Trans-Stilbene photoisomerization concentration effects

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.

Studies of solvent polar effects on the fluorescence spectrum of immobilized stilbenes indicated that the maximum wavelength of the fluorescence emission is not very sensitive to solvent polarity. The apparent local polarity of the medium in the vicinity of the stilbene label was estimated. The trans-cis photoisomerization kinetics of the stilbene derivatives in the immobilized and free state in a medium with different viscosity was monitored by fluorescence technique at constant illumination conditions. Investigation of the microviscosity effect on the photoisomerization of the immobilized and free stilbene label was carried out by changing the relative concentration of glycerin in a glycerin-water mixture used as a solvent. With an appropriate calibration, the microviscosity in the vicinity of the stilbene label was estimated. The apparent photoisomerization rate constant of the process was found to be 3-4 times less for the immobilized label than in a free state. 

An interesting result was also obtained (10) in a study of polyamides with stilbene residues in the chain backbone. In that case,an increasing polymer concentration led to a decreasing fluorescence intensity. This effect can be understood since the trans-cis isomerization and the emission from the excited stilbene moiety compete with each other. Thus, an increasing concentration of chain molecules which hinders the photoisomerization favors the fluorescence. This effect is observed long before the systems becomes glassy and it is, therefore, distinct from the enhancement of fluorescence in rigid media in dyes whose emission is quenched by internal motions of the excited molecules (11). 

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