Synthesis of poly(p-phenylene-vinylene) derivatives containing an oxadiazole pendant group and their applications to organic electronic devices

Article abstract of DOI:10.1166/jnn.2013.7291

Poly(p-phenylene vinylene) (PPV) derivatives with 2,5-diphenyl-1,3,4- oxadiazole-diyl (OXD) as the side chain, poly[2-{4-[5-(4-(heptyloxy)phenyl)-1,3, 4-oxadiazole-2-yl]phenyl-oxy}-1,4-phenylenevinylene] (OXH-PPV), poly[2-{4-[5-(4-(heptyloxy)phenyl)-1,3,4

Full text of DOI:10.1166/jnn.2013.7291

Copyright © 2013 American Scientific Publishers
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Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 3321–3330, 2013
Synthesis of Poly(p-phenylene-vinylene) Derivatives
Containing an Oxadiazole Pendant Group and Their
Applications to Organic Electronic Devices
Hyemi Lee1, Doojin Vak2, Kang-Jun Baeg2, Yoon-Chae Nah3ꢀ∗,
Dong-Yu Kim2ꢀ∗, and Yong-Young Noh1ꢀ∗
1Department of Chemical Engineering, Hanbat National University, Daejeon 305-719, Republic of Korea
2Heeger Center for Advanced Materials, Department of Nanobio Materials and Electronics, School of Materials
Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea
3School of Energy, Materials, and Chemical Engineering, Korea University of Technology and Education,
Cheonan 330-708, Republic of Korea
Poly(p-phenylene vinylene) (PPV) derivatives with 2,5-diphenyl-1,3,4-oxadiazole-diyl (OXD) as
the side chain, poly[2-{4-[5-(4-(heptyloxy)phenyl)-1,3,4-oxadiazole-2-yl]phenyl-oxy}-1,4-phenylene-
vinylene] (OXH-PPV), poly[2-{4-[5-(4-(heptyloxy)phenyl)-1,3,4-oxadia-zole-2-yl]phenyl-oxy}-1,4-phe-
nylenevinylene-co-2-methoxy-5-(2ꢀ-ethylhexyloxy)-p-phenylene vinylene] (OXH-PPV-co-MEH-PPV),
and poly[2-methoxy-5-(2ꢀ-ethylhexyl-oxy)-p-phenylene vinylene] (MEH-PPV), were synthesized via
a modified Gilch route. The electron-deficient oxadiazole moiety was introduced on the side chain
of the polymer backbone to increase the electron-affinity of the polymers. The electrolumines-
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cent (EL) properties of the resulting polymers as an active layer, were investigated by the fabrication
IP: 186.190.119.43 On: Tue, 05 Apr 2016 15:14:20
of single-layer LEDs and the devices using OXH-PPV-co-MEH-PPV showed better EL properties
Copyright: American Scientific Publishers
than those using pure MEH-PPV. Also, to investigate the switching properties of the resulting poly-
mers as an active layer, OFET devices were fabricated in a top-contact/bottom-gate configuration.
The resulting FETs exhibited typical p-channel characteristics, field-effect mobility of 6ꢀ5 × 10−4
–
7ꢀ0×10−5 cm2 V−1
s
−1, and on-off ratio of about 104–105.
Keywords: Poly(p-phenylene vinylene) (PPV) Derivatives, Oxadiazole, OLED, OTFT.
1. INTRODUCTION
∼ 1 cm2 V−1
s
−1.2 Instead of having a high performance
TFT function, however, most TFT active materials demon-
strate rather weak photoluminescence (PL) due to their
strong molecular packing, i.e., concentration quenching,
which means that they are difficult to use in light-emitting
applications. In unipolar light-emitting transistors, light
emission is restricted to a region very close to the contact
that injects the charge carriers having lower mobility.3–10
In contrast, an ambipolar light emitting transistor would
allow the electron–hole balance as well as the location of
the recombination zone between source and drain elec-
trode to be tuned by the gate voltage, hence improving
the quantum efficiency.11–18 Therefore, we need to sat-
isfy the demand and find organic materials that provide
both light-emitting and ambipolar transistor characteris-
tics. An efficient OLEFET device, as mentioned above,
requires balanced injection and transport of both electrons
and holes.19 However, most of the conjugated polymers
developed so far transport electrons much less efficiently
The interest for fusing organic light-emitting diodes
(OLEDs) and organic field-effect transistors (OFETs) to
produce organic light-emitting FETs (OLEFETs) has been
increasing and those demands aim to achieve simpli-
fied organic active matrix displays.1 To achieve efficient
OLEFETs, it is necessary to prepare adequate organic
materials with both efficient electroluminescence and tran-
sistor characteristics. However, it is still difficult to find
ideal candidates from well-established OLED materials,
since most OLED active materials have no high perfor-
mance FET characteristics probably due to their amor-
phous morphologies. Moreover, the recent progress in
OFETs has revealed that highly packed molecular thin
films with tight intermolecular ꢀ stacking have demon-
strated pronouncedly high carrier mobilities exceeding
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 5
1533-4880/2013/13/3321/010
doi:10.1166/jnn.2013.7291
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Synthesis of PPV Derivatives Containing an Oxadiazole Pendant Group
Lee et al.
than holes,20 which significantly decrease the efficiency of
the device. For improving the luminance efficiency, many
research groups have introduced electron transport moi-
eties on the side chain of the polymer backbones or poly-
mer main chains.21–26 These polymers showed enhanced
electron injection and improved balance in carrier mobility.
