Mechanistic Studies of Photobase Generation from Ammonium Tetraorganyl Borate Salts¹

Abstract

The photochemistry and photophysics of N,N,N -tributyl- N -acetonaphthone ammonium borate ( 1 ) and N,N,N -tributyl- N -acetobenzo[ b ]furan ammonium borate ( 2 ) have been investigated by steady-state spectroscopy, laser flash photolysis, and product analysis. Reaction of excited precursors leads to formation of tertiary amines by carbon−nitrogen bond cleavage. Laser flash photolysis and product analysis confirm homolytic carbon−nitrogen bond scission and the formation of respective acetyl radicals which, in acetonitrile, dimerize to form coupled products. In all cases, photodissociation occurs with such a short lifetime that it could not be measured by nanosecond flash photolysis. The quantum yield of disappearance was found to be higher for salts of the triphenylbutyl borate anion than for those of the tetraphenyl borate anion. A proposed mechanism involves electron transfer from borate to the excited acceptors as the primary photochemical step. The rate constants for excited-state quenching correlate with the free energy for electron transfer, as estimated from the excited-state energies and the reduction potentials of the acceptors.