Synthesis and characterisation of asymmetrical mesogenic materials based on 2,5-disubstituted-1,3,4-oxadiazole

ABSTRACT

New mesogenic homologous series bearing 1,3,4-oxadiazole ring with a nitro terminal group, 4-(5-(4-nitrophenyl)-1,3,4-oxadiazol-2-yl)phenyl 4-((4-methoxybenzylidene)amino)benzoate (G₁–G₁₁), were synthesised. Their chemical structures are identified by fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (¹H-NMR) and elemental analysis. The liquid crystalline properties of the series G_n and their precursory F_n were screened by differential scanning calorimetry and optical polarising microscopy (OPM). The compounds of the series G_n were screened by thermogravimetric analysis to observe their thermal stability. The target compounds (G_n) in this study, were displayed different liquid crystalline mesophase, the first two homologous (G₁ and G₂) did not show any liquid crystalline behaviour, the homologous (G₃–G₁₀) which have an alkoxy terminal group (n = 3–10) exhibited nematic phase, whilst the last derivative of the series (G₁₁), n = 12, displayed SmA phase. The mesomorphic properties of these derivatives were affected by the presence of the nitro group at the end of the molecules which was classified as a strong polar group. Also, the role of alkoxy terminal chain and the bent heterocyclic ring (1,3,4-oxadiazole) in the liquid crystalline properties of these molecules were debated.

Graphical Abstract

KEYWORDS:

• 1,3,4-oxadiazole
• liquid crystals
• mesomorphic properties
• nitro group

Acknowledgements

First, the authors would like to thank the staff of the Department of Chemistry, College of Science, University of Al-Mustansiriyah, for supplying us all the chemicals that were used to synthesise the compounds in this study. The authors also thank Dr Raghad A. Al-Salhi and Zainab K. M. Jawad (Al-Mustansiriyah University, Chem. Dept., for helping us in performing the mass and FT-IR spectroscopy, respectively). Also, the authors are very grateful to Dr Mohanad H. Masad (University of Al al-Bayt, Jordan) and Dr Dhafir T. A. Al-Heetimi (University of Baghdad, College of Education for Pure Sciences, Ibn Al-Haitham, Department of Chemistry) for helping in doing the ¹H-NMR, DSC and TGA analyses, respectively. As well, the authors appreciate and thank Arab Science & Technology Foundation (ASTF) for supporting optical polarising microscopy (OPM) with hot stage.

Disclosure statement

No potential conflict of interest was reported by the authors.