Modulatory Spin-flip of Triplet Excitons 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 ( ) 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 integrating diversiform electron-donating units with the MR skeleton. Compared with unsubstituted analogues, newly designed MR-TADF emitters exhibit significantly boosted values and close-to-unity photoluminescence quantum yield especially for BuCzBN-PXZ (2.5 × 10 s) and BuCzBN-Ph-PSeZ (2.1 × 10 s). Leveraging these exceptional properties, the maximum external quantum efficiency values of BuCzBN-PXZ- and BuCzBN-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.