Molecular biaxiality determines the helical structure - infrared measurements of the molecular order in the nematic twist-bend phase of difluoro terphenyl dimer.

Barbara Loska,Antoni Kocot,Georg H Mehl,Katarzyna Merkel,Chris Welch

doi:10.1039/d1cp00187f

Abstract

Fourier-transform infrared polarized spectroscopy was employed, to obtain the three components of the infrared absorbance for a series of bent-shaped dimers containing double fluorinated terphenyl core (DTC5Cn, n = 5, 7, 9, 11). The data were used to calculate both uniaxial and biaxial order parameters, for various molecular groups of dimers. The molecule bend was estimated based on the observed differences between the uniaxial order parameters for the terphenyl core and central hydrocarbon linker. The orientational order, distinctly reverses its monotonic trend of increase to decrease at the transition temperature, from the uniaxial nematic to the twist-bend nematic phase as result of the director tilt in latter/(twist-bend) phase. The molecular biaxiality, which is negligible in the nematic phase, starts increasing on entering the twist-bend nematic phase, following a sin-square relationships with the tilt angle. The local director curvature is found to be controlled by the molecular biaxiality parameter.

Highlights

An important leitmotif in the science of soft and liquid crystal materials is the understanding of the mutual relations between the shape of a molecule and macroscopic selforganization and the search for new possibilities of ordering based on the study of various molecules shapes [1-10]
By comparing the order parameters of the terphenyl core and the central alkyl linker, we examined the bend of the molecule in the nematic phase
In the nematic twist-bend (NTB) phase, it has been demonstrated that the temperature dependence of the S-order parameter distinctly reverses its monotonic trend of increase to decrease at the transition temperature from the nematic (N) to the twist-band (NTB) [26-31,40-42]

Summary

Introduction

An important leitmotif in the science of soft and liquid crystal materials is the understanding of the mutual relations between the shape of a molecule and macroscopic selforganization and the search for new possibilities of ordering based on the study of various molecules shapes [1-10]. By comparing the order parameters of the terphenyl core and the central alkyl linker, we examined the bend of the molecule in the nematic phase. Using the ratio of the NTB order parameter and the extrapolated trend from the nematic phase, the tilt angle for the different dimer segments can be calculated.

Results

Conclusion