Formation of smectic and columnar liquid crystalline phases by cyclotriveratrylene (CTV) and cyclotetraveratrylene (CTTV) derivatives incorporating calamitic structural units

Abstract

Novel liquid crystalline oligomesogens, which consist of five, six or eight calamitic 4-cyanobiphenyl, 2-alkyl-5-phenyl-1,3,4-thiadiazole or 5-octyl-2-phenylpyrimidine units covalently linked by spacers of different length with a cyclotribenzylene central core (CTV derivatives, 2,3,7,8,12,13-hexa-substituted 10,15-dihydro-5H-tribenzo[a,d,g]cyclononenes) or a cyclotetrabenzylene central core (CTTV derivatives, 2,3,6,7,10,11,14,15-octa-substituted 5,10,15,20-tetrahydrotetrabenzo[a,d,g,j]cyclododecenes) have been synthesized. These compounds were investigated by polarizing microscopy, differential scanning calorimetry and some of them were also studied by X-ray diffraction. Many of the CTV derivatives show liquid crystalline properties. The cyanobiphenyl derivatives incorporating long spacer units have enantiotropic S_A phases which can easily be supercooled. The thiadiazole derivatives and the pyrimidine derivatives have liquid crystalline phases only if the spacer units are rather short. Thiadiazole derivatives with an odd number of connecting atoms between the CTV core and the calamitic mesogens form S_A phases, whereas those with even-numbered spacers display columnar mesophases. We propose that the columnar mesophases observed for these oligomesogens do not result from their ability to adopt a nearly disc-like shape. Instead, they represent ribbon phases which result from a steric frustration caused by the different space filling of the central cone-like core and the rod-like mesogenic groups. A chevron-like (banana-like) average shape of the molecules with even-numbered spacers is assumed to be responsible for the formation of the ribbon phases.

Also a CTTV derivative with eight appended phenylthiadiazole mesogens displays a columnar mesophase. A nematic phase was found for the corresponding cyanobiphenyl derivative, whereas the pyrimidine derivative is not liquid crystalline.