Novel electroluminescent polymers with fluoro groups in vinylene units

Article abstract of DOI:10.1021/ma0493022

New electroluminescent polymers with fluoro groups in vinylene units, poly(p-phenylen-edifluorovinylene) (PPDFV) and poly(2-dimethyloctylsilyl-p-phenylenedifluorovinylene) (DMOS-PPDFV), have been synthesized by GILCH polymerization. These polymers have been used as the electroluminescent (EL) layers in double-layer light-emitting diodes (LEDs) (ITO/PEDOT/polymer/Al). PPDFV shows photoluminescence around λ _max = 580 nm (exciting wavelength, 410 nm) and yellow electroluminescence around λ_max = 565 nm. DMOS-PPDFV shows PL around λ_max = 495 nm and green EL around λ _max = 540 nm. The current-voltage-luminance (I-V-L) characteristics of the polymers show turn-on voltages of 3.0 V approximately. Two fluoro groups were introduced on every vinylene units of poly(p-phenylenevinylene) (PPV) and poly(2-dimethyloctylsilyl-1,4-phenylenevinylene) (DMOS-PPV) to give PPDFV and DMOS-PPDFV in an attempt to increase the electron affinity of the parent polymer. It was found that the efficiency of PLED of DMOS-PPDFV was about 7 times higher than that of DMOS-PPV.

Full text of DOI:10.1021/ma0493022

Macromolecules 2004, 37, 6711-6715
6711
Novel Electroluminescent Polymers with Fluoro Groups in Vinylene
Units
You n geu p J in ,† 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 ,§
Kw a n gh ee Lee,§ a n d Hon gsu k Su h *,†
Department of Chemistry and Physics, and Center for Plastic Information Systems, Pusan National
University, Pusan 609-735, Korea, and LG Electronics Institute of Technology, 16 Woomyeon-Dong,
Seocho-Gu, Seoul 137-724, Korea
Received April 9, 2004; Revised Manuscript Received J une 21, 2004
ABSTRACT: New electroluminescent polymers with fluoro groups in vinylene units, poly(p-phenylen-
edifluorovinylene) (PPDFV) and poly(2-dimethyloctylsilyl-p-phenylenedifluorovinylene) (DMOS-PPDFV),
have been synthesized by GILCH polymerization. These polymers have been used as the electrolumi-
nescent (EL) layers in double-layer light-emitting diodes (LEDs) (ITO/PEDOT/polymer/Al). PPDFV shows
photoluminescence around λmax ) 580 nm (exciting wavelength, 410 nm) and yellow electroluminescence
around λmax ) 565 nm. DMOS-PPDFV shows PL around λmax ) 495 nm and green EL around λmax ) 540
nm. The current-voltage-luminance (I-V-L) characteristics of the polymers show turn-on voltages of
3.0 V approximately. Two fluoro groups were introduced on every vinylene units of poly(p-phenylenevi-
nylene) (PPV) and poly(2-dimethyloctylsilyl-1,4-phenylenevinylene) (DMOS-PPV) to give PPDFV and
DMOS-PPDFV in an attempt to increase the electron affinity of the parent polymer. It was found that
the efficiency of PLED of DMOS-PPDFV was about 7 times higher than that of DMOS-PPV.
In tr od u ction
Exp er im en ta l Section
Gen er a l. Used all reagents were purchased from Aldrich
and used without further purification. Solvents were purified
by normal procedure 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
Caused by the prospective application as large-area
light-emitting diodes (LEDs),1-3 numerous reports about
PLEDs have been published since the discovery of
electroluminescence (EL) from a conjugated polymer,
poly(p-phenylenevinylene) (PPV).4 However, to be uti-
lized in the single-layer devices with high work function
metal as the cathode, PPV has the drawback that it is
a poor electron acceptor due to its high LUMO energy.
