Photoinduced anisotropy PIA

To sum up, during the irradiation, azobenzene groups with transition moments that are not initially perpendicular to the polarization direction of the laser light undergo a series of trans-cis-trans isomerization cycles accompanied by a change in orientation until they finally line up in directions approximately perpendicular to the polarization direction of the laser light (see Fig. 5.6). 

Photo-induced anisotropy (PIA) is quantitatively described by Eqs. (5-1) and (5-2), by An in terms of the induced birefringence and by the parameter S in terms of light absorption behavior. 

Chart 5.6 Chemical structure of pMNAP polymer used for the photogeneration of birefringence (see Fig. 5.7). 

A and and n and n denote the absorbances and the refractive indices at orientations parallel and perpendicular to the polarization plane of the exciting probe light, respectively. 

In recent years, optical dichroism and birefringence based on photo-induced trans-cis-trans isomerization of azobenzene groups has been observed with preoriented liquid-crystalline polymers [31-35] at temperatures above the glass transition temperature, and also with various amorphous polymers at temperatures well below the glass transition temperature. In the case of a polyimide (see Chart 5.7), a quasi-permanent orientation can be induced [36-38]. Here, the azobenzene groups are rather rigidly attached to the backbone and photoisomerization occurs at room temperature, i.e. 325 °C below the glass transition temperature, Tg = 350°C. This behavior is in accordance with the fact that the isomerization quantum yields of azobenzene compounds are very similar in solution and in polymer matrices 0 trans cis) 0,1 and 0(cis trans) 0.5. 

---