Emission decay, pyrene excimer

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.

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. 

Fig. 46 Pyrene monomer and excimer decay profiles in SDS micellar solutions [SDS] = 8.2 X 10 kmolm , [NaCl] = 10 kmolm , CMC = 1.5 x 10" kmolm", pyrene levels are indicated as the ratio of micellized SDS to added pyrene emission monitored at 383 nm for monomer and 480 nm for excimer. (A) Monomer emission for SDS/Py = 2160, (B) monomer emission for SDS/Py 108 (C) excimer emission for SDS/Py = 108...

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...

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.

It is concluded that below the transition temperature the monomer intensity increases with increasing temperature due to excimer dissociation to excited-state monomer. Above the transition temperature the excited-state equilibrium is apparently broken because the thermal energy is sufficient to activate excimer non-radiative decay by dissociation to ground-state monomer. Consequently, the monomer emission no longer increases with increasing temperature and the isobestic point disappears. Pyrene has been used as a fluorescent probe to monitor the conformational state of maleic acid with... 

Pyrene photophysics has produced the usual, and now to be expected, crop of papers on diverse topics. Aggregation in concentrated solutions has been evidenced by the Shpol ski effect and two photon excitation spectra have yielded new electronic state assignments. The maximum entropy method, mentioned earlier, for the determination of fluorescence lifetimes shows that in dipyrenylpropane the data for luminescence decay do not fit a simple 3-state model.This study adds more information upon a system hitherto open to much dispute. It is unlikely that the problem will be considered as solved. The presence of a ground state dimer of pyrene moieties has been shown by NMR in the bichromophoric molecules of (pyrenylcarboxyl) alkanesand also with racemic and meso dipyrenyl alkanes. Strong circular polarization in excimer emission has been detected from pairs of pyrene groups linked to a polypeptide chain. ... 

As far as the excimer decay kinetics of PAA in aqueous media is concerned, de Melo and coworkers [122,130,131] have investigated the time-resolved fluorescence from a series of samples modified with various amounts of pyrene and naphthalene, respectively. Even when the aromatic content was as low as 2mol%, excimer formation was evident in the steady-state spectra. The fluorescence decays were complex irrespective of the label and were best modeled by a triple-exponential function (as in Eq. 2.8) both when emission was sampled in the monomer and excimer regions. In contrast to the distribution of rate constants in the blob model [133,134], the authors favored a scheme that describes the decay kinetics in terms of discrete rate constants. The data were also consistent with previous schemes [124-127] that account for the presence of two distinct types of monomer in addition to that of excimer in macromolecular systems one monomer enjoys kinetic isolation and is unable to form excimers, whereas the second is able to participate in excimer formation within its fluorescence lifetime. The authors [130] concluded from both steady-state and time-resolved data that PAA undergoes a conformational change from a compact form in acidic solution to an open coil at high pH. Furthermore, as the... 

At high concentrations of pyrene (112), where there is a substantial probability of locating more than one pyrene molecule in the same micelle, excimer emission is the dominant photophysical process to follow excitation (Figure 10). The system may be treated quantitatively by Poisson distribution statistics and applied successfully to Si decay in surfactant (112). A similar treatment of naphthalene fluorescence in micelles has been presented. ... 

Due to the fact that the two emission bands of pyrene (monomer and excimer) are well separated, the monomer and excimer decays can be measured without mutual interference, and analysed with the two-state model (Elqs. 15.33-15.38). For the intermolecular case, the monomer lifetime is measured with pyrene at very low concentration (< 10 mol dm ), but for the intramolecular case a model... 

In Fig. 15.13, the fluorescence decays of l,l -dipyrenyldecane [lPy(10)lPy] in n-decane, are presented. In this compound, the two pyrene units are connected by a saturated carbon chain of 10 carbons, in this case only one excimer (more stable and non-parallel conformation) and one monomer exist this leads to a bi-expo-nential decay in which the sum of pre-exponential factors at the emission wavelengths of the excimer cancel out. 

Figure 12.8. Pyrene in a cetyltrimethylammonium chloride (CTAC) micelle at [CTAC] = 0.010 mol dm. A, [pyrene] = 7.5 x 10 mol dm B, 5.2 x 10 mol dm C, 1.0 x lO"" mol dm D, 2.08 x lO"" mol dm" , experimental points from transition experiment for monomer decay. Inset longer wavelength emission from the pyreme excimer obtained by steady-state experiment normalized to emission for A." ...

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