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

A series of azo-ester linked mesogen containing liquid crystalline acrylate compounds C1-C6 having different terminal groups (-F, -Cl, -Br, -OCH3, -OC2H5 and -OC3H7) 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 C1, C4 and C5 exhibited more stable E configuration with two bulky group in the opposite side of the N=N double bond motifs. The fluoro-substituted derivative (C1) is connected by the R-2(1)(5) type of C-H center dot center dot center dot O hydrogen bond motifs whereas the molecules of C4, and C5 are connected to each other by means cyclic R-2(2)(8) type of C-H center dot center dot center dot 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 (-OCH3, -OC2H5 and -OC3H7) 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 300-420nm with absorption maximum (lambda(max)) of nearly 370nm. [Graphics] .