Transition-Metal-Complexed Molecular Machine Prototypes

• Published in: Advanced materials (Weinheim) 2006-05, Vol.18 (10), p.1239-1250
• Main Authors: Bonnet, S., Collin, J.-P., Koizumi, M., Mobian, P., Sauvage, J.-P.
• Format: Article
• Language: English
• Subjects: Biomimetics; Catenanes; Electrochemistry; Metal-organic materials; Molecular machines; Rotaxanes
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.200502394

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. Transition metal‐containing catenanes and rotaxanes are ideally suited to building “molecular machines”. This review describes some examples of prototypes elaborated in Strasbourg, focusing on light‐driven systems using ruthenium(II) complexes. An electrochemically driven system and a linear rotaxane dimer whose behavior is reminiscent of that of muscles (see figure) are also presented.