Site-selective Fluorescence Spectra Research Articles

Thermal hysteresis in the luminescence ofYb3+ions inSr0.6Ba0.4Nb2O6

The optical spectra of ${\mathrm{Yb}}^{3+}$ ions in congruent, ${\mathrm{Sr}}_{0.6}{\mathrm{Ba}}_{0.4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{6}$ (SBN) crystals have been systematically investigated in a broad temperature range $(10\char21{}420\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. Laser site selective low temperature fluorescence spectra have shown that ${\mathrm{Yb}}^{3+}$ ions enter in four different cationic lattice sites. A thermal hysteresis in both bandwidth and luminescence intensity of ${\mathrm{Yb}}^{3+}$ ions has been detected in the temperature range $(300\char21{}400\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ around the ferro to paraelectric phase transition. This behavior has been explained as a result of the local structural modifications that take place around the ${\mathrm{Yb}}^{3+}$ ions when the crystal becomes nonpolar, which affect in a different way the luminescence of ${\mathrm{Yb}}^{3+}$ ions located in the different sites. Off-congruent SBN crystals activated with ${\mathrm{Yb}}^{3+}$ ions have been grown and investigated in order to examine the possibilities of extending the detected hysteretic behavior to other temperature ranges.

Fluorescence properties of Eu3+ : Y2SiO5 single crystal

Fluorescence properties of Eu3+ :Y2Si05 have been investigated. Transitions between 5D and 7Fwere were studied with transmission spectra, fluorescence spectra, photoluminescence excitation (or absorption) spectra and site selective fluorescence spectra. The X-ray powder diffraction pattern of Eu3+ :Y2Si05 shows that the crystal belong to monoclinic, and lattice’s constants a, b, c and β are obtained by a simulation with the measured diffraction angles.

Site-selective spectroscopy of iron stress pigment–protein complex CP 34

The synthesis of supplementary chlorophyll-binding proteins is a common response of various microorganisms to environmental stresses. The cyanobacteria Synechococcus elongatus grown under the lack of iron produces the protein called CP 34 which is generally considered to act as an auxiliary light harvesting complex in primary photosynthetic processes.A study of vibrations, electron–phonon coupling, and excitation energy transfer in CP 34 was based on systematic measurements of low temperature site selection fluorescence spectra. The spectral data obtained under non-selective blue excitation (into B absorption band of chlorophyll a) were compared to selectively red excited (into Qy absorption band of chlorophyll a) ones. The frequencies of normal vibrations were determined from site selection spectra excited at different wavelengths between 660 and 690nm. Their values obtained in CP 34 were compared to those observed in chlorophyll–protein complexes of reaction centrum of photosystem II.

Determination of single-site absorption spectrum in dye-doped disordered materials by double site-selective spectroscopy

By a combination of site-selective fluorescence-excitation and persistent spectral hole burning spectroscopies, the single-site absorption spectrum has been determined in dye-doped disordered materials for the first time. It has been found theoretically that the difference between the site-selective excitation spectra before and after hole burning by monochromatic light exactly gives the single-site absorption spectrum at very low temperatures. In the measurement of the excitation spectrum, the intensity of fluorescence from chlorin-doped polystyrene having the same wavelength as the exciting light has been obtained by rejecting the scattered laser light using a time gate with a time-correlated single-photon counting method. Site-selective fluorescence spectra before and after hole burning were also measured to compare their difference with the experimentally determined single-site absorption spectrum.

Site-selective emission spectra of Eu 3+:Ca 5(PO 4) 3F

The room temperature site-selective fluorescence and excitation spectra of Eu 3+:FAP powder samples as well as the decay times of the observed emission lines associated with the 5D 0→ 7F 0,1,2 transitions have been measured. In sample I, prepared by the thermally stimulated diffusion of Eu 2O 3 in the solid state, Eu 3+ reside only in the Ca(II) location which has C s symmetry. Experimental values for the Stark energy levels have been determined for three well-separated sites. Sites A and B exhibit very similar spectral features attributed to the presence of the charge-compensating O 2− ion in the F − position in the FAP lattice, whereas for Eu 3+ in site C the charge is most probably compensated by vacancies according to 3Ca 2+→2Eu 3++Vacancy substitution. In sample II, prepared by precipitation of Eu 3+(1%):FAP from solution, Eu 3+ substitute for calcium in the Ca(I) position exclusively and three sites with C 3 symmetry have been identified. Substitution of 3Ca 2+/2Eu 3+ is the most probable charge-compensation mechanism. Heating of sample II stimulates the migration of Eu 3+ from the Ca(I) position to the low symmetry Ca(II) site.

