Inch‐sized KNH2(CH2COO)2: a High‐Performance UV Nonlinear Optical Crystal Enabled by Novel Flexible π‐Conjugated Building Units, NH2(CH2COO)2−

Abstract High‐performance ultraviolet (UV) nonlinear optical (NLO) crystals must exhibit three critical properties: a large second‐harmonic generation (SHG) response, adequate birefringence, and a short UV cutoff edge. However, these key characteristics are often interdependent and challenging to achieve simultaneously within a single crystal. In this study, a novel non‐centrosymmetric compound, KNH 2 (CH 2 COO) 2 is introduced, synthesized from the innovative flexible π‐conjugated group NH 2 (CH 2 COO) 2 − . Benefiting from the excellent microscopic optical properties of the flexible π‐conjugated NH 2 (CH 2 COO) 2 − groups, despite the suboptimal arrangement of groups within the compound, KNH 2 (CH 2 COO) 2 still demonstrates a remarkable balance in its linear and nonlinear optical performance. This includes a strong SHG response of 3 times that of KDP, suitable birefringence of 0.088@1064 nm, and a short UV cutoff edge at 208 nm. Emphatically, high‐quality, inch‐sized single crystals (up to 60 × 46 × 30 mm 3 ) are successfully grown using a water solution technique, paving the way for practical applications of this promising material.