Metal-to-metal Charge Transfer Research Articles

A Mixed‐Valence {Fe13} Cluster Exhibiting Metal‐to‐Metal Charge‐Transfer‐Switched Spin Crossover

AbstractIt is promising and challenging to manipulate the electronic structures and functions of materials utilizing both metal‐to‐metal charge transfer (MMCT) and spin‐crossover (SCO) to tune the valence and spin states of metal ions. Herein, a metallocyanate building block is used to link with a FeII‐triazole moiety and generates a mixed‐valence complex {[(Tp4‐Me)FeIII(CN)3]9[FeII4(trz‐ph)6]}⋅[Ph3PMe]2⋅[(Tp4‐Me)FeIII(CN)3] (1; trz‐ph=4‐phenyl‐4H‐1,2,4‐triazole). Moreover, MMCT occurs between FeIII and one of the FeII sites after heat treatment, resulting in the generation of a new phase, {[(Tp4‐Me)FeII(CN)3][(Tp4‐Me)FeIII(CN)3]8 [FeIIIFeII3(trz‐ph)6]}⋅ [Ph3PMe]2⋅[(Tp4‐Me)FeIII(CN)3] (1 a). Structural and magnetic studies reveal that MMCT can tune the two‐step SCO behavior of 1 into one‐step SCO behavior of 1 a. Our work demonstrates that the integration of MMCT and SCO can provide a new alternative for manipulating functional spin‐transition materials with accessible multi‐electronic states.

Catalytic Decolorization of Rhodamine B, Congo Red, and Crystal Violet Dyes, with a Novel Niobium Oxide Anchored Molybdenum (Nb–O–Mo)

In this work, a new metal-to-metal charge transfer (MMCT) heterogeneous catalyst (Nb–O–Mo) was synthesized by a chemical grafting method under an inert atmosphere. The activity of the covalently anchored oxo-bridged Nb–O–Mo catalyst was estimated for decolorization of Rh B, congo red, and crystal violet dyes in an aqueous solution under fluorescent light. The catalyst was characterized via X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectrometer, Fourier-transform infrared, and FT-Raman. The catalytic decolorization was evaluated from the UV spectra of dyes in aqueous solution by changing different factors, including dye concentration, temperature, and catalyst loading. Decolorization percentages were 83%–89%, 86%–95%, 97%–99% for Rh B, Congo Red and Crystal Violet in 1 min at 298 K, showing the best performance among other catalysts. Decolorization efficiency for 50 ppm of Rh B was improved from 92% to 98%, with a temperature increase to 318 K.

Effective Sensitization of Eu3+ with Ce3+ by suppression of metal-to-metal charge transfer in composite structured TbF3 fluoride particles

Sensitization of Eu3+ with Ce3+ suffers from low emission intensities due to the well-known metal-to-metal charge transfer (MMCT), which effectively quenches the luminescence process. By separating sensitizer and activator spatially, reduced MMCT quenching and thus higher luminescence intensities can be expected. We chose TbF3 as host material, since it is well-known that Tb3+ acts as a mediator in the energy transfer (ET) between Ce3+ and Eu3+. Respective phosphor particles were prepared via precipitation synthesis. X-ray powder diffraction was used to determine the phase purity of the synthesized samples. Emission spectra, excitation spectra and diffuse reflectance spectra were measured to record the photoluminescence properties. Fluorescence lifetime measurements were performed to contrast the decay behavior between the co-doped and the composite approach. Especially by comparison of the emission spectra between the composite particles and the single co-doped phosphor particles it could be demonstrated that the composite approach significantly increases the luminescence intensities due to the spatial separation of Ce3+ and Eu3+.

