Nitrogen‐Embedded Multi‐Resonance Heteroaromatics with Prolonged Homogeneous Hexatomic Rings

Nitrogen‐containing polycyclic heteroaromatics have exhibited fascinating multi‐resonance (MR) characteristics for efficient narrowband emission, but strategies to bathochromic shift their emissions while maintaining the narrow bandwidths remain exclusive. Here, homogeneous hexatomic rings are introduced into nitrogen‐embedded MR skeletons to prolong the π‐conjugation length for low‐energy electronic transitions while retaining the non‐bonding character of the remaining parts. The proof‐of‐the‐concept emitters exhibit near unity photoluminescence quantum yields with peaks at 598 nm and 620 nm and small full‐width‐at‐half‐maximums of 28 nm and 31 nm, respectively. Optimal organic light‐emitting diodes exhibit a high external quantum efficiency of 18.2 %, negligible efficiency roll‐off, and ultra‐long lifetime with negligible degradation at an initial luminance of 10 000 cd m−2 after 94 hours. Red multi‐resonance emitters with high photoluminescence quantum yields and narrowband emission were demonstrated by introducing homogeneous hexatomic rings into nitrogen‐embedded polycyclic aromatic hydrocarbons. The resulting devices afforded state‐of‐the‐art performance with external quantum efficiency up to 18.2 % and superior operation stability.