Thermally activated delayed fluorescence materials based on acridin-9(10H)-one acceptor for organic light-emitting diodes

• Published in: Dyes and pigments 2022-11, Vol.207, p.110701, Article 110701
• Main Authors: Mei, Yongqiang, Liu, Di, Li, Jiuyan, Wang, Jiahui
• Format: Article
• Language: English
• Subjects: Acridone acceptor; Reverse intersystem crossing; Singlet and triplet energy splitting; Thermally activated delayed fluorescence materials; Trifluoromethyl
• ISSN: 0143-7208; 1873-3743
• DOI: 10.1016/j.dyepig.2022.110701

It is essential to manage the alignment of the locally excited (3LE) and charge-transfer triplet (3CT) and singlet (1CT) states of thermally activated delayed fluorescence (TADF) materials to accelerate reverse intersystem crossing (RISC) process. A series of donor-acceptor (D-A) type TADF emitters were developed by attaching acridine (DMAC) or phenoxazine (PXZ) donor at 3-site of acridin-9(10H)-one (i.e. acridone, AD) acceptor. Regardless of the donor, the lowest triplet excited states (T1) are always dominated by the 3LEA state with certain contribution from the 3CT state. By introducing methyl (Me) or trifluoromethyl (CF3) at 6-site of AD acceptor, the energy levels of 1CT, 3CT and 3LE states were tuned in different ways. When the donor is DMAC, incorporating Me on AD ring (3-DMAC-6-Me-AD) destructed the emission performance, while CF3 (3-DMAC-6-CF3-AD) strongly stimulated the TADF by pulling down the 1CT state level and reducing the energy splitting ΔEST to almost zero. In the case of strong donor PXZ, decorating acceptor with Me (3-PXZ-6-Me-AD) did not bring extinct effect on the TADF property. In particular, 3-DMAC-6-CF3-AD and 3-PXZ-6-Me-AD exhibited high rate constants of RISC (kRISC) of 10−6 and of radiation (kR) of 107 S−1, respectively. They achieved the maximum external quantum efficiency (EQEmax) of 21.6% and 23.3% in the doped OLEDs, and realized EQEmax of 14.7% and 17.6% with low efficiency roll-offs in non-doped devices. Highly rigid acridone is used as acceptor to construct donor-acceptor type thermally activated delayed fluorescence (TADF) emitters. Both incorporating trifluoromethyl on acceptor and adopting stronger donor can realize the energetically favorable 1CT ≈ 3CT ≈ 3LE status, increasing the kRISC from 105 s−1 to 106 s−1. High EQEs of 21.6%/14.7% and 23.3%/17.6% are achieved in doped and non-doped OLEDs. [Display omitted] •A series of thermally activated delayed fluorescence (TADF) emitters were developed using highly rigid acridone as acceptor.•Design strategies were focused on tuning the substituents on acceptor or varying donor strength, not changing twisting angles.•Energy level differences among 1CT, 3CT and 3LE states were reduced to facilitate the RISC to 106 s−1.•High EQEs of 21.6%/14.7% and 23.3%/17.6% were obtained in the doped and non-doped OLEDs.