Tuning photoluminescence and electroluminescence of multi-resonance thermally activated delayed fluorescence emitters through peripheral donor modification for highly efficient deep-blue OLEDs

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials have emerged as promising emitters for organic light-emitting diodes (OLEDs) due to their highly efficient narrow-band emission. Peripheral structure modification represents a simple and feasible strategy to improve the luminescence properties of MRTADF materials. Herein, we report two TADF materials, NBO-Ac and NBO-PXZ, which are synthesized by incorporating electron-donor groups with varying electron-donating abilities at the para-position of the boron atom in a boron/nitrogen/oxygen ternary-doped MR framework (NBO). Both theoretical and experimental studies demonstrate that modification of the peripheral donor groups effectively regulates the excited states, thereby influencing the photoluminescence and electroluminescence (EL) properties of these materials. Incorporating the moderate-strength donor dimethylacridine effectively accelerates the reverse intersystem crossing process by introducing intermediate long-range charge transfer (LRCT) states, while simultaneously preserving the narrow-band deep-blue emission from short-range charge transfer (SRCT) states, in comparison to the parent molecule NBO. In contrast, the incorporation of a stronger donor, phenoxazine, results in a significantly stabilized emissive singlet state with predominant LRCT characteristics, leading to broader and redshifted emission. Moreover, incorporating peripheral donor units can significantly increase the ratio of horizontal transition dipole orientation of these emitters in films. Consequently, both the NBO-Ac-based deep-blue OLEDs and the NBO-PXZbased sky-blue OLEDs exhibit EL performance far superior to that of the control NBO-based devices. The unsensitized and sensitized OLEDs using NBO-Ac as the emitter achieve EQEs of 32.0% and 33.6%, with Commission Internationale de L'Eclairage (CIE) coordinates of (0.134, 0.097) and (0.137, 0.106), respectively.