Does Topology Dictate the Incidence of the Twist‐Bend Phase? Insights Gained from Novel Unsymmetrical Bimesogens

Abstract

We prepared a significant number of unsymmetrical liquid‐crystalline dimers that exhibit the twist‐bend nematic phase; a state of matter that exhibits spontaneous breaking of mirror symmetry and, for some materials, a microsecond electrooptic response. A number of novel unsymmetrical bimesogens were synthesized and in comparing their thermal behaviour to previous literature examples, we have uncovered an unexpected relationship between the thermal stability of the nematic and NTB phases. This relationship demonstrates that molecular shape dictates the incidence of this fascinating phase of matter and leads us to speculate as to the existence of “twist‐bend nematic phases” on length scales beyond those of the molecule.

Free volume, molecular grains, self-organisation, and anisotropic entropy: machining materials

“…The data was taken from refs. [10,11,20,[22][23][24][25][26][27][28], and recent publications also support this analysis [29][30][31][32][33].…”

“…A linear dependency was found across a range of nearly 100 di-, tri-, tetra-, hexa-, oligo-and polymeric-mesogens (see Figure 3), composed of various functional groups and motifs, as shown in Figure 3. It should be noted that there is a little scattering of the data for some oligomers at high temperatures, which is due to decomposition [22,34]. The linear behaviour indicates that the chemical information for these materials is of little importance in affecting the condensation of the system in the formation of the twist-bend phase.…”

Developments in liquid-crystalline dimers and oligomers

“…We have found previously that dimers and bimesogens incorporating the DFBC unit have a strong tendency to exhibit the twist-bend nematic phase [63]. Increasing the aspect ratio of the mesogenic units of a dimesogen/bimesogen has been demonstrated previously to confer large increases to both the clearing point and, if present, the N TB -N transition temperature [31,39,64,65], and with this in mind, we prepared the two mixed biphenyl/terphenyl bimesogens 12 and 13. Compounds 11, 12 and 13 were found to exhibit enantiotropic nematic and N TB mesophases.…”

“…There has been something of resurgence in interest in liquid crystal dimers in recent years, due largely to the observation of thermodynamically stable twist-bend phases in these systems . This has motivated the synthesis of a number of novel compounds, and present experimental results indicate that the twist-bend phase is primarily driven by topology [39], with the thermal stability having a dependency on the intermesogen angle [40,41]. Several aspects of the N TB phase have been reviewed recently [28,32,42,43].…”

“…Several aspects of the N TB phase have been reviewed recently [28,32,42,43]. It has been demonstrated for materials with nonamethylene and heptamethylene central spacers that there is a linear relationship between T NTB−N and T N-Iso [39]. It has been observed that many functional groups -including the SF 5 groupcan be incorporated into the structure of a suitable dimer without loss of the N TB phase.…”