Short-wavelength Band Research Articles

Luminescence characteristics of the Pr3+ ion in SrB4O7 and SrB6O10

Spectral and kinetic characteristics of the luminescence and luminescence excitation spectra of polycrystalline SrB4O7:Pr (1%) and SrB6O10:Pr (1%) samples are studied at 150–170 K. The samples show an intense luminescence band in the vicinity of 405 nm (1 S 0→1 I 6 transitions of Pr3+) and shorter wavelength bands also assigned to transitions from the 1 S 0 level. The main luminescence decay constant is ∼2×10−7 s. The excitation spectra of the 1 S 0 luminescence in these crystals are significantly different. The SrB4O7:Pr crystal shows three well-resolved bands at 6.14, 6.55, and 6.91 eV in the region of the 4f 2→4f 15d transitions and a complex structure in the region of interband transitions (7.1–20 eV), whereas the SrB6O10:Pr crystal shows a weakly structured band at 6.31 eV and no excitation in the region of the interband transitions. The physical mechanisms that may be responsible for the observed features of the spectra are discussed.

The electrodeless discharge lamp: a prospective tool for photochemistry: Part 4. Temperature- and envelope material-dependent emission characteristics

This work extends our previous research on an original photochemical reactor—the electrodeless discharge lamp (EDL) inside a reaction mixture that generates ultraviolet radiation in the microwave (MW) field. This arrangement was found to be a straightforward solution for homogeneous as well as heterogeneous photochemical experiments that need to be carried out at higher temperatures. Here, we report the emission characteristics (250–600 nm) of EDL as a function of temperature, MW output power of the reactor, EDL envelope material, and properties of solvents used in photochemical reactions. Relative intensities of the individual emission peaks were found to be largely dependent on temperature (in the region of 35–174 °C): the short-wavelength bands (particularly the 254 nm peak) were suppressed with increasing temperature. Solvents absorbing MW significantly reduced the EDL emission intensity. It is concluded that the right choice of EDL envelope material and reaction conditions is essential for an efficient course of a photochemical process in this experimental arrangement.

A new ligand for an old lipocalin: induced circular dichroism spectra reveal binding of bilirubin to bovine β-lactoglobulin

This study reports that bilirubin–bovine β-lactoglobulin (BLG) complexes exhibit very characteristic induced circular dichroism (CD) spectra in the visible absorption region. Due to intramolecular chiral exciton coupling between the dipyrrinone chromophores, the long-wavelength negative and short-wavelength positive CD bands clearly prove that a single bilirubin molecule binds to BLG in a left-handed helical conformation (in pH 7.4 phosphate buffer Δ ϵ min is −54 M −1 cm −1 at 467 nm and Δ ϵ max is +48.5 M −1 cm −1 at 412 nm). The very low aqueous solubility and strong tendency of bilirubin molecules to aggregate around pH 7.4 meant that much more intense CD bands were measured at alkaline pH values owing to the increasing solubility of the ligand. Vanishing CD activity obtained upon titration of the complex with palmitic acid known to bind in the hydrophobic cavity of BLG indicates bilirubin to be bound at the open end mouth of the β-barrel. Reversible changes of the induced CD spectrum due to acidic pH shift of the sample solution lead to the same conclusion.

The Red-Edge Effects in Laurdan Solutions

The luminescence spectrum of laurdan in glycerol consists of two bands with maxima at 425 and 500 nm. The strong dependence of this spectrum on the energy of the excitation quanta is demonstrated. When, excited on the red-edge slope of the absorption band, it is investigated at various temperatures. The biggest red shift (about 10 nm) of λmax is found for the long wavelength band at low temperature, where the solvent forms a rigid matrix. A small shift, about 3 - 4 nm, and a change of the luminescence band shape occurs on increasing the temperature to 370 K. Simultaneously, intensity changes occur in the excitation spectra when the maximum intensity of the two emission bands is recorded. From the received data, the differential excitation spectrum has been determined. The additional absorption contributes to the luminescence of the short wavelength band. The deconvolution of the luminescence decay data obtained separately for the luminescence bands results in common components in the sub- and nanosecond time range with different participation. The experimental data testify the presence of an additional mechanism, responsible for the heterogeneity of the solution, which differs from the well-known solvent molecule reorientation. Torsion oscillation of the N(CH3)2 functional group in the laurdan molecule is necessary to take into account as well as the statistical properties of the laurdan solvate shells. The observed luminescence properties are explained taking into account the simultaneous existence of local-excited and charge transfer states in the solute molecules.