Photophysical studies of coordination polymers and composites based on heterometallic lanthanide succinate

This work reports the synthesis and characterization of heterometallic Tb/Eu lanthanide-succinate coordination polymers (CPs) under hydrothermal conditions. Compounds were obtained inserting different quantities of Eu3+ ions into the terbium succinate structure, named here xEuyTbSuc (x and y are the percentage of the respective lanthanide ions). Composites with these heterometallic CPs and activated carbon (CP@AC, AC = activated carbon) were obtained using one-pot hydrothermal reactions. Bimetallic CPs with or without presence of AC crystallize in monoclinic system, isostructural to [Ln2(Suc)3(H2O)2]•0.5H2O (Ln = Eu or Tb), already described in the literature, confirmed by X-ray powder diffraction (XRPD) data. Scanning electron microscopy (SEM) images show the CP crystals inside the carbon pores. Luminescence spectroscopy measurements (excitation, emission and lifetime) were conducted to explore photophysical properties of these new materials. Lifetime data indicates all Eu3+ ions are in the same chemical environment. Even at low concentration (1%), Eu3+ transitions (5D0→7FJ (J = 0–4)) were observed in the emission spectrum, along transitions related to the Tb3+ cations (5D4→7FJ (J = 6–3)), due to the metal-to-metal charge transfer (MMCT) between the lanthanide ions. Similar results are found in the composites. Increasing the Eu3+ concentration, a more intense MMCT process took place, suppressing the terbium emission.

Is Indenyl a Stronger or Weaker Electron Donor Ligand than Cyclopentadienyl? Opposing Effects of Indenyl Electron Density and Ring Slipping on Electrochemical Potentials

Complexes of the type RCp2Ti(C2Fc)2 (where RCp = cyclopentadienyl or a substituted cyclopentadienyl) have high molar absorptivity FeII to TiIV metal-to-metal charge transfer (MMCT) absorptions and ...

Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an FeII to TiIV Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States.

Time-resolved transient absorption spectroscopy and computational analysis of D-π-A complexes comprising FeII donors and TiIV acceptors with the general formula RCp2Ti(C2Fc)2 (where RCp = Cp*, Cp, and MeOOCCp) and TMSCp2Ti(C2Fc)(C2R) (where R = Ph or CF3) are reported. The transient absorption spectra are consistent with an FeIII/TiIII metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer (1MMCT) excited state for the FeII/TiIV complexes is not purely MMCT in nature but is contaminated with the higher-energy 1Fc (d-d) state. For the diferrocenyl complexes, RCp2Ti(C2Fc)2, the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the MeOOCCp complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased 1Fc character. The effect of CuI coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in Cp2Ti(C2Fc)2·CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of TiIV-3FeII-CuI.

Effect of Intermolecular Interactions on Metal‐to‐Metal Charge Transfer: A Combined Experimental and Theoretical Investigation

AbstractUnderstanding the effects of intermolecular interactions on metal‐to‐metal charge transfer (MMCT) is crucial to develop molecular devices by grafting MMCT‐based molecular arrays. Herein, we report a series of solvent‐free {Fe2Co2} compounds sharing the same cationic tetranuclear {[Fe(PzTp)(CN)3]2[Co(dpq)2]2}2+ (PzTp−=tetrakis(pyrazolyl)borate, dpq=dipyrido[3,2‐d:2′,3′‐f]quinoxaline) square units but having anions with different size, including BF4−, PF6−, OTf−, and [Fe(PzTp)(CN)3]−. Intermolecular π⋅⋅⋅π interactions between dpq ligands, which coordinate to cobalt ions in the {[Fe(PzTp)(CN)3]2[Co(dpq)2]2}2+ units, can be modulated by introducing different counterions, regulating the distortion of the CoN6 octahedron and ligand field around the cobalt ions. This change results in different MMCT behavior. Computational analyzes reveal the substantial role of the intermolecular interactions tuned by the presence of different counteranions on the MMCT behavior.

Effect of Intermolecular Interactions on Metal‐to‐Metal Charge Transfer: A Combined Experimental and Theoretical Investigation

Understanding the effects of intermolecular interactions on metal-to-metal charge transfer (MMCT) is crucial to develop molecular devices by grafting MMCT-based molecular arrays. Herein, we report a series of solvent-free {Fe2 Co2 } compounds sharing the same cationic tetranuclear {[Fe(PzTp)(CN)3 ]2 [Co(dpq)2 ]2 }2+ (PzTp- =tetrakis(pyrazolyl)borate, dpq=dipyrido[3,2-d:2',3'-f]quinoxaline) square units but having anions with different size, including BF4 - , PF6 - , OTf- , and [Fe(PzTp)(CN)3 ]- . Intermolecular π⋅⋅⋅π interactions between dpq ligands, which coordinate to cobalt ions in the {[Fe(PzTp)(CN)3 ]2 [Co(dpq)2 ]2 }2+ units, can be modulated by introducing different counterions, regulating the distortion of the CoN6 octahedron and ligand field around the cobalt ions. This change results in different MMCT behavior. Computational analyzes reveal the substantial role of the intermolecular interactions tuned by the presence of different counteranions on the MMCT behavior.

