Synthesis and electroluminescent properties of high-efficiency saturated red emitter based on copolymers from fluorene and 4,7-Di(4-hexylthien-2-yl)-2,1, 3-benzothiadiazole

Article abstract of DOI:10.1021/ma049204g

Novel soluble conjugated random copolymers are synthesized by palladium-catalyzed Suzuki coupling reaction from 9,9-dioctylfluorene (DOF) and 4,7-di(4-hexylthien-2-yl)-2,1,3-benzothiadiazole (DHTBT) with DHTBT composition varying from 1 to 50 mol % in the copolymer. All of the polymers are soluble in common organic solvents and are highly photoluminescent. Polyfluorene fluorescence is quenched completely at a DHTBT concentration as low as 1% in the solid film. The copolymer films are highly fluorescent under UV irradiation in contrast to its parent analogue, 4,7-di(thien-2-yl)-2,1,3-benzothiadiazole (PFO-DBT), without alkyl substitution on thiophene rings. Devices made up of these copolymers emit saturated red light. The emission peaks are shifted from 613 to 672 nm when the DHTBT content increases from 1 to 50%. The highest external quantum efficiency achieved in the device configuration ITO/PEDT/PVK/PFO-DHTBT/Ba/Al is 2.54% with luminous efficiency 1.45 cd/A for the copolymer with emission peak at 638 nm for 10% DHTBT content, among the highest values so far reported for saturated red polymer emitters.

Full text of DOI:10.1021/ma049204g

Macromolecules 2004, 37, 6299-6305
6299
Synthesis and Electroluminescent Properties of High-Efficiency
Saturated Red Emitter Based on Copolymers from Fluorene and
4,7-Di(4-hexylthien-2-yl)-2,1,3-benzothiadiazole
Qion g Hou , Qin gm ei Zh ou , Yon g Zh a n g, Wei Ya n g, Ren qia n g Ya n g, a n d Yon g Ca o*
Institute of Polymer Optoelectrionic Material and Devices, Key Laboratory of Specially Functional
Materials and Advanced Manufacturing Technology, South China University of Technology,
Guangzhou, China 510640
Received April 24, 2004; Revised Manuscript Received J une 19, 2004
ABSTRACT: Novel soluble conjugated random copolymers are synthesized by palladium-catalyzed Suzuki
coupling reaction from 9,9-dioctylfluorene (DOF) and 4,7-di(4-hexylthien-2-yl)-2,1,3-benzothiadiazole
(DHTBT) with DHTBT composition varying from 1 to 50 mol % in the copolymer. All of the polymers are
soluble in common organic solvents and are highly photoluminescent. Polyfluorene fluorescence is
quenched completely at a DHTBT concentration as low as 1% in the solid film. The copolymer films are
highly fluorescent under UV irradiation in contrast to its parent analogue, 4,7-di(thien-2-yl)-2,1,3-
benzothiadiazole (PFO-DBT), without alkyl substitution on thiophene rings. Devices made up of these
copolymers emit saturated red light. The emission peaks are shifted from 613 to 672 nm when the DHTBT
content increases from 1 to 50%. The highest external quantum efficiency achieved in the device
configuration ITO/PEDT/PVK/PFO-DHTBT/Ba/Al is 2.54% with luminous efficiency 1.45 cd/A for the
copolymer with emission peak at 638 nm for 10% DHTBT content, among the highest values so far reported
for saturated red polymer emitters.
