Showing papers on "Excimer published in 1999"

Photophysical characterization of dilute solutions and ordered thin films of alkyl-substituted polyfluorenes

Abstract: Absorbance and fluorescence spectra of dilute solutions and thin films of polyfluorene rigid-rod polymers 1-3 bearing two hexyl, octyl, or dodecyl groups at the 9-position define the effect of polymer chain interactions on excited state relaxation. Film morphology is controlled by annealing of 250 nm thick films. Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, in turn, a function of the length of the attached alkyl substituents. Alkyl substituents also affect the solubility of these polymeric liquid crystals in organic solvents; low solubility favors strong ground state aggregation, as is evidenced by a red-shifted absorption band. In the annealed films, aggregate and excimer formation is evidenced by a broadening of the absorbance band, an increase in the intensity of the low energy emission, the appearance of new long-lived fluorescent species, and structure-dependent changes in observed fluorescence quantum yields.

Sensing isothermal changes in the lateral pressure in model membranes using di-pyrenyl phosphatidylcholine

Abstract: In this work we present data from a homologous series of di-pyrenyl phosphatidylcholine (dipyPC) probes which can sense lateral pressure variations in the chain region of the amphiphilic membrane (lateral pressures are tangential to the interface). The dipyPC has pyrene moieties attached to the ends of equal length acyl chains on a phosphatidylcholine molecule. Ultraviolet stimulation produces both monomer and excimer fluorescence from pyrene. At low dilutions of dipyPC in model membranes the excimer signal is entirely intra-molecular and since it depends on the frequency with which the pyrene moieties are brought into close proximity, the relative intensity of the excimer to monomer signal, η, is a measure of the pressure. We synthesised or purchased dipyPC probes with the pyrene moieties attached to acyl chains having 4, 6, 8 and 10 carbon atoms and then measured η in fully hydrated bilayers composed of dioleoylphosphatidylcholine and dioleoylphosphatidylethanolamine (DOPC and DOPE respectively). Although the resolution of our measurements of lateral pressure as a function of distance into the monolayer was limited, we did observe a dip in the excimer signal in the region of the DOPC/DOPE cis double bond. As we isothermally increased the DOPE composition, and hence the desire for interfacial curvature, we observed, as expected, that the net excimer signal increased. However this net increase was apparently brought about by a transfer of pressure from the region around the glycerol backbone to the region near the chain ends, with the lateral pressure dropping above the cis double bond but increasing at a greater rate beyond the double bond.

One-dimensional modelling of low-frequency and high-pressure Xe barrier discharges for the design of excimer lamps

Abstract: A self-consistent one-dimensional modelling of a Xe gas discharge between electrodes covered with dielectric barrier is presented for power frequencies from 50 kHz to 1 MHz and gas pressures from 10 to 400 Torr. Spatiotemporal profiles of the concentration of electrons, ions, excited atoms and excimers are obtained. Excimers are mainly produced in the sheath regions. The efficiencies of spontaneous emission from excimers and resonance-state atoms increase with an increase in the input powers for gas pressures higher than 50 Torr. A characteristic period during which the barrier wall charge significantly influences the electric field in bulk region was found, and discharge properties in the period are discussed.

Chain length dependence of intramolecular excimer formation with 1,n-bis(1-pyrenylcarboxy)alkanes for n = 1-16, 22, and 32.

Abstract: Time-resolved and photostationary measurements in alkane solvents as a function of temperature are presented for the 1,n-bis(1-pyrenylcarboxy)alkanes (1PC(n)1PC, n = 1−16, 22, 32) with n methylene groups in the chain. For all compounds except 1PC(1)1PC intramolecular excimer fluorescence is observed. From fluorescence decay analysis and quenching data, the rate constant ka of excimer formation in the series 1PC(n)1PC is determined as a function of chain length. The value of ka, as well as that of the excimer-to-monomer fluorescence quantum yield ratio Φ‘/Φ, peaks at n = 5 and then gradually decreases with increasing n. These results are in clear contrast to those obtained previously with the 1,n-di(1-pyrenyl)alkanes (1Py(n)1Py), where Φ‘/Φ reaches its maximum at n = 3, with a sharp minimum (no excimer formation) at n = 7. This difference is attributed to a reduced steric hindrance at the excimer configuration (ka) and to dimer formation in the electronic ground state between the pyrenylcarboxy end groups ...

