Synthesis, thermal and luminescence properties of ortho-terphenyl derivatives with 1,3,4-oxadiazole moiety

Article abstract of DOI:10.1080/15421406.2011.599740

A series of compounds and two polymers, all consisting of o-terphenyl and 1,3,4- oxadiazole units, were synthesized. They were characterized by GC, FT/IR, NMR, elemental analysis, viscometry, DSC, POM, TGA, and absorption and emission spectrometry. In general, by introducing an o-phenyl central core to 2,5-diphenyl-1,3,4-oxadiazole backbone, the processablility of these materials was improved while maintaining a bluelight- emitting capability. The CIE coordinates of compounds with p-chloro, methoxy and t-butyl groups in EL devices were found to be (0.19, 0.16), (0.17, 0.13) and (0.18, 0.13), respectively, i.e., in the deep-blue region. Also, amorphous polymer with a m-phenylene unit, which shows good solubility in common organic solvents and excellent thermal stability, emits indigo-light at 415 nm on photoexcitation. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.599740

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Synthesis, Thermal and Luminescence
Properties of Ortho-Terphenyl
Derivatives with 1,3,4-Oxadiazole Moiety
E-Joon Choi a , Jun-Ha Hwang a , Eun-Kyoung Kwon a , Jung-Il Jin b
, Yong-Hwan Ahn c , Young-Chul Kim c , Jae-Woong Yu d , Wen Sheng
Shi e & Edward T. Samulski e
a Department of Polymer Science and Engineering , Kumoh National
Institute of Technology , Gumi , Gyungbuk , 730-701 , Korea
b Department of Chemistry , Korea University , Seoul , 136-713 ,
Korea
c Department of Chemical Engineering , Kyung Hee University ,
Yongin , Gyeonggido , 446-701 , Korea
d Center for Energy Materials Research, Korea Institute of Science
and Technology , Seoul , 136-791 , Korea
e Department of Chemistry , University of North Carolina at Chapel
Hill , Chapel Hill , North Carolina , 27599-3290 , USA
Published online: 18 Oct 2011.
To cite this article: E-Joon Choi , Jun-Ha Hwang , Eun-Kyoung Kwon , Jung-Il Jin , Yong-Hwan Ahn ,
Young-Chul Kim , Jae-Woong Yu , Wen Sheng Shi & Edward T. Samulski (2011) Synthesis, Thermal and
Luminescence Properties of Ortho-Terphenyl Derivatives with 1,3,4-Oxadiazole Moiety, Molecular
Crystals and Liquid Crystals, 550:1, 189-204, DOI: 10.1080/15421406.2011.599740
To link to this article: http://dx.doi.org/10.1080/15421406.2011.599740
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Mol. Cryst. Liq. Cryst., Vol. 550: pp. 189–204, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.599740
Synthesis, Thermal and Luminescence Properties
of Ortho-Terphenyl Derivatives
with 1,3,4-Oxadiazole Moiety
E-JOON CHOI,1,∗ JUN-HA HWANG,1 EUN-KYOUNG KWON,1
JUNG-IL JIN,2 YONG-HWAN AHN,3 YOUNG-CHUL KIM,3
JAE-WOONG YU,4 WEN SHENG SHI,5
AND EDWARD T. SAMULSKI5,∗
1Department of Polymer Science and Engineering, Kumoh National Institute of
Technology, Gumi, Gyungbuk 730-701, Korea
2Department of Chemistry, Korea University, Seoul 136-713, Korea
3Department of Chemical Engineering, Kyung Hee University, Yongin,
Gyeonggido 446-701, Korea
4Center for Energy Materials Research, Korea Institute of Science and
Technology, Seoul 136-791, Korea
5Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290, USA
A series of compounds and two polymers, all consisting of o-terphenyl and 1,3,4-
oxadiazole units, were synthesized. They were characterized by GC, FT/IR, NMR, ele-
mental analysis, viscometry, DSC, POM, TGA, and absorption and emission spectrome-
try. In general, by introducing an o-phenyl central core to 2,5-diphenyl-1,3,4-oxadiazole
backbone, the processablility of these materials was improved while maintaining a blue-
light-emitting capability. The CIE coordinates of compounds with p-chloro, methoxy and
t-butyl groups in EL devices were found to be (0.19, 0.16), (0.17, 0.13) and (0.18, 0.13),
respectively, i.e., in the deep-blue region. Also, amorphous polymer with a m-phenylene
unit, which shows good solubility in common organic solvents and excellent thermal
stability, emits indigo-light at 415 nm on photoexcitation.
Keywords blue-light-emitting materials; amorphous and soluble OLED; thermally
stable PLED; o-terphenyl derivatives; heteroaromatic moiety
1. Introduction
Organic luminescent materials have attracted much attention due to their potential applica-
tion in a wide range of electronic and optoelectronic devices [1–3]. The full color display
system requires three basic color materials for red, green, and blue emission [4–6]. A great
number of red, green, and blue emitting materials have been synthesized and the details
of structure-property relationships have been investigated [7]. However, pure blue light
∗Corresponding author. E-mail: ejchoi@kumoh.ac.kr (E-J. Choi), et@unc.edu (E. T. Samulski)
189

