Butadienes as novel photochromes for color tuning of cholesteric glasses: Influence of microscopic molecular reorganization within the helical superstructure

1-(Alkoxyphenyl)-4-(cyanophenyl)buta-1E,3E-dienes have been used as novel photoresponsive dopants for investigating light-induced changes in the pitch of a glass-forming cholesteric liquid crystal (CLC), dicholesteryl-10,12-docosadiynedioate. X-ray diffraction studies have helped to establish the role of microscopic changes of smectic domains within the helical superstructure, induced by the dopant molecules and their photoisomers, which result in changes in their macroscopic light-reflecting properties. Increasing concentration (1-12 wt.-%) of these dopants causes a dramatic red-shift in the wavelength of reflected light by the host CLC, extending it into the near-infrared region. The extent of red-shift depends strongly on the molecular length of the dopants and those possessing lengths between 25-28 Angstrom, namely 1-(octyloxyphenyl)-4-(cyanophenyl)buta-1E,3E-diene and 1-(decyloxyphenyl)-4-(cyanophenyl)buta-1E,3E-diene, exhibit the maximum shift (Deltalambda(max)=435 nm). Photoisomerization of these dopants leads to a blue-shift of the reflected light and this effect could be used to tune the light reflectivity of these mixtures over a large part of the visible region. The cholesteric pitch and hence the reflected light by these materials could be fixed in a glassy state by rapidly cooling them from their cholesteric temperatures to 0 degreesC. The efficacy of these materials for full-color photoimaging has also been demonstrated.