Showing papers on "Excimer published in 2003"

Microhollow cathode discharges

Abstract: By reducing the dimensions of hollow cathodes into the hundred micrometer range, stable, direct current, high (atmospheric) pressure glow discharges in rare gases, rare gas–halide mixtures and in air could be generated. The electron energy distribution in these microdischarges is non-Maxwellian, with a pronounced high-energy tail. The high electron energy together with the high gas density, which favors three-body collisions, is the reason for an efficient excimer generation in these microplasmas. Excimer efficiencies from 1% to 9% have been measured for argon, xenon, argon fluoride, and xenon chloride direct current excimer emitters, with a radiant excimer emittance of up to 2 W/cm2 for xenon. Adding small amounts of oxygen to argon has allowed us to generate vacuum ultraviolet line radiation at 130.5 nm with an efficiency approaching 1%. Pulsing xenon discharges with nanosecond electrical pulses has led to an increase in intensity to 15 W/cm2 and to a simultaneous increase in efficiency to more than 20%. Operating the discharges in an abnormal glow mode has allowed us to generate microdischarge arrays without individual ballast. Applications of these plasma arrays are excimer lamps and plasma reactors.

Excimer emission from microhollow cathode argon discharges

Abstract: Microhollow cathode discharges (MHCDs) operated in rare gases are sources of intense excimer emission. Of particular interest is argon, because of its relatively low cost and the short wavelength (128 nm) of its excimer emission. The measured internal efficiency, obtained in static argon at atmospheric pressure, was found to be on the order of 1%. Flowing argon through a direct current (DC) MHCD at atmospheric pressure caused the argon excimer internal efficiency to increase to 6%, indicating that the low efficiency in static argon is mainly due to impurities. Applying 10 ns pulses to the DC plasma resulted in an increase in excimer power from 30 mW DC to 180 mW peak power, at an efficiency of 5–6%. The increase in excimer power correlates with an increase in the electron density. For DC operation, electron densities of 1015 cm−3 were measured in atmospheric pressure argon micro-plasmas, which increased to values beyond 1016 cm−3 for nanosecond pulsed operation. This increase in electron density and excimer power is due to pulsed electron heating, an effect that has allowed us to raise the mean electron energy from 1 eV, for DC operation, to 2.25 eV in the pulsed mode.

Formation of triplet excimers and dimers in amorphous organic thin films and light emitting devices

Abstract: We examine triplet excimers in neat films of the phosphorescent molecule: platinum (II) (2-(4′,6′-difluorophenyl)pyridinato-N,C2) acetyl acetate (FPt1). This molecule was chosen because of its square planar structure, allowing it to facially pack in a crystal with an intermolecular separation of only 3.4±0.1 A, thereby facilitating excimer formation between adjacent molecules. The transient and steady-state photoluminescent and electroluminescent properties of FPt1 are used to understand the formation of triplet excimers ( 3 E 0 * ) and dimers ( 3 D * ) in amorphous films grown by thermal evaporation. We find that 3 E 0 * and 3 D * have peak emission intensities at 2.07 and 1.83 eV, and have lifetimes of (1.6±0.3) μs and (800±80) ns at room temperature, respectively. When films are optically pumped, triplet excitons form on monomers and then diffuse via thermally activated hopping to nearby excimer sites with an activation energy of (37±4) meV. We find that the energy transfer efficiency from the monomer to the excimer state approaches 100% at room temperature, and that the photoluminescence efficiency of the excimer is three times less than that of the monomer. Further, excitation of triplet aggregate states, or metal–metal dimers via the association of cations and anions of electrically pumped FPt1 is achieved in an organic light-emitting device, which achieves a maximum internal quantum efficiency of ∼14%. Electrical pumping also reveals dimer states unresolved in photoluminescent studies. The high efficiency of the triplet excimers of FPt1 is significant since dimeric species, especially triplets, are often associated with concentration quenching and low quantum yields.

