Nonsymmetrical cholesterol dimers constituting regioisomeric oxadiazole and thiadiazole cores: an investigation of the structure–property correlation

Abstract

Three series of chiral nonsymmetrical dimers were prepared by connecting promesogenic cholesterol to a bent structure derived from a substituted 1,3,4-oxadiazole or 1,2,4-oxadiazole or 1,3,4-thiadiazole moiety. These two mesogenic segments are interconnected through spacers of varying lengths and parity. The structures of the bent achiral unit were systematically varied with different central heterocyclic cores to understand the influence of bent angles on the thermal and gelation behavior. The bent angle of the achiral unit, which is determined by the heterocyclic core, has a major role in the stabilization of frustrated phases. Dimers based on the 1,3,4-oxadiazole unit with a more bent structure stabilized frustrated phases like blue phases and twist grain boundary phases. The bent system with a wider bent angle preferred to stabilize chiral nematic and smectic A phases. It is interesting to note that an increased bent structure reduced the mesophase stability as in the case of dimers based on the 1,3,4-oxadiazole unit, where many compounds exhibited monotropic phases. In the case of dimers with a wider bent angle, enantiotropic mesomorphism was observed. All the compounds showed blue light emission in the solution. Among these chiral dimers, only the compounds based on the 1,3,4-oxadiazole unit showed the gelation ability, which emphasizes how small structural changes like bent angle, dipole moment and the type of heteroatom in the heterocyclic unit affect the macroscopic self-assembly.