Reducing Efficiency Roll‐Off in Multi‐Resonant Thermally Activated Delayed Fluorescent OLEDs through Modulation of the Energy of the T ₂ State

Abstract The S ₁ state and high‐lying triplet excited states (Δ E _S1Tn ) offer insight into clarifying the mechanism of efficiency roll‐off of organic light‐emitting diodes (OLEDs). However, experimental detection of the Δ E _S1Tn is challenging due to Kasha's rule. Here, two emitters, PhCz‐O‐DiKTa and PhCz‐DiKTa , showing multi‐resonant thermally activated delayed fluorescence (MR‐TADF) are reported. By modulating the conjugation between the MR‐TADF DiKTa emissive center and donor substituent, emission directly from the T ₂ state is for the first time observed in MR‐TADF emitters. Single crystal and reduced density gradient analyses reveal the origin of the reduced observed concentration‐quenching results from weak CH···π and slipped π···π stacking interactions, which suppress nonradiative transitions. Theoretical and photophysical investigations reveal that the Δ E _S1T2 difference influences the reverse intersystem crossing rate. The OLEDs employing PhCz‐O‐DiKTa and PhCz‐DiKTa as emitters show maximum external quantum efficiencies (EQE _max ) of over 20%, but very different efficiency roll‐off behavior (54.5% vs 13.6% at 100 cd m ⁻² ). Thus, this design provides a possible solution to mitigating device efficiency roll‐off by designing MR‐TADF emitters with degenerate S ₁ and T ₂ states.