Porphyrin molecular tweezers for fullerenes

Abstract

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 C70 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) × 105 M-1, respectively. Binding of C60 and C70 in the clefts of syn-1 and syn-2 is clearly demonstrated by the crystal structures of C60 /syn-1, C70 /syn-1, C60 /syn-2, and C70 /syn-2 complexes. In the crystal structures with C70 , the directions of the long axis of C70 are found to be quite different: in the case of C70 /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-d8 also support these orientations even in solution.