Abstract
Self-assembled monolayers (SAMs) of tripod-shaped trithiols, consisting of an adamantane core with three CH2SH 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.
Highlights
The tight binding of a sulfur atom in an organic molecule to a metal gold surface provides a secure and facile way to prepare a self-assembled monolayer (SAM) [1,2,3,4], which allows a reliable means for interface functionalization
We further succeeded in preparing a trithiol with terminal ferrocene 2, in which the ferrocenyl group was connected with an adamantane tripod through a linear phenyleneethynylene linker [25,26,27]
A reversible cyclic voltammogram for the single-electron oxidation was obtained by capping the terminal thiophene ring with a phenyl group
Summary
The tight binding of a sulfur atom in an organic molecule to a metal gold surface provides a secure and facile way to prepare a self-assembled monolayer (SAM) [1,2,3,4], which allows a reliable means for interface functionalization. We synthesized dyads 3a and 3b, which consist of another electroactive molecule, bithiophene, connected with the adamantane tripod, and investigated the electrochemistry of their SAMs. SAMs of oligothiophenes with a monodentate anchor [20,35,36], bidentate anchor [37] or triarylmethyl tripod anchor [17,18,19,20,21,22] have been used in the construction of molecular electronic devices, such as photovoltaic cells and light-emitting diodes. Reduced surface coverages of tripods 3, if observed, could be good indicators for the presence of syn-linked bithiophene in SAMs (Figure 2)
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