The Divalent Lanthanoid Triflates Ln(CF3SO3)2(CH3CN) (Ln=Sm, Eu): Structure, Luminescence, and Magnetism.

The reaction of the trivalent lanthanoide triflates Ln(OTf)3 (Ln=Sm, Eu; OTf=CF3SO3 -) with the respective metals in acetonitrile leads to the Ln(II)-triflates Eu(OTf)2(CH3CN) (monoclinic, P21/n, Z=4, a=1053.54(1), b=610.28(5), c=1946.92(2) pm, ß =98.611(4)) and Sm(OTf)2(CH3CN) (monoclinic, P21/n, Z=4, a=1054.41(4), b=612.16(2), c=1952.65(7) pm, ß =98.524(2)). The isotypic strontium compound Sr(OTf)2(CH3CN) (monoclinic, P21/n, Z=4, a=1056.39(5), b=610.05(3), c=1950.1(1) pm, ß =98.900(2)°) has been obtained from SrCO3 and triflic acid. The compounds have been investigated by X-ray diffraction, vibrational spectroscopy, luminescence spectroscopy, cyclic voltammetry, thermal analysis, and magnetic measurements.

Impact of 1,10-Phenanthroline-Induced Intermediate Valence on the Luminesence of Divalent Europium Halides.

Starting from EuX2 (X = Cl, Br, I), we systematically investigated a variety of divalent europium complexes containing bidentate 1,10-phenanthroline (Phen) ligands. Depending on the Eu/Phen ratio, mono-, di-, and polynuclear complexes are formed, with the latter yielding one-dimensional ∞1[EuBr2(phen)] chains. Seven new divalent europium complexes, [Eu(phen)4(H2O)]Br2·2MeCN, [Eu(phen)4]I2·1.7Tol, [EuBr(phen)3]2Br2·4MeCN, [EuCl2(phen)2]2·2MeCN, [EuBr2(phen)2]2, [EuI2(phen)2]2, and [EuBr2(phen)]x, are presented in this work. All species show remarkable optical properties based on a partial electron transfer from the EuII center to the Phen ligand. The photophysical characterization is further supported by electrochemistry studies in order to describe the intermediate valence of the Eu center.

Thermally and Optically Activated Migration of Charge Carriers in Alkali Metal Sesquioxides.

The alkali metal sesquioxides A4 O6 (A=K, Rb, Cs) are mixed-valent with respect to oxygen and display several degrees of electronic and structural freedom, which give rise to diverse transport and ordering processes. We report on analyses of the respective underlying excitations by diffuse reflectance spectroscopy and thermally activated electron transport. Backed by DFT based band structure calculations we identify three possible mechanisms, inter valence charge transfer from peroxide to superoxide, excitation across the Jahn-Teller gap of tilted superoxide anions, and polaron migration. The activation energies as found by the three different approaches are in a rather narrow range of 0.62-0.89 eV for Rb4 O6 and 0.49-0.65 eV for Cs4 O6 , confirming opacity in the full range of visible light. The effect of the phase transition from cubic to tetragonal as demonstrated for the caesium representative corresponds to a marginal shift to higher activation energy.

Magnetism and Afterglow United: Synthesis of Novel Double Core-Shell Eu2+ -Doped Bifunctional Nanoparticles.

Afterglow-magnetic nanoparticles (NPs) offer enormous potential for bioimaging applications, as they can be manipulated by a magnetic field, as well as emitting light after irradiation with an excitation source, thus distinguishing themselves from fluorescent living cells. In this work, a novel double core-shell strategy is presented, uniting co-precipitation with combustion synthesis routes to combine an Fe3 O4 magnetic core (≈15 nm) with an afterglow SrAl2 O4 :Eu2+ ,Dy3+ outer coat (≈10 nm), and applying a SiO2 protective middle layer (≈16 nm) to reduce the luminescence quenching caused by the Fe core ions. The resulting Fe3 O4 @SiO2 @SrAl2 O4 :Eu2+ ,Dy3+ NPs emit green light attributed to the 4f6 5d1 →4f7 (8 S7/2 ) transition of Eu2+ under UV radiation and for a few seconds afterwards. This bifunctional nanocomposite can potentially be applied for the detection and separation of cells or diagnostically relevant molecules.

