Asymmetrically Fluorinated Phenyl 4-biphenylcarboxylate Motifs as a Design Principle for Enantiotropic Ferroelectric...

Understanding the molecular structures that exhibit ferroelectric liquid crystal (FLC) phases during both heating and cooling processes, i.e., enantiotropic FLC phases, remains a significant challenge. In this study, nine analogs based on a fluorinated phenyl 4-biphenylcarboxylate backbone are designed, and their FLC properties are evaluated. Of these, eight analogs exhibit enantiotropic FLC phases. One analog containing the 2-fluorobenzoate unit and a cyano terminal exhibits an enantiotropic ferroelectric nematic (N_F) phase at 119°C-190°C. In contrast, analogs incorporating a 2,6-difluorobenzoate unit show weakened or absent ferroelectric properties, whereas the introduction of a cyano terminal group leads to the emergence of an enantiotropic N_F phase. Quantum chemical calculations suggest that the planar orientation of the 2-fluorobenzoate unit effectively promotes intermolecular interactions in parallel arrangements rather than antiparallel ones, thereby stabilizing FLC phases. Furthermore, an analog containing an asymmetrically fluorinated 4-biphenylcarboxylate backbone exhibits an enantiotropic ferroelectric smectic A (SmA_F) phase at 92°C-124°C. These results provide strong evidence that structural motifs comprising fluorinated biphenyl and planar benzoate skeletons are effective in promoting enantiotropic FLC phases. This study highlights the critical role of the mesogenic core structure (backbone) and provides valuable insights for the molecular design of enantiotropic FLC materials.