Supramolecular chemistry

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An example of a supramolecular assembly reported by Jean-Marie Lehn and coworkers in Angew. Chem., Int. Ed. Engl. 1996, 35, 1838-1840.

An example of a mechanically-interlocked molecular architecture in this case a rotaxane reported by Stoddart and coworkers in the Eur. J. Org. Chem. 1998, 2565-2571.

An example of a host-guest chemistry reported by Sanders and coworkers in Angew. Chem., Int. Ed. Engl. 1995, 34, 1096-1099.

host-guest complex with a p-xylylenediammonium bound within a cucurbituril reported by Freeman in Acta. Crystallogr. B, 1984, 382-387.

Intramolecular self-assembly of a foldamer reported by Lehn and coworkers in Helv. Chim. Acta., 2003, 86, 1598-1624.

Supramolecular chemistry refers to the area of chemistry which focuses on the noncovalent bonding interactions of molecules ^[1]. Traditional organic synthesis involves the making and breaking of covalent bonds to construct a desired molecule. In contrast, supramolecular chemistry utilizes far weaker and reversible noncovalent interactions, such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi-pi interactions, and/or electrostatic effects to assemble molecules into multimolecular complexes. Important concepts that have been demonstrated by supramolecular chemistry include molecular self-assembly, molecular recognition, host-guest chemistry, mechanically-interlocked molecular architectures, and dynamic covalent chemistry.

[edit] History

The importance of supramolecular chemistry was recognized by the 1987 Nobel Prize for Chemistry which was awarded to Donald J. Cram, Jean-Marie Lehn, Charles J. Pedersen in recognition of their work in this area. The development of selective "host-guest" complexes in particular, in which a host molecule recognizes and selectively binds a certain guest, was cited as an important contribution.

Research in this area has it origins in biological systems which are highly dependent on noncovalent interactions to function. For instance, the important breakthrough that allowed the elucidation of the double helical structure of DNA occurred when it was realized that there were two separate strands of nucleotides connected through hydrogen bonds. The use of noncovalent bonds is essential to replication because they allow the strands to be separated and used to template new double stranded DNA.

[edit] Subdivisions

[edit] Molecular self-assembly

Molecular self-assembly is the assembly of molecules without guidance or management from an outside source. The molecules are directed to assemble through noncovalent interactions. There are two forms including both intermolecular self-assembly to form a supramolecular assembly and intramolecular self-assembly or folding as demonstrated by foldamers. Molecular self-assembly also allows construction of larger structures such as micelles, membranes, vesicles, liquid crystals, and is important to crystal engineering.

[edit] Molecular recognition

Molecular recognition the specific binding of a guest molecule to a complementary host molecule. The molecules are able to identify each other using noncovalent interactions. One of the key applications of this field is the construction of molecular sensors.

[edit] Host-guest chemistry

Host-guest chemistry is the study of complexes that are composed of two or more molecules held together in unique structural relationships by noncovalent bonds. There are a number of common host molecules such as cyclodextrins, calixarenes, cucurbiturils, porphyrins, crown ethers, and cryptands.

[edit] Mechanically-interlocked molecular architectures

Mechanically-interlocked molecular architectures are the connection of molecules not through traditional bonds, but instead as a consequence of their topology. Supramolecular chemistry is key to the efficient synthesis of the compounds. Examples of mechanically-interlocked molecular architectures including catenanes, rotaxanes, molecular knots, and molecular Borromean rings.

[edit] Dynamic covalent chemistry

In dynamic covalent chemistry covalent bonds are broken and formed in a reversible reaction under thermodynamic control. While covalent bonds are key to the process the system is directed by noncovalent forces to form the lowest energy structures.

[edit] Applications

Supramolecular chemistry and molecular self-assembly processes in particular have been applied to the development of new materials. Large structures can be readily accessed using bottom-up synthesis as they are composed of small molecules requiring fewer steps to synthesize. Thus most of the bottom-up approaches to nanotechnology are based on supramolecular chemistry.

Supramolecular chemistry is often pursued to develop new functions that cannot appear from a single molecule. These functions include magnetic properties, light responsiveness, catalytic activity, self-healing polymers, molecular sensors., etc. Supramolecular research has been applied to develop high-tech sensors, processes to treat radioactive waste, compact information storage devices for computers, high-performance catalysts for industrial processes, and contrast agents for CAT scans.

Supramolecular chemistry is also important to the development of new pharmaceutical therapies by understanding the interactions at a drug binding site. In addition, supramolecular systems have been designed to disrupt protein-protein interactions that are important to cellular function.

Research in supramolecular chemistry also has application in green chemistry where reactions have been developed which proceed in the solid state directed by non-covalent bonding. Such procedures are highly desirable since they reduce the need for solvents during the production of chemicals.

[edit] References