FT-IR Study on Liquid Crystal Phase Transitions of Thermotropic Hydrogen-Bonded Cubic Mesogenes, 1,2-Bis(4′-n-alkoxybenzoyl)hydrazines (BABH-n) and 4′-n-Alkoxy-3′-nitrobiphenyl-4-carboxlic acid (ANBC-n): Spectroscopic Evidence for Quasibinary Picture Model

Changes in intermolecular interactions and molecular geometry for two kinds of thermotropic cubic mesogenes, 4′-n-alkoxy-3′-nitrobiphenyl-4-carboxlic acid (denoted as ANBC-n, where n represents the number of carbon atoms in the alkoxy group) and 1,2-bis(4′-n-alkoxybenzoyl)hydrazines (BABH-n), at liquid crystal (LC) phase transitions were revealed utilizing the frequency shifts in Fourier transform infrared (FT-IR) bands as a guide. The ANBC-n and BABH-n form two kinds of bicontinuous cubic (Cubbi), Ia3d and Im3m types, and smectic LC phases depending on the length of the alkyl chain and temperature. In the present work, two kinds of phase transitions, i.e., smectic C ↔ Ia3d-Cubbi phase transition for the ANBC-16 and BABH-9 and Ia3d-Cubbi ↔ Im3m-Cubbi phase transition for the BABH-13 and BABH-16, were examined, and the experimental result was compared to the entropy changes predicted by the quasibinary picture model. In this model, it is postulated that the basic units in the BABH-n and ANBC-n, i.e., the “...