Design, Synthesis, and Characterization of New Imidazolium-Based Ionic Liquid Crystals and Ionic Liquid Crystal Monomers

Lily Alexandria Robertson, University of Colorado at Boulder


Ionic liquid crystals (ILCs) are molecules that combine the useful properties of ionic liquids (ILs) with the nanoscale self-assembled order of liquid crystals (LCs). This Ph.D. research focuses on two primary efforts: (a) to increase the IL-like character of new ILC molecules via incorporation of multiple imidazolium units, and (b) to enable the use of nanostructured materials over a range of environments via incorporation of multiple polymerizable groups to stabilize the formed LC phases via cross-linking.

Bicontinuous cubic LC phases have a distinctive 3D ordered nanostructure composed of two incompatible, intertwined domains with overall cubic symmetry. This feature makes these phases particularly useful and desirable for transport and uptake/release applications. A novel amphiphilic ILC platform based on a hexyl-bridged bis(imidazolium) headgroup connected to a single alkyl tail was developed that exhibited both bicontinuous cubic thermotropic LC (TLC) and lyotropic LC (LLC) phases. A total of fifteen compounds (CnX) with varying even tail lengths (Cn) and anions X = Br, BF4, or Tf2N were synthesized. Many of these compounds exhibited other TLC and LLC phases.

To obtain cross-linkable CnX analogs, a systematic study of the effect of different polymerizable groups on TLC phase formation was performed. A series of non-symmetric, monoimidazolium- based amphiphilic salts possessing two radically polymerizable groups at each end was synthesized. The structural features that favored TLC formation were investigated via the use of different polymerizable groups (e.g., n-alkyl-2,4-hexadiene, acrylate, and vinyl) and alkyl spacer lengths. Long alkyl tail spacers and less polar polymerizable groups favored TLC behavior. The unprecedented n-alkyl-2,4-hexadiene polymerizable group presented the best TLC phase behavior, similar to the alkyl-1,3-diene tails previously pioneered by our group. Successful TLC phase retention upon in situ cross-linking was also demonstrated. Finally, initial ionic conductivity measurements on the cross-linked nanostructures formed by these new ionic TLC monomers were performed.