X-Ray analysis and excited state dynamics in a new class of lanthanide mixed chelates of the type LnPhβ3·Phen (Ln = Sm, Eu, Gd, Tb)

Abstract

A new type of lanthanide complex with a derivative of β-diketone and phenanthroline, LnPhβ₃·Phen [where Phβ = CCl₃–C(O)–N–P(O)–(OCH₃)₂; Phen = 1,10-phenanthroline, Ln = Sm, Eu, Gd, Tb] was synthesised. The X-ray diffraction analysis of the Sm(III) complex shows that it is in the monoclinic system and P2₁/c space group. There are two different Sm³⁺ ions with insignificant differences between the Sm–O, –N distances (0.005 Å) and a CN = 8. Absorption, emission and emission excitation spectra at 293, 77 and 4.2 K were used to characterise the title compounds in the solid state and in solution. The luminescence spectra at 77 K for the Eu analogue are complex and two components in the ⁵D₀ → ⁷F₀ transition show that Eu³⁺ ions reside in two symmetry sites. The spectroscopic results correspond well to the crystal structure of the SmPhβ₃·Phen compound and confirm that the Sm³⁺, Eu³⁺ and Tb³⁺ monocrystals are isostructural. Efficient energy transfer sensitises the interion emission from ⁵D₄ or ⁵D₀ levels of Tb(III) and Eu(III) after ligand band excitation. Based on the splitting of the levels observed at 4.2 K in the single crystal spectra of Tb and Eu, as well as the absorption, phosphorescence and excitation spectra, energy level diagrams are proposed. The mechanism of energy transfer and excited state dynamics are discussed. Strong vibronic coupling was observed mainly in the ⁷F₀ → ⁵D₂ electronic transition. The IR spectra were used to analyse the vibronic components. Analysis of the emission spectra of a Eu³⁺-doped silica sol-gel sample was made and compared to those for the title monocrystals. The total spectral characteristics of the materials permits an evaluation of their potential applicability.