Dendritic boxes

Figure 13. I) Inclusion of guest molecules inside a dendritic box II) insertion of guest molecules into the dendritic interior.

Dendrimer synthesis, 26 787-788 breakthrough approaches in, 26 788 Dendritic box, 26 790 Dendritic metallic silver, 19 367—368 Dendritic nanoparticles, water-soluble, 26 796... 

In the preceding section, we reviewed the non-covalent dynamic encapsulation of guest probe molecules within dendrimer interiors. The second case, Scheme 2, involves the physical encapsulation of guest molecules wherein, guest molecules are locked inside dendritic containers (so-called dendritic boxes). This concept was originally proposed by Tomalia et al and referred to as unimolecular encapsulation [2]. More recent and well characterized examples have now been demonstrated by Meijer and co-workers [11-15]. 

These dendritic boxes (Figure 13.7) were synthesized by the conjugation of a chiral shell of protected amino acids onto a flexible polypropylene imine) dendrimer with 64 amino end groups. In solution, the shell was highly hydrogen-bonded and dense-packed, displaying a solid-phase behavior, which was indicated by the low NMR relaxation time of the surface groups [11]. 

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... 

Scheme 3 Two step opening of the dense shell of dendritic boxes...

Several studies have been devoted to determine the localization of end-groups in modified polypropylene imine) dendrimers. Goddard et al. [ 157] and Cavallo and Fraternali [158] investigated the properties of the dendritic box, a fifth-generation polypropylene imine) dendrimer functionalized with (t-BOC)-pro-tected L-phenylalanine residues (Figure 16.12a) [159]. 

Their results are indicative of a low-density region inside the dendrimer with a considerable end-group interaction in the shell. Similar results were obtained from CHARMm molecular mechanics calculations on the dendritic box (Figure 16.12b), which indicated a globular architecture in which the dendritic interior is... 

Figure 16.12 Chemical structure (a), and CHARMm molecular modeling picture (b), of the dendritic box [159]...

Earlier we found that the addition of alkyl-modified poly(propylene imine) dendrimers to polypropylene leads to fibers which can be dyed in conventional acid or disperse dyeing processes [3]. The alkyl chains make the additive compatible with the polypropylene matrix, while the polar core of the dendrimer can act as a receptor for the dye molecules. This host-guest behavior is analogous to the principle of the dendritic box as described by Meijer et al. [30] and elaborated by Baars et al. for dye extraction processes [31]. 

A light-controlled dendritic box was investigated in analogous fourth-generation POPAM dendrimers whose periphery alternately bore azobenzene and naphthyl moieties. Measurements of photoisomerisation at a wavelength of 365 nm in dichloromethane showed that not all azobenzene units in the E form are transformed into the Z form, but that, for example, four units remain in the E form and some 28 units isomerise to the Z form. At pH 7, six eosin molecules per host molecule were taken up by this mixture of isomers. On the other hand, the dendrimer having all azobenzene groups in the E form takes... 

However, irreversible inclusion can be accomplished through formation of a dendritic box by steric compression of the periphery (Fig. 6.16) - after covalent coupling to a sufficient number of large terminal groups. For the inclusion of dye molecules in such dendrimers, see also Section 8.3.6. 

Fig. 6.16 I rreversible encapsulation of guests (green) by compression of the surface to form a dendritic box (schematic, two-dimensional)...

Anionic dyes such as Bengal Rose and fluorescein are suitable as guest molecules for encapsulation since both are soluble on account of their acid groups and engage in electrostatic interactions with the - protonated - interior of the fixed dendrimer (Fig. 8.4). The fluorescent properties permit visualisation of inclusion of the surface-fixed dye molecule. The dendrimer thus acts as a dendritic box [23]. 

Jansen, J., Debrabandervandenberg, E. M. M., Meijer, E. W., Induced chirality of guest molecules encapsulated into a dendritic box. Recueil Des Travaux Chimiques Des Pays-Bas-Journal of the Royal Netherlands Chemical Society 1995, 114, 225-230. 

Scheme 4.8. Poly(propylene imine) dendrimers that have been terminally functionalized with rert-butoxycarbonyl protected phenylalanine units. These macromolecules were described as dendritic boxes due to their ability to entrap guests. R = benzyl (20) hydrogen atoms are omitted in the structures.

Meijer et al.123 constructed a series of novel dendritic boxes by surface modification of divergently prepared, amine-terminated poly(propyleneimine) dendrimers 24 (e.g., 30 Scheme 7.7) by attachment of A-protected chiral amino acid caps (see Section 4.3.1). 

Scheme 7.7. Construction of a dendritic box via peripheral modification by reaction with chiral amino protected activated esters. R = CH2C6H5 hydrogen atoms are omitted.

For a general review on supramolecular chemistry with dendrimers, the reader is referred to an excellent paper of Zimmerman et al. [18]. Because of our acquaintance with the polylpropylene imine) dendrimers, we will restrict ourselves in this Chapter to some examples of supramolecular behaviour of these systems as investigated in our laboratory. Three systems will be discussed (Figure 1) the dendritic box, which can encapsulate guest molecules, the polystyrene-poly(propy-lene imine) block copolymer superamphiphiles, and alkyl-decorated dendrimers, which function as unimolecular micelles, and show surprising aggregation behaviour. However, first the synthesis and properties of the poly(propylene imine) dendrimers will be discussed to demonstrate some typical dendrimer features. 

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