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The Tb(III) ion has the most intense luminescence of the trivalent lanthanide(III) ions. In contrast to Eu(III), where the two levels only include a single state, the high number of electronic states in the ground (7 F 6 ) and emitting ( 5 D4 ) levels makes detailed interpretations of the electronic structure—the crystal field— difficult. Here, luminescence emission and excitation spectra of Tb(III) complexes with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA, [Tb(DOTA)(H2O)]−), ethylenediaminetetraacetic acid (EDTA, [Tb(EDTA)(H2 O)3]−) and diethylenetriaminepentaacetic acid (DTPA, [Tb(DTPA)(H2O)]2−) as well as the Tb(III) aqua ion ([Tb(H2 O)9 ] 3+ ) were recorded at room temperature and in frozen solution. Using these data the electronic structure of the 5 D4 multiplets of Tb(III) was mapped by considering the transitions to the singly degenerate 7 F0 state. A detailed spectroscopic investigation was performed and it was found that the 5 D4 multiplet could accurately be described as a single band for [Tb(H2 O)9 ]3+ , [Tb(DOTA)(H 2O)]− and [Tb (EDTA)(H 2O)3 ]−. In contrast, for [Tb(DTPA)(H2 O)]2− two bands were needed. These results demonstrated the ability of describing the electronic structure of the emitting 5 D4 multiplet using emission spectra. This offers an avenue for investigating the relationship between molecular structure and luminescent properties in detailed photophysical studies of Tb(III) ion complexes.

The development of new materials for advanced technologies plays a significant role in addressing the global energy crisis. While the formation of solid-state material from aqueous solution is fundamental to many natural phenomena, and widely incorporated in various industrial processes, our understanding of the mechanisms remains limited. The properties of a material strongly depend on atomic structure, nanostructure, microstructure, and surface structure. Hence, understanding the mechanisms behind the formation of solid materials is crucial for gaining complete control over the process, and being able to tailor-make new materials with special properties for state-of-the-art technologies. Thus, scientists in various research fields have shown significant interest in studying the formation processes. This study aims to elucidate the mechanism involved in the formation of crystalline neodymium(III) sulfate by utilising the unique spectral properties of lanthanides, combined with total X-ray scattering. The formation process was investigated applying four different in situ techniques. Light scattering was utilised to follow the formation of particles on a macroscopic level, absorption and emission spectroscopy were used to track the changes in the coordination environment around the neodymium(III) ion, while total X-ray scattering allowed the observation of changes in the atomic structure in real space. By employing these techniques, insight into different aspects of the formation mechanism was gained.

The photophysics of a europium(III) complex of 1,4,7,10-tetraazacycododecane-1,4,7-triacetic acid-10-(2-methylene)-1-azathioxanthone was investigated in three buffer systems and at three pH values. The buffers—phosphate buffered saline (PBS), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and universal buffer (UB)—had no effect on the europium luminescence, but a lower overall emission intensity was determined in HEPES. It was found that this was due to quenching of the 1-azathioxanthone first excited singlet state by HEPES. The effect of pH on the photophysics of the complex was found to be minimal, and protonation of the pyridine nitrogen was found to be irrelevant. Even so, pH was shown to change the intensity ratio between 1-azathioxanthone fluorescence and europium luminescence. It was concluded that the full photophysics of a potential molecular probe should be investigated to achieve the best possible results in any application.

Halide recognition by supramolecular receptors and coordination complexes in water is a long-standing challenge. In this work, we report chloride binding in water and in competing media by pre-organised binuclear kinetically inert lanthanide complexes, bridged by flexible -(CH2)2- and -(CH2)3- spacers, forming [Ln2(DO3A)2C-2] and [Ln2(DO3A)2C-3], respectively. These hydrophilic, neutral lanthanide coordination complexes are shown to bind chloride with apparent association constants of up to 105 M−1 in water and in buffered systems. Hydroxide bridging was observed in these complexes at basic pH, which was proven to be overcome by chloride. Thus, these lanthanide complexes show promise towards chloride recognition in biology and beyond. The results described here have clearly identified a new area of anion coordination chemistry that is ripe for detailed exploration.

Two compounds, Na3[Eu(DPA)3] ⋅ 14H2O and [Eu(DPA)(HDPA)(H2O)2] ⋅ 4H2O, were created and the structure determined using single crystal X-ray diffraction. The single crystal luminescence properties were compared and related to the Eu3+ coordination geometry. The formation of single crystals from solutions of Eu(CF3SO3)3 and H2DPA was found change with the pH value of the H2DPA solution. Mixtures of Na3[Eu(DPA)3] ⋅ 14H2O and [Eu(DPA)(HDPA)(H2O)2] ⋅ 4H2O were observed with a pH ratio between the two structures. While visual inspection showed that all samples contained both Na3[Eu(DPA)3] ⋅ 14H2O and [Eu(DPA)(HDPA)(H2O)2] ⋅ 4H2O, the PXRD and luminescence data did not immediately reveal that the samples were pure. Having discovered that the samples were indeed mixtures, quantification was attempted by Rietveld refinement of the PXRD data, and the luminescence spectra were compared to those from single crystals. As the data was not found to reveal that the samples were mixtures, even though we knew that this was the case, we must urge caution when inferring structure-property relationships from powder samples. In this case we were able to isolate monophasic systems and do a comparative study, but this requires that the samples are identified as mixtures.

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.

