Covalently Modified Polyoxometalate Organic–Inorganic Hybrids for Visible-Light Photoactivation

Polyoxometalates (POMs) are anionic metal oxide nanoclusters with rich redox chemistry, making them promising candidates for photocatalysis. However, their strong UV-light absorption and rapid charge recombination hinder visible-light applications. This review focuses on covalent organic–inorganic hybridization as a modular strategy to engineer POMs for visible-light photoactivation. By grafting chromophoric ligands, metalloporphyrins, or organometallic complexes onto POM surfaces via robust covalent bonds (e.g., Si–C, P–C, C–C), two key photochemical pathways are enabled: (i) direct visible-light excitation of organic sensitizers followed by intramolecular charge transfer to/from POMs and (ii) modified ligand-to-metal charge transfer (LMCT) transitions in POMs via ligand-induced electronic structure perturbation. We discuss how organic ligands regulate POM frontier orbital energy levels (HOMO/LUMO), redox potentials, and photoresponse range, supported by experimental and density-functional theory (DFT) studies. We also review hybrid systems with organic photosensitizers (e.g., pyrene, boron dipyrromethene (BODIPY)), metalloporphyrins, and organometallic complexes (e.g., Ru(II), Ir(III)), emphasizing structure–activity relationships in electron-transfer efficiency, charge-separation lifetime, and catalytic performance (e.g., hydrogen evolution, selective oxidation). Finally, we outline current challenges and prospects for designing multifunctional POM hybrids with tailored visible-light photocatalytic properties.