A modular access to N-heterocycle-winged boron-nitrogen multiresonant emitters with narrowband deep-blue emissions

Abstract

Boron-nitrogen multiresonant thermally activated delayed fluorescence (BN MR-TADF) emitters have attracted widespread attention for high performance organic light-emitting diodes (OLEDs) due to their narrowband emissions and high efficiencies. However, emitters incorporating electron-deficient N-heterocycles are rarely reported and remain largely unexplored because of synthetic challenges under harsh borylation conditions. We present a protection/deprotection strategy using benzyl groups to overcome this restriction to offer a modifiable core molecule HBN, enabling late-stage modular introduction of electron-deficient N-heterocycles to furnish a variety of novel N-heterocycle-winged BN emitters, the HetBN series. It should be noted that a unique sequential two-step debenzylation protocol is developed and must be applied to remove both benzyl groups. As exemplified herein, four emitters, oPyBN, mPyBN, PymBN, and PhTzBN, which are inaccessible via conventional methods, were successfully prepared. They emit deep-blue photoluminescence with fine-tuneable wavelengths below 454 nm, high PLQYs above 90 %, narrow FWHMs under 30 nm, and fast exciton dynamics, demonstrating the beneficial effect of N-heterocycles in fine-tuning the photophysical properties of BN MR-TADF emitters. A preliminary OLED device fabricated with PhTzBN exhibited deep-blue emission at 444 nm with a narrow FWHM of 31 nm (0.19 eV), the Commission Internationale de l’Éclairage (CIE) coordinates of (0.16, 0.08), and a maximum external quantum efficiency (EQE) of 5.2 %. This study paves the way to modular synthesis of innovative N-heterocycle-winged BN MR-TADF emitters.