Transitions between liquid crystalline phases investigated by dielectric and infra-red spectroscopies

TL;DR

This study investigates phase transitions in a liquid crystalline compound using dielectric and infra-red spectroscopy, combined with density-functional theory, revealing detailed molecular interactions and phase behavior.

Contribution

It combines experimental spectroscopic techniques with computational modeling to analyze phase transitions and molecular interactions in liquid crystals, providing new insights into their structural dynamics.

Findings

Identified phase transition points via IR spectra analysis.

Determined the likely head-to-tail configuration in the crystal phase.

Observed slowing down of flip-flop relaxation at smectic phase transitions.

Abstract

The liquid crystalline 11OS5 compound, forming the nematic phase and a few smectic phases, is investigated by broadband dielectric spectroscopy and infra-red spectroscopy. The dielectric relaxation times, ionic conductivity, and positions of infra-red absorption bands corresponding to selected intra-molecular vibrations are determined as a function of temperature in the range from isotropic liquid to a crystal phase. The correlation coefficient matrix and k-means cluster analysis of infra-red spectra are tested for detection of phase transitions. The density-functional theory calculations are carried out for interpretation of experimental infra-red spectra. The performance of various basis sets and exchange-correlation functionals is compared, including both agreement of scaled calculated band positions with experimental values and computational time. The inter-molecular interactions in the crystal phase are inferred from the experimental IR spectra and density-functional theory calculations for dimers in head-to-head and head-to-tail configurations. The experimental temperature dependence of the C=O stretching band suggests that the head-to-tail configuration in a crystal phase is more likely. A significant slowing down of the flip-flop relaxation process is observed at the transition between the smectic C and hexagonal smectic X phases.