Luminescence anion sensing via modulation of MLCT emission from a naphthalimide–Ru(II)–polypyridyl complex

The selective sensing of fluoride (F−; an important breakdown product of some chemical warfare agents such as sarin) was achieved by observing quenching in the metal to ligand charge transfer (MLCT) emission of the 1,8-naphthalimide–ruthenium conjugate Ru–Nap-NH2, which occurs at long wavelengths in CH3CN, using steady state fluorescence spectroscopy. The F− recognition was also visible to the naked eye, with a clear colour change from yellow to red. The sensing mechanism is most likely initially via hydrogen bonding, between the anion and the amine, which, at higher concentrations, gives rise to deprotonation of the acidic 4-amino-1,8-naphthalimide moiety. However, counter ion effects may also be contributing to the overall emission changes. Other anions such as acetate, phosphate and chloride also give rise to quenching in the fluorescence emission with only minor changes in the UV–vis absorption spectra of Ru–Nap-NH2. Moreover, phosphate also gave rise to shifts in the λmax in the emission spectra.