[CdCl ₂ (4-Aminopyridine) ₂ ] _<i>n</i> : Synergistic Enhancement of Second-Harmonic Generation and Birefringence Enabled by a π-Conjugated Ligand for UV Nonlinear Optics

The integration of strong second-harmonic generation (SHG) response and high birefringence in a single crystalline material is highly desirable for advanced ultraviolet (UV) photonic applications, yet it remains a formidable challenge due to its conflicting structural prerequisites. Herein, a known organo-inorganic hybrid cadmium chloride, [CdCl₂(4-aminopyridine)₂]_<i>n</i>, has been found to achieve a balance of these properties. This compound exhibits a strong SHG response (5 × KDP), a large birefringence (0.368 at 546 nm), and a wide band gap (4.26 eV). Structural analysis combined with first-principles calculations reveals that the π-conjugated 4-aminopyridine (4-AP) ligand acts as a bifunctional module, serving as the primary source for both macroscopic SHG and birefringence. The synergistic effect between the aligned organic ligands and the one-dimensional inorganic [CdCl₄N₂] chains is identified as the key factor for the overall performance. This study offers insight into the rational design of UV NLO materials via functional ligand integration.