Showing papers on "Excimer published in 2001"

Photodimerization of anthracenes in fluid solutions: (part 2) mechanistic aspects of the photocycloaddition and of the photochemical and thermal cleavage

Abstract: One of the classics in photochemistry, the photodimerization of anthracenes can be considered as a paradigm of the photocycloaddition of non saturated hydrocarbons. The historical steps of the mechanistic studies are reviewed: based on fluorescence quenching, cyclization quantum yields measurement, the influence of dioxygen and solvents, they support a singlet state pathway; the dimerization rate constants are found to be generally high for reactions occurring within a few nanoseconds unless they are slowed down or inhibited by steric strain. In several cases, excimers have been demonstrated to be intermediates and it is shown that excimer fluorescence and cyclization are competitive processes. Another intermediate known as pericyclic minimum (or conical intersection) is postulated to form a sort of floppy cycloadduct where the reacting centres are at mutual distances shorter than in excimers and longer than in dimers. For intermolecular dimerizations, the triplet state is also reactive but through triplet–triplet annihilation in dilute solutions. Intramolecular photocycloadditions have also been carefully examined, for the role of multiple excimer formation, regioselectivity (9,10∶1′,4′ and 9,10∶1′,2′ cyclization) and solvent polarity. The triplet state reactivity is shown to lead to 4π + 2π or 4π + 4π cycloadducts, depending on geometric factors. In the latter case when intersystem crossing is favoured by the substituents, cyclization quantum yields as high as 0.65–0.72 have been observed. Photodissociation quantum yields are generally high and the reactions are partly adiabatic, leading to excimer and monomer fluorescence, but the major part follows another pathway not fully elucidated by flash photolysis. Thermodynamic and kinetic parameters for the thermal cleavage are given; they reveal a large gamut of stability for the photocycloadducts.

Origin of the stabilization energy of perylene excimer as studied by fluorescence and near-IR transient absorption spectroscopy

Abstract: The origin of the stabilization energy of perylene excimer in toluene has been studied. In order to measure the fluorescence spectra and its decay profiles under the condition free from the reabsorption effect, we designed a thin optical cell with a path length of about 10 μm. From the results, the photophysical parameters of perylene excimer such as the formation rate constant, the peak position and the lifetime and the quantum yield of fluorescence can be obtained. Using these values, the binding energy of perylene excimer can be evaluated as 0.44 eV from the temperature dependence of the transient absorption. By analyzing the band shape of the charge transfer absorption in near-IR wavelength range, the transfer integral V between a neutral excited state and an ion-pair state can be estimated as 0.37 eV. From these results, we conclude that the stabilization energy of perylene excimer consists of two components: exciton interaction (70%) and charge transfer interaction (30%). We also discuss the origin of the stabilization energy of other aromatic excimers on the basis of the results on perylene excimer.

A Study of Excimer Emission in Solutions of Poly(9,9-dioctylfluorene) Using Electrogenerated Chemiluminescence

Abstract: We report electrochemical studies and the observation of excimer emission from the conjugated polymer poly(9,9-dioctylfluorene) (PDOF) in benzene−MeCN solutions during electrogenerated chemiluminescence (ECL). The ECL emission spectrum resulting from the annihilation reaction between PDOF radical ions is characterized by two bands, one at 438 nm and a broad band centered at 610 nm. The ECL emission at shorter wavelengths is assigned to intrapolymer emission of PDOF, while the emission at longer wavelengths arises from the excited-state complex of the polymer formed in the radical annihilation reaction. The relative excimer emission from PDOF (the ratio of excimer ECL emission to intrapolymer emission) depends on polymer concentration.

White-light electroluminescence from soluble oxadiazole-containing phenylene vinylene ether-linkage copolymer

Abstract: We report a promising oxadiazole-containing phenylene vinylene ether-linkage copolymer, which can emit nearly white light from a single-layer light-emitting diode. The emission spectrum is composed of a red component originating from the new excited dimer in addition to the blue-green component from an individual lumophore and excimer. This excited dimer is formed under a strong electric field inside the diode and cannot be produced by photoexcitation, which is different from the excimer or exciplex that is often found both in photoluminescence and electroluminescence, and it is termed the “electromer.”

