Eu Complexes Research Articles

Structure-activity relationship approach toward the improved separation of actinides(III) over lanthanides(III) using amine-triamide ligands

The efficient separation of trivalent actinides over lanthanides is a major challenge for the partitioning and transmutation (P&T) strategy in spent nuclear fuel reprocessing due to their very similar properties. In this context, the present work reports three amine-triamide NTAamides (nitrilotriacetamides), to investigate the influence of ligand structure on the extraction of Am3+ over Eu3+. Among the three ligands, N,N-diethyl-N′,N′,N″,N″-tetraoctylnitrilotriacetamide shows higher extraction ability for Am3+ compared with that of N,N-dibutyl-N′,N′,N″,N″-tetraoctylnitrilotriacetamide and N,N,N′,N′,N″,N″-hexaoctylnitrilotriacetamide (NTAamide(n-Oct)) in the examined acid range of 0.01–0.50 mol/L HNO3. The structure–activity relationship study reveals that the two unsymmetrical NTAamides with varying alkyl substituted groups have higher extraction efficiency to Am3+ and relatively lower selectivity for Am3+ over Eu3+ than the symmetrical NTAamide(n-Oct) due to the steric hindrance effect. The complexation study by means of slope analysis, ESI-MS, and UV–vis titration suggests the formation of 1:2 metal/NTAamide extracted species. Meanwhile, the NTAamide is identified as a tetradentate ligand by FT-IR and XPS analyses. Furthermore, the analysis of Raman and TRLFS indicates that the NO3− and the H2O molecule do not participate in the inner coordination sphere of Eu3+. By combining the results of solvent extraction and complexation, a neutral complexation extraction model is proposed. Additionally, the stability constants (logβ) and thermodynamic parameters (ΔH, ΔS, and ΔG) regarding the complexation of Eu3+ with NTAamides are also presented.

Targeted, Molecular Europium(III) Probes Enable Luminescence-Guided Surgery and 1 Photon Post-Surgical Luminescence Microscopy of Solid Tumors.

Discrete luminescent lanthanide complexes represent a potential alternative to organic chromophores due to their tunability of optical properties, insensitivity to photobleaching, and large pseudo-Stokes shifts. Previously, we demonstrated that the lack of depth penetration of UV excitation required to sensitize discrete terbium and europium complexes can be overcome using Cherenkov radiation emitted by clinically employed radioisotopes in situ. Here, we show that the second-generation europium complexes [Eu(III)(pcta-PEPA2)] and [Eu(III)(tacn-pic-PEPA2)] (Φ = 57% and 76%, respectively) lower the limit of detection (LoD) to 1 nmol in the presence of 10 μCi of Cherenkov emitting isotopes, 18F and 68Ga. Bifunctionalization provides access to cysteine-linked peptide conjugates with comparable brightness and LoD. The conjugate, [Eu(tacn-(pic-PSMA)-PEPA2)], displays high binding affinity to prostate-specific membrane antigen (PSMA)-expressing PC-3 prostate cancer cells in vitro and can be visualized in the membrane-bound state using confocal microscopy. Biodistribution studies with the [86Y][Y(III)(tacn-(pic-PSMA)-PEPA2)] analogue in a mouse xenograft model were employed to study pharmacokinetics. Systemic administration of the targeted Cherenkov emitter, [68Ga][Ga(III)(PSMA-617)], followed by intratumoral injection or topical application of 20 or 10 nmol [Eu(III)(tacn-(pic-PSMA)-PEPA2)], respectively, in live mice resulted in statistically significant signal enhancement using conventional small animal imaging (620 nm bandpass filter). Optical imaging informed successful tumor resection. Ex vivo imaging of the fixed tumor tissue with 1 and 2 photon excitation further reveals the accumulation of the administered Eu(III) complex in target tissues. This work represents a significant step toward the application of luminescent lanthanide complexes for optical imaging in a clinical setting.

