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

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  • This paper is part of a special issue dedicated to Professor J. C. (Tito) Scaiano on the occasion of his 60th birthday.

ABSTRACT

Total rate constants of decay (kt) as a function of temperature from –45 to +65°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 (kf) and intersystem crossing (kisc) are temperature independent, that kic= 0 and that the rate constant of reaction (kr) is activated according to the Arrhenius expression. For compounds 1–3, values of kf and kisc were found to be independent of solvent for any given compound, but kr was consistently greater in TFE than AN. For the anisoles 4, the temperature effect was very small, indicating that kr did not compete with kf+kisc and suggesting that an activated intersystem crossing was the dominant temperature-dependent process. The kr, A and Ea 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.

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