Multicycle Desorption-Adsorption Voltammetry for Self-Assembled Mixed Monolayer Containing Ferrocenylthiol Molecules: A Discussion on Molecular Interaction in the Mixed Layer

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Self-assembled mixed monolayers (mixed SAMs) composed of 11-ferrocenyl-1-undecanethiol (11-FUT) and hexanethiol (HT) molecules were prepared through an exchange method and characterized by multicycle electrochemical desorption-adsorption voltammetry. The first cycles of the voltammograms showed a single desorption peak although the SAMs have a domain structure composed of compact 11-FUT molecules, indicating that some residual HT molecules were present in 11-FUT domains and that complete phase separation did not take place in the mixed SAMs. The structure, however, altered toward a mildly phase-separated structure when the mixed SAMs experienced desorption-adsorption cycle(s), and two separate reductive desorption peaks were found in the second cycles of voltammograms, which clearly evidenced that all mixed SAMs (before voltammetric analysis), including those prepared by prolonged exchange reactions, really contained both 11-FUT and HT molecules. This demonstrates the importance of such multicycle voltammetry for mixed SAMs that are usually characterized and discussed only in terms of the first reductive, or negative-going, scan of voltammograms. Even after the desorption-adsorption cycle(s), ferrocenyl groups in mixed SAMs underwent FeII/FeIII redox, while the intensity of redox peaks became weakened.
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