Luminescence Lifetime Measurements Research Articles

A New Luminescent Material, Sr2SnO4:Eu3+.

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The multichromophore approach: A case of temperature controlled switching between single and dual emission in Ru(II) polypyridyl complexes

A new series of ruthenium(II) complexes, based on 4′-(9-anthryl)-2,2′:6′,2″-terpyridine (an-tpy), have been synthesized from the Ru(III) precursor (an-tpy)RuCl 3 ( 4). These new Ru(II) complexes, [(an-tpy)Ru(tpy-pm-R)] (R = H, 3a; Cl, 3b; phenyl, 3c; p-bromophenyl, 3d), have extended π-conjugation through the 5′-substituted pyrimidyl group, and have been characterized by analytical and spectroscopic methods, and X-ray single crystal structure determination for 3b and 3d. Luminescence lifetime measurements have shown that the anthryl chromophore greatly increases room temperature (r.t.) excited-state lifetime, even though it is not directly connected to the ligand involved in the metal-to-ligand charge transfer ( 3MLCT) emitting state. This result demonstrates that an equilibrium is established between the anthryl triplet state ( 3an) and the 3MLCT state even though the two chromophores are physically separated by more than one nanometer. At low temperature in rigid matrix, such equilibrium does not take place and emission arises from both 3an and 3MLCT excited states. The temperature dependence of the excited-state equilibration induces a temperature switch from single (room temperature) to dual (77 K) emission behavior.

Auto-Assembling of Ditopic Macrocyclic Lanthanide Chelates with Transition-Metal Ions. Rigid Multimetallic High Relaxivity Contrast Agents for Magnetic Resonance Imaging

PhenHDO3A is a ditopic ligand featuring a tetraazacyclododecane unit substituted by three acetate arms and one 6-hydroxy-5,6-dihydro-1,10-phenanthroline group (PhenHDO3A = rel-10-[(5R,6R)-5,6-dihydro-6-hydroxy-1,10-phenantholin-5-yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid). This ligand was specially designed so as to obtain highly stable heteropolymetallic assemblies. PhenHDO3A has been prepared starting from phenanthroline epoxide and either a triprotected tetraazacyclododecane or tert-butyl triester of N,N',N' '-tetraazacyclododecane-triacetic acid. The latter yields PhenHDO3A in a single step. PhenHDO3A forms kinetically stable lanthanide complexes (acid-catalyzed kinetic constant kH = (1.2 +/- 0.2) x 10(-3) s(-1) M(-1)) whose solution structure has been deduced from a quantitative analysis of the paramagnetic shifts and the longitudinal relaxation times of the proton nuclei of YbPhenHDO3A. The alcohol group of the dihydro-phenanthroline unit remains coordinated to the encapsulated metal ion despite the steric crowding brought about by this group. Furthermore, the complexes are monohydrated, as shown by luminescence lifetime measurements on EuPhenHDO3A solutions. Relaxivity titrations at 20 MHz clearly indicate that the phenanthroline unit of GdPhenHDO3A is available for the spontaneous formation of highly stable tris complexes with the Fe2+ and Ni2+ ions. The water-exchange times and the rotational correlation times of GdPhenHDO3A and Fe(GdPhenHDO3A)32+ have been deduced from variable temperature 17O NMR studies and from nuclear relaxation dispersion curves. Despite rather slow water-exchange rates (taum0 = 1.0-1.2 x 10(-6) s), relaxivity gains of 90% have been observed upon the formation of the heterometallic tris complexes. The latter rotate about four times more slowly (taur0= 398 ps) than the monomeric unit (taur0 = 105 ps) and their relaxivity is, accordingly, twice as high. The relaxivity of the tris complexes between 10 and 50 MHz is comparable to relaxivities reported for Gd3+-containing dendrimers of much higher molecular weights. The high relaxivity of the tris-PhenHDO3A lanthanide complexes is attributed to their internal rigidity.

