Novel approaches to photocatalysis toward organic chemistry applications

The shift to renewable energy sources is a crucial step toward achieving a sustainable future, especially as the reduction of carbon dioxide (CO2) levels becomes essential for addressing the greenhouse effect and global climate change. For this reason, nowadays, appealing strategies set the goals to convert small molecules, such as water or CO2, into useful chemicals through redox reactions. In the reduction route, major efforts have focused on the reduction of protons to hydrogen and the reduction of carbon dioxide into valuable chemicals. Both pathways hold significant promise for addressing energy demands and reducing the reliance on fossil fuels. The counterpart of the reduction process is water oxidation (WO) to produce dioxygen but is hindered by high activation energy barriers. Overcoming these barriers has driven research into discovering new catalytic pathways under practical conditions. Advances in the field are focusing on the development of molecular catalysts, for example transition metal coordination complexes, which offer the potential to lower energy barriers and enhance reaction efficiency. Simultaneously, the integration of solar energy into these catalytic processes is gaining prominence as a renewable energy source. This approach, known as solar-driven catalysis, presents an appealing, sustainable alternative to conventional energy-intensive methods. This doctoral thesis is focused on the rethink of the photo-redox reactions described above. The new concepts that will be optimized, offer the possibility to enrich the current state of the art, both in terms of efficiency and of scalability. A breakthrough is expected from a re-design of artificial photosynthesis, expanding its scope also towards the functionalization of large molecules, by introducing new concepts for the involved redox transformations. Starting from Iridium and Ruthenium complexes as benchmark photocatalysts, as well-established standards in the field due to their proven effectiveness in facilitating photochemical reactions, the progress beyond state-of-art is to shift the focus toward the development of organic photosensitizers. These organic systems present several advantages, including wide availability, lower environmental impact and toxicity, and lower cost, making them attractive alternatives for sustainable photocatalysis. Since the target redox reactions involve the simultaneous management of electrons and protons, one of the core challenges in this shift is to design organic photosensitizers capable of performing proton-coupled electron transfer (PCET), a more efficient process that governs many essential redox reactions. The rationale of this thesis is also focused on a deep investigation into the underlying reaction mechanisms, an aspect that is often overlooked in the photosynthesis of organic compounds.