Raman and site-selective fluorescence spectra of model compounds for interchain interactions in poly(phenylenevinylene)

Nonresonant Raman spectra and site-selective fluorescence spectra (polyethylene matrix, 1.5 K) are reported for 1,4-distyrylbenzene, its tert-butyl and paracyclophane substituted derivatives, and the paracyclophane derivative bearing two distyrylbenzene chains in a pseudo- para orientation, a simple model for the interchain interactions in poly(phenylene-vinylene) films. The first three compounds show pronounced fluorescence line-narrowing when excited near the red edge of their absorption spectra. The bichromophoric compound exhibits no narrowing although its Raman spectrum, room-temperature fluorescence spectrum, and lifetime are similar to those of the other three molecules. This may reflect increased low-frequency Franck–Condon activity in the electronically delocalized bichromophoric system.

Fluorescence from molecules and aggregates in polycrystalline thin films of α-oligothiophenes

We report stationary fluorescence, fluorescence excitation spectra, site-selective fluorescence spectra, and femtosecond time-resolved fluorescence spectra of vacuum-evaporated α- quaterthiophene (T4) and α-quinquethiophene (T5) polycrystalline thin films at various temperatures. It is found that the fluorescence spectrum of each compound studied is a superposition of emission spectra originated from energetically and spatially separated fluorescent species or sites, which are suggested to be disordered-molecules, pre-aggregates, aggregates, and physical defects. The disordered molecules lying at grain boundaries display a solutionlike fluorescence decay feature. Multiple fluorescence spectra with identical vibrational developments from pre-aggregate species are observed in the T5 film. A long-lived fluorescence decay component observed in the T4 film is attributed to the dipole-allowed transition in the aggregates. A slow rise observed in the time-resolved fluorescence of the T4 film is interpreted in terms of energy transfer from the energetically high-lying species to the physical defects in crystal.

Fluorescence and Absorption Spectroscopy of the Weakly Fluorescent Chlorophyll a in Cytochrome b 6f of Synechocystis PCC6803

A spectroscopic characterization of the chlorophyll a (Chl) molecule in the monomeric cytochrome b6f complex (Cytb 6f) isolated from the cyanobacterium Synechocystis PCC6803 is presented. The fluorescence lifetime and quantum yield have been determined, and it is shown that Chl in Cytb 6f has an excited-state lifetime that is 20 times smaller than that of Chl in methanol. This shortening of the Chl excited state lifetime is not caused by an increased rate of intersystem crossing. Most probably it is due to quenching by a nearby amino acid. It is suggested that this quenching is a mechanism for preventing the formation of Chl triplets, which can lead to the formation of harmful singlet oxygen. Using site-selected fluorescence spectroscopy, detailed information on vibrational frequencies in both the ground and Q y excited states has been obtained. The vibrational frequencies indicate that the Chl molecule has one axial ligand bound to its central magnesium and accepts a hydrogen bond to its 13 1-keto carbonyl. The results show that the Chl binds to a well-defined pocket of the protein and experiences several close contacts with nearby amino acids. From the site-selected fluorescence spectra, it is further concluded that the electron-phonon coupling is moderately strong. Simulations of both the site-selected fluorescence spectra and the temperature dependence of absorption and fluorescence spectra are presented. These simulations indicate that the Huang-Rhys factor characterizing the electron-phonon coupling strength is between 0.6 and 0.9. The width of the Gaussian inhomogeneous distribution function is 210 ± 10 cm −1.

Relation between photoconduction and excimer formation in a conjugated oligomer

Photoconduction action spectra and site-selective fluorescence spectra have been measured in polystyrene blends of an oligomer of phenylenevinylene carrying cyano- and octyloxy-substituents (CNSB). Increasing the content from 20% to 80% (by weight) changes the fluorescence spectra from being monomer-like to excimer-like. Strong excimer emission correlates with efficient photoconduction measured in a sandwich cell configuration with indium tinoxide (ITO) and aluminium electrodes. The lowering of the photocurrent upon inserting a thin blocking silicon monoxide layer between the ITO and the sample in conjunction with the polarity dependence, as well as the dependence on temperature and electric field indicate that the charge carrier delivering process is the efficient dissociation of inter-chain excitations at the ITO electrode rather than bulk photoionization.

