Modulatory spin-flip of triplet excitons via diversiform electron-donating units for MR-TADF emitters towards solution-processed narrowband OLEDs†

Multiple resonance thermally activated delayed fluorescence (MR-TADF) molecules are emerging as promising candidates for high-resolution organic light-emitting diode (OLED) displays, but MR-TADF emitters always suffer from an unsatisfactory rate constant of reverse intersystem crossing (k_RISC) due to inherently low spin orbital coupling strength between excited singlet and triplet states. Herein, we systematically investigate the long-range charge transfer (LRCT) and heavy-atom effects on modulating the excited state natures and energy levels via integrating diversiform electron-donating units with the MR skeleton. Compared with unsubstituted analogues, newly designed MR-TADF emitters exhibit significantly boosted k_RISC values and close-to-unity photoluminescence quantum yield especially for tBuCzBN-PXZ (2.5 × 10⁵ s⁻¹) and tBuCzBN-Ph-PSeZ (2.1 × 10⁵ s⁻¹). Leveraging these exceptional properties, the maximum external quantum efficiency values of tBuCzBN-PXZ- and tBuCzBN-Ph-PSeZ-based solution-processed OLEDs can reach 21.3% and 19.4%, which are in the first tier of reported solution-processed MR-TADF binary OLEDs without employing additional sensitizers. This study provides a framework for modulating photoelectrical properties of MR-TADF emitters through fastidiously regulating LRCT and heavy-atom effects.