Double Sb‐N Coordination Enables Record‐High Birefringence in UV‐Transparent Crystals

Achieving high birefringence in ultraviolet (UV)-transparent crystals has remained a significant challenge due to the absence of systematic design methodologies. Herein, we present a novel organic-inorganic coordination strategy that combines π-conjugated organic ligands ([C₁₂H₈N₂]) with inorganic Sb-based polyhedra featuring stereochemically active lone pairs. This approach facilitates the formation of the first coplanar double Sb-N coordination framework, enabling the synthesis of two new birefringent hybrid crystals: (C₁₂H₈N₂)SbF₂(H₂PO₃) (PNSP) and (C₁₂H₈N₂)SbF₃ (PNSF). Notably, PNSF demonstrates an unprecedented birefringence of 0.79 at 546 nm, exceeding all known UV-transparent birefringent materials to date. These results not only expand the performance limits of birefringent hybrid crystals but also emphasize the pivotal role of stereochemically induced anisotropy in manipulating optical properties. Our study establishes a solid foundation for the rational design of next-generation optical materials, offering significant advancements for applications in cutting-edge photonic technologies, including UV polarizers and optical modulators.