Pyrene molecules

The pyrene molecule is transferred by irradiation to its cation radical [29]. This reacts at the oil/water interface by nucleophilic attack from the cyanide ion. Typically, the cyanated product remains in the organic phase. 

Pyrene fluorophores are also used as probes. Derivatives of pyrene show /.max/ Xem 340/376 nm, e 4.3 x 104 M 1 cm-1, and environmental sensitivity, this fluorophore can be used to report on RNA folding [102]. Pyrene also displays a long-lived excited state (x > 100 ns), which allows for an excited pyrene molecule to associate with a pyrene in the ground state. The resulting eximer exhibits a red-shift in fluorescence intensity (A,em 490 nm). This characteristic can be used to study important biomolecular processes, such as protein conformation [103]. 

In a film, however, molecular mobility is severely limited, so that excimer fluorescence must arise mainly from pairs or groups of pyrene molecules that were approximately in the excimer configuration when the film was cast. Thus, the intensity of the excimer emission is also an indication of the local concentration of pyrene in the cast film. If the pyrene aggregates, we expect that the excimer fluorescence would increase with aggregation. This system can be used to look at the aggregation of very low concentrations of a small molecule dye in a polymer film, and potentially detect molecular aggregation before it would be observable by other tech-... 

In order to study the molecular dynamics of the outer segments of a dendrimer, one pyrene moiety was selectively and covalently attached to one dendron of poly(aryl ester) dendrimers by Adams (in total three pyrene molecules per dendrimer) [24]. The fluorescence decay of pyrene in the THF solution of the labeled dendrimers provided details of the pyrene excimer formation, such as the excimer formation rate, the excimer decomposition rate constant and the equilibrium constant of the excimer formation. These parameters were utilized to evaluate the diffusional mobility of the dendrimer branches. 

Lee and Meisel incorporated Py, at levels of 10 M or more, into 1200 EW acid form samples that were swollen with water and with ferf-butyl alcohol. It was concluded based on the /3//1 value for water swollen samples that the Py molecules were located in the water clusters and were most likely near fluorocarbon—water interfaces. It was also concluded, based on both absorption and emission spectra, that the probes had strong interactions with the SO3 groups that were exchanged with Ag+ and Pb + cations in the case of water containing samples. Likewise, the pyrene molecules were rationalized as being surrounded by terf-butanol molecules in that case. However, excimer formation (due to the presence of adjacent pyrene molecules) in the ferf-butyl alcohol system suggested the loss of cluster morphology-... 

At low MEGA-n concentration, the II1/1 ratio is low (= 0.84) and almost constant suggesting that pyrene molecules reside in a rather polar environment, which is however not so polar as in an aqueous phase (III/I = 0.63). It may be considered that the very cohesive nature of the vesicular bilayer does not allow pyrene molecules to penetrate deep into the hydrophobic part, and they stay in peripheral solubilization sites. 

Eximer Fluorescence. Since Forster and Kasper discovered concentration-dependent long-wavelength emission resulting from association of an electronically excited pyrene molecule with another ground state pyrene molecule,39 the phenomenon of excimer fluorescence has been studied extensively.40 The mechanism for excimer formation and emission can be represented by... 

Fluorescence spectra and quantum yields of pyrene in supercritical CO2 have been determined systematically as functions of temperature, CO2 density, and pyrene concentration. Under near-critical conditions, contributions of the pyrene excimer emission in observed fluorescence spectra are abnormally large. The results cannot be explained in the context of the classical photophysical mechanism well established for pyrene in normal liquid solvents. The photophysical behavior of pyrene in a supercritical fluid is indeed unusual. The experimental results can be rationalized with a proposal that the local concentration of pyrene monomer in the vicinity of an excited pyrene molecule is higher than the bulk in a supercritical solvent environment. It is shown that the calculated ratios between the local and bulk concentrations deviate from unity more significantly under near-critical conditions (Sun and Bunker, 1995). 

