Synthesis and optoelectronic properties of luminescent copolyfluorenes slightly doped with thiophene chromophore

Article abstract of DOI:10.1016/j.polymer.2010.02.012

This paper describes the synthesis of new copolyfluorenes (P05-P5) slightly doped with 2,5-bis(2-phenyl-2-cyanovinyl)thiophene (GM, <3.4?mol%) and their application in electroluminescent (EL) devices. In film state, EL spectra of the copolyfluorenes are v

Full text of DOI:10.1016/j.polymer.2010.02.012

Polymer 51 (2010) 1555e1562
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Polymer
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Synthesis and optoelectronic properties of luminescent copolyfluorenes slightly
doped with thiophene chromophore
*
Wen-Fen Su, Tsui-Ting Chen, Yun Chen
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper describes the synthesis of new copolyfluorenes (P05eP5) slightly doped with 2,5-bis(2-
phenyl-2-cyanovinyl)thiophene (GM, <3.4 mol%) and their application in electroluminescent (EL)
devices. In film state, EL spectra of the copolyfluorenes are very different from photoluminescence (PL)
spectra, which have been ascribed to charge trapping in GM and energy transfer from fluorene segments
to GM chromophores. The maximum brightness and current efficiency of EL device from P05 (5230 cd/
m2, 0.65 cd/A) are significantly enhanced when compared with those from poly(9,9-dihexylfluorene)
(PF) (1310 cd/m2, 0.18 cd/A). The EL device using blend of P5 and PF (w/w ¼ 10/1) as emitting layer
exhibits near-white emission with CIE coordinate being (0.26, 0.32). The results demonstrate that the
copolyfluorenes slightly doped with GM chromophore are promising emitting materials for optoelec-
tronic devices.
Received 2 December 2009
Received in revised form
3 February 2010
Accepted 5 February 2010
Available online 12 February 2010
Keywords:
Copolyfluorenes
Electroluminescence
Charge trapping
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
polyfluorene backbone to adjust band-gap and to tune emission
color [12,13,16e23]. The
p-excessive thiophene ring reduced
In the last two decades, the synthesis of light-emitting polymers
have received intense attention after the discovery of electrolu-
minescence (EL) in conjugated polymers and the development of
polymer-light-emitting diodes (PLEDs) as display and lighting
applications [1e3]. A lot of conjugated polymers, such as poly(p-
phenylenevinylene) (PPV) [1], polythiophene (PT) [4], poly(p-phe-
nylene) (PPP) [5], and polyfluorene (PF) [6], and their derivatives
have been extensively investigated. Polythiophene (PT) and its
derivatives are promising candidates for electrically conductive and
optoelectronic materials due to their narrow band-gap, high
stability, high electrical conductivity, and attractive electronic and
optical properties [4,7e9]. Nevertheless, relatively low photo-
luminescence (PL) quantum efficiency of PT in the solid state [10],
typically 1e3% [11], has limited their applications in PLEDs. Hence,
PT with modified backbone structures by introducing phenylene
[12] and fluorene [13] groups were developed. It was reported that
for a dioctyl-phenyl substituted polythiophene derivative the PL
efficiency can be improved to 24% [11,14]; Liu et al. reported a series
of fluoreneethiophene copolymers showing PL efficiency in the
range of 23e40% [13]. Pei et al. increased the quantum efficiency of
polythiophene by incorporating fluorene moiety into backbone
[15]. Furthermore, the thiophene units had been incorporated into
oxidation potential of the fluorene moiety [24], which in turn raised
HOMO level of the copolyfluorenes to promote hole-injection. Cho
et al. further incorporated cyano group (CN) to prepare a series
copolyfluorenes containing red-emitting chromophore e 2,5-bis(2-
(50-bromothienyl)-1-cyanovinyl)-1-(200-ethylhexyloxy)-4-methoxy-
benzene [25,26], and found that substitution of thiophene by
phenylene resulted in improved EL device performance.
Polyfluorene (PF) and its derivatives are attractive blue-emitting
materials for PLEDs application because of their thermal, chemical
stability and high photoluminescence (PL) efficiency in solution
and solid states [27e30]. However, PF shows imbalance in charges
(electron and hole) injection and mobility when utilized as emit-
ting materials. This characteristic leads to low efficiency in its
polymeric light-emitting diodes (PLEDs), and because of this,
numerous attempts have been made to improve their performance
[16,21,31]. Meanwhile, the PFs with large band-gap can be used as
host material to blend with low band-gap chromophores.
Furthermore, fluorene can copolymerize with chromophores
having different band-gaps to obtain various emission colors. For
example, red-light emissions with CIE at (0.63, 0.37) were obtained
by copolymerizing fluorene with narrow band-gap monomer [31].
In this investigation, we designed a series of copolyfluorenes
containing a newlowband-gap chromophore e 2,5-bis(2-phenyl-2-
cyanovinyl)thiophene (GM). The chromophore contains only one
thiophene core that is attached with two 2-phenyl-2-cyanovinyl
groups. By this way, the content of heavy-atom (sulfur) can be
* Corresponding author. Tel.: þ886 6 2085843; fax: þ886 6 2344496.
