Trans photoisomerization

The first study in which a full CASSCE treatment was used for the non-adiabatic dynamics of a polyatomic system was a study on a model of the retinal chromophore [86]. The cis-trans photoisomerization of retinal is the primary event in vision, but despite much study the mechanism for this process is still unclear. The minimal model for retinal is l-cis-CjH NHj, which had been studied in an earlier quantum chemisti7 study [230]. There, it had been established that a conical intersection exists between the Si and So states with the cis-trans defining torsion angle at approximately a = 80° (cis is at 0°). Two... 

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. 

Under UV light irradiation, cis-trans photoisomerization of the ionophore in the membrane occurs. We assume that the cis and trans isomers are both present in the membrane and the cis isomer forms a 1 1 (ionophore-cation) complex with a stability constant, Am.cb- In this case, the surface charge density, and the phase boundary... 

Ben-Nun M, Molnar F, Schulten K, Martinez TJ (2000) The role of intersection topography in bond selectivity of cis-trans photoisomerization. Proc Natl Acad Sci USA 97 9379... 

Aromatic azo compounds such as azobenzene undergo cis-trans photoisomerization but do not decompose, due to the inherent instability of phenyl radicals. 

Voliani V, Bizzarri R, Nifosi R, Abbruzzetti S, Grandi E, Viappiani C, Beltram F (2008) Cis-trans photoisomerization of fluorescent-protein chromophores. J Phys Chem B 112 10714-10722... 

Nifosi R, Tozzini V (2006) Cis-trans photoisomerization of the chromophore in the green fluorescent protein variant E(2)GFP a molecular dynamics study. Chem Phys 323 358-368... 

Dienes and polyenes have been a subject of great interest due to their important role in biology, materials science and organic synthesis. The mechanism of vision involves cis-trans photoisomerization of 11 -civ-retinal, an aldehyde formed from a linear polyene. Moreover, this kind of molecule exhibits high linear and non-linear electrical and optical properties. Short polyenes are also involved in pericyclic reactions, one of the most important classes of organic reactions. 

Clomiphene citrate is used as a mixture of E (58) and Z (60) isomers to treat infertility. Photolysis of a chloroform solution with a high-pressure mercury lamp gave the expected phenanthrenes (59) and (61), which were separated and identified by GC-MS. Study of the separate isomers (58) and (60) indicated that rapid cis-trans photoisomerization preceded ring closure so that each gave a mixture of phenanthrenes (60) and (61) (Scheme 2.2) [52],... 

The conjugated diene dienoestrol (65) was irradiated at 254 nm in 90% aqueous methanol. Rotation and cis-trans photoisomerization gave (66) which underwent a photochemical [1, 5]sigmatropic rearrangement to give (67). Photocyclization followed by enol-keto tautomerism then gave the isolated dihydrophenanthrene dione (68) [56]. 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

A second example of the use of ionic chiral auxiliaries for asymmetric synthesis is found in the work of Chong et al. on the cis.trans photoisomerization of certain cyclopropane derivatives [33]. Based on the report by Zimmerman and Flechtner [34] that achiral tmns,trans-2,3-diphenyl-l-benzoylcyclopropane (35a, Scheme 7) undergoes very efficient (0=0.94) photoisomerization in solution to afford the racemic cis,trans isomer 36a, the correspondingp-carboxylic acid 35b was synthesized and treated with a variety of optically pure amines to give salts of general structure 35c (CA=chiral auxiliary). Irradiation of crystals of these salts followed by diazomethane workup yielded methyl ester 36d, which was analyzed by chiral HPLC for enantiomeric excess. The results are summarized in Table 3. 

Abstract After a brief introduction and summary of various methods of asymmetric induction in organic photochemistry, the main part of the review covers the solid-state ionic chiral auxiliary approach to asymmetric photochemical synthesis. Application of this technique to the Norrish type II reaction, as well as to the di-n-methane and oxa-di-n-methane photorearrangements, and the cis,trans-photoisomerization of diarylcyclopropane derivatives is presented and discussed. 

Stilbenes and Stilbene-Like Molecules, Cis-Trans Photoisomerization of... 

Studies by Nishiyama and Fujihara [149] utilizing azobenzene derivative (27) as isomerizable chromophores have demonstrated the importance of reaction cavity free volume in L-B films. The L-B films of amphiphilic derivative 4-octyl-4 -(3-carboxytrimethyleneoxy)-azobenzene (27) upon irradiation was found to be stable, no geometric isomerization of the azobennzene moiety occurred. This compound forms L-B films with water soluble polyallylamine 28 at an air-water interface. Reversible cis-trans photoisomerization occurs in the film containing 28. The reversible photoisomerization reaction in polyion complexed films is thought to occur because of the increased area per molecule provided in the film. The cross sections of molecule 27 in the pure film and in film containing 28 were estimated to be 0.28 and 0.39 nm2. Such an increased area per molecule... 

Azobenzene readily undergoes both sensitized and direct cis-trans photoisomerization. However, the sensitized photostationary cis/trans ratios with high-energy sensitizers do not predict those obtained upon direct irradiation, indicating that much of the direct photoisomerization occurs from states other than the lowest triplet.455 As with stilbene, cyclization of the cis isomer apparently is a singlet-state reaction.456... 

The rate constant for the cis- trans photoisomerization can be calculated from the observed time dependence. The rate constants obtained at various monolayer surface pressures under identical conditions of illumination are plotted vs surface pressure in Figure 3. 

Figure 3. Rate constants of cis - -trans photoisomerization obtained from enhanced reflection measurements vs. surface pressure of mixed monolayers of the thioindigo TI and arachidic acid (AA), molar ratio TI AA = 1 3. Subphase bidistilled water, 22° C.

W. J. Leigh and J. A. Postigo, /. Chem.Soc., Cbem. Commun., 24, 1836 (1993). The Role of Central Bond Torsional Motions in the Direct cis, 2 trans-Photoisomerization of Conjugated Dienes. 

Like proton transfer, photoisomerization is a fundamentally important photochemical process. The two most important forms of photoisomerization are valence isomerization and stereoisomerization. The latter is probably the most common photoinduced isomerization in supramolecular chemistry. It may occur in systems in which the photoactive component has unsaturated bonds which can be excited, and this effect may be exploited for optical switching applications. A number of interfacial supramolecular complexes capable of undergoing cis-trans photoisomerization have been studied from this perspective - some examples are outlined in Chapter 5. 

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