Colorimetric fluorescence probe detecting Hg2+ and OCl– based on intramolecular charge transfer and excited-state intramolecular proton transfer mechanisms

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The recognition and detection of Hg2+ and OCl– have been subjected to extensively attention, since these ions can cause extremely adverse effects on living body. Recently, a novel fluorescence probe molecule simultaneously detecting Hg2+ and OCl– ions had been researched in experiment, while a carefully molecular-level theoretical investigation was lacking. In this work, the photochemical and photophysical processes in excited state were researched for title species, the corresponding sensing mechanism was reported detailedly for the first time. The computational data of absorption and emission spectra agreed well with experimental results. The calculated hydrogen bond parameters and the visualized reduced density gradient isosurfaces indicated that the strengthening hydrogen bond in excited state facilitated the occurrence of the excited-state intramolecular proton transfer (ESIPT) reactions. The fluorescence signal that detected Hg2+ ion was also reassigned in our research, which originated from photoisomeric structure S-P instead of enol form S-N. The ESIPT mechanism was quantitatively explained by calculating the reactive Gibbs free energies. Finally, the frontier molecular orbitals and overlap between hole and electron analyses confirmed that the interaction between probe molecule and analytes impeded the excited-state intramolecular charge transfer (ESICT) behavior. Essential comprehending for ESICT and ESIPT properties were helpful in illuminating the probing mechanism, and to design an excellent fluorescence probe molecule.