Design, synthesis, and electroluminescent property of CN-poly(dihexylfluorenevinylene) for LEDs

Article abstract of DOI:10.1021/ma025862u

A new green electroluminescence polymer, CN-poly(dihexylfluorenevinylene) (CN-PDHFV), which denotes poly[(9,9-dihexyl-9H-fluorene-2,7-diyl)(1-cyanoethene-1,2-diyl)(9,9-dihexyl-9H-f luorene-2,7-diyl)(2-cyaoethene-1,2-diyl)], was synthesized by condensation polymerization utilizing the Knoevenagel reaction. The resulting polymer exhibits good solubility in common organic solvents such as chloroform, THF, and ODCB. The polymer is also easily cast on a glass plate to green film. The UV-vis spectrum of the polymer exhibits characteristically a broad absorption band at 440 nm. This polymer shows photoluminescence around λ_max = 535 nm (exciting wavelength 410 nm) and green electroluminescence around λ_max = 530 nm. The current-voltage-luminance (I-V-L) characteristics of the polymer show a turn-on voltage of 4.8 V and a brightness of 600 cd/m² at 5.8 V in the Al/polymer/PEDOT/ITO device. The highest efficiency was observed to be 0.85 lm/W at 5.6 V.

Full text of DOI:10.1021/ma025862u

6970
Macromolecules 2003, 36, 6970-6975
Articles
Design, Synthesis, and Electroluminescent Property of
CN-Poly(dihexylfluorenevinylene) for LEDs
You n geu p J in ,† J eon gm in J u ,† J in w oo Kim ,† Su n geu n Lee,‡ J in You n g Kim ,§
Su n g Heu m P a r k ,§ Se-Mo Son ,# Su n g-Ho J in , Kw a n gh ee Lee,§ a n d Hon gsu k Su h *,†
Department of Chemistry and Chemistry Institute for Functional Materials, Physics, and Chemistry
Education, Pusan National University, Pusan 609-735, Korea; LG Electronics Institute of Technology,
16 Woomyeon-Dong, Seocho-Gu, Seoul 137-724, Korea; and Division of Image Science & Information
Engineering, Pukyong University, Pusan 608-020, Korea
Received November 21, 2002; Revised Manuscript Received J uly 25, 2003
ABSTRACT: A new green electroluminescence polymer, CN-poly(dihexylfluorenevinylene) (CN-PDHFV),
which denotes poly[(9,9-dihexyl-9H-fluorene-2,7-diyl)(1-cyanoethene-1,2-diyl)(9,9-dihexyl-9H-fluorene-2,7-
diyl)(2-cyaoethene-1,2-diyl)], was synthesized by condensation polymerization utilizing the Knoevenagel
reaction. The resulting polymer exhibits good solubility in common organic solvents such as chloroform,
THF, and ODCB. The polymer is also easily cast on a glass plate to green film. The UV-vis spectrum of
the polymer exhibits characteristically a broad absorption band at 440 nm. This polymer shows
photoluminescence around λmax ) 535 nm (exciting wavelength 410 nm) and green electroluminescence
around λmax ) 530 nm. The current-voltage-luminance (I-V-L) characteristics of the polymer show a
turn-on voltage of 4.8 V and a brightness of 600 cd/m2 at 5.8 V in the Al/polymer/PEDOT/ITO device. The
highest efficiency was observed to be 0.85 lm/W at 5.6 V.
In tr od u ction
Since it was revealed that the electron injection is
more difficult than hole injection for the conjugated
polymers investigated so far, it has been necessary to
use polymers with increased electron affinity to reduce
the barrier to electron injection.15
In this paper, we report the synthesis and character-
ization of a new EL polymer, CN-poly(dihexylfluo-
renevinylene) (CN-PDHFV), which denotes poly[(9,9-
dihexyl-9H-fluorene-2,7-diyl)(1-cyanoethene-1,2-diyl)(9,9-
dihexyl-9H-fluorene-2,7-diyl)(2-cyaoethene-1,2-diyl)]. To
reduce the barrier to electron injection, a CN functional
group was incorporated into the known poly(dihexy-
lfluorenevinylene) (PDHFV). PDHFV was also synthe-
sized to be compared with CN-PDHFV. PDHFV was
synthesized via the Gilch reaction, which modified the
recently reported method.16
Because of the academic interest and potential ap-
plications as large-area light-emitting displays, much
attention have been focused at electroluminescence (EL)
devices based on organic thin layers.1-5 Since the
polymeric light-emitting diodes based on poly(p-phe-
nylenevinylene) (PPV) were reported by Burroughes et
al.,6 various kinds of conjugated polymers have been
developed for EL.7-11
In conjugated polymers, EL is well-known to be
generated by the injection of electrons from one elec-
trode and holes from the other, recombination, and
radiative decay of the excited state.