To overcome the imbalance of charge carrier injection
or mobility, the most frequently used solutions have
been either to use additional organic charge-transport-
ing layers between the emissive layer and one or both
of the electrodes5-7 or to adjust the energy band of the
polymer by introduction of electron-withdrawing groups
attached to the polymer backbone. It was shown by
Bredas et al. that introduction of electron-withdrawing
groups onto the arylene rings or the vinyl groups of PPV
lowers the HOMO and LUMO energies of the polymer,
thereby permitting the use of a higher work function
metal in the LED device.8 Abundant derivatives of PPV
have been reported with electron-withdrawing substit-
uents such as halide,9-13 cyano,14 trifluoromethyl,15 or
methylsulfonyl-phenyl16 on the arylene rings. Other
examples with electron-withdrawing groups on the
vinylene group of conjugated polymers have also been
reported.5,17-21 In the present paper, we report the
synthesis and electroluminescence properties of new EL
polymers, poly(p-phenylenedifluorovinylene) (PPDFV)
and poly(2-dimethyloctylsilyl-p-phenylenedifluorovi-
nylene) (DMOS-PPDFV), which contain two fluoro
groups in every vinylene units to reduce the barrier of
electron injection.
recorded with
a Varian CARY-5E UV/vis spectrophoto-
meter. Molecular weights and polydispersities of the polymers
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 spec-
troscopy.
For the EL experiment, poly(3,4-ethylenedioxythiophene)
(PEDOT) doped with poly(styrenesulfonate) (PSS), as the hole-
injection-transport layer, was introduced between the emissive
layer and ITO glass substrate cleaned by successive ultrasonic
treatments. The solution of PEDOT/PSS in aqueous isopropyl
alcohol was spin-coated on the surface-treated ITO substrate.
On top of the PEDOT layer, the emissive polymer film was
obtained by spin-casting 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 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
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 with 0.1 M tetra-
butylammonium tetrafluoroborate (TBAF, freshly distilled,
Aldrich) solution in acetonitrile. A platinum wire and a
Ag/AgNO3 electrode were used as the counter electrode and
reference electrode, respectively. Cyclic voltammetric waves
† Department of Chemistry and Center for Plastic Information
System, Pusan National University.
‡ LG Electronics Institute of Technology.
§ Department of Physics and Center for Plastic Information
System, Pusan National University.
* To whom correspondence should be addressed. E-mail: hssuh@
pusan.ac.kr.
10.1021/ma0493022 CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/10/2004

6712 J in et al.
Macromolecules, Vol. 37, No. 18, 2004
were produced by using a EG&G Parc model 273 potentiostat/
galvanostat at a constant scan rate of 100 mV/s.
The combined aqueous phase was extracted with diethyl ether
150 mL (3 × 50 mL). The combined organic layer was dried
with MgSO4, concentrated under reduced pressure, and puri-
fied by flash column chromatography (60 × 150 mm column,
SiO2, 100% of hexane) to give 22.4 g (81 mmol, 67%) of
dimethyloctylsilyl-p-xylene (5); Rf 0.36 (SiO2, hexane 100%).
1H NMR (200 MHz, CDCl3) δ (ppm): 7.28 (s, 1H), 7.09 (s, 2H),
2.43 (s, 3H), 1.41-1.14 (m, 12H), 0.97-0.75 (m, 5H), 0.33 (s,
6H).
Syn th esis of r,r-Dibr om o-p-xylen e (1). A stirred mixture
of 6 g (56.51 mmol) of p-xylene and 20.1 g (113.2 mmol) of
N-bromosuccinimide (NBS) in CCl4 (200 mL) at room temper-
ature was irradiated with the light source (300 W) for 1 h.
