Anion-induced ring-opening of fluorescein spirolactam: fluorescent OFF–ON

N-(2,7-Dichlorofluorescein)lactam-N′-phenylthiourea (L) was developed as a colorimetric and fluorescent chemosensor for anions such as AcO−, F− and H 2 PO 4 - . The addition of AcO− anion to the solution of L in CH3CN results in a distinct fluorescence “ON” observation as well as color change (from colorless to pink). The H-bonding interaction between AcO− anion and thiourea moiety of L induced the ring-opening of the spirolactam of fluorescein moiety, which gave rise to the dual chromo- and fluorogenic changes.