Modification of the wettability characteristics of polymethyl methacrylate (PMMA) by means of CO2, Nd:YAG, excimer and high power diode laser radiation

Abstract: The surface of the bio-material polymethyl methacrylate (PMMA) was treated with CO2, Nd:YAG, excimer and high power diode laser (HPDL) radiation. The laser radiation was found to effect varying degrees of change to the wettability characteristics of the material depending upon the laser used. It was observed that interaction with CO2, Nd:YAG and HPDL effected very little change to wettability characteristics of the PMMA. In contrast, interaction of the PMMA with excimer laser radiation resulted an increase in a marked improvement in the wettability characteristics. After excimer laser treatment the surface O2 content was found to have increased and the material was seen to be more polar in nature. The work has shown that the wettability characteristics of the PMMA could be controlled and/or modified with laser surface treatment. However, a wavelength dependence of the change of the wetting properties could not be deduced from the findings of this work.

Rb–He exciplex formation on helium nanodroplets

Abstract: Rubidium atoms have been deposited on helium nanodroplets and optical excitation and emission spectra of the 5 2P–5 2S transition have been measured and interpreted. After laser excitation of the 5 2P3/2 state, fluorescence from a Rb*He exciplex is observed while pumping the D1 (5 2P1/2–5 2S) line yields only emission from free rubidium atoms. This observation is in agreement with the predictions from a recent model by Reho et al. [J. Chem. Phys. 113, 9694 (2000)] about the extraction of an alkali–helium exciplex from the doped helium nanodroplet surface. A high barrier along the Hen–He–Rb* axis of the 1 2Π1/2 potential prevents desorption of Rb*He within the excited state lifetime, whereas the 1 2Π3/2 potential permits the exciplex extraction. The excitation spectrum, on the other hand, reflects the structure of the excited states 1 2Π1/2, 1 2Π3/2, and 2 2Σ1/2 of the HenRb complex whose potential surfaces will depend strongly on the alkali–He interaction dynamics near the droplet surface. For a heavy sur...

5′-bis-pyrenylated oligonucleotides displaying excimer fluorescence provide sensitive probes of RNA sequence and structure

Abstract: Oligonucleotide conjugates bearing two pyrene residues attached to 5′-phosphate through a phosphoramide bond were synthesised. Fluorescence spectra of the conjugates show a peak typical of monomer emission (λmax 382 nm) and a broad emission peak with λmax 476 nm, which indicates the excimer formation between the two pyrene residues. Conjugation of these two pyrene residues to the 5′-phosphate of oligonucleotides does not affect the stabilities of heteroduplexes formed by conjugates with the corresponding linear strands. A monomer fluorescence of the conjugates is considerably affected by the heteroduplex formation allowing the conjugates to be used as fluorescent hybridisation probes. The 5′-bis-pyrenylated oligonucleotides have been successfully used for investigation of affinity and kinetics of antisense oligonucleotides binding to the multidrug resistance gene 1 (PGY1/MDR1) mRNA. The changes of excimer fluorescence of the conjugates occurring during hybridisation depended on the structure of the binding sites: hybridisation to heavily structured parts of RNA resulted in quenching of the excimer fluorescence, while binding to RNA regions with a loose secondary structure was accompanied by an enhancement of the excimer fluorescence. Potentially, these conjugates may be considered as fluorescent probes for RNA structure investigation.

Segmental Mobility in Phosphorus-Containing Dendrimers. Studies by Fluorescent Spectroscopy

Abstract: Steady-state fluorescence spectra and decays of excitations of a phosphorus-containing dendrimer labeled with 12 internal labels and of an iminophosphorane model compound bearing two labels dissolved in acetonitrile, diglyme, 1,4-dioxane, triethylene glycol, and cyclohexanol revealed that the interior of dendrimers contained many solvent molecules, and movements of internally located pyrene labels were not reduced by interactions with the dendrimer core. This conclusion was based on the following findings. A ratio of the intensities of pyrene−pyrene excimer and pyrene monomer emissions (IE/IM) decreased with increasing solvent viscosity but, with exception of acetonitrile solutions, was very close for the dendrimers and for the model. Monomer and excimer emission intensity decays were fitted with two-exponential functions with time constants τ1 and τ2 characterizing formation of pyrene−pyrene excimers and recovery of pyrene moieties in their ground state, respectively. For solvent viscosities varying from...