Anion Sensing with Luminescent Tris(β-diketonato)europium(III) Complexes and Naked-Eye Detection of Fluoride Anion

Abstract Four kinds of tris(β-diketonato)europium(III) complexes were demonstrated to form highly coordinated complexes with inorganic anions and to exhibit anion-dependent luminescence. Their luminescence profiles were significantly dependent on the natures of β-diketonato ligands and external coordinative anions: Tris[3-(heptafluorobutyryl)-2-bornanonato]europium(III) complex gave F− anion-enhanced luminescence, and tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadionato)europium(III) complex responded to HSO4− and CH3CO2− anions. Although the employed europium complexes included similar β-diketonato ligands, UV and IR characterizations and luminescence lifetime measurements revealed that the structure of the β-diketonato ligand had significant influences on anion coordination and sensing behaviors of the europium complex. Since the observed anion-enhanced luminescence was high enough to be detected by the naked eye, visual detection of the F− anion in an aqueous sample was realized with the tris[3-(heptafluorobutyryl)-2-bornanonato]europium(III) complex.

Fluorescent Lifetime Quenching near d = 1.5 nm Gold Nanoparticles: Probing NSET Validity

The fluorescence behavior of molecular dyes at discrete distances from 1.5 nm diameter gold nanoparticles as a function of distance and energy is investigated. Photoluminescence and luminescence lifetime measurements both demonstrate quenching behavior consistent with 1/d(4) separation distance from dye to the surface of the nanoparticle. In agreement with the model of Persson and Lang, all experimental data show that energy transfer to the metal surface is the dominant quenching mechanism, and the radiative rate is unchanged throughout the experiment.

Luminescent studies of Ln(III) complexes with 4-amino-6-methylpicolinic acid N-oxide at 77 K

Spectroscopic properties of Ln(III) complexes with 4-amino-6-methylpicolinic acid N-oxide (ampicN–O) were characterized using absorption, fluorescence and phosphorescence spectra of the ligand as well as Eu(III) luminescence (intensity, lifetime measurements and selective excitation spectra in the range of 5D 0← 7F 0 transition) at 77 and 293 K. Based on emission behavior of ampicN–O and its Eu(III) and Gd(III) complexes the energy level of the ligand triplet state, mechanism of the ligand to metal energy transfer has been proposed and a role of the C–T state in this process analyzed. Detailed studies of the Eu(III) selective excitation spectra in the range of 5D 0← 7F 0 transition recorded for Eu/ampicN–O aqueous solution indicated equilibrium of three complex forms: ML, ML 2, ML 3.

Lifetime determination of materials that exhibit a stretched exponential luminescent decay

We report frequency- and time-domain luminescent lifetime measurements for materials that exhibit a broad spectrum of lifetimes, specifically, those for which the observed decay dynamics can be described by a stretched exponential function. The spectrum of lifetimes of such materials can be characterized by an average lifetime, and in principle this average can be extracted from either time- or frequency-domain measurements. In practice, this requires some care, because the extent to which the various states are excited depends on how long their lifetime is relative to the excitation pulse width or period. For the complex luminescent materials ZnS:Cu,Al and CdS quantum dots we compare the average lifetime obtained from frequency-domain data and from time-domain data under both pulsed laser excitation and steady-state, dc excitation. The agreement between the average measured lifetimes is good, but not perfect, showing that quoted average lifetimes for complex materials can be dependent on the measurement technique. Finally, a spectrum of lifetimes is given that gives rise to stretched exponential relaxation and this spectrum is used to compute frequency-domain data for a stretched exponential material.

Photoluminescent Properties of Eu(III) in the Composite Heterocyclic Ligands/Crown Ether Systems

Composite systems of Eu(phen) 2 Cl 3 (H 2 O) 2 , Eu(DN-bpy)phenCl3(H 2 O)2 and Eu(DB-bpy)phenCl3(H20)2 (DN-bpy: 4,4'-Dinonyl-2,2'-dipyridyl; DB-bpy: 4,4'-Di-tert-butyl-2,2'-dipyridyl) with crown ethers of Benzo-15-crown-5 (B15C5), Benzo-18-crown-6 (B18C6), 18-crown-6 (18C6), Dibenzo-18-crown-6 (DB18C6) and Dibenzo-24-crown-8 (DB24C8) were fabricated successfully and characterized by using photoluminescent spectroscopy and luminescent lifetime measurements. All composites formed show high luminescence mainly in red region. It was found that the heterocyclic ligands such as phen, DN-bpy and DB-bpy as well as the crown ethers have great influences on the photoluminescent properties of Eu 3 + ion. The environment around Eu 3 + ion in the composite systems changes greatly,presumably the variation of the first coordination sphere. The Eu 3 + ion occupies higher symmetrical environment and in more than one kind of symmetrical site in the composite systems studied in this work.

