Fluorescent anion sensors based on 4-amino-1,8-naphthalimide that employ the 4-amino N-H

The new charge neutral 4-amino-1,8-naphthalimide based anion sensors 2 and 3 bind to both acetate and dihydrogenphosphate with 1:1 stoichiometry through hydrogen bonding to both thiourea N-H atoms and in the case of dihydrogenphosphate, the naphthalimide 4 amino N-H group as well. This was clearly established from 1H NMR titration experiments with H2PO4- in DMSO-d6 where a substantial shift in the resonance for the naphthalimide N-H was observed concomitant with the expected migration of the thiourea N-H chemical shifts. The binding constants determined from the titration studies indicate that the new sensors bind H2PO4- more strongly than AcO−. Fluorescence titrations with sensor 3 indicate quenching of 59% and 36% upon addition of acetate and dihydrogenphosphate, respectively.