Construction of NIR and ratiometric fluorescent probe for Hg2+ based on a rhodamine-inspired dye platform

A rhodamine-inspired fluorescence dye (2) bearing 7-diethylaminocoumarin fluorophore was designed as a platform for the construction of an NIR and ratiometric fluorescent probe. The ring-open form of 2 shows NIR absorption and emission; however, its ring-closed form displays the visible absorption and emission because an intact 7-diethylaminocoumarin fluorophore was involved in the structure, providing the basis for an NIR and ratiometric fluorescent platform based on the spirocyclization-induced fluorescence switching mechanism. With the platform, we developed a novel NIR and ratiometric fluorescent probe R1, a thiolactone of 2, for sensing Hg2+, an environmentally and biologically concerned species. R1 displays an emission peak with the maximum at 480 nm, which is the typical emission of 7-diethylaminocoumarin moiety; however, upon addition of Hg2+ ions, the emission intensity at 480 nm gradually decreased with the simultaneous appearance of a new NIR emission band centred at 695 nm. Thus, the Hg2+-promoted ratiometric fluorescence response can be realized. The high selectivity towards Hg2+ over various cations and the high stability in a wide pH range of 1-12 indicate its potential for applications in biological systems. The subsequent cell imaging experiment revealed that R1 is cell permeable, and could be employed for ratiometric fluorescence imaging of Hg2+ in living cells.