Energy transfer from Eu(III) complexes to dyes in their mixed nanostructures. II

The effects of the concentration of a number of dyes in an aqueous solution and of the method of formation of mixed nanostructures of dyes and Eu(MBTA)₃phen (MBTA is p-methoxybenzoyltrifluoroacetone; phen is 1,10-phenanthroline) complexes that form these structures on the luminescence decay kinetics of Eu(III) ions are studied. It is shown that, in the concentration range 5–50 nM of Nile blue, the concentration dependences of the luminescence decay and of the decrease in τ _lum of Eu(III) nearly coincide and are nearly linear. The dependence of the ratio of I _lum of Eu(III) to the intensity of the sensitized delayed fluorescence of Nile blue on the delay time of the probe pulse is analyzed; it is found that the majority of dye molecules incorporated in nanostructures of Eu(MBTA)₃phen complexes emit sensitized delayed fluorescence with times 1–50 μs. Analysis of the effect of the structure of nanostructures on the quenching of I _lum of Eu(III) by the dye at different concentrations of Eu(III) shows that nanostructures of Eu(MBTA)₃phen and Eu(NTA)₃phen (NTA is 2-naphthoyltrifluoroacetone) complexes appear in the aqueous solution at a concentration of Eu(III) of 0.1 μM (the MBTA or NTA concentration is 50 μM, and the concentration of phen is 17 μM) and exist in the solution at the Eu(III) concentrations up to ～5 μM. This confirms the conclusion on the occurrence range of nanostructures of Ln complexes previously made based on the analysis of columinescence in these structures.     

Energy transfer from Eu(III) and Tb(III) complexes to dyes in their mixed nanostructures. I

The formation of nanostructures that consist of complexes of β-diketones with 1,10-phenanthroline and involve dyes of the polymethine, triphenylmethane, oxazine, and xanthene series is observed in aqueous solutions. It is found that nanostructures of complexes of Ln(III) ions and dyes are reliably observed at concentrations of Ln complexes from 0.5 to 5 μM and at dye concentrations above 5 nM. Nanostructures of complexes Eu(MBTA)₃phen, Eu(NTA)₃phen, Eu(PTA)₃phen, Tb(PTA)₃phen, Gd(MBTA)₃phen, and Lu(MBTA)₃phen with dyes are studied, where MBTA is n-methoxybenzoyltrifluoroacetone, NTA is naphthoyltrifluoroacetone, PTA is pivaloyltrifluoroacetone, and phen is 1,10-phenanthroline. It is shown that nanostructures formed can contain dye molecules not only inside a nanostructure of Ln complexes but also on its outer shell. It is proved that, at a dye concentration in the solution of the order of nanomole or higher, the formation of mixed nanostructures of Eu complexes and dyes whose S ₁ level is below the ⁵ D ₀ level of Eu(III) leads to the quenching of the luminescence of Eu(III) and gives rise to the sensitized luminescence of dyes. The energy transfer efficiency from Eu(III) ions to dye molecules is determined by the ability of these molecules to incorporate into nanostructures of Eu complexes. The effect of the formation of nanostructures on the shape and position of the spectra of luminescence and absorption of dyes is studied. Comparison of the sensitized luminescence intensities of Nile blue in structures of Eu, Lu, and Gd complexes shows that the greater part of the excitation energy of Eu complexes is transferred directly from ions to dye molecules according to the inductive-resonance energy transfer mechanism rather than by means of energy migration over singlet levels of organic ligands in complexes of a nanostructure.     

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.     

Energy transfer from the singlet levels of diketones and dyes to lanthanide ions in nanoparticles consisting of their diketonate complexes

