Mesoscopic supramolecular assembly

N. Kimizuka, S. Fujikawa, H. Kuwahara, T. Kimitake, A. Marsh, J.M. Lehn, Mesoscopic Supramolecular Assembly of a Janus Molecule and a Melamine Derivative via Complementary Hydrogen-Bonds , J. Chem. Soc., Chem. Commun., 2103 (1995)... 

This chapter describes supramolecular assemblies in mesoscopic dimension and their recent developments. It also compliments earlier reviews [21,22]. The mesoscopic supramolecular assemblies are defined as hierarchically self-assembled amphiphilic supramolecular structures whose ternary and the higher assembly structures are controlled through solvophilic-solvophobic interactions. Here, pairs of molecules brought by secondary interactions are designed that acquire amphiphilicity upon complexation. They become units of self-assembly and hierarchically grow into mesoscopic-scale supermolecules that are dispersed stably in aqueous or in organic media. 

Mesoscopic supramolecular assemblies are also obtainable in organic media from amphiphilic linear networks of complementary hydrogen-bonds. An equimolar mixture of bw-barbituric acid derivative 18 and 1 gave helical superstructures with a minimum width of 50 A (Figure 12(c), note that the components are achiral, but compound 18 displays molecular asymmetry) [86]. The observed thickness corresponds to the three-layered membrane structure schematically depicted in Figure 12(d). Soluble, tapelike supramolecular oligomers can be also prepared, by the covalent preorganization of melamine units [87]. 

The integration of molecular-recognition-directed self-assembly and chemistry of bilayer membranes has lead to the development of mesoscopic supramolecular assemblies. The impartment of amphiphilicity to supermolecules drives their hierarchical self-assembly. The solvophilic-solvophobic interactions play a pivotal role in the determination of the supramolecular architecture, and this is a distinct feature from the earlier supramolecular chemistry. The combinatorial supramolecular approach is also effective to develop functional mesoscopic assemblies. In addition, combination of supramolecular polymers and solvent engineering will give a new perspective in the design of mesoscopic materials. 

In this chapter, supramolecular chemistry related to developments in materials fabrication and functionalization at the mesoscale are discussed, with an emphasis on those systems based on organic-inorganic hybrid structures. The contents of this chapter are classified into (1) supramolecular chemistry within mesoscopic media, (2) supramolecular assembly at the mesoscale, and (3) supramolecular materials at the mesoscale. Despite this classification these topics have considerable similarities. 

This new family of mesoporous silica and aluminosilicate compounds were obtained by the introduction of supramolecular assemblies. Micellar aggregates, rather than molecular species, were used as structure-directing agents. Then, the growth of inorganic or hybrid networks templated by structured surfactant assemblies permitted the construction of novel types of nanostructured materials in the mesoscopic scale (2-100 nm) [110,113,117],... 

Besides these artificial approaches, biomolecules also provide powerful scaffolds to construct mesoscopic architectures (see Chapters 12 and 17). For example, well-designed nanoarchitectures are synthesized from oligonucleotides [130,131]. We also have recently reported that sticky-end-tagged three way junctions are formed from suitably designed three DNA stfands. They further hierarchically self-assemble into mesoscopic particles which possess cagelike architectures [132], This approach is simple and much easier than those devised for the previous DNA-based supermolecules. Supramolecular assemblies having such biomolecular components will find increased applications. 

Aksay and coworkers [20] produced mesoscopic patterning of oriented nan-ostructured silica thin films polymerized by a surfactant-templated sol-gel technique [21] in combination with a micromolding technique, which is another microfabrication technique without photolithography proposed by Xia and Whitesides [19]. A network pattern of microcapillaries (submicrometer scale) was transferred to an elastomeric PDMS stamp as a microreplica molding. An aqueous mixture of tetraethoxysilane and a cationic surfactant (CTAC cetyltri-methylammonium chloride) was introduced into the microcapillaries. After hydrolysis of tetraethoxysilane at the cationic interface of the tubular surfactant assemblies, a mesoscopic supramolecular structure hierarchically constituted from hexagonally packed nanoscopic tubules of silica was formed in the microscopic capillary. 

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