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, 5, based on a ferrocene-appended coumarin-quinoline platform has been designed and synthesized. Our designed probe is very specific toward Fe3+ via a reversible redox process, whereas it detects Cu2+ via irreversible oxidation. Interestingly, it exhibits differential affinity toward the Cu+ ion via complexation. High-resolution mass spectrometry, H-1 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 Fe3+ and Cu2+ ions, the fluorescence emission of probe 5 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 Fe3+ 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 Cut, which further complexes with 5. Thermodynamic calculations using the Weller equation along with density functional theory calculations validate the feasibility of the PET process. A unique combination of Fe3+, LAS, and Cu2+ ions has been used to produce a molecular system demonstrating combinational AND-OR logic operation.