Chromonic liquid crystalline phases

Abstract

Chromonic systems are lyotropic mesophases formed by soluble aromatic compounds. The basic structural units in these systems are stacks of molecules (rather than individual molecules or micellar assemblies). There are two common chromonic phases: a more dilute phase consisting of a nematic array of columns (the N phase) and a more concentrated phase in which the columns lie in a hexagonal array (the M phase). Chromonic phases are formed by a range of compounds, including drugs, dyes and nucleic acids, typically with three or four fused aromatic rings. They have distinctive optical textures and characteristic multi-peritectic phase diagrams (in contrast to the multi-eutectic phase diagrams of conventional amphiphiles). Many commercial dyes have proved to be chromonic, but the ability to form liquid crystalline phases has been incidental to their use. However, recent studies have shown that the combination of self-ordering, ease of alignment, sensitivity to changing conditions and additives, coupled with their optical and electro-optical properties, gives these systems unique and potentially valuable properties. It is expected that these will lead to a new generation of applications. It is predicted that there will be increased emphasis on the development of a range of sophisticated devices, either produced from, or actually incorporating chromonic phases. These include polarisers, optical compensators, light-harvesting devices, and biosensors for medical diagnosis.

Keywords:

• liquid crystals
• chromonic phases
• dyes
• organic electronics
• biosensors

Acknowledgements

I am heavily indebted to Professor R.J. Busby and Professor Gordon Tiddy for their encouragement and advice over many years. I thank Professor Mark Wilson for advice and permission to include a frame from his computer simulations of the N phase of Edicol Sunset Yellow and Dr Jim Henderson for help in interpreting the results of the coarse-grain modelling of sticky-ended cylinders. I am pleased to acknowledge the major contributions of Dr Jane Turner to the pioneering dye mesophase studies and the central role of Professor Terri Attwood in the initial characterisation of chromonic systems. Finally, I wish to put on record my gratitude to the anonymous referee for help and detailed critical advice.