Higher Excited States and Vibrationally Hot Excited States: How Important are they in Organic Photochemistry in Dense Media?
In photochemical reactions of organic molecules in fluid or rigid solutions, and even in the gas phase at moderate to high pressures, it is usually assumed without question that internal conversion to the lowest excited electronic state and thermal equilibration of vibrational motion occur much more rapidly than any chemical transformations, so that direct irradiation first produces cool S1, or possibly directly cool T1 if special structural features greatly enhance the rate of intersystem crossing relative to thermalization (e.g., heavy atoms), and sensitization produces cool T1, before any chemical reactions occur. The reacting species is therefore assumed to be thermalized S1 or T1, except perhaps if S2 (or T2 is so close in energy as to be thermally accessible, and kinetic schemes are set up accordingly. These assumptions represent an analogy to Kasha’s rule which states that thermalized S1 and T1 states are the only ones from which significant light emission is observed in dense media.
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- 1.C. M. Lok, J. Lugtenburg, J. Cornelisse, and E. Havinga, Tet. Lett., 4701 (1970).Google Scholar
- 2.A. Couture, K. Ho, M. Hoshino, P. de Mayo, R. Suau, and W. R. Ware, J. Am. Chem. Soc., 98, 6218 (1976); P. de Mayo, Acc. Chem. Res., 9, 52 (1976); and references therein.Google Scholar
- 6.For recent examples, see T. Wismonski-Knittel, G. Fischer, and E. Fischer, J. Chem. Soc., Perkin Trans. II, 1930 (1974); J. W. J. Gielen, H. J. C. Jacobs, and E. Havinga, Tet. Lett., 3751 (1976).Google Scholar
- 8.For a review, see R. Lesclaux and J. Joussot-Dubien, in “Organic Molecular Photophysics”, Vol. 1, J. B. Birks, Ed., Wiley, New York, N. Y., 1973.Google Scholar
- 11.J. Kolc and J. Michl, Abstracts, IVth IUPAC Symposium on Photochemistry, Baden-Baden, Germany, July 16–22, 1972, p. 167.Google Scholar
- 12.J. Kolc and J. Michl, J. Am. Chem. Soc. 95, 7391 (1973).Google Scholar
- 18.J. B. Birks, “Photophysics of Aromatic Molecules”, Wiley, New York, N. Y., 1970.Google Scholar
- 21.Not all such states are purely dissociative, as indicated by recent observations of weak Stokes-shifted fluorescence. See, e.g., P. Ausloos, Mol. Photochem., 4, 39 (1972); A. M. Halpern, Mol. Photochem., 5, 517 (1973).Google Scholar