Decay ratios, stilbene

The value of the pss calculated from Eq. (9.24) is 2.74 and the measured value<7> is 2.64. While this agreement could be taken as evidence in favor of a triplet mechanism, it could also mean that the twist singlet and twist triplet have similar geometries (see Figure 9.2), and therefore similar decay ratios. In this mechanism we have neglected fluorescence from fraws-stilbene and dihydrophenanthrene formation from cw-stilbene. (See Problem 4.)... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

Similar observations were made by Saltiel in a study of the isomerization of 2-pentene.14S In contrast to the stilbene system, a single decay ratio could not account for the results with different sensitizers (Table VI). This implies that different intermediates were produced with different sensitizers. 

In non-viscous organic solvents the fluorescence efficiency of most SNA derivatives is comparable to that of TS as are the yields of trans to ois photoisomerization (33,3A). The double chain derivatives, MSNA and MSN, show substantially higher fluorescence yields (0.2 0.02) in non-viscous solvents with a corresponding decrease in the trans to ois isomerization efficiency (3A). In the sensitized isomerization, the MSN derivatives investigated thus far show only photoisomerization but there is an increase in the triplet lifetime by a factor of 2 however there is no change in the decay ratio of the isomerization precursor and the sum of the benzophenone sensitized isomerization efficiencies of ASA is approximately unity. When water insoluble SNA or MSNA molecules are spread as a film, either pure or in mixtures with insoluble fatty acids, at the air-water interface, it is found that the films show comparable behavior on compression to those of pure fatty acid with an indicated area per molecule of the surfactant stilbenes very close to that of a linear fatty acid (oa. 202 /molecule) (35). This suggests strongly that the TS chromophore in these molecules offers suitable molecular dimensions to pack into a crystalline-like array of linear parafin chains. 

Figure 3.18. Time dependence of the peak position of the 1570 cm Raman band of Sj trans-stilbene in chloroform solution (filled triangle). The time dependence of the anti-Stokes/Stokes intensity ratio is also shown with open circles. The best fit of the peak position change with a single-exponential function is shown with a solid curve, while the best fit of the anti-Stokes/Stokes intensity ratio is shown with a dotted curve. The obtained lifetime for both single-exponential decay functions was 12ps. (Reprinted with permission from reference [78]. Copyright (1997) American Chemical Society.)...

Figure 28. Fluorescence decays and double exponential fits corresponding to the 20S cm"1 bands in the (from top to bottom) vlb = 1237, 1241, 1246,1249, and 1332 cm-1 spectra of jet-cooled c-stilbene. Given in the figure are the best-fit parameters for both the fast and slow lifetimes, and the ratio (F/S) of preexponential factors of fast versus slow fluorescence. All decays were obtained with R = 3.2 A. 

Two DHPs were obtained from tetramethoxystilbene [279]. Substitution by a nitro group in the meta or para position reduces distinctly [82], Saltiel et al. have questioned whether the values may be erroneous [105], On the basis of quenching measurements with azulene they proposed additional routes for bromostilbenes from c via excited states of DHP which may relax back to 3c or c. A consequence of a higher value of for the mechanism of cis -> tram isomerization is that the ratio of c decaying to the trans isomer may have to be reexamined. For trans-ct-bromostilbene and the / -phenyl substituted derivatives several photoreactions (e.g., debromination) compete with photocyclization [475]. Interestingly, no evidence for photocyclization could be found for several fluorinated stilbenes [481]. Rotamers can be distinguished in the cyclization of c/s-2,2 -DNE [482],... 

In view of the well-documented ability of Oj to function generally as an acceptor of triplet energy, it had been expected that its presence should also increase ([t]/[c])3 ratios for the triplet sensitized photoisomerization of stilbene-like olefins. However, as was first noted for nitrostilbenes, the quenching of the olefin triplets by O2 does not alter ([t]/[c])s (42,43). For the parent stilbene it was shown that when azulene is used as a quencher of stilbene triplets the slope of the ([t]/[c])s-vs. -[Az] plot is strongly attenuated by O2 see Figure 5 (41,44). Since the intercept of the azulene plot was not influenced by O2, it was reasoned that O2 deactivates p without changing the decay firaction 8 as predicted by Schemes 1 and 2 (44). To account for stilbene triplet deactivation by O2 without change in 8 two possibilities were considered ... 

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