Dipicrylamine as a colorimetric sensor for anions: experimental and computational studyElectronic supplementary information (ESI) available: Fig. S1-S10 (UV-Vis, mass, NMR and computationally optimized structures). See DOI: 10.1039/c4ra09099c

Dipicrylamine (2,4,6-2′,4′,6′-hexanitrodiphenylamine, DPA) has been used for the detection and extraction of metal ions, especially potassium; however, its capability as an anion sensor has not been reported to date. It contains a secondary amine (N-H), the proton of which can form H-bonds with anions. This property has been exploited to investigate the capability of DPA as an anion sensor. Out of the large number of anions used in this study, F − , OAc − , and H 2 PO 4 − exhibited strong interactions with sharp colour changes in acetonitrile. The DPA-anion recognition event was monitored by UV-vis, NMR and ESMS studies, apart from distinct colour changes detectable by the bare eye. A detailed investigation revealed that the anions first interact with the N-H proton through H-bonding and then deprotonation takes place forming a DPA − -TBA + (tetrabutylammonium) complex. The rate constants of these complex formation have been determined from time dependent UV-vis spectral change and the order of the observed rate constants is F − > OAc − > H 2 PO 4 − . For F − , the NMR and ESMS data indicated the interaction of F − with one of the carbons or its attached proton in one of the benzene rings. A computational study suggests that the F − ion binds with one of the phenyl carbons instead of the -CH hydrogen bond of DPA − . Dipicrylamine exhibited colorimetric sensing of F − , OAc − and H 2 PO 4 − , detectable by bared-eye, out of a large number of anions. Interestingly, F − binds with one of the phenyl carbon of dipicrylamine.