Photoluminescence modification of europium(III)-doped MAl2O4 (M = Zn, Mg) spinels induced by Ag@SiO2 core-shell nanoparticles

In recent years, there has been an increasing interest in developing new inorganic compounds with exceptional properties for advanced materials. Specifically, compounds containing europium have attracted much attention due to their luminescent properties. These compounds are used in electronics, biotechnology, medicine, and catalysis. Eu is known for its characteristic red emission, which can be influenced by the environment. This study investigates the surface-enhancement luminescence of europium-doped spinel oxides using modified surface with silver (Ag@SiO2 core-shell) nanoparticles as the enhancers. The europium-doped spinels were synthesized through a sol-gel method, and characterization techniques were used to analyze their structure and morphology. Photoluminescence spectra exhibited characteristic Eu3+ transitions, with the hypersensitive transition being the most prominent. The interaction with an Ag@SiO2 modified-surface led to a significant increase in photoluminescence. The study also analyzed the photoluminescence excitation and lifetimes of the oxides, leading to a 7.3-fold increase in photoluminescence. The improvements observed in the luminescence of these tailor-made materials show their potential interest in next-generation technologies.

Fine-Tuning Crystal Structures of Lead Bromide Perovskite Nanocrystals through Trace Cadmium(II) Doping for Efficient Color-Saturated Green LEDs

Decreasing perovskite nanocrystal size increases radiative recombination due to the quantum confinement effect, but also increases the Auger recombination rate which leads to carrier imbalance in the emitting layers of electroluminescent devices. Here, we overcome this trade-off by increasing the exciton effective mass without affecting the size, which is realized through the trace Cd2+ doping of formamidinium lead bromide perovskite nanocrystals. We observe an ~2.7 times increase in the exciton binding energy benefiting from a slight distortion of the [BX6]4- octahedra caused by doping in the case of that the Auger recombination rate is almost unchanged. As a result, bright color-saturated green emitting perovskite nanocrystals with a photoluminescence quantum yield of 96 % are obtained. Cd2+ doping also shifts up the energy levels of the nanocrystals, relative to the Fermi level so that heavily n-doped emitters convert into only slightly n-doped ones; this boosts the charge injection efficiency of the corresponding light-emitting diodes. The light-emitting devices based on those nanocrystals reached a high external quantum efficiency of 29.4 % corresponding to a current efficiency of 123 cd A-1, and showed dramatically improved device lifetime, with a narrow bandwidth of 22 nm and Commission Internationale de I'Eclairage coordinates of (0.20, 0.76) for color-saturated green emission for the electroluminescence peak centered at 534 nm, thus being fully compliant with the latest standard for wide color gamut displays.

Contrasting impact of coordination polyhedra and site symmetry on the electronic energy levels in nine-coordinated Eu(III) and Sm(III) crystals structures determined from single crystal luminescence spectra

Lanthanide luminescence is characterised by "forbidden" 4f-4f transitions and a complicated electronic structure. Our understanding of trivalent lanthanide(III) ion luminescence is centered on Eu3+ because absorbing and emitting transitions in Eu3+ occur from a single electronic energy level. In Sm3+ both absorbing and emitting multiplets have a larger multiplicity. A band arising in transitions from the first emitting state multiplet to the ground state multiplet will have nine lines for a Sm3+ complex. In this study, high-resolution emission and excitation spectra were used to determine the electronic energy levels for the lowest multiplet and first emitting multiplet in four Sm3+ compounds with either tricapped trigonal prismatic TTP or capped square antiprismatic cSAP coordination polyhedra but different site symmetry. This was achieved by the use of Boltzmann distribution population analysis and experimentally determined transition probabilities from emission and excitation spectra. Using this analysis it was possible to show the effect of changing three oxygen atoms with three nitrogen atoms in the donor set for two compounds with the same coordination polyhedra and site symmetry. This work celebrates the 40th anniversary of Kirby and Richardson's first report of [Eu(ODA)3]3- luminescence.

Step-wise changes in the excited state lifetime of [Eu(D2O)9]3+ and [Eu(DOTA)(D2O)]- as a function of the number of inner-sphere O-H oscillators

Lanthanide luminescence is dominated by quenching by high-energy oscillators in the chemical environment. The rate of non-radiative energy transfer to a single H2O molecule coordinated to a Eu3+ ion exceeds the usual rates of emission by an order of magnitude. We know these rates, but the details of these energy transfer processes are yet to be established. In this work, we study the quenching rates of [Eu(D2O)9]3+ and [Eu(DOTA)(D2O)]- in H2O/D2O mixtures by sequentially exchanging the deuterons with protons. Flash freezing the solutions allows us to identify species with various D/H contents in solution and thus to quantify the energy transfer processes to individual OH-oscillators. This is not possible in solution due to fast exchange in the ensembles present at room temperature. We conclude that the energy transfer processes are accurately described, predicted, and simulated using a step-wise addition of the rates of quenching by each O-H oscillator. This documents the sequential increase in the rate of the energy transfer processes in the quenching of lanthanide luminescence, and further provides a methodology to identify isotopic impurities in deuterated lanthanide systems in solution.

A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds

Trivalent europium (Eu3+) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu3+-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu3+:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu3+ ion, can directly affect the optical properties of Eu3+. We review the luminescence properties of Eu3+ ions in mixed-anion compounds of the oxynitride YSiO2N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the 5D0 → 7F0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu3+ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu3+-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu3+-doped phosphors are presented.

