Novel fluorene/trifluoromethylphenylene copolymers: Synthesis, spectra stability and electroluminescence

Article abstract of DOI:10.1016/j.dyepig.2012.01.019

A group of novel fluorene-based copolymers were synthesized and characterized. The trifluoromethylphenylene unit was introduced into the copolymer backbone and its content ratio was varied from 10 mol% to 50 mol%. The electronic bandgap of the copolymer increases regularly with increasing trifluoromethylphenylene ratio. In contrast to pure polyfluorene, the deep-blue fluorescence of these copolymers is quite stable and not contaminated by the well-known green emission associated with fluorenone defects. The copolymers were used as an emitting layer to fabricate organic light-emitting diodes with a pure blue electroluminescence with a CIE coordinate y ≤ 0.10 obtained for most copolymers. Whilst the presence of the trifluoromethylphenylene units in the copolymers seemed unfavorable for charge injection and device current, improved spectral purity and stability in both the photoluminescence and electroluminescence were noted and ascribed to the electron-withdrawing nature of the trifluoromethylphenylene units and the enhanced anti-autoxidation ability of the fluorene rings in these copolymers.

Full text of DOI:10.1016/j.dyepig.2012.01.019

Dyes and Pigments 94 (2012) 380e385
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel fluorene/trifluoromethylphenylene copolymers: Synthesis, spectra
stability and electroluminescence
Ting Zhang 1, Lijun Deng a 1, Renjie Wang a , Wei Zhou , Jiuyan Li a
a
,
,
,
1
b
,
*
a
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
School of Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 November 2011
Received in revised form
A group of novel fluorene-based copolymers were synthesized and characterized. The tri-
fluoromethylphenylene unit was introduced into the copolymer backbone and its content ratio was
varied from 10 mol% to 50 mol%. The electronic bandgap of the copolymer increases regularly with
increasing trifluoromethylphenylene ratio. In contrast to pure polyfluorene, the deep-blue fluorescence
of these copolymers is quite stable and not contaminated by the well-known green emission associated
with fluorenone defects. The copolymers were used as an emitting layer to fabricate organic light-
emitting diodes with a pure blue electroluminescence with a CIE coordinate y ꢀ 0.10 obtained for
most copolymers. Whilst the presence of the trifluoromethylphenylene units in the copolymers seemed
unfavorable for charge injection and device current, improved spectral purity and stability in both the
photoluminescence and electroluminescence were noted and ascribed to the electron-withdrawing
nature of the trifluoromethylphenylene units and the enhanced anti-autoxidation ability of the fluo-
rene rings in these copolymers.
18 January 2012
Accepted 20 January 2012
Available online 28 January 2012
Keywords:
Fluorene
Trifluoromethylphenylene comonomer
Copolymer
Deep-blue
Spectra stability
Organic light-emitting diodes
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Accordingly, polyfluorenes were carefully prepared without mon-
oalkylfluorene defects [10] and some attempts were also taken to
Polyfluorene (PF)-type polymers have been established as
promising blue light-emitting materials for application in organic
light-emitting diodes (OLEDs). They possess many merits such as
high fluorescent quantum yields, easy syntheses and structure
modification, good solubility in common organic solvents and
acceptable charge mobility [1e3]. However, an undesired low-
energy “green” band covering a broad spectral range from 500 to
remove the generated monoalkylfluorene impurities [11].
Furthermore, various approaches have been exploited to increase
the anti-oxidation ability of the 9-position carbon in fluorene
moieties and thus to suppress the green emission. One example
was to construct a rigid backbone in the fluorene moieties to inhibit
the oxidation of the 9-position carbon since the oxidation of the 9-
3
2
position carbon involves hybrid state conversion from sp to sp
6
00 nm is frequently generated in both photoluminescence (PL)
and thus the change of bond angles. In this way Park et al. devel-
oped a stable blue emitting PF derivative with CIE coordinates of
(0.17, 0.12) [12]. Another strategy was to decrease the electron
density at 9-position of fluorene ring by introducing certain
electron-deficient groups in the backbone or sidechains. In our
previous report [13], the fluorene copolymers with di(tert-butyl)
phenoxysulfonyl side groups showed stable blue PL even after
thermal annealing in air and exhibited pure blue EL. The anti-
oxidation behavior and improved spectral stability of these PFs
were suggested to benefit from the strong electron-withdrawing
nature of the sulphonate side groups. Similarly, the strong
electron-deficient 2,8-dihexyldibenzothiophene-S,S-dioxide-3,7-
diyl unit was also used as the building block to synthesize pure
blue light-emitting PF derivatives. However, the EL efficiency of
and electroluminescence (EL) of PFs during operation, which not
only damages the blue color purity and stability but also limits the
emission efficiency [4].
