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.     

Ln(III)-cored complexes based on 2-thenoyltrifluoroacetone ligand for near infrared emission: Energy transfer pathway and transient absorption behavior

Lanthanide(III)-cored complexes based on 2-thenoyltrifluoroacetone (TTA) ligand for near infrared (NIR) emission have been developed to investigate the energy transfer pathway from the antenna ligand to the Ln³⁺ ion. Their photophysical studies indicate the sensitization of Ln³⁺ luminescence by energy transfer through the excited triplet state of β-diketone ligand. Nanosecond (ns) transient absorption behavior of Ln³⁺-[TTA]₃(terpy) complexes at room temperature is explored. The triplet-triplet absorption spectrum for Gd³⁺-[TTA]₃(terpy) is observed under degassed condition, whereas it is hardly observed in Er³⁺-[TTA]₃(terpy) complex. The sensitizing process in Er³⁺-[TTA]₃(terpy), through the triplet state of TTA ligand to Er³⁺ ion, is also independent on the presence of oxygen. It indicates that the energy transfer rate through the excited triplet state of β-diketone ligand to Er³⁺ ion occurs approximately faster than that of the oxygen quenching rate.     

Hypersensitivity in the Luminescence and 4f?C4f Absorption Properties of Mono- and Dinuclear EuIII and ErIII Complexes Based on Fluorinated ??-Diketone and Diimine/ Bis-Diimine Ligands

The results of our investigation on the sensitized luminescence properties of three Eu(III) ??-diketonate complexes of the form [Eu₂(fod)₆(??-bpm)], [Eu(fod)₃(phen)] and [Eu(fod)₃(bpy)] and 4f?C4f absorption properties of their Er(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 non-aqueous solvents are presented. The Eu(III) complexes are highly luminescent and their luminescence properties (intensity and band shape) are sensitive to the changes in the inner coordination sphere of the Eu(III) ion. The luminescence intensity of the mononuclear complexes in pyridine is drastically decreased. The coordination structure of the complexes in pyridine is transformed into a more symmetrical one which results into a slow radiative rate of the emission from the complexes. The ancillary ligands, phen and bpy are found better co-sensitizers as compared to the bpm to sensitize Eu(III)-luminescence. The 4f?C4f absorption properties (oscillator strength and band shape) of the Er(III) complexes demonstrate that ⁴G_11/2 ?? ⁴I_11/2 and ²H_11/2 ?? ⁴I_15/2 hypersensitive transitions of Er(III) are very sensitive in some coordinating solvents which reflects complex?Csolvent interaction in solution. The hypersensitive transitions of [Er(fod)₃(phen)] remain unaffected in any of the solvents and this complex retains its bulk composition in solution. The erbium complexes as well as the Er(fod)₃ chelate are invaded by DMSO. This solvent enters the inner coordination sphere by replacing heterocyclic ligand and the complexes acquire similar structure [Er(fod)₃(DMSO)₂] in this solvent. The results reveal that the luminescence and absorption properties of lanthanide complexes in solution can be controlled by tuning the coordination structure through ancillary ligands and donor solvents. This work shall prove useful in designing new biological applications with such probes.     

The sensitivity of the lehn cryptand—europium and terbium (III) complexes to anions compared to a coordinatively saturated systems

Titrations of the Lehn cryptand [Ln⊂bpy.bpy.bpy]³⁺ complexes of both europium (III) and terbium (III) with fluoride and phosphate ions indicate that these anions displace water from the solvation sphere. In the case of the europium complex an enhancement of luminescence intensity is observed, whilst with the terbium complex a reduction in the emission intensity occurs. Described are the preparations of analogous, coordinatively saturated complexes that are inert to the influence of these anions.     

