Synthesis and mesomorphic properties of new azo H-shaped liquid crystal compounds

Using tartaric acid as the core, six azo H-shaped liquid crystal compounds M₁-M₆ with different terminal substituents were synthesised. All products were structurally well characterised by elemental analysis, ¹HNMR, and FT-IR. The phase behaviours were investigated by means of polarised optical microscopy and differential scanning calorimetry. M₁-M₆ were thermotropic enantiotropic nematic liquid crystals. They all showed schlieren texture in the process of heating up, and exhibited spherulite texture in the process of cooling down. M₁ and M₄ whose terminal substitution was nitro had the widest liquid crystal range in the same series of liquid crystal compounds, which were 135.1°C and 96.1°C, respectively, under heating cycle and 167.4°C and 112.7°C, respectively, under cooling cycle. The results show that the terminal substituents and flexible chain have important influence on the mesogenic phase and range. With the increase of flexibility, the mesogenic range of liquid crystal compounds becomes smaller when the end groups are the same. And when the flexible chain is the same, the greater the induced and polarised force of the terminal substituents, the better the mesogenic properties of the liquid crystal compounds.