Novel liquid-crystalline and amorphous materials containing oxadiazole and amine moieties for electroluminescent devices

Article abstract of DOI:10.1039/b005550f

Three 1,3,4-oxadiazole derivatives with an amine and an alkyl tail were designed and synthesized as novel electroluminescent materials; it was found that the length of alkyl tail and the structure of the amine strongly affect the phase structure of the oxadiazole derivatives.

Full text of DOI:10.1039/b005550f

Novel liquid-crystalline and amorphous materials containing oxadiazole and
amine moieties for electroluminescent devices
Hiroyuki Mochizuki,a Takahiro Hasui,a Masuki Kawamoto,a Takeshi Shiono,a Tomiki Ikeda,*a Chihaya
Adachi,b Yoshio Taniguchib and Yasuhiko Shirotac
a Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503,
Japan. E-mail: tikeda@res.titech.ac.jp
b Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda,
Nagano 386-8567, Japan
c
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565-0871,
Japan
Received (in Cambridge, UK) 12th July 2000, Accepted 10th August 2000
First published as an Advance Article on the web 13th September 2000
Three 1,3,4-oxadiazole derivatives with an amine and an
alkyl tail were designed and synthesized as novel electro-
luminescent materials; it was found that the length of alkyl
tail and the structure of the amine strongly affect the phase
structure of the oxadiazole derivatives.
route shown in Scheme 1. 4-(4-n-Alkylphenyl)benzoic acid was
reacted with an excess of thionyl chloride. The resulting 4-(4-n-
alkylphenyl)benzoyl chloride was reacted with aminophenyl-
tetrazole derivatives in dry pyridine at 135 °C, yielding the
products: 2-(4A-propylbiphenyl-4-yl)-5-(4-N,N-dimethylamino-
phenyl)-1,3,4-oxadiazole (3-OXD-Me), 2-(4A-heptylbiphenyl-
4-yl)-5-(4-N,N-dimethylaminophenyl)-1,3,4-oxadiazole
Functional organic materials have been the subject of recent
research work in relation to their optical and electronic
properties as well as their industrial applications in many fields
such as electroluminescent (EL) devices, transistors, batteries,
sensors, photoreceptors and displays.1 However, practical
products made of organic compounds are solely photoreceptors
and displays. Thus, the advantages of organic compounds are
not sufficiently utilized in practical materials and devices.
Liquid-crystalline (LC) phases and amorphous states of
organic materials have been known to show unique morphol-
ogy. LC materials are quite attractive in terms of possessing
both self-organizing capability and fluidity. On the other hand,
amorphous materials are promising in terms of their excellent
processability, flexibility, transparency, non-existence of grain
boundaries and isotropic properties. Recently, much attention
has been paid to low-molecular-weight materials which form
stable amorphous glasses above rt.2 It is of interest and of
significance to develop photo- and electroactive amorphous
molecular materials, which consist of p-electron systems and
have glass transition temperature (Tg) higher than rt, for use in
electronic devices.
(7-OXD-Me) and 2-(4A-heptylbiphenyl-4-yl)-5-(4-N,N-diph-
enylaminophenyl)-1,3,4-oxadiazole (7-OXD-Ph).7 These prod-
ucts were purified by column chromatography on silica gel,
followed by recrystallization from toluene–ethanol. The overall
yields were about 55%. The oxadiazole derivatives were
characterized as obtained by FT-IR, H NMR and elemental
analysis.
Thermotropic and LC behaviors were evaluated by means of
DSC (heating and cooling rate: 2 °C min21) and polarizing
microscopy. Fig. 1 shows DSC thermograms of 3-OXD-Me and
7-OXD-Me on the third cooling. It was found that 3-OXD-Me
showed only a sharp exothermic peak corresponding to a
melting point (Tm) at 174 °C. By contrast, 7-OXD-Me exhibited
two exothermic peaks at 143 and 138 °C. It seems that the peak
at 143 °C is due to isotropic (I)-nematic (N) phase transition and
another peak at 138 °C is due to N-crystal phase transition. Fig.
2 shows the textures observed with a polarizing microscope at
140 and 100 °C, respectively. Referring to the Schlieren texture,
the phase could be assigned to an N phase at 140 °C, and then
a typical crystal texture was observed at 100 °C. As 7-OXD-Me
showed no LC phase on heating but an N phase on cooling, it
seems to be a monotropic LC material.
1
In 1990, it was found that 2-(biphenyl-4-yl)-5-(4-tert-
butylphenyl)-1,3,4-oxadiazole (PBD) functioned very well as
an excellent electron transport material (ETM) in an organic
multilayer EL diode.3 After this report, many researchers began
to use various kinds of oxadiazole molecules to obtain high EL
performances. Furthermore, in recent studies of polymer light-
emitting diodes, the oxadiazole moieties were demonstrated to
possess high potential for electron transport.4
DSC thermograms of 7-OXD-Ph on the first and the second
heating are shown in Fig. 3. When the crystalline sample of
7-OXD-Ph obtained by recrystallization from ethanol–toluene