Influence of MgO codoping on the optical properties of Er3+-doped near-stoichiometric LiNbO3

Four near-stoichiometric lithium niobate (NSLN) crystals codoped with Er³⁺ (1 mol%) and MgO (0, 0.5, 1.0, and 2.0 mol%) were grown from K₂O-based flux in air using top seeded solution growth technique. The [Li]/[Nb] ratio, estimated from the blueshift of ultraviolet absorption edge, is 97.2% in NSLN:Er. MgO; codoping can increase the segregation coefficient of Er³⁺ in NSLN:Er:MgO crystal. The photorefractive damage threshold is enhanced by three orders of magnitude for NSLN:Er codoped with 1 mol% MgO, it coincides with the peak shift of OH⁻ absorption spectrum from 3481 to 3535 cm⁻¹. Judd–Ofelt theory based on absorption spectra is used to analyze the influence of MgO concentration on the Judd–Ofelt intensity parameter, transition strength, fluorescence branching ratio, and stimulated emission cross section. From the time-resolved emission spectra and the comparison among emission spectra, two Er³⁺ crystal-field sites are ascertained in NSLN:Er codoped with 2 mol% MgO, this coincides with the bimodal structure in X-ray photoelectron spectrometry spectra. The upconversion processes under pulse excitation is proposed based on the pump energy dependence and decay kinetics. The distribution of Er³⁺-clustered sites in NSLN:Er:MgO series is discussed based on the nonexponential decay curves monitored at 550 nm under two-photon excitation.     

Optical and electrochemical characteristics of ethylenediamine complexes of Pt(II) and Ir(III) with metalated 2-phenyl- and 2-naphthylbenzothiazole

The cyclometalated complexes [Pt(С^N)En]PF₆ and [Ir(C^N)₂En]PF₆ ((C^N)^– are deprotonated forms of 2-phenylbenzothiazole or 2-naphthylbenzothiazole and En is ethylenediamine) are studied by ¹Н NMR, IR, electronic absorption, and emission spectroscopy, as well as by voltammetry. Metalation of heterocyclic ligands leads to the formation of five-membered {M(C^N)} cycles in the composition of squareplanar Pt(II) complexes and octahedral Ir(III) complexes of the cis-С,С structure. A bathochromic shift of the metal-to-cyclometalated ligand charge transfer bands and a decrease in the potential difference between the single-electron waves of metal-centered oxidation and ligand-centered reduction of complexes upon substitution of 2-phenylbenzothiazole by 2-naphthylbenzothiazole and of Pt(II) by Ir(II) are shown. The phosphorescence of complexes in the visible region is assigned to the radiative transition from the metal-modified intraligand electronic excited state.     

Magnetic Properties of a New Er(III) Macrobicyclic Complex Studied by EPR

The macrobicyclic ligands of a Schiff base form complexes with various lanthanide metal cations, where the lanthanide ion can be coordinated in the mono- or binuclear systems. As a result of the Schiff-base condensation, a new erbium cryptate complex has been synthesized and investigated by electron paramagnetic resonance EPR technique. The measurements in the 3.9–300 K temperature range confirmed the presence of the 1:1 macrobicyclic Schiff base–Er³⁺ complex. Computer simulations of the registered EPR spectra have revealed the existence of low-symmetry crystal field at the Er(III) site. The complicated EPR spectra of Er(III) complex with an effective spin S = 1/2 in low magnetic fields (<200 mT) were observed only below 60 K. At 5 K the anisotropic g-factors and the integrated intensity reached local maximum values. In the 5–13 K temperature range the line width and g-factors remained constant, while the integrated intensity strongly decreased with increasing temperature. Above 13 K a strong increase of g _y -factor and ΔB _y line width was observed. The observed peak in integrated intensity indicates that the EPR signal is produced by the excited state that lies 6.3 K above the nonmagnetic ground state. Authors' address: Sławomir M. Kaczmarek, Institute of Physics, Szczecin University of Technology, Al. Piastów 48, 70-310 Szczecin, Poland     

Spectral and luminescent properties of mono- and binuclear cyclometalated complexes of Pd(II) and Pt(II) based on 4,6-diphenylpyrimidine with ethylenediamine and orthophenanthroline

