A new series of chiral nematic bimesogens for the flexoelectro-optic effect

A series of chiral bimesogenic compounds has been synthesized. The molecules comprise beta-estradiol as the central chiral unit, which is attached tcs a terminal 4'-cyanobiphenyl-4-yl group at C-17 by an alkanoate spacer containing 5, 6 or 11 carbon atoms. At C-3 the estradiol moiety is substituted with an omega-alkenyloxy chain, also containing 5, 6 or 11 carbon atoms. We will describe the synthesis and phase characterization of these materials. The compounds with 5 or 11 carbon atoms in the spacer separating the mesogens display chiral nematic mesophases with temperature ranges up to 40 degrees C,entering the isotropic state at about 100 degrees C. The pitch lengths of these pure materials are remarkably invariant with temperature. Their electro-optic behaviour has been characterized, with particular emphasis on their flexoelectric properties. The materials have the highest flexoelectric coefficients reported so far in any chiral nematic liquid crystal [with (e) over bar(kappa(-1)) > + 0.5 C N-1 m(-1)], with flexoelectro-optic response times, for 0 to 90% of transmitted intensity, of the order or less than 1 ms.