Columnar liquid crystalline imidazolium salts : Self-organized one-dimensional ion conductors

A series of columnar liquid crystalline (LC) imidazolium tetrafluoroborate salts, l-methyl-3-{3,4,5-tris(alkyloxy)-benzyl}imidazolium tetrafluoroborate (Im-n; n = 6-18), where n is the carbon number of the aliphatic alkyl chains, have been prepared and studied as anisotropic ion-conductors. Their self-assembled structures in the fluid columnar LC states have been examined by X-ray diffraction measurements. In the columnar phases, the imidazolium portions self-assemble into the inner part of the columns through nano-segregation between the ionic and non-ionic parts driven by electrostatic interactions. The columns can be oriented macroscopically in two directions by different methods: the direction parallel to a glass surface by mechanical shearing; the direction perpendicular to the modified surface of a glass substrate with 3-(aminopropyl)triethoxysilane. Anisotropic ionic conductivities have been measured for columnar ionic liquids Im-8 and Im-12 that are aligned parallel to the surface of the glass substrate. These columnar ionic liquids have been found to function as one-dimensional ion-conductors.