Electromechanics of a Redox-Active Rotaxane in a Monolayer Assembly on an Electrode

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Abstract
A rotaxane monolayer consisting of the cyclophane, cyclobis(paraquat-p-phenylene) (2), threaded on a “molecular string” that includes a π-donor diiminobenzene unit and stoppered by an adamantane unit is assembled on a Au electrode. The surface coverage of the electroactive cyclophane unit, E° = −0.43 V vs SCE, corresponds to 0.8 × 10-10 mol.cm-2. The cyclophane (2) is structurally localized on the molecular string by generating a π-donor−acceptor complex with the diiminobenzene units of the molecular string. The cyclophane (2) acts as a molecular shuttle, revealing electrochemically driven mechanical translocations along the molecular wire. Reduction of the cyclophane (2) to the respective biradical−dication results in its dissociation from the π-donor site, and the reduced cyclophane is translocated toward the electrode. Oxidation of the reduced cyclophane reorganizes 2 on the π-donor-diiminobenzene sites. The positions of the oxidized and reduced cyclophane units are characterized by chronoamperometric and impedance measurements. Using double-step chronoamperometric measurements the dynamics of the translocation of the cyclophane units on the molecular string is characterized. The reduced cyclophane moves toward the electrode with a rate constant corresponding to k1 = 320 s-1, whereas the translocation of the oxidized cyclophane from the electrode to the π-donor binding site proceeds with a rate constant of k2 = 80 s-1. Also, in situ electrochemical/contact angle measurements reveal that the electrochemically driven translocation of the cyclophane on the molecular string provides a means to reversibly control the hydrophilic and hydrophobic properties of the surface. The latter system demonstrates the translation of a molecular motion into the macroscopic motion of a water droplet.