The reaction mixture was filtered in order to remove generated
succinimide. The filtrate was concentrated in vacuo, and the
residue was purified by flash column chromatography (60 ×
150 mm column, SiO2, 100% of hexane) to give 9.9 g (67%) of
the desired final R,R′-dibromo-p-xylene (1), white solid; Rf 0.25
(SiO2, hexane 100%). 1H NMR (200 MHz, CDCl3) δ (ppm): 4.48
(s, 4H), 7.37 (s, 4H)
Syn th esis of r,r′-Diflu or o-p-xylen e (2). To a stirred
solution of 10 g (37.88 mmol) of R,R-dibromo-p-xylene (1) in
20 mL of tetrahydrofuran (THF) at room temperature, 98.5
mL (98.5 mmol) of 1.0 M TBAF (tetrabutylammonium fluoride)
in THF was added. After stirring for 24 h at 50 °C, the reaction
mixture was concentrated in vacuo to remove the solvent. After
adding hexane, the mixture was stirred for 1 h and filtered.
The filtrate was concentrated in vacuo and purified by column
chromatography to give 2.3 g (42.9%) of the desired R,R-
difluoro-p-xylene (2), yellow oil; Rf 0.28 (SiO2, hexane 100%).
1H NMR (200 MHz, acetone-d6) δ (ppm): 5.43 (d, 4H, J ) 49.4
Hz), 7.47 (s, 4H). 13C NMR (50 MHz, acetone-d6) δ (ppm): 84.90
(d, J ) 163.5 Hz), 128.80 (d, J ) 6.5 Hz), 138.13 (d, J ) 20
Hz).
Syn th esis of 1,4-Bis(br om oflu or om eth yl)ben zen e (3).
A stirred mixture of 716 mg (5.04 mmol) of R,R′-difluoro-p-
xylene (2) and 1.79 g (10.07 mmol) of NBS in CCl4 (20 mL) at
room temperature was irradiated with the light source (300
W) for 1 h. The reaction mixture was filtered in order to remove
generated succinimide. The filtrate was concentrated in vacuo
and purified by flash column chromatography (15 × 150 mm
column, SiO2, 100% of hexane) to give 688 mg (45.5%) of the
desired final 1,4-bis(bromofluoromethyl)benzene (3), white
solid; Rf 0.20 (SiO2, hexane 100%). 1H NMR (200 MHz, acetone-
d6) δ (ppm): 7.86 (d, 2H, J ) 48 Hz), 7.72 (s, 4H). 13C NMR
(50 MHz, acetone-d6) δ (ppm): 92.74 (d, J ) 249.5 Hz), 126.63
(d, J ) 6.5 Hz), 141.94 (d, J ) 20 Hz). Anal. Calcd for C8H6-
Br2F2: C, 32.04; H, 2.02; F, 12.67. Found: C, 31.88; H, 2.23;
F, 12.30.
Syn th esis of P oly(p-p h en ylen ed iflu or ovin ylen e) (P P -
DF V) (4). To a stirred solution of 781 mg (2.6 mmol) of 1,4-
bis(bromofluoromethyl)benzene (3) in 20 mL of THF at 40 °C
under argon was added, drop by drop, 62.4 mL (15.6 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 redissolved in 200 mL of DMF at 60 °C, cooled
to 40 °C, and reprecipitated by dropwise addition of 500 mL
methanol. The precipitated polymer was filtered and 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 200 mg (55.7%) of poly(p-phenylenedifluorovinylene)
(PPDFV) (4) as light yellow polymer fibers. 1H NMR (300 MHz,
DMF-d6) δ (ppm): 7.19 (brs). 13C NMR (75 MHz, DMF-d6) δ
(ppm): 68.3, 127.1, 129.5. Anal. Calcd for C8H4F2: C, 69.57;
H, 2.92; F, 27.51. Found: C, 68.82; H, 2.80; F, 26.04.
Syn th esis of Dim eth yloctylsilyl-p-xylen e (5). To a stirred
solution of 22.34 g (121 mmol) of 2-bromo-p-xylene in 50 mL
of THF was slowly added 75.6 mL (121 mmol) of 1.60 M
solution of n-C4H9Li in hexane at -78 °C. After 2 h at -78
°C, 25 g (121 mmol) of chlorodimethyloctylsilane was slowly
added to the reaction mixture at -78 °C over 2 h. After 3 h at
room temperature, the reaction mixture was quenched with
dilute aqueous hydrochloric acid solution. The oragnic layer
was separated and washed with water 150 mL (3 × 50 mL).
