<em>In-situ</em> STM Visualization of Molecular Structural Evolution during Electrochemical Oxidation Coupling of Para-Aminothiophenol on Au(111)

Para-aminothiophenol (PATP) is not only a widely-used probe molecule in the field of surface-enhanced Raman spectroscopy (SERS), but also an important model molecule for interfacial reactions, exhibiting distinct photo- and electro-oxidation pathways, so that PATP is selectively activated under different external fields. The evolution of products and intermediates during electrochemical oxidative coupling remains unknown. In this study, we have investigated the dynamic electrochemical oxidation process of PATP on the Au(111) surface using cyclic voltammetry, electrochemical (EC)-SERS, and in situ electrochemical scanning tunneling microscopy (EC-STM). The results showed that PATP forms an unsaturated adsorption coverage on Au(111) in an acidic solution. When the potential was positively shifted to the oxidation potential range, the cation radical generated by PATP oxidation alternated with PATP molecules in arrangement. Once the coupling reaction occurred, the adsorption structure of alternating arrangement was transformed into the stripe structure as observed in the STM images. Finally, the coverage on the Au(111) surface decreased and a √3x4√6 structure was formed. Accordingly, the oxidation reaction product 4'-mercapto-N-phenylquinone diimine (NPQD) would adopt the arrangement of mixed adsorption sites of bridge and hollow sites on Au(111). By determining the adsorption structure evolution of PATP during the electrooxidation process on the gold surface, the mechanism of interfacial electrochemical oxidation coupling reaction is revealed from a spatial perspective. This lays the foundation for further modulation and design of the interfacial reaction of aromatic amine molecules.