The position effect of nitro group on the anion sensing performance of benzimidazole–naphthalimide derivatives

Six benzimidazole–naphthalimide naked-eye probes containing nitrobenzoquinone have been designed and synthesized to study the positional effect of –NO 2 group on their anion recognition abilities in DMSO. The push–pull character of –NO 2 rendered the anion-binding site hydrazinium –NH more acidic and thus easier for hydrogen bonds or deprotonation. So disubstituted isomer responded to F − , CN − , AcO − and H 2 PO 4 − ; para-substituted isomer responded toward F − and CN − ; and ortho-substituted isomer responded to F − due to the forming of intramolecular hydrogen bond between oxygen in nitro and hydrogen in hydrazine. The ortho-substituted isomer demonstrated highly sensitive and selective detection toward F − with a distinct color change from orange to blue-green accompanied by UV–Vis absorption redshift and fluorescence turn-on; the detection limits were in the range of 7 × 10 −8 mol/L. Theoretical calculations and TBAOH titration confirmed the recognition mechanism was deprotonation process.