Important role played by interlayer steric interactions for the emergence of the ferroelectric phase in bent-core mesogens

Abstract

Two chiral bent-core mesogens (P7-O-PIMB5*, P6-O-PIMB4*) have been synthesized in addition to P8-O-PIMB6*, that is known to exhibit a stable ferroelectric phase, and their phase structures studied in order to clarify the effect of the interlayer steric interaction on the emergence of a ferroelectric ordering. Second harmonic generation (SHG) studies on P7-O-PIMB5* clearly reveal that the stable state is antiferroelectric and an electric-field-induced antiferroelectric–ferroelectric phase transition occurs with a sharp threshold and a hysteresis. Although P6-O-PIMB4* has no mesogenic phase, the inherent ferroelectric nature does exist as proved mainly by SHG and texture observation using binary mixtures with antiferroelectric Pn-O-PIMBs: P6-O-PIMB4*/P8-O-PIMB, a mixture with a similar chain length, exhibits both ferroelectric and antiferroelectric phases at higher and lower temperatures, respectively, in the B2 phase. In contrast, P6-O-PIMB4*/P7-O-PIMB and P6-O-PIMB4*/P12-O-PIMB show only a ferroelectric phase and an antiferroelectric phase, respectively. These results lead us to two important conclusions: (1) the terminal branching chains of P6-O-PIMB4* as well as P8-O-PIMB6* have ability to exhibit a ferroelectric order. (2) Longer terminal chains in the mixtures govern the interlayer molecular order. Thus, we conclude that the interlayer steric interaction plays a major role for the emergence of a variety of phase structures.