Isomeric Pentagonal Fusion and <i>π</i>‐Expanding of Nitrogen/Carbonyl‐Containing Multi‐Resonant Emitters for High‐Performance and Narrowband Organic Electroluminescence

Abstract Embedding nitrogen/carbonyl (N/C═O) units into rigid heterocyclic aromatic hydrocarbons creates novel multi‐resonant thermally activated delayed fluorescence (MR‐TADF) molecules. Nevertheless, the intrinsic short‐range charge transfer (SRCT) characteristics of N/C═O derivatives, as exemplified by quinolino[3,2,1‐ de ]acridine‐5,9‐dione (QAO), result in broad spectral bandwidths, posing challenging for achieving deep blue emission. Herein, a pentagonal cyclization and isomeric expansion strategy aimed is proposed at modulating SRCT characters and molecular symmetry to further narrow spectral bandwidths and regulate excited‐state energy levels. By fusing two 8 H ‐indolo[3,2,1‐ de ]acridin‐8‐one (IAO) skeletons via a central phenyl segment that cyclizes two nitrogen atoms, proof‐of‐concept emitters with reduced SRCT characteristics and narrow spectral widths are developed. These emitters achieved a wide color tuning range from deep blue (439 nm) to pure green (520 nm) and exhibited narrow spectral bandwidths of 17–24 nm (≈0.11 eV). The corresponding electroluminescence devices demonstrated bright deep‐blue, blue, and green emissions with narrow linewidths. Notably, sensitized deep‐blue/blue devices incorporating mTIAO and pTIAO emitters achieved exceptional external quantum efficiencies of 29.6% and 34.4%, respectively, representing the most efficient blue‐light N/C═O derivatives reported to date. Furthermore, the color index of mTIAO, measured at (0.147, 0.063), aligns perfectly with the blue National Television System Committee standard of (0.15, 0.06).