The Crucial Role of Solvent and Bound Oxygen on the Interplay of Cerium (III) and Cerium (IV) Chloride in a Photocatalytic Cycle for Organic Transformations

Abstract Here, we investigate the fundamental interactions between Ce (III) chloride, molecular oxygen, and various solvent environments to elucidate their collective impact on photocatalytic efficiency. Through comprehensive spectroscopic analyses and computational modeling, we discovered that in acetonitrile (MeCN), O 2 molecules directly bind to the metal complex using the sphere of action model with a separation distance of 2.35 Å. This binding leads to significant emission quenching of the 4f ← 5d transition, driven by an ultrafast electron transfer from Ce (III) to bound molecular oxygen, leading to Ce (IV) and O 2 •─ (superoxide radical anion) in approximately 1.0 ps, as confirmed by femtosecond transient absorption spectroscopy. However, this distinctive O 2 interaction is disrupted by the presence of strong oxidizing substrates, such as 4‐fluoroiodobenzene, or when the complex is dissolved in protic solvents like methanol (MeOH) or water (H 2 O). In these protic media, MeOH or H 2 O solvent molecules displace the Cl − ligands, and the extended hydrogen bonding network formed by MeOH or H 2 O ligand acts as an effective shield around the Ce (III) center. Our findings reveal how solvent selection can strategically modulate Ce (III) photochemistry, shifting the balance between emission‐dominated and redox‐active pathways, and providing a mechanistic foundation for optimizing cerium‐based photocatalytic systems.