Evaluation of Dynamic Features of Escherichia coli 16S Ribosomal RNA in Homogeneous Physiological Solution

There is no methodology for the estimation of the dynamic features of large-molecular-weight RNAs in homogeneous physiological media. In this report, a luminescence anisotropy-based method using a long-lifetime luminescent oligonucleotide probe for the estimation of the dynamic features of large-molecular-weight RNA is described. As a luminescent probe, Ru(II) complex-labeled oligonucleotides, which have a complementary sequence to the single-stranded regions of Escherichia coli 16S rRNA, were synthesized. After the hybridization of the probe to single-stranded regions of 16S rRNA, the segmental motions of the regions were evaluated by time-resolved luminescence anisotropy analysis. In 16S rRNA, the L2 site (323–332 nt) was found to be the most flexible among the seven sites chosen. From a comparison between the hybridization kinetics of oligonucleotides to these single-stranded regions and the rotational correlation times, it was suggested that the flexibility of the single-stranded region was closely correlated with the hybridization kinetics. Furthermore, results of the luminescence lifetime measurement and luminescence quenching experiments suggested that the highly flexible region was located on the surface of the 16S rRNA and that the less flexible region was located in the depths of 16S rRNA.

A new luminescent material, Sr 2SnO 4:Eu 3+

A novel red emitting phosphor, Eu 3+-doped Sr 2SnO 4, was prepared by the solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the formation of Sr 2SnO 4:Eu 3+. Field-emission scanning electron microscopy (FE-SEM) observation indicated a narrow size distribution of about 500 nm for the particles with a spheric-like shape. Photoluminescence measurements showed that the phosphor exhibited bright red emission at about 615 nm under UV excitation. The excellent luminescence properties make it possible as a good candidate for plasma display panels (PDP) application. Splitting of the 5D 0– 7F J transitions of the Sr 2SnO 4:Eu 3+ samples suggests that the Eu 3+ ions occupy two nonequivalent sites in the crystallite by substituting Sr 2+ and Sn 4+ ions. The luminescence lifetime measurement showed a bi-exponential decay, providing another evidence for the existence of two different environments for Eu 3+ ions.

Changes in Hydration of Lanthanide Ions on Binding to DNA in Aqueous Solution

The interaction of the trivalent lanthanides Ce(III), Eu(III), and Tb(III) with sodium deoxyribonucleic acid (DNA) in aqueous solution has been studied using their luminescence spectra and decays. Complexation with DNA is indicated by changes in luminescence intensity. In the system terbium(III)-DNA, changes in luminescence with pH are suggested to be due to the protonation of phosphate groups. The degree of hydration of Tb(III) on binding to DNA is followed by luminescence lifetime measurements in water and deuterium oxide solutions, and it is found that the lanthanide ion loses at least one hydration water on binding to long double stranded DNA at pH 4.7 and pH 7. Rather different behavior is observed on binding to long or short single stranded DNA, where six water molecules are lost, independent of pH. It is suggested that in this case the lanthanide probably binds to the bases of the DNA backbone. The DNA conformation seems to be an important factor in the binding. In addition, the isotopic effect on terbium luminescence lifetime may provide a useful method to distinguish between single and double stranded DNA. DSC results are consistent with cleavage of the double helix of DNA at pH 9 in the presence of terbium.

Non-destructive thermal barrier coating (TBC) damage assessment using laser-induced luminescence and infrared radiometry

A new non-destructive evaluation (NDE) approach for assessing damage in thermal barrier coating (TBC) systems is described. It is based on a combined measurement of the optical and thermal response due to photo-excitation, through the TBC, of Cr 3+ ions within the thermally grown oxide (TGO). This method has subsurface selectivity to the TBC/TGO interface, as it is based on using a laser wavelength to which the TBC is transparent but the TGO absorbs strongly, so that the energy of the laser is converted into both luminescence and heat within the TGO. Using mid-infrared detection, the subsequent change in temperature of the TGO is measured as heat is conducted away from the TGO into the adjacent alloy and TBC coating. Damage decreases the rate at which the heat decays away and can be quantified from measurements of the transient temperature changes. The luminescence signal may be used to quantify the compressive stress and the thickness of the TGO, using photoluminescence piezospectroscopy and luminescence lifetime measurements, respectively. Both the optical arrangement used for implementing this new approach and the results obtained from a series of thermally cycled TBCs are described.

