Self-assembly with postmodification

This process involves the covalent locking in of structures formed by reversible self-assembly. The irreversible, post-assembly step switches off the equilibrium process involved in the self-assembly. As we will see in the following sections, self assembly with covalent postmodification is involved in a range of biochemistry (e.g. insulin synthesis) and elegant abiotic supramolecular synthesis as in the formation of catenanes and knots. 

The use of metal ions as kinetic synthetic templates is extremely widespread, and is an excellent way in which to bring about the organisation of a number of reacting components in order to direct the geometry of the product. Because some metal ions, such as the transition metals, often have preferred coordination geometries (e.g. tetrahedral, square planar, octahedral etc), changes in metal ion may have a profound effect on the nature of the templated product. Metal-ion-templated syntheses may be classified more generally as examples of self-assembly with covalent postmodification. For example, the synthesis of the artificial siderophore 10.2 is effected by the use of an octahedral Fe3+ template.8 In this case, the macrobicyclic product is obtained as the Fe3+ complex from which it is difficult to separate. 

Fig. 5 Self-assembly with postmodification Preparative routes to a [2]catenane.

TMV particles illustrates, as the final product is thermodynamically selected, it is not necessarily kinetically robust. Consequently, synthetic chemists working to create functional architectures often adopted self-assembly with postmodification strategies. However, another strategy was also pursued—one in which construction by strict self-assembly is then followed by whole-system switching into the kinetic regime, thereby locking complex architectures into structurally resilient entities. 

One addition to the assembly lexicon added a layer of complexity to the above definition. Thus, one of the seven different classes of self-assembly originally proposed by Lindsey. — which are strict self-assembly processes (with or without a template) positioned in different chemical settings—is commonly known as "irreversible self-assembly." This term is used to describe two-step processes, whereby a strict self-assembly processes is followed by irreversible reactions that covalently knit together the array of subunits. As Whitesides noted, strictly speaking this term is a misnomer. Hence, along with other types of post-assembly modified self-assemblies, we categorize these processes as "self-assembly with covalent modification." Postmodification generally comes in the form of a series of covalent bond formation steps and is of less interest to us here. The crux of any self-assembly process is the self-assembly. 

Molecular self-assembly has been recognized as a powerful approach to designer soft materials with a nanoscopic structural precision [llj. However, self-assembled nanostructures are inherently subject to disruption with heating and exposure to solvents. The HBC nanotubes are not exceptional. Thus, for practical applications of the nanotubes, one has to consider postmodification of their nanostructures for covalent connection of the assembled HBC units. Because the inner and outer surfaces of the nanotubes are covered with TEG chains, incorporation of a polymerizable functionality into the TEG termini allows for the formation of surface polymerized nanotubes with an enhanced morphological stability. 

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