Electron interactions with excited atoms and molecules

Abstract: Elastic, inelastic, and superelastic scattering of electrons by an electron-impact ionization of excited atoms are reviewed and discussed and the role of the electric dipole polarizability in the interaction of slow electrons with excited atoms is indicated. The meager knowledge on the scattering of slow electrons from vibrationally and electronically excited molecules is also summarized and discussed, as well as the available information on excitation and ionization by electron impact of electronically excited molecules. The available data on the processes of electron attachment to vibrationally/rotationally and electronically excited molecules and the processes of electron detachment from energy-rich transient anions are also summarized and discussed.

Ground- and Excited-State Aggregation Properties of a Pyrene Derivative in Aqueous Media

Abstract: The aggregation properties of 4-(1-pyrene)butanoate in aqueous media have been quantitatively investigated. For the pyrene chromophore ground state, the monomer−dimer equilibrium was monitored using NMR methods, yielding a dimerization constant of 150 M-1 for pyrenebutanoate in alkaline water. In addition, the excited-state aggregation was studied by time-resolved emission spectroscopy. The excimer formation constant in water, which represents the equilibrium established by interaction of excited- and ground-state species, was determined to be 1.6 × 104 M-1 (about 100-fold larger than the ground-state dimerization constant). The role of solvent polarity and the influence of an aromatic cosolvent were examined in order to determine the types of interactions that contribute to the driving force for aggregation. The ground-state aggregation propensity of pyrenebutanoate was significantly diminished upon addition of methanol or pyridine to aqueous solutions, indicating that hydrophobic and/or π-stacking inter...

Xenon excimer emission from pulsed microhollow cathode discharges

Abstract: By applying electrical pulses of 20 ns duration to xenon microplasmas, generated by direct current microhollow cathode discharges, we were able to increase the xenon excimer emission by more than an order of magnitude over direct current discharge excimer emission For pulsed voltages in excess of 500 V, the optical power at 172 nm was found to increase exponentially with voltage Largest values obtained were 275 W of vacuum-ultraviolet optical power emitted from a single microhollow cathode discharge in 400 Torr xenon with a 750 V pulse applied to a discharge Highest radiative emittance was 152 W/cm2 The efficiency for excimer emission was found to increase linearly with pulsed voltages above 500 V reaching values of 20% at 750 V

Fluorescence Study of the Coil-Globule Transition of a PEO Chain in Toluene

Abstract: The cyclization kinetics of a poly(ethylene oxide) (PEO) chain (Mn = 3280; Mw/Mn = 1.05) labeled at both ends with pyrene was studied in toluene at several temperatures. Above 30 °C, the kinetics of pyrene excimer formation is well described by a two-state model. Below this temperature, a new excited species appears, which we identified as being a pyrene dimer. The appearance of the pyrene dimer was associated with a broad coil−globule transition. For lower temperatures, the kinetics of pyrene excimer and dimer formation can only be described by a three-states model. The fluorescence decay curves of the polymer in toluene were analyzed according to these models in order to obtain the rate constants. The activation energies of the cyclization and ring-opening processes were obtained from the Arrhenius plots of the rate constants. The activation energies for the cyclization processes (excimer and excited dimer formation) are close to the viscous flow activation energies. From the excimer dissociation rate c...

Mitomycin C reduces corneal light scattering after excimer keratectomy.

Abstract: Purpose.To evaluate the effect of intraoperative mitomycin C (MMC) on corneal light scattering after excimer laser keratectomy.Methods.Phototherapeutic keratectomy (PTK) was performed in 24 rabbit eyes. After 40-μm epithelial ablation, animals were divided into three groups. In group 1, filter paper

Monovision achieved with excimer laser refractive surgery.

Abstract: Presbyopia refers to the decrease in accommodative capacity that accompanies aging. By definition, presbyopia occurs when an individual’s accommodative power is no longer able to allow for sustained and comfortable near-vision work. The appearance of presbyopia is not defined by age or precise accommodative reserve, as people’s near-vision requirements differ. However, most people in their early forties begin to notice difficulties with performance of near-vision tasks. The presbyopic population in the United States is expected to double every 5 years until the year 2010. Monovision constitutes one means of addressing the problem of presbyopia. In monovision, one eye is optically corrected for distance vision and the other eye for near vision. The near-vision eye may be placed in focus at a reading distance (33 cm) or at an intermediate distance (for example, at 50 cm for computer use). The monovision approach has been successfully applied to presbyopic contact lens wearers. For contact lens wearers, single-vision contact lenses correct one eye for distance and undercorrect the other eye by 1 to 2 D. Refractive surgical procedures such as photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) serve to correct myopia by flattening the corneal curvature. The number of refractive surgery cases performed on a yearly basis continues to increase exponentially. Many refractive surgery patients are in the presbyopic or prepresbyopic age group, and these patients often experience difficulties with near vision