Electrogenerated Chemiluminescence. 76. Excited Singlet State Emission vs Excimer Emission in Ter(9,9-diarylfluorene)s

Abstract: The electrochemistry and electrogenerated chemiluminescence (ECL) of two ter(9,9-diarylfluorene)s, TDAFs, were investigated. The ECL from these new blue emitters produced by ion annihilation showed spectra with significant emission at longer wavelengths and was different from their fluorescence spectra. This was ascribed to the formation of excimers. Excimer emission was not observed in ECL during the reduction of the TDAFs with benzoyl peroxide (BPO) as a coreactant, where the ECL spectra were essentially the same as their fluorescence spectra, consistent with the requirement of ion annihilation for excimer production. Excimer emission is not seen in fluorescence, even at relatively high concentrations, because of the short lifetime of 1 TDAF*. The quantum yields, ECL, for the ECL by ion annihilation were 0.05% for TDAF-1 and 0.01% for TDAF-2. ECL with BPO as a coreactant produced 8% (TDAF-1) or 3% (TDAF-2) of the ECL intensity of Ru(bpy)3 2+ generated under the same conditions (0.5 mM TDAF or Ru(bpy)3 2+ /5 mM BPO).

Time-resolved and steady-state fluorescence studies of hydrophobically modified water-soluble polymers

Abstract: A comprehensive study of the photophysical behavior of poly(acrylic acids) randomly labeled with pyrene using steady-state and time-resolved fluorescence spectroscopy is presented. The influence of external factors, such as different solvents and pH in the aqueous solution, on the polymer photophysics has been investigated. These factors induce major changes in the polymer conformation, which are reflected in the fluorescence experiments. The random introduction of the hydrophobic pyrene groups along the macromolecule favors the coexistence of static (preformed dimers) and dynamic (excimers) monomer quenching phenomena together with a fraction of isolated monomers (not able to form excimer), as revealed by time-resolved and steady-state fluorescence data. In dioxane and methanol solvents, the global analysis of the fluorescence time profiles shows a rise time of similar to21-32 ns followed by a decay of similar to90-138 ns with an additional long r decay component with a low preexponential factor. This is consistent with the fact that in organic solvents such as methanol and dioxane, which are considered to be good solvents, the fraction of preassociated and isolated chromophores is highly reduced and excimer formation is essentially due to a dynamic mechanism. A kinetic scheme involving two types of monomers (M-A and M-B) and one excimer (E) is proposed. From the fluorescence decays it was possible to extract quantitatively the percentage of ground-state preformed dimers along with the percentage of isolated chromophores at room temperature. In addition, it is shown that the fraction of associated ground-state chromophores that can be excited is always larger than that of the isolated chromophores. The rate constants for excimer formation (k(a)), dissociation (k(d)), and deactivation (k(E)) were determined considering the absence and presence of preformed dimers. Additional photophysical and spectroscopic data consisting of wavelength shifts, peak- o-valley ratios and differences (obtained both from absorption and from excitation spectra collected at the monomer and excimer emission region), and the vibronic I-1/I-3 ratio in the pyrene monomer emission were found to be pH-dependent for the polymers in aqueous solution. Hence, by combining the results from steady-state fluorescence measurements with time-resolved fluorescence data, information is provided on how the chain conformation of the labeled PAA polymers changes depending on the solvent (water at different pH values or organic solvents). In water, the conformation changes from compact at low pH to an open polymer coil at high pH, whereas the polymers are in an extended state when in dioxane and methanol. (Less)

High-pressure effects on fluorescence of anthracene crystals

Abstract: Pressure-induced changes on the fluorescence spectra of anthracene crystals were examined up to 5 GPa. Changes in the spectra are demonstrated to be sensitive both to the sample morphology and to the compression conditions (e.g., hydrostaticity). Under hydrostatic pressure, there was no excimer fluorescence from anthracene single crystals. This result is in sharp contrast to previously published data. Because the normal fluorescence was not obscured by the excimer emission, pressure-induced energy shifts for vibronic bands could be determined. These shifts cannot be described adequately by available theoretical descriptions. Strong excimer fluorescence from anthracene crystals was observed only under nonhydrostatic conditions. This fluorescence is attributed to the emission from structural defects formed at the onset of plastic deformation in the crystals. The fluorescence spectra of polycrystalline samples exhibit a new weak band, identified as defect-related excimer emission. The intensity of this emiss...

Intercalating nucleic acids with pyrene nucleotide analogues as next-nearest neighbors for excimer fluorescence detection of single-point mutations under nonstringent hybridization conditions

Abstract: Fluorescence and hybridization specificity is reported for intercalating nucleic acids (INAs), which are here oligodeoxynucleotides (ODNs) synthesized with insertions using (S)-1-[bis(4-methoxyphenyl)(phenyl)methoxy]-3-[(pyren-1-yl)methoxy]propan-2-ol phosphoramidites. It is shown that an INA with two insertions placed as next-nearest neighbors can be used for discrimination between nucleic acids and their single-point mutants. Quenching of an excimer band at 480 nm is observed upon hybridizing to a complementary sequence, whereas the excimer band is present when the nucleobase pair between the two pyrene moieties is mismatched. It is the first example of a solution based on fluorescence detection of single-point mutants that uses excimer formation and does not rely on stringent hybridization conditions. Furthermore, it is shown that INAs with pyrene insertions retain their sequence specificity in thermal melting.

