E-Z photoisomerization of azobenzenes

The reversible E/Z photoisomerization of azobenzene upon UV/VIS irradiation, in methanolic solution, has been utilized to develop the first reusable chemical... 

A major feature of the azo group is its capability to isomerize this is the property used widely in photoresponsive organic thin films. The E-Z photoisomerization of azobenzene was detected by Flartley, who created the Z-form by irradiation of E-azobenzene. Generally, the E-forms of azo compounds are more stable than the Z-forms. The parent E-azobenzene is by ca. 50 kj moT the more thermodynamically stable isomer. Isomerization is the main photoreaction of most aromatic azo compounds. Other thermal- and photoreactions lose the competition with isomerization. 

It is weU known that E Z photoisomerization of azobenzene-containing LC molecules can lead to a nematic-to-isotropic transition as well as photochromism [167]. It was reported that UV irradiation of a nematic mixture doped by chiral azobenzene bent-core LCs leads to the N -I transition and shift in selective reflection band position of the N phase [177]. These chiral bent-core molecules can induce the N helix upon doping them into a nematic base mixture, and their helical twisting power (HTP) is given by P = UPC where P is the helical pitch length, and C is the concentration of a chiral dopant. The P value for the exclusively E isomer is maximum and decreases with the increase in the ratio of the Z isomer. UV irradiation causes E Z conversion, therefore increasing the helical pitch and shifting the selective reflection band of the N phase. 

Photoisomerization of azobenzene and its derivatives has been widely investigated in the frame of the study of materials, where it is taken advantage of their photochromic properties. They are ideal components of numerous molecular devices and functional materials. Typical examples of E-Z photoisomerization of azobenzenes were shown here. An aromatic polyazo compound (1) with five azobenzene units was constructed by using a dehydrochlorination reaction between p-hydro-jyazobenzene, 4,4 -dihydro grazobenzene and cyanuric chloride under very mild conditions. This compound exhibited typical reversible photoisomerization behavior. Similar polyazo compounds, such as 2,4,6-tri[4-(phenylazo)pheno3g ]-triazine (2), tris[4-(phenylazo)phenyl]amine (3), and aqueous azobenzene derivative (4) were also synthesized and characterized (Scheme 1). 

Recently, we synthesized a complicated switchable CD-based [3]rotaxane (Fig. 5) to simulate the abacus system [28]. The E- Z photoisomerization of azobenzene results in the shuttling of the two macrocycles close to NS stopper accompanied by... 

Photochemical butterfly-like E — Z photoisomerization of a bis(crown ether) azobenzene derivative 354 was found to be thermally reversible and the stereoisomers exhibit unique contrasting behaviour in the presence of metal ions.1108 The concentration of the Z-isomer in the photostationary state was noticeably enhanced by the addition of K+, Rb + or Cs +, because the corresponding Z-complex achieved a stable sandwich geometry (Scheme 6.162). As a result, the cations could be selectively extracted by the Z-derivative from an aqueous phase to an organic solvent (o-dichlorobenzene), whereas no complexation (i.e. no transfer) took place in the case of the E-isomer. 

Most azobenzene derivatives can be isomerized reversibly between the E and Z isomeric forms by light or heat. Some derivatives may show phototautomerism, photodimerization, photoreduction, and fading. The effects discussed in this chapter are based on E-Z photoisomerization of aromatic azobenzenes (Rau, 1990) also known in the literature as transits photoisomerization. 

Liquid crystallines as homolithic and heterolithic phloroglucinol-based trimeric materials including azobenzene units (5) and (6) were discussed (Scheme 2). Some other liquid crystalline materials, which showed interesting behavior such as holographic grating, were prepared and characterized. The E-Z photoisomerization of 3,3 -dicyanoazo-benzene on Bi(lll) surface was explained by electron transfer process. ... 