Syn th esis of 2-Dim eth yloctylsilyl-1,4-bis(br om om eth -
yl)ben zen e (6). A stirred mixture of 20 g (72.3 mmol) of
dimethyloctylsilyl-p-xylene (5) and 25.93 g (145 mmol) of NBS
in CCl4 (50 mL) at room temperature was irradiated with the
light source (300 W) for 1 h. The reaction mixture was filtered
in order to remove generated succinimide. The filtrate was
concentrated in vacuo, and the residue was purified by flash
column chromatography (60 × 150 mm column, SiO2, 100% of
hexane) to give 23.5 g (54.2 mmol, 75%) of 2-dimethyloctylsilyl-
1,4-bis(bromomethyl)benzene (6), colorless oil: Rf 0.32 (SiO2,
1
hexane 100%). H NMR (200 MHz, CDCl3) δ (ppm): 7.48 (s,
1H), 7.43 (s, 2H), 4.61 (s, 2H), 4.48 (s, 2H), 1.43-1.19 (m, 12H),
0.99-0.88 (m, 5H), 0.42 (s, 6H).
Syn th esis of 2-Dim eth yloctylsilyl-1,4-bis(flu or om eth -
yl)ben zen e (7). To a stirred solution of 10 g (23.0 mmol) of
2-dimethyloctylsilyl-1,4-bis(bromomethyl)benzene (6) in 20 mL
of THF at room temperature, 69.0 mL (69.0 mmol) of 1.0 M
solution of TBAF (tetrabutylammonium fluoride) in THF was
added. After stirring for 24 h at 50 °C, the reaction mixture
was concentrated in vacuo to remove solvent. After adding
hexane to the residue, the mixture was stirred for 1 h and
filtered. The filtrate was concentrated in vacuo and purified
by column chromatography (60 × 150 mm column, SiO2, 100%
of hexane) to give 2.0 g (19.2 mmol, 28%) of 2-dimethyloctyl-
silyl-1,4-bis(fluoromethyl)benzene (7), colorless oil; Rf 0.33
(SiO2, hexane 100%). 1H NMR (200 MHz, CDCl3) δ (ppm): 7.51
(s, 1H), 7.44 (s, 2H), 5.57 (d, 2H, J ) 48 Hz), 5.50 (d, 2H, J )
48 Hz), 1.43-1.19 (m, 12H), 0.99-0.81 (m, 5H), 0.32 (s, 6H).
Syn th esis of 2-Dim eth yloctylsilyl-1,4-bis(br om oflu o-
r om et h yl)b en zen e (8). A stirred mixture of 700 mg (2.24
mmol) of 2-dimethyloctylsilyl-1,4-bis(fluoromethyl)benzene (7)
and 800 mg (4.47 mmol) of NBS in CCl4 (50 mL) at room
temperature was irradiated with the light source (300 W) for
1 h. The reaction mixture was filtered in order to remove
generated succinimide. The filtrate was concentrated in vacuo,
and the residue was purified by flash column chromatography
(20 × 150 mm column, SiO2, 100% of hexane) to give 500 mg
(1.06 mmol, 48%) of 2-dimethyloctylsilyl-1,4-bis(bromofluoro-
methyl)benzene (8), colorless oil; Rf 0.32 (SiO2, hexane 100%).
1H NMR (200 MHz, CDCl3) δ (ppm): 7.87-7.27 (m, 5H), 1.43-
1.17 (m, 12H), 0.85 (m, 5H), 0.38 (s, 6H). 13C NMR (50 MHz,
CDCl3) δ (ppm): 145.97 (d, J ) 17.7 Hz), 139.47 (d, J ) 19.55
Hz), 136.37, 130.84 (d, J ) 6.3 Hz), 127.39 (d, J ) 5.5 Hz),
126.72, 126.60, 93.90, 88.87, 33.37, 31.86, 29.17, 23.77, 22.64,
16.35, 14.10. Anal. Calcd for C18H28Br2F2Si: C, 45.97; H, 6.00;
F, 8.08. Found: C, 45.66; H, 5.92; F, 7.81.