Bimetallic Eu/Tb Complexes for Ratiometric Temperature Sensing with Unusual Enhancement of Eu Luminescence with Temperature

In this paper we describe the results of the influence of temperature in the range of 280–340 K on the luminescence of bimetallic Eu/Tb complexes with N-heterocyclic ligand L based on 2,2′-bipyridyldicarboxylic acid in acetonitrile. The experiments were carried out for systems with various Eu/Tb ratios. The stability of the complexes of the ligand L with metal M (Eu or Tb) was determined using spectrophotometric titration in acetonitrile solutions. The LM complexes’ stability constants were found to be typical for these systems; however, the stability of Eu complex is slightly higher than that for Tb. Along with rising temperature, we observed a decrease in Tb emission intensity and, at the same time, an enhancement in Eu luminescence. An explanation of Eu luminescence enhancement involves the appearance of charge transfer states, bands of which can be observed in the Eu luminescence excitation spectra as difference spectra measured with two close temperatures. The unusual Eu luminescence enhancement upon heating was observed for the first time for the complex with tetradentate O,N-type heterocyclic diamide ligand L, while an inverse phenomenon was observed with the Tb luminescence. The Eu luminescence enhancement was found earlier for various carboxylate complex salts, but not for heterocyclic coordination complexes. This allows the construction of a ratiometric luminescent thermometer in the range of 280–340 K using the ratio of luminescence intensities for Eu and Tb. The stability constants for the individual Eu and Tb complexes help us to understand the equilibrium in L:Tb:Eu complex system and shed light on plausible speciation in solution.

Hydroxyl-group functionalized phenanthroline diimides as efficient masking agents for Am(III)/Eu(III) separation under harsh conditions

The separation of Lns(III) from radioactive Ans(III) in high-level liquid waste remains a formidable hydrometallurgical challenge. Water-soluble ligands are believed to be new frontiers in the search of efficient Lns/Ans separation ligands to close the nuclear fuel cycles and dealing with current existing nuclear waste. Currently, the development of hydrophilic ligands far lags behind their lipophilic counterparts due to their complicated synthetic procedures, inferior extraction performances, and acid tolerances. In this paper, we have showed a series of hydroxyl-group functionalized phenanthroline diimides were efficient masking agents for Am(III)/Eu(III) separation under high acidity (˃ 1 M HNO3). Record high SFEu(III)/Am(III) of 162 and 264 were observed for Phen-2DIC2OH and Phen-2DIC4OH in 1.25 M HNO3 which represents the best Eu(III)/Am(III) separation performance at this acidity. UV–vis absorption, NMR and TRLFS titrations were conducted to elucidate the predominant of 1:1 ligand/metal species under extraction conditions. X-ray data of both the ligand and Eu(III) complex together with DFT calculations revealed the superior extraction performances and selectivities. The current reported hydrophilic ligands were easy to prepare and readily to scale-up, acid tolerant and highly efficient, together with their CHON-compatible nature make them promising candidates in the development of advanced separation processes.

Highly luminescent ultra-thin films with rare earth for plasmonic applications

Amphiphilic complexes with luminescent rare earth metal ions suitable for Lanmuir-Blodgett (LB) deposition have been synthesized. LB monolayers with closely packed Eu complexes deposited directly on silver demonstrate significant far-field emission in contrast to the theoretical predictions of full quenching. Angular radiation and polarization patterns of the electric and magnetic dipole emission of Eu3+ point to a high excitation efficiency of surface plasmon polaritons. Different luminescent behavior of closely packed emitters in comparison to diluted systems is tentatively attributed to the collective state of emitters in LB layers formed via near-field coupling with surface plasmons.

Crystal structures and solid-state luminescent properties of Eu(III), Gd(III) and Tb(III) complexes with hexafluoroacetylacetone and 4′-substituted terpyridine ligands

Six mononuclear lanthanide complexes [Ln(hfac)3(ptpy)] (Ln(III) = Eu (1), Gd (2), Tb (3)) and [Ln(hfac)3(ftpy)] (Ln(III) = Eu (4), Gd (5), Tb (6)), synthesized with 1,1,1,5,5,5-hexafluoroacetylacetone (hfac) as a first ligand and 4′-phenyl-2,2′:6′,2″-terpyridine (ptpy) or 4′-furan-2,2′:6′,2″-terpyridine (ftpy) as a second ligand, were characterized by elemental analyses, infrared spectroscopy, single-crystal X-ray diffraction, and powder X-ray diffraction. Thermogravimetric analyses indicate that these ternary lanthanide complexes possess very good thermal stability. Solid-state samples of 1-Eu and 4-Eu exhibit characteristic red luminescence at CIE: 0.6721, 0.3276 and CIE: 0.6859, 0.3139, with absolute quantum yields of 59.95% and 83.14% and luminescence lifetimes of 0.819 and 0.727 ms, respectively. Solid-state samples of 3-Tb and 6-Tb display characteristic green luminescence at CIE: 0.2736, 0.7167 and CIE: 0.2674, 0.7219, with absolute quantum yields of 2.58% and 0.52% and luminescence lifetimes of 0.040 and 0.033 ms, respectively. The results suggest that terpyridine is a good sensitizer for Eu(III), while hfac is a good sensitizer for Tb(III). In solid-state, the synergistic sensitization effect of hfac and terpyridine is significant for Eu(III), but less for Tb(III). And the electron-donating substituent at the 4′-position make terpyridine more suitable for the sensitization of Eu(III), but unfavorable for Tb(III).