Steady-State and Time-Resolved Spectroscopic Characteristics of Novel Silicon-Containing Organopolymers

The spectroscopic characteristics of novel π-conjugated polymers containing four-coordinated silicon, acetylene groups and either 1,4-biphenylene or 2,7-fluorene in the main chain were investigated by steady-state and picosecond laser spectroscopy. The spectral features of absorption, fluorescence excitation spectra, fluorescence lifetime, and fluorescence polarization were explained by the existence of two kinds of inhomogeneously broadened electronic states formed in the disordered polymeric chain. The dynamics of photoinduced absorption was measured in the 400–900 nm spectral range with picosecond time resolution. The long-wavelength band with λmax ∼ 710 nm was ascribed to excited-state absorption from higher-lying electronic states created in short polymeric segments with essential conformational distortion of the subunits. The short-wavelength band with λmax ∼ 580 nm and a shoulder at 500 nm was interpreted as photoinduced absorption from a lower-lying state arisen in more planar, longer π-conjugated segments populated via direct excitation and energy migration between disordered segments of the polymeric chain. For the fluorene-containing polymer, the smaller Stokes shift and the greater degree of fluorescence polarization are consistent with more extensive electron delocalization along the backbone.

Enhanced substrate-induced coupling in two-dimensional gold nanoparticle arrays

The influence of substrate induced coupling on plasmon excitations is probed by means of visible and near-infrared extinction microspectroscopy on various arrays of gold oblate spheroidal particles deposited onto a 20-nm-thick gold film. At zero incidence angle and for an interparticle spacing smaller than 250 nm, the spectra exhibit two bands instead of the single one observed for similar particles but deposited on indium tin oxide coated glass. We suggest that the short-wavelength band proceeds from two simultaneous mechanisms: (i) Excitation of a surface-plasmon resonance localized on the particles. (ii) Generation at the gold-glass interface of a propagating surface-plasmon wave due to the fact that the gold particles can act as a grating coupler. Surface-enhanced Raman-scattering experiments performed on oblate spheroid arrays give arguments in favor of the attribution of the long-wavelength band to surface-plasmon resonance of an ensemble of strongly coupled particles. When increasing the spacing between particles of the array beyond 250 nm, extinction spectra display the emergence of a new band. Calculations suggest that this new band proceeds from the excitation of a surface-plasmon standing wave on the film due to Bragg scattering. This assignment to Bragg scattering is supported by the investigation of the effect of varying the incidence angle under both transverse magnetic and transverse electric polarizations.

On Measuring the Infrared Luminosity of Distant Galaxies with the [ITAL]Space Infrared Telescope Facility[/ITAL

The Space Infrared Telescope Facility (SIRTF) will revolutionize the study of dust-obscured star formation in distant galaxies. Although deep images from the Multiband Imaging Photometer for SIRTF (MIPS) will provide coverage at 24, 70, and 160 μm, the bulk of MIPS-detected objects may only have accurate photometry in the shorter wavelength bands because of the confusion noise. Therefore, we have explored the potential for constraining the total infrared (IR) fluxes of distant galaxies solely with the 24 μm flux density and for the combination of 24 and 70 μm data. We also discuss the inherent systematic uncertainties in making these transitions. Under the assumption that distant star-forming galaxies have IR spectral energy distributions (SEDs) that are represented somewhere in the local universe, the 24 μm data (plus optical and X-ray data to allow the redshift estimation and AGN rejection) constrain the total IR luminosity to within a factor of 2.5 for galaxies with 0.4 z 1.6. Incorporating the 70 μm data substantially improves this constraint by a factor 6. Lastly, we argue that if the shape of the IR SED is known (or well constrained, e.g., because of a high IR luminosity or a low ultraviolet/IR flux ratio), then the IR luminosity can be estimated with more certainty.

