Improving the Performance of Birefringent Crystals by Molecular Assembly Strategy in the Solar-Blind UV Region

Birefringent crystals are of great significance as the key element in the modulation of optical polarization state. However, there exists a mutual constraint between UV transparency and large birefringence, necessitating compromises. Herein, the micro-units that are beneficial to birefringence are selected and the crystal synthesis process is regulated by three molecular assembly strategies. First, the traditional π-conjugated groups are modified by introducing protons. Second, the π-conjugated groups are recombined through structural design, and different units are rationally assembled and oriented according to the beneficial way of birefringence. Third, inspired by the fluorination strategy in borate crystals, fluorine is added which can modify the chemical and function. The synthesis of a series of nine birefringent crystals, some of which (Rb₂HCO₃F·B(OH)₃, Rb₂C₂O₄·B(OH)₃, RbHC₂O₄) exhibit almost the best optical properties in respective compounds of similar structure, can be regarded as birefringent crystals of potential use, and validate the effectiveness of the strategies. To the knowledge, this is the first experimental study on how to modify and regulate the assembly and arrangement of molecules of the birefringent crystals in the solar-blind UV region (λ < 280 nm). This work provides a reference for accelerating the exploration of birefringent crystals.