An Extension of the Stern-Volmer Equation for Thermally Activated Delayed Fluorescence (TADF) Photocatalysts

Fluorescence quenching experiments are essential mechanistic tools in photoredox catalysis, allowing to elucidate the first step in the catalytic cycle that occurs after photon absorption. Thermally activated delayed fluorescence (TADF) photocatalysts, however, yield non-linear Stern-Volmer plots, thus requiring an adjustment to this widely used method to determine the efficiency of excited state quenching. Here, we derive an extension of the Stern-Volmer equation for TADF-fluorophores that considers quenching from both the singlet and triplet excited states, and experimentally verify it with fluorescence quenching experiments using the commonly employed TADF-photocatalyst 4CzIPN with three different quenchers in three solvents. The experimental data is perfectly described by this new equation, which in addition to the Stern-Volmer constants of quenching allows for the determination of the product of intersystem and reverse intersystem crossing quantum yields, a quantity that is independent of the quencher.