Femtosecond electron ejection in liquid acetonitrile: Evidence for cavity electrons and solvent anions

Excess electrons were studied in liquid acetonitrile at room temperature by femtosecond pump–probe spectroscopy. Using ≈200 fs, 265 nm laser pulses, electrons were ejected into the liquid by photodetachment from iodide ions and, in separate experiments, by photoionization of indole. A strong and broad absorption band with a maximum near 1400 nm was observed in both systems. A second absorption band was observed at wavelengths below 620 nm for iodide solutions, but was not seen in photoexcited indole due to overlapping excited state absorption. The bands are in good agreement with ones seen previously in nanosecond pulse radiolysis experiments [I. P. Bell, M. A. J. Rodgers, and H. D. Burrows, J. Chem. Soc., Faraday Trans. 1 73, 315 (1977)]. Bell, Rodgers, and Burrows assigned the visible and IR bands to absorption by acetonitrile dimer and monomer anions, respectively. Our results strongly question this interpretation. Instead, we assign the short-wavelength absorption band to a solvent-bound valence anion consisting of one or two acetonitrile molecules and the IR band to a solvated or cavity electron. Low-level quantum chemical calculations indicate that valence anion formation is strongly correlated with CCN bending, but do not provide a clear indication of whether a monomer or dimer valence anion is favored. The highly mobile cavity electron is scavenged by added chloroform at a bimolecular reaction rate of (1.02±0.03)×1011 M−1 s−1. The appearance of both absorption bands within our time resolution suggests that the two forms of the excess electron are produced by prompt reaction with the iodide charge-transfer-to-solvent (CTTS) excited state. In support of this mechanism, strong static scavenging by chloroform was observed at both visible and IR wavelengths. For iodide in acetonitrile, the signal in the IR decays biexponentially due to competition between geminate recombination of the cavity electron with the parent iodine atom and its reaction with the solvent. Geminate recombination between the solvated electron and the parent iodine atom occurs with a characteristic time constant of ≈30 ps, while additional solvent anions are formed in a slow reaction with a time constant of ≈260 ps. Approximately 30% of the solvated electrons photodetached from iodide undergo geminate recombination. There is no evidence for geminate reaction between the promptly formed solvent anion and iodine, suggesting that these species are formed at larger initial separation than the IR-absorbing cavity electron/iodine atom pair. In indole, geminate recombination occurs on a slower time scale of ≈135 ps.

Ultrasensitive fluorescent probe for the hydrophobic range of solvent polarities

The new 3-hydroxychromone derivative 2-(6-diethylaminobenzo[b]furan-2-yl)-3-hydroxychromone (FA) displays a dramatic solvent-dependent transformation of fluorescence spectra in the range of low-polarity solvents. The two well-separated emission bands change their relative intensities so that the short-wavelength band being of a very low intensity in hexane becomes dominant in the more polar ethyl acetate and trichloromethane. We suggest the participation in this effect of excited-state intramolecular proton transfer, which is characteristic for other 3-hydroxychromone and 3-hydroxyflavone derivatives, in the range of solvents of much higher polarities. Because of these unique properties, a number of spectroscopic parameters (positions of absorption and two fluorescence maxima, the ratio of their intensities and the fluorescence quantum yield) can be measured in this solvent range with multiparametric analysis of the data. In terms of solvent polarity, the shifts in both emission bands and their intensity ratio demonstrate a good correlation with empirical polarity scales E T N, P y and SPP, while the absorption spectra reveal some deviations for the tested oxygen-containing solvent molecules. A good cross-correlation is observed between fluorescence spectral shifts and the ratio of band intensities. The latter provides the means for a dramatic amplification of solvent response. Thus, a new approach for ultrasensitive scaling and probing the solvent polarity in the low-polararity range can be suggested. It involves very simple ratiometric measurements at two emission bands and can be posed for a variety of applications. We present examples of these applications for distinguishing of polarities between methylated benzene derivatives, for quantitative assay of polar impurities in low-polar solvents and for detection of the changes of solvent polarity as a function of temperature.

Mono-aromatic ring-fused versus adjacently di-aromatic ring-fused tetraazaporphyrins: regioselective synthesis and their spectroscopic and electrochemical properties.

