Distinct twist-bend nematic phase behaviors associated with the ester-linkage direction of thioether-linked liquid crystal dimers†
The twist-bend nematic phase (NTB) is a new spontaneous symmetry-breaking phenomenon observed in fluidic liquid crystal (LC) phases, which possesses a heliconical structure with a pitch ranging from several to tens of nanometers. Herein we demonstrate the distinct nano-to-macroscopic NTB phase behaviors associated with the ester-bond direction in two homologous series of sulfur-containing cyanobiphenyl-based LC dimers, viz. CBCOOnSCB and CBOCOnSCB (n = 2, 4, 6, 8, and 10). Both the series (excluding n = 2) formed NTB phases in which the homologues (n = 4 and 6) exhibited the NTB phases across a broad temperature range, which were observed to be stable even at room temperature and eventually formed NTB glasses. We found that both homologues (n = 4 and 6) displayed distinct phase-transition properties and optical textures with respect to the NTB phases. By performing tender resonant X-ray scattering measurements at the sulfur K-edge, we discovered their distinctly different nanoscopic helical pitch lengths. Within a similar shifted temperature, the pitches for CBOCOnSCB showed strong temperature dependence and were approximately double those of CBCOOnSCB, which exhibited significantly weaker temperature dependence. Compared to those of the representative twist-bend nematogenic dimers, the pitches of CBOCOnSCB and CBCOOnSCB are longer and shorter, respectively. It is assumed that the molecular bend (or the molecular biaxiality) of LC dimers strongly influences the precession angle of the heliconical helix, and hence the resulting pitch. These findings provide new insights into the molecular designs to modulate the nanoscale helical pitches of the NTB phases.