Hierarchical self-assembled structures

TERMINOLOGIES TO DESCRIBE HIERARCHICAL SELF-ASSEMBLED STRUCTURES... 

Fig. 3 Model of hierarchical self-assembly of chiral rodlike units. Local arrangements (c-f) and the corresponding global equilibrium conformations (c -f ) for the hierarchical self-assembling structures formed in solutions of chiral molecules (a), which have complementary donor and acceptor groups, shown by arrows, through which they interact and align to form tapes (c). The black and white surfaces of the rod (a) are reflected in the sides of the helical tape (c), which is chosen to curl toward the black side (c )- The outer sides of the twisted ribbon (d), of the flbril (e), and of the fiber (f) are all white. One of the fibrils in the fiber (f) is drawn in a darker shade for clarity, (e) and (f) show front views of the edges of fibrils and fibers, respectively. Reprinted with permission from [73]. Copyright 2001 National Academy of Sciences USA...

From Coordination Polymers to Hierarchical Self-Assembled Structures... 

Abstract In this review, novel hierarchical self-assembled structures based on reversible organo-metaUic supramolecular polymers are discussed. Firstly, we discuss recent advances in the field of coordination polymers, considering cases in which transition metal ions and bis- or multiligands are used to build up organo-metallic supramolecular polymers. Secondly, we review hierarchical self-assembled structures based on these coordination polymers, such as polyelectrolyte layer-by-layer films, capsules, complex coacervate core micelles and microemulsions, and nanoribbons. Finally, we give a short perspective on the formation of coordinationpolymeric hierarchical self-assembled structures. The implications of fundamental and applied research, as well as aspects of new technologies are also discussed. 

This review focuses on the most interesting reversible coordination polymers and their application in various hierarchical self-assembled structures, including thin films, microcapsules, micelles, microemulsions, and nanoribbons. These structures have in common that they are hierarchical assemblies containing metal-mediated reversible coordination polymers as a main component. The charges carried by the coordination polymers are utilized to interact with oppositely charged components, including nanoparticles, polyelectrolytes, block copolymers, and surfactants. Specific features of these objects, introduced by the coordination polymers, as well as the influence of additional salt are discussed. 

We have reviewed recent advances in the development of hierarchical self-assembly structures on the basis of coordination polymers. The judicious combination of reversible coordination polymers with oppositely charged species allows us to fabricate a variety of nanometer-sized self-assembled structures. Formation of films, hollow spheres, micelles, microemulsions, and nanoribbons has been observed. These secondary nanostructures can be used as new functional materials. 

Hierarchical self-assembly starts with the integration of individual components into complex structures, which in turn organize themselves to form higher-level architectures. This spontaneous assembly continues in a hierarchical way until the solid is completely built. This phenomenon-common to supramolecular chemistry and many biological systems- [53] produces solids with new properties that are not present in its original components. 

The above examples illustrate some of the various methods and motifs by which small molecules can undergo self-organization and hierarchical self-assembly to form complex materials with important properties. The self-assembly exhibited by small molecules often gives rise to extended order in one or more directions, over relatively long distances. Polymers also exist as extended structures over long distances and, as such, are an important venue for probing self-assembly on the macromolecular scale. 

Fig 1 a Schematic illustration of the formation of one-handed supercoiled structure by hierarchical self-assembly of phthalocyanine 1. b TEM image of left-handed supercoiled aggregates of 1. (Reprinted with permission from [35]. Copyright 1999 American Association for the Advancement of Science)... 

Fig. 7 Schematic representation of the formation of a helical columnar structure by hierarchical self-assembly of a chiral oligo(p-phenylene vinylene) 10 bearing ureido-s-triazine units in M-dodecane. (Reproduced with permission from [51]. Copyright 2005 American Chemical Society)...

Biological macromolecules such as DNA and proteins are typical polyelectrolytes, which further hierarchically self-assemble into complicated supramolecular structures such as coiled coil (helix bundle) superstructures, which are responsible for their sophisticated functions [152,153]. Therefore, with implications for biological superstructures and functions, the design and synthesis of supramolecular helical assemblies with a controlled helicity have attracted great interest. 

Scheme 10.3 Fabrication of drug-loaded pH-responsive three-layered onion-structured nanoparticles (3LNPs) via pH-controUed hierarchical self-assembly...

Finally, it needs to be mentioned that the mechanisms of supramolecular self-assembly apply to many systems more complex than simple surfactants and amphiphilic block copolymers. Supramolecular self-assembly and pattern formation is one of the crucial principles in nature and gives rise to hierarchical structure ranging from the length scale of a few nanometers to the macroscopic domain. The generation of intricate and very regular structures by templating supramolecularly self-assembled structures in the shape of LLC phases has proven extremely successful. Therefore, the utilization of the more complex systems found in nature (e.g. protein assemblies in viruses) is at the hands of mod-... 

Fig. 3 Hierarchical self-assembly of gold and silica NPs (n-Au and n-Si02, respectively) into hollow spheres with a two-layer shell structure. Reproduced from [32], with permission from the...

Hierarchical self-assembly occurs when the multiple levels of self-assembly occur, the primary building block has been previously self-assembled in another step which then becomes the secondary structure (Ozin et al., 2009). This process is repeated upward to many levels of hierarchy and is often seen in nature for seven or more levels (Cademartiri and Ozin, 2009). 

Miiller et al. [18] demonstrated that by carefully choosing the composition of triblock copolymers and adjusting the preparation techniques, precise hierarchical self-assembly of multicompartment micelles can be achieved, which can polymerize into micrometer structures (Fig. 5.10). 

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