Showing papers on "Excimer published in 2010"
TL;DR: In this article, the role of the self-assembled organogel scaffolds of π-gels derived from linear π systems in modulating the excited state properties such as excited state energy transfer, exciplex emission, phosphorescence, and aggregation-induced enhanced emission (AIEE), when compa...
Abstract: This article describes the recent progress made to the understanding of the excited state interactions in organogels derived from linear π-conjugated molecules. The soft organogel scaffolds of such systems create an excellent ambience for the self-organization of the molecules allowing the modulation of their excited state properties and thereby opening a new world of fascinating materials with interesting physical properties at nano- and macroscopic levels. Noncovalent interactions provide opportunity for the molecules to interact electronically within a space of defined size and shape. As a result, the photophysical properties of the molecules in solution and gel states are different from each other. In this perspective article we analyze the role of the self-assembled organogel scaffolds of π-gels derived from linear π-systems in modulating the excited state properties such as excited state energy transfer, exciplex emission, phosphorescence, and aggregation-induced enhanced emission (AIEE), when compa...
157 citations
TL;DR: A novel calix[4]arene derivative locked in the 1,3-alternate conformation bearing two pyrene and rhodamine fluorophores was synthesized as a selective sensor for the Hg(2+) ion and it was found that the pyrene excimer emissions are more effective in obtaining strong FRET bands than those by intermolecular π-π interactions.
Abstract: A novel calix[4]arene derivative locked in the 1,3-alternate conformation (2) bearing two pyrene and rhodamine fluorophores was synthesized as a selective sensor for the Hg2+ ion. The sensoring is based on FRET from pyrene excimer emissions to ring-opened rhodamine absorption upon complexation of the Hg2+ ion. Addition of Hg2+ to a mixed solution of 2 gave significantly enhanced fluorescence at ∼576 nm via FRET with excitation at 343 nm. We also found that the pyrene excimer emissions formed by the intramolecular π−π interactions are more effective in obtaining strong FRET bands than those by intermolecular π−π interactions.
142 citations
TL;DR: The structural basis of selective excimer formation in the presence of Ag(+) ions presents a viable approach for ratiometric detection of these ions.
Abstract: This Communication describes a novel adenine−pyrene conjugate (1) and its solid-state structure with silver and copper ions. Single-crystal studies of metal complexes of 1 offer insight into molecu...
75 citations
TL;DR: Experimental results suggest that it is feasible to alter the type of excimer formed by anthracene units attached to the PAMAM dendrimers through altering the propensity for ground-state aggregation, and increased pi conjugation in the molecular framework of anthracenes-substituted PAMam d endrimers leads to intense and exclusive excimer emission from Anthracene at room temperature.
Abstract: Lower generations of polyamidoamine (PAMAM) dendrimers were peripherally modified with anthracene moieties, and excimer emission from anthracene chromophores was investigated in an acetonitrile-water mixture at acidic and basic pH values. Results from fluorescence spectroscopic experiments suggest that 1) the propensity of anthracene-modified PAMAM dendrimers to aggregate in acetonitrile is substantial in the presence of 15-20 vol % of water, and 2) aggregate formation in anthracene-modified PAMAM dendrimers leads to unique morphologies in the ground state, where the anthracene units are pre-arranged to form stable excimers upon photoexcitation. Three types of anthracene excimers are generated in the system, with face-to-face, angular, and T-shaped geometry. The formation of different types of anthracene excimers was confirmed by steady-state and time-resolved fluorescence spectroscopic experiments. Experimental results further suggest that it is feasible to alter the type of excimer formed by anthracene units attached to the PAMAM dendrimers through altering the propensity for ground-state aggregation. Most excitingly, increased pi conjugation in the molecular framework of anthracene-substituted PAMAM dendrimers leads to intense and exclusive excimer emission from anthracene at room temperature.
73 citations
TL;DR: Fluorescence measurements are used to probe the state of aggregation and dynamics of rearrangement of pyrene, Hostasol Yellow (HosY), and amphotericin B (AmpB) in NP cores and show stability of the microphase-separated fluorophores, which is consistent with the Flory-Huggins theory calculations.
