Showing papers on "Excimer published in 2007"

Calix[4]arene-based, Hg2+ -induced intramolecular fluorescence resonance energy transfer chemosensor.

Abstract: A novel calix[4]arene-based chemosensor 1 based on Hg2+-induced fluorescence resonance energy transfer (FRET) was synthesized, and its sensing behavior toward metal ions was investigated by UV/vis and fluorescence spectroscopies. Addition of Hg2+ to a CH3CN solution of 1 gave a significantly enhanced fluorescence at ∼575 nm via energy transfer (FRET-ON) from the pyrenyl excimer to a ring-opened rhodamine moiety. In contrast, addition of Al3+ induced a distinct increase of pyrenyl excimer emission (∼475 nm), while no obvious FRET-ON phenomenon was observed. Different binding behaviors of 1 toward Hg2+ and Al3+ were also proposed for the interesting observation.

Intermolecular Hydrogen Bonds Induce Highly Emissive Excimers: Enhancement of Solid-State Luminescence

Abstract: Aggregation induced emission (AIE) is an amazing property for light emitting materials and has attracted much attention. Here, we report a new kind of AIE materials: fluorenone derivates 2,7-dip-tolyl-fluorenone (DTFO) and 2,7-bis(4-(tert-butylthio)phenyl)-fluorenone (DSFO). Strong light emissions with a large Stokes shift and long lifetime in the solid state originate from the formation of excimers. The crystal structure of DSFO shows that every two molecules are bound together even in the ground state by intermolecular hydrogen bonds and form a particular dimer. When this dimer is excited, it turns into an excimer without arrangement adjustment and likewise without repulsive interactions when the excimer decays back to the dimer; so, the nonradiative decay pathways that exist in common excimers are greatly reduced and thus induce a strongly enhanced luminescence in the solid state. OLED devices employing DTFO as light emitting layers are fabricated and evaluated.

A new pyrene-based fluorescent probe for the determination of critical micelle concentrations.

Abstract: A new pyrene-based fluorescent probe for the determination of critical micelle concentrations (CMC) is described. The title compound 1 is obtained in five steps, starting from pyrene. Fluorescence spectroscopic properties of 1 are studied in homogeneous organic solvents and aqueous micellar solutions. In a wide range of organic solvents, probe 1 exhibits a characteristic monomer emission of the pyrene fluorophore, with three distinct peak maxima at 382, 404, and 425 nm. The spectra change dramatically in aqueous solution, where no monomer emission of the pyrene fluorophore is detected. Instead, only strong excimer fluorescence with a broad, red-shifted emission band at lambda(max) = 465 nm is observed. In micellar aqueous solution, a superposition of the monomer and excimer emission is found. The appearance of the monomer emission in micellar solution can be explained on the basis of solubilization of 1 by the surfactant micelles. The ratio of the monomer to excimer fluorescence intensities of 1 is highly sensitive to changes in surfactant concentration. This renders 1 a versatile and sensitive probe molecule for studying the micellization of ionic and nonionic surfactants. For a representative selection of common surfactants, the critical micelle concentrations in aqueous solution are determined, showing excellent agreement with established literature data.

Combination of 308-nm xenon chloride excimer laser and topical calcipotriol in vitiligo

Abstract: BACKGROUND: A large variety of therapeutic agents are being used for the treatment of vitiligo, but treatment remains a challenge. Recently, monochromatic phototherapies such as 311-nm narrowband ultraviolet B therapy and 308-nm xenon chloride excimer laser have been reported to be an effective and safe therapeutic option in children and adult patients with vitiligo. Single reports stipulate that the addition of topically applied calcipotriol to phototherapy increases its effectiveness. OBJECTIVE: The purpose of the present pilot study was to determine if the addition of topical calcipotriol increases the efficacy of the 308-nm xenon chloride excimer in the treatment of vitiligo. METHODS: Ten patients with vitiligo with essentially bilateral symmetrical lesions were enrolled in this prospective right/left comparative, single-blinded trial conducted over a 15-month period. All patients received 308-nm XeCl excimer laser therapy three times weekly. Calcipotriol ointment (Daivonex) was applied to lesions on one side of the body twice daily. RESULTS: After 24 treatments (8 weeks), nine patients were evaluated. Eight patients showed evidence of repigmentation on both body sides, with no significant difference between the body side treated with calcipotriol and excimer laser and the side treated with excimer laser alone. The mean repigmentation rate was 22.4% (1-37%). CONCLUSION: The addition of calcipotriol ointment to 308-nm xenon chloride excimer laser phototherapy does not significantly enhance its efficacy. Small additive effects must be investigated in a larger trial.

