Record‐High Birefringence Achieved by Linearly Extended π‑Conjugated Building Blocks in Metal‐Free Sulfate

The advancement of birefringent materials is critical for next‐generation integrated photonics, yet conventional systems face intrinsic limitations in birefringence magnitude. Sulfate crystals offer a promising platform but have been hindered by their dependence on specific metal cations and insufficient optical anisotropy. Here, we report a molecular design strategy that overcomes these constraints by employing linearly extended π‐conjugated organic cations as high‐anisotropy building units. Through rational amino‐functionalization of C 12 H 8 N 2 , we engineer the novel cation [H 2 (C 12 H 10 N 4 )] 2 + , which exhibits an exceptional polarizability anisotropy of 156.7 a.u. Using a mild aqueous route, a metal‐free sulfate crystal, [H 2 (C 12 H 10 N 4 )]SO 4 ·3H 2 O was synthesized, which achieves a record birefringence of 0.981 at 550 nm, surpassing all known sulfate birefringent materials and outperforming commercial benchmarks and setting a new performance standard for birefringent sulfates. Structural analysis reveals that the cations align via hydrogen‐bond‐directed assembly with sulfate tetrahedra into a quasi‐two‐dimensional anisotropic structure, thereby maximizing the macroscopic optical response. This study demonstrates that incorporating linearly extended π‐conjugated organic building blocks into sulfate frameworks sever as a robust strategy for discovering highly birefringent crystals.