Eu(O2 C-C≡C-CO2 ): An EuII Containing Anhydrous Coordination Polymer with High Stability and Negative Thermal Expansion.

Anhydrous EuII -acetylenedicarboxylate (EuADC; ADC2- = - O2 C-C≡C-CO2 - ) was synthesized by reaction of EuBr2 with K2 ADC or H2 ADC in degassed water under oxygen-free conditions. EuADC crystallizes in the SrADC type structure (I41 /amd, Z=4) forming a 3D coordination polymer with a diamond-like arrangement of Eu2+ nodes (msw topology including the connecting ADC2- linkers). Deep orange coloured EuADC is stable in air and starts decomposing upon heating in an argon atmosphere only at 440 °C. Measurements of the magnetic susceptibilities (µeff =7.76 µB ) and 151 Eu Mössbauer spectra (δ=-13.25 mm s-1 at 78 K) confirm the existence of Eu2+ cations. Diffuse reflectance spectra indicate a direct optical band gap of Eg =2.64 eV (470 nm), which is in accordance with the orange colour of the material. Surprisingly, EuADC does not show any photoluminescence under irradiation with UV light of different wavelengths. Similar to SrADC, EuADC exhibits a negative thermal volume expansion below room temperature with a volume expansion coefficient αV =-9.4(12)×10-6  K-1 .

Underestimated Color Centers: Defects as Useful Reducing Agents in Lanthanide-Activated Luminescent Materials.

Inorganic hosts, such as SrB4 O7 or certain nitrides, intrinsically stabilize Eu2+ even when the dopant is an Eu3+ -based precursor and reducing conditions are not employed in the synthesis. Although this concept is well known in the synthesis of phosphorescent materials, the mechanistic details are scarcely understood. Herein, we demonstrate that trapped charge carriers, such as color centers, can also act as redox partners to stabilize certain oxidation states of activators. Eu-activated CsMgCl3 and CsMgBr3 are used as examples. Upon doping with EuCl3 and in the absence of reducing conditions during the synthesis, dominant cyan or green luminescence from Eu2+ ions was observed. Photoluminescence spectroscopy at 10 K revealed that the reduction is correlated to color centers localized at defects. Although defects are typically undesired in phosphors, we have shown that their role may be underestimated and they could be used on purpose in the preparation of selected inorganic phosphors.

A Guide to Brighter Phosphors-Linking Luminescence Properties to Doping Homogeneity Probed by NMR.

Crystalline powders of Ln3+ doped LaPO4 (Ln=Nd, Gd, Dy, Ho, Er, Tm, Yb) have been synthesized to serve in a case study for linking doping homogeneity as determined by NMR to luminescent properties. Samples obtained via different synthesis methods act as examples of homo- and inhomogeneous doping. The sample quality was verified by X-ray diffraction. The homogeneously doped samples show improved luminescent properties in terms of brightness and lifetime which is consistent with the interpretation that, NMR visibility curves probe the distribution of paramagnetic dopants on a similar length scale as necessary for an efficient energy transfer in crystalline phosphors i. e. between sensitizers and activators, and to killer sites. Thus "NMR homogeneity" as observed by visibility curves may serve as a tool to optimize luminescent materials.

Facile Ionic Liquid-Assisted Strategy for Direct Precipitation of Eu2+ -Activated Nanophosphors under Ambient Conditions.

This work describes a novel ionic liquid (IL)-assisted synthesis strategy for a direct and easy production of Eu2+ -doped nanoparticles (NPs), where ILs are also used as fluoride sources to avoid the use of elemental fluorine or toxic hydrofluoric acid. Up to now, the direct synthesis of Eu2+ -doped nanophosphors consisted of an enormous challenge, due to the oxidation to Eu3+ observed in hydrous solution, which is commonly used for the preparation of NPs, generating lattice defects and undesired particle growth or agglomeration by additional reducing steps at high temperatures. In contrast, ILs, unless containing ClO4- or NO3- anions, do not present an oxidizing character, allowing the direct precipitation of NPs, e.g., using Eu2+ containing starting materials. Here, the undoped and Eu2+ -doped BaFCl NPs have been prepared under atmospheric conditions for the first time using ILs as solvents and also as fluoride source, applying sonochemical and microwave-assisted approaches. In general, this method bears an enormous potential for an easy synthesis of fluoride materials compared to inconvenient solid-state methods. In addition, the IL plays the role of a strongly attached protecting shell which represents ≈7-8% of the total NPs weight.