Since the poly(fluorene) was applied in this field,
electrochemical properties of fluorene-containing poly-
mers including poly(dihexylfluorenevinylene) (PDHFV)
have attracted much attention. Fluorene-based poly-
mers are usually highly fluorescent, which is a required
aspect for electroluminescence applications.12-14 Balanc-
ing the rates of injection of electrons and holes from
opposite contacts into the device is required to achieve
high electroluminescence efficiency.
Exp er im en ta l Section
Gen er a l. All used reagents were purchased from Aldrich
and used without further purification. Solvents were purified
by normal procedures and handled under a moisture-free
atmosphere. 1H and 13C NMR spectra were recorded with a
Varian Gemini-200 (200 MHz) and Varian Gemini-500 (500
MHz) spectrometer, and chemical shifts were recorded in ppm
units with TMS as the internal standard. UV spectra were
recorded with a Varian Cary-5E UV/vis spectrophotometer.
Molecular weights and polydispersities of the polymer were
determined by gel permeation chromatography (GPC) analysis
with a polystyrene standard calibration. An Oriel InstaSpec
CCD detection systems of Busan Branch of Korea Basic
Science Institute was used for photoluminescence spectroscopy.
For the EL experiment, poly(3,4-ethylenedioxythiophene)
(PEDOT) doped with poly(styrenesulfonate) (PSS), as the hole-
injection-transport layer, was introduced between emissive
* To whom correspondence should be addressed: e-mail
hssuh@pusan.ac.kr.
† Department of Chemistry and Chemistry Institute for Func-
tional Materials, Pusan National University.
‡ LG Electronics Institute of Technology.
§ Department of Physics, Pusan National University.
#
Division of Image Science & Information Engineering, Puky-
ong University.
Department of Chemistry Education, Pusan National Uni-
versity.
10.1021/ma025862u CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/19/2003

Macromolecules, Vol. 36, No. 19, 2003
CN-Poly(dihexylfluorenevinylene) for LEDs 6971
layer and ITO glass substrate cleaned by successive ultrasonic
treatments. Isopropyl solution of the PEDOT/PSS was spin-
coated on the surface-treated ITO substrate. On top of the
PEDOT layer, the emissive polymer film was obtained by spin-
casting an ODCB (o-dichlorobenzene) solution of the polymer.
The emissive polymer thin film prepared had a uniform surface
with a thickness of around 110 nm. The emissive film was
dried in a vacuum, and aluminum electrodes were deposited
on the top of the polymer films through a mask by vacuum
evaporation at pressures below 10-7 Torr, yielding active areas
of 4 mm2. For the determination of device characteristics,
current-voltage (I-V) characteristics were measured using a
Keithley 236 source measure unit. All processing steps and
measurements mentioned above were carried out under air
and at room temperature.
To examine electrochemical properties of the resulting
polymer, the polymer film was cast from THF solution onto a
platinum plate as a working electrode with an area of 1 cm2.
Film thickness was controlled in the range of about 3 µm by
the amount of solution. After coating, the film adhering to the
electrode was dried in a vacuum oven for 10 h. The electro-
chemical measurements were performed on 0.1 M tetrabuty-
lammonium tetrafluoroborate (TBAF, freshly distilled, Aldrich)
solution in acetonitrile. Platinum wire and a Ag/AgNO3
electrode were used as a counter electrode and a reference
electrode, respectively. Cyclic voltammetric waves were pro-
duced by using a EG&G Parc model 273 potentiostat/gal-
vanostat at a constant scan rate of 100 mV/s.
temperature, and diluted with 200 mL of diethyl ether. The
solution was washed with 10 mL of a saturated aqueous
sodium chloride solution. The aqueous layer was extracted
with 2 × 50 mL of diethyl ether. The combined organic extract
was dried (MgSO4) and concentrated. The oily residue was
purified by flash chromatography (30 × 100 mm column, SiO2,
ethyl acetate:hexane ) 1:6) to give 1.89 g (89%) of diol 3, a
yellow oil: Rf 0.35 (SiO2, ethyl acetate:hexane ) 1:4). 1H NMR
(200 MHz, CDCl3): δ (ppm) 0.45-0.68 (m, 4H), 0.78 (t, 6H J
) 6.6 Hz), 0.90-1.20 (m, 12H), 1.69 (s, 2H), 1.90-1.98 (m, 4H),
4.77 (s, 4H), 7.31 (d, 4H, J ) 7.6 Hz), 7.66 (d, 2H, J ) 8.4 Hz).
13C NMR (50 MHz, CDCl3): δ (ppm) 13.91, 22.51, 23.68, 29.63,
31.42, 40.28, 54.93, 65.53, 119.56, 121.44, 125.69, 139.71,
140.28, 151.22.
