Cyclic metalloporphyrin dimers and tetramers: Tunable shape-selective hosts for fullerenes

Covalently linked cyclic metalloporphyrin dimers and tetramers have been demonstrated to be good shape-selective hosts for fullerene guests. The fullerene affinities of these hosts can readily be tuned by modulating the covalent linkage and the metal ions in the porphyrin subunits. A rigid Zn(porphyrin) dimer with conjugated bis(alkynyl) linkers exhibits a high selectivity towards C₇₀ over C₆₀ in toluene (K _a,C70/K_a,C60 = ∼28). For the host structures examined, a synergistic combination of rigidity in the linker and electropositive Al ions gives rise to the strongest binding of C₇₀. In the case of a bisected Zn(porphyrin) tetramer, two well-defined cavities exist; however, due to their comparatively small size, only one C₆₀ can be accommodated. Studies of fullerene binding as a function of metal ion in a porphyrin divider suggest that the right combination of shape and steric match is essential to exploit both van der Waals and local-charge/induced-dipole interactions.