Azobenzenes trans form

Three azobenzeneophane-type crown ethers in which the 4,4 positions of azobenzene are joined by a polyoxyethylene chain have been synthesized (Shinkai, Minami, Kusano Manabe, 1983). On irradiation with UV light, the ( ) (or trans) form (198) is isomerized to the (Z) (or cis) isomer (199). The ( ) isomer may be regenerated by heating, or by irradiation with visible light the interconversion is completely reversible. 

The simplest azo compound, azobenzene, exists as a mixture (Scheme 4-18) of a stable trans (4.19) and an unstable cis (4.20) form [38,39]. Formation of the cis isomer is induced by exposure to light, the quantum yield of the process depending upon the wavelength of the light employed [40]. The proportion of cis isomer can be appreciable in an equilibrium mixture. Thus a concentration of 24% of this unstable form builds up within a few hours when an acetic acid solution of azobenzene is exposed to sunlight in shallow white trays. Reversion to the trans form occurs readily on heating and is catalysed by a variety of substances that can function as electron donors or acceptors [41]-... 

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. 

Azobenzene [103-33-3] M 182.2, m 68°. Ordinary azobenzene is nearly all in the trans-form. It is partly converted into the cis-form on exposure to light [for isolation see Hartley JCS 633 1938, and for spectra of cis-and /runs-azobenzenes, see Winkel and Siebert B 74B 6701941], trans-Azobenzene is obtained by chromatography on alumina using 1 4 benzene/heptane or pet ether, and crystd from EtOH (after refluxing for several hours) or hexane. All operations should be carried out in diffuse red light or in the dark. 

In the phase separation process, however, it needed some induction period for the polymer to start the phase separation. Almost complete isomerization of the azobenzene pendant groups from the cis to the trans form is required to decrease the phase separation temperature below 19.5 °C. The phase separation process exhibited a non-linear response to the irradiation time or the number of photons. When the number of absorbed photons reached a critical value, the system underwent the phase separation and the polymer chain was shrunk. The photo-stimulated phase separation/dissolution cycle was not observed below 19.4 and above 26.0 °C. 

Azobenzene-appending p-CD (114) exhibits a positive circular dichroism band around 345 nm associated with the azobenzene 77-77 transtion for the trans form, whereas it exhibits strong positive and negative bands at 312 and 425 nm, respectively, after photoirradiation [104], The circular dichroism intensities of trans and cis forms of 114 decrease upon guest addition, and the analysis... 

Azobenzene with two (3-CD units (117) was prepared and found that it undergoes trans to cis photoisomerization with 66% cis at the photostationary state [107], The cis form returns to the original trans form with a half-life of 55 hr at 25°C. The azobenzene derivative was expected to bind one large molecule by the hands of two CD units, but so far no data were reported on this behavior. 

These azobenzene LCs display the liquid crystalline phase only when the azobenzene moiety is in the trans form, and no liquid crystalline phase at any temperature when the azobenzene moiety is in the cis form. In these azobenzene LC system, it was predicted that phase transition should be induced on essentially the same time-scale as the photochemical reaction of the photoresponsive moiety in each mesogen, if the photochemical reactions of a large number of mesogens were induced simultaneously by the use of a short laser pulse (Figure 7).1391 On the basis of such a new concept, the photoresponse of azobenzene LCs with the laser pulse was examined, and it was found that the N to I phase transition was induced in 200 xsJ39 40 This fast response, on the microsecond timescale, had been demonstrated for the first time in NLCs. From the viewpoint of application of LCs to photonic devices, such a fast response is quite encouraging. 

The first example of a photoresponsive [2]rotaxane, published in 1997 by Nakashima and co-workers, is one of those cases [61]. Molecular shuttle E/Z-224+ consists of an a-cyclodextrin macrocycle, and a tetracationic thread containing an azobiphenoxy moiety, very closely related to azobenzene, and two bipyridinium stations. The well-known E-Z isomerizations of azobenzenes and the ability of cyclodextrins to bind lipophylic compounds in water are exploited in this system to achieve shuttling. When the azobiphenoxy station is in its trans form, E-224+, the cyclodextrin encapsulates it preferentially over the more hydrophilic bipyridinium station (Scheme 12). 

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°. 

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