Rigidifying a Parent DA Motif Through Dual‐Core Merge‐Ring Engineering: Improved Charge Transfer Mode for Narrowband Deep‐Blue TADF Emitters with High EQE of 35.4% and CIE <i>y</i> Below 0.06

Conventional donor-acceptor (D-A) type thermally activated delayed fluorescence (TADF) materials face significant challenges in achieving deep-blue electroluminescence with narrow full-width at half maximum (FWHM). Herein, two deep-blue narrowband emitters, DPYDICz and DCNDICz, are developed by applying merge-ring engineering to rigidify dual-core D-A TADF motifs. This approach suppresses through-space charge transfer (TSCT) and enables short-range charge transfer (SR-CT), effectively balancing excited-state charge-transfer and local exciton contributions. The rigid molecular frameworks yield deep-blue emissions at 439 and 443 nm in toluene with narrow FWHMs of 17 and 21 nm, respectively, and high photoluminescence quantum yields exceeding 90% in doped films. When employed as terminal emitters in hyperfluorescence OLEDs sensitized by m4TCzBN, these materials achieve high external quantum efficiencies of 35.4% and 32.0% with deep-blue electroluminescence (CIEy of 0.06), narrow FWHMs (29-30 nm), and excellent operational stability (LT90 up to 54 h). This merge-ring engineering strategy offers a generalizable platform to convert conventional broad-emission TSCT-type D-A systems into narrowband SR-CT emitters, revitalizing classical D-A TADF materials for high-performance deep-blue OLED applications.