Two new thermally activated delayed fluorescent molecules incorporating spiro-acridine donors for sky-blue organic light emitting diode devices demonstrating an impressive 31% EQE

Organic light-emitting diodes (OLEDs) utilizing thermally activated delayed fluorescence (TADF) materials as the emitting layer are promising candidates for highly efficient light emission. However, they have traditionally faced challenges with achieving high luminance and high external quantum efficiencies (EQEs) in blue region. Here, we report on the design strategy of a donor-acceptor (D-A) type skeleton for highly efficient emitters, SpiroAC-MeTRZ and AdAC-MeTRZ, which incorporate SpiroAC and AdAC as donors and triazine as an acceptor, respectively. The introduction of methyl group on phenyl arms of the acceptor triazine is responsible for shallowing the lowest unoccupied molecular orbital (LUMO). SpiroAC-MeTRZ and AdAC-MeTRZ exhibit higher HOMO and LUMO values (and therefore a larger energy gap, Eg) compared to SpiroAC-TRZ and a-DMAC-TRZ. The photophysical properties of the materials were explored, revealing excellent TADF properties. Moreover, in the case of AdAC-MeTRZ, the emission spectrum exhibits dual fluorescence emission (DFE), which can be ascribed to quasi-axial conformation (QAC) and quasi-equatorial conformation (QEC). The OLED devices fabricated with these materials exhibited efficient performance, with maximum external quantum efficiency (EQE) of 31.4 % and 14.6 % at 484 nm and 472 nm for SpiroAC-MeTRZ and AdAC-MeTRZ, respectively.