Fusing acridine and benzofuran/benzothiophene as a novel hybrid donor for high-performance and low efficiency roll-off TADF OLEDs

Two novel acridine-benzofuran/benzothiophene hybrid donors of 34BxAc (x = F, 34BFAc: 13,13-dimethyl-8,13-dihydrobenzofuro[3,2-a]acridine; x = T, 34BTAc: 13,13-dimethyl-8,13-dihydrobenzo[4,5]thieno[3,2-a]acridine) were developed as aromatic fused donors by combining dimethyl acridine (Ac) with benzofuran and benzothiophene, respectively. As a comparison, the hybrid donors of 12BxAc (x = F, 12BFAc: 5,5-dimethyl-5,13-dihydrobenzofuro[3,2-c]acridine; x = T, 12BTAc: 5,5-dimethyl-5,13-dihydrobenzofuro[3,2-c]acridine) were also prepared. By attaching the TPPM (2,4,6-triphenylpyrimidine) acceptor to these fused hybrid donors (12BFAc, 12BTAc, 34BFAc and 34BTAc), 12BxAc-PM (12FAc-PM and 12BTAc-PM) and 34BxAc-PM (34BFAc-PM and 34BTAc-PM) were further designed and synthesized. It was revealed that 34BxAc-PM was better than 12BxAc-PM at shrinking the singlet-triplet energy gap and decay lifetime, and increasing the photoluminescence quantum yield. The sky-blue thermally activated delayed fluorescent organic light-emitting diodes (OLEDs) with 12BFAc-PM as a dopant just showed a maximum external quantum efficiency (EQE) of 12.9% and a CIE_x,y of (0.16, 0.29). The 12BTAc-PM-based device achieved a maximum EQE of 25.6%, however, the efficiency roll-off was relatively serious. In contrast, high-performance and low efficiency roll-off OLEDs can be developed using 34BxAc-PM. For instance, the 34BFAc-PM-based device realized a maximum EQE of 27.7%, and still a high EQE of 24.6% at 1000 cd m^-2 and 19.6 cd m^-2 at 5000 cd m^-2, respectively, indicative of the superiorities of the novel donors (34BFAc and 34BTAc).