[C(NH₂)₃]₂S₂O₆: A SBBO-Like Dithionate Crystal with Large Optical Anisotropy

Investigating ultraviolet (UV) birefringent crystals is a focal point of research in recent years. The development of superior birefringent materials faces substantial challenges, primarily due to the need to pinpoint optimal fundamental building blocks and to perfect their geometric arrangement within the crystal lattice. By selecting a planar π-conjugated [C(NH₂)₃] group and a staggered [S₂O₆] group, we have successfully synthesized an organic-inorganic hybrid crystal, [C(NH₂)₃]₂S₂O₆. Similar to the famous inorganic crystal Sr₂Be₂B₂O₇ (SBBO), [C(NH₂)₃]₂S₂O₆ exhibits a two-dimensional double-layered crystal structure connected by N-H···O hydrogen bonding. Due to the ideal spatial arrangements of the planar π-conjugated [C(NH₂)₃] group, this crystal displays a large birefringence (0.150 at 546 nm) among sulfate derivatives in the short-wave UV region. Meanwhile, [C(NH₂)₃]₂S₂O₆ has a large band gap of 5.23 eV. This work indicates that the [S₂O₆] unit, acting as a direct structural motif, is beneficial for the discovery of UV birefringent crystals.