ELECTRON-TRANSFER WITHIN RUTHENIUM(II) POLYPYRIDYL-(SALT BRIDGE)DIMETHYLANILINE ACCEPTOR-DONOR COMPLEXES

The kinetics of electron transfer from N,N'-dimethylaniline to electro nically excited Ru(II) polypyridyl acceptor complexes through an amidi nium-carboxylate salt bridge have been investigated. Acceptor-(salt br idge)-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-m ethyl-4'-amidinium-2,2'-bipyridine, Mebpy-COO- = 4-methyl-4'-carboxyla te-2,2'-bipyridine) to N,N'-dimethylaminobenzoate (DMA-COO-) and N,N'- dimethylaminobenzamidinium (DMA-amH(+)), respectively. The design of t he excited state structure of the Ru(II) polypyridyl complex is crucia l 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 modifica tion of the ancillary bipyridine ligands results in an MLCT excitation that places the electron on the ligands remote to the salt bridge, cl earing 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 brid ge. This behavior is opposite to that previously observed by us for th e oxidative quenching of Ru(II) polypyridyl complexes joined to accept ers via the same salt bridge; in this case, electron transfer out of t he Ru(II) complex is fast. These results suggest that the quenching me chanism of these types of Ru(II) polypyridyl assemblies is dominated b y the orientation of the salt bridge relative to the direction of elec tron transport. (C) 1997 Elsevier Science S.A.