In tr od u ction
and qunquethiphene-1′′,1′′-dioxide with emission peak
at around 666 nm and luminous efficiency 0.02 cd/A has
been reported by the Leclerc group.30,31 The Morgado
group has reported similar results with an alternating
copolymer from ethylhexylfluorene and 2,5-bis(2′-vinyl-
thienyl)thiophene.32 Significant enhancement in ef-
ficiency (up to 1.4%) with emission peak at 663 nm for
PFO-DBT copolymer was recently reported by our
group.28 Cambridge Display Technology (CDT) an-
nounced a saturated red emitter with a high external
quantum efficiency of 3% and a power efficiency of 1.2
lm/W with an emission peak at 650 nm, though the
detailed chemical structure was not disclosed.24 Re-
cently, we also reported the syntheses and properties
of a red emitter from fluorene and selen-containing
heterocycle: 2,1,3-naphthoselenadiazole (NSeD) with a
luminous efficiency about 0.9 cd/A and an emission peak
at 657 nm.25 As a summary of the results so far reported
in the scientific literature, the luminous efficiency of a
saturated emitter composed of fluorene and condensed
thiophene-related heterocycles is around 1 cd/A. The
results are not unexpected considering the low photo-
luminescence efficiencies of copolymers from fluorene
and thiophene-related heterocycles in the solid state.25,28
Recently, the commercial value of electroluminescent
conjugated polymers have absorbed considerable inter-
est due to their possible application in large area flat-
panel displays1 and other optoelectronic applications.2,3
A wide range of conjugated polymers as the emissive
layers such as poly(p-phenylenevinylene) (PPV) and
their derivatives,1,4-7 polythiophenes,8,9 and polyfluo-
renes10,11 have been developed and investigated in order
to obtain stable high-efficiency RGB emitters. Among
these conjugated polymers, polyfluorene (PFO) polymers
and their derivatives have been studied since poly(9,9-
di-n-hexylfluorene) (PDHF) was first reported as a blue-
light-emitting polymer by Yoshino et al. in 1991.12
Polyfluorene polymers and their derivatives have the
advantages of a highly efficient fluorescence quantum
yield, an excellent thermal and oxidative stability, and
good solubility in common organic solvents.13-15 As a
result, these materials have emerged as a very promis-
ing candidate for polymer light-emitting diodes.16,17
Emission color of polyfluorenes can be changed over an
entire visible region by introducing narrow-band-gap
comonomer into the polyfluorene backbone. Most widely
used narrow-band-gap comonomers are a variety of
aromatic heterocycles containing N or S atoms, such
as thiophene,18-21 benzothiadiazole,17 and thiophene-
containing heterocycles.22-28 Compared with a high-
efficiency green polyfluorene emitter based on copolymer
from fluorene and benzothiadiazole, the high-efficiency
polymer emitter with a saturated red emission remains
a great challenge. Saturated red polyfluorene emitter
was first synthesized by the Dow group reported in a
patent literature probably by alternating copolymeri-
zation of fluorene and 4,7-dithienyl-2,1,3-benzothia-
diazole (DBT).29 The alternating copolymer of fluorene
In this paper we report the synthesis and character-
ization of the copolymers derived from fluorene with 4,7-
di(4-hexylthien-2-yl)-2,1,3-benzothiadiazole (DHTBT)
(structures in Scheme 1) in under or up to 50% (molar)
in the polymer composition by the Suzuki coupling
reaction. The alkyl side chain in the 4-position of both
thiophene rings gives the material a good solubility.
More important, the PL efficiency of such copolymers
is significantly enhanced in the solid state in comparison
with its unsubstituted analogue, PFO-DBT. Devices
based on the copolymers emit a saturated red light. The
highest quantum efficiency achieved in device configu-
ration ITO/PEDT/PVK/PFO-DHTBT/Ba/Al was 2.54%
for the copolymer with emission peak at 638 nm for 10%
* Corresponding author: Ph 86-020-87114609; Fax 86-020-
87114535; e-mail poycao@scut.edu.cn.
10.1021/ma049204g CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/28/2004

6300 Hou et al.
Macromolecules, Vol. 37, No. 17, 2004
Sch em e 1. Syn th esis Rou te of Com on om er a n d Cop olym er s
raphy (neutral alumina, light petroleum) to give the product
as colorless oil (yield: 73%).
DHTBT content. The device performance (external
quantum and luminous efficiencies) is almost doubled
for a single-layer device as compared to that from its
parent analogue, PFO-DBT, of the same composition
without alkyl substitution at thiophene rings.
4,7-Di(4-h exyl-2-th ien yl)-2,1,3-ben zoth ia d ia zole (5).28,36
PdCl2(PPh3)2 was added to a solution of 4,7-dibromo-2,1,3-
benzothiadiazole and tributyl-(4-hexyl-2-thienyl)stannane in
THF. The mixture was refluxed in a nitrogen atmosphere for
6 h. After the removal of the solvent at a reduced pressure,
the residue was purified by column chromatography on silica
gel (eluent CH2Cl2/petroleum (60-90 °C, 1:1)). Recrystalliza-
tion from ethanol gave the compound (yield: 88%) as orange
needles; mp 76-78 °C. FAB-MS: 468. 1H NMR (500 MHz,
CDCl3) δ (ppm): 7.97 (dd, 2H), 7.82 (s, 2H), 7.04 (dd, 2H), 2.66
(m, 4H), 1.70 (m, 4H), 1.25-1.53 (m, 12H), 0.90 (t, 6H). 13C
NMR (100 MHz, CDCl3) δ (ppm): 126.15, 126.38, 127.21,
127.90, 128.42, 139.75, 153.02. Element Anal. Calcd for
Exp er im en ta l Section
Ma ter ia ls a n d Mea su r em en t. All reagents, unless other-
wise specified, were purchased from Aldrich, TCI, and Acros
and were used as they were received. All solvents were
carefully dried and purified under a nitrogen flow. All ma-
nipulations involving air-sensitive reagents were performed
in a dry argon atmosphere. 1H and 13C NMR spectra were
recorded on Varian Inova 500 or Bruker DRX 400 in deuter-
ated chloroform solution. Number-average (Mn) and weight-
C
26H32N2S3: C, 66.67%; H, 6.84%; N, 5.98%; S, 20.51%.
