J. Am. Chem. Soc. 1999, 121, 3651-3656
3651
Mechanistic Studies of Photochemical Silylene Extrusion from
2
,2-Diphenylhexamethyltrisilane
Takashi Miyazawa, Shin-ya Koshihara,‡,§ Chengyou Liu, Hideki Sakurai, and
‡
,§
‡
†,|
Mitsuo Kira*,
†,‡
Contribution from the Department of Chemistry, Graduate School of Science, Tohoku UniVersity,
Aoba-ku, Sendai 980-8578, Japan, and Photodynamics Research Center, the Institute of Physical and
Chemical Research, (RIKEN), 19-1399, Koeji, Nagamachi, Aoba-ku, Sendai 980-0952, Japan
ReceiVed September 21, 1998
Abstract: Photophysics and photochemistry of 2,2-diphenylhexamethyltrisilane (1) was investigated in detail.
Diphenyltrisilane 1 showed an intense fluorescence band assignable to the intramolecular charge-transfer (ICT)
band in a polar solvent. Whereas silylene extrusion and 1,3-silyl migration were the major photoreactions of
1
in a nonpolar solvent, solvolytic cleavage of the Si-Si bond occurred significantly in an ethanol-hexane
mixture; the ICT state is responsible for the latter reaction. Solvent and temperature dependence of the product
distribution has indicated that both the silylene extrusion and the 1,3-silyl migration take place from the nonpolar
but electronically different excited states of 1. On the basis of the photoreactions of 1 using a nonresonant
1
two-photon (NRTP) excitation method, it is suggested that the silylene extrusion occurs via the La state of 1
as a silyl-substituted benzene.
Introduction
respectively, in addition to a regular emission band from an
aromatic ππ* (LE, locally excited) state; we have revealed
recently that jet-cooled p-cyanophenylpentamethyldisilane shows
the ICT emission even in an isolated molecular condition.6d
Much attention has been focused also on the photochemical
Aryloligosilanes constitute a fascinating class of molecular
systems, exhibiting unique photophysical and photochemical
properties due to hyperconjugation between aromatic π orbitals
1
2
and high-lying Si-Si σ orbitals, low ionization potentials,
1
reactions and their mechanistic aspects of aryldisilanes, since
1
3
remarkable red-shift of the La bands and their stereoelectronic
they generate interesting reactive intermediates such as silylenes,
4
5,6
effects, and dual fluorescence in aryldisilanes. With close
6c
silenes, and silyl radicals. In our previous study of mechanisms
of photoreactions of p-trifluromethylphenylpentamethyldislane
in the presence of alcohols, we have revealed that the LE and
the ICT states participate in the photoreactions; 1,3-silyl
migration to give a silatriene intermediate, which is the most
predominant pathway in a nonpolar solvent, occurs via the LE
state, while significant direct solvolysis of the ICT state takes
place in an alcoholic solvent (Scheme 1).
similarity to so-called TICT molecules like p-dimethylami-
nobenzonitrile, aryldisilanes in solution show intramolecular
charge-transfer (ICT) emission bands, where the disilanyl group
and the aryl π system serve as an electron donor and an acceptor,
7
†
Tohoku University.
The Institute of Physical and Chemical Research.
Present address: Kanagawa Academy of Science and Technology, KSP
‡
§
East, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa, 213-0012, Japan.
|
Present address: Department of Industrial Chemistry, Faculty of Science
and Technology, Science University of Tokyo, Noda, Chiba 278-8510,
Japan.
Scheme 1
(1) For recent reviews, see: (a) Kira, M.; Miyazawa, T. In The Chemistry
of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John
Wiley: New York, 1998; Vol. 2, Chapter 22. (b) Brook, A. G. In The
Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.;
John Wiley: New York, 1998; Vol. 2, Chapter 21. (c) Brook, A. G. In The
Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.;
John Wiley: New York, 1989; Chapter 15. (d) Steinmetz, Chem. ReV. 1995,
9
5, 1527. (e) Sakurai, H. J. Organomet. Chem. 1980, 200, 261. (f) Ishikawa,
M.; Kumada, M. AdV. Organomet. Chem. 1981, 19, 51.
2) (a) Bock, H.; Alt, H. J. Am. Chem. Soc. 1970, 92, 1569. (b) Pitt, C.
(
G.; Carey, R. N.; Toren, E. C., Jr. J. Am. Chem. Soc. 1972, 94, 3806. (c)
Sakurai, H.; Kira, M. J. Am. Chem. Soc. 1974, 96, 791. (d) Sakurai, H.;
Kira, M. J. Am. Chem. Soc. 1975, 97, 4879. (e) Pitt, C. G.; Bock, H. J.
Chem. Soc., Chem. Commun. 1972, 28.
Whereas it has been well-known that photolysis of 2-aryl-
trisilanes provides a useful method for generation of the
1
,8-10
corresponding 2-arylsilylenes,
the 1,3-silyl migration to
(
3) Sakurai, H.; Kumada, M. Bull. Chem. Soc. Jpn. 1964, 37, 1894. (b)
Gilman, H.; Atwell, W. H.; Schwebke, G. L. J. Organomet. Chem. 1964,
, 369. (c) Hague, D. N.; Prince, R. H. Chem. Ind. (London) 1964, 1492.
give the corresponding silatriene occurs competitively, similar
to the photolysis of aryldisilanes (Scheme 2). No detailed
2
(
(
4) Sakurai, H.; Tasaka, S.; Kira, M. J. Am. Chem. Soc. 1972, 94, 3806.
5) (a) Shizuka, H.; Obuchi, M.; Ishikawa, M.; Kumada, M. J. Chem.
(6) (a) Sakurai, H.; Sugiyama, H.; Kira, M. J. Phys. Chem. 1990, 94,
1837. (b) Kira, M.; Miyazawa, M.; Mikami, N.; Sakurai, H. Organometallics
1991, 10, 3793. (c) Kira, M.; Miyazawa, T.; Sugiyama, H.; Yamaguchi,
M.; Sakurai, H. J. Am. Chem. Soc. 1993, 115, 3116. (d) Tajima, Y.;
Ishikawa, H.; Miyazawa, T.; Kira, M.; Mikami, N. J. Am. Chem. Soc. 1997,
119, 7400.
Soc., Chem. Commun. 1981, 405. (b) Shizuka, H.; Sato, Y.; Ishikawa, M.;
Kumada, M. J. Chem. Soc., Chem. Commun. 1982, 439. (c) Shizuka, M.;
Sato, Y.; Ueki, Y.; Ishikawa, M.; Kumada, M. J. Chem. Soc. Faraday Trans.
1
1984, 80, 341. (d) Shizuka, H.; Obuchi, M. Ishikawa, M.; Kumada, M.
J. Chem. Soc., Faraday Trans. 1 1984, 80, 383. (e) Hiratsuka, H. Mori, Y.;
Ishikawa, M.; Okazaki, K. Shizuka, H. J. Chem. Soc., Faraday Trans. 2
(7) For recent reviews, see: (a) Rettig, W. Angew. Chem., Int. Ed. Engl.
1986, 25, 971. (b) Bhattacharyya, K.; Chowdhury, M. Chem. ReV. 1993,
93, 507.
1
984, 81, 1665. (f) Shizuka, H. Pure Appl. Chem. 1993, 65, 1635.
1
0.1021/ja983370k CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/06/1999