Low quantum efficiency of μ-oxo iron bisporphyrin photocatalysts explained with femtosecond M-edge XANES

Abstract

Bridged μ-oxo iron porphyrins serve as photocatalysts for oxidative organic transformations, but suffer from low photon-to-product efficiency. This low photochemical quantum yield is most commonly attributed to the short lifetime of a disproportionated TPPFe(II)/TPPFe(IV)O state, but an alternate hypothesis suggests that the majority photoproduct is a catalytically inactive ligand-centered TPPFe(III)⁺/TPPFe(III)–O⁻ ion pair. We use femtosecond optical and extreme ultraviolet (XUV) spectroscopy to investigate the early photodynamics of the μ-oxo iron bisporphyrin (TPPFe)₂O and identify the primary loss mechanism. XUV spectroscopy probes 3p → 3d transitions, corresponding to M_2,3-edge XANES spectra, and is a distinctive probe of the metal oxidation state. Excitation of the mixed π–π*/ligand-to-metal charge transfer (LMCT) band results in the formation of an iron(II)/iron(III) LMCT state in tens of femtoseconds. This state decays on a subpicosecond timescale to the ligand-centred iron(III) ion pair state, and no TPPFe(IV)O species is observed within the sensitivity of the measurement. The lack of an iron(II)/iron(IV) XANES spectrum suggests that preferential formation of the inactive iron(III) ion pair state is a main cause of the low quantum yield of this and similar bisporphyrins.