要旨
This thesis is dedicated to a systematic study of the vibrational dynamics of representative thermotropic liquid crystals (LCs) in the spectral range from 1 to 7.5 THz. In particular, we measured the absorption spectra of two calamitic LCs, namely 4’-octyl-4-biphenylcarbonitrile (8CB), 4’-octyloxy-4- biphenylcarbonitrile (8OCB) and their mixtures, and two discotic LCs, hexabutoxytriphenylene (HAT4) and hexapentyloxytriphenylene (HAT5). These four molecules have been chosen to elucidate the role of specific atomic constituents and the molecular structure when inducing low-frequency vibrational modes. The first two are elongated and flexible, whereas the second is disk-like shaped and highly rigid, especially the core. We observed that 8OCB exhibited blue-shifting in low frequencies, spectral smoothing, and unclear differences at specific frequencies. Quantification of 8OCB’s peak at 7THz awaits future experiments. Strong interactions between 8CB and 8OCB, especially at higher 8CB concentrations and field strengths, were inferred. The non-linear activity was minimal in pure 8CB but pronounced in a 75% 8OCB mixture, suggesting a non-linear interaction-based cause. The experimental cases have been compared with DFT calculations considering only single molecules, thus neglecting any inter-molecular dynamics. On the other hand, the discotic molecules exhibited stronger absorbance in their extraordinary axis than in their ordinary axis, thus indicating higher molecular stiffness, the longer HATn’s alkoxy groups get.
