Efficiency Of Excimer Formation Research Articles

Effects of tunable circuit parameters on pulsed discharge and input-output characteristics of a XeCl excimer laser

High-pressure, pulsed discharge-pumped XeCl excimer lasers are operating in the ultraviolet range at a wavelength of 308 nm. In this paper, a one-dimensional fluid model coupled to a magnetic pulse compression (MPC) excitation circuit and a laser gain model is established for describing the discharge and laser output characteristics of XeCl excimer lasers. The effects of the peaking capacitor (the terminal of the MPC circuit) on the discharge properties and input-output characteristics are explored. The results reveal that plays a key role in suppressing discharge pulse oscillation, which in turn influences the peak value and duration of the laser pulse. By optimizing the capacitance, a maximum excimer formation efficiency of 6.6% and a maximum laser output efficiency of 5% (output energy of 1.26 J) are obtained when , where is the storage capacitor in the MPC circuit.

Highly Efficient Intramolecular Excimer Formation in a Disulfide-Linked Dipyrenyl Compound: Proton Recognition and Fluidity Assessment.

Intramolecular excimer formation has been utilized extensively in chemical sciences, especially to probe solvation within complex media as well as to assess physicochemical properties of the solubilizing milieu. Pyrene has been employed extensively as a fluorescence probe for this purpose due to its favorable multidimensional fluorescence properties. Termini-capped dipyrenyl scaffolds possessing various functionalities comprise the majority of such compounds. A new both end-tagged dipyrenyl compound DTP is designed and synthesized, which exhibits significantly high intramolecular excimer formation efficiency in polar solvents. The presence of a -NH-(CO)- and/or -S-S- functionality on the chain linking the two pyrenyl groups facilitates intramolecular excimer formation. Excited-state emission intensity decay reveals that the excimer formation exclusively takes place in the excited state with only one excimer conformation. The rate constant of excimer formation is found to be higher for DTP as compared to a similar compound with an alkyl backbone. The dependence of the excimer formation on the solvent (protic versus aprotic) as well as on temperature reveals further insights into the excimer formation process. The excimer formation by DTP is found to be highly sensitive to the presence of H+: the relative excimer formation efficiency decreases drastically in the presence of a small amount of H+ (∼10-5 M). Further, the recognition of protons by DTP via intramolecular excimer formation is also observed to be highly selective in nature. Based on the observation that both the excimer formation efficiency and kinetics depend on the viscosity of the solubilizing milieu, fluidity assessment of the (dimethyl sulfoxide + acetonitrile) mixture was carried out using DTP. Further, DTP is found to be an effective probe for the assessment of the amount of water in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide as well as in a deep eutectic solvent composed of choline chloride and urea in a 1:2 mol ratio. Highly efficient and rapid intramolecular excimer formation not only establishes DTP as a useful and versatile probe but also offers strategic pathways for designing effective excimer-forming compounds.

Fluorescence of pyrene and its derivatives to reveal constituent and composition dependent solvation within hydrophobic deep eutectic solvents.

Depending on their constituents and composition, deep eutectic solvents (DESs) may exhibit widely varying physicochemical properties. Based on the miscibility of water in a DES, it can broadly be classified as 'hydrophobic' or 'hydrophilic'. The polarity offered by hydrophobic DESs as compared to common organic solvents in the context of solute solubilization thus becomes of the utmost importance. A versatile fluorescence probe, pyrene (Py), its aldehyde derivative, pyrene-1-carboxaldehyde (PyCHO), and a termini end-tagged dipyrenyl polydimethylsiloxane polymer (Py-PDMS-Py) are employed to assess the solvation environment afforded by DESs composed of thymol (Thy), (-)-menthol (Men), and n-decanoic acid (DA). To decipher the effect of the constituents and composition on solute solvation, DESs composed of different pairs of constituents in different molar ratios, namely Thy : Men (1 : 1 and 1 : 2), DA : Men (1 : 1 and 1 : 2), and Thy : DA (2 : 1, 1 : 1, and 1 : 2), are investigated. Pyrene, through its band 1-to-band 3 emission intensity ratio (Py I1/I3), displays higher cybotactic region dipolarity in Thy-containing DESs due to the phenyl ring of Thy; the sensitivity of Py I1/I3 towards temperature is also higher in Thy DESs. The fluorescence lifetime of pyrene and its temperature dependence are higher in Men-containing DESs in comparison. Quenching of pyrene fluorescence by nitromethane in these DESs is dynamic in nature and the recovered bimolecular quenching rate constants (kq) reveal efficient diffusion of the fluorophore-quencher pair in these DESs as compared to other iso-viscous media. The kq adheres to the Stokes-Einstein relation, implying inherent homogeneity associated to these DESs. PyCHO emission spectra show a high energy structured band in Thy : Men DESs, whereas the band shifts bathochromically and becomes broad in DA-containing DESs. The PyCHO cybotactic region is relatively nonpolar in Thy : Men DESs as compared to Thy : DA and Men : DA DESs. The extent of intramolecular excimer formation by Py-PDMS-Py reveals these DESs to be "good" solvents for polymer solvation where DES-polymer interactions are maximized. The microviscosity (ημ) surrounding Py-PDMS-Py correlates with the bulk dynamic viscosity (ηbulk) within the investigated DESs, further corroborating the absence of microheterogeneity. Overall, the observations reveal the similarity of these hydrophobic DESs to common organic solvents in the context of solute solubilization.

