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 inte rnal activation barriers. To render these ordinarily fast rates more e asily measurable, we have employed low-defect-density, highly ordered pyrolytic graphite (HOPG) as an electrode material (see: Allred and Mc Creery, Anal. Chem. 1992, 64, 444). At the HOPG/aqueous solution inter face, we observe the systematic (exponential) increase of k(s) with in verse reactant size predicted by Marcus for electrochemical reactions whose barriers are primarily defined by solvent reorganizational effec ts. We also observe that rates can be significantly accelerated by del ocalizing electrons over multiple metal-centered trapping sites. The d egree of rate acceleration is quantitatively consistent with the exten t of solvent barrier lowering expected if electronic delocalization ef fectively increases the radius of the ET reaction site.