Sulfur-linked cyanoterphenyl-based liquid crystal dimers and the twist-bend nematic phase

The synthesis and characterisation of two series of cyanoterphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 34-{omega-[(4 '-cyano-[1,1 '-biphenyl]-4-yl)thio]alkyl}-[11,21:24,31-terphenyl]-14-carbonitriles (CBSnCT), and the 34-({omega-[(4 '-cyano-[1,1 '-biphenyl]-4-yl)thio]alkyl}oxy)-[11,21:24,31-terphenyl]-14-carbonitriles (CBSnOCT) are described. The odd members of both series show twist-bend nematic and nematic phases, whereas the even members exhibit only the nematic phase. This is consistent with the widely held view that molecular curvature is a prerequisite for the observation of the twist-bend nematic phase. The nematic-isotropic and twist-bend nematic-nematic transition temperatures are higher for the dimers containing cyanoterphenyl groups than for the corresponding cyanobiphenyl-based dimers. This change is more pronounced for the nematic-isotropic transition temperatures and is attributed to the enhanced interaction strength parameter associated with the cyanoterphenyl fragment whereas the molecular shapes, as governed by the spacer, are rather similar. The behaviour of CBS2CT appears somewhat anomalous and exhibits a higher value of the twist-bend nematic-nematic transition temperature than expected, and this is attributed to the presence of highly bent molecular conformations.