Guest molecule selective encapsulation

Dendrimer interior functional groups and cavities can retain guest molecules selectively, depending on the nature of the guest and the dendritic endoreceptors, the cavity size, the structure, and the chemical composition of the peripheric groups. Two main methods are known for the synthesis of metal nanoparticles inside dendrimers. The first method consists of the direct reduction of dendrimer-encapsulated metal ions (Scheme 9.4) the second method corresponds to the displacement of less-noble metal clusters with more noble elements [54]. 

Container molecules in general show an increasing number of applications and so do the container molecules based on imine type ligands. Many different shapes of open or nearly closed ones could already be synthesised. Those cages are known to encapsulate different types of guest molecules. This encapsulation can be selective and permanent or reversible. The container molecules described are also used for stabilisation of different compounds such as the allotrope P4. They can be used as gas or optical sensors. One of the described cages can also be opened and closed selectively. 

The diffusion of entrapped guest molecules out of dendritic boxes was un-measurably slow over a period of several months. However, Meijer and coworkers have demonstrated that the shape-selective liberation of encapsulated guests could be achieved by removing the closed shell in two steps (Scheme 3) [15]. First, hydrolysis of the tBOC groups of the dense shell with formic acid gave a partly open dendritic box . At this point, small guests such as p-nitroben-zoic acid and nitrophenol could diffuse out of the box by dialysis. Susequent and complete removal of the outer shell by refluxing with 12 AT HC1, lead to the... 

Although a number of impressive chiral, supramolecular architectures were not discussed in this chapter either due to their instability in solution or their inability to encapsulate guest molecules, they should not go unmentioned and are referenced to follow up for the interested reader. Some selected examples are (i) Saalfrank and coworkers octadecametallic square box composed of 52 achiral components with molecular D2 symmetry [160] (ii) Ward and coworkers truncated, cationic tetrahedral cage [Co Lis]244 [161] that encapsulates four tetrafluoroborate counter-ions in the solid state (iii) James... 

Fig. 4 Schematic representation of the selective encapsulation of various guest molecules in the hydrogen-bonded heterodimer 14...

The central cavity of the cylinder-shaped cyclodextrins behaves as an empty capsule It can accommodate so-called guest molecules of appropriate size, shape, and polarity. This "molecular encapsulation" can be utilized for stabilization and for enhancement of solubility of drugs, vitamins, flavors, etc., and utilizing the selectivity of the inclusion complexation, it can be applied for separation of substances, either by non-chromatographic methods, or chromatographic methods. 

We have discussed the synthesis of dendritic boxes possessing a unimolecular compartmented structure in which guest molecules are physically locked. Evidence is presented that the encapsulation is dominated by the architecture of the dendrimer and that some supramolecular ordering is present. Furthermore, a shape-selective liberation of guests can be accomplished by a two-step process. 

Dendrimers and dendrons are appealing types of nanoscale, highly branched, macromolecules, which, because of their structure and properties, have attracted the interest of many researches worldwide. The preceding text has tried to summarize the different selective supramolecular aspects about their properties, structure, potential diversity, and applications to nonspecialized scientists. An introduction of these dendritic structures has combined a short description of the structure and synthesis with some historical perspectives, followed by a classification of dendritic structures, as covalent and noncovalent entities. Emphases have been given to their host-guest capacity to encapsulate small molecules, ions, or nanoparticles, as well as to interact with themselves or other nano-objects. The continued investigation in many fields of these unique architectures has produced a wide variety of branched fractal constructs, which undoubtedly will continue to spark the imagination of future synthetic architects. 

Encapsulation of the guest molecule(s) within the hydrophobic nucroenviron-ment of the NR results in guest isolation from the bulk. This microenvironment can induce a specific conformation of the guest. Molecular recognition and selective encapsulation of the guest(s) is one of the key features of NRs, and the guest(s) of complementary shape, size, and chemical surface are expected to be encapsulated more easily than other molecules [5,7]. 

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