Study of the long-lived intermediate formed in the photoreduction of benzophenone by isopropyl alcohol

Abstract

The formation, structure and decay kinetics of the long-lived intermediate formed in the photoreduction of benzophenone by isopropyl alcohol in acetonitrile has been investigated. Laser flash photolysis and transient absorption measurements carried out with the benzophenone—isopropyl alcohol—acetonitrile and benzophenone—benzhydrol—acetonitrile systems indicated that the intermediate, generally called a “light-absorbing transient” (LAT), is formed by coupling of a diphenylketyl with a dimethylketyl radical. Dimerization of diphenylketyl radicals appeared to contribute very little to LAT formation. Study of the decay kinetics of the intermediate revealed that LAT actually consisted of a mixture of an ortho and a para isomer for which absorption spectra could be obtained. The ortho-LAT isomer decomposed faster, mainly by a benzophenone-enhanced process, and absorbed at longer wavelengths with a maximum at 358 nm, while para-LAT isomer decayed more slowly and had an absorption maximum at 317 nm. Additional investigations supplied absolute spectra for short-lived transients, i.e. for benzophenone and acetone triplet states and diphenylketyl and dimethylketyl radicals.