Azobenzenes, reorientation light

Figure 10 Reorientation of an azobenzene dye by photoisomerization with polarized light. Molecular representations of the lruns and cis forms of an azobenzene are represented by straight and bent shapes to illustrate how a trans-cis-trans cycle can effect a reorientation of the traru-azobenzene. If the inducing hght is polarized in the horizontal direction, the vertically oriented molecule will be unable to absorb the hght, and this orientation will therefore be photostable.

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. 

Photoreorientation of azobenzene chromophores by irradiation with polarized light is a very important photoinduced structural change. For azobenzene moieties, there is a widely accepted mechanism for the photo-reorientation The azobenzene moieties that are parallel with their long axis (and therefore with their transition dipole) to the electric field vector... 

Fig. 2 Schematic representation of the reorientation of azobenzene chromophores by excitation with linearly polarized light. Since both the cis and the trans state are excited by illumination with the writing wavelength, the molecules perform repeated reorientations until they end up with their transition dipole moments perpendicular to the light polarization. (Adapted with permission from [8]. Copyright Wiley-VCH Verlag GmbH Co. KGaA)...

For all the azo-polyglutamate LBK-polymer films, all the WGS experiments mentioned above were repeated by setting the UV and blue lights linearly polarized, in the plane of the sample, successively parallel and perpendicular to the dipping direction the same results were obtained for both UV and blue-light directions of polarization. In other words, no photoselection effect has been detected in these LBK structures. This is in contrast to the results obtained in a spin-cast film from the Pi.io polymer used in this study, and can be due to the subtle manner of the reorientation of the azobenzenes imposed by the LBK deposition technique, because efficient photoselection, following photoisomerization induced reorientation, can easily be achieved in spin-casted azo-polymer films [7-13]. This effect is discussed in the next section. 

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