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), 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 (C-60-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 C-60-Tripod in benzonitrile reached the order of 10(8)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 C-60-Tripod in N-(1-Zn)(3). The fluorescence of N-(1-Zn)(3) was almost completely quenched ( 97%) by complexation with C-60-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 C-60-Tripod is in direct contact with the porphyrin planes of N-(1Zn)(3) through fullerene-porphyrin pi-pi interactions. Thus, C-60-Tripod is accommodated in N-(1-Zn)(3) with a n-n 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 (approximate to 10(-7)m) in polar solvents such as benzonitrile. Additionally, complete fluorescence quenching of N-(1-Zn)(3) when accommodating C-60-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.