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 surfac e were characterized by cyclic voltammetry and in situ infrared reflection absorption spectroscopy (IRAS). The cyclic voltammogram showed a pair of sp ike-like cathodic and anodic waves ascribed to the one-electron redox react ion of the adsorbed species at potentials more positive than the main reduc tion wave. From the adsorption isotherm curves, the areas occupied by one m olecule (S-ads) were estimated to be ca. 83 and 71 Angstrom (2) for the oxi dant and the reductant, respectively, which. indicates that the adlayer of the reductant is more compact. On the other hand, some structural changes o f the adlayer depending on the electrode potential were confirmed from a se ries 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 n egative of the spike-like peaks, the HV cation radicals form a bilayer stru cture with their bipyridine planes parallel to the surface (the face-on con figuration). The theoretical S-ads value for the bilayer structure of HV wa s well consistent with the experimental value shown above. (C) 2000 Elsevie r Science S.A. All rights reserved.