Organic light-emitting diodes (OLEDs) are steadily becoming the dominant display technology across a range of consumer electronic markets such as smart watches, mobile phones, and televisions. This is thanks to their excellent power efficiency, that they are light in weight, they can produce both saturated colors necessary for ultra-high definition displays as well as true black, and they can be fabricated on a panoply of substrates, including those that are flexible and transparent. OLEDs are based on a multi-layer structure of organic semiconductors sandwiched between two electrodes, where the light-emission results from the electroluminescence of the emitter. The application of current to the device injects electrons and generates holes, which migrate through the device via a socalled hopping mechanism. When these recombine then an exciton is formed, and it is the radiative decay of the exciton that is responsible for the light that ultimately emanates from the OLED. Based on the statistics of spin multiplicity, 25% of the exciton formed by their recombination will have singlet multiplicity, and 75% will have triplet multiplicity. In fluorescent compounds the singlet exciton can decay radiatively, while the triplet exciton cannot be due to the spin forbidden nature of the T1àS0 transition. Both phosphorescent and thermally activated delayed fluorescent (TADF) materials, however, can harvest both singlet and triplet excitons to produce light but do so via distinct photophysical mechanisms. TADF materials have suitably small energy gaps between the S1 and T1 excited states (DEST) that enables triplet excitons to endothermically upconvert to singlets at ambient temperatures through a process called reverse inter-system crossing (RISC). The small DEST results from the small exchange integral where the HOMO and LUMO are localized on distinct parts of the emitter molecule. The most widely employed design is based on an an emitter with a strongly twisted donor-acceptor (D-A) conformation, where the HOMO will be mainly localized on the donor and the LUMO on the acceptor.