Asymmetric multiple resonance thermally activated delayed fluorescence emitters for sky-blue and pure blue electroluminescence

Since the first report of multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters employing a boron/nitrogen framework, a large number of MR-TADF materials have been developed due to the advantages of narrowband emission and high efficiency. In this study, two asymmetric MR-TADF molecules, denoted as Cz-SCz and BCz-SCz, were synthesized through strategic alteration of the pi-conjugated carbazole core. Compared with the previously reported MR-TADF emitters incorporating diphenylamine (DABNA-1) and carbazole (Cz-B) units in the para-carbon position of B-substituted phenyl rings, Cz-SCz and BCz-SCz were designed to finely adjust the luminescence by reducing the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. Cz-SCz and BCz-SCz show sky-blue and pure blue emission with peaks at 472 and 464 nm, respectively, accompanied by impressively narrow full width at half-maximum bands of 28 and 24 nm in toluene solutions. The corresponding doped organic light-emitting diodes exhibit electroluminescence peaking at 476 and 468 nm, and achieve maximum external quantum efficiencies of 16.1% and 20.5%, respectively. Notably, the BCz-SCz based device achieves electroluminescence in the coveted pure blue spectral region, attaining a CIEy coordinate below 0.15. Two asymmetric MR-TADF materials, Cz-SCz and BCz-SCz, exhibit sky-blue and pure blue emission with small FWHMs of 28 and 24 nm, respectively. OLEDs display EQEmax values of up to 20.5%, and that with BCz-SCz achieves pure blue emission with a CIEy coordinate below 0.15.