Photoresponsive shape changes

The photoresponsive behavior of a y-CD capped with an azobenzene moiety is very different from that of the fi-CD analog. Photoisomerization of the trans-AE regioisomer 129 led to a photostationary state containing only 47% cis isomer, which is much lower than that obtained with azobenzene as a free species or as a pendant (> 80%). This indicates that a stereochemical difficulty exists for trans to cis conversion. Since the cis photoisomer requires a distance of 6 A between the 4,4 sulfur atoms and the A and E glucose residues are located 9 A apart, it was concluded that the y-CD shape changes from a circle to an ellipse and that the cis isomer is... 

Azobenzenes are a well-known family of photochromic compounds that can experience trans-cis isomerization upon UV irradiation (Fig. 5.3a). The cis isomer can be driven back to the trans form eithCT by visible light or heat. The rod-like sttucture of trans form can stabilize calamitic LCs, while the cis form is bent and normally decreases the order parameters of LC phases. Owing to the dramatic shape change between the trans and cis isomers, azobenzenes were intensively investigated as mesogens or dopants in photoresponsive CLCs. 

Note 1 Examples of the changes in photosensitive polymers are a change in molecular shape (photoresponsive polymer), a change in its constitution (photoreactive polymer), and a reversible change in color (photochromic polymer). 

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. 

Photoresponsive molecules are required for the fabrication and photomodulation of photoresponsive CLC materials regardless of being employed as chiral mesogens, achiral LC host, or chiral/achiral dopants. The photoisomerization of molecules leads to change in molecular shape (geometry/conformation) and alter the bulk properties of LC material, which constitutes the basis for the photomodulation in chiral LCs [28, 29]. There are many types of photoresponsive molecules and some examples are shown in Fig. 5.4. 

Photoresponsive polymers are quite special polymers, able to respond to light and dark conditions and thus give rise to reversible variations in their structure and conformation. They can be obtained by introducing photochromic units, such as azobenzene or spiropyan group, into the macromolecules of polymeric compounds (Irie, 1990). Several papers have described hght-induced changes in the shape of polymers. 

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. 

Finally, it should be mentioned that the chiral additive used to create a chiral nematic phase does not have to be a mesogen itself - it has to be soluble at reasonable concentration levels to produce the desired pitch, but since it is now acting equally as an impurity, the normal phase transition properties of the host material may also be altered. Chiral nematic liquid crystals are highly sensitive in their optical properties, and may therefore be used as contaminant detectors. Equally photoresponsive additives, such as azo-based dyes, may be incorporated into a chiral nematic structure and on photoexcitation they change shape since they then behave as an impurity they can easily alter the pitch or selective reflection properties. Such effects are normally reversible because of the trans-cis and cis-trans back reactions available. This gives a simple imaging device. 

Cl8 and azo benzene groups. Upon irradiation, the increase of the polarity of the chromophore and/or its change in shape would affect the stability of these cross-links. Both SDS and BSA can bind hydrophobic molecules. It was not suiprising to find that an increase of the polarity was resulting in a decrease of the association. The latter polymer was the only one to exhibit marked photoresponse in the presence of BSA. A complete study has now confirmed that the presence of interpolymer associations was required for the photoresponsiveness to emerge (45). 

The synthesis of a photoresponsive, water-soluble azobenzenophane and the photoresponsive association properties for mono- and dicarboxylate guest molecules were studied by Shinkai and collaborators. It was found that the guest recognition ability of the cyclophane was affected by photo-irradiation, which induced a change in its cavity shape generated by ,Z-isomerism of the azobenzene moieties (Scheme 58). 

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