Novel modulated Hexatic Phases in Symmetric Liquid Crystal Dimers

Homologues of the dimeric $\alpha,\omega$-bis(4-$n$-alkylanilinebenzylidene-4'-oxy)alkanes ($m.$O$n$O$.m$) have been synthesised with spacer lengths $n$ ranging from 9 to 12 methylene units and with terminal alkyl chain lengths $m$ of 10, 12 and 14. Characterisation of these materials has been carried out by X-ray diffraction, differential scanning calorimetry and optical microscopy. In six of these compounds a novel modulated tilted hexatic phase, denoted S$_1$, has been identified, in which the smectic layers have a periodic modulation in their structure, analogous to the S$_{\tilde {\rm C}}$ ribbon phase. This modulated hexatic phase is only observed for dimers with an odd parity spacer, suggesting that the average modelular shape, believed to be bent for the odd spacer dimers but linear for the even ones, is responsible for its formation. The compound 10.O9O.10 exhibits a second modulated hexatic phase denoted S$_2$, which is monotropic, and which cannot be distinguished from the S$_1$ phase by polarizing microscopy or by calorimetry. These long chain dimeric liquid crystals have been found to have a strong tendency to form tilted hexatic phases; indeed two of the compounds studied here are the first dimers to undergo a transition directly from a tilted hexatic phase to the isotropic liquid