Intramolecular excimer formation and delayed fluorescence in sterically constrained pyrene dimers.

Abstract: The synthesis is described for a series of five molecular dyads comprising pyrene-based terminals covalently linked through a 1,3-disubstituted phenylene spacer. The extent of through-space communication between the pyrene units is modulated by steric interactions imposed by bulky moieties attached at the 6,8-positions of each pyrene unit. For the control compound, only hydrogen atoms occupy the 6,8 positions (DPI), whereas the remaining compounds incorporate ethynylene groups terminated with either triisopropylsilyl (DP2), 1-tert-butylbenzene (DP3), 2,6-di-tert-butylbenzene (DP4) or 1 -tert-butyl-3,5-dimethylbenzene (DP5) units. Each compound shows a mixture of monomer and excimer fluorescence in fluid solution at room temperature, but only monomer emission in a glassy matrix at 77 K. The ratio of monomer to excimer fluorescence depends markedly on the molecular structure; DPI is heavily biased in favour of the excimer and DP4 is enriched with monomer fluorescence. Photophysical properties, including laser induced and delayed fluorescence data, are reported for each compound. Delayed fluorescence occurs by both intramolecular and bimolecular steps, but these events take place on different timescales. The possibility is raised for using intramolecular triplet-triplet annihilation as a means of molecular imaging.

Dynamics of short as compared with long poly(acrylic acid) chains hydrophobically modified with pyrene, as followed by fluorescence techniques

Abstract: New low and high molecular weight poly(acrylic acid), PAA, 2000 g mol−1 and 450 000 g mol−1, respectively, were tagged with pyrene (low and high contents of probe) and its behaviour in solution was investigated using absorption and fluorescence (steady-state and time-resolved) techniques. Fluorescence data shows that the degree and level of intramolecular association strongly depends on the molecular weight. With the short pyrene-labeled PAA chains in aqueous solution, the excimer-to-monomer fluorescence ratio IE/IM decreases with the increase of pH, oppositely to the increase in the IE/IM ratio with the increase in pH previously observed with the long chain PAA. Time-resolved data suggest that excimer formation with the short pyrene-labeled PAA polymers (ca. 28 acrylic acid monomers per chain) in water is largely due to excitation of Ground State Dimers, GSD. The increment of pH, and the consequent gradual ionization of the carboxylic groups in the chain, initially increases the fraction of GSD, possibly due to the occurrence of special micelle-like chain conformations, inside which the pyrene units are accommodated. A further increase of the pH above the pKa values, resulting in the full ionization of carboxylic groups, apparently destabilizes such chain conformations, which leads to a pH effect on the photophysical properties identical to that of the long chain polymers. In water, the dynamic data shows the existence of two excimers coexisting with two monomer classes. In methanol and dioxane (good solvents for the pyrene probe) at room temperature, where one excimer and two monomers are present, all rate constants could be obtained, as well as the fractions of ground-state species. It is thus shown that different types of interactions are produced with small- and long-sized PAA polymers, i.e., the size of the polymer matters.