Properties of a new pyrazoline derivative and its application in electroluminescence

Abstract: As evaporated thin film and in solution, the absorption spectra, photoluminescence, electrochemistry and electroluminescence of the newly synthesized 1,3-diphenyl-5-pyren-2-yl-4,5-dihydro-1H-pyrazole (DPP) were investigated. The absorption spectra cover the whole near-ultraviolet region. At lower concentrations, the fluorescence emission is at 415 nm; at higher concentrations, a new face to face excimer emission appears on longer wavelengths; in the thin film state, the fluorescence emission becomes a single band peaking at 470 nm. Cyclic voltammetry of DPP and the hole transport material as evaporated thin films on ITO (indium tin oxide) was compared with that in solution and was used to determine the energy levels. The electric field strength dependent electroluminescent behavior of DPP was explained according to the energy levels by a tunneling mechanism, ruling out the possible cause by an excimer or exciplex formation. At a drive voltage of 18 V, the blue electroluminescence reached 2400 cd m –2 .

Excimer laser-induced hydroxyl radical formation and keratocyte death in vitro.

Abstract: PURPOSE. To characterize the type of reactive oxygen species (ROS) produced by excimer photoablation of aqueous solutions and to show the effects of ROS and antioxidants on corneal stromal cells in vitro. METHODS. Electron spin-resonance spectroscopy was performed using the spin-trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) for the detection of the superoxide anion and the hydroxyl radical in an acellular DMPO solution irradiated with the excimer laser. Hydroxyl radicals were produced by the Fenton reaction in vitro by the mixture of hydrogen peroxide and ferrous iron (Fe 2+ ), and the effects on cultured corneal fibroblasts were observed by fluorescent microscopy using the cell death marker, propidium iodide (PI) and TdT-mediated dUTP nick-end labeling (TUNEL). RESULTS. Excimer photoablation of a 1% DMPO solution produced a species-specific spin-trapping adduct for the hydroxyl radical (OH), but not for the superoxide anion or other unidentified free radical. The signals were inhibited dose dependently by the hydroxyl radical scavenger dimethylsulfoxide (DMSO) and an L-ascorbic acid analogue, EPCK-1. The production of OH in the supernatant of cultured rabbit corneal fibroblasts by the Fenton reaction caused an increase in PI (+) and TUNEL (+) cells by 90 minutes, which was significantly inhibited by the addition of DMSO. CONCLUSIONS. Hydroxyl radicals may be partly responsible for stromal fibroblast cell apoptosis after excimer photoablation.

Global modeling of a dielectric barrier discharge in Ne-Xe mixtures for an alternating current plasma display panel

Abstract: A global model of a dielectric barrier discharge in Ne–Xe mixtures for an alternating current plasma display panel was developed. This model was used to evaluate electron temperature, plasma density, densities of excited state atoms, wall charge density, current density, excimer density, and vacuum ultraviolet (VUV) intensity, and their gas composition-pressure dependencies, in order to analyze the mechanism of VUV radiation and discharge efficiency. The results show that the intensity ratio of 173 to 147 nm VUV is about a few percent. This means that the contribution of excimers is small in terms of VUV radiation. The maximum discharge efficiency was about 9% for Xe fraction in the range of 2%–12% and gas pressure in the range of 100–600 Torr. Discharge efficiency increases in the high Xe fraction and gas pressure region. The increase of the discharge efficiency is attributed to a decrease of discharge current and an increase of Xer*(3P1) excited state atom, due to the low electron temperature in the hig...

