Colorimetric detection of fluoride ions by anthraimidazoledione based sensors in the presence of Cu(II) ions

Anthraquinone based anion receptors have gained importance due to their colorimetric response on sensing a specific anion and the possibility of tuning this property by varying the conjugated moiety (the donor) to the diamine. In this work, we have synthesized and characterized four anthraimidazoledione compounds having 2,5-dihydroxy benzene, 4-(bis(2-chloroethyl) amino) benzene, imidazole and 4-methylthiazole moieties respectively (1-4). All of them were probed for their potential as anion sensors to study the effect of changes in the hydrogen bond donor-acceptor. The p-hydroquinone bound anthraimidazoledione (1) and thioimidazole bound anthraimidazoledione (4) were able to detect both F- and CN- in the presence of other anions Cl-, Br-, I-, H2PO4-, OAc-, NO3- and ClO4-. Both 1 and 4 could not differentiate F- from CN- and provided a similar response to both. The H-1 NMR studies of 1 and 4 with F-, showed the formation of [HF2](-)at 16.3 ppm and the F-19 NMR showed a sharp peak at -145 ppm in both cases. However, although there may be NMR evidence of [HF2](-) formation F- may not be detected colorimetrically if the CT band remains almost unchanged, as found for 3. The results emphasize that the change of a hetero atom in the donor moiety of an anthraimidazoledione may render a large difference in sensitivity. In the case of 4 selective detection of F- was possible in the presence of 0.5 equivalent of Cu2+ with the exhibition of a distinct green colour with a Delta lambda shift of ca. 50 nm in contrast to CN- which showed orange colouration with a Delta lambda shift of only 15 nm. In the presence of Cu2+ the F- detection limit was 0.038(5) ppm (below the WHO specified level) at a receptor concentration of 25 mu M.