190
E.-J. Choi et al.
emitting materials are still being sought after because of issues of color purity, thermal sta-
bility and fabrication problems. Recently reported blue emitting materials possess unique
molecular geometries such as heterocyclic [8,9], non-coplanar [10,11], branched [12,13],
and starburst [14–16] structures.
Aromatic polyoxadiazoles have excellent thermal and hydrolytic stability and fluoresce
in the blue-greenish range, but their application is rather limited due to poor solubility in
common organic solvents [17,18]. Although 1,3,4-oxadiazoles facilitate electron transport
and hole blocking [19,20] via the electron-withdrawing character of the 1,3,4-oxadiazole
ring, certain low molecular weight aromatic 1,3,4-derivatives have been used as blue emit-
ters. However, the applicability of the oxadiaxole moiety is limited because these materials
have a short lifetime, possibly as a result of crystallization or aggregation effects. To
overcome this problem, a variety of branched compounds, starburst structures, and small
dendrimers have been advanced [21–24]. These materials are amorphous and form stable
glasses, which minimizes the problems of crystallization. However, some attempts to im-
prove solubility of these materials occasionally interrupted the conjugation of the main
chain.
The principal purpose of our work was to improve the processability of conjugated
materials while maintaining the blue-light-emitting properties. For this reason, we have
synthesized a series of o-terphenylene derivatives with 1,3,4-oxadiazole moiety as blue-
light-emitting materials and two polymers based on corresponding model compounds.
The thermal, UV-Vis absorption, and photo- and electroluminescence properties of these
materials were characterized and the findings are reported herein.
2. Experimental Section
2.1. Chemicals
N,Nꢀ-Diphenyl-N,Nꢀ-bis(1-naphthyl)-(1,1ꢀ-biphenyl)-4,4ꢀ-diamine (α-NPB) and tris(8-
hydroxyquinoline) aluminium (Alq3) were purchased from Gracel Company; tereph-
thaloyl and isophthaloyl dihydrazides were purchased from TCI Company; 4-
methoxybenzhdrazide was purchased from Lancaster Company. Solvents such as carbon
disulfide, acetonitrile, phosphorus oxychloride and pyridine were purchased from Aldrich
Company. The other reactants were purchased from Aldrich Company and the solvents
from Fisher Company. 1,3,5-Tris(N-phenylbenzimidizol-2-yl)benzene (TPBI) was synthe-
sized according to the reported procedures [25]; all the commercial reagents were used
without further purification. Solvents such as thionyl chloride, dichloromethane and N,N-
dimethylacetamide were purified by standard methods; the other solvents were used without
further purification.
2.2. Instrumentation
NMR spectra of compounds 4a–4c and 4d–7b were recorded by a Brucker Advance
400 MHz and a Brucker DMX 200 MHz NMR spectrometer, respectively. IR spectra
were obtained using a Jasco 300E FT/IR spectrometer. To confirm the degree of purity of
compounds 4a–4c, a GC was performed using a Hewlett Packard 5890 Series II gas chro-
matograph and for compounds 4d–7b, elemental analyses were performed using a Ther-
mofinnigan EA1108 elemental analyzer. The phase transition temperatures were determined
by a differential scanning calorimeter (Seiko DSC 220C for compounds 4a–4c; DuPont

