Linear Cyano Substitution Unlocks Highly Birefringent Tricyanomethanide

Since the crystal structure of tricyanomethanide was first determined about half a century ago, its optical properties have rarely been explored. Herein, we report NaC(CN)3 as a high-performance birefringent crystal with high in-plane birefringence of Δn(011)exp = 0.36@550 nm. To the best of our knowledge, this value is among the highest for crystals whose birefringence originates from planar π-conjugated triangular units, and it exceeds that of commercial crystals such as α-BaB2O4 (0.122@532 nm) and CaCO3 (0.172@590 nm). First-principles calculations reveal that substituting O atoms in traditional π-conjugated triangles with polarizable linear −C≡N to construct “super-triangular” [C(CN)3]− significantly enhances the polarizability anisotropy. Their parallel alignment further maximizes the anisotropic electron-density distribution, amplifying the overall birefringence. This work not only highlights tricyanomethanide as a high-performance birefringent crystal for the first time but also provides a facile linear cyano substitution strategy for compact polarized optics.自约半个世纪前首次解析三氰基甲烷根（tricyanomethanide）的晶体结构以来，其光学特性鲜有研究。本文报道了一款高性能双折射晶体NaC(CN)₃，其面内双折射率Δn(011)exp=0.36@550 nm，数值优异。据我们所知，对于双折射源于平面π共轭三角形结构单元的晶体而言，该数值位居前列，且优于商用晶体α-偏硼酸钡（α-BaB₂O₄，0.122@532 nm）与碳酸钙（CaCO₃，0.172@590 nm）的双折射率。第一性原理计算（first-principles calculations）表明，将传统π共轭三角形结构中的氧原子替换为可极化的直线型氰基（−C≡N），以构建“超三角形”[C(CN)₃]⁻阴离子，可显著提升极化各向异性。这些结构单元的平行排布进一步最大化了各向异性电子密度分布，进而放大了整体双折射效应。本研究不仅首次确立了三氰基甲烷根基高性能双折射晶体的地位，同时为紧凑型偏振光学器件提供了一种简便易行的直线型氰基取代策略。