Examples of Directed Assembly

Fig. 9.17 Examples of self-assembly of nanoparticles by a) hydrophobic interactions via a shell of unfunctionalized n-alkanes. Depicted is a Schematic 2D Representation of the RS/ Au nanoparticle packing structure in the solid state. Domains or bundles of ordered al-kylthiolate chains on Au particles interdigitate into the chain domains of adjacent particles in order to compensate the free volume of the outer region of the alkyl shell (Reprinted with permission from [146] A. Badia, L. Cuc-cia, L. Demers, et al.,J. Am. Chem. Soc. 1997, 779, 2582-2592. Copyright 1997 American Chemical Society), b) Direct comparison of hydrophobic interactions and chemical bridg-...

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. 

Biology and organic chemistry are replete with examples of self-assembly. Examples include lipid bilayers, the DNA duplex, proteins in their correctly folded forms, self-assembled monolayers, and crystals. In MESA, we extend ideas abstracted from molecular recognition - shape recognition, chirality, directional interactions, hierarchy of bonds, and hydrophobicity - to the mesoscale. 

In certain cases, self-assembly methods can be employed to prepare multilayered thin films analogous to LB films. Typically, once the surface has been primed with a molecular adhesion layer, subsequent layers are assembled in a layer-by-layer fashion where the end group of the previously deposited layer directs the assembly of the next layer. Strong electrostatic or covalent interactions between the layers serve to stabilize the assemblies. The most notable examples of self-assembled multilayered films are those based upon metal phosphonates [21]. Although these multilayers are structurally analogous to LB films, their thermal and solvent stability makes them potentially more useful in many applications, including electron-transfer studies. 

Assisted and directed self-assembly are derivatives of thermodynamic and irreversible self-assembly in which an external agent or template either prevents the formation of non-functional intermediates (assisted self-assembly), or stabilizes key intermediates or products (directed self-assembly). The external agent need not appear in the final product. Numerous examples of directed self-assembly, in particular, have been described. Such systems provide important new pathways to novel structures. However, the factors controlling such processes are often not easily rationalized. 

Biological systems provide numerous examples of self-assembled objects. Owing to the relatively weak interactions involved, a self-assembled structure is much more sensitive and responsive to its environment than a more rigid structure held together by covalent bonds. Unlike processes involving simple surfactants, polymers, and nanoparticles, self-assembly processes in biological systems are usually directional and functional and often lead to the formation of extremely complex structures. For example, the three-dimensional structure adopted by a protein in solution is critical to the protein s function, and this structure is determined by both strong (covalent) and weak... 

The original paper describes this compound as an outer sphere complex in which the ligands are not coordinated directly to the tin atom but are held in position by hydrogen-bonds . The compound is a good example of self-assembly simultaneously involving different types of intermolecular force. 

Directed self-assembly. The common usage of this term has altered somewhat from its original definition. When first introduced, directed self-assembly referred to a templated process in which the template was not included in the final product. The definition has been diluted somewhat and now refers to a templated synthesis, regardless of whether the templating species is part of the product or not. Many of fhe synthetic examples of self-assembled systems are in fact examples of directed self-assembly, whereby templating agents are deliberately employed. 

The work outlined in the section above demonstrates that assembly of discrete supramolecular nanostructures is achievable through directed hydrophobic and electrostatic interactions alone. While some of the examples employ metals, this is to illustrate that their assembled systems fulfill a particular function they are not an integral part of the designs. The examples in this section employ metals as part of the assembly process. In many of the cases, assembly of metal ions or nanoparticles is directed by the peptides in others, it is the metal that influences organization of the peptides. Examples of peptide assemblies constructed from coiled-coil, collagen-like, and 6-structured tectons are all highlighted. 

Supramolecular assemblies including CNTs have proven difficult due to challenges associated with dispersing SWNTs in neat solvents and to the difficulty in controlling molecular orientation. In one example of supramolecular assembly, a direct current (DC) electric field was employed to induce surfactant (tetraoctlyammonium bromide) or polymer (Nafion) capped SWNTs to assemble into linear bundles. On the basis of the knowledge that fullerenes can experience charge-transfer interactions with porphyrins to form supramolecular assemblies, Kamat and coworkers determined that protonated porphyrins yielded ordered... 

Two recent publications provide additional examples of the CD control abilities of directed assembly of block copolymers, but also demonstrate for the first time the opportunity for resolution enhancement with block copolymer films directed to assemble on chemical patterns [81, 82]. In one case, cylinder-forming PS-h-PMMA was directed to assemble on a chemical pattern comprised of hexagonally arranged... 

In 2003, Barbas and coworkers described a one-pot synthesis of functionalized P-aminoalcohols from aldehydes, acetone, and dibenzyl azodicarboxylate [2], This enzyme-like direct asymmetric assembly process was catalyzed with 20 mol% of L-proline (l-Pto) and provided the optically active products 1. This was the first example of an assembly reaction that used directly both an aldehyde and a ketone as donors in a single vessel. The success of the assembly reaction can be attributed to the higher reactivity of aldehydes over ketones in the L-Pro-catalyzed a-amination. The reaction of propionaldehyde, acetone, and dibenzyl azodicarboxylate in acetonitrile produced the expected aminoalcohol 1 in 85% yield (Scheme 12.1). The two diastereomers were obtained with an anti/syn ratio of 54 46 and with an enantioselectivity of >99% for the anti product. The authors explored the scope of the assembly reaction using various aldehydic donors, and this transformation was applied to the expedient synthesis of a potent renin inhibitor. 

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