Polyfluorenes containing pyrazine units: Synthesis, photophysics and electroluminescence

Article abstract of DOI:10.1007/s11426-011-4238-6

A series of conjugated copolymers of 9,9-dioctylfluorene and symmetrical pyrazine unit (BY) were synthesized by Suzuki copolymerization and were used as novel light-emitting materials in PLEDs. Efficient energy transfer was observed in both thin film and

Full text of DOI:10.1007/s11426-011-4238-6

SCIENCE CHINA
Chemistry
April 2011 Vol.54 No.4: 656–665
doi: 10.1007/s11426-011-4238-6
• ARTICLES •
Polyfluorenes containing pyrazine units: Synthesis,
photophysics and electroluminescence
WANG Ming, LI Ying, XIE ZhiYuan & WANG LiXiang*
State Key Laboratory of Polymer Physics and Chemistry; Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China
Received November 30, 2010; accepted December 12, 2010
A series of conjugated copolymers of 9,9-dioctylfluorene and symmetrical pyrazine unit (BY) were synthesized by Suzuki co-
polymerization and were used as novel light-emitting materials in PLEDs. Efficient energy transfer was observed in both thin
film and solution. Compared with the lowest occupied molecular orbital (LUMO) energy level of the polyfluorenes ho-
mopolymer (PFO), the lower LUMO energy levels of copolymers indicated that the introduction of the BY unit would be
benefit to electron injection. The turn-on voltages of their single-layer electroluminescent (EL) devices (ITO/PEDOT/polymer/
LiF/Al) were at 6.1–4.0 V, which were much lower than that of PFO (7.0 V). The maximum brightness, current efficiency, and
external quantum efficiency of all PFBY copolymers were higher than those of the PFO homopolymer. The single-layer device
of PFBY5 was the best one in the copolymers, with a maximum brightness of 485 cd/m2, a current efficiency of 0.29 cd/A, and
an external quantum efficiency of 0.10%. The introduction of PVK and TPBI for the multilayer device of PFBY5 increased the
device efficiencies, which showed a maximum brightness of 3012 cd/m2, a maximum current efficiency of 1.81 cd/A, and an
external quantum efficiency of 0.66%.
pyrazine, polyfluorenes, energy transfer, PLED
PLEDs, because of their lower LUMO energy levels. Poly-
1 Introduction
fluorenes containing n-type units, such as benzothiadiazole,
naphthoselenadiazole, oxadiazole, quinoxaline [8–11], have
been reported. For example, Cao’s group has synthesized a
series of fluorene-co-dibenzothiophene-S,S-dioxide (FSO)
copolymers and investigated their electrochemistry and
electroluminescent performance. The n-type FSO unit low-
ers the LUMO energy level, balances the injection and
transportation both electron and hole in the polymers, and
therefore improves the EL device efficiencies. The maxi-
mum brightness (1292–6631 cd/m2) and luminous efficiency
(1.03–4.63 cd/A) of all the copolymers surpassed the PFO
homopolymer [12].
Polymer light-emitting diodes (PLEDs) based on conjugated
polymers have attracted a great interests of researchers due
to their potential application in large-area full-color flat
panel displays [1–3]. Polyfluorene and its derivatives have
been widely explored as the emissive materials for PLEDs
because of their high photoluminescence (PL) quantum ef-
ficiency, solution processibility, and good hole-transporting
properties [4–6]. However, electron injection and transport
are poor in polyfluorene homopolymers [7], which limits
their PLED efficiency greatly.
The introduction of n-type molecule in copolymers is an
effective method to improve electron injection properties of
In previous work, our group has synthesized a series of
n-type molecules based on unsymmetrical pyrazine units
(BYs) [13]. The LUMO energy levels decrease from 3.24
to 3.78 eV with an increasing number of pyrazine rings
*Corresponding author (email: lixiang@ciac.jl.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com
www.springerlink.com

