Emitter-Host Interactions of High-Efficiency Deep Blue Single-Gaussian Europium (II) Emitters

Eu(II) complexes are attractive emitters for deep-blue organic light-emitting diodes (OLEDs) due to their narrow, parity-allowed 4f-5d emission; however, their implementation in vacuum-processed OLEDs has remained limited. Here, we introduce a new molecular design concept for Eu(II) emitters, in which a crown-ether ligand is combined with carborate anions to define the coordination environment and improve steric shielding of the europium center. Based on this design, we present two emitters that combine narrow deep-blue photoluminescence with quantum yields approaching 90% and sufficient thermal stability for vacuum deposition. OLEDs incorporating these emitters exhibit electroluminescence at 456-458 nm, with spectral widths down to 36 nm and CIE coordinates as deep as (0.15, 0.06) and achieve a maximum external quantum efficiency above 12%. In order to find the pathway to maximum electroluminescence efficiency based on this emitter class, we study interactions between Eu(II) complexes and the host environment, based on density-functional theory and time-resolved experiments. We identify molecular design, steric shielding of the Eu(II) core, and energetic confinement of the excited 5d electron as key factors governing efficient luminescence, providing a roadmap for rational design of Eu(II) emitters. Together, these insights establish a foundation for higher-efficiency and deeper-blue OLEDs incorporating Eu(II) emitters.