Structural peculiarities and luminescence of europium dipivaloylmethanates with 2,2′-bipyridine derivatives. Polymorphism of [Eu(DPM)3Bpy]

Abstract A series of new dipivaloylmethanates of lanthanides with 2,2′-bipyridine [Ln(DPM)3Bpy] (Ln = Eu, Gd, Tb) and 4,4′-dimethylbipyridine [Ln(DPM)3DMBpy] (Ln = Eu, Tb) was synthesized and investigated using single-crystal X-ray diffraction and luminescence spectroscopy. Compounds with 4,4′-diphenylbipyridine [Ln(DPM)3DPhBpy] (Ln = Eu, Tb) as well as some lanthanide dipivaloylmethanates with Phen derivatives were also considered. The crystal structures of [Eu(DPM)3Bpy] and [Ln(DPM)3DMBpy] (Ln = Eu, Tb) belonging to the P 1 ¯ space group of symmetry, and the structures of [Ln(DPM)3Bpy] (Ln = Eu, Gd, Tb) related to the Pna21 space group were determined. The polymorphism of [Eu(DPM)3Bpy] was revealed. It is shown that the orthorhombic polymorph can transform into triclinic. The display of polymorphism in the luminescence spectra is considered. The influence of distortions of the Ln3+ nearest surroundings introduced by the bulky ancillary ligand on the luminescence spectra, the relative intensity of the electric-dipole 5D0-7F2 transition of the Eu3+ ion, and the lifetimes of the Eu3+ and Tb3+ emitting states is analyzed. A qualitative correlation between the measured lifetimes and the intensity of the 5D0-7F2 transition was observed for the series of dipivaloylmethanates. The relative contributions of the pure radiative and non-radiative processes to the decay rate of the 5D0 state of the Eu3+ ion are discussed. A significant effect of non-radiative processes on the lifetime, first of all, due to high-frequency vibrations of the DPM ligand was demonstrated.