π-conjugated organic molecule modified strategy to achieve high-performance metal nitrate birefringent crystal

Birefringent crystals that can modulate the polarization of light play an important role in modern scientific research. However, the birefringence of current commercial crystals is limited to inorganic compounds and the coefficients are generally lower than 0.3, rendering it challenging to fulfill stringent standard requirements. Therefore, developing the development of superior birefringent materials has emerged as a significant area of research. In this work, we synthesized a Hg-based nitrate Hg3O2(NO3)2·H2O, which is built by [(Hg3O2)2+]∞ layers and isolated NO3− anions. Hg3O2(NO3)2·H2O shows a large experimental birefringence (Δn = 0.25@546 nm). In order to improve birefringent property, we adopted π-conjugated organic molecule modified strategy to achieve high-performance metal nitrate birefringent crystals , and successfully synthesized a new Hg-based hybrid nitrate (CH5N3S)2Hg(NO3)2. The crystal structure of (CH5N3S)2Hg(NO3)2 is composed of [((CH5N3S)2Hg)2+]∞ units and isolated NO3− anions. Notably, it has an enhanced experimental birefringence (Δn = 0.32@546 nm), which is excellent among all metal nitrates. Structural analysis and theoretical calculations show that for Hg3O2(NO3)2·H2O, HgO2 and nitrate play a crucial role in optical anisotropy. For (CH5N3S)2Hg(NO3)2, the interaction between CH5N3S molecules and cations as well as nitrate play a crucial role in optical anisotropy. The research shows that the introduction of π-conjugated organic molecules is an effective strategy for developing high-performance birefringent materials.