3d Metal Ions Research Articles

3D coordination polymers constructed from d10 metal ions, flexible 1,2,4-triazole derivatives and aromatic tetracarboxylates: Syntheses, structures, thermal and luminescent properties

The reaction of mixed-ligands 1,2,4,5-benzene tetracarboxylic acid (H4btec) and 1,2-bis(4-amino-5-methylmercapto-1,2,4-triazole-3-yl)ethane (bamte) with zinc(II) nitrate hexahydrate under hydrothermal conditions led to the formation of the coordination polymer [Zn2 (bamte) (btec)]n (CP1), whereas the reaction of the same organic tetraacid with 4-amino-5-(pyridine-4-yl)-4H-1,2,4-triazole-3-thiol (Haptt) and cadmium (II) nitrate tetrahydrate under solvothermal conditions gave the coordination polymer [Cd3 (aptt)2 (btec)]n (CP2). Both CPs have been characterized by elemental analysis, FT−IR, powder X-ray diffraction as well as single-crystal X-ray diffraction. Both complexes possess three-dimensional (3D) networks based on two-dimensional MII-btec4-- layers connected by multidentate triazole moieties. CP1 shows a rarely observed binodal (4,6)-connected ‘‘fsc’’ coordination network with (44.610.8) (44.62) topology, whereas CP2 presents a new tetranodal (4,4,5,6)-connected 3D net with (43.62.8)2 (44.62) (44.66)2 (46.66.83) point symbol. Furthermore, the morphology, thermal stability, and solid-state luminescent property of both coordination polymers have been investigated.

Exploration of semiconducting properties of Zn(ii)- and Cd(ii)-based coordination polymers with dicarboxylate of a chair-type backbone

This study highlights the comparison of electrical properties and device applications of d10 metal ion based coordination polymers.

A study of corrosion processes in a demineralized aqueous medium in the presence of bacterial cells

Our earlier results indicate the feasibility of considering the microbiological contamination of closed and reversible cooling and water supply systems, given the effect of the microbial cells on the corrosion processes taking place there. The results show a possibility of transition metal ions acting as regulators and modifiers of microorganisms’ growth processes under conditions of an osmotic stress and starvation.The aim of the present work is to study the effect of transition d-metal ions (Zn2+ and Cr3+), both separately and in the presence of known antiskalants, oxyethylenediphosphonic acid (OEDFh) and nitrilotrimethylphosphonic acid (NTMFh), on the effectiveness of the corrosion protection and the course of the biological corrosion in a demineralized environment.Microorganisms belonging to different systematic affiliations were selected for the study of the biocorrosion processes: Bacillus sp. and Pseudomonas sp. These microorganisms belonged to different physiological groups of bacteria and had opposite types of life strategies. They differred significantly in their ability to survive under the influence of various factors. They could exist in closed systems of water consumption (with sharp temperature changes, oxygen starvation, a presence of toxic substances used for water conditioning). Bacillius and Pseudomonas strains were cultured on commonly used agar media, after which a cell suspension was prepared. For all bacterial strains used, the seeding dose was 108 cells‧cm-3. It was determined using turbidometric control methods and bacterial density standards. The effect of the microorganisms on the corrosion processes was followed in stationary bioreactors of a volume of 125 cm3. A solution of 0.1 % NaCl was added as a model medium to which corrosion inhibitors (Zn2+, Cr3+, OEDPh and NTMPh) were added at a concentration of 5 mg‧dm-3. At each stage of preparation, the suspension was thoroughly mixed, after which the metal samples were introduced to the reactor in the form of electrodes so that the entire surface of the electrode was completely immersed in the water. The metal samples were made of St 20 carbon steel. Each study of model water with metal plates was of 3 weeks duration. The intensity of the corrosion process was evaluated once a week. The rate of the biological corrosion was determined by the gravimetric method. The inhibition factor (j) and the degree of the corrosion protection (Z, %) of the samples in presence of corrosion inhibitors and bacteria were calculated. The negative value of this indicator indicated a change of the direction of the process, i.e. an activation of the corrosion process.The obtained data show that the presence of microorganisms significantly modifies the anti-corrosion effect of the heavy metal ions. The range offluctuations of the degree of the corrosion protection in presence of bacteria of the genus Pseudomonas is greater than that of the genus Bacillus. The dynamics of the protective effect in presence of both strains is characterized by significant changes (a decrease and a subsequent gradual increase of the degree of the corrosion protection). The highest intensity of the corrosion process is observed during the first week of the interaction in a model environment containing bacterial cells and anticorrosive agents. Then, the direction of the process changed and there was shown a slight protective effect at the 5-15 up to 36 % level. It is also demonstrated that the anti-corrosion compositions containing NTMPh provide a greater increase of the corrosion process rate in presence of microorganisms.The study of the presence of microorganisms on the corrosion proceeding demonstrates their ability to provide a significant effect. The bacteria can activate the corrosion process, which complicates the inhibitors behavior.

