Induction of ionic smectic C phases: a systematic study of alkyl-linked guanidinium-based liquid crystals

ABSTRACT

A series of ionic liquid crystals with an alkoxy biphenyl unit tethered via an alkyl spacer to a guanidinium head group were synthesised and the mesomorphic properties were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction (XRD; WAXS and SAXS). Whereas all symmetrical guanidinium chlorides with the same chain lengths in alkyl tail and spacer displayed enantiotropic SmA₂ phases, monotropic SmC₂ phases with 1–2 K temperature range were only formed for chain lengths ≥ C₁₀. Shifting the calamitic core more closely to the ionic head group by decreasing the tether length and simultaneously increasing the terminal alkyl chain improved the stability of both SmA and SmC phases considerably and led to enantiotropic SmC phases for the guanidinium chloride with C₁₄ alkyl tail and C₆ spacer. An even more pronounced effect was detected during anion exchange. Bromide, iodide, hexafluorophosphate, thiocyanate and triflate suppressed any SmC phase, whereas tetrafluoroborate behaved similar to chloride maintaining the SmC phase. However, acetate stabilised the SmC phase at the expense of the SmA phase. Based on temperature-dependant XRD measurements, a bilayer structure was proposed.

GRAPHICAL ABSTRACT

KEYWORDS:

• Ionic liquid crystal
• ILC
• smectic C
• mesophase
• biphenyl
• guanidinium

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary-material

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Additional information

Funding

Generous financial support by the Deutsche Forschungsgemeinschaft (grant number LA907/17-1), the Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-Württemberg, the Bundesministerium für Bildung und Forschung (shared instrumentation grant number 01 RI 05177) and the European Commission (ERASMUS fellowship for Nina Trbojevic) is gratefully acknowledged.