The Force Field of Porphycene: A Theoretical and Experimental Approach

The contribution deals with the theoretical and experimental examination of the force field of porphycene ([18]porphyrin-(2.0.2.0))a structural isomer of porphyrin. Special care was taken for a possible tautomerism of the hydrogen atoms in the cavity of the molecule. We report IR, Raman, UV, as well as site-selective fluorescence and fluorescence excitation spectra. Most spectra were measured under matrix isolation conditions. Quantum chemical calculations based on density functional theory methods were used to derive force fields and IR intensities for the most probable tautomeric forms.

The spectral assessment of the origin of the polaronic contribution to charge transport in a highly conjugated triarylamine donor molecule

Abstract Site-selective fluorescence spectra have been measured to investigate the origin of the polaronic contribution to charge transport in 4-[bis(4-ethylphenyl)amino]phenyl-N,N,N′,N′-tetrakis(4-ethylphenyl)-[1,1′:3′1″-terphenyl]-4,4″-diamine doped poly(styrene). The spectra feature an unusually large Stokes shift of σ = 750 cm−1 and remain broad over the entire range of excitation energies. This is a clear indication of a torsional mode of relaxation that can account for the polaronic contribution observed in charge transport experiments.

Inhomogeneous spectral broadening of the B820 subunit form of LH1

Site-selected fluorescence spectra of the B820 subunit of LH1 from Rhodospirillum rubrum G9 were measured at temperatures between 160 K and 4.2 K. A linear correlation between excitation wavelength and maximal emission wavelength was observed across the whole Q y absorption band of the B820 subunits, which persists at higher temperatures. This demonstrates that (a) the Q y absorbance band of the B820 subunit is inhomogeneously broadened; (b) the subunit consists of a strongly interacting dimer of BChl a; (c) no energy transfer occurs among these dimers. The temperature dependence of the shape of the emission spectrum confirms that the protein forms a ‘glass-like’ environment for the bacteriochlorophyll pigments. After reassociating the octyl-glucoside-solubilized B820 subunits into the reassociated B873 form, the emission wavelength is independent of the wavelength of excitation at 4.2 K. This implies that the reassociated B873 form consists of a large number of interacting pigment molecules. The nature and origin of the spectral inhomogeneity in the B820 subunits is discussed.

Resonance fluorescence spectra of dye-doped polymers

Site-selected fluorescence spectra have been measured for Mg-octaethylporphyrin doped in polystyrene and PMMA films at low temperatures by a time-correlated single-photon counting method under CW mode-locked dye-laser excitation into the lowest optical absorption band. A narrow resonance line accompanied by a broad sideband has been observed by rejecting the scattered laser light using a time gate. A single-site fluorescence spectrum has been obtained by the use of saturation effect of the laser-induced fluorescence. The density of phonon states weighted by the electron-phonon coupling coefficient has been determined, for the first time for a dye-polymer system, from the analysis of the obtained spectrum. Temperature dependence of the laser-induced fluorescence spectrum is compared with the simulation based on the weighted phonon-state density.

Measurements of the resonance fluorescence spectrum in dye-doped amorphous systems

Site-selected fluorescence spectra have been measured for dye molecules doped in polymer films at low temperatures under the excitation by a cw mode-locked dye laser. A time-correlated single-photon counting method has been employed for the detection. It has been found possible to obtain the resonance fluorescence spectrum by rejecting the scattered laser light using a time gate much after the termination of the exciting pulse. The distribution of the site energy in the sample has been determined from the excitation spectrum for the resonance fluorescence. The fluorescence. It is concluded that large deviation from the mirror sysmmetry relation is present between the single-site absorption and fluorescence spectra.

Intramolecular hydrogen bonding. X. Comparison of free jet and Shpol'skii matrix electronic spectra of 1,8-dihydroxyanthraquinone

Site-selected fluorescence and excitation spectra in n-octane Shpol'skii matrix, the free jet excitation spectrum, and the gas phase emission spectrum from the vibrationless level in S 1 are reported for 1,8-dihydroxyanthraquinone. The qualitative agreement of the gas phase and matrix spectra rules out any major influence of environment on the excited state proton transfer. However, the higher resolution of the jet excitation spectrum reveals important new and unique information on the excited state vibrational manifold. At about 600 cm −1 above the S 1 origin the excitation spectrum becomes very congested and the free jet rotational band contours also change. Combination bands and overtones of lower frequency fundamentals expected in this region are not found. Similar behavior occurs at 1100 cm −1 excess S 1 energy in 1,5-dihydroxyanthraquinone. We critically assess how reliably the excited state proton transfer potential function can be extracted from the spectra. One quantum of the OH stretching vibration is identified in the jet emission spectrum of 1,8-DHAQ.

