C 9 H 7 NBrX (X = Cl, Br, NO 3 ): Three Excellent Birefringent Crystals with Distinct Optical Anisotropy Regulated by Anions.

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.