Synthesis and properties of dibenzofuran- and dibenzothiophene-based mesogens: An effective access to liquid crystals with large negative dielectric anisotropy

[Display omitted] •Two series of dibenzofuran- and dibenzothiophene-based liquid crystals were designed and synthesised respectively.•All of the obtained mesogens possessed large negative dielectric anisotropy values.•The calculated orientation of the dipole moment in these compounds was almost perpendicular to their long molecular axis.•A new negative LC mixture with excellent low-temperature stability was formulated. Two series of dibenzofuran- and dibenzothiophene-based liquid crystals were designed and synthesised respectively. The structures of the intermediates and the target mesogens were confirmed by nuclear magnetic resonance and mass spectrometry. The melting points of dibenzofuran-based liquid crystals were lower than dibenzothiophene-based ones if the 3-position was long-chain alkoxy while a reversed result was obtained when cyclohexyl and tetrahydropyran were introduced. All of the designed dibenzofuran- and dibenzothiophene-based liquid crystals possessed large negative dielectric anisotropy (Δε) values when compared with the reference compounds, which proved the effectiveness of these two heterocycle core units in the improvement of the negative Δε. The DFT calculations found that the elevation of the negative Δε values might be owed to the orientation of the dipole moment in these new compounds was almost perpendicular to their long molecular axis. A new liquid crystal mixture LTNLCM-1 was finally formulated, which could be stored at −20 °C (−40 °C when filled in liquid crystal cell) for two weeks without any obvious precipitation, illustrating the vital function of the dibenzothiophene-based compounds in improvement of the low-temperature stability of liquid crystal mixtures.