The [M(N_^∧N)(Hdphpm)]ClO₄ and [(M(N_^∧N))₂(μ-dphpm)](ClO₄)₂ complexes (M = Pd(II), Pt(II); (N_^∧N) is ethylenediamine (En) and orthophenanthroline (Phen); Hdphpm^? and dphpm^2? are the mono- and bisdeprotonated forms of 4,6-diphenylpyrimidine) are obtained and characterized by ¹H NMR spectroscopy and electronic absorption and emission spectroscopy. The magnetic nonequivalence of protons of (N_^∧N) ligands is explained by a difference in the trans-effect of the carbanion and pyrimidine parts of the cyclometalated ligand. The long-wavelength absorption bands and the vibrationally structured luminescence bands of ethylenediamine complexes are attributed to optical transitions in the {M(Hdphpm)} and {M₂(μ-dphpm)} metal-complex fragments. The complexes with orthophenanthroline exhibit two low-energy optical transitions involving π* orbitals localized on the cyclometalated and chelating ligands; the difference in their energies depends on the metal and is much larger for Pt(II) than for Pd(II). It is found that the replacement of Pd(II) by Pt(II) in the [(M(phen))₂(μ-dphpm)]²⁺ complexes changes the direction of the photoexcitation energy degradation due to the energy transfer between the {M₂(μ-dphpm)} bridging fragment and peripheral phenanthroline ligands.     

Preparation and upconversion emission properties of TiO2 :Yb, Er inverse opals

Inverse opal photonic crystals of Y b³⁺, Er³⁺ co-doped TiO₂ (TiO₂:Yb, Er) were prepared by a self-assembly technique in combination with a sol-gel method. Upconversion (UC) luminescence properties of the inverse opals were investigated. The results show that photonic bandgap has significant influence on the upconversion emission of the TiO₂:Yb, Er inverse opal photonic crystals. Significant suppression of the upconversion emission was detected if the photonic bandgap overlapped with the Er³⁺ ions emission band.     

Optical, spectroscopic, and electrochemical properties of cyclometalated complexes of platinum metals based on 2-tolylpyridine and benzo[h]quinoline in ethylenediamine

Cyclometalated [M(C∧N)En]PF₆ (M = Pd(II), Pt(II)) and [M(C∧N)₂En]PF₆ (M = Rh(III), Ir(III)) complexes ((C∧N)^? corresponds to the deprotonated forms of 2-tolylpyridine and benzo[h]quinoline, and En is ethylenediamine) are studied by ¹H NMR spectroscopy, electronic absorption and emission spectroscopy, and voltammetry. Metalation of heterocyclic ligands leads to the formation of five-membered {M(C∧N)} cycles in the composition of square planar and octahedral complexes of the cis-C,C structure. Correlation of the energy positions of the long-wavelength metal-to-ligand charge-transfer absorption bands with the difference between the potentials of one-electron waves of metal-centered oxidation and ligand-centered reduction of complexes is shown. The phosphorescence of the complexes in the visible region of 469–524 nm is attributed to the radiative transition from the metal-modified intraligand excited state. The temperature quenching of the phosphorescence of complexes is attributed to the thermally activated population of metal-centered electronically excited states with subsequent nonradiative deactivation.     

DNA Binding and Photocleavage Study of Cationic Metalloporphyrins by Spectral Methods

The DNA binding and photocleavage specificities of the Zn(II), Cu(II), Co(III), Mn(III) complexes of 5,10,15-tris(1-methylpyridinium-4-yl)-20-(4-propionoxyphenyl)porphyrin have been studied by using a combination of absorption, fluorescence titration, surface-enhanced Raman spectroscopy (SERS), induced circular dichroism (ICD) spectroscopy, thermal DNA denaturation as well as gel electrophoresis experiment. It is found that Cu(II) porphyrin has comparable binding ability with the free base porphyrin while the axial-coordinated Zn(II), Co(III), and Mn(III) porphyrins have lower K_b because of the molecular steric hindrance. However, those metalloporphyrins with lower K_b have similar DNA cleavage efficiencies with the free base porphyrin. This could be best understood by the enhancement of the ¹O₂ productivity which may also result from the steric hindrance of the axial-coordinated metalloporphyrins.     

