Single-Molecule Measurements Probe Nanoscale Physics and Chemistry

Over the past decade, there has been tremendous progress in the measurement, modeling and understanding of structure-function relationships in single molecule circuits. Experimental techniques for reliable and reproducible single molecule junction measurements have led, in part, to this progress. In particular, the scanning tunneling microscope-based break-junction technique has enabled rapid, sequential measurement of large numbers of nanoscale junctions allowing a statistical analysis to readily distinguish reproducible characteristics. Although the break-junction technique is mostly used to measure electronic properties of single-molecule circuits, in this talk, I will demonstrate its versatile uses to understand both physical and chemical phenomena with single-molecule precision. In this talk, I will present results from a recent work where we demonstrate that molecular wires can behave as one-dimensional topological insulators. We focus on a family of oligophenylene-bridged bis(triarylamines) with tunable and stable (mono-/di-)radicaloid character. These wires can undergo one- and two-electron chemical oxidations to the corresponding monocation and dication, respectively. The oxidized wires exhibit high reversed conductance decay with increasing length, consistent with the expectation for the Su-Schrieffer-Heeger-type one-dimensional (1D) topological insulators. Importantly, we show that for one length, the dication displays a significantly high conductance greater reaching beyond 0.1 <i>G</i>₀ (2<i>e</i>²/<i>h</i>, the conductance quantum).