Found: C, 66.30%; H, 6.94%; N, 5.99%; S, 20.82%.
average (Mw) molecular weights were determined by
a
Waters GPC 2410 in tetrahydrofuran (THF) using a calibra-
tion curve of polystyrene standards. The elemental analysis
was performed on Vario EL elemental analysis instrument
(ELEMENTAR Co.). UV-vis absorption spectra were recorded
on a HP 8453 spectrophotometer. Cyclic voltammetry were
done by using a CHI660A electrochemical workstation with
platinum electrodes at a scan rate of 50 mV/s against a calomel
reference electrode with an anhydrous and nitrogen-saturated
solution of 0.1 mol/L tetrabutylammonium hexafluorophos-
phate (Bu4NPF6) in acetonitrile (CH3CN). The deposition of
polymers on the electrode was done by the evaporation of a
dilute chloroform solution. The PL quantum yields were
determined in integrating sphere IS080 (Labsphere) with 405
nm excitation of UV diode laser (CL-2000, crystal laser). PL
spectra were determined by Instaspec 4 CCD spectrophotom-
eter (Oriel) uner of 325 nm line of the HeCd laser.
2,7-Dibromo-9,9-dioctylfluorene (1),33 2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (2),33 and
4,7-dibromo-2,1,3-benzothiadiazole (3)34 were prepared follow-
ing the already published procedures.
4-(Hexyl-2-th ien yl)sta n n a n e (4).28,35 To a solution of
3-hexylthiophene in anhydrous THF at -30 °C, n-BuLi was
added dropwise and the mixture was stirred at this temper-
ature under N2 for 2 h. Then tributylchlorostannane was
added, and the mixture was stirred at -30 °C for 30 min. The
mixture was poured into saturated aqueous sodium hydrogen
carbonate, and the organic phase was separated and washed
with saturated aqueous brine and then dried over an anhy-
drous sodium sulfate. The solvent was removed at a reduced
pressure, and the residue was purified by column chromatog-
4,7-Bis(5-b r om o-4-h e xyl-2-t h ie n yl)-2,1,3-b e n zot h ia -
d ia zole (6). 4,7-Di(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole
was added into THF in nitrogen flow. After the solid dissolved
completely, N-bromosuccinimide (NBS) was added in one
portion. The reaction mixture was stirred at a room temper-
ature for 2 h, hexane was added into the mixture, the white
precipitate formed was filtered off, the filtrate was extracted
with ether, and the organic layer was washed with brine and
dried over anhydrous sodium sulfate. The solvent was removed
at a reduced pressure to give the product as a red solid (yield:
82%); mp 101-104 °C. FAB-MS: 626. 1H NMR (500 MHz,
CDCl3) δ (ppm): 7.75 (s, 2H), 7.69 (s, 2H), 2.63 (m, 4H), 1.67
(m, 4H), 1.33-1.40 (m, 12H), 0.90 (t, 6H). 13C NMR (100 MHz,
CDCl3) δ (ppm): 151.99, 142.98, 138.45, 127.94, 125.02, 124.51,
111.59, 31.68, 29.70, 29.03, 22.67, 14.14. Element Anal. Calcd
for C26H30N2S3Br2: C, 49.84%; H, 4.79%; N, 4.47%; S, 15.34%;
Br, 25.26%. Found: C, 50.61%; H, 5.26%; N, 4.37%; S, 14.86%;
Br, 24.9%.
P olym er Syn th esis.33 The synthesis of the copolymers was
carried out using palladium-catalyzed Suzuki couplings. Care-
fully purified 2,7-dibromo-9,9-dioctylfluorene (1), 2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (2), 4,7-
bis(5-bromo-4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (DHTBT)
(6), and (PPh3)4Pd(0) (0.5-1.5 mol %) and several of drops
Aliquat336 were dissolved in a mixture of toluene and aqueous
solution of 2 M Na2CO3. The mixture was first put in the argon
atmosphere and was refluxed with vigorous stirring for 48 h.
Then 2,7-dibromo-9,9-dioctylfluorene and 2-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene were added
to end-cap the polymer chain. The whole mixture was poured

Macromolecules, Vol. 37, No. 17, 2004
High-Efficiency Saturated Red Emitter 6301
into methanol. The resulted solid was filtered and washed with
acetone to remove oligomers and catalyst residues. Yield:
76%-85%. The resulted polymers were soluble in conven-
tional organic solvents (toluene, chloroform, and tetrahydro-
furan). The results of elemental analyses of sulfur and carbon
for each copolymer were used for the calculation of actual
copolymer composition. Element Anal. Found: for PFO-
DHTBT1: C, 88.73%; S, 0.66%; for PFO-DHTBT5: C, 87.30%;
S, 1.24%; for PFO-DHTBT10: C, 86.20%; S, 2.32%; for PFO-
DHTBT15: C, 84.52%; S, 3.38%; for PFO-DHTBT25: C,
80.84%; S, 5.56%; for PFO-DHTBT35: C, 77.94%; S, 8.30%;
for PFO-DHTBT50: C, 77.14%; S, 10.92%.
