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. Photoinduced changes of the ferroelectric properties of an azobenzene‐functionalized liquid‐crystalline polymer are investigated in detail using both photostationary and time‐resolved measurements (see figure). The results give an insight into the effect of trans–cis isomerization on the Curie temperature and ferroelectric properties and the molecular mechanisms leading to this “photoferroelectric effect”, respectively.