The undesired green emission band has been attributed to
either aggregate [5], excimer [6], or keto defects [4b]. After the
intensive debate on the true origin of this emission band, it is more
acceptable that it stems from the emissive fluorenone defects of
PFs, which are generated by chemical oxidation of the mono-
alkylfluorene impurities at the 9-position carbon during polymer
preparation or operation in air for a prolonged period [4b,7e9].
*
Corresponding author. Fax: þ86 411 84986233.
E-mail address: jiuyanli@dlut.edu.cn (J. Li).
Fax: þ86 411 84986233.
ꢁ
1
those polymers was only 0.24 cd A with blue color coordinates of
(0.18, 0.15) [14].
1
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.01.019

T. Zhang et al. / Dyes and Pigments 94 (2012) 380e385
381
Witha better understandingof theorigin of the greenemission in
fluorene-based polymers, it is possible to choose more appropriate
electron-withdrawing groups to avoid the formation of the keto
defects to obtain a pure blue emission. For this purpose, both the
substituent type and its content ratio are evidently two important
factors to determine the performance of the polyfluorene
derivatives. In this paper, we report the synthesis and properties of
a series of novel fluorene-based copolymers containing tri-
fluoromethylphenylene comonomer on the backbone (Scheme 1).
The trifluoromethylphenylene unit was selected as an important
building block with the expectation that the electron-withdrawing
nature of trifluoromethyl groups tunes the electron density at the 9-
further purification. 1H NMR spectra were recorded on a Varian
INOVA spectrometer (400 MHz). Number-average (M ) and weight-
average (M ) molecular weights were determined by GPC method
using a viscotek TDA 302 instrument with THF as retention solvent
and monodisperse polystyrenes (PS) as standards. Elemental
analysis was performed on a CarloeEriba 1106 elemental analyzer.
The UVevis absorption and fluorescence spectra measurements
were recorded on a PerkineElmer Lambda 35 UVeVisible spec-
traphotometer and a PerkineElmer LS55 fluorescence spectrom-
eter, respectively. The fluorescence quantum yields were
n
w
determined against quinine sulfate as the standard (
0.1 N H SO ) [17].
F
¼ 0.55 in
2
4
position carbon of fluorene rings through the
nels and thus increases the anti-oxidation ability of the polymers
15]. Although a phenylene unit has been used together with fluo-
p-conjugation chan-
2.2. Device fabrication and measurements
[
rene to construct copolymers [16], this is the first report of fluorene-
based copolymers containing trifluoromethylphenylene units on
the backbone. The content ratio of the trifluoromethylphenylene
ꢁ1
) were treated by
The pre-cleaned ITO glass substrates (30
U
,
UV-Ozone for 20 min. PEDOT/PSS (Bayer AG) was spin-coated on
pre-treated ITO substrates from aqueous dispersion and baked at
unit was varied from 10 mol% (PF-10CF
3
) gradually to 50 mol% (PF-
ꢂ
ꢁ1
1
20 C for 1 h. Subsequently the polymer solutions (12 mg mL in
5
0CF ). For comparison, poly(9,9-dioctylfluorene) (PFO) was also
3
chlorobenzene) were filtered through 0.45 m PTFE filter and spin-
m
synthesized and studied under the same conditions. The PL spectral
stability of these polymers were investigated by monitoring the
emission spectra before and after thermal annealing in air. These
polymers were also used as the emitting layer in fabricated OLEDs.
The results show that, the energy band gaps of these copolymers
were regularly tuned with the content ratio of the tri-
fluoromethylphenylene unit and all of these copolymers emit effi-
cient deep-blue fluorescence. Both the PL and the EL spectra of these
copolymers were never contaminated by a green emission band,
evenafter the samples were thermallyannealed in air for 2 h (for PL).