Formation of Defect-Dicubane-Type NiII 2LnIII 2 (Ln = Tb, Er) Clusters: Crystal Structures and Modeling of the Magnetic Properties.

In the field of molecular nanoclusters, cubane and defect-dicubane, or butterfly structures, are typical examples of tetranuclear metal core architectures. In this work, a halogenated and anionic Schiff-base ligand (L2–) is utilized as it is predisposed to chelate within a cluster core to both 3d and 4f metal ions, in different binding configurations (H2L = 4-chloro-2-(2-hydroxy-3-methoxybenzyliden amino)phenol). The phenolate oxygen atoms of the deprotonated ligand can act in μ-O and μ3-O bridging binding modes for the intramolecular assembly of metal ions. Based on that, two tetranuclear and isostructural compounds [Ni2Tb2(L)4(NO3)2(DMF)2]·2CH3CN (1) and [Ni2Er2(L)4(NO3)2(DMF)2]·0.5CH3CN (2) were synthesized and structurally characterized. Magnetic susceptibility and magnetization data indicate the occurrence of dominant intramolecular ferromagnetic interactions between the spin centers. Particular emphasis is given to the theoretical description of the magnetic behavior, taking into account the Ln–Ni and Ni–Ni coupling paths and the magnetic anisotropy of the LnIII and NiII ions. The study is distinguished for its discussion of two distinct models, whereby model A relies on the uniaxial B20 Stevens term describing the lanthanide anisotropy and model B is based on point-charge model calculations. Importantly, the physical meaning of the obtained parameters for both models was critically scrutinized.

Luminescent Ratiometric Thermometers Based on a 4f-3d Grafted Covalent Organic Framework to Locally Measure Temperature Gradients During Catalytic Reactions.

Covalent Organic Frameworks (COFs), an emerging class of crystalline porous materials, are a new type of support for grafting lanthanide ions (Ln3+ ), which can be employed as ratiometric luminescent thermometers. In this work we have shown that COFs co-grafted with lanthanide ions (Eu3+ , Tb3+ ) and Cu2+ (or potentially other d-metals) can synchronously be employed both as a nanothermometer and catalyst during a chemical reaction. The performance of the thermometer could be tuned by changing the grafted d-metal and solvent environment. As a proof of principle, the Glaser coupling reaction was investigated. We show that temperature can be precisely measured during the course of the catalytic reaction using luminescence thermometry. This concept could be potentially easily extended to other catalytic reactions by grafting other d-metal ions on the Ln@COF platform.

