Quinoxaline-containing polyfluorenes: Synthesis, photophysics, and stable blue electroluminescence

Article abstract of DOI:10.1021/ma048118d

Six new copolymers of 9,9′-dioctylfluorene and 2,3-bis(p-phenylene) quinoxaline were synthesized, characterized, and used as blue-emitting materials in light-emitting diodes (LEDs). The glass transition temperature increased from 67°C in poly(9,9′-dioctylfluorene) (PFO) to as high as 160 °C in the alternating copolymer. All the copolymers emitted blue light in dilute toluene solution (417-423 nm), with photoluminescence (PL) quantum yields that decreased from 72% to 26% with increasing quinoxaline content, and as thin films (429-439 nm). The PL lifetimes of solutions and thin films were 308-617 ps. Blue electroluminescence with Commission Internationale de L'Eclairage coordinates of (0.16, 0.06) was achieved. Luminances of 240-520 cd/m² varied with the quinoxaline content. External quantum efficiencies of up to 1% at brightnesses of over 100 cd/m² were obtained. Enhancement in LED performance by factors of 3-6 compared to that of PFO was achieved in a copolymer containing 5 mol % 2,3-bis(pphenylene)quinoxaline. Significant enhancements in LED brightness and external quantum efficiency by factors of up to 120 were observed in copolymers containing 15-50 mol % 2,3-bis(p-phenylene) quinoxaline when using bilayer LiF/Al cathodes instead of Al. These results demonstrate that composition can be used to optimize the light-emitting properties of electroluminescent copolymers and that the fluorene - quinoxaline copolymers are useful materials for developing stable blue LEDs.

Full text of DOI:10.1021/ma048118d

Macromolecules 2005, 38, 1553-1563
1553
Quinoxaline-Containing Polyfluorenes: Synthesis, Photophysics, and
Stable Blue Electroluminescence
Abhishek P. Kulkarni, Yan Zhu, and Samson A. Jenekhe*
Departments of Chemical Engineering and of Chemistry, University of Washington,
Seattle, Washington 98195
Received September 13, 2004; Revised Manuscript Received December 4, 2004
ABSTRACT: Six new copolymers of 9,9′-dioctylfluorene and 2,3-bis(p-phenylene)quinoxaline were
synthesized, characterized, and used as blue-emitting materials in light-emitting diodes (LEDs). The
glass transition temperature increased from 67 °C in poly(9,9′-dioctylfluorene) (PFO) to as high as 160
°
C in the alternating copolymer. All the copolymers emitted blue light in dilute toluene solution (417-
23 nm), with photoluminescence (PL) quantum yields that decreased from 72% to 26% with increasing
4
quinoxaline content, and as thin films (429-439 nm). The PL lifetimes of solutions and thin films were
3
08-617 ps. Blue electroluminescence with Commission Internationale de L’Eclairage coordinates of (0.16,
2
0
.06) was achieved. Luminances of 240-520 cd/m varied with the quinoxaline content. External quantum
2
efficiencies of up to 1% at brightnesses of over 100 cd/m were obtained. Enhancement in LED performance
by factors of 3-6 compared to that of PFO was achieved in a copolymer containing 5 mol % 2,3-bis(p-
phenylene)quinoxaline. Significant enhancements in LED brightness and external quantum efficiency
by factors of up to 120 were observed in copolymers containing 15-50 mol % 2,3-bis(p-phenylene)-
quinoxaline when using bilayer LiF/Al cathodes instead of Al. These results demonstrate that composition
can be used to optimize the light-emitting properties of electroluminescent copolymers and that the
fluorene-quinoxaline copolymers are useful materials for developing stable blue LEDs.
Introduction
acteristics by copolymerizing with an electron-deficient
n-type) building block while retaining the blue emis-
(
Organic light-emitting diodes (OLEDs) based on
organic semiconductors and conjugated polymers have
sion, with the aim of achieving stable blue OLEDs with
high brightness and EQE. One of the challenges of this
approach is to improve the electron-transport properties
while preserving the desired pure blue emission and
high luminescence quantum yields. We point out that
such an approach to improving the electron-transport
ability of blue-emitting polyfluorenes has also been
employed previously by using n-type building blocks
attracted much attention as candidates for the next
generation of full-color flat panel displays.1-13 OLED
displays in consumer products such as cell phones and
digital cameras are already on the market. Achievement
of stable blue electroluminescence with high external
quantum efficiency (EQE) remains an important chal-
lenge on the path toward full color polymer displays.
Polyfluorenes have emerged as very promising materials
for blue emission due to their unique combination of
high thermal stability, high hole mobilities, easy pro-
cessability, and high photoluminescence (PL) quantum
1
2a
12b
12c
such as oxadiazole,
quinoline,
pyridine,
and
quinoxaline,1
2b,13
either in the main chain or as pen-
dants to the polyfluorene backbone. However, studies
of the variation in electroluminescence of the copolymers
as a function of the copolymer composition were not
5
,6
yields in the solid state. However, a major problem
in using the poly(9,9′-dialkylfluorene)s as blue-emitting
materials in OLEDs is the undesirable lower energy
emission band centered at 530-540 nm that appears
under device operation, turning the pure blue emission
to blue-green color.
the pure blue emission from the polyfluorenes have been
performed, except in the recently reported fluorene-
12c
pyridine copolymers.
Systematic investigations of
such structure-property relationships of polyfluorene
copolymers are essential for a full understanding of the
role of a particular n-type moiety. We chose quinoxaline
as the n-type building block since it is known to have a
high electron affinity and good thermal stability and has
been successfully incorporated in polymers for use as
electron-transport materials in multilayer OLEDs.1
While there are a few previous reports on quinoxaline-
containing copolymers based on p-type emissive poly-
5
a,b,7
Many approaches to stabilize
8,9
explored with varying degrees of success. Due to their
high PL quantum yields, polyfluorenes have been
copolymerized with a host of other comonomers, both
p-type and n-type, to tune the emission color or enhance
the charge-injection and -transport properties in the
predominantly p-type poly(9,9′-dioctylfluorene) (PFO)
4,15
1
2b,13
10-13
mers such as polyfluorene
and poly(p-phenylene-
backbone.
