Azobenzene trans isomer

The second subject is concerned with cis and irans isomers. The trans isomer has the higher value (except for azobenzene) and the larger max.- This win be apparent from the data in Table XIII. 

If the photoequilibrium concentrations of the cis and trans isomers of the photoswitchable ionophore in the membrane bulk and their complexation stability constants for primary cations are known, the photoinduced change in the concentration of the complex cation in the membrane bulk can be estimated. If the same amount of change is assumed to occur for the concentration of the complex cation at the very surface of the membrane, the photoinduced change in the phase boundary potential may be correlated quantitatively to the amount of the primary cation permeated to or released from the membrane side of the interface under otherwise identical conditions. In such a manner, this type of photoswitchable ionophore may serve as a molecular probe to quantitatively correlate between the photoinduced changes in the phase boundary potential and the number of the primary cations permselectively extracted into the membrane side of the interface. Highly lipophilic derivatives of azobis(benzo-15-crown-5), 1 and 2, as well as reference compound 3 were used for this purpose (see Fig. 9 for the structures) [43]. Compared to azobenzene-modified crown ethers reported earlier [39 2], more distinct structural difference between the cis... 

A typical photochemical isomerization of the azobenzene amphiphile was found in an ethanol solution. A trans isomer converted to a cis isomer with ultraviolet irradiation. Back reaction from cis to trans was accelerated when a weak n-n absorption band of the cis isomer at ca.450nm was excited (Figure 21a). An alternative irradiation of uv and visible light to the ethanol solution gave reversible changes of the ji-ji transition between 355nm and 325nm attributed to the trans and cis isomers, respectively. 

Photochemical response of these liposomes is different from each other. With progression of trans - cis photoisomerization of azobenzene, ICD at the absorption band of the trans isomer decreases. As shown in Figure 4, depression in ICD is almost proportional to the amount of photoisomerization for the phase separated system. Photoisomerization in the domain of azobenzene aggregate proceeds independently from the rest of DPPC aggregate so that the depression in ICD corresponds to the concentration of remaining transazobenzene. When the two components are molecularly mixed, change of... 

For this puq)ose, the photoswitchable bis(crown ether)s 88 and 89 as well as the reference compound 90 have been synthesized. Compounds 88 and 89 are highly lipophilic derivatives of azobis(benzo-15-crown-5). The parent azobis crown ether was originally developed by Shinkai and its photoresponsive changes in complexation, extraction, and transport properties thoroughly examined. Compared to 87, more distinct structural difference between the cis and trans isomers can be expected for 88 and 89 because in the latter compounds the 15-crown-5 rings are directly attached to the azobenzene group. The photoequilibrium concentrations of the cis and trans forms and the photoinduced changes in the complexation constants for alkali metal ions are summarized in Table 7. 

In a dark room, to 50 ml of a 0.78 N solution of perbenzoic acid in chloroform is added with stirring, at room temperature, 1.5 gm (0.008 mole) of cis-azobenzene. The mixture is stirred for hr. Then it is extracted in turn with three 40 ml portions of 5 % aqueous sodium hydroxide solution and one 50 ml portion of water. The chloroform solution is rapidly dried with anhydrous sodium sulfate, filtered, and evaporated at room temperature under reduced pressure. The trans isomer is separated from the desired cis isomer by slurring and filtering the resulting solid six times with 20 ml portions of petroleum ether (b.p. 40°-60°C). The residue is the desired product yield approx. 0.75 gm (50%), m.p. 87°C. 

As an example of compounds containing —N=N— double bonds, photoisomerization of azobenzene has been extensively studied. Irradiation of azobenzene in neutral solutions is known to result in cis trans photoequilibration.102 Similar photochemical cis - trans isom-... 

With stilbene and azobenzene there are two isomers. In these cases a plane structure is realized for the //mf-isomers, in contrast to the cis forms where this is impossible on spatial grounds. The stability of the trans forms is a consequence of the extra resonance (extra R.E. /z -stilbene 7.0 kcal energy content of frmy-azobenzene 10 kcal lower than the cis form). This resonance also appears from the distances, stilbene, trans C—C 1.445 A, C=C 1.33 A azobenzene, trans C—N 1.415 A, cis C—N 1.46 A (sum of atomic distances 1.54 A and 1.47 A, respectively) in this latter compound the planes of the rings make an angle of 50°. 

Figure 16 shows a change in the absorption spectrum of the LB film of APT(8-12) with UV (365 nm) and visible (436 nm) photoirradiation. The strong band around 360 nm is due to the trans isomer of azobenzene. The absorption due to the local excitation of TCNQ polarized along the long axis is located at about 315 nm but is indiscernible in this spectrum since the transition moment of this band is oriented almost perpendicular to the film surface and the electric field of the light is parallel to the film surface [149]. 

With the irradiation of UV, the trans-to-cis photoisomerization of the azobenzene proceeds, which is seen by a decrease in the absorption assigned to the trans isomer and a simultaneous increase in the absorption assigned to the cis isomer in the region 400 to 500 nm. The conversion to the cis isomer in the photostationary state is estimated to be about 25% from the change in the absorbance of the trans isomer, which serves as a measure of the cis content. The formed cis isomer isomerizes back to the trans isomer with the irradiation of visible light. 

As mentioned above, the sol-to-gel phase transitions can be induced reversibly by trans-cis photoisomerization of the azobenzene groups. UV irradiation (330 < X < 380 nm) transforms a part of the trans isomers to the cis. As a consequence of the structural change, the gel state is switched to the sol. Visible irradiation (at X > 460 nm) isomerizes the cis isomers to their trans form and allows the gel to be re-established. The reversible photocontrol of the sol-to-gel phase transition can be monitored by CD spectroscopy. 

Interesting information concerning the CN bond is forthcoming from the x-ray data of Robertson and of Hampson and Robertson on the cis and trans isomers of azobenzene. The frans form VII is flat and... 

Section 3.2 of this chapter recalls the pure photochemical point of view of photoisomerization of azobenzene derivatives. Section 3.3 discusses the theory of photo-orientation by photoisomerization and gives analytical expressions for the measurement of coupled photoisomerization and photo-orientation parameters. Sections 3.4 and 3.5 review observations of photo-orientation in azobenzene and push-pull azobenzene derivatives, respectively. Among other things, these sections address photo-orientation in both cis and trans isomers and discuss the effect of trans<->cis cycling, i.e., the photochemical quantum yields, on photo-orientation. Section 3.6 discusses the effect of the symmetry of photochemical transitions on photo-orientation in spiropyran and diarylethene-type chromophores. Finally, I make some concluding observations in Section 3.7. 

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