Pyrene decay time

Nishimura, H., Yamaoka, T., Hattori, K., Matsui, A. and Mizuno, K. (1985) Wavelength-dependent decay times and time-dependent spectra of the singlet-exciton luminescence in anthracene crystals./. Phys. Soc. Jpn., 54, 4370-4381. Matsui, A. and Nishimura, H. (1980) Luminescence of free and self trapped excitons in pyrene. J. Phys. Soc. Jpn., 49, 657-663. 

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. 

Table I. Pyrene Monomer Decay Time on Decanol-Covered Si02 as a Function of Temperature...

Quenching Excimers and Exciplexes.—By measurements of decay times and fluorescence anisotropy of pyrene and the excimer in cellulose acetate films it has been found that the medium consists of spaces where small pyrene molecules have considerable freedom, Dissado and Walmsley have developed a complete theory of excimer formation and exciton-induced lattice distortion in crystals. Reference is made to data on 9-cyanoanthracene. The spectroscopy of chemically linked dimers of l,3-(l,l -dinaphthyl)propane in a... 

Dynamic Excimer Formation of Pyrene on Decanol-Covered Silica. The temperature dependence of the monomer and excimer fluorescence decays of Py adsorbed on silica gel with a monolayer coverage of 1-decanol has been studied by de Mayo et al. (39). The literature data for the two times T2 and x. of the double-exponent ial excimer decays are depicted in Fig. 9A. It is seen that the shorter decay time X2 approaches a value of about 60 ns upon lower ing the temperature, a value similar to the excimer lifetime of Py in methylcyclohexane (60), Fig. 9B. From the observation that the... 

Fig. 9. (A) Literature data (from ref. (39)) for the decay times and of the double-exponential excimer decay of pyrene...

The excited-state lifetime of the molecule in absence of any radiationless deeay processes is the natural fluorescence lifetime", r . The natural lifetime is a constant for a given molecule and given refraction index of the solvent. Because the absorbed energy can also be dissipated by internal conversion, the effective fluorescence lifetime, is shorter than the natural lifetime, The fluorescence quantum efficiency", i.e. the ratio of the number of emitted photons to absorbed photons, reflects the ratio of the radiative decay rate to the total decay rate. Most dyes of high quantum efficiency, such as laser dyes and fluorescence markers for biological samples, have natural fluorescence decay times of the order of 1 to 10 ns. There are a few exceptions, such as pyrene or coronene, with lifetimes of 400 ns and 200 ns, and rare-earth chelates with lifetimes in the ps range. 

A classic example of a three-state system is the intramolecular excimer formation with l,r-dipyrenylpropane [lPy(3)lPy], a dipyrenyl oligomer with three carbon atoms connecting the two pyrenes (see Fig. 15.14). Three species are observed one monomer and two excimers (sandwich-like and twisted conformations). It is worth noting that in the case of 2,2 -dipyrenylpropane [2Py(3)2Py] only one monomer and one excimer (less stable with a parallel sandwich-like geometry and decay time of 150 ns, [59]) are present, because the C2 symmetry of the pyrene-chain bond axis allows only one excimer conformation. Also, with the longer (ten carbon atoms chain) of l,r-dipyrenyldecane [lPy(10)lPy], see Rg. 15.13 above, only one monomer and one excimer are present, because the longer chain is sufficiently flexible to allow relaxation to the most stable conformation of the excimer (two-state system). 

In the vast majority of instances, the pyrene monomer fluorescence decays acquired with aqueous solutions of Py-WSPs, particularly when the pyrene labels are randomly incorporated into a WSP, are always multiexponential in nature. This experimental observation is a consequence of the distribution of distances between an excited pyrene and a ground-state pyrene or pyrene aggregate formed in water that leads to a distribution of rate constants for excimer formation by diffusion [30-32, 34]. Unfortunately, the analysis of multiexponential decays is notoriously difficult to handle and unless the decay times resulting from the photophysical processes are well resolved (i.e., separated by a factor of at least 2) as is the case for pyrene end-labeled short alkyl oligomers, the parameters retrieved from a triexponential fit should be considered with utmost caution [32]. Indeed early reports [38, 39] on the analysis of the fluorescence decays of Py-WSP based on the DMD model [40] or one of its variants introduced originally by Zachariasse to deal with the multiexponential decays of pyrene end-labeled oligomers yielded sets of parameters whose validity has been questioned [41]. 

The kinetic analysis of these multiexponential decays required a radical departure from the classic photophysical approach that attempted to assign a rate constant representing a specific photophysical process to each decay time. Instead, it was proposed that an excited pyrene label covalently attached onto a WSP would probe a restricted volume in the solution [34]. This proposal came from the consideraticMi that the motion of an excited pyrene label is strongly hindered as it must drag the polymer segment it is attached to in order to move through the crowded polymer coil and that its ability to form an excimer is limited by the time it remains excited since no excimer formation can take place upon encounter between two ground-state pyrenes. The volume probed by the excited pyrene label was referred to as a blob which could then be used to compartmentalize the polymeric ensemble of... 

Fluorescence Rise and Decay Curves. Both monomer and excimer fluorescence decay curves of the unirradiated film are nonexponential and the excimer fluorescence shows a slow rise component. This behavior is quite similar to the result reported for the PMMA film doped with pyrene. (23) A delay in the excimer formation process was interpreted as the time taken for the two molecules in the ground state dimer to form the excimer geometry. Dynamic data of the ablated area observed at 375 no (monomer fluorescence) and 500 nm (exciner fluorescence) are shown in Figure 5. When the laser fluence increased, the monomer fluorescence decay became slower. The slow rise of the excimer fluorescence disappeared and the decay became faster. 

