A Mechanism of the influence of Gd(III) and other Ln(III) ions on sensitized luminescence of Eu(III) and Tb(III) ions in aqueous and ethanol solutions: The role of hydroxyl bridges

We studied sensitization of Eu(III) and Tb(III) ions by molecules of 1,10-phenanthroline and 2,2-bipyridil in D₂O and d ₆-ethanol and the influence of Nd(III), Pr(III), Sm(III), Gd(III), and Ho(III) ions on the luminescence intensity I _lum and lifetime τ_lum of Eu(III) and Tb(III) in solutions. The stability constants of complexes of Eu(III) and Gd(III) with 2,2′-bipyridil are measured by spectrophotometric and luminescence methods. It is shown that luminescence of Eu(III) is quenched by Gd(III) ions at the ion concentration equal to 10^?2–10^?1 M, which is caused by competing between these ions for a sensitizer. At the concentration of Ln(III) ions equal to 10^?6?10^?3 M, the sensitized luminescence of Eu(III) and Tb(III) was quenched and τ_lum decreased in the presence of Nd(III) ions, whereas in the presence of Gd(III) the luminescence intensity increased. It is proved that a bridge that connects the two ions upon energy transfer is formed by hydroxyl groups. The intensity of luminescence of Eu(III) and Tb(III) in aqueous solutions and its lifetime decreased in the presence of hydroxyl groups, while upon addition of Gd(III) to these solutions these quantities were restored. We also found that the addition of Gd(III) to deoxygenated ethanol solutions of 2,2′-bipyridil and Eu(III) slows down photochemical and thermal reactions between bipyridil and Eu(III), resulting in the increase in the luminescence intensity of Eu(III).     

Sensitization of Luminescence of Europium Compounds on Solid Matrices by Terbium (III) Ions

The conditions of sensitization of the luminescence of Eu(III) by Tb(III) ions in complexes with inorganic (Na₂WO₄) and organic (nalidixic acid) ligands in sorbates on solid matrices (zeolite CaX and crystalline zirconium phosphate (ZrP)) and in coprecipitation with CaWO₄ have been investigated. It has been established that the maximum sensitization of the europium luminescence is attained in the case where Eu and Tb are present in a 1:0.5 ratio. In this case, the integral intensity of luminescence of Eu(III) (the band with _max = 612 nm) in the sorbate of its complex with nalidixic acid on ZrP accounts for more than 60% of the luminescence intensity of the industrial photoluminophor Y₂O₃:Eu (FL-612) possessing red luminescence, and the intensity of Tb(III) luminescence (the band with _max = 545 nm) accounts for about 40% of the luminescence intensity of the photoluminophor Gd₂O₂S:Tb possessing green luminescence.     

Synthesis and Fluorescence Properties of Lanthanide (III) Perchlorate Complexes with Naphthyl-Naphthalinesulphonylpropyl Sulfoxide

A ligand with double sulfinyl groups, naphthyl-naphthalinesulphonylpropyl sulfoxide(dinaphthyl disulfoxide, L), was synthesized by a new method and its several lanthanide (III) complexes were synthesized and characterized by element analysis, molar conductivity, coordination titration analysis, IR, TG-DTA, ¹HNMR and UV spectra. The composition of these complexes, were RE₂(ClO₄)₆·(L)₅·nH₂O (RE = La, Nd, Eu, Tb, Yb, n = 2 ∼ 6, L = C₁₀H₇SOC₃H₆SOC₁₀H₇). The fluorescent spectra illustrated that the Eu (III) complex had an excellent luminescence. It was supposed that the ligand was benefited for transferring the energy from ligand to the excitation state energy level (⁵D₀) of Eu (III). The Tb (III) complex displayed weak luminescence, which attributed to low energy transferring efficiency between the average triplet state energy level of ligand and the excited state (⁵D₄) of Tb (III). So the Eu (III) complex displayed a good antenna effect for luminescence. The phosphorescence spectra and the relationship between fluorescence lifetime and fluorescence intensity were also discussed.     

