From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding

The overall goal of this protocol is to provide a procedure for preparing new supramolecular ionic liquid crystals based on halogen bonding starting from non-mesomorphic building blocks. The halogen bond has become a tool for controlling aggregation self-assembly phenomena, and they can play a very important role in determining physical and chemical properties of biomaterials. The halogen bond is strong enough to overcome the lower field existing between perfluorocarbon and hydrocarbon compounds and can be exploited to introduce any fluorinated modules into a new supramolecular material.

Generally, people new to this method are suspicious because halogens are electron rich species. However, in covalently bound halogen atoms a region of positive electrostatic potential appears along the extension of the covalent bond. The ability of an ion to be involving in halogen bonding in the ideal directionality of the interaction gave us this idea of applying this method to anionic liquid crystal design.

To prepare the halogen-bonded complexes from solution, first dissolve 50 milligrams of the the imidazolium salt in 0.5 milliliters of acetonitrile. Next, dissolve 229 milligrams of perfluorooctyliodide in 0.5 milliliters of acetonitrile. Mix the two solutions in a vial.

Then, place the vial in a jar filled with paraffin oil and let the solvent diffuse slowly in the oil at room temperature. It remains challenging to anticipate the composition and the architecture of the final supramolecular structure, because the iodine anions can act as multidentate halogen bonding acceptor. However, the AC and x-ray diffraction analysis revealed that iodine anion preferentially binds to perfluorooctyl chains.

To prepare halogen-bonded complexes using the melt methodology, mix the imidazolium salt with appropriate iodo-perofluoroalkane in a clear borosilicate glass vial equipped with a magnetic stirring bar. Close the vial and place into an oil bath under vigorous stirring. Heat the reaction mixture at 70 degrees Celsius for 15 minutes.

Then, cool the mixture to room temperature. Perfluorinated molecules are highly volatile, therefore this methodology requires a sealed system to mitigate against the volatility of iodo-perfluoroalkane. Prepare a thin layer of the sample by placing a spatula tip amount of the supramolecular complex between two micro-cover glasses.

Place the sample in the hot stage of a polarized optical microscope between two cross-polarizers and heat the sample to melt. Finally, submit the sample to repeated heating and cooling cycles in order to promote phase transitions. DSC analysis of the complex obtained from the imidazolium salt and iodo-perfluorooctane was consistent with the preferred 1:2 stoichiometry.

The thermogram of the 1:1 complex shows the uncomplexed imidazolium salt. While, in the 1:3 complex there is excess pure iodo-perfluorooctane. The thermogram of the 1:2 complex shows a single peak distinct from the starting compounds, demonstrating that a new, pure crystalline species was formed.

The fluorine NMR spectrum of the complex between the imidazolium salt and iodo-perfluorooctane shows an up-field shift of 1.2 ppm for the difluoromethylene group adjacent to iodine, confirming non-covalent interactions involving halogen atoms as electrophilic species. Single crystal x-ray analysis of the complex between 1-ethyl-3-methylimidazolium iodide and iodo-perfluorooctane confirm that halogen bonding drives the formation of a trimeric supramolecular complex, where the iodide anion acts as a bidentate halogen bond acceptor binding two fluorinated chains. DSC and polarized optical microscopy revealed that all complexes melted at temperatures lower than 100 degrees Celsius and showed an antitropic liquid crystalline behavior with smectic B and smectic A phases.

Upon cooling, the smectic A, smectic B transition was identified by characteristic striations across the back of the thins. The supramolecular system reported in the paper demonstrates for the first time, the application of halogen bonding in the construction of supramolecular ionic liquid crystals. Thanks to an accurate supramolecular design based on highly directionality of the halogen bond and the fluoro-phobic effect, it is possible to obtain an antitropic liquid crystals based on a rigid, non-aromatic supramolecular halogen bonding synthons as a mesogenic core.

The supramolecular approach presented here represents an attractive platform for the design of new liquid crystalline materials and can provide the new opportunities for the development of sophisticated functional materials.