Oxidation-Induced Differentially Selective Turn-On Fluorescence via Photoinduced Electron Transfer Based on a Ferrocene-Appended Coumarin-Quinoline Platform: Application in Cascaded Molecular Logic.

Differentially selective molecular sensors that exhibit differential response toward multiple analytes are cost-effective and in high demand for various practical applications. A novel, highly differentially selective electrochemical and fluorescent chemosensor, , based on a ferrocene-appended coumarin-quinoline platform has been designed and synthesized. Our designed probe is very specific toward Fe via a reversible redox process, whereas it detects Cu via irreversible oxidation. Interestingly, it exhibits differential affinity toward the Cu ion via complexation. High-resolution mass spectrometry, H NMR titration, and IR spectral studies revealed the formation of a bidentate Cu complex involving an O atom of the amide group attached to the quinoline ring and a N atom of imine unit, and this observation was further supported by quantum-chemical calculations. The metal binding responses were further investigated by UV-vis, fluorescence spectroscopy, and electrochemical analysis. Upon the addition of Fe and Cu ions, the fluorescence emission of probe shows a "turn-on" signal due to inhibition of the photoinduced electron transfer (PET) process from a donor ferrocene unit to an excited-state fluorophore. The addition of sodium l-ascorbate (LAS) as a reducing agent causes fluorescence "turn off" for the Fe ion because of reemergence of the PET process but not for the Cu ion because it oxidizes the ferrocene unit to a ferrocenium ion with its concomitant reduction to Cu, which further complexes with . Thermodynamic calculations using the Weller equation along with density functional theory calculations validate the feasibility of the PET process. A unique combination of Fe, LAS, and Cu ions has been used to produce a molecular system demonstrating combinational "AND-OR" logic operation.