DCEa CHCl3
CBa 1,4-dioxane DMAca
a
Copolymers
Copolymers
Cz-
p
-B
Cz-
p
-B
Mn
Mw
PDI
Td
Tg
b
b
POF
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ þꢀ
þþ þꢀ
þþ þþ
þþ þþ
þþ þꢀ
þþ þþ
þþ þþ
þꢀ
þꢀ
þꢀ
þꢀ
þꢀ
þꢀ
þꢀ
unit (%)a unit (%)b (ꢂ103) (ꢂ103) (Mn/Mw
)
(ꢁC) (ꢁC)
PFCzPhB10
PFCzPhB30
PFCzPhB50
POF
e
e
19.2
16.3
12.0
11.7
14.6
9.3
38.1
38.5
26.3
29.3
36.5
21.1
18.2
1.98
2.36
2.19
2.51
2.49
2.28
2.16
432 100
PFCzPhB10
PFCzPhB30
PFCzPhB50
PFCzPhThB10 10
PFCzPhThB30 30
PFCzPhThB50 50
10
30
50
12
29
50
11
31
50
408
412
409
425 105
420 108
340 117
92
83
86
PFCzPhThB10 þþ
PFCzPhThB30 þþ
PFCzPhThB50 þþ
a
Organic solvents: Tetrahydrofuran (THF), 1,2-dichloroethane (DCE), chloroform
(CHCl3), chlorobenzene (CB), and dimethylacetamide (DMAc).
8.4
a
b
Molar ratio of carbazole unit in feed monomer mixture.
The solubility was measured in the concentration of 1 mg/mL; þþ: soluble at
b
Molar ratio of carbazole unit calculated from 1H NMR spectra.
room temperature; þꢀ: partially soluble at room temperature.
copolymers exhibited a blue-shift, which was also found in
previous reports on the analogous fluorene-carbazole copolymers
[32]. Such a hypsochromic shift is probably attributable to inter-
results of the copolymers confirmed that changing the content of
the 3,6-linked carbazole units could effectively manipulate the
maximal absorption wavelength of the copolymer. In addition, the
maximum absorption values of the copolymers in a thin film were
red-shifted slightly compared to those of the copolymers in DCE
ruption of the
by the meta-linkage between the fluorene unit and the carbazole-
-boron unit. This would result in the conjugation length of the
p-conjugation along the polymer backbone caused
p
solution. These results were attributed to the interactions and
stacking between the polymer chains.
pep
polymer being shortened [20]. Additionally, the absorption spectra
of PFCzPhB10 and PFCzPhThB10 were similar to that of POF due to
When irradiated with UV light at 350 nm, the PL emission colors
of the copolymers in solution and as thin films ranged from blue to
green. In dilute DCE solution, the main emission peaks of copoly-
mers PFCzPhB10, PFCzPhB30, and PFCzPhB50 were 417, 470, and
474 nm, respectively. As shown in Fig. 3, the PL spectrum of the
PFCzPhB10 showed maximum correspondence to the 0e0 transi-
tion at around 420 nm, with a well defined vibronic feature (0e1
transition) at around 436 nm, which is almost identical to that of
POF [11]. On the other hand, PFCzPhB30 showed a sky-blue emis-
sion peak at 470 nm, with a shoulder emission peak at 418 nm,
while PFCzPhB50 demonstrated a featureless vibronic PL spectrum,
the minimal content of carbazole-p-boron units in the backbone.
However, as the content of the carbazole-
p-boron units increased
to 50%, the spectral characteristics of PFCzPhB50 (lmax ¼ 344 nm)
and PFCzPhThB50 (lmax ¼ 356 nm) were assigned to the combined
electronic contribution from the carbazole and fluorene segments,
which indicated that the electronic configurations of both units in
the copolymers were almost mixed. Similar absorption results were
previously reported for a 3,6-linked carbazole-fluorene/spiro-
fluorene alternating copolymer [32]. Therefore, the absorption
Fig. 3. UVevis and PL spectra of the carbazole/fluorene copolymers in (a) DCE solution
and (b) thin films.
Fig. 2. TGA (a) and DSC (b) thermograms of the copolymers.

