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 (NCybC) 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 NCybC 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 NCybC 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 SmCsPA phase (Cs stands for a synclinic-type of molecular organization and PA 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′sPA phase). This journal is