Abstract: The state of aggregation of compounds, especially drugs, in the cores of nanoparticles (NPs) formed by rapid precipitation is a significant unresolved issue. The state can control the dissolution kinetics from the NP, bioavailability, and chemical stability of the compound. A block-copolymer-directed rapid precipitation process is used to form ≈100 nm NPs comprising mixtures of hydrophobic species including fluorescent probe molecules. Fluorescence measurements are used to probe the state of aggregation and dynamics of rearrangement of pyrene (Py), Hostasol Yellow (HosY), and amphotericin B (AmpB) in NP cores. The Flory-Huggins theory of mixing is used to predict the miscibility or phase separation of the fluorophores from the host NP core material (polystyrene, cholesterol, or polycaprolactone). For Py, excimer fluorescence shows an initial microphase separation in the polystyrene core. Over time the Py redistributes more uniformly with a decrease in excimer and increase in monomer fluorescence. The Flory-Huggins theory predicts the miscibility. For HosY, the fluorescence quenching is not time-dependent, thus indicating stability of the microphase-separated fluorophores, which is consistent with the Flory-Huggins theory calculations. For the drug compound AmpB, the amphiphilic character of the molecule creates unusual "anti-Ostwald" ripening behavior in which the size distribution decreases and narrows over time, and the fluorescence demonstrates an increased ordering in the NP core over time--opposite to the behavior observed for Py.
70 citations
70 citations
TL;DR: In this article, the authors examined the nature of excited states in Pt(II) organic complexes used as efficient phosphorescent emitters in organic light-emitting diodes (OLEDs).
58 citations
TL;DR: The close proximity of the pyrene labels in the complex induces pyrene excimer emission, which is observable by the unarmed eye, and Zn(2+)-cyclen-pyrene acts as luminescent chemosensor for PP(i) and fructose-1,6-bisphosphate in aqueous buffer.
Abstract: Pyrene labelled Zn2+-cyclen 1 and bis-Zn2+-bis-cyclen 2 complexes were synthesized. The reversible coordination at physiological pH of Zn2+-cyclens to phosphate anions and to imide moieties, as present in thymine and uracil nucleotides, is well known. In the presence of analytes bearing a phosphate and an imide or two phosphate groups the formation of a ternary complex consisting of two pyrene-labelled metal complexes and the analyte molecule, is observed. The close proximity of the pyrene labels in the complex induces pyrene excimer emission, which is observable by the unarmed eye. By this, the presence of UMP, UDP, UTP and TTP in buffered aqueous solution is signalled, while other nucleotides are not able to induce excimer emission. In the same way, Zn2+-cyclen-pyrene acts as luminescent chemosensor for PPi and fructose-1,6-bisphosphate in aqueous buffer.
48 citations
TL;DR: Results suggest that excimer formation between the pyrene-labeled ends is enhanced due to the branched nature of the dendrimer, which is consistent with the core-dense model.
Abstract: A series of pyrene-labeled dendrimers were prepared from generation n = 1 to n = 4 where the pyrenes were attached to the end groups of the dendrimers. Pyrene excimer formation was monitored by steady-state and time-resolved fluorescence spectroscopy as a function of generation number and in terms of the IE/IM ratio and the average rate constant of excimer formation . To account for the unconventional distribution of pyrene labels which were neither randomly distributed throughout the macromolecule nor limited to just two units which are the only two pyrene-labeling schemes that can be dealt with in a straightforward manner, a Model Free (MF) analysis was applied to the global analysis of the fluorescence decays. Within experimental error, the IE/IM ratios and obtained from, respectively, the steady-state fluorescence spectra and the time-resolved fluorescence decays were found to increase linearly with increasing generation number. This result is inconsistent with the fact that both the IE/IM rati...
46 citations
TL;DR: In this paper, the authors synthesized fluorescent copolymers of methyl methacrylate with 1-pyreneylmethyl methacylate with an atom transfer radical polymerization (ATRP).
