Calixarenes with fullerenes

Such specific ligand systems as catenanes [968], calixarenes [969], fullerenes [970a,b], and cyclodextrines [971], and functionalized dendrimers [972a-d] are of great interest. Thus, on the basis of fullerenes C60 and C80 were obtained metal-cyclo-pentadienyl and metal-carbonylcyclopentadienyl compounds with rj1 - and r 5-coordination bonds [970b]. 

Matching the convex shape of Ceo to a concave siuface has been used to gain control over the inherently weak association between a host moiety and a molecular guest. A built-in shape- and size-specific receptor site ensures efficient binding, but it also exerts a major challenge to find the right match. The following examples form remarkably stable composites with fullerenes nano-sized, bowl-shaped molecules or container molecules with curved, open-ended cavities, such as cyclodextrins, benzotri(benzonorbomadienes), calixarenes, calixnaphthalenes and cyclotriveratrylene. ... 

Fullerenes are also known to form the inclusion complexes with calixarenes. In the case of the water-soluble inclusion complex of Qo and calixarene (cationic homoox-acalix[3]arene), substantial interaction between fullerenes and the calixarene was observed in the ground state absorption spectrum [76]. Increase in the absorption intensity around 400-500 nm of C q in calixarene can be attributed to the charge-transfer complex formation due to the n -electron system of calixarene. Strong interaction between the calixarene and fullerenes was also observed in the excited states. The triplet absorption peak of 60 in the calixarene appeared at 545 nm, which is largely blueshifted compared to that of pristine Cgg- The triplet lifetime is as short as 50 ns. The substantial interaction between calixarene and included Qo was also observed in the singlet excited state as a large blueshift of the fluorescence peak. 

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. 

Low affinity to polar solvents and fullerenes aggregation in water limit their use in biologic systems. To increase water solubility of fullerenes, few ways are used solubilization with the use of some water-soluble polymers like PDT or polyvin-ilpyrrolidone, generation of complexes with cyclodextrines or calixarenes, and... 

It had been reported that fullerene Cgo forms a water-soluble complex with y-cyclodextrin by heating with an excess amount of y-cyclodextrin in water [10] or in a mixture of refluxing water and toluene for a long time, such as 30 h [ 11]. The isolated complex is considered to have the Cgo structure bicapped with y-cyclodextrin in a molar ratio of 1 2 [11], and the complex dissolved in water to give a solution of C o with a concentration of nearly 10 mol L 410,11 ]. Fullerene Qo was also solubilized in water by complexation with a sulfocalix[8] arene, i.e., calix-[8]aryloxy-49,50,51,52,53,54,55,56-octakis(propane-3-sulfonate). The concentration of this complex in water is estimated as 5x10 mol L [12]. Complex formation between fullerene and various calixarenes has also been reported [8]. 

Supramolecnlar derivatives containing crown ether [228], calixarenes [264, 265] or dendrimers [267, 279] have been synthesized with the precursors shown in Table 4.10. Fullerene-flavonoid [268] or the fuUerene glycoconjugate [258] derived from 237 and 235, respectively, are expected to show biological activity. 

Figure 14.8 Amphiphilic fullerene and calixarene derivatives with dendritic branches that form long-lived micelles (Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced by permission).

Fullerenes C60 and C70 form supramolecular adducts with a variety of molecules, such as crown ethers, ferrocene, calixarene, and hydroquinone. In the solid state, the intermolecular interactions may involve ionic interaction, hydrogen bonding, and van der Waals forces. Figure 14.2.9 shows a part of the structure of [K(18C6)]3-C6o-(C6H5CH3)3, in which Cgg is surrounded by a pair of [K+(18C6)] complexed cations. 

The calix[8]arene depicted in Fig. 2.18 can bind fullerenes (see Chap. 3) the fullerene soccer ball is trapped in the calix. Fullerenes are usually prepared as mixture of Ceo, C70, 75, and so on, and separating them is not always easy. The calix[8]arene has a cavity with an inner diameter of 1 nm, which is therefore suitable for Ceo, since it has a diameter of 0.7nm. When the calixarene is added to a toluene solution of a mixture of fullerenes, a 1 1 complex of the calixarene and Ceo selectively precipitates. Isolation of the precipitates followed by dispersion of them in chloroform results in the precipitation of dissociated Ceo. Repeating these processes results in Ceo with high purity. 

Films of fullerenes (C60 and C70) have shown interesting electrochemical behavior (45,46). Neither the C60 nor the completely reduced C60 him is a good electronic conductor, although a partly reduced him displayed enhanced conductivity (47). Recently, the same group reported investigations on a him of tm-butylcalix[8]arene-C60. As a consequence of the reduction of the inner fullerene, the latter escaped from the calixarene basket and soluble C6 0 species entered the acetonitrile solution, as observed in an SECM collection experiment with simultaneous measurements with a quartz crystal microbalance. 

Relatively few molecular separations have been studied from the utilitarian standpoint. One of these, the purification of fullerenes via 8 , is discussed on p. 170. In a reciprocal experiment the separation of 4 " , 6 , and 8 with a column using a chemically-bonded C o silica stationary phase has been re-ported. Chromatographic selectivity has been achieved for amino acid esters and alkali metal cations on silica-bonded calix[4]arene tetraesters, the structure of which has been explored by and Si-CP-MAS NMR. Silica-bonded calixarenes have also been used as packing materials for HPLC columns that are capable of separating disubstituted aromatics, peptides, and nucleosides. The HPLC separation of phenols using 6 ° as a constituent of the eluent has been described. ... 

The inclusion of buckminster fullerene Ceo into the cavity of a p-benzylcalix[5]-arene in toluene solution reported by Isaacs et al. [16], is associated with a dramatic partial molar volume change of AV = +195 cm mol which is consistent with the displacement of two toluene molecules from the cavity of the calixarene during complex formation (Fig. 11.5). 

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