Manipulation of Luminescence via Surface Site Occupation in Ln3+-Doped Nanocrystals

Ln3+-doped (Ln = lanthanide) nanocrystals are garnering strong interest for their potential as optical materials in various applications. For that reason, a thorough understanding of photophysical processes and ways to tune them in these materials is of great importance. This study, using Eu3+-doped Sr2YF7 as a well-suited model system, underscores the (not unexpected) significance of surface site occupation of Ln3+ and also challenges the prevailing views about their contribution to the luminescence of the system. High-temperature cation exchange and epitaxial shell growth allow nanocrystals to exclusively feature Eu3+ residing at the surface or in the interior, thereby separating their spectral responses. Meticulous experiments reveal that nanocrystals with high doping concentrations exhibit luminescence primarily from surface Eu3+, in contrast to the popular belief that luminescence from surface Ln3+ is largely negligible. The present study shows, on the one hand, the necessity to revise common ideas and also reveals the potential for manipulating the luminescence of such materials through an, until now, unperceived way of surface engineering.

Mapping the distribution of electronic states within the 5D4 and 7F6 levels of Tb3+ complexes with optical spectroscopy

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 (7F6) and emitting (5D4) 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)(H2O)3]-) and diethylenetriaminepentaacetic acid (DTPA, [Tb(DTPA)(H2O)]2-) as well as the Tb(III) aqua ion ([Tb(H2O)9]3+) were recorded at room temperature and in frozen solution. Using these data the electronic structure of the 5D4 multiplets of Tb(III) was mapped by considering the transitions to the singly degenerate 7F0 state. A detailed spectroscopic investigation was performed and it was found that the 5D4 multiplet could accurately be described as a single band for [Tb(H2O)9]3+, [Tb(DOTA)(H2O)]- and [Tb(EDTA)(H2O)3]-. In contrast, for [Tb(DTPA)(H2O)]2- two bands were needed. These results demonstrated the ability of describing the electronic structure of the emitting 5D4 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.

Design Rules, Accurate Enthalpy Prediction, and Synthesis of Stoichiometric Eu3+ Quantum Memory Candidates

Stoichiometric Eu3+ compounds have recently shown promise for building dense, optically addressable quantum memory as the cations' long nuclear spin coherence times and shielded 4f electron optical transitions provide reliable memory platforms. Implementing such a system, though, requires ultranarrow, inhomogeneous linewidth compounds. Finding this rare linewidth behavior within a wide range of potential chemical spaces remains difficult, and while exploratory synthesis is often guided by density functional theory (DFT) calculations, lanthanides' 4f electrons pose unique challenges for stability predictions. Here, we report DFT procedures that reliably reproduce known phase diagrams and correctly predict two experimentally realized quantum memory candidates. We are the first to synthesize the double perovskite halide Cs2NaEuF6. It is an air-stable compound with a calculated band gap of 5.0 eV that surrounds Eu3+ with mononuclidic elements, which are desirable for avoiding inhomogeneous linewidth broadening. We also analyze computational database entries to identify phosphates and iodates as the next generation of chemical spaces for stoichiometric quantum memory system studies. This work identifies new candidate platforms for exploring chemical effects on quantum memory candidates' inhomogeneous linewidth while also providing a framework for screening Eu3+ compound stability with DFT.

Enhancing the Circularly Polarized Luminescence of Europium Coordination Polymers by Doping a Chromophore Ligand into Superhelices

Lanthanide-based molecular materials showing efficient circularly polarized luminescence (CPL) activity with a high quantum yield are attractive due to their potential applications in data storage, optical sensors, and 3D displays. Herein we present an innovative method to achieve enhanced CPL activity and a high quantum yield by doping a chromophore ligand into a coordination polymer superhelix. A series of homochiral europium(III) phosphonates with a helical morphology were prepared with the molecular formula S-, R-[Eu(cyampH)3-3n(nempH)3n]·3H2O (S/R-Eu-n, n = 0-5%). The doping of chromophore ligand S- or R-nempH2 into superhelices of S/R-Eu-0% not only turned on the CPL activity with the dissymmetry factor |glum| on the order of 10-3 but also increased the quantum yield by about 14-fold. This work may shed light on the development of efficient CPL-active lanthanide-based coordination polymers for applications.

Equilibrium Thermodynamics of Macropa Complexes with Selected Metal Isotopes of Radiopharmaceutical Interest

To pursue the design of in vivo stable chelating systems for radiometals, a concise and straightforward method toolbox was developed combining NMR, isothermal titration calorimetry (ITC), and europium time-resolved laser-induced fluorescence spectroscopy (Eu-TRLFS). For this purpose, the macropa chelator was chosen, and Lu3+, La3+, Pb2+, Ra2+, and Ba2+ were chosen as radiopharmaceutically relevant metal ions. They differ in charge (2+ and 3+) and coordination properties (main group vs lanthanides). 1H NMR was used to determine four pKa values (±0.15; carboxylate functions, 2.40 and 3.13; amino functions, 6.80 and 7.73). Eu-TRLFS was used to validate the exclusive existence of the 1:1 Mn+/ligand complex in the chosen pH range at tracer level concentrations. ITC measurements were accomplished to determine the resulting stability constants of the desired complexes, with log K values ranging from 18.5 for the Pb-mcp complex to 7.3 for the Lu-mcp complex. Density-functional-theory-calculated structures nicely mirror the complexes' order of stabilities by bonding features. Radiolabeling with macropa using ligand concentrations from 10-3 to 10-6 M was accomplished by pointing out the complex formation and stability (212Pb > 133La > 131Ba ≈ 224Ra > 177Lu) by means of normal-phase thin-layer chromatography analyses.

