Photoresponsive ferroelectric liquid-crystalline polymers

The photoresponse of ferroelectric smectic side-chain liquid-crystalline (LC) polymers containing a photoisomerizable azobenzene derivative as a covalently linked photochromic side group is investigated. By static measurements in different photostationary states, the effect of trans-cis isomerization on the material's phase-transition temperatures and its ferroelectric properties (spontaneous electric polarization PS and director tilt angle θ) are analyzed. It turns out that the Curie temperature (transition SC* to SA) can be reversibly shifted by up to 17 °C. The molecular mechanism of this "photoferroelectric effect" is studied in detail using time-resolved measurements of the dye's optical absorbance, the director tilt angle, and the spontaneous polarization, which show a direct response of the ferroelectric parameters to the molecular isomerization. The kinetics of the thermal reisomerization of the azo dye in the LC matrix are evaluated. A comparison to the reisomerization reaction in isotropic solution (toluene) reveals a faster thermal relaxation of the dye in the LC phase. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.