Heterogeneous Electron‐Transfer Rates for the Reduction of Viologen Derivatives at Platinum and Bismuth Electrodes in Acetonitrile

The standard heterogeneous rate constants for the reduction of a series of viologen derivatives with a range of inter-ring torsion angles were measured at Bi and Pt electrodes. The electrode potentials for the first one-electron reduction of the viologens vary from -684 mV to -1070 mV vs. Ag/0.01 m Ag⁺; this enabled a comparison of the behaviour of metallic (Pt) and semi-metallic (Bi) electrodes over a wide range of applied potentials. The differential capacitance (6.5 μF cm⁻²) of Bi/MeCN,TBAPF₆ interfaces at the potential of zero charge (pzc=-0.60 V) is at least an order of magnitude greater than that calculated on the basis of the bulk Bi carrier density (3×10¹⁷ cm⁻³) and the differential capacitance (9.5 μF cm⁻²) of Pt/MeCN interfaces at their pzc (-0.43 V) is of the same order. The series of viologen derivatives exhibited simple one-electron redox behaviour and showed similar rate constants at Pt (1.8×10⁻⁴-1.6×10⁻³ cm s⁻¹) and Bi electrodes (1.1×10⁻⁴-1.9×10⁻³ cm s⁻¹) after application of the Frumkin correction. These results demonstrate that the density of states at the Bi surface is much higher than in bulk. Finally, the Frumkin-corrected standard rate constants were observed to be inversely correlated with the inter-ring torsion angle of the viologens.