Photoinduced Effects in Liquid Crystal Systems Containing Chiral α,β-Unsaturated Ketones

ABSTRACT

Photoinduced changes have been revealed and summarized for liquid crystalline (LC) systems with helical supramolecular structures induced with chiral α,β-unsa-turated ketones. The photoinduced inversion of a helix sign was ascertained for LC mixtures containing 3R-methylcyclohexanone arylidene derivatives. This effect is accompanied with an essential enhancement of the helical twist in the photostationary state and its invariability under the following prolonged UV irradiation. For ferroelectric LC, the photoinduced inversion of a spontaneous polarization sign and a concomitant increase in its absolute value and in the smectic tilt angle are observed. The E–Z photoisomerization of chiral compounds is established to be responsible for the effects discovered.

Keywords:

• chiral dopants
• induced cholesteric and ferroelectric liquid crystals
• photoinduced effects

Notes

*ν_max(ϵ) is the reciprocal wavelength (extinction) in the maximum of the long-wavelength ππ* band; φ _E (ϵ _{λ *} ) and φ _Z (ϵ _{λ *} ) are the quantum yields of forward (E–Z) and backward (Z–E) photoisomerizations (extinctions for isomers on the excitation wavelength); α _E is the E-isomer fraction in a photostationary mixture (on HPLC).

**Measured in octane for the Z-isomer of 2b.