An NIR Emissive Donor-π-Acceptor Dicyanomethylene-4H-Pyran Derivative as a Fluorescent Chemosensor System towards Copper (II) Detection

A novel donor-π-acceptor fluorescent dye as a chemosensor for Cu²⁺ ions is herein presented. The fluorophoric core consists of a 3,5-diphenyl-dicyanomethylene-4H-pyran (DCM), with extended styryl chains on positions 2 and 6, bearing terminal di-(2-picolyl)amine (DPA) groups for metal coordination. Optical characterization of the chemosensor dye reveals an absorption maximum at ca. 500 nm and a strong bathochromic shift in the emission, reaching ca. 750 nm in polar solvents. This solvatochromic behavior, which yields very large Stokes shifts (up to ~6700 cm⁻¹), is characteristic of the strong intramolecular Charge Transfer (CT) nature of this chromophoric system. While the chemosensor has demonstrated no changes in its optical properties over a wide pH range (2–12), a strong quenching effect was observed upon Cu²⁺ coordination, with a 1:1 binding stoichiometry, indicating that only one DPA unit is capable of effectively chelating Cu²⁺, rendering the second DPA motif inactive. The binding constant was determined to be 7.5 × 10⁷ M⁻¹, indicating a very high sensitivity, and an LOD of 90.1 nM. Competition assays have demonstrated that the chemosensor is highly selective towards Cu²⁺, even in the presence of excesses of other mono- and di-valent cations. Co²⁺ and Ni²⁺ proved to be the strongest interferents, particularly in the luminescent response. Paper test-strips prepared with the embedded sensor showed a fluorometric response in the presence of different copper (II) concentrations, which attested to the potential of this chemosensor to be used in the determination of Cu²⁺ content in aqueous media, for in-field applications.