A Diruthenium‐Based Mixed Spin Complex Ru25+(S=1/2)‐CN‐Ru25+(S=3/2)

AbstractAn unusual tetra‐nuclear linear cyanido‐bridged complex [Ru2(μ‐ap)4‐CN‐Ru2(μ‐ap)4](BPh4) (1) (ap=2‐anilinopyridinate) has been synthesized and well characterized. The crystallographic data, magnetic measurement, IR, EPR and theoretical calculation results demonstrate that complex 1 is the first example of mixed spin Ru25+‐based complex with uncommon electronic configurations of S=1/2 for the cyanido‐C bound Ru25+ and S=3/2 for the cyanido‐N bound Ru25+. This phenomenon can be understood by the theoretical calculation results that from the precursor Ru2(μ‐ap)4(CN) (S=3/2) to complex 1 the energy gap between π* and δ* orbitals of the cyanido‐C bound Ru25+ core increases from 0.57 to 1.61 eV due to the enhancement of asymmetrical π back‐bonding effect, but that of the cyanido‐N bound Ru25+ core is essential identical (0.56 eV). Besides, the analysis of UV/Vis‐NIR spectra suggests that there exists metal to metal charge transfer (MMCT) from the cyanido‐N bound Ru25+ (S=3/2) to the cyanido‐C bound Ru25+ (S=1/2), supported by the TDDFT calculations.

A Diruthenium-Based Mixed Spin Complex Ru2 5+ (S=1/2)-CN-Ru2 5+ (S=3/2).

An unusual tetra-nuclear linear cyanido-bridged complex [Ru2 (μ-ap)4 -CN-Ru2 (μ-ap)4 ](BPh4 ) (1) (ap=2-anilinopyridinate) has been synthesized and well characterized. The crystallographic data, magnetic measurement, IR, EPR and theoretical calculation results demonstrate that complex 1 is the first example of mixed spin Ru2 5+ -based complex with uncommon electronic configurations of S=1/2 for the cyanido-C bound Ru2 5+ and S=3/2 for the cyanido-N bound Ru2 5+ . This phenomenon can be understood by the theoretical calculation results that from the precursor Ru2 (μ-ap)4 (CN) (S=3/2) to complex 1 the energy gap between π* and δ* orbitals of the cyanido-C bound Ru2 5+ core increases from 0.57 to 1.61 eV due to the enhancement of asymmetrical π back-bonding effect, but that of the cyanido-N bound Ru2 5+ core is essential identical (0.56 eV). Besides, the analysis of UV/Vis-NIR spectra suggests that there exists metal to metal charge transfer (MMCT) from the cyanido-N bound Ru2 5+ (S=3/2) to the cyanido-C bound Ru2 5+ (S=1/2), supported by the TDDFT calculations.

Syntheses, crystal structures and MMCT properties of cyanide-bridged binuclear Ru–Fe complexes

In this report, two new binuclear mixed-valence (MV) compounds and their one-electron oxidation products, [MeCpLRu(μ-CN)Fe(dppe)Cp][PF6]n (L = dppe, (PPh3)2) (n = 1, 2), were synthesized and thoroughly characterized by single-crystal X-ray diffraction analysis. Complexes 12+ and 22+ are one-electron oxidation products of 11+ and 2+, respectively. The electronic absorption results indicate that there exists metal to metal charge transfer (MMCT) in the one-electron oxidation products, which is strongly supported by the theoretical calculations. Complexes 12+ and 22+ can be defined as Class II MV compounds. It is found that the MMCT band is obviously red-shifted on going from 12+ to 22+, because the donating electron ability of (PPh3)2 is stronger than that of dppe.

