Volume 69, Issue 2

Active waveguides can be applied in environmental sensors, telecommunication, medicine, computers and electronic convertors. The sol-gel technology allows formation of planar and fiber waveguides incorporating fluorescent organic and inorganic dopands. Several sol-gel matrices which enable the incorporation of lanthanides and their complexes and fluorescent organic molecules are presented. A short description of the theory of luminescence of lanthanides is outlined and examples of active waveguides known today are given. The mechanisms for infrared to visible light conversion are explained. Suggestions for practical approach to novel active waveguides are given. Increase of fluorescence as a result of an interaction of fluorescent species with metal nanoparticles is presented. Graphical Abstract Fluorescence-active Waveguides by the Sol-Gel Method. Theory and Application

The spectroscopic properties of the Mn 4+ ion (3d 3 ) in the double perovskite NaLaMgTeO 6 are reported in this work. Evidence is presented for the occupation by the Mn 4+ ion of both the six coordinated Mg 2+ and Te 6+ sites in the host structure. The Mn 4+ energy levels are calculated using the exchange charge model of crystal field theory for both occupied sites. The results of our calculations yield the crystal field splitting and Racah parameters of Dq = 2008 cm −1 , B = 790 cm −1 , C =2881 cm −1 , with C/B=3:65 (Mg 2+ site) and Dq=2008 cm −1 , B=790 cm −1 ,C =2949 cm −1 , with C/B = 3:73 (Te 6+ site). A cross-cutting comparative study of the variations in the crystal field splitting and the Racah parameters of the six-coordinated Mn 4+ ion in a series of materials with the perovskite structure are presented. Graphical Abstract Optical Spectroscopy and Crystal Field Studies of the Mn 4+ Ion (3d 3 ) in the Double Perovskite NaLaMgTeO 6

Colorless, platelet-shaped single crystals of Na 2 Y 3 Cl 3 [TeO 3 ] 4 were synthesized by a reaction of Y 2 O 3 and TeO 2 (molar ratio 3 : 8) in evacuated silica ampoules within five days at 775 °C with an excess of NaCl added as flux and reagent. The new quintenary compound crystallizes in the monoclinic space group C2/c with the lattice parameters a = 2380:69(14), b = 552:76(3), c = 1662:48(9) pm, β = 134:045(3)° and Z = 4. The crystal structure presents one of the rare oxotellurates(IV) containing isolated [TeO 3 ] 2− units, which could be verified by Raman spectroscopy. Doping with Eu 3+ (3 mass-%) at the Y 3+ sites in eightfold oxygen coordination leads to a strong orange-red luminescence of Na 2 Y 3 Cl 3 [TeO 3 ] 4 :Eu 3+ under UV excitation with a wavelength of λ = 254 nm. Beyond the electronic Eu 3+ transitions of the 4 f 6 state, the fluorescence excitation spectrum shows an intense broad charge-transfer band resulting from both O 2− →Eu 3+ ligand-to-metal charge-transfer (LMCT) as well as electronic Eu 3+ d→f and Te 4+ s→p transitions. The emission spectrum features the prominent f→f transitions of Eu 3+ -doped oxidic compounds. The optical band gaps of the undoped and doped samples were determined by diffuse reflectance spectroscopy revealing just minimal doping effects on the crystal structure. The optical band gaps could be assigned in the excitation spectrum in the charge-transfer broad-band region supporting the assumption of an inorganic antenna effect of the Te 4+ cations. Graphical Abstract Na 2 Y 3 Cl 3 [TeO 3 ] 4 : Synthesis, Crystal Structure and Spectroscopic Properties of the Bulk Material and its Luminescent Eu 3+ -doped Samples

