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Electrochemical and spectroscopic studies of 2-(2’-thienyl)pyridine on Au(111) Chung, Emily Susanne

Abstract

The adsorption behaviour of the bifunctional molecule 2-(2'-thienyl)pyridine (TP) on Au(111) was characterized under potential control in neutral and basic aqueous solution. Initial characterization of adsorbed TP was by cyclic voltammetry (CV) and ac voltammetry in aqueous solution. CV was also performed in organic media for comparison purposes. The electrochemical behaviour of TP was compared to that of pyridine, which has been well-characterized in the literature. CV and ac voltammetric characterizations of thiophene in aqueous solution were also performed within the same potential range for comparison to TP. It was found that TP underwent both capacitive changes similar to those observed in the presence of pyridine, and redox processes similar to those observed in the presence of thiophene. Chronocoulometric characterization of the capacitive changes showed that TP initially adsorbs at negative potentials as a low-coverage layer in which interaction with the Au surface is through the aromatic TC orbitals and the plane of the rings is parallel to the surface. Scanning to more positive potentials induces a pyridine-like two-dimensional phase transition to a more condensed phase in which the molecules are bonded to Au via the lone pairs of the N and/or S (likely both). The redox processes of TP were hypothesized to be an electrochemical α,α'-coupling of the thiophene rings, followed by an oxidative charging process; where subsequent reductive discharging gives the neutral dimer, 5,5'-bis(2-pyridyl)-2,2'-bithienyl (PTTP). PTTP was synthesized chemically for comparison to the product of TP electrochemical oxidation on the electrode. The photophysical properties of TP and PTTP in solution were used to interpret in situ fluorescence results obtained at a Au(111) electrode under potential control. Spectra obtained after holding the electrode at positive potentials in the presence of TP were identical to that of chemically synthesized PTTP, confirming the dimerization hypothesis. In addition, fluorescence microscopy images were consistent with the electrochemical results regarding other states of TP. This system demonstrates that multifunctional adsorbates can create chemically-tuned surfaces capable of switching between multiple states, and allow subtle comparisons of the influence of surface-adsorbate interactions between different functional groups.

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