A large optical anisotropy is the most important parameter of birefringent crystals. Integrating π‐conjugated groups with large polarizable anisotropy into target compounds is a common strategy for constructing brilliant birefringent crystals. However, the key problem is to enhance the density of the birefringence‐active units and further arrange them parallelly. In this study, three novel birefringent crystals, C9H7NBrX (X = Cl, Br, NO3), are successfully synthesized by introducing a new birefringence‐active [C9H7NBr]+ unit. Interestingly, these compounds feature similar layered structures but exhibit different optical anisotropies at 550 nm (0.277 for C9H7NBrCl, 0.328 for C9H7NBrBr, and 0.401 for C9H7NBrNO3) owing to the different anions in them. Particularly, the small trigonal planar NO3 anions perfectly fill the interstices of the π‐conjugated [C9H7NBr]+ groups with large optical anisotropy, with the resulting compound C9H7NBrNO3 showing superior optical properties compared to the others. The above findings provide strategies for designing new optical materials with large birefringence by matching birefringence‐active groups of different sizes. Additionally, a new theory for predicting and comparing the polarizability anisotropy of compounds is proposed, which would guide in exploring large birefringent crystals.

中文翻译：

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C9H7NBrX（X = Cl、Br、NO3）：三种优秀的双折射晶体，具有受阴离子调节的独特光学各向异性


大的光学各向异性是双折射晶体最重要的参数。将具有大极化各向异性的π共轭基团整合到目标化合物中是构建明亮双折射晶体的常见策略。然而，关键问题是提高双折射活性单元的密度并进一步平行排列。在本研究中，通过引入新的双折射活性[C9H7NBr]+单元，成功合成了三种新型双折射晶体C9H7NBrX（X = Cl、Br、NO3）。有趣的是，这些化合物具有相似的层状结构，但由于其中阴离子不同，在 550 nm 处表现出不同的光学各向异性（C9H7NBrCl 为 0.277，C9H7NBrBr 为 0.328，C9H7NBrNO3 为 0.401）。特别是，小的三角平面NO3阴离子完美地填充了具有大光学各向异性的π共轭[C9H7NBr]+基团的间隙，所得化合物C9H7NBrNO3与其他化合物相比表现出优异的光学性质。上述发现为通过匹配不同尺寸的双折射活性基团来设计具有大双折射的新型光学材料提供了策略。此外，提出了一种预测和比较化合物极化率各向异性的新理论，这将指导探索大型双折射晶体。