One of the most widely used electron transport moieties
electrode and Ag wire as the reference electrode immersed
in a 0.1 M solution of tetrabutylammonium-perchlorate
(Bu4NCl4ꢁ in acetonitrile solution as the electrolyte. The
molecular weight and polydispersity of the polymers in
THF solution were determined by gel permeation chro-
matography (GPC, Futecs, NS2001).
is aromatic 1,3,4-oxadiazole-based compounds with high
2.3. Synthesis
electron affinities.27
2.3.1. Synthesis of 4-Fluoro-Benzohydrazide, 1
For those purposes, in this paper, PPV derivatives both
homopolymers and their corresponding copolymers were
synthesized. The design is based on the consideration that
the PPV backbone is a good hole-transporting electro-
luminescent material, the electron-deficient moiety, 2,5-
diphenyl-1,3,4-oxadiazole-diyl (OXD), as the side chain,
provides a highly improved electron-transporting property
due to interchain interaction and efficient mixing between
OXD moieties with chromophores on the main chains, and
the linear alkoxy side group next to OXD unit provides
solubility and helps crystalline packing. Also, electroop-
tical properties and device performance could be easily
controlled by adjusting the feed ratio of the OXD content
through copolymerization.
In a 250 mL three-neck flask, hydrazine monohydrate
(35.04 g, 700 mmol) was dissolved under nitrogen
in 40 mL of ethanol. Ethyl-4-fluorobenzoate (16.81 g,
100 mmol) was added dropwise. The reaction mixture was
refluxed for 5 hrs. Upon completion, the reaction mixture
was recrystallized from methanol. A white crystal solid,
4-fluoro-benzohydrazide, 1 was obtained (12.80 g, 83%).
1H NMR (300 MHz, DMSO-d6ꢁ: ꢂ 4.46 (s, 2 H, NH2ꢁ,
7.25 (t, 2 H, J = 8ꢃ8 Hz, aromatic protons), 7.85 (dd, 2 H,
J = 5ꢃ5, 8.8 Hz, aromatic protons), 9.77 (s, 1 H, NH).
2.3.2. Synthesis of ꢄ4-heptylꢁOxybenzoyl-
4-Fluoro-Benzohydrazide, 2
In a 250 mL three-neck flask equipped with a nitrogen inlet,
4-fluoro-benzohydrazide, 1 (5.86 g, 38 mmol) and triethy-
lamine (6.07 g, 60 mmol) were dissolved in 200 mL of
2. EXPERIMENTAL DETAILS
2.1. Materials
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dichloromethane. 4-heptyloxy-benzoyl chloride (11.70 g,
IP: 186.190.119.43 On: Tue, 05 Apr 2016 15:14:20
Chemicals and solvents were reagents grades and pur-
chased from commercial sources (mostly from Sigma-
Aldrich Co.). THF were distilled to keep them anhydrous
before use. The other chemicals were used without further
purification. All reactions were carried out under a nitro-
gen atmosphere. Column chromatography was performed
using silica gel (Merck, 230–400 mesh).
Copyright: American Scientific Publishers
46 mmol) was added at a rate of 20 mL/h via a syringe
pump. The reaction mixture was then heated to reflux
for 1.5 hrs. After confirmation of the disappearance of
4-fluoro-benzohydrazide, 1, by TLC, the mixture was
cooled to room temperature, and then the reaction was
diluted with dichloromethane followed by extraction with
water and HCl (aq). The dichloromethane solution was
dried over MgSO4, and the dichloromethane was removed
under reduced pressure. The crude product was purified
by flash chromatography (hexane/ethyl acetate (8:2), sil-
ica gel) and recrystallized from dichloromethane/methanol
yielding (4-heptyl)-oxybenzoyl-4-fluoro-benzohydrazide, 2
(12.21 g, 86%). 1H NMR (300 MHz, CDCl3ꢁ: ꢂ 0.88
(t, J = 6ꢃ0 Hz, 3 H, CH3ꢁ, 1.30–1.80 (m, 10 H, 5CH2ꢅꢁ,
3.95 (t, J = 6ꢃ0 Hz, 2 H, OCH2ꢁ, 6.82–6,85, 7.0–7.04,
7.77–7.87 (m, 8 H, aromatic protons), 9.71, 10.12 (s, s,
2 H, NH).
2.2. Characterization and Measurements
1H NMR spectra data were obtained from a JEOL JNM-
LA300WB 300 MHz spectrometer with TMS as the
internal reference. Differential scanning calorimetry (DSC;
TA-2010) was conduꢁcted under a nitrogen atmosphere at
a heating rate of 10 C/min. Samples were scanned from
40 ꢁC to the decomposition temperature. Td was mea-
sured at a temperature of 5% weight loss for the polymer.
Thermo-gravimetric analysis (TGA; TA-2050) was also
performed under a nitrogen atmosphere. The UV/Visible
Spectra of the polymers were measured using a UV/Visible
Spectrophotometer (HP 8453). Photoluminescence (PL)
spectra of the polymers were obtained using an ACTON
spectrometer connected to a photo-multiplier tube (Acton
Research Co. SpectraPro-300i) with a xenon lamp as the
excitation source.
