Synthesis and optical and electroluminescent properties of novel conjugated copolymers derived from fluorene and benzoselenadiazole

Article abstract of DOI:10.1021/ma034134j

A novel series of light-emitting copolymers derived from 9,9-dioctylfluorene (DOF) and 2,1,3-benzoselenadiazole (BSeD) is prepared by means of palladium-catalyzed Suzuki coupling reaction. The feed ratios of DOF to BSeD were 50:50, 85:15, 92:8, and 98:2, respectively. All of the copolymers are soluble in common organic solvents and highly fluorescent in solid state. Devices from such copolymers emit orange-red light with λ_max = 570-600 nm. The maximal EL emissions of the devices slightly red-shifted gradually with increasing BSeD's contents. The maximal external quantum efficiency of the polymer light-emitting devices (PLED) reaches 1.0%, which indicates that this new seleno-containing EL polymer based on fluorene and benzoselenadiazole is a promising candidate for fabricating PLEDs.

Full text of DOI:10.1021/ma034134j

Macromolecules 2003, 36, 7453-7460
7453
Synthesis and Optical and Electroluminescent Properties of Novel
Conjugated Copolymers Derived from Fluorene and Benzoselenadiazole
Ren qia n g Ya n g, Ren yu Tia n , Qion g Hou , Wei Ya n g, a n d Yon g Ca o*
Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, China
Received J anuary 31, 2003; Revised Manuscript Received April 21, 2003
ABSTRACT: A novel series of light-emitting copolymers derived from 9,9-dioctylfluorene (DOF) and 2,1,3-
benzoselenadiazole (BSeD) is prepared by means of palladium-catalyzed Suzuki coupling reaction. The
feed ratios of DOF to BSeD were 50:50, 85:15, 92:8, and 98:2, respectively. All of the copolymers are
soluble in common organic solvents and highly fluorescent in solid state. Devices from such copolymers
emit orange-red light with λmax ) 570-600 nm. The maximal EL emissions of the devices slightly red-
shifted gradually with increasing BSeD’s contents. The maximal external quantum efficiency of the
polymer light-emitting devices (PLED) reaches 1.0%, which indicates that this new seleno-containing
EL polymer based on fluorene and benzoselenadiazole is a promising candidate for fabricating PLEDs.
In tr od u ction
parent chromophore.12 Regulating the HOMO and LUMO
energy levels permits the fine-tuning of charge injection
properties. In LED, the HOMO/LUMO energy difference
directly controls emission frequency, i.e., emission color.
Light-emitting polymers have revolutionized flat-
panel display technology. A large number of light-
emitting polymers have been introduced during the last
10 years.13 Polymers with aromatic or heterocyclic units
generally absorb light with wavelengths 300 to 600 nm
due to π-π* transitions.14 The color of light, quantum
efficiency (QE) of light emission, turn-on voltage, and
stability of the devices must be optimized for LEDs to
be applicable to commercial light-emitting devices.
There have been many attempts to improve the perfor-
mance of PLEDs. A high quantum efficient, pure, and
stable material is essential for commercial LED perfor-
mance, so many research groups have been developing
suitable materials. A series of electroluminescent ma-
terials has even been provided as the promising candi-
dates for high-resolution, full color, flat-panel polymeric
light-emitting diode (PLED) displays. Among those
polymers, polyfluorenes are an important class of elec-
troactive and photoactive material.15 In the last 4 years,
this research field has literally exploded because of
polyfluorenes’ exceptional optoelectronic properties for
applications in light-emitting diodes. Normally, poly-
fluorene homopolymers have large band gaps and emit
blue light. Emission color of polyfluorenes can be tuned
in entire visible region by incorporating narrow band
gap comonomer into polyfluorene backbone. Most widely
used narrow band gap comonomers are a variety of
aromatic heterocycles with S and N atoms.16,17
The notion that π-conjugated polymeric materials
should possess interesting and useful electronic and
optical properties is now well established in the scien-
tific literature.1 Many of these expectations have been
realized by the practical demonstration of electronic and
optoelectronic devices in which conjugated polymeric
materials are responsible for charge transport and/or
light generation.2 Examples include polymer light-
emitting diodes (LEDs),3 photovoltaic devices,4 and field-
effect transistors.5,6 The wide-ranging applications of
electroactive and photoactive conjugated polymers have
attracted great interest in the development of function-
alized polyacetylenes, polypyrroles, polythiophenes, poly-
anilines, polyfluorenes, etc.7 These materials combine
the excellent electrical and optical properties of metals
or inorganic materials with the advantages of polymeric
materials, such as low density, easy processability, and
synthetic tunability.8 The exceptional physical proper-
ties of conjugated polymers are mainly related to their
π-conjugated backbone which leads to a strong absorp-
tion in the UV-visible range and allows the generation
of stable and mobile charge carriers to have partial
oxidation or reduction.7,9 The electronic structure of
these conjugated polymers is modified by these revers-
ible redox reactions, leading also to new electronic states
within the band gap and associated electrochromic
properties.9,10 The electrical and optical activity of these
materials relies on the ability of the materials to
transport electrical charges: electrons and positive holes
through their structures. Such active organic materials,
such as oligomers, polymers, or smaller organic com-
pounds, have a commercial advantage over their inor-
ganic counterparts, such as silicon and gallium arsenide.