E-mail address: yunchen@mail.ncku.edu.tw (Y. Chen).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.02.012

1556
W.-F. Su et al. / Polymer 51 (2010) 1555e1562
reduced to enhance emitting performance. The resulting copoly-
fluorene (P05eP5) are slightly doped with GM chromophores
(<3.4 mol%). The copolymers exhibited yellowish-green electrolu-
minescence (EL) due tooverwhelmingenergy transferfromfluorene
segments to GM chromophores when its content was 2.3 mol%. The
performances of EL devices using the copolyfluorenes as active layer
surpassed that fabricated from PF. Doped GM chromophores also act
as charge trapping sites to promote the recombination of holes and
electrons, which is an essential step in EL processes. Besides, near-
white-light emission is obtainable by blending of the copoly-
fluorenes with poly(9,9-dihexylfluorene) (PF).
dissolution of the solids, it was added dropwise with an ethanol
solution of sodium ethoxide (21%) and allowed to react for 12 h. The
appearing precipitates were collected by filtration, washed
successively with distilled water and hydrochloric acid. The solids
were recrystallized from ethyl acetate and n-hexane to give orange
solids of GM. The yield was 63% (mp ¼ 169e170 ꢀC). FT-IR (KBr
pellet, cmꢁ1):
n
532, 685, 760, 809, 1236, 1448, 1498, 1587, 2210
7.38e7.47
(m, 6H, AreH), 7.61 (s, 2H, AreH), 7.66e7.68 (d, 4H, AreH, J ¼ 8 Hz),
7.80 (s, 2H, eCHe). 13C NMR (CDCl3, 400 MHz, ppm):
110.64,
(eC^N), 3035, 3057. 1H NMR (CDCl3, 400 MHz, ppm):
d
d
117.84, 125.83, 128.98, 129.21, 129.51, 129.77, 130.32, 131.94, 132.49,
133.43, 133.86, 136.44, 140.97. ELEM. ANAL. Calcd. for C22H14N2S (%):
C, 78.08; H, 4.17; N, 8.28; S, 9.47. Found: C, 78.00; H, 4.14; N, 8.27; S,
9.50.
2. Materials and methods
2.1. Materials and characterization
2.2.2. 2,5-Bis[2-(40-bromophenyl)-2-cyanovinyl]thiophene (M1)
The synthetic procedures were analogous to those employed in
the preparation of GM with the exception of using 4-bromophe-
nylacetonitrile (3) instead of phenylacetonitrile (1), and the product
was orange solid. The yield was 72% (mp ¼ 225e226 ꢀC). FT-IR (KBr
4-Bromophenylacetonitrile (Lancaster Co.), phenylacetonitrile
(TCI Co.), 9,9-dihexyl-2,7-dibromofluorene (Aldrich Co.), 9,9-dihex-
ylfluorene-2,7-bis(trimethyleneborate) (Aldrich Co.), Aliquat 336
(Alfa Aesar Co.), tetrakis(triphenylphosphine)palladium [Pd(PPh3)4]
(Acros Co.), methanol (ECHO Co.), toluene (Tedia Co.), chloroform
(CHCl3, Tedia Co.) and other solvents were HPLC grade reagents. All
reagents and solvents were used without further purification. All
compounds were identified by 1H NMR, 13C NMR, Fourier transform
infrared (FT-IR), and elemental analysis (EA). NMR spectra were
obtained on a Bruker AVANCE-400 NMR spectrometer with chemical
shifts reported in ppm using tetramethylsilane (TMS) as an internal
standard. The FT-IR spectrawere measured asKBrdisk using a Fourier
transform infrared spectrometer, model 7850 from Jasco. The
elemental analysis was carried out on a Heraus CHN-Rapid elemental
analyzer. The molecular weight and molecular weight distribution of
the copolymersweredeterminedbyagelpermeationchromatograph
(GPC) using chloroform (CHCl3) as eluent. The thermogravimetric
analysis (TGA) of the polymers was performed under nitrogen
atmosphere at a heating rate of 10 ꢀC/min using a PerkinElmer TGA-7
thermal analyzer. Thermalproperties of the polymers were measured
using a differential scanning calorimeter (DSC), Mettler Toledo DSC 1
Star System, under nitrogen atmosphere at a heating rate of 10 ꢀC/
min. Absorption spectra were measured with a Jasco V-550 spectro-
photometer and the photoluminescence (PL) spectra were obtained
using a Hitachi F-4500 fluorescence spectrophotometer. The cyclic
voltammograms of thepolymersweremeasuredwithavoltammetric
apparatus (model CV-50W from BAS) equipped with a three-elec-
trode cell. The cell was made up of a polymer-coated glassy carbon as
the working electrode, an Ag/AgCl electrode as the reference elec-
trode, and a platinum wire as the auxiliary electrode, immersing in
acetonitrile containing 0.1 M (n-Bu)4NClO4. The energy levels were
calculated using the ferrocene (FOC) value of ꢁ4.8 eV with respect to
vacuum level, which is defined as zero [32]. Morphology image of the
polymers' film were captured using atomic force microscope, Veeco/
Digital Instrument Scanning Probe Microscope (tapping mode) with
Nanoscope IIIa controller. The polymer solution (12 mg/mL in chlo-
robenzene) was prepared and filtered with a syringe filter (0.22-ìm
pore)to remove possibleparticle contaminants, and then spin-coated
on the top of cleaned ITO glass.
