Surface photochemistry of alkyl aryl ketones: energy transfer and the effect of a silica-gel surface on electronic states of excited molecules

Alkyl aryl ketones undergo Type II photoreaction both on a silica-gel surface and in solution. The amount of acetophenone produced from valerophenone increases linearly with increase in the amount of valerophenone loaded at coverage below ca 60%. The effect of the alkyl chain length on the Type II photoreactivity of alkyl phenyl ketones on the surface is weak. The relative quantum yields Phi(MeAP)/Phi(AP) and Phi(MeOAP)/Phi(AP) on the surface are lower than those in benzene containing 0.5 M pyridine. A silica-gel surface provides a polar reaction medium that makes the energy difference between the lowest n,pi* and the upper pi,pi* triplet states small and may cause the inversion of the nearby n,pi* and pi,pi* triplet states. The filter effect is trivial in surface photochemistry when all loaded molecules are in the monomolecular layer. The energy transfer processes are observed in the valerophenone-p-methylvalerophenone and valerophenone-p-methoxyvalenophonone systems in which the internal filter effects are so strong that the energy transfer processes cannot be observed directly in solution. Although the Stern-Volmer quenching kinetics would be applicable to the photoreaction on the surface, a correction for the probability of excitation of a molecule should be made. Copyright (C) 2000 John Wiley & Sons, Ltd.