Synthesis and self-assembly of halogen-bond donor–spacer–hydrogen-bond donor molecules: polymeric liquid crystals induced by combination of intermolecular halogen- and hydrogen-bonding interactions

Abstract

A series of linear molecules of 4-(n-(2,3,5,6-tetrafluoro-4-iodophenoxy)alkoxy)benzoic acid (4a–d) bearing a hydrogen-bond donor on one end of the molecule and a halogen-bond donor on another end was synthesised. Compounds 4a–d formed composite halogen- and hydrogen-bonded polymeric complexes with bis(4-trans-4′-stilbazolyl)-oligo(ethylene glycol) (5a-c), as verified by Fourier transform infrared spectroscopy and differential scanning calorimetry (DSC). The degree of polymerisation of complexes 4a–d/5a–c was estimated to be in the range of 11–25 using the Flory equation, suggesting the polymeric nature of the complexes. The complexes 4a–d/5a–c showed thermotropic mesophases upon cooling, confirmed by polarised optical microscopy and DSC. The liquid crystals exhibited significantly broader mesophase temperature ranges than those induced by the halogen bond alone, indicating that the introduction of a more flexible hydrogen-bonded mesogenic core in halogen-bonded polymeric assembly likely led to a marked improvement in the stability of the mesophase by way of modulating the flexibility of the polymeric chains. The novel halogen- and hydrogen-bonded assembly of this type is thought to provide an excellent model upon which to build supramolecular polymeric liquid crystals through a precise manipulation of the two types of non-covalent interactions.

Keywords:

• Liquid crystal
• halogen bond
• hydrogen bond
• supramolecular assembly

Acknowledgements

We gratefully acknowledge the financial support from Institute of Materials Research and Engineering (IMRE), Agency of Science, Technology and Research (A*-STAR).