pellet, cmꢁ1):
n
76, 538, 823, 1072, 1234, 1402, 1483, 1587, 2208
7.51e7.65
(m, 10H, AreH), 7.81 (s, 2H, eCHe). 13C NMR (CDCl3, 400 MHz,
ppm): 106.74, 114.06, 120.05, 128.92, 129.20, 129.28, 129.40,
(eC^N), 3035, 3089. 1H NMR (CDCl3, 400 MHz, ppm):
d
d
137.85. ELEM. ANAL. Calcd. for C22H12Br2N2S (%): C, 53.25; H, 2.44; N,
5.65; S, 6.46. Found: C, 53.19; H, 2.51; N, 5.65; S, 6.55.
2.3. Polymer synthesis (Scheme 2) [35]
Poly(9,9-dihexylfluorene) (PF) and copolyfluorenes (P05eP5)
were prepared from monomer 4, 5, and M1 by the palladium-cata-
lyzed Suzuki coupling reaction. The feed ratios of M1 were 0% and
0.5e5 mol% for PF and P05eP5, respectively. The general synthetic
procedures of the copolyfluorenes are described as follows: 9,9-
dihexylfluorene-2,7-bis(trimethyleneborate) (4), 9,9-dihexyl-2,7-
dibromofluorene (5), monomer M1, (PPh3)4Pd(0) and several drops
of Aliquat 336 were added to a mixture consisting of toluene and
aqueous solution of K2CO3 (2 M). The mixture was first purged with
nitrogen and then stirred rigorously at 90 ꢀC for 72 h. Then extra 4
and bromobenzenewere successivelyadded toend-capthe polymer
chain. After cooling to room temperature, the mixture was poured
into a largequantityof methanoland distilled water (v/v ¼ 10/1). The
appearing solid was collected by filtration and washed thoroughly
with methanol, followed by Soxhlet extraction with acetone to
remove trace oligomers and catalyst residues.
PF: FT-IR (KBr pellet, cmꢁ1):
n
1458 (eC]Ce), 2858, 2927, 2951
0.76e0.81 (m, 10H,
(eCH2e). 1H NMR (CDCl3, 400 MHz, ppm):
d
eCH2CH3), 1.14 (s, 12H, eCH2e), 2.12 (s, 4H, eCH2e), 7.67 (s, 4H,
AreH), 7.83e7.86 (d, 2H, AreH, J ¼ 12 Hz). ELEM. ANAL. Calcd. for
C25H32 (%): C, 90.30; H, 9.63. Found: C, 89.56; H, 9.76.
P05: FT-IR (KBr pellet, cmꢁ1):
n
1460 (eC]Ce), 2856, 2927, 2952
(eCH2e), 2213 (eCN). 1H NMR (CDCl3, 400 MHz, ppm):
d
0.78e0.81
(t, 10H, eCH2CH3, J ¼ 12 Hz), 1.14 (s, 12H, eCH2e), 2.12 (s, 4H,
eCH2e), 7.67e7.70 (m, 4H, AreH), 7.83e7.85 (m, 2H, AreH). ELEM
ANAL. found for C49H50N2S: C, 89.39%; H, 9.58%; N, <0.1%; S, <0.1%.
.
P1: FT-IR (KBr pellet, cmꢁ1):
n
1458 (eC]Ce), 2854, 2927, 2952
2.2. Synthesis of model compound GM and monomer M1 (Scheme 1)
(eCH2e), 2213 (eCN). 1H NMR (CDCl3, 400 MHz, ppm):
d
0.76e0.81
(t, 10H, eCH2CH3, J ¼ 20 Hz), 1.14e1.25 (d, 12H, eCH2e, J ¼ 44 Hz),
2.13 (s, 4H, eCH2e), 7.67e7.71 (m, 4H, AreH), 7.83e7.85 (m, 2H,
AreH). ELEM. ANAL. found for C49H50N2S: C, 88.38%; H, 9.48%; N,
<0.10%; S, <0.20%.
2.2.1. 2,5-Bis(2-phenyl-2-cyanovinyl)thiophene (GM)
The new monomer M1 and model compound GM were
synthesized by modifying the procedures described in previous
literatures [25,29,33,34]. For example, a mixture of 2,5-thio-
phenedicarboxaldehyde (2: 0.14 g, 1 mmol) and phenylacetonitrile
(1: 0.588 g, 3 mmol) in 10 mL methanol was stirred at room
temperature under nitrogen atmosphere. After complete
P3: FT-IR (KBr pellet, cmꢁ1):
n
1458 (eC]Ce), 2854, 2927, 2951
(eCH2e), 2212 (eCN). 1H NMR (CDCl3, 400 MHz, ppm):
d
0.78e0.81
(t, 10H, eCH2CH3, J ¼ 12 Hz), 1.14e1.25 (d, 12H, eCH2e, J ¼ 44 Hz),
2.12 (s, 4H, eCH2e), 7.67e7.71 (m, 4H, AreH), 7.84e7.86 (m, 2H,

W.-F. Su et al. / Polymer 51 (2010) 1555e1562
1557
S
S
+
O
O
CN
NC
CN
GM
1
2
S
Br
Br
S
+
Br
O
O
CN
NC
CN
3
2
M1
Scheme 1. Synthetic procedures of monomer M1 and model compound GM.