Fluorene-containing polymers have been
vinylene) (PPV),16 only alternating quinoxaline-fluo-
shown to emit efficiently across the entire visible
rene copolymers were reported,1
2b,13
and poor OLED
1
d,e
spectrum.
performance (0.013% EQE) was obtained using them as
blue emissive materials.1 Additionally, the prior al-
ternating quinoxaline-fluorene copolymers contained
linear rodlike 6,6′-bis(3-phenylquinoxaline) units in the
main chain, whereby the resultant thin film blue
emission had emission maxima at 455-460 nm with
poor Commission Internationale de L’Eclairage (CIE)
coordinates.1
Blue-emitting PFO is a p-type material that trans-
3b
ports holes with high mobility.6 However, electron
injection and transport in PFO is poor. Our motivation
for the present work was to synthesize polyfluorenes
with improved electron-injection and -transport char-
*
To whom correspondence should be addressed. E-mail:
2b,13
jenekhe@u.washington.edu.
1
0.1021/ma048118d CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/02/2005

1554 Kulkarni et al.
Macromolecules, Vol. 38, No. 5, 2005
1
Chart 1
50QXF Copolymer. H NMR (CDCl
3
): δ (ppm) 8.24 (br,
2
0
1
7
H), 7.65-7.80 (m, 16H), 2.08 (br, 4H), 1.10-1.20 (m, 20H),
-
1
.80 (m, 10H). FT-IR (film, cm ): 3060, 3033, 2926, 2853,
607, 1532, 1464, 1398, 1344, 1249, 1220, 1059, 1016, 977, 819,
60, 604.
Other copolymers showed NMR and FT-IR spectra similar
to those of 50QXF copolymer except that the relative intensi-
ties of the signals are different due to the different composition
of the copolymer. 2QXF showed an NMR spectrum similar to
that of PFO homopolymer due to the low concentration of
quinoxaline moiety.
Characterization. FT-IR spectra were taken on a Perkin-
1
Elmer 1720 FTIR spectrophotometer with NaCl plates.
H
NMR spectra were recorded on a Bruker-DRX499 at 500 MHz.
Gel permeation chromatography (GPC) analysis of the poly-
mers was performed on a Waters gel permeation chromato-
graph with Shodex gel columns and Waters model 150 C
refractive index detectors using THF as eluent and polystyrene
standards as reference. DSC analysis was performed on a TA
In this paper, we report the synthesis, photophysics,
and electroluminescence of a series of six new 2,3-bis-
p-phenylene)quinoxaline-containing poly(9,9′-dioctyl-
fluorene)s (QXFs) where the quinoxaline unit is at-
tached such that the polyfluorene backbone is kinked,
thereby improving the chance of retention of blue
emission with deep blue CIE coordinates. A series of
polyfluorene copolymers containing 2, 5, 15, 25, 33, and
Instrument Q100 differential scanning calorimeter under N
at a heating rate of 10 °C/min, and thermogravimetric analysis
TGA) was conducted with a TA Instrument Q50 thermogravi-
2
(
(
metric analyzer at a heating rate of 20 °C/min under a nitrogen
gas flow. UV-vis absorption spectra were recorded on a
Perkin-Elmer model Lambda 900 UV/vis/near-IR spectropho-
tometer. The PL emission spectra were obtained with a Photon
Technology International (PTI) Inc. model QM-2001-4 spec-
trofluorimeter.
5
2
0 mol % quinoxaline, denoted 2QXF, 5QXF, 15QXF,
5QXF, 33QXF, and 50QXF (Chart 1), respectively,
were synthesized by Suzuki coupling reaction and
systematically investigated. Structure-property rela-
tionships of the copolymers were investigated by dif-
ferential scanning calorimetry (DSC), optical absorption,
steady-state PL, time-resolved PL decay dynamics, PL
quantum yield measurements, and electroluminescence.
Systematic trends in the variation of relevant copolymer
properties such as the glass transition temperature and
PL quantum yields were observed. Bright blue elec-
troluminescence (EL) was obtained from the QXF
copolymers, demonstrating that they can be used as
stable blue electroluminescent materials in polymer
LEDs.
To measure the PL quantum yields (Φ
f
), polymer solutions
in spectral grade toluene were prepared. The concentration
-
5
(∼10 M) was adjusted so that the absorbance of the solution
-
5
would be lower than 0.1. A 10 M solution of 9,10-diphen-
1
7
f
ylanthracene in toluene (Φ ) 0.93) was used as a standard.
Time-Resolved Photoluminescence Decay Dynamics.
Fluorescence decays of the copolymers in solution and thin
films were obtained on a PTI model QM-2001-4 spectrofluo-
rimeter equipped with a Strobe Lifetime upgrade. The instru-
ment utilizes a nanosecond flash lamp filled with a high-purity
nitrogen/helium (30:70 v/v) mixture as the excitation source
and a stroboscopic detection system. The decay curves were
analyzed using a software package provided by the manufac-
2
turer. Reduced ø values, Durbin-Watson parameters, and
weighted residuals were used as the goodness-of-fit criteria.
All measurements were done in ambient conditions at room
temperature.
Experimental Section
Materials. All starting materials and reagents were pur-
chased from Aldrich and used without further purification. 2,3-
Bis(4-bromophenyl)quinoxaline was prepared according to the
Fabrication and Characterization of LEDs. Indium-
tin oxide (ITO) coated glass substrates (Delta Technologies
Ltd., Stillwater, MN) were cleaned sequentially in ultrasonic
baths of a 2-propanol/deionized water (1:1 volume) mixture,
toluene, deionized water, and acetone, and then dried at 60
1
6
literature method.
