Benzoxazole-terminated liquid crystals with large birefringence and negative dielectric anisotropy.

• The newly synthesized benzoxazole liquid crystals that display a wide nematic mesophase. • Target compounds show large birefringences (Δn) large negative Δ ε , which is an uncommon property for heterocyclic liquid crystals. • Target compounds can endow the commercial negative LC mixtures with large Δ n and FoM values. We synthesized heterocyclic liquid crystals with large birefringence and negative dielectric anisotropy, namely, 5–4-(2-(4-(alkoxy)phenyl)-2,3-difluorophenyl)benzo(d)oxazole derivatives (nPFO). These compounds display enantiotropic nematic mesophases with wide mesophase range of 45.9–84.0 °C (heating process) and 61.3–119.6 °C (cooling process). Compared with reference analogues of traditional benzoxazole compound 6PFB with different orientation of benzoxazole-terminated structure, the compounds nPFO roughly give lower melting points and clearing points because of their increased dihedral angle. Meanwhile, compounds nPFO show obviously large birefringence (Δn, 0.28–0.39 and 0.39–0.48 for experimental measurement and DFT theoretical calculation, respectively) due to the enhanced π-conjugation caused by the ethynyl bridge group in molecules. Benefitting from the larger perpendicular dipole moments and orientation angles, nPFO displays negative dielectric anisotropy (Δɛ) ranging from −3.60 to −4.10. These compounds can endow a commercial negative LC mixture the larger Δ n and higher values of figure of merit, suggesting they are suitable dopant to further improve the performance of some commercial LC mixtures. [ABSTRACT FROM AUTHOR]