We show that the occurrence of pure photoorientation by photoisomerization of azo dye in polymers is influenced by the polymer molecular structure, as well as by the photoisomerization quantum yields and the cis → trans thermal isomerization rate. These findings are rationalized by the study of the photoorientation of azo dye in a series of polyurethane polymers, each with distinct differences in the molecular structure of the unit building blocks. It will be shown that the thermally activated chromophore movement in these polymers is dictated by the polymer backbone. We also show how to quantify coupled photoisomerization and photoorientation of spectrally overlapping isomers in polymers, and we determine the photoisomerization quantum yields and the photoorientation movement of the chromophores in azo-polyurethanes.

A large photoinduced anisotropy ratio was observed in the push-pull azo dyes bonded covalently to polyurethane by polarized UV-vis spectroscopy. During irradiation with an Ar + laser, the absorbance perpendicular to the polarization direction of the laser light increases gradually even when the photoisomerization rate of trans to cis forms is equal to the reisomerization rate of cis to trans forms. The trans molecules are considered to reorient during the trans-cis-trans isomerization cycles. The process of photoinduced reorientation was also observed by polarized FT-IR spectroscopy. The dynamic behavior of trans and cis isomers is discussed in terms of orientation factors evaluated using the absorbance of some IR vibrational bands. The values of orientation factors for the trans and the cis isomers vary with irradiation time due to the reorientation. The reorientation process for the cis molecules was found to be essential to the reorientation for the trans molecules. On the other hand, according to the result of no variation of absorbance for the CdO group in urethane main chain, the urethane main chain may not move together with azo moieties by polarized light irradiation.

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