Linear Annulation Engineering of Indolocarbazole Multiple Resonance Emitter to Overcome Efficiency‐Stability‐Color Purity Trilemma in Deep‐Blue OLEDs

Abstract Deep‐blue emitters for organic light‐emitting diodes (OLEDs) still confront the critical challenge of balancing high efficiency, operational stability, and color purity, particularly for the ones with peak wavelengths (λ_max) ≤ 460 nm. Here, the study demonstrates deep‐blue devices featuring ultrapure emission (λ_max = 458 nm, full‐width at half‐maximum = 19 nm), high maximum external quantum efficiency of 34.3% with small roll‐off (26.9% at 1 000 cd m⁻²; 20.9% at 5 000 cd m⁻²), and long operational LT80 (time to 80% of the initial luminance) of 101 hours at 1,000 cd m⁻², being one of the longest lifetime among OLEDs with λ_max ≤ 460 nm and EQE >20%. This breakthrough stems from an indolocarbazole narrowband emitter employing a linear annulation strategy, which not only narrows spectral bandwidth while red‐shifting emission peak through multiple resonance framework extension, but also energetically and dynamically enhances device longevity via triplet energy reduction. Furthermore, strategic integration of steric hindrance on the emitting backbone suppresses intermolecular interactions and directs reactivity pathways. This emitter concurrently achieves a λ_max of 456 nm, FWHM of 15 nm and photoluminescence (PL) quantum yield of 98% in dilute toluene. The work highlights linear annulation engineering as a potential approach to resolve the efficiency‐stability‐color purity trilemma in deep‐blue OLEDs.