Seven members of the homologous series of the liquid crystal dimers, the bis(ω-(cholesteryloxycarbonyl)alkyl)disulfides, which contain a sulfur−sulfur link in the flexible spacer have been synthesised and their liquid crystal properties characterised. The dimers are referred to using the acronym Chol-n-SS-n-Chol in which n denotes the number of carbon atoms linking the cholesteryl-based groups and the sulfur atoms, and was varied between 3, 5, 6, 8, 10, 11 and 12. All seven homologues exhibit a chiral nematic phase and for the longest three members a smectic A phase was also observed. An odd−even effect is apparent in both the transition temperatures and the values of the entropy change associated with the chiral nematic−isotropic transition, ΔS_N*I /R, in which dimers with even values of n show the higher values. This is interpreted in terms of the average molecular shapes in which the C−S−S−C dihedral angle is around 90°. The values of ΔS_N*I/R shown by these dimers are very small for liquid crystal dimers and this is attributed to the increased molecular biaxiality arising from the C−S−S−C dihedral angle. The smectic A phase exhibited by the homologues with n = 10, 11 and 12 is proposed to consist of an intercalated arrangement of the dimers which is driven by the mismatch in cross-sectional areas of the cholesteryl-based groups and alkyl chains.

Six new non-symmetric dimers belonging to the family of compounds the (R,S)-(benzylidene-4ʹ-substituted-aniline)-2ʺ-methylbutyl-4ʹʹʹ-(4ʹʹʹʹ-phenyloxy) benzoate-oxy) hexanoates have been prepared and their thermal behaviour characterised. The terminal substituents on the 4-benzylideneaniline fragment are H, CH₃, C₂H₅, Cl, Br and I. All six dimers exhibit an enantiotropic nematic phase, and the efficiency of the terminal substituent in enhancing the nematic–isotropic transition temperature is largely consistent with that established for conventional low molar mass liquid crystals. This trend is discussed in terms of the size of the terminal substituent and its effect on the shape of the molecule. The halogen-substituted and ethyl-substituted dimers also exhibit smectic behaviour. For the halogen-substituted dimers, the driving force for smectic phase formation is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. In contrast, for the ethyl-substituted dimers, the driving force is thought to be the enhanced molecular inhomogeneity arising from the longer terminal chains. The properties of these new dimers are broadly similar to those of the corresponding dimers in which the ester linkage between the spacer and mesogenic unit is an ether link. This strongly suggests that the ether- and ester-linked materials have similar average molecular shapes.

The first series of liquid crystal dimers to contain an isoflavone-based moiety has been prepared, the α-(4'-methoxy-3-phenyl-4H-1-benzopyran-4-one-7-oxy)-ω-(4-methoxyazobenzene-4'-oxy)alkanes. All eight members of this homologous series exhibit an enantiotropic nematic phase. The nematic–isotropic transition temperatures and associated entropy changes exhibit a dramatic dependence on the length and parity of the flexible spacer. This characteristic behaviour for a homologous series of liquid crystal dimers has been interpreted in terms of how the flexible alkyl spacer controls the average molecular shape. A comparison of the transitional properties of this series with those of the symmetric parent dimers, the α,ω-bis(4-methoxyazobenzene-4'-oxy)alkanes, reveals that replacing a methoxyazobenzene unit by the isoflavone-based group reduces the melting point but increases the nematic–isotropic transition temperature. Three other non-symmetric dimers are reported: 1-(4'-methoxy-3-phenyl-4H-1-benzopyran-4-one-7-oxy)-9-(4-cyanobiphenyl-4'-oxy)nonane, 1-(4'-met-hoxy-3-phenyl-4H-1-benzopyran-4-one-7-oxy)-10-(4-nitroazobenzene-4'-oxy)decane and cholesteryl 11-(4'-methoxy-3-phenyl-4H-1-benzopyran-4-one-7-oxy)undecanoate. The former two show enantiotropic nematic behaviour while the latter exhibits an enantiotropic chiral nematic phase. Again, when compared with the parent symmetric dimer, a reduction in melting point is observed while the clearing temperature is increased. The trend in the melting points is accounted for in terms of the difficulties associated with packing the non-symmetric dimers into a crystalline lattice compared to the symmetric dimers. The physical significance of the increase in the clearing temperatures is not yet clear.

Ten new non-symmetric liquid crystal dimers belonging to the family of compounds α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-(2-methylbutyl-4′-(4″-phenyl)benzoateoxy)-alkanes have been synthesised and their transitional properties characterised. The dimers contain either a hexamethylene or octamethylene spacer, while the terminal substituents on the 4-benzylideneaniline fragment are H, CH₃, F, Cl and Br. The unsubstituted dimers are not liquid crystalline, while the remaining compounds exhibit enantiotropic nematic behaviour. The trends in the clearing temperatures, according to the chemical nature of the terminal substituent, are largely consistent with those established for conventional low molar mass liquid crystals. Three of the dimers also exhibit an intercalated smectic A phase, specifically the two bromo-substituted dimers and the chloro-substituted dimer containing a hexamethylene spacer. The driving force for the formation of this phase is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. The absence of smectic behaviour for the isosteric methyl-substituted dimers reveals that steric factors alone cannot stabilise the intercalated smectic A phase.

A series of six non-symmetric liquid crystal dimers α-(4-benzylidene-substituted-aniline-4′-oxy)-ω-[pentyl-4-(4′-phenyl)benzoateoxy]hexane have been synthesized and characterized. These dimers vary from one another in terms of the terminal substituent (H, CH₃, C₂H₅, F, Cl and Br) with different polarizabilities and molecular sizes. Investigation on the thermal behaviour shows that the dimer with a more polarizable terminal substituent of Br atom exhibits a smectic A phase as well as an enantiotropic nematic phase. Influence of the molecular size of the terminal substituent in stabilizing the liquid crystalline properties is also studied.