Design, Synthesis, and Thermotropic Liquid Crystal Behavior of Novel Tris(imidazolium) Ionic Organic Compounds

Schenkel, Magdalene Reynolds

Graduate Thesis Or Dissertation

Design, Synthesis, and Thermotropic Liquid Crystal Behavior of Novel Tris(imidazolium) Ionic Organic Compounds Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/5h73pw15s

Abstract

Thermotropic ionic liquid crystals (TILCs) are a relatively new field of liquid crystals (LCs) research. TILCs have the internal organization of traditionally uncharged LCs as well as many of the unique characteristics of organic molten salts known as room-temperature ionic liquids (RTILs). Within the latter's family of `designer solvents', which include many ionic moieties with varying advantages, imidazolium salts offer a cheap and modular ionic moiety to build ionic organic compounds with characteristics tailored to specific needs. The advances in these ionic liquid crystals (ILCs) have led to research in soft materials with improved gas separation, electrolytic, and reactive capabilities. However, the incorporated ionic units traditionally comprise very little of the overall architecture commonly found for ionic mesogens and are most often head or pendant groups, vestigial, or isolated from other ionic units present. In this thesis research, calamitic, symmetrical, organic ionic compounds bearing a linearly linked tris(imidazolium) salt core were designed, synthesized and analysed for thermotropic LC behaviour. This unprecedented TILC design allowed for an increase in the number of imidazolium units per molecule compared to the vast majority of imidazolium-based TILCs reported in the literature. This new modular TILC platform also allowed the systematic study of three major factors in the molecular for potentially controlling LC behaviour: the hydrophobic tail length, the type of counter-ion, and the spacer length within the polyionic core. Each of these tris(imidazolium) compounds prepared were structurally characterized and then observed through variable-temperature polarized light microscopy (PLM) to determine their thermotropic mesogenic behaviour. Those compounds that exhibited thermotropic LC behaviour were then analysed with powder X-ray diffraction (PXRD) to confirm the structures of the presented LC phases. From these systematic studies, it was found that:

1) The length of the n-alkyl tails peripherally must be equal to or longer than the length of the ionic core into to properly separate between hydrophobic and ionophilic sections.

2) BF₄^- anions permit shorter n-alkyl tails to produce TILC behaviour, and with wider mesogenic temperature ranges.

3) Only the longest tails in the series create TILCs when paired with Tf₂N⁻ anions, with higher order within the lamellar mesophase and with markedly smaller layers than with either BF₄^- or Br^- counter-ions.

4) Extending the inner-core spacers from hexyl to octyl created only three TILCs, suggesting either the imidazolium moieties were too far apart to create an ionic section, or the overall length of the ionic organic compounds were too long to allow mesogenic behaviour.

5) Shortening the inner-core spacers also produced no mesogens regardless of n-alkyl tails and counter-ion species, suggesting the moieties within the ionic core were spaced too closely to create an ionophilic section within the compound.

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