High-Efficiency Poly(p-phenylenevinylene)-Based Copolymers Containing an Oxadiazole Pendant Group for Light-Emitting Diodes

Article abstract of DOI:10.1021/ja036955+

A new series of high brightness and luminance efficient poly(p-phenylenevinylene) (PPV)-based electroluminescent (EL) polymers, poly[2-{4- [5-(4-(3,7-dimethyloctyloxy)phenyl)-1,3,4-oxadiazole-2-yl]phenyloxy}-1, 4-phenylenevinylene] (Oxa-PPV), poly[2-{2-((

Full text of DOI:10.1021/ja036955+

Published on Web 02/07/2004
High-Efficiency Poly(p-phenylenevinylene)-Based Copolymers
Containing an Oxadiazole Pendant Group for
Light-Emitting Diodes
Sung-Ho Jin,*,† Mock-Yeon Kim,† Jin Young Kim,‡ Kwanghee Lee,‡ and
Yeong-Soon Gal§
Contribution from the Departments of Chemistry Education and Physics, and Center for
Plastic Information System, Pusan National UniVersity, Busan 609-735, Korea, and
Polymer Chemistry Laboratory, Kyungil UniVersity, Hayang 712-701, Korea
Received June 28, 2003; E-mail: shjin@pusan.ac.kr
Abstract: A new series of high brightness and luminance efficient poly(p-phenylenevinylene) (PPV)-based
electroluminescent (EL) polymers, poly[2-{4-[5-(4-(3,7-dimethyloctyloxy)phenyl)-1,3,4-oxadiazole-2-yl]phen-
yloxy}-1,4-phenylenevinylene] (Oxa-PPV), poly[2-{2-((3,7-dimethyloctyl)oxy)phenoxy}-1,4-phenylenevi-
nylene] (DMOP-PPV), and their corresponding random copolymers, poly{[2-{4-[5-(4-(3,7-dimethyloctyloxy)-
phenyl)-1,3,4-oxadiazole-2-yl]phenyloxy}-1,4-phenylenevinylene]-co-[2-{2-((3,7-dimethyloctyl)oxy) phenoxy}-
1,4-phenylenevinylene]} (Oxa-PPV-co-DMOP-PPV), with an electron-deficient 1,3,4-oxadiazole unit on the
side groups, were synthesized through the Gilch polymerization method. The newly designed and
synthesized asymmetric molecular structures of Oxa-PPV, DMOP-PPV, and Oxa-PPV-co-DMOP-PPV were
completely soluble in common organic solvents, and defect-free optical thin film was easily spin-coated
onto the indium tin oxide (ITO) substrate. Oxa-PPV shows a high glass transition temperature (Tg), which
might be an advantage for long time operation of polymer light-emitting diodes (PLEDs). Double-layer LEDs
with an ITO/PEDOT/polymer/Al configuration were fabricated by using those polymers. Electrooptical
properties and device performance could be adjusted by introducing the Oxa-PPV content in the copolymers.
The emission colors could be tuned from green to yellowish-orange via intramolecular energy transfer.
The improved device performance of Oxa-PPV over DMOP-PPV and Oxa-PPV-co-DMOP-PPV may be
due to better electron injection and charge balance between holes and electrons and also efficient
intramolecular energy transfer from 1,3,4-oxadiazole units to PPV backbones. The maximum brightness
and the luminance efficiency of Oxa-PPV were up to 19395 cd/m2 at 14 V and 21.1 cd/A at 5930 cd/m2.
The maximum luminance efficiency of Oxa-PPV is ranked the highest value among the PPV derivatives to
date.
Introduction
efficiency of the device determined by the amount of charge
carrier injection, the probability of the capture of charges, and
Since the report of polymer light-emitting diodes (PLEDs)
based on poly(p-phenylenevinylene) (PPV) by the Cambridge
group, π-conjugated polymers have attracted much attention
because of their good ability to form thin films, good mechanical
properties, excellent luminescence, etc.1,2 Yet a problem of
π-conjugated polymers is that electron injection is much more
difficult than hole injection, resulting in the imbalance of rates
for electron and hole injection from negative and positive
contacts and a shift of the recombination zone toward the region
near the interface of the polymer/cathode.3 The luminance
the ratio of singlet excitons formed is limited.4-6 A balance of
the rates of injection of electrons and holes from opposite
contacts into the device is crucial in achieving high electrolu-
minescent efficiency.7,8 The use of a metal with a low work
function such as calcium, magnesium, or lithium as cathode
can lower the charge injection barrier at the cathode to improve
the luminance efficiency by balancing injected electrons and
holes.9-11 However, these metals have high chemical reactivity
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† Department of Chemistry Education and Center for Plastic Information
System, Pusan National University.
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Soc. 2001, 123, 2296-2307.
‡ Department of Physics and Center for Plastic Information System, Pusan
National University.
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A. B. Nature 1993, 365, 628-630.
§ Kyungil University.
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Schlatmann, B.; Friend, R. H.; Klapwijk, T. M.; Hadziioannou, G. AdV.
Mater. 1997, 9, 127-131.
