COMPARATIVE KINETIC STUDY OF THE REACTIVITY OF MOLECULES IN THEIR EXCITED SINGLET AND TRIPLET-STATES IN THE CASES OF AN ELECTRON ABSTRACTION AND AN INTRA-MOLECULAR PHOTOCYCLIZATION REACTION

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, k, and quenching, kG, of oxonine singlet and triplet states by EDTA in aqueous solution. The rate ks 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 l-(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. © IUPAC