Helical twisting power of steroidal solutes in cholesteric mesophases.

Abstract

Shifts in the nematic temperature of a compensated cholesteric liquid-crystal solvent, consisting of cholesteryl chloride and cholesteryl myristate, are caused by adding steroidal solutes. A helical twisting power P can be defined which is a molecular property; there is a linear dependence of the inverse of the solute concentration on the helical pitch. The method has the advantages of (1) allowing study of compounds which do not form mesophases themselves and (2) eliminating temperature effects. For a series of fatty acid esters of cholesterol, P decreases with decreasing number of carbon atoms in the ester chain, and shows a zigzag fine structure with P having higher values for odd-numbered carbon-atom chains. P, and therefore the helix, changes rotational senses if the ester chain is replaced by Cl, Br, or OH. Changes are also discussed associated with modification of the 17 side chain or the steroid ring system itself. A model is proposed attributing P to the molecular asymmetry at the 3β position of the steroid ring, which can cause macroscopic twist if two lever arms are present, one of which lacks rotational symmetry.