KF·SO2(NH2)2: the First Deep‐Ultraviolet Transparent Sulfonamide Fluoride Nonlinear Optical Crystal with Novel Cairo Pentagonal Structure

The sulfonamide group (SO2(NH2)2) shows great potential for deep-ultraviolet nonlinear optical (NLO) crystals due to its strong hyperpolarizability and anisotropy, but its neutrality and poor protonation limit its application. Here, we propose a new design strategy that introduces an ionic framework via halide salts to both stabilize sulfonamide molecules and control their orientation, thereby enabling desirable second-harmonic generation (SHG) and birefringence. Following this concept, the first sulfonamide fluoride, KF·SO2(NH2)2, was successfully synthesized, with SO2(NH2)2 units firmly anchored in the KF lattice by K─O ionic and N─H···F hydrogen bonds. Remarkably, this crystal exhibits a unique and compact Cairo pentagonal configuration. Benefiting from its well-ordered structure, the crystal exhibits a strong SHG response of 1.4 × KDP at 1064 nm, a large birefringence of 0.079@546 nm, and a short cutoff edge below 190 nm. This work opens new avenues for the design of advanced NLO crystals with a sulfonamide group.