Low dielectric loss of the isothiocyanate terphenyl and phenyl tolane liquid crystals by tuning their lateral methyl or fluorine substituents

Nematic liquid crystals are advantageous for microwave communication due to their tunable dielectric properties and broad frequency range. However, the relationship between the lateral substituents of liquid crystal compounds and their dielectric behaviour, especially dielectric loss, requires further investigation. In this study, two series of new high birefringent isothiocyanate phenyl tolane (<b>A1-A5</b>) and isothiocyanate terphenyl (<b>B1-B5</b>) liquid crystals with lateral methyl or fluorine substitutions were synthesised and characterised. Dielectric properties in the 9–31 GHz frequency range, as well as mesomorphic behaviour, were assessed. The findings revealed that compound <b>A5</b>, featuring 2,6-difluorine substitutions on the <b>P2</b> and <b>P3</b> phenyl rings of isothiocyanate phenyl tolane, exhibited the lowest dielectric loss of 0.00232 (tanδε_r⊥ decreases about 64.0%) and a higher material efficiency of 130.172 at 19 GHz (η increases about 64.4%) compared to reference compound <b>A1</b> with 2,6-difluorine substitutions on the <b>P3</b> phenyl ring. DFT calculations revealed a positive correlation between tanδεr_⊥ of <b>A1-A5</b> and <b>B1-B5</b> and the electrostatic potential at the -NCS end (∣<b>φ</b>_−NCS∣). In conclusion, this research further advances the design and synthesis of liquid crystal molecules with low dielectric loss and high material efficiency.