Wang M, et al. Sci China Chem April (2011) Vol.54 No.4
657
and the conjugation length of the molecules. By introducing
a pyrazine acceptor to the backbone of polyfluorenes, we
have synthesized a series of p-n diblock conjugated co-
polymers. The results also demonstrated that the acceptor
containing the pyrazine unit was a promising n-type build-
ing block for organic semiconductors [11].
In this work, we synthesized a series of new polyfluore-
nes containing the symmetrical BY unit (Chart 1) by Suzuki
polymerization and studied the photophysics, electrochemi-
cal and electroluminescence properties of copolymers. A
series of polyfluorene copolymers containing 1, 2, 5, and 10
mol% BY unit, denoted as PFBY1, PFBY2, PFBY5, and
PFBY10. The BY unit has been proved was a strong elec-
tron acceptor in previous paper and it’s LUMO energy level
was 3.27 eV [13]. Thus, the introduction of BY unit will
improve the electron injection property and EL performance
of polymers.
ysis system. IR spectra were obtained on FT-IR Bruker
Vertex 70 spectrometer at a nominal resolution of 2 cm1.
Polymer samples were prepared to be thin films on KBr.
Molecular weight and polydispersity of the polymer were
determined by gel permeation chromatography (GPC) on a
Waters 410 instrument with polystyrene as standards and
THF as eluent. Thermogravimetric analysis (TGA) meas-
urements were performed on a Perkin-Elmer series 7 analy-
sis systems under N2 at a heating rate 10 °C/min. UV-vis
absorption (UV) spectra were recorded with a PerkinElmer
Lambda35 UV-vis Spectrometer. Photoluminescence (PL)
spectra were obtained on a PerkinElmer LS50B Lumines-
cence spectrometer. The electroluminescence (EL) spectra,
Commission Internationale de L’Eclairage (CIE) coordi-
nates, Current-Voltage and Brighness-Voltage characteris-
tics of devices were measured with a Spectrascan PR650
spectrophotometer at the forward direction and a computer-
controlled Keithley 2400 under ambient condition.
Cyclic voltammetry (CV) was performed on a Chi660b
electrochemical analyzer with a three-electrode cell in 0.1 M
tetrabutylammonium perchlorate (Bu4NClO4) in dry di-
chloromethane solution under an argon atmosphere at a scan
rate of 100 mV/s and using ferrocene as standards. A glass
carbon disk (2-mm diameter) was used as working electrode
with a Pt wire as the counter electrode and an Ag/AgCl
electrode as the reference electrode.
2 Experimental
2.1 Materials
Pyrene-4,5,9,10-tetraones (1) [14], 9,9-dioctylfluorene-2-
boronic acid ester (5) [15], 9,9-dioctylfluorene-2,7-diboronic
acid ester (6), 9,9-dioctyl-2,7-dibromofluorene (7) [16] and
compound 3 [13, 17] were synthesized according to the re-
ported methods.
EL device fabrication
The ITO glass plates were degreased in an ultrasonic sol-
vent bath and then dried in a heating chamber at a tempera-
ture of 120 °C. The polymer light-emitting diodes with the
configuration of the organic light-emitting diodes (OLED)
were fabricated with the PFO and copolymers as the emis-
sive material. The PSS-doped PEDOT was spin-coated onto
the treated ITO at 3000 r/min for 1 min and heated for 1 h at
120 °C to obtain an approximate thickness of 40 nm. PVK
was spin-coated from its 5 mg/mL solution in toluene to
2.2 General experimental
Instruments
1H NMR spectra data were recorded on a Bruker Avance
300-MHz NMR spectrometer in CDCl3 with TMS standard.
Molecular mass spectra of intermediates were measured by
means of LDI-1700 MALDI-TOF mass spectroscopy. Ele-
mental analysis was performed by Bio-Rad elemental anal-
Chart 1 The structures of the BY, PFO, and PFBY copolymers.