Polymerization. The following general procedure was used
for the preparation of all the copolymers and PFO homopoly-
mer. To a three-necked flask were added 9,9-dioctylfluorene
°
C in a vacuum overnight. A 50 nm thick hole-injection layer
of poly(ethylenedioxythiophene) doped with poly(styrene-
sulfonate) (PEDOT) was spin-coated on top of ITO from a ∼1
wt % dispersion in water and dried at 200 °C for 15 min under
a vacuum. Poly(N-vinylcarbazole) (PVK) (Scientific Polymer
Products, Inc., Ontario, NY) was used as a hole-transport layer.
Thin films (∼15 nm) of PVK were deposited onto the PEDOT
layer by spin-coating from 1 wt % solutions in toluene and
dried in a vacuum at 60 °C for at least 8 h. Each QXF
copolymer solution in toluene was made at a concentration of
2
,7-bis(trimethyleneboronate) (558.4 mg, 1 mmol), 2,7-di-
bromofluorene and 2,3-bis(4-bromophenyl)quinoxaline (total
mmol), Aliquat 336 (122.4 mg), and toluene (15 mL). Once
1
all the monomers were dissolved, 2 M aqueous sodium carbon-
ate solution (10 mL) was added. The flask equipped with a
condenser was then evacuated and filled with argon several
3 4
times to remove traces of air. Pd(PPh ) (15 mg, 0.013 mmol)
was then added under an argon atmosphere. The mixture was
then stirred for 48 h at 110 °C under argon. The reaction
mixture was cooled to room temperature, and the organic layer
was separated, washed with water, and precipitated into
methanol. The crude polymer was filtered, washed with excess
methanol, and dried. The fibrous polymer was purified by a
Soxhlet extraction in acetone for 2 days. The resultant polymer
was further purified by being dissolved in THF and reprecipi-
tated into methanol several times. The pure copolymers were
obtained after drying under vacuum at 50 °C overnight (yield
1
0 mg/mL and then filtered through a 0.2 µm syringe filter.
Thin films (50 nm) of the copolymers were spin-coated from
these solutions onto the PVK layer and dried at 60 °C in a
vacuum overnight. Due to the solubility of PVK in toluene, it
is likely that the interface between the PVK and copolymer
layers is diffuse rather than being flat and well-defined.
However, overall film thicknesses of PVK + copolymer were
ca. 60-70 nm, which were measured by an Alpha-Step 500
surface profiler (KLA Tencor, Mountain View, CA). The
substrates were then transferred to an AUTO 306 vacuum
coater (BOC Edwards, Wilmington, MA), and 2 nm of LiF was
deposited from a molybdenum boat through a shadow mask
onto the polymer films. Finally, a 130-150 nm thick layer of
Al was evaporated without breaking the vacuum, typical
6
8-75%).
PFO Homopolymer. H NMR (CDCl
H), 7.70 (br, 4H), 2.15 (br, 4H), 1.16 (br, 20H), 0.84 (m, 10H).
1
3
): δ (ppm) 7.85 (br,
2
-
1
FT-IR (film, cm ): 3019, 2926, 2853, 1459, 1253, 885, 813,
56.
7

Macromolecules, Vol. 38, No. 5, 2005
Quinoxaline-Containing Polyfluorene Synthesis 1555
Scheme 1
Table 1. Molecular Weights and Thermal Properties of
Polymers
Tg b
Td c
polymer
PFO
Mn a
Mw/Mn a
(°C)
(°C)
23200
31900
25300
16800
12800
11300
7100
2.68
2.08
2.56
2.40
2.58
2.48
1.88
67
67
69
91
116
125
160
458
460
459
457
460
455
457
2
5
1
2
3
5
QXF
QXF
5QXF
5QXF
3QXF
0QXF
a
Molecular weights were determined by GPC using polystyrene
b
c
standards. Glass transition temperature. Onset decomposition
temperature measured by TGA under nitrogen.
-
6
evaporations being done at base pressures lower than 2 × 10
2
Torr. The resultant active area of each EL device was 0.2 cm .
EL spectra were obtained using a PTI QM-2001-4 spectro-
photometer. Current-voltage characteristics of the LEDs were
measured using an HP4155A semiconductor parameter ana-
lyzer (Yokogawa Hewlett-Packard, Tokyo). The luminance was
simultaneously measured using a model 370 optometer (UDT
Instruments, Baltimore, MD) equipped with a calibrated
luminance sensor head (model 211) and a 5× objective lens.
The device external quantum efficiencies were calculated using
Figure 1. 1HNMR spectra of PFO homopolymer and 50QXF
copolymer.
2a,7f,9a
procedures reported previously.
All the device fabrication
and characterization steps were done under ambient labora-
tory conditions.
observed in the spectra of all the copolymers except
2
QXF, where the concentration of the quinoxaline
moiety was too low. The multiple peaks in the range of
.65-7.80 ppm correspond to the other aromatic protons
Results and Discussion
7
Synthesis and Characterization. The synthetic
procedures used to prepare the monomer and polymers
are outlined in Scheme 1. The condensation of 4,4′-
dibromobenzil with 1,2-phenylenediamine afforded the
monomer 2,3-bis(4-bromophenyl)quinoxaline with high
in the quinoxaline and fluorene units. The resonances
at 2.08, 1.10, and 0.80 ppm are assigned to the aliphatic
protons in the octyl chains. FT-IR spectra of these
copolymers and PFO homopolymer also confirmed their
molecular structures. Representative FT-IR spectra of
copolymer 50QXF and PFO homopolymer are shown in
Figure 2. Compared to PFO homopolymer, the new
1
6
yield (68%). Random and alternating poly(9,9-dio-
ctylfluorene-co-quinoxaline) (QXF) copolymers were syn-
thesized by Suzuki coupling polymerization. The molar
ratio of quinoxaline moiety in the copolymers was
controlled by adjusting the molar ratio between 9,9′-
dioctyl-2,7-dibromofluorene and 2,3-bis(4-bromophenyl)-
quinoxaline while maintaining a 1:1 molar ratio be-
tween the dibromides and the bis(trimethyleneboronate).