Syn t h esis of 2,7-Bis(ch lor om et h yl)-9,9′-d ih exyl-9H -
flu or en e (4). To a stirred solution of 1.026 g (2.60 mmol) of
diol 3 in 10 mL of benzene at room temperature under argon
was added 1.89 mL of SOCl2 and heated at 40 °C for 3 h. The
reaction mixture was cooled and diluted with 2 mL of 50% HCl.
The organic layer was washed with 10 mL of a saturated
aqueous sodium chloride solution. The total aqueous extracts
were washed with 2 × 10 mL of ethyl acetate, and the
combined organic extract was dried (MgSO4) and concentrated.
The oily residue was purified by flash chromatography (30 ×
100 mm column, SiO2, ethyl acetate:hexane ) 1:10) to give
1.04 g (92%) of dichloride 4, a yellow solid: Rf 0.34 (SiO2,
hexane 100%). 1H NMR (500 MHz, CDCl3): δ (ppm) 0.50-
0.70 (m, 4H), 0.78 (t, 6H J ) 7.3 Hz), 1.03-1.20 (m, 12H),
1.94-1.97 (m, 4H), 4.68 (s, 4H), 7.45 (d, 4H, J ) 9 Hz), 7.66
(d, 2H, J ) 7.5 Hz). 13C NMR (125 MHz, CDCl3): δ (ppm)
13.86, 22.37, 23.57, 29.45, 31.24, 40.02, 46.63, 55.04, 119.88,
123.06, 127.44, 136.53, 140.63, 151.45.
Syn th esis of 2,7-Dibr om o-9,9′-dih exyl-9H-flu or en e (1).17
To a stirred solution of 5 g (15.43 mmol) of 2,7-dibromo-9H-
fluorene in 40 mL of DMSO under argon was added catalytic
amounts of triethylbenzylammonium chloride. After 1 h at 60
C, 6.37 g (38.60 mmol) of n-bromohexane was added to the
reaction mixture. After an additional 1 h at 60 C, the mixture
was treated with 25 mL of 50% aqueous NaOH, stirred for 5
h at room temperature, and diluted with 500 mL of ethyl
acetate. The organic layer was washed with 100 mL of a 1.0
M hydrochloric acid solution and 150 mL of water. The organic
layer was dried (MgSO4) and concentrated under reduced
pressure. The oily residue was purified by flash column
chromatography (30 × 150 mm column, SiO2, 100% of hexane)
to give 7.22 g (95.0%) of 2,7-dibromo-9,9′-dihexyl-9H-fluorene
Syn th esis of P oly(9,9′-d ih exylflu or en evin ylen e) (5)
Usin g th e Gilch Rea ction . To a stirred solution of 1.369 g
(3.24 mmol) of 2,7-bis(chloromethyl)-9,9′-dihexyl-9H-fluorene
(4) in 20 mL of THF at 40 °C under argon was added, drop by
drop, 97.06 mL (19.41 mmol) of a 0.25 M solution of potassium
tert-butoxide in THF using the dropping funnel over a period
of 1 h. During this addition, the reaction mixture had color
change from colorless via greenish to yellow, and the viscosity
increased significantly. After the addition was complete, the
reaction mixture was stirred for 10 h at room temperature.
The reaction mixture was slowly poured into 200 mL of
intensively stirred methanol. The precipitated polymer was
filtered off, washed with water, and dried under reduced
pressure at room temperature to generate 660 mg of the crude
polymer as yellow power. The resulting polymer was redis-
solved in 1.1 L of THF (60 °C), cooled to 40 °C, and reprecipi-
tated by dropwise addition of 1.2 L of methanol. The precipi-
tated polymer was dried at room temperature under reduced
pressure. This procedure was repeated once more using 1.0 L
of THF/1.0 L of methanol to generate 415 mg (56.5%) of poly-
(9,9′-dihexylfluorenevinylene) (5) as a light yellow polymer
fiber. 1H NMR (500 MHz, CDCl3): δ (ppm) 0.50-0.74 (m, 4H),
0.74-0.90 (m, 6H), 0.94-1.24 (m, 12H), 1.90-2.20 (m, 4H),
7.12 (d, 1H, J ) 33 Hz), 7.30 (d, 2H, J ) 10.5 Hz), 7.50-7.62
(m, 4H), 7.68 (d, 1H, J ) 33 Hz). 13C NMR (125 MHz, CDCl3):
δ (ppm) 13.93, 22.59, 23.74, 29. 7 6, 31.54, 40.58, 55.04, 119.59,
120.66, 122.94, 125.86, 128.67, 130.55, 140.66, 151.58.
