[Cu(dmp)(xantphos)]PF6, Eosin Y, 4CzIPN, 2CzPN and pDTCz-DPmS instead emit via thermally activated delayed fluorescence (TADF). After photoexcitation, the compound rapidly relaxes to its first singlet excited state (S1), after which fluorescence can occur, which in the case of TADF compounds is referred to as prompt fluorescence, with emission lifetimes on the order of nanoseconds. SOC is much less efficient in these compounds than in Ru and Ir complexes, however, the degree of state mixing is inversely proportional to the energy difference between the excited singlet and triplet states. These five compounds instead display a small energy gap, ΔEST, (i.e., < 0.2 eV) between the first singlet and triplet excited states (S1 and T1, respectively), thus enabling both ISC and reverse ISC (RISC) to occur. One of the manifestations of ISC/RISC in these compounds is the presence of delayed fluorescence, with emission lifetimes on the order of microseconds. A corollary is that both S1 and T1 states are populated. When ISC and RISC (kISC and kRISC, respectively) are fast relative to electron or energy transfer (kPET and kPEnT, respectively), a steady-state approximation may be assumed, and a Boltzmann distribution will govern the relative populations of the singlet and triplet excited states. This implies that the majority of excitons will exist in the triplet state owing to its lower energy. If kISC and kRISC are competitive or slower relative to kPET and kPEnT, then the relative populations of the excited states are determined by the relative magnitude of the rate constants. Regardless, as the energy gap between the S1 and T1 states is much smaller than the energy gap between these and the ground state of the substrates, then there should be a similar thermodynamic driving force for PET originating from either of these excited states. Most of the organic TADF PCs used for photocatalysis in the literature thus far