Aromatic tripodal receptors for (C60-Ih)[5,6]fullerene

Monotopic and ditopic tripodal benzene platforms featuring aromatic and perfluoroaromatic side-arms have been synthesized, and their binding properties toward C(60)-fullerene have been investigated by HPLC examining retention times on a fullerene-modified silica stationary phase, using highly polar eluants (acetonitrile and acetonitrile/water). By comparison of structurally homogeneous sets of receptors, a clear trend could be found, pointing to an increased retention for ditopic derivatives, in which binding can occur on both sides of the benzene platform, over their monotopic counterparts. Among the latter, monotopic receptors containing H-substituted aromatic residues showed stronger retention than their perfluorinated analogues. This effect was ascribed to the greater availability of the pi-electrons in a H-substituted aromatic ring with respect to the corresponding F-substituted counterpart in participating in a pi-pi interaction with the electron-poor surface of fullerene. Several NMR experiments aimed to investigate binding interactions in solution, using the much less polar solvents required by the fullerene solubility (1,1,2,2-tetrachloroethane, chloroform, toluene, and CS(2)), did not provide any evidence of binding interactions. We concluded that pi-pi interactions between fullerene and the investigated flexible tripodal receptors cannot compete with solvation in poorly polar solvents, and that the binding interactions observed by HPLC were essentially forced by the strongly polar eluant employed for the HPLC analysis.