Indolocarbazole-pyridine hybrid foldamers are strongly fluorescent in an extended random conformation, but the fluorescence is completely quenched upon folding to a helical conformation due to the compact stacking between aryl planes in the backbone. Anion binding disturbs the helical conformation, thus regenerating the fluorescence of the foldamers. This unique property has been utilized to develop a fluorescence turn-on probe for anions such as sulfate and fluoride.