Transition-metal-complexed molecular machine prototypes

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)(2+)(3) and Ru(terpy)(phen)(L)(2+) (terpy: terpyridine; phen: 1,10-phenanthroline) complexes display the universally used (MLCT)-M-3 (metal-to-ligand charge transfer) excited state and another interesting excited state, the (LF)-L-3 (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 (LF)-L-3 state, thus leading to ligand exchange in the coordination sphere of the ruthenium(ii) center.