In-situ X-ray measurements of light-controlled layer spacing in a smectic-A liquid crystal

The smectic-A phase of octylcyanobiphenyl (8CB) doped with a photosensitive azo-compound has been reported to exhibit structural changes and formation of periodic layers distortions upon exposure to low-level radiation. It is caused by changes in the smectic layer spacing triggered by light-induced molecular photoisomerization of the dopant azocompound. Our in-situ high resolution x-ray scattering study reveals that the light irradiation controls the smectic spacing. The exposure to UV light increases the layer spacing by as much as 0.5% (0.155 Angstrom for the studied mixture of 8CB and diheptylazobenzene 7AB at T = 24.00 degrees C). Irradiation by Be-Ne laser at 633 nm causes the contraction of the layers.