New Insights into Photo-Fenton Chemistry: The Overlooked Role of Excited Iron^III Species

Abstract

Direct photoreduction of Fe III is a widely recognized route for accelerating Fe III /Fe II cycle in photo-Fenton chemistry. However, most of the wavelengths covering the full spectral range are insufficient to supply enough photon energy for the direct reduction process. Herein, the hitherto neglected mechanism of Fe III reduction that the Fe III indirect reduction pathway initiated by light energy-dependent reactivity variation and reactive excited state (ES) was explored. Evolution of excited-state Fe III species (*Fe III ) resulting from metalcentered charge excitation (MCCE) of Fe III is experimentally verified using pulsed laser femtosecond transient absorption spectroscopy with UV−vis detection and theoretically verified by quantum chemical calculation. Intense photoinduced intravalence charge transition was observed at λ = 380 and 466 nm, revealing quartet 4 MCCE and doublet 2 MCCE and their exponential processes. Light energy-dependent variation of *Fe III reactivity was kinetically certified by fitting the apparent rate constant of the radical-chain sequence of photo-Fenton reactions. Covalency is found to compensate for the intravalence charge separation following photoexcitation of the metal center in the MCCE state of Fenton photosensitizer. The *Fe III is established as a model, demonstrating the intravalence hole delocalization in the ES can be leveraged for photo-Fenton reaction or other photocatalytic schemes based on electron transfer chemistry.