Green light for photoredox catalysis by corroles and porphyrins

Various monomeric and dimeric corroles and porphyrins chelating post-transition elements (phosphorus and gallium) have been prepared and shown to differ quite significantly in both the photophysical and redox properties. All were investigated as photoredox catalysts for halogenating quite reactive substrates, phenol and dimedone, by mineral acids. The strong absorbance of the corrole- and porphyrin-based complexes allows for exceptionally low catalyst loading and elucidation of the main variables that dictate performance. Brominating species are formed via both reductive and oxidative quenching by bromide and oxygen, respectively, and the dominant efficacy-determining factor was the match between the light-emitting and catalyst-adsorbing wavelengths. The best-performing complexes are also active when relatively low-energy (green light) sources are used, which paves the way for introducing them as photocatalysts for reactions where near-UV light cannot be used. Chlorination of phenol by aqueous HCl proceeds well via illumination by both near-UV and green light, even though chlorinating species may only be produced by the reductive quenching pathway.