A rare salamo-salophen type “on-off-on” fluorescent probe for relay recognition of Hg2+ and phosphate ions and its applications

A rare salamo-salophen type fluorescent probe SS with ICT effect was designed and synthesized, in which a salophen moiety with enlarged conjugation surface and a flexible long-chain salamo moiety were introduced into the probe SS molecule in order to make it strongly fluorescent and selectively recognize metal ions with larger radii. The fluorescence test showed that the probe SS has a bright blue fluorescence in DMF/H2O (v/v, 9:1) solution, and it is a relay fluorescent probe that recognizes Hg2+ and phosphate ions sequentially by switching ``on-off-on''. The introduction of a salophen moiety with an enlarged conjugation surface and a flexible long-chain salamo moiety in the design of this probe results in excellent fluorescence properties. Due to its relay recognition property, it has the advantages of simple operation and reduced workload for probe synthesis as well as versatility compared with single recognition probes. Probe SS showed highly selective and sensitive recognition of Hg2+ in DMF/H2O (v/v, 9:1) solution, and the identification mechanism was established through the study optical properties and high resolution mass spectrometry (HRMS), and verified further by DFT theoretical calculations. The SS-Hg2+ complex was also highly selective and sensitive to PO43−, with the lowest detection limits of the probe SS to Hg2+ and SS-Hg2+ complex to PO43− being 5.70 × 10−9 M and 2.44 × 10−8 M, respectively. Additionally, the SS probe was used successfully for the detection of real water samples, which is potentially applicable in the determination of Hg2+ and phosphate ions in the environment.