Lanthanide-cored fluorinated dendrimer complexes: synthesis and luminescence characterization

Eu³⁺-, Tb³⁺- and Er³⁺-cored dendrimer complexes were prepared by self-assembly of three fluorinated dendrons, each with a carboxylate anion focal point, around the lanthanide ion. Energy transfer from the peripheral fluorinated phenyl moieties of the dendrons to the lanthanide cation was evidenced spectroscopically for Eu³⁺- and Tb³⁺-cored dendrimer complexes in solution. The excitation of perfluorinated aromatic groups was found to decay with ca. 0.7 ns and a longer decay time 10-13 ns was related to the coordination at the Ln³⁺ focal point. Luminescence from the lanthanide core decays with lifetime in the range 1-1.5 ms over a wide concentration range (μM-mM), similar to the luminescence decay time of the corresponding acetate ion complexes in D₂O. The main quenching mechanism of the lanthanide emission appears to be due to vibrations among surrounding C-H bonds of the intermediate shell of the flexible dendrimer scaffold. Antenna effect and energy harvesting from the surface of the dendrimer and transfer to the core was the main mechanism for luminescecnce in the dendrimer complexes with lanthanide cations.     

Preparation and luminescence properties of hybrid materials containing lanthanide complexes covalently bonded to a terpyridine-functionalized silica matrix

New kinds of organic-inorganic hybrid materials consisting of lanthanide (Er³⁺, Eu³⁺, and Tb³⁺) complexes covalently bonded to a silica-based network have been obtained by a sol-gel approach. A new versatile compound containing terpyridine has been synthesized by 4′-p-aminophenyl-2,2′:6′,2″-terpyridine and 3-(triethoxysilyl)propyl isocyanate, which is used as the a ligand of lanthanide ions and also the siloxane network precursor. The obtained hybrid materials were characterized by FT-IR, TGA, DSC, near-infrared, and visible spectrofluorometer, as well as decay analysis. For the Hybrid-Er and Hybrid-Eu, excitation at the ligand absorption wavelength resulted in the typical near-IR luminescence (centered at around 1.54 μm) resulting from the ⁴I_13/2-⁴I_15/2 transition of Er³⁺ ions and strong visible region emission of the Eu³⁺ ions (⁵D₀-⁷F_J), which contributed to the efficient energy transfer from the ligands to the lanthanide ions. However, we have not found strong emission for the Hybrid-Tb. This indicated that the energy transfer did not take place in this system. A model of indirect excitation mechanism to explain the phenomenon was also suggested.     

Structure and photophysical behavior of 2,2′-bipyrimidine/lanthanide ion complexes in various environments

The photophysical behavior of 2,2′-bipyrimidine has been studied alone and in the presence of several lanthanide or other metal ions. This substance, which is employed as bridging ligand in homo- and hetero-dinuclear complexes, can form stable complexes with luminescent lanthanide ions like Eu³⁺ and Tb³⁺. Complexes precipitated from common solvents are crystalline with a structure that consists of discrete, centrosymmetric dinuclear entities with a planar ligand configuration. These complexes are strongly luminescent. Luminescence is sensitized by ligand-to-metal energy transfer. However, when the ligand and metal ions are mixed in an unconventional solvent, like a poly(ethylene glycol) oligomer, all reagents stay in solution and produce a different type of complex where only an enhanced ligand-centered fluorescence can be observed. It is possible that such fluorescence is emitted by 2,2′-bipyrimidine in a non-planar configuration. This behavior has also been observed with other heterocyclic ligands that can exist in different conformers, like terpyridine, and it may explain why some ligand-lanthanide complexes sometimes fail to sensitize efficient photoluminescence.     

Preparation and Time-Resolved Luminescence Bioassay Application of Multicolor Luminescent Lanthanide Nanoparticles

Because highly luminescent lanthanide compounds are limited to Eu³⁺ and Tb³⁺ compounds with red (Eu, ~615 nm) and green (Tb, ~545 nm) emission colors, the development and application of time-resolved luminescence bioassay technique using lanthanide-based multicolor luminescent biolabels have rarely been investigated. In this work, a series of lanthanide complexes covalently bound silica nanoparticles with an excitation maximum wavelength at 335 nm and red, orange, yellow and green emission colors has been prepared by co-binding different molar ratios of luminescent Eu³⁺–Tb³⁺ complexes with a ligand N,N,N¹,N¹-(4′-phenyl-2,2′:6′,2′′-terpyridine-6,6′′-diyl)bis(methylenenitrilo) tetrakis (acetic acid) inside the silica nanoparticles. The nanoparticles characterized by transmission electron microscopy and luminescence spectroscopy methods were used for streptavidin labeling, and time-resolved fluoroimmunoassay (TR-FIA) of human prostate-specific antigen (PSA) as well as time-resolved luminescence imaging detection of an environmental pathogen, Giardia lamblia. The results demonstrated the utility of the new multicolor luminescent lanthanide nanoparticles for time-resolved luminescence bioassays.     