A method which preferentially produces adjacently di-aromatic ring-substituted tetraazaporphyrins (TAPs) has been developed, and their electrochemical and spectroscopic properties have been studied and compared with those of the corresponding series of mono-aromatic ring-fused TAPs. Mono-aromatic ring-fused TAPs show a split Q-band, and the splitting energy increases with increasing size of the aromatic ring. In addition, for the split Q-bands, the relative intensity of the band at longer wavelength decreases with increasing molecular size of the fused aromatics, compared with the shorter wavelength band. In the di-aromatic ring-fused TAPs, this kind of splitting is not seen, and only a shift of the band is observed. The intensity and band position of the split or unsplit Q-bands are quantitatively evaluated by simultaneous band deconvolution analysis, using both electronic absorption and magnetic circular dichroism spectra. The preparation of these TAP compounds has made it possible to adjust the Q-band position in a stepwise manner between ca. 600 and 750 nm. The first reduction and oxidation potentials of the TAP ring shift negatively with increasing number and size of the fused aromatics. The extent of the shift is found to be very small for the LUMOs but significant for the HOMOs. These spectroscopic and electrochemical properties are almost perfectly reproduced by molecular orbital calculations within the framework of the Pariser-Parr-Pople approximation. In particular, a small variation of the LUMO level and large destabilization of the HOMO level on ring expansion are rationalized from the extent of stretch of molecular orbitals: i.e., since the LUMOs are localized in the central TAP moiety irrespective of the molecular size, while the HOMOs have appreciable coefficients even over the fused aromatics, the HOMO level destabilizes while the LUMO level remains constant with increasing molecular size. In one CoTAP derivative, Co(III/II) and the first ligand oxidation couples occur experimentally at the same potential.

Multi-component emission spectra in toluene–diamine systems

The fluorescence spectra of intermolecular and intramolecular toluene–amine and toluene–diamine systems were measured in THF. The fluorescence spectrum of toluene–N,N,N′,N′-tetramethyl-1,4-butanediamine (TMBD) mixtures was decomposed into four component bands with the shortest-wavelength band (band A) assigned to the excited toluene. Of the remaining bands, the short-wavelength (band B), the middle-wavelength (band C) and the long-wavelength band (band D) were assigned to a locally excited amine chromophore, an exciplex between toluene and an amino group, and an intramolecular excimer between two amino groups of the diamine, respectively. The fluorescence spectrum of toluene–TMBD mixtures in THF obtained upon nanosecond laser excitation showed a slight red shift with the time indicating that the contribution of band D increased with time. A reaction scheme and energy level diagram were proposed for this system. In toluene–dimethylethylamine mixtures bands A–C were observed. While for 4-phenyl-1-N,N-dimethylaminobutane bands A–C were also observed, for 3-phenyl-1-N,N-dimethylaminopropane only band C was observed. In toluene–N,N,N′,N′-tetramethyl-1,3-propanediamine mixtures, only bands A and D were observed, and for N,N,N′-trimethyl-N′-3-phenylpropyl-1,3-propanediamine bands C and D were observed. Emissions from intramolecular exciplexes and excimers in which two chromophores are separated by three methylene groups were predominantly observed.

Mercury Inhibits the Activity of the NADPH:Protochlorophyllide Oxidoreductase (POR)

The effect of Hg++ was studied on the arrangement and photoactivity of NADPH:protochlorophyllide oxidoreductase (POR) in homogenates of dark-grown wheat (Triticum aestivum L.) leaves. 77 K fluorescence emission spectra of the homogenates were recorded before and after the irradiation of the homogenates and the spectra were deconvoluted into Gaussian components. The mercury treatment caused a precipitation of the membrane particles, which was followed by a remarkable decrease of the fluorescence yield. 10-3 M Hg++ decreased the ratio of the 655 nm-emitting protochlorophyllide (Pchlide) form to the 633 nm-emitting form. 10-2 M Hg++ shifted the short wavelength band to 629-630 nm and a 655 nm form was observed which was inactive on irradiation. This inhibition may be caused by serious alteration of the enzyme structure resulting in the trans-localisation of NADPH within the active site of POR.

Efficient near-infrared polymer nanocrystal light-emitting diodes.

Conjugated polymers and indium arsenide-based nanocrystals were used to create near-infrared plastic light-emitting diodes. Emission was tunable from 1 to 1.3 micrometers--a range that effectively covers the short-wavelength telecommunications band--by means of the quantum confinement effects in the nanocrystals. The external efficiency value (photons out divided by electrons in) is approximately 0.5% (that is, >1% internal) and is mainly limited by device architecture. The near-infrared emission did not overlap the charge-induced absorption bands of the polymer.