Direct current planar excimer source

Abstract: Excimer emission at 172?nm was observed from xenon discharges generated between a perforated anode, with opening dimensions in the sub-millimetre range, and a planar cathode. A thin dielectric layer 100?250??m in thickness, with the same size opening as the anode, is aligned with the anode opening and used to separate the electrodes. Devices with this structure are referred to as cathode boundary layer (CBL) discharge or micro-hollow cathode discharge devices, depending on the surface structure of the cathode. The emission intensity and efficiency of these devices are pressure- and current-dependent. Typical power densities and internal efficiencies (ratio of excimer radiant power to electrical input power) are 0.5?1.5?W?cm?2 and 3?5%, respectively. In the current range between normal and abnormal mode operation, the CBL discharge shows regularly arranged filaments (self-organization). Optimum emission of the excimer radiation is observed at the transition from the normal glow mode to self-organization. The resistive current?voltage characteristic in the self-organization region allows the operation of multiple CBL devices in parallel without individual ballast, but with an excimer emission slightly off the maximum value. The measured decrease of the excimer emission to about 10% of its initial value after approximately 250?h of continuous operation seems to be caused by the increasing contamination of xenon, through minor leaks in the discharge chamber and/or the outgassing of chamber components. Refilling the chamber with fresh gas after such an extended operation resulted in full recovery of the discharge with respect to excimer emission. The results suggest the possibility of generating extended lifetime, intense, large area, planar excimer sources using CBL discharges in sealed discharge chambers including getters.

Correlating Pyrene Excimer Formation with Polymer Chain Dynamics in Solution. Possibilities and Limitations

Abstract: Four types of pyrene-labeled polystyrene samples (Py−PS) were prepared and the process of excimer formation between the pyrene labels was characterized by steady-state and time-resolved fluorescence to assess the effect the mode of pyrene incorporation into a polymer has on the kinetics of excimer formation. The pyrene label was incorporated into the PS backbone by either (1) reacting 1-pyrenemethoxide with a chloromethylated polystyrene backbone to yield the GrE−PS series, (2) copolymerizing styrene with 4-(1-pyrenyl)methoxymethylstyrene to yield the CoE−PS series, (3) copolymerizing styrene with N-(1-pyrenylmethyl)acrylamide to yield the CoA−PS series, or (4) polymerizing α,ω-dicarboxyl end-capped polystyrenes with l-lysine-1-pyrenemethylamide dihydrochloride to yield the ES−PS series. Steady-state and time-resolved fluorescence experiments demonstrated that the long and flexible linker of GrE−PS and CoE−PS enabled more efficient excimer formation than the short and rigid linker of CoA−PS, and that spac...

Effects of metal cation coordination on fluorescence properties of a diethylenetriamine bearing two end pyrene fragments.

Abstract: Fluorescence properties of a diethylenetriamine bearing two end pyrene fragments (L) have been studied in water, where effects of adding metal cations (Zn2+, Cd2+, Cu2+, Hg2+, Ag+) on the emission properties of L have been studied. Without metal cations, L shows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission intensity (IM) is strong at acidic pH but decreases with a pH increase because of an electron transfer (ET) from the unprotonated nitrogen atoms to the excited pyrene fragment. The excimer emission is due to the static excimer formed via a direct photoexcitation of the intramolecular ground-state dimer (GSD) of the end pyrene fragments. The excimer emission intensity (IE) is weak at acidic pH but increases with a pH increase because of the GSD stability increase associated with the deprotonation of the polyamine chain. Addition of metal cations leads to IM decrease, where chelation-driven IM enhancement does not occur even with diamagnetic Zn2+ and Cd2+ at any...

Solvent-driven multiply configurable on/off fluorescent indicator of the pH window: a diethylenetriamine bearing two end pyrene fragments.

Abstract: Fluorescence behaviors of a simple-structured molecule (L), a diethylenetriamine bearing two end pyrene fragments, have been investigated in water. Effects of adding a less-polar organic solvent (acetonitrile: MeCN) on the emission behaviors have been studied by means of steady-state and time-resolved fluorescence measurements. L dissolved in water shows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission shows an "on-off" intensity profile against the pH window (pH 2-12), whereas the excimer emission shows an "off-on" profile. Upon MeCN addition, the monomer emission maintains the "on-off" profile. In contrast, the "off-on" profile of the excimer emission is drastically changed: L shows two more types of profiles, "off-on-off-on" and "off-on-off", along with the MeCN concentration increase, thus behaving as a multiply configurable fluorescent indicator of the pH window. The MeCN-driven excimer emission switching of L is triggered by (i) the decrease in stability of the intramolecular ground-state dimer (GSD) formed between the end pyrene fragments, which suppresses the direct photoexcitation of GSD (suppression of the "static" excimer formation), leading to a decrease in the excimer emission intensity at basic pH; and (ii) the decrease in polarity of solution, which allows formation of a "dynamic" excimer via a monomer-to-excimer transition, resulting in an enhancement of the excimer emission intensity at acidic-neutral pH.

