Electroactive Rotaxanes and Catenanes

The described systems show the potentialities of electrochemical techniques (the various kinds of voltammetry, chronoamperometry, coulometry, impedance spectroscopy, spectro- and photo-electrochemistry) in characterizing complex systems such as redox-active rotaxanes and catenanes. They provide, indeed, a fingerprint of these systems giving fundamental information on (i) the spatial organization of the redox sites within the molecular and supramolecular structure, (ii) the entity of the interactions between such sites, and (iii) the kinetic and thermodynamic stabilities of the reduced/oxidized and charge-separated species. Electrochemistry is, therefore, a powerful tool to ‘read’ the state of the system. In suitable designed rotaxanes and catenanes, however, electrochemistry can play a more important role. By causing the occurrence of endoergonic heterogeneous electron-transfer processes electrochemistry can, indeed, provide the energy needed to modify the noncovalent interactions that stabilize a certain rotaxane and catenane structure promoting mechanical movements. In such cases electrochemistry plays the dual role of ‘writing’ and ‘reading’ the system: by means of electrons and/or holes it supplies the energy to make theses systems work as molecular machines, and by means of the various electrochemical techniques it is used for controlling and monitoring the operation performed by the system. Although the examples described here evidence that electrochemists have learned how to deal with increasingly complex molecular and supramolecular structures, it is important to notice that electrochemistry is only a part of the game: as the complexity of the systems studied increases, the contribution from many disciplines in a joint and collaborative effort is needed. For the intriguing structures here described this statement is particularly true. The goal of transforming molecular devices and machines into practically useful products requires, indeed, that people belonging to different fields, like chemistry, solid-state physics, biology, computer science, mathematics, materials sciences, etc., work together and learn a common language.