(C5N2H7)IO2F2 and (C3N6H8)(IO2F2)2 : Two new organic-inorganic hybrid fluoroiodate birefringent crystals

Birefringent crystals are anisotropic material commonly used in modern optical devices to obtain polarized light. Traditional commercial birefringence crystals, such as, MgF₂ and CaCO₃, have the problems of small birefringence and not easy to grow. Herein, two organic-inorganic hybrid birefringence crystals composed of π-conjugated organic groups and lone pair-containing (IO₂F₂)^- anions, namely, (C₅N₂H₇)IO₂F₂ and (C₃N₆H₈)(IO₂F₂)₂, were successfully grown by a simple evaporation method. In their structures, the π-conjugated organic cations and (IO₂F₂)^- anions are interconnected through hydrogen bonding and π-π stacking interactions into 3D supramolecular networks. Both of them exhibit large birefringence of 0.30 @ 550 nm and 0.23 @ 550 nm, superior to most of conventional birefringence materials. Moreover, UV-Vis-NIR diffuse reflectance spectra demonstrate that they have wide band gaps (4.06 eV and 4.10 eV) because of the presence of the F element with strong electronegativity. Theoretical analyses suggest that the parallel arrangement of the π conjugated cations resulted in large optical anisotropy. This work provides new promising candidates for UV birefringence materials.