Characterization of the Local Volume Probed by the Side-Chain Ends of Poly(oligo(ethylene glycol) 1-Pyrenemethyl ether methacrylate) Bottle Brushes in Solution Using Pyrene Excimer Fluorescence

Four poly(oligo(ethylene glycol) 1-pyrenemethyl ether methacrylate) [P(PyEGyMA), with y = 3, 5, 8, and 12] samples were synthesized, where each side chain was terminated with a 1-pyrenemethoxy derivative. The efficiency of excimer formation between an excited-state pyrene and a ground-state pyrene was used to assess the conformation of the PyEGy side chains in these polymeric bottle brushes by conducting time-resolved fluorescence measurements in tetrahydrofuran (THF), N,N-dimethylformamide (DMF), dioxane, and dimethylsulfoxide (DMSO). These experiments took advantage of the dependency of the average rate constant ⟨k⟩ of pyrene excimer formation (PEF) on the local pyrene concentration [Py]loc experienced by an excited pyrene bound to the P(PyEGyMA) samples. [Py]loc could be estimated theoretically by assuming that the EGy side chains adopted a Gaussian conformation. Linear plots of ⟨k⟩/fdiff as a function of [Py]loc, where fdiff is the molar fraction of pyrenyl labels forming excimers by diffusion and was introduced to account for residual pyrene aggregation, were obtained in all four solvents, with slopes that depended on solvent viscosity and the probability of PEF upon the diffusive encounter between two pyrenyl labels. Solvent-induced differences in PEF efficiency could be accounted for by determining the bimolecular rate constant kdiff[inter] for intermolecular PEF with the model compound 1-pyrenemethyl diethylene glycol methyl ether in the four solvents. Except for DMSO, which was too polar and led to the segregation of the pyrenyl labels close to the polymethacrylate backbone, the data obtained for all P(PyEGyMA) samples in THF, DMF, and dioxane collapsed on a master line, similar to the one obtained earlier with a series of pyrene-labeled dendrons, when plotting ⟨k⟩/(fdiff × kdiff[inter])-vs-[Py]loc. The ⟨k⟩/(fdiff × kdiff[inter])-vs-[Py]loc master line confirmed the direct relationship that exists between the PEF efficiency and [Py]loc, a relationship that could be used to probe the conformation of highly branched macromolecules in solution.

Excimer Formation Dynamics of Dipyrenyldecane in Structurally Different Ionic Liquids.

Ionic liquids, being composed of ions alone, may offer alternative pathways for molecular aggregation. These pathways could be controlled by the chemical structure of the cation and the anion of the ionic liquids. Intramolecular excimer formation dynamics of a bifluorophoric probe, 1,3-bis(1-pyrenyl)decane [1Py(10)1Py], where the fluorophoric pyrene moieties are separated by a long decyl chain, is investigated in seven different ionic liquids in 10-90 °C temperature range. The long alkyl separator allows for ample interaction with the solubilizing milieu prior to the formation of the excimer. The ionic liquids are composed of two sets, one having four ionic liquids of 1-butyl-3-methylimidazolium cation ([bmim+]) with different anions and the other having four ionic liquids of bis(trifluoromethylsulfonyl)imide anion ([Tf2N-]) with different cations. The excimer-to-monomer emission intensity ratio (IE/IM) is found to increase with increasing temperature in sigmoidal fashion. Chemical structure of the ionic liquid controls the excimer formation efficiency, as IE/IM values within ionic liquids with the same viscosities are found to be significantly different. The excited-state intensity decay kinetics of 1Py(10)1Py in ionic liquids do not adhere to a simplistic Birk's scheme, where only one excimer conformer forms after excitation. The apparent rate constants of excimer formation (ka) in highly viscous ionic liquids are an order of magnitude lower than those reported in organic solvents. In general, the higher the viscosity of the ionic liquid, the more sensitive is the ka to the temperature with higher activation energy, Ea. The trend in Ea is found to be similar to that for activation energy of the viscous flow (Ea,η). Stokes-Einstein relationship is not followed in [bmim+] ionic liquids; however, with the exception of [choline][Tf2N], it is found to be followed in [Tf2N-] ionic liquids suggesting the cyclization dynamics of 1Py(10)1Py to be diffusion-controlled and to depend on the viscosity of the ionic liquid irrespective of the identity of the cation. The dependence of ionic liquid structure on cyclization dynamics to form intramolecular excimer is amply highlighted.