Abstract: Well-defined fluorescent copolymers of methyl methacrylate with 1-pyreneylmethyl methacrylate were synthesized by atom transfer radical polymerization (ATRP). The random and block copolymer could be clearly distinguished by their glass-transition temperature (Tg) values, with a single Tg value (124 °C) for the random polymer, and two Tg values (115 and 158 °C) for the block copolymer. The emission spectra of the copolymers were different in excimer emission, allowing analysis of the ordering of the two polymers, by determining the ratio between excimer emission (IE) and monomer emission (IM). The fluorescence spectra of the random copolymer exhibited both monomer and excimer emission of pyrene with a IE/IM ratio of 1.20−1.39 at a concentration of 0.001−0.05 mg/mL. The block copolymer exhibited strong excimer emission with an emission quantum yield for the excimer (ΦE) of 42%. The IE/IM ratio from the block copolymer was >25, even in a very dilute solution. The ΦE value increased to 68% when the block copo...
43 citations
TL;DR: An amphiphilic asymmetric comb polymer with pendant pyrene groups and poly(N-isopropylacrylamide) (PNIPAM) side chains was synthesized based on click chemistry and reversible addition-fragmentation chain transfer polymerization and results all indicated successful synthesis of a well-defined comb polymer.
Abstract: An amphiphilic asymmetric comb polymer with pendant pyrene groups and poly(N-isopropylacrylamide) (PNIPAM) side chains was synthesized based on click chemistry and reversible addition−fragmentation chain transfer polymerization. Gel permeation chromatography, FTIR, and 1H NMR results all indicated successful synthesis of a well-defined comb polymer. The photophysical properties and self-assembly of the polymer in solution were studied by UV−vis spectroscopy, fluorescence technique, and transmission electron microscopy. The intensity ratio of the excimer peak (IE) to the monomer peak (IM) of the comb polymer in THF was used to monitor the formation of inter- or intramolecular excimers. At low polymer concentration, the value of IE/IM kept unchanged, indicating the formation of intramolecular excimer; at high polymer concentration, the value increased rapidly with concentration because of the formation of intermolecular excimer. The change of the intensity ratio of the first to the third vibronic band (I1/I...
TL;DR: IRIS can be efficiently performed in living corneal tissue and increasing the TPA of the tissue with Na-Fl increased both the scanning speeds and the magnitude of RI changes in a dose-dependent manner.
Abstract: Ultraviolet nanosecond excimer lasers and near infrared (NIR) femtosecond lasers are two forms of laser technology that have been adopted in ophthalmic practice for the purpose of altering corneal optics and cutting corneal flaps. Ultraviolet excimer lasers use single-photon absorption to ablate corneal tissue, changing the cornea's thickness and curvature and thus its refractive state.1,2 This technology takes advantage of the fact that the cornea naturally absorbs ultraviolet light to photoablate the tissue. Femtosecond lasers use NIR light that is naturally transmitted through the cornea with little one-photon absorption. To affect the cornea, femtosecond laser light must be focused inside the tissue. The resultant increase in laser intensity at the focal point causes localized, nonlinear, multiphoton absorption, allowing for a range of modifications within the tissue; because the absorption is nonlinear, the surrounding tissue is left virtually unaffected.3–5 NIR femtosecond lasers are now used clinically for corneal flap cutting.6–12 Such lasers use low-repetition-rate (kilohertz to several megahertz range) pulses that induce photodisruption and optical breakdown of the corneal tissue, accompanied by high-density microplasma and bubbles. The layer of damaged tissue created by the laser can then be used to separate a tissue flap from the rest of the cornea.
