Phosphorescent dyes harvest both singlet and triplet excitons to emit light from the triplet excited state resulting in an IQE of up to 100%. Most phosphorescent emitters, however, contain noble metals such as platinum or iridium, among the scarcest elements on Earth. Thermally activated delayed fluorescence (TADF) offers a different yet equally appealing strategy to triplet harvesting where 100% IQE is also possible in the device. In TADF emitters, dark triplet excitons are thermally upconverted into emissive singlets via reverse intersystem crossing (RISC). RISC is possible when the energy gap, DEST, between the lowest lying singlet and triplet excited states is of the order of the thermal energy (ca. < 0.02 eV), provided that spin-orbit coupling (SOC) between the two states is nonnegligible. To minimize ΔEST, the overlap between the HOMO and LUMO of the molecule must be reduced, localizing the two orbitals in separate electron-donating (for the HOMO) and electron-accepting (for the LUMO) parts of the molecule. This separation is most often obtained by enforcing a large dihedral angle between the electron donor and acceptor moieties. This strategy, however, leads to a reduction of the fluorescence efficiency and, according El Sayed rule, to a reduction of the spin-orbit coupling between the singlet and triplet states as well.