Potential-induced phase transition of benzoxazole-2-thiol, naphthaleneoxazole-2-thiol and anthraceneoxazole-2-thiol monolayers on gold electrodes

Abstract

In the present work we report the electrochemical characterization of self-assembled monolayers (SAMs) of N,O-heteroaromatic thiols, namely benzoxazole-2-thiol (1), naphthaleneoxazole-2-thiol (2) and anthraceneoxazole-2-thiol (3) on polycrystalline gold electrodes. SAMs were formed by immersion of pre-treated gold electrodes into ethanolic solutions of the title compounds. The modified electrodes exhibited a decrease of electroactivity, as found by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), in comparison to bare electrodes according to the length of the aromatic π-system. In addition, reductive desorption experiments confirmed that of the three compounds, 3 formed the most stable SAMs in agreement with EIS measurements which revealed better blocking properties against the redox couple in a test electrolyte indicating a charge transfer determined behaviour in the case of longer molecules. The charge transfer resistances revealed a trend towards higher resistances in the case of 3. When performing EIS at various starting potentials and different electrolyte concentrations, it was found that at certain critical potentials (Ecrit) a potential-induced structural change of the SAM occurred, with Ecrit(1) > Ecrit(2) > Ecrit(3). In the case of SAMs of 3 the nature of this alteration was investigated via scanning tunnelling microscopy and found to presumably be caused by an order-disorder-transition.