Much of the motivation for studying polymers stems
from the potential to tailor desirable optoelectronic
properties and processing characteristics by manipula-
tion of the primary chemical structure.11 Thus, it is easy
to raise or lower the HOMO and LUMO levels including
conjugation length control, as well as the introduction
of electron-donating or -withdrawing groups to the
Recently, we have reported a series of polyfluorene
copolymers prepared by Suzuki coupling of fluorene and
sulfur-containing heterocycles.18 The efficient energy
transfer due to exciton trapping on narrow band gap
heterocycle sites has been observed. Significant en-
hancement in the external quantum efficiencies of
electroluminescence by exciton confinement in the
random copolymers that have regions of different energy
gap within a single chain was reported first by Burn et
al.3d
In this contribution a series of novel copolymers
derived from substituted-fluorene and Se-containing
* To whom correspondence should be addressed. E-mail:
poycao@scut.edu.cn.
10.1021/ma034134j CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/03/2003

7454 Yang et al.
Macromolecules, Vol. 36, No. 20, 2003
Gen er a l P r oced u r e of P olym er iza tion .23 Carefully puri-
fied 2,7-dibromo-9,9-dioctylfluorene (2), 2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (3), 4,7-di-
bromo-2,1,3-benzoselenadiazole (4), (PPh3)4Pd0 (0.5-2.0 mol
%), and several drops of Aliquat 336 were dissolved in a
mixture of toluene and aqueous 2 M Na2CO3. The solvents
were further purified in advance. The solution was refluxed
with vigorous stirring for 3 days under argon atmosphere. At
the end of polymerization, 2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,9-dioctylfluorene was added to remove
bromine end groups and bromobenzene was added as a
monofunctional end-capping reagent to remove boronic ester
end groups, because boron and bromine units could quench
emission and contribute to excimer formation in LED applica-
tions.24 The whole mixture was then poured into methanol.
The precipitated material was recovered by filtration through
a funnel. The solid material was washed for 24 h using acetone
to remove oligomers and catalyst residues. The resulting
polymers were soluble in THF, CHCl3, and toluene. Yields:
∼65-80% respectively.
heterocycles, 2,1,3-benzoselenadiazole synthesized by
the palladium-catalyzed Suzuki coupling method, is
reported. In contrast to extensively investigated poly-
fluorene copolymers with S-containing heterocycless
thiophene9,15 and benzothiadiazole17,19sto the best of
our knowledge, this is a first report on electrolumines-
cent Se-containing conjugated polymer. Since the Se
atom has a much larger size and less electronegativity
than the S atom, it would have a more important
influence for the heteroatoms on the light-emitting
properties of the resulting copolymers. Complete syn-
thetic details and device characterization are presented.
The emission properties of Se-containing copolymers are
discussed in comparison with its sulfur analogue.
Exp er im en ta l Section
Gen er a l Deta ils. All manipulations involving air-sensitive
reagents were performed under an atmosphere of dry argon.
All reagents, unless otherwise specified, were obtained from
Aldrich, Acros, and TCI Chemical Co. and used as received.
P oly[2,7-(9,9-d ioct ylflu or en e)-a lt-4,7-(2,1,3-b en zosel-
en adiazole)] (5a). 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)-9,9-dioctylfluorene (3) (1.00 equiv) and 4,7-dibromo-
2,1,3-benzoselenadiazole (4) (1.00 equiv) were used in this
polymerization. Anal. Found: C, 61.38; H, 6.12; N, 3.63; Se,
10.21.
1
All the solvents were further purified before use. H and 13C
NMR spectra were recorded on a Varinan Inova 500 or Bruker
DRX 400 spectrometer operating at 500 and 100 MHz,
respectively, and were referred to tetramethylsilane. Analyti-
cal GPC was obtained using a Waters GPC 2410 in tetrahy-
drofuran via a calibration curve of polystyrene standards.
Elemental analyses were performed on Vario EL elemental
analysis instrument (Elementar Co.). The elemental selenium
analysis was recorded on a polarized Zeeman atomic absorp-
tion spectrophotometer, Hitachi Co.. UV-visible absorption
spectra were recorded on a HP 8453. The PL quantum yields
were determined on a Integrating Sphere IS080 with 325 nm
excitation from a HeCd laser (Mells Griod). EL efficiency and
brightness were carried out with calibrated silicon photodiode.
PL and EL spectra were recorded on Instaspec 4 CCD
spectrophotometer (Oriel Co.). Cyclic voltammetry was carried
out on a CHI660A electrochemical workstation with platinum
electrodes at a scan rate of 50 mV/s against calomel reference
electrode with nitrogen-saturated solution of 0.1 M tetrabut-
ylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile
(CH3CN).
