(C 8 H 6 BrN 2 O)NO 3 : A Hybrid Nonlinear Optical Crystal With an Appropriate Birefringence

Organic planar groups have recently been used to construct nonlinear optical materials because of their large hyperpolarizability, and then most of the products have birefringence which is unfavorable to phase matching. Here, a new organic–inorganic hybrid nonlinear optical crystal (C 8 H 6 BrN 2 O)NO 3 is synthesized by combining the π‐conjugated organic [C 8 H 6 BrN₂O] + cation and inorganic [NO 3 ] − anion as the fundamental building blocks. Surprisingly, it not only shows a large second‐harmonic generation response of 3.8 times that of the commercial K 2 HPO 4 crystals, but also has a moderate birefringence of 0.08 at 550 nm. Furthermore, it demonstrates high thermal stability up to 220 °C despite its hybrid material characteristics. Single crystal structure analysis and theoretical calculations confirm that the specific arrangement of NLO‐active [C 8 H 6 BrN₂O] + groups with halogen branch chains and strong hydrogen bonding between two π‐conjugated groups leads to moderate birefringence and large frequency‐doubling effect. These findings provide a method to explore hybrid nonlinear optical crystals.