Dynamic formation of Au-C anchors in molecular junctions

Terminal anchor groups play a key role in controlling the stability and electronic properties of molecular junctions. Single molecule junctions typically consist of two terminal anchors linking organic molecules to metal electrodes. Here, we show that p-terphenyl derivatives containing only a single terminal anchor exhibit conductance features similar to junctions with two terminal anchors, which arises due to dynamic Au-C bond formation due to a single electron oxidation event at the electrode. A set of p-terphenyl derivatives with one terminal anchor was prepared and characterized using automated chemical synthesis, single molecule electronics experiments, molecular dynamics (MD) simulations, and non-equilibrium Green’s function-density functional theory (NEGF-DFT) calculations. Our results show that 4-amino-p-terphenyl (PPP) exhibits a distinct and well-defined high conductance state that is greatly diminished or absent in other p-terphenyl derivatives with single terminal anchors, whereas a low conductance state is observed in all amino-p-terphenyl derivatives due to non-covalent dimeric interactions. The electronic properties of PPP are characterized using a combination of cyclic voltammetry, electrolysis, and electron spin resonance, revealing that the high conductance state in PPP arises due to robust Au-C bond formation facilitated by a single electron oxidation event and stabilized by a rigid resonance structure under an electric field. A series of control experiments with different anchor groups reveals the role of primary amines in forming dynamic linkages in molecular junctions. Overall, these results suggest that Au-C bond formation gives rise to high conductance pathways in organic molecules containing only one terminal anchor. Insights from this work can be leveraged in the design of molecular electronic devices, particularly in understanding the mechanisms of molecular binding and junction formation.