Parallel Alignment of π-Conjugated Anions in Hydroisocyanurates Enhancing Optical Anisotropy

Birefringent crystals with optical anisotropy, which are employed to modulate the polarization of light, play a vital role in many fields of the optical industry. In this Communication, two mixed alkali metal hydroisocyanurates, RbLi(H₂C₃N₃O₃)₂·2H₂O (I) and CsLi(H₂C₃N₃O₃)₂·2H₂O (II), were synthesized by a simple aqueous solution method, and they feature layered structures composed of _∞²[H₂C₃N₃O₃]^- ribbons through hydrogen bonds with the separation of water molecules and alkali cations. They exhibit a simultaneously short ultraviolet cutoff and giant birefringence, resulting from the perfect parallel alignment of π-conjugated (H₂C₃N₃O₃)^- anions. Interestingly, the first-principles calculations elucidated that the electronic states and optical properties are slightly different owing to the distinct p-π interaction between alkaline metals and the (H₂C₃N₃O₃)^- group despite the isostructural crystallographic lattice.