658
Wang M, et al. Sci China Chem April (2011) Vol.54 No.4
give a 20 nm thick film on PEDOT. This film was dried in a
vacuum at 80 °C for 1 h. The copolymer layer (about 70 nm)
was then spin-coated onto the PVK/PEDOT/ITO-coated
glass substrate in fresh toluene solution (10 mg/mL). Be-
cause of the partial solubility of PVK in toluene, the inter-
face between PVK and the copolymers was not flat; the
total thickness of the PVK/copolymer bilayers was meas-
ured instead to get the copolymer layer thickness. 20 nm
thick films of TPBI were obtained by evaporation from re-
sistively heated in a vacuum evaporator. Finally, a thin layer
of LiF (1 nm) followed by a layer of aluminum (100 nm)
was deposited in a vacuum thermal evaporator through a
shadow mask at a pressure of 3 × 104 – 4 × 104 Pa. All the
devices fabrication and characterization were carried out at
ambient conditions. The EL spectra, CIE coordinates, Current-
Voltage and Brightness-Voltage characteristics of the de-
vices were measured with a Spectrascan PR650 spectropho-
tometer at the forward direction and a computer-controlled
Keithley 2400 instrument.
mmol) in 10 mL toluene was added into the mixture. The
mixture was stirred 2 d at 90 °C and cooled to the room
temperature. The aqueous phase was extracted with CH2Cl2.
The CH2Cl2 extracts were combined with the organic phase
and washed with H2O. The solution was dried with anhy-
drous Na2SO4. The crude product was dried under vacuum
and purified by column chromatography on silica gel with
CH2Cl2 as the eluent to afford the product as a yellow solid
(0.38 g, 90%). 1H NMR (300 MHz, CDCl3):  (ppm) 9.75 (s,
4H), 8.31 (s, 4H), 8.11 (s, 2H), 8.02 (d, J = 8.0 Hz, 2H),
7.81 (d, J = 8.0 Hz, 2H), 7.76 (d, J = 7.2 Hz, 2H), 7.43 (m,
6H), 7.17 (d, J = 8.4 Hz, 8H), 6.81 (d, J = 8.4 Hz, 8H), 3.97
(t, J = 6.4 Hz, 8H), 2.24–2.04 (m, 8H), 1.83 (m, 8H),
1.62–0.65 (m, 128H). Elemental analysis calcd (%) for
C
150H190N4O4: C 85.26, H 9.06, N 3.03; found: C 85.49, H
8.89, N 2.78. [MALDI-TOF]: 2113 [M + H]+.
2.4 Polymerization
The polymers were synthesized by Suzuki coupling reaction
in an argon atmosphere. For example the synthesis of
PFBY10, to a stirred solution of 9,9-dioctylfluorene-2,7-
diboronic acid ester (0.2792 g, 0.5 mmol), 9,9-dioctyl-2,7-
dibromofluorene (0.2194 g, 0.4 mmol) and compound 4
(0.1495 g, 0.1 mmol) in 10 mL toluene and 5 mL 2 M
K2CO3 solution in water were added the catalysts Pd(PPh3)4
(5.8 mg, 5 mol‰) in 5 mL toluene and Aliquat 336 (0.1 g,
0.25 mmol). The mixture was stirred for 2 d at 90 °C and be
cooled to the room temperature. The solution was poured
into methanol. The precipitate was collected by filtration
and then dissolved in chloroform. The solution was washed
with water and dried with anhydrous Na2SO4 for 30 min.
After most of the solvent was removed, the residue was
poured into stirred methanol to obtain polymer fiber. The
reprecipitation procedure in chloroform/methanol was then
repeated three times. This polymer fiber was extracted with
acetone for 24 h. The final product was dried in vacuum.
Yield: 80%–88%. Random copolymers (PFBY) and the
corresponding poly(9,9-dioctylfluorene) (PFO) were synthe-
sized by Suzuki coupling polymerization.
2.3 Monomer synthesis
Synthesis of 2,7-dibromo-pyrene-4,5,9,10-tetraones (2)
A solution of pyrene-4,5,9,10-tetraones (3) (0.26 g, 1 mmol)
in 85% H2SO4 (11 mL). The solution was added n-bromo-
succinimide (NBS) (0.54 g, 3 mmol) after 10 min and was
stirred 3 h at room temperature. The mixture was poured
into 200 mL water and stirred 20 min. The precipitated was
filtered and dried under vacuum to afford compound 2 as a
yellow solid (0.36 g, 85 %). 1H NMR (300 MHz, DMSO-d6)
 (ppm): 8.37 (s, 0.14H), 8.33 (s, 4H).
Synthesis of compound 4
A solution of compound 3 (1.2 g, 2.1 mmol) and 2 (0.36 g,
0.85 mmol) in 30 mL m-cresol was degassed. The reaction
mixture was heated at 90 °C under nitrogen atmosphere for
20 h. The mixture was cooled to room temperature and
poured into 300 mL ethanol. The precipitate was filtered
and dried under vacuum. The crude product was purified by
column chromatography on silica gel with chloroform: pe-
troleum ether (1:1) as the eluent to afford the compound 4
as a yellow solid (0.6 g, 55 %). 1H NMR (300 MHz, CDCl3):
 (ppm): 8.97 (s, 4H), 8.01 (s, 4H), 7.11 (d, J=3.0 Hz, 8H),
6.79 (d, J = 3.0 Hz, 8H), 3.96 (t, J = 4.8 Hz, 8H), 1.83 (m,
8H), 1.57–1.30 (m, 56H), 0.92 (t, J=5.1 Hz, 12H). Elemental
Anal. calcd (%) for C92H108Br2N4O4: C 74.29, H 7.08, N
3.52; found: C 73.89, H 7.29, N 3.75. [MALDI- TOF]: 1495
[M + H]+.
PFO
1
Yield: 82%, H NMR (300 MHz, CDCl3)  (ppm): 7.85
(br, 2H), 7.68 (br, 4H), 2.15 (br, 4H), 1.15 (br, 20H), 0.81
(br, 10H). Calcd: C 89.63; H 10.37. Found: C 89.85; H
10.14.
PFBY10
Yield: 80%, 1H NMR (300 MHz, CDCl3)  (ppm): 8.58 (m,
4H), 8.23 (m, 4H), 7.84–7.68 (m, 54H), 6.87 (m, 8H), 4.00
(s, 8H), 2.13–1.15 (m, 316H), 0.91–0.60 (m, 66H). Calcd: C
87.79; H 9.70; N 1.16. Found: C 88.65; H 9.95; N 0.89.
Other copolymers showed NMR and FT-IR spectra
similar to PFBY10 copolymer.
Synthesis of model compound B2F
To a flask containing a degassed solution of 4 (0.30 g, 0.2
mmol) and 9,9-dioctylfluorene-2-boronic acid ester (0.21 g,
0.44 mmol) acid ester in 20 mL toluene added 10 mL of an
aqueous 2 M K2CO3 solution. The Pd(PPh3)4 (5 mg, 0.004

