Supramolecular chemistry

History

The existence of intermolecular forces was first postulated by host-guest chemistry. In the early twentieth century noncovalent bonds were understood in gradually more detail, with the hydrogen bond being described by Latimer and Rodebush in 1920.

The use of these principles led to an increasing understanding of microemulsions.

Eventually, chemists were able to take these concepts and apply them to synthetic systems. The breakthrough came in the 1960s with the synthesis of the Jean-Marie Lehn and Fritz Vogtle became active in synthesizing shape- and ion-selective receptors, and throughout the 1980s research in the area gathered a rapid pace with concepts such as mechanically-interlocked molecular architectures emerging.

The importance of supramolecular chemistry was established by the 1987 Nobel Prize for Chemistry which was awarded to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen in recognition of their work in this area.^[4] 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.

In the 1990s, supramolecular chemistry became even more sophisticated, with researchers such as dendrimers becoming involved in synthetic systems.

Control of supramolecular chemistry

Thermodynamics

Supramolecular chemistry deals with subtle interactions, and consequently control over the processes involved can require great precision. In particular, noncovalent bonds have low energies and often no chemical equilibrium equations show that the low bond energy results in a shift towards the breaking of supramolecular complexes at higher temperatures.

However, low temperatures can also be problematic to supramolecular processes. Supramolecular chemistry can require molecules to distort into thermodynamically disfavored conformations (e.g. during the "slipping" synthesis of molecular mechanics), and cooling the system would slow these processes.

Thus, thermodynamics is an important tool to design, control, and study supramolecular chemistry. Perhaps the most striking example is that of warm-blooded biological systems, which cease to operate entirely outside a very narrow termperature range.

Environment

The molecular environment around a supramolecular system is also of prime importance to its operation and stability. Many solvents have strong hydrogen bonding, electrostatic, and charge-transfer capabilities, and are therefore able to become involved in complex equilibria with the system, even breaking complexes completely. For this reason, the choice of solvent can be critical.

Concepts in supramolecular chemistry

Molecular self-assembly

crystal engineering.

Molecular recognition and complexation

catalysis.

Template-directed synthesis

Molecular recognition and self-assembly may be used with reactive species in order to pre-organize a system for a chemical reaction (to form one or more covalent bonds). It may be considered a special case of supramolecular stereochemistry. After the reaction has taken place, the template may remain in place, be forcibly removed, or may be "automatically" decomplexed on account of the different recognition properties of the reaction product. The template may be as simple as a single metal ion or may be extremely complex.

Mechanically-interlocked molecular architectures

molecular Borromean rings.

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.

Biomimetics

Many synthetic supramolecular systems are designed to copy functions of biological systems. These biomimetic architectures can be used to learn about both the biological model and the synthetic implementation. Examples include photoelectrochemical systems, catalytic systems, self-replication.

Imprinting

Molecular imprinting describes a process by which a host is constructed from small molecules using a suitable molecular species as a template. After construction, the template is removed leaving only the host. The template for host construction may be subtly different from the guest that the finished host bind. In its simplest form, imprinting utilizes only steric interactions, but more complex systems also incorporate hydrogen bonding and other interactions to improve binding strength and specificity.

Molecular machinery

Molecular machines are molecules or molecular assemblies that can perform functions such as linear or rotational movement, switching, and entrapment. These devices exist at the boundary between supramolecular chemistry and nanotechnology, and prototypes have been demonstrated using supramolecular concepts.

Building blocks of supramolecular chemistry

Supramolecular systems are rarely designed from first principles. Rather, chemists have a range of well-studied structural and functional building blocks that they are able to use to build up larger functional architectures. Many of these exist as whole families of similar units, from which the analog with the exact desired properties can be chosen.

Synthetic recognition motifs

• The pi-pi charge-transfer interactions of bipyridinium with dioxyarenes or diaminoarenes have been used extensively for the construction of mechanically interlocked systems and in crystal engineering.
• The use of crown ether binding with metal or ammonium cations is ubiquitous in supramolecular chemistry.
• The formation of carboxylic acid dimers and other simple hydrogen bonding interactions.
• The complexation of silver or other metal ions is of great utility in the construction of complex architectures of many individual molecules.
• The complexation of phthalocyanines around metal ions gives access to catalytic, photochemical and electrochemical properties as well as complexation. These units are used a great deal by nature.

Macrocycles

Macrocycles are very useful in supramolecular chemistry, as they provide whole cavities that can completely surround guest molecules and may be chemically modified to fine-tune their properties.

• crown ethers are readily synthesized in large quantities, and are therefore convenient for use in supramolecular systems.
• More complex cryptands can be synthesized to provide more taliored recognition properties.

Structural units

Many supramolecular systems require their components to have suitable spacing and conformations relative to each other, and therefore easily-employed structural units are required.

• Commonly used spacers and connecting groups include biphenyls and triphenyls, and simple alkyl chains. The chemistry for creating and connecting these units is very well understood.
• dendrimers offer nanometer-sized structure and encapsulation units.
• self-assembled monolayers and multilayers.

Photo-/electro-chemically active units

• phthalocyanines have highly tunable photochemical and electrochemical activity as well as the potential for forming complexes.
• Photochromic and photoisomerizable groups have the ability to change their shapes and properties (including binding properties) upon exposure to light.
• fullerenes, have also been utilized in supramolecular electrochemical devices.

Biologically-derived units

• The extremely strong complexation between biotin is instrumental in blood clotting, and has been used as the recognition motif to construct synthetic systems.
• The binding of enzymes with their cofactors has been used as a route to produce modified enzymes, electrically contacted enzymes, and even photoswitchable enzymes.
• DNA has been used both as a structural and as a functional unit in synthetic supramolecular systems.

Applications

Materials technology

Supramolecular chemistry and nanotechnology are based on supramolecular chemistry.

Catalysis

A major application of supramolecular chemistry is the design and understanding of catalysis. Noncovalent interactions are extremely important in catalysis, binding reactants into conformations suitable for reaction and lowering the transition state energy of reaction. Template-directed synthesis is a special case of supramolecular catalysis. Encapsulation systems such as micelles and dendrimers are also used in catalysis to create microenvironments suitable for reactions (or steps in reactions) to progress that is not possible to use on a macroscopic scale.

Medicine

Supramolecular chemistry is also important to the development of new pharmaceutical therapies by understanding the interactions at a drug binding site. The area of protein-protein interactions that are important to cellular function.

Data storage and processing

Supramolecular chemistry has been used to demonstrate computation functions on a molecular scale. In many cases, photonic or chemical signals have been used in these components, but electrical interfacing of these units has also been shown by supramolecular signal transduction devices. Data storage has been accomplished by the use of molecular logic gates have been demonstrated on a conceptual level. Even full-scale computations have been achieved by semi-synthetic DNA computers.

Green chemistry

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.

Other Devices and Functions

Supramolecular chemistry is often pursued to develop new functions that cannot appear from a single molecule. These functions also include CAT scans.

References