1H NMR (500 MHz, CDCl3), δ (ppm): 8.04 (2H, fluorene
ring), 7.92 (2H, fluorene ring), 7.79 (2H, fluorene ring), 7.61
(2H, benzoselenadiazole ring), 7.26 (benzene ring), 2.12 (4H,
H-alkyl), 1.25-1.11 (m, 20H, H-alkyl), 0.95 (t, 6H, H-alkyl),
0.81 (m, 4H, H-alkyl).
13C NMR (100 MHz, CDCl3), δ (ppm): 160.44 (C-benzosel-
enadiazole ring), 152.04, 141.22, 137.48, 135.85, 129.04, 128.76,
124.64, 120.23 (C-fluorene ring, C-benzene ring), 55.80 (C9-
fluorene ring), 40.61, 32.23, 30.54, 30.09, 29.68, 24.47, 23.01,
14.47 (C-alkyl).
P oly[2,7-(9,9-d ioct ylflu or en e)-co-4,7-(2,1,3-b en zosel-
en adiazole)] (5b). 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)-9,9-dioctylfluorene (3) (0.50 equiv), 2,7-dibromo-9,9-
dioctylfluorene (2) (0.35 equiv), and 4,7-dibromo-2,1,3-benzo-
selenadiazole (4) (0.15 equiv) were used in this polymerization.
Anal. Found: C, 84.96; H, 9.78; N, 1.10; Se, 3.62.
1H NMR (500 MHz, CDCl3), δ (ppm): 8.03 (2H, fluorene
ring), 7.89, 7.85, 7.80, 7.77, 7.68 (fluorene ring), 7.60 (2H,
benzoselenadiazole), 7.48, 7.37, 7.27 (benzene ring), 2.12 (4H,
H-alkyl), 1.25-1.14 (m, 20H, H-alkyl), 0.94 (t, 6H, H-alkyl),
0.82 (m, 4H, H-alkyl).
2,7-Dibr om oflu or en e (1) an d 2,7-dibr om o-9,9-dioctylflu -
or en e (2) were prepared according to the published proce-
dures.20
13C NMR (100 MHz, CDCl3), δ (ppm): 160.47 (C-benzosel-
enadiazole ring), 151.57, 140.35, 126.93, 120.36 (C-fluorene
ring, C-benzene ring), 55.35 (C9-fluorene ring), 40.63, 32.27,
30.60, 30.05, 29.88, 24.52, 22.97, 14.08 (C-alkyl).
2,7-Bis(4,4,5,5-tetr a m eth yl-1,3,2-d ioxa bor ola n -2-yl)-9,9-
d ioctylflu or en e (3) was prepared following the modified
procedure from 2,7-dibromo-9,9-dioctylfluorene (2).21 The re-
sulting boronic ester was recrystallized from THF and metha-
nol and then further purification by column chromatography
(silica gel, 10% ethyl acetate in hexane) to give the product
mentioned in the title as a white solid (3.1 g, 46%), mp 128-
131 °C. 1H NMR (500 MHz, CDCl3), δ (ppm): 7.80 (d, 2H,
fluorene ring), 7.74 (s, 2H, fluorene ring), 7.71 (d, 2H, fluorene
ring), 1.99 (m, 4H, H-alkyl), 1.39 (s, 24H, CH3), 1.22-1.00 (m,
P oly[2,7-(9,9-d ioct ylflu or en e)-co-4,7-(2,1,3-b en zosel-
en a dia zole)] (5c). 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)-9,9-dioctylfluorene (3) (0.50 equiv), 2,7-dibromo-9,9-
dioctylfluorene (2) (0.42 equiv), and 4,7-dibromo-2,1,3-benzo-
selenadiazole (4) (0.08 equiv) were used in this polymerization.
Anal. Found: C, 83.99; H, 9.84; N, 0.43; Se, 1.35.
20H, H-alkyl), 0.81 (t, 6H, H-alkyl), 0.56 (m, 4H, H-alkyl). 13
C
1H NMR (500 MHz, CDCl3), δ (ppm): 8.06 (2H, fluorene
ring), 7.94, 7.87, 7.80, 7.78, 7.72 (fluorene ring), 7.65 (2H,
benzoselenadiazole), 7.51, 7.40, 7.28 (benzene ring), 2.16 (4H,
H-alkyl), 1.29-1.17 (m, 20H, H-alkyl), 0.96 (t, 6H, H-alkyl),
0.85 (m, 4H, H-alkyl).
NMR (100 MHz, CDCl3), δ (ppm): 150.86, 144.30, 134.04,
129.29, 119.77 (fluorene ring), 84.11 (C-alkyl), 55.57 (C9,
fluorene ring), 40.49, 32.18, 30.33, 29.58, 25.33, 23.98, 22.99,
14.48 (C-alkyl). Anal. Calcd for C41H64O4B2: C, 76.74; H, 10.04.
