Spirocycle-incorporated triphenylamine derivatives as an advanced organic electroluminescent material

Article abstract of DOI:10.1246/cl.2000.192

For multi-color organic electroluminescent (EL) devices, new triphenylamine compounds attached to a spirocyclic framework were prepared from 2,7-bis(diphenylamino)-9-fluorenone. These amines showed exceedingly high Tg's or thermal stability as well as goo

Full text of DOI:10.1246/cl.2000.192

192
Chemistry Letters 2000
Spirocycle-Incorporated Triphenylamine Derivatives
as an Advanced Organic Electroluminescent Material
Makoto Kimura,* Shin-ichiro Inoue, Kou Shimada, Shizuo Tokito† Koji Noda† Yasunori Taga† and Yasuhiko Sawaki*
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603
†TOYOTA Central Research and Development Labs., Inc., Nagakute, Aichi 480-1192
(Received November 4, 1999; CL-990941)
For multi-color organic electroluminescent (EL) devices,
new triphenylamine compounds attached to a spirocyclic
framework were prepared from 2,7-bis(diphenylamino)-9-fluo-
renone. These amines showed exceedingly high Tg's or thermal
stability as well as good electrochemical properties and suffi-
cient EL characteristics, allowing practical application.
An organic electroluminescent (EL) devicesl has been ini-
tiated into practical use as a mono-color device and new mate-
rials thermally stable in particular are expected for a coming
multi-color device. Various triphenylamine derivatives have
been elaborated as the hole-transport material.2−8 For example,
N,N'-diphenyl-N,N'-di(m-tolyl)benzidine (TPD)3 shows excel-
lent EL characteristics, but is lacking the thermal stability as
illustrated by its glass transition temperature Tg as low as 60
°C; α-NPD4 a naphthalene substitute to TPD exhibited the sat-
isfactory Tg of 96 °C. Starburst shaped triphenylamine deriva-
tives invented by Shirota et al.5 have deserved the commercial
application owing to the capability to form good amorphous
solid films as well.
To improve the low thermal stability of TPD, we focused
on the spirocompounds like 1 and 2 in view of the high melting
points of the parent hydrocarbons and also recent application to
electronic materials.9,10 New triphenylamine compounds
incorporated into spiro-bi-9-fluorene (1) or a spiro-ketone (2)
were found to possess exceedingly high Tg's and EL character-
istics enough to allow practical use as the hole-transport mate-
rial of future organic EL devices.
The thermal properties of spiro-amines, 1 and 2, were
examined by the DSC analysis and the electrochemical ones by
cyclic voltammetry (CV). The electroluminescent characteris-
tics of 1 and 2 were obtained as a typical multi-layered
devices. As a reference compound, N,N,N',N'-tetraphenylben-
zidine (TPB) was used (Table l). The observed Tg's of 1 and 2
are all over 110 °C, exceeding those of TPD and TPB. The
present spirocompounds apparently have high Tg's enough for
practical use as compared to α-NPD and non-cyclic or related
amine derivatives.10 Therefore, the critical drawback of TPD
is resolved, and spiroamines 1 and 2 would be thermally stable
in keeping their solid thin films in EL devices.
Two or three successive waves of quasi-symmetry were
observed in each CV of 1 and 2. The first oxidation peaks of 1
and 2 appeared at 0.6-0.8 V vs. Ag/AgCl electrode. The wave
analysis indicates a reversible formation of cation radicals;14
e.g., the cathodic and anodic peak current ratio at the first oxi-
dation wave was 0.99 for 1b, whereas that for TPB being 0.89.
Thus, the cation radicals from 1 and 2 are electrochemically
stable in solution.
Preparation of 1 and 2 relied on the use of 2,7-
bis(diphenylamino)-9-fluorenone 3 (Scheme 1). This was
obtained from the requisite dibromide in 71 % yield by the
amination using palladium acetate/tri-tert-butylphosphine (the
Tosoh catalyst).11 Synthesis of spiro-bifluorenes 1a and 1b
were attained by coupling 3 with o-lithiobiphenyls, derived
from the corresponding bromides 4 with BuLi in ether, fol-
lowed by acid-treatment of the resulting crude alcohols in boil-
ing acetic acid. Spiroketonic amine 2 was obtained in 70% iso-
lated yield by heating 3 in triethyl phosphite;12 the one-pot
reaction involves apparently the pinacol coupling of 3 and sub-
sequent pinacol rearrangement of the resultant diol.13
Multi-layered EL devices for 1 and 2 were fabricated by
vapor deposition onto the transparent ITO glass substrate (1 60
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
193
2
3
4
5
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electrode Mg-Ag). By applying the electric voltage, a green
emission due to Alq3 was observed at 530 nm. For evaluating
EL characteristics, we measured the applied voltage required
for obtaining a practical level of luminance 100 cd/m2, i.e., V-
100 (the lower the better) together with the maximum lumi-
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V, clearly having the advantage of organic EL devices.
In conclusion, spirocyclic triphenylamines 1 and 2 pre-
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being expectable as a new hole-transport material.16 Based on
the simple synthesis, spirocyclic ketone derivative 2 is pro-
posed to be promissing for coming multi-color EL devices.
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15 Detailed studies on the device characteristics including EL
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