Stilbene isomerization, azulene quenching

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. 

More recently, however, Saltiel has shown that direct irradiation of the stilbenes produces isomerizing excited states that are quenched by azulene via singlet-singlet dipole-dipole interactions (see p. 693). Thus it now seems more likely that isomerization of unsubstituted stilbene, following direct excitation, takes place from the Sx states.54 />-Bromostilbene, which undergoes ISC more readily than stilbene, isomerizes from both its S and its T1 states.55... 

Stilbene 1 undergoes isomerization in the singlet excited state on direct irradiation [13,16-23]. The mechanism was established by J. Saltiel et al. from the experiments of the azulene effect on the photostationary state isomer composition [13,24,25]. Thus, usually, quenching experiments as well as observation of the transient spectra are necessary to distinguish the singlet and triplet mechanisms of isomerization of olefin on direct irradiation. Furthermore, triplet sensitization is necessary to study the isomerization in the triplet manifold. 

Other quenchers that have been used in the benzophenone-sensitized cis trans isomerization of stilbene are /9-carotene, oxygen, and di-tert-butyl nitroxide. /9-Carotene shifts the photostationary state to the trans side similar to the azulene effect [237]. In the presence of di-tert-butyl nitroxide, a radical quencher, the photostationary state is slightly shifted to the cis side Caldwell and Schwerzel [226] have suggested the involvement of the twisted triplet state and a quenching mechanism other than energy transfer, probably vibrational relaxation to the ground state caused by spin exchange. 

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],... 

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