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

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^o = -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.