Conduction properties of bipyridinium-functionalized molecular wires.

We present a detailed analysis of the coherent electron transport through a redox-active, 4,4'-bipyridinium (viologen)-functionalized molecular wire, which was studied in several recent experiments. Our calculations for the bare viologen predict conductances differing by 2 orders of magnitude depending on the contact geometry. For the alkyl-wired viologen unit, we obtain an exponential decay of the conductance with the wire length. Because this exponent also governs the conductance in the incoherent transport regime, comparison with experiments is legitimate and we find a good agreement. Furthermore, our calculations indicate that the experimentally observed conductance switching behavior is not amenable to an explanation inside a coherent transport picture. A possible incoherent mechanism is being discussed.