Amorphous and Soluble OLED and PLED Materials
191
TA910 DSC for compounds 4d–7b) and a polarizing optical microscope (POM: Nikon
Microphot-FX for compounds 4a–4c; Zeiss, Jenapol for compounds 4d–7b) equipped with
a hot stage (Linkam TMS 600 for compounds 4a–4c; Mettler FP82HT for compounds
4d–7b). DSC measurements were performed in a N2 atmosphere with heating and cooling
rates of 10◦C/min. Thermal stability of polymers was measured using a DuPont TGA 2950
in a N2 atmosphere with a heating rate of 20◦C/min. Solution viscosities of obtained poly-
mers were measured at 30◦C using a Ubbelohde-type viscometer. UV-visible spectra were
recorded using a Perkin Elmer Lambda 40 spectrometer for compounds 4a–4c, a Scinco S-
1100 spectrometer for compounds 4d–4g, and a Hewlett-Packard 8452A spectrometer for
polymers 7a and 7b. PL spectra were obtained by using a Spex 1403.35m double spectrom-
eter with a PTI xenon arc lamp and PMT detector for compounds 4a–4c, a Perkin Elmer
LS 50 spectrometer for compounds 4d–4g, and an Aminco Bowman Series 2 spectrometer
for polymers 7a and 7b. The absorption and emission spectra were measured in diluted
CHCl3 solutions with concentrations of about 1.0 × 10−5 mol/L and 1.0 × 10−4 mol/L,
respectively, at room temperature. Eelectroluminescence (EL) spectra of the EL devices
were measured with a JBS IVL-300 EL characterization system in an ambient atmosphere.
2.3. Syntheses of bis(5-phenyl-1,3,4-oxadiazole-2-diyl) 4,4ꢀꢀ-o-terphenyl (4a),
bis(5-(o-chlorophenyl)-1,3,4-oxadiazole-2-diyl) 4,4ꢀꢀ-o-terphenyl (4b), and
bis(5-(p-chlorophenyl)-1,3,4-oxadiazole-2-diyl) 4,4ꢀꢀ-o-terphenyl (4c)
The syntheses of compounds 4a–4c were carried out as previously reported [26].
2.4. Synthesis of bis(5-(p-methylphenyl)-1,3,4-oxadiazole-2-diyl) 4,4ꢀꢀ-o-terphenyl (4d)
o-Terphenyldicarboxylic acid (0.50 g, 1.57 mmol) was refluxed in 25 mL of SOCl2 for 4
h. Excess SOCl2 was removed by distillation under reduced pressure. The prepared 4,4ꢀ-
o-terphenyldicarbonyl dichloride (1) was dissolved in 20 mL of methylene chloride. To
this solution was added slowly a solution of p-toluic hydrazide (2d) (0.43 g, 3.14 mmol)
dissolved in 10 mL of pyridine (2:1 v/v) at room temperature under a dry N2-gas atmosphere,
while stirring vigorously. After stirring at 90◦C for 16 h, the mixture was concentrated by
evaporating the solvent under reduced pressure. The residue was cooled to room temperature
and poured into 250 mL of water. The resulting precipitate was collected by filtration,
thoroughly washed with water and dried under vacuum at 60◦C. The obtained intermediate
(3d) was used for the next synthetic step without further purification. The dihydrazide 3d
was then added to a mixture of phosphoryl chloride (10 mL) and acetonitrile (30 mL). The
reaction mixture was heated at 100◦C for 16 h, and then it was concentrated by evaporating
the solvent under reduced pressure. The residue was cooled to room temperature and
carefully submersed in 200 mL of ice-water to hydrolyze the residual POCl3. The resulting
solid was washed thoroughly with water, collected by filtration, dried under vacuum at
60◦C, and purified by using a column chromatography on silica gel with a mixture of
hexane and ethyl acetate (1:2 v/v). Further purification was conducted by re-precipitation:
dissolved in chloroform, filtered by a PTFE membrane filter, crystallized from hexane, and
dried under vacuum at 60◦C. Yield: 87%; TLC (silica gel, hexane/ethyl acetate = 1:2, v/v),
Rf 0.97. IR (KBr Pellet, cm−1): 3043 (aromatic =C-H, st), 1613, 1576 (aromatic C=C,
1
st), 1265, 1181, 1067 (C-O, st). H NMR (CDCl3, δ in ppm): 8.10–7.97 (8H, d, Ar-H),
7.56–7.51 (4H, s, Ar-H), 7.39–7.30 (8H, d, Ar-H). Elemental anal. Calcd for C36H26N4O2:
C, 79.12; H, 4.76; N, 10.26. Found: C, 78.06; H, 5.39; N, 10.89.

