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Exploration and Development of Novel Porphyrinoids with Intriguing Structural and Electronic Properties
Creation of organic molecules having novel structures and electronic systems still remains a central issue in current chemistry. Prof. Osuka has explored many such porphyrinoid molecules including geometry-controlled porphyrin dimers, extremely long and giant discrete porphyrin arrays, extremely ?-conjugated porphyrin tapes, giant porphyrin wheels, ring-expanded porphyrins, ring-contracted subporphyrins, and Mobius aromatic expanded porphyrins.
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Prof. Atsuhiro Osuka
Kyoto University
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1. Naphthalene-bridged Zn(II) porphyrin dimers were prepared and their Soret bands were found to exhibit band-splitting due to exciton coupling, reflecting the relative geometry of the two porphyrins. These porphyrin dimers have been used for studies on geometry-dependencies of electron and energy transfer reactions. As photosynthetic reaction center models, porphyrin oligomers bearing a diimide electron acceptor were explored, where the energy and electron transfer reactions of the natural photosynthetic reaction center were reproduced to provide a long-lived charge-separated state..
2. Ag(I)-promoted meso-meso coupling reactions of 5,15-diaryl Zn(II) porphyrin were found and used for the synthesis of extremely long linear arrays, three-dimensionally extending windmill porphyrin arrays, cyclic porphyrin wheels, and porphyrin boxes. Among these, meso-meso linked porphyrin 1024-mer still holds the fame of the man-made longest discrete molecule. As such, he has enhanced the level of bottom-up synthesis to micrometer size.
He also found that meso-meso singly linked porphyrin arrays were oxidatively converted to meso-meso β-β β-β triply linked porphyrin arrays that are fully π-conjugated. These fully conjugated porphyrin tapes exhibit their absorption bands deep into the infrared region. These porphyrin tapes were also shown to exhibit very large two-photon absorption cross-sections. A tetrameric porphyrin array bearing the central planar cyclooctatetraene (COT) unit was synthesized, which showed a distinct paratropic ring current owing to the central antiaromatic COT moiety..
3. A series of ring-expanded porphyrins was obtained from modified simple Rothemund reaction of pentafluorobenzaldehyde and pyrrole. New chemistry of these ring-expanded porphyrins has been explored, including unique coordination of multi-metal ions, large two-photon absorption cross-sections, novel transannular bridging reactions, and skeletal rearrangement reactions. An interesting reaction is the quantitative thermal splitting reaction of bis-Cu(II) complex of an octaphyrin into two molecules of Cu(II) porphyrins via a metathesis-like reaction. As a genuine ring-contracted porphyrin, subporphyrins were synthesized for the first time. Meso-aryl substituents of subporphyrins have been shown to rotate freely and thus have large impacts on the electronic properties of subporphyrins, which has been used for exploration of many interesting functional molecules.
4. The concept of Mobius aromaticity predicts an aromatic character for a twisted [4n]annulene lying on a Mobius strip in contrast to the ubiquitous Huckel aromaticity of [4n + 2]annulenes. Despite this interesting prediction, it has been very difficult to experimentally confirm this concept. It has been found that Möbius aromatic molecules are spontaneously formed from meso-aryl substituted expanded porphyrins upon metallation with group 10 metals. This was the first experimental confirmation of Mobius aromatic molecule, which was followed by findings that [28]hexaphyrins became Mobius aromatic species upon lowering temperature, and some expanded porphyrins became Mobius aromatic by protonation or intramolecular fusion reactins. In every case, the aromaticity of the molecules has been confirmed by diatropic ring currents, absorption features characteristic of aromatic expanded porphyrin, calculated nuclear independent chemical shifts (NICS), and large two-photon absorption cross sections. Through these experiments, the formation of Mobius aromatic molecules from expanded porphyrins has been demonstrated to be quite general.
Prof. Atsuhiro Osuka has continuously explored structurally and electronically novel porphyrinoids, which provide large impacts on organic chemistry, physical chemistry, and material chemistry owing to their novelty and high generality. His contribution is highly evaluated worldwide and thus is recognized to be worthy of The Chemical Society of Japan Award.
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