In a very different set of applications, NIR femtosecond lasers have also gained popularity as powerful tools for micromachining patterns into different, mostly nonbiological materials.13–17 When the femtosecond pulses are tightly focused into transparent bulk materials, the resultant nonlinear absorption and highly localized energy deposition alter the material's properties, allowing for the fabrication of different components and devices such as gratings, waveguides, and photonic crystals.18–24 The changes seen in these materials are usually the result of laser-induced two-photon polymerization,18–24 and recent research has shown that this polymerization process can be enhanced by doping bulk materials with photoinitiators or chromophores that have large two-photon absorption (TPA) cross sections.18–20,25
In 2008, our group showed for the first time, that it is possible to use a low-pulse-energy femtosecond laser to induce low-scattering-loss RI changes in lightly fixed, postmortem corneal and lens tissues.26 These modifications were attained with a 27-fs pulsed laser in the NIR (800 nm) with pulse energies titrated to fall below the optical breakdown threshold of the tissue (<0.5 nJ). This process, termed intratissue refractive index shaping (IRIS), causes long-standing refractive index changes that range between 0.005 and 0.01 in the cornea and 0.015 and 0.021 in the lens.26 However, these changes are achievable only with a slow scanning speed of 0.7 μm/s, which makes realistic clinical application of this method unlikely. Recently, Ding et al.18 reported that both coumarin-1 and fluorescein can be used to enhance the scanning speed and magnitude of RI changes attainable during micromachining in hydrogels, which like the cornea, possess a relatively high water content. Fluorescein is of particular interest, because it is already commonly used in ophthalmic practice for the identification of corneal abrasions and epithelial defects.27 It has also been shown to be safe when injected systemically to visualize retinal vasculature leaks and other abnormalities.27
Thus, in the present study, our goal was to test two hypotheses: (1) that IRIS can be performed in living, unfixed corneas with greater effectiveness than in our previous report of this phenomenon in fixed postmortem corneas, and (2) that the effects of IRIS can be significantly enhanced if the TPA capability of the living tissue is increased.
TL;DR: A new anthracene-based ligand shows emission shift and decrease of excimer band upon coordination to silver ions when crystallized from methanol when transformed into a second crystalline compound with excimer emission similar to the ligand alone.
TL;DR: The [6.6](9,10)anthracenophane 1 (Scheme 1) is a selective fluoroionophore for the detection of PdCl(2) with a large fluorescence enhancement factor (I/I(0) > 250).
TL;DR: The TDR spectra revealed the excitation-relaxation process of the biphenylylene moieties, including the relaxation of the twisted Frank-Condon (FC) state to the lowest singlet excited state (S(1)) with a time constant of 730 ± 95 fs, and efficient quenching of the S(1) state by excimer (E) formations with two time constants.
Abstract: The excited state dynamics of periodic mesoporous organosilica (powder) bearing biphenylylene moieties densely in the silica framework (Bp-PMO) is investigated for the first time using femtosecond time-resolved diffuse reflectance (TDR) and picosecond time-resolved fluorescence spectroscopies. The TDR spectra revealed the excitation-relaxation process of the biphenylylene moieties, including the relaxation of the twisted Frank-Condon (FC) state to the lowest singlet excited state (S1) with a time constant of 730 ± 95 fs, and efficient quenching of the S1 state by excimer (E) formations with two time constants of 7.0 ± 0.2 ps (E1: ca. 64%) and 170 ± 47 ps (E2 and E3: ca. 36%). The individual absorption spectra of the FC, S1, and E states were reconstructed by the TDR spectral analysis. The time-resolved fluorescence spectra showed that the excimers decayed with three time constants of 1.3 ± 0.2 ns (E1), 8.2 ± 0.7 ns (E2) and 27 ± 2 ns (E3). The fluorescence quantum yields of the excimers are suggested to be almost zero for E1, and unity for E2 and E3, which implies that the fluorescence quantum yield of Bp-PMO (ΦF = 0.38) can be explained by the fraction of the highly emissive excimers (E2 and E3). The excited state dynamics of Bp-PMO is quite different from those of a solution of 4,4′-bis-(triethoxysilyl)biphenyl precursor and a biphenyl molecular crystal (powder).
TL;DR: In this article, a series of (aminomethyl)anthracene derivatives with single alkyl chain substituent (octyl, dodecyl, octadecyl and p-butylaniline) were synthesized and self-assembly of the compounds was analyzed in dichloromethane and solid thin film.
