Photochemistry of substituted benzyldiphenylphosphine oxides

Photochemical reactions (lambda (irr) = 254 nm) of substituted benzyldiphen ylphosphine oxides la-e have been investigated in benzene and acetonitrile. alpha-Cleavage from the singlet excited state is proposed as the primary p rocess, and products formed both before and after escape of the primary int ermediates from the solvent cage result. Radicals 2 and 3 are observed by n anosecond flash photolysis following excitation of la at 266 nm in acetonit rile. Acetone sensitization of la,b fails to improve the efficiency of prod uct formation. The quantum yields of disappearance of la,b are unaffected b y O-2, and mechanisms for product formation from la in the presence of oxyg en are proposed.