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

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 >106 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.