Mechanical Deformation Distinguishes Tunneling Pathways in Molecular Junctions.

Developing a clearer understanding of electron tunneling through molecules is a central challenge in molecular electronics. Here we demonstrate the use of mechanical stretching to distinguish orbital pathways that facilitate tunneling in molecular junctions. Our experiments employ junctions based on self-assembled monolayers (SAMs) of homologous alkanethiols (C nT) and oligophenylene thiols (OPT n), which serve as prototypical examples of σ-bonded and π-bonded backbones, respectively. Surprisingly, molecular conductances ( Gmolecule) for stretched C nT SAMs have exactly the same length dependence as unstretched C nT SAMs in which molecular length is tuned by the number of CH2 repeat units, n. In contrast, OPT n SAMs exhibit a 10-fold-greater decrease in Gmolecule with molecular length for stretched versus unstretched cases. Experiment and theory show that these divergent results are explained by the dependence of the molecule-electrode electronic coupling Γ on strain and the spatial extent of the principal orbital facilitating tunneling. In particular, differences in the strain sensitivity of Γ versus the repeat-length ( n) sensitivity can be used to distinguish tunneling via delocalized orbitals versus localized orbitals. Angstrom-level tuning of interelectrode separation thus provides a strategy for examining the relationship between orbital localization or delocalization and electronic coupling in molecular junctions and therefore for distinguishing tunneling pathways.