Three Distinct Polar Phases, Isotropic, Nematic, and Smectic-A Phases, Formed from a Fluoro-Substituted Dimeric...

A dimeric molecule, di-5(3FM-C4T), with fluoro-substituted mesogenic cores composed of three-aromatic rings and linked by a pentamethylene spacer is prepared. Di-5(3FM-C4T) forms the ferroelectric nematic (N_F), ferroelectric smectic-A (SmAP_F), and polar isotropic (Iso_P) phases. The N_F phase is composed of molecules in U-shaped conformation that behave like polar rod-like molecules. The reversal spontaneous polarization (P_s) is approximately 8 μC cm^-2, which is extremely large and reflects the huge dipole moment (11.2 D) of the one-side mesogenic core. On the other hand, the SmAP_F phase is formed by bent-shaped molecules. The N_F-SmAP_F phase transition thus follows the conformational change of molecules. The reversal P_s of the SmAP_F phase is around 4 μC cm^-2, which is half of that in the N_F phase, and this is an expected value from the bent shape of the molecules. It is interesting that the highest temperature Iso_P phase still exhibits the polar structure and possibly retains some polar aggregation of molecules in small domains. The three polar phases exhibit the dielectric mode due to the collective polarization fluctuation at around 100 Hz, giving the high dielectric constant over 8000.