Synthesis and luminescent properties of fluorene copolymers bearing DCM pendants

Article abstract of DOI:10.1021/ma047947w

A series of copolymers, poly[(9,9-dioctylfluorene-2,7-diyl) -co-(4-dicyanomethylene-2-methyl-6-[4-(diphenylamino)styryl]-4H-pyran-4′, 4″-diyl], were synthesized by polymerizing 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene with mixtures of 2,7-dibromo-9,9-dioctylfluorene and 4-dicyanomethylene-2-methyl-6-[bis(4′-bromophenyl)amino]styryl]-4H-pyran (a DCM derivative) by the palladium-catalyzed Suzuki coupling reaction. The copolymers were characterized by molecular weight determination, elemental analysis, ¹H NMR, FT-IR spectroscopy, DSC, TGA, UV-vis spectroscopy, and photoluminescence (PL) and electroluminescence (EL) spectroscopy. The copolymers showed two absorption peaks at 380 and 485 nm, and the long-wavelength absorption increased with increasing the fraction of the DCM comonomer. The PL spectra of copolymers in chloroform solution displayed emission from both the main chain (420 nm) and DCM units (620 nm). In the solid state, however, PL spectra of copolymers showed only the long wavelength red emission at 620 nm with no trace of emission from the main chain, which implies a facile exciton migration or energy transfer to the lower energy sites from the fluorene part to the DCM part. This results in emission of only the red light originating from the latter segments. A study on time-resolved PL rise and decay of the polymers clearly supports the energy transfer mechanism. Light-emitting diode (LED) devices were fabricated to have the configuration of ITO (indiumtin oxide)/PEDOT/polymer/Li:Al alloy. EL spectra of the devices showed only red emissions as observed in the PL spectra of the polymers' thin films. EL efficiency decreased with increasing DCM contents. When a tris(8-hydroxyquinolinato)aluminum (Alq₃) layer was inserted between the emitting polymer layer and the cathode to make the ITO/PEDOT/polymer/Alq₃/Li:Al alloy configuration, the device efficiencies became much higher (～10^-2%) than those (5 × 10^-5-5 × 10^-3%) of single-layer devices.

Full text of DOI:10.1021/ma047947w

6336
Macromolecules 2005, 38, 6336-6345
Synthesis and Luminescent Properties of Fluorene Copolymers Bearing
DCM Pendants
†
†
†
,†
Cheol Hong Cheon, Sung-Hoon Joo, Kyungkon Kim, Jung-Il Jin,*
‡
†,‡
Hee-Won Shin, and Yong-Rok Kim
Division of Chemistry and Molecular Engineering and Center for Electro- and Photo-Responsive
Molecules, Korea University, Seoul 136-701, Korea, and Department of Chemistry, Yonsei University,
Seoul 120-749, Korea
Received October 5, 2004; Revised Manuscript Received May 24, 2005
ABSTRACT: A series of copolymers, poly[(9,9-dioctylfluorene-2,7-diyl)-co-(4-dicyanomethylene-2-methyl-
6
-[4-(diphenylamino)styryl]-4H-pyran-4′,4′′-diyl)], were synthesized by polymerizing 2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene with mixtures of 2,7-dibromo-9,9-dioctylfluorene
and 4-dicyanomethylene-2-methyl-6-[bis(4′-bromophenyl)amino]styryl]-4H-pyran (a DCM derivative) by
the palladium-catalyzed Suzuki coupling reaction. The copolymers were characterized by molecular weight
1
determination, elemental analysis, H NMR, FT-IR spectroscopy, DSC, TGA, UV-vis spectroscopy, and
photoluminescence (PL) and electroluminescence (EL) spectroscopy. The copolymers showed two absorption
peaks at 380 and 485 nm, and the long-wavelength absorption increased with increasing the fraction of
the DCM comonomer. The PL spectra of copolymers in chloroform solution displayed emission from both
the main chain (420 nm) and DCM units (620 nm). In the solid state, however, PL spectra of copolymers
showed only the long wavelength red emission at 620 nm with no trace of emission from the main chain,
which implies a facile exciton migration or energy transfer to the lower energy sites from the fluorene
part to the DCM part. This results in emission of only the red light originating from the latter segments.
A study on time-resolved PL rise and decay of the polymers clearly supports the energy transfer
mechanism. Light-emitting diode (LED) devices were fabricated to have the configuration of ITO (indium-
tin oxide)/PEDOT/polymer/Li:Al alloy. EL spectra of the devices showed only red emissions as observed
in the PL spectra of the polymers’ thin films. EL efficiency decreased with increasing DCM contents.
When a tris(8-hydroxyquinolinato)aluminum (Alq
3
) layer was inserted between the emitting polymer layer
/Li:Al alloy configuration, the device efficiencies
and the cathode to make the ITO/PEDOT/polymer/Alq
3
-
2
-5
-3
became much higher (∼10 %) than those (5 × 10 -5 × 10 %) of single-layer devices.
Introduction duction of a “kink” disorder along the conjugated
1
3
polymer chain, end-capping with a sterically hindered
groups14 or hole trapping moieties, and introduction
of bulky substituents at the 9-position of the fluorene
unit.1
1
Since the first report by the Cambridge group on the
15
electroluminescence (EL) of poly(p-phenylenevinylene)
(
PPV), enormous effort has been devoted to the syn-
6a,19
thesis of light-emitting, π-conjugated polymers be-
Meanwhile, attempts have been made to modify the
structure of PFs which would favor an energy transfer
from one part to the other in the molecules leading to
changes in electronic properties. Very often this ap-
proach enables tunability of luminescent colors. One
successful method is to dope the host PFs with a
chromophore with desired luminescent properties.20 The
cause of their potential applications as emitting materi-
2-6
als for electroluminescent displays.
Among the π-con-
jugated polymers, particularly a number of polyfluorene
polymers and their derivatives have been attracting
great of interest as very promising candidates for blue
light-emitting diode (LED) materials because poly-
fluorenes (PFs) exhibit a high luminescence efficiency,
good charge transport, excellent thermal and oxi-
dative stability, tunability of physical properties through
chemical modification and copolymerization, and good
other method is copolymerization with low-band-gap
comonomers.2
1-24
Energy transfer and trapping of ex-
citons or electrons between the chromophoric segments
7
-9
in conjugated polymer chains are known to be fast and
solubility in common organic solvents.
