Colorimetric and On-Off fluorescent chemosensor for fluoride ion based on diketopyrrolopyrrole

Four colorimetric and On-Off fluorescence sensors based on diketopyrrolopyrrole (DPP) were synthesized for detection of fluoride anion. The anion binding studies indicated that sensors 1-4 showed remarkable selectivity and specificity toward F^- ion over other anions. When binding with F^-, the sensors of thiourea derivatives 1 and 2 showed a bathochromic shift of 78 nm in UV-vis spectra, and the urea derivatives 3 and 4 showed a bathochromic shift of 118 nm. Addition of F^- ion to sensors 1-4 resulted in obvious quenching of fluorescence. The experimental results revealed that the formation of anionic species from the deprotonation of thiourea or urea moiety by the fluoride ion was responsible for the spectral changes. Calculations of the transition energies also verified such process. The spectral responses of sensors could be switched back and forth alternatively by adding F^- and HSO₄^- anions into the DMSO solution, which indicated that sensors 1-4 were stable and could be used in reversible manner.