Structural and dynamical investigation of glassforming smectogen by X-ray diffraction and infra-red spectroscopy aided by density functional theory calculations

TL;DR

This study combines X-ray diffraction, infrared spectroscopy, and density functional theory to analyze the molecular arrangement and phase behavior of a specific glassforming smectic liquid crystal, revealing insights into its structure and transitions.

Contribution

It provides a detailed molecular-level understanding of the smectic phases in a glassforming liquid crystal using combined experimental and theoretical methods, including DFT calculations.

Findings

Correlation length increases in supercooled state, indicating hexatic phase.

Infra-red spectra show splitting consistent with non-linear molecular shapes.

Glass transition subtly affects layer spacing and spectral features.

Abstract

Molecular arrangement in the chiral smectic phases of the glassforming (S)-4'-(1-methylheptylcarbonyl)biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]benzoate is investigated by X-ray diffraction. An increased correlation length of the positional short-range order in the supercooled state agrees with the previous assumption of the hexatic smectic phase. However, the registered X-ray diffraction patterns are not typical for the hexatic phases. Comparison of the smectic layer spacing and optical tilt angle indicates a strongly non-linear shape of molecules, which enables choice of the molecular models obtained by DFT calculations, used subsequently to interpret the infra-red spectra. The presumption of the hexatic smectic FA* or IA* phase is supported by the splitting of the absorption bands related to the C=O stretching in the supercooled state, which is absent in the smectic CA* phase above the melting temperature. The glass transition affects the temperature dependence of the smectic layer spacing but only subtly impacts the infra-red spectra. Application of the k-means cluster analysis enables distinction between the infra-red spectra below and above the glass transition temperature, but only for certain spectral ranges.