Symmetric liquid crystal dimers containing hydrazide groups: parity-dependent smectic structure, hydrogen bonding and substitution effect

Dimeric hydrazide derivatives with long terminal alkoxy chains, i.e. alpha,omega-bis[N-(4-hexadecyloxybenzoyl)-N'-(benzoyl-4'-oxy)hydrazine]alkanes (C16.n.C16, C16 indicates the terminal hexadecyloxy chain; n=3, 5, 6, 10, indicates the number of the carbon atoms in the spacer) were synthesised. The liquid crystalline properties were investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The dimers with shorter spacer (C16.n.C16, n=3, 5, 6) exhibit enantiotropic smectic phases, whereas C16.10.C16 with the longest spacer is non-mesomorphic. Furthermore, the smectic structure is parity dependent, i.e. C16.3.C16 and C16.5.C16 with odd spacer exhibit a monolayer smectic A phase, whereas C16.6.C16 with even spacer exhibits a monolayer smectic C phase, in which the incompatibility between the terminal chains, mesogenic groups and the spacers is considered to be the driving force. The results are compared with those for the terminally nitro-substituted series, alpha,omega-bis[N-(4-nitrobenzoyl)-N'-(benzoyl-4'-oxy)hydrazine]alkanes (Nn), which exhibit an intercalated smectic A phase. Temperature-dependent IR spectroscopic analysis on these two kind of dimers suggests that intermolecular hydrogen bonding, as well as the dipole-dipole interaction arising from the strong polar substituents (-NO2) synergistically drives the intercalated structure of Nn. Although hydrogen bonding exists in the monolayer smectic phase of C16.n.C16, microphase segregation is in favour of the monolayer smectic structure, preventing the formation of an intercalated structure.