Porphyrin-Based Conjugated Polymers as Intrinsic Semiconducting Photocatalysts for Robust H2 Generation under Visible Light

Up to now, conjugated organic polymers as organic semiconductors have received more and more attention on the direct conversion of water to hydrogen energy. Herein, we designed and synthesized a series of porphyrin-based conjugated polymers (Zn-ZnDETPP, Co-CoDETPP, and Zn-CoDETPP) with intrinsic semiconducting feature and broad spectral response ability. The Zn-CoDETPP polymer exhibits a photocatalytic H2 generation activity (43 μmol h–1) illuminated by λ ≥ 400 nm light and a record apparent quantum yield (7.36%) at λ = 400 nm monochromatic light among those porphyrin-based polymers reported previously, while the Zn-ZnDETPP and Co-CoDETPP only deliver a H2 generation activity of 8.0 and 33 μmol h–1 respectively. The characteristic results demonstrate that the CoDETPP porphyrin units as bridges in the Zn-CoDETPP polymer can act as an electron acceptor and promote the metal-to-metal charge transfer (MMCT) from zinc porphyrin to cobalt porphyrin units, which could lead to more effective and faster charge separation in the Zn-CoDETPP polymer for the H2 generation reaction. The present results open up a promising strategy applying porphyrin-based conjugated polymers as intrinsic semiconductors for efficient H2 generation and instruct us to efficiently convert solar energy into chemical energy by molecular designing.

New Members of SHG Active Dugganite Family, A3BC3D2O14 (A = Ba, Pb; B = Te, Sb; C = Al, Ga, Fe, Zn; D = Si, Ge, P, V): Synthesis, Structure, and Materials Properties.

A family of compounds, A3BC3D2O14 (A = Ba, Pb; B = Te, Sb; C = Al, Ga, Fe, Zn; D = Si, Ge, P, V), with the Dugganite structure was prepared employing traditional solid-state chemistry methods. PXRD and Rietveld refinement studies indicate that the compounds are stabilized in P321 space group (no. 150). The compounds are found to be SHG active with values ranging from 1.9 to 15.0 × KDP. The compounds exhibit high dielectric constants and low loss in our studies. The noncentrosymmetry related properties of the new Dugganites were understood by band structure calculations. We also explored the present Dugganite-structured oxides for the development of new inorganic colored materials by substituting Co2+, Ni2+, Cu2+, and Fe3+ in place of Zn2+. Thus, substitution of Co2+ and Fe3+ together tunes the blue color of the cobalt compound to blue-green color arising from metal-to-metal charge transfer (MMCT) of Fe3+ and Co2+ ions. The tetrahedrally coordinated Ni2+ in the Dugganite imparts a magenta color.

(INVITED) Ultraviolet and visible luminescence from bismuth doped materials

Bismuth is a non-toxic posttransition metal with a large number of possible valence states which, together with to its tendency to form clusters, make it a versatile but complex dopant for possible luminescence applications. The outer orbitals responsible for luminescence are unshielded and strongly influenced by the surrounding environment, so that their energies are host dependent. In this review article we surveyed models that have been developed to understand the optical properties of bismuth based materials and compiled a few case studies on the luminescence of bismuth doped materials, with some emphasis on trivalent Bi3+ and simple oxide hosts (e.g. CaO:Bi; Y2O3:Bi; ZnO:Bi; La2O3:Bi; SrO:Bi; YPO4:Bi and LaPO4:Bi). Special emphasis was given to the compilation of simplified energy diagrams under different conditions. For the most stable trivalent Bi3+ state it has been experimentally found by many researchers that the absorption spectra consist of medium, weak and strong bands in order of increasing energy (designated the A, B and C bands, respectively), which were interpreted in terms of transitions to the states associated with the 3P1, 3P2 and 1P1 levels of the excited 6s1p1 configuration. Traditionally above the C band energy, but before the host absorption, a further D band is generally encountered, which is attributed to a metal-to-metal charge transfer (MMCT) transition corresponding to the transfer of an electron from the Bi3+ ion to the bottom of the conduction band. Recent modelling of this MMCT band in different materials has clarified that it may occur at energies below the C band and be an important influence on excitation spectra at lower energies. The challenge of unambiguously identifying the nature of emissions is still an ongoing challenge, as illustrated by some of the case studies.

Manipulating Spin Transition To Achieve Switchable Multifunctions.

