Structural Anion Geometry-Regulated Molecular Alignment in [C <b> ₁₀ </b> H <b> ₈ </b> NO <b> ₂ </b> ]-Based Crystals for Enhancing Ultraviolet Birefringence

Rational design of functional crystalline materials through molecular-level control remains a pivotal challenge in materials chemistry. While cation engineering has been extensively studied, the systematic exploitation of anion geometry as a structural “template” to modulate macroscopic optical anisotropy remains underexplored. Herein, we implement an anion-driven structural evolution strategy by pairing a high-polarizability organic cation, [C10H8NO2]+, with anions of divergent geometries: spherical (Cl–), planar (NO3–), and polyhedral (SiF62–). This evolution in the anionic configuration successfully steers the crystal packing from large-angle staggered to perfectly parallel alignment. Consequently, three novel UV birefringent crystals─[C10H8NO2]Cl·H2O (1), [C10H8NO2]NO3 (2), and [C10H8NO2]2SiF6·2H2O (3)─were synthesized. Notably, compound 3 exhibits a giant birefringence of 0.79 at 550 nm, significantly outperforming compounds 1 (0.31) and 2 (0.33) while maintaining a short UV cutoff edge (354 nm). Structural-property analysis reveals that the dimensional evolution of anions effectively modulates the alignment of optically active groups and the intermolecular stacking, thereby governing the macroscopic optical anisotropy. This work provides a quantitative mechanism-driven framework for the targeted design of high-performance optical materials via anion engineering.