Mechanistic studies of photobase generation from ammonium tetraorganyl borate salts

The photochemistry and photophysics of N,N,N-tributyl-N-acetonaphthone ammo nium berate (1) and N,N,N-tributyl-N-acetobenzo[b]furan ammonium berate (2) have been investigated by steady-state spectroscopy, laser flash photolysi s, 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 t he 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 photoly sis. The quantum yield of disappearance was found to be higher for salts of the triphenylbutyl berate anion than for those of the tetraphenyl berate a nion. A proposed mechanism involves electron transfer from berate to the ex cited accepters as the primary photochemical step. The rate constants for e xcited-state quenching correlate with the free energy for electron transfer , as estimated from the excited-state energies and the reduction potentials of the acceptors.