Eu(iii) emission band changes caused by peripheral C–H/O hydrogen bonding

Abstract

We synthesized Eu(III) and Sm(III) complexes with tridentate phosphine oxide ligands, Eu(hfa)₃(TPPM) and Sm(hfa)₃(TPPM) (hfa: hexafluoroacetylacetonato, TPPM: tris(diphenylphosphinyl)methane), and we then examined their luminescent properties. In the complexes the Eu(III) and Sm(III) centres were fully surrounded by low-vibrational frequency ligands, which led to relatively high emission quantum yields (Φ_Eu = 30%, Φ_Sm = 4.7%). The X-ray single crystal structures of the Eu(hfa)₃(TPPM) revealed nona-coordinated Eu(III) complexes and C–H/O hydrogen bonding formations between the acidic hydrogen atom of the TPPM ligand and oxygen atoms of solvent molecules. The C–H/O hydrogen bonding slightly affected the coordination structure around the Eu(III) ion. Despite the seemingly small effect on the structural change, because the emission band profile of the ⁵D₀ → ⁷F₂ transition is sensitive to changes in the coordination environment of the Eu(III) complex, we observed a red shift in the emission spectral line.