Photoinduced Electron-Transfer Reactions:  Highly Efficient Cleavage of C−N Bonds and Photogeneration of Tertiary Amines

Abstract

The photocleavage of tertiary amine from tetraphenylborates complexed, via ammonium ions, to various chromophore electron acceptors has been studied by laser-flash photolysis. Electron transfer from the borate anions to acceptor excited states is determined to be the primary photochemical step leading to homolytic C−N bond scission and concomitant formation of the corresponding tertiary amine. This pathway was established by direct observation of p -benzoylbenzyl ( 7a ), p -acetylbenzyl ( 7b ), β-naphthylmethyl ( 7c ), and 7-methoxycoumarin-4-methyl ( 7d ) after laser-flash photolysis from N -(4-benzoyl)benzyl- N, N, N -tributylammonium tetraphenylborate ( 1a ), N -(4-acetyl)benzyl- N, N, N -trimethylammonium tetraphenylborate ( 1b ) N, N, N -tributyl- N -(2-methylnaphthalene)ammonium tetraphenylborate ( 1c ), and N, N, N -tributyl- N -(4-methyl-7-methoxycoumarin)ammonium tetraphenylborate ( 1d ), respectively. The presence of radicals ( 7a − 7d ) was also suggested by the reaction products. Comparison of the rate constants for quenching of the triplet states from excited ( 1a, 1b ) and singlet states from ( 1c, 1d ) reveals the efficiency of C−N bond cleavage. Quenching constants in the range of 10 9 −10 12 M -1 s -1 were observed. Differences are accounted for in terms of variations in the electron-accepting excited states and structural characteristics of the chromophore acceptors.