The major
of PFs, however, is that they display a
efficient,2
1-23
which shifts the wavelength of emitted
8
-15
drawback
light to a longer wavelength. Several groups studied
fluorescence properties of the copolymers containing
both the fluorene donor sites and other acceptor
moieties.2
red-shifted and diminished emission upon thermal
annealing or the passage of current. The source of
this long wavelength emission was initially believed
to be excimer emission from aggregates formed by
π-stacking of the polymer chains.1 Recently, the ori-
gin of the long wavelength emission, however, was
ascribed to ketonic defects incorporated in the poly-
1-24
6
The blending approach has an advantage by which
we can make a fine control of the nature and the amount
of the incorporated dye. However, the blends usually
suffer from long-term instability originating from the
possible phase separation and aggregation formation.
Direct incorporation of the low-band-gap comonomer
units into polymer main chains appears to be an
attractive strategy to overcome this drawback, although
very often one has to go through a multistep synthetic
route.
1
7
mer backbone.
Various approaches have been
adopted to suppress this undesired long wavelength
emission: copolymerization with anthracene,18 intro-
†
Korea University.
‡
Yonsei University.
*
Corresponding author. E-mail: jijin@korea.ac.kr.
1
0.1021/ma047947w CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/01/2005

Macromolecules, Vol. 38, No. 15, 2005
Fluorene Copolymers Bearing DCM Pendants 6337
Chart 1
-
3
In this investigation, we would like to report the
preparation and luminescence properties of a series of
pound 4 (1.3 g, 3.0 × 10 mol) in acetonitrile was added
-2
dropwise to a mixture of compound 3 (2.6 g, 1.5 × 10 mol)
and catalytic amount of piperidine in 150 mL of dry acetoni-
trile. After the reaction mixture was refluxed under a nitrogen
atmosphere for 24 h, it was cooled to room temperature. After
the solvent was removed by distillation, the crude product
obtained was purified by column chromatography on a silica
gel column using a mixture of hexane/ethyl acetate ) 1:6
dioctylfluorene copolymers containing moieties of the
DCM structure well-known for red-light emission.25-27
This article, in particular, discusses the synthesis of the
new fluorene-based copolymers whose structures are
shown above (Chart 1), energy transfer from the blue-
light-emitting fluorene segments to the DCM units, and
EL characteristics of LEDs fabricated from them. In the
copolymers the DCM moieties are linked to the fluorene
units through the triphenylamine (TPA) structure.
Among the three copolymers, PF-DCM 05 and
PF-DCM 15 are of random sequence, whereas PF-DCM
(
v/v) as an eluent. The product was dark red solid. Yield was
47% (0.83 g). H NMR (300 MHz, CDCl , ppm): δ 7.42 (m,
1
3
7H, Ar-H and -CH)), δ 7.00 (m, 6H, Ar-H), δ 6.65 (d, 1H,
pyran-H), δ 6.54 (d, 1H, -CH)), δ 6.52 (d, 1H, pyran-H),
δ2.4 (s, 3H, -CH
3 2 3
). Anal. Calcd for C29H19Br N O: C, 59.51;
H, 3.27; N, 7.18%. Found: C, 59.62; H, 3.31; N, 7.20%.
3
1
Poly[2,7-(9,9-dioctylfluorene)] (PF). Poly[2,7-(9,9-dio-
ctylfluorene)] (PF) was synthesized from a mixture of com-
pounds 1 and 2 by the well-known palladium-catalyzed Suzuki
coupling reaction according to the procedures reported in the
literature.
5
0
0 is of alternating sequence (Chart 1). The numbers
5, 15, and 50 stand for the mole percent of the DCM
monomer used in feed, respectively. Their actual com-
positions are described in the Results and Discussion.
Poly[2,7-(9,9-dioctylfluorene)-co-(1,4-phenylene-{â-(2-
Experimental Section
(4-dicyanomethylene-6-methyl-4H-pyran)-yl)styryl}imino-
31
Materials. Tetrahydrofuran (THF) was distilled over sodium/
1,4-phenylene)] (PF-DCM 05). A mixture of compounds 1
28
(0.365 g, 6.65 × 10-4 mol), 2 (0.474 g, 7.37 × 10-4 mol), and 5
2 5
benzophenone. Acetonitrile was distilled over P O . Dimeth-
-
5
ylformamide (DMF) was distilled over calcium hydride. Tet-
rakis(triphenylphosphine)palladium(0) was purchased from
Strem Chemicals (Newburyport, MA) and was used as re-
ceived. All other chemicals were purchased from Aldrich
Chemical Co. (Milwaukee, WI), TCI (Tokyo, Japan), or Acros
Chemicals (Geel, Belgium) and were used as received un-
less otherwise described. Poly(3,4-ethylenedioxythiophene)
(0.043 g, 7.37 × 10 mol) and tetrakis(triphenylphosphine)-
-6
palladium (8.5 mg, 7.37 × 10 mol) was added to a degassed
mixture of 5 mL of toluene and 4 mL of 2 M sodium carbonate
aqueous solution. The mixture was vigorously stirred at
85-90 °C for 48 h under a nitrogen atmosphere. After the
mixture was cooled to room temperature, it was poured into a
mixture of methanol and distilled water (10:1(v/v)) while being
stirred. Orange polymer precipitate was collected on a filter.
The recovered polymer was washed with methanol, water, and
then methanol followed by Soxhlet extraction with methanol
to remove oligomers and catalyst residue. The recovered yield
(
PEDOT) doped with poly(styrenesulfonate) with an electrical
-1
conductivity of 10 S cm was purchased from Aldrich Chemi-
cal Co. Tris(8-hydroxyquinolinato)aluminum (Alq
chased from TCI and used as received.
