Synthesis, liquid-crystalline properties, and photo-optical studies of photoresponsive oligomeric mesogens as dopants in a chiral glassy liquid crystal

We have prepared photoresponsive oligomers that have Molecular weights of ca. 4500, 8000, and 1.6 000 g mol(-1) via the free-radical polymerization of 4-[4-alkylplienylazo]phenoxyalkyl acrylates. All of the oligomers possess bilayer smectic A (SmA) and smectic B (SmB) phases. Increasing, the concentration of these oligomeric dopants in a glass-forming cholesteric liquid crystal causes a dramatic red-shift in the reflection wavelength. The pitch shifts are very dependent on the alkyl chain lengths and molecular weights of the dopants. The oligomer that contains octyl chains and an octyl spacer, and that has a molecular weight of 4500 g mol(-1) exhibits the largest shift in the reflection wavelength. UV exposure has been used to control the cholesteric reflection pitch of the oligomer-cholesteric glassy liquid-crystal mixture over the entire visible region of the electromagnetic spectrum and vitrifies the samples by rapid cooling from their cholesteric temperatures to 0 degrees C. Extremely stable, even at 70 degrees C, erasable, full-color images have been created using this host-guest mixture.