A Study of a Bistereogenic Mesogen for the Development of Orthoconic Antiferroelectric Liquid Crystal Materials

The first orthoconic antiferroelectric liquid crystals (OAFLCs), i.e. smectics where the optical director changes direction by 90° between neighboring layers, were mixtures of partially fluorinated monostereogenic compounds. They have successfully demonstrated the orthoconic properties (orientation-independent dark state between crossed polarizers) but suffer from too high polarization and too short helical pitch, necessitating very thin samples. Using an (S,R) bistereogenic OAFLC we have obtained the orientation-independent dark state in rather thick samples, but several other problems arise with this compound. The strongly first-order SmA*-SmC* _a transition produces defects leading to light leakage. In order to be switchable the sample must furthermore be mixed with chiral dopants, generally reducing the tilt angle as well as a shortening the helical pitch. Finally, a SmC* phase often appears in the phase sequence of the mixture, strongly dominating over the desired SmC _a * phase in thin cells.

Keywords:

• Orthoconic antiferroelectric liquid crystals
• high optical tilt
• defectgenerated light scattering
• chiral doping
• confinement effects
• supercooling

Acknowledgments

This work has been funded by the EU Research program SAMPA (Synclinic and Anticlinic Mesophases for Photonics Applications).

Notes

^a The optical director tilt referred to here is the ‘field-induced apparent tilt’ (half the angle between the two synclinic states).

^b The SmC_a* phase can be supercooled to room temperature where it is metastable for several days.

^c For the (S,S) & (R,R) compounds the polarization is immeasurable due to coexistence with the crystalline or the SmQ* phase in the whole temperature range of SmC_a*.