Tuning Single-Molecule Conductance in Metalloporphyrin-Based Wires via Supramolecular Interactions.

Abstract

Nature has developed amazing supramolecular constructs to deliver outstanding charge transport capabilities using metalloporphyrin-based supramolecular stacks.1 Here we are incorporating simple, naturally inspired supramolecular interactions via the axial complexation of metalloporphyrins into the formation of a single-molecule wire in a nanoscale gap to dissect the resulting electron pathways through the final chemical adduct. We observe that small structural changes in the axial coordinating linkers result in dramatic changes in the transport properties through the metalloporphyrin-based wire. The increased flexibility of a pyridine-4-yl-methanethiol ligand due to an extra methyl group as compared to a more rigid mercaptopyridine linker allows the former to adopt an unexpected highly conductive stacked structure between the two junction electrodes and the metalloporphyrin ring. DFT calculations reveal a molecular junction structure composed of a shifted stack of the three molecular backbones; the two pyridine ligands sandwiching the metalloporphyrin ring, which is stabilized by a combination of the porphyrin metal center coordinating the pyridinic N and the pyridine/porphyrin overlapping. Contrarily, the more rigid 4-mercaptopyridine ligand presents a more expected octahedral coordination of the metalloporphyrin metal center, leading to much lower conductance. Furthermore, we show that a mechanical forced imposed along the molecular wire axis results in a variety of more extended supramolecular structures between the pyridine linkers and the porphyrin ring spanning the tunneling gap and scoring relatively high conductance values. This works sets an example of the use of supramolecular chemistry in the construction of efficient molecular conduits towards the development of supramolecular electronics, a concept already exploited in natural organisms.