Naphthoimidazolium based ratiometric fluorescent probes for F⁻ and CN⁻, and anion-activated CO₂ sensing

The results of the current study show that fluoride and cyanide recognition by two naphthoimidazolium derivatives can be applied to CO₂ sensing. Among various anions, only fluoride and cyanide promote changes in absorption and fluorescence wavelegnth maxima of the naphthoimidazolium derivative in CH₃CN. The changes are attributed to (C–H)⁺—anion^- ionic hydrogen bonding interactions between the conjugated imidazolium salts and these anions. Especially important is the observations that the fluorescence emission maxima of the naphthoimidazolium derivative at 465 nm undergo hysochromic shifts to 375 nm upon addition of fluoride and cyanide exclusively. The naphthoimidazolium derivatives undergo highly selective changes in their fluoerecence maximum from 465 nm to 375 nm upon addition of CN⁻ even when 2% water is present in the CH₃CN solution. Moreover, exposure of the solution of CN⁻ activated naphthoimidazolium derivative to CO₂ promotes a red shift of emission maximum back to 465 nm. The results demonstrate that the naphthoimidazolium derivatives can be utilized for anion activated ratiometric sensing of CO₂.