Low-rate smartphone videoscopy for microsecond luminescence lifetime imaging with machine learning.

Time-resolved techniques have been widely used in time-gated and luminescence lifetime imaging. However, traditional time-resolved systems require expensive lab equipment such as high-speed excitation sources and detectors or complicated mechanical choppers to achieve high repetition rates. Here, we present a cost-effective and miniaturized smartphone lifetime imaging system integrated with a pulsed ultraviolet (UV) light-emitting diode (LED) for 2D luminescence lifetime imaging using a videoscopy-based virtual chopper (V-chopper) mechanism combined with machine learning. The V-chopper method generates a series of time-delayed images between excitation pulses and smartphone gating so that the luminescence lifetime can be measured at each pixel using a relatively low acquisition frame rate (e.g. 30 frames per second [fps]) without the need for excitation synchronization. Europium (Eu) complex dyes with different luminescent lifetimes ranging from microseconds to seconds were used to demonstrate and evaluate the principle of V-chopper on a 3D-printed smartphone microscopy platform. A convolutional neural network (CNN) model was developed to automatically distinguish the gated images in different decay cycles with an accuracy of >99.5%. The current smartphone V-chopper system can detect lifetime down to ∼75 µs utilizing the default phase shift between the smartphone video rate and excitation pulses and in principle can detect much shorter lifetimes by accurately programming the time delay. This V-chopper methodology has eliminated the need for the expensive and complicated instruments used in traditional time-resolved detection and can greatly expand the applications of time-resolved lifetime technologies.

Synthesis, Crystal Structure, and Optical Properties of Mononuclear Eu(III) and Tb(III) Complexes Containing a Chalcone Ligand

Chalcones are α,β-unsaturated ketones with great structural diversity and various applications. A chalcone produced by condensation of 2-acetylpyridine with 2-naphthaldehyde (L) was employed for synthesis of two mononuclear complexes: [Eu(L)(hfac)3(H2O)]·0.5CHCl3 and [Tb(L)(hfac)3], where hfac is the hexafluoroacetylacetonate anion. The chalcone and complexes were structurally characterized by single-crystal X-ray diffraction. The chalcone acts as a chelating bidentate ligand. Luminescent properties of the ligand L and the complexes were investigated in the solid state. For these heteroleptic mononuclear complexes, the emission of the Eu(III) and Tb(III) ions was influenced by the excitation wavelength.

Luminescent properties of Eu (III) complexes with carboxylate ligands and their light conversion films

This study reports the synthesis and characterization of four europium (III) complexes with different carboxylate ligands, namely 2-pyridine-acrylic acid (H2-PA), 2-pyridine-carboxylic acid (HPic), benzoic acid (HBen), and cinnamic acid (HCin), as well as a binuclear europium-lanthanum complex with HCin as the only ligand. The obtained complexes and the light conversion films prepared from them were extensively characterized by various techniques, including elemental analysis, infrared spectroscopy (IR), X-ray single crystal diffraction, thermogravimetric analysis (TGA), and fluorescence spectroscopy. Efforts were made to systemically investigate the structural, coordination, photophysical, and thermal properties of the luminescent materials, and elucidate the correlations between the ligand triplet energy levels and the luminescent properties of the complexes. The results reveal that the introduction of LaIII cannot only enhance the luminescent intensity of the light conversion agent and corresponding film, but also have obvious effect on enhancing the ultraviolet aging resistance of the light conversion film. Moreover, the proper doping concentration (0.5%) of the EuIII complex La-Eu-Cin in film may better balance the properties and cost.