AreH). ELEM. ANAL. found for C49H50N2S: C, 87.31%; H, 9.28%; N,
[25,29,33,34]. Monomer M1 and model GM were prepared with
high yields according to reported procedures and had been well
characterized. Poly(9,9-dihexylfluorene) (PF) and copolyfluorenes
P05eP5 were synthesized by the Suzuki coupling reaction using Pd
(PPh3)4 as the catalyst and Aliquat 336 as the phase-transfer
reagent in a mixture of toluene and aqueous K2CO3 (Scheme 2). The
feed ratios of monomer M1 in the preparation of copolymers P05,
P1, P3 and P5 were 0.5 mol%, 1 mol%, 3 mol% and 5 mol%, respec-
tively. Monomer 4 and mono-functional bromobenzene were used
as the end-capping agents after the polymerization. Chemical
structures of PF and P05eP5 were identified by 1H NMR, FT-IR
spectra, and elemental analysis data. The 1H NMR spectra of the
copolyfluorenes are similar to that of polyfluorene (PF) due to low
incorporated contents of the GM chromophore. Accordingly, chro-
mophore contents of P3 and P5 were estimated from elemental
analysis (EA) data (Table 1) to be 2.3 mol% and 3.4 mol%, which are
lower than feed compositions (3 mol% and 5 mol% respectively).
Clearly, the copolymerization reactivity of M1 is lower than 9,9-
dihexyl-2,7-dibromofluorene (5). However, the composition esti-
mation of P05 and P1 was not successful because their contents of
nitrogen and sulfur were lower than detection limit of the EA
instrument. The FT-IR spectra of PF and P05eP5 show character-
istic vibration band of cyano group at 2213 cmꢁ1, whose intensity is
gradually enhanced with increasing GM contents. All polymers are
soluble in common organic solvents such as toluene, chloroform
and chlorobenzene. The weight-average molecular weights (Mw) of
PF and P05eP5, determined by GPC using mono-disperse poly-
0.21%; S, 0.22%.
P5: FT-IR (KBr pellet,cmꢁ1):
n
1460 (eC]Ce), 2854, 2927, 2952
(eCH2e), 2211 (eCN). 1H NMR (CDCl3, 400 MHz, ppm):
d
0.78e0.80
(d, 10H, eCH2CH3, J ¼ 8 Hz), 1.14e1.33 (m, 12H, eCH2e), 2.12 (s, 4H,
eCH2e), 7.48e7.67 (m, 4H, AreH), 7.83e7.85 (m, 2H, AreH). ELEM
.
ANAL. found for C49H50N2S: C, 87.63%; H, 9.21%; N, 0.26%; S, 0.32%.
2.4. Fabrication and characterization of EL devices
Light-emitting devices (ITO/PEDOT:PSS/polymer/Ca/Al) were
fabricated by successive coating of hole-injection layer (PEDOT:PSS)
and active layer (emitting polymer) ontotransparent conductive ITO
glass, followed by vacuum deposition of calcium and aluminum as
cathode. The ITO glasses were pre-cleaned consecutively with
ultrasonic bath in neutraler reiniger/de-ionized water (1:10 volume)
mixture, de-ionized water, acetone and 2-propanol, and then dried
in vacuo. The PEDOT:PSS was first spin-coated on top of cleaned ITO
glass and dried at 150 ꢀC for 15 min. The polymer solution (12 mg/mL
in chlorobenzene) was prepared and filtered with a syringe filter
(0.22-ìm pore) to remove possible particle contaminants, and then
spin-coated onto the PEDOT:PSS layer to form as the emitting layer.
Finally, a thick layer of Ca (ca. 50 nm) and Al (ca. 100 nm) were
consecutively deposited as a cathode by thermal evaporation under
a vacuum of 2 ꢂ 10ꢁ6 Torr. The EL spectra and luminancee
currentevoltage characteristics of the devices were recorded using
a combination of Keithley power source (model 2400) and Ocean
Optics usb2000 fluorescence spectrophotometer.
