Counterion diffusivity measurements support ion atmosphere relaxation control of electron transfer rates in a semi-solid ruthenium complex molten salt

Molten salts of ruthenium complexes [Ru(bpy(CO2MePEG350)2)3] [X]2 have been prepared in which the counterion X is either perchlorate ion or mixtures of perchlorate and iodide ions in mole ratios of either 8:2 or 7:3. Cyclic voltammetry in dilute, fluid solutions and in undiluted, semi-solid films of the iodide-containing molten salt shows two well-resolved iodide oxidation peaks and an anodic Ru(II/III) wave. The average diffusion coefficients of the counterions were obtained by ionic conductivity impedance measurements, while that of iodide (as a surrogate for perchlorate ion transport) was measured directly using iodide voltammetry. Agreement between the conductivity-based and Faradaic counterion transport data provides a quantitative validation of previous use of ionic conductivity data in study of the ion atmosphere relaxation model for electron transfer rate control in semi-solid redox hybrid polyether melts. A second purpose of this report is examination of the ion atmosphere relaxation model with the Ru complex melts. In this model, electron transfer rate constants are controlled by post-electron transfer relaxation of counterion distribution around the donor-acceptor pair - else back-transfer occurs. The electron transfer rate constants as a result reflect counterion diffusion rates, not their intrinsic values. The Ru(III/II) electron transfer rate constants in the [Ru(bpy(CO2MePEG350)2)3][X]2 melts indeed vary linearly with counterion diffusion coefficients in a manner consistent with ion atmosphere relaxation electron transfer rate control.