Colorimetric Naphthalene-Based Thiosemicarbazide Anion Chemosensors with an Internal Charge Transfer Mechanism

A series of thiosemicarbazide anion chemosensors substituted with naphthalene and 4-nitrophenyl or phenyl units were synthesized. The molecules were characterized by using H-1,C-13 DEPT and N-15 NMR spectroscopy. The anion binding properties of compounds 1-4 were investigated by UV/Vis absorption and fluorescence spectroscopy in DMSO, DMSO/H2O (9:1 v/v), and acetonitrile with hydroxide, fluoride, acetate, hypophosphate, and chloride anions. Striking color changes were observed with nitro-containing chemosensors 2-4 with all anions, with the exception of chloride, which was attributed to an internal charge-transfer mechanism. H-1/C-13/N-15 NMR titration studies and DFT and charge distribution calculations support a mechanism involving deprotonation of the central N-H bond. Compound 4 with a methylene spacer between the naphthalene and thiosemicarbazide moieties was deliberately designed as a fluorescent photoinduced electron-transfer anion (PET) sensor. However, the methylene spacer did not prevent delocalization of the HOMO orbital between the planar aromatic ring and the thiosemicarbazide unit as expected on the basis of the fluorophore-spacer-receptor PET model.