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........ published in NEWSLETTER # 70
CURRENT CHALLENGES ON LARGE SUPRAMOLECULAR ASSEMBLIES
By Professor G. Tsoucaris, C.N.R.S. - University Paris-Sud (France)
The objectives of this Advanced Research Workshop (NATO ASI SERIES C519) were identifying areas and highlighting approaches by which large supramolecular (SM) assemblies can be achieved, as well as reviewing and combining methods to characterise and analyse such assemblies.
Novel Large SM Assemblies comprise synthetic entities with molecular weight as high as 15000. Further developments are foreseen at a higher level of organisation, such as between supermolecules or with electromagnetic fields in photochemical processes.
The difficulty in making large SM Assemblies and obtaining structural information has increased to such a level that a single technique is not sufficient. In this ARW, leading scientists both in SM synthesis and in structural characterisation gathered to examine current problems associated with the next generation of supramolecular compounds.
Upgrading and combining methodologies for structural characterisation is presently a main theme of active research:
- Direct Methods for crystal structures with 200-1000 atoms - Computer simulations for ionophores at interfaces - Studies on large clusters (nucleation) by hyperpolarised Xe NMR - Electrospray Mass Spectroscopy adapted to large Assemblies - Conformation changes (Dextran, Titin) from Single Molecule Force Spectrometry - Fabrication of SM assemblies by positioning individual molecules - Time Resolved Diffraction (picosecond) provides information on geometrical variations of molecules and complexes and on lattice behaviour during pulse laser illumination.
Synthesis pertains both to the chemical synthesis of components for SM assembly, and to the subsequent assembly process. Clearly the two steps are intimately linked. The latter is based on molecular recognition involving a large number of non-covalent interactions between components to drive and cooperatively assist the SM synthesis. The elaboration of multiple recognition "algorithms" concurrently employed (for instance, p-p and hydrogen bonds) has recently reached a high degree of sophistication in the sequence: molecules - supermolecule - supramolecular array - crystal or conglomerate.
As a general comment, the chapters of this book aim at a further bridging in areas of research or methods of investigation. Crystal structures are studied within the framework of creating and characterising organised entities in one, two and three dimensions. Quantum mechanics and other computational methods are bridged by crystallographic information. Individual molecules are now the object of experimental manipulation. This provides new information to be examined in conjunction with that obtained by collective methods (i.e. averaged information over a very large number of molecules) such as spectroscopy and diffraction. Atomic and mesoscopic scales are bridged by the synthesis of large synthetic supermolecules and, on the other side, by nanocrystals. Time dependent phenomena studied in the nanosecond and picosecond scale add a new dimension in the studies of formation and transformation of SM Assemblies.
Most of these bridged areas require vigorous progress in methodologies to meet the ever-increasing size, complexity and performance of the SM Assemblies.
Reference books: C473, C480, C484, C510, C519, C526, C527
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