Hydrophilic addends

The incorporation of the functionalized fullerene into a host molecule, such as a y-cyclodextrin or surfactants is an elegant way to bypass the aggregation of C6oC(COO )2-As demonstrated in studies with [60]fullerene this host can accommodate only a single fullerene molecule, which still has access to the solvent phase. The ground state spectrum of this guest-host complex shows the same narrow bands as, for example, monomeric C6oC(COOEt)2 or Cgo/Y-CD and clearly differs from the presumed CeoCfCOO )2 n cluster. 

The NIR fingerprint indicated also significant differences relative to the reduced y-CD-incorporated monomer ((C6o )(C4HioN )/y-CD). The spectral features of the fullerene cluster, for example, are much broader. This is further corroborated by lower yields of the reduced cluster compared to the reduced monomer. 

In summary, functionalization of Ceo shows that a single hydrophilic addend is not sufficient to prevent the strong hydrophobic 3-dimensional interactions among the fullerene moieties and the resulting tendency to form aggregates. This precludes formation of stable monomers of monofunctionalized fullerene derivatives in aqueous solution and leads to the irreversible formation of clusters. 

The systematic variation of the functionalizing addends, covalently linked to the fullerene core, has also been used to facilitate their incorporation into different vesicular matrices, and to probe these systems in electron transfer studies (50). 

Electron transfer to the electron-accepting fullerene moieties was probed by pulse radiolysis under strictly reductive conditions. The experiments showed significantly reduced lifetimes of the hydrated electron absorption around 700 nm suggesting reaction 28 to occur. Characteristic changes were also observed with respect to the fullerene n-radical anion band in the NIR. 

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Viable means of overcoming the insolubility of pristine fullerenes in polar solvents involve (i) incorporation of pristine fullerenes into the hydrophobic cavity of water-soluble host structures (e.g. cyclodextrin [77, 78], surfactants [79-82], and vesicles [81, 83, 84], or (ii) functionalization of the fullerene core with hydrophilic addends (e.g. C6oC(COO-)2, C60 [C(0CH2CH2)3CH3]2, C6o(C4HioN+), etc.) [2]... 

Because monofunctionalization of fullerenes shows that a single hydrophilic addend is insufficient to prevent the strong hydrophobic interactions among the compounds [89, 90, 93, 94], multiple functionalized derivatives were examined as water-soluble probes. In particular, introduction of a second hydrophilic ligand (e.g., pyrrolidinium salts or carboxylates) to the fullerene core enhances the surface coverage of the hydrophobic fullerene surface. In turn, it was expected that fullerene aggregation might be suppressed. It should be stated that these water-soluble derivatives are important alternatives to the y-CD-incorporated and surfactant-embedded fullerenes. 

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