2.3.3. Synthesis of 2-ꢆꢄheptyloxyꢁ-phenylꢇ-5-
ꢄ4-fluoro-phenylꢁ-1,3,4-Oxadiazole, 3
In a 250 mL three-neck flask, (4-heptyl)oxybenzoyl-4-
fluoro-benzohydrazide, 2 (2.64 g, 7.1 mmol), and thionyl
chloride (4.22 g, 35.5 mmol) were dissolved in 100 mL
of benzene. The mixture was stirred under nitrogen
and brought to reflux for 4 hrs. After confirmation
of the disappearance of (4-heptyl)oxybenzoyl-4-fluoro-
benzohydrazide, 2, by TLC, the mixture was cooled to
Electrochemical measurements of the polymers were
performed using cyclic voltametry (CV) at room tempera-
ture in a three-electrodes cell with Pt wire as the counter
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Lee et al.
Synthesis of PPV Derivatives Containing an Oxadiazole Pendant Group
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 ether was removed by evapo-
ration under reduced pressure. It was recrystallized from
dichloromethane/methanol to give 2.07 g (82%) of pure
product. 1H NMR (300 MHZ, CDCl3ꢁ: ꢂ 0.89 (t, J =
6ꢃ0 Hz, 3 H, CH3ꢁ, 1.32, 1.81 (m, 10 H, 5CH2ꢅꢁ, 4.03
(t, J = 6ꢃ0 Hz, 2 H, OCH2ꢁ, 7.03, 7.21, 8.02, 8.10 (m, 8 H,
aromatic protons).
4.54 (s, s, 4 H, 2CH2Br on aromatic ring), 6.99, 7.16, 7.44,
8.03–8.14 (m, 11 H, aromatic protons).
2.3.6. Synthesis of Polyꢈ2-ꢆ4-ꢈ5-(4-(heptyloxy)phenyl)-
1,3,4-oxadiazole-2-ylꢉphenyl-oxyꢇ-1,4-phenylene
vinyleneꢉ ꢄOXH-PPVꢁ
To a 100 mL Schlenk flask dried with stream of hot air and
flushed with nitrogen, 2-{4-[2,5-bis(bromomethyl) phen-
oxy]phenyl-5-{4-[heptyloxy]phenyl}-1,3,4-oxadiazole,
5
(0.306 g, 0.5 mmol) and 4-methoxy-phenol (1.24 mg,
2.0 mol%) were added and then it was dried and flushed
with nitrogen again, and dissolved in 15 mL of dry THF.
A solution of t-BuOK in THF (2.5 mL, 1.0 M) was then
added at a rate of 20 mL/h via a syringe pump. After
the complete addition of the base, the reaction mixture
was stirred at room temperature for 1 hr. The color of
the reaction mixture changed from colorless to yellowish,
then to orange, and also the viscosity increased. The
reaction mixture was poured into rapidly stirred 200 mL
of methanol to precipitate the polymer. The collected
polymer was Soxhlet-extracted with MeOH, Acetone,
Hexane, MC and chloroform. The resulting polymer was
redissolved in chloroform and reprecipitated in MeOH.
After filtration and drying under vacuum, a bright yellow-
2.3.4. Synthesis of 2-ꢆꢄheptyloxy)phenylꢇ-5-(4-ꢄ2,5-
dimethylphenoxyꢁphenylꢁ-1,3,4-Oxadia-Zole, 4
In a 250 mL three-neck flask equipped with a nitrogen inlet,
2-{(heptyloxy)-phenyl}-5-(4-fluoro-phenyl)-1,3,4-oxadia
-
zole, 3 (2.02 g, 5.7 mmol) and 2,5-dimethylphenol (2.09 g,
17.1 mmol) were dissolved in 80 mL of DMF. After addi-
tion of potassium tert-butoxide (1.92 g, 17.1 mmol), the
reaction mixture was refluxed for 4 h. After confirmation of
the disappearance of 2-{(heptyloxy)-phenyl}-5-(4-fluoro-
phenyl)-1,3,4-oxadiazole, 3, by TLC, the mixture was
cooled to room temperature, poured into excess water,
and extracted with ether. The organic phase was washed
neutrally, dried (MgSO4ꢁ and evaporated to dryness. The
product was recrystallized from methanol and washed
several times with hexane to give white crystals of
2-{(heptyloxy)phenyl}-5-(4-(2,5-dimethylphenoxy)phenyl)-
1
orange polymer was obtained (0.144 g, 64%). H NMR
(300 MHz, CDCl ꢁ: ꢂ 0.90 (br, 3 H, CH3ꢁ, 1.26, 1.79
(br, 10 H, 5CH2ꢁ, 4.01 (br, 2 H, –OCH2ꢁ, 6.99, 8.02
(br, 11 H, aromatic protons and vinylic protons).
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1
1,3,4oxadiazole, 4 (1.88 g, 72.5%). H NMR (300 MHz,
Copyright: American Scientific Publishers
CDCl3ꢁ: ꢂ 0.88 (t, J = 6ꢃ0 Hz, 3 H, CH3ꢁ, 1.31, 1.77
(m, 10 H, 5CH2ꢅꢁ, 2.15, 2.31 (s, s, 6 H, 2CH3 on aromatic
ring), 4.01 (t, J = 6ꢃ0 Hz, 2 H, OCH2ꢁ, 6.81, 7.00, 7.13,
8.03 (m, 11 H, aromatic protons).
Poly[2-{4-[5-(4-(heptyloxy)phenyl)-1,3, 4-oxadia-zole-2-yl]
phenyl-oxy}-1,4-phenylenevinylene-co-2-methoxy-5-(2ꢀ-
ethylhexyloxy)-p-phenylene vinylene] (OXH-PPV-co-
MEH-PPV) were prepared by varying the amount
of monomers
5
and ꢊ,ꢊꢀ-Dibromo-2-methoxy-5-(2-
ethylhexyl-oxy)xylene (for MEH-PPV) in the feed and
following the similar procedure described for OXH-PPV.