styrenes as
a calibration standard, are in the range of
2.4 ꢂ 104e4.2 ꢂ 104, with polydispersity index (PDI) being between
2.0 and 2.8 (Table 1). Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were employed to investi-
gate thermal behaviors of the polymers. As shown in Table 1, the
thermal decomposition temperature (Td) of PF and copolyfluorenes
P05eP5 are higher than 410 ꢀC. The residual weights of PF and
copolyfluorenes P05eP5 at 800 ꢀC in air were 1.3e4.5%, suggesting
3. Results and discussion
3.1. Synthesis and characterization
The synthetic schemes of dibromo monomer (M1) and model
compound of green chromophore (GM) are shown in Scheme 1
C6H13
C6H13
C6H13
C6H13
O
O
Br
Br
O
O
S
Br
Br
B
+
B
+
NC
CN
5
4
M1
In feed
PF x = 1
C6H13
C6H13
Pd(PPh3)4
P05 x = 0.995, y = 0.005
P1 x = 0.99, y = 0.01
P3 x = 0.97, y = 0.03
P5 x = 0.95, y = 0.05
S
2M K2CO3
toluene
y
x
NC
CN
Scheme 2. Synthetic procedures of poly(9,9-dihexylfluorene) (PF) and copolyfluorenes (P05eP5).

1558
W.-F. Su et al. / Polymer 51 (2010) 1555e1562
Table 1
Polymerization results and characterization of the polymers.
Yield
(%)
Mna (ꢂ104)
Mwa (ꢂ104)
PDIa
Tg (ꢀC)b
Td (ꢀC)c
y (%)d
WR (%)f
PF
70
71
72
73
72
1.83
1.47
1.08
1.04
1.01
4.18
2.91
2.39
2.66
2.84
2.28
1.98
2.21
2.56
2.81
96
94
94
97
107
421
420
417
410
410
0
51.6(2.6)
50.8(4.1)
52.1(4.5)
51.2(1.3)
49.7(4.4)
P05
P1
P3
P5
ee
ee
2.3
3.4
a
Mn, Mw, and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards in CHCl3.
Glass transition temperature (Tg) was measured by DSC at a heating rate of 10 ꢀC/min.
Thermal decomposition temperature (Td) at 5 wt% loss under nitrogen atmosphere.
The y values were actual molar percent of the GM chromophore in the copolyfluorenes, estimated from elemental analysis (EA) data.
Lower than measuring limit of the EA instrument.
Residual weight: remaining weight of the polymers upon heating to 800 ꢀC in nitrogen atmosphere; the values in the parentheses are those in air.
b
c
d
e
f
that slight carbonization might occur at high temperature.
Furthermore, the glass transition temperatures (Tg) of PF and
P05eP5 increase gradually from 94 ꢀC to 107 ꢀC with an increase in
GM composition. When fabricated as emitting layer in PLEDs,
emitting polymer with higher Tg is conducive to suppress
morphology deformation and degradation under high driving
voltage.
absorption peak reveals slight blue-shift (from 387 nm to 379 nm)
in going from solution to film state (Table 2). This is ascribed to
chain entanglement in film state as a result of concentrated poly-
mer solution used in film casting (12 mg polymer/1 mL chloro-
form). In our previous study, we found that the absorption maxima
were blue-shifted from 391 to 369 nm when the concentration was
increase from 6 to 30 mg/1 mL [36].
Fig. 2 shows film state photoluminescence (PL) spectra of PF and
P05eP5, in which the main emissions at ca. 425 nm and 535 nm are
attributed to fluorene segments and GM chromophores, respec-
tively. The solid state PL spectra of P3 and P5 containing high GM
contents (2.3 mol% and 3.4 mol%) are very different from those in
solution. The emission intensity at 535 nm is enhanced dramati-
cally, accompanied with simultaneous degeneration of emission at
425 nm. This enhancement of green emission is due to efficient
Förster energy transfer from fluorene segments to GM chromo-
phores, occurring intra- and intermolecularly in film state [21]. The
phenomenon is quite common in copolymers containing narrow
band-gap chromophores [26,37]. Moreover, the PL quantum effi-
ciencies of the copolyfluorenes are decreased from 0.86 (P05) to
0.52 (P5) with increasing GM contents (Table 2), estimated using PF
as reference standard (FPL ¼ 1). The heavy-atom sulfur in GM
chromophore inevitably increases the intersystem crossing process
from singlet state and triplet state that degrades PL efficiency.
3.2. Optical properties
Model compound GM was prepared to investigate the energy
transfer between fluorene segment and chromophore. As shown in
Fig. 1, the absorption peak of GM locates at 404 nm; the main
emission situates at 475 nm with a shoulder at ca. 500 nm.
Homopolyfluorene PF shows absorption and emission peaks at
387 nm and 419 nm, respectively. The absorption band of GM
overlaps extensively with the emission band of PF; therefore effi-
cient energy transfer from fluorene segment to GM chromophore
can be reasonably expected [26]. The optical properties of GM and
copolymers are summarized in Table 2. In solution state, the
absorption and PL spectra of P05eP5 show peaks at ca. 387 and
419 nm, respectively, which are contributed exclusively from flu-
orene segments. Moreover, both absorption and PL spectra of
P05eP5 are very similar to those of PF (Fig. S1 in Supporting
information), which is attributable to slight composition of GM
chromophore in main chain. Furthermore, their absorption spectra
in the film state are qualitatively similar to those in solution; the
only difference is slight enhancement of absorption at ca.
