Molecular Chirality Due to Twisted Conformation in a Bent-Shaped Liquid Crystal Studied by Polarized Ft-Ir Spectroscopy

Abstract

Microscopic polarized fourier-transform infrared (FT-IR) measurements were performed in the antiferroelectric B2 phase of a bent-shaped liquid crystal molecule. In homogeneously chiral domains, the absorbance maximum appeared when the transmission axis of a polarizer was parallel to the layer normal for the phenyl ring stretching and C-0-C asymmetric stretching and that for the C=O stretching was almost perpendicular to that for C-0-C in the absence of a field. When a dc field was applied, the maximum for C-0-C rotated by about 35". which is almost the same as the apparent tilt angle of the molecule. In contrast, the axis of maximum absorbance for C=O was neither parallel nor perpendicular to that of C-0-C under a field. It indicates that the two ester groups are twisted with respect to the central phenyl ring and are not perpendicular to the molecular long axis. Moreover, one of the twist conformations is chosen and the rotation around the molecular long axis is strongly hindered.

Solid-State 13C NMR Study of Chiral Twisted Conformation Attributable to Chirality in Smectic Phases of Achiral Banana-Shaped Molecules

“…The conformational chirality was first proposed on the basis of the 13 C NMR data, in which the NMR signal due to the carbonyl carbons appears as a doublet for both the B2 and B4 phases . FT-IR measurements also supported the existence of the chiral conformation. , In our previous studies, however, we have not been concerned with the heterogeneity of the structures of the B2 phase. Additionally, the resolution of the NMR spectrometer and the temperature control facilities were not sufficient to obtain qualitative data, especially in the high-temperature B2 phase.…”

Chiral Memory on Transition between the B2 and B4 Phases in an Achiral Banana-Shaped Molecular System

“…In fact, the bias rotational direction of the carbonyl carbons on the ester linkages between the central core and two side wings can make such a twist conformation . NMR , and IR data , also strongly support this assumption. This bias twisting of the carbonyl groups was also detected by time-resolved FTIR technique by Shilov et al In a recent study, Jakli et al have observed optically active domains with clear boundary in both anti-ferroelectric and ferroelectric transformed states in an all ester banana-shaped molecules.…”

“…One is that the carbonyl groups are relatively parallel to the layer and the other is perpendicular. Takezoe and co-workers have proven this specific molecular orientation in the B2 phase of the P-n-O-PIMB homologues by using FT-IR spectroscopy, , and they have found that such a molecule tilts in the layers with the carbonyl groups relatively parallel to the layer interface as shown in Figure d. The change of the molecular conformation by rotating of the carbonyl groups should be inevitably followed by the alternation of the tilt direction in the layer, which is supposed to have a high energy barrier.…”

Nature‐Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self‐Assembly to DNA Mesophase and Nanocolloids

“…The molecular conformational chirality in achiral bent‐core molecules has been theoretically predicted by the Monte Carlo simulations and density functional theory (DFT) . The existence of such molecular conformational chirality in achiral bent‐core molecules has been also verified by vibrational CD, nuclear magnetic resonance (NMR), and Fourier‐transform infrared (FTIR) absorption spectroscopy . It should be noted that Schenning et al reported the spontaneous symmetry breaking in nanoporous polymer particles of hydrogen bonded smectic liquid crystalline monomers, where equal quantities of enantiomeric single‐domain droplets with either positive or negative tilt of the molecular orientation within the concentric layers were observed.…”