A New “Switch-On” Fluorescence Chemosensor for Anions via Modulation of Intraligand and Metal-to-Ligand Charge-Transfer Emission in a Pd(II)-based Receptor

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In recent years there has been great interest in anion recognition and sensing due to the key roles played by anions during chemical, biological and environmental processes [1–13]. Fluorescence chemosensors are used to transduce binding events into a fluorescent signal, and many signaling mechanisms for the sensing of anions have been developed, such as photoinduced electron transfer (PET) [14–17], intramolecular charge transfer (ICT) [18, 19], excited-state proton transfer [20, 21], excimer/exciplex formation [22–24], competitive binding [25–27]and metalto-ligand charge-transfer (MLCT) [28, 29]. The development of simple and sensitive anion sensors continues to be an important field of research. When a fluorescent ligand is complexed to a metal ion, its fluorescence can be either quenched or enhanced by different metal ions [30]. In general, fluorescence quenching occurs with heavy metal ion complexation due to intramolecular energy transfer from the excited state of the ligand to the localized energy levels of the metal ion, or due to relaxation mechanisms of the heavy metal “paramagnetic effect” for intersystem crossing [31, 32]. Therefore, the fluorescence of intraligand (IL) transitions (between orbitals localized primarily on a coordinated ligand) is quenched by heavy metal ions and fluorescence due to metal-to-ligand charge-transfer (MLCT) transitions is not preferred. Here we describe a new fluorescence-enhanced anion sensor mechanism based on the modulation of IL and MLCT fluorescence emission from a fluorescent metal complex in the presence of certain anions. The syntheses of bis(1-benzylidene-4-phenylthiosemicarbazato)-palladium(II) (1) and bis(1-(4-(dimethylamino)benzylidene)-4-phenylthiosemicarbazato)palladium (II) (2, Scheme 1) are described, as well as their application as metal-based anion receptors. Complex 2 exhibits intraligand π–π* (IL) and MLCT dual fluorescence emissions in organic solvents at room temperature that can be modulated in the presence of anions. Complex 2 was found to act as an anion-triggered IL and MLCT emission enhanced chemosensor.