Dendrimers noncovalent interactions

While the convergent strategy, as described above, has been exceptionally successful, a considerable amount of effort has been devoted to improving its speed and synthetic efficiency. The remainder of this chapter reviews the major methodological developments and improvements in the accelerated, covalent, convergent synthesis of dendrimers. Dendrimers constructed via self-assembly and noncovalent interactions, as well as inorganic dendrimers [25], are beyond the scope of this brief chapter. 

This topic was partially covered in CHEC-II(1996) <1996CHEC-II(9)809> under the subentry Catenanes and Rotaxanes . In this section, emphasis is given to the design and construction (and to some extent, the properties) of supramolecular architectures derived from or incorporating crown ethers rather than to the synthesis of the crown ether component present in them. The crown ether rings described herein are either covalently linked (dendrimers), mechanically interlocked (rotaxanes, catenanes), or just bound by noncovalent interactions (pseudorotaxanes) to the rest of the supermolecule to which they belong. 

As a general statement it can be concluded that macromolecular conformations different fi om the predominant coil structure are still the exception. Defined spherical secondary-structures have not been obtained by means of noncovalent interactions, since there is no synthetic concept available, distinguishing between inter- and intra-molecular interactions. Formation of globular structures by linear macromolecules is still a privilege of biomolecules where intermolecular interactions are counteracted by well-coordinated intermolecular interactions [50,51]. Synthetic nanospheres can be obtained by the stepwise synthesis of dendrimers [15] or by polymerization of microgels [52] (see below). 

Supramolecular chemistry that invokes weak and noncovalent interactions between a dendrimer and molecules of interest is of great significance, and has been investigated using a diverse range of methodologies. For example, dendrimers can be designed to... 

Dendrimer offers excellent host-guest property, which leads to the noncovalent interactions for the encapsnlation of drug molecules (Figure 15.4). The core is hydrophobic, while the shell is hydrophilic in dendrimers. This analogous micelle-like behavior makes it a promising carrier for drugs and... 

Supramolecular chemistry relies on noncovalent interactions between small molecules to self-assemble into molecular assemblies. These interactions range from hydrogen-bonding, hydrophobic, and electrostatic interactions to template-directed self-assembly. Both dendrimers and supramolecular stmctures can be obtained by the use of supramolecular approaches (Figure 13). An ideal approach to constmct neat dendrimers using supramolecular chemistry and from nondendritic building blocks was elucidated by Franz et This topic can be revisited in numerous reviews and book chapters. ... 

An interesting work by Kim showed that the periphery of the dendrimers can be reversibly manipulated through noncovalent interactions (40). They used Cucurbituril as the molecular bead that encapsulates the amino butane peripheral units (Fig. 11). The pseudorotaxane-terminated dendrimers not only gave a... 

An elegant experiment by Crooks showed that the hydrophobic modification of PAMAM dendrimers by noncovalent interactions could also result in macromolecules that behave like inverted micelles (118). The spontaneous assembly between the fatty acids and amino periphery of the PAMAM dendrimer was driven by ionic interactions (Fig. 32). These dendrimers were shown to be capable of extracting hydrophilic dyes such as methyl orange fi om water into toluene. Similarly, these dendrimers were also shown to be excellent molecular containers for catalytically active metal nanoparticles. The inverted micellar nature of various dendrimers have been used by Crooks and others for the preparation of a variety of nanoparticles (119-122). A related macromolecule, but an architecture with less of a control, is a hyperbranched polymer. Hydrophobically modified hyperbranched polymers have also been shown to be capable of acting as inverted micelles (123,124). 

Figure 5 Structure of the modified PPI dendrimer synthesized Kaneda and coworkCTs exhibiting the proposed noncovalent interactions between the tertiary amines within the dendrimer and the palladium phosphine catalyst. ...

The concept for the design of supramolecular liquid crystals and supramolecular polymers has opened new fields in materials and polymer science, which are ever expanding. New stable and dynamic structures are generated by self-organization of these materials. Related functional polymeric materials such as dendrimers [140-143], block copolymers [144], polymer blends [145,146], rotaxanes [147-149], anisotropic gels [150-152], metallo-supramolecular polymers [153,154], nanoobjects [155,156] as well as supramolecular polymers are also obtained by self-assembly of multicomponents through noncovalent interactions. 

Because of the peculiar electrochemical behavior (a single cyclic voltammetric wave characterized by remarkable electrochemical and chemical reversibility), dendrimers terminated with ferrocene-type units can be profitable used as exoreceptors, provided that they contain a group able of interacting through noncovalent bonds with the species to be recognized. Furthermore, such a group has to be located near the ferrocene units to sufficiently perturb their electrochemical response as a consequence of the interaction with the guest species. 

The groups of Reek and Meijer have also applied the noncovalent approach for catalyst anchoring to a dendrimer support [62], Phosphine functionalized ligands were attached to the periphery of polypropylene imine) dendrimers via combined ionic interactions and H-bonding using a specific binding motif that is complementary to that of the support (Fig. 6). 

Noncovalently Functionalized Dendrimers Based on Multiple Interactions... 

Fig. 4 The first supramolecular dendritic catalyst in which 32 phosphine ligands are noncovalently anchored to the functionalized dendrimer by hydrogen bonds and ionic interactions...

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