2.3.5. Synthesis of 2-ꢆ4-ꢈ2,5-Bisꢄbromomethylꢁ
phenoxyꢉphenyl-5-ꢆ4-ꢈheptyloxyꢉphenylꢇ-
1,3,4-Oxadiazole, 5
2.4. Fabrication of EL Devices
A mixture of 1.87 g of 2-{(heptyloxy)phenyl}-5-(4-(2,5-
dimethylphenoxy)phenyl)-1,3,4-oxadiazole, 4 (4.1mmol)
and 2.04 g of N-bromo-succinimide (NBS) (11.48 mmol)
was dissolved in 70 mL of benzene. A catalytic amount of
benzoyl peroxide was added as an initiator. The reaction
mixture was refluxed for 4 hrs under a nitrogen atmo-
sphere. The completion of the reaction was indicated by
the appearance of succinimide on the surface of the reac-
tion solution. A solution was obtained after filtration of
the succinimide. The solution was then poured into water
and extracted with dichloromethane. The organic layer
was collected and dried using MgSO4, and the solvent
was removed by evaporation under reduced pressure. The
crude product was purified by column chromatography
(hexane/ethyl acetate (9.5:0.5), silica gel) affording 0.97 g
(39%) of a slightly yellowish powder. 1H NMR (300 MHz,
CDCl3ꢁ: ꢂ 0.89 (t, J = 6ꢃ0 Hz, 3 H, CH3ꢁ, 1.32, 1.81
(m, 10 H, 5CH2ꢁ, 4.03 (t, J = 6ꢃ0 Hz, 2 H, OCH2ꢁ, 4.39,
The OLED devices had the configuration ITO/PEDOT:PSS
(30 nm)/polymer (70 nm)/Al (150 nm). The thickness
of all organic layers was determined by surface profiler
(ET3000i, Kosaka Laboratory Ltd.). The OLED devices
were fabricated on the glass substrate, pre-coated with
indium-tin-oxide (ITO), which was cleaned in an ultra-
sonic bath with acetone, detergent, deionized water, and
iso-propanol and surface treatment was performed by UV-
ozone before use. The hole injecting poly(3,4-ethylene
dioxythiphene) (PEDOT) were spin-coated on the ITO
ꢁ
with a thickness of 30 nm. PEDOT was baked at 120 C
for 5 min under N2. Then a layer of emitting polymers
were spin-coated from chlorobenzene solution (1 wt%) and
ꢁ
baked at 150 C for 30 min under N2. Finally, a Al metal
electrode was deposited by thermal evaporation at a pres-
sure of 10−5 Torr through a shadow mask. The pixel size
was 4 mm2.
J. Nanosci. Nanotechnol. 13, 3321–3330, 2013
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Synthesis of PPV Derivatives Containing an Oxadiazole Pendant Group
Lee et al.
2.5. Fabrication of EL Devices
polymerization as an initiator to control molecular weight
and to circumvent the problem of self-condensation
observed for 4-tert-butylbenzyl chloride. Hsieh et al. have
shown that 4-tert-butylbenzyl chloride can be used as
an anionic initiator to reduce Mn, but exact control of
molecular weight was limited due to self-condensation of
the benzyl chloride.28
Most of the PPV homopolymers with electron-
withdrawing 1,3,4-oxadiazole units in side chain or main
chains are limited in their application, because of their
poor solubility and gelation during polymerization.29ꢅ30
Therefore, to improve the processibility for the device fab-
rication, copolymerization was carried out with various
initial monomer concentrations according to the similar
method of homopolymerization. The resulting copoly-
mers, OXH-PPV-co-MEH-PPV with various monomers
feed ratios showed good solubility behavior in com-
mon organic solvents, such as chloroform, chlorobenzene,
toluene, THF, xylene, etc.
The OTFT devices were fabricated in a top-contact/
bottom-gate configuration. The commercially available
300 nm thick SiO2/Si (n+) wafer was used as a sub-
strate after cleaning with acetone, iso-propanol, and deion-
ized water. The surface of SiO2/Si substrate was treated
with hexamethyldisilane to form a hydrophobic mono-
layer surface. Then, polymers 6c-6e were spin coated from
0.5 wt% solution in chlorobenzene with a thickness of
about 6ꢁ0–80 nm. The spin-coated films were annealed
at 110 C for 30 min in N2-purged glove box. Finally,
gold (50 nm) electrode was thermally vapor deposited.
A shadow mask was used to define the source and drain
electrodes. The channel width and length were 1.5 mm and
50 ꢋm, respectively. All the measurements were carried
out in an N2-purged glove box.
3. RESULTS AND DISCUSSION
The TGA thermograms, which were measured at a tem-
perature of 5% weight loss for OXH-PPV, MEH-PPV,
and OXH-PPV-co-MEH-PPV, showed a thermal stability
3.1. Synthesis and Characterization
Figure 1 shows the synthetic route for the 1,3,4-oxadiazole
containing monomer, corresponding homopolymer, and
their copolymers with various compositions of MEH-PPV.
The synthetic routes were a little bit complicated, but all
of the intermediate compounds were obtained with high
yields.
ꢁ
from 294 to 371 C. The glass transitions for these poly-
mers was not detected up to degradation temperature and
melting temperature by differential scanning calorimetry,
which means that the polymers have amorphous structures.
