Choh

CHjOH

Fig. 10.20. (a) A perspective view of the truncated cone shape adopted by an a-cyclodextrin molecule. The OH groups situated at the rims of the cone make this molecule soluble in aqueous solutions, (b) The chemical structure of a /3-cyclodextrin molecule consisting of seven glucose units [10.9].

Fig. 10.21. (a) A 2:1 -y-CDrQo complex as seen from the side of the two cyclodextrins (CDs), (b) An end view showing the CD cavity filled with C«, [10.64],

The calixarene molecule is another cup-shaped molecule containing phenol units which, interestingly, forms a 1:1 (rather than a 2:1) complex with C60 as shown in Fig. 10.22 [10.65], where R is a functional group and p denotes "para." Top views of the p-R-calix[8]arene and the p-R-calix[6]arene molecules are shown in Figs. 10.22(a) and (b), respectively [10.64], The schematic "ball and socket" nanostructure for calix[8]arene-C60 is shown in Fig. 10.22(c), where the term nanostructure refers to ball and socket structures on the size scale of a few nm or less. In Fig. 10.22 it is seen that C^, can be suspended by a van der Waals interaction over the truncated cone of the

Fig. 10.21. (a) A 2:1 -y-CDrQo complex as seen from the side of the two cyclodextrins (CDs), (b) An end view showing the CD cavity filled with C«, [10.64],

Fig. 10.22. Planar projections of the chemical structure for the molecules: (a) p-R-calix-[8]arene and (b) /?-R-calix[6]arene. (c) Diagrammatic representation of a "ball and socket" nanostructure where the ball is CM and the socket is a calixarene molecule [10.65].

Fig. 10.22. Planar projections of the chemical structure for the molecules: (a) p-R-calix-[8]arene and (b) /?-R-calix[6]arene. (c) Diagrammatic representation of a "ball and socket" nanostructure where the ball is CM and the socket is a calixarene molecule [10.65].

/?-R-calix[8]arene molecule. Furthermore, a p-i-butyl-calix[8]arene complex with C70 has also been reported [10.65], where p stands for para and denotes a location for the complex opposite the oxygen atom and "/-butyl" denotes (CH3)3C. Complexation of /?-i-butyl-calix[8]arene with a mixture of a toluene extract of crude fullerene soot, followed by a series of recrystal-lizations, yielded >99.5% pure C60 [10.65], showing a use of complexation reactions for the separation of C60 from soot.

Another example of structural stabilization through weak intermolecular interactions is the formation of a 2:1 complex between C60 and bis(ethylene-dithio)tetrathiafulvalene (ET). Evidence of charge transfer between C60 and the ET molecule was given by UV spectroscopy and x-ray crystallography [10.66]. The authors suggest that charge transfer occurs between a p orbital of the sulfur atoms in the ET molecule and a valence tt orbital of

Fig. 10.23. Triclinic structure P1 (a = 9.899 A, b = 10.366 A, c = 11.342 A, a = 95.65°, 0 = 90.96°, y = 118.33°, and cell volume = 1017.1 A3) of the host-guest compound Qoiferrocene)^ Eight ferrocene molecules (C5H5-Fe-C5H5) are shown in the figure surrounding a Cw molecule. Evidence for weak charge transfer between the ferrocene and CM molecules has been reported [10.67].

the C50 [10.66]. Another example of stabilization through intermolecular interaction without formal charge transfer is provided by C60 and ferrocene complexes [10.67] (C5H5-Fe-C5H5, i.e., two parallel cyclopentane rings with a central Fe atom located midway between the rings). The weak ferrocene-C60 charge transfer complex is shown schematically in Fig. 10.23.

10.10.2. Polymerization

Polymeric materials contain macromolecules that are built from smaller molecular units, called monomers, which are linked together in a regular array by covalent bonds. The physical properties of these polymeric materials depend both on the properties of an average macromolecule and on the way in which these macromolecules bind together in the mixture, albeit by a van der Waals interaction or via covalent cross-linking. Examples of polymers involving C60 in the polymer chain and in polymer side chains have been reported and are discussed below after a few preliminary remarks on polymer terminology and structure.

Two common forms of polymers are the "homopolymers" and "copolymers" (see Fig. 10.24). The homopolymers are formed from a single monomer (X), and in the simplest case they bond end to end to form long-chain macromolecules (-X-X-X-X-). Copolymerization occurs when a mixture of two or more monomer units (X and Y) bond together (polymerize) so that both units appear in the polymer chain. Random bonding seldom occurs; i.e., a tendency toward an ordered structure is observed, in which X and Y monomeric units usually alternate in the chain (-X-Y-X-Y-). Ordered "graft" and "block" copolymers are also common.

10.10. Host-Guest Complexes and Polymerization -X-X-X-X-X-X-X-X-X-X- Homopolymer

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