NMR results: for PFO-DHTBT50: 1H NMR (500 MHz,
CDCl3) δ (ppm): 8.07 (s, 2H), 7.89 (s, 2H), 7.77 (d, 2H), 7.53
(s, 4H), 2.81 (s, 4H), 2.06 (s, 4H), 1.75 (s, 4H), 1.53 (s, 4H),
1.13-1.40 (m, 32H), 0.79-0.89 (m, 12H). 13C NMR (100 MHz,
CDCl3) δ (ppm): 152.74, 151.34, 140.38, 139.89, 137.70, 137.15,
133.35, 130.46, 128.25, 125.79, 125.28, 123.60, 119.94, 118.99,
55.31, 43.13, 31.81, 31.12, 30.16, 29.36, 24.05, 22.66, 14.07.
LED F a br ica tion a n d Ch a r a cter iza tion . Polymers were
dissolved in p-xylene and filtered through a 0.45 µm filter.
Patterned indium tin oxide (ITO)-coated glass substrates were
cleaned with acetone, detergent, distilled water, and 2-pro-
panol, subsequently in an ultrasonic bath. After treatment
with oxygen plasma, 150 nm of poly(3,4-ethylenedioxythio-
phene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS)
(Batron-P 4083, Bayer AG) was spin-coated onto the ITO
substrate followed by drying in a vacuum oven at 80 °C for 8
h. For some devices, poly(vinylcarbazole) (PVK, Aldrich) from
1,1,2,2-tetrachloroethane solution was coated on top of a dried
PEDOT-PSS layer subsequently. A thin film of electrolumi-
nescent polymer was coated onto the anode by spin-casting
inside a drybox. The film thickness of the active layers was
around 70 nm, as measured with an Alfa Step 500 surface
profiler (Tencor). We note that since PVK is not soluble in
p-xylene, PVK remains untouched during spinning top DHTBT
layer. A thin layer of Ba (4-5 nm) and subsequently 200 nm
layers of Al were vacuum-evaporated subsequently on the top
of an EL polymer layer under a vacuum of 1 × 10-4 Pa. Device
performances were measured inside a drybox. Current-
voltage (I-V) characteristics were recorded with a Keithley
236 source meter. EL spectra were recorded by Oriel Instaspec
IV CCD spectrograph. Luminance was measured by a PR 705
photometer (Photo Research). The external quantum efficien-
cies were determined by a Si photodiode with calibration in
an integrating sphere (IS080, Labsphere).
F igu r e 1. (a) UV-vis absorption spectra for the copolymers
in CHCl3 solution. (b) UV-vis absorption spectra for the
copolymers in thin film.
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/DHTBT
Mn
in the feed
in the
copolymers
(×103) Mw/Mn composition copolymersa
PFO-DHTBT1
PFO-DHTBT5
PFO-DHTBT10
PFO-DHTBT15
PFO-DHTBT25
PFO-DHTBT35
PFO-DHTBT50
16
15
67
18
3
2.7
2.5
2.9
2.3
2.0
1.9
2.1
99/1
95/5
97.3/2.7
94.9/5.1
90.3/9.7
85.7/14.3
75.7/24.3
62.9/37.1
51.3/48.7
Resu lts a n d Discu ssion
90/10
85/15
75/25
65/35
50/50
Syn th esis a n d Ch a r a cter iza tion of Cop olym er s.
The general synthetic routes toward the monomers and
polymers are outlined in Scheme 1. Monomer 5 was
synthesized via Stille coupling method from monomer
3 and monomer 4. Monomer 6 was prepared from
monomer 5 and NBS in THF. To the best of our knowl-
edge, monomer 6, 4,7-bis(5-bromo-4-hexyl-2-thienyl)-
2,1,3-benzothiadiazole, is synthesized for the first time.
Through the synthetic route in Scheme 1, conjugated
copolymers of different compositions were prepared from
2,7-dibromo-9,9-dioctylfluorene (DOF), 2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluo-
rene, and narrow-band-gap comonomer 4,7-bis(5-bromo-
4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (DHTBT) using
palladium-catalyzed Suzuki coupling methods. The
comonomer feed molar ratios of DOF to DHTBT are
99:1, 95:5, 90:10, 85:15, 75:25,65:35, and 50:50, respec-
tively, and the corresponding copolymers were named
PFO-DHTBT1, PFO-DHTBT5, PFO-DHTBT10, PFO-
DHTBT15, PFO-DHTBT25, PFO-DHTBT35, and
PFO-DHTBT50, respectively. For these copolymers,
n-octyl substituents in the 9-position of fluorene and
n-hexyl substituents in the 4-position of thiophene were
employed to improve the solubility of the resulted
4
40
a
Calculated from results of elemental analysis.