400e440 nm region with increasing GM contents. However, the
3.3. Electrochemical properties [38,39]
Cyclic voltammetry was used to investigate the reduc-
tioneoxidation behaviors of the copolyfluorenes to comprehend
their electrochemical properties. The HOMO and LUMO energy
levels of the polymer films can be estimated from the cyclic
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Table 2
PF
GM
Optical properties of GM and copolymers.
Solutiona
Film
UVevis lmax PL lmax
(nm)
O
fPL UVevis lmax PL lmax
Stokes shift
(nm)
c
(nm)b
(nm)
(nm)b
GM 404
PF 387
475,500
419, 441
e
e
e
e
1
379
426, 451, 47
481
P05 386
P1 387
P3 387
P5 387
419, 441 0.86 379
419, 441 0.85 379
419, 441 0.58 379
419, 441 0.52 381
425,
454,483
47
425, 451, 47
483
425, 451, 157
535
425, 451, 161
539
300
400
500
600
700
Wavelength (nm)
a
In chloroform (1 ꢂ 10ꢁ5 M).
b
c
Fig. 1. Absorption and photoluminescence (lex: 385 nm) spectra of PF and GM in CHCl3
solution.
The excitation wavelength was 385 nm.
Stokes shift ¼ PL(film)/nm ꢁ UV(film)/nm.

W.-F. Su et al. / Polymer 51 (2010) 1555e1562
1559
-2.73
PF
P05
P1
P3
P5
1.0
0.8
0.6
0.4
0.2
0.0
Ca/Al
-3.41
PF
GM
PEDOT
unit: eV
-5.62
-5.66
Fig. 3. Energy level diagrams of PF and GM.
400
450
500
550
600
650
700
Wavelength (nm)
3.4. Electroluminescent properties
Fig. 2. Photoluminescence spectra (lex: 385 nm) of PF and copolyfluorenes P05eP5 in
the film state.
Double-layer electroluminescent (EL) devices having a structure
of ITO/PEDOT:PSS/emitting layer/Ca(50 nm)/Al(100 nm) were
fabricated to investigate the optoelectronic properties and perfor-
mances of the copolyfluorenes. Fig. 4 shows the current density
versus bias and brightness versus bias curves (JeBeV) of the EL
devices, with the corresponding characteristic data summarized in
Table 4. The turn-on voltages (at 10 cd/m2) are in the range of
5.3e7.8 V; the maximum brightness and maximum current effi-
ciency are 2000e5230 cd/m2 and 0.43e0.65 cd/A, respectively. The
brightnessevoltageeluminance (BeV) curves shift to higher volt-
ages with increasing GM composition, suggesting that the GM
chromophores dominate the charge trapping processes [21]. Both
maximum brightness and maximum current efficiency of P05eP5
are superior to those of PF (1310 cd/m2, 0.18 cd/A). The P05 device
exhibits the best performance, with the maximum brightness and
current efficiency being 5230 cd/m2 and 0.65 cd/A, respectively.
Clearly, incorporating small amount of GM chromophore signifi-
cantly enhances the device performance. This is attributable to
enhanced recombination of electrons and holes at GM chromo-
phore which act as charge trapping sites. Hence, our device results
obviously demonstrate that GM chromophore can increase
maximum brightness of the EL devices. However, the brightness of
P05eP5 devices decrease from 5230 cd/m2 (P05) to 2000 cd/m2
(P5) as GM content on the backbone increases, due mainly to the
heavy-atom effect (sulfur in GM chromophore) [41]. Therefore, it is
necessary to control GM contents to obtain efficient EL devices. The
AFM images of the films cast from PF and copolyfluorenes P05eP5,
in which the root-mean-square (rms) roughness of the copoly-
fluorenes increases from 1.24 to 1.83 with increasing GM contents
(Fig. S3). Therefore, both rough morphology and heavy-atom effect
degrade device brightness and efficiency at high GM contents.
voltammograms (CV) using the equations EHOMO ¼ ꢁ(Eox þ 4.8) eV
and ELUMO ¼ ꢁ(Ered þ 4.8) eV, respectively, where Eox and Ered are
the onset oxidation and onset reduction potentials, respectively,
relative to the ferrocene/ferrocenium couple. The polymer films
coated on a glassy carbon working electrode, supported in 0.10 M
tetra-n-butylammonium perchlorate (n-Bu)4NClO4 in anhydrous
acetonitrile, were measured at a scanning rate of 100 mV/s. The
cyclic voltammograms of GM, PF and P5 are shown in the Fig. S2
and the corresponding electrochemical data are summarized in
Table 3. The HOMO and LUMO levels of GM are estimated to be
ꢁ5.62 and ꢁ3.41 eV, respectively, from which the electrochemical
band-gap (Eegl) of GM is estimated to be 2.21 eV. The estimated
LUMO levels of the copolyfluorenes (P05eP5) decrease from
ꢁ2.77 eV to ꢁ2.89 eV with an increase in GM contents (to 3.4 mol
%). The LUMO levels are lower than ꢁ2.73 eV of PF. On the
contrary, the HOMO levels remain almost unchanged at ꢁ5.64 eV,
which are close to ꢁ5.66 eV of PF. Therefore, by incorporating GM
chromophores, consisting of narrow band-gap thiophene and
electron affinitive cyano groups, the band-gap of copolyfluorenes
decreases from 2.93 eV (PF) to 2.75 eV (P5) [40]. However, the
LUMO level of GM (ꢁ3.41 eV) is much lower than that of PF
(ꢁ2.73 eV); therefore, the energy barrier of electron injection can
be effectively reduced by the incorporation of GM chromophore
[40]. Fig. 3 shows the energy band diagrams of PF and GM. The
HOMO and LUMO levels of GM lie between those of PF, suggesting
that GM chromophore acts as a charge trapping site for the
copolyfluorenes [21]. Therefore, electrons and holes tend to
recombine mainly in GM chromophores and charge trapping could
be the major mechanism for the EL process.
