Plugging the 3 MC Sink in Ru II ‐Based Photocatalysts

Quantum chemical methods have been utilised to explore the kinetics and thermodynamics of a prominent charge recombination pathway in a series of Ru II based molecular photocatalysts. Selective tuning of the Ru II coordination sphere, replacing the tbbpy ligands of the hydrogen evolving parent photocatalyst with electron rich, biimidazole based ligands, promotes unidirectional charge transfer towards the bridging ligand during initial photoexcitation. These electronic effects are also significant in the triplet manifold, where the predicted rate of the undesired deactivation process from the 3 MLCT state on the bridging ligand to a 3 MC state on the ruthenium centre, is decreased relative to the parent complex, by 1–2 orders of magnitude, alongside a decrease in electronic coupling. This design methodology could be utilised to promote targeted (light‐driven) electron transfer pathways, as well as to potentially reduce 3 MC deactivation pathways in commonly used polypyridyl‐based Ru II photocentres, thus enhancing the quantum efficiency of light driven catalysis.