Organic lighting-emitting diodes (OLEDs) have emerged as an exciting display technology that has steadily gained market share in a number of consumer electronic markets, from mobile phones and smart watches to televisions and monitors. The quality of an OLED depends on three key factors: its stability, its efficiency and its colour purity. All of these parameters are linked in part to the intrinsic properties of the emitter. Two strategies to achieve high colour purity in the device are to (1) employ a narrowband emissive material and/or (2) employ color filters to select out the desired emission. The use of color filters leads inevitably to light being lost by absorption, and a lower efficiency device, thus there is a strong desire to develop narrowband emitters. A high-efficiency device also requires the use of emitter materials that can efficiently convert both singlet and triplet excitons into light. Two classes of emitters are capable of realizing 100% internal quantum efficiency (IQE) in the device, these are phosphorescent compounds and compounds that emit via thermally activated delayed fluorescence (TADF). Organic TADF emitters harvest both singlet and triplet excitons via an endothermic reverse intersystem crossing (RISC) process that converts non-emissive triplet excitons into singlets prior to light generation. RISC is enabled when the energy gap between the lowest-lying singlet and triplet excited states (ΔEST) is sufficiently small. A molecular design that showcases a small exchange integral between molecular orbitals related to the excited state is responsible for the small ΔEST in the emitter. This is typically accomplished using a twisted structure containing donor (D) and acceptor (A) fragments. However, the charge-transfer (CT) excited state and the large reorganization energy in the excited state lead to broad emission, characterized by large full width half-maximum (FWHM) > 80 nm. The corresponding TADF OLEDs do not show the desired high color purity.