Hot electron-induced electrogenerated chemiluminescence of rare earth(III) chelates at oxide-covered aluminum electrodes

Aromatic Gd(III) and Y(III) chelates produce ligand-centered emissions during cathodic pulse polarization of oxide-covered aluminum electrodes, while the corresponding Tb(III) chelates produce metal-centered⁵D₄ →⁷F_j emissions. It was observed that a redox-inert paramagnetic heavy lanthanoid ion, Gd(III), seems to enhance strongly intersystem crossing in the excited ligand and direct the deexcitation toward a triplet-state emission, while a lighter diamagnetic Y(III) ion directs the photophysical processes toward a singlet-state emission of the ligand. The luminescence lifetime of Y(III) chelates was too short to be measured with our apparatus, but the luminescence lifetime of Gd(III) chelates was between 20 and 70 μs. The mechanisms of the ECL processes are discussed in detail. Preliminary results of electrogenerated triplet-state emission of one of the ligands used in this work have been published in a letter elsewhere [9].     

Luminescence properties of lanthanide(III) ions in concentrated carbonate solution

Luminescence properties of lanthanide(III) ions (Ln = Nd, Sm, Eu, Gd, Tb, Dy and Tm) were investigated by measuring the excitation and emission spectra, and emission lifetimes in H₂O and D₂O solutions of 3 moll^?1 K₂CO₃, where anionic tetra-carbonate complexes, [Ln(CO₃)₄]^5- were the predominant species.

Electronic transitions of the carbonato complex corresponding to both the excitation and emission spectra were assigned from the energy level diagrams of Ln(III) and compared with those of the aqua ion. Enhancement of emission intensity of the complex was observed at particular excitation transitions of Eu(III), Gd(III) and Tb(III), and at particular emission transitions of Sm(III), Eu(III), Dy(III) and Tm(III). The enhancement at the emission transition was estimated quantitatively as a branching ratio from the lowest emitting state of Ln(III), and discussed in terms of hypersensitivity.

Emission lifetimes of the carbonato complexes were all longer than those of aqua ions in H₂O solution, while the lifetimes of the complexes for Eu(III) and Tb(III) shorter than those in D₂O solution. The difference in non-radiative decay constants for the excited complex in H₂O and D₂O solutions was found to be proportional to an exponential of the energy gap of Ln(III). The lifetime ratio between the H₂O and D₂O solutions showed the order of Sm > Dy > Eu > Tb, corresponding to the opposite order of the energy gap. These were discussed in terms of an energy gap law, i.e. a relationship between the energy gap of Ln(III) and vibration energies of the ligand or water molecules.     

Effect of the medium and the formation of nanostructures on deexcitation of electronic excitation of Eu(III) and Tb(III) chelates

The intensity I _lum and lifetime τ_lum of the luminescence of complexes of Eu(III) and Tb(III) ions with β-diketones and o-phenanthroline in water-ethanol solutions of these ligands have been analyzed as functions of the concentrations of ligand, luminescing lanthanide ions, and added ions causing columinescence and of the solvent deuteration. It is shown that the formation of nanostructures from Ln complexes and their coarsening leads to an increase in τ_lum of Eu(III) and Tb(III) and that this increase is due to the suppression of both photochemical deexcitation of these ions and transfer of their electronic excitation energy to OH vibrations of water molecules. The disappearance of the dependence of I _lum of Eu(III) on deuteration of water-ethanol solutions of n-methoxybenzoyltrifluoracetone + o-phenanthroline caused by adding Gd(III) ions is explained by the shift of the equilibrium of formation of complexes of Ln chelates to neutral hydrophoblic forms corresponding to the formation of nanostructures of these chelates in the solution. The differences in effect of La(III) and Gd(III) ions on I _lum and τ_lum of Eu(III) and Tb(III) complexes are explained. It is shown that the widely discussed effect of columinescence not only results from the energy migration in mixed structures of Eu or Tb complexes and Gd complexes but is also due to a large extent to the decrease in τ_lum of Eu(III) or Tb(III) caused by their incorporation into nanostructures.     

Synthesis and Photoluminescent Properties of Lanthanides Acetoacetanilide Complexes

This work reports on the photoluminescent properties of three new lanthanide complexes with acetoacetanilide (aaa), a β-diketonate ligand. The complexes have the general molecular formulae [RE(aaa)₃(H₂O)], they are soluble in organic solvents such as ethanol and chloroform and insoluble in water. The energy of the triplet state was determined at about 4,700 cm^?1 higher than the ⁵D₄ emitting level of the Tb(III) ion, leading to an absolute quantum yield of 22 % for the [Tb(aaa)₃(H₂O)] complex. The photoluminescent properties were studied and the luminescence parameters of the [Eu(aaa)₃(H₂O)] complex were experimentally determined. The photostabilities of the complexes under continuous UV irradiation were measured and the data indicate low stability of the [Tb(aaa)₃(H₂O)] complex when the system is excited at the band attributed to energy transfer from the ligand to terbium(III) ion. However, its photostability is significantly improved under inert atmosphere.     

