One Shift, Huge Impact: Isomeric Engineering Unlocks Pure Organic Ultra-Narrowband Dual Functional Ultraviolet Emitters for High-Performance OLEDs

Crafting boron (B)-free, purely organic narrowband ultraviolet (UV) emitters for organic light-emitting diodes (OLEDs) remains a significant challenge. Herein, a simple regioisomeric engineering strategy by integrating the rigid indolo[3,2,1-jk]carbazole into carbazole at different positions were unveiled. Conjugation isolation and steric effects enabled tunable emission from deep-blue to UV and ultra-narrow FWHM. The C3-isomer (3CzICz) exhibited broad blue emission, while C1- and C4-isomers (1CzICz, 4CzICz) delivered UV-emission (λem ≤ 385 nm) with ultra-narrow FWHM ≤ 17 nm. These materials function as both emitters and hosts owing to their wide bandgap and high triplet energies. Notably, 4CzICz as an emitter demonstrated outstanding performance with a EQE of 4.2% and UV-emission (CIEy ≈ 0.028). As a host for green phosphorescent organic light-emitting diodes (PHOLEDs), it exhibited a high EQE of 25.0% with minimal efficiency roll-off (5.0%). This study provides key insights for designing efficient B-free, narrowband organic luminophores for next-generation OLEDs.