Effect of position of the lateral fluoro substituent on the mesophase behaviour of aryl 4-alkoxyphenylazo benzoates in pure and binary mixtures

Five groups of 4-substituted phenyl 4?-(2?- (or 3″-) substituted-4?-alkoxyphenylazo) benzoates (In_a-c to Vn_a-c) were investigated in which, within each group, the length of the terminal alkoxy group varies between 8 and 16 carbons, while the other terminal substituent, X, is a polar group that alternatively changed between the electron-donating CH₃O and the electron-withdrawing Br group, in addition to the un-substituted analogue (X = H). The lateral substituent (Y) in the five groups I–V varies, respectively, between H, 3-CH₃, 2-CH₃, 3-F and 2-F. Their mesophase stabilities were determined by differential scanning calorimetry and phases identified by polarised light microscopy. The two newly prepared groups of compounds (IVn_a-c and Vn_a-c) are structurally characterised by infrared, ¹H-NMR, mass spectroscopy, thermogravimetric and elemental analyses. Binary phase diagrams were constructed for each pair of isomers from groups IV and V bearing the same wing substituents but the lateral F is attached to different positions (2? or 3″).