Most copolymers exhibited stable deep-blue EL with CIE coordinate
y ꢀ 0.10 in their OLEDs. The luminance and efficiency of these new
OLEDs are better than or comparable with those reported poly-
fluorene derivatives [12,13,14]. Furthermore, the EL performance of
these copolymers depends on the ratio of the tri-
fluoromethylphenylene unit, the efficiency decreases with
increasing trifluoromethylphenylene ratio.
coated on PEDOT/PSS film. The thickness of polymer films was
controlled to 40 nm by adjusting the spin rate. The substrate was
transferred into a vacuum chamber to deposit the TPBI layer with
ꢁ6
a base pressure less than 10 torr. Finally, the device fabrication
was completed by thermal deposition of LiF (10 nm) and then
capping with Al (100 nm) cathode. The emitting area of each pixel is
2
determined by overlapping of the two electrodes(9 mm ). The EL
spectra, CIE coordinates, and current-voltage-luminance relation-
ships of devices were measured with computer-controlled Spec-
trascan PR 705 photometer and a Keithley 236 source-measure-
unit. All the measurements were carried out at room temperature.
2.3. Synthesis of polymers
The monomers 1 and 2 were synthesized according to the
literature methods [18], 3 was purchased from Alfa Aesar and used
without further purification. The following general procedure was
used for the preparation of all the polymers. To a 50 mL two-necked
flask was added the appropriate diboronate (1 equiv), the appro-
2
. Experimental
3 4
priate dibromide (1 equiv), Pd(PPh ) (0.005 equiv) and toluene
2
.1. Materials and methods
(15 mL). The flask equipped with a condenser was then evacuated
and filled with argon for three times to remove traces of air. The
mixture was stirred at 85 C for 5 min before an aqueous tetrae-
ꢂ
Chemicals, reagents and solvents from commercial sources are
of analytical or spectroscopy grade and used as received without
thylammonium hydroxide solution (20%, 4.3 equiv) was added. The
Scheme 1. Synthetic routes of polymers.

382
T. Zhang et al. / Dyes and Pigments 94 (2012) 380e385
ꢂ
mixture was then stirred for 24 h at 85 C under nitrogen. Bro-
mobenzene (1 equiv) was added to the mixture and stirred for 12 h.
Phenylboronic acid (1 equiv) was added to the mixture after which
the mixture was stirred for 12 h. The mixture was cooled to room
temperature and dropped by pipette into a stirred solution of
methanol (100 mL) in a beaker. The precipitate was isolated and
extracted for 12 h in a Soxlet apparatus with acetone, the resulting
polymers were collected and dried under reduced pressure.
PFO: Light yellow powder (yield: 81.5%). GPC (THF, PS stan-
polymers were end-capped with phenyl groups by adding bro-
mobenzene and phenylboronic acid successively upon cessation of
the polymerization. Monomers 1 and 2 were synthesized according
to literature methods as shown in Scheme 1 [18]. The copolymers
were obtained in high yield typically over 80% and have good
solubility in common organic solvents such as tetrahydrofuran,
dichloromethane and toluene. The number average molecular
weights (M ) of these copolymers were determined by gel-
n
permeation chromatography (GPC) using polystyrene (PS) stan-
dard and ranged from 22,360 to 9020. The corresponding poly-
ꢁ1
ꢁ1
1
dard): M
NMR (400 MHz, CDCl
.25e1.15 (m, CH ), 0.84e0.80 (m, CH
PF-50CF
standard): M
n
¼ 202,215 g mol , M
w
¼ 269,000 g mol , PDI ¼ 1.33; H
7.84e7.32 (m, ArH), 2.12 (br, CH ),
þCH );
3
, TMS):
d
2
dispersity indexes (PDI, PDI ¼ M
w n
/M ) were from 1.67 to 1.11 for the
1
2
2
3
new polymers. The actual ratio of 9,9-dioctylfluorene (DOF) to tri-
fluoromethylphenylene (TFMP) in the copolymers was calculated
from elemental analysis. The details of the calculation method are
shown in the Supporting information. A brief summary of these
parameters is listed in Table 1.
3
: White fibrous solid (yield: 93.0%). GPC (THF, PS
ꢁ1
ꢁ1
n
¼ 22,360 g mol , M
H NMR (400 MHz, CDCl , TMS):
.12 (br, CH ), 1.23e1.12 (m, CH
Elemental analysis found: C 79.30, H 8.00.