Spectroscopic and TDDFT studies of N-phenyl-N′-(3-triazolyl)thiourea) compounds with transition metal ions

Reaction of N-phenyl-N′-(3-triazolyl)thiourea) (H3L) with M(ClO4)2 (M = Co(II) (1), Ni(II) (2) and Cu(II) (3)) afforded [M(H2L)2] complexes, which were characterized experimentally and theoretically using different analytical, and spectral tools. The susceptibility of Staphylococcus aureus and Escherichia coli bacteria towards H3L and its complexes was evaluated. The thiourea ligand coordinates to the 3d-metal ions via C–S−, and triazole nitrogen yielding coordination compounds between the tetrahedral, and square-planar geometries (“flattened” tetrahedron, D2d symmetry). Full geometry optimization, vibrational analyses, and natural bond orbital analyses of the proposed conformations of 1–3 were executed at B3LYP/def2-SVP to gain some knowledge about the local minima structures, natural charge of the coordinated metal ion, electronic configuration as well as the hybridization of M–L bonds. The electronic structures of the local minimum structures of 1–3 were investigated by time-dependent density functional theory calculations. Coordination of the thiourea ligand to Co(II) and Cu(II) ions did alter the toxicity against the tested microbes, while chelation of Ni(II) ion gave rise to inactive species.

Dithiol-Dithione Tautomerism of 2,3-Pyrazinedithiol in the Synthesis of Copper and Silver Coordination Compounds.

A promising strategy for new electrically conductive coordination polymers is the combination of d10 metal ions, which tolerate short metal···metal distances, with dithiolene linkers, known for their "non-innocent" redox behavior. This study explores the coordination chemistry of 2,3-pyrazinedithiol (H2pdt) toward Cu+ and Ag+ ions, highlighting similarities and differences. The synthetic approach, starting with the fully protonated ligand, allowed the isolation of a homoleptic bis(dithiolene) complex with formal CuI atoms, [Cu(H2pdt)2]Cl (1). This complex was further transformed to a 1D coordination polymer with short metal···metal distances, 1D[Cu(Hpdt)] (2Cu). The larger Ag+ ion directly built up a very similar coordination polymer, 1D[Ag(Hpdt)] (2Ag), without any appearance of an intermediate metal complex. The coordination polymer 1D[Cu(H2pdt)I] (4), like complex 1, bears fully protonated H2pdt ligands in their dithione form. Upon heating, both compounds underwent auto-oxidation coupled with a dehydrogenation of the ligand to form the open-shell neutral copper(II) complex [Cu(Hpdt)2] (3) and the coordination polymer 1D[Cu2I2(H2pdt)(Hpdt)] (5), respectively. For all presented compounds, crystal structures are discussed in-depth. Furthermore, properties of 1, 3, and those of the three 1D coordination polymers, 2Ag, 2Cu, and 4, were investigated by UV-vis-NIR spectroscopy, cyclic voltammetry, and variable-temperature magnetic susceptibility, and direct current (dc)-conductivity measurements. The experimental results are compared and discussed with the aid of DFT simulations.

Synthesis and identification of 1,1,3,3-tetra-[1,2-dihydro-1,5-dimethyl-2-phenyl-3-oxopyrazole]propane

Pyrazole derivatives are of practical value as analytical reagents, dyes and medicinal substances. It is known that 1,2-dihydro-1,5-dimethyl-2-phenylpyrazol-3H-one (antipyrine) and its bidentate derivatives are good analytical reagents for d-metal ions (Cd2+, Fe3+, Sc3+, Ti4+, Zn2+, Co2+, etc.). The chelating ability of bi-, tri- and tetradentate antipyrine derivatives make them promising organic ligands for the synthesis of organometallic coordination polymers. Therefore, synthesis of new tetradentate derivatives of 1,2-dihydro-1,5-dimethyl-2-phenyl-3Hpyrazole-one seems to be a highly relevant research task. The aim of this work was to synthesize and study a new substance – 1,1,3,3-tetra-[1,2-dihydro-1,5-dimethyl-2-phenyl-3-oxopyrazole]propane (tetraantipyryl-propane). A review of existing methods for the synthesis of antipyrine derivatives through condensation of 1,2-dihydro-1,5-dimethyl-2-phenyl-3H-pyrazol-3-one with aliphatic and aromatic aldehydes was carried out. A method was developed for the preparation and purification of 1,1,3,3-tetra-[1,2-dihydro-1,5-dimethyl-2-phenyl-3-oxopyrazole]propane by the reaction of electrophilic substitution of the protonated form of propandial (malonic dialdehyde) in the aromatic ring of the oxopyrazole heterocycle with a yield of 17.1%. The resulting compound is a white crystalline substance, readily soluble in organic polar and non-polar solvents, but insoluble in water. The purity of the recrystallized synthesis product was confirmed by measuring its melting point and conducting a qualitative reaction with sodium nitrite to detect an unsubstituted oxopyrazole cycle in the reaction substrate. The structure of 1,1,3,3-tetra-[1,2-dihydro-1,5-dimethyl-2-phenyl-3-oxopyrazole]propane was established by mass spectrometry and Raman spectroscopy.