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2474
J. AM. CHEM. SOC. 2004, 126, 2474-2480
10.1021/ja036955+ CCC: $27.50 © 2004 American Chemical Society

High-Efficiency Poly(p-phenylenevinylene)-Based Copolymers
A R T I C L E S
and then potassium tert-butoxide (18.4 g, 164 mmol) was added at
room temperature with stirring. The mixture was heated to 150 °C and
kept at reflux for 10 h. The reaction mixture was cooled to room
temperature, poured into excess water, and extracted with ether. The
combined organic layers were washed several times further with water,
dried over anhydrous MgSO4, and filtered. The solvent was removed
by evaporation under reduced pressure. The product was purified by
column chromatography on silica gel using hexane/dichloromethane
(1:1) as an eluent to give ethyl 4-(2,5-dimethylphenoxy)benzoate, 1
(25 g, 68%). 1H NMR (CDCl3, δ ppm): 1.34-1.41 (t, 3H, CH3), 2.12,
2.30 (s, 6H, 2CH3 on aromatic ring), 4.29-4.40 (q, 2H, -OCH2-),
6.79, 6.84-6.89, 6.92-6.96, 7.13-7.17, 7.96-8.00 (m, 7H, aromatic
protons).
to oxygen and moisture, which limits their practicality. Multi-
layer devices have an electron transport layer (ETL) between a
light-emitting layer and a negative electrode and have been
found to be more efficient than single-layer devices.12 However,
multilayer devices require careful selection of solvent so that
the solution of an electron transport material will not damage
the previous light-emitting layer. Besides, the method could
cause space charges and tunneling of accumulated holes13,14 and
increase the turn-on voltage. Another strategy to improve the
device efficiency is use of a blend of electron transport material
with EL polymer. The unavoidable drawback to this approach
is that phase separation15,16 between the two different kinds of
materials might happen, which could accelerate a rate of
recrystallization or aggregate formation by generated heat during
the device operation. Therefore, the lifetime and stability of the
devices are reduced. To solve the problems of previous methods
for improving the luminance efficiency, many research groups
have introduced electron transport moieties on the side chain
of polymer backbones or polymer main chains.17-22 One of the
most widely used electron transport moieties is aromatic 1,3,4-
oxadiazole-based compounds with high electron affinities, which
facilitate electron transport and injection. For such a purpose,
we designed and synthesized novel PPV derivatives containing
1,3,4-oxadiazole pendant groups. In our previous work, we have
synthesized PPV derivatives with alkylsilylphenyl and alkylox-
yphenoxy groups as side chains and poly(9,9-di-n-octylfluore-
nyl-2,7-vinylene) (PFV) derivatives, which showed the high
performance of PLED.23-27 In this paper, we report the synthesis
and characterization of PPV derivatives with a 1,3,4-oxadiazole
moiety as a side chain. The 1,3,4-oxadiazole units are incor-
porated between the phenoxy and alkoxyphenyl substituents to
improve the solubility and EL characteristics. The resulting EL
polymers were synthesized by the Gilch polymerization method
for high molecular weight, easy purification, narrow polydis-
persity, and good thermal stability.
Synthesis of 4-(2,5-Dimethylphenoxy)benzohydrazide, 2. In a 250
mL three-neck flask, hydrazine monohydrate (26 g, 518 mmol) was
dissolved in 30 mL of ethanol. Ethyl 4-(2,5-dimethylphenoxy)benzoate,
1 (20 g, 74 mmol), was added dropwise and stirred at 90 °C for 17 h.
After confirmation of the disappearance of ethyl 4-(2,5-dimethylphe-
noxy)benzoate, 1, by TLC, the mixture was cooled to room temperature
and then recrystallized from methanol. A crystal product was washed
several times further with hexane. A white crystal product, 4-(2,5-
1
dimethylphenoxy)benzohydrazide, 2, was obtained (14 g, 75%). H
NMR (CDCl3, δ ppm): 2.12, 2.30 (s, 6H, 2CH3 on aromatic ring),
4.0-4.2 (b, 2H, NH2), 7.20-7.40 (b, 1H, NH), 6.78, 6.84-6.96, 7.13-
7.17, 7.65-7.72 (m, 7H, aromatic protons).
Synthesis of N-{4-[(3,7-Dimethyloctyl)oxy]benzoyl}-4-(2,3-di-
methylphenoxy)benzohydrazide, 3. In a 250 mL three-neck flask,
4-(2,5-dimethylphenoxy)benzohydrazide, 2 (4.4 g, 17 mmol), and
triethylamine (1.7 g, 17 mmol) were dissolved in 50 mL of dichlo-
romethane, and 4-(3,7-dimethyloctyloxy)benzoyl chloride (5.1 g, 17
mmol) was added dropwise. The mixture was stirred at room temper-
ature for 4 h and then extracted with water and dichloromethane several
times. The organic layers were dried over anhydrous MgSO4 and
filtered. The solvent was removed by evaporation under reduced
pressure. The product was recrystallized from methanol and washed
several times with hexane to give white crystals of N-{4-[(3,7-
dimethyloctyl)oxy]benzoyl}-4-(2,3-dimethylphenoxy)benzohy-
drazide, 3 (8.5 g, 97%). 1H NMR (CDCl3, δ ppm): 0.84-0.88 (d, 6H,
2CH3), 0.92-0.95 (d, 3H, CH3), 1.16-1.85 (m, 10H, 4CH2, 2CH), 2.11,
2.30 (s, 6H, 2CH3 on aromatic ring), 3.98-4.05 (t, 2H, OCH2), 6.78-
6.96, 7.12-7.16, 7.78-7.83 (m, 11H, aromatic protons), 9.35-9.48
(d, d, 2H, NH).
