elaboration of artificial molecular presses able to capture, compress and release other molecular species

Since the beginning of the 90s, the field of "molecular machines" has experienced a spectacular development. Books have even been devoted to this new area of research.[1] From the early days, to nowadays, spectacular developments can be noticed, related to the fabrication of real photomechanical devices and information storage devices. Catenanes and rotaxanes are particularly important in relation to molecular machines. Since the beginning of the 80s, our group has been interested in such interlocking or threaded ring systems.[2] The present proposal is concerned with new molecular assemblies belonging to the general family of "molecular presses". With this general term, we designate molecular ensembles containing two "plates" that can be brought closer or moved away from one another by gliding them along one or two axes in a controlled manner. Between the "plates" of the "press", it will be possible to imprison various organic molecules and to compress them or to allow them to relax and elongate. The nature of a "molecular press" is that of a dynamic multirotaxane, since rotaxanes are well suited to realize controlled molecular motions. In the first project we plan to prepare a new rotaxane, a [3]rotaxane consisting of two rings threaded by a rod. Each ring is attached to a porphyrin, the "plate" element of the "molecular press". The porphyrins attached to the macrocycle can be metallated and can trap molecules either by coordination bond, π-π as well as donor-acceptor interactions. The rod is composed of two side-by-side bipyridines at the centre of the rail and of two bulky groups acting as stoppers at both extremities. The principle of assembling the components, the rail and the macrocycles, is based on the template effect of Cu(I) that we use generally in the group. Cu(I) forms very stable 4-coordinate pseudo tetrahedral complexes with bidentate chelates so that in the Cu(I) [3]rotaxane the distance between the two pendant porphyrins is strictly controlled. Upon removal of Cu(I), the two macrocycles are free to rotate around the rod and to glide along it. Starting from the free rotaxane, an aromatic molecule can be trapped between the porphyrins of the macrocycles and be expelled upon metallation with Cu(I). Otherwise, the metallated porphyrins of the free rotaxane can also coordinate a ditopic ligand in an extended situation. Upon metallation with Cu(I), the motion of the macrocycles towards the central chelates will compress the ligand. The [3]rotaxane is therefore a "molecular press" in which the position of the macrocycles can be controlled by two elements: the porphyrins and the Cu(I). The second project is to build a more sophisticated system, a three-control "molecular press", in which the motion of the two rings from the centre of the rod towards its extremities is controlled by a redox signal. Therefore a new rod will be designed in which two side-by-side bipyridine chelates at the centre of the rod will be linked to two external terdentate ligands, terpyridines. Translation motion of the rings will be based on the rearrangement of the coordination sphere of Cu(I) upon oxidation as already used with success in our group with simpler systems. If both extremities of a long linear ligand are coordinated to the metallated porphyrins, the motion of the macrocycles upon oxidation of Cu(I) to Cu(II) will extend the linear fragment whereas reduction of Cu(II) should compress it. The system will thus work reversibly as a "molecular press" under an electrochemical stimulus. Extensions of these projects such as building two-dimensional dynamic systems and fast moving "presses" are described. In the field of artificial molecular machines and motors, an important motivation is certainly the synthetic challenge that the elaboration of such systems represents. It is also very challenging to reproduce some of the simplest functions of the natural biological motors using synthetic molecular systems. As far as practical applications are concerned, several possibilities can be explored. Information storage and processing at the molecular level is for the moment the most popular field of applications but other ambitious and futuristic practical outcomes could be considered such as the fabrication of molecular devices able to perform various functions: catalysis, transport of molecules or ions through a membrane, sort different molecules, act as valves or pumps, just to cite a few. These various functions have numerous possible applications in the fields of biological and medicinal chemistry. 1. V. Balzani, M. Venturi and A. Credi , Molecular Devices and Machines, Wiley-VCH, Weinheim, (2003). J.-P. Sauvage, Molecular Machines and Motors, Structure & Bonding, Springer, Berlin, Heidelberg, vol 99 (2001). 2. J.-P. Sauvage, C. Dietrich-Buchecker eds., Molecular Catenanes, Rotaxanes and Knots, Wiley-VCH, Weinheim, (1999).