Molecular Recognition in the Solid State

CyDs have a cylindrical cavity with a depth of about 8 A and a diameter specific for each CyD. Therefore, CyDs include molecules or groups with a size and shape suitable to fit into the cavity. This leads to guest selectivity in complex formation. CyDs recognize not only the molecular size but also the molecular shape that is, CyDs differentiate between geometrical isomers and selectively include the specific one that has a shape suitable for accommodation in the cavity. Since CyDs consist of optically active o-glucose units, they form a pair diastereoisomers with the racemic compound. Therefore, as discussed in Chapters 5 and 6, CyDs are potent reagents for chiral resolution. 

---

CRYSTAL ENGINEERING AS MOLECULAR RECOGNITION IN THE SOLID STATE... 

Throughout the cross-polarization pulse sequence, a number of competing relaxation processes are occurring simultaneously. The recognition and understanding of these relaxation processes are critical in order to apply CP pulse sequences for quantitative solid state NMR data acquisition or ascertaining molecular motions occurring in the solid state. 

It was quickly recognized that chirality would play an important role in discotic liquid crystals, not only for the possibility of creating cholesteric and ferroelectric liquid crystals but also as a tool for studying the self-assembly of these molecules as a whole, both in solution and in the solid state. However, initial studies revealed that expression of chirality in discotic liquid crystals was not as straightforward as for liquid crystals derived from calamitic molecules. More recently, with the increase in interest in self-assembly and molecular recognition, considerably more attention has been directed to the study of chiral discotics and their assemblies in solution. The objective of this chapter is... 

The exploitation of the reactivity of molecular crystals lies close to the origins of crystal engineering and is at the heart of the pioneering work of Schmidt [47a]. The idea is that of organizing molecules in the solid state using the principles of molecular recognition and self-assembly. Successful results have been obtained with bimolecular reactions, particularly [2+2] photoreactivity and cyclisation [47b,c]. Another important area is that of host-guest chemistry. 

The all-important finding that carboxylate-appended sapphyrins can self-assem-ble was confirmed in the case of the sapphyrin monocarboxylate 6. ° For both compounds the self-assembly phenomena were shown to take place not only in the solid state, but also in solution and in the gas phase (see Section 3.3). This meant that the carboxylate-binding properties of the sapphyrins could be used as the key molecular recognition basis for engendering the spontaneous self-assembly of appropriately designed supramolecular ensembles. 

Figure 3 Schematic representation of the formation of inclusion complexes based on the recognition of a guest molecule by a host receptor (a), of a clathrate based on the inclusion of guest molecules within cavities generated upon packing of clathrands in the solid state (b), and of a 1-D inclusion molecular network named koilate formed through interconnection of hollow tectons (koilands) by connector molecules (c).

---