Wang M, et al. Sci China Chem April (2011) Vol.54 No.4
659
toluene, dichloromethane and tetrahydrofuran (THF).
3 Results and discussion
3.1 Synthesis and structure
The chemical structures of the polymers were verified by
1
1H NMR and FT-IR spectra. Representative H NMR spec-
tra of copolymer PFBY10, PFO, and compound 4 were
shown in Figure 1. The small peaks at 8.58, 8.23, and 6.87
ppm, which belong to the aromatic ring protons in the
compound 4, and the peak at 4.0 ppm, which was assigned
to the aliphatic protons (OCH2). The peaks in the range
of 7.68–7.84 ppm correspond to the aromatic ring protons in
fluorene units. The FT-IR spectra of PFO and PFBY10 also
confirmed their molecular structures. The FT-IR spectra of
PFO and PFBY10 were shown in Figure 2. The new vibra-
tional bands at 1608 cm1 and 1514 cm1 were assigned to
stretching vibrations of C=N group in the pyrazine ring. The
number-average molecular weights (Mn) of these polymers
were determined by gel permeation chromatography (GPC)
The synthetic procedures are outlined in Scheme 1. We first
reported the synthesis of 2,7-dibromo-pyrene-4,5,9,10-
tetraones in 85% H2SO4 and NBS system. The method was
simple, convenient and high yield. However, the poor solu-
bility of crude product 2 leads to difficulty in purification.
Through the reaction of compound 2 and 3, we obtained the
key monomer 4 with good solubility, which can be purified
by column chromatography on silica gel easily. Then, the
model compound B2F and copolymers were synthesized by
the Suzuki coupling reactions. The molar ratio of BY in the
copolymers was controlled by adjusting the molar ratio of 4,
6, and 7. All polymers have good solubility and can be dis-
solved in common organic solvents, such as chloroform,
Scheme 1 Synthetic routes of model compound B2F and the resulting polymers.

