Cooperative Self-Assembly of Dimer Junctions Driven by π Stacking Leads to Conductance Enhancement

We demonstrate enhanced electronic transport through dimer molecular junctions, which self-assemble between two gold electrodes in π-π stabilized binding configurations. Single molecule junction conductance measurements show that benzimidazole molecules assemble into dimer junctions with a per-molecule conductance that is higher than that in monomer junctions. Density functional theory calculations reveal that parallel stacking of two benzimidazoles between electrodes is the most energetically favorable due to the large π system. Imidazole is smaller and has greater conformational freedom to access different stacking angles. Transport calculations confirm that the conductance enhancement of benzimidazole dimers results from the changed binding geometry of dimers on gold, which is stabilized and made energetically accessible by intermolecular π stacking. We engineer imidazole derivatives with higher monomer conductance than benzimidazole and large intermolecular interaction that promote cooperative in situ assembly of more transparent dimer junctions and suggest at the potential of molecular devices based on self-assembled molecular layers.