PF-33CF : White powder (yield: 83.3%). GPC (THF, PS standard):
¼ 10,573 g mol , M
400 MHz, CDCl , TMS):
CH ), 1.14 (m, CH ), 0.86e0.80 (m, CH
found: C 82.69, H 8.96.
PF-25CF : White powder (yield: 86.3%). GPC (THF, PS standard):
¼ 12,162 g mol , M
400 MHz, CDCl , TMS):
CH ), 1.14 (m, CH ), 0.83e0.80 (m, CH
found: C 85.57, H 9.58.
PF-20CF : Light yellow powder (yield: 90.3%). GPC (THF, PS
standard): M
NMR (400 MHz, CDCl
.12 (br, CH ), 1.20e1.14 (m, CH
Elemental analysis found: C 86.28, H 9.94.
PF-10CF : Light yellow powder (yield: 92.3%). GPC (THF, PS
standard): M
NMR (400 MHz, CDCl
.12 (br, CH ), 1.25e1.14 (m, CH
Elemental analysis found: C 87.33, H 10.09.
w
¼ 35,944 g mol , PDI ¼ 1.67;
1
3
d 8.08 (s, ArH), 7.89e7.40 (m, ArH),
2
2
2
), 0.86e0.78 (m, CH
2
þCH
3
);
3
3.2. Photophysical properties
ꢁ1
ꢁ1
1
M
n
w
¼ 12,065 g mol , PDI ¼ 1.14; H NMR
8.08 (s, ArH), 7.91e7.40 (m, ArH), 2.12 (br,
þCH ); Elemental analysis
(
3
d
The photophysical properties of these polymers were investi-
gated by means of electronic absorption and photoluminescence
(PL) spectra in dilute dichloromethane solutions and solid films on
quartz plates. As shown in Fig. 1a, all the copolymers exhibit
2
2
2
3
3
ꢁ
1
ꢁ1
1
M
n
w
¼ 14,392 g mol , PDI ¼ 1.18; H NMR
8.08 (s, ArH), 7.89e7.41 (m, ArH), 2.12 (br,
þCH ); Elemental analysis
structureless absorption with the peaks located from 345 to
*
(
3
d
381 nm, which are ascribed to the
pe
p
transition of the polymer
2
2
2
3
backbone. With gradually increasing the ratio of the tri-
fluoromethylphenylene unit from zero (PFO) to 50 mol% in PF-
3
50CF , the absorption spectrum reveals a general trend of a blue
3
ꢁ1
ꢁ1
1
n
¼ 10,508 g mol , M
, TMS): 8.08 (s, ArH), 7.90e7.41 (m, ArH),
), 0.83e0.80 (m, CH );
þCH
w
¼ 11,645 g mol , PDI ¼ 1.11; H
shift.
This
indicates
that
the
introduction
of
tri-
3
d
fluoromethylphenylene units reduces the p-electron conjugation
2
2
2
2
3
extent along the polymer backbone mainly due to the less copla-
narity of the adjacent fluorene and trifluoromethylphenylene rings
in comparison with the two fluorene rings in PFO, thus resulting in
a blue shift of absorption peaks and the increase of the highest
occupied molecular orbital (HOMO)-the lowest unoccupied
molecular orbital (LUMO) energy gap of the individual molecules
[19]. Upon photoexcitation at the absorption maximum, all these
polymers emit strong deep-blue fluorescence. As shown in Fig. 1b,
the fluorescence spectra of these polymers show three well-
resolved vibronic bands. For example, the three bands for PF-
3
ꢁ
1
ꢁ1
1
n
¼ 9020 g mol , M
, TMS): 8.08 (s, ArH), 7.89e7.41 (m, ArH),
), 0.83e0.80 (m, CH );
þCH
w
¼ 10,606 g mol , PDI ¼ 1.17; H
3
d
2
2
2
2
3
3
. Results and discussion
3.1. Synthesis
3
25CF are located at 415, 437, and 471 nm, and those for PFO are at
417, 440, and 475 nm. The three fluorescence bands of each poly-
The general synthetic routes used to access these fluorene-
mer can be ascribed to the electronic transitions from the lowest
vibronic level of the ground state (S0,0) to different vibronic levels of
the first singlet excited state (S1,0, S1,1, and S1,2). In consistence with
the absorption spectra, a blue shift is also observed in the PL spectra
of these polymers when increasing the ratio of tri-
fluoromethylphenylene units. A blue shift of 19 nm was detected for
based copolymers and the reference polymer PFO are outlined in
Scheme 1. The key step was a Suzuki polycoupling reaction of
monomer 2 (1 equiv), with 1 and 3 (totally 1 equiv) in the appro-
priate molar ratios ([3] ¼ 0, PFO; [1] ¼ 0, PF-50CF
PF-33CF ; [1]/[3] ¼ 1.5, PF-20CF
; [1]/[3] ¼ 1, PF-25CF
PF-10CF ). The reaction was performed in toluene solution with
Pd(PPh ] (0.005 equiv) as catalyst and aqueous tetraethy-
3
; [1]/[3] ¼ 0.5,
3
3
3
; [1]/[3] ¼ 4,
3
3
the emission peak from PFO to PF-50CF .
