Pyrene fluorescence lifetime

The probe molecule pyrene (-10"6 M) was used in time-resolved fluorescence quenching experiments using a single photon counting apparatus, cetylpiridinium chloride (CpyC, 10"3 M) being introduced as a quencher of the pyrene fluorescence[ll-13]. All the experiments were performed at 303K. From these fluorescence studies the micelle aggregation number (N) and the pyrene fluorescence lifetime (x) were obtained [14]. 

Systems CTAB/SKVNaOH, CTAC/Si02/NaOH, and CTAB/SiO/TMAOH/MeOH Values of the pyrene fluorescence lifetime (t) and micelle aggregation number (N). ... 

In the presence of D4R species as silica source (second set of experiments Table 1), the aggregation numbers, with and without silica (experiments 11 and 10, respectively), are characteristic of spherical micelles. Moreover, the addition of D4R units has no effect on the pyrene fluorescence lifetime (x), which means that there is no Br /silicate exchange in this micelles-containing system. As it is well known [21], the presence of methanol leads to a decrease of the aggregation number (compare experiments 9 and 10). 

The formation mechanism of the material was also studied by fluorescence techniques, and more particularly by time-resolved fluorescence [24]. In these experiments, the synthesis temperature was lowered down to 60 °C in order to slow down the kinetics of precipitation. The value of the aggregation number (N = 104) at the beginning of the experiment indicates that the micelles in the precursor solution have a quasi-spherical shape (axial ratio equal to 1.8) and interact with urea molecules. The pyrene fluorescence lifetime decreases when aluminum nitrate is added to a solution containing SDS and urea. Consequently, nitrate anions, which are fluorescence quenchers, are at the micelle surface. 

In these solutions, K-(A -X)/(Ai-X2), F-K/(Ai-A2), A-(Ai X)/(X-Y), X-ki+k2+k3, Y-k4+k5, and Aj and A2 are given by Eqn. 16. For several well known (30) limiting cases, A3 and A2 are equivalent to and r2, the lifetimes of the pyrene singlet state and excited state complexes, respectively (see Eqns. 9-11). Activation parameters for pyrene excimer formation were calculated by two Independent methods. Since kx+k2 is known to be virtually temperature independent and k4 and ky are negligible(31), the ratios of fluorescent intensity maxima from the pyrene excimer and monomer maxima (Ig/Itl) the inverse of temperature yield the activation energy for pyrene excimer formation, E3. A similar experiment for the pyrene-CA system was not possible since its exciplex is not emissive. Activation parameters for the excimer and exciplex were also obtained from temperature and phase dependent pyrene fluorescent lifetime data. In the 1iquid-crystalline and isotropic phases of M, all pyrene decays were single exponential and the excimer decays could be expressed as the difference between two exponentials. 

The pyrene fluorescence lifetime, Xq, Table 1 and Figure 4, shows a decrease with... 

Fig. 7.2 Micelle aggregation number N (O), pyrene fluorescence lifetime tq (A), and fluoresecnece intensity ratio /1//3 ( ) as a function of lithium perfluorooctane-sulfonate concentration at 25 C. (From Ref. 17. Reproduced by permission of Academic Press, Inc.)...

Muto et al. [252] measured pyrene fluorescence lifetime Tq and the ratio IiHt, of the intensities of the first vibronic and the third vibronic band of the monomeric pyrene. The pyrene fluorescence data revealed the existence of a single type of mixed micelle in solutions of LiDS-LiFOS, LiFOS-hexaoxyethylene glycol do-decyl ether, or LiFOS-octaoxyethylene glycol dodecyl ether mixtures. The lifetime and the intensity ratio of vibronic peaks have been used to determine the cmc of fluorinated surfactant micelles [253]. However, the solubility of pyrene in micelles of fluorinated surfactants is not adequate for determining the micelle aggregation number [253,254]. 

We find that the fluorescence yield of freshly prepared covalent (+)-anti-BaPDE-DNA adducts in oxygen-free solutions is 66+2 lower than the yield of the tetraol 7,8,9,10-tetrahydroxytetrahydro-benzo(a)pyrene (BaPT) in the absence of DNA. Since the fluorescence lifetime of BaPT under these conditions is 200ns, the mean fluorescence lifetime of the adducts (see reference T7) can be estimated to have a lower limit of 3ns, which is close to the mean value of 0.52x1.6 + 0.42x4.0 = 2.7 ns estimated from the two short fluorescence components of Undeman et al (10). 