The corresponding polyfluorene homopolymer, PFO, was
also prepared under the same conditions for comparison.
All the copolymers were soluble in organic solvents such
as chloroform, toluene, and tetrahydrofuran (THF). The
number-average molecular weights (Mn) of these poly-
mers were determined by GPC against polystyrene
standards to be 7100-31900 with a polydispersity index
of 1.88-2.68 (Table 1).
-
1
vibrational bands at 1607 and 1532 cm in all the
copolymers are due to stretching vibrations of the
CdN group in the quinoxaline ring.
The thermal properties of the polymers were evalu-
ated by TGA and DSC, and the results are summarized
in Table 1. TGA revealed that the onset decomposition
temperatures of the copolymers and PFO homopolymer
under nitrogen were in the range of 455-460 °C,
indicative of good thermal stability. The DSC second-
heating scans of the copolymers and PFO homopolymer
are shown in Figure 3. PFO homopolymer exhibits three
transitions: a glass transition (T ) 67 °C), a crystal-
g
lization exothermic peak (94 °C), and a melting peak
(152 °C). These typical thermal characteristics in PFO
b,18
The chemical structures of the polymers were verified
are similar to those of previous reports.8
Similar
1
1
by H NMR and FT-IR spectra. The H NMR spectra of
the copolymers and PFO homopolymer were in good
agreement with the structures of the polymers. Repre-
thermally induced phase transitions were observed for
copolymer 2QXF, but were not as well resolved as in
PFO. The DSC traces of all the other copolymers
containing greater than 2 mol % quinoxaline showed
no crystallization or melting peaks, but only glass
transitions. The glass transition temperatures of these
copolymers are in the range of 69-160 °C, increasing
1
sentative H NMR spectra of copolymer 50QXF and PFO
homopolymer are shown in Figure 1. A small peak at
8
1
.24 ppm due to the two protons (labeled “o” in Figure
) near nitrogen atoms in the quinoxaline unit was

1556 Kulkarni et al.
Macromolecules, Vol. 38, No. 5, 2005
Figure 2. FT-IR spectra of PFO homopolymer and 50QXF
copolymer.
-
5
Figure 4. (a) Normalized optical absorption spectra of 10
M solutions of PFO and QXF copolymers in toluene. (b) PL
emission spectra of 10 M solutions of the polymers in toluene.
The excitation wavelength was 380 nm.
-5
of the copolymers suggests increased disruption of
π-conjugation caused by the increasing amounts of the
bent 2,3-bis(p-phenylene)quinoxaline linkages in the
PFO backbone. Also, the absorption onsets of the spectra
shift to higher energy in going from 2QXF to 50QXF,
indicating shortened conjugation lengths. The incorpo-
ration of such kinked linkages in polymer backbones has
been used successfully in poly(p-phenylenevinylene)
backbones to control the conjugation length of the
polymer and tune the emission colors.19
Figure 3. Second-heating DSC curves of the polymers with
a heating rate of 10 °C/min in nitrogen.
steadily from the 67 °C Tg in PFO to 160 °C in 50QXF.
This clearly indicates that the incorporation of the rigid
quinoxaline unit into the polyfluorene backbone reduces
the segmental mobility and effectively suppresses the
tendency of the polymer chains to densely pack and
crystallize. This improved thermal resistance of the
copolymers bodes well for stable blue emission from
LEDs made from them.
The PL emission spectra under 380 nm excitation of
PFO and the copolymers in dilute toluene solution are
shown in Figure 4b. The PFO emission spectrum has
the typical vibronic progression with the 0-0 PL emis-
sion band located at 417 nm and the 0-1 transition at
Photophysical Properties. The normalized optical
absorption and PL emission spectra of the PFO homo-
-
5
5a
polymer and the six copolymers in dilute (10 M)
toluene solution are shown in Figure 4. PFO has an
absorption maximum at 388 nm corresponding to the
π-π* transition of the polyfluorene backbone. All the
quinoxaline-containing copolymers have blue-shifted
absorption maxima compared to PFO. The absorption
maximum is progressively blue shifted with increasing
quinoxaline content, from 383 nm in 2QXF to 365 nm
in 50QXF. The full width at half-maximum (fwhm) of
the absorption spectra increases with increasing qui-
noxaline content, suggesting greater inhomogeneous
broadening due to the local disorder induced by the
presence of kinked 2,3-bis(p-phenylene)quinoxaline link-
ages. The gradual blue shift in the absorption maxima
440 nm. With increasing quinoxaline content, there
is a slight red shift in the 0-0 emission band to 423
nm in 50QXF with simultaneous loss in vibronic struc-
ture of the emission spectra. However, the emission of
all the copolymers remains blue. The 0-1 emission band
at 440 nm in PFO becomes a shoulder in copolymers
containing greater than 15 mol % quinoxaline. This loss
of the well-resolved structure of the emission spectra
in the copolymers suggests lack of intrachain ordering
due to the many kinks in the copolymer backbone. The
steady increase in Stokes shift from PFO to 50QXF
suggests greater planarization of the backbone in the
excited state due to intramolecular charge transfer in
the donor-acceptor-type fluorene-quinoxaline copoly-

Macromolecules, Vol. 38, No. 5, 2005
Quinoxaline-Containing Polyfluorene Synthesis 1557
Table 2. Photophysical Properties of Polymers
λa,maxa in λf,maxb in
λa,maxa in a λf,maxb in a
solution
(nm)
solution
(nm)
thin film
(nm)
thin film
(nm)
polymer
PFO
Φf c
388
383
382
379
376
373
365
417
417
418
419
421
422
423
0.72
0.69
0.62
0.48
0.41
0.35
0.26
383
382
382
380
377
374
373
439
439
429
431
435
436
437
2
5
1
2
3
5
QXF
QXF
5QXF
5QXF
3QXF
0QXF
a
The absorption maximum in toluene solution or in a thin film.
b
c
The PL emission maximum in toluene solution or in a thin film.
