SOLVENT DYNAMICS EFFECTS ON HETEROGENEOUS ELECTRON-TRANSFER RATE CONSTANTS OF COBALT TRIS(BIPYRIDINE)

This paper presents microelectrode voltammetry-derived heterogeneous e lectron transfer kinetic rates k(ET) for the redox couple [Co(bpy)(3)] (2+/3+) in a series of solvents for which (tau(L)) longitudinal relaxa tion values are known (four polar monomeric solvents and four oligomer ic polyether solvents, CH3O-(CH2CH2O)(n)-CH3 where n = 1, 2, 3, and 4) and one, a higher oligomer (n = 8, MPEG-400), for which tau(L) is est imated. tau(L) ranges from 0.2 to 38 ps. The results show that k(ET) v aries inversely with tau(L), and according to other modes of analysis, as predicted for control of the energy barrier-crossing rate by the d ynamics of solvent dipolar relaxation. Additionally, the observed k(ET ) is proportional to the diffusion coefficient D-Co of [Co(bpy)(3)](2), which is rationalized by the mutual connection of D-Co and k(ET) to the solvent viscosity. D-Co, k(ET), and viscosity were also measured as a function of electrolyte concentration in MPEG-400 which allowed e xtension of the overall solvent viscosity range. The [Co(bpy)(3)](2+/3 +) rate constant in these media was also proportional to D-Co, indicat ing solvent dynamics control over a time scale range of ca. 500-fold, larger than any previously reported. Experiments at constant viscosity but varied electrolyte concentration demonstrated the absence of stro ng double layer or ion pairing influences on the reaction rate.