Discovery of the K4 Structure Formed by a Triangular π Radical Anion

Asato Mizuno, Yoshiaki Shuku, Rie Suizu, Michio M. Matsushita, Masahisa Tsuchiizu, Daniel Reta Mañeru, Francesc Illas, Vincent Robert, Kunio Awaga

    Research output: Contribution to journalArticlepeer-review


    The K4 structure was theoretically predicted for trivalent chemical species, such as sp2 carbon. However, since attempts to synthesize the K4 carbon have not succeeded, this allotrope has been regarded as a crystal form that might not exist in nature. In the present work, we carried out electrochemical crystallization of the radical anion salts of a triangular molecule, naphthalene diimide (NDI)-Δ, using various electrolytes. X-ray crystal analysis of the obtained crystals revealed the K4 structure, which was formed by the unique intermolecular π overlap directed toward three directions from the triangular-shape NDI-Δ radical anions. Electron paramagnetic resonance and static magnetic measurements confirmed the radical anion state of NDI-Δ and indicated an antiferromagnetic intermolecular interaction with the Weiss constant of θ = -10 K. The band structure calculation suggested characteristic features of the present material, such as a metallic ground state, Dirac cones, and flat bands.

    Original languageEnglish
    Pages (from-to)7612-7615
    Number of pages4
    JournalJournal of the American Chemical Society
    Issue number24
    Publication statusPublished - 24 Jun 2015


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