Green Synthesis of A2 SiF6 (A=Li-Cs) Nanoparticles using Ionic Liquids as Solvents and as Fluorine Sources: A Simple Approach without HF.

In this Communication, nanoparticles of the fluoridosilicates A2 SiF6 (A=Li, Na, K, Rb, Cs), which are extremely promising host lattices for future LEDs, are presented for the first time. The preparation method we introduce here is a very simple and energy and time saving one, moreover the usage of toxic HF or elemental fluorine is avoided. In detail, the ionic liquid [Bmim]PF6 was used both as solvent and fluoride source in an ionothermally assisted microwave synthesis. The small size of the so-obtained nanoparticles is of huge relevance for their applications as thin films or for the coverage of surfaces, for example in next-generation white LEDs upon doping with Mn4+ .

Decay times of the spin-forbidden and spin-enabled transitions of Yb2+ doped in CsCaX3 and CsSrX3 (X = Cl, Br, I).

In this paper, a systematic study of the decay times of the spin-enabled and spin-forbidden transitions of Yb2+ doped into the halidoperovskites CsMX3 (M = Ca, Sr; X = Cl, Br, I) is presented. The spin-forbidden transitions are characterized by ms decay times, which are typical for Yb2+. On the contrary, the spin-enabled transitions show much shorter decay times in the range of µs and have so far only been rarely observed. These results allow detailed conclusions about systematics of the decay times of Yb2+ doped in similar compounds and their correlation to the local structure of the coordination sphere of Yb2+ as well as the role of vibrational interaction between the excited high spin (HS) and low spin (LS) states. The halidoperovskites are ideally suited as host lattices in this context and may work as text book examples due to their comparable structures, which allows a detailed interpretation of the decay times in relation to the local structure. An understanding of the impact of the composition and structure of the host material on the decay times of Yb2+ will be of relevance for future applications of this activator in scintillators or lighting materials.

ABiO2X (A = Cd, Ca, Sr, Ba, Pb; X = halogen) Sillen X1 Series: Polymorphism Versus Optical Properties.

The Sillen X1 series of Bi(3+)A(2+)O2X (A = Cd, Ca, Sr, Ba, Pb; X = Cl, Br, I) compounds is composed of three main crystallographic types, namely, the tetragonal form (space group (S.G.) I4/mmm), the orthorhombic form (S.G. Cmcm), and the monoclinic form (S.G. P21/m). Because of Bi(3+)/A(2+) disorder the Bi(3+) based photoluminescence (PL) of the tetragonal polytypes is quenched at room temperature (RT). In the two other ordered forms, the Bi-O-Bi connectivity is different but limited, such that bluish/greenish emission occurs at RT in the monoclinic CdBiO2Cl and CaBiO2Cl and orthorhombic SrBiO2Cl and BaBiO2Cl phases. The crystal structure of BaBiO2Br was refined in the orthorhombic Cmcm space group and also shows RT emission. Focusing on the RT luminescent activity as a key parameter, the PL active compounds were investigated by means of density functional theory calculations and UV-visible reflectance spectroscopy. The influence of A and X ions on the excitation energy is discussed by analyzing the A-O-Bi and Bi-X bonding schemes and gives some insights for rational tuning of both the excitation and emission energies.

Photoluminescence properties of Yb(2+) ions doped in the perovskites CsCaX3 and CsSrX3 (X = Cl, Br, and I) - a comparative study.