Relaxation Dynamics in the Excited States of LDS-821 in Solution

Relaxation dynamics in the excited electronic state (S1) of LDS-821 have been studied in aprotic and alcoholic solvents with pico- and subpicosecond time resolution using pump−supercontinuum probe transient absorption technique. Steady-state absorption and fluorescence as well as the time-resolved transient absorption/stimulated emission spectra and the temporal dynamics monitored at different wavelengths mainly reveal the features of a two-mode kinetic process in the S1 stateconversion of the locally excited (LE) state to the twisted intramolecular charge-transfer (TICT) state. A short (<1 ps) rise time of stimulated emission monitored at the shorter wavelength band, is the signature of the barrierless skeletal stretching motion along the “valley-like” region of the potential energy surface (PES) of the S1 state in the Franck−Condon (FC) region to attain a metastable untwisted configuration, called the LE state. The decay time of the LE state is equal to the rise time measured at the longer wavelength ba...

Infrared [ITAL]L[/ITAL]-Band Observations of the Trapezium Cluster: A Census of Circumstellar Disks and Candidate Protostars

We report the results of a sensitive near-infrared JHKL imaging survey of the Trapezium cluster in Orion. We use the JHKL colors to obtain a census of infrared excess stars in the cluster. Of (391) stars brighter than 12th magnitude in the K and L bands, 80% ± 7% are found to exhibit detectable infrared excess on the J-H, K-L color-color diagram. Examination of a subsample of 285 of these stars with published spectral types yields a slightly higher infrared excess fraction of 85%. We find that 97% of the optical proplyds in the cluster exhibit excess in the JHKL color-color diagram indicating that the most likely origin of the observed infrared excesses is from circumstellar disks. We interpret these results to indicate that the fraction of stars in the cluster with circumstellar disks is between 80%–85%, confirming earlier published suggestions of a high disk fraction for this young cluster. Moreover, we find that the probability of finding an infrared excess around a star is independent of stellar mass over essentially the entire range of the stellar mass function down to the hydrogen burning limit. Consequently, the vast majority of stars in the Trapezium cluster appear to have been born with circumstellar disks and the potential to subsequently form planetary systems, despite formation within the environment of a rich and dense stellar cluster. We identify 78 stars in our sample characterized by K-L colors suggestive of deeply embedded objects. The spatial distribution of these objects differs from that of the rest of the cluster members and is similar to that of the dense molecular cloud ridge behind the cluster. About half of these objects are detected in the short wavelength (J and H) bands, and these are found to be characterized by extreme infrared excess. This suggests that many of these sources could be protostellar in nature. If even a modest fraction (i.e., ~50%) of these objects are protostars, then star formation could be continuing in the molecular ridge at a rate comparable to that which produced the foreground Trapezium cluster.

Emission spectrum of concentrated dye solutions excited by high-power laser radiation

Emission spectra of concentrated (~1018–1019 cm–3) ethanol solutions of Rhodamine C excited by high-power (~1025–1026 cm–2s–1) laser radiation were measured. The emission spectrum consists of two narrow bands (with half-widths of ~200–300 cm–1). The long-wavelength band was interpreted as cooperative spontaneous emission, and the short-wavelength band was assigned to the amplified spontaneous emission. The ratio of their intensities depends on the dye concentration and the pump power density. The ratio of band intensities is assumed to be determined by the dephasing rate of excited molecules responsible for cooperative spontaneous emission.

Electromodulation of fluorescence in hole-transporting materials (TPD, TAPC) for organic light-emitting diodes

