Photostationary state example

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... 

Many studies of benzene-sensitized isomerizations have been reported. One interesting example is provided by the behavior of cis,trans,trans-1,5,9-cyclododecatriene. With benzene photosensitization it is isomerized to the M-trans and the cis,cis,trans isomers, and a new, probably bicyclic, product is also formed in small yield.274 Another study employed a large number of sensitizers, many of which apparently reacted with the triene, and no particular trend was evident other than that the original isomer remained as the major geometric isomer in the photostationary state, with varying amounts of the all-trans and cis,cis,trans isomers being formed with different sensitizers.275... 

Shinkai et al.111-151 synthesized a series of azobis(benzocrown ethers) called butterfly crown ethers , of which compounds 9 and 10 are examples. Their photoresponsive molecular motion resembles that of a flying butterfly. It was found that the proportion of their Z forms at the photostationary state increases remarkably with increasing concentration of Rb+ and Cs+, which interact with two crown rings in a 1 2 sandwich fashion. This is clearly due to the bridge effect of the metal cations with the two crowns, results that support the view that the Z forms make an intramolecular 1 2 complex with these metal cations. As expected, the Z forms extracted alkali metal cations with large ion radii more efficiently than did the corresponding E forms. In particular, the photoirradiation effect on 9 is quite remarkable for example, ( )-9 (n= 2) extracts Na+ 5.6 times more efficiently than (Z)-9 (n= 2), whereas (Z)-9(n= 2) extracts K+ 42.5 times more efficiently than ( )-9(n= 2). l ... 

Differential interactions between cations in zeolites and the products of a photoreaction may result in selectivity. One such example is the selective photoisomerization of nms-l,2-diphenylcyclopropane to the cis isomer [136]. Triplet sensitization of 1,2-diphenylcyclopropane in solution results in a photostationary state mixture consisting of 55% cis and 45% trans isomers. When the same process is carried out within NaY zeolite the cis isomer is formed in excess of 95%. The preference for the cis isomer within NaY is attributed to the preferential binding of the cation to the cis... 

Another example is the thermal and photochemical cis-trans isomerization of Cp2Fe2(CO)2( -CO)( -Sip-TolH).25 In this case, both cis(H) and trans isomers can be isolated at full purity by flash chromatography. Interconversion between these isomers occurs both thermally and photochemi-cally in cyclohexane-d12, and the composition in the thermal equilibrium state (cis(H) trans = 2 98 at 25°C) is extremely different from that in the photostationary state (cis(H) trans = 70 30). Kinetics of the thermal isomerization in decalin afforded the activation parameters shown in Eq. (58). The large negative activation entropies imply that this reaction also... 

Rearrangements.—Various examples of Z,E-photoisomeri7ation about the carbon-nitrogen double bond have been reported. The E-esters (1) have been prepared by irradiation of the Z-isomers (2) in pentane-benzene, and quantum yields for the photochemically induced interconversions of the four stereoisomers of l-(methoxyimino)-3-phenylprop-2-ene have been determined. Particular interest has been shown in the isomerization of azines the E — Z photoisomerization of benzophenone-9-anthraldehyde azine is quenched by dyes with the appropriate singlet energy," with the quantum yield for this transformation being influenced by solvent polarity." The composition of the photostationary state has been determined for a number of arylazines "... 

The photochemical ring opening of palustric ester (90), an example of a [l,2]-aimulated cyclohexa-diene, has been achieved wherein a 1 1 photostationary state mixture of diene (90) and triene (91) was obtained. Upon heating for 4 h (162 C), the photo-mixture regenerates palustric ester (90). A related interconversion was observed for a series of photochromic cyclohexadienes of the xanthenone, dibenzo-furan and acridone types, exemplified by (92) -> (93). °... 

The effect of solvent polarity on interpreted qualitatively Because the highly polar excited states enter into strong interaction with polar solvents, their tendency toward geometric isomerization (which requires complete reorganization of the solvate structure) is naturally very low. Data from Schulte-Frohlinde and co-workers [5] show substantial and different polar solvents, for example, /V./V -dimethylformamide (DMF) or ethanol, photostationary state is correspondingly shifted to the trans side [152], With... 

Many other examples of the photostationary states are known. The decomposition of NO 2 occurs photochemically below400 nm. 

Another example of a geometrical photoisomerization process influenced by CD is the cis-trans photoconversion of cyclooctene in solid P-CD complexes. After prolonged irradiation of the cis-cyclooctene complex, an apparent photostationary state with a trans/cis ratio of 0.47 was detected, which was considerably smaller than that obtained in homogeneous fluid solution (0.96). This result is explained by the reduced rotational mobility of the guest in the inclusion complex. The asymmetric CD environment however, is not able to induce optical activity in the photoproduct, which shows an enantiomeric excess of only 0.24%. In this study, light of 185 nm, also absorbed by the CD, was used [302]. 

This is a relatively crude approximation. However, a graph of the change in absorbance versus time gives a linear slope for the beginning of the reaction under these conditions with a deviation less than 1% in comparison to the correct evaluation. These factors can be calibrated by other actinometers as for example the Parker solution [121]. The values given in Table 5.4 can be used to determine some of the lines of a mercury arc. The dashed line in the table indicates that for the measurement of the 254 nm line the other band of the azobenzene solution has to be taken. A trans solution must be preirradiated at 313 nm to bring the solution into the photostationary state. 

When aryl groups are substituted on the nitrogen of the C=N bond, the E isomers are stable at room temperature and undergo photoisomerization to Z isomers, which thermally revert to the E isomers [70]. For example, ( )-15b isomerizes to (Z)-15b on direct as well as triplet sensitization, giving photostationary state mixtures of Z and E isomers, and (Z)-15b reverts to... 

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