Solvent Dynamics Effects on Heterogeneous Electron Transfer Rate Constants of Cobalt Tris(bipyridine)

This paper presents microelectrode voltammetry-derived heterogeneous electron transfer kinetic rates k ET for the redox couple [Co(bpy) 3 ] 2+/3+ in a series of solvents for which (τ L ) longitudinal relaxation values are known (four polar monomeric solvents and four oligomeric polyether solvents, CH 3 O−(CH 2 CH 2 O) n −CH 3 where n = 1, 2, 3, and 4) and one, a higher oligomer ( n = 8, MPEG-400), for which τ L is estimated. τ L ranges from 0.2 to 38 ps. The results show that k ET varies inversely with τ L, and according to other modes of analysis, as predicted for control of the energy barrier-crossing rate by the dynamics 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 extension of the overall solvent viscosity range. The [Co(bpy) 3 ] 2+/3+ rate constant in these media was also proportional to D Co, indicating 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 strong double layer or ion pairing influences on the reaction rate.