VII-C-1 Effects of Alkyl Chain as a Spacer on Electrochemical Reaction and SEIRA Spectra for Self-Assembled Monolayer Having Anthraquinone Redox Center

Self-assembled monolayers (SAMs) of 1-mercaptoanthraquinone (AQSC0), bis(3-(1-anthraquinonylthio)propyl)disulfide (AQSC3) and bis(6-(1-anthraquinononylthio)hexyl)disulfide (AQSC6), on a gold electrode were investigated by cyclic voltammetry and surface enhanced infrared absorption spectroscopy (SEIRAS). A redox wave was observed for all the anthraquinone (AQ) derivatives and the redox potential shifted ca. –60 mV/pH in the pH region from 1 to 7. The peak separation of the voltammograms depended on the spacer length of the AQ derivatives, and the heterogeneous electron transfer rates of AQSC0, AQSC3 and AQSC6 at pH 1 were 7.5, 2.3 × 10–1, and 3.1 × 10–3 s–1, respectively. For the SEIRA spectra of AQSC0 and AQSC3, the C=O stretching and C–C stretching bands of AQ ring were clearly observed, suggesting that the AQ ring plane is perpendicular or tilted to the electrode surface. The SEIRA signal of the AQ derivatives decreased with an increase in the alkyl chain length, indicating taht the AQSC3 and AQSC6 molecules were adsorbed on the electrode not by the sulfide S atoms but by the thiolate S atoms.