Syn th esis of 9,9′-Dih exyl-9H-flu or en e-2,7-d ica r ba ld e-
h yd e (6). To a stirred solution of 2.28 g (5.80 mmol) of the
diol 3 in 15 mL of DMF at 0 °C under a argon atmosphere
was added 5.22 g (13.87 mmol) of pyridinium dichromate. After
4 h at 0 °C, the reaction mixture was treated with 1 mL of
water, diluted with 100 mL of diethyl ether, washed with 2 ×
10 mL of water, and dried (MgSO4). After removal of the
solvent under reduced pressure, the oily residue was purified
by flash chromatography (30 × 100 mm column, SiO2, ethyl
acetate:hexane ) 1:5) to give 1.88 g (83%) of aldehyde 6, a
1
(c), white crystals; mp 61 °C, Rf 0.57 (SiO2, hexane 100%). H
NMR (200 MHz, CDCl3): δ (ppm) 0.45-0.68 (m, 4H), 0.77 (t,
6H J ) 6.6 Hz), 0.90-1.20 (m, 12H), 1.86-1.95 (m, 4H), 7.43-
7.54 (m, 6H). 13C NMR (50 MHz, CDCl3): δ (ppm) 13.98, 22.57,
23.63, 29.56, 31.45, 40.18, 55.66, 121.11, 121.45, 126.15,
130.13, 139.04, 152.53.
Syn th esis of Dieth yl 9,9′-Dih exyl-9H-flu or en e-2,7-d i-
ca r boxyla te (2). To a solution of 7.80 g (25 mmol) of
2,7-dibromo-9,9′-dihexyl-9H-fluorene (1) in 20 mL of EtOH at
room temperature, 6.06 g (60 mmol) of triethlamine (Et3N),
44.3 mg (0.25 mmol) of PdCl2, and 131.7 mg (0.50 mmol) of
Ph3P were added. The reaction mixture was placed in a 100
mL SUS-316 stainless steel autoclave under 5.1 MPa of carbon
monoxide. Afterheating at 150 °C for 5 h the reaction mixture
was washed with 100 mL of a 1.0 M hydrochloric acid solution.
After removal of the solvent under reduced pressure, the oily
residue was purified by flash column chromatography (60 ×
150 mm column, SiO2, ethyl acetate:hexane ) 1:10) to give
11.85 g (99.0%) of diethyl 9,9′-dihexyl-9H-fluorene-2,7,-dicar-
boxylate (2), yellow oils; Rf 0.20 (SiO2, ethyl acetate:hexane )
1:10). 1H NMR (200 MHz, CDCl3): δ (ppm) 0.42-0.63 (m, 4H),
0.74 (t, 6H J ) 6.6 Hz), 0.90-1.20 (m, 12H), 1.99-2.08 (m,
4H), 7.78 (d, 2H, J ) 7.6 Hz), 8.06 (d, 4H, J ) 11.4 Hz). 13C
NMR (50 MHz, CDCl3): δ (ppm) 13.94, 14.39, 22.50, 23.67,
29.53, 31.44, 40.04, 55.50, 61.07, 120.23, 124.10, 128.79,
129.97, 144.39, 151.78, 166.88.
Syn th esis of (9,9′-Dih exyl-7-h yd r oxym eth yl-9H-flu o-
r en e-2-yl)-m eth a n ol (3). To a stirred solution of 2.57 g (5.38
mmol) of diethyl 9,9′-dihexyl-9H-fluorene-2,7-dicarboxylate (2)
in 10 mL of tetrahydrofuran at -78 C under argon was added
26.9 mL (26.9 mmol) of a 1 M solution of diisobutylaluminum
hydride in THF. After 3 h at -78 C, the reaction mixture was
cautiously treated with 3 mL of methanol, warmed to room
1
yellow solid: Rf 0.35 (SiO2, ethyl acetate:hexane ) 1:10). H
NMR (200 MHz, CDCl3): δ (ppm) 0.40-0.64 (m, 4H), 0.74 (t,
6H J ) 6.6 Hz), 0.80-1.20 (m, 12H), 2.03-2.12 (m, 4H), 7.92
(s, 6H), 10.11 (s, 2H). 13C NMR (50 MHz, CDCl3): δ (ppm)
13.91, 22.47, 23.74, 29.47, 31.40, 40.02, 55.56, 121.30, 123.34,
130.31, 136.39, 145.59, 152.84, 192.19.

