Structure and photoluminescence of dimeric europium and terbium pivalates with 1,10-phenanthroline derivatives

The relationship between the spectroscopic and structural features of europium, terbium, and gadolinium pivalates [Ln(Piv)₃L], where Piv = (CH₃)₃C-COO^–, L = 1,10-phenanthroline and its 4-methyl-, 4,7-dimethyl-, 3,4,7,8-tetramethyl-, 5-amino-, and 5-nitro- derivatives, was studied. Crystal structures of seven new dimeric lanthanide pivalates were determined by X-ray diffraction methods. The luminescence spectra of Eu and Tb compounds, the phosphorescence spectra of Gd compounds, and the lifetimes of the electronic states ⁵D₀ (Eu³⁺) and ⁵D₄ (Tb³⁺) at 77 and 295 K were investigated. The efficiency of excitation energy transfer to Eu³⁺ and Tb³⁺ ions was analyzed depending on the relative position of the lowest excited singlet S₁ and triplet T₁ states of the ligands and resonance levels of the Ln³⁺ ions. The Judd-Ofelt intensity parameters Ω_λ for europium pivalates, the rates of radiative and nonradiative processes, and other spectroscopic characteristics were calculated. In most europium pivalates, multiphonon relaxation is the only nonradiative process contributing to the decay rate of the ⁵D₀ state. At 77 K, the calculated quantum efficiency and quantum yield of Eu³⁺ luminescence in such compounds vary within the range of 55–62 %. It was shown that the intensity of the f-f transitions of the Eu³⁺ ion in pivalates is associated predominantly with the dynamic coupling (DC) mechanism. In most terbium pivalates, in addition to multiphonon relaxation affecting the ⁵D₄ decay, the back transfer energy ⁵D₄-T₁ is also active, which leads to a reduction in the lifetime of the ⁵D₄ state at 295 K, as well as a sharp decrease in the luminescence intensity. The absence of ⁵D₄-T₁ back transfer in Tb pivalate with Phen and the high luminescence intensity are caused by the strong Tb-ligands binding.