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

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.K4结构（K4 structure）最初被理论预测可存在于三价化学物种中，如sp²碳（sp² carbon）。然而由于合成K4碳的尝试均未成功，该同素异形体一度被认为是自然界中可能不存在的晶体形态。本研究中，我们通过多种电解质实现了三角形分子萘二酰亚胺（NDI）-Δ的自由基阴离子盐的电化学结晶。对所得晶体的X射线晶体分析表明，其结构为K4结构：该结构由三角形的NDI-Δ自由基阴离子以沿三个方向定向的独特分子间π重叠作用构成。电子顺磁共振（Electron Paramagnetic Resonance）与静态磁测量证实了NDI-Δ的自由基阴离子态，并表明其分子间存在反铁磁相互作用，外斯常数θ = −10 K。能带结构计算表明，该材料具有金属基态、狄拉克锥（Dirac cones）和平带等特征性质。