The Role of Terminal Groups in Nonchiral Rod-Like Compounds on the Formation of Polar Fluids

The emergence of ferroelectric mesophases in nonchiral liquid crystals (LCs) has sparked fundamental interest in the molecular mechanisms governing polarity. In this study, we investigate how terminal molecular groups influence the formation and stability of polar phases by analyzing six compounds from three homologous series. Specifically, we compare synthesized homologues with a nitro group, which predominantly exhibit polar mesophases, to previously reported structurally related analogs containing either a cyano group or a fluorine atom as a terminal fragment. Density functional theory calculations provide insights into electronic surface potential (ESP) distributions, revealing alternating regions of positive and negative charge density along the molecular axis, consistent with Madhusudana's model of polar phase stabilization. We propose the ESP-derived parameter quantifying terminal electrostatic charge, revealing a direct correlation between the negative-to-positive charge ratio at the molecular termini and the formation of ferroelectric or antiferroelectric mesophases. To validate this hypothesis, we analyze the molecular structure–mesomorphic behavior relationship of other known nonchiral compounds that exhibit polar phases, demonstrating the critical role of terminal groups in determining mesophase polarity. Our findings enhance the understanding of the molecular origins of ferroelectricity in nonchiral LCs, paving the way for the rational design of next-generation functional polar soft materials.