Alkylthio- and alkyl-substituted asymmetric diphenyldiacetylene-based liquid crystals: phase transitions, mesophase and single-crystal structures, and birefringence
Asymmetric diphenyldiacetylenes (DPDAs) bearing a fixed hexylthio group and different alkyl groups with carbon numbers (n) of 0–8 and 12 at each termini (or 4- and 4ʹ- positions), abbreviated as 6S–DPDA–n, were synthesised and their phase transition behaviour and birefringence were evaluated. It was found that alkyl-unsubstituted and methyl-substituted analogues (6S–DPDA–0 and 6S–DPDA–1, respectively) do not show any mesophases, whereas 6S–DPDA–n (n ≥ 2) form enantiotropic liquid crystal phases. Wherein, the nematic (N) phase for 6S–DPDA–4 is supercooled to room temperature, and 6S–DPDA–12 forms not only N phase but also highly ordered mesophase under room temperature. In light of the fact that symmetrically alkylthio-possessing 6S–DPDA–S6, 6S–DPDA–0 and 6S–DPDA–1 are not mesogenic, the present results strongly suggest that an alkyl group opposite to an alkylthio group plays a vital role to induce mesophases. Single-crystal X-ray analysis revealed anti-parallel packed molecules in the lattice. 6S–DPDA–7 have higher birefringence than 7–DPDA–7 being the symmetric alkyl counterpart, over the entire range of each N phase, which is supported by the enhanced polarisabilities based on density functional theory calculation.