Hierarchical self-assembly approach

When the coordination bonds that hold these assemblies together are both stable and inert, their formation from the components occurs under kinetic rather than thermodynamic control and it is thus disputable if they can be truly defined as supramolecular systems. On the other hand, owing to their kinetic stability, some of these species can be further exploited as building blocks in the construction of higher order architectures through a hierarchical self-assembly approach (see for example the molecular sandwiches 13-15). Through this modular approach, multichromophore systems become easily accessible on demand, with a limited synthetic effort. 

The hierarchical self-assembly approach has been proposed as a valuable method in many examples of macromolecules nanodesign. Nanoparticles assembly at... 

Jenekhe and coworkers showed that solution-cast micellar PPQ-Z -PS films consisted of multilayers with the hexagonally ordered arrays of spherical holes (Jenekhe and Chen, 1998, 1999). The hierarchical self-assembly approach to the microporous solids with a hollow core, a... 

Polymeric cylinders prepared via the self-assembly approach represent a versatile and powerful soft template for the fabrication of 1-D hybrid or inorganic nanomaterials with hierarchical architectures and complex functionaHties. 

Figure 23 Illustration of hierarchical self-assembly of a C6-functionalized hexamer 80 and polyanionic dendrimer 81 into fibers using anion pairing approach, (a and b) TEM image of the fibers (c) SAED pattern from fiber (d) molecular model of packing. Adapted with permission from Ref 08AMi38i. Copyright 2008 WiLEY-VCH Verlag GmbH Co. KGaA, Weinheim.

Progress in heterocyclic metaUosupramolecular constmction is reviewed by James Ludlow and George Newkome (University of Akron) in Chapter 6. They discuss in particular hierarchical self-assembly and interconversion between supramolecular structures. In Chapter 7, Mati Karelson (University of Tartu) and Dimitar Dobchev (Tallin University of Technology) describe the QSAR approach applied to heterocycles where large descriptor spaces... 

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. 

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. 

A different approach to the synthesis of nanosized macromolecules through hierarchical self-assembly is based on Layer-by-Layer (LbL) chemistry. LbL allows the deposition of ultra thin films whose thickness can be controlled by the chemical structure of the molecules and number of deposited layers. The interactions between layers can be ionic, covalent, hydrogen-bonding, and charge-transfer, depending upon the nature of the polymer used in the preparation.The layer-by-layer assembly of an electroactive polymer nanocomposite thin film of cationic linear poly(ethyleneimine) and Prussian Blue nanoparticles, has been exploided... 

The successful approach to smart responsive particles was further transformed into the principle of hierarchical self-assembly in the system when the particle... 

Figure 4.3 Schematic representation of the self-assemble, then polymerize approach for the preparation of hierarchically structured conjugated polymers.

The description of structure in complex chemical systems necessarily involves a hierarchical approach we first analyse microstructure (at the atomic level), then mesostructure (the molecular level) and so on. This approach is essential in many biological systems, since self-assembly in the formation of biological structures often takes place at many levels. This phenomenon is particularly pronounced in the complex structures formed by amphiphilic proteins that spontaneously associate in water. For example myosin molecules associate into thick threads in an aqueous solution. Actin can be transformed in a similar way from a monomeric molecular solution into helical double strands by adjusting the pH and ionic strength of the aqueous medium. The superstructure in muscle represents a higher level of organisation of such threads into an arrangement of infinite two-dimensional periodicity. 

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