Reactivity of organic photocatalysts displaying thermally activated delayed fluorescence (TADF): rationalizing unexpected differences between rates of quenching of the lowest singlet and triplet states

Cyanoarene chromophores exhibiting thermally activated delayed fluorescence (TADF) are increasingly used in photoredox catalysis. At high concentrations of organic substrates, which are typically employed in preparative synthesis, the primary photoinduced electron transfer (PeT) steps in the photocatalytic processes can involve both singlet (S₁) and triplet (T₁) excited states of TADF chromophores, despite very short lifetimes (nanoseconds) of the former. However, the difference between the reactivities of these states is not well understood, while being critically important for the photocatalytic process. In this work, three representative TADF chromophores were examined in reductive and oxidative PeT quenching reactions. First, using kinetic simulations, we assert that Stern-Volmer quenching plots based on the experimentally measured prompt and delayed fluorescence lifetimes, but not integrated intensities, yield accurate bimolecular rate constants for the PeT quenching reactions involving S₁ and T₁ excited states. Secondly, experimental measurements of prompt and delayed fluorescence reveal significantly higher quenching constants for reductive quenching of S₁ compared to T₁ states, while for oxidative quenching the rate constants are nearly equal. Electronic structure calculations provide insight into the difference between the PeT rates for reductive quenching, suggesting that it might stem from the different spatial hole-electron distributions in S₁ and T₁ states. Taken together, our findings bring crucial information about the photocatalytic process involving TADF chromophores that should aid the design of the next-generation of TADF photocatalysts.