PL quantum yield in 10 M toluene solution.
-5
mers.20 The main photophysical properties of all the
polymers are collected in Table 2.
Figure 5a shows the normalized optical absorption
spectra of thin films of PFO and the six QXF copolymers
after drying at 60 °C in a vacuum. The trends observed
with increasing quinoxaline content in terms of the
spectral shapes, peak positions, and fwhm are similar
to those of the absorption spectra in dilute solution. PFO
has an absorption maximum at 383 nm along with a
distinct shoulder at ∼430 nm. This absorption feature
has been attributed to the â-phase of PFO, which is
comprised of a sequence of intrachain ordered dioct-
2
1
ylfluorene repeat units. This low-energy transition
completely disappears in the absorption spectra of all
the QXF copolymers. This again confirms that the bent
2,3-bis(p-phenylene)quinoxaline linkages introduce dis-
order in the polyfluorene backbones and thus prevent
ordered intrachain conformations. The distinct similar-
ity between the thin film absorption spectra and the
dilute solution spectra suggests comparable ground-
state electronic structures of the QXF copolymers with
no significant aggregation in the condensed state. Un-
like the blue shift in the absorption onset observed in
the solution spectra of the copolymers with increasing
quinoxaline content, the absorption edges of the thin
film spectra are all identical. This means that the same
segments with extended delocalization are present in
the copolymers in the solid state, giving rise to similar
bulk electronic structures in all the copolymers. The
optical band gaps derived from the absorption edge of
the thin film spectra gave similar values of 2.9-3.0 eV
for all copolymers.
Figure 5b shows the PL emission spectra of the
copolymer thin films dried at 60 °C under 380 nm
excitation. The PFO homopolymer spectrum shows the
typical well-resolved vibronic structure with the 0-0
transition at 439 nm, followed by the 0-1 and 0-2
transitions at 464 and 493 nm, respectively. Copoly-
mer 2QXF shows the 0-0 transition at 424 nm along
with the dominant 0-1 transition at 439 nm. Further
increase in the quinoxaline fraction in the copolymers
results in a gradual loss of the vibronic structure, with
the spectrum of 50QXF becoming broad enough to
envelope the structured PFO emission spectrum. How-
ever, all the QXF copolymers retain the blue emission
of the parent PFO backbone and are thus candidates
for blue electroluminescence in LEDs.
5
a
Figure 5. (a) Normalized optical absorption spectra of thin
films of PFO and QXF copolymers. (b) PL emission spectra of
the polymer thin films dried at 60 °C in a vacuum. (c)
Normalized PL emission spectra (with respect to the 0-0
emission band) of the polymer thin films annealed at 200 °C
in air for 1.5 h. The excitation wavelength was 380 nm.
sion spectra (with respect to the 0-0 emission band) of
these annealed films are shown in Figure 5c. The PL
emission spectrum of the PFO film dried at 60 °C is also
shown for comparison. The annealed PFO film shows
the signature broad green emission band at 530 nm,
whose origin has been a subject of controversy in the
literature.7 The intensity of this broad low-energy
emission band relative to the blue emission band
steadily decreases with increasing quinoxaline content
in the copolymer. Besides, the emission peak blue shifts
The spectral instability in blue LEDs based on poly-
fluorenes under constant applied stress remains a
5
a,b,7
continual problem.
To test the stability of the
quinoxaline copolymers toward thermal oxidation, the
copolymer thin films were annealed at 200 °C in air for
1
.5 h to accelerate degradation. The normalized emis-

1558 Kulkarni et al.
Macromolecules, Vol. 38, No. 5, 2005
from 530 nm in PFO to 521 nm in 25QXF, appearing
only as a shoulder in the spectrum of 33QXF and almost
disappearing in the 50QXF emission spectrum. Given
the recent conclusive evidence that this additional green
emission in polyfluorenes is from isolated fluorenone
defects on the polyfluorene chains,7c,f the observed
decrease in the intensity of the emission band suggests
lower concentration of keto defects in the QXF copoly-
mers. The electron-deficient quinoxaline moieties in the
copolymers withdraw electron density from adjacent
fluorene units and thereby reduce the susceptibility to
oxidation at the C-9 position of the 9,9′-dioctylfluorene
units. Given the increase in the Tg of the copolymers
with increasing quinoxaline content (Table 1) and the
lack of any crystallization/melt transitions in the qui-
noxaline copolymers (Figure 3), the copolymer thin films
will be in a more disordered state relative to the PFO
homopolymer thin film after the 200 °C heating and
subsequent cooling to room temperature. This will
minimize, but not completely eliminate, the interchain-
assisted excitation energy migration to the fluorenone
defects with increasing quinoxaline content in the
copolymers. Thus, even if the same concentration of
fluorenone defects are present in all the annealed
copolymer thin films, the reduced interchain energy
transfer to those defects could lead to the observed lower
intensity of the fluorenone emission band in the high-
Tg 50QXF copolymer versus the low-Tg 5QXF copolymer.
The blue shift in the peak position of this fluorenone
emission band could be a result of the variation in the
local polarity of the medium surrounding the keto
defect, which will vary depending on the quinoxaline
fraction in the copolymer. Due to the strong polar
character of the fluorenone emission band, significant
red shifts have been observed in highly polar environ-
Table 3. Fluorescence Decay Lifetimes of Polymer
Solutions and Thin Filmsa
τ1 (nm)
(A1, %)
τ2 (nm)
(A2, %)
polymer
fitb
ø2
D-Wc
Solution
PFO
F1
F1
F1
F1
F1
F1
F1
0.425
1.472
1.33
1.266
1.271
1.067
1.386
1.311
1.538
1.71
2.011
1.639
2.017
1.645
1.364
2QXF
0.421
0.445
0.548
0.527
0.617
0.606
5
1
2
3
5
QXF
5QXF
5QXF
3QXF
0QXF
Thin Film
PFO
2QXF
5QXF
5QXF
5QXF
3QXF
0QXF
F1
F2
F2
F2
F2
F2
F2
0.394
1.376
1.304
1.798
1.272
2.6
1.313
0.998
1.526
1.611
1.316
1.48
0.395 (99.3)
0.373 (99.5)
0.339 (99.2)
0.528 (98.3)
0.448 (98.5)
0.308 (99.7)
2.37 (0.7)
2.97 (0.5)
2.78 (0.8)
3.22 (1.7)
2.95 (1.5)
4.04 (0.3)
1
2
3
5
1.19
1.579
1.823
a
The excitation wavelength was 381 nm, and the PL emission
maxima were monitored in each case. The concentration of all
-
5
b
polymer solutions was 10 M in toluene. F1 ) single-exponential
c
fit; F2 ) biexponential fit. Durbin-Watson parameter for the fits.
