Experimental Data on Photoisomerization

The Si-photoisomerization of ris-stilbene was investigated by femtosecond pump-probe absorption spectroscopy in compressed solvents [79]. The viscosity dependence confirmed the existence of two pathways of the reaction. One showed an inverse viscosity dependence and led to trans-stilbene, the other one indicated no viscosity dependence and led to dihydrophenathrene. 

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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... 

Figure A3.6.5. Photoisomerization rate constant of (ran.s -stilbene m n-pentane versus inverse of the self-diflfrision coefficient. Points represent experimental data, the dashed curve is a model calculation based on an...

Calculation results on a possible unified approach fit the experimental data (Figure 2) (99H(50)1115). Ab initio calculations showed a different level in the Dewar thiophene (02JPP(A)(149)31). More recently, the possible presence of a cage thiophene structure into the photoisomerization has been studied in order to explain some results that cannot be rationalized by using the above-described approach (09H(78)737). 

In comparing these results with experimental data it has to be remembered that in contrast to ketones, azo compounds can also undergo photochemical trans-cis isomerizations. (Cf. Section 7.1.7.) In the gas phase n-> r excitation results in photodissociation with nearly unit quantum efficiency. At higher pressures, however, and especially in solution, this reaction almost completely disappears and photoisomerization dominates. The latter is observed even at liquid nitrogen temperatures. This is understandable if it is accepted that photodissociation proceeds in the gas phase as a hot ground-state reaction. According to Figure 7.18, it has to overcome a barrier in the excited state and is therefore not observed in solution. For the trans-cis isomerization, on the other hand, no excited-state barrier is to be expected from the results in Section 7.1.7. 

The E Z photoisomerization of stilbene has been studied in considerable detail. We use the customary labels T (for trans) to designate structures near that of ( )-stilbene, c (for cis) for those near (Z)-stilbene and p (for perpendicular) for those near the transition-state geometry for E Zisomerization in the ground state. Figure 5.20 shows schematic potential energy surfaces (PESs) for S0, Si and Ti that are based largely on the experimental data discussed below. 

ITO-coated cell (8- to 10- tm gap) is subjected to holographically induced photopolymerization, grating can be formed with alternating regions of polymer and LC droplets. Figure 11.29 is a schematic illustration of the optical switching mechanism based on the reversible photoisomerization of azobenzene dopant mixed with the LC host (in droplets) as well as the supporting experimental data. 

A series of four platinum acetylide complexes that contain 4-ethynylstilbene (4-ES) ligands have been subjected to a detailed photochemical and photophysical investigation [83]. Using absorption, variable temperature photoluminescence, and transient absorption spectroscopy, UV-vis absorption, and NMR spectroscopy, it was shown that these compounds undergo trans-cis photoisomerization from the triplet excited state. The obtained experimental data indicated that in all of the complexes, excitation led to a high yield of a 3n,n excited state that is localized on... 

Theoretical considerations and existing experimental data indicate that under certain conditions, the rate of photoisomerization strongly depends on the microviscosity around the isomerized molecule and upon the effect of steric hindrance. In a viscous medium, the apparent rate constant of trans-cis photoisomerization ki o is controlled by the reorganization rate of the process in the medium (Equations 4.2 and 4.3). This method was used for the measurement of fluidity of biological membranes and microviscosity of a specific site of a protein. On this theoretical basis, fluorescence-photochrome immunoassay (FPHIA)] were used. 

Ab initio studies on the photochemical isomerization of thiazole derivatives have been reported. The results are in keeping with data from experimental and semi-empirical methods. An alternative mechanism for 2-phenylthiazole photoisomerization has been proposed (Scheme 5) <2002T8037>. 

Effects of Temperature and Viscosity. As can be seen from Tables 5a-e, the sum of and d>c, does not equal unity under most experimental conditions although it approaches unity in some cases. On the basis of these data it appears rather illusory therefore, to suggest whether or not cis z trans photoisomerization proceeds via a common intermediate [89, 90, 92, 99],... 

The primary photoreactions in the PYP have been experimentally studied both in the protein and in solution environments. The initial structural change of PYP was directly observed by time-resolved x-ray crystallography. Ultrafast fluorescence spectroscopy was performed on the initial process of the photoreaction of PYP. It was shown that the photoisomerization reaction of the PYP chromophore is completed within 1 ps. The initial photoreaction processes were analyzed by using time-resolved spectroscopic data. Although the reaction of the PYP chromophore in solution environments has been studied by several groups, the characterization of the dynamics in these environments is not fully understood. 

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