Methyl-Induced Ring-Locking Strategy for Concentration-Independent MR-TADF Emitters toward High-Performance OLEDs with BT. 2020 Blue Gamut

Abstract

Realizing deep-blue multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters that simultaneously offer high efficiency and resistance to aggregation-caused quenching (ACQ) remains formidably challenging, severely limiting their practical application in BT.2020-compliant OLEDs. Herein, we report a series of high-performance deep-blue MR-TADF emitters with outstanding anti-ACQ characteristics via strategic peripheral methylation. By systematically varying the quantity and regiochemistry of methyl substituents on a DABNA-NP scaffold, we establish a comprehensive structureproperty-performance relationship governing molecular rigidity and nonradiative decay processes. Photophysical and theoretical calculations analyses reveal that di-ortho-methyl substitution induces an effective "ring-locking" effect, significantly restricting peripheral phenyl rotations and low-frequency vibrations, thereby suppressing nonradiative relaxation without perturbing frontier molecular orbitals. Consequently, the three emitters (2M-BN, 3M-BN, 4M-BN) realize narrow deep-blue emission with high PLQYs of 95-99% in 1 wt% doped films. Notably, all emitters display excellent resistance to ACQ, maintaining stable emission profiles over a wide doping range (1-15 wt%). OLEDs based on 2M-BN consistently achieve high external quantum efficiencies of 23.3-26.9% while fully satisfying the BT.2020 blue standard (Commission Internationale de l' Éclairage y coordinate, CIEy = 0.040-0.046). This work demonstrates a clear mechanistic link between steric methylation and vibrational confinement, providing a general molecular design principle for concentrationindependent deep-blue MR-TADF emitters.