The influence of 0.1 mol-% Eu 2+ doping in a transparent MgAl 2 O 4 ceramic on the optical properties has been investigated. The goal was to prepare first an Eu 3+ -doped spinel by a conventional sintering process for a transparent ceramic followed by a reductive densification step yielding an Eu 2+ -doped product. Commercially available spinel powder was doped with 0.1 mol-% europium using a nitrate salt and afterwards compacted into green bodies. In the following process the specimens were pre-sintered and post-densified using hot isostatic pressing (HIP) in an argon atmosphere. The transparency of the disks was measured optically using UV/Vis and fluorescence spectroscopy. After optimization of the sintering conditions the resulting Eu 2+ -doped spinel showed a transparency of 77:4% and a conversion of the UV light with a wavelength of 260 nm into blue light with a wavelength of 450 nm. Graphical Abstract Preparation and Characterization of Transparent, Photoluminescent MgAl 2 O 4 :Eu 2+ Ceramics

Lu 2 O 3 :Tb,Hf ceramics were investigated as energy storage materials. It was shown that X-rays caused a significant thermoluminescence with a main band around 250 °C at a heating rate of 4:8 °Cs −1 . 780 nm radiation released up to about 75% of the stored energy, while 420 nm light liberated all the stored energy in the form of green photons produced by the Tb 3+ ions. Graphical Abstract Energy Recovery from Lu 2 O 3 :Tb,Hf Ceramic Storage Phosphors

Similar to many other Eu 2+ ,RE 3+ -co-doped persistent luminescence materials, for Sr 2 MgSi 2 O 7 :Eu 2+ ,RE 3+ the initial intensity and duration of persistent luminescence was also found to depend critically on the rare-earth (RE) co-doping. An enhancement of 1 - 2 orders of magnitude in these properties could be obtained by Dy 3+ co-doping whereas total quenching of persistent luminescence resulted from the use of Sm 3+ and Yb 3+ . To solve this drastic disparity, the effects of the individual RE 3+ ions were studied with thermoluminescence (TL) spectroscopy to derive information about the formation of traps storing the excitation energy. The charge compensation defects were concluded to be the origin of the complex TL glow curve structure. The tuning of the band gap of the Sr 2 MgSi 2 O 7 host and especially the position of the bottom of the conduction band due to the Eu 2+ ,RE 3+ co-doping was measured with the synchrotron radiation vacuum UV (VUV) excitation spectra of the Eu2+ dopant. The model based on the evolution of the band gap energy with RE 3+ co-doping was found to explain the intensity and duration of the persistent luminescence.

A series of Sm 3+ -activated molybdates Li 3 Ba 2 (La 1−x Sm x ) 3 (MoO 4 ) 8 with 0<̲x<̲1 (0% to 100% Sm 3+ ) have been prepared by the conventional solid-state synthesis method, and their optical properties were investigated. Reflection, excitation and emission spectra were recorded and put in relation to the various [Xe]4 f 5 → [Xe]4 f 5 transitions of Sm 3+ . The positions of the charge transfer bands of Sm 3+ and Mo 6+ were resolved by Gaussian peak fitting. Emission spectra recorded at 100 K revealed the Stark sublevels of the Sm 3+ energy levels. Time-dependent emission measurements of the 4 G 5/2 → 6 H 9/2 transition were performed to disentangle the influence of temperature and activator concentration on the decay constants. The results are discussed in the context of the structure of the host material. Sm 3+ occupies two different crystallographic sites at higher activator concentrations, which results in a bi-exponential decay curve. Temperature-dependent emission spectra were recorded to determine the thermal quenching behavior of the material. Internal and external quantum efficiencies (IQE and EQE) have been calculated. The IQE is independent of temperature, while the emission intensity strongly decreases at temperatures higher than 400 K. It is concluded that the photon escape efficiency in Li 3 Ba 2 La 3 (MoO 4 ) 8 correlates with temperature. An EQE of 44% was achieved for the 2% Sm 3+ sample, which is comparatively high for Sm 3+ . Color points and luminous efficacies were calculated. The color point is independent of the Sm 3+ concentration, but a blue-shift was observed with increasing temperature. This shift may be caused by lattice expansion and a subsequent decrease of spin-orbit coupling. Graphical Abstract Synthesis and Optical Properties of Li 3 Ba 2 La 3 (MoO 4 ) 8 :Sm 3+ Powders for pcLEDs

