Photochemically Inducible and Erasable Dichroism by Molecular Reorientation of an Azobenzene Pendant Unit in a Polyimide Langmuir‒Blodgett Film

Polyimide Langmuir‒Blodgett (LB) film possessing a photoresponsive pendant unit was prepared by a “precursor method”, where a polyamic acid alkylamine salt was used as the precursor for the polyimide. An azobenzene unit was incorporated onto the polymer backbone as a side chain group in order to allow for a trans-cis photoisomerization to take place. The conformation of the azobenzene unit in the LB films was revealed by means of FTIR reflection‒absorption (RA) spectroscopy. The specific orientation of the azobenzene unit affected the in-plane anisotropic photoisomerization during exposure to linearly polarized UV light. When the azobenzene units were lying flat on the surface plane, optical dichroism was induced in the polyimide LB film due to the reorientation of azobenzene units after anisotropic trans‒cis‒trans isomeric conversion. The reoriented conformation was stable at room temperature but could be randomized by irradiation with nonpolarized UV light. Further, this anisotropy displayed an angular independence to the polarization of the incident UV light.