Table 3
Electrochemical potentials of the GM and copolymers.
Polymer
Eonset(ox) vs. FOC (V)a
Eonset(red) vs. FOC (V)a
EHOMO (eV)b
ELUMO (eV)b
Eegl (eV)c
Eogpt (eV)d
GM
PF
P05
P1
P3
P5
0.82
0.86
0.84
0.84
0.84
0.84
ꢁ1.39
ꢁ2.07
ꢁ2.03
ꢁ2.01
ꢁ2.00
ꢁ1.91
ꢁ5.62
ꢁ5.66
ꢁ5.64
ꢁ5.64
ꢁ5.64
ꢁ5.64
ꢁ3.41
ꢁ2.73
ꢁ2.77
ꢁ2.79
ꢁ2.80
ꢁ2.89
2.21
2.93
2.87
2.85
2.84
2.75
e
2.93
2.95
2.93
2.93
2.83
a
EFOC ¼ 0.47 V vs. Ag/AgCl.
b
c
EHOMO ¼ ꢁ(Eonset(ox), FOC þ 4.8 eV); ELUMO ¼ ꢁ(Eonset(red), FOC þ 4.8 eV).
Eg ¼ |;LUMO ꢁ HOMO|;.
d
Band gaps obtained from onset absorption (lonset): Eogpt ¼ 1240/lonset
.

1560
W.-F. Su et al. / Polymer 51 (2010) 1555e1562
6000
5000
4000
3000
2000
1000
1.0
0.8
0.6
0.4
0.2
0.0
PF
P05
P1
P3
P5
PF
P05
P1
P3
P5
0
400
500
600
700
PF
800
Wavelength (nm)
P05
P1
Fig. 5. Emission spectra of the electroluminescent devices using PF or copolyfluorenes
(P05eP5) as emitting layer. Device structure: [ITO/PEDOT:PSS/polymer/Ca(50 nm)/Al
(100 nm)].
P3
600
400
200
0
P5
chromophore (ca. 530 nm) in P3 and P5 is attributed to simulta-
neous energy transfer (from fluorene segments to GM chromo-
phores) and charge trapping in GM chromophore. This
phenomenon is coincident with the optical and electrochemical
results mentioned above. However, P05 and P1 devices reveal both
blue and green emission peaks due to incomplete energy transfer.
0
2
4
6
8
10
12
Voltage (V)
Device A (PF:P5 = 5:1)
Device B (PF:P5 = 10:1)
Device C (PF:P5 = 15:1)
Fig. 4. Brightnessevoltage and current densityevoltage characteristics of the EL
devices using PFeP5 as emitting layer [ITO/PEDOT:PSS/polymer/Ca(50 nm)/Al
(100 nm)].
3000
2000
1000
Fig. 5 shows the emission spectra of the electroluminescent (EL)
devices. The EL spectrum of PF is similar to its PL spectrum, with
a major emission peak at ca. 429 nm. The other emission peak at
450 nm and shoulder at 484 nm correspond to S10 / S01 and
S10 / S02 vibronic transitions, respectively. However, the EL spectra
of P05eP5 devices are obviously different from that of PF. The
emission peaks of P05 and P1 shift to about 488 nm, and those of
P3 and P5 shift further to ca. 530 nm. In particular, P3 and P5 show
yellowish-green emissions because of higher GM composition in
main chain (3.4 mol%). Moreover, the emission intensity at
400e500 nm from fluorene segments degenerates quickly with
increasing GM contents. The overwhelming emission of GM
0
Device A (PF:P5 = 5:1)
Device B (PF:P5 = 10:1)
Device C (PF:P5 = 15:1)
600
Table 4
Electroluminescent properties of the electroluminescent devices.
400
200
0
Devicea
Vonb (V)
Lmax
(cd/
m2)
LEmax
Emissione
lem, nm)
CIE
c
d
(cd/A)
(
coordinates
(x, y)f
PF
4.6
5.3
5.9
6.6
7.8
1310
5230
3530
2610
2000
0.18
0.65
0.43
0.44
0.48
429, 450, 484
429, 453, 487
429, 454, 488
530, 559s
(0.17, 0.11)
(0.18, 0.20)
(0.20, 0.22)
(0.30, 0.41)
(0.35, 0.52)
P05
P1
P3
P5
533, 559s
0
2
4
6
8
10
12
a
Device structure: [ITO/PEDOT:PSS/polymer/Ca(50 nm)/Al(100 nm)].
