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 (231 references).

Liquid crystal dimers and higher oligomers: between monomers and polymers

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 dimers and oligomers

Four series of symmetric, dimeric molecules with flexible α,ω-diiminoalkylene spacers have been synthesised and characterised by polarising optical microscopy, differential scanning calorimetry, and X-ray diffraction. The effects on the mesomorphism of the various alkylene spacer (number of methylene units m = 5 to 8) as well as the terminal alkoxy chain length (n = 4 to 14) on the mesomorphic properties have been studied. The compounds with an even-numbered spacer display, in addition to a nematic phase, various smectic phases whose nature depends on the ratio between the length of the terminal chain and the length of the spacer (n/m). A modulated SmA phase was found for the ratio n/m = 1 to 1.6 and a SmC/crystal G polymorphism emerges at n/m > 1.7. In contrast, all liquid-crystalline dimers with odd-numbered spacers display an intercalated B6 phase regardless of the chain length. The preference for intercalated structures is discussed based on dipolar considerations following electronic structure calculations using a model molecule.

Intercalated liquid-crystalline phases formed by symmetric dimers with an α,ω-diiminoalkylene spacer

CORRIE T. IMRIE* School of Engineering and Physical Sciences, Meston Building, University of Aberdeen, Old Aberdeen, Scotland, UK A Commentary on the paper “Non‐symmetric dimeric liquid crystals. T...

Non‐symmetric liquid crystal dimers: how to make molecules intercalate

Bent-shaped molecules based on the oxadiazole central core with various side wings and terminal chain groups have been synthesized, and their liquid-crystalline behavior was investigated by optical microscopic, X-ray, and electrooptic measurements. These molecules exhibit liquid-crystal polymorphism including both the calamitic and banana phases. Such a characteristic polymorphism is attributable to the larger bend angle of the oxadiazole core compared to that of the resorcinol core used in conventional banana molecules. Only one type of banana phase, designated as the Bx phase, is formed. It appears upon cooling from the nematic and smectic liquid crystals and exhibits chiral domains with a very weak birefringence (apparently optically isotropic). By applying an electric field, the Bx phase is altered to a high-birefringence B2 phase with a homochiral SmCAPA structure that exhibits an antiferroelectric response. From detailed analyses of the optical texture and X-ray patterns through the transformation f...

Low-birefringent, chiral banana phase below calamitic nematic and/or smectic C phases in oxadiazole derivatives.

We have prepared two series of dimeric compounds, which comprise the schiff's base mesogen, alkyl spacer with the carbon number of 5 and alkyl or alkoxy tail with the carbon number of m = 4∼16. In these compounds, we observed three types of smectic liquid crystals, the single layer phase with the tail group randomly mixing with the spacer, the bilayer phase in which the segregation of the spacer and tail groups takes place and so two mesogenic layers are included within a repeat unit, the frustrated smectic phase in which the density modulation appears along the layer as well as the layer normal. The bilayer phase was found to be antiferroelectric and the frustrated smectic phase was considered to result from the two dimensional escape from the dipolar interaction. The phase behaviour with m and the structure and properties of each phase will be described in detail.

Tatuya Izumi

Papers

Smectic mesophase properties of dimeric compounds. 2. Distinct formation of smectic structures with antiferroelectric ordering and frustration