Kinetics of radical formation and decay in photooxidation of 4-halophenols sensitized by 4-carboxybenzophenone in aqueous solutions

Kinetics of formation and recombination of radicals formed by quenching of the triplet state of 4-carboxybenzophenone (CB) with para-substituted phenol derivatives RC6H4OH (R OMe, H, Cl, Br, 1) in aqueous solutions was studied by nanosecond laser photolysis. At pH 5.4, quenching proceeds with high rate constants ((1-3) (.) 10(9) L mol(-1) s(-1)) through electron transfer to form the radical anion CB center dot- and radical cation RC6H4OH center dot. The latter is transformed into the phenoxyl radical within <= 10 ns. At pH <= 8, the CB center dot- radical anion is protonated in a phosphate buffer with the rate constant increasing from 4 (.) 10(6) to 15 (.) 10(6) s(-1) with a decrease in the pH from 8 to 5.4. The yield of radicals decreases from 100 to 13% as the atomic weight of halogen in the RC6H4OH molecule increases due to an increase in the probability of recombination of the primary triplet radical pair in the solvent cage and partial intersystem crossing in an encounter complex ((CB)-C-3, RC6H4OH). The effect of heavy atom is also observed in the kinetics of volume recombination of the radicals, the magnitude of effect corresponds to the acceleration of the primary recombination of the triplet radical pair.