6972 J in et al.
Macromolecules, Vol. 36, No. 19, 2003
Sch em e 1. Syn th etic Rou tes for Mon om er a n d P olym er of P DHF V
Sch em e 2. Syn th etic Rou tes for Mon om er a n d P olym er of CN-P DHF V
Syn t h esis of 2,7-Bis(b r om om et h yl)-9,9′-d ih exyl-9H -
flu or en e (7). To a stirred solution of 2.18 g (5.52 mmol) of
diol 3 in 20 mL of benzene at 0 °C under argon was added,
drop by drop, 1.57 mL (16.57 mmol) of PBr3. After stirring at
45 °C for 6 h, the reaction mixture was cooled and diluted with
5 mL of water. The aqueous layer was separated and extracted
with 2 × 10 mL of CHCl3. The combined organic extract was
dried (MgSO4) and concentrated. The oily residue was purified
by flash chromatography (40 × 150 mm column, SiO2, 100%
hexane) to give 2.79 g (97%) of dibromide 7, colorless oil: Rf
0.35 (SiO2, ethyl acetate:hexane ) 1:8). 1H NMR (200 MHz,
CDCl3): δ (ppm) 0.45-0.68 (m, 4H), 0.78 (t, 6H J ) 6.6 Hz),
0.90-1.20 (m, 12H), 1.89-1.98 (m, 4H), 4.60 (s, 4H), 7.34 (d,
4H, J ) 2.2 Hz), 7.64 (d, 2H, J ) 8.0 Hz). 13C NMR (50 MHz,
CDCl3): δ (ppm) 13.99, 22.48, 23.62, 29.53, 31.35, 34.47, 40.07,
55.12, 120.04, 123.64, 127.99, 136.86, 140.72, 151.63.
(1 M). After refluxing for 12 h, the reaction mixture was
treated with 30 mL of MeOH and cooled to 0 °C over 5 min.
The resulting solid (0.50 g) obtained was then filtered off and
purified by Soxhlet extraction with methanol to generate 0.58
g (80.6%) of poly[(9,9-dihexyl-9H-fluorene-2,7-diyl)(1-cyanoet-
hene-1,2-diyl)(9,9-dihexyl-9H-fluorene-2,7-diyl)(2-cyaoethene-
1,2-diyl)] (CN-PDHFV) (9), yellow powders. 1H NMR (200
MHz, CDCl3): δ (ppm) 0.50-0.90 (m, 10H), 0.90-1.30 (m,
12H), 1.90-2.20 (m, 4H), 7.60-8.10 (m, 7H). 13C NMR (50
MHz, CDCl3): δ (ppm) 13.98, 22.58, 23.80, 29.69, 31.47, 40.29,
55.57, 111.43, 118.53, 120.24, 120.63, 125.21, 133.54, 134.11,
142.55, 152.25.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion . The general syn-
thetic routes toward the monomers and polymers are
outlined in Scheme 1 and Scheme 2. The PDHFV
monomer, 2,7-bis(chloromethyl)-9,9′-dihexyl-9H-fluo-
rene (4), was synthesized using commercially available
2,7-dibromo-9H-fluorene as the starting material, which
was converted to 2,7-dibromo-9,9′-dihexyl-9H-fluorene
(1) by catalytic amounts of triethylbenzylammonium
chloride, 50% aqueous NaOH, and 1-bromohexane.
Diethyl-9,9′-dihexyl-9H-fluorene-2,7-dicarboxylate (2)
was prepared by using ethanol, triethylamine, and
palladium complex catalyst under high pressures. Re-
duction with diisobutylaluminum hydride (DIBAL) and
subsequent reaction with thionyl chloride afforded 2,7-
bis(chloromethyl)-9,9′-dihexyl-9H-fluorene (4). The PDH-
FV monomer 4 was polymerized by the Gilch polymer-
ization with potassium tert-butoxide in THF to generate
the polymer PDHFV (5). The monomer CN-PDHFV
was synthesized using (9,9′-dihexyl-7-hydroxy-methyl-
9H-fluorene-2-yl)methanol (3), which was oxidized to
9,9′-dihexyl-9H-fluorene-2,7-dicarbaldehyde (6) by py-
ridinium dichromate (PDC). And the diol 6 was bromi-
Syn th esis of (7-Cya n om eth yl-9,9′-d ih exyl-9H-flu or en e-
2-yl)a ceton itr ile (8). To a stirring solution of 0.39 g (0.89
mmol) of 2,7-bis(bromomethyl)-9,9′-dihexyl-9H-fluorene (7) in
10 mL of acetonitrile at room temperature under argon were
added 0.36 mL (2.67 mmol) of trimethylsilyl cyanide and 2.67
mL (2.67 mmol) of tetrabutylammonium fluoride (TBAF). The
light yellow reaction mixture was concentrated under reduced
pressure. The oily residue was purified by flash chromatog-
raphy (30 × 70 mm column, SiO2, methylene chloride:hexane
) 1:9) to give 0.33 g (90%) of acetonitrile 8, a yellow solid: Rf
1
0.35 (SiO2, ethyl acetate:hexane ) 1:10). H NMR (200 MHz,
CDCl3): δ (ppm) 0.40-0.64 (m, 4H), 0.74 (t, 6H J ) 6.6 Hz),
0.90-1.20 (m, 12H), 1.90-1.99 (m, 4H), 3.84 (s, 4H), 7.28 (d,
4H, J ) 7.4 Hz), 7.67 (d, 2H, J ) 8.4 Hz). 13C NMR (50 MHz,
CDCl3): δ (ppm) 13.57, 22.08, 23.28, 23.34, 29.12, 30.96, 39.72,
54.94, 117.76, 119.95, 122.04, 126.39, 128.72, 139.77, 151.39.