was heated, 7-OXD-Ph exhibited Tm at 166 °C to give an I
phase. 7-OXD-Ph in the I phase was then cooled by standing in
Introduction of the oxadiazole moieties to polymer main
chains and to mesogens of LC compounds is expected to tune
EL efficiencies and electron transport properties. EL polymers
with p–n diblock structures were reported, in which thiophene
and 1,3,4-oxadiazole moieties connected alternately to form
fully conjugated rigid-rod polymers: one has been successfully
used to fabricate single layer EL devices showing blue
emission.5 Likewise, LC compounds containing oxadiazole
moieties were reported to exhibit a high electron transport
capability and blue EL emission.6
In this study, we report the synthesis and properties of a novel
class of oxadiazole derivatives for photo- and electroactive
materials, emphasizing that the length and structure of the
introduced moieties strongly affect the phase structure of the
oxadiazole derivatives.
The oxadiazole derivatives in this study were prepared
starting from 4-(4-n-alkylphenyl)benzoic acid by the synthetic
Scheme 1 Synthetic route for compounds used in this study with
abbreviations.
DOI: 10.1039/b005550f
Chem. Commun., 2000, 1923–1924
This journal is © The Royal Society of Chemistry 2000
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Fig. 4 X-Ray diffraction patterns of 7-OXD-Ph at 23 and 80 °C.
peak owing to crystallization appeared at around 60 °C.
Subsequently, another crystallization occurred at 87 °C and the
crystal melted at 166 °C. These phenomena of crystallization
from the melt were confirmed with a polarizing microscope,
however, the structure of each crystal was beyond identifica-
tion.
Fig. 1 DSC thermograms on the third cooling. (a), 3-OXD-Me; (b), 7-OXD-
Me.
The formation of an amorphous glass was evidenced in the X-
ray diffraction patterns of 7-OXD-Ph at rt (23 °C) and 80 °C, as
exhibited in Fig. 4. 7-OXD-Ph showed no sharp signals but
only broad halos, when cooled from an I state to 23 °C,
indicating that 7-OXD-Ph is in an amorphous state. On the
other hand, several sharp signals appeared at 80 °C, suggesting
that 7-OXD-Ph is in a crystalline state. 7-OXD-Ph did not
show an LC phase but an amorphous state; steric hindrance
between two clusters of benzene rings of 7-OXD-Ph may
contribute to formation of the amorphous glass.
All materials investigated in this work emitted strong blue
fluorescence with high quantum yields (0.81–0.91); emission
peaks of 3-OXD-Me and 7-OXD-Me were 420 nm, and that of
7-OXD-Ph was 436 nm. It is assumed that these compounds
show an excellent EL performance. The present study will
enable us to perform molecular design of various compounds
based on oxadiazole for developing photo- and electroactive
materials. These materials are expected to find potential
application as functional materials, e.g. charge transport and
luminescent materials for photoreceptors and electrolumines-
cent devices.
In summary, 1,3,4-oxadiazole derivatives with amine and
alkyl tails were designed and prepared as novel EL materials:
the phase structures and optical properties of the oxadiazole
derivatives were investigated in this study. We found that the
length of the alkyl tail and the structure of the amine strongly
affected the phase structure of the oxadiazole derivatives.
Fig. 2 Polarizing optical micrographs of the texture of 7-OXD-Me. (a), at
140 °C (N phase); (b), 100 °C (crystalline phase).
Notes and references
1 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913; J. H.
Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay,
R. H. Friends, P. L. Burn and A. B. Holmes, Nature, 1990, 347, 539; A. J.
Epstei, MRS Bulletin, June 1997, 16; G. P. Wiederrecht, A. B. Yoon and
M. R. Wasielewski, Science, 1995, 270, 1794; T. Ikeda and O. Tsutsumi,
Science, 1995, 268, 1873.
2 Y. Shirota, T. Kabata and N. Noma, Chem. Lett., 1989, 1145; A. Higuchi,
H. Inada, T. Kabata and Y. Shirota, Adv. Mater., 1991, 3, 549; H. Inada
and Y. Shirota, J. Mater. Chem., 1993, 3, 319; W. Ishikawa, K. Noguchi,
K. Kuwabara and Y. Shirota, Adv. Mater., 1993, 5, 559; K. Katsuma and
Y. Shirota, Adv. Mater., 1998, 10, 223.
3 C. Adachi, T. Tsutsui and S. Saito, Appl. Phys. Lett., 1990, 56, 799; C.
Adachi, T. Tsutsui and S. Saito, Appl. Phys. Lett., 1990, 57, 513.
4 C. Hosokawa, N. Kawasaki, S. Sakamoto and T. Kuzumoto, Appl. Phys.
Lett., 1992, 62, 2503; Q. Pei and Y. Yang, Chem. Mater., 1995, 7,
1568.
Fig. 3 DSC thermograms of 7-OXD-Ph. (a), on the first heating; (b), on the
second heating.
5 W. Huang, H. Meng, W. Yu, J. Gao and A. J. Heeger, Adv. Mater., 1998,
10, 593.
6 H. Tokuhisa, M. Era and T. Tsutsui, Appl. Phys. Lett., 1998, 72, 2639.
7 N. Tamoto, C. Adachi and K. Nagai, Chem. Mater., 1997, 9, 1077.
air, and it formed spontaneously the supercooled liquid state,
which changed into the amorphous glassy state. On the second
heating, Tg was observed at 27 °C, and then the first exothermic
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Chem. Commun., 2000, 1923–1924