Abstract:A new fluorescent chemosensor bearing two imidazolium groups was designed and investigated by DFT/TDDFT method. The fluoride-sensing mechanism of the chemosensor was studied by the geometry optimization, two-dimensional potential energy surface (PES) scan, absorption/emission spectra simulation, and frontier molecular orbital (FMO) analysis. The calculations show that this chemosensor displays an emission band at 407 nm. PES scan confirmed that the excited state proton transfer (ESPT) process of the chemosensor-fluoride complex is barrierless. The ESPT process took place in the C-H•••F hydrogen bond with C-H moiety acting as a proton donor and the fluoride anion acting as a proton acceptor. This process proved that addition of fluoride anion could lead to the formation of the carbene form of the chemosensor.Due to the nπ-type transition mode obtained by FMO, the carbene form has a red-shift emission band at 523 nm.