Electroclinic effect in low molar mass organosiloxane liquid crystals

Monomesogen low molar mass organosiloxane liquid-crystal molecules consist of a single mesogenic moiety attached via an alkyl chain to a short siloxane moiety. When the material is cooled from the isotropic phase, the siloxane moieties micro-separate from the mesogenic moieties and aggregate in planes that acts as an effective polymer backbone. The material is therefore in a smectic phase with alternating siloxane-rich layers and mesogen-rich layers. With a suitable design of the mesogenic moiety one can achieve a certain degree of decoupling between the mesogenic-rich layers. Such materials can be expected to display de Vries smectic A phases. In this paper we present monomesogen low molar mass organosiloxane liquid-crystal materials that display a wide temperature range de Vries smectic A* phase. The field dependence of the electric field-induced tilt of the optic axis and the field dependence of the birefringence have been measured at several temperatures. One of the materials displays a transition from de Vries smectic A* to smectic C* at low temperatures.