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Chemistry Letters Vol.36, No.5 (2007)
Hexaphenylbenzene Derivatives for Blue Organic Light-emitting Devices
Soichi Watanabe1 and Junji Kidoꢀ1;2
1Graduate School of Science and Engineering, Yamagata University, Yonezawa 992-8510
2Research Institute for Organic Electronics, Yonezawa 992-1128
(Received December 27, 2006; CL-061516; E-mail: kid@yz.yamagata-u.ac.jp)
Novel hexaphenylbenzene derivatives having high triplet
energy levels were synthesized and used in blue organic electro-
derivatives having high triplet excited energy (T1) levels for
the blue phosphorescent complex-based OLEDs.
phosphorescent devices. The device with a blue phosphorescent
emitter, iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2 ]-
picolinate, exhibited a high external quantum efficiency of
11% (24 cd/A) and a high power efficiency of 12 lm/W at
100 cd/m2.
The structures of the hexaphenylbenzene derivatives and
the synthetic scheme are shown in Scheme 1.7 A mixture of
(p-bromophenyl)acetic acid (1) and magnesium oxide as a
heating medium were heated to 340 ꢁC in reduced pressure
2–4 Torr for 30 min. After cooling at room temperature, 1,3-
bis(p-bromophenyl)-2-propanone (2) was extracted with EtOH
in 14% yield. 2,5-Bis(p-bromophenyl)-3,4-diphenylcyclopenta-
dion (3) was prepared by a condensation reaction of 2 with
benzyl and potassium hydroxide in EtOH at 70 ꢁC for 15 min
in 92% yield. 4,400-Dibromophenyl-20,30,50,60-tetraphenyl-p-
terphenyl was prepared by the Diels–Alder reaction of 3 with
diphenylacetylene in 77% yield. Finally, 5a: CzTT and 5b:
TATT were synthesized by the palladium-catalyzed coupling
reaction8 in 96 and 86% yield, respectively. Each of the crude
materials was purified by silica-gel column chromatography
and further purified by thermal gradient sublimation under N2
gas stream. The structures of 1–5 were characterized by 1H NMR
and MS spectroscopy.9 Hexaphenylbenzene derivatives show no
glass-transition temperature (Tg) up to 300 ꢁC by differential
scanning calorimetry analysis.
0
Organic light-emitting devices (OLEDs) have been expect-
ed to be the next generation flat panel displays and solid-state
lighting devices.1–3 Recently, extremely high efficiencies have
been observed by using phosphorescent metal complexes as
emitting centers. This is due to the fact that both singlet and trip-
let excited states are involved in the luminescence process at
room temperature. Thus, internal quantum efficiencies could
reach 100%.4
Compared with the conventional host and carrier transport
materials, such as CBP, the extension of the ꢀ conjugation
of CzTT and TATT is reduced owing to the twisted structure
of the bulky hexaphenylbenzene unit. Thus, wide HOMO–
LUMO energy gaps and high T1 levels are expected for
hexaphenylbenzene derivatives. The optical energy gaps or
HOMO–LUMO energy gaps, of CzTT and TATT were deter-
mined from UV–vis absorption edges using a Shimadzu UV-
2200A spectrophotometer to be 3.49 and 3.33 eV, respectively.
a) MgO 1.1 equiv., 340 °C, 2−4 Torr, 30 min.
b) Benzyl 1.2 equiv., 1.5 M KOH in EtOH, 70 °C, 15 min.
c) Diphenylacetylene 2.0 equiv., Benzophenone, 300 °C 20 min.
d) N-Carbazole 2.2 equiv., Pd(OAc)2, P(t-Bu)3, NaOtBu 6 equiv., Toluene, reflux, 24 h
e) Diphenylamine 2.2 equiv., Pd(OAc)2, P(t-Bu)3, NaOtBu 6 equiv., Toluene, reflux, 24 h
Scheme 1. Syntheses of CzTT and TATT.
Phosphorescent metal complexes require properly chosen
wide-energy-gap host materials and carrier transporting materi-
als so that the triplet excited energy at the metal complex can
be confined. For instance, a blue OLED, with phosphrescent
0
iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2 ]-picolinate
(FIrpic) as a guest emitter and 4,40-N,N0-carbazolylbiphenyl
(CBP) as a host, exhibits rather low efficiencies such as an
EQE of 6.0%.5 This is due to the fact that the host material,
CBP, does not have sufficiently high triplet energy level to con-
fine the triplet excited energy of the emitting center.5,6 In this
study, we designed and synthesized novel hexaphenylbenzene
Figure 1. Normalized UV–vis absorption spectra CBP (open
circle), CzTT (open triangle), and TATT (open square). Normal-
ized fluorescent spectra CBP with Ex. = 331 nm (closed circle)
CzTT with Ex. = 297 nm (closed triangle) and TATT with
Ex. = 331 nm (closed square).
Copyright Ó 2007 The Chemical Society of Japan