Experimental Section
Synthesis of Ethyl 4-(2,5-Dimethylphenoxy)benzoate, 1. In a 250
mL three-neck flask, 2,5-dimethylphenol (16.7 g, 137 mmol) and ethyl
4-fluorobenzoate (23 g, 137 mmol) were dissolved in 100 mL of DMF,
Synthesis of 2-{4-[(3,7-Dimethyloctyl)oxy]phenyl}-5-(4-(2,5-di-
methylphenoxy)phenyl-1,3,4-oxadiazole, 4. To a 250 mL three-neck
flask were added N-{4-[(3,7-dimethyloctyl)oxy]benzoyl}-4-(2,3-di-
methylphenoxy)benzohydrazide, 3 (4 g, 7.7 mmol), 50 mL of benzene,
and SOCl2 (3.7 g, 31 mmol), and the mixture was refluxed at 120 °C
for 4 h and then cooled to room temperature. The solvent and SOCl2
were removed by evaporation under reduced pressure, and then the
reaction mixture was poured into excess water and extracted with
chloroform. The combined organic layers were washed several times
further with water, dried over anhydrous MgSO4, and filtered. The
solvent was removed by evaporation under reduced pressure. The crude
product was purified by column chromatography on silica gel using
hexane as an eluent to give 4 (3.7 g, 95%). 1H NMR (CDCl3, δ ppm):
0.85-0.89 (d, 6H, 2CH3), 0.94-0.97 (d, 3H, CH3), 1.18-1.88 (m, 10H,
4CH2, 2CH), 2.15, 2.31 (s, 6H, 2CH3 on aromatic ring), 4.03-4.10 (t,
2H, OCH2), 6.82, 6.93-7.03, 7.14-7.19, 8.01-8.08 (m, 11H, aromatic
protons).
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Sci., Part A: Polym. Chem. 2000, 38, 4185-4193.
Synthesis of 2-{4-[2,5-Bis(bromomethyl)phenoxy]phenyl-5-{4-
[(3,7-dimethyloctyl)oxy]phenyl}-1,3,4-oxadiazole, 5. To a 250 mL
three-neck flask were added 2-{4-[(3,7-dimethyloctyl)oxy]phenyl}-5-
(4-(2,5-dimethylphenoxy)phenyl-1,3,4-oxadiazole, 4 (5.8 g, 11.6 mmol),
N-bromosuccinimide (NBS) (4.6 g, 25.6 mmol), catalytic amounts of
benzoyl peroxide (BPO), and 100 mL of benzene. The mixture was
refluxed and stirred until succinimide was on top of the solution. After
(24) Jin, S. H.; Jang, M. S.; Suh, H. S.; Cho, H. N.; Lee, J. H.; Gal, Y. S.
Chem. Mater. 2002, 14, 643-650.
(25) Jin, S. H.; Kang, S. Y.; Yeom, I. S.; Kim, J. Y.; Park, S. H.; Lee, K. H.;
Gal, Y. S.; Cho, H. N. Chem. Mater. 2002, 14, 5090-5097.
(26) Jin, S. H.; Park, H. J.; Kim, J. Y.; Lee, K.; Lee, S. P.; Moon, D. K.; Lee,
H. J.; Gal,Y. S. Macromolecules 2002, 35, 7532-7534.
(27) Jin, S. H.; Kang, S. Y.; Kim, M. Y.; Yoon, U. C.; Kim, J. Y.; Lee, K. H.;
Gal, Y. S. Macromolecules 2003, 36, 3841-3847.
9
J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004 2475

A R T I C L E S
Kim et al.
Scheme 1. Synthetic Routes for the Monomer and Polymers
confirmation of the disappearance of the 2-{4-[(3,7-dimethyloctyl)oxy]-
phenyl}-5-(4-(2,5-dimethylphenoxy)phenyl-1,3,4-oxadiazole, 4, by TLC,
the mixture was cooled to room temperature. The succinimide was
filtered off, and the mother liquor was poured into excess water and
extracted with dichloromethane. The combined organic layers were
washed several times further with water, dried over anhydrous MgSO4,
and filtered. The solvent was removed by evaporation under reduced
pressure. The product was purified by column chromatography on silica
gel using hexane to give 2-{4-[2,5-bis(bromomethyl)phenoxy]phenyl-
5{4-[(3,7-dimethyloctyl)oxy]phenyl}-1,3,4-oxadiazole, 5 (3.2 g, 41%).