Syn th esis of P oly(2-d im eth yloctylsilyl-p-p h en ylen ed i-
flu or ovin ylen e) (DMOS-P P DF V) (9). To a stirred solution
of 400 mg (0.85 mmol) of 1,4-bis(bromofluoromethyl)benzene
(8) in 20 mL of THF at 40 °C under argon was added, drop by
drop, 20.4 mL (5.10 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 green power. The resulting polymer was redis-
solved in 100 mL of THF (60 °C), cooled to 40 °C, and
reprecipitated by dropwise addition of 500 mL of methanol.
The precipitated 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 97 mg

Macromolecules, Vol. 37, No. 18, 2004
Novel Electroluminescent Polymers 6713
Sch em e 1. Syn th etic Rou tes for Mon om er a n d
P olym er of P P DF V
Sch em e 2. Syn th etic Rou tes for Mon om er a n d
P olym er of DMOS-P P DF V
F igu r e 1. TGA of PPDFV and DMOS-PPDFV.
Ta ble 1. P olym er iza tion Resu lts of P P DF V a n d
DMOS-P P DF V
a
a
polymers
PPDFV
DMOS-PPDFV
yield (%)
Mn
Mw
PDIa
55.7
19.3
161 000
28 000
287 000
78 000
1.77
2.79
a
Mn, Mw, and PDI of the polymers were determined by gel
permeation chromatography using polystyrene standards.
(37%) of poly(2-dimethyloctylsilyl-p-phenylenedifluorovinylene)
(DMOS-PPDFV) (9) as a light green polymer power. 1H NMR
(300 MHz, CDCl3) δ (ppm): 7.41-7.17 (brs, 3H, aromatic
proton), 1.45-0.96 (brs, 14H, (CH2)7), 0.74 (brs, 3H, CH3), 0.35
(brs, 6H, Si(CH3)2). Anal. Calcd for C18H26F2Si: C, 70.08; H,
8.50; F, 12.32. Found: C, 69.09; H, 8.32; F, 11.58.
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. For the preparation
of PPDFV, p-xylene was brominated using NBS and
light source (300 W) to generate R,R′-dibromo-p-xylene
(1). The resulting dibromide was fluorinated with tet-
rabutylammonium fluoride (TBAF)22 and then bromi-
nated23 again using NBS and light source to generate
1,4-bis(bromofluoromethyl)benzene (3), the monomer for
PPDFV. The DMOS-PPDFV was also synthesized in
five steps. 2-Bromo-p-xylene, as the starting material,
was coupled with chlorodimethyloctylsilane using n-
butyllithium in THF to generate dimethyloctylsilyl-p-
xylene, which was brominated, fluorinated, and then
brominated to generate 1,4-bis(bromofluoromethyl)-2-
dimethyloctylsilylbenzene (8), the monomer for DMOS-
PPDFV. The structure and purity of the monomers were
F igu r e 2. (a) UV-vis absorption and (b) PL spectra of
PPDFV, DMOS-PPV, and DMOS-PPDFV.
age molecular weight (Mh n) and the weight-average
molecular weight (Mh w) of the PPDFV were 161 000, and
287 000 with polydispersity of 1.77 as determined by
GPC using DMF as the eluent and polystyrene as the
standard. Mh n and Mh w of the DMOS-PPDFV were
28 000 and 78 000 with polydispersity of 2.79 as deter-
mined by GPC using THF as the eluent. The thermal
properties of the polymers were determined by thermal
gravimetric analysis (TGA) under a nitrogen atmo-
sphere at a heating rate of 10 °C /min. PPDFV and
DMOS-PPDFV lose less than 5% of their weights on
heating to 550 and 350 °C, respectively. The TGA
thermograms of the polymers are shown in Figure 1.
The polymers did not show either an endo or exo curve
in DSC thermograms due to side chain scission or
thermal cross-linking until the temperature of initial
decomposition.
