Kinetics of fast reactions of triplet states and radicals under photolysis of 4,4′-dimethylbenzophenone in the presence of 4-halophenols in micellar solutions of sodium dodecyl sulfate in magnetic field

Abstract

Quenching kinetics of the 4,4´ dimethylbenzophenone triplet state with para substituted phenol derivatives RC 6 H 4 OH (R = Н, F, Сl, Br, I) was studied by nanosecond laser photolysis in aqueous micellar solutions of sodium dodecyl sulfate. The kinetic data were processed in the framework of a model with the Poisson distribution of phenols between micelles. The partition constants of RC 6 H 4 OH between the aqueous and micellar phases and the rate constants of their escape from a micelle and quenching of the 4,4´ dimethylbenzophenone triplet state with phenols in micelles were obtained. The quenching proceeds with high rate constants through hydrogen atom transfer to form the ketyl and phenoxyl radicals (no radicals are formed in the case of 4 iodophenol), which then recombine in a micelle or escape into the outer aqueous volume. The application of an external magnetic field retards radical pair recombination in a micelle and increases the fraction of radicals escaped into the aqueous phase. The quantum yield of radical pairs decreases 2.5 fold, and the rate of their recombination in micelles in creases 2.5 fold on going from 4 chloro to 4 bromophenol. This is caused by the acceleration of triplet radical pair recombination in the solvent cage.Key words: 4,4´ dimethylbenzophenone, 4 halophenol, laser photolysis, triplet states, hydrogen atom transfer, radical recombination, heavy atom effect, magnetic effect.Researchers are interested in the kinetics of fast pho toinduced reactions in systems containing carbonyl and phenol compounds in micellar solutions due to pro nounced magnetic and spin effects for the recombination of triplet radical pairs (RPs) formed in these reactions. 1-15 Hydrophobic substituted phenols almost completely lo calized in the micellar phase were predominantly used in earlier studied systems. Neither the kinetics of primary processes of quenching of triplet states of carbonyl com pound with phenols and RP formation, nor the kinetic behavior of RPs at different phenol concentrations in micellar solutions were studied. Several kinetic regulari ties observed for RP recombination in micelles, in par ticular, internal heavy atom effect, assume that the corre sponding effect can appear during the primary step. 3,5-10 The effect of a heavy atom on the primary step of radical formation in carbonyl-phenol systems was repeatedly observed in homogeneous solutions. This effect depends on the parameters of the medium. 16-18 It is of interest to study the kinetics of all the processes of radical formation and recombination during photosensitized oxidation of phenols in micellar solutions. A study of the quenching kinetics of the triplet state of a photosensitizer localized in the micellar phase by molecules of hydrophilic phenols, which are distributed between the aqueous and micellar phases, also provides information on the partition con stants. 19 The kinetics of fast processes during photooxida tion of water soluble halosubstituted phenols is also worthy of attention because of the problem of industria...

Decay Kinetics of Benzophenone Triplets and Corresponding Free Radicals in Soft and Rigid Polymers Studied by Laser Flash Photolysis

“…The hard matrix of PS holds radicals together long enough to allow such recombination. The characteristic time of recombination of similar triplet contact RPs in liquids or in microheterogeneous media is assumed to be less than 1 μs .…”