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

A V-shaped compound incorporating two bipyridinium units, which emanate fro m a central hydrophilic core and bear hydrophobic tetraarylmethane-based st oppers at each end, was designed and synthesized. In a thermodynamically co ntrolled self-assembly process in solution, either one or two 1,4-dioxybenz ene-based macrocyclic polyethers can be slipped over the bulky stoppers of the V-shaped compound, affording either a [2]rotaxane or a [3]rotaxane, res pectively. The parent V-shaped compound and the two rotaxanes incorporate t wo redox-active bipyridinium units that can be reduced reversibly and two r edox-active phenoxy groups in the stoppers that can be oxidized irreversibl y. Furthermore, these three compounds have amphiphilic character and, as a result, form stable monolayers at the air/water interface. Langmuir-Blodget t monolayers of these compounds were sandwiched between two electrodes to a fford molecule-based solid-state switches. In forward bias mode, the I-V ch aracteristics of the junction are reversible, but upon application of a suf ficient reverse bias the junction resistance is irreversibly decreased, the reby switching the device. As a result, the current flowing through the dev ice 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 pro perties determined in solution for the three compounds. Initially, current flow at forward bias is determined by resonant tunneling through the molecu lar LUMO states associated with the bipyridium units. The irreversible decr ease in current that occurs at reverse biases suggests a similarity to the solution-phase oxidation of the phenoxy groups.