Biaxial and antiferroelectric structure of the orthogonal smectic phase of a bent-shaped molecule and helical structure in a chiral mixture system.

We examined the biaxial and antiferroelectric properties in the Smectic-AP(A) (Sm-AP(A)) phase of bent-shaped DC-S-8. The biaxiality, which results from the existence of a secondary director, was well established from birefringence observations in the homeotropically aligned Sm-AP(A). By entering into Sm-AP(A) phase, the birefringence (Δn, difference between two refractive indices of short axes) continuously increased from 0 to 0.02 with decreasing temperature. The antiferroelectric switching and second harmonic generation (SHG) activity on the field-on state were also observed in the Sm-AP(A) phase, and the evaluated spontaneous polarization (P(S)) value strongly depended on temperature. The temperature dependence of Δn and P(S) resembles each other and follows Haller's approximation, showing that the biaxiality is due to polar packing in which the molecules are preferentially packed with their bent direction arranged in the same direction, and that the phase transition of Sm-AP(A) to Sm-A is second order. The biaxiality was further examined in chiral Sm-AP(A)(). Doping with chiral components induced the helical twisting of the secondary director in the Sm-AP(A)() phase, which was confirmed by observing the reflection of the circular dichroism (CD) bands in the homeotropically aligned cell. The helical pitch of Sm-AP(A)(*) is tunable in the range of 300-700 nm wavelength with a variation in the chiral content of 5 to 10 weight (wt)%.