Electric Field Effects on Absorption and Fluorescence Spectra of Trimethylsilyl- and Trimethylsilylethynyl-Substituted Compounds of Pyrene in a PMMA Film

Abstract: External electric field effects on absorption and fluorescence spectra of 1,3,6,8-tetrakis(trimethylsilyl)pyrene and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TMSPy and TMS(E)Py, respectively) have been examined in a poly(methyl methacrylate) (PMMA) film at various concentrations at various temperatures. TMS(E)Py preferentially forms an aggregate in a PMMA film, as the concentration increases, indicating that the acetylenic groups enhance the π−π interactions between pyrene molecules. The change in molecular polarizability following excitation has been determined both for the monomer and for the aggregate, based on the electroabsorption spectra. The change in molecular polarizability following emission has also been determined in both compounds, based on the electrofluorescence spectra. TMSPy exhibits two excimer fluorescence emissions at high concentrations which are ascribed to the partially overlapping excimer and the sandwich-type excimer, respectively, besides the monomer fluorescence emitted fr...

Excited-state interactions in flurbiprofen-tryptophan dyads.

Abstract: Fluorescence and laser-flash photolysis measurements have been performed on two pairs of diastereomeric dyads that contain the nonsteroidal anti-inflammatory drug (S)- or (R)-flurbiprofen (FBP) and (S)-tryptophan (Trp), which is a relevant amino acid present in site I of human serum albumin. The fluorescence spectra were obtained when subjected to excitation at 266 nm, where approximately 60% of the light is absorbed by FBP and approximately 40% is absorbed by Trp; the most remarkable feature observed in all dyads was a dramatic fluorescence quenching, and the residual emission was assigned to the Trp chromophore. In addition, an exciplex emission was observed as a broad band between 380 and 500 nm, especially in the case of the (R,S) diastereomers. The fluorescence lifetimes (tauF) at lambdaem=340 nm were clearly shorter in the dyads than in Trp-derived model compounds; in contrast, the values of tauF at lambdaem=440 nm (exciplex) were much longer. On the other hand, the typical FBP triplet-triplet transient absorption spectrum was obtained when subjected to laser-flash photolysis, although the signals were less intense than when FBP was directly excited under the same conditions. The main photophysical events in FBP-Trp dyads can be summarized as follows: (1) most of the energy provided by the incident radiation at 266 nm reaches the excited singlet state of Trp (1Trp*), either via direct absorption by this chromophore or by singlet singlet energy transfer from excited FBP (1FBP*); (2) a minor, yet stereoselective deactivation of 1FBP* leads to detectable exciplexes and/or radical ion pairs; (3) the main process observed is intramolecular 1Trp* quenching; and (4) the first triplet excited-state of FBP can be populated by triplet-triplet energy transfer from excited Trp or by back-electron transfer within the charge-separated states.

White light emission from single layer poly (n-vinylcarbazole) polymeric light-emitting devices by mixing singlet and triplet excimer emissions.

Abstract: White light electroluminescence (EL) was obtained by mixing emission from singlet and triplet excimers from a single poly (n-vinylcarbazole) (PVK) spin cast layer after irradiation of the solution with UV light. With increased UV light irradiation, the intensity from the triplet excimer (red-630 nm) of PVK increased compared with that of the singlet excimer (blue-460 nm) due to an increased population of both adjacent benzene rings being aligned with one another (fully overlapping) versus only one of the adjacent benzene rings being aligned (partially overlapping). The emission color changed from blue to white with increased UV irradiation time while the EL brightness and current density decreased and the turn-on voltage increased.

