DIFFUSE REFLECTANCE LASER FLASH-PHOTOLYSIS STUDIES OF REACTIONS OF TRIPLET BENZOPHENONE WITH HYDROGEN DONORS ON SILICA

The reactions of triplet benzophenone adsorbed on silica with several hydrogen donors have been examined by using diffuse reflectance laser flash photolysis techniques. The triplet is efficiently quenched by substrates such as cyclohexane, benzhydrol, and diphenylmethane with the concomitant formation of a transient that shows an absorption maximum at 545 nm and that is assigned to the diphenylketyl radical. The triplet decays with exponential kinetics in the presence of these substrates, with rate constants that increase linearly with the hydrogen donor concentration. Rate constants for hydrogen abstraction of 3.9 x 10(14) and 4.4 x 10(14) dm2/(mol s) are measured for diphenylmethane and benzhydrol, respectively. These reactions are over an order of magnitude slower than energy transfer from triplet benzophenone to naphthalene, for which a rate constant of 6.4 x 10(15) dm2/(mol s) has been measured. The rate constants for decay of triplet benzophenone are also decreased at higher ketone loadings or in the presence of moderate amounts of inert substrates such as benzoic acid or phenyl benzoate.