An UV Transparent Metal‐Free Sulfate With Giant Birefringence Enabled by Optimal Structural Anisotropy

ABSTRACT Sulfate crystals have attracted growing interest due to their appealing characteristics and potential use in optical materials, particularly as birefringent materials. The development of birefringent sulfate crystals has long been limited to sulfates containing specific metal cations. In this work, we report a metal‐free sulfate birefringent crystal, [H 2 C 6 N 7 (NH 2 ) 3 ]SO 4 ·2H 2 O, which is designed by chemically assembling an overlooked anisotropic building block [H 2 C 6 N 7 (NH 2 ) 3 ] 2+ with the sulfate tetrahedron. The title compound exhibits remarkable birefringence of 0.46 at 550 nm, which surpasses a wide variety of the newly developed metal sulfate birefringent crystals as well as the commercially used birefringent crystals. Furthermore, [H 2 C 6 N 7 (NH 2 ) 3 ]SO 4 ·2H 2 O possesses a wide optical band gap, outperforming state‐of‐the‐art metal sulfate birefringent crystals. Structural analysis reveals that the hydrogen bonding‐driven assembly of [H 2 C 6 N 7 (NH 2 ) 3 ] 2+ and tetrahedral [SO 4 ] 2− modules facilitates perfect alignment of the crucial [H 2 C 6 N 7 (NH 2 ) 3 ] 2+ units and the formation of quasi‐two‐dimensional architectures, resulting in optimal structural anisotropy and thus maximizing the birefringence of the title compound. Our findings open up new possibilities for metal‐free sulfate solids as promising birefringent materials.