Characterization of the Association Level of Pyrene-Labeled HASEs by Fluorescence

Abstract: The hydrophobic associations of three hydrophobically modified alkali swellable emulsion polymers (HASE) were studied by fluorescence. The chromophore pyrene was used as the hydrophobic pendant. The level of association was characterized qualitatively and quantitatively by monitoring the fluorescence behavior of the pyrene monomer and excimer. Excimer formation occurs mostly by diffusion in an organic solvent (tetrahydrofuran, THF) where the pyrene moieties are not associated. In aqueous solutions where pyrene is insoluble, excimer formation occurs mostly via direct excitation of ground-state pyrene aggregates. The level of association of these pyrene-labeled HASEs was determined quantitatively by analyzing the monomer and excimer fluorescence decays using a blob model. It was found that 58% of the hydrophobic pendants were associated in aqueous solutions, but that only 5% of them were associated in THF.

Excimer Formation in a Naphthalene-Labeled Dendrimer†

Abstract: Steady-state and time-resolved fluorescence measurements were conducted on newly synthesized Frechet-type dendrimers with naphthalene peripheral groups in homogeneous and sodium dodecyl sulfate (SDS) micellar solutions. Intense excimer emission was observed for the third-generation dendrimer 3 but not for the first-generation dendrimer 1a. This different spectroscopic behavior was attributed to the geometry of dendrimer 3, where excimeric interactions between adjacent π-stacked naphthyl groups are present.

Influence of laser fluence and irradiation timing of F2 laser on ablation properties of fused silica in F2-KrF excimer laser multi-wavelength excitation process

Abstract: A collinear irradiation system of F2 and KrF excimer lasers for high-quality and high-efficiency ablation of hard materials by the F2 and KrF excimer lasers’ multi-wavelength excitation process has been developed. This system achieves well-defined micropatterning of fused silica with little thermal influence and little debris deposition. In addition, the dependence of ablation rate on various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number is examined to investigate the role of the F2 laser in this process. The multi-wavelength excitation effect is strongly affected by the irradiation timing, and an extremely high ablation rate of over 30 nm/pulse is obtained between -10 ns and 10 ns of the delay time of F2 laser irradiation. The KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing F2 laser fluence. Moreover, the ablation rate shows a linear increase with the logarithm of KrF excimer laser fluence when the F2 laser is simultaneously irradiated, while single KrF excimer laser ablation shows a nonlinear increase. The ablation mechanism is discussed based on these results.

Fluorescent molecular sensory system based on bis pyrene-modified γ-cyclodextrin dimer for steroids and endocrine disruptors

Abstract: A γ-cyclodextrin dimer modified with two pyrene moieties, 6-(2-pyrenebutylate-aminoethyl)pyrenebutylate-amino-6-deoxy-bis-γ-cyclodextrin, has been synthesized in the presence of N,N′-dicyclohexycarbodiimide from γ-cyclodextrin dimer linked with ethylenediamine at an upper rim. The sensing ability and binding property of the titled dimer were investigated for bile acids and endocrine disruptors. This cyclodextrin dimer showed both monomer and excimer fluorescence; the guest-induced emissions were observed as increases or decreases depending on the guest. The values ΔIm/I0m and ΔIex/I0ex, where I0m and Im are fluorescence intensities of monomer emission in the absence and presence of a guest and I0ex and Iex are those of excimer emission and ΔIm and ΔIex were Im - I0m and Iex - I0ex, respectively, were used as a parameter of sensitivity. This host exhibited highly sensitive molecular recognition ability for bile acids and endocrine disruptors, in which the sensing parameters obtained from monomer emission were plus or minus values, whereas the parameters obtained as excimer emission were minus ones. The behavior of the appended moieties of the host during a host-guest complexation was studied by induced circular dichroism (ICD) and fluorescence spectra. The ICD intensities of the titled dimer were decreased upon an addition of a guest. The guest-induced variations in the fluorescence and ICD intensity suggest that the appended moieties move by altering the spatial relationship in the hydrophobic space between two linked cyclodextrins.

