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Energy Transfer Followed by Electron Transfer in a Porphyrin Macrocycle and Central Acceptor Ligand: A Model for a Photosynthetic Composite of the Light-Harvesting Complex and Reaction Center

Authors

  • Yusuke Kuramochi Dr.,

    1. Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101 (Japan), Fax: (+81) 774-38-3508
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  • Atula S. D. Sandanayaka Dr.,

    1. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai, 980-8577 (Japan)
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  • Akiharu Satake Prof.,

    1. Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101 (Japan), Fax: (+81) 774-38-3508
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  • Yasuyuki Araki Prof.,

    1. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai, 980-8577 (Japan)
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  • Kazuya Ogawa Prof.,

    1. Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101 (Japan), Fax: (+81) 774-38-3508
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  • Osamu Ito Prof.,

    1. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai, 980-8577 (Japan)
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  • Yoshiaki Kobuke Prof.

    1. Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101 (Japan), Fax: (+81) 774-38-3508
    2. Present address: Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011 (Japan)
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Abstract

A mimic for nature's solar cells: Simple mixing of nonaporphyrin macrocycle N-(1–Zn)3 and acceptor ligand C60–ZnP–Tripod affords a supramolecular architecture (see scheme), in which the excitation energy collected by the macrocycle is transferred efficiently to the central ZnP acceptor, inducing charge separation between the ZnP and C60 sites.

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A system that models a photosynthetic composite of the light-harvesting complex and reaction center is reported in which light energy collected by cyclic antenna porphyrins is transferred to a central energy-acceptor porphyrin, followed by photoinduced electron transfer to a fullerene positioned above the ring plane. Pyridyl tripodal ligands appended with bis(phenylethynyl)porphyrinatozinc(II) (ZnP–Tripod) and additional fulleropyrrolidine moieties (C60–ZnP–Tripod) were synthesized as the reaction center units. The tripodal ligand was strongly accommodated by the light-harvesting porphyrin macrocycle N-(1–Zn)3 (1–Zn=trisporphyrinatozinc(II)) by using three-point coordination of pyridyl to uncoordinated porphyrinatozinc sites to afford a stable 1:1 composite. The binding constants for ZnP–Tripod and C60–ZnP–Tripod in benzonitrile were estimated from steady-state fluorescence titrations to be 1.4×107 and 1.6×107M−1, respectively. The steady-state fluorescence titration, fluorescence lifetime, and transient absorption studies revealed that in both composites the excitation energy collected by the nine porphyrins of N-(1–Zn)3 was efficiently transferred to a ZnP moiety by means of a through-space mechanism with a quantum yield of approximately 90 %. Furthermore, in the composite with C60–ZnP–Tripod, the converged energy at the ZnP moiety induced electron transfer to the C60 moiety, which afforded the stable charge-separated state (ΦCS>90 %).

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