[
3
)
4
From the wavelength difference between the absorption
lammonium hydroxide (4.3 equiv.) as an emulsifying base. The
maximum and fluorescence maximum, the Stokes shifts of these
Table 1
Structural characterization, thermal and photophysical data of the polymers.
abs
em
F
(%)a
Polymer
Yield
%)
M
n
M
w
/M
n
DOF/TFMP
in feed
DOF/TFMP
in polymer
l
l
max
max
c
b
b
(
(nm)
(nm)
PFO
81.5
93
83.3
86.3
90.3
92.3
202,215
22,360
10,573
12,162
10,508
9020
1.33
1.67
1.14
1.18
1.11
1.17
/
/
387/384
345/345
361/361
370/371
376/377
381/382
417/432
398/404
413/420
414/422
415/424
416/424
0.63
0.73
0.72
0.63
0.67
0.71
PF-50CF
PF-33CF
PF-25CF
PF-20CF
PF-10CF
3
3
3
3
3
50/50
67/33
75/25
80/20
90/10
44/56
58/42
72/28
77/23
84/16
a
Measured in CH
2
Cl
2
solutions with quinine sulfate as the standard (
F
¼ 0.55 in 0.1 N H
2 4
SO ).
b
c
The data before the slash are for dilute dicholoromethane solutions, and the data after the slash are for solid films.
Calculation method of DOF/TFMP is shown in Supporting Information.

T. Zhang et al. / Dyes and Pigments 94 (2012) 380e385
383
a
a
1
0
.0
.8
PF-50CF3
PF-33CF3
PF-33CF3
Before
1
.0
50
100
150
0
.8
PF-25CF3
PF-20CF3
0.6
0
0
0
0
.6
.4
.2
PF-10CF3
PFO
0
0
0
.4
.2
.0
3
50
400
450
500
550
600
650
.0
2
50
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
b
1
.0
.8
.6
PFO
Before
b
1
.0
.8
.6
.4
.2
.0
PF-50CF3
PF-33CF3
PF-25CF3
PF-20CF3
50
0
0
1
1
00
50
0
0
0
0
0
PF-10CF3
0.4
PFO
0
0
.2
.0
3
50
400
450
500
550
600
650
Wavelength (nm)
3
50
400
450
500
550
600
650
Wavelength (nm)
3
Fig. 2. Fluorescence spectra of (a) PF-33CF and (b) PFO films before and after thermal
annealing in air at various temperatures.
Fig. 1. Absorption (a) and fluorescence (b) spectra of polymers in dilute dichloro-
methane solutions.
analogs before and after annealing are provided in Fig. S1 in the
supporting information. It is obvious that the PL spectra of copol-
ymers after thermal annealing are almost identical to those before
treatment except for the discernable spectral position change that
is probably due to physical change, either instability of measuring
environment or aggregation effect in thin films upon thermal
treatment [13,20]. The absence of any new emission band in the
copolymer films excludes the formation of any new emissive
copolymers were determined in the range of 35 nme53 nm, which
are lightly larger than that of PFO (30 nm). A larger Stokes shift is
usually desired for a light-emitting material since it means
a weaker self-absorption and more efficient light output in OLEDs.