1H NMR (CDCl3, δ ppm): 0.84-0.88 (d, 6H, 2CH3), 0.93-0.96 (d,
3H, CH3), 1.17-1.88 (m, 10H, 4CH2, 2CH), 4.06-4.12 (t, 2H, OCH2),
4.40, 4.54 (s, 4H, 2CH2Br on aromatic ring), 6.97-7.03, 7.11-7.21,
7.43-7.48, 8.01-8.14 (m, 11H, aromatic protons). 13C NMR (CDCl3,
δ ppm): 19.7, 22.6, 27.0, 28.0, 29.9, 32.0, 36.0, 37.3, 39.2, 66.7, 115.0,
116.1, 118.8, 119.5, 120.1, 125.3, 128.7, 129.7, 132.0, 140.4, 154.2,
159.6, 162.0. Anal. Calcd for C32H36Br2N2O3: C, 58.55; H, 5.53; N,
4.27. Found: C, 58.14; H, 5.86; N, 4.01.
a 100 mL Schlenk flask dried with a stream of hot air and flushed
with N2 was added monomer, 5 (0.3 g, 0.5 mmol), and then it was
dried and flushed with N2 again and dissolved in 30 mL of dry toluene.
A solution of potassium tert-butoxide in THF (2.7 mL, 1.0 M) was
then added at a rate of 5.5 mL/h via syringe pump. After complete
addition of base, the reaction was stirred at room temperature for 4 h.
To terminate the polymerization reaction, monobrominated byproduct,
2-{4-[5-(bromomethyl)-2-methylphenoxy]phenyl}-5-{4-[(3,7-dimethyl-
octyl)oxy]phenyl}-1,3,4-oxadiazole, was added to the reaction mixture
and further stirred for 1 h. The reaction mixture changed color from
colorless to yellow to orange, and the viscosity increased. The reaction
mixture was poured into 300 mL of methanol to precipitate the polymer.
The collected polymer was further purified by Soxhlet extraction
through methanol to remove the low molecular weight oligomers and
inorganic impurities. The resulting polymer was redissolved in chlo-
roform and washed several times further with water. The solvent
moieties were evaporated with a rotary evaporator and again repre-
cipitated into 300 mL of methanol. The precipitated polymer was
collected by suction filtration and dried in a vacuum oven to yield 0.16
Synthesis of Poly[2-{4-[5-(4-(3,7-dimethyloctyloxy)phenyl)-1,3,4-
oxadiazole-2-yl]phenyloxy}-1,4-phenylenevinylene] (Oxa-PPV). To
1
mg (70%) of Oxa-PPV. H NMR (CDCl3, δ ppm): 0.72-1.00 (br,
9
2476 J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004

High-Efficiency Poly(p-phenylenevinylene)-Based Copolymers
A R T I C L E S
Table 1. Polymerization Results and Thermal Properties of
Oxa-PPV, DMOP-PPV, and Oxa-PPV-co-DMOP-PPV
9H, 3CH3), 1.02-1.40 (br, 6H, (CH2)3), 1.57 (br, 4H, 2CH, -CH2-),
3.90-4.12 (br, 2H, -OCH2), 6.55-7.40, 7.80-8.15 (br, 13H, aromatic
protons and vinylic protons). Anal. Calcd for C32H34N2O3: C, 77.70;
H, 6.93; N, 5.66. Found: C, 75.66; H, 7.25; N, 5.31.
Poly[2-{2-((3,7-dimethyloctyl)oxy)phenoxy}-1,4-phenylenevinyl-
ene], DMOP-PPV, and their corresponding random copolymers, poly-
{[2-{4-[5-(4-(3,7-dimethyloctyloxy)phenyl)-1,3,4-oxadiazole-2-yl]phe-
nyloxy}-1,4-phenylenevinylene]-co-[2-{2-((3,7-dimethyloctyl) oxy)-
phenoxy}-1,4-phenylenevinylene]}, Oxa-PPV-co-DMOP-PPV, with
various feed ratios of DMOP-PPV content were synthesized and purified
by a method similar to that used for Oxa-PPV.