Excimer Luminescence From Nonresonantly Excited Pyrene and Perylene Molecules in Solution

By nonresonant excitation below the absorption edge of pyrene and perylene molecules in solution, efficient excimer luminescence is observed. For perylene in solution, there are two kinds of nonresonant excimer emission that closely resemble Y- and E-emissions in perylene crystal and perylene in polymer. The concentration dependence of the photon flux density of monomer and excimer emission is reasonably explained by a simple model with a few fitting parameters based on the ordinary excimer formation process, in which the excited-state monomer interacts with a ground-state monomer. Observed quantum efficiency of excimer formation is by orders of magnitude higher than that expected if a monomer is excited from thermally populated vibrational levels. The experimental evidence for direct resonant excitation of excimers in perylene nanocrystals is obtained from the absence of anti-Stokes emission. This is due to a symmetry-breaking effect close to crystal surface, suggesting that excimers are formed through a more efficient process in solution, such as resonant excitation of weakly interacting molecules with a symmetry-broken intermolecular configuration.

Excimer formation of 6-(1-pyrenyl)hexyl-11(1-pyrenyl)undecanoate within an ionic liquid and cosolvent-modified ionic liquid mixture

The ring closing metathesis within an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]), a 'green' solvent tetraethylene glycol (TEG), and their equimolar mixture is investigated through the intramolecular excimer formation of 6-(1-pyrenyl)hexyl-11-(1-pyrenyl)undecanoate [1]. The excimer formation efficiency correlates directly with the inverse of the bulk viscosity of the systems. A simplistic double exponential decay model based on Birk's scheme is enough to fit the excited state intensity decay of 1 in these liquids. This behavior is in stark contrast to that reported in organic solvents and in scCO(2), where three exponentials are required to fit the fluorescence decay. The activation energies of the excimer formation (E(a)), viscous flow (E(a,η(bulk))), and microviscosity (E(a,η(μ))) were estimated from the temperature dependence. E(a) and E(a,η(μ)) are significantly higher for the equimolar mixture as compared to that for [bmim][PF(6)] or TEG, whereas E(a,η(bulk)) is close to the average. This highlights the important outcome that the cyclization dynamics scales with the microviscosity rather than the bulk viscosity of the systems investigated.

Fluorescence studies of a series of monodisperse telechelic α,ω-dipyrenyl poly(N-isopropylacrylamide)s in ethanol and in water

Steady-state and time-resolved fluorescence measurements were performed on solutions in ethanol and in water (23 °C) of a series of poly(N-isopropylacrylamide)s labeled with a pyrenyl group at each chain end (Py2-PNIPAM) and ranging in molecular weight (Mn) from 5 900 to 44 500 g·mol−1. Water and ethanol are of similar solvent quality towards the PNIPAM chain. The pyrene labels, in contrast, are soluble in ethanol but not in water. The efficiency of excimer formation for Py2-PNIPAM samples in ethanol decreased with increasing chain length, a trend typical of pyrene end-labeled polymers in good solvents. The ratio IE/IM, where IE is the Py excimer emission intensity and IM is the Py monomer emission intensity, scaled as Mn−1.4, where Mn is the number-average molecular weight of the sample. The kinetics of excimer formation were more complex for aqueous Py2-PNIPAM solutions, as a consequence of pyrene–pyrene association prior to excitation. The excimer time-dependent profiles exhibited significantly faster rise times, compared with the situation in ethanol, and they could not be fitted with the traditional Birks scheme. The results are discussed in the context of the solution properties of telechelic amphiphilic PNIPAMs and are compared with data gathered previously in studies of dipyrenyl end-labeled poly(ethylene oxides), the only other polymers soluble in water and organic solvents subjected to similar studies in the past.

