Europium(iii) dipicolinate complexes have been a model system in lanthanide solution chemistry for decades, here it is investigated in unprecedented detail.

The structure of lanthanide(III) ions in solutions high in nitrate has been debated since the early days of lanthanide coordination chemistry. The structure and properties of lanthanides in these solutions are essential in industrial rare-earth separation, as well as in the fundamental solution chemistry of these elements. Pending decades of debate, it was established that nitrate is bidentate and coordinates in the inner sphere, and complexes have been observed with as many as four nitrates coordinated to a single lanthanide(III) center in nonaqueous solutions. We revisit the interactions between nitrate and europium(III) in methanol using optical spectroscopy, X-ray total scattering, and the current understanding of europium(III) photophysics. By a combination of direct and indirect methods to probe the structure, it was found that four distinct species from Eu(MeOH) 93+ to [Eu(MeOH) 3 (NO 3 ) 3 ] are present in solutions containing from 0 to 2 M NO 3 − ions. It was shown that the changes in transition probabilities together with high-resolution spectra can provide information on speciation and how the minute changes in ligand field affect the microstates. By a comparison to total Xray scattering, it was concluded that the optical spectra alone allow the constitution and symmetry of the europium(III) species to be determined. Most notably, the minute changes in the all oxygen atom coordination imply significant changes in the optical properties of the europium(III) center.

Lanthanide luminescence has been treated separate from molecular photophysics, although the underlying phenomena are the same. As the optical transitions observed in the trivalent lanthanide ions are forbidden, they do belong to the group that molecular photophysics has yet to conquer, yet the experimental descriptors remain valid. Herein, the luminescence quantum yields (ϕ lum ), luminescence lifetimes (τ obs ), oscillator strengths (f), and the rates of nonradiative (k nr ) and radiative (k r ≡ A) deactivation of [Eu(H 2 O) 9 ] 3+ were determined. Further, it was shown that instead of a full photophysical characterization, it is possible to relate changes in transition probabilities to the relative parameter A rel , which does not require reference data. While A rel does not afford comparisons between experiments, it resolves emission intensity changes due to emitter properties from intensity changes due to environmental effects and differences in the number of photons absorbed. When working with fluorescence this may seem trivial; when working with lanthanide luminescence it is not.

The wide range of optical and magnetic properties of lanthanide­(III) ions is associated with their intricate electronic structures which, in contrast to lighter elements, is characterized by strong relativistic effects and spin–orbit coupling. Nevertheless, computational methods are now capable of describing the ladder of electronic energy levels of the simpler trivalent lanthanide ions, as well as the lowest energy term of most of the series. The electronic energy levels result from electron configurations that are first split by spin–orbit coupling into groups of energy levels denoted by the corresponding Russell–Saunders terms. Each of these groups are then split by the ligand field into the actual electronic energy levels known as microstates or sometimes m J levels. The ligand-field splitting directly informs on the coordination geometry and is a valuable tool for determining the structure and thus correlating the structure and properties of metal complexes in solution. The issue with lanthanide complexes is that the determination of complex structures from ligand-field splitting remains a very challenging task. In this paper, the optical spectraabsorption, luminescence excitation, and luminescence emissionof ytterbium­(III) solvates were recorded in water, methanol, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF). The electronic energy levels, that is, the microstates, were resolved experimentally. Subsequently, density functional theory calculations were used to model the structures of the solvates, and ab initio relativistic complete active space self-consistent field calculations (CASSCF) were employed to obtain the microstates of the possible structures of each solvate. By comparing the experimental and theoretical data, it was possible to determine both the coordination number and solution structure of each solvate. In water, methanol, and N,N-dimethylformamide, the solvates were found to be eight-coordinated and have a square antiprismatic coordination geometry. In DMSO, the speciation was found to be more complicated. The robust methodology developed for comparing experimental spectra and computational results allows the solution structures of homoleptic lanthanide complexes to be determined.

Lanthanide luminescence has been treated separate from molecular photophysics, although the underlying phenomena are the same. As the optical transitions observed in the trivalent lanthanide ions are forbidden, they do belong to the group that molecular photophysics have yet to conquer, yet the experimental descriptors remains valid. Determining these have proven challenging as full control/knowledge of sample composition is a prerequisite. This has been achieved, and here the luminescence quantum yields (ϕlum), luminescence lifetimes (τobs), oscillator strengths (f ), and the rates of non-radiative (knr) and radiative (kr ≡ A) deactivation of [Eu(H2O)9]3+ was determined for the trigonal tricapped prismatic (TTP) coordination geometry. Further, it was shown that instead of a full photophysical characterization, it is possible to relate changes in transition probabilities to the relative parameter Arel, which does not require reference data. While Arel does not afford comparisons between experiments, it resolves emission intensity changes due to emitter properties—changes in A—from intensity changes due to environmental effects—changes in knr, and differences in the number of photons absorbed. When working with fluorescence this may seem trivial, when working with lanthanide luminescence it is not.