Metal Cation Engineered UV Birefringent Crystals in π‐Conjugated Sulfonates with Record‐High Optical Anisotropy

Exploring excellent UV birefringent crystals is very urgent for use in advanced optical technologies to manipulate light polarization of great significance. Planar π-conjugated groups are of great interest to researchers in the exploration of optical materials with pronounced birefringence (∆n). However, the systematic understanding of cation and anion groups-regulated structure-property relationships requires further investigation. Herein, metal cation engineering is applied to achieve controlled synthesis of novel compounds, enabling precise coordination modulation and structure-property correlation studies. In this work, eight new 3-pyridinesulfonate crystals with the formula A(3-C₅H₄NSO₃)_x·yH₂O (A = Li, Na, Mg, Ca, Zn, Sr, and Ba; x = 1 and 2, y = 0, 4, and 6) and Na(3-C₅H₄NSO₃)·2NaBF₄·(3-C₅H₄NSO₃H) are synthesized and characterized. Notably, these crystals exhibit strong optical anisotropy, with birefringence (∆n) values ranging from 0.106 to 0.345 (measured at 546 nm) and short UV cut-off edges below 280 nm. Specifically, Li(3-pySO₃) exhibits an exceptionally large birefringence (∆n = 0.345 at 546 nm), which represents the highest value within the metal-containing 3-pyridinesulfonate system and surpasses that of most commercially available birefringent crystals. Moreover, this analysis of the eight crystals elucidates a comprehensive cation-structure-birefringence correlation framework, thereby establishing a cation-driven design principle for high-performance birefringent crystals.