The energy transfer from the S ₁ levels of p-phenylbenzoyltrifluoroacetone (PhBTA) and dyes to different Ln³⁺ ions is studied in nanoparticles (NPs) composed of complexes of this diketone with Ln³⁺ and 1,10-phenanthroline (phen) and doped with dye molecules. The quenching rate constants in the NPs consisting from complexes of Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Ho³⁺, Er³⁺, and Tm³⁺ are determined from the data on the quenching of sensitized (cofluorescence) and ordinary fluorescence of coumarin 30 (C30) and rhodamine 6G (R6G). The quenching rate constants vary from ≤5 × 10¹¹ to 10¹³ s^?1 for the fluorescence quenching of PhBTA by different Ln³⁺ ions, while the quenching of dye fluorescence occurs at rates of the order of 10⁹ s^?1. In the case of complexes with the Pr³⁺ ions, the fluorescence quenching of PhBTA in NPs composed of its complexes is accompanied by sensitized luminescence of Pr³⁺. The quenching observed is due to a nonradiative energy transfer from the S ₁ states of ligands and dyes to these ions. It is shown that in NPs composed of complexes with Eu³⁺, Yb³⁺, and Sm³⁺ the cofluorescence of C30 is quenched via the electron-transfer mechanism. The study of quenching of cofluorescence and fluorescence of dyes in NPs composed of mixed complexes of La³⁺ and Nd³⁺ (Ho³⁺) shows that the observed quenching of fluorescence and cofluorescence is governed mainly by the quenching of the S ₁ state of dyes when the Nd³⁺ (Ho³⁺) content does not exceed 5–10% and by the quenching of the S ₁ state of a ligand when the Nd³⁺ (Ho³⁺) content exceeds 50%. It is assumed that the high rate constant of energy transfer from the S ₁ level of ligands to ions Pr³⁺, Nd³⁺, Ho³⁺, Er³⁺, and Tm³⁺ in NPs composed of beta-diketonate complexes is caused by exchange interactions.     

The influence of deuteration of complex organic molecules on their fluorescence quantum yield (a review)

Data on the influence of deuteration of organic molecules and/or solvent on their fluorescence quantum yield are summarized. It is demonstrated that the effect of deuteration is normal, i.e., deuteration increases the fluorescence quantum yield due to decrease in the internal conversion probability, for organic molecules exhibiting fluorescence in the red region of spectrum, in which the internal conversion competes with the emission of fluorescence. In the case in which the probability of internal conversion is low, i.e., energy of level S₁ ≥ 16000–20000 cm^–1, the intersystem crossing probability from the S₁ state to the set of triplet levels depends on the relative position of S₁ and the nearest triplet level. In so doing, the effect of deuteration can be either normal or anomalous, depending on whether the resonance between the S₁ level and the level nearest to it, the T_n level, improves or deteriorates when these levels shift as a result of deuteration. In dilute vapors, cooled supersonic jets, and crystalline matrices, including Shpolskii matrices, the effect of deuteration at helium temperatures depends on exact resonance between the interacting levels. In the case of dilute vapors and cooled jets, the effect also depends on the vibronic level being excited. Deuteration of OH and NH groups, as a rule, slows proton transfer in the S₁ state of the molecule that occurs with their involvement, leading to normal effect of deuteration.     

Sensitization of fluorescence of dye molecules in nanoparticles of metal complexes

We studied the dependence of absorption and fluorescence spectra of complexes of Al, In, Sc, Y, and La with dibenzoylmethane and naphthoyltrifluoroacetone, as well as the dependence of sensitized fluorescence of dyes in nanoparticles of these complexes, in relation to the water pH, the ratio between ions and diketones, and the ion selection. We showed that the ability of complexes of ions to form nanoparticles that efficiently sensitize dye molecules incorporated into them is determined by stability constants of these ions with organic ligands and by their ability to compete with the formation of hydroxy complexes of these ions. We found that nanoparticles consist of diketonates of different compositions and that Nile red incorporated into nanoparticles is an indicator of the dependence of the composition of nanoparticles on the selection of the central ion of complexes and conditions of their formation. We revealed that complexes M(diketone)(OH)₂ self-assemble into nanoparticles with an admixture of dye molecules and efficiently sensitize dyes upon excitation into absorption bands of complexes. We showed that, at concentrations of rhodamine 6G in water smaller than 50 nM, the use of a solution that contains 50 μM of Al(III), In(III), or Sc(III) + 50 μM of naphthoyltrifluoroacetone makes it possible to increase the sensitivity of the luminescence analysis by 20-fold for the presence of rhodamine 6G in an aqueous solution.     

The mechanism of sensitized anti-stokes luminescence in crystals with adsorbed dyes

We have analyzed regular features of sensitization processes of crystals AgCl, AgCl_0.95I_0.05, and Zn_0.6Cd_0.4S to processes of a low-threshold (10^?3–10^?4 W/cm²) two-quantum excitation of anti-Stokes luminescence by adsorbed molecules and by J and H aggregates of dyes of different classes. The excitation centers of this luminescence are complex and consist of dye molecules and few-atom silver clusters adsorbed nearby. Luminescence-excitation processes involve stages of photoexcitation of adsorbed dye molecules; resonance transfer of electronic excitation energy to adsorbed atoms and few-atom silver clusters, levels of which lie near the middle of the band gap; and their subsequent photoionization.     

