Study of ferro- and anti-ferroelectric polar order in mesophases exhibited by bent-core mesogens

Abstract

The polar nature of three pure homologous bent-core compounds derived from 4-cyanoresorcinol bisbenzoates as a central core unit were studied based on dielectric spectroscopy measurements and electro-optical investigation. All these compounds exhibited a highly correlated nematic phase (N_CybC) followed by a phase (CybC) comprising elongated but not completely fused strings of cybotactic clusters. Additionally, the higher homologous compounds exhibited one or more tilted smectic phases (SmC_(I), SmC_(II)). Based on all the investigated results, the N_CybC phase was found to be quite different from the usual nematics, consisting of tiny polar clusters, which in effect revealed a ferroelectric polarization within the medium. Moreover, the lower temperature SmC phases (SmC_(I) and SmC_(II)) characterized an anti-ferroelectric type of molecular dipolar ordering in adjacent layers. The intermediate CybC phase accomplished the transition from the ferroelectric-type N_CybC phase to the anti-ferroelectric type of the SmC_(I) phase, where the cybotactic clusters were elongated. The SmC_(II) phase was a SmC phase with an enhanced packing density in comparison to the SmC_(I) phase, which seems like a more solid-like state or glassy state. The field reversal polarization technique and the simultaneous observation of the optical textures corroborated the polar nature of the mesophases. These outcomes were explained by considering the structural changes of the molecules by increasing the chain length and the combined effect of their association in the dielectric parameters of the respective mesophases. Finally, it was concluded that the SmC_(I) phase should be designated as SmC_sP_A phase (C_s stands for a synclinic-type of molecular organization and P_A corresponds to an anti-ferroelectric ordering of the polar orientation in neighboring layers), where the SmC_(II) phase was characterized by a quite solid-like state or close-packed SmC phase with a similar type of synclinic anti-ferroelectric ordering (SmC′_sP_A phase).