Dynamic Au-C anchors in molecular junctions under oriented external electric fields

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 under oriented external electric fields (OEEFs). 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 radical-based rigid resonating structure under OEEFs. A series of control experiments on junctions with different anchor groups reveals the role of primary amines in forming dynamic linkages under OEEFs. Overall, these results suggest that OEEFs can trigger Au-C bond formation leading 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.