Eu3+ Site Distribution and Local Distortion of Photoluminescent Ca3 WO6 :(Eu3+ , K+ ) Double Perovskites as High-Color-Purity Red Phosphors

In this study, Eu3+ and K+ co-doped Ca3 WO6 double perovskite, a high-color-purity red phosphor, is quantitatively investigated using synchrotron X-ray diffraction Rietveld refinement, energy dispersive X-ray spectroscopy, and high-resolution PL spectroscopy. The Eu3+ fluorescence line-narrowing (FLN) results, used to estimate the Eu3+ occupation at given sites (so-called A and B sites) in the host crystal (A2 BMO6 ; A, B = Ca; M = W), reveal that the Eu3+ ions have a twin distribution in both the A and B sites with high asymmetry ratios of ΛA  = 9.7 and ΛB  = 10.7, respectively. More interestingly, at lower Eu3+ doping levels, the ions are predominantly located at the B sites (≈75%), indicating that the high color purity of Ca3 WO6 :(Eu3+ , K+ ) is mainly caused by the high asymmetry ratios of the Eu3+ sites. Rietveld refinement combined with the FLN technique provides more reliable results for increasing the Eu3+ substitution at the A site with Eu3+ and K+ doping concentrations, which lower the lattice energy of Ca3 WO6 :(Eu3+ , K+ ). The structural distortions owing to K+ co-doping in terms of the quadratic elongation and bond-angle variance exhibit good correlations with the Ca(Eu)O12 A-site cuboctahedra and Ca(Eu)O6 B-site octahedra, partially accounting for the higher Λ values.

Eu3+ ions as a crystal-field probe for low-symmetry sites in doped phosphors - a case study: Eu3+ at triclinic sites in Li6RE(BO3)3 (RE = Y, Gd), YBO3 and ZnO and at trigonal sites in YAl3(BO3)4

We present crystal-field (CF) calculations of energy levels (Ei) of Eu3+ ions doped in various hosts aimed at exploring the low-symmetry properties of CF parameters (CFPs) and reliability of CFP modelling with decreasing site symmetry. The hosts studied are: Li6Y(BO3)3, Li6Gd(BO3)3, YBO3, and ZnO with Eu3+ at triclinic sites; YAl3(BO3)4 with Eu3+ ions at trigonal D3 symmetry. Two independent CFP modelling approaches utilizing the hosts' structural data are employed: the exchange charge model (ECM) and the superposition model (SPM). We adopt the Eu3+ actual site symmetry and not the approximated one. The Ei values calculated using CFPs modelled by the ECM and SPM mutually agree with the observed ones. For triclinic symmetry, the ECM/CFPs and SPM/CFPs were numerically distinct, yet turned out to be physically equivalent yielding identical rotational invariants, Sk (k = 2, 4, 6) and Ei. For trigonal symmetry, both CFP sets agree numerically, thus Sk and Ei are identical. This disparity poses a dilemma, since the modified crystallographic axis system was used in both approaches. The standardization of the triclinic CFPs using the 3DD package was performed to solve this dilemma. It has enabled discussing standardization aspects in experimental and computed CFP sets and elucidating intricate low-symmetry aspects inherent in CFP sets. Understanding of low-symmetry aspects in CF studies may bring about a better interpretation of the spectroscopic and magnetic properties of rare-earth ion doped host crystals. Thus, our study could provide more deep insights into the importance of clear definitions of axis systems and adequate treatment of actual site symmetry in the modelling of CFPs for low-symmetry cases which is essential for technological applications and engineering of rare-earth activated phosphor materials.

Exploring the Influence of ZnF2 on Zinc-Tellurite Glass: Unveiling Changes in OH Content, Structure, and Optical Properties

Tellurite glasses have garnered considerable interest as optical host materials due to their advantageous properties, including low processing temperature, high resistance to corrosion and crystallization, and excellent solubility for rare earth ions. However, their applicability in the infrared (IR) region is limited by the absorption of species with distinct vibrations. The incorporation of fluorides has emerged as a promising approach to reduce hydroxyl (OH) absorption during the precursor melting process. In this study, we investigated the influence of ZnF2 on a glass matrix composed of TeO2-ZnO-Na2O, resulting in notable changes in the glass structure and optical properties, with Eu3+ serving as an environmental optical probe. The samples underwent comprehensive structural, thermal, and optical characterization. Structural analyses encompassed 19F and 125Te nuclear magnetic resonance (NMR), with the latter being complemented by mathematical simulations, and these findings were consistent with observations from Raman scattering. The main findings indicate an enhancement in thermal stability, modifications in the Te-O connectivity, and a reduction in emission intensity attributed to the effects of ligand polarizability and symmetry changes around Eu3+. Additionally, the fluorotellurite matrices exhibited a shift in the absorption edge toward higher energies, accompanied by a decrease in mid-IR absorptions, thereby expanding the transparency window. As a result, these glass matrices hold substantial potential for applications across various regions of the electromagnetic spectrum, including optical fiber drawing and the development of solid-state emitting materials.