Syn th esis of P oly[(9,9-d ih exyl-9H-flu or en e-2,7-d iyl)(1-
cya n oeth en e-1,2-d iyl)(9,9-d ih exyl-9H-flu or en e-2,7-d iyl)-
(2-cya oeth en e-1,2-d iyl)] (CN-P DHF V) (9). To a stirred
solution of 0.32 g (0.81 mmol) of 8 and 0.33 g (0.81 mmol) of 6
in 23.3 mL of THF and 11.7 mL of MeOH at reflux (65-66
°C) under argon was added 0.41 mL of Bu4NOH in methanol

Macromolecules, Vol. 36, No. 19, 2003
CN-Poly(dihexylfluorenevinylene) for LEDs 6973
F igu r e 2. Photoluminescence spectra of PDHFV and CN-
PDHFV.
F igu r e 1. UV-vis absorption spectra of PDHFV and CN-
PDHFV.
Ta ble 1. P olym er iza tion Resu lts of P DHF V a n d
onset wavelengths of polymers were 486 and 521 nm,
which correspond to band gaps of 2.55 and 2.38 eV. The
PL spectra of polymers all consist of a typical vibroni-
cally structured band comprising a maximum, a shoul-
der, and a tail. The PL spectrum of the PDHFV thin
film exhibits a maximum at 505 nm and two shoulder
at 480 and 550 nm, and the PL spectrum of the CN-
PDHFV thin film exhibits a maximum at 535 nm and
a shoulder at 500 nm, which are red-shifted about 30
nm relative to the corresponding features measured
from PDHFV (exciting wavelength 410 nm).
CN-P DHF V
a
a
polymers
yield (%)
Mh n
Mh w
PDIa
PDHFV
CN-PDHFV
56.5
80.6
39 300
7 800
69 600
12 100
2.28
1.54
a
Mh n, Mh w, and PDI of the polymers were determined by gel
permeation chromatography using polystyrene standards.
nated with PBr3 in benzene to generate 2,7-bis-
(bromomethyl)-9,9′-dihexyl-9H-fluorene (7). The result-
ing dibromide was cyanized with trimethylsilylcyanide
by the nucleophilic substitution reaction. The new
conjugate polymer, CN-PDHFV, was synthesized using
the Knoevenagel condensation reaction, which were
Electr och em ica l P r op er ties of th e P olym er s. The
cyclic voltammetry was used to investigate the electro-
chemical behavior of the polymers. The CV was per-
formed in a solution of tetrabutylammonium tetraflu-
oroborate (Bu4NBF4) (0.10 M) in acetonitrile at a scan
rate of 100 mV/s at room temperature under the
protection of argon. A platinum electrode (∼0.05 cm2)
coated with a thin polymer film was used as the working
electrode. Pt wire and Ag/AgNO3 electrode were used
as the counter electrode and reference electrode, respec-
tively. All measurements were calibrated against an
internal standard, ferrocene (Fc), which has the IP value
1
carried out in THF/MeOH mixtures (2/3, /3 in volume)
with tetrabutylammonium hydoxide as catalyzer at 65
°C, in a similar way as was reported by the Cambridge
group15 for the preparation of cyano-substituted poly-
(phenylenevinylene) (CN-PPV).
The resulting polymers are highly soluble in common
organic solvents including THF, CHCl3, CH2Cl2, ODCB,
etc., and easily cast on a glass plate to give bright
greenish-blue and green thin films for PDHFV and CN-
PDHFV, respectively. The number-average molecular
weights (Mh n) and the weight-average molecular weights
(Mh w) of the resulting polymers, determined by GPC
using polystyrene standard, were in the range of 7800-
39 300 and 12 100-69 600 with a polydispersity index
of 1.54-2.28. The polymerization results of PDHFV and
CN-PDHFV polymers are summarized in Table 1.
Thermal properties of the synthesized polymers were
evaluated by the means of TGA under a nitrogen
atmosphere. The weight losses of the polymer were less
than 5% in heating to about 450 °C. The polymers did
not show either an endo or exo curve in DSC thermo-
grams due to side chain scission or thermal cross-linking
until the temperature of initial decomposition.
