Monomer lifetime

Birks 68) has proposed that the only change between the unexcited and excited pyrene pair is a reduction in the interplanar distance from 3.53 to 3.37 A, i.e. that the pyrene excimer is not a completely eclipsed sandwich pair either in solution or in the crystal. This proposal is consistent with the observed similarity of the excimer band position for the crystal and solution environment, and with the emission of excimer fluorescence from the crystal even at 4 K. For naphthalene, the greater separation and the nonparallel structure of nearest-neighbor pairs in the crystal apparently prohibits the formation of the sandwich excimer during the naphthalene singlet monomer lifetime. Thus, no excimer fluorescence is observed from defect-free naphthalene crystals. 

The final expression is in the form of the Stern-Volmer equation, where tj, is the monomer lifetime in dilute solution and te is assumed to be excimer lifetime in infinitely dilute solution. 

We also determined the temperature dependence of the emission lifetimes, as shown in Figure 10. It is remarkable that the excimer decay lifetime is almost completely temperature independent, showing no detectable rise time, while the monomer lifetime decreases rapidly with temperature. An Arrhenius plot of the monomer lifetime data is shown in Figure 11. It gives, within experimental error, the same activation energy as that for the monomer intensity. Figure 9. 

Figure 11. Arrhenius fit to the monomer lifetime data of Figure 10.

Hence, the pyrene monomer lifetime is decreased accord, ing to the Stem-Volmer equation, where the quencher concentration is replaced by the ground-state pyrene monomer concentration. 

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... 

Using the same approximations as in the Aniansson and Wall treatment of the surfactant exchange process the monomer lifetime % has been shown to be given by ... 

The onset of mobility, detected at the beginning of the decrease of the monomer lifetime, occurs about - 20°C in PB, that is 70 C above the static reference temperature Tg measured at 1 Hz. This shift illustrates the time-temperature superposition principle the motion is observed at the dynamic glass transition which corresponds to the fre-... 

Fig, 3 - Temperature evolution of the monomer lifetime of diphant in polybutadiene and styrene-butadiene copolymers and of the model compound ( ) in polybutadiene... 

The data obtained for diphant in PB ana its copol3nners show clearly that at a given temperature, the monomer lifetime, and consequently the rate of conformational change of the probe, is affected by the host matrix. 

Our objective in choosing to study these particular probes in polymer matrices is that the monomer lifetime of pyrene is over twenty times longer than that of the carhazolyl or the phenyl-anthryl group. The lifetime of the excited pyrene group is 184 ns in meso-DIPP and 235 ns in meso-BIPEE in a PB film at - 90°C, so that the two molecules are appropriate probes for investigation of the relaxation phenomena in the time range 10 - 10 s. 

The onset of mobility, accurately detected on account of the long monomer lifetime, occurs at about - 50°C for both compounds dispersed... 

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