Potential Gradient Effects on Electron Transfer Reactions Mediated by Quantum Capacitive States

This study investigates the effect of the potential gradient on electron transfer (ET) reactions occurring in redox-active monolayers self-assembled on metallic electrodes. Redox centers within the monolayer serve as quantum capacitive (Cq) energy levels (E = e2/Cq) that mediate ET from free redox couples (in the electrolyte) to the electrode. Different from diffusion-controlled ET reactions where the current collector forms a junction with the free redox couples, Cq-mediated ET reactions are diffusionless and hence exhibit quantized resistance limits of Rq ∝ h/e2 ∼ 25.8 kΩ that follows quantum electrodynamics principles. This diffusionless ET dynamics relies on the energy level alignment with the redox molecules of the monolayer and electrolyte environment that ultimately governs the ET rate dynamics with a rate constant of ν ∝ e2/hCq ∝ 1/RqCq. Changes in the acidity (inducing potential gradient) of the redox-active monolayer due to the deprotonation of free Fc–COOH redox-couple groups significantly impact the charging behavior of the E = e2/Cq states, thus affecting the energy alignment between anchored and free redox species. That the acidity of the monolayer affects Cq-mediated ET efficiency is a piece of valuable information for designing suitable energy alignment between man-made Cq interfaces and redox couples present in electrolytic solvents.