Synthesis and characterisation of thermotropic liquid-crystalline properties of azomethine dimers

Abstract

A series of azomethine dimers were prepared by condensation reactions of benzaldehyde, biphenylcarboxaldehyde and 9-anthraldehyde with various aromatic diamines of varying flexibility in ethanol in the presence of tosic acid. Their chemical structures were determined by Fourier transform infrared and ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopies, as well as elemental analysis. Their thermal properties were also examined by using a number of experimental techniques, including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarising optical microscopy (POM) and variable temperature X-ray diffraction (VTXRD). Azomethine dimer, prepared from benzaldehyde and 1,9-bis(4-aminophenoxy)nonane, exhibited a monotropic, nematic liquid-crystalline (LC) phase. The majority of the azomethine dimers containing biphenyl moieties exhibited enantiotropic, nematic LC phase on melting at relatively low temperatures, since they developed typical Schlieren, threaded or marbled textures in their LC phase. They also had accessible isotropisation temperatures well below their decomposition temperatures. Azomethine dimers containing anthracene moieties did not exhibit LC properties, but exhibited polymorphism as determined by POM and VTXRD in two cases. All of these azomethine dimers in the series had excellent thermal stability that was in the broad range of temperatures of 307–400°C depending on their degrees of aromaticity index.

Keywords:

• azomethine dimers
• synthesis
• thermotropic
• liquid-crystalline phase
• nematic
• Schlieren texture

Acknowledgements

PKB acknowledges the University of Nevada Las Vegas (UNLV) for the New Investigation Award (NIA), the Planning Initiative Award (PIA) and the Applied Research Initiative (ARI) grants; the donors of the Petroleum Research Fund, administered by the American Chemical Society; and an award (CCSA# CC5589) from the Research Corporation for the support of this research. AKN acknowledges the Graduate College (UNLV) for providing him with a Nevada Stars Graduate Assistantship for the period of 2006–2008. HDM acknowledges Texas A&M International University (TAMIU) for a mini-research grant and the Welch Foundation for a summer research grant (BS0040). Use of Advanced Photon Source (APS) is supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract No. W-31-109-Eng-38. The Midwest Universities Collaborative Access Team's sector at the APS is supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, through the Ames Laboratory under Contract No. W-7405-Eng-82. This research is supported, in part, by the National Science Foundation grant DMR-03-12792.

Notes

This article is dedicated to Professor Timothy C.K. Su with best wishes on the occasion of his 65th birthday.