Blue-emitting ionic multi-resonance emitters for efficient narrowband light-emitting electrochemical cells
Light-emitting electrochemical cells (LECs) are promising for low-cost, solution-processed display and lighting applications, yet achieving high efficiency and color purity remains challenging. Here, 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/nitrogen-doped polycyclic skeleton, the emitters retain the narrowband emission and high photoluminescence quantum yield (PLQY) of the MR core while gaining ionic character. The design exploits two types of nitrogen atoms in the imidazolium unit: the pyrrolic N at the 1-position forms a para-B-π-N linkage, elevating excited-state energy levels and blue-shifting emission; the pyridinic N at the 3-position provides a quaternization site, yielding intrinsically ionic emitters compatible with ionic hosts. The emitters exhibit blue emission with narrow full-width at half-maximum of 26–27 nm and high PLQYs of 95%–97% in solid-state films. LECs based on these emitters achieve narrowband blue electroluminescence with CIE coordinates of (0.12, 0.26) and a maximum external quantum efficiency (EQE) of 4.6%, representing the first narrowband blue LECs based on intrinsically ionic MR emitters. This work demonstrates a viable molecular design strategy for high-color-purity LECs, addressing the long-standing trade-off between efficiency and color purity in this technology.