Photochromic LC–polymer composites containing azobenzene chromophores with thermally stable Z-isomers

Abstract

Novel types of photochromic liquid crystalline (LC) polymer-based composites containing chiral photochromic azobenzene moieties were elaborated. For this purpose, a new chiral azobenzene-containing methacrylic monomer was synthesized. On one of the benzene rings of the azobenzene chromophore, there are methyl substituents in both positions ortho to the azo group. This substitution significantly increases the thermal stability of the photoinduced Z-form. The first type of novel photosensitive composite is a glass cell filled with cholesteric polymer-stabilized layers produced by thermal polymerization of a mixture containing a synthesized chiral photochromic azobenzene monomer, a nematic mixture of cyclohexane derivatives, mesogenic diacrylate and a thermal initiator. UV-irradiation of such samples leads to E–Z isomerization of the photochromic groups of the azobenzene monomer units, accompanied by a shift of the selective light reflection peak to a long wavelength spectral region. This process is thermally and photochemically reversible; the kinetics of the selective light reflection shift was studied. The second type of novel LC composite was obtained by the introduction of the same cholesteric photochromic mixture into porous stretched polyethylene (PE) films. Due to the highly anisotropic porous structure of PE, a uniaxially aligned nematic phase was obtained inside the pores. Irradiation with UV and visible light provides the possibility of dichroism and birefringence photocontrol in the obtained LC composite films. The possibility of photo-optical image recording on the prepared composite was demonstrated. It is noteworthy that the images recorded by the novel composite types have extremely high thermal stability (weeks) in comparison with those based on other azobenzene derivatives.