A selective hydrolytic and restructuring approach through a Schiff base design on a coumarin platform for "turn-on" fluorogenic sensing of Zn2+

A new Schiff base, CMD, designed based on a coumarin platform was synthesized and fully characterized through single crystal X-ray diffraction studies. CMD underwent selective Zn2+-triggered hydrolysis in ethanolic medium followed by restructuring of its fragments, resulting in a "turn-on" green fluorogenic response. This response was confirmed through various physico-chemical measurements along with single crystal X-ray diffraction studies. This selective hydrolytic fluorogenic event was exploited for the successful optical detection and live cell imaging of Zn2+ in SiHa cells. The above restructured products were characterized as two new Schiff bases, viz. CM and NSA, of which NSA was highly fluorescent (green). Hence, the formation of this green fluorogenic product accounted for the above fluorogenic "turn-on" sensing of Zn2+ with a sub-nanomolar detection limit. Spectroscopic evidence along with mass determinations indicated that the Zn-CMD ensemble took the form of CM-Zn-CM in solution, supporting our above proposal of hydrolysis and restructuring. However, the X-ray diffraction studies of the Zn-CMD ensemble further revealed it to consist of NSA and CM-Zn-CM', where CM' is yet another new Schiff base formed in situ during the process of developing single crystals.