660
Wang M, et al. Sci China Chem April (2011) Vol.54 No.4
using polystyrene as the standard in THF. The molecular
weights of all polymers were 4.5 × 104–9.0 × 104 with a
polydispersity index of 2.2–3.4 (Table 1). The thermal
properties of the polymers were evaluated by thermogra-
vimetric analysis (TGA) and the results were summarized in
Table 1. The onset decomposition temperatures of the
PFBY copolymers and PFO homopolymer were in range of
394–438 °C, which indicated their good thermal stability
(Figure 3).
3.2 Photophysical
Figure 4 showed the normalized absorption spectra of B2F,
Figure 3 The thermogravimetric analysis (TGA) of the PFBY copoly-
mers and PFO homopolymer under nitrogen at a heating rate 10 °C/min.
Figure 1 The 1H NMR spectra of compound 4, PFO, and PFBY10.
Table 1 The molecular weight and thermal properties of the resulting
polymers
Figure 4 Normalized optical absorption spectra of 105 M toluene solu-
tions of model compound B2F, PFO and PFBY copolymers.
a)
Polymer
PFO
Mna
Mw/Mn
2.5
Td (°C) b)
438
8.9 × 104
9.0 × 104
7.4 × 104
7.4 × 104
4.5 × 104
PFO and four PFBY copolymers in toluene solutions (105 M).
PFO has an absorption maximum at 389 nm corresponding
to the -* transition of poly-fluorene backbone. The main
absorption band progressively blue-shifts from 386 nm in
PFBY1 to 378 nm in PFBY10, suggesting the decrease of
the effective conjugation length [18]. A new absorption
band at 446 nm was observed for all copolymers and in-
creased with increasing BY amount. The same absorption
band also was found for the model compound B2F in tolu-
ene solution. Thus, it can be assigned to the B2F unit in
copolymers.
PFBY1
PFBY2
PFBY5
PFBY10
2.7
412
3.4
404
2.7
397
2.2
394
a) Molecular weights were determined by GPC using polystyrene as a
standard in THF. b) Onset decomposition temperature measured by TGA
under nitrogen.
Figure 5(a) showed the normalized emission spectra of
PFO and copolymers in toluene solutions (105 M). The PL
spectra of PFO showed two blue emission peaks (417 nm
and 439 nm). For all the copolymers, the blue emission
steadily decreased and a new emission peak at 470–490 nm
increased with increasing BY content in the copolymers.
The similar emission peak can be observed from the PL
spectra of model compound B2F, suggesting the new emis-
sion peak could also be attributed to B2F moieties in co-
polymers. Considering the absorption band of B2F (446 nm)
have good overlap with the blue emission of PFO (417 and
439 nm), energy transfer from the conjugated main chain to
Figure 2 The FT-IR spectra of PFO and PFBY10.

