(C5N2H7)IO2F2 and (C3N6H8)(IO2F2)2: two new organic–inorganic hybrid fluoroiodate birefringent crystals

Abstract

Birefringent crystals are anisotropic materials commonly used in modern optical devices to obtain polarized light. The traditional commercial birefringent crystals, such as MgF₂ and CaCO₃, have low birefringence and are not easy to grow. Herein, two organic–inorganic hybrid birefringent 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 values of 0.30 @ 550 nm and 0.23 @ 550 nm, which are superior to the values of most of the conventional birefringent 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 birefringent materials.