Highly Efficient Solution-Processed Thermally Activated Delayed Fluorescence Emitter Base on Fused Difluoroboron Ketoiminate Acceptor: C/N Switch to Realize Effective Modulation of Luminescence Behavior

Thermally activated delayed fluorescent (TADF) materials based tetracoordinate boron acceptors hold great potential for organic light emitting diodes (OLEDs) applications, however, these emitters still suffer from low luminescent efficiency (ΦPL) and large device efficiency roll-off. Herein, we design and synthesize two D-A type emitters based on two fused boron ketoiminate (FBKI and aza-FBKI) acceptors and a 9,9-dimethylacridine (DMAC) donor. Through changing the carbon atom to nitrogen atom in the fused boron ketoiminate units, a conventional fluorescent molecule (DMAC-FBKI) can be transformed into a highly efficient TADF molecule (DMAC-aza-FBKI) due to the dramatically reduced ΔEST (0.46 to 0.04 eV). DMAC-aza-FBKI exhibits a short delay lifetime of 2.2 μs and a large reverse intersystem crossing rate constant (kRISC ~1.5×106 s-1), which is attributed to the rather close energy levels of its 3LE, 3CT and 1CT states based on theoretical calculations. Furthermore, comparing with DMAC-FBKI (ΦPL~44%), DMAC-aza-FBKI has a higher ΦPL of 82% due to the existence of intramolecular hydrogen bonding. Consequently, the corresponding electroluminescent device achieves the highest external quantum efficiency (EQE) of 16.2% and an efficiency roll-off of only 9.3% at 1000 cd m−2, which are much better than those of the device based on the TADF-inactive DMAC-FBKI.