feed ratio
(5:6)
M a
yield
(%)
DSC
w
b
polymer
Oxa-PPV
Oxa-PPV-co-DMOP-
PPV (84:16)c
(×104)
PDIa
(T )
g
TGA
100:0
90:10
9.7
69
1.4
4.9
70
68
171
169
290
290
Oxa-PPV-co-DMOP-
PPV (64:36)
Oxa-PPV-co-DMOP-
PPV (51:49)
Oxa-PPV-co-DMOP-
PPV (28:72)
Oxa-PPV-co-DMOP-
PPV (7:93)
DMOP-PPV
70:30
50:50
30:70
10:90
0:100
38
35
59
39
18
6.5
7.8
8.6
5.9
4.8
66
72
79
76
70
126
132
158
102
252
300
280
370
380
320
Results and Discussion
Synthesis and Characterization. Molecules and polymers
with 1,3,4-oxadiazole derivatives are the classes of electron
injection and hole blocking materials. The synthetic routes for
the 1,3,4-oxadiazole containing monomer, corresponding ho-
mopolymer, and their copolymers with various compositions
of DMOP-PPV are depicted in Scheme 1. Monomer with a
1,3,4-oxadiazole side chain was synthesized in five steps. Ethyl
4-(2,5-dimethylphenoxy)benzoate, 1, was synthesized by the
reaction of ethyl 4-fluorobenzoate with 2,5-dimethylphenol in
the presence of potassium tert-butoxide in DMF solvent. The
ester compound, 1, was treated with hydrazine monohydrate to
give the hydrazide, 2, compound. Condensation of hydrazide,
2, with 2,7-dimethyloctyloxybenzoyl chloride yielded the bis-
(dihydrazide) derivative, 3, in almost quantitative yield. Ring
closure of the bis(dihydrazide) derivative with thionyl chloride
in benzene solvent led to the 1,3,4-oxadiazole unit as a pendant
group, 4. The newly designed monomer was obtained via
bromination of compound 4 in the presence of AIBN and was
purified by column chromatography. The synthesis of 1,4-bis-
(bromomethyl)-2-{2-[(3,7-dimethyloctyl)oxy] phenoxy}benzene,
6, was described elsewhere.25 The synthetic strategy of the
present monomer, 5, is introduction of the 1,3,4-oxadiazole unit
as an ether linkage to improve the solubility of the corresponding
EL polymers. The comonomer, 6, was also linked to the 3,7-
dimethyloctyloxyphenyl group in the bis(bromomethyl)benzene
backbone via an oxygen atom, which induced the molecular
similarity between the two monomers. Homopolymerization of
monomer 5 was performed with an excess of potassium tert-
butoxide in dry toluene under nitrogen atmosphere. To improve
the device performance of PLEDs and tune the electrooptical
properties of Oxa-PPV, we copolymerized the 2-{4-[2,5-bis-
(bromomethyl)phenoxy]phenyl-5{4-[(3,7-dimethyloctyl)oxy]-
phenyl}-1,3,4-oxadiazole, 5, with various feed ratios of
1,4-bis(bromomethyl)-2-{2-[(3,7-dimethyloctyl)oxy]phenoxy}-
benzene, 6, in THF. During the polymerization, the viscosity
of the reaction mixture was increased without any precipitation,
and intense fluorescent light was observed. A small amount of
monobrominated byproduct, 2-{4-[5-(bromomethyl)-2-meth-
ylphenoxy]phenyl}-5-{4-[(3,7-dimethyloctyl)oxy]phenyl}-1,3,4-
oxadiazole, during the synthesis of monomer 5, was added to
the polymerization mixture to end-cap the polymer chain. Most
of the light-emitting homopolymers with electron-withdrawing
1,3,4-oxadiazole units in side or main chains exhibit poor
solubility that limits their application.28,29 The resulting EL
polymers were completely soluble in common organic solvents,
a Mw and PDI values of the polymers were determined by GPC using
polystyrene standards. b TGA was measured at a temperature of 5% weight
loss for the polymers. c Composition ratio was determined from the ratios
of phenyl protons on 1,3,4-oxadiazole and methylene next to the oxygen
atom.
such as chloroform, THF, chlorobenzene, and 1,2-dichloroben-
zene. The good solubility behavior of the present EL polymers,
Oxa-PPV and Oxa-PPV-co-DMOP-PPV, is due to the easily
free rotation between the PPV backbone and side chain 1,3,4-
oxadiazole units, which were linked via the oxygen atom and
bent type 3,7-dimethyloctyloxy substituent to the phenoxy side
group on DMOP-PPV. Most of the conjugated polymers tend
to be p-type semiconductors with a much greater tendency for
injecting and transporting holes than electrons. A balanced
injection and transport of holes and electrons is crucial in
achieving high quantum efficiency.
Table 1 summarizes the polymerization results, molecular
weights, and thermal data of the Oxa-PPV, DMOP-PPV, and
their copolymers, Oxa-PPV-co-DMOP-PPV. The weight aver-
age molecular weight (Mw) and polydispersity of the present
polymers were found to be in the range of (9.7-69) × 104 and
1.4-8.6, respectively. The structure and thermal properties of
the polymers were identified by H,13C NMR spectroscopy,
1
TGA, and DSC thermograms. Bromobenzylic proton peaks of
monomer 5 showed two peaks at 4.40 and 4.54 ppm. These
two splitting peaks are due to the asymmetric molecular structure
by introduction of one electron-withdrawing 1,3,4-oxadiazole
substituent into the 1,4-bis(bromomethyl)benzene unit, which
gave different acidities of benzylic protons. These peaks
disappeared during the polymerization, and new vinylic proton
peaks appeared at 6.5-7.3 ppm together with phenyl proton
peaks. Four phenyl proton and methylene proton peaks next to
the 1,3,4-oxadiazole ring and oxygen atom appeared at 8 and
4.1 ppm, respectively. The compositions of copolymers were
calculated from the comparison of these two peaks. The TGA
thermograms, which were measured at a temperature of 5%
weight loss for Oxa-PPV, DMOP-PPV, and Oxa-PPV-co-
DMOP-PPV, revealed a thermal stability from 290 to 380 °C.
It is difficult to detect the Tg value for most of the dialkyloxy-
substituted PPV derivatives such as MEH-PPV and BEH-PPV.
