@article{oai:nagasaki-u.repo.nii.ac.jp:00006998,
 author = {Higashi, Tomohiro and Sagara, Takamasa},
 journal = {Electrochimica Acta},
 month = {Dec},
 note = {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., Electrochimica Acta, 114, pp.105-115; 2013},
 pages = {105--115},
 title = {Remarkable effect of bromide ion upon two-dimensional faradaic phase transition of dibenzyl viologen on an HOPG electrode surface: Emergence of two-step transition},
 volume = {114},
 year = {2013}
}