Host-guest molecular assemblies

A cucurbituril macropolycycle composed of six units, and a side-on view of the cucurbituril acting as a host to an organic guest cation. 

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With reference to hosts and a guest, molecular assemblies have to conform to certain circumstances, generally called complementary relationships. They involve both steric and electronic terms. The objects may be achieved by the use of properly chosen sensor groups and by a suitably tailored basic skeleton as exemplified by the present scissor- or roof-shaped host molecules. From the point of view of the introductory thoughts of this chapter (cf. Sect. 3.1), it is a matter of consideration to see how consistent the scissor or the roof simile is in the light of crystal structures. 

Figure 15.6 Schematic diagram of the azobenzene nanotube assembly on the complementary a-CD/Au substrates via host-guest molecular recognition and light-induced nanotube detachment and attachment on the a-CD surfaces.20 (Reprinted with permission from I. A. Baneijee et al., J. Am. Chem. Soc. 2003,125, 9542-9543. Copyright 2003 American Chemical Society.)...

The main aim in host-guest chemistry is to construct molecular receptors by a self-assembly process so that such receptors could, to some extent, gain molecular recognition capability. The goal of such molecular recognition capability is to either mimic or block a biological effect caused by molecular interactions [157]. 

Keywords. Rotaxane dendrimers. Host-guest interaction, Recognition, Self-assembly, Supra-molecular chemistry... 

Other examples of organized molecular assemblies of interest for photocatalysis are (1) PC-A, PC-D or D-PC-A molecules where PC, A and D fragments are separated by rigid bridges (2) host-guest complexes (3) micelles and microemulsions (4) surfactant monolayers or bilayers attached to solid surfaces, and (5) polyelectrolytes [19]. 

Molecular imprinting can be accomplished in two ways (a), the self assembly approach and (b), the preorganisation approach3. The first involves host guest complexes produced from weak intermolecular interactions (such as ionic or hydrophobic interaction, hydrogen bonding) between the analyte molecule and the functional monomers. The self assembled complexes are spontaneously formed in the liquid phase and are sterically fixed by polymerisation. After extraction of the analyte, vacant recognition sites specific for the imprint are established. Monomers used for self assembly are methacrylic acid, vinylpyridine and dimethylamino methacrylate. 

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. 

Most network structures are effective host structures for small guest molecules, often the solvent. Exceptions arise when there is a high degree of interpenetration, i.e. where two or more networks are entangled [7]. This type of host-guest behaviour is not intrinsic to the molecular components themselves, but occurs in cavities or clefts created by the assembly of the network structure. Mol-... 

Using information obtained from X-ray crystallography, we have described the structure of a chiral, spherical molecular assembly held together by 60 hydrogen bonds. [10] The host, which conforms to the structure of a snub cube, self-assembles in apolar media and encapsulates guest species within a cavity that possesses an internal volume of approximately 1.4 nm3. 

Noncovalent interactions play a special role in synthetic procedures used to assemble various types of supramolecular species. These syntheses rely on the stabilization provided by non-covalent interactions between recognition sites incorporated within precursors. Various types of non-covalent interactions can be used as a recognition motif utilized to guide the synthesis.Targeted synthesis of macro- and supramolecular structures of various sizes, shapes, and functionality has now become possible. Supramolecular chemistry offers incredible applications in various fields such as medicinal chemistry (drug delivery systems),host-guest chemistry,catalysis,and molecular electronics. ... 

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