3
) was pur-
2
9
1
3
was 73% (0.42 g). H NMR (300 MHz, CDCl , ppm): δ 7.86
Synthesis. 2,7-Dibromo-9,9-dioctylfluorene (1) and 2,7-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluo-
(br, 2H, Ar-H), δ 7.68 (br, 4H, Ar-H), δ 7.44 (br, 5/95 × 7H,
Ar-H and -CH)), δ 7.33 (m, 5/95 × 4H, Ar-H), δ 7.19-7.21
(br, 5/95 × 2H, Ar-H), δ 6.54-6.67 (m, 5/95 × 3H, pyran-H
2
9
rene (2) were prepared according to the procedures reported
in the literature, and 4-dicyanomethylene-2,6-dimethyl-4H-
pyran (3) was synthesized from 2,6-dimethyl-4-pyrone as
and -CH)), δ 2.42 (s, 5/95 × 3H, -CH
3
), δ 2.12 (br, 4H,
3
0
2 2 6 3 3 2 2
-CH (CH ) CH ), δ 1.40 (s, -C(CH ) -), δ 1.14 (m, 20H, -CH -
described by Woods.
4
-[Bis(4′-bromophenyl)amino]benzaldehyde (4). A 2.5
(CH
2 5 2 3 2 2 5 2 3
) CH CH ), δ 0.79-0.84 (br, 10H, -CH (CH ) CH CH ).
-
2
Anal. Calcd for C H
29 38.95 0.15 0.05: C, 89.20; H, 10.06; N, 0.54%.
O
N
M n-butyllithium solution (4.6 mL, 1.1 × 10 mol) in hexane
was added slowly to a solution of tris(4-bromophenyl)amine
Found: C, 89.20; H, 9.89; N, 0.59%.
-2
31
(
5.0 g, 1.1 × 10 mol) in 30 mL of dry THF. After the reaction
PF-DCM 15. This polymer was prepared by the same
-
2
mixture was stirred at -78 °C for 1 h, 4.0 mL (5.2 × 10 mol)
of dimethylformamide (DMF) was added under a nitrogen
atmosphere. The mixture was stirred for another 1 h and
warmed to room temperature. After being kept for 1 h at room
temperature, the reaction mixture was poured into distilled
water. The aqueous solution was extracted with diethyl ether.
The organic layer was separated and then dried over anhy-
drous magnesium sulfate. After removing the solvent, the
product was purified by column chromatography on a silica
gel column using a mixture of hexane/ethyl acetate ) 1:15
manner as described above for the preparation of PF-DCM 05.
-
4
A mixture of compounds 1 (0.283 g, 5.16 × 10 mol), 2 (0.474
-
4
-4
g, 7.37 × 10 mol), and 5 (0.130 g, 2.21 × 10 mol) was
polymerized in the presence of tetrakis(triphenylphosphine)-
-
6
palladium (8.5 mg, 7.37 × 10 mol). Polymer obtained was a
1
red solid. The yield was 74% (0.43 g). H NMR (300 MHz,
3
CDCl , ppm): δ 7.86 (br, 2H, Ar-H), δ 7.68 (br, 4H, Ar-H), δ
7.44 (br, 15/95 × 7H, Ar-H and -CH)), δ 7.33 (m, 15/95 ×
4H, Ar-H), δ 7.19-7.21 (br, 15/95 × 2H, Ar-H), δ 6.54-6.67
(m, 15/95 × 3H, pyran-H and -CH)), δ 2.42 (s, 15/95 × 3H,
(
v/v) as an eluent. The product was green solid. Yield was 51%
-CH
3
), δ 2.12 (br, 4H, -CH
2
(CH
CH
). Anal. Calcd for C29
2
)
2
6
CH
3 3 2
), δ 1.40 (s, -C(CH ) -
1
(
2.3 g). H NMR (300 MHz, CDCl
CHO), δ 7.70 (d, 2H, Ar-H), δ 7.44 (d, 4H, Ar-H), δ 7.00
m, 6H, Ar-H). Anal. Calcd for C19 13BrNO: C, 52.93; H, 3.04;
N, 3.25%. Found: C, 52.81; H, 2.99; N, 3.29%.
-Dicyanomethylene-2-methyl-6-{[bis(4′-bromophen-
yl)amino]styryl}-4H-pyran (5). A solution (50 mL) of com-
3
, ppm): δ 9.84 (s, 1H,
), δ 1.14 (m, 20H, -CH
-CH (CH CH CH
2
(CH
2
)
5
CH
3
), δ 0.79-0.84 (br, 10H,
-
(
2
2
)
5
2
3
H
36.85
N
0.45
O
0.15: C,
H
88.63; H, 9.42; N, 1.60%. Found: C, 87.97; H, 9.55; N, 1.50%.
Poly[2,7-(9,9-dioctylfluorene)-alt-(1,4-phenylene-{â-(2-
(4-dicyanomethylene-6-methyl-4H-pyran)-yl) styryl}-
4
3
1
imino-1,4-phenylene)] (PF-DCM 50). A mixture of com-

6338 Cheon et al.
Macromolecules, Vol. 38, No. 15, 2005
Scheme 1. Synthetic Route to the Monomers
-
4
pounds 2 (0.474 g, 7.37 × 10 mol) and 5 (0.431 g, 7.37 ×
model 818-UV) was used to measure the device light out-
put in microwatts. Luminance was measured by a Minolta
LS-100 luminance meter. The thickness of polymers was
determined by a TENCOR P-10 surface profiler. The redox
-
4
1
0
mol) was polymerized in the presence of tetrakis(tri-
-
6
phenylphosphine)palladium (8.5 mg, 7.37 × 10 mol) in
the same manner as described above in the preparation of
32
PF-DCM 05 and PF-DCM 15. PF-DCM 50 was a dark red solid.
properties of polymers were examined by cyclic voltammetry.
