Finding Short-Wavelength Birefringent Crystals with Large Optical Anisotropy Activated by π-Conjugated [C(NH2)3] Units

Abstract

Looking for ultraviolet (UV) crystals with large birefringence that are easy to grow, and nontoxic, has always been a hotspot in the field of optical materials. Thus, finding novel chromophores and discovering new candidates that are constructed by them enable us to obtain high performance short-wavelength birefringent crystals. Here, we synthesized five guanidine-based crystals with large birefringence (calcd. 0.072–0.371@532 nm) and short UV cutoff edges (201–254 nm). Among them, [C(NH2)3]HC2O4·H2O possesses a giant birefringence of Δn = 0.371@532 nm, which is even larger than that of most of the commercialized UV birefringent crystals. Moreover, these crystals can be obtained by a simple aqueous solution volatilization method, and they have unique advantages including low cost, easy to grow, and free of toxic reactants. Further analysis confirmed that the large birefringence in them originates from the π-conjugated [C(NH2)3] units, which implies that the guanidine-based compound is a promising system for exploring UV and even deep-UV materials with large birefringence.