Electron transfer rates at Bi and Pt of viologens

The standard heterogeneous rate constants for the reduction of a series of viologen derivatives with a range of inter–ring torsion an- gles 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/0.01M 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 (6.5 μF cm−2 ) of Bi/MeCN,TBAPF6 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 (3 × 1017 cm−3 ) and the differential capacitance (9.5 μF cm−2) of Pt/MeCN interfaces at their pzc (-0.43V) is of the same order. The series of vi- ologen 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 den- sity 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.