A rich structural chemistry in p-conjugated hydroisocyanurates: layered structures of A2B (H2C3N3O3)4 • nH2O (A = K, Rb, Cs; B = Mg, Ca; n=4, 10) with high ultraviolet transparency and strong optical anisotropy

Ultraviolet (UV) transparent birefringent crystals are indeed indispensable for modern optoelectronics and polarizer devices. Herein, mixed alkali/alkali-earth metal hydroisocyanurates A(2)B(H2C3N3O3)(4)nH(2)O (A = K, Rb, Cs; B = Mg, Ca; n = 4, 10) were synthesized and their thermal, vibrational and optical properties were characterized in detail. Although they crystallize in different crystallographic point groups, all compounds feature quasi-two-dimensional layered structures built by the 2 infinity[H2C3N3O3](-) ribbons through hydrogen bonds, separated by water molecules and cations. Benefiting from the delocalized pi-conjugated bonds and planar structural configuration of (H2C3N3O3)(-) groups, they show a concurrently short ultraviolet absorption edge (lambda(cut-off) approximate to 230 nm) and strong optical anisotropy (Delta n approximate to 0.37 at 800 nm), twice larger than that of the benchmark UV birefringent crystal calcite.