Adsorption structure of the heptyl viologen cation radical on a mercury electrode surface: voltammetric and in situ infrared reflection absorption spectroscopic studies

The adsorption states of heptyl viologen (HV) on a mercury electrode surface were characterized by cyclic voltammetry and in situ infrared reflection absorption spectroscopy (IRAS). The cyclic voltammogram showed a pair of spike-like cathodic and anodic waves ascribed to the one-electron redox reaction of the adsorbed species at potentials more positive than the main reduction wave. From the adsorption isotherm curves, the areas occupied by one molecule (Sads) were estimated to be ca. 83 and 71 إ2 for the oxidant and the reductant, respectively, which indicates that the adlayer of the reductant is more compact. On the other hand, some structural changes of the adlayer depending on the electrode potential were confirmed from a series of the IRA spectra obtained on the mercury-amalgamated gold electrode; at a potential well positive of the spike-like peaks, the HV dications are considered to be present in a liquid-like state, where the molecules near the electrode surface are randomly oriented. In the potential region just negative of the spike-like peaks, the HV cation radicals form a bilayer structure with their bipyridine planes parallel to the surface (the face-on configuration). The theoretical Sads value for the bilayer structure of HV was well consistent with the experimental value shown above.