Directed metalation approach to ferroelectric liquid crystals with chiral fluorenol cores: The effect of restricted rotation and hydrogen bonding on polar order.

摘要

The Directed ortho Metalation (DoM), cross coupling and Directed Remote Metalation (DreM) reactions have been combined in the synthesis of a series of ferroelectric liquid crystals (FLCs) with chiral fluorenol core structures. The synthetic methodology employed allowed variation of the tail and core structures, for the preparation of a homologous series of diastereomeric fluorenols (1.131a--e), an enantiomeric fluorenol (1.133), and diastereomeric 1-fluoro- and 4-azafluorenols (1.135a and b) from their fluorenone precursors (1.130a--e, 1.132, 1.135a and b, respectively). Variable temperature FT-IR measurements suggest that the SmC* formed by the fluorenols 1.131, 1.133 and 1.35 are stabilized by intermolecular hydrogen bonding between the fluorenol and an ester carbonyl in the side-chain.; Differences in spontaneous polarization (PS) between the pure diastereomers (R,R)- (+106 nC/cm2) and (S,R)-1.131d (+183 nC/cm2) are consistent with a contribution to PS from the fluorenol core. Conformational analysis suggests that the conformational bias of the fluorenols in the SmC* phase is very subtle and is consistent with the differences in PS between the diastereomers. A more complete conformational analysis of (R)-1.133 suggests that there are only very subtle differences in conformations with different polar orientation. The absence of a clear conformational preference and the fact that (R)-1.133 has an observable PS, suggests that intermolecular hydrogen bonding may play a role in increasing polar order via a cooperative effect. Dilution experiments support this finding and suggest that dimerization of the fluorenol mesogens is responsible for an increase in polar order.; Substitution ortho to the chiral side chain in fluorenones 1.134a and b promoted steric coupling of the cores to the side chains, which caused a dramatic increase in PS, as compared to unsubstituted derivatives 1.130a--e. In the fluorenol series, differences in the ferroelectric behaviour of 1.105a and 1.105b with respect to 1.101d are attributed to intermolecular hydrogen bonding which appears to affect the conformation of the cores in unexpected ways.