Inhibition of Interregional Charge Transfer Transition in Asymmetrical Heteroatomic Framework Enables BT.2020 Deep‐Blue Electroluminescence with EQE Exceeding 40%

Multi-boron/nitrogen/oxygen (B/N/O)-embedded polyaromatics featuring the multiple resonance (MR) effect are popular photoluminescent emitters that meet the BT.2020 blue standard, while they often fail to achieve this in organic light-emitting diodes (OLEDs) because the uneven electron-donating properties of N/O atoms make them sensitive to their surrounding environments. Herein, taking such an emitter as the prototype and developing two triple-borylated isomers (TBNO-1 and TBNO-2) via π-extensions and heteroatom regulations. Both emitters display highly efficient thermally activated delay fluorescence properties and nearly identical narrowband emission with CIE-y coordinates far exceeding the BT.2020 standard in toluene as the prototype does due to their similar core patterns. On the other hand, their emission spectra differ in other environments due to their different interregional charge transfer (IRCT) characters. Importantly, due to the strategically opposed oxygen atoms, TBNO-2 demonstrates uniformly delocalized wavefunctions and a much-suppressed IRCT contribution than TBNO-1, and thus exhibits minimal spectral variations across diverse environments. In OLEDs, while both emitters can afford impressive external quantum efficiency exceeding 40%, only TBNO-2 can afford electroluminescence satisfying the BT.2020 standard with CIE-y coordinate of 0.044. This work offers valuable insights for finely modulating the photophysical properties of MR emitters to realize BT.2020 blue electroluminescence.