Optical and electrochemical properties of cyclometalated Pd(II) and Pt(II) complexes with a metal-metal chemical bond

Cyclometalated [M(C^N)(μ-(N-S))]₂ complexes ((M = Pd(II), Pt(II)), (C^N)^? are the deprotonated forms of 2-tolylpyridine and benzo[h]quinoline, and (N-S)^? are pyridine-2-thiolate and benzothiazole-2-thiolate ions) are studied by ¹H NMR, IR, electronic-absorption, and emission spectroscopy, as well as by voltammetry. It is shown that the formation of the metal-metal chemical bond and the σ _dz2 ^* orbital as a HOMO of complexes leads to the long-wavelength spin-allowed (410–512 nm) and spin-forbidden (595–673 nm) optical transitions σ _dz2 ^* ?π _(C^N) ^* in the absorption and phosphorescence spectra, as well as to the two-electron and successive one-electron oxidation with the formation of binuclear Pt(III) and Pd(III) complexes. The substitution of Pt(II) by Pd(II) is characterized by hypso- and bathochromic shifts of the spin-allowed and forbidden σ _dz2 ^* ?π _(C^N) ^* optical transitions in the absorption and phosphorescence spectra of complexes, by phosphorescence quenching of Pd(II) complexes in liquid solutions, and by an anodic shift of the oxidation potential of Pd(II) complexes compared with Pt(II) complexes.     

Photonic band gap and upconversion emission properties of Yb,Er co-doped lead lanthanum titanate inverse opal photonic crystals

Upconversion (UC) emission properties of Yb, Er co-doped lead lanthanum titanate inverse opal photonic crystals were investigated. The results showed that the photonic band gap has a significant influence on the upconversion emission of Yb, Er co-doped lead lanthanum titanate inverse opal photonic crystals. Significant suppression of the upconversion emission was detected if the photonic band gap overlapped with the Er³⁺ ions emission band.     

Novel Tb(III) complexes with two different structures of phosphine oxides and their properties

Novel Tb(III) complexes with two different structures of phosphine oxide ligands and three β-diketonates were synthesized. Characteristic of the excitation spectra mainly depend on the substituents of β-diketonates, and remarkable correlation among molecular structures, maximum wavelengths and emission intensity could be found. Namely, the shorter the maximum wavelengths when electron-donating groups are substituted in β-diketonates, the larger the emission intensity. These results were explained on the basis of extent of back energy transfer from excited Tb(III) ions to the ligand. From the analysis of the branching ratio of emission spectra, that is defined as the ratio of the intensity of the electric-dipole transition to the magnetic-dipole transition, complex 2 with β-diketonates having small substituent (methyl group) has more asymmetric ligand fields than others. It was found that introduction of two different phosphine oxide was effective for increasing solubility of Tb(III) complexes.     

Effect of Er3+ doping in SnO2 semiconductor nanoparticles synthesized by solgel technique

Pure and Er³⁺ doped SnO₂ semiconductor nanoparticles have been synthesized by solgel technique. The X-ray diffraction patterns show peaks corresponding to tetragonal structure of SnO₂. No Er related impurity peaks could be observed. From the TEM micrographs average crystallite size was estimated to be 12 nm. The UV–visible absorption spectra of SnO₂:Er showed blue shift in the absorption shoulder compared with the spectra of undoped SnO₂ sample. Photoluminescence emission intensity of SnO₂:Er nanoparticles was found to be quenched with increasing concentration of Er³⁺ ions. The electron spin resonance (ESR) analysis of Er doped SnO₂ nanoparticles indicated Er in 3 + state with g = 2.     

Emission characteristics, crystalline phase and composition of vapor-transport-equilibrated Er:LiNbO3 crystal codoped with 6 mol% MgO

Polarized downconversion, 980-nm-upconversion and near-infrared emission characteristics of vapor-transport-equilibrated (VTEed) bulk Er (0.4 mol%)/MgO (6 mol%)-codoped LiNbO₃ crystals were investigated. The downconversion and upconversion visible emissions display similar VTE effects including the drop of emission intensity and the weakening of polarization dependence. At 0.98 and 1.5 μm regions, the VTE has a weak effect on the emission intensity, but a strong effect on the spectral shape. The crystalline phases in these bulk Er/Mg-codoped VTE crystals are determined by comparing their infrared emission characteristics with those of pure ErNbO₄ powder and locally Er-doped MgO (4.5 mol%):LiNbO₃ crystal. The results show that the Er³⁺ ions present in these bulk Er/Mg-codoped VTE crystals as a mixture of Er:LiNbO₃ and ErNbO₄ phases. The percentages of the ErNbO₄ phase contained in these VTE crystals were evaluated from the 1531 and 1536 nm characteristic absorption areas. The contents of constituent elements were determined by chemical analysis.     

