From Molecular to Supramolecular Chemistry

Chemistry is the science of matter and of its transformations, and life is its highest expression. It provides structures endowed with properties and develops processes for the synthesis of structures. It plays a primordial role in our understanding of material phenomena, in our capability to act upon them, to modify them, to control them and to invent new expressions of them. 

Chemistry is also a science of transfers, a communication centre and a relay between the simple and the complex, between the laws of physics and the rules of life, between the basic and the applied. If it is thus defined in its interdisciplinary relationships, it is also defined in itself, by its object and its method. 

In its method, chemistry is a science of interactions, of transformations and of models. In its object, the molecule and the material, chemistry expresses its creativity. Chemical synthesis has the power to produce new molecules and new materials with new properties. New indeed, because they did not exist before being created by the recomposition of atomic arrangements into novel and infinitely varied combinations and structures [1.1]. 

For more than 150 years, since the synthesis of urea by Friedrich Wohler in 1828 [1.2], molecular chemistry has developed a vast array of highly sophisticated and powerful methods for the construction of ever more complex molecular structures by the making or breaking of covalent bonds between atoms in a controlled and precise fashion. 

Wohler s urea to the synthesis of Vitamin Bi2 by Robert B. Woodward [1.3] and Albert Eschenmoser [1.4] assisted by a hundred or so collaborators  

---

Fig. 1. From molecular to supramolecular chemistry molecules, supermolecules, molecular and supramolecular devices.

The emergence of the concepts of supramolecular chemistry has led to the introduction of new terms or to the reappropriation of old ones, describing either classes of compounds or types of properties. As the field affirmed itself, the passage from molecular to supramolecular chemistry brought about a change in objects and goals from structures and properties towards systems and functions. 

Turro NJ. From molecular chemistry to supramolecular chemistry to superdupermolecu-lar chemistry. Controlling covalent bond formation through non-covalent and magnetic interactions. Chem Commun 2002 2279-92. 

Over the past years the accuracy of molecular structure determination has increased. This is due not only to improved experimental and computational facilities, and to the combined application of various techniques, but also to a better understanding of the physical meaning of structural information from different sources. Critical assessment of structural information facilitates its application to investigate intramolecular and intermolecular interactions and their consequences on the rest of the molecular structure. Supramolecular chemistry poses new challenges to accurate molecular structure determination at various levels of complexity of chemical systems. 

Since reference was made to supramolecular chemistry, perhaps it is use fill to remember some definitions. Supramolecular chemistry is the chemistry of molecular assemblies and of the intermolecular bond. It is the chemistry beyond the molecule and deals with organized entities of higher complexity that result from the association oftwo or more chemical species held together by intermolecular forces ... 

On the way from molecular to macromolecular and supramolecular chemistry, researchers have realized increasingly complex functional materials however, these still lag behind compared to highly sophisticated macroscopic architectures with precise and predefined structure that are encountered in biological systems. In this review, we have summarized the first steps chemists have taken toward the construction of macromolecular architectures with defined three-dimensional structures. In the past decade, polymers with defined secondary structure mimicking one or more aspects from biological systems have been prepared. Issues important for the programmed organization of synthetic macromolecules that have... 

Clearly, molecular recognition processes are the prototypical supramolecular reactions on which the other aspects are based. Without molecular recognition, there are no template effects, no self-assembly, and certainly no self-replication. In contrast to opinions sometimes encountered among chemists from other areas, supramolecular chemistry did not come to a halt with the examination of hosts and guests and their interactions. Sophisticated molecular devices are available which not only are based on, but go far beyond mere molecular recognition. 

A variety of monomers can be trapped in the inclusion spaces at the molecular level and polymerized under suitable conditions. Such a reaction is called inclusion polymerization. " The study of inclusion polymerization started soon after the discovery of a honeycomb structure of urea inclusion compoxmds. The early study aimed to obtain highly stereoregular and asymmetric polymers in the spaces. Further studies brought about a profound understanding of the space effects from various viewpoints. Now. inclusion polymerization is classified between bulk or solution polymerization and solid state polymerization. In other words, it may be situated as low-dimensional and space-dependent polymerizations. Such a polymerization closely relates to supramolecular chemistry from a viewpoint of molecular information and expression. 

Fig. 1 Coinparison between the scope of molecular and supramolecular chemistry according to Lehn. (From Ref. [1].) (View this art in color at www.dekker.com.)...

The integration of molecular-recognition-directed self-assembly and chemistry of bilayer membranes has lead to the development of mesoscopic supramolecular assemblies. The impartment of amphiphilicity to supermolecules drives their hierarchical self-assembly. The solvophilic-solvophobic interactions play a pivotal role in the determination of the supramolecular architecture, and this is a distinct feature from the earlier supramolecular chemistry. The combinatorial supramolecular approach is also effective to develop functional mesoscopic assemblies. In addition, combination of supramolecular polymers and solvent engineering will give a new perspective in the design of mesoscopic materials. 

---