Narrowband Green Thermally-activated Delayed Fluorescence Emitters Based on Bis-fused Indolo[3,2,1-jk]carbazole with Peripheral Donor Decoration

Fused indolo[3,2,1-jk]carbazole (fICZ) framework has emerged as a new paradigm for developing narrowband emitters, but red-shifting the emission and triggering reverse intersystem crossing (RISC) for thermally-activated delayed fluorescence (TADF) have remained challenges. Here, narrowband green TADF emitters based on a fICZ framework are reported with a peripheral donor decoration strategy. By attaching electron-donating diphenylamino-type groups onto the highest occupied molecular orbital (HOMO)-dominating positions of bis-fused indolo[3,2,1-jk]carbazole, the HOMO is largely destabilized and the emission is markedly red-shifted (by > 70 nm) with retained narrow bandwidth. In toluene solution, the emitters show green emission with emission maximums at 500−523 nm and small full-widths at half maximums at 26−28 nm. Importantly, the donor decoration narrows the singlet-triplet energy gap and introduces a higher-lying long-range charge-transfer triplet state having considerable spin-orbital coupling with the emissive singlet state, which behaves as an intermediate triplet state to facilitate RISC. In doped films, the emitters show efficient narrowband green TADF emission. Solution-processed organic light-emitting diodes (OLEDs) using the emitters afford green emission close to the NTSC standard and high external quantum efficiencies up to 24.7%, which are among the most efficient solution-processed narrowband green OLEDs.