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

Abstract

Nature has developed supramolecular constructs to deliver outstanding charge-transport capabilities using metalloporphyrin-based supramolecular arrays. Herein we incorporate simple, naturally inspired supramolecular interactions via the axial complexation of metalloporphyrins into the formation of a single-molecule wire in a nanoscale gap. Small structural changes in the axial coordinating linkers result in dramatic changes in the transport properties of 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 4-pyridinethiol linker, allows the pyridine-4-yl-methanethiol ligand 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 two pyridinic linkers and the metalloporphyrin ring. In contrast, the more rigid 4-mercaptopyridine ligand presents a more classical lifted octahedral coordination of the metalloporphyrin metal center, leading to a longer electron pathway of lower conductance. This works opens to supramolecular electronics, a concept already exploited in natural organisms.

Keywords: biomolecular electronics; density functional calculations; metalloporphyrins; single-molecule junctions; supramolecular electronics.

Figure 1

Schematic representation of the supramolecular architecture used to form metalloporphyrin molecular junctions in a STM tunneling gap using pyridine‐4‐yl‐methanethiol (PyrMT) and 4‐pyridinethiol (PyrT) linkers.

Figure 2

1D semi‐log conductance histograms of the Co‐DPP junctions using PyrMT (a) and PyrT (b) functionalized junction electrodes. The most probable conductance values are extracted from Gaussian fits of the peaks. The insets show representative individual current traces used to build the 1D histograms (see corresponding 2D histograms in Supporting Information (SI) Section S2). Feature I shows strong correlation (consecutive appearance) with both features II and III (see extended discussion on junction dynamics in SI Section S2 and S3). Counts have been normalized versus the total amount of counts. The applied bias voltage was set to +7.5 mV.

Figure 3

DFT‐optimized structures for the PyrMT (a) and PyrT (b) ligands adsorbed on Au(111). c) Schematic representation of both lying down and lifted Co‐DPP/linkers (PyrMT and PyrT) geometries, together with thicknesses determined from theoretical cases (solid lines) and ellipsometry values (stripped lines). Optimized full junction structures ascribed to the highest conductance signatures labelled as I in Figure 2 a, b, for the PyrMT (d) and PyrT (e) Co‐DPP/linker systems.

Figure 4

1D semi‐log conductance histograms of the Co‐P/PyrMT (a), P/PyrMT (b), Co‐P/PyrT (c), and P/PyrT (d) systems. The most probable conductance values are extracted from Gaussian fits of the peaks. The insets show representative individual current traces used to build the 1D histograms. Counts have been normalized versus the total amount of counts. The applied bias voltages were set to +7.5 mV (a,b,d) and +15 mV (c).

Figure 5

DFT‐optimized structures of the low conductance signatures for the Co‐DPP/PyrMT system a) labelled as II in Figure 2 a and b) labelled III in Figure 2 a c) the lowest conductance signature for the Co‐P/PyrMT system labelled as II in Figure 4 a, and d) two equally probable configurations for the lowest conductance signature of the Co‐DPP/PyrT labelled as II in Figure 2 b.

Figure 6

Schematic diagram of the supramolecular landscape for all studied molecular junctions. The conductance values are represented on the x‐axis in G₀ units (solid lines are the experimental values, stripped lines the theoretical values) for both linkers (PyrMT and PyrT) and the four studied porphyrin systems (Co‐DPP, Co‐P, DPP, and P). Schematic representations of the structural models confirmed by DFT are drawn for each conductance signature. The color legend represents equivalent interactions across all junctions: blue corresponds to the both pyridine linkers interacting with the metal center, green is one pyridine interacting with the metal center and the other with a side phenyl group, red is both pyridine interacting with side phenyl rings, and yellow corresponds to two pyridine linkers interacting with the porphyrin pyrrolic ring.