Structural differences in self-assembled monolayers of anthraquinone derivatives on silver and gold electrodes studied by cyclic voltammetry and in situ SERS spectroscopy

Abstract

Self-assembled monolayers (SAMs) of 1-anthraquinonylbutylsulfide (1-AQS) and 2-anthraquinonylbutylsulfide (2-AQS) on silver and gold electrodes were investigated by cyclic voltammetry and in situ surface enhanced Raman spectroscopy (SERS). A reversible redox wave was observed for 1-AQS and the redox potential shifted ca. −30 and −60 mV pH −1 at silver and gold electrodes, respectively, in the pH region 5–9. On silver electrodes, the electrochemical properties of 1-AQS were similar to those of 2-AQS. In addition, the Raman spectrum of 1-AQS powder is very similar in intensity and peak positions to that of 2-AQS powder. However, in situ SERS measurements showed that the 1-AQS molecule was tilted relative to the silver surface normal and no change in its orientation took place between oxidized and reduced forms. The oxidized 2-AQS molecule was shown to be adsorbed perpendicular to the silver surface, but on reduction at −0.6 V the orientation of the anthraquinone plane changed. On gold electrodes, no clear SERS spectrum was obtained for the SAM of 1-AQS, suggesting a flat orientation of the anthraquinone plane relative to the electrode surface. SERS spectra of 2-AQS on the gold surfaces gave clear spectra, suggesting a perpendicular orientation of the anthraquinone plane relative to the gold surface independent of the applied potential. These differences in orientation are ascribed to the position of the butylsulfide moiety on the anthraquinone ring at gold and silver electrodes.