Remarkable effect of bromide ion upon two-dimensional faradaic phase transition of dibenzyl viologen on an HOPG electrode surface: Emergence of two-step transition

Abstract

We found that dibenzyl viologen (dBV) on an HOPG electrode undergoes a two-step first order faradaic phase transition at high concentrations of bromide ion (Br−). Results of voltammetric and electroreflectance measurements were used to describe the mechanism of the two-step transition processes. When [Br−]>180mM, the transition step at less negative potential was ascribed to a phase transition between a gas-like adsorption layer of dBV dication (dBV2+) and a mesophase of dBV radical cation (dBV+). Most likely, the mesophase is a two-dimensional (2D) ordered phase composed of co-adsorbed dBV+ and Br− where both are in direct contact with the HOPG surface. The transition step at more negative potential was ascribed to a phase transition between the dBV+ Br− mesophase and a 2D condensed phase of dBV+. In the condensed phase being denser than the mesophase, dBV+ molecules are π-stacked due to face-to-face interaction between bipyridinium radical cations. This transition step involves also a reduction process of dBV2+ to dBV+ followed by its incorporation into the condensed phase. The two-step transition was not observed in KCl solution of any concentration, either in KBr solution of [Br−]<75mM. Other viologens examined, including benzyl–heptyl viologen, did not exhibit such a two-step transition but single-step one. The nature of the transition, especially in the [Br−] range from 75 to 180mM, was closely analyzed.