Supramolecular hosts calixarenes

The main supramolecular self-assembled species involved in analytical chemistry are micelles (direct and reversed), microemulsions (oil/water and water/oil), liposomes, and vesicles, Langmuir-Blodgett films composed of diphilic surfactant molecules or ions. They can form in aqueous, nonaqueous liquid media and on the surface. The other species involved in supramolecular analytical chemistry are molecules-receptors such as calixarenes, cyclodextrins, cyclophanes, cyclopeptides, crown ethers etc. Furthermore, new supramolecular host-guest systems arise due to analytical reaction or process. 

CONTENTS Preface. George W. Gokel. Cryptophanes Receptors for Tetrahedral Molecules, Andre Collett, Jean-Pierre Dutasta and Benedict Lozach. Inclusion Polymerization in Steroidal Canal Complexes, Kiichi Takemoto, Mikiji Miyata. Functionalized Tetraazamacrocycles Ligands with Many Aspects, Thomas A. Kaden. Calixarenes as the Third Supramolecular Host, Seiji Shinkai, Kyushu University, Japan. Fluorescent Chemosensors for Metal and Non-Metal Ions in Aqueous Solutions Based on the Chief Paradigm, Anthony W. Czamik. Index. 

It is also possible to build metal-polypyridines into supramolecular hosts and receptors. For example, one of the bpy ligands may be a part of a crown ether ring [34], or it may be capped with a calixarene [33, 257] attached at the 4,4 positions. Electrochemical behavior of the metal-polypyridine unit is then affected by an interaction with a guest , e.g. an alkali metal cation. 

Calixarenes and Resorcinarenes 12.16.1.3.1 Introduction to calixarenes as supramolecular hosts... 

Supramolecular Host-Guest Co-assembly of Amphiphilic Calixarene... 

Calixarenes are the most easily derivatized of the common supramolecular host systems, and tens of thousands of calixarene derivatives have been synthesized. In order to focus this discussion. Sect. 23.2 of this chapter will present selected examples of calixarenes binding to amino acids, peptides, and proteins, with some examples that provide general lessons in calixarene chemistry and biomolec-ular recognition. A Table that serves as an index for all reported equilibrium constants, including solution conditions, is presented at the end of Sect. 23.2. Section 23.3 of the Chapter will report on recent efforts to use calixarenes in both host-guest and scaffold modes as the basis for new biomolecular technologies. 

Shinkai, S. Calixarenes as Third Supramolecular Host In Advances in Supramolecular Chemistry-, JAI Press, 1993 vol. 3, p. 97. 

Combining the supramolecular host-guest properties of calixarenes, the optical properties of calixarene-modified CdSe/ZnS semiconductor QDs were controlled for the optical detection of neurotransmitter (acetylcholine), Ceo and mercury ions [65-70]. SC[4,6]A was selected to prepare highly fluorescent, stable and water-soluble CdSe QDs by simple, rapid ligand exchange route [71]. Then, this kind of QDs were used to construct Au electrode with enhanced photocurrents systems. [72] CdTe QDs in silica spheres coated with calix[4]arene were also synthesized as luminescent probes for pesticides and PAHs [64]. 

We described the development of calixarenes and pillarenes on surfaces and the applications of these hybrid materials. The selected surfaces include NPs, metal surface. Si surface, electrode, porous materials, and carbon materials. These multivariant hybrid materials combine the supramolecular host-guest properties of calixarene/pillarene hosts with the unique surface properties of other entities, expand the applications in recognition, stabilization, self-assembly, dispersion, electrode, controlled drug release, sensing, separation and absorption. This design principle holds great promise for the design and application of new desired materials in future. 

