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

Abstract

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–H⋯F hydrogen bonds, resulting in a fixed molecular conformation that enhances molecular rigidity and avoids intermolecular π–π 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 × 10⁵ s⁻¹versus 5.50 × 10⁴ 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.