The effect of terminal substituents on crystal structure, mesophase behaviour and optical property of azo-ester linked materials

<p>A series of azo-ester linked mesogen containing liquid crystalline acrylate compounds <b>C1-C6</b> having different terminal groups (–F, –Cl, –Br, –OCH<sub>3</sub>, –OC<sub>2</sub>H<sub>5</sub> and –OC<sub>3</sub>H<sub>7</sub>) were successfully synthesised and characterised. The chemical structure, purity, thermal stability, mesophase behaviour and optical property of the synthesised compounds were investigated by different instrumental techniques. X-ray crystal structure showed that compounds <b>C1</b>, <b>C4</b> and <b>C5</b> exhibited more stable <i>E</i> configuration with two bulky group in the opposite side of the N=N double bond motifs. The fluoro-substituted derivative (<b>C1</b>) is connected by the R<sup>1</sup><sub>2</sub>(5) type of C–H…O hydrogen bond motifs whereas the molecules of <b>C4</b>, and <b>C5</b> are connected to each other by means cyclic R<sup>2</sup><sub>2</sub>(8) type of C–H…O hydrogen bond motifs. Thermogravimetric study revealed that the investigated compounds exhibited excellent thermal stability. All the compounds showed enantiotropic liquid crystal (LC) phase behaviour and the mesophase formation was greatly influenced by the terminal substituents. Alkoxy (–OCH<sub>3</sub>, –OC<sub>2</sub>H<sub>5</sub> and –OC<sub>3</sub>H<sub>7</sub>) substituted compounds exhibited greater mesophase stability than those of halogen (–F, –Cl and –Br) terminated derivatives. UV-vis spectroscopic study revealed that the investigated compounds exhibited a broad absorption band around 300–420 nm with absorption maximum (<i>λ</i><sub>max</sub>) of nearly 370 nm.</p>