Scheme 1. Literature reported proposed mechanism for the decarboxylative addition of N-Cbz-Pro to diethyl maleate. 1 pDTCz-DPmS could also photocatalyze this reaction but at a lower yield of 64% (Table ). We investigated the mechanism of the photoreaction to provide insight into the origin of the differences in yields compared to the previously reported photocatalysts. We did not observe any quenching of the prompt fluorescence upon addition of the reagents. To our surprise, we observed a strong quenching of the delayed fluorescence of pDTCz-DPmS in degassed DMF solution upon addition of diethyl maleate (kq = 7.0 x 10 8 M -1 s -1 , Figure ). Upon addition of N-Cbz-Pro, quenching was observed only in the presence of K2HPO4 and only after a few hours stirring (due to the limited solubility of the base in DMF). The efficiency of quenching of pDTCz-DPmS was evaluated at the same concentrations used in the reaction conditions for diethyl maleate and N-Cbz-Pro (in the presence of K2HPO4): 85% of the excited states are deactivated by diethyl maleate quenching, 8% by N-Cbz-Pro quenching and 7%