Luminescence studies on lanthanide ions (Eu, Dy and Tb) doped YAG:Ce nano-phosphors

Yttrium aluminum garnet nanoparticles both undoped and doped with lanthanide ions (Ce³⁺, Eu³⁺, Dy³⁺ and Tb³⁺) having average size around 30 (±3 nm) nm were prepared by glycine nitrate combustion method followed by annealing at a relatively low temperature of 800 °C. Increase in the annealing temperature has been found to improve the luminescence intensity and for 1200 °C heated samples there exists strong energy transfer from Tb³⁺ to Ce³⁺ ions in YAG:Ce(2%),Tb(2%) nanoparticles as revealed by luminescence studies. Co-doping the YAG:Ce nanoparticles with Eu³⁺ results in significant decrease in the emission intensity of both Ce³⁺ and Eu³⁺ ions and this has been attributed to the oxidation of Ce³⁺ to Ce⁴⁺ and reduction of Eu³⁺ to Eu²⁺ ions. Dy³⁺ co-doping did not have any effect on the Ce³⁺ emission as there is no energy transfer between Dy³⁺ and Ce³⁺ ions.     

Analysis of guanosine-5′-monophosphate at nM level based on the co-luminescence effect of Tb-Gd-GMP system

This paper first found the co-luminescence effect of guanosine-5′-monophosphate (GMP) system. Experiment showed that La³⁺, Gd³⁺, Sc³⁺ and Y³⁺ all could enhance the luminescence of Tb-GMP system, among which Gd³⁺ has the greatest enhancement. Under the optimum conditions, the enhanced intensity of the system is in proportion to the concentration of GMP in the range from 3×10⁻⁸ to 3×10⁻⁵ mol/l. The detection limit is 3.9×10⁻⁹ mol/l. In the study of mechanism, we propose that both Tb and Gd complexes can form bigger netlike compound by the link of the oxygen bridge, through which the energy absorbed by Gd complex may transfer to Tb³⁺ in Tb complex.     

Luminescence properties of Eu in gadolinium molybdate β′-Gd2Mo3O12 phosphors

In this paper, we report the synthesis and photoluminescence (PL) properties of β′-Gd₂Mo₃O₁₂ doped with Eu³⁺ ions. The relationship between Eu³⁺ luminescence versus concentration and temperature is discussed. In order to investigate the mechanism of concentration quenching, luminescence decay curves are measured and the Inokuti–Hirayama model is used to analyze them. The activation energy for the thermal quenching is estimated by the Arrhenius fitting. The emission spectrum of β′-Gd₂Mo₃O₁₂ exhibits the strongest emission peak at 614.5 nm due to electric–dipole transition. The excitation spectrum shows several sets of lines in the range of 350–425 nm which are associated with the typical intra-configurational 4f⁶ transitions of Eu³⁺. The spectral positions of these lines match well with the emission spectra of near-UV LEDs, which makes the phosphor find a potential application for white light-emitting-diodes.     

Sensitized luminescence of Eu and Tb macrocyclic complexes bearing benzophenone antennae

Sensitized luminescence behavior of lanthanide (Ln=Eu³⁺, Tb³⁺) macrocyclic cyclen (1,4,7,10-tetraazacyclododecane) complexes bearing one or four benzophenone (BP) moieties as antenna (LnL1 and LnL4) has been studied in water. Despite higher molar extinction coefficient of EuL4 owing to four antennae, it shows only one-thirtieth the luminescence intensity of EuL1. Energy level of triplet excited-state of BP antenna (E_T) is only a few kJ mol⁻¹ higher than that of ⁵D₂ excited-state of Eu³⁺, thus promoting a back energy transfer (BET) from ⁵D₂ of Eu³⁺ to ground-state BP antennae. On EuL4 bearing four antennae, BET occurs more rapidly than that on EuL1, thus exhibiting much weaker luminescence. For Tb complexes, the energy gap between E_T of BP antenna and ⁵D₄ excited state of Tb³⁺ is large enough (>13 kJ mol⁻¹), such that practically no BET occurs. The luminescence intensity of TbL4 is, however, lower (two-third) than that of TbL1. Time-resolved luminescence measurement reveals that hydration number of Tb³⁺ within TbL4 is twice that within TbL1. This is because the structural distortion of ligands on TbL4, caused by an intramolecular dipole-dipole interaction among the BP antennae, allows coordination of higher number of H₂O molecules to Tb³⁺, thus leading to a strong Tb luminescence quenching via O-H oscillators.     