However, a distinct Tg value for Oxa-PPV, DMOP-PPV, and
Oxa-PPV-co-DMOP-PPV was observed from 102 to 252 °C,
and no melting transition was observed, which indicate that the
polymers have amorphous structures. The high Tg values of
present EL polymers prevent the deformation of polymer
morphology and degradation of the polymer emitting layer by
applied electric fields of LEDs.
(28) Huang, W.; Meng, H.; Yu, W. L.; Ga, J.; Heeger, A. J. AdV. Mater. 1998,
10, 593-596.
(29) Chen, Z. K.; Meng, H.; Lai, Y. H.; Huang, W. Macromolecules 1999, 32,
4351-4358.
9
J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004 2477

A R T I C L E S
Kim et al.
Figure 3. Hypothesized energy diagram of ITO/PEDOT/polymer/Al
devices, Oxa-PPV, (a) Oxa-PPV-co-DMOP-PPV (84:16), (b) Oxa-PPV-
co-DMOP-PPV (64:36), (c) Oxa-PPV-co-DMOP-PPV (51:49), (d) Oxa-
PPV-co-DMOP-PPV (28:72), (e) Oxa-PPV-co-DMOP-PPV (7:93), and
DMOP-PPV.
Figure 1. UV-visible absorption spectra of Oxa-PPV, DMOP-PPV, and
DMOP-PPV-co-Oxa-PPV in the solid state.
PPV are observed at 520 and 539 nm. However, as the Oxa-
PPV content was increased in the copolymer systems, the
emission peak is red-shifted from 520 to 573 nm. The photo-
luminescence excitation (PLE) spectra of Oxa-PPV, DMOP-
PPV, and Oxa-PPV-co-DMOP-PPV were monitored at the λmax
of the emission spectrum of each polymer. The PLE spectra of
Oxa-PPV, DMOP-PPV, and Oxa-PPV-co-DMOP-PPV are
similar to the absorption spectrum. These phenomena indicated
that the efficient energy transfer occurs from the 1,3,4-
oxadiazole units to the PPV main chain.
Cyclic voltammogram (CV) is a useful method for measuring
electrochemical behaviors and evaluation of the relative HOMO,
LUMO energy levels and the band gap of a polymer. During
the anodic scan, the oxidation onset potentials of Oxa-PPV,
DMOP-PPV, and Oxa-PPV-co-DMOP-PPV are in the range
of 0.80-0.92 and exhibited irreversible p-doping processes.
From the onset potential for the oxidation, the HOMO energy
level of the polymers was estimated regarding the energy level
of the FOC reference.30 Figure 3 shows the hypothesized energy
diagrams of the ITO/PEDOT/polymer/Al device fabricated in
this work. HOMO energy levels of the present EL polymers
are similar, and the values are about 5.43-5.57 eV. The LUMO
energy level was calculated from the values of the band gap
and HOMO energy level. The LUMO energy level of DMOP-
PPV is 3.08 eV. However, as the 1,3,4-oxadiazole units in the
copolymers were increased, the LUMO energy level was not
increased. The LUMO energy level of 3.21 eV in Oxa-PPV is
higher than that of CN-PPV (3.02 eV) and alkyl- or alkoxy-
substituted PPV derivatives.31 This means that the electrons are
better injected into Oxa-PPV from the cathode metal than into
CN-PPV and Oxa-PPV has an expected higher luminance
efficiency than PPV derivatives when it is used as an emitting
layer in PLEDs. The electrooptical data of the Oxa-PPV,
DMOP-PPV, and Oxa-PPV-co-DMOP-PPV are summarized in
Table 2.
Figure 2. Photoluminescence spectra of Oxa-PPV, DMOP-PPV, and
DMOP-PPV-co-Oxa-PPV in the solid state.
UV-Visible Absorption, Photoluminescence Spectroscopy,
and Electrochemical Properties. Figure 1 shows the optical
absorption spectra of Oxa-PPV, DMOP-PPV, and Oxa-PPV-
co-DMOP-PPV with various compositions of DMOP-PPV in
the solid state. The absorption spectrum of Oxa-PPV exhibited
two peaks at 441 and 303 nm, which were contributed from
π-conjugated main chains and 1,3,4-oxadiazole units, respec-
tively. As the DMOP-PPV content is increased, the absorption
maximum peak of the 1,3,4-oxadiazole units is decreased and
the absorption λmax of the π-π* transition of conjugated main
chains is statistically increased. The band gap of Oxa-PPV,
DMOP-PPV, and Oxa-PPV-co-DMOP-PPV, taken from the
absorption edge spectrum, is quite different according to the
compositions of copolymers. Due to the electron-withdrawing
1,3,4-oxadiazole unit, the LUMO binding energy of the
copolymers is decreased, which gave the narrow band gap as
compared to the DMOP-PPV. The band gap of Oxa-PPV,
DMOP-PPV, and Oxa-PPV-co-DMOP-PPV is about 2.41-
2.34 eV. Figure 2 shows emission spectra of the Oxa-PPV,
DMOP-PPV, and Oxa-PPV-co-DMOP-PPV thin films. When
the polymer films were excited at the maximum absorption
wavelength of each polymer, two emission peaks of DMOP-
Electroluminescence Properties and Current Density-
Voltage-Luminescence (J-V-L) Characteristics. To de-
crease the operating voltage and smooth the surface roughness
(30) Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F.
