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Series of heterocyclic compounds containing ethynyl group, 2-[4-[2-[4-(alkoxy)phenyl]ethynyl]phenyl] -benzoxazole derivatives (nPEPBx), are prepared and investigated. They display enantiotropic nematic and smectic mesophases with large mesophase ranges of 30–133°C (heating) and 31–141°C (cooling). Compared with reference analogues without ethynyl group, the compounds nPEPBx roughly give lower melting points and much higher nematic mesophase stability because of the introduction of triple carbon-carbon bond. Meanwhile, they show much higher birefringence (Δn, 0.50–0.65 and 0.55–0.68 for experimental and calculated values with density functional theory (DFT), respectively) than common tolane-based liquid crystals, which indicates that the introduction of benzoxazole unit is an effective method to enhance Δn values of the rod-like molecule. It is concluded that nPEPBx has a potential application in liquid crystal mixture as a dopant to enhance the performance of the host mixture. [ABSTRACT FROM AUTHOR]

Terminal vinyl-based benzoxazole liquid crystalline compounds, 2-(3-fluoro-4ʹ-alkoxy-1,1ʹ-biphenyl −4-yl)-5-(2-propenyloxymethyl)-benzoxazole (nPPF(2)BP), were synthesised and their structures were confirmed by infrared (IR) spectra, proton nuclear magnetic resonance (1H-NMR) spectra, gas chromatography with electron impact-mass spectrometry (GC/EI-MS), matrix-assisted laser desorption/ionisation-time of flight (MALDI-TOF) mass spectrometry and elemental analysis (EA). The compounds show enantiotropic smectic/nematic phases with mesophase ranges are 71–97 °C and 87–136°C on heating and cooling processes for nPPF(2)BP, respectively. They give low melting points due to lateral fluoro substituent and flexible terminal 2-propenyloxymethyl chain. It is found that the compounds nPPF(2)BP with shorter alkoxy chain (n = 3, 4) exhibit a wide range of nematic mesophase, which is ascribed to enhanced π–π interaction caused by terminal vinyl moiety, whereas further elongation of the terminal alkoxy chain results in supressing nematic phase and increasing smectic mesophase. Compared with methyl terminated analogues, 2-propenyloxymethyl terminated compounds nPPF(2)BP display much lower melting points and wider or comparable mesophase range both in heating and cooling. [ABSTRACT FROM AUTHOR]