Singlet and triplet excitons are formed in an electroluminescent (EL) device in the ratio 1:3 due to the Fermionic nature of holes and electrons. Commercial displays use two classes of emitters for the primary colors. Phosphorescent emitters are used for green and red, whereas triplet-triplet annihilation (TTA) using purely organic emitters are used for the blue emitter as there is at present not a sufficiently stable blue phosphorescent OLED. OLEDs with TTA emitters, however, can only achieve a maximum 62.5% internal quantum efficiency (IQE), thus, there is still room for improvement of the overall efficiency of the device. Not surprisingly, there is a huge effort undertaken by both academia and industry to develop stable and high efficiency blue emitters that can harvest 100% IQE in the device. Furthermore, deep blue OLED light sources can find application beyond displays, such as for sterilization, and dental and dermatological treatments. Purely organic thermally activated delayed fluorescent (TADF) emitters provide a tantalizing solution as TADF OLEDs can achieve up to 100% IQE. However, donor-acceptor (D-A) TADF compounds show very broad emission due to the long-range chargetransfer (LRCT) character of the excited state and the broad range of accessible geometries in the excited state as the D and A units are connected through single bonds. This results in OLEDs that show poor color purity. A solution to this apparent weakness of D-A TADF emitters was advanced by Hatakeyama et al. who demonstrated how p-and n-doped nanographenes, termed multi-resonant TADF (MR-TADF) emitters, could also exhibit TADF but with much narrower emission profiles. The narrow emission spectra were rationalized in terms of the rigid structure of these compounds together with the short-range charge transfer (SRCT) nature of the emissive excited state. The potential of MR-TADF compounds to act as pure blue OLED emitters was first exemplified by the DABNA series (Figure ). 12 DABNA-1 presents a photoluminescence maximum, lPL, of 460 nm, a full width at half maximum, FWHM, of 30 nm, a high photoluminescence quantum yield, ΦPL, of 88%, and a moderate singlet-triplet energy gap, ΔEST, of 180 meV in 1 wt% mCBP host. The corresponding OLED showed pure blue emission at moderate efficiency (lEL = 460 nm, FWHM = 30 nm, EQEmax = 13.5%, CIEY = 0.09). The same group reported a π-extended version of DABNA-1, v-DABNA, which still represents the pinnacle of blue MR-TADF emitter design. This emitter exhibited efficient narrowband blue emission (lPL = 467 nm, FWHM = 18 nm, ΦPL = 90%) and a very small ΔEST of 17 meV in 1 wt% DOBNA-OAr host. The OLED showed an impressive performance with an EQEmax as high as 34.4% at lEL of 469 nm, a FWHM of 18 nm and CIE coordinates of (0.12, 0.11). Indeed, compared to the OLED with DABNA-1, there is a remarkable improvement in the device performance with v-DABNA. The same group modified the structure of v-DABNA by replacing one of the nitrogen atoms with a less electron-donating oxygen atom as in v-DABNA-OMe. This emitter maintained efficient and narrowband blue emission (lPL = 464 nm, FWHM = 24 nm in 1 wt% DABNA-OAr). The OLED with this derivative showed a slightly blue-shifted EL of 465 nm (FWHM = 23 nm, EQEmax = 29.5%).