Synthesis and characterisation of ionic liquid crystals based on substituted pyridinium cations

ABSTRACT

Novel ionic liquid crystals based on pyridinium and methylpyridinium cations with a dodecyl chain and bromide as anion have been prepared and characterised. [C₁₂Pyr][Br] and [C₁₂-n-Pic][Br] (n = 2, 3 or 4) were studied in order to understand the influence of the methyl group position in the mesomorphic behaviour. Neat microwave-assisted reactions were carried out to synthesised [C₁₂Pyr][Br], [C₁₂-3-Pic][Br] and [C₁₂-4-Pic][Br] as a sustainable approach, leading to significantly reduced reaction times (35 min vs. 48 h in conventional method). All prepared compounds were characterised by spectroscopic techniques (¹H and ¹³C-NMR; ATR-FTIR), Elemental Analysis, Differential Scanning Calorimetry (DSC), Polarised Optical Microscopy (POM) and X-Ray Powder Diffraction (XRD). It was observed that the presence of water highly influences the phase transformations. Nevertheless, thermotropic smectic phases were detected for all materials with a layer spacing between 20 and 35 Å, compatible with a bilayer interdigitated or tilted lamellar arrangement of the cation’s alkyl chain. This work also indicates new routes for preparing ionic liquid crystals with various functionalities ruled by the synthesis design.

GRAPHICAL ABSTRACT

KEYWORDS:

• Substituted pyridinium
• ionic liquid crystals
• microwave-assisted reactions
• Differential Scanning Calorimetry (DSC)
• Polarised Optical Microscopy (POM)
• X-Ray Powder Diffraction (XRD)

Acknowledgments

This work was supported by the Associate Laboratory for Green Chemistry LAQV (UID/QUI/50006/2019), i3N (UID/CTM/50025/2019) and CeFEMAS (UID/CTM/04540/2019), which are financed by national funds from FCT-MCTES and by FEDER funds through the COMPETE 2020 Program. The authors also thank the National Funds through FCT-MCTES and POR Lisboa 2020, under the projects numbers POCI-01-0145-FEDER-007688, PTDC/CTM-REF/30529/2017 (NanoCell2SEC) and Action European Topology Interdisciplinary Action (EUTOPIA CA17139). A. F. M. Santos acknowledges FCT-MCTES for the PhD Grant (SFRH/BD/132551/2017).

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental data

Supplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2022.2070783.

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.