A theoretical study of the ground and first excited singlet state proton transfer reaction in isolated 7-azaindole–water complexes

Abstract: A systematic study of the proton transfer in the 7-azaindole–water clusters ( 7 -AI ( H 2 O ) n ; n=1–4 ) in both the ground and first excited singlet electronic states is undertaken. DFT(B3LYP) calculations for the ground electronic state shows that the more stable geometry of the initial normal tautomer presents a cyclic set of hydrogen bonds that links the two nitrogen atoms of the base across the waters. For the n=4 cluster the water molecules adopt a double ring structure so that two cycles of hydrogen bonds are found there. From this structure full tautomerization implies only one transition state so that a concerted but non-synchronous process is predicted by our theoretical calculations. This behavior is found both in the ground and the excited states where CIS geometry optimizations and TD(B3LYP) energy calculations are performed. The difference between both states is the height of the energy barrier that is much lower in the excited state. Another clear difference between both electronic states is that full tautomerization is an endergonic process in the ground state whereas it is clearly exergonic (then favorable) in the excited state. This is so because electronic excitation implies a charge transfer from the five-member cycle to the six-member one of 7-azaindole so that the proton transfer from the pyrrolic side to the pyridinic one is favored. These results clearly indicate that full tautomerization will not likely occur in the ground state but it will be quite easy (and fast) in the excited state. Reaction is already feasible in the S 1 1:1 complex but it is faster in the 1:2 complex. However the reaction slows again for the 1:3 complex and, finally, reaches a new maximum for the largest cluster studied here, the n=4 case. These results, which are in agreement with experimental data, are explained in terms of the number of hydrogen bonds that are involved in the transfer. The proton transfer through a ring formed by the substrate and two water molecules is found to be the more efficient one, at least in this system.

Argon excimer emission from high-pressure microdischarges in metal capillaries

Abstract: We report on argon excimer emission from high-pressure microdischarges formed inside metal capillaries with or without gas flow. Excimer emission intensity from a single tube increases linearly with gas pressure between 400 and 1000 Torr. Higher discharge current also results in initial intensity gains until gas heating causes saturation or intensity drop. Argon flow through the discharge intensifies emission perhaps by gas cooling. Emission intensity was found to be additive in prealigned dual microdischarges, suggesting that an array of microdischarges could produce a high-intensity excimer source.

Crystal Structure and Excimer Fluorescence of Some Benzoylacetonatoboron Difluorides: Stacking Factor

Abstract: The comparative study of the crystal structures of benzoylacetonato- (1), p-toluylacetonato- (2) and p-ethylbenzoylacetonatoboron (3) difluoride was carried out. Correlation of the luminescence properties of the complexes 1–3 and their crystal structures was revealed. The excimer formation in these compounds occurred the most efficiently for the complexes 1 characterized by the formation of stacks of molecules, unlike the complex 3, where individual dimers behaved as excimer-forming centers. The maximal fluorescence intensity was observed for the complex 2, which had the highest structural ordering.

Solute exchange in Synperonic surfactant micelles

Abstract: We describe experiments that examine the rates of solute exchange in micelles formed from the surfactants Synperonic A7 and Synperonic A50. These surfactants are linear alkane (mixed C13 and C15) ethoxylates with mean degrees of ethoxylation of 6.5 for A7 and 53.7 for A50. The solute is gycerol-1,2-distearate-3-pyrenebutyrate, 1. Solutions containing on average 0.4 molecules of 1 per micelle show both excimer and monomer fluorescence. Excimer emission originates from micelles containing at least two pyrene chromophores. In A50, 1 forms aggregates that do not exchange on a time scale of weeks. When solutions of A7 are treated with an excess of empty micelles, the excimer fluorescence decays with a pseudo-first-order rate that depends on the concentration of empty micelles. Because 1 is so insoluble in water, its exchange involves either fusion of two micelles to form a short-lived supermicelle, or fragmentation of a micelle into submicelles, which then grow back to form normal micelles. The corresponding r...

