Metal porphyrins commonly function as photocatalysts in nature. As such, there has been much work done to develop artificial porphyrin photocatalysts. Upon irradiation with visible light, one such catalyst, µ-oxo iron bis-porphyrins, has been found to catalyze the oxidation of hydrocarbons with high yield. The bis-porphyrin has an ligand to metal charge transfer (LMCT) excited state, which reduces one of the metal centers from Fe(III) to Fe(II). Then, the molecule disproportionates into two porphyrins – one with the reduced Fe(II) center and the other with an Fe(IV) center, with the oxygen bound to it. The Fe(IV) porphyrin will oxidize a hydrocarbon, and then recombine with the Fe(II) center to remake the catalyst. However, the photonic quantum yield of this system is quite low. It was initially thought that this was because the Fe(II) and Fe(IV) porphyrins recombine before any catalysis can occur, so synthesis of new catalysts focused on keeping them apart long enough for a reaction to occur. However, it has been proposed that the majority of the excited molecules follow a different, catalytically inactive pathway. In this proposed pathway, after the LMCT excitation, the oxidation and reduction occur on the ligand instead of the metals, and the molecule disproportionates into two Fe(III) centered porphyrins, which then recombine into the bis-porphyrin without performing any catalysis. If this is the case, synthetic efforts should be re-directed to making bis-porphyrins that favor the catalytic pathway over the inactive one. Here we perform ultrafast K-edge x-ray absorption near edge structure, x-ray emission spectroscopy, and x-ray scattering experiments at an x-ray free electron laser facility to determine which pathway the catalyst follows.

Determining the photocatalytic pathway of µ-oxo iron bis-porphyrin with transient K-edge spectroscopy

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