Solvent-dependent dual-luminescence properties of a europium complex with helical π-conjugated ligands

A europium(iii) complex with a large π-conjugated helical ligand, tris(hexafluoroacetylacetonato)europium(iii)bis((pentahelicenyl)diphenylphosphine oxide) (Eu(hfa)₃(HPO)₂), exhibits dual luminescence excited at the π-π* transition band. In this study, the solvent-dependent photophysical properties of Eu(hfa)₃(HPO)₂ are reported. The ground-state properties of the ligand and the Eu(iii) complex are assessed from electronic absorption spectra, magnetic circular dichroism spectra, and density functional theory calculations. The excited-state properties of these compounds are evaluated using luminescence spectra, excitation spectra, luminescence lifetimes, and luminescence quantum yields depending on solvents, which are CHCl₃, C₂H₅Br, C₂H₅I, CH₃COCH₃, and CD₃COCD₃. Under all of the examined conditions, the intensity ratios of dual luminescence originating from the ligand and the Eu(iii) ion were strongly dependent on the properties of the solvent. The solvent dependence of the luminescence ratio is discussed in terms of the external heavy atom effect.