copolymers. At the end of polymerization, 2,7-dibromo-
9,9-dioctylfluorene was added to remove boronic acid
ester end groups, 2-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)-9,9-dioctylfluorene, as a monofunctional end-
capping reagent was added to remove bromine end
groups because boron and bromine units could quench
emission and contribute to red shift of EL emission in
PLEDs.37,38 The starting monomer molar ratios have
been adjusted in order to investigate the effect of
copolymer composition on the physical and optical
properties. The actual molar ratio of DOF/DHTBT in
the copolymer calculated from elemental analysis is very
close to the feed ratio as listed in Table 1. Incorporation
of DHTBT into the polyfluorene main chain was also
monitored by UV spectroscopy (λmax(oligofluorene seg-
ments) ) 380 nm, λmax(incorporated DHTBT) ) 520 nm)
(Figure 1). The number-average molecule weights (Mn)
of the synthesized copolymers, PFO-DHTBT1 to PFO-

6302 Hou et al.
Macromolecules, Vol. 37, No. 17, 2004
Ta ble 2. UV-vis P r op er ties a n d Electr och em ica l P r op er ties of th e Cop olym er s
optical band gapa/eV
copolymers
λ
abs,max/nm
for DOF segment
for DHTBT unit
Eox/V
1.36
HOMO/eV
LUMOb/eV
PFO-DHTBT1
PFO-DHTBT5
PFO-DHTBT10
PFO-DHTBT15
PFO-DHTBT25
PFO-DHTBT35
PFO-DHTBT50
383
2.91
2.94
2.91
2.94
2.95
2.94
2.94
-5.76
-5.68
-5.68
-5.64
-5.62
-5.62
-5.60
2.85
-3.59
-3.61
-3.58
-3.56
-3.59
-3.54
382, 520
381, 520
378, 520
371, 522
369, 526
370, 525
2.09
2.07
2.06
2.06
2.03
2.06
1.28
1.38,1.28
1.35,1.24
1.38,1.22
1.36,1.22
1.35, 1.20
a
b
Estimated from the onset wavelength of optical absorption in solid film. Calculated from HOMO level and the onset of optical
absorption of DHTBT unit.
DHTBT50, were determined by gel permeation chro-
matography (GPC) using a polystyrene standard (Table
1). A polydispersity index (Mw/Mn) was around 19-2.9
(Table 1), typical for a polycondensation reaction. As
shown in Scheme 1, the particular advantage of polym-
erization by Suzuki coupling is that each individual
narrow-band-gap unit is separated from both sides by
wide-band-gap segments for copolymers with the nar-
row-band-gap component under or equal to 50 mol %
in the copolymer. In this case we could expect the
narrow-band-gap unit functions as an exciton trap
which allows most efficient intramolecular energy trans-
fer from fluorene segment to DHTBT unit as shown in
other random copolymers synthesized via Suzuki cou-
pling.25,28,39
Op tica l a n d Electr och em ica l P r op er ties. The
absorption spectra of polymers PFO-DHTBT in CHCl3
solution and in solid state are shown in parts a and b
of Figure 1, respectively. All the copolymers showed
similar absorption spectra in the solution (Figure 1a)
and in the solid films (Figure 1b). The absorption
spectra of the copolymers measured in thin films consist
of two absorption peaks both in the solution and in solid
states. The absorption peak appeared at ca. 380 nm in
the solid films and is consistent with those reported for
poly(9,9-dihexylfluorene) homopolymer (388 nm).40 This
peak can be attributed to the fluorene segment in the
copolymer. An extralong wavelength absorption band
at ca. 520 nm, the intensity of which increases with
increasing of DHTBT content, can be attributed to the
DHTBT unit incorporated into the polyfluorene main
chain. In the case of the copolymer containing 1% of
DHTBT in the copolymer, the presence of the DHTBT
unit is not apparent from the absorption spectrum in
the solid film (Figure 1b). However, a weak 520 nm peak
is clearly seen in the solution spectrum of 1% DHTBT
copolymer (Figure 1a).
The electrochemical properties of the copolymers were
investigated by cyclic voltammetry (CV) in order to
gauge their electronic properties. All the electrochemical
data for PFO-DHTBT obtained from the films are listed
in Table 2. Except the copolymers with 1% and 5%
DHTBT, we can record two p-doping waves. However,
we failed to detect n-doping process in these copolymers.
