Increased Molecular Conductance in Oligophenylene Wires by Thermally Enhanced Dihedral Planarization

<b>Understanding and manipulating the electron transport within single-molecule wires are key for the development of molecular-scale devices. Significant efforts have been made to control electron transport properties with a variety of parameters including chemical designs of molecular three-dimensional conformations. As electron transport occurs much faster than intramolecular rotations, the average conductance of single-molecule wire is assumed to relate to its energy-minimum molecular conformation. Moreover, in a coherent tunneling regime, electron transport through a single-molecule wire has been theoretically established as a temperature-independent process. As such, the influence of temperature-driven dynamic molecular conformation on electron transport has not been widely investigated. Here, we demonstrate how the dynamic molecular conformation influences the electron transport through oligophenylene wires within a coherent tunneling regime. Using a custom-built variable-temperature scanning tunneling microscopy break-junction (STM-BJ) instrument, we find that the conductance of oligophenylene wires can be manipulated via subtle temperature control (296-328 K). Our DFT-based calculations disclose that thermally accelerated intramolecular rotations allow the oligophenylene wires to have a higher probability of being in a planar conformation compared to lower temperatures. The thermally enhanced planarization substantially increases the time-averaged electron tunneling probability, as the tunneling occurs primarily through π-orbitals. These calculations are consistent with the observation that more rotational pivot points in longer oligophenylene molecular wires lead to larger temperature dependence on conductance. These findings show that molecular conductance within coherent and off-resonant electron transport regimes can be controlled by manipulating dynamic molecular structures.</b>