Azobenzene birefringence

C Wang, H Fei, Y Qiu, Y Yang, Z Wei, Y Tian, Y Chen, and Y Zhao, Photoinduced birefringence and reversible optical storage in liquid-crystalline azobenzene side-chain polymers, Appl. Phys. Lett., 74 19-21, 1999. 

If one makes use of the rather limited information available and given above one may infer that a tilt of between 20° and 30° is normal for straight chain azobenzene derivatives when deposited as LB films, even when a homeotropic phase exists. Such a structure can only be produced in a rather loosely packed film. At the moment it is an open question whether monolayers of these materials exist in the hexatic phase as is the case for fatty acids or whether the structure more nearly corresponds to the smectic-A phase. In the case of the birefringent phase described by Jones et al. [151] it was shown that, once this phase was established, further layers deposited by the LB technique go down in an epitaxial manner. 

As mentioned in Section 5.1, the reorientation of azobenzene photo-chromic moieties under illumination with polarized light and the birefringence that is induced from it are well known and covered in many publications. 

In polyurethane 33, the azobenzene moieties are separated from each other along the polymer backbone by isophorone units and have the free volume necessary for isomerization and molecular reorientation. This polymer s azobenzene moieties can be photoisomerized readily in mixed LBK films, as demonstrated by measuring the optically induced birefringence that originates from the photoreorientation of the chromophore upon polarized irradiation see Section 6.5.2). 

Wu, Y., Natansohn, A., Rochon, P. (2001). Photoinduced birefringence and surface relief gratings in novel polyurethanes with azobenzene groups in the main chain. Macromolecules 34. 7822-7828. 

Lagugne Labarthet, E, Rochon, P., Natansohn, A. (1999). Polarization analysis of diffracted orders from a birefringence grating recorded on azobenzene-containing polymer. Appl. PIrys. Left. 75, 1377-1379. 

Labarthet, R L., Buffeteau, T., and Sourisseau, C. Analyses of the diffraction efficiencies, birefringence, and surface relief gratings on azobenzene-containing polymer films. /. Phys. Chem. B 1998, 102, pp. 2654-2662. 

The photoselection in azobenzene polymers (Figure 11) is rather a general phenomenon.77-84 Systematic studies have been conducted by Natansohn and coworkers on the photoinduced molecular reorientation of azobenzene units to induce birefringence and dichroism of thin films of acrylate amorphous polymers with Dispose Red (DR) pendant residues, which have been attracting extensive... 

S. Ivanov, I. Yakovlev, S. Kostromin, V Shivaev, L. Lasker, J. Stumpe, and D. Kreysig, Laser-induced birefringence in homeotropic films of photochromic comb-shaped liquid-crystalline copolymers with azobenzene moieties at different temperatures, Makromol. Chem. Rapid... 

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