192
E.-J. Choi et al.
2.5. Synthesis of bis(5-(p-methoxyphenyl)-1,3,4-oxadiazole-2-diyl)
4,4ꢀꢀ-o-terphenyl (4e)
Compound 4e was synthesized similarly to the compound 4d. Yield: 72%; TLC (silica gel,
hexane/ethyl acetate = 1:2, v/v), Rf 0.88. IR (KBr Pellet, cm−1): 3012 (aromatic =C-H,
st), 1611, 1496 (aromatic C=C, st), 1299, 1255, 1171, 1023 (C-O, st). 1H NMR (CDCl3,
δ in ppm): 8.10–7.96 (8H, d, Ar-H), 7.53–7.48 (4H, s, Ar-H), 7.07–6.96 (8H, d, Ar-H).
Elemental anal. Calcd for C36H26N4O4: C, 74.74; H, 4.50; N, 9.69. Found: C, 69.49; H,
4.98; N, 9.84.
2.6. Synthesis of bis(5-(p-butyloxyphenyl)-1,3,4-oxadiazole-2-diyl)
4,4ꢀꢀ-o-terphenyl (4f)
Compound 4f was synthesized similarly to the compound 4d. Yield: 88%; TLC (silica gel,
hexane/ethyl acetate = 1:2, v/v), Rf 0.97. IR (KBr Pellet, cm−1): 3050 (aromatic =C-H, st),
1611, 1555, 1523 (aromatic C=C, st), 1299, 1256, 1170, 1023 (C-O, st). 1H NMR (CDCl3,
δ in ppm): 8.10–7.96 (8H, d, Ar-H), 7.53–7.48 (4H, s, Ar-H), 7.07–6.96 (8H, d, Ar-H).
Elemental anal. Calcd for C36H26N4O4: C, 74.74; H, 4.50; N, 9.69. Found: C, 69.49; H,
4.98; N, 9.84.
2.7. Synthesis of bis(5-(p-nitrophenyl)-1,3,4-oxadiazole-2-diyl) 4,4ꢀꢀ-o-terphenyl (4g)
Compound 4g was synthesized similarly to the compound 4d. Yield: 89%; TLC (silica gel,
hexane/ethyl acetate = 1:2, v/v), Rf 0.98. IR (KBr Pellet, cm−1): 3042 (aromatic =C-H, st),
29.43 (C-H, st), 1612, 1556 (aromatic C=C, st), 1211, 1076 (C-O, st). 1H NMR (CDCl3,
δ in ppm): 8.11–7.98 (8H, d, Ar-H), 7.58–7.49 (4H, s, Ar-H), 7.39–7.31 (8H, d, Ar-H).
Elemental anal. Calcd for C42H38N4O2: C, 80.00; H, 6.03; N, 8.89. Found: C, 78.72; H,
6.16; N, 8.92.
2.8. Synthesis of Poly(1,3,4-oxadiazole-2,5-diyl-4,4ꢀꢀ-o-terphenyl-1,3,4-
oxadiazole-2,5-diyl-m-phenylene) (7a)
Diacid dichloride (1) (1.57 mmol) was dissolved in 10 mL of N,N-dimethylacetamide
(DMAc) in a nitrogen atmosphere. To this solution was added a solution of isophthalic
dihydrazide (5) (0.30 g, 1.57 mmol) dissolved in DMAc (20 mL) by use of a syringe at
room temperature, resulting in the production of hydrochloric acid gas. After agitation at
120◦C for 16 hr, the reaction mixture was cooled to room temperature and poured into
methanol (250 mL) to produce a precipitate. The resulting solid of precursor (6a) was
filtered, washed several times with water and methanol, and dried under vacuum at 60◦C
to use in the next synthetic step. IR (KBr Pellet, cm−1): 3470 (N-H, st), 3043 (aromatic
=C-H, st), 1645 (C=O, st), 1528, 1474 (aromatic C=C, st), 1266 (C-N, st).
The poly(hydrazide) 6a was added to a mixture of acetonitrile (30 ml) and phosphorus
oxychloride (10 ml). After refluxing with vigorous stirring at 90◦C for 16 hr, the mixture
was cooled to room temperature and excess acetonitrile was removed at 50◦C in a vacuum.
Trace amounts of POCl3 was hydrolyzed by adding ice water, and then precipitated in
methanol. The resulting solid product was filtered, washed several times with water and
methanol, and dried in a vacuum at 60◦C. Yield: 70%. IR (KBr Pellet, cm−1): 3053 (aromatic
=C-H, st), 1612, 1545, 1471 (aromatic C=C, st), 1249, 1183 (C-O, st), 1073 (=C-O-C= ,

