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

Decoupling the trade‐off between second‐harmonic generation (SHG) and birefringence (Δn) while maintaining a wide bandgap (Eg) 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—[(C5H6N2O2)(C5H5N2O2)ZnCl (I), (C5H6N2O2)ZnCl2(II), and (C5H6N2O2)ZnBr2 (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 [ZnO2X2] polyhedron, while halogen substitution (Cl → Br) amplified polarizability gradients. These advancements resulted in a stepwise amplified SHG response (0.4→3.5→8.2×KH2PO4, abbreviated as KDP), controlled Δn, wide Eg (>5.0 eV), and remarkable thermal stability (>250°C). Notably, (C5H6N2O2)ZnBr2 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 (Eg>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.