Investigations of the Nematic Phase Structure and Biaxiality of Selected Oxadiazole Mesogens Employing an Optimized Force Field

Abstract

A unique class of bent-core nematics, based on the bis-phenyl-oxadiazole motif, have attracted considerable attention due to their unusual properties, including the possibility of forming the elusive biaxial nematic phase, a phase with significant potential for technological applications. A summary of current research into biaxial nematics is given, including experimental evidence,controversies,and the different computational models used to study this phase. Fully atomistic molecular
dynamic simulations were employed to study the differences and relationships in the mesophase molecular organization of four closely related oxadiazole bent-core molecules.As an accurate force field is essential to model liquid crystal systems, it was first found necessary to partially re-parametrize the General Amber Force Field, (GAFF), to accurately reproduce phase transitions of liquid crystal mesogens. Dramatic improvements of phase transition temperature predictions for a number of liquid crystals were achieved with the new force field, compared to the
original GAFF predictions.

Using the improved force field, GAFF-LCFF, the uniaxial and biaxial orientational order parameters were deduced for the four oxadiazole derivatives.These were found to be in good agreement with experimental data, where available. Small
differences in the magnitude of the biaxial order parameters were found between the four oxadiazole systems in their respective nematic phases, with the bent-core mesogen, C5-Ph-ODBP-Ph-OC12 displaying the largest values. The simulations confirm that the nematic phase biaxiality is predominantly local and not macroscopic, and do not support the presence of large cybotactic clusters with inherent biaxial order.

The atomistic simulations enabled the distinct differences in structure and molecular organization in the nematic phase of the four systems to be identified and analyzed, and the simulations were found to accurately represent a range of experimental observables, including the manifestation of enhanced local biaxial correlations for a
trimethlylated oxadiazole based mesogen.

The study also provides the novel result of the first simulation insight into the local structure of the dark conglomerate (DC) phase, and shows evidence of pretran-
sitional fluctuations relating to the onset of the DC phase in the bent-core mesogen C5-Ph-ODBP-Ph-OC12. A number of explanations linking key molecular chemical and structural features to mesophase behaviour have also been proposed.