Photoisomerization pressure effects

Figure 6. Pressure effects in the photoisomerization of trans-stilbene.

This chapter is organized as follows. Section 4.2 addresses the study of photoisomerization and photoinduced orientation of azobenzene molecules at the molecular level in SAMs of azo-silane molecules. Section 4.3 discusses photoinduced effects in supramolecular assemblies, i.e., LBK multilayer structures containing azobenzene molecules, and compares the photoinduced movement of azobenzenes in these structures to that observed in spin-cast films. Section 4.4 focuses on the isomerization and sub-Tg photoinduced orientation in a series of very high Tg (up to 350°C) nonlinear optical polyimide and thermoplastic donor-embedded polyurethane polymers containing azo dye, especially focusing on polymer structure-Tg-photoinduced molecular movement relationships. Section 4.5 describes pressure effects on photoisomerization and photo-orientation in films of a PMMA polymer containing azo dye. Finally, we make some concluding remarks in Section 4.6. 

Kaneda, M., Asaoka, S., Ikeda, H., Mori, X, Wada, X, and Inoue, Y, Discontinuous pressure effect upon enantiodifferentiating photoisomerization of cyclooctene, /. Chem. Soc., Chem. Commun., 1272, 2002. 

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... 

Meyer A, Schroeder J and Troe J 1999 Photoisomerization of f/ a/rs-stilbene in moderately compressed gases pressure-dependent effective barriers J. Phys. Chem. A 103 10 528-39... 

The key step in their-approach was asymmetric photoisomerization of the a, 3-unsaturated (Z)-ketone precursor 103b in diethyl ( + )-Lg-tartrate. The bromide 103a obtained by N-bromo-succinimide bromination of the (Z)[8.8] precursor 64b, was converted into the a,P-unsaturated (Z)-ketone 103b by the routine synthetic procedures. Irradiation in a hexane solution with a medium pressure Hg lamp effected the photoisomerization of the (Z)-precursor 103b to afford a 1 5.5 mixture of (Z)-( )[8.8] ketones. After these preliminary experiments, a neat solution of 103 b in diethyl ( + )-Lg-tartrate was irradiated for 3 h. Preparative GLC of the resulting 1 7 mixture of (Z) and ( )[8.8] ketones produced a 38 % yield of ( )[8.8] ketone 104 enriched in the (—)-enantiomer, [a]n4 —13° (hexane). 

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. 

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. 

The above result unequivocally demonstrates that the seemingly weak solvation effect can play a decisive role, which controls and even switches the stereochemical outcome of the enantiodifferentiating photoisomerization. Temperature, pressure, and solvation, which function as environmental factors to control the enantiodifferentiation in the excited state, are all entropic in nature. Probably the key is the critical control of the weak interactions involved in the exciplex intermediate, as with the biological and supramolecular interactions in... 

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-... 

Figure 5 The effect of pressure on the enantioselectivity of the asymmetric photoisomerization of cyclooctene using several benzene(poly)carboxylate sensitizers (see Fig. 3 for sensitizer structure).

The effect of pressure was studied for a series of such photoisomerization reactions [123] and the volumes of activation for the photochemical reaction, AV, estimated from the plot of ln[/(l — )] versus pressure, are summarized in Table 9. In most cases, the absolute values of AV are small and are difficult to interpret. Dissociation of Cl- or Br would result in charge creation and an associated volume decrease due to the increase in electrostriction. Thus the intrinsic volume increase associated with ligand dissociation from the ES can be partially or totally compensated for by the volume collapse due to the increase in electrostriction. This is not a complication when NH3 is the leaving group, as demonstrated by the data in Table 3, and discussed in more detail in Section IV. The overall values of AV are such that they support the operation of a dissociative mechanism,... 

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 crown ether (74) has been used to determine the mode of cis to trans isomerization of derivatives of azobenzene. The process occurs via an inversion mechanism rather than a rotation mechanism, as was shown by the small effect of pressure on the course of the reaction. The azo system (75) also photoisomerizes by inversion in this case, steric factors exclude the possibility of a rotation mechanism. ... 

---