Porphyrin molecular tweezers for fullerenes

Facing zinc bisporphyrins connected with diethanoanthracene and diethanonaphthacene (syn-1 and syn-2) are prepared by double [3+1] porphyrin synthesis of tripyrranedicarbaldehyde with syn-5,11-dimethoxy-4,6,10,12- tetrahydro-4,12;6,10-diethanoanthracene[2,3-c;7,8-c′]dipyrrole and syn-4,7,11,14- tetrahydro-4,14;7,11-diethanonaphthacene[2,3-c;8,9-c′] dipyrrole. These bisporphyrins contain large clefts with different sizes capable for complexation with fullerenes such as C₆₀ and C₇₀. These designed syn-oriented bisporphyrins serve as effective and selective molecular tweezers for C₇₀. Binding affinity of the zinc bisporphyrins, namely, syn-1 and syn-2, towards C₆₀ and C ₇₀ in solution is determined by employing UV-vis spectrophotometric technique. Values of binding constants (K) in toluene for the non-covalent complexes of syn-1 with C₆₀ and C₇₀ are estimated to be 3.1(4) × 10⁴ and 5.0(2) × 10⁵ M^-1, respectively, and those of syn-2 with C₆₀ and C₇₀ are enumerated to be 2.1(4) × 10⁴ and 1.70(13) × 10 ⁵ M^-1, respectively. Binding of C₆₀ and C ₇₀ in the clefts of syn-1 and syn-2 is clearly demonstrated by the crystal structures of C₆₀/syn-1, C₇₀/syn-1, C ₆₀/syn-2, and C₇₀/syn-2 complexes. In the crystal structures with C₇₀, the directions of the long axis of C ₇₀ are found to be quite different: in the case of C ₇₀/syn-1 complex the axis lies perpendicularly to the line connecting two zinc atoms; however, for the C₇₀/syn-2 complex, the axis occupies in-plane to the same line. Both the clefts of syn-1 and syn-2 are induced to fit the included fullerenes by domed out-of-plane distortion of porphyrin rings. Moreover, the bicyclo[2.2.2]octadiene moieties of syn-1 are widened by complexation with the fullerenes, while the same moieties of syn-2 are narrowed by the complexation. Therefore, syn-1 catches the fullerenes at the shallow part of the cleft. On the other hand, syn-2 catches the fullerenes at the bottom part of the cleft. NMR experiments of the complexes in THF-d ₈ also support these orientations even in solution.