Table I summarizes the polymerization results and
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In the first step, 4-fluoro-benzohydrazide, 1, was syn-
IP: 186.190.119.43 On: Tue, 05 Apr 2016 15:14:20
molecular weights of the homopolymers and copolymers.
thesized by the reaction of ethyl-4-fluorobenzoate with
hydrazine monohydrate to give the hydrazide. Condensa-
tion of hydrazide, 1, with 4-heptyloxy-benzoyl chloride
Copyright: American Scientific Publishers
The number average molecular weight (Mnꢁ and poly-
dispersity values of the present polymers were in the range
of 3.5∼16.9×104 and 1.32∼3.74, respectively.
yielded
(4-heptyl)oxybenzoyl-4-fluoro-benzohydrazide,
2. To enhance crystalline property of the polymers,
linear alkoxy side chain, 4-heptyloxy group, was intro-
duced at the hydrazide group. Compound 3 was synthe-
sized by dehydrative cyclization of compound 2 using
SOCl2in benzene. Compound 3 was reacted with 2,5-
dimethyl- phenol in the presence of potassium tert-butoxide
in DMF solvent to produce 2-{(heptyloxy)phenyl}-5-
(4-(2,5-dimethylphenoxy)phenyl)-1,3,4-oxadiazole, 4.
Finally, compound 4 was brominated bezylically using
N-bromo-succinimide (NBS) and the free-radical gen-
erator, benzoyl peroxide and was purified by column
chromatography. All of the synthetic steps produced
products in 72.5–86% yield, with the exception of the
last step that yielded the monomer, 5, only in the yield
of 39%. 2-{4-[2,5-bis(bromomethyl)phenoxy]phenyl-5-
{4-[heptyloxy]phenyl}-1,3,4-oxadiazole, 526 and ꢊ,ꢊꢀ-
3.2. Optical and Electrochemical Properties
Figure 2 shows the UV-vis spectra of OXH-PPV, OXH-
PPV-co-MEH-PPV, and MEH-PPV (6a–6e) in the solution
and film state. Polymers, 6a–6e, had a common feature in
their spectra, absorption centered around 304–334 nm and
461–498 nm in the solution state and 307–338 nm and
441–506 nm in the film state, respectively. The absorp-
tion in the shorter wavelength region is attributed to the
oxadiazole pendant,31 and those in the longer wavelength
region are attributed to the ꢀ–ꢀ∗ transitions of the main
chains. In Figure 2, as the MEH-PPV content decreased
(that is, as the oxadiazole unit increased), the absorption
maximum (ꢌmaxꢁ peak of the 1,3,4-oxadiazole units which
represented in the shorter wavelength gradually increased.
From this result, it could be assumed that composition
ratios (m:n) of copolymers have similar values as com-
pared with feed ratios of monomers by comparing the
maximum intensity of the absorption band of the 1,3,4-
oxadiazole. And, the absorption position for the backbone
ꢀ–ꢀ∗ transition was slightly blue-shifted as increasing
the content of repeat unit of polymer 6a, OXH-PPV. This
could be explained by a steric effect exerted by the bulky
dibromo-2-methoxy-5-(2-ethylhexyl-oxy)xylene27
were
synthesized according to the procedures from the litera-
ture. The homopolymerization of 5 and copolymerization
with ꢊ,ꢊꢀ-dibromo-2-methoxy-5-(2-ethylhexyl-oxy)xylene
were performed with an excess of potassium tert-
butoxide in THF at room temperature over a period of
1 hr under a nitrogen atmosphere via a modified Gilch
route. 4-methoxy-phenol (2 mol%) was also used in the
3324
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Lee et al.
Synthesis of PPV Derivatives Containing an Oxadiazole Pendant Group
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Copyright: American Scientific Publishers
Fig. 1. Synthetic scheme for OXH-PPV and its co-polymers with MEH-PPV.
pendant groups present in every repeat unit. That is, it is
ascribed to a partial destruction of the ꢀ-localization along
the backbone by the bulky substituents that disrupt the
coplanarity of the ꢀ-conjugated backbone.17
The band gap of OXH-PPV, MEH-PPV, and OXH-PPV-
co-MEH-PPV, taken from the absorption edge spectrum,
was slightly different according to the compositions of
copolymers about 2.03–2.31.
Table I. Molecular weight and thermal properties of OXH-PPV, MEH-PPV, and OXH-PPV-co-MEH-PPV.
a
a
b
b
c
Polymer
Mn (g/mol)
Mw (g/mol)
PDIa
Tg (ꢁC)
Tm (ꢁC)
Td (ꢁC)
∗
∗
∗
∗
∗
∗
∗
∗
OXH-PPV
–
–
–
–
–
–
–
–
–
–
–
294
320
368
371
309
OXH-PPV-co-MEH-PPV (75:25)
OXH-PPV-co-MEH-PPV (50:50)
OXH-PPV-co-MEH-PPV (25:75)
MEH-PPV
3ꢃ8×104
3ꢃ5×104
12ꢃ6×104
16ꢃ9×104
5ꢃ0×104
1ꢃ3×105
4ꢃ1×105
4ꢃ6×105
1.32
3.74
3.28
2.73
∗
∗
–
–
Notes: aMolecular weight and polydispersity index (PDI) were estimated by GPC against polystylene standards; bGlass transition temperature and melting temperature were
measured by DSC under nitrogen; cTemperature of 5% weight loss were measured by TGA; ∗Tg and Tm were not detected.
