Pyrene intensity

A Cu(II)-induced perturbation of pyrene fluorescence has been utilized to create a sensor for glutamate [388], A 2 2 1 Cu2+ 3-CD pyrene complex is formed by the noncovalent assembly of the constituents the site of Cu(II) binding is unknown. The pyrene emission resulting from complexation of the lu-mophore to 3-CD is effectively quenched by the addition of Cu(II). A 500-fold enhancement in pyrene intensity is observed upon the addition of 1.87 M glutamate, which is presumed to extract Cu(II) from the 2 2 1 complex. The precise nature of the quenching and restoration mechanisms is currently unknown. 

The following data was recorded for the phosphorescence intensity for several standard solutions of benzo[a]pyrene. 

Emission spectra at these points are shown in Figure 8.2d. The band shapes were independent of the excitation intensity from 0.1 to 2.0 nJ pulse . The spectrum of the anthracene crystal with vibronic structures is ascribed to the fluorescence originating from the free exdton in the crystalline phase [1, 2], while the broad emission spectra of the pyrene microcrystal centered at 470 nm and that of the perylene microcrystal centered at 605 nm are, respectively, ascribed to the self-trapped exciton in the crystalline phase of pyrene and that of the a-type perylene crystal. These spectra clearly show that the femtosecond NIR pulse can produce excited singlet states in these microcrystals. 

Figure 8.2e shows the dependence of the fluorescence intensity on the excitation power of the NIR light for the microcrystals measured with a 20x objective. In this plot, both axes are given in logarithmic scales. The slope of the dependence for the perylene crystal is 2.8, indicating that three-photon absorption is responsible for the florescence. On the other hand, slopes for the perylene and anthracene crystals are 3.9 for anthracene and 4.3 for pyrene, respectively. In these cases, four-photon absorption resulted in the formation of emissive excited states in the crystals. These orders of the multiphoton absorption are consistent with the absorption-band edges for each crystal. The four-photon absorption cross section for the anthracene crystal was estimated to be 4.0 x 10 cm s photons by comparing the four-photon induced fluorescence intensity of the crystal with the two-photon induced fluorescence intensity of the reference system (see ref. [3] for more detailed information). 

The first observations of P-type delayed fluorescence arose from the photoluminescence of organic vapors.<15) It was reported that phenanthrene, anthracene, perylene, and pyrene vapors all exhibited two-component emission spectra. One of these was found to have a short lifetime characteristic of prompt fluorescence while the other was much longer lived. For phenanthrene it was observed that the ratio of the intensity of the longer lived emission to that of the total emission increased with increasing phenanthrene vapor... 

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]. 

Different aromatic hydrocarbons (naphthalene, pyrene and some others) can form excimers, and these reactions are accompanying by an appearance of the second emission band shifted to the red-edge of the spectrum. Pyrene in cyclohexane (CH) at small concentrations 10-5-10-4 M has structured vibronic emission band near 430 nm. With the growth of concentration, the second smooth fluorescence band appears near 480 nm, and the intensity of this band increases with the pyrene concentration. At high pyrene concentration of 10 2 M, this band belonging to excimers dominates in the spectrum. After the act of emission, excimers disintegrate into two molecules as the ground state of such complex is unstable. 

The biphenyl, naphthalene, pyrene, and triphenylene adducts display intense room-temperature phosphorescence.237 238 These observations indicate the occurrence of a mercury heavy atom effect, which promotes... 

Recent advances in PAH carcinogenesis research over the past decade have led to identification of diol epoxide metabolites as the principal active forms of the PAH investigated to date Q,2). Benzo-(a)pyrene (BP) has been most intensively investigated, and it has been demonstrated that a diol epoxide metabolite anti-BPDE is the active intermediate which binds covalently to DNA in human and other mammalian tissues 0,4). Anti-BPDE was also demonstrated to be a powerful mutagen in both bacterial and mammalian cells (15) These findings stimulated an outpouring of research directed towards elucidation of the molecular mechanism of PAH carcinogenesis. 