Found: C, 76.44; H, 9.90.
4,7-Dibr om o-2,1,3-b en zoselen a d ia zole (4).22 Bromine
(3.2 g, 0.02 mol) was added to a solution of 2,1,3-benzoselena-
diazole (1.83 g, 0.01 mol) and silver sulfate (3.12 g, 0.01 mol)
in concentrated sulfuric acid (20 mL). The mixture was shaken
at a room temperature for 1.25 h, the precipitate of silver
bromide was filtered off, and the filtrate poured into ice-water.
The precipitate, from ethyl acetate (500 mL), gave 4,7-dibromo-
2,1,3-benzoselenadiazole (2.05 g, 60%) as golden yellow needles,
mp 285-287 °C. 1H NMR (500 MHz, CDCl3), δ (ppm): 7.63
(s, 2H, phenylene ring).13C NMR (100 MHz, CDCl3), δ (ppm):
157.2, 132.1, 116.5. Anal. Calcd for C6H2Br2N2Se: C, 21.1; H,
0.6; Br, 46.9; N, 8.2. Found: C, 21.4; H, 0.9; Br, 46.9; N, 7.8.
13C NMR (100 MHz, CDCl3), δ (ppm): 160.48 (C-benzosel-
enadiazole ring), 152.23, 140.94, 140.44, 126.58, 121.92, 120.38
(C-fluorene ring, C-benzene ring), 55.76 (C9-fluorene ring),
40.79, 32.19, 30.44, 30.11, 29.61, 24.34, 22.99, 14.44 (C-alkyl).
P oly[2,7-(9,9-d ioct ylflu or en e)-co-4,7-(2,1,3-b en zosel-
en adiazole)] (5d). 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)-9,9-dioctylfluorene (3) (0.50 equiv), 2,7-dibromo-9,9-
dioctylfluorene (2) (0.48 equiv), and 4,7-dibromo-2,1,3-benzo-
selenadiazole (4) (0.02 equiv) were used in this polymerization.
Anal. Found: C, 87.97; H, 10.14; N, <0.3; Se, 0.46.
1H NMR (500 MHz, CDCl3), δ (ppm): 8.02 (2H, fluorene
ring), 7.85, 7.70, 7.68 (fluorene ring), 7.59 (2H, benzoselena-
diazole), 7.49, 7.37, 7.26 (benzene ring), 2.13 (4H, H-alkyl),

Macromolecules, Vol. 36, No. 20, 2003
Novel Conjugated Copolymers 7455
Sch em e 1. Syn th etic Rou te of th e Cop olym er s
Ta ble 1. Molecu la r Weigh ts of th e Cop olym er s a n d Cop olym er Com p osition Deter m in ed by Elem en ta l An a lysis
DOF/ BSeD
in the feed
composition
in the
in the
copolymers
Mn (×103)a
Mw/Mn
copolymersb
copolymersc
DOF 50/ BSeD 50 (5a )
DOF 85/ BSeD 15 (5b)
DOF 92/ BSeD 8 (5c)
DOF 98/ BSeD 2 (5d )
16
32
28
26
2.04
3.15
2.78
2.56
50:50
85:15
92:8
60:40
84:16
93.5:6.5
97.8:2.2
53.5:46.5
86:14
94:6
98:2
a
b
Measured by gel permeation chromatography (GPC) referenced to polystyrene standards. Calculated based on elemental analysis
of Se and carbon (Se was measured on a polarized Zeeman atomic absorption spectrophotometer). c Calculated based on elemental analysis
of N and carbon content (N was measured on a Vario EL elemental analysis instrument (Elementar Co.)).
1.25-1.14 (m, 20H, H-alkyl), 0.83 (t, 6H, H-alkyl), 0.69 (m,
4H, H-alkyl).
13C NMR (100 MHz, CDCl3), δ (ppm): 160.42 (C-benzosel-
enadiazole), 152.22, 140.92, 140.43, 126.56, 121.91, 120.34 (C-
fluorene ring, C-benzene ring), 55.75 (C9-fluorene ring), 40.78,
32.20, 30.43, 30.09, 29.60, 24.32, 23.00, 14.43 (C-alkyl).
both sides of each segments. In comparison, 5a has a
regular alternating A-B structure. The resulting co-
polymers are completely soluble in CHCl3, THF, and
toluene. On the basis of GPC, the number-average
molecular weights of these polymers are relatively high,
from 16 000 to 32 000 with a polydispersity index
(Mw/Mn) from 2.0 to 3.15, which is consistent with a
polycondensation reaction. The selenium contents of
these polymers measured by atomic absorption spec-
troscopy are close to the feed ratios (Table 1).