Abstract: A series of (aminomethyl)anthracene (AMA) derivatives with single alkyl chain substituent (octyl, dodecyl, octadecyl and p-butylaniline) were synthesized and self-assembly of the compounds was analyzed in dichloromethane and solid thin film. Upon heat treatment of the solid thin films, the AMA derivatives exhibit plastic crystalline phase which can be preserved in a glassy state through rapid cooling of the system to room temperature. In the glassy state, the AMA derivatives are arranged in rectangular discotic columnar fashion, where the face-to-face distance between two anthracene moieties is 3.9 A. Steady state and time resolved fluorescence studies were carried out in dichloromethane, which indicate that a fraction of the AMA derivatives form anthracene excimer, upon excitation at 360 nm. Conversely, photoexcitation of the solid thin film at 360 nm leads to the exclusive formation of anthracene excimers due to the close proximity of anthracene units in the discotic columnar arrangement. Furthermore, the self-assembled AMA derivatives were utilized as effective templates for in situ prepared Ag nanoparticles. While the AMA derivatives stabilize the Ag nanoparticles effectively, the excimer emission intensity was quenched in the presence of silver nanoparticles. The experimental results from the present study provide a unique approach for generating anthracene excimer emission from solid thin films at room temperature, which is a rare observation.
TL;DR: Two series of geometrically-related dyads are discussed based on the difluoroborondipyrromethene (Bodipy) unit, and incorporating covalently attached hydroquinone/quinone groups, which show various levels of fluorescence depending on the oxidation state of the appended group and the substitution pattern.
Abstract: Two series of geometrically-related dyads are discussed based on the difluoroborondipyrromethene (Bodipy) unit, and incorporating covalently attached hydroquinone/quinone groups. These units are anchored directly, or via a phenylene spacer, to the Bodipy core at the meso position in one series (BD-MHQ, BD-MQ, BD-MPHQ, BD-MPQ), but for the second series the attachment site is the 2-position (BD-SHQ, BD-SQ, BD-SPHQ, BD-SPQ). The compounds show various levels of fluorescence depending on the oxidation state of the appended group and the substitution pattern. In non-polar solvents such as toluene, diethyl ether and dichlorobenzene, the S(1) state deactivation of the Bodipy unit in BD-SPQ and BD-MPQ is dominated by (1, 3)exciplex formation, which has not been reported for Bodipy derivatives so far. In the latter molecule, the decay of the exciplex is divided between population of the Bodipy triplet state (13 %-21 %) and ground state reformation. This partitioning is not seen for the side-on substituted derivative, BD-SPQ, and only ground state reformation is observed following decay of the exciplex. This difference in behavior is explained by the radical-pair inter-system-crossing mechanism, which more effectively operates in BD-MPQ because of the orthogonality of the donor-acceptor units. In the more polar solvent CH(3)CN all the quinone derivatives show fast formation of the charge-separated state (k(CS)) followed by slower charge recombination (k(CR)). The ratio k(CS)/k(CR)
TL;DR: A femtosecond pump-probe mass and photoelectron spectra for adenine (A) and microhydrated A(m)(H(2)O)(n) clusters are presented and a nanosecond lifetime from excimer states in pi-stacked clusters is observed.
Abstract: We present femtosecond pump–probe mass and photoelectron spectra for adenine (A) and microhydrated Am(H2O)n clusters. Three distinct relaxation processes of photoexcited electronic states were distinguished: in unhydrated A, relaxation of the optically bright ππ* state occurred via the dark nπ* state with respective lifetimes of <0.1 and 1.3 ps. In microhydrated clusters A(H2O)n, relaxation via the nπ* state is quenched by a faster relaxation process, probably involving πσ* states. For the predominantly hydrogen-bonded adenine dimer (A2), excited state relaxation is dominated by monomer-like processes. When the adenine dimer is clustered with several water molecules, we observe a nanosecond lifetime from excimer states in π-stacked clusters. From the electron spectra we estimate adiabatic ionization potentials of 8.32 eV (A), 8.27 eV (A(H2O)1), 8.19 eV (A(H2O)2), 8.10 eV (A(H2O)3), 8.18 eV (A2), and 8.0 eV (A2(H2O)3–5).
TL;DR: Four new linear oligoesters containing a diphenylacetylene unit were prepared by fragment coupling sequences and the ion channel forming ability of the compounds was investigated, establishing that all compounds are able to form channels, but that regular step changes in conductance depend on the location of the Diphenyl acetylene unit within the oligoester and on the electrolyte.
