Host-guest chemistry principles

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. 

If the periphery of a POPAM dendrimer bears azobenzene units then dye molecules can be included as guests in the dendrimer scaffold (see also host/ guest chemistry in Section 6.2.3) [23]. Here the E- and Z-isomers (or their enriched versions) differ in their capacity for accommodating guest molecules. In principle, guest molecule inclusion can be controlled (switched) in this way (Fig. 5.21). 

The paper is organized as follows. Sect. 2 describes the general principles and recent results for the synthesis of cyclotriveratrylenes Sect. 3 describes some applications of these compounds to host-guest chemistry. This latter section is devoted principally to the cryptophanes, and to host molecules containing one CTV unit that have recently been described. Sec. 4 presents some prospective work in the field of material sciences, i.e., ferroelectric liquid crystals and organic three-dimensional charge transfer salts. 

The host-guest chemistry described above has incorporated, in an initial step, activation of the host molecule. In principle, we may envisage an exchange in the chemical roles of host and guest. For example, on may consider keto derivatives of DCA with the keto group directed towards the channel such as the 6-keto and 16-keto... 

When greater selectivity is required in the solid-state reaction, host-guest chemistry techniques can be applied efficaciously. Reaction in the solid state of the guest compound as its inclusion complex crystal with a chiral host can give an optically active reaction product. Various host compounds have been designed by us to follow this simple principle. 

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