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 y Below 0.06

ABSTRACT 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 (CIE y 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.