LiO4 tetrahedra lock the alignment of π-conjugated layers to maximize optical anisotropy in metal hydroisocyanurates†
Abstract
Parallel and compact alignments of π-conjugated anions in a crystal are of importance to maximize the optical anisotropy of crystalline materials. Our attempts to design colossal birefringence in metal hydroisocyanurates M2N(H2C3N3O3)4·6H2O (M/N = Li/Ca and Na/Ba) were successful using cation coordination control. In particular, Li2Ca(H2C3N3O3)4·6H2O (LCHCY) shows the largest birefringence (Δn = 0.407@800 nm) among all the reported metal cyanurates and hydroisocyanurates, even larger than that of 2D BN layers, due to the reinforced lock effect of basal oxygens from LiO4 tetrahedra. LCHCY is a novel promising ultraviolet birefringent crystal with a short ultraviolet cut-off and a wide transmission range.