1
The yield was 70% (0.44 g). H NMR (300 MHz, CDCl
3
, ppm):
The polymer thin films were spin-coated on a platinum plate
2
δ 7.86 (br, 2H, Ar-H), δ 7.68 (br, 4H, Ar-H), δ 7.44 (br, 7H,
Ar-H and -CH)), δ 7.33 (m, 4H, Ar-H), δ 7.19-7.21 (br,
(1.5 cm ) using chlorobenzene as the solvent. The electrolyte
solution employed was 0.10 M tetrabutylammonium tetrafluo-
2
2
H, Ar-H), δ 6.54-6.67 (m, 3H, pyran-H and -CH)), δ
4 4 3
roborate (Bu NBF ) in acetonitrile. The Ag/AgNO and Pt wire
.42 (s, 3H, -CH
-), δ 1.14 (m, 20H, -CH
.79-0.84 (br, 10H, -CH (CH CH CH
3
), δ 2.12 (br, 4H, -CH
2
(CH
2
2
)
2
6
CH
CH
). Anal. Calcd for
3
), δ 1.40
(600 µm in diameter) electrodes were utilized as reference
and counter electrodes, respectively. The scan rate was 50 mV/
s. For time-resolved photoluminescence (PL) analysis, all
samples were excited by 315 nm pulses generated from a
Raman shifter which was filled with 18 atm of methane gas
and pumped by the fourth harmonic (266 nm, fwhm 20 ps, 10
Hz) of a hybrid mode-locked Nd:YAG laser (Continuum,
Leopard D10). The PL kinetic profiles were obtained using a
picosecond streak camera (Optronis, SCMU-ST-S20) connected
with a spectrometer (CVI, DKSP240) and a CCD system
(Optronis, SCRU-SE-S). The observed PL decay curves were
analyzed by a nonlinear least-squares iterative deconvolution
method.
(
s, -C(CH
3
)
2
(CH
)
5
2
CH ), δ
3
0
C
2
)
2 5
2
3
H N
29 29.5 1.5
O
0.5: C, 85.56; H, 7.31; N, 5.16%. Found: C, 85.19;
H, 7.22; N, 5.07%.
1
Measurements. H NMR (300 MHz) and IR spectra were
recorded on a Varian AM 300 spectrometer and on a Bomem
MB FT-IR instrument, respectively. Elemental analyses were
performed by the Center for Organic Reactions, Sogang
University, Seoul, Korea, using an Eager 200 elemental
analyzer. The purity of intermediates was confirmed by TLC
on silica gel plates (MERCK, silica gel 60 F254) with a UV lamp
(
254 or 365 nm) and a visualization reagent. Molecular weights
of polymers were determined by gel permeation chromatog-
raphy (GPC; Waters) equipped with RI 2410 detector and
Styragel HR52 column using polystyrene as standard. THF
was employed as an eluent. Thermal properties were studied
under a nitrogen atmosphere on a Mettler DSC 821 instru-
ment. Thermogravimetric analysis (TGA) was performed un-
der a nitrogen atmosphere at the heating/cooling rate of 10
Fabrication of LED Devices. The devices have a structure
of ITO/PEDOT (20 nm)/polymer (80-130 nm)/Li:Al (100 nm)
3
or ITO/PEDOT (20 nm)/polymer (80-130 nm)/Alq (3 nm)/
Li:Al (100 nm). The ITO-coated glasses were cleaned as
33
described earlier. The conducting PEDOT layer (20 nm thick)
was spin-coated onto the ITO-coated glasses in an argon
atmosphere using a Laurell spin-coater. The emitting polymer
layer then was spin-coated onto the PEDOT layer using the
polymer solution in chlorobenzene. Finally, Li:Al alloy (100
nm, 2 wt % Li) electrodes were vacuum-deposited using a VPC-
260 (ULVAC, Japan) vacuum coater and a CRTM-6000 thick-
ness monitor (ULVAC, Japan) at a deposition rate of 5 Å/s
°
C/min on a Mettler TGA 50 thermogravimetric analyzer. The
UV-vis spectra of polymers were obtained on a Hewlett-
Packard 8452A diode array spectrophotometer. PL and EL
spectra of polymers were acquired on an AMINCO-Bowman
series 2 luminescence spectrometer. The current and lumi-
nescence intensity as a function of applied field were measured
using an assembly consisting of PC-based dc power supply
-
5
onto the polymer layer at the pressure of 2.0 × 10 Torr. For
3
bilayer devices, the Alq layer was vacuum-deposited under a
-5
(
Hewlett-Packard 6623A) and a digital multimeter (Hewlett-
reduced pressure of 2.0 × 10 Torr onto the emitting polymer
Packard 34401). A light power meter (Newport Instrument,
layer. Then, Li:Al alloy (100 nm) electrodes were deposited onto

Macromolecules, Vol. 38, No. 15, 2005
Fluorene Copolymers Bearing DCM Pendants 6339
Scheme 2. Synthesis of Polyfluorene Homo-(PF) and Copolymers (PF-DCMs)
Table 1. General Properties of Polyfluorene Homo-(PF)
and Copolymers (PF-DCMs)
polydispersity indices of the four polymers were esti-
mated to be 4.8, 2.6, 6.6, and 3.3, respectively. It appears
that the reactivity of compound 5, which carries the
DCM pendant, is much lower than that of compound 1.
Not only the actual contents (Table 1) of the monomer
PF-DCM PF-DCM PF-DCM
PF
05
15
50
Mna
11000
53000
4.8
83
5600
14000
2.6
73
4.5
98
411
5400
35000
6.6
74
12.8
2300
7500
3.3
70
50.0
116
379
5
unit incorporated into the copolymers are lower than
Mwa
in feed, but also molecular weights (Table 1) of copoly-
mers are significantly reduced when compared with the
molecular weight of PF. The actual compositions esti-
PDI
b
polymer yield (%)
y (%)c
Tg (°C)d
Td (°C)
97
421
1
mated by their H NMR spectra of the two random
397
copolymers (PF-DCM 05 and PF-DCM 15: refer to
Scheme 2) prepared using the dibromo monomers
(compound 1:compound 5) at feed compositions of m:n
a
All values were obtained by GPC measurement with polysty-
b
rene as the calibration standard (eluent: tetrahydrofuran). Re-
covered yields. c The y values were actual fractions of the DCM
)
95:5.0 and 85:15 in molar ratio are m:n ) 95.5:4.5
monomer unit in PF-DCM 05 and PF-DCM 15, as determined by
and 87.2:12.8, respectively. Among the three copoly-
mers, PF-DCM 50 is unique in that it is of alternating
sequence of fluorene and DCM moieties, whereas the
other two polymers are of random sequence. We note
that the molecular weight of PF-DCM 50 is the lowest,
which again suggests that DCM monomer 5 has a lower
reactivity than monomer 1.