lifetime of PFO homopolymer is comparable to the
literature values, 520 ps.22 With increasing quinoxaline
content from 2QXF to 50QXF, the lifetime of the blue
emission generally increases. Knowing the fluorescence
quantum yields in dilute solution and these fluorescence
lifetimes, the natural radiative lifetimes (τ0) of the
emission bands can be estimated. The τ0 values were
found to steadily increase in going from PFO (590 ps)
to copolymer 50QXF (2.33 ns), suggesting slower radia-
tive rate constants and increased channels for non-
radiative decay. The increasing number of kinks in the
QXF copolymer backbones with increasing quinoxaline
fraction could be potential sites for efficient exciton
quenching, leading to the observed steady decrease in
Φf values (Table 2).
7f
ments. Nonetheless, the improved spectral stability in
the QXF copolymers relative to the PFO homopolymer
augurs well for spectrally stable blue EL from LEDs
made from the copolymers.
In going from the polymer dilute solutions to thin
films, it is generally believed that interchain species
such as aggregates or excimers play a more dominant
The fluorescence quantum yields (Φf) of all the
-
5
polymers in dilute (10 M) toluene solution are listed
in Table 2. The highest Φf value of 72% was observed
for PFO homopolymer, with the Φf values steadily
decreasing with increasing quinoxaline fraction in the
copolymers. At low quinoxaline content, 2QXF and
23
role in the PL emission. The fluorescence of a PFO
homopolymer thin film was well-described by single-
exponential decay with a short lifetime of 394 ps,
slightly shorter than that in solution. The increased
number of nonradiative decay sites such as polymer
chain ends tends to generally reduce the lifetime in thin
films. The lifetimes of the thin film QXF copolymers are
also shorter than that in solution, ranging from 308 to
5
QXF, the Φf value remains high at 0.62-0.69. The
lowest Φf value obtained was 26% for copolymer 50QXF.
One likely reason for the decreasing Φf is the greater
intramolecular charge transfer with increasing quinoxa-
5
28 ps. However, the emissions are now described by
2
0
line content. Although solid-state Φf values were not
calculated, the relative intensities in the thin film PL
emission spectra suggested a trend in Φf values similar
to that observed in solution. To further understand the
nature of the emission from these quinoxaline-contain-
ing polyfluorenes, we investigated their fluorescence
decays in both dilute solution and thin films as dis-
cussed next.
biexponential decays, unlike the single-exponential fits
in solution. Attempts to make single-exponential fits to
2
the thin film decays gave very high ø values of greater
than 10. Biexponential decays led to substantially better
fits to the data. For all copolymer thin films, the
dominant emitting species had short lifetimes of less
than 1 ns, which can be assigned to intrachain excitons.
A very minor contribution (<2%) of a longer lived
species with lifetimes ranging from ∼2 to 4 ns is noted
in all copolymers. Although unambiguous assignment
of this 2-4 ns species is not possible, it is thought to be
related to some interchain phenomenon such as ag-
gregates. Nonetheless, the fact that more than 98% of
the contribution to the blue emission comes from the
short-lived species suggests that the degree of interchain
interactions is minimal in thin films of the quinoxaline-
containing polyfluorenes. Representative decay curves
Time-Resolved PL Decay Dynamics. The fluores-
cence decay parameters of the polymers in solution and
in thin films are collected in Table 3. The PL emission
maximum of the copolymers was monitored in each case.
-5
In dilute toluene solution (10 M), the decay of the blue
emission band for PFO and all the copolymers was found
to be single-exponential, with lifetimes ranging from 421
to 617 ps. Given the lack of interchain interactions in
dilute solution, this emission is assigned to intrachain
singlet excitons on the polymer backbones. The observed
-
5
of copolymer 50QXF in toluene solution (10 M) and

Macromolecules, Vol. 38, No. 5, 2005
Quinoxaline-Containing Polyfluorene Synthesis 1559
Table 4. Device Characteristics of Type I LEDs:
ITO/PEDOT/PVK/Polymer/Al
current
voltage
(V)
density
brightness CIE 1931 EQEb
2
2
(x, y)a
polymer
PFO
(J, mA/cm ) (L, cd/m )
(%)
15.6
10.3
16.4
17.3
14.5
9.2
500
336
500
500
500
240
312
190
300
560
150
60
(0.16, 0.06) 0.053
(0.16, 0.06) 0.30
(0.16, 0.05) 0.35
(0.16, 0.07) 0.07
(0.16, 0.05) 0.04
(0.16, 0.07) 0.01
(0.16, 0.08) 0.002
2
5
1
2
3
5
QXF
QXF
5QXF
5QXF
3QXF
0QXF
12
9.4
2
a
The CIE coordinates are calculated at the maximum EQE.
b
The maximum EQE is calculated at the given voltage, current
density, and brightness.