Furancarboxylate Coordination Polymers of Gd3+ and Eu3+: Synthesis, Structural Variations, and Biological Properties

A series of polymer complexes of Gd(III) and Eu(III) with 3-furancarboxylic (HFur) and 5-nitro-2-furancarboxylic (HNfur) acids differed in the composition and coligands presented by solvent molecules(CH3OH/C2H5OH/H2O) is synthesized: [Gd(Fur)3(CH3OH)(C2H5OH)]n (I), [Gd(Nfur)3(CH3OH)2]n·CH3CN (II), [Eu(Fur)3(C2H5OH)]n (III), and [Eu(Nfur)3(H2O)2]n·3CH3CN (IV). According to the X-raydiffraction (XRD) data, all complexes are 1D coordination polymers in which the lanthanide cation has thecoordination number 8 (LnO8) to form the environment as a doubly augmented triangular prism (I, II) or asquare antiprism (III, IV). The supramolecular levels of the polymers are stabilized due to intra- and intermolecularhydrogen bonds between the coordinated solvent molecules and O atoms of the chelate-boundanions of the acid and via two types of noncovalent C–H…O and N–O…π interactions that significantly contributeto an additional stabilization of the crystal packings. The biological properties of complexes I, II, andIV are studied with respect to the model nonpathogenic strain Mycolicibacterium smegmatis.

Waterborne rare earth complexes/polyurethane systems with UV shielding and tunable fluorescence for anti-counterfeiting

Fluorescent polymer materials integrated with rare-earth complexes demonstrate promising multifunctional capabilities. However, their complex synthesis procedures and prohibitive cost currently constrain large-scale utilization. Herein, rare earth complexes Eu(PBA)3Phen and Tb(PBA)3Phen were synthesized by coordinating Eu3+, Tb3+ with p-hydroxybenzoic acid and phenanthroline, which were then physically incorporated into waterborne polyurethane (WPU) to obtain Eu-WPU and Eu/Tb-WPU composites. The as-prepared Eu/Tb-WPU had tunable fluorescence, whose fluorescent colors from green to red were achieved under ultraviolet excitation. Compared to Eu(PBA)3Phen (0.27 ms; 0.22 %), the Eu-WPU exhibited a prolonged fluorescence lifetime(1.08 ms) and high fluorescence quantum yield (10.7 %). In addition, Eu-WPU had a wider UV shielding range of 200–289 nm than WPU (200–222 nm). The prepared Eu/Tb-WPU exhibited rich fluorescent emission colors and can be used for information security encryption. Therefore, these Eu-WPU and Eu/Tb-WPU have good application prospects in ultraviolet shielding and fluorescence anti-counterfeiting.

A Series of Novel 3D Coordination Polymers Based on the Quinoline-2,4-dicarboxylate Building Block and Lanthanide(III) Ions-Temperature Dependence Investigations.

A series of novel 3D coordination polymers [Ln2(Qdca)3(H2O)x]·yH2O (x = 3 or 4, y = 0-4) assembled from selected lanthanide ions (Ln(III) = Nd, Eu, Tb, and Er) and a non-explored quinoline-2,4-dicarboxylate building block (Qdca2- = C11H5NO42-) were prepared under hydrothermal conditions at temperatures of 100, 120, and 150 °C. Generally, an increase in synthesis temperature resulted in structural transformations and the formation of more hydrated compounds. The metal complexes were characterized by elemental analysis, single-crystal and powder X-ray diffraction methods, thermal analysis (TG-DSC), ATR/FTIR, UV/Vis, and luminescence spectroscopy. The structural variety of three-dimensional coordination polymers can be ascribed to the temperature effect, which enforces the diversity of quinoline-2,4-dicarboxylate ligand denticity and conformation. The Qdca2- ligand only behaves as a bridging or bridging-chelating building block binding two to five metal centers with seven different coordination modes arising mainly from different carboxylate group coordination types. The presence of water molecules in the structures of complexes is crucial for their stability. The removal of both coordinated and non-coordinated water molecules leads to the disintegration and combustion of metal-organic frameworks to the appropriate lanthanide oxides. The luminescence features of complexes, quantum yield, and luminescent lifetimes were measured and analyzed. Only the Eu complexes show emission in the VIS region, whereas Nd and Er complexes emit in the NIR range. The luminescence properties of complexes were correlated with the crystal structures of the investigated complexes.

