Self-sensitized photolysis of N-(1-naphthoyl)-N-phenyl-O-(benzoyl-substituted benzoyl)hydroxylamines

Article abstract of DOI:10.1246/bcsj.69.2603

The mechanism of intramolecular triplet-triplet (T-T) energy transfer and subsequent reaction in N,O-diacylhydroxylamines was investigated using the model compounds N-( 1 -naphthoyl)-N-phenyl-O-(benzoyl-substituted benzoyl)hydroxylamines (NPB) with self-sensitization abilities. An examination of the UV absorption and phosphorescence behavior as well as of energy-minimized conformations of these relatively flexible model compounds established that T-T energy transfer from the benzophenone chromophore to the naphthoyl chromophore occurs in a nearly unit efficiency exhibiting only phosphorescence derived from the latter chromophore in both methanol-ethanol (1 : 1 v/v) and 2-chlorobutane at 77 K and is more likely to proceed by a "through-space" mechanism than by a "through-bond" mechanism. The self-sensitized photolysis of NPB with 366 nm light in methanol at room temperature was found to give the fragmentation products, N-phenyl-1-naphthalenecarboxamide (PNA), benzophenone (BP), and benzoyl-substituted benzoic acids (BBA), whereas no BBA was detected in the photolysis in 1,2-dichloroethane and acetonitrile. The finding that the reaction of NPB is efficiently quenched by trans-stilbene according to the Stern-Volmer equation in both methanol and 1,2-dichloroethane indicates that all the products come from the first excited triplet state of the naphthoyl chromophore. On the other hand, the enhanced hydrogen bonding ability of the medium resulted in an increase in the quantum yield for the formation of BBA (Φ_BBA) accompanied by a decrease in Φ_BA holding the magnitude of Φ_BBA+Φ_BA nearly constant. But neither Φ_PNA nor the quantum yield for the disappearance of NPB was subject to such a hydrogen-bonding effect. This intriguing result was explained in terms of a mechanism in which the N-O bond cleavage in triplet NPB gives a vibrationally excited triplet radical pair whose relaxation is very slow compared to decarboxylation of the caged benzoyl-substituted benzoyloxyl radical in 1,2-dichloroethane. Solvation of this vibrationally hot radical pair through hydrogen bonding substantially promotes its relaxation eventually affording BBA.