Journal of Optical Materials

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 → ⁷F_3,4 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.