Birefringence is a fundamental parameter used to quantify the optical anisotropy of crystals, which is important for optical materials to achieve light polarization or angle phase-matching. Especially, excellent birefringent materials can control polarized light and are indispensable materials to realize the functions of various linear optical devices, fiber optic sensors, and advanced optical communication systems. Manipulating the functional module ordering to maximize anisotropy is crucial in the design of birefringent materials. However, there exist significant challenges in constraining the spatial freedom of modules and achieving an optimal structural arrangement.

Recently, researchers from the Research Center for Crystal Materials of Xinjiang Technical Institute of Physics and Chemistry proposed a strategy to design deep-UV birefringent materials that achieve functional module ordering via weak interactions. Following this strategy, four compounds CN₄H₇SO₃CF₃, CN₄H₇SO₃CH₃, C(NH₂)₃SO₃CH₃, and C(NH₂)₃SO₃CF₃ were identified as high-performance candidates for deep-UV birefringent materials. The UV cutoff edges of CN₄H₇SO₃CF₃, CN₄H₇SO₃CH₃, and C(NH₂)₃SO₃CH₃ crystals are 183, 198 and 195 nm, respectively. CN₄H₇SO₃CF₃ exhibits the shortest cutoff edge among reported guanidinium/aminoguanidinium-based compounds. The birefringence of CN₄H₇SO₃CF₃, CN₄H₇SO₃CH₃, C(NH₂)₃SO₃CH₃, and C(NH₂)₃SO₃CF₃ is 0.149, 0.107, 0.144, and 0.146 at 546 nm, respectively. Remarkably, CN₄H₇SO₃CF₃ exhibits the largest birefringence (0.149 @ 546 nm, 0.395 @200 nm) in the deep-UV region among reported sulfates and sulfate derivatives. This study not only discovers new deep-UV birefringent materials but also provides an upgraded strategy for optimizing optical anisotropy to achieve efficient birefringence.

The results have been published in Angewandte Chemie International Edition. Xinjiang Technical Institute of Physics and Chemistry was the only completed unit, with Zhihua Yang and Shilie Pan as corresponding authors, and Huan Zhou as the first author. The research was supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences.

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