For copolymers with DHTBT content equal to or over
10%, the first onset of p-doping wave decreases with the
increasing of DHTBT content from 1.28 V for PFO-
DHTBT10 to 1.20 V for PFO-DHTBT50. The second
onset of oxidation process is almost constant for all
copolymers at about 1.35-1.38 V. Since the oxidation
potential for polyfluorene homopolymer was observed
typically at 1.4 V,41 the second oxidation wave can be
assigned to the oxidation process for the fluorene
segments in the copolymers. The first oxidation wave
observed at 1.20-1.28 V in the copolymers must be
attributed to the oxidation process for the DHTBT unit.
For PFO-DHTBT1 only one peak can be observed in
the voltammogram. The position of the onset of this
peak is the same (1.36 V) as the second onset (DOF
segments) in copolymer with high DHTBT content.
HOMO level is calculated according to the empirical
formula (EHOMO ) -e(Eox + 4.4 eV).42 LUMO level is
estimated from the optical band gap. HOMO and LUMO
levels are listed in Table 2. HOMO levels responsible
for DHTBT unit slightly decreases with the increasing
of DHTBT content and becomes 5.60 eV for PFO-
DHTBT50. Keep in mind that a single DHTBT unit is
isolated from both sides by fluorene segments, and the
change in HOMO levels for DHTBT units must be
originated from interchain interaction. Two well-defined
oxidation processes in the cyclic voltammograms and
the similarity of the onset of oxidation waves for DOF
segment in the copolymers and homopolymer demon-
strate again that the electronic states of both compo-
nents, DOF and DHTBT, in the copolymers are not
mixed. This is consistent very well with the two sepa-
rated absorption peaks in the UV-vis spectra (Figure
1).
P h otolu m in escen t P r op er ties. Parts a and b of
Figure 2 show the photoluminescent (PL) spectra of the
polymers PFO-DHTBT by the excitation of the 325 nm
line of the HeCd laser in CHCl3 solution and in thin
film, respectively. In the thin solid film, polyfluorene
emission was completely quenched, and PL emission
consists exclusively of DHTBT emission at DHTBT
concentration as low as 1% (Figure 2b). PL emission
peaks are gradually red-shifted with the increasing of
DHTBT content in the copolymers, from 629 nm for the
copolymer PFO-DHTBT1 to 669 nm for the alternating
copolymer PFO-DHTBT50. In the CHCl3 solution
(Figure 2a) with the copolymer concentration of 1 × 10-5
mol/L (by repeat unit), excluding copolymer PFO-
DHTBT50, all the copolymers show two PL emission
consisting of two emission features peaked at around
420 and 655 nm. An intensity of the emission at around
420 nm decreases with the increasing of DHTBT content
in the copolymers while the intensity of the red-emitting
peak increases with the increasing of DHTBT content.
Therefore, 420 nm emission can be assigned to the
emission of DOF segment in the copolymer, and 655 nm
emission can be assigned to the emission of DHTBT unit
(Figure 2a). For copolymers with DHTBT content equal
to or over 5%, the PL emission is dominated by DHTBT
unit, and an emission from the PFO segments gradually
decreases with the increase of the DHTBT content.
Figure 3a shows the concentration dependence of PL
profile of PFO-DHTBT1 (1% DHTBT in copolymer) in
CHCl3 solution. With the increasing of the DHTBT1
concentration in the solution, the intensity of 420 nm
peak (responsible for DOF emission) decreases while the

Macromolecules, Vol. 37, No. 17, 2004
High-Efficiency Saturated Red Emitter 6303
F igu r e 2. (a) PL spectra of the copolymers in the CHCl3
solution in the concentration of 1 × 10-5 mol/L under 325 nm
excitation. (b) PL spectra of the copolymers in thin film under
325 nm excitation.
F igu r e 3. (a) Concentration dependence of PL profile of PFO-
DHTBT1 (1% DHTBT in copolymer) in CHCl3 solution. (b)
Critical concentration (in log scale) as a function of molar ratio
of DHTBT in the copolymers.