Amorphous and Soluble OLED and PLED Materials
193
st). Elemental anal. Calcd for C28H16N4O2: C, 76.36; H, 3.64; N 12.73. Found: C, 71.27;
H, 3.90; N, 12.29.
2.9. Synthesis of Poly(1,3,4-oxadiazole-2,5-diyl-4,4ꢀꢀ-o-terphenyl-1,3,4-oxadiazole-2,5-
diyl-p-phenylene) (7b)
Precursor 6b was synthesized using the same method as precursor 6a except a mixture of
LiCl (5 wt%) and DMAc (30 ml) was used as a polymerization solvent. IR (KBr Pellet,
cm−1): 3477 (N-H, st), 3039 (aromatic =C-H, st), 1650(C = O, st), 1533, 1473 (aromatic
C=C, st), 1272 (C-N, st).
Polymer 7a was synthesized using the same method as polymer 7b. Yield: 76%. IR
(KBr Pellet, cm−1): 3053 (aromatic =C-H, st), 1612, 1570, 1499, 1474 (aromatic C=C,
st), 1274, 1186 (C-O, st), 1073 (=C-O-C= , st). Elemental anal. Calcd for C28H16N4O2:
C, 76.36; H, 3.64; N, 12.73. Found: C, 70.48; H, 4.12; N, 11.81.
2.10. Device Preparation
EL devices with the configuration of [ITO/α-NPB(40 nm)/compounds 4c, 4e and 4f(30
nm)/TPBI(10 nm)/Alq3(30 nm)/LiF(1 nm)/Al(100 nm)] were prepared. For the devices,
α-NPB was used as the hole transport layer (HTL), and TPBI and Alq3 were used as
the electron transport layer (ETL). Indium tin oxide (ITO) coated glass substrates (Asahi
Glass Co., Ltd.) with a surface resistance of 10 ꢀ/sq were patterned by using a standard
photolithography method and then cleaned by sonication in trichloroethylene, acetone,
deionized water, and isopropyl alcohol. Thermal evaporation of organic materials and
metals was carried out in a high vacuum condition (2 × 10−6 Torr) and the deposition
rates were 1.0, 0.1, and 5–6 Å/s for the organic layer, the LiF layer, and the Al cathode,
respectively. The deposition rate and thickness of each layer was regulated using a calibrated
quartz crystal microbalance. The active emitting area of the device was 3 × 3 mm2.
Also EL devices with the configurations of [ITO(15 ꢀ)/CuPC(30 nm)/compounds
4d, 4e and 4f(80 nm)/LiF:Al(100 nm)] and [ITO(15 ꢀ)/PEDOT(30 nm)/polymer 7a(80
nm)/LiF:Al(100 nm)] were prepared. In the devices, copper phthalocyanine (CuPC) and
poly(3,4-ethylenedioxythiophene-2,5-diyl) (PEDOT) were used as the hole transport (HTL)
and the injection layer (HIL) for the compounds and the polymer, respectively. Deposition
rate of compounds 4d and 4e was 0.5–6.0 Å/s at 77–78◦C, and that of compound 4f was
1.0–4.4 Å/s at 117–169◦C. Thin film of polymer 7a was obtained by spin-coating: the
solution (40 µl) of the polymer (40 mg) in TCE (1 mL) was spin-coated and then vacuum-
dried at 150◦C for 2 h. Obtained films were used to measure the absorption and emission
spectra. PEDOT was spin-coated onto the polymer layer and dried at 100◦C for 2 h.
3. Results and Discussion
3.1. Synthesis and Thermal Properties
The synthetic route to compounds and polymers is shown in Scheme 1: the 1,3,4-oxadiazoles
(4 or 7) were prepared by cyclodehydration of the hydrazide precursors (3 or 6) which were
obtained by reaction of 4,4ꢀꢀ-o-terphenyldicarbonyl dichloride (1) and benzhydrazide (2 or
5). The structures of all compounds were identified using IR and NMR spectrometry. All of
the resultant spectra were in accordance with expected values. The purities of compounds
4a–4c reported by us [26] were confirmed by using TLC and GC methods. As a result, they

194
E.-J. Choi et al.
ClOC
COCl
1
O
C
O
C
O
C
Ar
5
H2NHN
NHNH2
NHNH2
Pyridine
-HCl
DMAc
-HCl
X
2
C
O
HNHN
C
O
C
O
NHNH
C
O
Ar
C
O
C
NHNH
C
O
C
O
NHNH
X
X
O
3
n
6
POCl3
CH3CN
POCl3
CH3CN
O
O
O
O
N
N
N
N
N
N
N
N
Ar
X
X
n
4a: X = H
4b: X = o-Cl
4c: X = p-Cl
4d: X = p-CH3
7a
: Ar =
7b
: Ar =
4e
: X = p-CH3O
4f
: X = p-t-Butyl
4g
: X = p-NO2
Scheme 1. Synthetic route to compounds and polymers.
showed a single Rf value in TLC plate as well as a sharp peak with a single elusion time
in GC chromatographs. The purities of compounds 4d–4g and polymers 7a and 7b were
confirmed by using an elemental analysis, and the data was within tolerable uncertainties.
Representative differential scanning calorimetric (DSC) thermograms are presented in
Fig. 1, and the results of the analysis are summarized in Table 1. On the first heating scan,
the as-prepared sample of compound 4a showed a melting (Fig. 1a); upon cooling at a rate
of 10◦C/min, the compound did not recrystallize but showed a glass transition (Fig. 1g) [26].
On the second heating scan, the compound exhibited a Tg at 82◦C without melting (Fig. 1b).
Compound 4a was annealed by heating at the onset of the melting temperature endotherm
in order to confirm whether this amorphous property is an intrinsic characteristic. After the
annealing, the Tm and Tg were almost unchanged, but the magnitude of melting endotherm
was decreased significantly from 32 kJ mol−1 to 0.5 kJ mol−1 (Fig. 1c). Again, during
cooling, the compound would not recrystallize (Fig. 1h). This indicates that compound
4a only can readily crystallize by precipitation from a non-solvent, whereas once it has

