Paradigm Shift in Triplet-Triplet Energy Transfer Catalysis: Towards Earth Abundant Transition Metals and Low Photon Energies

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Quo vadis, photocatalysis? The catalytic activation of substrates by triplet-triplet energy transfer (TTEnT) represents a key technology in the context of organic synthesis towards efficient, selective and mild use of (visible) light. At the same time, however, the field faces crucial challenges in terms of sustainability and limited photonic energy. These problems are addressed in this interdisciplinary project proposal, which is built on main contributions from synthetic (Glorius) and physical chemistry (Guldi). Building on previous work on the rationalization of TTEnT processes and the design of optimized TTEnT catalysts, two closely interwoven work packages have been developed to initiate a paradigm shift in the field of TTEnT processes:The focus of the first work package is on the development of sustainable catalysts that avoid the need for rare noble metals (Ru, Ir). To this end, an integrated pipeline of computational modeling, synthesis, and photophysical characterization of catalysts will be realized. Under systematic optimization of the carbene-based ligand motifs, emissive cobalt(III) complexes are to be obtained, whose TTEnT activity will be subsequently evaluated. Transient midIR spectroscopy represents a key technology in this regard, allowing for a detailed characterization of the underlying molecular processes. The second work package addresses the use of two-photon processes to unlock high-energy excited states under mild conditions. One focus will be the development of efficient Vis→UV upconversion cascades based on triplet-triplet annihilation processes. For this purpose, a combinatorial evaluation of potential sensitizer-annihilator-catalyst triads will be performed to ultimately enable TTEnT activation of substrates. This evaluation goes hand in hand with photophysical characterization using time-resolved spectroscopy, as well as the investigation of external stimuli such as magnetic fields. In the long term, these two work packages not only contribute to a deeper understanding of TTEnT processes, but also open multiple perspectives in the context of organic synthesis and beyond – owing to the paradigm shift towards sustainable systems and low excitation energies.