New Six-Membered pH-Insensitive Rhodamine Spirocycle in Selective Sensing of Cu2+ through C–C Bond Cleavage and Its Application in Cell Imaging

A new rhodamine-based chemosensor 1 with a six-membered spirocyclic ring has been synthesized, which exhibits excellent pH stability and shows selective “turn-on” fluorescent detection of Cu2+ ions over a series of other metal ions including Cu+ ions. The expansion of spirocycle improves the stability and selectivity of the chemosensors in sensing of metal ions. Till today only few rhodamine structures R1–R5 with thiourea-, hydrazine amide-, or pyrrole-decorated six-membered spirocyclic rings are known that exhibit metal-ion sensing via C–N bond cleavage of the spiro ring. In this context, rhodamine compound that responds to the metal ion through C–C bond cleavage of the six-membered spiro ring is completely unknown. The present example is a first-time report that demonstrates selective sensing of Cu2+ ions through C–C bond cleavage over the conventional existing systems in the literature. The chemosensor 1 is cell permeable and can detect Cu2+ in live cells using confocal microscopy in the biologically relevant pH range with high photostability.