Ionic liquid crystals based on viologen dimers: tuning the mesomorphism by varying the conformational freedom of the ionic layer

ABSTRACT

We investigated the liquid crystal behaviour of newly synthesised bistriflimide salts of symmetric viologen dimers. A smectic A phase was observed for intermediate spacer lengths and for relatively long lateral alkyl chains. The systems were characterised by thermal analysis, polarised optical microscopy, X-ray scattering and solid-state NMR. An intermediate ordered smectic phase was also exhibited by the compounds (except for systems with very short lateral chains) consisting of molten layers of alkyl chains and partially ordered ionic layers. These results, relating to the mesomorphic behaviour of viologen salts, are qualitatively compared to those of the more common imidazolium salts, highlighting the importance of the conformational degrees of freedom of the anions and of the cationic core. It appears that fine tuning of the conformational degrees of freedom of the ionic layer is an important component of mesophase stabilisation.

Graphical Abstract

KEYWORDS:

• Ionic liquid crystals
• viologens
• 4-4ʹ-bipyridinium

Acknowledgements

We thank Moreno Lelli for helpful discussions, Renato Schiesari for help in the setting up of SS-NMR experiments and Roberta Saini for help in the setting up of the DSC measurements. We thank the CNR-CAS bilateral agreement 2014-2016 for support.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Additional information

Funding

Financial support from MIUR (PRIN 2010N3T9M4, FIRB RBAP11C58Y) and Fondazione CARIPARO (Progetti di Eccellenza ‘Nano-Mode’ 2010, Varian 400 spectrometer) is gratefully acknowledged.