RE(H2C3N3O3)2·(OH)·xH2O (RE = La, Y and Gd): potential UV birefringent materials with strong optical anisotropy originating from the (H2C3N3O3)- group.

A new series of dihydro-isocyanurates, namely, RE(H2C3N3O3)2·(OH)·xH2O (RE = La, Y and Gd) was readily obtained by a mild hydrothermal technique using lithium hydroxide as a deprotonation agent. La(H2C3N3O3)2·OH·2H2O and Re(H2C3N3O3)2·OH·5H2O (Re = Y, Gd) crystallized in the centrosymmetric (CS) space groups P21/c and P1[combining macron], respectively. Their intricate 3D frameworks were built by cationic polyhedrons (LaO8N/ReO8) and (H2C3N3O3)- groups. The experiments and theoretical studies demonstrated that these compounds possessed short ultraviolet cut-off edges (≤240 nm) and large birefringences (Δn≥ 0.257@546.1 nm). In addition, the theoretical studies further revealed that the (H2C3N3O3)2- group plays a pivotal role in strengthening the optical anisotropy for all compounds.