Achieving Ultimate Narrowband and Ultrapure Blue Organic Light‐Emitting Diodes Based on Polycyclo‐Heteraborin Multi‐Resonance Delayed‐Fluorescence Emitters

To achieve an ultimate wide color gamut for ultrahigh‐definition displays, there is great demand for the development of organic light‐emitting diodes (OLEDs) enabling monochromatic, ultrapure blue electroluminescence (EL). Herein, high‐efficiency and ultrapure blue OLEDs based on polycyclo‐heteraborin multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials, BOBO‐Z, BOBS‐Z, and BSBS‐Z, are reported. The key to the design of the present luminophores is the exquisite combination and interplay of multiple boron, nitrogen, oxygen, and sulfur heteroatoms embedded in a fused polycyclic π‐system. Comprehensive photophysical and computational investigations of this family of MR‐TADF materials reveal that the systematic implementation of chalcogen (oxygen and sulfur) atoms can finely modulate the emission color while maintaining a narrow bandwidth, as well as the spin‐flipping rates between the excited singlet and triplet states. Consequently, OLEDs based on BOBO‐Z, BOBS‐Z, and BSBS‐Z demonstrate narrowband and ultrapure blue EL emission, with peaks at 445–463 nm and full width at half maxima of 18–23 nm, leading to Commission Internationale de l'Éclairage‐y coordinates in the range of 0.04–0.08. Particularly, for OLEDs incorporating sulfur‐doped BOBS‐Z and BSBS‐Z, notably high maximum external EL quantum efficiencies of 26.9% and 26.8%, respectively, and small efficiency roll‐offs are achieved concurrently. High‐efficiency ultrapure blue delayed‐fluorescence materials are produced by the strategic doping of multiple boron, nitrogen, oxygen, and sulfur atoms into the fused polycyclo‐heteraborin π‐system. These new fluorophores allow organic light‐emitting diodes to demonstrate ideal narrowband and ultrapure blue electroluminescence, meeting the requirements for modern ultrahigh‐definition displays.