Comparative kinetic study of the reactivity of molecules i n their excited singlet and triplet states in the cases of an electron abstraction and an intramolecular photocyclization reaction

Abstract

Summary Two photochemical reactions are considered, illustrating quantitatively the differences in rate constants for chemical and physical deactivating processes from excited singlet and triplet states, in each case, for the identical photochemical reaction. Firstly we have determined the rate constants of electron abstraction, ke, and quenching, kG, of oxonine singlet and triplet states by EDTA in aqueous solution. The rate kSe for the singlet state, is found to be about ten times larger than kTe for the triplet state, whereas kSG is 103 to 104 faster than kTG. Secondly, in the case of the intramolecular photocyclization of 1-(orthodiphenyl) −1 phenyl ethylene in cyclohexane, a common intermediate X is found for the singlet and triplet pathways. The rate constant kSX is found to be 150 times faster than an apparent rate kTX whereas the non radiative (nr) rate constant kSnr is about 103 times faster than kTnr. In conclusion it is cleared that the measurement of rate constants can really provide the necessary information concerning the difference of reactivity between singlet and triplet pathways of photochemical reactions. However kinetics only yields the values of the rate constant for each step considered in the scheme. This means that we can only have confidence in elementary rate constant for reaction scheme that have been established very carefully.