UC San Diego

Investigations into the dynamics of picosecond electron transfer in a series of mixed valence systems of the type [Ru₃([mu]₃-O)(OAc)₆(py)(CO)-([mu]₂-BL)- Ru₃([mu]₃- O)(OAc)₆(py)(CO)]⁻¹, where BL = 1,4-pyrazine or 4,4'- bipyridine and py = 4-dimethylaminopyridine, pyridine, or 4-cyanopyridine are described. Solvent and temperature dependence into the rate of ground state intramolecular electron transfer is probed by infrared analysis of [nu](CO) bandshapes where simulated rate constants yield to rates ranging from 4 E 11 to 3 E 12 s-1. Correlations between rate constants and solvent properties including solvent reorganization energy, optical and static dielectric constants, microscopic solvent polarity, viscosity, principal rotational moments of inertia, and solvent dipolar relaxation times, have been examined. Correlations revealed a marked lack of dependence on electron transfer rates with respect to solvent thermodynamic parameters, and a strong dependence on solvent dynamic parameters. This is consistent with electron transfers having very low activation barriers that approach zero. Temperature dependent studies revealed electron transfer rates accelerated as the freezing points of solvent solutions were approached with a sharp increase in the rate of electron transfer upon freezing. This has been attributed to a localized-to-delocalized transition in these mixed valence ions at the solvent phase transition. This non-Arrhenius behavior is explained in terms of decoupling the slower solvent motions involved in the frequency factor, [nu]N, which weights faster vibrational promoter modes that increase the value of [nu]N. Solvent and temperature dependence of optically induced intramolecular electron transfer is probed by analysis of intervalence charge transfer bands in NIR spectra. The application of a semi-classical three-state model for mixed valency best describes the electronic spectra wherein is the appearance of two intervalence bands; a band which has metal-to-metal-charge-transfer character and another having metal-to-ligand-charge- transfer character. This three-state model fully captures the observed spectroscopic behavior where the MBCT transition increases in energy and the MMCT band decreases in energy as electronic communication increases through the series of mixed valence ions. The solvent and temperature dependence of the MBCT and MMCT electronic transitions is found to persist as coalescence of infrared vibrational spectra suggest ground state delocalization on the vibrational timescale. The solvent and temperature dependence of the MBCT and MMCT electronic transitions defines the mixed valence complexes as lying at the borderline of delocalization. Fine tuning the electronic coupling in the series of dimers has allowed for the resolution of a full Class II, early Class II/III, late Class II/III to Class III systems and the influence of solvent dynamics in each regime. These investigations have prompted the redefinition of borderline Class II/III mixed valency to account for outer sphere (solvent) contributions to electron transfer; in nearly delocalized systems, solvent dynamics localized otherwise delocalized electronic ground states. Further, studies explore the origins and dynamics behind spectral coalescence of vibrational [nu] (CO) bandshapes in [Ru₃([mu]₃- O)(OAc)₆(py)(CO)-([mu]₂-BL)- Ru₃([mu]₃-O)(OAc)₆(py)(CO)]⁻¹ systems and a picosecond isomerization in square pyramidal Ru(S₂C₄F₆)(P(C₆H₅)₃)₂(CO) system