Thermally activated delayed fluorescence (TADF) is a photophysical mechanism that manifests in dual emission, a rapid prompt fluorescence from singlet excitons that decay radiatively, and a slow delayed fluorescence that occurs as a result of the thermal up-conversion of triplet excitons into singlets via reverse intersystem crossing (RISC) prior to light generation. The efficient harvesting both singlet and triplet excitons to produce light emission make TADF materials particularly attractive as emitters for electroluminescent devices as, like phosphorescent devices, can achieve 100% internal quantum efficiency (IQE). Most of the reported TADF emitters employ a strongly twisted donor (D) acceptor (A) structure, which leads to localization of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) on the D and A, respectively, and thus to a small singlet-triplet energy gap (ΔEST). A sufficiently small ΔEST is necessary to enable endothermic upconversion of triplet excitons to singlets by RISC. The apparent paradox in TADF emitter design is that the design requirements for efficient RISC are in contrast to those to attain high photoluminescence quantum yield (ΦPL), which require significant orbital overlap to maximize the oscillator strength of the transition. Balancing these two criteria in the design of high-performance orange-red emitters becomes that much more difficult as radiative decay decreases while non-radiative decay increases due to the energy-gap law. The general molecular design principles for red/deep red D-A TADF emitters are: 1) Use of strong donor and acceptor motifs is a common strategy, affording a shallow HOMO and a deep LUMO for D and A, respectively; and 2) Construction of a rigid and typically planar fused ring donor and/or acceptor for the suppression of nonradiative transitions. Within this model, most of the acceptors are derivatives of either acenaphtho [1,2b]pyrazine-8,9-dicarbonitrile (APDC), heterocyclic quinoxaline-6,7-dicarbonitrile (QCN), 11,12dicyanodibenzo[a,c]phenazine (CNBPz), 2,3-dicyanodibenzo[f,h]quinoxaline (CNBQx), or dibenzo[a,c]-phenazine-3,6-dicarbonitrile (PZCN). Nitrogen-doped polycyclic aromatic hydrocarbons (N-PAH), like dibenzo[a,c]phenazine (BP), have proved particularly popular as moieties in orange-red TADF emitters (Figure ). Zhao et al. first reported BP-based TADF compounds, containing one-to-three 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor moieties (Figure ). The orange-red OLEDs with 3DMAC-BP, an emitter with three DMAC donors situated at the 3-, 6-, and 11-positions of the BP, showed a maximum external quantum efficiency (EQEmax) of 22.0% with an electroluminescence peaking at lEL = 606 nm. Yang et al. reported a related compound using the BP acceptor, DMAC-11-DPPZ with the DMAC donor attached at the 11-position of the BP acceptor, which emits at lPL of 576 nm, has a photoluminescence quantum yield, FPL, of 57% and a delayed lifetime, τd, of 1.53 µs in 10 wt% doped films in CBP. The optimized OLED with DMAC-11-DPPZ displays an orange emission at 588 nm with the EQEmax of 23.8%. respectively. The same group reported an orange-yellow TADF material DPPZ-DMAC, where the related DPPZ acceptor contains two additional nitrogen atoms compared to BP. This compound emits at ~590 nm, has a smaller ΔEST of 0.01 eV, and a higher ΦPL of 91.6% in 6 wt% doped films in CBP, As excepted, SAF-2NP shows similar photophysics as DPPZ-DMAC but has a slightly blue-shifted emission at lPL of ~580 nm and a much higher ΦPL of 99%. The SAF-2NP-based OLED demonstrated a very high EQEmax of 32.5% with an lEL at 576 nm, an efficiency linked to the high ΦPL of 99% and horizontally oriented transition dipole (Q//) of 85% of the emitter in the host matrix. When DPPZ is coupled with a triphenylamine (TPA) donor, as in oTPA-DPPZ, the emission shifts to the red at 605 nm in the 30 wt% doped films (FPL = 75%; ΔEST of 0.07 eV). The OLEDs with oTPA-DPPZ showed an EQEmax of 18.5% at λEL of 600 nm. Through adjusting the position of the TPA groups, a "T-shape" deep-red TADF emitter, pTPA-DPPZ, was obtained. Compared to oTPA-DPPZ, the rational spatial arrangement of D and A groups in pTPA-DPPZ accelerated radiative decay from the singlet state by 90-fold, giving rise to a FPL of 87% in the neat film. The corresponding OLED, having a simplified bilayer non-doped structure, showed an EQEmax of 12.3% at lEL of 652 nm.