The field of molecular machines, i.e. multicomponent systems able to undergo large amplitude motions under the action of an external signal, has experienced a spectacular development since the beginning of the 1990s. Transition metal complexes have played an important role in this context, often as components of catenanes and rotaxanes. The present tutorial review will discuss a few systems of this type, taken from the contributions of our group or from others. The stimulus responsible for the controlled motion of the machine can be chemical, electrochemical, or photochemical. Examples of these three categories will be considered.

In the course of the last decade, many dynamic molecular systems, whose movements are controlled externally, have been elaborated. These compounds are generally referred to as “molecular machines”. Transition‐metal‐containing catenanes and rotaxanes are ideally suited to build such systems. In the present article, we will discuss a few examples of molecular machines elaborated and studied in Strasbourg. In the first section, we will discuss an electrochemically driven system consisting of a fast‐moving pirouetting rotaxane. The second section will be devoted to a linear rotaxane dimer whose behavior is reminiscent of muscle movement, in the sense that it can stretch and contract. In the rest of this article, the focus will be mostly on light‐driven machines consisting of ruthenium(II)‐complexed rotaxanes, catenanes, and scorpionates. In the case of rotaxanes and catenanes, the synthetic approach is based on the template effect of an octahedral ruthenium(II) center. Two polydentate ligands are incorporated in an axis or in a ring, affording the precursor to the rotaxane or the catenane, respectively. Ru(diimine)32+ and Ru(terpy)(phen)(L)2+ (terpy: terpyridine; phen: 1,10‐phenanthroline) complexes display the universally used 3MLCT (metal‐to‐ligand charge transfer) excited state and another interesting excited state, the 3LF (ligand field) state, which is strongly dissociative. By taking advantage of this latter state, it has been possible to propose a new family of molecular machines, which are set in motion by populating the dissociative 3LF state, thus leading to ligand exchange in the coordination sphere of the ruthenium(II) center.

A marked difference is observed in the photochemistry of [2]catenanes containing a complexed ruthenium(II) center. The photochemical reactions result in a decoordination process and large amplitude motions through the formation of a strongly dissociative ligand‐field excited state (see example, L=CH3CN or Cl−), with the rate dependent on the ring size. The back reaction is performed thermally.