Dual-Mode Multiple Ion Sensing via Analyte-Specific Modulation of Keto-Enol Tautomerization of an ESIPT Active Pyrene Derivative: Experimental Findings and Computational Rationalization.

A pyrene-based e xcited - state intramolecular proton transfer (ESIPT) active probe PMHMP was synthesized, characterized, and employed for the ppb-level, dual-mode, and high-fidelity detection of Cu 2+ (LOD: 7.8 ppb) and Zn 2+ ions (LOD: 4.2 ppb) in acetonitrile medium. The colorless solution of PMHMP turned yellow upon the addition of Cu 2+ , suggesting its ratiometric, naked-eye sensing. On the contrary, Zn 2+ ions displayed concentration-dependent fluorescence rise till a 0.5 mole fraction and subsequent quenching. Mechanistic investigations indicated the formation of a 1:2 exciplex (Zn 2+ : PMHMP ) at a lower concentration of Zn 2+ , which eventually turned into a more stable 1:1 (Zn 2+ : PMHMP ) complex with an additional amount of Zn 2+ ions. However, in both cases, it was observed that the hydroxyl group and the nitrogen atom of the azomethine unit were involved in the metal ion coordination, which eventually altered the ESIPT emission. Furthermore, a green-fluorescent 2:1 PMHMP -Zn 2+ complex was developed and additionally employed for the fluorimetric analysis of both Cu 2+ and H 2 PO 4 - ions. The Cu 2+ ion, owing to its higher binding affinity for PMHMP , could replace the Zn 2+ ion from the preformed complex. On the other hand, H 2 PO 4 - formed a tertiary adduct with the Zn 2+ -complex, leading to a distinguishable optical signal. Furthermore, extensive and organized density functional theory calculations were performed to explore the ESIPT behavior of PMHMP and the geometrical and electronic properties of the metal complexes.