Enhancement of anti-stokes sensitized luminescence in AgCl(I) crystals in the presence of silver nanoparticles

We have observed a significant increase in the intensity with anti-Stokes excitation of recombination luminescence in AgCl(I) microcrystals sensitized by methylene blue molecules in the presence of silver nanoparticles.     

Laser power influence on Raman spectra of ZnO(Co) nanoparticles

Influence of laser power on nanocrystalline samples of ZnO(Co) prepared by commonly used wet chemistry method followed by calcination was investigated. Previous confirmation of the existence of ZnO and Co₃O₄ phases was based on the X-ray diffraction measurements. Here we report the experimental spectra of non-resonant Raman scattering in the range between 100 cm⁻¹ and 1600 cm^–1, for a series of samples irradiated with four different laser power densities. The laser power density has different influence on relative intensity of peaks that belong to ZnO phase than on those corresponding to Co₃O₄ phase. Both peak types show characteristic broadening and red shift toward lower frequencies. The laser power densities used in our study did not cause thermal destruction in any of the investigated samples. Laser-induced local heating effects in samples caused formation of cobalt dimers on the surface of Co₃O₄.     

Photon distribution functions of fluorescence photons from single nanoparticles: Three different photon counting methods

Mathematical expressions are derived for three photon distribution functions w _N ^M (T), w _N ^Z (T), and w _N ^O (T) corresponding to three different methods for counting fluorescence photons from a single nanoparticle excited by continuous laser radiation. Each of the three functions is expressed in terms of Poisson functions, which makes it possible to pass in the w _N ^M (T), w _N ^Z (T), and w _N ^O functions from N multiple integrals to single or double integrals. This not only eases the numerical calculation of the photon distribution, but it also makes it possible to find that, for each exponential process in the dynamics of a nanoparticle, there is a Poisson function in the photon distribution function. All three photon counting methods yield the same photon distribution for continuous fluorescence and different photon distributions for blinking fluorescence.     

The irradiation influence on the properties of silver sulfide (Ag2S) colloidal nanoparticles

The aqueous solutions of different stability containing silver sulfide (Ag₂S) nanoparticles are studied. The stable, transparent, and turbid solutions have been subjected to daylight for 7 months, to ultraviolet and laser irradiation, as well as to an electron beam. Solar radiation is found to favor the Ag₂S reduction to Ag and/or the formation of Ag₂S/Ag hybrid nanoparticles in the solution. At a high amount of hybrid nanoparticles, the exciton–plasmon interaction causes asymmetry in the absorption spectra. The exposure of Ag₂S particles precipitated from the solution with the electron beam leads to the reversible growth of Ag threads. The possible exciton–plasmon interplay mechanisms in Ag₂S/Ag hybrid nanoparticles are considered. The physical mechanisms of the changing Ag₂S stoichiometry, the formation of metallic Ag and Ag₂S/Ag hybrid nanoparticles are the generation of hot carriers and the energy transfer (exciton–plasmon interaction) in a metal–semiconductor hybrid nanosystem are elucidated, as well.

     

Time-resolved fluorescence of cationic dyes covalently bound to poly(methacrylic acid) in rigid media

Atactic poly(methacrylic acid) labeled with acridine and Nile blue (NB) were studied by photophysical techniques in bulk solid state and in solution-cast films over different surfaces (glass, ITO, and polymethylmethacrylate). In the systems with both dyes, energy transfer from acridine to NB occurs with an efficiency depending on the type of substrate (solid or film). The films are more disordered fluorescent rigid media than the bulk chromophoric or bichromophoric polymers, and this effect is ascribed to inhomogeneous distribution of the dyes in the film. This effect enhances dye bimolecular interactions and increases the energy transfer rates between acridine donor and NB acceptor. Bimodal distributions of donor fluorescence lifetimes are observed.     

