A large number of natural and/or artificial compounds – biomolecules, drugs, agrochemicals or functional materials – contain heteroatomic sulfur in their backbones.[1–3] Consequently, targeted strategies under preferably mild synthetic conditions are crucial in the context of synthesizing novel compounds with either unprecedented or fine-tailored properties. To this end, photochemistry constitutes a unique platform for precise synthetic tools. But, many (organic) substrates lack sufficient absorption throughout the visible range of the solar spectrum. Thus, a crucial bottleneck of light-driven chemical transformations is the need for substrate activation by means of harsh UV-light. The yields of transformations, which are induced by UV-light, are low and impacted by the formation of unwanted side-products including polymers etc. Here, radical intermediates produced by UV-light initiation are detrimental. An additional concern is the rather poor functional group tolerance throughout these reactions. A promising approach to optimize the photon energy, which is necessary for substrate activation, is based on photocatalysis. Not only transition metal complexes, but also organic molecules stand out as suitable photocatalysts. A common pathway by which the photocatalysts engage in the substrate activation is electron donation or acceptance. In the present work, the focus is on a much less explored and understood pathway of photocatalysis, that is, triplet energy transfer catalysis.[1,6] In essence, upon photoexcitation the high-energy and long-lived triplet excited state of the photocatalysts are transferred in a Dexter-type fashion to the substrates of choice. We highlight recent progress in understanding the modus operandi of substrate photoactivation. This knowledge is used as starting point to design appropriate metal-complexes with optimized catalytic activity to visible-light activate organic substrates.[1,6]
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