Electrochemistry of the Self-Assembled Monolayers of Dyads Consisting of Tripod-Shaped Trithiol and Bithiophene on Gold

Self-assembled monolayers (SAMs) of tripod-shaped trithiols, consisting of an adamantane core with three CH(2)SH legs and a bithiophene group, were prepared on a Au(111) surface. Adsorption in a tripod-like fashion was supported by polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS) of the SAMs, which indicated the absence of free SH groups. Cyclic voltammetry showed an irreversible cathodic wave due to reductive desorption. The SAM also showed an anodic wave due to the single-electron oxidation of the bithiophene moiety without concomitant desorption of the molecules. Although oxidation was irreversible in the absence of a protecting group, it became reversible with the introduction of a terminal phenyl group. The charge of the oxidation was one-third that of the reductive desorption, confirming a three-point adsorption. The surface coverage was ca. 50% of that expected for the anti bithiophene conformation, which suggested that an increase in the surface area per molecule had been caused by the presence of an energetically high-lying syn conformer. In accordance with this, the line shape of the oxidation wave suggested an electrostatic repulsive interaction between neighboring molecules.