b,b-Directly Linked Porphyrin Rings: Synthesis, Photophysical Properties and Fullerene Binding

Cyclic porphyrin oligomers have been studied as models for photosynthetic light-harvesting antenna complexes and as potential receptors for supramolecular chemistry. Here we report the synthesis of unprecedented b,b-directly linked cyclic zinc porphyrin oligomers, the trimer (CP3) and tetramer (CP4), by Yamamoto coupling of a 2,3-dibromoporphyrin precursor. Their 3D structures were confirmed by NMR spectroscopy, mass spectrometry and single-crystal X-ray diffraction analysis. The minimum-energy geometries of CP3 and CP4 have propeller and saddle shapes, respectively, as calculated by density functional theory (DFT). Their different geometries result in distinct photophysical and electrochemical properties. The smaller dihedral angles between the porphyrin units in CP3, compared with CP4, results in stronger π-conjugation, splitting the UV-vis absorption bands and shifting them to longer wavelengths. Analysis of the crystallographic bond lengths indicates that the central benzene ring of the CP3 is partially aromatic (HOMA 0.52), whereas the central cyclooctatetraene ring of the CP4 is non-aromatic (HOMA –0.02). The saddle-shaped structure of CP4 makes it a ditopic receptor for fullerenes, with affinity constants of (1.1 +/- 0.4) × 10^5 M–1 for C70 and (2.2 x 0.1) × 10^4 M–1 for C60, respectively, in toluene solution at 298 K. The formation of a 1:2 complex with C60 is confirmed by NMR titration and single crystal X-ray diffraction.