A π-conjugated organic molecule-modified strategy to achieve high-performance metal nitrate birefringent crystals†

Abstract

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, making it challenging to fulfill stringent standard requirements. Therefore, 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 with [(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 properties, we adopted a 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 interactions 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.