Selective Supramolecular Porphyrin/Fullerene Interactions1

Naturally assembling cocrystallates of C60 and C70 fullerenes with tetraphenylporphyrins (H2TPP·C60·3 toluene, 1; H2T3,5-dibutylPP·C60, 2; H2T3,5-dimethylPP·1.5C60·2 toluene, 3; H2TpivPP·C60, 4; H2T3,5-dimethylPP·C70·4 toluene, 5; ZnTPP·C70, 6; NiT4-methylPP·2C70·2 toluene, 7) show unusually short porphyrin/fullerene contacts (2.7−3.0 Å) compared with typical π−π interactions (3.0−3.5 Å). In the C60 structures, an electron-rich, 6:6 ring juncture, C−C bond lies over the center of the porphyrin ring. In the C70 structures, the ellipsoidal fullerene makes porphyrin contact at its equator rather than its poles; a carbon atom from three fused six-membered rings lies closest to the center of the porphyrin. These structures provide an explanation for the manner in which tetraphenylporphyrin-appended silica stationary phases effect the chromatographic separation of fullerenes. The interaction of the curved π surface of a fullerene with the planar π surface of a porphyrin, without the need for matching convex with concave surfaces, represents a new recognition element in supramolecular chemistry. NMR measurements show that this interaction persists in toluene solution, suggesting a simple way to assemble van der Waals complexes of donor−acceptor chromophores.