Synthesis and Gelation Behavior of a Pyrene-Containing Glucose Derivative

A pyrene-containing glucose derivative(PSDAPG) was synthesized and characterized. Its gelation behavior in 36 solvents was evaluated and the compound geled 16 of the solvents tested. Interestingly,PSDAPG was a super gelator for 1-decanol and its lowest gelation concentration(LGC) was 7.0×10-4 g·mL-1. PSDAPG was found to behave as a "bifunctional gelator"as it gels in both water and organic solvents. Scanning electron microscopy(SEM) ,Fourier transform infrared spectroscopy(FTIR) ,nuclear magnetic resonance(1H NMR) spectroscopy and fluorescence spectroscopy reveal that the gelator adopts different supramolecular network structures in different solvents. Additionally,hydrophobic and π-π stacking interactions exist between the pyrene structures of the gelator molecules. Hydrogen bonding between the glucose residues also plays a crucial role in the spontaneous formation of gel networks. Fluorescence emission spectra of PSDAPG in the solution state and in the gel state were characterized by monomer emission and excimer emission. Profiles of the two types of spectra were quite different. We found that the monomer emission increased and the excimer emission decreased as the gelation process proceeded. This indicates that formation of gel networks greatly suppresses the mobility of pyrene units in the gelator and results in lower excimer formation efficiency via Birks′ scheme.

The microenvironment in polymer solids with added plasticizers. An attempt to measure the size and distribution of free volume in poly(methyl methacrylate) solids

Various amounts of n-alkylbenzenes (C n : C 6H 5–C n H 2 n+1 ( n = 3–16)) were doped into poly(methyl methacrylate) (PMMA) films, and the emission and thermal properties of each film were measured in detail together with their solid-state 13C NMR spectra. The aim of the present work was to estimate the size distribution of free volume in amorphous regions of polymer solids heavily doped with plasticizers by using C n as models of a plasticizer. The excimer fluorescence yields of C n in PMMA were found to depend on both the amount of C n and the length of the alkyl chains of C n , although the fluorescence spectra of all of the C n s in dilute fluid solution were almost the same. The quantitative analysis showed: (1) C n with n ⩾ 12 induces a phase separation in PMMA, in which almost all of the C n molecules are in a separated phase, and thus they cannot penetrate regions in which PMMA chains are aggregated. This means that C n with n ⩾ 12 cannot enlarge the space between PMMA chains. (2) Smaller C n ( n < 11) can enter free volumes between PMMA chains that correspond to their molecular size, but they can enlarge them only to a limited extent. Thus, the amount by which plasticization can increase the free volume of PMMA is limited by the size of the dopant and the inherent free volume of the polymer matrix. (3) The efficiency of excimer formation was found to reveal the maximum amount of C n that could fit in the free volume of PMMA. Thus our fluorescence measurements showed that PMMA solids that were plasticized to their limit had a free volume that was larger than the volume occupied by all the conformers of C 5 and smaller than the volume occupied by almost all the conformers of C 12. In conclusion, we were able to obtain information on plasticization and to demonstrate a method of monitoring microenvironments in polymer solids after they have been doped with plasticizers.

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

AbstractSolubilization environments afforded by dilute aqueous solutions of poly(sulfonyl maleic anhydride‐co‐dodecyl vinyl ether) [PSMADVE, 1 wt.‐%] 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 II/IIII 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 o‐xylene, toluene, and butyl benzoate. The solvation microenvironment afforded by the PSMADVE solutions is more hydrophobic than that observed within sodium dodecyl sulfate micellar solution. Additionally, 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 intramolecular 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 pH 7 could be due to probe ground state heterogeneity, i.e., the existence of probe moieties in the ground state where 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. Structures of fluorescence probes pyrene and 1,3‐bis(1‐pyrenyl)propane.imageStructures of fluorescence probes pyrene and 1,3‐bis(1‐pyrenyl)propane.

