| Summary: | A series of fluorescent coumarin derivatives <b>2a</b>−<b>e</b> were systematically designed, synthesized and studied for their Cu<sup>2+</sup> sensing performance in aqueous media. The sensitivities and selectivities of the on-to-off fluorescent Cu<sup>2+</sup> sensing signal were in direct correlation with the relative arrangements of the heteroatoms within the coordinating moieties of these coumarins. Probes <b>2b</b> and <b>2d</b> exhibited Cu<sup>2+</sup> concentration dependent and selective fluorescence quenching, with linear ranges of 0−80 μM and 0−10 μM, and limits of detection of 0.14 μM and 0.38 μM, respectively. Structural changes of <b>2b</b> upon Cu<sup>2+</sup> coordination were followed by fluorescence titration, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), mass spectrometry, and single crystal X-ray diffraction on the isolated Cu<sup>2+</sup>-coumarin complex. The results revealed a 1:1 stoichiometry between <b>2b</b> and Cu<sup>2+</sup>, and that the essential structural features for Cu<sup>2+</sup>-selective coordination are the coumarin C=O and a three-bond distance between the amide NH and heterocyclic N. Probe <b>2b</b> was also used to determine copper (II) levels in aqueous soil extracts, with recovery rates over 80% when compared to the standard soil analysis method: inductively coupled plasma-mass spectrometry (ICP-MS).
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