Ionic liquid crystals containing pyridinium and pyrazolium units: mesomorphism and acidochromic behaviors

Ionic liquid crystals are soft materials whose mesomorphic behavior can be strategically modulated as a function of their components, considering both the design of the organic cation and the nature of the inorganic anion. A new pyridylpyrazole derivative has been synthesized and used as a precursor for obtaining ionic salts that contain the corresponding dicationic fragment and different counterions. Most salts behave as liquid crystal materials exhibiting smectic A mesophases at temperatures ranging between 99 and 295 °C, as well as luminescence behavior in solution and/or the solid state. Interestingly, the related chloride salt emits blue light in the solid state under excitation at 261 nm, in contrast to the absence of the emission of the initial pyridylpyrazole derivative. This turn-on fluorescence response has been exploited to fabricate polymer supports that show a bright blue emission under exposure to hydrochloric acid.