Photoluminescence, optical absorption and hypersensitivity in mono- and dinuclear lanthanide (Tb and Ho) β-diketonate complexes with diimines and bis-diimine bridging ligand

The photoluminescence properties of three Tb(III) complexes of the form [Tb₂(fod)₆(μ-bpm)], [Tb(fod)₃(phen)] and [Tb(fod)₃(bpy)] and optical absorption properties of their Ho(III) analogues (fod=anion of 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione, bpm=2,2′-bipyrimidine, phen=1,10-phenanthroline and bpy=2,2′-bipyridyl) in a series of solvents are presented. The luminescence of the complexes is sensitive to changes in environment (ligand/solvent) around Tb(III) and co-sensitization of the ancillary ligands. The enhancement of the luminescence intensity in coordinating solvents is attributed to the transformation of eight-coordination into less symmetric nine-coordination structure around Tb(III). Among phen and bpy, the phen is better co-sensitizer while bpm has been observed as poor co-sensitizer. The enhancement of the oscillator strength of ⁵G₆←⁵I₈ hypersensitive transition in the 4f-4f absorption in some coordinating solvents is attributed to decrease in the symmetry of the field around Ho(III) ion. The [Ho(fod)₃(phen)] is inert towards the solvents and retains its bulk structure and composition in solution. The transformation of the holmium complexes in DMSO into [Ho(fod)₃(DMSO)₂] species is found. The results reveal that the luminescence and 4f-4f absorption properties of lanthanide complexes in solution can be modulated by tuning the coordination structure through ancillary ligands and donor solvents.     

Synthesis and Fluorescence Properties of Lanthanide (III) Perchlorate Complexes with Bis(benzoylmethyl) Sulfoxide

A ligand with two carbonyl groups and one sulfinyl group has been synthesized by a new method and its several lanthanide (III) complexes were synthesized and characterized by element analysis, molar conductivity, coordination titration analysis, IR, TG-DSC, ¹H NMR and UV spectra. The results indicated that the composition of these complexes is REL₅(ClO₄)₃·3H₂O (RE = La(III), Pr(III), Eu(III), Tb(III), Yb(III), L = C₆H₅COCH₂SOCH₂COC₆H₅). The fluorescent spectra illustrate that both the Tb (III) and Eu (III) complexes display characteristic metal-centered fluorescence in solid state, indicating the ligand favors energy transfer to the excitation state energy level of them. However, the Tb (III) complex displays more effective luminescence than the Eu (III) complex, which is attributed to especial effectively in transferring energy from the average triplet energy level of the ligands (T) onto the excited state (⁵D₄) of Tb (III) than that (⁵D₀) of Eu (III), showing a good antenna effect for Tb(III) luminescence. The phosphorescence spectra and the relationship between fluorescence lifetimes and fluorescence intensities were also discussed.     

Variation in the Luminescence Lifetimes of Eucl3·nH2O(N=0,L,2,3,6)

Eu³⁺ ion emission spectra and luminescence lifetimes were investigated for EuCl₃ -nH₂O (n=0,1,2,3,6). Each compound exhibited a characteristic set of emission bands and a specific luminescence lifetime. The number of water molecules and chloride ions coordinated to the Eu³⁺ ion in these materials was estimated from the observed lifetimes, spectroscopic implications, and expected lanthanide coordination numbers. Approximation of the observed luminescence decay constant for each material was possible through the use of arithmetic terms associated with both the complexed water molecules and the complexed chloride ions which make up the inner-coordination sphere of the Eu³⁺ ion.     

Luminescence of dipicolinic complexes of lanthanide ions

The luminescence decay times τ_lum of the complexes of the ions Tb(III), Eu(III), Sm(III), Dy(III), and Yb(III) with dipicolinic acid (DPA) dissolved in protonated and deuterated water, methanol, and dimethyl sulfoxide are measured. The values of τ_lum for crystals H₃[Ln(DPA)₃]·nH₂O and their aqueous solutions coincide, which points to the identity of the environment in the nearest spheres of an ion in both cases. A comparison of τ_lum of solutions of the complexes in H₂O and D₂O, as well as in CH₃OH, CH₃OD, CD₃OD, DMSO-h ₆, and DMSO-d ₆ shows that the molecular groups in the second and third spheres of an ion, exhibiting high-frequency vibrations, have a noticeable effect on the rate constants of nonradiative transitions k _nr in the ion. From this comparison, some inferences on the structure of the solvate shell of the Ln(DPA) ₃ ^3? complexes in the solvents used are made. The contributions to k _nr of Eu(III), Tb(III), Sm(III), Dy(III), Nd(III), and Yb(III) made by OH and CH groups located at different distances from the ion are estimated. It is demonstrated that the dependence of k _nr on the distance to the OH and CH groups is steeper for the Eu(III) and Tb(III) ions than for the remaining ions.     

