C9H7NBrX (X = Cl, Br, NO3): Three Excellent Birefringent Crystals with Distinct Optical Anisotropy Regulated by Anions

Abstract 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, C 9 H 7 NBrX (X = Cl, Br, NO 3 ), are successfully synthesized by introducing a new birefringence‐active [C 9 H 7 NBr] + unit. Interestingly, these compounds feature similar layered structures but exhibit different optical anisotropies at 550 nm (0.277 for C 9 H 7 NBrCl, 0.328 for C 9 H 7 NBrBr, and 0.401 for C 9 H 7 NBrNO 3 ) owing to the different anions in them. Particularly, the small trigonal planar NO 3 anions perfectly fill the interstices of the π‐conjugated [C 9 H 7 NBr] + groups with large optical anisotropy, with the resulting compound C 9 H 7 NBrNO 3 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.