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.