Electromechanics of a redox-active rotaxane in a monolayer assembly on an electrode

A rotaxane monolayer consisting of the cyclophane, cyclobis(paraquat-p-phenylene) (2), threaded on a "molecular string" that includes a pi-donor diiminobenzene unit and stoppered by an adamantane unit is assembled on a Au electrode. The surface coverage of the electroactive cyclophane unit, E degrees = -0.43 V vs SCE, corresponds to 0.8 x 10(-10) mol.cm(-2). The cyclophane (2) is structurally localized on the molecular string by generating a pi-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 pi-donor site, and the reduced cyclophane is translocated toward the electrode. Oxidation of the reduced cyclophane reorganizes 2 on the pi-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 k(1) = 320 s(-1), whereas the translocation of the oxidized cyclophane from the electrode to the pi-donor binding site proceeds with a rate constant of k(2) = 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.