Methods Polystyrene nanoparticles (PS NPs) were fabricated from an emulsion of PS/toluene in water using several surfactants, and purified via dialysis in a simple procedure. The synthesis process was carried out at room temperature, without hazardous chemicals, and with a workload of 5 h. All relevant parameters–surfactant type, component concentration, solvent volume, polymer chain length, sonication time–were varied and the effect on the size of the resulting PS NPs were determined. A robust PS NP synthesis procedure was developed, repeated, and tested by three independent researches. The procedure was up-scaled to prove the applicability of the method and the NPs were prepared with four different hydrophobic dyes. Results All products were found to be comparable, and we conclude that the method can provide PS NPs with or without dye dopants with an average diameter of 25 nm in a reproducible size distribution.

Optical sensors hold the promise of providing the coupling between the tangible and the digital world that we are currently experiencing. The core of optical sensor development lies in materials development, where specific requirements of opposing physicochemical properties create a significant obstacle. The sensor material must provide dye retention, while ensuring porosity for analyte transport. The sensor material must provide hydrophobic pockets for dyes to ensure high signal intensity, while remaining fully hydrophilic to measure in water. We have previously reported optical sensors, where we compromised on sensor manufacturing by using a double-layer composite. Here, we report a composite organically modified sol–gel (ORMOSIL) polymer, where polystyrene (PS) nanoparticles (NPs) have been incorporated. This allows all the opposing requirements on optical sensor materials to be fulfilled, and by introducing a hydrophobic reference dye in the fully hydrophobic compartments of the sensor material. We show that we can incorporate any hydrophobic fluorophore in this material, even those which are suffering from quenching in water. In this work, PS NPs with 1,13-dimethoxyquinacridinium (DMQA) were immobilized in a composite sol–gel material with pH responsive diazaoxatriangulenium (DAOTA) dyes prior to curing. The multicomponent sensor composite was cured on a polycarbonate hemiwicking substrate, and the resulting fluorescence intensity ratiometric optical pH sensor was shown to have excellent performance. We expect that this type of composite sensor materials will allow the creation of next generation industrial chemosensors.

Lanthanide luminescence fascinates with a complicated electronic structure and "forbidden" transitions. By studying the photophysics of lanthanide(III) solvates, a close to ideal average coordination geometry can be used to map both electronic energy levels and transition probabilities. Some lanthanide(III) ions are simpler to study than others, and samarium(III) belongs to the more difficult ones. The 4f5system has numerous absorption and emission lines in the visible and infrared parts of the spectrum and in this work, the energy levels giving rise to these transitions were mapped, the transition probability between them was calculated, and it was shown that the electronic structures of the samarium(III) solvates in DMSO, MeOH, and water are different.

As part of moving our optical pH and dissolved oxygen (DO) optical chemosensors toward industrial applications, we decided to explore a many-sensors-in-one principle. It was tested if physical segregation of the optical sensor components in a single sensor polymer could remove cross-talk and quenching. It was found that a design concept with an oxygen-responsive dye in polymer nanoparticles and a pH-responsive dye in an organically modified siloxane polymer resulted in a robust pH/O2 dual optical sensor. Individually, the O2-sensitive nanoparticles, a known component for optical DO sensing, and the pH sensor are operational. Thus, it was decided to test if nanoparticles enclosed within the pH-sensitive responsive sol-gel (i) could work together if segregated and (ii) could operate with a single intensity signal that is without a reference signal; developments within industrial optical sensor technology indicate that this should be feasible. The prototype optode produced in this work was shown to have a negligible drift over 60 h, bulk diffusion-limited DO response, and independent response to pH and O2. On the individual optode, pH calibration was found to show the expected sigmoidal shape and pKa, while the complexity of the calibration function for the DO signal was significant. While the engineering of the sensor device, optics, and hardware are not robust enough to attempt generic sensor calibration, it was decided to demonstrate the design concept in simple fermentation experiments. We conclude that the dual sensor design with the physical segregation of components is viable.

Our theoretical treatment of electronic structures in coordination complexes often rests on assumptions of symmetry. Experiments rarely provide fully symmetric systems to study. In solutions, fluctuations in solvation, variations in conformations, and even changes in constitution occur and complicate the picture. In crystals, lattice distortion, energy transfer, and phonon quenching play a role, but we are able to identify distinct symmetries. Yet the question remains: How is the real symmetry in a crystal compared to ideal symmetries?

Lanthanides are found in critical applications from display technology to renewable energy. Often, these rare earth elements are used as alloys or functional materials, yet access to them is through solution processes. In aqueous solutions, the rare earths are found predominantly as trivalent ions and charge balance dictates that counterions are present. The fast ligand exchange and lack of directional bonding in lanthanide complexes have led to questions regarding the speciation of Ln3+ solvates in the presence of various counterions and the distinction between innocent = non-coordinating and non-innocent = coordinating counterions. There is limited agreement as to which group counterions belong to, which led to this report. By using Eu3+ luminescence, it was possible to clearly distinguish between coordinating and non-coordinating ions. To interpret the results, it was required to bridge the descriptions of ion pairing and coordination. The data—in the form of Eu3+ luminescence spectra and luminescence lifetimes from solutions with varying concentrations of acetate, chloride, nitrate, sulfate, perchlorate and triflate—was contrasted to those obtained with ethylenediaminetetraacetic acid (EDTA4−), which allowed for the distinction between three Ln3+–anion interaction types. It was possible to conclude which counterions are truly innocent (e.g. ClO4− and OTf−) and which clearly coordinate (e.g. NO3− and AcO−). Finally, a considerable amount of data from systems studied under similar conditions allowed the minimum perturbation arising from the inner sphere or outer sphere coordination in Eu3+ complexes to be identified.