Unusual fluorescein prototropism within aqueous acidic 1-butyl-3-methylimidazolium tetrafluoroborate solution.

Fluorescence emission of the cationic form of fluorescein is observed from dilute aqueous acidic solutions in the presence of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate which is otherwise observed only from aqueous solutions of very high acidity.