Combined Spectroscopic Ellipsometry and Voltammetry of Tetradecylmethyl Viologen Films on Gold

Abstract

Dynamic measurements of changes in composition and conformation during adsorption of tetradecylmethyl viologen (C14MV) on a gold electrode were made using spectroscopic ellipsometry. Film thickness measurements indicated that a monolayer is initially formed in which the viologen species is in the upright extended position and that conversion to a tilted conformation occurs during equilibration to the final monolayer film. Subsequent reduction of the film as electrode potential was scanned from −0.2 to −0.9 V resulted in linear growth to a multilayer film in which dications (C14MV2+) were reduced to radical (C14MV•+) and neutral (C14MV0) species. A gradual increase of the film charge-to-thickness ratio and of the film index of refraction during reduction indicated densification of the deposit. The pattern in the variation of film extinction coefficient with electrode potential indicated that radical dimers were the dominant species in the potential range corresponding to the first reduction step. During the reverse potential scan, the radical film detached due to faster reoxidation of the layers adjacent to the electrode surface.

Reorientation of Tetradecylmethyl Viologen on Gold upon Coadsorption of Decanethiol and Its Mediation of Electron Transfer to Nitrate Reductase

“…The initial voltammetric response showed redox peaks, but after approximately three scans, subsequent voltammograms showed no redox peaks in the potential range from −0.43 to −0.68 V (see Figure b), indicating only the decrease in double-layer capacitance with respect to the electrodes that had no exposure to viologen (Figure a). In contrast, our previous results showed well-defined current peaks in the voltammograms obtained in dilute C 14 MV solution .…”

“…In the discussion above, it was noted that the parallel orientation of the bipyridinium moiety relative to the electrode plane does not favor appearance of this vibration in the SERS spectrum, suggesting that this is the prevailing position when C 14 MV is adsorbed without the decanethiol. Such an arrangement was found to be electroinactive in this work, in contrast to the case when viologen was present in solution 5 SERS spectra for (a) C 14 MV adsorbed alone on a bare gold SERS-active electrode and (b) C 14 MV with coadsorbed decanethiol on gold.…”

“…All of these additional bands are characteristic of the radical monocation, which is somewhat unexpected because the spectra were recorded at an electrode potential characteristic of fully oxidized species. We showed earlier, by spectroscopic ellipsometry, the transient presence of the monocation radical species on a gold electrode at this same potential (−0.3 V). The current data therefore provide additional evidence for the existence of radical molecules in the adsorbed film, the effect being more pronounced with coadsorbed decanethiol.…”

“…We estimate coverage of about 30%, as determined by coulometry and using a surface area of 40 Å 2 /molecule for C 14 MV adsorbed in the end-on, upright position. The midpoint potential for C 14 MV with coadsorbed decanethiol (−0.53 V vs Ag/AgCl, sat KCl) was shifted about 70 mV from that for C 14 MV in solution (−0.46 V) 1 Potentiodynamic curves recorded on a SERS-active gold electrode in 0.1 M KCl + 0.1 M KHPO 4 buffer, pH 7.4: (a) bare gold; (b) after 2 h exposure to 10 mM tetradecylmethyl viologen (C 14 MV) solution with subsequent washing before the potential scan.…”

“…Electrodes consisted of glass slides coated with 300 nm thermally evaporated gold (99.99%, Johnson−Matthey) on a Cr underlayer for better adhesion. These slides were stored in distilled water prior to use and were flame-cleaned according to ref . Following cleaning, electrodes were placed in 10-mM C 14 MV solution for 2 h under open circuit conditions.…”