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Synthesis of PPV Derivatives Containing an Oxadiazole Pendant Group
Lee et al.
(a)
(a)
OXH-PPV
OXH-PPV
OXH-co-MEH-PPV (75:25)
OXH-co-MEH-PPV (50:50)
OXH-co-MEH-PPV (25:75)
MEH-PPV
OXH-co-MEH-PPV (75:25)
OXH-co-MEH-PPV (50:50)
OXH-co-MEH-PPV (25:75)
MEH-PPV
500
600
700
800
Wavelength (nm)
300
400
500
600
700
800
Wavelength (nm)
(b)
OXH-PPV
(b)
OXH-PPV
OXH-co-MEH-PPV (75:25)
OXH-co-MEH-PPV (50:50)
OXH-co-MEH-PPV (25:75)
MEH-PPV
OXH-co-MEH-PPV (75:25)
OXH-co-MEH-PPV (50:50)
OXH-co-MEH-PPV (25:75)
MEH-PPV
500
800
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Wavelength (nm)
300
400
500
600
700
800
Wavelength (nm)
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Copyright: American Scientific Publishers
Fig. 3. Photoluminescence spectra of OXH-PPV, MEH-PPV, and OXH-
PPV-co-MEH-PPV in (a) the solution and (b) the film.
Fig. 2. UV-visible absorption spectra of OXH-PPV, MEH-PPV, and
OXH-PPV-co-MEH-PPV in (a) the solution and (b) the film.
3.3. Device Characterizations
Figure 3 shows the photoluminescence spectra of OXH-
PPV, OXH-PPV-co-MEH-PPV, and MEH-PPV. The poly-
mer films and solutions were excited by the light at the
absorption ꢌmax of each polymer. OXH-PPV-co-MEH-PPV
and MEH-PPV shows similar emission maximum peaks
at about 615, 606, 608, and 610 nm. However, the emis-
sion peak of OXH-PPV was slightly blue shifted about
31 nm in the solution state and 37 nm in the film state,
respectively. It could be seen that such blue-shift was not
contributed from OXD groups but was from the reduced
content of electron-donating oxygen atom attached to the
phenylene ring in the main chain. The characteristic values
are listed in Table II.
3.3.1. OLED Devices
To estimate the emission properties of the resulting
polymers as an active layer, devices were fabricated
with the configuration ITO/PEDOT:PSS (30 nm)/Polymer
(70 nm)/Al (150 nm). Figure 4 shows the current
density–voltage–luminance (J–V –L) characteristics of the
ITO/PEDOT:PSS/Polymer (6a–6e)/Al devices. The turn-on
voltage for the light emission varied from approximately
6.9 V for MEH-PPV to 9.8‘V for OXH-PPV-co-MEH-PPV
(75:25). The current densities of the polymers increased
exponentially with an increasing forward bias.
The current–luminescence efficiency and current-
quantum efficiency curves of ITO/PEDOT:PSS/Polymer
(6b–6e)/Al devices are shown in Figure 5. The lumines-
cence intensities of OXH-PPV, MEH-PPV, and OXH-PPV-
co-MEH-PPV are exponentially increased with an increase
in voltage. The maximum luminescence of MEH-PPV is
41 cd/m2 at 11.8 V. However, the maximum luminescence
of copolymers increased with an increase in the feed ratios
of OXH-PPV, and the maximum luminescence of OXH-
PPV-co-MEH-PPV (50:50) and OXH-PPV-co-MEH-PPV
(25:75) is 175 cd/m2 and 87 cd/m2, respectively.
The oxidation potential of the polymers was measured
and calibrated by comparison of the half-wave potential
of ferrocene/ferrocenium redox couple. The highest occu-
pied molecular orbital (HOMO) of the resulting polymers
was 5.36, 4.96, 4.94, 4.91, and 4.88 eV, respectively. The
lowest unoccupied molecular orbital (LUMO) levels were
obtained from the values of the band gap calculated from
UV-Vis spectra and HOMO level. The band gap of OXH-
PPV, OXH-PPV-co-MEH-PPV, and MEH-PPV were in the
range of 2.03–2.31 eV. The electrochemical characteristics
of the present polymers are also summarized in Table II.
3326
J. Nanosci. Nanotechnol. 13, 3321–3330, 2013

Lee et al.
Table II. Optical properties and energy levels of OXH-PPV, MEH-PPV, and OXH-PPV-co-MEH-PPV.
UV absorbance PL
Synthesis of PPV Derivatives Containing an Oxadiazole Pendant Group
a
Polymer
ꢌma (sol)
ꢌmax (film)
ꢌmx (sol)
ꢌma (film)
Eg (eV)
HOMOb (eV)
LUMO (eV)
OXH-PPV
304, 461
304, 467
304, 483
305, 496
334, 498
307, 441
309, 461
318, 474
320, 499
338, 506
560
600
585
588
591
573
615
606
608
610
2.31
2.12
2.03
2.10
2.09
5.36
4.96
4.94
4.91
4.88
3.05
2.84
2.91
2.81
2.79
OXH-co-MEH-PPV (75:25)
OXH-co-MEH-PPV (50:50)
OXH-co-MEH-PPV (25:75)
MEH-PPV
Notes: aDetermined from the edge of the red end (onset) of absorption spectrum; bDetermined from the onset of the oxidation curve of cyclic voltammetry measurement.