Y.-H. Chen et al. / Polymer 52 (2011) 976e986
983
Table 3
reflecting their different electronic features. Accordingly, upon
Optical properties of the carbazole/fluorene copolymers.
increasing the content of the CzPhB moiety, the PL spectra of the
CzPhB/fluorene copolymers revealed a relatively intensive emis-
sion peak at around 470 nm in dilute DCE solutions. This result can
be explained as follows: partial ICT from fluorene segments to the
UV
max
UV
max
PL
max
PL
Copolymers
l
(nm)a
l
(nm)b
l
(nm)c
l
(nm)d
Ffl (%)e
max
POF
382
376
365
344
391
384
372
351
384
370
352
420, 442, 470 440, 460, 484 58
417, 439, 468 420, 436
418, 470
474
441, 466, 498 439, 462
PFCzPhB10
PFCzPhB30
PFCzPhB50
PFCzPhThB10 378
PFCzPhThB30 366
PFCzPhThB50 356
60
63
53
72
69
65
422, 435
431
carbazole-p-boron units occurred with an increasing content of
3,6-linked carbazole-
p
-boron units in the polymer main chain [50].
As a result, the relative intensity of the emission peak at 470 nm
was strikingly enhanced. Such ICT behavior had been observed
in other conjugated polymers with donoreacceptor segments [51].
In DCE solution, the main emission peaks of copolymers
PFCzPhThB10, PFCzPhThB30, and PFCzPhThB50 were at 440, 495,
and 503 nm, respectively. The PL emission maximum values of the
CzPhThB/fluorene copolymers exhibited a large red-shift compared
to those of the CzPhB/fluorene copolymers. This PL trend was also
observed for the copolymers in the thin film state. Such differences
in PL spectra between CzPhB/fluorene and the CzPhThB/fluorene
copolymers may be attributed to incorporation of the thiophene
moiety at the 9-position of carbazole, which not only enhances the
strength of the ICT effect via its high delocalization properties, but
also extend the side-chain conjugation length at the 9-position of
the carbazole [52]. On the other hand, thin films of CzPhB/fluorene
and CzPhThB/fluorene copolymers showed small red-shifts in their
419, 495
501
456
464
a
Maximal absorption wavelength of the copolymers in DCE.
Maximal absorption wavelength of the copolymers as solid film.
Maximal PL wavelength of the copolymers in DCE.
Maximal PL wavelength of the copolymers as solid film.
PL quantum efficiency (Ffl) of the copolymers in DCE.
b
c
d
e
order or packing density. An aggregation or excimer was likely
formed among the polymer chains, which led to the occurrence of
an inter-chain exciton emission at longer wavelengths. However,
a relatively lower intensity of green emission peak was observed for
the CzPhB/fluorene and CzPhThB/fluorene copolymers with higher
carbazole moiety contents. A greater PL stability was observed for
both the CzPhB/fluorene and CzPhThB/fluorene copolymers
compared to that of the POF homopolymer. This was attributed to
suppression of the polymer chain packing and aggregation due to
the presence of kink linkages between the fluorene unit and
PL maxima upon increasing the carbazole-p-boron unit content.
This could be due to less -stacking aggregation in the polymer
p
carbazole-p-boron unit and/or the presence of non-coplanar
backbone because of the meta-linkage of the carbazole and/or the
non-planar structures of the dimesitylboryl groups. Furthermore,
with increasing carbazole-p-boron unit content, PL maxima of the
dimesitylboryl groups as pendants [41,42]. Therefore, better PL
stability was obtained for the carbazole/fluorene copolymer with
higher carbazole units.
copolymers in DCE solution exhibited a largely red-shifted as
compared to those of the copolymers as thin films. This is due to the
solvatochromic effect of the copolymers in polar organic solvent
[47]. Lambert et al. reported that the PL spectra of N-p-(diarylboryl)
phenyl-substituted 3,6-linked polycarbazoles show significant
solvatochromic shifts in polar organic solvents with respect to the
PL spectra of the copolymers as thin films [47]. Therefore, the large
red-shift was possibly caused by the dipoleedipole interaction
between the polar solvent (DCE) and bipolar side chains of CzPhB/
fluorene and CzPhThB/fluorene copolymers.
