Headgroups versus symmetry in congruent ion pairs: which one does the job in mesomorphic aryl guanidinium and aryl imidazolium sulphonates?

Abstract

Ionic liquid crystals based on congruent ion pairs composed of mesogenic cations and anions of similar shape provide an attractive tool for the tuning of mesophase properties. Here, the effect of the number and symmetry of lipophilic side chains and the type of head group on the phase type and thermal mesophase properties was probed by the synthesis and investigation of two series of novel guanidinium and imidazolium sulphonates and compared with the corresponding iodides. Their mesomorphic properties were examined via differential scanning calorimetry, optical polarising microscopy and X-ray diffraction. While derivatives bearing only one alkoxy chain in either cation or anion with up to three alkoxy chains in total within the ion pairs display smectic A mesophases, hexagonal columnar mesophases were observed for all other compounds with four or five alkoxy chains totally irrespective of the head group. However, with increasing steric bulk, i.e. with a total of six alkoxy chains, the symmetry of the aryl moiety in the anion as well as the type of head group becomes relevant, resulting in hexagonal columnar or plastic phases for guanidinium sulphonates with symmetrical anions, while those with unsymmetrical anions were non-mesomorphic. In contrast, the corresponding imidazolium sulphonates displayed cubic phases for the combinations of symmetrical cation/symmetrical anion and symmetrical cation/unsymmetrical anion. If both ions are unsymmetrically substituted, the imidazolium sulphonate displayed a hexagonal columnar phase. The results further demonstrate the utility of the congruent ion pairs for tailor-made ionic liquid crystals.

Keywords:

• ionic liquid crystals
• imidazolium
• guanidinium
• sulphonates
• mesophases
• smectic, columnar
• cubic
• congruent ion pairs

Acknowledgements

Generous financial support by the Studienstiftung des Deutschen Volkes (fellowship for M.B.), the Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-Württemberg, the Bundesministerium für Bildung und Forschung (shared instrumentation grant) and the Fonds der Chemischen Industrie is gratefully acknowledged. We would like to thank Professor Thomas Schleid for providing the TGA equipment.

Correction Statement

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

Supplemental data

Supplemental data for this article can be accessed here.

Electronic Supplementary Information. Experimental procedures and analytic data. DSC data. TGA data. NMR spectra. This information is available free of charge via the Internet at http://pubs.acs.org/journal/jacsat.

Scheme 1. Structures of mesogenic cations with either spherical (1) or congruently-shaped (2) anions.

Scheme 2. Synthesis of acyclic (5) and cyclic (6) guanidinium chlorides.

Scheme 3. Reaction conditions: (i) HSO₃Cl, CHCl₃, 0°C→rt, 12 h; (ii) KOH, MeOH, 60°C, 30 min; (iii) POCl₃, DMF, hexane, rt, 3 h; (iv) NaOMe, CH₂Cl₂, 0°C, 15 min (10b) or 5 min (10c); (v) BuLi, THF, −78°C, 30 min; (vi) TMSCl, −78°C→rt, 2 h; (vii) ClSO₃TMS, dioxane, 0°C→rt, 1 h; (viii) KOH, MeOH, 50°C, 1 h; (ix) POCl₃, DMF, hexane, rt, 3 h; (x) NaOMe, MeOH, CH₂Cl₂, 0°C, 15 min.

Scheme 4. Synthesis of guanidinium sulphonates Gua m/n, Guc m/n and guanidinium iodides Gua m/I, Guc m/I.

Scheme 5. Reaction conditions: (i) NBS, THF, rt, 12 h; (ii) KBr, H₂O₂, HOAc, CH₂Cl₂, rt, 1 h; (iii) imidazole, K₂CO₃, 20 mol% CuI, DMSO, 140°C, 5 d.

Scheme 6. Synthesis of imidazolium sulphonates Im m/n and imidazolium iodides Im m/I.