Resu lts a n d Discu ssion
Syn th esis a n d Ch em ica l Ch a r a cter iza tion . 2,7-
Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dio-
ctylfluorene was prepared by routine method.21 4,7-
Dibromo-2,1,3-benzoselenadiazole was prepared by
bromination of benzoselenadiazole.22 Following the syn-
thetic route in Scheme 1, conjugated copolymers derived
from 2,7-dibromo-9,9-dioctylfluorene (2), 2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluo-
rene (3), and 4,7-dibromo-2,1,3-benzoselenadiazole (4)
have been prepared by using palladium-catalyzed
Suzuki coupling methods. The utilization of 2,7-bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylflu-
orene (3) simplified their purification by column chro-
matography combined with the advantage of the presence
of a protecting group on the boronic acid moieties. The
comonomer ratios of DOF to BSeD are 50:50, 85:15, 92:
8, and 98:2, and the corresponding copolymers are
named 5a -d , respectively. The intent to adjust the
starting monomer ratios is to investigate the effects of
various copolymer compositions on the LEDs’ electrical
and optical properties. When the concentration of BSeD
in the copolymers is less than 50%, as the characteristic
of Suzuki coupling reaction, the obtained polymers
5b-d have random copolymers of a kind of special
structure that consists of the fluorene segments of
different lengths separated by a single BSeD unit from
The starting monomer ratios have been adjusted in
order to investigate the effect of copolymer composition
on the physical and optical properties. The actual ratio
of DOF to BSeD in the copolymer can be estimated by
elemental analysis. The calculated DOF/BSeD ratio
from elemental analysis of Se or N and C is listed in
Table 1, and it is very close to the feed ratio. A slightly
different result for the alternating copolymers (DOF/
BSeD50) calculated from Se content measured by atomic
absorption spectra might be due to low sensitivity for
samples with high Se content. The value calculated from
N content gives a more reasonable result. Incorporating
of BSeD into the polyfluorene main chain is also
consistent with the absorption spectra, where the
intensity of the 470 nm peak corresponding to BSeD
absorption (shown below) increases with increasing
BSeD content in the copolymers.
IR spectra of copolymers were examined and consis-
1
tent with designed structure. The H NMR (500 MHz,
CDCl3) spectra of the copolymers show that ca. 8.05-
7.70 ppm represent the H atoms on the fluorene ring,
2.12-0.70 ppm are for the H atoms at the substituted
octyl on C9 position of the fluorene ring, and about 7.60

7456 Yang et al.
Macromolecules, Vol. 36, No. 20, 2003
Ta ble 2. UV-Vis Absor p tion , Electr och em ica l, a n d P h otolu m in escen t P r op er ties of th e Cop olym er s in Solid Sta te F ilm s
optical band gap/eV photoluminescence
copolymers
λ(abs)max/nm
PFOa
BSeDb
Eox/V
Ered/V
HOMO/eV
LUMO/eV
λ
max/nm
QE/%
DOF50/BSeD50 (5a )
DOF85/BSeD15 (5b)
DOF92/BSeD8 (5c)
DOF98/BSeD2 (5d )
342, 487
375, 469
374, 468
382
3.18
2.88
2.88
2.88
2.18
2.32
2.30
1.39
1.30
1.31
1.28
-2.26
-2.20
-2.19
-2.18
-5.79
-5.70
-5.71
-5.68
-2.14
-2.20
-2.21
-2.22
595
579
563
568
16
52
21
27
a
Estimated from the onset of absorption peak responsible for fluorene segment. b Estimated from the onset of absorption peak responsible
for BSeD unit.
F igu r e 2. Cyclic voltammograms of the copolymers films on
glassy carbon in 0.1 mol/L Bu4NPF6, CH3CN solution.
that a new band structure forms due to band alterna-
tion. Two distinguished absorption features for the
copolymers demonstrate that the electronic configura-
tion of both components, DOF and BSeD, in the copoly-
mers are not mixed.
The electrochemical behavior of the copolymers was
investigated by cyclic voltammetry (CV). The positions
of CV figures of the copolymers are very similar except
alternating copolymer (DOF/BSeD50) (Figure 2). Table
2 summarizes oxidation and reduction potentials de-
rived from the onset of oxidation waves in the cyclic
voltammograms of the copolymers. The onset of oxida-
tion process is almost constant at about 1.28-1.32 V
for the copolymers except alternating copolymer. This
peak is attributed to p-doping of DOF segments. The
onset of n-doping waves of DOF segments occurs at
-(2.17-2.20) V except for the alternating copolymer.
The oxidation and reduction potentials of the alternat-
ing copolymer, Eox ) 1.39 V and Ered ) -2.14 V, are
very close to the data reported for polyfluorene ho-
mopolymer, Eox ) 1.4 V and Ip ) 5.8 eV.25 Unfortu-
nately, we were unable to record the p- and n-doping of
the BSeD unit after many trials. HOMO and LUMO
levels calculated according to an empirical formula
(EHOMO ) -e(Eox + 4.4) eV and ELUMO ) -e(Ered + 4.4)
eV)26 are also listed in Table 2. The oxidation and
reduction processes in the cyclic voltammograms and
the similarity of HOMO and LUMO levels values for
DOF segment in the copolymers and for polyfluorene
homopolymer show that the electronic configuration of
both DOF segments are not changed significantly by
incorporation of BSeD unit in the copolymers. This is
quite consistent with the two separated absorption
peaks observed in the UV-visible spectra (Figure 1).