Table 5. Device Characteristics of Type II LEDs:
ITO/PEDOT/PVK/Polymer/LiF/Ala
current
voltage
(V)
density
brightness CIE 1931
EQE
(%)
2
2
b
polymer
PFO
(J, mA/cm ) (L, cd/m )
(x, y)
8.7 (7.1) 185 (46)
10.0 (9.1) 176 (87)
10.5 (9.0) 207 (65)
360 (106) (0.16, 0.05) 0.60 (0.66)
330 (180) (0.16, 0.04) 0.72 (0.78)
520 (217) (0.16, 0.05) 0.73 (1.03)
306 (128) (0.16, 0.08) 0.25 (0.35)
250 (105) (0.16, 0.06) 0.44 (0.64)
270 (210) (0.16, 0.06) 0.19 (0.30)
240 (130) (0.16, 0.08) 0.25 (0.34)
2
5
1
2
3
5
QXF
QXF
5QXF 10.4 (9.0) 257 (79)
5QXF
3QXF
8.7 (6.9) 193 (55)
9.8 (8.4) 356 (186)
0QXF 10.0 (8.4) 250 (102)
a
The values in parentheses correspond to those for maximum
b
EQE. The CIE coordinates are calculated at the maximum EQE.
between the fluorene units and the 2,3-bis(p-phenylene)-
quinoxaline moieties is poor due to the bent linkages
in the current QXF copolymers.
Figure 6. (a) Fluorescence decay curves of the PL emission
-
5
peak of 50QXF in 10 M toluene solution and in a thin film
with 381 nm excitation. Open symbols represent the actual
data, and the solid lines are fits to the data. (b) Weighted
residuals for the single-exponential fit corresponding to the
solution decay curve.
Using a thin PVK buffer layer (15 nm), we made two
sets of devices of the type ITO/PEDOT/PVK/polymer/
cathode, one with pure Al as the cathode (type I) and
the other with LiF/Al serving as the electron-injecting
electrode (type II). The relevant device parameters of
type I and type II LEDs are summarized in Tables 4
and 5, respectively. Figure 7 shows the normalized EL
spectra of type II LEDs based on PFO and the QXF
copolymers. Expectedly, almost identical EL spectra
were observed for the type I LEDs (not shown). The only
major difference between the two types of devices is that
the operating voltages of type II LEDs are much lower
than those of type I LEDs as noted from Tables 4 and
5. Although some interlayer mixing is bound to occur
at the PVK/polymer interface due to common solubility
in the spin-coating solvent (toluene), the EL spectra of
the LEDs are similar to the thin film PL spectra of the
corresponding copolymers (Figure 5b). The EL spectra
of Figure 7 clearly show that the EL emission stays blue
at all operating voltages of the devices for all the QXF
copolymers. The CIE coordinates25 of the EL emission
of the QXF copolymers do not vary with operating
voltage and are identical to those of the deep-blue EL
from PFO homopolymer at ∼(0.16, 0.07).
in a thin film with the corresponding exponential fits
are shown in Figure 6a along with the plot of weighted
residuals for the fit (Figure 6b).
Electroluminescent Properties. Initial investiga-
tions of the EL properties of the quinoxaline-containing
polyfluorenes were made by fabricating single-layer
LEDs of the type ITO/PEDOT/polymer/Al. However,
blue EL with poor performance was obtained with
2
maximum brightness of only ∼40-50 cd/m . Besides,
there were a lot of nonemissive dark spots on the active
area of the pixel in all cases, which led us to believe
that doping of the QXF copolymers at the PEDOT/
polymer interface caused significant quenching of the
luminescence. Such charge-transfer and doping pro-
cesses have been observed at the PEDOT interface.24 It
was thought that the addition of a buffer layer such as
PVK which could also serve as a hole-transport/electron-
blocking layer would help to mitigate this problem.
Besides, the ionization potential (IP) of PFO is very
large at 5.8 eV, leading to a substantial barrier for hole
injection even from the PEDOT layer (φ ) 5.2 eV). In
the QXF copolymers, it is likely that the HOMO levels
could be even lower lying due to the presence of the 2,3-
bis(p-phenylene)quinoxaline groups. We investigated
the electrochemical properties of the copolymers by
cyclic voltammetry; however, no significant difference
was observed between the redox waves of PFO and those
of the QXF copolymers (not shown). Even for copolymer
The current density-voltage (J-V) and luminance-
voltage (L-V) characteristics of type I LEDs (no LiF
layer) are shown in parts a and b, respectively, of Figure
8. The turn-on voltages (voltage at which EL emission
is visible to the eye) for the homopolymer PFO diode
and the copolymer LEDs are comparable at ∼6.0 V.
There is no discernible variation in the turn-on voltage
with copolymer composition to suggest that electron
injection is significantly enhanced in the QXF copoly-
mers containing n-type quinoxaline moieties. LEDs
based on 25QXF, 33QXF, and 50QXF showed current
spikes in the region below the turn-on voltages. The fact
5
0QXF, it was difficult to resolve any differences in
either the oxidation or reduction waves, although it is
expected that the quinoxaline groups would improve
electron transport in the copolymers compared to PFO.
This indicates that electron delocalization (conjugation)

1560 Kulkarni et al.
Macromolecules, Vol. 38, No. 5, 2005
Figure 7. Normalized EL spectra of LEDs of the type ITO/PEDOT/PVK/polymer/LiF/Al: (a) PFO, (b) 2QXF, (c) 5QXF, (d) 15QXF,
e) 33QXF, and (f) 50QXF.
(
2
that such spikes are not observed for the copolymers
containing lower quinoxaline contents suggests that
they could be associated with the filling of interface
states or trap states related to the quinoxaline moi-
16.4 V and maximum EQE of 0.35% (500 mA/cm , 560
2
cd/m ). Copolymer devices with quinoxaline fraction
greater than 15 mol % had very low brightnesses and
EQEs. For example, the 50QXF copolymer diode had a
2
6
2
eties. The traps could be associated with structural
defects in the copolymer backbone caused by the kinked
quinoxaline linkages. Figure 8b shows the variation in
brightness of the type I LEDs as a function of the
applied voltage. The PFO homopolymer diode had a
low brightness of 2 cd/m and a poor EQE of 0.002%.
