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Exploring local currents in molecular junctions

Abstract

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.

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Figure 1: Different surfaces across which the current can be calculated.
Figure 2: The local transmission at the Fermi energy (top) and total transmission as a function of energy (bottom) for two commonly studied molecules.
Figure 3: The local π transmission contributions through a meta-substituted phenyl ring at a selection of energies (top), and a plot of the π transmission (dashed line) and total transmission (solid line) across a large energy range (bottom).
Figure 4: The σ (left), π (centre) and total (right) local transmission for a meta-substituted phenyl system at an energy where the π system dominates (top) and where the σ system dominates (bottom).
Figure 5: The local π transmission contributions through a cross-conjugated molecule at a selection of energies (top) and a plot of the π transmission (dashed line) and total transmission (solid line) across a large energy range (bottom).
Figure 6: The local transmission in syn-1,8-octanedithiol at a selection of energies (top) and a plot of the total transmission across a large energy range (bottom).

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Acknowledgements

The authors would like to thank David Q. Andrews for very helpful comments. This work was funded by NSF-Chemistry, NSF-MRSEC, and ONR-Chemistry. C.H. would like to thank the German Research Foundation (DFG) for generous support through a postdoctoral research fellowship. T.H. thanks the Carlsberg foundation (Carlsbergfondet) for generous support.

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G.C.S. performed calculations and analysis for the paper and Supplementary Information and wrote the paper. C.H. performed calculations and analysis for the Supplementary Information. T.H. wrote the theory section of the Supplementary Information. V.M. assisted in project design. M.A.R. edited the paper and provided supervision.

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Correspondence to Gemma C. Solomon.

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Solomon, G., Herrmann, C., Hansen, T. et al. Exploring local currents in molecular junctions. Nature Chem 2, 223–228 (2010). https://doi.org/10.1038/nchem.546

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