Probing the Limits: Ultraslow Diffusion and Heterogeneous Electron Transfers in Redox Polyether Hybrid Cobalt Bipyridine Molten Salts

This paper describes microelectrode voltammetry measurements of serf-diffusion coefficients and of heterogeneous Co(II/III) electron-transfer rate constants (k(super o)) in undiluted molten salts of three cobalt tris(bipyridine) perchlorate complexes in which the bipyridine ligands are 'tailed' with poly(propylene oxide) and poly(ethylene oxide) oligomers. The self-diffusion coefficients are measured with potential step chronoamperometry and range from 10(super-12) to 10(super -17) cm(super 2)/s, while the quasi-reversible reaction rate constants are measured using cyclic voltammetry and small potential steps and range from 10(super -7) to 10(super -12) cm/s. The k(super o) measurements are unusual in that when rate constants become smaller, the reaction remains quasi-reversible, because of concurrently decreasing self-diffusion rates. The measurements are, furthermore, accomplished in the face of uncompensated resistances that range from mega- to gigaohms, which is made possible by the combination of microelectrode properties and small diffusivities. The melt in which serf-diffusion and k(super o) values are smallest is at a temperature below its nominal glassing transition and in the regime of molecule-scale diffusion profiles.