Studies on influence of light on fluorescence of Tris-(8-hydroxyquinoline)aluminum thin films

Tris-(8-hydroxyquinoline)aluminum (Alq₃) thin films, the most widely used electron transport/emissive material in the organic electroluminescent (EL) devices, have been deposited on glass substrates by thermal evaporation process. Alq₃ thin films were exposed to light for various time periods under normal ambient. The fluorescence of as-prepared and light exposed Alq₃ thin films and formation of luminescent quencher have been studied using fluorescence, Mass, MALDI-ToF-MS, ¹H & ¹³C NMR, and FT-IR spectroscopy. It is observed that among the three 8-hydroxyquinoline (HQ) units in Alq₃ molecule, one HQ unit is affected during the light exposure in the normal ambient. It is found that the affected resultant Alq₃ molecule containing the carbonyl group acts as fluorescent quencher and the energy of excitons in the Alq₃ molecule in the light exposed Alq₃ thin films can be non-radiatively transferred to the neighboring fluorescent quencher, quenching the fluorescence of light exposed Alq₃ thin films in the normal ambient.     

Magnetic field effects on the yields of dyes in sensitized transformation of diphenylamine in micellar systems

A strong influence (an increase in the rate of formation of the diphenylbenzidine dication by up to three times) of an external magnetic field on the yields of colored products was observed when micellar solutions of benzophenone and diphenylamine derivatives were UV irradiated in the presence of acids. A mechanism of the phenomenon was suggested. The mechanism by based on the concept of the simultaneous occurrence of two processes, (1) geminate recombination of ketyl and aminyl radicals with the formation of triarylmethane dye and (2) bulk recombination of two radical cations produced in the protonation of aminyl radicals with the formation of the diphenylbenzidine dication. Magnetic field is a factor determining the rate of the triplet-singlet conversion of radical pairs formed in the photoreduction of the ketone by amine and, therefore, the ratio between the stationary concentrations of aminyl radicals and radical cations. The influence of the structure of ketones, the ratio between the reagents and surfactant, and the pH value on the dye yield and the scale of magnetic-spin effects were analyzed.     

无序二元合金（NixCu1-x）表面CO吸附及对表面偏析的影响

根据计算机编程构造出了存在和不存在表面偏析的无序二元合金Ni_xCu_1-x（x=0.4）的原子集团模型，然后按覆盖度θ=0.5，构造出了CO表面吸附的模型 ，应用Recursion方法计算了CO在（Ni_xCu_1-x）（存在偏析和不存在偏析时）合金表面不同位置（顶位和芯位）吸附的电子结构 .由此得出：1)CO在顶位吸附时较稳定；2)CO吸附使合金表面态密度峰降低，带宽加宽，使d轨道的局域性变弱；3)CO的吸附抑制了Cu 关键词： 化学吸附 表面偏析 Recursion方法 态密度     

Studies on Cu(II), Zn(II), Ni(II) and Co(II) complexes derived from two dipeptide Schiff Bases

Some new metal complexes of two dipeptide Schiff bases derived from salicylaldehyde and dipeptides such as glycyl-DL-alanine and glycyl-DL-phenylalanine have been synthesized and characterized by elemental analysis, molar conductance, IR, UV spectra, TG and DTA studies. The COO stretching bands in IR spectra suggest that the carboxylate acts as a monodentate group when binding with metal. The ligands are coordinated to the central metal as tetradentate ligands. The bonding sites are the carboxylate oxygen, imino nitrogen, amide nitrogen and phenolic oxygen.     

Evaluation of influence of anisotropic diffusion near a wall in near-field fluorescence correlation spectroscopy of nanoparticles

Fluorescence correlation spectroscopy (FCS) of colloidal nanoparticles using a near-field fiber probe was numerically simulated. The near-wall dynamics was simulated by accounting for the anisotropic mobility of nanoparticles owing to hydrodynamic interaction with a wall (Stokes viscous force). By comparing the simulation results with theoretical model calculations, we found that the influence of anisotropic diffusion is insignificant in near-field FCS autocorrelation analysis.     

Evidence for two different adsorption sites of CO on Pt(111) from infrared reflection spectroscopy

The adsorption of CO on Pt(111) surfaces has been studied under clean conditions by a highly surface sensitive double-beam infrared reflection spectroscopy (IRS). In contrast to results of other authors two stretching vibrations of adsorbed CO rather than one are detected near 2100cm⁻¹ and 1870cm⁻¹. This is in agreement with recent findings in high-resolution electron energy loss spectroscopy (ELS). The results are discussed in terms of two adsorption sites: CO adsorbed in on-top positions and double coordinated on bridging sites, respectively. Furthermore, a precursor state and a preferential adsorption in islands at low coverage is taken into account.