Photoinduced textural and optical changes in a cholesteric copolymer with azobenzene-containing side groups

Textural changes induced by UV irradiation of planarly oriented films of a cholesteric copolymer were investigated. The copolymer is composed of photosensitive ethoxyazobenzene and chiral cholesterol-containing side groups, and displays a chiral nematic phase with a left-handed helical supramolecular structure in the temperature range 84-128degreesC. UV light with a wavelength of 365 nm leads to a deformation of the planar texture at temperatures higher than 100degreesC and to a reduction of the absorbance peak for left-handed circularly polarized light. Polarizing optical microscopy revealed the formation of a focal-conic texture in the irradiated regions. The texture induced by irradiation is very stable but the initial planarly oriented state can be recovered by mechanical stress. The observed phenomenon is attributed to the E-Z photoisomerization of azobenzene groups leading apparently to the formation of a thin isotropic layer in the films which in turn causes a destabilization of the planar texture. At temperatures below 100degreesC the textural transformations are accompanied by small shifts of the selective reflection maxima to longer wavelengths. The kinetics of the observed phenomenon were studied in detail and the influence of different parameters, such as light intensity and temperature, examined.