The effects of fluoride anions on tryptophan-derived isomeric urea ligands (UT1–UT3) were examined using UV-vis, fluorescence and ¹H NMR spectroscopy. Remarkably, the addition of TBAF triggered unusual enhancement of fluorescence for UT1, which emanated from intramolecular charge transfer following fluoride-induced deprotonation of the indole NH proton.