Photoisomerization geometrical

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). 

Both Z and E isomeric enol lactones undergo photoisomerization to yield mixtures of isomers (5,14,87) in which the thermodynamically more stable one prevails. It is the Z form in hydrastine series (5) and the E isomer in the more hindered narcotine series (87). Relative stabilities of isomeric enol lactones (98 versus 99 and 101 versus 102) were determined by comparing their rates of methanolysis. Keto esters of type 126 were formed (87). It turned out that both ( )-N-methylhydrastine enol lactone (99) and (Z)-narceine enol lactone (101) solvolyzed faster than their geometric partners. 

The less stable isomers were obtained from the more stable ones by photoisomerization. (Z)-Fumaridine (113) when exposed to sunlight was isomerized to a separable 3 2 mixture of geometric isomers (5). The Z form of narceine imide (116) is unstable and in daylight equilibrates easily to a mixture of Z and E forms (123). 

Figure 9. Schematic reprsentation of a classical trajectory moving on the Si and So energy surfaces of the H2—(CH) -NHt trans cis photoisomerization, starting near the planar Franck-Condon geometry. The geometric coordinates are (a) torsion of the C2—C3 and C3 C4 bonds and (b) asymmetric stretching coupled with pyramidalization. Both Si and So intersect at a conical intersection (Si/S0 Cl) located near the minimum of the Si surface (Min-C ) where the C2C3C4N5 torsion angle is 104°. [Reproduced with permission from [87], Copyright 2000 Amercian Chemical Society],...

A quick survey of the photochemistry of the different complexes described above shows that the mechanism of photoactivation and the subsequent nature of the observed photoproducts varies from complex to complex and from one geometric isomer to another. Photochemical pathways often involve a combination of photosubstitution, photoisomerization, and photoreduction steps. In general, photolysis is rather slow in water and many different products are obtained if the complex is irradiated alone. The presence of nucleophilic biomolecules, on the other hand, can have a major influence, as photoreduction is usually rapid and accompanied by simpler reaction pathways. NMR methods... 

Optically active benzene(poly)carboxamides and benzene(poly)carboxy-lates were used by Inoue and co-workers as sensitizers for the geometrical photoisomerization of (Z)-cyclooctene and (Z,Z)-cyclooctadienes in various solvents at different temperatures. Under energy-transfer conditions, enantiomeric excesses up to 64% ee in unpolar solvents like pentane were reported. The use of polar solvents diminished the product ee s due to the intervention of a free or solvent-separated radical ion pair generated through the electron transfer from the substrate to the excited chiral sensitizer (Scheme 58) [105-109]. 

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. 

The loss of stereochemical memory in the non-vertical excited state implies that stereospecific, concerted reactions of an alkene singlet state may take place from the vertical state. Of particular importance is that the change from cis or trans geometry to something in between opens up a route for converting one geometrical isomer of an alkene to another, and this is a photoisomerization reaction that will be described in the next section. 

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... 

The stilbenes absorb light strongly, and their excited singlets cross over to triplets fairly efficiently, from which state photoisomerization occurs. The lowest energy T - S0 absorption bands measured for cis-and rnm-stilbene are at 57 and 48-50 kcal, respectively. However, it is quite likely again that both spectroscopically formed triplets relax very rapidly to a common twisted triplet which decays to both geometric isomers. 

So far, various kinds of polymers which change their conformation reversibly by photoirradiation have been reported [1-6]. The polymers contain pendant or backbone photoisomerizable chromophores, and the molecular property changes, such as geometrical structure or dipole moment changes, control the conformation. The polymers change their conformation in proportion to the number of photoisomerized chromophores. Thus, when the polymers contain more photoisomerizable chromophores and absorb more photons, the conformation changes more. Physical and chemical properties associated with the conformation changes also vary with the number of absorbed photons. 

Carotenoids can be converted into mixtures of geometrical isomers under appropriate conditions, the most common being iodine catalyzed photoisomerization. This produces an equilibrium mixture of isomers, in general the all-trans isomers predominates. These isomers in an isomeric mixture cannot be measured separately by simple spectrophotometric determination. The usual method of subsequent measurement would be chromatographic separation, diode-array detection, and spectral analysis. In the absence of any definitive data on extinction coefficients for cfv-isomcrs, they are quantified against the all-trans isomer. Modem procedures involve the direct synthesis of c/.v-carotcnoids. 

Properties which change concomitantly with diarylethene derivative photoisomerization are geometrical structures, electronic structures, refractive indices, and chiral properties (when the molecules have chiral substituents). Table 1 shows how the above property changes are applied to various photoswitching molecular systems. Details of these photoswitching functions are described in Sections 2.3 to 2.6. 

In the rather short history of organic photochemistry, the geometrical E-Z photoisomerization has been exceptionally intensively studied for half a century and a number of reviews have been published [11-18], Although the geometrical isomerization of alkenes can be effected thermally, catalytically, and photochemically, one of the unique features of photoisomerization is that the photostationary EfZ ratio is independent from the ground-state thermodynamics but is instead governed by the excited-state potential surfaces, which enables the thermodynamically less-stable isomers... 

Geometrical photoisomerization of (Z)-l-methylcyclo-octene 59aZ can be performed by direct, singlet-sensitized, and triplet-sensitized irradiations. The photostationary EjZ ratio is 0.30 upon direct irradiation... 

The M-L bond weakening in the MC excited state could also lead to photoisomerization, which can yield different categories of isomeric complexes, eg geometrical, linkage, optical, spin isomers. The most popular geometrical photoisomerization has already been discussed above in section 6.2.2, and illustrated in Figure 6.4 [33,41],... 

The reactions of alkenes and related compounds are grouped here into nine sections. The first five deal essentially with photoisomerization processes—geometrical isomerization about a carbon—carbon double bond, concerted (electrocydic) cycfization, concerted shifts (usually of hydrogen) along the -system, thedi-jr-methane reaction. 

Photoisomerization. Studies have been reported for the photoisomerization of cis- to tra y-2,3-diphenylpiperazine,250 cis- to trails-1,4-dimethyl -2,3-diphenylpiperazine,250 and between possible geometric forms of 2-[2-(naph-thalen-2-yl)vinyl]pyrazine (240)66 or related compounds.1236... 

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