Optimal arrangement of π-conjugated anionic groups in hydro-isocyanurates leads to large optical anisotropy and second-harmonic generation effect

Birefringent and nonlinear optical (NLO) crystals play indispensable roles in modern optoelectronics and information communication. Hydro-isocyanurates contain pi-conjugated building anions [HxC3N3O3](x-3)(x= 0-3) with strong anisotropic first-order polarizability and large second-order susceptibility, and thus they are proposed as an emerging source of birefringent and NLO crystals. Here, we conducted a systematical exploration of alkaline earth-metal hydro-isocyanurates and successfully obtained eight new compounds: Mg(H2C3N3O3)(2)center dot 8H(2)O (I), Mg(H2C3N3O3)(2)center dot 6H(2)O (II), Ca(H2C3N3O3)(2)center dot 3H(2)O (III), alpha-Sr(H2C3N3O3)(2)center dot 2H(2)O (IV), beta-Sr(H2C3N3O3)(2)center dot 2H(2)O (V), SrHC3N3O3(VI), SrHC3N3O3 center dot 2.5H(2)O (VII) and Ba(H2C3N3O3)(2)center dot 2H(2)O (VIII). Most of them possess larger birefringence (>0.24) than commercial ultraviolet birefringent crystals such as calcite (similar to 0.16). In particular, among all the reported hydro-isocyanurates, acentricVIIexhibits a giant NLO effect (d(15)= 5 pm V-1) and desirable birefringence (Delta n= 0.341 @ 800 nm) to realize a short phase-matching wavelength, due to the optimal arrangement of anionic groups.