The spin transition of metal ions involves interconversion between electron configurations exhibiting considerably different functions and plays a substantial role in the chemical, physical, and biological fields. The photoinduced spin transition offers a promising approach to tune various physical properties with high spatial and temporal resolutions for producing smart multifunctional materials not only to explore their basic science but also to satisfy the demands of the next-generation photoswitchable-molecule-based devices. Therefore, it is attracting considerable interest to utilize photoinduced spin transition to simultaneously tune multifunctions. However, two issues are challenging in obtaining reversible and swift manipulation of functions: (1) the interconversion between different electron configurations of photoresponsive units should be reversibly switched via photoinduced spin transition; (2) effective coupling should be built between the photoresponsive and functional units to produce photoswitchable functions utilizing photoinduced spin transition. In this Account, we will review our recent advances in the usage of spin transition of metal ions as actuators for tuning the magnetic, dielectric, fluorescence, and mechanical properties, wherein the role of a photoswitchable spin transition is highlighted. We mainly focus on the study of two spin-transition categories, including spin-crossover (SCO) of one metal ion and metal-to-metal charge transfer (MMCT). Initially, we will describe a strategy for developing photoinduced reversible SCO and MMCT. The role of flexible intermolecular interactions, in particular, π···π interactions, is discussed with respect to a photoinduced reversible MMCT. Then, the SCO and MMCT units were assembled using metallocyanate building blocks to form a chain, wherein the spin states, anisotropy, and magnetic coupling interactions can be photoswitched to tune the single-chain magnet behavior. Besides magnetic properties, the photoinduced spin transition that is associated with the concomitant changing of charge distribution, bond lengths, and absorption spectra can be utilized to tune the multifunctions. Therefore, the transfer of an electron from a central cobalt site to one of the two iron sites in linear trinuclear Fe2Co compounds resulted in the transformation of a centrosymmetric nonpolar molecule into an asymmetric polar molecule, and the molecular electric dipole and dielectric properties can be reversibly switched. Moreover, the spin transition usually involved significant expansion or contraction of the coordination sphere of metal ions because of the population/depopulation of the antibonding eg orbitals. Therefore, colossal positive and negative thermal expansion behaviors were achieved in a layered compound by manipulating the spin-transition process and the rotation of the functional units, thereby providing a strategy for synthesizing phototunable nanomotors. Photoinduced spin transition can also be used to modulate the fluorescence properties by controlling the energy transfer between the fluorescent ligands and the metal sites showing SCO. Finally, we will provide a perspective and detail the remaining challenges that are associated with this research area. We believe that an increasing number of fascinating photoswitchable SCO and MMCT systems will emerge in the near future and that the materials exhibiting various properties and functions that can be manipulated using photoinduced spin transition will provide novel opportunities for the development of smart multifunctional materials and devices.

Tunable trap depth for persistent luminescence by cationic substitution in Pr3+:K1−xNaxNbO3 perovskites

AbstractThe development of efficient red‐emitting persistent phosphor is still an ongoing challenge. In the search of persistent materials in red range, Pr3+ is a good candidate owing to its transitions between 1D2 and 3H4 state at about 612 nm. In this paper, we investigated the red persistent properties of Pr3+‐doped perovskite oxide ABO3, (A: K, Na and B: Nb), which can be elaborated as large single crystal. KNbO3:Pr3+ appears to have weak photoluminescence and no persistent luminescence. However, the cationic substituted compounds K1−xNaxNbO3:Pr3+ (x = 0, 0.4, 0.5, 0.7 0.9, 1) exhibit intense persistent luminescence, which increases steadily with increase in Na content. We correlated persistence behavior with the position of Metal‐to‐Metal Charge Transfer (MMCT) band, which plays crucial role in tuning of the trap depth. The MMCT band position decreases with the addition of Na contents and the thermoluminescence peak shifts toward higher temperature indicating the formation of deeper traps. This is in good agreement with the enhancement of the persistent luminescence suggesting that a proper tailoring of MMCT is needed to design efficient Pr3+‐persistent phosphors with better performance. A detailed analysis on the trap depth, bandgap energy, and persistent luminescence properties is reported tuning the composition in the K1−xNaxNbO3:Pr powder.