The PL quantum efficiencies (Ffl) of the carbazole/fluorene
copolymers in DCE are summarized inTable 4. The Ffl values of these
copolymers were measured in dilute DCE solution by comparing
their emission with that of a standard solution of 9,10-diphenylan-
thracene in cyclohexane (Ffl ¼ 0.90) at room temperature. Accord-
ingly, the Ffl values of POF, PFCzPhB10, PFCzPhB30, and PFCzPhB50
were 58, 60, 63, and 53%, respectively, while the Ffl values of
PFCzPhThB10, PFCzPhThB30, and PFCzPhThB50 were 72, 69, and
65%, respectively. The CzPhB/fluorene and CzPhThB/fluorene
copolymers show higher Ffl values than the POF. This was attributed
to the dual fluorescence arising from a fluorescent delocalized state
of the POF backbone and from a fluorescent ICT between the
dimesitylboron and carbazole groups. However, the Ffl values of the
copolymers decreased upon increasing the carbazole derivative
content (CzPhB or CzPhThB) in the polymer main chain. Increasing
the high meta-linkage carbazole moiety content results in inter-
3.3. Electrochemical properties
CV was employed to investigate the electrochemical behavior
and to estimate the HOMO and LUMO energy levels of the copoly-
mers. The HOMO levels of the copolymers were calculated from the
ox
onset potential of oxidation (E
) byassuming the absolute energy
onset
level of ferrocene at ꢀ4.8 eV below the vacuum level. The LUMO
levels were calculated from the HOMO energy level and
the absorption edge [55,56]. The oxidation and reduction behaviors
of the copolymers are shown in Fig. 5. The CV curves of all copoly-
mers showed broad spectra with single quasi-reversible oxidation
and reduction waves. The electrochemical properties of the copol-
ymers are summarized in Table 4. The E
ox
values were observed at
onset
about 1.01, 0.84, and 0.78 V for PFCzPhB10, PFCzPhB30, and
PFCzPhB50, respectively. Moreover, the optical band gaps (Egs) of
copolymers PFCzPhB10, PFCzPhB30, and PFCzPhB50, determined
from the absorption edge, were 2.89, 2.91, and 2.96 eV, respectively.
Therefore, the LUMO levels of PFCzPhB10, PFCzPhB30, and
PFCzPhB50 were 2.82, 2.63, and 2.53 eV, respectively, while the
HOMO levels were 5.72, 5.55 and 5.49 eV. In addition, the values
ox
of
E
for PFCzPhThB10, PFCzPhThB30, and PFCzPhThB50
onset
were about 1.04, 0.87 and 0.85 V, respectively. Moreover, Egs of
copolymers PFCzPhThB10, PFCzPhThB30, and PFCzPhThB50 were
Table 4
ruption of the p-conjugation along the polymer backbone, and thus,
Electrochemical properties of the carbazole/fluorene copolymers.
the effective conjugation length of the polymer is shortened.
Therefore, the Ffl values of the copolymers decreased slightly with
increasing carbazole derivative content
The PL stability of the CzPhB/fluorene and CzPhThB/fluorene
copolymers as solid films was further investigated by thermal
annealing at 200 ꢁC for 1 h. As shown in Fig. 4, the annealed
polymer film exhibited a new broad green emission peak at about
528 nm, which was attributed the formation of low-energy excimer
aggregates at high temperature [53,54]. High thermal energy cau-
ses the chain conformation to be altered to a higher inter-chain
Copolymers
Eox (V)a
LUMO (eV)
HOMO (eV)
Egopt (eV)b
POF
1.08
1.01
0.84
0.78
1.04
0.87
0.85
ꢀ2.96
ꢀ2.82
ꢀ2.63
ꢀ2.53
ꢀ2.93
ꢀ2.78
ꢀ2.19
ꢀ5.78
ꢀ5.72
ꢀ5.55
ꢀ5.49
ꢀ5.75
ꢀ5.58
ꢀ5.56
2.82
2.89
2.91
2.96
2.82
2.80
2.77
PFCzPhB10
PFCzPhB30
PFCzPhB50
PFCzPhThB10
PFCzPhThB30
PFCzPhThB50
a
Onset oxidation potential versus Ag/AgNO3.
Estimated from the UVevis absorption edge.
b

984
Y.-H. Chen et al. / Polymer 52 (2011) 976e986
Fig. 6. EL spectra of the carbazole/fluorene copolymer-based devices.
Fig. 4. PL spectra of the carbazole/fluorene copolymers as thin films after being treated
at 200 ꢁC for 1 h.
attributed to improved coplanarity between the phenyl/thiophene-
linked carbazole moiety and polymer backbone. Apart from that,
higher LUMO and HOMO levels were observed for the carbazole/
fluorene copolymers compared to the POF homopolymer due to
incorporation of the electron-rich unit into the polymer backbone.
A higher HOMO level is favorable for hole-injection to the light-
emitting layer from an ITO transparent anode [20].
