Eleven series of dimeric liquid crystals, namely the α,ω-bis(4-n-alkylanilinebenzylidine-4′-oxy)alkanes, have been synthesized and their transitional properties characterized. The number of methylene groups in the flexible core was varied from one to twelve while the length of the terminal alkyl chains was increased from a methyl group to a decyl substituent. The rich smectic polymorphism of this family of compounds was studied using X-ray diffraction and polarizing microscopy; it includes several examples of rare phase transitions, for example, smectic F-smectic A and crystal G-isotropic, as well as novel modulated hexatic phases based on a two dimensional lattice. The transitional properties of a given series, where the length of the terminal chain is held constant while the length and parity of the flexible spacer is varied, exhibit a pronounced alternation. For example, the entropy change at the nematic-isotropic transition exhibits a strong alternation and this has been interpreted in terms of a corresponding alternation in the long range orientational order at the transition. Increasing the spacer length for a given terminal chain promotes nematic behaviour while increasing the terminal chain length for a given spacer promotes smectic behaviour. A simple relationship relating the occurrence of smectic behaviour to the molecular structure is deduced, namely that for smectic properties to be observed the length of the terminal chains must be greater than half the length of the spacer. We discuss this observation in terms of microphase separation into three distinct regions, the aromatic moieties, the terminal chains and the spacers.

Six series of a family of non-symmetric dimers, the α-(4-cyanobiphenyl-4′-yloxy)-ω-(4-n-alkylanilinebenzylidene-4′-oxy)alkanes, have been prepared. In three of the series, the number of methylene groups in the flexible alkyl spacer has been varied from 3 to 12 while the terminal alkyl chain is held constant at either 2, 6 or 10. In the remaining series, the effect of increasing the terminal chain length has been studied by holding the spacer length constant at 3, 4 or 5 methylene units while varying the length of the terminal chain from 0 to 10 carbon atoms. The phase behaviour and transitional properties of these series have been investigated using optical microscopy and differential scanning calorimetry, while the structures of the mesophases have been studied using X-ray diffraction on both powder and aligned samples. This study has revealed novel phase behaviour involving intercalated smectic phases stabilized by the mixed mesogenic group interaction. We have identified the first examples of intercalated smectic C, smectic I, crystal B and crystal J phases and have assigned the layer stacking sequences in the crystal B phases. An extension to the current nomenclature used to describe smectic phases is proposed in order to include these new structural modifications such that, for example, SA_c , refers to the intercalated smectic A phase. The specific interaction between the two unlike groups has a much smaller influence on the nematic-isotropic transition temperatures than it has on the structures of the smectic phases. Quantitative estimates of the nematic-isotropic transition temperatures based on the Marcelja-Luckhurst theory provide further evidence for a specific interaction between the unlike mesogenic groups. Finally, the thermal behaviour of binary mixtures of the analogous symmetric dimers is also found to be governed by the mixed core interaction and intercalated smectic phases are observed in these mixtures.