11974
J. Am. Chem. Soc. 1996, 118, 11974-11975
Silole Derivatives as Efficient Electron Transporting
Materials
,
†
‡
‡
Kohei Tamao,* Manabu Uchida, Takenori Izumizawa,
,‡
†
Kenji Furukawa,* and Shigehiro Yamaguchi
Institute for Chemical Research
Kyoto UniVersity, Uji, Kyoto 611, Japan
Yokohama Research Center
Chisso Corporation, Kanazawa-ku
Yokohama 236, Japan
ReceiVed August 13, 1996
Figure 1. Comparison of HOMO and LUMO energy levels based on
ab initio calculations at RHF/6-31G* level of theory.
Organic electroluminescent (EL) devices, generally composed
of thin multilayers of hole transporting, emissive, and electron
transporting (ET) materials sandwiched between two elec-
trodes,1 are enjoying a great deal of interest because of their
possible application as large-area flat panel displays. One of
the major current subjects in this field is the development of
efficient ET materials.3 In principle, the efficiency of EL
depends on the quantities of injected carriers, holes, and
electrons as well as the quantum yield of the emissive material.
A balanced injection of holes and electrons is required to realize
an efficient EL. However, the efficiency of the electron
injection for the devices reported so far seems to be inferior to
that of the hole injection, due to the large differences between
the Fermi levels of cathode metals and the LUMO levels of the
ET materials. High electron affinity may thus be the first
requisite for the design of ET materials.4 Although the low-
lying LUMO levels would be readily achieved by the introduc-
tion of electron-withdrawing groups into π-conjugated systems,
these traditional structural modifications might often result in
the undesired formation of charge transfer complexes or
exciplexes with HT or emissive materials. Consequently, the
representative ET materials widely used so far are rather
restricted to only a few types of compounds, such as CdN
Chart 1
,2
5
double bond-containing heterocyles, metal-quinolinol com-
plexes,6 and cyano-substituted poly(p-phenylenevinylene)s.
Conceptually, new efficient ET materials are still being devel-
oped.
7
nent for efficient ET materials. The high electron accepting
ability of the silole ring has long been experimentally known.10
It has recently been theoretically demonstrated that the low-
lying LUMO energy level of the silole ring is ascribed to the
σ*-π* conjugation between the σ* orbital of the exocyclic σ
bonds on silicon and the π* orbital of the butadiene moiety in
We now introduce a silicon-containing cyclic π-electron
system, silole (silacyclopentadiene),8 as a novel core compo-
,9
†
Kyoto University.
Chisso Corporation.
‡
(
1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. Tang,
C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610.
1
1,12
(
(
2) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1990, 57, 531.
3) (a) Strukelj, M.; Papadimitrakopoulos, F.; Miller, T. M.; Rothberg,
the ring.
This feature is conspicuous by comparison, as
shown in Figure 1, of the calculated HOMO and LUMO energy
levels of the silole ring with those of some nitrogen-containing
L. J. Science 1995, 267, 1969. (b) Strukelj, M.; Miller, T. M.; Papadimi-
trakopoulos, F.; Son, S. J. Am. Chem. Soc. 1995, 117, 11976.
1
3
(
4) It has recently been pointed out, however, that there seems to be no
cyclic compounds, most of which have been used as core
5
,14
direct correlations between the reduction potentials of the ET materials and
the performance of the devices. See ref 3.
components of conventional ET materials.
The silole ring
has the lowest LUMO energy level among them. It was thus
anticipated that new efficient ET materials would be realized
by using the silole ring as a core component.
(
5) Representative efficient ET materials reported so far and their
pertinent references: 1,3,4-Oxadiazole: (a) Adachi, C.; Tsutsui, T.; Saito,
S. Appl. Phys. Lett. 1989, 55, 1489. (b) Adachi, C.; Tsutsui, T.; Saito, S.
Appl. Phys. Lett. 1990, 56, 799. (c) Hamada, Y.; Adachi, C.; Tsutsui, T.;
Saito, S. Jpn. J. Appl. Phys. 1992, 31, 1812. (d) Pei, Q.; Yang, Y. AdV.
Mater. 1995, 7, 559. (e) Li, X.-C.; Cacialli, F.; Giles, M.; Gr u¨ ner, J.; Friend,
R. H.; Holmes, A. B.; Moratti, S. C.; Yong, T. M. AdV. Mater. 1995, 7,
(8) Dubac, J.; Laporterie, A.; Manuel, G. Chem. ReV. 1990, 90, 215.
(9) Silole-containing π-conjugated compounds: (a) Tamao, K.; Yamagu-
chi, S.; Shiozaki, M.; Nakagawa, Y.; Ito, Y. J. Am. Chem. Soc. 1992, 114,
5867. (b) Tamao, K.; Yamaguchi, S.; Ito, Y.; Matsuzaki, Y.; Yamabe, T.;
Fukushima, M.; Mori, S. Macromolecules 1995, 28, 8668. (c) Tamao, K.;
Ohno. S.; Yamaguchi, S. J. Chem. Soc., Chem. Commun. 1996, 1873.
(10) (a) Janzen, E. G.; Pickett, J. B.; Atwell, W. H. J. Organomet. Chem.
1967, 10, 6. (b) O’Brien, D. H.; Breeden, D. L. J. Am. Chem. Soc. 1981,
103, 3237.
8
98. (f) Li, X.-C.; Holmes, A. B.; Kraft, A.; Moratti, S. C.; Spencer, B. C.
W.; Cacialli, F.; Gr u¨ ner, J.; Friend, R. H. J. Chem. Soc., Chem. Commun.
995, 2211. (g) Berggren, M.; Granstr o¨ m, M.; Ingan a¨ s, O.; Andersson, M.
AdV. Mater. 1995, 7, 900. (h) Kido, J.; Harada, G.; Nagai, K. Chem. Lett.
996, 161. See also refs 2 and 3. 1,2,4-Triazole: (i) Kido, J. Ohtani, C.;
Hongawa, K.; Okuyama, K.; Nagai, K. Jpn. J. Appl. Phys. 1993, 32, L917.
j) Kido, J.; Kimura, M.; Nagai, K. Science 1995, 267, 1332.
6) Tris(8-quinolinolato)aluminum(III) Alq and its derivatives: Hamada,
Y. Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata, K. Chem. Lett. 1993,
05. See also ref 1.
7) Cyano-substituted poly(p-phenylenevinylene): a) Greenham, N. C.
Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature
993, 365, 628. b) Moratti, S. C.; Cervini, R.; Holmes, A. B.; Baigent, D.
R.; Friend, R. H.; Greenham, H. C.; Gr u¨ ner, J.; Hamer, P. J. Synth. Metal
995, 71, 2117.
1
1
(
(11) Khabashesku, V. N.; Balaji, V.; Boganov, S. E.; Nefedov, O. M.;
Michl, J. J. Am. Chem. Soc. 1994, 116, 320.
(
(12) (a) Tamao, K.; Yamaguchi, S. Pure Appl. Chem. 1996, 68, 139. (b)
Yamaguchi, S.; Tamao, K. Bull. Chem. Soc. Jpn. 1996, 69, 2327.
(13) Ab initio calculations were carried out by using the Gaussian 92
program at the RHF/6-31G* level of theory, in which geometries of those
compounds were fully optimized.
9
(
1
(14) Strukelj, M.; Jordan, R. H.; Dodabalapur, A. J. Am. Chem. Soc.
1996, 118, 1213.
1
S0002-7863(96)02829-6 CCC: $12.00 © 1996 American Chemical Society