PHOTOINDUCED INTER-ION-PAIR AND INTRA-ION-PAIR ELECTRON-TRANSFER REACTIONS IN N-(P-BENZOYLBENZYL)-N,N,N-TRI-N-BUTYLAMMONIUM TRIPHENYL-N-BUTYLBORATE AND N-(P-BENZOYLBENZYL)-N,N,N-TRI-N-BUTYLAMMONIUM TRIPHENYL-N-GALLATE SALTS

The photoreduction of N-(p-benzoylbenzyl)-N,N,N-tri-n-butylammonium tr iphenyl-n-butylborate (1) and gallate (2) was studied using nano- and picosecond laser flash photolysis. An electron transfer reaction from the berate or the gallate counteranion to the excited triplet state of the benzophenone moiety (BP(3)) was demonstrated. This reaction lead s to the formation of benzophenone radical anion (6) and the boranyl r adical, 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 s) indicates an intramolecular process and that the compo unds exist as tight ion pairs. The addition of 1% MeCN increases the t riplet lifetime up to 1.2 ns. The formation of a solvent-separated ion pair was suggested in this solvent mixture. However, in the polar sol vent MeCN, 1 and 2 exist partially as free ions as determined by their dissociation constants. A diffusion-controlled intermolecular electro n transfer process was shown in this solvent. The p-benzoylbenzyl radi cal (BPCH2(.)), most likely formed by the C-N cleavage bond in the rad ical anion (6), was identified as an intermedate in the reaction pathw ay. After the initial electron transfer reaction, this produced tribut ylamine. The quantum yield obtained for the reaction of 1, 10(-2) M in MeCN, is 0.5 +/- 0.05. Triplet quenching as well as coupling of the r adical anion (6) with the butyl radical deriving from decomposition of the berate was suggested to compete with the C-N cleavage reaction.