Breaking the SHG-stability trade-off in UV nonlinear optical materials by polar-layer-dense-locking.

In ultraviolet (UV) nonlinear optical (NLO) metal-organic complexes (MOCs), simultaneously achieving a strong second-harmonic generation (SHG) response and high thermal and water stability has remained a persistent bottleneck. Herein, a "polar-layer-dense-locking" strategy is utilized to synthesize a 3D cadmium-based coordination polymer, Cd(imc) (imc2- = imidazole-4-carboxylate), breaking the long-standing SHG-stability trade-off. Employing the rigid, π-conjugated imc2- ligand with large molecular hyperpolarizability (|β max|) as the "nonlinear source", this strategy constructs high-performance 2D layers via µ2-κ2-N, O dual-bridging coordination. This architecture enforces a cooperative alignment of ligand dipoles, efficiently translating microscopic |β max| into an amplified macroscopic SHG response. The 3D topological network is achieved via strong Cd-O coordination bonds and dense interlayer packing, imparting exceptional structural stability. Cd(imc) exhibits a giant SHG response (10.0 × KDP), ultrahigh thermal stability (up to 470 °C), and remarkable water resistance (retains crystallinity for >180 days in water at room temperature and >30 days at 100 °C), alongside a wide bandgap (4.28 eV). It also exhibits good phase-matchability, making it a practical candidate for UV frequency conversion. This "polar-layer-dense-locking" paradigm establishes a synergistic mechanism to decouple the SHG-stability constraints in UV NLO materials.