Azopyridine-based hydrogen-bonded liquid crystals with thioether tail

The first examples of azopyridine-based hydrogen-bonded liquid crystals (HBLCs) with thioether tail are reported. For this purpose, new HBLCs were designed and synthesised using 4-octylthiobenzoic acid as the proton donor and two different types of 4-alkoxyazopyridines as the proton acceptors. The first group of HBLCs has no lateral substitution, whereas the second has a lateral fluorine atom next to the alkoxy chain attached to the azopyridine derivative. Therefore, it was possible to investigate not only the effect of thioalkyl tail but also the influence of the aromatic core fluorination on the phase behaviour of the newly synthesised HBLCs. Hydrogen bond formation between the proton donor and the proton acceptors was confirmed using FTIR spectroscopy and X-ray diffraction (XRD), while the liquid crystalline behaviour of the HBLCs was investigated using differential scanning calorimetry (DSC) and polarised light microscope (PLM). It was found that all the reported HBLCs are mesomorphic exhibiting smectic A (SmA) and/or smectic C (SmC) phases depending on the length of the alkoxy chain as well as the presence or absence of the lateral fluorine atom. The possibility of photo-switching of the reported HBLCs under UV light irradiation was demonstrated for some selected examples in solution as well as in the bulk state. [GRAPHICAL ABSTRACT]