Wang M, et al. Sci China Chem April (2011) Vol.54 No.4
661
LUMO orbital was localized on the BY unit and the HOMO
energy level was on polymer main chain, which also sug-
gested the existence of charge transfer in B2F.
Figure 7(a) showed the normalized absorption spectra of
PFO and copolymers in thin films. As increasing BY con-
tent, the change trend of the spectral shapes and peak posi-
tions are similar to those of the absorption spectra in dilute
solution. The main absorption band was also blue-shifted
from 385 nm in PFO homopolymer to 377 nm in PFBY10.
The intensity of the new peak at 446 nm was also increased
with increasing BY content. The absorption onsets (471 nm)
of the four copolymers were red-shifted compared to that of
PFO (414 nm). The optical band gaps of all copolymers
were 2.63 eV, which were calculated from the absorption
edge of the thin film spectra. The PL emission spectra of the
copolymers and PFO in thin films were shown in Figure
7(b). The PL spectra of copolymers in thin film were dif-
ferent from those in solutions. Besides the PFBY1, the blue
emission peak of all copolymers was disappeared in thin
film and only the green emission around 511 nm was found,
suggesting much more efficient energy transfer for copoly-
mers in thin film than in solutions. At the same time, the PL
spectra were steadily red-shifted with increasing BY content,
possibly due to the larger ICT effect of the excited state in
the copolymers [7]. The PL quantum yields of copolymers
and PFO in film were 23.1%, 18.3%, 11.5%, 11.4% and
49% for PFBY1, PFBY2, PFBY5, PFBY10 and PFO re-
spectively. The PL quantum yields of copolymers in film
progressively reduced from 23.1% in PFBY1 to 11.4% in
PFBY10, indicating the existence of aggregation.
Figure 5 (a) Normalized PL emission spectra of 105 M solutions of B2F,
PFO and PFBY copolymers in toluene and (b) normalized PL emission
spectra of 105 M solutions of B2F in toluene, THF, acetone and DMF.
the B2F moieties would happen. Moreover, energy transfer
was more and more complete as the BY content increased.
The PL quantum yields of copolymers and PFO in toluene
solution were 62%, 49%, 30%, 19% and 90% for PFBY1,
PFBY2, PFBY5, PFBY10 and PFO, respectively. The
emission characteristics of B2F in dilute solution were in-
vestigated in different solutions (Figure 5(b)). As the sol-
vent polarity increased, the emission maximum peak was
red-shifted from 471 to 509 nm indicating an intramolecular
charge transfer (ICT) excited state with a large dipole mo-
ment [19]. The geometry of model compound B2F was op-
timized by density functional theory calculations at the
B3LYP/6-31G* level [20]. Figure 6 indicated that its
Figure 6 HOMO and LUMO molecular orbitals of B2F.
Figure 7 (a) Normalized optical absorption and (b) normalized PL emission spectra of thin films of PFO and PFBY copolymers.

