Recognition Preference of Rhodamine-Thiospirolactams for Mercury(II) in Aqueous Solution

This work presents the design, syntheses, photophysical properties and Hg2+-binding of the red-emitting rhodamine derivatives RS1, RS2, and RS3 with different coordination ability and different spatial effects that derived from rhodamine thiohydrazone chromophores and respective carboxaldehydes (benzaldehyde, pyridine-2-carboxaldehyde, ferrocenecarboxaldehyde). Chemosensors RS2 and RS3 afford turn-on fluorescence enhancement and display high brightness in water with the EC50 for Hg2+ of 0.5 ppb. The fluorescence intensities are nearly proportional to the amount of Hg2+ at ppb level, when employing 100 nM probes in water. The fluorescence responses of these two chemosensors are Hg(II) specific, and the probes are selective for Hg(II) over alkali, alkaline earth metals, divalent first-row transition metal ions, and Group 12 congeners Zn(II) and Cd(II), as well as heavy metals Pb(II) and Ag(I). X-ray crystal structure analyses exhibit the thioether derivative of the spirolactone in these compounds. Hg-II-specific binding in water would make the opening of the spirolactam ring and consequently causes the appearance of strong absorption at visible range, and the obvious and characteristic color change from colorless to pink. Compared to the thioamides, the improved selectivity for Hg2+ is attributed to the poorer coordination affinity of the thioether over other interference metal ions.