Fabrication and transport properties of single-molecule-thick electrochemical junctions

A V-shaped compound incorporating two bipyridinium units, which emanate from a central hydrophilic core and bear hydrophobic tetraarylmethane-based stoppers at each end, was designed and synthesized. In a thermodynamically controlled self-assembly process in solution, either one or two 1,4-dioxybenzene-based macrocyclic polyethers can be slipped over the bulky stoppers of the V-shaped compound, affording either a [2]rotaxane or a [3]rotaxane, respectively. The parent V-shaped compound and the two rotaxanes incorporate two redox-active bipyridinium units that can be reduced reversibly and two redox-active phenoxy groups in the stoppers that can be oxidized irreversibly. Furthermore, these three compounds have amphiphilic character and, as a result, form stable monolayers at the air/water interface. Langmuir-Blodgett monolayers of these compounds were sandwiched between two electrodes to afford molecule-based solid-state switches. In forward bias mode, the I-V characteristics of the junction are reversible, but upon application of a sufficient reverse bias the junction resistance is irreversibly decreased, thereby switching the device. As a result, the current flowing through the device at forward bias voltages is lowered by a factor of 60-80. The behavior of the solid-state devices can be interpreted on the basis of the redox properties determined in solution for the three compounds. Initially, current flow at forward bias is determined by resonant tunneling through the molecular LUMO states associated with the bipyridium units. The irreversible decrease in current that occurs at reverse biases suggests a similarity to the solution-phase oxidation of the phenoxy groups.