Hydrogen-bonded supramolecular complexes formed between isophthalic acid and pyridine-based derivatives

Two types of supramolecular liquid crystals were prepared through the formation of double hydrogen-bonded complexes between isophthalic acid (A) and two different groups of pyridine-based derivatives ( I n and I _a-e ). The first group of the base, I n (molecular formula 4-C_nH_2n+1OC₆H₄COOC₆H₄-N=N-C₅H₄N) homologues differ from each other by the number of carbon atoms (n) in the alkoxy chain, which varies between 8, 10, 12 and 14 carbons. The second group of the pyridine-based derivatives, I _a-e (molecular formula 4-X-C₆H₄COOC₆H₄-N=N-C₅H₄N) analogues differ from each other by the terminal polar substituent, X, that changes between OCH₃, CH₃, H, NO₂ and Br groups. In this manner two different groups of complexes are formed, one of them is A : 2I n, (Group A ), and the other is A : 2I _a-e , (Group B ). All complexes were investigated for their mesophase behaviour by differential scanning calorimetry and polarised light microscopy. The formation of 1:2 hydrogen-bonded complexes was confirmed by FTIR spectroscopy and binary phase diagrams. Most complexes A and B show nematic and/or SmA phases. X-ray diffraction of the SmA phase of a representative complex of type A indicates a layer distance corresponding to only half of the length of the H-bonded complexes which is interpreted by a phase structure where these complexes adopt a U-shape which intercalate and form non-polar SmA phases.