A Highly Selective Two‐Photon Fluorescent Probe for Detection of Cadmium(II) Based on Intramolecular Electron Transfer and its Imaging in Living Cells

A new quinoline‐based probe was designed that shows one‐photon ratiometric and two‐photon off–on changes upon detecting Cd 2+ . It exhibits fluorescence emission at 407 nm originating from quinoline groups in Tris‐HCl (25 m M , pH 7.40), H 2 O/EtOH (8:2, v/v). Coordination with Cd 2+ causes quenching of the emission at 407 nm and simultaneously yields a remarkable redshift of the emission maximum to 500 nm with an isoemissive point at 439 nm owing to an intramolecular charge‐transfer mechanism. Thus, dual‐emission ratiometric measurement with a large redshift (Δ λ =93 nm) and significant changes in the ratio ( F 500 / F 439 ) of the emission intensity ( R / R 0 up to 27) is established. Moreover, the sensor H 2 L displays excellent selectivity response, high sensitive fluorescence enhancement, and strong binding ability to Cd 2+ . Coordination properties of H 2 L towards Cd 2+ were fully investigated by absorption/fluorescence spectroscopy, which indicated the formation of a 2:1 H 2 L/Cd 2+ complex. All complexes were characterized by X‐ray crystallography, and TD‐DFT calculations were performed to understand the origin of optical selectivity shown by H 2 L. Two‐photon fluorescence microscopy experiments have demonstrated that H 2 L could be used in live cells for the detection of Cd 2+ .