Boron–nitrogen framework substituted anthracenes as “hot exciton” fluorophores for efficient sky-blue narrowband emissive OLEDs with full-width at half-maximum below 30 nm

Abstract

Due to the small energy gap between the high-lying triplet state T_n (n ≥ 2) and the singlet state S_m (m ≥ 1), “hot exciton” fluorescent materials are capable of harvesting triplet excitons through high-lying reverse intersystem crossing (hRISC) and achieve high efficiencies in organic light-emitting diodes (OLEDs). Herein, we report the development of three new narrowband blue fluorophores—AnBNCz, AnBNCz-CN, and AnBNCz-OMe—by integrating the multiple-resonance (MR) boronitride compound BNCz with the conventional “hot exciton” scaffold anthracene (An). The results of photophysical experiments reveal that the three fluorophores exhibit narrowband sky-blue emissions centered at 483 nm, characterized by a full width at half-maximum (FWHM) of merely 24 nm and photoluminescence quantum yields (PLQY) exceeding 70% in dilute toluene. Owing to the steric hindrance effect of the anthracene unit, the electroluminescence (EL) spectra of these fluorophore-based doped devices remain unchanged with varying doping concentrations, still maintaining narrowband emissions. With an optimized doping concentration, AnBNCz-OMe-based devices demonstrate sky-blue emissions centered at 492 nm with a maximum external quantum efficiency (EQE_max) of up to 7.6%, and their associated EL FWHM values are less than 28 nm. The corresponding exciton utilization efficiency (EUE) is estimated to exceed 50%, surpassing the theoretical maximum limit. Additional theoretical calculation studies indicate that the incorporation of BNCz and An units results in “hot exciton” energy-level alignment within these fluorophores, and further experimental evidence confirms the presence of effective hRISC processes, elucidating the high EUE in the EL process. This study provides a viable strategy for constructing “hot exciton” materials with high color purity.