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Light‐Harvesting Supramolecular Porphyrin Macrocycle Accommodating a Fullerene–Tripodal Ligand
Author(s) -
Kuramochi Yusuke,
Satake Akiharu,
Itou Mitsunari,
Ogawa Kazuya,
Araki Yasuyuki,
Ito Osamu,
Kobuke Yoshiaki
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200701720
Subject(s) - chemistry , tripodal ligand , porphyrin , tripod (photography) , moiety , fullerene , ligand (biochemistry) , photochemistry , stereochemistry , supramolecular chemistry , crystallography , organic chemistry , crystal structure , biochemistry , physics , optics , receptor
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 (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‐( 1 –Zn) 3 through fullerene–porphyrin π–π interactions. Thus, C 60 –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 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.