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

•First order faradaic phase transition of dibenzyl viologen (dBV) at HOPG was studied.•At high Br− concentration, dBV exhibits two-step phase transitions.•As the first reduction step, a dBV+ Br− mesophase emerges at [Br−] > 75 mM.•As the second, phase transition from the mesophase to a 2D condensed phase occurs.•Remarkable effect of Br− upon the transition enabled us to model the mesophase.

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−] > 180 mM, 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−] < 75 mM. 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 180 mM, was closely analyzed.

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