, therefore overestimating the amount of singlet state quenching if the data was analyzed using equation 6 instead. For the same reason, linear plots might be obtained if dioxygen exclusion is insufficiently performed, which likewise do not yield accurate fitted constants. For each solvent, the data obtained for the three different quenchers can be fitted with the same value of Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 (in Figure , a global fit was performed, with Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 as a shared parameter between the data of the three quenchers in the same solvent, see SI for individual fits per quencher). This is expected, as this constant is a property of the fluorophore (in a certain solvent) and does not depend on the quencher. In contrast, the fitted value for Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 does differ when changing the solvent due to the dependence of the unimolecular rate constants (e.g., 𝑘 𝐼𝑆𝐶 and 𝑘 𝑅𝐼𝑆𝐶 ) on the solvent polarity. The value for Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 obtained through fluorescence quenching experiments closely matches that from fitting the TCSPC data (see Table ), further confirming that the equation 10 describes well the experimental data and that therefore intrinsic properties of the TADF-fluorophore can be obtained by simple steady-state fluorescence quenching experiments. Additionally, the precise value the property Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 can be obtained to greater accuracy using quenching experiments than through TCSPC experiments by performing global fitting on datasets with more than one quencher. Using a combination of steady-state fluorescence quenching, and a single TCSPC experiment in the absence of quencher, the quenching rate constants 𝑘 𝑞𝑆 and 𝑘 𝑞𝑇 can be determined, since 𝑘 𝑆 𝑡𝑜𝑡 and 𝑘 𝑇 𝑡𝑜𝑡 can be obtained from the biexponential fit of the TCSPC decay, vide supra. Table lists the quenching rate constants, showing that quenching of both the S1 and T1 state of 4CzIPN is very efficient, and close to the diffusion limit, with only small variation between the two, even though the Stern-Volmer constants 𝐾 𝑆𝑉 𝑆 and 𝐾 𝑆𝑉 𝑇 are two orders of magnitude different. The data of a fluorescent quenching experiment cannot be fitted correctly by any other combination of constants using different values for Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 , therefore excluding overparameterization of the threeparameter equation, see Figure . The higher Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 , the more TADF-character the fluorophore possesses. In contrast, if this parameter is equal to 0, we obtain the normal Stern-Volmer equation (6), and the fluorophore only emits from the initial singlet state (i.e., it does not demonstrate TADF). Closer to a value of 1, the fluorophore has a rapidly established equilibrium of S1 and T1 excited states and the data shows a very clear curvature in the Stern-Volmer plot. Indeed, when we compare the curves in Figure , the data obtained in toluene has a more pronounced curvature compared to the data in acetonitrile (Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 = 0.70 vs. 0.29, respectively). Figure further demonstrates the effect of simulating a change in Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 while keeping 𝐾 𝑆𝑉 𝑆 and 𝐾 𝑆𝑉 𝑇 constant, showing more pronounced curvature at higher values. The limits of equation 10 were further scrutinized by employing a MR-TADF (multiple resonance) fluorophore (QAO, also called DiKTa, see Figure ), which has a much smaller value for Φ 𝐼𝑆𝐶 0 Φ 𝑅𝐼𝑆𝐶 0 , limited by the low efficiency of intersystem crossing, Φ 𝐼𝑆𝐶 0 = 0.03 (see SI). Even at such low efficiency of TADF, a small amount of curvature was still observed in the Stern-Volmer plots (see SI), highlighting the importance of considering both excited states in fluorescence quenching for any photocatalyst displaying TADF. In conclusion, an extension of the Stern-Volmer equation was derived to describe the non-linear behavior introduced by the complex excited state system of TADF-photocatalysts that are currently experiencing a rapid growth in their application in photoredox catalysis. This new model comprises three constants: the Stern-Volmer constants for quenching of the singlet and triplet excited states, and the quantum yield of successive ISC and RISC. The model was experimentally verified by using three quenchers in three different solvents, showing congruent behavior with rate constants matching those that were independently obtained from time-correlated single photon counting experiments. Supporting Information. The following Supporting Information is available free of charge. The PDF contains the following information: mathematical derivation of the equations used, general methods, and TCSPC data and fits.