Abstract: Four new linear oligoesters containing a diphenylacetylene unit were prepared by fragment coupling sequences and the ion channel forming ability of the compounds was investigated. Activity in vesicles was very strongly controlled by overall length; the longest compound was effectively inactive. Planar bilayer studies established that all compounds are able to form channels, but that regular step changes in conductance depend on the location of the diphenylacetylene unit within the oligoester and on the electrolyte. The intrinsic fluorescence of the diphenylacetylene unit was used to probe aggregation and membrane localization. Both monomer (320 nm) and excimer (380 nm) emissions are quenched by copper ions; quenching of the excimer emission from an aqueous aggregate is very efficient. Time-dependent changes in the intensities of monomer and excimer emission show slow transfer of diphenylacetylene units from an aqueous aggregate to a membrane-bound monomer with subsequent growth of emission from a membrane-bound excimer. The latter species is not quenched by aqueous copper ions. The implications of these species and processes for the mechanism of ion channel formation by simple oligoesters are discussed.
TL;DR: In this article, the response to ultrashort laser pulses of two stacked thymine molecules has been studied by semiclassical dynamics simulation with laser radiation explicitly incorporated, and the simulation finds that the deformation of the pyrimidine ring plays an important role in cleavin...
Abstract: Response to ultrashort laser pulses of two stacked thymine molecules has been studied by semiclassical dynamics simulation with laser radiation explicitly incorporated. The laser pulses used to excite the thymine molecule have a 25 (fwhm) fs Gaussian shape with a photon energy of 4.0 eV. Simulation follows two different reaction paths produced by the laser pulses with two different fluences. In one reaction, the stacked thymine molecules form a cyclobutane pyrimidine dimer, which is the main course of photoinduced DNA damage, and the formation of two chemical bonds linking two thymines occurs nonsynchronously after the excimer decays to electronic ground state. In the other reaction, only one bond is formed between the two thymine molecules. In the second reaction, the bond breaks about 50 fs after formation, and then the two molecules move away from each other. This reaction leads to the DNA damage repair. The simulation finds that the deformation of the pyrimidine ring plays an important role in cleavin...
TL;DR: The results provide a theoretical basis for the effectiveness of UVB irradiation, including excimer light and excimer laser, in the treatment of vitiligo.
Abstract: Narrow-band ultraviolet B (NB-UVB; 310-315 nm) is commonly used to treat vitiligo, but the mechanisms underlying the therapeutic effect have not been understood completely. Here, we evaluated the effects of broadband UVB (BB-UVB), NB-UVB, 308 nm xenon chloride (XeCl) excimer laser, and 308 nm XeCl excimer light on the expression of endothelin-1 (ET-1) in a cultured human epidermal tissue model. Our results provide a theoretical basis for the effectiveness of UVB irradiation, including excimer light and excimer laser, in the treatment of vitiligo.
TL;DR: In this paper, bichromophoric pyrene probes with different linkage between pyrene units were compared in chloroform, cyclohexane and methanol, as well as in the polymer matrices polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC).
Abstract: Spectral properties of bichromophoric pyrene probes with different linkage between pyrene units were compared in chloroform, cyclohexane and methanol, as well as in the polymer matrices polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC). The derivatives under study were pyrene, di(1-pyrenylmethyl) ether (DiPyM), 4-(1-pyrenemethoxymethyl) phenylethene (PyS) and 2,3-di(4-(1-pyrenemethoxymethyl)phenyl)butane (DiPyS). The absorption spectra of the probes exhibited well-resolved bands that were only slightly influenced by substitution and medium. Parent pyrene and PyS, exhibited well-resolved monomer fluorescence in all media, which indicated a weak solvent effect. DiPyM and DiPyS exhibited strong excimer fluorescence in solutions of low viscosity. The influence of the viscosity on the ratio of excimer and monomer emission Φ exc / Φ em of DiPyS and DiPyM in diethylene glycol was also studied. In polymer matrices, only monomer emission of DiPyM and DiPyS was observed. However, the lifetime of monomer emission was influenced by the medium.
TL;DR: The structural and ultrastructural evaluation of corneas treated with ablation frequencies of 200 Hz, 500 Hz, and 1000 Hz showed no specific side effects associated with the high repetition rates, indicating the clinical potential in refractive surgery of excimer lasers with a repetition rate of 1000 Hz.
