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[prime]-bipyridine)[sub 2]Ru(4-CH[sub 3]-2,2[prime]-bipyridine-4[prime]) (CH[sub 2])[sub n](4,4[prime]-bipyridinium-CH[sub 3])[sup 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 [open quotes]through bond[close quotes] electron transfer pathway for short chains and a [open quotes]through solvent[close quotes] 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 [open quotes]through bond[open quotes] 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[sub et] = A exp(-[beta]r[sub DA]), where r[sub DA] is the donor-acceptor distance) are 1.38 and 0.66 [angstrom][sup [minus]1] for forward and back ET, respectively. However, correction of k[sub ET] for the distance dependence of the solvent reorganization energy gives similar values (1.0-1.2 [angstrom][sup [minus]1]) ofmore » [beta] for the two ET reactions. In this case, [beta] describes the distance dependence of [vert bar]V[vert bar][sup 2] (V = electronic coupling matrix element) rather than that of k[sub ET]. 44 refs., 9 figs., 3 tabs.« less