Abstract
ABSTRACTThe twist-bend phase (NTB) is most commonly observed in materials with a gross-bent shape: dimers; bent-cores; bent-oligomers. We had suggested previously that the bend-angle of such systems effectively dictates the relative thermal stability of the NTB phase. However, our earlier paper relied on the use of a single energy-minimum conformer and so failed to capture any information about flexibility and conformational distribution. In the present work, we revisit our hypothesis and examine a second set of dimers with varying linking groups and spacer composition. We have improved on our earlier work by studying the conformational landscape of each material, allowing average bend-angles to be determined as well as the conformer distribution. We observe that the stability of the NTB phase exhibits a strong dependence not only on the Boltzmann-weighted average bend-angle (rather than just a static conformer), but also on the distribution of conformers. To a lesser extent, the flexibility of the spacer appears important. Ultimately, this work satisfies both theoretical treatments and our initial experimental study and demonstrates the importance of molecular bend to the NTB phase.
Highlights
The discovery of the twist-bend phase (NTB) has given fresh impetus to the study of dimeric liquid crystals [1,2,3,4,5,6,7,8,9,10,11,12,13]
We present a new set of liquid-crystalline dimesogens with varying linking-group and spacer compositions; in addition to studying the thermal behaviour by microscopy, calorimetry and X-ray scattering, we studied the conformational landscape of each material computationally, allowing us to obtain the bend-angle as a weighted average of many conformers
Compounds 1–10 were studied by a combination of polarised optical microscopy, differential scanning calorimetry (DSC) and – for selected materials – small-angle X-ray scattering (SAXS)
Summary
The discovery of the twist-bend phase (NTB) has given fresh impetus to the study of dimeric liquid crystals [1,2,3,4,5,6,7,8,9,10,11,12,13]. In our previous demonstration of the relationship between the dimer bend-angle and the thermal stability of the twist-bend phase, we relied on a single conformer This fails to capture any information concerning the conformational landscape and makes rather large assumptions, despite being shown to be the dominant conformer by 1D 1H NOESY NMR [36]. In this present work, we present a new set of liquid-crystalline dimesogens with varying linking-group and spacer compositions; in addition to studying the thermal behaviour by microscopy, calorimetry and X-ray scattering, we studied the conformational landscape of each material computationally, allowing us to obtain the bend-angle as a weighted average of many conformers. The transition temperatures of the obtained material only marginally different to those reported previously [24]
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