The MR-TADF Emitters: A Novel Modification Strategy via 4,4′-Di <i>tert</i> -butyl-diphenylamine for Effective Spectral Narrowing

Multiresonance thermally activated delayed fluorescence (MR-TADF) materials, merging high exciton utilization efficiency with intrinsically narrowband emission, have attracted considerable attention as next-generation emitters for high-performance OLEDs. However, conventional strategies to extend their emission wavelength often result in spectral broadening, thereby compromising color purity and limiting their applicability in long-wavelength light-emitting devices. For this, we propose a site-specific substitution strategy targeting the 4-position of the carbazole unit within the <b>CzBN</b>-based framework. By introducing a 4,4'-di<i>tert</i>-butyl-diphenylamine substituent at this position, two MR emitters, <b>MR-asym1</b> and <b>MR-asym2</b>, were designed and synthesized. Both compounds exhibit blue-green fluorescence with emission peaks at 486 and 489 nm, along with full widths at half-maximum (fwhm's) of 20 and 22 nm, respectively. The photophysical measurements confirm that this approach enables a slight emission redshift of approximately 10 nm, accompanied by a reduced fwhm of 2 nm, thereby achieving simultaneous spectral fine-tuning and enhanced color purity. Density functional theory (DFT) and natural transition orbital (NTO) analyses reveal substantial excited-state involvement of the donor substituent. At the same time, the reorganization energies (λ) and the root-mean-square deviation (RMSD) values further suggest a modulation of the excited-state relaxation process. Meanwhile, for the first time, the effects of site modification within the <b>CzBN</b>-based framework on the emission wavelength have been summarized. The OLED devices fabricated using these emitters demonstrate outstanding performance. Specifically, Device B, based on <b>MR-asym2</b>, exhibits electroluminescence at 501 nm with a 24 nm fwhm, along with a maximum external quantum efficiency (EQE_max) of 22.8% and an efficiency roll-off of 15.0% at 1000 cd m^-2. These results offer a viable molecular design strategy for achieving both emission tunability and high device efficiency, and provide valuable insights into the development of high-color-purity MR-TADF materials.