2.89, 2.91, and 2.96 eV, respectively. Hence, the LUMO levels of
PFCzPhThB10, PFCzPhThB30, and PFCzPhThB50 were 2.93, 2.78, and
2.79 eV, respectively, while the HOMO levels were 5.75, 5.78, and
5.56 eV. Using the same approach, the LUMO and HOMO levels of
POF were about 2.90 and 5.76 eV, respectively. The CV results indi-
ox
cated that the E
values of the copolymers decreased upon
onset
increasing the content of the electron-rich carbazole derivative.
ox
Moreover, with the same carbazole unit content, higher E
values
onset
3.4. EL properties of carbazole/fluorene copolymers-based devices
For the carbazole/fluorene copolymers, the HOMO levels
increased with increasing carbazole-p-boron unit content, and thus
facilitated hole-injection into the copolymer-based light-emitting
layer from the indium tin oxide anode. However, the LUMO energy
were observed for the CzPhThB/fluorene copolymers compared to
those of the CzPhB/fluorene copolymers. When compared to the
phenyl-linked CzPhB moiety, better conjugation of the phenyl/
thiophene linkage results in a poor electro-donating capacity of the
ox
CzPhThB moiety. Therefore, higher E
values were observed for
onset
the CzPhThB/fluorene copolymers. In addition, the Egs of CzPhB/
fluorene copolymers were increased by increasing the CzPhB unit
content, while the Egs of the CzPhThB/fluorene copolymers were
decreased upon increasing CzPhThB content. For the CzPhB/fluo-
rene-based copolymers, the meta/kink-linkage between the fluo-
levels were also enhanced upon increasing the carbazole-p-boron
unit content. A higher LUMO level will result in the poor electron
injection to the light-emitting layer from the cathode. Therefore,
multilayer devices were fabricated with the configuration of ITO/
PEDOT/copolymer/TPBI (30 nm)/LiF(1 nm)/Al (120 nm) in this
work. An electron injection/transporting layer of TPBI was inserted
into the light-emitting layer and cathode interface for better elec-
tron and hole charge balance. The EL spectra of the CzPhB/fluorene
and CzPhThB/fluorene copolymer-based devices are shown in
Fig. 6. The EL emission maximum wavelength, full width at half-
maximum (fwhm) of EL, and CIE coordinates of the carbazole/flu-
orene copolymer-based devices are summarized in Table 5. The EL
spectra showed a main emission peak at around 426e438 and
442e466 nm, respectively, for the CzPhB/fluorene and CzPhThB/
fluorene copolymer-based devices, without any shoulder emission
of an excimer or exciplex at around 500 nm. Not much difference
was observed for the maximal EL emission wavelengths of the
PLEDs in comparison with the maximal PL emission wavelengths of
rene unit and carbazole-p-boron units resists p-conjugation along
the polymer backbone [20]. Moreover, the presence of the dimesi-
tylboryl group reduced the coplanarity of the carbazole-based
pendant and polymer backbone. Therefore, the Egs of the copoly-
mers were increased with increasing CzPhB unit content. Never-
theless, the coplanarity between the pendant and backbone would
increase because of the enhancement of the effective conjugation
length and rigidity of the pendants. A decrease of the Egs of
CzPhThB/fluorene copolymers with increasing pendant content was
Table 5
Electroluminescence properties of the carbazole/fluorene copolymers-based
devices.
EL
max
Copolymers
l
fwhm Von (V)a Max
Max
CIE (x, y)b
(nm) (nm)
luminance efficiency
(cd/m2)
(cd/A)
POF
436
426
430
438
22
48
52
60
58
56
70
6.5
6.0
8.0
8.5
6.5
5.5
6.5
130
101
92
0.23
0.18
0.26
0.22
0.51
0.34
0.19
(0.17, 0.08)
(0.17, 0.09)
(0.16, 0.07)
(0.18, 0.13)
(0.16, 0.11)
(0.16, 0.13)
(0.21, 0.21)
PFCzPhB10
PFCzPhB30
PFCzPhB50
PFCzPhThB10 442
PFCzPhThB30 454
PFCzPhThB50 466
94
445
414
288
a
Turn-on voltage at 1 cd/m2.
CIE (x, y): Commission Internationale de L’Eclairage coordinates.
b
Fig. 5. CV spectra of the carbazole/fluorene copolymers in the thin film state.

Y.-H. Chen et al. / Polymer 52 (2011) 976e986
985
Fig. 7. Current density versus voltage of the carbazole/fluorene copolymer-based
devices.