Calixarenes, which are macrocyclic compounds, are one of the best building blocks to design molecular hosts in supramolecular chemistry [158]. Synthesis of calix[4]arenes, which have been adamantylated, has been reported [105, 109]. In calix[4]arenes, adamantane or its ester/carboxylic acid derivatives were introduced as substituents (Fig. 29). The purpose of this synthesis was to learn how to employ the flexible chemistry of adamantane in order to construct different kinds of molecular hosts. The X-ray structure analysis of p-(l-adamantyl)thiacalix[4]arene [109] demonstrated that it contained four CHCI3 molecules, one of which was located inside the host molecule cavity, and the host molecule assumed the cone-like conformational shape (Fig. 30). 

The dynamics of a supramolecular system are defined by the association and dissociation rate constants of the various components of the system. The time-scale for the dynamic events is influenced by the size (length-scale) and by the complexity of the system. The fastest time for an event to occur in solution is limited by the diffusion of the various components to form encounter complexes. This diffusion limit provides an estimate for the shortest time scale required for kinetic measurements. The diffusion of a small molecule in water over a distance of 1 nm, which is the length-scale for the size of small host systems such as CDs or calixarenes, is 3 ns at room temperature. In general terms, one can define that mobility within host systems can occur on time scales shorter than nanoseconds, while the association/dissociation processes are expected to occur in nanoseconds or on longer time scales. The complexity of a system also influences its dynamics, since various kinetic events can occur over different time scales. An increase in complexity can be related to an increase in the number of building blocks within the system, or complexity can be related to the presence of more than one binding site. 

The network structures to be discussed will all involved hydrogen bonding as the supramolecular synthon. It should be noted however that other interactions such as coordinate bonds and host-guest interactions may also organise host molecules into network structures. Coordination polymers constructed from molecular hosts may involve functionalised calixarenes [8-11], cyclotriveratrylene [12], or cucurbituril [13]. Calixarenes have also been used to build up network structures via host-guest interactions [14,15]. It is also notable that volatile species may be trapped within the solid state lattice of calix[4] arene with a structure entirely composed of van der Waals interactions [16]. 

In principle, there are four basic strategies to compensate for the repulsive effects between the hydrophobic fullerene surface and water (a) encapsulation in the internal hydrophobic moiety of water-soluble hosts like cyclodextrins (Andersson et al., 1992 Murthy and Geckeler, 2001), calixarenes (Kunsagi-Mate et al., 2004) or cyclotriveratrylenes (Rio and Nierengarten, 2002) (b) supramolecular or covalent incorporation of fullerenes or derivatives into water-soluble polymers (Giacalone and Martin, 2006) or biomolecules like proteins (Pellarini et al., 2001 Yang et al., 2007) (c) suspension with the aid of appropriate surfactants and (d) direct exohe-dral functionalization in order to introduce hydrophilic moieties. 

Supramolecular catalysis may also involve the combination of a host cavity and a metal active site as in the bis(diphenylphosphino)calix[4]arene nickel(II) complex 12.40 which acts as an efficient catalyst for ethylene and propylene polymerisation, and in tandem with zirconocene dichloride, for the formation of linear low-density polyethylene. In the latter case the complex gives very little branching - a significant advantage. The key to the effectiveness of the catalyst involves calixarene-induced changes in the bite angle at the Ni(II) centre, which is square planar in the active form of the catalyst.29... 

In summary, our approach of using cyclopeptides with natural amino acids and 3-aminobenzoic acid subunits for the development of macrocydic receptors has afforded remarkably efficient hosts. The cation affinity of 4b, for example, exceeds that of many calixarene derivatives. Even more interesting is the high anion affinity of 5 in aqueous solution. By introdudng additional functional groups such as car-boxylates to the periphery of the cavity, we recently also obtained cydopeptides that interact with neutral substrates, for example, carbohydrates [25]. Our peptides therefore represent a versatile dass of artificial receptor that should prove useful in supramolecular and bioorganic chemistry. 

Calixarenes are widely used platforms in supramolecular chemistry and by ring closing of acyclic thiophene precursors in the final step a thiophene ring can be included into the calixarene moiety <1999H(51)2807>. The isomers 167-169 can be isolated serving as good hosts for T-methylpyridinium iodide. 

---