Solvent dynamical control of ultrafast ground state electron transfer: Implications for class II-III mixed valency

We relate the solvent and temperature dependence of the rates of intramolecular electron transfer (ET) of mixed valence complexes of the type {[Ru₃O(OAc)₆(CO)(L)]₂-BL}^-1, where L = pyridyl ligand and BL = pyrazine. Complexes were reduced chemically or electrochemically to obtain the mixed valence anions in seven solvents: acetonitrile, methylene chloride, dimethylformamide, tetrahydrofuran, dimethylsulfoxide, chloroform, and hexamethylphosphoramide. Rate constants for intramolecular ET were estimated by simulating the observed degree of ν(CO) IR band shape coalescence in the mixed valence state. Correlations between rate constants for ET and solvent properties including static dielectric constant, optical dielectric constant, the quantity 1/ε_op - 1/ε_S, microscopic solvent polarity, viscosity, cardinal rotational moments of inertia, and solvent relaxation times were examined. In the temperature study, the complexes displayed a sharp increase in the k _et as the freezing points of the solvents methylene chloride and acetonitrile were approached. The solvent phase transition causes a localized-to-delocalized transition in the mixed valence ions and an acceleration in the rate of ET. This is explained in terms of decoupling the slower solvent motions involved in the frequency factor ν_N which increases the value of ν_N. The observed solvent and temperature dependence of the k_et for these complexes is used in order to formulate a new definition for Robin-Day class II-III mixed valence compounds. Specifically, it is proposed that class II-III compounds are those for which thermodynamic properties of the solvent exert no control over k_et, but the dynamic properties of the solvent still influence k_et.