Acid-Catalyzed Rearrangement Reaction for Single-Molecule Junction Formation

Single-molecule junction formation is not possible for hydrazobenzenes due to the absence of anchoring groups for making contact with the Au electrodes. In stark contrast, with the addition of acid, the junction formation is immediate and robust, monitored in real-time via the emergence of the 1.3×10^-3 G₀ conductance peak by the scanning tunneling microscope-based break junction method. We propose that this single-molecule junction is benzidine attaching to Au electrodes through NH₂→Au dative interactions, which is a rearrangement reaction product of hydrazobenzene, supported by measurements of ex situ synthesized benzidine and high-performance liquid chromatography analysis. Critically, we show that in forming the metal-molecule-metal junctions under an applied voltage in a nonpolar solvent, the reaction products possibly become deprotonated, which otherwise requires an addition of base. We find that for synthesized -NH₃ ⁺ terminated compounds, -NH₃ ⁺ is also possibly converted into -NH₂ upon the junction formation. These results underscore a new acid-catalyzed method for single-molecule device formation.