Photocatalytic Mechanisms of Organic Thermally Activated Delayed Fluorescence Compounds.

Reverse intersystem crossing (RISC) has become possible by minimizing the energy gap between the first excited singlet (S ₁ ) and triplet state (T ₁ ), which facilitates the thermally activated delayed fluorescence (TADF). Due to the small singlet-triplet energy gap, the S ₁ and T ₁ states exhibit comparable redox reactivity, leading organic TADF compounds to be potent photocatalysts. Here, we report such TADF compounds with multiple donor units designed as an efficient photocatalyst for the direct C(sp ³ )-H carbamoylation of saturated aza-heterocycles. The results obtained by photophysical investigations and chemical calculations confirm that both the S ₁ and T ₁ states are involved in the photocatalysis cycle, with the fast spin-flip from the S ₁ to triplet states being a crucial factor in the enhancement of catalytic performance. The findings will be beneficial for the design of novel, efficient organic photocatalysis with TADF characteristics and aid in the development of organic photocatalysis.