Transition metal-complexed catenanes and rotaxanes in motion: Towards molecular machines

In the course of the last decade, many dynamic molecular systems, for which the movements are controlled from the outside, 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 review 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 paragraph will be devoted to a linear rotaxane dimer whose behaviour is reminiscent of muscles, in the sense that it can be stretched or contracted, and in the rest of this review article, we will mostly focus on light-driven machines, consisting of ruthenium(II)-complexed rotaxanes or catenanes. For these latter systems, the synthetic approach is based on the template effect of an octahedral ruthenium(II) centre. Two 1,10-phenanthroline ligands are incorporated in an axis or in a ring, affording the precursor to the rotaxane or the catenane, respectively. image 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) centre. Finally, the potential of the Ru(terpy)(phen) core in the frame of light-driven molecular machines will be briefly discussed.