Intramolecular Excimer Formation in Short- and Long-chainlength di(9-anthryl) Bichromophoric Compounds and Relation to Ground State Properties

Abstract: The ground state conformational, UV absorption and fluorescence spectral as well as decay time data for a series of polymethylene- and one polyoxyethylene-linked di(9-anthracenes) in solution are compared, for chainlengths varying from n=0 (directly linked system A0A, 9,9′-bianthryl) to n=∞ (A∞, reference system 9-decylanthracene). 1H NMR spectroscopy of A2A to A11A shows that there is no clearly preferred ground state conformation of the anthracene moieties. The crystal structure of A2A was solved and the anti conformation of the anthracene units was established. The UV spectra show a redshift interpreted within the Forster exciton model (predominance of extended conformations). Fluorescence spectra for (CH2)n-bridged systems show no clear excimer band for short-chain systems but redshifted excimer emission for A4A to A9A. Loss of structure and fluorescence band shape changes as well as biexponential decay times allow the conclusion that excimer formation occurs in all chained systems. For a related polyoxyethylene-chained system, the excimer band is substantial, probably a consequence of the increased flexibility of the chain and moderate photoreactivity. Solvent polarity changes are strong for A0A [formation of a twisted intramolecular charge transfer (TICT) state] but also significant in the systems A1A and A2A. This is rationalized by the possiblity for competitive formation of an excimer-type (parallel but tilted chromophore arrangement) and a TICT-type (near perpendicular) conformation. For AnA (n>2) the solvent polarity was found to have virtually no influence.

Electroluminescent Behavior of a Homologous Series of Phenylenevinylene Oligomers

Abstract: Finally, the film emission spectra of both the DEHF/PER and the DEHF/CNSt copolymers are unaffected bythermal annealing at 200˚C for 48 h, suggesting that noemissive excimer is formed in the annealed copolymers.The dramatic effect of annealing on the DEHF homopoly-mer is shown in Figure 3b. Although excimer formation isless pervasive in the branched DEHF derivative relative tothe di-n-hexylfluorene (DHF) homopolymer, a broad andunstructured longer-wavelength excimer emission (500–550 nm) that increases with annealing time and tempera-ture is observed.In summary, the synthesis of thermally stable, easily pro-cessable random copolymers of 9,9-di-2¢-ethylhexylfluor-ene and either perylene or a-cyanostilbene can be accom-plished by nickel-mediated coupling reaction. Here, thebranched DEHF units provide adequate solubility whenunsubstituted perylene and a-cyanostilbene units are incor-porated into the conjugated main chain. We have demon-strated that formation of excimers in thin films of poly-(fluorene) derivatives upon thermal annealing can be sup-pressed by incorporation of lower-bandgap chromophoricsegments, which serve as efficient excitonic energy trapsupon excitation of the poly(fluorene) chromophoric seg-ments. This study provides indirect evidence of the possiblerole of the anthracene subunits in DHF/ANT copolymersin suppressing excimer formation.

Mass Spectrometry of Liquid Aniline Aerosol Particles by IR/UV Laser Irradiation.

Abstract: The first results are reported from a new single-particle two-color laser time-of-flight mass spectrometer, incorporating a combination of infrared (CO2) and UV (excimer) laser irradiation. This combination of lasers has the capability to effectively separate the desorption or evaporation step from the ionization step, thereby greatly improving the analytical capabilities of such an instrument. The results on liquid aerosols, such as aniline, show that prior evaporation of the aerosol particle with the IR laser increases the ion signal produced by the excimer laser by more than 2 orders of magnitude. In the case of nitrobenzene aerosols, the excimer laser alone produces no ions, while a very large signal is observed when the aerosol is first irradiated with the CO2 laser. A simple model, based on the Coulomb explosion of the ionized aerosol, is used to estimate the number of ions generated by the excimer laser (∼105 ions). Experimental evidence based on the observed time delay of protonated aniline parent...