P h otolu m in escen ce P r op er ties. The photolumi-
nescent (PL) spectra of the copolymers by excitation of
325 nm line of the HeCd laser in thin film are com-
pletely different from those of polyfluorene homopolymer
(Figure 3). Polyfluorene homopolymer has double peak
at about 426 and 439 nm due to the fluorene segment
with a small shoulder at around 490 nm due to vibronic
F igu r e 1. (a) UV-visible absorption spectra for the copoly-
mers in the solid-state films. (b) UV-visible absorption spectra
for the copolymers in the CHCl3 solution.
ppm represents the H atoms on the benzoselenadiazole
ring. From the 13C NMR (100 MHz, CDCl3) spectra of
the copolymers, about 160 ppm is the C atoms of
benzoselenadiazole ring, 152-120 ppm represent C
atoms on the fluorene and benzoselenadiazole rings, ca.
55 ppm is C9 on fluorene ring, and 40-14 ppm are C
atoms at the substituted octyl on C9 position of the
fluorene ring. 1H NMR and 13C NMR show that the
copolymers are right on target molecular structures
containing alkyl-substituted fluorene and benzoselena-
diazole.
Op tica l P r op er ties a n d Electr och em ica l Ch a r -
a cter istics. The UV-visible absorption properties of
the conjugated polymers based on fluorene and ben-
zoselenadiazole are presented in Table 2. The absorption
spectra of PFO-BSeD copolymers in the solid film show
two absorption peaks at ca. 380 and 470 nm. The 380
nm peak is due to the PFO segment, and the 470 nm
peak is due to the BSeD unit (Figure 1a). A long
wavelength peak for 2% BSeD copolymer is not appar-
ent in solid-state film, but it appears clear in the
chloroform solution spectra (Figure 1b). With increasing
BSeD concentration in the copolymers, the intensity of
the long wavelength peak increases. For the alternating
copolymer, the absorption feature at 380 nm due to the
PFO segment disappeared completely. This indicates

Macromolecules, Vol. 36, No. 20, 2003
Novel Conjugated Copolymers 7457
F igu r e 3. PL spectra for the copolymers in the thin films.
structure (Figure 3, solid line). Once the BSeD unit is
incorporated into the polyfluorene main chain, the PL
emission was changed dramatically; that is, the PL
emission due to the fluorene segment completely disap-
peared (Figure 3, dotted line). PL emission consists
exclusively of BSeD unit emission at around 568-595
nm depending on the BSeD content even though the
BSeD concentration is only 2% in the copolymer (Figure
3). The shapes of the PL spectra for copolymers with
different BSeD content are very similar, and the PL
peaks are significantly red-shifted with increasing BSeD
content in the copolymers, from λmax ) 568 nm for the
copolymer with 2% BSeD content to 595 nm for the
alternating copolymer. Since no such large red shift was
observed for absorption spectra of corresponding copoly-
mers, the significant red-shift in PL emission indicates
that the Stokes shift was increasing with the increase
in BSeD content in the copolymers. This fact implies
that the chain became more rigid with increasing BSeD
content in the copolymers. Similar increases in the
Stokes shift between the absorption and emission
spectra were reported for MEH-PPV at lower temper-
atures, and it was interpreted as the increase in the
conjugation length at lowering the temperature.27
The complete disappearance of fluorene emission and
the appearance of new emissions due to the BSeD unit
indicate that the BSeD unit serves as a powerful trap
in the copolymer chain. As we reported previously for
polyfluorene copolymers prepared by Suzuki coupling
of fluorene and narrow band gap sulfur-containing
heterocycles, the trapping mechanism is especially
favorable, since both HOMO and LUMO levels of the
narrow band gap unit are located within the band gap
of the host segment.18 Although we were unable to
detect the oxidation wave due to the BSeD unit in the
DOF-BSeD copolymers; however, two distinct oxidation
peaks (1.24 V for the PFO segment, 0.97 V for the BTDZ
unit) were recorded in cyclic voltammograms of sulfur
analogue, poly(fluorene-co-benzothiadiazole) (PFO-
BTDZ). This fact indicates unambiguously that both
HOMO and LUMO levels of BTDZ are located within
the band gap of the polyfluorene host. Considering the
similarity in absorption, PL, and EL spectra of both
types of copolymers and also that BSeD has a more
narrow gap than BTDZ, we could assume that both
HOMO and LUMO levels of BSeD unit are located
within the band gap of the PFO host segment. In
results, the excitation energy on the fluorene segments
is transferred to the BSeD unit with very high efficiency.
