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

In nature, the elements of the inorganic part of the periodic table are found in three forms: metals, ions in salts & minerals, and ions in solution. The ions may be coordinated to simple or complicated ligands. They may form purely electrostatic or partially covalent bonds. A common trend is that the more covalent bonds an element form, the more we know of its physicochemical properties. The rare earths form purely electrostatic bonds, thus, our understanding of the solution chemistry of these elements is limited-yet important. Most rare earth elements used today pass through hydrometallurgical processes that rely on the solution chemistry of these elements, even through the critical applications are in alloys and functional materials. Through developments in optical spectroscopy, total X-ray scattering, and quantum chemical methods we are posed to remedy this situation: we are ready to create predictive structure-property relationships in the field of lanthanide solution chemistry. The scope of this review is to summarise the state-of-the-art for neodymium(III), to go through the structure-property relationships that are in use. In the form of NdFeB magnets, neodymium plays a crucial role in green energy production and electric propulsion. As a 4f3 ion in solution it is also one of the simpler rare earth ions, and the Nd(III) ion has characteristic optical properties that can be exploited as a handle in physicochemical studies. Here, we start with a critical review of the current concepts used to relate structure and electronic energy levels. We follow with our suggested approach of using the methodology from molecular photophysics to relate optical properties and structure, and conclude with selected literature examples.

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

Solution chemistry of the lanthanide(III) ions is unexplored and relevant: extraction and recycling processes exclusively operate in solution, MRI is a solution-phase method, and bioassays are done in solution. However, the molecular structure of the lanthanide(III) ions in solution is poorly described, especially for the near-IR (NIR)-emitting lanthanides, as these are difficult to investigate using optical tools, which has limited the availability of experimental data. Here we report a custom-built spectrometer dedicated to investigation of lanthanide(III) luminescence in the NIR region. Absorption, luminescence excitation, and luminescence spectra of five complexes of europium(III) and neodymium(III) were acquired. The obtained spectra display high spectral resolution and high signal-to-noise ratios. Using the high-quality data, a method for determining the electronic structure for the thermal ground states and emitting states is proposed. It combines Boltzmann distributions with population analysis and uses the experimentally determined relative transition probabilities from both excitation and emission data. The method was tested on the five europium(III) complexes and was used to resolve the electronic structures of the ground state and the emitting state of neodymium(III) in five different solution complexes. This is the first step toward correlating optical spectra with chemical structure in solution for NIR-emitting lanthanide complexes.

A high-sensitivity rapid acquisition spectrometer for lanthanide(III) luminescence.

Detecting luminescence beyond 750-800 nm becomes problematic as most conventional detectors are less sensitive in this range, and as simple corrections stops being accurate. Lanthanide luminescence occurs in narrow bands across the spectrum from 350-2000 nm. The most emissive lanthanide(III) ions have bands from 450 nm to 850 nm, some with additional bands in the NIR. Investigating NIR bands are hard, but the difficulties already start at 700 nm. In general, the photon flux from lanthanide(III) emitters is not great, and the bands beyond 700 nm are very weak, we therefore decided to build a spectrometer based on cameras for microscopy with single-photon detection capabilities. This was found to allieviate all limitations and to allow for fast and efficient recording of luminescence spectra in the range from 450 to 950 nm. The spectrometer characteristics were investigated and the performance was benchmarked against two commercial spectrometers. We conclude that this spectrometer is ideal for investigating lanthanide luminescence, and all other emitters with emission in the target range.

The effect of weighted averages when determining the speciation and structure-property relationships of europium(iii) dipicolinate complexes.

Lanthanide(iii) coordination chemistry in solution is inherently complicated by the lack of directional interactions and rapid ligand exchange. The latter can be eliminated in kinetically inert complexes, but remains a challenge in complexes between lanthanide(iii) ions and smaller ligands. As multiple conformations and partial decomplexation is an issue even with multidentate ligands, it will influence the observed solution properties of complexes of smaller ligands common in the field of f-elements coordination chemistry such as acetylacetonates and dipicolinates. Here, europium(iii) complexes with one, two and three dipicolinates were investigated in a series of 13 samples, where the composition was varied from 0 to 3 equivalents of dipicolinate. While the results did show the formation of three distinct europium(iii) dipicolinate complexes confirming the literature data on the system, clear discrepancies in speciation related properties were evident when comparing the results from absorption and luminescence spectroscopy. It was concluded that the difference is due to the difference in time constant of the two experiments. Furthermore, it is shown that the information obtained from luminescence arises from a weigthed average, and with discepancies between the observed and actual concentration exceeding 25%, it is advised that the weighted averages are taken into consideration when reporting on solution properties of lanthanide(iii) complexes. From the resolved optical spectra of [Eu(H2O)9]3+, [Eu(DPA)(H2O)6]+, [Eu(DPA)2(H2O)3]-, and [Eu(DPA)3]3-, the excited energy levels and transition probabilities are determined, and it was concluded that both transition probabilities and ligand field effects on the microstates are different in all four species.

Using Polarized Spectroscopy to Investigate Order in Thin-Films of Ionic Self-Assembled Materials Based on Azo-Dyes.

Three series of ionic self-assembled materials based on anionic azo-dyes and cationic benzalkonium surfactants were synthesized and thin films were prepared by spin-casting. These thin films appear isotropic when investigated with polarized optical microscopy, although they are highly anisotropic. Here, three series of homologous materials were studied to rationalize this observation. Investigating thin films of ordered molecular materials relies to a large extent on advanced experimental methods and large research infrastructure. A statement that in particular is true for thin films with nanoscopic order, where X-ray reflectometry, X-ray and neutron scattering, electron microscopy and atom force microscopy (AFM) has to be used to elucidate film morphology and the underlying molecular structure. Here, the thin films were investigated using AFM, optical microscopy and polarized absorption spectroscopy. It was shown that by using numerical method for treating the polarized absorption spectroscopy data, the molecular structure can be elucidated. Further, it was shown that polarized optical spectroscopy is a general tool that allows determination of the molecular order in thin films. Finally, it was found that full control of thermal history and rigorous control of the ionic self-assembly conditions are required to reproducibly make these materials of high nanoscopic order. Similarly, the conditions for spin-casting are shown to be determining for the overall thin film morphology, while molecular order is maintained.