Emission intensity of terbium(III) bound to benzene-carboxylic acid derivatives

The emission intensity of Tb³⁺ bound to phenol, catechol, resorcinol, benzoic acid, pyridine, nicotonic acid, phthalic acid, salicylic acid, and picolinic acid has been studied. Relative intensities of Tb³⁺ emission indicate that formation of a chelate ring is necessary to observe enhancement of the emission relative to that of Tb³⁺ alone in aqueous solution. The pH dependence of Tb³⁺ emission when bound to phthalic, salicylic, and picolinic acids were examined in detail and evidence was found which indicated that the geometry of the Tb³⁺ ion undergoes several changes as the solution pH is raised. The emission intensities and pH dependences are also found to depend on the ratio of ligand to metal, and this observation support the presence of polynuclear lanthanide complexes existing in solution.     

A luminescent dinuclear Eu(III) complex based on 2,8-bis(4′,4′,4′,-trifluoro-1′,3′-dioxobutyl)-dibenzothiophene for light-emitting diodes

A dinuclear Eu (III) complex Eu₂(dbt)₃·4H₂O was synthesized, where H₂dbt was 2,8-bis(4′,4′,4′,-trifluoro-1′,3′-dioxobutyl)-dibenzothiophene. The complex emits the characteristic red luminescence of Eu³⁺ ion due to the ⁵D₀→⁷F_J(J=0-4) transitions under 395 nm-light excitation with a luminescent quantum efficiency of 17%. The complex is thermally stable up to 280 °C. It was found that the complex can be effectively excited by a 395 nm-emitting InGaN chip. Bright red light was obtained using the complex as light color-conversion material.     

Design and Synthesis of a Terbium(III) Complex-Based Luminescence Probe for Time-Gated Luminescence Detection of Mercury(II) Ions

Time-gated luminescence detection technique using lanthanide complexes as luminescent probes is a useful and highly sensitive method. However, the effective application of this technique is limited by the lack of the target-responsive luminescent lanthanide complexes that can specifically recognize various analytes in aqueous solutions. In this work, a dual-functional ligand that can form a stable complex with Tb³⁺ and specifically recognize Hg²⁺ ions in aqueous solutions, N,N,N ¹ ,N ¹ -{[2,6-bis(3′-aminomethyl-1′-pyrazolyl)-4-[N,N-bis(3″,6″-dithiaoctyl)-aminomethyl]- pyridine]} tetrakis(acetic acid) (BBAPTA), has been designed and synthesized. The luminescence of its Tb³⁺ complex is weak, but can be effectively enhanced upon reaction with Hg²⁺ ions in aqueous solutions. The luminescence response investigations of BBAPTA-Tb³⁺ to various metal ions indicate that the complex has a good luminescence sensing selectivity for Hg²⁺ ions, but not for other metal ions. Thus a highly sensitive time-gated luminescence detection method for Hg²⁺ ions was developed by using BBAPTA-Tb³⁺ as a luminescent probe. The dose-dependent luminescence enhancement of the probe shows a good linearity with a detection limit of 17 nM for Hg²⁺ ions. These results demonstrated the efficacy and advantages of the new Tb³⁺ complex-based luminescence probe for the sensitive and selective detection of Hg²⁺ ions.     

In situ sol-gel composition of multicomponent hybrid precursor to hexagon-like Zn2SiO4:Tb microcrystalline phosphors with different silicate sources

Zn₂SiO₄ doped with Tb³⁺ were in situ synthesized by a modified sol-gel technology with the assembly hybrid precursor employed four different silicate sources, i.e. 3-aminopropyl-trimethoxysilane (APMS), 3-aminopropyl-triethoxysilane (APES), 3-aminopropyl-methyl-diethoxysilane (APMES) and tetraethoxysilane (TEOS), respectively. The SEM result shows that there exist some novel unexpected micromorphological structures of hexagon-like with the dimension of 0.5-1.0 μm. The photoluminescent properties of Zn₂SiO₄:Tb³⁺ phosphors have been studied as a function of Tb³⁺ doping concentration. Cross-relaxation process between identical Tb³⁺ ions results in the quenching of the ⁵D₃ emission for high concentration sample.     