Excitons are confined and recombined in BSeD unit
isolated from both sides by fluorene segment and emit
orange-red light corresponding to BSeD emission. En-
ergy transfer from fluorene segment to BSeD unit may
F igu r e 4. (a) PL spectra of the copolymers in the chloroform
solution at the concentration of 1 × 10-5 mol/L. (b) PL spectra
for the copolymer 5d (DOF 98/BSeD 2) in the chloroform
solution in different concentration.
happen via intra- or intermolecular mechanism. To
elucidate the relative role of intrachain and interchain
energy transfer, we investigated the concentration
dependence of PL spectra of DOF-BSeD copolymers in
the solution. In Figure 4a, PL spectra of the DOF-BSeD
copolymers in very dilute concentration of 1 × 10-5
mol/L (sufficiently low to avoid a strong interchain
interaction between polymer chains in the solution)
were taken with 325 nm excitation from a HeCd laser.
As can be seen from Figure 4a, for copolymers with high
BSeD content, DOF/BSeD15 and DOF/BSeD50, polymer
solutions emit exclusively BSeD emission. This indicates
complete energy transfer from the fluorene segment to
BSeD unit when an intramolecular trapping mechanism
takes place. However, for DOF/BSeD2 and DOF/BSeD8,
along with the main emission peak in the red region, a
small emission peak with low intensity due to the
fluorene segment can be observed in the PL spectra.
Compared with the PL spectra in solid state films
(Figure 3) where fluorene emission is completely disap-
peared, the fact unambiguously indicates the energy
transfer in such concentrations is incomplete for copoly-
mers with low BSeD content (e8%). We also note that,
in the case of DOF 98/BSeD2 copolymer with a number-
average molecular weight of 2.6 × 104 (Table 1),
statistically, only two out of three copolymer chains
contain one BSeD unit; the interchain energy transfer
must take place in the solid state film in order to
depress PL emission from fluorene segment completely,
though the intramolecular trapping mechanism is obvi-
ously dominant. Figure 4b shows the PL spectra in
chloroform solution in different concentration for DOF98/
BSeD2 where the DOF emission disappears at a con-
centration of g1 × 10-4 mol/L (Figure 4b). This thres-
hold concentration decreases with increasing BSeD
content, being 1 × 10-6 mol/L for DOF 85/BSeD15
(Table 3). Table 3 lists threshold concentrations in
solution for each copolymer where emission from DOF
segment is completely depressed. A very low threshold

7458 Yang et al.
Macromolecules, Vol. 36, No. 20, 2003
Ta ble 3. Th r esh old Con cen tr a tion of Cop olym er s a t
Wh ich F lu or escen ce of F lu or en e Segm en t J u st Ap p ea r s
for comparison. In the case of PL spectra in the solid
film (Figure 3), EL emission due to the fluorene segment
was completely disappeared even though the BSeD
concentration is only 2% in the copolymer. Similar to
PL emission in the solid state, EL emission consists
exclusively of BSeD unit emission with an emission
peak at around 573-594 nm depending on the BSeD
content. EL peaks are also red-shifted with increasing
narrow band gap content as in PL emission. Table 4 lists
emission maximum and chromaticity coordinate for
devices from each copolymer. It is noted that excimer
emission appearing in devices from polyfluorene ho-
mopolymer (which is absent in the PL spectra of PF
homopolymer (Figure 3)) was quenched completely in
the copolymers, starting from a BSeD loading as low
as 2%. Incorporation of only 2% BSeD units into
copolymers chain depresses completely both the fluorene
host emission (λmax ) 420 nm) and the wide excimer
emission (λmax ) 540 nm) indicates that intrachain
energy trapping process (from fluorene segment to BSeD
unit) must be very fast and intrachain energy trapping
should be more efficient than interchain interaction
between fluorene segments. This indicates again that
intramolecular energy transfer is important in the
devices made from such copolymers.
For the copolymers mentioned above, energy transfer
probably occurs mainly within the polymer chain via a
intramolecular trapping mechanism. Similar behavior
was reported by us for the copolymer of fluorene with
4,7-dithien-2-yl-2,1,3-benzothiadiazole18 and poly(3,6-
carbazole) with benzothiadiazole synthesized by Suzuki
coupling.28 It indicates that intramolecular energy trap-
ping in such copolymers is a very quick and efficient
process, where an individual narrow band gap unit
separated by random fluorene segment from both sides
serves as a efficient trap for generated excitons on the
fluorene host. As a result, the generated excitons are
efficiently confined and recombined on BSeD sites, and
copolymers emit exclusively emission of narrow band
gap component.
copolymers
threshold concentration/(mol/L)
DOF 50/ BSeD 50 (5a )
DOF 85/ BSeD 15 (5b)
DOF 92/ BSeD 8 (5c)
DOF 98/ BSeD 2 (5d )
1 × 10-8
1 × 10-6
1 × 10-5
5 × 10-5
F igu r e 5. EL spectra for the copolymers in the films.
concentration for completed depression of fluorene host
emission seems to suggest that the intramolecular
trapping mechanism plays a major role in such copoly-
mers in the solution. However, the increase in the
threshold concentration with decreasing the BSeD
content in the copolymer (Table 3) indicates that the
interchain energy transfer also plays a certain role in
the energy transfer from the excited state of the fluorene
segment to the BSeD unit. Such a copolymer system
serves as an ideal prototype for investigation of intra-
and interchain polymer interaction in the polymer
solution.
