Azobenzenes photomechanical effects

Menzel1811 more recently described the properties and behavior of monolayers prepared from azobenzene-containing poly(L-glutamate)s possessing the structures XIX and XX (n= 2) shown in Scheme 11. These monolayers showed photomechanical effects opposite to those described above for azo-modified poly(L-lysine)s. In fact, they expanded when exposed to UV light (trans—>tis isomerization), and shrank when exposed to visible light (ds— trans isomerization). The expansion was found to be smaller than expected from comparison of the monolayer isotherms obtained from irradiated and nonirradiated solutions. This was attributed to the trans-cis photoconversion of the azo units, which occurs with lower yields in monolayers... 

The corresponding fatty acid could not be photoisomerized in the LBK film. By attaching the azobenzene chromophore to the hydrophilic backbone, however, the free volume in LBK films was increased and photoisomerization was possible (i.e., 50 to 70% cts-isomer compared to 0% for the nontethered azobenzene amphiphile and 90% cis-isomer in solution). However, concomitant with the increased free volume, there is a decrease in the orientational order of the chromophores. These polymers have been widely used as command surfaces to control the orientation of liquid crystals and to investigate the photomechanical effect. ... 

Another class of polymers equipped with azobenzene moieties comprises a-helical polypeptides, in particular pQly(L-glutamate)s and poIy(L-lysine)s. In solution, these azobenzene-modified polypeptides can undergo photoinduced helix-coil transitions. Polypeptides partially (30 to 50%) substituted with azobenzene moieties are surface active and form stable monolayers. Because of the partial substitution, there is sufficient free volume, and the azobenzene moieties can be isomerized in the monolayer. The photoisomerization changes the area per molecule, and the monolayer shows a photomechanical effect. LBK films of a photosensitive poly(L-lysine) with 31 mol... 

Menzel, H. Langmuir-Blodgett films of photochromic polyglutamates. 7. The photomechanical effect in monolayers of polyglutamates with azobenzene moieties in the side chains. Macromol. Chem. Phys., 195, 3747 (1994). 

Seki, T., and Tamaki, T. Photomechanical effect in monolayers of azobenzene side ch.iin polymers. Chem. Lett., 1739 (1993). 

The photoinduced deformation phenomenon of materials is called a photomechanical effect, and it has been so far reported for photoresponsive polymer films and gels [35-43]. When azobenzene is isomerized from the trans form to the cis form, the length of the molecule is shortened from 0.90 to 0.55 nm. The size change of the molecule on photoirradiation is expected to alter the shape of the polymers which contain the azobenzene molecules. However, it is not the case in polymer systems. The transformation in polymer films does not change the polymer shape because of the large free volumes of the polymer bulk. Suitable organization or assembly of the molecules is required for the photoinduced deformation of materials. 

PHOTODEFORMABLE MATERIALS AND PHOTOMECHANICAL EFFECTS BASED ON AZOBENZENE-CONTAINING POLYMERS AND... 

Eisenbach (1980) investigated the photomechanical effect of poly(ethyl acrylate) networks (3) cross-linked with azobenzene moieties and observed that the polymer network contracted upon exposure to UV light (caused by the trans-cis isomerization of the azobenzene cross-links) and expanded upon irradiation of visible light (caused by cis-trans back-isomerization Fig. 3.5). This photomechanical effect is mainly due to the conformational change of the azobenzene crosslinks by the trans-cis isomerization of the azobenzene chromophore. However, the degree of deformation was small (0.2%). 

Figure 3.5. Schematic representation of photomechanical effect in a polyfethyl acryiate) network with azobenzene cross-links upon irradiation. f= Force. Source Eisenbach, 1980.

Matejka et al. synthesized several types of photochromic polymers based on a copolymer of maleic anhydride and styrene with azobenzene moieties both in the side chains and cross-links of the polymer network (Matejka et al., 1981 Matejka and Dusek, 1981 Matejka et al., 1979). The photomechanical effect was enhanced by an increase in the content of photochromic groups, and the photoinduced contraction of the sample amounted to 1% for a polymer with 5.4mol% azobenzene moieties. 

Yu Y, Maeda T, Mamiya J, Ikeda T. 2007. Photomechanical effects of ferroelectric liquid crystalline elastomers containing azobenzene chromophores. Angew Chem Int Ed Engl 46 881 883. 

Fig. 5.13 Photomechanical effects. Schematic illustration of UV-light-induced bending of a cross-linked liquid-crystalline polymer film containing azobenzene groups. Light is absorbed at the upper surface layer of the film and causes anisotropic contraction. Adapted from Ikeda et al. [51] with permission from Wiley-VCH.

Incorporation of light-sensitive functional groups such as azobenzene or triph-enylmethane leuco derivatives enabled SCP with responsiveness towards light [153-158]. Here, the light-induced changes of shape were based on electrostatic repulsion, on photomechanical effects, or light-stimulated phase transitions. 

The transfer of the geometrical change caused by photoisomerization on the molecular level to macroscopic shape changes was demonstrated in different concepts. While azo-dye loaded nylon filament fabrics showed shrinkage of approximately 0.1% after irradiation under load [162], the incorporation of azobenzene-containing crosslinkers in poly(ethyl acrylate) network films enhanced this photomechanical effect to 0.25% [163]. This is a significant difference to... 

Yu, Y Maeda, T. Mamiya, J. Ikeda, T. Photomechanical effects of ferroelectric liquid-crystalhne elastomers containing azobenzene chromophores. Angew. Chem. Int. Ed. 2007, 46, 881-883. 

On the other hand, LSCEs that contain isomerisable azobenzenes as light-sensitive molecules can also undergo anisotropic macroscopic deformations upon exposure to non-polarised light of the appropriate wavelength (i.e. photomechanical effect, Fig. 18.3b). Photo-mechanical effect in nematic LSCEs was first observed by Finkelmann et al. (2001) and, since then, it has been deeply investigated from both the theoretical and the experimental point of view. 

A photomechanical effect involves a reversible contraction and expansion of an oriented rubbery film after irradiation, based on the change in the dimension of chromophores embedded in the polymer networks. This effect was first described, by Agolini and Gay [5], for a partly crystalline polyimide [see structure (2)] with azo-aromatic chromophores in the backbone. They reported a contraction of the polymer film at 200°C of 0.5% by UV irradiation, which in principle could cause isomerization of azobenzene chromophores from the trans- to cw-form. A correlation between the isomerization and the film contraction was not obtained. At this high temperature, however, the content of cw-isomer in the photostationary state is believed to be very low because of rapid back-reaction from the cis- to trans-form. At the same time, the rapid thermal reaction made it difficult to measure any correlation between the contraction and the isomerization. It seems that the isomerization may not be the sole origin of the contraction of this system. 

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