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

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.