Unexpected highly selective fluorescence 'turn-on' and ratiometric detection of Hg2+ based on fluorescein platform

Methylated fluorescein 1 was explored for fluorescence 'turn-on' and ratiometric detection of Hg2+ in THF and CH2Cl2/MeOH (v/v=9:1), respectively, with unexpected high selectivity. In the presence of Hg2+, characteristic structured absorption band of 1 diminished and a new sharp band appeared at 445 nm. Meanwhile a blue shifted and enhanced emission was observed. The ratio of the fluorescence intensity at 559 and 478 nm increased linearly with [Hg2+], and solution color changing from yellow to cyan under irradiation at 365 nm in CH2Cl2/MeOH. Job plot indicated a 1:1 stoichiometry for 1-Hg2+ complex in solution. 1H NMR titration and IR spectra suggested the coordination of carbonyl group in xanthene moiety to Hg2+, affording its spectral behavior. Compound 2 bearing two triazolyl amino esters in place of methyl group showed quite similar behavior to Hg2+, which indicated that substituents did not interfere with the specific binding behavior of fluorescein platform. Our work presents a new way to explore xanthene dyes as new chemosensors by modulating electron density on the xanthene ring through non-covalent interactions with carbonyl group.