Shape photoresponsive

Kongkanand A, Tvrdy K, Takechi K, Kuno M, Kamat PV (2008) Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe-Ti02 architecture. J Am Chem Soc 130 4007-4015... 

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. 

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... 

Irie M. 1986. Photoresponsive pol5mer. Reversible bending of rod shaped acrylamide gels in an electric field. Macromolecules 19 2890 2892. 

White TJ (2012) Light to work transduction and shape memory in glassy, photoresponsive macromolecular systems trends and opportunities. J Polym Sci B50 877... 

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. 

Similar to the phase transition from nematic to isotropic phase induced by azobenzene molecules, the trans-cis isomerization also destabilize the SmC phase composed of calamitic mesogens and lower the Curie point, which is a transition temperature where the SmC will transform from ferroelectric to non-ferroelectric. Some examples have been reported with an early demonstration by Ikeda et al. [130-135]. For example, a photoresponsive SmC was formulated by doping 3 mol % of 4,4 -disubstituted azobenzene 29 into a FLC host 27 and the UV irradiation at 260 nm resulted in the lowering of Curie point and the coercive force Ec required to switch the SSFLC due to the destabilization of bent shape cis isomers [130] (Fig. 5.23). When the electric field was close to Ec before irradiation, the flip of polarization of SmC was achieved. It is noteworthy that its response time 500 ps is much faster than normally observed for photochemical N-I phase transitions [131]. 

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. 

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