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, 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.