Energy- and Electron-Transfer Pathways in the Triplet-Sensitized Photolysis of N-(1-Naphthoyl)-O-(p-toluoyl)-N-phenylhydroxylamine with Substituted Benzophenones

Article abstract of DOI:10.1246/bcsj.65.1950

Quantum-yield and thermodynamic analyses of the triplet-sensitized photolysis of the title hydroxylamine (NT, 1) with 4,4'-bis(dimethylamino)benzophenone (BAB), 4-dimethylaminobenzophenone (DAB), 4,4'-dimethoxybenzophenone (DMB), and 3,3'-bis(trifluoromethyl)benzophenone (BFB) suggest that the BAB- and DAB-sensitized reactions proceed by a preferential electron-transfer pathway within a triplet-exciplex intermediate, formed between the triplet-state sensitizer and the ground-state NT, to give N-phenyl-1-naphthalenecarboxamide (2) and p-toluic acid (3).On the other hand, the energy transfer pathway in this exciplex intermediate, eventually forming 2 and 3, as well as toluene (4), predominates for reactions sensitized with DMB and BFB having fewer electron-donating substituents than does the dimethylamino group of BAB and DAB.The finding that benzil, both excited singlet and triplet states of which lie below those of NT, brings about a sensitized photolysis to afford the same fragmentation products, provides strong evidence for the existence of a triplet-exciplex intermediate.In addition, evidence concerning the operation of an electron-transfer mechanism in our sensitized photolysis comes from an observation that the photolysis of NT sensitized with N,N,N',N'-tetramethylbenzidine, the excited state of which is one of the most powerful one-electron reductants, gives the same product distribution as does that obtained by the BAB- and DAB-sensitized reactions.