Directed Metalation-Cross Coupling Route to Ferroelectric Liquid Crystals with a Chiral Fluorenol Core: The Effect of Intermolecular Hydrogen Bonding on Polar Order

Abstract

The chiral fluorenol mesogen (R)-2-(1-octyloxy)-7-((4-undecyloxybenzoyl)oxy)fluoren-9-ol ((R)-3) was synthesized using a combined directed metalation-cross coupling strategy. The SmC* liquid crystal phase formed by the fluorenol mesogen is more stable and has a wider temperature range than that formed by the fluorenone presursor, which may be ascribed to intermolecular hydrogen bonding. The spontaneous polarization (PS) of (R)-3 at 10 K below the SmC*-I phase transition temperature is −10.7 nC/cm2. Molecular modeling based on the Boulder model suggests that the intrinsic conformational bias favoring one orientation of the fluorenol dipole moment along the polar axis of the SmC* phase is very subtle and implies that self-assembly via hydrogen bonding may play a role in enhancing polar order. Results from FT-IR spectroscopy, dilution with achiral SmC additives, and deuterium exchange experiments suggest that the spontaneous polarization is enhanced by the formation of fluorenol dimers via OH−OC hydrogen bonding. Such self-assembly should increase the rotational order about the long molecular axis and, therefore, the orientational bias of the fluorenol transverse dipole moment along the polar axis that gives rise to PS.