Low temperature fluorescence of porphyrins: Chlorin methylester derivatives

Low temperature fluorescence spectra of chlorin-e6-trimethylester, 7-cyano-chlorin-e6-trimethylester, mesoisochlorin-e4-dimethylester, 13-acetyl-mesoisochlorin-e4-dimethylester and 20-chloro-mesoisochlorin-e4-dimethylester in Shpolskii n-octane and non-Shpolskii methanol + + dichloromethane matrices at 5 K are reported. The role of both laser and conventional excitation together with matrix-pigment interaction are discussed. Especially, Shpolskii and site-selection fluorescence spectra were measured and their vibrational analysis is presented yielding frequencies of normal vibrations in the ground and the first excited singlet state.

Site-Selective Shpol'skii Spectrometry of Sulfur-, Oxygen-, and Nitrogen-Containing Aromatic Compounds in Complex Samples

Laser excited Shpol'skii spectrometry (LESS) was utilized to directly determine nitrogen (N-), oxygen (O-), and sulfur (S-) heterocyclic compounds in solvent refined coal (SRC-II), petroleum crude oil, and carbon black. Characteristic quasilinear LESS excitation and emission spectra of the heterocyclic compounds are presented for the first time under site-selective conditions. Deuterated analogs of dibenzothiophene and dibenzofuran were utilized for quantitation. Site-selective fluorescence spectra of aminopyrene derivatives of polycyclic aromatic compounds (PAC) are also presented for the first time. The potential for utilizing the LESS technique in critical environment and biological studies for the direct determination of N-, O-, and S-heterocyclic compounds and substitutional derivatives of parent PAC has been demonstrated.

Site-selective fluorescence and hole-burning spectroscopy of MTHF glasses doped with tetracene or pentacene

Site-selective fluorescence spectra of tetracene (TC) doped in a MTHF glass have been recorded as a function of excitation energy. Based upon the true inhomogeneous absorption profile of the TC S 1 ← S 0 origin determined via vibrational ZP emission intensities the true phonon wing has been evaluated employing an iterative numerical spectral convolution procedure. It is found that both phonon spectra and Debye-Waller factor are virtually constant (α = 0.55 ± 0.08) across the inhomogeneous absorption band as are hole-burning yields in MTHF:TC glasses if probed in the zero-phonon line fluorescence excitation mode. It is concluded that the distributions of both the rate constant for NPHB and the strength of electron-phonon coupling are uncorrelated with the electronic site excitation energies.

Site-selected fluorescence spectra of (n-C4H9)4NUO2(NO3)3 in poly(methyl methacrylate)

The emission spectrum of (n-C4H9)4NUO2(NO3)3 in poly(methyl methacrylate) at ca. 8 K has been measured using three laser lines for excitation. The three emission spectra differ in the energy of the origins and hence in the energy of associated progressions, but not in energy separations. These differences correlate with the energy shift of the exciting laser line from the maximum of the solvent-broadened absorption band. This shows that the site-selection process is operative and that site integrity is maintained over quite large excitation–emission energy differences. The intensity distribution in the emission spectrum also depends on this energy shift, and indicates that sites corresponding to the absorption band maximum interact least with the polymer matrix. Similar results for UO2(NO3)2·6H2O are presented.

Conformation of biphenyl in a cyclohexane host at 10 K

The site-selected fluorescence, phosphorescence, and luminescence excitation spectra of biphenyl in a cyclohexane Shpol'skii matrix have been measured. There are three principal sites. The relative population of the sites depends on the freezing protocol. The S 1 ↔ S 0 0&.sbnd;0 band, a forbidden gg transition in D 2h planar biphenyl, is weakly observed for one of the sites. For this site the zero-phonon lines in fluorescence and phosphorescence are accompanied by prominent sidebands but such sidebands are nearly completely absent in the excitation spectrum. The other two sites have negligibly weak 00 bands in fluorescence and minimal sidebands. We attribute the sidebands to translational and/or librational motion of biphenyl in the cyclohexane cage.