Green multi-resonance thermally activated delayed fluorescence emitters containing phenoxazine units with highly efficient electroluminescence

Highly efficient multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with narrowband emissions based on a boron/nitrogen (B/N) framework are crucial for next-generation full-color displays with high color purity. In this work, we develop a simple molecular design strategy for MR-TADF materials based on a skeleton of phenoxazine units and amplification at the para-position of the boron atom by different electron-donating groups (phenoxazine/tert-butylcarbazole). Two novel phenoxazine-fused MR-TADF materials, TPXZBN and DPXZCZBN, show green emissions with sharp peaks at 502 nm and 500 nm, with small full-width at half maximum (FWHM) bandwidths of 33 nm and 32 nm, respectively. TPXZBN and DPXZCZBN also exhibit small singlet-triplet state energy gaps (ΔEsts) of 0.16 eV and 0.13 eV, with high photoluminescence quantum yields (PLQYs) of 91% and 90% in toluene solutions, and 99% and 94% in doped films, respectively. The corresponding organic light-emitting diodes (OLEDs) based on TPXZBN and DPXZCZBN exhibited maximum external quantum efficiencies (EQEmaxs) of 21.3% and 19.8%, respectively, accompanied by small efficiency roll-offs, with EQEs of 17.2% and 18.8% at 100 cd m-2, and 17.4% and 19.6% at 1000 cd m-2, respectively. Notably, the small electroluminescence FWHMs of the OLEDs still remained at 37 nm and 36 nm, respectively.