Photocatalytic\nMechanisms of Organic Thermally Activated\nDelayed Fluorescence Compounds

Reverse intersystem crossing (RISC) has become possible\nby minimizing\nthe energy gap between the first excited singlet (S<sub>1</sub>) and\ntriplet state (T<sub>1</sub>), which facilitates the thermally activated\ndelayed fluorescence (TADF). Due to the small singlet–triplet\nenergy gap, the S<sub>1</sub> and T<sub>1</sub> states exhibit comparable\nredox reactivity, leading organic TADF compounds to be potent photocatalysts.\nHere, we report such TADF compounds with multiple donor units designed\nas an efficient photocatalyst for the direct C(sp<sup>3</sup>)-H carbamoylation\nof saturated aza-heterocycles. The results obtained by photophysical\ninvestigations and chemical calculations confirm that both the S<sub>1</sub> and T<sub>1</sub> states are involved in the photocatalysis\ncycle, with the fast spin-flip from the S<sub>1</sub> to triplet states\nbeing a crucial factor in the enhancement of catalytic performance.\nThe findings will be beneficial for the design of novel, efficient\norganic photocatalysis with TADF characteristics and aid in the development\nof organic photocatalysis.