Larger assemblies, calixarenes

It should be mentioned, that larger assemblies consisting of nine calixarenes are available with the calixarene-monocyanurate 139,208a while the double-calixarene 140 forms a double-box with 136 consisting of 15 molecules (3 x 140 and 9 x 136).283 Polymeric aggregates are formed between bis-melamine and bis-cyanurate calix[4]arenes.284 In all these cases, chiral induction still remains to be explored. 

The inclusion of guests is not the only interesting aspect of the dimers formed by tetra-urea calixarenes. Numerous other self-assembled capsules are known, larger in volume, different in shape, and able to include more than one guest molecule, and various aspects of guest inclusion have been extensively studied [45]. 

A complete survey on calixarenes was far beyond the scope of this chapter. We therefore concentrated mainly on the chemistry, namely the synthesis and the (basic) chemical modification of calixarenes, which form the basis for aU fnrther stndies. Fnrther interesting aspects, such as inherent chirality of calixarenes, catalytic or biomimetic effects , larger structures formed via self-assembly in solution or in mono- and multilayers could be stressed only shortly. Important properties snch as complexation of cations , anions and of nentral gnests , including fnUerenes , conld not be treated at all. The same is tme for applications arising from these properties in snch different areas as sensor techniqnes , chromatographic separations or treatment of nuclear wastes . In all these cases, the reader is referred to special reviews. 

The nitrogenous base with caffeine has the larger binding constant for the porphyrin. Together with the sulfonic acid groups on the calixarene (see Fig. 7.7, no. 6), the hydrophilic amino acid-based substituents (Fig. 7.7, no. 5) make the supramolecular assembly water soluble up to 4-5 mM. Figure 7.7 shows an example of a complex that was prepared with either Co or Zn as central metal ion [44]. 

In the last decades many reports [2, 89] on the solid state structures of the larger calixarenes have shown intruiguing self-assembling properties. A special attention has been also devoted to the solid state structure of their metal complexes. 

Fig. 34.15 (a) Calixarenes 5 and 8 used in the synthesis and self-assembly of Co NP rings (b) energy-filtered TEM image of Co NPs with CoO shells [37] (c) initial mixture of Co NPs stabilized with 5 in toluene deposited onto TEM substrate, with larger (>20 nm) NPs assembled into nanorings [18] (d) size-refined Co NPs (27 4 nm) stabilized by 5 in toluene, deposited onto TEM substrate [37] (Reprinted with permissions from the American Chemical Society and the Royal Society of Chemistry)... 

Simultaneous, multiple ester formation was employed to assemble larger multitopic assemblies. For example, self-assembled molecular capsules via boronate esterification have been demonstrated. Two calix[3]arenes, one having the upper rim functionalized with diols, the other functionalized with boronic acids, were combined in the presence of a templating agent to form the heterodimeric capsule (Fig. 25). Extensive NMR analysis was performed to show that the two calixarenes in solution by themselves do not interact. It is only when tetramethyl ammonium acetate is added that the capsule forms. NMR diffusion studies have further demonstrated that the ammonium cation is included within the cavity. In addition, reversible decomposition-reconstruction of the capsule was observed in response to pH. 

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