Time-Resolved Resonance Raman and Surface-Enhanced Resonance Raman Scattering Study on Monocation Radical Formation Processes of Heptyl Viologen at Silver Electrode Surfaces
Heptyl viologen (HV2+ (X−)2, X−= a counter anion; see Figure 1) has been known to perform a one-electron redox reaction with half-wave potential near -0.4 V (vs. Ag/AgCl). Monocation radical (HV+· X−), which is formed on stepping electrode potential to a value negative of -0.4 V, is insoluble in water and forms a red-purple film giving a broad band centered around 550 nm on an electrode. On reversing the potential to a value positive of -0.4 V, the colored film disappears. This property can be applied to electrochromic display devices. Therefore, a number of studies have been made to elucidate the mechanism of the film formation process. [1, 2] These studies have proved that the initial film formation consists of nucleation, nuclear growth and subsequent film formation processes. In the present paper we applied time-resolved resonance Raman scattering (RRS) and surface-enhanced resonance Raman scattering (SERRS) spectroscopies to analyze these processes. Time-resolved RRS spectra have already been reported by Osawa and Suetaka  for HV+· films on a Pt electrode; the spectra, however, were observed with time resolution of 5 msec and recorded after the initial processes had already terminated. So, we confined the measurement mainly within several milliseconds since the application of reduction potential.
KeywordsTransient Current Benzyl Viologen Resonance Raman Scattering Film Formation Process SERRS Band
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