Synthesis, characterization, in silico, and in vitro biological screening of coordination compounds with 1,2,4-triazine based biocompatible ligands and selected 3d-metal ions

A bidentate Schiff base ligand, MHMMT, obtained from 1,2,4-triazine derivative and 4-hydroxy-3-methoxy benzaldehyde and its Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) complexes were synthesised in ethanolic media and characterized by various analytical techniques like elemental analyses, magnetic susceptibility measurements, FTIR, UV-VIS, proton NMR, ESR, spectroscopic and thermogravimetric studies. Various geometries like a tetrahedral for Co(II) and Zn(II) complexes, an octahedral for Fe(III) and Ni(II) complexes, and square planar for Cu(II) complex has been assigned. For all metals complexes except Co(II), a 2:1 ligand to metal ratio is observed, while Co(II) complex has a 1:1 ratio. In accordance with the probable activity spectra of substances as obtained from PASS analysis, in vitro α-amylase inhibition studies by starch-iodine method for ligand and complexes except that of Fe(III) and anticancer screening against human breast cancer cell lines MCF-7 using MTT assay for Fe(III) complex were conducted. The tested compounds were found to be good α-amylase inhibitors, characteristically similar to most of the antidiabetic drugs. Among the compounds, Cu(II) complex exhibited the highest α-amylase inhibitory activity. Furthermore, ligand and complexes were also exposed to in vitro antimicrobial activities, drug-likeness, bioactivity score prediction by Molinspiration software. Molecular docking analysis of selected compounds on α-amylase and VEGFR-2 kinase were carried out for confirming the experimental observations.

Mitigation of grain boundary resistance in La2/3-xLi3xTiO3 perovskite as an electrolyte for solid-state Li-ion batteries

In this work, we report that modification of the chemical composition of grain boundaries of La2/3-xLi3xTiO3 double perovskite, one of the most promising Li-ion conducting solid electrolytes, can be a convenient and versatile way of controlling the space charge potential, leading to a mitigated electrical resistance of the grain boundaries. Two groups of additives are investigated: lithium-enriching agents (Li3BO3, LiF) and 3d metal ions (Co2+, Cu2+), both expected to reduce the Schottky barrier. It is observed that Li-containing additives work effectively at a higher sintering temperature of 1250 °C. Regarding copper, it shows a much stronger positive impact at lower temperature, 1150 °C, while the addition of cobalt is always detrimental. Despite overall complex behavior, it is documented that the decreased space charge potential plays a more important role in the improvement of lithium conduction than the thickness of the grain boundaries. Among the proposed additives, modification of La2/3-xLi3xTiO3 by 2 mol.% Cu2+ results in the space charge potential reduction by 32 mV in relation to the reference sample, and the grain boundary specific conductivity increase by 80%, as measured at 30 °C. Introduced additive allows to obtain a similar effect on the conductivity as elevating the sintering temperature, which can facilitate manufacturing procedure.Graphic abstract

Azo-Label Heterometal-Organic Rhomboids Exhibiting Photoswitchable NIR Luminescence in Crystalline State.