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Wang M, et al. Sci China Chem April (2011) Vol.54 No.4
3.3 Electrochemical
of devices in multilayer devices.
The electrochemical properties of copolymers and B2F
were studied by a three-electrode electrochemical cell with
Bu4NClO4 (0.1 M) as electrolyte and Ag/AgCl as reference
electrode. The HOMO and LUMO energy level were calcu-
3.4 Electroluminescent
The single-layer PLEDs devices with the structure of ITO/
PEDOT:PSS (40 nm)/polymer (70 nm)/LiF (1 nm)/Al (100
nm) (type I) were fabricated to investigate the EL properties
of the polymers [19]. The much more stable LiF/Al cathode
was chosen to replace very active Ca/Al cathode due to the
better electron injection properties of these copolymers.
Their EL spectra under different voltages were shown in
Figure 9. The EL emission maxima of all the copolymers
were from 488 to 500 nm. The EL spectral shapes of the
devices of these polymers kept almost unchanged at differ-
ent voltages [23]. Table 2 summarizes the EL properties of
the single-layer PLEDs.
lated according to the formula EHOMO = [4.8  EFOC +Eoonxset
]
and ELUMO = [4.8  EFOC + Eornesdet] eV [21]. The onset oxida-
tion potential (EFOC) of ferrocene (the standard) was meas-
ured to be 0.45 eV. The cyclic voltammetry of B2F exhib-
ited two reversible reduction processes, however only irre-
versible oxidation process was observed. The onset reduc-
tion and oxidation potentials were 1.18 and 1.57 eV for
B2F. Thus, the LUMO and HOMO energy levels of B2F
were 3.17 and 5.92 eV, respectively. The HOMO level of
PFBY10 was 5.77 eV by calculation from the onset oxida-
tion potential. However no reduction process of PFBY co-
polymers was found, perhaps because of the low content of
BY units [22]. Thus, their LUMO energy levels (around
3.14 eV) were calculated from the corresponding HOMO
energy level and the band gap. The LUMO energy levels of
PFBY copolymers were much lower than PFO (2.17 eV)
[11], confirmed that the importing of the BY units was
benefit to improve the electron injection. The energy levels
of PFO and PFBY copolymers were shown in Figure 8.
However, the barrier of electron injection is still higher than
that of the hole injection, suggesting the introduction of an
electron transport layer will much improve the performance
Figure 8 Energy levels of PFO and PFBY copolymers.
Figure 9 The EL spectra of type I diodes (ITO/PEDOT/polymer/LiF/Al) based on PFBY copolymers under 6, 7, 8, 9 and 10 V.

Wang M, et al. Sci China Chem April (2011) Vol.54 No.4
663
Figure 10 showed the current density-voltage-brightness
and current efficiency-current density characteristics of sin-
gle-layer devices. As shown in Figure 10(a), the turn-on
voltages were at 4.0–6.1 V for all devices of copolymers,
which were lower than that of PFO (7.0 V). The turn-on
voltages decreased with the increasing BY content, sug-
gesting better electron injection of copolymers. At the same
voltage, the current density of the copolymer devices were
higher than the PFO homopolymer diode, suggesting a bet-
ter charge recombination efficiency in the copolymers due
to the presence of the BY units. The PFO homopolymer
diode had a maximum brightness of 74.5 cd/m2, a maximum
current efficiency of 0.06 cd/A and an external quantum
efficiency (EQE) of 0.041%. The brightness and current
efficiency of four copolymers diodes were higher than PFO
devices. The device of PFBY5 was the best one among the
copolymers, with a maximum brightness of 485 cd/m2, a
maximum current efficiency of 0.29 cd/A and an external
quantum efficiency (EQE) of 0.10%. These results clearly
demonstrated that the introduction of BY unit can stabilize
the EL spectra and improve luminance and current effi-
ciency.
To improve the device performance, PVK (20 nm) hole-
transport layer and TPBI (20 nm) electron-transport layer
were used to construct multilayer diodes of PFBY5:type II
(ITO/PEDOT/PVK/PFBY5/LiF/Al), type III (ITO/PEDOT/
PFBY5/TPBI/LiF/Al) and type IV (ITO/PEDOT/PVK/
PFBY5/TPBI/LiF/Al). Table 3 summarizes the relevant EL
properties of multilayer devices. Figure 11 showed the EL
spectra of multilayer diodes. The EL spectra were stable
when the voltage increases and the turn-on voltages were at
4.5–5.0 V. The current density-voltage-brightness and cur-
rent efficiency-current density characteristics of multilayer
diodes were displayed in Figure 12. When the PVK was
Table 2 EL performance data of the single-layer devices
Polymer
PFO
Von a) (V) Maximum brightness (cd/m2)
Current efficiency (cd/A)
CIE coordinate b)
(0.17，0.11)
(0.19，0.40)
(0.21，0.45)
(0.21，0.48)
(0.23，0.50)
EQE (%)
0.04

