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

Abstract

ABSTRACT 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 (NO 3 • ) 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 NO 3 • , 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.