The effect of molecular bend on the properties of bimesogenic materials

A series of compounds were synthesised belonging to two families of unusually shaped
liquid-crystals, a series of dimers, also called bimesogens, with varying molecular
architecture, as well as a series of so-called “pseudo bent-core” materials, wherein the
molecule is in possession of a mostly rigid core, with one possible site of flexibility in the
central unit.
The properties of the pseudo bent core materials were found to be strongly dependent on
the shape of the central unit, ranging from non-mesogenic for those materials with
sterically bulky central cores, to nematic for cores with a group that promotes linearity,
to lamellar and even the relatively uncommon dark conglomerate phase in materials that
are more bent.
For the flexible bimesogens the behaviour was much more consistent, with all of the phase
behaviour displayed being nematic in nature, with some exhibiting the hotly debated
twist-bend nematic (NTB) phase. It was found that the phase behaviour, particularly the
occurrence of the NTB phase was strongly dependent on both the terminal unit;
conjugating polar groups gave more stable phases than non-conjugating, and the nature
and length of the spacer. With methylene spacer providing a more stable NTB phase, and
the stability of the phase behaviour generally increasing with spacer length. Mixture
studies indicated that the primary barrier to ether linked compounds forming the NTB
phase was their high melting point. Mixtures with a chiral dopant also provided some
insight into the NTB phase, and allowed for the observation of the first direct isotropic-
NTB transition.
This thesis is concluded with a discussion on the relationship between molecular bend
and flexibility and the phase behaviour of these materials, relating the phase behaviour of
both the bimesogens and the “pseudo bent-cores” to their ability to twist and reshape to
effectively pack into certain mesophases.