Photochromism is an important strategy for realizing reversible light-controllable fluorescence switching. In spite of several reports on fluorescence switching via a photochromic process, the success of photochromic multimetallic complexes reversibly showing fluorescence switching in the solid or crystalline state has been limited for their application importance. Here, we report a photoswitchable near-infrared (NIR) fluorescence based on photochromism in the azo-label 3d/4f heterometal-organic rhomboids, azo-Zn2Ln2 (Ln = Eu (1), Yb (2), and Er (3)), in the crystalline state. An individual metallorhomboid contains up to four azobenzene fragments, which is prepared via the three-component assembly of a trans-azobenzene-grafted multifunctional ligand, and 3d and 4f metal ions. The photoisomerization quantum yields of azo-Zn2Ln2 complexes can be retained or even higher when compared to the free ligand due to the modification of electronic structure. The impressive crystalline-state NIR luminescence is observed for the complexes of azo-Zn2Yb2 (2) and azo-Zn2Er2 (3) at room temperature. Intriguingly, the switchable NIR luminescence can be effectively regulated by photochromism in the crystalline state. These features endow the self-assembly of the 3d/4f metallorhomboid with synergetic multifunctional behavior between photochromism and NIR luminescence.

Modulating Magnetic and Photoluminescence Properties in 2-Aminonicotinate-Based Bifunctional Coordination Polymers by Merging 3d Metal Ions.

Herein, the synthesis and study of bifunctional coordination polymers (CPs) with both magnetic and photoluminescence properties, derived from a heterometallic environment, are reported. As a starting point, three isostructural monometallic CPs with the formula [M(μ-2ani)2 ]n (MII =Mn (1Mn ), Co (3Co ) and Ni (4Ni ); 2ani=2-aminonicotinate), crystallise as chiral 2D-layered structures stacked by means of supramolecular interactions. These compounds show high thermal stability in the solid state (above 350 °C), despite which, in aqueous solution, compound 1Mn is shown to partially transform into a novel 1D chain CP with the formula [Mn(2ani)2 (μ-H2 O)2 ]n (2Mn ). A study of the direct current (dc) magnetic properties of 1Mn , 3Co and 4Ni reveals a spin-canted structure derived from antisymmetric antiferromagnetic weak exchanges along the chiral network (as confirmed by DFT calculations) and magnetic anisotropy of the ions, in such a way that long-range ordering is observed with variable magnitude for the spin carriers. Moreover, compounds 3Co and 4Ni show no frequency-dependent alternating current (ac) susceptibility curves under zero dc field; this is characteristic behaviour of a glassy state that may be partially supressed for 3Co by applying an external dc field. To overcome long-range magnetic ordering, CoII ions are diluted in a diamagnetic ZnII -based matrix, which enables single-molecule magnet behaviour. Interestingly, this strategy allows a bifunctional Cox Zn1-x 2ani material, which is imbued with a strong photoluminescent emitting capacity, as characterised by an intense blue light followed by a green afterglow, to be obtained.

Two 3d-4f metal-organic frameworks as fluorescent sensor array for the discrimination of phosphates based on different response patterns

Both organic and inorganic phosphates play important roles in biological activities. Considering the conventional methods are not very useful for identifying them from their analogues, the sensor array strategy has emerged in recent years, which is able to distinguish chemical species with similar structure or chemical properties. We propose a fluorescence sensor array based on two 3d-4f metal-organic frameworks (MOFs) by recording their direct one-step fluorescence changes with the addition of various phosphates. With the introduction of d-metal ions in the sensor unit synthesis process, two 3d-4f MOFs display higher sensitivity and noticeable fluorescence differences to five phosphates than the homometallic MOFs. Phosphates show diverse affinities to sensor units, forming the basis of differential sensing. The signals are much easier to collect, enabling the sensor array to simply accomplish the identification. Through statistical analysis, the sensor array is proved to be excellent in discriminating individual phosphate and mixture. Discrimination of five phosphates is successfully performed in serum samples to illustrate the practicability of the sensor array. This work presents an efficient and reliable approach for simultaneous detection of different phosphates, which can be extended to devise other array-based sensing methods.

