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.[1] 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,[3] 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.

[1] H. Seo, M. H. Katcher, and T. F. Jamison, Nat. Chem., 2017, 456, 9, 453–456, DOI: 10.1038/nchem.2690
[2] S. Yanagida et al., J. Phys. Chem., 1992, 96, 11, 4437-4442, DOI: 10.1021/j100190a057
[3] P. Anastas and N. Eghbali, Chem. Soc. Rev., 2010, 39, 1, 301-312, DOI: 10.1039/B918763B
[4] E. Speckmeier, T. G. Fischer, and K. Zeitler, J. Am. Chem. Soc., 2018, 140, 45, 15353–15365, DOI: 10.1021/jacs.8b08933
[5] C. Fischer, C. Kerzig, B. Zilate, O. S. Wenger, and C. Sparr, ACS Catal., 2020, 10, 1, 210-215, DOI: 10.1021/acscatal.9b03606