(333k) Using Redox Chemistry for Dynamic Anchor Groups in Molecular Electronics

Molecular junctions consist of three key components: an anchor, electrode and molecular bridge. Traditionally, the field of single molecule electronics has focused on studying organic molecules with two, or more, anchors capable of binding to the metal electrodes. Here, we show that redox chemistry can be used to generate a dynamic terminal anchor group to form robust linkages to metal electrodes for molecular electronics. We characterize the electronic properties of a class of amino-p-terphenyl derivatives using a combination of automated chemical synthesis, single molecule and bulk scale experiments, molecular modeling, and machine learning. Interestingly, 4-amino-p-terphenyl (4-ATP) shows a distinct and well-defined high conductance state that is diminished or absent in all related amino-p-terphenyl derivatives. Our results indicate that the high conductance state in 4-ATP arises due to a radical-based rigid resonating structure, which involves the formation of a robust Au-C bond. Bulk scale experiments including cyclic voltammetry, electrolysis, spectroelectrochemistry, and electron spin resonance are used to fully characterize the high conductance state, which confirms the existence of the radical species. Our results further show a low conductance state in all amino-p-terphenyl derivatives, which is attributed to pi-stacked dimeric interactions facilitating through-space electron transport. Molecular dynamics (MD) simulations are further used to elucidate the pi-stacking behavior in these molecules, with binding energy below 1 k_BT, which corroborates the temperature-dependent nature of the low conductance state. Overall, our work enhances the understanding of non-covalent dimeric interactions in single anchored organic molecules and suggests that 4-ATP is a unique molecule capable of forming a rigid radical-based structure, which can result in robust and controlled conductance in the molecular junction. Insights from our work can be leveraged in the design of molecular electronics, wherein molecular conductance remains stable over operational timescales, thus stabilizing device performance.