Decoupling the SHG‐Birefringence Trade‐off in UV NLO Materials via Triple‐Control Coordination Engineering: Achieving Synergistic Performance Optimization

Abstract Decoupling the trade‐off between second‐harmonic generation (SHG) and birefringence ( Δn ) while maintaining a wide bandgap ( E g ) is a critical challenge in the field of ultraviolet (UV) nonlinear optical (NLO) materials. In this work, we introduce a novel “triple‐control coordination engineering” strategy that successfully addresses this challenge. By integrating dihedral angle modulation, coordination mode switching, and polarizability engineering, we have synthesized three zinc halides—[(C 5 H 6 N 2 O 2 )(C 5 H 5 N 2 O 2 )ZnCl ( I ), (C 5 H 6 N 2 O 2 )ZnCl 2 ( II ), and (C 5 H 6 N 2 O 2 )ZnBr 2 ( III )]—that exhibit exceptional UV NLO performance. Through dynamic C─C─N bond rotation, we achieved precise control over the dihedral angle (81°→84°), effectively modulating optical anisotropy (Δn = 0.048–0.071@1064 nm). pH/temperature‐driven coordination mode evolution enabled complete alignment of the [ZnO 2 X 2 ] polyhedron, while halogen substitution (Cl → Br) amplified polarizability gradients. These advancements resulted in a stepwise amplified SHG response (0.4→3.5→8.2 × KH 2 PO 4 , abbreviated as KDP), controlled Δn , wide E g (>5.0 eV), and remarkable thermal stability (>250 °C). Notably, (C 5 H 6 N 2 O 2 )ZnBr 2 stands out as the first zinc‐based halide to simultaneously achieve large SHG (>8 × KDP), moderate birefringence (0.05< Δn < 0.1), and a wide bandgap ( E g > 5.0 eV). This work not only provides a paradigm‐shifting approach to materials design but also paves the way for the development of UV NLO materials with superior comprehensive performance.