A Photoluminescence Study of Eu3+, Tb3+, Ce3+ Emission in Doped Crystals of Strontium-Barium Fluoride Borate Solid Solution Ba4-xSr3+x(BO3)4-yF2+3y (BSBF)

The present study is aimed at unveiling the luminescence potential of Ba4-xSr3+x(BO3)4-yF2+3y (BSBF) crystals doped with Eu3+, Tb3+, and Ce3+. Owing to the incongruent melting character of the phase, the NaF compound was used as a solvent for BSBF crystal growth. The structure of BSBF: Eu3+ with Eu2O3 concentration of about 0.7(3) wt% was solved in the non-centrosymmetric point group P63mc. The presence of Eu2O3 in BSBF: Eu3+ leads to a shift of the absorption edge from 225 nm to 320 nm. The photoluminescence properties of the BSBF: Ce3+, BSBF: Tb3+, BSBF: Eu3+, and BSBF: Eu3+, Tb3+, Ce3+ crystals have been studied. The unusual feature of europium emission in BSBF is the intensively manifested 5D0→7F0 transition at about 574 nm, which is the strongest for BSBF: Eu3+ at 370 nm excitation and for BSBF: Eu3+, Tb3+, Ce3+ at 300 nm and 370 nm excitations. No evidence of Tb3+→Eu3+ energy transfer was found for BSBF: Eu3+, Tb3+, Ce3+. The PL spectra of BSBF: Eu3+ at 77 and 300 K are similar with CIE chromaticity coordinates of (0.617; 0.378) at 300 nm excitation and (0.634; 0.359) at 395 nm excitation and low correlated color temperature which implies application prospects in the field of lightning. Due to the high intensity of 5D0→7F0 Eu3+ transition at 370 nm excitation, the BSBF: Eu3+ emission is yellow-shifted.

Negative Thermal Expansion in the Polymorphic Modification of Double Sulfate β-AEu(SO4)2 (A-Rb+, Cs+)

New polymorphic modifications of double sulfates β-AEu(SO₄)₂ (A-Rb⁺, Cs⁺) were obtained by the hydrothermal method, the structure of which differs significantly from the monoclinic modifications obtained earlier by solid-state methods. According to single-crystal diffraction data, it was found that the compounds crystallize in the orthorhombic system, space group Pnna, with parameters β-RbEu(SO₄)₂: a = 9.4667(4) Å, b = 13.0786(5) Å, c = 5.3760(2) Å, V = 665.61(5) ų; β-CsEu(SO4)2: a = 9.5278(5) Å, b = 13.8385(7) Å, c = 5.3783(3) Å, V = 709.13(7) ų. The asymmetric part of the unit cell contains one-half Rb⁺/Cs⁺ ion, one-half Eu³⁺ ion, both in special sites, and one SO₄^2- ion. Both compounds exhibit nonlinear negative thermal expansion. According to the X-ray structural analysis and theoretical calculations, the polarizing effect of the alkali metal ion has a decisive influence on the demonstration of this phenomenon. Experimental indirect band gaps of β-Rb and β-Cs are 4.05 and 4.11 eV, respectively, while the direct band gaps are 4.48 and 4.54 eV, respectively. The best agreement with theoretical calculations is obtained using the ABINIT package employing PAW pseudopotentials with hybrid PBE0 functional, while norm-conserving pseudopotentials used in the frame of CASTEP code and LCAO approach in the Crystal package gave worse agreement. The properties of alkali ions also significantly affect the luminescent properties of the compounds, which leads to a strong temperature dependence of the intensity of the ⁵D₀ → ⁷F₄ transition in β-CsEu(SO₄)₂ in contrast to much weaker dependence of this kind in β-RbEu(SO₄)₂.

Highly efficient light harvesting of a Eu(iii) complex in a host-guest film by triplet sensitization

Trivalent lanthanide complexes are attractive light emitters owing to their ideal high color purity. Sensitization using ligands with high absorption efficiency is a powerful approach to enhancing photoluminescence intensity. However, the development of antenna ligands that can be used for sensitization is limited due to difficulties in controlling the coordination structures of lanthanides. When compared to conventional luminescent Eu(iii) complexes, a system composed of triazine-based host molecules and Eu(hfa)₃(TPPO)₂ (hfa: hexafluoroacetylacetonato and TPPO: triphenylphosphine oxide) significantly increased total photoluminescence intensity. Energy transfer from the host molecules to the Eu(iii) ion occurs via triplet states over several molecules, according to time-resolved spectroscopic studies, with nearly 100% efficiency. Our discovery paves the way for efficient light harvesting of Eu(iii) complexes with simple fabrication using a solution process.

Novel Red Phosphor of Gd3+, Sm3+ co-Activated AgxGd((2-x)/3)-0.3-ySmyEu3+0.30☐(1-2x-2y)/3WO4 Scheelites for LED Lighting

Gd³⁺ and Sm³⁺ co-activation, the effect of cation substitutions and the creation of cation vacancies in the scheelite-type framework are investigated as factors influencing luminescence properties. Ag_xGd_{((2-x)/3)-0.3-y}Sm_yEu³⁺_0.3☐_(1-2x)/3WO₄ (x = 0.50, 0.286, 0.20; y = 0.01, 0.02, 0.03, 0.3) scheelite-type phases (A_xGS_yE) have been synthesized by a solid-state method. A powder X-ray diffraction study of A_xGS_yE (x = 0.286, 0.2; y = 0.01, 0.02, 0.03) shows that the crystal structures have an incommensurately modulated character similar to other cation-deficient scheelite-related phases. Luminescence properties have been evaluated under near-ultraviolet (n-UV) light. The photoluminescence excitation spectra of A_xGS_yE demonstrate the strongest absorption at 395 nm, which matches well with commercially available UV-emitting GaN-based LED chips. Gd³⁺ and Sm³⁺ co-activation leads to a notable decreasing intensity of the charge transfer band in comparison with Gd³⁺ single-doped phases. The main absorption is the ⁷F₀ → ⁵L₆ transition of Eu³⁺ at 395 nm and the ⁶H_5/2 → ⁴F_7/2 transition of Sm³⁺ at 405 nm. The photoluminescence emission spectra of all the samples indicate intense red emission due to the ⁵D₀ → ⁷F₂ transition of Eu³⁺. The intensity of the ⁵D₀ → ⁷F₂ emission increases from ~2 times (x = 0.2, y = 0.01 and x = 0.286, y = 0.02) to ~4 times (x = 0.5, y = 0.01) in the Gd³⁺ and Sm³⁺ co-doped samples. The integral emission intensity of Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄ in the red visible spectral range (the ⁵D₀ → ⁷F₂ transition) is higher by ~20% than that of the commercially used red phosphor of Gd₂O₂S:Eu³⁺. A thermal quenching study of the luminescence of the Eu³⁺ emission reveals the influence of the structure of compounds and the Sm³⁺ concentration on the temperature dependence and behavior of the synthesized crystals. Ag_0.286Gd_0.252Sm_0.02Eu_0.30WO₄ and Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄, with the incommensurately modulated (3 + 1)D monoclinic structure, are very attractive as near-UV converting phosphors applied as red-emitting phosphors for LEDs.