Electric-field-modulated fluorescence (EMF) was measured in films of N, N ′-diphenyl- N, N ′-bis(3-methylphenyl)-1,1 ′-biphenyl-4,4 ′-diamine (TPD) and 1,1 ′-bis(di-4-tolylaminophenyl) cyclohexane (TAPC), the diamine compounds commonly used as hole-transporting materials in organic light-emitting diodes. External electric fields of 2×10 6 V/cm reduce the integral emission intensity from about 0.3% down to 10% dependent on the excitation photon energy and spectral emission range. The analysis of the experimental data provides evidence that the EMF in TPD is due to the field imposed changes of the dissociation rate of localized molecular excited states. The observed difference in the EMF for the short-wavelength (monomolecular) and long-wavelength (excimer) emission bands of TAPC can be ascribed to the different field response of these two types of emitting species. All these results are consistent with a kinetic scheme that includes formation of molecular excited states, their localization and relaxation into charge pairs within extended trapping domains, and formation of excimer states at a field-dependent rate mediated by the field-assisted dissociation of the charge pairs. The 3D-Onsager theory of dissociation satisfactorily explains the electric field and excitation wavelength dependence of EMF in TPD and that for the excimer emission of TAPC. The quadratic Stark effect on fluorescence quantum yield must have been invoked to understand the EMF characteristics of monomolecular emission of TAPC.

Bathymetry sediment thickness and crustal structure from satellite gravity data

Analysis of satellite gravity data offers an opportunity to rapidly evaluate the sedimentary structure of frontier basins. The accurate and evenly distributed measurements of the gravity field determined from satellite orbits contain information about the bathymetry, sediment thickness and crustal structure of the world’s oceans. These data have been used by various authors, e.g., Smith and Sandwell (1994), to predict bathymetry for ocean basins where sediment cover is thin and where crustal structure is well known. In these areas, over a short wavelength band (15-160 km), the gravity field is highly correlated with the bathymetry. Along continental margins, however, the gravity field is complicated by sedimentary basins and changes in crustal structure.The method presented here differs from previous techniques for analysis of satellite gravity data by inverting simultaneously for water, sediment and crustal thickness instead of decomposing spectral bands in the gravity field. The inversion is constrained by interpretations of seismic and other data, and by assumptions about the nature and spatial variability of the interfaces. The seabed, basement and crust/mantle boundaries are defined by a series of triangular facets, whose size varies as the amount of constraining data changes. The sediment/basement boundary and the base of the crust are defined by larger facets than those defining the bathymetry. Conditioning equations smooth the interfaces, make the depths tend towards the input values and ensure that the interfaces do not intersect. In areas of sparse bathymetric and crustal structural control a degree of ambiguity between longwavelength shallow structures and short-wavelength deeper features inevitably remains.This technique has been used to identify the presence and location of several sedimentary basins in an area 1250 by 1000 km west of New Zealand.

Tunable two-dimensional photonic crystals using liquid crystal infiltration

The photonic band gap of a two-dimensional photonic crystal is continuously tuned using the temperature dependent refractive index of a liquid crystal. Liquid crystal $E7$ was infiltrated into the air pores of a macroporous silicon photonic crystal with a triangular lattice pitch of 1.58 $\ensuremath{\mu}$m and a band gap wavelength range of 3.3--5.7 \ensuremath{\mu}m. After infiltration, the band gap for the H polarized field shifted dramatically to 4.4--6.0 \ensuremath{\mu}m while that of the E-polarized field collapsed. As the sample was heated to the nematic-isotropic phase transition temperature of the liquid crystal $(59\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}),$ the short-wavelength band edge of the H gap shifted by as much as 70 nm while the long-wavelength edge was constant within experimental error. Band structure calculations incorporating the temperature dependence of the liquid crystal birefringence can account for our results and also point to an escaped-radial alignment of the liquid crystal in the nematic phase.

Effects of chromophore interaction in photophysics and photochemistry of cyanine dyes

Spectral and fluorescent properties of ketocyanine dyes (polyenic bis‐ω, ω′‐aminoketones) and cation‐anionic polymethine dyes of various structures were studied. The symmetric ketocyanines were shown to have a long‐wavelength absorption band bathochromically shifted in comparison with that of the asymmetric ketocyanines with the same total length of the polyenic chain. The nonlinear ketocyanines exhibit the additional short‐wavelength band in their absorption spectra, which can be more intense than the longwavelength band. The absorption spectra of ion pairs of cation‐anionic dyes with overlapping cation and anion bands contain a new intense short‐wavelength band inactive in fluorescence excitation. These spectral peculiarities are explained on the basis of chromophore interaction model. It has also been shown that the T1 levels of ketocyanine chromophores do not essentially interact with each other in a ketocyanine molecule in nonpolar solvents; in polar solvents this interaction becomes appreciable due to lowering the potential barrier for conjugation.