pH‐Regulated Ring‐Opening/Closing Transformation in Polyoxometalates Enables High‐Performance Nonlinear Optical Crystals

Abstract Inorganic polyoxometalate (POM) crystals are a promising class of materials with considerable potential in modern laser science and technology. However, the limited understanding of POM formation mechanisms and the unclear structure‐property relationship between POMs and nonlinear optics (NLO) have hindered the rational design and controllable synthesis of high‐performance inorganic POM‐based NLO crystals. Herein, we report the successful synthesis of two novel polar heteropolymolybdates, (NH 4 ) 4 [Mo 5 O 15 (SeO 3 ) 2 ]·2H 2 O (NMSeO‐5) and (NH 4 ) 8 [Mo 6 O 18 (SeO 3 ) 4 ]·5H 2 O (NMSeO‐6). Supported by theoretical calculations and experimental verification, we demonstrate, for the first time, pH‐regulated ring‐opening and ring‐closing transformations in an inorganic POM system and propose a mechanistic pathway for their formation. Specifically, NMSeO‐5, containing highly symmetrical Strandberg‐type [Mo 5 O 15 (SeO 3 ) 2 ] 2− clusters, undergoes structural transformation under strongly acidic conditions (H 2 SeO 3 ) into NMSeO‐6, which features a lower‐symmetry, Reuleaux triangular [Mo 6 O 18 (SeO 3 ) 4 ] 8− cluster. Remarkably, this transformation results in a dramatic enhancement of the second‐harmonic generation response from 0.07 × KDP (NMSeO‐5) to 10 × KDP (NMSeO‐6). This work provides not only mechanistic insight into POM structural evolution but also establishes a paradigm for the design of next‐generation, high‐performance inorganic POM‐based NLO materials.