Preparation and characterization of Tb and Tb(sal)3·nH2O doped PC:PMMA blend

Tb doped polycarbonate:poly(methyl methacrylate) (Tb-PC:PMMA) blend was prepared with varying proportions of PC and PMMA. Thermal and spectroscopic properties of the doped polymer have been investigated employing Fourier Transform Infrared (FTIR) absorption and differential scanning calorimetric (DSC) techniques. PC:PMMA blend (with 10 wt% PC and 90 wt% PMMA) shows better miscibility. Optical properties of the dopant Tb³⁺ ions have been investigated using UV-vis absorption and fluorescence excited by 355 nm radiation. It is seen that luminescence intensity of Tb³⁺ ion depends on PC:PMMA ratio and on Tb³⁺ ion concentration. Concentration quenching is seen for TbCl₃·6H₂O concentration larger than 4 wt%. Addition of salicylic acid to the polymer blend increases the luminescence from Tb³⁺ ions. Luminescence decay curve analysis affirms the non-radiative energy transfer from salicylic acid to Tb³⁺ ions, which is identified as the reason behind this enhancement.     

Spectroscopic Study on the Photophysical Properties of Lanthanide Complexes with Long Chain Mono-Docosyl Phthalate

Ortho phthalic anhydride was modified with long chain alcohol (1-docosanol) to its corresponding monodocosyl phthalate (22-Phth). Subsequently, three novel lanthanide (Eu³⁺, Tb³⁺, and Dy³⁺) complexes with the long chain monodocosyl phthalate were synthesized and characterized by elemental analysis and Infrared spectra. The photophysical properties of these complexes were studied in detail with ultraviolet-visible absorption spectra, low temperature phosphorescence spectra and fluorescent spectra. The triplet state energy of 22-Phth was determined to be around 25,000 cm⁻¹ from the maximum phosphorescent peak at 400 nm, suggesting 22-Phth is suitable for the sensitization of the luminescence of Eu³⁺, Tb³⁺, and Dy³⁺. The fluorescence excitation and emission spectra for these lanthanide complexes of the three ligands take agreement with the above predict from energy match principle.     

Synchrotron and laser excitation of luminescence in PbWO<Subscript>4</Subscript>:Tb crystals at different temperatures

The effect of temperature on the spectral luminescence characteristics of PbWO₄:Tb³⁺ crystals with synchrotron and laser excitation is studied. If PbWO₄:Tb³⁺ is excited by synchrotron radiation with λ = 88 nm at 300 K, a faint recombination luminescence of the impurity terbium is observed against the matrix luminescence. When the temperature is reduced to 8 K, the luminescence intensity of PbWO₄:Tb³⁺ increases by roughly an order of magnitude and the characteristic luminescence of the unactivated crystal is observed. Excitation of PbWO₄:Tb³⁺ by a nitrogen laser at 300 K leads to the appearance of emission from Tb³⁺ ions. At 90 K, a faint matrix luminescence is observed in addition to the activator emission. The formation of the luminescence excitation spectra for wavelengths of 60–320 nm is analyzed and the nature of the emission bands is discussed.     

Enhanced electroluminescence of Eu by Tb in complexes Tb1−xEux(TTA)3Dipy

A series of rare earth ternary compounds of Tb_1−xEu_x(TTA)₃Dipy (HTTA=thenoyltrifluoroacetone, Dipy=2,2′-dipyridyl) have been synthesized, and the characteristics of the compounds have been performed by DTA-TG, IR, UV and fluorescence spectroscopy. Photoluminescence measurements indicated that the complexes of Eu(III) emit strong red luminescence under UV radiation. IR spectra suggest that complexes have been successfully synthesized, and TG curves indicate that the complexes are stable up to a temperature of about 220 °C. The Eu complex was blended with poly(N-vinylcarbazole) (PVK) and spin coated into films, and electroluminescence devices with the structure of Indium Tin Oxide (ITO)/PVK:Tb_1−xEu_x(TTA)₃Dipy/BCP(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)/aluminum quinoline (AlQ)/Al were fabricated, the luminescence of Eu³⁺ complexes enhances after doping with Tb³⁺. Therefore, it may be an effective method to improve the EL intensity of the lanthanide complex.     

