Synthesis, Characterization, and Optical Properties of Cationic Europium (III)-Based Complexes for Application in Light-Emitting Electrochemical Cells

Cationic lanthanide luminophores remain underexplored compared with their neutral analogues, despite attractive features for solution-processed electroluminescent devices. Here we report the synthesis and characterization of five europium(III) cationic complexes of the type [Eu(L^{N,N})2(L^{O,O})2][PF6], where L^{N,N} = 2,2′-bipyridine, 1,10-phenanthroline, or bathophenanthroline and L^{O,O} = acetylacetonate (acac) or hexafluoroacetylacetonate (hfac). These complexes are obtained in good yield via one-pot reactions from EuCl3·6H2O, β-diketonates, and diimine ligands, and are confirmed by FT-IR and RAMAN spectroscopies, elemental analysis, and single-crystal X-ray diffraction to possess well-defined EuO4N4 coordination spheres with PF6 counterions. UV–Vis spectra show intense ligand-centered π–π* bands below ~300 nm and weak visible tails, consistent with efficient antenna sensitization and negligible direct Eu(III) f–f absorption. All complexes exhibit characteristic Eu(III) emission between ~575 and 725 nm, dominated by the 5D0 → 7F2 transition near 610–620 nm for all complexes and 5D0 → 7F3,4 transitions for 4 and 5, with similar line patterns but markedly different intensities. Replacing acac with hfac dramatically enhances performance with the quantum yields increasing from 15–18% to 85–93%, and excited-state lifetimes from 1.01–1.53 ms to 2.50–3.25 ms. These gains are attributed to improved ligand triplet to Eu(III) 5D0 energy matching and reduced multiphonon relaxation, establishing these cationic Eu(III) complexes as promising emitters for solution-processable photonic devices.