A homologues series of 12 new asymmetric fluorinated azobenzene-Schiff Bases, 2-((E)-(3-chloro-4-fluorophenyl)diazenyl)-5-(((E)-arylidene) amino) anilines (Aa-l) of three rings consist of different alkyloxy groups varying carbon chain length n = 2 to n = 18 at one end and the other end of the compounds is linked to polar atoms like chlorine and fluorine on an aromatic ring at position 3 and 4 respectively were synthesised. The proposed structures of all new mesogens (Aa-l) were confirmed by FT-IR, ¹H-NMR, ¹³C-NMR, ¹⁹F NMR and elemental analysis. The mesomorphic behaviour, optical properties and thermal stability of all new compounds have been investigated by polarised optical microscopy (POM), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Characteristic textures of smectic A phase were observed for all new mesogenic compounds. The photo-isomerisation property of azo compounds under UV light was also studied. Further, density functional theory (DFT) calculations were carried out to investigate thermal and geometrical parameters of all newly prepared mesogens. Moreover, the relationship between the calculated and experimentally measured parameters has been correlated to understand the effect of the structural parameters on the mesomorphic behaviour. GRAPHICAL ABSTRACT

A new series of Schiff base liquid crystal have been prepared and studied. Schiff bases of p-alkyl aniline derivatives and 4-phenyl pyridine-4′-carbaldehyde were prepared. The terminal alkyl groups substituting aniline are of varied chain length, namely C8, C12 and C14. The structures of the compounds were elucidated by 1H NMR and 13C NMR. The mesomorphic thermal and optical characteristics of the samples were determined via differential thermal analysis (DSC) and polarization optical microscopy (POM). All compounds exhibit enantiotropic dimorphic mesophase behaviour, referred to as smectic X1 (SmX1) and smectic X2 (Sm A). Experimental results obtained for the mesophases were correlated with density functional theory (DFT) theoretical calculations. The results of the new series are further compared to two series of compounds bearing pyridine (two ring Schiff bases) and biphenyl, respectively, in their mesogens. The series of compounds of one pyridine ring are generally not mesomorphic. The results indicate that the alkyl chain length has a strong impact on the mesomorphic characteristics and thermal stabilities of the different mesophases. As a trend, the temperature ranges of both of smectic mesophases of all compounds are higher in new compounds bearing the 4-phenyl pyridine moiety. In addition, the total mesophase range is generally higher in the new compounds when compared to their biphenyl analogues. Finally, theoretical DFT calculations were performed to illustrate the experimental finding of the mesomorphic behaviour in terms of the molecular geometry and aromaticity, π–π stacking and LOL-π.