Colorimetric sensing of cyanide anion in aqueous media with a fluorescein–spiropyran conjugate

A fluorescein–spiropyran conjugate (2) behaves as a receptor for colorimetric sensing of cyanide anion (CN−) in aqueous media under irradiation of UV light. The compound 2, which exists as a spirocycle-closed (SP) form in the dark condition, is isomerized to the spirocycle-opened merocyanine (MC) form upon irradiation of UV light and shows absorption bands at 467 and 568 nm. Addition of CN− to the solution leads to a decrease in these bands and an appearance of new absorption band at 512 nm, via a nucleophilic interaction between CN− and the spirocarbon of MC form. This absorption change occurs selectively with CN− and enables ratiometric quantification of CN− by absorption analysis.