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)₂Ru(4-CH₃-2,2'-bipyridine-4')(CH₂)_n(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 β-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 β values (defined by k_ET = A exp(-βr_DA), where r_DA is the donor-acceptor distance) are 1.38 and 0.66 Å^-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 Å^-1) of β for the two ET reactions. In this case, β describes the distance dependence of |V|² (V = electronic coupling matrix element) rather than that of k_ET.