Seven Europium(III) Complexes in Solution – The Importance of Reporting Data When Investigating Luminescence Spectra and Electronic Structure

Abstract

Despite significant effort, the complexity of europium(III) luminescence in general -and sample composition in particular -has precluded the publication of standard spectra of specific complexes in aqueous solution. Further, low spectral resolution, weighted averages with contributions from several species, as well as poor reproduction hampers the use of literature data, with the note that high quality data from e. g. Bünzli, Latva, Carnall, Parker, Heller, Balzani and Sabatini is from the preelectronic era. Here, luminescence spectra of Eu.DOTA, Eu.EDTA, Eu.DPA 1 , Eu.DPA 2 . Eu.DPA 3 , Eu.DTPA and the Eu(III) aqua ion was determined in samples with known pH and conductivity. The ligand concentrations were varied in a buffered medium (MES) with a significant salt background. In this manner, full complexation could be ensured, while pH and ionic strength were monitored. The spectra corresponding to the complexes were extracted, and the electronic structure of the ground state 7 F J manifold was investigated and contrasted to literature data. All spectroscopic data is made available and we suggest this becomes part of the standard procedure when investigating lanthanide(III) complexes.

Electronic Structure of Neodymium(III) and Europium(III) Resolved in Solution Using High-Resolution Optical Spectroscopy and Population Analysis

“…From this it can be seen that Eu·DOTA displays different emitting species while Eu·DPA and Eu·EDTA do not. In previous work, speciation studies of these three complexes were carried out by recording the lifetimes of europium­(III) at different concentration equivalents of the three ligands. , Here it is found that Eu·DOTA indeed has multiple lifetimes that can be assigned to the different forms of different symmetry, c-SAP and TSAP, that these complexes are known to possess in solution. , Eu·EDTA primarily displays a speciation between the aqua ion and the EDTA complex. At 1.2 equiv of EDTA, as used in this work, mostly the EDTA complex was observed, although the aqua ion cannot be completely dismissed.…”

“…In previous work, speciation studies of these three complexes were carried out by recording the lifetimes of europium(III) at different concentration equivalents of the three ligands. 42,62 Here it is found that Eu•DOTA indeed has multiple lifetimes that can be assigned to the different forms of different symmetry, c-SAP and TSAP, that these complexes are known to possess in solution. 43,63 Eu•EDTA primarily displays a speciation between the aqua ion and the EDTA complex.…”

Invisible strings. The first single crystal of the cTSAP form of [Eu(DOTA)(H2O)]− has an electronic structure similar to one of the reported cSAP forms

“…3 In all spectra, the 5 D 0 → 7 F 0 is present as expected for [Eu(DOTA)(H 2 O)] − complexes because the crystal field does not have mirror symmetry when nitrogen and oxygen donor atoms are coordinating to the metal ion. 36 A high symmetry fourfold rotation axis persists along the capping water molecule, which can be seen in the 5 D 0 → 7 F 1 transition, which is split in three when the symmetry is lowered. 37 The emission spectra in Fig.…”

Optical spectroscopy as a tool for studying the solution chemistry of neodymium(iii)

“…Little is known of lanthanide(III) solution structure, and the constitution has been debated. 32,35,177 At present this leaves two routes to be investigated: kinetically inert complexes, 178 and simple solvates. For the former the constitution is unchanged when they are dissolved, for the latter the sample composition can be controlled in a manner that ensures that the constitution is known.…”