Manipulating Metal-to-Metal Charge Transfer for Materials with Switchable Functionality.

Metal-to-metal charge transfer (MMCT) describes electron transfer between metal ions, to generate valence isomers with markedly different electronic configurations. In particular, MMCT changes the spin states of single-metal sites and the coupling interactions between them, while also changing the symmetry in charge distribution. The result is a drastic change in both magnetic and electric properties of the affected material. Moreover, MMCT causes significant variation in bond length and absorption spectra, and induces unusual thermal expansion and photochromic behavior. Thus, materials demonstrating MMCT in response to external stimuli are excellent candidates for switchable multifunctional devices with synergistic responses. In this Minireview, recent progress in utilizing MMCT units as actuators to tune magnetic, electric, thermal expansion, and photochromic properties in cyanide-bridged systems is highlighted, and emphasis is given to the remaining challenges and future perspectives in the field.

Photoinduced Electron Transfer from ZrOCo Binuclear Light Absorber to Pyridine Elucidated by Transient Optical and Infrared Spectroscopy

All-inorganic heterobinculear units can be excited to a metal-to-metal charge transfer (MMCT) state by visible and ultraviolet light and form the core of multicomponent artificial photosystems that rely on conjugated molecular wires to transmit electrons and holes across ultrathin silica separation membranes to catalytic sites. To understand electron transfer from a heterobinuclear unit to an organic molecule, model systems consisting of pyridine interacting with ZrIVOCoII or TiIVOCoII sites on silica nanoparticles were assembled and spectroscopically characterized. Transient optical absorption spectroscopy combined with infrared analysis revealed that visible light excitation of assemblies in which pyridine interacts with the Zr center of the ZrOCo unit results in sub-10 ns separation by electron transfer to the organic moiety. Rapid-scan FT-IR spectroscopy supported by density functional theory confirmed the identity of the pyridine radical anion charge-separated state. This state shows a lifetime of 22...

Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts.

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic polymers, with the aim of photo-activating oxygen-evolving manganese oxide catalysts, which are important components in artificial photosynthesis. The cross-linking technique was applied to obtain a self-standing membrane with a high PW12O403- content. Incorporation and structure retention of PW12O403- within the polymer matrix were confirmed by FT-IR and micro-Raman spectroscopy, and optical characteristics were investigated by UV-Vis spectroscopy, which revealed successful construction of the metal-to-metal charge transfer (MMCT) unit. After deposition of MnOx oxygen evolving catalysts, photocurrent measurements under visible light irradiation verified the sequential charge transfer, Mn → MMCT unit → electrode, andthe photocurrent intensity was consistent with the redox potential of the donor metal (Ce or Co). This method provides a new strategy for preparing integrated systems involving catalysts and photon-absorption parts for use with photo-functional materials.

Suppression of metal-to-metal charge transfer quenching in Ce3+ and Eu3+ comprising garnets by core-shell structure

This work concerns a novel Eu3+ activated phosphor sensitized by Ce3+ in order to obtain absorption in the blue spectral range in combination with line emission in the red spectral range. It is well known that this approach suffers from metal-to-metal charge transfer (MMCT) quenching between Ce3+ and Eu3+. To reduce this quenching mechanism in Tb3Al5O12 (TAG), the sensitizer and activator were spatially separated by creating core-shell particles. Various powder samples including core-shell and non-core-shell particles were prepared via precipitation synthesis and post-annealing steps. The phase purity of the synthesized particles was determined by X-ray powder diffraction. Scanning electron microscope images have been taken to study the particle morphology and shell formation. It turned out that nearly uniform shells could be achieved by precipitation synthesis with urea as precipitant. The photoluminescence properties were investigated by recording emission spectra as well as diffuse reflectance spectra. Furthermore, temperature dependent emission spectra and fluorescence lifetimes were measured to compare thermal quenching and decay behavior of the samples. With around 60%, the external quantum efficiencies (EQE) of TAG:Ce3+ and TAG:Ce3+@TAG:Eu3+ core-shell particles are almost identical. Since the EQE of non-core-shell TAG:Ce3+,Eu3+ is lower than 5%, it could be demonstrated that the core-shell approach significantly increases the EQE by spatial separation of Ce3+ and Eu3+.