Pyrene absorption

Pyrene absorption and emission spectra exhibit much vibronic structure. If pyrene is adsorbed on silica gel pretreated so as to have only isolated silanol groups, the spectra are considerably... 

Hawkins and Reifenrath made in vitro percutaneous absorption measurements using neat compoimds applied to skin and allowed to evaporate to air circulated over the apphed dose. For studies on benzo(a)pyrene absorption without sodium azide (NaNj), they foimd that mouse skin (6% of dose absorbed) was more permeable than human skin (1% of dose absorbed), which was more permeable than pig skin (0.6% of dose absorbed). In a similar study using 11 compounds [DDT, benzo(a)pyrene, fluocinonide acetonide, progesterone, hndane, testosterone, par-athion, diisopropyl fluorophosphonate, malathion, benzoic acid, and caffeine], they found that there was no statistically significant difference between human and pig skin. 

The y-CD - pyrene (1 2) complex shows an exciton coupling type of circular dichroism in the pyrene absorption regions, which indicates that the two pyrene molecules included in the y-CD cavity are facing each other, taking S-helical relationship. On the other hand, designed guests which have two naphthyl moieties at both ends show enhanced excimer fluorescence in the presence of y-CD, taking a folded form in the y-CD cavity (Type Circular dichroism spectrum of... 

Foggi P, Pettini L, Santa I, Righini R and Califano S 1995 Transient absorption and vibrational relaxation dynamios of the lowest exoited singlet state of pyrene in solution J. Phys. Chem. 99 7439-45... 

Skin is also important as an occupational exposure route. Lipid-soluble solvents often penetrate the skin, especially as a liquid. Not only solvents, but also many pesticides are, in fact, preferentially absorbed into the body through the skin. The ease of penetration depends on the molecular size of the compound, and the characteristics of the skin, in addition to the lipid solubility and polarity of the compounds. Absorption of chemicals is especially effective in such areas of the skin as the face and scrotum. Even though solid materials do not usually readily penetrate the skin, there are exceptions (e.g., benzo(Lt)pyrene and chlorophenols) to this rule. 

The investigation by Becker et al. (1977 b) also included work on the effect of pyrene added as electron donor. Pyrene has an absorption maximum at 335 nm (e = 55000 M-1cm-1, in petroleum). Much more hydro-de-diazoniation takes place in the presence of pyrene with irradiation at 365 nm, and even more on irradiation with light of wavelength <313 nm. Photoexcited pyrene has a half-life of 300 ns and is able to transfer an electron to the diazonium ion. This electron transfer is diffusion-controlled (k= (2-3) X 1010 m 1s 1, Becker et al., 1977a). The radical pairs formed (ArN2 S +) can be detected by 13C- and 15N-CIDNP experiments (Becker et al., 1983, and papers cited there). 

Figure 3 depicts profiles of Total PAH fluxes vs. time (36). The following polycyclic hydrocarbons have been determined by high performance liquid chromatography, variable wavelength absorption detection Naphthalene, acenaphthylene, 7,12-dimethylbenzanthracene, 2-methylnaphtalene, fluorene, acenaphtene, phenanthrene, 2,3-dimethylnaphtalene, anthracene, fluoranthene, 1-methylphenanthrene, pyrene, 2,3-benzofluorene, triphenylene, benz(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, perylene, benzo(e)pyrene, 1,2,3,4-dibenzanthracene, benzo(a)pyrene, and 1,2,5,6-dibenzanthracene. 

The optical absorption spectra and the first reduction potentials are virtually independent of the number of pyrene units present in the molecule, as a result of the specific stereoelectronic situation. Since the orbital coefficients of the bridgehead centers are almost zero, the rings are electronically decoupled. Thus, oligopyrenes differ significantly from oligo(pflrfl-phenylene)s (OPVs). 

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). 

Figure 7.4 shows the absorption spectrum of pyrene, evaluated with Equation 7.8 (above), and the Kubelka-Munk spectrum of pyrene (below), evaluated by use of Equation 7.7. 

The pyrene-like aromatic chromophore of BaPDE is characterized by a prominent and characteristic absorption spectrum in the A 310-360 nm spectral region, and a fluorescence emission in the X 370-460 nm range. These properties are sensitive to the local microenvironment of the pyrenyl chromophore, and spectroscopic techniques are thus useful in studies of the structures of the DNA adducts and in monitoring the reaction pathways of BaPDE and its hydrolysis products in DNA solutions. 