The fluorescence quantum yields of these copolymers were
measured in dilute dichloromethane solutions using quinine
sulfate (
ymers showed high PL quantum yields (
which are either comparable with or slightly higher than that of
PFO (
F
¼ 0.55 in 0.1 N H
2
SO
4
) as a reference [17]. These copol-
ꢂ
species, even after annealing at such a high temperature of 150 C
for 2 h. However, under the same conditions, the PFO film exhibited
F
¼ 0.63e0.73), most of
the well-established green emission band from 500 to 600 nm after
F
¼ 0.63). The high fluorescence quantum yields of the
ꢂ
annealing at and over 100 C. Furthermore, the intensity of the
present copolymers indicate that they may be promising blue light-
emitting materials for application in OLEDs.
green band became higher with increasing temperature. It is
obvious that the introduction of the trifluoromethylphenylene
units to the copolymer main chain prevented the formation of the
green emission band and improved the color purity and stability of
their blue fluorescence.
3.3. Spectral stability
In order to evaluate the spectral stability of the copolymers,
copolymer films were made by spin-coating chlorobenzene solu-
tions onto quartz plates and then thermal annealing in air at
various temperatures for 2 h. After the films were cooled to the
ambient temperature, PL spectra were measured. PFO films were
also treated and tested under same conditions for comparison.
3.4. Electroluminescence properties of OLEDs
Based on their pure blue fluorescence and relatively high
quantum yields, the copolymers were used as the emitting layer to
fabricate OLEDs with a configuration of indium tin oxide (ITO)/
PEDOT:PSS (40 nm)/polymers (40 nm)/TPBI (20 nm)/LiF (1 nm)/Al
(200 nm). The fresh PFO sample was also used to fabricate OLEDs
3
Fig. 2 shows the fluorescence spectra of PF-33CF as an example
and PFO films before and after annealing. The PL spectra of other

384
T. Zhang et al. / Dyes and Pigments 94 (2012) 380e385
3
00
250
00
150
00
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
PF-50CF3
PF-50CF3
PF-33CF3
PF-25CF3
PF-20CF3
PF-10CF3
PF-33CF3
PF-25CF3
2
PF-20CF3
PF-10CF3
1
5
0
0
0
2
4
6
8
10 12 14 16 18
400
500
600
700
Voltage (V)
Wavelength (nm)
Fig. 4. The current density-voltage characteristics for the polymers based OLEDs.
Fig. 3. EL spectra for the copolymers based OLEDs.
but not in PL of the fresh PF films. There have been similar reports
that the green emission from the fluorenone is stronger in EL than
in PL [9,21]. This is because the low-energy keto defects can be
excited not only by energy transfer from the fluorene moieties but
also by direct charge trapping in the EL device made of PFO, while
only by energy transfer in PL of PFO films. Evidently the presence of
these trifluoromethylphenylene units in these copolymers also
guaranteed the deep-blue color purity and stability of their EL,
which is usually more sensitive to the purity of light-emitting
material than PL. It is suggested that the improved spectral purity
and stability observed in both PL and EL of these blue light-emitting
copolymers benefit from the presence of trifluoromethylphenylene
units which take away electron density from the 9-position carbon
and investigated for comparison. In these devices, PEDOT:PSS
poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)) acts as
(
hole injection layer (HIL), TPBI (1,3,5-tris(phenyl-2-benzimidazoyl)
benzene) as the electron-transporting (ETL) and hole-blocking
layer (HBL). The chemical structures of PEDOT:PSS and TPBI are
shown in Fig. S2 in the supporting information. The copolymer
based devices emitted bright deep-blue light with emission peaks
at around 422 nm, as shown by the EL spectra in Fig. 3. The EL
spectra of copolymers are similar to the PL of films with almost
identical spectral profiles and peak wavelengths. For copolymers
with CF
spectra are only 46, 43, 44, and 45 nm for PF-10CF
5CF , and PF-33CF , respectively. As shown by the data in Table 2,
the copolymers with CF
3
ratio ꢀ 33%, the full width at half maximum of their EL
3
, PF-20CF , PF-
3
2
3
3
through
p-conjugation channels due to its appropriate electron-
3
ratio ꢀ 33% exhibited excellent CIE coor-
withdrawing nature. There have been similar reports to enhance
the anti-autoxidation ability and consequent the spectral stability
of fluorene-based polymers by incorporating electron-deficient
groups in either the main chain or side chain [13,22].
