Ion Atmosphere Relaxation Control of Electron Transfer Dynamics in a Plasticized Carbon Dioxide Redox Polyether Melt

The sorption of CO₂ into the highly viscous, semisolid hybrid redox polyether melt, [Co(phenanthroline)₃](MePEG-SO₃)₂, where MePEG-SO₃ is a MW 350 polyether-tailed sulfonate anion, remarkably accelerates charge transport in this molten salt material. Electrochemical measurements show that as CO₂ pressure is increased from 0 to 800 psi (54 atm) at 23 °C, the physical diffusion coefficient D_PHYS of the Co(II) species, the rate constant k_EX for Co(II/I) electron self-exchange, and the physical diffusion coefficient of the counterion D_COUNTERION all increase, from 4.3 × 10^-10 to 6.4 × 10^-9 cm²/s, 4.1 × 10⁶ to 1.6 × 10⁷ M^-1 s^-1, and 3.3 × 10^-9 to 1.6 × 10^-8 cm²/s, respectively. Plots of log(k_EX) versus log(D_PHYS) and of log(k_EX) versus log(D_COUNTERION) are linear, showing that electron self-exchange rate constants are closely associated with processes that also govern D_PHYS and D_COUNTERION. Slopes of the plots are 0.68 and 0.98, respectively, indicating a better linear correlation between k_EX and D_COUNTERION. The evidence indicates that k_EX can be controlled by relaxation of the counterion atmosphere about the Co complexes in the semisolid redox polyether melts. Because the counterion relaxation is in turn controlled by polyether “solvent” fluctuations, this is a new form of solvent dynamics control of electron transfer.