How Do Intermolecular Interactions Evolve at the Nematic to Twist–Bent Phase Transition?

Abstract

Polarized beam infrared (IR) spectroscopy provides valuable information on changes in the orientation of samples in nematic phases, especially on the role of intermolecular interactions in forming the periodically modulated twist–bent phase. Infrared absorbance measurements and quantum chemistry calculations based on the density functional theory (DFT) were performed to investigate the structure and how the molecules interact in the nematic (N) and twist–bend (NTB) phases of thioether dimers. The nematic twist–bend phase observed significant changes in the mean IR absorbance. On cooling, the transition from the N phase to the NTB phase was found to be accompanied by a marked decrease in absorbance for longitudinal dipoles. Then, with further cooling, the absorbance of the transverse dipoles increased, indicating that transverse dipoles became correlated in parallel. To investigate the influence of the closest neighbors, DFT calculations were performed. As a result of the optimization of the molecular cores system, we observed changes in the square of the transition dipoles, which well corresponds to absorbance changes observed in the IR spectra. Interactions of molecules dominated by pairing were observed, as well as the axial shift of the core to each other.

Dielectric Study of Liquid Crystal Dimers: Probing the Orientational Order and Molecular Interactions in Nematic and Twist-Bend Nematic Phases

“…We simply interpreted this fact as an apparent growth of the potential barrier, q , which separated the two minima along the n direction after entering the N TB phase. This finding corresponded well with the orientational correlations effect of the longitudinal dipoles, which was revealed by IR spectroscopy …”

“…Both of the longer linker dimers, CBSC7SCB and CBSC7OCB, exhibited a smaller/slower increase of the potential barriers that extended in the range of 20–30 K, after which they reached their saturation, while the samples underwent vitrification. It is worth mentioning that the barriers increased in two steps: the first step corresponded well to an orientational correlations effect of the longitudinal dipoles ( g ∥ < 1) along the major axis of anisotropy, and the second step appeared at the temperature at which the interactions along the perpendicular axis became important ( g ⊥ > 1) that was revealed by IR spectroscopy …”

“…This finding corresponded well with the orientational correlations effect of the longitudinal dipoles, which was revealed by IR spectroscopy. 16 Figure 6 shows the apparent increase of the potential barrier, q, for symmetric and asymmetric dimers on entering the N TB phase. Overall, the q barrier grew from about 12 kJ/mol (in N phase) up to about 18 kJ/mol in the N TB phase;, however, the temperature trends for the shorter dimers (n = 5) are clearly different than for the longer ones (n = 7).…”

“…However, the specific structure of the mesophase still remains a matter of debate. − Although the basic features of this phase are now established, the nature of this phase is still discussed controversially. The existing materials do not behave fully in line with the prevailing models. ,,, Besides, there have been significant theoretical and simulation efforts toward developing an understating of the interactions and underlying mechanisms that account for the origin of such nematic–nematic (N–N) transitions, with most recent results indicating steric interactions and the primary role of the molecular shape as the driving force for the phase formation. ,,− …”

“…Helical structures, which have been the subject of great interest in the field of molecular sciences partly because of their potential applications in electro-optical devices, have emerged as one of the most important subgroups of hierarchical structures. 5−8 Even when chiral nematic phase has a pitch of few molecular lengths, the director twists and bends are generally accepted, as has been confirmed by several experiments: polarizing optical microscopy (POM), 9 freeze fracture transmission electron microscopy, 10,11 atomic force microscopy, 11 nuclear magnetic resonance (NMR), 12,13 Raman scattering, 14 IR spectroscopy (IR), 15,16 and non-resonant hard X-ray scattering, 17,18 as well as C K-edge and Se K-edge resonant X-ray scattering. 19,20 The chirality of the phase is measured directly using the synchrotron circular dichroism of aligned samples.…”