1
confirmed by H NMR and 13C NMR. The polymers of
these monomers were prepared by the Gilch reaction,24
with an excess amount of potassium tert-butoxide in
THF at 0 °C for 24 h under an Ar atmosphere. Known
poly(2-dimethyloctylsilyl-p-phenylenevinylene (DMOS-
PPV)25 was also synthesized, using a similar method,
to be compared with DMOS-PPDFV.The resulting
PPDFV, brittle dark-yellow polymer was soluble in
organic solvents such as dimethylformamide (DMF) and
o-dichlorobenzene (ODCB). The emissive polymer film
was obtained by spin-casting an ODCB solution of
PPDFV. The DMOS-PPDFV was soluble in various
organic solvents such as chloroform, chlorobenzene,
THF, dichloromethane, and ODCB. The number-aver-
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 PPDFV and DMOS-PPDFV as thin films are
shown in Figure 2. The thin films were prepared by
spin-coating on quartz plates from the polymer solutions

6714 J in et al.
Macromolecules, Vol. 37, No. 18, 2004
F igu r e 3. Energy band diagram of PPDFV, DMOS-PPV, and
DMOS-PPDFV.
F igu r e 4. Electroluminescence (EL) spectra of PPDFV,
DMOS-PPV, and DMOS-PPDFV with a configuration of ITO/
PEDOT:PSS/polymer/Al.
with o-dichlorobenzene (ODCB). The PPDFV exhibit
absorption spectra with a maximum peak of 390 nm,
which are blue-shifted about 30 nm relative to PPV,3
attributed to the π-π* transition of the conjugated
backbones. DMOS-PPDFV has a strong absorption
band at around 385 nm, which is attributed to the π-π*
transition of the conjugated segments. The absorption
onset wavelengths of PPDFV and DMOS-PPDFV were
512 and 457 nm, which correspond to band gaps of 2.42
and 2.71 eV. The PL spectra of the polymers all consist
of a typical vibronically structured band comprising a
maximum, a shoulder, and a tail.
reference electrode, respectively. All measurements
were calibrated against an internal standard, ferrocene
(Fc), which has the IP value (-4.8 eV) of the Fc/Fc+ redox
system.27 All of the polymers exhibit irreversible pro-
cesses in an oxidation scan. The band gap and HOMO
level are 2.42 and 5.47 eV for PPDFV, 2.7l and 5.51 eV
for DMOS-PPDFV, and 2.34 and 5.04 eV for DMOS-
PPV. The HOMO and LUMO energy levels of DMOS-
PPDFV are lower than those of DMOS-PPV, which can
be attributed to the introduction of the electron-
withdrawing fluoro groups. The higher work function
of the LUMO of DMOS-PPDFV as compared to that of
DMOS-PPV indicates that the electron injection pro-
cess is easier in DMOS-PPDFV than in DMOS-PPV.
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 normalized electrolumines-
cence (EL) spectra and the current-voltage character-
istics of the ITO/PEDOT/polymers/Al device are shown
in Figure 4. The PEDOT:PSS was spin-coated from
aqueous solution with isopropyl alcohol (10 wt %) 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 the
aluminum electrode were deposited on the top of the
polymer film through a mask by vacuum evaporation
at pressure below about 10-6 mbar, yielding an active
area of 4 mm2. The current-voltage (I-V) characteris-
tics were measured using a Keithley 236 source measure
unit. All processing steps and measurements mentioned
above were carried out under air and at room temper-
ature.
The EL spectra of PPDFV and DMOS-PPDFV ex-
hibit maximum peaks at 565 and 540 nm, which
correspond to yellow and green light, respectively.
