The low-temperature phase of matter known as the twist-bend nematic phase is a highly studied area in liquid crystal chemistry. Mesogen denotes the part of a liquid crystal that is responsible for its properties. Certain mesogenic molecules can pack into bent structures, either twist or splay, called twist-bent nematic or splay-bend nematic. The twist-bent nematic phase was first discovered for a liquid crystal dimer (1,7-bis-4-(4’cyno-biphenyl) heptane; CB7CB; pictured below). The molecule contains two mesogenic units linked by a flexible spacer.
Only a few twist-bend nematogens are known; the structure-property relationship involved is not well understood. Recent work has explored how polar functional groups can lead to antiparallel pairing in unsymmetrical bimesogenic liquid crystals.
Rrichard J. Mandle and John W. Goodby, University of York, UK, have extended this work to new unsymmetrical polar bimesogens that feature polar functional groups. The team synthesized many bimesogens with one polar and one non-polar functionality. The mesogen units consist of substituted phenyl benzoates, separated by a nonamethylene spacer and methylene linking groups. Using this approach, the team was able to synthesize new materials that demonstrate the twist-bend nematic phase.
The researchers observed a linear relationship between the nematic to isotropic transition temperature and the twist-bend nematic to nematic transition temperature. This demonstrates that the molecular shape or topology of the mesogenic units dictates the occurrence of the twist-bend nematic phase. In molecules with different chemical linkers, the bend angles and shapes of the molecules are different, and no such relationship exists. According to the researchers, the results may lead to the discovery of twist-bend nematic phases for even larger molecules.
- Does Topology Dictate the Incidence of the Twist-Bend Phase? Insights Gained from Novel Unsymmetrical Bimesogens,
Richard J. Mandle, John W. Goodby,
Chem. Eur. J. 2016.