Kinetic Models for Thermally Activated Delayed Fluorescence Photocatalysts: From Time-Resolved Spectroscopy to Photoreactions under Continuous Irradiation

Photocatalysts displaying thermally activated delayed fluorescence (TADF) display complex excited-state kinetics. Several approximations have been proposed to extract radiative and non-radiative rate constants from experimentally accessible observables such as prompt and delayed fluorescence lifetimes and quantum yields. However, many of these approaches implicitly neglect the distinctive TADF character of the chromophore, and the kinetics of excited-state quenching in TADF systems has not been examined in detail. Here, we analyze the quenching kinetics of S ₁ and T ₁ excited states of TADF chromophores. Using analytical modeling and numerical simulations, we show that under pulsed excitation Stern-Volmer analysis is generally valid, unless very strong singlet-triplet coupling dominates and new analytical expressions are provided. Under continuous irradiation, quencher depletion leads to more complex kinetics described by quasi-steady-state treatments. Importantly, triplet quenching is generally more productive than singlet quenching because of different cage escape yields, and the highest quenching rate constant does not necessarily yield the maximum photochemical quantum yield. These results highlight the need for rigorous kinetic modeling to optimize TADF-based photochemical reactions.