Polymer photoinduced anisotropy

The effect of the structure of the polymer backbone on photo-orientation can be seen from the dynamic behavior as well as from the steady-state values of the photoinduced anisotropy in all azo-PURs. The photo-orientation dynamics of PUR-2 resemble but also contrast with those of PUR-1. In PUR-2, AbSi exceeds AbsQ, but not quite, as is the case for PUR-1, and the photostationary-state anisotropy is smaller than that of PUR-1, as can be seen in Figure 4.20. PUR-1 and PUR-2 exhibit exactly the same extinction coefficient at the analysis wavelength because they have the same azo chromophore furthermore, the rate of the cis- trans thermal isomerization is nearly the same in both polymers. The seemingly small difference into the... 

Zhang X, Wang C, Pan X et al (2009) Nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic Uquid-crystallme polymer. Opt Commun 283 146-150... 

Pan X, Xiao S, Wang C et al (2009) Photoinduced anisotropy in an azo-containing ionic liquid-crystalline polymer. Opt Commun 282 763-768... 

There are a large number of publications on photoinduced anisotropy in supramolecular-ordered azobenzene-containing systems, such as liquid crystals (Kreuzer et al., 2000 Szabados et al., 1998), liquid crystalline polymers (Rosen-hauer et al., 2001 Han et al., 2000), LbL (Geue et al., 1997 Stumpe et al., 1996), self-assembled monolayers (Sekkat et al., 1995), and adsorbed azobenezene molecules (West et al., 2001). But mass transport resulting in SRG was only reported till recently in glassy systems. 

Note that the sol-gel materials based on polysiloxane have been known for their durability and good optical quality. If functionalized with azobenzene moieties, they exhibit nonlinear optical properties and photoinduced anisotropy. There are a few examples of SRG inscribed onto materials (Kim et al., 2006b Chaput et al., 2000 Frey et al., 2000 Darracq et al., 1998). The gratings could be inscribed only in freshly prepared films, whereas in aged films, the cross-linking of the polymer completely hindered SRG formation. 

One of the most interesting photoresponsive properties of azo polymers is the photoinduced birefringence and dichroism (Xie et al., 1993). The photoinduced anisotropy is caused by the disparity of the repeated trans-cis isomerization of azo chromophores under linear polarized light irradiation. The most efficient excitation occurs in the polarization direction, which can force the chromophores to continually change their orientation and to be eventually stabilized at the direction perpendicular to the polarization (Natansohn and Rochon, 2002). The effect shows potential applications in areas such as reversible optical data storage, optical switching and sensors. 

Figure 20 Schematic representation of (a) the appearance of photoinduced anisotropy in films of azobenzene-containing polymers leading to orientation of photochromic and mesogenic groups (b) trans-cis-trans isomerization of azobenzene groups and (c) change in the direction of director n relative to the electric vector of the light wavelength, E, during photo-orientation.

The first example of halogen-bonded liquid crystal with additional functional properties appeared in 2012 when an azobenzene-based system capable of combining a high degree of photoinduced anisotropy and exceptionally efficient SRG formation was reported [12]. The structure of the complex is shown in Fig. 7. The design is rather unusual because of the lack of flexible alkyl chains, which have been reported to suppress the formation of SRGs in LC polymers [56]. 

Kawatsuki N, Goto K, Yamamoto T (2001) Photoinduced anisotropy and photoalignment of nematic liquid crystals by a novel polymer liquid crystal with a coumarin-containing side group. Liq Cryst 28 1171-1176... 

Blinov, L. M., Kozlovsky, M. V., Ozaki, M., Skarp, K., and Yoshino, K. Photoinduced diefaroism and optical anisotropy in a liquid crystalline azobenzene side-chain polymer caused by anisotropic angular distribution of trans and cis isomers./. Appl. Phys. 84, 3860 (1998). 

T. Fischer, L. Lasker, J. Stumpe, and S. G. Kostromin, Photoinduced optical anisotropy in films of photochromic liquid crystalline polymers, Photochem. Photobiol. A 80, 453 159... 

L. Lasker, T. Fischer, J. Stumpe, S. Kostromin, S. Ivanov, V. Shivaev, and R. Ruhmann, Photoinduced optical anisotropy in thin films of amorphous photochromic side chain polymers. Mol. Cryst. Liq. Cryst. 246, 347-350 (1994). 

The development of H-bonded complexes is now considered. An influence of UV irradiation on optical properties of low-molecular-weight azobenzene-con-taining material (Fig. 2.4b) has been investigated (Aoki et ah, 2000) on the basis of such interactions. The first observation of photoinduced optical anisotropy in H-bonded complexes of azobenzene dyes and copolymers (Fig. 2.4b) has been recently demonstrated (Medvedev et ah, 2005). In this case, the induced anisotropy was stable, and the maximum dichroic ratio of 2 has been observed. A kinetics of the induction of birefringence (maximum value of ca. 0.01) in one of these complexes is shown in Fig. 2.5. An influence of H-bonding on the mesomorphic and photoorientation properties was recently demonstrated (Cui and Zhao, 2004). In this approach, the amorphous azopyridine side-chain polymer was converted into liquid crystalline polymers through self-assembly with a series of commercially available, aliphatic, and aromatic carboxylic acids (Fig. 2.4d). 

The measurement of photoinduced birefringence revealed very different anisotropy. Considering the numerous compounds that could readily be complexed with this type of polymer, including chiral adds, phenols, and metals, the approach of using side-chain azopyridine polymers offers the possibility to produce a large number of new photoactive liquid crystalline materials without exhaustive synthetic efforts and with new properties to exploit. And really the approach has been exploited in recent works. The materials similar to that... 

---