Changing the Nature of the Chelating Ligand of Tetracoordinate Boron-Containing PAH Multi-resonant Thermally Activated Delayed Fluorescence Emitters Tunes the Emission from Green to Deep Red

Multi-resonant thermally activated delayed fluorescence (MR-TADF) materials have attracted considerable attention recently. The molecular design frequently incorporates cycloboration. However, to the best of our knowledge MR-TADF compounds containing nitrogen chelation to boron is still unknown. Reported herein is a new class of tetracoordinate boron-containing MR-TADF emitters bearing a C^N^C- and N^N^N-chelating ligands. We demonstrate that the replacement of B−C covalent bond in C^N^C-chelating ligand by B−N covalent bond affords a regioisomer, which dramatically influences the optoelectronic properties of the molecule. The resulting N^N^N-chelating compounds show bathochromically shifted absorption and emission spectra relative to C^N^C-chelating compounds. The incorporation of tert-butylcarbazole group to the 4-position of the pyridine significantly enhances both the thermal stability and the reverse intersystem crossing rate, yet has a negligible effect on the emission properties. Consequently, high-performance hyperfluorescence organic light-emitting diodes (HF-OLEDs) that utilize these molecules as green and yellow-green emitters show maximum external quantum efficiency (ηext) of 11.5% and 25.1%, and a suppressed efficiency roll-off with ηext of 10.2% and 18.7% at a luminance of 1000 cd m−2, respectively.