Genomic epidemiology sheds light on the emergence and spread of Mycobacterium bovis Eu2 Clonal Complex in Portugal

ABSTRACT Animal tuberculosis (TB) remains a serious concern for animal and human health. Mycobacterium bovis circulates in multi-host systems, dominated by the European 2 clonal complex (Eu2) in Iberia. In this work, we use genomic epidemiology to infer the emergence, spread, and spatiotemporal patterns of Eu2 in the official epidemiological risk area of animal TB in Portugal. Phylogenetic analysis of 144 M. bovis whole-genome sequences from cattle, wild boar, and red deer, representing the 2002–2021 period, distinguished three Eu2 clades that evolved independently. The major Eu2 clade underwent phylodynamic inferences to estimate the time and location of outbreaks, host transitions, and spatial diffusion as well. The origin of this Eu2 clade was attributed to the red deer population in the Castelo Branco district, near the border with Spain. Most host transitions were intraspecific (80%), while interspecific transmissions between wildlife species (wild boar-red deer), and between wild boar and cattle, were highly supported. Phylogeographic reconstruction evidenced that most transitions (82%) occur within municipalities, highlighting local transmission corridors. Our study indicates that M. bovis continues to spread at the cattle-wildlife interface within the animal TB hotspot area, possibly driven by the foraging behaviour of wild boar near agricultural lands. Red deer seems to be an important driver of TB within wildlife hosts, while the wild boar links the multi-host wildlife community and livestock. This work highlights the value of combining genomic epidemiology with phylodynamic inference to resolve host jumps and spatial patterns of M. bovis, providing real-time clues about points of intervention.

Synthesis of a europium complex with ultrahigh quantum yield and its application as an invisible luminescent additive in pigments for preservation of cultural relics

A europium coordination compound [Eu2(ppip)2(PhCOO)6] (ppip is 1,2-diphenylimidazo[4,5-f][1,10]phenanthroline) has been synthesized and structurally characterized. In this symmetric dinuclear complex of octa-coordinated Eu(III) atom, both hanging phenyl rings in the ppip are twisted dramatically from the imidazophenanthroline part, leading to strong edge-to-face (T-type) C–H∙∙∙π interaction. This pale yellow stable complex possesses an ultrahigh luminescence quantum yield (84.2 ± 0.2%) under excitation at 365 nm, and hence is explored to be utilized in the preservation pigments for traditional Chinese paintings as an luminescent additive, which looks invisible under normal light, but exhibits bright red luminescence under UV excitation at a rather low percentage (0.02% wt).

Luminescent Papers with Asymmetric Complexes of Eu(III) and Tb(III) in Polymeric Matrices and Suggested Combinations for Color Tuning.

Complexes of lanthanide ions, such as Eu(III) (red light emission) and Tb(III) (green light emission), with proper ligands can be highly luminescent and color-tunable, also attaining yellow and orange emission under UV radiation. The ligands employed in this work were poly(sodium acrylate), working as polymeric matrix, and 1,10-phenanthroline, taking advantage of its antenna effect. Possibilities of color display were further enhanced by incorporating a cationic polyfluorene with blue emission. This strategy allowed for obtaining cyan and magenta, besides the aforementioned colors. Uncoated cellulose paper was impregnated with the resulting luminescent inks, observing a strong hypsochromic shift in excitation wavelength upon drying. Hence, while a cheap UV-A lamp sufficed to reveal the polyfluorene's blue emission, shorter wavelengths were necessary to visualize the emission due to lanthanide ions as well. The capacity to reveal, with UV-C radiation, a full-color image that remains invisible under natural light is undoubtedly useful for anti-counterfeiting applications. Furthermore, both lanthanide ion complexes and polyfluorenes were shown to have their luminescence quenched by Cu(II) ions and nitroarenes, respectively.

Molecular layer deposition to grow a luminescent metalorganic monolayer on inorganic substrate: Breaking monopoly of solution routes to self-assembled monolayer

A gas-phase route such as molecular layer deposition has been applied for the first time to create a monolayer of metalorganic Eu(III) molecules on NiO oxide layer. The surface of the as-deposited NiO layer is activated using H2O/O2 vapors, and then functionalized by anchoring molecules of the Eu complex working at different temperatures to favor the chemisorption process. The effects of the activation process and of the MLD temperature on the chemisorption of the Eu complex were evaluated through in-depth X-ray photoelectron and luminescence spectroscopy studies. These analyses allowed to define the stoichiometry of the Eu moiety chemisorbed on the surface and the optimization of an impressive strategy to the synthesis of hybrid metalorganic/inorganic systems. The hybrid system's luminescence properties have proved the monolayer's energy conversion phenomena and produced further information about the local environment around the Eu(III) anchored as a complex on the NiO surface.

