Electrochemical in situ FT-IRRAS studies of a self-assembled monolayer of 2-(11-mercaptoundecyl)hydroquinone

Abstract

The potential-dependent structure change and irreversible anodic decomposition reaction of a 2-(11-mercaptoundecyl)hydroquinone (H₂QC₁₁SH) monolayer on a gold electrode surface in 0.1 mol l^–1 HClO₄ solution has been investigated by electrochemical in situ FT-IRRAS. A number of bands with good signal-to-noise ratios have been observed at 1700–1100 cm^–1 using p-polarization measurement but no bands were observed by s-polarization measurement when the potential was less than +1.2 V. The bands in this region corresponded well to the redox reaction of the terminal quinone group in the monolayer. The rate of this redox reaction was slow and its kinetics were discussed using the results obtained by the time-dependent FT-IRRAS measurements. When the electrode potential became more positive than +1.4 V, anodic current was present corresponding to oxidative decomposition of the monolayer, the IR bands at 1700–1100 cm^–1 decreased in the p-polarization spectra and the bands at this frequency appeared in the s-polarization spectra. Bands at 2920 and 2850 cm^–1 due to the C—H stretch and those at 1380 and 2342 cm^–1 were also observed both by p- and s-polarization in this potential region. The bands at 1380 and 2342 cm^–1 are attributed to the SO₂ stretch of alkylsulfonic acid, R—SO₃H, and CO₂, respectively. The results suggest that the monolayer was oxidized by anodic cleavage of an S—Au bond to form R—SO₃H which possibly existed within the partially decomposed monolayer and was further oxidized to CO₂.