Double photoisomerization

Vaveliuk, P., Scafifardi, L.B., and Duchowicz, R., Kinetic analysis of double photoisomerization of the DTDCl cyanine dye,/. Phys. Chem., 100,11630,1996. 

In the case of cycloheptenone and larger rings, the main initial photoproducts are the -cycloalkenones produced by photoisomerization. In the case of the seven- and eight-membered rings, the double bonds are sufficiently strained that rapid reactions follow. In nonnucleophilic solvents dimerization occurs, whereas in nucleophilic solvents addition occurs. ... 

The non-preservation of cis stereochemistry of dienophiles 24 and 26 in the adducts 25 and 27 is due to a cis-trans photoisomerization of the double bond and to the concerted suprafacial Diels-Alder cycloaddition of diene to the ground state of trans dienophiles. 

A detailed analysis of photoisomerization of the nitrone group in the nitroxyl radical 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-3-oxide-l-oxyl, based on double electron-nuclear resonance methods, has shown that in the absence of oxygen the photochemical reaction occurs without affecting the radical center. The... 

The E,Z-photoisomerization of previtamin D to tachysterol has also received recent attention. Jacobs and coworkers examined the process in various solvents at 92 K and found evidence for the formation of a triene intermediate which converts thermally (Ea ca 6.5 kcal mol 1) to the more stable tEc rotamer of tachysterol (tEc-T equation 58)230. The rate of this conversion is viscosity dependent. They identified this intermediate as the cEc rotamer, produced by selective excitation of the cZc rotamer of previtamin D. In a re-examination of the low temperature ,Z-photoisomerization of previtamin D as a function of excitation wavelength, Fuss and coworkers have suggested an alternative mechanism, in which tEc-1 is produced directly from cZc-P and cEc-T directly from tZc-P (equation 59)103. This mechanism involves isomerization about both the central double bond and one of its associated single bonds—the hula-twist mechanism of Liu and Browne101 — and involves a smaller volume change than the conventional mechanism for ,Z-isomerization. The vitamin D system has also been the subject of recent theoretical study by Bemardi, Robb and Olivucci and their co workers232. 

In contrast to cyclization and rearrangement as the unimolecular reaction, the EZ isomerization of olefins is difficult due to a drastic and unenviable change in the size and shape of the occupied space by substituents on the double bond during isomerization in the crystalline state. Some (Z,Z)-muconic derivatives provide a geometrical isomer as the photoproduct in a high yield, but not a polymer, under UV irradiation in the crystalline state, as is described in the Introduction (Scheme 1 and Table 1). This isomerization is a crystal-to-crystal reaction with an excellent selectivity, which is completely different from ordinary photoisomerizations. 

Diphenyl-1,3-butadiene. The excited-state behavior of this diene differs significantly from stilbene and is the subject of a review. Unlike tS in which the lowest vertical excited singlet state is the 1 B state and S2 is the 2 Ag state in solution, these two excited states lie very close to each other in all-trans-1,4-diphenyl-1,3-butadiene (DPB). The additional carbon-carbon double bond introduces a new conformational equilibrium involving the s-trans and s-cis rota-mers. Most spectroscopic studies in solution have concluded that the l B state is S. The DPB compound has a low quantum yield for photoisomerization, so the use of DPB in time-resolved spectroscopic studies on photoisomerization, especially those that monitor only fluorescence decay, needs to be considered cautiously and critically. 

Isomerization of unsaturated nitrogen-containing organic compounds, containing C=C double bonds, has been investigated less intensely than the isomerization of other related unsaturated compounds. Photoisomerization has been reported upon irradiation of a benzene solution of aqueous solution of its hydrochloride, and of the methiodide salt under nitrogen atmosphere. The corresponding cis-isomers are formed.308,309... 

As an example of compounds containing —N=N— double bonds, photoisomerization of azobenzene has been extensively studied. Irradiation of azobenzene in neutral solutions is known to result in cis trans photoequilibration.102 Similar photochemical cis - trans isom-... 

H. Hamaguchi I would like to comment on the stilbene photoisomerization in solution. We recently found an interesting linear relationship between the dephasing time of the central double-bond stretch vibration of Si franj-stilbene, which was measured by time-resolved Raman spectroscopy, and the rate of isomerization in various solutions. Although the linear relationship has not been established in an extensive range of the isomerization rate, I can point out that the vibrational dephasing time measured by Raman spectroscopy is an important source of information on the solvent-induced vibrational dynamics relevant to the reaction dynamics in solution. 

The demonstration that both double bonds of the cross-conjugated dienone system are not necessary for photoisomerization considerably extends the potential utility of these rearrangements because of the greater availability of a,/3-unsaturated ketones. It is safe to predict that photochemical rearrangements of a,/3-unsaturated ketones will be subject to intense investigation in the immediate future. 

Photoisomerization of E-C=C double bonds substituted with heteroaromatic groups that allow intramolecular hydrogen bonding were found to afford one-way E - Z... 

In non-viscous organic solvents the fluorescence efficiency of most SNA derivatives is comparable to that of TS as are the yields of trans to ois photoisomerization (33,3A). The double chain derivatives, MSNA and MSN, show substantially higher fluorescence yields (0.2 0.02) in non-viscous solvents with a corresponding decrease in the trans to ois isomerization efficiency (3A). In the sensitized isomerization, the MSN derivatives investigated thus far show only photoisomerization but there is an increase in the triplet lifetime by a factor of 2 however there is no change in the decay ratio of the isomerization precursor and the sum of the benzophenone sensitized isomerization efficiencies of ASA is approximately unity. When water insoluble SNA or MSNA molecules are spread as a film, either pure or in mixtures with insoluble fatty acids, at the air-water interface, it is found that the films show comparable behavior on compression to those of pure fatty acid with an indicated area per molecule of the surfactant stilbenes very close to that of a linear fatty acid (oa. 202 /molecule) (35). This suggests strongly that the TS chromophore in these molecules offers suitable molecular dimensions to pack into a crystalline-like array of linear parafin chains. 

For compounds such as 63, a more complex rearrangement replaces the decarbonyla-tion reaction [65]. This photoisomerization yields lactone 64 by a-cleavage at C-2, C-3 position (Norrish I) and hydrogen transfer from C-l to C-3 followed by a stereoselective nucleophilic attack at the carbonyl group by the terminal carbon of the electron-rich double bond and final ring closure (Scheme 34). 

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

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