Selective Extraction of C70 by a Tetragonal Prismatic Porphyrin Cage

Along with the advent of supramolecular chemistry, research on fullerene receptors based on noncovalent bonding interactions has attracted a lot of attention. Here, we present the design and synthesis of a cationic molecular cage: a cyclophane composed of two tetraphenylporphyrins, bridged face-to-face by four viologen units in a rhomboid prismatic manner. The large cavity inside the cage, as well as the favorable donor-acceptor interactions between the porphyrin panels and the fullerene guests, enables the cage to be an excellent fullerene receptor. The 1:1 host-guest complexes formed between the cage and both C60 and C70 were characterized in solution by HRMS and NMR, UV-vis and fluorescence spectroscopies, and confirmed in the solid state by single-crystal X-ray diffraction analyses. The results from solution studies reveal that the cage has a much stronger binding for C70 than for C60, resulting in a selective extraction of C70 from a C60-enriched fullerene mixture (C60/C70 = 10/1), demonstrating the potential of the cage as an attractive receptor for fullerene separation.