Op tica l a n d P h otolu m in escen ce P r op er ties. The
UV-vis absorption spectra and photoluminescence spec-
tra of PDHFV and CN-PDHFV as thin films are shown
in Figure 1 and Figure 2.
The thin films were prepared by spin-coating on
quartz plates from the polymer solutions in o-dichlo-
robenzene (ODCB). The PDHFV exhibit absorption
spectra with a maximum peak of 420 nm attributed to
the π-π* transition of the conjugated backbones and a
shoulder at 370 nm due to the π-π* transition of
fluorene units. CN-PDHFV has a strong absorption
band at around 440 nm which is attributed to the π-π*
transition of the conjugated segments. The absorption
+
(-4.8 eV) of the Fc/Fc redox system.18
All of the polymers exhibit irreversible processes in
an oxidation scan. The oxidation onsets of the polymers
were estimated to be 0.6 and 0.7 V for PDHFV and CN-
PDHFV, respectively, which correspond to HOMO en-
ergy levels of 5.4 and 5.5 eV.19,20 The LUMO energy
levels of polymers can be calculated with the HOMO
and optical band gap. The LUMO energy levels of
PDHFV and CN-PDHFV were thus determined to be
2.85 and 3.12 eV, respectively. The HOMO and LUMO
energy levels of CN-PDHFV are lower than that of
PDHFV, which can be attributed to the introduction of
the electron-withdrawing cyano group. The barrier
height were found to be 1.35 and 1.08 eV at the interface
of the Al (4.2 eV)/LUMO state for the electron. The red
shift in the spectroscopic features of CN-PDHFV was
caused by the greater increase of the work function of
the LUMO (from 2.85 to 3.12 eV) as compared to that
of the HOMO (from 5.40 to 5.50 eV). The Cambridge
group have reported that the cyano-substituted PPV
lowered the HOMO and LUMO levels by 0.6 and 0.9
eV, respectively, compared with those of PPV. When
indium tin oxide coated with a PPV layer, which helps
to localize charge at the interface between the PPV and
the cyano-substituted PPV, was used, they were able
to achieve high internal efficiencies (photons emitted
per electrons injected) of up to 4%.15

6974 J in et al.
Macromolecules, Vol. 36, No. 19, 2003
F igu r e 3. Energy band diagram of PDHFV and CN-PDHFV.
F igu r e 5. Current-voltage-luminescence (I-V-L) charac-
teristics of PLEDs of (a) PDHFV and (b) CN-PDHFV with a
configuration of ITO/PEDOT:PSS/polymer/Al.
F igu r e 4. Electroluminescence (EL) spectra of PDHFV and
CN-PDHFV with a configuration of ITO/PEDOT:PSS/polymer/
Al.
Electr olu m in escen t P r op er ties a n d Cu r r en t-
Volta ge-Lu m in a n ce. The double-layered polymer
light-emitting diodes (PLEDs) with the configuration
ITO/PEDOT/polymer/Al were fabricated to investigate
the electroluminescent properties and the current-
voltage-luminance characteristics of PDHFV and CN-
PDHFV. Polymer films of thickness approximately 110
nm were spin-cast onto a PEDOT layer which had been
precast on the ITO substrate. The forward bias current
was obtained when the ITO electrode was positively
biased and the aluminum electrode was negative. The
electroluminescence from the polymer in the device was
greenish-blue for PDHFV and green for CN-PDHFV.
As shown in Figure 4, the EL spectra of PDHFV and
CN-PDHFV show maximum peaks at 505 and 535 nm,
respectively. These features are similar to those ob-
served in the PL spectra of the corresponding polymer
films. The voltage-current density characteristics of the
devices fabricated from PDHFV and CN-PDHFV are
shown in Figure 5. In the forward bias, the turn-on
voltages of PDHFV and CN-PDHFV are 5.0 and 4.8
V, respectively. The maximum brightness of CN-
PDHFV is 600 cd/m2 at 8 V. However, PDHFV reaches
a brightness of 300 cd/m2 at 8 V. As shown in Figure 3,
the more negative energy of the LUMO of CN-PDHFV
indicates the electron injection process is easier in CN-
PDHFV than in PDHFV. From this result, a higher
quantum efficiency of CN-PDHFV as compared to that
of PDHFV can be expected due to its improved electron
injection ability from the cathode. Figure 6 shows the
EL efficiency of CN-PDHFV is higher than that of
PDHFV. The highest efficiency of CN-PDHFV was
observed to be 0.85 lm/W at 5.6 V, which was higher
than 0.29 lm/W at 6.0 V of PDHFV.
