Orthogonal Steric Modulation to Suppress Concentration Quenching and Spectral Broadening in Multi-Resonance TADF Emitters

Abstract

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters have been recognized as promising candidates for simultaneously achieving high efficiency and excellent color purity in organic light-emitting diodes (OLEDs).However, their intrinsically planar and rigid frameworks render the device performance highly sensitive to the doping concentration, posing a major obstacle to practical applications. Herein, three novel MR-TADF emitters, o-1O-DtCzBN, o-2O-DtCzBN, and m-2O-DtCzBN, are designed by appending phenyl linkers to sterically shield the DtBuCzB framework with bulky 4-tert-butylphenoxy substituents. By varying the substitution pattern on the phenyl linker, intermolecular interactions can be effectively modulated, leading to distinct variations in device performance. In particular, o-2O-DtCzBN, featuring ortho-disubstituted 4-tert-butylphenoxy, exhibits a near-orthogonal conformation of steric shielding substituents relative to the MR framework, which imposes substantial steric hindrance and effectively suppresses intermolecular interactions. Consequently, o-2O-DtCzBN shows a high photoluminescence quantum yield (Φ PL ) of 94% in a 2 wt% doped film and still retains 82.3% at 20 wt%. The corresponding non-sensitized OLEDs maintain a consistently narrow full width at half maximum (FWHM) of 27 nm across the 2-20 wt% doping range, with a maximum external quantum efficiency (EQEmax ) of 25.9% at a 5 wt% doping concentration.