Wavelength calibration tool for narrow band excimer lasers

Abstract: An excimer laser system having a precisely calibratable absolute emission wavelength is provided wherein at least one source of reference light is used. The reference light may be a laser such as a HeNe laser or a cathode lamp such as a hollow Pt, As, C, or Fe cathode lamp. The reference light and the excimer laser beam are directed along substantially the same optical path. The beams are broadened and recollimated by beam expanding optics. The broadened beams impinge upon a dispersive element, preferably an echelle grating, and various orders for each incident wavelength are dispersed. The beams are refocused onto a position sensitive detector such as a CCD camera. Different orders of one or more lines of known wavelength of the reference light and a line from the excimer laser emission are also incident at the detector simultaneously. An absolute wavelength calibration of the excimer laser emission is precisely determinable based on a position of said excimer laser emission relative to positions of the one or more lines of the reference light at the detector. The grating constant of the preferred grating and the detector are such that calibration of the central wavelength of the narrowed excimer laser emission is performable when the central wavelength is anywhere within the broadband emission spectrum of the excimer laser. A processor calculates the absolute wavelength of the excimer laser emission. Embodiments are provided for refractive optic, catadioptric, and reflective imaging systems.

"Orientation teeth" in non-mechanical laser corneal trephination for penetrating keratoplasty: 2.94 µm Er:YAG v 193 nm ArF excimer laser

Abstract: Background/aims—“Orientation teeth” at the donor trephination margin and correspondent “notches” at the host margin facilitate graft orientation and avoid “horizontal torsion” induced by asymmetric suture placement. In this study the quality and reproducibility of these structures created by non-mechanical laser corneal trephination were compared using two laser emissions. Methods—The procedure was performed in 20 enucleated pigs’ eyes using open metal masks with eight “orientation teeth/ notches”(0.3 × 0.15 mm,base × height),an automated globe rotation device, and either a 193 nm ArF excimer laser or a Q switched 2.94 µm Er:YAG laser. “Teeth/ notches”were analysed by planimetry and scanning electron microscopy (SEM). Results—Mean size was 0.30 (0.027) × 0.16 (0.017) mm for “teeth” and 0.30 (0.035) × 0.15 (0.021) mm for “notches” (excimer), and 0.31 (0.022) × 0.16 (0.015) mm and 0.30 (0.031) × 0.14 (0.021) mm respectively (Er:YAG). Overall, variability of notches was higher than that of teeth. By SEM, comparable cut regularity and sustained ablation profile were observed with both lasers. However, the corneal surface at the cut edge appeared slightly elevated (<35 µm) in the Er:YAG group. Conclusion—Orientation teeth/notches resembling those obtained with the excimer laser can be created using the Q switched Er:YAG laser, with potential advantages of lower costs, convenient equipment size, and solid state safety.

Dynamics of Excimer Formation and Relaxation in the T-Shaped Benzene Dimer

Abstract: The excited-state dynamics of the T-shaped van der Waals (vdW) dimer of benzene has been studied by probing fluorescence after S1 excitation. Excimer fluorescence is observed when the dimer is excited into the S1 origin and the ν6 vibronic level. The results demonstrate that the excimer formation proceeds by tunneling through an energy barrier between vdW and excimer potential wells. Use of a pump−probe fluorescence depletion technique showed that the excimer absorbs strongly at 500 nm, consistent with the result in solution. Evidence is observed for an equilibrium between the vdW dimer and excimer states in the ν6 excitation. These results are explained by invoking an excimer of a parallel stacked geometry, having a charge-transfer character. The excimer formation dynamics of the benzene dimer is discussed based on comparison with those of other aromatic dimers and cluster systems.

Human corneal ablation threshold using the 193-nm ArF excimer laser.

Abstract: PURPOSE. To determine the human corneal threshold ablation energy density for the 193-nm ArF excimer laser, approximating clinical conditions. METHODS. The VISX Star (Santa Clara, CA) 193-nm argon fluoride excimer laser was used to ablate the cornea in human eye bank eyes under clinical conditions. Corneas were exposed to energy densities of 10, 20, 30, 35, 40, 45, and 140 to 160 mJ/cm 2 . Corneas were fixed for light and transmission electron microscopy immediately after laser exposure. RESULTS. Different ablation thresholds for various corneal structural elements were observed. The ablation threshold for the collagen in the corneal stroma was determined to be 30 mJ/cm 2 . Keratocytes had ablation thresholds of 40 mJ/cm 2 . These different ablation thresholds accounted for the production of stromal peaks and valleys, with the keratocytes atop the peaks. CONCLUSIONS. Different corneal structural elements have different ablation threshold energy densities.

