Artificial molecular shuttles: from concepts to devices

The bottom-up construction and operation of mechanical machines of molecular size is a topic of great interest in nanoscience, and a fascinating challenge of nanotechnology. Researchers in this field are stimulated and inspired by the progress of molecular biology that has begun to reveal the secrets of the natural nanomachines which constitute the material base of life. In an artificial context, the most common implementation of the molecular machine concept is represented by molecular shuttles. They are rotaxanes equipped with (at least) two distinct recognition sites on the axle portion of their dumbbell-shaped component, in which the interlocked macrocyclic component can be displaced from one site to the other in a controlled manner as a result of an external stimulation. Here we present the strategies at the basis of the design of molecular shuttles, illustrate the operation of a selected number of examples working in solution and on surfaces, and describe recent attempts of constructing functional devices and materials based on molecular shuttles. Open problems and perspectives of these kinds of systems are also critically discussed.