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Supramolecular Chemistry

Supramolecular chemistry is a relatively new field of chemistry which focuses quite literally on going "beyond" molecular chemistry. It can be described as the study of systems which contain more than one molecule, and it aims to understand the structure, function, and properties of these assemblies. Interest in supramolecular chemistry arose when chemistry had become a relatively mature subject and the synthesis and properties of molecular compounds had become well understood. The domain of supramolecular chemistry came of age when Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen were jointly awarded the Nobel Prize for Chemistry in 1987 in recognition of their work on "host-guest" assemblies (in which a host molecule recognises and selectively binds a certain guest). Other examples of supramolecular systems include biological membranes, polynuclear metal complexes, liquid crystals, and molecule-based crystals. Even a cell can be envisaged as a (very complex!) supramolecular system and indeed recent research has targeted assemblies involving biopolymers such as nucleic acids, and proteins.

A supramolecular assembly is a multi-component system of atoms, ions, and/or molecules which are held together by non-covalent interactions such as hydrogen bonds, van der Waals forces, pi-pi interactions, and/or electrostatic effects. The latter mode of bonding is particularly important for assemblies involving metal ions. These various bonding interactions are far weaker than covalent bonding (which are the kind of bonds which hold molecules together) therefore supramolecular assemblies are usually far less stable than molecular compounds (for example, they can be more suceptible to breaking apart at high temperatures or if they are mixed with acid).

How are supramolecular assemblies formed? The most straightforward method is to use self-assembly techniques, which is to say that you mix your components under given set of conditions (solvent, temperature, pH etc.) then cross your fingers and hope for the best. Of course, most chemists try to approach things a little more rationally than this and attempt to design synthetic routes based on predicted interactions between the various components of the mixture. Happily, self-assembly processes often converge on a single product out of the coutless possible combinations of your starting materials. This product is formed because it is the thermodynamically most stable arrangement of the constituent entities. Chemists get very excited about self-assembly as they see it as being a very efficient and reliable method for the "bottom-up" synthesis of new materials (as opposed to current "top-down" methods for producing, for example, computer chips).

During the self-assembly process, inevitably some of your starting materials will go down the 'wrong track' towards other products. It therefore turns out to be very useful that the bonding interactions between the components of these assemblies are quite weak. This is because the 'wrong' products are easily dis-assembled and the components can quickly recombine in the 'right' way to form the most stable assembly. This 'reversibility' is one of the key features of supramolecular synthesis and it contrasts with the situation in conventional molecular synthesis involving covalent bonds. In molecular synthesis, a reaction which goes down the 'wrong pathway' often ends up at a dead end and the material which is formed must in the end be separated from the desired product.

One of the major goals of supramolecular chemists is the synthesis of supramolecular assemblies which have new functions that cannot appear from a single molecule or ion. These functions are based on novel magnetic properties, light responsiveness, catalytic activity, fluorescence, redox properties, etc., of supramolecular systems. These useful properties may lead to the application of these assemblies as—and this is a list of random examples—high-tech sensors for pollutants in air or water, compact information storage devices for next-generation computers, as high-performance catalysts in industrial processes, or as contrast agents for CAT scans. Supramolecular chemistry is intimately related to nanotechnology, and many promising nanotech devices are based on the principles of supramolecular chemistry.