The Yb(2+)-doped perovskite derivatives CsMX3 (M = Ca and Sr; X = Cl, Br, and I) are ideal systems for obtaining a detailed insight into the structure-luminescence relationship of divalent lanthanides. The investigation of the respective photoluminescence properties yielded two emission bands in the violet and blue spectral range for all compounds, which are assigned to the spin-allowed and spin-forbidden 5d-4f transitions, respectively. The impact on their energetic positions is dependent on both the covalency of the Yb(2+)-halide bond and the corresponding bond length in agreement with expectations. The excitation spectra provide a detailed fine structure at low temperatures and can be partly interpreted separating the 4f(13) core from the 5d electron in the excited state. The local crystal field in CsSrI3:Yb(2+) provides a special case due to the trigonal distortion induced by the crystal structure that is clearly evident in the luminescence features of Yb(2+). The structure-property relationship of several spectroscopic key quantities of Yb(2+) in this series of halides is analyzed in detail and parallels the properties of Eu(2+) ions doped in the given perovskites.

A Highly Triflated Rare-Earth Ion in [Eu(O3SCF3)8](5-).

The reaction of Eu2O3 with fuming nitric acid, trifluormethanesulfonic acid, and its anhydride in torch-sealed glass ampoules at 120 °C gave the europium compound (NO)5[Eu(O3SCF3)8] (orthorhombic, Fddd, Z = 16, a = 1932.69(4), b = 2878.44(7), c = 2955.12(7) pm, V = 16439.7(7) Å(3)). The compound exhibits the [Eu(O3SCF3)8](5-) anion showing for the first time a lanthanide ion that is exclusively coordinated by eight triflate anions. The anion has been further investigated by DFT calculations, which also allowed clear assignment of the vibrational spectra. Moreover, magnetochemical and luminescence measurements gave additional insight into the properties of this complex. The luminescence spectra revealed that the Eu(3+) ions are in a pseudo D4d symmetric environment.

Prospecting Lighting Applications with Ligand Field Tools and Density Functional Theory: A First-Principles Account of the 4f(7)-4f(6)5d(1) Luminescence of CsMgBr3:Eu(2+).

The most efficient way to provide domestic lighting nowadays is by light-emitting diodes (LEDs) technology combined with phosphors shifting the blue and UV emission toward a desirable sunlight spectrum. A route in the quest for warm-white light goes toward the discovery and tuning of the lanthanide-based phosphors, a difficult task, in experimental and technical respects. A proper theoretical approach, which is also complicated at the conceptual level and in computing efforts, is however a profitable complement, offering valuable structure-property rationale as a guideline in the search of the best materials. The Eu(2+)-based systems are the prototypes for ideal phosphors, exhibiting a wide range of visible light emission. Using the ligand field concepts in conjunction with density functional theory (DFT), conducted in nonroutine manner, we develop a nonempirical procedure to investigate the 4f(7)-4f(6)5d(1) luminescence of Eu(2+) in the environment of arbitrary ligands, applied here on the CsMgBr3:Eu(2+)-doped material. Providing a salient methodology for the extraction of the relevant ligand field and related parameters from DFT calculations and encompassing the bottleneck of handling large matrices in a model Hamiltonian based on the whole set of 33,462 states, we obtained an excellent match with the experimental spectrum, from first-principles, without any fit or adjustment. This proves that the ligand field density functional theory methodology can be used in the assessment of new materials and rational property design.

Multidimensional open-frameworks: combinations of one-dimensional channels and two-dimensional layers in novel BI/M oxo-chlorides.

Here we discuss the synthesis and characterization of three novel bismuth oxo-chlorides ([Bi6Na0.5O7.5][Na0.5Cl3]channel[Cl]layer; [Bi17PbO22][Cl6]channel[Cl3]layer; [Bi9(Pb0.2Mn0.8)O12][Cl3]channel [Cl2]layer) which all show an original multidimensional crystal structure. It is formed of two-dimensional (2D)-layered blocks separated by Cl(-) layers. The blocks are porous with triangular one-dimensional (1D)-Cl(-) channels with various section sizes. This multidimensional feature is unique in the field of Bi and Pb oxo-halides, while so far only 1D or 2D halides units have been reported. The stability of the framework is allowed by Bi(3+)/M(n+) aliovalent substitution to balance charge neutrality. The channel and tunnel walls are formed by edge-sharing O(Bi,M)4 oxocentered tetrahedra, while the triangular tunnel junctions are achieved by O(Bi,M)5 pyramids. The three compounds are rather stable, but only [Bi6Na0.5O7.5][Na0.5Cl3]tunnel[Cl]layer was obtain as a single-phase material so that its photoluminecence properties have been investigated. It shows an unusual red bright luminescence with a maximum at 14150 cm(-1) at low temperatures due to Bi(3+) transitions that are well explained by the Bi-Cl bonding scheme.