Method and device for determining the oil concentration in liquids by means of fluorescence excitation with an excimer lamp

Abstract: The present invention relates to a method and a device for fluorescence measurement of oil residues in water. The core of the invention is to use a powerful, narrow-band excimer lamp to excite intensive fluorescence radiation to detect the latter with a photodiode or the like, and to determine the oil concentration therefrom. The excimer lamp can be designed as a flat radiator a hollow cylindrical inner radiator or a tubular radiator, and can be combined with a through-flow or free fall measuring cell. The excimer lamp is advantageously operated in a pulsed or modulated fashion, the fluorescence radiation is measured at a right angle to the exciting radiation, the emitted and/or transmitted exciting light is monitored with the detectors, and a redundant measuring arrangement with a plurality of excimer lamps and fluorescence detectors is provided. The sensor is distinguished by reliability, freedom from maintenance and a long service life.

Fluorescence Polarization Studies of Inclusion Complexes between β-Cyclodextrin and 1-Chloronaphthalene in Aqueous and Aqueous d-Glucose Solutions

Abstract: In aqueous and aqueous d-glucose solutions, 1-chloronaphthalene (1CN) has been found to form a 2:2 inclusion complex with β-cyclodextrin (β-CD), which is generated by the self-association of 1:1 β-CD−1CN inclusion complexes. The 2:2 inclusion complex is responsible for the excimer fluorescence of 1CN. Equilibrium constants for the formation of the 1:1 and 2:2 inclusion complexes in aqueous solutions have been evaluated from a simulation of the 1CN excimer fluorescence intensity which is a function of β-CD concentration. When a wavelength of 315 nm has been selected as an excitation light, the degree of polarization has been maximized at the wavelengths of the 1CN monomer fluorescence band, and is decreased at the 1CN excimer fluorescence band. The decrease in the degree of polarization is due to the long lifetime of the excimer fluorescence. The results of the fluorescence polarization are consistent with the theory that the transition moment of the 1CN excimer fluorescence is parallel to that of the mono...

KrF excimer laser ablation of thin Cr film on glass substrate

Abstract: In order to understand the removal mechanism and seek the optimal processing conditions, KrF excimer laser ablation of Cr films deposited on glass substrates is investigated The surface morphology of the laser-irradiated spot is examined by scanning electron microscopy, and the ablation rate is also obtained from the energy-dispersive X-ray spectrometry of SEM The measured single-shot ablation rate is found to be about two times the result of numerical analysis based on a surface vaporization model and heat conduction theory Surface morphology examination indicates that the Cr film is removed by the sequence of cracking-melting-surface vaporization-melt expulsion by plasma recoil, and that the outmost ripple of the diffraction pattern gives a strong effect on the morphology of molten Cr during the melting and vaporization processes Although removal quality is divided into film damage, partial removal, excellent removal, and glass damage, EDXS shows that about 14% of Cr remains even at the excellent removal condition Optimal fluence range which gives excellent removal, good precision of pattern size, and few droplets and cracks, is so narrow that the process conditions for patterning should be established carefully From this study, it is found that thinner films are well ablated by excimer lasers

Fluorescence quenching by reversible excimer formation: Kinetic study on the basis of generalized, non-Markovian rate equations

Abstract: The kinetics of fluorescence quenching by reversible excimer formation has been studied theoretically on the basis of generalized, non-Markovian rate equations derived from a diffusion-kinetic hierarchy approach in the low-reactant density limit. It is demonstrated that, in contrast to the case of reversible excitation transfer [W. Naumann, J. Chem. Phys. 110, 3926 (1999)], compact rate kernel expressions can be derived not only for contact encounters but also for more realistic, longer-range reactivities. Given as functionals of so-called phenomenological excimer formation and dissociation coefficients, the rate kernel expressions allow a critical assessment of approximations based on time-scale separation arguments. An effective excimer formation coefficient is defined, which leads to simple and physically transparent formulas for the Laplace transforms of the excited monomer and excimer concentrations. The effect of the back reaction on the total fluorescence yield is discussed. The study also includes a critical comparison to analogous results obtained for the case of reversible energy transfer quenching.

