Heliconical smectic phases with transversal polar order†

Abstract

Liquid crystalline (LC) materials integrating polar order and mirror symmetry-broken helical superstructures are of growing interest for advanced applications. We report a new series of achiral bent-core LCs based on 4-cyanoresorcinol bis-terephthalate cores with end-chains ranging from OC₂H₅ to OC₂₀H₂₃. For chains ≥OC₆H₁₃, all compounds form smectic phases which exhibit a paraelectric-to-(anti)ferroelectric transition with Curie–Weiss type divergence, accompanied by an onset of molecular tilt leading to a SmA–SmC phase transition. Increasing chain length induces a tilt correlation crossover from anticlinic (alternating, SmC_a) to synclinic (uniform, SmC_s) ordering. Notably, a stable heliconical smectic phase with a helical axis perpendicular to the polar layers (Sm(CP)^hel) emerges near this crossover. Application of an alternating electric field further expands the Sm(CP)^hel stability range, replacing the SmC_s phase and vanishing near SmC_a. The helical superstructure – spanning C₁₀ to C₂₀ chains and a temperature range up to 80 K – is attributed to synergistic effects of the polar cyano apex, weak molecular bending, and transient helicity. These transversely polarized heliconical phases complement the recently reported longitudinally polarized analogues, offering new pathways for designing chiral LCs from achiral molecules.