Development of Supramolecular Photocatalytic Systems

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Longer lifetimes of the charge-separated (CS) state than that of the natural photosynthetic reaction center have been achieved by using suitable chromophores and redox components, namely zinc porphyrins (or zinc chlorins) and fullerenes, which are linked with covalent bonds. A remarkably long lived electron-transfer state as compared with the natural photosynthetic reaction center has also been attained by using a simple donor-acceptor dyad, 9-mesityl-10-methylacridinium ion. However, construction of artificial photosynthetic model systems containing multiple photosynthetic reaction centers has remained as a great challenge because of the synthetic difficulty. We have developed multiple photosynthetic reaction centers composed of light-harvesting multi-porphyrin units and charge-separation units, which have been combined by non-covalent bonds such as π-π-interaction and coordination to attain long-lived CS states. First, multi-porphyrin systems were developed using porphyrin dendrimers … More and porphyrin-peptide oligomers. Then, they are combined with the reaction center units with electron acceptors by non-covalent bonds. For example, a π-extended viologen derivative can form a supramolecular complex with free base porphyrin dendrimers. The energy migration occurs efficiently among the porphyrin units, followed by efficient electron transfer. The lifetime of CS state becomes longer with increasing generation of porphyrin dendrimer. This indicates that the CS lifetime is elongated by the hole migration among porphyrin units. The longest CS lifetime was determined as 1.3 ms for the supramolecular complex of a free base porphyrin dendrimer, which contains 16 porphyrin units, with a π-extended viologen derivative in PhCN at 298 K. Fulleropyrrolidine bearing a pyridine coordinating ligand (C_<60>py) or imidazole ligand (C_<60>Im) also forms supramolecular complexes with zinc porphyrin dendrimers and zinc porphyrin-peptide oligomers. The CS states of the supramolecular complexes were detected as the transient absorption spectra by laser flash photolysis measurements. The longest CS lifetime was attained as 1.7 ms for the supramolecular complex of a zinc porphyrin-peptide hexadecamer, which contains 16 porphyrin units, with C_<60>py in benzonitrile (PhCN) at 298 K. The organization of such porphyrin and fullerene molecules has been applied to construct efficient light energy conversion systems such as photovoltaic devices. Less