π-π and σ-π Interactions in α, ω-di-(9-anthryl) and di-(1-naphthyl) oligosilanes studied by time-resolved fluorescence in solution

Abstract: The photophysical properties of two series of oligosilanes, (1-naphthyl)-(SiMe2)n-(1-naphthyl) (1−5) and (9-anthryl)-(SiMe2)n-(9-anthryl) (6−10) with n = 1−4 and 6, were investigated. In these compounds, two types of interactions, a π−π interaction between two aromatic groups and a σ−π interaction between aromatics and a silicon chain unit, were observed. Intramolecular excimer emission was observed in cyclohexane when n ≥ 2. The strongest excimer emission of 2 and 7 is different from the Hirayama rule (n = 3) proposed for carbon analogues and also shows that intramolecular cycloaddition is minor. The time-resolved fluorescence spectra of an anthryl series revealed that the time constant of excimer formation varied depending on the chain length (82−152 ps). In the cases of 4, 5, and 10, charge-transfer (CT) emission was observed in acetonitrile or THF. The time constant of the CT-state formation for 10 was relatively slow (45 ps), which may indicate a conformational change. Monomer emission from the local...

Optical oxygen sensor based on controlling the excimer formation of pyrene-1-butylic acid chemisorption layer onto nano-porous anodic oxidized aluminium plate by myristic acid

Abstract: Controlling the excimer formation of pyrene-1-butylic acid (PBA) chemisorption layer onto nano-porous anodic oxidized aluminium plate by myristic acid layer was attempted and the optical oxygen-sensitivity using PBA and myristic acid co-chemisorption layer was controlled by varying the molar ratio of myristic acid to PBA. When the [myristic acid]/[PBA] value increased, the fluorescence intensities at 376 and 396 nm attributed to monomer emission increased and the intensity at 474 nm attributed to excimer emission decreased, indicating that the excimer formation between PBA molecules onto nano-porous anodic oxidized aluminium plate was suppressed by the myristic acid layer. The oxygen-sensitivity is changed by controlling the excimer formation of PBA by co-chemisorption of myristic acid.

Energetics and Dynamics of Naphthalene Polyaminic Derivatives. Influence of Structural Design in the Balance Static vs Dynamic Excimer Formation

Abstract: Two new fluorescent macrocyclic structures bearing two naphthalene (Np) units at both ends of a cyclic polyaminic chain were investigated with potentiometric, fluorescence (steady-state and time-resolved) and laser flash photolysis techniques. The fluorescence emission studies show the presence of an excimer species whose formation depends on the protonation state of the polyamine chains implying the existence of a bending movement (occurring in both the ground and in the first singlet excited state), which allows the two naphthalene units to approach and interact. For comparison purposes, one bis-chromophoric compound containing a rigid chain (piperazine unit) was also investigated. Its emission spectra shows a unique band decaying single exponentially thus showing that no excimer is formed. With the two new ligands, excimer formation occurs in all situations even at very acidic pH values when the protonation of the polyamine bridges is extensive. Coexistence of ground-state dimers with dynamic excimers was established based on steady-state and timeresolved fluorescence data. The energetics of excimer formation and dissociation were determined in ethanol and water. Different methods of decay analysis (independent decay deconvolution, global analysis and excimer deconvolution with monomer) were used to extract the kinetic (rate constants for excimer formation, dissociation, and decay) and thermodynamic parameters. In ethanol and acidified ethanol:water mixtures, an additional short decay time was found to exist and assigned to a dimer, whose presence is assumed to be responsible by the decrease in activation energy for excimer formation in this solvent. The results are globally discussed in terms of the small architectural differences that can induce significant changes in the photophysical behavior of the three studied compounds.

Excimer Formation in the Interlayer Region of Arene-Derivatized Zirconium Phosphonates

Abstract: Photophysical investigations are reported for two arene-derivatized zirconium phosphonates (layered solids). The chromophore groups that are attached within the interlayer region of these materials are seen to form excimer pairs. Whereas the naphthalene-containing system exhibits both monomer and excimer fluorescence, the potentially greater overlap of chromophores for the system containing anthracene-pendant groups causes that compound to only show excimer fluorescence.

