Conformational change in 4-pyridineethanethiolate self-assembled monolayers on Au(111) driven by protonation/deprotonation in electrolyte solutions

Abstract

Self-assembled monolayers (SAMs) of 4-pyridineethanethiol (4-PyEtS) formed on a Au(111) surface were investigated in aqueous electrolyte solutions by cyclic voltammetry and in situ scanning tunneling microscopy (STM). On the basis of reductive desorption of 4-PyEtS on Au(111), the reduction peak potential was found to be −0.86 V vs.Ag/AgCl (sat. KCl). The surface excess was calculated from the reductive desorption peak area to be 5.4 × 10⁻¹⁰ mol cm⁻². High-resolution STM images revealed that a p(5 ×√3 R–30°) lattice including two molecules was found in 0.1 M NaClO₄, whereas the 4-PyEtS-SAMs formed a p(10 ×√3 R–30°) lattice constituting four molecules in 0.05 M HClO₄. The transition from a p(10 ×√3 R–30°) to a p(5 ×√3 R–30°) in the 4-PyEtS-SAMs was directly observed by changing pH of the electrolyte solution, indicating that the conformational change in 4-PyEtS-SAMs is driven by protonation/deprotonation. The present results obtained by STM observation are also supported by the clear pH dependence of the pyridine moiety in the surface-enhanced infrared spectrum.