On photochemical isomerization

Hong, J.-D., Park, E.-S., and Jung, B.-D. (1999). Studies on photochemical isomerization of azobenzene in self-assembled boloamphiphilic monolayers. Mol. Cryst. Liq. Cryst.. Sci. Technol. Sect. A, 327, 119. 

Hong JD, Park ES, Park AL.1999. Effects of added salt on photochemical isomerization of azobenzene in alternate multilayer assemblies bipolar amphiphile polyelectrol5he. Langmuir 15 6515 6521. 

The photochemical behavior of 1,2,4-oxadiazoles has been investigated experimentally under a wide variety of conditions (2009OBC4337) and computational studies have tried to rationalize some of its aspect, focusing on photochemical isomerization (2014CPC2712). A generally applicable scheme considers the involvement of a very reactive open-chain spedes as the primary... 

Reviews on photochemical isomerization reactions may be found in the following references. 

Among the isomeric alkanes of molecular formula C5H12, identify the one that on photochemical chlorination yields... 

Also in this case the relative energy of all the possible intermediates involved in the photochemical isomerization was calculated (OOOJOC2494). The results are collected in Fig. 2. Also in this case the sensitized irradiation involves the formation of the biradicals. We have to note, however, that the fission of O—Cq, bond in the triplet state of the molecule is not so favored as in furan. The process should be quite inefficient. The corresponding biradicals show the same energy as that in the triplet state. In this case, then, the formation of a biradical should depend on the activation energy. 

FtG. 2. Relative energy of the excited states of 2-methylfuraii and of some reactive intermediates. [Reprinted with permission from M. D Auria. Ab initio study on the photochemical isomerization of furan derivatives. J. Org. Chem. 65, 2494-2498 (2000)]. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Field studies on the transformation of endrin in the atmosphere were not located in the available literature. Photochemical isomerization of endrin, primarily to the pentacyclic ketone commonly called delta ketoendrin or endrin ketone, was observed after exposure of thin layers of solid endrin on glass to sunlight (Burton and Pollard 1974). Minor amounts of endrin aldehyde were also formed in this reaction. Results of seasonal studies indicated that this isomerization would proceed with a half-life (first-order kinetics) of 5-9 days in intense summer sunlight, with complete conversion to the pentacyclic ketone in 15-19 days. Knoevenagel and Himmelreich (1976) reported that photodegradation of solid endrin in the laboratory... 

Rosen, J.D., Sutherland, J., and Lipton, G.R. The photochemical isomerization of dieldrin and endrin and effects on toxicity. Bull. Environ. Contam. Toxicol, 1(4) 133-140, 1966. 

Tetrahedrane (CR)4 with bulky tert. butyl substituents R has been synthesized by G. Maier and coworkers in 1978 (28). In their already classic investigations, an interesting valence isomerization has been observed on melting, the C4 cluster rearranges to the thermodynamically more stable cyclobutadiene derivative, which on photochemical energy uptake forms again the tetrahedrane (28) ... 

Extensive work has been done to determine and understand the factors controlling diastereoselectivity in the cycloaddition of nitrile oxides to alkenes but very little is known about nitrile ylides in this regard. Work on their reactions with alkenes that are geminally disubstituted with electron-withdrawing groups (e.g., 187) has illustrated some of the difficulties in such studies. When the imidoyl chloride-base route was used to generate the nitrile ylides it was found that the products 188 epimerized under the reaction conditions. When the azirine route was used, the reaction was complicated by the photochemical isomerization of the dipolarophiles (96,97). Thus, in both cases, it proved impossible to determine the kinetic product ratio. 

The effect of association on photochemical reactivities of stilbenes and alkyl cinnamates having poor solubilities in water has been demonstrated recently (Schemes 37 and 38) [140, 205,206]. Even at concentrations of trans-stilbene in water as low as 10 6 M, dimerization occurs efficiently. The ratios of dimers were similar to those obtained in benzene when the initial stilbene concentration is high. However, in organic solvents, geometric isomerization is the only reaction observed at low stilbene concentrations. Similar behavior... 

Thus photochemical isomerization of benzene is known to occur. The yield based on isomerization of 1,3,5-trideuterobenzene is wavelength dependent and increases with decreasing wavelength,87 but is relatively small. 

The photochemical isomerization of PtClPh(PEt3)2 in acetonitrile solvent occurs at 254, 280 and 313 nm. For cis- trans isomerization the quantum yield is close to 0.1 at all wavelengths, but for trans- cis isomerization the quantum yield varies from 0.5 to <10-3 on changing A from 254 to 313 nm.584... 

The effect of substitution of the ethane linkage on the isomerization quantum yields is less dramatic, though the course of the photochemical reaction can be affected by the nature of the substituents (cf. also Section II.C). Both l,2-di-9-anthrylethanol (0F = 0.04) and its acetate (F = 0.03) undergo photochemical intramolecular An+An cycloaddition with a quantum yield of 0.35 [51], However, irradiation of meso-l,2-di-9-... 

The above properties (a and b) are interpreted by Cohen and Schmidt (21) on the basis of a detailed crystallographic study of photochromic and thermochromic anils. They conclude that photochromic crystals involve structures in which the central portion of adjacent molecules are essentially isolated from one another, so that, to a first approximation the energetics are that of an isolated molecule. On the other hand, when the alignment of the molecular dipoles is such as to give strong intermolecular interactions then the transition to the quinoid form requires much less energy and can occur thermally. For crystals in which thermochromism occurs, the photochemical isomerization is still possible but the reverse reaction is so rapid that no buildup of color is observed. In fact, fluorescence measurements on the thermochromic 5 -chlorosalicylidene-aniline (Fig. 5) indicate that photochemical isomerization precedes the luminescence process via the photochromic route ... 

Aoyama et al. in relation to their studies on photochemical synthesis of (3-lactams [91] reported the synthesis of 4-spirocyclopropylazetidin-2-one [92] via photocycloaddition of 4-thioxoazetidin-2-one to alkenes followed by subsequent desulfurization. A solution of 1-isopropyl-3-phenyl-4-thioxoazetidin-2-one 70 and 1,1-diphenylethylene in benzene on irradiation with a high pressure mercury lamp afforded a [2 + 2] adduct 72 (R = Ph), in 67% yield which, on desulfurization with Raney-nickel [93] in anhydrous ethanol gave two isomeric... 

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