Selective fluorescent PET sensing of fluoride (F−) using naphthalimide–thiourea and –urea conjugates

The thiourea and urea functionalised 4-amino-1,8-naphthalimide sensors 1–3, based on the fluorophore–spacer–receptor principle, were synthesised in high yield in three steps. The sensors were shown to signal selectively the detection of fluoride in the fluorescence emission spectrum in DMSO. On all occasions the emission was quenched due to enhanced photoinduced electron transfer quenching (PET) from the receptor to the excited state of the fluorophore upon recognition of F−, particularly for the thiourea sensors 1 and 2. In comparison, the changes in the absorption spectra were minor for all three, even after the addition of 80–100 equiv of F−. The sensing of acetate or dihydrogenphosphate gave rise to only ∼5–20% quenching.