In the present work, we have examined poly(N-vinylcarbazole) (abbreviated hereafter as PVCz) and pyrene-doped poly(aethyl methacrylate) (PMMA) films by using a tine-resolved fluorescence spectroscopic aethod. Fluorescence spectra and their dynanic behavior of the forner fila were elucidated with a high intensity laser pulse and a streak camera, which nakes it possible to neasure dynaaics just upon laser ablation. This aethod reveals aolecular and electronic aspects of laser ablation phenomena (17). For the latter fila a laser pulse with weak intensity was used for characterizing the ablated and Basked areas. On the basis of these results, we demonstrate a high potential of fluorescence spectroscopy in aolecular studies on laser ablation and consider its mechanism. Experimental... 

Figure 6. Comparison of excimer intensity for free and tagged pyrene in novolac films. Film composition ( ) free pyrene in novolac (A) pyrene tagged novolac (o) 19.6 mol% pyrene tagged novolac mixed with untagged novolac. The inset is an expansion of the graph for low pyrene concentrations.

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-... 

The emission due to pyrene at lower wavelength decreases in intensity. 

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

In some aromatic molecules that have a high degree of symmetry, i.e. with a minimum D2h symmetry (e.g. benzene, triphenylene, naphthalene, pyrene, coronene), the first singlet absorption (So —> Si) may be symmetry forbidden61 and the corresponding oscillator strength is weak. The intensities of the various forbidden vibronic bands are highly sensitive to solvent polarity (Ham effect). In polar solvents, the intensity of the 0-0 band increases at the expense of the others. 

The relative changes in intensity of the vibronic bands in the pyrene fluorescence spectrum has its origin in the extent of vibronic coupling between the weakly allowed first excited state and the strongly allowed second excited state. Dipole-induced dipole interactions between the solvent and pyrene play a major role. The polarity of the solvent determines the extent to which an induced dipole moment is formed by vibrational distortions of the nuclear coordinates of pyrene (Karpovich and Blanchard, 1995). 

Fig. 7.8. Fluorescence spectra of pyrene in hexane, n-butanol, methanol and acetonitrile showing the polarity dependence of vibronic band intensities (excitation wavelength 310 nm) (reproduced with permission from Kalyanasun-daran and Thomas, 1977b).

Tab. 7.4. Solvent dependence of the ratio / //m of the fluorescence intensities of the first and third vibronic bands in the fluorescence spectrum of pyrene.

In contrast, the Py scale, based on the relative intensities of vibronic bands of pyrene, appears to be relatively insensitive to hydrogen bonding ability of solvents. 

Kalyanasundaran K. and Thomas J. K. (1977b) Environmental Effects on Vibronic Band Intensities in Pyrene Monomer Fluorescence and their Application in Studies of Micellar Systems, J. Am. Chem. Soc. 99, 2039-2044. 

In multicomponent systems A"0 can be written as a sum of the individual absorption coefficients A ot = 2TA , where each AT,(A ) depends in a different way on the wavelength. If one or more of the components are fluorescent, their excitation spectra are mutually attenuated by absorption filters of the other compounds. This effect is included in Eqs. (8.27) and (8.28) so that examples like that of Figure 8.4 can be quantified. The two fluorescent components are monomeric an aggregated pyrene, Mi and Mn. The fluorescence spectra of these species are clearly different from each other but the absorption spectra overlap strongly. Thus the excitation spectrum of the minority component M is totally distorted by the Mi filter (absorption maxima of Mi appear as a minima in the excitation spectrum ofM see Figure 8.4, top). In transparent samples this effect can be reduced by dilution. However, this method is not very efficient in scattering media as can be seen by solving Eqs. (8.27 and 8.28) for bSd — 0. Only the limit d 0 will produce the desired relation where fluorescence intensity and absorption coefficient of the fluorophore are linearly proportional to each other in a multicomponent system. 

Figure 8.23. Lateral distribution of the fluorescence intensity underline irradiation. Sample pyrene/silica gel covered with a fused silica plate of 2 mm thickness (the sample is the same as in Figure 8.2).

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