Mechanism of the Direct and Triplet-sensitized Photolysis of N-(1-Naphthoyl)-O-(p-toluoyl)-N-phenylhydroxylamine

Abstract

Abstract Quantum yields (Φ) were determined for benzophenone-sensitized and direct photolysis of the title hydroxylamine (NT). Analysis of linear Stern-Volmer plots of the reciprocal of Φ against the reciprocal of NT concentration reveals that the sensitized decomposition of NT takes place via its first triplet excited state (T1) produced by triplet-triplet energy transfer from benzophenone to NT. The fact that the quantum yields for the direct photolysis are independent of NT concentration indicates no occurrence of both self-quenching and induced decomposition of NT by the photochemically generated radicals. Closer examination of the micellar and solvent viscosity effects on the quantum yields for the direct photolysis with 313-nm light shows the existence of at least two spin-correlated (singlet) radical pairs: One is a contact radical pair whose recombination eventually yields 1,3-toluoyloxyl-migrated product and the other a solvent-separated radical pair from which 1,5-toluoyloxyl-migrated product is derived in competition with diffusive escape from the solvent or micelle cage. Exclusive occurrence of the direct photolysis from the first singlet excited state (S1) was explained on the basis of the large energy gap between S1 and T1 (31 kcal mol−1) which is likely to make intersystem crossing from S1 to T1 very inefficient.