Luminescence properties of compounds of europium(III) with quinaldic acid and phosphor-containing neutral ligands

Luminescent complex mixed-ligand compounds of europium(III) with quinaldic acid and phosphor- containing neutral ligands have been obtained. Their composition and structure have been determined. The thermal and spectral-luminescent properties of the obtained complex mixed-ligand compounds of europium( III) have been studied. It is shown that, during thermolysis, a water molecule and neutral ligand are detached in two stages with endothermic effects. It is established that quinaldinate ion is coordinated to europium(III) ion in a bidentate fashion. The Stark structure of the ⁵D₀–⁷F_j (j = 0, 1, 2) transitions in low-temperature luminescence spectra of complex compounds of europium(III) has been analyzed.     

Microwave-assisted synthesis of (tris-acetylacetonato)(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)terbium(III) complex with outstanding high green luminescence

Microwave-assisted synthesis method enabled the preparation of the (tris-acetylacetonate)(2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinate) terbium(III) (Tb(acac)₃(dmdpphen)) complex with outstanding high green luminescence. This method is appropriate for green chemistry and energy-saving processes. The Tb(acac)₃(dmdpphen) complex has good thermal stability. Emission peaks are assigned to the f-f transitions of the Tb³⁺. The Tb(acac)₃(dmdpphen) complex is expected to be used in functional materials of electronic products.     

Spectroscopic studies of lanthanide(III) ion complexes with diethyl(phthalimidomethyl) phosphonate

Complexation and photophysical properties of complexes of lanthanide ions, Ln(III), with diethyl(phthalimidomethyl)phosphonate ligand, DPIP, were studied. Interactions between Ln(III) and DPIP were investigated using Nd(III) absorption and Eu(III) and Tb(III) luminescence (emission and excitation) spectra, recorded in acetonitrile solution containing different counter ions (NO₃^-, Cl^- and ClO₄^-). Results of the absorption spectroscopy have shown that counter ions play a significant role in the complexation of Ln(III)/DPIP complexes. Studies of luminescence spectra of Eu(III) and Tb(III) ions proved that the formation of Ln(III)/DPIP complexes of stoichiometry Ln:L=1:3 is preferred in solution. Based on the results of elemental analysis, Nd(III) absorption spectra and IR and NMR data, it was shown that the DPIP ligand binds Ln(III) ions via oxygen from phosphoryl group, forming complexes of a general formula Ln(DPIP)₃(NO₃)₃·H₂O, in which the NO₃^- ions are coordinated with the metal ion as bidentate ligands. Luminescent properties and energy transfer, from the ligand to Ln(III) ions in the complexes formed, were studied based on the emission and excitation spectra of Eu(III) and Tb(III). Their luminescent lifetimes and emission quantum yields were also measured.     

Synthesis,Photoluminescence Behavior of Green Light Emitting Tb(III) Complexes and Mechanistic Investigation of Energy Transfer Process

A series of five new terbium(III) ion complexes with 4,4-difluoro-1-phenylbutane-1,3-dione (HDPBD) and anciliary ligands was synthesized. The composition and properties of complexes were analyzed by elemental analysis, IR, NMR, powder X-ray diffaraction, TG-DTG and photoluminescence spectroscopy. These complexes exhibited ligand sensitized green emission at 546 nm associated with ⁵D₄?→?⁷F₅ transitions of terbium ion in the emission spectra. The photoluminescence study manifested that the organic ligands act as antenna and facilitate the absorbed energy to emitting levels of Tb(III) ion efficiently. The enhanced luminescence intensity and decay time of ternary C2-C5 complexes observed due to synergistic effect of anciliary ligands. The CIE color coordinates of complexes came under the green region of chromaticity diagram. The mechanistic investigation of intramolecular energy transfer in the complexes was discussed in detail. These terbium(III) complexes can be thrivingly used as one of the green component in light emitting material and in display devices.

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Graphical Abstract Illustrate the sensitization process of the Tb ion and intramolecular energy transfer process in the Tb³⁺ complex.