Pellicle for ArF excimer laser photolithography

Abstract: We have developed pellicle for ArF excimer laser lithography. Especially we have studied light resistivity against ArFexcimer laser. We have selected fluoropolymer for ArF pellicle material because of its high transmission against deep uvlight.Transmission of the pellicle film at wavelength A. 193nm is over 99.5% at peak value from sinusoidal transmissionspectrum. Lifetime of our pellicle film against the ArF excimer laser irradiation is estimated for total exposure energy70,000J/cm2. Number of 300mm wafer processed within the lifetime becomes 480,000 wafers from the total exposureenergy 70,000J/cm2.Degradation mechanism ofpellicle film caused by the ArF excimer laser irradiation has been investigated. The degradationmechanism is interpreted as following. Pellicle film is first etched from its surface by the ArF excimer laser irradiation.This etching ofthe pellicle film causes the film thickness reduction and roughens the surface of the pellicle film. Thus thetransmission is reduced. The pellicle film material, which is fluoropolymer, however, has not changed on chemical basis.Fluorination of the pellicle film material, i.e., fluoropolymer, has improved its light resistivity against ArF excimer laser.Further fluorination ofpellicle film material is expected to improve more its light resistivity against ArF excimer laser.Keywords: pellicle, ArF excimer laser, lithography, fluoropolymer, transmission, lifetime, fluorination

In-Cylinder NO-LIF Imaging in a Realistic GDI Engine Using KrF Excimer Laser Excitation

Abstract: The formation of nitric oxide in a transparent direct injection gasoline engine was studied experimentally using two different schemes of laser-induced fluorescence (LIF) with KrF excimer (248 nm) excitation. With detection of the fluorescence shifted towards the red, strong interference from fluorescence of partially burned fuel was found. With blue-shifted fluorescence, interference was minimized allowing selective detection of NO. Possibilities of quantifying NO fluorescence intensities in inhomogeneous combustion are discussed.

Photodegradation of pyrene-labeled polystyrene adsorbed on silica surface in chloroform