The work presented is mainly focused on synthesis and study of structural and optical properties of microcrystalline Nd 3+ -doped monoclinic dilanthanum dimolybdate at both room and cyrogenic temperatures (4K and 77 K). These compounds might be useful for application in the future as optical materials and also as transparent ceramics when the structure is cubic. The Nd3+-doped phases with monoclinic structure (α-form, space group P21, unit cell parameters a = 7:1426, b = 7:1544, c = 7:1618 Å and β = 89:538°) were observed for a concentration of the optically active ions equal to 5%. When the concentration of the Nd 3+ ions is higher than 15%, a cubic structure is formed (β-form, space group P2 1 3, with the lattice parameter a = 7:155±0:005 Å). A series of Nd 3+ - doped La 2 Mo 2 O 9 phases with different concentration of Nd 3+ were prepared using conventional solid-state reactions. The formation of phase-pure Nd 3+ -doped La 2 Mo 2 O 9 has been monitored by powder X-ray diffraction, DSC, SEM, Raman, and FT-IR absorption techniques. High-resolution absorption and emission spectra, as well as the dynamics of the Nd 3+ excited states characterized by decay time measurements were recorded from room temperature to 4 K. At least two slightly different crystallographic sites are available for the Nd 3+ ions. First results show that this new Nd 3+ -doped monoclinic La 2 Mo 2 O 9 molybdate phosphor is promising for applications of ultra-short pulse lasers. Graphical Abstract Development of Nd 3+ -doped Monoclinic Dimolybdates La 2 Mo 2 O 9 as Optical Materials

The double phosphate KLuP 2 O 7 doped with Pr 3+ ions was prepared by solid-state reaction. The material obtained was characterized by powder X-ray diffraction. The luminescence spectroscopy and the excited state dynamics of this material were studied upon excitation with VUV synchrotron radiation. The 5d-4 f emission of Pr 3+ upon both direct and band gap excitation was detected and assigned. The decay kinetics of the Pr 3+ 5d-4 f emission is characterized by a decay time of about 20 ns and is nearly temperature independent within the range 8 - 300 K. Both dynamics and energy transfer peculiarities revealed in the study suggested good potentials for application of KLuP 2 O 7 :Pr 3+ as a fast scintillator material. Graphical Abstract Synchrotron Radiation Study of Interconfigurational 5d-4f Luminescence of Pr 3+ in KLuP 2 O 7

Due to their ease of fabrication, chemical stability and optical transparency polydimethylsiloxanederived silicones ([O-Si(CH 3 ) 2 ] ∞ ) are excellent matrices to enable optical functions. We here report on the luminescence of silicone hybrids with red-emitting europium diketonate complexes, which have not been described previously in this matrix. The problem of too low solubility of the pure complexes has been resolved by co-coordination with trioctylphosphine oxide (TOPO), which permits complex concentrations of up to 5×10 −3 mol L −1 , at the same time maintaining complete transparency. Quantum efficiencies in excess of 60% could thus be obtained for benzoyltrifluoroacetonates, and near 50% for thenoyltrifluoroacetonates. These high efficiencies have been confirmed by room-temperature life time measurements, which displayed straight single-exponential decay behavior for both complexes independent of their concentration in the silicone. Graphical Abstract Efficient Red Emission from Europium Chelate-Silicone Host-Guest Hybrids

