Transition metal-complexed catenanes and rotaxanes as light-driven molecular machines prototypes

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." Our group has made and studied catenanes and rotaxanes since the early 80s. An important class of molecular machines is based on these interlocking or threaded ring compounds. Although a "molecular muscle" is briefly discussed in the present review article, we mostly focus on light-driven machines, consisting of ruthenium(II)-complexed rotaxanes or catenanes. The synthetic approach is based on the template effect of an octahedral ruthenium(II) centres. Two 1,10-phenanthroline ligands are incorporated in an axis or in a ring, affording the precursor to the rotaxane or the catenane, respectively. Ru(diimine)(3)(2+) 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. In the past, the (LF)-L-3 state has mostly been considered as detrimental and thus, its reactivity has not been frequently utilised. 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) centre.