Photosensitized Stilbene Isomerization

Herkstroeter and Hammond found support for this postulate from a flash photolysis study. They were able to measure directly the rate of sensitizer quenching (energy transfer) by cis- and trons-stilbene. When a sensitizer triplet had insufficient excitation energy to promote troiu-stilbene to its triplet state, the energy deficiency could be supplied as an activation energy. The decrease in transfer rate as a function of excitation energy of the sensitizer is given by 

However, c -stilbene does not behave as a classical triplet energy acceptor inasmuch as the rate of energy transfer is much larger than would be predicted from Eq. (9.7) for donors with triplet energies less than 57 kcal/mole. This is then additional information consistent with Eq. (9.5). 

The initial quantum yields for cis- to traR. stilbene isomerization (Oo-.t) and for trans to cis isomerization (4 t o) consistent with Hammond s postulate that isomerization takes place from a common state, most likely the twisted or phantom triplet state  

The following generalizations were made for donors that transfer their energy at a diffusion-controlled rate (high-energy sensitizers)  

A short summary of quantum yield data is given in Table 9.1. Equations (9.11) and (9.12) can be easily derived from the suggested mechanism  

---

Figure 9.3. Variation of the stationary states in photosensitized isomerization of stilbenes with various sensitizers. [From G. S. Hammond, Kayaku to Kdgyd (Tokyo) 18, 1464 (1965).]...

Fig. 3. Photostationary states obtained in the photosensitized isomerization of stilbene as a function of triplet energy of the sensitizer. (From Wagner and Hammond.1)...

The photosensitized isomerization of ethyler.es and stilbenes has been studied in detail by Hammond and his group and it has been found that in each case both the cis-trans and the trans-cis isomerizations occur with equal efficiency. A common (tc, jt ) triplet state is assumed to be the intermediate in each case. This triplet state is referred to as the perpendicular triplet state and was termed a phantom triplet state by Hammond. The energy of the perpendicular triplet state is expected to be low since in this configuration the overlap between the re and rc orbitals is minimized. The potential energy of S0, Tx and Sx states as a function of twist angle is given in Figure 7.7... 

The photoisomerization of stilbene is one of the most extensively studied photoreactions (25). Solvent effects have been thoroughly investigated for both the direct and photosensitized isomerizations, and a model has been developed which attributes these effects to solvent viscosity (26). Increased viscosity inhibits direct photoisomerization of the cis isomer, but facilitates that of trans-stilbene. As a result, the cis/trans ratio of the photostationary state increases with increasing solvent viscosity. The wide range of viscosities which are attainable by pressure manipulation of supercritical carbon dioxide provides an excellent opportunity to probe the effect of viscosity on stilbene photochemistry in the same solvent. 

The discussion of cis-trans photoisomerization of alkenes, styrene, stilbene, and dienes has served to introduce some important ideas about the interpretation of photochemical reactions. We see that thermal barriers are usually low, so that reactions are very fast. Because excited states are open-shell species, they present new kinds of structures, such as the twisted and pyramidalized CIs that are associated with both isomerization and rearrangement of alkenes. However, we will also see familiar structural units as we continue our discussion of photochemical reactions. Thus the triplet diradical involved in photosensitized isomerization of dienes is not an unanticipated species, given what we have learned about the stabilization of allylic radicals. 

The role of the triplet state in the cis-trans isomerization of stilbenes effected by photosensitizers, such as acetophenone, benzophenone, or anthraquinone, which have large So Ti excitation energies, was first revealed in Ref. [65]. Theoretical considerations and experimental data on intermolecular triplet-triplet energy transfer leading to the sensitized stilbene photoisomerization are described in Section 4.2.2. It was shown that data on positional dependence of the heavy-atom effect on the cis-trans photoisomerization of bromostilbenes were consistent with the fact that, in contrast to the para position, the meta position is near a node in the highest occupied and the lowest unoccupied MO of stilbene [66]. According to [67], internal and external heavy-atom effects induce phosphorescence in frans-stilbene... 

---