An Efficient <i>D‐π‐A‐π‐D</i> ‐Type Fluorescent Probe for Selective and Sensitive Detection of Cu ²⁺ and CN ⁻ Ions Through Fluorescence On–Off Responses

A D-A-D-type fluorescent molecular probe 4 has been designed and synthesized. The solvatochromic and photophysical properties of probe 4 were studied in different solvent systems. Probe 4, upon interaction with different classes of ions in a partial aqueous medium, showed sensitivity for Cu²⁺ and CN^- ions in which the intensity of the probe due to the intramolecular charge transfer (ICT) process diminished upon formation of a stable complex, 4.Cu²⁺ and deprotonation of -OH functions, respectively. Job's plot analysis revealed a 1:1 (Cu²⁺) and 1:2 (CN^-) stoichiometry. Probe 4 displayed a naked eye-sensitive fluorescence response to detect both ions in solution with different rates of reaction and a limit of detection in the nanomolar range. The electrodynamic properties of the probe displayed redox behavior upon interaction with both ions. The variation in the ICT process was further assessed by the density functional theory (DFT) method. The probe has been utilized to detect the ions on test paper strips and in real water samples with a good recovery percentage. Also, probe 4 was successfully applied to estimate the Cu²⁺ ion concentration in natural grass extract solution. The molecular probe showed good applicability in on-site detection of both ions using a smartphone-based RGB analysis technique.