Discontinuous change in the helical pitch of cholesteric liquid crystals by photoisomerization of a chiral azobenzene molecule

This paper describes the discontinuous change in the helical pitch of a cholesteric liquid crystal (ChLC) by means of the photoisomerization of chiral azobenzene molecules under homogenous alignment conditions. A mixture of E44, R811 and Azo was prepared in the ratio 68/28/4, respectively. R811 and Azo have opposite twisting abilities such that they induce right- and left-handed helices, respectively when added to E44. The mixture was injected into a glass cell having a 2 or 5 mm cell gap, and treated for homogeneous molecular orientation. The wavelength of selective reflection from the ChLC was shifted to shorter wavelengths by the trans-cis photoisomerization of Azo. The change in the helical pitch was not only discontinuous, but also dependent on the cell thickness. The discontinuous change in the helical pitch was estimated to be almost the same as the half turn of the helical pitch in each cell gap, and was dependent on the number of helical half pitches in the glass cell. The homogeneous alignment condition affects the photochemical change in the helical structure of the ChLC system.