Time-Resolved Resonance Raman and Surface-Enhanced Resonance Raman Scattering Study on Monocation Radical Formation Processes of Heptyl Viologen at Silver Electrode Surfaces

Abstract

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 [3] 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.