0.08
0.06
0.04
0.02
0.00
(a)
The device performance characteristics of the resulting
polymers in ITO/PEDOT:PSS/polymer/Al devices are tab-
ulated in Table III.
OXH-co-MEH-PPV (75:25)
OXH-co-MEH-PPV (50:50)
OXH-co-MEH-PPV (25:75)
MEH-PPV
Figure 5 shows the luminance efficiency OXH-PPV,
MEH-PPV,and OXH-PPV-co-MEH-PPV as a function
of current density. 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
OXH-PPV-co-MEHPPV (75:25) OXH-PPV-co-MEH-PPV
(50:50), and OXH-PPV-co-MEH-PPV (25:75) were found
to be 1.25, 1.39, and 1.49 at the interface of the Al/LUMO
state for electron injection. Thus, easily injected electrons
were recombined with the holes in the emitting layer to
give high luminance efficiency. These results showed that
electron injection was enhanced and balance in carrier
0
50
100
150
200
Current density (mA/cm2)
0.04
(b)
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OXH-co-MEH-PPV (75:25)
0.03
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OXH-co-MEH-PPV (50:50)
OXH-co-MEH-PPV (25:75)
MEH-PPV
Copyright: American Scientific Publishers
(a)
OXH-PPV
OXH-co-MEH-PPV (75:25)
OXH-co-MEH-PPV (50:50)
OXH-co-MEH-PPV (25:75)
MEH-PPV
1500
1200
900
0.02
0.01
0.00
600
0
50
100
150
200
Current density (mA/cm2)
300
Fig. 5. (a) Luminance efficiency and (b) quantum efficiency as a func-
tion of current density of OXH-co-MEH-PPV (75:25, 50:50, 25:75) and
MEH-PPV.
0
2
4
6
8
10 12 14 16 18 20 22
Voltage (V)
(b)
OXH-PPV
OXH-co-MEH-PPV (75:25)
OXH-co-MEH-PPV (50:50)
OXH-co-MEH-PPV (25:75)
MEH-PPV
Table III. EL device performance characteristics of the resulting poly-
mers in ITO/ PEDOT: PSS/polymer/Al devices.
100
10
1
a
b
Turn-on
(V)
Quantum
efficiency (%)
LEmax
,
Lmax ,
Polymer
(cd/A)
cd/m2 (@V)
OXH-PPV
5.4
9.8
–
–
7 (8.4)
43 (18.2)
OXH-PPV-co-
MEH (75:25)
OXH-PPV-co-
MEH (50:50)
OXH-PPV-co-
MEH (25:75)
MEH-PPV
0.025
0.026
7.9
6.9
6.9
0.043
0.020
0.010
0.082
0.028
0.013
175 (16.2)
87 (12.2)
41 (11.8)
0
2
4
6
8
10
12
14
16
18
20
Voltage (V)
Fig. 4. (a) J–V and (b) V –L characteristics of OXH-co-MEH-PPV
(75:25, 50:50, 25:75) and MEH-PPV.
Notes: aMaximum luminescence efficiency; bMaximum luminescence.
J. Nanosci. Nanotechnol. 13, 3321–3330, 2013
3327

Synthesis of PPV Derivatives Containing an Oxadiazole Pendant Group
Lee et al.
–7
mobility was improved by introducing electron transport
moieties (OXD) on the side chain of the polymer back-
bones.
(a)
10
3.0x10–4
2.5x10–4
2.0x10–4
1.5x10–4
1.0x10–4
10–8
10–9
Vd = –140 V
3.3.2. OFET Devices
To investigate the switching properties of the resulting
polymers as an active layer, devices were fabricated in
a top-contact/bottom-gate configuration according to the
layer sequence. The fundamental output and transfer char-
acteristics of the devices are shown in Figures 6 and 7,
respectively. In Figure 6, the output plots (the drain cur-
rent Id as a function of the drain voltage Vdꢁ showed typ-
ical p-channel organic FETs, being initially linear with
10–10
10–11
–40 V
5.0x10–5
0.0
–140 –120 –100 –80 –60 –40 –20
V
[V]
g
10–6
10–7
10–8
10–9
(b)
8.0x10–4
6.0x10–4
4.0x10–4
Vd = –140 V
(a)
–1.2x10–8
Vg = –100 V
–40 V
2.0x10–4
0.0
10–10
10–11
–1.0x10–8
–8.0x10–9
–140 –120 –100 –80
Vg[V]
–60
–40
–20
–6.0x10–9
–80 V
–4.0x10–9
10–6
10–7
10–8
(c)
1.2x10–3
1.0x10–3
8.0x10–3
6.0x10–3
4.0x10–3
–2.0x10–9
0.0
–60 V
V = –120 V
d
0
–20
–40
–60
–80
–100
V [V]
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–40 V
Copyright: American Scientific Publishers
–3.5x10–8
–3.0x10–8
–2.5x10–8
–2.0x10–8
–1.5x10–8
–1.0x10–8
(b)
10–10
2.0x10–3
0.0
Vg = –100 V
10–11
–140 –120 –100 –80
Vg[V]
–60
–40
–20
Fig. 7. The transfer characteristics that were obtained; (a) OXH-co-
MEH-PPV (50:50), (b) OXH-co-MEH-PPV (25:75), and (c) MEH-PPV.
