Transitions between Liquid Crystalline Phases Investigated by Dielectric and Infrared Spectroscopies

The liquid crystalline 11OS5 compound, forming the nematic phase and a few smectic phases, is investigated by broadband dielectric spectroscopy and infrared spectroscopy. The dielectric relaxation times, ionic conductivity, and positions of infrared absorption bands corresponding to selected intramolecular vibrations are determined as a function of temperature in the range from an isotropic liquid to a crystal phase. The correlation coefficient matrix and k-means cluster analysis of infrared spectra are tested for detection of phase transitions. The density-functional theory calculations are carried out for interpretation of experimental infrared 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 intermolecular 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 the 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.