New fluorinated azo/schiff base liquid crystals: synthesis, characterisation, mesomorphic study and DFT calculations

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.