655 nm peak increases. Polyfluorene emission was
completely quenched when the concentration of PFO-
DHTBT1 is equal to or greater than 1 × 10-2 mol/L
(Figure 3a). We define a concentration at which poly-
fluorene fluorescence (420 nm) was completely quenched
as a critical concentration in the copolymer solution
(determined practically as a concentration at which PL
emission at 420 nm with intensity plotted in a logarithm
scale is completely disappeared). Figure 3b shows the
curve of critical concentration (in log scale) vs molar
ratio of DHTBT comonomer in the copolymer in the
solution. The critical concentration of PFO-DHTBT
decreases gently with the increasing of the DHTBT
content in the copolymers in the solution from 10-2
mol/L for PFO-DHTBT1 to 10-5 mol/L for PFO-
DHTBT50. For comparison, in Figure 3b we also in-
cluded critical concentration vs DBT content for PFO-
DBT copolymers which has no alkyl substitution in the
thiophene rings.28 As can be seen from Figure 3b, the
critical concentration of PFO-DBT copolymers shows
significantly larger dependence on the molar ratio of
narrow-band-gap component. This fact indicates that
the alkyl substitution on the thiophene rings in DHTBT
greatly reduces the interchain interaction of PFO-
DHTBT copolymers in comparison with that of PFO-
DBT. We have reported previously that PFO-DBT
copolymers shows very low PL efficiency in the solid
state films, and the PL efficiency decreases very quickly
with the increasing of DBT content (Table 3).28 This fact
was thought to be the results of increasing interchain
interaction with the increase of DBT content in the
copolymers.28 It is suggested that interchain interaction
Ta ble 3. P L Em ission P ea k a n d P L Efficien cy of
P F O-DHTBT (in Com p a r ison w ith P F O-DBT) Th in
Solid F ilm s
photo-
photo-
luminescence
luminescence
λPl,max
nm
/
QEPL
(%)a
λPl,max
nm
/
QEPL
(%)b
copolymers
copolymers
PFO-DHTBT1
PFO-DHTBT5
PFO-DHTBT10
PFO-DHTBT15
PFO-DHTBT25
PFO-DHTBT35
PFO-DHTBT50
629
636
647
642
670
678
669
88.2 PFO-DBT1
74.6 PFO-DBT5
68.6 PFO-DBT10
69.6 PFO-DBT15
50.4 PFO-DBT25
43.8 PFO-DBT35
19.0
635
651
655
678
678
685
11.4
12.5
8.6
7.9
5.2
4
a
b
Obtained under 405 nm excitation. Obtained under 325 nm
excitation.
should be significantly reduced for PFO-DHTBT due
to alkyl substitution on thiophene rings in comparison
with its parent analogue, PFO-DBT. As a result, we
can expect that PFO-DHTBT copolymer might have the
enhanced PL efficiency in the solid state. Table 3 lists
the PL efficiencies obtained with 405 nm excitation. To
make a comparison, PL efficiency of PFO-DBT28 is
listed in Table 3, too. Copolymers of PFO-DHTBT have
much higher PL efficiency in the solid film than those
of PFO-DBT copolymers. This fact indicates that the
introduction of the long alkyl chain into the thiophene
ring prevents a close packing of polymer chains and
thereby reduces significantly concentration quenching
due to the interchain interaction in PFO-DHTBT
copolymers. The fact that PL emission of PFO-DHTBT

6304 Hou et al.
Macromolecules, Vol. 37, No. 17, 2004
F igu r e 5. Current-voltage and luminance-voltage curve of
copolymers PFO-DHTBT10 with device structure of ITO/
PEDT/PVK/PFO-DHTBT/Ba/Al.
F igu r e 4. EL spectra of devices from the copolymers.
Ta ble 4. Device P er for m a n ce of P F O-DHTBT
Cop olym er s in Device Con figu r a tion :
ITO/P EDT/P F O-DHTBT/Ba /Al
Ta ble 5. Device P er for m a n ce fr om P F O-DHTBT
Cop olym er w ith P VK a s a n An od e Bu ffer (in Device
Con figu r a tion : ITO/P EDT/P VK/ P F O-DHTBT/Ba /Al)
device performance
mA cd
cm-2 m-2 cd/A
chromaticity
coordinates
Qext/
λEl,max
nm
/
device performance
chromaticity
coordinates
Qext/
copolymers
V
%
x
y
λEls,max
nm
/
mA cd
cm-2 m-2 Cd/A
%
x
y
PFO-DHTBT1
PFO-DHTBT5
613 8.5 22.7 84.6 0.37 0.65 0.60 0.38
625 8.0 25.1 146.9 0.58 1.02 0.65 0.35
copolymers
V
PFO-DHTBT5
PFO-DHTBT10
PFO-DHTBT15
634
638
647
15.0 33.3 366 1.1
10.6 28.5 413 1.45 2.54 0.66 0.34
9.6 32.8 392 1.19 2.09 0.67 0.33
1.93 0.66 0.34
PFO-DHTBT10 629 5.5 41.1 257.5 0.63 1.10 0.66 0.34
PFO-DHTBT15 634 5.0 42.3 350.9 0.83 1.45 0.66 0.34
PFO-DHTBT25 672 4.0 34.3 43.2 0.13 0.42 0.67 0.33
PFO-DHTBT35 671 3.5 28.3 26.0 0.09 0.31 0.68 0.32
PFO-DHTBT50 669 4.5 26.3 19.7 0.08 0.25 0.68 0.32
the PL emission of corresponding copolymers. EL peaks
are also red-shifted with the increasing of DHTBT
content as in PL emission. EL emission peaks are
shifting from 613 nm for PFO-DHTBT1 to 669 nm for
PFO-DHTBT50 (Figure 4 and Table 4). The chromatic-
ity coordinates changed from x ) 0.60, y ) 0.38 for
PFO-DHTBT1 to x ) 0.68, y ) 0.32 for PFO-
DHTBT50 (Table 4). The great red shift in EL emission
with the increase in DHTBT content indicates an
aggregation between DHTBT units of neighboring chains.
