Liquid crystalline hydrazones revisited: dipolar interactions vs hydrogen bonding affecting mesomorphic properties

In order to understand the role of dipolar interactions vs. H-bonding, a series hydrazones were synthesised from 4-alkoxy-, 3,4-dialkoxy- or 3,4,5-trialkoxybenzaldehydes and phenyl, bromo- nitrophenylhydrazine, respectively. Their mesomorphic properties investigated by differential scanning calorimetry (DSC), polarising optical microscopy (POM), X-ray diffraction (WAXS, SAXS) compared with known members. Only those derived 3,4,5-trisalkoxybenzaldehyde either meta, meta-dinitro- ortho, para-dinitrophenylhydrazine displayed hexagonal columnar mesophases. All other derivatives non-mesomorphic, even when H-bonds present. Dipole moments various nitro-substituted experimentally determined dielectric measurements supported theoretical DFT calculations, which indicated that mesophase formation is mostly governed strong dipole moment further enforced intramolecular H-bonding.