Peter A. Henderson

Liquid crystal dimers and higher oligomers: between monomers and polymers

The underlying theme of this Critical Review is the relationship between molecular structure and liquid crystalline behaviour in a class of materials referred to as liquid crystal oligomers. For the purposes of this review, a liquid crystal oligomer will be defined as consisting of molecules composed of semi-rigid mesogenic units connected via flexible spacers. Much of the review will be devoted to structure-property relationships in the simplest oligomers, namely dimers, in which just two mesogenic units are connected by a single spacer. Along the way we will see how this molecular architecture has been exploited to address issues in a range of quite different areas and has given rise to potential applications for these materials. On the whole, only compounds in which the mesogenic units are linked essentially in a linear fashion will be considered while structures such as liquid crystal dendrimers and tetrapodes fall outside the scope of this review. The review will be of interest not only to scientists working directly in this area but in particular to those interested in understanding the relationships between structure and properties in polymers, and those designing materials for new applications.

Liquid crystal dimers and oligomers

Abstract A liquid crystal dimer is composed of molecules containing two mesogenic groups linked via a flexible spacer. Initial interest in these materials stemmed from their ability to act as model compounds for semi-flexible main chain liquid crystal polymers but are now of fundamental interest in their own right because their behaviour is significantly different to that of conventional low molar mass liquid crystals. Recently research has begun to focus also on higher monodisperse oligomers such as trimers and tetramers consisting of molecules containing either three or four mesogenic units, respectively, linked via flexible spacers. In this review the most recent developments in our understanding of structure–property relationships in liquid crystal dimers and higher oligomers is discussed.

Liquid crystal oligomers: going beyond dimers

This review focuses on structure-property relationships in liquid crystal oligomers, which consist of molecules containing two or more mesogenic units linked via flexible spacers essentially in a linear fashion and so does not consider, for example, liquid crystal dendrimers and tetrapodes. Previous reviews have tended to focus mainly on liquid crystal dimers in which just two mesogenic units are interconnected by a single spacer. By contrast, this review is largely devoted to higher oligomers such as liquid crystal trimers and tetramers containing three or four mesogenic units connected by two or three spacers, respectively.

Methylene-linked liquid crystal dimers and the twist-bend nematic phase

The transitional properties of three methylene-linked liquid crystal dimers are reported, namely, 1,5-bis(4-cyanoanilinebenzylidene-4′-yl)pentane (CN-5-CN), 1,5-bis(4-methoxyanilinebenzylidene-4′-yl)pentane (1O-5-O1), and 1,5-bis(4-ethoxyanilinebenzylidene-4′-yl)pentane (2O-5-O2). Each dimer exhibits two monotropic mesophases. The higher temperature mesophase is a normal nematic phase while the lower temperature phase is assigned as a twist-bend nematic phase. The assignment of the twist-bend nematic phase was based on the strong similarities in the optical textures observed to those reported recently for a structurally similar dimer. The complete miscibility of the mesophases exhibited by CN-5-CN and 1O-5-O1 has been established. The analogous hexamethylene-linked dimers exhibit only the normal nematic phase as do the corresponding ether-linked dimers. A review of the literature reveals another five methylene-linked odd-membered dimers that exhibit a nematic–nematic transition and, in each, the lower temperature nematic phase exhibits similar properties to those reported for the twist-bend nematic phase. The formation of this new nematic phase has been attributed to a negative bend elastic constant which results from the bent geometry of methylene-linked odd-membered dimers.

Methylene-linked liquid crystal dimers

A range of symmetric liquid crystal dimers which differ in the nature of the link, either ether or methylene, between the spacer and mesogenic units has been prepared and their transitional properties characterized. The nematic-isotropic transition temperature, T NI, and the associated entropy change, ΔS NI/R, are sensitive to the chemical nature of this link. Specifically, T NI falls on replacing ether links with methylene links for both odd and even members although this reduction is more pronounced for odd members. In comparison, ΔS NI/R increases on changing ether links for methylene links for even dimers, but decreases for odd-membered dimers. These observations are completely in accord with the predictions of a model developed by Luckhurst and co-workers in which the difference between the ether-linked and methylenelinked dimers rests exclusively in their shapes. Furthermore, the highly non-linear pentamethylenelinked dimers show a greater tendency to exhibit smectic behaviour; this is interpreted in terms of molecular packing giving rise to an alternating smectic phase.

