PHOTOSENSITIZED ELECTRON-TRANSFER FROM STERICALLY HINDERED AMINES TO THE BENZOPHENONE TRIPLET AND ITS REVERSION IN SOLVENTS OF DIFFERENT POLARITY

The reductive quenching of the benzophenone triplet by 2,2,6,6-tetrame thyl- and 1,2,2,6,6-pentamethyl-piperidine was studied by laser flash photolysis. The rate constants for the quenching process were obtained in solvents of different polarity, acetonitrile, methanol, acetone, n -butanol, t-butanol, butyl chloride, chloroform, 1,4-dioxane, cyclohex ane and benzene. The contact ion pair primarily formed, decays by prot on transfer under the formation of the benzophenone ketyl radical, bac k electron transfer and charge separation. In our experiments no indic ation for the free solvated anion radical was found. The ketyl radical yields vary with solvent polarity, showing a minimum for the solvent butyl chloride. We explain this behaviour in terms of the solvent depe ndence of the back electron transfer process. Rate constants were dete rmined for all reactions studied. (C) 1998 Elsevier Science B.V. All r ights reserved.