Phenoxazine-Sensitized CO2-to-CO Reduction with an Iron Porphyrin Catalyst: A Redox Properties-Catalytic Performance Study

We have evaluated six phenoxazine derivatives as visible light photosensitizers for the photochemical reduction of CO2 to CO with an iron porphyrin catalyst in organic media. The phenoxazine core was functionalized with electron-donating or -withdrawing groups to modify the photophysical properties. Both singlet and triplet excited state potentials of the sensitizers spanned several hundred mV range and the ground state oxidation potentials spanned 250 mV. We observed that no correlation can be established between the production of CO and the excited state potential of the phenoxazine, which determines the driving force for electron transfer to the catalyst. On the contrary, a clear correlation can be made between the oxidation potentials of the phenoxazine and the production of CO. This observation indicates that the process was limited by photosensitizer regeneration and highlights the fact that electron transfers not directly related to the catalyst activation could play a key role in homogeneous photocatalytic systems.

Automated summary

We have evaluated six phenoxazine derivatives as visible light photosensitizers for the photochemical reduction of CO2 to CO with an iron porphyrin catalyst in organic media. The phenoxazine core was functionalized with electron-donating or -withdrawing groups to modify the photophysical properties. Our observation shows that there is no correlation between the production of CO and the excited state potential of the phenoxazine, but a clear correlation can be made between the oxidation potentials of the phenoxazine and the production of CO. This indicates that the process was limited by photosensitizer regeneration and that electron transfers not directly related to the catalyst activation could play a key role in homogeneous photocatalytic systems.