Charge-by-Charge Ion Pairing Preserves Fluorescence Emission

In this issue of Chem, Flood and co-workers demonstrate spatially and electronically isolated cationic dyes in crystalline charge-by-charge ion-pairing systems based on sandwiched [2 + 1]-type anion complexes of a π-electronic cyanostar macrocycle; these dyes exhibit fluorescent colors and intensities similar to those in solution.

Interaction of pyridoxal-derived hydrazones with anions and Co2+, Co3+, Ni2+, Zn2+ cations

ABSTRACT Present paper reports on the interactions between four hydrazones derived from pyridoxal and hydrazides of 2-furoic, thiophene-2-carboxylic, pyrazinoic and pyridine-3-carboxylic acids and 10 different anions (F−, Cl−, I−, PO4 3-, HPO4 2-, H2PO4 3-, NO3 −, SO3 2-, SO4 2-, CH3COO−) and three cations (Co2+, Ni2+, Zn2+). Hydrazones are sensitive towards anion presence in non-buffered DMSO only, thus, indicating that the proton transfer from ligand towards anion is responsible for the changes in colour, UV-Vis and fluorescent spectra. Buffered solutions of ligand mixture with any anion are indistinguishable from free hydrazone. The addition of d-metal ions to pyridoxal-derived hydrazones dissolved in buffered DMSO changes the solution colour, UV-Vis and fluorescent spectra. Conditional rate constants of Co2+ oxidation, as well as the apparent stability constants of Co2+, Co3+ complexes with Schiff bases, were determined from kinetic experiments, while Ni2+, Zn2+ complexes stability constants were calculated from UV-Vis and fluorimetry titration data.

Resonance Raman Optical Activity Shows Unusual Structural Sensitivity for Systems in Resonance with Multiple Excited States: Vitamin B12 Case.

In this work, cobalamins with different upper axial substituents and a cobalamin derivative with a ring modification were studied using chiroptical spectroscopies, in particular resonance Raman optical activity (RROA), to shed light on the influence of structural modifications on RROA spectra in these strongly chiral systems in resonance with multiple excited states at 532 nm excitation. We have demonstrated that for these unique systems RROA possesses augmented structural specificity, surpassing resonance Raman spectroscopy and enabling at the same time measurement of cobalamins at fairy low concentrations of ∼10–5 mol dm–3. The enhanced structural specificity of RROA is a result of bisignate spectra due to resonance via more than one electronic state. The observation of increased structural capability of RROA for cobalamins opens a new perspective for studying chiral properties of other biological systems incorporating d-metal ions.

Subcomponent Self-Assembly of Covalent Metallacycles Templated by Catalytically Active Seven-Coordinate Transition Metal Centers

Coordination geometries of transition metals play vital roles in the self-assembly process of supramolecular coordination complexes. Herein, seven-coordinate 3d metal ions were applied as templates and catalytically active sites for subcomponent self-assembly that resulted in a new category of covalent metallacycles. Single-crystal structures showed that the sizes, configurations, and functionalization of covalent metallacycles could be tuned by the selection of rigid dihydrazide, transition metal ions, and prefunctionalized subcomponents, respectively. Moreover, metallacycles decorated with carboxylic groups could be employed as precursors to prepare aerogels through hierarchical self-assembly, which also exhibited high catalytic activity for cycloaddition of CO2 into cyclic carbonates.

Metal Complexes of Pharmacophore-Containing Pectin with d-Elements Ions (Cu2+, Co2+, and Mn2+)

Spectral methods have been used to study the complexation of apple pectin modified with organic pharmacophores (nicotinic, salicylic, 5-aminosalicylic, or anthranilic acid) with Cu2+, Co2+, and Mn2+ ions in an aqueous solution. The composition and stability series of the metal complexes, corresponding to the empirical Irving–Williams series, have been established. The standard thermodynamic parameters (ΔH°, ΔG°, and ΔS°) of the complexation process have been calculated. It has been shown by means of 13C NMR and IR spectroscopy that the interaction of pharmacophore-containing pectins with d-metal ions (Cu2+, Co2+, Mn2+) occurs via the carbonyl and hydroxyl groups of the modified polymer matrices. The viscosity and thermal properties of the metal complexes have been determined.

