A great interest of molecular electronics comes from its change in electronic structure through external stimuli, which provides functionality at the single-molecule level. Mechanically-controlled break junction (MCBJ) is a great tool for characterizing molecular properties and their response to different stimuli including light, solvent and importantly, mechanical deformation1. In our recent MCBJ experiment, we showed that the conductance of a spring-like molecule can be mechanically tuned up to an order of magnitude at room temperature2. The physical origin of such feature is a consequence of destructive quantum interference between the frontier orbitals. This indicates not only a possible application for mechanical sensors based on this class of molecules, but also demonstrates a good example of quantum interference effect in single molecules. Following this intriguing result, we investigate other properties of this molecule, such as I-V characteristics at low temperature and thermopower. We are also exploring other molecules with similar spring-like structures, where quantum interference effect is expected to manifest. @en

The bowl-shaped, 3-fold interlinked porphyrin dimer 2 was obtained in respectable yields during macrocyclization attempts. Its bicyclic structure, consisting of a macrocycle made of a pair of acetylene interlinked tetraphenylporphyrins which are additionally linked by a C-C bond interlinking two pyrrole subunits, has been confirmed spectroscopically (2D-NMR, UV/vis, HR-MALDI-ToF MS). Late-stage functionalization provided the structural analogue 1 with acetyl-protected terminal thiol anchor groups enabling single molecule transport investigations in a mechanically controlled break junction experiment. The formation of single-molecule junctions was observed, displaying large variations in the observed conductance values pointing at a rich diversity in the molecular junctions.

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An appealing feature of molecular electronics is the possibility of inducing changes in the orbital structure through external stimuli. This can provide functionality on the single-molecule level that can be employed for sensing or switching purposes if the associated conductance changes are sizable upon application of the stimuli. Here, we show that the room-temperature conductance of a spring-like molecule can be mechanically controlled up to an order of magnitude by compressing or elongating it. Quantum-chemistry calculations indicate that the large conductance variations are the result of destructive quantum interference effects between the frontier orbitals that can be lifted by applying either compressive or tensile strain to the molecule. When periodically modulating the electrode separation, a conductance modulation at double the driving frequency is observed, providing a direct proof for the presence of quantum interference. Furthermore, oscillations in the conductance occur when the stress built up in the molecule is high enough to allow the anchoring groups to move along the surface in a stick-slip-like fashion. The mechanical control of quantum interference effects results in the largest-gauge factor reported for single-molecule devices up to now, which may open the door for applications in, e.g., a nanoscale mechanosensitive sensing device that is functional at room temperature.

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