Supramolecular assemblies, host-guest interaction

Studies on molecular recognition by artificial receptors are thus one of the most important approaches to such characterization in relation to supramolecular chemistry [4]. Functional simulation of intracellular receptors in aqueous media has been actively carried out with attention to various noncovalent host-guest interactions, such as hydrophobic, electrostatic, hydrogen-bonding, charge-transfer, and van der Waals modes [5-10]. On the other hand, molecular recognition by artificial cell-surface receptors embedded in supramolecular assemblies has been scarcely studied up to the present time, except for channel-linked receptors [11-13]. 

Like the currently popular area, called nanoscience , the field of supramolecular chemistry has rather hazy boundaries. Indeed, both areas now share much common ground in terms of the types of systems that are considered. From the beginning, electrochemistry, which provides a powerful complement to spectroscopic techniques, has played an important role in characterizing such systems and this very useful book goes considerably beyond the volume on this same topic by Kaifer and Gomez-Kaifer that was published about 10 years ago. Some of the classic supramolecular chemistry topics such as rotaxanes, catenanes, host-guest interactions, dendrimers, and self-assembled monolayers remain, but now with important extensions into the realms of fullerenes, carbon nanotubes, and biomolecules, like DNA. 

NIL patterns were also used for the assembly of nanoparticles via supramole-cular host-guest interactions.95 The NIL-patterned substrate was functionalized with CD SAMs via a three-step synthesis process. The fabrication of 3D nanostructures was achieved by the alternating assembly of multivalent guest-functionalized dendrimers and CD-fnnctionalized Au nanoparticles.88 This methodology can be applied to various nanoparticles, regardless of their size and core material. For instance, CD-functionalized silica and polystyrene nanoparticles were adsorbed onto NIL-patterned CD SAMs with preadsorbed guest-fnnctionalized dendrimers.60 92 Recently, Huskens et al. demonstrated the supramolecular LbL assembly of 3D multicomponent nanostructures of nanoparticles by alternating assembly steps of complementary ferrocenyl-functionalized silica nanoparticles and different kinds of host-fnnctionalized nanoparticles (see Fig. 13.8).66... 

While supramolecular chemistry of porphyrins has been extensively studied over the last few decades, related study of phthalocyanines is still in its infancy. Despite the great potential of phthalocyanine-based supramolecular arrays in various applications as mentioned earlier, only a handful systems have been reported so far. Self-assembled systems held by hydrogen bonding, donor-acceptor interactions, and host-guest interactions are still very rare. There is certainly much room for further investigation in the chemistry of this important class of compounds, particularly on their structural and functional aspects. We hope this article can provide the grounding for further studies. 

Figure 6.6. Host-guest interaction in supramolecular assemblies of Type A.

It is important to note that weak hydrogen bonds must not be neglected in multi-valent host-guest interactions as their contribution to the interaction energy can be decisive and can thus determine the structure of a supramolecular assembly (see Refs. [184, 188, 189] for general accounts on weak hydrogen bonds, Ref. [190] for weak C-H F-C interactions, and Refs. [59, 175] for weak C-H O= contacts). 

Supramolecular spherical assemblies of NPs with photoresponsive adhesion/dispersal behaviour were also obtained in a ternary system hierarchically combining the host-guest interaction of different types of CDs toward porphyrin and azobenzene. The inclusion complexation of an azobenzene modified water soluble porphyrin (1) with phthalo-cyanine-grafted permethyl (3-CDs (2) could be reversibly cross-linked to relatively larger nanospheres with naphthyl bridged bis(a-CD)s (3). The large spheres (12 -3) turned reversibly to small-sized particles (1 2) upon photoisomerization of the azoaromatic group in 1 (Fig. 13). 

The last type of supramolecular assembly to be discussed in this chapter is that mediated by host-guest interactions. Formed in a variety of systems, host-guest chemistry revolves around the fit of a small guest molecule into the interior of a larger host molecnle via complementary... 

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