Photoswitchable smectic liquid-crystalline elastomers

We succeeded in the synthesis of azo side chain containing polysiloxanes with broad smectic C* and A phases. In these polymers the phase transition temperatures can be shifted reversibly by up to 17 degrees C by irradiation with UV (cis) or VIS (trans) light. Thin films of these polymers in the smectic phase (both on substrates and as free-standing films) orient perfectly in a homeotropic manner. As a consequence, the azo chromophores do no longer absorb during a perpendicular illumination with light (dichroism), It is thus possible to crosslink these films photochemically to prepare photoswitchable smectic LC elastomers.