Twist‐Bend Nematic Glasses: The Synthesis and Characterisation of Pyrene‐based Nonsymmetric Dimers

A selection of pyrene‐based liquid crystal dimers have been prepared, containing either methylene‐ether or diether linked spacers of varying length and parity. All the diether linked materials, CBO n O.Py ( n =5, 6, 11, 12), exhibit conventional nematic and smectic A phases, with the exception of CBO11O.Py which is exclusively nematic. The methylene‐ether linked dimer, CB n O.Py, with an even‐membered spacer ( n =5) was solely nematogenic, but odd‐members ( n =6, 8, 10) exhibited both nematic and twist‐bend nematic phases. Replacement of the cyanobiphenyl fragment by cyanoterphenyl giving CT6O.Py, gave elevated melting and nematic‐isotropic transition temperatures, and SmA and SmC A phases were observed on cooling the nematic phase. Intermolecular face‐to‐face associations of the pyrene moieties drive glass formation, and all these materials have a glass transition temperature at or above room temperature. The stability of the glassy twist‐bend nematic phase allowed for its study using AFM, and the helical pitch length, P TB , was measured as 6.3 and 6.7 nm for CB6O.Py and CB8O.Py, respectively. These values are comparable to the shortest pitch of a twist‐bend nematic phase measured to date.