1
H NMR analysis. d The temperature revealing 5 wt % loss.
3
the Alq layer under the same condition. The device efficiency
and electroluminescence spectra were obtained by the same
3
3
method as described in the literature.
Results and Discussion
Thermogravimetric analysis performed in a nitrogen
atmosphere shows that thermal stability of the present
polymers is fairly good. The temperature revealing 5 wt
General Properties of Polymers. Synthetic routes
to the preparation of monomers and polymers can be
found in Schemes 1 and 2. The monomers 1, 2, and 5
could be prepared, as shown in Scheme 1, using known
synthetic reactions. Synthetic details were described in
the Experimental Section. All the polymers, PF and the
three copolymers, were prepared by the Suzuki coupling
reaction between 2,7-bis(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolan-2-yl)-9,9-octylfluorene (2) and functionalized
dibromoaromatic compounds (compounds 1 and 5; refer
to Scheme 1). They are soluble at room temperature in
common organic solvents such as chloroform, tetrahy-
drofuran (THF), toluene, xylene, and chlorobenzene.
Table 1 summarizes the polymerization results and
general properties of the four polymers prepared in this
investigation. The number-average molecular weight
%
loss decreased from 421 °C for PF to 379 °C for
PF-DCM 50. The corresponding temperatures of the
other two copolymers lie in between. The presence of
the DCM units in the main chain of copolymers appears
to lower the initial decomposition temperature. The
glass transition temperatures (Tgs) of the polymers
obtained by differential scanning calorimetry (DSC) are
31
9
7 °C for PF, 98 °C for PF-DCM 05, and 116 °C for
PF-DCM 50. The relatively lower Tg values than ex-
pected for these polyaromatic polymers can be ascribed
to the relatively low DP and the presence of long alkyl
groups in the fluorene units. The glass transition
temperature of PF-DCM 15, however, could not be
clearly identified on its thermogram. It appears that
the presence of the polar DCM moieties in the co-
polymers increases the Tg value due to increased inter-
chain interactions and bulkiness of the polar pendant
groups, both of which are expected to reduce segmental
motions.
(
M ) of the polymers determined by GPC against poly-
n
3
styrene standard were 11 × 10 (DP ) 28) for PF, 5.6
3
3
×
10 (DP ) 14) for PF-DCM 05, 5.4 × 10 (DP ) 14)
3
for PF-DCM 15, and 2.3 × 10 (DP ) 6) for PF-DCM
5
0. The eluent used was tetrahydrofuran (THF). The

6340 Cheon et al.
Macromolecules, Vol. 38, No. 15, 2005
Figure 3. Emission spectra of PF, PF-DCM 05, PF-DCM 15,
and PF-DCM 50 (100 nm thick) before and after annealing
at 200 °C for 4 h. The designations “ba” and “aa” respec-
tively refer to before annealing and after annealing at 200 °C
for 4 h.
Figure 1. (a) UV-vis absorption and (b) photoluminescence
DCM unit.26,27 As the fraction of the DCM moiety
increases, the absorption maxima (λmax) of the first peak
are slightly blue-shifted because introduction of the
triphenylamine segments of the DCM units into the
main chains interrupts π-conjugation of the backbone.
The λmax value of this peak moves from 390 nm for PF
to 384 nm for PF-DCM 05, 383 nm for PF-DCM 15, and
-5
(
PL) spectra of the polymer solutions in chloroform (1.0 × 10
M in repeating unit). All the PL spectra were recorded at the
excitation wavelength of 370 nm, but the excitation wavelength
of PF-DCM 50 was 361 nm.
3
61 nm for PF-DCM 50. The optical band gaps (Egs) of
the backbone π-systems estimated from the UV-vis
absorption edges (435 nm for PF, 434 nm for PF-DCM
0
5, 431 nm for PF-DCM 15, and 415 nm for PF-DCM
0) are 2.85 (PF), 2.86 (PF-DCM 05), 2.88 (PF-DCM 15),
5
and 2.99 eV (PF-DCM 50). Because the DCM moiety is
incorporated into the PF main chain via the triphenyl-
amine unit, it is not directly involved in the main chain
conjugated structure, and the triphenylamine moiety
acts as a generator of “kink” disorder. This kink effect
leads to the increased Eg values of the copolymers with
increasing the content of the DCM unit. At the same
time, the absorption band position of the DCM moiety
is rather independent of its content in the polymers due
to the localization of its own π-system.
As shown in Figure 1, PL spectra of the copolymers
obtained in chloroform solution at the excitation wave-
length of 370 nm (excitation wavelength of PF-DCM 50
was 361 nm) display two characteristic emission bands
at about 420 and 620 nm. Blue emission is considered
to be arisen from the polyfluorenyl chromophoric seg-
ments disrupted by the DCM units as observed in the
PL spectrum of the PF homopolymer, and the additional
longer wavelength emission band is from the DCM
units. However, the effect of incorporating the DCM
units into the main chain on the luminescent properties
is more dramatic in thin films. As shown in Figure 2,
in all the copolymers no blue emission from the main
chain, but only a longer wavelength reddish emission
from the DCM units is observed. These results can be
explained by easier exciton migration or energy transfer
Figure 2. (a) UV-vis absorption and (b) photoluminescence
(
PL) spectra of the polymer films (100 nm thick). All the PL
spectra were recorded at the excitation wavelength of 370 nm,
but the excitation wavelength of PF-DCM 50 was 361 nm.
Spectroscopic Properties. Figures 1 and 2 display
the absorption and photoluminescence (PL) spectra of
-
5
the four polymers both in chloroform solution (1.0 × 10
M in repeating unit) and in thin films (100 nm thick).
