Low-Energy Conformational Gating in π-Conjugated Molecular Junctions

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Abstract

Oligo(phenylene ethynylene) is widely used as a molecular conductor in molecular electronics. Phenylene units connected by two carbon–carbon triple bonds at the para position are known to have a low energy barrier (less than 10 kJ/mol) for rotation around the axis of the molecular wire. π orbital localization accompanied with the rotation of the phenylene unit due to the reduction of the π–π coupling strongly influences the electron transport through the molecular wires. Here we demonstrate low-energy conformational gating in oligo(phenylene ethynylene)-based molecular wires connected between contacts. We could achieve an on–off ratio of over 104 by rotating the phenylene units out of the plane of the molecule, which requires only 90 meV in total energy. This result implies that studies of electron transport using stationary geometries may require that special attention be paid to the relationship between molecular fluctuations and conductance.

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