F igu r e 6. Efficiency of PLEDs of PDHFV and CN-PDHFV
with a configuration of ITO/PEDOT:PSS/polymer/Al.
Con clu sion
A new electroluminescence polymer, CN-poly(di-
hexylfluorenevinylene) (CN-PDHFV), was synthesized
by condensation polymerization utilizing the Knoevena-
gel reaction. The resulting polymer exhibits good solu-
bility in common organic solvents such as chloroform,
THF, and ODCB. This polymer shows photolumines-
cence around λmax ) 535 nm (exciting wavelength 410
nm) and green electroluminescence around λmax ) 530
nm. The current-voltage-luminance (I-V-L) charac-
teristics of the polymer shows a turn-on voltage of 4.8
V and a brightness of 600 cd/m2 at 5.8 V in the
Al/polymer/PEDOT/ITO device. Poly(dihexylfluorenevi-
nylene) (PDHFV) was also prepared to be compared
with CN-PDHFV. PDHFV was synthesized by the
Gilch reaction, which modified the recently reported
method.16 The monomer of PDHFV was synthesized by
using 2,7-dibromo-9H-fluorene as the starting material
for the purpose of synthesizing pure 2,7-bis(chlorom-
ethyl)-9,9′-dihexyl-9H-fluorene. The highest efficiency

Macromolecules, Vol. 36, No. 19, 2003
CN-Poly(dihexylfluorenevinylene) for LEDs 6975
(10) Cimrova, V.; Remmers, M.; Neher, D.; Wegner, G. Adv. Mater.
1996, 8, 146.
of CN-PDHFV was observed to be 0.85 lm/W at 5.6 V,
which was higher than that of PDHFV due to the
improved electron injection ability from the cathode
caused by the cyano functionality.
(11) Greenham, N. C.; Friend, R. H. Solid State Phys. 1996, 49,
1.
(12) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. J pn. J . Appl.
Phys. 1991, 30, L1941.
Ack n ow led gm en t. This work was supported by
Korea Research Foundation Grant (KRF-2001-042-D-
0039) (H. Suh).
(13) Pei, Q.; Yang, Y. J . Am. Chem. Soc. 1996, 118, 7416.
(14) Cho, H. N.; Kim, J . K.; Kim, D. Y.; Kim, C. Y.; Song, N. W.;
Kim, D. Macromolecules 1999, 32, 1476.
(15) (a) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend,
R. H.; Holmes, A. B. Nature (London) 1993, 365, 628. (b)
Moratti, S. C.; Cervini, R.; Holmes, A. B.; Baigent, D. R.;
Friend, R. H.; Greenham, N. C.; Gru¨ner, J .; Hamer, P. J .
Synth. Met. 1995, 71, 2117.
Refer en ces a n d Notes
(1) Tang, W. C.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(2) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1990, 56,
799.
(3) Kido, J .; Kimura, M.; Nagai, K. Science 1995, 267, 1332.
(4) Dodavalapur, A.; Rotherverg, L. J .; Miller, T. M. Electron.
Lett. 1994, 30, 1000.
(5) Yan, M.; Rotherberg, L. J .; Papadimitralopoules, F.; Galvin,
M. E.; Miller, T. M. Phys. Rev. Lett. 1994, 73, 744.
(6) Burroughes, J . H.; Bradley, D. D. C.; Brown, A. R.; Marks,
R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B.
Nature (London) 1990, 347, 539.
(7) Braun, D.; Heeger, A. J . Appl. Phys. Lett. 1991, 58, 1982.
(8) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri,
N.; Heeger, A. J . Nature (London) 1992, 352, 477.
(9) Burn, P. L.; Holmes, A. B.; Kraft, A.; Bradley, D. D. C.; Brown,
A. R.; Friend, R. H.; Gymer, R. W. Nature (London) 1992,
356, 47.
(16) J in, S.; Park, H.; Kim, J . Y.; Lee, K.; Lee, S.; Moon, D.; Lee,
H.; Gal, Y.; Macromolecules 2002, 35, 7532.
(17) Miller, R. D.; Scott, J . C.; Kreyenschmidt, M.; Kwak, J .; Lee,
V.; Fuhrer, T.; Kang, S. Polym. Prepr. 1997, 38, 421.
(18) Pommerehne, J .; Vestweber, H.; Guss, W.; Mahrt, R. F.;
Bassler, H.; Porsch, M.; Daub, J . Adv. Mater. 1995, 7, 551.
(19) Bredas, J . L.; Silbey, R.; Boudreux, D. S.; Chance, R. R. J .
Am. Chem. Soc. 1983, 105, 6555.
(20) DeLeeuw, D. M.; Simenon, M. M.; Brown, A. B.; Einerhand,
R. E. F. Synth. Met. 1997, 87, 53.
MA025862U