An impressive example of a molecular machine was presented by Willner et al. (Figure 89.9). Using the base l-ethyl-3(3-dimethylaminopropyl)carbodiimide (EDC), a super molecule attached at a gold electrode was synthesized in a stepwise fashion. Here E- Z photoisomerization of the azobenzene moiety is a necessary step already in the synthetic procedure. By switching photolytic wavelengths between 320-380 and >420 nm, respectively, the movement of the shuttle is activated between the -azobenzene station site and an alkyl-chain railway component The distance between the redox active ferrocene label from the electrode is indicated by a chronoamperic response. 

A bis(boronic acid)-based receptor bearing an azobenzene moiety has been synthesized. The photocontrol of the saccharide-binding properties by the change of distance between two boronic acid groups was found to be possible by E,Z-photoisomerization of the azobenzene unit ° (Scheme 60). 

Scheme 12 Photochemically driven shuttling movement of an a-cyclodextrin in an azobenzene-containing thread through reversible E/Z photoisomerization [61]...

Z- E isomerization yield is nearly temperature-independent (Figure 1.10) or increases at low temperature, with only a small difference for excitation to the two lowest-excited states. So obviously, the E —> Z photoisomerization— after irradiation to the (n,7C ) state as well as the Z —> E isomerization— proceeds even at low temperature and in frozen solvents. In solid matrices, fast and slowly isomerizing molecules are observed on it —> it excitation. The fast process has a quantum yield of < = 0.14 that is temperature independent down to 4 K. With strong lasers, photoisomerization in the E —> Z direction have been exploited, even at 4 K in hole burning experiments. Thus, azobenzene photoisomerization cannot be frozen out. 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE. ... 

The interesting dual behaviour of porphyrin derivatives is demonstrated by intramolecular electron transfer between an electron-rich tris(4-methylphenyl)porphyrin zinc complex (ZnTPP) and an electron-deficient tris(4-methylphenyl)octafluoroporphyrin (TPOFP) (Teflon porphyrin) free base connected via an azobenzene moiety (438 Scheme 6.209).1250 Upon photolysis at >440 nm, E —> Z photoisomerization (Section 6.4.1) occurs. (Z)-438 can revert to ( )-438 only thermally, because the excited state of the Z-isomer is efficiently quenched by intramolecular electron transfer from ZnTPP to TPOFP, whereas no such interaction is possible in the -isomer because the chromophores are too remote. This approach has been suggested for photocontrolled molecular electronics. 

Photoisomerization of azobenzenes has been widely developed as photochromic compounds. Lots of papers sriU appeared in this subject from basic chemisty to applications. Some examples were described. Harada et al., reported the effect of cyclodextrin derivatives bearing photo-isomerized stilbene moiety. Single molecule conductance of E- and Z-4, 4 -(ethane-l,2-diyl)dibenzoic acids (E-l and Z-1) was determined using scanning tunnelling microscopy (STM). The dynamics of photoisomerization of E- and Z-urocanic acids (E- and Z-2) driven by Hg-free... 

Photo-stimulated phase transformations in LCs and their applications have been widely investigated [167]. Photoisomerization of azobenzene is the most commonly used approach to achieving this goal because azobenzenes exhibit the fastest, most efficient, fully reversible, cleanest, and most robust photoisomerization. Azobenzene exists as two tmns (or E) and cis (or Z) geometric isomers. The trans isomer is energetically more stable than the cis isomer, and they show different maximum absorption wavelength (Xmax = 350 nm for E and 440 nm for Z) in their absorption... 

FIGURE 6.17 A facile photoisomerization of azobenzene between the trans (E)- and the cis (Z)-isomer upon irradiating with light of different wavelengths at 350 nm and 440 nm, respectively. 

The most widely investigated photochromic system is the azobenzene chromophore (see Figure 5.6). The azobenzene possesses two distinct conformations, i.e. the (E)- and (Z)-states. Illumination of light in the near-ultraviolet region (at around 350 nm) induces an (E)-(Z) photoisomerization, while exposure to blue light reverses this isomerization. Such a reversible photoisomerization can take place with little degradation after thousands of switching cycles. The azobenzene chromophore can be incorporated in an amphiphile and... 

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