Boosting emission efficiency and suppressing device-efficiency roll-off for TADF emitters by modulating molecular conformation and intra-intermolecular interactions

Thermally activated delayed fluorescent (TADF) materials that can exhibit both high emission efficiency and efficient reverse intersystem crossing (RISC) are highly desirable for achieving high-performance electroluminescence. However, attaining these two characteristics in TADF emitters is a challenging task that requires adherence to strict design principles. Here, we report two novel TADF emitters (CF3-Pym-DMAC and Ph-Pym-DMAC) featuring an asymmetric donor-acceptor-donor (D-A-D) molecular conformation, utilizing dimethylacridine donors and substituted-pyrimidine acceptors. The presence of different substituent groups, namely trifluoromethyl and phenyl, on pyrimidine acceptors can lead to significant variations in molecular conformations, packing modes, excited-state nature and luminescence properties. Specifically, the trifluoromethyl group forms strong intra-/intermolecular C-HMIDLINE HORIZONTAL ELLIPSISF hydrogen bonds, resulting in a fixed molecular conformation that enhances molecular rigidity and avoids intermolecular pi-pi stacking. Moreover, the substitution effect regulates the excited state alignments of these TADF molecules. As a result, compared to the phenyl-substituted molecule Ph-Pym-DMAC, the trifluoromethyl-substituted molecule CF3-Pym-DMAC exhibits a significantly higher PLQY (0.91 versus 0.36) and a much larger RISC rate (5.84 x 10(5) s(-1)versus 5.50 x 10(4) s(-1)). The substitution effect on these TADF emitters ultimately results in a more than threefold increase in maximum external quantum efficiency (EQE(max): 25.0% versus 6.7%) and a substantial reduction in efficiency roll-off for the TADF-OLEDs.