Here, Φ 𝑝𝑓 0 is the quantum yield of prompt fluorescence, and Φ 𝐼𝑆𝐶 0 and Φ 𝑅𝐼𝑆𝐶 0 are the quantum yields of intersystem crossing and reverse intersystem crossing, respectively. Having defined the quantum yield of fluorescence we can consider bimolecular reactions occurring from both the singlet and the triplet excited states (Figure ). The mathematical description of such reactions is identical for electron transfer and energy transfer reactions. Each possible reaction has its corresponding rate constant and depend on the concentration of the quenching compound, i.e., 𝑘 𝑞𝑆 [Q] or 𝑘 𝑞𝑇 [Q] for reactions of compound Q with the singlet or triplet excited state, respectively. The quantum yields defined above are all affected by these reactions, and their definitions in the presence of quencher Q are as follows: , Φ 𝐼𝑆𝐶 (𝑄) = 𝑘 𝐼𝑆𝐶 𝑘 𝑆 𝑡𝑜𝑡 +𝑘 𝑞𝑆 , Φ 𝑅𝐼𝑆𝐶 (𝑄) = 𝑘 𝑅𝐼𝑆𝐶 𝑘 𝑇 𝑡𝑜𝑡 +𝑘 𝑞𝑇 (9) Since the ratio of the emission intensities is equal to the ratio of fluorescence quantum yields, we divide Φ 𝑓 0 by Φ 𝑓 (𝑄) to derive the extension of the Stern-Volmer equation (see SI): The resulting fitted constants are found in Table . The data in Figure clearly deviates from the typical linear relationship of the Stern-Volmer equation ( ), which can be rationalized considering the presence of two excited states where it is easier to quench the triplet state than the singlet state due to the much longer lifetime of the former. As such, efficient triplet-state quenching leads to a considerably larger Stern-Volmer constant for the triplet state 𝐾 𝑆𝑉 𝑇 than for the singlet state 𝐾 𝑆𝑉 𝑆 (cf. values in Table ). Upon increasing the concentration of quencher, initially the quenching is very effective followed by more moderate quenching.