A self-assembled monolayer of 1-dodecanethiol (C12SH) on a gold electrode was prepared by dipping a polished bare electrode into an ethanolic solution of C12SH (1.0 mmol dm-3) for 10 min. The obtained modified electrode blocked the direct electron-transfer reaction of [Fe(CN)6]3- and [Fe(CN)12]4- with the underlying electrode. However, a pH-dependent redox compound, 4,4′-biphenyldiol (1) in solution, was permeable into the monolayer and gave well-defined cyclic voltammograms at both higher and lower pHs. Using this chemical-recognition ability of the modified electrode together with the formal potential shift of 1 according to the Nernst equation, pH-modulated vectorial electron transfer reactions were examined. At pH 3, cyclic voltammograms at the modified electrode in a 1 + |Fe(CN)6]4- aqueous solution revealed that one-way electron flow from |Fe(CN)]4- to the electrode via 1 (oxidized form) occurred. On the contrary, at higher pHs (pH 9 - 11), the vectorial electron flow at the electrode in 1 + [Fe(CN)6]3- solution was reversed. A triggered change ('switching') between the anodic and cathodic vectorial electron transfer reactions by changing the pH was also possible at the C12SH-modified electrode in the presence of 1, [Fe(CN)6]4- and [Fe(CN)6]3- in solution. This finding may offer possibilities for the construction of functional molecular devices based on the tuning of rectified electron transfer reactions.
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