Photochemical transformations. 46. Photophysics and photochemistry of some compounds undergoing light-induced solvolysis and Wagner-Meerwein rearrangement

Abstract

Absorption maxima, fluorescence maxima, and fluorescence quantum yields are reported for a variety of 7-substituted and 7,8-disubstituted dibenzobicyclo(2.2.2)octadienes and veratrolobenzobicyclo(2.2.2)octadienes in cyclohexane and/or acetonitrile. Singlet lifetimes have been computed from these data. Those compounds that are reactive toward photosolvolysis and/or Wagner-Meerwin photorearrangement have fluorescence yields (and lifetimes) 10/sup -2/-10/sup -4/ times those of the corresponding compounds containing only hydrogens on the saturated bridges. Compounds that are photoinert but have an electron-attracting group on the bridge have lifetimes about one-tenth those of unsubstituted compounds. These results and the variations in fluorescence maximum wavelengths are rationalized by assumptions that there is some degree of charge transfer from aromatic ring to C-X bond in the photoexcited state and that this favors decay to the ground state. When more or less complete intramolecular electron transfer to a zwitterionic biradical intermediate is exergonic, lifetimes in the 4-80-ps range are completed.