Ethynyl Substituents as Rheostatic Triggers in Single-Molecule Junctions

Miniaturised mechanoresistive devices, where charge transport efficiency can be modulated by mechanical displacement, are an important class of electromechanical systems that promise unique sensing accuracy and sensitivity. Single-molecule junctions have emerged in the last decade as the ultimately scaled down system, operating in the quantum realm. Several strategies have been developed to impart mechanoresistivity to a molecular backbone, but all these require structural modifications to the conducting backbone, to introduce either alternative, short-circuiting anchoring points to the electrode, or conformationally flexible moieties. Here, we show a synthetically accessible way to impart mechanoresistivity, by adding silyl-protected ethynyl functional groups that extend the π-system orthogonally to the transport axis. Junctions fabricated with ethynyl-extended acenes demonstrate strong and reproducible mechanoresistivity, with log-linear dependence of conductance on displacement and high conductance modulation amplitude (>10^2 per nanometre). Analytical and Density Functional modelling demonstrate that the compressed junctions are better coupled to the electrodes, due to overlap between the extended π-system and the density of states of the Au electrodes. As ethynyl moieties are trivial to attach to a variety of substrates, we expect this strategy to be widely applicable as a synthetically advantageous way to impart mechanoresistivity – or simply stronger coupling to the electrodes – to a wide variety of molecular wires.