Hard-and-soft phosphinoxide receptors for f-element binding: structure and photophysical properties of europium(iii) complexes

Abstract

New phosphinoyl-containing tetradentate heterocycles preorganised for metal ion binding were designed and prepared in high yields. The X-ray structures of two allied phosphinoyl-bearing 2,2′-bipyridyl and phenanthroline ligands, as well as closely related structures of 2,6-bis(diphenylphosphinoyl)pyridine and 9-(diphenylphosphinoyl)-1,10-phenanthroline-2-one, are reported. Complexes of nitrates of several lanthanides and trifluoroacetate of Eu(III) with two phosphinoyl-bearing 2,2′-bipyridyl and phenanthroline ligands were isolated and characterised. The first structures of lanthanide complexes with phosphinoyl-bearing 2,2′-bipyridyl and phenanthroline ligands are reported. The nature of the counter-ion is crucial for the coordination environment of the metal ion. The photophysical properties of the complexes differring in both the nature of the ligand and counter-ion were investigated. The photophysical properties of the complexes are strongly ligand- and counter-ion-dependent. Absorbance and luminescence excitation spectra of complexes showed main peaks in the UV range which correspond to the absorption of light by the ligand and these are ligand-dependent. Luminescence spectra of complexes show typical europium emission in the red region with a high quantum yield, which corresponds to the transitions ⁵D₀ → ⁷F_J (J = 0–6). The value of deviation of the components of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions from the inversion centre shows a larger dependence on the counter-ion than on the nature of the ligand. The value of the luminescence quantum yield is larger for europium complexes with 2,2′-bipyridyl-based ligands and NO₃ counter-ions than for complexes with phenanthroline-based ligands and NO₃ counter-ions. A low dependence of the luminescence lifetime of Eu complexes on the nature of the ligand has been demonstrated: values in the solid state were in the range 1.1–2.0 ms.