Synthesis and Characterization of New-Type Ionic Cocrystals with Cyanuric Acid and Alkali Halides: AX·(H3C3N3O3)2 (A = Rb, Cs; X = Cl, Br)

Recently, neutral cyanurate acid and negative (iso-)cyanurates (HxC3N3O3)(3–x)– (x = 0–2) with strong conjugated π-bonds have been proposed as a new possible type of structural motifs for exploring high-performance birefringent materials. Especially, ionic cocrystals (ICCs) with neutral H3C3N3O3 molecules have attracted much attention due to their extended ultraviolet transparent range and concurrently improved birefringence. Herein, three ICC single crystals, RbCl·(H3C3N3O3)2 (I), CsCl·(H3C3N3O3)2 (II), and RbBr·(H3C3N3O3)2 (III), combining cyanurate acid and alkali halides have been designed and synthesized in ethanol solution. They simultaneously exhibit wide band gaps more than 5.4 eV, large birefringence (Δnexp ≈ 0.096–0.115, Δncal ≈ 0.071–0.105 at 800 nm), and high thermal stability above 325 °C. In addition, theoretical calculations have also been performed to evaluate the relationship of the molecular arrangement and their anisotropic optical properties. This work suggests that cyanurate-based cocrystals could be a promising structural family for the discovery of ultraviolet birefringent crystals.