dinate y ꢀ 0.1, which is quite close to the pure blue color coordinate
y value of 0.08. Moreover, the EL spectra and the CIE coordinates of
the copolymers are insensitive to the driving voltage, indicating
a stable blue emission. It should be noted that the EL spectrum of
The current density-voltage (JeV) characteristics of these
PF-50CF
3
device is broader, especially in the long-wavelength
copolymers based OLEDs are illustrated in Fig. 4. The PF-10CF
3
)
region, than its PL spectrum and the EL of other analogs, resulting
in the CIE coordinates of (0.176,0.142). It is evident that the EL
spectra of all these copolymers with trifluoromethylphenylene
units in the backbones are not contaminated by a green emission
band even at high driving voltages. However, the EL spectra of the
PFO device (Fig. S3 in the supporting information) always consists
of the blue parts at 438, 466 and 494 nm, which correspond to the
fine-structured bands in PL of the fresh PFO films, and a broad
green component with a peak at 520 nm, even at a low driving
voltage of 5 V. The intensity of the green emission band increased
with the driving voltage. It should be noted that the two long-
wavelength blue bands also ascended along with the green band.
The green emission could be observed in EL of the fresh PFO device,
ꢁ
2
device has a low turn-on voltage (to deliver a brightness of 1 cd m
of 3.8 V, the turn-on voltage increased gradually to 6.2 V for the PF-
3
3CF containing device, and further increased to 10 V for PF-50CF .
3
3
The J-V curves of these OLEDs exhibited a regular variation in spite of
the identical device configuration. With gradual increase of the
content ratio of trifluoromethylphenylene units in the copolymer,
the turn-on voltage increased gradually and the current density at
a given voltage of the corresponding OLED decreased accordingly,
which implies a charge injection limited behavior for the copolymer
based OLEDs. This is probably because the highest occupied
molecularorbital (HOMO) shifts to a lower level with introduction of
the trifluoromethylphenylene units in the copolymer backbone and
thus the hole injection barrier at PEDOT:PSS/copolymer interface
is increased and the higher the content ratio of the tri-
fluoromethylphenylene units, the lower the HOMO level of the
corresponding copolymer. As a result, the turn-on voltage is regu-
larly increased with the trifluoromethylphenylene units. The
copolymer devices exhibited maximum luminance ranging from
Table 2
EL performance of the polymers based OLEDs.
ꢁ2
ext (%)b
Polymer
V
on (V)a
4.8
10
L (cd m )b
h
l
max (nm)
CIE (x, y)
PFO
452 (14 V)
155 (16 V)
200 (14 V)
167 (13 V)
158 (10 V)
130 (8 V)
0.53 (6 V)
0.082 (14 V)
0.11 (8 V)
0.13 (6 V)
0.15 (5 V)
0.24 (4.5 V)
438
422
420
422
424
424
0.179,0.210
0.176,0.142
0.168,0.090
0.166,0.098
0.168,0.102
0.164,0.106
PF-50CF
PF-33CF
PF-25CF
PF-20CF
PF-10CF
3
3
3
3
3
ꢁ2
6.2
5
4.1
3.8
1
30 to 200 cd m , and forward viewingexternal quantum efficiency
from 0.08% to 0.24%, which are comparable with or better than the
data for some polyfluorene derivatives reported in the literature
[
12e14]. In comparison with other analogs, the worse performance
for the PF-50CF device, including the broader EL spectra, the lower
maximum brightness even at higher voltage, and the lower
a
ꢁ2
Turn-on voltage, recorded at 1 cd m
.
b
3
Maximum values of the devices. The data in the parentheses are the voltages at
which the data are recorded.

T. Zhang et al. / Dyes and Pigments 94 (2012) 380e385
385
efficiency, is most likely caused by the less efficient charge injection
and the consequent unbalanced charges in the emitting layer. The EL
performance data for all these copolymers and PFO are summarized
inTable 2. Although the luminance and efficiencies of the copolymer
devices are lower than those of PFO, the copolymers are advanta-
geous in possessing better color coordinates that are more close to
those (0.14, 0.08) of the standard blue color. The present work
provides an effective strategy to obtain stable deep-blue emitting
fluorene-based copolymers by introducing suitable electron-
deficient groups in the polymer backbone.
[3] Ego C, Grimsdale AC, Uckert F, Yu G, Srdanov G, Müllen K. Triphenylamine-
substituted polyfluorene-a stable blue-emitter with improved charge injec-
tion for light-emitting diodes. Adv Mater 2002;14:809e11.
