Temperature effects, arrhenius activation parameters and rate constants for the photochemical reactivity of cyano, boronato, trifluoromethyl and methoxy substituted toluenes in the excited singlet state.
Total rate constants of decay (k(t)) as a function of temperature from -45 to +65 degrees C for the compounds 1 and 2 in AN and TFE and 3 and 4 in AN have been determined by fluorescence lifetime measurements. The data have been fit to an equation that assumes that the rate constants of fluorescence (k(f)) and intersystem crossing (k(isc)) are temperature independent, that k(ic) = 0 and that the rate constant of reaction (k(r)) is activated according to the Arrhenius expression. For compounds 1-3, values of k(f) and k(isc) were found to be independent of solvent for any given compound, but k(r) was consistently greater in TFE than AN. For the anisoles 4, the temperature effect was very small, indicating that k(r) did not compete with k(f) + k(isc) and suggesting that an activated intersystem crossing was the dominant temperature-dependent process. The k(r), A and E(a) values obtained for compounds 1-3 were rationalized in terms of their known photochemistry, phototransposition reactions in AN and photoadditions in TFE. The critical reactive intermediate in all cases is a bicycle[3.1.0]hexenyl biradical/zwitterion that is formed in an activated process from S1. This reactive intermediate returns to starting material faster than it rearranges, and therefore an activated internal conversion is a major pathway for deactivation of S1.
Gonzalez
Carlos
C
Department of Chemistry, Dalhousie University, Halifax, NS, Canada B3H 4J3.
Pincock
James
J