Two mono- and dinuclear Eu(iii) enantiomeric pairs based on chiral bis-bidentate bridging ligands: synthesis, structures, luminescent and ferroelectric properties

Abstract

Using the enantiomeric bis-bidentate bridging ligands (+)/(−)-2,5-bis(4,5-pinene-2-pyridyl)pyrazine (L_S/L_R) and depending on the ratio control of reactants, two mono- and dinuclear Eu(III)-based enantiomeric pairs with the formulae Eu(dbm)₃L_R/S·2H₂O (L_R in R-1, L_S in S-1 and dbm = dibenzoylmethanato) and Eu₂(dbm)₆L_R/S·H₂O (L_R in R-2 and L_S in S-2) have been stereoselectively synthesized and structurally characterized. The circular dichroic (CD) spectra confirmed their chiroptical activities and enantiomeric natures. The homochiral dinuclear species represents the first example of a polynuclear lanthanide β-diketonate complexes with circular dichroic and crystallographic evidences. The photoluminescent properties studies revealed that both mono- and dinuclear Eu(III) complexes exhibited the characteristic red emissions of Eu(III) ions in the solid state (at 77 K and 300 K) and CH₂Cl₂ solution. Notably, the photophysical properties of the mononuclear enantiomers were superior to the dinuclear species. Interestingly, R-2 displayed a ferroelectric property at room temperature, which was not observed for R-1 due to the lack of crystalline polarity. R/S-2 are the first examples of homochiral polynuclear lanthanide complexes with luminescence and ferroelectric properties, being potential multifunctional materials.