Amorphous and Soluble OLED and PLED Materials
195
Table 1. Transition temperatures and enthalpy changes of compounds
DSC
Scans
Tg
aTc
aꢁHc
Tm
ꢁHm
[kJ mol−1
]
Compounds
X
H
[◦C]
[◦C] [kJ mol−1
]
[◦C]
4a
1st heating
cooling
(84)b
197c(197)b 32c(0.5)b
83c(79)b
2nd heating 82(90)b
4b
4c
o-Cl 1st heating
cooling
77
65
74
110
183
36
43
215
284
49
52
2nd heating
p-Cl
heating
cooling
heating
heating
heating
heating
4d
4e
4f
CH3
CH3O
t-Bu
165
162
d
d
d
214
4g
NO2
>250e
aTc and ꢁHc stand for the recrystallization temperature and the associated enthalpy change,
b
c
respectively. Data in parenthesis was obtained after annealing for 12 h at 170◦C. The data of
d
e
the reference 26 was included for comparison. Not available. Melting coincides with thermal
decomposition.
Figure 1. DSC curves of compounds 4a and 4b (heating and cooling rates = 10◦C/min): 4a (as-
prepared): (a) 1st heating, (b) 2nd heating, and (g) 1st cooling scans; 4a (annealed at 170◦C for 12
h): (c) 1st heating, (d) 2nd heating, and (h) 1st cooling scans; 4b (as-prepared): (e) 1st heating, (f)
2nd heating, and (i) 1st cooling scans.

196
E.-J. Choi et al.
Table 2. General properties of polymers
ηinh
[dL/g]
Tg
aTd
Residue at
Polymers
Ar
[◦C]
[◦C]
600◦C [wt%]
7a
7b
m-Ph
p-Ph
0.30b
0.35c
247
243
430
420
65
36
aTd stands for the initial decomposition temperature. bMeasured using a 0.5 g/dL solution of CHCl3
at 30◦C. cMeasured using a 0.5 g/dL solution of H2SO4 at 30◦C.
Figure 2. TGA curves of (a) precursor 6a, (b) precursor 6b, (c) polymer 7a, and (d) polymer 7b
(heating rate = 20◦C/min).
melted, the compound cannot recrystallize easily in its bulk state. On the first heating
scan, the as-prepared sample of compound 4b exhibited three thermal transitions: a glass
transition, crystallization, and melting (Fig. 1e). Similar to 4a, once it was melted, 4b
would not recrystallize easily (Fig. 1, f and i). An increase in conformational variability
due to the o-chloro-substitution may account for the unique crystallization behavior of
4b. In Table 1, the Tg of 4b is not higher than that of 4a, indicating that the libration is
mainly correlated with reorientation of the entire bent conformation rather than regional
steric hindrance due to o-substitution by the chlorine atom. Compounds 4c–4f were normal
crystalline compounds with Tm’s in the range of 162–284◦C. However, compound 4g with
a nitro group exhibited thermal decomposition at about 250◦C after melting.
Table 2 presents the general properties of the polymers. Poly(1,3,4-oxadiazole) with
a nonlinear m-phenylene unit has better solubility in common organic solvents than that
of poly(1,3,4-oxadiazole) with a linear p-phenylene unit. The inherent viscosities (ηinh
)
of polymers 7a and 7b are 0.30 and 0.35, respectively, indicating that the polymers do
not have a very high molecular weight. The polymers have a glass transition temperature
(Tg) without melting according to their DSC thermograms, indicating their amorphous
nature. Note that even though the value of solution viscosity for polymer 7a was lower
than polymer 7b, the Tg of the former polymer with the nonlinear m-phenylene moiety is
higher than the latter polymer with the linear p-phenylene moiety. The thermal stability of
the polymers in a nitrogen atmosphere was studied by TGA. Fig. 2 shows that the thermal