Symmetric and non-symmetric chiral liquid crystal dimers

The synthesis and characterisation of three sets of non-symmetric liquid crystal dimers consisting of a cholesteryl-based unit and either 4-methoxybiphenyl, 4-cyanobiphenyl or (S)-2-methylbutyl 4′oxybiphenyl-4-carboxylate are described. The transitional properties of these non-symmetric dimers are compared to those of the corresponding parent symmetric dimers. The symmetric dimers exhibit only chiral nematic or nematic phase behaviour. By contrast, members of the non-symmetric dimer series containing either 4-cyanobiphenyl or (S)-2-methylbutyl 4′-oxybiphenyl-4-carboxylate groups exhibit smectic behaviour. The former series show an interdigitated smectic A phase while for the latter the structure of the smectic A phase depends on the length of the flexible spacer. Specifically, for short spacer lengths a monolayer arrangement is seen while for long spacers an intercalated smectic A phase is formed. For an intermediate spacer length, the small-angle X-ray scattering pattern for the intercalated smectic A phase reveals short-range incommensurate structural fluctuations associated with the monolayer smectic A phase. The driving force for the formation of the intercalated phase may be an electrostatic interaction between the electron rich carbonyl linking group attached to the cholesteryl unit and the electron deficient aromatic unit while the monolayer arrangement may be stabilised by the van der Waals interactions between the cholesteryl unit and the alkyl chains. Blue phases are observed only for a small number of these non-symmetric dimers and these all contain an odd-membered spacer. This is in accord with the rather general observation that blue phases are observed for odd-membered non-symmetric dimers and not their even-membered counterparts.

Cholesteryl-based liquid crystal dimers containing a sulfur–sulfur link in the flexible spacer

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, ΔSN*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 ΔSN*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.

Oligomeric liquid crystals: From monomers to trimers

A homologous series of linear liquid crystal trimers, the 4,4′-bis[ω-(4-methoxyazobenzene-4′-yloxy)alkoxy]azobenzenes, has been synthesized and characterized. The transitional properties of the trimers are compared with those of the corresponding series of dimers, the α,ω-bis(4-methoxyazobenzene-4′-oxy)alkanes, and monomers, the 4-methoxy-4′-alkoxyazobenzenes. Characteristically pronounced odd-even effects were seen for the transitional properties of both dimers and trimers on varying the spacer lengths. The clearing temperatures of the trimers were higher than those of the corresponding dimers, but as the length of the flexible spacers was increased this difference became rather small. The ratios of T NI, and ΔS NI/R for monomer:dimer and dimer:trimer are discussed. These are very similar to reported values for similar materials, suggesting that there may be a rather general relationship between the transitional properties of liquid crystal oligomers as the number of mesogenic units is increased.

Non-symmetric chiral liquid crystal trimers

Two sets of non-symmetric chiral liquid crystal trimers have been synthesised and characterised. These consist of molecules containing a cholesteryl-based group, a 4-methoxyazobenzene unit and a central benzylideneaniline group, interconnected in a linear fashion via two flexible spacers. The length of the spacer connecting the 4-methoxyazobenzene moiety to the benzylideneaniline group is held constant at either five or six methylene units while the length of the second spacer is varied. All 12 trimers exhibited an enantiotropic chiral nematic phase. In addition, a smectic A phase was observed for both sets of trimers when the second spacer contained three, five or seven methylene units. The chiral nematic–isotropic transition temperatures and associated entropy changes depend critically on the parity of the flexible spacers. This behaviour is interpreted in terms of the average molecular shape and how this is controlled by the flexible spacers. The ratio of the smectic periodicity to the estimated all-trans molecular length for each of the trimers was approximately 0.3, implying a triply intercalated arrangement in which each layer contains all three mesogenic units. The driving force for the formation of this phase is considered to be electrostatic interactions between electron rich and electron deficient segments of the molecules.

Liquid crystal dimers and the twist-bend nematic phase. The preparation and characterisation of the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates

Eleven members of the homologous series of liquid crystal dimers, the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates, have been synthesised and their transitional properties characterised. These dimers consist of two cyanobiphenyl units connected by an alkyl spacer attached via ester linkages. All eleven members exhibit exclusively nematic behaviour. The nematic–isotropic transition temperatures, TNI, and associated entropy changes, ∆SNI/R, exhibit pronounced alternations as the length and parity of the spacer is varied; this is characteristic behaviour of liquid crystal dimers. The transitional properties of the ester-linked dimers are compared with the corresponding materials having either ether, methylene or carbonate linkages between the spacer and mesogenic units. For short spacer lengths and both odd- and even-membered dimers, the ester-linked materials show the highest values of TNI and the methylene-linked the lowest. For longer spacer lengths, TNI of the carbonate-linked dimers fall between those of the corresponding ester- and ether-linked dimers. The ether-linked materials show the largest alternation in ∆SNI/R on varying spacer length and the carbonate-linked dimers the lowest. This behaviour is interpreted in terms of the molecular geometry and it is suggested that the ether- and ester-linked odd-membered dimers have rather similar shapes. A phase diagram constructed using binary mixtures of the pentyl member of this ester-linked series and the known twist-bend nematogen, 1,7-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB), is presented. The twist-bend nematic–nematic transition temperature of the mixtures shows a striking convex curvature as the concentration of CB7CB is decreased, and so it is not possible to estimate a virtual twist-bend nematic–nematic transition temperature for the ester-linked material.