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

[GRAPHICS] Phenyl-1H-anthra[1,2-d]imidazole-6,11-dione (1) and its derivatives (2 and 3) have been investigated 14 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 H-1 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 [(LHF)-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.