ELECTRON-TRANSFER REACTIONS OF RUTHENIUM TRISBIPYRIDYL-VIOLOGEN DONOR-ACCEPTOR MOLECULES - COMPARISON OF THE DISTANCE DEPENDENCE OF ELECTRON-TRANSFER RATES IN THE NORMAL AND MARCUS INVERTED REGIONS

The rates of photoinduced forward and thermal back electron transfer (ET) in a series of donor-acceptor molecules (2, 2'-bipyridine)(2)Ru(4-CH3-2,2'-bipyridine-4') (CH2),(4,4'-bipyridinium-CH3)4+ (n = 1-5, 7, 8) were studied by flash photolysis/transient absorbance techniques. The rate of intramolecular forward ET (MLCT quenching) in acetonitrile varies exponentially with the number of carbon atoms in the spacer chain up to n = 5 and is roughly constant for n = 5, 7, 8, consistent with a predominantly ''through bond'' electron transfer pathway for short chains and a ''through solvent'' pathway for longer chains. Encapsulation of the spacer chain by beta-cyclodextrin molecules slows the rate of forward ET for n = 7, 8, consistent with a ''through bond'' ET pathway. The rate of back ET, which occurs in the Marcus inverted region, also varies exponentially with n, but more weakly than the forward ET rates. Apparent beta values (defined by k(ET) = A exp(-beta r(DA)), where r(DA) is the donor-acceptor distance) are 1.38 and 0.66 Angstrom(-1) for forward and back ET, respectively. However, correction of k(ET) for the distance dependence of the solvent reorganization energy gives similar values (1.0-1.2 Angstrom(-1)) of beta for the two ET reactions. In this case, beta describes the distance dependence of V/(2) (V = electronic coupling matrix element) rather than that of k(ET).