Beyond the Diffusion Limit: Preassociation Enhanced Photon Upconversion and Photocatalysis Sensitized by Iron Complex

Photoactive iron complexes present highly desirable and accessible alternatives to the well-known noble metal complex-based photocatalysts, thanks to iron’s high abundance, low toxicity, and minimal environmental impact. Recent breakthroughs with luminescent 3d5 FeIII complexes show that their doublet ligand-to-metal charge transfer (2LMCT) excited states readily facilitate electron transfer and photoredox catalysis. However, their utilization in energy transfer-based photophysics and photochemistry remains underexplored. This study demonstrates that the luminescent [Fe(phtmeimb)2]PF6 complex (FeIII) sensitizes efficient doublet-triplet energy transfer to 9,10-bis((triisopropylsilyl)ethynyl)anthracene (AnTIPS) that is photoredox active. This process benefits greatly from their ground state preassociation, ultimately achieving green-to-blue upconversion. The FeIII/AnTIPS upconversion pair facilitated photoredox catalytic dehalogenation of phenacyl halides with high product yields of 66–74% under green light irradiation. Encapsulation of the FeIII/AnTIPS pair onto polystyrene nanoparticles gave water-stable hybrid nanocatalysts PS(FeIII/AnTIPS) with high photostability, which enabled near-quantitative photocatalytic polymerization of acrylate monomers in an aqueous environment. For these photochemical transformations, the FeIII/AnTIPS upconversion pair exhibits substantially higher catalytic activity than the FeIII complex or AnTIPS alone under identical reaction conditions. This work establishes a powerful photocatalytic platform based on iron complex-sensitized photon upconversion for demanding chemical reactions and facilitates iron-based hybrid nanocatalysts for aqueous photocatalysis, marking an important step toward emerging energy conversion technologies.