Electron transfer within ruthenium(II) polypyridyl-(salt bridge)dimethylaniline acceptor-donor complexes

The kinetics of electron transfer from N,N'-dimethylaniline to electronically excited Ru(II) polypyridyl acceptor complexes through an amidinium-carboxylate salt bridge have been investigated. Acceptor-(salt bridge)-donor complexes are formed by the 1:1 association of [(4,4'-X-2,2'-bipyridine)(2)Ru-II(Mebpy-amH(+))](3+) and [(4,4'-X-2,2'-bipyridine)(2)Ru-II(Mebpy-COO-)](+) (X = diethylcarboxy, X = H, Mebpy-amH(+) = 4-methyl-4'-amidinium-2,2'-bipyridine, Mebpy-COO- = 4-methyl-4'-carboxylate-2,2'-bipyridine) to N,N'-dimethylaminobenzoate (DMA-COO-) and N,N'-dimethylaminobenzamidinium (DMA-amH(+)), respectively. The design of the excited state structure of the Ru(II) polypyridyl complex is crucial to a proper kinetics study of the electron transfer reaction through the salt bridge of these complexes. The reaction is photoinitiated by the metal-to-ligand charge (MLCT) transfer excitation. Ester modification of the ancillary bipyridine ligands results in an MLCT excitation that places the electron on the ligands remote to the salt bridge, clearing the way for very fast electron transfer from DMA into the Ru(II) center via the salt bridge. We observe facile electron transfer when the Ru(II) complex is attached to the amidinium-side of the salt bridge. This behavior is opposite to that previously observed by us for the oxidative quenching of Ru(II) polypyridyl complexes joined to accepters via the same salt bridge; in this case, electron transfer out of the Ru(II) complex is fast. These results suggest that the quenching mechanism of these types of Ru(II) polypyridyl assemblies is dominated by the orientation of the salt bridge relative to the direction of electron transport. (C) 1997 Elsevier Science S.A.