     

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.     

Design, synthesis and characterization of a highly luminescent Eu-complex monomer featuring thenoyltrifluoroacetone and 5-acryloxyethoxymethyl-8-hydroxyquinoline

A multi-functional ligand, 5-acryloxyethoxymethyl-8-hydroxyquinoline (Hamq), was synthesized, which contained a polymerizable C=C double bond for the copolymerization with other vinyl monomers and acted as photon antenna able to transfer energy to Eu³⁺ ions effectively. The triplet state energy of Hamq was determined to be 22,370 cm⁻¹ via the phosphorescence spectra of Hamq and its gadolinium complex. The title complex monomer Eu(tta)₂(amq) was prepared by coordination reaction of Hamq with europium isopropoxide and 2-thenoyltrifluoroacetone (Htta) in dry organic solvents under argon atmosphere and characterized by elemental analysis and IR spectrum. The photophysical properties of the complex were studied in detail with UV-vis, luminescence spectra, luminescence lifetime and quantum yield. The complex exhibited nearly monochromatic red emission at 612 nm, a remarkable luminescence quantum yield at room temperature (30.6%) upon ligand excitation and a long ⁵D₀ lifetime (389 μs), which indicated that the ligand Hamq could sensitize the luminescence of Eu(III) ion efficiently in Eu(tta)₂(amq), resulting in a strong luminescence of its copolymer poly[MMA-co-Eu(TTA)₂(amq)] under UV excitation. The excellent luminescence properties of the complex made it not only a promising light-conversion molecular device but also an excellent luminescent monomer.     

水杨酸酯-铽络合物的发光及其向铕离子的能量传递

本文应用稳态和瞬态光谱技术探讨了稀土离子铽与水杨酸甲酯,苯酯所形成络合物的发光特性,它们向铕离子的能量传递过程;以及pH值对络合物的发光和能量传递的影响。络合物的发光比络合前铽离子的发光增强了几至十几倍;介质的pH变化影响到络合物的发光强度和能量传递效率,中性介质(pH:6～8)下的量传递效率最高。     

Syntheses and Fluorescence Properties of Two Novel Lanthanide (III) Perchlorate Complexes with Bis(benzylsulfinyl)methane

A novel ligand with double sulfinyl groups, bis(benzylsulfinyl)methane, was synthesized by a new method and its two lanthanide (III) complexes were synthesized and characterized by element analysis, molar conductivity, coordination titration analysis, IR, TG-DSC, ¹HNMR and UV spectra. The results indicated that the composition of these complexes was REL_2.5(ClO₄)₃·3H₂O (RE = Tb (III), Dy (III), L = C₆H₅CH₂SOCH₂SOCH₂C₆H₅). The FT-IR results revealed that the perchlorate group was bonded with the lanthanide ion by the oxygen atoms, and the coordination was bidentate. The fluorescent spectra illustrated that both the Tb (III) and Dy (III) complexes displayed characteristic fluorescence in solid state, especially for the Tb (III) complex, the peak of ⁵D₄ → ⁷ F₅ of the Tb (III) ion in 544 nm was stronger than that of others. It indicated that the Tb (III) complex could emit purer green fluorescence. By analysis fluorescence and phosphorescence spectra, it was found that the ligand had the advantage to absorb energy and transfer it to the Tb (III) and Dy (III) ions. The phosphorescence spectra and fluorescence lifetimes of the complexes were also measured.     

Luminescence of praseodymium (III) in solutions of mixed-ligand complexes with β-diketones

We have found that it is possible to enhance the luminescence of Pr(III) in solutions of complexes with β-diketones: acetylacetone derivatives containing fluoroalkyl substituents of different lengths and structures. We have established that in the presence of organic solvents, second (additional) ligands, and surfactants shielding the central ion from the quenching effect of the water molecules (OH oscillators), the intensity of luminescence for Pr(III) increases by a factor of 65, 38, and 45 respectively. In this case, cationic surfactants form ionic associates with Pr(III) β-diketonates, with incorporation of one more β-diketone molecule (ratio Pr:β-diketonate:surfactant = 1:4:1). As a result of suppression of intramolecular energy losses in solutions of Pr(III) β-diketonates, it is possible to observe its rather intense luminescence at 605 nm (the transition ¹D₂ → ³H₄), 612 nm (³P₀ → ³H₆), and 646 nm (³P₀ → 3F₂). __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 746–750, November–December, 2006.