Synthetic quartz glass substrate for excimer lasers and making method

Abstract: When a synthetic quartz glass substrate is prepared from a synthetic quartz glass block, (I) the block has a hydrogen molecule concentration of 5×1017-1×1019 molecules/cm3, (II) the substrate has a hydrogen molecule concentration of 5×1015-5×1017 molecules/cm3, (III) the substrate has an in-plane variation of its internal transmittance at 193.4 nm which is up to 0.2%, and (IV) the substrate has an internal transmittance of at least 99.6% at 193.4 nm. The synthetic quartz glass substrate has a high transmittance and a uniform transmittance distribution, and is adapted for use with excimer lasers, particularly ArF excimer lasers.

Excimer state of a conjugate polymer (MEH-PPV) in liquid solutions

Abstract: The conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is an important electrooptical material, which is useable as an excellent laser media. In this paper, we present the absorption and fluorescence spectral characteristics of MEH-PPV in a few organic solvents. The paper shows that MEH-PPV exists in an excimeric state in benzene, and that the laser action as shown here arises from a liquid excimeric state.

Highly selective fluorescent signaling for Al3+ in bispyrenyl polyether

Abstract: A series of bispyrenyl-polyether have been synthesized and investigated as a fluorescent chemosensor for metal ions. The results showed that bispyrenyl-polyether system is selective towards Al 3+ ion over other ions tested. In free ligand, excited at 343 nm, it displays a strong excimer emission at around 475 nm with a weak monomer emission at 375 nm. A ratiometry of monomer (375 nm) increase and excimer (475 nm) quenching was shown only when Al 3+ ion is bound to ligand, because two facing pyrene groups form a less efficient overlap of π-π stacking compared with that of free ligand.

Excited-state energy transfers in single-walled carbon nanotubes functionalized with tethered pyrenes

Abstract: Single-walled carbon nanotubes were co-functionalized with pyrenemethanol and 3,5-dihexadecanyloxybenzyl alcohol in different ratios with the purpose of altering the content of pyrene moieties tethered to the nanotube surface. The functionalized nanotube samples were characterized by using established instrumental techniques. The absorption and emission results of the samples suggest that there are significant “intramolecular” excited-state energy transfers from both pyrene monomer and excimer to the linked nanotube, though the energy transfer efficiencies may be different between the monomer and excimer. The excimer formation can be limited by reducing the pyrenemethanol fraction to simplify the excited state processes, but contributions from the luminescence of the well-functionalized carbon nanotubes in the same wavelength region becomes an additional complication. Mechanistic implications of the photophysical results are discussed.

Intramolecular Delayed Fluorescence as a Tool for Imaging Science: Synthesis and Photophysical Properties of a First-Generation Emitter

Abstract: The synthesis of a molecular dyad comprising two pyrene-based terminals covalently linked via a Hantzsch 1,4-dihydropyridine is described. The dyad is sufficiently flexible to allow the end groups to approach each other in fluid solution, as is evident from the appearance of excimer fluorescence. Transient absorption spectroscopy indicates that both intra- and intermolecular triplet−triplet annihilation (TTA) takes place at modest laser intensities and leads to delayed fluorescence. The spectral distribution of the delayed fluorescence signal matches that of the excimer. The kinetics of TTA and the overall yield of delayed fluorescence are considered in terms of the molecule adopting disparate conformations that interconvert slowly. Consideration is also given to the possible application of such delayed fluorescence in imaging technology.

Discharge transitions between glow-like and filamentary in a xenon/chlorine-filled barrier discharge lamp

Abstract: An asymmetric electric voltage pulse source (2–20 kHz, Vp–p: 0–20 kV) was applied to stimulate excimer radiation by a dielectric barrier discharge in a binary gas mixture of chlorine (10.8 Torr)/xenon at high pressure (~460 Torr). When the source frequency increases continuously from 2.0 to 12.0 kHz or in reverse under a fixed output voltage of Vp–p (13.8 kV), it is observed that the discharge modes excited by this pulse source transmit between glow-like and filamentary, and we observe a mixed mode with filaments randomly distributed in the diffuse-like background in the narrow frequency range 7.0–8.0 kHz. It is argued that the reasons for the discharge transition could be the frequency and the time derivative of the voltage waveform of the power source. It is also observed that there is an obvious difference in the emission spectral profiles and the energy efficiency of excimer emission for the two discharge modes.