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 phot ochemical step leading to homolytic C-N bond scission and concomitant formation of the corresponding tertiary amine. This pathway was establ ished by direct observation of p-benzoylbenzyl (7a), p-acetylbenzyl (7 b), beta-naphthylmethyl (7c), and 7-methoxycoumarin-4-methyl (7d) afte r laser-flash photolysis from N-(4-benzoyl)benzyl-N,N,N-tributylammoni um tetraphenylborate (1a), N-(4-acetyl)benzyl-N,N,N-trimethylammonium tetraphenylborate (1b) N,N,N-tributyl-N-(2-methylnaphthalene)ammonium tetraphenylborate (1c), and ,N-tributyl-N-(4-methyl-7-methoxycoumarin) ammonium tetraphenylborate (1d), respectively. The presence of radical s (7a-7d) was also suggested by the reaction products. Comparison of t he rate constants for quenching of the tripler states from excited (1a , 1b) and singlet states from (1c, 1d) reveals the efficiency of C-N b ond 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 characteristi cs of the chromophore accepters.