title: Faradaic phase transition of dibenzyl viologen on an HOPG electrode surface studied by in situ electrochemical STM and electroreflectance spectroscopy.
abstract: Phase transitions of an adsorption layer of dibenzyl viologen (dBV) as a typical diaryl viologen on a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode are described using voltammetry, in situ electrochemical scanning tunneling microscopy (EC-STM), and electroreflectance (ER) spectroscopy. A monolayer redox process at less negative potential than the bulk redox process was found to be the first-order faradaic phase transition between a gaslike adsorption layer of dication (dBV(2+)) and a 2D condensed monolayer of radical cation (dBV(•+)). Comparison of the results of cyclic voltammetry and potential step chronoamperometry was made with those of heptyl viologen (HV), which also undergoes a faradaic phase transition of the first order. It suggested that the contribution of intermolecular π-π interaction between benzyl groups of dBV to the phase transition is minor and apparently equivalent to interchain interaction between the heptyl chains of HV. In situ EC-STM images of the 2D condensed monolayer demonstrated stripe patterns of the rows of dBV(•+) molecules forming 3-fold rotationally symmetric domains. The results of the ER measurements also revealed that the orientation of the longitudinal molecular axis of the bipyridinium moiety of dBV(•+) molecules lying flat on the HOPG electrode surface, most likely with a side-on configuration.
authors.affiliation: Department of Molecular Science, Graduate School of Science and Technology, Nagasaki University, Nagasaki 852-8521, Japan.
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Higashi
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Shigemitsu
authors.forename: Takamasa
authors.initials: T
authors.lastname: Sagara