When an ion-pair recombines, it may form an excited state which can luminesce. The intensity of luminescence is a direct monitor of the competing ratio of recombination and luminescence. With steady-state conditions, the luminescence intensity is proportional to the rate of recombination. For instance, Morrow et al. [380] have radio lysed solutions of pyrene in cyclohexane. Solvated electrons and pyrene cations are produced. On recombination, an excited singlet state is produced which can fluoresce. If two pyrene molecules are in (or near) contact when one or other molecule is in the excited singlet state, then excimer fluorescence may be observed. The intensity of fluorescence can be decreased by application of an electric field, since fewer ion-pairs recombine to form the excited state. Jarnagin [381] and Holroyd and Russell [382] have photoionised iVjA iV. iV -tetramethyl-p-phenylenedia-mine (TMPD) with light (of photon energy 5.5—6 eV) in hydrocarbon solvents and measured the photocurrent at various electric field strengths. 

The influence of the nonuniform character of the interior of zeolites on the photophysics of adsorbed guest molecules has been observed. Pyrene molecules included in zeolite faujasites show both monomer and excimer emission [232,233]. As in the case of silica surfaces, the excitation spectra of the emission corresponding to the monomer and the excimer differ (Figure 36), suggesting that there are at least two independent sites, each responsible for monomer emission and excimer emission. Time-resolved emission studies of pyrene included in Na + -X and Na + -Y (<0.1 molecule per cage) indicate... 

Figure 36. Diffuse reflectance and excitation spectra of pyrene included in Na+X. Note the monomer and excimer emissions possess different excitation spectra. This difference may be the result of nonuniform distribution of pyrene molecules within cages.

Electron tunneling between organic species was first detected, by direct kinetic experiments, for reactions of the biphenyl anion radical with naphthalene and pyrene [11] and triphenylethylene [12], As is known, upon irradiating vitreous solutions containing biphenyl or pyrene, Py, these acceptors react with electrons to form Ph2 and Py with characteristic optical spectra [13]. Ph2 particles have been found [11] to enter into the electron exchange reactions at 77 K with naphthalene, Nh, and pyrene molecules in mixtures of ethyl alcohol and diethyl ether (2 1). 

As has been previously reported, when pyrene is adsorbed on silica gel there is evidence for ground state association which is not present In solution or the vapor phase.9-13 but which has been described as being present when pyrene is dissolved in a plastic medium. This is also a manifestation of surface inhomogeneity -some sites enhance the tendency to form a ground state bimolecular complex, whereas other sites contain isolated pyrene molecules. The interaction differences are sufficient to yield significant spectral shifts in absorption and the ground state complex emits with the characteristic pyrene excimer fluorescence. Fig. 5 shows a typical set of spectra Illustrating this association and Fig. 6 presents evidence that this observation Is not due to microcrystal formation. 

Enhanced excimer emission was also observed from PBAC bound to a-ZrP [20], Excimer formation from pyrene is well known in aqueous solutions [54], As in the case with AMAC, excimer formation is increased with PBAC concentration (Fig. 16) due to increased local concentrations but with two significant differences. Hydrophobic interactions between the pyrene molecules favor the aggregation of PBAC even at moderate coverages and the PBAC singlet excited state is much longer lived ( 200 nsec) than that of AMAC ( 10 nsec) these factors, in turn, promote excimer formation even at low loadings. The broad, red-shifted fluorescence band with a peak centered around 470 nm, characteristic of the pyrene excimer emission, is evident in Fig. 16. Rapid formation of the excimer at low coverages is also evident from the plot of the ratio of emission intensities at... 

The formation of novel membrane-like materials based on LDHs has also been established. These materials are of relevance to separation and membrane technologies. A LiAl-LDH containing myrisate or hexanoate anions, for example, has the ability to partition pyrene from a methanol/water solution containing the polycyclic aromatic [132], No sorption of pyrene was observed for a LiAl(succinate) LDH, however, and this was attributed to a sieving effect of this compound towards the pyrene molecule. The sorption of polyaromatic hydrocarbons, such as pyrene, is important from an environmental perspective. 

In a mixture containing a 2 1 molar ratio of 1 and II the NDI proton signal of 1 and all the protons of II are shifted upheld by 0.09 and 0.08 ppm, respectively, while in a similar mixture containing 1-ester and II the same protons are shifted upheld by only 0.04 ppm (Fig. 37). This behaviour is consistent with the formation of an inclusion complex between 1 and II, where the pyrene molecules are shielded by the naphthalene aromatic cores of the NDIs, which in turn are shielded by the pyrenes complexed inside the nanotube. Spectrum d in Fig. 37 shows that, although there is some... 

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