KF·[SO 2 (NH 2 ) 2 ]: the First Deep‐Ultraviolet Transparent Sulfonamide Fluoride Nonlinear Optical Crystal with Novel Cairo Pentagonal Structure

Abstract The sulfonamide group (SO 2 (NH 2 ) 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·[SO 2 (NH 2 ) 2 ], was successfully synthesized, with SO 2 (NH 2 ) 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.