Self-assembling capsules

Fig. 2 (a) The ethylene-bridged cavitand 3 is not suited for capsule self-assembly with Pd(dppp) (CF3S03)2. (b) Self-assembly of the methylene-bridged cavitand 4 to the dimeric capsule 5... 

Figure 5.12 shows the Rebek softballs. Two C-shaped monomers can form a capsule self-assembled through a seam of hydrogen bonds. Inside the cavity, the... 

These molecular capsules self-assemble through a range of forces such as metal-ligand interactions, hydrophobic interactions, hydrogen bonding, and dynamic covalent chemistry (Figure 8.2). Their lifetimes vary from milliseconds to days. 

CorbeUini F, Knegtel RMA, Grootenhuis PDJ, Crego-Calama M, Reinhoudt DN (2005) Water-soluble molecular capsules self-assembly and binding properties. ChemEurJ 11 298-307... 

Macroheterocycles as self-assembling cavities and capsules 97CRV1647. 

There are a variety of routes currently utilized to fabricate a wide range of hollow capsules of various compositions. Among the more traditional methods are nozzle reactor processes, emnlsion/phase-separation procednres (often combined with sol-gel processing), and sacrificial core techniques [78], Self-assembly is an elegant and attractive approach for the preparation of hollow capsules. Vesicles [79,80], dendrimers [81,82], and block hollow copolymer spheres [83,84] are all examples of self-assembled hollow containers that are promising for the encapsnlation of various materials. 

The foregoing results demonstrate that the thickness of the capsule wall can be controlled at the nanometer level by varying the number of deposition cycles, while the shell size and shape are predetermined by the dimensions of the templating colloid employed. This approach has recently been used to produce hollow iron oxide, magnetic, and heterocomposite capsules [108], The fabrication of these and related capsules is expected to open up new areas of applications, particularly since the technology of self-assembly and colloidal templating allows unprecedented control over the geometry, size, diameter, wall thickness, and composition of the hollow capsules. This provides a means to tailor then-properties to meet the criteria of certain applications. 

Crossley (89) described the self-assembly of a spherical cage-like structure made up of two zinc porphyrin dimers bound to a tetramine ligand. The 2 1 complex is stable at 10-6m concentrations, but addition of excess of ligand causes dissociation of the capsule with formation of a 1 1 complex. 

The formation of polymeric capsules can also be achieved by the cross-linking of self-assembled amphiphilic block copolymers [85]. The hydrophobic section of the polymer in an aqueous solution will tend to aggregate on the interior of the micelle, whereas the hydrophilic ends will form the outer shell of the micelle. If the hydrophilic end is appropriately functionalized, it can be cross-linked, giving a polymeric shell. The overarching concept is shown in Figure 5.10. 

The most versatile method to prepare such hollow capsules is self-assembly [203-205, 214, 215]. Owing to their amphiphilic nature and molecular geometry, lipid-based amphiphiles can aggregate into spherical closed bilayer structures in water so-called liposomes. It is quite reasonable that the hollow sphere structure of liposomes makes them suitable as precursors for the preparation of more functional capsules via modification of the surfaces with polymers and ligand molecules [205, 216, 217]. Indeed, numerous studies based on liposomes in this context have been performed [205, 209, 213]. 

Recently, it has been found that the outcome of some cycloadditions can be altered remarkably when performed inside the cavity of cyclodextrins (288), self-assembled molecular capsules (289), or coordination cages (290). This fact intrigued us greatly and stimulated our interest in the Diels-Alder reactivity of the calixarene-like [M2(L19)(L )]+ complexes bearing unsaturated carboxylate coligands L (215). 

Self-assembly is a thermodynamically controlled process. The formation of the capsules and the exchange of guest molecules proceed within seconds to hours, sometimes days, but finally, an equilibrium is reached which is governed by a finely balanced interplay of enthalpy and entropy. A... 

J. Kang, J. Rebek, Jr, Entropically driven binding in a self-assembling molecular capsule , Nature 1996, 382, 239-241. 

C. A. Schalley, R, K. Castellano, M. S. Brody, D. M. Rudkevich, G. Siuzdak, J. Rebek, Jr, Investigating Molecular Recognition by Mass Spectrometry Characterization of Calixarene-Based Self-Assembling Capsule Hosts with Charged Guests , J. Am. Chern Soc, 1999, 121, 4568-4579. 

J. M. Rivera, T. Martin, J. Rebek, Jr, Chiral Spaces Dissymmetric Capsules Through Self-Assembly , Science 1998, 279,1021-1023. 

The design and synthesis of supramolecular architectures with parallel control over shape and dimensions is a challenging task in current organic chemistry [13, 14], The information stored at a molecular level plays a key role in the process of self-assembly. Recent examples of nanoscopic supramolecular complexes from outside the dendrimer held include hydrogen-bonded rosettes [15,16], polymers [17], sandwiches [18, 19] and other complexes [20-22], helicates [23], grids [24], mushrooms [25], capsules [26] and spheres [27]. 

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