Tuning the optical and magnetic properties of lanthanide single-ion magnets using nitro-functionalized trispyrazolylborate ligands

We report the synthesis, crystal structures, photophysical and magnetic properties of 11 novel lanthanide complexes with the asymmetrically functionalized trispyrazolylborate ligand 4-nitrotrispyrazolylborate, 4-NO₂Tp^-: [Ln(4-NO₂Tp)₃] (Ln = La-Dy, except Pm). In-depth photophysical characterization of the ligands via luminescence, reflectance and absorption spectroscopic techniques, decay lifetimes, quantum yields supported by time-dependent density functional theory (TD-DFT) and natural bond order (NBO) analysis reveal that n-NO₂Tp^- ligands are dominated by intra-ligand charge transfer (ILCT) transitions and that second-sphere interactions are critical to the stabilization of the T₁ state of n-NO₂Tp^- ligands and hence their ability to sensitize Ln³⁺ emission. The luminescence properties of the complexes indicate that 4-NO₂Tp^- is a poor sensitizer of Ln³⁺ emission, unlike 3-NO₂Tp^-. Moreover, [Nd(4-NO₂Tp)₃] (crystallized as a hexane solvate) displays single-molecule magnet (SMM) properties, with longer relaxation times and larger barrier than the non-functionalized [NdTp₃], attributed to the addition of the NO₂-group and subsequent rigidification of the molecular structure.

Eu3+ and Tb3+ coordination compounds with phenyl-containing carbacylamidophosphates: comparison with selected Ln3+ β-diketonates

Materials based on Eu³⁺ and Tb³⁺ coordination compounds are of great interest due to their strong red and green luminescence. Appropriate selection of ligands plays a huge role in optimizing their photophysical properties. Another very helpful instrument for such optimization is theoretical modelling, which permits the prediction of the emissive properties of materials through intramolecular energy transfer analysis. The ligands that allow for achieving high efficiency of Eu³⁺ and Tb³⁺ emissions include carbacylamidophosphates (CAPh, HL). In this brief review, we summarize recent research for lanthanides CAPh-based coordination compounds of general formulas Cat[LnL]₄, [LnL₃Q] and [Ln(HL)₃(NO₃)₃], where Cat⁺ = Cs⁺, NEt4⁺, PPh₄ ⁺ and Q = 1,10-phenanthroline, 2,2-bipyridine or triphenylphosphine oxide, involving the use of thermal gravimetric analysis, X-ray analysis, and absorption and luminescence spectroscopy. We carried out a comparison with selected Ln³⁺ β-diketonates. Possibilities and developments of theoretical calculations on energy transfer rates are also presented.

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.

Synthesis of fluorescent chalcones, photophysical properties, quantitative structure-activity relationship and their biological application

The development of fluorescent pigments is an area of interest in several research fields due to their high sensitivity. In the current study-eight known and three new N,N-dimethylamino-chalcones (12a-k) were synthesized with good yields using the Claisen-Schmidt reaction. For each molecular system, the photophysical properties, including the maximum absorption wavelength (λ_Absorption), molar absorption coefficient (ε), maximum excitation wavelength (λ_Excitation), maximum emission wavelength (λ_Emission), Stokes Shift (Δλ), fluorescence quantum yield (Φ_fl), fluorescence lifetime (τ_fl), radiative and non-radiative rate constants (k_R and k_NR, respectively) were evaluated. Variations in each of these properties were analyzed depending on the substituents present on each compound. To relate the chemical structures of the synthesized compounds to their photophysical properties, Hansch analysis (2D-QSPR) was applied. As a result of Hansch analysis, we found different photophysical properties related to molecular orbitals and the energy of their derivatives (Highest Occupied Molecular Orbital-HOMO, Lowest Unoccupied Molecular Orbital-LUMO, Difference between LUMO-HOMO-ΔLH, Chemical potential-µ, Hardness-η, Softness-S, and electrophilic global index-ω) as well as to the atomic charges on atoms C₅, C_α, C_β, and CO. The application of this type of analysis has made it possible to understand and subsequently design new molecules with defined photophysical properties. Finally, the compounds were use as fluorescent pigment to get living cell imaging on breast cancer cells, obtaining the compound 12a as promissory alternative.

