Structure, luminescence, and raman spectroscopy of europium and terbium dipivaloylmethanates and other β-diketonates with 2,2′-bipyridine

The study reveals the relationship between structural and spectroscopic characteristics of lanthanide dipivaloylmethanates with 2,2′-bipyridine derivatives [Ln(DPM) 3 L] (Ln = Eu, Tb; L = 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, 4,4′-diphenyl-2,2′-bipyridine) and fluorinated β-diketonates [Ln(β) 3 Bpy] (β = TTFA, TFAA, BTFA) with the CN (Ln 3+ ) = 8 and the LnN 2 O 6 coordination polyhedron in the form of a square antiprism. We discuss structural regularities resulting from the difference between Eu–O and Eu–N bond lengths in β-diketonates due to the changes in donor-acceptor properties of substituents and the steric factor. The effect of the main distortion Δ = r (Eu–N) – r (Eu–O) of the Eu coordination polyhedron on the luminescence spectra and vibrational Raman spectra is considered. A qualitative correlation is demonstrated between the relative intensity of the 5 D 0 – 7 F 2 (Eu 3+ ) electric dipole transition, which characterizes the degree of distortion of the nearest environment of the Eu 3+ ion, and the lifetimes of excited electronic states 5 D 0 (Eu 3+ ) and 5 D 4 (Tb 3+ ). The contribution of nonradiative processes caused by high-frequency vibrations of ligands to the decay rate of 5 D 0 (Eu 3+ ) and 5 D 4 (Tb 3+ ) levels is discussed. The Raman spectra are analyzed in terms of studying the changes in vibrational frequencies of the Bpy molecule depending on the distortion Δ of the Eu coordination polyhedron.