Dendritic hosts

Dendritic hosts can be used in aqueous solution to encapsulate water-soluble fluorescent probes. Changes in the photophysical properties of these encapsulated probes are useful to understand the properties of the microenvironment created by the dendritic interior. For example, adamantyl-terminated poly(pro-pylene amine) dendrimers from the first to the fifth generation (36 represents the third generation) can be dissolved in water at pH<7 in the presence of -cyclodextrin because of encapsulation of the hydrophobic adamantyl residue inside the /1-cyclodextrin cavity and the presence of protonated tertiary amine units inside the dendrimer [72]. Under these experimental conditions, 8-anifi-... 

Fig. 13. Newkome s dendritic host with internal H-bonding sites...

The possibilities for encapsulating guest molecules in dendritic hosts were first proposed by Maciejewski in 1982 [143], In 1990, Tomalia presented evidence for unimolecular encapsulation of guest molecules in dendrimers and pointed out that it was one of the possible future research areas in dendrimer chemistry [164],... 

Zimmerman and Moore recently reported on dendrimers with hydrogen bonding units at the focal point [187]. Two classes of dendritic hosts were synthesized with naphthyridine units in the core capable of hydrogen bonding benzamidinium derivatives (Figure 16.14). 

The field in which the interaction between the dendritic host and the guest molecule(s) can be classified as electrostatic is very elaborate and therefore the focus in this section will be on the interaction of organic acids with dendrimers. 

Excitation energy transfer in dendritic host-guest donor-acceptor systems. Chem Phys Chem 3 1005-1013... 

H-NMR studies and analysis of sensor responses in the CD spectrum - after addition of sugar as guest to the dendrocleft host - show that the ability of this dendritic host to enantioselectively recognise monosaccharides decreases with increasing generation number of the dendron surrounding the chiral core unit. 

In contrast, the diastereoselectivity of the dendritic host increases. This indicates that on shielding of the chiral core unit with sterically more demanding dendrons in higher-generation dendroclefts the monosaccharide guests are no longer bound in the immediate vicinity of the chiral core unit instead, a less specific host/guest interaction takes place with the dendrimer scaffold. 

While the previous sections have largely addressed variation of the dendrimer periphery by covalent transfunctionalisation, an alternative concept is based on modification of the dendrimer surface by non-covalent interactions [18]. Selective interactions of guest molecules with dendritic hosts depend upon the nature of both the dendrimer core and the dendrimer shell. 

Inclusion of guest species in dendritic host molecules 6.2.3.1 Dendrimers with multiple receptor units... 

A poly(propylenamine) dendrimer (11, Fig. 6.37) functionalised with poly-(N-isopropylacrylamide) (PIPAAm) (see Section 4.1.2) was used as dendritic host for anionic cobalt(II)-phthalocyanine complexes (a, b) as guests, which are held together by supramolecular (electrostatic and hydrophobic) interactions [57]. These dendritic complexes were investigated as catalysts in the above-mentioned oxidation of thiols, where they show a remarkable temperature dependence the reaction rate suddenly increases above 34°C. One attempted explanation assumes that the dendritic arms undergo phase separation and contraction above the Lower Critical Solubility Temperature (LCST). At this temperature the phthalocyanine complex site is more readily accessible for substrates and the reaction rate is therefore higher. 

Fig. 3 Theoretical retention curves for catalysts noncovalently anchored to a functionalized dendritic host with various association constants concentration guest catalyst ([Guest]) in the reactor as function of the residence time (Nr). Calculation parameters [Host] = 3.60 x 10-3 M, [Guest]start = 2.80 x 1CT3 M, retention guest catalyst = 96%, retention host = 100%...

In addition to simple metallic nanostructures, more complex intermetallic species have also been synthesized through the introduction of more than one metal. For instance, bimetallic nanoclusters may be generated via three routes within a dendritic host (Figure 6.33). In addition to already being proven for core-shell nanoclusters, this route should also be amenable for the growth of trimetallic nanostructures for interesting catalytic applications. 

Figure 6.33. Schematic of the three methods used to generate bimetallic nanoclusters within a dendritic host. Reproduced with permission from Scott, R. W. J. Wilson, O. M. Crooks, R. M. J. Phys. Chem. B 2004,109, 692. Copyright 2004 American Chemical Society.

Archut A, Azzellini GC, Balzani V, Cola LD, Vogtle F. 1998. Toward photoswitchable dendritic hosts interation between azobenzene functionalized dendrimers and eosin. J Am Chem Soc 120 12187 12191. 

Supramolecular hyperbranched polymers have been developed that utilize molecular recognition to facilitate selective binding of a guest molecule or molecules by a (dendritic) host or self-assembly that involves numerous weak noncovalent interactions to generate large functional assemblies. Examples of both these classes of supramolecular hyperbranched polymers are examined in the following sections. 

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