Cd[C(NH₂)₂O]₂Cl₂: A Urea-Based Open-Framework Crystal with Concurrent Short UV Cutoff Edge and Large Birefringence

High-performance birefringent crystals are crucial for advancing modern optics. Regrettably, conventional birefringent crystals are usually limited by narrow transmission windows or insufficient birefringence. Recognizing that birefringence originates from structural anisotropy, we herein assembled anisotropic π-conjugated urea ligands through a cadmium-based open framework. By leveraging the distorted Cd-based octahedra and urea-derived hydrogen-bonding network, a novel birefringent crystal, Cd[C(NH₂)₂O]₂Cl₂, was successfully obtained. Cd[C(NH₂)₂O]₂Cl₂ was characterized by a characteristic 4 × 4 × 6 open-framework skeleton, wherein the repeating building unit {Cd[C(NH₂)₂O]₂Cl₄} is constructed by the oxygen sharing between distorted CdCl₄O₂ octahedra and planar [C(NH₂)₂O] groups. Notably, Cd[C(NH₂)₂O]₂Cl₂ achieved an excellent balance between wide bandgap and substantial optical anisotropy by exhibiting a short UV cutoff edge of 201 nm and a birefringence of 0.117@546 nm, which should benefit practical applications.