Adsorption and excimer formation of 4-(1-pyrenyl)-butyltrimethylammonium bromide on non-porous silica nanoparticles in aqueous dispersions

Adsorption and excimer formation of 4-(1-pyrenyl)-butyltrimethylammonium bromide (PyB) on non-porous silica nanoparticles (Aerosil) have been studied in aqueous dispersions. The efficiency of excimer formation is remarkably enhanced on going from the homogeneous aqueous solution to the silica dispersion. This is ascribed to the effective adsorption of PyB onto the silica surfaces. Adsorption isotherm data are not fitted to a simple Langmuir binding model, suggesting cluster formation of PyB molecules on the silica surfaces. From the dependence of PyB excimer fluorescence intensity on the surface density of the probe on the silica surface, it is elucidated that PyB molecules are inhomogeneously distributed on the surfaces. Excitation spectra and fluorescence decay curves reveal that the excimers of PyB on Aerosil surfaces are mainly originated from ground-state dimers, i.e. dynamic processes through lateral diffusion of the probe on the surface are less important in the present case.

Photoreactions on latex surfaces: Excimer formation of 1-pyrenemethanol on polystyrene latex particles

Excimer formation from 1-pyrenemethanol (PyM) and several other pyrene derivatives is investigated in aqueous dispersions of polystyrene (PS) latex, as well as in homogeneous solutions. The efficiency of excimer formation is markedly enhanced on going from the homogeneous aqueous solutions to the particle surface in the presence of the PS latex. This is attributed to adsorption of the probes onto the latex surfaces. The peak height ratio between the excimer and monomer fluorescence bands ( I E/ I M) of PyM is measured as a function of the surface density of the probe. The ratio takes a maximum when the surface density is about 0.3 molecules nm −2. Analysis of various spectral and kinetic data suggest that the probes are inhomogeneously distributed on the latex surface and that a significant portion of the probes self-associate into ground state dimeric forms.

Numerical modeling of a XeCl laser excited by microwave discharge

, 169 (1987). Reasonably good agreements in the peak output power and laser efficiency have been achieved. Model calculations also predict that an efficiency as high as 2.7% can be obtained once the conditions of the above-mentioned experiments have been optimized. From the consideration that the skin depth effectively limits the absorption length of the microwave pumping and hence the excitable volume, it is concluded that high input power densities (>2 MW/cm3) and higher gas pressures (between 3 and 10 atm) are the preferable conditions to achieve higher efficiency. Preliminary calculations on CCl4 containing XeCl gas mixtures show that improvement in laser efficiency by several folds may be achieved as a result of the higher intrinsic efficiency of excimer formation.

Studies of the antenna effect in polymer molecules 29. Isomerization of provitamin D 3 photosensitized by polymers containing pendant naphthalene groups

The photosensitized isomerization of provitamin D 3 was studied in THF solutions of styrene-2-vinylnapthalene copolymers, poly(2-vinylnaphthalene poly(2-isopropenylnaphthalene) and poly(acenaphthalene). The effect of efficiency of excimer formation and energy migration on the quantum efficiency of previtamin formation was demonstrated.

Effects of unsaturation and curvature on the transverse distribution of intramolecular dynamics of dipyrenyl lipids

The roles of acyl chain unsaturation and curvature in the excimer formation efficiency (EFE) of site-specific conjugated pyrene molecules in lipid membranes have been investigated by steady-state and time-resolved fluorescence spectroscopy. Six 1-2-(pyrenyl-n-acyl)-phosphatidylcholine (dipy(n)PC) probes, with pyrenyl chains of varying methylene units n from 4 to 14 carbons, were incorporated separately into dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylethanolamine (DOPE) lipid membranes at 0.1 mol%. Both the excimer-to-monomer fluorescence intensity ratio and association-to-dissociation rate constant ratio of conjugated pyrenes were used to quantify EFE. At all temperatures (T = 0-30 degrees C) and for n = 4 and 6, the EFE for DOPE was always smaller than EFE for DOPC. At T < 10 degrees C (where DOPE and DOPC are in the liquid crystalline L alpha phase) and for n > 8, the EFE for curvature frustrated DOPE was significantly greater than EFE for nonfrustrated DOPC (control), and the difference increased gradually with n. At T> 18 degrees C (where DOPE is in the inverted hexagonal H(II) phase and DOPC is in the L alpha phase) and for n > 8, EFE for the curvature-relaxed DOPE was again smaller than the EFE for DOPC control. The contributions of splay conformation and internal dynamics of pyrenyl chains to EFE were examined separately using a lattice model. Our results suggest that i) the cis double bonds of the host lipid matrix strongly perturb both the conformation and dynamics of conjugated pyrenes at the specific location around n = 8, and ii) the lateral stress at the upper part (n < 8) of the curvature frustrated bilayer membranes (DOPE) may be significantly relaxed once the membrane surface adopts a favorable negative interfacial curvature.

