[C(NH<sub>2</sub>)<sub>3</sub>]<sub>2</sub>S<sub>2</sub>O<sub>6</sub>: A SBBO-Like\nDithionate Crystal with Large Optical\nAnisotropy

Investigating ultraviolet (UV) birefringent crystals\nis a focal\npoint of research in recent years. The development of superior birefringent\nmaterials faces substantial challenges, primarily due to the need\nto pinpoint optimal fundamental building blocks and to perfect their\ngeometric arrangement within the crystal lattice. By selecting a planar\nπ-conjugated [C(NH<sub>2</sub>)<sub>3</sub>] group and a staggered\n[S<sub>2</sub>O<sub>6</sub>] group, we have successfully synthesized\nan organic–inorganic hybrid crystal, [C(NH<sub>2</sub>)<sub>3</sub>]<sub>2</sub>S<sub>2</sub>O<sub>6</sub>. Similar to the famous\ninorganic crystal Sr<sub>2</sub>Be<sub>2</sub>B<sub>2</sub>O<sub>7</sub> (SBBO), [C(NH<sub>2</sub>)<sub>3</sub>]<sub>2</sub>S<sub>2</sub>O<sub>6</sub> exhibits a two-dimensional double-layered crystal structure\nconnected by N–H···O hydrogen bonding. Due to\nthe ideal spatial arrangements of the planar π-conjugated [C(NH<sub>2</sub>)<sub>3</sub>] group, this crystal displays a large birefringence\n(0.150 at 546 nm) among sulfate derivatives in the short-wave UV region.\nMeanwhile, [C(NH<sub>2</sub>)<sub>3</sub>]<sub>2</sub>S<sub>2</sub>O<sub>6</sub> has a large band gap of 5.23 eV. This work indicates\nthat the [S<sub>2</sub>O<sub>6</sub>] unit, acting as a direct structural\nmotif, is beneficial for the discovery of UV birefringent crystals.