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4] (a) Setayesh S, Grimsdale AC, Weil T, Enkelmann V, Müllen K, Meghdadi F,
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[
[
[
[
4
. Conclusions
In conclusion, a group of fluorene-based copolymers containing
various ratios of trifluoromethylphenylene units in the backbone
were synthesized and characterized as deep-blue light-emitting
materials. In comparison with pure polyfluorene, the introduction
of trifluoromethylphenylene units in these copolymers not only
increased the fluorescence quantum yields, but also improved the
spectral purity and stability of the blue fluorescence. The well-
established green emission band at 520 nm from fluorenone
defects was never detected for these copolymers even when they
[
10] Cho SY, Grimsdale AC, Jones DJ, Watkins SE, Holmes AB. Polyfluorenes without
monoalkylfluorene defects. J Am Chem Soc 2007;129:11910e1.
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color purity and stability in a blue emitting polyfluorene by monomer puri-
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12] Park SH, Jin YG, Kim JY, Kim SH, Kim JW, Suh HS, et al. A blue-light-emitting
polymer with a rigid backbone for enhanced color stability. Adv Funct Mater
2007;17:3063e8.
[
ꢂ
were thermal annealed in air at 150 C. The EL devices fabricated by
[
these copolymers exhibit stable deep-blue emission with CIE
coordinate y ꢀ 0.10. The adequate electron-withdrawing nature of
the trifluoromethylphenylene units is suggested to be, at least
partially, responsible for the enhanced anti-autoxidation ability and
improved spectral purity and stability for these copolymers. The
present study provides a promising molecular design strategy to
tune the anti-autoxidation ability and consequent spectral purity of
light-emitting materials by incorporating electron-deficient groups
such as trifluoromethylphenylene.
[13] Li JY, Ziegler A, Wegner G. Substituent effect to prevent autoxidation and
improve spectral stability in blue light-emitting polyfluorenes. Chem Eur J
2005;11:4450e7.
[
14] Kamtekar KT, Vaughan HL, Lyons BP, Monkman AP, Pandya SU, Bryce MR.
Synthesis and spectroscopy of poly(9,9-dioctylfluorene-2,7-diyl-co-2,8-
dihexyldibenzothiophene-S, S-dioxide-3,7-diyl)s: solution-processable, deep-
blue emitters with a high triplet energy. Macromolecules 2010;43:4481e8.
15] Kameshima H, Nemoto N, Endo T. Synthesis and properties of fluorene-based
fluorinated polymers. J Polym Sci Part A Polym Chem 2001;39:3143e50.
16] Zeng G, Yu WL, Chua SJ, Huang W. Spectral and thermal spectral stability
study for fluorene-based conjugated polymers. Macromolecules 2002;35:
6907e14.
[
[
Acknowledgments
0
[
17] Si ZJ, Shao Y, Li CX, Liu Q. Synthesis and fluorescence study of sodium-2-(4 -
0
0
dimethyl-aminocinnamicacyl)-3,3-(1 ,3 -alkylenedithio) acrylate.
007;124:365e9.
J Lumin
We thank the National Natural Science Foundation of China
2
(
20923006, and 21072026) and the Ministry of Education for the
[
[
18] Jo J, Chi CY, Höger S, Weger G, Yoon DY. Synthesis and characterization of
monodisperse oligofluorenes. Chem Eur J 2004;10:2681e8.
19] Wand R, Wand WZ, Yand GZ, Liu TX, Yu JS, Jiang YD. Synthesis and charac-
terization of highly stable blue-light-emitting hyperbranched conjugated
polymers. J Polym Sci Part A: Polym Chem 2008;46:790e802.
New Century Excellent Talents in University (Grant NCET-08-0074)
for financial support of this work.
Appendix. Supplementary data
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doped blue organic light-emitting diodes based on fluorene derivatives with
high thermal stability. Adv Funct Mater 2005;15:1716e21;
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.dyepig.2012.01.019.
(b) Lenhard JR, Hein BR. Effects of J-aggregation on the redox levels of
a cyanine dye. J Phys Chem 1996;100:17287e96.
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21] (a) Lupton JM. On-chain defect emission in conjugated polymers-comment on
exciton dissociation dynamics in a conjugated polymer containing aggregate
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