Sulfur-based ferroelectric nematic liquid crystals†

We report a structure–property relationship in sulfur-based chemical structures for thermotropic ferroelectric nematic (N_F) materials. We synthesized a wide variety of sulfur-containing thioether-, thioester-, and thiophene-based analogs and their oxygen-based counterparts of an archetypal ferroelectric nematogen RM734. Compound 2, having the 4-terminal methylthio (SMe) group instead of the methoxy (OMe) group of RM734, shows the nematic (N), unknown N (N_X), and N_F phases upon cooling. In contrast, 6 with SMe at the inner lateral 2-position only exhibts a monotropic N phase. A thioester analog 7 exhibits the N and N_F phases. Without the 4-terminal OMe, 10 having SMe at the lateral 3-position shows the enantiotropic N and monotropic N_X phases, whereas 12 and 13 with SMe and OMe, respectively, at the 2-position form the monotropic N_F phase directly from the isotropic (Iso) phase. In addition, thiophene-besed compounds (14, 15, 18, and 19) exhibit the N_F phase (and N_X phase for 14 and 15) regardless of the presence of the terminal chain groups. The N_F phases of 18 and 19 with OMe at the inner 3-position and no chain group, respectively, directly appear from the Iso phase. Compounds 20 and 21 with the ester direction opposite from RM734 only show enantiotropic N phases. The ferroelectric nature is characterized by dielectric measurement, second harmonic generation, and spontaneous polarization analysis.