Visible luminescence of nanocrystalline AlN:Er thin film by co-deposition of AlN,Er, and SiO2

We report a visible luminescence of Er³⁺ ions in an amorphous-nanocrystalline AlN:Er thin film prepared by co-deposition using AlN, Er, and SiO₂ targets. A PL emission spectrum of Er³⁺ in the AlN:Er film annealed at 750 °C showed a strong bluish green emission of Er³⁺ in the amorphous-nanocrystalline AlN:Er thin film, which is attributed to the intra-4fEr³⁺ transitions of ²H_11/2 → ⁴I_15/2 and ⁴F_7/2 → ⁴I_15/2. It was found that crystallite diameters were between 3 and 5 nm by high-resolution transmission electron microscopy. The occurrence of the strong Er³⁺ emission in the annealed AlN:Er thin film with a mixture of amorphous and nanocrystalline phases may be contributed to an increase in the number of excitation Er³⁺ centers and a presence of oxygen related to Er³⁺ excitation and recombination process in the AlN:Er thin film.     

1.54 μm emission mechanism of Er-doped zinc oxide thin films

Zinc oxide (ZnO) and Er-doped zinc oxide (ZnO:Er) thin films were formed by pulsed laser deposition, and characterized by photoluminescence (PL) and X-ray diffraction (XRD) in order to clarify the 1.54 μm emission mechanism in the ZnO:Er films. Er ions were excited indirectly by the 325 nm line of a He-Cd laser, and the comparison of the ultraviolet to infrared PL data of ZnO and ZnO:Er films showed that the 1.54 μm emission of Er³⁺ in ZnO:Er film appears at the expense of the band edge emission and the defect emission of ZnO. The crystallinity of the films was varied with the substrate temperature and post-annealing, and it was found that the intensity of the 1.54 μm emission is strongly related with the crystallinity of the films. There are three processes leading to the 1.54 μm emission; absorption of excitation energy by the ZnO host, energy transfer from ZnO to Er ions, and radiative relaxation inside Er ions, and it is suggested that the crystallinity plays an important role in the first two processes.     

Mixed-ligand complexes of Pt(II) and Pd(II) with 2-phenylbenzothiazole

The mixed-ligand cyclometalated [M(Bt)(μ-Cl)]₂ and [(M(N∧N))(Bt)]⁺ complexes (M = Pd(II), Pt(II); Bt^? is the deprotonated form of 2-phenylbenzothiazole; and ( N∧N) is ethylenediamine (En) and orthophenanthroline (Phen)) are studied and described by ¹H NMR spectroscopy, electronic absorption and emission spectroscopy, and voltammetry. The one-electron reduction of complexes is attributed to the electron transfer to the π * orbitals of both diimine and cyclometalated ligands. The long-wavelength absorption bands and vibrationally structured luminescence bands are assigned to optical transitions that are localized mainly on the M(Bt) metal-complex fragment.     

The influence of Zn2+ ion on the 1.5 μm laser properties of LiNbO3 crystal heavily doped with Er3+ ion

The spectral characteristics of 1.54 μm emission in a series of Zn/Er:LiNbO₃ crystals with heavy Er content and variable Zn content were reported. The inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the concentration of Er³⁺ ion in the crystal. The absorption and emission spectra of Zn/Er:LiNbO₃ crystals were measured. Based on Judd–Ofelt theory, the spectral parameters such as intensity parameters Ω_t (t=2, 4 and 6), transition strengths, radiative transition probabilities, radiative lifetime and fluorescence branching ratio have been obtained in Zn/Er:LiNbO₃ system. The emission cross section corresponding to ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ion was obtained according to Füchtbauer–Ladenburg theory. The gain cross section of Er:LiNbO₃ crystal codoped with 6 mol% Zn²⁺ ions were also discussed in this work.