Supramolecular phenomena

Lehn describes Supramolecular Chemistry as the chemistry of inter-molecular bonds, which involves recognition, transformation and translocation of information [1] beyond the elementary structures of individual molecules. Rapidly expanding at the interdisciplinary frontiers of chemical science with physical and biological phenomena, supramolecular chemistry has opened up a broad range of activities to create and fabricate diverse nanoscale architectures via recognition, which impUes the storage and read-out of molecular... 

Liquid crystal behavior is a genuine supramolecular phenomenon based on the existence of extended weak interactions (dipole-dipole, dispersion forces, hydrogen bonding) between molecules. For the former two to be important enough, it is usually necessary for the molecules to have anisotropic shapes, able to pack efficiently so that these weak interactions can accumulate and co-operate, so as to keep the molecules associated in a preferred orientation, but free enough to move and slide, as they are not connected by rigid bonds. 

At shorter distances, particularly those characteristic of H-bonded and other charge-transfer complexes, the concepts of partial covalency, resonance, and chemical forces must be extended to intramolecular species. In such cases the distinction between, e.g., the covalent bond and the H-bond may become completely arbitrary. The concept of supramolecular clusters as fundamental chemical units presents challenges both to theory and to standard methods of structural characterization. Fortunately, the quantal theory of donor-acceptor interactions follows parallel lines for intramolecular and intermolecular cases, allowing seamless description of molecular and supramolecular bonding in a unified conceptual framework. In this sense, supramolecular aggregation under ambient thermal conditions should be considered a true chemical phenomenon. 

For many chemists LCs are mysterious and complex materials, their very definition defying a simple understanding. The basic idea behind this discussion of stereochemistry in LCs is that molecules and LCs represent the same phenomenon. Liquid crystals are supermolecules in a different way than are supramolecular assemblies. Indeed, LCs can be composed of supramolecular... 

The supramolecular transformation from sphere to cylinder is supported by X-ray data indicating that the spherical polymers adopt a cubic phase, whereas the cylindrical polymers adopt a hexagonal phase [23b]. Further studies involving a library of dendritic macromonomers led to the conclusion that the effect of DP on polymer shape is a general phenomenon [24], More recently, scanning... 

A clear explanation for this observation was not offered. However, as a consequence of the space group symmetry, the crystals differ in their tertiary level of supramolecular architecture (see Fig. 13 and Fig. 14). The authors note that this difference affords a potential non-polar cleavage plane in form I but not in form II. As it is form I that preferentially formed in the presence of non-polar solvents, it is possible that this may play a role in the phenomenon. 

Finally, we wish to emphasize that, to the best of our knowledge, only the present theory can consistently explain the whole molecular recognition systems and present a global and unified view to understand the sophisticated supramolecular interactions in chemistry and biology. It is also said that the molecular recognition phenomenon through cooperative weak interactions is synonymous to entropy-governed chemistry. 

Supramolecular isomerism Supramolecular isomerism has been defined by Zaworotko64 as the existence of more than one type of network superstructure for the same molecular building blocks, and hence he adds that it is therefore related to structural isomerism at the molecular level. In cases where the molecular building blocks are capable of forming more than one type of supramolecular synthon then supramolecular isomerism is identical to polymorphism. Zaworotko defines another kind of supramolecular isomerism, however, in which the same building blocks exhibit different network architectures or superstructures. We will see examples of this phenomenon in chapter 9, particularly regarding interpenetrated networks. 

As one example, the observation of hydrogen bonding [7] in natural systems such as peptide helices and DNA base pairs led to a theoretical understanding of this phenomenon. This understanding has permitted the use of hydrogen bonding in synthesis, leading to the preparation of such diverse structures as Rebek s capsules [8], Lehn s supramolecular polymers [9], and Whitesides rosettes [10]. 

Hierarchical self-assembly starts with the integration of individual components into complex structures, which in turn organize themselves to form higher-level architectures. This spontaneous assembly continues in a hierarchical way until the solid is completely built. This phenomenon-common to supramolecular chemistry and many biological systems- [53] produces solids with new properties that are not present in its original components. 

Photostimulated molecular motion is an important photophysical phenomenon frequently exploited in molecular switches. The molecular electronic rearrangements accompanying optical excitation may stimulate nuclear rearrangement of the excited species. Like electron and energy transfer, such processes compete with radiative events and therefore reduce the measured lifetime and quantum yield of emission. The most important nuclear rearrangements in supramolecular species are proton transfer and photoisomerization. 

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