Solvent extraction of Eu(III), Am(III) and Cm(III) by N,N,N’,N’-tetrabutyldiglycolamide: Can the diluent participate in the extraction?

The complexation of Eu(III), Am(III) and Cm(III) during the extraction of them by N,N,N’,N’-terabutyldiglycolamide in 9 diluents from nitric acid solutions was examined. The composition of the extracted species was determined by the slope analysis. The Kex. values were calculated for all the systems studied. The influence of the nature of the diluent on the extraction of the listed metal ions was studied. The complex of Am(NO3) with N,N,N’,N’-tetraethyldiglycolamide was isolated in crystalline form and characterized by X-ray diffraction.

Comprehensive study of functional properties and electrochemical performance of layered lanthanum nickelate substituted with rare-earth elements

The present work is devoted to the comparative study of the absolute oxygen content, thermal expansion, electrical conductivity, and electrochemical performance of the La1.6Ln0.4NiO4+δ (Ln = La, Pr, Nd, Sm, Eu) complex oxides. The thermal expansion coefficients, obtained for the La1.6Ln0.4NiO4+δ compact samples, have approved thermomechanical compatibility of the materials with the Sm0.2Ce0.8O1.9 electrolyte. The highest conductivity value is found to be 107 S cm−1 at 420 °C for the La1.6Sm0.4NiO4+δ sample. The results of scanning electron microscopy, performed after the cycle of measurements on two-layer electrodes with the La1.6Ln0.4NiO4+δ functional layers and the LaNi0.6Fe0.4O3−δ (LNF) collector layers, have demonstrated the stability of the electrode structure under the measurement conditions and the maintenance of high average porosity for both layers. The minimum polarization resistance value, equal to 0.64 Ω cm2 at 700 °C, has been measured for the La1.6Eu0.4NiO4+δ electrode sintered at 1200 °C. Comprehensive distribution function of relaxation times (DFRT or DRT) analyses of the impedance data has been performed to establish the rate-limiting stages of the electrode reaction. Oxygen kinetic parameters calculated in the framework of the Adler-Lane-Steele (ALS) model with considering the microstructural parameters are shown to correlate well with those obtained from the isotope exchange data.

Accurate Evaluation of Dispersion Energies at Coupled Cluster Level to Understand the Substituent Effects in Am(III) and Eu(III) Complexes.

The effect of cyclic and aromatic substituents on the complexation behavior of phosphine oxide ligands with Am(III) and Eu(III) was investigated at density functional theory (DFT) and domain-based local pair natural orbital coupled-cluster (DLPNO-CC) levels. Combining DFT with accurate coupled cluster methods, we have evaluated the dispersion energy contributions to the complexation energies for trivalent Am and Eu complexes for the first time. Irrespective of the nature of substituents on the P atom, the electronic structure of the P═O group remains identical in all of the ligands. The study reveals the importance of dispersion interactions during complexation and is estimated to be more significant for Am(III) than for Eu(III) complexes.

Light- and Chemical-Doping-Induced Magnetic Behavior of Eu Molecular Systems.

Variable temperature electron paramagnetic resonance (VT-EPR) was used to investigate the role of the environment and oxidation states of several coordinated Eu compounds. We find that while Eu(III) chelating complexes are diamagnetic, simple chemical reduction results in the formation of paramagnetic species. In agreement with the distorted D3h symmetry of Eu molecular complexes investigated in this study, the EPR spectrum of reduced complexes showed axially symmetric signals (g⊥ = 2.001 and g∥ = 1.994) that were successfully simulated with two Eu isotopes with nuclear spin 5/2 (151Eu and 153Eu with 48% and 52% natural abundance, respectively) and nuclear g-factors 151Eu/153Eu = 2.27. Illumination of water-soluble complex Eu(dipic)3 at 4 K led to the ligand-to-metal charge transfer (LMCT) that resulted in the formation of Eu(II) in a rhombic environment (gx = 2.006, gy = 1.995, gz = 1.988). The existence of LMCT affects the luminescence of Eu(dipic)3, and pre-reduction of the complex to Eu(II)(dipic)3 reversibly reduces red luminescence with the appearance of a weak CT blue luminescence. Furthermore, encapsulation of a large portion of the dipic ligand with Cucurbit[7]uril, a pumpkin-shaped macrocycle, inhibited ligand-to-metal charge transfer, preventing the formation of Eu(II) upon illumination.