Exploring local currents in molecular junctions

Electron transfer through molecules is an ubiquitous process underlying the function of biological systems and synthetic devices. The electronic coupling between components varies with the structure of the molecular bridge, often in classically unintuitive ways, as determined by its quantum electronic structure. Considerable efforts in electron-transfer theory have yielded models that are useful conceptually and provide quantitative means to understand transfer rates in terms of local contributions. Here we show how a description of the local currents within a bridging molecule bound to metallic electrodes can provide chemical insight into current flow. In particular, we show that through-space, as opposed to through-bond, terms dominate in a surprising number of instances, and that interference effects can be characterized by the reversal of ring currents. Together these ideas have implications for the design of molecular electronic devices, in particular for the ways in which substituent effects may be used for maximum impact.