Towards molecular machines and motors based on transition metal complexes

Biological motors and machines are multicomponent assemblies undergoing large-amplitude geometrical changes or leading to the locomotion of one of the components, under the action of either an external stimulus (pH change, redox process, light pulse, etc.) or a chemical gradient. Many examples are known of proteins which undergo important shape modifications (folding-defolding) after a signal has been sent to the protein. Threaded or interlocking rings are ideally suited to the construction of fully artificial molecular motors. If a ring is threaded onto a rod, it can either rotate around the axle or undergo a translation movement. Similarly, in catenanes (i.e. interlocking ring multicomponent systems), a ring can glide at will and spin within another ring. Several examples of such compounds have been elaborated and studied in recent years, using threaded and interlocking molecules. Our group has proposed several molecular assemblies acting as 'machines.' They are based on transition metal complexes and the systems are set in motion by sending an electrochemical or photochemical signal. A recent contribution from our team describes a doubly threaded compound which can be contracted or stretched at will. By sending a chemical signal to the molecule, one can readily convert the contracted compound [copper(I) complex] to the extended situation (zinc complex). The back reaction is also easily carried out and it is quantitative. Potentially, the system is ready to react to an electrochemical stimulus and lead to the same movement. The present molecular assembly is thus reminiscent of skeletal muscles. Copyright (C) 2002 John Wiley Sons, Ltd.