An Extension\nof the Stern–Volmer Equation for\nThermally Activated Delayed Fluorescence (TADF) Photocatalysts

Fluorescence quenching experiments are essential mechanistic\ntools\nin photoredox catalysis, allowing one to elucidate the first step\nin the catalytic cycle that occurs after photon absorption. Thermally\nactivated delayed fluorescence (TADF) photocatalysts, however, yield\nnonlinear Stern–Volmer plots, thus requiring an adjustment\nto this widely used method to determine the efficiency of excited\nstate quenching. Here, we derive an extension of the Stern–Volmer\nequation for TADF fluorophores that considers quenching from both\nthe singlet and triplet excited states and experimentally verify it\nwith fluorescence quenching experiments using the commonly employed\nTADF-photocatalyst 4CzIPN, and multiple-resonance TADF-photocatalyst\nQAO with three different quenchers in four solvents. The experimental\ndata are perfectly described by this new equation, which in addition\nto the Stern–Volmer quenching constants allows for the determination\nof the product of intersystem and reverse intersystem crossing quantum\nyields, a quantity that is independent of the quencher.