Highly efficient solution-processable four-coordinate boron compound: A thermally activated delayed fluorescence emitter with short-lived phosphorescence for OLEDs with small efficiency roll-off

Although great research progress on thermally activated delayed fluorescence (TADF) emitters has been made, highly-efficient solution-processable TADF emitters with short TADF decay time and additional radiative decay path are rarely reported. Herein, we developed a novel four-coordinate boron compound Spiro-PMB-DI with excellent solubility of over 1700 mg mL−1 in chlorobenzene, using spiroboron as acceptor and rigid triazatruxene (DI) as donor. Due to the small singlet and triplet energy gap (ΔEST), as well as the hybridized local and charge transfer character for T1 state and the large spin–orbit coupling (SOC) matrix elements, Spiro-PMB-DI possesses an efficient phosphorescence path in addition to the TADF path at ambient temperature. This is evidenced by the short delay lifetime and phosphorescence lifetime, both of which occur on the microsecond timescale. Consequently, the fast reverse intersystem crossing (rISC) rate of 1.47 × 106 s−1 and phosphorescence rate of 4.57 × 104 s−1 are observed. Combining the high photoluminescence quantum yield (PLQY) of 81.8 %, Spiro-PMB-DI can function as efficient emitter in solution-processed OLEDs, providing a high external quantum efficiency (EQE) up to 24.86 % with a small efficiency roll-off. Additionally, by introducing orange and red phosphorescent materials as terminal emitters, a high EQE of 37.15 % are achieved for solution-processed phosphorescent OLEDs, which is nearly four times higher than the one without Spiro-PMB-DI sensitizer.