The voltage-current density characteristics of the
devices fabricated from PPDFV and DMOS-PPDFV are
shown in Figure 5. For PPDFV and DMOS-PPDFV, the
turn-on voltages are approximately 3-4 V, and the
current densities increase in an exponential manner
with increasing forward bias, which is typical of diode
characteristic. The maximum brightness of DMOS-
PPDFV is 750 cd/m2 at 7.5 V. However, PPDFV reaches
a brightness of 95 cd/m2 at 7.0 V. As shown in Figure 3,
the higher work function of the LUMO of DMOS-
PPDFV as compared to that of LUMO of DMOS-PPV
indicates the electron injection process is easier in
DMOS-PPDFV than in DMOS-PPV. From this result,
higher quantum efficiency of DMOS-PPDFV as com-
The PL spectrum of the PPDFV thin film exhibits a
maximum at 580 nm, which is red-shifted about 40 nm
relative to that of PPV.3 PPDFV shows interesting
spectral properties as was reported to other fluorine-
substituted PPV derivatives. The PPVs with fluorine
substitution on the phenylene units have UV absorption
maximum peaks that are blue-shifted relative to that
of PPV but have PL emissions that are substantially
red-shifted relative to that of PPV.9,11,13 These results
are not well understood, but red-shifted PL emission of
fluorine substituted PPVs clearly reflect the electronic
effects of fluorine substitution. A reasonable suggestion
is that the excitons generated in fluorine-substituted
PPVs may be trapped in shallow trap states associated
with charge separation between polymer backbone and
highly electronegative fluorine atoms. Therefore, this
donor-acceptor pair recombination process exhibits a
longer wavelength emission in fluorine-substituted
PPVs.3 On the other hand, the PL spectrum of the
DMOS-PPDFV thin film exhibits a maximum at 495
nm, which is blue-shifted about 30 nm relative to that
of DMOS-PPV. The blue shift in the spectroscopic
features of DMOS-PPDFV as compared to that of
DMOS-PPV indicates the decrease of the effective
conjugation length by the inclusion of the fluoro group
at the vinyl position. The effective conjugation length
of DMOS-PPDFV was decreased by the possible steric
effect between the fluoro group and the bulky trialkyl-
silyl group.
Electr och em ica l P r op er ties of th e P olym er s. The
energy band diagrams, as shown in Figure 3, of poly-
mers were determined from the band gaps, which were
estimated from the absorption edges, and the HOMO
energy levels, which were estimated from cyclic volta-
mmetry.26 The CV was performed with a solution of
tetrabutylammonium tetrafluoroborate (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. A Pt wire and a Ag/
AgNO3 electrode were used as the counter electrode and

Macromolecules, Vol. 37, No. 18, 2004
Novel Electroluminescent Polymers 6715
polymers have low turn-on voltages of 3-4 V in the Al/
polymer/PEDOT/ITO device. The highest efficiency of
PPDFV was observed to be 0.47 cd/A at 4.6 V, and that
of DMOS-PPDFV was observed to be 2.7 cd/A at 6.5
V, which was higher than that of DMOS-PPV (0.39
cd/A at 7.0 V) due to the improved electron injection
ability from the cathode caused by the fluoro functional-
ity.
Ack n ow led gm en t. This work was supported by the
Ministry of Information & Communications, Korea,
under the Information Technology Research Center
(ITRC) Support Program (H. Suh).
Refer en ces a n d Notes
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pared to that of DMOS-PPV can be expected due to its
improved electron injection ability from the cathode.
Figure 6 shows the EL efficiency of DMOS-PPDFV
is higher than that of DMOS-PPV. The luminescence
efficiency of the polymer LEDs with PPDFV and DMOS-
PPDFV at room temperature were about 0.47 cd/A at
4.6 V and 2.7 cd/A at 6.5 V, respectively.
Con clu sion
We have synthesized, by the Gilch reactions, new PPV
derivatives, poly(p-phenylenedifluorovinylene) (PPDFV)
and poly(2-dimethyloctylsilyl-1,4-phenylene-difluorovi-
nylene) (DMOS-PPDFV), with fluoro groups in vi-
nylene units. These polymers have quite good thermal
stability. The polymer LEDs of PPDFV and DMOS-
PPDFV emit bright yellow and green light with maxi-
mum peak around 565 and 540 nm, respectively. Both
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