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)-(1 × 1) 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 V 2+ to the radical cation state gives rise to a 50% increase of the junction conductance. Increasing the length of the alkyl spacer units leads to a tunnelling decay constant β CH2 = 5.9–6.1 nm −1 . This value is significantly lower than β 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 µS.