Heterometallic europium disiloxanediolates: synthesis, structural diversity, and photoluminescence properties.

This contribution presents a full account of a structurally diverse class of heterometallic europium disiloxanediolates. The synthetic protocol involves in situ metalation of (HO)SiPh2OSiPh2(OH) (1) with either (n)BuLi or KN(SiMe3)2 followed by treatment with EuCl3 in suitable solvents such as 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF). Reaction of EuCl3 with 2 equiv of (LiO)SiPh2OSiPh2(OLi) in DME afforded the Eu(III) bis(disiloxanediolate) "ate" complex [{(Ph2SiO)2O}2{Li(DME)}3]EuCl2 (2), which upon attempted reduction with Zn gave the tris(disiloxanediolate) [{(Ph2SiO)2O}3{Li(DME)}3]Eu (3). Treatment of EuCl3 with (LiO)SiPh2OSiPh2(OLi) in a molar ratio of 1:2 yielded both the ate complex [{(Ph2SiO)2O}3Li{Li(THF)2}{Li(THF)}]EuCl·Li(THF)3 (4) and the LiCl-free europium(III) complex [{(Ph2SiO)2O}2{Li(THF)2}2]EuCl (5). Compound 5 was found to exhibit a brilliant red triboluminescence. When (KO)SiPh2OSiPh2(OK) was used as starting material in a 3:1 reaction with EuCl3, the Eu(III) tris(disiloxanediolate) [{(Ph2SiO)2O}3{K(DME)}3]Eu (6) was isolated. Attempted ligand transfer between 5 and (DAD(Dipp))2Ba(DME) (DAD(Dipp) = N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene) afforded the unique mixed-valent Eu(III)/Eu(II) disiloxanediolate cluster [(Ph2SiO)2O]6Eu(II)4Eu(III)2Li4O2Cl2 (7). All new complexes were structurally characterized by X-ray diffraction. Photoluminescence studies were carried out for complex 5 showing an excellent color quality, due to the strong (5)D0→(7)F2 transition, but a weak antenna effect.

Unusual photoluminescence properties of the 3D mixed-lanthanide-organic frameworks induced by dimeric structures: a theoretical and experimental approach.

The present work describes a complementary experimental and theoretical investigation of the spectroscopic properties of the four isostructural 3D Ln-MOFs (wherein PDC = pyrazole-3,5-dicarboxylate, [La2(PDC)3(H2O)4]·2H2O (1), [(La0.9Eu0.1)2(PDC)3(H2O)4]·2H2O (2), [(La0.9Tb0.1)2(PDC)3(H2O)4]·2H2O (3) and [(La0.9Eu0.5Tb0.5)2(PDC)3(H2O)4]·2H2O (4)). The experimental data and theoretical calculations show that the singular photophysical properties presented by these Ln-MOFs are induced by strong interaction between the Ln(3+) ions.

Physical properties of superbulky lanthanide metallocenes: synthesis and extraordinary luminescence of [Eu(II)(Cp(BIG))2] (Cp(BIG) = (4-nBu-C6H4)5-cyclopentadienyl).