Excimer and Exciplex Formation in van der Waals Dimers of Toluene and Benzene

Abstract: Structural isomerization dynamics of photoexcited van der Waals dimers of toluene and benzene have been investigated by means of fluorescence and mass spectroscopic methods. Two isomeric forms of the heterodimer complex formed between toluene and benzene, assigned to parallel displaced and T-shaped configurations, are found to rearrange into exciplexes when excited in the S1 state with a small amount of vibrational energy. The toluene homodimer also reveals similar excess energy dependence on excimer formation. The excited-state dynamics are contrasted with those of the T-shaped benzene homodimer where no vibrational energy is required for structural isomerization. The excess energy dependence observed for the toluene complexes is attributed to a steric requirement of sandwich-type excimer geometry. Dissociation behavior of these exciplex/excimer complexes has been measured by photodepletion spectroscopy and compared with that of the benzene excimer. These excited-state complexes are found to strongly abs...

Observation of exciplex emission from the mixed dimer of naphthalene and 2-methoxynaphthalene: A laser-induced fluorescence study in supersonic jet

Abstract: Exciplex formation in the van der Waals mixed dimer of naphthalene and 2-methoxynaphthalene has been investigated by using laser-induced fluorescence spectroscopy in supersonic jet. In contrast to the barrierless excimer formation in the S1 state of naphthalene homodimer, the exciplex formation in the mixed dimer occurs with a vibrational energy barrier of 466 cm−1. The energy gap between the S1 states of two molecules in the mixed dimer is 1000 cm−1. Therefore, the contribution of the exciton resonance interaction to the stability of the exciplex state is considered to be insignificant. Charge transfer interaction and the mixing of the charge transfer state with the locally excited state due to orbital overlap between two molecules at suitable geometry are proposed to be the decisive factors in formation of the exciplex.

Phase Separation of Poly(N-isopropylacrylamide) in Mixtures of Water and Methanol: A Spectroscopic Study of the Phase-Transition Process with a Polymer Tagged with a Fluorescent Dye and a Spin Label

Abstract: The thermoreversible phase transition of poly(N-isopropylacrylamide) randomly labeled with a spin label, 4-amino-2,2',6,6'-tetramethylpiperidine 1-oxide (TEMPO), and a fluorescent dye, 4-(pyren-1-yl)butyl (PNIPAM-Py-T), in different H2O/MeOH mixtures was studied by turbidimetry, continuous-wave electron paramagnetic resonance spectroscopy (CW-EPR), and fluorescence spectroscopy. The macroscopic phase diagram of PNIPAM-Py-T in H2O/MeOH measured by turbidimetry was identical to those of poly(Nisopropylacrylamide) (PNIPAM) and of TEMPO-labeled PNIPAM (PNIPAM-T) in H2O/MeOH mixtures. However, distinct differences among the three polymers were detected in their solvent-dependent EPR and fluorescence-spectroscopic properties. The EPR spectra were analyzed in terms of the isotropic hyperfine coupling constants, which monitor the variation in environmental polarity of the radical labels occurring for the conformational transitions of the polymer as a function of temperature, as well as the correlation time for reorientation motion, the increase of which is indicative of the increased viscosity of the radical environment and interactions occurring between the radical and other surface groups of the precipitated polymer, if compared to the soluble polymer. The fluorescence of Py in PNIPAM-Py-T displayed contributions from isolated excited pyrenes (monomer emission) and from preformed pyrene ground-state aggregates (excimer emission). The quantum efficiencies of monomer and excimer emission were monitored as a function of solvent composition. By the two experimental approaches, we demonstrate the profound influence of the PNIPAMattached pyrene units in increasing the hydrophobicity of the nanodomains formed upon heat-induced precipitation of PNIPAM-Py-T.

Infrared fluorescence of Xe 2 molecules in electron'proton beam excited pure Xe gas and in an Ar'Xe gas mixture

Abstract: We report experimental results of proton- and electron-beam-induced near-infrared fluorescence in high-pressure Xe gas and in a 90% Ar–10% Xe gaseous mixture at room temperature. The investigated wavelength band spans the range 0.7⩽λ⩽1.8 μm. In the previously unexplored range for λ⩾1.05 μm we have detected a broad continuum near-infrared fluorescence centered at λ≈1.3 μm. The continuum shifts towards longer wavelengths as pressure is increased up to 1.5 MPa. The shift is quantitatively different in pure Xe or in the mixture, but qualitatively similar. We believe that this continuum is produced by a bound-free Xe2 excimer transition in a way similar to the well-known first and second vacuum ultraviolet continua of noble gas excimers. The pressure-dependent shift can then be explained by the interaction of the outer electron in the excimer with the atoms of the host gas.