Nanocomposites combining functional nanoparticles and transparent polymers allow for stabilization of filler properties over long periods of time while retaining transparency of the polymer matrix. Here we employ CuInS 2 /ZnS quantum dots (QDs), ternary visible- and NIR-emitting semiconductors as wavelength-tunable luminescent fillers. Luminescence in the near infrared (NIR) is of particular interest in medicine which allows deep penetration into human tissue enabling in vivo diagnostics and treatment, while visible emitters may serve as color converters in displays or lighting. To stabilize the optical properties of QDs and prevent agglomeration, polymethyl metacrylate (PMMA) was chosen as a matrix. These novel polymer nanocomposites (PNCs) show good optical properties and stability under ambient conditions, and can be easily deposited over large areas. High-quality QDs and hydrophobic functionalization with long-chain hydrocarbons are a prerequisite for embedding into a PMMA matrix. Transparent PNC films without visible scattering losses were obtained for 1 wt-% QD loading with respect to the polymer. Partial transparency is retained up to 10 wt-% QD loading and vanishes rapidly at higher loading. Luminescence properties increase up to 5 wt-% and then decrease rapidly due to QD agglomeration and reabsorption between adjacent particles. Potential applications include converter materials for medical applications, laser layers, displays and white LEDs. Graphical Abstract Transparent CuInS 2 PMMA Nanocomposites Luminescent in the Visible and NIR Region

Different sol-gel strategies based on functionalization of ZrO 2 :Eu microparticles with 1,10- phenanthroline (phen) and incorporation of colloidal Eu(phen) 2 (NO 3 ) 3 into zirconia have been used to obtain hybrid sol-gel composites with controlled optical properties. The process leads to materials with quantum yields of about 48% monitoring the 615 nm emission line at 350 nm excitation. Excitation/ luminescence spectroscopy, diffuse reflectance spectroscopy and X-ray diffraction have been used to characterize the hybrid zirconia composites. Graphical Abstract Functionalization of Sol-Gel Zirconia Composites with Europium Complexes

The [RE(NTA)(H 2 O) 2 ]⋅(H 2 O) rare-earth complexes (RE 3+ : Eu, Gd, Tb and NTA: nitrilotriacetate) were synthesized and characterized by elemental analysis, thermogravimetry, X-ray diffraction, and infrared spectroscopy. The complexes show high crystallinity and thermostability. The photoluminescence behavior was studied based on the excitation and emission spectra, and luminescence decay curves. The compounds exhibit red (Eu 3+ ), green (Tb 3+ ) and UV (Gd 3+ ) monochromatic emissions under UV excitation. The NTA ligand acts as luminescence sensitizer in the ligand-to- RE 3+ intramolecular energy transfer process, owing to the fact that the first excited triplet state (T1: 29 000 cm −1 ) is located above of the 5 D 0 (Eu 3+ ) and 5 D 4 (Tb 3+ ) emitting levels. The experimental intensity parameters Ωλ (λ: 2 and 4) for the Eu 3+ NTA complex were calculated, and the results are discussed. The Gd 3+ -NTA complex exhibits a high-intensity emission band in the UV region (32 000 cm −1 ) assigned to the 6 P 7/2 → 8 S 7/2 intraconfigurational transition. Graphical Abstract Red (Eu 3+ ), Green (Tb 3+ ) and Ultraviolet (Gd 3+ ) Emitting Nitrilotriacetate Complexes Prepared by One-step Synthesis

New ternary Eu(III) 5-(2-pyridyl-1-oxide)tetrazolate complexes with phosphine oxide co-ligands have been synthesized and characterized by elemental analysis, and IR and 1 H NMR spectroscopic techniques. The analytical data revealed that these complexes are mononuclear, and the central Eu(III) ion is coordinated by three oxygen and three nitrogen atoms of the tetrazolate and two oxygen atoms from the phosphine oxide ligands. The ancillary ligands increased remarkably the luminescence efficiency of the Eu(III) tetrazolate. Graphical Abstract Photoluminescent Tetrazolate-based Eu(III) Complexes: An Outstanding Impact of Aromatic Phosphine Oxide Co-ligands on the Photoluminescence Quantum Yields

