Abstract
We present structural studies of a dimeric compound composed of a central heptyl spacer linking two mesogens consisting of terphenyl units at which two adjacent fluoro groups are attached to each central ring. The terminal rings are linked to pentyl chains as terminal groups. The material exhibits a nematic-nematic transition and a low temperature modulated phase. The higher temperature nematic phase was found to exhibit an anomaly of the bend elastic constant similar to that of the dimers with N-Ntb phase sequence, and the physical properties of the low-temperature nematic phase are similar to those of the known Ntb materials. The structure of the low-temperature modulated smectic/columnar phase is described together with its ability to form freely suspended films and fibres. The relation of the modulated structure to the fibre formation and to the appearance of the labyrinthine instability in freely-suspended films is discussed.
Highlights
Ntb phase despite the fact that the mesogens do not possess any chiral groups.[8,22,23,24] Measurements of the electroclinic effect in CB7CB25 suggested, and freeze-fracture transmission electron microscopy observations in thin films[10,11] proved the presence of a very short pitch helix in the range of 8–12 nm
Besides the fundamental interest of exploring new kinds of selfassembly, the unique combination of optical, dielectric and elastic properties of these new nematic phases makes them appealing for use in potential applications, such as tuneable diffraction gratings[31] or full-colour electrically tuneable selective light reflection displays.[32]
We demonstrate that the high-temperature nematic phase exhibits an anomaly of the bend elastic constant similar to that in the N phase of flexible dimers with underlying Ntb phase, and where the physical properties of the low-temperature nematic phase are similar to those of the known Ntb-materials
Summary
Ntb phase despite the fact that the mesogens do not possess any chiral groups.[8,22,23,24] Measurements of the electroclinic effect in CB7CB25 suggested, and freeze-fracture transmission electron microscopy observations in thin films[10,11] proved the presence of a very short pitch helix in the range of 8–12 nm. Materials combining a variety of molecular modifications with respect to the original cyanobiphenyls have been reported, including studies of the combination of polar and apolar mesogens in the dimeric units, the influence of the nature of linking or terminal chains and their length,[9,27,33,34,35] hydrogen-bonding driven dimers[36] and asymmetric dimers with chiral groups, which show the cholesteric analogue of the Ntb phase and remarkably give rise to multiple sequential N–N transitions.[36,37] This variety of new materials gave rise to richer mesomorphism which, in some cases, involve additional low temperature new smectic phases.[27,35,38].
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