Organic light-emitting diodes (OLEDs) is a technology for generating light from electricity using organic molecules. The first generation of OLEDs were based on fluorescent organic molecules with a maximum efficiency of 25% due to spin statistics. As the transition T 1 → S 0 , from the lowest excited triplet state to the singlet ground state, is spin-forbidden, the OLED molecule can only emit efficiently from its first excited state of singlet multiplicity, S 1 . The second generation of OLEDs exploited ways of increasing the rate of this spin-forbidden phosphorescence. The third generation of OLEDs are based on Thermally Activated Delayed Fluorescence (TADF), where the S 1 state is partially populated from a near-lying T 1 state. For this to happen, the gap Δ𝐸 ST between the states needs to be sufficiently small so that the thermal activation competes with non-radiative decay processes from T 1 . However, as the T 1 population is still substantial, problems with stability and lower than ideal quantum yields due to non-radiative decay persist. The logical step for the next generation of OLEDs is to emit directly from an S 1 state that lies below the T 1 state, potentially converting all of the electrically generated excitons into photons, i.e., 100% internal quantum efficiency, while avoiding decay reactions from the T 1 state.