Many physical and chemical effects, however, can lead to a deviation from linearity such as static quenching, high viscosity, and inner-filter effects. Interestingly, TADF photocatalysts also exhibit non-linear Stern-Volmer plots portraying a negative curvature. Within the photoredox catalysis community, however, these fluorophores are typically treated as if they were photocatalysts with a single excited emissive state, which can lead to incorrect interpretations of the luminescence quenching studies. In this work, we derive and experimentally verify an extension of the Stern-Volmer equation that fully takes into account the rapid singlet-triplet interconversions that define the excited state of TADF photocatalysts, and considers quenching from both the singlet and triplet excited state (Figure ). We first consider the quantum yield of fluorescence, since it is proportional to the luminescence intensity I.