Density Functional Study on the Deprotonation and Binding Mechanism of Imidazole on Gold Electrodes in an Aqueous Environment

Abstract

Imidazole molecules have broad application in molecular electronic devices constructed with gold electrodes, but the formation mechanism of gold–imidazole–gold molecular junctions has not been elucidated so far. Herein, we carry out density functional theory (DFT) calculations to investigate the deprotonation and binding mechanism of imidazole on gold electrodes in water. Bulk solvation effects are simulated by a polarized continuum model (PCM) augmented with explicit water molecules. Our calculations show that the deprotonation of an imidazole molecule weakly adsorbed on a gold surface via its pyrrole nitrogen atom is an exothermic reaction with an energy barrier of ∼0.2 eV. Such a proton transfer process is catalyzed by the strong gold–imidazole attraction presented in both the transition state and the product, which not only lowers the energy barrier height but also stabilizes the product. The deprotonated imidazole molecule covalently binds to the gold electrode, and the negative charge resides mainly on the gold surface converting it into an imidazole radical. The subsequent hydrogen evolution reaction neutralizes the negatively charged gold electrode and further enhances the Au–N covalent bond formed at the gold–imidazole interface. At this point, the imidazole radical adsorbed on gold is sufficiently stable and also has a second binding site (the pyridine nitrogen atom) ready for the formation of gold–imidazole–gold junction when a second gold electrode approaches. Our findings provide an explanation for the formation and stability of gold–imidazole–gold molecular junctions and are also helpful for understanding other deprotonation processes in water, especially in the presence of metal electrodes.