Two types of DNA binding sites. Two different spectroscopically distinct types of binding sites have been identified utilizing absorption, fluorescence and linear dichroism data on non-covalent (6), and covalent (7) pyrene-like metabolite model compound-DNA complexes. 

Site I is characterized by a relatively large red shift of 10 nm in the absorption maxima (relative to the aqueous solution spectra), exhibiting maxima at 337 and 354 nm, and a negative AA spectrum all of these properties are consistent with an intercalation-complex geometry in which the planar pyrene ring-system is nearly parallel to the planes of the DNA bases. 

Site II is characterized by a relatively small 2-3 nm red shift in the absorption spectrum and a positive AA spectrum. In this conformation, the planes of the pyrene moeities tend to align parallel rather than perpendicular to the axis of the DNA helix. 

Figure 7. Absorption spectra in 15% methanol at 23°C of trans-7,8-dihydroxy-7,8-dihydrobenzo[5]pyrene in native DNA at concentrations of 0.0, 8.0 x l0 5, 1.6 x 10 2.4 x 10, 3.2 x 10 and 4.0 x 10 M. The broken line shows a spectrum in the presence of 3.2 x 10 M DNA and 3.2 x 10 M spermine. (Reproduced with permission from Ref. 15. Copyright 1985, Alan R. Liss.)...

Table IV also contains results of UV absorption studies of hydroxylation effects on the DNA intercalative binding of ben-zo[a]pyrene metabolites and metabolite model compounds. The most important feature of these results is that hydrolysis of BPDE to BPT causes a four-fold reduction in the intercalation association constant. Of all the BP derivatives studied, the tetrol has the lowest binding constant for intercalation. The small binding constant of the tetrol compared with BPDE, coupled with the DNA catalyzed hydrolysis of BPDE to the tetrol may provide a detoxification pathway for removal of a portion of unreacted intercalated BPDE.

Table IV. Comparison of Results from UV Absorption Studies of the Intercalation Binding Constants for Benzo[a]pyrene Metabolites and Metabolite Model Compounds into DNA...

Since the same dye molecules can serve as both donors and acceptors and the transfer efficiency depends on the spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor, this efficiency also depends on the Stokes shift [53]. Involvement of these effects depends strongly on the properties of the dye. Fluoresceins and rhodamines exhibit high homo-FRET efficiency and self-quenching pyrene and perylene derivatives, high homo-FRET but little self-quenching and luminescent metal complexes may not exhibit homo-FRET at all because of their very strong Stokes shifts. 

The width of a band in the absorption or emission spectrum of a fluorophore located in a particular microenvironment is a result of two effects homogeneous and inhomogeneous broadening. Homogeneous broadening is due to the existence of a continuous set of vibrational sublevels in each electronic state. Absorption and emission spectra of moderately large and rigid fluorophores in solution could therefore be almost structureless at room temperature. However, in some cases, many of the vibrational modes are not active, neither in absorption nor in emission, so that a dear vibrational structure is observed (e.g. naphthalene, pyrene). 

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 Na+ sensor M-9 has a structure analogous to that of compound E-4, but instead of two identical pyrene fluorophores, it contains two different fluorophores with a pyrene group and an anthroyloxy group. Resonance energy transfer (see Chapter 9) from the former to the latter is then possible because of the spectral overlap between the fluorescence spectrum of the pyrene moiety and the absorption spectrum of the anthroyloxy moiety. Upon addition of Na+ to a solution of M-9 in a mixture of MeOH and THF (15 1 v/v), the fluorescence of the anthroyloxy group increases significantly compared with that of the pyrene group, which permits a ratiometric measurement. 

Estimates of oCbiend using a rule-of-mixtures relationship are 3.0 X 102 and 7.2 X 103 cm lor 0.2 and 5.0% polyimide, respectively. This dependence of the optimum absorption coefficient (in terms of ablation rate), OVx on fluence is consistent with the observations of Chuang et al.6% for ablation of several UV-transparent (at 308 nm) polymers sensitized with low-molecular-weight dopants, e.g., PMMA doped with pyrene. For the pyrene-PMMA system, Chuang et al.6S reported maximum etch rates for 1.2 J/cm2 at a = 7 X 102 cm 1. It should not be expected that different dopant-matrix systems would yield the same optimum absorption coefficient for a given fluence level since the thermal properties for different polymers may vary significantly. 

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