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

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(10 H)-one (i.e. acridone, AD) acceptor. Regardless of the donor, the lowest triplet excited states (T 1) are always dominated by the 3LE A state with certain contribution from the 3CT state. By introducing methyl (Me) or trifluoromethyl (CF 3) 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 CF 3 (3-DMAC-6-CF 3 -AD) strongly stimulated the TADF by pulling down the 1CT state level and reducing the energy splitting Δ E ST 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-CF 3 -AD and 3-PXZ-6-Me-AD exhibited high rate constants of RISC (k RISC) of 10−6 and of radiation (k R) of 107 S−1, respectively. They achieved the maximum external quantum efficiency (EQE max) of 21.6% and 23.3% in the doped OLEDs, and realized EQE max 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 k RISC 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. [ABSTRACT FROM AUTHOR]