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

초록

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, (NH4)4[Mo5O15(SeO3)2]<middle dot>2H2O (NMSeO-5) and (NH4)8[Mo6O18(SeO3)4]<middle dot>5H2O (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 [Mo5O15(SeO3)2]2- clusters, undergoes structural transformation under strongly acidic conditions (H2SeO3) into NMSeO-6, which features a lower-symmetry, Reuleaux triangular [Mo6O18(SeO3)4]8- cluster. Remarkably, this transformation results in a dramatic enhancement of the second-harmonic generation response from 0.07 x KDP (NMSeO-5) to 10 x 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.

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