Polar [<scp>NH</scp>(<scp>CH₂SO₃</scp>)₂]²⁻ Dimers Enable the First Promising Iminodimethanesulfonate with Nonlinear Optical Properties and Wide <scp>UV</scp> Transparency

Comprehensive Summary Non‐π‐conjugated groups have recently emerged as excellent fundamental building blocks (FBUs) for UV nonlinear optical (NLO) crystals, due to their ability to achieve wide transparency. However, their generally high crystallographic symmetry often results in limited optical anisotropy, making few resultant crystals phase‐matchable in the UV wavelength range. In this work, we introduce the polar [NH(CH 2 SO 3 ) 2 ] 2− dimer as a novel non‐π‐conjugated FBU, and synthesize the first NLO iminodimethanesulfonate crystal, namely Cs 2 NH(CH 2 SO 3 ) 2 ·H 2 O. Experimental results demonstrate that this crystal exhibits second‐harmonic generation (SHG) responses with 0.8 times of KDP, moderate birefringence of 0.044@550 nm, and broad UV transparency (&lt; 200 nm), enabling phase‐matchable SHG at the key UV wavelength of 266 nm. First‐principles calculations confirm that polar [NH(CH 2 SO 3 ) 2 ] 2− dimers contribute to the optical properties of Cs 2 NH(CH 2 SO 3 ) 2 ·H 2 O. This work highlights an excellent UV NLO‐active FBU for exploring high‐performance UV NLO crystals.