Selective recognition of fluoride by using a benzobisimidazolium derivative through aggregation-induced fluorescence

A new benzobisimidazolium derivative (1) bearing four naphthalene moieties was synthesized and demonstrated as an F^– ion-selective fluorescent chemosensor. The fluorescence of 1 in acetonitrile (CH₃CN) is significantly stronger with F^– and acetate (CH₃CO₂^–), but not with other anions (Cl^–, Br^–, I^–, HSO₄^–, and H₂PO₄^–). The fluorescence of 1 is enhanced selectively with F^– in the presence of a small amount of water. Our DFT calculations indicate that the electrostatic interactions between the positively charged benzobisimidazolium moieties and F^– play an important role in the formation of stable aggregates. The formation of stable aggregates of 1 with F^– in CH₃CN is a key step for the selective sensing of F^–, and the fluorescence of the aggregates is further enhanced in a mixture of 95% CH₃CN and 5% water, which can be attributed to the aggregation-induced emission.