Turn-on voltage at 10 cd/m2.
Maximum luminance.
b
c
d
e
f
Voltage (V)
Maximum luminance efficiency.
Fig. 6. Brightnessevoltage and current densityevoltage characteristics of the devices
using blends of PF and P5 as emitting layer. Device structure: [ITO/PEDOT:PSS/polymer
blend/Ca(50 nm)/Al(100 nm)].
Electroluminescent spectra ca. 1000 cd/m2.
The 1931 CIE coordinates at ca. 1000 cd/m2.

W.-F. Su et al. / Polymer 51 (2010) 1555e1562
1561
Table 5
4. Conclusions
Electroluminescent properties of blend devices.
Devicea Emitting layera
(weight ratio)
Von Lmax
(V) (cd/m2) (cd/A)
LEmax
Emissione
lem, nm)
CIE
b
c
d
We successfully synthesized
a
series of copolyfluorenes
(
coordinates
(P05eP5) slightly doped with 2,5-bis(2-phenyl-2-cyanovinyl)
thiophene (GM) chromophore (0e3.4 mol%) in main chain. In
solution, their absorption and PL spectra were similar to those of PF
(peaked at 425 nm) due to low GM contents. However, in film state
the PL spectra showed two emission peaks at ca. 425 nm and
535 nm attributed to fluorene segments and GM chromophores,
respectively. The 535 nm emission intensity increased gradually
with increasing GM contents. The electrochemical band-gap
decreased from 2.87 eV to 2.75 eV with increasing GM contents
(w3.4 mol%). Besides, the HOMO (ꢁ5.62 eV) and LUMO (ꢁ3.41 eV)
of GM chromophore were within those of PF (ꢁ5.66 eV, ꢁ2.73 eV),
suggesting that GM acts as a charge trapping sites when used as
active layer in an EL device. The P05eP5 revealed electrolumines-
cent performance (2000e5230 cd/m2, 0.43e0.65 cd/A) superior to
PF (1310 cd/m2, 0.18 cd/A), indicating that GM chromophore effi-
ciently improves the charge recombination. Moreover, near-white-
light-emitting devices were realized by blending the P5 with PF (w/
w ¼ 10/1), with the CIE coordinates being (0.26, 0.32). Slight
incorporation of GM chromophore into polyfluorene is an effective
way to improve EL performance.
(x, y)f
A
B
C
PF:P5 (5/1)
PF:P5 (10/1)
PF:P5 (15/1)
6.9 2730
6.1 1730
5.6 1300
0.49
0.36
0.27
429, 457,
494, 523
429, 453,
491, 526
429, 451,
488, 525
(0.28, 0.39)
(0.26, 0.32)
(0.22, 0.25)
a
b
c
d
e
f
Device structure: [ITO/PEDOT:PSS/polymer blend/Ca(50 nm)/Al(100 nm)].
Turn-on voltage at 10 cd/m2.
Maximum luminance.
Maximum luminance efficiency.
Electroluminescent spectra ca. 1000 cd/m2.
The 1931 CIE coordinates at ca. 1000 cd/m2.
To tune the emission color by controlling the energy transfer
from PF segments to GM chromophores, we fabricated EL devices
using blends of PF and P5 as active layer. The configuration of the
blend devices was ITO/PEDOT:PSS/polymer blend/Ca (50 nm)/Al
(100 nm). Fig. 6 shows the current density and brightness versus
bias curves (JeBeV) of the blend devices (A, B, and C), with the
corresponding characteristic data summarized in Table 5. The
weight ratios of PF over P5 in the devices A, B, and C are 5/1, 10/1,
and 15/1, respectively. The maximum brightness and maximum
current efficiency are 1300e2730 cd/m2 and 0.27e0.49 cd/A
respectively. The root-mean-square (rms) roughness of the blend
devices (A, B, and C) was in the range of 1.43e1.5 nm that reveals no
obvious phase separation. Similar molecular structures between PF
and P5 are probably the attribution of this phase homogeneity [42].
Moreover, the blend devices (A, B, and C) show broad emission
bands at 400e700 nm (Fig. 7), in which the blue and yellowish-
green emissions are attributed to the PF segments and GM chro-
mophores respectively. The 1931 CIE coordinates of the emitted
light from devices A, B, and C are (0.28, 0.39), (0.26, 0.32), and (0.22,
0.25). It is noteworthy that the CIE coordinate of device B (0.26,
0.32) appears near-white to the naked eye [21]. To conclude, the
emission color of the copolyfluorenes containing GM chromophore
can be readily tuned by controlling the chromophore contents at
the synthesis stage or by simple blending with homopolyfluorene.
Acknowledgment
The authors thank the National Science Council of Taiwan for the
financial aid through project NSC 98-2221-E-006-002-MY3.
Appendix. Supplementary data
Supplementary data associated with this article can be found in
the on-line version, at doi:10.1016/j.polymer.2010.02.012.
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