“Medium-ring” strategy enables high-performance narrowband pure-blue multi-resonance emitters: boost provided by a unique perpendicular geometry

]azepine (TBA) donor onto BCz-BN enabled the development of a pure-blue narrowband emitter, TBA-BCz-BN, and induced a significant blue shift of emission from 483 nm to 468 nm and a decrease in the singlet-triplet energy gap from 0.17 eV to 0.14 eV. The corresponding TBA-BCz-BN-based OLED exhibited a high external quantum efficiency of 30.8%.