PHOTOINDUCED ELECTRON-TRANSFER BETWEEN SULFUR-CONTAINING CARBOXYLIC-ACIDS AND THE 4-CARBOXYBENZOPHENONE TRIPLET-STATE IN AQUEOUS-SOLUTION

The mechanism of photoinduced electron transfer between sulfur-contain ing carboxylic acids and the 4-carboxybenzophenone (CB) triplet state in aqueous solution was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for que nching of the CB triplet state by six sulfur-containing acids, with va rying numbers of COO- groups and varying locations with respect to the sulfur atom, were found to be in the range (0.3-2.1) X 10(9) M(-1) s( -1) depending on the charge of the acid molecule. The observation of k etyl radical anions and intermolecular (S therefore S)-bonded radical cations of some of the acids was direct evidence for the participation of electron transfer in the mechanism of quenching. An additional abs orption band at approximately 410 nm in the transient absorption spect ra for some of the acids was assigned to intramolecularly (S therefore O)-bonded species (for acids for which a five-member ring structure w as sterically favorable). Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of CO2 formation and CB disappearance from the steady-state measurements were determin ed. The values of these quantum yields clearly indicated that the diff usion apart (escape of the radical ions) of the charge-transfer comple x, formed as a primary photochemical step, is the main photochemical p athway (contribution of similar to 90%). Competing processes of proton transfer and back electron transfer within the CT complex gave only m inor contributions to these yields. A detailed mechanism of the CB-sen sitized photooxidation of sulfur-containing carboxylic acids is propos ed, discussed, and compared with that for sulfur-containing amino acid s in aqueous solution.