Potential- And anion-controlled organization of 2-mercapto-5-benzimidazolesulfonate on the Au(111) electrode in acidic media

Electrodes modified by thiol molecules with conjugated heterocycles can lead to improved activities in electroanalysis and electrodeposition. However, the intricate interactions among thiol molecules and solution species at the electrified interface can perplex the configuration and organization of thiol admolecules. The purpose of the current study was to use in situ scanning tunneling microscopy (STM) to gain insights into the adsorption of 2-mercapto-5-benzimidazolesulfonate (MBIS) on Au(111) under potential control. MBIS self-assembled on the Au(111) electrode so efficiently that a long-range-ordered adlattice, characterized as Au(111)−(3√3 × 3√3)R30°−MBIS, was formed within 5 min at −0.1 V (vs Ag/AgCl) in 0.1 M H₂SO₄ and HClO₄ containing dilute MBIS (1−5 μM). The MBIS molecules in this ordered structure were imaged as elongated protrusions, suggesting an edge-on molecular orientation with the S- and protonated N-ends tethered to the Au substrate. Real-time STM imaging revealed that the MBIS admolecule reoriented from the edge-on to face-on adsorption mode, as the potential was shifted positively. The high electric field at the interface was responsible for the reorientation of the polar MBIS molecule. Different MBIS surface structures were seen in HCl and H₂SO₄, indicating the important contribution of the anion in organizing this interface. The MBIS adlayer on Au(111) was disordered in HCl but was more stable against cathodic stripping than in H₂SO₄ and HClO₄