The effect of the terminal linking atom on the appearance of twist-bend nematic and smectic phases

The synthesis and characterization of five new members of both the CB6O·Om (6O defining the spacer as hexyloxy) and CBO5O·m (O5O defining the spacer as oxypentyloxy) series are reported which possess extended terminal alkyl chains. Two new series of dimers are also reported in which the linking group between the terminal alkyl chain and mesogenic unit is varied, namely the CBO5O·Om and CBO5O·Sm series. All four series exhibit both the twist-bend nematic phase (N_TB) as well as a rarely reported twist-bend smectic C phase (SmC_TB-SH). X-ray diffraction measurements showed that the SmC_TB-SH phase had a partially interdigitated bilayer packing arrangement in agreement with previous literature reports. The phase behaviour between these four series and the previously reported CB6O·m and CB6O·Sm series are compared. The N_TB phase is observed when the terminal alkyl chains are short whereas when the terminal alkyl chains are long there is a predisposition towards more smectic-like behaviour such as the SmC_TB-SH phase. The dimers which contain an oxygen link in the terminal chain have higher values of T_NTBN than those with a sulfur or methylene link. Additionally, the dimers with an 6O spacer, in general, had slightly higher values than those with O5O spacers. Interestingly, the SmC_TB-SH phase did follow the same trend with regards to the effect of the terminal linking group but the reverse trend with regards to the spacer.