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 C(70) over C(60) 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 C(70). In the case of a bisected Zn(porphyrin) tetramer, two well-defined cavities exist; however, due to their comparatively small size, only one C(60) 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.