The time-resolved PL decays in mCP show a nanosecond prompt emission and a microsecond long delayed emission at room temperature (Table ). 3TPA-DiKTA possesses a shorter delayed lifetime, tDF, of 131 μs in comparison to 3DPA-DiKTa (tDF of 323 μs) in 2wt% mCP doped films (Figures ). The temperature-dependent time-resolved PL decays are shown in Figure . The prompt emission is insensitive to temperature while the delayed emission is thermally activated, the latter behaviour consistent with TADF. The S1/T1 energies were determined from the onsets of the prompt fluorescence and phosphorescence spectra of 2 wt% mCP doped films of 3TPA-DiKTa and 3DPA-DiKTa at 77 K (Table ). The DEST value is 0.13 eV for 3TPA-DiKTa and 0.20 eV for 3DPA-DiKTa (Figure ). The former is smaller than that of DiKTa (0.20 eV) and accompanied by a shorter tDF, which is desirable for a TADF material. Based on the promising optoelectronic properties, we next proceeded to fabricate vacuumdeposited OLEDs using 3TPA-DiKTa and 3DPA-DiKTa as emitters. As shown in Figure and Figure , the optimised OLED stack (device B) consisted of: indium-tin-oxide (ITO, 112 nm)/ 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN) (5 nm)/ 1,1-bis[(di-4tolylamino)phenyl]cyclohexane (TAPC) (40 nm)/tris(4-carbazoyl-9-ylphenyl)amine (TCTA) (10 nm)/ 1,3-bis(N-carbazolyl)benzene (mCP) (10 nm)/emissive layer (20 nm)/ 1,3,5-tri[(3pyridyl)-phen-3-yl]benzene (TmPyPB) (50 nm)/ LiF (0.6 nm)/ Al (100 nm), where HATCN is the hole injection layer (HIL), TAPC and TCTA play the role of hole transporting layers (HTL) and mCP acts as an electron/exciton blocking layer (EBL). TmPyPB acts both as an electron transport layer and a hole blocking layer due to its deep HOMO (-6.7 eV), and LiF acts as The EL spectra are similar to their corresponding PL spectra in the mCP doped thin film (Figure ) reflecting an emission from the same SRCT excited state. In comparison to the previously reported OLED with DiKTa (lEL = 465 nm, FWHM = 39 nm), both of the devices showed a red-shifted emission with a slightly broader electroluminescence. As shown in Table and DiKTa-based MR-TADF emitters (Figure ). We note that, besides the DiKTa based red MR-TADF OLEDs, there are only a small number of red MR-TADF OLEDs. These include emitters based on TOAT such as mBDPA-TOAT (lEL = 600 nm, FWHM = 45 nm, EQEmax =17.3%), pBDPA-TOAT (lEL = 624 nm, FWHM = 62 nm, EQEmax = 11.3%), and 5 (lEL = 595 nm, FWHM = 45 nm, EQEmax = 2%), based on BCz such as BBCZ-tert-butyl (5) (lEL = 615 nm, FWHM = 26 nm, EQEmax = 22%) and boron-nitrogen embedded polycyclic heteroaromatics such as R-BN (lEL = 664 nm, FWHM = 48 nm, EQEmax = 28.1%) and R-BNT (lEL = 686 nm, FWHM = 49 nm, EQEmax = 27.6%). The corresponding Commission Internationale de l'Éclairage (CIE) coordinates are (0.409, 0.577) and (0.633, 0.365) for the devices with 3TPA-DiKTa and 3DPA-DiKTa, respectively. The 3TPA-DiKTa based device showed a much higher EQEmax in comparison to the device with 3DPA-DiKTa as well as other reported DiKTa based devices (Table ). Considering the FPL of the evaporated 2 wt% mCP doped films (Table ) and assuming 25% out coupling efficiency, the EQEmax of the 3TPA-DiKTa device is expected to be 23.3%, which is much lower than the observed EQEmax of 30.8% whilst the calculated EQEmax of the 3DPA-DiKTa device is 15.0%, and very close to the observed EQEmax of 16.7%. One potential explanation of the much-improved 3TPA-DiKTa OLED efficiency would be that the transition dipole moment (TDM) of the emitter is preferentially horizontally oriented, parallel to the substrate surface. We therefore measured the orientation of the TDM of the emitters in 2 wt% evaporated doped films in mCP of 50 nm thickness, emulating the thickness in the device. The angular dependent PL measurement results are shown in Figure and supporting refractive index measurements and modelling are shown in Figures . The anisotropy factors, a, which is defined by the ratio of emitted power by vertical dipoles to total emitted power by all dipoles and extracted from the p-polarized emission, were found to be 0.189 for 3TPA-DiKTa, and 0.278 for 3DPA-DiKTa; notably, previously reported DiKTa derivatives showed anisotropy factors close to isotropic at 0.33. The relatively horizontal transition dipole moment orientation in 3TPA-DiKTa is due to its high molecular weight and higher degree of planarity in contrast to 3DPA-DiKTa (based on the DFT calculated optimized structure in the gas phase). Considering the device structure and the measured anisotropic factors, the corresponding simulated out-coupling efficiency calculated for 3TPA-DiKTa is 28.3% and for 3DPA-DiKTA is 20.5%. Combining the measured FPL of the films and the simulated outcoupling efficiencies, the calculated EQEmax values are 26.4% for 3TPA-DiKTa and 12.3% for 3DPA-DiKTa, which are slightly lower than the observed values. Therefore, the emitter orientation alone cannot explain this inconsistency. Similar higher than expected efficiencies of OLEDs have also been reported in many other MR-TADF device studies. Although the origin of the higher-than-expected EQEmax is not clear here, we can envision that a potential cause is the Purcell effect associated with the reflective electrodes in the OLED stack, which can result in a higher than expected IQE. A second potential explanation could be microcavity effects in the OLED stack leading to light emission that is directed forwards more than for a Lambertian emitter and hence increasing the apparent EQE when measured from the forward direction. An important contribution to the outstanding EQE of the 3TPA-DikTa-is horizontal alignment of the transition dipole moment of the emitter in the evaporated film. These results demonstrate that simple decoration of the DiKTa acceptor with a TPA and DPA substituent is an effective approach to attaining efficient green/red TADF OLEDs.