Flash-photolysis study of the 4-(methyl sulphonate)-benzophenone ketyl radical anion in alcohol–water mixtures
Abstract
Irradiation of sodium 4-(methyl sulphonate)-benzophenone (BzPS), a water-soluble benzophenone, using ultraviolet (UV)–visible light in solutions containing an aliphatic alcohol and water results in the formation of the intermediate ketyl radical anion, BzPS˙–. The photochemistry of this species was studied in several alcohol–water solvent systems. The molar extinction coefficient of BzPS˙–(measured at its wavelength of maximum absorbance) and the pKa of BzPSH˙(the conjugate acid of BzPS˙–) in propan-2-ol–water solutions were determined to be (1.1 ± 0.1)× 103 m2 mol–1 and 11.8 ± 0.1, respectively. These values are independent of the solvent alcohol concentration in the range 10–50%(v/v) propan-2-ol–water. The absorption maximum of BzPS˙– shifts from ca. 600 to 618 nm over this concentration range. The rate constant for the recombination of the BzPS˙– species to form the sulphonated pinacol depends upon the type of alcohol used in the solvent and increases as the percentage of water in the solution is increased. The results are explained in terms of the preferential solvation of the primary solvation shell surrounding the BzPS˙– radical. The recombination rate constants are ca. 103 times lower than that of the benzophenone ketyl radical anion in 50%(v/v) propan-2-ol–water and decrease with increasing pH of the solvent medium.