where substitution next to the oxygen of the dibenzofuran induced a large conformational distortion. This, coupled with the ortho-connection between the donor and acceptor, leads to shortened delayed lifetimes and small ΔEST, both of which were ascribed to the TSCT character of the emissive excited state. BCzTRZDBF, TCzTRZDBF and IDCzTRZDBF possess small ΔEST of less than 0.1 eV yet with high FPL of 82.4, 86.3 and 85.4%, respectively, as 5 wt% in mCBPTRZ films. The best device was obtained with TCzTRZDBF as the emitter and showed an EQEmax of 23.5% with CIE coordinates of (0.27, 0.57) and FWHM of 80 nm. Like TCzTRZDBF, another analogue BCzTRZDBF, with a less bulky dendritic donor, also exhibited a similar EQEmax of 20.1% and comparable CIE coordinates of (0.24, 0.52) and FWHM of 84 nm. However, the device with IDCzTRZDBF showed a much worse performance with an EQEmax of only 12.2%, a CIE coordinates of (0.22, 0.48) and FWHM of 81 nm. Based on the fact that IDCzTRZDBF has a FPL of 85.4%, the unexpected lower efficiency may be attributed to the unoptimized device configuration for IDCzTRZDBFb which has the largest HOMO-LUMO gap and the deepest HOMO level among the three compounds.