Time-Resolved Mechanistic Depiction of Photoinduced CO₂ Reduction Catalysis on a Urea-Modified Iron Porphyrin

The development of functional artificial photosynthetic devices relies on the understanding of mechanistic aspects involved in specialized photocatalysts. Modified iron porphyrins have long been explored as efficient catalysts for the light-induced reduction of carbon dioxide (CO₂) towards solar fuels. In spite of the advancements in homogeneous catalysis, the development of the next generation of catalysts requires a complete understanding of the fundamental photoinduced processes taking place prior to and after activation of the substrate by the catalyst. In this work, we employ a state-of-the-art nanosecond optical transient absorption spectroscopic setup with a double excitation capability to induce charge accumulation and trigger the reduction of CO₂ to carbon monoxide (CO). Our biomimetic system is composed of a urea-modified iron(III) tetraphenylporphyrin (UrFe^III) catalyst, the prototypical [Ru(bpy)₃]²⁺ (bpy=2,2'-bipyridine) used as a photosensitizer, and sodium ascorbate as an electron donor. Under inert atmosphere, we show that two electrons can be successively accumulated on the catalyst as the fates of the photogenerated UrFe^II and UrFe^I reduced species are tracked. In the presence of CO₂, the catalytic cycle is kick-started providing further evidence on CO₂ activation by the UrFe catalyst in its formal Fe^I oxidation state.