Tuning of the H-bonding ability of imidazole N-H towards the colorimetric sensing of fluoride and cyanide ions as their sodium salts in water

Imidazole functionalized receptors, 2-R-1H-naphtho[2,3-d]imidazole-4,9-dione (2a-h), containing naphthoquinone as a chromogenic signalling unit have been synthesized from the reaction of 2,3-diaminonaphthoquinone and different aldehydes. These receptors showed a color change upon addition of fluoride and cyanide ions in DMSO with a bathochromic shift of the characteristic intramolecular charge transfer (ICT) transition band. No color change was observed upon addition of other anions such as Cl-, Br-, I-, NO3-, AcO- and H2PO4-. H-1 NMR and electrochemical studies revealed that these receptors sense fluoride and cyanide ion via the formation of H-bond with the imidazole N-H moiety. Electronic and spectrofluorimetric studies indicated that the binding constants of these receptors with F- and CN- ions were in the order of similar to 10(6). The results of the spectral studies indicated that, by changing the R group in the receptor, the acidity of the imidazole N-H can be varied from delta(H) 13.70 (for isopropyl) to 14.94 ppm (for thiophene). Theoretical calculations based on Density Functional Theory showed that the HOMO-LUMO energy gap for the ICT transition corroborate the results of the spectral studies. Receptor 2f (R = thiophene) was also able to detect fluoride and cyanide ions as their sodium salts in aqueous solution with a visual color change.