Characterization of siloxane/cholesterol based liquid crystalline materials by X-ray diffraction and molecular modeling

Liquid crystalline materials exhibit a state of aggregation that is intermediate between the crystalline solid and the isotropic liquid. A series of liquid crystalline cholesteryl esters and liquid crystalline compounds composed of siloxane backbones substituted with cholesteryl esters were studied by X-ray diffraction and molecular modeling. Some of these materials have been processed into ordered, optically clear films which are of potential use as matrices for optical devices.;The crystal structures of two cholesteryl esters, cholesteryl-4-vinylbenzoate (C4VB) and cholesteryl-4-octeneoxybenzoate (C4OB), were solved by X-ray diffraction. C4VB crystallizes in a primitive orthorhombic unit cell (space group P2;Molecular modeling calculations were undertaken to examine the low energy conformations and molecular flexibility of a series of cholesteryl ester substituted cyclosiloxane-based liquid crystals. Preliminary studies examined the effect of an electrostatic energy term (in the Tripos force field) on modeling siloxane and cholesterol based materials. Modifications to the Si atom force field parameter were also made. Molecular dynamics calculations were undertaken on individual cholesteryl ester substituted cyclotetrasiloxanes simulated within a cylindrical potential boundary. Cyclotetrasiloxanes substituted with C4OB units exhibited closer lateral packing of substituents than C4VB substituted cyclotetrasiloxanes. This behavior was attributed to the added flexibility of C4OB units, as the longer spacer group decoupled the substituents from the cyclosiloxane backbone. The cyclosiloxane moiety was found to be highly flexible during molecular dynamics simulations and did not present a barrier to rotation of the pendant cholesteryl units.