Photoinduced Chirality in Thin Films of Achiral Polymer Liquid Crystals Containing Azobenzene Chromophores

Circular dichroism and circular birefringence were induced in thin films of achiral polymer liquid crystals containing 7 or 15 mol % azobenzene chromophores by irradiation with circularly polarized light (CPL). Circular dichroism was observed not only for the azobenzene moieties but also for the nonphotoactive mesogens, cyanobiphenyl groups, due to a cooperative motion. CPL with opposite handedness produces enantiomeric structures, and a chiroptical switch can be achieved by alternating irradiation with left and right CPL. The level of photoinduced chirality depends on the incorporation methods (doped or chemically bound) of the azobenzene chromophores, the amount of azobenzene units, and the sample temperature. The photoinduced circular dichroism can be erased by heating the films above the clearing temperatures or by annealing the films in the liquid-crystalline phase.