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

Abstract 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 (C₃H₂O₄)²⁻ FBU, resulting in the development of four new polar‐enhancing FBUs: (C₃(CH₃)₂O₄)²⁻, (C₃F₂O₄)²⁻, (C₃H(OH)O₄)²⁻, and (C₃HFO₄)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 [Li₂C₃(CH₃)₂O₄]₂·3H₂O 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.