One electron oxidations or reductions of many substrates require very high redox potentials. The p-terphenyl radical anion is a highly reactive intermediate and known as photoredox catalyst for energy-demanding substrate activation reactions. However, its usage requires not only hazardous UV light (<320 nm mostly from mercury-based light sources) for the direct generation, but also unsustainable solvents and high catalyst loadings.[1,2] Aiming to find more sustainable reaction conditions, a novel strategy to access the radical anion of a tailor-made water-soluble p-terphenyl derivative has been investigated and exploited for photocatalytic applications.
Double sulfonation of p-terphenyl ensured the usage of water as “green” solvent. Incorporating an SO2-bridge into the terphenyl backbone shifted the absorption spectrum towards longer wavelengths allowing direct excitation and subsequent radical anion formation with a conventional violet/UVA LED (390 nm).
Detailed spectroscopic and photophysical investigations revealed the importance of the triplet excited state of our novel catalyst. This excited species can either undergo triplet-triplet energy transfer with suitable substrates or reductive quenching with ascorbate (vitamin C) to yield a reactive radical anion, thereby initiating different types of organo-catalyzed photoreactions[4,5] under environmentally friendly conditions.
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