Multi-channel sensing of trivalent metal ions using a simple ferrocenyl Schiff base probe with AIE property

A simple ferrocene-based Schiff base (probe L) exhibiting aggregation-induced emission (AIE) effect has been synthesized and investigated. The probe L showed selective detection for trivalent metal ions (Al3+, Cr3+and Fe3+) over a range of monovalent and divalent metal ions in methanol via multi-channel of detection using colorimetry, fluorescence and electrochemistry. Upon binding with trivalent metal ions, color change from faint yellow to pink for Al3+, Cr3+and Fe3+ was observed by the naked-eye, which was consistent with the change of UV–Vis absorption spectra. In addition, probe L showed a dramatic off-on fluorescence emission along with a large Stocks-shift of about 33 nm in the emission wavelength. The fluorescence enhancement with a blue shift may be attributed to the combined effect of rotation inhibition and an extended intramolecular charge transfer (ICT) mechanism. Moreover, electrochemical detection demonstrated that the oxidation potential difference between probe L and probe l-metal complexes was 0.028 V (Al3+), 0.044 V (Cr3+) and 0.015 V (Fe3+), respectively, indicating that trivalent metal ions induced charge transfer from the ligand to the metal (LMCT) and reduced the electron density of ferrocene moiety. Therefore, Al3+, Cr3+and Fe3+ can be selectively detected using multi-channel of detection techniques. Finally, 1H NMR titration study confirmed that two molecules of probe L bind with one metal ion through the nitrogen atom and oxygen atom of the probe L.