Polyoxometalate (POM)-based crystalline hybrid photochromic materials

As discrete anionic metal–oxygen clusters, polyoxometalates (POMs) demonstrate immense advantages as photochromic materials owing to their good redox properties and photo-activity, as well as excellent stability. However, single inorganic POM materials exhibit a poor coloration rate. The increasing interest in this field focuses on realizing the design of diversified POM-based crystalline hybrid photochromic materials (PCHPMs) and exploring applications based on the electron-rich features of POMs. Alkylammonium, carboxylic acid, sulfonium cations, spiropyran, naphthalene diimide, and viologen ligands have been used to construct PCHPMs with a good photochromic property and reversibility. In particular, recent studies have reported that the introduction of electron-deficient viologen ligands affords a new category of hybrid materials as well as a new photochromic mechanism. In this review, by comparing different PCHPMs, recent advances on PCHPMs are summarized, especially in terms of the types of structures, new photochromic mechanisms, and applications, including photocatalysis, ink-free rewritable printing, and sensors. This paper can offer valuable guidance for researchers investigating POM-based hybrid photochromic materials.