On the impact of aromatic core fluorination in hydrogen-bonded liquid crystals

Herein we report the impact of fluorine substitution on the liquid crystalline (LC) self-assembly of supramolecular hydrogen-bonded rod-like architectures. This was systematically investigated by introducing fluorine atom at different positions or with different numbers on the investigated supramolecules. Therefore, eight different groups of hydrogen-bonded LCs (HBLCs) were designed and synthesized in which four 4-hexyloxybenzoic acid derivatives without or with fluorine substitution were used as proton donors. The proton acceptors are fluorinated or nonfluorinated alkyloxyazopyridine derivatives. The hydrogen bond formation between the complementary components was proved using FTIR and 1H NMR spectroscopy. All HBLCs were investigated for their mesomorphic behaviour using various tools such as differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD). Depending on the position and number of fluorine atoms different LC phases were observed including nematic, orthogonal non-tilted smectic A (SmA) or tilted smectic C (SmC) phases in addition to an unknown X phase. Depending on the position of the fluorine substitution, it was proved from the XRD investigations that different types of cybotactic nematic phases (NCyb) are exhibited by the reported HBLCs. Because of cis–trans photoisomerization under light irradiation of the reported HBLCs, their photo responsivity was investigated in solutions as well as between the different LC phases. This report provides key insights into the structure–property relationships of HBLCs, which might be of interest for optical storage device applications.