Towards the biaxial nematic phase through molecular design

In this article, the search for the elusive biaxial nematic phase (NB) in liquid crystals is considered. The structure of the phase is described along with theoretical and computational work which suggests how it might be realised. An overview of the work of the Exeter group in this area is then given showing the different approaches adopted and illustrating how one of these has led to a new type of amphiphilicity based on shape.     

Influence of mesogenic core polarity and position of chains attachment on columnar phase stability

Series of new Ni(II) metalomesogens of triangular molecular shape and forming Col_h liquid crystalline (LC) phase were synthesised and described. Using in the molecular core the barbituric moieties that contain carbonyl or thiocarbonyl groups causes strong polarisation of the molecules and creates a permanent dipole moment μ, which was confirmed by quantum mechanical calculations. The relationship between molecular dipole moment and self-organisation of molecules into the columnar phase was considered. The position of alkyl and alkoxy chains substituted at phenyl ring that affects LC phase formation seems to be connected with planar conformation of the attached chains. These can broaden the mesogenic core and stabilise the Col_h mesophase.     

From smectic to columnar phase of polypedal liquid crystals based on tetrathiafulvalene/1,3-dithiol-2-thione and cholesterol

Bipedal 1,3-dithiole-2-thiones attached two cholesteryl through a ω-thioalkanoyloxy spacer of varying length were synthesized. The bipedals were easily transformed to the appropriate tetrapedal tetrathiafulvalene derivatives by self-coupling reaction in net triethyl phosphite. All the synthesized compounds exhibit mesogenic phases in a wide temperature region, no crystallization but vitrifying to form glassy mesogens during cooling from the isotropic melt. The liquid crystals with shorter spacer (n=2-6) exhibited only a smectic A phase and those with the longest spacer (n=7) exhibited only a hexagonal columnar. In these series, the molecular packing depended on the length of spacers.     

Influence of halogen substituent on the mesomorphic properties of five-ring banana-shaped molecules with azobenzene wings

Three new series of bent-shaped molecules with 4-chlororesorcinol, 4-bromoresorcinol or 4-fluororesorcinol as the central unit, and azobenzene with different alkoxy chain length as side arms were synthesised. The mesophase behaviour was investigated by polarising optical microscopy, and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction (XRD) studies. It is found that almost all of the materials prepared are monotropic liquid crystalline. Depending on the substituent at the central unit and on the chain length nematic phases, B₆ phases, a B₄-like dark conglomerate phase and a modulated/undulated anticlinic SmC phase were found. As a unique feature, upon reducing the chain length a transition from nematic to B₆-type smectic phases was observed, which is reverse to usually observed phase sequences. The UV–vis absorption spectroscopy was also performed to study the effect of light-induced trans–cis-isomerisation on the prepared compounds.     

The binary system composed of a bent-core compound forming a B7 phase and a nematogenic calamitic compound

The isobaric phase diagram for a binary system of liquid crystals is presented. One mixing component is a bent-core mesogen forming a B₇ phase and the other one is a nematogenic calamitic compound. The occurring mesophases have been identified by their optical textures as well as by X-ray investigations. The B₇ phase exists down to about 60 mol% of the bent-core compound A / 40 mol% of the calamitic compound B. Two additional mesophases are induced in the mixed phase region. At medium concentrations (around 50 mol%) an oblique columnar phase appears which shows a similar X-ray pattern to the B₇ phase but an utterly different nucleation on cooling the isotropic liquid and a completely dissimilar texture. At concentrations below 35 mol% A, a further phase is induced which possesses an irregularly modulated layer structure. This phase can be regarded as an intermediate state between the regularly modulated Col_r (B₁) phase and the intercalated B₆ phase. It transforms on cooling into the phase with a regularly modulated structure.     

The influence of sodium chloride and urea on chromonic liquid crystals formed by CI Acid Red 266

Chromonic liquid crystals are currently receiving renewed interest with particular attention on the Edicol Sunset Yellow (ESY)/water system, which forms columnar nematic and hexagonal phases. CI Acid red 266 is structurally fairly similar to ESY and also forms columnar nematic and hexagonal phases but at much lower concentrations (>1%). In this study, we have examined the influence of sodium chloride and urea on chromonic liquid crystals formed by CI acid red 266. The techniques employed were polarising microscopy, X-ray diffraction and ²H NMR. Sodium chloride moves the concentration at which mesophases form to higher values. Once formed, the mesophases are stable to slightly higher temperatures. Screening of the interstack electrostatic repulsions by added electrolyte appears to be responsible for the changes. Urea can be added in fairly large concentrations (up to 25 wt%) without significant changes in mesophase stability. X-ray diffraction measurements show that there is little change in the aggregate structure with added urea. NMR measurements on urea and water ordering show that urea has much larger order parameters than water. Both order parameters are much smaller than values reported for ESY, but this is simply because of the lower dye concentrations. The larger order parameters for urea appear to arise from some intercalation of urea into the acid red 266 stacks. There is no evidence for changes in ‘water structure’ by the addition of urea.