Synthesis and photophysical properties of water-soluble sulfonato-Salen-type Schiff bases and their applications of fluorescence sensors for Cu2+ in water and living cells

A series of water-soluble sulfonato-Salen-type ligands derived from different diamines including 1,2-ethylenediamine (Et-1–Et-4), 1,2-cyclohexanediamine (Cy-1 and Cy-2), 1,2-phenylenediamine (Ph-1–Ph-3 and PhMe-1–PhMe-4), and dicyano-1,2-ethenediamine (CN-1) has been designed and prepared. Sulfonate groups of ligands ensure good stability and solubility in water without affecting their excited state properties. These ligands exhibit strong UV/Vis-absorption and blue, green, or orange fluorescence. Time-dependent-density functional theory calculations have been undertaken to reveal the influence of ligand nature, especially sulfonate groups, on the frontier molecular orbitals. Since their fluorescence is selectively quenched by Cu2+, the sulfonato-Salen-type ligands can be used as highly selective and sensitive turn-off fluorescence sensors for the detection of Cu2+ in water and fluorescence imaging in living cells.

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► Sulfonate groups ensure good stability and solubility in water.
► Sulfonate groups have little effect on the photophysical properties.
► This is confirmed by the TD-DFT calculations and experimental results.
► The strong blue, green, or orange fluorescence is selectively quenched by Cu2+.
► The ligands are sensitive fluorescence sensors for Cu2+ in water and living cells.