Controlling charge transport in single molecules using electrochemical gate.

We have studied charge transport through single molecules covalently bound to two gold electrodes in electrolytes by applying a voltage between the two electrodes and a reference electrode (gate). This electrochemical gating can effectively control the current through the molecules, depending on the electronic properties of the molecules. For electrochemically inactive molecules, such as 4,4'-bipyridine and 1,4'-benzenedithiol, the gate voltage influences the transport current only slightly (less than 30%). This lack of significant gate effect is attributed to the large LUMO-HOMO gaps of the molecules and the screening of the gate field by the two electrodes. For nitro-oligo(phenylene ethynylene) (OPE-NO2), which undergoes multiple irreversible reductions at negative gate voltages, the current through the molecules can be modulated several folds by the gate. This gate effect is irreversible and associated with the reduction of the NO2 group to different products that have different electron withdrawing capabilities from the conjugate backbone of the molecule. The most interesting molecules are perylene tetracarboxylic diimide compounds (PTCDI), which exhibit fully reversible redox reactions. The current through PTCDI can be reversibly varied and controlled over three orders of magnitude with the gate. Such a large gate effect is related to a redox state-mediated electron transport process.