β-(C3H7N6)2Cl2·H2O and (C3H7N6)F·H2O: two UV birefringent crystals induced by uniformly aligned structural groups

Abstract

Planar π-conjugated groups are of great interest to researchers in the exploration of optical materials with large birefringence. The key point is to control the orientation and spatial density of birefringence-active groups. Here, two metal-free UV birefringent crystals, β-(C₃H₇N₆)₂Cl₂·H₂O and (C₃H₇N₆)F·H₂O, were synthesized using an aqueous solution evaporation technique. They feature a two-dimensional layered structure composed of π-conjugated planar [C₃H₇N₆]⁺ units with the same orientation. As a result, the title compounds exhibit a large birefringence (0.33–0.38@550 nm), not only exceeding that of most of the melamine-based materials but also being larger than that of the commercial birefringent materials. Moreover, UV-Vis-NIR diffuse reflectance spectra and thermal stability analysis demonstrate that (C₃H₇N₆)F·H₂O exhibits a shorter UV absorption edge and higher thermal stability because of the more electronegative F element. The first-principles calculations elucidate the origin of their optical properties. The effect of group packing density and orientation on the birefringence in melamine-based compounds has been discussed in detail for the first time. The arrangement of groups regulated by halogen atoms will inspire the exploration of new birefringent materials with large optical anisotropy.