Thus, the device performance varied significantly with
quinoxaline content in the copolymers. These results
clearly show that composition is in fact a very powerful
tool to optimize the properties of electroluminescent
copolymers. An alternating copolymer may not neces-
sarily be the optimum composition; superior properties
could be achieved from the same building blocks at more
dilute copolymer compositions.
2
maximum brightness of 190 cd/m at 15.6 V and a
2
2
maximum EQE of 0.053% (500 mA/cm , 190 cd/m )
(
Table 4). The best performance among the copolymers
was given by 5QXF with a brightness of 560 cd/m at
2

Macromolecules, Vol. 38, No. 5, 2005
Quinoxaline-Containing Polyfluorene Synthesis 1561
Figure 9. (a) Current density-voltage characteristics of type
II polymer LEDs. (b) Luminance-voltage characteristics of the
LEDs in (a). The inset shows a schematic of the device.
Nonetheless, the observed device performance is better
than or comparable to that of the best blue-emitting
polymer LEDs based on polyfluorenes.9 Copolymer
LEDs containing greater than 5% quinoxaline had lower
brightnesses and EQEs than the PFO device.
,12
The variations in the maximum luminance and EQE
of both type I and type II LEDs with quinoxaline content
in the copolymer are shown in parts a and b, respec-
tively, of Figure 10. Given that the PL quantum yields
of the copolymers decrease steadily with increasing
quinoxaline content, the observed variation in the
performance for type I LEDs would not be surprising.
Both the brightness and the EQE drop significantly with
increasing quinoxaline content from 5 to 50 mol %.
Enhancement in performance by factors of 3-6 com-
pared to that of PFO is seen for 5QXF, most likely due
to a balance in charge carrier injection and transport.
However, the variation in performance for type II LEDs
is completely different from that for type I LEDs. The
best LED performance was still obtained from 5QXF
with a maximum EQE of 1% as shown in Figure 10b.
In copolymers containing greater than 5 mol % qui-
noxaline, the brightness and EQE do not drop off
significantly as in the type I LEDs. In fact, enhance-
ments in brightness by factors of 2, 4, 23, and 120 are
seen for type II LEDs based on 15QXF, 25QXF, 33QXF,
and 50QXF, respectively, compared to type I devices.
The most likely explanation is the reduction in cathode-
induced fluorescence quenching sites at the cathode/
polymer interface.28 It is well-known that thermal
evaporation of Al on PFO films leads to covalent Al-C
Figure 8. (a) Current density-voltage characteristics of type
I polymer LEDs. (b) Luminance-voltage characteristics of the
LEDs in (a). The inset shows a schematic of the device.
Parts a and b of Figure 9 show the current density-
voltage (J-V) and luminance-voltage (L-V) charac-
teristics of type II LEDs (with a LiF layer), respectively.
Since the overall film thickness of the PVK/polymer
bilayer is comparable in all the LEDs, the applied
voltage and the electric field are equivalent. The turn-
on voltage for the PFO homopolymer diode and the
copolymer LEDs is lower at ∼4.5 V, and the operating
voltages are lower by ∼3-5 V. It is well-known that LiF
enhances electron injection from the cathode and leads
2
7
to lower turn-on and drive voltages. Superior perfor-
mance was obtained in the type II LEDs at lower
operating voltages as expected. The PFO homopolymer
2
diode had a maximum brightness of 360 cd/m at 8.7 V
2
and maximum EQE of 0.60% (7.1 V, 106 cd/ m ). The
maximum brightness of the 2QXF copolymer LED is
2
slightly lower at 330 cd/m , while the best performance
2
is given by 5QXF with a brightness of 520 cd/m at 10.5
2
V and maximum EQE of 1.03% (9.0 V, 217 cd/m ). This
represents enhancement in performance by a factor of
less than 2 compared to that of the PFO homopolymer.

1562 Kulkarni et al.
Macromolecules, Vol. 38, No. 5, 2005
Conclusions
A series of five random copolymers and one alternat-
ing copolymer of 9,9′-dioctylfluorene and 2,3-bis(p-
phenylene)quinoxaline were synthesized and success-
fully used as blue-emitting materials in polymer LEDs.
Systematic variations in the polymer properties were
seen such as a steady increase in the glass transition
temperature from 67 to as high as 160 °C and a decrease
in fluorescence quantum yield from 72% to 26% with
increasing quinoxaline content from 2 to 50 mol % in
the copolymers. The copolymers emit blue light both in
dilute solution (417-423 nm) and in the solid state
(
429-439 nm). Time-resolved fluorescence decay analy-
sis gave short lifetimes of 308-617 ps in both dilute
solution and thin films, with no significant interchain
aggregation. Blue electroluminescence with CIE coor-
dinates of (0.16, 0.06) and varying performance was
achieved from copolymer LEDs of the type ITO/PEDOT/
PVK/copolymer/cathode, both with and without a LiF
buffer layer, with maximum luminances of 240-520 cd/
2
m and external quantum efficiencies of up to 1% at
brightnesses over 100 cd/m . Our results showed that
2
the copolymer containing 5 mol % 2,3-bis(p-phenylene)-
quinoxaline had a significant enhancement in electrolu-
minescence compared to either the PFO homopolymer
or any other copolymer. These results clearly demon-
strate that composition is a powerful synthetic tool to
optimize the electroluminescent properties of light-
emitting copolymers. The improved thermal oxidative
stability of the quinoxaline-containing copolymers sug-
gests that the fluorene-quinoxaline copolymers are
promising materials for stable blue polymer LEDs.
Acknowledgment. This research was supported by
the Air Force Office of Scientific Research (Grant
F49620-03-1-0162) and the Army Research Office TOPS
MURI (Grant DAAD19-01-1-0676).
Figure 10. Semilogarithmic plot of maximum (a) luminance
and (b) external quantum efficiency of polymer LEDs versus
quinoxaline content in the copolymer.
References and Notes
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2
8a
PFO backbone. We believe that, in the present QXF
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