Replacement of the meso methine carbon atoms of porphyrins with heteroatoms is a powerful strategy for tuning their optical and electronic properties. In particular, 5-oxaporphyrin is an attractive target due to its importance as an intermediate in heme catabolism. In this work, we describe the synthesis and properties of a free-base 5-oxaporphyrinium cation, which was prepared by the ring-closure of a bilindione with trifluoromethanesulfonic anhydride. This free-base 5-oxaporphyrinium cation exhibits dual fluorescence originating from its unique NH tautomerism. In contrast to normal porphyrins, the cis form of the 5-oxaporphyrinium cation is more stable than the trans form due to the effective delocalization of its positive charge. We thus demonstrate here that meso-modified heteroporphyrins enable the manipulation of NH tautomerism in porphyrinic macrocycles.
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We would like to express our gratitude to Prof. Shigehiro Yamaguchi and Keiji Kajiwara (Nagoya University) for the help with fluorescence lifetime measurement. This work was supported by JSPS KAKENHI grants JP17H01190 (H.S.) and JP19KK0138 (H.S.). H.S. also acknowledges the Sumitomo Foundation for financial support. A.T. expresses her gratitude for a JSPS Research Fellowship for Young Scientists (JP20J11437) and the “Graduate Program of Transformative Chem‐Bio Research” at Nagoya University, supported by MEXT (WISE Program). The work at Yonsei University was supported by a National Research Foundation of Korea (NRF) grant (2020R1A5A1019141) funded by the Korean government (MSIT).
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