Chiral Selenium-Integrated Multi-Resonant Thermally Activated Delayed Fluorescent Emitters Showing Improved Reverse Intersystem Crossing Rate.

Nitrogen/carbonyl (N/C═O) based multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters are attractive due to their bright, narrowband emission and the ease with which they can be synthesized. However, their photophysics typically suffer from slow reverse intersystem crossing (RISC) because of their relatively large singlet-triplet energy gap (ΔEST). Thus, the organic light-emitting diodes (OLEDs) with these emitters typically show severe efficiency roll-off. Here, two MR-TADF emitters DiKTaSe and tBuCz-DiKTaSe have been designed and synthesized. The introduction of selenium in the form of an annelated benzoselenophene enhances spin-orbit coupling and increases the RISC rate. The twisted ortho-substituted tert-butylcarbazole moiety in tBuCz-DiKTaSe helps to suppress aggregation-caused quenching of the emission in films. In addition, the large size of the selenium atom and long C─Se bonds induce helical chirality in both DiKTaSe and tBuCz-DiKTaSe. Finally, the OLEDs with DiKTaSe showed maximum external quantum efficiency (EQEmax) of 22.7% while OLEDs with tBuCz-DiKTaSe showed a higher EQEmax of 27.8% and less-pronounced efficiency roll-off, with EQE at 100 cd m-2 (EQE100)/ EQE at 1000 cd m-2 (EQE1000) of 23.5/12.5%. These efficiency values are amongst the highest of devices employing DiKTa-based emitters. Our work provides key insight into how to judiciously employ heavy atoms to increase the performance of the emitter and the device.