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

Abstract

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.