Tetra‐N‐Fused Indolocarbazole Multiple Resonance Emitters Enabling Ultra‐Narrowband Pure Green Emission with High Efficiency and Stability

Abstract Developing efficient narrowband emitters beyond the blue region remains challenging for indolocarbazole‐based multiple resonance (ICz‐MR) systems, primarily due to the inherent trade‐off between spectral red‐shifting and linewidth broadening. To address this, we pioneer tetra‐N‐fused ICz isomers ( l ‐N4ICz and s ‐N4ICz) with four consecutive para ‐positioned nitrogen atoms, forming extended π‐systems via alternating six‐/five‐membered ring fusion. The linear isomer l ‐N4ICz achieves sharp green photoluminescence peaking at 506 nm with a full width at half maxima (FWHM) of merely 14 nm—surpassing the bent analogue—alongside suppressed spectral shoulders and higher photoluminescence efficiency. Theoretical studies reveal the critical role of orbital symmetry engineering of adjacent segments in governing the optoelectronic properties of isomers. Organic light‐emitting diodes with l ‐N4ICz deliver pure‐green electroluminescence with an ultra‐narrow FWHM of 19 nm and the first chromaticity y ‐coordinate reaching 0.7 among ICz‐MR systems, alongside a peak external quantum efficiency of 30.1%, which remains at >20% even under extremely high luminance over 200 000 cd m −2 . The same device also sets a benchmark long operational lifetime of 2327 h to decay to 90% of the initial luminance of 1000 cd m −2 . These findings highlight the great potential of multi‐N‐fused ICz‐MR structures for highly efficient, stable, and narrow electroluminescence.