Colorimetric and ratiometric fluorescence sensing of fluoride: Tuning selectivity in proton transfer

Phenyl-1H-anthra[1,2-d]imidazole-6,11-dione (1) and its derivatives (2 and 3) have been investigated as new colorimetric and ratiometric fluorescent chemosensors for fluoride. Acute spectral responses of 1 and 3 to fluoride in acetonitrile have been observed: an approximately 100 nm red shift in absorption and fluorescence emission and a very large ratiometric fluorescent response (R_max/R_min is 88 for sensor 1 and 548 for sensor 3). From the changes in the absorption, fluorescence, and ¹H NMR titration spectra, proton-transfer mechanisms have been deduced. In ground states, a two-step process has been observed: first, the formation of the sensor-fluoride hydrogen-bond complex [LH⋯F]^- and then the fluoride-induced deprotonation of the complex to form L^- and FHF^-. In excited states, the excited-state intermolecular proton-transfer made a contribution to the deprotonation. The selectivity for F^- can be tuned by electron push-pull properties of the substituents on the phenyl para position of the sensors. Sensor 1 shows the best selectivity. The excellent selectivity of 1 for F^- is attributed to the fitness in the acidity of its NH-group, which is tuned to be able to distinguish the subtle difference in the affinity of F^-, CH₃CO₂^-, and H₂PO₄^- to proton.