Unveiling Catalytic Sites in a Typical Hydrogen Photogeneration System Consisting of Semiconductor Quantum Dots and 3d-Metal Ions.

Semiconductor quantum dots (QDs) in conjunction with non-noble 3d-metal ions (e.g., Fe3+, Co2+, and Ni2+) have emerged as an extremely efficient, facile, and cost-effective means of solar-driven hydrogen (H2) evolution. However, the exact structural change of the active sites under realistic conditions remains elusive, and the mechanism of H2 evolution behind the remarkable activity is poorly understood. Here, we successfully track the structural variation of the catalytic sites in the typical H2 photogeneration system consisting of CdSe/CdS QDs and 3d-metal ions (i.e., Ni2+ used here). That is, the nickel precursor of Ni(OAc)2 changes to Ni(H2O)62+ in neutral H2O and eventually transforms to Ni(OH)2 nanosheets in alkaline media. Furthermore, the in operando spectroscopic techniques of electron paramagnetic resonance and X-ray absorption spectroscopy reveal the photoinduced transformation of Ni(OH)2 to a defective structure [Nix0/Ni1-x(OH)2], which acts as the real catalytic species of H2 photogeneration. Density functional theory (DFT) calculations further indicate that the surface Ni-vacancies (VNi) on the Ni(OH)2 nanosheets enhance the adsorption and dissociation of H2O molecules to enhance the local proton concentration, while the Ni0 clusters behave as H2-evolution sites, thereby synergistically promoting the activity of H2 photogeneration in alkaline media.

Pressure-induced variation of persistent luminescence characteristics in Y3Al5-xGaxO12:Ce3+-M3+ (M = Yb, and Cr) phosphors: opposite trend of trap depth for 4f and 3d metal ions.

Changing the electronic structure of materials by pressure and the accompanying changes in optical properties have attracted scientific interest. We have reported that the energy position of the conduction band (CB) bottom and the crystal field splitting of the Ce3+:5d excited level in Y3Al5-xGaxO12:Ce3+ are changed by applying pressure, which results in the red shifting of the Ce3+:5d → 4f luminescence and the increase of the quenching temperature. We also reported dramatic improvement of the persistent luminescence performance by either Cr3+ or Yb3+ codoping into the Y3Al5-xGaxO12:Ce3+ phosphors. The different trap depths formed by Cr3+ and Yb3+ affect the initial persistent luminescence intensity and the persistent luminescence duration. In this study, the effect of pressure on the persistent luminescence performance was investigated. For the Y3AlGa4O12:Ce3+-Yb3+ phosphor, the slope of persistent luminescence decay curve becomes more gentle with increasing pressure, while for the Y3AlGa4O12:Ce3+-Cr3+ phosphor the slope becomes steeper. These results indicate that the trap depth of Yb3+ becomes deeper and that of Cr3+ becomes shallower with increasing pressure. Based on the pressure-dependence of the luminescence quenching and the trap depth change estimated from the decay slopes, the relative electronic energies of the CB bottom and the Yb2+ (4f14) or Cr2+ (3d4) levels are discussed. The CB bottom energy is increased relative to the ground 1S0 state of Yb2+ with increasing pressure, which results in deepening of the electron trap depth of the Yb2+ state. The opposite tendency of the Cr3+ codoped sample was described by a decreasing tendency of the energy gap between the CB bottom and the Cr2+:eg level, the relative energy level of which is increased by the increase of the crystal field with increasing pressure in the garnet host material, where the electron-trapping Cr2+ ions take the high spin state (t32ge1g) rather than the low-spin state (t42g).