Abstract: Adsorption of polymers onto particles has been a subject of study because of its importance in the behavior of suspensions.1,2 To understand the adsorption/desorption process in detail, studies have been carried out with model systems. Some studies have employed polystyrene (PS) adsorbed onto metals, silica, glass, and mica.3-7 In previous work, we reported on the use of pyrene-labeled PS to study the dynamics of the rearrangement of adsorbed PS after exposing the PS adsorbed on a silica surface to desorption conditions, i.e., after replacing the bulk solution containing unabsorbed PS with pure solvent.8 Pyrene has been used widely in the literature as a probe of structure and dynamics in macromolecular systems.9-11 From the fluorescence emission properties of pyrene labels, such as the I3/I1 ratio, the excimerto-monomer ratio (Ie/Im), and fluorescence polarization value, one obtains information about the conformation, mobility, and dielectric environment of the pyrene probe within the macromolecule being studied. In this note, we report on the photodegradation of the pyrene labels on PS in chloroform (CHCl3) and adsorbed on silica surface contacting with CHCl3 caused by excitation light under conditions typical for photophysical studies of polymers. We feel this effect should be brought to the attention of the community because of the common use of pyrene as a probe.12,13 As published in a previous paper,8 after the adsorption of labeled PS onto a silica surface has proceeded for a predetermined time in dilute PS/CHCl3 solution, replacing the bulk polymer solution with pure solvent CHCl3 causes a progressive relative increase in excimer emission intensity with time, i.e., an increase in Ie/Im ratio over time. In this work, we observe that the increase in Ie/Im ratio was dependent on the total exposure time of the sample to the excitation light in the fluorescence spectrometer. The synthesis and characterization of the pyrenelabeled PS were reported previously.8 The polymer used has a molecular weight of 50K and label density of 15 pyrene labels per chain. Porous silica samples were obtained as gifts from Dupont and had average pore sizes of 57 or 122 A. Solvent CHCl3 was distilled before use. The preparation of adsorbed, pyrene-labeled PS layers on silica was described before.8 The bulk polymer concentration for adsorption was 0.1 g/L in CHCl3. After adsorption for a certain time, the bulk polymer solution was replaced with pure CHCl3 by rinsing the solid and transferring it quickly into a fluorescence cell with a 2 mm path length containing pure CHCl3. The fluorescence emission and excitation spectra were taken on a SPEX Fluorolog 1680 0.22 m double spectrometer with a 450 W xenon lamp using front face excitation. The excitation light beam was directed onto the polymer/silica sediment pack, not the supernatant. First, the effect of the excitation light on the pyrene labels of the polymer adsorbed on a porous silica was investigated. Typical excitation spectra obtained with successive scans are shown in Figure 1. The excitation spectrum monitored at 480 nm (maximum emission wavelength for pyrene excimer) has several broad unexpected bands in the range 350-450 nm when compared to other reports.13 It is well-known that the excitation spectra of the preassociated pyrene molecules have similar vibrational structure as the pyrene monomer excitation spectra but are 1-4 nm red-shifted.13 The observed bands do not correspond to a static excimer of pyrene, which is expected when pyrene molecules are preassociated in the ground state. It was speculated that these bands were caused by other species that emit in the same spectral region as the pyrene excimer. At this point, we decided to investigate whether photodegradation products formed during the spectral collection process while the sample was being irradiated in the sample chamber of the fluorescence spectrometer was the cause of the observation. On the basis of this hypothesis, it was found that the intensity of the excitation spectra of pyrene labels changed dramatically with cumulative exposure to the instrument excitation light under conditions typical for photophysical studies. The typical observation is that intensities in the monomer excitation spectra decrease with the number of scans taken, while that in the excitation spectra monitored at the excimer emission wavelength increase dramatically, especially in the region of 350-450 nm. These changes could be caused by rearrangement of polymer chains on the surface upon exposure to pure solvent, as suggested earlier, or by photodegradation in the instrument as suggested here. To determine which mechanism was operating, an adsorbed layer of labeled PS on silica was prepared at conditions similar to that described in ref 8 and exposed to solvent for 2 days in complete absence of light, and then its fluorescence excitation spectra were acquired. This sample showed changes in its fluorescence excitation spectra similar to those shown Figure 1 which were taken within minutes after sample preparation. Since, after 2 days under desorption conditions, chain rearrangements should have reached an equilibrium, photodegradation of labels is strongly suspected as the cause of spectrum changes rather than conformational changes for which the labels maintain their chemical integrity. Similar spectral and intensity changes were observed for all the silica samples used in ref 8 and for three other pyrene-labeled PS with different molecular weight and different labeling densities.14 For comparison, we tested pyrene-labeled PS in CHCl3 solution under the same conditions in the fluorescence instrument with the results shown in Figure 2. In the fluorescence emission spectra with excitation at 330 nm, multiple scans caused an intensity decrease * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Chemical Engineering and Applied Chemistry. 4151 Macromolecules 1999, 32, 4151-4153

Rational design of an optical sensing system for multivalent proteins

Abstract: A generic design principle for detection of multivalent interactions is described. A phospholipid bilayer consisting of natural and pyrene-derivatized phosphatidylcholines is used as both a supporting biomimetic surface and part of a signal transduction element. The pyrene excimer formed in the surface can act as fluorescence donor, and DABCY/BODIPY-FL covalently attached to receptor (GM1) can act as acceptors. Aggregation of the acceptor-tagged receptors resulting from multivalent binding of CT induces a decrease in efficiency of fluorescence quenching of the pyrene excimer by DABCY or energy transfer from pyrene excimer to BODIPY-FL. In the case using fluorescent acceptors that can undergo distance-dependent fluorescence self-quenching, combination of the lower energy transfer efficiency from the excimer and the acceptor`s self-quenching capability make acceptor fluorescence go down even further by the binding. This scheme can achieve signal amplification and high surface density of the optical transduction elements, which, in return, require relatively small surface area.