Electroreductive deposition of anthraquinone derivative attached Au clusters: Optical properties and scanning tunneling microscopy observation of the electrodeposited cluster film

Anthraquinone derivative modified Au clusters with a 2.2 nm core diameter prepared by a substitution reaction of octyl thiolate-covered Au clusters with 1-( 1,8-dithiaoctyl)anthracene-9, 10-dione (AQS) underwent two-step one-electron reduction in aprotic solvents, and the second reduction potential was significantly more negative than that of free AQS. The magnitude of this potential shift became larger for the clusters in association with increase in the number of exchanged anthraquinone-terminated thiolates due to the electronic interaction between the adjacent anthraquinone moieties on the Au cluster surface. By electrochemical reduction of the anthraquinone moieties, a redox-active Au cluster film was formed on the electrode. The energy of the surface plasmon band of the film on ITO was shifted with the change in the solvent refractive index from n(d)(20) = 1.33 to 1.62, and the V-shaped profile of the energy shift vs nd20 plots is inexplicable by Mie theory. The band was also blue-shifted by 12 nm by changing the potential in the negative direction between -1.0 and -2.0 V vs Ag/Ag+ in Bu4NClO4-MeCN. The scanning tunneling microscopy image revealed that spherical Au clusters aggregated to form an organized assembly at the early stage of the electrodeposition process.