Photoinduced Inter- and Intra-Ion-Pair Electron Transfer Reactions in N-(p-Benzoylbenzyl)-N,N,N-tri-n-butylammonium Triphenyl-n-butylborate and -Gallate Salts

Abstract

The photoreduction of N-(p-benzoylbenzyl)-N,N,N-tri-n-butylammonium triphenyl-n-butylborate (1) and gallate (2) was studied using nano- and picosecond laser flash photolysis. An electron transfer reaction from the borate or the gallate counteranion to the excited triplet state of the benzophenone moiety (BP*3) was demonstrated. This reaction leads to the formation of benzophenone radical anion (6) and the boranyl radical, the latter of which dissociates rapidly to form butyl radical. The electron transfer rate was found to depend on the polarity of the solvent. In neat benzene, the short lifetime of the triplet obtained (300 ± 150 ps) indicates an intramolecular process and that the compounds exist as tight ion pairs. The addition of 1% MeCN increases the triplet lifetime up to 1.2 ns. The formation of a solvent-separated ion pair was suggested in this solvent mixture. However, in the polar solvent MeCN, 1 and 2 exist partially as free ions as determined by their dissociation constants. A diffusion-contro...