Solvational barriers to interfacial electron transfer: Minimization via valence delocalization

Standard rate constants (k_s) for interfacial electron transfer (ET) have been obtained for several redox couples featuring very small internal activation barriers. To render these ordinarily fast rates more easily measurable, we have employed low-defect-density, highly ordered pyrolytic graphite (HOPG) as an electrode material (see: Allred and McCreery, Anal. Chem. 1992, 64, 444). At the HOPG/aqueous solution interface, we observe the systematic (exponential) increase of k_s with inverse reactant size predicted by Marcus for electrochemical reactions whose barriers are primarily defined by solvent reorganizational effects. We also observe that rates can be significantly accelerated by delocalizing electrons over multiple metal-centered trapping sites. The degree of rate acceleration is quantitatively consistent with the extent of solvent barrier lowering expected if electronic delocalization effectively increases the radius of the ET reaction site.