According to the UV-vis absorption spectra shown in
Figures 1 and 2, the homo- and copolymers display
similar absorption spectral features both in chloroform
solution and in the solid state. All the copolymers
exhibit two major absorptions: one around 380 nm and
the other over a longer wavelength region (400-550
nm). The first absorptions in the short wavelength
region are originated from the π-π* transitions of main
chain π-conjugated systems and the second from the
in the solid state from the polyfluorenyl segments to
lower energy sites containing the DCM unit.2
2,23
It also
should be noted that the blue emission region by the
polyfluorene segments overlaps significantly with the
absorption region by the DCM moieties. In other words,
there exist spectral overlaps for the copolymers, which
is expected to favor energy transfer between the two

Macromolecules, Vol. 38, No. 15, 2005
Fluorene Copolymers Bearing DCM Pendants 6341
Table 2. Optical Properties and Quantum Yield of Polyfluorene Homo-(PF) and Copolymers (PF-DCMs)
λ max (solution; 1.0 × 10- M)a
5
λ max (films)a
polymers
PF
PF-DCM 05
PF-DCM 15
PF-DCM 50
abs
em
abs
em
Eg (eV)b
ΦPLc
390 (-)
418 (-)
383 (-)
418 (-)
423 (598)
- (613)
- (632)
2.85
2.86
2.88
2.99
1.00
384 (485)
383 (485)
361 (477)
417 (598)
417 (613)
420 (606)
383 (484)
383 (485)
361 (477)
0.609
0.234
0.084
a
The data in the parentheses are the absorption and emission wavelengths by the DCM unit. b Eg stands for the band-gap energy
c
estimated from the onset wavelength of optical absorption. Photoluminescence quantum yields in chloroform solution determined relative
to coumarin 307.
3
6b
Figure 4. Time-resolved PL decay profiles of PF, PF-DCM 05, PF-DCM 15, and PF-DCM 50 in chloroform solution detected (a)
at 416 nm and (b) detected at 610 nm with the excitation of 315 nm.
structural elements, and the emission maxima of the
copolymers gradually shift to a longer wavelength from
further information on the energy transfer dynamics of
the present copolymers. Figure 4a presents the PL decay
profiles of PF, PF-DCM 05, and PF-DCM 15 in chloro-
form solution at the detection wavelength of 416 nm.
We found that the samples did not show any detection
wavelength dependence within the 416 nm band. Figure
5
98 nm for PF-DCM 05 to 632 nm for PF-DCM 50 as
2
3b,c
the DCM content increases.
We observed that the homopolymer, PF, shows an
additional red-shifted emission8-15 or a so-called green
4
b exhibits the PL decay profiles of the copolymers
band at 530 nm when the polymer was subjected to a
thermal annealing at temperatures above its Tg. How-
ever, all the present copolymers containing the DCM
moieties display little change (Figure 3) in their emis-
sion even after annealing at 200 °C for 4 h. This
annealing temperature is much higher than their
respective Tgs. We believe that, as we alluded above,
the presence of the triphenylamine units reduces the
linearity of the main chain conformation and, thus,
suppresses effective molecular packing. In Table 2, we
present a summary of spectroscopic properties of all the
polymers. According to the data given in the table, the
PL quantum efficiency in solution decreases signifi-
cantly with increasing the fraction of the DCM unit in
copolymers. The luminescence efficiency decreases due
to so-called “concentration quenching”,34 owing to the
formation of weakly emissive aggregates of intra-
and/or intramolecular nature. There also are trapping
or charge-transfer mechanisms proposed for such donor-
(
PF-DCM 05, PF-DCM 15, PF-DCM 50) in chloroform
solution monitored at 610 nm where fluorescence is
originated from the DCM moiety. The observed PL
decay profiles were well fitted to the single- or double-
exponential function. In Table 3, the fitted values for
the rise and decay components of the exponential
functions are summarized for the polymer samples in
chloroform solution. Figure 4b and Table 3 clearly
demonstrate that the copolymers revealed rise time in
contrast to the homopolymer, PF, that did not so.
As shown in Table 3, the PL decay of the homopoly-
mer (PF) exhibits a single-exponential decay with the
lifetime of 370 ps. On the other hand, the copolymers
of PF-DCM 05 and PF-DCM 15 follow the double-
exponential decay consisting of a fast and a slow
component at the detection wavelength of 416 nm,
which is the emission wavelength from the polyfluorene
moiety. The fast decay components (38-50 ps) at 416
nm show a good correlation with the rise components
(32-47 ps) of the copolymers at the detection wave-
length of 610 nm, which corresponds to the emission
2
3,35-37
acceptor type copolymer systems.
Time-Resolved PL Rise and Decay. We have
employed the time-resolved PL spectroscopy to obtain

6342 Cheon et al.
Macromolecules, Vol. 38, No. 15, 2005
Table 3. PL Lifetime and Relative Amplitude of the Polymers in Chloroform Solution
sample
PF
PF-DCM 05
PF-DCM 15
PF-DCM 50
λ (nm)
rise τ1 (ps)a
35 ((3)
decay τ2 (ps)/A2a
decay τ3 (ps)/A3a
370 ((5)
416
416
45 ((5)/0.17 ((0.02)
43 ((5)/0.39 ((0.02)
372 ((5)/0.83 ((0.02)
1320 ((10)
6
10
416
10
610
395 ((5)/0.61 ((0.02)
1300 ((10)
1450 ((10)
6
43 ((3)
44 ((3)
a
I(t) ) -A1 exp(-t/τ1) + A2 exp(-t/τ2) + A3 exp(-t/τ3), where I(t), A, and τ are the time-dependent fluorescence intensity, relative
amplitude, and lifetime, respectively. The excitation wavelength for all decays was 315 nm.
Figure 5. Time-resolved PL decay profiles of PF, PF-DCM 05, PF-DCM 15, and PF-DCM 50 films detected at 439, 608, 623, and
640 nm with the excitation of 315 nm.
