Selective supramolecular porphyrin/fullerene interactions

Naturally assembling cocrystallates of C60 and C70 fullerenes with tetraphenylporphyrins (H2TPP · C60 · 3 toluene, 1; H2T(3,5-dibutyl)PP · C60, 2; H2T(3,5-dimethyl)PP · 1.5C60 · 2 toluene, 3; H2T(piv)PP · C60, 4; H2T(3,5-dimethyl)PP · C70 · 4 toluene, 5; ZnTPP · C70, 6; NiT(4-methyl)PP · 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