Thermotropic liquid crystal behaviour of gemini imidazolium-based ionic amphiphiles

Abstract

Thermotropic ionic liquid crystals (LCs) are useful for a number of applications such as anisotropic ion transport and as organised reaction media/solvents because of their ordered fluid properties and intrinsic charge units. A large number of different ionic LC architectures are known, but only a handful of examples of gemini (i.e. paired or dimeric) ionic LCs have been prepared and studied. In this work, a series of 20 new symmetric, imidazolium-based, gemini cationic LCs containing two bridged imidazolium cations and two pendant alkyl chains was synthesised, and the thermotropic LC behaviours were characterised. The imidazolium unit provides a highly tunable and modular platform for the design and synthesis of gemini cationic LCs which offers excellent structure control. As expected, the thermotropic LC properties of these new amphilphilic, gemini ionic LCs were found to be strongly influenced by the length of the spacer between the imidazolium units, the length of the pendant alkyl tails, and the nature of the anion. Smectic A (SmA) thermotropic LC phases were observed in more than half of the gemini imidazolium LC systems studied.

Keywords:

• gemini imidazolium salts
• thermotropic ionic liquid crystals
• synthesis of imidazole derivatives
• nanostructured materials
• smectic phases

Acknowledgements

Financial support for this research from the US Army Research Office (AB07CBT010), the Defense Threat Reduction Agency (HDTRA1-08-1-0028) and the National Science Foundation (DMR-0552399 and CBET-08054553) is gratefully acknowledged.