Red‐to‐Blue Upconversion Nanoparticles Catalyze Aqueous Radical Polymerization

ABSTRACT Sensitized triplet‐triplet annihilation upconversion (sTTA‐UC) is an emerging strategy for driving photoredox catalysis using low‐energy red or near‐infrared (NIR) light. However, its application in aqueous environment is often hindered by the poor solubility of requisite chromophores. Here, we address this challenge with water‐stable sTTA‐UC nanoparticles that encapsulate a hydrophobic red‐to‐blue upconversion pair within 100 nm‐sized polystyrene (PS) nanoparticles. These nanoparticles exhibit high colloidal stability in water without chromophore leakage and sufficient photostability for photocatalysis. They serve as highly active heterogeneous photoredox catalysts, successfully mediating the aqueous polymerization of various acrylate monomers under red light irradiation to form free‐standing polymer gels with high conversion yields. This work establishes our photorobust and water‐stable sTTA‐UC nanoparticles as effective heterogeneous photocatalysts for polymer synthesis with red light, offering a versatile and attractive platform for chemical transformations in aqueous environment.