Syntheses and properties of imidazopyridine-based ionic liquids

We have synthesized imidazopyridine-type ionic liquids and evaluated their properties. [C_nimpy][Br] (n = 4, 6, 8, 10), in which an alkyl group was introduced at the N₁ position of imidazo[1,2-a]pyridine, were all solid at room temperature. Exchange of the counteranions of [C_nimpy][Br] with N(CN)₂⁻ and FSI⁻ gave the ionic liquids [C_nimpy][N(CN)₂] and [C_nimpy][FSI] in the liquid state at room temperature. In particular, the DSC thermogram of [C₁₀impy][FSI] showed a clear melting point at 22 °C. In [C_nimpy][TFSI], ionic liquids consisting of cations with short alkyl chain lengths (n = 4,6) were solids at room temperature, while those with long alkyl chain lengths (n = 8,10) were liquids. [C_nimpy][FSI] has lower viscosity than other anion-consisting ionic liquids which showed a liquid state at room temperature. [C_nimpy][FSI] has lower viscosity than other anion-containing ionic liquids. In particular, [C₄impy][FSI], with its relatively short alkyl chain length, exhibited a viscosity of 101 mPa·s at 25 °C, which renders it suitable for practical applications. [C₄impy] [FSI] (50 μM acetonitrile solution) showed fluorescence with maximum absorption at 336 nm. Its fluorescence wavelength was shorter than that of imidazo[1,2-a]pyridine, whereas its fluorescence quantum yield (Φ_F 0.39) was higher. Therefore, it is expected to be used as a composite material by employing FRET with a substance that exhibits fluorescence in the visible region.