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 -684mV to -1070mV vs. Ag/0.01m 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 (6.5Fcm(-2)) of Bi/MeCN,TBAPF(6) interfaces at the potential of zero charge (pzc=-0.60V) is at least an order of magnitude greater than that calculated on the basis of the bulk Bi carrier density (3x10(17)cm(-3)) and the differential capacitance (9.5Fcm(-2)) of Pt/MeCN interfaces at their pzc (-0.43V) is of the same order. The series of viologen derivatives exhibited simple one-electron redox behaviour and showed similar rate constants at Pt (1.8x10(-4)-1.6x10(-3)cms(-1)) and Bi electrodes (1.1x10(-4)-1.9x10(-3)cms(-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.