Modification of photochemical pathways of sensitized oxidation of phenylthioacetic acid: Effects of solvent and tetrabutylammonium salt

The mechanisms of triplet-sensitized electron transfer from thioether-containing aromatic carboxylic acids were investigated using steady state and laser flash photolysis in acetonitrile and aqueous solutions. Benzophenone (BP) and 4-carboxybenzophenone were used as the triplet sensitizers, and phenylthioacetic acid and its tetrabutylammonium salt were used as the electron-donating quenchers. In aqueous solution, (BP−⋯>S+) radical pairs (formed in the electron-transfer quenching of the triplet state of BP) decayed mainly via a charge-separation reaction leading to the formation of ketyl radical anions (BP−) and sulfur-centered radical zwitterions (>S+). The later transients underwent fast decarboxylation leading to the formation of alkyl-type radicals. In acetonitrile solutions, however, proton transfer between the radical ions was observed with the formation of ketyl radicals (BPH). It was shown that the carboxylic and methylene groups in the phenylthioacetic acid moiety within the radical-ion pair are responsible for the proton-transfer reactions. The presence of counter cations from the tetrabutylammonium salt of phenylthioacetic acid can significantly modify the decay pathways of the photochemically produced radical-ion pairs in acetonitrile. In this case, the reaction leads to the formation of [BP−⋯N+(C4H9)4] transients that can undergo Hofmann elimination leading to the formation of an alkene and tributylamine. These results indicate that the photochemical pathways (primary and secondary reactions) for the sensitized oxidation of phenylthioacetic acid depend on its state of ionization (that can be modified by the solvent used), properties of solvent, and the presence of associated counter cations from the tetrabutylammonium salt of phenylthioacetic acid.