Photoinduced electron-transfer reactions: Highly efficient cleavage of C-N bonds and photogeneration of tertiary amines

The photocleavage of tertiary amine from tetraphenylborates complexed, via ammonium ions, to various chromophore electron accepters has been studied by laser-flash photolysis. Electron transfer from the berate 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), beta-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 tripler 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 accepters.