Introduction of 5,6-dihydro-4H-cyclopenta[b]thiophene core unit into phenyl-tolane: Expanding the mesophase range and increasing the birefringence and dielectric anisotropy of liquid crystal

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ABSTRACT Six novel phenyl-tolane type liquid crystals based on 5,6-dihydro-4 H-cyclopenta[b]thiophene core were designed and synthesised by Sonogashira coupling reactions in high yields. Two reference compounds were then used to clarify the effects of this new core on the mesomorphic behaviours and physical properties of phenyl-tolane type mesogens and found that these new core-based mesogens possessed wider mesophase ranges. Both the birefringence (Δn) and dielectric anisotropy (Δε) values were increased after the employment of this new core, offering an alternative option for the design and synthesis of high-performance phenyl-tolanes. The geometry, ultraviolet absorption, polarisability (α) and dipole moment (μ) of two representative compounds and their references were calculated by density functional theory to correlate the structure–property relationship of the new mesogens. The calculated energy minimum structures of the new mesogens showed well linearity and the molecular length/width ratio was increased, which would benefit the formation of the nematic phase. The orders of the calculated polarisability (α) and dipole moment (μ) of two representative compounds and their references were in accordance with the sequences of the measured birefringence (Δn) and dielectric anisotropy (Δε) values, respectively, which might shed light on the correlation between microscopic molecular structure and macroscopic physical properties.