Tb 2 (bpdc) 3 and Eu 2 (bpdc) 3 nanoparticles (bpdc: 2,2ʹ-bipyridine-4,4ʹ-dicarboxylate) have been prepared via straightforward precipitation from aqueous solution. The nanoparticles exhibit mean diameters of 41(5) nm (Tb 2 (bpdc) 3 ) and 56(4) nm (Eu 2 (bpdc) 3 ) and show a very good colloidal stability in aqueous suspension. Particle size and chemical composition have been characterized based on electron microscopy, X-ray diffraction, infrared spectroscopy and thermogravimetry. Photoluminescence validates an efficient excitation of Tb 3+ /Eu 3+ via the bpdc ligand as an antenna that leads to intense characteristic green and red emissions. The absolute quantum yields of Tb 2 (bpdc) 3 and Eu 2 (bpdc) 3 have been determined at 28 and 12%, respectively. Although rare-earth metal-based photoluminescence is typically quenched in water due to vibronic loss processes (v(O-H)), here, the antenna effect and the shielding of the metal centers via the bpdc ligand are very efficient, allowing for an intense green and red emission of the Tb 2 (bpdc) 3 and Eu 2 (bpdc) 3 nanoparticles even in aqueous suspension. Graphical Abstract Tb 2 (bpdc) 3 and Eu 2 (bpdc) 3 Nanoparticles (bpdc: 2,2ʹ-Bipyridine-4,4ʹ- dicarboxylate) and Their Luminescence

The series of luminescent monomeric lanthanide thiazole complexes [LnCl 3 (thz) 4 ]⋅0.5thz (Ln = Sm, Eu, Gd, Tb, Dy; thz=1,3-thiazole) has been synthesised and characterised by powder and singlecrystal X-ray diffraction, IR and photoluminescence spectroscopy, DTA/TG as well as elemental analysis. The colourless compounds exhibit photoluminescence in the visible region with varying quantum efficiencies up to QY = 48% for [TbCl 3 (thz) 4 ]⋅0.5thz. Both, the lanthanide ions as well as the thiazole ligand contribute to the luminescence. Excitation can be achieved via intra-4 f transitions and by exciting the ligand, emission is observed mainly from the lanthanide ions again by 4 f transitions. Thiazole can transfer energy to the lanthanide ions, which further feeds the lanthanide emission by an efficient antenna effect even at room temperature. The lanthanide ions show pentagonalbipyramidal coordination by three chloride anions and four N atoms of 1,3-thiazole, which leads to a strong 5 D 0 → 7 F 4 transition for europium. Significant differences arise as compared to thiophene complexes because no sulphur atom is involved in the metal coordination, as the thiazole ligand is solely coordinated via its nitrogen function. Graphical Abstract 1,3-Thiazole as Suitable Antenna Ligand for Lanthanide Photoluminescence in [LnCl 3 (thz) 4 ]·0.5thz, Ln = Sm, Eu, Gd, Tb, Dy

This work describes the synthesis and photophysical characterization of Ir(III) and Ru(II) complexes bearing terminal amino groups, which act as anchoring units for the attachment to quantum dots, QDs. The photophysical properties of the metal complexes in combination with different types of QDs, allows directional photoinduced processes in the assemblies. In particular, we show photoinduced energy transfer from the luminescent excited Ir(III) unit to the CdTe nanocrystals, with an efficiency of 40%. The directionality was then inverted by employing an emitting Ru(II) complex as energy acceptor, in combination with photoluminescent CdSe/ZnS quantum dots. The efficiency of the photoinduced energy transfer from the nanocrystals to the Ru(II) center was estimated to be as high as 75%. This work provides model systems for nanoassemblies based on quantum dots and metal complexes for optoelectronic applications, and as active light-harvesting systems. Graphical Abstract Bidirectional Photoinduced Energy Transfer in Nanoassemblies of Quantum Dots and Luminescent Metal Complexes