Synth. Met. 1997, 87, 53-59.
(31) Cervini, R.; Li, X. C.; Spencer, G. W. C.; Holmes, A. B.; Moratti, S. C.;
Friend, R. H. Synth. Met. 1997, 84, 359-360.
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High-Efficiency Poly(p-phenylenevinylene)-Based Copolymers
A R T I C L E S
Table 2. Absorption, PL, and EL Data of Oxa-PPV, DMOP-PPV,
and Oxa-PPV-co-DMOP-PPV
absmax
(nm)
PLmax
(nm)
ELmax
(nm)
E
HOMO LUMO
g
polymer
Oxa-PPV
(eV)a (eV)b (eV)
441, 303 542,c 573
443 542,c 573
591 2.36 5.57 3.21
570 2.36 5.56 3.20
Oxa-PPV-co-DMOP-
PPV (84:16)
Oxa-PPV-co-DMOP-
PPV (64:36)
Oxa-PPV-co-DMOP-
451 535, 571 534, 560 2.34 5.44 3.10
457 533, 568 534, 560 2.35 5.48 3.13
PPV (51:49)
Oxa-PPV-co-DMOP-
PPV (28:72)
Oxa-PPV-co-DMOP-
PPV (7:93)
456
537 528,c 561 2.37 5.43 3.06
449 520,c 558 528,c 561 2.41 5.55 3.14
454 520, 539 513, 539 2.41 5.49 3.08
DMOP-PPV
a Determined from the edge of the absorption spectrum. b Determined
from the onset of the anodic scan of cyclic voltammetry. c Shoulder.
of the indium tin oxide (ITO) electrode, the hole injection-
transport layer, poly(3,4-ethylenedioxythiophene) doped with
poly(styrenesulfonate) (PEDOT/PSS), was spin-coated onto the
surface-treated ITO substrate and dried on a hot plate for 30
min at 110 °C. The emissive layer was successively built up
by spin casting on the top of the PEDOT layer. An aluminum
electrode was deposited by evaporation at a pressure of 10-6
Torr. The current density-voltage (a) and luminescence-
voltage (b) characteristics of ITO/PEDOT/polymer/Al devices
are shown in Figure 4. The turn-on voltages of ITO/PEDOT
(25 nm)/DMOP-PPV (80 nm)/Al and ITO/PEDOT (25 nm)/
Oxa-PPV (80 nm)/Al devices are about 2.5 and 5 V, respec-
tively. The copolymers showed a turn-on voltage similar to that
of Oxa-PPV. The luminescence intensities of Oxa-PPV, DMOP-
PPV, and Oxa-PPV-co-DMOP-PPV are exponentially increased
with an increase in voltage. The maximum luminescence (Lmax
)
of DMOP-PPV is 1840 cd/m2 at 10 V. However, the maximum
luminescence of copolymers dramatically increased with an
increase in the feed ratios of Oxa-PPV, and the maximum
luminescence of Oxa-PPV-co-DMOP-PPV (84:16), Oxa-PPV-
co-DMOP-PPV (64:36), and Oxa-PPV-co-DMOP-PPV (51:49)
is about 6500 cd/m2. Oxa-PPV with an Al electrode shows a
maximum luminescence of 19395 cd/m2 at a voltage of 14 V.
The device performance characteristics of double-layer devices
are tabulated in Table 3.
Figure 4. Current density-voltage-luminescence (J-V-L) characteristics
of ITO/PEDOT/polymer/Al devices.
DMOP-PPV (84:16) was 41 times higher than that of DMOP-
PPV and reached about 10 cd/A at 1760 cd/m2 and 17 mA/
cm2. The luminance efficiency of Oxa-PPV containing 1,3,4-
oxadiazole side groups was 21.1 cd/A at 5930 cd/m2 and 29
mA/cm2 based on double-layer PLEDs using an Al electrode
and was 88 times higher than that of DMOP-PPV. The highest
luminance efficiency of copolymers with a relatively low current
density is due to the high LUMO energy level as compared to
that of conventional PPV derivatives. The barrier heights of Oxa-
PPV and Oxa-PPV-co-DMOP-PPV (84:16) were found to be
1.09 and 1.1 eV at the interface of the Al/LUMO state for
electron injection. Thus, easily injected electrons were recom-
bined with the holes in the emitting layer to give high luminance
efficiency. The Oxa-PPV system also seems to form another
charge transport channel (mainly for electrons) via wave-
function overlapping of π-electrons along the 1,3,4-oxadiazole
pendants itself as well as original transport (mainly for holes)
along the PPV backbone. In fact, the suggestion of new
π-electron channels is strongly supported by the observation
of a broad spectral feature around 300 nm in the absorption
spectrum of Oxa-PPV, which reflects π-electron wave-function
overlapping along the 1,3,4-oxadiazole units. Therefore, we
consider that the excellent performance of Oxa-PPV is also
attributed to enhanced electron transport along new transport
channels as well as easier electron injection in this system.