Table 4. PL Lifetime and Relative Amplitude of the
Polymer Films
wavelength of the DCM moiety. Therefore, it can be
suggested that the S1 state energy transfer occurs at
the time scale of ∼45 ps from the polyfluorene parts to
the DCM moieties in chloroform solution. The slow
components (372, 395 ps) of PF-DCM 05 and PF-DCM
λ
sample
PF
(nm)
decay τ1 (ps)/A1a
decay τ2 (ps)/A2a
439 110 ((5)
PF-DCM 05 608 56 ((5)/0.64 ((0.02) 409 ((10)/0.36 ((0.02)
PF-DCM 15 623 57 ((5)/0.70 ((0.02) 416 ((10)/0.30 ((0.02)
PF-DCM 50 640 50 ((5)/0.69 ((0.02) 340 ((10)/0.31 ((0.02)
1
5 detected at 416 nm are attributed to the intrinsic
decays of the polyfluorene parts in the backbone without
energy transfer, which is the same as the decay profile
of the homopolymer, PF. And it is noted that the
amplitudes of the slower decay components are much
greater than those of the faster decay components. The
other slow components of 1.3-1.4 ns at the 610 nm
detection are responsible for the intrinsic lifetimes of
the DCM moiety.
Figure 5a,b displays the PL decay profiles of the
homopolymer (PF) and the copolymers in thin films (100
nm thick). As shown in Table 4, the homopolymer film
exhibits a single-exponential decay with the lifetime of
a
I(t) ) A1 exp(-t/τ1) + A2 exp(-t/τ2), where I(t), A, and τ are
the time-dependent fluorescence intensity, relative amplitude, and
lifetime, respectively. The excitation wavelength for all decays was
3
15 nm.
shorter PL lifetimes of films are known to be due to the
various fast nonradiative decay processes such as exci-
ton trapping by defect sites via enhanced interchain
interactions.38 In particular, the rise components of the
copolymer films at ∼620 nm responsible for the DCM
emission are not observed, when compared with the
emissions from their solutions, within our detection
resolution limit of ∼10 ps in the utilized collection
window. It implies that, in the film states, the S1 state
energy-transfer process occurs much faster than the
process in the solution. This must be due to the much
better contact between polyfluorene parts and DCM
moieties in solid films than in solutions. In addition,
there is an interesting point to be noted: the fast and
slow decay time constants of films are not much differ-
ent from those of solutions, but the amplitudes of the
fast decay components are much greater than those of
∼
110 ps at the detection wavelength of 439 nm, and the
copolymer films (PF-DCM 05, PF-DCM 15, PF-DCM 50)
follow the double-exponential decay consisting of the
fast components (45-62 ps) and the slow components
(330-426 ps) at the detection wavelengths of 608, 623,
and 640 nm, respectively. All the films experienced a
slight detection wavelength dependence of the lifetime
within the 10-40 ps range, which is ascribed to the
structural inhomogeneities existing in the films. The
samples in film exhibit much shorter PL lifetimes
compared with those in chloroform solution. Such

Macromolecules, Vol. 38, No. 15, 2005
Fluorene Copolymers Bearing DCM Pendants 6343
Table 5. Band-Gap Characteristics of the Polymers As
Determined from CV Data and UV-Vis Absorption Edges
band
oxidation onset (V) vs HOMO LUMO
sample
PF
PF-DCM 05
PF-DCM 15
PF-DCM 50
gap (eV)
Ag/AgNO3 (0.10 M)
(eV)
(eV)
2.85
2.86
2.88
2.99
1.39
1.37
1.32
0.89
5.79
5.77
5.72
5.29
2.94
2.91
2.84
2.30
the slow decay components, which is in contrast to the
case of solutions. This must be one of the important
factors causing lower PL efficiencies of solid films when
compared with solution.
Electrochemical Properties. The redox behaviors
of the polymers were investigated by cyclic voltammetry
3
2
32
(
CV). From CV, we could determine the ionization
potentials (IPs), i.e., the HOMO levels, of the polymer
backbones. The oxidative onset potential was taken as
the IP value. The LUMO levels then were estimated
from the HOMO levels and optical band gaps (Egs) of
the backbones determined from the absorption edges of
their UV-vis absorption spectra. The work function of
Figure 6. Electroluminescence spectra of single-layer devices
of ITO/PEDOT (20 nm)/Polymer/Li:Al (100 nm).
3
9
ITO was taken to be 4.8 eV. The results are sum-
marized in Table 5. The HOMO level of the main chain
of the polymers steadily decreases from 5.79 eV for PF
to 5.29 eV for PF-DCM 50 with increasing the fraction
4
0
of DCM moiety. As to the LUMO level, it also
decreases from 2.94 eV for PF to 2.84 eV for PF-DCM
15 with increasing the content of the DCM unit followed
by a decrease to 2.30 eV with a further increase in the
DCM content in PF-DCM 50. The latter significant
decrease, however, should be more of the result of
the alternating sequence of the two repeating units in
PF-DCM 50.
Electrical and Electroluminescence Properties
of LED Devices. Electroluminescence of the LED
devices constructed with the four polymers was studied
in detail. The device configuration was ITO/PEDOT (20
nm)/polymer (80 -130 nm)/Li:Al (100 nm). The elec-
troluminescence (EL) spectra of the devices are pre-
sented in Figure 6. The EL spectra of polymers are very
similar to the corresponding PL spectra of their thin
films, which indicates that the same luminescence
mechanism is involved in both EL and PL. Unfortu-
nately, we could not obtain the EL spectrum of PF-DCM
Figure 7. (a) Dependence of current density on electrical field
and (b) external quantum efficiencies of single-layer LED
devices.
50 due to its low light output and poor devices stability.
Figure 7 shows the I-V characteristics of the four
devices. The turn-on electric field decreases from 1.95
MV/cm for the PF device to 0.94 MV/cm for the PF-DCM
LUMO levels (Table 5) of the corresponding polymers,
we reach a conclusion that the larger the difference
between the work function of lithium metal (2.9 eV)41
and the LUMO level of a polymer, the lower the external
quantum efficiency. This indicates that the energy
barrier for electron injection from the cathode controls
the external quantum efficiency of the present devices,
which is also consistent with above statement that the
major carrier is hole and the minor carrier is electron.