Characterization of alumina surfaces by fluorescence spectroscopy : Part 2. Photophysics of a bound pyrene derivative as a probe of the spatial distribution of reactive hydroxyl groups

Abstract: In order to characterize and compare the spatial distribution of hydroxyl groups on γ- and δ-alumina surfaces, a pyrene derivative was covalently grafted on these groups and studied by steady-state and time-resolved fluorescence spectroscopy. When the pyrene probes are close enough, they can form excimers with a characteristic emission band. The relative intensities of the monomer and excimer bands provide a global information on the proximity of pyrene probes, whereas time-resolved fluorescence experiments offer a more detailed characterization of the surface in terms of heterogeneity. The fluorescence decay curves were satisfactorily analyzed in terms of stretched exponentials that account for the distribution of decay times resulting from the distribution of distances between the pyrene probes that can form excimer, and the distribution of the energies of interaction between the alumina surface and the pyrene monomers or excimers. Comparison of samples having the same surface coverage of pyrene probes allowed us to conclude that there are indeed significant differences in the characteristics of γ- and δ-alumina surfaces. In particular, the reactive hydroxyl groups are not homogeneously distributed on alumina surfaces but they are rather clustered into regions of high density. The fraction of these reactive groups belonging to the clustered regions is more than four times larger for δ-alumina than for for γ-alumina.

Characterization of Solvation Environment Provided by Dilute Poly(sulfonyl maleic anhydride-co-dodecyl vinyl ether) Solutions at Various pH Using Pyrene and 1,3-Bis(1-pyrenyl)propane as Fluorescence Probes

Abstract: Solubilization environments afforded by dilute aqueous solutions of poly(sulfonyl maleic anhydride-co-dodecyl vinyl ether) [PSMADVE, 1wt.-%] at pH 4, 7, and 10 are investigated using pyrene and 1,3-bis(1-pyrenyl)propane as fluorescence probes. While pyrene is a common polarity probe, intramolecular excimer formation in 1,3-bis(1-pyrenyl)propane makes it an interesting cybotactic region microviscosity probe. Results for pyrene I I /I III indicate that the most hydrophobic solubilization site is afforded at pH 4 followed by pH 10 and pH 7. The dipolarity indicated by pyrene is similar to that observed in m- and -oxylene, toluene, and butyl benzoate. The solvation microenvironment afforded by the PSMADVE solutions is more hydrophobic than that observed within sodium dodecyl sulfate micellar solution. Additionnally, further investigation of the relative size of the solubilization site revealed that the polymer solutions promoted intermolecular pyrene excimer formation at relatively low pyrene concentrations, indicating the possibility of localized concentration enhancement within the solvation pockets and/or compartmentalization of the solute molecules. This observation was most prominent at pH 10 and least at pH 7. 1,3-Bis(1-pyrenyl)propane intra-molecular excimer formation efficiency at pH 4 is slightly higher than that for neat ethylene glycol, while at pH 10 is similar to that observed in 1-octanol or dimethyl sulfoxide. Anomalously high excimer-to-monomer intensity ratio at pH7 could be due to probe ground state heterogeneity, i.e., the existence of probe moieties in the ground state were the two pyrenyl groups are already aggregated. This is demonstrated by the fluorescence excitation data. Presence of the ground state aggregated form may give rise to anomalously high excimer formation efficiency.

Direct initiation of the photopolymerization of acrylates by short-wavelength excimer UV radiation

Abstract: Investigations on the direct initiation of the photopolymerization of acrylates by irradiation with short-wavelength monochromatic UV light using the 222 nm emission of a KrCl * excimer lamp are reported. The reactivity of various acrylates was studied by real-time FTIR-ATR spectroscopy. Laser photolysis experiments and accompanying quantum chemical calculations were performed in order to propose a possible mechanism of initiation.

Intramolecular excimer fluorescence from folded ground state rotamers of N,N'dimethyl-N,N'-dipyrenylurea protophanes

Abstract: The molecular structure, absorption, and fluorescence spectra of N,N'-dimethyl-N,N'-di-1-pyrenylurea and N,N'-dimethyl-N,N'-di-2-pyrenylurea have been investigated and compared to the properties of...

Soft x-ray excited optical luminescence from poly(N-vinylcarbazole)

Abstract: X-ray excited optical luminescence (XEOL) using tunable soft x rays from a synchrotron light source, together with x-ray absorption fine structure spectroscopy has been used to study the electronic structure and optical properties of thin films of poly(N-vinylcarbazole). It is found that carbon core level excitation enhances the formation of excimers emitting at 380 and 410 nm. A third excimer at 310 nm is also noted. In addition, excitations across the C K edge and the N K edge show noticeably different optical response. These results are interpreted in terms of the site specificity of the XEOL technique.