title: Light-harvesting supramolecular porphyrin macrocycle accommodating a fullerene-tripodal ligand.
abstract: 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 (approximately 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 pi-pi interactions. Thus, C(60)-Tripod is accommodated in N-(1-Zn)(3) with a pi-pi 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 ( approximately 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.
authors.affiliation: Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Kuramochi
authors.forename: Akiharu
authors.initials: A
authors.lastname: Satake
authors.forename: Mitsunari
authors.initials: M
authors.lastname: Itou
authors.forename: Kazuya
authors.initials: K
authors.lastname: Ogawa
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Araki
authors.forename: Osamu
authors.initials: O
authors.lastname: Ito
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Kobuke
mesh_terms: D037741:Fullerenes / Q000737:chemistry* / Q000528:radiation effects; D008024:Ligands; D008027:Light; D047028:Macrocyclic Compounds / Q000138:chemical synthesis / Q000737:chemistry* / Q000528:radiation effects; D046911:Macromolecular Substances / Q000138:chemical synthesis / Q000737:chemistry / Q000528:radiation effects; D009682:Magnetic Resonance Spectroscopy / Q000379:methods / Q000592:standards; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry* / Q000528:radiation effects; D011166:Porphyrins / Q000737:chemistry* / Q000528:radiation effects; D012015:Reference Standards; D013237:Stereoisomerism; D013997:Time Factors; D015032:Zinc / Q000737:chemistry / Q000528:radiation effects
chemical_list: D037741:Fullerenes; D008024:Ligands; D047028:Macrocyclic Compounds; D046911:Macromolecular Substances; D009942:Organometallic Compounds; D011166:Porphyrins; D015032:Zinc