For small molecule dye-doped polymer LEDs, it is
typically observed a big difference between PL and EL
spectra at a low doping concentration region, for ex-
ample for devices fabricated with blend films from Eu
complex/CNPPP,44 PtOEP/PFO,45 and Ir complex doped
PFO and PVK.46 For these systems host PL emissions
were quenched completely only at a doping concentra-
tion of g4-8%, much higher than the doping concentra-
tion of 1-2%, at which the host EL emission is com-
pletely quenched. For copolymers mentioned in this
paper, the host emission has been quenched at DHTBT
content as low as 1% for both PL and EL emission. This
fact seems to indicate that, in contrast to the dye-doped
polymer system where energy transfer is intermolecu-
lar, the energy transfer of PFO-DHTBT copolymers
occurred mainly within polymer chain via intramolecu-
lar trapping mechanism along conjugated polymer
chain. As a result, for copolymers PFO-DHTBT, the
generated excitons are efficiently confined on the
DHTBT site, copolymers emit exclusively emission of
the narrow-band-gap component, DHTBT.
is around 5-10 nm blue-shifted (Table 3) than that of
PFO-DBT copolymers of same composition indicates
also less interchain interaction in the PFO-DHTBT
copolymers. We note that PL efficiency of PFO-DHTBT
decreases with the increasing of the DHTBT content in
the copolymers from 88% for PFO-DHTBT1 to around
19% for PFO-DHTBT50. Like DBT in PFO-DBT
copolymer, the DHTBT unit in the PFO-DHTBT co-
polymer serves as a quenching channel (both radiative
and nonradiative) in these copolymers. The PL efficiency
measurement is consistent well with the dependence of
the critical concentration on DHTBT content shown in
Figure 3b.
Electr olu m in escen t P r op er ties. The EL perfor-
mance of the copolymers was examined in the device
configuration of ITO/PEDT/PFO-DHTBT/Ba/Al and
ITO/PEDT/PVK/PFO-DHTBT/Ba/Al. The typical EL
spectra from such device are shown in Figure 4. The
EL spectrum of polyfluorene homopolymer is also shown
in Figure 4 for comparison. We note that due to the
introduction of a small amount of (from DHTBT content
1%) of DHTBT into polyfluorene chain, PFO emission
at around 430 nm and a broad excimer emission43
peaked at around 530 nm observed typically for poly-
fluorenene homopolymers are completely quenched
(Figure 4). We note that recently several groups at-
tributed this long wavelength emission shoulder to the
fluorenone defects introduced by photooxidation and
thermal oxidization.44-46 For devices fabricated from
PFO-DHTBT copolymers of all compositions, starting
from DHTBT content as low as 1%, only a saturated
red emission responsible for DHTBT unit has been
observed. This fact indicates that energy transfer from
DOF segment to DHTBT unit is very efficient and quick
process which suppress both radiative decay from DOF
segment and excimer emission. EL peaks are generally
10 nm blue-shifted (Figures 2 and 4) in comparison with
Preliminary device data are promising. The external
EL efficiencies in our standard device configuration ITO/
PEDT/PFO-DHBT/Ba/Al with PEDT intralayer vary
with copolymer compositions. The device performance
is presented in Table 4. The external quantum efficien-
cies increase initially with the DHTBT content, reaching
the maximum for copolymer with 15% DHTBT content,
and then gradually decrease. The highest external EL

Macromolecules, Vol. 37, No. 17, 2004
High-Efficiency Saturated Red Emitter 6305
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1.45%, and the luminous efficiency was 0.83 cd/A at the
luminance of 351 cd/m2 for copolymer with PFO-
DHTBT15. As indicated by the CV measurement, the
HOMO level of PFO-DHTBT copolymers is located
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We synthesized a series of new conjugated copolymers
PFO-DHTBT, composed of random or alternating 9,9-
dioctylfluorene and DHTBT, via a palladium-catalyzed
Suzuki coupling reaction. All the obtained copolymers
have excellent solubility in common organic solvents,
such as CHCl3, THF, and toluene. The efficient energy
transfer due to exciton trapping on narrow-band-gap
DHTBT sites has been observed. PL emission from
fluorene segments was completely quenched at DHTBT
concentration as low as 1% in the solid-state film. The
copolymers are highly fluorescent at 620-660 nm under
UV irradiation. A double-layer LED device with these
polymers as the emitting layer shows a saturated red
emission. The highest external quantum efficiency,
2.54%, and luminous efficiency, 1.45 cd/A, which peaked
at around 630 nm are reached for the device fabricated
with copolymer of 10% DHTBT content. This is among
the highest values so far reported for saturated red
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MA049204G