Amorphous and Soluble OLED and PLED Materials
197
stability of polymers could be enhanced by cyclodehydration of the hydrazide prepolymers
to poly(1,3,4-oxadiazole)s. In Table 2, the decomposition temperatures (Td) of poly(1,3,4-
oxadiazole)s emphasize the good thermal stability of the polymers even those with a highly
nonlinear o-terphenyl core.
3.2. Absorption and Photoluminescence Properties
The absorption spectra of compounds 4a–4f in CHCl3 solution and polymer 7a are shown
in Fig. 3a. The absorption maximums of the compounds span 281–324 nm depending on
the substituent, and that of polymer 7a was 299 nm (see Table 3), substantially red-shifted
in comparison with biphenyl (246 nm) and p-terphenyl (278 nm) [27]. Considering the fact
that the unsubstituted 2,5-diphenyl-1,3,4-oxadiazole (DPO) shows an optical absorption
peak at 284 nm [5f], we suggest that a highly extended delocalized system which typically
dominates the system’s absorption was formed throughout the o-terphenyl core. The ab-
sorption maxima of compounds 4e with an electron donor, 4g with an electron acceptor,
and 4d and 4f with alkyl groups were observed at 303, 324, 294, and 299 nm, respectively;
all were substantially red-shifted in comparison with those of 4a (286 nm) without substi-
tution, and 4b (287 nm), 4c (281 nm) with a Cl-substituent. As can be seen from Fig. 3a, the
nitro group exhibits the largest red-shift with the most distinctive shoulder absorption. This
can be accounted for the increased band gap due to its electron withdrawing character. On
the other hand, polymer 7a showed an absorption maximum at 295 nm. However, spectra of
polymer 7b could not be measured because of poor solubility in common organic solvents.
The PL spectra of compounds 4a–4f in CHCl3 solution, and polymer 7a are shown in
Fig. 3b; three of them show the vibronic feature. The PL maxima of these samples occurred
near 398–405 nm (see Table 3), which was substantially red-shifted in comparison with
that of DPO (365 nm). As a result, found were the materials emitting light in the violet
and indigo region; no emission from compound 4g with a nitro group was observed when
photoexcited, probably due to exiton quenching.
Table 3. Physical properties of compounds and polymers
CHCl3 Solution
Film on Glass
aPLmax
Sample
Code
UVmax
PLmax
UVmax
bELmax
[nm]
X or Ar
[nm]
[nm]
[nm]
[nm]
4a
4b
4c
4d
4e
4f
H
286
287
281
294
303
398,447c
401,450c
398,447c
391
d
d
308
403,452c(408,456c)
401,460c(401)
403,446c(403)
408,466c
d
d
451
o-Cl
p-Cl
CH3
CH3O
t-Bu
NO2
m-Ph
269c,310
308
400
403
431
410
454(440)
442
299
298
4g
7a
324,278c
299
405
297
415,464c
b
aData in parenthesis was for crystal samples, while remaining data for glassy samples. Data in
parenthesis was taken from the device of [ITO/CUPC/compound/LiF: Al], while remaining data from
the device of [ITO/α-NPB/compound/TPBi/Alq3/LiF/Al]. cShoulder peak. dNot available.

198
E.-J. Choi et al.
Figure 3. (a) UV-Vis absorption and (b) PL spectra of compounds and a polymer in CHCl3 solution
at room temperature.
The PL spectra of compounds 4a and 4b in different physical states are compared in
Fig. 4. Glassy samples were prepared by rapidly cooling the melts to room temperature;
crystalline samples were prepared by adhering the solid to an indium plate using a glass
“rolling pin”. For compound 4a, the peak wavelength at 398 nm in solution shifted to 403
nm in the glassy state, and to 408 nm in the crystalline state, respectively. These red-shifts
imply that the π–orbital overlapping increases because the mobility of conjugated bond
remains low in the glassy and the crystalline states with a low degree of crystallinity due
to molecular packing constraints of highly bent o-terphenyl groups as well as the DPO
backbone retaining its rigidity. However, as can be seen in Fig. 4, the spectra of compound
4b differ insignificantly with the physical state.

Amorphous and Soluble OLED and PLED Materials
199
Figure 4. PL spectra of compounds 4a and 4b at different physical states: (a) CHCl3 solution, (b)
glassy and (c) crystal samples of 4a; (d) CHCl3 solution, (e) glassy and (f) crystal samples of 4b.
The absorption and emission spectra of the solid state of compounds 4c–4f and polymer
7a are shown in Fig. 5. Films on glass were prepared by spin-coating from a CHCl3 solution.
A similar red-shift trend found in solutions was observed and depended on substituents.
Generally, due to aggregation in the crystalline states, spectra of samples were substantially
red-shifted in comparison with those observed in solutions. The PL maximum of compound
4e with a methoxy group revealed the highest red-shift of all of the samples, indicating
a morphological changes potentially stemming from interactions between the electron
accepting 1,3,4-oxadiazole and the electron donating methoxy groups.
3.3. Electroluminescence Properties
The current density versus voltage (J-V) characteristics of the devices based on compounds
4d–4f and polymer 7a are compared in Fig. 6a. The EL threshold voltage of the devices
with compounds 4d, 4e and 4f was 1.0, 1.3 and 0.8 MV cm−1, respectively. Energy levels
of compounds 4c, 4e and 4f from ultraviolet photoelectron spectroscopy (UPS) have been
determined: the HOMO energy levels of compounds 4c, 4e, and 4f are 6.08, 6.05 and
5.75 eV, respectively, and the LUMO levels are 2.49, 2.49, and 2.16 eV, respectively. The
band energy diagrams of the devices with different materials and structures are compared
in Fig. 7. It is expected that hole injection from the anode to compound 4f will be more
favorable than to compound 4e. Therefore the current density of the devices with compounds
4d and 4f with an alkyl group is higher than that of compound 4e with an alkoxy group as
shown in Fig. 6a. As a result, the driving voltage required to obtain the same current density
is lower for compounds 4d and 4f than for compound 4e. However, a high current density
does not necessarily guarantee a high quantum efficiency of the EL device, especially when
the hole current prevails over the electron current leading to largely unbalanced charge
carrier injection. The device with compound 4e can give a higher quantum efficiency
than those with compounds 4d and 4f if hole and electron injection is better balanced
at the same current density (see Fig. 6b). In addition, compound 4e has an intramolecular

