Ferryl species electronic state

Photoinduced electron-transfer in the opposite direction was demonstrated upon irradiation of the Ru(bpy)3 +-Mb system in the presence of Co +(NH3)5Cl as a sacrificial electron acceptor (Figure 44B) [244]. The photochemical reaction results in the formation of ferryl species (i.e., Fe(IV)-heme), with the intermediate formation of the porphyrin cation radical (as demonstrated using laser flash photolysis [237]). The electron-transfer cascade includes the primary oxidative quenching of the excited chromophore, Ru(bpy)3"+, by Co +(NH3)5Cl to yield Ru(bpy)3 + [E° = +1.01 V vs. SCE). The resulting oxidant efficiently takes an electron from the porphyrin ring (fcet = 8.5 x 10 s ) and the porphyrin cation radical produced further oxidizes the central iron atom, converting it from the Fe(III) state to the Fe(IV) state (/cet = 4.0 x 10 s at pH 7.5). 

The reported experimental data, DFT-derived structural electronic and energetic information, and mechanistic interpretations, lead to a consistent picture, which is in fine with observations with other ligand systems, although there is some disagreement in terms of possible interpretations. However, there are a number of open questions (e.g., regarding the spin state of the ferryl species, which for both the tetra- and pentadentate ligand-based systems is predicted by DFT to be close to the spin cross-over limit) (175). Therefore, some of the current interpretations might be revised in the future. 

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