Liquid crystal dimers and oligomers: Experiment and theory

Liquid crystal dimers and trimers exhibit a range of unusual properties which are fascinating in their own right and which also parallel those found for semi‐flexible main‐chain liquid crystal polymers. This close similarity suggests that dimers and oligomers should form good model systems for their polymeric counterparts and evidence is presented in support of this view. The transitional properties of a range of dimers, all containing the cyanobiphenyl mesogenic group, are presented to illustrate their dependence on the length and parity of the spacer as well as the nature of its link to the mesogenic groups. It is shown that the odd‐even effect thought to characterise dimers docs not always occur and can also vanish when rod‐like mesogens are replaced by disclike groups. Theories, of varying degrees of sophistication, are presented with which to understand the transitional behaviour of the dimers. Of particular interest is a generic model which seems to represent the essential features of the liquid crystallinity of dimers. An especially important characteristic of liquid crystals is their orientational order. The results of NMR studies of this for dimers are described and shown to lead to a novel model of the molecular organisation in the nematic phases of dimers. This simple picture is found to be in accord with the more complete molecular field theories. Finally, the behaviour of liquid crystal trimers is compared with that of dimers, this comparison suggests that much of the essential physics for the polymeric systems is contained within that of the dimers. However, trimers do allow the introduction of novel structural features not possible within dimers. The behaviour of such systems should provide a uniquely valuable test bed for molecular theories of liquid crystal oligomers.