Trans -Stilbene, photoisomerization

Figure A3.6.8. Isothemis of k ) for tran.s-stilbene photoisomerization in n-hexane at temperatures between...

Schroeder J, Schwarzer D, Troe J and Voss F 1990 Cluster and barrier effects in the temperature and pressure dependence of the photoisomerization of trans.stilbene J. Chem. Phys. 93 2393-404... 

Sensitivity to the magnetic held strength has also been observed in the process of photoisomerization of tran -stilbene (tS) into c -stilbene (cSt) in the presence of pyrene (P). The reaction was run in a solution with acetonitrile, DMSO, or hexafluorobenzene being a solvent. During the reaction, spin polarization was established (Lyoshina et al. 1980). The spin polarization was explained by the following sequence of the elementary acts ... 

Diphenylethene (Stilbene). This molecule has been the subject of many photophysical and photochemical investigations and the subject of several reviews. It is the prototypical alkene for studies of photoisomerization. Transient spectroscopic measurements in the picosecond time domain have been performed on electronically excited tran -stilbene in a wide range of environments. Selections from these studies are described here. 

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

Organic Chemistry in Supercritical Fluid Solvents Photoisomerization of trans- Stilbene... 

Investigations of organic reactions in supercritical solvents are subject to several constraints, one attributable to supercritical fluid properties and others imposed for interpretive and experimental simplicity. Because supercritical fluid properties are affected by changes in temperature, a reaction should be selected which does not require heat for initiation and is not highly exothermic. Additionally, for experimental simplicity and clarity of interpretation, a clean, well-understood reaction should be chosen and one should expect an experimentally observable response to changes in pressure. Finally, a unimolecular reaction which produces a single product obviates the complication of controlling the concentrations of two reactants and simplifies product analysis. The photoisomerization of trans-stilbene meets these requirements. 

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. 

Photoisomerizations were carried out using a 450 watt Hanovia medium pressure mercury source, and each reaction was followed to the photo-stationary state by gas chromatographic determination of the cis/trans ratios. Twice recrystallized trans-stilbene (Aldrich), spectrophotometric grade cyclohexane, and dry, high purity carbon dioxide were used in these experiments. 

A widely used method for assessing supercritical fluid phenomena consists of comparing physical and chemical behavior above the critical point with corresponding behavior in the subcritical liquid. Because this approach (unrealistically) seeks to observe discontinuous behavior between states, the results of such experiments are often ambiguous. In the present study, we have compared the photoisomerization of trans-stilbene in subcritical and supercritical C02 and, to model liquid behavior, we have also carried out these isomer-izations in cyclohexane. In all three systems, the effects of temperature and concentration on the cis/trans ratio were compared and, for the C02 systems, the effect of pressure on this photostationary ratio was also probed. The results from these experiments are shown in Tables I and II and are plotted in Figures 4 through 6. 

Figure 4. Concentration effects in the photoisomerization of trans-stilbene.

Schroeder J, Troe J, Vohringer P. Photoisomerization of trans-stilbene in compressed solvents Kramers-turnover and solvent induced barrier shift. Z Phys Chem 1995 188 287-306. 

Saltiel, J., Waller, A.S., and Sears, D. F., Jr. (1992) Dynamics of cis-stilbene photoisomerization the adiabatic pathway to excited trans-stilbene, J. Photochem. Photobiol., A 65, 29-40. 

Stilbene is well known to undergo a trans-cis photoisomerization upon irradiation with UV light.1 In addition to this isomerization, stilbene shows a photo-cyclization reaction to produce dihydrophenanthrene. 

The trans-cis photoisomerization of the stilbene-type ligand in the Re(I) diimine complexes has been reported (55). 

Solvent reorientation and isomerization of trans-stilbene in alkane solutions has been studied by ps time scale anisotropic absorption and polarization239 Coupling of solute and solvent decreases as the size of the solvent molecules increases. The applicability of currently favoured models for the activated barrier crossing in the photoisomerization of stilbene is discussed, A method for measuring quantum yields in the photoisomerization of trans-stilbene gives high accuracy without use of a chemical actinometer . Evidence has been found for dynamic solvent effects on the photoisomerization of 4,4 -dimethoxystilbene in which the effects of temperature and hydrostatic pressure were made in n-alkane and n-alkyl alcohol. A ps laser time-resolved study fits frequency dependent solvent shifts but gives results inconsistent with the free volume model. Photophysical and theoretical studies of trans and 9-... 

The photoisomerization of cis-and trans-stilbenes has been studied in ionic liquids and evidence is presented that the mechanism depends on the particular ionic liquid used. In basic N-butylpyridinium chloride/AlCb the photoequilibration involves stilbene radical cations, whereas in basic l-ethyl-3-methyl-imidazolium chloride/AlCb the process occurs via the standard singlet state photoisomerization mechanism. It has been found that the tetra-O-acetylribo-flavin (126) sensitized dehydrogenation of substituted benzyl alcohols, giving the... 

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... 

A different type of environmental effect is manifested in the photoisomerization of trans-stilbene complexed to cyclodextrin. In picosecond laser studies a single decay in a-cyclodextrin shows binding occurs at one site, whilst in the (3-forra... 

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