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 (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-[(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 E_g (>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 (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.