Design and Mechanism of Rare-Earth Singlet Oxygen Sensing: An Experimental and Quantum Chemical Approach

The sensitive detection of singlet oxygen (¹O₂) is one key issue in various photochemical analyses, reactions, and processes; it is indispensable for designing catalysts for photodynamic therapies. Corresponding fluorescence-based organic ¹O₂ monitor luminophores may be equipped with rare-earth complexes with several intrinsic advantages. The design of the necessary ligands being a tedious, time-consuming effort, often involving empirical guesswork, we decided to support our experimental work with quantum chemical calculations. Hence, next to the experimental core, this paper suggests the additional use of time-dependent density functional theory (TDDFT) on suitable, free β-diketonate ligands to devise corresponding Eu³⁺ complexes as ¹O₂ probes eventually; the free ligand calculations obviously allow profoundly reduced computational efforts. Novel β-diketonate-substituted dimethyl anthracene complexes of Eu³⁺, Tb³⁺, and Gd³⁺ and their endoperoxidized descendants were thus synthesized, compared to known related complexes and analyzed with regard to their electronic characteristics; in addition, spectroscopy of a Eu³⁺ complex with ancillary epoxiphenanthroline for subsequent attachment to biological substrates featuring -NH₂ or -SH groups was included. The spectroscopic determination of the decisive lowest triplet (T₁) states of the Gd complexes could be matched by the Tamm-Dancoff approximation (TDA)/TDDFT calculations on the free ligands satisfactorily if suitable functionals were applied. Most significantly, the results suffice to describe the luminescence "switch-on" mechanism of this complex in the presence of ¹O₂.

Luminescent Lanthanide Complexes for Effective Oxygen-Sensing and Singlet Oxygen Generation

Oxygen quantification using luminescence has attracted considerable attention in various fields, including environmental monitoring and clinical analysis. Among the reported luminophores, trivalent lanthanide complexes have displayed characteristic narrow emission bands with high brightness. This bright emission is based on photo-sensitized energy transfer via organic triplet states. The organic triplet states in lanthanide complexes effectively react with the triplet oxygen, enabling oxygen quantification by lanthanide luminescence. Some Tb^III and Eu^III complexes with slow deactivation processes have also formed the excited state equilibrium, thus resulting in the emission-lifetime based oxygen sensing property. The combination of Tb^III /Eu^III emission, Eu^III /Sm^III emission, Eu^III /ligand phosphorescence, and ligand fluorescence/ligand phosphorescence provide the ratiometric oxygen-sensing properties. Moreover, the reaction generates singlet oxygen species which exhibit numerous applications in the photo-medical field. The ligands with large π-conjugated aromatic systems, such as porphyrin, phthalocyanine, and polyaromatic compounds, induces highly efficient oxygen generation. The combination of effective luminescence with singlet-oxygen generation by the lanthanide complexes render them suitable for photo-driven theranostics. This review summarizes the research progress of lanthanide complexes with efficient oxygen-sensing and singlet-oxygen generation properties.

4f → 3d sensitization: a luminescent EuII-MnII heteronuclear complex with a near-unity quantum yield

A new heteronuclear Eu^II-Mn^II complex [Eu(N2O6)]MnBr4 (N2O6 = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is designed and synthesized, which shows an intense green emission from Mn^II with a near-unity photoluminescence quantum yield. Measurement of excited-state dynamics demonstrated the sensitization process from Eu^II to Mn^II, which represents the first example of f → d molecular sensitization. Due to the large optical absorption cross-section of the Eu^II center, [Eu(N2O6)]MnBr4 shows an emission intensity 7 to 2500 times stronger than that of the Sr^II-Mn^II control complex [Sr(N2O6)]MnBr4 upon the excitation of near ultraviolet to blue light.

X-ray Excited Optical Luminescence of Eu in Diamond Crystals Synthesized at High Pressure High Temperature

Powder diamonds with integrated europium atoms were synthesized at high pressure (7.7 GPa) and temperature (1800 °C) from a mixture of pentaerythritol with pyrolyzate of diphthalocyanine (C₆₄H₃₂N₁₆Eu) being a special precursor. In diamonds prepared by X-ray fluorescence spectroscopy, we have found a concentration of Eu atoms of 51 ± 5 ppm that is by two orders of magnitude greater than that in natural and synthetic diamonds. X-ray diffraction, SEM, X-ray exited optical luminescence, and Raman and IR spectroscopy have confirmed the formation of high-quality diamond monocrystals containing Eu and a substantial amount of nitrogen (~500 ppm). Numerical simulation has allowed us to determine the energy cost of 5.8 eV needed for the incorporation of a single Eu atom with adjacent vacancy into growing diamond crystal (528 carbons).

Preparation of Eu(III) Complexes Containing Maleopimaric Acid Anhydride with Ultra-narrow and Efficient Fluorescence Emission for High Determination on Gallic Acid in Acetonitrile Solution

Two novel Eu(III) complexes constructed by maleopimaric acid anhydride (MPA) and 2,2'-bipyridyl (Bpy) / 1, 10-phenanthroline (Phen), named as MPA-Bpy-Eu / MPA-Phen-Eu, have been synthesized by a one-step precipitation method. FTIR, UV, TG, elemental analysis, XPS, and ESI-MS revealed the successful coordination of Eu(III) ions with MPA and Bpy / Phen through Eu-O and Eu-N bonds, respectively. Introducing MPA into coordinate structure increased the electron cloud density in 3d_5/2 orbit of Eu(III) ions, enhanced the absolute quantum yields of MPA-Bpy-Eu (89.27%) and MPA-Phen-Eu (94.41%), and decreased the full width at half maxima of ⁵D₀ → ⁷F₂ transitions of MPA-Bpy-Eu (2.23 nm) and MPA-Phen-Eu (2.93 nm), respectively. The fluorescence quenching experiments showed that there was a good linear relationship between the relative fluorescence intensity of MPA-Bpy-Eu and gallic acid (GA) concentration in acetonitrile solution in the range of 0 to 8.41 × 10^-2 mM. The results of UV-vis spectra, the fluorescence lifetimes, and the quantum yields demonstrated that the dynamic quenching model played a major role in the quenching process of GA on MPA-Bpy-Eu. The quenching process of GA on MPA-Phen-Eu involved dual fluorescence quenching mechanistic pathways, including static and dynamic models.