Fig. 9. Current efficiency versus current density of the carbazole/fluorene copolymer-
based devices.
the copolymers in the solid state. Nevertheless, the fwhm values of
the EL emission peaks were increased upon increasing the carba-
zole-p-dimesitylborane content in the CzPhB/fluorene and
devices showed the better EL performances than the POF-based
devices, especially for the CzPhThB/fluorene copolymer-based
devices. The better EL performance of the carbazole/fluorene copol-
ymer-based devices was attributed to dual fluorescence arising from
CzPhThB/fluorene copolymer-based devices. This is because partial
emission was contributed from the light-emitting side-chain
conjugated pendants. The CIE coordinates of the PFCzPhB10-,
PFCzPhB30- and PFCzPhB50-based devices were (0.17, 0.09), (0.16,
0.07), and (0.18, 0.13), respectively, while the CIE coordinates of
PFCzPhThB10, PFCzPhThB30 and PFCzPhThB50 were (0.16, 0.11),
(0.16, 0.13), and (0.21, 0.21), respectively. High color purities of the
PLEDs indicated that these carbazole/fluorene copolymers are good
candidates for use as blue-emitting layers in PLEDs.
The EL properties of the CzPhB/fluorene and CzPhThB/fluorene
copolymer-based devices areshowninFigs. 7e9. The turn-on voltages
of the CzPhB/fluorene and CzPhThB/fluorene copolymer-based
devices were about 6.0e8.5 V and 5.5e6.5 V, respectively. The
maximum brightness of the CzPhB/fluorene and CzPhThB/fluorene
copolymer-based devices were about 92e101 cd/m2 and 288e445 cd/
m2, while the current efficiencies were about 0.18e0.26 cd/A and
0.19e0.51 cd/A, respectively. Moreover, the POF-based device showed
a maximum brightness of 130 cd/m2 and current efficiency of 0.23 cd/
A. The EL performances of POF-based device were poorer than those
reported in the literature [11,25]. This is attributed to the low molec-
ular weight of POF. Low molecular weight led to the poor thin film
quality and low brightness and small current efficiency of the POF-
based device [54]. Clearly, the carbazole/fluorene copolymer-based
the polymer backbone and the carbazole-p-boron unit. The reduction
of polymer chain aggregation via incorporation of the carbazole-
p-
boron unit into the polymer backbone is another reason for the better
EL performance of the devices. In addition, higher brightness and
larger current efficiency were observed for the CzPhThB/fluorene
copolymer-based devices compared to those of the CzPhB/fluorene
copolymer-based devices. This is because the phenyl/thiophene-
linked CzPhThB unit possesses a longer effective conjugation length
than the phenyl-linked CzPhB unit, and thus better EL performances
were observed with the CzPhThB/fluorene copolymer-based devices.
Apartfromthat, lowervaluesofbrightnessandcurrentefficiency were
observed for the devices fabricated from the CzPhThB/fluorene
copolymers with higher CzPhThB contents. Interruption of the
p-conjugation along the polymer backbone via the incorporation of
a high meta-linkage carbazole moiety content occurs in these cases,
and thus the EL performances of the PLEDs are reduced.
4. Conclusion
A series of novel carbazole/fluorene copolymers with dimesi-
tylboron pendants were synthesized by Suzuki coupling. Excellent
thermal stability was observed for the copolymers due to incorpo-
ration of the rigid carbazole-p-boron (CzPhB and CzPhThB)
pendants in the fluorene-based backbone. The CzPhB/fluorene and
CzPhThB/fluorene copolymers showed a higher PL quantum effi-
ciency than the POF. Moreover, higher brightness and larger current
efficiency were observed for the CzPhB/fluorene and CzPhThB/flu-
orene copolymer-based devices compared to the POF-based devices,
which was attributed to the dual fluorescence arising from
the polymer backbone and the carbazole-p-boron pendants. The
reduction of polymer chain aggregation via incorporation of the
rigid pendants into the polymer backbone is another reason for
the better EL performance of the carbazole/fluorene copolymer-
based devices. In addition, the phenyl/thiophene-linked CzPhThB
unit possesses a longer effective conjugation length than the
phenyl-linked CzPhB unit, and thus the CzPhThB/fluorene copol-
ymer-based devices had better EL performance than the CzPhB/
fluorene copolymer-based devices. It was concluded that the EL
properties of the copolymer-based devices were strongly dependant
on the effective conjugation length and content of the carbazole-
Fig. 8. Luminescence versus voltage of the carbazole/fluorene copolymer-based
devices (applied voltage: 11 V).
p-boron pendant.

986
Y.-H. Chen et al. / Polymer 52 (2011) 976e986
[23] Vak D, Jo J, Ghim J, Chun C, Lim B, Heeger AJ, Kim DY. Macromolecules
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