max (nm)
424
7
74.5
238
365
485
112
0.06
0.12
0.17
0.29
0.15
PFBY1
PFBY2
PFBY5
PFBY10
488
6.1
5.8
4.6
4
0.05
492
0.07
500
0.10
500
0.05
a) Turn-on voltage. b) Measured at the voltage of 8 V.
Figure 10 Current density-voltage-brightness (a) and current efficiency-current density (b) for the PFO and PFBY copolymers. In Figure 10(a), current density-
voltage curves (solid symbol) and brightness-voltage curves (open symbol).
Table 3 EL performance data of the devices with different configuration based on PFBY5.
Device configuration
Type I
Vona) (V) Maximum brightness (cd/m2)
Current efficiency (cd/A)
CIE coordinatesb)
(0.21，0.48)
(0.21，0.47)
(0.20，0.47)
(0.20，0.47)
EQE (%)
0.10

max (nm)
500
4.5
4.5
4.5
5.0
485
628
0.29
0.30
1.61
1.81
Type II
500
0.11
Type III
500
2980
3012
0.58
Type IV
500
0.66
a) Turn-on voltage. b) Measured at the voltage of 8 V.

664
Wang M, et al. Sci China Chem April (2011) Vol.54 No.4
Figure 11 The EL spectra of different device structures for PFBY copolymers under 6, 7, 8, 9 and 10 V.
Figure 12 EL performance data of the devices with different configuration based on PFBY5.
introduced in the device (type II), there was only a little
4 Conclusions
change for current efficiency (0.30 cd/A), the brightness
(628 cd/m2) and the external quantum efficiencies (0.11%).
We synthesized a series of polyfluorenes containing BY
However, the introduction of TPBI significantly improved
the performance. The maximum current efficiency, the
maximum brightness, and the external quantum efficiencies
of type III were 1.61 cd/A, 2980 cd/m2 and 0.58%. The
multilayer devices containing PVK and TPBI further im-
proved the EL performance. The maximum current effi-
ciency, the maximum brightness, and the external quantum
efficiencies reached 1.81 cd/A, 3012 cd/m2 and 0.66%.
Overall, the EL performances of multilayer devices were
significantly improved compared to the corresponding single-
layer device.
units by Suzuki polymerization. Energy transfer from the
polymer main chain to the B2F moieties was more efficient
in thin film than that in solution. For the copolymers, the
blue emission of PFO steadily decreased and a new green
emission increased in intensity with increasing the BY con-
tent in solution. Complete energy transfer was achieved in
film besides PFBY1 copolymer. LUMO energy levels of
these copolymers and PFO were 3.14 and 2.17 eV, sug-
gesting that the introduction of BY unit would facilitate
electron injection in PLED. Based on the single-layer EL
device (ITO/PEDOT/polymer/LiF/Al), we found that the

Wang M, et al. Sci China Chem April (2011) Vol.54 No.4
665
DR. Synthesis and electroluminescence properties of fluorene-co-
diketopyrrolopyrrole-co-phenothiazine polymers. Polymer, 2000, 51:
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EL performance of PFBY copolymers surpassed PFO due to
the presence of the electron acceptor BY unit. The best effi-
ciency among the copolymers was given by PFBY5, with a
maximum brightness of 485 cd/m2, a maximum current ef-
ficiency of 0.29 cd/A, and an external quantum efficiencies
of 0.10 %. In multilayer devices of PFBY5, the introduction
of PVK (20 nm) and TPBI (20 nm) further greatly improved
EL performance. The type IV diodes gave the best per-
formance, with an onset voltage of 5.0 V, a current effi-
ciency of 1.81 cd/A, a maximum brightness of 3012 cd/m2,
and an external quantum efficiencies of 0.66% with the CIE
coordinates of (0.20, 0.47). All these results clearly demon-
strated that the introduction of the BY unit in the polyfluo-
rene can reduce barrier of electron injection and effectively
improve their EL performance.
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