In order to test further the applicability of 1-pyrene carboxaldehyde as a fluorescent probe, we applied Keh and Valeur s method (4) to determine average micellar sizes of sulfonate A and B micelles. This method is based on the assumption that the motion of a probe molecule is coupled to that of the micelle, and that the micellar hydrodynamic volumes are the same in two apolar solvents of different viscosities. For our purposes, time averaged anisotropies of these systems were measured in two n-alkanes hexane and nonane. The fluorescence lifetime of 1-pyrene carboxaldehyde with the two sulfonates in both these solvents was found to be approximately 5 ns. The micellar sizes (diameter) calculated for sulfonates A and B were 53 5A and 82 lOA, respectively. Since these micelles possesed solid polar cores, they were probably more tightly bound than typical inverted micelles such as those of aerosol OT. Hence, it was expected that the probe molecules would not perturb the micelles to an extent which would substantially affect the micellar sizes measured. 

Polarity Variation in Sulfonate Micelles. Other workers have established a correlation between the fluorescence lifetime of pyrene in solution and the polarity of the solvent medium (9). Polar media quench the excited electronic state of pyrene and hence shorten its fluorescence lifetime. We applied this principle to measure the polarity variation within the micelles of sulfonates A and B. 

Figure 5. Plots of the fluorescence lifetime of pyrene as a function of distance from the polar core of the micelles of sulfonates A and B in heptane solutions.

Pyrene carboxaldehyde has utility as a fluorescent probe in some Inverted micellar systems containing solubilized Inorganic species in the polar core. Its fluorescence lifetime is ca. 5 ns thus it is an appropriate probe for measuring micellar sizes which are approximately lOOA. 

As Fig. 15b illustrates, the graphical relation appears to be linear for an interaction number of 3 to 4, if A 1. Alternatively, for A = 1, linearity is evident (Fig. 15c) when the interaction number is 5 to 6. Thus a large value of A is compatible with the smallest interaction number. Excimer formation occurs within the fluorescence lifetime, about 8 nsec. Within that time the pyrene-labeled amine side chains must approach within about 4 A of each other. For the 5.3% pyrenylpolyethylenimine derivative in ethanol, where no ground-state association occurs, the effective local concentration of pyrene on the polymer matrix is about 10-2 M, as calculated from excimer fluorescence. In aqueous solution, where clusters form within the polymer matrix, the effective local concentration of pyrene adduct must be even greater. The quantitative assessment of fluorescence intensities (Fig. 15) points to a minimum interaction number of 3 to 4 pyrenyl-labeled amine side chains, within the 8 nsec lifetime. Since A 1, it appears from (12) that kDM(A) kMD + kD. Thus excimer formation must be very rapid in the polymer environment. We can conclude, therefore, that the primary-amine side chains of poly-ethylenimine are very flexible and mobile. 

Concentration of pyrene, c Lifetime of delayed fluorescence, msec. ... 

Depending on the number of these cycles the final oxygen concentration is determined by the residual pressure of the pump. Quantitative analysis of traces of oxygen in liquids can be done by the measurements of long-lived luminescence lifetimes (e.g. pyrene fluorescence) and applying the Stern-Volmer equation. 

Fluorescence techniques have been used with great success in the study of PEO-fe-PSt micelles [64]. In this study, the effect of polymer concentration on the fluorescence of pyrene present in water at saturation was studied. Three features of the absorption and emission spectra change when micellization occurs. First, the low-energy band of the (S2-So) transition is shifted from 332.5 to 338 nm. Second, the lifetime of the pyrene fluorescence decay increases from 200 to ca. 350 ns, accompanied by a corresponding increase in the fluorescence quantum yield. Third, the vibrational fine structure changes, as the transfer of pyrene from a polar environment to a nonpolar one suppresses the permissibility of the symmetry-forbidden (0,0) band. 

The overall oxygen sensitivity exhibited by an optical sensor is basically predefined by the Stern-Volmer constant Ksv. The sensitivity of the final optical oxygen sensor increases with Ksv [65]. Generally, high Ksv values are provided by the Pd- and Pt-porphyrin complexes, by Ru(dpp)3, and by pyrene. Fluorescence quenching by oxygen not only affects the fluorescence intensity of the dye, but also has an influence on its lifetime r (Fig. 6) ... 

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