Liquid crystal phases generated by supramolecular self-assembly of biforked amphiphilic imidazoles

Abstract

Functional groups with the capability of hydrogen bonding are widely used in the molecular design and preparation of liquid crystalline supramolecular systems, a rapidly growing area of materials showing a high sensitivity towards external stimuli. A series of novel imidazole-containing Schiff's bases replenishing the family of supramolecular liquid crystals has been synthesised and characterised by proton nuclear magnetic resonance, Fourier transform infrared, and ultraviolet–visible spectroscopy, and elemental analyses. Variation of lengths of the terminal alkyl substituents in the obtained amphiphilic imidazoles within 6, 8, 10, 12, 14 and 16 carbon atoms leads to significant changes in their thermal behaviour, micro-segregation and supramolecular self-assembly. Lower homologues were non-mesomorphic, while intermediate members of the homologous series exhibited monotropic bilayered smectic and columnar mesophases. A higher homologue with 16 carbon atoms has an increased trend towards crystallisation of the aliphatic chains and did not exhibit mesomorphism again. The liquid crystalline mesophases were identified and investigated by polarised optical microscopy, differential scanning calorimetry, X-ray diffraction and thermal emission microscopy methods. According to X-ray diffraction characteristics, the smectic mesophase has a bilayered structure where the hydrophilic imidazole groups form a continuous hydrogen bonded network. The interface curvature created by the second alkyl chain leads to the appearance of columnar nanostructures in homologues with 12 and 14 aliphatic carbon atoms.

Keywords:

• imidazole
• liquid crystals
• Schiff's base
• hydrogen bond
• supramolecular assembling
• amphiphiles

Acknowledgements

This work has been supported by a grant from the Korea Science and Engineering Foundation (KOSEF) through the Center for Bioactive Molecular Hybrids (CBMH), and the program Brain Korea 21 (BK-21). Pohang Accelerator Laboratory is acknowledged for providing X-ray beam from their synchrotron facilities.