Designing Strong Polarity and High Configurational Entropy Flexible Units toward Excellent Ultraviolet Nonlinear Optical Materials

Polar materials play a crucial role in numerous essential fields; however, efficiently synthesizing them remains a significant challenge. In this study, a viable approach is proposed to enhance the probability of discovering polar materials by developing new functional basic units (FBUs) with strong polarity and high configurational entropy. To validate this approach, a site‐modification strategy is applied to the high configurational entropy flexible (C 3 H 2 O 4 ) 2− FBU, resulting in the development of four new polar‐enhancing FBUs: (C 3 (CH 3 ) 2 O 4 ) 2− , (C 3 F 2 O 4 ) 2− , (C 3 H(OH)O 4 ) 2− , and (C 3 HFO 4 ) 2− . Moreover, 11 new compounds through temperature‐driven conformational changes in FBUs are successfully synthesized. Among these, the proportion of polar structures reaches an impressive 54%. Remarkably, all of these novel polar compounds exhibit outstanding nonlinear optical (NLO) properties. The [Li 2 C 3 (CH 3 ) 2 O 4 ] 2 ·3H 2 O demonstrates the rare coexistence of a moderate birefringence (0.071@514 nm), short UV cutoff edge (210 nm), and the strongest second harmonic generation (SHG) effect (6.5 × KDP) among ionic‐organic fourth harmonic generation crystals. Preliminary laser experiments have also proven its potential practical value as a fourth harmonic generation crystal. This research not only discovers a variety of exceptional UV NLO crystals but also provides a feasible method to increase the likelihood of finding polar materials.