Light-Harvesting Supramolecular Porphyrin Macrocycle Accommodating a Fullerene–Tripodal Ligand

Trisporphyrinatozinc(II) (1–Zn) with imidazolyl groups at both ends of the porphyrin self-assembles exclusively into a light-harvesting cyclic trimer (N-(1–Zn)3) through complementary coordination of imidazolyl to zinc(II). Because only the two terminal porphyrins in 1–Zn are employed in ring formation, macrocycle N-(1–Zn)3 leaves three uncoordinated porphyrinatozinc(II) groups as a scaffold that can accommodate ligands into the central pore. A pyridyl tripodal ligand with an appended fullerene connected through an amide linkage (C60–Tripod) was synthesized by coupling tripodal ligand 3 with pyrrolidine-modified fullerene, and this ligand was incorporated into N-(1–Zn)3. The binding constant for C60–Tripod in benzonitrile reached the order of 108 M−1. This value is ten times larger than those of pyridyl tetrapodal ligand 2 and tripodal ligand 3. This behavior suggests that the fullerene moiety contributes to enhance the binding of C60–Tripod in N-(1–Zn)3. The fluorescence of N-(1–Zn)3 was almost completely quenched (≈97 %) by complexation with C60–Tripod, without any indication of the formation of charge-separated species or a triplet excited state of either porphyrin or fullerene in the transient absorption spectra. These observations are explained by the idea that the fullerene moiety of C60–Tripod is in direct contact with the porphyrin planes of N-(1–Zn)3 through fullerene–porphyrin π–π interactions. Thus, C60–Tripod is accommodated in N-(1–Zn)3 with a π–π interaction and two pyridyl coordinations. The cooperative interaction achieves a sufficiently high affinity for quantitative and specific introduction of one equivalent of tripodal guest into the antenna ring, even under dilute conditions (≈10−7 M) in polar solvents such as benzonitrile. Additionally, complete fluorescence quenching of N-(1–Zn)3 when accommodating C60–Tripod demonstrates that all of the excitation energy collected by the nine porphyrins migrates rapidly over the macrocycle and then converges efficiently on the fullerene moiety by electron transfer.