Heterometallic Europium(III)–Lutetium(III) Terephthalates as Bright Luminescent Antenna MOFs

A new series of luminescent heterometallic europium(III)–lutetium(III) terephthalate metal–organic frameworks, namely (Eu_xLu_1−x)₂bdc₃·nH₂O, was synthesized using a direct reaction in a water solution. At the Eu³⁺ concentration of 1–40 at %, the MOFs were formed as a binary mixture of the (Eu_xLu_1−x)₂bdc₃ and (Eu_xLu_1−x)₂bdc₃·4H₂O crystalline phases, where the Ln₂bdc₃·4H₂O crystalline phase was enriched by europium(III) ions. At an Eu³⁺ concentration of more than 40 at %, only one crystalline phase was formed: (Eu_xLu_1−x)₂bdc₃·4H₂O. All MOFs containing Eu³⁺ exhibited sensitization of bright Eu³⁺-centered luminescence upon the 280 nm excitation into a ¹ππ* excited state of the terephthalate ion. The fine structure of the emission spectra of Eu^{3+ 5}D₀-⁷F_J (J = 0–4) significantly depended on the Eu³⁺ concentration. The luminescence quantum yield of Eu³⁺ was significantly larger for Eu-Lu terephthalates containing a low concentration of Eu³⁺ due to the absence of Eu-Eu energy migration and the presence of the Ln₂bdc₃ crystalline phase with a significantly smaller nonradiative decay rate compared to the Ln₂bdc₃·4H₂O.

Excitation-Dependent Photoluminescence of BaZrO3:Eu3+ Crystals

The elucidation of local structure, excitation-dependent spectroscopy, and defect engineering in lanthanide ion-doped phosphors was a focal point of research. In this work, we have studied Eu³⁺-doped BaZrO₃ (BZOE) submicron crystals that were synthesized by a molten salt method. The BZOE crystals show orange-red emission tunability under the host and dopant excitations at 279 nm and 395 nm, respectively, and the difference is determined in terms of the asymmetry ratio, Stark splitting, and intensity of the uncommon ⁵D₀ → ⁷F₀ transition. These distinct spectral features remain unaltered under different excitations for the BZOE crystals with Eu³⁺ concentrations of 0-10.0%. The 2.0% Eu³⁺-doped BZOE crystals display the best optical performance in terms of excitation/emission intensity, lifetime, and quantum yield. The X-ray absorption near the edge structure spectral data suggest europium, barium, and zirconium ions to be stabilized in +3, +2, and +4 oxidation states, respectively. The extended X-ray absorption fine structure spectral analysis confirms that, below 2.0% doping, the Eu³⁺ ions occupy the six-coordinated Zr⁴⁺ sites. This work gives complete information about the BZOE phosphor in terms of the dopant oxidation state, the local structure, the excitation-dependent photoluminescence (PL), the concentration-dependent PL, and the origin of PL. Such a complete photophysical analysis opens up a new pathway in perovskite research in the area of phosphors and scintillators with tunable properties.

Achieving circularly polarized luminescence and large piezoelectric response in hybrid rare-earth double perovskite by a chirality induction strategy

Chirality, an intrinsic property of nature, has received increased attention in chemistry, biology, and materials science because it can induce optical rotation, ferroelectricity, nonlinear optical response, and other unique properties. Here, by introducing chirality into hybrid rare-earth double perovskites (HREDPs), we successfully designed and synthesized a pair of enantiomeric three-dimensional (3D) HREDPs, [(R)-N-methyl-3-hydroxylquinuclidinium]₂RbEu(NO₃)₆ (R1) and [(S)-N-methyl-3-hydroxylquinuclidinium]₂RbEu(NO₃)₆ (S1), which possess ferroelasticity, multiaxial ferroelectricity, high quantum yields (84.71% and 83.55%, respectively), and long fluorescence lifetimes (5.404 and 5.256 ms, respectively). Notably, the introduction of chirality induces the coupling of multiaxial ferroelectricity and ferroelasticity, which brings about a satisfactory large piezoelectric response (103 and 101 pC N^-1 for R1 and S1, respectively). Moreover, in combination with the chirality and outstanding photoluminescence properties, circularly polarized luminescence (CPL) was first realized in HREDPs. This work sheds light on the design strategy of molecule-based materials with a large piezoelectric response and excellent CPL activity, and will inspire researchers to further explore the role of chirality in the construction of novel multifunctional materials.

