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

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.