Abstract: Purpose To evaluate the possible side effects on human and porcine cadaver eyes induced by excimer laser ablation with 3 ablation frequencies Setting Augenklinik, Klinikum rechts der Isar, Technische Universitaet Muenchen, Munich, Germany Methods Central epithelial abrasion was performed on porcine and human donor eyes Next, the porcine eyes were photoablated (−900 diopters) at 1 of 3 frequencies (200 Hz, 500 Hz, or 1000 Hz) using a prototype 1000 Hz excimer laser The human eyes were ablated at 1000 Hz The surface of the stroma as well as the structure and ultrastructure of the corneal cells and stroma were examined using light microscopy, transmission electron microscopy, and scanning electron microscopy (SEM) Special attention was given to the detection of potential thermal damage and the evaluation of corneal smoothness Results Histopathologic examination showed very low to almost no differences between the 3 repetition rates In all cases, SEM showed an equally smooth surface Conclusions The structural and ultrastructural evaluation of corneas treated with ablation frequencies of 200 Hz, 500 Hz, and 1000 Hz showed no specific side effects associated with the high repetition rates The ablation quality was comparable in the 3 frequency groups However, the treatment time was significantly less with a high repetition rate, indicating the clinical potential in refractive surgery of excimer lasers with a repetition rate of 1000 Hz Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned Additional disclosures are found in the footnotes
TL;DR: Pulsed excimer lasers are the by far strongest and most efficient laser sources in the ultraviolet spectral region as mentioned in this paper, and they have a lateral resolution of 1 μm, which is achievable with excimer-laser based processing systems as well as a large field size covered within each laser shot.
Abstract: Pulsed excimer lasers are the by far strongest and most efficient laser sources in the ultraviolet spectral region. On account of the unique lateral resolution of 1 μm, which is achievable with excimer-laser based processing systems as well as of the large field size covered within each laser shot, excimer lasers are indispensable laser sources in pulsed-laser deposition, high-precision marking, low-temperature surface annealing and large-area micropatterning.
TL;DR: Evidence from (1)H NMR and variable temperature NMR (VT-NMR) of 1 in comparison to model compounds bearing only one triphenylene unit suggests that there is an equilibrium between the open conformer and the intramolecularly pi-pi interacting closed conformer in solution.
Abstract: A diacetylenic molecular hinge bearing two ethynyltriphenylene units (1) has been synthesized. Evidence from 1H NMR and variable temperature NMR (VT-NMR) of 1 in comparison to model compounds bearing only one triphenylene unit suggests that there is an equilibrium between the open conformer and the intramolecularly π–π interacting closed conformer in solution (equilibrium constant K = 6.5 at 298 K in CDCl3) arising from the rotation of the diacetylenic hinge. Unusual fluorescence emission observed from 1 has been assigned to excimer formation arising from intramolecularly π–π interacting triphenylene units in the excited state. Steady state and picosecond time resolved fluorescence spectra of 1 were nearly identical and corresponded to intramolecular excimer emission.
TL;DR: The results suggest that optical excitation establishes an equilibration between two molecular states of different spectroscopic properties and lifetimes that are coupled only via the excited state as a gateway.
Abstract: The photophysics of pyrene attached to an adenine base within RNA single strands and duplexes is examined with respect to the position of the pyrene within the strand and the number of pyrenes attached to one duplex. Compounds with pyrenes intercalating sequence specifically are examined, as well as a doubly modified compound, where the two pyrenes are located close enough to each other for significant excimer interaction. Femtosecond transient absorption measurements and time correlated single photon counting measurements allow a thorough examination of the local influences on the pyrene photophysics. Our results suggest that optical excitation establishes an equilibration between two molecular states of different spectroscopic properties and lifetimes that are coupled only via the excited state as a gateway. One of them is a neutral pyrene-adenine excited state, S*, while the second one is connected to an excited charge transfer state, S*(CT). In all compounds, an ultrafast sub-ps decay from a higher excited state into the lowest excited state S* occurs, and an excited charge transfer species S*(CT) is formed within picoseconds. The fluorescence behavior of the pyrene-modified adenine, however, is strongly dependent on RNA conformation. Both S* and S*(CT) states are fluorescent, and decay within hundreds of picoseconds and approximately 2 ns, respectively. The ratio between S* and S*(CT) fluorescence depends strongly on pyrene intercalation, and it is found that the S* state is quenched selectively upon intercalation of the pyrene into RNA. The doubly modified duplex exhibits an additional fluorescent state with a lifetime of 18.7 ns, which is associated with the pyrene excimer state. This state coexists with a significant population of the pyrene monomer, since the characteristic features of the latter can still be observed. Formation of the excimer occurs on femtosecond time scales. The pyrene label thus provides a sensitive tool to monitor the local structural dynamics of RNA with the chromophore acting as a molecular beacon.
