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.