Rotaxanes are molecular architectures ideally suited for building integrated, multicomponent modular systems, displaying novel chemical and physical properties. In this new field of functional rotaxanes, those incorporating transition metals, which are incidentally used as synthetic templates, are particularly attractive for their photophysical and electronic properties as well as their dynamic behaviour. It is believed that they will provide, in the future, the basic elements for constructing nanoscale machines and motors.

The authors describe their study of molecular systems suited to the fabrication of machines and (rotory or linear) motors at the molecular level. They indicate that a future application of these molecular ‘muscles’ could be in the area of information storage and processing.

In the course of the last 20 years, our view on rotaxanes and catenanes has completely changed. Copper(I)-templated strategies, in particular, have allowed us to prepare catenanes on a real preparative scale, in a few chemical steps from commercially available compounds. A particularly significant improvement was the introduction of the recently developed ring-closing metathesis reaction, using Grubbs catalyst. The dynamic properties of rotaxanes and catenanes has been exploited to construct molecular systems for which one component can be set in motion under the action of an external signal, while the other components can be considered as motionless (artificial molecular “machines ”and “motors ”). A particularly representative example is that of a rotaxane dimer, whose overall length can be controlled chemically: A metal exchange reaction (Cu^I/Zn^II) triggers a reversible contraction/stretching process of the same molecular assembly, in a way reminiscent of the functioning of biological muscles.