– 80 V
– 60 V
–5.0x10–9
0.0
Vd and then saturating at higher Vd. The transfer charac-
teristics of the devices were depicted as the Id values at
Vd = −40 V to −140 V plotted against the various gate
voltages Vg (Fig. 7). The basic parameters in OFETs such
as field-effect mobility (ꢋFETꢁ, threshold voltage (Vthꢁ,
and on-off ratio (Ion/Ioff ꢁ are obtained by gradual chan-
nel approximation listed in Table IV. The homo-polymer
OXH-PPV showed no FET characteristics and MEH-PPV
had relatively high ꢋFET of 6ꢃ5 × 10−4 cm2/Vs. Copoly-
mers between OXH-PPV and MEH-PPV also exhibited
enhanced ꢋFET from 2ꢃ3 × 10−6 to 3ꢃ7 × 10−4 cm2/Vs
by increasing the MEH-PPV content from 25% to 75%,
respectively. It was mainly attributed that higher HOMO
level by incorporation of electron-deficient OXD pendent
unit hindered from efficient hole carrier injection.
0
–20
–40
–60
–80
–100
Vd [V]
(c)
Vg = –100 V
–2.5x10–7
–2.0x10–7
–1.5x10–7
–1.0x10–7
–80 V
–60 V
–5.0x10–8
0.0
0
–20
–40
–60
–80
–100
Vd [V]
Moreover, molecular ordering and ꢀ–ꢀ overlap interac-
tion between the adjacent molecules were also presumably
deteriorated due to big-sized OXD pendent group directly
in polymer backbone.
Fig. 6. The output characteristics that were obtained; (a) OXH-
co-MEH-PPV (50:50), (b) OXH-PPV-co-MEH-PPV (25:75), and
(c) MEH-PPV.
3328
J. Nanosci. Nanotechnol. 13, 3321–3330, 2013

Lee et al.
Synthesis of PPV Derivatives Containing an Oxadiazole Pendant Group
Table IV. Fundamental OFET properties that were obtained in this
experiment; mobility, threshold voltage, and on/off ratio, where W =
1ꢃ5 mm and L = 50 ꢋm.
were successfully fabricated. The improved device perfor-
mance of OXH-PPV and OXH-PPV-co-MEH-PPV over
MEH-PPV may be due to better electron injection and
charge balance between holes and electrons and efficient
Mobility
(cm2/Vs)
Threshold
voltage VTh (V)
On/off
ratio (Ion/Ioff )
Polymer
intramolecular energy transfer from OXD units to the
PPV backbones as well. The maximum luminescence and
the luminescene efficiency of the polymers were up to
175 cd/m2 and 0.082 cd/A at 16.2 V, respectively. And,
to investigate the switching properties of the resulting
polymers as an active layer, devices were fabricated in a
top-contact/bottom-gate configuration. The resulting FETs
exhibited typical p-channel characteristics. The mobilities
in the saturation regime were in the range of about 6ꢃ5×
10−4∼7ꢃ0 × 10−5 cm2/Vs, and on-off ratios were about
104∼105.
OXH-PPV
OXH-PPV-co-MEH-
PPV (75:25)
OXH-PPV-co-MEH-
PPV (50:50)
OXH-PPV-co-MEH-
PPV (25:75)
–
–
−46
–
2ꢃ3×10−6
∼ 102
7ꢃ0×10−5
3ꢃ7×10−4
6ꢃ5×10−4
−50
−38
−41
∼ 104
∼ 105
∼ 104
MEH-PPV
Although the OXH-PPV, OXH-PPV-co-MEH-PPV, and
MEH-PPV materials showed no ambipolar charge trans-
port behavior, the electron-rich (PPV moiety) and electron-
deficient (OXD) moieties in the polymer backbone enabled
to be potentially ambipolar charge transport. In fact,
most of ꢀ-conjugated organic semiconductors have intrin-
sically ambipolar or bipolar charge transport capabil-
ity, but the n-channel properties frequently hided from
extrinsic effects, such as dielectrics, passivation, and/or
source/drain contact. It is noted that no n-channel prop-
erties of those materials observed because the OFET
devices in present study had bottom gate (SiO2 dielec-
tric and doped Si gate) and top-contact (Au) structure.
From these results, the present copolymers showed
OLED and OFET characteristics that is required for effi-
cient OLEFET devices. Therefore, copolymer systems of
these materials could be new candidates for active materi-
als in OLEFETs.
Acknowledgment: This work was supported by the
Industrial Strategic Technology Development Program
(KI002104, Development of Fundamental Technolo-
gies for Flexible Combined-Function Organic Electronic
Device) funded by the Ministry of Knowledge Economy
(MKE, Korea), and the Industrial-Educational Cooperation
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The SiO2 is commonly known as strong electron traps
Program between Korea University and Samsung Electron-
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due to its hydroxyl (–O–H) group on SiO2 surface, and
the high work function (∼ − 5ꢃ1 eV) Au electrode also
lead to unbalanced electron and hole injection efficiency.
We believe that other device structure, such as top-gate
(hydroxyl-free polymer dielectrics such as poly(methyl
methacrylate))/bottom-contact (Au) or bottom-gate (BCB-
coated or SAM-treated SiO2ꢁ/dissimilar top electrodes
(Au and Ca), enable to show the ambipolar properties.
The MEH-PPV OFET already reported ambipolar charge
transport17 and applied to OLEFETs.33 High performance
ambipolar OFETs and high efficiency light-emitting tran-
sistors are currently under study in our group.
Copyright: American Scientific Publishers
ics and we thank the staff of KBSI for technical assistance.
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Copyright: American Scientific Publishers
3330
J. Nanosci. Nanotechnol. 13, 3321–3330, 2013