Solvent and Ligand Control of Long-Lived Charge-Separated States in Cerium(IV) Photocatalysis

Cerium(IV) complexes are powerful photooxidants with broad applications in organic synthesis and environmental remediation. We employ broadband transient absorption spectroscopy spanning in time (fs-µs) to map the influence of solvent and ligand identity on the excited-state dynamics of cerium(IV) ammonium nitrate (CAN). In acetonitrile, photoexcitation triggers an LMCT event that generates a remarkably long-lived (τ = 40 µs) charge-separated state comprising free nitrate radicals (NO₃ ^•) and a reduced Ce(III) center. In protic solvents, productive photochemistry is suppressed: methanol drives ultrafast deactivation (τ = 1.5 ps) arising from its mixed Ce(IV) coordination environment, whereas complete nitrate displacement in water enforces rapid geminate recombination (τ < 100 ps). Crucially, exchanging the nitrate ligand for chloride in acetonitrile generates a more stable charge-separated state (τ > 100 µs) involving chloride radicals (Cl^•), whose interaction with toluene is kinetically faster (τ = 4 ns vs. τ = 10 µs) than that of NO₃ ^•, consistent with the thermodynamically favorable adduct formation confirmed by DFT calculations. This enhanced reactivity translates into dramatically improved photocatalytic yields across a range of aromatic substrates. This work offers a rational basis for the design of more efficient earth-abundant photocatalysts.