Lewis-acid mediated reactivity in single-molecule junctions

Recent work has shown that solution-based chemical reactions at a gold electrode can be monitored using molecular conductance and driven by extrinsic stimuli, such as an electric field or electrochemical potential. However, the intrinsic properties of the nanostructured interface may be expected to play important additional functions that are not yet well understood. Here we study single-molecule junctions formed from components having the same molecular backbone functionalized with 12 different sulfur-based linker groups to determine the influence of sulfur-substituent on junction conductance and stability. Remarkably, we find evidence for in situ S–C(sp3) bond breaking, and C(sp2)–C(sp3) bond forming, reactions that are consistent with the ex situ transformations expected for those systems in the presence of a Lewis acid. We also find we approach the limits of substituent influence on the conductance of physisorbed sulfur-bound junctions. As an illustrative example, we show that a tert-butylthio-functionalized 4,4’-biphenyl precursor can form both chemisorbed (Au−S) junctions, consistent with heterolytic S−C(sp3) bond cleavage to generate the stable tert-butyl carbocation, as well as physisorbed junctions that are >1 order of magnitude lower than analogous junctions comprising cyclic “locked” thioether contacts. Our findings are supported by a systematic analysis of model thioether components comprising different simple hydrocarbon substituents of intermediate size, which do not form chemisorbed contacts and further clarify the inverse relationship between conductance and substituent steric bulk. First-principles calculations based on density-functional theory also confirm that bulky sulfur-substituents increase the probability of forming junction geometries with reduced electronic coupling between the electrode and π-conjugated molecular backbone. Together, this work helps to rationalize the dual roles that linker molecular structure and metal electrode Lewis character play in mediating interfacial reactions in break-junction experiments. The insights provided may be applied to advance our understanding of heterogeneous chemical processes or develop new molecular electronic components with improved or novel functionality.