NASICON-type Na3.6Lu1.8-x(PO4)3:xEu3+ phosphors: structure and luminescence

Na_3.6Lu_1.8-x(PO₄)₃:xEu³⁺ phosphors were synthesized by a high-temperature solid-state reaction. A powder X-ray diffraction study revealed that homogeneous solid solutions with a NASICON-type structure were formed at 0 ≤ x ≤ 0.7. The Na_3.6Lu_1.8(PO₄)₃ structure was refined from the powder X-ray diffraction data and the cation distribution in the lattice sites of the NASICON-type structure was revealed. The refinement indicates structural disorder caused by the displacement of a part of Lu cations along the c axis inside the (Lu/Na)O₆ octahedra that is confirmed by the broadened emission lines of Eu³⁺, which substitutes Lu cations. The highest Eu³⁺ luminescence intensity is found in Na_3.6Lu_1.8-x(PO₄)₃:xEu³⁺ for x = 0.5, whereas a further increase of the Eu³⁺ content leads to concentration quenching that is shown to occur due to the dipole-dipole interaction. An enhanced temperature stability of the Eu³⁺ emission was observed at the excitation energy of 3.23 eV. At this excitation energy, thermal quenching of the emission caused by the ⁷F₀ → ⁵L₇ transitions is compensated by the intensity increase of the emission related to the ⁷F₁ → ⁵G_J transitions, which occurs due to the increase of the ⁷F₁ level population, induced by a temperature rise.

We are never ever getting (back to) ideal symmetry: structure and luminescence in a ten-coordinated europium(III) sulfate crystal

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?

Arel: Investigating [Eu(H2O)9]3+ Photophysics and Creating a Method to Bypass Luminescence Quantum Yield Determinations

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(H₂O)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.

Tumor-Microenvironment-Responsive Biodegradable Nanoagents Based on Lanthanide Nucleotide Self-Assemblies toward Precise Cancer Therapy

Stimuli-responsive nanoagents, which simultaneously satisfy normal tissue clearance and tumor-specific responsive treatment, are highly attractive for precise cancer theranostics. Herein, we develop a unique template-induced self-assembly strategy for the exquisitely controlled synthesis of self-assembled lanthanide (Ln³⁺ ) nucleotide nanoparticles (LNNPs) with amorphous structure and tunable size from sub-5 nm to 105 nm. By virtue of the low-temperature (10 K) and high-resolution spectroscopy, the local site symmetry of Ln³⁺ in LNNPs is unraveled for the first time. The proposed LNNPs are further demonstrated to possess the ability for highly efficient loading and tumor-microenvironment-responsive release of doxorubicin. Particularly, sub-5 nm LNNPs not only exhibit excellent biocompatibility and predominant renal-clearance performance, but also enable efficient tumor retention. These findings reveal the great potential of LNNPs as a new generation of therapeutic platform to overcome the dilemma between efficient therapy and long-term toxicity of nanoagents for future clinical applications.

Photoluminescence of the Eu3+-Activated YxLu1−xNbO4 (x = 0, 0.25, 0.5, 0.75, 1) Solid-Solution Phosphors

Eu3+-doped YxLu1−xNbO4 (x = 0, 0.25, 0.5, 0.75, 1) were prepared by the solid-state reaction method. YNbO4:Eu3+ and LuNbO4:Eu3+ crystallize as beta-Fergusonite (SG no. 15) in 1–10 μm diameter particles. Photoluminescence emission spectra show a slight linear variation of emission energies and intensities with the solid-solution composition in terms of Y/Lu content. The energy difference between Stark sublevels of 5D0→7F1 emission increases, while the asymmetry ratio decreases with the composition. From the dispersion relations of pure YNbO4 and LuNbO4, the refractive index values for each concentration and emission wavelength are estimated. The Ω2 Judd–Ofelt parameter shows a linear increase from 6.75 to 7.48 × 10−20 cm2 from x = 0 to 1, respectively, and Ω4 from 2.69 to 2.95 × 10−20 cm2. The lowest non-radiative deexcitation rate was observed with x = 1, and thus LuNbO4:Eu3+ is more efficient phosphor than YNbO4:Eu3+.

Luminescent properties of neodymium, samarium and europium niobate and tantalate thin films

Thin films of lanthanide orthoniobate LnNbO₄ (LnNO) and orthotantalate LnTaO₄ (LnTO), (Ln = Nd, Sm, Eu) were fabricated by sol-gel method with subsequent spin-coating on PbZrO₃ /Al₂ O₃ substrate and annealing at 1000°C. X-ray diffraction patterns showed monoclinic M-LnNbO₄ or M´-LnTaO₄ , which coexists with orthorhombic or tetragonal phase. X-ray photoelectron spectroscopy demonstrate presence of Nd³⁺ , Sm³⁺ /Sm²⁺ and Eu³⁺ /Eu²⁺ ions. Luminescence properties of polymorphic films were investigated. Excitation spectra of PbZrO₃ interlayer represented broad bands at 410 and 550 nm, that were assigned to charge transfer bands (CTB). In all films, CTB broad band at around 275 nm related to charge transfer transition of Ln³⁺ → O^2- and NbO₄ ^3- or TaO₄ ^3- groups. In excitation spectra, ⁴ I_9/2 → ⁴ G_5/2 (Nd³⁺ ), ⁶ H_5/2 → ⁶ P_3/2 (Sm³⁺ ) and ⁷ F₀ → ⁵ L₆ (Eu³⁺ ) transitions (at 585, 402 and 395 nm), respectively are found to be more intense than any other Ln³⁺ transition. The emission spectra showed narrow and intense bands at 1065, 600 and 614 nm ascribed to Nd³⁺ , Sm³⁺ and Eu³⁺ 4f-f intraconfigurational transition ⁴ F_3/2 → ⁴ I_11/2 , ⁴ G_5/2 → ⁶ H_7/2 and ⁵ D₀ → ⁷ F₂ , respectively. The excellent luminescencent properties of films make them new potential group for visible and/or near-infrared applications such as sensors and imaging equipments.