Controlling excited-state prototropism via the acidity of ionic liquids

Abstract

Proton transfer involving acridine dissolved in neat ionic liquids (ILs) as a function of temperature is studied with the help of electronic absorbance and steady-state fluorescence, as well as time-resolved fluorescence, and is compared with the proton transfer observed within water and common organic solvents. The acidity of the IL cation is found to control the excited-state proton transfer involving acridine. Fluorescence corresponding to the protonated form of acridine is observed in the temperature range of 10–90 °C when the solubilizing IL contains an imidazolium cation possessing C2–H functionality. This is attributed to the acidity associated with the C2 proton of the imidazolium cation. When the C2 proton of the imidazolium cation is substituted with a methyl group in the IL (C2–CH₃ substituted imidazolium ILs), the fluorescence from the dissolved acridine is found to depend on temperature. The fluorescence corresponds to neutral acridine at room and lower temperatures, whereas it is again from the cationic form of acridine at slightly higher temperatures. This is attributed to the weaker acidity associated with the C2–CH₃ protons of the imidazolium cation as compared to the C2–H proton; the acidity of the C2–CH₃ protons is found to increase with increasing temperature. In contrast, the observed fluorescence corresponds exclusively to the neutral acridine in the temperature range of 10–90 °C when the solubilizing IL contains a pyrrolidinium cation possessing no such acidic protons. Control of the excited-state proton transfer using the acidity of the cation of the solubilizing IL is amply demonstrated.