Azobenzenes memory

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Photochemical switching of the phase transition is also found in the polyion complex film. Figure 29 shows reversible cycles of the absorption at 370nm by the coupling of the thermal and photoinduced phase transition of the complex film with carboxymethylcellulose 8. In conclusion, we indicate that the immobilized bilayer membranes containing the azobenzene chromophore are available to the erasable memory materials based on the phase transition triggered by thermal and photochemical processes. The polyion complex technique is clearly shown to be a very useful method for materialization of the immobilized bilayer membranes. 

The process of reorientation during cis—>trans thermal isomerization can be seen at the value of in Equation 3.11, which shows that the cis anisotropy does not contribute to the trans anisotropy if the trans isomer loses total memory of the orientation in the cis isomer Q2 = 0). It is informative to note that in the realistic physical case—i.e., the case of the azobenzene molecule chemically attached to a polymer, where the cis and trans diffusion rates are negligible in comparison to the cis— trans isomerization rate—the relaxation of the cis and trans anisotropy, AA and can be written respectively in the form ... 

For all Azo-PURs, the quantum yields of the forth, i.e., trans—>cis, are small compared to those of the back, i.e., cis—>trans, isomerization—a feature that shows that the azo-chromophore is often in the trans form during trans<->cis cycling. For PUR-1, trans isomerizes to cis about 4 times for every 1000 photons absorbed, and once in the cis, it isomerizes back to the trans for about 2 absorbed photons. In addition, the rate of cis—>trans thermal isomerization is quite high 0.45 s Q 1 shows that upon isomerization, the azo-chromophore rotates in a manner that maximizes molecular nonpolar orientation during isomerization in other words, it maximizes the second-order Legendre polynomial, i.e., the second moment, of the distribution of the isomeric reorientation. Q 1 also shows that the chromophore retains full memory of its orientation before isomerization and does not shake indiscriminately before it relaxes otherwise, it would be Q 0. The fact that the azo-chromophore moves, i.e., rotates, and retains full orientational memory after isomerization dictates that it reorients only by a well-defined, discrete angle upon isomerization. Next, I discuss photo-orientation processes in chromophores that isomerize by cyclization, a process that differs from the isomeric shape change of azobenzene derivatives. 

Chen et al. reported novel photoswitches using C2-symmetric dibenzo-suberane-based helicenes with a tetraline-based fragment. Photoisomerization of the diastereomerically pure (P)-from (34) led to virtually exclusive formation of the (M )-form (35) at 290 nm irradiation. Upon 330 nm irradiation, (35) isomerized to (34). This photoswitch in a nematic liquid crystal was applicable to a cholesteric mesophase with modulated pitches, reversible helical senses, and a switch memory of ternary logic. Furthermore, the helicene (36)-(39) with bis-azobenzene chromophores acted as highly diastereoselective chirochromic (92 8 to 3 97) and photo-chromic (>99 <1 to 19 81) switches both in solutions and in the nematic liquid crystal. ... 

In addition to electric potential, light can also be used to trigger switching properties of surfaces and polymers. Application of ultraviolet (UV) light to these materials may result in reversible changes in characteristics such as hydrophilic-ity/hydrophobicity, structural arrangement, and shape. Commonly utilized photoresponsive materials include azobenzene molecules, spiropyran molecules, and shape-memory polymers. 

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