200
E.-J. Choi et al.
Figure 5. (a) UV-Vis absorption and (b) PL spectra of compounds and a polymer in solid state at
room temperature.
charge-transfer complex between the electron-withdrawing 1,3,4-oxadiazole moiety and the
electron-releasing 4-methoxy-phenyl moiety. It is probable that the intramolecular donor-
acceptor (D-A) type complex formed in compound 4e can trap holes as well as electrons
more efficiently than compounds 4d and 4f. Therefore, we can suggest that compound 4e
is the best deep blue light-emitting material among the compounds in spite of having the
highest driving threshold voltage.
In Fig. 8, the EL spectra of the devices with compounds 4c, 4e and 4f show deep-blue
EL emissions with a peak at 440–454 nm (see Table 3). The general appearance of the EL
spectra of these compounds was relatively red-shifted in comparison with the PL spectra

Amorphous and Soluble OLED and PLED Materials
201
Figure 6. (a) Current density on applied electric field strength in EL devices of
[ITO/CUPC/4d–4f/LiF:Al] and [ITO/PEDOT/7a/LiF:Al]. (b) External quantum efficiencies on ap-
plied electric field strength in EL device of [ITO/CUPC/4d–4f /LiF:Al].
Figure 7. Schematic energy band diagrams of the energy band for two different devices: (a) [ITO/α-
NPB/4c, 4e and 4f/TPBi/Alq3/LiF/Al]; (b) [ITO/CUPC/4e and 4f/LiF:Al].

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E.-J. Choi et al.
Figure 8. EL spectra of the devices with compounds 4c, 4e and 4f: [ITO/α-NPB/(a) 4c, (b) 4e or
(c) 4f /TPBi/Alq3/LiF/Al] and [ITO/CUPC/(d) 4e/LiF:Al].
in solid state. Previously, it was reported that the second substituent in DPO influences the
emission wavelength: usingan electron donor, thematerial emits inthe blue; withanelectron
acceptor in the green spectral range [18f]. However, compound 4g with a nitro group does
not luminescence at all. Compound 4e showed a different EL maximum depending on the
configuration of the EL device, maintaining the blue-light-emitting property. The deep blue
OLED device using 4c, 4e, and 4f as an EML has a maximum luminance of 240, 1120
and 220 cd/m2, respectively. The CIE coordinates of compounds 4c, 4e, and 4f in the EL
devices with the configuration of [ITO/α-NPB/compound/TPBi/Alq3/LiF/Al] were found
to be (0.19, 0.16), (0.17, 0.13) and (0.18, 0.13), respectively, i.e., in the deep-blue region.
4. Conclusions
A series of o-terphenyl-1,3,4-oxadiazole derivatives and two poly(o-terphenyl-1,3,4-
oxadiazole)s were synthesized by a dehydration reaction of hydrazide precursors. The
o-chloro- and unsubstituted compounds form glasses with a Tg in the range of 65–90◦C
depending on the thermal history: The PL spectrum of the unsubstituted compound in
a glass is closer to the spectrum in solution than in the crystal. The absorption and the
PL maximums of compounds with a substituent such as methyl, methoxy, and t-butyl
groups were substantially red-shifted in comparison to those of compounds with o-chloro-
and p-chloro-substituents or without a substituent. The absorption maximum of the com-
pound with a nitro group exhibited the largest red-shift, but this compound did not emit
light after electrical excitation nor photoexcitation. The absorption and PL maxima of the
m-phenylene-based polymer were red-shifted in comparison with the compounds, which
emitted indigo-light (λPL,max = 415 nm). Moreover, EL maxima of compounds with methyl,

Amorphous and Soluble OLED and PLED Materials
203
methoxy and t-butyl groups were found at deep-blue-light region (λEL,max = 440–454 nm).
According to thermal analysis, the two polymers possess amorphous characteristics as
well as good thermal stability. Moreover, the m-phenylene-based polymer revealed good
solubility in general organic solvents and emits indigo-light at 415 nm on photoexcitation.
Acknowledgement
This work was supported by Grant No. R01-2005-000-10852-0, from the Basic Research
Program of the Korean Science & Engineering Foundation and in part by NSF Grant
DMR-9971143.
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