Fluorescence Properties, Induced-Fit Guest Binding and Molecular Recognition Abilities of Modified γ-Cyclodextrins Bearing Two Pyrene Moieties

Abstract: γ-Cyclodextrin (γ-CD) derivatives bearing two pyrene moieties at AB, AC, AD, and AE glucose units (1–4), were prepared as chemosensors for detecting molecules by their guest-responsive excimer emission intensity. All these hosts exhibited predominant excimer emission and remarkable induced circular dichroism (ICD) in a 20% dimethyl sulfoxide aqueous solution. The excimer emission and the ICD intensities mostly decreased upon addition of a variety of organic guests. The result indicates that 1–4 undergo an induced fit conformational change upon complex formation, excluding the pyrene moieties from inside to outside the γ-CD cavity in guest binding. The guest-induced fluorescence variations were analyzed by 1 : 1 host–guest stoichiometry and the obtained binding constants were roughly correlated with the guest-induced fluorescence variations. The binding and fluorescence data revealed that 1–4 have remarkable molecular recognition abilities for bile acids and other guests.

Poly(methyl methacrylate) model study of optical surface quality after excimer laser photorefractive keratectomy.

Abstract: Purpose: To evaluate lenses produced by excimer laser ablation of poly(methyl methacrylate) (PMMA) plates. Setting: University research laboratory. Methods: Two Nidek EC-5000 scanning-slit excimer laser systems were used to ablate plane-parallel plates of PMMA. The ablated lenses were examined by focimetry, interferometry, and mechanical surface profiling. Results: The spherical optical powers of the lenses matched the expected values, but the cylindrical powers were generally lower than intended. Interferometry revealed marked irregularity in the surface of negative corrections, which often had a positive “island” at their center. Positive corrections were generally smoother. These findings were supported by the results of mechanical profiling. Contrast sensitivity measurements carried out when observing through ablated lenses whose power had been neutralized with a suitable spectacle lens of opposite sign confirmed that the surface irregularities of the ablated lenses markedly reduced contrast sensitivity over a range of spatial frequencies. Conclusion: Improvements in beam delivery systems seem desirable.

Polyamine Linear Chains Bearing Two Identical Terminal Aromatic Units. Evidence for a Photo Induced Bending Movement

Abstract: Several chemosensors bearing a fluorescent unit at both ends of a linear polyamine chain were synthesised. The protonation as well as the association constants with Cu2+ and Zn2+ were determined by potentiometry in 0.15 mol dm−3 NaCl at 298.1 K. In the case of 1,16-bis(1-naphthylmethyl)-1,4,7,10,13,16-hexaazadecane hexahydrochloride (L1), formation of an excimer emission in aqueous acidic solutions was observed. The system was characterized by steady state fluorescence emission and by time resolved fluorescence. In the ground state the molecule is expected to adopt a more or less linear conformation, while in the excited state a bending movement of the chain must occur in order to allow the excimer emission. The system can be viewed as an elementary machine driven by light.

Maleated Ethylene-Propylene Random Copolymers: Determination of the Microstructure and Association Level by Fluorescence Spectroscopy

Abstract: A maleated ethylene-propylene (EP) random copolymer was labeled with the fluorescent dye 1-pyrenebutanoicacid hydrazide to yield EP−Py. Fluorescence spectroscopy was used to determine the microstructure of EP−Py (i.e., the distribution of the pendants along the chain) in THF where little association is expected to occur between the polar pendants. In hexane, an apolar solvent, associations take place between the polar pendants and the association level of EP−Py (i.e., fractions of pendants, which are associated and unassociated) was estimated by analyzing quantitatively both the monomer and the excimer fluorescence decays. The ability of pyrene to form an excimer was used to carry out this study. The excimer formation process was monitored by using a combination of two models, namely the sequential model and the blob model. Both monomer and excimer decays were analyzed in order to completely describe the distribution of pyrene groups. It was found that 70% of all pyrene groups are located at small distanc...