Fluorescence ON/OFF switching Zn2+ sensor based on pyridine–pyridone scaffold

We report a novel series of fluorescent Zn2+ sensors based on a pyridine–pyridone core structure, which functions as both the chelating part for Zn2+ and as the fluorophore. Pyridine–pyridone derivatives obtained by replacement of an electron-donating group with an electronic-withdrawing group at the 3-position of the pyridone ring showed almost no background fluorescence because electron transfer to the fluorophore was inhibited. However, these derivatives showed strong fluorescence with Zn2+. Coordination with Zn2+ greatly influenced the electron transfer of the fluorophore, and, as a result, enabled fluorescence ON/OFF switching. In these sensors, 4-[4-(methylsulfanyl)-2-oxo-6-(pyridin-2-yl)-1,2-dihydropyridin-3-yl]benzoic acid (5) displayed excellent properties: small molecular weight (MW = 338), good water solubility, large Stokes shift (132 nm), and fluorescence ON/OFF switching with Zn2+. In addition, fluorescence images of Zn2+ with 5 in living cells show that the new fluorescent sensor is useful for biological applications.