Complex molecular dynamics of symmetric model discotic liquid crystals: comparison of hexakis(hepta-alkanoyloxy)triphenylene (HOT6) with hexakis(hexa-alkyloxy)triphenylene (HAT6)

Abstract

This study investigates the complex molecular dynamics of discotic liquid crystals (DLCs) by comparing two structurally similar compounds: hexakis(hepta-alkanoyloxy)triphenylene (HOT6) and hexakis(hexa-alkyloxy)triphenylene (HAT6) having the same triphenyl core and the same length of the alkyl side chain. The difference of both materials is that the alkyl chain is linked by an oxygen bridge to the triphenylene core for HAT6 and by a ester group for HOT6. Using a combination of broadband dielectric spectroscopy, differential scanning calorimetry, X-ray scattering, and neutron scattering techniques, the research explores the glass transition phenomena and relaxation processes in these materials. HOT6, featuring ester linkages, exhibits distinct dynamic behavior compared to HAT6, including two separate glass transitions indicated by the α₁- and α₂-relaxation found by dielectric spectroscopy which are assigned to the glassy dynamics of the alkyl side chain in the intercolumnar space and that of the columns, respectively. The study reveals that the ester group in HOT6 leads to increased molecular rigidity and altered packing in the intercolumnar space, as evidenced by X-ray scattering and the vibrational density of states. Neutron scattering confirms localized methyl group rotations and a further relaxation process which relates to the γ-relaxation revealed by dielectric spectroscopy. The findings contribute to a deeper understanding of glassy dynamics in partially ordered systems and highlight the influence of molecular architecture on relaxation behavior in DLCs.