Sensitized luminescence through nanoscopic effects of ZnO encapsulated in SiO2:Tb sol gel derived phosphor

Terbium (1 mol%) doped ZnO-SiO₂ binary system was prepared by a sol-gel process. Nanoscopic effects of ZnO on the photoluminescence (PL) and the cathodoluminescence (CL) properties were studied. Defects emission from ZnO nanoparticles was measured at 560 nm and the line emission from Tb³⁺ ions in SiO₂:Tb³⁺ and ZnO-SiO₂:Tb³⁺ with a major peak at 542 nm was measured. The PL excitation wavelength for 542 nm Tb³⁺ emission was measured at ∼320 nm in both SiO₂:Tb³⁺ and ZnO-SiO₂:Tb³⁺. The CL data showed quenched luminescence of the ZnO nanoparticles at 560 nm from a composite of ZnO-SiO₂:Tb³⁺ and a subsequent increase in 542 nm emission from the Tb³⁺ ions. This suggests that energy was transferred from the ZnO nanoparticles to enhance the green emission of the Tb³⁺ ions. The PL and CL properties of ZnO-SiO₂:Tb³⁺ binary system and possible mechanism for energy transfer from the ZnO nanoparticles to Tb³⁺ ions are discussed.     

Ionic liquid-assisted hydrothermal synthesis of dendrite-like NaY(MoO4)2:Tb phosphor

Micro-sized NaY(MoO₄)₂:Tb³⁺ phosphors with dendritic morphology was synthesized by a ionic liquid-assisted hydrothermal process. X-ray diffraction (XRD) indicated that the as-prepared product is pure tetragonal phase of NaY(MoO₄)₂. Field emission scanning electron microscopy (FE-SEM) images showed that the as-prepared NaY(MoO₄)₂:Tb³⁺ phosphors have dendritic morphology. The photoluminescent (PL) spectra displayed that the as-prepared NaY(MoO₄)₂:Tb³⁺ phosphors show a stronger green emission with main emission wavelength 545 nm corresponding to the ⁵D₄→⁷F₅ transition of Tb³⁺ ion, and the optimal Tb³⁺ doping concentration for obtaining maximum emission intensity was confirmed to be 10 mol%. Based on Van Uitert's and Dexter's models the electric dipole–dipole (D–D) interaction was confirmed to be responsible for the concentration quenching of ⁵D₄ fluorescence of Tb³⁺ in the NaY(MoO₄)₂:Tb³⁺ phosphors. The intrinsic radiative transition lifetime of ⁵D₄ level is found to be 0.703 ms.     

Study on the color tunability and energy transfer mechanism in Tb3+, Sm3+ co-doped LiLaSiO4 phosphors

A several of LiLaSiO₄: xTb³⁺, ySm³⁺ (LLSO) phosphors were synthesized by high-temperature solid-phase reaction. Through SEM, XRD and fluorescence spectrometer, the phase, morphology, luminescence properties and energy transfer of the samples were systematically analyzed and discussed. Under an excitation of 378 nm wavelength, LLSO: xTb³⁺ phosphors emit green light, and the concentration quenching point of Tb³⁺ ions was x = 0.08. In LLSO: xTb³⁺, ySm³⁺ phosphors, When Sm³⁺ ions doping molar mass fraction increases, the fluorescence intensity of Tb³⁺ ion decreases while the fluorescence intensity of Sm³⁺ ions first strengthen and then weaken. The concentration quenching point of Sm³⁺ ions was y = 0.04. By changing the proportion of Sm³⁺ and Tb³⁺ ions, the luminous color can be adjusted from green to red. There is effective energy transfer between Tb³⁺→Sm³⁺. The molar mass fraction of doping Sm³⁺ ions is y = 0.10, the energy transfer efficiency reaches 96.67%. The energy transfer mechanism is the quadrupole-quadrupole interaction. The quantum yield is 22.34%. Therefore, LLSO: xTb³⁺, ySm³⁺ phosphors have certain potential application value in the field of ultraviolet-near ultraviolet white LEDs.     

Photoluminescence investigations of Zn2SiO4 co-doped with Eu and Tb ions

Zinc silicate phosphors co-doped with Eu³⁺ ions and also with both Eu³⁺ and Tb³⁺ ions were prepared by high temperature solid state reaction in air or reducing atmosphere. The luminescence characteristics of the prepared phosphors were investigated. While in the samples prepared in air, Eu³⁺ emission was found to be dominant over Tb³⁺ emission, in the samples prepared in reducing atmosphere, intense Eu²⁺ emission at 448 nm was found to be predominant over narrow Tb³⁺ emission. Luminescence studies showed that Eu³⁺ ions occupy asymmetric sites in Zn₂SiO₄ lattice. The intense f-f absorption peak of Eu³⁺ at 395 nm observed in these phosphors suggests their potential as red emitting phosphors for near ultra-violet light emitting diodes.