(CNH)IOF and (CNH)(IOF): two new organic-inorganic hybrid fluoroiodate birefringent crystals.

Birefringent crystals are anisotropic materials commonly used in modern optical devices to obtain polarized light. The traditional commercial birefringent crystals, such as MgF and CaCO, 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 (IOF) anions, namely, (CNH)IOF and (CNH)(IOF), were successfully grown by a simple evaporation method. In their structures, the π-conjugated organic cations and (IOF) 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.