INTERMOLECULAR AND INTRAMOLECULAR OXIDATIVE QUENCHING OF MIXED-LIGAND TRIS(BIPYRIDYL)RUTHENIUM(II) COMPLEXES BY METHYL VIOLOGEN

The rate constants for photoinduced electron transfer, as well as thermal charge recombination, were measured in a series of [(4,4'-R(2)-2,2'-bipyridine)(2)Ru-II(4'-(CH3)-2,2'-bipyridine-4- (CONHR'))] (R' = (CH2)(x)MV(2+)) systems, in which a tris(bipyridyl)ruthenium(II) chromophore was covalently linked to a 4,4'-bipyridinium (MV(2+)) electron acceptor. The nature of R(R = H, CH3, COO-, COOH, CONHCH(CH3)2) and the number (x = 2, 3) of intervening methylene units were varied to tune the chromophore's electronic properties, including the pi* orbital energies of the 4,4'-R(2)-2,2'-bipyridine ligands and donor-acceptor separation distance, respectively. For a given donor-acceptor distance, x, and similar driving force, the rate constants for forward electron transfer were nearly 60 (x = 3) to 400 (x = 2) times smaller in complexes in which the two 4,4'-R(2)-2,2'-bipyridine ligands were R-substituted with electron-withdrawing functional groups (R = CONHCH(CH3)(2)) Charge recombination from the reduced viologen acceptor to the oxidized metal center occurs in the Marcus inverted region, with the rate constants (k(b)) decreasing with increasing magnitude of driving force. The kinetics of the bimolecular oxidative quenching of the electronically excited state of these mixed ligand tris(bipyridyl)ruthenium(II) complexes (R' = CH(CH3)(2)) by methyl viologen was also characterized in homogeneous aqueous solution, and the escape efficiencies were measured for separation of the redox products from the solvent cage.