Dual Emission and Low-Temperature Afterglow in Xanthone-Dibenzoazepine for High EQE Host-Guest OLEDs with Low-Efficiency Roll-Off.

Research has been driven to demonstrate organic light-emitting diodes (OLEDs) with high efficiency, and in the quest for new materials, thermally activated delayed fluorescence (TADF) emitters have been employed. Preparation of donor-acceptor (D-A) π-conjugates is a useful guideline for developing TADF emitters. TADF emitters have shown excellent progress and high maximum external quantum efficiency (EQE) for OLEDs in the recent past; however, they suffer with substantial roll-off resulting in a decrease in their efficiency. In order to have efficient OLED emitters with less efficiency roll-off, we designed a xanthone-amine derivative with twisted electron-rich dibenzoazepine having limited rotation at the donor-acceptor bond. shows solvent polarity-dependent fluorescence in the range of 441- 597 nm having a lifetime below 10 ns. At 77 K in Me-THF, a triplet at 557 nm was observed having a decay lifetime of 0.75 s and an afterglow for about 6 s. In powder, it shows dual emission, i.e., fluorescence (490 and 6 ns) and phosphorescence (530 nm and 192 μs) at ambient conditions. The energy difference between the singlet and triplet energy levels of is found to be 0.18 eV in the powder sample. Its blend in 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) showed delayed fluorescence with a lifetime of 118 μs at 300 K, while it reduced to 84 μs at 150 K. These observations suggest the TADF nature of in its CBP blend. OLED devices of showing a turn-on voltage of 2.8 V and a EQE of 12% were successfully fabricated. In the doped films of (5 wt %) with CBP, a maximum luminescence of 5980 Cd/m at a current density of 70 mA/cm was obtained, resulting in devices with low-efficiency roll-off (2.75%).