Blue-emitting ionic multi-resonance emitters for efficient narrowband light-emitting electrochemical cells

Light-emitting electrochemical cells (LECs), featuring simple device structures and low-cost solution processability, have garnered considerable attention for display and lighting applications, but they face challenges in achieving high efficiency and high color purity. Herein, we report two ionic multi-resonance (MR) emitters with narrowband blue emission for high-color-purity LECs, by covalently bonding an imidazolium functional group into a boron (B)/nitrogen (N)-doped polycyclic skeleton. This design not only maintains narrowband emission and high photoluminescence quantum yield (PLQY) of the MR skeleton, but also harnesses the functions of two kinds of N atoms (pyrrolic and pyridinic N atoms) within the imidazolium unit. On the one hand, the 1-position pyrrolic N atom with electron-rich character forms a para-B-π-N linkage with the MR skeleton, elevating the excited-state energy levels and blue-shifting the emission band. On the other hand, the 3-position pyridinic N atom provides a quaternization site to form intrinsically ionic emitters compatible with ionic hosts. Such emitters exhibit blue emission with narrow full-widths at half-maximum of 26–27 nm, and high PLQYs of 95%–97% in solid-state films. LECs based on these emitters exhibit narrowband blue emission with CIE coordinates of (0.12, 0.26) and a maximum external quantum efficiency of 4.6%, representing the first narrowband blue LECs based on intrinsically ionic MR emitters.