We measured the time-dependent anisotropy of 1-pyrene carboxaldehyde in sulfonate A and B systems. The results are shown in Figure 2. Relaxation times determined from the unconvoluted anisotropy decays for sulfonates A and B in heptane solution were found to be 7 ns and 28 ns, respectively. 

The internal rotational relaxation times of 1-pyrene carboxaldehyde in sulfonate systems may offer some indication of the extent of probe binding to the inverted micelle. In the absence of any background fluorescence interference to the time-dependent anisotropy decay profile, the internal rotational relaxation time should correlate with the strength of binding with the polar material in the polar core. However, spectral interference from the aromatic moieties of sulfonates is substantial, so that the values of internal rotational relaxation time can only be used for qualitative comparison. 

Even taken qualitatively, these reactivity data have important toxicological as well as chemical implications regarding the composition of PAHs and PACs in and on the surfaces of aerosols in polluted air parcels, both near-source and during transport (downwind). Thus, under certain conditions (e.g., daytime, summer season, and high oxidant levels) over a period of hours BaP concentrations in ambient air could be expected to decay dramatically as a result of reactions, while those of the benzofluoranthenes and indeno[l,2,3-cabsolute concentrations also change as a result of dilution of the air parcel caused by increased mixing depth over time and transport. However, impacts of such physical processes are minimized if one considers ratios of concentrations of reactive to nonre-... 

FIGURE 10.30 Percent conversion-time profiles for the decay of 5 PAHs in diesel exhaust particulate matter (Dp = 0.5 p.m) collected on glass fiber filters and exposed to 1.5 ppm of ozone in air under Hi-Vol sampling conditions. Half-lives (dotted line) decrease in order of the Nielsen (1984) electrophilic reactivity scale (Table 10.30) BaP, benzo[u]pyrene BghiP, benzo[ghi]pclyIcne BeP, benzolelpyrene IndP, indeno[l,2,3-cd]pyrene BkFI, benzolk]fluoranthene (adapted from Van Vaeck and Van Cauwenberghe, 1984). 

Fig. 9. Decay of luminescence with time. Ordinate In (luminescence intensity) one division = 0.25. Abscissa time one division = 0.0003 sec. for curves (a) and (6) 0.0005 sec. for curve (c) 1.0 sec. for curve (d) and 0.1 sec. for curve (e). (a) and (6) Delayed fluorescence of pyrene monomer and dimer in ethanol at +23°C. (c) Delayed fluorescence of naphthalene in ethanol at —23°C. (d) Triplet-singlet phosphorescence of 10-W phenanthrene in EPA at 77°K. (e) Delayed fluorescence of 10-lAf phenanthrene in EPA at 77°K.

Figure 7.32 Kinetics of luminescence of pyrene following laser flash excitation. L, laser pulse profile M, monomer emission, E, excimer emission rise and decay. Horizontal axis, time in ns vertical axis, light intensity in arbitrary units. The three kinetic curves are normalized to a common maximum...

The importance of surface charge is amply demonstrated by the reductive quenching of surfactant [Ru(bipy)3]2+ derivatives by pyrene-N,AT-dimethylaniline (DMA). Figure 13 shows the decay of photogenerated [Ru(bipy)3]+ in the millisecond time domain for various different micellar environments and for free MeCN solution. Apparently the crucial step is the repulsion of DMA1 by the micelle and hence CTAC provides considerably more efficient charge separation than does SDS.328... 

Figure 8 shows a pair of typical time-resolved fluorescence decay traces for 100 / M pyrene in supercritical CO2 (Tr = 1.02 pr = 1.17). Note that the ordinate is logarithmic. The upper and lower panels show results for selective observation in the monomer (400 +. 10 nm) and excimer (460 + 10 nm) regions of the pyrene emission spectrum. Several interesting features are apparent from these traces. First, both decay processes are not single exponential. Second, the excimer emission has a significant contribution from a species that "grows in" between 30 - 75 ns this is a result of the excimer taking time to form (i.e., k in Figure 1). Third, the fits between the experimental data and the model shown in Figure 1 are good. Detailed analysis of these decay traces (10,11,21-26) yields the entire ensemble of photophysical kinetic parameters for the pyrene excimer in supercritical C02.

Figure 8. Time-resolved fluorescence decay traces for 100 fiM pyrene in supercritical C02. Tr = 1.02 pr = 1.17. Upper and lower panels represent monomer (400 nm) and excimer (460 nm) emission, respectively.

Work conducted by Tiller and Jones (1997) demonstrated that the fluorescence of PAHs decayed over time under both under anoxic and oxic conditions. Typically, however, the presence of dissolved oxygen had a more pronounced influence on baseline fluorescence decay for all the PAHs studied. Moreover, certain PAHs (pyrene and anthracene) were more susceptible to this phenomenon than others. To date a mechanism to explain this phenomenon has not been identified, but it is probably a combination of complex pathways including the reaction of the analyte with reactive oxygen species formed from the excited triplet state DOM and the direct photolysis of the analyte by the excitation light source. Thus, the application of fluorescence quenching for measuring Kdom is probably limited to systems, which can be analyzed under anoxic conditions. 

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