Spectroscopy and energy transfer characteristics of PLZT:Tm 3+ transparent ceramics

Thulium-doped PLZT transparent ceramic is proposed as a new material for photonic applications in the near-infrared wavelength region (1.8 and 1.44 μm). In this work, its structural and spectroscopic characteristics are investigated through ground state absorption, luminescence and excited state lifetime measurements. The results are discussed in terms of energy transfer processes among Tm 3+ ions as the cross relaxation 3H 4, 3H 6 → 3F 4, 3F 4 and the energy migrations 3H 4, 3H 6 → 3H 6, 3H 4 and 3F 4, 3H 6 → 3H 6, 3F 4. Excellent thermal and mechanical properties associated with desirable spectroscopic characteristics suggest that PLZT:Tm 3+ is a potential laser active medium, worth of further development and investigations.

Mononuclear and Dinuclear Complexes of Isoeilatin

This work describes the synthesis and characterization of mononuclear and dinuclear Ru(II) and Os(II) complexes based on the symmetrical bridging ligand isoeilatin (1). The crystal structure of 1.[HCl]2 consists of layers of tightly pi-stacked molecules of the biprotonated isoeilatin. The mononuclear complexes [Ru(bpy)2(ieil)]2+ (2(2+)) and [Os(bpy)2(ieil)]2+ (3(2+)) form discrete dimers in solution held together by face-selective pi-stacking interactions via the isoeilatin ligand. Coordination of a second metal fragment does not hinder the pi-stacking completely, as demonstrated by the concentration dependence of the 1H NMR spectra of the dinuclear complexes [{Ru(bpy)2}2{mu-ieil}]4+ (4(4+)), [{Os(bpy)2}2{mu-ieil}]4+ (5(4+)), and [{Ru(bpy)2}{mu-ieil}{Os(bpy)2}]4+ (6(4+)) and supported by the solid-state structure of meso-4.[Cl]4. The bridging isoeilatin ligand conserves its planarity even upon coordination of a second metal fragment, as demonstrated in the solid-state structures of meso-4.[Cl]4, meso-4.[PF6]4, and meso-5.[PF6]4. All of the dinuclear complexes exhibit a preference (3/2-3/1) for the formation of the heterochiral as opposed to the homochiral diastereoisomer. Absorption spectra of the mononuclear complexes feature a low-lying dpi(M) --> pi*iel MLCT band around 600 nm that shifts to beyond 700 nm upon coordination of a second metal fragment. Cyclic and square-wave voltammetry measurements of the complexes exhibit two isoeilatin-based reduction waves that are substantially anodically shifted compared to [M(bpy)3]2+ (M = Ru, Os). Luminescence spectra, quantum yields, and lifetime measurements at room temperature and at 77 K demonstrate that the complexes exhibit 3MLCT emission that occurs in the IR region between 950 and 1300 nm. Both the electrochemical and photophysical data are consistent with the low-lying pi orbital of the isoeilatin ligand. The dinuclear complexes exhibit two reversible, well-resolved, metal-centered oxidation waves, despite the chemical equivalence of the two metal centers, indicating a significant metal-metal interaction mediated by the bridging isoeilatin ligand.

A novel red emitting phosphor Ca 2SnO 4: Eu 3+

A novel red emitting phosphor, Eu 3+-doped Ca 2SnO 4, was prepared by the solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the formation of Ca 2SnO 4: Eu 3+. Field-emission scanning electron-microscopy (FE-SEM) observation indicated a narrow size-distribution of about 500 nm for the particles with spherical shape. Photoluminescence measurements indicated that the phosphor exhibits bright red emission at about 615 nm under UV excitation. The excellent luminescence properties make it possible as a good candidate for plasma display panels (PDP) application. Splitting of the 5 D 0– 7 F J transitions of Ca 2SnO 4: Eu 3+ suggests that the Eu 3+ ions occupied two nonequivalent sites in the crystallite. The luminescence lifetime measurement showed a bi-exponential decay, providing other evidence for the existence of two different environments for Eu 3+ ions.

