Photoisomerization continued stilbenes

Photoisomerization and Related Processes - The photoisomerization of stilbene and the related photophysics of this molecule must surely be the most thoroughly examined of all single photochemical systems. Nevertheless further investigations continue... 

The rate constant with the form of eq.(S) can be obtained only when reaction tal place as a result of sequential two (or more) steps. It is easy to prove eq.(S) for a single intermediate state M, but in this case the a value in eq.(6) reduces to unity. It is when M is continuously distributed [described by a continuous variable X in eq.(4)] that a becomes smaller than unity.This form of eq.(S) has been used also by Troe for describing the pressure dependence of photoisomerization rates of stilbenes in solvents, without specifying sequential two steps underlying it. 

Squires, Venier, and Aida (1983) describe an experimental technique they use to study the effect of solvent viscosity on the cisitrans ratio of stilbene irradiated in supercritical CO2. They use a dynamic flow technique similar to that described in chapter 4. In their system trau5-stilbene is coated onto glass beads, which are then packed into a high-pressure column. Supercritical CO2 flows through the column and solubilizes some of the trans-stilbene. The C02-stilbene phase is continuously irradiated with ultraviolet light as it flows through a quartz photoreactor at a fixed temperature and pressure. As the solvent viscosity increases, the photoisomerization of the cis isomer is inhibited while that of the trans isomer is facilitated. We should expect to see the cisitrans ratio of stilbene vary as the density of CO2 varies. This viscosity effect is clearly shown in figure 11.11. While there is a small effect of pressure on the... 

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. 

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