Therefore, imbalance in the injection of the positive
holes and electrons leads to the diminished efficiencies.
It is also well-known42 that balance in the mobility of
the opposite carriers is important for improved efficien-
cies of EL devices. We, however, presently do not have
information on the carriers’ mobility of these polymers.
In addition, fluorescence quenching induced by the
charge-transfer mechanism may be applicable to EL of
the present copolymers consisting of electron-donor and
-acceptor segments.
5
0 device. The other two copolymers exhibit the turn-
on electric field of 1.60 and 1.22 MV/cm, respectively,
with the value being lower for the copolymer containing
a higher level of the DCM unit. Comparison of the
values of turn-on electric field of the devices with the
HOMO levels (Table 5) of the corresponding polymers
leads to the conclusion that the smaller the difference
between the work function of the ITO anode and the
HOMO level of a polymer, the lower the turn-on electric
field becomes. This, in turn, implies that easier hole
injection from the anode controls the turn-on electric
field of the present devices, which should hold true only
when the major carrier is the positive hole. Moreover,
we observed that the external quantum efficiency of the
devices dramatically decreased with increasing the
-
2
DCM content. It decreased from about 3 × 10 % for
-
5
the blue-emitter PF to 4 × 10 % for the red-emitter
PF-DCM 50. From the comparison of the values of
external quantum efficiency of the devices with the
When we inserted a very thin layer (3 nm thick) of
Alq3,43 the well-known electron injecting and transport-
ing materials, between the emitting polymer layers and

6344 Cheon et al.
Macromolecules, Vol. 38, No. 15, 2005
transport by the presence of the Alq3 layer leads to an
improved electron-hole recombination and thus to an
improved EL efficiency.
Conclusion
We have described the synthesis and luminescence
properties of a series of new polyfluorene copolymers
bearing DCM moieties as pendants. The polymers were
prepared by the well-known palladium-catalyzed Suzuki
coupling reaction. They show good solubility at room
temperature in common organic solvents such as chlo-
roform, tetrahydrofuran (THF), toluene, xylene, and
chlorobenzene. They exhibit moderately high glass
transition temperatures (>97 °C) and also good thermal
stability. UV-vis absorption spectra of three copolymers
showed two major absorptions by the main chain (380
nm) and also by the DCM units (485 nm). Although in
chloroform solution the copolymers displayed simulta-
neous PL emissions at 420 and 610 nm from both
structural components, in the solid state they exhibited
a quite dramatic change in emission spectra. The
copolymers showed only a long wavelength emission at
Figure 8. Electroluminescence spectra of bilayer devices: ITO/
PEDOT (20 nm)/Polymer/Alq (3 nm)/Li:Al (100 nm).
3
620 nm originated from the DCM moieties with no trace
of emission from the main chain. We also observed that
light emission of the LED devices fabricated with them
was solely from the segments containing the low-band-
gap structure, i.e., the DCM moiety. This is interpreted
by an efficient energy transfer or exciton migration from
the polyfluorene main chain to the low-band-gap trap
sites or the acceptor DCM moieties. This energy transfer
could be unequivocally supported by a time-resolved PL
decay study. The PL efficiencies of the copolymers in
solution decreased significantly with increasing the
fraction of DCM unit, which is attributed to concentra-
tion quenching at high DCM content. Copolymerization
with the DCM monomer suppresses an undesired long
wavelength emission and leads to a stabilized emission
from these copolymers due to fast energy transfer from
the fluorene moiety to the DCM unit. The tripheny-
lamine moiety of the DCM comonomer unit incorporated
into the main chain disrupts the linear structure of the
conjugated polyfluorene backbone, which, in turn, in-
creases the band-gap energy of the copolymers. This
leads to an enhanced hole injection from the anode, but
to a diminished electron injection from the cathode to
the emitting layer. EL efficiency of PLED devices
dramatically decreases with increasing the content of
the DCM moiety. When we fabricated bilayer devices
inserting a thin layer of electron transporting Alq3 layer
between the emitting polymer layer and the cathode,
we observed significantly improved EL efficiencies,
which is conjectured to be due to improved injection of
the electrons to the active layer. This investigation
provides us with excellent examples of polymers that
could demonstrate facile excited energy transfer from
one structural part to the other.
Figure 9. (a) Dependence of current density on electrical field
and (b) external quantum efficiencies of the bilayer LED
devices.
the cathode (device configuration: ITO/PEDOT(20 nm)/
polymer(100-130 nm)/Alq3(3 nm)/Li:Al(100 nm)), we
obtained EL spectra (Figure 8) very similar to their PL
spectra with no trace of emission from the Alq3 layer.
This indicates that the excitons are effectively formed
in the emitting polymer layer through combination
between the injected positive holes into the polymer and
electrons transported through the Alq3 layer from the
cathode. Figure 9a shows the I-V characteristics of the
bilayer devices. The turn-on electric field of the bilayer
device was almost same as that of single-layer device.
However, the efficiency (Figure 9b) of the bilayer de-
Acknowledgment. This work was supported by the
Korea Science and Engineering Foundation through
CRM of Korea University. C.H.C. and S.-H.J. were the
Brain Korea 21 Fellow supported by the Ministry of
Education and Human Resources, Korea. Y.-R.K. thanks
to National Research Laboratory grant (Grant M1-0302-
-
2
-2
vices was 2.1 × 10 % for PF-DCM 05, 4.2 × 10 % for
-
2
PF-DCM 15, and 1.0 × 10 % for PF-DCM 50. Maxi-
2
mum brightness of the bilayer devices was 65 cd/m at
2
2
2
.82 MV/cm (1.28 mA/mm ) for PF-DCM 05, 70 cd/m
2
at 2.23 MV/cm (1.56 mA/mm ) for PF-DCM 15, and 60
0
0-0027) for financial support.
2
2
cd/m at 1.80 MV/cm (6.86 mA/mm ) for PF-DCM 50.
These values are much higher than those (Figure 7b)
of single-layer devices. Especially, the increase for the
PF-DCM 50 device was more than 2 orders of magni-
tude. This implies that an efficient electron injection and
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