It is worthwhile to also note that, in contrast to PL
emission in the solid state (Figure 3), PL peaks almost
do not change with the BSeD content in the copolymer
solution (Figure 4a) and peak emission always appears
at around 600 nm. As mentioned above, PL spectra in
the solid film are red-shifting with increasing BSeD
content (Figure 3). This fact seems to support the above
argument about the possible explanation of red shift of
PL emissions in the solid state, since chain rigidity of
the DOF-BSeD copolymer could not change signifi-
cantly in the dilute solution; therefore, the Stokes shift
does not change in the solution.
Listed in Table 2 are the PL efficiency and the
maximum emission wavelength of the copolymers in
thin films. The copolymer containing 15% BSeD (5b)
has the maximal efficiency of 52%. The efficiency of the
alternating polymer, 5a , decreases to 16%.
Electr olu m in escen t P r op er ties. A double-layer
device was fabricated in the configuration, ITO/PEDT
(or PVK)/DOF-BSeD/Ba/Al. The EL spectra from such
a device are presented in Figure 5, EL spectra of device
with polyfluorene homopolymer also shown in Figure 5
Preliminary device data are promising. The external
EL quantum efficiency (QE) reached 1.0% for copolymer
of DOF/BSeD15 with peak EL emission at ca. 580 nm
for devices with PVK as hole transport layer (Table 4).
When PEDT-PSS was used, the device efficiencies are
much lower. The results indicate that a relatively high
energy barrier exists for hole injection in this case. This
is quite consistent with HOMO levels of PFO-BSeD
copolymers (5.7-5.8 eV, Table 2) and work function of
hole transport layers: PVK, 5.8 eV, and PEDT-PSS,
4.8-5.0 eV.29 Efforts at further improvement of the
PLEDs’ efficiency with DOF/BSeD copolymers are in
progress and will be reported on elsewhere.
We have synthesized a PFO copolymer with benzo-
thiadiazole (BTDZ), a sulfur analogue of benzoselena-
diazole (BSeD) in a different composition. In Figure 6,
Ta ble 4. Device P er for m a n ces of th e Cop olym er s
device performancesa
copolymers
hole transport layer
λ
Elsmax/nm
V/V
I/mA cm-2
cd/m-2
QE/% chromaticity (x) coordinate (y)
DOF 50/ BSeD 50 (5a )
PVK
PEDT
PVK
PEDT
PVK
PEDT
600
594
582
580
574
573
13.9
11.0
13.7
9.8
29.0
22.1
5
5
5
5
5
5
147
16
345
156
278
148
0.42
0.05
1.0
0.44
0.81
0.43
0.706
0.705
0.698
0.696
0.689
0.683
0.292
0.295
0.300
0.304
0.314
0.316
DOF 85/ BSeD 15 (5b)
DOF 98/ BSeD 2 (5d )
a
Device structure: ITO/hole transport layer/PFO-BSeD/Ba/Al; active area 0.15 cm2.

Macromolecules, Vol. 36, No. 20, 2003
Novel Conjugated Copolymers 7459
shift of the emission of Se-containing copolymers in
comparison with its sulfur analogue is related to the
narrower π-π* gap of the BSeD unit. Stronger inter-
chain interaction between BSeD units neighboring
copolymer chains could contribute to the red shift of
emission peak too. Thus, these two types of copolymers
serve as good prototypes for the investigation of intra-
and interchain interaction in the EL polymers. Prelimi-
nary device performance is encouraging. A device fab-
ricated with copolymer with 15% BSeD content shows
the highest EL and PL efficiency. Part of the purposes
of the current work is to develop a new class of highly
efficient red-emitting polymer. Preliminary results pre-
sented in this paper by using the Se-containing hetero-
cycle in replacement of a well-investigated S analogue
seems to provide a way to reach this goal. Synthesis of
a series of new Se-containing EL polymers is in progress
and will be reported on in subsequent publications.
F igu r e 6. EL spectra for DOF 85/BSeD 15 and DOF 85/BTDZ
15 in thin films.
Ta ble 5. Device P er for m a n ces of th e Cop olym er s
Com p a r ison betw een BSeD a n d its Su lfu r An a logu e
BTDZa
Ack n ow led gm en t. This work was supported by
research grants from MOST National Research Project
(No. 2002CB613402) and the National Natural Science
Foundation of China (Project No. 29992530-6).
photoluminescence electroluminescence
copolymers
λ
max/nm Qmax/%
λ
max/nm
QEext/%
DOF 85/ BSeD 15 (5b)
DOF 85/ BTDZ 15
579
538
52
60
582
530
1.0
1.5
a
Device structure: ITO/pvk/copolymer/Ba/Al; active area 0.15
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