Selective supramolecular porphyrin/fullerene interactions

Naturally assembling cocrystallates of C-60 and C-70 fullerenes with tetraphenylporphyrins (H2TPP . C-60. 3 toluene, 1; H2T3,5-dibutylPP . C-60, 2; H2T3,5-dimethylPP . 1.5C(60). 2 toluene, 3; H2TpivPP . C-60, 4; H2T3,5-dimethylPP . C-70. 4 toluene, 5; ZnTPP . C-70, 6; NiT4-methylPP . 2C(70). 2 toluene, 7) show unusually short porphyrin/fullerene contacts (2.7-3.0 Angstrom) compared with typical pi-pi interactions (3.0-3.5 Angstrom). In the C-60 structures,an electron-rich, 6:6 ring juncture, C-C bond lies over the center of the porphyrin ring. In the C-70 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 pi surface of a fullerene with the planar pi 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.