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

Abstract

Birefringent and nonlinear optical (NLO) crystals play indispensable roles in modern optoelectronics and information communication. Hydro-isocyanurates contain π-conjugated building anions [H_xC₃N₃O₃]^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)₂·8H₂O (I), Mg(H₂C₃N₃O₃)₂·6H₂O (II), Ca(H₂C₃N₃O₃)₂·3H₂O (III), α-Sr(H₂C₃N₃O₃)₂·2H₂O (IV), β-Sr(H₂C₃N₃O₃)₂·2H₂O (V), SrHC₃N₃O₃(VI), SrHC₃N₃O₃·2.5H₂O (VII) and Ba(H₂C₃N₃O₃)₂·2H₂O (VIII). Most of them possess larger birefringence (>0.24) than commercial ultraviolet birefringent crystals such as calcite (∼0.16). In particular, among all the reported hydro-isocyanurates, acentric VII exhibits a giant NLO effect (d₁₅ = 5 pm V⁻¹) and desirable birefringence (Δn = 0.341 @ 800 nm) to realize a short phase-matching wavelength, due to the optimal arrangement of anionic groups.