Simple bisthiocarbonohydrazones as sensitive, selective, colorimetric, and switch-on fluorescent chemosensors for fluoride anions

Bisthiocarbonohydrazones are found to be a class of sensitive, selective, ratiometric, and colorimetric chemosensors for anions such as fluoride (F-) or acetate (Ac-). The sensitivities, or the binding constants of the sensors with anions, were found to be strongly dependent on the substituents appended on the pi-conjugation framework, the delocalization bridge CH-N the aromatic moiety, and the hetero atom in the C=X group (X=O, S) of the sensors. Single-crystal structures and H-1 NMR titration analysis shows that the -CH=N- moiety is a hydrogen-bond donor, and it is proposed that an additional CH center dot center dot center dot F hydrogen bond is formed for the sensors in the presence F-. A sensor bearing anthracenyl groups is demonstrated as a switch-on fluorescent chemosensor for F- and Ac-. The recognition of F- in acetonitrile (MeCN) by a sensor with nitrophenyl substituents is tolerant to MeOH (MeCN/MeOH=10:1, v/v) and water (MeCN/H2O=30:1, v/v); at these solvent ratios the absorption intensity of the sensor-F- complex solution at maximal absorption wavelength was attenuated to half of the original value in pure MeCN.