TIME-RESOLVED SURFACE-ENHANCED RESONANCE RAMAN-SCATTERING STUDY OF THE REDUCTION PROCESS OF HEPTYLVIOLOGEN MONOCATION RADICAL FILM ON SILVER ELECTRODE SURFACES

Time-resolved SERRS spectroscopy was applied to elucidate the mechanis m of the reduction process of a heptylviologen monocation radical film to a neutral species on Ag electrode surfaces under conditions. The f ilm deposited on Ag electrodes at -0.65 V (vs. Ag/AgCl) consists of di mers, (HV+.)2. On application of a step potential from -0.65 to -1.2 V , the radical dimer is converted to a neutral species, HV0. The time-r esolved spectra measured as a function of time after application of th e step potential indicates that on the electrode immersed in KBr solut ions (0.3 and 3 mol l-1) the radical dimer is at first converted to an intermediate state, which is a surface complex of a monocation radica l monomer and a Br- ion (the radical monomer in a type B state), and t hen reduced to the neutral species. The time-resolved spectra proved a lso the existence of a disproportionation reaction, i.e. 2HV+. (type B ) --> HV2+ + HV0. The increase in the KBr concentration (0.3 --> 3 mol l-1) stabilizes the intermediate surface complex, causing an apprecia ble decrease in the reduction rate from (HV+.)2 to HV0. The reduction process on a silver electrode in 0.3 mol l-1 Na2SO4 consists of two re action paths; one is a direct conversion from (HV+.)2 to HV0 and anoth er is a path through a radical monomer, which gives SERRS features app reciably different from those of type B. The first process proceeds mu ch faster than that on the electrode in the KBr solutions.