Birefringent crystalline materials play a crucial role in advanced optical technologies, including optical isolators, electro-optic modulators, and polarization beam splitters, due to their ability to precisely control light polarization states. However, the rational design and controlled synthesis of high-performance birefringent materials with a birefringence value (Δn > 0.3) remains a substantial hurdle. To achieve superior optical anisotropy, it is essential to identify birefringent-active functional groups with strong polarizability anisotropy and optimize their spatial orientation within the crystal lattice.

Recently, researchers from the Crystal Materials Research Center at the Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, proposed a design strategy that enhances optical anisotropy in low-dimensional structures by intralayer hydrogen bonding modulation and optimizing anionic frameworks. Through this approach, they successfully synthesized a series of 2-aminopyrazine based birefringent crystals, four of which exhibited birefringence values of 0.489, 0.490, 0.594, and 0.658 at 546 nm. The significant enhancement in birefringence was attributed to structural dimensional transitions and the tendency of protonated 2-aminopyrazine groups to form low-dimensional frameworks under the influence of hydrogen bonding. The intralayer [N−H···O], [O−H···N], and [N−H···F] hydrogen bonds facilitate the perfect coplanar arrangement (θ = 0 °)  of birefringent active units results in an enhanced in-plane optical anisotropy. Furthermore, theoretical calculations confirmed that sequential anionic substitutions induced variations in optical polarizability, thereby improving the linear optical properties of the birefringent materials.

This study introduces a novel birefringent functional group and provides both theoretical and experimental guidance for the design and synthesis of high-birefringence compounds within low-dimensional frameworks.

The findings have been published in Materials Horizons under the title "Optimizing Optical Anisotropy in Low-Dimensional Structures via Intralayer Hydrogen Bonding Modulation and Anionic Substitution." The Xinjiang Technical Institute of Physics and Chemistry is the sole corresponding institution for this research. Professors Xueling Hou and Shilie Pan are the corresponding authors, while master's student Muhammad Arif is the first author. This study was supported by grants from the National Natural Science Foundation of China and the Chinese Academy of Sciences.

Figure: The formation of low-dimensional aminopyrazine based structures with enhanced birefringence is attributed to the optimized coplanarity of birefringent-active units, meticulously regulated by intralayer [N−H···X] hydrogen bonds.