Photoisomerism, cis-trans

Photosubstitution at ruthenium(II)-bipyridyl complexes has been described for the polymer-bound complex [Ru (bipy)2 (pvp)2f", where pvp is poly(4-vinylpyridine). Reactivity is lower here than for equivalent monomeric complexes. " Photoisomerization cis trans) of [Ru(bipy)2(OH2)2] has been studied at the surface of hectorite, a layer silicate, and the results compared with those for aqueous solution. " ... 

Nishiyama Kand Fujihira M 1988 Cis-trans reversible photoisomerization of an amphiphilio azobenzene derivative in its pure LB film prepared as polyion oomplexes with polyallylamine Chem. Lett. 1257-60... 

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... 

Photochromism Based on Geometric Isomerism. The simplest examples of a photochromic reaction involving reversible cis-trans isomerization is the photoisomerization of azobenzene [103-33-3] C22H2QN2 (16). 

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. 

Category 4. Photoisomerization. The most common reaction in this category is photochemical cis-trans isomerization." For example, cw-stilbene can be... 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

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... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

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 effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

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. 

A photochemical reaction in which the reactant is converted into an isomer. The most common example of such a reaction is photochemical cis-trans isomerization. Another example of photoisomerization is the effect of light on cholesta-3,5-diene. 

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... 

Trans-23 displays photoisomerization behavior typical of an azobenzene (Figure 5.8). Irradiation of the bis-barium complex at 370 nm in EtOH/MeCN (65/35, v/v) leads after 40 s to a stationary state - quasi-cis-28-Ba2 - in which the cis/trans ratio is as high as 95 5. In turn, irradiation ofthe quasi-cis mixture leads to a... 

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