TL;DR: Evaluating endothelial changes after excimer laser-assisted lamellar keratoplasty showed that the Corneal Lamellar Ablation for Transplantation procedure induced mild changes to the endothelium in corneas, but serious damage in one eye with a residual bed thickness of 72 microm.
TL;DR: In this paper, a high-pressure argon plasma excited by a highcurrent pulsed volume discharge was investigated and it was found that the spectrum of the UV-visible photorecombination continuum is sensitive to discharge constriction.
Abstract: Results of investigation of high-pressure argon plasma excited by a high-current pulsed volume discharge are presented. The plasma diagnostics employs spatiotemporal dependences of the emission intensity in the VUV — visible range. A homogenous discharge is observed at pressures up to 10 atm. It is found that the spectrum of the UV — visible photorecombination continuum is sensitive to the discharge constriction. Change in the shape of the spectrum is caused bythe change of the type of positive charge carriers upon passing of the discharge from the uniform phase (molecular Ar2+ ions) to the arc phase (atomic Ar+ ions). Experimental data and model calculations show that the electron heating after the main excitation pulse is a highly undesirable process. It slows down the recombination flow in the plasma, which results in stretching of all the kinetic processes for all excited components in time, and hence in a decrease in the peak values of their concentrations. Electron collision-induced mixing effi-ciently converts the reservoir of long-lived Ar2* molecules in the triplet state into rapidly emitting singlet excimers. It is this mechanism that dominates the production of singlet Ar2* excimer molecules. The threshold concentration needed to obtain lasing at a wavelength of 127 nm on Ar2* excimers (1Σ+u(v=0)) was, according to calculations, about 5×1015 cm-3 for the gain 0.05 cm-1. This concentration can be achieved in the case of homogeneous pulsed discharge pumping with the peak electron concentration 2.×1016 cm-3 at the argon pressure 10 atm.
TL;DR: In this paper, single microcrystals of pyrene have been studied by steady state and time resolved fluorescence microscopy, and the fluorescence spectra of micro-crystals exhibit vibrational structure unlike the broad spectrum observed in pyrene excimer.
TL;DR: Intramolecular excimer formation of di-9H-fluoren-9-yldimethylsilane in various solvents was studied by means of steady-state and time-resolved fluorescence spectroscopies, revealing that excitation energy is significantly decreased with decreasing the interchromophore separation and the dihedral angle in a fluorene dimer, as an indication that the excimer state emitting the significantly red-shifted fluorescence corresponds to a sandwich
Abstract: Intramolecular excimer formation of di-9H-fluoren-9-yldimethylsilane in various solvents was studied by means of steady-state and time-resolved fluorescence spectroscopies. Solvent viscosity effect on the kinetics for the monomer-excimer transition was found to be very slight, indicating that the transformation is accompanied by a slight conformational change presumably involving a transition from a near face-to-face conformer to a true sandwich conformer. The excitation energies and the corresponding oscillator strengths were derived with the time-dependent B3LYP/cc-pVTZ method. The simulated absorption spectrum involving the variation of the oscillator strengths in terms of the wavelengths accords with the experimental absorption pattern. The TD DFT calculations also reveal that excitation energy is significantly decreased with decreasing the interchromophore separation and the dihedral angle in a fluorene dimer, as an indication that the excimer state emitting the significantly red-shifted fluorescence corresponds to a sandwich conformer.