Fused pi-conjugated imidazolium liquid crystals: synthesis, self-organization, and fluorescence properties

Fused pi-conjugated imidazolium compounds bearing two or three long alkoxy chains and various counter anions were systematically prepared following our reported synthetic route. On the basis of DSC measurements and POM observations, the combination of three alkoxy chains longer than the dodecyl group and moderately coordinating anions was important to obtain an enantiotropic mesophase. VT-XRD measurements coupled with a single crystal X-ray structure of a model compound revealed a hexagonal columnar structure supported by ionic interactions. The regularity of the self-organized nanostructure was governed by the alkoxy chain length and the coordinating ability of the counter anion. In the solution state, UV-vis and fluorescence peak maxima were influenced by the number of alkoxy chains and independent of the chain length and anion character. On the contrary, the alkoxy chain length and the coordinating ability of the counter anion had a large impact on the fluorescence spectra in the solid state. A longer wavelength emission at around 560 nm was observed for compounds with the highly regular hexagonal columnar structure. From the result of fluorescence lifetime measurements, the interaction between fused pi-conjugated imidazolium cations was suggested.