Lanthanide complexation with 1,3,5-triamino 2,4,6-trihydroxycyclohexane

The protonation constants of 1,3,5-triamino-2,4,6-trihydroxycyclohexane (taci), at 25 °C in I = 1.00 M (NaClO 4) were determined to be: p Ka 1, 5.57 (0.08); p Ka 2, 7.45 (0.02); p Ka 3, 9.05 (0.04). The log of the stability constants, log β 302, at 25°C in I = 1.00 M (NaClO 4) for formation of [ Ln ( H - 3 taci ) 2 3 + ] were measured by potentiometry to be: Nd(III), 25.33 (0.09); Eu(III), 26.42 (0.06); Tm(III), 30.07 (0.10); Lu(III), 33.68 (0.07) ; Y(III), 28.59 (0.07). 1H NMR spectra were consistent with formation of a single complex from pcH 6 to 10. Laser fluorescence measurements of the 7F o– 5D o transition of Eu(III) complexed by taci indicated a single complexed species. The shift in this peak relative to that of Eu 3+(aq) was significantly greater than the values reported for the complexes of other organic ligands with Eu(III). Luminescence lifetime measurements indicated two water molecules bound to each of the Eu(III) cations in the taci complex.

Studies on photoluminescent properties of Eu(III) ions in composite europium complex/polymer systems

Europium complex Eu(DB-bpy)phenCl 3(H 2O) 2 ( phen: 1,10-phenanthroline, DB-bpy: 4,4′-di- tert-butyl-2,2′-dipyridyl) was doped in poly(methyl methacrylate) (PMMA) and poly(vinyl pyrrolidone) (PVP). The composite systems with various molar ratios of C O groups/Eu 3+ ions were characterized by photoluminescent (PL) spectroscopy, X-ray diffractometry (XRD) and luminescent lifetime measurements. It was found that Eu 3+ ions have more than one kind of symmetrical sites in Eu/PMMA composites and the PL spectra of Eu(III) change with the compositions. However, the composites of Eu/PVP with different molar ratios show similar PL spectra, indicating different influence of matrices on luminescent properties.

Optical properties of single InAs quantum dots in close proximity to surfaces

The optical properties of single InAs∕GaAs quantum dots (QDs) were studied as a function of their distance from the air∕GaAs interface. A short-period superlattice structure allows us to controllably shorten the distance between the QDs and the surface in 6-nm steps. The QD luminescence intensity and lifetime measurements show that quantum tunneling effect results in a sharp reduction in fluorescence efficiency and lifetime when the wetting-layer–surface distance is within 9 nm. For distances between 15 and 40 nm, broadening of the photoluminescence linewidths of single QDs was observed. Since exciton recombination time and efficiency are in this case unchanged with respect to bulk QDs, the observed line broadening is most likely due to dephasing or spectral diffusion processes.

Spectroscopic study of floating zone technique-grown Nd3+-doped CaMoO4

Optical spectroscopy studies of Nd 3+ -doped scheelite calcium molybdate (CaMeO 4 ) single crystal grown by a floating zone-like technique was performed using Judd-Ofelt analysis, besides luminescence data and lifetime measurements in the visible and near infrared domains. The excited state absorption in cross section units of this material was measured by two different methods for all expected infrared emission. By using calculated intensity parameters Ω t (t = 2, 4, 6), it were determined branching ratios, emission cross-sections, and lifetimes for all related transitions. The observed single excited state absorption calibrated in absorption cross-section units was found too small to disturb the laser efficiency in the 1030-1100 nm spectral range. A comparison of main spectroscopic parameters of this material with those of other Nd 3+ -doped crystals is presented.

Iridium luminophore complexes for unimolecular oxygen sensors.

In this study, a series of novel luminescent cyclometalated Ir(III) complexes has been synthesized and evaluated for use in unimolecular oxygen-sensing materials. The complexes Ir(C6)(2)(vacac), 1, Ir(ppy)(2)(vacac), 2, fac-Ir(ppy)(2)(vppy), 3, and mer-Ir(ppy)(2)(vppy), 4, where C6 = Coumarin 6, vacac = allylacetoacetate, ppy = 2-phenylpyridine, and vppy = 2-(4-vinylphenyl)pyridine, all have pendent vinyl or allyl groups for polymer attachment via the hydrosilation reaction. These luminophore complexes were characterized by NMR, absorption, and emission spectroscopy, luminescence lifetime and quantum yield measurements, elemental analysis, and cyclic voltammetry. Complex 1 was structurally characterized using X-ray crystallography, and a series of 1-D ((1)H, (13)C) and 2-D ((1)H-(1)H, (1)H-(13)C) NMR experiments were used to resolve the solution structure of 4. Complexes 1 and 3 displayed the longest luminescence lifetimes and largest quantum efficiencies in solution (tau = 6.0 micros, phi = 0.22 for 1; tau = 0.4 micros, phi = 0.2 for 3) and, as result, are the most promising candidates for future luminescence-quenching-based oxygen-sensing studies.