Conductance of redox-active single molecular junctions: an electrochemical approach

The conductance of molecular junctions formed of N, N'-bis(n-thioalkyl)4,4'-bipyridinium bromides or alkanedithiols between a gold (Au) scanning tunnelling microscope tip and a Au(111)-(1x1) electrode has been studied at electrified solid/liquid interfaces. A statistical analysis based on large sets of individual current - distance traces was applied to obtain the electrical conductance of single junctions. The one-electron reduction of the viologen moiety from the dication V2+ to the radical cation state V+. gives rise to a 50% increase of the junction conductance. Increasing the length of the alkyl spacer units leads to a tunnelling decay constant ss(CH2) = 5.9-6.1 nm(-1). This value is significantly lower than ss(CH2) = 8.2 nm(-1) estimated for molecular junctions of alkanedithiols. The difference is attributed to conformational changes within the two junctions. The contact conductance was estimated to 10 mu S.