Accordingly, introduction of 1,3,4-oxadiazole units into the PPV
Figure 5 shows the EL spectra of ITO/PEDOT/polymer/Al
devices. The EL spectra are similar to their PL spectra. This
result indicates that the EL and PL phenomena originated from
the same excited state. The maximum EL peak of Oxa-PPV is
red-shifted by 18 nm as compared to the PL spectrum. This
change in EL spectrum is due to the injected current density,
which results in an extended effective conjugation length. To
investigate the color purity, chromaticity coordinates using the
Commision International l’Eclairage (CIE) (1931) color match-
ing function were converted from the EL spectrum, and the
results are listed in Table 3. The emission colors of Oxa-PPV
and DMOP-PPV at the CIE coordinates of x ) 0.50, y ) 0.47
and x ) 0.36, y ) 0.55 are yellowish-orange and green. By
adjusting the feed ratios of Oxa-PPV in the copolymers, we
could tune the emission colors from green to yellowish-orange.
Figure 6 shows the luminance efficiency of Oxa-PPV,
DMOP-PPV, and Oxa-PPV-co-DMOP-PPV as a function of
current density. As the 1,3,4-oxadiazole content in the copoly-
mers was increased, the luminance efficiency of Oxa-PPV-co-
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J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004 2479

A R T I C L E S
Kim et al.
Table 3. Device Performance Characteristics of Oxa-PPV, DMOP-PPV, and Oxa-PPV-co-DMOP-PPV
2
polymer
turn-on (V)
LEmax,a cd/A
Lmaxb, cd/m (V)
CIE (x, y)c
Oxa-PPV
Oxa-PPV-co-DMOP-
PPV (84:16)
5
7
21.1 (29 mA, 5930 cd/m2)
10.0 (17 mA, 1760 cd/m2)
19395 (14 V)
6447 (18 V)
(0.50, 0.47)
(0.47, 0.50)
Oxa-PPV-co-DMOP-
PPV (64:36)
Oxa-PPV-co-DMOP-
PPV (51:49)
Oxa-PPV-co-DMOP-
PPV (28:72)
Oxa-PPV-co-DMOP-
PPV (7:93)
5.5
5.5
6
7.6 (26 mA, 1960 cd/m2)
0.62 (119 mA, 740 cd/m2)
2.2 (10.9 mA, 233 cd/m2)
1.54 (42.5 mA, 640 cd/m2)
0.24 (65.6 mA, 164 cd/m2)
6430 (12 V)
6370 (13 V)
377 (12 V)
980 (14 V)
1840 (10 V)
(0.44, 0.52)
(0.43, 0.53)
(0.44, 0.52)
(0.45, 0.51)
(0.36, 0.55)
6.5
2.5
DMOP-PPV
a Maximum luminescence. b Maximum luminance efficiency. c Calculated from the EL spectrum.
Figure 6. Luminance efficiency as a function of current characteristics of
ITO/PEDOT/polymer/Al devices.
Figure 5. Electroluminescence spectra of ITO/PEDOT/polymer/Al devices.
backbone via oxygen linkage and asymmetric molecular design
of Oxa-PPV and Oxa-PPV-co-DMOP-PPV are responsible for
the outstanding device performance.
were in the range of 2.5-7 V. As the Oxa-PPV content was
increased in copolymers, the device performance was signifi-
cantly increased as compared to that of DMOP-PPV. The
maximum brightness and luminance efficiency of the present
EL polymers were 19395 cd/m2 at 14 V and 21.1 cd/A at 5930
cd/m2. The improved device performance of Oxa-PPV over
those of DMOP-PPV and Oxa-PPV-co-DMOP-PPV is due to
the better electron injection and efficient energy transfer from
the modified 1,3,4-oxadiazole side group to the PPV main chain.
This is the first report concerning the high brightness and high
luminance efficiency at high operating luminescence even
though the Al electrode and these polymers are promising
materials for PLEDs.
Conclusions
This study focused on the design and synthesis of asymmetric
PPV-based homopolymers as well as copolymers containing
new chemically modified 1,3,4-oxadiazole derivatives through
Gilch polymerization of PLEDs. The resulting EL polymers with
high molecular weights exhibited good solubility in conventional
organic solvents, which make high-quality optical thin films,
and high glass transition temperatures. Electrooptical properties
and device performance can be easily controlled by properly
adjusting the feed ratios of the Oxa-PPV content in copolymers.
From the absorption and emission spectra, these polymers emit
green to yellowish-orange with a band gap of 2.34-2.41 eV.
Introduction of an electron-deficient 1,3,4-oxadiazole unit into
the PPV backbone as the side chain enhances the electron
affinity and increases the LUMO energy level, which facilitate
the improvement of device performance. Turn-on voltages of
the present EL polymers with ITO/PEDOT/polymer/Al devices
Acknowledgment. This work was supported by a Korea
Research Foundation Grant (KRF-2002-015-CS0028).
Supporting Information Available: Experimental spectra
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA036955+
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2480 J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004