The superbulky deca-aryleuropocene [Eu(Cp(BIG))2], Cp(BIG) = (4-nBu-C6H4)5-cyclopentadienyl, was prepared by reaction of [Eu(dmat)2(thf)2], DMAT = 2-Me2N-α-Me3Si-benzyl, with two equivalents of Cp(BIG)H. Recrystallizyation from cold hexane gave the product with a surprisingly bright and efficient orange emission (45% quantum yield). The crystal structure is isomorphic to those of [M(Cp(BIG))2] (M = Sm, Yb, Ca, Ba) and shows the typical distortions that arise from Cp(BIG)⋅⋅⋅Cp(BIG) attraction as well as excessively large displacement parameter for the heavy Eu atom (U(eq) = 0.075). In order to gain information on the true oxidation state of the central metal in superbulky metallocenes [M(Cp(BIG))2] (M = Sm, Eu, Yb), several physical analyses have been applied. Temperature-dependent magnetic susceptibility data of [Yb(Cp(BIG))2] show diamagnetism, indicating stable divalent ytterbium. Temperature-dependent (151)Eu Mössbauer effect spectroscopic examination of [Eu(Cp(BIG))2] was examined over the temperature range 93-215 K and the hyperfine and dynamical properties of the Eu(II) species are discussed in detail. The mean square amplitude of vibration of the Eu atom as a function of temperature was determined and compared to the value extracted from the single-crystal X-ray data at 203 K. The large difference in these two values was ascribed to the presence of static disorder and/or the presence of low-frequency torsional and librational modes in [Eu(Cp(BIG))2]. X-ray absorbance near edge spectroscopy (XANES) showed that all three [Ln(Cp(BIG))2] (Ln = Sm, Eu, Yb) compounds are divalent. The XANES white-line spectra are at 8.3, 7.3, and 7.8 eV, for Sm, Eu, and Yb, respectively, lower than the Ln2O3 standards. No XANES temperature dependence was found from room temperature to 100 K. XANES also showed that the [Ln(Cp(BIG))2] complexes had less trivalent impurity than a [EuI2(thf)x] standard. The complex [Eu(Cp(BIG))2] shows already at room temperature strong orange photoluminescence (quantum yield: 45 %): excitation at 412 nm (24,270 cm(-1)) gives a symmetrical single band in the emission spectrum at 606 nm (νmax =16495 cm(-1), FWHM: 2090 cm(-1), Stokes-shift: 2140 cm(-1)), which is assigned to a 4f(6)5d(1) → 4f(7) transition of Eu(II). These remarkable values compare well to those for Eu(II)-doped ionic host lattices and are likely caused by the rigidity of the [Eu(Cp(BIG))2] complex. Sharp emission signals, typical for Eu(III), are not visible.

Glowing selenates: novel alkaline earth nanoparticles.

Up to now, no selenate nanoparticles have been described even though much research has been done on respective oxidic compounds such as sulfates and phosphates. For the first time, alkaline earth selenates of the composition MSeO4 (M = Ca, Sr, Ba) were synthesized as nanoparticles or nanorods as described in this publication. For this purpose, a micro emulsion method was applied using CTAB as a surfactant. Using X-ray diffraction measurements (XRD) phase purity of the materials could be proven. Furthermore, the nanoparticles were analyzed by raster electron microscopy (REM) and dynamic light scattering (DLS) measurements. Finally, the products were doped with small amounts of Eu3+ to obtain luminescent materials. Successful doping was demonstrated by luminescence investigations in the region of 18 000 to 14 000 cm-1 (550-715 nm). Incorporation of Eu3+ led to strong red-emitting nanoparticles. Low temperature measurements at 10 K allowed conclusions about the site symmetry of Eu3+ ions located on the alkaline earth sites.

Surface-anchored MOF-based photonic antennae.

The loading of a metal-organic framework (MOF), [Cu(3)(btc)(2)xH(2)O] HKUST-1, with europium ß-diketonate complexes is studied with the goal to using the porous molecular framework as a photonic antenna. Whereas loading of HKUST-1 powder particles produced via the conventional solvothermal synthesis method was strongly hindered, for HKUST-1 SURMOFs, thin MOF films fabricated using the liquid phase epitaxy method, a high filling factor can be achieved. The optical properties of the HKUST-1-MOFs before and after loading were analysed with the aid of luminescence spectroscopy. Careful analysis of the absorption spectra reveals the presence of an effective energy transfer between the HKUST-1 framework and the Eu(3+) centers.