Common molecular weight dependence of excimer formation and 13C spin-lattice relaxation in fluid solution of poly(α-methylstyrene)

Poly(α-methylstyrene) (PMS) with a variety of chain lengths was investigated by both fluorescence measurements and 13C nuclear magnetic relaxation measurements. The molecular weight dependence of excimer formation efficiency agress well with that of the spin-lattice relaxation time, indicating that it is due to the molecular weight dependence of local motion.

Static pyrene excimer formation at hydrophobic zirconium phosphate surfaces: The salt effect

4-(1-Pyrene)butylamine hydrochloride (PBAC) binds to layered zirconium phosphate (BAZrP) in the presence of butylamine hydrochloride, with a high affinity. Electronic absorption and emission spectra of the pyrenyl chromophore change dramatically upon binding to the layered phosphate and provide spectroscopic signatures for the microenvironment of the pyrenyl chromophore. For example, BAZrP-promoted excimer emission from PBAC was observed only in a narrow range of BAZrP and PBAC concentrations. The intensity of the excimer emission was very high at low concentrations of BAZrP and decreased dramatically at high concentrations of BAZrP. The intensity of the excimer emission also depended upon the ionic strength of the medium. For example, addition of sodium chloride to a suspension of PBAC and BAZrP dramatically enhanced the excimer formation. This result is in contrast to our previous observations with 9-anthracenemethylamine hydrochloride bound to BAZrP where a decrease in the anthracene excimer emission was noted with increased NACl concentration. The strong role of electrostatic and hydrophobic interactions on the binding of the probe as well as on the efficiency of excimer formation are clearly indicated by the current observations. Fluorescence quenching studies with iodide anion show that the intensity of excimer emission is influenced by both the changes in the ionic strength as well as the iodide quenching effect. The relative magnitudes of these two effects depended upon the concentration of the iodide. Excimer emission was not observed when PBAC binds to DNA or to polyelectrolytes, highlighting the unique role of the layered phosphates on the probe excited state dynamics.

Pyrene excimer formation in individual oil droplets dispersed in gelatin matrixes: space- and time-resolved fluorescence spectroscopy

Pyrene excimer formation in individual liquid paraffin droplets dispersed in aqueous gelatin matrices was studied by three-dimensional space- and time-resolved fluorescence spectroscopy. The pyrene excimer formation efficiency in the droplets was constant irrespective of the droplet size. The parameters of the excimer formation were determined for each droplet and compared with those observed in bulk pyrene/liquid paraffin solution. The results were discussed on the basis of the depth profiles and the dynamics of pyrene fluorescence in individual liquid paraffin droplets

The use of STM for biological purposes

The roles of acyl chain unsaturation and curvature in the excimer formation efficiency (EFE) of site-specific conjugated pyrene molecules in lipid membranes have been investigated by steady-state and time-resolved fluorescence spectroscopy. Six 1–2-(pyrenyl-n-acyl)-phosphatidylcholine (dipy(n)PC) probes, with pyrenyl chains of varying methylene units n from 4 to 14 carbons, were incorporated separately into dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylethanolamine (DOPE) lipid membranes at 0.1 mol%. Both the excimer-to-monomer fluorescence intensity ratio and association-to-dissociation rate constant ratio of conjugated pyrenes were used to quantify EFE. At all temperatures (T = 0–30 degrees C) and for n = 4 and 6, the EFE for DOPE was always smaller than EFE for DOPC. At T < 10 degrees C (where DOPE and DOPC are in the liquid crystalline L alpha phase) and for n > 8, the EFE for curvature frustrated DOPE was significantly greater than EFE for nonfrustrated DOPC (control), and the difference increased gradually with n. At T> 18 degrees C (where DOPE is in the inverted hexagonal H(II) phase and DOPC is in the L alpha phase) and for n > 8, EFE for the curvature-relaxed DOPE was again smaller than the EFE for DOPC control. The contributions of splay conformation and internal dynamics of pyrenyl chains to EFE were examined separately using a lattice model. Our results suggest that i) the cis double bonds of the host lipid matrix strongly perturb both the conformation and dynamics of conjugated pyrenes at the specific location around n = 8, and ii) the lateral stress at the upper part (n < 8) of the curvature frustrated bilayer membranes (DOPE) may be significantly relaxed once the membrane surface adopts a favorable negative interfacial curvature.