Novel Ferroelectric and Antiferroelectric Smectic and Columnar Mesophases in Fluorinated Symmetrical Bent-Core Compounds

We report the synthesis and mesomorphic properties of new bent-core (BC) mesogens exhibiting novel antiferroelectric switching columnar phases with rectangular as well as oblique lattices. The homologues with short chains show typical nonswitchable rectangular columnar (B₁) phase and a ferroelectric switching smectic phase is obtained for the middle homologues. Interestingly, with increasing chain length, the switchable columnar phases are induced and the X-ray diffraction pattern with well-oriented samples indicates the presence of rectangular and oblique lattices. On the basis of the experimental observations, possible models have been proposed for these phases. Field-induced SmC_SP_F and M_x phases are also observed in the ferroelectric and antiferroelectric rectangular columnar phases, at high and low temperatures, respectively. Switching chiral domains of opposite handedness as well as a change in the sense of chirality within a domain could also be observed for the first time for the ferroelectric phase. Two different switching mechanisms in the ferroelectric and antiferroelectric columnar phases have also been observed. In the presently investigated two series of compounds, the electrooptical switching changes from ferroelectric to antiferroelectric behavior with increasing chain length.