The sensing mechanism of the N-Phenyl-N′-(3-quinolinyl)urea (PQU) chemosensor for fluoride anion has been investigated by density functional theory/time-dependent density function theory. The double intermolecular hydrogen bonds are formed between the three anions (X−F−, AcO−, Cl−) and the urea fragment of PQU. In the S0 states, the HbX− hydrogen bonds are slightly stronger than the HaX− hydrogen bonds and the fluoride-induced deprotonation occurs at the NHb position rather than at the NHa position. Consequently, the absorption peaks, including an intramolecular charge transfer transition and a ππ* transition, are significantly red-shifted. Thermodynamic calculations confirm that the deprotonation in the ground state is favorable in energy only when excess fluoride anion exists. Along with the S0 → S1 transition, the HaX− hydrogen bonds strengthen and the HbX− hydrogen bonds weaken. However, the emission spectra of [PQU-Hb]−, instead of [PQU-Ha]−, are observed upon addition of fluoride anion. © 2013 Wiley Periodicals, Inc.