Heavy‐Atom‐Embedded Narrowband Multiple‐Resonance Thermally Activated Delayed Fluorescence Emitters With Aromaticity Guided by Hückel's and Baird's Rules

ABSTRACT Multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters featuring rapid reverse intersystem crossing (RISC) are highly desirable for efficient triplet harvesting. Conventional heavy‐atom strategies often enhance RISC at the expense of spectral broadening, particularly when heavy atoms are embedded in 8π‐electron six‐membered rings, where aromaticity reversal between Hückel's and Baird's rules induces structural reorganization. Here, we report a design strategy through peripheral fusion of heavy‐atom‐containing five‐membered aromatic rings into a classical multiple‐resonance framework BCzBN. The rigid aromatic 6π‐electron rings maintain planarity in both ground and excited states for small reorganization energy, which effectively suppresses structural relaxation‐induced spectral broadening, while simultaneously enhancing spin–orbit coupling (SOC). The resulting emitters achieve narrowband pure‐green electroluminescence with a 27 nm full‐width at half‐maximum (FWHM) and Commission Internationale de l′Éclairage y ‐coordinate of 0.72, together with a RISC rate >10 6 s − 1 . Optimized organic light‐emitting diode devices show a maximum external quantum efficiency (EQE) of 31.3% with negligible efficiency roll‐off, maintaining EQEs of 31.2% and 25.6% at 1000 and 10 000 cd m − 2 , respectively. This work demonstrates the critical role of π‐electron counting in heavy‐atom integration and provides a general design principle for high‐performance MR‐TADF materials that concurrently achieve narrow emission and fast RISC kinetics.