Since 1987, when the first viable electroluminescent device was reported by Tang and Van Slyke, significant progress has been made in terms of materials development such that organic light-emitting diodes (OLEDs) have now been commercialized in both the solid-state lighting and display markets. The current mature OLED products heavily rely on the phosphorescent emitters that contain noble metals such as iridium because these materials permit 100% utilization of the electrogenerated singlet and triplet excitons to produce light. The breakthrough of thermally activated delayed fluorescence (TADF) materials in 2012 by Adachi and coworkers provides an alternative route to reach equally efficient OLEDs, and without the use of these scarce metals. To date, thousands of TADF materials have been reported and their use in red, green and blue OLEDs has been demonstrated. Importantly, there are now many examples of devices that show high external quantum efficiency (EQE), device stability and color purity, these reports demonstrate that TADF OLEDs are attractive alternatives to phosphorescent devices. Indeed, the first commercialized TADF OLED devices are now on the market. For instance, one of the best red TADF OLED, which uses TPA-PZCN as the emitter, has realized a record high maximum EQE (EQEmax) of 28.1% with an electroluminescence maximum, lEL, of 648 nm. For green OLEDs, the use of CzDBA as the emitter resulted in a device with an EQEmax of 37.8% with the lEL at 528 nm. Blue OLED employing TDBA-DI as the emitter showed a comparable high EQEmax of 38.2% with CIE coordinates of (0.15, 0.28), and an OLED using a multiple resonance TADF (MR-TADF) emitter, ν-DABNA, achieved an EQEmax of 34.4% with a lEL at 469 nm and a full-width at half-maximum (FWHM) of only 18 nm. Figure . Timeline of the development of 1,3,5-triazine-based TADF materials for OLEDs All these achievements in terms of device performance are inextricably linked to the development of high-performance highly twisted donor-acceptor emitter architectures. The vast majority of electron-donor groups used within TADF emitters are hole-transporting Nheterocycles and are typically chosen from carbazole and its derivatives (e.g., bicarbazole, benzofurocarbazole, thienocarbaozle, indolocarbazole), arylamine, acridan, phenoxazine, phenothiazine, and phenazine, and their derivatives. There is more structural diversity in the electron-acceptor moiety, with popular motifs containing borane, sulfone, ketone, pyrimidine, benzonitrile, phthalonitrile, triazole, oxadiazole, thiadiazole, benzothiazole, benzooxazole, quinoxaline, anthraquinone, heptazine, and of course, triazine. In particular, 1,3,5-triazine has been one of the most popular electron-deficient heteroaromatic acceptors used in green and blue TADF emitters and is also a popular component used in host materials design due to its moderate electron affinity with the LUMO values in the range of -2.7 to 3.1 eV. Most commonly, the 1,3,5-triazine is decorated by three phenyl groups attached to the 2, 4, 6position of triazine in a triphenyltriazine structure (TRZ). There are also examples of triazinecontaining materials where the electron-donor is directly attached to the triazine. However, in many of these materials, as there are negligible steric interactions between the triazine and the donor, thus leading to a predominantly co-planar conformation, there is significant overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), thereby resulting in a correspondingly large singlet and triplet energy splitting (DEST) and no TADF. For example, DPhCzT, a compound containing a carbazole directly coupled to a diphenyltriazine, was reported as a host material, but did not show any TADF properties due to its large DEST of 0.3 eV (Figure ).